J. Phys. Chem. C 2007, 111, 2401-2408
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Green Synthesis and Characterization of Anisotropic Uniform Single-Crystal r-MoO3 Nanostructures Liang Fang,†,‡ Yuying Shu,† Aiqin Wang,† and Tao Zhang*,† State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China, and Graduate School of the Chinese Academy of Sciences, Beijing 100049, China ReceiVed: September 6, 2006; In Final Form: December 4, 2006
Anisotropic uniform single-crystal nanostructures of R-MoO3 have been synthesized successfully via a novel green and facile approach, i.e., decomposition and condensation of peroxomolybdic acid under hydrothermal conditions. The structure and morphology of the products were characterized by means of X-ray diffraction, transmission electron microscopy, selected area electron diffraction, high-resolution transmission electron microscopy, scanning electron microscopy, thermogravimetric/differential thermal analysis, temperature programmed decomposition-mass spectrometry, and Fourier transform infrared spectroscopy. It has been found that the formation of R-MoO3 proceeds at hydrothermal temperatures higher than 83.5 °C and that of MoO2.67(O2)0.33‚0.75H2O is at 81.5 °C with the 0.9 mol/L molybdenum solution. The as-synthesized uniform nanostructures grow preferentially along [001], and the dimensions are 200-330 nm in width, 60-90 nm in thickness, and up to 10 µm in length during time spans from 20 to 45 h at 170 °C. The structure and morphology of R-MoO3 show a weak dependence on the molybdenum concentrations of 0.2-0.9 mol/L, while the growth in the b-axis direction can be enhanced distinctly and specifically by the addition of nitric acid to the initial peroxomolybdic acid solution. The critical point in temperature (81.5-83.5 °C) to form hydrate and oxide is discussed, and one possible mechanism is proposed.
1. Introduction In recent years, the development of anisotropic nanostructured materials has attracted more and more attention in materials research, because nanoscale materials have inherent physicochemical properties which differ remarkably from those of their respective bulk phases, and their anisotropic morphologies facilitate functionalization, alignment, and self-assembly processes of materials with regard to potential applications in the fabrication of functional nanodevices.1,2 Besides carbon nanotubes, so far, anisotropic nanostructures (including nanotubes, nanowires, and nanorods) of elements, oxides, nitrides, and metal chalcogenides have been extensively synthesized and characterized.3-5 Among the nanomaterials researched, molybdenum trioxide has attracted considerable interest, and its distinctive properties enable it to function widely as an active component in supported catalysts,6,7 chemical sensors,8 and cathodes of rechargeable ion batteries.9-11 Lately, R-molybdenum trioxide (R-MoO3) with nanorod morphology has been utilized as positive templates for metal oxide deposition12-15 and that with nanobelt morphology as a field emitter.16 Substantial advancements have been achieved with respect to the preparation of molybdenum oxides, especially R-MoO3, with anisotropic morphologies. One-dimensional nanoscale arrays, composites, tubes, and rods have been made available by physical methods17-21 and the host/hard template method.22-26 By use of the soft chemical method, especially the hydrothermal/ solvothermal synthesis, molybdenum trioxide presents many fascinating morphologies. In the presence of different structure* To whom correspondence should be addressed. E-mail: taozhang@ dicp.ac.cn. Tel: +86-411-84379015. Fax: +86-411-84691570. † Dalian Institute of Chemical Physics, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.
directing organic templates or inorganic salts, nanoscopic fibers,27 hierarchical structures,28 multilamellar mesostructures,29 and morphology-controllable nanostructures30 have been formed. Without templates, on the other hand, nanoribbons/nanorods and nanobelts have been fabricated successfully by different acidification processes under hydrothermal conditions.31,32 An efficient approach to make sub-micrometer fibers has been established from the one-step acidic or neutral solvothermal procedure of MoO3‚2H2O within a wide parameter window.33,34 In the template-free synthesis of anisotropic R-MoO3 nanostructures, some involved the environmentally unfavorable perchloric acid/perchlorate,32 others involved aging of the precursor solution31 or the preparation of a solid precursor,33,34 both of which required a prolonged time of approximately 1 month.31,35,36 It is, therefore, significant and attractive to develop a green and efficient template-free synthesis of anisotropic R-MoO3 nanostructures. Oxo peroxo complexes of molybdenum(VI), which are environmentally satisfactory oxidants, have become significant catalysts of oxygen-transfer reactions,37,38 and for the preparation of electrochromic thin films,39 molybdosilicate mesoporous molecular sieves,40-42 hollow molybdenum trioxide nanospheres,43,44 and highly ordered mixed-valent molybdenum oxides.45 In this paper, we present a novel green and efficient hydrothermal synthetic method for the preparation of anisotropic uniform single-crystal R-MoO3 nanostructures via decomposition and condensation of peroxomolybdic acid, one of the oxo peroxo complexes of molybdenum. The effect of synthetic aspects in the hydrothermal process (temperature, time, concentration, and retaining high acidity) on the morphology of nanorods is addressed systematically in this paper.
