Carbon dots (CDs)/g-C3N4 is a promising photocatalyst to split water for H2 production; however, the synthesis of CDs/g-C3N4 is usually rigorous and involves multiple steps, which limits its practical application. In this study, a facile hydrothermal approach was developed to prepare CDs/g-C3N4 photocatalysts using L-ascorbic acid and g-C3N4 as the precursors. Upon in situ thermal polymerization of L-ascorbic acid on the g-C3N4 surface, the carbon dots were homogeneously and solidly modified on the g-C3N4 surface. The CDs/g-C3N4 photocatalysts showed higher photocatalytic performance for H2 production than g-C3N4 under UV light irradiation using lactic acid as the sacrificial agent. The improved photocatalytic performance of CDs/g-C3N4 was mainly attributed to rapid interfacial charge transfer. After a Pt co-catalyst was loaded, the Pt-CDs/g-C3N4 catalyst formed exhibited a further improved photocatalytic performance for H2 production and could even split pure water to produce H2. Considering our present economic and facile synthetic approach for the modification of carbon dots on the surface of g-C3N4 photocatalysts, the as-prepared CDs/g-C3N4 photocatalysts will be promising for practical use in water splitting.

Recently, Ag3PO4 has been demonstrated to be a new kind of material with high visible-light photocatalytic performance for the decomposition of various organic species. To further improve the photocatalytic activity of Ag3PO4, hole cocatalyst modification is a promising approach via the rapid transfer of photogenerated holes for effective oxidation reaction. In this work, Co-Pi as a hole cocatalyst was successfully modified on the Ag3PO4 surface by an in situ photodeposition method (referred to as CoPi/Ag3PO4). The results showed that the photocatalytic activity of CoPi/Ag3PO4 was greatly improved compared with that of Ag3PO4. Especially, CoPi/Ag3PO4 (0.3 wt%) reached the highest photocatalytic rate constant (k = 9.2 × 10−2 min−1), a value larger than that of Ag3PO4 (k = 1.4 × 10−2 min−1) by a factor of 6.6. However, it was further found that more accumulated electrons resulted in an obvious deactivation of Ag3PO4 due to the rapid transfer of holes by the Co-Pi cocatalyst, resulting in an obviously decreased photocatalytic performance during repeated tests. To enhance the performance stability of CoPi/Ag3PO4, the Cu(II) electron-cocatalyst was further loaded onto its surface to prepare the CoPi–Cu(II)/Ag3PO4 photocatalyst. The resultant CoPi–Cu(II)/Ag3PO4 not only indicated a much higher photocatalytic activity than CoPi/Ag3PO4, but also maintained the excellent stability, which was ascribed to the synergistic effect of Co-Pi as a hole cocatalyst and Cu(II) as an electron cocatalyst. This work may provide new insight for the development of highly stable and efficient photocatalysts for the degradation of organic pollutants.

For an efficient photocatalytic system, the rapid orientation transfer of photogenerated electron–hole pairs inside the photocatalyst and their effective interfacial catalytic reactions are significantly critical for achieving a high photocatalytic performance. However, it is quite difficult for a general photocatalyst to realize the crucial functions. In this study, the above idea was easily realized via a coupling strategy of crystal-facet engineering and spatially separated cocatalyst modification, namely, a TiO2 single-crystal photocatalyst with spatially separated Ag and F− bi-cocatalysts (Ag/F–TiO2). In this case, the F ions (as a hole cocatalyst) and Ag nanoparticles (as an electron cocatalyst) were selectively modified on the hole-rich (001) and electron-rich (101) facets of TiO2 single crystals, respectively. Photocatalytic results demonstrated that the resultant spatially separated Ag/F–TiO2 photocatalyst exhibited an obviously higher photocatalytic performance than pure TiO2, single-cocatalyst modified TiO2 (F–TiO2 and Ag/TiO2) and randomly Ag-deposited TiO2 (Ag/F–TiO2(R)). The main reason for the enhanced photocatalytic activity can be attributed to the excellent synergistic effect of orientation transfer of photogenerated charges and their rapid interfacial reaction via the efficient coupling strategy of crystal-facet engineering and cocatalyst modification, namely, the TiO2 single crystal structure can self-induce the orientation transfer of photogenerated charges to different crystal facets, while the spatially separated cocatalysts function as the effective active sites for the rapid interfacial catalytic reactions of those spatially separated charges (Ag nanoparticles on the (101) facets work as the active centres for oxygen-reduction reactions, and F ions on the (001) facets serve as the active sites for oxidation reactions of organic substances). The present coupling strategy of crystal-facet engineering and cocatalyst modification may also provide new ideas for the design and preparation of other highly efficient semiconductor photocatalysts.

