Recent progress in highly efficient Ag-based visible-light photocatalysts

Abstract

As one of the most promising and efficient approaches for remediating the deterioration of natural environments, semiconductor-based photocatalysis has received considerable attention. To date, numerous efforts have been focused to explore novel materials for highly efficient photocatalysis under visible light or sunlight irradiation. Among them, Ag-based compounds are emerging to be a promising candidate because of their excellent visible light-responsive photoelectrochemical properties. This review summarizes the recent progress in the design and fabrication of Ag-compound-based semiconductor photocatalysts and their applications in the photocatalytic decomposition of organic molecules. Initially, the mechanisms of the related photocatalytic reactions will be discussed, and then we will highlight some of the recent progresses in Ag-based micro- or nano-structured material fabrication that exhibit enhanced photocatalytic performance. These novel and highly efficient photocatalysts mainly include Ag₂O, Ag₂S, AgX (X = Cl, Br, I), Ag₂CO₃ and Ag₃PO₄. We expect that the present tutorial review will provide insights in the direction of the future visible-light photocatalyst design.

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1. Introduction

Over the past several decades, the ever-increasing environmental pollution and fossil energy shortage have substantially attracted global attention. To solve these problems and to facilitate the sustainable development of human society, it is imperative to develop available technologies for remediating the deterioration of natural environments^1,2 and to generate alternative clean energy.^3–5 As a potential solution, semiconductor-based photocatalysis has attracted significant interest, especially since 1972, when Fujishima and Honda discovered that a water molecule could split into H₂ and O₂ on a titania (TiO₂) electrode under UV light irradiation.⁶ Therefore, photocatalysis can be regarded as a light-driven reaction, which is an environment-friendly and efficient approach for dealing with environmental pollution^7,8 and producing hydrogen energy.⁹

The unique electronic structure of semiconductor materials, consisting of a filled valence band (VB) and an empty conduction band (CB), plays a crucial role in the photoelectrochemical processes mentioned above. By absorbing photon energy equal to or higher than its band gap energy, the electrons in the VB are promoted to the CB, leaving an equal number of vacant sites (holes) in the VB. The photo-generated electrons and holes migrate to the surface of semiconductor and react with the adsorbed electron acceptors and electron donors, respectively. Therefore, these relevant reduction and oxidation reactions constitute the fundamental mechanism of semiconductor photochemistry. Theoretically, the semiconductor materials that meet the above mentioned requirements could be potentially used as photocatalysts for photoelectrochemical reactions. To date, more than 190 different semiconductors have been demonstrated as suitable photocatalysts for photoelectrochemical applications.⁹ Among them, TiO₂ still remains the most widely studied photocatalyst. However, TiO₂ only responds to high-energy UV light owing to its wide band gap. As known, one of the most fascinating aspects of photocatalysis technology is rooted in the possibility of employing sunlight, which is the most reliable, abundant and ‘green’ energy source under mild conditions. However, UV light only accounts for less than 5% of the solar energy reaching to the earth, which limits the efficient utilization of solar energy and thus the real-world applications of TiO₂. Because the major component of solar radiation is visible light, developing an efficient visible-light-responsive photocatalysts has thus become one of the most important objectives.¹⁰ However, such a pursuit remains to be a great challenge.

There are a variety of strategies that can be used to improve the usage of solar energy to make the photocatalysis technology cost-effective. Extensive research has been carried out for improving the photocatalytic activity of TiO₂, for example, through doping with foreign elements^11–13 to modulate the energy band gap, combining with noble metals¹⁴ or other semiconductors,¹⁵ and adding quantum dots^16,17 or dyes¹⁸ for light sensitization. Although these strategies, to some extent, could improve the visible-light photocatalytic performance of TiO₂, the activities and efficiencies pertaining to the utilization of solar irradiation over these state-of-art TiO₂-based semiconductor materials are still not satisfactory from the practical application point of view. Therefore, the exploration and creation of new semiconductor materials that can be used as highly efficient visible-light photocatalysts are highly desirable.

Very recently, a series of Ag-based compounds including Ag₂O,¹⁹ Ag₂S,²⁰ AgX (X = Cl, Br, I),^21–23 Ag₂CO₃ (ref. 24) and Ag₃PO₄ (ref. 25) have been demonstrated to possess excellent photoelectrochemical activities under the illumination of visible light or sunlight. In particular, a significantly high apparent quantum yield of nearly 90% under visible light (λ = 420 nm) have been achieved with Ag₃PO₄ as the photocatalyst for the evolution of O₂ during water photolysis.²⁵ Because many important results have been reported on the Ag-based semiconductor photocatalysts over the past a few years, it is indeed a great opportunity to summarize the recent cutting-edge developments in visible-light photocatalysts. In this review article, we will focus on the recent achievements in the Ag based photocatalysts from the standpoint of photoelectrochemistry, highlighting the recent progress in the fabrication, modification and applications of these types of photocatalysts. In addition, the mechanisms of the related photocatalytic reactions will also be discussed.

2. Principles of Ag-based photocatalysis

In essence, photocatalysis is the initiation or acceleration of specific reduction and oxidation reactions with the assistance of irradiated semiconductors. Ag-based compounds are a type of photosensitive materials that tend to easily decompose under illumination. For example, a silver halide particle can generate an electron and a hole on absorbing a photon, and then the photo-generated electron can reduce Ag⁺ to Ag⁰ atom. After absorbing more photons, a cluster of silver atoms is deposited on the silver halide particle. Because of their low stability under sunlight, Ag-based compounds, in particular silver halides, are extensively used in photography; however, until recently, these compounds are seldom used as photocatalysts.

