Highly Visible-Light-Responseive Photocatalytic AgCl

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Journal of Nanoscience and Nanotechnology Vol. 13, 1–6, 2013

Highly Visible-Light-Responseive Photocatalytic AgCl/BiOCl Hetero-Nanostructures Synthesized by a Chemical Coprecipitation Method Jia Liang, Gang-Qiang Zhu∗ , Peng Liu, and Cai Xu School of Physics and Information Technology, Shaanxi Normal University, Xi’an 710062, P. R. China AgCl/BiOCl heteronanostructures were synthesized by a room-temperature chemical coprecipitation method. The as-obtained products were characterized by energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and UV-Vis diffuse spectra, which show the structures, morphologies, and optical properties. The results revealed that the absorption edge of AgCl/BiOCl shifted towards visible light regions. Meanwhile, the AgCl/BiOCl heteronanostructures showed better photocatalytic properties than the pure BiOCl to degrade rhodamine B and the 5% AgCl/BiOCl showed the best photocatalytic ability, which completely decomposed the target molecules in 17 minites with the visible-light illumination. The formation of heteronanostructures might improve the separation of photogenerated electrons and holes derived from the coupling effect of BiOCl and AgCl heteroarchitectures, which was regarded as the main reason for the high photocatalytic activity.

Keywords: Photocatalytic, Visible Light, Heterostructure, Degradation.

Along with the development of industry and economy of human society, environmental problems are becoming more and more serious. It is necessary to solve the problems by different methods. Since Fujishima and Honda reported the evolution of oxygen and hydrogen from a TiO2 electrode under the irradiation of light in 1972,1 2 photocatalysis has been regarded as one of the most effective and economical ways to solve the environmental problems. Originally, TiO2 is the best choices due to its relatively high efficiency, low cost, non-toxicity, and high stability.3–8 However, because of its large band gap (3.0–3.2 eV), TiO2 is only responsive to ultraviolet irradiation, which greatly limits its practical applications.7–9 Furthermore, the lack of effective surface area and low transfer rate of charge carriers also hamper the photocatalyst developments. Given all those reasons, it is expectative to develop novel photocatalysts with high photocatalytic activities under visible light. Recently, BiOCl has drawn considerable attention due to its remarkable photocatalytic activities for degradation of organic compounds with UV and visible light.9–12 Because of its high crystallinity and ultrathin thickness, it is well ∗

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known that this structure could be beneficial for reducing the recombination opportunities of the photogenerated electron–hole pairs. Thus photogenerated electrons and holes could transfer to the surface rapidly to degrade the organic molecules. Zhang et al.12 13 revealed that BiOCl exhibited better performance on photocatalytic degradation of MO than TiO2 . This bandgap of BiOCl (Eg = 34 eV) is wider than that of TiO2 (Eg = 32 eV). Perhaps BiOCl has stronger oxidative ability due to VB potential of BiOCl which is more positive than that of TiO2 . However, the application of BiOCl is limited by its large band gap energy (3.4 eV for anatase), which limits its photoresponse only to the ultraviolet (UV) region. However, only a small ultraviolet (UV) fraction (< 4%) of the total solar spectrum reaching the surface of the earth.9 Therefore, how to make BiOCl responsive to visible light in photocatalysis becomes an important subject for developing the BiOCl-based photocatalysts. Recently, silver halides have been widely investigated and applied as a highly efficient visible-light photocatalysts.14–19 Huang et al. reported that Ag@AgCl photocatalysts showed highly efficient and stable for the degradation of methyl orange under the visiblelight irradiation.14 Kakuta et al. found that AgBr was not destroyed by successive UV irradiation after the formation of Ag species in the early stage of the irradiation.16 Hu et al. also reported the high efficiency and stability

1533-4880/2013/13/001/006

doi:10.1166/jnn.2013.8043

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1. INTRODUCTION

Highly Visible-Light-Responseive Photocatalytic AgCl/BiOCl Hetero-Nanostructures

of Ag–AgI photocatalyst supported on mesoporous alumina for the photodegradation of toxic pollutants under visible-light irradiation.15 All these results suggested that the silver halides could be a highly efficient and promising photocatalyst for the degradation of various organics. Herein, AgCl/BiOCl heteronanostructures were synthesized with high photocatalytic activities. In this paper, the AgCl/BiOCl prepared by a chemical coprecipitation method exhibite higher photocatalytic activities for RhB degradation under visible light irradiation in 17 min. A chemical coprecipitation method was employed in our experiments, which is a simple way that can provide both perfect crystalline quality and large surface areas. The optimized compositions and components have been obtained and systematically studied. Moreover, the origin of the high photocatalytic activity for the heterostructure was also discussed.