10.1021/jp065791r CCC: $37.00 © 2007 American Chemical Society Published on Web 01/23/2007
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2. Experimental Section Materials Preparation. Ammonium heptamolybdenum tetrahydrate (AHM; (NH4)6Mo7O24‚4H2O, 99.0%, Tianjin KAIDA Chemical Plant of Tianjin No. 4 Chemical Reagent Factory) and 30% aqueous hydrogen peroxide (analytical grade, Shenyang No. 5 Reagent Factory) were purchased and used as received. The synthesis was based on the preparation of the peroxomolybdic acid solution and subsequent treatment of a hydrothermal reaction. AHM was calcined at 500 °C for 4 h to obtain MoO3 powder (the commercial MoO3 is satisfactory and in this case calcined MoO3 powder was used only for convenience). In a typical procedure, calcined MoO3 powder (1.44 g, 10 mmol) reacted with 30% aqueous H2O2 (11.0 mL) and dissolved completely after stirring for 6 h at 30 °C, forming the transparent orange-green peroxomolybdic acid solution (mother liquor, the molybdenum concentration is 0.9 mol/L). (If the mother liquor was allowed to stand for several days at room temperature without hydrothermal synthesis, a deep yellow sample was formed from the solution.46,47) For each experimental run, the above peroxomolybdic acid solution was diluted with deionized water or 2 mol/L nitric acid solution to a certain molybdenum concentration. The resultant precursor solution was transferred into a Teflon-lined stainless steel autoclave until it was 50-60% filled. The autoclave was then sealed and maintained at 70-220 °C for 1-45 h (In the 70-85 °C range, heating was carried out with the autoclave immersed completely in a thermostated water bath. The temperature was controlled by a Beckman temperature controller and determined by a thermometer with the minimum scale of 0.5 °C. In the 90-220 °C range, heating was carried out in an electric oven). After cooling naturally to room temperature in ambient surroundings, the precipitate/product was collected after centrifuging, thoroughly rinsed with deionized water, and dried at 40 °C. Materials Characterization. Crystallographic information of samples was investigated with X-ray powder diffraction (XRD) using a Rigaku D/max-RB 12 kW X-ray diffractometer operated at 30 kV and 80 mA, with graphite monochromatized Cu KR radiation (λ ) 1.5406 Å). The morphology of crystal samples was examined with transmission electron microscopy (TEM) using a JEOL JEM-2000EX microscope operated at 120 kV, high-resolution transmission electron microscopy (HRTEM) using a FEI Tecnai G2 F30 S-Twin microscope operated at 300 kV, and scanning electron microscopy (SEM) using a JEOL JSM-6360LV operated at 30 kV. For TEM/HRTEM imaging studies, the material suspended in ethanol was deposited on a polymer foil supported on a copper grid. For SEM examination, the sample was sputter-coated with gold. Thermogravimetric and differential thermal analysis (TG/DTA) was performed with a Shimadzu Thermal Analyzer DT-20B to obtain the total content of water of crystallization and peroxo group in deposits obtained at 81.5 °C. The TG/DTA measurement was carried out at a heating rate of 15 °C min-1 between 20 and 350 °C in an inert atmosphere (nitrogen, 30 mL min-1). For temperatureprogrammed decomposition-mass spectrometry (TPDE-MS) examination, O2 (m/e ) 32) in the decomposed gas of the sample in a nitrogen atmosphere was monitored by a mass spectrograph (Omnistar GSD 301 O3, Pfeiffer Vacuum, D-35614 Asslar), and the quantity of O2 was determined by the external standard method (Supporting Information). Infrared spectra of pressed wafers in the region of the stretching vibrations, between 1050 and 400 cm-1, were recorded with the KBr pellets technique in a Fourier transform infrared spectrometer (Bruker Equinox 55) at a resolution of 4 cm-1, and 64 scans were accumulated for each sample.
