The Effect of Sintering Temperature on Mesoporous Structure of WO3 Doped Tio2 Powders

In this study, WO3 doped TiO2 powders were synthesized via sol-gel method combined with a hydrothermal process. The effect of sintering temperature on mesoporous structure and catalytic activities of these powders were investigated. The physical analysis via X-ray diffraction indicates that prepared samples are a mixture of anatase and rutile TiO2 phases. X-ray peak analysis is used to evaluate the crystallite size and lattice strain by the Williamson-Hall analysis. Considering all the reflections of the anatase phase the lattice strain ranging from c = 9.505 to c = 9.548 is calculated, suggesting that microstrain decreases when calcination temperature increases. N2 adsorption-desorption analysis shows that the surface area and pore volume decrease with increasing temperature and that WOxTiO2 powders primarily consist of mesopores. Sintering temperature induced a change in textural properties causing a systematic shift towards larger mesopores. Simultaneously, photoactivity in decolorization of methyl orange increases with increasing calcination temperature up to 700 °C, followed by significant decrease with its further increase.


Introduction
TiO 2 is an excellent material for environmental applications, including photodegradation of various pollutants and purification of water and air [1-3]. Conventional photocatalytic processes for organic decomposition in waste waters use TiO 2 powder as photocatalyst. Finely dispersed TiO 2 particles are usually suspended in wastewater and placed under UV irradiation [4,5]. When TiO 2 is illuminated by UV light, electrons and holes are photogenerated within, thus oxidizing the organic compounds dissolved in water. To increase the overall efficiency of this process it is very important to develop methods for broadening the spectrum of absorbed light in the visible range and increase the photocatalytic properties of TiO 2 in both UV and visible range [6].
To improve the photocatalytic efficiency of such materials many efforts have been made, including various synthesis pathways and subsequent modification of TiO 2 powder properties. Coupling TiO 2 with other semiconducting materials is considered beneficial because coupling two semiconductors with different redox energy levels can increase the separation of their corresponding conduction and valence bands [7]. So far, the most successful results have been achieved through the loading of metal or metal oxides on the surface of TiO 2 [8][9][10]. Generally, metal oxides such as WO 3 and MoO 3 proved as excellent candidates for coupling with TiO 2 , due to increased photocatalytic activity of [11,12].
The structural and microstructural, morphological and photocatalytic properties of TiO 2 nanocrystals are strongly dependent on the synthesis process [13] and calcination temperature [14,15].
Sol-gel processing has become one of the most successful techniques for preparation of TiO 2 powders and gels [16,17]. Nevertheless, TiO 2 materials obtained by sol-gel processing are either amorphous or not well crystallized and consequently, they must undergo a suiTab. treatment to become active photocatalysts. In order to control crystallinity of TiO 2 thermal treatment is usually applied but the temperature must be carefully selected.
The temperature of anatase to rutile phase transition is around 600 °C, depending on different factors such as defects, impurities and grain size. The relationship between the microstructure and elastic properties of the rutile phase has been widely discussed [18][19][20], while only a few studies have been conducted for the anatase phase [21].
X-ray powder diffraction (XRD) analysis is a simple and powerful tool to estimate the crystallite size and lattice strain. This is achieved by Williamson-Hall analysis that is a simplified method where the size and strain broadening are deconvoluted by considering the peak width as a function of 2θ [22]. These analyses are employed for estimating crystallite size and lattice strain in the present study. When microstrain is neglected, the coherently diffracted domain size can simply be evaluated by the Debye -Scherrer equation [23].
In this study, WO x -TiO 2 powders were prepared by sol-gel method combined with a hydrothermal treatment. WO x -TiO 2 powders with varying amount of anatase and rutile phases are obtained by carefully controlling the sintering temperature. By combining two analytical techniques (XRD and N 2 adsorption-desorption), we determined the correlation between the change of phase composition, nanoparticle growth, and textural properties with sintering temperature. The strain associated with calcination of WO x -TiO 2 samples at 500, 600, 650, 700, 750 and 800 °C due to lattice deformation was estimated by a modified form of Williamson-Hall model. In order to estimate the photocatalytic activity of WO x -TiO 2 powders, methyl orange was employed in the photocatalytic tests.

Experimental 2.1. WO x -TiO 2 powders synthesis
Titanium (IV) isopropoxide and tungstophosphoric acid were used for the synthesis of WO x -TiO 2 powders in this study. All chemicals were of analytical grade. Amorphous powders were prepared by a sol-gel method. For the synthesis of samples, titanium isopropoxide was slowly dissolved in isopropyl alcohol. After that, an appropriate amount of H 3 PW 12 O 40 (20 %) was dissolved in water and then added into solution drop by drop. The resulting mixture was heated to 45 °C until homogeneous hydrogel was formed. This hydrogel was subsequently heated to 200 °C at a heating rate of 2 °C/min and after that washed with hot water three times. The wet gel was annealed at 500, 600, 650, 700, 750 and 800 °C for 3 hours, which resulted in the formation of WO x -TiO 2 powders.

