Graphene Oxide Hybrids as Highly

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Tetrahedral Silver Phosphate/Graphene Oxide Hybrids as Highly Efficient Visible Light Photocatalysts with Excellent Cyclic Stability Yaxin Liu, Dongzhi Yang, Ruomeng Yu, Jin Qu, Yongzheng Shi, Hongfei Li, and Zhong-Zhen Yu J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 21 Oct 2017 Downloaded from http://pubs.acs.org on October 22, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

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Tetrahedral Silver Phosphate/Graphene Oxide Hybrids as

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Highly Efficient Visible Light Photocatalysts with Excellent

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Cyclic Stability

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Yaxin Liu,a,b Dongzhi Yang,a* Ruomeng Yu,a,b Jin Qu,a Yongzheng Shi,a,b Hongfei Li,b and

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Zhong-Zhen Yua,b*

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a

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Engineering, Beijing University of Chemical Technology, Beijing 100029, China

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b

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Chemical Technology, Beijing 100029, China

State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and

Beijing Key Laboratory of Advanced Functional Polymer Composites, Beijing University of

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ABSTRACT: The degradation efficiency and recyclability of photocatalysts are the key for

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their practical applications. Tetrahedral silver phosphate (Ag3PO4) is a superior visible-light

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photocatalyst, while graphene oxide (GO) sheets with high specific surface area and abundant

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functional groups are expected to further enhance the photocatalytic efficiency and improve

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the recyclability of Ag3PO4. Herein, we demonstrate an eco-friendly and kinetically controlled

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approach to synthesize Ag3PO4/GO hybrids. Tetrahedral Ag3PO4 are grown in situ on the GO

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sheets in a mixed solvents, and their microstructures are controlled by the slow dissolution

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and ionization of H3PO4, and the adjustment of the volume ratios of ethanol/water solvents.

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The hybrid with 5 wt% of GO exhibits an extraordinary photocatalytic efficiency and

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satisfactory recyclability for the degradation of organic dyes. Approximately 99% of

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methylene blue could be degraded in 4 min, and the degradation percentage is still as high as

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97% even after 5 cycles of photocatalytic degradations. The mechanism of reinforcement of 1

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the photocatalytic performance was also studied. This hybridization of tetrahedral Ag3PO4

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with GO sheets provides an efficient solution to the photocorrosion of Ag3PO4 and is an

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efficient approach for synthesizing Ag3PO4-based semiconducting hybrids as highly efficient

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and recyclable photocatalysts.

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INTRODUCTION

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Semiconducting photocatalysis as a promising solution to the issues of energy shortage and

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environmental deterioration has received much attention over the years.1-5 However, the

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semiconducting materials available so far are generally limited by narrow light-response

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range or insufficient charge separation ability.6,7 Therefore, much effort has been focused on

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developing photocatalysts with high catalytic efficiency and narrow band gaps for absorbing

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more visible light (400-600 nm).8-11

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Silver phosphate (Ag3PO4) was used as an efficient n-type semiconducting photocatalyst

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for the first time by Ye et al.12 Its suitable band gap of 2.36 eV, high quantum efficiency (up to

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90%), and high surface energy result in considerably higher photo-oxidative capabilities for

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O2 evolution from water as compared to standard visible-light driven photocatalysts including

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BiVO4 and WO3, and lead to excellent photocatalytic activities for the degradation of organic

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dyes under visible light irradiation.13-16 Among several types of Ag3PO4 morphologies,

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tetrahedron with high exposed {111} facet exhibited considerably higher surface energy and

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much smaller hole mass than either the cube with {110} facet or the rhombic dodecahedron

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with {100} facet, and it thus presents the highest photocatalytic efficiency.15-17 However,

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Ag3PO4 usually suffers from its poor stability under a prolonged light illumination due to the

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serious photocorrosion, and Ag+ could be reduced to metallic Ag by the photogenerated

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electrons. It is therefore necessary to synthesize tetrahedral Ag3PO4 with both excellent

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photocatalytic efficiency and satisfactory stability.

