g-C3N4 in degradation of sulfamethoxazole

Accepted Manuscript Z-scheme mechanism of photogenerated carriers for hybrid photocatalyst Ag3PO4/g-C3N4 in degradation of sulfamethoxazole Li Zhou, Wei Zhang, Ling Chen, Huiping Deng PII: DOI: Reference:

S0021-9797(16)30835-9 http://dx.doi.org/10.1016/j.jcis.2016.10.068 YJCIS 21701

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Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

10 August 2016 24 October 2016 25 October 2016

Please cite this article as: L. Zhou, W. Zhang, L. Chen, H. Deng, Z-scheme mechanism of photogenerated carriers for hybrid photocatalyst Ag3PO4/g-C3N4 in degradation of sulfamethoxazole, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis.2016.10.068

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Z-scheme mechanism of photogenerated carriers for hybrid photocatalyst Ag3PO4/g-C3N4 in degradation of sulfamethoxazole Li Zhoua,b, Wei Zhangb , Ling Chenb, Huiping Dengb*

ARTICLE INFO

ABSTRACT

Article history: Received

Composite or hybrid photocatalysts are gaininig increasing interests due to the unique and enhanced photocatalytic activity. In this study, Ag3PO 4/gC3N4 with different ratios of Ag3PO4 and g-C3N4 were synthesized using a facile in situ precipitation method. The photocatalysts were characterized by transmission electron microscopy (TEM), X-ray diffraction pattern (XRD), Fourier transform infrared spectra (FTIR), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) and ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS). The photocatalytic performance was evaluated by the degradation of sulfamethoxazole (SMX), a model antibiotic Keywords: compound, under visible light irradiation. It was found that the composite silver phosphate photocatalyst Ag3PO4/g-C3N4 with a mass ratio of Ag3PO4:g-C3N4 of 98:2 g-C3N4 exhibited a higher photocatalytic activity than Ag3PO4/g-C3N4 with the mass Ag ratio of 2:98 for the degradation of SMX. When Ag3PO4 was the primary part stability photocatalytic mechanism of Ag3PO4/g-C3N4 photocatalyst, the migration of photogenerated electronhole showed a Z-scheme mechanism with photongenerated holes on the valance band of Ag3PO4 to oxidize pollutants. The separation mechanism was investigated by the photoluminescence technique and the scavengering of reactive oxygen species. photocatalysts hold great promises, many issues still hamper the practical applications, including fast 1. Introduction recombination of photo-generated charge carriers and  In the past few years, photocatalysis through stability. Lately, photocatalysts hybridized with a semiconductors has extensively been studied and π-conjugated structure material has proven to be utilized in the degradation of the organic pollutants[1-4]. effective for enhancing visible light-responsive There has been a growing interest lately in the photocatalytic activity[1]. In comparison with the other development of composite or hybrid photocatalysts for π-conjugated structures, g-C3N4 is a soft polymer that water treatment under visible light irradiation[5]. can easily be decorated on the surface of Compared to single-phase photocatalysts, multiphotocatalysts[13]. In addition, g-C3N4 with a high component systems, such as TiO2/Bi2WO6[6], standard reduction potential (-1.15 eV vs. normal CuS/ZnO[7], Co3O4/Ag3PO4[8] and hydrogen electrode or NHE) makes up the insufficiency AgBr/Ag3PO4/TiO2[9], with heterostructures usually of Ag3PO4 with lower reduction potential (+0.45 eV vs. possess enhanced visible light photocatalytic activity. NHE) to produce more oxygen oxidation species (e.g., One of the possible reasons is that the heterojunction hydroxyl radicals). However, the photocatalytic between different semiconductors with matching energy performance of the Ag3PO4/g-C3N4 composite is not band structures can promote efficient separation of fully understood. Some studies reported the mass ratio photogenerated charge carriers and sometimes enhance of 75-98% Ag3PO4 in Ag3PO4/g-C3N4 achieved the solar light absorption in the visible region[10]. highest decomposition rates for organic dyes[14-18], Ag3PO4-based photocatalysts have attracted whereas some others reported 67-83.3% g-C3N4 in considerable attentions due to their excellent visibleAg3PO4/g-C3N4 exhibited the highest photocatalytic light-driven photocatalytic activity for degradation of activities in dye removal[19, 20]. Clearly, the organic pollutants[11, 12]. Although Ag3PO4-based mechanisms of hybridization for Ag3PO4/g-C3N4 remain elusive. One of the common mechanisms for hybrid a College of Environmental State Key Laboratory of Pollution structures is the direct band-band transfer[10, 15, 17-21], Control and Resource Reuse (Tongji University), Shanghai,China in which photogenerated electrons in the conduction b Key Laboratory of Yangtze River Water Environment, Ministry band (CB) of g-C3N4 transferred into the CB of Ag3PO4 of Education, Shanghai, China . and meanwhile the photogenerated holes in the valence * Corresponding author. Tel.: +86 18817365817; Email address: band (VB) of Ag3PO4 transferred into the VB of g-C3N4. [email protected], [email protected] In this system, it is the holes in the VB of g-C3N4 that 1

