Scientific paper
Synthesis and Structural Characterization of Nano-sized
Copper Tungstate Particles
Dragana J. Jovanovic, Ivana Lj. Validzic, Miodrag Mitric and Jovan M. Nedeljkovic*
Vinca Institute of Nuclear Sciences, University of Belgrade, P.O. Box 522, 11001 Belgrade, Serbia
* Corresponding author: E-mail: jovned@vinca.rs; phone:+(381)112438906; fax:+(381)112447382
Received: 09-03-2011
Abstract
The nano-sized copper tungstate (CuWO4) was prepared by precipitation method in the presence of non-ionic copolymer surfactant (polyoxyethylene-polyoxypropylene block copolymer) and consequent annealing at low temperature (400 °C). The scanning electron microscopy (SEM) indicated formation of spherical CuWO4 particles in the size range from 10 to 90 nm. The thermogravimetric analysis was used to study dehydration processes. The X-ray diffraction analysis undoubtedly confirmed formation of triclinic CuWO4 and the refinement of the diffraction data showed that CuWO4 powder belongs to the distorted tungstate type of structure with space group P1. The structure of the CuWO4 can be described as infinite zigzag chains formed by edge-sharing alternating [W-O6] and [Cu-O6] octahedra. Indirect and direct band-gap energies of CuWO4 (2.3 and 3.5 eV, respectively) were determined using optical measurements.
Keywords: Copper tungstate; rietveld refinement; optical properties; block copolymer
1. Introduction
Transition metal tungstates are important family of inorganic materials that have a significant application potential in various fields.1 Copper tungstate (CuWO4) is a well-known semiconductor with potential technological applications in scintillator detectors, laser hosts, photoanodes, optical fibers, etc.2-4 Because of that, a wide range of studies such as photoelectrochemical investigations,5 crystal growth,6,7 electrical and electrochemical characterizations,8,9 have been performed.
The most common techniques of forming CuWO4 are solid-state synthesis and liquid precipitation method. Solidstate synthesis of CuWO4 was achieved by heating an intimate mixture of equimolar proportions of CuO and WO3,10-12 as well as CuCl2 and Na2WO4 up to temperatures of 850 °C.7 In solution, hydrated CuWO4 can be obtained by precipitation from corresponding salts, followed by annealing at temperatures up to 800 °C in order to obtain dehydrated crystalline CuWO4.11-13 It should be kept in mind that limiting temperature for synthesis of CuWO4 is 935 °C, i.e., starting temperature of thermal degradation of CuWO4.14
In this paper, we used colloidal chemistry approach for preparation of hydrated CuWO4. The stabilizer (poly-
oxyethylene-polyoxypropylene block copolymer) was present in solution in order to suppress particle's growth from nano to micron size domain. To the best of our knowledge, the soft solution methods that include the assistance of block copolymers have been applied only for other tungstates.15,16 In the second step, annealing of hydrated CuWO4, performed at 400 °C, led to the formation of crystalline CuWO4 powder. Structural and optical characterization of the CuWO4 powder was performed. Special attention was paid to the refinement of the crystal structure of the synthesized material.
2. Experimental
All chemicals (Na2WO4 ■ 2H2O (99% Riedel-de Haen), CuCl2 ■ 2H2O (99% Merck), non-ionic copolymer surfactant Pluronic F68 (Polyoxyethylene-polyoxypropy-lene block copolymer, Mn ~ 8400 (Aldrich)) were of the highest purity available and they were used without further purification.
Typically, 50 ml of 0.1 M Na2WO4 ■ 2H2O solution was mixed with 100 ml of copolymer solution (10 g/L). The pH of the solution was adjusted to 3 using concentrat-
ed HCl solution. Then, under vigorous stirring, 50 ml of 1.5 M CuCl2 ■ 2H2O was added drop by drop. After that, the mixture was refluxed at 80 °C for 90 minutes. During the reflux, precipitation of hydrated CuWO4 took place. The hydrated CuWO4 was separated from solvent containing copolymer immediately after synthesis by using ultra-centrifugation, then washed several times with ethanol and distilled water, and finally annealed at 400 °C for 48 hours in order to produce golden yellow crystalline CuWO4 powder.
