Preparation and Characterization of Highly Efficient and Stable Visible-Light-Responsive Photocatalyst AgBr/Ag3PO4

AgBr/Ag 3 PO 4 photocatalyst was synthesized using a facile coprecipitation method. The photocatalyst was characterized by Xray powder diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), BrunauerEmmett-Teller (BET) surface areas, and photoluminescence (PL) technique. The activity of the photocatalyst was evaluated by the degradation of methyl orange (MO) and rhodamine B (RhB). The results showed that the prepared AgBr/Ag 3 PO 4 exhibited excellent performance and much higher photocatalytic activity than the single one under visible-light irradiation. The optimum mole ratio of Br/P in AgBr/Ag 3 PO 4 samples is 0.3.The prepared AgBr/Ag 3 PO 4 photocatalyst was transformed to Ag/AgBr/Ag 3 PO 4 system with excellent property and good stability in the photocatalytic process. The possible mechanisms of the enhanced photocatalytic activity for the AgBr/Ag 3 PO 4 were also discussed in detail.


Introduction
Photocatalysis is a promising technology for the treatment of contaminants, especially for the removal of organic pollutants with solar energy [1][2][3][4][5][6].To date, TiO 2 has undoubtedly been proven to be the most excellent photocatalyst for the decomposition of many organic compounds.However, TiO 2 photocatalyst has a wide bandgap (i.e., 3.2 eV for anatase and 3.0 eV for rutile) and hence absorbs only the UV light, which accounts for only 4% of the total sunlight, to generate charge carriers for promoting the surface redox reactions.Due to this inherent property of TiO 2 [7][8][9][10][11][12][13][14], its practical applications are rather limited.To effectively utilize the visible light that constitutes 43% of the total sunlight, it is important to find a photocatalyst that is active and efficient under visiblelight illumination [15].Consequently, much effort has been devoted to developing visible-light-driven photocatalysts in order to utilize solar energy and indoor light efficiently in current photocatalysis research field.
Silver halides (AgX, X = Cl, Br, I) are photosensitive materials extensively used as source materials in photographic films.When absorbing a photon, silver halide particle may generate an electron and a hole, so AgX can be used as a potential photocatalyst.But the photoinduced electrons will combine with interstitial Ag + ions to form a cluster of Ag 0 atoms within the silver halide particle, which results in the instability of AgX under light irradiation [16].Therefore, pure AgX is seldom used as a photocatalyst.Recently, Kakuta et al. [17] observed that Ag 0 species are formed on the surface of AgBr in the early stage of the reaction and AgBr in the AgBr/SiO 2 photocatalyst is not destroyed under successive UV illumination.The separation of the photoexcited electron-hole pairs may occur in the presence of Ag 0 species on the surface of the photocatalyst.It is considered that the silver nanoparticles formed on silver halide particles might be expected to be a stable photocatalyst under visible-light illumination.At the same time, Ag 0 species formed on the surface of the photocatalysts may produce the effect of surface plasmon resonance (SPR) [15,18].Since the results were reported by Kakuta et al. [17], to improve the photostability of AgX, numerous studies have attempted to form composite photocatalysts by loading AgX particles on different materials, such as SiO 2 [19], TiO 2 , Al 2 O 3 [20][21][22], Al-MCM-41 [23], Y-zeolite [24], Fe 3 O 4 [25], BiOI [26], H 2 WO 4 [27,28], and Bi 2 WO 6 [29,30].The results showed that the Ag 0 species on the surface of the catalyst indeed enhanced the interfacial charge transfer and the stability of AgX.This expectation leads us to prepare a new stable photocatalyst under visiblelight, which will greatly improve its current performance and open up new applications.
Recently, Ag 3 PO 4 has attracted considerable attention as a new visible-light photocatalyst, and it is a pale yellow semiconductor with a bandgap of 2.45 eV [16,[31][32][33].The results in the pieces of literature demonstrated that Ag 3 PO 4 was a potential photocatalyst in decomposing organics and exhibited outstanding oxygen evolution rate and excellent antibacterial activity under visible-light irradiation.It is known that the conduction band (CB) and valence band (VB) levels of AgBr (2.50 eV) [34] are 0.06 eV and 2.56 eV, respectively, and the CB and VB of Ag 3 PO 4 (2.45 eV) are 0.45 eV and 2.90 eV [16,31], respectively.It is clear that the CB and VB of Ag 3 PO 4 , respectively, lie below those of AgBr, which is favorable for the efficient separation of photoinduced electrons and holes.Furthermore, AgBr/Ag 3 PO 4 can be easily transformed into an Ag/AgBr/Ag 3 PO 4 system in the early stage of the photocatalytic reaction, in which Ag nanoparticles will play an important role.So, AgBr is considered to be an appropriate choice to improve the photocatalytic activity for the Ag 3 PO 4 photocatalyst.To the best of our knowledge, however, the study of the composite photocatalyst AgBr/Ag 3 PO 4 has rarely been reported.
In the study, a novel AgBr/Ag 3 PO 4 was constructed and synthesized by a facile coprecipitation method.Methyl orange (MO) and rhodamine B (RhB) were used as model pollutants to evaluate the photocatalytic activity of the AgBr/Ag 3 PO 4 composites under visible-light irradiation ( > 400 nm).The optimum mole ratio of Br/P in AgBr/Ag 3 PO 4 system and the stability of the photocatalyst were also investigated.More importantly, different scavengers were introduced to the photocatalytic reaction system to explore the roles of different reactive species, and the reaction mechanism was discussed in detail.

