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A novel core–shell like composite In2O3@CaIn2O4 for efficient degradation of Methylene Blue by visible light
Applied Catalysis A: General 321 (2007) 1–6 www.elsevier.com/locate/apcata
A novel core–shell like composite In2O3@CaIn2O4 for efficient degradation of Methylene Blue by visible light Wen Ku Chang *, K. Koteswara Rao, Hua Cing Kuo, Jen Fong Cai, Ming Show Wong Department of Materials Science and Engineering, National Dong Hwa University, Hualien 974, Taiwan, ROC Received 12 September 2006; received in revised form 16 December 2006; accepted 27 December 2006 Available online 14 January 2007
Abstract In the process of preparing a visible light photocatalyst CaIn2O4, a novel core shell like composite In2O3@CaIn2O4 was observed at intermediate calcination temperatures, during the solid-state reaction between ball milled powders of CaCO3 and In2O3. The composition, crystallinity and photo absorption of the coupled phases obtained in the temperature range 873–1323 K, were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV–vis diffused reflectance spectroscopy. The coupled composite phases In2O3@CaIn2O4 showed superior visible light induced photocatalytic degradation of Methylene Blue (MB) compared to single phase CaIn2O4. Selective charge separation and efficient charge transport at the interface when illuminated are considered to be the possible reasons for the enhanced photocatalytic performance of this composite catalyst. # 2007 Elsevier B.V. All rights reserved. Keywords: Photocatalyst; Core shell; Photocatalytic degradation of Methylene Blue; Crystallinity
1. Introduction ‘Visible light driven photocatalysis’ and ‘photo electrochemical conversion of solar energy’ are the prime focus of present research efforts, directed towards pollution-free environment and energy [1–3]. Beginning with the photocatalytic splitting of water into H2 and O2 by TiO2 under UV light [4], extensive studies were carried out on many other semiconducting oxides like WO3, ZnO2, Fe2O3, In2O3, SnO2, etc. for various photocatalytic and photovoltaic applications [5,6]. The relatively high band gap energy of these simple oxides required structural modification, for example, incorporation of dopant atoms to induce band gap narrowing so as to absorb visible light and show photoactivity [7,8]. Apart from TiO2 and other metal oxides, many single phased multicomponent oxides like RbPb2Nb3O10, In2O3(ZnO)m, BiVO4, InMO4 (M = V, Nb, Ta), M2.5VMoO8 (M = Mg, Zn), MIn2O4 (M = Ca, Sr, Ba), etc. studied recently [9–14], are also found to be active for decomposition of water (into H2 or O2) or degradation of organic contaminants under visible light
* Corresponding author. Tel.: +886 3 863 4217; fax: +886 3 863 4200. E-mail address: [email protected] (W.K. Chang). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.12.021
illumination. The electronic structures of these materials calculated by density functional theory (DFT) implied the role of transition/p-block metal’s d-orbital (in conduction band) and O 2p orbital (in valence band), in suitably tuning the band structure for either H2 or O2 evolution from water respectively [12]. While the approaches for band gap narrowing were successful to a large extent, the recombination of photogenerated electron–hole pairs inside the semiconductor still remains a major impediment for efficient performance of the photocatalyst [5]. Moreover, the band gap narrowing would also in principle, put a limitation on the positions of conduction band minima (CBM) and valence band maxima (VBM), that determine the redox power of photo-excited electrons and holes. The reducing or oxidizing power of the semiconductor greatly depends on (i) the relative positions of CBM and VBM; (ii) recombination rates of the charged species (e , h+); (iii) transport of these species to the surface of semiconductor for redox processes. To minimize the fast recombination of photoexcited e –h+ pairs before their eventual participation in redox processes, coupling of two semiconducting oxides or a metal and metal oxide with a difference in their CBM was considered to be a good strategy [15]. A great deal of work is already reported on such coupled catalysts in colloid systems by
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Kamat’s group [15–17] and others [18,19], specially using CdS, ZnS, CdSe, WO3, SiO2 in combination with TiO2, ZnO, Ag or Au. Selective diffusion and trapping of photo generated electrons was observed when SnO2 or Ag is capped with TiO2 in the form of core–shell structures (SnO2@TiO2 and Ag@TiO2), resulting in accumulation of electrons in SnO2/ Ag (core) and concentration of holes at the surface of TiO2 (shell) [16,17]. Such charge separation processes significantly improved the photocatalytic properties. Recently, a nanocomposite of ZnFe2O4 (band gap of 1.9 eV) and TiO2 was attempted and it showed promising result for photodegradation of phenol [20]. Similar complex oxides having small band gap, present a great scope for obtaining novel composite systems with enhanced photocatalytic activity, when alloyed with a simple semiconducting oxide. A recent example of this concept was demonstrated by Ye group [21], who has synthesized Cr-doped Ba2In2O5/In2O3 by solid-state reaction, possessing high activity for water splitting. Here, both Ba2In2O5 and In2O3 are semiconducting oxides and their conduction band minimums (CBMs) have a difference for required coupling. For an ohmic contact between the two semiconductors, the individually synthesized components were calcined together at high temperatures. Sometimes, this might result in new solid solution or alloy formation at the interface, as was observed for above example. As an alternative, we attempted at synthesizing a semiconducting visible light photocatalyst CaIn2O4 [14] from CaCO3 and In2O3 by sequential calcinations, in the hope that both In2O3 and the CaIn2O4 would co-exist as a composite, at an intermediate calcination step before the final phase formation. This approach would inherently retain the ohmic contact between both the semiconducting oxides, which is necessary for efficient charge transport at the interface, along with the realization of charge separation due to coupling. As expected, a novel core–shell like composite In2O3@CaIn2O4 was formed with a superior photocatalytic performance. The characterization of the compositions formed during CaIn2O4 synthesis and their catalytic performance towards Methylene Blue degradation, form the central part of this study.
