The Surface Modification of Ag3PO4 using Tetrachloroaurate(III) and Metallic Au for Enhanced Photocatalytic Activity

The improvement of Ag3PO4 photocatalytic activity was successful by incorporating tetrachloroaurate(III) (AuCl4 ) and metallic Au on the surface of Ag3PO4. The photocatalysts were synthesized using the coprecipitation and chemisorption method. Coprecipitation of Ag3PO4 was carried out under ethanol-water solution using the starting material of AgNO3 and Na2HPO4.12H2O. AuCl4 − ion and metallic Au were incorporated on the surface of Ag3PO4 using a chemisorption method under auric acid solution. The photocatalysts were characterized using XRD, DRS, SEM, and XPS. The AuCl4 − ion and metallic Au were simultaneously incorporated on the Ag3PO4 surface. The high photocatalytic activity might be caused by increasing the separation of hole and electron due to capturing photogenerated electrons by metallic Au and Au(III) as electron acceptors. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).


Introduction
Recently, the Au dopant-modified photocatalysts have a tremendous great attention for improving the photocatalytic properties. The photocatalyst of Au/TiO2 [1-3], Au/ZnO [4,5], Au/WO3 [6], Au/SrTiO3 [7], Au/g-C3N4 [8], Au/CdS [9] has been successfully designed for improvement the photocatalytic properties. The incorporation of Au in TiO2 can reduce Ti 4+ to Ti 3+ that effectively enhances the stability and activity [3]. This design also generates a strong plasmon resonance absorption and improves photocatalytic activities. The metallic and ionic Au can be incorporated on the surface of TiO2. The metallic Au can produce the highest Schottky barrier which facilitates the electron capture, whereas the ionic Au (Au + and Au 3+ ) can facilitate electron transfer [10]. Both metallic and ionic Au dopants in TiO2 inhibit the recombination of electron and hole pairs. The Schottky barrier in the interfacial of metal-ZnO can also be formed by Au, this mechanism can enhance the separation of photogenerated carriers at the ZnO surface [11]. Schottky barriers can increase photoreaction through trapping and prolonging the life of the electrons [12]. However, these modifications have still drawbacks of photogenerated electron and holes recombination. Therefore, many researchers have developed the synthesis into a multi-component design such as a ternary composite of Au dopant in a photocatalyst.
The ternary composite of Au dopant, such as Au-CoFe2O4/MoS2 [13], g-C3N4/Au/ZnIn2S4 [14], rGO/NiWO4@Au [15], Au/CuO/Co3O4 [16], and Au/TiO2@CNTs [17], have been successfully synthesized. The Au has a significant role in the improvement of photocatalytic in these composites. The Au in the Au-CoFe2O4/MoS2 and g-C3N4/Au/ZnIn2S4 can effectively act as a facilitator of the Z-scheme mechanism [13,14]. This mechanism maximizes the reaction in the valence band and conduction band, hence improving the photocatalytic activity. The electrons on the surface of Au in the composite of rGO/NiWO4@Au can also act as electron donors, increase the electron transfer and the quantum efficiency leading to high catalytic activity [15]. The Au in Au/CuO/Co3O4 effectively mediates an electron trapping that improves the O2 generation [16]. Plasmonic resonance, the powerful properties of metallic Au, can also be designed in Au/TiO2@CNTs [17]. This plasmon can have a synergetic effect with CNTs that enhances the transport rates of electrons and holes. However, these multi-components of Au-based photocatalysts have still produced a low quantum efficiency. Therefore, an alternative photocatalyst should be expected.
Ag3PO4 is a new photocatalyst that has high quantum efficiency and narrow bandgap energy [18], could be promising as a highly active photocatalyst. Many researchers have focused on modifying this catalyst to improve its properties. The surface modification of the Ag3PO4 using Au has been reported [19][20][21]. Au in the Ag3PO4 inhibited the recombination of photogenerated electrons and holes and decreased the photo-corrosion of the Ag3PO4 [19]. The Auincorporated Ag3PO4 heterostructure showed highly productive photocatalytic activities [20]. These properties might be induced by the double effect of plasmonic Au nanoparticles and Ag3PO4 photoexcitation that generate converting hydroxyl groups into hydroxyl radicals [20]. The Au nanoparticle incorporated on Ag3PO4/TiO2 heterojunctions showed an excellent activity of 10.2 times higher improvement [21]. Based on the literature reports above, both metallic Au and ionic Au have been shown to increase photocatalytic activity. Therefore, it is promising to modify the surface of Ag3PO4 using the mixed Au states of metallic Au and ionic Au.
Besides the incorporation of Au on the surface of Ag3PO4, the defect formation on Ag3PO4 can enhance the photocatalytic properties. The defect sites on Ag3PO4 can be initiated under ethanol-water coprecipitation [22]. These defects lead to enhance absorption in the visible region resulting in high photocatalytic activity. A combination of this preparation with Au incorporating might improve the photocatalytic activity significantly.
Herein, the incorporation of AuCl4 − and metallic Au on the surface of Ag3PO4 has been successfully created using the coprecipitation and chemisorption method. The first step is the coprecipitation of Ag3PO4 under ethanol-water solution using the material of AgNO3 and Na2HPO4.12H2O, and the second step is the chemisorption of AuCl4 − ion on Ag3PO4 under sonication. This design aims to improve the photocatalytic activity of Ag3PO4 through an Au and AuCl4 − . Incorporation of these species on the Ag3PO4 surface can increase the electron capture and prevent photogenerated electronhole recombination, leading to high photocatalytic activity.