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Very recently, graphene oxide (GO) has been considered as a promising candidate for

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improving the efficiency of visible-light-driven photocatalysts, and a few of hybrids have

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already been reported, including TiO2/GO,18,19 CsPbBr3/GO,20 Ag/AgX/GO21 and ZnO/GO.22

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Generally, GO sheets with high specific surface area and active sites could facilitate the

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attachment of inorganic nanomaterials,23 enhance the adsorption of organic dyes and benefit

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the charge separation and transfer. Additionally, the sensitization of GO could improve the

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visible light absorption and utilization. Therefore, the combination of GO with Ag3PO4 could

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not only enhance the photocatalytic performance for the degradation of organic pollutants, but

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also significantly improve the stabilities of Ag3PO4 during the photocatalytic degradation

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process.

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Herein, we firstly demonstrate an environmentally friendly and kinetic-controlled

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electrostatic self-assembly approach for the fabrication of Ag3PO4/GO hybrids by growing in

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situ the tetrahedral Ag3PO4 nanoparticles on GO sheets in EtOH/H2O mixed solvents. The

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resultant tetrahedral Ag3PO4/GO hybrid exhibits an extraordinary photocatalytic efficiency

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and an excellent recyclability for the degradation of organic dyes with an excellent

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degradation percentage of 97% even after 5 cycles. The mechanisms for the significantly

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improved photodegradation efficiencies are also discussed in details.

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EXPERIMENTAL SECTION

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Materials. Graphite flakes (300 mesh) were provided by Qingdao Huadong Graphite

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Factory (Shandong, China). Silver nitrate (AgNO3), phosphoric acid (H3PO4), disodium

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ethylene diamine tetra-acetate (EDTA), p-benzoquinone (BQ), n-butanol (n-BuOH),

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methylene blue (MB), Rhodamine B (RhB) and methyl orange (MO) were supplied by

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Aladdin Chemical Reagent Co. Ltd. (Beijing, China). All chemicals are of analytical grade

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and were used as received without further purification.

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Synthesis of Ag3PO4/GO Hybrids. Graphite oxide was synthesized by oxidation

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of natural graphite using a modified Hummers method.24,25 Ag3PO4/GO hybrids were

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synthesized with a kinetic-controlled approach in mixed solvents: An appropriate amount of

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graphite oxide was exfoliated and dispersed by ultrasonication in a mixture of EtOH/H2O

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solvents with different volume ratios to obtain a stable suspension of GO. AgNO3 (1.02 g)

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was added to the GO suspension with magnetic stirring for 12 h, and then H3PO4 (0.196 g)

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was added dropwise with stirring for 0.5 h. The resultant was collected by centrifuging, rinsed

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with ethanol 5 times, dried under vacuum at 60 oC overnight, and finally denoted as

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Ag3PO4/GO-x, where x (%) is the weight percentage of GO in the hybrid.

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Characterization. X-ray diffraction (XRD) patterns were acquired using a Rigaku

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D/Max 2500 X-ray diffractometer with Cu Kα radiation (λ=1.54Å) at an operating voltage of

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20 kV and an operating current of 10 mA. The microstructures of the hybrids were observed

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with both Hitachi S-4700 scanning electron microscope (SEM) and JEOL JEM-3010

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transmission electron microscope (TEM). Chemical compositions of the GO and its hybrids

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were characterized using a ThermoVG RSCAKAB 250X high-resolution X-ray photoelectron

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spectroscopy (XPS). Raman spectra were obtained with a Renishaw inVia Raman microscope

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at an excitation wavelength of 514 nm. UV-vis diffuse reflectance absorption spectra were

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recorded on a UV-vis spectrophotometer equipped with an integrating sphere using BaSO4 as

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the reflectance standard. Photoluminescence (PL) spectra were acquired on an F-5400

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fluorescence spectrophotometer with an excitation wavelength of 300 nm. The scanning speed,

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photomultiplier voltage, excitation and emission slits were 240 nm min-1, 700 V, 2.5 nm and

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2.5 nm, respectively.

