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Origin of Activity and Stability Enhancement for Ag3PO4 Photocatalyst after Calcination Pengyu Dong 1, *, Guihua Hou 1 , Chao Liu 2 , Xinjiang Zhang 2 , Hao Tian 2 , Fenghua Xu 2 , Xinguo Xi 3 and Rong Shao 3, * 1 2

3

*

Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province, Yancheng Institute of Technology, Yancheng 224051, China; [email protected] School of Materials Engineering, Yancheng Institute of Technology, Yancheng 224051, China; [email protected] (C.L.); [email protected] (X.Z.); [email protected] (H.T.); [email protected] (F.X.) Jiangsu Collaborative Innovation Center for Ecological Building Materials and Environmental Protection Equipments, Yancheng Institute of Technology, Yancheng 224051, China; [email protected] Correspondence: [email protected] (P.D.); [email protected] (R.S.); Tel.: +86-515-8829-8923 (P.D.); +86-515-8816-8011 (R.S.)

Academic Editor: Greta Ricarda Patzke Received: 30 September 2016; Accepted: 23 November 2016; Published: 29 November 2016

Abstract: Pristine Ag3 PO4 microspheres were synthesized by a co-precipitation method, followed by being calcined at different temperatures to obtain a series of calcined Ag3 PO4 photocatalysts. This work aims to investigate the origin of activity and stability enhancement for Ag3 PO4 photocatalyst after calcination based on the systematical analyses of the structures, morphologies, chemical states of elements, oxygen defects, optical absorption properties, separation and transfer of photogenerated electron-hole pairs, and active species. The results indicate that oxygen vacancies (VO ¨) are created and metallic silver nanoparticles (Ag NPs) are formed by the reaction of partial Ag+ in Ag3 PO4 semiconductor with the thermally excited electrons from Ag3 PO4 and then deposited on the surface of Ag3 PO4 microspheres during the calcination process. Among the calcined Ag3 PO4 samples, the Ag3 PO4 -200 sample exhibits the best photocatalytic activity and greatly enhanced photocatalytic stability for photodegradation of methylene blue (MB) solution under visible light irradiation. Oxygen vacancies play a significantly positive role in the enhancement of photocatalytic activity, while metallic Ag has a very important effect on improving the photocatalytic stability. Overall, the present work provides some powerful evidences and a deep understanding on the origin of activity and stability enhancement for the Ag3 PO4 photocatalyst after calcination. Keywords: Ag3 PO4 ; photocatalysis; oxygen vacancies

1. Introduction Photocatalysis using semiconductors has attracted considerable attention due to its potential applications in solving energy supply and environmental pollution problems [1]. Much effort has been devoted to exploit new and efficient visible light-driven photocatalysts [2,3]. In particular, a breakthrough was made by Ye et al., who used silver phosphate (Ag3 PO4 ) semiconductor as the visible light photocatalyst for the oxidation of water as well as the photodecomposition of organic compounds. The results indicated that Ag3 PO4 semiconductor had an extremely higher visible light activity than commercial TiO2−x Nx [4,5]. Zhu et al. investigated the origin of photocatalytic activation of Ag3 PO4 via first-principle density functional theory, showing that Ag3 PO4 had a large dispersion of conduction band (CB) and the inductive effect of PO4 3− , favoring the separation of electron-hole pairs [6]. In the past several years, various strategies have been developed to further improve the

Materials 2016, 9, 968; doi:10.3390/ma9120968

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photocatalytic activity and stability of Ag3 PO4 under visible light irradiation. Coupling of Ag3 PO4 with other semiconductors or noble metals is an effective approach to promote the charge separation efficiency of Ag3 PO4 , and thus improve the photocatalytic activity. Some coupled systems such as Ag3 PO4 /TiO2 [7–9], Ag3 PO4 /AgX (X = Cl, Br, I) [10], Ag3 PO4 /Ag [11–13], Ag3 PO4 /g-C3 N4 [14,15] and Ag3 PO4 /graphene [16–19] have recently been developed to improve the photocatalytic activity of Ag3 PO4 . On the other hand, morphological engineering design is another effective approach to modify the photocatalytic activity of Ag3 PO4 . For instance, our research group synthesized Ag3 PO4 crystals with various morphologies such as branch, tetrapod, nanorod, and triangular prism [20] and tetrahedral Ag3 PO4 mesocrystals [21]. Additionally, other groups also prepared cubic sub-microcrystals [22], colloidal nanocrystals [23], hierarchical porous microcubes [24], tetrapod microcrystals [25–27], and two-dimensional dendritic nanostructures [28] of Ag3 PO4 . Nevertheless, researchers are still trying their best to explore novel and facile ways to further improve the photocatalytic activity and stability of Ag3 PO4 . Recently, we found that the photocatalytic activity of Ag3 PO4 was highly enhanced after calcination process in the air. However, we do not want to avoid the fact that two papers were recently published (nearly) simultaneously. Chong et al. prepared Ag3 PO4 by using AgNO3 as the precursor, and found that Ag3 PO4 annealed at 400 ◦ C showed the best photocatalytic degradation performance under visible light due to the synergistic effect of crystallinity, oxygen vacancies and specific surface [29]. Nevertheless, it lacked the direct evidence for the formation of oxygen vacancies as well as the content of oxygen vacancies, and the detailed photocatalytic mechanism was not discussed. On the contrary, Yan et al. reported that the thermal treatment yielded many Ag vacancies within Ag3 PO4 lattices, and both metallic Ag NPs and Ag vacancies in annealed Ag3 PO4 samples were beneficial to the enhancement of photocatalytic activity toward methyl orange (MO) degradation under visible light irradiation [30]. However, no corresponding direct evidence was introduced to prove the presence of Ag vacancies within Ag3 PO4 lattices. Therefore, some interesting questions naturally arise: What makes the contrary behavior? What are the substantial changes of Ag3 PO4 during the calcination process? What is the evidence? In this work, the substantial changes happened to Ag3 PO4 after calcination were investigated based on the systematical analyses of the structures, morphologies, chemical states of elements, oxygen defects (including the direct and indirect evidences for the formation of oxygen vacancies and the content of oxygen vacancies), optical absorption properties, separation and transfer of photogenerated electron-hole pairs, and active species. The reasons for the enhancement of photocatalytic activity and stability of calcined Ag3 PO4 series samples were analyzed and revealed. Furthermore, the transfer pathways of photo-induced carriers and degradation mechanism were proposed and discussed in detail. 2. Experimental 2.1. Synthesis The calcined Ag3 PO4 photocatalysts were synthesized as follows. Firstly, the pristine Ag3 PO4 sample was prepared by a co-precipitation method in the dark at room temperature, according to a procedure described in the literature [4]. Typically, 1 g Na2 HPO4 was dissolved into 250 mL deionized water to form a transparent solution. Then, 13.5 mL CH3 COOAg solution (0.15 M) was dropped into the above Na2 HPO4 solution under stirring. After stirring for 0.5 h, the obtained sample was filtered, washed with deionized water and ethanol, and dried at 80 ◦ C overnight in vacuum. Secondly, the as-obtained Ag3 PO4 precursor was placed in quartz boat and calcined in muffle furnace at a heating rate of 1 ◦ C/min for 1 h at 100, 200, 300, and 400 ◦ C in air atmosphere, respectively. The final Ag3 PO4 photocatalysts were denoted as Ag3 PO4 -T, where T refers to the calcination temperature (100, 200, 300, and 400 ◦ C).


