High-efficiency visible-light-driven Ag3PO4 photocatalysts modified by conjugated polyvinyl alcohol derivatives

Due to its high-efficiency Ag3PO4 serves as a photocatalyst driven by visible light, which in this study was prepared through surface modification using a small amount of conjugated polyvinyl alcohol derivatives (CDPVA) via a simple chemisorption and heat treatment approach. The as-prepared CDPVA/Ag3PO4 composite photocatalysts were characterized via a varieties of analyses. The photocatalytic performance of the as-prepared composite photocatalysts was estimated through the photodegradation of methyl orange solution and the photoreduction of aqueous Cr(VI) solution with visible light. Results showed that the introduction of a trivial part of CDPVA on the surface of Ag3PO4 particles did not change their crystallinity and sizes but significantly reduced the aggregation of particles, strengthened the visible-light absorbance, and produced a more efficient separation of the photogenerated electron–hole pairs in the investigated composite photocatalysts. The visible-light photocatalysis of the composites exhibited a higher stability and activity than pure Ag3PO4. The visible-light photocatalysis of the composites exhibited an initial rise and a later reduction with increase in CDPVA content in the composites and heat treatment temperature and time. The synthetic photocatalysts exhibited the strongest visible-light photocatalysis when the CDPVA to Ag3PO4 mass ratio, heat treatment temperature, and treatment time were 1: 8000, 180 °C, and 1 h respectively. The mechanism for visible-light photocatalysis of the CDPVA/Ag3PO4 composites was also investigated. The solubility of Ag3PO4 in water environment was significantly decreased by the surface modification of CDPVA.


Introduction
During the past decades, more and more focus has been paid to the design and synthesis of highly visible-lightdriven photocatalysts because of their practical potential during production process of hydrogen from photocatalytic water splitting, decrease of CO 2 into reusable chemical fuel, and alleviation of pollution under solar irradiation [1][2][3][4][5][6]. Highly efficient photocatalytic activity was achieved with Silver orthophosphate (Ag 3 PO 4 ) in O 2 production from photooxidated water and photodegradated organic pollutants under visible-light irradiation. It could accomplish an impressive quantum efficiency as high as 90% with wavelengths above 420 nm, which implies a quite low rate of recombinated electron-hole pairs [5][6][7][8][9][10]. Therefore, Ag 3 PO 4 -based photocatalysts belongs to a rather promising solution for the environmental contamination and energy crisis [11]. However, several drawbacks generally restrict the large-scale application of Ag 3 PO 4 photocatalyst. Firstly, photocatalytic activity for Ag 3 PO 4 under the visible-light is featured with a low stability during photocatalysis because of massive transformation of Ag + into Ag°by photogenerated electrons. Second, Ag 3 PO 4 easily dissolves in water media owing to its large solubility, producing a decrease in visible-light driven photocatalytic activity [12].
A series of methods were proposed to address the limitations of using Ag 3 PO 4 ; such methods include noble metal deposition (Ag, Au, and Pt) [13][14][15], composites with other semiconductors [TiO 2 , ZnO, AgX (X=Cl, Br, I), CdS, and g-C 3 N 4 ] [ [16][17][18][19][20][21][22][23], modification by carbon materials (carbon quantum dots, carbon nanotubers, and grapheme and its derivatives) [24][25][26][27][28][29][30]. Given the excellent electron-capturing ability of Ag, Au, and Pt nanoparticles, the photogenerated electrons in Ag 3 PO 4 can be easily transferred to these nanoparticles, thereby producing a more efficient separation of photogenerated electron-hole pairs and decreasing the amount of the photogenerated electrons on the Ag 3 PO 4 surface; the use of these particles can thus enhance photocatalytic activity driven by visible light and decrease the reduction probability of Ag + to form Ag°. Ag 3 PO 4 photocatalysts modified by the other semiconductors or carbon materials display enhanced photocatalytic activity driven by visible light because of the well band gap match between Ag 3 PO 4 and other semiconductors or the excellent electron transfer ability of carbon materials. However, these noble metal nanoparticles, other semiconductors, or carbon materials cannot uniformly and continuously cover the Ag 3 PO 4 surface to decrease the transfer of Ag + to Ag and the solubility of Ag 3 PO 4 in water; as such, improvement in the chemical composition stability and photocatalytic stability in the water environment is hindered. Therefore, developing a novel strategy for preparing Ag 3 PO 4 -based photocatalysts with high stability and activity and stable chemical composition in water media is a valuable research direction.
Polyvinyl alcohol (PVA) with flexible macromolecular chains is a widely used water-soluble polymer. Conjugated derivatives from PVA (CDPVA) can be easily formed with heated PVA to a temperature range of 180°C-270°C (scheme 1); these conjugated derivatives exhibit physical and chemical properties similar to those of typical organic semiconductors, such as polyaniline and polythiophene [31,32]. The conjugated PVA derivatives can be used for enhancing visible-light photocatalytic performance to modify inorganic semiconductors. Therefore, surface modification of Ag 3 PO 4 particles by a little CDPVA could evidently enhance photocatalytic performance driven by visible light, including stability and activity.
In this work, Ag 3 PO 4 particles were dispersed in PVA solution to adsorb PVA macromolecules to the adsorption-desorption equilibrium. After rapid filtration and drying, the PVA molecules uniformly and continuously covered on the Ag 3 PO 4 surface. The obtained PVA/Ag 3 PO 4 composites were treated at 180°C to prepare CDPVA/Ag 3 PO 4 composite photocatalysts. The photocatalysts were evaluated with XRD, Raman , XPS , SEM , UV-vis DRS , PL, and EIS analyses. Visible-light photocatalytic performance was studied by evaluating the photodegradation of MO (methyl orange ) solution and the photoreduction of aqueous Cr(VI) solution irradiated by visible light. The solubility of pure Ag 3 PO 4 and CDPVA/Ag 3 PO 4 composites in water was then tested, and the possible photocatalytic mechanism governing the composite was studies through several experiments of reactive species trapping.

