Synthesis and characterization of aurivillius phase Bi 5 Ti 3 FeO 15 layered perovskite for visible light photocatalysis

Transcription

1 Paper Synthesis and characterization of aurivillius phase Bi 5 Ti 3 FeO 15 layered perovskite for visible light photocatalysis Jum Suk JANG, Sang Su YOON, * Pramod H. BORSE, ** Kwon Taek LIM, *** Tae Eun HONG, **** Euh Duck JEONG, **** Ok-Sang JUNG, * Yoon Bo SHIM * and Hyun Gyu KIM ****, Department of Mechanical Engineering, University of Colorado, Boulder, CO 80309, USA * Department of Chemistry (BK21), Pusan National University, San 30, Jangjeon-2-dong, Geumjeong-gu, Busan , Korea ** Center for Nanomaterials, International Advanced Research Center for Powder Metallurgy and New Materials (ARC International), Balapur PO, Hyderabad, AP, , India *** Department of Imaging System Engineering, Pukyong National University, San 100, Yongdang-dong, Nam-gu, Busan , Korea **** Busan Center, Korea Basic Science Institute, San 1275, Jisa-dong, Gangseo-gu, Busan , Korea An aurivillius phase Layered perovskite Bi 5FeTi 3O 15 system was synthesized for first time by the solid-state reaction method yielding a single phase Bi 5FeTi 3O 15 at only 1030 C. The electronic band structure and optical properties as well as the crystallization behavior of the materials were studied by using UV-DRS and XRD. The as-prepared particles those were the agglomerates of 25 nm crystals, exhibited the band gap of 2.38 ev (525 nm). Their visible light (λ 420 nm) photocatalytic activity as studied by the photo-decomposition of isopropyl alcohol (IPA) was found to be much higher than that of well-known TiO 2 xn x photocatalyst The Ceramic Society of Japan. All rights reserved. Key-words : Bi 5Ti 3FeO 15, Aurivillius phase, SSR reaction, Visible light, Photocurrent, Photocatalytic activity [Received May 22, 2009; Accepted August 20, 2009] Introduction Corresponding author: H. G. Kim; hhgkim@kbsi.re.kr Photocatalysts convert solar energy into clean hydrogen energy by splitting water, and by decomposing the harmful organic and inorganic pollutants. The remarkable progress over the last decade in the photocatalysis was limited to ultraviolet (UV) light spectrum, although the visible light is far more abundant and a more useful region of the solar spectrum. Thus, the development of visible light photocatalysts has become an important topic in the photocatalysis research today. Despite many efforts, 1) 4) the design of the materials showing a high activity under visible light is still a major challenge in the field. One promising approach to develop new photocatalysts is the tuning of the optical properties of UV light active catalysts by substitutional doping, as demonstrated in LaTiO 2N, 5) TaON, 6) Y 2Ta 2O 5N 2, 7) Sr 2Nb 2O 7 xn x, 8) TiO 2 xc x, 9) 11) 12) or TiO 2 xn x for anionic doping, and in metal ion-implanted TiO 2, 13) TiO 2 or SrTiO 3:Sb/ Cr/ M (Ru, Rh, Ir, Pt, Pd), 14),15) BaSnO 3:Pb 16),17) and La 2Ti 2O 7:Cr/ Fe, 18) for cationic doping. However, this kind of substitutional doping, in most of the cases, resulted in a small absorption in the visible light region with the original band gap absorption intact, rather than a total red-shift of the band gap energy. In search of the high activity visible light photocatalysts, we have recently discovered that a novel oxide photocatalyst, PbBi 2Nb 2O 9, an Aurivillius-phase perovskite, is an efficient photocatalyst for viz. the photo-production of O 2 or H 2 from water, isopropyl alcohol (IPA) degradation to CO 2, and photogeneration of current, all under visible light. 