Structure, Optical, and Catalytic Properties of Novel Hexagonal

Oct 28, 2010 - Again, this suggests the better crystallinity arising from the annealing, being consistent with the XRD results. The peaks positioned a...
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6202 Chem. Mater. 2010, 22, 6202–6208 DOI:10.1021/cm102703s

Structure, Optical, and Catalytic Properties of Novel Hexagonal Metastable h-MoO3 Nano- and Microrods Synthesized with Modified Liquid-Phase Processes Wenzhi Pan,† Ruiyuan Tian,† Hao Jin,† Yanjun Guo,† Liping Zhang,† Xiaochun Wu,† Lina Zhang,†,‡ Zhihua Han,†,‡ Guangyao Liu,†,§ Jianbo Li,†,§ Guanghui Rao,†,§ Hanfu Wang,*,† and Weiguo Chu*,† National Center for Nanoscience and Technology, P. R. China, ‡Tsinghua - Foxconn Nanotechnology Research Center, Tsinghua University, P. R. China, and §Institute of Physics, Chinese Academy of Sciences, P. R. China

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Received September 20, 2010. Revised Manuscript Received October 13, 2010

Single crystalline h-MoO3 nano- and microrods were successfully synthesized using modified liquid-phase processes with concentrated HNO3 and H2SO4. Their X-ray powder diffraction (XRD) data were unambiguously indexed based on a hexagonal structure with the lattice constants a ≈ 10.57 and c ≈ 3.72 A˚ instead of a = 10.53 and c = 14.98 A˚ (JCPDS 21-0569) usually adopted. Rietveld refinements of the XRD data were pioneeringly performed based on the (Na 3 2H2O)Mo5.33[H4.5]0.67O18 structure with the space group of P63/m regardless of Hþ and Naþ. Nanorods synthesized under different conditions show different sizes and aspect ratios. Annealing at 300 °C for 3 h significantly improves the crystallinity and phase purity of as-synthesized h-MoO3 rods, which is evidenced by sharpening of peaks in micro-Raman spectra with no shift. An irreversible transition from h-MoO3 to R-MoO3 occurring between 413 and 436 °C can be triggered by irradiation of either electrons or laser with high energies or powers as well. The turning points on both differential thermal analysis (DTA) and thermogravimetry (TG) curves show presence of water molecules interacted differently with the lattice which escape at different temperatures. h-MoO3 rods reduce the temperatures of soot oxidation to 482-490 °C, much higher than its structural transition temperatures. This makes it simply suitable for catalyzing reactions taking place at temperatures lower than the transition temperatures, say, as the catalyst of the selective oxidation of methanol. 1. Introduction Oxide nanostructures have been extensively explored due to their potential applications in various fields, such as sensors, catalysis, field emission, energy storage and conversion, optoelectronics, magneto-electronics, magnetooptics, and microelectronics, etc.1-8 So far, oxide nanostructures are focused mostly on transition metal oxides, *Corresponding authors. W.C.: [email protected]; H.W.: wanghf@ nanoctr.cn.

(1) Hansen, B. J; Kouklin, N; Lu, G. H; Lin, I. K.; Chen, J. H.; Zhang, X. J. Phys. Chem. C 2010, 114, 2440–2447. (2) Chu, W. G.; Wang, H. F.; Guo, Y. J.; Zhang, L. N.; Han, Z. H.; Li, Q. Q.; Fan, S. S. Inorg. Chem. 2009, 48, 1243–1249. (3) Yan, Y. G.; Zhou, L. X.; Han, Z. D.; Zhang, Y. J. Phys. Chem. C 2010, 114, 3932–3936. (4) Ning, J. J.; Jiang, T.; Men, K. K.; Dai, Q. Q.; Li, D. M.; Wei, Y. J.; Liu, B. B.; Chen, G.; Zou, B.; Zou, G. T. J. Phys. Chem. C 2009, 113, 14140–14144. (5) Singamaneni, S.; Gupta, M.; Yang, R. S.; Tomczak, M. M.; Naik, R. R.; Wang, Z. L.; Tsukruk, V. V. ACS Nano 2009, 3, 2593–2600. (6) Goto, K.; Tanaka, H.; Kawai, T. J. Appl. Phys. 2009, 105, 064301– 064303. (7) Caicedo, J. M.; Taboada, E.; Hrabovsky, D.; Lopez-Garcia, M.; Herranz, G.; Roig, A.; Blanco, A.; Lopez, C.; Fontcuberta, J. J. Magn. Magn. Mater. 2010, 322, 1494–1496. (8) Calzada, M. L.; Torres, M.; Ricote, J.; Pardo, L. J. Nanopart. Res. 2009, 11, 1227–1233. (9) Ivanova, O. S.; Zamborini, F. P. J. Am. Chem. Soc. 2010, 132 70–72.