oxided pollutants and that electrons in the CB of Ag3PO4 that produced reactive oxygen specices (ROS) during photocatalytic processes[15]. The Z-scheme electron transfer is another common mechanism, where the photoexcited electrons in the CB of Ag3PO4 and photoexcited holes in the VB of g-C3N4 combine directly[21] or through Ag particles[16, 22-24] contacted with both Ag3PO4 and g-C3N4, leaving holes in the VB of Ag3PO4 and electrons in the CB of g-C3N4 to exhibit high oxidizing and ruducing power. Most of the previous studies have used dyes such as Rhodamine B (RhB)[19, 20]and Methyl Orange (MO)[15, 16] as the target pollutants to evaluate the photocatalytic activity of Ag3PO4/g-C3N4. However, dye compounds may not be suitable since the dye degradation is also affected through photosensitization that transfer excited electrons from dyes to photocatalysts, which could contribute to the dye discoloration. To avoid this issue, other emerging trace level water contaminants such as sulfamethoxazole (SMX) could be selected for photocatalytic evalution. SMX is a model antibiotic compound with wide pharmaceutical applications and environmental relevance[25]. More importantly, the light absorption at wavelengths above 320 nm by SMX is negligible, which makes it an ideal candidate to investigate photocatalytic properties of visible-light responsive photocatalysts[26]. In the present work, hybrid Ag3PO4/g-C3N4 photocatalysts with different mass ratios of g-C3N4 and Ag3PO4 were sythesized through an in situ precipitation method with rigorous material characterization. Photocatalytic activity of Ag3PO4/g-C3N4 was evaluated by monitoring the degradation of SMX under visible light irradiation (>400 nm). Besdies photodegradation, the adsorption of SMX on the catalysts and catalyst stability in aqueous suspension were also investigated. Possible photocatalytic degradation mechansims was proposed by investigating the influence of ROS scavengers on photocatalytic activities of Ag3PO4/gC3N4.

