Article pubs.acs.org/JPCC
Vacuum Topotactic Conversion Route to Mesoporous Orthorhombic MoO3 Nanowire Bundles with Enhanced Electrochemical Performance Zhengqiu Yuan,† Lulu Si,† Denghu Wei,† Lei Hu,† Yongchun Zhu,*,‡ Xiaona Li,† and Yitai Qian*,†,‡,§ †
Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China Hefei National Laboratory for Physical Science at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China § School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, P.R. China ‡
S Supporting Information *
ABSTRACT: The growth of mesoporous bundles composed of orthorhombic MoO3 nanowires with diameters ranging from 10 to 30 nm and lengths of up to 2 μm by topotactic chemical transformation from triclinic α-MoO3·H2O nanorods under vacuum condition at 260 °C is achieved. During the process of vacuum topotactic transformation, the nanorod frameworks of the precursor α-MoO3·H2O can be preserved. The crystal structures, molecular structures, morphologies, and growth behavior of the precursory, intermediate and final products are characterized using powder X-ray diffraction (PXRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and selected-area electron diffraction (SAED). Detailed studies of the mechanism of the mesoporous MoO3 nanowire bundles formation indicate topotactic nucleation and oriented growth of the well-organized orthorhombic MoO3 nanowires inside the nanorod frameworks. MoO3 nanocrystals prefer [001] epitaxial growth direction of triclinic α-MoO3·H2O nanorods due to the structural matching of [001] α-MoO3·H2O//[100] MoO3. The electrochemical measurement of the mesoporous MoO3 nanowire bundles indicates that their galvanostatic Li storage performance can be significantly improved. The high reversible capacities of 954.8 mA h g−1 can be retained over 150 cycles. The topotactic growth under vacuum based on the crystal structural relationship of hydrated metal oxide and related metal oxide will provide an effective and all-purpose route to controlled preparation of novel micro/ nanostructured oxides (such as V2O5 and WO3 nanowires, etc.) with enhanced properties (energy storage/conversion, organic electronics, catalysis, gas-sensor, and so on).
1. INTRODUCTION Transition metal oxides (TMOs) such as molybdenum trioxide (MoO3), tungsten trioxide (WO3), and vanadium pentoxide (V2O5) have aroused much extensive interest due to their variety of crystalline phases and fascinating properties during the past few years.1,2 For instance, the ready reversible incorporation of alkali ions or protons into their lattices can form electronically conducting bronzes.3 These insertion reactions take place with the formation of a large free energy, which has led to their use as the cathodes of lithium batteries. Concomitant with the ion insertion, electrons are introduced into the conduction bands giving rise to their dramatic colors, which can be related to the concentration of conduction electrons.4 This ready coloration under ambient conditions led to their proposed and actual use.5,6 With the development of nanotechnology, nanostuctrues have received great attention. Novel physical phenomena and improved properties appear as the scale of the building blocks © 2014 American Chemical Society
approaches the nanoscale. h-MoO3 nanobelt synthesized by simple hydrothermal route exhibited a stable electrochromic performance responding to electrical impulse in a very short time.6 Few-layer V2O5 nanosheets with high-rate transportation of lithium ions and electrons have been synthesized via a simple and scalable liquid exfoliation technique.7 MoO3 nanoparticles with diameters of 5−20 nm showed a durable Li storage capacity of 630 mA h g−1 for 150 cycles at high rate.8 In particular, nanowires are one of these building blocks that possess several practical, distinct properties, such as electronic radial transport, short ions transport path, crystallinity, and well-controlled dimensional composition; this is favorable for organizing the nanoscale building blocks into assemblies and, ultimately, useful systems. However, the crystallography Received: October 25, 2013 Revised: February 7, 2014 Published: February 7, 2014 5091