photocatalytic degradation of MB was studied under visible light illumination. The crystallinity and crystal phase of the samples calcined at different temperatures were determined by powder X-ray diffraction (XRD) method (Rigaku DMAX-2500 diffract˚ ) with an ometer) using Cu Ka radiation (l = 1.54178 A accelerating voltage and emission current of 40 kVand 50 mA, respectively, in the 2u range of 10–908. Analytical transmission electron microscopy (ATEM, JEOL JEM 3010) was used to examine the crystallinity, shape and size of the particles. The surface area of the samples was determined by BET (Brunauer, Emmett and Teller) measurement of nitrogen adsorption at 77 K (Quantachrome NOVA 4000e) after the pretreatment at 573 K for 2 h. UV–vis diffuse reflectance spectra of the samples was measured by using a Hitachi 3300H UV–vis spectrometer in the measurement range of 300–700 nm. Thermogravimetrical analysis (TGA) of the sample mixture was carried out from room temperature to 1173 K. For the photocatalytic test of MB degradation, a light source of 50 W Xe arc lamp with the true power density of 1 W/cm2 on the reaction cell surface is used. The reaction was carried out with 50 mg of powdered photocatalyst suspended in 33 mL of MB solution in a Pyrex glass cell. MB solution was prepared by taking a required amount of MB powder and dissolving it in distilled water such that its concentration was 10 ppm. Commercially available Degussa P-25 was used as a bench mark for comparison. All experiments were conducted at room temperature in air. The degradation of MB solution was analyzed by following the absorption at 664 nm wavelength using a UV–vis spectrometer. 3. Results and discussion The powder XRD patterns of the calcined samples (873– 1323 K) are presented in Fig. 1. The phases of the calcination product are indicated against each pattern. From the figure it is observed that the ball milled starting precursors, CaCO3 and
2. Experimental The polycrystalline samples of CaIn2O4 and the intermediate composite phases were prepared by a solid-state reaction method. Highly pure chemicals (CaCO3 and In2O3) were used as starting materials and were dried at 673 K before using them. The stoichiometric amounts of precursors were mixed and ball milled for 24 h. The mixed powder was dried and passed through 325-mesh sieve. In order to investigate the best conditions for photocatalytic activity, small lots of this powder were calcined each at different reaction temperature (873–1327 K) for 12 h. To obtain the phase pure CaIn2O4 with high crystallinity, the sample calcined at 1327 K is further annealed for 24 h and its photocatalytic activity is taken as reference for comparison. Other samples were used as such for characterization by powder XRD, TEM, BET surface area, UV–vis absorption spectroscopy and
Fig. 1. Powder XRD patterns of ball milled CaCO3 and In2O3 mixture at various calcination temperatures.
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In2O3, without calcination is simply a mixture of the two components. Since the ball milling is not of high energy it could only result in finer particle nature of the two powders. After calcining at 873 K, there is no significant change in the XRD pattern. However at 973 K, peaks corresponding to CaIn2O4 were observed and its content has further improved at 1073 K calcination temperature. In both these samples In2O3 peaks are also observed. At 1173 K the intensity of peaks corresponding to In2O3 has decreased drastically indicating its presence only in a minor proportion. The sample calcined and annealed at 1323 K is almost pure CaIn2O4 single phase, except for a trace of In2O3. The earlier reports of solid-state synthesis [14,22] also required temperatures more than 1423 K for a highly crystalline phase of CaIn2O4 and had small amount of In2O3 impurity phase. CaCO3 usually decomposes into CaO and CO2 upon calcination. To know the temperature of decomposition in the present study, thermogravimetric analysis (TGA) was carried out for the sample of mixed CaCO3 and In2O3 powders, with a heating rate of 10 K/min (the figure is provided in supporting information). The onset of weight loss for the powders is observed from 948 K and reached saturation above 1073 K. The partially decomposed CaCO3 at 973 K was found to be almost completely decomposed into CaO and CO2 by around 1073 K, probably due to the fine particle nature of it upon ball milling. In the XRD patterns, peaks corresponding to CaO were observed at calcination temperatures of 973 and 1073 K. The nucleation of CaIn2O4 phase should have started around 948 K with the formed CaO and the In2O3 present. This is evident from the presence of CaIn2O4 in the XRD patterns of 973 K calcined sample. The absence of CaCO3 peaks at both 973 and 1073 K suggests that most of it has decomposed into CaO. The XRD patterns also reveal the decreased peak intensities of CaO, In2O3 and the simultaneous increased intensities of CaIn2O4 peaks at 1073 K, compared to the calcination temperature of 973 K. This supports the presence of more CaIn2O4 phase and less In2O3 at 1073 K than 973 K. At the same time, an ohmic contact between the phases CaIn2O4 and In2O3 exists due to their coupled nature at these intermediate calcination temperatures. The composite nature of phases is retained till 1173 K, with varying amounts of each phase in the composite. While the presence of both semiconducting oxides In2O3 and CaIn2O4 in substantial amounts is clearly evident from XRD, the nature of their coupling can only be understood from electron microscopy. ATEM images of a representative sample (obtained at 973 K) are shown in Fig. 2. Nano particulate nature of the sample is clearly observed with an average size of 50–75 nm in Fig. 2a. Moreover, the sample shows a distinct grey outer region and a dark inner region for each particle indicating a core–shell nature of coupling of the phases. In Fig. 2b, a central dark core region surrounded by a grey shell is clearly observed for a single particle of the obtained phase at 973 K calcined sample. Solid-state synthesis by calcination should primarily give a coupled composite, as the probability of total encapsulation of reacted product (CaIn2O4) over one of the components (In2O3) is not absolute. However, such a possibility cannot be over ruled
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Fig. 2. (a) ATEM image of sample powder calcined at 1073 K showing the coupled composite phases; (b) single particle ATEM image of sample (a) with core–shell geometry of coupled In2O3 (core) and CaIn2O4 (shell).
