Modified g-C3N4 Photocatalyst Research Articles

Simultaneous bulk and surface modifications of g-C3N4 via supercritical CO2-assisted post-treatment towards enhanced photocatalytic activity

A high-temperature supercritical CO2 (ScCO2)-assisted post-treatment strategy is developed to simultaneously modify the bulk and surface structures of graphitic carbon nitride (g-C3N4) using ScCO2 as penetrant and a small amount of methanol/acetonitrile mixture as modifiers. Specifically, the product of modified g-C3N4 is obtained through the post-treatment of pristine g-C3N4 in a homogeneous ScCO2/methanol/acetonitrile system, continuously heating to reach a target temperature (330 °C, 15.2 MPa) without a residence time. With the assistance of ScCO2, organic molecules penetrate the interlayers of g-C3N4 and subsequently modify the in-plane structure of g-C3N4. Methanol induces the partial transformation of g-C3N4 into a carbon nitride with a poly(heptazine imide)-like structure and with abundant methyl and hydroxyl groups. Simultaneously, acetonitrile polymerizes into 2,4,6-trimethyl-1,3,5-triazine, which is then covalently grafted onto the surface of modified g-C3N4. Remarkably, the entire post-treatment process is completed within 5 min, achieving a final product yield of 92.3 %. The modified g-C3N4 photocatalyst achieves an H2-evolution rate of 2126.1 μmol·h−1·g−1 and a CO2-to-CO conversion rate of 874.1 µmol·h−1·g−1. This work provides a green, efficient, high-yield, and scalable strategy for preparing layered and porous non-metallic photocatalysts using supercritical fluids technology.

Synergistically boosted photocatalytic production of hydrogen peroxide via protonation and oxygen doping on graphitic carbon nitride

Photocatalytic synthesis of hydrogen peroxide (H2O2) using modified graphitic carbon nitride (g-C3N4) has been recognized as a promising green and sustainable process. However, the effect of g-C3N4 modification on the surface properties of photocatalytic surface reactions still needs extended exploration. In this work, we design and synthesize a protonation induced oxygen doping in g-C3N4 (pCN1), which shows uniquely curled nanosheet structures in microscopic level. Under visible light irradiation, the photocatalytic H2O2 production rate of pCN1 reaches 707 μmol g−1 h−1, which is approximately eight times higher than that of primitive g-C3N4. The enhanced photocatalytic performance is attributed to the synergistic effect of protonation and oxygen doping, which not only dramatically facilitates the adsorption of oxygen molecules and the desorption of H2O2 product, but also promotes the charge separation. In addition, more abundant protons supplied via protonation process can combine with the adjacent oxygen atoms on catalyst surface, thus promoting the formation of OOH* intermediate and improving the H2O2 yield. This work provides a facile and effective strategy to prepare highly-efficient modified g-C3N4 photocatalyst for the clean and convenient synthesis of H2O2.

Photodegradation of ciprofloxacin antibiotic in water by using ZnO-doped g-C3N4 photocatalyst

Ciprofloxacin antibiotic (CIP) is one of the antibiotics with the highest rate of antibiotic resistance, if used and managed improperly, can have a negative impact on the ecosystem. In this research, ZnO modified g-C3N4 photocatalyst was prepared and applied for the decomposition of CIP antibiotic compounds in water. The removal performance of CIP by using ZnO/g-C3N4 reached 93.8% under pH 8.0 and an increasing amount of catalyst could improve the degradation performance of the pollutant. The modified ZnO/g-C3N4 completely oxidized CIP at a low concentration of 1 mg L−1 and the CIP removal efficiency slightly decreases (around 13%) at a high level of pollutant (20 mg L−1). The degradation rate of CIP by doped sample ZnO/g-C3N4 was 4.9 times faster than that of undoped g-C3N4. The doped catalyst ZnO/g-C3N4 also displayed high reusability for decomposition of CIP with 89.8% efficiency remaining after 3 cycles. The radical species including ·OH, ·O2− and h+ are important in the CIP degradation process. In addition, the proposed mechanism for CIP degradation by visible light-assisted ZnO/g-C3N4 was claimed.

