One-Step Hydrothermal Synthesis of TiO2@MoO3 CoreâShell

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One-Step Hydrothermal Synthesis of TiO@MoO Core-Shell Nanomaterial# Microstructure, Growth Mechanism, and Improved Photochromic Property Ning Li, Yamei Li, Wenjing Li, Shidong Ji, and Ping Jin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10752 • Publication Date (Web): 14 Jan 2016 Downloaded from http://pubs.acs.org on January 20, 2016

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The Journal of Physical Chemistry

One-step Hydrothermal Synthesis of TiO2＠MoO3 Core-Shell Nanomaterial: Microstructure, Growth mechanism, and Improved Photochromic Property Ning Li†, Yamei Li‡, Wenjing Li†, Shidong Ji†, Ping Jin*,†, § †

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai

institute of Ceramics, Chinese Acamdemy of Sciences, Dingxi 1295, Changning, Shanghai, 200050, China ‡

Biofunctional Catalyst Research Team, Riken, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

§

National Institute of Advanced Industrial Science and Technology (AIST), Moriyama, Nagoya

463-8560, Japan

ABSTRACT: Photochromic TiO2@MoO3 core-shell (TM) nanopowder was synthesized by a one-step

hydrothermal

method

and

characterized

with

XRD,

TEM,

Raman

and

spectrophotometer. The nanopowder has a very small particle size of 5-10 nm in diameter, and a well-defined core-shell structure with the tight interface, that is, an anatase TiO2 core is tightly surrounded with amorphous MoO3, to form an ideal heterojunction structure. The growth mechanism was proposed with a surface-induced nucleation of amorphous MoO3 from a wellmixed precursor, where the mutual inhibition between core and shell led to the confined core size.

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The photochromic performance of TiO2@MoO3 was evaluated in solution as well as casted thin film on glass using a spectrophotometer, with a comparison to that of single MoO3. A significant enhancement in the photochromic properties was demonstrated for TM, that is, the change of the absorbance in water at 600 nm is 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. Meanwhile the samples in ethanol showed much stronger photochromic coloration efficiency than that in water by 66.6% enhancement in the absorption at 600 nm. Such a significant photochromic enhancement is considered to the formation of a relatively ideal heterojunction at the interface, leading to an efficient electron-hole separation. The size effect of TM provides a high specific surface area for cation insertion/extraction and diffusion and decreases the distance of electron transfer. Furthermore, a shift of excitation wavelengths from UV to visible was also observed due to the appearance of extra defect bands by Mo doping into TiO2 in their interface, which was supported by XRD and Raman measurements. The enhanced photochromic performance of TiO2@MoO3 is expected to be used for color displays and smart window.

1. INTRODUCTION As a wide bandgap n-type semiconductor, MoO3 has shown promising applications in optical memories1, 2, photocatalysis3-5, energy-storage6, 7, chemical sensors8, 9 and photochromic (PC) optical modulation devices10. These applications mainly benefit from the structural flexibility of MoO3 and its reversible ion intercalation/deintercalation property. MoO3 is its intrinsic layered nature that readily accommodates large quantities of positive ions, resulting in potential bandgap manipulations. Additionally, the layered structure also offers superior charge transfer11. On this basis, amorphous MoO3 with partly layer structure shows better photochromic properties than those crystalline ones, due to more defective sites available for charge transfer12, 13.


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However, amorphous MoO3 still has many problems to be used as an efficient PC material. The photochromism of MoO3 was limited by its large band gap energy (3.0-3.1 eV)14 and the recombination of the photogenerated e-/h+ pairs, resulting in low absorptivity and low conversion efficiency of photon energy. The band gap energy of MoO3 is beyond the most energy-intensive region of the solar spectrum (centered at 2.6 eV), which indicates that this material can only absorb ultraviolet (UV) light. To make MoO3 suitable for practical photochromic films, intensive research has been carried out towards improving its chromogenic properties through synthesis of composite films containing MoO3 and another semiconducting oxide to hinder the recombination of photogenerated electron- hole pairs15, 16. Ahmed et al. reported that MoO3-WO3 composites showed coloration under visible light with more intense absorption when compared to pure MoO317. The photochromic performance of MoO3 was also enhanced by forming a double layer MoO3/CdS/glass system which was justified by an increased color center concentration18. Particularly, the combination of anatase TiO2 and α-MoO3 (band gap of 2.9 eV) on the porous vycor glass resulted in heterostructure material with band gap of ca. 2.7 eV, due to the rearrangement of the valence and conduction bands19. Based on this model, the TiO2 can absorb light in the visible region through interfacial charge transfer to generate electron–hole pairs, and the excited electrons were efficiently transferred from conduction band of TiO2 to that of MoO3, owing to the rectifying heterostructure20. More importantly, the coupled heterojunction could increase the photo catalytic efficiency by enhancing the charge separation, elongating the lifetime of the charge carriers, and extending their photoresponse range21, 22. However, it’s still an important issue that how such photochromic heterojunction can be deliberately manipulated on a microscopic level. Thus, the integration of nanomaterial with controllable interfaces on the microscale (especially nanoscale) is a key scientific and technological issue.


