J. Phys. Chem. C 2007, 111, 1101-1104
1101
Direct Mechanochemical Synthesis of Nanocrystalline NiWO4 Maria N. Mancheva,*,† Reni S. Iordanova,† Dimitar G. Klissurski,† Georgi T. Tyuliev,‡ and Boris N. Kunev‡ Institute of General and Inorganic Chemistry, Bulgarian Academy of Science, “Acad. G. BoncheV” str., bl.11, Sofia 1113, Bulgaria and Institute of Catalysis, Bulgarian Academy of Science, “Acad. G. BoncheV” str., bl.11, Sofia 1113, Bulgaria ReceiVed: August 7, 2006; In Final Form: October 18, 2006
The possibilities of mechanochemical activation as a successful route for direct synthesis of NiWO4 have been studied. A stoichiometric mixture of NiO and WO3 in a 1:1 molar ratio was subjected to intense mechanical treatment in air using a planetary ball mill for different periods of time. The phase and structural transformations were monitored by X-ray diffraction (XRD), infrared spectroscopy (IR), and X-ray photoelectron spectroscopy (XPS). It was found that 7.5 h of milling of the reagents leads to complete crystallization of single nanostructured phase NiWO4 at room temperature.
Introduction AWO4 (A ) Ni, Co, Cd, Fe, or Cu) compounds are important materials due to their use as hydrotreating catalysts, scintillator detectors, humidity sensors, and solid-state laser hosts and in optical fiber applications.1-5 They crystallize in scheelite- or wolframite-type structures depending of the ionic radius of the bivalent cations.6-8 NiWO4 crystallizes in a wolframite structure, space group P2/c. The coordination around tungsten and nickel atoms is six, giving ribbons of like-filled octahedra sharing edges but unlike octahedra share corners.9 In this way W2O8 structural units are formed in NiWO4 similar to those of the CdWO4 compound.10 Usually single crystalline NiWO4 can be prepared by solidstate synthesis at high temperatures,11,12 by coprecipitation from aqueous solutions of soluble salts,13,14 and by microwaveassisted synthesis.15 This work deals with mechanochemical activation of NiO and WO3 in order to obtain directly nanocrystalline and single-phase products. The advantages of this approach are that the use of voluminous solutions and complicated operations as well as the sintering of the final product can be avoided. Experimental Methods A stoichiometric mixture of reagent grade NiO (BDH) and WO3 (Merck) in a 1:1 molar ratio was subjected to 2.5-10 h of intense mechanical treatment in air using a planetary ball mill (Fritsch No. 7). Both container and balls were of stainless steel. The balls to powder weight ratio was 10:1. For a comparison, direct heating at 800 °C according to ref 12 carried out the solid-state reaction between NiO and WO3 in the same molar ratio. The phase and structural transformations were monitored by X-ray diffraction (XRD) and infrared spectroscopy (IR). Powder XRD patterns were registered at room temperature with a TUR M62 diffractometer using Co KR radiation in the 5° < θ < 55° range. The crystallite sizes were calculated using the Scherrer * To whom correspondence should be addressed. Tel: + 359 2 979 35 88. Fax: +359 2 870 50 24. E-mail:
[email protected]. † Institute of General and Inorganic Chemistry. ‡ Institute of Catalysis.
formula for the (100) peak of NiWO4. Infrared spectra were registered in the range 1200-400 cm-1 on a Nicolet-320 FTIR spectrometer using the KBr pellet technique. The specific surface area of the NiWO4 was measured using a modified BET method. The samples obtained after 2.5, 5, and 10 h of milling time were analyzed by X-ray photoelectron spectroscopy (XPS). The XPS measurements were carried out in the UHV chamber of an ESCALAB-MkII (VG Scientific) electron spectrometer using Mg KR excitation with a total instrumental resolution of ∼1 eV. Energy calibration was performed, taking the C1s line at 285 eV as a reference. Surface atomic concentrations were evaluated using Scofield’s ionization cross-sections with no corrections for λ (the mean free path of photoelectrons) and the analyzer transmission function. Results and Discussion XRD Analysis. Phase identification of the milled sample was carried out using powder X-ray diffraction. Figure 1 shows the XRD patterns of the initial mixture before and after mechanical treatment for different periods of time. The initial XRD patterns exhibit all peaks corresponding to NiO (JCPDS card 78-0429) and WO3 (JCPDS card 43-1035) precursors (Figure 1a). After a milling time of 2.5 h, part of the diffraction lines of NiO and WO3 disappeared and the intensity of some lines decreased. Simultaneously, new diffraction peaks were observed as the principal peaks of the monoclinic NiWO4 phase (JCPDS card 72-1189, d ) 4.60, 3.56, and 2.88 Å). It is an indication that structural transformations in the mixture of solid reagents occur even at the early stage of mechanical activation, and the interaction between NiO and WO3 takes place. Complete interaction was achieved in 7.5 h. The XRD patterns did not show diffraction lines of the initial compounds (Figure 1d). The change in the XRD pattern was not observed after a longer milling time of 10 h, which suggests good structural stability of NiWO4 (Figure 1e). The calculated crystallite size of NiWO4 obtained by mechanochemical treatment was 30 nm with a specific surface area of 3 m2/g. Figure 2 shows the X-ray diffraction patterns of the sample prepared by a classical solid-state reaction. The synthesis was carried out following the procedure described by Jacob.12
10.1021/jp065071k CCC: $37.00 © 2007 American Chemical Society Published on Web 12/08/2006
1102 J. Phys. Chem. C, Vol. 111, No. 3, 2007
Mancheva et al.
