Enhanced Photocatalytic Activity of Porous Nanostructed TiO2 Thin Film Prepared by Sol-Gel Process

Journal of Chemistry, Vol. 42 (2), P. 1 - 4, 2004 Enhanced Photocatalytic Activity of Porous Nanostructed TiO2 Thin Film Prepared by Sol-Gel Process Received 25-6-2003 T. T. Duc1, T. X. Hoai1, N. T. Hue2, N. L. Lam1, N. T. Tinh1 1Institute of Applied Physics and Scientific Instrument, VAST 2Institute of Chemistry, VAST Abstract TiO2 thin film with nanoporous structure are prepared from colloidal suspension containing polyethylen glycol (PEG) via Sol-Gel method. The effect of PEG addition to the precursor solution on the surface structures and photocatalytic activity of the resultant thin films are studied. The larger the amount of PEG added to to the precursor solution, the larger the size and number of pores produced in the resultant films when PEG added to the gel films decomposed completely during heat-treatment. The photocurrent of such porous thin film is found to increase with increasing amount of PEG up to 30% weigh of TiO2. Photocatalytic degradation experiments show that eosin, methyl orange is efficiently decolorized in the presence of the TiO2 films by exposing its aqueous solution to UV light and the suitable surface structures remarkably enhance the photocatalytic activity of TiO2 films. 1. Introduction In recent years, one of the most active areas in heterogeneous photocatalysis is environmen- tal remendiation with the photocatalysis. Many organic pollutants can be degraded and ultima- tely mineralized using TiO2 photocatalysts under UV irradiation [1]. Photocatalytic chemical reactions occurring on the surface of semi- conductor materials depend on the process, which start from the absorption of light and results in attainment of photogenerated electrons and holes in the surface. The illuminated semiconductor surface is regarded as a producer of hydroxyl radicals *OH. These and other highly oxidizing initial products of this indirect photochemical reaction go on to attack oxidi- zable contaminants. The current bottleneck in photocatalysis lies in its low quantum yield, which depends on the ratio of the surface charge carrier transfer rate to the electron-hole (e-/ h+) recombination rate. Particle size is an important parameter for catalysis in general since it directly impacts the specific surface area of a catalyst. With a smaller particle size, the number of active surface sites increases and so does the surface charge carrier transfer rate in photo- catalysis. Most of the studies related to the photodegradation reaction have been carried out using suspensions of powdered TiO2 in aqueous solutions. However, their removal from water is difficult and recent research has focussed on the preparation of highly photoactive immobilized catalysts for water treatment. In this context, we have paid much attention in preparing nanoporous TiO2 thin film on glass plates and silicagel granules by a sol-gel process and studying its photocatalytic ability for the degradation of some typical toxic pollutants such as eosin and methyl orange, whose complete degradation is of great environmental concern. 2. Experimental details Preparation of nanoporous TiO2 films The TiO2 colloidal suspensions for TiO2 films are prepared by hydrolysis of titanium isopropoxide as described in a previous paper [2]. Various amounts of polyethylene glycol (PEG) are then respectively added to the above colloidal suspensions and served as precursor solutions (PSA Sol). The average molecular weight of PEG is 2000. The TiO2 films are formed on glass substrates and silicagel granules from the precursor solutions . The substrates coated with gel films are heat treated at 500oC for 1h in air using an electric oven. The thickness of the TiO2 films is adjusted by repeating the cycle from dipping to heat treatment. Characterization of nanoporous TiO2 films Crystallinity of the TiO2 films is identified by X-ray diffraction (XRD) with the diffracto- meter D-5000 (Siemens). The X-ray diffraction patterns indicate that the film crystallites consist of pure anatase with no rutile traces for the films treated at 5000C. The heat treatment is monitored using a TG-DTA machine (model TAS-100, Japan). The surface morphology and thickness of TiO2 films are observed using Atomic Force Microscopy AFM and SEM. Fig.1a depicts a three-dimensional AFM images of TiO2 films prepared from the colloidal suspension contai- ning PEG. It reveals a highly roughed and nanostructured photography in comparision with conventional sol-gel TiO2 film prepared from PSB sol [3] ( Fig. 1b ) Photocurrent measurements It has been used as a simple method for probing the activity of different photocatalysts. The photocurrent measurements were performed in a conventional three-electrode cell consisting of a quartz cuvette. The working electrode was TiO2 thin film electrode, the counter electrode was a platinum foil and Ag/AgCl electrode was reference electrode. The electrolytes were prepared from grade NaOH, CH3OH and bidistillated water. The photocurrent measure- ments were carried out with HP-Data Acquisi- tion (USA). Potentiostat and Programmable Electrochemical Analyzer were used to per- form photocurrent-voltage characteristics. The electrode was illuminated with the full output of 100 W Hg lamp. Photocatalytic measurement TiO2 films are settled in aqueous eosin with a concentration of 30 ppm and 100 ppm in a quartz cells (10 mm ì10 mm ì30 mm). The UV lamp (40 W) is used as a light source. The concentration of aqueous eosin is determined with a UV-visible spectrophotometer by measu- ring the absorbance at 535 nm. 3. Result and discussion Fig. 2a shows the photocurrent-voltage curves for a conventional anatase film prepared from Fig.1: AFM images of nanostructed TiO2 films Fig. 1b: PSB Sol, Anatase, size 50 nmFig. 1a: PSA Sol, Anatase, size 25 nm PSB sol, recorded in 0.1M aq. NaOH solution with and without addition of 0.1M of CH3OH. Band gap irradiation (< 400 nm) of the TiO2 film in aq. NaOH results in the oxidation of water (OH- ions) by photogenerated holes. Addition of methanol to the electrolyte leads to a shift of photocurrent onset towards negative potentials. This kind of behavior, typical of a species undergoing rapid hole capture at a photoanode surface, is consistent with the ability of methanol to compete more efficiently than OH- ions with surface electron-hole recombination. Moderate increase of saturation photocurrent, observed in fig.2a following addition of methanol to the solution is caused by photocurent – doubling effect [1]. Organic compound CH3OH adsorved on the surface of TiO2 are oxidized by photogenerated holes into the intermediates CH3O*, that is able to inject electrons into the conduction band of TiO2 causing the increase of the photocurrent. By way of contrast, analogous photocurrent-voltage curves, obtained for nanoporous TiO2 films prepared from the colloidal suspension containing PEG, reveal entirely different behaviour (Fig. 2b). In the case of 0.1mol NaOH containing 0.1 mol of CH3OH , the photocurrent rises quite sharply with increasing potential in the anodic direction and reaches the saturation much more rapid than for the conventional sol/gel anatase TiO2 film. Nanostructured materials contain a significant amount of grain boundaries, but the surface recombination is not predominant in this small size particle system. This anomalous behavior could be explained on the basis that the electrolyte makes contact with individual crystal in the porous film. Due to differences in the overall kinetic for electrons and holes, the holes quickly reaches the surface of the crystallites and oxidize the species in the solution, the electrons left behind will be stored in the conduction band of the crystallites causing higher photocurrent. Photoelectrochemical mea- surements have shown that both nanoporous TiO2 films (from PSA