Efficient generation of singlet oxygen on modified g-C3N4 photocatalyst for preferential oxidation of targeted organic pollutants

• A simple one-step method has been developed to chemically modify C 3 N 4 . • The modified C 3 N 4 can be used for efficient degradation of organic pollutants. • Singlet oxygen is the dominant active oxidative specie generated on modified C 3 N 4 . • Preferential oxidation of target organic pollutants is achieved with modified C 3 N 4 . • This work highlights the application of singlet oxygen generated in photocatalysis. Graphitic carbon nitride (g-C 3 N 4 ) is an emerging photocatalyst for treatment of organic pollutants. However, pristine g-C 3 N 4 exhibits poor photocatalytic activity due to the inefficient generation of reactive species. Here, chemically multi-modified g-C 3 N 4 was prepared through a facile thermal polymerization approach. The S, K-doped/alkalized g-C 3 N 4 product can generate singlet oxygen ( 1 O 2 ) efficiently. This powerful yet selective oxidant is found to be effective for the oxidation of bisphenol A (BPA). The degradation rate of BPA on S, K-doped/alkalized g-C 3 N 4 reaches 0.61 h −1 at pH 8 under visible-light irradiation, 4.1 times of pristine g-C 3 N 4 . As alkaline generation of · OH is unfavorable, 1 O 2 could be a more reliable oxidative specie for degradation of pollutants in alkaline waters. Furthermore, taking advantage of the selective oxidation nature of 1 O 2 , preferential treatment of targeted organic pollutants becomes possible. Such an approach can prevent the production of undesirable toxic intermediates which may be inevitable during · OH-based photocatalysis. This study provides a novel photocatalytic approach for preferential oxidation of targeted compounds in a complex matrix by using 1 O 2 as the active oxidative specie.

Bimetal-modified g-C3N4 photocatalyst for promoting hydrogen production coupled with selective oxidation of biomass derivative

The solar-driven coproduction of hydrogen (H2) fuel and value-added chemicals from biomass derivatives represents a promising strategy to address the energy and environment issues. Herein, the Ni-Au bimetal modified g-C3N4 photocatalyst was synthesized by a step-by-step photodeposition method and used to promote the photocatalytic coproduction of H2 and furfural from furfural-alcohol aqueous solution. In the designed Ni-Au/g-C3N4 photocatalyst, g-C3N4 was the main light-harvesting material for generating electron-hole pairs which were separated efficiently by metal/g-C3N4 interface, plasmonic Au nanoparticles further increased the light utilization efficiency, and Ni acted as both electron trap and the active sites for H2 evolution. Beneficial from the synergy of three components, the Ni-Au/g-C3N4 photocatalyst exhibited 3-folds activity of Au/CN sample toward photocatalytic coproduction of H2 and furfural. Both photoluminescence spectroscopy and photoelectrochemical measurements demonstrated the more efficient separation and transfer of photogenerated charge carriers in Ni-Au/g-C3N4 photocatalyst. In situ electron spin resonance revealed the generations of hydroxyl radical and carbon center radical in the photocatalytic reaction. Based on the systematic characterizations, a photocatalytic mechanism was proposed and discussed. This work provides a rational construction of photocatalysts for coproduction of H2 and value-added chemicals from biomass derivatives as well as explores the synergistic effect of bimetallic cocatalyst on enhancing the photocatalysis.

Fabrication of MIL-Fe (53)/modified g-C3N4 photocatalyst synergy H2O2 for degradation of tetracycline