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particularly on those with relatively simple ingredients and structures.9-12 Of great interest is molybdenum trioxide because it has different crystalline phases with open structures which make it possible to be applied in different fields.13-15 Molybdenum trioxide is revealed to have several polymorphs: the well-known thermodynamically stable R-MoO 3 (space group Pnma), 16 metastable β-MoO3 (P21/c),17 ε-MoO3 (P21/m),18 and hexagonal metastable h-MoO3 (P63/m).19 However, all the polymorphs reported are basically constructed merely in different ways based on the building block of MoO6 (10) Rao, P. M.; Zheng, X. L. Nanoletters 2009, 9, 3001–3006. (11) Sayle, T. X. T.; Maphanga, R. R.; Ngoepe, P. E.; Sayle, D. C. J. Am. Chem. Soc. 2009, 131, 6161–6173. (12) Yan, B.; Liao, L.; You, Y. M.; Xu, X. J.; Zheng, Z.; Shen, Z. X.; Ma, J.; Tong, L. M.; Yu, T. Adv. Mater. 2009, 21, 2436–2440. (13) Wang, J. F.; Rose, K. C.; Lieber, C. M. J. Phys. Chem. B 1999, 103, 8405–8409. (14) Hamelmann, F.; Gesheva, K.; Ivanova, T.; Szekeres, A.; Abroshev, M.; Heinzmann J. Optoelectron. Adv. Mater. 2005, 7, 393–399. (15) Mestle, G. Top. Catal. 2006, 38, 69–82. (16) Kihlborg, L. Ark. Kemi 1963, 21, 357–364. (17) Parise, J. B.; McCarron, E. M.; Sleight, A. W.; Prince, E. Mater. Sci. Forum 1988, 27, 85–88. (18) McCarron, E. M.; Calabrese, J. C. J. Solid State Chem. 1991, 91, 121–125. (19) Caiger, N. A.; Crouch-Baker, S.; Dickens, P. G.; James, G. S. J. Solid State Chem. 1987, 67, 369–373.

