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hydroxyapatite composite
Materials Research Bulletin 46 (2011) 2283–2287
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Characterisation and photocatalytic activity of structure-controlled spherical granules of an anatase/hydroxyapatite composite Masanobu Kamitakahara *, Osamu Kawaguchi, Noriaki Watanabe, Koji Ioku Graduate School of Environmental Studies, Tohoku University, 6-6-20 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 July 2011 Received in revised form 18 August 2011 Accepted 30 August 2011 Available online 6 September 2011
Materials that can purify the environments are desirable. Anatase (TiO2) has received attention because it is stable and can decompose organic substances because of its photocatalytic activity. To make use of anatase effectively, we deposited nano-sized anatase particles on porous hydroxyapatite (HA) ceramics composed of rod-shaped particles. Spherical porous HA granules composed of rod-shaped HA particles were prepared using a hydrothermal process. The granules were soaked in a solution containing a watersoluble titanium complex and then hydrothermally treated. Nano-sized anatase particles were deposited on each rod-shaped HA particle. The anatase/HA granules composed of rod-shaped HA particles showed higher photocatalytic activity than those composed of globular HA particles. The granules are expected to be useful as an environment-purifying material with high manageability and photocatalytic activity. ß 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Ceramics A. Composites C. X-ray diffraction D. Catalytic properties
1. Introduction Environment pollution is a serious problem and materials that can purify the environment are desirable. Anatase (TiO2) has received attention as an environmental purification material because it is stable and can decompose organic substances as the result of its photocatalytic activity [1–3]. When anatase is irradiated with ultraviolet (UV) light, radicals are formed on the surface, and these radicals cause the organic substances on the anatase surface to decompose. To make use of anatase particles effectively, it is important to immobilise them without diminishing their catalytic ability. It is not easy to immobilise anatase particles on polymeric materials because the polymeric materials are degraded by the photocatalytic activity of anatase. We deposited anatase particles on porous hydroxyapatite (HA, Ca10(PO4)6(OH)2) ceramics because HA ceramics are very stable and have high adsorption abilities [4,5]. Several researchers reported the usefulness of HA as a catalyst support [6,7]. There have also been reports on composite materials composed of anatase and HA [8–11], but these composites are in the shape of films or particles, so their application is limited. We previously successfully prepared anatase/HA composite granules by hydrothermal treatment and revealed that the granules have the ability to decompose methylene blue in a liquid system and acetaldehyde in a gas system [12,13]. The spherical, granular shape provides easy
* Corresponding author. Tel.: +81 22 795 7375; fax: +81 22 795 7375. E-mail address: [email protected] (M. Kamitakahara). 0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2011.08.059
handling. However, the fixation of anatase depended on the mechanical locking of the anatase particles to the HA porous structure. We speculated that the microstructures of HA ceramics would affect the efficiency of the photocatalytic activity of anatase because the microstructures govern the adsorption and diffusion of substances. A hydrothermal method can provide porous HA ceramics composed of rod-shaped HA particles under mild conditions [14–17]. The rod-shaped HA particles are expected to contribute to the toughness of the granules because of the entanglement of the particles. Moreover, large spaces formed by the rod-shaped particles would provide a location in which organic substances can adsorb and still be exposed to UV light. If nanosized anatase particles can be deposited on rod-shaped HA particles without losing the microstructure of the granules, the advantages of the structure would be put to use. In the deposition of the nano-sized anatase particles, a water-soluble titanium complex was used [18,19]. The water-soluble titanium complex was decomposed, and then titania particles were formed under hydrothermal conditions. Sujaridworakun et al. reported that nano-sized anatase particles can be deposited on HA particles [11]. We expected that nano-sized anatase particles could be deposited preferentially on rod-shaped HA particles if the hydrothermal conditions were controlled to allow the deposition of anatase with preventing homogeneous deposition. We prepared spherical porous granules composed of rodshaped HA particles by the hydrothermal treatment of alphatricalcium phosphate (a-TCP, Ca3(PO4)2). These granules were soaked in a solution containing the water-soluble titanium
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2. Materials and methods
Ltd., Japan) and 10 cm3 of 30 mass% H2O2 solution (Wako Pure Chemical Industries, Ltd., Japan), and then, this solution was diluted with distilled water to obtain a solution with a titanium concentration of 10 mg cm3, based on the method previously reported [18]. The obtained HA granules (0.2 g) were soaked in 4 cm3 of the obtained water-soluble titanium complex solution and were hydrothermally treated at 200 8C for 6 h to deposit anatase on the surface of the HA granules.