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Photocatalytic Degradation Experiments. For a typical photocatalytic

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degradation experiment, 20 mg of the catalyst was added into 20 mL aqueous solution of MB

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(50 ppm), and the suspension was magnetically stirred in the dark for 30 min to ensure the

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adsorption-desorption equilibrium of MB on the surface of the catalyst before its irradiation.

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A 350 W Xe arc lamp equipped with a cutoff filter (λ> 400 nm) was employed as a

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visible-light source, which was 20 cm away from the photocatalytic reactor. At a regular time

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interval, 1 mL of the suspension was taken out, diluted and centrifuged at 8000 rpm for 5 min.

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The supernatants were analyzed to measure the concentration of MB and total organic carbon

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(TOC) with a Shimadzu UV-2600 spectrophotometer and Vario TOC analyser, respectively.

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The degradations of RhB and MO were also carried out using the same procedure. The

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degradation experiments were repeated for 5 cycles to evaluate the stability of the

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Ag3PO4/GO hybrids. After each cycle, the hybrid was washed with ethanol and deionized

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water, and fresh MB solution was used for next cycle.

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Furthermore, comparative photocatalytic experiments were conducted to explore the

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photocatalytic mechanism, in which EDTA, BQ and n-BuOH were individually added to the

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MB solution under similar conditions. The degradation rate was calculated using the

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following formula: ln (C0/Ct) = k t, where k (min-1) is the rate constant at time t, and C0 (ppm)

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and Ct (ppm) represent the concentrations of dyes before the degradation and at the

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degradation time t, respectively.26 The degradation efficiency of the photocatalysts can be

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estimated by the f value. f = [(C0/Ct) V]/(mca t), where V, mCa. and t are the volume of dye

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(mL), the mass of photocatalyst (mg), and the degradation time (min), respectively.

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RESULTS AND DISCUSSION

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Microstructures of the Synthesized Ag3PO4/GO Hybrids. To synthesize Ag3PO4/GO

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photocatalysts with high charge transfer ability and catalytic efficiency, a kinetic-controlled

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approach at ambient temperature is employed to tailor the in situ growth of tetrahedral

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Ag3PO4 nanoparticles on GO sheets (Scheme 1). Figure 1a shows XRD patterns of

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Ag3PO4/GO hybrids prepared with different EtOH/H2O volume ratios. It is clear that the

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diffraction peaks of the hybrids can be indexed to body-centered cubic crystalline Ag3PO4

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(JCPDS No. 06-0505), and there are variations on the intensity ratios between the {222} and

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{200} planes. The different morphologies correspond to different intensity ratios.27 For

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spheroidal Ag3PO4 in the deionized water, its intensity ratio is 1.61. With increasing the

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volume ratio of ethanol, the intensity ratio gradually decreases to 1.03, similar to a reported

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value for tetrahedron. 27

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Scheme 1. Schematic illustrating the in situ growth of tetrahedral Ag3PO4 on GO sheets.

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Figure 1. XRD patterns of Ag3PO4/GO hybrids prepared with (a) different EtOH/H2O volume

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ratios and (b) different GO contents; SEM images of Ag3PO4/GO hybrids prepared in

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different solvents: (c) H2O only, (d) EtOH/H2O = 8/1 (v/v), and (e) EtOH only.

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Figure 1c-e show typical morphologies of the Ag3PO4/GO hybrids prepared at different

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solvents. The morphology of Ag3PO4 can be controlled by varying the volume ratios of

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ethanol to water. In the absence of ethanol, rapid ionization occurs in water, and a large

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amount of Ag+ in the system leads to free nucleation of Ag3PO4 with a similar growth rate of

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the crystal face, resulting in the formation of spherical Ag3PO4 (Figure 1c). The negatively

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charged functional groups on GO sheets provide reactive and anchoring sites for nucleation

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and growth of Ag3PO4 crystals by electrostatic self-assembly in the aqueous solution. With 7



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increasing the ethanol ratios, the rates of nucleation and initial growth of Ag3PO4 are

