Triple-Channel Charge Transfer over W18O49/Au/g-C3N4 Z-Scheme Photocatalysts for Achieving Broad-Spectrum Solar Hydrogen Production

Development of a novel Z-scheme CoxNi1-xTiO3/CdS (x = 0.5) photocatalyst for the efficient degradation of organic pollutants via a visible-light-driven photocatalytic process

Herein, for the first time, we reported the synthesis of a novel Z-scheme CoxNi1-xTiO3/CdS (x = 0.5) heterojunction photocatalyst and the investigation of its visible-light-driven photocatalytic performance toward degradation of methylene blue (MB). The developed photocatalyst was structurally characterized by applying X-Ray diffraction analysis (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET), differential reflectance spectroscopy (DRS), and photoluminescence (PL) techniques. The results indicated the formation of a highly porous structure with improved visible light adsorption capacity, favorable for the catalytic activity. At an optimum condition of 10 mg/L of MB and 300 mg/L of catalyst, the ternary photocatalyst demonstrated a MB removal efficiency of 99 % after 75 min of the treatment process. The radical trapping experiments unveiled that hydroxyl and superoxide radicals were two main reactive species formed under visible light, while the valance holes possessed an insignificant role. The synergetic impact of the CoxNi1-xTiO3 (x = 0.5) and CdS on the photodegradation of MB over the as-prepared CoxNi1-xTiO3/CdS (x = 0.5) photocatalyst through Z-scheme photocatalysis was indicated by the results of the mechanism studies. The percentage impact of the treatment time, MB concentration, the ratio of CoxNi1-xTiO3/CdS (x = 0.5), and the dosage of catalyst using analysis of the CCD modeling was obtained as 47.04, 16.67, 7.22 and 0.87 %, respectively. Furthermore, the as-synthesized photocatalyst possessed high recyclability and photostability with only a 3 % decline in activity after four repetitive cycles.

Visible-Light-Driven Inactivation of Bacteria and H2 Generation Catalyzed by Oxygen-Vacancy-Rich One-Dimensional/Two-Dimensional W18O49/g-C3N4 Z-Scheme Heterostructures

Z-scheme heterostructure-based photocatalysts consist of a reduction photocatalyst and an oxidation photocatalyst, enabling them to possess a high capacity for both reduction and oxidation. However, the coupling reaction between photocatalytic H2 generation through water reduction and sterilization using Z-scheme systems has been rarely reported. Herein, 1D W18O49 nanowires embedded over 2D g-C3N4 nanosheets are well-constructed as an integrated Z-scheme heterojunction. Experimental results and density functional theory calculations not only demonstrate the achievement of efficient interfacial charge separation and transport, leading to prolonged lifetime of photogenerated charge carriers, but also directly confirm the mechanism of Z-scheme charge transfer. As expected, the optimized W18O49/g-C3N4 nanostructure exhibits superior photocatalytic sterilization activity against Staphylococcus aureus as well as excellent H2 generation performance under visible-light irradiation (λ ≥ 420 nm). Due to its nontoxic nature, W18O49/g-C3N4 holds great potential in eradicating bacterial infections in living organisms.

Manipulation of interfacial charge dynamics for metal-organic frameworks toward advanced photocatalytic applications

Compared to other known materials, metal-organic frameworks (MOFs) have the highest surface area and the lowest densities; as a result, MOFs are advantageous in numerous technological applications, especially in the area of photocatalysis. Photocatalysis shows tantalizing potential to fulfill global energy demands, reduce greenhouse effects, and resolve environmental contamination problems. To exploit highly active photocatalysts, it is important to determine the fate of photoexcited charge carriers and identify the most decisive charge transfer pathway. Methods to modulate charge dynamics and manipulate carrier behaviors may pave a new avenue for the intelligent design of MOF-based photocatalysts for widespread applications. By summarizing the recent developments in the modulation of interfacial charge dynamics for MOF-based photocatalysts, this minireview can deliver inspiring insights to help researchers harness the merits of MOFs and create versatile photocatalytic systems.