The preferential adsorption of targeted contaminants on a photocatalyst surface is highly required to realize its photocatalytic selective decomposition in a complex system. To realize the tunable preferential adsorption, altering the surface charge or polarity property of photocatalysts has widely been reported. However, it is quite difficult for a modified photocatalyst to realize the simultaneously preferential adsorption for both cationic and anionic dyes. In this study, to realize the selective adsorption for both cationic and anionic dyes on a photocatalyst surface, the negative reduced graphene oxide (rGO) nanosheets and positive phenylamine (PhNH2) molecules are successfully loaded on the TiO2 surface (PhNH2/rGO-TiO2) with spatially separated adsorption sites, where the negative rGO and positive PhNH2 molecules work as the preferential adsorption sites for cationic and anionic dyes, respectively. It was interesting to find that although all the TiO2 samples (including the naked TiO2, PhNH2/TiO2, rGO-TiO2, and PhNH2/rGO-TiO2) clearly showed a better adsorption performance for cationic dyes than anionic dyes, only the PhNH2/rGO-TiO2 with spatially separated adsorption-active sites exhibited an opposite photocatalytic selectivity, namely, the naked TiO2, PhNH2/TiO2, and rGO-TiO2 showed a preferential decomposition for cationic dyes, while the resultant PhNH2/rGO-TiO2 exhibited an excellently selective decomposition for anionic dyes. In addition, the resultant PhNH2/rGO-TiO2 photocatalyst not only realizes the tunable photocatalytic selectivity but also can completely and sequentially decompose the opposite cationic and anionic dyes.Keywords: adsorption active sites; graphene; photocatalysis; photocatalytic selectivity; preferential adsorption

Crystalline TiO2 is a well-known oxide which can be used to improve the photocatalytic performance of other photocatalytic materials by a semiconductor-coupling strategy. However, compared with crystalline TiO2, amorphous TiO2-modified semiconductors have seldom been reported. In this study, amorphous TiO2 (referred to as Ti(IV)) as a hole cocatalyst was used to modify the photocatalytic performance of a Bi2WO6 photocatalyst by a facile wet-chemical method, where metallic Pt as the electron cocatalyst was also coated on the Bi2WO6 surface to promote the interfacial electron transfer. It was found that the dual-cocatalyst modified Ti(IV)–Pt/Bi2WO6 photocatalyst exhibited an obviously higher photocatalytic performance than the blank Bi2WO6 and single-cocatalyst modified Pt/Bi2WO6 and Ti(IV)/Bi2WO6. Based on the present experimental results, we propose a synergistic effect of amorphous Ti(IV) and Pt to illustrate the enhanced photocatalytic activity of the Ti(IV)–Pt/Bi2WO6 photocatalyst, namely amorphous Ti(IV) works as a hole cocatalyst to rapidly transfer the photogenerated holes in the valence band of Bi2WO6, while Pt acts as an electron cocatalyst to rapidly transfer the photogenerated electrons on its conduction band. As a consequence, the transfer rate and the interfacial catalytic reaction of photogenerated electrons and holes were simultaneously accelerated, which resulted in improved photocatalytic performance of the Ti(IV)–Pt/Bi2WO6 photocatalyst. The above synergistic effect mechanism in Ti(IV)-modified Bi2WO6 photocatalysts can further be demonstrated by using a low-cost Fe(III) or Cu(II) electron cocatalyst. The present study suggests that amorphous Ti(IV) can act as a new and effective hole cocatalyst for the enhanced photocatalytic performance of photocatalysts, which provides an approach for the design and development of high-performance visible-light photocatalysts with amorphous oxides.

