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Hydrothermal Synthesis of r-MoO3 Nanorods via Acidification of Ammonium Heptamolybdate Tetrahydrate Xiong Wen Lou and Hua Chun Zeng* Department of Chemical and Environmental Engineering, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 Received May 29, 2002. Revised Manuscript Received August 15, 2002
One-dimensional nanostructures of orthorhombic molybdenum trioxide (R-MoO3) have been synthesized in the forms of ribbons or rods via acidification under hydrothermal conditions at 140-200 °C. The reaction path has been revealed with our kinetic investigations, which shows the following sequence: (i) from the starting compound (NH4)6Mo7O24‚4H2O to (ii) formation of intermediate compound ((NH4)2O)0.0866‚MoO3‚0.231H2O, and then to (iii) final R-MoO3 nanoribbons or nanorods in 100% phase purity. The optimal growth temperature is in the range of 170-180 °C under the current experimental settings. At higher reaction temperatures, this transformation can be accelerated, but with poorer crystal morphological homogeneity. It has been found that the dimensions of these rectangular nanorods are about 50 nm in thickness, 150-300 nm (mean value at 200 nm) in width, and a few tens of micrometers in length. The crystal morphology can be further altered with inorganic salts such as NaNO3, KNO3, Mg(NO3)2, and Al(NO3)3. Using an H2S/H2 stream, the above-prepared R-MoO3 nanorods can be converted completely to 2H-MoS2 nanorods at 600 °C. The original rodlike morphology is well-retained, although the aspect ratio of the oxide template is reduced upon the sulfidation treatment.
Introduction In recent years, development of one-dimensional (1D) materials has become a focal area in nanostructured materials research, owing to their special characteristics which differ from those of respective bulk crystals.1-15 These highly anisotropic 1D materials include elemental carbon, metals, semiconductor alloys, sulfides, oxides, hydroxides, and so forth.1-15 Among the important * To whom correspondence should be addressed. E-mail: chezhc@ nus.edu.sg. (1) Iijima, S. Nature 1991, 354, 56. (2) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (3) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisato, A. P. Nature 2000, 404, 59. (4) Gudiksen, M. S.; Wang, J.; Lieber, C. M. J. Phys. Chem. B 2001, 105, 4062. (5) Wu, Y.; Yang, P. J. Am. Chem. Soc. 2001, 123, 3165. (6) Tenne, R.; Margulis, L.; Genut, M.; Hodes, G. Nature 1993, 360, 444. (7) Feldman, Y.; Wasserman, E.; Srolovitz, D. J.; Tenne, R. Science 1995, 267, 222. (8) Jun, Y.-W.; Lee, S.-M.; Kang, N.-J.; Cheon, J. J. Am. Chem. Soc. 2001, 123, 5150. (9) Ajayan, P. M.; Stephan, O.; Redlich, P.; Colliex, C. Nature 1995, 375, 564. (10) Satishkumar, B. C.; Govindaraj, A.; Vogl, E. M.; Basumallick, L.; Rao, C. N. R. J. Mater. Res. 1997, 12, 604. (11) Satishkumar, B. C.; Govindaraj, A.; Nath, M.; Rao, C. N. R. J. Mater. Chem. 2000, 10, 2115. (12) Aggarwal, S.; Monga, A. P.; Perusse, S. R.; Ramesh, R.; Ballaaarotto, V.; Williams, E. D.; Chalamala, B. R.; Wei, Y.; Reuss, R. H. Science 2000, 287, 2235. (13) Huang, M. H.; Wu, Y. Y.; Feick, H.; Tran, N.; Weber, E.; Yang, P. D. Adv. Mater. 2001, 13, 113. (14) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Webber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (15) Li, Y.; Sui, M.; Ding, Y.; Zhang, G.; Zhuang, J.; Wang, C. Adv. Mater. 2000, 12, 818.
