Structural, Photocatalytic Activity and Photovoltaic Properties of Mesoporous Anatase Titania Nanopowders Prepared by Hydrothermal Method

Transcription

1 Structural, Photocatalytic ctivity and Photovoltaic Properties of Mesoporous natase Titania Nanopowders Prepared by Hydrothermal Method Sorapong Pavasupree 1,2, Jaturong Jitputti 1, Supachai Ngamsinlapasathian 1, Yoshikazu Suzuki 1 and Susumu Yoshikawa 1,* 1 Institute of dvanced Energy, Kyoto University, Uji, Kyoto, , Japan 2 Department of Materials and Metallurgical Engineering, Faculty of Engineering, ajamangala University of Technology Thanyaburi, Klong 6, Pathumthani, 1211, Thailand bstract: Mesoporous anatase TiO 2 nanopowder was synthesized by hydrothermal method at 13 o C for 12 h. The samples characterized by XD, SEM, TEM, SED, HTEM, and BET surface area. The as-synthesized sample with narrow pore size distribution had average pore diameter about 3-4 nm. The specific BET surface area of the as-synthesized sample was about 193 m 2 /g, higher than the previous works (ref. 8-1, 8 m 2 /g) about 2 times. Mesoporous anatase TiO 2 nanopowders (prepared by this study) showed higher photocatalytic activity than the nanorods TiO 2 (prepared as ref. 6), nanofibers TiO 2 (prepared as ref. 3-4), mesoporous TiO 2 (prepared as ref. 8-1), and commercial TiO 2 nanoparticles (P-25, JC-1, and JC-3). The solar energy conversion efficiency ( ) of the cell using the mesoporous anatase TiO 2 was about 6.3 % with Jsc of m/cm 2, Voc of.72 V and ff of.676; while of the cell using P-25 reached 5.82 % with Jsc of m/cm 2, Voc of.74 V and ff of.649. Keywords: Mesoporous, natase, TiO 2, Hydrothermal, Photocatalytic ctivity, Dye-sensitized Solar Cell 1. INTODUCTION In the past decade, mesoporous materials have been of great interest as catalysts because of their unique textural and structural characteristics. Much effort has concentrated on the important metal oxides such as TiO 2, SnO 2, VO 2, and ZnO. mong them, TiO 2 and TiO 2 -derived materials are of importance for utilizing solar energy and environmental purification. TiO 2 has been widely used for various applications such as a semiconductor in dye-sensitized solar cell, water treatment materials, catalysts, gas sensors, and so on [1]. Functional properties of TiO 2 are influenced by many factors such as crystallinity, particle size, surface area, and preparation. Hydrothermal synthesis has become one of the most important and promising new material fabrication method for nanoscale materials and nanotechnology [2-7]. In our previous works, mesoporous TiO 2 were synthesized by a modified sol-gel method, however, the prepared mesoporous TiO 2 had surface area about 8 m 2 /g [8-1]. In this study, mesoporous anatase TiO 2 nanopowders with narrow pore size distribution (pore size about 3-4 nm) and higher surface area (about 193 m 2 /g) has been synthesized, which shows high photocatalytic activity and high performance in dye-sensitized solar cell. The detail microstructure, photocatalytic, and photovoltaic properties will be reported. 2. METHODOLOGY 2.1 Synthesis Titanium (IV) butoxide (ldrich) was mixed with the same mole of acetyacetone (C, Nacalai Tesque, Inc., Japan) to slowdown the hydrolysis and the condensation reactions [11-13]. Subsequently, distilled water 4 ml was added in the solution, and the solution was stirred at room temperature for 5 min. fter kept stirring, the solution was put into a Teflon-lined stainless steel autoclave and heated at 13 o C for 12 h with stirring condition. fter the autoclave was naturally cooled to room temperature, the obtained product was washed with 2-propanol and distilled water, followed by drying at 1 o C for 12 h. 2.2 Characterization The crystalline structure of the samples was evaluated by X-ray diffraction (XD, IGKU INT 21). The microstructure of the prepared materials was analyzed by scanning electron microscopy (SEM, JEOL JSM-65FE), transmission electron microscopy (TEM, JEOL JEM-2CX), and selected-area electron diffraction (SED). The Brunauer-Emmett-Teller (BET) specific surface area was determined by the nitrogen adsorption (BEL Japan, BELSOP-18 Plus). 