It should be emphasized that identical synthetic procedure was used for a wide range of concentration ratio between reactants, but the pure CuWO4 phase was obtained in reaction with huge excess of Cu2+ ions compared to WO42- ([Cu2+]/[WO42-] = 15). The appearance of small amounts of impurities was noticed at equimolar concentrations of reactants or with slight excess of Cu2+ ions. Tungsten (VI) oxide was main impurity in the case of equimolar concentration of reactants, while with the increase of excess of Cu2+ ions sodium pyrotungstate appears to be a characteristic impurity.
The scanning electron microscopy (SEM) was performed using JEOL JSM-6460LV instrument (Tokyo, Japan). The CuWO4 sample was coated with thin layer of gold deposited by sputtering process.
Thermogravimetric (TG) and differential thermal analysis (DTA) measurements in air atmosphere were performed on dried, but not annealed CuWO4 sample using SETARAM SETSYS Evolution-1750 instrument (heating rate 10 °C min1).
The X-ray Powder Diffraction (XRPD) patterns of investigated samples were obtained on a Philips PW-1050 automated diffractometer using Ni-filtered CuKa radiation (operated at 40 kV and 30 mA). A fixed 1° divergence and 0.1° receiving slits were used. Diffraction data for structural analysis were collected in the 20 range from 10 to 120°, with 0.02° steps and 12s exposition per step. Structural analysis was performed by using the KOALA-RIE computing program based on the Rietveld full profile refinement method.17,18 Samples for XRPD measurements were prepared using the standard protocol.19
The absorption spectra of dispersed CuWO4 particles in water were measured using Thermo Scientific Evolution 600 UV-Vis spectrophotometer.
3. Results and Discussion
Typical SEM image of the CuWO4 powder annealed at 400 °C for 48 hours is shown in Figure 1. The CuWO4 particles are mostly spherical in the size range from 10 to 90 nm. So far, synthesis of agglomerated micrometer in size copper tungstate particles was reported,11,13,20 but, recently, Sen21 and Montini22 developed synthetic procedure for preparation of CuWO4 in nano- size regime. The poly-oxyethylene-polyoxypropylene block copolymer have
been used to synthesize very small noble metal nanoparti-cles,23,24 so that its presence on the particle surface prevented growth of CuWO4 from nano to micron size domain.
Figure 1. Typical SEM image of the CuWO4 particles.
Simultaneously measured, TG and DTA curves of dried, but not annealed, CuWO4 sample are shown in Figure 2. The total mass loss observed at temperatures higher than 400 °C is slightly smaller than 12%. At low temperatures (below 200 °C), the TG measurements reveled that the mass loss is little bit smaller than 10%. Assuming the following mechanism of dehydration process of CuWO4 ■ 2H2O:
CUWO4 ■ 2H2O(s}:
: CUWO4 ■ H2O(s) + H2O(g) (1)
CUWO4 ■ H2O(s) = CuWO4(s) + H2O(g)
(2)
the observed mass loss corresponds well to the calculated value of the mass loss for dehydration of two water molecules (10.38%). The additional mass loss at higher tem-
Figure 2. TG and DTA curves of copper tungstate dehydrate (CuWO4 ■ 2H2O).
peratures is most likely consequence of the oxidation in air traces of copolymer.
The DTA data are in agreement with TG measurements. Strong endothermic peak (around 100 °C) accompanied with shoulder (around 200 °C) corresponds to the dehydration of two water molecules, while two exothermic peaks at higher temperatures (around 250 and 400 °C) are most likely due to oxidation of copolymer traces.