• Fe-MOF enhanced the adsorption property of the compound. • Photocatalysts showed high degradation efficiency of tetracycline (TC). • The added H 2 O 2 formed a synergistic effect with the compound. • Fe 3+ and H 2 O 2 formed Fenton-like system enabling the rapid separation of e - and h + . • The GC–MS illustrated the degradation intermediates and possible pathway. An improved porous g-C 3 N 4 material was prepared by calcination with equal quality of melamine and cyanuric acid. The improved g-C 3 N 4 was recorded as CM and an appropriate amount of MIL-Fe (53) was loaded on the CM to synthesize Fe-MOF/CM (denoted as x% Fe-MOF/CM, which x was the mass of MIL-Fe (53) for a given amount of CM) by self-assembly synthesis method. Photocatalytic degradation experiments showed that the 3 % Fe-MOF/CM-H 2 O 2 system can degrade 100 % tetracycline (10 ppm) within 60 min. The efficiency of this system to degrade tetracycline was about 3.6 times than that of pure g-C 3 N 4 . More importantly, the 3 % Fe-MOF/CM-H 2 O 2 photocatalyst system still has excellent degradation effect on high concentration of TC (30 ppm), which can reach about 100 % within 60 min. This system exhibited high degradation efficiency of TC than g-C 3 N 4 and CM, especially under the high concentration of TC condition. The reason could ascribe the addition of Fe-MOF enhanced the adsorption performance of the material. Moreover, Fe 3+ and H 2 O 2 formed a Fenton-like system, which enabled the rapid separation of e - and h + . Finally, the possible degradation pathway was suggested through the intermediates identified by GC–MS. The photocatalytic mechanism would provide some suggestions for degradation of TC in waste water.

Metal-free four-in-one modification of g-C3N4 for superior photocatalytic CO2 reduction and H2 evolution

Utilization of g-C3N4 as a single photocatalyst material without combination with other semiconductor remains challenging. Herein, we report a facile green method for synthesizing a metal free modified g-C3N4 photocatalyst. The modification process combines four different strategies in a one-pot thermal reaction: non-metal doping, porosity generation, functionalization with amino groups, and thermal oxidation etching. The as-prepared amino-functionalized ultrathin nanoporous boron-doped g-C3N4 exhibited a high specific surface area of 143.2 m2 g−1 which resulted in abundant adsorption sites for CO2 and water molecules. The surface amino groups act as Lewis basic sites to adsorb acidic CO2 molecules, which can also serve as active sites to facilitate hydrogen generation. Besides, the simultaneous use of ammonium chloride as a dynamic gas bubble template along with thermal oxidation etching efficiently boosts the delamination of the g-C3N4 layers to produce ultrathin sheets; this leads to stronger light–matter interactions and efficient charge generation. Consequently, the newly modified g-C3N4 achieved selective gas-phase CO2 reduction into CO with a production yield of 21.95 µmol g−1, in the absence of any cocatalyst. Moreover, a high hydrogen generation rate of 3800 µmol g-1h−1 and prominent apparent quantum yield of 10.6% were recorded. This work opens up a new avenue to explore different rational modifications of g-C3N4 nanosheets for the efficient production of clean energy.

Bifunctional template-mediated synthesis of porous ordered g-C3N4 decorated with potassium and cyano groups for effective photocatalytic H2O2 evolution from dual-electron O2 reduction

The photocatalytic method to produce hydrogen peroxide (H2O2) as a substitute for fossil fuels has become a hot research topic. We herein propose cyano defect and potassium intercalation modified g-C3N4 photocatalysts in which the porous structure and crystallinity can be maintained relatively well. Simultaneously, the photocatalysts possess adjustable band structure, enhanced carrier transfer efficiency and highly selective dual-electron O2 reduction. The physicochemical properties and reaction mechanisms are well studied through the effective combination of experiments and theoretical calculations. The H2O2 yield of KDCN-0.2 (278.9 μmol L-1h−1) is approximately 4.3 times higher than that of original CN (65.2 μmol L-1h−1), and the energy conversion efficiency is also higher than many current CN-based photocatalysts. This work deepens the understanding of the mechanism of photocatalytic O2 reduction for H2O2 production and points out the promising direction for the design of novel CN-based photocatalysts with ideal properties in energy conversion and environmental applications.