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procedure, 0.48 g (5 mmol) of pure molybdenum power was slowly added into a 50-mL beaker containing 10 mL of H2O2 aqueous solution (30% w/w) with strong stirring. The feeding rate of Mo power should be much slower because the reaction of Mo and H2O2 is a strong exothermic reaction with the product of MoO2(OH)(OOH). Then the yellowish white precipitate was obtained with the above transparent and yellow solution under the influence of ultrasonic for about 1 day. The chemical reaction can be formulated as shown in the equation MoO2(OH)(OOH) → α-MoO3·H2O + 1/2O2.The product was collected, washed with deionized water and pure alcohol, respectively, for several times, and finally dried in a vacuum at 60 °C overnight. In addition, in our experiments, the 0.48 g is an optimal value. When the dosage of molybdenum power is greater than 0.48 g, another phase which will be reported in our next work coexists in the final product; otherwise, the reaction time for the formation of the α-MoO3·H2O product increase significantly (several days, even one week) and the reaction efficiency become very low. 2.2. Synthesis of α-MoO3 Nanowire Bundles. The onedimensional α-MoO3 nanowire bundles were synthesized by a vacuum technique from the precursor. The as-obtained αMoO3·H2O nanorods were heat treated at 260 °C in vacuum for 10 h. After cooling to room temperature naturally in vacuum, the final products were obtained for subsequent characterization and testing. 2.3. Sample Characterization. Powder X-ray diffraction (PXRD) measurements were determined by a Bruker D8 advanced X-ray diffractmeter. The scanning electron microscopy (SEM) images were taken by using a field-emitting scanning electron microscope (FESEM, JEOL-JSM-6700F). The transmission electron microscopy (TEM) images, highresolution transmission electron microscopy (HRTEM) images, and the selected-area electron diffraction (SAED) patterns were taken on a JEOL-2010 transmission electron microscope with an accelerating voltage of 200 kV. Raman spectrum was carried out on a JY LABRAM-HR confocal laser micro-Raman spectrometer using Ar+ laser excitation with a wavelength of 514.5 nm. 2.4. Electrochemical Test. The active materials (α-MoO3 nanowire bundles), acetylene black and poly(vinylidene fluoride) with a ratio of 70:15:15 (weight ratio) were mixed homogeneously with N-methyl-pyrrolidone as a solvent, the obtained slurry was pasted on Cu foil and dried at 110 °C for 10 h in vacuum. The coin cells (size: 2016) were then assembled in an argon-filled glovebox. The cells were composed of lithium foil (anode), Celgard 2400 (separator), and 1 M LiPF6 in a mixed solvent of ethylene carbonate and dimethyl carbonate (1:1 volume ratio) (electrolyte), α-MoO3 nanowire bundles (cathode)∥1 M LiPF6 in EC: DMC (1:1 volume ratio) (electrolyte)∥lithium foil (anode). The galvanostatic charge and discharge were controlled between 0.001 and 3.0 V on a LAND-CT2001A instrument at room temperature. The loading mass of active material is about 2 mg per foil. To confirm the stable and reliable electrochemical performances, at least 30 different LIBs were fabricated from α-MoO3 nanowire bundles and α-MoO3 nanobelts, respectively, at the same test condition.