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2. EXPERIMENTAL DETAILS AgCl/BiOCl heteronanostructures were synthesized by a room-temperature coprecipitation method in our experiments. Chlorinated bismuth (BiCl3 ), silver nitrate (AgNO3 ), diluted hydrochloric acid and diluted aqua ammonia were employed as starting materials. First, the BiCl3 and AgNO3 were dissolved into diluted hydrochloric acid and deionized water, respectively, then mixed together. Aqueous ammonia was added with constant stirring to the above solution until pH was adjusted to 10 to ensure complete precipitation. Then the precipitates were washed by deionized water for several times and dried in an oven at 60  C. Respectively, samples were labeled as x% AgCl/BiOCl, where x% is the wt ratio of AgCl to BiOCl. In order to investigate the role of compositions on the catalytic activities of AgCl/BiOCl heteronanostructures, pure BiOCl nanostructures are also prepared with the same process. Various characterizations have been employed to test the structures and morphologies of our samples including X-ray diffraction (XRD, Model D/Max2550, Rigaku, Japan), transmission electron microscopy (TEM). Energy-dispersive spectroscopy (EDS) was employed to determine the actual Ag/Bi ratios in the products. The diffuse reflectance spectra were measured on a UV-Vis spectrophotometer equipped with an integrating sphere in the wavelength range of 200–800 nm. Photocatalytic properties were evaluated by the bleaching of rhodamine B (RhB) dye solvated in water, which is recognized as one of the most standard methods for the evaluation of photoactivate activity. In a typical bleaching test of RhB, 50 mg of as-synthesized powder was suspended in 50 mL of RhB aqueous solution (pH = 7) with the initial concentration of 20 mg/L in a glass reactor with a 30 cm2 cross section and 10 cm height. The reactor was then kept in the dark with agitation for 60 min to obtain an adsorption equilibrium, prior to light irradiation by a 400 W 2

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metal halide halogen lamp (Lambda 950, Perkin-Elmer, USA) with 420 nm filter. The efficiency of the degradation processes was evaluated by monitoring the dye decolorization at the maximum absorption around  = 553 nm as a function of irradiation time in the separated RhB solution with a UV-vis spectrophotometer (Shimadzu UV 2550).

3. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of pure BiOCl and AgCl/BiOCl with different wt ratios of AgCl contents. All the diffraction peaks in Figure 1(a) can be well assigned to the tetragonal structure of BiOCl (space group P4/nmm (129), JCPDS 82-0425) and all the diffraction peaks in Figures 1(b)–(e) can be well assigned to the tetragonal structure of BiOCl (space group P4/nmm (129) and JCPDS 82-0425) and cubic phase AgCl (space group Fm-3m (225), JCPDS 85-1355). The peak at 2 = 12058, 24.229, 25.967, 32.654, 33.528, 34.936, 36.608, 41.015, 46.793, 48.474, 49.776, 53.357, 54.262, 58.742, 60.736, 68.316, 75.121 and 77.651, which are marked with “#”, are assigned to the (001), (002), (101), (110), (102), (111), (003), (112), (200), (201), (113), (202), (211), (212), (203), (220), (214) and (310) planes of BiOCl. Respectively, the peak at 2 = 27957, 46.322 and 57.562, which are marked with “∗ ”, are assigned to the (111), (220) and (222) planes of cubic phase of AgCl crystal. The intensities of diffraction peaks of AgCl increase with increasing the AgCl contents in AgCl/BiOCl. AgCl/BiOCl showed some degree of anisotropic peak strengthening and the peak intensities were stronger than those of pure BiOCl, implying that AgCl might be favorable for the crystallization of BiOCl. Furthermore, the strong and sharp diffraction peaks reflected a high degree of crystallization. No other phases were found in AgCl/BiOCl composites, suggesting that there is no appreciable chemical reaction between BiOCl and AgCl. The microstructure of the pure BiOCl and 5% AgCl/ BiOCl composite samples was characterized by the TEM

Fig. 1. XRD patterns of (a) BiOCl, (b) 3 wt% AgCl/BiOCl, (c) 5 wt% AgCl/BiOCl, (d) 7 wt% AgCl/BiOCl, (e) 9 wt% AgCl/BiOCl.

