CoCO3 hierarchical structure embedded on g-C3N4 nanosheets to assemble 3D/2D Z-scheme heterojunction towards efficiently and stably photocatalytic hydrogen production

Structure and interface control of heterojunction is usually a challenging issue to improve the photocatalytic performance. Herein, a new 3D/2D CoCO₃/g-C₃N₄ heterojunction is assembled by embedding hexahedral CoCO₃ on g-C₃N₄ nanosheets. The unique step-like hierarchical structure of CoCO₃, the formed built-in electric field and Z-scheme charge transfer behavior at the interface jointly drive the high-efficient separation of photogenerated carriers to boost the photocatalytic H₂ production. It achieves the optimal H₂ production rate that is almost 2.6 times than g-C₃N₄, apparent quantum efficiency (AQE) of 10.1% at 400 nm and continuous running of 60 h over the 3D/2D CoCO₃/g-C₃N₄ heterojunction. This work endows a fresh structural control strategy for the fabrication of 3D/2D Z-scheme heterojunction to improve the photocatalytic H₂ production performance.     

PtS2/g-C3N4 van der Waals heterostructure: A direct Z-scheme photocatalyst with high optical absorption,solar-to-hydrogen efficiency and catalytic activity

Using two-dimensional semiconductors to build heterojunction as photocatalyst for water splitting is an important green and clean energy technology and has wide development prospects. Here, the monolayered PtS₂ and g-C₃N₄ are used to build the direct Z-scheme van der Waals (vdW) heterostructure, and the structure, electrical, Bader charge, optical properties and solar-to-hydrogen efficiency are calculated in detail through first-principle calculations. The direct Z-scheme PtS₂/g-C₃N₄ vdW heterostructure has an inherent type-II band alignment that enables it to reduce the photogenerated carriers aggregation, and it also possesses a decent band edge position to fully induce the redox reactions of decomposed water. The charge density shows that PtS₂ monolayer is negatively charged while g-C₃N₄ monolayer is positively charged, and the interface potential drop of PtS₂/g-C₃N₄ vdW heterostructure forms a built-in electric field with the direction from g-C₃N₄ to PtS₂. The PtS₂/g-C₃N₄ vdW heterostructure has suitable optical property, outstanding solar-to-hydrogen efficiency, high catalytic activity and thus a promising application prospect for photocatalytic water splitting.     

Construction of g-C3N4/BCN two-dimensional heterojunction photoanode for enhanced photoelectrochemical water splitting

Two-dimensional heterojunction g-C₃N₄/BCN was constructed via thermal polymerization process. The formed two-dimensional heterostructure could enhance the interfacial contact area between BCN and porous g-C₃N₄ as well as shorten the photogenerated charge carriers transfer time and distance. The two-dimensional g-C₃N₄/BCN heterojunction photoanode shows enhanced photoelectrochemical (PEC) performance for water splitting under visible-light irradiation, which primarily originates from the improved charge transfer and separation, and prolonged lifetime of electrons. Under the visible light irradiation, the g-C₃N₄/BCN heterojunction sample yields a photocurrent density of ∼0.62 mA cm⁻² at 1.23 V vs. RHE, which is about eight times as many as that of CN (0.08 mA cm⁻²) electrode at the same conditions. In addition, the possible electron transfer model and mechanism of PEC water splitting for H₂ evolution have been discussed.     

Electron transfer via a carbon channel for efficient Z-scheme solar hydrogen production

Z-scheme photocatalysis provides a promising solution to photocatalytic solar water splitting, yet restricted by inferior interfacial charge transfer. Here, we demonstrate a Z-scheme composite photocatalyst made of Fe₂O₃, a carbon layer, and g-C₃N₄ that can achieve efficient hydrogen generation from solar water decomposition. The success relies on in-situ preparation of core-shell Fe₂O₃@C structure at the surface of g-C₃N₄. Carbon as an intermediate layer thus acts as a bridge that significantly accelerates the migration of photogenerated electrons from Fe₂O₃ conduction band to g-C₃N₄ valence band. As a result, the highest rate of H₂ generation reaches 5.26 mmol h⁻¹g⁻¹. This activity is approximately 33-time greater than that achieved over pristine g-C₃N₄ and about 4-time larger than that obtained over a Fe₂O₃/g-C₃N₄ heterojunction without internal carbon layer. This work explicates the potential insight of the composite and paves a promising way to engineer the charge transfer behavior.     