2.2.
Preparation of AgBr/Ag 3 PO 4 Photocatalyst.AgBr/Ag 3 PO 4 was prepared by the coprecipitation method in a dark room to facilitate the experimental manipulation and prevent the decomposition of AgBr.In a typical procedure, 1.42 g of Na 2 HPO 4 dispersed in 500 mL of deionized water was placed in a 1000 mL pyrex glass beaker.Then, 0.119 g of KBr in 100 mL of deionized water was added to the above suspension and stirred magnetically for 2 h.Subsequently, 5.3 g of AgNO 3 in 100 mL of deionized water was quickly added to the mixture.The resulted suspension was vigorously stirred for 12 h.The product was filtered and washed with absolute ethanol and deionized water for several times and dried at 60 ∘ C for 24 h.Finally, the obtained 0.10-AgBr/Ag 3 PO 4 with theoretical Br/P molar ratio of 0.10 : 1 was collected.Varying the amount of AgBr, different AgBr/Ag 3 PO 4 photocatalysts were prepared, respectively, and defined as BP-0.1,BP-0.3, BP-0.5, and BP-0.7.Pure Ag 3 PO 4 and AgBr samples were prepared using the same method but with only one kind of anion in the solution (PO 4 3− or Br − ).scanning voltages was employed to observe the morphologies of as-prepared catalysts.UV-Vis diffuse reflectance spectroscopy (DRS) measurements were carried out using a UV 3600 (SHIMADZU, Japan) UV-Vis-NIR spectrophotometer equipped with an integrating sphere attachment.The analysis range was from 200 to 700 nm, and BaSO 4 was used as a reflectance standard.Fluorescence emission spectra were recorded on a JASCO FP-6500 type fluorescence spectrophotometer with 315 nm excited source over a wavelength range of 350-600 nm.The Brunauer-Emmett-Teller (BET) surface areas were measured using a Micromeritics ASAP 2020 N 2physisorption method at 77 K.