In 1999, Kakuta and co-workers reported AgBr/SiO₂ photocatalyzed H₂ production from CH₃OH/H₂O solution under UV irradiation.²⁶ They observed that the Ag⁰ species were only generated at the initial stage of the reaction, and AgBr did not break further even after successive UV irradiation. As suggested, the metallic Ag could trap the photo-induced electrons, which inhibited the further reduction of Ag⁺ and promoted charge separation after coming into contact with a photoexcited semiconductor. Thus, the resulting Ag/Ag salts composites are stable under irradiation because the photo-induced electrons get enriched in the Ag nanoparticles (NPs) on the surface rather than being transferred to the Ag salts. The self-stabilization mechanism has been highlighted by many researchers. For instance, metallic Ag arises from the partial photodecomposition of Ag salts by illumination, and enhances the structure stability of the Ag salts. In general, this mechanism holds true for almost all the Ag-compound-based photocatalysts including Ag₂O, AgX (X = Cl, Br, I), Ag₂CO₃ and Ag₃PO₄.

In addition to the stability, Ag-compound-based photocatalysts exhibit a high photocatalytic efficiency. As is well known, the noble metal nanoparticles (NPs), such as Au NPs and Ag NPs, show efficient plasmon resonance in the visible region because of the localized surface plasmon resonance (LSPR) effect,^27,28 thus causing the strong absorption of sunlight. The LSPR effect of noble metal NPs has been utilized to improve the performance of semiconductor photocatalysts and to develop new plasmonic photocatalysts.^29–33 In addition, the excellent conductivity of Ag NPs can promote the electron transfer such that it suppresses electron–hole recombination, and thus enhance the interfacial charge transfer. Therefore, Ag NPs supported on silver salts particles might be expected to be highly efficient and stable visible-light photocatalysts.²¹ The underlying mechanisms will be discussed below. Studies on Ag-compound photocatalytic mechanism under visible light irradiation have been carried out recently. In the case of wide band gap materials, for which the visible light energy does not suffice, such as AgCl (direct band gap of 5.6 eV and indirect band gap of 3.25 eV), Ag NPs facilitate the photocatalysis, and a novel mechanism of Ag plasmonic photocatalysis has also been proposed.²¹ It has been suggested that a photon is absorbed by a Ag NP, leading to the separation of an electron and a hole because of the dipolar character of the surface plasmon state of Ag NPs. The AgCl particle is negatively charged because its surface is mostly terminated by Cl⁻ ions. Consequently, free electrons in the Ag NPs are polarized by the AgCl core and, as a result, the regions of negative and positive charges in the Ag NPs are far from and close to the Ag/AgCl interface, respectively. Therefore, the electron–hole separation and interfacial charge transfer could be efficiently facilitated by the synergetic effect between the dipolar characters of the surface plasmon state of Ag NPs and the polarization field provided by the AgCl core. As a result, the electrons are enriched on the surface of Ag NPs, which are far from the AgCl core, thus preventing the reduction of AgCl, and eventually leading to a highly stable composite material. Moreover, the holes diffuse into the AgCl core and promote the oxidation of Cl⁻ ions to Cl⁰ atoms that are very reactive towards the oxidizing organic molecules. The electrons are then trapped by O₂ dissolved in the solution to form the reactive oxygen species that can also chemically oxidize the organic compounds in the photocatalytic process. All these factors contribute to a high catalytic efficiency of AgCl photocatalyst.

For the narrow band gap materials such as Ag₂O (1.2 eV), Ag₂CO₃ (2.30 eV), AgBr (2.69 eV), and Ag₃PO₄ (direct bandgap of 2.43 eV and indirect bandgap of 2.36 eV), the conventional photocatalytic process occurs due to the fact that these semiconductor materials can be directly excited under visible light illumination. For example, when an Ag₂O photocatalyst is irradiated with visible light, a number of electron–hole pairs are generated; given that the more positive potential of Ag⁺/Ag (0.7991 V, vs. SHE) compared with O₂/HO₂ (−0.046 V, vs. SHE), the photo-induced electrons in the CB preferably combine with Ag⁺ ions to form Ag NPs in the early stage of the reaction. The photo-induced electrons enriched on the as-formed Ag NPs are expected to be trapped by the O₂ dissolved in the solution to form the reactive oxygen species, and the photo-induced holes in the VB directly oxidize the organic substances (see Fig. 1).¹⁹ It has been expected that the novel Ag plasmonic photocatalysis mechanism may also coexist in these photocatalytic systems.

Fig. 1 Schematic diagram showing the self-stabilizing process of the Ag₂O photocatalyst and photocatalytic mechanism under visible light irradiation (h = holes, e⁻ = electrons) (from ref. 19).

3. Synthesis and modification of Ag-compound based photocatalysts

3.1 Ag compound-based photocatalysts

To improve the utilization of solar energy, considerable effort has been devoted towards the development of new visible-light-sensitive photocatalysts with highly efficient photoelectrochemical performance. Recently, a series of Ag compounds have been studied and have been demonstrated to be novel photocatalysts with excellent performance under visible light irradiation.

3.1.1 Ag/Ag oxosalts photocatalysts. Wang et al. synthesized Ag₂O powder via a typical precipitation reaction of silver nitrate and sodium hydroxide at room temperature.¹⁹ Under fluorescent light irradiation, the Ag₂O sample exhibited a higher photocatalytic activity toward the degradation of methyl orange (MO) than the typical visible-light photocatalyst N–TiO₂. Further investigation revealed that the Ag₂O photocatalyst could also effectively decolorize rhodamine B (RhB) in an aqueous solution under fluorescent light irradiation and decompose a colourless phenol solution under visible light irradiation, thus demonstrating that the Ag₂O material could be used as a new type of potential visible-light-responsive photocatalyst.