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controlled by the dipolar solvent of ethanol, thus facilitating the formation of the tetrahedron

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with an exposed {111} facet (Figure 1d, S1, S2). Regular tetrahedrons are mostly achieved at

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an EtOH/H2O volume ratio of 8/1. Ethanol plays a critical role in controlling the reaction rate

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and making the tetrahedral Ag3PO4 grow on GO sheets. However, in the absence of deionized

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water, the negative charge of GO is insufficient to provide nucleation sites for Ag3PO4 growth,

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and hence Ag3PO4 nanoparticles are isolated from the GO sheets (Figure 1e).

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To reveal the influence of GO contents on Ag3PO4 microstructure, Figure 1b shows XRD

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patterns of the Ag3PO4/GO hybrids with different GO contents. All the samples share the

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same diffraction peaks of body-centered cubic crystalline Ag3PO4. The intensity ratios of the

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hybrids closely match the value of tetrahedron, suggesting that the content of GO has little

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influence on Ag3PO4. No other trace peaks at 11o associated with GO could be observed,

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indicating that the presence of Ag3PO4 restrains the GO sheets from stacking and in turn

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inhibits the growth and aggregation of Ag3PO4.

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To further ascertain the tetrahedron formation, Figure 2a-e show TEM images of the

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Ag3PO4/GO-5 hybrid formed at different times. Ag3PO4 nanoparticles with the diameter of

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~15 nm aggregate together to form a ribbon first, and then evolve to a tetrahedron. The red

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dash line indicates the boundary of the tetrahedron. Figure 2f exhibits tetrahedral Ag3PO4

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nanoparticles attached onto GO sheets, which cannot be detached even under mild sonication.

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The Ag3PO4 phase is composed of tetrahedrons with sizes from several hundred nanometers

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to 2.5 microns. The Ag3PO4 nanoparticles with dimension of ~10 nm exhibit a good

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distribution on the GO sheets, which ensures the high specific surface area of the

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nanoparticles and is beneficial for transferring photoelectrons to the surface, thereby

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improving the reaction efficiency. There are many mesoporous in the Ag3PO4/GO hybrid with

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an average pore diameters ~25 nm, and the specific surface area of Ag3PO4/GO-5 is 15.0 m2

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g-1 (Figure S3). In addition, the morphology of the Ag3PO4/GO-5 hybrid was investigated by

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SEM observation and elemental mapping (Figure 2g). The distributions of O, Ag and P

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elements have similar distributions, indicating the definite existence of Ag3PO4 in the hybrid.

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The distribution of C element illustrates that the tetrahedral Ag3PO4 nanoparticles are covered

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with straticulate GO sheets.

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Figure 2. TEM images of Ag3PO4/GO-5 hybrids formed at different times: (a) 1 min, (b) 5

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min, (c) 10 min, (d) 15 min, (e) 20 min, and (f) 30 min. The red dash lines indicate the

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boundary of Ag3PO4. Inset in (f) is a magnified image of tetrahedral Ag3PO4/GO-5 hybrid. (g) 9



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SEM and elemental mapping images of the Ag3PO4/GO-5 hybrid.

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Raman spectra of Ag3PO4 and its hybrids support the close integration between GO and

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Ag3PO4 in the hybrids (Figure 3). Typically, two apparent bands at ~1358 cm−1 (D band) and

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~1590 cm−1 (G band) for the graphitized structures are observed, suggesting the presence of

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GO in the hybrids. In addition, the peaks at 907 and 996 cm-1 correspond to the symmetric

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and asymmetric stretching vibrations of (PO4)3-, respectively.28,29 Interestingly, neat Ag3PO4

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exhibits a predominant symmetric stretching vibrations. When the content of GO increases to

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2 wt.%, the asymmetric stretching vibration of (PO4)3- becomes more intense. Upon

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increasing the GO content to 10 wt.%, the asymmetric stretching vibration becomes the main

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vibration form, reflecting the close bonding between GO and Ag3PO4.

