Study on the Preparation of TiO2-SiO2 Composites from Rice Husk Ash

In this paper, TiO2-SiO2 composites with different SiO2 contents were prepared by coprecipitation method usin g rice husk ash as raw material. The effects of the concentration of alkaline leaching solution and reaction temperature on the extraction rate of SiO2 from rice husk ash were investigated. The optimum formation conditions of T iO2-SiO2 composites were explored, an d the microstructure and properties of the composites were analyzed. The experimental results showed that the extraction rate of SiO2 from rice husk ash reaches the maximum when the concentration of alkaline leaching solution is 1.5 mol/L and the reaction temperature is 90°C . When the pH value is 7, the TiO2-SiO2 composites can achieve the optimum formation condition. TiO2-SiO2 composites are composed of approximately spherical particles with obvious pore structure of the surface and piling up in fluffy state. The Si atoms enter the crystal structure of TiO2 with T i-O-Si bonds connected to T i atoms, which inhibits the growth of TiO2 grains and leads to the increase of the transition temperature of T iO2 from anatase to rutile.


Introduction
In recent years, nanoscale TiO2 has been widely used in the fields of seawater desalination, photocatalysis and solar cells for its high weather ability and excellent optical and mechanical properties [1][2][3] .However, the pure TiO2 has many defects such as higher price, smaller specific surface area and weaker acid, leading thatthe development demand of many fields is limited [4] . Therefore, in order to overcome these defects and improve the physical and chemical properties of TiO2, it is usually combined with SiO2, Fe2O3or CeO2 to form composite o xide materials with lo w density and high porosity [5 -6] . Among them, SiO2 has high bonding strength, large specific surface area and good thermal stability [7] ,in addition, TiO2-SiO2 composites after adding SiO2 can have excellent properties of TiO2 and SiO2 at the same time [8] . Therefore, TiO2-SiO2 co mposites have become the focus of research. At present, most laboratories apply sol-gel or coprecipitation to synthesis TiO2-SiO2 composites that were made fro m titaniu m butoxide and tetraethyl o rthosilicate [9][10] .However, the hydrolysis rate of ester organic compounds is faster, the experimental conditions are demanding, and the price of themare relatively high, so the industrial production of TiO2-SiO2 composites are limited in so me extent. It is found that there are a lot of amorphous SiO2 in rice husk ash from agricultural waste [11] ,and the alkaline solution of SiO2 is easy to react with Ti(SO4)2 solution [12] .
Accordingly, TiO2-SiO2 co mposites can be prepared by cheap rice husk ash and Ti(SO4)2 as precursors, which can not only rationalize the utilization of agricu ltural waste, but also protect the environment and create higher commercial value [13] .
In this paper, cheap and readily available rice husk ash and Ti(SO4)2 were used as silicon source and titanium source respectively to prepare TiO2-SiO2 co mposites, which were applied to coprecipitation method to adjust the pH value by the ammonia tit ration. The microstructure and properties of TiO2-SiO2 co mposites were studied by means of BET, SEM, FT-IR and XRD.

Preparation of TiO2-SiO2 Composites
First, the rice husk ash and alkaline leach ing solution are heated and mixed in a flask, and the sodiu m silicate solution is obtained through filtration. Then add appropriate concentration of Ti(SO4)2 solution slowly and regulate pH value through ammonia water in it. Last, TiO2-SiO2 composites with different SiO2 contents were obtained after filtration, drying and calcining in muffle furnace (FR-1236). The prepared samples were represented by TiO2-SiO2(x), where x was the content of SiO2 in TiO2-SiO2 composites.

Characterization of TiO2-SiO2 Composites
Micro meritics's TriStar 3020 high speed automatic specific surface area and pore analy zer was used for the specific surface area and pore structure analysis (BET) of the TiO2-SiO2 co mposites. The specific surface area is calculated by Brunauer-Emmett-Teller and the pore size distribution is obtained by measuring the N2 desorption isotherm and calculating it by Barrett-Joyner-Halenda (BJH); The crystal structure and morphology of TiO2-SiO2 composites were analyzed by XRD-7000 X-ray diffraction analy zer produced by Shimadzu Corporation. Cu is the target. Ka is the rad iation source.
The tube voltage is 40KV and the scanning range is 10~70 degrees; The FTIR-8400S infrared spectrometer was used for analyzing the TiO2-SiO2 co mposites by Fourier infrared spectroscopy (FT-IR). The samp le was dried, ground and pressed test and the test range was 400-1400cm -1 ; The QUANTA 600F scanning electron microscope (SEM) of FEI co mpany was used for analyzing the microstructure of the surface of TiO2-SiO2 composites.