In addition, the theoretical studies of electronic band structure suggest that changing the crystal structure, and thus adjusting the bandgap by the incorporation of p-block element into a simple narrow bandgap oxide is an effective strategy.²⁵ Ag₂O is a semiconductor with a narrow band gap of 1.3 eV. Therefore, adding p-block elements into Ag₂O may provide a new approach for the development of new visible-light-sensitive photocatalysts. As expected, the yellow green Ag₂CO₃ powder with a larger band gap of 2.30 eV was prepared by a simple precipitation reaction between NaHCO₃ and AgNO₃.²⁴ The as-formed Ag₂CO₃ powder showed a high catalytic activity towards the decomposition of many types of dyes, including RhB, MO and methylene blue (MB). However, the photocatalytic activity gradually decreased in the cyclic experiments owing to the decomposition of the formed Ag₂CO₃ under light irradiation. A possible approach to inhibit the photocorrosion of Ag₂CO₃ was proposed by Dai and co-workers.³⁴ They found that the photoactivity of Ag₂CO₃ could be enhanced with the presence of AgNO₃. In this photocatalytic system, AgNO₃ acted as a scavenger for electrons to effectively separate the photo-generated electron–hole pairs owing to the lower potential of Ag/AgNO₃ than that of Ag/Ag₂CO₃, thus inhibiting the photocorrosion of Ag₂CO₃.

The other successful example was the adoption of non-metal P elements to tune the low-lying CB of Ag₂O. The golden coloured Ag₃PO₄ powder prepared by a simple ion-change method could efficiently absorb solar energy at a wavelength shorter than 530 nm (Fig. 2A and C).²⁵ Under visible light illumination, the Ag₃PO₄ powder displays extremely high photooxidative abilities for the evolution of O₂ from water (Fig. 2B) and for the decomposition of MB dye (Fig. 2D). In particular, the high apparent quantum yields achieved at wavelength in the region from 400 to 480 nm (Fig. 2C) were obviously higher than the values reported to date. A comparative study of the electronic structures of Ag₃PO₄ and Ag₂O revealed that CB was constructed mainly through the Ag s states in Ag₃PO₄, and through the Ag d states in Ag₂O.³⁵ The excellent photocatalytic performance of Ag₃PO₄ was partly attributed to the highly dispersive Ag s-Ag s bands without the localized d states at CB, realizing a small effective mass of the electron, and thus eliminating the carrier recombination.^35,36

Fig. 2 (A) SEM image of the prepared Ag₃PO₄ powders. (B) O₂ evolution from aqueous AgNO₃ solutions under illumination (λ > 400 nm) on various semiconductor powders, respectively. (I) Ag₃PO₄: 636 μmol h⁻¹; (II) BiVO₄: 246 μmol h⁻¹; (III) WO₃: 72 μmol h⁻¹. (C) Ultraviolet-visible diffuse reflectance spectrum and apparent quantum yields of the Ag₃PO₄ semiconductor plotted as a function of wavelength of the incident light. Apparent quantum yields were plotted at the centre wavelengths of the band-pass filters, with error bars showing the deviation of the wavelengths (Δλ = ±15 nm). (D) Variation of methylene blue concentration as a function of illumination time under visible light (λ > 400 nm) (I) with powder samples of Ag₃PO₄ (a), TiO_2−xN_x (b), BiVO₄ (c) and methylene blue photolysis (d), and under various monochromatic visible lights with Ag₃PO₄ (II) at wavelengths of λ = 420.4 nm (Δλ = ±14.9 nm) (e) and λ = 639.3 nm (Δλ = ±16.2 nm) (f). The inset shows the color changes of the methylene blue solutions corresponding to the four filled square points in (a) (from ref. 25).

Furthermore, the photocatalytic activities of various silver oxosalts, Ag_x(XO_y)_z, viz. Ag₃AsO₄, Ag₂CO₃, Ag₃PO₄, Ag₂SO₄, and Ag₂SeO₄, have also been systematically investigated.³⁷ The results revealed that the optical band gaps of silver oxosalts linearly increased with the increasing charge-to-size (Z/r) ratio of the central atom X, whereas the surface charge of the silver oxosalts decreased with the increasing Z/r ratio. Except for Ag₂SO₄, all the silver oxosalts were fairly active under visible light irradiation, and the overall photocatalytic efficiency decreased in the order Ag₃PO₄ > Ag₃AsO₄ > Ag₂CO₃ > Ag₂SeO₄ > Ag₂SO₄, which roughly matched with the relative orders of their band gaps, suggesting the Z/r ratio of the central atom provides a facile and effective measure for predicting the optical band gap and photocatalytic performance of silver oxosalts.

3.1.2 Ag/AgX photocatalysts. In the Ag-compound based photocatalytic systems, silver halides, such as AgCl, AgBr and AgI are the other type of extensively studied photocatalysts. Considering that silver halides can spontaneously decompose to metallic Ag on light irradiation, and the resulting metallic Ag plays a key role in stabilizing the silver halides, the AgX particles with Ag NPs modified on their surface (referred to as Ag@AgX particles for convenience) might be expected to be stable photocatalysts.²¹ This expectation has been demonstrated by recent reports on the synthesis of Ag@AgX particles. For example, Wang and co-workers produced Ag@AgCl particles by two steps: they first treated Ag₂MoO₄ with HCl to form AgCl particles and then reduced some of the Ag⁺ ions via a photo-reduction method.²¹ In the photo-reduction process, the Ag⁺ ions were used as scavengers of photo-generated electrons to form Ag NPs with diameters of 20–150 nm on the surface of AgCl particles. In addition, a chemical reduction method, by using sodium borohydride (NaBH₄) as the reduction agent, was also employed to prepare Ag@AgCl nanocubes.³⁸ Subsequently, An et al. developed an efficient one-pot approach for the synthesis of hybrid nanoparticles composed of AgCl cores coated with Ag nanograins, involving a precipitation reaction, followed by polyol reduction at a temperature of 160 °C.²² Another one-pot hydrothermal method involving the ionic liquid 1-octyl-3-methylimidazolium chloride ([Omim]Cl), which acted as both a precursor and a reducing reagent, was also reported.³⁹ All the obtained Ag@AgCl particles showed a strong adsorption in the visible region because of the plasmon resonance of Ag NPs, and exhibited high activity and durability towards the degradation of the dye molecules such as MO and MB under visible light or sunlight irradiation.