Photocatalytic Activity Test.
The photocatalytic degradations of MO and RhB were adopted to evaluate the photocatalytic activity of the samples in a photoreaction apparatus [35].A 500 W Xe lamp (Institute of Electric Light Source, Beijing) was used as a light source with a 400 nm cut-off filter (Instrument Company of Nantong, China) to provide visiblelight irradiation.In each experiment, the reaction suspension containing 0.05 g photocatalyst and 25 mL dye solution was put in the 100 mL beaker, and a magnetic stirrer was used to stir the reaction solution.The initial concentrations of MO and RhB were 10 mg/L and 4.0 mg/L, respectively.And the initial pH value of solution is 7.0.The distance between the light source and the surface of the reaction solution is 11 cm.Prior to illumination, the suspension was magnetically stirred in the dark for 30 min to reach adsorption-desorption equilibrium of the dyes on catalyst surfaces.At the given time intervals, 5 mL of the suspension was collected, centrifuged, and filtered through a 0.2 m millipore filter to remove the catalyst particles.The concentration of catalystfree dye solution is determined spectrophotometrically.The photocatalytic activity of the samples is calculated from the following expression [35]: where  is the photocatalytic efficiency;  0 is the concentration of reactant before illumination (mg/L);  is the concentration of reactant after illumination time  (mg/L).3.1.4.DRS Analysis.UV-Vis diffuse reflectance spectroscopy was carried out to investigate the optical properties of the samples.Figure 5 shows UV-Vis diffuse reflectance spectra of different photocatalysts.From Figure 5(a), it can be seen that, compared with pure AgBr, the absorption wavelength range of the BP-0.3 photocatalyst is extended greatly towards visible light.In theory, because the absorption wavelength range is extended greatly towards visible light, the formation rate of electron-hole pairs on the photocatalyst surface also increases greatly, resulting in the photocatalyst exhibiting higher photocatalytic activity.The absorption wavelength range of the BP-0.3 photocatalyst is extended greatly towards visible light, which may be attributed to the formation of the heterojunction between AgBr and Ag 3 PO 4 .A similar result was also reported [37].

Results and Discussion
From Figure 5(a), it is clear that the absorption edge of the prepared Ag 3 PO 4 and AgBr samples is at about 506 nm and 496 nm in the spectrum, respectively.It is known that the bandgap energy of the photocatalysts can be calculated by the following equation [38,39]: In this equation, , ℎ, V, , and   are absorption coefficient, Planck constant, light frequency, proportionality, and bandgap energy, respectively;  is determined from the type of optical transition of a semiconductor ( = 1 for direct transition and  = 4 for indirect transition).The values of  for AgBr and Ag 3 PO 4 are 4 [40] and 1 [41], respectively.By applying this equation, the bandgap of AgBr and Ag 3 PO 4 is 2.50 eV and 2.45 eV, respectively, which agrees well with the previous reports [16,31,32,34].
The band edge positions of CB and VB of semiconductor can be determined with a simple approach.The valance band edge ( VB ) and conduction band edge ( CB ) of a semiconductor at the point of zero charge (pH ZPC ) can be predicted by the following equation [42]: where  is the absolute electronegativity of the semiconductor, expressed as the geometric mean of the absolute electronegativity of the constituent atoms, which is defined as the arithmetic mean of the atomic electron affinity and the first ionization energy.  is the energy of free electrons on the hydrogen scale (likely 4.5 eV), and   is the bandgap energy of the semiconductor.The  values for AgBr and Ag 3 PO 4 are 5.81 and 6.17 eV, and the  VB of AgBr and Ag 3 PO 4 are calculated to be 2.56 and 2.9 eV, respectively.Thus, the  CB of AgBr and Ag 3 PO 4 are estimated to be 0.06 and 0.45 eV, respectively.The UV-Vis diffuse reflectance spectra of the used photocatalyst (BP-0.3)are shown in Figure 5(b).It is clear that, compared with the fresh BP0.3 photocatalyst, there is much stronger absorption in the visible region, and with the increase in the recycling runs, the absorption wavelength range and the absorption intensity increase gradually.It may be resulted from the strong surface plasmon resonance (SPR) [43,44] effect of Ag nanometer particles (NPs) scattering on the surface of AgBr and Ag 3 PO 4 particles.The gradually enhanced absorption in the visible region from 1 to 5 recycling runs shows that the amount of Ag NPs increases slowly in the photocatalytic reaction.The increased content of metal Ag will affect the size of Ag, which further affects the separation of photoinduced carriers, the SPR effect of Ag NPs, and the corresponding photocatalytic activities [15].process of MO and RhB follows first-order kinetics equation.