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Figure 3. Raman spectra of Ag3PO4, and Ag3PO4/GO hybrids.

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Figure 4a shows the UV-vis absorbance spectra of Ag3PO4/GO hybrids. It is seen that neat

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Ag3PO4 absorbs visible light with a wavelength less than 525 nm, corresponding to a band 10


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gap of 2.36 eV. The introduction of GO increases the absorption of the hybrids, and its

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absorbance increases with increasing the GO content, which can be explained by the

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sensitization effect of GO. To study the effect of GO on the separation efficiency of

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photogenerated electrons (e-) and holes (h+) originating from Ag3PO4, PL spectra of the

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Ag3PO4/GO hybrids are acquired using an excitation wavelength of 300 nm (Figure 4b). The

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strong emission band centered at ~530 nm results from the recombination of photogenerated

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electrons and holes, and the photon energy is approximately equal to the band gap of Ag3PO4.

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A small emission band at ~450 nm is observed, originating from the self-trapped excition and

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the recombination of charge-transfer transition between the O2p orbital and the empty d orbital

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of the central Ag+.29 The PL intensities are evidently decreased with the introduction of GO. It

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is widely accepted that a lower PL intensity corresponds to a lower recombination rate of

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e−/h+ pairs. These results suggest that the migration of electrons from Ag3PO4 to GO sheets

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prevents the direct recombination of e− and h+, leading to a longer lifetime for e− and h+,

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which is expected to provide superior photocatalytic activity. Nevertheless, it is noted that

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further increasing the content of GO has little influence on the charge transfer.

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Figure 4. (a) UV-vis absorbance spectra and (b) PL spectra of neat Ag3PO4 and its hybrids.

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Photocatalytic Performances and Mechanisms. To investigate the effect of GO

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on the photocatalytic activities in details, the degradation of MB, a model pollutant, is

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conducted to evaluate the degradation ability of Ag3PO4/GO hybrids under the visible light

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irradiation. It is seen that the adsorption equilibrium reaches within 30 min (Figure S4). Neat

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Ag3PO4 reveals poor adsorption ability for MB in dark, whereas it displays a higher

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degradation efficiency of 99% in 9 min under irradiation. Increasing the content of GO from

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2% to 10% results in an obvious enhancement in adsorption ability. It is mainly contributed

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by the oxygen-containing groups and nonoxidized conjugated regions on GO sheets serving

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as active adsorption sites (Figure S5). Under the irradiation, the Ag3PO4/GO-2 hybrid

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achieves 99% degradation of MB in 5 min; Ag3PO4/GO-5 hybrid needs 4 min, followed by 3

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min for Ag3PO4/GO-8 and Ag3PO4/GO-10 hybrids (Figure 5a).

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Figure 5. (a) Photocatalytic activities of Ag3PO4/GO hybrids with different contents of GO

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for MB degradation under visible light irradiation; (b) Photocatalytic activities of

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Ag3PO4/GO-5 for degrading MB, RhB and MO under visible light irradiation; (c)

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Photocatalytic activities of Ag3PO4/GO hybrids for MB degradation in the presence of

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different scavengers; (d) Cyclic degradation curves of Ag3PO4 and Ag3PO4/GO-5 hybrid for

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degradation of MB.

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Figure S6 shows the relative changes of MB concentration with 20 mg of Ag3PO4/GO-5

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hybrid as the photocatalyst. 50 ppm of MB is readily decomposed in 4 min, whereas 13 min is

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needed for the 80 ppm of MB. For 100 ppm MB, the rate of removal is far slower than that

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observed at 80 ppm. Considering the high efficiency of the catalyst, it is necessary to

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determine the optimal concentration of the dyes and the catalyst dosage. To eliminate the

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photosensitization of dyes, colorless organic compound of phenol is used for evaluating the

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photocatalytic of as-prepared hybrid (Figure S7). Among the photocatalysts, Ag3PO4/GO-8

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hybrid shows the highest photocatalytic efficiency because of the enhanced visible light

15

absorption and electron/hole separation, and it achieves 90% degradation of phenol in 18 min,

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which is much slower than the degradation of MB. Phenol molecule is composed of a

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hydroxyl group directly attached to the benzene ring, which is an enol type structure.

