Catalysis Communications

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1 Catalysis Communications 13 (2011) Contents lists available at ScienceDirect Catalysis Communications journal homepage: Short Communication Novel BiOI/BiOBr heterojunction photocatalysts with enhanced visible light photocatalytic properties Jing Cao, Benyan Xu, Bangde Luo, Haili Lin, Shifu Chen College of Chemistry and Materials Science, Huaibei Normal University, Anhui, Huaibei, , PR China article info abstract Article history: Received 19 April 2011 Received in revised form 4 June 2011 Accepted 22 June 2011 Available online 30 June 2011 Keywords: Photocatalysis BiOI/BiOBr Heterojunction Methyl orange A novel heterojunction photocatalyst BiOI/BiOBr was synthesized by a simple modified deposition precipitation method. Several characterization tools including XRD, SEM, HRTEM and UV vis DRS were employed to study the phase structures, morphologies and optical properties of the samples. BiOI/BiOBr exhibited higher photocatalytic activity than single BiOI and BiOBr for the degradation of methyl orange (MO) under visible light (λn420 nm). This result can be due to the formation of the heterojunction between BiOI and BiOBr, which can separate photogenerated carriers efficiently. The photocatalytic mechanism study demonstrates that O 2 and h + are the main reactive species while OH can be negligible Elsevier B.V. All rights reserved. 1. Introduction Since Fujishima and Honda announced TiO 2 photochemical electrode for splitting water in 1972 [1], the study on the development of photocatalysts used for the removal of environmental pollutants and the utilization of solar energy has attracted increasing attention [2 4]. As a traditional photocatalyst, TiO 2 has been widely used in the photocatalytic reactions [5 7]. However, due to its large band gap ( ev), TiO 2 can only exhibit excellent photocatalytic activity by ultraviolet illumination which occupies less than 4% of the solar spectrum. Therefore, the development of novel photocatalysts under visible light is indispensable. In recent years, new type of semiconductor materials of bismuth oxyhalides (BiOX, X=Cl, Br, I) has drawn extensive interests of researchers. BiOX compounds with layered tetragonal matlockite structures [8] represent good optical, electrical and magnetic properties, and have been widely applied in the potential fields of catalysis, ionic conductors, photochromic devices, ferroelectric materials, pigments and solar cells [9,10]. Especially, BiOX perform as a family of promising photocatalysts due to their good photocatalytic activities under both ultraviolet and visible light irradiation. Although different methods [11 14] have been employed to prepare pure BiOX samples for the enhanced photocatalytic activities, it is still badly Corresponding authors. Tel.: ; fax: addresses: caojing@mail.ipc.ac.cn (J. Cao), chshifu@chnu.edu.cn (S. Chen). needed to further improve their photocatalytic efficiency for practical applications. It is well known that semiconductor combination, which can construct a heterojunction between two different semiconductors with matching energy band gaps, is an efficient way for the separation of photoinduced carries, and thus raises the photocatalytic efficiency of heterostructured semiconductor. Previously, many heterostructured semiconductors concerned BiOX, such as NaBiO 3 /BiOCl [15], BiOCl/Bi 2 O 3 [16], BiOI/TiO 2 [17], AgI/BiOI [18], BiOI/Bi 2 O 3 [19], Fe 3 O 4 /BiOCl [20] and WO 3 /BiOCl [21], have been reported and demonstrated the enhanced photocatalytic activities. Therefore, it is a promising way to improve the photocatalytic activities of BiOX by construction of heterojunction. Among the BiOX family, BiOI possesses the smallest band gap (E g = ev) and strong absorption under visible light [13,18,22,23]. BiOBr is also visible light responsive (E g = ev) and relatively stable [12 14,24]. In addition, their matching band structures are conducive to the construction of heterojunction between BiOI and BiOBr, which will facilitate the separation of electron hole pairs and thus improve the photocatalytic activities of BiOX. In the present study, we prepared BiOI/BiOBr with heterostructure via a facile modified deposition precipitation method. It is very different from the solid solution xbiobr/(1 x)bioi photocatalysts [23]. The photocatalytic performance of BiOI/BiOBr was evaluated by the degradation of methyl orange (MO). We also characterized the BiOI/BiOBr heterojunction structures. Moreover, a photocatalytic mechanism was also proposed based on the heterojunction structure and the reactive species detection in detail /$ see front matter 2011 Elsevier B.V. All rights reserved. doi: /j.catcom