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In recent years, much of the research on semiconductor nanomaterial has focused on semiconductor based core-shell nanoparticles (CSNs), since CSNs can combine the advantages from both core and shell compartments and modulate properties by tuning the interfacial microstructure, which could favor the applications in optical materials, heterogeneous catalysts, and solar energy conversion materials17,

21, 23-26

. Based on these merits, an enhanced

photochromic performance can be expected21 if MoO3–TiO2 composites have: i) a nanocrystalline feature with high surface area, from which both the enhanced charge carrier generation in anatase TiO2 and the elevated charge transfer/ion intercalation kinetics in nanocrystalline MoO3, could be expected; and ii) a core-shell structure that can provide a large interfacial area for redox reactions and facilitate ion and electron transport through the heterojunction interface. With higher surface area, CSN may demonstrate better performances than conventional metal oxides in applications such as energy conversion and storage, catalysis, sensing, adsorption, and separation, etc. Elder et al. fabricated TiO2/MoO3 core/shell nanoparticles by a co-nucleation of metal oxide clusters at the surface of surfactant micelles and found the decrease of photoabsorption energy due to the electron transition from the valence band of TiO2 to the conduction band of MoO323, but the preparation procedure was relatively complex with multiple steps. The gas sensing properties of TiO2/α-MoO3 core/shell nanoparticles fabricated by a modified wet-chemical method were also enhanced, due to the heterojunction barrier formed at the perfect interface between them27. As for photocatalytic property, the apparent first-order rate constant for the degradation of RHB by p-MoO3 nanostructures/n-TiO2 nanofibers with heterojunction was two times that of TiO2 nanofibers26. However, much less attention has been paid so far to the photochromism of TiO2/ amorphous MoO3 CSN.


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Hydrothermal synthesis has numerous advantages including simplicity, low cost, and flexibility with respect to ideal shape and size28, which has been effectively applied to prepare high performance single phase nanopowder such as TiO229, MoO328, and WO330. We expect that core-shell nanoparticles synthesized by one-pot hydrothermal method could lead to tight contact interface, high specific surface area with tunable morphologies, thus resulting in the improvement of their properties. In this paper, the photochromic properties of core-shell TiO2@MoO3 nanospheres synthesized via a one-step hydrothermal method were studied for the first time, to the best of our knowledge. The composite was consisted of anatase TiO2 and amorphous MoO3 with perfect interface and high specific surface area. The core-shell TiO2@MoO3 nanospheres exhibited remarkable photochromic properties: the samples dispersed in alcohol solvent were colored to deep blue only after 4 min irradiation. For sample TiO2/MoO3= 9:1 (denoted as TM-10) suspended in water with concentration of 3 wt%, the remarkable change of absorbance at 600 nm is 10.85, which was 30 times larger than that of α-MoO3 (only 0.354) with the same concentration. Meanwhile, in comparison with the pure MoO3, the excitation wavelength of TiO2@MoO3 CSN extended from ultraviolet to visible light and the optical modulation range expanded from visible to nearinfrared (400 to1400 nm). Furthermore, the possible mechanism for the improved photochromic property was proposed in the perspective of heterojunction effects. 2. EXPERIMENTAL SECTION 2.1. Preparation of TiO2@MoO3 nanocomposites. TiO2@MoO3 core-shell structured nanopowder was synthesized by one-step hydrothermal method. All chemical reagents were of analytical grade and bought from Aladdin Reagent Co. China. A typical synthesis procedure was as follows. Ammonium molybdate acidic aqueous solution (pH~1) was firstly prepared by


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tuning the pH using HCl. 1 mL of TiCl4 was added to the above mentioned ammonium molybdate acid aqueous solution (pH~1) under stirring for 10 min. The pH value of the obtained solution was then adjusted to about 8 by ammonium aqueous solution, forming a white precipitate suspension. The precipitate suspension were filtered and washed twice with water, then re-dispersed in 40 mL deionized water (DIW). Subsequently, 5 g hydrogen peroxide (30 wt%) aqueous solution was added, and the mixture solution was finally transferred into a 100 mL autoclave Teflon vessel and treated at 150 ℃ for 6 h. The obtained pale powders were filtered and washed twice with DIW, and dried at 70 ℃ in air for 6-8 h. The samples with different Mo/Ti atomic ratios were prepared by tuning the Mo content from 0 at% to 50 at% with fixed amount of TiCl4, as listed in Table 1. Table 1. Samples and experimental conditions Sample

TM-5

TM-10

TM-15

TM-25

TM-50

TiO2

Mo/at%

5

10

15

25

50

0

(NH4)6Mo7O24/g

0.083

0.176

0.280

0.529

1.588

0

TiCl4/mL

1

1

1

1

1

1

2.2. Characterizations. The crystal phase of the as-prepared powders were determined by xray diffraction (XRD) with a Cu-Kα radiation of 1.541 Å in wavelength and settings of 40 mA and 40 kV at a scanning rate of 5 °/min in the 2 θ range from 10-85 º. The microscopic morphology was obtained using a field-emission scanning electron microscope (FE-SEM, HITACHIS-3400, Japan) at an acceleration voltage of 15 kV. The composition and microstructures of the powders were investigated by transmission electron microscopy (TEM, JEOL2010) with an energy-dispersive spectrometer (EDS) attachment (HITACHIH-800, Japan).