Synthesis of Photocatalysts
The chemicals of AgNO3 (ACS, ISO, Reag. Ph Eur; Merck), Na2HPO4.12H2O (ISO, Reag. Ph Eur, Merck), HAuCl4.4H2O (purity: 99.9%, Shandong Honrel Co. Ltd), and ethanol (absolute for analysis, Merck), were used for the synthesis of samples. The samples of APO (Ag3PO4) and E-APO (Ag3PO4 prepared under ethanol solution) were synthesized using the coprecipitation method [22,23]. To prepare the E-APO sample, the AgNO3 (0.85 g) was dissolved in 200 mL of ethanol solution (50% of ethanol in water). The Na2HPO4 solution was prepared by dissolving 1.8 g of Na2HPO4.12H2O in 50 mL of deionized water. AgNO3 in ethanol solution was slowly added (dropwise) by Na2HPO4 solution. The precipitate of E-APO was filtered and washed with deionized water and subsequently dried in an oven at 60 °C for 4 h. The sample of APO as control was synthesized with a similar procedure but without ethanol. The APO/Au (Au in Ag3PO4) and E-APO/Au (Au in Ag3PO4 prepared under ethanol solution) were synthesized using the chemisorption method. Firstly, the Au solutions were prepared using HAu-Cl4.4H2O dissolved in water with a concentration of 1 mg/mL. The 0.5 gram of APO and E-APO powder were each mixed with 10 mL of water to obtain the suspension. The Au solu-tions (10 mL) were added to the suspension and sonicated for 15 minutes. The products were filtered and washed with water and dried in an oven at 60 °C for 4 h.

Catalysts Characterizations
All samples were characterized using X-ray diffraction (XRD, Bruker D2 Phaser) with Cu K radiation, =1.5418 Å. Absorption spectra were measured using diffuse reflectance spectroscopy (DRS, JASCO V-670). Morphology and particle size of E-APO/Au were investigated using SEM. The binding energies of Ag3d, P2p, O1s, and Au4f of E-APO/Au were investigated using XPS (ULVAC PHI 5600, ULVAC PHI Co., Ltd), calibrated by internal reference of the adventitious carbon.

Photocatalytic Activities
The photocatalytic activities were evaluated under visible light irradiation (Ranpo, LED Spotlight 3 W, blue light) [23]. The catalysts of 0.1 g were added to 100 mL of RhB solution (10 mg/L) then equilibrated for 20 minutes. The sample of RhB solution (5 mL) was withdrawn and centrifuged at 5000 rpm to separate the solution from the catalyst and the concentration was measured using the spectrophotometer. The mechanism of photocatalyst in the surface of E-APO/Au was investigated using benzoquinone (BQ), ammonium oxalate (AO), and isopropyl alcohol (IPA) as a scavenger of O2• − , h + , and •OH, respectively [22].