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octahedron (see Figure S1, Supporting Information). Among the above four crystalline phases, the stable R-MoO3 phase with multiform nanostructures has been extensively studied whereas the hexagonal metastable h-MoO3 phase is much less known. A very salient structural feature for h-MoO3 is the presence of a tunnel (∼ 3.0 A˚ in diameter) running along the c direction probably with some cations or water molecules inside, which is enclosed by twelve MoO6 octahedra linked by sharing corners along the a and b directions as well as edges along the c direction (see Figure S1, Supporting Information). Most researchers take h-MoO3 as the structure described in JCPDS 21-0569 (lattice constants: a=10.53 and c=14.87 A˚; primitive: P or space group: P63/m).20-25 However, to the best of our knowledge, no detailed structural analysis is reported, such as structural refinements on XRD data based on the above or other models. It is well recognized that metastable phases usually show some novel or enhanced properties but are considered more difficult to synthesize as compared to the stable ones. In the case of MoO3, a variety of methods are developed for the synthesis of R-MoO3 with diversified nanostructures.26-31 In sharp contrast, far fewer methods are reported to synthesize h-MoO3 and its hydrates due to its metastability. Previously, large cations were believed to be indispensable for the synthesis of h-MoO3 in that they can stabilize its structure upon being incorporated into the tunnel.32 Presently, two methods were employed to synthesize crystalline h-MoO3 successfully without large cations incorporated: one is the hot liquid-phase process,22,23 and the other is the liquid-phase process followed by autoclaving and vacuum heat-treatment.20,24,25 Within the above two methods either concentrated HCl or HNO3 were used as the promoter of formation of h-MoO3.20,22,23,25 In this paper a simple and effective liquid-phase process was developed to synthesize hexagonal h-MoO3 nanorods. Also, concentrated H2SO4 was attempted to synthesize h-MoO3 microrods successfully. Rietveld refinements of XRD data of h-MoO3 nano- and microrods were performed based on a hexagonal structure (space group: (20) Song, J. M.; Ni, X. M.; Zhang, D. G.; Zheng, H. G. Solid State Sci. 2006, 8, 1164–1167. (21) Komaba, S.; Kumagai, N.; Kumagai, R.; Kumagai, N.; Yashiro, H. Solid State Ionics 2002, 152 - 153, 319–326. (22) Atuchin, V. V.; Gavrilova, T. A.; Kostrovsky, V. G.; Pokrovsky, L. D.; Troitskaia, I. B. Inorg. Mater. 2008, 44, 622–627. (23) Ramana, C. V.; Atuchin, V. V.; Troitskaia, I. B.; Gromilov, S. A.; Kostrovsky, V. G. Solid State Commun. 2009, 149, 6–9. (24) Zheng, L.; Xu, Y.; Jin, D.; Xie, Y. Chem. Mater. 2009, 21, 5681– 5690. (25) Song, J. M.; Ni, X. M.; Gao, L. S.; Zheng, H. G. Mater. Chem. Phys. 2007, 102, 245–248. (26) Li, X. L.; Liu, J. F.; Li, Y. D. Appl. Phys. Lett. 2002, 81, 4832–4834. (27) Patzke, G. R.; Michailovski, A.; Krumeich, F.; Nesper, R.; Grunwaldt, J. D.; Baiker, A. Chem. Mater. 2004, 16, 1126–1134. (28) Lou, X. W.; Zeng, H. C. Chem. Mater. 2002, 14, 4781–4789. (29) Song, R. Q.; Wu, A. W.; Deng, B.; Fang, Y. P. J. Phys. Chem. B 2005, 109, 22758–22766. (30) Li, Y. B.; Bando, Y. Chem. Phys. Lett. 2002, 364, 484–488. (31) Chu, W. G.; Zhang, L. N.; Wang, H. F.; Han, Z. H.; Han, D.; Li, Q. Q.; Fan, S. S. J. Mater. Res. 2007, 22, 1609–1617. (32) Guo, J. D.; Zavalij, P.; Whittingham, M. S. J. Solid State Chem. 1995, 117, 323–332.

