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Materials Research

versão impressa ISSN 1516-1439versão On-line ISSN 1980-5373

Mat. Res. vol.18 no.5 São Carlos set./out. 2015

https://doi.org/10.1590/1516-1439.346614 

Articles

One-step Synthesis of Ag3PO4/Ag Photocatalyst with Visible-light Photocatalytic Activity

Kai Huang a  

Yaohui Lv a   *  

Wei Zhang a   *  

Shanyun Sun a  

Bin Yang a  

Fangli Chi a  

Songlin Ran a  

Xianguo Liu a  

aSchool of Materials Science and Engineering, Anhui Key Laboratory of Metal Materials and Processing, Anhui University of Technology, 243002, Maanshan, Anhui, P. R. China


Abstract

A highly efficient photocatalyst Ag3PO4/Ag was prepared by a one-step low temperature chemical bath method. The reflectance spectra (DRS) indicated Ag3PO4/Ag had strong absorption in visible light region. The synthesized Ag3PO4/Ag photocatalyst was used as the efficient photocatalysts for the photocatalytic degradation of Rhodamine B (RhB) under visible-light illumination which showed almost complete degradation (~ 98%) of RhB dye in 90 min. The negative surface of Ag3PO4/Ag photocatalyst also promoted the degradation of a cationic dye like methylene bule (MB; 78% in 90min), while the performance against an anionic dye like methyl orange was poorer (MO; 40% in 90 min). Compared to the pure Ag3PO4 photocatalyst, the Ag3PO4/Ag photocatalyst showed the enhanced visible light photocatalytic informance. The excellent photocatalytic performance is mainly ascribed to the surface plasmon resonance of Ag nanoparticles and a large negative charge of PO43- ions.

Keywords:  Ag3PO4/Ag; chemical synthesis; photocatalysis; visible light

1 Introduction

Semiconductor-based photocatalysts have attracted considerable attention due to their potentially promising avenue for solving current environment and energy problems, by using the abundant solar light1. Of the well-known photocatalysts, titanium oxide (TiO2) has undoubtedly proven to be the most promising material, due to its low toxicity, earth abundance, chemical and thermal stability, and resistance to photocorrosion2. Unfortunately, this commonly used photocatalyst, with a relatively wide band gap (3-3.2 eV), can only absorb a small fraction of solar energy (< 5%), which limits its practical applications. Therefore, development of more efficient photocatalysts is urgent and indispensable.

Recently, it has been demonstrated that plasmonic nanostructures of noble metals (mainly silver and gold) show significant promising in the field of photocatalysis3. A series of new hybrid photocatalysts on the basis of surface plasmon resonance (SPR) of metal nanoparticles4,5, namely, plasmonic photocatalysts6, were developed to decompose various organic pollutants under visible-light irradiation. Tobaldi et al.7,8 have reported that the incorporation of Ag NPs can significantly improve the photocatalytic activity of TiO2 nanopowders. Chen et al.9 also revealed that the SPR effect of Au nanoparticles could induce visible-light-driven photocatalytic activity of Au/ZrO2 and Au/SiO2 photocatalyst. Ag/silver halide structure has been developed as a visible-light photocatalyst to enhance the photocatalytic activity of semiconductor, such as Ag/AgCl[10], Ag/AgBr[11] and Ag/AgI[12]. These catalysts displayed high photocatalytic activity and good stability under visible light due to the localized surface plasmon resonance (LSPR) effect of silver nanoparticles produced on the surface of silver halide.

Silver orthophosphate (Ag3PO4), a new type of photocatalyst reported by Ye et al.13-15, has been demonstrated to show excellent photocatalytic ability for O2 evolution from water as well as great photodecomposition of organic compounds. Interestingly, in order to promote the charge separation efficiency of the Ag3PO4 semiconductor, selective growth of Ag3PO4 submicron-cubes on Ag nanowires to construct necklace-like hetero-photocatalysts and two-dimensional dendritic Ag3PO4 nanostructures was demonstrated by this group16,17. The effect of particle size18,19, pH value20 and morphology21,22 of silver phosphate on its photocatalytic activity have also been investigated. Unfortunately, one major limitation of this novel photocatalyst is the instability upon photo-illumination, since it is easily corroded by the photogenerated electrons (4Ag3PO4+6H2O+12h++12e12Ag+4H3PO4+3O2)[13]. Therefore, it is highly desirable to develop effective strategies to improve the stability of the Ag3PO4 photocatalyst. Recent reports indicated that AgX (X=Cl, Br, I) nanoshells on the surface of Ag3PO4 can enhance their photocatalytic properties and stability23. Furthermore, carbon quantum dots24, graphene oxide25,26, TiO2[27],Bi2MoO6[28], SnO2[29] and Fe3O4[30] were successfully used to form Ag3PO4 based hybrid nanostructures for getting enhanced photocatalytic activity and stability. The origin of photocatalytic performance of Ag3PO4 using first-principles density functional theory was investigated31-33. The excellent photocatalytic performance of Ag3PO4 is partly attributed to the highly dispersive band structure of the conduction-band minimum (CBM) resulting from Ag s-Ag s hybridization without localized d states.

