Research Paper
Journal of Chemical Engineering of Japan, Vol. 49, No. 4, pp. 366–371, 2016
Preparation, Characterization and Photocatalytic Properties of BiPO4 Decorated with Ag/AgBr Hongchao Ma, Guoliang Yang, Yinghuan Fu, Chun Ma, Xiaoli Dong and Xiufang Zhang School of Chemistry Engineering & Material, Dalian Polytechnic University, No. 1 Qinggongyuan, Ganjinzi District, Dalian 116034, P. R. China Keywords: Photodegradation, KN-R, Ag/AgBr Modification, BiPO4 The present study investigates rod-like BiPO4 prepared by a hydrothermal process using bismuth nitrate and sodium dihydrogen phosphate as raw materials. Herein, Ag–AgBr was deposited onto the surface of BiPO4 via a facile precipitation–photoreduction technique. The structure, morphology, composition and optical property of as-synthesized samples were characterized by XRD, SEM, TEM, XPS, DRS and PL techniques. The results showed that introduction of Ag–AgBr did not change the structure and morphology of BiPO4, but extended its optical absorption to visible region. Moreover, the introduction of Ag–AgBr shifted binding energy of elements of BiPO4 to lower values, which implied that a strong interaction exists between Ag–AgBr and BiPO4. The photocatalytic tests showed that the Ag/AgBr/BiPO4 composites possess higher photocatalytic activity for degradation of anthraquinone dye (reactive brilliant blue KN-R) under simulated sunlight, as compared to that of pure BiPO4. The improvement of photocatalytic activity for Ag/AgBr/BiPO4 composites could be ascribed to their good optical absorption and the synergistic action between Ag–AgBr and BiPO4 substrate (the synergistic action effectively retarded recombination of photogenerated carriers in hetero-structured Ag/AgBr/BiPO4).
Introduction To solve the problems of environmental pollution, photocatalysis technology has attracted much attention owing to its versatility, low-cost and being environmentally benign in the past decades (Zhang et al., 2001; Deng et al., 2008; Hernández-Alonso et al., 2009; Ibhadon and Fitzpatrick, 2013; Keane et al., 2014). Phosphate materials have excellent catalytic performance, and ion exchange capacity because of their abundant P-O coordination polyhedron and open frame structure (Yu and Xu, 2003). In recent years, phosphate materials (especially BiPO4) have been widely used as photocatalyst in the photocatalysis field due to its high photoreactivity (Pan and Zhu, 2010; Pan et al., 2012a; Lv et al., 2014). However, BiPO4 has limitations of lightabsorption (can only absorb ultraviolet light which accounts for just a tiny portion of the sunlight) and quick recombination of electron-hole pairs (Qi et al., 2012; Afzal et al., 2013). To develop visible-light driven photocatalysts, many efforts have focused to improve the utilization efficiency of sunlight and reduce the recombination rate of electron-hole pairs, such as ion doping (Rana et al., 2006; Rawat et al., 2007a, 2007b; Venkatasubramanian et al., 2008; Fulekar et al., 2014; Liu et al., 2014), semiconductor coupling (Rana et al., 2005a, 2005b; Pan et al., 2012b; Cao et al., 2013), noble metal depositing (Papp et al., 1993; Sclafani and
Herrmann, 1998; Pawinrat et al., 2009) and exploring new efficient photocatalytic materials (Gao and Wang, 2013). Meanwhile, among these approaches, forming silver/silver halides (Ag/AgX; X=Cl, Br, I) heterostructure have received more attention recently owing to their excellent catalytic activity, such as Ag/AgCl/Bi2MoO6 (Zhang et al., 2015), Ag/AgBr/AgIn(MoO4)2 (Yan et al., 2015), Ag/AgBr/TiO2 (Hu et al., 2006; Wang et al., 2012), Ag/AgBr/g-C3N4 (Xu et al., 2013), Ag/AgBr/ZnWO4 (Li et al., 2014) and Ag– AgI/Al2O3 (Hu et al., 2010). This paper presents an experimental study done on Ag/AgBr/BiPO4 photocatalysts synthesized by a two-stage strategy. It is found that the Ag/AgBr/BiPO4 composites exhibited excellent photocatalytic activities, which can be ascribed to the formation of heterostructure and the surface plasmon resonance of Ag.
1. Experimental 1.1 Reagents and materials Bismuth nitrate pentahydrate, sodium dihydrogen phosphate, potassium bromide, silver nitrate, and reactive brilliant blue KN-R were purchased from Tianjin Chemical Reagents Company. All of these reagents were of AR grade and used without further purification.