layered transition metal oxides and chalcogenides, V2O5, WO3, MoO3, MoS2, and WS2 have been extensively investigated. The interest in fabricating these layered materials into 1D nanostructures is based on their many technological applications. For example, MoO3 and its derivatives are widely used in industry as catalysts, display devices, sensors, smart windows, lubricants, battery electrodes, and nanostructured materials in the latest context.16-33 In particular, MoO3 has been prepared into the forms of carbon-metal-oxide nanocom(16) Sabu, K. R.; Rao, K. V.; Nair, C. G. R. Indian J. Chem. 1994, 33B, 1053. (17) Fournier, M.; Aouissi, A.; Rocchiccioli-Deltcheff, C. J. Chem. Soc., Chem. Commun. 1994, (3), 307. (18) Hayashi, H.; Sugiyama, S.; Masaoka, N.; Shigemoto, N. Ind. Eng. Chem. Res. 1995, 34, 137. (19) Gu¨nther, S.; Marsi, M.; Kolmakov, A.; Kiskinova, M.; Noeske, M.; Taglauer, E.; Mestl, G.; Schubert, U. A.; Knozinger, H. J. Phys. Chem. B 1997, 101, 10004. (20) Takenaka, S.; Tanaka, T.; Funabiki, T.; Yoshida, S. J. Phys. Chem. B 1998, 102, 2960. (21) Thorne, R. E. Phys. Today 1996, 49, 42. (22) Tagaya, H.; Ara, K.; Kadokawa, J. I.; Karasu, M.; Chiba, K. J. Mater. Chem. 1994, 4, 551. (23) Wang, J.; Rose, K. C.; Lieber, C. M. J. Phys. Chem. B 1999, 103, 8405. (24) Carcia, P. F.; McCarron, E. M., III. Thin Solid Films 1987, 155, 53. (25) Yang, Y. A.; Cao, Y. W.; Loo, B. H.; Yao, J. N. J. Phys. Chem. B 1998, 102, 9392. (26) Kerr, T. A.; Leroux, F.; Nazar, L. F. Chem. Mater. 1998, 10, 2588. (27) Tachibana, H.; Yamanaka, Y.; Sakai, H.; Abe, M.; Matsumoto, M. Chem. Mater. 2000, 12, 854. (28) Brenner, J.; Marshall, C. L.; Ellis, L.; Tomczyk, N.; Heising, J.; Kanatzidis, M. Chem. Mater. 1998, 10, 1244. (29) Ollivier, P. J.; Kovtyukhova, N. I.; Keller, S. W.; Mallouk, T. E. Chem. Commun. 1998, 1563. (30) Chhowalla, M.; Amaratunga, G. A. J. Nature 2000, 407, 164.
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posites, nanotubes, and nanorods using carbon nanotubes (CNTs) as a host material or template.9-11 Nanorods of orthorhombic MoO3 (R-MoO3) with the dimensions of 80-150 nm in diameter and 5-15 µm in length have been prepared at 600 °C with a CNT template method.11 Very recently, a template-directed synthesis of R-MoO3‚H2O (water intercalated in the interlayer space of R-MoO3) has been developed using lamellar molybdenum oxide amines (with long alkyl chains) as precursors. The as-prepared R-MoO3‚H2O or anhydrous MoO3 (after removal of water at 400 °C in air) nanofibers obtained with this novel method are up to 15-µm long with their diameters ranging from 50 to 150 nm.34 There are two basic polytypes of MoO3. The first one, orthorhombic MoO3 (R-type), is a thermodynamically stable phase, and the second one, metastable monoclinic MoO3 (β-type), has a ReO3-type structure. The most important structure characteristic of R-MoO3 is its structural anisotropy,35,36 where highly asymmetrical MoO6 octahedra are interconnected with their edges along [001] direction and interlinked with their corners along [100], resulting in a so-called double-layer planar structure. An alternate stack of these double-layered sheets along [010] will lead to the formation of R-MoO3, where van der Waals interactions are the major binding means among the piled sheets.35 One might further take advantage of the intrinsic structural anisotropy of R-MoO3 for the synthesis of other low-dimensional nanostructures.37-39 In contrast, the structural anisotropy within a single “molecular” sheet of R-MoO3 does not exist in many layered compounds. For example, graphite, boron nitride, metal hydroxides, and transition-metal dichalcogenides are structurally isotropic for their “molecular” sheets.40 Among them, MoS2 (with two major polytypes: hexagonal 2H-MoS2 and rhombohedral 3R-MoS2) is an important transition-metal dichalcogenide worth being explored further for its related low-dimensional nanostructures. It has been well-known that MoS2 can be converted efficiently from pristine MoO3 solid particles or vaporphase clusters by sulfurization,41-48 for instance, using (31) Remskar, M.; Mrzel, A.; Skraba, Z.; Jesih, A.; Ceh, M.; Demsar, J.; Stadelmann, P.; Levy, F.; Mihailovic, D. Science 2001, 292, 479. (32) Chen, J.; Kuriyama, N.; Yuan, H.; Takeshita, H. T.; Sakai, T. J. Am. Chem. Soc. 2001, 123, 11813. (33) Zak, A.; Feldman, Y.; Lyakhovitskaya, V.; Leitus, G.; PopovitzBiro, R.; Wachtel, E.; Cohen, H.; Reich, S.; Tenne, R. J. Am. Chem. Soc. 2002, 124, 4747. (34) Niederberger, M.; Krumeich, F.; Muhr, H.-J.; Mu¨ller, M.; Nesper, R. J. Mater. Chem. 2001, 11, 1941. (35) Kihlborg, L. Ark. Kemi 1963, 21, 357. (36) (a) Chen, M.; Waghmare, U. V.; Friend, C. M.; Kaxiras, E. J. Chem. Phys. 1998, 109, 6854. (b) Queeney, K. T.; Friend, C. M. J. Phys. Chem. B 2000, 104, 409. (37) Zeng, H. C. Inorg. Chem. 1998, 37, 1967. (38) Zeng, H. C.; Sheu, C. W.; Hia, H. C. Chem. Mater. 