Materials and instruments
Materials and Instruments are shown in S1.  (1: 8000) particles possess similar shape and size to those of pure Ag 3 PO 4 but with smooth surface and decreased agglomeration because of the presence of the CDPVA layer on the Ag 3 PO 4 surface.

XRD and Raman spectra
The XRD patterns of CDPVA/Ag 3 PO 4 (1: 8000) and pure Ag 3 PO 4 particles are shown in figure 2(a). Diffraction spectrum for pure Ag 3 PO 4 exhibit clears 4 peaks at 20.93, 29.76, 33.37, and 36.66°, which correspond to the planes (110), (200), (210), and (211) for cubic Ag 3 PO 4 phase (JCPDS Card No. 01-089-7399), respectively. CDPVA/Ag 3 PO 4 (1: 8000) presents the same XRD spectrum as that of pure Ag 3 PO 4 , revealing that CDPVA coated on Ag 3 PO 4 particles surface produces little alteration of Ag 3 PO 4 crystal structure. The absence of characteristic peaks of CDPVA is attributable to its low crystallinity and poor content in the composites. Figure 2(b) displays spectra for CDPVA/Ag 3 PO 4 synthetic photocatalysts and pure Ag 3 PO 4 particles through the Raman. Pure Ag 3 PO 4 exhibits dual bands, namely, a stronger narrow one at 912 cm −1 and a weak broader one at 1012 cm −1 , which are associated with the antisymmetric and symmetric stretching vibrations of (PO 4 ) 3− , respectively [33,34]. The Raman characteristic peaks of the CDPVA/Ag 3 PO 4 composite photocatalysts at 910 and 1012 cm −1 weaken compared with those of pure Ag 3 PO 4 owing to the existence of the CDPVA layer on the Ag 3 PO 4 surface; the peaks further weaken with increasing CDPVA content on the composite surface.   Meanwhile, two new characteristic peaks at ca. 1350 and 1589 cm −1 are visible in Raman spectrum of the CDPVA/Ag 3 PO 4 composites, which can be related to the C=C stretching vibration of the conjugated polymers CDPVA [35]. The results reveal the existence of Ag 3 PO 4 and CDPVA in the CDPVA/Ag 3 PO 4 composites.