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P63/m) with the indexed lattice constants of a ≈ 10.57 and c ≈ 3.72 A˚ instead of a = 10.53 and c = 14.87 A˚ ≈ 4c (JCPDS 21-0569). Their structure, optical properties, and catalytic properties for soot oxidation were explored in detail. 2. Experimental Section Eight kinds of h-MoO3 samples labeled A-H were prepared. Sample A was made by dropwise adding 4.5 M/L concentrated nitric acid into a solution of 10 mL of ammonium heptamolybdate tetrahydrate with a molybdenum ion molar concentration of 0.5 M/L under stirring, followed by supersonicating for 50 min until significant precipitates formed. The precipitates were then washed several times with deionized water, centrifuged, and finally dried overnight at 30 °C. Samples B and C were prepared in a similar way the solutions were heated in hot water baths at 60 and 85 °C for 20 min under continuous stirring instead of supersonicating, respectively. Concentrated H2SO4 (1.5 M/L) instead of HNO3 was used for the preparation of sample D with the solution heated in a hot water bath at 100 °C for 20 min. For comparison a part of samples A-D each thus obtained was annealed at 300 °C for 3 h, labeled E-H, respectively. The morphology and structure of samples were examined using both field emission electron scanning microscopy (FEESM, Sirion 200, FEI, USA) and high-resolution transmission electron microscopy (FEHRTEM, Tecnai 200 F20 G2, FEI, USA) equipped with energy dispersive spectroscopy of X-ray (EDS) for check of the chemical constituents. TEM and HRTEM observations were made at a voltage of 200 kV. XRD data of most of the samples were recorded on a Philips X’Pert Pro diffractometer (PANalytical, The Netherlands) using a Cu KR radiation with 45 kV and 40 mA, an angular range of 6-80°, a step size of 0.017°, and a sampling time of 0.5 s. The divergence slit was 1/2° and the antiscattering slit was 1°. For better Rietveld refinements higher quality of XRD data for sample F were collected on a higher power diffractometer (Rigaku D/max 2500, Japan) using Cu KR radiation at a power of 45 kV  250 mA, a graphite monochromator, and a scan mode of 2θ-θ. The scan range was from 6° to 120°, with a step size of 0.02° and a sampling time of 1 s. In the experiments, the divergence slit was 1° and the receiving slit was 0.3 mm. Also, X-ray photoelectron spectra (XPS) with wide and fine scans were acquired for further check of sample purity and determination of valence of molybdenum ion using an ESCALab250 electron spectrometer from Thermo Scientific Corporation with monochromatic 150 W Al KR radiation. The base pressure was about 6.5  10-10 mbar. The spectra were calibrated using the C1s line at 284.8 eV from alkyl or adventious carbon. Optical properties of samples were studied via micro-Raman spectroscopy (Renishaw Invia, UK) and ultraviolet absorption spectroscopy (Perkin-Elmer, Wellesley, MA). The microRaman spectra were recorded at room temperature with an argon ion laser operating at 514 nm and a resolution of 1 cm-1. Catalytic performance of all samples for soot combustion was tested using TG and DTA (Diamond TG - DTA, Perkin-Elmer, USA). Printex U (Degussa, Germany) carbon black was taken as model soot for catalytic experiments. The weight of samples used for TG and DTA experiments was around 3 mg. The weight ratio between h-MoO3 and soot in the mixtures was 3:1. Both TG and DTA experiments were carried out in a 20% O2/ Ar gas mixture with a scan rate of 5 °C/min.

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3. Results and Discussion 3.1. Hexagonal Metastable Structure of h -MoO3 Nano- and Microrods. The relative openness of the hexagonal structure of h-MoO3 makes it very attractive for intercalation chemistry, catalysis, and other applications. Therefore, we employed the modified liquid-phase processes to synthesize h-MoO3 nano- and microrods with concentrated HNO3 and H2SO4 at different temperatures. Here, for clarity, only the XRD patterns of samples A, B, E, and F are shown in Figure 1. For better comparison those of all the samples are presented in Figure S2 of Supporting Information. All the diffraction peaks for different samples are seen to be rather consistent with one another. Minor impurities in samples A-D are evidenced by some extra weak peaks in the XRD patterns in Figure 1a and Figure S1a. However, the purity and crystallinity of samples is ameliorated by annealing, as demonstrated by disappearance of the extra weak peaks in Figure 1b and Figure S2b. Intriguingly, careful comparison reveals that either the peak positions or the interplanar spacings in our case resemble those reported in refs 20-25 (see Tables, Supporting Information), but in their cases a hexagonal symmetry with a = 10.53 and c=14.87 A˚ (JCPDS 21-0569) was taken to explain their XRD data without performing Rietveld refinements. We tried the hexagonal symmetry with the lattice constants similar to those reported in JCPDS card 21-0569 to index the XRD reflections under study but failed. Nevertheless, the XRD reflections can unambiguously be indexed as the hexagonal symmetry with the lattice constants of a ≈ 10.57 and c ≈ 3.72 A˚ for the annealed samples with the De Wolff figures of merit greater than those for the assynthesized samples (see Tables S1 and S2, Supporting Information). In addition, all the reflections reported in JCPDS 21-0569 were successfully indexed again as the hexagonal symmetry with a=10.55 and c=3.72 A˚ with F30=8 (0.046) instead of a=10.53 and c=14.87 A˚ with F30=2 (0.068). To reveal the structural details further we adopted the hexagonal (Na 3 2H2O)Mo5.33[H4.5]0.67O18 structure and a simple hexagonal structure with space group P63/m as the initial models to refine the XRD data of all the h-MoO3 samples using the FULLPROF program without considering the incorporation of the cations, i.e., Naþ (see Tables, Supporting Information) and Hþ.33,34 The experimental, simulated, and different XRD patterns for samples A, B, E, and F are displayed in Figure 1, and those for all the samples are displayed in Figure S2 of Supporting Information for comparison. The refinement results indicate that the structure based on the hexagonal (Na 3 2H2O)Mo5.33[H4.5]0.67O18 model well describes the structures of h-MoO3, in particular those for the annealed samples. In Figure 1b the simple structural model much more poorly describes the h-MoO3 structure compared to (33) McCarron, E. M.; Thomas, D. M.; Calabrese, J. C. Inorg. Chem. 1987, 26, 370–373. (34) Rodriguez-Carvajal, J. Fullprof 2004 (version 2008); Laboratoire Leon Brillouin (CEA-CNRS), France.