1998, 10, 974. (39) Balakumar, S.; Zeng, H. C. J. Cryst. Growth 1998, 194, 195. (40) Sampanthar, J. T.; Zeng, H. C. J. Am. Chem. Soc. 2002, 124, 6668. (41) Dickinson, R. G.; Pauling, L. J. Am. Chem. Soc. 1923, 45, 1466. (42) Bell, R. E.; Herfert, R. E. J. Am. Chem. Soc. 1957, 79, 3351. (43) Jellinek, R.; Bauerand, G.; Muller, H. Nature 1960, 185, 376. (44) Wickman, F. E.; Smith, D. K. Am. Mineral. 1970, 55, 1843. (45) Viswanath, R. N.; Ramasamy, S. J. Mater. Sci. 1990, 25, 5029. (46) Tenne, R.; Homyonfer, M.; Feldman, Y. Chem. Mater. 1998, 10, 3225, and references therein. (47) Cristol, S.; Paul, J. F.; Payen, E.; Bougeard, D.; Clemendot, S.; Hutschka, F. J. Phys. Chem. B 2000, 104, 11220.
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a reducing stream of H2S/H2 or H2S/H2/N2 gas at elevated temperatures.49-52 If 1D nanorods of MoO3 (layer anisotropic) could be prepared, it would then imply that a further extension to the preparation of 1D nanorods of MoS2 (layer isotropic) is plausible because the 1D MoO3 can be treated as a solid precursor for the subsequent sulfidation. In this article, as a part of our recent efforts on the investigation of low-dimensional materials,40,53 we report a simple hydrothermal method for preparation of nanostructured R-MoO3 with morphology control on either ribbons or rods at only 140200 °C. 2H-MoS2 nanorods indeed can be prepared from the resultant MoO3 nanorods through sulfidation conversion. Experimental Section Materials Preparation. The R-MoO3 nanoribbons and nanorods were synthesized by a hydrothermal route. Ammonium heptamolybdate tetrahydrate (AHM; (NH4)6Mo7O24‚ 4H2O) and nitric acid were the two starting reagents. A saturated solution of precursor compound AHM was prepared at room temperature and acidified to pH around 5 using a 2.2 M nitric acid solution. (Note that Mo7O246- is predominant isopolymolybdate in the pH range of 4.5-6 at room temperature.) There were two types of the acidified AHM precursor solutions. The first one was used immediately after preparation (“fresh” solution). The second type of acidified precursor solution was stored at room temperature for ≈1 month in a tightly capped volumetric flask before use (“aged” solution). For each run of experiment in Table 1, 5.0-10.0 mL of the above solution was diluted with deionized water (0-15.0 mL) and then further acidified using the 2.2 M nitric acid (5.010.0 mL) to a total of 15.0-30.0 mL in volume. The resultant solution, together with some white precipitate, was transferred to a Teflon-lined stainless steel autoclave and heated at 140200 °C for 5-62 h. The product precipitate was filtered and rinsed with deionized water, followed by drying at 62 °C for 5 h. The dried sample was fibrous and pale yellowish. The above as-grown R-MoO3 crystals were further used as metal oxide precursors for sulfidation investigation. The reactions were carried out in a three-necked flask reactor (350-400 °C) or a tubular quartz reactor (i.d. ) 4 mm, 400600 °C) using a H2S/H2 stream (H2S 5 mol % + H2 95 mol %) under normal atmospheric pressure. A small gas flow rate of 20-40 mL min-1 (Brooks 5950) was used to reduce a concentration gradient of the input H2S/H2 stream across the sample zone (40 mg of the prepared R-MoO3). The temperature of the reactors was monitored with a K-type thermocouple. The offgas was introduced to a ZnSO4 solution to remove unreacted H2S before it was vented into the atmosphere. Materials Characterization. Crystallographic information of samples including sulfurized ones was investigated with X-ray diffraction (XRD; Shimadzu XRD-6000, Cu KR, λ ) 1.5406 Å). Crystal sizes of the samples were estimated from full-width at half-maximums (fwhm’s) of some intense XRD diffraction peaks using Scherrer’s method.54 Elemental analysis for C, H, and N contents in an intermediate crystal phase (48) Park, K. T.; Kong, J.; Klier, K. J. Phys. Chem. B 2000, 104, 3145. (49) Whitehurst, D. H.; Isoda, T.; Mochida, I. Adv. Catal. 1998, 42, 345. (50) Kushmerick, J. G.; Kandel, S. A.; Han, P.; Johnson, J. A.; Weiss, P. S. J. Phys. Chem. 2000, 104, 2980. (51) Feldman, Y.; Frey, G. L.; Homyonfer, M.; Lyakhovitskaya, V.; Margulis, L.; Cohen, H.; Hodes, G.; Hutchison, J. L.; Tenne, R. J. Am. Chem. Soc. 1996, 118, 5362. (52) Zak, A.; Feldman, Y.; Alperovich, V.; Rosentsveig, R.; Tenne, R. J. Am. Chem. Soc. 2000, 122, 11108. (53) (a) Xu, R.; Zeng, H. C. Submitted to J. Phys. Chem. B. (b) Wei, X. M.; Zeng, H. C. Submitted to Chem. Mater. (54) Cheetham, A. K.; Day, P. Solid-State Chemistry: Techniques; Clarendon Press: Oxford, 1987; p 79.