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However, due to the stability of the benzene ring, such a structure is hardly converted to a

19

ketone structure by electron or chemical bonds. So, it takes more time to degrade phenol than

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MB. Table S1 lists a comparison of the reaction rate constants and the photocatalytic

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degradation efficiencies (f values) for degradation of dyes caused by 1 mg of Ag3PO4/GO

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photocatalyst within 1 min. Among all the hybrids, Ag3PO4/GO-8 hybrid shows the highest

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photocatalytic degradation ability with a rate constant of 1.22 min-1 and an f value of

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1.65×10-2 mg min-1 mg-1. Nevertheless, the partial dyes adsorbed on the isolated GO sheets

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could not be completely degraded, causing a decrease in the degradation efficiency during the

4

recycling (Figure S8). To eliminate the effect of adsorption and reflect the stability of Ag3PO4

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in the recycling process, Ag3PO4-GO-5 is chosen as the sample for the recycling stability

6

experiments.

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TOC was monitored to investigate the mineralization of MB during the photocatalytic

8

reactions. After 7 min of photodegradation, the TOC removal percentages of MB in the

9

presence of neat Ag3PO4 and Ag3PO4/GO-5 hybrid are 72.7% and 55.5%, respectively (Figure

10

S9). It is noticed that the TOC removal percentage is lower than the discoloration percentage

11

of dyes. It is believed that the MB molecules are destroyed into some colorless intermediates,

12

not completely mineralized to inorganic molecules. Additionally, the TOC removal percentage

13

for pure Ag3PO4 is less than that for Ag3PO4/GO-5 hybrid, which is consistent with the

14

photocatalytic degradation results. Furthermore, the Ag3PO4/GO-5 hybrid exhibits stronger

15

photocurrent response for each light-on/off event than pure Ag3PO4 (Figure S10a), which

16

confirms high photo-activity of Ag3PO4/GO hybrid under visible light. The smaller arc radius

17

on the EIS plot of Ag3PO4/GO-5 corresponds to the lower transmission resistance and more

18

effective separation of the photogenerated e-/h+ (Figure S10b).

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Figure 5b shows the catalytic degradation of RhB and MO by Ag3PO4/GO-5 hybrid under

20

similar measurement conditions. Interestingly, the Ag3PO4/GO hybrid requires 5 min to

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degrade 99% of RhB and 4 min to degrade 99% of MB; In contrast, only 88% of degradation

22

is observed for MO over 7 min, which can be explained by the selective adsorption of GO.

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Because the surface of GO gathers many negative charges when the suspending of GO in

2

aqueous solution, the cationic dyes of RhB and MB are preferentially adsorbed on the GO

3

sheets and degraded. Differently, as MO is a typical anionic dye with the same electric charge

4

as GO (Figure S11),30 the hybrid exhibits poor adsorption and thus low degradation efficiency.

5

Figure S12 shows the time profiles of the absorbance spectra of MB, MO and RhB during

6

their degradations over Ag3PO4/GO-5 hybrid under visible light irradiation. The apparent blue

7

shift of the maximum absorption peak for the three dyes can be ascribed to the intermediates

8

generated during the demethylated process.31

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To further study the photocatalytic performance, Table 1 compares photocatalytic activities

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of the Ag3PO4/GO hybrids with those of other photocatalysts reported in the literature. The

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rate constants for MB and RhB degradation over Ag3PO4/GO-5 hybrid are 0.75 and 0.78 min-1,

12

and their photocatalytic degradation efficiencies are 1.25×10-2 and 9.90×10-3 mg min-1 mg-1,

13

respectively. These values are 100-fold higher than the corresponding values reported for

14

Ag3PO4/graphene catalyst,32 and much higher than other Ag3PO4 semiconducting hybrids.33-36

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Table 1. Comparison of the photocatalytic activity of Ag3PO4/GO hybrids with those reported

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in the literature. Catalyst Ag3PO4/CeO2 Ag3PO4/g-C3N4 Ag3PO4/MoS2 Ag3PO4/Ag Ag3PO4/Graphene Ag3PO4/GO-5 Ag3PO4/GO-8 Ag3PO4/GO-5

Cdye (ppm) MB MB RhB RhB MB MB MB RhB

10 10 10 20 6.4 50 50 50

Cca. (mg·mL-1)

t (min)

DP (%)

k (min-1)

f (mg min-1 mg-1)

Ref.