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To identify the roles of metallic Ag and oxygen vacancies, Ag/Ag3 PO4 composite was prepared by photodeposited method using the pristine Ag3 PO4 as the precursor. The synthesis details are as follows. The as-obtained Ag3 PO4 precursor was dispersed into the CH3 COOAg solution and irradiated with a 300 W Xe lamp for 1 h. Then the photocatalyst was filtered, washed with deionized water and ethanol, and dried at 80 ◦ C overnight, which was marked as Ag/Ag3 PO4 -PD. 2.2. Characterization The phase of the as-prepared samples was identified using Rigaku D/max-2400 X-ray diffractometer (XRD, Rigaku, Tokyo, Japan) with Ni-filtered Cu Kα radiation at room temperature. Thermogravimetric and differential scanning calorimetric analyses (TG-DSC) were performed with a NETZSCH DSC 200 F3 under an air flow, and the measurements were conducted from room temperature to 600 ◦ C at a heating rate of 10 ◦ C/min. Brunauer-Emmett-Teller (BET) surface area measurements were carried out using a Micromeritics ASAP 2000 system (Micromeritics Instrument Corp., Atlanta, GA, USA). The morphology was investigated by scanning electron microscopy (SEM, JSM-7500F, JEOL, Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) measurements were carried out to investigate the surface chemical compositions and states with Al Kα X-ray (hν = 1486.6 eV) radiation source (K-Alpha, Thermo Electron, Waltham, MA, USA). The binding energies (BE) were referenced to the adventitious C 1s peak (284.6 eV) which was used as an internal standard to take into account charging effects. Electron paramagnetic resonance (EPR) spectra were recorded on a JEOL JES-FA200 EPR spectrometer (JEOL, Tokyo, Japan) at room temperature. A combination of Gaussian and Lorentzian functions was used to fit the curves. Photoluminescence (PL) spectra were recorded on a 48000DSCF luminescence spectrometer (SLM Corp., Edison, NJ, USA) at room temperature by using a continuous-wave 325 nm He-Cd laser as the excitation source. UV-Vis diffuse reflectance spectra (DRS) were obtained on a UV-Vis spectrophotometer (UV-2600, Shimadzu, Kyoto, Japan) using BaSO4 as reference. The samples were pressed into a thin disk and fixed in a homemade quartz cell. The electrochemical impedance spectra (EIS) and Mott-Schottky (MS) plots of the as-prepared photocatalysts were measured on an electrochemical analyzer (CHI660E, Shanghai Chenhua Instruments Co., Ltd., Shanghai, China), and the details are described in Supplementary Materials. 2.3. Photocatalytic Tests According to the test method of photocatalytic materials for purification of water solution (GB/T 23762-2009, China) [31], the visible light photocatalytic activity of as-prepared samples was evaluated by degradation of MB in aqueous solution. The test details are as follows. Firstly, 0.10 g of catalyst was suspended in an MB aqueous solution (200 mL, 10 mg/L) in the dark for 1 h to gain the adsorption-desorption equilibrium. After the adsorption equilibrium of MB, the photocatalytic degradation reaction was carried out in a beaker with a circulating water system to remove the thermal effect of light. Light from a 300 W Xe lamp (HSX-UV300, Beijing NBET Technology Co. Ltd., Beijing, China), passed through a UV light filter film (to remove radiation with λ < 420 nm), was focused onto the reaction cell. When the light was turned on, at given time intervals, approximately 4 mL of the reaction suspension was sampled and thoroughly separated by means of high-speed centrifugation (18,000 rpm for 10 min) to reduce the experimental errors as much as possible. Then, clear and transparent filtrate was analyzed by recording the maximum absorbance at 664 nm in the UV-visible spectrum of MB. The degradation efficiency at time t was determined from the value of Ct /C0 , where C0 is the initial concentration and Ct is the concentration of MB at time t. The photocatalytic stability of pristine Ag3 PO4 and Ag3 PO4 -200 was also investigated by checking the cycling runs in photocatalytic degradation of MB. Four consecutive cycles were completed and each cycle lasted for 14 min. After each cycle, the photocatalyst was filtrated and washed thoroughly with deionized water, and then fresh MB solution (10 mg/L) was added again for the next cycling run. The Ag3 PO4 -200 after second cycle was marked as the used Ag3 PO4 -200 sample.


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The2016, active species capturing experiments were carried out to study the photocatalytic mechanism. Materials 9, 968 4 of 17 Different radical scavengers such as ethylenediaminetetraacetic acid disodium salt (EDTA-2Na, were added to the MB aqueous1 solution, to capture various active species. 1mmol/L) mmol/L), tert-butyl alcohol (t-BuOH, mmol/L)sequentially, and 1,4-benzoquinone (BQ, 1 mmol/L) were Then, the remaining experimental processes were similar various to the above test. added to the MB aqueous solution, sequentially, to capture activephotocatalytic species. Then,activity the remaining experimental processes were similar to the above photocatalytic activity test. 3. Results and Discussion 3. Results and Discussion 3.1. Phenomenon of the Enhanced Photocatalytic Activity and Stability 3.1. Phenomenon of the Enhanced Photocatalytic Activity and Stability Figure 1 shows the photocatalytic activities of pristine Ag3PO4 and Ag3PO4-T (T = 100, 200, 300 Figure 1 shows thelight photocatalytic of pristine Ag3 PO Ag3 PO4 -Tunder (T = 100, 200,light 300 4 and and 400) under visible irradiation.activities First of all, MB solution was illuminated visible and 400) under visible light irradiation. First of all, MB solution was illuminated under visible light irradiation in the absence of photocatalyst and the result indicates that the self-degradation of MB is irradiation the absence of photocatalyst andefficiency the resultof indicates that the self-degradation of3PO MB4-T is neglectable.inThe photocatalytic degradation MB over pristine Ag3PO4 and Ag neglectable. The photocatalytic degradation efficiency of MB over pristine Ag PO and Ag PO -T 3 4 3 4. The 4 follows the order of Ag3PO4-200 > Ag3PO4-100 > Ag3PO4-300 ≈ Ag3PO4-400 > pristine Ag3PO follows of Ag3intensity PO4 -200 > > Ag3 PO ≈ Ag > pristine Ag3 PO4 . 3 PO 4 -100 4 -300 3 PO4 -400 variationthe oforder absorption of Ag MB dye solutions over these samples at different irradiation The variation of absorption intensity Materials of MB dyeFigure solutions these samples at different irradiation times was recorded (Supplementary S1), over which strongly supports the above result. times was recorded (Supplementary Materials Figure S1), which strongly supports the above result. Clearly, the photocatalytic activity of calcined Ag3PO4 samples is much higher than that of pristine Clearly, photocatalytic activity of4calcined Ag3 PO is much higher than activity that of pristine 4 samples Ag3PO4.the More strikingly, the Ag3PO -200 sample shows the highest photocatalytic among Ag PO . More strikingly, the Ag PO -200 sample shows the highest photocatalytic activity among 3 4 3 4 these calcined Ag3PO4 series samples, with the decomposition of about 98% MB within 4 min, while these calcined with the of about 98% MB within 4 min, 3 PO 4 series samples, only 40% MB Ag was decomposed by pristine Agdecomposition 3PO4 within 4 min, even, only 80% MB was while only 40% was Ag decomposed by pristine Ag3 PO4 within 4 min, even, only 80% MB decomposed by MB pristine 3PO4 after 14 min under visible light irradiation. It is obvious thatwas the decomposed by pristine Ag PO after 14 min under visible light irradiation. It is obvious that the 3 3PO 4 4 was significantly enhanced after calcination. This enhancement photocatalytic activity of Ag photocatalytic activity Ag3 POinterest, enhanced after calcination. This enhancement 4 was significantly strategy aroused ourofgreat which requires neither the combination with other strategy aroused our great interest, which requires neither the combination with other semiconductor semiconductor nor the design of morphology engineering. Anyway, this phenomenon of the nor the design of morphology engineering. this phenomenon photocatalytic enhanced photocatalytic activity suggests Anyway, that something happened of tothe Agenhanced 3PO4 during the simple activity suggests that something happened to Ag3 PO4 during the simple calcination process. calcination process.

Figure 1. 1. Photocatalytic Photocatalyticdegradation degradationofof MB over pristine 4 and Ag3PO4-T (T = 100, 200, 300 Figure MB over pristine Ag3Ag PO34PO and Ag3 PO4 -T (T = 100, 200, 300 and and 400) samples under visible light irradiation. 400) samples under visible light irradiation.

To investigate the photocatalytic stability, pristine Ag3PO4 and Ag3PO4-200 were selected and To investigate the photocatalytic stability, pristine Ag3 PO4 and Ag3 PO4 -200 were selected and used to carry out the cycling runs in photocatalytic degradation of MB under visible light used to carry out the cycling runs in photocatalytic degradation of MB under visible light irradiation, irradiation, as displayed in Figure 2. It is observed that the Ag3PO4-200 still maintains a high as displayed in Figure 2. It is observed that the Ag3 PO4 -200 still maintains a high photocatalytic activity photocatalytic activity even after four cycles. In contrast, the pristine Ag3PO4 shows a significant even after four cycles. In contrast, the pristine Ag3 PO4 shows a significant decrease in its photocatalytic decrease in its photocatalytic degradation efficiency, and almost no MB degradation occurs over degradation efficiency, and almost no MB degradation occurs over pristine Ag3 PO4 after four cycles. pristine Ag3PO4 after four cycles. This result clearly indicates that the Ag3PO4-200 sample possesses This result clearly indicates that the Ag3 PO4 -200 sample possesses excellent photocatalytic stability. excellent photocatalytic stability.