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Table 1. Conditions of Some Representative Experiments Investigated in This Work run no.
precursor (mL)
DI water (mL) + salt
HNO3 (mL)
temp (°C)
time (h)
pH (after reaction)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
10 (fresh)a 10 (fresh) 10 (aged)c 10 (aged) 10 (fresh) 10 (aged) 0.6 gd 10 (aged) 10 (aged) 10 (aged) 5 (aged) 5 (aged) 5 (aged) 5 (aged) 5 (aged) 5 (aged) 5 (aged) 5 (aged) 10 (aged) 10 (aged) 10 (aged)
15 13 15 10 10 10 10 10 10 10 10 0 0 0 0 0 0 + 5 ge 0 + 15 g 10 + 4 g 10 + 10 g 10 + 20 g
5b 7 5 10 10 10 6 10 10 10 10 10 10 10 10 10 10 10 10 10 10
180 180 180 180 180 140 170 180 180 180 200 200 200 200 200 170 170 170 170 170 170
40 40 30 40 40 24 20 20 30 40 5 7 9 12 16 34 43 62 40 43 50
4.0 2.5 0.5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
a Freshly prepared acidified AHM precursor solution (see Experimental Section). b Nitric acid with a concentration of 2.2 M. c Acidified AHM precursor solution after 1 month’s aging (see Experimental Section). d Crystal sample of intermediate phase (prepared from run 5). e Weight of NaNO3 salt added in the reactions.
was carried out with a Perkin-Elmer 2400 CHN analyzer. Thermogravimetric analysis and differential thermogravimetric analysis (TGA/DTG, Shimadzu TGA-50) was performed to obtain the metal (oxide) content in this intermediate solid compound. The TGA/DTG measurement was carried out at a heating rate of 5 °C min-1 in an inert atmosphere (nitrogen, 50 mL min-1).55,56 The morphology of crystal samples was examined before and after the sulfidation reactions with scanning electron microscopy (SEM, JSM-5600LV, 15 kV). High-resolution analytical transmission electron microscopy (TEM, JEM-2010, 200 kV) was also used to examine crystalline R-MoO3 and its sulfurized products. The specimens for TEM imaging study were prepared by suspending solid samples in acetone.40
Results and Discussion Acidification and Aging of Precursor Solution. To convert isopolymolybdate anions Mo7O246- to neutral Mo7O21 (or R-MoO3), excess divalent oxygen anions must be removed. Stoichiometrically, three divalent oxygen anions per Mo7O246- must be combined with protons from the acidic medium:
Mo7O246- + 6H+ ) 7MoO3 + 3H2O
(1)
In the above overall equilibrium, high concentrations of both Mo7O246- and H+ would shift the reaction to the right, although many intermediate steps (and thus compounds/phases) may exist. It is thus anticipated that the formation of R-MoO3 should have a strong dependence on the acid concentration (protons provided) and reaction time. Figure 1 displays a set of XRD patterns accounting for these tested samples (Table 1). For run 1 to run 3, the final pH values of the reacted solutions are generally (55) Ji, L.; Lin, J.; Zeng, H. C. J. Phys. Chem. B 2000, 104, 1783. (56) Xu, Z. P.; Zeng, H. C. Chem. Mater. 2001, 13, 4564.
