Synthesis, Functional Modifications, and Diversified Applications of

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Review

Synthesis, functional modifications and diversified applications of molybdenum oxides micro-/nanocrystals: A review Haoqi Ren, Shaodong Sun, Jie Cui, and Xifei Li Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00894 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on September 1, 2018

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Crystal Growth & Design

Synthesis, functional modifications and diversified applications of molybdenum oxides micro/nanocrystals: A review Haoqi Ren,a Shaodong Sun,a* Jie Cuia and Xifei Lib* a

Shaanxi Province Key Laboratory for Electrical Materials and Infiltration Technology, School

of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, ShaanXi, People’s Republic of China. b

Institute of Advanced Electrochemical Energy, Xi’an University of Technology, Xi’an 710048,

ShaanXi, People’s Republic of China. KEYWORDS: Molybdenum oxides; synthesis; modifications; applications

1

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

As a significant n-type semiconductor, MoOx (2 ≤ x ≤ 3) micro-/nanostructures with well-tuned shapes, sizes, crystalline phases and compositions have attracted great attention in the past decades due to remarkable performances in numerous fields of photodevices, energy storage and conversion, gas sensing and catalysts. Additionally, the cheapness, non-toxic and eco-friendly natures as well as semiconducting properties also endow them with competitive advantages for their practical applications. However, it is still lacking of a comprehensive review on morphological MoOx micro-/nanostructures so far. Hence, it is necessary to thoroughly summarize the recent advances made in function-oriented MoOx-based architectures. In the review, we have highlighted the progress in diversified MoOx micro-/nanostructures, including the general synthetic strategies for the synthesis of 0D, 1D, 2D and 3D MoOx micro/nanostructures, the modification (such as doping and hybridization) of MoOx-based composites for enhanced performances in diversified applications (including photodetectors, photothermal therapy, SERS, supercapacitors, ion batteries, solar cells, gas sensors, multi-phase catalysts, photodegradation, HER and chemical template). Finally, we briefly summarize the present issues and share promising research perspectives emerging from the fascinating MoOx materials.

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1. Introduction

Environmental and energy problems have become more serious since 20th century, thus scientists put more attention on new materials which can help to alleviate or solve those problems through developing innovative clean energy or the treatment for pollution.1−10 Transitional metal oxides usually have a good performance on energy storage, photocatalysis or fuel cells.11−20 Molybdenum oxide (MoOx, 2 ≤ x ≤ 3), a kind of metal oxide with n-type semiconducting and nontoxic nature, has attracted a lot of attention for its diverse functions.21,22 The performance of MoOx material mainly depends on its compositions and structures. For example, the change in the stoichiometric ratio of the molybdenum oxide from 1:3 to 1:2 induces quite different physicochemical properties. As for the optical property, MoO3 usually seems to be white color, while MoO2 presents dark blue or even black color, allowing a different light absorption.23 In fact, the oxygen vacancies in the crystal of MoOx result in a variation of the electrical conductivity as well as controllable band gap in a range of 2.8−3.6 eV.24 Moreover, the structural diversity of MoOx allows that it can be extensively applied into energy conversion and storage fields like lithium ion batteries or catalysts for hydrogen evolution reaction (HER).25,26 Moreover, MoOx can also be used in gas sensors and photodevices due to its localized surface plasmon resonance (LSPR) effect as well.27,28 Therefore, large efforts have been made to obtain the MoOx materials with appealing properties for the applications in the fields of catalysts, gas sensing, energy conversion, and photodevices by engineering their sizes, crystalline phases, morphologies and even the components.

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To accomplish controllable sizes or morphologies of MoOx, multiple synthetic strategies have been developed, including hydrothermal/solvothermal method, template assisted method, chemical vapor deposition (CVD) method and so on.29−32 To date, extensive research has been reported on the synthesis of morphological MoOx in multi-dimensions such as zero-dimensional (0D) quantum dots (QDs),33−36 one-dimensional (1D) structure such as nanorods and nanotubes,37−71 two-dimensional (2D) nanosheets,72−85 three-dimensional (3D) architectures.86−95 To a large extent, different geometric configuration of MoOx materials always determines their applications. For example, 0D MoOx QDs usually have strong LSPR effects due to the quantum confinement effect, which can be used for photodevices33,36. 1D MoOx nanorods often keep porous structures that will benefit the charge/mass migration, leading to an enhanced performance in energy storage devices. Additionally, hollow structures are regarded as one of the most promising architectures for drug delivery, ion batteries and gas sensors, because of their large specific surface area, low density, and superior transport.4,5,94 In short, the properties will decide the performances through engineering the structures by various strategies. Therefore, the development of novel micro-/nanostructured MoOx with controllable composition, crystal facet and structural block as well as defect is still a hot research topic.

Furthermore, many strategies have been used to improve the performance of MoOx-based micro-/nanostructures by tailoring their interfacial and electronic structures through doping76,96−99 and hybridization.100−125 For instance, hydrogen-doping MoO3 usually exhibits a narrower band gap as well as lower resistance, which will be traits in photocatalysis or electrochemical reactions.76,96 Besides, it has been reported that the MoOx/noble metal 4


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nanocomposite can display a strong LSPR effect, which can enhance the sensitivity of photodetectors and the surface-enhanced Raman scattering (SERS). MoOx/carbon materials are also extensively employed into supercapacitors or lithium ion batteries, because they not only improve the capacitance of the device, but also keep a much better durability by relieving the deterioration of the micro-/nanostructures. Consequently, there is a tremendous need to develop efficient synthetic strategies for the construction of the desired composite architectures and make clear the synergetic mechanisms.

Though lots of literatures have been reported on MoOx micro-/nanostructures, a thorough overview on diverse MoOx micro-/nanostructures is still lacking so far. Hence, we will present a comprehensive review on the important advances in function-directed MoOx-based micro/nanostructures (see Figure 1). First, we give a brief introduction on the crystalline structures and fundamental electrical as well as optical properties of MoOx materials. Next, we mainly focus on the diverse synthetic strategies for the preparation of multi-dimensional (including 0D, 1D, 2D, and 3D) micro-/nanostructured MoOx materials with well-defined morphologies, sizes, crystalline phases, and compositions according to the reaction parameters. Then, the modifications (such as doping and hybridization) on MoOx-based composites for enhanced performances are discussed. Finally, we highlight the various applications of MoOx-based materials (including optical applications,126−133 energy storage and conversion devices,134−154 gas sensors,29,94,97,155−156

catalysts46,59,93,125,157−163

and

chemical

template

for

synthesizing

molybdenum sulfides or carbides164−168). Several current challenges and promising perspectives of MoOx-based materials that need to be addressed in future research are also presented. 5



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2. Intrinsic property

2.1 Crystal structure

MoOx with a small x value (2 ≤ x ≤ 3) exists in different crystalline phases ranging from MoO3 to MoO2. Pure MoO3 can involve in different polymorphs (see Figure 2), namely the thermodynamically stable orthorhombic phase (α-MoO3), the metastable monoclinic phase (βMoO3, similar to a ReO3-type structure), and the hexagonal phase (h-MoO3). 29,30,63,164,169,170 The stable α-MoO3 and metastable h-MoO3 phases are the commonly formed phases during crystallization. In fact, all the reported polymorphs are basically constructed in different ways based on the building block of MoO6.73 The arrangement of the MoO6 octahedra (formation of any phase) depends on the external conditions such as temperature, pressure and impurities.169,170 At high temperature, MoO6 octahedra favorably arrange in the α-MoO3 phase, in which MoO3 crystal has a layered structure with the orthorhombic symmetry (a = 3.963 Å, b = 13.86 Å, c = 3.696 Å) and rectangular architecture. In the crystal structure of α-MoO3, double-layers of MoO6 octahedra repeat in the ab plane along the b axis and arrange in an (ABA) manner to form a layered framework (see Figure 2a). Unlike graphite, boron nitride, metal hydroxides or other isotropic layered materials, α-MoO3 keeps a structural anisotropy, which allows the formation of special architecture such as single-wall nanotube.43,171 One of the most striking features of αMoO3 is that the oxygen coordination about the Mo atoms is asymmetric: the length of Mo−O in the MoO6 varies from 1.67 to 2.33 Å, which allows MoO3 to possess a double-layer planar structure and stack together as a result of van der Waals interactions.164,172,173 The layered 6


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structure leads to a relatively higher tolerance to nonstoichiometry such as the unusual pentavalent ion Mo5+, which exhibits high affinity for oxygen atoms.29

As for the β-MoO3, the MoO6 octahedral units share corner oxygen atoms in the direction of the c axis, and edge sharing occurs in the direction of the a axis (see Figure. 2b).39,174 The MoO6 octahedra in β-MoO3 crystal do not crystalize in double layers either or occur in zigzag rows along the [001] plane but construct a monolic structure.11 During the temperature ranging from 370 to 400 oC, a transformation from β to α phase will take place spontaneously.158,175 Besides, β-MoO3 has superior catalytic properties for some reactions when compared with α-MoO3, which will be discussed in later section.

In addition, the structure of h-MoO3 is composed of zigzag chains of MoO6 octahedra linked to each other by corner sharing along the c axis (see Figure 2c). Meanwhile, the h-MoO3 phase (shares corners and edges to form pillars of MoO6 octahedra) dominates in the presence of cations, such as H+, Na+, K+, or NH4+. It is apparent that between these octahedra, the extended tunnels can serve as conduits and intercalation sites for mobile ions. The tunnel (~3.0 Å in diameter) running along the c direction probably accommodates some cations or water molecules insides, which is enclosed by twelve MoO6 octahedra linked by sharing corners along the a and b axis as well as edges along the c axis.44 Hence, the phase can often be formulated as (A2O)x·MoO3·(H2O)y, where A represents an alkali-metal ion or ammonium ion and the exact values of x and y are decided by the preparations.39,176 Indeed, a hexagonal phase of MoO3 can normally be obtained when the concentration of the ammonium and water molecules vary in the 7



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range of 0.09 < x < 0.20 and 0.18 < y < 0.6. It has been reported that 1.6 Li+/Mo ions could be intercalated into the h-MoO3 nanostructures due to the good accommodation for alkali ions while only 1.5 Li+/Mo ions can be introduced into α-MoO3 in an electrochemical experiments.177 It is believed that large cations in the MoO3 phase can help stabilize the structure of h-MoO3 upon being incorporated into the tunnel, which is quite meaningful for the selective synthesis of hMoO3.44,178 It is worth being mentioned that the electronic structure of h-MoO3 can be described by using a semiconductor model with hydrogen as dopant. In such model, the MoO3 structure cell remains unaltered and defines the overall electronic structure, but the wide band gap is perturbed by new band gap states due to the intercalation of hydrogen atoms.179 Borgschulte et al. proposed that the HxMoO3, a form of MoO3 intercalated with hydrogen, would slowly decompose into MoO2 and H2O, which could be evidenced by the fast reduction of Mo6+ into Mo5+ states and slow but simultaneous formation of Mo4+ states. Furthermore, several reaction pathways during the hydrogen reduction can take place simultaneously and the preference of the pathway depends on the environmental conditions.126

Furthermore, MoO3 polymorphs can exist in several forms of crystalline hydrates. For instance, dihydrated MoO3 (MoO3·2H2O) is found to be monoclinic, monohydrated MoO3 (MoO3·H2O) exists in both triclinic (white appearance) and monoclinic (yellow appearance), and hemihydrated MoO3 (MoO3·0.5H2O) exists as monoclinic and orthorhombic phases.123,180,181 Additionally, MoO3 hydrates will transform into α-MoO3 when heated to 350−400 oC. However, it requires an additional annealing procedure to obtain high crystalline MoO3 at relatively high temperature.44 8


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Notably, MoO3 can be transformed into substoichiometric molybdenum oxides when H+ ions intercalate into the layered structure.182−185 The H+ ions can mainly bind to edge-shared oxygen and terminal oxygen atoms, resulting in the formation of the intermediate OH2 groups.184 Such groups are relatively unstable and will be released from the crystal lattice under light irradiation or reducing gas treatment, leading to the formation of oxygen vacancies.182,183 Generally, The family of substoichiometric molybdenum oxides includes MoOx with substoichiometric levels ( x ) in the range of 2 < x < 3, which can present various properties due to the different amount of oxygen vacancies. The substoichiometric molybdenum oxides are usually categorized into the series MonO3n−m+1 such as Mo17O47 and Mo18O52, possinging a metable stable structure based on the crystallographic shear of MoO3.11,17 Magnéli series are special series recognized as MonO3n−1 such as Mo4O11 and Mo8O23, which possess a ReO3 type caused by the collapse after futher reduction. In regard to MoO2, it also has three polymorphic forms, named as hexagonal phase, tetragonal phase and monolic phase.87,186,187 The MoO2 usually shows a monolic structure (see Figure 2d), while the hexagonal phase is quite unstable.109 The monolic lattice of MoO2 crystal is usually viewed as a deformed rutile structure with a = 5.537 Å, b = 4.859 Å, c = 5.607 Å. The rutile structure exhibits a tetragonal orientation, where the MoO6 octahedra chains share opposite edges along the c-axis. It’s similar to the structure of MoO3 that the Mo is surrounded by six oxygen atoms and in this monolic lattice each oxygen atom is surrounded by three metal atoms arranged at the corners of an equilateral triangle. 11 In the MoO2 crystal, Mo-Mo bonding exists

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between the atoms, which allows a high occupation of Mo 4d states per cation and lead to the unique electrical properties.17

2.2 Electrical and optical property

Among various transition metal oxides, MoOx with metallic conductivity (8.8 × 10−5 Ω·cm at 300 K for bulk MoO2 vs 5.4 × 10−6 Ω·cm for metallic Mo) and high theoretical capacity (838 mAh·g−1 for MoO2 and 1117 mAh·g−1 for MoO3) have received great attention because of the existence of oxygen vacancies.41,64,87,104 The main reason for the metallic conductivity of MoO2 is that the MoO6 octahedron units in MoO2 are partly linked with Mo–Mo bonds, which leads to the part of the Mo electrons being delocalized in the conduction band. Additionally, the oxygen vacancies allow the metal oxide to keep a lower resistance and more active sites in catalysis.76 The molybdenum oxides with low oxygen vacancies such as Mo9O26 and Mo8O23 exhibit semiconducting behavior, while other phases such as Mo4O11 are quasi-metallic and MoO2 is metallic.76,182,184

Moreover,

nanostructured

MoOx

possesses

special

physicochemical

characteristics such as low thermal dynamic stability, high specific surface area and high electrochemical activity, because of its diverse structures and availability of multiple oxidation states, which can be grown as single crystals exposed with certain crystal planes.86, 109

In addition, MoOx often exhibits different colors when it has different amount of oxygen vacancies. For example, MoO3 with low concentration of oxygen vacancies is in white color, while MoOx with high concentration of oxygen vacancies often presents blue or dark blue. A typical color evolution of MoOx (2 ≤ x ≤ 3) is given in Figure 3a.23 It can be explained as the 10


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crystal rearrangements of the surface Mo−O units during reduction process, resulting in a disordered surface with huge oxygen vacancies, leading to an enhanced visible light absorption. The LSPR wavelengths are usually related to the oxidized state of MoOx, in other words, the amount of oxygen vacancies (see Figure 3b and Figure 3c). The chromogenic response of MoO3 shows a stronger and more uniform absorption of visible light in its colored state and also exhibits a better open-circuit memory than most of transition-metal oxides.57 Indeed, MoO3 is more akin to the sensitivity in human visual perception, meaning that MoO3 possesses a significant color change efficiency.63 It is widely accepted that, in photochemically colored MoO3, the optical absorption is due to the intervalence charge transfer between Mo6+ and Mo5+.72,188,189 The layered structure of MoO3 guarantees a good band-gap manipulation for its accommodation for mass of positive ions as well. However, the photochromic properties of amorphous MoO3 films show faster response times for coloration than those of crystalline films. Thus, amorphous MoO3 with a partly layered structure shows better photochromic property than the crystalline one because of more defective sites available for charge transfer,40,41 which can be characterized by Raman spectrum. Raman peaks reveals that the three Mo−O stretching modes (singly, doubly, and triply coordinated oxygen) are observed to shift reversibly during the coloration-bleaching cycles (see Figure 3d).72 Moreover, the MoOx QDs exhibit special optical property due to the quantum effects, for instance, the emission peaks will have a red shift when increasing the excitation wavelength (see Figure 3e),36 which can be explained by the inhomogeneity of chemical components and polydispersity of the lateral sizes. Moreover, a dual absorption located in both visible and near infrared region (at 768 and 1023 nm, respectively)

11



can be observed in Figure 3f, due to a synergetic effect of quantum effects and presence of oxygen vacancies.36,184

3. Controllable synthesis of diversified MoOx micro-/nanostructures

3.1 Basic synthetic principle

3.1.1 Crystal growth theory.

Facet-dependent property of MoO3 catalyst is significant for uncovering their relationship between structure and performance. For example, it has been reported that formaldehyde formed mainly over the (010) plane on a flat-needle α-MoO3 in an oxidation process of methanol, while the selectivity to acrolein became higher on the (100) planes of α-MoO3 when oxidizing propylene.190,191 Hence, the morphology-controlled synthesis of MoOx significantly changed physicochemical properties through preferential appearance of specific crystal planes. Thermodynamically, the crystal growth is according with the fact that the system tends to possess a minimization of the total surface energy, which would affect the nucleation and growth manner.

The surface energy varies on different surfaces, which has the significant influence on the growth of nanocrystal and potential applications like catalysis. Agarwal and coworkers calculated the surface energies by creating slab and ribbon modeling.192 In such model, the ribbons, a typical 2D materials that extend to infinity in one direction, were exfoliated from the slabs. The energy of ribbon edge was different from that of the original slab surface (see Figure 12


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4). The chemical reactivity then could be evaluated by the formation of oxygen vacancies: better oxidant faces of a slab or the edge of a two-dimensional ribbon tended to form oxygen vacancies. After a series of calculation, they claimed that the properties of ribbon edges were different from those of the corresponding slab surfaces, to be more specific, the surface energies of slabs were in the order of (010) < (100) < (101) < (001), whereas the edge energies of ribbons were in the order (100) ≈ (101) < (001). Moreover, the (001) and (101) facets of slabs faces had the lowest oxygen-vacancy formation energies and the (010) facets of slabs had the highest ones, while the (101) facets of ribbons had the lowest vacancy formation energies. Also, among the structures studied, they found that the (101) edges of ribbons was the most reactive and the (010) surfaces of slabs was the least reactive. Hence, in order to design a MoO3 ribbon into a milder oxidant, the length of the (101) edge should be minimized.

Indeed, the morphology control can be achieved by the introduction of capping agents or templates in a liquid phase reaction. The species of capping agent, e.g. surfactant agents, polymer molecules and inorganic ions, will definitely influence the growth process of MoOx, thus leading to a selective exposure of specific crystal facets. Furthermore, the value of pH also has great influence on the morphology control and facets selectivity. For example, the growth rate of (010) facet was slower than that of the other facets with an additive of P123. As a consequence, the as-prepared MoO3 nanobelts may possess two (010) flat planes, two (100) side planes as well as two (001) end planes.69 Notably, when the concentration of P123 ranged from 0.025–0.1 g/mL, it would facilitate the growth rate of the (010) facet, and then the nanobelts were formed (see Figure 5a). However, with increasing the amount of P123, the MoO3 13



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assembled into microflowers (see Figure 5b). The possible function of P123 might act as a template, and the schematic illustration of the illustration is shown in Figure 5c. Firstly, P123 molecules aggregated into micelles which was absorbed on the surface of α-MoO3 nuclei and inhibited the growth of the (100) and (010) facets. Consequently, the P123 can decrease the width and the thickness of the nanobelts compared with the MoO3 synthesized without any additives. Furthermore, the controlling of pH value also becomes an effective method that can help adjust the ratio of facets and morphology. At a pH value of 1.59, bulk h-MoO3 rods and αMoO3 nanobelts were formed at the same time. While decreasing pH to 0.92–1.1, it was constructed into microflowers composed of smaller size of nanobelts caused by the disappearance of h-MoO3.

3.1.2 General synthesis and characterizations.

The

synthetic

approach

of

MoOx

nanomaterials

can

be

mainly

classified

into

hydrothermal/solvothermal route, vapor deposition, template-assisted method, sonochemical strategy and so on. In this part, we will briefly list the most common synthetic method of MoOx.