Direct Z-scheme CoS/g-C3N4 heterojunction with NiS co-catalyst for efficient photocatalytic hydrogen generation

Facilitating the separation of photoexcited electron-hole pairs and enhancing the migration of photogenerated carriers are essential in photocatalytic reaction. CoS/g-C₃N₄/NiS ternary photocatalyst was prepared by hydrothermal and physical stirring methods. The optimized ternary composite achieved a hydrogen yield of 1.93 mmol g^?1 h^?1, 12.8 times that of bare g-C₃N₄, with an AQE of 16.4% at 420 nm. The enhanced photocatalytic activity of CoS/g-C₃N₄/NiS was mainly ascribed to the synergistic interaction between the Z-scheme heterojunction constructed by CoS and g-C₃N₄ and the NiS co-catalyst featuring a large amount of hydrogen precipitation sites, which realized the efficient separation and migration of photogenerated carriers. In addition, the CoS/g-C₃N₄/NiS heterojunction-co-catalyst system exhibited excellent photocatalytic stability and recyclability.     

Z-scheme MoO3-2D SnS nanosheets heterojunction assisted g-C3N4 composite for enhanced photocatalytic hydrogen evolutions

In this work, the 2D SnS/g-C₃N₄ nanosheets have been successfully prepared by a facile ultrasonic and microwave heating approach, which formed intimate interfacial contact and suitable energy band structure. The optimized sample displayed enhanced photocatalytic hydrogen evolution from water assisted with Pt co-catalyst, which is much higher than that of pure g-C₃N₄. After loaded with MoO₃ particles, the stability of photocatalysts displayed significate improvement due to the formed Z-scheme heterojunction. With the characterization, the enhanced hydrogen evolution reaction (HER) performance might be ascribed to the improved light-harvesting capability of the composite, lowered charge-transfer resistance, increased electrical conductivity and the co-catalyst effect of SnS. This study provides insights about SnS assisted HER photocatalysts and a new strategy to improve the stability of metal sulfides photocatalysts.     

1D/2D carbon-doped nanowire/ultra-thin nanosheet g-C3N4 isotype heterojunction for effective and durable photocatalytic H2 evolution

It is still challenging to design effective g-C₃N₄ photocatalysts with high separation efficiency of photo-generated charges and strong visible light absorption. Herein, a simple, template-free and “bottom-up” strategy has been developed to prepare 1D/2D g-C₃N₄ isotype heterojunction composed of carbon-doped nanowires and ultra-thin nanosheets. The ethanediamine (EE) grafted on melamine ensures the growth of 1D g-C₃N₄ nanowires with high carbon doping, and the ultra-thin g-C₃N₄ nanosheets were produced through HCl-assisted hydrothermal strategy. The apparent grain boundary between 2D nanosheets and 1D carbon-doped nanowires manifested the formation of the isotype heterojunction. The built-in electric field provide strong driving force for photogenerated carriers separation. Meanwhile, the doping carbon in g-C₃N₄ nanowires promotes visible light absorption. As a result, the photocatalytic H₂ evolution activity of 1D/2D g-C₃N₄ isotype heterojunction is 8.2 time that of the pristine g-C₃N₄, and an excellent stability is also obtained. This work provides a promising strategy to construct isotype heterojunction with different morphologies for effective photocatalytic H₂ evolution.     

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.     