Figure 1. XRD patterns of solid products prepared under different experimental conditions (Table 1). Patterns of runs 4 and 7 are standard R-MoO3 phase, while those of runs 5 and 6 are single-phase hexagonal ((NH4)2O)x‚MoO3‚yH2O.
greater than 1, which indicates that most of the acid added has been consumed. In this agreement, the XRD patterns reveal that the solids have not been converted to R-MoO3 phase yet, except in run 3, where the characteristics of R-MoO3 are emerging.35,57 While no R-MoO3 diffraction peaks are identified in the samples prepared from run 1 and run 2, four main peaks in the patterns are numbered for the convenience of discussion. It can be concluded that peaks 1, 3, and 4 and peaks 2 and 3 belong to two different phases because their relative intensities change as more protons are provided (more nitric acid in run 2, Table 1). This type of XRD pattern still shows common features of ammonium molybdates and molybdenum oxides (including R-MoO3),57 noting that peak 3 is observed in all the patterns. The history of AHM precursor solution is crucial in making final R-MoO3, which is compared in the experiments of run 1 and run 3. Using an aged AHM precursor solution, R-MoO3 can be formed with a shorter reaction time while other reaction conditions were kept identical. For run 4 to run 7, pH values of solution after reaction indicate a strong acidic synthetic condition. In Figure 2, two crystal morphologies from run 4 and run 5 are shown. It is clearly evidenced that phase-pure nanorods of R-MoO3 can be prepared with an aged precursor solution whereas an intermediate phase of ammonium molybdates (the assignment of this phase will be given in next subsection) is formed with a freshly prepared precursor solution under exactly the same hydrothermal conditions (Table 1). The as-prepared R-MoO3 exhibits rodlike morphology with the width at around 200 nm. A close examination on the growing tips indicates that (57) Joint Committee on Powder Diffraction Standards, International Centre for Diffraction Data, Card No. 35-0609, Swarthmore, PA, 1996.
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Figure 3. TGA scan for the intermediate crystal phase ((NH4)2O)x‚MoO3‚yH2O (from run 5) with a heating rate of 5 °C/min under N2 at a flow rate of 50 mL min-1.
Figure 4. XRD patterns of solid products prepared from runs 8-10 (180 °C, Table 1), with standard references of the intermediate crystal phase ((NH4)2O)x‚MoO3‚yH2O and R-MoO3 phase.
Figure 2. SEM crystal morphologies of samples prepared under the same experimental condition but with different histories of acidified AHM precursor solutions ((a) run 4 versus (b) run 5, Table 1).
an overall rectangular morphology (i.e., single crystal facets) is well-maintained, which will be further addressed in later TEM investigations. The detected intermediate compound from runs 5 and 6 is in the form of hexagonal prismatic morphology. This single-crystalline phase, in fact, is formed throughout the acidification reactions, and it can be traced back in the solid mixtures generated in runs 1-3 (XRD patterns, Figure 1). In experiment run 7, as a further example, the crystal phase (from run 5) is transformed completely into final R-MoO3 nanorods. Identification of Intermediate Phase. The structure and composition of the intermediate compound (run 5) were determined with CHN elemental analysis, TGA technique, and XRD method. As this compound was produced in a highly oxidative environment (in the presence of HNO3), it is reasonable to assume the highest oxidation state of VI for molybdenum is maintained throughout the reactions. Considering thermal decomposition of AHM and the aqueous environment in the synthesis, a general formula of (NH4)2O‚mMoO3‚ nH2O or ((NH4)2O)x‚MoO3‚yH2O can be employed according to many known compounds of this type in the literature.58,59 Indeed, the diffraction pattern of run 5 sample (as well as the same diffraction patterns in other samples) can be assigned perfectly to the ((NH4)2O)x‚ MoO3‚yH2O phase (hexagonal system: space group P63/
m).58 The values of m and n determined from CHN results are m ) 11.55 and n ) 2.67 or x ) 0.0866 and y ) 0.231 for easy comparison with the literature data (e.g., x ) 0.0892 and y ) 0.312).58 This compositional analysis was further verified with TGA. As shown in Figure 3, TGA scan indicates a weight loss of 5.66% over 60-450 °C (excluding physical adsorption of water before 60 °C), which is identical to the theoretical value of 5.680%, noting that the compound is converted completely to R-MoO3 at 420 °C. Therefore, the determined formula for this intermediate phase is evidently correct. Kinetic Investigations on the Growth Process. This subsection starts to investigate the kinetic factors associated with this process. In run 8 to run 10 (Table 1), the temperature was fixed at 180 °C while the reaction time was varied in the range of 20-40 h. From the XRD patterns of Figure 4, it is noted that (020) and (060) reflections for R-MoO3 in run 8 are very weak though detectable.57 The sample from run 8 can then be assigned to a solid mixture comprising ((NH4)2O)0.0866‚ MoO3‚0.231H2O as a predominant phase and R-MoO3 as a minor phase.57-59 The SEM image of Figure 5 indeed shows that the majority of crystallites in this solid mixture are in microscale hexagonal prismatic morphology with some tiny fibrous R-MoO3 needles (58) Joint Committee on Powder Diffraction Standards, International Centre for Diffraction Data, Card No. 83-1175, Swarthmore, PA, 1996. (59) Joint Committee on Powder Diffraction Standards, International Centre for Diffraction Data, Card No. 23-0786, Swarthmore, PA, 1996.