sol) and the transparent TiO2 films with nanocrystalline structure (from PSB sol) have a rather good photocatalytic activity. Fig. 2c shows that the activity of TiO2 photocatalyst increases with the amount of added PEG up to 30% weight of TiO2. Higher amount of PEG increases in opacity and light scattering of TiO2 thick films, leading to a decrease in the passage of irradiation through the films. It has been well demonstrated that photo- degradation of the organic compounds will occur when TiO2 is illuminated by UV light in present of water containing dissolved oxygen and organic compounds. Eosin and methyl orange are attached by hydroxyl radical and generates organic radicals or some other intermediates. Eventually all the parent com- pounds and intermediates are oxidized into CO2, SO4 2- and NO3 - [1]. The decolorization was evaluated by decrease in absorptance of dye with irradiation time. Under dark conditions without light illumination, the content of Eosin and Methyl Fig.2: Photocurrent-voltage curves of TiO2 films (2a) (2b) (2c) orange does not change for every measurement using various TiO2 films. Illumination with 234 nm UV lamp in the absence of TiO2 films results in UV decolorization (curve UV in Figure 3). The degradation is much more effficient with presence of both illumination and TiO2 films (curves TiO2(PSA) and TiO2(PSB) in Figure 3). Figure 4 shows that the dye of 30 ppm eosin solution decreases rapidly with irradiation time and is totally transparent after 2 hours illumina- tion. The results reveal that TiO2 film coated on silicagel granules can be a good photocatalytic material for water treatment 4. Conclusion Nanoporous TiO2 films have been prepared on glass substrates and silicagel granules from the colloidal suspension (sol PSA) containing PEG via Sol-Gel method. The surface structure and photocatalytic activity of the nanoporous TiO2 films thus obtained has been investigated in comparision with trasparent TiO2 films prepared from alcohol sol PSB. The results obtained are summaried as follows: 1. There is an obvious increase of the photocurrent of TiO2 film (increase of photo- oxidation of organic compound methanol) with increasing amount of added PEG (increasing of the porousness of the films). 2. The photocatalytic activity of TiO2 films depends on such factors as crystalline size, pore size, the surface area and transmitance of the films. 3. Nanostructed TiO2 thin films exhibits the potential to be effective in decomposition of toxic organic compounds. Acknowledgements: This work is financially supported by the Natural Science Foundation and Vietnamese Academy of Science and Technonoly. References 1. Amy L. Linsebigler. Chem. Rev. Vol. 95, P. 735 - 758 (1995) 2. T. T. Duc, D. X. Dung, N. T. Tinh, and T. X. Hoai. J. Chem., Vol. 37, No. 4 (1999). 3. T. T. Duc, P. T. X. Binh, N. T. Tinh, and T. X. Hoai. Proceedings of VGS5, P. 272. Fig. 3: Decrease of absorbance at 535 nm of 100 ppm Eosin with irradiation time Fig. 4: Decomposion of 30 ppm of osin solution by 365nm lamp (40 W) and TiO2/ SiO2 0 0.1 0.2 0.3 1 2 3 4 5 6 7 Irradiation time Abs (%) U V TiO2(PSA) TiO2(PSB) Tóm tắt : MIng mỏng TiO2 sol-gel có cấu trúc nano xốp đP đQợc chế tạo từ dung dịch huyền phù có chứa polyethylen glycol (PEG). BIi nIy trình bIy kết quả nghiên cứu sự ảnh hQởng của việc đQa thêm PEG vIo sol đến cấu trúc bề mặt vI hoạt tính quang xúc tác của mIng TiO2 đP chế tạo. Polyethylen glycol trong mIng sẽ bị phân huỷ hoIn toIn sau khi sử lý nhiệt vI để lại các lỗ rỗng trong mIng. Kích thQớc vI số lQợng của chúng tỷ lệ với lQợng PEG đQa vIo sol. Dòng quang điện của mIng tăng đến tỷ lệ của PEG lI 30% trọng lQợng của TiO2. Các thí nghiệm phân huỷ quang xúc tác cho thấy dung dịch eosin vI methyl da cam khi chứa mIng mỏng TiO2 nano xốp vI đQợc chiếu bằng ánh sáng UV sẽ bị phân huỷ hoIn toIn vI mất mIu. Kết quả nghiên cứu cho thấy cấu trúc bề mặt thích hợp sẽ lIm tăng hoạt tính quang xúc tác của mIng TiO2.