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Figure 2. Cycling curves of photocatalytic degradation of MB solution in the presence of pristine Figure 2. Cycling curves of photocatalytic degradation of MB solution in the presence of pristine Figure Cycling of photocatalytic degradation of MB solution in the presence of pristine Ag3PO42.and Ag3POcurves 4-200, respectively. Ag3PO 4 and Ag3PO4-200, respectively. Ag PO and Ag PO -200, respectively. 3 4 3 4

3.2. Investigation of Substantial Changes 3.2. Investigation of Substantial Changes 3.2. Investigation of Substantial Changes In order to make clear the origin of the enhanced photocatalytic activity and stability for In order to make clear the origin of the enhanced photocatalytic activity and stability for In 4order make clearXRD, the origin of theBET, enhanced and stability for Ag3 PO4 Ag3PO aftertocalcination, TG-DSC, SEM,photocatalytic XPS, EPR, PLactivity and UV-Vis DRS technologies Ag3PO4 after calcination, XRD, TG-DSC, BET, SEM, XPS, EPR, PL and UV-Vis DRS technologies after calcination, XRD, TG-DSC, BET, SEM, XPS, EPR, PL and UV-Vis DRS technologies were employed. were employed. were employed. It It is is well-known well-known that that aa high high calcination calcination temperature temperature always always improves improves the the crystallization crystallization of of It is well-known that a high calcination temperature always improves the crystallization of photocatalysts [32]. In order to examine whether the crystallinity is enhanced after calcination process, photocatalysts [32]. In order to examine whether the crystallinity is enhanced after calcination photocatalysts [32]. In order to examine whether the crystallinity is enhanced after calcination the as-prepared samples samples were identified by XRD. shown in Figure 3, the3,XRD pattern of the process, the as-prepared were identified byAs XRD. As shown in Figure the XRD pattern of process, the as-prepared samples were identified by XRD. As shown in Figure 3, the XRD pattern of as-prepared pristine Ag3Ag PO34PO is4 in good accordance with the the as-prepared pristine is in good accordance with thestandard standarddata data(JCPDS (JCPDSNo. No. 06-0505), 06-0505), the as-prepared pristine Ag3PO4 is in good accordance with the standard data (JCPDS No. 06-0505), indicating the formation formation of ofthe thepure pureAg Ag with a body-centered cubic structure. 3 PO 4 phase indicating the 3PO 4 phase with a body-centered cubic (bcc)(bcc) structure. The indicating the formation of the pure Ag3PO4 phase with a body-centered cubic (bcc) structure. The The intense and well-defined diffraction peaks suggest that pristine Ag PO sample is well crystallized. 3 4 4sample is well crystallized. intense and well-defined diffraction peaks suggest that pristine Ag3PO intense and well-defined diffraction peaks suggest that pristine Ag3PO4 sample is well crystallized. After thethe crystal structure of the samples is retained, showingshowing a good thermal Aftercalcination calcinationininair,air, crystal structure ofresulted the resulted samples is retained, a good After calcination in air, the crystal structure of the resulted samples is retained, showing a good stability of the pristine Ag PO . However, it should be noted that the peak intensity of Ag 3 4 Ag3PO4. However, it should be noted that the peak intensity 3 PO4 -T thermal stability of the pristine of thermal stability of the pristine Ag3PO4. However, it should be noted that the peak intensity of samples does not obviously as expected. Especially, samples of Agsamples PO44-200, -300 3 PO4 -200, Ag3PO4-T samples does notincrease obviously increase as expected.the Especially, the of Ag Ag33PO Ag3PO4-T samples does not obviously increase as expected. Especially, the samples of Ag3PO4-200, and Ag43-300 PO4and -400Ag show decreased intensities, meaning that the crystallinity of these Ag3PO 3PO4slightly -400 show slightly peak decreased peak intensities, meaning that the crystallinity Ag3PO4-300 and Ag3PO4-400 show slightly decreased peak intensities, meaning that the crystallinity samples is slightly reduced. of these samples is slightly reduced. of these samples is slightly reduced.

Figure 3. XRD patterns of pristine Ag3PO4 and Ag3PO4-T (T = 100, 200, 300 and 400) (the standard Figure 3. AgAg AgAg (T (T = 100, 200,200, 300300 andand 400)400) (the standard data 3. XRD XRDpatterns patternsofofpristine pristine 3PO 4 and 3PO 4-T = 100, (the standard 3 PO 4 and 3 PO 4 -T data for body-centered cubic Ag3PO4 (JCPDS No. 06-0505) are also presented at the bottom for for body-centered cubic Ag (JCPDS No. 06-0505) also presented the bottom data for body-centered cubic 3PO4 (JCPDS No. are 06-0505) are also at presented atfor thecomparison). bottom for 3 PO4Ag comparison). comparison).

Figure S2 presents TG-DSC curves of pristine Ag3 PO4 measured in air atmosphere. The first weight loss below 150 ◦ C is attributed to the release of free water (ca. 0.7 wt %). The second weight loss (ca. 1.2 wt %) ranging from 150 to 371 ◦ C is due to the dehydration of pristine Ag3 PO4 . A sharp

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Figure S2 presents TG-DSC curves of pristine Ag3PO4 measured in air atmosphere. The first

◦ C is observed in the DSC curve, which can be attributed to the melting of endothermic at 150 528°C weight losspeak below is attributed to the release of free water (ca. 0.7 wt %). The second weight Ag3loss PO4(ca. instead the phase from change This with the XRD results. 1.2 wtof%) ranging 150[33]. to 371 °C result is due is to consistent the dehydration of pristine Ag3PO4. A sharp endothermic 528 °C is observed DSC curve, can betoattributed melting of of the BET surfacepeak areasatof as-prepared Ag3in POthe werewhich measured estimate to thethe influence 4 samples Ag3PO 4 instead the phase change [33]. This result isitconsistent withthe theBET XRDsurface results. areas of pristine surface area on theofphotocatalytic activity. However, is found that areas as-prepared Ag3PO4 samples were measured to estimate the influence of Ag3 PO4BET , Ag3surface PO4 -100, Agof 3 PO4 -200, Ag3 PO4 -300, and Ag3 PO4 -400 are relatively low, which are 1.02, the surface area on the photocatalytic activity. it isthe found the BET areasas of the 2 0.83, 0.52, 0.31, and 0.16 m /g, respectively. It However, is clear that BETthat surface areasurface decreases pristine Ag 3PO4, Ag3PO4-100, Ag3PO4-200, Ag3PO4-300, and Ag3PO4-400 are relatively low, which calcination temperature rises. Therefore, it is considered that the BET surface area is not a positive are 1.02, 0.83, 0.52, 0.31, and 0.16 m2/g, respectively. It is clear that the BET surface area decreases as factor for the enhanced activity of calcined Ag3 PO4 samples. the calcination temperature rises. Therefore, it is considered that the BET surface area is not a Figure 4a–j shows SEM images of the pristine and calcined Ag3 PO4 samples. As shown in positive factor for the enhanced activity of calcined Ag3PO4 samples. Figure 4a,b, the pristine Ag3 PO4 consists of spherical particles with an estimated average diameter of Figure 4a–j shows SEM images of the pristine and calcined Ag3PO4 samples. As shown in ~0.8–1.2 Compared pristine Ag ofPO be clearly visible that some nanoparticles with 4 , it canparticles Figureµm. 4a,b, the pristinewith Ag3PO 4 consists 3 spherical with an estimated average diameter of size~0.8–1.2 of ~5–10 nm are adorned on the surface of Ag PO microspheres (Figure 4c,d) for Ag PO 3 be4clearly visible that some nanoparticles 3 with 4 -100. μm. Compared with pristine Ag3PO4, it can A similar structure is maintained in the samples of Ag PO -200 and Ag PO -300. Moreover, it 3 4 3 4c,d) 4 for Ag3PO4-100. Acan size of ~5–10 nm are adorned on the surface of Ag3PO4 microspheres (Figure be seen that the amount of nanoparticles on the surface increases the calcination similar structure is maintained in the samples of Ag 3PO4-200 and Agas 3PO 4-300. Moreover,temperature it can be rises, indicating that theofthermal treatment promotes the grainasgrowth. However, for Ag3 PO seen that the amount nanoparticles on the surface increases the calcination temperature rises, 4 -400, indicating that the thermal treatment promotes the grain growth. However, for Ag 3 PO 4 -400, large large particles are linked end-to-end and thus self-assembled into a chain-structure as the higher particles are end-to-end and thus self-assembled into a chain-structure as all thethe higher temperature maylinked promote the disorder movement of fine particles [34], and almost surface temperature may promote the disorder movement of fine particles [34], and almost all the surface of of Ag PO is covered with sintered nanoparticles. Figure 4k,l shows the SEM images of the reused 3 4 Ag 3PO4 is covered with sintered nanoparticles. Figure 4k,l shows the SEM images of the reused Ag3 PO4 -200 sample. It can be seen that the surface of Ag3 PO4 particles becomes unshaped and Ag3PO 4-200 sample. It can be seen that the surface of Ag3PO4 particles becomes unshaped and rough, rough, and the nanoparticles are assembled on the surface of Ag3 PO4 . In addition, the quantity of and the nanoparticles are assembled on the surface of Ag3PO4. In addition, the quantity of nanoparticles was not significantly increased compared with fresh Ag3 PO4 -200 sample (Figure 4e,f). nanoparticles was not significantly increased compared with fresh Ag3PO4-200 sample (Figure 4e,f). These nanoparticles with a size of ~5–10 nm, however, are hard to probe by high resolution transmission These nanoparticles with a size of ~5–10 nm, however, are hard to probe by high resolution electron microscopy (HRTEM) because the structure of Ag3 PO4 materials could be destroyed by transmission electron microscopy (HRTEM) because the structure of Ag3PO4 materials could be high-energy beams electron during measurements [20,35]. destroyedelectron by high-energy beams during measurements [20,35].