XPS
The valence state and chemical composition of various species could be determined by means of XPS measurement [36,37]. Peak strength is affected by the sample preparation, the resolution of the instrument and the method of background subtraction. The XPS survey spectra (figures 3(a) and (b)) show that the Ag 3 PO 4 surface contains Ag (2.4%), P (3.4%), O (18.6%), and C (75.7%) and the surface of CDPVA/Ag 3 PO 4 (1: 8000, 180°C, 1 h) predominantly contains Ag (2.0%), P (2.1%), O (17.2%), and C (78.7%), indicating that the contents of Ag, P, and O decrease with increasing C content. Hence, CDPVA adheres to the surface of Ag 3 PO 4 particles, consistent with the Raman result.
The spectrum decomposition was performed using the XPS PEAK 4.1 program with Gaussian functions after subtraction of a Shirley background. Measurements for Ag 3d, O 1 s, P 2p, and C 1 s core levels were performed to clarify the chemical state of Ag, P, O, and C. Figure 3(c) reveals dual peaks of 367.8 and 373.6 eV because of Ag 3d 3/2 and Ag 3d 5/2 , implying that Ag + exists in CDPVA/Ag 3 PO 4 composite photocatalysts [38]. The O 1 s XPS spectrum in figure 3(d) displays two characteristic peaks. The peak spectrum corresponding to 531.7 eV could be associated with the bridge of the Ag-O-P and P-O-P groups, and the peak of 533.1 eV could be assigned to P=O and C-O [39,40]. The XPS peak of P 2p is located at 133.7 eV (figure 3(e)), which is possibly due to P 5+ in PO 4 3− [41,42]. , which is associated with the C=C bonds in CDPVA [45]. The result further confirms the co-existence of the conjugated CDPVA in the investigated composites.  4(a)), and their color continuously darkens. With increasing temperature and time of heat treatment, the visible-light absorption intensity of the CDPVA /Ag 3 PO 4 composite photocatalysts is enhanced firstly and then weakens; their absorption intensity reaches the highest value with heat treatment temperature of 180°C and time of 1 h. Without sufficient temperature or adequate time, the thermal elimination reaction of PVA macromolecules is insufficient, resulting in lack of perfect conjugate structure. When the temperature is too high or the time is too long, side reactions, such as chain breaking, chain transfer, and peroxidation will occur, leading to decreased conjugated degree of CDPVA [31,32].

Optical and electrical properties
PL spectrum offers a method to evaluate the efficiency of separation for the photogenerated electron-hole pairs in semiconductors [46]. Figure 5(a) exhibits the PL spectra for pure Ag 3 PO 4 and CDPVA/Ag 3 PO 4 (1: 8000). Pure Ag 3 PO 4 presents higher PL emission than CDPVA/Ag 3 PO 4 (1: 8000), implying less combined pairs of the photogenerated electron-hole. The reduced recombination probability in CDPVA/Ag 3 PO 4 composite photocatalysts results from strong mobility of electron of spatially extended π-bonding conjugated system, thereby promoting the visible-light photocatalytic activity. EIS technique has extensive application in investigation on the efficiency of charge transfer and separated pairs of photogenerated electron-hole [47,48]. According to the EIS Nynquist plots, the FTO/CDPVA/Ag 3 PO 4 (1: 8000) electrode has a significantly smaller arc radius than FTO/Ag 3 PO 4 ( figure 5(b)), revealing a higher separation efficiency for CDPVA/Ag 3 PO 4 composite photocatalysts compared to that for pure Ag 3 PO 4 particles. The result agrees well with the PL spectrum analysis. Figure 6 shows MO photodegradation catalyzed by pure Ag 3 PO 4 and CDPVA/Ag 3 PO 4 composite photocatalysts prepared under different conditions with irradiated light. The deterioration percentage of MO with CDPVA/Ag 3 PO 4 composite photocatalysts is larger compared to that of pure Ag 3 PO 4 , indicating the trivial amount of CDPVA can enhance the visible-light photocatalytic activity of Ag 3 PO 4 photocatalysts. Hence, the optimal preparation conditions are 1:8000 mass ratios of PVA and Ag 3 PO 4 , treatment temperature of 180°C, and treatment time of 1 h. Figure 6(d) exhibits the correlation between photodegradation time and ln(c 0 /c) with pure Ag 3 PO 4 and CDPVA/Ag 3 PO 4 composite photocatalysts with different mass ratios. The plot shows obviously first-order kinetic reaction, as well as its kinetics is determined by:

Photocatalytic activity
where k denotes the rate constant for degradation; c corresponds to pollution concentration at instant t, and c 0 represents absorption equilibrium concentration of MO [49]. CDPVA/Ag 3 PO 4 (1: 8000) has a rate constant approximately 6.2 larger than pure Ag 3 PO 4 , in case of the same experiment conditions. The UV-vis absorption spectra of MO solution degraded by pure Ag 3 PO 4 and CDPVA/Ag 3 PO 4 (1: 8000) with visible-light were recorded, as presented in figures 7(a) and (b). With prolonged irradiation time, the maximum absorption of the solution with pure Ag 3 PO 4 decreases slightly, and that of the solution with CDPVA/Ag 3 PO 4 (1: 8000) rapidly decreases and even disappears when irradiated for more than 90 min. The efficiency of MO degraded by CDPVA/Ag 3 PO 4 composite photocatalysts is superior compared to that of pure Ag 3 PO 4 under the same experimental conditions. Figure S1 is available online at stacks.iop.org/MRX/6/125558/mmedia presents the High Performance Liquid Chromatography (HPLC) patterns of MO solution degraded by pure Ag 3 PO 4 (a) and CDPVA/Ag 3 PO 4 (1: 8000)(b). The characteristic peak of MO with a retention time of about 4.0 min decreases gradually, indicating that the MO content decreases with prolonged reaction. In the case of CDPVA/Ag 3 PO 4 composite photocatalysts, the intensity of the peak rapidly decreases, and then disappears, but no other peaks appear. Hence, both decolorization and mineralization of MO are also efficiently. Figure 8 offers the photocatalytic decrease of Cr(VI) (a) and its first-order kinetic curves (b). The Cr(VI) concentration with CDPVA/Ag 3 PO 4 (1: 8000) decreases faster than that of CDPVA and Ag 3 PO 4 , and the rate constant (k) of CDPVA/Ag 3 PO 4 (1: 8000) is twice that of pure Ag 3 PO 4 under the same experimental conditions.

Stability of the photocatalyst
It is of great importance that a photocatalyst possesses a high stability, particularly for its use and estimation. The recycling photocatalytic deterioration of MO over pure Ag 3 PO 4 and CDPVA/Ag 3 PO 4 (1: 8000) was performed to evaluate photocatalytic stability. As illustrated by figure 9, the photocatalytic activity of CDPVA/Ag 3 PO 4 (1: 8000) slightly decreases with increasing number of recycling runs, and that of pure Ag 3 PO 4 becomes evidently deteriorated. The efficiency of photocatalytic deterioration of MO over CDPVA/Ag 3 PO 4 (1: 8000) reaches 70% within 75 min after six recycling runs, indicating its photocatalytic stability.
The XRD patterns of the two photocatalysts were acquired after the recycling photocatalytic experiments, as illustrated by figure10. The XRD pattern of the recycled CDPVA/Ag 3 PO 4 (1: 8000) is similar to that of the original one. However, some new peaks attributed to Ag are found in the XRD pattern of pure Ag 3 PO 4 , and the Ag content is ca. 1.3%, indicating that pure Ag 3 PO 4 particles have slight photocorrosion during photocatalysis. We can infer from the two partial enlargements that CDPVA/Ag 3 PO 4 (1: 8000) composite photocatalysts are more stable than pure Ag 3 PO 4 .
The conductivity of an electrolyte solution offers an estimation of its concentration, thus representing the dissolution condition of the photocatalysts to a certain extent. Generally, electrolyte solution with high concentrations presents high conductivity. Mettler Toledo FE3 conductivity meter was used for measurement of  the conductivities of the suspensions , and the measured data are presented in figure S2. The conductivities of the suspensions containing CDPVA/Ag 3 PO 4 composite photocatalysts are much smaller compared to those of suspensions containing pure Ag 3 PO 4 , implies a lower solubility of the CDPVA/Ag 3 PO 4 composite photocatalysts compared to that of pure Ag 3 PO 4 because of the protective effect of CDPVA on the surface. This property makes great sense for large-scale application of Ag 3 PO 4 photocatalysts in practice.