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The Raman spectra of the composite samples were measured using a Raman microscope (Renishaw in Via) with a 514 nm laser source at an input power of 1 mW. Furthermore, optical properties were characterized by UV-Vis spectrometer (HITACHI U-3010) with scanning range of 350 nm~1400 nm for the sample solution prepared by dispersing the sample powder into the solution, and more wider range of 350 nm~2600 nm was applied for the sample films by coating the composite powder on the glass using resin binders. The scanning speed is 600 nm per min. 3. RESULTS AND DISCUSSION

Figure 1. XRD patterns for the series TM-x compounds. a) full extent, b) the enlarged dominating peak at 2θ≈25° 3.1. Structure and morphology analysis. The TiO2 @MoO3 composites with MoO3 molar ratio of x%, named as TM-x, were synthesized by hydrothermal method. Detailed experimental parameters of the samples were listed in Table 1. Figure.1 (a) depicted the XRD patterns of these products with different Ti/Mo ratio. All the diffraction peaks could be well indexed to the anatase TiO2 and no peaks ascribing to molybdenum oxides could be identified, which implied that in all the TM composites TiO2 was well crystallized in anatase phase whereas MoO3 was amorphous or doped in the TiO2 lattice, which will be further discussed. Compared with TiO2,


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the dominating diffraction peaks of TM-5 at ~ 25° shifted slightly to the higher angle as shown in Fig. 2(b). According to Bragg equation 2dsinθ=nλ, such a lattice contraction may indicate the substitutional doping nature of Mo in TiO2, because the ionic radius of Mo+6 (0.62Å ) is much smaller than the Ti4+ (0.68Å )31. Then the diffraction peak at ~ 25° shifted slightly to the lower angle with the Mo content increasing, which is consistent with the reported GGA (generalized gradient approximation) calculation32, due to Mo-O bond length increasing with elevated Mo doping ratio. But this diffraction peak shifted slightly to the higher angle when Mo content x>25. As the Mo content x (x%25, the diffraction peak intensity was sharper and higher at in Figure. 1(b). TM-25 sample owned the weakest diffraction peak intensity, may indicating the maximum doping level of Mo in anatase lattice.

Figure 2. High resolution TEM images for the series of TM-x core-shell composites: a)TiO2; b) TM-5; c) TM-10, P: EDS pattern; d) TM-15, R: SAED pattern of TM-15; e) TM-25; f) TM-50.


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The TEM images (Fig. 2) exhibited the core-shell microstructure of the prepared TiO2@MoO3 composites. From the HRTEM image in Fig. 2(b), it can be observed that the lattice fringes of 0.35 nm was in agreement with the interplanar distance of anatase (101) planes. The indexed SAED (Selected area electron diffraction) pattern rings in Fig. 2R implied that the core TiO2 was crystallized. From the HRTEM images in Fig. 2(c), (e) and (f), the uniformly coated layer was amorphous and the TiO2 outermost layer lattice was ambiguous and bonded by amorphous layers with different thickness. Thus the nanocomposites were uniform with tight interfaces, exhibiting a significant feature for the core-shell structure. In contrast, TiO2 had obvious boundaries between particles, but the core-shell nanoparticles of TM-x connected together without clear particle boundary. The connection was caused by interaction between the surface dangling bonds Mo-O (H) of amorphous MoO3, which was also demonstrated in Fig. 4, resulting in agglomeration. Meanwhile, the nanoparticles were analyzed using Raman spectroscopy (Figure 3) by monitoring the E2g band (dominating vibration mode for anatase TiO2) and the bands located at 820 ~ 996 cm-1 (vibrational modes of a-MoO3) attributed to the Mo-O-Mo stretching of corner sharing MoO6 octahedron and the symmetric Mo=O stretching mode in orthorhombic a-MoO3 19, 33, 34

. In all the samples, the Mo=O band of MoO3 was identified (Fig. 3), indicating MoO3

existed. Compared with the XRD results and the TEM image, MoO3 is amorphous. Thus, TiO2@MoO3 core-shell nanostructures were consisted of the core of Mo-doped anatase TiO2 and the shell of amorphous MoO3. In Fig. 3(b), when Mo content (25, the shell thickness reaches to a maximum and the excess MoO3 was assembled into spherical particles as shown in Fig. 4(f1).


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Figure 4. (b1), (d1), (e1) and (f1) showed the TEM (HITICHI H-800) images for the assynthesized core-shell MoO3-TiO2 nanopowder, respectively; (b2), (d2), (e2) and (a2) show the statistical particle size distribution of TM=5, TM=15, TM=25 and TiO2 respectively. The circled parts in e1 and f1 denoted the aggregation of the particles. 3.2. Growth mechanism of the core-shell TiO2@MoO3 structure. The formation and the growth mechanism of the TiO2@MoO3 core-shell structure was of fundamental importance to understand how such a well-organized heterostructure could be formed. Here we proposed the following mechanism, which was schematically depicted in Fig.5. Firstly, TiCl4 was hydrolyzed by adding ammonia and peptized into titanium colloids by condensation. During hydrolysis, the Mo7O246- ions joint with [Ti(OH)x] octahedral units in the bonding network, resulting in the formation of the TiOp(OH)m(Mo7O24)q precursor with well-mixed molecular structure, as shown in process I . The general reaction was described below: TiCl4+(x+6y)NH3·H2O+yH6Mo7O24 → Ti(OH)x(Mo7O24)y+6yH2O+4Cl-+(x+6y)NH4+ (Hydrolysis)

(I)

Such a well-mixed molecular structure was in agreement with the finding that Mo was partially doped into the oxide network based on the previous XRD and Raman analysis. During the


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reaction, NH4+, as stabilizing agent, attracted precursor clusters [Mo7O24]6- together to finally form the shell36-38. Meanwhile the slow dropwise addition of NH3·H2O under vigorous stirring was

important

for

nucleating

and

precipitating

of

hydrolyzed

species

on

the

TiOp(OH)m(Mo7O24)q to ensure a homogenous core-shell gel. Ti(OH)x(Mo7O24)y → nTiO(x-m)/2(OH)m7yMoO3-z(OH)3y-7yz+[(x-m-3y+7yz)/2]H2O (Polymerization and condensation)