the addtion of 5 mL H 3PO4 (85%) that was dissolved in 25 mL ethanol with intensive stirring for 3 h to form a homegeneous suspension. The obtained precipitates were seperated by a centrifuge (10,000 rpm for 30 min) followed by washing with Milli-Q water and ethanol for several times and was freeze-dried under vacuum to obtain Ag3PO4/g-C3N4. Different mass ratios of the Ag3PO4 to g-C3N4 samples were obtained by changing the mass of g-C3N4, which were labeled as Ag3PO4/2.1%g-C3N4 (AGPCNA) and Ag3PO4/98%gC3N4 (AGPCNB), respectively. According to the previous studies, graphene and g-C3N4 have similar lamellar structure and large surface area[28]. For comparison with respect to the effects on photocatalytic degradation, the same procedure of AGPCNA synthesis was used to fabricate Ag3PO4/graphene (AGPG) with 2.1% graphene as detailed elsewhere[29]. 2.2 Catalyst Characterization The morphology was characterized by TEM (CM200FEG, Philips, the Netherlands). The approach to prepare the sample for TEM analysis is to disperse a very small quantity of sample in a small volume of ethanol by sonicating, then a small drop of colloidal suspension (usually about 5 μl) is pipetted onto a TEM grid and simply allowed to dry at room temprerature. The grid can then be directly observed in a TEM once the medium is evaporated. FTIR spectra were obtained using a Nicolet 5700 FTIR spectrometer (Thermo Electron Scientific Instrument Co. U.S.A) equipped with a pyroelectric deuterated triglycine sulfate (DTGS) detector. The FT-IR controlled by OMNIC sofeware and dataset was collected between 4000 and 650 cm-1 by coaddition of 32 scans at a resolution of 4 cm-1. equipped with a KBr beam splitter (KBr, FTIR grade) at room temperature. XRD patterns were recorded using a Bruker D8 Advance X-ray diffractrometer with CuKαradiation (Optik GmbH, Ettlingen, Germany). XPS experiments were carried out on a RBD upgraded PHI5000C ESCA system (Perkin Elmer) with Mg-Kα radiation (h=1253.6 eV). Ultraviolet-visible diffuse reflectance spectra (UV-vis DRS) were measured using a Shimadzu UV-2550 spectrometer with BaSO4 as a reference. BET was used to measure the specific surface area of prepared photocatalysts. BET adsorption and desorption isotherms were obtained using a surface area analyzer (Micromeritics TriStar 3000 series) with sample de-gassing at 150 ℃ under vacuum (0.2 mmHg) for 5 hour. Photoluminescence (PL) emission spectra were recorded on a HORIBA MAX-4 type fluorescence spectrophotometer.

2. Experimental 2.1 Catalyst preparation All chemicals used in this work were analytical grade without further purification. The polymeric gC3N4 was prepared by direct heating of sulfuric acid treated melamine at 550 oC in a muffle furnace for 4 h in an alumina crucible with a cover at a heating rate of 4 o C/min[27]. As-prepared light yellow g-C3N4 powder was then crushed into fine powder and dispersed into 50 mL N,N-dimethyformamide (DMF) with sonicating for 2 h. Then, 0.849 g AgNO3 was added into the above solution under vigorous stirring for 10 min, followed by

2.3 Photocatalytic degradation of SMX The photocatalytic experiments for SMX removal were conducted with a 300 W xenon lamp equipped 2

with a cut-off filter (λ>400 nm). The light intensity on the surface of the suspension was kept at 138.7 mW/cm2. The measurement of photocatalytic oxidation of SMX was performed in a 250 mL beaker (8 cm diameter) with a cooling system on the bottom that was placed on a magnetic stirrer to maintain the constant solution temperature of 25±2 oC. In a typical experiment, 5 mg of photocatalysts were suspended in the SMX solution (1 mg/L, 100 mL) by ultrasonication for 5 min. Before irradiation, the suspensions were magnetically stirred in the dark at 250 rpm for 30 min to achieve adsorptiondesorption equilibrium. Then the lamp was switched on to initiate the photocatalytic reaction which was kept for 60 min. During the experimental process, water samples were withdrawn at selected times and filtered with 0.45 μm syringe filters to remove suspended photocatalysts for analysis of SMX.

with phosphoric acid. Analysis was performed under isocratic conditions at a flow rate of 1 mL/min, with an injection volume of 10 μL and oven temperature of 25 o C. The wavelength of the UV detector was set at 270 nm. 3. Results and discussion 3.1 Catalyst characterization

(b)

(a)