2.1. Preparation of HA granules
2.3. Characterisation of the samples
Spherical porous granules composed of rod-shaped HA particles (Rod-HA) were prepared by a hydrothermal process [17]. A flowchart of the procedure and a schematic illustration of the hydrothermal treatment are shown in Fig. 1. a-TCP powder (Taihei Chemical Industrial Co., Ltd., Japan) was mixed with a 10 mass% gelatine solution to obtain a slurry. The gelatine was purchased from Wako Pure Chemical Industries, Ltd., Japan. The mixing mass ratio of powder to solution was 1/5. The slurry was dropped in liquid N2. The frozen spherical granules were collected and then dried at 4 8C for 5 d in a refrigerator. The obtained a-TCP/gelatine granules were heated at 1200 8C for 10 min to obtain a-TCP granules. The a-TCP granules were set in a Teflon1-lined stainless steel autoclave with distilled water and were hydrothermally treated under saturated water vapour at 120 8C for 24 h. As a reference material, spherical porous granules composed of globular HA particles (Glob-HA) were prepared by a sintering method. HA powder (Ube Material Industries, Ltd., Japan) was mixed with a 10 mass% gelatine solution to obtain a slurry. The mixing mass ratio of powder to solution was 1/5. The slurry was dropped in liquid N2. The frozen spherical granules were collected and then dried at 4 8C for 5 d in a refrigerator. The obtained HA/ gelatine granules were heated at 1000 8C for 6 h to remove the gelatine and sinter the particles.
The phases of the samples were examined by X-ray diffraction (XRD, RINT2200VL, Rigaku Co., Japan). The morphologies of the samples were observed using a digital microscope (VHX100, Keyence Co., Japan) and a scanning electron microscope (SEM, S4100, Hitachi, Ltd., Japan). For the SEM observations, a thin coating of Pt was deposited on the surfaces of the samples. The microstructure of the sample was observed by a transmission electron microscope (TEM, HF-2000, Hitachi, Ltd., Japan) equipped with an energy dispersive X-ray spectrometer (EDX, NORAN Instruments Inc., USA). In the preparation of the specimen for the TEM observation, the surface of the granule was scratched, and particles were obtained. The obtained particles were dispersed in ethanol and loaded on a grid for TEM observation. The specific surface areas of the samples were determined by a BET method using nitrogen gas adsorption (AUTOSORB-1, AS-1MP/XP, Yuasa Ionics Inc., Japan).
complex to prepare spherical anatase/HA granules. The photocatalytic activity of the anatase/HA granules was determined by measuring the decomposition of methylene blue (MB) under ultraviolet (UV) irradiation. This data was used to determine the potential of spherical anatase/HA granules as an environmentpurifying material.
2.2. Preparation of anatase/HA granules These HA porous granules were soaked in a solution containing a water-soluble titanium complex and were then hydrothermally treated to obtain anatase/HA granules (TiO2/Rod-HA and TiO2/ Glob-HA, respectively). The water-soluble titanium complex solution was prepared by dissolving 0.25 g of titanium (Wako Pure Chemical Industries, Ltd., Japan) in a mixture of 2.5 cm3 of 28 mass% NH3 aqueous solution (Wako Pure Chemical Industries,
2.4. Evaluation of photocatalytic activity The photocatalytic activity of the anatase/HA granules was evaluated by examining the decomposition of methylene blue (MB, Merck KGaA, Germany), which is often used as a model substance. The anatase/HA granules (0.2 g) were soaked in 15 cm3 of 4 ppm MB aqueous solution and incubated for 30 min to reach adsorption equilibrium. After adsorption equilibrium was reached, UV light (LUV-16, AS ONE Co., Ltd., Japan) was used to irradiate the granules. The change in the MB concentration was examined by measuring the absorbance at 664 nm using a UV-VIS spectrophotometer (Sefi IUV-1240, As One Co., Ltd., Japan). The schematic illustration of the apparatus is shown in Fig. 2. As a blank test, the changes in the MB concentration of the system not containing samples were also examined. In the present study, the decrease in concentration (%) was calculated based on the concentrations for
Fig. 1. Procedure for the preparation of Rod-HA.
M. Kamitakahara et al. / Materials Research Bulletin 46 (2011) 2283–2287
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Black light
Methylene blue solution (4 ppm, 15 cm3) Sample (0.2 g) Stirring bar Magnetic stirrer Fig. 2. Procedure for the evaluation of photocatalytic activity.
the tests with and without the sample, csample and cblank, respectively: Decrease in concentration ¼
cblank csample 100 cblank
(1)
3. Results and discussion
Fig. 3. Appearance of Rod-HA.
Fig. 3 shows the photograph of the sample Rod-HA. The morphology and size of the sample Glob-HA were almost same as those of Rod-HA. Spherical granules of about 2 mm in diameter were obtained for both Rod-HA and Glob-HA. The droplets of the slurry were immediately frozen in liquid N2, and frozen spherical granules were formed. As the frozen spherical granules contain gelatine, the granules could keep their spherical shape even at 4 8C after melting. The drying process at 4 8C in a refrigerator was useful in preventing the destruction of the spherical shape during heating at 1200 8C. Fig. 4 shows SEM images of the surfaces of samples Rod-HA and Glob-HA. Rod-HA and Glob-HA were composed of micrometersized, rod-shaped particles and globular-shaped particles, respectively. Although a small amount of monetite (CaHPO4) phase was detected by XRD in Rod-HA, the dominant phase was HA. In GlobHA, only the HA phase was detected. By using the hydrothermal process, spherical HA granules composed of rod-shaped particles were obtained. We could control the microstructure of the HA spheres. The rod-shaped HA particles are expected to contribute to the toughness of the granules because of the entanglement of the particles. We previously reported that we can obtain rod-like HA particles by the hydrothermal treatment of a-TCP [14–17]. In the hydrothermal treatment, a-TCP is reacted with water to form HA, accompanied by the release of phosphoric acid. This released phosphoric acid provides the acidic conditions during the reaction, and the acidic conditions should have resulted in the formation of monetite, which is stable under acidic conditions, in the Rod-HA sample.
