International Journal of Photoenergy

Volume 2014 (2014), Article ID 637532, 7 pages

http://dx.doi.org/10.1155/2014/637532

Research Article

Photocatalytic Activity of Hierarchically Structured TiO₂ Films Synthesized by Chemical Vapor Deposition

Heon Lee,¹ Sung Hoon Park,¹ Young-Kwon Park,² Sun-Jae Kim,³ Byung-Hoon Kim,⁴ Jaehong Lee,⁵ and Sang-Chul Jung¹

¹Department of Environmental Engineering, Sunchon National University, Sunchon, Jeonnam 540-742, Republic of Korea

²School of Environmental Engineering, University of Seoul, Seoul 130-743, Republic of Korea

³Faculty of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 143-747, Republic of Korea

⁴Department of Dental Materials, School of Dentistry, Chosun University, Gwangju 501-759, Republic of Korea

⁵Department of Architecture Engineering, Sunchon National University, Sunchon, Jeonnam 540-742, Republic of Korea

Received 28 January 2014; Accepted 7 February 2014; Published 10 March 2014

Academic Editor: Kangle Lv

Copyright © 2014 Heon Lee et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Hierarchically structured TiO₂ photocatalyst films were synthesized using low-pressure metal-organic chemical vapor deposition (LPMOCVD) method to examine their photocatalytic activity. The thickness of the TiO₂ films increased proportionally with increasing deposition time. The TiO₂ film synthesized at 773 K showed a hierarchical structure composed of vertically grown laminar (112)-oriented anatase crystals. With increasing deposition time, the grain became larger and the morphology became sharper. In the initial CVD stage, small particular crystals were formed, above which sequential growth of layers of columnars with increasing size took place, forming hierarchical structure. The hierarchically structured TiO₂ film exhibited much higher photocatalytic activity than unhierarchically structured TiO₂ film. The photocatalytic activity increased with increasing film thickness.

1. Introduction

Photocatalysis is a process in which a semiconducting material absorbs light energy more than or equal to its band gap, thereby generating holes and electrons, which can further generate free radicals on surface [1, 2]. The resulting free radicals are very efficient oxidizers of organic matter [3, 4]. TiO₂ has been considered to be an effective photocatalyst due to its large surface area [5, 6]. Recently, many efforts have been concentrated on maximizing the catalytic performance of TiO₂-based photocatalysts [7–10]. It has been proved that highly crystalline TiO₂ materials have higher photocatalytic activities than their amorphous counterparts because of a higher rate of photogeneration of electron-hole pairs. Furthermore, a high surface area is also one of the critical factors in maximizing the amount of redox reaction sites available on the catalyst surface [11].

Hierarchical structured materials provide photocatalyst with large surface area in wide applications including catalysis [12]. Such hierarchical structure can promote chemical reactions occurring on surface more effectively [13, 14]. Therefore, the high reaction activity can be achieved with hierarchical structured TiO₂ photoatalyst films. A variety of synthesis techniques, including sol-gel methods [15] and chemical vapor deposition (CVD) growth [16], have been used to form hierarchically structured TiO₂. Among these, CVD is considered a promising method to prepare high-quality thin films over large surface area with a well-controlled composition and low defect density [5, 11].

In this paper, we prepared anatase crystal structure TiO₂ photocatalyst films with different morphologies using CVD and determined the effects of film morphology on photocatalytic activity. We also investigated the change in TiO₂ film morphology during the growth by CVD process.

2. Experimental Procedure

2.1. Preparation of LPMOCVD-Grown TiO₂ Photocatalyst Films

For the analysis of the crystal structure, TiO₂ photocatalyst films were grown on silicon substrate, and also on alumina balls, using a LPMOCVD apparatus, with titanium tetraisopropoxide (Ti[OCH(CH₃)₂]₄, TTIP) as a reagent. Details of the apparatus have been described in our previous paper [11]. The LPMOCVD conditions used for the preparation of TiO₂ films were as follows: total flow rate of gas fed to the reactor of 1500 sccm, oxygen concentration at the reactor inlet of 50 mol%, operating pressure of 1 torr, and deposition temperature of 673 K and 773 K. The morphology of the films grown on the Si substrate was evaluated using a field emission-scanning electron microscope (FESEM, Hitachi, S-4700). The crystal structure was characterized by X-ray diffraction (Max Science, MPX3).