As an analogous photocatalyst, Ag@AgBr prepared via ion-exchange reaction and subsequent photo-induced reduction method showed even higher photocatalytic activity than Ag@AgCl under visible light irradiation. The higher photocatalytic efficiency of AgBr might be due to the lower electron affinity of Br⁰ relative to Cl⁰, making it easier for Br⁻ to combine with a hole, in addition to the smaller band gap of AgBr relative to AgCl.⁴⁰ Moreover, the quick absorption of MO molecule on the photocatalyst arising from the complexation of Ag⁺ ions with N in the MO molecules also played an important role in accelerating the degradation rate.⁴¹ Because AgBr could also be directly excited by visible light, a new insight into the photocatalytic mechanism of Ag@AgBr involving interesting synergistic effect between plasmonic Ag photocatalysis and the conventional AgBr-based semiconductor photocatalysis was proposed by Jiang and coworkers.⁴² Under daylight illumination, SPR-excited electrons could be produced and enriched at the surface of Ag NPs. The increased electron density lifted the Fermi energy level of Ag, and the SPR electrons spontaneously transferred from Ag NPs to the CB of AgBr. In addition, it was suggested that the synergistic effect arising from SPR-induced local electric field enhancement in the Ag NPs could accelerate the formation of electron–hole pairs in the semiconductor.^43,44 In this case, more electrons were generated in the CB of AgBr under daylight irradiation. These photo-generated electrons in AgBr, along with the injected SPR electrons from Ag NPs, could then initiate the catalytic reaction and enhance the photocatalytic activity.

Similarly, a plasmonic photocatalyst Ag–AgI supported on an insulating solid mesoporous alumina (Ag–AgI/Al₂O₃) was prepared by Hu and coworkers.⁴⁵ They first deposited AgI onto Al₂O₃ support using a deposition–precipitation method, and then reduced some Ag⁺ ions on the surface of AgI particles to produce Ag NPs via a photocatalytic reduction method. The photocatalytic activity of AgI/Al₂O₃ was greatly enhanced when it was further loaded with Ag NPs. Under visible light irradiation, the obtained Ag–AgI/Al₂O₃ photocatalyst could effectively degrade the pollutant phenolic compounds including 2-chlorophenol (2-CP), 2,4-dichlorophenol (2,4-DCP) and trichlorophenol (TCP). The plasmonic photocatalytic mechanism was proposed based on a two-electron transfer process from the plasmon-excited Ag NPs to AgI and from 2-CP to the Ag NPs. It should be noted that the transfer of SPR electrons to the CB of AgBr⁴² or AgI⁴⁵ was considerably different from the other reports in the case of Ag@AgBr plasmonic photocatalyst,^40,41 where the photo-generated electrons were supposed to enrich on the Ag NPs and be far away from AgBr to prevent the reduction of Ag⁺ in AgBr, according to the self-stability mechanism. More studies are still needed to elucidate the photocatalytic mechanism of these photocatalysts.

Although many types of Ag@Ag salts have been proved to be promising candidates for the development of visible-light photocatalysts with high efficiency, the photocatalytic properties of these types of catalysts closely depend on the type of the Ag salts. The photocatalytic activities can be tuned by altering the anions of the Ag salts. To understand how the photocatalytic performance of the Ag salts depends on the negatively charged ions in their salts, Huang et al. designed and fabricated a series of Ag@Ag salts photocatalysts with different anions, such as Cl⁻, Br⁻, I⁻, CrO₄²⁻, PO₄³⁻, PW₁₂O₄₀³⁻ and SiW₁₂O₄₀⁴⁻.⁴⁶ They systematically compared the photocatalytic performance of these catalysts for the decomposition of MO under visible light illumination, and concluded that higher stability and higher charged anions led to a stronger photocatalytic ability.

3.2 Morphology control of Ag-based photocatalysis

Because the photocatalytic reactions are typically surface-based processes, the performance of the semiconductor materials is strongly dependent on their morphologies. Therefore, it would be a promising way to further improve the photocatalytic activity via the shape-controlled synthesis of photocatalytic materials. Because of the distinct electronic, optical and chemical properties, one-dimensional (1-D) nanostructures (including wires, belts, rods and tubes) have received considerable attention for photocatalytic applications.^47–50 Among them, the 1-D coaxial hetero-nanostructures with modulated compositions and interfaces have attracted particular interest because of their synergetic effects on the enhancement of photocatalytic performance.^51,52 Recently, Bi et al. synthesized Ag/AgCl core–shell nanowire heterostructures through the in situ oxidation of Ag nanowires by FeCl₃ (Fig. 3A and C).⁵³ The obtained Ag/AgCl core–shell nanowires could efficiently decompose MO dye under visible light irradiation, and the optimized ratio of Ag to AgCl was 8 : 92 for the most optimal photocatalytic activity. Moreover, the AgCl nanowires could be feasibly modified with Au NPs (Fig. 3D–F).⁵⁴ The Au NPs were generated from the reduction of Au precursor by Fe²⁺, which was derived from the reduction of FeCl₃ using the Ag nanowires. The formed Au NPs on the surface of AgCl nanowires enhanced the stability and the visible light absorption, and the resulting composite AgCl:Au nanowires exhibited good photocatalytic activity toward the decomposition of the MB dye under visible light illumination.

Fig. 3 (A) Schematic illustration of the in situ oxidation process for Ag/AgCl core–shell nanowires. (B, C) SEM images of Ag nanowires and as-prepared Ag/AgCl core–shell nanowires (from ref. 53). (D) Schematic illustration of the major steps involved in the synthesis of AgCl:Au by templating against Ag nanowires (the white lines represent grain boundaries in the AgCl nanowire). (E and F) SEM images of the as-synthesized AgCl:Au nanowires at different magnifications (from ref. 54).