Received on April 15, 2015; accepted on September 11, 2015 DOI: 10.1252/jcej.15we064 Correspondence concerning this article should be addressed to Y. Fu (E-mail address:
[email protected]) or X. Dong (E-mail address:
[email protected]).
1.2 Simple synthesis BiPO4 was synthesized by the hydrothermal method. In a typical experiment, Bi(NO3)3 ·5H2O (3 mmol) and NaH2PO4 (3 mmol) were mixed with 60 mL deionized water under magnetic stirring for 1 h, and then the mixture was transferred into a 100 mL stainless steel autoclave. The autoclave
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was sealed and maintained at 180°C for 6 h, and was then allowed to cool to room temperature. The as-obtained samples were sufficiently washed with absolute ethanol and deionized water, collected by filtration and dried at 60°C for 4 h. The preparation of Ag/AgBr/BiPO4 composites by precipitation–photoreduction process was as follows. Firstly, BiPO4 particles (2 mmol) were dispersed into KBr aqueous solution (0.01 mol/L) with a certain volume, and then the mixture was ultrasonically dispersed for 10 min. Meanwhile, an AgNO3 aqueous solution (0.01 mol/L) was added dropwise into the above solution under magnetic stirring in the dark. The obtained solid was sufficiently washed with absolute ethanol and deionized water, and collected by filtration. Finally, the above solid again dispersed into deionized water to obtain Ag/AgBr/BiPO4 composite under the illuminate of Xe lamp (300 W) for 4 h. 1.3 Characterization Scanning electron microscopy (SEM) was performed with a JSM-6460LV (JEOL Ltd.). X-Ray diffraction (XRD) patterns were measured in the range of 2θ=10–80° by continuously scanning on a XRD-6100 (Shimadzu Corp.) X-ray diffractometer with graphite monochromatized CuKα radiation (λ=1.5418 Å). UV-visible diffuse reflectance spectra were recorded with a Cary-100 spectrophotometer (The United States Varian Technology Co., Ltd.) and barium sulfate was used as a standard. The surface structure of the as-prepared samples was determined by X-ray photoelectron spectroscopy (XPS), and was performed using a VG EscaLab 250 SYSTEM (Thermo Fisher Scientific Inc.) with Al Kα radiation (1,486.6 eV). The C1s photoelectron peak (binding energy at 284.6 eV) was used as the energy reference. Transmission electron microscopy (TEM) images were obtained on a TECNAI G2 F20 (FEI) microscope at an accelerating voltage of 200 kV. The photoelectrochemical measurements were performed in a standard three-electrode system using the glassy carbon electrode loading catalyst as the working electrode, a platinum foil as a counter electrode and Ag/AgCl electrode as reference electrode. The electrolyte was 0.1 mol/L Na2SO4 solution. The linear scanning voltammetry (LSV) can test their photoelectrochemical activity (Ko et al., 2011). The electrode potential and working current was controlled with an electrochemical analyzer (CHI660D Shanghai Chen Hua Instrument Co., Ltd.). 1.4 Measurement of photocatalytic activity The photocatalytic activities of the samples were evaluated by the photodegradation of anthraquinone dye (reactive brilliant blue KN-R) in an aqueous solution. An amount of 100 mL of anthraquinone dye aqueous solution with a concentration of 30 mg/L was mixed with 1 g/L of catalysts in the reactor, which was exposed to illumination of simulated sunlight. The simulated sunlight was obtained by a 300 W xenon lamp (CEL-HXF300, Beijing Ceaulight Co., Ltd.). The average light intensity of 80 mW cm−2 was obtained and reached the sample. The light source was located on the top of the container by suspension. The suspension Vol. 49 No. 4 2016
Fig. 1 XRD patterns of Ag/AgBr/BiPO4 samples with different content of Ag/AgBr nanoparticles
containing reactive brilliant blue KN-R and photocatalyst was magnetically stirred under a dark condition for 30 min till an adsorption–desorption equilibrium was established. Samples were then taken out from the reactor at regular time intervals and centrifuged immediately for separation of any suspended solid. The concentration of reactive brilliant blue K-NR after illumination was determined using a UVvisible spectrophotometer (UV-1800PC, Shanghai Meipuda Instruments Co., Ltd.) at λ=592 nm. The photocatalytic degradation rate D of reactive brilliant blue K-NR can be estimated according to Eq. (1). D = ( A − A t ) / A ×100%
(1)
Here, A is the initial absorbance of reactive brilliant blue K-NR, t is the reaction time and At is the absorbance at time t.