2 64 J. Cao et al. / Catalysis Communications 13 (2011) Experimental 2.1. Materials and preparation All the reagents are of analytical purity from Sinopharm Chemical Reagent Co., Ltd. and used for the experiment without further purification. Deionized water was used throughout this study. In a typical synthetic route, Bi(NO 3 ) 3 5H 2 O ( mol) was dissolved in 400 ml deionized water in which the ph value was adjusted to 1.96 with glacial acetic acid. Then KI ( mol) was added to the Bi (NO 3 ) 3 solution drop by drop with stirring. Meanwhile, a brick red precipitate was formed. After a period of time, NaBr ( mol) was added slowly to the above solution with constant stirring. Subsequently, the ph value of the reaction solution was adjusted to 3.0 by ammonia water. The resulting suspension was then kept at 85 C with stirring for 12 h. Finally, the precipitates BiOI/BiOBr with 60% of molar percentage ratio of I to total Bi (I/Bi) were collected, washed and dried at 80 C in air. Samples with different molar percentage ratios (from 20% to 80%) were prepared to investigate the effect of compositions on photocatalytic performances. For comparison, pure BiOI and BiOBr powders were also synthesized Characterization of catalyst X-ray diffraction (XRD) patterns were carried out at room temperature using a BRUKER D8 ADVANCE X-ray powder diffractometer with Cu Kα radiation (λ = Å) and a scanning speed of 10 /min. The accelerating voltage and emission current were 40 kv and 40 ma, respectively. The morphologies of as prepared catalysts were obtained with FEI Sirion 200 field emission scanning electron microscope (SEM) with 5.00 kv scanning voltages. High-resolution transmission electron microscopy (HRTEM) images were performed with JEOL-2011 high-resolution transmission electron microscope, using an accelerating voltage of 200 kv. NOVA 2000e surface areas and pore size analyzer (Quantachrome Instruments) was used to measure the Brunauer Emmett Teller (BET) surface areas of the samples at liquid nitrogen temperature (77.3 K). UV vis diffuse reflectance spectroscopy measurements were carried out using a TU-1901 UV vis spectrophotometer (Beijing Purkinje General Instrument Co., Ltd.) equipped with an integrating sphere attachment. Fluorescence emission spectra were recorded on a JASCO FP-6500 type fluorescence spectrophotometer with 260 nm excited source over a wavelength range of nm. concentration of TA was set at M in M NaOH solution. The sample was collected every 60 min and measured on a JASCO FP-6500 type fluorescence spectrophotometer after centrifugation. The excitation wavelength used was 315 nm. 3. Results and discussion 3.1. Characterization of BiOI/BiOBr XRD analysis was carried out to investigate the phase structures of the catalysts. The XRD patterns of the samples are given in Fig. 1(a). It shows that all the samples were well crystallized and all the diffraction peaks can well be indexed to the tetragonal BiOBr (JCPDS file no ) and tetragonal BiOI (JCPDS file no ), respectively. In addition, the intensities of diffraction peaks of BiOI were strengthened gradually with increasing the BiOI amounts in the BiOI/BiOBr, whereas those of BiOBr decreased simultaneously. The average crystalline sizes of BiOI in the BiOI/BiOBr composites were calculated to be 21.1, 21.3, 23.7 and 23.9 nm for 20%, 40%, 60% and 80% BiOI/BiOBr, respectively, according to the Scherrer formula [25]: L = Kλ β cos θ B where L is taken as crystalline size, K is a constant equals to 0.9, λ is Å, β is the FWHM measured in radians on the 2θ scale, and θ B is the Bragg angle for the diffraction peaks. In the same way, the average crystalline sizes of BiOBr in the BiOI/BiOBr were worked out to be 24.0, 24.8, 25.4 and 24.7 nm for 20%, 40%, 60% and 80% BiOI/BiOBr accordingly. ð1þ 2.3. Evaluation of photocatalytic activities under visible light The photocatalytic activities of as prepared composites BiOI/BiOBr were evaluated by degradation of MO under visible light irradiation (λn420 nm) from 500 W Xe lamp (Institute of Electric Light Source, Beijing). Light passed through annular quartz tube with running water and a 420 nm cutoff filter (Instrument Company of Nantong, China) before entering the reactor. In a typical experiment, 0.10 g catalysts were added to 50 ml of a MO solution (10 mg/l). Prior to illumination, the suspension was magnetically stirred in the dark for 20 min to establish an adsorption desorption equilibrium between the photocatalysts and MO. Then, the suspension was exposed to visible light irradiation under magnetic stirring. At stated time intervals, 5 ml analytical suspension was sampled, centrifuged and analyzed using 722 s spectrophotometer. The characteristic absorption peak of MO at 464 nm was measured to determine the extent of MO degradation. The reference sample was deionized water. Relevant reactive species were evaluated by adding a quantity of different scavengers in a manner similar to the above photodegradation experiment. Terephthalic acid photoluminescence probing technique (TA-PL) was employed to detect OH radicals, in which a basic TA solution was added to the reactor instead of MO and the Fig. 1. (a) XRD patterns of the samples; (b) UV vis diffuse reflectance spectra of the samples. The inset shows the band gaps (E g ) of the samples.