Fig. 5 shows SEM images of the samples TiO2/Rod-HA and TiO2/ Glob-HA. Deposits were observed on the surfaces of Rod-HA and Glob-HA after the hydrothermal treatment in the solution of the water-soluble titanium complex. The deposits, which were much smaller than the HA particles, were formed on the surfaces of the particles without filling the space within the granule. In the XRD analysis of both TiO2/Rod-HA and TiO2/Glob-HA, not only diffraction lines due to HA but also a weak diffraction line, probably due to anatase, was detected. The XRD pattern of the sample TiO2/Rod-HA is shown in Fig. 6. In TiO2/Rod-HA, the diffraction line due to monetite disappeared. The monetite remaining in Rod-HA was reacted with water in a solution of the water-soluble titanium complex and was completely changed into HA. As the peak intensity of anatase was weak and only one distinct diffraction line was confirmed in the XRD analysis, TEM observation was conducted to confirm the deposition of anatase on the surfaces of the HA particles. The TEM photograph and electron diffraction of the TiO2/Rod-HA sample are shown in Fig. 7. From the element analysis, Ca, P and Ti were detected. Nano-sized particles were deposited on the rod-shaped particles. The nano-sized particles were identified as anatase based on electron diffraction. The titanium complex decomposed under the hydrothermal conditions to deposit nano-sized anatase particles on the HA particles. This result is consistent with the XRD and SEM results. Despite the preparation processing required to image the specimen with TEM, we could confirm that nano-sized anatase particles were attached on the HA particles. Thus, the anatase
Fig. 4. SEM images of HA granules.
M. Kamitakahara et al. / Materials Research Bulletin 46 (2011) 2283–2287
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Fig. 5. SEM images of TiO2/HA granules.
particles were attached to the HA particles strongly enough for handling. The nano-sized anatase particles are speculated to be deposited on HA surfaces through heterogeneous nucleation. These results indicate that anatase/HA composite granules in which nano-sized anatase particles were deposited on HA particles were successfully prepared. Table 1 shows the specific surface
HA
Intensity / a.u.
Anatase
areas of the samples determined by nitrogen gas adsorption. RodHA showed a larger specific surface area than Glob-HA. The specific surface areas increased after the deposition of anatase particles on HA particles for both of the samples. The decrease in concentration of MB in the photocatalytic activity test is shown in Fig. 8. The concentration change almost ended at 30 min when the adsorption test was conducted as a preliminary experiment, and the UV irradiation was started after the samples had been soaked in the MB solution for 30 min. The transverse of the graph indicates the time after UV irradiation. Therefore, the decrease in concentration at time 0 is due to the adsorption by the samples. TiO2/Rod-HA showed higher adsorption ability than TiO2/Glob-HA. This can be explained by the large specific surface area of TiO2/Rod-HA. Although the specific surface area of TiO2/Rod-HA was increased after the anatase precipitation, TiO2/Rod-HA showed lower adsorption ability than Rod-HA. This implies that anatase shows lower adsorption ability than HA. A continuous decrease in concentration was observed for TiO2/RodHA and TiO2/Glob-HA with irradiation time, while the continuous concentration decrease was not observed for Rod-HA. The
Table 1 Specific surface areas of samples.
20
30
40
2θ / º (CuKα) Fig. 6. XRD pattern of TiO2/Rod-HA.
Sample
Specific surface area (m2 g1)
Rod-HA Glob-HA TiO2/Rod-HA TiO2/Glob-HA
4.9 3.0 5.9 3.3
Decrease in concentration / %
60
TiO2 /Rod-HA 40
20 TiO2 /Glob-HA Rod-HA 0 0
1
Irradiation time / h Fig. 7. TEM image and electron diffraction of TiO2/Rod-HA.
Fig. 8. Decrease in concentration of MB after UV irradiation.
2
Decomposition rate / μg.m-2 . h-1
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was easier in the pores and the area irradiated by UV light was larger for TiO2/Rod-HA, thus resulting in effective MB decomposition. Granules composed of rod-shaped HA particles would be a useful support for anatase.
8 6
4. Conclusions
4
The anatase/HA granules composed of rod-shaped HA particles on which nano-sized anatase particles were deposited showed high photocatalytic ability under UV irradiation. The granules are expected to be useful as an environmental-purifying material with high manageability and photocatalytic activity.