2. Results and Discussion The phase structure of as-prepared samples was investigated by XRD technique. Figure 1(a) shows that all samples exhibit some characteristic diffraction peaks which correspond to the standard data for BiPO4 with P21/n spacegroup (JCPDS No. 15-0767). The XRD results indicate that Ag–AgBr modified BiPO4 samples possess high crystallinity which is similar to pure BiPO4. Obviously, the modification of Ag–AgBr did not change the crystallinity of BiPO4. Figure 1(b) (from local magnification of Figure 1(a)) shows that there are two peaks located at 2θ values of 30.94° and 44.33° in Ag/AgBr/BiPO4 samples. The two peaks can be at367
Fig. 2 SEM and TEM images of Ag/AgBr/BiPO4–7 at% and pure BiPO4: (a) SEM image of pure BiPO4; (b) SEM image of Ag/AgBr/BiPO4–7 at%; (c) TEM image of pure BiPO4; (d)–(f) TEM images of Ag/AgBr/BiPO4–7 at%
tributed to (2 0 0) and (2 2 0) crystal planes of AgBr (JCPDS No. 06-0438), respectively. However, Ag0 diffraction peaks are not detected in Ag/AgBr/BiPO4 samples, which may be ascribed to Ag0 having concentrations beyond the detection capacity of the diffractometer. However, the existence of Ag0 will be demonstrated by the following XPS analysis. The SEM images of pure BiPO4 and Ag/AgBr/BiPO4 samples are shown in Figures 2(a) and (b), respectively. Figure 2(a) shows that most of the BiPO4 crystal particles are rodlike with length ranging from 800 nm to 3 µm. As shown in Figure 2(b), the morphology of Ag/AgBr/BiPO4 sample is similar to that of pure BiPO4 particles. This implies that the introduction of Ag/AgBr does not change the morphology of BiPO4 crystal. Furthermore, the observation of TEM for pure BiPO4 and Ag/AgBr/BiPO4 samples was carried out and is shown in Figures 2(c)–(f). A relative smooth surface can be observed for pure BiPO4 (see Figure 2(c)). Conversely, Figures 2(d)–(f) show a lot of nanoparticles adhered on the surface of Ag/AgBr/BiPO4 sample. It can be concluded that the mentioned nanoparticles may be Ag/AgBr nanoparticles because their morphologies are clearly different from that of rod-like BiPO4 crystals. Considering the presence of metal Ag/AgBr nanoclusters on the surface of BiPO4, 368
a great amount of hetero-interfaces may be constructed in the Ag/AgBr/BiPO4 composites (Xu et al., 2013). Thus, these heterojunctions could facilitate the charge transfer, and hence improved the photocatalysis efficiency. The XPS technique was used to investigate the surface information of pure BiPO4 and Ag/AgBr/BiPO4 samples. Figure 3(a) shows a representative survey spectrum for the BiPO4 and Ag/AgBr/BiPO4 samples. In Figure 3(a), the survey spectrum indicates that the elemental compositions on the surface of the pure BiPO4 sample are Bi, P and O, and the element compositions on the surface of the Ag/AgBr/BiPO4 sample are Ag, Br, Bi, P and O. The high-resolution XPS spectra of the samples are exhibited in Figures 3(b)–(e). The binding energy of Br 3d (see Figure 3(b)) located at about 69.2 eV is consistent with that of Br− in AgBr.34 The high-resolution XPS spectra of Ag 3d is shown in Figure 3(c), and the Ag 3d spectra of Ag/AgBr consisted of two individual peaks located at about 367.1 and 373.8 eV, which are attributed to Ag 3d3/2 and Ag 3d5/2, respectively. The Ag 3d5/2 (or 3d3/2) peak is further split into two different components at 366.9 and 368.1 eV (or at 372.8 and 374.3 eV), respectively. The peaks at 368.1 (or 374.3 eV) can be attributed to metal Ag; whereas, the peaks Journal of Chemical Engineering of Japan
Fig. 3 XPS spectra of Ag/AgBr/BiPO4 and pure BiPO4: (a) XPS survey spectra of samples; (b) Br3d; (c) Ag3d;(d) Bi4f;(e) O1s; (d) P2p