Enhanced photocatalytic nitrogen fixation performance of g-C3N4 under the burning explosion effect

A series of novel sodium persulfate (Na2S2O8) modified g-C3N4 photocatalysts were prepared in this study to enhance nitrogen fixation. Characterization results found that the type of precursors and the mass ratio affect the crystal structure, morphology, and optical properties of the modified g-C3N4. The photocatalysts exhibited considerable nitrogen vacancies (NVs) depending on the nature of the different precursors and their mass ratios to Na2S2O8. In addition, the production rate of ammonia of the modified g-C3N4 was significantly higher than that of pure g-C3N4 under optimal conditions. The four cycles show modified g-C3N4 excellent stability. Combined with the results of pH test, the possible mechanism of the photocatalytic nitrogen fixation process using modified g-C3N4 was proposed.

Polyethyleneimine induced highly dispersed Ag nanoparticles over g-C3N4 nanosheets for efficient photocatalytic and antibacterial performance

Improving the efficiency of Ag nanoparticles (NPs) in an antibacterial system remains a problem to be solved. Herein, a simple polyethyleneimine (PEI) modification strategy has been developed for realizing the high dispersion of Ag NPs with average size of 10 nm over two dimensional nanosheets of g-C3N4 photocatalyst, and the as-prepared ternary Ag/PEI/CN hybrid demonstrates enhanced bifunctional performance of antibiotics (e.g., tetracycline, TC) degradation and antibacterial activity. The enhanced mechanism is investigated and proposed to be a synergy effect between reactive oxygen-containing species (ROS, e.g., •OH and O2•−) derived from PEI modified g-C3N4 photocatalyst and slow released Ag+ ions generated by the highly dispersed Ag NPs as antimicrobial agent. Furthermore, the surface plasmonic effect of Ag NPs and coupling of PEI can increase light absorption and accelerate the electron-hole separation and mobility. Consequently, more availability of ROS along with holes and Ag+ ions can accelerate the death of bacteria via destructing biomolecules. As a result, under visible-light irradiation, Ag/PEI/CN hybrid presents greatly improved antibacterial activity against E. coli, including a low MIC100% of 30 ppm and MBC97% of 10 ppm and enhanced photocatalytic performance towards TC degradation (~80% TC can be degraded within 60 min).

FeCo alloy@N-doped graphitized carbon as an efficient cocatalyst for enhanced photocatalytic H2 evolution by inducing accelerated charge transfer

Abstract Cocatalysts play important roles in improving the activity and stability of most photocatalysts. It is of great significance to develop economical, efficient and stable cocatalysts. Herein, using Na2CoFe(CN)6 complex as precursor, a novel noble-metal-free FeCo@NGC cocatalyst (nano-FeCo alloy@N-doped graphitized carbon) is fabricated by a simple pyrolysis method. Coupling with g-C3N4, the optimal FeCo@NGC/g-C3N4 receives a boosted visible light driven photocatalytic H2 evolution rate of 42.2 μmol h−1, which is even higher than that of 1.0 wt% Pt modified g-C3N4 photocatalyst. Based on the results of density functional theory (DFT) calculations and practical experiment measurements, such outstanding photocatalytic performance of FeCo@NGC/g-C3N4 is mainly attributed to two aspects. One is the accelerated charge transfer behavior, induced by a photogenerated electrons secondary transfer performance on the surface of FeCo alloy nanoparticles. The other is related to the adjustment of H adsorption energy (approaching the standard hydrogen electrode potential) by the presence of external NGC thin layer. Both factors play key roles in the H2 evolution reaction. Such outstanding performance highlights an enormous potential of developing noble-metal-free bimetallic nano-alloy as inexpensive and efficient cocatalysts for solar applications.

Enhanced carriers separation efficiency in g-C3N4 modified with sulfonic groups for efficient photocatalytic Cr(VI) reduction

Rapid carrier transport and high carrier separation efficiency are key factors in improving photocatalytic performance. Herein, a novel sulfonic groups modified g-C3N4 photocatalyst was constructed by solution impregnation and evaporation method. The strong electronegativity of the sulfonic acid group provides a guide for the migration of electrons to the surface, thereby promoting the transport and separation of photogenerated carriers. In addition to that, the sulfonic acid group can also achieve spatial separation of carriers on the surface and serve as an active site. Based on the above merits, the sulfonic-acid-modified g-C3N4 exhibited a significantly enhanced visible light reduction Cr(VI) activity which is 2.9 times higher than that of pure g-C3N4.The resent work provides a new strategy for the surface modification of photocatalysts, which contributes to construct high performance photocatalysts.