•Suspended rGO/g-C3N4 was synthesized via a facile in situ hydrothermal route.•Pt-rGO/g-C3N4 exhibited an improved photocatalytic H2-evolution performance.•The synergistic effect of rGO and Pt causes the enhanced photocatalytic performance.The combination of graphene and semiconductor photocatalysts such as graphitic carbon nitride (g-C3N4) has been widely demonstrated as an effective strategy to improve the H2-evolution performance of photocatalysts. However, owing to the hydrophobicity of graphene, the green synthesis of well-suspended graphene/g-C3N4 in the absence of surfactant or additives is still a challenge. In this study, the suspended reduced-graphene-oxide-modified g-C3N4 (rGO/g-C3N4) photocatalysts were fabricated through an in situ hydrothermal synthesis route, which includes a facile preparation of well-coupled GO/g-C3N4 precursor and its following in situ transformation to form the rGO/g-C3N4 by a hydrothermal reduction method. It was found that the obtained rGO/g-C3N4 photocatalysts showed a cotton-like structure and could be well suspended in water owing to the well coupling between rGO and g-C3N4 nanosheets, which is quite favorable for the photocatalytic reactions in an aqueous solution system. After loading Pt cocatalyst as the active site for H2 evolution reaction, the rGO/g-C3N4 composite photocatalysts exhibited an obviously improved performance (ca. 23.7%) by the addition of a small amount of rGO (0.08%), compared with the pure g-C3N4. The improved photocatalytic H2-evolution performance of rGO/g-C3N4 composite can be attributed the synergistic effect of rGO nanosheets and Pt nanoparticles, namely, rGO nanosheet acts as an electron-transfer mediator to rapidly capture the photogenerated electrons from the g-C3N4 and then transfer to the Pt cocatalyst, while the Pt nanoparticle functions as a reduction active site to promote the H2-evolution reaction effectively. Considering its green preparation and high photocatalytic performance, the present synthesis route of suspended rGO/g-C3N4 may provide new insights into design and fabrication for other composite materials with various potential applications.The suspended rGO/g-C3N4 photocatalysts were synthesized via a facile in situ hydrothermal route and exhibited an improved photocatalytic H2-evolution performance owing to the synergistic effect of rGO electron-transfer mediator and Pt reduction active sites.

CdS is one of the most well-known and important visible-light photocatalytic materials for water splitting to produce hydrogen energy. Owing to its serious photocorrosion property (poor photoinduced stability), however, CdS photocatalyst can unavoidably be oxidized to form S0 by its photogenerated holes, causing an obviously decreased photocatalytic performance. In this study, to improve the photoinduced stability of CdS photocatalyst, amorphous TiO2 (referred to as Ti(IV)) as a hole cocatalyst was successfully loaded on the CdS surface to prepare Ti(IV)/CdS photocatalysts. It was found that the resultant Ti(IV)/CdS photocatalyst exhibited an obviously enhanced photocatalytic stability, namely, its deactivation rate clearly decreased from 37.9% to 13.5% after five cycles of photocatalytic reactions. However, its corresponding photocatalytic activity only showed a very limited increase (ca. 37.4%) compared with the naked CdS. To further improve its photocatalytic performance, the amorphous Ni(II) as an electron cocatalyst was subsequently modified on the Ti(IV)/CdS surface to prepare the dual amorphous-cocatalyst modified Ti(IV)–Ni(II)/CdS photocatalyst. In this case, the resultant Ti(IV)–Ni(II)/CdS photocatalyst not only exhibited a significantly improved photocatalytic activity and stability, but also could maintain the excellent photoinduced stability of CdS surface structure. Based on the experimental results, a synergistic effect of dual amorphous Ti(IV)–Ni(II) cocatalysts is proposed, namely, the amorphous Ti(IV) works as a hole-cocatalyst to rapidly capture the photogenerated holes from CdS surface, causing the less oxidation of surface lattice S2– ions in CdS, while the amorphous Ni(II) functions as an electron-cocatalyst to rapidly transfer the photogenerated electrons and then promote their following interfacial H2-evolution reaction. Compared with the traditional noble metal cocatalysts (such as Pt and RuO2), the present amorphous Ti(IV) and Ni(II) cocatalysts are apparently low-cost, nontoxic, and earth-abundant, which can widely be applied in the design and development of highly efficient photocatalytic materials.