Inspired by this study, Ag/Ag3PO4 was successfully synthesized as highly efficient and stable plasmonic photocatalyst34-36. However, most of the reported synthesis methods require surfactant, such as using pyridine, or in relatively complicated process. Here, we design a one-pot, simple experimental approach to prepare the Ag3PO4/Ag photocatalyst to improve the stability of Ag3PO4. The photocatalytic activity evaluation was carried out by decomposing methyl orange (MO), rhodamine (RhB) and methyl blue (MB) under visible irradiation at room temperature. Based on the experimental results, the photocatalytic reaction mechanism of the Ag3PO4/Ag plasmonic photocatalyst was discussed.

2 Experimental

2.1 Preparation of Ag3PO4/Ag photocatalyst

All the reagents used in this work were in analytical grade without further purification, and purchased from Shanghai Reagents Company (Shanghai, China). Bare Ag3PO4 powder samples were prepared by the simple ion-exchange method. In a typical synthesis, 0.018 mol AgNO3 was dissolved in 100 mL of DI water. Na2HPO4 aqueous solution (0.12 M) was added drop by drop to the solution, under magnetic stirring, until the initial white colour changed to yellow. The resulting products were washed with DI water for several times, and finally dried at 70 °C for 5 h in a vacuum.

The Ag3PO4/Ag plasmonic photocatalysts were prepared by a one-step low temperature chemical bath method. In a typical synthetic route, 0.005 mol of silver nitrate (AgNO3) was added to 50 ml ethylene glycol (EG) under vigorous stirring, and the resultant solution was marked as solution A. The solution which was marked as solution B results from the dissolution of Na2HPO4·12H2O (0.001 mol) in 50 ml EG under vigorous stirring. When solutions A and B became clear, they were mixed together and stirred for 2 h until to assure homogeneity. The solution was heated in an oil bath at 160 °C for 30 min. The resulting precipitates were washed with DI and absolute ethanol several times and then dried at 70 °C for 5 h. A schematic diagram of the catalyst preparation process is shown in Figure 1.

Figure 1 A schematic diagram of the preparation processof Ag3PO4/Ag photocatalyst. 

2.2 Characterization of photocatalysts

The X-ray powder diffraction (XRD) patterns of the as-prepared catalysts were characterized by using a Bruker D8 advance powder X-ray diffractometer with Cu Kα radiation (λ=1.54056 Å) in the 2θ range from 20° to 70°. Field-emission scanning electron microscope (FESEM, JEOL-6300F) was employed to characterize the morphologies and size of the synthesized samples. UV-vis diffuse reflectance spectra were obtained for the dry pressed disk samples by using a Shimadzu UV 2550 recording spectrophotometer, which was equipped with an integrating sphere, and BaSO4 was used as a reference.

A Zeta potential analyzer, (Zetapals, Brookhaven Instruments Corporation, USA) was used to characterize the electrokinetic properties of the Ag3PO4/Ag photocatalysts. For the Zeta potential measurement, an Ag3PO4/Ag photocatalyst suspension of 2 mg/ml concentration was prepared by dispersing Ag3PO4/Ag powder in deionized water under continuous magnetic stirring. The effect of pH (in the range of 5-11) on the electrokinetic properties was investigated by measuring the zeta potential under distinct pH values, changed by using 0.1 M HNO3 (in the acidic range) and 0.1 M (alkaline range) solutions.

2.3 Evaluation of photocatalytic performance

MO (anionic dye), MB and RhB (cationic dyes) were selected as model chemicals to evaluate the activity and properties of the Ag3PO4/Ag photocatalyst. A 300 W Xe arc lamp (PLS-SXE300, Beijing Trusttech Co., Ltd.) equipped with an ultraviolet cutoff filter to provide visible light was used as the light source. The distance between it and the photocatalyst was 50 cm. The light intensity reaching the samples was measured using a radiometer and was found to be approximately 35 W m–2 in the visible-light range. In a typical experiment, 250 ml aqueous suspensions of RhB (10 mg/L) and 100 mg of photocatalyst powders were placed in a test tube. Prior to irradiation, the suspensions were magnetically stirred in the dark for 30 min to establish adsorption/desorption equilibrium between the dye and the surface of the catalyst under room air equilibrated conditions. At given irradiation time intervals, about 5 ml of the suspension was collected and centrifuged to remove the catalyst particulates for analysis. The residual dye absorption property was detected using a UV-vis spectrophotometer (Hitachi UV-3100)

