Latvian Journal of Chemistry, No 1/2, 2012, DOI: /v

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1 Latvian Journal of Chemistry, No 1/2, 2012, DOI: /v PHOTOCATALYTIC ACTIVITY OF ZnWO 4 NANOPARTICLES PREPARED BY COMBUSTION SYNTHESIS J. Grabis, D. Jankovica, M. Kodols, D. Rašmane RTU Institute of Inorganic Chemistry, 34 Miera str., Salapils, LV-2169, Latvia grabis@nki.lv The influence of the organic fuel (ethylene glycol, citric acid and ethanol) and calcination temperature on the characteristic and photo catalytic activity of ZnWO 4 nanopowders prepared by combustion synthesis have been studied. The pure crystalline zinc tungstate nanoparticles were obtained after calcination at 600 o C independently on the used organic fuel. The highest specific surface area of 23.4 m 2 /g and 27.4 m 2 /g and the smallest crystallite size of 38 nm and 24 nm were obtained using ethylene glycol with and without HNO 3 and additive, respectively. ZnWO 4 nanoparticles with the smallest crystallite size exhibited the higher photodegradation activity of Methylene blue under UV irradiation. Key words: zinc tungstate, photocatalyst, combustion synthesis, methylene blue decomposition INTRODUCTION Nanosize zinc tungstate particles are promising photocatalytic and luminescent materials, X-ray scintillators and flame retardants. Active ZnWO 4 nanoparticles with different size have been prepared by the wet chemical methods such as co-precipitation [1], hydrothermal [2, 3], molten salt [4] or mechanochemical [5] synthesis. Photocatalytic and luminescent activity of the prepared zinc tungstate nanoparticles were shown to depended on their crystallinity, size and specific surface area but results were somewhat contradictory. Accordingly to [1], high photocatalytic activity had the relatively large ZnWO 4 particles with grain size of about nm and specific surface area of 6 m 2 /g prepared by the coprecipitation of Zn(NO 3 ) 2 6H 2 O and Na 2 WO 4 2H 2 O and calcinated at 500 o C for 4 h. The photocatalytic activity of ZnWO 4 nanoparticles prepared by hydrothermal synthesis reached the maximal value after calcination at 450 o C when crystallite size was of nm and specific surface area amounted to m 2 /g [3]. At higher annealing temperature the photocatalytic activity of samples decreased because the specific surface was low and grain size of particles reached nm. Obviously beside grain size and crystallinity the photocatalytic activity of ZnWO 4 nanoparticles depended also on purity and structure defects determined by the preparation method. Among wet chemical preparation methods of nanoparticles, combustion synthesis due to simplicity and energetic efficiency were very promising but data concerning the influence of the process parameters on the ZnWO 4 photocatalytic activity were insufficient. 93

2 In the present work, the influence of the organic fuel used in the combustion synthesis of ZnWO 4 and calcinations temperature on the phase formation, specific surface area, crystallite size and photocatalytic activity were studied. EXPERIMENTAL Commercial tungsten powder (purity 99.9%) was dissolved in H 2 O 2 and mixed with zinc acetate aqueous solution during 15 minutes (Fig. 1). After this, the calculated amounts of citric acid, ethylene glycol or ethanol as organic fuel and HNO 3 (0.7) were added. Separate syntheses were carried out without adding HNO 3. The reactants were stirred and heated in air for 3 4 h up to 250 o C until the burning of the formed viscous gel was started. Calcination at o C for 2 h in air was performed to eliminate the admixture of ash and to improve crystallinity of ZnWO 4. Fig. 1. Preparation of ZnWO 4. Phase and chemical composition of the products were controlled by XRD (D8 Advance, Bruker AXS), X-ray spectroscopy (S4 Pioneer, Bruker AXS). The crystallite size was calculated from broadening of diffraction maxima using software Topas 3 (Bruker AXS). The specific surface area of powders was determined by the argon adsorption desorption method. Photocatalytic activity of ZnWO 4 nanoparticles was evaluated by degradation of Methylene blue (MB) under UV radiation (Hg lamp, 120 W). The aqueous solution of MB ( g/l) with ZnWO 4 nanoparticles was introduced in a quartz reactor. The suspension was treated by ultrasound for 10 min and then it was stirred in dark for 30 min. The MB degradation was studied by irradiation of Hg at the distance of 110 mm for 3 h. The samples of suspension for analysis were taken each 10 min. Before the analysis the ZnWO 4 particles were removed from suspension by centrifugation. The degradation of MB was determined by measuring the light absorption by solution at the wavelength of 662 nm using Jenway-6300 spectrophotometer. The role of UV radiation in degradation of MB was evaluated by illumination of the solution in the absence of nanoparticles. The absorption of MB on the surface of ZnWO 4 nanoparticles was determined by investigation of the change of optical density of solution in dark. 94