Superior Interfacial Contact Yields Efficient Electron Transfer Rate and Enhanced Solar Photocatalytic Hydrogen Generation in M/C3N4 Schottky Junctions

Various literature studies (Table 6) have reported that dispersion of metal nanoparticles (NPs) on graphitic carbon nitride g-C3N4 (M/CN) has considerably improved the photocatalytic hydrogen yield. It is understood that metal NPs create active sites on the surface of CN and act as a cocatalyst. However, the precise changes induced by different metal NPs on the surface of CN still elude us. Here, we report a thorough understanding and comparison of the morphology, metal-support interactions, interfacial charge transfer kinetics, and band characteristics in different M/CN (M = Pt, Pd, Au, Ag, Cu) correlated with photocatalytic activity. Among all metals, Pt/CN was found to be the best performer both under sunlight and UV-visible irradiation. Under sunlight, maximum H2@ 2.7 mmol/h/g was observed over Pt/CN followed by Pd/CN > Au/CN > Ag/CN > Cu/CN ≈ CN. The present study revealed that among all metals, Pt formed superior interfacial contact with g-C3N4 as compared to other metals. The maximum Schottky barrier height (Φb,Pt) of 0.66 V was observed at Pt/CN followed by Φb,Au/CN (0.46 V) and Φb,Pd/CN (0.05 V). The presence of electron-deficient Pt in Pt-XPS, decrease in the intensity of d-DOS of Pt near the Fermi level in VB-XPS, increase in CB tail states, and cathodic shift in Vfb in MS plots sufficiently confirmed strong metal-support interactions in Pt/CN. Due to the SPR effect, Au and Ag NPs suffered from agglomeration and poor dispersion during photodeposition. Finely dispersed Pt NPs (2-4 nm, 53% dispersion) successfully competed with shallow/deep trap states and drove the photogenerated electrons to active metallic sites in a drastically reduced time period as investigated by femtosecond transient absorption spectroscopy. Typically, an interfacial electron transfer rate, KIET,avg, of 2.5 × 1010 s-1 was observed for Pt/CN, while 0.087 × 1010 s-1 was observed in Au/CN. Band alignment/potentials at M/CN Schottky junctions were derived and most favorable in Pt/CN with CB tail states much above the water reduction potential; however, in the case of Pd, these extend much below the H+/H2 potential and hence behave like deep trap states. Thus, in Pd/CN (τ0 = 4200 ps, 49%) and Ag/CN (3870 ps, 53%), electron deep trapping dominates over charge transfer to active sites. The present study will help in designing futuristic new cocatalyst-photocatalyst systems.

1D/1D W18O49/Cd0.9Zn0.1S S-scheme heterojunction with spatial charge separation for high-yield photocatalytic H2 evolution

Semiconductor photocatalytic water splitting is a green way to convert solar energy into chemical energy, but the recombination of electron and hole pairs and the low utilization of sunlight restrict the development of photocatalytic technology. By comparing the morphologies and hydrogen production properties of different proportions of solid solutions (Cd_xZn_1-xS), one-dimensional (1D) Cd_0.9Zn_0.1S nanorods (NRs) with the best photocatalytic properties are obtained. In addition, 1D W₁₈O₄₉ nanowires are assembled on the surface of 1D Cd_0.9Zn_0.1S NRs to construct a novel 1D/1D step-scheme (S-scheme) W₁₈O₄₉/Cd_0.9Zn_0.1S heterojunction photocatalyst. The W₁₈O₄₉/Cd_0.9Zn_0.1S heterojunction expands the optical absorption capacity of Cd_0.9Zn_0.1S NRs to provide more energy for the photoexcitation of electrons. The optimal hydrogen production rate of W₁₈O₄₉/Cd_0.9Zn_0.1S NRs with W₁₈O₄₉ content of 9 wt% is as high as 66.3 mmol·h^-1·g^-1, which is 5.7 times and 1.6 times higher than that of Cd_0.9Zn_0.1S NRs and 1 wt% Pt/Cd_0.9Zn_0.1S NRs. The apparent quantum efficiency (AQE) of 9 wt% W₁₈O₄₉/Cd_0.9Zn_0.1S reaches 56.0 % and 25.9 % under light wavelength irradiation at 370 and 456 nm, respectively. After the 20 h cycle stability test, the activity of photocatalytic hydrogen evolution does not decrease, due that the severe photo-corrosion of Cd_0.9Zn_0.1S NRs is efficiently inhibited. This work not only provides a simple and controllable synthesis method for the preparation of heterojunction structure, but also opens up a new way to improve the hydrogen evolution activity and stability of sulfur compounds.