in the calcination of fine particles with one of the components (CaCO3) being more reactive due to decomposition at calcination temperature. We consider that small particles of CaO are formed during decomposition of CaCO3 and a layer of such particles reacts with relatively bigger In2O3 particles at the surface, forming a layer of CaIn2O4 over In2O3. The coupling of reaction product CaIn2O4 with the unreacted In2O3 would mostly be a capping of CaIn2O4 over In2O3 (core–shell), though
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both coupling modes are expected between these semiconducting phases, i.e. a simple contact based and a core–shell type. The CaO would be present either at the interface or the outer surface of these coupled photocatalysts. In either mode of coupling, a large surface of CaIn2O4 in ohmic contact with In2O3, is always available for photo oxidation of MB under visible light illumination. To ascertain the composition of outer shell and inner core, the ATEM image taken for the sample calcined at 973 K (Fig. 2b) was probed by EDS and it indicated a relatively higher at.% of In at the core. This supports the presence of In2O3 at the core for core–shell geometry of coupled composite. Previous reports of photocatalysts with core–shell geometry were mainly colloidal suspensions obtained by solution processing in steps [16] or by a reverse micelles method [23]. The composites reported by Ye group using solid-state method [21] and Yuan and Zhang [20] are predominantly contact coupled with negligible core–shell geometry. So the solid-state method adopted in this study, forming core–shell type coupling of product (CaIn2O4) and one of the reactant (In2O3) offers an added advantage, especially for photocatalysis. Fig. 3 shows the UV–vis diffuse reflectance spectra of all samples. The absorption profiles of samples calcined at 1173 and 1323 K are similar to the ones reported earlier by solidstate method [14,22]. These samples are mainly CaIn2O4 and have two absorption peaks in the spectra. The peak at higher wavelength is similar to the pure In2O3 absorption peak and the band gap of CaIn2O4 is slightly higher than that of In2O3 [22]. For the samples calcined below 1073 K, there is a clear shift in absorption towards higher wavelength. The intensity of absorption in the UV region also improved. Moreover, unlike two peaks observed for the samples calcined at 1173 and 1323 K, the absorption edge for these samples is more linear. This could be due to the well-hybridized absorption edges of both In2O3 and CaIn2O4 which exist as coupled composites below 1073 K of calcination temperature. Such a compatibility of band gaps and band edges was also observed in coupled Cr– Ba2In2O5/In2O3 composite [21]. The absorption spectra of
mixed CaCO3 and In2O3 sample is essentially that of In2O3 as the CaCO3 is transparent in the visible region. The photo absorption ability of coupled composite phases (samples calcined below 1073 K) in the visible region, with an ohmic contact between them should enable better charge separation and transport, resulting in efficient photodegradation of MB. For evaluating the activity of samples calcined at different temperatures, the photocatalytic degradation of MB is tested under visible light illumination. MB degradation in the presence of Degussa P-25 and simple photolysis of MB was also carried out for a comparison. The results of variation in MB concentration with reaction time are presented in Fig. 4. All the samples showed a degradation of MB under visible light and the rate of degradation varied for each photocatalyst. A remarkable performance was shown by the sample calcined at 973 K which degraded MB completely after 40 min of irradiation. This result is a great improvement when compared with the activity of single phase CaIn2O4 reported earlier [24], even by taking into consideration the amount of catalyst, power of light source and the concentration of MB used in the reported experiment. Moreover, this sample and the samples calcined at 1073 and 873 K also showed better photoactivity when compared with the samples calcined at 1173 and 1323 K (pure CaIn2O4 phase). The superior photoactivity of coupled composites obtained at lower calcination temperatures (973, 1073 K) in comparison to the MB degradation efficiency of P-25 or the MB photolysis (without using any photocatalyst–photobleaching of MB) is very encouraging and is explained in the following discussion. In general the primary factors that influence a photocatalytic reaction are (i) the adsorption ability of the reactant on the catalyst surface; (ii) the absorption ability of the catalyst in the available light energy region; (iii) the efficient separation and transport of light induced electrons and holes in the catalyst. For good adsorption of any reactant a high surface area of catalyst is desirable. The surface areas of all calcined samples (873– 1323 K) obtained by BET are in the range 0.85–1.23 m2/g and the surface area of P-25 is 50.13 m2/g. Since the surface area of calcined products is much smaller than P-25 and varies little
Fig. 3. UV–vis diffuse reflectance spectra of samples calcined at various temperatures.
Fig. 4. Photocatalytic degradation of MB by the samples calcined at different temperatures and P-25. Photolysis of MB is also shown.
