GaN/Surface-modified graphitic carbon nitride heterojunction: Promising photocatalytic hydrogen evolution materials

The coupling of two-dimensional (2D) layered materials is an effective way to realize photocatalytic hydrogen production. Herein, using first-principles calculations, the photocatalytic properties of GaN/CNs heterojunctions formed by two different graphite-like carbon nitride materials and GaN monolayer are discussed in detail. The results show that the GaN/C₂N heterojunction can promote the effective separation of photogenerated electron and hole pairs, which is attributed to its type-II band orientation and high carrier mobility. However, the low overpotential of GaN/C₂N for photocatalytic hydrogen production limits the photocatalytic performance. On this basis, we adjust the CBM position of the GaN/C₂N heterojunction by applying an electric field to enhance its hydrogen evolution capability. In addition, the GaN/g-C₃N₄ is a type-I heterojunction, which is suitable for the field of optoelectronic devices. This work broadens the field of vision for the preparation of highly efficient photocatalysts.     

NiSe2/Cd0.5Zn0.5S as a type-II heterojunction photocatalyst for enhanced photocatalytic hydrogen evolution

A designed type-II heterojunction photocatalyst, NiSe₂/Cd_0.5Zn_0.5S (NiSe₂/CZS), was successfully synthesized and it exhibits outstanding photocatalytic hydrogen evolution performance. The optimal loading amount of NiSe₂ on Cd_0.5Zn_0.5S is 13 wt %, and the corresponding hydrogen production rate is approximately 121.01 mmol g^?1 h^?1 under visible light. The heterojunction structure between Cd_0.5Zn_0.5S and NiSe₂ promoted the separation of photogenerated electron-hole pairs, effectively suppressed the photogenerated carrier recombination and endowed the material with excellent interfacial charge transfer properties, thus improving the photocatalytic performance.     

NiSe as an effective co-catalyst coupled with TiO2 for enhanced photocatalytic hydrogen evolution

Construction of semiconductor heterojunctions can effectively accelerate the separation of photo-induced charge carriers and thereby enhance photocatalytic activity. Here, NiSe was used as an effective co-catalyst to construct an active NiSe/TiO₂ heterojunction for improving the photocatalytic H₂ production of TiO₂. The resultant 10%NiSe/TiO₂ heterojunction exhibited 11 times higher photocatalytic H₂-production activity than that of bare TiO₂. The NiSe/TiO₂ heterojunction and the photo-reduction of partial Ni²⁺ to Ni⁰ notably accelerated the separation and transfer of photo-excited electron-hole pairs, and thus resulted in obvious improvement of H₂-evolution activity. This work holds promise for the application of NiSe in photocatalysis as a high-efficiency photocatalytic cocatalyst.     

ZnCdS/NiAl hydrotalcite S-scheme heterojunction for efficient photocatalytic hydrogen evolution

ZnCdS/NiAl hydrotalcite S-scheme heterojunction with highly effective photocatalytic hydrogen evolution activity was devised and prepared by a simple solution-based mixing way. Layered double hydroxide (LDH), also called hydrotalcite-like compound, is composed of adjustable metal cations and exchangeable anions between layers. The hydrogen evolution performance of ZnCdS/NiAl LDH is about 7 times that of ZnCdS and 130 times that of NiAl LDH. Because the rod-shaped ZnCdS and the layered NiAl LDH can construct close interface contact. This interface contact helps to accelerate charge transfer, thereby achieving more effective photocatalytic hydrogen evolution. The S-scheme ZnCdS/NiAl LDH heterojunction catalyst shows excellent hydrogen evolution and good stability, which not only gets benefits from the prominent performances of the cob-like ZnCdS and the layered NiAl LDH but also the matching bandgap structure for them. The configuration of the S-scheme ZnCdS/NiAl LDH heterojunction catalyst accelerates the rapid charge movement and inhibits the recombination of charge carriers, thereby greatly enhancing visible-light-driven water splitting, which is corroborated by the PL spectrum, I-T, LSV, EIS, MottSchottky and UV–vis DRS studies, etc.     