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Figure 6. XRD patterns of solid products prepared from runs 11-15 (200 °C, Table 1). Patterns of runs 11 and 12 include two phases: ((NH4)2O)x‚MoO3‚yH2O and R-MoO3 (with bolded indices).
Figure 5. SEM crystal morphologies of samples prepared from runs 8-10 (180 °C, Table 1): (a) run 8, (b) run 9, and (c) run 10.
scattered among them. For run 9, whose reaction time was increased to 30 h, the (0k0) reflections become very strong, revealing that most of the intermediate compound has been converted to R-MoO3 nanorods, although the peaks of the ((NH4)2O)0.0866‚ MoO3‚0.231H2O phase can still be observed. With a further increase in reaction time to 40 h in run 10, the intermediate phase is no longer observed and R-MoO3 is the sole product formed in the hydrothermal synthesis. The amount of ((NH4)2O)0.0866‚MoO3‚0.231H2O for total R-MoO3 product is mainly determined in the first
part of hydrothermal reactions. This is evidenced with the observation of a nearly constant yield of 15-20% on the basis of molybdenum in these syntheses. Additional direct evidence is that the amount of solid products from run 9 and run 10 (30 h versus 40 h) does not differ much, which indicates that the total R-MoO3 product in these two runs had been predetermined by the amount of ((NH4)2O)0.0866‚MoO3‚0.231H2O formed in the first 20 h (equivalent to run 8). In good agreement with the XRD data, the intermediate crystal phase in run 9 is indeed negligible. As further confirmed in the SEM image shown in Figure 5b, tiny dissolving hexagonal prismatic crystallites are hardly observed. Nonetheless, there is a significant morphological alternation between the two samples (Figure 5b,c). With the reaction time increased to 40 h in run 10, the R-MoO3 fibers become more rigid, showing a rodlike morphology. This change can be referred to as the normal crystallographic dimension of R-MoO3 in the following sequence: [001] > [100] > [010] (A closer examination of these R-MoO3 nanofibers with TEM will be presented shortly).60-62 With this information, the difference in crystal rigidity of the R-MoO3 fibers can be further addressed. In the first 20 h (run 8), the formation of the intermediate crystal ((NH4)2O)0.0866‚MoO3‚ 0.231H2O is largely completed. In the next 10 h (run 9: 20 + 10 h), the growth is along the preferential direction [001] so as to produce long and thin nanoribbons. For the final 10 h (run 10: 30 + 10 h), growths along [100] and [010] become noticeable. As a result, both thicknesses along [010] and widths along [100] of the original nanoribbons are increased, giving more rigid R-MoO3 nanorods as a sole final product in run 10. The average width and length of R-MoO3 ribbons produced after 30 h of reaction are in the ranges of 150450 nm and a few tens of micrometers, respectively (run (60) Hsu, Z. Y.; Zeng, H. C. J. Phys. Chem. B 2000, 104, 11891. (61) Zeng, H. C.; Ng, W. K.; Cheong, L. H.; Xie, F.; Xu, R. J. Phys. Chem. B 2001, 105, 7178. (62) Zeng, H. C.; Xie, F.; Wong, K. C.; Mitchell, K. A. R. Chem. Mater. 2002, 14, 1788.
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Figure 7. SEM crystal morphologies of samples prepared from runs 11-15 (200 °C, Table 1): (a) run 12, (b) details on fiber part in (a), (c) run 13, and (d) run 14.