Figure 4. SEM images of: pristine Ag3PO4 (a,b); Ag3PO4-100 (c,d); Ag3PO4-200 (e,f); Ag3PO4-300 (g,h);

Figure 4. SEM images of: pristine Ag3 PO4 (a,b); Ag3 PO4 -100 (c,d); Ag3 PO4 -200 (e,f); Ag3 PO4 -300 (g,h); Ag3PO4-400 (i,j); and the reused Ag3PO4-200 sample (k,l). Ag3 PO4 -400 (i,j); and the reused Ag3 PO4 -200 sample (k,l).

In order to make these nanoparticles clear, XPS spectra were employed to further analyze the In order to make nanoparticles XPS spectra employed to further analyze surface structure andthese composition. It can clear, be seen from Figurewere S3 that the XPS survey spectra with the peak identification for all samples are similar, which contain the elements of C, O, Ag, and P, and surface structure and composition. It can be seen from Figure S3 that the XPS survey spectrano with

peak identification for all samples are similar, which contain the elements of C, O, Ag, and P, and no


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obvious impurities Materials 2016, 9, 968 can be detected. The C peak is attributed to adventitious hydrocarbon from 7 of 17 the XPS instrument. obvious impurities can 5a–e, be detected. The C peak is attributed to adventitious hydrocarbon from the As shown in Figure the high-resolution XPS spectra of Ag 3d region of the as-prepared XPS instrument. samples consist of two individual peaks at ~374 and ~368 eV, which can be attributed to Ag 3d3/2 and shown in Figure 5a–e, the high-resolution XPS spectra of Ag 3d region of the as-prepared Ag 3d5/2Asbinding energies, respectively [8,36]. Since the peaks are asymmetrical, the Ag 3d peaks are samples consist of two individual peaks at ~374 and ~368 eV, which can be attributed to Ag 3d3/2 further divided into four different peaks at 374.2, 375.4, 368.2 and 369.4 eV, respectively [37]. The peaks and Ag 3d5/2 binding energies, respectively [8,36]. Since the peaks are asymmetrical, the Ag 3d at 375.4 and 369.4 eV can be ascribed to metal Ag0 , while the signals at 368.2 and 374.2 eV are associated peaks are further divided into four different peaks at 374.2, 375.4, 368.2 and 369.4 eV, respectively with[37]. Ag+The ionspeaks [37]. at This result indicates the presence of metallic Ag,the butsignals it could eliminate 375.4 andclearly 369.4 eV can be ascribed to metal Ag0, while at not 368.2 and the existence of Ag O. Moreover, as shown in Figure 5f, it is observed that the peak area attributed + 2 374.2 eV are associated with Ag ions [37]. This result clearly indicates the presence of metallic Ag, 0 0 content for the to Ag increased with thethe increase ofofcalcination temperature. surface Ag but is it could not eliminate existence Ag2O. Moreover, as shownThe in Figure 5f, it is observed that 0 is increased pristine Ag3 PO Ag3 PO4 -100, Ag3 PO -300, and is calculated be about the peak area to AgAg the4increase of Ag calcination temperature. Theto surface 4 , attributed 3 PO4 -200, with 3 PO4 -400, 0 content the and pristine Agrespectively, 3PO4, Ag3PO4-100, Ag3POthat 4-200, 3PO4-300, and Ag is 1.7%,Ag1.8%, 1.9%,for 2.2%, 2.6%, indicating theAgsurface content of3PO Ag40-400, increases to be about 1.7%, 1.8%, 1.9%, 2.2%, being and 2.6%, respectively, thatFurthermore, the surface it withcalculated the increase of the calcination temperature, accordance with indicating SEM results. content of Ag0 increases with theAg increase ofprobed the calcination being accordance should be mentioned that metallic are not by XRDtemperature, patterns because the amountwith of Ag0 SEM results. Furthermore, it should be mentioned that metallic Ag are not probed by XRD patterns in the calcined samples is below the detection limit (5 mol %) of the XRD analysis [38], as confirmed 0 in the calcined samples is below the detection limit (5 mol %) of the XRD because the amount by the calculated resultof inAg Figure 5f. In addition, the reused Ag3 PO4 -200 sample was collected after a analysis [38], as confirmed by the calculated result in Figure 5f. In addition, the reused Ag3PO4-200 second run and characterized by XPS technique. In Figure S4a, it can be seen that the Ag 3d peaks are sample was collected after a second run and characterized by XPS technique. In Figure S4a, it can be further divided into four different peaks at 374.2, 375.4, 368.2 and 369.4 eV, respectively. The peaks at seen that the Ag 3d peaks are further divided into four different peaks at 374.2, 375.4, 368.2 and 375.4369.4 and eV, 369.4 eV can be ascribed to metal Ag0 [37], indicating the presence of metal Ag in the reused respectively. The peaks at 375.4 and 369.4 eV can be ascribed to metal Ag0 [37], indicating 0 content for the reused Ag PO -200 sample was calculated to Ag3 PO sample. Moreover, 3 4 the4 -200 presence of metal Ag in the the Ag reused Ag3PO4-200 sample. Moreover, the Ag0 content for the be about 2.08%, which is close was to that for freshtoAg sample (1.9%), as shown Figure 4 -200 reused Ag3PO 4-200 sample calculated be3 PO about 2.08%, which is close to thatinfor freshS4b. ThisAg result that thereas is shown no obvious photocorrosion phenomenon for there the fresh 3PO4indicates -200 sample (1.9%), in Figure S4b. This result indicates that is noAg obvious 3 PO4 -200 sample during photocatalytic process. photocorrosion phenomenon for the fresh Ag3PO4-200 sample during photocatalytic process.

Figure 5. High-resolution XPS spectra Ag3d 3dregion regionof of pristine pristine Ag AgAg 3POPO 4-T (T = 100, Figure 5. High-resolution XPS spectra ofofAg Ag33PO PO4 4(a); (a);and and 3 4 -T (T = 100, 0 to the total area of all Ag3d 300 and (b–e); the ratios of peak attributed to 0Ag 200, 200, 300 and 400)400) (b–e); andand the ratios of peak areaarea attributed to Ag to the total area of all Ag3d peaks peakssamples for these(f) samples (f) (AP denotes for these (AP denotes Ag3 PO4Ag in3PO (f)).4 in (f)).

Figure 6a–e shows the O1s peaks of pristine Ag3PO4 and Ag3PO4-T (T = 100, 200, 300 and 400).

Figure the O1sare peaks of pristine Agat3 PO 100, 200, 531.5 300 and 3 PO4 and 4 -T± (T The O1s 6a–e peaksshows of all samples divided into fourAg different peaks 530.8 0.1=(O1 peak), ± 0.1400). The O1s peaks of all samples are divided into four different peaks at 530.8 ± 0.1 (O1 peak), 531.5 ± 0.1 (O2 peak), 532.3 ± 0.1 (O3 peak) and 533.2 ± 0.1 eV (O4 peak), respectively. The O1 peak can be (O2 attributed peak), 532.3 ± 0.1 (O3 peak) and 533.2 ± 0.1 eV (O4 peak), respectively. The O1 peak can to the non-bridging (P=O) oxygen atoms [39,40]. The O2 peak is associated with oxygen be attributed to the non-bridging (P=O) oxygen atoms [39,40]. The O2 peak is associated with oxygen