2) It is much more active than recently disclosed TiO 2 xn 19) x in several photocatalytic reactions. Layered perovskite phase of the Aurivillius (Bi 2O 2) 2+ (A n 1 B no 3n+1) 2 was discovered for the first time by Aurivillius and Subbarao who described them by a general formula in the form 20),21) of: A m 1Bi 2B mo 3m+3 where A = Ba, Sr, Ca, Bi, Pb, B = Ti, Nb, Ta, Mo, W, Fe, Mn. The family of material is a crucial visible light active photocatalyst particularly for the decomposition of toxic compounds and water splitting. Sun et al. reported that the Bi 5Ti 3FeO 15 showed meaningful photocatalytic activity for the decomposition of Rhodamine B and acetaldehyde under visible light irradiation. 22) But, this system still lacks further studies in context to the visible light active photocatalysis. Herein we report the synthesis and crystallization behavior of the layered perovskite Bi 5Ti 3FeO 15 Aurivillius-phase during the solid state reaction (SSR) and further characterize the optical property of Bi 5Ti 3FeO 15 by UV-vis diffuse reflectance spectroscopy. To our knowledge, there has been no report on the hard synthetic condition and crystallization behavior of the layered perovskite Bi 5Ti 3FeO 15 Aurivillius-phase. We also investigated the photocatalytic and the photoelectrochemical performance of this material for photo-current generation under visible light (λ 420 nm). 2. Experimental procedure 2.1 Synthesis of Bi 5Ti 3FeO 15 Bi 5Ti 3FeO 15 was synthesized by the conventional SSR method. The stoichiometric amounts of Bi 2O 3 (99.99%, Aldrich Co., Ltd.), TiO 2 (99.99%, Aldrich Co., Ltd.) and Fe 2O 3 (99%, Aldrich Co., Ltd.) were mixed and ground in methanol. The pelletized 2009 The Ceramic Society of Japan

2 JCS-Japan powders were calcined at C for 5 h in a static furnace. On the other hand, for the purpose of comparison, TiO 2 xn x particles was also prepared by the hydrolytic synthesis method (HSM) 19) Accordingly, an aqueous ammonium hydroxide solution (an ammonia content of 28 30% (99.99%, Aldrich Co., Ltd.)) was slowly added dropwise to 20% titanium (III) chloride solution (TiCl 3, Kanto Chemical Co., Inc., contained 0.01% iron as the major impurity) kept in an ice bath, for 30 min under N 2 atmosphere, while continuously stirring the mixture. The suspension was stirred for 5 h to complete the reaction. After the completion of the reaction, the precipitate was filtered in air and washed several times with the deionized water. The filtered powder was dried at 70 C for 3 4 h in a convection oven. The sample obtained at this stage was an amorphous precipitate powder containing traces of ammonia and titanium. Further this sample was calcined at 400 C for 2 h under air in an electric furnace to obtain the crystalline powder of TiO 2 xn x. 2.2 Characterization The Bi 5Ti 3FeO 15 particles were characterized using X-ray diffraction (XRD) using Mac Science Company Ltd., M18XHF X- ray Diffractometer. The XRD results were compared with the Joint Committee Powder Diffraction Standards (JCPDS) data for the phase identification of different samples. The optical properties viz. the band gap energy of the as-prepared powders was studied by UV-Visible diffuse reflectance spectrometer (Shimadzu, Co., UV 2401). The morphological studies were carried out by using scanning electron microscopy (SEM, Hitachi, Ltd., S 2460N). The BET surface area was estimated by the N 2 adsorption in a constant volume adsorption apparatus (Micrometrics, ASAP 2012). 