2 0
TiO2/Rod-HA
TiO2/Glob-HA
Fig. 9. MB decomposition rate per unit surface area of the samples.
continuous decrease in concentration of MB is due to the decomposition of MB by the photocatalytic activity of anatase. TiO2/Rod-HA showed higher decomposition ability than TiO2/ Glob-HA. A linear relationship between the decrease in concentration and time was obtained during 0–1.5 h, and the decomposition rate of MB was calculated during this time. The decomposition rate for TiO2/Rod-HA was about six times higher than that of TiO2/Glob-HA when the amounts of the samples were same. As the specific surface area of TiO2/Rod-HA is larger than TiO2/Glob-HA, it can be speculated that this large surface area provides many active sites and the higher photocatalytic activity. However, the higher photocatalytic activity of TiO2/Rod-HA cannot be explained by only the surface area. The decomposition rate of MB per unit surface area was also calculated and the results are shown in Fig. 9. Even considering the decomposition rate of MB per unit surface area, the decomposition rate for TiO2/Rod-HA was about three times higher than that of TiO2/Glob-HA. These results indicate that the sample TiO2/Rod-HA showed higher photocatalytic activity. The properties of the nano-sized anatase particles themselves in TiO2/Rod-HA and TiO2/Glob-HA are speculated to be almost the same because the same water-soluble titanium complex solution and hydrothermal conditions were used. Therefore, we speculated that the difference in the decomposition efficiency of MB was also due to the difference in the microstructure of the HA granules that support the anatase particles. When we observed the microstructure of the HA granules, large spaces were observed among rod-shaped particles for TiO2/Rod-HA. On the other hand, there were smaller spaces among the globular particles in TiO2/Glob-HA. It is speculated that the diffusion of MB
Acknowledgements The authors thank Prof. Kakihana of Tohoku University for kind advice on the experiment and Prof. Tohji and Mr. Miyazaki of Tohoku University for experimental help. References [1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69– 96. [2] O. Carp, C.L. Huisman, A. Reller, Prog. Solid State Chem. 32 (2004) 33–177. [3] A. Fujishima, X.T. Zhang, D.A. Tryk, Surf. Sci. Rep. 63 (2008) 515–582. [4] T. Kawasaki, S. Takahashi, K. Ikeda, Eur. J. Biochem. 152 (1985) 361–371. [5] T. Kawai, C. Ohtsuki, M. Kamitakahara, M. Tanihara, T. Miyazaki, Y. Sakaguchi, S. Konagaya, Environ. Sci. Tech. 40 (2006) 4281–4285. [6] A. Venugopal, M.S. Scurrell, Appl. Catal. A-Gen. 245 (2003) 137–147. [7] K. Mori, T. Hara, T. Mizugaki, K. Ebitani, K. Kaneda, J. Am. Chem. Soc. 126 (2004) 10657–10666. [8] K. Teraoka, T. Nonami, Y. Yokogawa, H. Taoda, T. Kameyama, J. Mater. Res. 15 (2000) 1243–1244. [9] T. Nonami, H. Hase, K. Funakoshi, Catal. Today 96 (2004) 113–118. [10] A. Nakajima, K. Takakuwa, Y. Kameshima, M. Hagiwara, S. Sato, Y. Yamamoto, N. Yoshida, T. Watanabe, K. Okada, J. Photochem. Photobiol. A-Chem. 177 (2006) 94– 99. [11] P. Sujaridworakun, F. Koh, T. Fujiwara, D. Pongkao, A. Ahniyaz, M. Yoshimura, Mater. Sci. Eng. C 25 (2005) 87–91. [12] S. Ji, S. Murakami, M. Kamitakahara, K. Ioku, Mater. Res. Bull. 44 (2009) 768–774. [13] M. Kamitakahara, S. Murakami, N. Watanabe, S. Ji, H. Nishikawa, K. Ioku, J. Ceram. Soc. Jpn. 117 (2009) 1172–1174. [14] K. Ioku, G. Kawachi, S. Sasaki, H. Fujimori, S. Goto, J. Mater. Sci. 41 (2006) 1341– 1344. [15] K. Ioku, S. Yamauchi, H. Fujimori, S. Goto, M. Yoshimura, Solid State Ionics 151 (2002) 147–150. [16] M. Kamitakahara, C. Ohtsuki, G. Kawachi, D. Wang, K. Ioku, J. Ceram. Soc. Jpn. 116 (2008) 6–9. [17] K. Ioku, M. Kamitakahara, N. Watanabe, O. Kawaguchi, S. Murakami, T. Ikeda, Key Eng. Mater. 396–398 (2009) 645–648. [18] M. Kakihana, M. Tada, M. Shiro, V. Petrykin, M. Osada, Y. Nakamura, Inorg. Chem. 40 (2001) 891–894. [19] K. Tomita, M. Kobayashi, V. Petrykin, S. Yin, T. Sato, M. Yoshimura, M. Kakihana, J. Mater. Sci. 43 (2008) 2217–2221.