A facile hydrothermal/solvothermal method with advantages of low cost and high yield has been widely used in one-pot synthesis of 1D, 2D, 3D MoOx nanostructures. The method applying high pressure and temperature shows great advantage in synthesizing diversities of morphology via choosing solvent and surfactants. The presence of surfactant can help stabilize nanodots with tiny sizes, specific high-energy facets in 1D and 2D nanostructures as well as 3D structures with certain building blocks. Meanwhile, the pH and other parameters of the solution can also have 14


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giant influence on the morphology, which will be discussed in detail later. Moreover, vapor deposition, including chemical and physical vapor deposition, is one method that can be used to obtain MoOx films without any template or surfactant, and the depth of MoOx films with ultrathin nanosheet building blocks (< 10 nm) can be controlled well by prolonging or shortening the reaction time. Additionally, the template-assisted strategy including soft-template assisted and hard-template assisted method is one of the most effective methods to synthesize MoOx micro/nanostructures with controllable sizes, morphologies, and even hollow/porous features. For soft template, it is feasible to control the size of products by adjusting the size of micelles, while easy for hard template method to inherit the morphology of the template after removal of the template. In the next part, we will offer a detailed discussion on the corresponding formation mechanisms by using some typical examples.

Notably, it is necessary to characterize the MoOx materials with appropriate technologies to obtain the information about elemental composition, crystal phase, microstructure, electronic and optical properties. Hence, to achieve such goal, lots of modern techniques for both physical and chemical

characterizations

of

MoOx

micro-/nanostructures

have

been

applied.

The

characterization of MoOx micro-/nanostructures about the structural information like crystal phase or morphology could be accomplished by using a wide variety of techniques, such as Xray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM) high resolution TEM (HRTEM), energy dispersive Xray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), Raman spectrum and so on. To detect the electronic structure, ultraviolet-visible diffuse reflectance spectroscopy (UV-vis 15



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DRS), X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES) and ultraviolet photoelectron spectroscopy (UPS) are widely used in calculating the band energy of materials. As for the optical properties, ultraviolet-visible absorption spectroscopy and photo-luminescence (PL) are the most common strategies.

3.2 Morphology-controlled synthesis of MoOx micro-/nanostructures

Based on the above crystal growth theory and general synthetic strategies, numerous architectures with controllable sizes or morphologies have been reported, which provides new perspectives to optimize the structure, composition or even hybrids to further expand their usages. In this section, we review the synthetic strategies applied in the controllable synthesis of MoOx micro-/nanostructures and summarize the advances made in tuning the size, phase, and morphology of MoOx micro-/nanostructures, including 0D, 1D, 2D and 3D architectures. The synthesis pathways and morphology of some MoOx micro-/nanostructures are given in Table 1.

3.2.1 Zero-dimensional (0D) nanodots. Quantum dots (QDs) represent ultra-small dot-like nanocrystals with all three dimensions in several nanometers. MoOx QDs have mainly been applied into photothermal therapy for its unique size-dependent optical properties due to the quantum effects. Since MoOx QDs have been less reported, the development of new synthetic strategies for synthesizing MoOx QDs is necessary. Herein, we will summarize the recent advances in the synthesis of MoOx QDs.

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Hydrothermal method can achieve the synthesis of MoOx QDs by using capping agents to limit the growth of final product. Ding et al. developed a hydrothermal method to synthesize MoOx quantum dots (QDs) via using chitosan (Mw = 2000) as additive at 80oC.33 In such work, the chitosan acted as both reducing agent and capping agent. It was observed that the MoOx dots had a uniform size around 2.5 nm and was likely to be the reduced species of Mo13O33, which displayed a stronger LSPR for multifunctional theranostics.

As for the HxMoO3 precursors, most of the intercalated H+ ions interact with edge-shared oxygen and terminal oxygen atoms in MoO6, leading to the formation of intermediate groups. When taking some measures like thermal treatment or ultraviolet (UV) irradiation, the bond between intermediate groups and MoO3 will break up, and then water will eventually get out of the crystal lattice and leave oxygen vacancy sites, resulting in the formation of MoOx QDs. For example, it has been reported that the synthesis of MoOx QDs can be achieved by a photoetching method.35 When MoO3 nanoparticles (see Fig 6a) absorbed photons, their sizes would decrease to form QDs (see Figure 6b) until their bandgap energy was consistent with the energy corresponding to the UV light. Hence, the sizes of MoOx QDs could be controlled by selecting the wavelength of irradiation light. Moreover, the concentration of oxygen vacancies could also be well adjusted by increasing the reaction time (see Figure 6c) in water/ethanol solvent. Interestingly, the as-obtained MoOx QDs in water/ethanol and N-methyl-2-pyrrolidinone (NMP) solvent showed quite differently in morphology and size, which might arise from different mechanisms. In water/ethanol solvent, the water could combine with photo-excited holes and produce H+ ions, involving H+ intercalation mechanism. Nevertheless, in NMP, as no H+ ions 17



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could be produced, a photocorrosion mechanism was more reasonable for the morphology evolution of QDs. A schematic illustration of the overall morphology evolution from MoO3 precursor to QDs in different solvent is shown in Figure 6d.

In addition, an integration technique of molecular intercalation and thermal exfoliation can also be applied to the preparation of MoO3 QDs because of the layered structures of MoO3, which is similar to that applied to graphene QDs from graphene oxide. According to the strategy, MoO3 QDs (in an average size of 3.8 nm) possessing irregular shapes and being rich in defects could be successfully synthesized.34 A schematic illustration of the mechanism of transforming MoO3 nanosheets to MoO3 QDs in different solvents using intercalation and exfoliation method is shown in Figure 6e. First, alkylamine was inserted into the space between the MoO3 layers, namely an intercalation process. Next, after a rapid heating, the decomposition of alkylamine would separate the layers of MoO3 and make them aggregate into small nanoparticles. Finally, a sonication treatment could help the MoO3 QDs detach from each other. Alkylamine not only served as a matter that increased the length of interlayer and impaired the van der Waals force, but also as a decomposing agent to break up the chemical bonds in MoO3 layer by rapidly increasing gas pressure during the decomposition. It is worth being mentioned that the exfoliation temperature as well as reaction time will determine the properties of products.

Based on the above two intercalation methods, different mechanisms in the formation of MoOx QDs can be summerized. With regard to the UV photoetching procedure, H+ ions intercalated into the MoO3 nanosheets to form HxMoO3, which broke up into small pieces and 18


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releases H2O. Such process involved a transformation from MoO3 to MoOx QDs.35 In contrast, MoO3 QDs were prepared by the molecule-assisted thermal exfoliation method through breaking the van der Waals forces in the layered structure of MoO3 powders.34 MoO3 QDs were detached from rough MoO3 particles that were assembled spontaneously after thermal exfoliation. Notably, the existence of oxygen defects was believed to be helpful in the scission of the layered structures in both two methods. Therefore, it is proposed that MoOx QDs with tunable composition can be synthesized according to the integrated exfoliation and photoetching.

3.2.2 One-dimensional (1D) architectures. The term “1D nanostructure” is mainly used to describe structures of which one dimension is in the size range of 1− 100 nm and the growth is along one direction. Generally, 1D architectures can be classified into nanowires, nanotubes, nanorods and so on, which usually perform well in many fields due to their unique structural traits. Taking lithium ion battery as an example, 1D nanowires can not only provide good conduction pathways for electrons along with shorter path for lithium diffusion but also offer a diffusion channel for Li+ ions.31,41,141

Nanorods. MoO3 nanorods are formed from the dissolution of an intermediate hydrated crystal, and the crystal growth rates along different directions are in the following order: [001] > [100] > [010].164 Therefore, there is a great tendency to form 1D nanostructure during the reaction process. Especially, 1D MoOx nanorods can be synthesized by hydrothermal methods,37,39,50,53, 64,69,101,122,137,141,164,193

which generally involve the precursors such as ammonium heptamolybdate

tetrahydrate (AHM, (NH4)6Mo7O24·4H2O), molybdenum powder, sodium molybdate dihydrate 19



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(Na2MoO4·2H2O) and organic molybdenum salt.64 A typical synthesis approach is shown as follows. Firstly, AHM was dissolved in a mixed solution of 65 % HNO3 and deionized H2O. After fully dissolved, the reaction solution was heated at 160−200 oC.137 Finally nanorods with controllable sizes were synthesized. Lou et al. revealed a possible formation mechanism of MoO3 during the hydrothermal procedure,164 which can be proposed as follows: (1) AHM transformed to an intermediate ((NH4)2O)0.0866·MoO3·0.231H2O, (2) then the intermediate compound transformed into the final pure α-MoO3 nanorods.

Under different pH condition, there would be different polymeric forms of Mo species.50 For example, when the pH was upper than 6, the colorless [MoO4]2− existed in the solution. The [MoO4]2− transformed into [Mo7O24]6− and [Mo8O26]4− (1< pH 500), high charge carrier mobility (1100 cm2/(Vs)) and tunable bandgap.28,73,198,199 Herein, we will introduce the recent achievements in the synthesis of 2D MoOx nanostructure.

Nanosheets. Diversities of approaches, including exfoliation, solvo-/hydrothermal method, vapor deposition, have been reported in the synthesis of 2D MoOx materials. Therefore, some typical synthetic strategies applied to prepare nanosheets are given as follows.

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As MoO3 possesses a layered structure, exfoliation is considered as a facile method to obtain 2D nanostructures, especially nanosheets. Exfoliation method can be divided into several approaches such as mechanical exfoliation, solvent-assisted exfoliation and chemical exfoliation.

First, MoO3 nanosheets can be acquired from MoO3 microsheets by a mechanical method.67,73 Kumar et al. reported a two-step procedure to prepare MoO3-II nanosheets from h-MoO3. The first annealing step involved the rearrangement of the octahedron chains, which transformed hMoO3 into MoO3-II layered framework. Then the microsheets was exfoliated into nanosheets mechanically by scotch tape, of which the thickness was only 1.4 nm corresponding to merely 2 double layers of the MoO6 octahedra in MoO3-II. Notably, MoO3-II nanosheets obtained by such method not only showed a thin-layered structure no more than several layers but also presented a high crystalline quality. It has also reported that the mechanical exfoliation can be combined with a thermal evaporation process of MoO3 crystals to obtain 2D nanosheets.79 The as-prepared ultrathin MoO3 nanosheets had a 4 to 20 double-layers structure corresponding to the thickness of 2.8 to 14 nm, which also proved the effectiveness of mechanical exfoliation.

Second, liquid-phase exfoliation is also adopted to produce single- and few-layered materials by a simple grinding-assisted sonication exfoliation in either organic solvents or aqueous surfactant solutions.78,80 In fact, grinding-assisted sonication exfoliation was one of the most attractive methods due to its high yield without using any hazardous reagents. The 2D structure will be stabilized in the presence of surfactant, and thereby aggregation can be effectively avoided. The exfoliation of 2D MoO3 could be achieved in varieties of solvents such as DMF, 31



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dimethyl sulfoxide (DMSO) and some mixed solvents.80 Notably, it showed an interesting result that it exhibited a best synergistic outcome when synthesized in a combination of two relatively poor solvents. Furthermore, 2D α-MoO3 nanoplates could be prepared by mixing molybdic acid (MoO3·H2O) powders and n-octylamine at room temperature under sonication.78 It revealed that the solvent would influence the formation rate of molybdate according to an acid-base mechanism, where an intercalation and reorganization of octylamine happened in order to separate the MoO3 layered structure. Therefore, solvent species were a key factor on the formation of 2D MoO3 micro-/nanostructure when applying an exfoliation in liquid phase. However, non-polar solvents usually perform badly because they are hard to be involved in the acid-base mechanism.

Third, soft-chemical exfoliation has been regarded as a simple and mild route to obtain unilamellar MoO2 nanosheets.200 Sugaya et al. mixed a protonated form of Na0.9Mo2O4 with AHM solution to yield a dark-green suspension of monodispersed MoO2 which was in a restacked form. Since there was a cation exchange during the process, the residues of Na+ ions can be a positive factor in later application like ion batteries due to the enhanced charge transport. Notably, the restacked process of nanosheets after exfoliation was a favorable factor in energy storage, because more Li+ ions were allowed to intercalate into the interlayer gallery, resulting from the electrochemically active surfaces on the both side of nanosheets.

Solvothermal/hydrothermal methods were also applied in the synthesis of MoOx nanosheets. For example, MoOx nanosheets could be synthesized with the thickness of 20−30 nm by using 32


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ethanol as the solvent and Mo powder and H2O2 as the precursors.74 Typical SEM and TEM images of Mo MoOx nanosheets can be seen in Figure 10a. The approach allows the formation of oxygen vacancies in MoOx nanosheets, which enhances electric conductivity and ions diffusion coefficient, leading to a better performance in supercapacitors or ion batteries. Moreover, Cheng et al. have reported a facile non-aqueous access to prepare well-defined MoOx nanosheets without any surfactants, which displayed strong LSPR in the visible light region.77 MoOx nanosheets were prepared by oxidizing metal molybdenum powders with hydrogen peroxide followed by a solvothermal treatment in an ethanol solution. The phase and morphology evolution of molybdenum oxide nanostructures as well as the LSPR tunability can be achieved by controlling synthetic temperatures and solvents. With reaction temperature decreased, the LSPR frequency of the MoOx nanosheets could be tuned from visible (680 nm) to near-IR (950 nm) by tailoring the oxygen vacancies. Similarly, a red shift in the LSPR wavelengths could be seen at about 870−950 nm resulting from the usage of 1-butanol and 2-propanoal as the solvents, respectively. Additionally, Cui’s group synthesized porous MoO2 nanosheets with mesopores and large specific surface area on nickel foam.26 They used AMT as precursors while sodium dodecyl sulfate (SDS) as additives. The Brunauer-Emmett-Teller (BET) results of the MoO2 porous nanosheets presented that it possessed pores in the average size of around 23 nm and surface area of 25.7 m2/g. Interestingly, they found that the introduction of SDS resulted in the formation of nanosized MoO2 sheets, since it could prevent the overgrowth of MoO2 due to the capping effect.

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Vapor deposition is a general method to get film materials. Hence, it can use a vapor deposition to obtain MoO2 nanoflakes.85,201 MoO2 flakes were synthesized on a substrate of SiO2/Si through a chemical vapor deposition method using MoO2, Mo4O11, Mo9O26 as precusors.201 It was observed that a disproportionation reaction contributed to the formation of MoO2 flake other than the physical vapor deposition process. Therefore, it can be used to synthesize large-size MoO2 in micro-scale with a non-reducing method when choosing MoO2 as precursors. Additionally, ultrathin MoO2 nanosheets with thickness of 5−10 nm could be synthesized in a similar way.85 The nanosheets possessed high crystal quality as well as a much better electro-conductivity up to 475 S/cm at room temperature.

Nanoplates. As mentioned, vapor deposition has a preference to grow 2D nanostructures, which can also be employed into the synthesis of MoO3 nanoplates. For instance, hexagonal and truncated hexagonal shaped MoO3 nanoplates could be synthesized by using a simple vapor deposition method without using any catalyst or template,30 and it was proposed that the MoO3 molecular would condense into nuclei onto the substrate and grow faster along [001] than [100] direction, thus it finally stretched hexagonal plates along the [001] direction (see Figure 10b). In this work, the as-prepared MoO3 plates exhibited quite differently in Raman spectrum compared with bulk MoO3, which might arouse from a size-dependent effect that played an important role in the lattice vibrational property.

Besides, liquid-phase reaction is a general synthetic route for the MoO3 nanoplates. For instance, Tang et al. reported a sol-gel method to synthesize MoO3 nanoplates with Mo powder 34


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oxidized in H2O2 as precursor. The as-prepared MoO3 consisted of 2-dimensional nanoplates, which was in a size of 1 µm × 1 µm × 100 nm.84 A further study about the function of additives in the formation of MoO3 nanoplates has been reported. The α-MoO3 nanoplates were also synthesized by a chemical precipitation method with the aid of organic acids.81 The species of organic acid had significant impact on the controllable synthesis of MoO3 nanostructures, and it could be observed that the hexagonal plates were facilely synthesized in citric acid and tartaric acid environment, whereas ethylenediaminetetraacetic acid (EDTA) led to the formation of both nanorods and nanoplates. The above morphology evolution could be attributed to the structural alternation of organic ligands.

3.2.4 Three-dimensional (3D) micro-/nanostructures. 3D architectures including hierarchical architectures and hollow architectures allow MoOx materials possess more sophisticated structures, which will enhance specific surface area and provide more facets with active sites to optimize their chemical activities. In the subsection, we will mainly highlight the diversity of 3D MoOx micro-/nanostructures.

Ordered mesoporous structure. Mesoporous structure can not only minimize solid-state diffusion lengths for both ions and electrons due to the nano-sized walls (< 10 nm) but also enhance the electrochemical or photocatalytic properties because of their intrinsic high surface areas.87,202

Mesoporous MoO3 structure can be prepared by a soft-template method involving an evaporation-induced self-assembly (EISA) process.21,89 Brezesinski et al. synthesized the 35



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mesoporous MoO3 films by evaporating an ethanol solution containing sol-gel precursor and structure-directing agent,89 which resulted in a co-assembly to form a mesostructured inorganic/organic composite. Finally, a relatively pure α-MoO3 was prepared after removing soft-template by directly annealing in air. The TEM images of mesoporous MoO3 are given in Figure 11a. A porous cubic architecture without apparent structural defects can be observed. Meanwhile, it can be confirmed that the pores on the surface are open by AFM image (see Figure 11b). Moreover, MoOx nanoparticles with a strong LSPR effect could be synthesized by combining EISA process and subsequent hydrogen reduction with F127 (EO106PO70EO106) as soft templates,21 in which F127 acted as a surfactant to limit the growth of MoO3 nuclei, leading to high surface areas compared to the commercial MoO3 (30 to 1.32 m2/g).

Generally, it is hard to directly obtain mesoporous MoO2 from mesoporous MoO3 due to structure damage caused by the high volume shrinkage (about 36 %).87 Hard-template method can be effectively applied into the synthesis of mesoporous MoO2 without any structure damage. Shi et al. filled a normal phosphomolybdic acid (PMA) precursor into mesoporous silica KIT-6 templates and reduced into crystalline MoO2.87 The TEM image shown in Figure 11c suggests the as-prepared MoO3 has ordered mesostructure in the whole particle domain, while the SEM image in Figure 11d suggests that the surface of MoO2 was clean, indicating that almost all of the PMA is filled into the KIT-6 template and transformed in situ to MoO2.The as-prepared MoO2 products could inherit the mesoporous feature and 3D bicontinuously cubic symmetry morphology, which would be an advantage in ion batteries.

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Micro-/nanoflowers. Micro-/nanoflowers are special kinds of hierarchical structures which can be regarded as compositions of certain structural blockings like nanorods or nanosheets.61,69 MoOx micro-/nanoflowers can expose more active sites as well as specific facets by controlling the experimental parameters, which are meaningful in catalysis or lithium batteries.203 In order to synthesize MoOx micro-/nanoflowers, various strategies have been applied as follows.

Hydrothermal/solvothermal strategy is a widely-used method to synthesize 3D hierarchical MoO3 nanomaterials such as nano- or micro- flower-like structures due to the assembly of MoO3 rods as we have stated above.95 For instance, MoO2 nanoflowers were obtained by a solvothermal route using a mixture of molybdenyl acetylacetonate, n-butyl alcohol and nitric acid, followed with a calcination process. At the initial stage of the chemical reaction, small nuclei would firstly be formed, and then they would grow upto a structural unit (nanorod or nanosheet). Finally, structural units would attach together along the preferential orientation to form the flower-like morphology. Besides, Shen and coworkers synthesized nanoflower-like MoO2·2H2O with nanosheet building blocks in a hydrothermal condition.92 They deposited the precursor (AHM) on commercial nickel foam which played a role not only as a substrate but also a reactant in such a reaction. The hydrothermal mechanism can be expressed as follows.