properties of the layered structures of vanadium, molybdenum, and tungsten oxides result in the difficulty of preparing MoO3, V2O5, and WO3 one-dimensional (1-D) nanowires. Their layered structures contain predominantly MO6 octahedra, with tetrahedra being less common. These octahedra normally share corners and/or edges to form two-dimensional sheet structures, which are very favorable for the formation of two-dimensional nanostructures, such as nanobelt, nanosheet, etc.7−15 In 2000, the electrode position method was used by Zach et al. to prepare MoOx nanowires with the diameters ranging from 20 nm to 1.3 μm for the first time.16 Then, MoO3, V2O5, and WO3 nanowires with diameters of 10−200 nm were synthesized using carbon nanotubes as templates.17 Up to the present, limited success in synthesizing MoO 3, V2 O5, and WO3 superfine nanowires have been obtained. So searching for a simple and template-free method for preparing MoO3, V2O5, and WO3 superfine nanowires in a large scale is still a great challenge. It is worth mentioning that the isolated MO6 octahedra chains sharing corners and double chains of strongly distorted octahedra sharing two common edges with each other commonly exist in their hydration states. For example, triclinic monohydrate MoO3·H2O is made from isolated double chains of strongly distorted [MoO5 (H2O)] octahedra sharing two common edges with each other. The [MoO5 (H2O)] octahedra are much distorted in such a way that the local structure can be approximated by chains of corner-linked MoO4 tetrahedra and isolated water molecules.18 In the process of hydrated metal oxides transforming into thermodynamic stability anhydrous phases, the isolated 1-D MO6 octahedra chains are connected with each other via chemical bonds or van der Waals interaction. Can we prepare the MoO3, V2O5, and WO3 nanowires by inhibiting and/or destroying the connecting of 1-D MO6 octahedra chains in the radial direction based on the dehydration of their hydrated metal oxides? We choose MoO3 to demonstrate the concept in consideration of the limited success in preparing MoO3 nanowires and their expanding technological applications in various fields. Herein, we report the fabrication of unique mesoporous orthorhombic MoO3 nanowire bundles by a vacuum thermal decomposition from precursory α-MoO3·H2O nanorods based on the 1-D double chains of MO6 octahedra sharing two common edges for two and the topotactic relationship between the (001) α-MoO3·H2O and (100) MoO3 crystal planes. During the mesoporous MoO3 nanowire bundles formation, the nanorod frameworks could be well preserved, as was expected based on the reported work.19 As an example, we show that the as-prepared mesoporous orthorhombic MoO3 nanowire bundles exhibit enhanced electrochemical performances when evaluated as anode materials for lithium-ion batteries (LIBs). The method of preparing nanowires could be extended to other two TMOs, V2O5, and WO3. This topotactic growth under vacuum based on the crystal structural relationship of hydrated metal oxide and related metal oxide will provide an effective and all-purpose route to controlled preparation of novel micro/nanostructured oxides with enhanced properties (energy storage/conversion, organic electronics, catalysis, gas-sensor, and so on).
2. EXPERIMENTAL SECTION 2.1. Synthesis of Triclinic α-MoO3·H2O Nanorods. The precursor triclinic molybdenum oxide monohydrate α-MoO3· H2O nanorods were synthesized from peroxomolybdate precursor solutions via a sonochemical process. In a simple
3. RESULTS AND DISCUSSION 3.1. Triclinic Monohydrate Precursor α-MoO3·H2O Nanorods. The reaction of molybdenum powder and hydrogen peroxide aqueous solution can yield a traditional 5092