Figure 1. The change in color of products formed at different hydrothermal temperatures: (a) 81.5 °C, the yellow green final liquid and the yellow solid product; (b) 83.5 °C, the original suspended product with a weak pink luster; (c) 170 °C, the original white suspended product.
Figure 2. XRD patterns of solid products prepared from the 0.9 mol/L molybdenum solution for 45 h at different hydrothermal temperatures: (a) 81.5 °C; (b) 81.5 and 83.5 °C two-step hydrothermal procedure (vide infra); (c) 83.5 °C; (d) 120 °C; (e) 170 °C.
3. Results and Discussion Formation and Characterization of R-MoO3 Nanorods at Different Temperatures. MoO3 reacts with aqueous hydrogen peroxide to generate the soluble low-condensed peroxomolybdic acid after stirring at 30 °C for 6 h. With excess peroxide in the mother liquor, oxo-peroxo species are mainly monomer MoO(O2)2(H2O)2 and dimer [Mo2O3(O2)4(H2O)2]2-.41 At adequate temperatures, these species will decompose and condense simultaneously or consecutively to form R-MoO3. It is thus anticipated that the formation of R-MoO3 should have a dependence on reaction temperature. Below 81.5 °C of the hydrothermal temperature for 45 h with the 0.9 mol/L molybdenum solution, the yellow solid product and yellow-green liquid were obtained in the liner at the same time (Figure 1a). When the temperature was increased to 83.5 °C, abruptly only the suspended product was formed (Figure 1b). When the temperature was increased to 120 °C and 170 °C, only the suspensions were still collected and the amount of
R-MoO3 Nanostructures
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Figure 3. TG/DTA of the sample obtained from the 0.9 mol/L molybdenum solution at 81.5 °C for 45 h with a heating rate of 15 °C min-1 under nitrogen at a flow rate of 30 mL min-1. The sample weight in TG analysis was 9.24 mg, while that in DTA was 26.36 mg.
Figure 4. IR spectra of (a) the deep yellow sample formed from the mother liquor which stood for several days at room temperature, and samples prepared from the 0.9 mol/L molybdenum solution for 45 h at different hydrothermal temperatures: (b) 70 °C; (c) 81.5 °C; (d) 83.5 °C; (e) 170 °C.
solid products increased (Figure 1c). When the temperature was too high, for example, 220 °C, the Teflon liner, and even the stainless steel autoclave could be corrupted by the highly oxidative acidic precursor solution. Figure 2 displays a set of XRD patterns associated with investigated samples. The XRD patterns of samples obtained under 81.5 °C were similar in our experimental system. Their diffraction peaks were broad and no characteristic diffraction peaks of R-MoO3 were identified in these samples. In this paper, the structure and composition of the compound obtained at 81.5 °C were investigated in detail, and the XRD pattern of this sample was shown (Figure 2a), while the XRD pattern of the sample obtained at 83.5 °C was quite different from that of the sample obtained at 81.5 °C. The characteristic peaks of the sample at 81.5 °C vanished and those of R-MoO3 emerged (orthorhombic system, space group Pbnm, JCPDS card No. 350609) (Figure 2c). From the XRD pattern it was clearly shown that phase-pure R-MoO3 can be prepared when the hydrothermal temperature is higher than 83.5 °C. As higher temperatures were