20NH4+ + Mo7O246− + 7Ni + 4H2O → 20NH3 + 7MoO2·2H2O + 7Ni2+

(1)

The as-obtained nanoflower-like MoO2·2H2O consisting of many nanosheets covered the surface of nickel foam uniformly and possessed a high surface area, leading to the exposure of more active sites. 37



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In general, in order to directly fabricate oxygen-deficient metal oxides in a hydrothermal reaction, some reductants or inert gas (such as oleylamine, or ethanol) are always needed to prevent the complete oxidation of Mo and endowed the formation of the composition-tuned MoOx products. However, the existence of surfactants generally results in some unpredictable cytotoxic effects, which also lowers their surface feasibility, thereby restricting their large-scale applications, especially in the fields of sensing and catalysis. Hence, a surfactant-free route is necessary and has attracted great attention. 3D hierarchical MoO3 nanostructures can be synthesized under a precisely pH-controlled circumstance via a sacrificing VO2 nanoarray template of which the crystal structures and elemental similarities were close to those of MoO3.47 The SEM images of VO2 nanoarray template and MoO3 nanostructures can be seen in Figure 12a and 12b, respectively. A possible formation mechanism was as follows. Firstly, VO2 nanoarray was gradually dissolved in the acidic (HCl) solution and thus transformed into VO2+·5H2O. At the same time, MoO42− was protonated and dehydrated to form MoO3, which deposited on the VO2 nanostructure surface. With the reaction time prolonged, the formation of MoO3 with original wall-like structure was accompanied by the dissolution of VO2 templates. The schematic illustration of the reaction mechanism is depicted in Fig 12c. Notably, the method could be extended to the controllable synthesis of novel hierarchical nanostructures using low-valence metal oxides templates in various shapes.

Micro-/nanospheres. As one type of promising architecture, MoOx micro-/nanospheres with large surface area and good surface permeability for charge and mass transport, have been widely investigated, which can be used in the fields of energy storage conversion, catalysis, 38


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sensing, and bio-medicine.4,5,17,94 So far, the strategies for preparing MoOx micro-/nanospheres can also be categorized into two parts: template-assisted method and template-free method.

Organic soft templates are of great advantage because they can be easily removed and a great deal of ligands and block copolymers has been used as matrix scaffolding materials because their nanoscopic structures can be manipulated. A controllable synthesis of α-MoO3 with PMo12 precursor could also be conducted through the adjustment of soft template species, such as chitosan (CS) for microspheres, cetyltrimethyl ammonium bromide (CTAB) for microbelts and a mixture of CS and CTAB for nanorods, respectively.135 The chitosan offered a spherical template and PMo12 assembled into the template to form PMo12-CS complex, while CTAB tended to form bilayer sheets due to high CTAB concentration, resulting in the formation of microbelts of MoO3. The mechanism can be seen in Figure 13a and the SEM images of MoO3 microspheres and microbelts are given in Figure 13b and 13c, respectively. A triblock copolymers system (B20-5000 (E45B14E45)) could be established to synthesize MoO3 hollow nanospheres via a decomposition of MoO2(OH)(OOH). Unlike other system, the E45B14E45 can be dissolved in water, which avoided the structural damage after removing organic polymers through heating treatment. More importantly, the as-prepared MoO3 hollow nanospheres were found to be in a uniform size distribution which were controlled accurately by choosing suitable micellar cores.88

Figure 13d shows a typical growth process for the interconnected MoO2 core/shell microcapsules with a template-free strategy using molybdenum(IV) tetrachloride oxide.25 The original small crystallites on the surface of MoO2 spheres would act as nucleation seeds for the 39



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subsequent recrystallization and voids could occur and the thickness of the shell increased due to the outward diffusion of solutes through the permeable shell. Meanwhile, nanoparticles could be self-assembled with adjacent ones by sharing a common crystallographic orientation. The unique structure of MoO2 exhibited several merits such as a core/shell structure with interconnected framework, carbon-free and porous shell. Such a structure showed the giant potential applications in drug delivery, catalysis or ion batteries as well as supercapacitors. Additionally, the MoO2 nanorods could grow upon the surface of the core/shell structure by prolonging the solvothermal time (see Figure 13e), resulting in the formation of 1D electronic pathway for efficient charge transport.

4. Functional modifications

Functional modifications are meaningful to uncover the relationship between

the

interfacial/electronic structures and the enhanced performances. In general, the modifications of MoOx are categorized into two classifications: (i) doping and (ii) hybridization. Doping is an effective method to optimize the optical or electrical properties; while hybridization is an alternative route for acquiring excellent or unprecedented physicochemical properties, which can be ascribed to the synergistic effect between host material and active component. Currently, modified MoOx with well-controlled compositions, morphologies and sizes have been rationally prepared with the rapid progress in nanotechnology. However, a systematic review of MoOxbased materials has less been demonstrated so far.

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In the section, we firstly review the significant achievements made in the doped MoOx. Then, we briefly summarize on the advances in different kinds of hybrid MoOx-based heterogeneous architectures, including hybrid MoOx/semiconductor, hybrid MoOx/metal and hybrid MoOx/carbon nanostructures.

4.1 Doping

4.1.1 Nonmetal-doping. Hydrogen (H)-doping MoO3 usually exists in a form of HxMoO3, which will definitely change the lattice structure. Generally, three thermodynamically stable phases can be identified as HxMoO3 by their hydrogen content, including type 1 (0.23 < x < 0.40), 2 (0.85 < x < 1.04), and 3 (1.55 < x < 1.72).204 The different concentration of H+ will change the band structure and other physiochemical properties of MoO3 accompanied by a phase transition. For instance, it is observed that the H+ intercalation can enhance the conductivity of the bronzes.204206

Pure MoO3 exhibits a resistance of 1012 Ω, while the H-doped MoO3 shows a much smaller

value of about 104 Ω, aroused from the improvement of carrier concentration.76 Notably, the resistance of MoO3 is related to the environmental temperature in some degree. At 380 K, a metal-to-insulator transition occurred for H-doped MoO3, while a metal-to-semiconductor transition happened at 180 K, allowing that H-doped MoO3 possessed a temperature-dependent resistance. Additionally, in the doping process, hydrogen atoms could form binds with oxygen atoms when they were intercalated into the layered structure consisted of MoO6 octahedral double slabs.184 Moreover, in such a structure, electrons of hydrogen atom were surplus as well as move freely within the layers of MoO3 at a low hydrogen concentration, resulting in a slight 41



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decrease of binding energy.96 However, when increasing the concentration of hydrogen in HxMoO3, the excess electron will lead to a reduction reaction, in which some of Mo6+ cations can be reduced to Mo5+. Besides, H-doping MoO3 shows great advantages in tunable LSPR effects.204,205 Kalantar-zadeh’s group revealed that the plasmon resonance wavelength of HxMoO3 would vary in a big range in visible-near infrared region based on the composition of bronzes, in other words, the intercalated H+ concentration.206 Hence, the plasmonic HxMoO3 can show a great potential in heterogeneous catalysis or high-performance biosensors.

The p-block elements including S, N, P and so on are often doped into inorganic materials to tailor their electronic structure, leading to an enhancement of electrical conductivity or increase in catalytically active site. Herein, we will discuss some typical doping systems of MoOx in this subsection. S-doped MoOx could easily be obtained via oxidizing MoS2 in a mixture of Ar and O2, where the content percent of O2 varied from 0 to 40 %.207 By controlling the amount of O2 in the reaction chamber, S-doped MoOx with different amount of oxygen vacancies was obtained. Additionally, S doped in the lattice can tune hole-transport properties by introducing p-type materials, which effectively hindered carrier recombination at the interface when holes transfered through the valence band. As for N-doped MoO3, it could be synthesized by heating MoO3 commercial powder in NH3 atmosphere.208 The as-obtained N-doped MoO3 was further treated to prepare freestanding MoO3 or N-doped MoO3 single layers. The nitrogen which was confined in MoO3 atomic-layers would not only decrease interfacial electron transfer resistance but also increasing the concentration of divacancy which serve as active sites to trigger HER. An intercalation method has also been applied to synthesize P-doped MoOx nanosheets.209 The P 42


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doped MoOx nanosheet showed an excellent HER performance with a low overpotential and fast electron transfer because the electronegativity of P element was relatively low, which benefited the kinetics of the adsorption and desorption of protons. In addition, a fascinating merit of the Pdoped MoOx is that it can be applied over a wide pH range from acidic to alkaline environment, allowing the use under various working conditions.

4.1.2 Metal doping. A series of metal elements can be doped into MoO3 to enhance the physicochemical properties. Small alkali ions (including Li+, Na+ and K+) can be doped into the lattice of MoOx, leading to the improved electronic properties such as electronic transport and extended response spectral range.210-213 For instance, K-enriched MoO3 had a shallow electron donor level due to the intercalation of foreign atoms, which allowed an ultrasensitively phototransistive process with a sub-millisecond level photocurrent response.210 As for Na doping, the molybdenum oxide glasses doped with sodium ions exhibited a better ionic conduction than bare molybdenum oxide glasses, which might be caused by the larger ionic conductivity of sodium ions.211A similar result can be found in Li doping, which is beneficial for lithium batteries.212,213 Notably, electrical relaxations were different in LixMoO3, which might be ascribed to polarizations at different scales. Meanwhile, the electronic conduction was also relevant to the size- and morphology-effect. 212

Transition metals are often used as dopants which can modify the properties based on the doping species. For instance, mixed oxides of molybdenum and titanium are regarded as a promising material because of alteration of structure and multiple valance states that determine 43



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electrochromic performance to some extent. Mahajan et al. reported MoO3 doped with titanium were obtained by a spray pyrolysis technique.214 Interestingly, MoO3 transformed from polycrystalline into amorphous phase when the doping amount increased, leading to a promotion in the charge capacity, chromic properties and electrochemical stability. The improvement can be attributed to the amorphous spongy morphology of the doped samples that favors feasible intercalation and deintercalation processes. Notably, the phenomenon could be also observed in Nb-doped MoO3.215

Some late-transition metals have been applied into the doping of MoOx. For instance, Cddoped MoO3 nanobelts could be synthesized through a hydrothermal route using Cd (NO3)2·6H2O and AHM as precursors,97 which almost remained the pristine rod-like configuration of the un-doped one. The band gap of Cd-doped MoO3 was slightly lower than that of pure MoO3 nanobelts. Interestingly, with increasing the amount of Cd, the band gap continued decreasing. The change of band gap with the dopant amount followed well with the semiconductor-transition metal theory in which the band gap will decrease sharply when the impurity concentration was higher than the Mott transition concentration. Similarly, Fe-doped MoO3 can also be achieved by hydrothermal treatment of the mixture of MoO3 and Fe(NO3)3 in a H2O2 solution.99 It can be directly observed the doped MoO3 appears yellowish, while the undoped MoO3 nanorods show grayish white, which indicates the solution of Fe3+ in the crystal.

In addition, rare earth element can be doped into the lattice of MoOx to tune the electronic structures. For instance, Ce-doped MoO3 could be synthesized by an impregnation44


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decomposition method using Ce(NO3)3·6H2O as additive.98 The XRD showed a slight shift, indicating a volumetric increase of MoO3 unit cell. It suggested that the doped Ce had altered the size of crystal after intercalation due to its bigger size than Mo atoms.174 Notably, superfluous Ce would form CeO2 on the surface of MoO3 when the amount of Ce was up to 20 %, which restricted the growth of crystal, leading to a decrease in crystal size. Moreover, the band gap of the doped MoO3 can be even lower than either pure MoO3 or CeO2 and result in a better photocatalytic activity. It was also proposed Eu-doped MoO3 showed a better performance in photodegradation of methyl blue.216

Besides, it was also reported that indium could be doped into MoO3.217 The MoO3 and In2O3 precursors were mixed and deposited on glass or polyimide substrates through vapor deposition. The as-prepared indium-doped molybdenum oxide transformed into p-type semiconductor and exhibited a high optical transparency with a high band gap (around 3.8 eV). Moreover, the existence of indium decreased the resistance significantly (6.3 × 10−3 Ω·cm) even after bending for several cycles, which was a good advantage in photovoltaic devices. The work also opens new opportunities in designing Ga- or Tl-doped MoO3 materials with impressive performances.

Based on the above discussions, the previous achievement is limited to only a few elements, such as hydrogen, nitrogen, sulfur, Cd, In, Eu, Ce or Fe. Therefore, it is still a challenge to develop the selective doping of alternative heteroatoms or co-doping for enhanced performances and new strategies for the synthesis of novel doped-MoOx.

4.2 Hybridization 45



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The controllable synthesis of heterogeneous semiconductor-based composites with unique structures or certain components is an effective way to pursue excellent performance and provides opportunities for expansion in practical fields. So far, considerable efforts have been employed to design hybrid MoOx-based micro-/nanostructures with diversities of heterogeneous interfaces to optimize the properties for applications. Generally, the hybrid MoOx structures are classified into three species based on the materials used in the hybridization, including hybrid metal/MoOx, hybrid semiconductor/MoOx, and hybrid carbon/MoOx nanostructures. Hence, we summarize the synthetic strategies and formation mechanisms of the above-mentioned hybrid MoOx-based composites.

4.2.1 Hybrid metal/MoOx. MoOx has exhibited a better LSPR than the stoichiometric MoO3, which can be a merit when applied into photodetectors or SERS.83 Compared with single MoOx, a heterogeneous structure of metal/MoOx is promising in the photocatalysis, which can enhance the light absorption and visible light driven photocatalytic efficiency. Noble metal, such as Pd, Au and Pt, can be anchored onto the surface of MoOx in order to enhance the catalytic activity.62,100,219-224 Pd/MoOx can be a promising material in photocatalysis or catalytic hydrogenation reaction.100,219,220 A general solution-processed method could be applied to grow Pd nanoparticles, of which the size was merely around 3 nm (see Figure 14a).100 The Pd/MoOx hybrid was synthesized by a typical impregnation-reduction method with excess NaBH4 as the reducing agent. The synthetic route is given in Figure 14b. After anchoring the noble metal particles onto the MoOx nanosheets, it exhibited a strong LSPR peak in the visible light spectrum and showed a satisfied HER efficiency. Notably, Pd nanoparticles with certain facets can be 46


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obtained by an in-situ growth method.219 With the function of CTAB and PVP, the Pd nanotetrahedrons can dispersed well on the MoOx nanosheets, which is beneficial for the chemoselective hydrogenation reactions via electron donating effect and avoid aggregation through the reaction. Au/MoOx have also been widely applied in SERS, gas sensing and electrocatalysis.62,221,222 It’s reported that the gold nanoparticles can lead to the formation of Schottky barriers between the interfaces of Au and MoO3, which results in the enhancement of selectivity in gas sensing. Moreover, it can be fulfilled to obtain even Au nanocrystals with certain facets by a sonochemical method. The as-prepared Au/MoOx showed improved efficiency and selectivity in experiment. Notably, the results suggested high-index facets of Au nanocrystals performed better than the low-index ones, which was meaningful to reveal the relationship between the facets and activities. As for Pt nanoparticles, a Pt-MoO3-rGO system is often used as an electrocatalyst of methanol oxidation.223,224 In such a system, MoO3 not only help to disperse the Pt nanoparticles on rGO well as a linker but also reduce the size of Pt as a stabilizer.223 Li et al. made a further exploration in the function of structural and interfacial effects.224 In fact, there was an obvious electron delocalization among Pt 4f, Mo 3d orbitals and rGO π-conjugated ligands, leading to great synergistic effects that enhance the activity of electrooxidation.

4.2.2 Hybrid semiconductor/MoOx. Lots of research has been carried out to enhance electrochemical, photochromic and electrochromic properties of MoOx through synthesis of composite films containing MoOx and another semiconducting oxide to hinder the recombination of photogenerated electron−hole pairs.102,225 The rearrangement of the valence and conduction 47



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bands will tune the band gap of composites as well as avoid the recombination of excited electrons and holes as well as enhance the charge separation, elongating the lifetime of the charge carriers and extending their photoresponse range.102,226,227

MoOx can be a good substrate for the growth of other metal oxide. For example, TiO2 is a typical metal oxide which can combine with MoOx very well due to the match of lattice.37 Both α-MoO3 and anatase TiO2 consist of metal-centered oxygen octahedrons MO6 (M = Mo and Ti). Hence, the epitaxial growth of these two surfaces can be achieved; as a result, TiO2 can work as a capping agent in order to limit the growth of α-MoO3 on (001). The growth process can be seen in Figure 15a. It can be observed that the TiO2 capped on the end of MoO3 and α-MoO3 would have long MoO3 arms outsides after a second growth (see Figure 15b), which could be detached to obtain MoOx ultrathin nanowires by sonication method.37 By adjusting the reaction condition, TiO2 could also form arrays on the (010) surface of MoO3 template. The different ratio of TiF4/EDTA and TiF4/MoO3 would decide the morphology of MoO3 and TiO2.228 A one-step hydrothermal method can be applied in order to synthesized the TiO2@MoO3 (TM) core/shell nanostructure.102 Through the hydrothermal process, the light absorbance of TM core/shell nanostructure in water at 600 nm was about 30 times larger than that of α-MoO3, and a 20 % transmittance regulation at 500−800 nm was obtained for the TM-based thin film. Furthermore, the samples in ethanol even showed much stronger photochromic coloration efficiency than that in water by 66.6 % enhancement in the absorption at 600 nm, which exhibit great potential in smart windows and color displays.

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Tungsten oxides usually have similar properties with MoOx like good electrical conductivity and wide band gap as well as a high resemblance in crystal structure, which makes it possible to form WxMoyO3.103,229,230 WxMoyO3 can exhibit strong LSPR in the visible light region due to the oxygen vacancy by controlling reaction solvent and synthetic temperature. The oxygen vacancy can be regarded as the result of a mutual doping of molybdenum ions and tungsten ions and the LSPR peaks will red-shifted with decreased intensity when the molar ratio of of Mo/W is changed from 1:1 to 2:1 and 1:2.229 Additionally, Li et al. reported a hybridization of molybdenum oxide with tungsten molybdenum oxide nanowires (MoO3-W0.71Mo0.29O3) as reversible energy storing smart windows, which exhibited better reversibility and enhanced electrochemical behavior than pure MoO3.230 The possible reason for the improved performance might be that the MoO3-W0.71Mo0.29O3 hybrid film could be fully bleached during the dynamic cycling, in which the intercalated lithium ions was easily extracted from the hybrid without the large capacity loss typical of pure MoO3 films.

Metal sulfide/MoOx heterogeneous nanostructures are another topic that has attracted a lot of attention.104,106,107 For example, researchers have tried to replace a carbon-based secondary phase by MoS2 for maximizing the energy density of the lithium ions batteries electrode, since carbon allotropes will add unnecessary volume to the system.104 In addition, the activity of pure MoS2 in HER is restricted by low density of active sites and their poor electrical conductivity, as the MoS2 layer is always stacked. The framework of MoOx will help expose tremendous active sites of terminal disulfur of S22− (in MoS2) as well as facilitate an interfacial charge transport for the proton reduction. Furthermore, the hybrid structure can protect MoOx in an acidic environment 49



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effectively, which promise the lifetime of the hybrid materials.106 A sulfurization step is often taken to synthesize MoS2/MoOx system. Xiao et al. have reported an in-situ growth method to fabricate nanocarved hybrids of molybdenum oxides and molybdenum disulfides named threedimensional (3D) MoS2 nanomasks.105 The illustration of synthetic route is seen in Figure 15c. After a second sulfuration, the specific areas of MoO2@MoS2 were significantly improved and more porous structures appeared, resulting from the introduction of MoS2. The as-obtained MoO2@MoS2 had an average width of 90 nm with the exposure of (111) facets of monoclinic MoO2 and the (002) crystal planes of hexagonal MoS2 (see Figure 15d). Besides, one kind of solid-phase method could be used to obtain MoS2/MoO2 from MoO3 and S powder.104 Moreover, Lee’s group has decorated CdS nanoribbons with MoO3 nanodots to fabricate photovoltaic devices on flexible substrates. MoO3 nanodots was prepared by dropcasting or spin-coating the MoO3 solution (0.1 wt%) on the CdS nanoribbons. The MoO3 spontaneously precipitated on CdS nanoribbons during solution evaporation.107 Intensive charge transfer between CdS nanoribbons and MoO3 nanodots were investigated and selective deposition of MoO3 on one-half parts of the CdS achieved p−n homojunctions, leading to a better performance in optoelectronic devices.