Controllable fabrication of a red phosphorus modified g-C3N4 photocatalyst with strong interfacial binding for the efficient removal of organic pollutants

The development of low-cost and efficient photocatalysts is pursued to promote the large-scale application of photocatalysis in environmental remediation. In this study, cost-effective g-C3N4-RP (red phosphorus) heterostructures were controllably fabricated, in which sodium hypophosphite was utilized as a new phosphorus source. The formed RP exhibited a particle morphology with the size of 60–100 nm. These nanoparticles were strongly bonded to the g-C3N4 surface by forming P-N chemical bonds. When compared with pristine g-C3N4, the as-synthesized g-C3N4-RP hybrids exhibited synergistically improved adsorption and photocatalytic activity for the removal of Rhodamine B (RhB) pollutant in aqueous solution. The improved adsorption for g-C3N4-RP resulted from the higher surface area, presence of the manufactured larger delocalized system, and more negatively charged surface when RP was introduced. Meanwhile, the improved performance of the photocatalysis was a result of the strengthened light adsorption, improved separation and transfer of photogenerated electron-hole pairs, and efficient surface reaction. A typical Type II heterostructure was confirmed to be formed at the interface to accelerate the charge carrier transfer between g-C3N4 and RP. The h+ and •O2⁻ radicals were determined to be the major oxide species for RhB degradation. Finally, a possible RhB degradation mechanism was revealed in the g-C3N4-RP photocatalytic system. We believe that this study is of importance to accelerate the development of low-cost and effective catalysts for wastewater remediation by photocatalysis.

Synthesis and photo-catalytic activity of porous g-C3N4: Promotion effect of nitrogen vacancy in H2 evolution and pollutant degradation reactions

Different density nitrogen vacancy modified g-C3N4 photocatalysts were successfully prepared, and investigated their photocatalytic performance for H2 evolution and pollutant degradation. In this system, the increase of nitrogen vacancy density could enable the conduction band position of resultant g-C3N4 to negative shift compared to previous g-C3N4. Meanwhile, various positive factors were also introduced in this system, such as larger surface areas, the improvement of light response capacity, effective separation of photogenerated charge, resulting in resultant g-C3N4 photocatalysts exhibiting evident improvement of photocatalytic performance for H2 evolution and pollutant degradation. Obviously, this work provides physico-chemical insights for the nature of the promotion effect of nitrogen vacancies.

A New and stable Mo-Mo2C modified g-C3N4 photocatalyst for efficient visible light photocatalytic H2 production

Design and preparation of highly efficient and stable cocatalysts are critical to the improvement of photocatalyst performance. A traditional cocatalyst consists of metal nanoparticles for the separation of photo-induced electron-hole pairs and for the reduction of protons. In this research we report a metal-semiconductor composite cocatalyst to increase light adsorption and to effectively enhance proton reduction capacity. A molybdenum rich molybdenum carbide (Mo-Mo2C) based noble-metal-free metal/semiconductor cocatalyst was loaded onto graphitic carbon nitride (g-C3N4) for highly efficient photocatalytic H2 evolution from water. The Mo-Mo2C was synthesized via a temperature-programmed reaction using (NH4)6Mo7O24·4H2O as a precursor. The cocatalyst loaded 2.0 wt.% Mo-Mo2C/g-C3N4 composite photocatalyst has demonstrated excellent photocatalytic performance. The hydrogen evolution rate for the 2.0 wt.% Mo-Mo2C/g-C3N4 nanocomposites can be as high as 219.7 μmol h−1 g−1, which is 440 times higher than that of g-C3N4 alone and 90% as high as 0.5 wt.% Pt/g-C3N4 photocatalyst (244.1 μmol h−1 g−1). Due to strong synergetic effects between Mo and Mo2C nanoparticles, this rate is 11.47 and 3.60 times higher than those for 2.0 wt.% Mo/g-C3N4 (19.1 μmol h−1 g−1) and 2.0 wt.% Mo2C/g-C3N4 (60.9 μmol h−1 g−1) photocatalysts respectively. Moreover, the 2.0 wt.% Mo-Mo2C/g-C3N4 catalyst is significantly stable for application in photocatalytic hydrogen evolution, with an apparent quantum efficiency of 8.3%—one of the highest noble-metal-free efficiencies reported in literature. All results indicate that metal/semiconductor composites can serve as highly efficient cocatalysts for photocatalytic hydrogen evolution from water reduction.