used, the intensities of characteristic peaks of R-MoO3 increased (Figure 2d). At 170 °C, the intensities of reflection peaks of (020), (040), and (060) became very strong, revealing that R-MoO3 appeared with strong preferred orientation of [001] (Figure 2e), as the normal crystallographic dimension of R-MoO3 is in the sequence: [001] > [100] > [010].31 The powder diffraction patterns of R-MoO3 display the intensities different from the JCPDS data (parts c-e of Figure 2). This deviation is mainly due to the preferred orientation of R-MoO3 nanorods on the flat sample holder used in the XRD investigation. The structure and composition of the compound obtained at 81.5 °C were determined with XRD, TG/DTA, TPDE-MS, and FTIR techniques. As this sample was prepared in an oxidative environment due to addition of excessive H2O2, it can be assumed that in this sample the oxidation state for molybdenum is the highest valence VI. Considering the aqueous environment and strong coordination capacity of the peroxo group, a nominal formula of MoO3-x(O2)x‚yH2O (0 < x < 1) can be employed. In definite forms of molybdenum base compounds, R-MoO3‚ H2O (triclinic system, space group P1h, JCPDS card No. 261449) could be found to be the closest match to the diffraction pattern of the sample obtained at 81.5 °C, but a certain deviation of d values of some diffraction peaks was displayed (Figure 2a). This compositional analysis was further verified with TG/ DTA and TPDE-MS results. As shown in Figure 3, DTA scanning demonstrates that the endothermic peak ended at 250 °C and TG analysis indicates a weight loss of 11.6% from 150 to 250 °C. Moreover, with TPDE-MS investigation it is indicated that a weight loss of 3.3% was from O2, which the peroxo complex is decomposed into (Supporting Information). Combining TG/DTA with TPDE-MS results, a nominal formula of MoO2.67(O2)0.33‚0.75H2O can be employed in the sample obtained at 81.5 °C. In the literature,45 yellow R-MoO3‚H2O was obtained from the peroxo-polymolybdate solution48 under autogenous thermal ultrasound irradiation. The reason for the different components is the significantly higher efficiency of ultrasound irradiation than our hydrothermal reaction for eliminating the coordinated peroxo group around molybdenum. IR spectra and the frequencies (cm-1) for our samples prepared from the 0.9 mol/L molybdenum solution for 45 h at 70, 81.5, 83.5, and 170 °C are shown in Figure 4 and Table 1. As a contrast, data for the deep yellow sample formed from the mother liquor which stood for several days at room temperature is also shown. Though there is some peroxide in samples obtained below 81.5 °C, the peaks centered at 929 and 852 cm-1 (shown simultaneously in the sample formed at room temperature and which are assigned to the distinct O-O stretching vibration of the peroxo structure of the molybdenum peroxo complex) are not simultaneously shown in those samples (Figure 4a).47,49 The IR stretching vibrations of the sample obtained at
TABLE 1: IR Frequencies (cm-1) for the Deep Yellow Sample Formed from the Mother Liquor which Stood for Several Days at Room Temperature and Samples Prepared from the 0.9 mol/L Molybdenum Solution for 45 h at Different Hydrothermal Temperatures IR frequencies (cm-1) for samples 25 °C
70 °C
81.5 °C
83.5 °C
170 °C
976 852, 929
931, 961
929, 957, 982
998
998
617, 762
625, 762
868
821, 870
554
544, 569
572
561
631 521 a
assignmenta
ref
ν OdMo ν O-O ν OMo2 νas OMo νs OMo ν OMo3
47, 50 47, 49 47, 50 47 47 50
Assignment: OdMo involves terminal oxygen O, OMo2 double bridging oxygens O(3) and O(3′), and OMo3 triple bridging oxygens O(2) and O(2′) according to ref 50. ν indicates stretching vibrations, νs symmetric stretching vibrations, and νas antisymmetric stretching vibrations.
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Figure 5. TEM images showing the crystal morphologies of samples prepared from the 0.9 mol/L molybdenum solution for 45 h at: (a) 120 °C; (b) 170 °C; (c) 170 °C. Inset: a SAED pattern, the [010] zone axis diffraction, which reveals the [001] growth direction of the left-bottom R-MoO3 nanorod, together with TEM information. (d) SEM image of lateral planes of the sample obtained at 170 °C; (e) HRTEM image of the sample obtained at 170 °C.