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= 91.7500°, β = 104.3600°, γ = 65.6600°, space group P-1, Z = 2, and JCPDS no. 26-1449). The result of the thermogravimetric (TG) analysis (Figure 1b) of the precursor reveals that the mass decrease of the yellowish white product started at about 82.58 °C and was finished at around 254.02 °C. The total mass loss is about 11.07%, very corresponding to the mass of a crystal water molecule from triclinic molybdenum trioxide monohydrate (theoretical value, 11.12%). The as-obtained precursor is further confirmed by Raman spectroscopy. The Raman spectrum of the as-obtained sample is shown in Figure 1c, which shows the typical Raman scattering bands of αMoO3·H2O.22 The stretching frequencies at 968.9 and 925.3 cm−1 are well attributed to the symmetric stretching vibrations of OMoO unit, whereas the next two narrow Raman lines at 694.5 and 401.3 cm−1 are ascribed to the intermediate bridging O−Mo−O bond of α-MoO3·H2O. Many Raman bands appear below 250 cm−1, such as 247.4, 217.1, 201.9, 171.5, 137.7, and 124.5 cm−1, correspond certainly to deformation and lattice modes. Thus, the yellowish white precursor can be primarily identified as triclinic monohydrate α-MoO3·H2O. The morphology of the as-synthesized α-MoO3·H2O nanorods is examined by SEM. Figure 2a,b show low-magnification and high-magnification SEM images of as-synthesized α-MoO3· H2O. The as-obtained products are composed of a great quantity of nanorods and a small amount of nanoparticles. The diameters of the nanorods range from 200 to 500 nm, and their lengths vary between 3 and 5 μm (Figure 2a). The elongated nanoparticles with the size of hundreds of nanometers are observed, as shown in Figure 2b. It is clear that the α-MoO3· H2O nanorods may have improved physicochemical properties due to the highly oriented growth behavior, thus enhancing their practical applications. The morphology and structure of the α-MoO3·H2O nanorods were investigated in detail by transmission electron microscopy (TEM), selected area electron diffraction (SAED), and high-resolution TEM (HRTEM). Figure 2c shows the TEM image of the α-MoO3·H2O nanorod, which is a solid and smooth nanorod with a diameter of ∼200 nm. The SAED pattern (inset of Figure 2c) obviously supports its single crystal nature and confirms the nanorod is grown along the [140] direction in length. The parallelogram SAED pattern (inset of Figure 2c) with the [001] zone axis demonstrates that the larger surface and side surface of the nanorod are enclosed by (001) and (100) lattice planes, respectively. The confirmed growth direction of each surface of the nanorod is schematically illustrated on the nanorod of Figure 2c. We try our best to obtain the corresponding HRTEM image of the α-MoO3·H2O nanorod, but it is difficult due to the awful instability under the stronger electron irradiation. Based on the above analysis, Figure 2d shows the crystal structure of the (001) lattice plane of the precursor α-MoO3·H2O nanorod, in which the isolated double chains of strongly distorted [MoO5(H2O)]-octahedra sharing two common edges with each other are approximatively aligned parallel to the smooth outer side edge of the nanorod. 3.2. Vacuum Thermal Decomposition of α-MoO3·H2O. The crystal structural transformation from triclinic molybdenum trioxide monohydrate to orthorhombic molybdenum trioxide has been studied by ex situ PXRD and Raman spectroscopy in details in vacuum at various temperatures from 110 to 220 °C. Figure 3 shows the PXRD patterns of vacuum thermal decomposition dehydration products from the precursor at different temperatures. The main features of the
peroxomolybdate MoO2(OH)(OOH)20,21 aqueous solution. Under the influence of ultrasonic, the yellowish white product was precipitated from the peroxomolybdate aqueous solution. The powder X-ray diffraction (PXRD) pattern of the asobtained precursor (Figure 1a) is well consistent with the reported PXRD pattern of the triclinic monohydrate α-MoO3· H2O (anorthic cell, a = 6.5530 Å, b = 7.3720 Å, c = 3.7070 Å, α
Figure 1. (a) Powder X-ray diffraction (PXRD) pattern, (b) thermogravimetric (TG) analysis, and (c) Raman spectrum of the precursor triclinic monohydrate α-MoO3·H2O nanorods. 5093
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Figure 2. (a) Typical low-magnification SEM image of the initial α-MoO3·H2O product, (b) an enlarged SEM image corresponding to the area enclosed by the white square in panel a. (c) Low-magnification TEM image of α-MoO3·H2O nanorod viewed from the broad plane. (d) Schematic illustration showing the crystal structure of the area enclosed by the black square in panel c.