3 RESULTS AND DISCUSSION Combustion synthesis resulted in obtaining fine grey powder independently on used organic fuel indicating on presence of ashes. The XRD patterns (Fig. 2) of as-prepared powders by using ethylene glycol or citric acid as organic fuel could be indexed to pure phase of monoclinic sanmartinite and they were in agreement with standard data (ICPDS No ) ZnWO 4 WO Counts, a.u Theta Fig. 2. XRD patterns of the ZnWO 4 nanoparticles prepared by using ethanol (1), citric acid (2) and ethylene glycol (3) as fuel. The high degree of crystallinity of ZnWO 4 could be explained by high temperature of combustion process. The as-prepared powders in the presence of ethanol as organic fuel in accordance with XRD analysis data, besides ZnWO 4 phase contained the negligible amount of WO 3 phase. With the increase of the annealing temperature, the colour of samples became white-yellow, the XRD patterns of ZnWO 4 were sharpened and their intensity increased indicating that ash was burnt out and the possible X-ray amorphous phase was crystallized. All samples calcinated at 600 o C for 2 hours contained only pure crystalline sanmartinite. The crystallite size of the prepared zinc tungstate depended on the calcination temperature, used organic fuel and the ratio Me/fuel (Fig. 3). Increase of the Me/organic fuel ratio in the presence of HNO 3 additive from 0.20 to 0.50 increased crystallite size of ZnWO 4 approximately 1.8 times independently on the nature of the used fuel. The smallest crystallite size of 38 nm and specific surface area of 23.4 m 2 /g were estimated for ZnWO 4 nanoparticles prepared using ethylene glycol as fuel at the ratio Me/fuel = 0.2 and fuel/no 3 = 0.7, and by calcination at 600 o C for 2 hours. The decrease of Me/fuel ratio till 0.1 had the insignificant effect on the change of specific surface area and crystallite size of ZnWO 4 particles prepared with additives of HNO 3 (Table 1). The further decrease of crystallite size to 24 nm and increase of specific surface area up to 27.4 m 2 /g were reached by using alcohol as fuel and without introducing HNO 3 in suspension (Table 1). 95

4 70 Crystallie Cristallie size, nm ethylene glycol citric acid ethanol Ratio Me/organic fuel Fig. 3. Dependence of the crystallite size of ZnWO 4 calcinated at 600 o C for 2 h on the ratio of metal/organic fuel. Table 1. Characteristics of ZnWO 4 nanoparticles prepared by using ethanol as fuel Sample Me/fuel ratio Fuel/NO 3 - ratio Calcination temperature, o C Specific surface area, m 2 /g Crystaliite size, nm without HNO without HNO The specific surface area of ZnWO 4 depended strongly on the calcination temperature (Fig. 4) The increase of the calcinations temperature from 400 o C up to 800 o C causes the decrease of the specific surface area approximately 2 3 times depending on the nature of the used organic fuel. Specific surface area, m 2 /g ethylene glycol citric acid ethanol Calculation temperature, o C Fig. 4. Dependence of ZnWO 4 specific surface area on the calcination temperature and the nature of used organic fuel. Although the decrease of the specific surface area of all the prepared samples was not so dramatic as for powders prepared by combining hydrothermal process followed by annealing treatment [3] where the increase of the 96