(2)

The dehydration, dissolution and recrystallization processes took place under heat treatment (150℃) in process II. The precursor lost water to form tetragonal TiO2 units in eq (2). And [MoO6] octahedral units were connected by vertex sharing or edge sharing arrangement to form amorphous MoO3 shell. Based on previous hydrothermal synthesis of pristine MoO3 crystals, the MoO3 tends to grow in one-dimensional mode to form a rod or fiber like structure, owing to the low surface energy of the facets parallel to the c tunnels. However in our system, the mixed precursor restrained such an oriented growth of MoO3, probably because the as-grown TiO2 decreased the surface energy of MoO3 and favored the irregular precipitation of MoO3. Meanwhile, the TiO2 cores were coated by [MoO6]x octahedral units, which inhibited the growing of TiO2 core. So the core and the shell both hindered each other to grow up. In the interface, MoO3 absorbed tightly on TiO2 by forming chemical bond Ti-O-Mo. With the reaction proceeded, the nearby core/shell nanoparticles tended to connect with each other through the interaction between [MoO6] dangling bonds. Therefore, TM-x has a clear core-shell interface but a vague boundary between nanoparticles due to the bond connection, as shown in the TEM images.


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Figure 5. the formation and growth mechanism of TM-x core-shell nanopowder. 3.3. Photochromic performance Photochromic properties of TM-x powders and TM-x film were evaluated and the influence of the core-shell structure on the photochromism, such as photoefficiency, irradiation time, and excitation wavelength were investigated systematically. 3.3.1. The photochromic properties of core-shell TM-x in solution. In this work, we observed the reversible color change in response to on-and-off of UV irradiation utilizing the colloidal suspension of the prepared TM-x dispersed in different solvents. The reference sample was α-MoO3, and the PC performance was introduced in Fig. S1. UV-vis absorption spectra of the suspension for virgin and colored states were contrastively depicted in Fig. 6, 7, and 8. As shown in Fig. 6(a), 0.4 wt% TM-10 in the water exhibited much stronger photochromic efficiency than that of α-MoO3. The milky suspension of TM-10 turned visibly blue after UV irradiation for 8 min in water and 4 min in ethanol, whereas the color change could hardly be seen in pure MoO3 sample. The change in absorbance was apparently seen across the visible and


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near-infrared range from 400 nm to 1300 nm. At wavelength of 600 nm, TM-10 dispersed in water showed a dramatic change of absorbance (10.85), which was about 30 times stronger than that of α-MoO3 (only 0.354) in Fig. 6(a). The remarkable enhancement was very impressive for MoO3。

Figure 6. The photochromic response of 0.3 wt% TM-10 solution in different solvents: a) water solution, 8min irradiated, solid line: 0.3 wt% TM-10, dash line: 0.3 wt% α-MoO3. b) ethanol solution, 4min irradiated, solid line: 0.3 wt% TM-10,dash line: 0.3 wt% α-MoO3. As for the solvent effect, the samples in ethanol showed much stronger photochromic coloration efficiency than that in water (Fig. 6(b)) by 66.6% enhancement in the absorption at 600 nm. During PC transition, the more protons diffuse into the lattice, the more hydrogen molybdenum bronze will be formed, resulting in stronger coloration. The amount of protons available depends on the amount and nature of the adsorbed solvent such as H2O, C2H5OH on the particles. Alcohol can form protons easier because of the greater electronegativity than water, so TiO2@MoO3 alcohol solution has larger photochromic efficiency than aqueous suspension39, 40

. Based on this solvent effect, the following measurements were conducted in ethanol medium.


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The photochromic sensitivity of MoO3 sample is limited by its bandgap energy, corresponding to the near-UV range. For efficient utilization of solar energy and laser sources that cover a broad wavelength range from mid-IR to blue, it is important to extend the coloration response to the visible-light region. Here it is demonstrated that the visible-light coloration in MoO3 can be achieved by combining it with a photoresponsive semiconductor TiO2 as shown in Fig. 7. 0.3 wt% TM-10/EtOH solution was irradiated under different wavelength excitation light by inserting optical filters for about 8 min. TM-10 irradiated by UV light (λ~ 365 nm) exhibited visibly blue and had the obvious change of the absorbance (Fig. 7(a)), meanwhile, TM-10 irradiated by visible light (λ ≥ 420 nm) also exhibited a color change as shown in Fig. 7(b), however, with relatively small absorption contrast, which indicated that UV photons work more efficiently on the PC coloration. The excitation wavelength extended from UV light to visible light. Compared with different content TM-x (0.05 wt% under ethanol), TM-25 has the largest change of absorption under the same irradiation time (4 min) as shown in Fig. 8. As the Mo content increased in the range of 0 to 25%, the change of absorption increased, and when the Mo content was more than 25 %, the PC performance was deteriorated.


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Figure 7. The photochromic response of 0.3 wt% TM-10 EtOH solution in different irradiated wavelength and different irradiated time. Sample was irradiated with a) UV-light (λ ~ 365±10 nm); and b) with visible light (λ ≥ 420 nm), dash line is reference sample: 0.3 wt% α-MoO3, inset: partial enlarged view of (b).

Figure 8. The photochromic response of 0.05 wt% TM-x EtOH solution for different Mo content.