Characterization of Photocatalysts
The structure of photocatalysts was investigated using the XRD and the results are displayed in Figure 1A. All samples showed a similar structure of body-centered cubic Ag3PO4 (JCPDS no.06-0505) [24]. Due to a very small concentration of Au, the (111) plane of Au was not observed in the XRD. The doublet peak was observed at (210) plane at the highest peak (2θ) of 33.4074°, 33.4479°, 33.3668°, 33.3668° for APO, E-APO, APO/Au, and E-APO/Au, respectively ( Figure 1B). The shift of (210) peaks was observed after incorporating the Au into the Ag3PO4. Both APO/Au and E-APO/Au exhibited a lower 2θ compared to the Au-untreated photocatalysts. The d-space (Å) of the (210) plane can be measured from the XRD data, resulting in 2.6821, 2.6790, 2.6853, and 2.6853 for APO, E-APO, APO/Au, and E-APO/Au, respectively. The d-space of E-APO is slightly lower than APO, indicating that the preparation with ethanol-water might form a silver vacancy. The changed polarity of solvent can affect the coprecipitation of Ag3PO4 leading The intensity ratio of (210)/(110)  to a defect formation of silver vacancy. This phenomenon can lead to a shrinkage of the lattice. In contrast, incorporating Au on the Ag3PO4 slightly increases the d-space, indicating that the Au was successfully incorporated in Ag3PO4 leading to the expansion of the lattice. The (210)/(110) intensity ratio of 7.2, 7.6, 8.4, 8.0 can be found in the samples of APO, E-APO, APO/Au, and E-APO/Au, respectively. The Au-treated samples showed a higher intensity ratio of (210)/(110) plane, indicating that the incorporation of Au might change the plane atomic composition of the crystal. The XRD data of Ag3PO4 prepared under different treatments can be summarized in Table 1. The morphology of E-APO/Au was also investigated under SEM, showing that a spherical and irregular shape with the particle size distribution of 100-400 nm was created (Figure 2).
To investigating the effect of different preparation, the spectra absorptions of APO, E-APO, APO/Au, and E-APO/Au are scanned and the results are displayed in Figure 3. The absorption of E-APO along the visible region has higher than that of APO, indicating the defect of silver vacancy might be generated in E-APO that is consistent with the previous result [22]. The incorporating Au in both APO/Au and E-APO/Au generated a broad absorption peak at 520 to 640 nm (insert picture of Figure 3). These absorptions might originate from the surface plasmon resonance of Au metallic nanocluster in Ag3PO4 [25]. Due to the small amount of metallic Au on the surface of Ag3PO4, these plasmon absorptions are not so strong.
The optical bandgap of the samples can be estimated using Tauc's relation [26] using the following Equation (1): (1) where,  is the absorption coefficient, h is Planck's constant, ν is the transition frequency, A is a constant, Eg is the bandgap energy, and the exponent n represents the nature of transitions. The bandgap energy is determined from the linear extrapolation of (hν) 2 versus hν (n=1/2 for direct allowed transition). The results showed that the optical bandgap energies of APO, E-APO, APO/Au, and E-APO/Au are 2.40 eV, 2.37 eV, 2.42 eV, 2.42 eV respectively ( Table 1). The decrease of bandgap in the E-APO sample might be caused by the defect of silver vacancy [22], whereas the increase of bandgap in APO/Au and E-APO/Au might be caused by Au incorporation on the surface.
The XPS profile of the Ag3PO4 (E-APO/Au) was displayed in Figure 4. The peak of 373.9 and 367.9 eV originates from the Ag3d3/2 and Ag3d5/2 respectively, indicating that the state of silver is Ag + (Figure 4a) and no Ag metallic was formed [27]. The P2p1/2 and P2p3/2 were also observed at 133.4 eV and 132.4 eV, indicating that the P +5 existed in the surface of Ag3PO4 (Figure 4b) [28]. There are two types of oxygen at 532.2. eV and 530.5 eV which are assigned to the oxygen of hydroxyl in the surface and oxygen in the lattice, respectively ( Figure  4c) [29]. The Au incorporation was observed in XPS spectra with an atomic percent of 3.34% (2.12% metal Au 0 and 1.22% ion Au 3+ ). There are two states of Au in the samples suggesting that both metallic and ionic were formed in the surface as shown in Figure 4d. The 88.4 eV and 84.7 eV originate from the Au4f5/2 and Au4f7/2 respectively with a spin−orbital doublet split-    ting of 3.7 eV that confirms the existence of Au 0 states [27]. Whereas, the Au 3+ found at 89.9 eV and 86.2 eV came from the Au4f5/2 and Au4f7/2, respectively [30].