0.38 0.50 0.25 1.00 2.50 1.00 1.00 1.00

6 30 30 15 20 4 3 5

99 97 100 100 98 99 99 99

0.73 0.12 0.13 0.75 1.22 0.78

4.39×10-3 6.40×10-4 1.33×10-3 1.33×10-3 1.24×10-4 1.25×10-2 1.65×10-2 9.90×10-3

[33] [34] [35] [36] [32] This work This work This work

15



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Page 16 of 27

1

Cdye: the concentration of dyes (ppm); Cca.: the concentration of the catalysts (mg mL-1); t: the

2

degradation time (min); DP: the degradation percentage of dyes (%); k: the rate constant

3

(min-1); f: the photocatalytic degradation efficiency of dyes caused by 1 mg of catalyst within

4

1 minute (mg min-1 mg-1).

5

To study the photocatalytic degradation mechanisms of the tetrahedral Ag3PO4/GO hybrids,

6

we explore the active species responsible for the degradation of MB by adding different

7

radical scavengers (Figure 5c). The introduction of 2 mM of EDTA (a scavenger of h+) results

8

in a fast deactivation of the Ag3PO4/GO photocatalyst; 10 mM of EDTA presents the same

9

inhibitory effect. Differently, the presence of n-BuOH (a scavenger of ·OH radical) has no

10

deleterious effect on the photocatalytic activity. In addition, when 2 mM of BQ (·O2− radical

11

scavenger) is added, the degradation percentage of MB reaches 75.7% in 4 min, and the

12

degradation percentage decreases to 55.4% in 4 min at a higher concentration of BQ (10 mM).

13

These results indicate that h+ is the main active species assisted by the ·O2− radical, and ·OH

14

has only a minimal effect on the degradation of MB under visible light irradiation.

15

As shown in Figure 5d, the degradation percentage of MB over neat Ag3PO4 becomes 18%

16

only after 5 cycles. Compared to the significantly decreased degradation percentage of neat

17

Ag3PO4, there is almost no obvious change in degradation percentage for Ag3PO4/GO hybrids,

18

indicating its favorable recyclability. In addition, the XPS spectra (Figure 6a) and XRD

19

patterns (Figure 6b) of the Ag3PO4/GO hybrid before and after the photocatalytic experiments

20

also confirm the inhibitory effect of GO on the photocorrosion of Ag3PO4. Thanks to the

21

intensive interaction between Ag3PO4 and GO sheets, the peak at 373.85 eV associated with

22

Ag 3d5/2 of Ag+ shifts to 373.65 eV, and the peak at 367.85 eV associated with Ag 3d3/2 of Ag+ 16


Page 17 of 27

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1

shifts to 367.65 eV.34,37,38 The same peak position of the Ag3PO4/GO hybrid indicates its

2

structural stability after the 5 photodegradation cycles. The new peaks at 374.02 and 368.15

3

eV correspond to the Ag 3d5/2 and Ag 3d5/2 of Ag0,39 and the intensities of the two peaks are

4

more significant for neat Ag3PO4, which is in good agreement with the diffraction peaks of

5

metallic Ag after the 5 cycles (Figure 6b). In contrast, only small peaks of metallic Ag is

6

observed for the Ag3PO4/GO hybrid after the 5 cycles, which is also an evidence of the

7

improved stability of the Ag3PO4/GO hybrid.