does not change the principal features of the MoO6 octahedra with the removal of interlamellar water. Meanwhile, up to 150 °C, with the removal of interlamellar water, the molecular structures of the dehydration derivatives α-MoO3•xH2O bear a close similarity to that of the triclinic molybdenum oxide monohydrate α-MoO3·H2O as a result of much the same Raman spectrum, as shown in Figure 4a. At 165 °C, the subtle changes of the crystal structure and molecular structure of the dehydration product have been observed from the XRD pattern and Raman spectra, as shown in Figures 3d and 4b, respectively. The characteristic diffraction peaks of (021) and (002) for the orthorhombic MoO3 are clearly observed in the PXRD pattern of the 165 °C sample (Figure 3d). In addition, the well-defined line at 820.2 cm−1 in the Raman spectra deriving from the stretching vibrations of units Mo−O−Mo of orthorhombic MoO3 appears for 165 °C sample (Figure 4b).23 The removal of overmuch coordinated water results in the damage of the long-range order of the α-MoO3·H2O crystal structure and the change of the molecular structure. When annealed at 180 °C in vacuum, the obtained Raman spectra (Figure 4c) shows the typical Raman scattering bands of orthorhombic MoO3 at 994.9, 820.2, 666.3, 471.9, 374.7, 338.3, 286.4, 244.1, 214.9, 197.2, 152.3, and 124.9 cm−1, indicating that the molecular structure has changed to be the orthorhombic MoO3 molecular structure.23 While, there are still some other peaks, such as two broad peaks (14.9°and 40.6°) in Figure 3e, which may be ascribed to an unknown dehydrated intermediate phase. Above 180 °C, marked structural changes in the end of the dehydration process are observed. The PXRD patterns (Figure
Figure 3. PXRD patterns of (a) precursor α-MoO3·H2O nanorods and (b−g) intermediate products by annealing the precursor in vacuum at temperatures of 110 (b), 150 (c), 165 (d), 180 (e), 220 (f), and 260 °C (g) for 10 h. The indexed diffraction peaks are designated for orthorhombic MoO3 (JCPDS card No. 05−0508). Two circled peaks in PXRD pattern e may be ascribed to an unknown intermediate phase.
PXRD patterns of the as-obtained sample remain unchanged up to 150 °C (Figure 3a−c), indicating that the previous dehydration is a topotactic process without any significant structural change. Therefore, the previous dehydration process 5094
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Figure 4. Raman spectra of the products by calcining the precursor αMoO3·H2O nanorods at temperatures of 150 (a), 165 (b), 180 (c), and 260 °C (d), respectively, for 10 h. The indexed peaks are designated for orthorhombic MoO3.
3e) are consistent with the molybdenum oxides having infinite sheets of corner-shared MoO6 octahedra.24 The orthorhombic structure was formed (Figure 3f) above 220 °C with crystal lattice parameters a = 3.962 Å, b = 13.85 Å, and c = 3.697 Å (JCPDS no. 05-0508, SG Pbnm). The morphologies of the samples calcined at different temperature are shown in Figure 5. After 110 °C annealed in vacuum for 10 h, the size of the nanorods (Figure 5a) become bigger and the nanorods display greatly uniform distribution of widths (300−600 nm) and lengths (4−7 μm). Figure 5b shows the closer examination of the material, revealing that the shape of the nanorods is a rectangular pole. The surfaces of the nanorods turn smoother than those of the precursor and the elongated nanoparticles disappear. The Ostwald ripening mechanism,25 a main path of crystal growth, can really help us to understand the reason for the nanoparticles disappearing. In the 110 °C annealing, within an ensemble of as-obtained products with nanorods and nanoparticles the large nanorods will grow at the cost of the small nanoparticles. For 150 °C sample, the lengths of the nanorods range from 8 to over 12 μm (Figure 5c,d). At the same time, the nanorods slightly bend. Up to 150 °C, the change of the morphology is almost attributed to the Ostwald ripening. Actually, the dehydration process has been happening naturally in the internal structure of the α-MoO3·H2O when annealed in vacuum, although the morphology of the products has not been changed because of the dehydration process ranging from room temperature to 150 °C. After annealed at 165 °C in vacuum for 10 h, the nanorods of the annealed sample bend severely (Figure 5e). At the same time, the obvious filamentous stripes are observed on the surfaces of the nanorods (Figure 5f). With the annealing temperature rising to 180 °C, an interesting morphology is obtained. Figure 5g shows the low-magnification SEM image of the product obtained via vacuum thermal decomposition at 180 °C. The very puffed and porous nanorods, with the lengths of most of the crystals being about 2 μm and the average diameter reaching 300 nm, are observed in Figure 5g. Compared to the as-obtained precursor, the lengths of the puffed nanorods are shorter due to the fracture of the as-obtained nanorods in the dehydration process. Figure 5h clearly shows the fracture surface of the nanorod. The microstructure of the puffed
Figure 5. SEM images of the annealed products in vacuum at various temperatures of 110 °C (a and b), 150 °C (c and d), 165 °C (e and f), 180 °C (g and h), and 220 °C (i and j), respectively, for 10 h.