Figure 1. Experimental, simulated, and different XRD patterns of as-synthesized (a) and annealed h-MoO3 samples (b). Fs and F are the results based on the simple structure and the (Na 3 2H2O)Mo5.33[H4.5]0.67O18 structure, respectively.

the (Na 3 2H2O)Mo5.33[H4.5]0.67O18 model, as shown by Fs and F. For the simple model Rwp=22.2%, Rexp=5.63%, RB = 14.6%, RF = 9.52%, which shows a much worse Rietveld refinement result as opposed to those for the (Na 3 2H2O)Mo5.33[H4.5]0.67O18 model (see Table S2). The agreements between experiment and theory for the assynthesized samples A-D are not as good as those for the annealed ones though the positions of most peaks (except for some weak ones from minor impurities) are quite close, as shown by Figure 1 and Figure S2 and Tables in Supporting Information. The relatively poorer agreements for samples A-D could be ascribed to the interplay of their minor impurities and poorer crystallinity. Figure 2 presents SEM images of h-MoO3 synthesized with different recipes. All the h-MoO3 products are seen to be rod-like with a hexagonal cross section, as clearly revealed by the inset in Figure 2a. h-MoO3 rods for samples A-C are mainly 100-300 nm thick and several micrometers long, whereas those for sample D are around 2 μm thick and about 10 μm long. Those nanorods for sample A are relatively uniform in thickness and have larger aspect ratios compared to those for samples B and C. As the temperature of solutions for HNO3 is increased both the size and nonuniformity of nanorods tend to increase, and their aspect ratios get lower. In contrast, for H2SO4 h-MoO3 rods are uniform in both size and morphology, and their aspect ratios appear to have a narrower distribution. Again, numerous microrods are found to have defects such as voids, as shown in Figure 2d and h. The subsequent annealing causes no size change but leads to an edge sharpening instead, which could be attributed to the better crystallization. The lattice constants and cell volumes for samples A-H are displayed in Figure 3. The annealing leads to a small contraction along the c axis, a relatively large

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Figure 2. SEM images of as-synthesized (a-d) and annealed h-MoO3 (e-h).

Figure 3. Lattice constants and volumes of cell of as-synthesized and annealed h-MoO3.

expansion along the a axis, and a slight increase in cell volume, in accordance with ref 32. Reasonably, the lattice is considered to contain no NH4þ as no better agreements between experiment and theory were obtained with NH4þ considered upon refining on one side. On the other side, samples were checked with both EDS and XPS, but no traces of nitrogen and sulfur were observed, as revealed in Figure 4 and Figure S3a. Furthermore, the lattice constant a, for all samples, ∼10.57 A˚ is considerably higher than ∼10.53 A˚ for the ammonium stabilized h-MoO3 samples. However, a small amount of water in the tunnel of the structure could not be excluded, which is evidenced by the smaller R factors upon refinement by taking some oxygen in the tunnel into consideration. The escape of a portion of water molecules (∼ 3.3 A˚) in the tunnel (∼ 3.0 A˚) by the annealing could lead to the expansion along a due to the easier deformation as a consequence of corner