3 Results and Discussions

The crystalline structure of the as-prepared sample has been examined, as shown in Figure 2. Figure 2a shows the typical powder X-ray diffraction (XRD) pattern of the product. It can be clearly seen that the diffraction peaks can be indexed to the pure body-centered cubic (bcc) structure of Ag3PO4 with cell constant of a= 6.013 Å (JCPDS no. 84-512). Figure 2b is a typical XRD pattern of the as-obtained Ag3PO4/Ag sample. All the diffraction peaks in this pattern can be categorized into two sets. The diffraction peaks that marked with “▼” can be readily indexed to cubic Ag3PO4 (JCPDS no. 84-512), while the peak labeled with “★” can be indexed to cubic phase Ag (JCPDS no. 87-717). No other crystalline impurities can be observed. The strong diffraction peaks of Ag indicate the good crystallinity degree of Ag in the Ag3PO4/Ag plasmonic photocatalyst.

Figure 2 XRD patterns of the (a) Ag3PO4 photocatalyst and (b) Ag3PO4/Ag photocatalyst. 

The morphology and size of the pure Ag3PO4 and the Ag3PO4/Ag were investigated with SEM and TEM techniques. Figure 3a depicts a typical SEM micrograph of the pure Ag3PO4 sample, revealing the spherical morphology with uniform size of about 800-900 nm. Figure 3b shows a SEM image of the prepared Ag3PO4/Ag photocatalyst. It can be seen that Ag3PO4/Ag photocatalyst is composed of many tiny spherical Ag3PO4 nanoparticles dispersed on the surface of the spherical Ag particles. Figure 3c shows the representative TEM micrographs of Ag3PO4/Ag photocatalyst. As shown in Figure 3c, the TEM image confirms that a large quantity of Ag3PO4 nanocrystals are attached onto the surface of the spherical Ag, and the size of Ag3PO4 nanoparticles is mostly in the range of 100-200 nm.

Figure 3 SEM images of (a) Ag3PO4, (b) Ag3PO4/Ag and TEM image of (c) of Ag3PO4/Ag photocatalyst. 

The UV-Vis diffuse reflectance spectra of as-prepared Ag3PO4and Ag3PO4/Ag photocatalysts are shown in Figure 4. It is observed that pure Ag3PO4could absorb visible light with a wavelength shorter than 530 nm (shown in Figure 4a), in agreement with the results previously reported15. However, for Ag3PO4/Ag plasmonic photocatalyst, except for a much higher absorbance than Ag3PO4 at the range of 500-800 nm, a new broad absorbance peak at 320 nm has been observed, which should be ascribed to the metallic Ag particles (shown in Figure 4b). Remarkable absorption enhancement in visible light region is beneficial for improving photocatalytic activity in this irradiation region.

Figure 4 UV-vis absorption spectrum of (a) Ag3PO4photocatalyst and (b) Ag3PO4/Ag photocatalyst. 

The photocatalytic performances of Ag3PO4/Ag plasmonic photocatalyst and of pure Ag3PO4 particles for the RhB degradation have been studied and are compared in Figure 5. As shown in this figure, it can be clearly seen that the Ag/Ag3PO4 composite shows a higher photocatalytic activity for the decomposition of RhB compared with Ag3PO4photocatalyst. Furthermore, the photocatalytic stability of Ag3PO4/Ag was investigated by recycling in the repeated RhB degradation experiments. As shown in Figure 6, the RhB dye is quickly bleached after every RhB decomposition experiments, and Ag3PO4/Ag photocatalysts are stable enough during the repeated experiments without exhibiting any significant loss of photocatalytic activity. Therefore, the as-prepared Ag3PO4/Ag composites can work as effective photocatalysts for organic compounds degradation with good stability in the absence of electron acceptors. To elucidate the photocatalytic reaction mechanism, the main species including h+, •O2− and •OH involved in the photocatalytic process was examined. The method was applied according to the Xiang’s report without any modifications37. The results indicated that h+ and •O2− were the main reactive oxidizing species in photocatalytic reaction process of Ag3PO4/Ag composites.

Figure 5 Photocatalytic activities of Ag3PO4/Ag composites and pure Ag3PO4 particles for RhB degradation under visible-light irradiation (λ> 420 nm). 

Figure 6 The repeated bleaching of RhB over recycled Ag3PO4/Ag photocatalysts under visible light. 