9). With the additional 10 h for reactions, the width increases only slightly to the range of 180-500 nm with a mean value situated around 300 nm (run 10). As reported earlier, most of the intermediate crystals have been transformed to the R-MoO3 nanoribbons after 30 h, and this fact determines relatively small increases in average width and thickness by ≈30 and 3 nm (a nominal value estimated by XRD method; see final subsection) during the final 10 h of reaction. The second set of kinetic experiments was conducted at 200 °C. Reported in Table 1 and Figure 6 are five experiments (runs 11-15) with shorter reaction times and a higher content of nitric acid. A similar transformation process from the intermediate crystal ((NH4)2O)0.0866‚MoO3‚0.231H2O to the final R-MoO3 can be achieved, despite the reaction time used here being much shorter than that at 180 °C. This phase evolution could be partly attributed to the fact that the reaction rate is increased at higher temperatures. However, it should be noted that the acid content in this set of experiments was double that used in the previous set, which is also a critical factor to reaction kinetics (eq 1). Therefore, the fast transformation observed in Figure 6 stems largely from the above two factors. In this series of samples, coexistence of the intermediate crystals and forming R-MoO3 nanoribbons is elucidated in SEM images of Figure 7. The intermediate product is dissolving, as indicated in bulletlike endings of hexagonal prismatic crystallites (run 12). With a longer reaction time, this transitional phase is no longer observable, leaving only final uniform R-MoO3 nanorods (run 13). As evidenced in run 14 (12 h), the majority of R-MoO3 nanorods are restructured into much shorter crystal platelets with only a few micrometers in length. On the other hand, our further experiments by adding
NaNO3 (0-20 g, runs 16-21, Table 1) to the hydrothermal reactions have indicated some negative effects on the growth under similar reaction conditions, as the R-MoO3 nanorods produced are generally not as uniform in width and easily agglomerated into short bundles. With a small amount of salt added (e.g., NaNO3, 4 g), however, the rectangular R-MoO3 nanorods can be altered to a grass-like morphology with reducing sharp ends. In general, adding inorganic salts to the hydrothermal synthesis will reduce the overall reaction rate and broaden the width distribution of the R-MoO3 rods. Apart from the NaNO3, other nitrate salts including KNO3, Mg(NO3)2, and Al(NO3)3 have also been tested in this work. All of them give very similar effects on the growth. Sulfidation of r-MoO3 Nanofibers. The above R-MoO3 nanorods were further reacted under a H2S/H2 stream at 350-600 °C. This treatment converts R-MoO3 to certain reduced-state compounds such as HxMoO3, MoO3-x, MoO2, MoO2-xSx, and finally MoS2.51,52,61-63 In Figure 8, some of these intermediate phases have been detected. Sulfidation at 350 °C only results in partial conversion of R-MoO3 since prolonged reactions at this temperature do not change the proportion of final products much (pattern A versus pattern B). By comparing pure-phase compounds in the literature,57,64,65 both patterns can be assigned to a transitional solid mixture comprising MoO3-x, MoO2, and MoS2-xOx.64 Similar observation has also been obtained for the sulfidation reactions at 400 °C. The XRD pattern for a sulfurized sample reacted at 600 °C for only 4 h bears (63) de Jong, A. M.; Borg, H. J.; van Ijzendoorn, L. J.; Soudant, V. G. F. M.; de Beer, V. H. J.; van Veen, J. A. R.; Niemantsverdriet, J. W. J. Phys. Chem. 1993, 97, 6477.
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Figure 8. XRD patterns of reduced and/or sulfurized solid products under different sulfidation conditions (pattern A: 350 °C, 5 h, H2S/H2 flow rate 30 mL min-1; pattern B: 350 °C, 10 h, H2S/H2 flow rate 30 mL min-1; pattern C: 600 °C, 10 h, H2S/H2 flow rate 20 mL min-1).
close resemblance to that reacted for 10 h, indicating that MoO3 to MoS2 is greatly accelerated at this temperature. The reflection peaks indexed with (002), (100), (103), and (110) belong to the 2H-MoS2 structure (pattern C).65 The conversion process is presumably similar to the proposed mechanism for gas-phase production of MoS2-nested inorganic fullerenes and nanotubes.51,52 The transformation of MoO3 to 2H-MoS2 commences as adsorbed S atom replaces O of the MoO3, followed by continuous diffusion of H2S through outer layers via defects and gradual substitution of lattice O2- by S2-. Since this transformation is essentially mass-diffusioncontrolled, the reaction temperature has a more profound impact on kinetic processes than reaction time. The direct sulfidation of R-MoO3 in a H2S/H2 atmosphere investigated in this work is to take advantage of the initial ribbon/rod morphologies of R-MoO3 in fabricating one-dimensional MoS2 nanostructures. It has been known that the initial morphology of MoO3 determines the final shapes of IF-MoS2 in a gas-phase reaction.51,52 Sulfidation of small MoO3 particles or supported polymolybdate species usually leads to bulk MoS2 or to clusters comprised of MoS2 slabs. In the present work, the initial morphology of MoO3 is wellretained, which is elucidated in Figure 9. Structural Characterization of MoO3 and MoS2 Nanorods. The R-MoO3 nanocrystallites prepared with this method show an extremely large aspect ratio (defined as length along [001] to width along [100]: L[001]/W[100]) of more than 100 (see Figure 2a and Figure 5b,c). Figure 10 gives a typical TEM image of assynthesized R-MoO3 nanorods (run 16). The elongated platelet-like morphology is further confirmed by a uniform degree of grayness observed in the rectangular nanorod contrast. As indicated, the nanorods are perfectly straight with a constant width. Due to the gravity effect, the majority of nanorods landed selectively on their largest (010) planes under free sedimentation in acetone fluid during TEM specimen preparation (see Experimental Section); i.e., the nanorods in this image (64) Joint Committee on Powder Diffraction Standards, International Centre for Diffraction Data, Card No. 33-0929, Swarthmore, PA, 1996. (65) Joint Committee on Powder Diffraction Standards, International Centre for Diffraction Data, Card No. 37-1492, Swarthmore, PA, 1996.