ZnCr LDH nanosheets modified graphitic carbon nitride for enhanced photocatalytic hydrogen production

ZnCr layered double hydroxides (ZnCr LDH) nanosheets modified graphitic carbon nitride (g-C₃N₄) nanohybrids were fabricated via a self-assembly procedure through electrostatic interaction between these two components. Such 2D-2D inorganic-organic hybrid material was employed for photocatalytic hydrogen production under visible light for the first time. The physical and photophysical properties of the hybrid nanocomposites were investigated to reveal the effect of ZnCr LDH nanosheets on the photocatalytic activities of g-C₃N₄. It was found that 1 wt% ZnCr LDH nanosheets modified g-C₃N₄ was optimal for the formation of intimate interfacial contact. The visible light photocatalytic H₂ production activity over g-C₃N₄ was enhanced about 2.8 times after ZnCr LDH nanosheets modification. The significant enhancement in photocatalytic performance for ZnCr LDH/g-C₃N₄ heterojunction should be attributed to the promoted charge transfer and separation efficiency, resulting from the intimate interfacial contact and Type II band alignment between ZnCr LDH and g-C₃N₄.     

Two dimensional N-doped ZnO-graphitic carbon nitride nanosheets heterojunctions with enhanced photocatalytic hydrogen evolution

The effective separation of photogenerated charge carriers, their transport and interfacial contact is of great significance for excellent performance of semiconductor based photocatalysts. Herein, we report the fabrication of two dimensional (2D) nanosheets heterojunction comprising of N-doped ZnO nanosheets loaded over graphitic carbon nitride (g-C₃N₄) nanosheets for enhanced photocatalytic hydrogen evolution. The prepared 2D-2D heterojunctions with varying amount of g-C₃N₄ nanosheets have been characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS) techniques. The optimized heterojunction photocatalyst with 30 wt% of g-C₃N₄ nanosheets (NZCN30) exhibit hydrogen evolution rate of 18836 μmol h^?1 g_cat^?1 in presence of Na₂S and Na₂SO₃ as sacrificial agents under simulated solar light irradiation. The enhanced photocatalytic performance of NZCN30 heterojunction has been supported well by photoluminescence and photoelectrochemical investigations, which shows the minimum recombination rate and high photoinduced current density, respectively. In addition, the existence of 2D-2D interfacial contact plays a major role in enhanced H₂ evolution by high face-to-face contact surface area for separation of photogenerated charge carriers in space which facilitate their transfer for H₂ generation. This work paves way for the development of 2D-2D heterojunctions for diverse applications.     

Preparation,structure and application of g-C3N4/BiOX composite photocatalyst

Graphite-like carbon nitride (g-C₃N₄) has attracted great attention for pollutant degradation and clean energy production. The heterojunctions of bismuth halide (BiOX, X = Cl, Br, I) and g-C₃N₄ are proposed to overcome the shortcomings of the g-C₃N₄ photocatalyst, such as low charge separation rate and high charge recombination rate. This review paper systematically discusses the progress in synthesis, structure, and applications of heterojunction photocatalytic composites made of g-C₃N₄ and BiOX based on the understanding of their photocatalytic reaction mechanism. We clarify and summarize structural mechanisms of a single and compound semiconductor to reveal the factors that affect photocatalytic performances. We discuss and compare advantages and disadvantages of versatile preparation processes. Particularly, we focus on the understanding of the structure and characteristics of type II, Z-type, n-n, and p-n heterojunctions and their applications, specifically in pollutant degradation, H₂ production by water splitting, CO₂ reduction, and medical sterilization. The future prospects of g-C₃N₄/BiOX composites are also discussed from aspects of their preparation, application, and research methods. This work may offer a good avenue and data reference to develop novel g-C₃N₄ photocatalytic materials to meet the ever-increasing environmental pollution and energy shortage.     