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vacancies or defects within the matrix, which are originally occupied by O2− ions [41–44], and the content of oxygen vacancies or defects depends theoriginally intensityoccupied or the area peak [42]. and In addition, vacancies or defects within the matrix, whichonare by of O2−this ions [41–44], the 2 − it was reported that the peak attributed O in Ag O was located at 528.8 eVthis [45],peak which much content of oxygen vacancies or defectstodepends on 2the intensity or the area of [42].is In 2− 2− loweraddition, than theit value of O2 peak (531.5 ± 0.1 eV) in our study. Then, it can be concluded that O was reported that the peak attributed to O in Ag2O was located at 528.8 eV [45], which is due to vacancies and not of toO2 thepeak formation Ag O4 peaks higher binding energies is oxygen much lower than the value (531.5 ±of0.1 eV) ourand study. Then, itatcan be concluded that 2 O.inO3 2− O assigned is due to to oxygen vacanciesoxygen and not atoms to the formation Agthe 2O. structure O3 and O4ofpeaks at higher can be the bridging (P–O–Ag)ofin Ag3 PO the specific 4 and binding energies can besuch assigned to the bridging oxygen atoms molecules, (P–O–Ag) inrespectively the structure [46]. of AgAdditionally, 3PO4 and the absorbed oxygen, as adsorbed oxygen and water as specific absorbed oxygen, such as adsorbed oxygen and water molecules, respectively [46]. shown in Figure 6f, the ratio of the O2 peak area to the total area of all O1s peaks (O1 + O2 + O3 + O4) Additionally, as shown Figure 6f, the of the O2 peak area to theand totalthe area of all first rapidly increases with in the increase of ratio the calcination temperature, ratio forO1s Agpeaks 3 PO4 -200 (O1 + O2 + O3 + O4) first rapidly increases with the increase of the calcination temperature, and the reaches maximum of 11.4%, which is circa six times higher than that of the pristine Ag3 PO4 sample ratio for Ag3PO4-200 reaches maximum of 11.4%, which is circa six times higher than that of the (1.8%). This indicates that Ag3 PO4 -200 has the highest density related to deficient oxygen, which is pristine Ag3PO4 sample (1.8%). This indicates that Ag3PO4-200 has the highest density related to attributed to oxygen, the release of is oxygen from the calcination giving rise to the 4 during deficient which attributed to Ag the3 PO release of oxygen from Ag3POprocess, 4 during the calcination ¨ ¨ formation of V on the surface of Ag PO [29]. V could create shallow donor states below CB and O process, giving rise to the formation3of V4O˙˙ on theO surface of Ag3PO4 [29]. VO˙˙ could create shallow increase thestates electron concentration, thus leading toconcentration, the enhancement the electrical conductivity donor below CB and increase the electron thus of leading to the enhancement of and the increase of active sites [47]. the electrical conductivity and the increase of active sites [47].

Figure 6. High-resolution XPS spectra of O1s region of pristine Ag3PO4 (a); and Ag3PO4-T (T = 100,

Figure 6. High-resolution XPS spectra of O1s region of pristine Ag3 PO4 (a); and Ag3 PO4 -T (T = 100, 200, 300 and 400) (b–e); and the ratios of the O2 peak area to the total area of all O1s peaks for these 200, 300 and 400) (b–e); and the ratios of the O2 peak area to the total area of all O1s peaks for these samples (f) (AP denotes Ag3PO4 in (f)). samples (f) (AP denotes Ag3 PO4 in (f)).

Given the SEM results and the analyses of high-resolution XPS spectra of Ag 3d and O1s regions of pristine Ag3POand 4 and Aganalyses 3PO4-T (Tof = high-resolution 100, 200, 300 andXPS 400),spectra it is reasonable assume Given the SEM results the of Ag 3dtoand O1sthat regions Ag (NPs) deposited on the surface of Ag 3PO4 300 microspheres. Moreover, it is obvious that metal of pristine Agare PO and Ag PO -T (T = 100, 200, and 400), it is reasonable to assume that Ag 3 4 3 4 0 in the on as-prepared Agof 3PO4 samples including the pristine Ag3PO4, which might result (NPs)Ag areexists deposited the surface Ag3 PO4 microspheres. Moreover, it is obvious that metal Ag0 theas-prepared “self-corrosion” of pristine Ag3PO4 during the period of preservation in the dark [48]. existsfrom in the Ag3 PO 4 samples including the pristine Ag3 PO4 , which might result from the Nevertheless, the fact that the surface Ag0 content increases with the increase of the calcination “self-corrosion” of pristine Ag3 PO4 during the period of preservation in the dark [48]. Nevertheless, the temperature could originate from the thermally decomposition during calcination process. During fact that the surface Ag0 content increases with the increase of the calcination temperature could the heat treatment, Ag3PO4 semiconductor is activated at high temperatures, and thus electrons are originate from thethe thermally decomposition during process. During theInheat excited from valence band (VB) into the CB ofcalcination Ag3PO4, leaving holes behind. our treatment, work, Ag3 PO semiconductor is activated at high temperatures, and thus electrons are excited the 4 because CH3COOAg was used as the raw material, the residual CH3COO− species on the Agfrom 3PO4 valence band (VB) into the ofwith Ag3 PO holes behind. our work, CH3 COOAg 4 , leaving surface may capture and CB react thermally excited holes, whichIn will inhibit thebecause recombination of − species on the Ag PO surface may capture and was used as the raw material, the residual CH3 COO thermally excited charge carriers and further promote the reduction reaction between electrons and 3 4

react with thermally excited holes, which will inhibit the recombination of thermally excited charge carriers and further promote the reduction reaction between electrons and Ag+ ions within Ag3 PO4

Materials 2016, 9, 968 Materials 2016, 9, 968

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matrix, resulting in the formation of metallic Ag in the air [30]. Simultaneously, molecular O2 in air condition adsorbed act asinanthe electron scavenger to trapAg theinthermally excited Ag+ ions within on AgAg 3PO34PO matrix, resulting formation of metallic the air [30]. 4 can also − Simultaneously, O2 radicals in air condition adsorbed on which Ag3POis 4 can also act as anreaction electron[30]. electrons to form themolecular superoxide (•O2 ) active specie, the predominant scavenger to trap thermally condition, excited electrons to form the superoxide radicals (•Omainly 2−) active specie, However, under the the oxygen-free the thermally excited electrons should participate which is the predominant reaction [30]. Ag; However, under the with oxygen-free condition, the air thermally in the reduction of Ag+ ions into metallic thus, compared that annealed in the condition, participate in the reductionAg of[30]. Ag+ Anyway, ions into metallic metallic Ag; thus,also the excited calcinedelectrons samplesshould shouldmainly have higher content of metallic Ag was compared with that annealed in the air condition, the calcined samples should have higher content formed in the presence of air. of metallic Ag [30]. Anyway, metallic Ag was also formed in the presence of air. EPR is considered to be a sensitive and direct method to monitor the behaviors of oxygen EPR is considered to be a sensitive and direct method to monitor the behaviors of oxygen defects [48]. The EPR spectra of the pristine and calcined Ag3 PO4 samples are displayed in Figure 7. defects [48]. The EPR spectra of the pristine and calcined Ag3PO4 samples are displayed in Figure 7. The peak at g = 2.001–2.004 has been reported previously and it can be attributed to natural surface The peak at g = 2.001–2.004 has been reported previously and it can be attributed to natural surface oxygen vacancies [49,50]. Notably, Ag3Ag PO3PO has much higher intensity of the EPR signal at g factor 4 -200 oxygen vacancies [49,50]. Notably, 4-200 has much higher intensity of the EPR signal at g of ~2.003 for the oxygen vacancy than both pristine and other calcined Ag3 POAg 3 PO4 Ag 4 samples, factor of ~2.003 for the oxygen vacancy than both Ag pristine 3PO4 and other calcined 3PO4 implying the increase of oxygen vacancies in the sample of Ag PO -200, which is consistent with the 3 of4 Ag3PO4-200, which is consistent samples, implying the increase of oxygen vacancies in the sample O1swith XPSthe results. O1s XPS results.

Figure 7. EPR spectra of pristine Ag3PO4 and Ag3PO4-T (T = 100, 200, 300 and 400) recorded at

Figure 7. EPR spectra of pristine Ag3 PO4 and Ag3 PO4 -T (T = 100, 200, 300 and 400) recorded room-temperature. at room-temperature.