2000 nm

2.4 Stability of photocatalysts

(c)

To test the stability and reusability of Ag3PO4/gC3N4 composites, cyclic experiments of SMX photodegradation were conducted. To reduce the effect of lost photocatalysts, the higher concentrations of photocatalysts and SMX (Ccatalyst=0.25 g/L, CSMX=5 mg/L) were used to conduct the cycling experiments. After each cycle, the photocatalyst was filtrated with 0.2 μm membrane (collected photocatalysts by filters were also recovered) and washed thoroughly with Milli-Q water. The fresh SMX solution (5 mg/L) was added to begin the next cyclic experiment. Three consecutive cycles were completed and each cycle lasted for 120 min consisting of 30 min adsorption and 90 min photocatalytic reaction.

g-C3N(d) 4

Fig. 1 TEM images of the (a) g-C3N4, (b) Ag3PO4, (c) AGPCNB and (d) AGPCNA.

Fig. 1a and 1b show the TEM images of the pure g-C3N4 and Ag3PO4 samples respectively. It is clearly seen that the morphology of g-C3N4 was smooth, thin and flat sheets and as-prepared Ag3PO4 particles have an irregular spherical morphology with about 300-800 nm. Fig. 1c and 1d show that Ag3PO4 particles were attached to the g-C3N4 sheets. As shown in Fig. 1c, the g-C3N4 was well overlaid on the surface of Ag3PO4 particles in the AGPCNB composite. However, as shown in Fig. 1d, the TEM image of AGPCNA displays that the sample consisted of isolated Ag3PO4 particle aggregates and little g-C3N4 sheets attached on the Ag3PO4 particles. Despite of the different content of gC3N4 and Ag3PO4, there is a direct contact between them in this bulk heterojunction, which may favor the charge transfer.

2.5 Effect of reactive species scavengers To deterimine the effect of ROS in the degradation process, iso-propanol (10 mM), benzoquinone (0.5 μM ， sodium azide (0.5 μM) and disodium ethylenediamintetraacetate (Na2-EDTA) (5mM) as scavengers were added respectively to the SMX solutions (1 mg/L) containing 0.05 g/L AGPCNA. The other experimental process was the same with photodegradation experimental process. 2.6 Analytical method The SMX concentration was quantified by HPLCUV supplied by Agilent. The HPLC column used was a Teknokroma C-18 Tracer Extrasil ODS2 (250 mm×4.6 mm). The mobile phase was a mixture of 40% acetonitrile and 60% water which was adjusted at pH=3

3

(a)

(b)

(c)

(d)

Fig. 2 (a) XRD patterns and (b) FTIR spectra of g-C3N4, Ag3PO4, AGPCNA and AGPCNB XPS peaks of (c) Ag 3d and (d) P 2p in AGPCNA

The XRD patterns of g-C3N4, Ag3PO4, AGPCNA and AGPCNB are presented in Fig. 2a. All the peaks of Ag3PO4 can be readily indexed to the cubic structure of Ag3PO4 (JCPDS No. 06-0505). No characteristic peak of g-C3N4 was observed in the patterns of AGPCNA, which might be ascribed to the low quantity of g-C3N4 in the composite and its relative weaker crystallinity. Pure g-C3N4 showed its characteristic peaks at 27.4o and 13.0o, which can be indexed to the (002) and (100) diffraction planes of the graphite-like carbon nitride (JCPDS No. 87-1526)[30]. There are only crystalline g-C3N4 peaks in the AGPCNB composite photocatalyst, and the peak intensity decreased slightly than pure g-C3N4 probably because of the presence of Ag3PO4. Fig. 2b shows the FTIR spectra of g-C3N4, Ag3PO4, AGPCNA and AGPCNB. Pure g-C3N4 and AGPCNB exhibited several strong characteristic peaks at 808 cm-1 and in the range of 1,200-1,700 cm-1, which may be