2.2. Evaluation of Photocatalytic Activity

The photocatalytic activities of the TiO₂ films were evaluated by the photocatalytic decomposition of an aqueous solution of bromothymol blue (BTB) in a recirculation type annular tube photoreactor. Details of the photoreactor have been described in our previous paper [11]. The photocatalyst balls were placed in the annular space of the reactor. A UV-A lamp (Philips, TL4W/05, with the UV output power of 0.2 W which is most intensive at 365 nm) was used as a light source. The decomposition rate was evaluated as a function of the irradiation time from the change in the BTB concentration at the reactor outlet. The concentration of BTB was measured by the absorbance at , using a spectrophotometer (UV-1601, Shimadzu). The initial BTB concentration was about  mol/liter, with 500 cm³ of solution circulated in the reactor at a flow rate of 200 cm³/min. Specific surface area of TiO₂ films is very small so that it was measured by a BET adsorption apparatus with differential tensimeter of a symmetrical design (Belsorp-18plus, BEL Japan, Inc.) at liquid nitrogen temperature by adsorbing krypton gas (99.995%) to the film. Films with different temperatures grown on 1 gram of the small glass beads were pretreated by heating the samples up to 383 K for two hours in flowing helium gas (99.9999%) under vacuum.

3. Results and Discussion

3.1. Preparation of TiO₂ Films by CVD

TiO₂ films with different morphologies were synthesized to examine the effects of the film morphology on photocatalytic activity. The morphology of crystal structure of TiO₂ films synthesized by CVD varies depending on CVD reaction temperature. A previous study reported that TiO₂ film with hierarchical structure could be obtained when the CVD process temperature was high [5]. Therefore, we synthesized TiO₂ films at two different temperatures: 773 K to obtain hierarchical structure and 673 K to obtain nonhierarchical structure. Figure 1 compares the time evolution of TiO₂ film thickness observed at two CVD temperatures. The film thickness was proportional to the deposition time at both temperatures. The film thickness obtained at 773 K was 5.5 times higher than that obtained at 673 K.

Figure 1: Thickness of TiO₂ films as a function of deposition times.

One method that can be used to increase the film growth rate at a given CVD temperature is to increase the TTIP dose rate by reducing CVD operating pressure or increasing source evaporator temperature. The supply of excessive quantity of TTIP, however, may lead to the generation of airborne TiO₂ particles by gas-phase reaction among reactive species with high concentration. The concurrent deposition of these TiO₂ aerosol particles reduces the film density, and, hence, the photocatalytic activity of the film. Therefore, in this study, caution was given so that the TTIP concentration was suppressed to prohibit the generation of aerosol particles.

TiO₂ has three crystal phases: brookite, anatase, and rutile; the latter two crystal phases have been widely adopted in photocatalysis, solar cells, and self-cleaning. A previous study reported that TiO₂ film was formed with amorphous structure below 577 K, with anatase crystal structure between 673 K and 823 K, with anatase-rutile combined crystal structure between 873 K and 1023 K, and with rutile crystal structure at and above 1073 K [11]. In this study, TiO₂ films were synthesized at 673 K and 773 K to obtain anatase crystal phase. Figure 2 shows the X-ray diffraction patterns of the TiO₂ films grown at the two deposition temperatures. Although TiO₂ films with anatase crystal structure were synthesized at both temperatures, the film grown at 773 K was (112)-oriented, whereas the one grown at 673 K was (211)- and (118)-oriented.

Figure 2: X-ray diffraction of anatase TiO₂ films prepared by CVD method under different deposition temperatures.

Figure 3 shows the FESEM images of a TiO₂ film prepared at 673 K using the CVD method: surface morphology (a) and cross-section morphology (b). Grains deposited irregularly with the size of 200300 nm were observed. The surface and cross-section morphology of the film grown at 673 K revealed that the film structure was not amorphous but crystalline. However, the crystalline structure was not hierarchical.

(a)

(b)

Figure 3: Surface and cross-sectional morphology of TiO₂ films prepared by the CVD methods. Deposition time: 2 hours; deposition temperature: 673 K.

Figure 4 shows the FESEM images of the TiO₂ film prepared at 773 K: low-resolution surface morphology (a), high-resolution surface morphology (b), and cross-section morphology (c). In the low-resolution surface image (a), 0.51.0 μm sized grains deposited irregularly were observed. The high-resolution surface image (b) reveals that each grain is composed of sheet-like (laminar) crystals. Generally, this laminar crystal structure is observed in vermiculite. The cross-section morphology (c) shows that the TiO₂ film with laminar crystal structure has grown vertically from substrate. The columnar crystals of the film found near substrate were small, whereas larger columnar crystals were observed at distant locations from the substrate. This layered structure with different crystal sizes is called hierarchical structure. Figure 4(c) shows that the structure of the TiO₂ film synthesized at 773 K is hierarchical with the columnar crystal size increasing along the -axis.