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with calcination temperature, it would not have contributed significantly for the photocatalytic activity of calcined samples. But the better activity of P-25 compared to CaIn2O4 and simple photolysis of MB dye is perhaps due to the high surface area of P-25 and the presence of a small UV component in the Xe lamp used for photocatalysis experiments. The higher activity of sample calcined at 873 K with respect to CaIn2O4 phase or P-25 is, however unclear now. It could be perhaps due to the formation of a minor CaIn2O4 phase on the In2O3, to result in a composite having better activity, similar to the samples calcined at 973 and 1073 K. However, the amount of CaIn2O4 is less than the detection limit of powder XRD and hence could not be observed in the XRD pattern. The enhanced photocatalytic degradation of MB by the samples calcined at 973 and 1073 K can be modeled on the coupling of band structures as shown in Fig. 5. CaIn2O4 is a visible light photocatalyst absorbing wavelengths shorter than 480 nm with an indirect band gap for optical transitions [14]. In2O3 is an n-type semiconducting transparent oxide with an indirect band gap of about 2.5 eV. For both the contact type and the core–shell like geometry of coupled semiconducting oxides In2O3 and CaIn2O4, the photo-excited electrons in the CBM of CaIn2O4 gets injected efficiently into the lower lying CBM of In2O3 due to the ohmic contact between the phases [21]. In2O3 has a band gap similar to SnO2 and hence can trap the photo-excited electrons efficiently; analogous to the SnO2 core in SnO2 coupled TiO2 colloids reported earlier [16]. With the selective trapping of electrons in In2O3 and the availability of holes in CaIn2O4 it becomes highly favorable for photo oxidation of MB by the core shell geometry of coupling. When the coupling is not an ideal core shell type, the separation of electrons and holes (into respective In2O3 and CaIn2O4 sides of contact based coupling), still leaves enough CaIn2O4 surface for photo oxidation of MB. Based on the photocatalysis results and the ATEM pictures obtained, we propose that a core–shell like composite is predominantly formed at the calcination temperature of 973 K. Along with core–shell geometry, another important factor for better activity of sample calcined at 973 K is the large interface area between the core (In2O3)
Fig. 5. Band structures model of coupled-photocatalysts in contact type and core–shell type geometries.
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and the shell (CaIn2O4). Here, the thickness of shell is small and the interface it forms with the core is quite large. After calcining at 1073 K the thickness of shell increases, there by decreasing the interface area between shell and core. The large interface area is highly favorable for efficient transport of photo induced charges across the interface and promotes the photocatalytic activity. Moreover, the relatively small volume of core in the sample calcined at 1073 K becomes less favorable for further injection of electrons upon charge separation. In the coupled catalysts reported earlier [25,26], for example, WO3/TiO2 or ZnO/SnO2, there existed an optimum amount of component with the low lying conduction band (WO3 or SnO2, in this case) that is required for best photocatalytic performance upon coupling. So, we consider that the core–shell geometry obtained at 973 K is more optimal in terms of CaIn2O4 and In2O3 amounts as well as the interface between them and hence shows better photodegradation ability than the sample calcined at 1073 K. The presence of unreacted CaO at these temperatures is perhaps not detrimental for photoactivity. Considerable degradation of MB observed by simple photolysis, suggests that MB dye also assists the resultant photocatalytic activity when adsorbed on CaIn2O4. It was reported [24] that when the light source has wavelengths greater than 480 nm, the dye gets sensitized and transfers its excited electrons to conduction band of CaI2O4. In the process the dye also gets oxidized via many intermediates. So the suitable coupling of band structures by In2O3, CaIn2O4 and MB dye is considered to be the operative mechanism for efficient degradation of MB for the samples calcined at 973 and 1073 K compared to pure CaIn2O4. 4. Conclusion Solid-state synthesis of visible light photocatalyst CaIn2O4 from CaCO3 and In2O3 resulted in core–shell like coupled composites (In2O3@CaIn2O4) at lower calcination temperatures. To address the charge recombination in photocatalysts, this strategy of obtaining coupled catalysts with inherent ohmic contact between the phases is more suitable. The coupled phase showed an enhanced photocatalytic degradation of MB in the visible light compared to CaIn2O4. The better performance of sample calcined at 973 K than a sample calcined at other temperatures is due to the optimum thickness of CaIn2O4 shell and In2O3 core, providing high interface area. The selective trapping of electrons by In2O3 is based on the band structures of the coupled semiconductors. The low lying conduction band minimum of In2O3 compared to CaIn2O4 is favorable for efficient charge separation at the interface of coupling. The core–shell geometry of coupling leaves more surface of CaIn2O4 with holes after the charge separation to selectively oxidize Methylene Blue. A better control of the reacting materials in terms of stoichiometry and the processing methods adopted to get ideal core–shell geometries would further optimize the photocatalytic performance of this coupled catalyst.
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Acknowledgement The authors are thankful for the financial support of National Science Council of Taiwan ROC under grant No. NSC 94-2120M-259-001. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.apcata.2006.12.021. References [1] A. Mills, S. Le Hunte, J. Photochem. Photobiol. A: Chem. 108 (1997) 1. [2] D.A. Tryk, A. Fujishima, K. Honda, Electrochim. Acta 45 (2000) 2363. [3] M. Kaneko, I. Okura, Photocatalysis: Science and Technology, Kodansha/ Springer Verlag, Tokyo/Germany, 2003. [4] A. Fujishima, K. Honda, Nature 238 (1972) 37. [5] M.R. Hoffman, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [6] A. Hagfeldt, M. Gra¨tzel, Chem. Rev. 95 (1995) 49. [7] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. [8] S.U.M. Khan, M. Al-Shahry, W.B. Ingler Jr., Science 297 (2002) 2243.