In addition, the redox reactions involving Ag nanowires have also been employed for synthesizing novel 2-D dendritic Ag₃PO₄ nanostructures. For example, Bi et al. produced 2-D dendritic Ag₃PO₄ nanostructures by directly reacting Ag nanowires with H₂O₂ and Na₂HPO₄ in an aqueous solution at room temperature.⁵⁵ They used poly(vinylpyrrolidone) (PVP) as the capping agent, which got selectively adsorbed on the surfaces of Ag₃PO₄ nanoclusters, and thus changed their growth direction. Both PVP and Ag nanowires played crucial roles in the successful preparation of dendritic Ag₃PO₄ nanostructures. Under visible light irradiation, the novel 2-D dendritic Ag₃PO₄ exhibited considerably higher photocatalytic activities as compared to irregular Ag₃PO₄ nanocrystals and N-doped TiO₂ catalyst that are used for the degradation of RhB dye.⁵⁵

In addition to their role as facile templates for the preparation of novel nanostructures, Ag nanowires can also be coupled with semiconductor materials to further enhance the charge separation efficiency because of their high conductivity. For instance, the necklace-like hetero-photocatalysts have been constructed by selectively growing Ag₃PO₄ submicro-cubes on Ag nanowires.⁵⁶ As shown in the SEM image in Fig. 4A, a single-crystal Ag nanowire drilled through the two parallel {100} facets of each Ag₃PO₄ submicro-cube and joined them together along the longitudinal axis. The density of the Ag₃PO₄ cubes on the Ag nanowires could be readily tailored by adjusting the concentration of [Ag(NH₃)₂]⁺ complex (Fig. 4B–D). This novel hetero-structure with proper Ag₃PO₄ density exhibited considerably higher activities than both pure Ag₃PO₄ cubes and Ag nanowires, towards the degradation of RhB (Fig. 4E) and MO (Fig. 4F) under visible light irradiation.

Fig. 4 (A) SEM image of the Ag nanowire/Ag₃PO₄ cube necklace-like heterostructure. (B–D) SEM images of the as-prepared Ag/Ag₃PO₄ heterostructures obtained using different morphologies by adjusting the concentration of [Ag(NH₃)₂]⁺ complex (B: 0.05 M, C: 0.1 M, D: 0.2 M). (E and F) Photocatalytic activities of Ag nanowire/Ag₃PO₄ cube necklace-like heterostructures, Ag nanowires, and pure Ag₃PO₄ cubes for RhB and MO degradation under visible-light irradiation (λ > 420 nm) (wire/cube-1: sample B; wire/cube-2: sample C; wire/cube-3: sample A; wire/cube-4: sample D) (from ref. 56).

In addition to the above described studies, porous micro- or nano-structured materials with unique properties, such as large surface areas, low density, surface permeability, and light-trapping effects, have been widely applied in various fields.^57,58 The porous structures may be beneficial for the photocatalytic process because of more active sites and less resistance to mass transfer during the reaction.⁵⁹ Very recently, Liang et al. synthesized uniform hierarchical Ag₃PO₄ porous microcubes using a one-step reaction.⁶⁰ In their study, trisodium citrate was used as structure directing agent, crystal growth modifier and aggregation-orienting agent for the formation of a porous microstructure. Under the irradiation of visible light, the porous Ag₃PO₄ microcubes exhibited a superior photocatalytic performance for decomposing RhB as compared to the solid Ag₃PO₄ particles and commercial P25 TiO₂ powders. They claimed that the narrow bandgap, large surface-to-volume ratio, more active sites and synergistic effect of hierarchical porous structure contributed to the enhanced photocatalytic activity. Moreover, porous Ag₂S–Ag heterostructure nanotubes were successfully prepared via a one-pot microwave-assisted method (Fig. 5A–C).²⁰ The Ag content in the composite significantly influenced the photocatalytic performance, which could be easily tuned by varying the thioacetamide concentration. The as-prepared hybrid structures with a moderate Ag₂S/Ag molar ratio displayed excellent photocatalytic activity for the degradation of MO (Fig. 5D) and reduction of aqueous Cr^VI (Fig. 5E) under visible light irradiation.

Fig. 5 (A) Illustration of the fabrication of porous Ag₂S–Ag heterostructure nanotubes by a one-pot microwave-assisted method. (B) SEM images of the as-prepared porous Ag₂S–Ag heterostructure nanotubes; (C) TEM image of the porous Ag₂S–Ag heterostructure nanotubes (inset: a high-magnification image). (D) Photocatalytic degradation of MO, and (E) photocatalytic reduction of Cr^VI in the presence of different photocatalysts. C is the concentration of MO after light irradiation for a certain period, and C_o is the concentration of the MO after reaching adsorption/desorption equilibrium in the dark (from ref. 20).