Photocatalytic Activity of AgBr/Ag
According to the kinetics model, the rate constant k of the different Ag 3 PO 4 /AgBr samples and pure Ag 3 PO 4 is calculated and illustrated in Figure 8.The results demonstrate that the optimum sample is BP-0.3 with the maximal degradation rate constant of 0.1737 min −1 for RhB and 0.0456 min −1 for MO, which is about 2 and 3 times that of pure Ag 3 PO 4 , respectively.
The BET surface areas of Ag 3 PO 4 , BP-0.1, BP-0.3, BP-0.5, and BP-0.7 are 2.3, 2.6, 3.1, 3.7, and 3.7 m 2 /g, respectively.It is clear that the BET surface areas of the samples are increased gradually with the increase in the amount of AgBr.It is obvious that the increased photocatalytic activity is not in obvious correspondence with the BET surface area of the samples.Therefore, the enhanced photocatalytic activity of the samples can only be ascribed to the presence of AgBr.

Stability of the Photocatalyst. The catalyst's lifetime
is an important parameter of the photocatalytic reaction process, so it is essential to evaluate the stability of the catalyst for practical application.As shown in Figure 9(a), the repetition tests reveal that the photocatalytic degradation efficiency of MO for BP-0.3 sample decreases by 12.3% after 5-time cycle experiments, and after 2-time cycle experiments, the photocatalytic degradation efficiency of MO is obviously decreased, which indicates that AgBr/Ag 3 PO 4 has high stability under visible-light irradiation.The stability of AgBr/Ag 3 PO 4 was also studied through the degradation of RhB (Figure 9(b)).The result shows that the photocatalytic activity of AgBr/Ag 3 PO 4 is not obviously changed and only little decrease after 5-time cycles.It further confirms the good stability of the AgBr/Ag 3 PO 4 photocatalyst.From Figure 9, it is clear that, though AgBr/Ag 3 PO 4 is not very stable at the initial reaction process under visible-light irradiation, the formed Ag/AgBr/Ag 3 PO 4 system can effectively retain its activity due to the efficient transfer of photoexcited electrons by Ag nanoparticles [45].
The high photocatalytic activity and good stability are closely related to the efficient separation of photoexcited electron-hole pairs derived from the matching band potentials between AgBr and Ag 3 PO 4 , as well as the surface plasmon resonance of Ag nanoparticles formed on the surface of the photocatalyst during the photocatalytic reaction process.The presence of metal Ag can restrain the further decomposition of AgBr under visible-light irradiation conditions [46,47].Besides, Br 0 atoms produced by AgBr under visiblelight irradiation are the reactive radical species to degrade MO and RhB and then turn to Br − , because the holes in the VB of AgBr can oxidize Br − ions to Br 0 atoms.If the number of Br 0 atoms from AgBr reaches a certain amount, Br − ions to Br 0 atoms will balance on the surface of the photocatalyst.Consider Therefore, the obtained AgBr-Ag-Ag 3 PO 4 nanojunction photocatalyst is a stable and effective photocatalyst under the experimental conditions.