Figure 9. (a) Transmission spectra of the film of TM-25 (0.3 wt%) with different thickness (c represents the number of layer that was coated), solid line: before colored, dash line: after colored. Inset images demonstrated the evolution of images of samples before and after


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colored.(b) showed the absorbance contrast curves of TM-25-3C (abbrev. TM-3C) versus photon energy under irradiation of xenon lamp. 3.3.2. The photochromic properties of core-shell TM-x film. The photochromic properties of core-shell TM-x films were investigated by coating the colloidal suspension on transparent glass. Fig.9 displayed the transmittance spectra of 3 wt% TM-25 film before and after PC transition. The coloration range of the films with different layers was 400-1400 nm, mainly focused in visible light, which made the film deep blue colored, due to the small polar absorption of amorphous MoO3. It can be seen that the as-prepared TM-25 film was almost colorless and transparent. When irradiated by UV for 2 min, the film displayed a blue color immediately. As the thickness of the film increased, the transmittance decreased with higher PC efficiency. The film with 4C (C represents the number of layer that was coated) in 600 nm can reach to a large transmittance change by about 22%. Meanwhile, the absorbance modulation of TM-25 mainly focuse on 1.89ev in Fig. 9(b), that is, TM-x mainly modulate the visible light whereas with little effect in infrared light, which may be of significant importance for future applications, where visual modulation could be achieved without modulation of heat through infrared part. Although the core-shell structure improves the photochromic efficiency, the bleaching rate has not improved. Its Multi cycle photochromic performance was putted into Fig.S2 of the supplementary information. TM-25 was bleached after several days.


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Figure 10. Schematic illustration of steps involved in photochromism of TiO2-MoO3 core-shell structure 3.3.3. Discussion. In comparison with pure MoO3 and other reported MoO3 samples, TM-x exhibited much better photochromic properties so far, ascribing to its unique heterostructure. When TiO2 was coated by MoO3, the heterostructure effect between MoO3 and TiO2 was formed, as was schematically represented in Fig. 10. Meanwhile, TM-x with one-dimensional (1D) nanostructures works well in solar energy conversion because they have a long axis to absorb incident sunlight yet a short radial distance for separation of photogenerated charge carriers41. MoO3 and TiO2 are n-type semiconductors and can both be excited by UV-light, which can generate holes and electrons. For the TiO2@MoO3 heterostructure, the photogenerated holes can transfer from the MoO3 valence band to the TiO2 valence band and the electrons can transfer from the TiO2 conduction band to the MoO3 conduction band42. The hole can weaken the H-O bond of adsorbed water molecules and cause the water molecules to decompose into proton and highly reactive oxygen radials. After the protons diffuse into the lattice, the photogenerated electrons injected into the conduction band of MoO3 and react with MoO3 and the counter ions (i.e. protons) to form the blue-colored hydrogen molybdenum bronze (HxMoxVMo1-xVIO3),


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resulting from the occupation of the photogenerated electrons on the t2g of MoO3 to lead to the formation of Mo5+ 43. As a result, the photogenerated carriers can be effectively separated and result in improved coloration performance because of the interfacial junction44. The core-shell structure realized high absorption as we expected. Meanwhile, according to the TEM analysis, the small particle size of TiO2 is only 5~10nm, and the amorphous MoO3 shell thickness is only several nanometers. For size effect45, it has two advantages: (1) it decreases the distance of electron-transfer, which increases the rate of coloration and decreases the time of coloration. (2) it has high surface area in Table 2, which contributes to the higher charge transfer and ion intercalation kinetics, and can favor high photochromic efficiency. Table 2. Elemental analysis (mol% Mo) and Surface area for the TM-x core-shell material TM-x

mol% Mo

surface area(m2/g)

5

4

123.132

15

9

184.576

25

13

197.022

50

21

262.458

According to the equation αhv = B(hv - Eg)2

(3)

the value of Eg is found to about 3.07 eV (400 nm). This alteration of the bandgap structure was induced by the inherently


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Figure11. (a) Plots of (αhv)1/2 as a function of photon energy for TM-10,α-MoO3,and pure TiO2, according to the UV-VIS spectrum.(b) The photoluminescence (PL) properties of TiO2, MoO3 and TM-10 were measured using a Xe lamp as the excitation source at room temperature narrower bandgap of pure MoO3 (3 eV) due to the heterojunction11. However, in Fig. 7, TM-10 can excite by visible light, that is, the band gap of TM-10 need to become narrower than 2.94 eV (420 nm). So the visible-chromism of TM-10 is not caused by the heterostructure, but probably ascribes to the inter-band in TiO2 formed by Mo doping. When Mo is doped into the lattice of TiO2, they can distort the host structure with intercalation, particularly through expansion of the van der Waals gap, and can also add discrete electronic inter-band states of their own between the valence band and conduction band, which decreases the effective band gap46. Meanwhile, the Photoluminescence (PL) properties of TiO2, MoO3 and TM-10 were measured using a Xe lamp as the excitation source at room temperature. The intensity of photoluminescence spectrum directly affects the recombination rate of photogenerated electron-hole pairs. Compared with TiO2 and MoO3, TM-10 with heterojunction hinder the recombination of photogenerated electron-hole pairs effectively, resulting in the improved PC property. When the excitation