3.3 Facet-controlled engineering of Ag-based photocatalysis

Moreover, the different facets of a single crystal possess distinctive chemical and physical properties, and the exposed highly reactive facets generally exhibit considerably higher catalytic activities.⁶¹ For example, the highly active facet {001} of TiO₂ crystals exhibited significantly greater photocatalytic performance than other common stable facets. The photocatalytic activities of TiO₂ crystals could be tuned by varying the percentage of {001} facet, and the TiO₂ with a higher percentage of {001} facet exhibited more effective photocatalytic performance.⁶² Nevertheless, the high energy surfaces rapidly diminished during the crystal growth process to minimize surface energy. Generally, it is favorable to introduce certain ions or surfactants for the formation of highly reactive facets because they can markedly reduce the surface energy during the crystal growth process. For example, Wang et al. developed a facile precipitation reaction using PVP as a capping agent for the synthesis of AgBr nanoplates with exposed {111} facet.⁶³ They found that both the AgBr-based photocatalysts showed better photocatalytic performance than Ag₃PO₄ under visible light irradiation, and the rate of MO decomposition over AgBr nanoplates was four times faster than that of the irregular AgBr particles. The enhanced photocatalytic activities of AgBr/Ag nanoplates were rationally explained by the density functional theory calculation results, which showed that the surface energy of the AgBr {111} surface (ca. 1.253 J m⁻²) was higher than that of AgBr {100} (ca. 0.495 J m⁻²) and {110} (ca. 0.561 J m⁻²). This result indicated that the {111} facet of AgBr was more reactive than the {110} and {100} facets, and thus it was significantly favourable for enhancing the photocatalytic activity. In addition, the five distinct morphologies of Ag₂O microcrystals, including cubic, octahedral, rhombic dodecahedral, rhombicuboctahedral and polyhedra with 18 faces, were successfully prepared by controlling the type or concentration of complexing agents.⁶⁴ Under the visible light illumination, they exhibited facet-dependent photocatalytic activities for the degradation of MO dye, and the order of degradation rate was in accordance with that of the Ag₂O surface energies ({100} > {110} > {111}). The cubic Ag₃PO₄ crystals were also shown to possess higher catalytic activity with better photoelectric properties than the spherical particles for the degradation of MB dye and photoelectric conversion under visible light irradiation, which might be attributed to their novel cubic structure and exposed {100} facets.⁶⁵

The capping agents adsorbed on the semiconductor crystals is another factor to be considered because they mask the reactive surface and have to be removed before the photocatalytic applications. Therefore, it is desirable to develop controlled synthesis methods for semiconductor materials with exposed reactive facets and clean surfaces. Successful examples for the synthesis of photocatalysts with highly reactive facets exposed but without capping agents were recently reported. Ye and co-workers fabricated two forms of single-crystalline Ag₃PO₄, i.e., the rhombic dodecahedrons enclosed by 12 well-defined {110} facets (Fig. 6A) and the cubes bound by the {100} facets (Fig. 6B), using CH₃COOAg and [Ag(NH₃)₂]⁺ complex as the silver ion precursors in aqueous solutions at room temperature, respectively.⁶⁶ The rhombic dodecahedral Ag₃PO₄ exhibited higher photocatalytic activity for the degradation of MO and RhB dyes than the cubes under the identical conditions, indicating that the {110} facet was more reactive than the {100} facet. The result was consistent with the higher surface energy of 1.31 J m⁻² for the {110} facet as compared to 1.12 J m⁻² for the {100} facet. Moreover, the tetrapod-shaped Ag₃PO₄ microcrystals (Fig. 6C and D) with an increased percentage of exposed {110} facets were prepared through a facile precipitation route in aqueous solution⁶⁷ or via a simple hydrothermal method⁶⁸ without any surfactant. The Ag₃PO₄ microcrystals were three-dimensional tetrapods, and the arms were hexagonal prisms. Under visible light irradiation, both Ag₃PO₄-based photocatalysts exhibited obvious superior photocatalytic performance than the reference N–TiO₂, and the photodegradation rate of MO and RhB dyes over the Ag₃PO₄ tetrapods was faster than that of the irregular Ag₃PO₄ particles. In addition, the tetrahedral submicro-crystals (Fig. 6E),⁶⁹ and concave trisoctahedral Ag₃PO₄ microcrystals (Fig. 6F)⁷⁰ were also synthesized; they exhibited an enhanced photocatalytic performance.

Fig. 6 SEM images of Ag₃PO₄ sub-microcrystals with different morphologies: (A) rhombic dodecahedron, (B) cube (from ref. 66); (C and D) tetrapod at low and high magnifications (from ref. 67); (E) tetrahedron (from ref. 69); (F) trisoctahedron (from ref. 70).

3.4 Composite photocatalysts

Semiconductor photocatalysis could be regarded as a type of system engineering, and a series of processes such as optoelectronic conversion, electrons and holes separation, surface/interface catalysis, mass transfer (such as adsorption/desorption, diffusion, etc.), and post-treatment or recycling of the photocatalyst are involved. Thus, all the relevant functions need to be optimized into the photocatalytic system. However, it is difficult to satisfactorily fulfill all the abovementioned tasks by a single composition. Therefore, a combination with other advanced materials to design composite photocatalysts with an integrated function could offer a promising solution.

To facilitate the electron–hole pair separation in semiconductor photocatalysts to further improve their photoelectrochemical activities, numerous studies have been recently performed to organize the semiconductor–semiconductor composite hierarchical structures.^71,72 Among various semiconductor photocatalysts, TiO₂ and ZnO are still the most widely used photocatalysts because of their excellent properties, such as high photocatalytic efficiency, good stability, low cost, and nontoxicity. The main drawback of these conventional photocatalysts is the lack of visible light utilization because of their large band gap. To overcome this problem, many attempts have been made to enhance the photocatalytic efficiency and visible light utilization using the combinations of narrow band gap semiconductors.^73–76 Recently, researchers have found that Ag-based compounds, such as Ag₂O, AgX (X = Cl, Br, I), and Ag₃PO₄, are excellent candidates to couple with TiO₂ or ZnO. This effectively improves the photocatalytic efficiency of these conventional photocatalysts. The synergetic effects of both carrier separation and photocatalytic efficiency in these heterostructures have been studied in recent years. In most of the cases, the Ag-based compounds serve to be a visible light sensitizer, which promote the interfacial electron transfer process, and thus reduce the charge recombination in the semiconductor. The typical examples of such systems include Ag₂O/TiO₂,⁷⁷ Ag₂O/ZnO,⁷⁸ AgX/TiO₂,^79–82 AgX/ZnO,^83–85 Ag₃PO₄/TiO₂,⁸⁶ Ag₃PO₄/ZnO,⁸⁷ and Ag₃VO₄/TiO_2.⁸⁸

In addition to the conventional large band gap semiconductors, such as TiO₂ and ZnO, some small band gap semiconductors, such as Bi₂O₃,⁸⁹ Bi₂₀TiO₃₂,⁹⁰ BiVO₄,⁹¹ BiOX (X = Cl, Br)⁹² and WO₃ (ref. 93) have also been incorporated with Ag-based high-efficient photocatalysts to construct hierarchical photocatalysts. Because of the efficient separation and transfer of photogenerated electron–hole pairs between these two semiconductors, all the hierarchical photocatalysts with an optimized mass ratio show superior photocatalytic performance as compared to each of the single semiconductor-based photocatalysts.