Compared to oxides, the nitrides of transition metal have many impressing properties like high conductivity, high chemical stability, mechanical strength as well as high melting points. Herein, hollow MoO2 coated with Mo2N was synthesized via a gas reaction with NH3, which exhibited well in ion batteries.108 Hierarchical hollow nanostructures with pores on the surface are shown in Figure 16a and 16b. The commercial MoO3 powder was first reduced into MoO2 and then transformed into Mo2N with a NH3 treatment (see Figure 16c). The Mo2N coating layer not only 50


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acts as an active material offering additional capacitance but also as a protective layer enhancing the stability in ion batteries. It has pointed out a potential research direction in free-carbon coating layer of MoOx. 4.2.3 Hybrid carbon/MoOx. Carbon materials, including active carbon, graphene frameworks or carbon nanotubes, exhibit superior electrical conductivity, large surface area, structural flexibility and chemical stability. Moreover, it can host nanostructured electrode materials for energy applications. Graphene oxide (GO) or other carbon materials are often used as support to enhance the electrochemical performances, disperse the MoOx better or restrict the volume variation.109−112 The hybridization of elastic and conductive carbon and MoOx can directly enhance the structural integrity; therefore, the cycling stability comes better. Moreover, carbon coating has been widely used to prevent the exfoliation of active materials and improve the electrical conductivity of electrode materials.111 Carbon nanotubes (CNTs) have been widely used for enhancing the photocatalytic performance as they can provide extra specific area and help maintain the structure of transition metal oxides. Shakir and his coworkers obtained MoO3/functionalized multi-wall carbon nanotubes (MWCNTs) through a hydrothermal route.113 The MoO3/MWCNTs containing 20 wt% MWCNTs exhibited large surface area of 117 m2/g with a wide pore size in the range between 2 and 100 nm. Furthermore, the MoO3/MWCNTs displayed a combined synergetic effect, resulting in an enhancement of harvesting visible light and a red-shift phenomenon. Notably, it is vital to adjust the ratio of MoO3 and MWCNTs to optimize the performance, since the black color of MWCNTs will shield the light absorption leading to a fast reduction in the photocatalytic activity. 51



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Graphite is one of the most common and commercial anode materials; however, it shows a low capacity (372 mAh·g−1), which restricts the usage of graphite. To enhance the specific capacity, MoO3 is used to form MoO3/C nanocomposites for its a high theoretical capacity of 1117 mAh·g−1. Tao et al. presented a ball-milling method to downsize MoO3 into nanoparticles as small as 2−10 nm following by dispersing them homogeneously on the graphite. The agglomerate showed good cycling stability in lithium ion batteries (LIBs) (94 % retention of the theoretical capacity after 120 cycles), and more importantly no additional conductive agent such as carbon black was required in such an electrode.114 The graphene layers can play an important role as a support for anchoring nanoparticles and work as a highly conductive matrix for good contact. Moreover, graphene layers prevent the volume expansion/contraction and the aggregation of nanoparticles effectively during charge and discharge processes. At the same time, the intercalated nanoparticles can efficiently prevent the restacking of graphene sheets, which promises a high surface area and a better performance.111 MoO2/GO composites can be applied as anode material for lithium-ion batteries. The precursor solution consisted of H2O2 and Mo powder was mixed with GO and ethylene glycol by stirring and sonication, accompanied with a solvothermal treatment to obtain MoO2/GO composites. The charge transfer resistance of MoO2/GO (44 Ω) was much lower than that of MoO2 (330 Ω), and an initial discharge capacity of as high as 817.6 mAh·g−1 was achieved when cycling at 100 mA·g−1, which was almost two times of the capacity of pure MoO2.109 Flawed MoO2 can be transformed on graphene substrate, which was facilitated by the abundance of electrons on graphene.74 Additionally, the emergence of flaws was ascribed to the intrinsic structure defects showed in MoO3 during the process of reduction. To strengthen the combination between 52


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reduced-graphene oxide (rGO) and MoO3, poly(diallyldimethylammonium chloride) (PDDA) was used, since it could transform rGO into positive charged state and attach the negative charged MoO3 onto the surface by electrostatic attraction. Huang’s group has reported a largescale fabrication of unprecedented self-assembled hierarchical MoO2/graphene nanoarchitectures through a facile solution-based method combined with a subsequent reduction process, which is given in Figure 17a.111 The as-prepared MoO2 nanocrystallites of ca. 30−80 nm were wrapped in the graphene layers and aggregated into rods. The SEM image of the product is shown in Figure 17b. α-MoO3 nanoribbons could be modified by aminopropyltrimethoxysilane (APS) to obtain a nanocomposite of graphene encapsulated α-MoO3 nanoribbons due to the electrostatic force between the positively charged MoO3@APS and negatively charged graphene which drove a self-assembly process.112 The kind of structure showed great advantage in accommodating the volume expansion in the cycling process, avoiding the aggregation of MoO3 and enhancing the electrical conductivity as well as improving the rate of transportation of the charge carriers. Hence, these merits led to a better performance in LIBs. Additionally, MoO2@C could be synthesized by a hydrothermal procedure by replacing the GO with other carbon source like ethylene glycol.110 Carbon thermal reduction method is considered as a rational and largescalable synthetic route for fabricating low valence state materials. Additionally, MoO2/ordered mesoporous carbon nanocomposite was synthesized by utilizing ordered mesoporours carbon as a nanoreactor and phosphomolybdic acid as Mo source.115 MoO2 nanoparticles were homogeneously dispersed into the channels of the ordered mesoporous carbon in a size of 4 to 10 nm. Moreover, the as-prepared MoO2/C could accommodate well for the strain and volume variations and provide a shorter pathway for ion and electron transport. 53



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Recently, 3D network graphene is quite hot for it is able to support further hierarchical nanocomposites growth as well as offer a high electrical conductivity property of graphene. It means that nanomaterials can be directly deposited onto the surfaces of 3D network graphene foam to create binder-free electrodes which have a much higher electrical conductivity and avoid capacity losses when active material exfoliates from the current collector.66 Besides, CVD method can be used to deposit MoO2 into the 3D network graphene framework via the reduction of α-MoO3 nanobelts, which were directly used as LIB electrodes without binder or further treatment. MoO2@C could be obtained by reducing MoO3 precursors with oleic acid at high temperature without any additional carbon sources or templates.116 The procedure could fulfill simultaneous reduction and carbon ultrathin film (several nanometers) formation under a relatively mild condition with the surface area significantly increased. It was also reported using cotton cloth as a carbon matrix, ultrafine MoO2 less than 2 nm in size can be obtained through an impregnation–reduction–carbonization method.117 The precursor PMA not only played a role as the Mo source but also prevented the agglomeration of carbon matrix for the carbon could be easily stacked and melts at 500 oC. In addition, functional groups on the surface of the cotton like −OH enable the impregnation of PMA as well as influence the shape of MoO2 nanoparticles. Morphosynthesis has become one of most important techniques to synthesize material because it can maintain unique and complex architectures at different length scales from nature materials which usually has hierarchical configuration from the nanometer to the macroscopic scale. It can be accomplished growing nanostructured inorganic components into functional inorganic nanomaterials though a self-assembly procedure. For example, cotton is of great advantage for it is consisting of polysaccharide chains which are arranged into crystalline and amorphous regions 54


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composed of porous structure in a range of few nanometers in width and located in the crystallites. A morphogenetic route was presented to fabricate a batten-like MoO2 monolith via using PMA as precursors on the cotton texture, which acted as both a template and a stabilizer.91 The scheme of synthetic is given in Figure 17c. It was observed the MoO2 monolith had hierarchical porosity and an interconnected framework, indicating that it possessed a similar topology to that of the MoO3 replica. The SEM image of the final product is shown Figure 17d. Moreover, it was observed the MoO2 monolith had hierarchical porosity and an interconnected framework, and more importantly, the scale and microstructure of the final MoO2 product could be manipulated by modifying the cellulose templates with many textile fabrication techniques such as “spinning’’, ‘‘weaving’’ and ‘‘tailoring’’ etc. Nanoporous MoO2/carbon nanowires can be synthesized via carbonizing a hybrid structure of organic–inorganic nanowires of MoOx/amine. A typical synthetic route of MoO2/carbon nanowires can be achieved by using ammonium heptamolybdate and aniline as precursors in an acidic solution.118 Through a further calcination in inert gas atmosphere, the precursor can be carbonized but remain the 1D morphology. More importantly, the system can be extended to synthesize many other nanoarchitectures of MoO2/C with quantities of organic–inorganic hybrid precursors available. For instance, MoO2/C can be obtained via employing MoOx/1,6hexanediamine nanowires to be precursors by a similar procedure. Surfactants are generally used in soft template method to accomplish a self-assembly process and a following carbonization can transform surfactants into carbon even with porous structures. A typical synthesis of a C@MoO2 hollow yolk–shell structure was achieved via a soft template method.119 In the method, CTAB and sucrose were used as the soft template, AHM as the metal 55



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source, and ammonium carbonate as an additive. The schematic illustration of the synthesis is given in Figure 18a. The CTAB would form micelle in water and aggregate into a worm-like morphology. Addition of sucrose would lead to the formation of spherical template and AHM would then enter into the template. After a carbonization, a yolk-shell structure can be obtained. The TEM image is shown in Figure 18b. We can find though there is thin hollow space between the shell and yolk, they are partially interconnected by interparticle interactions. Such method provides a new perspective that the MoO2 can play as a shell, which may show unique properties and expose more active sites. Additionally, a one-pot hydrothermal synthesis of uniform MoO2@C nanospheres was delivered via using AHM as precursors and PVP as capping agents to prevent overgrowth of the MoO2 nanoparticles as well as over-agglomeration during the assembly.120 Utilizing the assembly of PVP to obtain a micelle template, uniform MoO2@carbon hollow nanospheres were fabricated through a soft-template method.121 A detailed synthetic route is given in Figure 19. AHM molecules would enter the inner space of the hollow micelle of PVP and a further carbonization process would not only degrade PVP into carbon but also play as an Ostwald ripening process to obtain larger MoO2 nanoparticles. Organic macromolecular materials such as polyaniline (PANI), polypyrrole (PPy), polythiophene (PT) and so on has shown good conducting properties, and thus they are paid great attention to and regarded as widely-used modifier. Taking PPy as an example, PPy is one kind of conducting polymers which show good conductivity and chemical stability. Since it can not only act as a conducting agent but also prevent the structural collapse of metal oxide caused by volume expansion during the cycling, it plays an important role in modifying electrode materials for supercapacitors or batteries.122,231 For instance, a nanocomposite of PPy-coated 56


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MoO3 nanobelts could be synthesized through an in situ oxidative polymerization method.122 Similarly, PANI can be also obtained via an oxidative polymerization procedure. Kumar et al. synthesized polyaniline/h-MoO3 hollow nanorods by decorating PANI onto hollow MoO3 with the presence of oxidant FeCl3.123 The SEM and TEM images of hollow h-MoO3 nanorods and the polyaniline/h-MoO3 hollow nanorods are given in Figure 20a and 20b, respectively. It can be observed that the as-prepared polyaniline/h-MoO3 hollow nanorods possess a core/shell structure. The synthetic route is given in Figure 20c. It has also been reported that PANI/MoO3 was synthesized through a hydrothermal method by heating the mixture of PANI and MoO3 sols from a proton exchange resin of (NH4)6Mo7O24·4H2O solution.124 Interestingly, the existence of polyaniline also affected the morphology of MoO3. Pure MoO3 nanobelts would grow separately, whereas 0.05 mol% PANI/MoO3 nanobelts had a flower-like morphology because of high reactivity of polyaniline. Hybrid of pyrazine and MoO3 nanorods can also be achieved through hydrothermal route.58 A self-assemble process occured in the hydrothermal process that the pyrazine intercalated into the MoO3, since the length of the polymers was comparable to the van de Waals layer gap cell by sharing its nitrogen atoms in place of oxygen at Mo=O sites. Besides, when the temperature of the hydrothermal process increased, the nanorods aggregated and encouraged more intercalation as well as led to the formation of plate-like morphology. Graphitic carbon nitride (g-C3N4) has been regarded as a promising material in water splitting or photodegradation of organic pollutants under visible light. However, the efficiency still needs great promotion, which can be achieved by coupling with other semiconductors. MoO3 has shown that it can enhance photocatalytic performance when combined with g-C3N4 because of the adjusted band gaps after recombination. A mixing-calcination method can be applied into the 57



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synthesis of MoO3/g-C3N4.125 The heterojunction structure not only promoted the separation of photogenerated electron–hole pairs and minimized recombination of the photo-excited charges during reaction. In addition, the presence of molybdenum blue in the hybrid can help capture more visible light photons, which is beneficial for photocatalytic reaction. 5. Applications 5.1 Photodevices 5.1.1 Photodetectors. MoOx is a well-known transition metal oxide that possesses good electrochromic or photochromic properties. The color change properties of MoOx, resulting from different band gap states and ion intercalations, shows a good signal that it will be a promising sensors or organic photovoltaic cells. Because of the small atomic diameter and the ambivalent character of ions or hydrogen, ions or hydrogen molecular intercalations are regarded as an additional donor or acceptor of electrons which provides various optical and electronic effects exhibiting giant potential in photodecvices.126 MoO3 is a quite promising material for ultraviolet photodetectors (UVPDs) due to the wide band gap corresponding to a wavelength of 388 nm, which is located in the UV spectral range. In fact, 2D MoO3 nanosheets are quite popular in making flexible UVPDs. Compared with quantum dots or 1D, 2D materials possess higher flexibility and carrier mobility. It is of great advantage that MoO3 UVPDs can work without any optical filter which will lower the light intensity and result in the generation of optical response for its high selectivity in ultraviolet region.67 Moreover, it is found that dangling bonds, serving as recombination centers for the photo-generated carriers, do not exist in 2D MoO3 nanosheets, which is beneficial for photo- detection.67,111 Zheng et al. reported a UVPD fabricated by MoO3

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nanosheets had a high responsivity of 183 mA/W, which exhibited excellent stability and reproducibility under different mechanical deformations. However, both the poor light adsorption in visible light region (λ > 400 nm) and the weak light response owing to the wide band gap of MoO3 restricted the application in optoelectronic nanodevices utilizing solar energies. MoOx exhibited strong LSPR absorbance for its oxygen vacancies and the LSPR tunability can be achieved by altering its stoichiometric compositions. It is generally believed that the formation of mid-gap states in the wide bandgap semiconductors such as WOx or MoOx can response to not only UV region but also the visible or even nearinfrared region (NIR), and hence significantly improves the efficiency of the optoelectronic devices and photocatalysts.28 Hydrogen intercalation is regarded as a facile method to adjust the band gap of MoOx. After the intercalation of hydrogen, the electron of hydrogen gets into the conduction band forming protons and “valence band-like Mo5+ states”, leading to the change of band gap.126 Xiang et al. have reported that the responsibility and external quantum efficiency of the photodetector could reach as high as 56 A/W and 10200 % after annealing in a hydrogen atmosphere under the illumination of visible light at 680 nm, while the intrinsic MoO3 showed no response at all. A schematic illustration of MoO3 for photocurrent measurement is given in Figure 21a. The photocurrent of as-prepared MoOx can be induced and decreased by switching on and off the 660 nm laser (see Figure 21b). External quantum efficiency (EQE) is one vital criterion to evaluate the quality of photodetectors that is the number of electrons detected per incident photon. It is notable that the EQE represents the sensitivity and can be determined as high as 16300 % for the wavelength of 420 nm (see Figure 21c). As displayed in the energy level diagram of MoO3 before and after annealing in Figure 21d, it can be seen that the gap states 59



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induced by annealing in a reducing gas provide many possible routes for electrons to be excited from gap states to the conduction band, which significantly improve the photoresponse of MoOx in a wide spectrum region.28 However, the investigation of the application of MoOx in photodetectors under visible light is still rarely reported in the literatures, and more effort should be employed in the field.

5.1.2 Phototherapy. Photothermal therapy has gained immense attention in the cure of cancer for its accuracy and efficiency. Moreover, a theranostic system of image-guided phototherapy is regarded as a promising technique in disease diagonsis and treatment because it can enhance accuracy and visualization during the treatment. The mechanism of photothermal therapy is that materials will absorb light irradiation and transform them into heat, which can have impact on the cells in biological organ or tissues in certain area with precise control, resulting in the death of cancer cell. As it requires a good light absorption even under a depth of skin and fat to produce enough heat, the material is thought to be must possess a good NIR absorption.232 Therefore, MoOx nanocrystals with LSPR in visible and NIR regions are promising materials for phototherapy.

It has been reported that MoOx QDs could be used in photoacoustic (PA) imaging-guided photothermal (PT)/photodynamic (PD) combined cancer treatment.33 MoOx QDs exhibited a good NIR performance and converted the NIR photoenergy into thermal energy under irradiation fast and efficiently (the photothermal conversion efficiency induced by 880 nm laser raised up to 25.5 % for a 0.45 ml (1 mg/mL) of MoOx solution), as shown in Figure 22a, which was quite 60


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useful in PT applications. Besides, MoOx generated singlet O by the inducement of NIR, which produced PD efficiency. Moreover, it displayed a good chemical stability under laser irradiation heating in both vitro and in vivo groups. The sizes of tumor cells have been investigated to evaluate the effect of various treatments, which is shown Figure 22b. The MoOx QDs with NIR irradiation showed good ability to eliminate cancer cells. Notably, MoOx QDs showed a negligible cytotoxic effect in dark, while it could present remarkable cytotoxicity to cancer cells under NIR laser irradiation.

Carbon coating is a promising strategy because C/MoOx possesses the LSPR originated from oxygen vacancies of MoOx as well as keeps a good photostability and biocompatiblity due to the ultrathin carbon coating nanoarchitecture. Liu et al. presented a method to synthesize MoO2 coated with ultrathin carbon layer (1−2 nm) through a solvothermal route.127 It could induce a significant change in the temperature from 22 to 53 oC in five minutes when the solution containing 1 mg/mL as-synthesized C/MoO2 nanoparticles was lighted under laser irradiation (0.6 W/cm2). Meanwhile, the thermal images of the interested regions can be recorded using an infrared camera due to the thermal effect brought by C/MoO2 nanoparticles (see Figure 22c). Modified by thiol functionalized polyethylene glycol (SH-PEG), the above C/MoO2 exhibited a good biocompatibility in several cells. In vitro ablation, MDA-MB-231 cells (human mammary epithelial cell line) and cancer cells were used to test the ablation capacity. The cancer cells showed unaffected under 808 nm laser irradiation unless the power density was higher than 0.6 W/cm2, and the cells proliferation ability increased when the concentration of C/MoO2 got down. In a vivo experiment, it also showed a great temperature increase (more than 20 oC), which was 61



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high enough to kill tumor cells. Notably, all of the tumors were eliminated after seven days NIR irradiation treatment owing to the photothermal ablation induced from the nanoparticles without any recurrence. The results prove that the photothermal therapy can be a promising treatment to cancers.

According to the Parkin’s work, MoO2 displayed a stronger absorption around 600−1000 nm especially at about 800 nm. Furthermore, an 808-nm laser heat conversion of MoO2 was also calculated to be 62.1 %, which was much higher than that of other semiconductors like WS2 nanosheets (32.8 %) or Cu2−xSe (22 %).132 Obviously, the existence of oxygen vacancies allows the LSPR which enhance the NIR light absorption significantly. However, one current problem of MoOx for photothermal therapy is that the oxidation leads to a giant decay in performance, because the prior absorption peak will disappear when the amount of oxygen vacancies decreases.128 Hence, it is important to retain the oxygen vacancies in keeping high efficiency of MoOx during the treatment.