ascribed to the breathing mode of triazine unites and typical stretching vibration of CN heterocycles respectively[30]. In the spectra of Ag3PO4 and AGPCNA, the two peaks at 1,012 cm-1 (asymmetric stretching of P-O-P groups) and 855 cm-1 (symmetric stretching vibration of P-O-P rings) can be assigned to molecular vibrations of the phosphate (PO43-)[31, 32], another characteristic frequency of PO43-, phosphoryl (P=O) at 1,300-1,450 cm-1 was also found in both spectra of Ag3PO4 and AGPCNA. Fig. 2c shows the peaks of Ag3d in AGPCNA. The peaks centered at 368.05 and 374.08 eV corresponded to Ag 3d5/2 and Ag 3d3/2 of Ag+ in Ag3PO4[33], respectively. In Fig. 2d, the P 2p peak of AGPCNA appreared at 132.8 eV, suggesting that the phosphorus in the sample existed in pentavalent oxidation state (P5+)[34]. The XRD and XPS results both confirmed that no Ag0 was formed during the hybrid compound preparation.

4

(a)

(c)

(b)

Fig. 3 (a) UV-vis spectra and (b)(c)band gap energies of g-C3N4, Ag3PO4, AGPCNA, AGPCNB and AGP-G

The indirect band gap of AGPCNA is the most lowest among all the five semiconductors, indicating that the AGPCNA can enhance the photocatalytic activitiy by faciliating the electron transfer.

The light-absorbance of g-C3N4, Ag3PO4, AGPCNA, AGPCNB and AGP-G was shown in Fig. 3. The absorption edge of pure g-C3N4 is at about 450 nm, while pure Ag3PO4 has a broader absorption in the visible region with an absorption edge at about 550 nm. The light-absorbance in 350-700 nm range was remarkably higher for AGPCNA and AGPG than that for pure Ag3PO4. In the AGPCNB with loading of Ag3PO4 there is a small shift in the band edge position to a higher wavelength as compared with pure g-C3N4. The above results suggest that the heterostructured AGPCNA would result in the efficient utilization of visible light and exhibit high photocatalytic activities. Moreover, the bad gap (Eg) of the semiconductor can be caculated by the equation below[18, 21, 24, 35]: αhν=A(hν- Eg)n/2 where α, h, ν, A and Eg are absorption coefficient, Planck constant, light frequency, a constant and band gap energy, respectively. According to the Fig.3 (b) (c), while (αhν)1/2 or (αhν)2 versus hν, the indirect band gaps of AGP, AGPG and AGPCNA are 2.20, 1.93 and 1.76 eV and the direct band gaps of gC3N4 and AGPCNB are 2.93 and 2.90 eV, respectively. 5

3.2 Photocatalytic performance

AGPCNA photodecomposition was higher than both AGPG and pure Ag3PO4, which indicates that the adsorption capacity may not be the main factor for the photocatalytic performance. Fig. 4b shows the kinetic data for SMX photocatalytic degradation with five catalysts under visible light irradiation. The relationship between ln(C0e / CT ) ( C0e , the initial concentration of SMX after adsorption; CT, the concentration of SMX after irradiation) and irradiation time was linear, indicative of a first-order kinetics. The slopes of these lines are the apparent rate constants for the different photocatalysts. The obtained values of apparent rate constants were 0.003 min-1 for g-C3N4, 0.006 min-1 for AGPCNB, 0.042 min-1 for AGPG, 0.046 min-1 for Ag3PO4 and 0.063 min-1 for AGPCNA. AGPCNA with Ag3PO4 as the main composition exhibited the highest photocatalytic activity for the decomposition of SMX. Our result is opposite to the observation that the Ag3PO4/g-C3N4 with 0.5%-5% Ag3PO4 exhibited the highest photocatalytic activities and when Ag3PO4 was the main part of Ag3PO4/g-C3N4 (93%-99%), their photocatalytic activities were reduced slightly compared with pure Ag3PO4 for the degradation of dyes.[21]