3 J. Cao et al. / Catalysis Communications 13 (2011) UV vis diffuse reflectance spectra (DRS) of different photocatalysts are shown in Fig. 1(b). It can be observed that BiOBr absorbed visible light slightly while the absorption edge of BiOI extended nearly to the whole spectra of visible light. Moreover, the absorption edges of BiOI/BiOBr composites had a monotonic red shift with increasing the BiOI amounts because of the strong visible light response of BiOI. Based on their absorption spectra, the band gap energy (E g )ofa semiconductor can be calculated from the following equation [26]: ahv = A hv E g ð2þ n = 2 where α, ν, E g and A are absorption coefficient, light frequency, band gap energy, and a constant, respectively. Among them, n is determined by the type of optical transition of a semiconductor (n=1 for direct transition and n =4 for indirect transition). The values of n for BiOI and BiOBr are 4 and 4 respectively [11,18]. According to Eq. (2), the E g of BiOI and BiOBr is determined from a plot of (ahv) 1/2 versus (hv) and obtained to be 1.72 ev and 2.75 ev, separately, which is shown in the inset of Fig. 1(b). Moreover, the E g of BiOI/BiOBr composites is located from 1.72 to 2.75 ev. Fig. 2 shows the SEM images of BiOBr, BiOI, and BiOI/BiOBr powders. The image in Fig. 2(a) displays that BiOBr was consisted of plenty of microspheres with diameters ranging from 12 to 24 μm. The exterior surfaces of the microspheres were not clearly smooth but contained an extensive growth of sheetlike structures, as shown in the inset of Fig. 2(a). Fig. 2(b) exhibits that the pure BiOI was of smooth plate structure with diameters in the range of μm. In Fig. 2(c), the BiOI/BiOBr was of anomalous cloddy shape, which displays not only the nature of smooth plates of BiOI, but also has the microsphere character of BiOBr. Fig. 2(d)showstheHRTEMimageofthe60% BiOI/BiOBr heterostructure. It can be observed that two sets of different lattice images are found with d spaces of nm and nm, corresponding to the (212) plane of tetragonal BiOBr and the (122) plane of tetragonal BiOI, respectively, which are in good accordance with the results of the XRD patterns shown in Fig. 1(a). It further confirms that BiOI/BiOBr heterojunction was formed in the composites. Moreover, based on the nitrogen adsorption desorption isotherms, the BET surface areas (A BET ) of BiOBr, BiOI, 20%, 40%, 60% and 80% BiOI/BiOBr were calculated to be , , , , and m 2 /g, respectively. The results obtained shows that the A BET of all the samples did not have obvious difference, which implies that A BET will have little effect on the photocatalytic performance of different samples Photocatalytic activity The photocatalytic activity of the BiOI/BiOBr for the decomposition of MO is displayed in Fig. 3. The dark adsorptions of MO on different catalysts cannot be observed after 20 min. In Fig. 3, it shows that all the BiOI/BiOBr composites exhibited higher photocatalytic activities than single BiOBr and BiOI. When the amount of BiOI in BiOI/BiOBr increased from 20% to 60% with the similar A BET, the MO photodegradation efficiency gradually increased to 63.1% under visible light irradiation for 300 min, which may be due to the intense visible light response of BiOI and the heterojunction formed between BiOI and BiOBr. However, the 80% BiOI/BiOBr with the highest A BET could only degrade 40% of MO, less photocatalytically active than 60% BiOI/BiOBr because of the decrease of the heterojunction interface, which would induce the recombination of numerous photoinduced electrons and holes on the surface of excess BiOI with narrow band gap. This result displays that 80% BiOI/BiOBr with the highest A BET did not represent a b c d nm BiOI (122) nm BiOBr (212) Fig. 2. SEM images of (a) BiOBr, (b) BiOI and (c) BiOI/BiOBr; (d) HRTEM image of BiOI/BiOBr.