Compared with the well-known three-dimensional Bi2WO6 nanosheet-assembled nanostructures, the free-standing two-dimensional porous Bi2WO6 nanosheets have seldom been reported. The possible reason is that Bi2WO6 nanosheets with a high surface-to-volume ratio usually tend to self-assemble or aggregate to form microspheres to reduce their surface energy. To prevent their aggregation, in this study, a new and facile self-assembled route, which includes the in situ ion-exchange reaction of Na2WO4 solution with the Bi(NO3)3 solid powder and the following high-temperature calcination, has been successfully developed to prepare the free-standing porous Bi2WO6 nanosheets. The ion-exchange reaction between the Bi(NO3)3 solid and Na2WO4 solution can in situ produce amorphous Bi2WO6 nanosheets, while the high-temperature calcination (500 °C) causes the formation of homogeneously porous structures in individual nanosheets during their phase transformation from amorphous to crystalline. The resultant porous nanosheets are composed of one-layer Bi2WO6 nanoparticles with a size of 30–50 nm, and there is a strong coupling interface among these nanoparticles. Photocatalytic experimental results suggest that the resultant porous Pt/Bi2WO6 nanosheets show a high photocatalytic performance for the decomposition of phenol solution. Considering their facile preparation, the present synthetic route may provide new insights for the design and fabrication of other nanostructured materials with various potential applications.

•Ag2S nanocrystal-sensitized Ag8W4O16 nanorods were prepared by a facile method.•The particle size and amount of the Ag2S nanocrystals can be well controlled.•Ag2S/Ag8W4O16 photocatalyst showed high visible-light photocatalytic activity.•The quantum effect of Ag2S nanocrystal contributed to the high performance.Narrow band-gap (NBG) Ag2S nanocrystals (NCs) attaching on the surface of wide band-gap (WBG) Ag8W4O16 nanorods were prepared by employing a facile in situ anion exchange method with the reaction between S2− and WO42-, and the photocatalytic activity was evaluated by the photocatalytic decolorization of methyl orange solution under visible-light irradiation. It was found that in situ anion exchange could uniformly deposit Ag2S NCs on the surface of Ag8W4O16 nanorods, controllably adjust the size, distribution and amount of Ag2S NCs, and solidly connect Ag2S NCs to the Ag8W4O16 nanorods via the replacement of S2− in the solution with lattice WO42- on the Ag8W4O16 surface. The photocatalytic results indicated that the as-prepared Ag2S/Ag8W4O16 composite photocatalysts exhibited obviously higher activity compared with the pure Ag8W4O16 and N-TiO2 photocatalysts. On the basis of band structures of Ag2S and Ag8W4O16 semiconductors and the quantum size effect of Ag2S NCs, a possible photocatalytic mechanism about the Ag2S nanocrystal-sensitized Ag8W4O16 nanorods was proposed to account for the effective visible-light photocatalytic activities. This present work may provide some insight into the design of novel and high-efficiency NBG semiconductor NCs coupled with WBG semiconductor composite photocatalysts.Graphical abstract