g-C3N4-wrapped nickel doped zinc oxide/carbon core-double shell microspheres for high-performance photocatalytic hydrogen production

The development of efficient heterojunctions with enhanced photocatalytic properties is considered a promising approach for photocatalytic hydrogen production. In this study, graphitic carbon nitride (g-C₃N₄)-wrapped nickel-doped zinc oxide/carbon (Ni-ZnO@C/g-C₃N₄) core-double shell heterojunctions with unique core-double shell structures were employed as efficient photocatalysts through an innovative approach. Ni doping can enhance the intensity and range of visible light absorption in ZnO, and the carbon core coupled with the hollow double-shell structure can accelerate the charge transfer rate and improve the photon utilization efficiency. Meanwhile, the construction of the Z-scheme heterojunction extended the electron-hole pair transport path. In addition, the Z-scheme charge-transfer mechanism of Ni-ZnO@C/g-C₃N₄ under simulated sunlight was verified by photoluminescence (PL) and electron spin resonance (ESR) experiments. As a result, the obtained photocatalyst acquired a high hydrogen evolution rate of 336.08 μmol g^-1h^-1, which is 36.49 times higher than that of pristine ZnO. Overall, this work may provide a pathway for the construction of highly efficient photocatalysts with unique core-double shell structures.

The Advanced Synthesis of MOFs-Based Materials in Photocatalytic HER in Recent Three Years

Since the advent of metal–organic frameworks (MOFs), researchers have paid extensive attention to MOFs due to their determined structural composition, controllable pore size, and diverse physical and chemical properties. Photocatalysis, as a significant application of MOFs catalysts, has developed rapidly in recent years and become a research hotspot continuously. Various methods and approaches to construct and modify MOFs and their derivatives can not only affect the structure and morphology, but also largely determine their properties. Herein, we summarize the advanced synthesis of MOFs-based materials in the field of the photocatalytic decomposition of water to produce hydrogen in the recent three years. The main contents include the overview of the novel synthesis strategies in four aspects: internal modification and structure optimization of MOFs materials, MOFs/semiconductor composites, MOFs/COFs-based hybrids, and MOFs-derived materials. In addition, the problems and challenges faced in this direction and the future development goals were also discussed. We hope this review will help deepen the reader’s understanding and promote continued high-quality development in this field.

Construction and excellent photoelectric synergistic anticorrosion performance of Z-scheme carbon nitride/tungsten oxide heterojunctions

The use of heterojunctions for metal corrosion protection is a highly innovative and challenging task. Based on the composition and structure of tungsten oxide-based heterojunctions, Z-scheme heterojunctions were designed and synthesized by the electrostatic self-assembly method using energy band-matched g-C₃N₄ and WO₃ materials. Applied in the field of anticorrosion, they overcame the problems of poor reduction ability and transmission inefficiency of traditional materials. The Z-scheme heterojunctions ensured unidirectional electron transfer, while the aggregation of the retained strongly reduced electrons on the surface of the iron substrates provided a strong driving force for retarding corrosion occurrence. Meanwhile, the inherent shielding properties of the two-dimensional material g-C₃N₄ and the confinement of chloride ions as an electroactive layer hindered the penetration of the corrosive solution. After being corroded for 72 h, the corrosion impedance of the g-C₃N₄/WO₃ heterojunction system was improved by 640.11% compared with the epoxy resin coating. In addition, the g-C₃N₄/W₁₈O₄₉ heterojunction was synthesized by using mixed valence tungsten oxide, which overcame the problems of photogenerated electron yield and lifetime, and enhanced the anticorrosion performance compared with a single g-C₃N₄ phase. This research provided ideas for designing efficient and environmentally friendly heterojunction anticorrosion materials.