2.3 Photocatalytic activity measurement The photocatalytic activity was measured through the formation rate of I 3 - due to the oxidation photo-reaction of I - to I 2 in excess I - conditions [11-14]. reaction system was set up by adding 5 mg of a sample powder into 1 ml of.2 M of potassium iodide aqueous solution then stirred and irradiated with UV light (15 W, Vilber Lourmat VL-115L, with a maximum emission at about 365 nm) at room temperature. fter the irradiation of 15, 3, 45, and 6 min, the suspension was withdrawn and centrifuged. fter the clear supernatant was diluted ten times, the concentration of liberated I 3 - ions was monitored by the absorbance at 288 nm, using an UV Vis spectrophotometer (Shimadzu UV 245). The molar extinction coefficient was determined to be 4. x 1 4 (cm mol/l) -1. No I 3 - formation was observed when the experiments were conducted in the dark or in the absence of the TiO 2 samples. For comparison, 3 difference prepared TiO 2 (nanorods TiO 2 prepared as ref. 6, nanofibers TiO 2 prepared as ref. 3-4, and mesoporous TiO 2 prepared as ref. 8-1) and 3 difference commercially available nanoparticles TiO 2 powders, P-25 (Nippon erosil Co., Ltd., Japan), JC-1 and JC-3 (The Catalysis Society of Japan) were tested. 2.4 Dye-sensitized solar cell measurement TiO 2 electrodes were prepared as follows; 1 g of TiO 2 powder was mixed with.1 ml of C, and was ground mechanically. During vigorous stirring, 5 ml of mixture of water and ethanol (1:1, in vol %) was added and.4 ml of polyoxethylene (1) Corresponding author: s-yoshi@iae.kyoto-u.ac.jp

2 octylphenyl ether (Triton x-1) was added to facilitate spreading of the paste on the substrate. The obtained colloidal paste was coated on fluorine-doped SnO 2 conducting glass (FTO, sheet resistance 15 /, sahi glass Co., Ltd.) by squeegee technique. fter coating, each layer was dried at room temperature and then annealed at 4 o C for 5 min. The coating process was repeated to obtain thick films. The resulting films were sintered at 45 o C for 2 h in air. Sintered TiO 2 electrodes were soaked in.3 mm of ruthenium (II) dye (known as N719, Solaronix) in a t-butanol/ acetonitrile (1:1, in vol %) solution. The electrodes were washed with acetonitrile, dried, and immediately used for measuring photovoltaic properties. The electrolyte was composed of.6 M dimethylpropylimidazolium iodide,.1 M lithium iodide (LiI),.5 M iodide (I 2 ), and.5 M 4-tert-butylpyridine in acetonitrile. The thickness of the TiO 2 films was measured with a Tencor lpha-step Profiler. Photocurrent-voltage curve was measured under simulated solar light (CEP-2, Bunkoh-Keiki, M 1.5, 1 mw/cm 2 ). The light intensity of the illumination source was calibrated by using a standard silicon photodiode (BS52, Bunkoh-Keiki). The amount of adsorbed dye was determined by desorbing the dye from the titania surface into a mixed solution of.1 M NaOH and ethanol (1:1 in volume fraction) and measuring its absorption spectrum. The concentration of adsorbed dye was analyzed by UVvis spectrophotometer (UV-245 SHIMDZU). 3. ESULTS ND DISCUSSION [17] 3.1 Characterization results Fig. 2 shows the X-ray diffraction pattern of the as-synthesized sample. The peaks were rather sharp, which indicated the obtained TiO 2 had relatively high crystallinity, and attributable to the anatase phase. Fig. 3 gives the nitrogen adsorption isotherm and the pore size distribution of the as-synthesized sample. The isotherm shows a typical IUPC type IV pattern with inflection of nitrogen adsorbed volume at P/P about.5 (type H 2 hysteresis loop), indicating the existence of mesopores. The pore size distribution of the sample, as shown in the inset of Fig. 3, showed that the as-synthesized sample had average pore diameter about 4-5 nm. The BET surface area and pore volume of sample are about 193 m 2 /g and.286 cm 3 /g, respectively (higher than the previous works (ref. 8-1, 8 m 2 /g) about 2 times). SEM and TEM images of the as-synthesized sample showed