(a)

(b)

3.3 Stability of photocatalysts

Fig. 4 (a) Adsorption abilities and photocatalytic activities of g-C3N4, Ag3PO4, AGPCNA, AGPCNB and AGP-G for the SMX removal under visible light (λ>400 nm) irradiation (CSMX=1 mg/L, Ccatalysts=0.05 g/L) (b) Plots of ln(C0e / CT ) versus irradiation time for SMX representing the fit using a pseudo-firstorder reaction rate (CSMX=1mg/L, Ccatalysts=0.05g/L)

Fig. 4a shows the decomposition of SMX without photocatalysts did not occur under visible light. After 30 min adsorption in the dark and 90 min photodecomposition under visible light, the g-C3N4 removed only 27.4% of SMX in the solution and AGPCNB showed higher adsorption ability and photocatalytic degradation rate of 42.1%. Ag3PO4dominated photocatalysts had substaintially increased the SMX degradation compared to those with g-C3N4dominated photocatalysts. AGPG showed the highest adsorption of SMX in the dark, leading to approximately 40% of decline in the SMX concentations within 30 min. The AGPG with graphene and AGPCNA with g-C3N4 both exhibited higher adsorption capacity than pure Ag3PO4, which agrees with the order of their specific surface areas (1.10 m2/g for Ag3PO 4 <2.02 m2/g for AGPCNA<2.17 m2/g AGPG). Howeve, rthe removal rate of SMX by

Fig. 5 Repeated adsorption and photocatalytic degradation of AGPCNA under visible light

6

photoinduced electron-hole[36]. As shown in Fig.7, it is obviously shown that the PL intensity follows the order of AGPCNA
Fig. 6 XRD patterns of AGPCNA before and after recycle use

To evaluate the photocatalytic stability of AGPCNA, the used catalyst was collected and reused in three successive SMX degradation experiments, respectively. As shown in Fig. 5, the adsorption capacity and photocatalytic activities of AGPCNA remained almost the same in the first two successive experimental runs (totals 4 hours) but decreased appreciably in the third run. The removal percentage after adsorption and photodecomposition decreased from almost 100% to approximate 60% at the end of third run. Except the loss of collected photocatalysts, the minor changes in XRD patterns (Fig. 6) of AGPCNA before and after use indicates that the structure or integrity of photocatalyst changed during photocatalytic processes, leading to the decrease of photocatalytic activities. Compared with the XRD pattern of original AGPCNA, the additional peaks corresponding to Ag (JCPDS No. 04-0783) appeared at 38.1° and 64.2° respectively in the XRD pattern of after used AGPCNA, which confirmed the formation of Ag during photocatalytic processes.

Fig. 7 PL emission spectra of the as-prepared samples at room temperature

Fig. 8 Photocatalytic activities of AGPCNA in the reactive species scavengering experiments under visible light

3.4 Mechanism analysis It is generally accepted that the high surface area, good light adsorption capability and high separation efficiency of electron-hole pairs are beneficial for the performance of photocatalysts. In this case of AGPCNA, the surface area was not the key factor for its photocatalystic activities. The UV-vis spectra indicates that the photoadsorption performance of AGPCNA has been increased than pure Ag3PO 4 because of the presence of g-C3N4. In addition to the improved light adsorption capability, the high separation efficiency of electron-hole pairs could also increase the photocatalytic activity of AGPCNA. Fig. 7 exhibits the PL emission spectra of Ag3PO4, g-C3N4, AGPCNA and AGPCNB. Generally, a higher/lower PL intensity demonstrates a higher/lower recombination rate of