nanorods looks like a weathered tree trunk without water (Figure 5h). After 220 °C vacuum heat-treated, the puffed nanorods continue to grow in filamentation (Figure 5i,j). The above changes of the morphologies, such as fracture and filamentation, are mainly ascribed to the removal of the overmuch crystal water in α-MoO3·H2O and the crystal structural relationship between the triclinic hydrated and orthorhombic dehydrated phases. The details are discussed in the mechanism part below. 3.3. Mesoporous Orthorhombic MoO3 Nanowire Bundles. Succedent thermal treatment in vacuum was realized at 260 °C. Powder XRD pattern (Figure 3g) of the final product has identical peaks, which can be perfectly indexed to orthorhombic MoO3 (JCPDS card no. 05-0508, see Figure S4 5095
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the the Supporting Information). With increasing temperature, a higher crystalline product is obtained. At the same time, no diffraction peaks of other phases were detected, indicating that the final product is of high purity. It is clear that the α-MoO3· H2O nanorods are converted into orthorhombic MoO3 above 260 °C, which is in accordance with the TG curve (Figure 1b) and the typical orthorhombic MoO3 Raman spectrum as shown in Figure 4d. SEM and TEM images (Figure 6) show the maintained nanorod morphology and porous nanowire bundles characteristics of the final product obtained at 260 °C in vacuum. From the SEM images (Figure 6a,b), we can see that a small amount of nanowires with uniform dispersion are peeled from the surfaces of the puffed nanorods. Figure 6c shows the tip of the orthorhombic MoO3 nanowire bundles, indicating that the MoO3 bundles are composed of vast numbers of nanowires ranging from 10 to 30 nm in diameter from the inside out and porous. The MoO3 with a nanowire bundle shape has a great part of the atoms exposed at surfaces. Thus, they are good candidates for applications to catalysis because the exposed atoms are accessible to reactant molecules.26 Therefore, the 260 °C sample with small scale dimension are expected to be promising with regard to the development of new catalytic materials. Detailed structural characterization was carried out for the 260 °C sample by TEM and HRTEM. Figure 6d shows the typical TEM image of the main part of final MoO3 nanorod, further illustrating its porous nanowire bundle feature. The related HRTEM images as shown in Figure 6e,f indicate a higher crystalline character. Figure 6e shows the HRTEM of surface edges of a nanobundle with a considerably distinct lattice spacing of 0.38 nm that corresponds to the value of the (110) planes of orthorhombic MoO3. The HRTEM of the mainbody of nanowires as shown in Figure 6f shows a considerably distinct lattice spacing of 0.72 nm, corresponding to that of the (020) planes of orthorhombic MoO3. The observed (110) and (020) lattice planes are approximatively aligned parallel to the outer side edge of the nanobundle, demonstrating a clear growth behavior of the nanowire: (1) the nanowire is grown along the [001] direction in length; (2) the larger- and side-surface are enclosed by (100) and (110) crystal lattice planes, respectively. Remarkably, the growth behavior of this MoO3 nanowire is different from the previous published works.7−15,27,28 The typical orthorhombic MoO3 nanobelt is grown along the [001] direction and enclosed by (010) lattice plane on large surface (Figure S1, Supporting Information).10,28 Compared to the typical nanobelt, more entrances (open channels, [100] direction) of the interlayer between the [MoO6] octahedron bilayers are distributed on the large surface of the MoO3 nanowires, which is beneficial for guest (e.g., Li+) intercalation in kinetics. Because the Li+ ions can rapidly move into the bulk of orthorhombic MoO3 crystal in the [100] direction, the mesoporous orthorhombic MoO3 nanowire bundles with highly crystalline are expected to be used as a high-rate cathode material in LIBs and related work will be published elsewhere. Figure 6g shows the crystal structure of the (100) lattice plane of the orthorhombic MoO3, enclosed by a white square in Figure 6f, in which unique layers and zigzag chains are approximatively aligned parallel to the outer side edge of the nanobundle. 