Moreover, the Ag-based semiconductor composite hierarchical nanostructures also exhibit significantly improved stability. For example, enhanced photocatalytic properties and stabilities were achieved by the epitaxial growth of AgX (X = Cl, Br, I) nanoshells on the surfaces of rhombic dodecahedral Ag₃PO₄ crystals.⁹⁴ In the AgX/Ag₃PO₄ core–shell heterostructures, the AgX nanoshell was in intimate contact with the outer surfaces of Ag₃PO₄ crystals. Because AgX nanoshell was considerably less soluble in an aqueous solution, it could effectively protect the Ag₃PO₄ core-crystals from dissolution during the photocatalytic process. Thus, their structural stabilities were greatly improved. In addition, the energy bands of AgX semiconductors (with an exception of AgCl) and Ag₃PO₄ crystals efficiently match. This could promote the transfer and separation of photo-excited electron–hole pairs through the hetero-junctions. Similarly, this strategy for enhancing the efficiency and stability of photocatalysts was also applied to the AgBr/Ag₂CO₃ hybrid materials.⁹⁵

In addition to the abovementioned photocatalysts, cheap and abundant carbon materials (carbon nanotubes, graphene and its derivatives, carbon quantum dots) have been also introduced to enhance the photoelectrochemical activity of Ag-compound-based photocatalysts because of their unique structure and remarkable properties. Carbon nanotubes (CNTs) are 1-D structures based on carbon layers with sp² bonds rolled into cylindrical tubes. they have been frequently employed as a catalyst carrier owing to their hollow structure, as well as their large specific surface area.^96,97 In addition, CNTs can serve as an electron attracting reservoir to reduce the electron–hole recombination, and thus improve the performance of the resultant composite photocatalysts.⁹⁸ Recently, Xu et al. synthesized the CNTs-loaded Ag/AgBr via a simple one-step hydrothermal method.⁹⁹ The obtained CNT/Ag/AgBr hybrid composite with low CNT content (<4.1 at%) showed better photocatalytic ability for the degradation of MO dye than pure Ag/AgBr. Subsequently, Ag/AgX (X = Cl, Br, I) loaded on CNT composites with improved photoactivity for 2,4,6-tribromophenol (TBP) degradation in aqueous phase under visible light was prepared.¹⁰⁰ The main reason for the photocatalytic performance enhancement was attributed to the loaded CNTs, which promoted the separation of electron–hole pairs in the hybrid photocatalysts. Furthermore, Zhai et al. developed a new type of photocatalytic system based on Pickering emulsions.¹⁰¹ The Pickering emulsions were formed by self-assembling Ag₃PO₄-multiwalled carbon nanotubes (MWNTs) nanohybrid at the water/oil interface (Fig. 7A). Under visible light irradiation, the as-formed photocatalytic system exhibited superior photocatalytic capability compared to raw Ag₃PO₄ nanoparticles in the solution for the decomposition of MB dye (Fig. 7B), as well as for the evolution of O₂ from water (Fig. 7C). It was elucidated that the excellent features of Pickering emulsions (including enlarged active surface areas and facilitated separation of products from reactants at the water/oil interface), together with promoted charge separation by CNTs, contributed to the enhanced photocatalytic activity.¹⁰¹

Fig. 7 (A) Schematic of the Pickering emulsion-based photocatalytic system formed by self-assembling Ag₃PO₄-MWNT nanohybrid at the water/oil interface. (B) First-order linear transforms of MB decomposition in Pickering emulsion-based system (black) and solution-dispersed system (red). (C) O₂ evolution under illumination in solution-dispersed system and Pickering emulsion-based system. Inset: O₂ yield in the solution-dispersed system (black curve) and Pickering emulsion-based system (red curve) (from ref. 101).

Another more popular carbonaceous material, graphene, has attracted a significant attention since its discovery because of its outstanding mechanical, thermal, optical, and electrical properties. Recently, numerous attempts have been made to design and prepare graphene-based semiconductor photocatalysts.^102,103 In particular, some research have been carried out to combine graphene or graphene oxide (GO) with Ag-containing compounds to enhance their photocatalytic performance. For instance, the GO enwrapped Ag/AgX (X = Br, Cl) nanocomposites were prepared in an oil/water system at room temperature and used as a highly efficient visible-light plasmonic photocatalyst (Fig. 8A–C).¹⁰⁴ The improved adsorption affinity of Ag/AgX/GO towards MO molecules was achieved by the hybridization of Ag/AgX with GO nanosheets owing to the non-covalent intermolecular π–π interactions between the MO molecules and GO-based hybrids. These hybrid photocatalysts exhibited better visible-light photocatalytic activities for the degradation of MO dyes compared to the bare Ag/AgX composite (Fig. 8D and E). Similarly, the graphene sheets-grafted Ag@AgCl composite was prepared via deposition–precipitation reaction followed by photoreduction method.¹⁰⁵ Under visible light irradiation, the composite catalyst displayed a four-fold enhancement in the photodegradation of RhB compared to bare Ag/AgCl nanoparticles. The obvious enhancement of photocatalytic activity was attributed to the effective charge transfer from SPR-excited Ag NPs to GO, which suppressed the charge recombination during photocatalytic process. In addition, the composites of other Ag-based photocatalysts and graphene or GO, such as Ag₂O-GO,¹⁰⁶ Ag₃PO₄-GO,^107–110 and Ag₃PO₄-graphene,^111,112 have also been prepared and used as efficient visible-light responsive photocatalysts for the decomposition of organic pollutants in water. The improved photocatalytic activity was mainly attributed to the enlarged surface area, enhanced adsorption of organic substance, and more efficient separation of the photogenerated electron–hole pairs. Moreover, the presence of graphene or GO sheets could effectively tailor the size of the Ag-compound particles, and smaller particles were generally obtained in these hybrid composites.^{104,105,110,111} The smaller size of the Ag-compound particles in the composites facilitated charge transfer, and thus suppressed the recombination of electron–hole pairs, which also contributed to the enhanced photocatalytic performance.

