Article pubs.acs.org/cm
Topotactic Phase Transformation of Hexagonal MoO3 to Layered MoO3‑II and Its Two-Dimensional (2D) Nanosheets Vipin Kumar, Afriyanti Sumboja, Jiangxin Wang, Venkateswarlu Bhavanasi, Viet Cuong Nguyen, and Pooi See Lee* School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore S Supporting Information *
ABSTRACT: This study reports a novel strategy to obtain a layered metastable phase of molybdenum trioxide (MoO3). The metastable phase in the layered framework, i.e., MoO3-II, was a result of the topotactic phase transformation of hexagonal-MoO3 (h-MoO3) into MoO3-II. This transformation was realized through the removal of the water and ammonium molecules from the framework of h-MoO3. Next, a facile exfoliation strategy was adopted to prepare MoO3-II nanosheets, consisting of double-layers of MoO6 octahedra. The as-exfoliated MoO3-II nanosheets had measured thicknesses of 1.4, 2.4, and 11.2 nm, as quantified using atomic force microscopy (AFM), equivalent to 2, 3, and 16 doublelayers of the MoO6 octahedra in MoO3-II, respectively, after taking into account their respective van der Waals gaps (thickness of a MoO3-II double-layer ∼0.709 nm). This work is the first attempt to synthesize MoO3-II nanosheets and shed light on their formation mechanism.
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Na+, etc.) in electrochemical applications.12 α-MoO3 has been widely studied in various morphologies, including nanoparticles, nanosheets, nanobelts, nanorods, nanowires, and nanotubes. MoO6 octahedra are the basic building blocks for all types of polymorphs (stable as well as metastable; e.g., α-MoO3 and MoO3-II) of MoO3. 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. In contrast, in MoO3-II, the layers arrange in an (AAA) manner and grow along the c axis, with a reduced bond length along the b axis (compared to α-MoO3).16 To date, several strategies to form 2D α-MoO3 have been reported, including the use of cation-intercalation-assisted exfoliation,12 scotch tape exfoliation using CVD-grown nanosheets as the base material,17 organic-molecule-assisted exfoliation under hydrothermal conditions18 and grinding (in acetonitrile) followed by probe sonication (in ethanol/water) of bulk MoO3.19 Intriguing research on the development of 2D nanosheets of MoO3 is in progress due to its large permittivity value (>500),20 high charge carrier mobility (1100 cm2 /(V s)),19 tunable band structure (2−3 eV),21 relative abundance and eco-friendliness. α-MoO3 is not the only layered phase of MoO3. A layered structure can also be obtained in its metastable polymorph, i.e., MoO3-II. This metastable phase of MoO3 in a layered
INTRODUCTION Two-dimensional (2D) nanomaterials are considered ideal candidates for many future applications becaues of their atomic thickness and infinite length in the vertical and lateral directions, respectively. Layered materials are varied and unexploited source of 2D nanomaterials with exotic physical and chemical properties relevant to electronics, catalysis, and energy storage applications.1−3 Various nanostructures can be created using 2D nanosheets of transition metal dichalcogenides (TMD) and transition metal oxides (TMO) as building blocks.4−9 In a true 2D nanosheet of TMD or TMO, a layer of metal atoms is sandwiched by two layers of chalcogenide or oxide atoms, which engage in covalent bonding within these trilayer arrangements, whereas the adjacent trilayers are connected via weak van der Waals or electrostatic interactions. More than 10 TMO 2D nanosheets have been reported in the past few years, varying from insulators (HfS2 or Ti3O7) to semiconductors (MoS2 or MoO3) to semimetals (TiSe2 or MnO2) and true metals (NbS2 or RuO2).10−15 The nanosheets developed from TMD or TMO possess superior properties, e.g., a high value of dielectric constant (high-K), nonzero band gap and thermal stability in air.8,9 However, the fabrication or construction of these nanosheets is hampered by the lack of a facile route to synthesize them in substantial quantities in mono- or multilayer form. Therefore, there is an impending need to find a simple yet effective method to fabricate true 2D nanosheets of layered materials. Because of the interspacing (∼6 Å) between consecutive layers, α-MoO3 can serve as a host for electrolyte ions (H+, Li+, © 2014 American Chemical Society
Received: May 6, 2014 Revised: September 1, 2014 Published: September 15, 2014 5533