3.4. Topotactic Formation Mechanism of Nanowire Bundles. Based on the above observations and analysis, it is suggested that the formation of MoO3 nanowire bundles with porous structures is ascribed to an in situ α-MoO3·H2O-to-
Figure 6. (a) Typical low-magnification and (b) high-magnification SEM images of orthorhombic MoO3 nanowire bundles. (c) The enlarged SEM image of the tip of a MoO3 nanowire bundle. (d) A typical TEM image of the MoO3 nanowire bundle. (e and f) HRTEM images of the side edge and a part of the broad surface, respectively, of the MoO3 nanowire bundle. (g) Schematic illustration showing the crystal structure of the area enclosed by the white square in panel f.
MoO3 topotactic conversion with the crystal relationship of [001] α-MoO3·H2O//[100] MoO3 (Figures 2d, 7b, 6g, and 5096
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Figure 7. (a) Schematic illustration of the process of porous orthorhombic MoO3 nanowire bundles formation. The successfully oriented growth of orthorhombic MoO3 nanowires from a triclinic α-MoO3·H2O nanorod is ascribed to the close 3D structural match between them, as indicated by their crystal structures. Comparative structural diagrams depicting the water removal mechanism from triclinic α-MoO3·H2O to form orthorhombic MoO3: (b and c) initial triclinic α-MoO3·H2O, (d) dehydrated MoO3 and double-chains of octahedra to form new chemical bonds, and (e and f) the final orthorhombic MoO3 crystal structure.
Figure 8. N2-adsorption (black line)−desorption (read line) isotherm and pore size distribution curves (inset) of the MoO3 nanowire bundles.
7f). The single-crystal triclinic α-MoO3·H2O nanorods are employed as an in situ solid-state template, precursor, and support during the process of the topotactic transition. The processes of topotactic crystallization and details of mesoporous MoO3 nanowire bundles from α-MoO3·H2O nanorods can be schematically illustrated in Figure 7. The mesoporous MoO3 nanowire bundles formation can be expressed as a threestep process: (1) orthorhombic MoO3 nucleates in situ epitaxially on α-MoO3·H2O nanorods with a preference for [001] α-MoO3·H2O//[100] MoO3 (Figure 3d); (2) the annealing leads to the growth of orthorhombic MoO3 along this specific direction topotactically with a 3D crystal structural
relationship, as illustrated in Figure 7a; (3) the thermal decomposition of the precursor causes weight loss and volume shrinking, and consequently, porous texture and nanowires are yielded. An important feature of these nanowire bundles is that the direction of nanowires is along the length of the nanorod. From the structural information of the α-MoO3·H2O and MoO3, the double chains of [MoO6]-octahedra sharing two common edges with each other in them are all aligned parallel to the length of the nanorod, as illustrated in Figures 2d and 6g. At the same time, the structure of [MoO6]-octahedra double chains are preserved in the process of vacuum thermal decomposition, which is confirmed by the obvious lattice 5097
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The Journal of Physical Chemistry C
Article