Solid material obtained in the reflux reaction between sodium tungstate with huge excess of copper (II) chloride, in the presence of non-ionic copolymer surfactant (polyoxyethylene-polyoxypropylene block copoly-mer), thoroughly washed and annealed at 400 °C was analyzed using the XRPD measurements. It should be pointed out that annealing temperature is close to the proposed crystallization temperature of CuWO4 (410 °C).13
The starting parameters in the least-squares refinement were taken from Kihlborg25 and the refinement of the diffraction data showed that CuWO4 powder belongs to the triclinic distorted wolframite type of structure. The intensity data were evaluated assuming the Voigt peak shape, and in this refinement 410 independe-nt reflections were used. The refinement was done in the P1 space group in (Cu, Zn)WO4 structural type where all ions occupy
Figure 3. Final Rietveld plot of the CuWO4 powder.
general crystallographic positions 2i with local symmetry 1. The least squares refinement were made by varying 35 parameters: one parameter for the scale factor, one zero point, six lattice constants, four for the description of the background, 18 fractional ionic coordinates, three isotrop-
Table 1. The structure of CuWO4 with space group P1 and unit cell dimensions a = 4.70887(10) Â, b = 5.84412(12) Â, c = 4.88457(9) Â and a = 91.65183(121), P = 92.50790(124), y = 82.80525(118).
Atom	Position	X(a(X))	Y(o(Y))	Z(a(Z))	B(a(B))
W	2i	0.02049(24)	0.17364(16)	0.25426(26)	0.57(2)
Cu	2i	0.49744(71)	0.65811(46)	0.24645(65)	1.03(6)
O(1)	2i	0.25241(210)	0.34472(126)	0.42174(192)	0.85(10)
O(2)	2i	0.20829(186)	0.87575(128)	0.43315(189)	0.85(10)
O(3)	2i	0.73701(219)	0.37421(138)	0.10117(195)	0.85(10)
O(4)	2i	0.78033(183)	0.90519(129)	0.04858(191)	0.85(10)
Table 2. Interatomic distances (A) and bond angles (degrees) in CuWO4. The standard deviations are in brackets.
CuO6 octahedra
Cu-O(2)	1.99(5)	O(1)-Cu-O(1)	85.7(18)	O(2)-Cu-O(4)	103.4(15)
-O(1)	1.97(5)	O(1)-Cu-O(2)	86.1(17)	O(3)-Cu-O(3)	81.5(17)
-O(3)	1.92(4)	O(1)-Cu-O(3)	77.9(17)	O(3)-Cu-O(4)	92.3(15)
-O(3)	2.00(4)	O(1)-Cu-O(3)	88.3(17)	O(3)-Cu-O(4)	93.4(16)
-O(4)	2.32(4)	O(1)-Cu-O(2)	89.0(19)	O(1)-Cu-O(4)	169.8(14)
-O(1)	2.29(4)	O(1)-Cu-O(3)	86.6(18)	O(1)-Cu-O(3)	167.6(16)
		O(1)-Cu-O(4)	90.7(17)	O(2)-Cu-O(3)	163.7(18)
		O(2)-Cu-O(3)	101.4(18)		
		WO6	octahedra		
W-O(1)	1.86(5)	O(1)-W-O(2)	96.7(18)	O(2)-W-O(4)	81.8(17)
-O(4)	1.83(4)	O(1)-W-O(2)	101(2)	O(3)-W-O(4)	86.8(15)
-O(3)	1.87(4)	O(1)-W-O(3)	101.7(18)	O(3)-W-O(4)	99.0(18)
-O(2)	1.91(5)	O(1)-W-O(4)	98.3(18)	O(4)-W-O(4)	77.2(15)
-O(2)	2.07(4)	O(2)-W-O(2)	72.3(18)	O(1)-W-O(4)	171.0(16)
-O(4)	2.22(3)	O(2)-W-O(4)	75.9(16)	O(2)-W-O(3)	156.4(18)
		O(2)-W-O(4)	92.8(17)	O(2)-W-O(4)	156.8(16)
		O(2)-W-O(3)	89.7(19)		
ic thermal displacement factors, microstrain and crystal size. The distortion in CuWO4 is mainly reflected in the deviation of y from 90°. Although the synthesis was performed in the presence of huge excess of Cu2+ ions, no traces of any other crystalline phase were noticed in the sample due to thorough post-synthetic washing treatment.