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framework was first evaluated from the thermodynamically stable phase of MoO3 under high temperature and pressure, in which α-MoO3 decreased in lattice parameters and increased in density, resulting in a high packing efficiency of MoO3-II.16 The topotactic dehydration of monohydrated MoO3 (MoO3 H2O) and hemihydrated MoO3 (MoO3 0.5H2O) could also lead to the formation of a MoO3-II phase, earlier identified as the highpressure phase of α-MoO3.22 A reaction is said to be topotactic if the lattice of the as-formed product possesses one or a few crystallographically equivalent and definite orientations with respect to the lattice of the parent crystal. Therefore, it was believed that the amount of water molecules and mode of sharing of the MoO6 octahedron chains (edge- and cornersharing) inside the framework plays a vital role in achieving the MoO3-II phase. Furthermore, transformation of α-MoO3 into MoO3-II must involve breakage of the Mo−O bond followed by rotation and translation by a 1/2 unit cell along the b axis. The Mo−O1 bond of α-MoO3 can be broken by the application of high pressure only, as demonstrated recently.23,16 Less is known about this phase of MoO3 than the other polymorphs because it is an intermediary metastable phase in the evolution to the thermodynamically stable phase, i.e., α-MoO3. Therefore, it has been considered to be difficult to achieve. MoO6 octahedra are the basic building blocks used to construct MoO3 in various phases. The arrangement of the MoO6 octahedra (formation of any phase) depends on the external conditions, e.g., temperature, pressure and impurities. At high temperature, MoO6 octahedra favorably arrange in the α-MoO3 phase (sharing corners and edges to form layers of the MoO6 octahedra in an ABA arrangement).17 Meanwhile, the hMoO3 phase (shares corners and edges to form pillars of MoO6 octahedra) dominates in the presence of cations, such as H+, Na+, K+, or NH4+.24,25 Here, we introduce a route to produce the MoO3-II phase (sharing corners and edges to form layers of MoO6 octahedra in an AAA arrangement), and then prepare MoO3-II nanosheets, using a two-step procedure, as depicted in Scheme 1. The MoO3 pillar-like structure involves the corner-
exfoliated (because of the layered structure of MoO3-II) to obtain MoO3-II nanosheets.
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EXPERIMENTAL SECTION
Chemicals Used. In this study, Mo powder (99.99%), H2O2 (30%), HNO3 (conc.), thiourea crystal, and KOH were used as received from Alfa Aesar and Sigma-Aldrich, respectively, without further purification. Synthesis of Hexagonal-MoO3 (h-MoO3) Nanorods. h-MoO3 nanorods were synthesized as described in our previous report.27 Briefly, 10 mL of H2O2 was added dropwise into a 20 mL glass vial containing 500 mg of Mo powder (poly peroxomolybdic solution pH ∼ 1.5). A solution of 0.88 M thiourea prepared in 0.5 M KOH was mixed dropwise into the as-prepared poly peroxomolybdic solution and left stirring. The yellow solution was then transferred to an oven fixed at 100 °C and left for 18 h. The product was washed thoroughly with ethanol and DI water several times and dried at 70 °C. Fabrication of MoO3-II Nanosheets. The as-prepared h-MoO3 nanorods were annealed at 530 °C for 3 h in air atmosphere, to realize the transformation into MoO3-II microsheets. MoO3-II microsheets were then exfoliated into nanosheets using scotch tape (3 M Scotch). This procedure was repeated 10−20 times until a faint reflection of MoO3-II was observed on the tape and then transferred onto a previously cleaned SiO2/Si substrate.
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MATERIAL CHARACTERIZATIONS Structural Characterization. The product was characterized using powder X-ray diffractometry (Shimadzu XRD − 6000, Cu Kα radiation λ = 1.54 Å; power, 2 kW) at a scan rate of 1°/min in the 2θ range of 10° - 60°. Scanning electron microscopy (FESEM; JEOL, JSM 7600F Thermal FEG, JSM 6340F cold cathode FEG) and Transmission electron microscopy (TEM; JEOL, JEM 2010 and JEM 2100F) were used to analyze the microstructure of MoO3-II microsheets and nanosheets. Thermal analysis was illustrated using thermogravimetric analysis (TGA)/Differential scanning calorimetry (DSC)/differential thermal analysis (DTA) (STA - 449 C, max temperature 1500 °C), optical microscopy and Raman spectroscopy was performed using a polarizing microscope (LABOPHOT2-POL & Nikon E600, Model Sony and Nikon) and 488 nm wavelength laser light (Witec, Model WiTec alpha 300 SR − separation between experimental data points ±1.0 cm−1), respectively. Atomic force microscopy (AFM) (NT MDT, NTegra) was used to evaluate the thickness of the asprepared nanosheets.