sharing. The smaller contraction along c could be ascribed to the interplay of the harder distortion due to edge sharing and of minimum energy gains by reducing the increase of cell volume as much as possible, as indicated by only a maximum increase of ∼1.3 A˚3 for all samples. To gain insights into the crystalline structure of individual h-MoO3 rods, both TEM and HRTEM observations were made on numerous samples. Here, shown in Figure 4 are TEM, electron diffraction (ED), HRTEM images, and EDS for samples A and E. The elements of C, O, Mo, and Cu were detected with EDS in which C and Cu come from the holey grids, and no traces of N and S were found for all the samples. The combination of TEM and ED allows one to determine the growth direction of onedimensional nanostructures and to evaluate their crystallinity. All h-MoO3 nano- and microrods synthesized with different recipes are found to be single crystalline with a

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Figure 5. UV-vis absorption spectra of as-synthesized samples A-D and annealed samples H-G.

Figure 4. TEM, ED, and HRTEM images of the as-synthesized h-MoO3 sample A (a) and (b), and of the corresponding annealed sample E (c) and (d).

[001] growth direction, as indicated in Figure 4. The growth along [001] can be readily understood in terms of the stronger bondings of MoO6 octahedra along c due to the zigzag structure formed by sharing edges. From HRTEM images the (001) and (110) interplanar spacings were derived to be ∼3.7 and ∼5.3 A˚, respectively, in good agreement with the values calculated from the refinement results. It is worthwhile to point out that electron irradiation for certain time would damage single crystalline h-MoO3 rods to amorphous or polycrystalline ones having the R-MoO3 structure, probably depending on irradiation energy, dosage, and duration. This is also demonstrated by the following Raman spectroscopy. The extra energy supply could alter the h-MoO3 structure, suggesting its metastability. 3.2. Optical Properties of h-MoO3 Rods. The electronic structure features of a semiconductor could usually manifest themselves in the optical absorption properties revealed by UV-vis absorption spectra. The UV-vis absorption spectra of samples A-D, G, and H are presented in Figure 5. Both nanorods and microrods of hMoO3 are shown to have the apparent photoabsorption properties, particularly for the wavelengths shorter than 420 nm. It is well established that at absorption edge for indirect transition the optical absorption coefficient R varies with the photo energy hv according to the expression of (rhv)1/2 µ (hv-Eg) where Eg is the band gap energy. By extrapolating the linear parts on the plots of (rhv)1/2 versus hv to (rhv)1/2 = 0,35 as shown in the inset in Figure 5, Eg for the samples were derived to range from 2.92 to 3.05 eV, in good agreement with 3.017 eV of h-MoO3 nanorods with thicknesses around 500 nm and (35) Toyoda, T.; Nakanishi, H.; Endo, S.; Irie, T. J. Phys. D: Appl. Phys. 1985, 18, 747–751.

larger than 2.64 eV of h-MoO3 nanobelts with thicknesses around 20-30 nm.23,24 A much smaller optical bandgap was also observed for R-MoO3 nanobelts compared to that of prism-like R-MoO3 particles.36 It is usually thought that the reduction of size may result in a wider bandgap due to quantum size confinement according to the relationship of the value of blue-shifting being inversely proportional to the square of the size.37 Contrarily, the value for h-MoO3 nanobelts reported in ref 24 smaller than those of h-MoO3 rods in this study may arise from the larger red-shifting due to the formation of surface states of h-MoO3 nanobelts overwhelmingly counteracting the blue-shifting caused by the reduction of sizes. The absorption curves in Figure 5 show no additional absorption traces or splitting of bands due to localized levels in the bandgaps, indicative of the high quality of h-MoO3 nano- and microrods from the perspective of oxygen vacancies and metal impurities. In addition, the annealing causes no significant changes of the optical bandgaps. RT micro-Raman spectra for samples A-H are shown in Figure 6. In particular, for sample A the spectra were taken with different laser powers in order to probe the possible influence of power on its structure. All samples are found to have very similar peaks in both position and relative intensity. The peaks are positioned at 134, 173, 217, 250, 312, 395, 412, 489, 690, 887, 901, 916, and 980 cm-1, which agree well with those reported in ref 22. It is apparent that the peaks for samples E-H are sharper than those for samples A-D, as shown more clearly by the splitting away of the peak at 916 cm-1, and the far sharper peak at 980 cm-1. Again, this suggests the better crystallinity arising from the annealing, being consistent with the XRD results. The peaks positioned at 134, 217, 250, and 395 cm-1 correspond well to those of R-MoO3, which can be assigned to the translational rigid MoO4 chain mode (B3g, Tc), the rotational rigid MoO4 chain mode (Ag, Rc), the OdModO twist (B3g), and the O; Mo;O scissor (B1g), respectively.38,39 No corresponding peaks in position were observed for h-MoO3 and R-MoO3 within the range of bands higher than 400 cm-1. (36) Xia, T.; Li, Q.; Liu, X. D.; Meng, J.; Cao, X. Q. J. Phys. Chem. B 2006, 110, 2006–2012. (37) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226–13239. (38) Py, M. A.; Schmid, P. E.; Vallin, J. T. Nuovo Cimento B 1977, 38, 271–279. (39) Py, M. A.; Maschke, K. Physica B 1981, 105, 370–374.

