Synthesis of Nanoparticles of Mixed Oxides Containing Titanium Cerium Silver and Silicon: Phase Transformation

Problem statement: Titanium dioxide is a well known material using wi th self-cleaning application. This is due to its properties after ex posed to UV light: Superhydrophilicity and photocatalysis. The addition of other oxides could prolong these properties without continuing exposed to UV. However, titanium dioxide phase is essential . In order to obtain these properties, titanium dioxide needs to be in anatase phase. Approach: In this Research, phase transformation temperature s of mixed oxide of titanium dioxide and other oxides were determined. Samples of different mixed oxides were prepared by sol gel method. The first sample contained 30%TiO 2 and 70%SiO2. The second sample contained 30%TiO 2, 15%CeO2 and 55%SiO2. The last sample contained 30%TiO 2, 15%CeO2 and 55%SiO2. Each sample was separately calcined at various t emperatures from 350850°C stepping by 50 °C and followed by grinding and sieving to obtain in the form of powders. Then, each powder was characterized for its microstructur e and phases of titanium dioxide, crystallite size by X-Ray diffraction. Results: The results from XRD showed that for a 30%TiO 2/SiO2, an increase in calcined temperatures from 350-850°C (increasing by 50°C) increased average crystallite sizes of titanium dioxide (from 5.1-11.8 nm). Also, titanium dioxide phase found in the samples was only anatase. For a 30%TiO 2/15%CeO2/SiO2, an increase in calcined temperatures was not affe ct to the structure of the samples and XRD patterns seemed to be in an amorphous structure. Finally, for a 30%TiO2/15%AgO/SiO2, titanium dioxide was found in an anatase phase at 350°C until 650°C. Then, at calcined temperatures greater than 700°C, rutile phase started appearing in the structure while anatase phase peaks slowly declined with an increas ing in calcined temperatures. This result showed that using these mixed oxides to coat over glass sl ides or mirrors for self-cleaning purposes, the fil ms should be treated at certain temperatures to obtain an anatase phase. Conclusion: Phase transformation of titanium dioxide depends on calcined temperature s. The addition of other oxides such as silica, cerium or silver can effectively suppress the anata se-rutile phase transformation and resulted in an increase of anatase-rutile phase transformation tem peratures.


INTRODUCTION
Titanium dioxide is of great interest owning to its applications related to photo-splitting of water, photocatalyst, photovoltaic devices and so on (Besor et al., 2009;Meen et al., 2009). Moreover, its thin film can induce hydrophilic surface with a water contact angle of 0-5° under UV light irradiation. This is so called "superhydrophilicity". This property of TiO 2 thin film makes it possible to be utilized for many applications such as anti-fogging or self-cleaning mirrors (Liu et al., 2008). Phases of titanium dioxide affected to superhydrophilic property of the films. The anatase phase in TiO 2 thin films resulted in superhydrophilic property (Kontos et al., 2007). Therefore, phases of titanium dioxide are of importance to their applications. The other oxides cooperated with titanium dioxide enhanced photocatalytic and superhydrophilic properties (Houmard et al., 2007). However, the addition of other oxides affected the anatase-rutile phase transformation temperatures (Kumar et al., 1999). In this research, the effect of calcined temperatures to titanium dioxide phase transformation and crystalline sizes of titanium dioxide mixed with others was studied. The amount of titanium dioxide was kept constant at 30% wt and the rest is other oxides.

MATERIALS AND METHODS
Sample preparation: Mixed oxide of titanium dioxide with other oxides was prepared by sol gel method. Titanium isopropoxide obtained from Sigma Aldrich was a titanium precursor. For a 30%TiO 2 /SiO 2 preparation, it began with dissolving the desire amount of Tetraethylorthosilicate (TEOS) into ethanol. The solution was stirring for 30 min. and then a stoichiometric amount of water was added into the solution to obtain hydrolysis reaction. The solution was continuing stirring. The known amount of nitric acid was added to peptize the solution. Then, the desire amount of titanium isopropoxide was added into the solution. The solution was kept stirring for 30 min. The obtained solution was aged for overnight and is called "sol". The sol was heated until it became gel. The obtained gel was aged overnight. After aging, the gel was dried at 110°C followed by calcined at different temperatures from 350-850°C (increased by 50°C). The final oxide was ground and sieved to 100 mesh.
For a 30%TiO 2 /15%CeO 2 /SiO 2 and a 30%TiO 2 /15%AgO/SiO 2 preparation, the same procedure was used. It was started with dissolving the desire amount of Tetraethylorthosilicate (TEOS) into ethanol. The solution was stirring for 30 min. and then a stoichiometric amount of water was added into the solution to obtain hydrolysis reaction. The solution was continuing stirring. The known amount of nitric acid was added to peptize the solution. The known amount of cerium nitrate or silver nitrate was added into the solution. The obtained solution was stirring for 30 min. and then the known amount of titanium isopropoxide was added and the solution was kept stirring for another 30 min. to obtain the uniformity. The process of drying and calcining was followed the procedure of a 30TiO 2 /SiO 2 preparation.
Sample characterization: Average crystalline sizes of each oxide were determined by Scherrer's equation using the X-ray line broadening from X-ray diffraction, Bruker AXS model D 8 Discover equipped with a CuKα radiation with a nickel filter. Diffraction intensity was measured in the 2 theta ranges between 20 and 85°, with a step of 0.02° for 8 sec point −1 .