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Highly Visible-Light-Responseive Photocatalytic AgCl/BiOCl Hetero-Nanostructures

images. The panoramic view of pure BiOCl shown in Figure 2(a) indicates that as-synthesized products are composed of a lamellar structure. The size of the flake is between 75 and 200 nm, which is thicker than that of the 5% AgCl/BiOCl composite sample as shown in Figure 2(d). The size of the flake in Figure 2(d) is between 50 and 100 nm. To further investigate the structure of the pure BiOCl, high-resolution TEM (HRTEM) was displayed in Figure 2(b). It shows a HRTEM image of a single BiOCl plate-like nanostructure, which indicates that BiOCl plate-like nanostructure was formed as a single-crystal and the lattice fringe of 0.275 nm in the observed crystallites agrees well with the (110) lattice plane. On the other side, it also shows that the BiOCl nanoplates were well-crystallized and had a high order J. Nanosci. Nanotechnol. 13, 1–6, 2013

Fig. 3. UV-vis diffuses reflectance spectra of all samples with different wt ratio: (a) BiOCl, (b) 3% AgCl/BiOCl, (c) 5% AgCl/BiOCl, (d) 7% AgCl/BiOCl, (e) 9% AgCl/BiOCl.

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Fig. 2. (a) TEM images of sample BiOCl; (b) HRTEM images for pure BiOCl; (c) EDX images of sample BiOCl; (d) TEM images of sample 5% AgCl/BiOCl; (e) high-magnification TEM image of 5% AgCl/BiOCl; (f) EDX images of sample 5% AgCl/BiOCl.

Highly Visible-Light-Responseive Photocatalytic AgCl/BiOCl Hetero-Nanostructures

edge of BiOCl occurs at about 362 nm, and the band-gap energy is estimated to be 3.42 eV by the formula g = 1 2398/Eg ,20 which is slightly larger than that of TiO2 (3.23 eV). Pure silver chloride has a direct band gap of 2.93 eV.20 Not only the absorption edge of AgCl/BiOCl shifted towards visible light region but also strengthened in the UV region, resulting in the improvement of photocatalytic activity under UV illumination. Figure 4 shows quantitative XPS analysis of 5 wt% AgCl/BiOCl. Figure 4(a) shows the wide scans XPS spectra of the 5 wt% AgCl/BiOCl composites. It displays the binding energies for Bi4d5, Bi4d3, Bi4f7, Bi5d, Ag3d, C1s, Cl2p and O1s of the as-prepared nanostructures. One can see that the sample contains only C, O, Bi, Ag and Cl elements. Carbon can be ascribed to

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of crystallinity. High-magnification TEM image for 5% AgCl/BiOCl composite sample as shown in Figure 2(e), which confirms that the flower-like products also consist of lots of nanoplates with the thick of about 7 nm. Figure 2(e) showed some ordered spots corresponding to AgCl nanoparticles, which were attached to BiOCl nanoplates. EDS was used to identify the composition of nanostructures. Figures 2(c) and (f) showed the EDS analyses of pure BiOCl and 5% AgCl/BiOCl composite samples. The pure BiOCl sample consisted of Bi, O, and Cl. 5% AgCl/BiOCl composite consisted of Bi, O, Cl, and Ag, which was in good agreement with the XRD result as shown in Figure 2. Figure 3 indicated the UV-Vis diffuse reflectance spectra of BiOCl and AgCl/BiOCl composite. The absorption

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Fig. 4. Wide scans XPS spectra for sample 5wt% AgCl/BiOCl composites (a), high-resolution XPS spectra Ag (b), Bi (c), Cl (d) and O (e) of the 5 wt% AgCl/BiOCl composites.

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Highly Visible-Light-Responseive Photocatalytic AgCl/BiOCl Hetero-Nanostructures

Fig. 5. (a) Degradation profiles of RhB over the samples with pure BiOCl. (b) UV-vis spectra and photograph images of RhB at different irradiation time over 5% AgCl/BiOCl composite. (c) UV-vis spectra and photograph images of RhB at different irradiation time over pure BiOCl and samples with different wt% AgCl. (d) Kinetic linear simulation curves of RhB photocatalytic degradation with samples with different wt% AgCl sample: 0 wt%, 3 wt%, 5 wt%, 7 wt% and 9 wt%.

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RhB molecular was indeed decomposed in the reaction process. Figure 5(c) shows UV-vis spectra and photograph images of RhB at different irradiation time over 5% AgCl/BiOCl composite, in which pure BiOCl demonstrated inefficient degradation rate due to the poor specific surface area. Although the proportion of silver chloride is very small, there was an amazing degradation ratio that cannot be neglected. For all AgCl/BiOCl samples, the photocatalytic activities were enhanced compared with that of pure BiOCl, which might be due to the formation of heterojunction between BiOCl and AgCl, resulting in efficient separation of photogenerated holes and electrons. Moreover, the mesoporous structures and the corresponding large special surface areas also contributed greatly to the enhancement. The best photocatalytic degradation efficiency was observed when 5% AgCl/BiOCl powder was adopted. Note that 5% AgCl/BiOCl would be used in all photocatalytic degradation experiments as an efficient photocatalyst in the following sections, if not otherwise stated. In order to quantitatively understand the reaction kinetics of the RhB degradation in our experiments, the pseudo-first order model as expressed by Eq. (1) was applied − lnC/C0  = Kt in which C0 and C are the concentrations of RhB in solution at time 0 (the time to obtain adsorption–desorption equilibrium) and t, respectively, and K is the pseudo-first order rate constant. 5