Results of the final Rietveld refinements (the unit cell dimensions) are presented in Table 1 and the final Rietveld plot is depicted in Figure 3. The crystallite size and micro strain of the whole pattern are 27.58 nm and 0.005%, respectively. The structure of copper tungstate was refined down to the R-factor of 5.4%. Values of estimated standard deviations as well as reliability factors confirmed that these data are reliable and that structure was well refined. Based on the fixed and refined fraction coordinates of the ions, interatomic distances (metal-oxygen) as well as bond angles (at the metal atoms and the corresponding oxygen-oxygen separation) were determined in CuWO4 octahedrons and presented in Table 2 with their estimated standard deviations. WO6 octahedra are slightly distorted in wolframite with W-O distances ranging from 1.83 to 222k, while the CuO6 octahedra have a pseudo tetragonally elongated geometry with four planar Cu-O distances from 1.92 to 2.00 k and two axial Cu-O distances around 2.3k. The obtained coordinates of CuWO4 are in good agreement with data reported by Kihlborg.25
The CuWO4 is an indirect band gap semiconductor. The most of the band gap energy data were obtained using optical measurements with CuWO4 single crystals.
Two groups of data for direct band gap energy can be found in literature: 3.88 eV and much lower values in the range from 2 to 2.3 eV.8'26'27 Recently, Pandey et al.28 determined both, indirect and direct, band gap energies (1.9 and 2.1 eV, respectively) using CuWO4 thin films prepared from precursor synthesized by precipitation method and annealed in temperature range from 350 to 450 °C. Absorption spectra of colloids consisting of semiconductor nanoparticles that negligible scatter light are frequently used for band gap determination. In order to determine band gap energy, absorption spectrum of slightly turbid CuWO4 dispersion was analyzed using the following well-known relation:
a = k(hv)-1(hv-Eg)n
(3)
Figure 4. Tauc's plots for determination of band gap energies.
where k is the constant, hv is the photon energy and n is equal to 1 for direct band gap and 4 for indirect band gap. Plots of (ahv)2 and (ahv)1/2 vs. (hv) for the CuWO4 are shown in Figure 4. As can be seen, both plots are linear. The values obtained by extrapolating the straight portion to energy axis at zero absorption coefficients gave the direct and indirect band gaps of 3.5 and 2.3 eV, respectively. The obtained band gap energy values are in reasonable agreement with already published data in literature.
To conclude, a new synthetic procedure for preparation of copper tungstate particles in nanometer size domain under mild experimental conditions was developed. Also, experimental conditions for preparation of CuWO4 without impurities were found and structural characterization was performed. Obtained material was optically characterized and direct and indirect band gap energies were determined. The extension of this approach for synthesis of other tungstate family members is under way in our laboratory.
4. Acknowledgments
Financial support for this study was granted by the Ministry of Science and Technological Development of the Republic of Serbia (Projects 172056, 45020 and 45015).
5. References
1.	B. L. Chamberland, J. A. Kafalas, J. B. Goodenough, Inorg. Chem. 1977,16, 44-46.
2.	R. H. Gillette, Rev. Sci. Instrum. 1950, 21, 294-301.
3.	L. G. VanUitert, S. T. Preziosi, J. Appl. Phys. 1962, 33, 2908-2909.
4.	R. Bharati, R. Shanker, R. A. Singh, Pramana, 1980, 14, 449-454.
5.	J. P. Doumerc, J. Hejtmanek, J. P. Chaminade, M. Pouchard, M. Krussanova, Phys. Stat. Solidi (a), 1984, 82, 285-294.
6.	L. G. VanUitert, R. B. Soden, J. Appl. Phys. I960, 31, 328-330.
7.	S. K. Arora, T. Mathew, N. M. Batra, J. Crystal Growth,
1988,	88, 379-382.