Enhanced optical absorption and photocatalytic water splitting of g-C3N4/TiO2 heterostructure through C&B codoping: A hybrid DFT study

Our theoretical research indicate that the electric field are generated in the direction of (C doped) TiO₂ (101) surface to (B-doped) g-C₃N₄ monolayer for the pristine, C and B doped g-C₃N₄/TiO₂, and higher band-edge potential on the (C doped) TiO₂ (101) surface are observed compared to (B-doped) g-C₃N₄ monolayer. Thus, the pristine (2.591 eV), C-doped (2.663 eV) and B-doped (2.339 eV) g-C₃N₄/TiO₂ are Z-scheme heterostructures, which promotes charge separation and retains a prominent redox ability. After C doping, the C 2p energy level is introduced which facilitate the separation of photoexcited carriers. The B-doped g-C₃N₄/TiO₂ has a reduced bandgap and the mixing of B 2p and N 2p energy levels, promoting the red-shift of the optical absorption edge. The C&B codoped g-C₃N₄/TiO₂ follows type-II charge transfer mode because of their synergistic effect in C and B atoms, which changes the direction of the built-in electric field. It also has a narrow bandgap (1.309 eV) and effectively separate electron-hole pairs leading to strong optical absorption ability in the range of 360 nm–460 nm. The band-edges matching of the semiconductor photocatalyst and the direction of the built-in electric field jointly determine whether the charges are selected to be Z-scheme or II-type transfer mode. Based on g-C₃N₄/TiO₂ for C or/and B (co)doping, their different charge transfer modes have been established and they are expected to show promising photocatalytic water splitting performance.     

An efficient Z-scheme (Cr,B) codoped g-C3N4/BiVO4 photocatalyst for water splitting: A hybrid DFT study

A systematic theoretical research on the geometrical, electronic, optical, charge transfer, and photocatalytic mechanisms of pure, Cr-doped, B-doped, and (Cr, B) codoped g-C₃N₄/BiVO₄ heterostructures using a hybrid density functional approach has been carried out. The face-to-face g-C₃N₄/BiVO₄ composed of two-dimensional materials of g-C₃N₄ and BiVO₄ (010) surface, can introduce a built-in electric field, which promotes interface charge transfer and prevents the electron-hole pair recombination, and causing g-C₃N₄ monolayer with negative charge and BiVO₄ (010) surface with positive charge. Under visible light irradiation, electrons are excited to the conduction band minimum (CBM) of the BiVO₄ (010) surface undergoing the hydrogen evolution reaction (HER), while the holes remain in the valence band maximum (VBM) of g-C₃N₄ monolayer aiding the oxygen evolution reaction (OER). The band edge potentials of BiVO₄ (010) surface is higher than that of g-C₃N₄ monolayer, which ensures a stronger redox reaction potential and therefore belongs to a typical Z-scheme heterostructure. In addition, the Cr or/and B (co)doping introduces the Cr-3d or/and B-2p states to reduce the bandgap and generate impurity levels, thus enhancing solar energy utilization rate and expanding the optical absorption in the visible-light range. The optical absorption intensity of the (Cr, B) codoped g-C₃N₄/BiVO₄ is superior to pure and Cr or B doped g-C₃N₄/BiVO₄, confiriming the synergistic effect of Cr-3d and B-2p states. Thus, this research is helpful to design a novel and potential Z-scheme photocatalyst useful for the photocatalytic water splitting.     

Microbial coupled photocatalytic fuel cell with a double Z-scheme g-C3N4/ZnO/Bi4O5Br2 cathode for the degradation of different organic pollutants

A novel heterostructure of g-C₃N₄/ZnO/Bi₄O₅Br₂ (ZB-3) was designed, and used in the microbial coupled photocatalytic fuel cell (MPFC). It can effectively improve electron utilization efficiency and pollutant degradation using this double Z-scheme heterojunction structure. The current–time (I–t) curves demonstrated that the current density of ZB-3 was higher than that of ZnO, ZnO/Bi₄O₅Br₂ (ZB-1), g-C₃N₄/ZnO (ZB-2). Electrochemical impedance spectroscopy (EIS) indicated ZB-3 possessed the minimum charge-transfer resistance. This MPFC for degrading rhodamine B (RhB) and tetracycline (TC) under different conditions were developed using these materials. Even in the dark condition, MPFC with g-C₃N₄/ZnO/Bi₄O₅Br₂ demonstrated 93% and 82% degradation efficiency for RhB and TC, respectively. Furthermore, the electron transport mechanism of the MPFC and ZB-3 were proposed. It paves the approach for more efficient pollutant degradation via MFC photocatalysis.     