70 °C resemble the typical ones appeared in R-MoO3‚H2O except for a very broad absorption band centered at about 762 cm-1 and a defined absorption at 617 cm-1 (Figure 4b).50 The two bands might have the same quality with the bands of the vibration of Mo-O-Mo obtained after the evacuation of the molybdenum peroxo complex.47 On the other hand, the two absorptions might involve the vibration of the OMo2 entity of β-MoO3‚H2O.50 When the hydrothermal temperature is increased from 70 °C to 81.5 °C, IR spectra differ in the following ways (Figure 4c): (i) One new absorption band with the maximum at 982 cm-1 appears, which is a typical one of the stretching vibrations of the OdMo unit in MoO3‚0.5H2O.50 (ii) The shoulder at 569 cm-1 and strong absorption at 544 cm-1 are derived from the broad band at 554 cm-1, which reveal more or less the characteristic stretching vibrations of OMo3 units merging in MoO3‚0.5H2O.50 The IR spectroscopic study, and the XRD, TG/DTA, and TPDE-MS results indicate that the sample obtained at 81.5 °C has a very complicated structure, which could be considered as, to some content, the peroxo
structure-modified R-MoO3‚H2O. Furthermore, within the sample, some local structures have the characteristic Mo-O-Mo units and have the similar Mo-O bond network in MoO3‚0.5H2O. When the hydrothermal temperature is increased to 83.5 °C, the IR spectrum of the sample has considerably different characteristics compared with the spectra of samples obtained below the critical temperature range. As shown in parts d and e of Figure 4, the IR spectra of samples prepared at 83.5 °C and 170 °C are identical, in accordance with the typical IR spectrum of R-MoO3.50 In Figure 5, the typical crystal morphologies of as-synthesized samples at hydrothermal temperatures of 120 and 170 °C are shown. The product obtained at 120 °C is from 180 to 300 nm in width and about 1 to 3 µm in length, and the particle dimensions have a broad distribution (Figure 5a). While the asprepared R-MoO3 at 170 °C exhibits a rodlike structure with rectangular tops on a large scale, the width of which is between 200 and 330 nm, the majority is around 250 nm, and the length is from 6 to 10 µm. As indicated in Figure 5b, the nanorods are
R-MoO3 Nanostructures
Figure 6. TEM images displaying the influence of the reaction time on crystal morphologies of samples prepared from the 0.9 mol/L molybdenum solution at 170 °C for: (a, b) 1 h, (c, d) 5 h, and (e, f) 20 h. The crystal directions of the R-MoO3 nanorods are shown according to the respective SAED patterns. Inset: SAED pattern of the R-MoO3 nanorod.
perfectly straight with a defined and constant width. The selected area electron diffraction (SAED) pattern (inset of Figure 5c) recorded perpendicular to the anisotropic growth axis of an individual nanorod is attributed to the [010] zone axis diffraction, and, combining with the TEM image, the SAED pattern indicates that the well-crystallized single-crystal nanorods grew along the [001] orientation of R-MoO3. Because of gravity, the majority of the nanorods landed selectively on (010) base planes under free sedimentation in the preparation of the specimen for TEM imaging study.31 The image of the scanning electron microscope (SEM) shows that the thickness of nanorods was about 60-90 nm (Figure 5d). By HRTEM, the fine structure of the nanorods was further observed. In the HRTEM image (Figure 5e), the crystal lattice fringes corresponding to d001 (0.36 nm) and d100 (0.38 nm) can be easily detected. In some areas of Figure 5e, those corresponding to d002 and d200 can also be identified. Formation of R-MoO3 Nanorods at Different Times and Concentrations. Then we investigated the time-dependence of the size of nanorods in this process. When the temperature was fixed at 170 °C, the formation of R-MoO3 proceeded quite
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Figure 7. TEM images of R-MoO3 nanorods synthesized at 170 °C for 45 h from the precursor solutions (diluted by deionized water from the mother liquor) when the molybdenum concentration was (a) 0.2 mol/L and (b) 0.036 mol/L.
rapidly, so the reaction time was varied in the range of 1-45 h (1, 5, 20, and 45 h). From the XRD patterns, it was noted that all XRD peaks in each pattern exhibited strong (0k0) reflections, patterns were not very different from each other in the time scale of the ex situ experiments, and no other intermediates were found (Figure S1, Supporting Information). Figure 6 shows the changes in morphology of samples as time increased. At the early stage of the hydrothermal process (1 h), short particles (180-250 nm in width, 1-2.2 µm in length) with irregular edges at their tops were generated in the form of nanorods grown along [001] (parts a and b of Figure 6). When time was increased to 5 h, particles (180-250 nm in width, 1.5-3.5 µm in length) in the TEM image of parts c and d of Figure 6 exhibited a flat fibrous morphology with smooth rectangular ends of R-MoO3. As the reaction time was increased further, the length of nanorods along [001] in the hydrothermal synthesis increased, while the width along [100] increased less. The average width along [100] and length along [001] of R-MoO3 nanorods produced after 20 h of reaction were in the ranges of 250 nm and about 4.5-7.5 µm, respectively (parts e and f of Figure 6). This means that the growth along the crystallographic direction [001] of R-MoO3 is much faster than those along the other two directions under these conditions. In a word, R-MoO3 nanorods could be synthesized efficiently by our method, and to some
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Figure 9. TEM images showing the crystal morphologies of samples prepared from the mother liquor for 45 h: (a, b) MoO2.67(O2)0.33‚ 0.75H2O; (c, d) R-MoO3. Figure 8. Images of R-MoO3 nanorods grown at 170 °C for 45 h from the precursor solution with 0.2 mol/L molybdenum diluted by 2 mol/L HNO3 from the mother liquor. (a) Low magnified TEM image, (b) high magnified TEM image. Inset: SAED of the upper R-MoO3 nanorod. (c) SEM image.