It is well-known that the band-band PL signals of semiconductor materials are caused by the It is well-known that the band-band PL signals semiconductor materials caused by recombination of photoinduced charge carriers. In of general, a lower intensity of are band-band PL the signals indicates a decrease in the recombination rate of photogenerated carriers recombination of photoinduced charge carriers. In general, a lower intensity ofcharge band-band PL[51]. signals However, the excitonic PL signals mainly from surface oxygen and However, defects of the indicates a decrease in the recombination rateresult of photogenerated charge vacancies carriers [51]. semiconductors [52], because the oxygen vacancies as well as defects can easily bind photo-induced excitonic PL signals mainly result from surface oxygen vacancies and defects of semiconductors [52], electrons form excitons andasthe exciton energy can leveleasily can bebind formed near the bottom of the CB because the to oxygen vacancies well as defects photo-induced electrons to so form that PL signal can easily occur. The larger the content of oxygen vacancy or defect, the stronger the excitons and the exciton energy level can be formed near the bottom of the CB so that PL signal PL signal [52]. As shown in Figure 8, the strong emission peak is located at about 488 nm for the can easily occur. The larger the content of oxygen vacancy or defect, the stronger the PL signal [52]. as-prepared samples, which could be attributed to the band edge binding excitons resulting from the As shown in Figure 8, the strong emission peak is located at about 488 nm for the as-prepared samples, surface oxygen vacancies and defects [53]. In addition, the band-band PL peak at about 530 nm which could be attributed to the band edge binding excitons resulting from the surface oxygen (corresponding to the band gap of ~2.4 eV for Ag3PO4 semiconductor) cannot be found [4]. vacancies and defects [53]. Inofaddition, band-band peak at about 530 follows nm (corresponding Furthermore, the intensity excitonic the PL signal for thePL as-prepared samples the order of to the Ag band gap of ~2.4 eV for Ag3 PO semiconductor) be4 sample. found [4]. the intensity 44-400 3PO4-200 > Ag3PO4-300 > Ag 3PO > Ag3PO4-100cannot > Ag3PO ThisFurthermore, order is consistent with of excitonic PL signal for the as-prepared samples follows the order of Ag PO -200 > 3 4 impliesAg 3 PO 4 -300 the content of oxygen vacancies obtained from the O1s XPS spectra. The result that the > Ag Agsignal, > Ag3the POsurface This vacancy order isconcentration. consistent with the contentit of oxygen stronger the>PL the larger oxygen Additionally, can be 3 PO4 -400 3 PO4 -100 4 sample. vacancies obtained the O1s XPS spectra. resultfor implies that theofstronger the PL signal, concluded that thefrom PL spectra provide an indirectThe evidence the formation oxygen vacancies. Figure 9 showsoxygen UV-visible DRS ofconcentration. pristine Ag3POAdditionally, 4 and Ag3PO4-Tit(T = 100, 200, 300 andthat 400). It PL the larger the surface vacancy can be concluded the can be observed that all Ag 3 PO 4 samples show similar absorption plot. The pristine Ag 3 PO 4 has a spectra provide an indirect evidence for the formation of oxygen vacancies. broader absorption in the visible region with an absorption edge at ~530 nm, which is attributed to Figure 9 shows UV-visible DRS of pristine Ag3 PO4 and Ag3 PO4 -T (T = 100, 200, 300 and 400). thebe charge transfer of Ag 3PO4 from VB to CB. As seen from Figure S5, the estimated It can observed thatresponse all Ag3 PO 4 samples show similar absorption plot. The pristine Ag3 PO4 has indirect band gap of pristine Ag3PO4 is 2.44 eV, and the thermal treatment decreased the energy gap a broader absorption in the visible region with an absorption edge at ~530 nm, which is attributed of Ag3PO4-T (T = 100, 200, 300 and 400) series sample from 2.44 eV to 2.41 eV, resulting in an

to the charge transfer response of Ag3 PO4 from VB to CB. As seen from Figure S5, the estimated


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indirect band gap of pristine Ag3 PO4 is 2.44 eV, and the thermal treatment decreased the energy gap of Materials 2016, 9, 968 10 of 17 Ag3 PO4 -T (T = 100, 200, 300 and 400) series sample from 2.44 eV to 2.41 eV, resulting in an expanded lightexpanded response. The band gap narrowing calcination may be caused the formation of Materials 2016, 9, 968 10oxygen of 17 light response. The band gap upon narrowing upon calcination may beby caused by the formation of oxygen vacancies, local state below Ag 3 PO 4 . In addition, the localized surface vacancies, which createwhich a localcreate stateabelow CB of Ag3CB POof . In addition, the localized surface plasmon 4 expanded light response. The gapobserved. narrowing upon calcination may be caused bythe the formation plasmon resonance (LSPR) ofband metallic Ag is not Itobserved. It might result from the factmetallic that theAg0 resonance (LSPR) of metallic Ag is not might result from the fact that 0 content in of oxygen vacancies, which create a local state below CB of Ag 3PO 4. 200, In addition, the localized metallic Ag the pristine Ag 3 PO 4 and Ag 3 PO 4 -T (T = 100, 300 and 400) samples is very content in the pristine Ag3 PO4 and Ag3 PO4 -T (T = 100, 200, 300 and 400) samples is very low surface according plasmon resonance (LSPR) of metallic Ag is not observed. It might result from of thethese fact samples that the low according to the calculation from high-resolution XPS spectra of Ag 3d region to the calculation from high-resolution XPS spectra of Ag 3d region of these samples (Figure 5f). 0 metallic5f). Ag0The content in the pristine Ag3PO 4 and Ag3PO 4-Tnot (T =exhibit 100, 200, and 400) samples is very (Figure metallic quite low content will the300 plasmon the 0 The metallic Ag with quiteAg lowwith content will not exhibit the plasmon absorption inabsorption the UV-Visinspectra. low according to the calculation from high-resolution XPS spectra of Ag 3d region of these samples UV-Vis spectra. Similar phenomenon was also reported by Bi et al. [13], who synthesized the Similar phenomenon was also reported by Bi et al. [13], who synthesized the Ag/Ag3 PO4 necklace-like 0 with quite (Figure3PO 5f).4 necklace-like The metallic Ag low not exhibit the plasmon absorption in the Ag/Ag heterostructure andcontent did not will observe the plasmon absorption in the UV-Vis heterostructure and did not observe the plasmon absorption in Bi theetUV-Vis spectra. UV-Vis spectra. Similar phenomenon was also reported by al. [13], who synthesized the spectra. Based on the above analyses of O1s XPS spectra, EPR, PL and DRS, it is clear that oxygen vacancies Ag/Ag 3PO4 on necklace-like didspectra, not observe absorption the oxygen UV-Vis Based the above heterostructure analyses of O1sand XPS EPR, the PL plasmon and DRS, it is clearinthat are generated during the calcination process, and the Ag3 POand sample shows the highest content of 4 -200 spectra. are vacancies generated during the calcination process, the Ag3PO 4-200 sample shows the oxygen vacancies. Based on the above analyses highest content of oxygen vacancies.of O1s XPS spectra, EPR, PL and DRS, it is clear that oxygen vacancies are generated during the calcination process, and the Ag3PO4-200 sample shows the highest content of oxygen vacancies.

Figure 8. PL spectra of the as-prepared samples (excitation wavelength: 325 nm).

Figure 8. PL spectra of the as-prepared samples (excitation wavelength: 325 nm). Figure 8. PL spectra of the as-prepared samples (excitation wavelength: 325 nm).

Figure 9. UV-Vis DRS of pristine Ag3PO4 and Ag3PO4-T (T = 100, 200, 300 and 400). Figure UV-Vis DRS ofReaction pristine Ag3PO4 and Ag3PO4-T (T = 100, 200, 300 and 400). 3.3. Mechanism of the9.Photocatalytic Figure 9. UV-Vis DRS of pristine Ag3 PO4 and Ag3 PO4 -T (T = 100, 200, 300 and 400). EIS analysis is a powerful tool to investigate the transfer process of the photogenerated charges. 3.3. Mechanism of the Photocatalytic Reaction Figure 10 displays EIS Nyquist plots of pristine and calcined Ag3PO4 samples. In Nyquist plots, the 3.3. Mechanism of the Photocatalytic Reaction EIS analysis is a powerful tool to investigate thethe transfer ofprocess, the photogenerated charges. semicircle or the arch at high frequency represents chargeprocess transfer and the diameter of EIS analysis is a powerful tool to investigate the transfer process of the photogenerated charges. Figure 10 displays EIS Nyquist plots of pristine and calcined Ag 3 PO 4 samples. In Nyquist plots, the the semicircle or the arch reflects the mobility of charges; that is, the smaller the diameter of the Figure 10 displays EIS plots of pristine andClearly, calcined In Nyquist plots, semicircle or thefaster archNyquist at high frequency represents the charge transfer process, the diameter of the 3 PO4 samples. semicircle, the the electrons transfer [54]. theAg diameters of theand semicircle for the semicircle or the arch at high frequency represents the charge transfer process, and the diameter of the semicircle or the arch reflects the mobility of charges; that is, the smaller the diameter of the electrodes prepared by the calcined Ag3PO4 series samples are all much smaller than by the pristine the semicircle, thearch faster thetheelectrons transfer [54]. ofdiameter the semicircle for the the semicircle the reflects the mobility of charges; that is, the the smaller themobility. semicircle, Ag 3PO4or , indicating that calcination process canClearly, enhance thediameters electron Asofathe result, electrodes prepared by the calcined Ag3PO4 series samples are all much smaller than by the pristine Ag3PO4, indicating that the calcination process can enhance the electron mobility. As a result, the