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the adventitious hydrocarbon from XPS instrument itself. Figures 4(b)–(e) shows high-resolution XPS spectra of the 5 wt% AgCl/BiOCl composites. The peaks centered at 167.5 eV and 161.25 eV are assigned to Bi4f5/2 and Bi4f7/2, respectively. Cl 2p, Ag3d5/2 and Ag3d3/2 orbit show the peaks at 201.5 eV, 370 eV and 376 eV, respectively, in the Figure 4(d). The high-resolution XPS spectra of the O 1s peaks can be deconvoluted into oxygen in lattice (O2− ) at binding energy of 531.2 eV and surface adsorbed oxygen (O− ) at 532.8 eV (Fig. 4(e)). All of these results gave the insight that the heteroarchitectures were composed of BiOCl and AgCl. In order to investigate the photocatalytic efficiency of as-prepared powders, the photocatalytic degradation experiments were performed in the condition of catalysts of 1.0 g/L and RhB initial concentration of 20 mg/L, under 17 min visible light irradiation. The results are shown in Figure 5. The temporal evolution of the absorption spectral changes during the photocatalytic degradation of RhB by the pure BiOCl and 5 wt% AgCl/BiOCl are shown in Figures 5(a)–(b). It is found that the absorption peak of the solution decreases slowly in the case of BiOCl sample. By contrast, in the presence of AgCl/BiOCl catalyst (Fig. 5(b)), it shows a faster decrease in the absorbance of RhB solution. The color of the suspension changed to colorless after 17 min, which indicates that the

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4. CONCLUSIONS

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Scheme 1. The reaction mechanism for the degradation of rhodamine B with AgCl/BiOCl under visible light.

The pseudo-first order reaction rate constants K for as-prepared pure RhB, pure BiOCl nanoplates, 3%, 5%, 7% and 9% wt AgCl/BiOCl composite, calculated based on data in Figure 5(d), are 0, 0.01309, 0.03885, 0.04585, 0.05288 and 0.0573 min−1 , respectively. As shown in Figure 5(d), it can be inferred that the order for the photodegradation rates is: 5% wt AgCl/BiOCl > 9%wt AgCl/BiOCl > 7%wt AgCl/BiOCl > 3% wt AgCl/BiOCl > pure BiOCl >pure RhB. So the phase composition of the photocatalysts AgCl/BiOCl shows its strong influence on the RhB degradation. The most plausible photosensitization mechanism for the degradation of the RhB under visible light irradiation is proposed, as displayed in Scheme 1. Both the CB bottom and the VB top of BiOCl lay below the CB bottom and VB top of AgCl, respectively. When they were coupled together to form a heterostructure, the adsorbed RhB dye molecules on the BiOCl nanostructures absorbed the light energy to produce singlet and triplet states (denoted as RhB∗ ) and the electron injection from the excited states of the absorbed dye molecules into the conduction band of BiOCl resulted in the conversion of RhB to the radical cation •RhB+ and the formation of BiOCl (e-) (as shown in Scheme 1). At the same time, the AgCl could be excited under visible light irradiation and the generated electrons in the AgCl were then migrated to the conduction band (CB) of BiOCl. Moreover, due to the high crystallinity of the AgCl, the resistance of electron transportation was very low and reduced the electron–hole recombination. The CB of BiOCl is more negative than that of AgCl, which is beneficial for reducing their recombination. Then, BiOCl (e− ) were captured by O2 to yield O2− and H2 O2 , and then to hydroxyl radicals. On the other hand, the photogenerated holes in AgCl are powerful oxidative species, which can activate some unsaturated organic pollutants, leading to their subsequent decomposition. The photogenerated holes have been demonstrated to play a major role for the degradation of organic compounds over the AgCl/BiOCl catalysts.

AgCl/BiOCl heteronanostructures were synthesized by a simple chemical coprecipitation method. The optimized composition is obtained by the systematic investigation of the photocatalytic activities of the series of samples with different AgCl/BiOCl wt ratios in the starting materials. The interface and the energy band structure of the AgCl/BiOCl composite were also investigated. The AgCl/BiOCl heteronanostructures are proven to be efficient in the separation of photogenerated holes and electrons and have a high photocatalytic activities based on our systematic analysis. The designing of the semiconductor heteronano-structures is regarded as a potential way in the development of future photocatalysts. Acknowledgment: This work was supported by the National Natural Science Foundation of China (Program No. 51102160) and the Fundamental Research Funds for the Central Universities (Program No. GK201102027).

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Received: 2 August 2012. Accepted: 26 December 2012. 6

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