degree their length could be adjusted by controlling the hydrothermal time. Subsequently, the influence of the concentrations of the peroxomolybdic acid solutions on morphologies of products was explored. The mother liquor was a strong acid, with a pH value close to 0. When the solution was diluted to a certain extent, the acidity of the solution was weakened. For instance, when the molybdenum concentration was 0.45, 0.2, and 0.036 mol/ L, the pH value of the solution increased to 0.8, 1.1, and 1.9, respectively. It was found that the structure of the resultant nanorods remained essentially identical when the molybdenum concentration of the precursor solution decreased from 0.9 to 0.2 mol/L. The morphologies remained essentially unchanged while all other reaction conditions were kept identical except that the pH value of the precursor solution increased accordingly. In the case of the molybdenum concentration of 0.2 mol/L, the nanorods had a width between 200 and 330 nm and a length from 5.5 to 7.5 µm (Figure 7a). When the molybdenum concentration of the precursor solution was too low, such as 0.036 mol/L, the width and length distribution of R-MoO3 rods were quite broadened (570 nm-1.3 µm in width, longer than 15 µm in length) and the rods did not remain uniform (Figure 7b). Thus, R-MoO3 nanorods could still be synthesized easily even though the molybdenum concentration was not strictly controlled. Formation of R-MoO3 Nanorods in Exotic Acidic Additive. The acidity of the sole peroxomolybdic acid solution varied throughout the hydrothermal process, and the pH value increased from about 0 to around 2.5 when the molybdenum concentration was 0.9 mol/L in the precursor solution. It seemed to suggest that the pH value of the precursor solution could affect the morphology of nanorods. The effect of an acidic additive on the rod morphology was also investigated. The molybdenum concentration of the mother liquor was diluted from 0.9 to 0.2 mol/L by a 2 mol/L HNO3 solution, which replaced the role of
deionized water to sustain the pH value of the precursor solution throughout the hydrothermal process. Figure 8 illustrates how the rod dimensions depended on the acid additive. The nanorods were 170-330 nm in width, 4.5-7.5 µm in length (parts a and b of Figure 8), and about 150 nm in thickness (Figure 8c). When contrasted with the sample obtained from the mother liquor (parts b-d of Figure 5) and the one attained from the precursor solution diluted by deionized water (Figure 7a), the thickness of the rods obtained from the precursor solution diluted by 2 mol/L HNO3 solution had increased significantly while the two other dimensions hardly changed. It turned out that 2 mol/L HNO3 (a strong oxidative acid) instead of deionized water enhanced the growth of nanorods along the direction of the [010] axis much more than it did the growth of nanorods along the [100] axis and the [001] axis. The effect of various acids, such as HNO3, on the morphology of MoO3 nanorods has been investigated with MoO3‚2H2O as the molybdenum source by Patzke et al.33 SEM images show that the diameters of MoO3 nanorods are controlled into nanoscale when weak organic acids are added, whereas strong inorganic acids tune nanoscale rods to overall microscale dimensions.33 In our synthetic system, the synthesis of MoO3 is an acid-consuming process. With the presence of HNO3, the increase in growth rate and the subsequent dimensional enhancement of MoO3 nanorods are favorable due to the kinetic enhancement of using a higher acid concentration. However, the reason for the specific and distinct dimensional enhancement is unclear, and we will investigate this interesting effect in detail soon. The Relationship of Products Obtained at 81.5 and 83.5 °C and One Possible Mechanism. In consideration of the fact that a surprising critical point appeared in the hydrothermal temperature range of 81.5-83.5 °C, we conducted a series of experiments to investigate the relationship of products obtained at 81.5 and 83.5 °C. The yellow product, which was obtained for 45 h at the hydrothermal temperature of 81.5 °C, was thoroughly rinsed with deionized water, and then immersed with 11 mL of deionized water. The autoclave was then sealed and maintained at 83.5 °C for another 45 h. After the 81.5 and 83.5 °C two-step hydrothermal procedure, the product remained yellow and was not converted to R-MoO3 (Figure 2b). While