An impressive result was found in the calculation of the Ag/P atomic ratio from the XPS data. The Ag/P atomic ratio of 3.76 was observed in E-APO/Au. This value was higher compared to the Ag3PO4 (2.80) and defect-Ag3PO4 (2.49) that were previously reported [23]. The high ratio of Ag/P suggesting that the phosphate was substituted by Au and AuCl4 -. Interestingly, the atomic ratio of Ag/P in E-APO/Au was higher than the incorporating platinum complexes on the defect-Ag3PO4 (2.97) [23]. This higher ratio of Ag/P in E-APO/Au might be induced by metallic Au formation on the lattice of Ag3PO4 which replaced much higher phosphate ion. Therefore, phosphate deficiency in E-APO/Au was created.
Based on XPS analysis, the formation of AuCl4 − and Au 0 can be explained. The compound of HAuCl4.4H2O can be ionized into AuCl4 − ions in water. AuCl4 − ions can substitute PO4 3− ions of Ag3PO4 on the surface. This unbalanced charge species substitution might easily induce electron transfers. During this process, the photoreduction of AuCl4 − in low intensity of lightroom might occur, resulting in Au 0 . Therefore, both AuCl4 − and Au 0 existed on the surface of Ag3PO4 as observed in XPS. Figure 5A showed the photocatalytic activity of APO, E-APO, APO/Au, and E-APO/Au. The photocatalytic activity rates are studied using the pseudo-first-order kinetics, ln(C0/Ct)=kt, where C0 and Ct are the RhB concentration at times 0 and t, respectively, and k is the rate constant [22]. After plotting ln(C0/Ct) versus t, all reactions followed the pseudo-first-order kinetics with the rate constant of 0.126 min −1 , 0.223 min −1 , 0.162 min −1 , and 0.391 min −1 for the samples of APO, E-APO, APO/Au, and E-APO/Au, respectively ( Figure 5B). The ethanol treatment in the preparation of samples enhances catalytic activities. The incorporation of Au on APO increases the catalytic activity as found in the APO/Au sample however it is lower than the E-APO sample. The lower activity might be caused by the lower absorption of APO/Au in the visible region due to decreased amount of electron excitation on the surface. The treatment of Au on E-APO significantly improves the photocatalytic activity as found in E-APO/Au. It was increased 3 times higher compared to the APO. The highest activity of E-APO/Au might be caused by the dual effect of Au 0 and AuCl4 -that acted as effective electron acceptors on the surface of Ag3PO4.
The results of the reaction mechanism study were shown in Figures 5C and 5D. The reactions in the surface were suppressed under the addition of AO and BQ, indicating that the role of hole and superoxide radicals were more prominent in the reaction. Based on this investigation, the reaction mechanism can be summarized in reactions (5-13). h + +H2O → •OH + H + (12) •OH + RhB → CO2 + H2O + NO3 − + NH4 + (13) The proposed mechanism in the surface of E-APO/Au can be illustrated and shown in Figure 6. Under visible light irradiation, the photo-excited electrons are injected into the conduction band of Ag3PO4. At the same time, the Au(III) ion acts as an electron acceptor and carries out the redox reaction on the surface of the photocatalyst, producing superoxide radicals. Figure 6. Proposed mechanism of photocatalytic reaction on the surface of E-APO/Au. Meanwhile, the metallic Au incorporated on the surface of Ag3PO4 generated the highest Schottky barrier that also facilitated the electron transfer [10]. The trapped electron is transferred to the adsorbed oxygen, producing superoxide radicals. These phenomena promote charge separation leading to high photocatalytic activity. The excellent photocatalytic activity is ascribed to efficient electron transfer from the conduction band of Ag3PO4 to Au(III) complexes chemically bonded surface and metallic Au, which brings to high charge separation, resulting in higher quantities of O2• − radical anions [35]. The O2• − radical anions under excess electron can produce H2O2, resulting in much higher •OH radicals. These species have very important roles in the oxidation of RhB into CO2, H2O, NO3 − , and NH4 + [36].

Conclusions
The metallic Au nanoparticles and AuCl4 − complexes ions were successfully incorporated on the surface of Ag3PO4. The ion of AuCl4 − complex substituted the PO4 3− of Ag3PO4 surface. The substitution was effectively on Ag3PO4 that synthesized under ethanol-water solution. The high photocatalytic activity of Au/AuCl4 − Ag3PO4 was mainly caused by metallic Au and Au(III) complex as electron acceptors.