Usually, cocatalyst modification of photocatalysts is an efficient approach to enhance the photocatalytic performance by promoting effective separation of photogenerated electrons and holes. It is highly required to explore new and effective cocatalysts to further enhance the photocatalytic performance of photocatalytic materials. In the present work, Cu(II) cocatalyst was successfully loaded on the surface of various Ag-based compounds (such as AgCl, Ag3PO4, AgBr, AgI, Ag2CO3, and Ag2O) by a simple impregnation route, and their photocatalytic activity of Cu(II)/Ag-based photocatalysts was evaluated by the photocatalytic decolorization of methyl orange and photocatalytic decomposition of phenol solution under visible-light illumination. As one of the typical photosensitive Ag-based compounds, the photocatalytic activity of AgCl could be greatly improved by optimizing the amount of Cu(II) cocatalyst, and the highest photocatalytic performance of the resulted Cu(II)/AgCl was higher than that of the unmodified AgCl by a factor of 2.1. Significantly, the Cu(II) was demonstrated to be a general and effective cocatalyst to improve the visible-light photocatalytic performance of other various photosensitive Ag-based compounds (such as AgBr, AgI, Ag3PO4, Ag2CO3, and Ag2O) in addition to the AgCl photocatalyst. Based on the present results, it is proposed that the Cu(II) cocatalyst functions as electron scavengers to quickly capture photogenerated electrons from the excited photocatalysts and then works as reduction active sites to reduce O2 effectively, resulting in an effective separation of photogenerated electrons and holes. Compared with the expensive noble metal cocatalyst (such as Pt, Au, and Pd), the present promising Cu(II) cocatalyst can be considered to be one of the perfect cocatalysts for the smart preparation of various highly efficient photocatalysts in view of its abundance and low cost.

To alleviate the main limitations of lithium ion diffusion rate and poor electronic conductivity for LiFePO4 cathode material, it is desirable to synthesize nano-size LiFePO4 material due to its enhanced electronic and lithium ion transport rates and thus an improved high-rate performance. However, our previous synthesized LiFePO4 nanorods only exhibited low high-rate and slightly unstable cycle performance. Possible reasons are the poor crystallization and Fe2+ oxidation of LiFePO4 nanorods prepared by hydrothermal method. In this paper, LiFePO4 nanorods were simply dealt with at 700 °C for 4 h under the protection of Ar and H2 mixture gas. The electrochemical properties of LiFePO4/Li cells were investigated by galvanostatic test and cyclic voltammetry (CV). The experimental results indicated that the annealed LiFePO4 nanorods delivered an excellent cycling stability and obviously improved capacity of 150 mA·h·g−1 at 1C, and even 122 mA·h·g−1 at 5C.

•Fe(III) cocatalyst was loaded on the TiO2−xNx surface by an impregnation method.•Fe(III)/TiO2−xNx showed an enhanced visible-light photocatalytic activity.•Fe(III) cocatalyst promoted the rapid separation of photogenerated charges.•The advantage of Fe(III) as a new cocatalyst is its low cost and abundant resource.Nitrogen doped TiO2 (TiO2−xNx), one of the most promising visible-light photocatalytic materials, exhibits a low photocatalytic activity owing to the rapid recombination of photogenerated electrons and holes, which is caused by the isolated energy level of N 2p formed above the valence band of TiO2. To reduce the recombination of photogenerated charges and further improve the photocatalytic performance of TiO2−xNx, in this study, Fe(III) cocatalyst was loaded on the surface of TiO2−xNx photocatalyst by a simple impregnation method. It was found that after surface modification by Fe(III) cocatalyst, the photocatalytic performance of all the TiO2−xNx photocatalysts could be greatly improved for the decomposition of MO under visible-light irradiation. Especially, the Fe(III)/TiO1.981N0.019 photocatalyst showed the highest photocatalytic activity and its rate constant k was ca. 52.5 × 10−4 min−1, which is higher than that of the corresponding TiO1.981N0.019 photocatalyst (9.9 × 10−4 min−1) by a factor of 5.3 times. Based on the present experimental results, the Fe(III) was thought to be an effective cocatalyst to significantly improve the electron transfer from the conduction band of TiO2−xNx to the adsorbed oxygen, thus facilitating the rapid separation of photogenerated charges and greatly enhancing the photocatalytic activity. In addition, the direct interfacial charge transfer from the N 2p isolated energy level to Fe(III) cocatalyst can also contribute to the enhanced photocatalytic efficiency of TiO2−xNx. Considering the abundant Fe resources and its low cost, the Fe(III)-cocatalyst modification can be regarded as an effective approach for rational design and development of high-performance visible-light photocatalysts in practical applications.