4 66 J. Cao et al. / Catalysis Communications 13 (2011) Fig. 3. MO photodegradation over the catalysts under visible light irradiation. the highest photocatalytic activity among the samples, which suggests that the changes in the photocatalytic activity of BiOI/BiOBr result from the heterojunction rather than the A BET. Therefore, the 60% BiOI/BiOBr heterostructure showed the best photocatalytic activity. To further understand the reaction kinetics of MO degradation, the apparent pseudo-first-order model [22] expressed by Eq. (3) was applied in our experiments: ln C 0 C = k appt where k app is the apparent pseudo-first-order rate constant (min 1 ), C is MO concentration in aqueous solution at time t (mg/l), and C 0 is initial MO concentration (mg/l). Via the first-order linear fit from the data of Fig. 3, the k app of 60% BiOI/BiOBr was obtained at the maximal degradation rate of min 1, greatly higher than those of BiOI and BiOBr. The result shows that the BiOI/BiOBr heterojunction composite is a much more effective photocatalyst than single BiOI and BiOBr Mechanism Generally, three possible reaction mechanisms [27] are suspected to be involved in dye photodegradation by a semiconductor: a photolysis process, a dye photosensitization process, or a photocatalytic process. For the photolysis process, a photoinduced electron on the induced dye directly reacts with O 2 to produce a singlet oxygen atom that can work as an oxidant for the pure dye's photolysis [27,28]. In our experiment, MO degradation by the photolysis process upon visible light in the blank experiment was not observable, as shown in Fig. 3. This means that MO is a kind of structure-stable dye, thus, the MO decomposition by the photolysis mechanism is negligible. For a dye photosensitization process, the energy of irradiation light can stimulate the dye to form photoinduced electrons which transfer to the conduction band of the catalyst that absorbs the dye, and subsequently react with O 2 to generate O 2 oxidant [27 29]. As previous studies [28,30] shown, the properties of the dye, such as the structural stability of the dye, the adsorbability of the dye on catalyst surface, and the absorbance of the dye, are responsible for a dye photosensitization mechanism. In this study, almost no changes in the MO concentration over different samples can be detected in 20 min of dark adsorption experiment before the photocatalytic reactions. The slight MO adsorptions on the catalyst were not benefit to the transfer of charge carriers between the dye and the catalyst surfaces in the dye photosensitization process. In addition, the photosensitization of MO can be evaluated through selecting a photocatalyst whose responsive wavelength is lower than 464 nm [29]. Since TiO 2 with the large band gap of 3.20 ev (λ=387.5 nm) cannot respond to visible light it can ð3þ be adopted as a representative photocatalyst [28] to explore the photosensitization process under visible light irradiation (λn420 nm), as shown in Fig. 3. The result displays that no MO was degraded after 300 min of illumination. Presumably the photosensitization processes taking place over BiOBr, BiOI and BiOI/BiOBr are the same as that over TiO 2, that is to say, the photosensitization mechanism in the MO decomposition is also neglectable as well as the photolysis mechanism. Therefore, it is believed that the MO degradation is initiated almost by a photocatalytic process. As is known, the photocatalysts are excited to generate electron hole pairs directly after illumination in the photocatalytic oxidation (PCO) process. Moreover, the photocatalytic efficiency mainly depends on the recombination rate or lifetime of the photogenerated electron hole pairs. The faster recombination occurs, the less time is for the chemical reactions. Therefore, photoluminescence (PL) spectrum was utilized to investigate the recombination rate of the photogenerated electron hole pairs. The fluorescence emission spectra of BiOI/BiOBr, BiOI, and BiOBr were obtained respectively using an ultraviolet light with a 260 nm wavelength as the excitation source. In general, a decrease in the recombination rate gives rise to a low PL intensity [31]. In Fig. 4(a), the lowest relative PL intensity of 60% BiOI/BiOBr sample suggests that it has the lowest recombination rate of electron hole pairs, which results in the highest photocatalytic activity of 60% BiOI/BiOBr, as shown in Fig. 3. Herein, the photocatalytic mechanism of BiOI/BiOBr composites was investigated in detail. First, the effects of various radical scavengers on the degradation of MO were examined under visible light irradiation, as shown in Fig. 4(b). The comparison experiment was also performed with no quencher under otherwise identical conditions. It can be observed that the degradation of MO was obviously depressed by AO ((NH 4 ) 2 C 2 O 4, a quencher of h + [32]) and BQ (benzoquinone, a quencher of O 2 [33]), while the addition of IPA (isopropanol, a quencher of OH [33]) almost did not effect the reaction. Then, terephthalic acid photoluminescence probing technique (TA-PL) [34] was employed to further detect OH radicals (Fig. 4(c)). No PL signal at about 425 nm was observed with increasing irradiation time, which demonstrates that no OH radicals were formed during the PCO process. The above results indicate that h + and O 2 radicals are the main reactive species for the degradation of MO. The formation of O 2 could be depicted as Eq. (4): e + O 2 O 2 : Furthermore, the band structure of the photocatalyst is responsible for the efficient generation and separation process of the electron hole pairs [16]. The band positions of BiOI and BiOBr can be calculated by the following empirical formulas [23]: E VB = X E e +0:5E g E CB = E VB E g where E VB is the valence band (VB) potentials, E CB is the conduction band (CB) potential, X is the electronegativity of the semiconductor (which is the geometric mean of the electronegativity of the constituent atoms), E e is the energy of free electrons on the hydrogen scale (~4.5 ev). Based on all of the above information, the band structure diagram of BiOI/BiOBr was presented in Fig. 5. Both BiOI and BiOBr can be simultaneously excited to form electron hole pairs under visible light (λ N420 nm, energy less than 2.95 ev). The CB bottom of BiOI (0.58 ev) lies below that of BiOBr (0.30 ev), while the VB top of BiOI (2.30 ev) is higher than that of BiOBr (3.05 ev). It seems that the band structures of BiOI and BiOBr are unfavorable for the separation of the electron hole pairs in the BiOI/BiOBr composite photocatalyst. However, electrons in the VB of BiOI could be excited up to a higher ð4þ ð5þ ð6þ

5 J. Cao et al. / Catalysis Communications 13 (2011) Fig. 5. Schematic band structure of BiOI/BiOBr. separated efficiently on the heterojunction interface formed between BiOI and BiOBr. As a result, the photocatalytic activity of BiOI/BiOBr composites is much higher than that of single BiOI and BiOBr (Fig. 3). Moreover, the VB potential of BiOI (2.30 ev) is less positive than the standard reduction potential of OH/OHˉ (2.38 ev) [18], suggesting that the h + on the surface of BiOI cannot oxidize OHˉ into OH. Therefore, it is theoretically reasonable that the photocatalytic degradation of MO could be attributed to the reaction with h + directly rather than OH radicals. In addition, it seems that the CB potential of BiOBr (0.1 ev) is not negative enough to reduce the O 2 to the O 2 radicals because the single-electron reduction potential of O 2 is ev [35], yet, this cannot exclude the formation of O 2 radicals by photogenerated electrons from the higher levels of conduction bands. The experimental evidence to evaluate the role of reactive species has also demonstrated the existence of O 2 radicals. 4. Conclusion In summary, the heterojunction photocatalysts BiOI/BiOBr prepared via a simple modified deposition precipitation method exhibited higher photocatalytic activity than the individual BiOI and BiOBr for the degradation of MO under visible light. The morphological and optical studies reveal that the formation of the BiOI/BiOBr heterojunction plays a key role on the efficient separation of carriers for the enhancement of photocatalytic activity. Moreover, O 2 and h + are proved to be the dominant reactive species in the BiOI/BiOBr photocatalysis. Fig. 4. (a) PL spectra of samples; (b) k app values of 60% BiOI/BiOBr with different quenchers; (c) OH trapping PL spectral changes observed during irradiation of 60% BiOI/BiOBr in a M terephthalic acid solution (excitation at 315 nm). potential edge ( 0.65 ev) under visible light illumination with energy less than 2.95 ev (λn420 nm), while that of BiOBr could be only excited up to 0.10 ev. The reformed CB edge potential of BiOI ( 0.65 ev) is more negative than that of BiOBr (0.1 ev). Consequently, the photogenerated electrons could easily transfer from the reformed CB of BiOI to that of BiOBr, and then react with O 2 adsorbed on the surface of BiOI/BiOBr to produce O 2 that decompose MO, meanwhile the holes can also move in the opposite direction from the VB of BiOBr to that of BiOI. Therefore, the electron hole pairs are Acknowledgments This work was financially supported by the Natural Science Foundation of China (no ), Youth Foundation of Huaibei Normal University (no ) and Anhui Key Laboratory of Energetic Materials (no. KLEM ). References [1] A. Fujishima, K. Honda, Nature 238 (1972) [2] A.L. Linsebigler, G. Lu, J.T. Yates, Chem. Rev. 95 (1995) [3] Z. Zou, J. Ye, K. Sayama, H. Arakawa, Nature 414 (2001) [4] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) [5] A. Mill, S.L. Hunte, J. Photochem. Photobiol. A 108 (1997) [6] E.J. Wolfrum, J. Huang, D.M. Blake, P.C. Maness, Z. Huang, J. Fiest, W.A. Jacoby, Environ. Sci. Technol. 36 (2002) [7] Y. Zang, R. Farnood, Appl. Catal. B 57 (2004)

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