nanoparticles size about 5 nm (Fig. 4 (a-b)).the electron diffraction pattern shown in the inset of (Fig. 4(c)) supported that the as-synthesized sample was anatase-type TiO 2. The lattice fringes of the nanorods and the nanoparticles appearing in the image (d =.35 nm) also allowed for the identification of the anatase phase (Fig. 4 (d)). HTEM images of the as-synthesized sample with clear lattice fringes, again confirming its high crystallinity. Fig. 5 show the summarized results (SEM, TEM, SED, and BET surface area) of the prepared TiO 2 nanopowders (nanorods TiO 2, nanofibers TiO 2, mesoporous TiO 2 ) and commercial TiO 2 nanopowders (P-25, JC-1, JC-3). The BET surface area of the nanorods TiO 2 (Fig. 5 (a), anatase structure with diameter 6-8 nm and the lengths of 3-6 nm), the nanofibers TiO 2 (Fig. 5 (b), anatase structure with diameter 2-1 nm and the lengths of several ten micrometers), and mesoporous TiO 2 (Fig. 5 (c), anatase structure with particles diameter 7-15 nm) were about 19, 1, and 8 m 2 /g, respectively. The BET surface area of the commercial TiO 2, P-25 (Fig. 5 (d), anatase 7 % and rutile 3 % structure with particles diameter 3-5 nm), JC-1 (Fig. 5 (e), anatase structure with particles diameter 15-3 nm), JC-3 (Fig. 5 (f), rutile structure with particles diameter 2-5 nm) were about 56, 71, and 48 m 2 /g, respectively. 3.2 Photocatalytic activity results The I 3 - concentration at 6 min of the irradiation period of the nanorods/nanoparticles TiO 2 were about 2.32 x 1-4 M (Fig. 6), which is higher than that of other synthesized powders (nanorods TiO 2, nanofibers TiO 2, mesoporous TiO 2 ) and also that of 3 commercially available titania nanomaterials, P-25, JC-1, and JC-3 which exhibit I 3 - concentration about 1.5 x 1-4 M,.66 x 1-4 M, and.25 x 1-4 M, respectively. The introduction of mesopore into titania photocatalyst substantially improved the photocatalytic performance [15]. 3.3 Dye-sensitized solar cell results Fig. 7 shows comparison between photocurrent voltage characteristics of the cell using the mesoporous anatase TiO 2 nanopowders (thinckness = 1. m) and P-25 (thinckness = 13.8 m). The solar energy conversion efficiency of the cell using the mesoporous anatase TiO 2 nanopowders was about 6.3 % with Jsc of m/cm 2, Voc of.72 V and ff of.676; while of the cell using P- 25 reached 5.82 % with Jsc of m/cm 2, Voc of.74 V and ff of.649. Higher current density and efficiency might be attributed to higher amount of adsorbed dye (1.21 x 1-8 mol/cm 2 for the mesoporous anatase TiO 2 nanopowders and 5.68 x 1-8 mol/cm 2 for P-25), owing to larger surface area of the mesoporous anatase TiO 2 nanopowders (Table 1). Fig. 8 (a, c) depict the nitrogen adsorption-desorption isotherms and pore size distributions of the mesoporous anatase TiO 2 nanopowders calcined at 45 o C for 2 h. The isotherms exhibit typical type IV pattern with hysteresis loop, characteristic of mesoporous material according to the classification of IUPC. sharp increase in adsorption volume of N 2 was observed and located in the P/P range of This sharp increase can be imputable to the capillary condensation, indicating the good homogeneity of the sample and fairly small pore size since the P/P position of the inflection point is related to the pore dimension. The pore size distribution obtained by BJH approach (Fig. 8 (c)) is noticeably narrow, confirming good quality of the sample. In comparison, the isotherm of the P-25 after calcined at 45 o C for 2 h is also shown in Fig. 8 (b). It can be seen that the isotherm elucidates the typical IUPC type II pattern, revealing the absence of mesoporous structure in the commercial nanoparticles P-25. The BET surface area of the mesoporous anatase TiO 2 nanopowders and P-25 were around 11 and 54 m 2 /g, respectively. Second, electron transport of anatase structure faster than rutile structure [16]. The crystalline structure of the mesoporous anatase TiO 2 nanopowders calcined at 45 o C for 2 h was anatase, while P-25 calcined at 45 o C for 2 h was a mixture of anatase and rutile (Fig. 9).