Fig. 8 (A) Illustration of the preparation of Ag/AgX/GO hybrid nanocomposites via an oil/water system at room temperature. (B and C) SEM images of the synthesized Ag/AgBr/GO and Ag/AgCl/GO, respectively. (D and E) Photocatalytic photodegradation of MO molecules under visible light irradiation (D: Ag/AgBr (a) and Ag/AgBr/GO (b); E: Ag/AgCl (a) and Ag/AgCl/GO (b) nanospecies) (from ref. 104).

Very recently, carbon quantum dots (CQDs) were introduced into Ag₃PO₄ photocatalyst for the photocatalytic decomposition of MO in water under visible light irradiation.¹¹³ In comparison to Ag₃PO₄ particles, both CQDs/Ag₃PO₄ and CQDs/Ag/Ag₃PO₄ displayed enhanced photocatalytic activities and good structural stabilities. The improved efficiency could be attributed to three effects: First, the dissolution and photocorrosion of Ag₃PO₄ could be prevented by CQDs on the surface of Ag₃PO₄ based on their insolubility and photoinduced electron transfer properties. Second, the excellent upconverted photoluminescence behaviour of CQDs could enable the complex systems to effectively utilize the full spectrum of sunlight. Finally, CQDs could act as an electron reservoir to facilitate the transport of the photogenerated electrons, and thus hinder the electron–hole pair recombination in the coupling system.

It is a known fact that reducing the size of photocatalyst is generally beneficial for surface-dependent photocatalysis because this leads to a higher specific surface area and more reactive sites. The emergence of nano-photocatalysts has offered efficient ways for promoting photocatalytic efficiency.³⁶ In the nano-photocatalyst system, the carriers migrating to the surface have extremely short distances to travel, and thus they reduce the recombination possibility for photo-excited electron–hole pairs. For example, the Ag₃PO₄ nanocrystals with particle size ranging from 8 to 16 nm possessed superior catalytic activity for the photodecomposition of MB under visible light irradiation as compared to Ag₃PO₄ microcrystals.¹¹⁴ However, for practical applications in aqueous media, the recycling of nano-sized photocatalysts from the reaction system is important. However, this is extremely difficult to achieve because of the small size of the catalysts employed. One of the feasible solutions is to develop magnetic photocatalytic materials. Recently, Li et al. reported a facile and fast approach for the synthesis of magnetically separable Ag₃PO₄–Fe₃O₄ sub-micrometre composites.¹¹⁵ These composites could effectively decompose the MB dye under visible light irradiation, which could be collected by an external magnet after the reaction and reused for the next cycle. In addition, Ag-AgX/Fe₃O₄@SiO₂ (X = Cl, Br or I) was also prepared by immobilizing photocatalytically active components on silica-coated magnetic Fe₃O₄ nanoparticles.^116–118 They exhibited enhanced performance towards the decomposition of organic molecules under visible light illumination, and could be easily recovered owing to their paramagnetic property.

4. Conclusion and perspectives

In summary, we have summarized recent developments on engineering a series of Ag-containing compounds, such as Ag₂O, Ag₂S, AgX (X = Cl, Br, I), Ag₂CO₃, and Ag₃PO₄, as highly efficient photocatalysts for decomposing organic substance under the illumination of visible light or sunlight. The photocatalytic performance could be rationally tuned through morphology and facet-controlled processes. Furthermore, after combining with other functional materials, Ag-compound-based hybrids show even better photocatalytic activities because these additional functional materials can introduce new or improved properties, such as extended light absorption, enhanced charge separation and transportation properties, enhanced structure stability, improved adsorption affinity, and enhanced solar energy conversion.

Ag-compound-based photocatalysts have attracted significant attention because of their high photocatalytic activities under visible light irradiation and potential applications in environmental remediation. Although considerable advances in this area have already been made over the past few years, the studies in this field are still in the initial stage. A number of challenges still exist and further developments of more Ag-based photocatalysts are required. Moreover, Ag salts are unstable under irradiation. Although the self-stability mechanism of Ag/Ag salts under illumination has been proposed and supported by some experimental evidences, the distribution of the Ag NPs on the surface of Ag salts and the exact proportion of metallic Ag required to stabilize the composite is not properly understood. Moreover, the size, shape and ratio of Ag NPs in the composite significantly influence the photocatalytic performance. Thus, the facile reduction method needs to be developed to realize the size, shape and number control of Ag NPs in the formation process. In addition, the detailed mechanism of the photocatalytic reaction is still unclear, and the proposed possible mechanism is under debate. More work as well as theoretical investigations should be carried out to obtain a better understanding of the photocatalytic process on these Ag-based compounds. Finally, the scarcity and high cost of Ag salts would be an obvious limitation for practical applications. How to easily recover the robust high performance photocatalysts from the reaction system for subsequent use appears to be an important issue. Nevertheless, the exploration of Ag-compound based materials for photocatalytic applications is rapidly progressing at present, and we believe that the present challenges will be gradually overcome. Furthermore, the rapid development of materials science over the past few years has created several kinds of advanced materials. By incorporating these advanced materials with Ag-compound photocatalysts, the performance of the catalysts can be significantly improved.

Acknowledgements

This work was financially supported by the NSF of China (21210007, 91213305, and 91232000 for L.M., and 21305129 for G. L.), the National Basic Research Program of China (the 973 programs 2010CB33502), and Chinese Academy of Sciences.

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