Scheme 1. Schematic Illustration of the Steps Involved in the Preparation of MoO3-II Nanosheets
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RESULTS AND DISCUSSION X-ray diffraction patterns of the h-MoO3 nanorods and asprepared MoO3-II microsheets are represented in Figure 1a. The diffraction patterns are readily indexed to the hexagonal and monoclinic phases of MoO3 with the space groups P63/m and P21/m, respectively (ICDD # 29−0115 and 47−1320). From the XRD analysis, the cell parameters a = 10.54 Å, c = 3.72 Å and a = 3.95 Å, b = 3.68 Å, c = 7.09 Å and the cell volumes V = 359.37 Å3 and V = 100.47 Å3 are determined for h-MoO3 and MoO3-II, respectively.26,16 The hexagonal phase is represented by (M)x MoO3 (H2O)y, where M is the framework-stabilizing ion (Na+, K+, NH4+, etc.) and x and y are the variables (0 < x < 1, 0 < y < 1).24 XRD analysis shows that a definite crystallographic relationship exists between the lattice parameters of the two phases, i.e., bMoO3‑II ∼ ch‑MoO3 and (b + c)MoO3‑II ∼ ah‑MoO3, which indicates that the two phases, i.e., h-MoO3 and MoO3-II, are crystallographically correlated with each other. The structure of MoO3-II consists of layers of
as well as edge-sharing of the octahedra via O1 and O3 atoms along the a axis (of the other chains) and c axis (of the same chain), respectively. In the first step, the dehydration and deammonization of h-MoO3 (a phase of (NH4)0.21 MoO3 (H2O)0.26) nanorods was carried out to produce MoO3-II microsheets. This step involves the rearrangement of the octahedron chains. The new octahedra share corners and edges with the adjacent octahedra in the horizontal and vertical planes, respectively, and form a layered framework of MoO3-II. The as-prepared MoO3-II microsheets were then mechanically 5534
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The transformation of monohydrated and hemihydrated MoO3 into MoO3-II has been reported previously,22 but the transformation of h-MoO 3 (a phase of (NH 4 ) 0 . 2 1 MoO3(H2O)0.26) into MoO3-II has not been realized before. This transformation is dependent on the impurity content, e.g., the content of water molecules within MoO3 H2O and MoO30.5H2O. The transformation of h-MoO3 nanorods into MoO3-II microsheets began at 450 °C (the exothermic peak indicates the beginning of solid-state reactions or crystallization; Figure S1 in the Supporting Information indicates partial phase formation) and was completed at 530 °C (presence of an endothermic peak in Figure 1b, signifies the phase formation of MoO3-II). This is corroborated by the XRD patterns of the samples, which were recorded before and after annealing at 530 °C, as seen in Figure 1a. Subsequent annealing at 650 °C results in the α-MoO3 phase (the thermodynamically stable phase of MoO3). The appearance of a broad endothermic peak at 650 °C indicates that a large amount of energy is required, to realize the formation of α-MoO3. A strong endothermic peak at ∼780 °C designates the melting or sublimation of the material. XRD patterns of the samples annealed at 450, 530, and 650 °C are shown in the Supporting Information, Figure S1. It should be noted that no polymorphic phase was identified throughout the transformation temperature interval (ΔT ∼ 100 °C), as seen in the DSC analysis in Figure 1b. The morphology of the nanorods before and after annealing at 530 °C was evaluated using FESEM, as presented in Figure 2a, b, respectively. It is believed that the microsheets are the result of the decomposition and recrystallization of the nanorods (as shown in DSC analysis, Figure 1b). The inset of Figure 2a shows a magnified view of the h-MoO3 nanorods. FESEM micrographs of h-MoO3 nanorods annealed at 450 and 650 °C are shown in the Supporting Information, Figure S2. The mechanism of the transformation from the h-MoO3 phase to the MoO 3 -II phase will be discussed from thermodynamic as well as crystallographic perspectives following the schematic illustration proposed in a and b in Figure 3. Upon the annealing (at temperature ∼140 °C) of the h-MoO3 nanorods, water molecules start to digress from the lattice of the nanorods (as shown in the first TGA decomposition step, Figure 1b). A continuous increment in the temperature leads to the removal of the ammonium molecules (as seen in the second TGA decomposition step, Figure 1b) at approximately 395 °C, and the expulsion of ammonium molecules breaks the hexagonal framework (as indicted by an exothermic peak in the DSC analysis, Figure
Figure 1. (a) Comparison of the XRD patterns of MoO3 powder before (h-MoO3) and after (MoO3-II) annealing at 530 °C, showing distinct phase formation. Inset: the crystal structure of MoO3-II, illustrating the layered framework structure of MoO3-II. A minute fraction of the parent phase (h-MoO3) was found in the diffraction (red diamond) spectra of MoO3-II. (b) DSC/TGA results for h-MoO3 nanorod powder, showing the dehydration and deammonization of hexagonal nanorods, which leads to the formation of a new phase of MoO3.
