Fabrication of robust M/Ag₃PO₄ (M = Pt, Pd, Au) Schottky-type heterostructures for improved visible-light photocatalysis

Abstract

M/Ag₃PO₄ (M = Pt, Pd, Au) Schottky-type heterostructures were successfully fabricated by a chemical deposition route using NaBH₄ as a reduction agent. The structure and optical properties of the as-synthesized samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectroscopy, and electrochemical techniques. The photocatalytic activity was evaluated by the decomposition of dyes (methyl orange, methylene blue, and rhodamine B) under visible light irradiation (λ > 420 nm). These noble metals as nanoparticles were highly dispersed on the surface of Ag₃PO₄ polyhedrons. The light absorption of Ag₃PO₄ in both the UV and visible regions was extensively increased upon noble metal deposition. The photocurrent response over M/Ag₃PO₄ electrodes was much higher than that of pure Ag₃PO₄ and followed the decreased order of Pt > Pd > Au. The metallic nanoparticles deposited on Ag₃PO₄ could promote the transfer of photo-generated electrons, which not only inhibited the recombination of electrons and holes effectively, but also suppressed the photocorrosion of Ag₃PO₄, leading to a significant increase in photocatalytic activity and stability. On the basis of radical-trapping experiments and the PL technique, h⁺ and ˙O₂⁻ were well-established to correspond to the quick photo-degradation of dyes and a possible mechanism was proposed.

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1. Introduction

The photocatalytic decontamination of wastewater containing organic pollutants in the presence of semiconductors has attracted increasing interest and been recognized as a “green” and efficient strategy because it can directly utilize sunlight as the energy source.¹ Unfortunately, the most widely studied semiconductor photocatalyst TiO₂ can only utilize a small fraction of solar energy and this restricts its practical application. To better utilize the visible light accounting for 43% of solar energy, two general strategies have been reported. The first one is modifying TiO₂ by conventional methods such as metals/nonmetals doping,^2–4 noble metal deposition,⁵ dye sensitization,⁶ semiconductor composite,⁷ and so forth. The other one is developing new photocatalysts such as In_1−xNi_xTaO₄ (x = 0–0.2),⁸ BiVO₄,⁹ SnIn₄S₈,¹⁰ In(OH)_yS_z,¹¹ g-C₃N₄,¹² Ag/AgX (X = Cl, Br, I),^13–15 ZnS–ZnO,¹⁶ etc. that can yield high reactivity under visible light irradiation.

Among these novel photocatalysts, silver orthophosphate (Ag₃PO₄) has been widely considered the most promising material in energy conversion and environmental remediation due to its high efficiency, i.e. it can achieve a quantum efficiency of up to 90% for O₂ evolution at wavelengths longer than 420 nm.¹⁷ However, this novel Ag₃PO₄ photocatalyst is still facing the same challenges encountered by most photocatalysts, such as limited photocatalytic efficiency associated with the fast recombination of photo-generated charge carriers (electrons and holes). Moreover, Ag₃PO₄ is also suffered from stability issue in practical applications because it is easily photocorroded and decomposed to weakly active Ag if no electron acceptor is supplied.^17,18 A variety of strategies, such as formation of semiconductor composites (AgX/Ag₃PO₄ (X = Cl, Br, I),^18,19 Ag₃PO₄/TiO₂,²⁰ Fe₃O₄/Ag₃PO₄,²¹ Ag₃PO₄/SnO₂,²² Ag₃PO₄/In(OH)₃,²³ g-C₃N₄/Ag₃PO₄,²⁴ Ag₃PO₄/Bi₂MoO₆ ²⁵) and surface modification (graphene oxide/Ag₃PO₄,²⁶ carbon quantum dots/Ag₃PO₄,²⁷ Ag₃PO₄–graphene,^28,29 Ag₃PO₄/multiwalled carbon nanotube³⁰) have been employed to reduce the recombination of charge carriers and improve the structural stability. Nevertheless, semiconductor composites require specific band positions of the dual semiconductors and surface modification highly depends on the surface-electric properties (such as electron-accepting and -transport) of the introduced components. On the other hand, many studies have shown that metallic silver (Ag⁰) nanoparticles formed on the surface of Ag₃PO₄ may act as electron acceptors to enhance the charge separation because the high Schottky barrier at the Ag/Ag₃PO₄ interface could induce efficient interfacial charge transfer.^31,32 In addition to Ag, some other noble metals (such as Pt, Au, Pd) have also been recognized as well-known electron acceptors to reduce the recombination rate of photoinduced carriers in noble metal/semiconductor composite system. However, the reported noble metal deposition effects on photocatalytic activity have not been always positive.³³ Thus, by rational deposition of noble metals on semiconductor to constitute metal/semiconductor heterostructures and reveal the unique role of noble metal in promoting the photocatalytic performance is highly desirable.