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the faster the electrons transfer [54]. Clearly, the diameters of the semicircle for the electrodes prepared by the calcined Ag PO4 series samples are all much smaller than by the pristine Ag3 PO4 , indicating that Materials 2016, 9, 968 3 11 of 17 Materials 2016, 9, 968 11 of 17 the calcination process can enhance the electron mobility. As a result, the recombination of electron-hole pairs can be prevented to a certain extent. Among the electrodes prepared the calcined Ag3 PO4 recombination of electron-hole pairs can be prevented to a certain extent.byAmong the electrodes recombination of electron-hole pairs canshows be prevented to asemicircle, certain extent. Among the electrodes series samples, the Ag PO -200 electrode the smallest suggesting that Ag PO 3 4 Ag3PO4 series samples, the Ag3PO4-200 electrode shows the3 smallest 4 -200 prepared by the calcined prepared by the calcined Ag 3PO4 series samples, the Ag3PO4-200 electrode shows the smallest owns a moresuggesting effective separation pairs and a faster charge semicircle, that Ag3of POphotogenerated 4-200 owns a electron-hole more effective separation of interfacial photogenerated semicircle, suggesting that supports Ag3PO4-200 owns aphotocatalytic more effective separationefficiency of photogenerated transfer. This result strongly the highest degradation of MB over electron-hole pairs and a faster interfacial charge transfer. This result strongly supports the highest electron-hole pairs and a faster interfacial charge transfer. This result strongly supports the highest Ag displayed inefficiency Figure 1. of MB over Ag3PO4-200, as displayed in Figure 1. 3 PO4 -200, asdegradation photocatalytic photocatalytic degradation efficiency of MB Ag3PO4-200, as displayed in Figure 1. Figure 11 shows Mott-Schottky plots for over the prepared by pristine and calcined Ag3 PO4 Figure 11 shows Mott-Schottky plots for electrodes the electrodes prepared by pristine and calcined Figure 11 shows Mott-Schottky plots for the electrodes prepared by pristine and calcined samples. All samples have a positive slope in the Mott-Schottky plots, as expected for the n-type Ag3PO4 samples. All samples have a positive slope in the Mott-Schottky plots, as expected for the Ag 3PO4 samples. All samples have a positive slope in the Mott-Schottky plots, as expected for the semiconductors. Additionally, it is noteworthy that thethat slopes the linear region the calcined n-type semiconductors. Additionally, it is noteworthy theofslopes of the linearfor region for the n-type semiconductors. Additionally, it is noteworthy that theAg slopes the lineararegion for the Ag are much smaller thatthan for that pristine ,ofsuggesting higher donor 3 PO4 series 3 PO4Ag calcined Ag3POsamples 4 series samples are much than smaller for pristine 3PO4, suggesting a higher calcined[55]. Ag3PO 4 series samples are much smaller than that for pristine Ag3PO4, suggesting a higher density Generally, the higher donor the fasterthe thefaster photocatalytic degradation rate [56]. donor density [55]. Generally, the the higher thedensity, donor density, the photocatalytic degradation donor density [55]. Generally, the higher the donor density, the faster the photocatalytic degradation Therefore, the Mott-Schottky results further confirm thatconfirm the calcination greatly promotes the rate [56]. Therefore, the Mott-Schottky results further that theprocess calcination process greatly rate [56]. Therefore, the significantly Mott-Schottky results further of confirm that the calcination process greatly charge transfer, favoring the improvement the photocatalytic activity. promotes the charge transfer, favoring significantly the improvement of the photocatalytic activity. promotes the charge transfer, favoring significantly the improvement of the photocatalytic activity.

Figure 10. EIS EIS Nyquistplots plots of the electrodes prepared by pristine and calcined Agsamples 3PO4 samples Figure of of thethe electrodes prepared by pristine and calcined Ag3 POAg under 4 3PO4 samples Figure 10. 10. EISNyquist Nyquist plots electrodes prepared by pristine and calcined under visible light irradiation. visible light irradiation. under visible light irradiation.

Figure 11. Mott-Schottky (MS) plots of the electrodes prepared by pristine and calcined Ag3PO4 Figure Mott-Schottky (MS) ofelectrodes the electrodes prepared by pristine and calcined 3PO4 Figure 11.11. Mott-Schottky (MS) plotsplots of the prepared by pristine and calcined Ag3 PO4Ag samples. samples. samples.

A succession of control experiments was performed to understand the role of the photoexcited A succession of control experiments was performed to understand the role of the photoexcited active species in the photocatalytic degradation of MB dye. Herein, EDTA-2Na [57], t-BuOH [58] active species in the photocatalytic degradation of MB dye. Herein, EDTA-2Na [57], t-BuOH [58] and BQ [59] were used as scavengers of the trapped holes (h+), hydroxyl radical (•OH) and •O2−, and BQ [59] were used as scavengers of the trapped holes (h+), hydroxyl radical (•OH) and •O2−, respectively. Figure 12 presents the photocatalytic activity of Ag3PO4-200 upon the addition of respectively. Figure 12 presents the photocatalytic activity of Ag3PO4-200 upon the addition of different scavengers. The degradation efficiency greatly decreases to 6.9%, 55.1%, and 98.2% with the different scavengers. The degradation efficiency greatly decreases to 6.9%, 55.1%, and 98.2% with the addition of EDTA-2Na, t-BuOH and BQ, respectively. Thus, the role of active species in the addition of EDTA-2Na, t-BuOH and BQ, respectively. Thus, the role of active species in the degradation of MB dye follows the order of h+ > •OH > •O2−. This result indicates that h+ and •OH


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A succession of control experiments was performed to understand the role of the photoexcited active species in the photocatalytic degradation of MB dye. Herein, EDTA-2Na [57], t-BuOH [58] and BQ [59] were used as scavengers of the trapped holes (h+ ), hydroxyl radical (•OH) and •O2 − , respectively. Figure 12 presents the photocatalytic activity of Ag3 PO4 -200 upon the addition of different scavengers. The degradation efficiency greatly decreases to 6.9%, 55.1%, and 98.2% with the addition of EDTA-2Na, t-BuOH and BQ, respectively. Thus, the role of active species in the degradation of Materials 2016, 9, 968 12 of 17 MB dye follows the order of h+ > •OH > •O2 − . This result indicates that h+ and •OH are the most − crucial whilespecies there iswhile little there or nois•O the2− photocatalytic degradationdegradation of MB in the 2 during are the species most crucial little or no •O during the photocatalytic of Ag PO -200 system. 3 4 MB in the Ag3PO4-200 system. From both metallic AgAg andand oxygen vacancies are are present in the From the theabove aboveanalysis, analysis,ititisisclear clearthat that both metallic oxygen vacancies present in calcined Ag3 PO samples. However, it is difficult to trace beneficial effect back to back eitherto ofeither them or the calcined Ag43PO 4 samples. However, it is difficult tothe trace the beneficial effect of both identify roles of both species, the uncalcined Ag3 PO4 was theused precursor themoforthem. both To of them. Tothe identify the roles of both species, the uncalcined Agused 3PO4 as was as the and metallic Ag was photodeposited on it non-thermally, marked as Ag/Ag PO -PD. Then, oxygen 3 4 as Ag/Ag3PO precursor and metallic Ag was photodeposited on it non-thermally, marked 4-PD. vacancies would likely not be formed. Then, oxygen vacancies would likely not be formed. Figure Figure 13 13 shows shows aa comparison comparison of of the the photocatalytic photocatalytic activity activity over over pristine pristine Ag Ag33PO PO44,, Ag/Ag PO4-PD, and Ag Ag33PO PO4-200 samples under under visible visible light light irradiation. is found found that 4 -PD, and 4 -200 samples Ag/Ag33PO irradiation. ItIt is that the the photocatalytic degradation efficiency of Ag/Ag PO -PD is lower than that of pristine Ag PO and 3 4 3 4 photocatalytic degradation efficiency of Ag/Ag3PO4-PD is lower than that of pristine Ag3PO4 and Ag -200, indicating indicating that that metallic metallic Ag Ag has has aa negative Ag33PO PO44-200, negative effect effect on on the the photocatalytic photocatalytic activity. activity. Thus, Thus, itit can can be be concluded concluded that that oxygen oxygen vacancies vacancies play play aa significantly significantly positive positive role role in in the the enhancement enhancement of of photocatalytic activity for the calcined Ag PO samples. However, it should be pointed out that photocatalytic activity for the calcined Ag33PO44 samples. However, it should be pointed out that metallic metallic Ag Ag plays plays an an important important role role in in the the enhancement enhancement of of photocatalytic photocatalytic stability stability for for the the calcined calcined Ag samples because because the the metallic metallic Ag Ag can can serve serve as Ag33PO PO4 samples as aa good good electron electron acceptor acceptor for for facilitating facilitating quick quick ++ ions electron transfer from Ag PO [13] as far as possible instead of remaining in the position of Ag electron transfer from Ag33PO44 [13] as far as possible instead of remaining in the position of Ag ions in lattice, leading in Ag Ag33PO PO4 lattice, leading to to the the inhibition inhibition of of photocorrosion photocorrosion of of Ag Ag33PO PO44 [11,12]. [11,12]. This This explains explains why why the Ag PO -200 sample exhibits improved photocatalytic stability. 3 4 the Ag3PO -200 sample exhibits improved photocatalytic stability.