RESULTS AND DISCUSSION
As been known, phase transformation of titanium dioxide depends on the calcination temperatures. For pure titanium dioxide, it was found that an increase in calcined temperatures led to changes of anatase to rutile. Qingju et al. (2002) observed the anatase occurred at the most when the sample was calcined at temperature of 450°C and further increasing in calcined temperature rutile phase appeared. At calcined temperature of 800°C, titanium dioxide was completely in the form of rutile. In this research, the study of titanium dioxide phase transformation in the mixed oxide of titanium and other was investigated. The results were showed as following: Phase and average crystallite size of a 30%TiO 2 /SiO 2 : Samples containing 30%TiO 2 and 70%SiO 2 were prepared by sol gel. The obtained solid powders were calcined at different temperatures from 350-850°C with increments of 50°C. All samples were analyzed for their phase and average crystallite sizes by X-Ray Diffraction method (XRD). The results were shown in Fig. 1. XRD measurements were performed in order to verify the occurrence of TiO 2 phases at differences in calcined temperatures. Fig. 1 shows the XRD diffraction patterns obtained with 2θ = 20-80°. * represents peak positions of TiO 2 anatase phase at 25.4, 38.1, 48.1, 54.8, 62.5 and 75.1°. # represents standard peak position of TiO 2 rutile phase at 27.4, 36.1, 41.2, 56.6, 69.1 and 69.9°. These data were obtained from the references of spinel code 00-021-1272 and 01-076-0317, respectively, attached with the XRD instrument. Figure 1a is an XRD pattern of the sample calcined at 350°C. As can be seen, the XRD pattern was quite smooth except at 25.4° appearing as a small hill. An increasing of calcined temperatures increased intensity of peaks at this position and the peaks were getting sharp as been seen in Fig.1ac. Further increasing in calcined temperatures to 500°C increased a number of peak positions matching with peak positions of TiO 2 anatase phase. The peak positions were found at 25.4, 38.1, 48.1, 54.8 62.5 and 53.9°. These peak positions appeared in the samples calcined at 500-850°C and the intensities of each peaks increased with increasing of calcined temperatures. These results indicated that at low calcined temperatures, it was an amorphous structure appearing in the sample. An increase in calcined temperatures increased crystallinity of titanium dioxide and anatase phase was appeared in the samples calcined at temperature 450-850°C. Interestingly, no rutile phase was observed in the all samples. Kumar et al. (1999) prepared a 90%TiO 2 /SiO 2 by cohydrolysis method and the samples were calcined at 800 and 1000°C. They found that TiO 2 was in only anatase phase for both calcined temperatures while a sample contained 95% TiO 2 and 5% SiO 2 , TiO 2 was in anatase phase at calcined temperature of 800°C and TiO 2 was found in rutile phase at calcined temperature of 1000°C. The addition of silica in the samples resulted in an increase of anatase-rutile phase transformation temperature.
From XRD results, average crystallite sizes of TiO 2 were calculated using Scherer equation. The results were showed in Table 1.
As in Table 1, average crystallite sizes of titanium dioxide increased with an increase of calcined temperatures. The XRD pattern of a sample calcined at 350°C was a bump and this could be described as either an amorphous structure of the oxide or a crystallite structure and the particles were well dispersed in the samples. An increase of calcined temperatures from 350-850°C increased an average crystallite size of titanium dioxide from less than 3-11.1 nm. The sample calcined at 850°C has the largest crystallite size of 11.1 nm. This was due to the agglomeration of TiO 2 at high temperatures (Tian et al., 2009).
Phase and average crystallite size of a 30%TiO 2 /15%CeO 2 /SiO 2 : The samples contained 30% TiO 2 , 15% CeO 2 and 55% SiO 2 were calcined at different temperatures. The results from XRD showed in Fig. 2. XRD measurements were performed in order to verify the occurrence of TiO 2 phases at differences in calcite temperatures. Figure 2 shows the XRD diffraction patterns obtained with 2θ = 20-80. ▪ represents peak positions of TiO 2 anatase phase at 25.4, 38.1, 48.1, 54.8, 62.5 and 75.1°. ▲ represents standard peak position of TiO 2 rutile phase at 27.4, 36.1, 41.2, 56.6, 69.1 and 69.9°. These data were obtained from the references of    spinel code 00-021-1272 and 01-076-0317, respectively, attached with the XRD instrument. As can be observed from the XRD patterns of all samples, there were no peak positions appeared except the bumps at 2θ = 25°. It means that either all particles are well dispersed in the samples or there was an amorphous structure existing in the samples. However, an increase in calcined temperatures was not affected to the structure at all. Further investigation was conducted with EDX to determine the chemical composition in the sample. The results showed in Fig. 3. As can be seen in Fig. 3, the results indicated the presence of titanium, cerium, silicon and oxygen in the sample. The composition of each atom was reported in Table 2.