[9] J. Yoshimura, Y. Ebina, J. Kondo, K. Domen, J. Phys. Chem. 97 (1993) 1970. [10] A. Kudo, I. Mikami, Chem. Lett. (1998) 1027. [11] A. Kudo, K. Ueda, H. Kato, I. Mikami, Catal. Lett. 53 (1998) 229. [12] J. Ye, Z. Zou, H. Arakawa, M. Oshikiri, M. Shimoda, A. Matsushita, T. Shishido, J. Photochem. Photobiol. A: Chem. 148 (2002) 79. [13] D. Wang, Z. Zou, J. Ye, Catal. Today 93–95 (2004) 891. [14] J. Tang, Z. Zou, M. Katagiri, T. Kako, J. Ye, Catal. Today 93–95 (2004) 885. [15] P.V. Kamat, Chem. Rev. 93 (1993) 267. [16] I. Bedja, P.V. Kamat, J. Phys. Chem. 99 (1995) 9182. [17] T. Hirakawa, P.V. Kamat, J. Am. Chem. Soc. 127 (2005) 3928. [18] L. Spanhel, H. Weller, A. Henglein, J. Am. Chem. Soc. 109 (1987) 6632. [19] T. Ung, L.M. Liz-Marzan, P. Mulvaney, J. Phys. Chem. B 103 (1999) 6770. [20] Z. Yuan, L. Zhang, J. Mater. Chem. 11 (2001) 1265. [21] D. Wang, Z. Zou, J. Ye, Chem. Mater. 17 (2005) 3255. [22] S. Esther Dali, V.V.S.S. Sai Sundar, M. Jayachandran, M.J. Chockalingam, J. Mater. Sci. Lett. 17 (1998) 619. [23] B. Pal, T. Torimoto, K. Iwasaki, T. Shibayama, H. Takahashi, B. Ohtani, J. Appl. Electrochem. 35 (2005) 751. [24] J. Tang, Z. Zou, J. Ye, Chem. Mater. 16 (2004) 1644. [25] X.Z. Li, F.B. Li, C.L. Yang, W.K. Ge, J. Photochem. Photobiol. A: Chem. 141 (2001) 209. [26] W. Cun, Z. Jincai, W. Xinming, M. Bixian, S. Guoying, P. Pingan, F. Jiamo, Appl. Catal. B: Environ. 39 (2002) 269.
A novel core–shell like composite In2O3@CaIn2O4 for efficient degradation of Methylene Blue by visible light Wen Ku Chang *, K. Koteswara Rao, Hua Cing Kuo, Jen Fong Cai, Ming Show Wong Department of Materials Science and Engineering, National Dong Hwa University, Hualien 974, Taiwan, ROC Received 12 September 2006; received in revised form 16 December 2006; accepted 27 December 2006 Available online 14 January 2007
Abstract In the process of preparing a visible light photocatalyst CaIn2O4, a novel core shell like composite In2O3@CaIn2O4 was observed at intermediate calcination temperatures, during the solid-state reaction between ball milled powders of CaCO3 and In2O3. The composition, crystallinity and photo absorption of the coupled phases obtained in the temperature range 873–1323 K, were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV–vis diffused reflectance spectroscopy. The coupled composite phases In2O3@CaIn2O4 showed superior visible light induced photocatalytic degradation of Methylene Blue (MB) compared to single phase CaIn2O4. Selective charge separation and efficient charge transport at the interface when illuminated are considered to be the possible reasons for the enhanced photocatalytic performance of this composite catalyst. # 2007 Elsevier B.V. All rights reserved. Keywords: Photocatalyst; Core shell; Photocatalytic degradation of Methylene Blue; Crystallinity
1. Introduction ‘Visible light driven photocatalysis’ and ‘photo electrochemical conversion of solar energy’ are the prime focus of present research efforts, directed towards pollution-free environment and energy [1–3]. Beginning with the photocatalytic splitting of water into H2 and O2 by TiO2 under UV light [4], extensive studies were carried out on many other semiconducting oxides like WO3, ZnO2, Fe2O3, In2O3, SnO2, etc. for various photocatalytic and photovoltaic applications [5,6]. The relatively high band gap energy of these simple oxides required structural modification, for example, incorporation of dopant atoms to induce band gap narrowing so as to absorb visible light and show photoactivity [7,8]. Apart from TiO2 and other metal oxides, many single phased multicomponent oxides like RbPb2Nb3O10, In2O3(ZnO)m, BiVO4, InMO4 (M = V, Nb, Ta), M2.5VMoO8 (M = Mg, Zn), MIn2O4 (M = Ca, Sr, Ba), etc. studied recently [9–14], are also found to be active for decomposition of water (into H2 or O2) or degradation of organic contaminants under visible light
* Corresponding author. Tel.: +886 3 863 4217; fax: +886 3 863 4200. E-mail address: [email protected] (W.K. Chang). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.12.021
illumination. The electronic structures of these materials calculated by density functional theory (DFT) implied the role of transition/p-block metal’s d-orbital (in conduction band) and O 2p orbital (in valence band), in suitably tuning the band structure for either H2 or O2 evolution from water respectively [12]. While the approaches for band gap narrowing were successful to a large extent, the recombination of photogenerated electron–hole pairs inside the semiconductor still remains a major impediment for efficient performance of the photocatalyst [5]. Moreover, the band gap narrowing would also in principle, put a limitation on the positions of conduction band minima (CBM) and valence band maxima (VBM), that determine the redox power of photo-excited electrons and holes. The reducing or oxidizing power of the semiconductor greatly depends on (i) the relative positions of CBM and VBM; (ii) recombination rates of the charged species (e , h+); (iii) transport of these species to the surface of semiconductor for redox processes. To minimize the fast recombination of photoexcited e –h+ pairs before their eventual participation in redox processes, coupling of two semiconducting oxides or a metal and metal oxide with a difference in their CBM was considered to be a good strategy [15]. A great deal of work is already reported on such coupled catalysts in colloid systems by
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Kamat’s group [15–17] and others [18,19], specially using CdS, ZnS, CdSe, WO3, SiO2 in combination with TiO2, ZnO, Ag or Au. Selective diffusion and trapping of photo generated electrons was observed when SnO2 or Ag is capped with TiO2 in the form of core–shell structures (SnO2@TiO2 and Ag@TiO2), resulting in accumulation of electrons in SnO2/ Ag (core) and concentration of holes at the surface of TiO2 (shell) [16,17]. Such charge separation processes significantly improved the photocatalytic properties. Recently, a nanocomposite of ZnFe2O4 (band gap of 1.9 eV) and TiO2 was attempted and it showed promising result for photodegradation of phenol [20]. Similar complex oxides having small band gap, present a great scope for obtaining novel composite systems with enhanced photocatalytic activity, when alloyed with a simple semiconducting oxide. A recent example of this concept was demonstrated by Ye group [21], who has synthesized Cr-doped Ba2In2O5/In2O3 by solid-state reaction, possessing high activity for water splitting. Here, both Ba2In2O5 and In2O3 are semiconducting oxides and their conduction band minimums (CBMs) have a difference for required coupling. For an ohmic contact between the two semiconductors, the individually synthesized components were calcined together at high temperatures. Sometimes, this might result in new solid solution or alloy formation at the interface, as was observed for above example. As an alternative, we attempted at synthesizing a semiconducting visible light photocatalyst CaIn2O4 [14] from CaCO3 and In2O3 by sequential calcinations, in the hope that both In2O3 and the CaIn2O4 would co-exist as a composite, at an intermediate calcination step before the final phase formation. This approach would inherently retain the ohmic contact between both the semiconducting oxides, which is necessary for efficient charge transport at the interface, along with the realization of charge separation due to coupling. As expected, a novel core–shell like composite In2O3@CaIn2O4 was formed with a superior photocatalytic performance. The characterization of the compositions formed during CaIn2O4 synthesis and their catalytic performance towards Methylene Blue degradation, form the central part of this study.