5.1.3 SERS. SERS has been known as a promising and powerful analytical technique, utilizing a Raman scattering enhanced phenomenon when molecules are adsorbed on nanomaterials. It is believed that LSPR will be a vital factor that results in a strong electromagnetic field enhancement near the surfaces of the nanostructure. As stated above, MoOx with oxygen vacancies can exhibit strong LSPR, suggesting that it has a great potential in SERS. For instance, Zhang et al. have observed that MoOx QDs displayed an enhancement factor up to 106 and a low the detection limit down to 10−9 M, which can even be comparable to the noble metal.36 The 62


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MoOx QDs can show LSPR in both visible region and NIR due to the single-layered morphology, few-crystalline structure and oxygen vacancies. Indeed, the oxygen vacancies allow more free electrons as well as strengthen the interaction with Rodanmine 6G (R6G) via vibronic coupling, while free crystalline leads to a disorder structure making the system metastable to low down the work function and have a strong LSPR effect. Moreover, a stronger charge transfer effect between MoOx QDs and probe molecules also leads to the outstanding performance as well. It is worth being mentioned that the surfactant-free method also endows the MoOx QDs without much interference SERS performance. Wu et al. have a more profound understanding in the function of oxygen vacancies.128 To illuminate how the oxygen vacancies work in the Raman scattering, they established a model named “effective electric current model” to describe the photo-induced charge transfer process. To prove the model, a series of MoOx with different amount of oxygen vacancies were prepared and the MoO3 turned from non-SERs active to SERS active by increasing the oxygen vacancies. The introduction of oxygen vacancies would add new defect levels in the band gap of the α-MoO3 nanobelts and then enhance the SERS performance. Such model can explain the functions of oxygen vacancies in SERS. MoO2 possessed a high enhancement factor (EF) up to 1.10 × 107, which was much higher than that of MoO3, according with the trend after introducing vacancies.82 As we can see in Figure 23a, the Raman peaks of R6G are significantly strengthened by molybdenum oxides, especially MoO2. Take Peak 1 (P1) and Peak 3 (P3) shown in Figure 23a into consideration, the EF can be calculated and it’s shown in Figure 23b. It can be seen that it exhibits a better EF in a lower concentration of R6G. Meanwhile, a unique MoOx@MoO3 structure would improve the sensitivity by preventing the leaky free carrier concentration and photocatalytic degradation by MoO3 isolated shell.82 63



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Many strategies, such as anchoring noble metal and combining molecule-imprinting polymer, improve the SERS performance besides introducing oxygen vacancies have been attempted. For example, a combination of MoO3 and noble metal like Ag or Au are also used because of the strong LSPR effect of noble metals.129,130 It was reported that Au monolayer was assembled on the MoO3 film with silver-grating film on it with a compact and uniform distribution.130 With the existence of Au and Ag, a strong resonance in the near field is observed for the coupling of noble metals. In addition, polymers are used into composites to enhance the device performance.131 Wang et al. successfully synthesized a MoO3 nanorod core with a uniform 4-nm moleculeimprinting polymethacrylic acid shell (MIP) named MoO3@MIP. The MoO3@MIP not only presented a good stability and relatively high EF (1.6 × 104) but also displayed a good selectivity, described as aim and shoot. Though the concentration of methyl blue is much lower than that of crystal violet, the signal of methyl blue can always be clearly detected by using MoO3@MIP (see Figure 23c). Moreover, the MoO3@MIP showed a good recyclability after cleaning the absorbed methyl blue on the surface and the intensity can almost remain as before reflected in Figure 23d.

5.2 Energy storage and conversion

5.2.1 Supercapacitors. The layered structure of MoOx facilitates ions intercalation with enhanced charge storage for both faradic and nonfaradic system. A massive amount of electrolyte ions (0.28 Å for H+) can insert into the interlayer space originating from an inter-layer spacing (6 Å) in the layered structure of α-MoO3.109 The charge storage may undergo as 64


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following processes:89 (1) Cations can react with the surface of electroactive material and then form various compositions or phases. (2) Cations can insert into van der Waals gaps of layered or tunnel materials. (3) Cations can be electrochemically adsorbed onto the surface of a material through charge-transfer processes. MoO3 follows the intercalation process which provides a high pseudocapacity resulting in a high energy density. As an example of NaCl as electrolyte, the mechanism pseudocapacitor of MoO3 may work as follows (see equation (2)):

(MoVIO3)surface + Na+ + e−↔2(MoVO3Na+)surface

(2)

Therefore, MoO3 has been used as a pseudocapacitive (intercalation pseudocapacitance) electrode material for electrochemical capacitors due to its high theoretical specific capacitance (∼2700 F·g−1) and fast faradic redox reaction kinetics.123 However, two main challenges may be addressed urgently. First, the poor electronic conductivity severely restricts the supercapacitor performance of MoO3. Second, the material exhibits poor cycling stability due to the irreversible structural damages during the first cycle.

In order to mitigate these problems, a mass of research has been made on nanostructural of MoOx by tuning the shapes, phase and sizes or forming nanocomposites with carbon materials. Micro-/nanostructures MoOx will influence the electrical properties such as electron transport ability, which will later reflect on the performance of devices. 1D nanostructure possesses some advantages for supercapacitors, for example, the porous structure and shortened electron transport pathway. 1D, 2D and 3D α-MoO3 can be synthesized controllably via a hydrothermal method.135 Among the three types of structures, 1D α-MoO3 nanorods showed the best 65



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performance with almost 2−4 times higher capacitance than other nanostructures. The specific capacitance of 1D MoO3 nanorods was 214 F·g−1 at a current density of 0.1 A·g−1, while those of 2D microbelts and 3D microspheres were 94 and 168 F·g−1, respectively. The improvement was attributed to the high specific area possessed by MoO3 nanorods. Furthermore, it was reported that ultra-long 1D MoO3 nanowires electrode showed an excellent cycling performance.48 Even after 600 cycles, no obvious capacitance loss occurred and it remained a more than 80 % capacitance of the initial capacitance. Indeed, the good electrochemical performance of MoO3 nanowires may be ascribed to the 1D nanowires of MoO3 which offers amounts of fast electrontransport pathways to the current collector as well as providing large specific surface area, thus resulting in rapid electron transfer.

Hollow micro-/nanostructures have also received massive attention owing to their porous structures, inner hollow architectures and high surface areas.119,233−235 The enhancement in the electrochemical performance is owing to increased surface area originating from the formation of inner hollow space and the structural resistance to damage during the redox cycle which ensure the lifetime of electrical device. For instance, yolk-shell structural C@MoO2 displayed good faradic behavior with a specific capacitance of 188 C·g−1 at the current density of 0.5 A·g−1 as well as 78% retention of capacity after 5000 circles which was much higher than those of pure MoO2.119 Moreover, polyaniline-h-MoO3 hollow nanorods in a core/shell structure showed improvement in the specific capacitance (270 F·g−1) when compared to the pristine h-MoO3 hollow nanorods (126 F·g−1) or polyaniline (180 F·g−1) at a current density of 1 A·g−1 with enhanced cycling stability.129 66


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Additionally, hierarchical porous structures can offer more electrolyte access to the electrochemical sites and reduce diffusion pathway lengths. Brezesinski et al. have proposed that the mesoporous films of iso-oriented α-MoO3 owned a higher charge capacitance (605 C·g−1) than those of either mesoporous amorphous material or non-porous crystalline MoO3 (330 C·g−1 and 215 C·g−1, respectively) at the same sweep rate and the promotion might arise from an intercalation pseudocapacitance.88 Zheng et al. reported that the porous MoO2 nanowires exhibited a high areal capacitance of 424.4 mF/cm2 at a current density of 4 mA/cm2.49 Moreover, it showed that no obvious capacitance decrease occurred after 10000 cycles at many scan rates. Notably, the MoO2/carbon cloth electrode also delivered an ultrahigh rate capability, maintaining even a quite high capacitance of 320 mF/cm2 at a huge current density of 20 mA/cm2.

Related to the intercalation, the amount of oxygen vacancies on the surface of MoOx is another vital factor that influences the performance of supercapacitors. A reduced oxidation state exhibits a relatively lower specific capacitance, and a significant improvement of the cycling stability can be observed, It is believed that with oxygen vacancies introduced as shallow donors, the carrier concentration is also increased, leading to a better electrical conductivity.134,236 A kind of αMoO2.87 was designed to deliver higher capacitor-like charge storage performance oin comparison to fully oxidized α-MoO3 (75 % versus 37 %).134 The α-MoO3 structure could be retained during the insertion and removal of Li+ ions, since the oxygen vacancies were introduced. The introduction of oxygen vacancies could certainly lead to the improved electrochemical kinetics by increasing the conductivity and enlarging interlayer space 67



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encouraging the lithiation. Another reason for the high specific capacitance is that the reversible formation of a significant amount of Mo4+ following lithiation. Notably, the 2D nanostructures like nanosheets or nanoplates provide a better accommodation for Li+ ions to insert into the interlayer space. Hence, 2D MoOx often performs well to provide both high energy density and power density. For instance, an aqueous supercapacitor of MoO3 nanoplates was fabricated to deliver a high energy density of 45 Wh·kg−1 at 450 W·kg−1 and maintain 29 Wh·kg−1 at 2 kW·kg−1.84

It should be noted that the hexagonal phase of MoO3 has shown great potential to be a host material that can allow ions in the electrolyte to diffuse to all possible direction because of the availability of the various intercalation sites, including hexagonal window (HW), trigonal cavity (TC) and four-coordinated square window (SW). Thus, h-MoO3 possesses a relatively higher capacitance and better performance. Kumar et al. reported that the as-prepared h-MoO3 in pyramidal morphology exhibited high specific capacitance (230 F·g−1) at a current density of 0.25 A·g−1 with a quite low resistance around 5 Ω. Moreover, it displayed a good capacitance retention keeping 74 % of original capacitance after 3000 cycles.32

Coating or hybridization is often regarded as an effective strategy to enhance the performance of supercapacitors of metal oxides. Liu et al. reported that PPy could increase the life time and the conductivity of active materials at the expense of a slight loss of the initial capacitance, compared with virginal MoO3. The energy density also reached up to even 12 Wh·kg−1 at 3

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kWh·g−1. Moreover, there was only 17 % capacitance loss which mainly happened at the initial several cycles through 600 cycles, suggesting an excellent stability.

5.2.2 Ion batteries. Electronically conducting bronzes (AxMO3, A = H+, Na+ and so on) can be formed with the fast-reversible incorporation of alkali ions or protons into MoO3 lattices. These insertion reactions will occur with the formation of a large free energy, which has resulted in their use as the electrodes of lithium batteries. Thus, the MoOx materials can also be utilized in LIBs (See Table 2),25,31,45,50,64,83,91,104,105,111,137−139 because they exhibit some attractive properties to be host materials for lithium storage, such as the multiple valence states that allows a complex battery reaction, low electrical resistivity, high electrochemical activity toward lithium as well as affordable cost.41,66,109,110,115,140,141,144 For example, one of the great advantages of MoO2 is that during the insertion phase transition process (monoclinic 1 → orthorhombic → monoclinic 2) the volume change is merely 11%, much less than Si (~300 %).64 In addition, the high density (6.5 g/cm−3 for MoO2) of MoOx enables it to store more energy compared with that of graphite (2.3 g/cm−3), which allows the batteries to be smaller and more portable.31 Moreover, MoO2 anode material is a kind of safe electrode materials due to its higher Li insertion voltage compared with that of commercial graphite anode.

However, MoOx bulks suffer from poor cyclic stability and rate capability, resulting from a serious pulverization problem that will cause an electrical conduct pathway blocking problem and a sluggish lithiation process. Therefore, researchers have afforded downsizing, preintercalating Na+, Li+ or other ions, and hybridization to shorten the distance of Li+ 69



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transportation, facilitates the electronic transport and buffers the volume variation.41,111,140 Nanosizing can be an effective way to improve specific surface areas, shorten diffusion distances, and provide extra active sites.111 For example, bulk MoO2 is known to react with Li+ via an insertion/extraction processes, with a theoretical capacity of 209 mAh·g−1, while the nanostructured MoO2 exhibits a theoretical capacity of 838 mAh·g−1, about four times higher than that of bulk MoO2.100 Furthermore, it has been reported that core–shell and hollow nanostructures will significantly enhance the performance of LIBs.25 For instance, interconnected core/shell MoO2 material, as designed to deliver the initial specific capacity of up to 749.3 mAh·g−1 at 1 C and still maintained 83 % of the initial capacity after 50 cycles owing to the unchanged structure. Surprisingly, the core–shell MoO2 hierarchical microcapsules exhibited much better performance than pure MoO2 and could even be comparable to the MoO2/C nanocomposites. Besides, carbon materials such as carbon nanotubes, graphene, etc. are often composited with MoOx in order to mitigate the volume change and improve the electron conductivity.

Forming oxygen vacancies can also be an effective method to enhance the performance of batteries, which is similar to the MoOx-based supercapacitors. MoOx has varieties of valence states with different amount of oxygen vacancies on its surface. In principle, the localized states below the conduction band can be allowed by the existence of oxygen vacancies. In LIBs, oxygen vacancies can lead to changes in the electronic structure of metal oxides during the charging/discharging processes and enhance the conductivity, which will affect the energetics for electron and ion transport. Moreover, the oxygen vacancies on the interface between electrolyte 70


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and electrode can possibly provide more electrochemical active sites, facilitate the phase transition in lithiation process by modified surface thermodynamics and stabilize the structure, which can preserve the integrity of the electrode surface morphology.237,238 Sun et al. synthesized MoOx nanobelts with different quantities of oxygen vacancies through a sauna reaction and test their performance in LIBs.23 The H2 produced by the reaction of the carbon cloth and water vapor to reduce MoO3 under proper temperature and pressure, and the richness of oxygen vacancies depended on the reaction time. In the work, it was observed that a great enhancement of the performance aroused from the introduction of oxygen vacancies. For instance, the asobtained MoOx nanobelts remained a reversible capacity of 400 mAh·g−1 at 1 A·g−1 after 200 cycles without obvious capacity loss. Furthermore, MoOx exhibited a better rate performance than MoO3. The specific capacities of MoOx were 400 and 267 mAh·g−1 at the current densities of 100 and 200 mA·g−1, while those of the pristine MoO3 nanobelts were 123 and 84 mAh·g−1, respectively.

In a sodium ion battery, the oxygen vacancies of MoOx can enhance the electric conductivity and Na-ion diffusion coefficient.74 Li’s work revealed how oxygen vacancies of MoOx influenced the performance of the sodium ion batteries.142 Among MoO2.5, MoO2.97 and MoO3, it can be interestingly found that MoO2.97 possessing only a few oxygen vacancies exhibited the best specific capacity (176.6 mAh·g−1 at 50 mA·g−1) and cycling stability, showing merely 8 % capacity loss over 2000 cycles compared with the highest capacity at the 500th cycle. Additionally, according to the voltage profile of MoO2.5, a two-plateau characteristic is discovered unlike a sloping feature typically observed in orthorhombic α-MoO3, suggesting the 71



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stabilized function of oxygen vacancies during the cycling process. Nevertheless, in-depth study may be required to demonstrate the relationship between the oxygen vacancies and performance.

To better design ion batteries based on MoOx materials, a deep mechanism understanding of MoOx need to be explored. The mechanisms may vary because different types of active materials have different lithium-intercalation ability. For example, bulk MoO3 only involves the intercalation process, while nanostructure MoO3 involves both intercalation and transformation. However, many mechanisms concerning lithiation and delithiation processes still keep ambiguous as there has not been much direct evidence. Most work has proved that the MoOx can be transformed into Mo and Li2O.109,140 Furthermore, Xia et al. proposed that after the formation of Mo and Li2O, Mo nanograins could continue changing into crystalline Li1.66Mo0.66O2 along with the disappearance of Li2O and size shrink, followed by the conversion to amorphous Li2MoO3 during the cycling process.141

Interestingly, in the first cycle, the first lithiation capacity of a MoOx system may far exceed the theoretical capacity of MoOx, coming up with a sharply dropped second capacity and coulombic efficiency. To be more specific, during the first lithiation cycle, α-MoO3 undergoes an irreversible phase transition around 2.8V (versus Li/LiC) leading to limited cycling lifetimes.143 It is because of the formation of Li2O and the irreversible Li ions intercalating into the crystal lattice, as well as the other irreversible electrochemical processes such as electrolyte decomposition and inevitable formation of solid-electrolyte interface (SEI) layer.7,45 Xia et al. reported that the reversible capacity of MoO2 nanosheets could be up to 1516 mAh·g−1 after 100 72


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cycles at 100 mA·g−1, while 489 mAh·g−1 at 1000 mA·g−1, which is higher than the theoretical capacity.71 A possible explanation is the formation of metallic Li around LixMoO2 significantly enhances the measured value, which can be supported by the observation of Li storage in the MoO2.

Another advantage is that MoOx can be utilized as both cathode and anode materials (400 mAh·g−1 for MoO3 cathodes, while 1000 mAh·g−1 for MoO3 anodes).65,114,137,144,177 For instance, Zhou et al. found that when the MoO3 nanobelts were used as cathodes material, the cathode showed good stability with high coulombic efficiencies between 96 and 101 % for 50 cycles. It was worth being mentioned that the charge-transfer resistance decreased after the first cycle and become even smaller after the 50th cycle.65 The enhancement of conductivity of the electrode could be attributed to the formation of LixMoO3 caused by the incomplete deintercalation of Li+ during cycling. Chu et al. reported that the MoO3 nanowire bundles could reach a relative high capacity of 195 mAh·g−1 at 100 mA·g−1, though the materials faced big problems in cycling stability with merely 37 % capacity retention after 100 cycles.56 In contrast, large quantities of work have been reported on MoOx anode materials which displays ultrahigh capacity or coulombic efficiency. The MoO2 nanorods synthesized by Guo et al. showed a high reversible Li storage capacity of 830 mAh·g−1 (99 % of the theoretical capacity) and an excellent cycling performance as an anode for Li ion batteries after 29 cycles.31 Yang et al. also reported a flowerlike MoO3 performed well as anode materials in LIBs due to the large surface area.54 The MoO3 nanoflower anode could even reach up to a high capacity of 1020 mAh·g−1 and maintain 810 mAh·g−1 (78 % retention) after 100 cycles at a current density of 550 mA·g−1. 73



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As mentioned above, hybrids of MoOx materials have been regarded as ideal electrochemical active materials. Semiconductors has been applied into composition with MoOx. Manthiram and coworkers synthesized W0.4Mo0.6O3 which exhibited an initial discharge capacity of 1060 mAh·g−1, even higher than the capacity expected for the intercalation of lithium (898 mAh·g−1) into W0.4Mo0.6O3 to form metallic W and Mo. Unfortunately, it showed a fast capacity fade after only 50 cycles. To solve the problem, a kind of carbon-decorated WOx/MoO2 nanorods was applied to exhibit a stable capacity retention due to the existence of carbon buffer layer. Interestingly, upon cycling, W0.4Mo0.6O3 involved a delithiation process of Li2O and an oxidation process of W and Mo, while the carbon-decorated WOx/MoO2 nanorods may not be transformed into Mo metal, and no Li2O can be found by any detection.103 Besides, the MoS2/MoO2 nanonetworks was reported to deliver a reversible discharge capacity of 1233 mAh·g−1, with only 5 % degradation after 80 cycles. Meanwhile, the capacities are 1158 and 826 mAh·g−1 at the discharging rates of 200 and 500 mA·g−1, respectively.104 Additionally, Liu et al. reported a hollow structure of MoO2 coated with Mo2N shown both good cycling stability and high capacity, because the Mo2N significantly increased the performance.108 It exhibited good rate capability with 660 mAh·g−1 at 1 A·g−1, 548 mAh·g−1 at 2 A·g−1, 415 mAh·g−1 at 5 A·g−1, respectively, while the bare hollow MoO3 merely showed a reversible capacity of 109 mAh·g−1 at 5 A·g−1. In addition, it could maintain 91 % of the initial capacity after 100 cycles, suggesting a great promotion in cycling stability. Meanwhile a significant decrease has been shown in the resistance from 165 to 50 Ω. All enhancements can be attributed to the Mo2N coating which not only bring a protection of the oxidation of MoO2 as well as a higher conductive network but also mitigate mechanical degradation to keep the good stability of electrodes. 74


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Carbon/MoOx has been considered as a feasible way to enhance the performance of LIBs. The carbon/ MoOx not only strengthens the integrity of the active material and the current collector avoiding an exfoliation, but also exhibits a high conductivity than that of pure MoOx. Lu et al. successfully synthesized a kind of graphene nanosheets encapsulated α-MoO3 nanoribbons which exhibited a high specific capacity (823 mAh·g−1 after 70 cycles at 200 mA·g−1), as well as an extraordinary cycling stability with more than 754 mAh·g−1 after 200 cycles at 1000 mA·g−1 accompanied with good rate ability.112 It showed better performance compared with MoO3 simply grown on graphene or other similar hybrids of MoO3.The encapsulated structure enwrapping the MoO3 compensates for the volume expansion, thereby resulting in prolonging the cycling lifetime. Meanwhile, the existence of α-MoO3 separated the graphene layer and prohibited the restacking of those graphenes. Furthermore, the graphene with a high conductivity could provide additional surface area and increase the contact with electrolyte and ions diffusion which effectively facilitates the electron and ions transport. Consequently, the special structure of graphene nanosheets encapsulated α-MoO3 nanoribbons performed well in LIBs. An ultrafine MoO2 around 2 nm on the carbon matrix was reported to keep good cycling stability.117 The specific discharge capacity of MoO2/C was as high as 1207 mAh·g−1 in the first cycle and 734 mAh·g−1 over 350 cycles at 50 mA·g−1, while a specific capacity of 602 mAh·g−1 could be remained at 100 mA·g−1 after 600 cycles. The coloumbic efficiency could also keep around 100 % during the cycling process, as ultrafine MoO2 nanoparticles embedded in the carbon matrix showed great resistance to the volume change and accelerated the charge transfer kinetics, ion diffusion by the synergistic effect of the nanocomposite. MoO3/polymer materials have also been used as anode materials for LIBs. Mohan et al. reported that though the MoO3/polyanilines 75



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showed a lower initial capacity of 228 mAh·g−1 compared with that of pure MoO3 (276 mAh·g−1) at 30.7 mA·g−1, the composite could work within 1.0−4.0 V with a better stability.124 The lower capacity was because of the reduced specific surface area with smaller pore sizes, while the increase of stability is due to the enhancement of insertion/extraction of Li+ ions.