MoO6 octahedra, connected along the a axis (100) and growing along the c axis (001) to form a layered framework structure, which resembles the structure of α-MoO3 (growing along the b axis (010) and connecting along the c axis (001)). A single unit cell of MoO3-II consists of two layers of MoO6 octahedra or a single double-layer with a fundamental thickness of 0.709 nm including the respective van der Waals gaps,16 as represented in the inset of Figure 1a. The fundamental thickness of a single double-layer of MoO3-II was found to be ∼57% that corresponding to α-MoO3 (∼1.3 nm), indicating the high packing efficiency in MoO3-II.16
Figure 2. FESEM micrographs of h-MoO3 nanorods (a) before and (b) after annealing at 530 °C. Image b clearly shows the rectangular shape of the microsheets with a lateral size of 5 μm × 10 μm and thickness of 1.5 μm, which corresponds to ∼2138 stacks of double-layers (red arrows in the image). The inset in image a shows a magnified view of the h-MoO3 nanorods (scale bar ∼100 nm). 5535
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affected by the facets of the nanorods.30 Due to the removal of water and ammonium molecules (dehydration and deammonization) from the lattice of h-MoO3, the hexagonal framework starts to destabilize and gradually changes into a pseudomorph phase (mixed phase) at 450 °C (XRD pattern shown in the Supporting Information, Figure S1) and then to pure MoO3-II phase at 530 °C (as revealed by XRD and DSC analyses, Figure 1). From the DSC analysis, the enthalpies of formation of MoO3-II and α-MoO3 were determined to be approximately −350.2 and −501.2 kJ/mol, respectively. The enthalpy of formation for MoO3-II is much lower, indicating the metastability of this intermediate phase. In addition to the phase changes, the morphology of the product also changes (from a nanorods to microsheet-like morphology) to minimize the increase in the free energy. Dehydration and deammoniazation are the principal requirements for realizing the transformation from the h-MoO3 phase to the MoO3-II phase. Attributed to the removal of water and ammonium molecules from the lattice of h-MoO3, the displacement of the molybdenum atoms within individual octahedra occurs in such a way that the condensation of the octahedron chains forms a layered structure with a (AAA) stacking sequence. The MoO3 octahedra (in either phase) consist of three different types of oxygen atoms: O1, O2, and O3. The framework structure of h-MoO3 involves the corner- as well as edge-sharing of the octahedra via O1 and O3 atoms along the a axis (with the octahedra of the other chain) and c axis (with the octahedra of the same chain), respectively, realizing a pillarlike structure. The arrangement of the octahedra in h-MoO3 along the c axis is represented in Figure
Figure 3. (a) Schematic illustration of the steps involved in the topotactic phase transformation of h-MoO3 nanorods into MoO3-II microsheets and (b) arrangement of the octahedra in the h-MoO3 and MoO3-II framework. The red balls, black balls and purple balls in Figure 3 (b) correspond to the O1, O2, and O3 atoms, respectively, in the MoO6 octahedra (orange boxes).