Scavengering ROS was used to experimentally determine and elucidate the posible photocatalytic degradation pathways or mechanisms. To probe different oxidative species in the three photocatalytic systems, iso-propanol, benzoquinone, sodium azide and Na2-EDTA were added separately to the SMX solutions ([SMX]=1 mg/L, [catalysts]=0.05 g/L) as scavengers of hydroxyl radicals (·OH), superoxide radicals (O2·-), singlet oxygen (1O2) and hole (h+), respectively. The results in Fig. 8 show that the addition of iso-propanol, benzoquinone and sodium azide had negligible effects on the photodegradation rates of SMX. The results indicate that OH·, O2·- and 1O2 do not appear to be the main reactive oxygen species in the photocatalytic process. The addition of Na2-EDTA caused a rapid 7

termination of photocatalysis for AGPCNA photocatalyst. One possibility is that EDTA anion could deposit on the surface of the photocatalyst and block the reaction sites as a hole trap, which implies that the photogenerated holes could play a major role in the surface interactions and oxidation of SMX in the solution. Fig. 9 illustrates the schematics of possible charge separation, migration and degradation processes under visible light irradiation. The Ag nanoparticles act as the charge conductor in the visible-light-driven AGPCNA Z-scheme system. Under visible light irradiation, both Ag3PO4 and g-C3N4 are excited, and the photogenerated electrons and holes are in their conduction and valence band, respectively. The conduction band (CB) and valence band (VB) potentials of Ag3PO4 and g-C3N4 are +0.45, -1.15 V and +2.9, +1.6 V (as shown on the left Y axis of Fig. 9, marked in blue color for Ag3PO4 and in red color for g-C3N4 separately) versus a normal hydrogen electrode (NHE), respectively. The CB potential of Ag3PO4 is more negative than the Fermin level of metallic Ag[37], and the VB potential of g-C3N4 is more positive than the Fermi level of Ag[38]. According to the band potential and Fermi level, there are two photogenerated electron-hole transmission channels (internal and external) in the visible-lightdriven Z-scheme AGPCNA system. In the internal transmission channel, the electrons in the CB of Ag3PO4 shift to metallic Ag and the holes in the VB of g-C3N4 move to metallic Ag simultaneously. The transferred electrons and hole combine with each other and release

heat. Therefore, the internal charge transmission enhances the separation of photogererated electron-hole pairs in the AGPCNA system. In the external transmission channel, the VB potential of Ag3PO4 is more positive than the oxidation potential of SMX (0.9 V)[39, 40] and (·OH/OH-) (1.89V/NHE)[41], indicating that the photogenerated holes have a strong oxidative ability and they can oxide SMX to CO2 or other intermediates[42, 43]. The potential difference between VB of Ag3PO4 and ·OH/H2O (2.68 V/NHE) [44](ΔE=0.22 V) was small compared with the difference between the potential of VB of Ag3PO4 and the oxidation potential of SMX and ·OH/OH(ΔE1=2.0V, ΔE2=1.01V). The difference indicats that the holes would likely oxidize SMX preferentially instead of reacting with adsorbed H 2O to form ·OH. The solution in this scanvering experiment was at a neutral pH such that the ·OH produced from OH- oxdiation should be negligible. That is why the addition of isopropanol as ·OH scavenger had little effect of photocatalytic activity of AGPCNA. On the other hand, the CB potential of g-C3N4 is so negative to reduce Ag+ to Ag0 (+0.8V/NHE) and O2 in the solution to other oxygen species. The reduction potential of O2 to O2·-(0.33 V/NHE) and H2O2 (+0.695 V/NHE)[45] was positive than the potential of CB of g-C3N4, so the photoexcited electrons may be efficiently transferred to O2 to form O2· - and H2O2[46]. When O2·- contact with h+ on the surface ot catalysts, the electrons transfer occurred to form 1O2, which has been verified by scavengering experiments results.