Furthermore, to demonstrate the photocatalytic activities of the synthesized Ag3PO4/Ag plasmonic photocatalyst for the degradation of distinct organic pollutants, we conducted photocatalytic degradation experiments of RhB, MB and MO. Figure 7a shows the adsorption spectrum of RhB dye over the prepared Ag3PO4/Ag photocatalyst. As seen from the observed UV-vis, the absorption peak characteristic of this dye (554 nm) decreases gradually with the increasing of the illumination time. After 90 min irradiation, RhB color was destained by about 98%. The adsorption spectrum of MB in aqueous solution under the same condition is shown in Figure 7b. It indicates that the concentration of MB is decreased as the irradiation time increasing by measuring the intensity of characteristic absorption peak (665 nm), and MB is degraded 78% after 90 min. Finally, discoloration of MO in aqueous solution was tested, by measuring the attenuation of adsorption peak (465 nm) intensity over irradiation time. Again, the color attenuation was observed (Figure 7c), but in a less overall extent (about 40% after 90 min of irradiation) than registered with the previous dyes. Differences in discoloration efficiencies for RhB, MB, and MO dyes can be compared in Figure 7d. The photocatalyst now prepared shows much better performance against cationic dyes (RhB and MB) than to discoloring the anionic dye (MO) solution.

Figure 7 Variations in adsorption spectra of organics dye solution in the presence of the Ag3PO4/Ag photocatalyst irradiated under visible light for different time: (a) RhB, (b) MB, (c) MO, and (d) photocatalytic degradation rate of RhB, MB, and MO. 

Attempting to clarify that difference, zeta potential measurements for a series of aqueous Ag3PO4/Ag suspensions of variable pH values were performed, as shown in Figure 8. The negative values of zeta potential reveal that the as-obtained particles show negative surface charge. Thus, the cationic dye molecules (RhB or MB) could be easily absorbed onto the catalyst surface by electrostatic attraction forces, and charge transfer is facilitated. By contrast, anionic dye molecules (MO) tend to be repulsed and as consequence the action of Ag3PO4/Ag plasmonic photocatalyst is less effective.

Figure 8 Effect of pH on the zeta potential of Ag3PO4/Ag composites in aqueous solution (T= 25 °C). 

Based on these results, a possible mechanism is proposed to explain that the enhanced photocatalytic activity and stability of the Ag3PO4/Ag plasmonic photocatalyst is due to the synergistic effects between the Ag3PO4 nanoparticles and the Ag nanoparticles, as shown in Figure 9. First, the deposition of Ag3PO4 nanoparticles on the surface of the Ag nanoparticles can effectively protect Ag3PO4 from dissolution, since Ag nanoparticles can retain/store electrons. The photogenerated electrons can be transferred to Ag0 nanopaticles instead of remaining in Ag3PO4 lattice. This will inhibits the reduction of Ag3PO4, and then will reduce the molecular oxygen to form the O2- active species. Second, The LSPR produced by the collective oscillations of surface electrons on Ag nanoparticles could enhance the local inner electromagnetic field. The electrons generated by the Ag3PO4 could be separated efficiency with the help of the local electromagnetic field34. Finally, PO43- ions own large negative charge and will attract positive holes while repelling electrons. Then, photogenerated holes tend to remain on the surface of Ag3PO4 nanoparticles. Meanwhile, PO43- ions have strong bonding affinity for H2O molecules. As is well known, H2O molecules could be easily adsorbed on the surface and then be oxidized by the holes to OH· radicals, being these active apecies to oxidize the dye molecule into carbon dioxide molecules. The above aspects together contributed to the enhanced photocatalytic activity and improved stability of the Ag3PO4/Ag plasmonic photocatalyst compared to pure Ag3PO4 particles.

Figure 9 Schematic illustrations of the photocatalytic mechanism of the Ag3PO4/Ag photocatalyst. 

4 Conclusions

This work developed a facile approach for the synthesis of Ag3PO4/Ag plasmonic photocatalyst. The novel material shows poorer performance in discoloring anionic dye solutions, such as MO dye, in comparison to cationic dyes (RhB and MB). This behavior can be well explained in terms of the exposed negative surface of the Ag3PO4/Ag photocatalyst. High efficiency of Ag3PO4/Ag photocatalyst arises from LSPR of silver nanoparticles and large negative charge of PO43- ions. It is a promising candidate for the removal of hazardous organic materials from wastewater.

Acknowledgements

This research was financially supported by the Natural Science Foundation of Anhui Provincial Education Depart-ment (KJ2015A085), the Natural Science Foundation of Anhui Province (1308085QE73) and the National Natural Science Foundation of China (Nos. 51302002 and 51201002).

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Received: November 06, 2014; Revised: May 17, 2015

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