In the present contribution, we report robust M/Ag₃PO₄ (M = Pt, Pd, Au) Schottky-type heterostructures synthesized via a simple chemical deposition route using NaBH₄ as reduction agent. Various dyes including methyl orange (MO), methylene blue (MB), and rhodamine B (RhB) were chosen as model pollutants to evaluate the photocatalytic activity in aqueous solution under visible light irradiation. The effects of metal type, metal content, as well as the deposition method on the structure and photocatalytic performance of M/Ag₃PO₄ were systemically investigated. The main active species and the possible photocatalytic mechanism for dyes degradation over M/Ag₃PO₄ heterostructures were proposed on the basis of the experimental results.

2. Experimental section

2.1 Synthesis of Ag₃PO₄ photocatalyst

Ag₃PO₄ photocatalyst was prepared by a simple precipitation method. In a typical synthesis, NaH₂PO₄ aqueous solution (100 mL, 0.2 M) was added dropwise to the aqueous solution of AgNO₃ (100 mL, 0.6 M) under stirring, and the resultant golden yellow precipitates were collected by filtration, washed with distilled water repeatedly, and dried at 60 °C overnight. SnIn₄S₈ and N-TiO₂ as reference photocatalysts were prepared according to the previous literatures.^10,34

2.2 Chemical deposition synthesis of M/Ag₃PO₄ composites

In a typical synthesis, 0.5 g of as-prepared Ag₃PO₄ was dispersed in 10 mL of deionized water and sonicated for 5 min. Then a certain amount of noble metal precursor (H₂PtCl₆, K₂PdCl₆, and HAuCl₄) solution was added to the Ag₃PO₄ dispersed solution. After magnetically stirring for 5 min, the solution was subsequently reduced with a double amount of NaBH₄ solution (0.01 M) to produce M/Ag₃PO₄, which was then filtered, washed and dried.

2.3 Photo-reduction synthesis of M/Ag₃PO₄ composites

For comparison, Pt/Ag₃PO₄ composites with various contents were prepared by a photo-reduction route. Briefly, after H₂PtCl₆ solution was added into the Ag₃PO₄ suspension solution and magnetically stirring for 60 min, the resulting solution was irradiated with a 300 W halogen lamp (Philips Electronics) equipped with a composited cut-off filter (400 nm < λ < 800 nm) for 2 h to produce Pt/Ag₃PO₄, which was then filtered, washed and dried.

2.4 Sample characterizations

X-ray diffraction patterns (XRD) were collected on a Rigaku MinFlex II equipped with Cu Kα irradiation. Morphologies and microstructures of the samples were observed by field emission scanning electron microscope (FE-SEM) (Hitachi SU-8000) and high-resolution transmission electron microscope (HRTEM) (JEM-2010). X-ray photoelectron spectroscopy (XPS) analysis was conducted on an ESCALAB 250 photoelectron spectroscope (Thermo Fisher Scientific) at 3.0 × 10⁻¹⁰ mbar with monochromatic Al Kα radiation (E = 1486.2 eV), the binding energy was corrected with reference to the C 1s peak (284.6 eV) for each sample. A Varian Cary 500 Scan UV/vis system was used to obtain the optical absorption spectra of the samples over a range of 200–800 nm.

2.5 Evaluation of photocatalytic activity

Photocatalytic experiments were performed in an aqueous solution at ambient temperature. A 300 W halogen lamp (Philips Electronics) equipped with cutoff filters (400 nm < λ < 800 nm) was used as the visible light source. The system was cooled by a fan and circulating water to maintain at room temperature. Briefly, 80 mg of photocatalyst was suspended in 80 mL aqueous solution of dye (MO: 10 ppm; RhB: 10⁻⁵ M, and MB: 10⁻⁵ M). Prior to irradiation, the suspension was magnetically stirred in dark for 1 h to establish an adsorption–desorption equilibrium. A 3 mL aliquot was taken at several minutes intervals during the experiment and centrifuged to remove the powders. The residual concentration of dye was analyzed on a Perkin-Elmer UV WinLab Lambda 35 spectrophotometer. The degradation percentage is reported as C/C₀, where C₀ is the initial concentration of dye, and C represents the corresponding concentration at a certain time interval. The stability was tested as follows: after each dye degradation reaction, the suspension was filtered and the solids were washed with water and dried at 80 °C in air for 4 h. Then the regenerated product was employed to degrade a new dye aqueous solution for another test under the same visible light irradiation. The total organic carbon (TOC) measurement was performed on a Shimadzu TOC-4100 analyzer. The generation of hydroxyl radicals was investigated using the PL (F-4600 type) technique, in which a basic TA solution including 5 × 10⁻³ M terephthalic acid (TA) and 0.01 M NaOH was added to the reactor. The excitation wavelength was 312 nm.