5 temperature from 400 o C up to 550 o C caused the reduction of the specific surface area from 43,16 to 3.50 m 2 /g. Obviously, the presence of the ashes in the powders prepared by combustion inhibited the formation of the agglomerates and the grain growth of ZnWO 4 during annealing. The maximum specific surface area was characteristic of the samples prepared in the absence of HNO 3, because in this case the lower burning intensity and the temperature of the combustion process reduced the growth of particles. The photocatalytic activity of the as-prepared ZnWO 4 powders despite the presence of the crystalline sanmartinite phase was low due to the presence of ash. Additional calcination of samples increased the photocatalytic activity gradually. The photocatalytic activity reached the maximum at the temperature of 600 o C, when the admixture of ash was removed and ZnWO 4 was fully crystallized. The photocatalytic activity of calcinated samples depended strongly on the specific surface area value (Fig. 5) and the crystallite size. The 94% and 43% of MB was degradated during 3 h by ZnWO 4 particles having specific surface area of 27.4 and 11.3 m 2 /g, respectively, under UV radiation. The photocatalytic activity of ZnWO 4 possessing specific surface area of 6.6 m 2 /g was very low. Degree of degradation m2/g m 2 /g 25.0 m2/g m 2 /g 17.8 m2/g m 2 /g 11.3 m2/g m 2 /g 6.6 m2/g m 2 /g only UV UV Time, min Fig. 5. Degradation of Methylene blue by ZnWO 4 nanoparticles under UV radiation depending on exposure and the specific surface area. CONCLUSIONS 1. The combustion synthesis ensures preparation of the single phase ZnWO 4 nanoparticles possessing specific surface area in the range of 6 27 m 2 /g and the crystallite size in range of nm after calcination at o C depending on the nature of the used fuel and the Me/fuel ratio. 2. The synthesis of ZnWO 4 nanoparticles using ethylene glycol and HNO 3 or ethanol without the additive of HNO 3 as well as decrease of the ratio Me/fuel from 0.5 to 0.1 causes the reduction the crystallite size to 38 and 24 nm, respectively, after calcination at 600 o C. 3. ZnWO 4 nanoparticles with the highest specific surface area exhibit the highest photocatalytic activity in degradation of MB solution under UV irradiation. R E F E R E N C E S 1. Huang, G., Zhu, Y. (2007). Synthesis and photocatalytic performance of ZnWO 4 catalyst. Mat. Sci. Eng. B, 139,

6 2. Fu-Shan Wen, Xu Zhao, Hua Huo, Lie-Shing Chen, E. Shu-Lin, Lia-Hua Zhang (2002). Hydrothermal synthesis and photoluminiscent properties of ZnWO 4 and Eu 3+ -doped ZnWO 4. Mat. Lett., 55, Huang, G., Zhang, C., Zhu, Y. (2007). ZnWO 4 photocatalyst with high activity for degradation of organic contaminents. J. Alloys Comp., 432, Ran, S., Gao, L. (2006). Synthesis of nanocrystalline ZnWO 4 via molten salt route and its photoluminiscence. Chem Lett., 35 (11), Dodd, A., McKinley, A., Tsuzuki, T., Sounders, M. (2009). Mechanical synthesis of nanoparticulate ZnO ZnWO 4 powders and their photocatalytic activity. J. Eur. Ceram. Soc., 29, DEGŠANĀ IEGŪTU ZnWO 4 NANODAĻIŅU FOTOKATALĪTISKĀ AKTIVITĀTE J. Grabis, D. Jankoviča, M. Kodols, D. Rašmane K O P S A V I L K U M S Pētīta degšanā iegūtā cinka volframāta ZnWO 4 nanodaļiņu veidošanās dažādās kalcinēšanas temperatūrās, lietojot dažādus organiskos degšanas aģentus (etilēnglikols, citronskābe, etanols), un tā fotokatalītiskā aktivitāte. Tīras kristāliskas cinka volframāta nanodaļiņas iegūtas 600 o C temperatūrā, neatkarīgi no lietotā degšanas aģenta. Lietojot par degšanas aģentu etilēnglikolu kopā ar HNO 3 vai etanolu bez slāpekļskābes piedevas, iegūti nanopulveri ar lielu īpatnējo virsmu (23,4 m 2 /g un 27,4 m 2 /g attiecīgi) un maziem kristalītu izmēriem (38 nm un 24 nm). ZnWO 4 nanodaļiņas ar mazākiem kristalītu izmēriem izraisa straujāku metilēnzilā sadalīšanu ultravioletā starojuma iedarbībā. Iesniegts