8.	S. K. Arora, T. Mathew, N. M. Batra, J. Phys. Chem. Solids,
1989,	50, 665-668.
9.	S. K. Arora, T. Mathew, Phys. Stat. Solidi (a), 1989, 116, 405-413.
10.	M. V. Susie, Y. M. Solonin, J. Mater. Sci. 1988, 23, 267-271.
11.	L. P. Dorfman, D. L. Houck, M. J. Scheithauer, J. N. Dann, H. O. Fassett, J. Mater. Res. 2001, 16, 1096-1102.
12.	O. Y. Khyzhun, T. Strunskus, S. Cramm, Y. M. Solonin, J. Alloys. Compd. 2005, 389, 14-20.
13.	S. M. Montemayor, A. F. Fuentes, Ceramics International, 2004, 30, 393-340.
14.	E. Tomaszewicz, J. Typek, S. M. Kaczmarek, J. Therm. Anal. Calorim. 2009, 98, 409-421.
15.	F. Zhang, M.Y. Sfeir, J. A. Misewich, S. S. Wong, Chem. Mater. 2008, 20, 5500-5512.
16.	S. H. Yu, M. Antonietti, H. Colfen, M. Giersig, Angew. Chem. Int. Ed. 2002, 41, 2356-2360.
17.	R. W. Cheary, A. A. Coelho, J. Appl. Crystallogr. 1992, 25, 109-121.
18.	H. Rietveld, J. Appl. Crystallogr. 1969, 2, 65-71.
19.	V. K. Pecharsky, P. Y. Zavalij, Fundamentals of powder diffraction and structural characterization of materials, Chapter 3 Springer, Berlin, 2005.
20.	Y. Li, S. Yu, Int. Journal of Refracrory Metals & Hard Materials, 2008, 26, 540-548.
21.	A. Sen, P. Pramanik, J. Eur. Cerm. Soc. 2001, 21, 745-750.
22.	T. Montini, V. Gombac, A. Hameed, L. Felisari, G. Adami, P. Fornasiero, Chem. Phys. Lett. 2010, 498, 113-119.
23.	K. Holmberg, J. Coll. Int. Sci. 2004, 274, 355-364.
24.	M. Andersson, V. Alfredsson, P. Kjellin, A. E. C. Palmqvist, Nano Letters, 2002, 2, 1403-1407.
25.	L. Kihlborg, E. Gebert, Acta Cryst B, 1970 26, 1020-1026.
26.	M. Ch. Lux-Steiner, Springer Proceedings in Physics, Polycrystalline Semiconductors II1991, 54, 420-431.
27.	R. L. Perales, J. R. Fuertes, D. Errandonea, D. M. Garcia, A. Segura, EPL, 2008, 83, 37002-37007.
28.	P. K. Pandey, N. S. Bhave, R. B. Kharat, Electrochim Acta, 2006, 51, 4659-4664.
Povzetek
Bakrov volframat nanometrske velikosti smo sintetizirali z obarjalno metodo v prisotnosti površinsko aktivnega kopolimera in segrevanjem pri 400 °C. Z vrstično elektronsko mikroskopijo smo potrdili nastanek okroglih delcev velikosti med 10 nm in 90 nm. S termogravimetrično analizo smo spremljali proces dehidratacije. Z rentgensko analizo smo potrdili nastanek triklinskega CuWO4. Prilagajanje modela eksperimentalnim podatkom je potrdilo, da ima prah strukturo volframatnega tipa s prostorsko skupino P1. Strukturo lahko opišemo kot neskončne verige, sestavljene iz izmeničnih oktaedrov [W-O6] in [Cu-O6] s skupnimi robovi. Z optičnimi meritvami smo določili vrednosti energije prepovedanega pasu (2.3 in 3.5 eV).