SiO2 from rice husk ash
The composition of rice husk ash showed that the content of SiO2 in rice husk ash reached 37.5% and the rests are carbon and a small amount of metal [8] . Its component analysis results are shown in Table 1 3    Fig. 3 The effect of pH value on the generation of TiO2 and

SiO2
The pH value is a key factor affecting the generation of TiO2 and SiO2. By ad justing the pH of the solution, it can not only determine the ratio of TiO2 to SiO2 in TiO2-SiO2 composites, but also obtain the optimal generation condition of TiO2-SiO2 composites. As shown in Fig. 3, when pH is 10, white silica sol begins to form in sodium silicate solution. With the decrease of pH, more and more white co llo id forms in the solution, because of the H + first reacting with SiO3 2in the solution [13] ,which indicates that the quality of SiO2 is bigger and bigger.
When the pH of solution is 2, the mass of SiO2 reaches the maximu m value, which indicates that SiO2 is more suitable for generation in acid solution. When the pH value is 2, wh ite titanic acid begins to be produced in In order to investigate the effect of SiO2 content on the surface structure of TiO2-SiO2 co mposites, the specific surface area and pore structure of the composites were analyzed in this paper, as shown in Table 2.
It can be seen from the table that the specific surface area and pore volume of pure TiO2 are the smallest. With the increase of SiO2 content, the specific surface area of TiO2-SiO2 composites is larger and larger, and the pore volume also increases gradually. In the TiO2-SiO2(0.8), the content of SiO2 is h igh and Si ato ms affect the formation of Ti atoms, resulting in the comp lete collapse of the framework of TiO2. Therefore, the pore volume decreases. When the content of SiO2 is 100%, the specific surface area and pore volume reach the maximu m value.
In the TiO2-SiO2 co mposites, the pores of SiO2 particles are occupied by the TiO2 powders, so the specific surface area and pore volu me decrease rapidly when the content of TiO2 powders are high [14] . Therefore, the specific surface area and pore volu me are relatively higher, wh ich are in agreement with the analysis results of Table 2.

FT-IR analysis of TiO2-SiO2 Composites
In order to determine the form of SiO2 and TiO2 in to the stretching vibration of the Si-O-Si bond [15] . It has been proved that the infrared absorption peak of 920~970cm -1 is the characteristic absorption peak of the Ti-O-Si bond [16] . being inhibited and the grain size becomes s maller [17] .
Therefore, The Ti ato m transforms fro m octahedral coordination to tetrahedral coordination gradually [18] .

Analysis of XRD test results
In order to study the effect of SiO2 content and calcination temperature on the crystal structure of TiO2, different kinds of samples were tested by XRD respectively. The results are shown in Fig. 7. XRD test chart of TiO2-SiO2 co mposites with different SiO2 contents at 600℃ are shown in Fig. 7(a).
When the content of SiO2 is less than 20%, the diffract ion peak of anataseTiO2 exists only in TiO2-SiO2 co mposites, and the peak value is highest and the width is narrowest.
With the addition of SiO2, the characteristic peaks of anataseTiO2 gradually generalize and the peaks value gradually decrease, but the characteristic peak of SiO2 gradually becomes obvious. When the content of SiO2 is higher than 50%, the characteristic peaks of anataseTiO2 disappear, and a steamed bread shaped peak is found near 22.5 degrees, indicat ing that the TiO2-SiO2composites was mainly in the form o f amo rphous SiO2 [19] . Therefore, as SiO2 content increases, the crystallinity of TiO2 in TiO2-SiO2 composites decreases gradually. TiO2 and TiO2-SiO2(0.5) co mposites at 600~1000 temperature. As can be seen from Fig. 7(b), when the temperature is 700℃ , the pure TiO2 begins to appear weak rutile characteristic diffraction peaks, indicating that part of the anataseTiO2 changes to the rutile structure gradually. When the temperature rises to 900℃ , the characteristic diffraction peak of anatase is disappeared, and TiO2 transforms into rutile structure completely. As is shown in Fig. 7(c), co mpared with pure TiO2, the phase transformation temperature of TiO2-SiO2(0.5) composites is higher. When the temperature rises to 900℃ , the rutile characteristic diffract ion peak appears.
Therefore, the addition of SiO2 increases the transition temperature of TiO2 fro m anatase to rutile and also improves its thermal stability. Some studies have shown that the transformat ion of TiO2 fro m anatase to rutile is caused by lattice stress and cell volume [20] .The smaller the cell volu me is, the higher the transition temperature is. Accordingly, with the increase of SiO2 content, it can be speculated that the grain size of TiO2-SiO2 composites decreases gradually, wh ich is consistent with the results of SEM and FT-IR.