R-MoO3 Nanostructures the mother liquor converted swiftly to R-MoO3 under the same hydrothermal conditions except at 83.5 °C all the time. Furthermore, the morphologies of the two products were quite different in terms of TEM images. Rods of MoO2.67(O2)0.33‚ 0.75H2O had widths from 100 to 130 nm and lengths of about 6 µm (parts a and b of Figure 9), while R-MoO3 had widths between 200 and 300 nm, and a length about 2.5 µm (parts c and d of Figure 9). The hydrate contrasted sharply with the corresponding oxide not only in the degree of grayness but also in the structure. Compared with the former with smooth rectangular tops, the latter consisted of nanorods with 45 nm in width attaching to each other. On the basis of the above three aspects, it was suggested that when the temperature was 83.5 °C R-MoO3 could not arise from the dissolution-crystallization and/or topotactic dehydration of the bulk MoO2.67(O2)0.33‚ 0.75H2O. That is when the temperature was 83.5 °C, bulk MoO2.67(O2)0.33‚0.75H2O was not the intermediate of R-MoO3. At two different temperatures, products could precipitate from the supersaturated solutions containing corresponding precursors. According to the above synthesis and characterization results, the following possible mechanism is proposed to explain the critical point in temperature (81.5-83.5 °C) for forming different compositions. As the peroxide decomposes, the molybdenum species condense to chainlike clusters at the low peroxide concentration from low condensed monomer and dimer species formed at the high peroxide concentration.48 When thermal vibration cannot break up the O-H bond in the coordinated water and the peroxo structure around molybdenum, chainlike clusters condense to the isolated double chain configuration seen in R-MoO3‚H2O, or even isolated layer configuration like that seen in MoO3‚0.5H2O at some local sites. The chains or layers are linked together by hydrogen bonding.51,52 Moreover, the undecomposed peroxo structure could hinder the condensation of molybdenum oxide framework from the steric viewpoint. Once thermal vibration transforms the coordinated water around molybdenum to an oxo group and the peroxo structure around molybdenum is decomposed completely, chainlike clusters condense to a lamellar double layer structure emerging in R-MoO3 and the layers are held together only by van der Waals forces. Thus, different compositions are formed from distinct building blocks which are determined by temperature diversity and are linked by altered intermolecular bonding. 4. Conclusions With this novel synthetic route, i.e., using “green” decomposition and condensation reactions in a peroxomolybdic acid solution and using cheap commercial precursors with no participation of templates or catalysts, anisotropic uniform single-crystal R-MoO3 nanostructures can be synthesized easily and efficiently. The hydrothermal temperature, time, and solution composition are the key factors in controlling the structure and morphology of the products. It is indicated that the formation of R-MoO3 proceeds at reaction temperatures higher than 83.5 °C, and that of MoO2.67(O2)0.33‚0.75H2O at 81.5 °C when the molybdenum concentration is 0.9 mol/L in the precursor solution. The dimensions of the final rectangular nanorods are 200-330 nm in width, 60-90 nm in thickness, and up to 10 µm in length under the optimal reaction conditions at 170 °C for 20-45 h. The structure and morphology of R-MoO3 show a weak dependence on the concentrations of molybdenum (between 0.2 and 0.9 mol/L) in the precursor solutions. The addition of nitric acid to the mother liquor is an efficient method for enhancing the growth especially along the [010] direction.