WO x -TiO 2 powders characterization
The phase structure of samples was analyzed by X-ray diffraction method, using a Rigaku Ultima IV diffractometer in Bragg-Brentano geometry, with Ni-filtered CuK α radiation (40 kV, 30 mA, λ=1.54178 Å). The structural and microstructural parameters of TiO 2 samples were estimated by Williamson-Hall (WH) plots [22]. N 2 adsorption-desorption isotherms of all samples were measured with an automatic adsorption apparatus (Sorptomatic 1990 Thermo Finnigan) at 77 K. The specific surface areas (S BET ) were calculated by fitting the adsorption data to Brunauer-Emmett-Teller (BET) equation [24] and the pore size distributions were calculated by the Barrett-Joyner-Halenda method [25].

Photocatalytic tests
Photocatalytic degradation of methyl orange (MO) was carried out in an open cylindrical thermostated Pyrex cell of 6.8 cm in diameter, corresponding to the surface area accessible to the light of 36.3 cm 2 . The experiments were performed with 100 mL solution containing 8 mg/L MO and 100 mg of WO x -TiO 2 or pure TiO 2 powders. Irradiation of the solutions was performed under the lamp that simulates solar radiation (Solimed BH Quarzlampen), with a power consumption of 300 W, housed 25 cm above the top surface of the solution. Illumination intensity on the top of the photocatalytic reactor was 850 lx. Prior to illumination, the suspensions were magnetically stirred in the dark for 30 min to achieve adsorption-desorption equilibrium. Aliquots of suspensions were collected at different time intervals for a total of 150 min. The aliquots were filtered through a 0.20 μm syringe membrane filter into standard quartz cuvettes with an optical path of 1 cm and directed to UV-Vis spectrometer (Thermo Electron Nicolet Evolution 500) to check the degradation of MO via its absorption peak at 464 nm. These absorption data were used in the determination of degradation of MO through comparison with the absorbance at a certain time as a percentage of the initial absorbance.