Figure 1. XRD patterns of NiWO4 obtained by mechanochemical synthesis: (a) nonactivated sample; (b) mechanical activation for 2.5 h; (c) mechanical activation for 5 h; (d) mechanical activation for 7.5 h; (e) mechanical activation for 10 h. Figure 3. IR spectra of the sample prepared by mechochemical synthesis: (a) nonactivated sample; (b) mechanical activation for 2.5 h; (c) mechanical activation for 5 h; (d) mechanical activation for 7.5 h; (e) mechanical activation for 10 h.
Figure 2. XRD patterns of NiWO4 obtained by solid-state synthesis at 800 °C for different time: (a) for 27 h and (b) for 50 h.
Heating at 800 °C for 27 h led to partial interaction between the initial oxides (Figure 2a). Even the increase in time of the solid-state reaction up to 50 h did not cause full interaction. Some diffraction lines of NiO (JCPDS card 78-0429, d ) 2.41, 2.08, and 1.48 Å) and WO3 (JCPDS card 43-1035, d ) 3.84, 3.76, and 3.64 Å) remained in the XRD patterns (Figure 2b). It is worth nothing that mechanical treatment is more appropriate for the preparation of NiWO4 as compared to the solid-state synthesis. Infrared Analysis. The formation of NiWO4 has been confirmed by IR spectroscopy. In the IR spectrum of the nonactivated sample, there are bands characteristic of WO6 and NiO6 polyhedra building WO3 and NiO oxides, respectively (Figure 3a). The band at 960 cm-1 corresponds to the stretching modes of the WdO terminal bond present in each octahedron of WO3. The bands at 815 and 750 cm-1 are assigned to stretching vibrations of the W-O-W bringing bonds.16,17 The absorption bands below 500 cm-1 correspond to stretching
Figure 4. X-ray photoelectron spectra of the Ni2p level: (a) mechanical activation sample for 2.5 h; (b) mechanical activation sample for 5 h; (c) mechanical activation sample for 10 h.
vibrations of the NiO6 polyhedra building NiO oxide.18,19 After mechanical activation (2.5 h), the band at 960 cm-1 disappears, the bands at 815 and 750 cm-1 are broadened, and there intensity decreases (Figure 3b). This fact is a result of destruction of the long-range order of the reagents. The appearance of bands at 870, 685, and 530 cm-1 evidence that during mechanical activation formation of new bonds starts. Figure 3, panels c and d, presents the IR spectrum, which is typical of NiWO4.20 The
Synthesis of Nanocrystalline NiWO4
J. Phys. Chem. C, Vol. 111, No. 3, 2007 1103
TABLE 1: Binding Energies of NiWO4 Samples Obtained after Different Milling Times (in eV)a W4f7/2 sample NiWO4 obtained after 2.5 h milling time. NiWO4 obtained after 5 h milling time. NiWO4 obtained after 10 h milling time.
O1s
Ni2p3/2 LBEb
HBEc
LBEb
HBEc
856.3
36.4 (77%) 36.4 (80%) 36.41 (90%)
531.1 (93%) 531.2 (92%) 531.25 (79%)
533.1 (7%) 533.1 (8%) 533.5 (10%) 532.8 (11%)
856.1 856.2
35.46 (23%) 35.44 (20%) 35.40 (10%)
a The values in parentheses indicate the relative amount of different components. b Lower binding energy component (LBE). c Higher binding energy component (HBE).
TABLE 2: Surface Composition (at. %) Estimated from the XPS Intensities sample NiWO4 obtained after 2.5 h milling time. NiWO4 obtained after 5 h milling time. NiWO4 obtained after 10 h milling time.