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Characterisation and photocatalytic activity of structure-controlled spherical granules of an anatase/hydroxyapatite composite Masanobu Kamitakahara *, Osamu Kawaguchi, Noriaki Watanabe, Koji Ioku Graduate School of Environmental Studies, Tohoku University, 6-6-20 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 July 2011 Received in revised form 18 August 2011 Accepted 30 August 2011 Available online 6 September 2011
Materials that can purify the environments are desirable. Anatase (TiO2) has received attention because it is stable and can decompose organic substances because of its photocatalytic activity. To make use of anatase effectively, we deposited nano-sized anatase particles on porous hydroxyapatite (HA) ceramics composed of rod-shaped particles. Spherical porous HA granules composed of rod-shaped HA particles were prepared using a hydrothermal process. The granules were soaked in a solution containing a watersoluble titanium complex and then hydrothermally treated. Nano-sized anatase particles were deposited on each rod-shaped HA particle. The anatase/HA granules composed of rod-shaped HA particles showed higher photocatalytic activity than those composed of globular HA particles. The granules are expected to be useful as an environment-purifying material with high manageability and photocatalytic activity. ß 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Ceramics A. Composites C. X-ray diffraction D. Catalytic properties
1. Introduction Environment pollution is a serious problem and materials that can purify the environment are desirable. Anatase (TiO2) has received attention as an environmental purification material because it is stable and can decompose organic substances as the result of its photocatalytic activity [1–3]. When anatase is irradiated with ultraviolet (UV) light, radicals are formed on the surface, and these radicals cause the organic substances on the anatase surface to decompose. To make use of anatase particles effectively, it is important to immobilise them without diminishing their catalytic ability. It is not easy to immobilise anatase particles on polymeric materials because the polymeric materials are degraded by the photocatalytic activity of anatase. We deposited anatase particles on porous hydroxyapatite (HA, Ca10(PO4)6(OH)2) ceramics because HA ceramics are very stable and have high adsorption abilities [4,5]. Several researchers reported the usefulness of HA as a catalyst support [6,7]. There have also been reports on composite materials composed of anatase and HA [8–11], but these composites are in the shape of films or particles, so their application is limited. We previously successfully prepared anatase/HA composite granules by hydrothermal treatment and revealed that the granules have the ability to decompose methylene blue in a liquid system and acetaldehyde in a gas system [12,13]. The spherical, granular shape provides easy
* Corresponding author. Tel.: +81 22 795 7375; fax: +81 22 795 7375. E-mail address: [email protected] (M. Kamitakahara). 0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2011.08.059
handling. However, the fixation of anatase depended on the mechanical locking of the anatase particles to the HA porous structure. We speculated that the microstructures of HA ceramics would affect the efficiency of the photocatalytic activity of anatase because the microstructures govern the adsorption and diffusion of substances. A hydrothermal method can provide porous HA ceramics composed of rod-shaped HA particles under mild conditions [14–17]. The rod-shaped HA particles are expected to contribute to the toughness of the granules because of the entanglement of the particles. Moreover, large spaces formed by the rod-shaped particles would provide a location in which organic substances can adsorb and still be exposed to UV light. If nanosized anatase particles can be deposited on rod-shaped HA particles without losing the microstructure of the granules, the advantages of the structure would be put to use. In the deposition of the nano-sized anatase particles, a water-soluble titanium complex was used [18,19]. The water-soluble titanium complex was decomposed, and then titania particles were formed under hydrothermal conditions. Sujaridworakun et al. reported that nano-sized anatase particles can be deposited on HA particles [11]. We expected that nano-sized anatase particles could be deposited preferentially on rod-shaped HA particles if the hydrothermal conditions were controlled to allow the deposition of anatase with preventing homogeneous deposition. We prepared spherical porous granules composed of rodshaped HA particles by the hydrothermal treatment of alphatricalcium phosphate (a-TCP, Ca3(PO4)2). These granules were soaked in a solution containing the water-soluble titanium
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2. Materials and methods
Ltd., Japan) and 10 cm3 of 30 mass% H2O2 solution (Wako Pure Chemical Industries, Ltd., Japan), and then, this solution was diluted with distilled water to obtain a solution with a titanium concentration of 10 mg cm3, based on the method previously reported [18]. The obtained HA granules (0.2 g) were soaked in 4 cm3 of the obtained water-soluble titanium complex solution and were hydrothermally treated at 200 8C for 6 h to deposit anatase on the surface of the HA granules.