at 366.9 (or 372.8 eV) can be attributed to the Ag+ existing in AgBr (Wang et al., 2011b; Yan et al., 2013). This result also confirms that Ag/AgBr/BiPO4 sample contains elemental silver. It is obvious that the binding energy of Bi 4f, O 1s and P 2p shows a negative shift (Figures 3(d)–(f)), which indicated that the Bi, O and P chemical environment in the Ag/AgBr/BiPO4 was changed. It also suggests the existence of the interaction between Ag/AgBr and BiPO4 (Pan et al., 2012b; Xu et al., 2013). Figure 4 shows the UV-vis absorption spectrum of pure BiPO4 and Ag/AgBr/BiPO4 samples. It is found that pure BiPO4 exhibits weak absorption in the visible region. Compared with pure BiPO4, Ag/AgBr/BiPO4 samples display obvious absorption in the visible region, and the absorption intensity increases with the increase of Ag/AgBr content (Wang et al., 2011a). The better absorption of Ag/AgBr/BiPO4 samples in the visible region can be attributed to the plasmon resonance of silver nanoparticles deposited on the surface of BiPO4 particle (Chekroun et al., 2012). The PL emission spectrum of the samples was measured with excitation wavelength of λ=260 nm, and is shown in Figure 5. All samples exhibit a broad emission centered around 370 nm, which can be attributed to the recombinaVol. 49 No. 4 2016
Fig. 4 UV-visible diffuse reflectance spectra of the as-synthesized samples
tion of photogenerated electron-hole pairs by excitation of UV light (Liang et al., 2001). PL intensity of BiPO4 is decreased by introducing Ag/AgBr, which indicates introducing Ag/AgBr can evidently inhibit the recombination of photogenerated electron-hole pairs due to formation of Ag/AgBr/BiPO4 heterostructure (Cao et al., 2013). Thus, the introducing of Ag/AgBr should be favorable for improving photocatalytic activity of BiPO4. Furthermore, it is well 369
Fig. 5 PL spectra of the as-synthesized samples
Fig. 7 Photocatalytic degradation of KN-R under simulated sunlight
Figure 7 shows that the reactant is not decomposed completely at a later stage of photoreaction. This phenomenon may be attributed to the fact that hardly degradable compounds are produced with the progression of the reaction. Nevertheless, the decomposition of as-formed intermediate products mainly depends on the ability of the prepared catalyst.
Conclusions Fig. 6 Photocurrent–voltage characteristics of the obtained photocatalysts
known that the photocurrent curve can reflect the ability of electron-hole separation and electronic transmission under light irradiation (Kim et al., 2005). The photocurrent curves of BiPO4 and Ag/AgBr/BiPO4 samples are shown in Figure 6. It is found that the photocurrent is significantly enhanced at the same anodic potential after introducing Ag/AgBr. The photocurrent measurement again confirms that introducing Ag/AgBr can inhibit the recombination of photogenerated electron-hole pairs, which agrees with above stated results of PL. The photocatalytic activity of the catalysts is investigated by monitoring the decolorization of the KN-R solution under simulated sunlight irradiation. The decolorization rate of all samples is shown in Figure 7. It can be seen that the pure BiPO4 sample did not exhibit photocatalytic activity for decolorization of KN-R. Furthermore, it can be seen that the photocatalytic activity of BiPO4 is gradually improved with the increase of Ag/AgBr content until Ag/AgBr/BiPO4-7% reached a maximum decolorization rate (88.6%). The enhanced photocatalytic activity of BiPO4 can be attributed to the introduction of Ag/AgBr, which extends the light absorption of BiPO4 to the visible region and formation of Ag/AgBr/BiPO4 heterostructure improves the separation of photogenerated electron-hole pairs. However, excessive addition of Ag/AgBr can reduce the photocatalytic activity of BiPO4 because excessive Ag/AgBr can turn to an electron-hole recombination center and increase the probability of electron-hole recombination. It is noteworthy that 370
A series of novel Ag/AgBr/BiPO4 photocatalysts were successfully synthesized via a two-stage strategy. It is found that the introduction of Ag/AgBr effectively improves the photocatalytic activity under simulated sunlight irradiation, as compared to that of pure BiPO4. This can be attributed to the formation of heterostructure between Ag–AgBr and BiPO4 and the plasmon resonance of metallic Ag deposited on the surface of AgBr and BiPO4. This research provides a simple approach and new opportunity for enhancing photoreactivity of BiPO4. Acknowledgements This work was supported by The National Natural Science Foundation of China (21476033), Program for Liaoning Excellent Talents in University (LR2014013) and Cultivation Program for Excellent Talents of Science and Technology Department of Liaoning Province (No. 201402610).
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