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wavelength is 320 nm, the emission peaks of TM-10 consist of one UV emission at 410nm, two blue luminescence emissions at 440nm and 466nm, and one green luminescence emission at about 525nm47-49, implying the nanopowder can be excited by visible light. In Table 2, the specific surface areas change with the photoefficiency of TM-x, showing a strong dependency on microstructure and PC property. As the Mo content increased in the range of 0 to 25%, the particle size decreased, causing an increase in the specific surface area of the core-shell composite and thus an improvement in the coloration. When the Mo content is more than 25 %, the coloration decreased but the specific surface area increased. From TEM analysis, the heterostructure of TM-50 has destroyed, and the excess MoO3 will assemble into amorphous MoO3 nanospheres, which will show much weaker PC performance, therefore the total PC performance for the same mass concentration will be getting lower. It demonstrates the coated MoO3 layer of the core-shell structure has the saturation thickness. However, the specific surface area of TM-50 is larger than that of MT-25, due to the interface exposed to the surface. From these results and analysis, we can propose an explanation for the chromic behavior of the core-shell structure based on the coupled behavior of structural and electronic properties. The heterostructure and the Mo dopant between MoO3 and TiO2 are equally important for the PC property. The heterostructure effectively separates the photogenerated electron-hole. The Mo dopant distorts the structure and form inter-band to the visible-chromism. The little particles of TM provide a high surface area and short electron-transfer distance, due to the size effect. 4. Conclusions TiO2@MoO3 core-shell nanopowder was successfully synthesized by a one-step hydrothermal method and the significance of the core-shell heterojunction structure on photochromism enhancement was demonstrated in detail. The composite particles owned tight core/shell


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interface and small particle size (5-10 nm), which endowed the particle with high specific surface area and rich interfacial sites for charge transfer. Enhanced photochromic properties of the as-prepared TM-x were investigated in comparison with the conventional pure α-MoO3. The change of the absorbance at 600nm in TM-10 in water is about 30 times larger than that of αMoO3 and the film can regulate the transmittance by about 20% at 500-800 nm wavelength region, which was shown to be very large absorbance change. The excitation wavelength expanded from UV light to visible light, because of the defect band formed by Mo doping in TiO2 lattice, which was verified by XRD and Raman spectroscopy. The exciting performances of the as-prepared core-shell TiO2@MoO3 are expected to become available in the foreseeable future for wide applications in the areas of color displays and smart window devices. ASSOCIATED CONTENT Supporting Information. The preparetion method of α-MoO3, and its structure and morphology analysis by xrd and sem images. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This study was financially supported by the high-tech project of MOST (2012AA030305), the national sci-tech support plan the National Natural Science Foundation of China (NSFC, No.: 51272273, 51172265, 51372264), and the Science and Technology Commission of Shanghai Municipality (STCSM, No.: 13PJ1409000, 13NM1402200). REFERENCES (1) Zhang, G. J.; Yang, W. S.; Yao, J. N. Thermally Enhanced Visible-light Photochromism of Phosphomolybdic Acid-polyvinylpyrrolidone Hybrid Films. Adv. Funct. Mater. 2005, 15 (8), 1255-1259. (2) Yao, J. N.; Hashimoto, K.; Fujishima, A. Photochromism Induced in An Eletrolytically Pretreated MoO3 Thin-film by Visible-light. Nature. 1992, 355 (6361), 624-626. (3) Shakir, I.; Shahid, M.; Kang, D. J. MoO3 and Cu0.33MoO3 Nanorods for Unprecedented UV/Visible Light Photocatalysis. Chem. Commun. 2010, 46 (24), 4324-4326. (4) Miyauchi, M.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Photocatalysis and Photoinduced Hydrophilicity of Various Metal Oxide Thin Films. Chem. Mater. 2002, 14 (6), 2812-2816. (5) Shen, Z.; Chen, G.; Yu, Y.; Wang, Q.; Zhou, C.; Hao, L.; Li, Y.; He, L.; Mu, R. Sonochemistry Synthesis of Nanocrystals Embedded in A MoO3-CdS Core-shell Photocatalyst with Enhanced Hydrogen Production and Photodegradation. J. Mater. Chem. 2012, 22 (37), 19646-19651. (6) Cao, X.; Zheng, B.; Shi, W.; Yang, J.; Fan, Z.; Luo, Z.; Rui, X.; Chen, B.; Yan, Q.; Zhang, H. Reduced Graphene Oxide-wrapped MoO3 Composites Prepared by Using Metal-Organic


23


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

Page 24 of 30

Frameworks as Precursor for All-solid-state Flexible Supercapacitors. Adv. Mater. 2015, 27 (32), 4695-4701. (7) Brezesinski, T.; Wang, J.; Tolbert, S. H.; Dunn, B. Ordered Mesoporous Alpha-MoO3 with Iso-oriented Nanocrystalline Walls for Thin-film Pseudocapacitors. Nature Materials. 2010, 9 (2), 146-151. (8) Wen, X.; Yang, W.; Ding, Y.; Niu, S.; Wang, Z. L. Piezoresistive Effect in MoO3 Nanobelts and Its Application in Strain-enhanced Oxygen Sensors. Nano. Res. 2014, 7 (2), 180189. (9) Taurino, A. M.; Forleo, A.; Francioso, L.; Siciliano, P.; Stalder, M.; Nesper, R. Synthesis, Electrical Characterization, and Gas Sensing Properties of Molybdenum Oxide Nanorods. Appl. Phys. Lett. 2006, 88 (15). (10)

Wang,

Z.

C.;

Hu,

X.

F.;

Helmersson,

U.