fringes aligned parallel to the length of the nanorod of 180 °C sample (Figure S2). The interaction force of the double chains in the [100] direction of α-MoO3·H2O and the [020] direction of MoO3 (the two direction are all perpendicular to the length of the nanorod) are hydrogen-bond and van der Waals interactions, respectively. During the water loss and the volume shrinking process, only the hydrogen-bond and van der Waals interaction forces can be easily disrupted, resulting in the filamentation along the length of the nanorod. A small amount of nanowires peeled from the surfaces of nanowire bundles as shown in Figure 6b,d further confirm the above filamentation mechanism. This topotactic transformation and growth based on the relationship of the crystal structures presents a viable route to special nanostructures and mesoporous materials. In addition, the vacuum condition is very important for the formation of the MoO3 nanowire bundles, which may effectively inhibit and/or destroy the connecting of 1-D MO6 octahedra chains in the radial direction in the dehydration process. To reveal the role of the vacuum condition in preparing MoO3 nanowire bundles, a control experiment is carried out by treating the precursor α-MoO3·H2O nanorods at 260 °C in air condition. The solid orthorhombic MoO3 nanorods without any traces of the filamentation are obtained after 260 °C heat-treated in air, as shown in the Supporting Information (Figure S3). From structural comparison of both hydrated triclinic αMoO3·H2O (Figure 7b,c) and dehydrated orthorhombic MoO3 (Figure 7e,f) phases in detail, a dehydration reaction mechanism could be proposed. The departure of the crystal water from the hydrated phase triclinic α-MoO3·H2O induces a collapse of its structure (Figure 7d). Then, new bonds are forming between oxygen atoms at the edges of octahedron and the closest transition metal cation (from the following octahedra chain) to compensate the vacancies (white spheres in Figure 7d) created by departure of the crystal water (see Figure 7d,e). Finally, the orthorhombic MoO3 is obtained (Figure 7f). 3.5. Porous Property and Electrochemical Performance. The Brunauer−Emmett−Teller (BET) method was used to study the surface-area data of the final product MoO3 nanowire bundles from the topotactic transformation of precursor solid α-MoO3·H2O nanorods in vacuum. The N2adsorption desorption isotherms of the nanowire bundles sample can be categorized as type IV with hysteresis loops (Figure 8). The BET surface-area data is calculated to be about 23.80 m2 g−1. The pore size distribution curve (inset in Figure 8) suggests most pores are around 13 nm for the MoO3 nanowire bundles, which is in accord with the observation from the SEM and TEM images (Figure 6). The electrochemical properties of porous MoO3 nanowire bundles and the orthorhombic MoO3 nanobelts were evaluated under galvanostatic testing conditions at a current of 200 mA g−1 in the voltage window ranging from 1 mV to 3.0 V (vs Li+/ Li) at room temperature (25 °C). The right vertical dot line in Figure 9a indicates the theoretical limit for MoO3 electrode that undergoes a conversion reaction to completely produce metal Mo and Li2O. For the MoO3 nanowire bundles, the first lithiation capacity (1988 mA h g−1) greatly exceed the theoretical capacity (1117 mA h g−1). However, the subsequent charge (first charge) capacity is 1078 mA h g−1, with very low Coulombic efficiency 54.2%. The large loss of the capacity in the first cycle is mainly ascribed to the formation of Li2O and the irreversible Li ions intercalating into the crystal lattice, as
Figure 9. Front two cycles voltage profiles of (a) the MoO3 nanowire bundles and (b) the MoO3 nanobelts at the current density of 200 mA g−1 in the voltage window of 0.001−3.0 V (vs Li+/Li) at room temperature (#, discharge potential). Vertical lines indicate the expected capacities for the intercalation reaction and a full reduction of MoO3, respectively [(1) Addition (>1.5 V), MoO3 + xLi+ + xe− → LixMoO3 (x = ∼ 1.4); (2) conversion (1.5 V), MoO3 + xLi+ + xe− → LixMoO3 (x = ∼1.4); (2) conversion (