Proposed Photocatalytic Mechanism
3.3.1.Roles of Reactive Species.The photocatalytic mechanism was investigated for the excellent photocatalytic property of the prepared AgBr/Ag 3 PO 4 .It is generally accepted that the dyes and organic pollutants can be photodegraded via photocatalytic oxidation process.A large number of main reactive species including h + , • OH, and • O 2 − are involved in the photocatalytic oxidation process.Therefore, the effects of some scavengers on the degradation of RhB and MO were examined in an attempt to elucidate the reaction mechanism.As an • O 2 − scavenger, benzoquinone (BQ) was added to the reaction system.Isopropanol (IPA) was introduced as the scavenger of • OH, and ammonium oxalate (AO) was adopted to quench h + [40].As a consequence of quenching, photocatalytic oxidation reaction is partly suppressed, and  is lowered.The more  is reduced by scavengers, the more important the role the corresponding oxidizing species play in the photocatalytic oxidation reaction.The effects of a series of scavengers on the degradation efficiency of MO and RhB are shown in Figure 10.It can be seen from Figure 6 that the degradation efficiencies of the AgBr/Ag 3 PO 4 (BP0.3)photocatalyst for RhB and MO are 82.4% and 89.8%, respectively, before the quenchers are added.However, the photodegradation efficiencies of RhB and MO are reduced to 32.1% and 36.5%,respectively, after adding ammonium oxalate (AO).Adding benzoquinone (BQ), the photodegradation efficiencies for RhB and MO are decreased to 45.3% and 48.2%, respectively.However, there is almost no obvious change for the photodegradation of RhB and MO after mixing the isopropanol (IPA).According to the above experimental results, it can be clearly seen that • O 2 − and h + are the main reactive species in the photocatalytic oxidation process of RhB and MO, whereas the effect of • OH can be negligible in this process.
To further research whether • OH was formed on the surface of AgBr/Ag 3 PO 4 (BP-0.3)photocatalyst under visible light illumination, a photoluminescence (PL) technique with terephthalic acid as a probe molecule was carried out.The detailed experimental procedures have been reported in our earlier reports [48,49].The PL emission spectra excited at 315 nm from TA solution suspension with AgBr/Ag 3 PO 4 were measured every 10 min.The results are shown in Figure 11.It can be seen that no obvious PL signal at about 425 nm is observed, demonstrating that no • OH is formed in the photocatalytic oxidation process, which agrees well with the results of IPA quenching.In summary, the main reactive species involved in the degradation of MO   edge (−0.39 eV) under visible-light illumination with energy less than 2.95 eV [45,46,50].These electrons will further react with O 2 to generate reactive

3. 1 .
Characterization of AgBr/Ag 3 PO 4 Photocatalyst 3.1.1.XRD Analysis.In order to determine the crystal phase composition and the crystallite size of the photocatalyst, XRD study of the samples was carried out.Figure1shows the XRD patterns of different photocatalysts.The patterns show that Ag 3 PO 4 is a body-centered cubic structure (JCPDS no.06-0505) and AgBr is face-centered cubic crystal (JCPDS no.06-0438).The result is in accordance with the previous report[16].As shown in Figure1, it can be seen that when the amount of AgBr is 0.1, the diffraction peaks of AgBr can be found in XRD patterns.And with further increase of AgBr concentration, the intensity of diffraction peaks of AgBr increases remarkably, whereas the intensity of diffraction peaks of Ag 3 PO 4 decreases simultaneously.It demonstrates that the AgBr particles are evenly dispersed on the surface of Ag 3 PO 4 particles.The calculation from the Scherrer equation shows that the diameter of Ag 3 PO 4 in the composite photocatalyst is not obviously changed compared with that of pure Ag 3 PO 4 .The average crystallite size of Ag 3 PO 4 is about 30 nm according to the main peak (210) of Ag 3 PO 4 , and the calculated crystallite size of AgBr is about 53.5 nm according to the main peak (200) of AgBr.No other new crystal phases are found in the patterns.In addition, XRD analysis was also carried out for the used AgBr/Ag 3 PO 4 photocatalyst.The result is shown in Figure2.It is clear that a new diffraction peak at 38.07 ∘ (JCPDS no.04-0783) assigned to Ag was found in the used BP0.1 after one and five recycling runs compared with the fresh BP0.1.However, the diffraction peak at 38.07 ∘ is not obviously changed between the sample used one time and five times.It is suggested that the photocatalyst is stable in the experimental conditions.The color of the fresh photocatalyst is faint yellow, and the color of the used photocatalyst becomes a little darker.This demonstrates that the fresh AgBr/Ag 3 PO 4 photocatalyst has been changed into Ag/AgBr/Ag 3 PO 4 photocatalyst after photocatalytic reaction.3.1.2.XPS Analysis.To further confirm the existence of Ag in the used AgBr/Ag 3 PO 4 sample, the used BP-0.1 sample was examined by XPS.The results are shown in Figure3.