2.3 Photoelectrochemical and photocatalytic activities The Bi 5Ti 3FeO 15 particles were used for the studies on the photocurrent generation and photocatalytic degradation of IPA under visible light irradiation (λ 420 nm). For the photoelectrochemical measurements, 11) 25 mg of the photocatalyst was suspended in distilled water (75 ml) containing sodium acetate (0.1 M) and Fe 3+ (FeCl 3 6H 2O, 0.1 mm) as an electron donor and an acceptor, respectively; the suspension ph was adjusted to 1.4 with HClO 4 to maintain the Fe 3+ state. A counter electrode platinum plate ( mm) with both sides exposed to the solution, a reference saturated calomel electrode (SCE), and platinum gauze was immersed in the reactor as working (collector) electrode for the study. With the continuous N 2 purging of the suspension, photocurrents were measured by applying a potential (+0.6 V vs. SCE) to the Pt working electrode using a potentiostat (EG&G). The photocatalytic decomposition of the gaseous IPA under visible light irradiation was performed in a closed gas circulation system at room temperature using a high pressure Hg lamp (Oriel, 500 W). The UV-radiation was filtered by using a cutoff filter (λ 420 nm). About 200 ppm of gaseous isopropyl alcohol (IPA) was injected into a 500-mL Pyrex reaction cell filled with air and 0.3 g of photocatalyst. The concentration of the reaction products (CO 2) was determined by a gas chromatograph equipped with a thermal conductivity detector and a molecular sieve 5-Å column. Before reaction, 1 wt% of Pt was deposited on the photocatalysts by photodeposition method under visible light (λ 420 nm). 2. Results and discussion Figure 1 shows the XRD patterns of Bi 5Ti 3FeO 15 samples synthesized by grinding and sintering the stoichiometric mixtures of Bi 2O 3, TiO 2 and Fe 2O 3 at various temperatures (ranging from 650 C to 1100 C) for 5 h in a static furnace. The Bi 5Ti 3FeO 15 Aurivillius phase formation was observed only in the sample calcined at 1030 C. The samples calcined at the temperatures lower than 1030 C, showed impurity phases. The average crystallite size of the Bi 5Ti 3FeO 15 particles was estimated from the full width at half maxima (FWHM) of the main XRD peak (see Fig. 1, a typical sample prepared at 1030 C) by using the Scherrer s equation 23) : D = 0.9λ /Bcosθ (1) where λ is the wavelength of X-ray radiation (λ = nm), B is FWHM of the peak (in radians) corrected for the instrumental broadening, θ is the Bragg angle, and D is the crystallite size (Å). The average crystallite sizes were found to be 25 nm as shown in Table 1. On the other hand, the average sizes of the TiO 2 (P25), TiO 2 xn x particles were found to be ca. 54 nm, 50 nm, respectively. 19) The crystal structure of the layered perovskite Bi 5Ti 3FeO 15 with Aurivillius phase is shown in Fig. 2 for the sake of understanding. Bi 5Ti 3FeO 15 has a general formula of [Bi 2O 2] 2+ [B m 1M mo 3m+1] 2, where B is the 12-fold coordinated cation with a low valence in the perovskite sublattice, M denotes Fig. 1. XRD patterns of Bi 5Ti 3FeO 15 samples calcined at various temperatures for 4 h; (a) 650 C, (b) 800 C, (c) 900 C, (d)1000 C, (e) 1030 C, (f) 1060 C, (g) 1090 C, (f) 1100 C. Table 1. BET Surface Areas, Crystal Size and Band Gap of Bi 5Ti 3FeO 15, TiO 2 xn x and TiO 2 (P25) Samples Photocatalyst BET Surface Area (m 2 /g) Crystallite size (nm) Band gap (ev) Bi 5Ti 3FeO (2.12) TiO 2 xn x TiO 2 (P25)

3 JCS-Japan Jang et al.: Synthesis and characterization of aurivillius phase Bi 5Ti 3FeO 15 layered perovskite for visible light photocatalysis Fig. 3. SEM images of Bi 5Ti 3FeO 15 prepared by solid state reaction at (a) 1000 C and (b) 1030 C, for 5 h. Fig. 2. The schematic of the crystal structure of layered perovskite Bi m+1ti 3Fe m 3O 3m+3 Aurivillius phase with m = 4. Fig. 4. UV-Vis diffuse reflectance spectra of (a) TiO 2 xn x and (b) Bi 5Ti 3FeO 15 calcined at 1030 C. The inset shows the schematic band structures for Bi 5Ti 3FeO 15. the octahedral site occupied by the ions with high valence, and m is the number of perovskite layers between the [Bi 2O 2] 2+ layers. Thus the perovskite sheets of Bi 5Ti 3FeO 15 are composed of MO 6 octahedrons, 12-fold