•RGO-cocatalyst modification and AgCl nanonization were simultaneously realized.•AgCl nanoparticles with a size of 20–200 nm are tightly grafted on the rGO surface.•Ag/AgCl-rGO photocatalyst shows an excellent photocatalytic activity.•Both rGO cocatalyst and AgCl nanonization contribute to an improved performance.Usually, cocatalyst modification and nanonization of photocatalytic materials have been demonstrated to be two kinds of effective strategies to improve the photocatalytic performance. For the well-known Ag/AgCl photocatalyst, it is difficult to obtain AgCl nanoparticles by a conventional precipitation reaction in aqueous solutions. It is highly required to develop a facile and effective strategy to simultaneously realize the cocatalyst modification and nanonization of Ag/AgCl photocatalysts. In this study, cocatalyst modification and nanonization of Ag/AgCl photocatalyst were simultaneously realized via a facile reduction–reoxidization route by using graphene oxide (GO) as the cocatalyst modifier. It was found that the chemical reduction of both Ag+ and GO by NaBH4 leaded to the formation of nanoscale Ag grafted on the reduced GO (rGO), whereas the following in situ reoxidization of metallic Ag in FeCl3 solution resulted in the final formation of well-dispersed Ag/AgCl nanoparticles on the rGO surface. Owing to a good encapsulation of Ag nanoparticles by rGO nanosheets, the resultant AgCl nanoparticles could be easily controlled to be 20–200 nm and were tightly grafted on the rGO cocatalyst surface. The photocatalytic experimental results indicated that all the Ag/AgCl-rGO (1–5 wt% rGO) nanocomposites exhibited a much higher photocatalytic decomposition of phenol than the Ag/AgCl under visible light irradiation, and the Ag/AgCl-rGO (3 wt% rGO) showed the highest performance. The enhanced photocatalytic activity of Ag/AgCl-rGO can be attributed to the cooperation effect of rGO nanosheet cocatalyst promoting the effective transfer of photogenerated electrons, and the nanonization of AgCl particles that provide more surface active sites for the decomposition of organic substances. This work may provide new insights into the fabrication of high-performance visible-light photocatalytic materials.

Recently, Ag3PO4 has been demonstrated to be a new kind of material with high visible-light photocatalytic performance for the decomposition of various organic species. To further improve the photocatalytic activity of Ag3PO4, hole cocatalyst modification is a promising approach via the rapid transfer of photogenerated holes for effective oxidation reaction. In this work, Co-Pi as a hole cocatalyst was successfully modified on the Ag3PO4 surface by an in situ photodeposition method (referred to as CoPi/Ag3PO4). The results showed that the photocatalytic activity of CoPi/Ag3PO4 was greatly improved compared with that of Ag3PO4. Especially, CoPi/Ag3PO4 (0.3 wt%) reached the highest photocatalytic rate constant (k = 9.2 × 10−2 min−1), a value larger than that of Ag3PO4 (k = 1.4 × 10−2 min−1) by a factor of 6.6. However, it was further found that more accumulated electrons resulted in an obvious deactivation of Ag3PO4 due to the rapid transfer of holes by the Co-Pi cocatalyst, resulting in an obviously decreased photocatalytic performance during repeated tests. To enhance the performance stability of CoPi/Ag3PO4, the Cu(II) electron-cocatalyst was further loaded onto its surface to prepare the CoPi–Cu(II)/Ag3PO4 photocatalyst. The resultant CoPi–Cu(II)/Ag3PO4 not only indicated a much higher photocatalytic activity than CoPi/Ag3PO4, but also maintained the excellent stability, which was ascribed to the synergistic effect of Co-Pi as a hole cocatalyst and Cu(II) as an electron cocatalyst. This work may provide new insight for the development of highly stable and efficient photocatalysts for the degradation of organic pollutants.