2.6 Photoelectrochemical measurement

Photoelectrochemical measurements were conducted with an epsilon (BAS) electrochemical workstation. A 300 W Xe-arc lamp equipped with cutoff filters (420 nm ≤ λ ≤ 800 nm) was used as a visible light source. A standard three-electrode cell with a work electrode (as-prepared photocatalyst), a platinum wire as counter electrode, and a standard calomel electrode as reference electrode were used in the photoelectric studies. 0.1 M Na₂SO₄ was used as the electrolyte solution. All electrochemical potentials are reported vs. NHE.

3. Results and discussion

3.1 Characterization of M/Ag₃PO₄ (M = Pt, Pd, Au) composites

M/Ag₃PO₄ (M = Pt, Pd, Au) Schottky-type composites were synthesized by a chemical deposition route using NaBH₄ as reduction agent. XRD patterns for the as-synthesized composites are shown in Fig. 1. It can be clearly seen that the diffraction peaks of the neat and deposited Ag₃PO₄ samples are all readily indexed to the body-centered cubic structure of Ag₃PO₄ (JCPDS no. 06-0505). No characteristic diffraction peaks for noble metals are observed in the patterns due to the low amount (0.1 wt%) and the possible high dispersion of noble metal particles on Ag₃PO₄. Taking into consideration the used noble metal precursor and the solubility of Ag₃PO₄, it is expected that the reaction conditions may favor the formation of secondary AgCl phase just like the reported work for ZnAg₃SbO₄.³⁵ However, in the present XRD patterns, no diffraction peaks assigned to AgCl are found. This might be due to the fact that the insoluble noble metal particles deposited on Ag₃PO₄ surface can effectively protect Ag₃PO₄ from dissolution in aqueous solution. Careful analysis of these XRD patterns indicates that all samples display a good crystallinity and noble metal deposition has negligible effect on the crystallinity of the composites.

Fig. 1 XRD patterns of as-synthesized M/Ag₃PO₄.

As photo-reduction is an alternative route to prepare most noble metal/semiconductor photocatalysts and exclusively for those Ag-based photosensitive materials, such as Ag/AgX (X = Cl, Br, I),^13–15 Ag/Ag₃PO₄,^17,31,32 Ag/Ag₂O,³⁶ and etc., in the present study, we also synthesized Pt/Ag₃PO₄ by photo-reduction route. For comparison, the pristine Ag₃PO₄ and the Pt/Ag₃PO₄ synthesized by chemical deposition route were also irradiated under the same visible light. As shown in Fig. 2a, when pure Ag₃PO₄ was exposed to visible light irradiation, the diffraction peaks corresponding to metallic Ag are observed obviously and their intensities are even much stronger than that of Ag₃PO₄, suggesting the partial decomposition of Ag₃PO₄ to Ag. A recent work by Wang et al. has found that when Ag₃PO₄ was used as a photocatalyst without the use of AgNO₃ as sacrificial reagent, it would completely convert into weakly active Ag under successive light irradiation.³⁷ From this viewpoint, it seems that a highly stable and efficient Ag/Ag₃PO₄ photocatalyst like reported Ag/AgX (X = Cl, Br, I) can not be achieved via a photo-reduction route. Interestingly, the introduction of H₂PtCl₆ into the Ag₃PO₄ system could inhibit the decomposition of Ag₃PO₄ to some extent. As seen from Fig. 2b, the diffraction peaks of Ag remarkably decreased as compared to the directly exposed Ag₃PO₄ in Fig. 2a. However, this inhibition to photo-reduction is much weaker than that in chemical deposition. From Fig. 1c, it is demonstrated that when chemically deposited Pt/Ag₃PO₄ composite was suffered from the same light irradiation, no typical diffraction peaks corresponding to metallic Ag were observed. This might be due to the efficient transfer of photogenerated electrons in Ag₃PO₄ to noble metal Pt at their interfaces, which greatly avoided the reaction between silver ions and photogenerated electrons.

Fig. 2 XRD patterns of (a) Ag₃PO₄ after visible light irradiation, (b) Pt/Ag₃PO₄ synthesized by photo-reduction route, and (c) Pt/Ag₃PO₄ synthesized by chemical deposition route after visible light irradiation. The content of noble metal is 0.1 wt%.