1b).27 The nanorods merge with each other as a result of surface diffusion (coalescence phenomenon). Below the melting point, surface diffusion as well as the movement and subsequent annihilation of grain boundaries is expected to be the governing mechanism for the coalescence of the nanorods.28,29 It is also assumed that the rate of coalescence is
Figure 4. (a) Optical microscopy image of the as-exfoliated MoO3-II nanosheets showing the formation of a 2-fold double-layer (2 DL), 5-fold double-layer (5 DL) and multifold double-layer (ML) of MoO6 octahedra in MoO3-II. (b−d) Corresponding AFM topography images reveal the thickness of the as-exfoliated nanosheets. The inset in the micrographs shows the thickness profile of the MoO3-II nanosheets. 5536
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Figure 5. (a) Transmission electron microscopy (TEM) image, (b) high-resolution TEM (HRTEM) image, (c) FFT pattern along the [31̅ 1]̅ zone axis and (d) unit cell of MoO3-II exposing the (3̅11̅) plane. Red and green balls depict Mo and O atoms, respectively. The inset of figure d shows a magnified HRTEM image with the corresponding Mo and O atom positions.
low as 1.4 nm, as shown in Figure 4b, which is equivalent to the thickness of two double-layers (2 DL) of MoO6 octahedra in MoO3-II (the fundamental thickness of two double-layers is 0.709 nm). Panels c and d in Figure 4 how that the thicknesses of the as-exfoliated nanosheets are 2.4 and 11.2 nm, corresponding to 3 double-layers (3 DL) and 16 double-layers (16 DL) of MoO6 octahedra, respectively. In conjunction with the AFM evaluation of the nanosheets thickness, HRTEM analysis was also conducted, and the results are shown in the Supporting Information, Figure S4. To evaluate the crystallographic relationship between two polymorphs of MoO 3 , we used transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), and the results are shown in Figure 5a, b, respectively. From the HRTEM analysis, we found that the interspacings between two fringes are 0.22 and 0.33 nm, corresponding to the (1̅03) and (011) plane of MoO3-II, respectively. The apparent fringes in the HRTEM micrograph reveal the good crystalline quality of the as-obtained nanosheets. The crystallographic relationship can also be established, based on the interplanar spacing relationship, i.e., d(−103) MoO3‑II ∼ d(221) h‑MoO3 and d(011) MoO3‑II ∼ d(210) h‑MoO3. The crystallographic relationship between the h-MoO3 phase and the MoO3II phase proves that the two phases are topotactically correlated. Images c and d in Figure 5 show the fast Fourier transformation (FFT) of the HRTEM image exposing the (3̅11̅) plane of MoO3-II and the atomic representation of MoO3-II in the (3̅11̅) plane, respectively. Figure 6a shows the Raman spectra of the as-exfoliated nanosheets. In the Raman spectra, three intense bands appeared at 660, 815, and 987 cm−1, corresponding to MoO3-II, and a band originating from the Si substrate was observed at 512 cm−1.20 Interestingly, as the thickness of the nanosheets decreased to ∼11.2 nm, a few new bands appeared
3b. During the dehydration and deammonization of h-MoO3, the breaking of low-energy Mo−OH2 bonds as well as the rearrangement of the octahedra ocuur (due to the breakage of Mo−O3−Mo and Mo−O1−Mo bonds). In addition to the rearrangement, a reorientation of the MoO6 octahedra occurs such that that the octahedra of the new phase share corners via Mo−O2−Mo (with the octahedra of the same chain) bonds and edges via Mo−O1−Mo (with the octahedra of other chains) bonds, and form a layered framework of the MoO6 octahedra growing along the c axis, as shown in Figure 3b. We believe that the transformation from the h-MoO3 phase to the MoO3-II phase is energetically favorable because of the common growth axis of the two polymorphs, i.e., the c axis. MoO3-II can be considered an intermediary phase of MoO3 in the evolution of thermodynamically stable α-MoO3, as further increment in the temperature or energy leads to the formation of α-MoO3. Additional energy is required to realize the formation of α-MoO3 because one octahedra layer in MoO3-II must be rotated 180° about the stacking axis with respect to the other octahedra layer and grow along the b axis with the distorted octahedra (the arrangement of the octahedra in the αMoO3 and MoO3-II phases is shown in the Supporting Information, Figure S3). Subsequently, the as-prepared MoO3-II microsheets were exfoliated to create MoO3-II nanosheets. Figure 4a presents the optical micrographs of the MoO3-II nanosheets on a SiO2/Si substrate, varying in thinckness from few double-layers (DL) to multilayers (ML). In the optical micrograph, the nanosheets appearing faintly black correspond to thin double-layers (∼2DL), whereas those that are blue and green are slightly thicker (>5DL to ML). To quantify the exact thickness of the nanosheets, we conducted AFM measurements. AFM topography and height measurements were conducted on MoO3-II/SiO2/Si. AFM was able to resolve thicknesses as 5537
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sheets produced using this synthetic approach exhibit high crystalline quality. The lowest feature size of the nanosheets that could be detected by AFM was 1.4 nm, which is equivalent to two double-layers (2 DL) of MoO6 octahedra in MoO3-II. Raman analysis reveals that as the thickness of the nanosheets decreased, the Mo−O2−Mo stretching along the a axis was suppressed significantly. As a result of their true 2D character (atomic thickness and relatively long in length) and excellent crystalline quality, this class of nanomaterials offers a vast array of possibilities for the further development of next-generation lightweight and transparent optical or electrical devices with robust chemical and environmental stability. The strategy for the MoO3-II nanosheets synthesis presented in this work paves a path to produce layered materials using the hydrothermal product as the starting material.