Photoelectrochemical study on charge separation mechanisms of Bi2WO6 quantum dots decorated g-C3N4

In this paper, the Bi₂WO₆ quantum dots (QDs) decorated g-C₃N₄ nanoplates were successfully synthesized via a one-step hydrothermal method. The morphology of the Bi₂WO₆ could be tuned from regular nanoplates to quantum dots. Remarkably, the Bi₂WO₆ QDs coupled with g-C₃N₄ not only prevented the aggregation, but also decreased the size of Bi₂WO₆ QDs about 3.5 nm. Meanwhile, the charge separation mechanisms of Bi₂WO₆ QDs/g-C₃N₄ photocatalyst were investigated by electrochemical impedance spectra, Mott-Schottky and linear voltammetry scans. As a result, the photoelectrochemical (PEC) experiments provided forceful evidence for the charge separation mechanism of the Bi₂WO₆ QDs/g-C₃N₄ Z-scheme. The Z-scheme system not only accelerated the separation efficiency of charge, but also improved the ability of PEC water splitting at measured 1.23 V vs. RHE.     

Synthesis of g-C3N4/WO3-carbon microsphere composites for photocatalytic hydrogen production

In this paper, a g-C₃N₄/WO₃-carbon microsphere composite-based photocatalyst was successfully prepared by a one-pot thermal synthesis method for sunlight driven decomposition of water to produce hydrogen. The results show that the g-C₃N₄/WO₃-carbon microspheres had better photocatalytic properties and stability. Under visible light and sunlight irradiation, the hydrogen production efficiency of the photocatalytic decomposition of water was 107.75 times and 70.54 times greater than that of pure g-C₃N₄, respectively. The experimental and characterization results show that g-C₃N₄ and WO₃ formed a Z-scheme heterojunction on the surface of the g-C₃N₄/WO₃-carbon microsphere composite-based photocatalyst. Carbon microspheres modified on g-C₃N₄ nanosheets and WO₃ had good conductivity and promoted the transfer of photogenerated electrons in g-C₃N₄ nanosheets. The addition of carbon microspheres increased the specific surface area of the composite photocatalyst. The g-C₃N₄/WO₃-carbon microsphere composite-based photocatalyst showed strong adaptability to the fluctuating light intensity, providing feasibility for industrialized mass production.     

A novel CoSeO3 photocatalyst assisting g-C3N4 in enhancing hydrogen evolution through Z scheme mode

A novel CoSeO₃/g-C₃N₄ composite photocatalyst with Z-scheme heterostructure is constructed through electrostatic self-assembly to be utilized in photocatalytic hydrogen evolution. The optimal photocatalytic H₂ evolution rate of CoSeO₃/g-C₃N₄ hybrids and apparent quantum yield (AQY) have raised about 65.4 times under full light irradiation with no noble metal cocatalyst loading than that of pure g-C₃N₄. The CoSeO₃ semiconductor is firstly prepared for assisting to elevate the photocatalytic hydrogen evolution activity. After combining with g-C₃N₄, CoSeO₃/g-C₃N₄ hybrids with a sheet-sheet structure enhance the contact area with water and broaden the light absorption region as well as reduce transfer resistance of carriers. Moreover, the photo generated carriers possess a typical direct Z scheme transmission, which decreases the recombination of electrons and holes. This work offers a new choice for constructing a Z scheme heterostructure to apply in photocatalytic water reduction, and offers a deep view to explain the elevated photocatalytic activity.     