Ni (%) (from Ni2p)
W (%) (from W4f)
O (%) (from O1s)
7
24
69
8
23
69
14
18
67
observed absorption bands are in good agreement with IR data on other crystalline phases belonging to the wolframite structure.10,21,22 The vibrational spectra of compounds containing Me2O8 (Me ) Mo or W) structural units have been analyzed using Factor group analysis and the correlation method.21-23 Taking into account structural data and vibrational spectra of these compounds, we attributed the vibrational bands of the NiWO4. The bands at 870 and 820 cm-1 are due to vibration of the WO2 entity present in the W2O8 groups. The bands at 685 and 620 cm-1 are typical of a two-oxygen bridge (W2O2) and are due to the asymmetric stretching of the same units.10,21-23 The vibrations of the NiO6 polyhedra are in the absorption range below 500 cm-1.19 XPS Analysis. The NiWO4 synthesized for various times of mechanical treatment (2.5, 5, and 10 h) was studied by X-ray photoelectron spectroscopy (XPS). The Ni2p3/2, W4f7/2, and O1s binding energy values along with the atomic concentrations evaluated from the peak intensities are presented in Tables 1 and 2. The binding energies of the Ni2p3/2 line in the samples investigated range from 856.3 to 856.1 eV and are in good agreement with the literature data for Ni2+ in mixed oxides.24-26 The W4f line for samples obtained after 2.5 and 5 h of mechanical treatment is broadened slightly asymmertic on the side of the low binding energies and is for that reason presented as the sum of two doublets having values for W4f7/2 of 36.41 and 35.46 eV. This indicates that, in addition to the 6+ oxidation state of tungsten, 5+ also exists with lower binding energy.24,27-30 According to the X-ray phase analysis (XRD), NiWO4 obtained after 2.5 and 5 h of mechanical treatment contains some amounts of unreacted WO3 and NiO. The amount of W ions in 5+ oxidation state is 23, 20, and 10% after 2.5, 5, and 10 h milling time, respectively. The WO3 that has not reacted is probably present as a thin film on the surface of the NiWO4 particles formed after a shorter milling time (2.5 and 5 h).30 The symmetrical W4f line of NiWO4 obtained after a longer mechanical treatment (10 h) indicates complete interaction between WO3 and NiO oxides. Under equal experimental conditions, a standard WO3 sample is not reduced. Sun et al.30 reported partial reduction to take place in WO3 as a thin film. The W ions in the 5+ oxidation state may presumably be due
Figure 5. X-ray photoelectron spectra of the W4f level: (a) mechanical activation sample for 2.5 h; (b) mechanical activation sample for 5 h; (c) mechanical activation sample for 10 h.
Figure 6. X-ray photoelectron spectra of the O1s level: (a) mechanical activation sample for 2.5 h; (b) mechanical activation sample for 5 h; (c) mechanical activation sample for 10 h.
to partial reduction under the effect of X-rays during the XPS analysis and/or due to the formation of defects in the structure of NiWO4. X-ray photoelectron spectroscopy has confirmed that the W ions are of a high oxidation degree in NiWO 4 synthesized by direct mechanochemical treatment. As one can see from the peak fitting in Figure 5, panels a and b, the O1s line exhibits a higher binding energy shoulder, assigned to lowly charged oxygen and/or surface defects or adsorbed water.31,32 The O1s line of the sample obtained by 10 h of milling time is present as the sum of three lines (Figure 5c). The third line centered at 532.8 eV may be assigned to formation of a defect structure in the NiWO4 due to the excessive mechanical treatment.
1104 J. Phys. Chem. C, Vol. 111, No. 3, 2007 As one can see from Table 2 XPS data confirmed present of the some amounts of unreacted WO3 in the samples prepared by 2.5 and 5 h of milling time. The ratio between Ni, W, and O for the sample obtained after 10 h of milling time is very close to the stoichiometic for NiWO4. Conclusion It is established that mechanochemical treatment of NiO and WO3 is very appropriate for the synthesis of nickel tungstate. The mechanical activation led to precursors that were more reactive and capable of faster chemical reactions. Monophase and nanocrystalline NiWO4 powders are successfully synthesized after 7.5 h of milling time at room temperature. References and Notes (1) Carel, W.; Eijk, E. Nucl. Instrum. Methods Phys. ReV. A 1997, 392, 285. (2) Sundaram, R.; Nagaraja, K. S. Mater. Res. Bull. 2004, 39, 557. (3) Wang, H.; Medina, F. D.; Zhou, Y. D.; Zhang, Q. N. Phys. ReV. B 1992, 45, 10356. (4) Quintana-Melgoza, J. M.; Gomez-Cortes, A.; Avalos-Borja, M. React. Kinet. Catal. Lett. 2002, 76, 131. (5) Scheffer, B.; Molhoek, P.; Moulijn, J. A. Appl. Catal. 1989, 46, 11. (6) Yu, S. H.; Liu, B.; Mo, M. S.; Huang, J. H.; Liu, X. M.; Qian, Y. T. AdV. Funct. Mater. 2003, 13, 639. (7) Mohosoev, M. V.; Bazarova, J. G. Complex Oxides of Molybdenum and Tungsten with Oxides of I-IV Group Elements; Nauka: Moskow, 1990. (8) Sleight, A. W. Acta Cryst. B 1972, 28, 2899. (9) Smith, G. W.; Ibers, J. Acta Cryst. 1965, 19, 269. (10) Daturi, M.; Busca, G.; Borel, M. M.; Leclaire, A.; Piaggio, P. J. Phys. Chem. B 1997, 101, 4358.
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