The morphology of the M/Ag₃PO₄ Schottky-type samples synthesized by the chemical deposition route is presented in Fig. 3. Fig. 3a shows a typical SEM image of the neat Ag₃PO₄ sample. The products consist of irregular polyhedral crystals with average size of ca. 1.5–5 μm. It is clearly seen that the surface of these Ag₃PO₄ crystals is quite clean, further indicating that the energy beam of SEM in the present work is much low and would not decompose Ag₃PO₄ to small Ag nanoparticles. After noble metal deposition, as shown in Fig. 3b–d, the obtained M/Ag₃PO₄ composites inherit the morphology and crystal size of the initiated polyhedral Ag₃PO₄ crystals. However, when we have a careful observation on the crystal surface, it can be seen that some small particles with size of several ten nanometers are dispersed on the surface of Ag₃PO₄ crystals (inset in Fig. 3b). As compared to pure Ag₃PO₄ crystals, these small nanoparticles should be the reduced Pt, Pd, and Au nanoparticles. To investigate the microstructure of these smaller nanoparticles, High-resolution TEM (HRTEM) images of M/Ag₃PO₄ samples are presented in Fig. 4. The results show the characteristic lattice fingers of 0.214 nm for Ag₃PO₄ and 0.227, 0.224, and 0.235 nm for Pt, Pd, and Au nanoparticles, which can be indexed as the (111) plane of face-centered cubic (fcc) structures of noble metal (Pt, Pd, and Au) nanoparticles, respectively. It is also observed that the smaller Pt, Pd, and Au nanoparticles are distributed on the surface of Ag₃PO₄ particles with an intimate interfacial contact.

Fig. 3 SEM images of (a) Ag₃PO₄ (b) Pt/Ag₃PO₄, (c) Pd/Ag₃PO₄, and (d) Au/Ag₃PO₄ synthesized by chemical deposition route. The content of noble metal is 0.1 wt%.

Fig. 4 HRTEM images of (a) Pt/Ag₃PO₄, (b) Pd/Ag₃PO₄, and (c) Au/Ag₃PO₄ synthesized by chemical deposition route. The content of noble metal is 0.1 wt%.

The elemental composition and chemical states of the M/Ag₃PO₄ Schottky-type composites were further analyzed by XPS. The full spectra of the M/Ag₃PO₄ composites and the pure Ag₃PO₄ reference are given in Fig. S1 (ESI†). The spectra demonstrate the predominant presence of Pt, Pd, and Au elements in addition to Ag, O, and P. Both Ag₃PO₄ and M/Ag₃PO₄ composites display a peak located at ca. 132 eV, which can be assigned to the characteristic P 2p.²⁴ Fig. 4a shows the high resolution Ag 3d spectra of M/Ag₃PO₄ and Ag₃PO₄, the profile of which exhibits two peaks centered at 373.65 and 367.65 eV. According to the previous reports,^31,38 these two peaks can be attributed to characteristic Ag⁺ in Ag₃PO₄. The same binding energy of the Ag₃PO₄ and M/Ag₃PO₄ composites suggests that noble metal deposition has no effect on the surface structure of Ag₃PO₄ or no formation of the Ag–Pt (Pd, Au) alloy.³⁹ To further investigate the chemical states of noble metals, the fine XPS spectra of Pt 4f, Pd 3d, and Au 4f are shown in Fig. 5b–d. It is clear that their binding energies are respectively located at 74.2 and 70.9 eV, 340.8 and 335.5 eV, 88.1 and 84.3 eV, which can be mainly attributed to the metallic Pt, Pd, and Au.^40–42 The XPS results combined with the HRTEM results confirm that the noble metals in this study are all in metallic states, suggesting that the chemical deposition route is a better strategy to fabricate M/Ag₃PO₄ heterostructures as compared to photo-reduction route.

Fig. 5 XPS spectra of M/Ag₃PO₄ and pure Ag₃PO₄. (a) Ag 3d, (b) Pt 4f, (c) Pd 3d and (d) Au 4f.

3.2 UV-vis absorption spectra

The light absorption properties of pure Ag₃PO₄ and M/Ag₃PO₄ Schottky-type heterostructures were analyzed by UV-vis DRS and the results are illustrated in Fig. 6. The absorption spectrum of pure Ag₃PO₄ indicates that it can absorb solar energy with a wavelength shorter than ∼530 nm, corresponding to 2.5 eV of band gap energy, which coincides with other work.¹⁷ In contrast to Ag₃PO₄, M/Ag₃PO₄ samples exhibit marked absorption enhancement in both the UV and visible light region, which can be attributed to the surface plasmon resonance (SPR) of noble metal nanoparticles formed on the surface of Ag₃PO₄. This enhancement in light absorption for M/Ag₃PO₄ may also support the successful deposition of noble metal nanoparticles on the surface of Ag₃PO₄.