Recently, myriad of efforts has been made to fabricate bind-free electrodes achieved by directly depositing MoOx on frameworks, since the performance will enhance caused by the decreased resistance without any binders. For instance, a MoO2/3D graphene framework nanocomposite was directly used as a binder-free anode electrode without any additives or binders, which delivered high reversible capacities of 975.4 and 537.3 mAh·g−1 at the current densities of 50 and 1000 mA·g−1. More importantly, the electrode also showed an increased capacity from 763.7 to 986.9 mAh·g−1 after 150 discharge/charge cycles at a current density of 200 mA·g−1, which might be explained by the synergistic effects of MoO2 nanoparticles and 3D net-work graphene layers.66 Additionally, Wang et al. reported the MoO2@C could exhibit a high capacity of 1034 mAh·g−1 at 0.1 A·g−1. And even at a superior current density of 22 A·g−1, a reversible capacity of 155 mAh·g−1 still remained. Furthermore, it displayed a good stability during hundreds of cycles which was attributed to good accommodation for volume expansion by mesoporous structure as well as excellent conductivity.116 Furthermore, it kept 50–80 % of the theoretical capacity at 0.5–3 C while most MoOx only showed good rate performance at a low rate (0.05–0.2 C). Interestingly, a hierarchically nanostructured ‘‘cellulose’’ MoO2 monolith was fabricated into anodes in lithium ion batteries without any binder.91 It exhibited a high specific capacity of 719.1 mAh·g−1 at 200 mA·g−1 after 20 cycles, much higher than other anodes 76


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fabricated of MoO2 materials (for example, MoO2 nanoparticles reduced by ethanol vapor: 318 mAh·g−1 at 5.0 mA/cm2; MoO2/carbon hybrid nanowires converted from the organic-inorganic hybrid precursors: 595.7 mAh·g−1 at 200 mA·g−1). However, the bind-free method usually applies Ni foam or carbon as structural framework. Hence, few hierarchical or multipledimension morphologies of MoOx can be synthesized by some approaches, which remains to be improved.

In brief, lots of efforts have been made in ion batteries for providing low electrical resistivity as well as high reversible capacities. Nevertheless, a greater basic understanding of the mechanism is still in urgent need, since it can offer guidance in designing MoOx material with better performance in the field of energy storage. Additionally, although it has been found that oxygen vacancies can be beneficial of enhancing the performances, most recent reports have focused on the MoO3 and MoO2 in the lithium batteries, while MoOx with medium valence state that can combine both the advantages of MoO3 and MoO2 are rarely reported in literature. Therefore, some further improvement may be required to explore in the near future.

5.2.3 Solar cells. Organic solar cells (OSCs) are one of the most common energy conversion devices which are quite promising for its fascinating properties of eco-friendly, low-cost and light weight. Heterojunction polymer solar cells (PSCs) have been regarded as a kind of OSCs which has relatively high power conversion efficiency (PCE) even exceeding 10 % due to low band gap possessed by the polymers with deep highest occupied molecular orbital (HOMO) energy level.145,239,240 However, it is still a big challenge that the polymers hole transport layers 77



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(HTLs), which usually behaves acidic, will attack the oxygen in ITO layer leading to a further structural deterioration. One undergoing measure to prolong the lifetime as well as inhibit electrode oxidation is to establish an intercalated layer as an electron selective interlayer. MoO3 has shown giant potential as it is both an extraordinary hole injection material and excellent hole transport material that can form ohmic contact with quantities of organic hole transporting materials.147,148,151 Moreover. MoOx shows potential that it can enhance the PCE via LSPR, which can be attributed to the improved light absorption and enhanced conductivity.146 In addition, the work function of MoO3 is around 5.3−5.7 eV fitting quite well with the optimum HOMO level of many polymers used in PSCs and it may have a synergistic effect with metal layers when the MoO3 is used as a top interlayer. Figure 24a is a schematic of the structure of solar cells, where MoO3 play as an HTL in varieties of forms.150 Taking MoO3/AgAl alloy anode as an example, the existence of MoO3 helped form an AlOx interlayer that not only prevented the diffusion of Ag atoms into organic layer also increased the built-in potential across the active layer as well as established a stable organic/anode contact to impede the moisture encroachment.145

Besides, the presence of MoO3 seems to have an additional influence on the performance of the OSCs that it can keep interaction with some electron donor layers. Dou et al. fabricated a solar cell employing squaraine (SQ) donor and fullerene acceptor as well as a MoO3 electron transport layer.149 When a MoO3 layer was applied in a 6-nm SQ layer system, a significant increase in the VOC could be observed when introducing the MoO3 layer. The phenomenon can

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be explained by a chemical interaction between SQ and MoO3 layers which influences the electronic conduct and recombination behavior.

Furthermore, a relationship between the sizes of MoO3 particles and the morphologies of the hole transport layers has also been investigated.150 They have successfully synthesized 120nm and 15-nm MoO3 nanoparticles through a solution-process method named d-(MoO3)120 and d-(MoO3)15, respectively. It was observed that MoO3 with smaller size had a lower surface energy which led to an intimate contact with the active layer and then improved the PCE of the solar cells from 2.06 to 3.35 %, even higher than the PCE of pure PEDOT: PSS (around 3.04 %). Notably, when a bilayer of evaporated MoO3/PEDOT:PSS was applied in the device, the exciton generation rate in the active layer could be promoted accompanied with the light intensity.

As mentioned, a bilayer strategy using polymer:MoO3 layers is often adopted because of feasible fabrication, excellent film forming ability and promoted device performance. Diversities of HTLs can be obtained with various synthetic routes which may exhibit differential performance on the solar cells. For instance, a core/shell structure was synthesized by mixing MoO3 nanoparticles ink together with PEDOT:PSS.148 As MoO3 had a preference to connect with PSS, the core/shell structure with PEDOT being a core and MoO3:PSS a shell could be formed through an assembly process. Such architecture ensured HTLs a better adhesion to photoactive layers caused by the presence of MoO3 and inhibited the aggregation of MoO3 nanoparticles, thus yielding a smooth surface and a better performance. Notably, a high blended ratio of MoO3 caused a decrease in the device performance in spite of a better stability, which 79



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could be aroused from the change in both the resistance and the morphology. To enhance the Jsc or PCE of the solar cells, decoration is a widely-used strategy compositing MoO3 with other semiconductors such as MoS2 or CdSe.152,153 For instance, Qin et al. proposed an in-situ synthetic process to obtain a double layer of MoO3:MoS2 from MoS2. Using such method, the changes of energy level could be adjusted by the deposition temperature to some extent and then showed p-type conductive behavior. Smaller electron affinity achieved by such structure also effectively blocked the electron from the exciton dissociation, resulting in a better hole-transfer process and an increase in the Jsc.

MoO3 has also been used in dye sensitized solar cells (DSSCs) as a reverse electron recombination barrier layers (RBLs). The MoO3 film will influence the electron–hole capture process and then suppress the interfacial recombination by facilitating efficient charge transport and blocking the leaky holes at FTO/TiO2 interface. To evidence that, a 15 % promotion in the PCE was observed compared with that of solar cell without MoO3 RBLs.154 Moreover, the MoO3 thickness influenced the fill factor and Jsc as well, which was attributed to the photo-electron collection at the FTO electrode. In addition, although theoretically MoO3 can absorb more dyes with more pores, the porosity may cause a diffusion of the liquid electrolyte to the bulk and thus lead to a TiO2/N719/electrolyte junction bringing about a possible electrical short. The valance state also affects the performance. When at a lower surface state, it can be much more feasible to go through an electron capture process by trapping photo-generated electrons at the FTO/TiO2 interface. Therefore, both the morphology and the defects of MoO3 have great impact on the device performance in DSSCs. To make a reasonable explanation for the function MoO3 layer, 80


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the energy alignment is analyzed in Figure 24. Figure 24b shows the energy band of the FTO, TiO2, N719 and electrolyte before exciton generation, while Figure 24c shows several physical processes occur under light. Process 1, 2, 3, 4 and 5 are corresponding to exciton generation in N719, electron-injection by N719 to EC of the TiO2, diffusion, collection at FTO, and transport in the FTO, respectively. As shown in Fig 24d and 24e, the introduction of MoO3 layer changed in FTO/TiO2 interface and thus the density and activity of defects on the interface, leading to the prohibition of electron–hole recombination.

Notably, Dong et al. proposed an ultrafast laser-assisted synthesis route of hydrogenated molybdenum oxides which was used into fabrication of solar cells.151 The hydrogen molybdenum bronzes displayed good performance in light trapping in the flexible topilluminated OSCs and possessed high WF as well as electrical conductivity controlled by laser energy, causing an increase in short-circuit current compared to the device with PEDOT:PSS layers. Furthermore, it shows a good mechanical stability of PCE which has no significant decay after 5000 bending cycles.

Briefly, MoO3 has been used mainly as hole transport layer in solar cell because of its semiconducting properties. The existence of MoO3 can effectively prevent PEDOT:PSS layer from deterioration and have a WF matching to the HOMO level of many kinds of polymers, which enhances the performance of solar cells and prolongs the lifetime of solar cells. However, there is still much left to do in order to improve the PCE that mainly restricts the practical use of solar cells. 81



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5.3 Gas sensors

Gas sensors are one kind of important devices under many circumstances to detect toxic or flammable and explosive gas or monitor air quality. MoO3 semiconductor is regarded as a good candidate to be gas sensors for the compatibility, low cost, high sensitivity and simplicity, which can be used to detect many kinds of gas like H2S or ethanol due to its intrinsic semiconductiveproperties.27 The detection mechanism of some gases may be described as follow:

H2S + 3O−(ads) → SO2 + H2O +3e−

(3)

C2H5OH + 6O−(ads) → 2CO2 + 3H2O + 6e−

(4)

CO + O−(ads) → CO2 + e−

(5)

H2 + O−(ads) → H2O +e−

(6)

Where the O−(ads) represents that chemisorbed oxygen species on the surfaces of MoO3.181 The equations (3)−(6) suggest that the surface oxygen groups will interact with various kinds of gas and react with them following by a change in resistance. In other words, when the surface of gas sensor is exposed to the reducing gas, reducing molecules (such as H2S) can react with oxygen ions absorbed on the surface, leading to the production of more free electrons for improving conductivity.

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Notably, it can be achieved to adjust the sensitivities and response rates by altering the sizes, morphologies as well as the compositions of components. 1D semiconductor metal oxides have been widely used as gas sensing materials due to their large specific surface areas.241 [001]oriented α-MoO3 nanoribbons have been used onto hydrogen sensing. The sensitivity, response speed, and recovery speed of the sensor improved with the increasing hydrothermal temperature.29 It is reported that a MoO3 film consisting of rhombic molybdenum trioxide with a (010) texture showed a good performance on detecting NH3 at the operating temperatures between 400 and 450 oC.155 Besides, α-MoO3 nanorods exhibited high sensitivity to CO, which showed a highest response (239.6) to 40 ppm CO at an operating temperature of 292 oC.53 They found that the annealing temperature would influence the performance of sensors, because the enhancement of crystallinity resulted in an increase of response and meanwhile the specific area decreased with crystallite size increasing. Additionally, it was believed that oxygen vacancies played an important role in gas sensing. A possible mechanism is speculated as follows:242

OO → VO2+ + Oi2−

(7)

CO + Oi2− → CO2 + 2e−

(8)

Where OO is the lattice oxygen, VO2+ the oxygen vacancy in the lattice, and Oi2− the interstitial oxygen. When the CO interacted with the surface of MoO3, it would be oxidized into CO2 and resulted in an increase in the concentration of oxygen vacancies, causing a decrease in resistance.

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MoO3 nanosheets tend to be partially reduced forming MoOx which maintains the structure at high temperature, thus the oxygen-deficient MoOx nanosheets are strong adsorbents to organic molecules, which allows them the best candidates in chemical sensors for volatile organic compounds. 2D-MoO3 nanosheets were far more effective than bulk MoO3 powder for chemical sensor application due to an increased surface area and reactive sites.96 Alsaif et al. also reported the hydrogen sensor based on 2D α-MoO3 nanoflakes which behaved fast responses and high sensitivity to 1 % of H2 gas at 50 oC.156 The gas sensing mechanism is given in Figure 25. H2 will react with MoO3 and form H2O, which lead to a change in the resistance. Notably the reaction can be reversible which allow the recovery of devices. Ethanol sensors fabricated of α-MoO3 nanoplates exhibited a high sensitivity of 44–58 at a concentration of 800 ppm operating at 260– 400 oC and their response times were less than 15 s.78 Moreover, the selectivity of ethanol was also so good that it could easily display a good discrimination between ethanol and other organic vapors, since it almost had no response to formaldehyde, isopropanol, benzene during the whole operating temperature region.

Hollow nanostructures show a great advantage in gas sensors that they are more favorable to the diffusion of gas molecules which can effectively reduce the response/recovery time. For instance, the bulk MoO3 only owned the specific surface area of 0.85 m2/g, whereas that of a MoO3 hollow microspheres could reach up to 48.2 m2/g.94 Due to the enhanced surface area, the hollow MoO3 microspheres can get the maximum sensitivity to 500 ppm NH3 of about 20 at an operating temperature of 270 oC. Moreover, it showed a fast response and recovery time which was 5 and 30 s, respectively. One exciting thing is that the gas sensors fabricated of the hollow 84


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MoO3 microspheres have the potential to behave a good thermal stability in a relatively broad temperature range, as a complete ionization of electrons of the donor level will occur.

It should be mentioned that doped MoOx usually shows a better performance for gas sensing. Cd-doped MoO3 showed three times higher response to H2S than that of pure MoO3 as well as small cross-sensing to other reducing gases, like CO, methanol or NH3.97 Obviously, the enhancement of performance of gas sensor could be attributed to the defects brought by Cd that influenced the surface characteristics of semiconductors, though excess dopants would impede a carrier transport leading to a decrease the performance of gas sensing.

In short, MoOx has been fabricated to sensors of large quantities of gases in the operation temperature range. The sensitivity and selectivity to gas can be adjusted by applying various morphologies and compositions.

5.4 Catalysts

5.4.1 Multi-phase catalysis. Noble metals like Pt, Au or Pd have shown excellent properties in decomposing methanol into H2due to the strong catalytic activity on special facets. Removing the generated H2 continuously in the system will help promote the thermodynamic driving force which will result in an increase in the conversion and H2 yield. As MoO3 is a good kind of metal oxide which can store H atom in a form of HxMO3, it does well in decreasing the amount of H atoms in system and keeping metallic surface free from H atoms. Martins et al. compared the performance of several Pt/metal oxides support catalysts like Pt-ZrO2, Pt/SiO2 and Pt-Al2O3 with 85



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or without the presence of MoO3.157 The selectivity of producing ethylene is in following sequence: Al2O3> SiO2> ZrO2. However, it is altered with the presence of MoO3 into SiO2> Al2O3> ZrO2. The presence of MoO3 significantly improves the C–C nucleation capability of Pt/SiO2 catalyst during methane chemisorption, which is supposed to enhance the mobility of C ad-species, resulting in the formation of ethane and propane.

The catalytic mechanism is widely accepted as a Lewis acid center mechanism. Hydrogen molecules was dissociated on Pt or other noble metal catalyst sites and formed H atoms that spilt over and diffused on the surface. The spiltover hydrogen atom would combine with a Lewis acid site and an H+ by donating an electron. Then the H+ ion was stabilized by O atoms nearby. Subsequently, The Lewis acid site reacted with another spiltover hydrogen atom forming an H−ion which was bound to a Lewis acid site.159 Hence, the amount of Lewis acid center can determine the performance of catalysts. However, one of the problem α-MoO3 faces in chemisorption and reaction of methanol is that the major (010) crystal face in the layered solidstate structure of α-MoO3 is coordinatively-saturated, so that it owns a low coverage of an important intermediate Mo(OCH3), thus resulting in a lower reaction activity. In contrast, βMoO3 has more surface adsorption sites for methoxy groups, leading to a better and catalytic activity and selectivity of methanol transformed from into formaldehyde. The methanol conversion at 523 K over β-MoO3 is almost 19 times higher than that over α-MoO3, which can be attributed to a larger number of the Lewis acid sites on the surface accommodated for the methanol.158

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MoO3 has also been used into the oxidization of small organic molecules. Li et al. reported the facet-dependent catalytic activity of ethanol oxidization.69 It was observed that the α-MoO3 nanobelts exhibited a higher activity to acetaldehyde at the expense of a lower selectivity compared with the microflowers, mainly due to the large exposure of the (010) planes, considering the similar active sites for the oxidation they possess.

5.4.2 Photodegradation of dyes. For a good phtocatalyst that can exhibit high reactivity and stability under natural sunlight and especially visible light irradiations, it requires that the band gap of a photocatalyst material has better be narrower or less than 3.0 eV and a percentage ionic character between 20 and 30 %. MoOx is a kind of promising material that meets such requirements with its band-gap easily being controlled by the amount of oxygen vacancies. Furthermore, a combination of MoOx and semiconductors or noble metals, such as Pt/MoO3 and Au/MoO3, will enhance the performance of photodevices or photocatalysts as well because of the formation of Schottky barrier and change in Fermi levels. Moreover, the reduced band-gap and increased surface area also contributes to the enhancement.59

Cationic methylene blue (MB) dye, possessing two absorption bands at 293 (π–π*) and 664 (n–π*) nm in aqueous solution, represents one kind of general organic pollutants existed in waste water. Therefore, the degradation of such kind of dyes becomes a hot field, especially photocatalytic degradation of organic dyes which shows giant potential because of the low cost of solar energy. Electrons and holes are located into the conduction and valence bands of MoO3 during the photocatalytic process, as a result of the photovoltaic effect. Afterwards, O2 and H2O 87



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could be attached to the surface of MoO3 and then interact with holes or electrons to produce active oxidants O2− and OH·, respectively. Consequently, the active oxidants continue the chain reaction and turn into small molecules like CO2 and H2O. In other words, MB molecules are degraded into small molecules such as CO2 and H2O through the direct oxidation of MB by these active oxidants.220 Li et al. compared the catalytic activity of α-MoO3 nanobelts with that of hMoO3 microrods using photo-catalytic degradation reaction of MB. In the first 180 min, the degradation of MB over the h-MoO3 microrods was 97 %, while that of α-MoO3 nanobelts was 80 %. Obviously, h-MoO3 showed a higher photodegradation efficiency.46 Besides, MoO3 synthesized with the presence of F− showed a better photodegradation of MB (57 %) than that of MoO3 without F− (34 %), resulting from the different morphology of samples that caused a different exposure of crystal planes.93 However, it displayed a significant decrease in stability which might be ascribed to the active sites of the faces covered by MB partial decomposition products.