Photocatalytic mechanism of CDPVA /Ag 3 PO 4 photocatalyst
Carrier trapping experiments were conducted to elucidate the governing mechanism of reaction. The influences of DMSO (dimethylsulfoxide , a typical e − scavenger) [50], EDTA ( ethylene diamine tetraacetic acid, a hole scavenger) [51], TBA (tert butyl alcohol, a hydroxyl radicals ·OH scavenger) [52,53] and p-BQ (p-benzoquinone, a typical ·O 2 − scavenger) [54] on MO photodegradation with CDPVA/Ag 3 PO 4 (1: 8000) were studied, and figure 11 provides the results. DMSO produces a small increase of the photodegradation rate for MO, implying a negligible effect of e − on MO degradation. Meanwhile, the addition of TBA slightly suppresses the photodegradation of MO, suggesting that ·OH is not the active species. EDTA obviously inhibits MO photodegradation, revealing that h + is the primary active species for MO degradation. In the presence of p-BQ, the photodegradation of MO was inhibited compared with no scavenger at the same conditions, indicating the main roles of ·O 2 − for MO degradation.
Calculation for The HOMO-LUMO gap of CDPVA is given by the equation [55]: where A stands for a constant parameter and n=1 for a direct transition and n = 4 for an indirect. The magnitudes of E g and n can be identified with the subsequent steps [43]: firstly, ln(αhν) is plotted against ln(hν − E g ) by using an means of an approximation of E g . And then n is determined based on the slope of the straightest line near the band edge. With such approach, n = 1 for CDPVA, and the band gap is ca. 1.87 eV, as determined through an extrapolation the linear line, as shown in figure 12(a). The HOMO potential of CDPVA  determined from the XPS valence band spectra ( figure 12(b)   electrons of CDPVA transfer to the CB of Ag 3 PO 4 according to the traditional dual-charge transfer mode, then the mounting electrons in the CB of Ag 3 PO 4 would decrease Ag + to Ag. The trace Ag might acts as a charge transmission bridge. Due to the CB potential of Ag 3 PO 4 is more negative than the Fermi level of metallic Ag, the photogenerated electrons in the CB of Ag 3 PO 4 shift to metallic Ag. Simultaneously, the holes in the VB of CDPVA move to metallic Ag and combine with the electrons [54,58]. The silver content is stable and does not increase because of combining of h + and e − . This type of charge transmission leads to accumulation of abundant electrons in the LUMO of CDPVA in reduction reaction and the participation of holes in the VB of Ag 3 PO 4 in oxidation reaction. The major reactions during MO photodegradation can be presented as follows.  decreased rate of recombined pairs for photogenerated electron-hole. The underlying photocatalytic mechanism could be Z scheme, as confirmed by the carrier trapping experiments and analysis of the Ag content in the photocatalyst after recycling experiments. The results could shed some light on the design of Z-scheme heterostructures with engineered band structure and provide significant understanding in solar visible-lightdriven catalysis.