Fig. 6 UV-vis diffuse reflectance spectra of Ag₃PO₄ and M/Ag₃PO₄.

3.3 Photocatalytic performance

The photocatalytic activity of the obtained M/Ag₃PO₄ Schottky-type composites was evaluated in terms of the degradation of various organic pollutants (such as MO, MB and RhB) under visible light irradiation (λ > 420 nm). For comparison, the degradation of pollutants over visible-light photocatalyst N-TiO₂ and ternary SnIn₄S₈ was also performed under the same conditions. The degradation curves of anionic dye MO over different photocatalysts as a function of visible-light irradiation time are firstly plotted in Fig. 7a. The blank experiment without any photocatalyst shows that the self-photolysis of MO under visible light irradiation could be neglected. The thermal effect could also be excluded by cooling the reaction system to room temperature. From Fig. 7a, it could be observed that 85% of MO was decomposed within 15 min of irradiation over the pure Ag₃PO₄. After noble metal deposition, the photocatalytic activity of the M/Ag₃PO₄ heterostructures was greatly enhanced. Among all the samples, the Pt/Ag₃PO₄ heterostructures exhibited the highest photocatalytic activity toward the degradation of MO. Within 8 min of light irradiation, MO was degraded completely, which is far more than the negligible activity over N-doped TiO₂ and SnIn₄S₈ photocatalysts. The ability of noble metals to promote activity of Ag₃PO₄ follows the order of Pt > Pd > Au. We can further describe and compare the photocatalytic activity of these photocatalysts by defining rate constant as the photocatalytic ability. In many previous results, the degradation of dyes can be ascribed to follow a pseudo-first-order kinetics model, ln(C_t/C₀) = −kt, where C₀ and C_t are the initial concentration of the dye solution and the concentration at time t, respectively, and k is the kinetic constant. As displayed in Fig. 7b, the Pt/Ag₃PO₄ heterostructures have much higher rate constant (ca. 1.45965 min⁻¹) than Pd/Ag₃PO₄ (ca. 0.21653 min⁻¹) and Au/Ag₃PO₄ (ca. 0.15914 min⁻¹) and is approximately 16.5 times higher than that of the pure Ag₃PO₄ (ca. 0.08838 min⁻¹). In addition to noble metal types, the metal content also influenced the photocatalytic activity of M/Ag₃PO₄. Fig. 7c shows the rate of MO degradation as a function of Pt content. It is clearly seen that the rate of MO degradation quickly increased to 1.03245 min⁻¹ when 0.05 wt% of Pt was loaded on the Ag₃PO₄ surface. The optimal Pt/Ag₃PO₄ mass ratio for photocatalytic degradation of MO was about 0.1%. Further increasing Pt resulted in the decrease of photocatalytic activity; however, this activity is still higher than that over Ag₃PO₄. As Pt/Ag₃PO₄ could be prepared by both chemical deposition and photo-reduction routes, we also compared their photocatalytic activity. Both photocatalysts with the same Pt content exhibited much higher photocatalytic activity than bare Ag₃PO₄ (Fig. S2†), but the sample prepared by chemical deposition is superior to the counterpart synthesized by photo-reduction route. The lower activity for the later might be due to the appearance of larger Ag clusters on the Ag₃PO₄ surface greatly shuttering the incident light and decreasing the interfacial charge transfer between metal and semiconductor.³⁶

Fig. 7 (a) Photocatalytic degradation curves of MO and (b) the kinetics over different photocatalysts, (c) comparison of the apparent reaction rate constant of Pt/Ag₃PO₄ with different Pt content in the photocatalytic degradation of MO, and (d) photocatalytic degradation curves and TOC (insert) of MB, RhB and MO over Pt/Ag₃PO₄ photocatalyst.