From Table 2, the sample contained 30% TiO 2 , 15% CeO 2 and 55% SiO 2 calcined at 450° was analyzed for their chemical composition using EDX. The results indicated that there were 19.6% Ti, 27.5% Si, 11.8% Ce and 41.0% O in the sample. Comparing these data with those of calculating from the precursors during the preparation, it was found that there were 17.9% Ti, 25.7% Si, 12.2% Ce and 44.1% O and these brought about to errors less than 10% for each element. The differences in results from preparation and those from EDX may cause from the loss of chemicals during preparation. Consequently, the results from EDX confirmed that all chemicals were present in the samples even though no peak positions of TiO 2 and CeO 2 were observed from XRD. Phase and average crystallite size of a 30%TiO 2 /15%AgO/SiO 2 : The samples contained 30% TiO 2 , 15% AgO and 55% SiO 2 were calcined at different temperatures. The results from XRD showed in Fig. 4. XRD measurements were performed in order to verify the occurrence of TiO 2 phases at differences in calcined temperatures. Figure 4 shows the XRD diffraction patterns obtained with 2θ = 15-80°. * represents peak positions of Ag at 21.8,38.1,44.3,64.5 and 77.4°. ▪ represents peak positions of TiO 2 anatase phase at 25.4, 38.1, 48.1°, 54.8° 62.5° and 75.1°. ▲ represents standard peak position of TiO 2 rutile phase at 27.4, 36.1, 41.2, 56.6, 69.1 and 69.9°. These data were obtained from the references of spinel code 01-089-3722, 00-021-1272 and 01-076-0317, respectively, attached with the XRD instrument. In Fig. 2a, XRD pattern of the sample calcined at 350°C indicated an amorphous structure of the oxide. An increase in calcined temperatures increased the peak intensity and appearing of peaks at different angles. Most likely, these peaks were peak positions of TiO 2 anatase phase. Up until the calcined temperature of 500°C, peak positions of Ag appeared in the XRD patterns. Each peak intensity increased with an increase of calcined temperatures until calcined temperatures of 700°C, rutile phase appeared. Further increasing in calcined temperatures increased peak intensities of all compounds. These results indicated that the anataserutile phase transformation depended on calcined temperatures. Moreover, for a sample of titanium dioxide cooperated with other oxides, the phase transformation temperatures were higher that that of pure compound (Guan, 2005).
From XRD results, average crystallite sizes of TiO 2 were calculated using Scherer equation. The results were showed in Table 2.
As can be seen from Table 2, average crystallite sizes increased with an increase of calcined temperatures for all compounds. The sample calcined at 800°C has the biggest crystallite sizes for anatase, rutile and silver. For the samples containing anatase and rutile phases, average crystallite size of anatase was smaller than that of rutile at any calcined temperatures. Ag appeared in the sample calcined at temperatures greater that 400°C. Interestingly, the preparation method was sol gel. Theoretically, the final product from this method is oxide. This was due to the condensation reaction (Brinker et al., 1990). However, in this experiment, the amount of water added during the hydrolysis reaction was less that the stoichiometric amount. That could be the reason for obtaining Ag instead of Ag 2 O.

CONCLUSION
• Phase transformation of titanium dioxide depends on calcined temperatures • The addition of other oxides such as silica, cerium or silver can effectively suppress the anatase-rutile phase transformation and resulted in an increase of anatase-rutile phase transformation temperatures • Anatase phase was only species found in 30%TiO 2 /SiO 2 samples calcined at temperatures up to 850°C • The addition of cerium oxide in mixed oxide of titanium and silica led to well dispersion of all oxides in the samples • Average crystallite sizes increased with an increase in calcined temperatures • The addition of silver in mixed oxide of titanium and silica lowered the anatase-rutile phase transformation temperatures to 700°C