In-situ constructing cobalt incorporated nitrogen-doped carbon/CdS heterojunction with efficient interfacial charge transfer for photocatalytic hydrogen evolution

Designing an efficient heterojunction interface is an effective way to promote the electrons' transfer and improve the photocatalytic H₂ evolution performance. In this work, a novel hollow hybrid system of Co@NC/CdS has been fabricated and constructed. CdS nanospheres are anchored on the hollow-structured cobalt incorporated nitrogen-doped carbon (Co@NC) through a one-pot in-situ chemical deposition approach, forming an intimate interface and establishing an excellent channel to improve the electrons transfer and charge carriers separation between CdS and Co@NC cocatalyst, which immensely promotes the photocatalytic activity. The rate of photocatalytic H₂ evolution over hollow structured Co@NC/CdS heterojunction can be achieved 8.2 mmol g^?1 h^?1, which is about 45 times of pristine CdS nanospheres. The photocatalytic H₂ evolution mechanism has been investigated by the techniques of photoluminescence (PL) spectra, photocurrent-time (i-t) curves, electrochemical impedance spectroscopy (EIS) etc. This work aims to provide a new way in developing of high-performance advanced 3D heterojunction for photocatalytic hydrogen evolution.     

Co3O4/CeO2 p-n heterojunction construction and application for efficient photocatalytic hydrogen evolution

The construction of p-n type heterojunction is an effective way to enhance the efficiency of photocatalytic hydrogen evolution. In this work, Co₃O₄/CeO₂ p-n heterojunction was construct by a simple hydrothermal method. This heterojunction mainly uses the internal electric field formed and accelerate the separation of electrons and holes in the opposite direction. In addition, according to SEM and TEM characterization, it was found that the granular cobalt oxide nanoparticles prepared by in-situ hydrothermal method were firmly and uniformly dispersed in cerium oxide, which effectively increased the active sites of hydrogen evolution. And combined with the BET results, it shows that the growth of cobalt oxide effectively increases the specific surface area and increases the active sites for hydrogen evolution. By exploring the hydrogen evolution capacity of different ratios of the complex, the test results showed that in all different ratios of the catalyst, CC-0.16 showed the best performance, and the hydrogen production efficiency reached 2298.52 μmol g⁻¹h⁻¹, which was 71 times that of nanobelt CeO₂ and 2.72 times that of Co₃O₄. According to the characterization results, the photocatalytic water splitting mechanism of the p-n heterojunction was proposed, and the charge transfer mechanism in the process was discussed in depth.     

2D NiCo2S4 decorated on ZnIn2S4 formed S-scheme heterojunction for photocatalytic hydrogen production

The hydrothermal preparation of NiCo₂S₄/ZnIn₂S₄ photocatalysts with different mass ratios is studied. Ni-cobalt bimetallic sulfide nanosheets are grown on zinc-indium bimetallic sulfide to form a compact heterojunction. First, both NiCo₂S₄ and ZnIn₂S₄ exhibit N-type semiconductor characteristics, a heterojunction formed by both can reduce the surface reaction energy barrier and use its synergy, strengthening the charge self-diffusion between the two semiconductors, it means the formation of a strong electric field. From the electron transfer path and band structure, NiCo₂S₄/ZnIn₂S₄ has S-scheme heterojunction characteristics. NiCo₂S₄ is a reducing photocatalyst (RP), and ZnIn₂S₄ is an oxidative photocatalyst (OP). Under the action of built-in electric field (BIEF), strong photogenic electrons and holes exist in CB of RP (NiCo₂S₄) and VB of OP (ZnIn₂S₄). Thus, the overall redox capacity of the NiCo₂S₄/ZnIn₂S₄ heterojunction is enhanced. Using visible light, the composite material can be used for photocatalytic hydrogen production. It is further shown that the composite material has a good effect in photocatalytic hydrogen production under the sensitizer eosin Y (EY) system. The optimal hydrogen production is about 221.75 μmol when the mass ratios of NiCo₂S₄/ZnIn₂S₄ is 20%, and the photocatalytic activity of the composite is about 47 times that of ZnIn₂S₄. Notably, the stability of the composites is the better. A reasonable photo-catalytic mechanism is proposed based on the band gap and photoelectrochemical properties of heterojunction.     