(a)

(b)

(c)

Figure 4: Surface and cross-sectional morphology of TiO₂ films prepared by the CVD methods. Deposition time: 4 hours; deposition temperature: 773 K.

Figure 5 shows the FESEM images of the surface of TiO₂ films prepared with different deposition times. Flat surface composed of tiny particles was observed when the deposition time was 1 min. TiO₂ grains with laminar structure were observed when the deposition time was 10 min or longer. The grain size increased with increasing deposition time. The grain morphology also became sharper with increasing deposition time. In a previous study [5], the BET analysis showed that the grain size and specific surface area increased rapidly in the early TiO₂ film growth phase of CVD process, whereas the grain size and surface area increasing rates became lower after two hours, which is in good agreement with the hierarchical structure with increasing columnar crystal size observed in Figure 4.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 5: Surface morphology of TiO₂ films grown by CVD at different deposition times.

3.2. Mechanism of Hierarchical Growth of TiO₂ Film

As shown in Figures 4 and 5, small particular crystals were observed in the initial growth stage (in the bottom part of film), while larger columnar crystals were observed above these particles. A mechanism for the growth of hierarchically structured TiO₂ film by CVD is suggested in Figure 6. In the initial stage, small particular crystals are formed on the substrate via TiO₂ nucleation, nuclei growth, and coalescence. Then, small-sized columnar grows on these particles, followed by sequential growth of larger and larger columnaris constructing hierarchical structure. After the deposition time of two hours (after the TiO₂ film grew thicker than 3 μm), the TiO₂ crystal width remained almost uniform (about 0.50.7 μm). These changes may correspond to Kolmogorov’s “geometrical selection” of crystals [17]. In conclusion, TiO₂ film with hierarchical anatase crystal structure is synthesized by CVD at 773 K until a certain deposition time (or up to a certain crystal thickness), beyond which the crystal width remains unchanged.

Figure 6: Proposed growth mechanism for the hierarchically structured TiO₂ films by CVD.

3.3. Evaluation of Photocatalytic Reaction Activity

The TiO₂ films synthesized in this study were used for the photocatalytic decomposition of BTB. Figure 7 shows the BTB concentration decay as a function of the irradiation time. The decomposition of BTB via the photocatalytic reaction in the presence of TiO₂ photocatalyst could be approximated with a pseudo-first-order reaction model:

where is the BTB concentration at time , the initial concentration, and the overall rate constant.

Figure 7: Photocatalytic decomposition of BTB on the CVD grown TiO₂ film with different deposition temperatures.

The hierarchically structured TiO₂ photocatalyst grown at 773 K showed much higher catalytic activity than the unhierarchically structured TiO₂ photocatalyst grown at 673 K. This result can be attributed to the larger surface area of the hierarchically structured TiO₂ photocatalyst. BET analysis showed that the specific surface area of hierarchically structured TiO₂ photocatalyst was 0.1373 m²/g, whereas that of unhierarchically structured TiO₂ film was 0.0615 m²/g.

Figure 8 compares the BTB decomposition rate constants obtained with the hierarchically structured TiO₂ photocatalysts prepared at 773 K with different deposition times (with different thicknesses). The overall rate constant, , was determined from the slopes of the lines in Figure 7 using linear regression analysis. The rate constant increased with increasing deposition time. This can be attributed to the increase in the surface area of the photocatalyst film with increasing film thickness. A previous study [5] also reported that the decomposition rate of methylene blue increased with increasing TiO₂ film deposition time, which was attributed to the increase in the surface area of TiO₂ film. The slowdown of the increase in film surface area after the deposition time of two hours (after the TiO₂ film grew thicker than 3 μm), leading to the slowdown of the decomposition of methylene blue, was also reported in that study. The results of the present study also showed that the photocatalytic activity of the hierarchically structured TiO₂ photocatalyst film synthesized using CVD method increased with increasing deposition time (with increasing film thickness) due to increasing specific surface area.

Figure 8: Effect of hierarchically structured TiO₂ film thickness on the apparent first-order rate constant.

4. Conclusions

The conclusions obtained in this study with the hierarchically structured TiO₂ photocatalyst films synthesized using the CVD method are as follows.

(1)TiO₂ film thickness increased proportionally with deposition time.(2)The TiO₂ film synthesized at 773 K showed (112)-oriented anatase crystal structure.(3)In the TiO₂ film synthesized at 773 K, laminar crystal TiO₂ grew vertically from the substrate forming a hierarchical structure.(4)With increasing deposition time, the grain size increased and the morphology became sharper.(5)In the initial stage of CVD, small particular crystals are formed, above which sequential growth of columnaris with increasing size takes place leading to a hierarchical structure. After the deposition time of two hours, or after TiO₂ film grew thicker than 3 μm, the width of TiO₂ crystalline columnar remains uniform.(6)The photocatalytic activity of the hierarchically structured TiO₂ film grown at 773 K was much higher than that of unhierarchically structured TiO₂ film. The photocatalytic activity increased with increasing film thickness.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2013R1A1A2A10004797).