Nitrogen vacancies modified graphitic carbon nitride: Scalable and one-step fabrication with efficient visible-light-driven hydrogen evolution

Graphitic carbon nitride (g-C3N4) with nitrogen vacancies (CN-x) was fabricated by a straightforward one-step N2H4·H2O-assisted thermal polymerization method, avoiding high-energy consumption and complex post-treatment. 100-fold precursor amplification experiment proved that such a simple one-step method could achieve scalable synthesis. Experiment results show that nitrogen vacancies, introduced into CN-x architecture by removing N atoms in the C-containing triazine rings (CNC) (N2C) sites, cause bandgap narrowing and generate downward-shifted midgap states under the conduction band (CB) edge, which consequently extend the visible-light absorption and inhibit recombination of electrons and holes. As a result, under optimal amount of N2H4·H2O added, CN-75 shows a highest H2 evolution rate of 8171.4 μmol·h−1·g−1 and 3895.1 μmol·h−1·g−1 under λ > 420 nm and λ > 470 nm visible-light irradiation, respectively, far higher than that of pristine g-C3N4 (481.6 μmol·h−1·g−1, λ > 420 nm and 148.6 μmol·h−1·g−1, λ > 470 nm). Moreover, stability assays show that the as-prepared CN-75 possesses satisfactory light and structure stability after ten cycling runs (4 h per cycling). This work offers a simple and effective route for fabricating high-performance nitrogen vacancies modified g-C3N4 photocatalyst on a large scale.

Photocatalytic reductive dechlorination of 2-chlorodibenzo-p-dioxin by Pd modified g-C3N4 photocatalysts under UV–vis irradiation: Efficacy, kinetics and mechanism

Polychlorinated dibenzo-p-dioxins (PCDDs), as a group of notorious anthropogenic environmental toxicants, are arguably ubiquitous in nature. In this study, we investigated the photocatalytic reductive dechlorination of 2-chlorodibenzo-p-dioxin (2-CDD) over Pd/g-C3N4 catalysts under UV–vis irradiation. The g-C3N4 and a series of Pd/g-C3N4 catalysts were prepared by thermal polymerization and mechanical mixing-illumination method and characterized by XRD, TEM, BET, SEM and UV–vis DRS analyses. Among all the samples, the Pd/g-C3N4 (5 wt%) yielded the optimal dechlorination activity with a total 2-CDD conversion of 54% within 4 h, and 76% of those converted 2-CDD were evolved to dibenzo-p-dioxin (DD). The kinetics of dechlorination could be described as pseudo-first-order decay model (R2 > 0.84). Corresponding rate constants (k) increased from 0.052 to 0.17 h−1 with Pd contents up to 5 wt% and decreased to 0.13 h−1 with a 10 wt% of Pd. The enhanced activities originated from the surface plasmonic resonance (SPR) effect of Pd nanoparticles and the formation of Schottky barrier between Pd and g-C3N4, which extend the spectrum responsive range and suppress the charge recombination of g-C3N4. This is the first report on the photocatalytic reductive removal of PCDDs and may provide a new approach for PCDDs pollution control.