One-pot in-situ hydrothermal synthesis of ternary In2S3/Nb2O5/Nb2C Schottky/S-scheme integrated heterojunction for efficient photocatalytic hydrogen production

MXene-derived photocatalysts continue to fascinate the research community in developing photo-driven green and sustainable fuel production. However, the efficiency of MXene-derived photocatalyst is still low due to the wide bandgap and high recombination rate of photo-excited charge carriers. Here, we have synthesized the Nb₂C MXene-derived ternary photocatalyst via one-pot in-situ hydrothermal method for photocatalytic hydrogen (H₂) evolution. The partial oxidation of Nb₂C MXene into Nb₂O₅ nanorods and coupling with In₂S₃ nanoparticles via in-situ chemical anchoring were the key factors toward high efficiency and long-term stability during photocatalytic H₂ evolution. The optimized ternary photocatalyst composite manifested the highest H₂ evolution efficiency at 68.8 µmol g^-1 h^-1, which was 11 and 7.5 times higher than the Nb₂O₅/Nb₂C (NNC) and pure In₂S₃ photocatalyst, respectively. Moreover, the photocatalytic stability of the optimized ternary photocatalyst composite was analyzed for five consecutive cycles, and above 87% activity retention was observed even after the fifth cycle without any obvious decline. The separation efficiency of photoexcited charge carriers could be attributed to the synergic effect of the In₂S₃/Nb₂O₅ heterojunction and the redox reactions at different sites of the composite. More importantly, the participation of Schottky junction and S-scheme heterojunction charge transfer for the obtained novel ternary photocatalyst was evaluated via ultraviolet photoelectron spectroscopy (UPS) and electron paramagnetic resonance (EPR). This research will provide additional insight into the extended potential of MXene-derived ternary photocatalysts towards efficient H₂ production to meet future global energy demands.

Bifunctional 2D/2D g-C3N4/BiO2-x nanosheets heterojunction for bacterial disinfection mechanisms under visible and near-infrared light irradiation

The efficient deployment of visible and near-infrared (NIR) light for photocatalytic disinfection is of great concern a matter. Herein, we report a specific bifunctional 2D/2D g-C₃N₄/BiO_2-x nanosheets heterojunction, prepared through a self-assembly approach. Delightfully, the obtained 2D/2D heterojunctions exhibited satisfactory photocatalytic disinfection performance towards Escherichia coli K-12 (E. coli K-12) under visible light irradiation, which was credited to the Z-scheme interfacial heterojunction facilitating the migration of photogenerated carries. The photoactivity enhancement driven by NIR light illumination was ascribed to the cooperative synergy effect of photothermal effect and "hot electrons", engineering efficient charge transfer. Intriguingly, the carboxyl groups emerged on g-C₃N₄ nanosheets contributed a vital role in establishing the enhanced photocatalytic reaction. Moreover, the disinfection mechanism was systematically described. The cell membrane was destroyed, evidenced by the generation of lipid peroxidation reaction and loss of energy metabolism. Subsequently, the damage of defense enzymes and release of intracellular constituents announced the irreversible death of E. coli K-12. Interestingly enough, considerable microbial community shifts of surface water were observed after visible and NIR light exposure, highlighting the critical feature of disinfection process in shaping microbial communities. The authors believe that this work gives a fresh light on the feasibility of heterostructures-enabled disinfection processes.

Gold@Carbon Nitride Yolk and Core-Shell Nanohybrids

Graphitic carbon nitride (g-C₃N₄) is a promising conjugated polymer with visible light responsiveness and numerous intriguing characteristics that make it highly beneficial for a myriad of potential applications. A novel design and universal approach for the fabrication of unique plasmonic g-C₃N₄ nanoscale hybrids, with well-controlled morphology, is presented. A single gold nanoprism is encapsulated within dense or hollow g-C₃N₄ spheres for the formation of Au@g-C₃N₄ core-shell and Au@g-C₃N₄ yolk-shell nanohybrids. Au nanoprisms were chosen duo to the strong (visible range) plasmon resonances and electromagnetic field hotspots formed at their sharp corners. The incorporation of Au nanoprisms into the g-C₃N₄ nanospheres results in a dramatic ∼threefold rise in the emission of plasmonic g-C₃N₄ yolk-shell nanohybrids and ∼3.6-fold enhancement of the photocurrent density obtained from the plasmonic g-C₃N₄ core-shell nanohybrids, when compared with the g-C₃N₄ hollow nanospheres. Hence, these hybrids can potentially benefit applications in the areas spanning from solar energy harvesting to biomedical imaging and theranostics.