ZIF-L-derived porous C-doped ZnO/CdS graded nanorods with Z-scheme heterojunctions for enhanced photocatalytic hydrogen evolution

This study presents a novel approach for synthesizing C–ZnO/CdS graded nanorods derived from metal–organic frameworks (MOFs) that can be applied as a catalyst for photocatalytic hydrogen evolution from pure water. Porous C-doped ZnO was prepared by a self-template method using imidazole-like metal–organic backbone (ZIF-L) as a precursor through a two-step calcination method. CdS nanoparticles were deposited on ZIF-L surface by chemical deposition. The two-step calcination method introduced elemental C, and the unique architecture of ZIF-L played an essential role in forming the hierarchical structure of the porous ZnO nanorods. Compared with other ZnO/CdS catalysts, the C-doped ZnO/CdS graded nanorods exhibited remarkable photocatalytic activity for hydrogen production. The highest hydrogen production rate of 20.25 mmol g^?1 h^?1 with an apparent quantum yield (AQY) of 24.7% at 365 nm obtained over C–ZnO/CdS with Pt as co-catalyst, which was 24.4 and 65.3 times higher than that over CdS (0.83 mmol g^?1 h^?1) and ZnO (0.31 mmol g^?1 h^?1), respectively. This outcome was attributed to (i) the formation of Z-scheme heterojunction that significantly promoted the separation and migration of photogenerated electron–hole pairs; (ii) C doping that reduced the bandgap of ZnO and broadened its spectral response range; and (iii) the ordered arrangement of porous nanorods that effectively reduced the recombination rate of the electron–hole pairs.     

CuInS2-based photocatalysts for photocatalytic hydrogen evolution via water splitting

The realization of efficient photocatalytic hydrogen evolution (PHE) significantly depends on the development of durable and effective semiconductor photocatalysts. Copper indium sulfide (CuInS₂) is an emerging ternary chalcogenide semiconductor material for solar-to-chemical energy application, because it possesses a suitable bandgap, environment-friendly elements, and a low melting point. CuInS₂-based semiconductor photocatalysts have been investigated for PHE via water splitting, but current PHE performance still has difficulty in meeting commercial application requirements and needs to be further improved. In this review, the basic semiconductor properties of CuInS₂, including its crystal and band structures, are introduced, and its PHE mechanism is discussed in detail. The PHE performance of CuInS₂-based photocatalysts is systematically discussed, with a focus on morphology, engineered structure, and heterojunction construction. Finally, issues and challenges currently encountered in the PHE application of CuInS₂-based photocatalysts and their possible solutions are presented.     

Facile fabrication of mesoporous biochar/ZnFe2O4 composite with enhanced visible-light photocatalytic hydrogen evolution

A promising biochar/ZnFe₂O₄ (BZF) composite has been synthesized to improve the efficiency of visible-light-driven H₂ evolution via a simple microwave hydrothermal method. The materials were investigated through diverse characterization means including XRD, FTIR, SEM, BET, XPS, VSM, UV–vis/DRS, PL, EIS. Different ratios of BZF composites expressed enhanced photocatalytic H₂ evolution performance over pure ZnFe₂O₄. Especially, biochar/ZnFe₂O₄ catalysts with 5:1 mass ratio (BZF-5) attained the optimal H₂ evolution rate, which is around 6 times higher than that of pure ZnFe₂O₄. Biochar acts as an electron mediator can effectively promote the separation of electron-hole pairs to enhance the rate of photocatalytic hydrogen evolution. Moreover, Eosin Y, photocatalyst and TEOA have synergistic effects accounted for enhanced photocatalytic performance in reaction system. Three cyclic runs for the photocatalytic H₂ evolution on BZF-5 sample illustrated its good stability and sustainable reusability.     