Synthesis of g-C3N4 by different precursors under burning explosion effect and its photocatalytic degradation for tylosin

The use of abundant sunlight for semiconductor photo-degradation of antibiotics is an ideal way to solve global water pollution. Here, the sodium nitrate modified photocatalysts exhibits enhanced photocatalytic efficiency for degradation of tylosin under simulated sunlight irradiation over g-C3N4 alone. In addition, the pure g-C3N4 and sodium nitrate modified g-C3N4 photocatalysts were investigated in terms of their crystal structures, morphologies, optical properties by using XRD, SEM, TEM, Raman, FTIR, UV–vis and XPS. On the basis of these results, the morphology dependence of the visible light absorption and the photocatalytic efficiency under simulated sunlight irradiation has been systematically investigated. It was found that the type of precursors and the molar ratio of sodium nitrate have an evident impact on the crystal structure of g-C3N4, and photocatalytic performance due to varied reaction pathways and degree of condensation. The photocatalytic activity evaluated under simulated sunlight indicates that the as-synthesized photocatalysts is effective in obtaining the energy of solar spectrum and transforming it into the chemical energy for tylosin degradation.

0D (MoS2)/2D (g-C3N4) heterojunctions in Z-scheme for enhanced photocatalytic and electrochemical hydrogen evolution

MoS2 quantum dots (MSQDs) with high and stable dispersion in water were prepared via a facile one-pot hydrothermal process. The MSQDs were then applied to decorate graphitic carbon nitride (g-C3N4, CN) nanosheets to obtain modified g-C3N4 photocatalysts (MSQD-CN). Compared to pristine g-C3N4, the hybrid photocatalysts showed a slight red shift and stronger light absorption with remarkably improved photocatalytic activity in water splitting to generate hydrogen. The hydrogen-evolution rate over 0.2 wt% MSQD-CN increased by 1.3 and 8.1 times as high as that of 0.2 wt% Pt-CN and g-C3N4, respectively. With deposition of 2 wt% Pt as a cocatalyst, 5 wt% MSQD-CN exhibited the highest photocatalytic efficiency with an average hydrogen evolution reaction (HER) rate of 577 μmol h−1 g−1. Photoluminescence spectra (PL) and photoelectrochemical measurements inferred that MSQDs introduction drastically promoted the electron transfer for more efficient separation of charge carriers, which could lower HER overpotential barriers and enhance the electrical conductivity. In addition, the well-matched band potentials of the MSQD-CN hybrid with an intimate contact interface of p-n heterojunction also inhibited the recombination of photo-generated carriers, leading to enhanced photocatalytic HER performance. A direct Z-scheme charge transfer mechanism of the MSQD-CN hybrid was proposed to further elaborate the synergistic effect between MSQDs, Pt and g-C3N4. This work underlines the importance of heterojunction interface and presents a feasible protocol for rational construction of g-C3N4 based photocatalysts for various photocatalytic applications.

Enhanced selective photocatalytic reduction of CO2 to CH4 over plasmonic Au modified g-C3N4 photocatalyst under UV–vis light irradiation

A series of Au-g-C3N4 (Au-CN) catalysts were prepared through a NaBH4-reduction method using g-C3N4 (CN) from pyrolysis of urea as precursor. The catalysts’ surface area, crystal structure, surface morphology, chemical state, functional group composition and optical properties were characterized by X-ray diffraction, transmission electron microscope, X-ray photoelectron spectroscopy, ultraviolet visible (UV–vis) diffuse reflectance spectra, fourier transform infrared, photoluminescence and transient photocurrent analysis. The carbon dioxide (CO2) photoreduction activities under ultraviolet visible (UV–vis) light irradiation were significantly enhanced when gold (Au) was loaded on the surface of CN. 2Au-CN catalyst with Au to CN mole ratio of 2% showed the best catalytic activity. After 2 h UV–vis light irradiation, the methane (CH4) yield over the 2Au-CN catalyst was 9.1 times higher than that over the pure CN. The CH4 selectivity also greatly improved for the 2Au-CN compared to the CN. The deposited Au nanoparticles facilitated the separation of electron-hole pairs on the CN surface. Moreover, the surface plasmon resonance effect of Au further promoted the generation of hot electrons and visible light absorption. Therefore, Au loading significantly improved CO2 photoreduction performance of CN under UV–vis light irradiation.