Fig. 9 Schematic diagram of the charge separation and transfer in photodegardation of SMX over the AGPCNA catalysts under visible light

A facile chemical precipitation approach was employed to synthesize hybrid Ag3PO 4/g-C3N4 4. Conclusions 8

photocatalysts. AGPCNA with 97.9% Ag3PO4 as the main composition exhibited better photocatalytic activity than AGPCNB with 98% g-C3N4 as the main composition during the decomposition of SMX. The hybrid photocatalyst AGPCNA showed stable photocatalytic activities in at least two successive experimental runs (totals 4 hours). During the photocatalytic processes of AGPCNA, the in situ generated metallic Ag acted as the recombination center of photogenerated electrons from Ag3PO4 and holes from g-C3N4 keeping the active holes on the VB of Ag3PO4 for SMX oxidation. The transfer of photoexcited electrons and holes obeyed the direct Zscheme mechanism, which had been proved by PL technique and ROS scavengering experiment. This research highlights the distinguished roles of the core material, Ag3PO4, the supporting sheets, g-C3N4 and the metallic Ag in composite photocatalyst Ag3PO4/g-C3N4.

heterojunction photocatalyst, Appl. Catal. B-Environ., 181 (2016) 707715. [9] X. Wang, M. Utsumi, Y.N. Yang, D.W. Li, Y.X. Zhao, Z.Y. Zhang, C.P. Feng, N. Sugiura, J.J.Y. Cheng, Degradation of microcystin-LR by highly efficient AgBr/Ag3PO4/TiO2 heterojunction photocatalyst under simulated solar light irradiation, Appl. Surf. Sci., 325 (2015) 1-12. [10] N.N. Wang, Y. Zhou, C.H. Chen, L.Y. Cheng, H.M. Ding, A gC3N4 supported graphene oxide/Ag3PO4 composite with remarkably enhanced photocatalytic activity under visible light, Catal. Commun., 73 (2016) 74-79. [11] G.F. Huang, Z.L. Ma, W.Q. Huang, Y. Tian, C. Jiao, Z.M. Yang, Z. Wan, A.L. Pan, Ag3PO4 Semiconductor Photocatalyst: Possibilities and Challenges, J. Nanomater., (2013) 1-8. [12] X. Chen, Y. Dai, X. Wang, Methods and mechanism for improvement of photocatalytic activity and stability of Ag3PO4: A review, J. Alloy. Compd., 649 (2015) 910-932. [13] X. Bai, R. Zong, C. Li, D. Liu, Y. Liu, Y. Zhu, Enhancement of visible photocatalytic activity via Ag@C3N4 core–shell plasmonic composite, Appl. Catal. B:Environ., 147 (2014) 82-91.

Ackowledgements

[14] S. Kumar, T. Surendar, A. Baruah, V. Shanker, Synthesis of a novel and stable g-C3N4-Ag3PO4 hybrid nanocomposite photocatalyst and

This work was financially supported by Chinese Ministry of Construction Foundation for Major Special projects of Water Pollution Control and Management of Science & Technology (No. 2014ZX07405002D) and the Foundation of State Key Laboratory of Pollution Control and Resource Reuse (Tongji University), China (No. CRRY15003).

study of the photocatalytic activity under visible light irradiation, J. Mater. Chem. A, 1 (2013) 5333-5340. [15] Z. Xiu, H. Bo, Y. Wu, X. Hao, Graphite-like C3N4 modified Ag3PO4 nanoparticles with highly enhanced photocatalytic activities under visible light irradiation, Appl. Surf. Sci., 289 (2014) 394-399. [16] H. Katsumata, T. Sakai, T. Suzuki, S. Kaneco, Highly Efficient Photocatalytic Activity of g-C3N4/Ag3PO4 Hybrid Photocatalysts through Z-Scheme Photocatalytic Mechanism under Visible Light, Ind.

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10

Graphical abstract

Ag3PO4

g-C3N4

Ag3PO4/2.1%g-C3N4 When Ag3PO4 was the primary part of Ag3PO4/g-C3N4, the migration of photogenerated electron-hole showed a Zscheme mechanism.

11