coordinated Bi 3+ and are 4-layer thick unit consisting of a disordered Ti 4+ to Fe 3+ (3:1 ratio) M sites as shown in Fig ) Figure 3 shows a scanning electron microscopy (SEM) image of the Bi 5Ti 3FeO 15 particles calcined at 1000 and 1030 C respectively. In both the cases the particles are found to lie in the size range of μm. The correlation of XRD and SEM indicates that the single phase Bi 5Ti 3FeO 15 particles observed in SEM are the agglomerates of smaller particles having an average size of 25 nm as estimated by XRD. The above characterization indicates successful synthesis of the single phase Bi 5Ti 3FeO 15 perovskite system at 1030 C. Figure 4 shows UV-vis diffuse reflectance spectra for the Bi 5Ti 3FeO 15 samples prepared at 1030 C and for the TiO 2 xn x samples. The spectra were used to estimate the band gap energy of these materials. The Bi 5Ti 3FeO 15 showed two absorption edges around 520 nm and 590 nm, respectively due to the electron excitation from valence band to the d-orbitals of Ti and Fe as shown in the inset of Fig. 4. The finer details are explained in the later section. TiO 2 xn x also showed two absorption edges; the main edge due to the oxide at 390 nm and a shoulder due to the nitride at 451 nm. 18) The colors of these materials were dark brown and yellow indicating that these materials could indeed absorb the visible light. The most widely used model for the characterization of the semiconductors was developed by Kubelka and Munk. 24),25) It allows the calculation of the absorption coefficient α by the measurement of the diffuse reflectance from the powdered mixture containing absorbing and scattering media. A limiting assumption is that the particles making up the layer must be much smaller than the total thickness. Both absorbing and scattering media must be uniformly distributed through the scattering layer. For the semi-infinite samples, the main result of the theory can be described by the following formula 26) ( ) 2 1 Rd α f ( Rd )= =, (2) 2Rd s where f (R) is the Kubelka Munk function, R d is the diffuse reflectance from the sample, and s is the scattering coefficient. 1270

4 JCS-Japan If the dependence of the scattering coefficient s on the wavelength of the incident light is neglected, the Kubelka Munk function is directly proportional to the absorption coefficient α. The optical bandgap, E g, of the materials was calculated on the basis of the optical spectral absorption using the well-known formula, ( ) m Ahν Eg α = (3) hν where A is a constant, hν is the incident photon energy, and m depends on the nature of band transition, m = 1/2 for direct and m = 2 for indirect allowed transitions. Thus, E g can be obtained by extrapolating the linear portion of the modified Kubelka Munk function, {f (Rd)hν} 1/m, Vs. the energy of exciting light, hν, to zero. The band gaps estimated from the above analysis (figures not shown) of UV-vis spectra of Bi 5Ti 3FeO 15 and TiO 2 xn x were 2.38 (2.12) ev and 2.76 ev, respectively (see Table 1). These band gap energies are greater than the theoretical energy required for the water splitting (λ > 1.23 ev), and thus are suitable for the role of visible light photocatalysts. Sun et al. reported that the valence bands of Bi 5Ti 3FeO 15 are mainly composed of O 2p + Fe t 2g + Bi 6s hybrid orbitals, while the conduction bands are mainly composed of the Ti 3d + Fe e g orbits. 