photocatalytic degradation of MB was studied under visible light illumination. The crystallinity and crystal phase of the samples calcined at different temperatures were determined by powder X-ray diffraction (XRD) method (Rigaku DMAX-2500 diffract˚ ) with an ometer) using Cu Ka radiation (l = 1.54178 A accelerating voltage and emission current of 40 kVand 50 mA, respectively, in the 2u range of 10–908. Analytical transmission electron microscopy (ATEM, JEOL JEM 3010) was used to examine the crystallinity, shape and size of the particles. The surface area of the samples was determined by BET (Brunauer, Emmett and Teller) measurement of nitrogen adsorption at 77 K (Quantachrome NOVA 4000e) after the pretreatment at 573 K for 2 h. UV–vis diffuse reflectance spectra of the samples was measured by using a Hitachi 3300H UV–vis spectrometer in the measurement range of 300–700 nm. Thermogravimetrical analysis (TGA) of the sample mixture was carried out from room temperature to 1173 K. For the photocatalytic test of MB degradation, a light source of 50 W Xe arc lamp with the true power density of 1 W/cm2 on the reaction cell surface is used. The reaction was carried out with 50 mg of powdered photocatalyst suspended in 33 mL of MB solution in a Pyrex glass cell. MB solution was prepared by taking a required amount of MB powder and dissolving it in distilled water such that its concentration was 10 ppm. Commercially available Degussa P-25 was used as a bench mark for comparison. All experiments were conducted at room temperature in air. The degradation of MB solution was analyzed by following the absorption at 664 nm wavelength using a UV–vis spectrometer. 3. Results and discussion The powder XRD patterns of the calcined samples (873– 1323 K) are presented in Fig. 1. The phases of the calcination product are indicated against each pattern. From the figure it is observed that the ball milled starting precursors, CaCO3 and
2. Experimental The polycrystalline samples of CaIn2O4 and the intermediate composite phases were prepared by a solid-state reaction method. Highly pure chemicals (CaCO3 and In2O3) were used as starting materials and were dried at 673 K before using them. The stoichiometric amounts of precursors were mixed and ball milled for 24 h. The mixed powder was dried and passed through 325-mesh sieve. In order to investigate the best conditions for photocatalytic activity, small lots of this powder were calcined each at different reaction temperature (873–1327 K) for 12 h. To obtain the phase pure CaIn2O4 with high crystallinity, the sample calcined at 1327 K is further annealed for 24 h and its photocatalytic activity is taken as reference for comparison. Other samples were used as such for characterization by powder XRD, TEM, BET surface area, UV–vis absorption spectroscopy and
Fig. 1. Powder XRD patterns of ball milled CaCO3 and In2O3 mixture at various calcination temperatures.
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In2O3, without calcination is simply a mixture of the two components. Since the ball milling is not of high energy it could only result in finer particle nature of the two powders. After calcining at 873 K, there is no significant change in the XRD pattern. However at 973 K, peaks corresponding to CaIn2O4 were observed and its content has further improved at 1073 K calcination temperature. In both these samples In2O3 peaks are also observed. At 1173 K the intensity of peaks corresponding to In2O3 has decreased drastically indicating its presence only in a minor proportion. The sample calcined and annealed at 1323 K is almost pure CaIn2O4 single phase, except for a trace of In2O3. The earlier reports of solid-state synthesis [14,22] also required temperatures more than 1423 K for a highly crystalline phase of CaIn2O4 and had small amount of In2O3 impurity phase. CaCO3 usually decomposes into CaO and CO2 upon calcination. To know the temperature of decomposition in the present study, thermogravimetric analysis (TGA) was carried out for the sample of mixed CaCO3 and In2O3 powders, with a heating rate of 10 K/min (the figure is provided in supporting information). The onset of weight loss for the powders is observed from 948 K and reached saturation above 1073 K. The partially decomposed CaCO3 at 973 K was found to be almost completely decomposed into CaO and CO2 by around 1073 K, probably due to the fine particle nature of it upon ball milling. In the XRD patterns, peaks corresponding to CaO were observed at calcination temperatures of 973 and 1073 K. The nucleation of CaIn2O4 phase should have started around 948 K with the formed CaO and the In2O3 present. This is evident from the presence of CaIn2O4 in the XRD patterns of 973 K calcined sample. The absence of CaCO3 peaks at both 973 and 1073 K suggests that most of it has decomposed into CaO. The XRD patterns also reveal the decreased peak intensities of CaO, In2O3 and the simultaneous increased intensities of CaIn2O4 peaks at 1073 K, compared to the calcination temperature of 973 K. This supports the presence of more CaIn2O4 phase and less In2O3 at 1073 K than 973 K. At the same time, an ohmic contact between the phases CaIn2O4 and In2O3 exists due to their coupled nature at these intermediate calcination temperatures. The composite nature of phases is retained till 1173 K, with varying amounts of each phase in the composite. While the presence of both semiconducting oxides In2O3 and CaIn2O4 in substantial amounts is clearly evident from XRD, the nature of their coupling can only be understood from electron microscopy. ATEM images of a representative sample (obtained at 973 K) are shown in Fig. 2. Nano particulate nature of the sample is clearly observed with an average size of 50–75 nm in Fig. 2a. Moreover, the sample shows a distinct grey outer region and a dark inner region for each particle indicating a core–shell nature of coupling of the phases. In Fig. 2b, a central dark core region surrounded by a grey shell is clearly observed for a single particle of the obtained phase at 973 K calcined sample. Solid-state synthesis by calcination should primarily give a coupled composite, as the probability of total encapsulation of reacted product (CaIn2O4) over one of the components (In2O3) is not absolute. However, such a possibility cannot be over ruled
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Fig. 2. (a) ATEM image of sample powder calcined at 1073 K showing the coupled composite phases; (b) single particle ATEM image of sample (a) with core–shell geometry of coupled In2O3 (core) and CaIn2O4 (shell).