A visible-light-driven heterojunction for enhanced photocatalytic water splitting over Ta2O5 modified g-C3N4 photocatalyst

The photocatalytic water splitting for generation of clean hydrogen energy has received increasingly attention in the field of photocatalysis. In this study, the Ta₂O₅/g-C₃N₄ heterojunctions were successfully fabricated via a simple one-step heating strategy. The photocatalytic activity of as-prepared photocatalysts were evaluated by water splitting for hydrogen evolution under visible-light irradiation (λ > 420 nm). Compared to the pristine g-C₃N₄, the obtained heterojunctions exhibited remarkably improved hydrogen production performance. It was found that the 7.5%TO/CN heterojunction presented the best photocatalytic hydrogen evolution efficiency, which was about 4.2 times higher than that of pure g-C₃N₄. Moreover, the 7.5%TO/CN sample also displayed excellent photochemical stability even after 20 h photocatalytic test. By further experimental study, the enhanced photocatalytic activity is mainly attributed to the significantly improve the interfacial charge separation in the heterojunction between g-C₃N₄ and Ta₂O₅. This work provides a facile approach to design g-C₃N₄-based photocatalyst and develops an efficient visible-light-driven heterojunction for application in solar energy conversion.     

Synthesis of MOF/MoS2 composite photocatalysts with enhanced photocatalytic performance for hydrogen evolution from water splitting

With the shortage of global fossil energy and the increasing crisis of environmental deterioration, hydrogen energy has become an environmentally benign alternative as a clean energy source. In most studies on photocatalytic hydrogen production, novel photocatalytic material has played an important role to enhance the hydrogen production rate. In this study, the optimal conditions for the synthesis of MoS₂ were established through series of characterizations with 190 °C calcination temperature and 1 wt% PEG surfactant addition. The best conditions for synthesizing MOF include copper nitrate as the copper precursor, 30% ultrasonic amplitude, and 240 °C calcination temperature. After adding 1 wt% MOF in MOS₂, a flower-like structure with small particle size, uniform distribution, regularity, and large surface pores, has been formed, where its unit is modified with many rough, porous, and high specific surface area octahedral structures. In addition, 1MOF/MOS₂ has the most negative conduction band edge (?0.135 V), the smallest charge transfer resistance (R_ct = 1.78 Ω), the largest photo current (11.1 mA/cm²), the lowest PL spectral peak intensity, and excellent photocatalytic stability. The above morphological features and optical properties can significantly form more active sites, enhance the electron transfer rate, and inhibit the electron-hole recombination, thus making the MOF/MOS₂ composite photocatalyst achieve the maximum hydrogen production capacity (626.3 μmol g^?1 h^?1).     

Designing of a novel Mn0.2Cd0.8S@ZnO heterostructure with Type-II charge transfer path for efficient photocatalytic hydrogen evolution reaction

The development of photocatalysts with efficient hydrogen evolution activity has been the goal for sustainable hydrogen production. In this work, heterojunction composite photocatalyst is formed by hydrothermal coupling of ZnO and Mn_0.2Cd_0.8S. Compared with pure ZnO and Mn_0.2Cd_0.8S, the composite photocatalyst has the ability to provide more abundant active sites and better photogenerated carriers separation efficiency. The optimized composite photocatalyst shows a 9.36-fold increase in hydrogen evolution activity (4297.99 μmol g^?1 h^?1) compared to Mn_0.2Cd_0.8S (459.31 μmol g^?1 h^?1) and exhibits excellent cycling stability. Density functional theory calculations identifies Type-II charge transfer path in the composite photocatalyst, achieving effective separation in space of photogenerated electrons from holes and suppressing recombination within the semiconductor. The results show that the construction of Type-II heterojunction in this work achieves a significant enhancement of the hydrogen evolution activity of the photocatalyst by constructing carrier transport channels at the contact interface of the heterojunction.     