J. Phys. Chem. C, Vol. 111, No. 6, 2007 2407 Acknowledgment. This work was supported by the National Natural Science Foundation of China for Distinguished Young Investigators (Nos. 20325620). We gratefully acknowledge Dr. Xingyun Huang for the XRD experiments and Dr. Xiu-Ying Gao for the TG/DTA examination. Supporting Information Available: The determination of the nominal formula of the sample obtained at 81.5 °C and XRD patterns of solid products prepared from the 0.9 mol/L molybdenum solution at 170 °C for different time (Figure S1). The material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Patzke, G. R.; Krumeich, F.; Nesper, R. Angew. Chem., Int. Ed. 2002, 41, 2446. (2) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, H. Q. AdV. Mater. 2003, 15, 353. (3) Rao, C. N. R.; Nath, M. Dalton Trans. 2003, 1. (4) Rao, C. N. R.; Deepak, F. L.; Gundiah, G.; Govindaraj, A. Prog. Solid State Chem. 2003, 31, 5. (5) Tenne, R.; Rao, C. N. R. Phil. Trans. R. Soc. Lond. A 2004, 362, 2099. (6) Oyama, S. T.; Zhang, W. J. Am. Chem. Soc. 1996, 118, 7173. (7) Liu, H. F.; Liu, R. S.; Liew, K. Y.; Johnson, R. E.; Lunsford, J. H. J. Am. Chem. Soc. 1984, 106, 4117. (8) Galatsis, K.; Li, Y. X.; Wlodarski, W.; Comini, E.; Sberveglieri, G.; Cantalini, C.; Santucci, S.; Passacantando, M. Sens. Actuators, B 2002, 83, 276. (9) Julien, C.; Nazri, G. A. Solid State Ionics 1994, 68, 111. (10) Julien, C.; Nazri, G. A.; Guesdon, J. P.; Gorenstein, A.; Khelfa, A.; Hussain, O. M. Solid State Ionics 1994, 73, 319. (11) Li, W. Y.; Cheng, F. Y.; Tao, Z. L.; Chen, J. J. Phys. Chem. B 2006, 110, 119. (12) Yang, H. G.; Zeng, H. C. Chem. Mater. 2003, 15, 3113. (13) Yang, H. G.; Zeng, H. C. J. Phys. Chem. B 2004, 108, 819. (14) Liu, B.; Zeng, H. C. J. Phys. Chem. B 2004, 108, 5867. (15) Lou, X. W.; Zeng, H. C. J. Am. Chem. Soc. 2003, 125, 2697. (16) Li, Y. B.; Bando, Y.; Golberg, D.; Kurashima, K. Appl. Phys. Lett. 2002, 81, 5048. (17) Zhou, J.; Xu, N. S.; Deng, S. Z.; Chen, J.; She, J. C.; Wang, Z. L. AdV. Mater. 2003, 15, 1835. (18) Zhou, J.; Deng, S. Z.; Xu, N. S.; Chen, J.; She, J. C. Appl. Phys. Lett. 2003, 83, 2653. (19) Li, Y. B.; Bando, Y. S. Chem. Phys. Lett. 2002, 364, 484. (20) Zhao, Y.; Liu, J. G.; Zhou, Y.; Zhang, Z. J.; Xu, Y. H.; Naramoto, H.; Yamamoto, S. J. Phys.: Condens. Matter. 2003, 15, L547. (21) Liu, J. G.; Zhang, Z. J.; Pan, C. Y.; Zhao, Y.; Su, X.; Zhou, Y.; Yu, D. P. Mater. Lett. 2004, 58, 3812. (22) Chen, Y. K.; Green, M. L. H.; Tsang, S. C. Chem. Commun. 1996, 2489. (23) Chen, Y. K.; Chu, A.; Cook, J.; Green, M. L. H.; Harris, P. J. F.; Heesom, R.; Humphries, M.; Sloan, J.; Tsang, S. C.; Turner, J. F. C. J. Mater. Chem. 1997, 7, 545. (24) Satishkumar, B. C.; Govindaraj, A.; Vogl, E. M.; Basumallick, L.; Rao, C. N. R. J. Mater. Res. 1997, 12, 604. (25) Satishkumar, B. C.; Govindaraj, A.; Nath, M.; Rao, C. N. R. J. Mater. Chem. 2000, 10, 2115. (26) Ogihara, H.; Takenaka, S.; Yamanaka, I.; Tanabe, E.; Genseki, A.; Otsuka, K. Chem. Lett. 2005, 34, 1428. (27) Niederberger, M.; Krumeich, F.; Muhr, H. J.; Mu¨ller, M.; Nesper, R. J. Mater. Chem. 2001, 11, 1941. (28) Wang, S. T.; Zhang, Y. G.; Ma, X. C.; Wang, W. Z.; Li, X. B.; Zhang, Z. D.; Qian, Y. T. Solid State Commun. 2005, 136, 283. (29) Song, R. Q.; Xu, A. W.; Deng, B.; Fang, Y. P. J. Phys. Chem. B 2005, 109, 22758. (30) Xia, T.; Li, Q.; Liu, X. D.; Meng, J.; Cao, X. Q. J. Phys. Chem. B 2006, 110, 2006. (31) Lou, X. W.; Zeng, H. C. Chem. Mater. 2002, 14, 4781. (32) Li, X. L.; Liu, J. F.; Li, Y. D. Appl. Phys. Lett. 2002, 81, 4832.
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