Peroxo

Sol-gel

Preparation:

Photochromic/electrochromic Properties of Mo-Ti Oxide Gels and Thin Films. J. Mater. Chem. 2000, 10 (10), 2396-2400. (11) Di Yao, D.; Field, M. R.; O'Mullane, A. P.; Kalantar-zadeh, K.; Ou, J. Z. Electrochromic Properties of TiO2 Nanotubes Coated with Electrodeposited MoO3. Nanoscale. 2013, 5 (21), 10353-10359. (12) Ajito, K.; Nagahara, L. A.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. Study of The Photochromic Properties Of Amorphous MoO3 Films Using Raman Microscopy. J. Phys. ChemUs. 1995, 99 (44), 16383-16388.


24

Page 25 of 30

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


(13) Bechinger, C.; Wirth, E.; Leiderer, P. Photochromic Coloration of WO3 with Visible Light. Appl. Phys. Lett. 1996, 68 (20), 2834-2836. (14) Chu, C. W.; Li, S. H.; Chen, C. W.; Shrotriya, V.; Yang, Y. High-performance Organic Thin-film Transistors with Metal oxide/metal Bilayer Electrode. Appl. Phys. Lett. 2005, 87 (19). (15) He, T.; Yao, J. N. Photochromism in Composite and Hybrid Materials based on Transition-metal Oxides and Polyoxometalates. Prog. Mater. Sci. 2006, 51 (6), 810-879. (16) Liu, S.; Zhang, N.; Xu, Y.-J. Core-Shell Structured Nanocomposites for Photocatalytic Selective Organic Transformations. Part. Part. Syst. Charact. 2014, 31 (5), 540-556. (17) Ahmed, H. M. F.; Begum, N. S. Synthesis and Characterization of MoO3–WO3 Composite Thin Films by Liquid Phase Deposition Technique: Investigation of Its Photochromic Properties. B. Mater. Sci. 2013, 36 (1), 45-49. (18) Rao, K. S.; Madhuri, K. V.; Uthanna, S.; Hussain, O. M.; Julien, C. Photochromic Properties of Double Layer CdS/MoO3 Nano-structured Films. Mat. Sci. Eng. B-Solid. 2003, 100 (1), 79-86. (19) Santos, E. B.; de Souza E Silva, J. M.; Mazali, I. O. Raman Spectroscopy as A Tool for The Elucidation of Nanoparticles with Core-shell Structure of TiO2 and MoO3. Vibrational Spectroscopy. 2010, 54 (2), 89-92. (20) Chopoori.Ja; Dorion, G. H.; Model, F. S. Photochromism of Metal Oxides. I. Light Sensitivity Of MoO3 or WO3 Coprecipitated with TiO2. J. Inorg. Nucl. Chem. 1966, 28 (1), 83-&.


25


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

Page 26 of 30

(21) Wu, L.; Xing, J.; Hou, Y.; Xiao, F. Y.; Li, Z.; Yang, H. G. Fabrication of Regular ZnO/TiO2 Heterojunctions with Enhanced Photocatalytic Properties. Chem-Eur J. 2013, 19 (26), 8393-8396. (22) Weng, B.; Zhang, X.; Zhang, N.; Tang, Z.-R.; Xu, Y.-J. Two-Dimensional MoS2 Nanosheet-Coated Bi2S3 Discoids: Synthesis, Formation Mechanism, and Photocatalytic Application. Langmuir. 2015, 31 (14), 4314-4322. (23) Elder. S. H. TiO2- (MoO3) Core-Shell Materials. J. Am. Chem. Soc. 2000, 122 5138-5146. (24) Yang, X. L.; Dai, W. L.; Guo, C. W.; Chen, H.; Cao, Y.; Li, H. X.; He, H. Y.; Fan, K. N. Synthesis of Novel Core-shell Structured WO3/TiO2 Spheroids and Its Application in The Catalytic Oxidation of Cyclopentene to Glutaraldehyde by Aqueous H2O2. J. Catal. 2005, 234 (2), 438-450. (25) Zhang, N.; Liu, S.; Xu, Y. J. Recent Progress on Metal Core@Semiconductor Shell Nanocomposites as A Promising Type of Photocatalyst. Nanoscale. 2012, 4 (7), 2227-2238. (26) Lu, M.; Shao, C.; Wang, K.; Lu, N.; Zhang, X.; Zhang, P.; Zhang, M.; Li, X.; Liu, Y. pMoO3 Nanostructures/n-TiO2 Nanofiber Heterojunctions: Controlled Fabrication and Enhanced Photocatalytic Properties. Acs. Appl. Mater. Inter. 2014, 6 (12), 9004-9012. (27) Chen, Y. J.; Xiao, G.; Wang, T. S.; Zhang, F.; Ma, Y.; Gao, P.; Zhu, C. L.; Zhang, E.; Xu, Z.; Li, Q. H. Alpha-MoO3/TiO2 Core/shell Nanorods: Controlled-synthesis and Low-temperature Gas Sensing Properties. Sensor. Actuat. B-Chem. 2011, 155 (1), 270-277.