Doping can also help enhance the degradation efficiency. Ce-doped MoO3 exhibited a 10 times higher degradation activity of methylene blue than pristine MoO3.98 To be more specific, 87 % of MB could be degraded by 5 % Ce-doped MoO3 under visible light irradiation in 50 minutes and 71 % of can be degrade under light-off condition. In the photocatalysis reaction, CeO2 not only reduced the band gap to allow more light absorption but also enhanced the electron conductivity by establishing an efficient separation of charges. The light-off degradation mechanism could be explained as follow:

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2MoO3 + MB → R + + H2O + Mo2O5

(9)

Mo2O5 + 2CeO2 → Ce2O3 + 2MoO3

(10)

Ce2O3 + 1/2O2 → 2CeO2

(11)

The CeO2 acted as a vital catalyst in the redox cycle and the improved activity under light irradiation, which could be regarded as synergetic effect of both photocatalytic mechanism and redox cycle mechanism.

Hybridization is an alternative strategy to optimize the photodegradation efficiency, since they can improve light absorption or adjust the charge transfer by heterostructure. It has been reported that MoO3/g-C3N4 enhanced the photocatalytic performance compared with simple component MoO3 or g-C3N4. The MoO3/g-C3N4 composite induced an efficient degradation of MB (95 %) in 2.5 h under visible light irradiation, which was much higher than that of g-C3N4 (49 %), MoO3 (32 %) and a MoO3/g-C3N4 mechanical mixture (41 %).125 A possible reason can be proposed that a blue shift of light absorption occurring in the composite of MoO3/g-C3N4 will allow more light absorption as well as be excited by more light protons, and meanwhile, the composite has the electron–hole pairs recombined to improve the efficiency. MoO3/MWCNT nanocomposites accomplished rapid degradation of total organic carbon (TOC) under UV light, with a high efficiency up to 95 % in 15 minutes.113 In contrast, when using visible light, though it displayed a fast initial decomposition rate and 60 % of the carbon atoms were degraded into CO2 during the decolorization of toluidine blue O in 20 minutes, complete TOC removal could not be fully 89



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accomplished before 40 hours as a result of stable intermediates formed during the photodegradation.

Orange II dye has also been used as targeted dyes to evaluate the catalytic activities of MoO3 through a catalytic ozonation process. A much faster degradation will occur when using MoO3 to assist the photocatalysis. Color removal was fully completed in 30 min in MoO3 catalyzed ozonation experiments color removal, whereas it could only achieve about 50 % in the absence of MoO3, which can be attributed to the facilitated kinetics of ozone decomposition resulting in breaking of azo bond in the dye molecule.55

5.4.3 Hydrogen evolution reaction. Hydrogen has been regarded as one of the most promising novel energy sources due to the clean, sustainable and renewable properties. To date, two effective approaches including electrocatalysis and photocatalysis have been applied to the hydrogen production.

Electro-catalytic HER. MoOx is regarded as a low-cost metallic semiconductor, which possesses high stability and high metallic-like electrical conductivity. To some extent, it can be a potential substitute of the noble metal as a hydrogen evolution reaction electrocatalyst.40,160

In theory, three possible steps are suggested for the evolution of H+ to H2, as described below:243,244

Volmer step: H+ + e− → Hads (Tafel slope: ~ 120 mV)

(12)

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Heyrovsky step: H+ + e− + Hads → H2 (Tafel slope: ~ 40 mV)

(13)

Tafel step: Hads + Hads → H2 (Tafel slope: ~ 30 mV)

(14)

To lower the overpotential of HER, superb electrocatalysts are required to have to possess excellent conductivity to facilitate electron transport, thus many strategies are taken to modify the electrocatalysts. First, MoO3 can hybrid with semiconductor with high HER activities. For instance, the electronic conductivity of MoS2 can be enhanced by using MoO3 as substrate and synthesized a MoS2/NiS/MoO3 catalyst via gas-phase sulfurization, which exhibited that in alkaline medium the HER overpotential was only about 91 mV for the current density of 10 mA/cm2 and the robust long-term stability during was more than 20 h.160 3D MoS2/MoO2 hybrid materials was reported to exhibit great activity in HER attributed to the small onset overpotential of 142 mV, a largest cathodic current density of 85 mA/cm2, a low Tafel slope of 35.6 mV/dec and robust electrochemical durability.106 Second, carbon matrix can help enhance the stability and thus offer a better performance. The MoO2/rGO composite exhibited a Tafel slope of ~68 mV per decade in HER, and the onset potential was −0.16 V (vs. RHE). Furthermore, the MoO2/rGO showed excellent persistence that it maintained about ~ 80 % of its original activity after 500 cyclic tests, and still keep ~ 70 % after 1000 cycles.68 Moreover, the direct growth of MoO3 on nickel foam can offer a new perspective of binder-free strategy, which increase the exposure of active sites. MoO2/Ni foams could be used as a HER electrocatalyst in alkaline medium with excellent stabilities that it can maintain 94.3 % of its initial current density after 25 h.92 The overpotentials of MoO2/NFs were merely about 55 and 80 mV when current densities 91



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reached up to 10 and 20 mA/cm2, respectively. Moreover, hydrogen bubbles were observed on the surface of electrodes after keeping a static overpotential of 10 mV for several minutes and the Rct of MoO2/ Ni (2.4 Ω) was much lower compared with that of Ni foams (approximately 80 Ω). According to the work of Cui’s group, they used the template to synthesize porous MoO2 nanosheets/Ni foam which showed quite low overpotential (27 and 40 mV to reach up to −10 and −20 mA/cm2, respectively) as well as low Tafel slope.26 Meanwhile, it also exhibited good performance in OER. Compared with the poor performance of compact MoO2, a conclusion could be achieved that the high specific area and more active exposed sites were vital in the excellent performance in the water splitting. In addition, the binder-free method also contributed to the high catalytic activity.

Photo-catalytic HER. MoOx can not only be an electrocatalyst for HER but also a good photocatalyst. Ammonia borane (NH3BH3), containing as much as 19.6 wt% of hydrogen and exceeding the energy-carrier capabilities of gasoline, is an attractive candidate for nextgeneration hydrogen-storage materials.161 Without catalysts, NH3BH3 is quite stable in water and no H2 is generated, even when exposed to visible light irradiation. The dehydrogenation of ammonia borane to produce H2 can be described as follows:77,162

H3NBH3 + 2H2O → 3H2 + NH4+ + BO2−

(15)

Shi et al. reported a MoOx material reached a maximum rate up to 2.10 µmol/min at the beginning of reaction at room temperature, which was much faster than pure MoO3 due to the LSPR adsorption of MoOx nanoparticles.162 The probable mechanism of NH3BH3 92


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dehydrogenation was speculated as a three-step procedure.21 First, an activated complex species formed as a result of the interaction between the metal particle surface and NH3BH3. Second the B–N bond was dissociated after attack by a H2O molecule. Finally, the resulting BH3 intermediate hydrolyzed, and then the H2 was evolved accompanied by the formation of BO2− ions. Interestingly, the catalytic activity of MoOx for the hydrolytic dehydrogenation of NH3BH3 exhibited a morphology-dependent property.90 It was found that flower-like microstructure exhibited the best catalytic activity than those of the schistose- and nanorod-morphology, which can be attributed to a synergistic effect of LSPR and surface area.

To enhance the activity of the catalysts, lots of efforts have been made in the design of morphology or hybrid structure. Yamashita’s group reported a Pd/MoOx material that could exhibited that the H2 yield rate of the Pd/MoOx hybrid under visible-light illumination (2.0 mL/min) was found to be four times over that in the dark condition in an aqueous NH3BH3 soultion.100 They also reported one kind of MoOx nanosheets obtained by surfactant-free nonaqueous synthetic procedure exhibited good performance (5.74 mol%/min) on the hydrogengeneration activity of NH3BH3 under visible light irradiation, which even displayed a two-fold higher initial H2 yield rate than the Ag/SBA-15 samples with the best activity (2.67 mol%/min).77

Hybridization is another effective way to improve the photocatalytic performance.101,245,246 For instance, Shen et al. synthesized CdS nanocrystal embedded in MoO3@CdS core/shell structure, exhibiting a large enhancement in photocatalytic activity under visible light irradiation.245 In this 93



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system, CdS mainly harvested the light, while MoO3 acted as a core and worked as electron acceptor. Meanwhile, the core/shell structures not only offered a better conjunction but also decreased the electron-hole recombination. Additionally, organic semiconductor such as polyimide (PI) can also be used into the photo-catalytic HER in a methanol-water solution.237 The MoO3 flakes anchored on PI could induce light absorption and limit the recombination of charge carriers, leading to a better performance. The photogenerated electrons from conduction band of PI could transfer to the surface of MoO3 to reduce water, while the photogenerated holes at the valence band of PI was the center of methanol oxidation.246

Hydrolysis reaction of active metal like Mg or Al has attracted a lot of attention, because of its low density, excellent activity, low cost and more importantly the eco-friendliness of the hydrolysis product. Take Mg as an example, the hydrolysis reaction can be described as:

Mg + 2H2O → Mg(OH)2 + H2 △Hr = −354 kJ/mol

(16)

The main problem the promising catalyst faces is that the formed Mg(OH)2 will grow directly on the catalyst and form a thin film, thus leading to a significant decrease in the rate of H2 generation. Mg combined with Mo or its compounds (MoS2, MoO2 and MoO3) can act as quite promising constituents of hydrogen generation system in order to achieve satisfactory hydrolysis performance and overcome the shortage of low stability of catalyst at the same time. Huang et al. developed a HER system using Mg/Mo or its compounds (MoS2, MoO2 and MoO3) for catalyzing hydrolysis of seawater.163 It suggested that with the small amounts of the compounds of Mo added, the activation energies for hydrolysis decreased significantly from 63.9 to 27.6, 94


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20.4, 14.3 and 12.1 kJ/mol as involving Mo, MoS2, MoO2 and MoO3, respectively. Moreover, the composites of Mg/Mo compounds yielded around 90 % H2 in ten minutes and relative stable cycle performance.

In brief, MoOx has shown a good activity in HER both in photocatalytic and electrocatalytic reaction due to the surface state on the catalysts. However, most photocatalytic process was finished to decompose NH3BH3. A further exploration to photocatalytic water splitting is still untapped but meaningful.

5.5 Chemical templates

As mentioned, MoOx can be synthesized in multi-dimensional micro-/nanostructure and exhibit great diversity in morphologies (see Table 1), thereby showing the potential in acting as an alternative hard template, especially sacrificed template for the further controllable synthesis of molybdenum chalcogenides and cabrides. In a typical synthesis, the method for the MoOx template-assisted synthesis of other materials follows several steps: (i) the synthesis of welldefined MoOx templates with unique structures, and (ii) the synthesis of target materials on the above templates by sulfurization or carbonization treatment.

It has been well-known that MoS2 can be converted efficiently from pristine MoO3 solid particles or vapor phase clusters by sulfurization.164,168 The α-MoO3 can change into certain reduced-state compounds such as HxMoO3, MoOx, MoO2, MoO2−xSx, and finally MoS2 according to the time and temperature of H2S/H2 stream treatment. Lin et al. transformed MoO3 into a 95



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wafer-scale MoS2 through a two-step procedure.165 In the first step, MoO3 was reduced into layered MoO2 by H2/Ar. Next, a further sulfurization was achieved by heating MoO2 with sulfur powder at 1000 oC. A uniform thin layer MoS2 around 2 nm could be easily obtained, which benefited the application in flexible electronics and optoelectronics. It was also reported that though one-step direct sulfurization of MoO3 with sulfur at 1000 oC could be fulfilled, the asobtained MoS2 films displayed a great deficiency in electrical carrier mobility, which was even several orders of magnitude lower than that of MoS2 from a two-step thermal process. Furthermore, special architectures of MoS2 like hollow spheres were synthesized from MoO3 belts by a hydrothermal method with a tetrabutyl ammonium bromide template.166 It was because the metal oxo anion interacted with the surfactant due to the charge on the surface that the MoO3 could be assembled into a hollow sphere structure during the hydrothermal process.

Mo2C has attracted great attention because of their high structural stability, high melting points and giant potential in catalysis such as CO2 hydrogenation or alcohol synthesis. It was reported that a direct conversion of MoO3 to cubic molybdenum carbides was fulfilled via using H2/toluene through a temperature-programmed reaction at 670 K.167 Interestingly, the Mo2C was easily stabilized in a cubic phase that possessed molybdenum vacancies without any structural stabilizing agent. Furthermore, the as-obtained molybdenum carbides had smaller particle sizes but larger specific area, which was of great advantage in accelerating the rate of catalysis to some extent.

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Briefly, it can be concluded that the various compositional MoOx micro-/nanostructures, combined with different template-based synthetic strategies, can provide a perspective route to design and synthesize a number of new hybrid composites. To date, the transformation to MoS2 has been relatively mature; however, it is still lack of reports about other Mo chalcogenides and nitrides in literatures. Additionally, most efforts have focused on the anions exchange while rare has been done in the cation exchange.

6. Conclusions and perspective

In summary, MoOx is one of the most promising materials extensively applied into versatile applications, including photodetectors, photothermal therapy, SERS, supercapacitors, ion batteries, solar cells, gas sensing, multi-phase catalysts, photodegradation and HER, due to its unique electrical or optical property, controllable morphology and size, crystalline phase and even function-directed modification. Herein, we have thoroughly summarized the recent advances in the synthetic strategies and formation mechanisms, the morphological and compositional diversities, the modification strategies including doping and hybridization as well as current applications based on the diverse MoOx-based materials. The progress made in the controllable synthesis of MoOx micro-/nanostructures with modified properties are quite valuable to provide new chance and experience for the further investigation in designing novel MoOxbased composites with enhanced performances. Although giant progress has been made over the past decades, as stated in the review, it still requires further investigation to get a comprehensive

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knowledge of MoOx, which can help us to obtain devices with better performances using innovative MoOx nanomaterials.

Considering the synthetic engineering of MoOx micro-/nano-structures, most researchers focused on tuning the morphology, size, crystalline phase and composition. Moreover, a precise control of electronic structure as well as surface states like oxygen vacancies or surface/interfacial atomic arrangement is of huge demand. As for the synthesis of 0D MoOx materials, MoO2 QDs have rarely been synthesized without any aid of support, meanwhile the synthesis of MoOx QDs always requires surfactant or polymer templates, which has underlying impact on the fabricated devices. Hence, novel surfactant-free strategies are worth exploring. With regard to 1D MoOx nanomaterials, it has been well-discussed as the tendency to form MoO3 nanorods or nanobelts in spite that only a few additives can be used to synthesize nanotubes or other porous structures. Besides, it has been reported on the synthesis of MoOx nanorods, but ultrathin ones (diameter size < 100 nm) is still a big challenge. In short, there still can be much improvement in increasing the specific surface area as well as quantum effect. As for 2D MoOx nanomaterials, MoOx nanosheets exposed with certain (020) facets have been easily obtained through an exfoliation method from the MoO3 nano-/microbelt precursors. Notably, when using a liquid phase method, surfactant is imperative to keep the nanosheets from aggregation, which will be an adverse factor in later application. As for 3D MoOx materials, it is a great challenge to synthesize polyhedral structure of MoOx, since it is quite meaningful to explore the relationship between the special facets and performance. Therefore, it shows great value in synthesizing the architecture with more exposed facets. Moreover, new methods should 98


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be developed for synthesizing hollow and hierarchical nanostructures possessing controllable and uniform sizes and tunable compositions as well. Except for the dimensional properties, the polymorphics phase can also influence the properties and performances. Although the h-MoO3 behaves more instability than α-MoO3, it possesses a much more porous structure which may exhibit better performance in lithium batteries or catalysts. Therefore, it is still essential to investigate the preparation of MoOx nanocrystals with the desired compositions. In addition, it should be noted that oxygen vacancies are also a vital factor that can determine the electronic structure to some extent, which is essential in tuning the physiochemical properties.

In order to further enhance the performances of MoOx micro-/nanostructures, the functional modifications (including doping and hybridization) have been investigated. As for doping, the previous achievement is restricted to only a few elements (such as hydrogen, nitrogen, sulfur, Cd, Ce or Fe), therefore, it is still a challenge to explore the selective doping of more heteroatoms for improved performances and new strategies for the preparation of novel dopedMoOx. As for hybridization, the integration of noble metal, metal oxides as well as carbon materials with MoOx are extensively developed in the past years, but the controllable-tuning of the active components or the crystal facets of MoOx supports is still in its infancy. Furthermore, it should be taken more attention into the theoretical calculation of the doping electronic structures and hybridizing interfacial structures for understanding the intrinsic mechanism comprehensively.

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In regard to the current applications, MoOx can be generally used in the fields of optical applications, energy conversion devices, gas sensors and catalysts by designing the morphology or adopting hybrid MoOx nanostructures (such as metal/MoOx, semiconductor/MoOx and carbon/MoOx). Because of the presence of oxygen vacancies in MoOx, it can be observed LSPR effects in near infrared region for photocatalysis and photothermal therapy. However, it still requires lots of investigations to reveal the exact relationship between oxygen vacancies and enhanced performances. As for energy storage and conversion devices, there is far way to go ahead, since it has a bottleneck to assure both stability and capacitance. Besides, a better design is still required for solar cells to improve the power conversion efficiency for the practical use. With regard to gas sensors, though the MoOx exhibits good response to various gases, it is still necessary to fulfill better discrimination and low the working temperature. Moreover, MoOx has been widely applied as the electocatalyst for HER, while the photocatalytic performance of HER is worth being paid more attention and lots of efforts can be done to enhance the activity. In addition, the application of MoOx templates for preparing MoX2 (X= S, Se and Te) or multicomponent heterogeneous nanostructures should be further highlighted.

In summary, the review has covered the recent progress made in the synthetic strategies and formation mechanisms, the modifications, and their potential applications as well as intrinsic properties, which can be in favor of establishing a basic knowledge foundation of MoOx materials and thus promote the development of novel function-directed MoOx-based micro/nanostructures. In short, MoOx is emerging as ideal candidates in many fields, while more

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encouraging studies are highly desirable to have a better knowledge of MoOx materials in order to utilize them into real applications.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (S. D. Sun); [email protected]（X. F. Li）

Author Contributions H. Q. Ren conceived the project and contributed to the collection of the figures and the writing o f the whole paper. S. D. Sun contributed to designing the whole content of the manuscript and re vised the paper. J. Cui mainly contributed to design and revision of the figures in the manuscript. X. F. Li contributed to provide the suggestion and revise the paper. All the authors discussed the results and commented on the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Science Foundation of China (NSFC No. U1502274 and 51302213), the National High Technology Research and Development Program of China (863 Program, No.2015AA034304), the Pivot Innovation Team of Shaanxi Electric Materials and Infiltration Technique (2012KCT-25), the Hundred Talent Program of Shaanxi Province. The authors also thank Prof. Shengchun Yang for revising the manuscript. 101



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FIGURES

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Figure 1 Summary of the advances in MoOx-based micro-/nanostructures.

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Figure 2 Atomic structures of different crystalline phases of MoOx. (a) α-MoO3. (b) β-MoO3. (c) h-MoO3. (d) MoO2. The red sphere is oxygen atom.

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Figure 3 (a) Optical images of MoOx with different amount of vacancies, reprinted with permission from ref. 23. Copyright 2017 Elsevier. (b) UV/Vis-NIR diffuse reflectance spectra of the MoOx sample and commercial MoO3 and (c) The LSPR wavelength of the products prepared by using different ratios of Mo (VI)/Mo (V) in the synthesis. Reprinted with permission from ref. 162. Copyright 2016 Wiley. (d) Raman spectra for MoO3 crystalline powder and vacuumdeposited amorphous MoO3 film. Reprinted with permission from ref. 72. Copyright 1995 ACS. (e) PL spectra of MoOx QDs under different excitation wavelengths and (f) UV-vis-NIR spectrum of MoOx QDs. Reprinted with permission from ref. 36. Copyright 2018 RSC.

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Figure 4 Schematic illustration of the transformation pathway of ribbon from slab. Reprinted with permission from ref. 192. Copyright 2016 ACS.

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Figure 5 (a) SEM image of MoO3 nanobelts without P123. (b) SEM image of MoO3 nanoflowers with P123. (c) Illustration of possible growth model of the MoO3 nanobelts and microflowers with the aid of P123. Reprinted with permission from ref. 69. Copyright 2013 RSC.

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Figure 6 (a) TEM image of the precursor of MoO3 nanosheets in photo-etching method. (b) TEM image of the MoOx QDs in photo-etching method. (c) Photo images of products synthesized with different irradiation time in photo-etching method. (d) Schematic illustration of the mechanism of transforming MoO3 nanosheets to MoOx QDs in different solvents using photo-etching method. Reprinted with permission from ref. 35. Copyright 2016 RSC. (e) Schematic illustration of the mechanism of transforming MoO3 nanosheets to MoO3 QDs in different solvents using intercalation and exfoliation method. Reprinted with permission from ref. 34. Copyright 2016 RSC.