2.1. Preparation of HA granules
2.3. Characterisation of the samples
Spherical porous granules composed of rod-shaped HA particles (Rod-HA) were prepared by a hydrothermal process [17]. A flowchart of the procedure and a schematic illustration of the hydrothermal treatment are shown in Fig. 1. a-TCP powder (Taihei Chemical Industrial Co., Ltd., Japan) was mixed with a 10 mass% gelatine solution to obtain a slurry. The gelatine was purchased from Wako Pure Chemical Industries, Ltd., Japan. The mixing mass ratio of powder to solution was 1/5. The slurry was dropped in liquid N2. The frozen spherical granules were collected and then dried at 4 8C for 5 d in a refrigerator. The obtained a-TCP/gelatine granules were heated at 1200 8C for 10 min to obtain a-TCP granules. The a-TCP granules were set in a Teflon1-lined stainless steel autoclave with distilled water and were hydrothermally treated under saturated water vapour at 120 8C for 24 h. As a reference material, spherical porous granules composed of globular HA particles (Glob-HA) were prepared by a sintering method. HA powder (Ube Material Industries, Ltd., Japan) was mixed with a 10 mass% gelatine solution to obtain a slurry. The mixing mass ratio of powder to solution was 1/5. The slurry was dropped in liquid N2. The frozen spherical granules were collected and then dried at 4 8C for 5 d in a refrigerator. The obtained HA/ gelatine granules were heated at 1000 8C for 6 h to remove the gelatine and sinter the particles.
The phases of the samples were examined by X-ray diffraction (XRD, RINT2200VL, Rigaku Co., Japan). The morphologies of the samples were observed using a digital microscope (VHX100, Keyence Co., Japan) and a scanning electron microscope (SEM, S4100, Hitachi, Ltd., Japan). For the SEM observations, a thin coating of Pt was deposited on the surfaces of the samples. The microstructure of the sample was observed by a transmission electron microscope (TEM, HF-2000, Hitachi, Ltd., Japan) equipped with an energy dispersive X-ray spectrometer (EDX, NORAN Instruments Inc., USA). In the preparation of the specimen for the TEM observation, the surface of the granule was scratched, and particles were obtained. The obtained particles were dispersed in ethanol and loaded on a grid for TEM observation. The specific surface areas of the samples were determined by a BET method using nitrogen gas adsorption (AUTOSORB-1, AS-1MP/XP, Yuasa Ionics Inc., Japan).
complex to prepare spherical anatase/HA granules. The photocatalytic activity of the anatase/HA granules was determined by measuring the decomposition of methylene blue (MB) under ultraviolet (UV) irradiation. This data was used to determine the potential of spherical anatase/HA granules as an environmentpurifying material.
2.2. Preparation of anatase/HA granules These HA porous granules were soaked in a solution containing a water-soluble titanium complex and were then hydrothermally treated to obtain anatase/HA granules (TiO2/Rod-HA and TiO2/ Glob-HA, respectively). The water-soluble titanium complex solution was prepared by dissolving 0.25 g of titanium (Wako Pure Chemical Industries, Ltd., Japan) in a mixture of 2.5 cm3 of 28 mass% NH3 aqueous solution (Wako Pure Chemical Industries,
2.4. Evaluation of photocatalytic activity The photocatalytic activity of the anatase/HA granules was evaluated by examining the decomposition of methylene blue (MB, Merck KGaA, Germany), which is often used as a model substance. The anatase/HA granules (0.2 g) were soaked in 15 cm3 of 4 ppm MB aqueous solution and incubated for 30 min to reach adsorption equilibrium. After adsorption equilibrium was reached, UV light (LUV-16, AS ONE Co., Ltd., Japan) was used to irradiate the granules. The change in the MB concentration was examined by measuring the absorbance at 664 nm using a UV-VIS spectrophotometer (Sefi IUV-1240, As One Co., Ltd., Japan). The schematic illustration of the apparatus is shown in Fig. 2. As a blank test, the changes in the MB concentration of the system not containing samples were also examined. In the present study, the decrease in concentration (%) was calculated based on the concentrations for
Fig. 1. Procedure for the preparation of Rod-HA.
M. Kamitakahara et al. / Materials Research Bulletin 46 (2011) 2283–2287
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Black light
Methylene blue solution (4 ppm, 15 cm3) Sample (0.2 g) Stirring bar Magnetic stirrer Fig. 2. Procedure for the evaluation of photocatalytic activity.
the tests with and without the sample, csample and cblank, respectively: Decrease in concentration ¼
cblank csample 100 cblank
(1)
3. Results and discussion
Fig. 3. Appearance of Rod-HA.
Fig. 3 shows the photograph of the sample Rod-HA. The morphology and size of the sample Glob-HA were almost same as those of Rod-HA. Spherical granules of about 2 mm in diameter were obtained for both Rod-HA and Glob-HA. The droplets of the slurry were immediately frozen in liquid N2, and frozen spherical granules were formed. As the frozen spherical granules contain gelatine, the granules could keep their spherical shape even at 4 8C after melting. The drying process at 4 8C in a refrigerator was useful in preventing the destruction of the spherical shape during heating at 1200 8C. Fig. 4 shows SEM images of the surfaces of samples Rod-HA and Glob-HA. Rod-HA and Glob-HA were composed of micrometersized, rod-shaped particles and globular-shaped particles, respectively. Although a small amount of monetite (CaHPO4) phase was detected by XRD in Rod-HA, the dominant phase was HA. In GlobHA, only the HA phase was detected. By using the hydrothermal process, spherical HA granules composed of rod-shaped particles were obtained. We could control the microstructure of the HA spheres. The rod-shaped HA particles are expected to contribute to the toughness of the granules because of the entanglement of the particles. We previously reported that we can obtain rod-like HA particles by the hydrothermal treatment of a-TCP [14–17]. In the hydrothermal treatment, a-TCP is reacted with water to form HA, accompanied by the release of phosphoric acid. This released phosphoric acid provides the acidic conditions during the reaction, and the acidic conditions should have resulted in the formation of monetite, which is stable under acidic conditions, in the Rod-HA sample.