In view of the promising photocatalytic performance of M/Ag₃PO₄ toward degradation of anionic dye (MO) in aqueous solution, we extended the photocatalysts for decomposing cationic dyes (MB, RhB). Fig. 7d displays the evolution of the degradation of MB, RhB and MO over Pt/Ag₃PO₄ under identical light irradiation. As can be seen, both MB and RhB could be completely decomposed by Pt/Ag₃PO₄ within 1 min of light irradiation. From the degradation process, the higher degradation rate is achieved for the cationic dye as compared to the anionic dye. This discrepancy in activity may be related to the structure of dyes which has been explained in our previous report.¹⁰ Moreover, it has been proved that there are plenty of hydroxyl groups on the Ag₃PO₄ surface, suggesting that Ag₃PO₄ inherits negative surface charge.⁴³ Thus the cationic dye could be easily adsorbed onto the catalyst surface by the electrostatic field force, and charge transfer is facilitated. To further investigate the performance of M/Ag₃PO₄ heterostructures, the TOC experiments, which reflect the general concentration of organics in solution, were used to evaluate the ability of photocatalysts for mineralization of dyes. As shown in inset of Fig. 7d, the mineralization yield of MO over Pt/Ag₃PO₄ could reach about 80%, while that of RhB and MB is nearly 100% after the same period of irradiation.

In addition to photocatalytic activity, stability is another important parameter determining the practical application of photocatalysts, especially for those Ag-based photosensitive materials. Therefore, we investigated the repeated degradation of MO over Ag₃PO₄ and Pt/Ag₃PO₄ under identical conditions. It is clear from Fig. 8(a and b) that Pt/Ag₃PO₄ heterostructures exhibited much more stable photocatalytic activity than bare Ag₃PO₄. During the repeated six experiments, the degradation curve of MO was nearly overlapping for each run over Pt/Ag₃PO₄ while the bare Ag₃PO₄ displayed an obvious deactivation. This is consistent with the recent work by Wang et al.,³⁷ who found that the photocatalytic activity of Ag₃PO₄ deteriorated and almost deactivated during the consecutive cycles. They attributed the lost activity of Ag₃PO₄ to its gradual decomposition to weakly active Ag nanoparticles. In the present work, we also performed the XRD and XPS experiments to investigate the structural change of photocatalysts before and after photocatalytic reaction. As shown in Fig. 8c, the XRD patterns of the fresh and used Pt/Ag₃PO₄ were almost the same while obvious diffraction peaks corresponding to metallic Ag appeared in the used Ag₃PO₄ sample, indicating that the crystal structure of Pt/Ag₃PO₄ had no obvious change. In addition, from the detailed XPS spectra (Fig. 8d–f), the binding energies of Pt 4f and Ag 3d of the used Pt/Ag₃PO₄ had also no peak shift compared to those of the fresh sample, inferring that the chemical states of Pt and Ag elements in Pt/Ag₃PO₄ did not change during the reaction process. In contrast, the Ag 3d peaks of used Ag₃PO₄ displayed positive shift with respect to fresh sample due to the formation of metallic Ag.³¹ Therefore, it can be concluded that deposition noble metal onto Ag₃PO₄ surface via chemical deposition route can not only enhance the visible light photocatalytic performance of Ag₃PO₄, but also give rise to activity durability upon effectively inhibiting the photodecomposition of Ag₃PO₄. From XRD and XPS spectra, the emergence of metallic Ag in the used Ag₃PO₄ confirmed the formation of Ag/Ag₃PO₄ heterostructures. Although Ag nanoparticles deposited on Ag₃PO₄ could act as electron trapping centers to prevent photogenerated electron–hole pairs from recombination, the activity of Ag/Ag₃PO₄ heterostructures was highly dependent on the Ag content.⁴⁴ Moreover, as compared to M/Ag₃PO₄ (M = Pt, Pd, Au) and pure Ag₃PO₄, the Ag/Ag₃PO₄ heterostructures showed much lower photocatalytic activity (Fig. 7a and 8a), which might be due to the formation of Ag adlayers on the surface of Ag₃PO₄ shielding its light absorption.^37,44

Fig. 8 The recycled photocatalytic degradation curves of MO (a and b), XRD patterns (c) and XPS spectra of Ag₃PO₄ and Pt/Ag₃PO₄ before and after photocatalytic reaction (c–f).

All of the above experimental results demonstrate that noble metals played key roles in improving the photocatalytic activity and stability of Ag₃PO₄ photocatalyst. Considering the fact that the noble metals usually act as electrons capture to effectively promote the separation ratio of photoinduced electrons and holes in metal/semiconductor composites, we have investigated the performance of the rate and transfer of photogenerated charge carriers of Ag₃PO₄ and M/Ag₃PO₄ samples by photoelectrochemical analysis. As mirrored in Fig. 9, an obvious transient photocurrent response was observed in Ag₃PO₄ electrode in the initial stage, however, this photocurrent signal gradually decreased with the prolonged irradiation time, indirectly proving the structural instability of Ag₃PO₄. In contrast, the generated photocurrent responses over all noble metal/Ag₃PO₄ electrodes were much higher and reproducible, suggesting that the more efficient separation and longer lifetime of photoexcited electron–hole pairs of M/Ag₃PO₄ than those of Ag₃PO₄. Noted that the order of photocurrent intensity over noble metal/Ag₃PO₄ electrodes is Pt > Pd > Au, which is well consistent with the activity order. This result proves that noble metal can efficiently scavenge the excited-state electrons and thus promote charge separation, which causes the highly efficient degradation of organic pollutants.