Results and discussion
Physical properties of WO x -TiO 2 powders often vary with preparation history and post-treatment. Therefore, as a first step, it is necessary to determine the structure of prepared samples. XRD patterns of prepared WO x -TiO 2 powders ( Fig. 1) without annealing feature weak broad peaks in the positions corresponding to characteristic peaks of the anatase phase. This is an indication of poorly crystallized and/or mostly amorphous solids. After annealing from 500 to 800 °C all diffraction peaks become more intense, which is related to crystallization and particle growth. However, when the annealing temperature was elevated to 700 °C, small rutile peaks are observed, indicating the onset of anatase-to-rutile transformation. In previously published papers, the beginning of the transformation of anatase-to-rutile was observed in the range 400-1200 °C [26,27], depending on the utilization of different raw materials, processing methods and methods for determination of the transition temperature. It was also observed that when annealing temperature goes above 600 °C, the Keggin molecule breaks up to form WO 3 species. The orthorhombic WO 3 and a non-stoichiometric form of tungsten oxide (WO 2.92 ) were observed in the samples annealed at 700 and 800 °C. The variation of anatase-to-rutile ratio of WO x -TiO 2 powders are correlated with annealing temperature and presented in Tab. I. Anatase (101) peak at 2θ = 25.48° and rutile (110) peak at 2θ = 27.58° were analyzed using the equation [28]: where X is the weight fraction of rutile, and I A and I R are the X-ray intensities of anatase and rutile peaks, respectively.
As can be seen, the anatase content estimated from equation 1 decreases from 100 to 82 % with increasing sintering temperature up to 800 °C. Also, results indicate that at 700, 750 and 800 °C all WO x -TiO 2 samples contain much lower fraction of rutile than pure TiO 2 exposed to the same temperature treatment, which is usually transformed to rutile phase by over 90 % [26]. Obviously, WO 3 hindered the phase transformation from anatase to rutile during sintering. Annealing at elevated temperatures can induce the sintering of crystals, resulting in an increase in the crystallite size. Gaining enough energy could position the crystallite in proper equilibrium sites resulting in improved crystallinity and degree of orientation [29]. The value of lattice parameters a = b and c of anatase phase in samples calcined at various temperatures as a function of the growth of crystallites are shown at Fig. 2. The continuous increase of the unit cell ratio (c/a) occurs with increasing calcination temperature and concurrent crystallite growth, mainly originates from stretching in the direction of the c-axis and, to a lesser extent, in the small shortening of a-axis (insert of Fig. 2). This particular value of c-axis together with a relatively weak variation of a-axis has been identified for nanocrystalline TiO 2 [30]. XRD pattern can be utilized to evaluate the narrowing of peaks with the growth of the crystallites and lattice strain. If strain contribution is neglected, the particle size can be estimated from the Scherrer equation, while better estimation of size and strain can be made from WH method. Williamson and Hall proposed a method for deconvolution of size and strain broadening based on the peak width as the function of 2 θ. If a linear fit is obtained from the equation: it is possible to derive the crystallite size (D XRD ) from the intercept, and micro-strain (ε) from the slope. K is a constant equal to 0.94 for spherically shaped particles, λ is the wavelength of the x-ray (1.54178 Å for CuK α radiation) and θ is the peak center. Figure 3(a)-(f) represents the W-H plot of annealed WO x -TiO 2 samples. The experimental data points are shown with symbols and fitted data points are shown with the straight line. As can be seen from Fig. 3, the microstrain gradually increases with increasing calcination temperature up to 700 °C and then decreases at higher temperatures. On the other hand, the particle size progressively increases with increasing calcination temperature (Tab. I). Dynamics of all these changes should be considered through mechanism of the process. It should be kept in mind that above 650 °C simultaneously three processes occur; sintering, anatase-to-rutile phase transformation, and decomposition of Keggin anion to WO x species. Pore size distribution, BET surface area, and pore volume were obtained by N 2 adsorption analysis (Tab. II). BET surface area and pore volume of WO x -TiO 2 samples decreases as the calcination temperature increases, which is in accordance with sintering process. It has been reported [31] that the main contribution to the surface area comes from small pores, while macropores have smaller contribution. Smaller pores have narrow, while larger pores have broader pore size distribution. Obviously, small micropores (3.7 nm) did not change while the larger ones continuously increased from 8.8 to 21 nm, upon thermal treatment (Tab. II). Besides, with raising the sintering temperature, the relative amount of smaller sized mesopores decreased gradually and disappears at 800 °C, while the relative amount of the larger ones constantly increases.
Tab. II Textural properties of WO x -TiO 2 powders after calcination at different temperatures.  Comparison of microstructural properties (specific surface area -S BET and pore sized BET ) and crystallite size -D XRD as a function of thermal treatment is represented in Fig. 4. With increasing sintering temperature the average crystallite size of anatase increases, while specific surface area of WO x -TiO 2 decreases. Also, WO x -TiO 2 samples (calcined from 500 to 700 °C) have similar total pore volume and average pore size at corresponding crystallite size. These observations indicate that samples undergo similar particle growth and aggregation during the thermal treatment.
Considering that calcination temperature is an important parameter that affects the structure and photocatalytic activity of TiO 2 [31], WO x -TiO 2 samples prepared at different calcination temperatures were tested for photocatalytic degradation of MO under visible-light illumination. Also, in order to compare the degree of decomposition of MO over WO x -TiO 2 samples we presented results for pure TiO 2 calcinated at 500 o C (Fig. 5a). TiO 2 (500) sample showed higher photocatalytic activity towards MO degradation than TiO 2 samples calcinated at high temperatures, 600, 650, 700, 750, and 800 o C. For this reason, TiO 2 (500) sample was selected as a reference material. Observed photodegradation rate of MO over pure TiO 2 and WO x -TiO 2 samples show that increasing calcination temperature from 500 to 700 °C leads to an increase of photocatalytic activity. Further increase of the calcination temperature to 800 °C leads to significant reduction of BET surface area and pore volume (Tab. II), resulting in noTab. drop of MO degradation rate. It is also observed that an increase of irradiation time rapidly decreases the concentration of MO, suggesting fast photodegradation of MO on the surface of WO x -TiO 2 powders. The kinetic data fit well to the Langmuir Hinshelwood model: where k is apparent reaction constant rate (min -1 ), C 0 and C are initial and reaction concentration of MO, respectively and t is time of irradiation. Kinetic data are displayed in Fig. 5b and Tab. III.
Tab. III Rate constants of TiO 2 (500) and WOx-TiO 2 photocatalysts calcined at different temperature. a k app. -Apparent first order constant b k app. -Apparent first order constant per unit of specific surface area As the data show, the apparent first -order rate constants can be placed in the following order WO x -TiO 2 (700)<WO x -TiO 2 (650)<TiO 2 (500)<WO x -TiO 2 (600)<WO x -TiO 2 (500)<WO x -TiO 2 (800). The same order is obtained by dividing the apparent rate constant by specific surface area. These values point out the significance of exposure of active sites on their role in photocatalytic reaction. Although the specific surface area continuously drops with increasing calcination temperature, specific activity of the catalysts increases up to 700 C as the result of complex structural changes. Thus, the synthesized WO x -TiO 2 (700) powder can be used as a viable UV/visible-light driven photocatalyst for the degradation of azo-dye.

Conclusions
Nanocrystalline WO x -TiO 2 powders were prepared by a sol-gel method combined with hydrothermal treatment. The XRD analysis revealed that WO x -TiO 2 powders mostly consist of anatase phase with minor parts of rutile and tungsten oxides. The rise of calcination temperature caused an increase of crystallinity, crystallite and pore size, while specific surface area decreases continuously. All samples exhibited bimodal pore size distribution; smaller pores (3.7 nm) remain almost unchanged within the whole temperature range while larger mesopores were shifted towards large values, from 8.8 to 21 nm. Photocatalytic activity increases with calcination temperature up to 700 °C despite the lowering of the specific surface area, mainly due to increased pore size.