Heterostructured boron doped nanodiamonds@g-C3N4 nanocomposites with enhanced photocatalytic capability under visible light irradiation

Boron doped nanodiamonds (BDND) were coupled with graphitic carbon nitride (g-C₃N₄) nanosheets to form a heterojunction via a facile pyrolysis approach. The BDND@g-C₃N₄ heterojunction exhibits enhanced visible-light absorbance, improved charge generation/separation efficiency and prolonged lifetime of carriers, which lead to the enhanced photocatalytic activities for the hydrogen evolution and organic pollution under visible-light irradiation. The optimal H₂ evolution rate and apparent quantum efficiency at 420 nm of the BDND@g-C₃N₄ heterojunction is 96.3 μmol h⁻¹ and 6.91%, which is about 5 and 2 times higher than those of pristine g-C₃N₄ nanosheets (18.2 μmol h⁻¹ and 3.92%). No obvious decrease in hydrogen generation rate is observed in the recycling experiment due to the high photo-stabilization of the BDND@g-C₃N₄ composite. The degradation kinetic rate constant of organic pollution of the BDND@g-C₃N₄ structure is 0.1075 min⁻¹, which is 3 times higher compared to pristine g-C₃N₄. This work may provide a promising route to construct highly efficient non-metal photocatalysts for hydrogen evolution and organic pollution degradation under visible light irradiation.     

Redox couple mediated charge carrier separation in g-C3N4/CuO photocatalyst for enhanced photocatalytic H2 production

Graphitic carbon nitride (g-C₃N₄) is one of the promising two-dimensional metal-free photocatalysts for solar water splitting. Regrettably, the fast electron-hole pair recombination of g-C₃N₄ reduces their photocatalytic water splitting efficiency. In this work, we have synthesized the CuO/g-C₃N₄ heterojunction via wet impregnation followed by a calcination method for photocatalytic H₂ production. The formation of CuO/g-C₃N₄ heterojunction was confirmed by XRD, UV–vis and PL studies. Notably, the formation of heterojunction not only improved the optical absorption towards visible region and also enhanced the carrier generation and separation as confirmed by PL and photocurrent studies. The photocatalytic H₂ production results revealed that CuO/g-C₃N₄ photocatalyst demonstrated the increased photocatalytic H₂ production rate than bare g-C₃N₄. The maximum H₂ production rate was obtained with 4 wt % CuO loaded g-C₃N₄ photocatalyst. Importantly, the rate of H₂ production was further improved by introducing simple redox couple Co²⁺/Co³⁺. Addition of Co²⁺ during photocatalytic H₂ production shuttled the photogenerated holes by a reversible conversion of Co²⁺ to Co³⁺ with accomplishing water oxidation. The effective shuttling of photogenerated holes decreased the election-hole pair recombination and thereby enhancing the photocatalytic H₂ production rate. It is worth to mention that the addition of Co²⁺ with 4 wt % CuO/g-C₃N₄ photocatalyst showed ∼7.5 and ∼2.0 folds enhanced photocatalytic H₂ production rate than bare g-C₃N₄/Co²⁺ and CuO/g-C₃N₄ photocatalysts. Thus, we strongly believe that the present simple redox couple mediated charge carrier separation without using noble metals may provide a new idea to reduce the recombination rate.     

g-C3N4/B doped g-C3N4 quantum dots heterojunction photocatalysts for hydrogen evolution under visible light

Developing high activity and eco-friendly photocatalysts for water splitting is still a challenge in solar energy conversion. In this paper, B doped g-C₃N₄ quantum dots (BCNQDs) were prepared via a facile molten salt method using melamine and boron oxide as precursors. By introducing BCNQDs onto the surface of g-C₃N₄, g-C₃N₄/BCNQDs heterojunction was constructed via hydrothermal treatment. The resulting g-C₃N₄/BCNQDs heterojunction exhibits enhanced hydrogen evolution performance for water splitting under visible light irradiation. The mechanism underlying the improved photocatalytic activity was explored and discussed based on the formation of heterojunction between g-C₃N₄ and BCNQDs with well-matched band structure.