Mn0.2Cd0.8S nanorods assembled with 0D CoWO4 nanoparticles formed p-n heterojunction for efficient photocatalytic hydrogen evolution

In this work, p-type semiconductor CoWO₄ nanoparticles was successfully anchored on the surface of n-type Mn_0.2Cd_0.8S nanorods, and p-n type heterostructure photocatalysts with low cost and high efficiency were synthesized. By optimizing the loading of CoWO₄ nanoparticles, the hydrogen evolution rate of the compound can reach a maximum, the peak is 408 μmol/5 h. Under visible light irradiation, it is equivalent to 3.61 times pure Mn_0.2Cd_0.8S. In addition, the composite catalyst has favorable durability and structural stability. In terms of morphology, the combination of nanorods (S_BET = 24 m²g^-1) and nanoparticles (S_BET = 12 m²g^-1) increased the specific surface area of the composite catalyst (S_BET = 31 m²g^-1), thus the composite exposed more active sites. In terms of heterojunction, the conduction bands of two semiconductors were determined by UV–vis diffuse reflection and Mott-Schottky, and the construction of p-n heterojunction was verified. The results of photochemical experiment further indicate that the built-in electric field in the p-n type heterostructure not only accelerates the electron-hole pair transfer, but vastly enhances the carrier average lifetime. In this paper, the morphology of the photocatalyst was modified and p-n heterojunction was constructed. The result is that the performance of Mn_0.2Cd_0.8S MCS was improved and the possible mechanism of photocatalytic hydrogen evolution was proposed.     

Direct Z-scheme ZnIn2S4/LaNiO3 nanohybrid with enhanced photocatalytic performance for H2 evolution

The direct Z-scheme ZnIn₂S₄/LaNiO₃ nanohybrid based on ZnIn₂S₄ nanosheets and LaNiO₃ cubes was synthesized by a facile hydrothermal method. The ZnIn₂S₄/LaNiO₃ nanohybrid showed improved photocatalytic H₂ evolution and stability. The photocatalytic H₂ evolution activity of ZnIn₂S₄/LaNiO₃ nanohybrid is 3-fold enhanced than that of bare ZnIn₂S₄. The enhanced performance of ZnIn₂S₄/LaNiO₃ nanohybrid is mainly ascribed to the formation of heterojunction between LaNiO₃ and ZnIn₂S₄. The heterojunction can facilitate charge transport on the interface between LaNiO₃ and ZnIn₂S₄ and suppress the recombination of photo-generated charge carriers over ZnIn₂S₄/LaNiO₃ nanohybrid, which were well demonstrated by photoelectrochemical tests. Moreover, the direct Z-scheme photocatalytic reaction mechanism was proposed to elucidate the improved performance of ZnIn₂S₄/LaNiO₃ nanohybrid photocatalyst. This study may provide some guidance on the construction of direct Z-scheme photocatalytic system for photocatalytic H₂ evolution.     

In situ W/O Co-doped hollow carbon nitride tubular structures with enhanced visible-light-driven photocatalytic performance for hydrogen evolution

Heteroatom co-doping has been considered as an effective strategy to simultaneously overcome intrinsic shortcomings of g-C₃N₄ to achieve enhanced photocatalytic properties, in which the involved dopants could play its role in altering electronic structure, optical absorption and charge separation of the catalyst. Herein, W/O co-doped hollow g-C₃N₄ tubular structures are successfully obtained for the first time via a one-step thermal decomposition. By W/O co-doping, architecture of g-C₃N₄ is able to be modulated with enhanced optical absorption towards visible region. In addition, narrowed band gap and restrained charge recombination are conducive for the excitation of electron-hole pairs and transportation. Photocatalytic water splitting tests indicate that the co-doped hollow tubular g-C₃N₄ structures enable superior activity for generating hydrogen up to 403.57 μmol g^?1 h^?1 driven by visible light, nearly 2.5 times as high as that of pristine g-C₃N₄. This work presents a rational strategy to design co-doped g-C₃N₄ as an efficient visible-light-driven photocatalyst.     