3 Intensity (a.u.) (11) (4) (2) (15) (21) (24) (116) (22) (215) dsorbed amount / ml(stp) ḡ dv/d / mm 3 nm -1 g Pore radius / nm adsorption desorption Cu-K 2 (degree).5 1 elative Pressure P/P Fig. 1 (a) X-ray diffraction pattern of the as-synthesized mesoporous anatase TiO 2 nanopowders. (b) Nitrogen adsorption isotherm pattern of the as-synthesized mesoporous anatase TiO 2 nanopowders, and the pore size distribution of the sample with pore diameter about 4-5 nm (inset) (a) (b) (c) (15) (2) (4) (11) (d) Fig. 2 (a) SEM, (b) TEM, (c) SED, and (d) HTEM images of the as-synthesized mesoporous anatase TiO 2 nanopowders

4 SEM images TEM, SED images (a) Nanorods TiO 2 (19 m 2 /g) (d) P-25 (57 m 2 /g) (b) Nanofibers TiO 2 (1 m 2 /g) (e) JC-1 (71 m 2 /g) (c) Mesoporous TiO 2 (8 m 2 /g) (f) JC-3 (48 m 2 /g) Fig. 3 SEM, TEM, SED, and BET surface area results of the prepared and commercial TiO 2. (a) nanorods TiO 2 prepared by hydrothermal at 17 o C for 72 h as ref. 6. (b) nanofibers TiO 2 prepared by hydrothermal at 15 o C for 72 h as ref (c) mesoporous TiO 2 prepared by sol-gel at 8 o C for 7 days as ref (d-f) the commercial TiO 2 (P-25, JC-1, and JC-3)

5 3 (a) Concentration of I 3 - x 1-4 (M) Irradiation time (min) Mesoporous TiO2 (Hydrothermal, 193 m 2 /g) Mesoporous TiO2 (Sol-gel, 8 m 2 /g) P-25 (56 m 2 /g) JC-1 (71 m 2 /g) JC-3 (48 m 2 /g) Nanorods TiO2 (19 m 2 /g) Nanofibers TiO2 (1 m 2 /g) No sample, No light (b) 2 Photocurrent density (m/cm ) Nanoparticles TiO 2 Jsc = m/cm 2 Voc =.72 V, FF =.676, = 6.3 %, thinkness = 1. m P-25 Jsc = m/cm 2 Voc =.74 V, FF =.649 = 5.82 %, thinkness = 13.8 m Voltage (V) Fig. 4 (a) Photocatalytic activity (I 3 - concentration) of the mesoporous anatase TiO 2 nanopowders (prepared by this study, hydrothermal method), the nanorods TiO 2 (prepared as ref. 6), the nanofibers TiO 2 (prepared as ref. 3-4), the mesoporous TiO 2 (prepared as ref. 8-1, sol-gel method) and commercial TiO 2 (P-25, JC-1, and JC-3). (b) The comparison between photocurrentvoltage characteristic of a typical dye sensitized solar cells fabricated by the mesoporous anatase TiO 2 nanopowders and P-25 dsorbed amount / ml(stp) ḡ dv/d / mm 3 nm -1 g Pore radius / nm Mesoporous 45 o C 2h (a) P o C 2h.5 1 elative Pressure P/P Intensity (a.u.) = natase TiO 2 = utile TiO 2 Mesoporous TiO 2 45 o C, 2 h (b) P o C, 2 h Cu-K 2 (degree) Fig. 8 (a) Nitrogen adsorption isotherm pattern of the mesoporous anatase TiO 2 nanopowders calcined at 45 o C for 2 h, BET surface are 134 m 2 /g), P-25 (45 o C for 2 h, BET surface are 54 m 2 /g), and the pore size distribution of the mesoporous anatase TiO 2 nanopowders with pore diameter about 6-1 nm (inset). (b) X-ray diffraction patterns of the mesoporous anatase TiO 2 nanopowders calcined at 45 o C for 2 h and P-25 calcined at 45 o C for 2 h Table 1 BET surface area and amount of adsorbed dye of the mesoporous anatase TiO 2 nanopowders (prepared by this study) calcined at 45 o C for 2 h and P-25 calcined at 45 o C for 2 h Samples BET surface area (m 2 /g) mount of adsorbed dye (mol/cm 2 ) Mesoporous TiO x 1-8 P x CONCLUSION In summary, high surface area (about 18 m 2 /g) anatase TiO 2 with mesoporous structure (pore diameter about 3-4 nm) were synthesized by hydrothermal method at 13 o C for 12 h. Mesoporous anatase TiO 2 nanopowders (prepared by this study) showed higher photocatalytic activity than the nanorods TiO 2, nanofibers TiO 2, mesoporous TiO 2 (prepared by sol-gel method), and commercial TiO 2 (P-25, JC-1, and JC-3). The of the cell using mesoporous anatase TiO 2 nanopowders (prepared by this study) was about 6.3 %, while of the cell using P-25 reached 5.82 %. This synthesis method providing a simple route to fabricate mesoporous anatase TiO 2 nanopowders with mild condition.