Figure 6 :
Figure 6: (a) The degradation process of MO with different photocatalysts.(b) The degradation process of RhB with different photocatalysts.

Figure 7 :− 1 )Figure 8 :
Figure 7: (a) The first-order kinetics of MO degradation with different photocatalysts.(b) The first-order kinetics of RhB degradation with different photocatalysts.
• O 2 − ( Θ (O 2 / • O 2 − ) = −0.33 eV versus NHE) that induced the degradation of MO and RhB.The holes in the VB of AgBr oxidize Br − ions to Br 0 atoms, which are the reactive radical species to degrade MO and RhB.In summary, MO and RhB were decomposed by Ag/AgBr/Ag 3 PO 4 systems under visible-light irradiation through • O 2 − , Br 0 , and direct h + oxidation pathway.Based on the analyses, the proposed schematic diagram of photoexcited electron-hole separation process is shown in Figure 12.
) displays the degradation of MO by the different photocatalysts.It is clear that the BP-0.3 photocatalyst exhibits the highest catalytic activity.The degradation efficiency of MO is 56.3%, 60.2%, 89.8%, 82.3%, and 78.2% for (or RhB) are h + and • O 2 − .3.3.2.Proposed Mechanism.Based on bandgap structure of the prepared AgBr/Ag 3 PO 4 and the effects of scavengers, a possible mechanism of AgBr/Ag 3 PO 4 photocatalyst for degradation dyes was proposed.The AgBr/Ag 3 PO 4 photocatalyst was transformed into a plasmonic Z-scheme mechanism of Ag 3 PO 4 /Ag/AgBr system during the photocatalytic oxidation process.In other words, the AgBr/Ag 3 PO 4 photocatalyst was firstly transformed into the Ag 3 PO 4 /Ag/AgBr photocatalyst under visible-light irradiation.And then, AgBr, Ag, and Ag 3 PO 4 can be simultaneously excited and produce photogenerated electrons and holes.The plasmon-induced electrons of Ag nanoparticles are injected into the CB of AgBr, while the holes remain on the Ag nanoparticles.As for Ag 3 PO 4 , the photogenerated electrons move to the Ag nanoparticles to recombine with the plasmon-induced holes produced by plasmonic absorption of Ag nanoparticles, while the holes in the VB of Ag 3 PO 4 may oxidize MO and RhB directly.Besides, it was reported that electrons in the CB of AgBr could probably be excited up to higher potential 3 PO 4 exhibited excellent performance for the degradation of MO and RhB and displayed a much higher photocatalytic activity than the single one under visible-light irradiation.The optimum mole ratio of Br/P in AgBr/Ag 3 PO 4 samples is 0.3.The AgBr/Ag 3 PO 4 photocatalyst was transformed to Ag/AgBr/Ag 3 PO 4 photocatalyst quickly, and the formed Ag/AgBr/Ag 3 PO 4 photocatalyst remained with high photocatalytic property and good stability in the photocatalytic process.The reason is attributed to the efficient separation of photoexcited electron-hole pairs of the photocatalyst.The degradation of MO and RhB for the AgBr/Ag 3 PO 4 photocatalyst is mainly via • O 2 − , Br 0 , and direct h + oxidation process.The AgBr/Ag 3 PO 4 photocatalyst may be a promising, efficient, and stable photocatalyst for new applications in environmental purification under visible-light irradiation.