8 9

Figure 6. (a) XPS spectra of Ag 3d for neat Ag3PO4 and Ag3PO4/GO-5 hybrid before and

10

after the photocatalytic degradation; (b) XRD patterns of neat Ag3PO4 and Ag3PO4/GO-5

11

hybrid after the 5 photocatalytic degradation cycles.

12

On the basis of the above analyses, a photocatalytic degradation process is proposed to

13

explain the high photocatalytic degradation efficiency and satisfactory stability of Ag3PO4/GO

14

hybrids (Scheme 2). The dyes are easily adsorbed on the surface of the photocatalyst due to

15

the electrostatic interaction and the π-π stacking with GO component. Ag3PO4 is excited to

16

generate e- at the conduction band (CB, 0.45 eV) and h+ at the VB (VB, 2.45 eV) under the

17



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Page 18 of 27

1

visible light irradiation. Because of the almost zero or very small band gap width and

2

electrical conduction, GO could serve as an effective acceptor of the photo-excited electrons,

3

hence, the efficient electron transformation from Ag3PO4 to GO sheets keeps the electrons

4

away from Ag3PO4, which decreases the reduction rate of Ag+ to metallic Ag in the

5

photocatalytic process and suggests a superior stability and recyclability of the Ag3PO4/GO

6

hybrid. GO (e-) reacts with O2 adsorbed on the surface of GO to produce ·O2−, which

7

continues to oxidize the adsorbed dyes producing harmless compounds of CO2 and H2O. The

8

photogenerated holes with powerful oxidizing ability directly decompose dyes into inorganic

9

micromolecules.36 Therefore, Ag3PO4/GO hybrids exhibit a high photocatalytic degradation

10

efficiency of dyes.

11 12

Scheme 2. Schematic illustrating the possible photocatalytic degradation mechanism of the

13

Ag3PO4/GO hybrid.

14

CONCLUSION

18


Page 19 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

Tetrahedral Ag3PO4/GO hybrids are synthesized by in situ growth of tetrahedral Ag3PO4

2

nanoparticles on GO sheets using a kinetic-controlled and electrostatic self-assembly

3

approach in mixed solvents. The introduction of GO significantly improves the photocatalytic

4

activity of Ag3PO4, and the optimal content of GO is found to be 5 wt%. Over 99% of MB

5

degradation is achieved in 4 min for the Ag3PO4/GO-5 hybrid, and its photocatalytic

6

efficiency is significantly higher than those of other Ag3PO4 based semiconducting

7

composites. More importantly, the Ag3PO4/GO-5 hybrid exhibits a satisfactory recyclability,

8

and its degradation percentage of MB still reaches 97% even after 5 cycles of photocatalytic

9

degradation. This study provides an efficient solution to the photocorrosion of Ag3PO4 and is

10

an efficient approach for synthesizing novel Ag3PO4-based semiconducting hybrids with

11

excellent photocatalytic degradation efficiency and satisfactory recyclability.

12

ASSOCIATED CONTENT

13

Supporting Information

14

SEM images of Ag3PO4/GO hybrids; nitrogen adsorption-desorption, and adsorption kinetics

15

of Ag3PO4/GO hybrids; photocatalytic degradation of MB and phenol; cyclic degradation

16

curves of Ag3PO4/GO-8 hybrid; TOC removal of MB; photocurrent transient response, and

17

electrochemical impedance spectroscopy of Ag3PO4 and Ag3PO4/GO-5 hybrid; UV-vis spectra

18

of Ag3PO4/GO-5 hybrid; and molecular structures of different dyes.

19

AUTHOR INFORMATION

20

Corresponding Authors

21

E-mail: [email protected] (D. Yang); [email protected] (Z.-Z. Yu)

22

Notes 19



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Page 20 of 27

1

The authors declare no competing financial interest.

2

ACKNOWLEDGEMENT

3

Financial support from the National Natural Science Foundation of China (51273015,

4

51533001, 51521062) and the Fundamental Research Funds for the Central Universities

5

(YS201402) is gratefully acknowledged.

6

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TOC Graphic

2

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