Figure 6. (a) Raman spectra of the as-exfoliated nanosheets, showing that the intensity of the Raman bands decreases and a few bands disappear completely as the nanosheet thickness decreases. (b) Schematic representation of the MoO6 octahedron in MoO3-II.
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ASSOCIATED CONTENT
S Supporting Information *
at 430and 614 cm−1, while a shift of 20 cm−1 was observed for the band that originally appeared at 987 cm−1. As the thickness decreased from 11.2 to 2.4 to 1.4 nm, the entire family of the bands disappeared, except for the band positioned at 967 cm−1. The Raman bands observed at 660, 815, and 987 cm−1 in our study are considered to be the characteristic bands of MoO3-II, indicating the different arrangement of the MoO6 octahedra in MoO3-II compared to α-MoO3 (the bands at 665, 819, and 995 cm−1 are the fingerprint of α-MoO3).31 Figure 6b shows the schematics of the octahedron of MoO3/1O2/2O1/3-II, which contains one Mo atom and three nonequivalent O atoms, namely, O3/1, O2/2, and O1/3. The Raman band at 987 cm−1 is assigned to the stretching mode of the terminal bond (Mo−O3) of bulk MoO3-II. A shift of 20 cm−1 is observed in the spectra as the thickness of nanosheets decreased, which is attributable to the random stretching of MoO6 octahedra along the b axis. The Raman band at 817 cm−1 (in the case of bulk and 11.2 nm thick nanosheets) is assigned to Mo−O2Mo strentching along the a axis, which clearly reveals the presence of the MoO3-II phase.32 However, this band was suppressed significantly, with decreasing nanosheet thickness, revealing the low-dimensional characteristics of the nanosheets. A band at 664 cm−1 reflects the Mo−O1 stretching mode in the ab plane. The negligible intensity of Mo−O1 could be found in the spectra of the 1.4 nm sample, which suggests restriction of the vibration perpendicular to the c axis (growth axis of the MoO3II framework). The band at 279 cm−1 can be assigned to the wagging mode of O1MoO1 bonds. The transformation from the h-MoO3 phase to the MoO3-II phase is a topotactic reaction, in which the arrangement of the octahedra plays a vital role in constructing a layered framework. Attributed to the layered structure of MoO3-II, facile production of nanosheets can be realized using mechanical exfoliation. The as-fabricated MoO3-II nanosheets are promising for use in various applications, such as field effect transistors, photodetectors and energy storage devices. Due to the high packing efficiency in MoO3-II, these nanosheets strongly likely to be chemically and mechanically robust compared to other polymorphs of MoO3, such as α-MoO3.
XRD patterns of the samples annealed at 450, 530, and 650 °C; FESEM micrographs of the samples annealed at at 450 and 650 °C; depictions of the arrangement of the MoO6 octahedra in the MoO3-II and α-MoO3 phases of MoO3; HRTEM evaluation of the as-obtained nanosheets. This material is available free of charge via the Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
All authors participated in the writing of the manuscript and have approved the final version. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Vipin Kumar acknowledges the research scholarship awarded by Nanyang Technological University and Temasek Laboratories @ NTU, Singapore.
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REFERENCES
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CONCLUSIONS In conclusion, we have demonstrated the topotactic phase transformation of h-MoO3 nanorods into MoO3-II microsheets, which involves the rearrangement of MoO6 octahedra and a strategy to create MoO3-II nanosheets. The MoO3-II nano5538
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Chemistry of Materials
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dx.doi.org/10.1021/cm502558t | Chem. Mater. 2014, 26, 5533−5539
