Ag/TiO2 sol prepared by a sol–gel method and its photocatalytic activity

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Abstract

Ag/TiO2 sol with narrow particle size distribution was synthesized using TiCl4 as the starting material. TiCl4 was converted to Ti(OH)4 gel. The Ag/TiO2 sol was prepared by a process where H2O2 was added and then heated at 90–97 °C. After condensation reaction and crystallization, a transparent sol with suspended Ag/TiO2 was formed. Ag/TiO2 was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, contact angle analysis, and X-ray photoelectron spectroscopy. The photocatalytic properties of Ag/TiO2 film were evaluated by degradation of methylene blue in aqueous solution under UV light irradiation. The suspended Ag/TiO2 particles were rhombus primary particles with the major axis ca. 40 nm and the minor axis ca. 10 nm. Ag nanoparticles were well dispersed on TiO2 and the particle size was only 1–2 nm. Ag could restrain the recombination of photo-generated electrons and holes effectively. Transparent thin films could be obtained through dip-coating glass substrate in the sol. The thin film had strong hydrophilicity after being illuminated by UV light. Ag/TiO2 film showed a significant increase in photocatalytic activity compared to the TiO2 film. The high amount of surface hydroxyls on Ag/TiO2 film also played an important role in its photocatalytic activity.

Highlights

► [TiO(H2O2)]2+ complex was used to fabricate TiO2 film. ► Ag doping was achieved in-situ during the formation of TiO2 nanoparticles. ► Ag doped TiO2 film showed better photocatalytic performance than TiO2 film. ► Surface hydroxyl groups played key roles during the photocatalytic process.

Introduction

Heterogeneous photocatalytic oxidation is a promising technique for the complete oxidation of dilute organic pollutants in waste gas stream and waste water. Titanium dioxide (TiO2) has attracted much interest due to its advantages such as strong resistance to the corrosion of chemicals and photo, low operational temperature, low cost, and low energy consumption [1]. The organic pollutants can be fully oxidized at room temperature with TiO2 catalysts in air when irradiated by UV or near-UV light. The photons excite electrons of TiO2 from valence band to the conduction band. The resulting electron/hole pairs then migrate to the surface and initiate redox reactions with adsorbed organics [2], [3], [4], [5]. The high degree of recombination between photo-generated electrons and holes is a major rate-limiting factor that controlled the photocatalytic efficiency [6]. Doping metal onto TiO2 has been proved to be an effective strategy to improve the catalytic process. Pt and Ag deposited on TiO2 film using the CVD (chemical vapor deposition) method have been studied by many researchers and Ag/TiO2 showed a higher photocatalytic activity [7], [8].

Sol–gel process is one of the methods generally employed for the preparation of TiO2. This method is very suitable for fabricating transparent film. Several researchers synthesized Ag/TiO2 sol by adding silver compound to modify the sol–gel process [9], [10], [11]. Depending on the synthetic approach they used, TiO2 samples with various physical and chemical properties were obtained. By the sol–gel method one could obtain TiO2 nanoparticles with high homogeneity, and particle size could be easily tuned. Jagadale et al. [9] used titanium tetraisopropoxide, hydrogen peroxide and ethylmethylamine to synthesize Ag/TiO2 composite catalyst by sol–gel method. The titanium–hydrogen peroxide complex was synthesized, slowly thickened with time, and transformed into a gel. It was dried by IR, subsequently calcined at 300 °C, and TiO2 nanoparticles were obtained. Strong acids, such as HNO3 [10] and HCl [12], were used as peptizing agents and as condensation catalysts to prepare crystalline TiO2. In order to synthesize a neutral solution, H2O2 is a better choice to use in the peptization step [13]. In order to synthesize TiO2 sol with nano-sized particles, it is required to prevent the materials from aggregation at high temperature. The sol can be dip-coated onto the glass substrates directly without further calcination [14], [15], [16], [17]. Doping of metals could be achieved during the synthesis. The dispersion of nanoparticles and the interaction between the dopant and TiO2 are apparently dependent on the preparation conditions [18], [19], [20].

In a previous paper [21], one of the authors has reported the preparation of TiO2 thin film by TiCl4. TiCl4 was converted to Ti(OH)4 and H2O2 then used as the peptizing agent to form a TiO2 sol. By this method, nanosized TiO2 with high crystallinity was suspended in water without using any surfactant. In addition, the TiO2 sol was in a neutral solution, instead of an acid solution. In this study, Ag/TiO2 sol was prepared by the above method. Ag was added to enhance the activity. Ag/TiO2 was then coated on a glass substrate to form thin film on glass. The photocatalytic activity of the Ag/TiO2 film for the photodegradation of methylene blue (MB) was also studied under UV light irradiation.

Section snippets

Synthesis of Ag–TiO2 sol

The typical procedure of preparing Ag–TiO2 sol was as follows. 300 mL 0.5 M HCl aqueous solution was loaded into a 500 mL beaker, 5.0 g TiCl4 was added in a dropwise manner under magnetic stirring, the beaker was kept in an ice bath to maintain the temperature at ∼0 °C, 30 min later, and 1 M NH3·H2O aqueous solution was added into the above solution. The color of the gel became ivory immediately, indicating the formation of hydrated titanium oxide gel, Ti(OH)4. The pH of the solution was kept at 8.0.

Formation process of Ag/TiO2 film

Ti4+ was converted to Ti(OH)4 gel with the addition of NH3·H2O. The as-prepared Ti(OH)4 was re-dispersed in water and reacted with H2O2; two reactions would take place:Ti(OH)4TiO2++2OH+H2OTiO2++H2O2[TiO(H2O2)]2+

As described in reaction (2), the hydrogen peroxide showed strong coordination ability with TiO2+, the reaction (1) shifted to right-hand side, and Ti(OH)4 was gradually dissolved. Heating of the solution caused the decomposition of H2O2, which made the equilibrium of reaction (2)

Conclusion

Ag-doped TiO2 film was prepared by a sol–gel process using TiCl4 as the starting material and H2O2 as the peptizing agent. Ti(OH)4 gel was firstly synthesized by adding NH3·H2O to TiCl4 solution. H2O2 was then added to form the [TiO(H2O2)]2+ complex. Ag doping was achieved by adding AgNO3 solution in this stage. The solution was heated at 90 °C and TiO2 nanoparticles were formed gradually. The slow decomposition of the [TiO(H2O2)]2+ complex resulted in the formation of the homogeneous nucleation

Acknowledgment

This research was supported by the National Science Council, Taiwan.

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