In situ growing Cu2(OH)2CO3 on oxidized carbon nitride with enhanced photocatalytic hydrogen evolution and pollutant degradation

A novel composite has been successfully synthesized in situ via a coprecipitation method about the coupling of Cu₂(OH)₂CO₃ with oxidized carbon nitride (O-g-C₃N₄) forming Cu₂(OH)₂CO₃/O-g-C₃N₄ (CuCN) heterojunction structure. The as-prepared composites were characterized by diverse means. The CuCN composite with 3:5 mass ratio of Cu₂(OH)₂CO₃ to O-g-C₃N₄ (60CuCN) presented an extremely excellent photocatalytic activity. The photocatalytic H₂ evolution of 60CuCN was around 23.26 and 44.62 times higher than that of g-C₃N₄ and Cu₂(OH)₂CO₃, respectively. The photocatalytic degradation malachite green (MG) rate of 60CuCN was up to 91%, which was around 2.2 and 4.8 times as much as that of g-C₃N₄ and Cu₂(OH)₂CO₃, respectively. These results are mainly attributed to the structure property of O-g-C₃N₄ and the heterojunction structure of the composite, which could effectively accelerate the separation and transfer rate of photogenerated electrons and holes. The holes (h⁺) and superoxide radicals (·O₂⁻) played a dominant role in photocatalytic degradation MG reaction.     

Promoted interfacial charge transfer by coral-like nickel diselenide for enhanced photocatalytic hydrogen evolution over carbon nitride nanosheet

Seeking an efficient and non-precious co-catalyst for g-C₃N₄ (CN) remains a great demanding to achieve high photocatalytic hydrogen generation performance. Herein, a composite photocatalyst with high efficiency was prepared by modifying CN with coral-like NiSe₂. The optimal hydrogen evolution rate of 643.16 μmol g^?1 h^?1 is from NiSe₂/CN-5 under visible light. Superior light absorption and interfacial charge transfer properties including suppressed photogenerated carrier recombination and efficient separation of photogenerated electron-hole pairs have been observed, which account for the enhanced photocatalytic performance of CN.     

Construction of 3D/3D heterojunction between new noble metal free ZnIn2S4 and non-inert metal NiMoO4 for enhanced hydrogen evolution performance under visible light

Promoting the separation of electron and hole plays an important role in photocatalytic hydrogen production. However, single semiconductor materials cannot fully realize their potential due to the rapid recombination of photogenerated carriers. Therefore, in this experiment, a new photocatalyst ZnIn₂S₄/NiMoO₄ was prepared by using an electrostatic self-assembly method, which greatly improved the electron-hole recombination phenomenon. After 5 h reaction under visible light irradiation, ZIS/NMO-3 composite catalyst prepared in ethanol showed the best photocatalytic activity, and the hydrogen evolution capacity reached 173.09 μmol. The hydrogen evolution capacity of ZIS/NMO-3 was 2.47 and 25.83 times that of short rod-like NiMoO₄ and microflower-like spherical ZnIn₂S₄, respectively. Through some physical characterization and electrochemical experiments, it can be seen that NiMoO₄ and ZnIn₂S₄ have good composability. Meanwhile, the composite catalyst ZnIn₂S₄/NiMoO₄-3 has high current response characteristics. It can be seen from the fluorescence emission spectra that the composite catalyst presents the lowest peak value, which indicates that ZIS/NMO-3 can effectively inhibit the recombination of photogenerated electrons and holes. When ZnIn₂S₄ is loaded on NiMoO₄, the separation of photogenerated carrier will be accelerated due to the formation of heterojunction, thus improving the photocatalytic activity. At the same time, the large specific surface area will also provide more abundant active sites for the composite catalyst, which provides a good condition for photocatalytic hydrogen production. This work provides an efficient, uncomplicated and feasible method for the synthesis of ZIS/NMO-3 composite catalyst with excellent properties.