21) The photoelectron excitation from the hybridized O 2p + Fe t 2g + Bi 6s orbitals to the Fe e g orbital in the Bi 5Ti 3FeO 15 should be responsible for the absorption of visible light photons as shown in inset of the Fig. 4. The other photoelectronic excitation from the O 2p + Fe t2g + Bi 6s hybrid orbitals to the Ti 3d orbital should correspond to the absorption of UV light photons (Inset in Fig. 4). Their result is consistent with the UV-vis diffuse reflectance spectrum shown here in Fig. 4. Visible light active photocatalysts should generate photocurrents upon absorption of the photons (Inset in Fig. 5). To investigate the photocurrent generation of visible light active photocatalysts, aqueous suspensions of the materials containing acetate (donor) and Fe 3+ (acceptor) were illuminated with visible light (λ 420 nm). Figure 5 shows the photocurrents generated over TiO 2 (P25), TiO 2 xn x and Bi 5Ti 3FeO 15 calcined at 1030 C. In general the unmodified TiO 2 with a wide band gap (3.2 ev) under irradiation, as well as all other catalysts in the dark could not generate any photocurrent as expected. But, these materials show (except TiO 2) the photocurrents generated under visible light irradiation, indicating that they possess a visible responsive small band gap. Bi 5Ti 3FeO 15 generated photocurrent approximately 9 times higher than the TiO 2 xn x. The generation of photocurrent is the initial critical step for photocatalytic reactions upon light irradiation, and the rate of photocatalytic reactions can be directly correlated with the photoelectrochemical property of the material. To investigate the photocatalytic ability of the as-prepared samples we tested for the photo-degradation of IPA, a common model compound under visible light irradiation. Figure 6 shows the rate of CO 2 evolution from the photo-degradation reaction over TiO 2 (P25), TiO 2 xn x and Bi 5Ti 3FeO 15 samples. The concentration of CO 2 increased steadily with irradiation time and the decomposition rate of IPA was ca mol/h under visible light irradiation. The production of CO 2 gas stopped when light was turned off and restarted at the same rate when light was turned on again. The photocatalytic activity of TiO 2 xn x for IPA decomposition was 25% less than that of Bi 5Ti 3FeO 15, while TiO 2 (P25), which couldn t absorb visible light didn t show any photocatalytic activity. It is well in accord with the result of UV- Vis diffuse reflectance spectroscopy. Therefore, the superiority Fig. 5. Photocurrents generated under visible light (λ 420 nm) irradiation from (a) TiO 2(P25), (b) TiO 2 xn x and (c) Bi 5Ti 3FeO 15 material suspensions with acetate and Fe 3+, as an electron donor or acceptor, respectively. Experimental conditions: photocatalysts = g/75 ml; acetate = 0.1 M; Fe 3+ = 0.1 mm; continuous N 2 purging; ph = 1.4; E app = 0.6 V (vs SCE); Pt plate ( mm), Pt-gauze as working and counter electrodes respectively. The Insert shows the principle of photocurrent measurement in the power type system. Fig. 6. Time courses of CO 2 evolution from IPA decomposition over various materials under visible light irradiation (λ 420 nm) in presence of 1 g photocatalyst (a) TiO 2 (P25) (- -), (b) TiO 2 xn x (- -) and (c) Bi 5Ti 3FeO 15 (- -); IPA concentration, 200 ppm in air. of the Bi 5Ti 3FeO 15 is expected from the photocurrent generation behavior and thus confirmed in the present study. 4. Conclusions Aurivillius phase layered perovskite Bi 5Ti 3FeO 15 system was successfully synthesized for first time by solid state reaction method. Single crystalline Bi 5Ti 3FeO 15 can merely be formed at 1030 C by SSR method. Photocatalytic activity of Bi 5Ti 3FeO 15 was found to be much higher than that of TiO 2 xn x for IPA photo degradation under visible light (λ 420 nm). It can be concluded that this layered perovskite material with Aurivillius phase can be used for the decomposition of toxic or hazard gases under visible light irradiation. Acknowledgement This work has been supported by KBSI grant T29320, Hydrogen Energy R&D Center, MKE-RTI , Korea. 1271

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