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Figure 6. Micro-Raman spectra of as-synthesized and annealed samples (a), and of sample A with laser irradiation with different powers, indicating a phase transition from h-MoO3 to R-MoO3.

Figure 6b clearly shows the changes of peaks of h-MoO3 with increased laser power. The peaks at 690, 887, 901, and 916 cm-1 tend to move toward the side of lower bands while the band at 980 cm-1 moves to the opposite direction instead. Apart from this, some changes could as well be observed for the lower bands, as revealed by Figure 6b. The peaks at 124, 148, 194, 214, 239, 285, 334, 376, 470, 663, 818, and 997 cm-1 are the characteristic ones for R-MoO3.31,38-40 By reducing the power to the low enough values that do not cause the phase transition, the spectra were taken again and all the spectroscopic characteristics of R-MoO3 still remain, as shown in Figure 6b. This unambiguously indicates that the irradiation of laser with higher powers triggers an irreversible switch from h-MoO3 to R-MoO3 quickly, as also observed by TEM aforementioned. 3.3. Catalytic Performance of h-MoO3 Rods for Soot Oxidation. MoO3 is considered a typical catalyst capable of catalyzing the oxidation of many kinds of organic substances.41-45 Catalytic properties of h-MoO3 nanoand microrods for soot combustion were also tested using the DTA and TG techniques. The TG and DTA curves for samples A-D are displayed in Figure 7a and b, respectively, and those of the mixtures of samples A-H and soot in Figure 7c and d, respectively. For samples A-D one can find several turning points on both the TG and DTA curves, as indicated by the arrows. These turning points corresponding to the temperatures of about (40) Dieterle, M.; Weinberg, G.; Mestl, G. Phys. Chem. Chem. Phys. 2002, 4, 812–821. (41) Chen, M.; Friend, M.; Kaxiras, E. J. Am. Chem. Soc. 2001, 123, 2224–2230. (42) Onal, I.; Duzenli, D.; Seubsai, A.; Chandorkar, J. G.; Umbarkar, S. B. Top. Catal. 2010, 53, 92–99. (43) Kim, H. Y.; Lee, H. M.; Pala, R.; Ganesh, S.; Metiu, H. J. Phys. Chem. 2009, 113, 16083–16093. (44) Wang, F.; Ueda, W. Chem. - Eur. J. 2009, 15, 742–753. (45) Christodoulakis, A.; Boghosian, S. J. Catal. 2008, 260, 178–187.

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Figure 7. TG and DTA curves of as-synthesized samples (a) and (b), and of the mixtures of as-synthesized and annealed h-MoO3 with soot (c) and (d).