Fig. 9 Photocurrent observed from Ag₃PO₄ and M/Ag₃PO₄ (the content of noble metal is 0.1 wt%) electrodes in an electrolyte solution under visible light.

3.4 Photocatalytic mechanism

To get insight into the activity and stability enhancement of noble metal deposited Ag₃PO₄ and further to reveal the photocatalytic mechanism, radical and hole trapping experiments were performed to detect the main oxidative species during the photocatalytic process. The photogenerated electron–hole pairs in the photocatalytic process were detected through trapping experiments of radicals and holes by using tert-butyl alcohol (TBA, ˙OH scavenger), ammonium oxalate (AO, h⁺ scavenger), and benzoquinone (BQ, ˙O₂⁻ scavenger), respectively. As shown in Fig. 10a, compared with the addition of TBA and BQ, the photocatalytic activity for MO degradation in the pure Ag₃PO₄ system was greatly suppressed when AO was introduced. This indicates that holes were the main active oxidizing species involved in the photoreaction process. From Fig. 10b, it is observed that holes were also the main active species in the Pt/Ag₃PO₄ system as similar activity inhibition for MO degradation was observed with the addition of AO. Moreover, ˙O₂⁻ may also serve as active species in this reaction as MO degradation was slightly inhibited by the addition of BQ. Since the conduction band (CB) edge potential of Ag₃PO₄ (0.45 V vs. NHE) is more positive than E(O₂/O₂˙⁻) (−0.33 V), which means that the photogenerated electrons in the CB of Ag₃PO₄ cannot reduce O₂ to yield ˙O₂⁻. Thus, the formed ˙O₂⁻ upon noble metal deposition may also serve as an indirect evidence to prove the efficient transfer of photoinduced electrons from Ag₃PO₄ to noble metal, similar to the observed results in Ag₃PO₄–graphene composites.²⁸ The PL technology with TA as a probe molecule also proves that no ˙OH species were generated in Pt/Ag₃PO₄ composites (Fig. S3†). On the basis of the results, it is concluded that the direct hole transfer mainly governs the photocatalytic process in both Ag₃PO₄ and Pt/Ag₃PO₄ system while electrons could react with adsorbed O₂ to form ˙O₂⁻ after deposition of noble metal onto Ag₃PO₄.

Fig. 10 The plots of photogenerated carrier trapping in the system of photo-degradation of MO by (a) Ag₃PO₄ and (b) Pt/Ag₃PO₄ under visible light irradiation.

On the basis of the above experimental results and previously reported work,^13,17,31,45 a schematic diagram of the band structure and the charge transfer at the interface of M/Ag₃PO₄ Schottky-type heterostructures is proposed in Scheme 1. In principle, when a semiconductor is contact with a metal, a Schottky barrier would be formed at their interface due to the difference in work functions.⁴⁶ According to the equation E(eV) = −4.5 − E_NHE(V), where E is the work function potential of the conduction band (CB) and valence band (VB) versus a vacuum, and E_NHE is the CB and VB potential versus a normal hydrogen electrode (NHE), the work function potential of the CB and VB for Ag₃PO₄ can be calculated to be about −4.95 and −7.40 eV, respectively.⁴⁷ Because Ag₃PO₄ is an n-type semiconductor, its Fermi level should be close to the CB edge (4.95 eV), and thus Ag₃PO₄ possess the lower work function than that of Au (5.1 eV), Pd (5.12 eV), and especially Pt (5.65 eV).⁴⁸ When Ag₃PO₄ contacts noble metal to form heterostructures, the electrons flow from Ag₃PO₄ to noble metal until the two systems attain equilibrium and form the new Fermi level. At the same time, the bands of Ag₃PO₄ bend upward toward the surface and an internal electric field directed from Ag₃PO₄ to noble metal is established. Therefore, the interface of noble metal and Ag₃PO₄ can form a Schottky barrier. When noble metal/Ag₃PO₄ heterostructures were exposed to visible light, electrons in the VB of Ag₃PO₄ could be excited up to the CB and directionally flew into noble metal across the interface of noble metal/Ag₃PO₄, leaving the holes on the Ag₃PO₄ VB. Additionally, the internal electric field within the heterostructures interface further accelerated the transfer of electron–hole pairs and prolonged the lifetime of the excited electrons and holes during the transfer process, as verified by photocurrent studies. In this way, the recombination rate of photogenerated electrons and holes over M/Ag₃PO₄ was effectively suppressed. Meanwhile, these well-separated electrons on noble metal nanoparticles were trapped by the available surface oxygen molecules to initiate the yield of ˙O₂⁻ reactive species to decompose the organic pollutant while the holes left on the VB of Ag₃PO₄ induced some oxidation process directly (Scheme 2). As discussed above, it can be concluded that the enhancement of the photocatalytic activity and stability of M/Ag₃PO₄ heterostructures should be mainly attributed to the successful and fast transfer of photogenerated electrons from Ag₃PO₄ to noble metals. Moreover, the insoluble noble metal layer on the surface of Ag₃PO₄ can also effectively protect Ag₃PO₄ from dissolution in aqueous solution, thus the structural stability of M/Ag₃PO₄ heterostructures can be greatly enhanced during the photocatalytic process.