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Figure 7 (a) A typical TEM image of MoO3 nanorods. Reprinted with permission from ref. 189. Copyright 2007 ACS. (b) SEM image of hierarchical MoO2 nanorods and (c) TEM image of hierarchical MoO2 nanorods Reprinted with permission from ref. 40. Copyright ACS 2017. (d) A typical SEM image of MoO3 nanowire arrays. Reprinted with permission from ref. 41. Copyright ACS 2012. (e) A typical TEM image of MoO3 nanotubes. Reprinted with permission from ref. 43. Copyright 2008 ACS. (f) A typical SEM image of MoO3 nanowire bundles. Reprinted with permission from ref. 45. Copyright 2014 ACS.

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Figure 8 (a) SEM image of MoO3 nanoribbons, and the inset is the corresponding SEM image of individual nanoribbon. Reprinted with permission from ref. 29. Copyright 2015 ACS. (b) TEM image of MoO3 nanobelts, and the inset is the corresponding SEM image. Reprinted with permission from ref. 63. Copyright 2009 ACS.

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Figure 9 (a) SEM image of α-MoO3 without NaNO3. (b) SEM image of h-MoO3 with 7.49 mol/L NaNO3. (c) SEM image of h-MoO3 before transformation. (d) SEM image of α-MoO3 after transformation. (e) The proposed mechanism for the mutual transformation between α-MoO3 and h-MoO3. Reprinted with permission from ref. 46. Copyright 2017 RSC.

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Figure 10 (a) SEM image of MoOx nanosheets, and the inset is the corresponding TEM image. Reprinted with permission from ref. 74. Copyright 2015 Wiley. (b) SEM image of MoO3 hexagonal nanoplates, and the inset is the corresponding TEM image Reprinted with permission from ref. 30. Copyright 2009 ACS.

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Figure 11 (a) Low-magnification TEM image of mesoporous α-MoO3 with highly oriented crystalline walls, and the inset is the corresponding high-magnification TEM image. (b) Tappingmode AFM image of the top surface and the inset is a 3D-AFM image at a tilt of 50o; the contrast covers height variations in the 1–5 nm range. Reprinted with permission from ref. 89. Copyright 2010 Nature Publishing Group. (c) and (d) TEM and SEM image of the mesoporous MoO2 material, respectively. Reprinted with permission from ref. 87. Copyright 2009 ACS. (c) SEM image of the hierarchical VO2 nanostructure. (d) SEM image of the hierarchical MoO3 nanostructure grown on the template. (e) Schematic illustration of the VO2 and MoO3 nanostructures and the growth mechanism of the hierarchical MoO3 nanostructure. Reprinted with permission from ref. 47. Copyright 2014 RSC.

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Figure 12 (a) SEM image of the hierarchical VO2 nanostructure. (b) SEM image of the hierarchical MoO3 nanostructure grown on the template. (c) Schematic illustration of the VO2 and MoO3 nanostructures and the growth mechanism of the hierarchical MoO3 nanostructure. Reprinted with permission from ref. 47. Copyright 2014 RSC.

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Figure 13 (a) Schematic illustration of the formation process of α-MoO3 microspheres and αMoO3 microbelts. (b) SEM images of α-MoO3 microspheres and (c) SEM images of α-MoO3 microbelts. Reprinted with permission from ref. 135. Copyright 2013 RSC. (d) Schematic illustration of the evolution process of core–shell MoO2 hierarchical microcapsules and (e) SEM image of the corresponding product of core–shell MoO2. Reprinted with permission from ref. 25. Copyright 2012 RSC.

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Figure 14 (a) TEM image of the Pd/MoOx and the inset is a particle size distribution of Pd. And the insert is the corresponding size-distribution diagram. (b) Schematic illustration of synthetic procedure of the Pd/MoOx. Reprinted with permission from ref. 100. Copyright 2015 Wiley.

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Figure 15 (a) Flowchart of synthesis of fork like MoO3 nanostructures via manipulating growth directions by TiO2 capping agent and (b) TEM image of TiO2 caps and fork like MoO3 nanorods. Reprinted with permission from ref. 37. Copyright 2003 ACS. (c) Schematic illustration of the in-situ synthesis of MoO2@MoS2 and (d) TEM image of MoO2@MoS2 nanobelts. Reprinted with permission from ref. 105. Copyright 2016 ACS.

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Figure 16 (a) Low-magnification SEM image of Mo2N nanolayer coated MoO2. (b) High magnification SEM image of Mo2N nanolayer coated MoO2. (c) Schematic illustration of the formation of Mo2N nanolayer coated MoO2 hollow nanostructures. Reprinted with permission from ref. 108. Copyright 2013 RSC.

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Figure 17 (a) Schematic representation of the fabrication process of the MoO2/graphene. (b) FESEM images of the MoO2/graphene material. Reprinted with permission from ref. 111. Copyright 2011 ACS. (c) Schematic illustration of hierarchical MoO2 formation on cotton cloth. (d) SEM image of the hierarchical MoO2. Reprinted with permission from ref. 91. Copyright 2011 RSC.

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Figure 18 (a) Schematic illustration of the formation of C@MoO2 hollow yolk-shell nanospheres and (b) TEM image of C@MoO2 hollow yolk-shell nanospheres. Reprinted with permission from ref. 119. Copyright 2017 RSC.

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Figure 19 Schematic illustration of the formation of MoO2@C hollow nanospheres. Reprinted with permission from ref. 121. Copyright 2015 RSC.

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Figure 20 (a) SEM image of h-MoO3 hollow nanorods and the inset is the corresponding TEM image. (b) SEM image of h-MoO3 hollow nanorods@PANI and the inset is the corresponding TEM image. (c) Schematic illustration of the formation of h-MoO3 hollow nanorods@PANI. Reprinted with permission from ref. 123. Copyright 2015 RSC.

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Figure 21 (a) Schematic illustration of MoO3 for photocurrent measurement. (b) Timedependent photoresponse of MoO3 device before and after annealing in reducing gas under 660 nm laser illumination at 0.1 V bias voltage. (c) Plot of the response and EQE versus light wavelength. (d) Schematic diagram of the energy level for MoO3 film before and after annealing. Reprinted with permission from ref. 28. Copyright 2014 Nature Publishing Group.

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Figure 22 (a) NIR irradiation induced temperature elevation of MoOx QD aqueous solution with different concentrations, using an 880 nm irradiation of 2 W/cm2. (b) Representative photos of the mice and tumors under various treatments after 14 days. Reprinted with permission from ref. 33. Copyright 2017 RSC. (c) Thermal infrared images of tumor-bearing mice treated with PEGylated C/MoO2 nanoparticles as well as only PBS, nanoparticles injection or laser irradiation, using an 808 nm NIR laser irradiation of 0.6 W/cm2 at different time intervals. Reprinted with permission from ref.127. Copyright 2015 RSC.

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Figure 23 (a) Raman spectra of R6G aqueous solution mixed with MoO2, MoOx and MoO3 NPs dispersion at the concentration of 5 × 10−5 M and bare silicon slice. (b) The average Raman EFs estimated by selecting P1 and P3 characteristic peaks of R6G with 5 × 10−5, 5 × 10−6 and 5 × 10−7 M concentrations. Reprinted with permission from ref. 82. Copyright 2018 RSC. (c) SERS spectra of mixtures of 10−5 M CV and MB (10−6 to 5 × 10−6 M). (d) SERS spectra of 10−5 M MB on MoO3@MIPs before and after self-cleaning for 4 cycles. Reprinted with permission from ref. 131. Copyright 2017 RSC. (e) and (f) Proposed mechanism for SERS enhancement depending on nanoshell isolated electromagnetic enhancing mechanism for MoOx@MoO3 nanosheets. Reprinted with permission from ref. 133. Copyright 2015 RSC.

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Figure 24 (a) Schematic illustration of structure for the PSCs with versatile MoO3 HTLs. Reprinted with permission from ref. 150. Copyright 2014 ACS. (b) Energy band diagrams of the FTO, TiO2, N719 and electrolyte before contact formation and (c) energy level alignment at FTO/TiO2/N719/electrolyte interfaces after contact formation. (d) Energy levels representing MoO3 RBL at FTO/TiO2 interface and (e) the alignment facilitating the charge transport through MoO3. Reprinted with permission from ref. 154. Copyright 2017 RSC.

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Figure 25 Schematic diagram of the H2 sensing mechanism of MoO3 nanoribbons. Reprinted with permission from ref. 29. Copyright 2015 ACS.

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TABLES. Table 1. Synthesis pathways and morphologies of different MoOx nanostructures Precursor

Solvent

Additive/Surfactant

Synthetic Method

Template

Morphology

Ref.

(NH4)6Mo7O24

H2O

Nitric acid

Hydrothermal method

None

MoO3 nanobelts

23

MoO3 nanobelts

none

none

Sauna Reaction

none

MoOxnanobelts

23

molybdenum(IV) tetrachloride

acetophen one

none

Solvothermal method

none

Interconnected core/shell MoO2

25

(NH4)6Mo7O24·4 H2O

H2O

Sodium sulfate

Wet chemical method

Ni foam

Porous MoO2 nanosheets

26

molybdenum foil

none

none

Thermal oxidization

none

MoO3 nanobelts

28

Na2MoO4·2H2O

H2O

HNO3

Hydrothermal

None

α-MoO3 nanoribbons

29

none

Truncated Hexagonal MoO3

30

oxide dodecyl

Method Mo plates

none

none

Vapor deposition

Nanoplates H3PMo12O40

none

none

Hard templateassisted method

SBA-15

MoO2 nanorod

31

peroxomolybdic solution

HMTA solution

none

Solvothermal method

none

pyramidal nanorod, prismatic

32

nanorod and hexagonal nanoplate-like h-MoO3 Molybdenum trioxide

ethanol

n-butylamine

Intercalation/e xfoliation method

none

MoOx quantum dots

34

165



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Molybdenum trioxide

Water/eth anol

none

UV-assisted method

none

MoOx quantum dots

35

ammonium molybdate

H2O

thiourea

Wet chemical method/dialys is and a lyophilization

none

MoOx quantum dots

36

process (NH4)6Mo7O24· 4H2O

H2O

Nitric acid

Hydrothermal method

None

α-MoO3 nanorods

37

peroxomolybdic

H2O

none

Hydrothermal method

None

α-MoO3 nanorods

38

acid sol (Mo powder in H2O2) None H2O

(NH4)6Mo7O24·4H 2O

HCl

ethanol

Hydrothermal method

39

Agglomeration and nonhexagonal rods

Solvothermal method

HNO3

heptane

Widely distributed hexagonal rods

Solvothermal method

HNO3

Distorted hexagonal rods

MoO2(acac)2

[BMIM][ Tf2N]

None

Ion Liquidsassisted method

None

1DHierarchical MoO2

40

Mo filaments

none

None

Hot-filament Chemical vapor deposition

None

MoOx nanowires arrays

41

MoO3·2H2O

H2O

Acetic acid

Hydrothermal method

None

α-MoO3

42

submicrometer fibers molybdic

H2O

dodecanethiol

(NH4)6Mo7O24·4H 2O

H2O

Concentrate acid

MoO2(OH)(OOH

H2 O

none

Hydrothermal metod

none

Single-walled nanotubes

43

Chemical percitipation

none

h-MoO3 nanorods and microrods

44

Sonochemical

none

α-MoO3·H2O

45

acid nitric

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)

α-MoO3·H2O nanorods

method

none

none

Calcination

nanorods

none

α-MoO3

45

nanowire bundles peroxomolybdic acid

H2O

NaNO3

Hydrothermal method

none

h-MoO3 microrods

46

h-MoO3 microrods

H2O

none

Hydrothermal method

none

α-MoO3 nanobelt

46

Na2MoO4·2H2O

H2O

HCl

wet chemistry method

VO2 nanowires arrays

MoO3 nanorod

47


H2O

chitosan

Hydrothermal method/dialys is and a lyophilization

arrays

none

MoOx quantum dots

48

process MoO3 nanowires

none

none

Hydrogen reduction

none

porous MoO2 nanowire

49

polymeric

H2O

HNO3

Hydrothermal method

none

ɑ-MoO3 nanofibers

50

Na2MoO4

DMF

water

Templateassisited method

PS-b-PVPb-PEO micelles

MoO3 nanotubes

51


H2O

thiourea

Hydrothermal method

none

pyramidal and prismatic hexagonal MoO3

52

nitrosyl-complex of molybdenum(II)

nanorods (NH4)6Mo7O24

H2O

Nitric acid

Hydrothermal method

none

ɑ-MoO3 nanorods

53

ammonium heptamolybdate tertrahydrate

H2O

HCl

Sonochemical method

none

MoO3 nanoflowers and nanorods

54

ammonium heptamolybdate

H2O

HNO3

Hydrothermal method/Micro wave-assisted

none

MoO3 nanoparticles

55

167



tetrahydrate

method/ Sonochemical method

Page 168 of 173

and nanorods

(NH4)6Mo7O24·4 H2O

H2O

HCl, aniline

Selfassembled/cal cination method

none

MoO3 nanowire bundles

56

hydrated molybdic acid

Ammoniu m solution

HCl

Chemical precipitation/ Hydrothermal method

none

h-MoO3 microrods

57

Na2MoO4

H2O

HCl

Hydrothermal method

none

α-MoO3 nanorods

58

α-MoO3 nanorods

H2O

pyrazine

Hydrothermal method

none

MoO3 microsheets

58

(NH4)6Mo7O24·4 H2O

H2O

HNO3

Chemical precipitation

none

h-MoO3 microrods and microflowers

59

(NH4)6Mo7O24·4 H2O

H2O

NH4NO3, citric acid

Molten method

salt

none

MoO3 fibers and belts

60

(NH4)6Mo7O24·4 H2O

H2O

sodium dodecyl sulfate, urea, HNO3

Sonochemical method

none

MoO3 nanorods

61

ammonium

H2O

HNO3

Hydrothermal method

none

MoO3 nanowires

62

molybdenyl acetylacetonate

H2O

Nitric acid

Hydrothermal method

none

Ultralong MoO3 nanobelts

64

peroxomolybdic acid solution.

H2O

none

Hydrothermal method

none

α-MoO3 nanobelts

65

Na2MoO4·2H2O

H2O

NaCl/HCl

Hydrothermal method

none

α-MoO3 nanobelts

66

MoO3 powder

none

none

Physical vapor deposition

none

MoO3 nanobelts

67

α-MoO3 nanobelt

none

none

Hydrogen reduction

Graphene

Flawed belts

Na2MoO4·2H2O

H2O

HCl, P123

Hydrothermal method

none

ɑ-MoO3 nanobelts

molybdate

MoO2

68

69

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Na2MoO4·2H2O

H2O

Sodium oleate, HCl

Chemical precipitation

none

MoO3 nanobelts

70


H2O

none

Hydrothermal method

none

MoO2 ribbons

71

MoO3 powder

none

none

Physical vapor deposition

none

Amorphous MoO3 film

72

poly peroxomolybdic solution

H2O

Thiourea, KOH

Hydrothermal method

none

h-MoO3 nanorods

73

h-MoO3 nanorods

none

none

Anneal and exfoliation

MoO3-II nanosheets

73

peroxomolybdic acid solution

ethanol

none

Solvothermal method

none

MoOx nanosheets

74

ammonium

none

polyethylene glycols (PEGs)

Sintering method

none

1D nanobelts, 3D nanoparticles and 2D nanomultilayer s MoO3

75

H2O

Nitric acid

Hydrothermal method

none

MoO3 nanosheets

76

ethanol

none

Solvothermal method

none

MoOx nanosheets

77

none

molybdate tetrahydrate

ammonium heptamolybdenu m tetrahydrate

Mo powder H2O2

in

169



Page 170 of 173

MoO3·H2O powders

Ethanol

n-octylamine

calcination

none

MoO3 nanoplates

78

MoO3 powder

none

none

Thermal evaporation and mechanical exfoliation

none

MoO3 nanosheets

79

MoO3 powder

ethanol/w ater solution

none

Sonochemical method

none

MoOx nanosheets

80

tartaric acid or citric acid or

calcination method

none

MoO3 hexagonal nanoplates and nanorods

81

Solvothermal method

none

MoO2 nanosheets

82

water ethanol iso-propyl alcohol DMF DMSO ammonium heptamolybdate

H2O

tetra hydrate

MoS2

ethylene diamine tetra acetic acid ethanol

H2O2

MoOx nanoparticles MoO3

Water/eth ylene glycol

none

solvothermal method

none

MoO2 nanosheets

83

peroxomolybdic

H2O

none

Sol-gel method

none

MoO3 nanoplates

84

MoO3

none

none

Chemical vapor deposition

none

MoO2 nanosheets

85

phosphomolybdic acid

ethanol

none

calcination

Cotton cloth

hierarchical MoO2

91

(NH4)6Mo7O24·4 H2O

H2O

none

Hydrothemal method/ Hydrogen

Ni foam

MoO2 nanoflowers

92

acid sol

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reduction Mo plate

water– ethylene glycol solution

H3PO4, NH4F

electrochemic al anodization

none

Maze-like MoO3

93

MoOCl4

acetophen one

benzoic acid

Solvothermal method

none

MoO3 microspheres

94

molybdenylacetyl acetonate

n-butyl alcohol solution

HNO3

Solvothermal method

none

MoO3 microflowers

95

Phosphomolybdic acid

H2O, acetic acid solution

CTAB, chitosan

Hydrothermal method

none

microsphere, microbelt

135

Na2MoO4·2H2O

H2O

HCl, HNO3

Cation exchange/ Evaporation method

none

β-MoO3

158

MoO3/Na2MoO4

none

none

semi-open flux growth method

None

α-MoO3 layered crystal

172

and nanorod MoO3

chemical etching

Table 2. The typical lithium ion batteries based on various MoOx nanostructures. Type

Active material

Revesible Capacities

1D MoOx nanostructures

MoO2 Nanorods


α-MoO3 Nanobelts:

830 mAhg−1 between 3.0 and 0.01 V for anode 264 mAhg−1 at 30 mAg−1 and 176 mAhg−1 at 5000 mAg−1

Colume efficienciesin first cycles none

∼74% in the first cycle at 5000 mAg−1

Cycling stability

reference

830 mAhg−1 after 29 circles

31

114 mAhg−1 after 50 cycles at 5000 mAg−1

65

171




Semiconductor/MoOx nanostructures

Interconnected core/shell MoO2 MoO3 nanoflowers

81% in the 1st cycle at 1C

none

25

99% from the second circle

810 mAhg−1 after 100 cycles at 550 mAg−1 1163 mAhg−1 at 100 mAg−1 after 80 cycles. 91% retention after 100 cycles

54

Sulfur Refines MoO2

1233 mAhg−1 at 100 mAg−1 for anode

none

Mo2N coated MoO2 hollow structure

898 mAhg−1 in the voltage range of 0.01–3.0 V 975.4 mAhg−1 at 50 mAg−1 for anode

none

823 mAhg−1 at 200 mAg−1

99% after several cycles

3D graphene supported MoO2

C/MoOx nanostructures

For cathode 623.8 mAhg−1 at 1C 1041.6 mAhg−1 at 550 mAg−1 for anode

Graphene nanosheets encapsulated α-MoO3 nanoribbons

Page 172 of 173

79.3% cycle

in

first

986.9 mAhg−1 at 100 mAg−1 after 150 cycles 754 mAhg−1 after 200 cycles at 1000 mAg−1 (a capacity fading rate of only 0.1% per cycle)

104

108

66

112

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For Table of Contents Use Only

Synthesis, functional modifications and diversified applications of molybdenum oxides micro-/nanocrystals: A review Haoqi Ren, Shaodong Sun,* Jie Cui and Xifei Li*

In this review, we have highlighted the progress in diversified MoOx micro-/nanostructures, including that in the general synthetic strategies for the synthesis of 0D, 1D, 2D and 3D MoOx micro-/nanostructures, the modification (such as doping and hybridization) of MoOx-based composites for enhanced performances, and their various applications Furthermore, the present issues and promising perspective are also given.

173


Synthesis, Functional Modifications, and Diversified Applications of

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