Figure 12. The photocatalytic activity of Ag3PO4-200 for the degradation of MB under visible light Figure 12. The photocatalytic activity of Ag3 PO4 -200 for the degradation of MB under visible light upon the addition of different scavengers. upon the addition of different scavengers.

Figure 13. Photocatalytic degradation of MB over pristine Ag3PO4, Ag/Ag3PO4-PD, and Ag3PO4-200 under visible light irradiation.

Figure 12. The photocatalytic activity of Ag3PO4-200 for the degradation of MB under visible light 13 of 17 upon the addition of different scavengers.


Figure 13. 13. Photocatalytic Photocatalyticdegradation degradationofofMB MBover overpristine pristine 3PO4, Ag/Ag3PO4-PD, and Ag3PO4-200 Figure AgAg 3 PO4 , Ag/Ag3 PO4 -PD, and Ag3 PO4 -200 under visible visiblelight lightirradiation. irradiation. under

Since Ag3PO4 has a low CB level (+0.45 V vs. a normal hydrogen electrode (NHE)) [4,60], which Since Ag3 PO4 has a low CB level (+0.45 V vs. a normal hydrogen electrode (NHE)) [4,60], which is is more positive than the potential for the single-electron reduction of oxygen (E(O2/•O2−) ≈ −0.33 V more positive than the potential for the single-electron reduction of oxygen (E(O2 /•O2 − ) ≈ −0.33 V vs. NHE)2016, [61,62], 9, 968 the electrons transferred to the surface of Ag NPs could not react with dissolved 13 of 17 vs.Materials NHE) [61,62], the electrons transferred to the surface of Ag NPs could not react with dissolved oxygen to generate the active groups of •O2 −−, but leaving a large number of h+ + in the VB of Ag3 PO4 , oxygen to generate the active groups of •O2 , but leaving a large number of h in the VB of Ag3PO4, as as shown in Scheme 1a. This proposed mechanism is confirmed by the photogenerated carriers shown in Scheme 1a. This proposed mechanism is confirmed by the photogenerated carriers trapping experiment electronson onthe thesurface surfaceofofAg AgNPs NPsmight might react with trapping experiment(Figure (Figure12). 12). The The excess excess electrons react with + in the aqueous solution of MB to form H O by multi-electron reduction oxygen molecules and H + 2 2 oxygen molecules and H in the aqueous solution of MB to form H2O2 by multi-electron reduction + − process (O(O 2H +0.682 V vs. NHE) 2 +2 + 2O 2 (aq), process 2H+++2e 2e− ==HH2O 2 (aq), +0.682 V vs. NHE) [63].[63].

Scheme 1. (a) Proposed transfer pathways of photoinduced carriers based on the energy levels of Scheme 1. (a) Proposed transfer pathways of photoinduced carriers based on the energy levels of Ag3PO4 and oxygen vacancies; and (b) a schematic illustration of MB photodegradation for the Ag3 PO4 and oxygen vacancies; and (b) a schematic illustration of MB photodegradation for the calcined calcined Ag3PO4 samples. Ag3 PO4 samples.

Furthermore, the oxygen vacancies might form a local state below CB of Ag3PO4, leading to Furthermore, thegap oxygen vacancies might formofa the localvisible-light state below CB of Ag narrowing of band and thus the enhancement response (as3 PO confirmed by to 4 , leading narrowing of band gapirradiated and thus the enhancement ofathe visible-light response by UV-Vis UV-Vis DRS). Once under visible light, majority of electrons are (as ableconfirmed to jump from the VB to the irradiated local state formed by oxygen in Ag PO4, and are leave h+ to in jump the VB. During the to DRS). Once under visible light,vacancies a majority of 3electrons able from the VB +defects of photocatalytic reaction, the formed oxygen vacancies and can become centers to theprocess local state formed by oxygen vacancies in Ag PO , and leave h in the VB. During the process 3 4 capture photo-induced electrons, so the recombination of photo-induced electrons and holes can be of photocatalytic reaction, the formed oxygen vacancies and defects can become centers to capture effectively inhibited [64], leading to the enhancement of photocatalytic activity. a photo-induced electrons, so the recombination of photo-induced electrons and holes Additionally, can be effectively large number of h+ with high oxidation potential (+3.0 V vs.activity. NHE) [65] are produced and take part of in h+ inhibited [64], leading to the enhancement of photocatalytic Additionally, a large number thehigh photocatalytic degradation reaction. This is[65] consistent with the measured results the mainly with oxidation potential (+3.0 V vs. NHE) are produced and take part in theofphotocatalytic oxidative species. Of course, some of h+ can also react with OH− group or H2O molecules to produce •OH radicals, which are responsible for the degradation of organic MB pollutant due to their high oxidation capacity, as illustrated in Scheme 1b. 4. Conclusions


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degradation reaction. This is consistent with the measured results of the mainly oxidative species. Of course, some of h+ can also react with OH− group or H2 O molecules to produce •OH radicals, which are responsible for the degradation of organic MB pollutant due to their high oxidation capacity, as illustrated in Scheme 1b. 4. Conclusions The calcined Ag3 PO4 photocatalysts with VO ¨ have been successfully synthesized via a facile calcination method. During the calcination, Metallic Ag NPs are generated due to the reduction of Ag+ in Ag3 PO4 semiconductor with the thermally excited electrons, and deposited on the surface of Ag3 PO4 microspheres with the formation of the closely contacted interface between two components which is beneficial for the rapid transfer of the photo-excited electrons from Ag3 PO4 to Ag NPs, and thus inhibiting the photocorrosion of Ag3 PO4 that means the enhancement of photocatalytic stability. Notably, the Ag3 PO4 -200 sample owns the highest amount of VO ¨, which could inhibit the recombination of photo-induced electron-hole pairs by acting as centers to capture photoinduced electrons and enhance the visible-light response by creating a shallow donor state below CB of Ag3 PO4 , leading to the enhancement of the electrical conductivity and the increase of active sites and then the highest photocatalytic activity. Moreover, it is found that a large number of h+ and •OH play a crucial role in the photocatalytic degradation of MB, while there is little or no •O2 − generated during the photocatalytic degradation of MB. In brief, the present work provides insight for the design of novel efficient visible-light photocatalysts. Supplementary Materials: The following are available online at www.mdpi.com/1996-1944/9/12/968/s1. Figure S1: UV-Vis absorption spectra of MB solutions separated from: pristine Ag3 PO4 (a); Ag3 PO4 -100 (b); Ag3 PO4 -200 (c); Ag3 PO4 -300 (d); and Ag3 PO4 -400 (e) suspensions during illumination, Figure S2: TG-DSC curves of pristine Ag3 PO4 , Figure S3: The survey XPS spectra of pristine Ag3 PO4 and Ag3 PO4 -T (T = 100, 200, 300 and 400), Figure S4: (a) High-resolution XPS spectrum of Ag 3d region of the reused Ag3 PO4 -200 sample; and (b) a comparison of Ag0 content for fresh Ag3 PO4 -200 sample and the reused Ag3 PO4 -200 sample, Figure S5: Plots of (αhν)1/2 versus photon energy (hν) of pristine Ag3 PO4 (a); Ag3 PO4 -100 (b); Ag3 PO4 -200 (c); Ag3 PO4 -300 (d); and Ag3 PO4 -400 (e). Acknowledgments: This work is financially supported by the National Natural Science Foundation of China (Grant Nos. 21403184 and 21301149), Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant No. 14KJB150025), China Postdoctoral Science Foundation (No. 2014M561622), Jiangsu Collaborative Innovation Center for Ecological Building Materials and Environmental Protection Equipments (No. CP201502, and GX2015102), Natural Science Foundation of Jiangsu Province (BK20150428 and BK20160434), and National Key Research and Development Project of China (No. 2016YFC0209202). Author Contributions: Pengyu Dong, Guihua Hou and Hao Tian participated in the experimental work, literature review, data analysis, and manuscript preparation. Chao Liu, Xinjiang Zhang, Fenghua Xu and Xinguo Xi provided suggestions on the concept, research methodology, as well as participated in data analysis and manuscript preparation. Rong Shao provided suggestions on the revision of original manuscript as well as participated in data analysis. All authors have read and approved the final manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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