Compared with the well-known three-dimensional Bi2WO6 nanosheet-assembled nanostructures, the free-standing two-dimensional porous Bi2WO6 nanosheets have seldom been reported. The possible reason is that Bi2WO6 nanosheets with a high surface-to-volume ratio usually tend to self-assemble or aggregate to form microspheres to reduce their surface energy. To prevent their aggregation, in this study, a new and facile self-assembled route, which includes the in situ ion-exchange reaction of Na2WO4 solution with the Bi(NO3)3 solid powder and the following high-temperature calcination, has been successfully developed to prepare the free-standing porous Bi2WO6 nanosheets. The ion-exchange reaction between the Bi(NO3)3 solid and Na2WO4 solution can in situ produce amorphous Bi2WO6 nanosheets, while the high-temperature calcination (500 °C) causes the formation of homogeneously porous structures in individual nanosheets during their phase transformation from amorphous to crystalline. The resultant porous nanosheets are composed of one-layer Bi2WO6 nanoparticles with a size of 30–50 nm, and there is a strong coupling interface among these nanoparticles. Photocatalytic experimental results suggest that the resultant porous Pt/Bi2WO6 nanosheets show a high photocatalytic performance for the decomposition of phenol solution. Considering their facile preparation, the present synthetic route may provide new insights for the design and fabrication of other nanostructured materials with various potential applications.

For an efficient photocatalytic system, the rapid orientation transfer of photogenerated electron–hole pairs inside the photocatalyst and their effective interfacial catalytic reactions are significantly critical for achieving a high photocatalytic performance. However, it is quite difficult for a general photocatalyst to realize the crucial functions. In this study, the above idea was easily realized via a coupling strategy of crystal-facet engineering and spatially separated cocatalyst modification, namely, a TiO2 single-crystal photocatalyst with spatially separated Ag and F− bi-cocatalysts (Ag/F–TiO2). In this case, the F ions (as a hole cocatalyst) and Ag nanoparticles (as an electron cocatalyst) were selectively modified on the hole-rich (001) and electron-rich (101) facets of TiO2 single crystals, respectively. Photocatalytic results demonstrated that the resultant spatially separated Ag/F–TiO2 photocatalyst exhibited an obviously higher photocatalytic performance than pure TiO2, single-cocatalyst modified TiO2 (F–TiO2 and Ag/TiO2) and randomly Ag-deposited TiO2 (Ag/F–TiO2(R)). The main reason for the enhanced photocatalytic activity can be attributed to the excellent synergistic effect of orientation transfer of photogenerated charges and their rapid interfacial reaction via the efficient coupling strategy of crystal-facet engineering and cocatalyst modification, namely, the TiO2 single crystal structure can self-induce the orientation transfer of photogenerated charges to different crystal facets, while the spatially separated cocatalysts function as the effective active sites for the rapid interfacial catalytic reactions of those spatially separated charges (Ag nanoparticles on the (101) facets work as the active centres for oxygen-reduction reactions, and F ions on the (001) facets serve as the active sites for oxidation reactions of organic substances). The present coupling strategy of crystal-facet engineering and cocatalyst modification may also provide new ideas for the design and preparation of other highly efficient semiconductor photocatalysts.

Carbon dots (CDs)/g-C3N4 is a promising photocatalyst to split water for H2 production; however, the synthesis of CDs/g-C3N4 is usually rigorous and involves multiple steps, which limits its practical application. In this study, a facile hydrothermal approach was developed to prepare CDs/g-C3N4 photocatalysts using L-ascorbic acid and g-C3N4 as the precursors. Upon in situ thermal polymerization of L-ascorbic acid on the g-C3N4 surface, the carbon dots were homogeneously and solidly modified on the g-C3N4 surface. The CDs/g-C3N4 photocatalysts showed higher photocatalytic performance for H2 production than g-C3N4 under UV light irradiation using lactic acid as the sacrificial agent. The improved photocatalytic performance of CDs/g-C3N4 was mainly attributed to rapid interfacial charge transfer. After a Pt co-catalyst was loaded, the Pt-CDs/g-C3N4 catalyst formed exhibited a further improved photocatalytic performance for H2 production and could even split pure water to produce H2. Considering our present economic and facile synthetic approach for the modification of carbon dots on the surface of g-C3N4 photocatalysts, the as-prepared CDs/g-C3N4 photocatalysts will be promising for practical use in water splitting.