in the calcination of fine particles with one of the components (CaCO3) being more reactive due to decomposition at calcination temperature. We consider that small particles of CaO are formed during decomposition of CaCO3 and a layer of such particles reacts with relatively bigger In2O3 particles at the surface, forming a layer of CaIn2O4 over In2O3. The coupling of reaction product CaIn2O4 with the unreacted In2O3 would mostly be a capping of CaIn2O4 over In2O3 (core–shell), though
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both coupling modes are expected between these semiconducting phases, i.e. a simple contact based and a core–shell type. The CaO would be present either at the interface or the outer surface of these coupled photocatalysts. In either mode of coupling, a large surface of CaIn2O4 in ohmic contact with In2O3, is always available for photo oxidation of MB under visible light illumination. To ascertain the composition of outer shell and inner core, the ATEM image taken for the sample calcined at 973 K (Fig. 2b) was probed by EDS and it indicated a relatively higher at.% of In at the core. This supports the presence of In2O3 at the core for core–shell geometry of coupled composite. Previous reports of photocatalysts with core–shell geometry were mainly colloidal suspensions obtained by solution processing in steps [16] or by a reverse micelles method [23]. The composites reported by Ye group using solid-state method [21] and Yuan and Zhang [20] are predominantly contact coupled with negligible core–shell geometry. So the solid-state method adopted in this study, forming core–shell type coupling of product (CaIn2O4) and one of the reactant (In2O3) offers an added advantage, especially for photocatalysis. Fig. 3 shows the UV–vis diffuse reflectance spectra of all samples. The absorption profiles of samples calcined at 1173 and 1323 K are similar to the ones reported earlier by solidstate method [14,22]. These samples are mainly CaIn2O4 and have two absorption peaks in the spectra. The peak at higher wavelength is similar to the pure In2O3 absorption peak and the band gap of CaIn2O4 is slightly higher than that of In2O3 [22]. For the samples calcined below 1073 K, there is a clear shift in absorption towards higher wavelength. The intensity of absorption in the UV region also improved. Moreover, unlike two peaks observed for the samples calcined at 1173 and 1323 K, the absorption edge for these samples is more linear. This could be due to the well-hybridized absorption edges of both In2O3 and CaIn2O4 which exist as coupled composites below 1073 K of calcination temperature. Such a compatibility of band gaps and band edges was also observed in coupled Cr– Ba2In2O5/In2O3 composite [21]. The absorption spectra of
mixed CaCO3 and In2O3 sample is essentially that of In2O3 as the CaCO3 is transparent in the visible region. The photo absorption ability of coupled composite phases (samples calcined below 1073 K) in the visible region, with an ohmic contact between them should enable better charge separation and transport, resulting in efficient photodegradation of MB. For evaluating the activity of samples calcined at different temperatures, the photocatalytic degradation of MB is tested under visible light illumination. MB degradation in the presence of Degussa P-25 and simple photolysis of MB was also carried out for a comparison. The results of variation in MB concentration with reaction time are presented in Fig. 4. All the samples showed a degradation of MB under visible light and the rate of degradation varied for each photocatalyst. A remarkable performance was shown by the sample calcined at 973 K which degraded MB completely after 40 min of irradiation. This result is a great improvement when compared with the activity of single phase CaIn2O4 reported earlier [24], even by taking into consideration the amount of catalyst, power of light source and the concentration of MB used in the reported experiment. Moreover, this sample and the samples calcined at 1073 and 873 K also showed better photoactivity when compared with the samples calcined at 1173 and 1323 K (pure CaIn2O4 phase). The superior photoactivity of coupled composites obtained at lower calcination temperatures (973, 1073 K) in comparison to the MB degradation efficiency of P-25 or the MB photolysis (without using any photocatalyst–photobleaching of MB) is very encouraging and is explained in the following discussion. In general the primary factors that influence a photocatalytic reaction are (i) the adsorption ability of the reactant on the catalyst surface; (ii) the absorption ability of the catalyst in the available light energy region; (iii) the efficient separation and transport of light induced electrons and holes in the catalyst. For good adsorption of any reactant a high surface area of catalyst is desirable. The surface areas of all calcined samples (873– 1323 K) obtained by BET are in the range 0.85–1.23 m2/g and the surface area of P-25 is 50.13 m2/g. Since the surface area of calcined products is much smaller than P-25 and varies little
Fig. 3. UV–vis diffuse reflectance spectra of samples calcined at various temperatures.
Fig. 4. Photocatalytic degradation of MB by the samples calcined at different temperatures and P-25. Photolysis of MB is also shown.