References

M. R. Hoffmann, S. T. Martin, W. Choi, and D. W. Bahnemann, “Environmental applications of semiconductor photocatalysis,” Chemical Reviews, vol. 95, no. 1, pp. 69–96, 1995.

I. Poulios, E. Micropoulou, R. Panou, and E. Kostopoulou, “Photooxidation of eosin Y in the presence of semiconducting oxides,” Applied Catalysis B, vol. 41, no. 4, pp. 345–355, 2003.

C. A. Silva, L. M. Madeira, R. A. Boaventura, and C. A. Costa, “Photo-oxidation of cork manufacturing wastewater,” Chemosphere, vol. 55, no. 1, pp. 19–26, 2004.

S. T. Martin, H. Herrmann, W. Choi, and M. R. Hoffmann, “Time-resolved microwave conductivity. 1. TiO₂ photoreactivity and size quantization,” Journal of the Chemical Society, Faraday Transactions, vol. 90, no. 21, pp. 3315–3322, 1994.

S.-C. Jung, S.-J. Kim, N. Imaishi, and Y.-I. Cho, “Effect of TiO₂ thin film thickness and specific surface area by low-pressure metal-organic chemical vapor deposition on photocatalytic activities,” Applied Catalysis B, vol. 55, no. 4, pp. 253–257, 2005.

J.-M. Herrmann, “Heterogeneous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants,” Catalysis Today, vol. 53, no. 1, pp. 115–129, 1999.

K. Nagaveni, G. Sivalingam, M. S. Hegde, and G. Madras, “Solar photocatalytic degradation of dyes: high activity of combustion synthesized nano TiO₂,” Applied Catalysis B, vol. 48, no. 2, pp. 83–93, 2004.

S.-J. Kim, S.-C. Kim, S.-G. Seo et al., “Photocatalyzed destruction of organic dyes using microwave/UV/O₃/H₂O₂/TiO₂ oxidation system,” Catalysis Today, vol. 164, no. 1, pp. 384–390, 2011.

S. C. Jung, “The microwave-assisted photo-catalytic degradation of organic dyes,” Water Science and Technology, vol. 63, no. 7, pp. 1491–1498, 2011.

H. Lee, S. H. Park, S. J. Kim et al., “The effect of combined processes for advanced oxidation of organic dye using CVD TiO₂ film photo-catalysts,” Progress in Organic Coatings, vol. 74, no. 4, pp. 758–763, 2012.

S.-C. Jung and N. Imaishi, “Preparation, crystal structure, and photocatalytic activity of TiO₂ films by chemical vapor deposition,” Korean Journal of Chemical Engineering, vol. 18, no. 6, pp. 867–872, 2001.

H. Bai, Z. Liu, and D. D. Sun, “Hierarchically multifunctional TiO₂ nano-thorn membrane for water purification,” Chemical Communications, vol. 46, no. 35, pp. 6542–6544, 2010.

H. Bai, J. Juay, Z. Liu, X. Song, S. S. Lee, and D. D. Sun, “Hierarchical SrTiO₃/TiO₂ nanofibers heterostructures with high efficiency in photocatalytic H₂ generation,” Applied Catalysis B, vol. 125, pp. 367–374, 2012.

H. Bai, Z. Liu, and D. D. Sun, “The design of a hierarchical photocatalyst inspired by natural forest and its usage on hydrogen generation,” International Journal of Hydrogen Energy, vol. 37, no. 19, pp. 13998–14008, 2012.

Z. Liu, D. D. Sun, P. Guo, and J. O. Leckie, “One-step fabrication and high photocatalytic activity of porous TiO₂ hollow aggregates by using a low-temperature hydrothermal method without templates,” Chemistry—A European Journal, vol. 13, no. 6, pp. 1851–1855, 2007.

S.-C. Jung, B.-H. Kim, S.-J. Kim, N. Imaishi, and Y.-I. Cho, “Characterization of a TiO₂ photocatalyst film deposited by CVD and its photocatalytic activity,” Chemical Vapor Deposition, vol. 11, no. 3, pp. 137–141, 2005.

A. N. Kolmogorov, “Geometric selection of crystals,” Doklady Akademii Nauk SSSR, vol. 65, pp. 681–684, 1949.