Fig. 5 shows SEM images of the samples TiO2/Rod-HA and TiO2/ Glob-HA. Deposits were observed on the surfaces of Rod-HA and Glob-HA after the hydrothermal treatment in the solution of the water-soluble titanium complex. The deposits, which were much smaller than the HA particles, were formed on the surfaces of the particles without filling the space within the granule. In the XRD analysis of both TiO2/Rod-HA and TiO2/Glob-HA, not only diffraction lines due to HA but also a weak diffraction line, probably due to anatase, was detected. The XRD pattern of the sample TiO2/Rod-HA is shown in Fig. 6. In TiO2/Rod-HA, the diffraction line due to monetite disappeared. The monetite remaining in Rod-HA was reacted with water in a solution of the water-soluble titanium complex and was completely changed into HA. As the peak intensity of anatase was weak and only one distinct diffraction line was confirmed in the XRD analysis, TEM observation was conducted to confirm the deposition of anatase on the surfaces of the HA particles. The TEM photograph and electron diffraction of the TiO2/Rod-HA sample are shown in Fig. 7. From the element analysis, Ca, P and Ti were detected. Nano-sized particles were deposited on the rod-shaped particles. The nano-sized particles were identified as anatase based on electron diffraction. The titanium complex decomposed under the hydrothermal conditions to deposit nano-sized anatase particles on the HA particles. This result is consistent with the XRD and SEM results. Despite the preparation processing required to image the specimen with TEM, we could confirm that nano-sized anatase particles were attached on the HA particles. Thus, the anatase
Fig. 4. SEM images of HA granules.
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Fig. 5. SEM images of TiO2/HA granules.
particles were attached to the HA particles strongly enough for handling. The nano-sized anatase particles are speculated to be deposited on HA surfaces through heterogeneous nucleation. These results indicate that anatase/HA composite granules in which nano-sized anatase particles were deposited on HA particles were successfully prepared. Table 1 shows the specific surface
HA
Intensity / a.u.
Anatase
areas of the samples determined by nitrogen gas adsorption. RodHA showed a larger specific surface area than Glob-HA. The specific surface areas increased after the deposition of anatase particles on HA particles for both of the samples. The decrease in concentration of MB in the photocatalytic activity test is shown in Fig. 8. The concentration change almost ended at 30 min when the adsorption test was conducted as a preliminary experiment, and the UV irradiation was started after the samples had been soaked in the MB solution for 30 min. The transverse of the graph indicates the time after UV irradiation. Therefore, the decrease in concentration at time 0 is due to the adsorption by the samples. TiO2/Rod-HA showed higher adsorption ability than TiO2/Glob-HA. This can be explained by the large specific surface area of TiO2/Rod-HA. Although the specific surface area of TiO2/Rod-HA was increased after the anatase precipitation, TiO2/Rod-HA showed lower adsorption ability than Rod-HA. This implies that anatase shows lower adsorption ability than HA. A continuous decrease in concentration was observed for TiO2/RodHA and TiO2/Glob-HA with irradiation time, while the continuous concentration decrease was not observed for Rod-HA. The
Table 1 Specific surface areas of samples.
20
30
40
2θ / º (CuKα) Fig. 6. XRD pattern of TiO2/Rod-HA.
Sample
Specific surface area (m2 g1)
Rod-HA Glob-HA TiO2/Rod-HA TiO2/Glob-HA
4.9 3.0 5.9 3.3
Decrease in concentration / %
60
TiO2 /Rod-HA 40
20 TiO2 /Glob-HA Rod-HA 0 0
1
Irradiation time / h Fig. 7. TEM image and electron diffraction of TiO2/Rod-HA.
Fig. 8. Decrease in concentration of MB after UV irradiation.
2
Decomposition rate / μg.m-2 . h-1
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10
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was easier in the pores and the area irradiated by UV light was larger for TiO2/Rod-HA, thus resulting in effective MB decomposition. Granules composed of rod-shaped HA particles would be a useful support for anatase.
8 6
4. Conclusions
4
The anatase/HA granules composed of rod-shaped HA particles on which nano-sized anatase particles were deposited showed high photocatalytic ability under UV irradiation. The granules are expected to be useful as an environmental-purifying material with high manageability and photocatalytic activity.