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with calcination temperature, it would not have contributed significantly for the photocatalytic activity of calcined samples. But the better activity of P-25 compared to CaIn2O4 and simple photolysis of MB dye is perhaps due to the high surface area of P-25 and the presence of a small UV component in the Xe lamp used for photocatalysis experiments. The higher activity of sample calcined at 873 K with respect to CaIn2O4 phase or P-25 is, however unclear now. It could be perhaps due to the formation of a minor CaIn2O4 phase on the In2O3, to result in a composite having better activity, similar to the samples calcined at 973 and 1073 K. However, the amount of CaIn2O4 is less than the detection limit of powder XRD and hence could not be observed in the XRD pattern. The enhanced photocatalytic degradation of MB by the samples calcined at 973 and 1073 K can be modeled on the coupling of band structures as shown in Fig. 5. CaIn2O4 is a visible light photocatalyst absorbing wavelengths shorter than 480 nm with an indirect band gap for optical transitions [14]. In2O3 is an n-type semiconducting transparent oxide with an indirect band gap of about 2.5 eV. For both the contact type and the core–shell like geometry of coupled semiconducting oxides In2O3 and CaIn2O4, the photo-excited electrons in the CBM of CaIn2O4 gets injected efficiently into the lower lying CBM of In2O3 due to the ohmic contact between the phases [21]. In2O3 has a band gap similar to SnO2 and hence can trap the photo-excited electrons efficiently; analogous to the SnO2 core in SnO2 coupled TiO2 colloids reported earlier [16]. With the selective trapping of electrons in In2O3 and the availability of holes in CaIn2O4 it becomes highly favorable for photo oxidation of MB by the core shell geometry of coupling. When the coupling is not an ideal core shell type, the separation of electrons and holes (into respective In2O3 and CaIn2O4 sides of contact based coupling), still leaves enough CaIn2O4 surface for photo oxidation of MB. Based on the photocatalysis results and the ATEM pictures obtained, we propose that a core–shell like composite is predominantly formed at the calcination temperature of 973 K. Along with core–shell geometry, another important factor for better activity of sample calcined at 973 K is the large interface area between the core (In2O3)
Fig. 5. Band structures model of coupled-photocatalysts in contact type and core–shell type geometries.
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and the shell (CaIn2O4). Here, the thickness of shell is small and the interface it forms with the core is quite large. After calcining at 1073 K the thickness of shell increases, there by decreasing the interface area between shell and core. The large interface area is highly favorable for efficient transport of photo induced charges across the interface and promotes the photocatalytic activity. Moreover, the relatively small volume of core in the sample calcined at 1073 K becomes less favorable for further injection of electrons upon charge separation. In the coupled catalysts reported earlier [25,26], for example, WO3/TiO2 or ZnO/SnO2, there existed an optimum amount of component with the low lying conduction band (WO3 or SnO2, in this case) that is required for best photocatalytic performance upon coupling. So, we consider that the core–shell geometry obtained at 973 K is more optimal in terms of CaIn2O4 and In2O3 amounts as well as the interface between them and hence shows better photodegradation ability than the sample calcined at 1073 K. The presence of unreacted CaO at these temperatures is perhaps not detrimental for photoactivity. Considerable degradation of MB observed by simple photolysis, suggests that MB dye also assists the resultant photocatalytic activity when adsorbed on CaIn2O4. It was reported [24] that when the light source has wavelengths greater than 480 nm, the dye gets sensitized and transfers its excited electrons to conduction band of CaI2O4. In the process the dye also gets oxidized via many intermediates. So the suitable coupling of band structures by In2O3, CaIn2O4 and MB dye is considered to be the operative mechanism for efficient degradation of MB for the samples calcined at 973 and 1073 K compared to pure CaIn2O4. 4. Conclusion Solid-state synthesis of visible light photocatalyst CaIn2O4 from CaCO3 and In2O3 resulted in core–shell like coupled composites (In2O3@CaIn2O4) at lower calcination temperatures. To address the charge recombination in photocatalysts, this strategy of obtaining coupled catalysts with inherent ohmic contact between the phases is more suitable. The coupled phase showed an enhanced photocatalytic degradation of MB in the visible light compared to CaIn2O4. The better performance of sample calcined at 973 K than a sample calcined at other temperatures is due to the optimum thickness of CaIn2O4 shell and In2O3 core, providing high interface area. The selective trapping of electrons by In2O3 is based on the band structures of the coupled semiconductors. The low lying conduction band minimum of In2O3 compared to CaIn2O4 is favorable for efficient charge separation at the interface of coupling. The core–shell geometry of coupling leaves more surface of CaIn2O4 with holes after the charge separation to selectively oxidize Methylene Blue. A better control of the reacting materials in terms of stoichiometry and the processing methods adopted to get ideal core–shell geometries would further optimize the photocatalytic performance of this coupled catalyst.
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Acknowledgement The authors are thankful for the financial support of National Science Council of Taiwan ROC under grant No. NSC 94-2120M-259-001. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.apcata.2006.12.021. References [1] A. Mills, S. Le Hunte, J. Photochem. Photobiol. A: Chem. 108 (1997) 1. [2] D.A. Tryk, A. Fujishima, K. Honda, Electrochim. Acta 45 (2000) 2363. [3] M. Kaneko, I. Okura, Photocatalysis: Science and Technology, Kodansha/ Springer Verlag, Tokyo/Germany, 2003. [4] A. Fujishima, K. Honda, Nature 238 (1972) 37. [5] M.R. Hoffman, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [6] A. Hagfeldt, M. Gra¨tzel, Chem. Rev. 95 (1995) 49. [7] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. [8] S.U.M. Khan, M. Al-Shahry, W.B. Ingler Jr., Science 297 (2002) 2243.
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