Scheme 1 Schematic diagram of band structure as well as the charge transfer between noble metal and Ag₃PO₄; Φ is the work function, E₀ the vacuum level, E_C the conduction band of Ag₃PO₄, E_V the valence band of Ag₃PO₄, E_g the band gap of Ag₃PO₄, E_f the Fermi levels of noble metal and Ag₃PO₄, E_f(new) the Fermi level of M/Ag₃PO₄ heterostructures, E the internal electric field formed at the interface of M/Ag₃PO₄ Schottky-type heterostructures.

Scheme 2 The proposed photocatalytic mechanism of M/Ag₃PO₄ Schottky-type heterostructures in organic solution under visible light irradiation.

It is also of great significance to understand the different photocatalytic activity among Pt/Ag₃PO₄, Pd/Ag₃PO₄, and Au/Ag₃PO₄. According to the different work functions of the noble metals,⁴⁸ it is observed that the height of Fermi energy levels in the reducing order of Pt > Pd > Au (Scheme 1). As the Schottky barrier formed between metal (or semiconductor) and semiconductor is highly dependent on the difference in work functions, therefore, the higher Fermi level of Pt than that of Pd or Au and the larger difference in work function between Pt and Ag₃PO₄ may facilitate the transfer of photo-generated electrons from Ag₃PO₄ to Pt at their interfaces, resulting in the photocatalytic activity order of Pt/Ag₃PO₄ > Pd/Ag₃PO₄ > Au/Ag₃PO₄. Moreover, in contrast to Pt, Pd, and Au, Ag possesses a lower work function (4.26 eV) than Ag₃PO₄. Thereby, if Ag contacts with Ag₃PO₄, the electrons will transfer from Ag to Ag₃PO₄ as this transfer process is thermodynamically favorable and is similar to Ag/BiPO₄ heterostructures.⁴⁵ As a result, over Ag/Ag₃PO₄ heterostructures, the electrons are primarily accumulated on Ag₃PO₄ surface, on which the silver ions would be reduced by photo-generated electrons and thus Ag₃PO₄ was gradually decomposed to metallic Ag during consecutive photocatalytic process.³⁶

4. Conclusions

In summary, M/Ag₃PO₄ (M = Pt, Au, Pd) Schottky-type heterostructures have been successfully fabricated by a chemical deposition route, during which the decomposition of Ag₃PO₄ to weakly active metallic Ag was greatly inhibited. These M/Ag₃PO₄ heterostructures have exhibited superior photocatalytic performance to pure Ag₃PO₄ toward dyes degradation in the order of Pt > Pd > Au with a low amount (0.1 wt%). The enhancement of photocatalytic activity and stability of M/Ag₃PO₄ is mainly attributed to the successful and fast transfer of photogenerated electrons from Ag₃PO₄ to noble metals driven by Schottky barrier effect. Radical-trapping experiments and PL technique confirmed that h⁺ and ˙O₂⁻ in M/Ag₃PO₄ system played great roles toward the organic dyes degradation. This work not only greatly promotes the development of M/Ag₃PO₄ heterostructures for practical applications, but should also inspire the exploration of similar synthetic route to prepare and stabilize other highly efficient but easily photocorroded Ag-based photocatalysts, such as M/AgX (X = Cl, Br, I), M/Ag₂O, M/Ag₂S, M/Ag₂MoO₄, M/Ag₂WO₄, etc., which may have potential applications in environmental remediation and water splitting.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (no. 21103193, 21275089, and 21303094), Doctoral Foundation of Shandong Province (BS2013NJ013) and Scientific Research Foundation of Qufu Normal University (BSQD20110116).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06254j

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