2 0
TiO2/Rod-HA
TiO2/Glob-HA
Fig. 9. MB decomposition rate per unit surface area of the samples.
continuous decrease in concentration of MB is due to the decomposition of MB by the photocatalytic activity of anatase. TiO2/Rod-HA showed higher decomposition ability than TiO2/ Glob-HA. A linear relationship between the decrease in concentration and time was obtained during 0–1.5 h, and the decomposition rate of MB was calculated during this time. The decomposition rate for TiO2/Rod-HA was about six times higher than that of TiO2/Glob-HA when the amounts of the samples were same. As the specific surface area of TiO2/Rod-HA is larger than TiO2/Glob-HA, it can be speculated that this large surface area provides many active sites and the higher photocatalytic activity. However, the higher photocatalytic activity of TiO2/Rod-HA cannot be explained by only the surface area. The decomposition rate of MB per unit surface area was also calculated and the results are shown in Fig. 9. Even considering the decomposition rate of MB per unit surface area, the decomposition rate for TiO2/Rod-HA was about three times higher than that of TiO2/Glob-HA. These results indicate that the sample TiO2/Rod-HA showed higher photocatalytic activity. The properties of the nano-sized anatase particles themselves in TiO2/Rod-HA and TiO2/Glob-HA are speculated to be almost the same because the same water-soluble titanium complex solution and hydrothermal conditions were used. Therefore, we speculated that the difference in the decomposition efficiency of MB was also due to the difference in the microstructure of the HA granules that support the anatase particles. When we observed the microstructure of the HA granules, large spaces were observed among rod-shaped particles for TiO2/Rod-HA. On the other hand, there were smaller spaces among the globular particles in TiO2/Glob-HA. It is speculated that the diffusion of MB
Acknowledgements The authors thank Prof. Kakihana of Tohoku University for kind advice on the experiment and Prof. Tohji and Mr. Miyazaki of Tohoku University for experimental help. References [1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69– 96. [2] O. Carp, C.L. Huisman, A. Reller, Prog. Solid State Chem. 32 (2004) 33–177. [3] A. Fujishima, X.T. Zhang, D.A. Tryk, Surf. Sci. Rep. 63 (2008) 515–582. [4] T. Kawasaki, S. Takahashi, K. Ikeda, Eur. J. Biochem. 152 (1985) 361–371. [5] T. Kawai, C. Ohtsuki, M. Kamitakahara, M. Tanihara, T. Miyazaki, Y. Sakaguchi, S. Konagaya, Environ. Sci. Tech. 40 (2006) 4281–4285. [6] A. Venugopal, M.S. Scurrell, Appl. Catal. A-Gen. 245 (2003) 137–147. [7] K. Mori, T. Hara, T. Mizugaki, K. Ebitani, K. Kaneda, J. Am. Chem. Soc. 126 (2004) 10657–10666. [8] K. Teraoka, T. Nonami, Y. Yokogawa, H. Taoda, T. Kameyama, J. Mater. Res. 15 (2000) 1243–1244. [9] T. Nonami, H. Hase, K. Funakoshi, Catal. Today 96 (2004) 113–118. [10] A. Nakajima, K. Takakuwa, Y. Kameshima, M. Hagiwara, S. Sato, Y. Yamamoto, N. Yoshida, T. Watanabe, K. Okada, J. Photochem. Photobiol. A-Chem. 177 (2006) 94– 99. [11] P. Sujaridworakun, F. Koh, T. Fujiwara, D. Pongkao, A. Ahniyaz, M. Yoshimura, Mater. Sci. Eng. C 25 (2005) 87–91. [12] S. Ji, S. Murakami, M. Kamitakahara, K. Ioku, Mater. Res. Bull. 44 (2009) 768–774. [13] M. Kamitakahara, S. Murakami, N. Watanabe, S. Ji, H. Nishikawa, K. Ioku, J. Ceram. Soc. Jpn. 117 (2009) 1172–1174. [14] K. Ioku, G. Kawachi, S. Sasaki, H. Fujimori, S. Goto, J. Mater. Sci. 41 (2006) 1341– 1344. [15] K. Ioku, S. Yamauchi, H. Fujimori, S. Goto, M. Yoshimura, Solid State Ionics 151 (2002) 147–150. [16] M. Kamitakahara, C. Ohtsuki, G. Kawachi, D. Wang, K. Ioku, J. Ceram. Soc. Jpn. 116 (2008) 6–9. [17] K. Ioku, M. Kamitakahara, N. Watanabe, O. Kawaguchi, S. Murakami, T. Ikeda, Key Eng. Mater. 396–398 (2009) 645–648. [18] M. Kakihana, M. Tada, M. Shiro, V. Petrykin, M. Osada, Y. Nakamura, Inorg. Chem. 40 (2001) 891–894. [19] K. Tomita, M. Kobayashi, V. Petrykin, S. Yin, T. Sato, M. Yoshimura, M. Kakihana, J. Mater. Sci. 43 (2008) 2217–2221.