100, 200, and 400 °C may arise from the losses of water involved which have different interactions with the lattice of h-MoO3, i.e., the water molecules physically adsorbed on the surface, the ones with weak interactions and the ones in the tunnel, respectively. If the temperatures of around 200 and 400 °C are taken as the starting and completing points of escape of the water molecules in the tunnel, respectively, the molecular formula is derived to be MoO3 3 0.25H2O based on the TG curves of samples A, C, and D. Because sample B for the TG experiment was quite moist the heating kinetics may be changed due to the presence of too much water physically adsorbed on its surface, as shown in Figure 7a. From the DTA curves the peaks of heat flow of samples A-D correspond to the temperatures ranging from 413 to 436 °C, indicative of the phase transition from h-MoO3 to R-MoO3.23,24 The transition temperatures slightly higher than that of h-MoO3 nanobelts, 400 °C, suggest that h-MoO3 nano- and microrods in this study is relatively more stable compared to h-MoO3 nanobelts with tens of nanometers in thickness.24 The better stability may facilitate easier applications of h-MoO3. The peak temperatures of samples A-H catalyzing soot combustion were derived from both the TG and DTA curves, ranging from 482 to 490 °C, lower than 600 °C, the oxidation temperature of soot.2 This indicates that the preparation conditions and morphologies of samples under study have no significant influences on the temperature of soot oxidation. We should bear in mind that the weak peaks observed prior to the strong peaks on the DTA curves are considered to originate from the transition of h-MoO3 to R-MoO3, suggesting the occurrence of the phase transition before the catalytic reaction of soot oxidation initiates significantly. Thus, it is R-MoO3 instead of h-MoO3 that catalyzes the soot combustion. Consequently, h-MoO 3 can simply be used as catalysts that are capable of catalyzing the reactions at temperatures lower than the temperature of the transition

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of h-MoO3 to R-MoO3, say, as the catalyst of the selective oxidation of methanol.46 4. Conclusions Single crystalline h-MoO3 nano- and microrods with the hexagonal metastable structure were successfully synthesized using the modified liquid-phase processes with concentrated HNO3 and H2SO4, respectively. Both nano- and microrods were identified as the hexagonal structure with lattice constants a ≈ 10.57 and c ≈ 3.72 A˚ instead of a = 10.53 and c = 14.98 A˚ adopted by other researchers usually. All the XRD data were successfully refined based on the (Na 3 2H 2O)Mo5.33[H 4.5]0.67 O18 structure with the space group of P63/m without Naþ considered. With increased temperatures of solutions for HNO3 nanorods tend to increase and get nonuniform in size, and to become lower in aspect ratio. All rods are determined to grow in the direction of [001] which can be attributed to the stronger bonding of MoO6 octahedra along c due to the zigzag structure formed by sharing edges. The annealing at 300 °C for 3 h has no significant influence on the morphology and size of h-MoO3 rods, but improves their crystallinity and phase purity, and results in the expansion along the a axis, the contraction along the c axis, and the slight increase of the volume of unit cell. This could be attributed to the escape of a (46) Briand, L. E.; Jehng, J. M.; Cornaglia, L.; Hirt, A. M.; Wachs, I. E. Catal. Today 2003, 78, 257–268.

Pan et al.

portion of larger water molecules (∼ 3.3 A˚ in diameter) in the tunnel (∼ 3.0 A˚ in diameter). The optical bandgaps for all samples are determined to range from 2.92 to 3.05 eV which are not closely related to the preparation recipes and not markedly influenced by the annealing. The sharper peaks of micro-Raman spectra for the annealed samples result from their better crystallinity compared to those for the as-synthesized ones. The irradiation of both electrons with high energies and laser with high powers causes the irreversible switch of h-MoO3 to R-MoO3. The turning points on both the DTA and TG curves corresponding to different temperatures imply the presence of water molecules with different interactions with h-MoO3, i.e., the water molecules physically adsorbed on the surface, the ones with weak interactions, and the ones in the tunnel. The temperatures at which the phase transition occurs are determined to range from 413 to 436 °C. The peak temperatures of h-MoO3 catalyzing soot oxidation are derived to lie between 482 and 490 °C, significantly higher than its transition temperatures, which makes it simply suitable for catalyzing the reactions taking place at temperatures lower than the transition temperatures. Acknowledgment. This work was funded by the Ministry of Science and Technology of China, Project 973 under Grant 2006CB932602. Supporting Information Available: Four figures and three tables (pdf). This material is available free of charge via the Internet at http://pubs.acs.org.