6 5. CKNOWLEDGMENTS The authors would like to express gratitude to Prof. S. Isoda and Prof. H. Kurata, Institute for Chemical esearch, Kyoto University for the use of TEM apparatus Prof. T. Yoko, Institute for Chemical esearch, Kyoto University for the use of XD equipment. Prof. Mochizuki at IST for the kind supply of Pt counter electrode. We are also grateful to thegeomatec Co. Ltd. for providing a part of conducting glass. This work was supported by grant-in-aids from the Ministry of Education, Science Sports, and Culture of Japan under the 21 COE program, the Nanotechnology Support Project, NEDO under high-performance dye-sensitized solar cell project, and from JSPS research fellow. 6. EFEENCES [1] Fujishima,., ao, T. N. and Tryk, D.. (2) Titanium dioxide photocatalysis, J. Photochem. Photobiol. C:Photochem. ev., 1, pp [2] Pavasupree, S., Suzuki, Y., Kitiyanan,., Pivsa-rt, S. and Yoshikawa, S. (25) Synthesis and characterization of vanadium oxides nanorods, J. Solid State Chem., 178, pp [3] Suzuki, Y., Pavasupree, S., Yoshikawa, S. and Kawahata,. (25) Natural rutile-derived titanate nanofibers prepared by direct hydrothermal processing, J. Mater. es., 2, pp [4] Pavasupre, S., Suzuki, Y., Yoshikawa, S. and Kawahata,. (25) Synthesis of titanate, TiO 2 (B), and anatase TiO 2 nanofibers from natural rutile sand, J. Solid State Chem., 178, pp [5] Pavasupree, S., Suzuki, Y., Pivsa-rt, S. and Yoshikawa, S. (25) Synthesis and characterization of nanoporous, nanorods, nanowires metal oxides, Sci. Tech. dv. Mater., 6, pp [6] Pavasupree, S., Ngamsinlapasathian, S., Nakajima, M., Suzuki, Y. and Yoshikawa, S. (26) Synthesis, characterization, photocatalytic activity and dye-sensitized solar cell performance of nanorods/nanoparticles TiO 2 with mesoporous structure, J. Photochem. Photobio. : Chem., in press. [7] Pavasupree, S., Ngamsinlapasathian, S., Suzuki Y. and Yoshikawa, S. (26) Synthesis and dye-sensitized solar cell performance of nanorods/nanoparticles TiO 2 from high surface area nanosheet TiO 2, J. Nanosci. Nanotech., in press. [8] Pavasupree, S., Suzuki, Y., Pivsa-rt, S. and Yoshikawa, S. (25) Preparation and characterization of mesoporous MO 2 (M=Ti, Ce, Zr, and Hf) nanopowders by a modified sol-gel method, Ceram. Int., 31, pp [9] Pavasupree, S., Suzuki, Y., Pivsa-rt, S. and Yoshikawa, S. (25) Preparation and characterization of mesoporous TiO 2 CeO 2 nanopowders respond to visible wavelength, J. Solid State Chem., 178, pp [1] Kitiyanan,., Ngamsinlapasathian, S., Pavasupree, S. and Yoshikawa, S. (25) The preparation and characterization of nanostructured TiO 2 ZrO 2 mixed oxide electrode for efficient dye-sensitized solar cells, J. Solid State Chem., 178, pp [11] dachi, M., Murata, Y., Harada, M. and Yoshikawa, S. (2) Formation of titania nanotubes with high photo-catalytic activity, Chem.Lett., 8, pp [12] dachi, M., Okada, I., Ngamsinlapasathian, S., Murata, Y. and Yoshikawa, S. (22) Dye-sensitized solar cell using semiconductor thin film composed of titania nanotubes, Electrochemistry, 7, pp [13] dachi, M., Murata, Y., Okada, I. and Yoshikawa, S. (23) Formation of titania nanotubes and applications for dyesensitized solar cells, J. Electrochem. Soc., 15, (8), pp. G488-G493. [14] Sakulkhaemaruethai, S., Pavasupree, S., Suzuki, Y., and Yoshikawa, S. (25) Photocatalytic activity of titania nanocrystals prepared by surfactant-assisted templating method effect of calcination conditions, Mater. Lett., 59, pp [15] Sreethawong, T., Suzuki, Y. and Yoshikawa, S. (25) Synthesis, characterization, and photocatalytic activity for hydrogen evolution of nanocrystalline mesoporous titania prepared by surfactant-assisted templating sol-gel process, J. Solid State Chem., 178, pp [16] Park, N.-G., van de Lagemaat, J. and Frank,.J. (2) Comparison of dye-sensitized rutile- and anatase-based TiO 2 solar cells, J. Phys. Chem. B, 14, pp [17] Pavasupree, S., Jitputti, J., Ngamsinlapasathian, S., Suzuki, Y. and Yoshikawa, S. (26) Hydrothermal synthesis, characterization, photocatalytic activity and dye-sensitized solar cell performance of mesoporous anatase TiO 2 nanopowders, in contribution.