Figure 9. SEM crystal morphologies of 2H-MoS2 nanorods converted from R-MoO3 nanorods (also refer to pattern C of Figure 8: 600 °C, 10 h, H2S/H2 flow rate 20 mL min-1).
are observed along the [010] direction of R-MoO3. On the basis of image statistics, the width ([100] direction) of the nanorods is ranged from 150 to 300 nm while the majority have a width of 200 nm. A close examination of the nanorod in Figure 10c indicates that there are two tiers of (010) planes in this nanorod. This type of on-plane growth is commonly observed in macro- and microscopic growths of R-MoO3 single crystals.37-39 The nominal dimensions along the [010] axis for the nanoribbons (run 9) and nanorods (run 10) are 38 and 41 nm (with Debye-Scherrer formula), respectively. Our TEM images also are in broad agreement with the above nominal XRD results (as an example, a [100]-standing R-MoO3 nanorod with darker contrast is detailed in Figure 10d). During the conversion of R-MoO3 to 2H-MoS2, the specific area per Mo atom in the respective basal planes changes from 0.0732 nm2/Mo to 0.0865 nm2/Mo.62 This implies that there would be a lateral expansion (18%) when the 2H-MoS2 is generated. Furthermore, it is known that the intersheet distances of R-MoO3 and 2H-MoS2 are 0.693 and 0.617 nm, respectively, which corresponds to an 11% reduction in thickness of the nanorods when this conversion takes place. Figure 11 displays some representative TEM images of the as-sulfurized 2H-MoS2 nanorods. By comparing them with those in Figure 10, it is apparent that the nanorod morphology of the pristine R-MoO3 is still wellretained. The majority of 2H-MoS2 nanorods have a
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Figure 10. TEM images of pristine R-MoO3 nanorods prepared from run 16 (170 °C, Table 1). Scale bars in (b), (c), and (d) all are equal to 0.2 µm.
uniform width and sound crystallinity. In particular, the 2H-MoS2 nanorods have a similar width distribution with the R-MoO3 ones, although the average aspect ratio is greatly reduced. The shortening in [001] direction is due to substantial breakage of long R-MoO3 nanorods during the sulfidation, resulting from an accumulation of lattice expansion along the [001] direction. The thickness of the 2H-MoS2 nanorods is estimated from a leaning nanorod (exceptionally slim, with a dark contrast) in Figure 11a. A magnified image reveals that the thickness of the nanorods is in the range of 50-55 nm (excluding the light-contrasted side which is induced by leaning projection). The side view of 2H-MoS2 nanorods is further examined in Figure 11b along the direction parallel to the (002) planes. The interslab distance is about 0.65 ( 0.04 nm, which is in excellent agreement with the literature data for the bulk 2H-MoS2 (0.62 nm).46-48 Conclusions In summary, with different acidification treatments of precursor compound ammonium heptamolybdate tetrahydrate, nanoribbons or nanorods of orthorhombic molybdenum trioxide (R-MoO3) can be synthesized with excellent morphological homogeneity under hydrothermal conditions at 140-200 °C. On the basis of the findings of this work, the above synthetic process can be further divided into the following stages: the starting acidified (NH4)6Mo7O24‚4H2O is first converted into an intermediate crystal phase ((NH4)2O)0.0866‚MoO3‚ 0.231H2O. This resultant intermediate compound is then decomposed and grown during a prolonged reaction into final products of R-MoO3 nanoribbons or nanorods with 100% phase purity. At higher reaction temperatures (e.g., 200 °C versus 180 °C) and acid content, the time for the above transformation can be significantly shortened, although poorer morphological control is observed. The dimensions of final rectangular nanorods
Figure 11. TEM images of 2H-MoS2 nanorods converted from R-MoO3 nanorods (the same as those in Figure 9). Scale bars in the two inserts are both equal to 100 nm.
Hydrothermal Synthesis of R-MoO3 Nanorods
are about 50 nm in thickness, 150-300 nm in width, and a few tens of micrometers in length under the optimal reaction conditions at 170-180 °C. The aspect ratio of this 1D material is well above 100, and the crystal morphology can be further controlled with a secondary crystal growth (Ostwald-ripening mechanism) and with addition of common nitrate salts such as NaNO3, KNO3, Mg(NO3)2, and Al(NO3)3. Under a H2S/H2 atmosphere, the hydrothermally prepared R-MoO3 nanorods can be converted to pure 2H-MoS2 nanorods via gas-solid reactions at 600 °C for 4 h, upon
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which the original rodlike morphology is well-preserved, although the original aspect ratio of R-MoO3 precursor is reduced pronouncedly, owing to lattice parameter variation between the two solid compounds. Acknowledgment. The authors gratefully acknowledge research funding (R-279-000-064-112 and A/C50384) co-supported by the Ministry of Education and the National Science and Technology Board, Singapore. CM0206237