26

Page 27 of 30

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


(28) Shen, Y.; Hu, F.; Yang, Y.; Xiao, Y.; Yan, P.; Li, Z. Effect of Formaldehyde on The Photochromic Properties of Ordered Molybdenum Oxide Thin Films Produced by Hydrothermal Process. Surf. Coat. Tech. 2014, 240, 393-398. (29) Chen, X.; Mao, S. S. Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications. Chem. Rev. 2007, 107 (7), 2891-2959. (30) Huang, R.; Shen, Y.; Zhao, L.; Yan, M. Y. Effect of Hydrothermal Temperature on Structure and Photochromic Properties of WO3 Powder. Adv. Powder. Technol. 2012, 23 (2), 211-214. (31) Dean, J. A. Lange's Chemistry Handbook Version 13th. Lange's Chemistry Handbook Version 13th. 1999, 3-118. (32) Khan, M.; Xu, J.; Chen, N.; Cao, W. Electronic and Optical Properties of Pure and Mo Doped Anatase TiO2 using GGA and GGA Plus U Calculations. Physica. B. 2012, 407 (17), 3610-3616. (33) Silveira, J. V.; Batista, J. A.; Saraiva, G. D.; Mendes Filho, J.; Souza Filho, A. G.; Hu, S.; Wang, X. Temperature Dependent Behavior of Single Walled MoO3 Nanotubes: A Raman Spectroscopy Study. Vibrational Spectroscopy. 2010, 54 (2), 179-183. (34) Balaji, S.; Djaoued, Y.; Robichaud, J. Phonon Confinement Studies in Nanocrystalline Anatase-TiO2 Thin Films by Micro Raman Spectroscopy. J. Raman. Spectrosc. 2006, 37 (12), 1416-1422. (35) Kelly, S.; Pollak, F. H.; Tomkiewicz, M. Raman Spectroscopy as A Morphological Probe for TiO2 Aerogels. J. Phys. Chem. B. 1997, 101 (14), 2730-2734.


27


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

Page 28 of 30

(36) Chithambararaj, A.; Sanjini, N. S.; Bose, A. C.; Velmathi, S. Flower-like Hierarchical hMoO3: New Findings of Efficient Visible Light Driven Nano Photocatalyst for Methylene Blue Degradation. Catal. Sci. Technol. 2013, 3 (5), 1405-1414. (37) Chithambararaj, A.; Sanjini, N. S.; Velmathi. S.; Bose. A. C. Preparation of h-MoO3 and Alpha-MoO3 Nanocrystals: Comparative Study on Photocatalytic Degradation of Methylene Blue under Visible Light Irradiation. Phys. Chem. Chem. Phys. 2013, 15 (35), 14761-14769. (38) Zheng, L.; Xu, Y.; Jin, D.; Xie, Y. Novel Metastable Hexagonal MoO3 Nanobelts: Synthesis, Photochromic, and Electrochromic Properties. Chem. Mater. 2009, 21 (23), 56815690. (39) Yao, J. N.; Loo, B. H.; Hashimoto, K.; Fujishima, A. Photochromic Characteristics of Mixed WO3-MoO3 Thin-Films in Alcohol Vapors. Berichte Der Bunsen-Gesellschaft-Physical Chemistry Chemical Physics. 1991, 95 (5), 554-556. (40) He, T.; Yao, J. N. Photochromic Materials Based on Tungsten Oxide. J. Mater. Chem. 2007, 17 (43), 4547-4557. (41) Liu. S. Q., Tang. Z. R., Sun. Y. G., Colmenares. J. C. and Xu. Y. J., One-dimensionbased Spatially Ordered Architectures for Solar Energy Conversion. Chem. Soc. Rev. 2015, 44. (42) Lu, B.; Li, X.; Wang, T.; Xie, E.; Xu, Z. WO3 Nanoparticles Decorated on Both Sidewalls of Highly Porous TiO2 Nanotubes to Improve UV and Visible-light Photocatalysis. J. Mater. Chem. A. 2013, 1 (12), 3900-3906.


28

Page 29 of 30

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


(43) He, Y. P.; Wu, Z. Y.; Fu, L. M.; Li, C. R.; Miao, Y. M.; Cao, L.; Fan, H. M.; Zou, B. S. Photochromism and Size Effect of WO3 and WO3-TiO2 Aqueous Sol. Chem. Mater. 2003, 15 (21), 4039-4045. (44) Zhang, N.; Yang, M.-Q.; Liu, S.; Sun, Y.; Xu, Y.-J. Waltzing with the Versatile Platform of Graphene to Synthesize Composite Photocatalysts. Chem. Rev. 2015, 115 (18), 10307-10377. (45) Alivisatos, A. P. Semiconductor Clusters, Nanocrystals, and Quantum Dots. Science. 1996, 271 (5251), 933-937. (46) Wang, M.; Koski, K. J. Reversible Chemochromic MoO3 Nanoribbons through Zerovalent Metal Intercalation. Acs. Nano. 2015, 9 (3), 3226-3233. (47) Wang, J.; Lee, P. S.; Ma, J. Synthesis, Growth Mechanism and Room-temperature Blue Luminescence Emission of Uniform WO3 Nanosheets with W As Starting Material. J. Cryst. Growth. 2009, 311 (2), 316-319. (48) Su, X.; Li, Y.; Jian, J.; Wang, J. In Situ Etching WO3 Nanoplates: Hydrothermal Synthesis, Photoluminescence and Gas Sensor Properties. Mater. Res. Bull. 2010, 45 (12), 19601963. (49) Boukhachem, A.; Kamoun, O.; Mrabet, C.; Mannai, C.; Zouaghi, N.; Yumak, A.; Boubaker, K.; Amlouk, M. Structural, Optical, Vibrational and Photoluminescence Studies of Sn-doped MoO3 Sprayed Thin Films. Mater. Res. Bull. 2015, 72 252-263. Table of Contents Graphic


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