Architecture and Kinetic Studies of Photocatalytic H2O2 Generation and H2 Evolution through Regulation of Spatial Charge Transfer via Z-Scheme Path over a (001) Facet Engineered TiO2@MXene/B-g-C3N4 Ternary Hybrid

A review on recent advances in g-C3N4-MXene nanocomposites for photocatalytic applications

The solutions for environmental remediation and renewable energy generation have intensified the exploration of efficient photocatalytic materials. Recently, the composites of g-C3N4and MXene have gained considerable interest for their potential applications in photocatalysis. In the g-C3N4-MXene composite, the g-C3N4possesses unique physical, chemical, and optical properties to increase visible light absorption. At the same time, MXene improves conductivity, adsorption of reactant molecules or the active sites, and charge transfer properties. Combining the unique physico-chemical properties of MXene and g-C3N4, the resulting composite exhibits superior photo-responsive behavior and is critical in photocatalytic reactions. Furthermore, the g-C3N4-MXene composite exhibits stability and recyclability, making it a promising candidate for sustainable and scalable photocatalytic material in environmental remediation. This review offers an in-depth analysis of the development and design of g-C3N4-MXene composites through diverse synthesis procedures and a comprehensive analysis of their application in carbon dioxide (CO2) reduction, photocatalytic degradation, water splitting processes, mainly hydrogen (H2) generation, H2O2production, N2fixation, and NOxremoval. The charge transfer mechanism of g-C3N4-MXene composite for photocatalytic application has also been discussed. This review provides insights into the photocatalytic capabilities of g-C3N4-MXene composites, showing their potential to address current environmental challenges and establish a robust foundation for sustainable energy conversion technologies.

A Schottky/Z-Scheme Hybrid for Augmented Photocatalytic H2 and H2O2 Production

The prodigious employment of fossil fuels to conquer the global energy demand is becoming a dreadful threat to the human society. This predicament is appealing for a potent photocatalyst that can generate alternate energy sources via solar to chemical energy conversion. With this interest, we have fabricated a ternary heterostructure of Ti3C2 nanosheet modified g-C3N4/Bi2O3 (MCNRBO) Z-scheme photocatalyst through self-assembly process. The morphological analysis clearly evidenced the close interfacial interaction between g-C3N4 nanorod, Bi2O3 and Ti3C2 nanosheets. The oxygen vacancy created on Bi2O3 surface, as suggested by XPS and EPR analysis, supported the Z-scheme heterojunction formation between g-C3N4 nanorod and Bi2O3 nanosheets. The collaborative effect of Z-scheme and Schottky junction significantly reduced charge transfer resistance promoting separation efficiency of excitons as indicated from PL and EIS analysis. The potential of MCNRBO towards photocatalytic application was investigated by H2O2 and H2 evolution reaction. A superior photocatalytic H2O2 and H2 production rate for MCNRBO is observed, which are respectively around 5 and 18 folds higher as compared to pristine CNR nanorod. The present work encourages for the development of a noble, eco-benign and immensely efficient dual heterojunction based photocatalyst, which can acts as saviour of human society from energy crisis.

Construction of a BiOCl/Bi2O2CO3 S-Scheme Heterojunction Photocatalyst via Sharing [Bi2O2]2+ Slabs with Enhanced Photocatalytic H2O2 Production Performance

The construction of a close contact interface is key to enhancing the photocatalytic activity in heterojunctions. In the work, the BiOCl/Bi2O2CO3 of sharing [Bi2O2]2+ slabs S-scheme heterojunction was prepared by a HCl in situ etching method. The optimal composite photocatalyst could accomplish sizable productivity of H2O2 to 2562.95 μmol g-1 h-1 under simulated solar irradiation, higher than that of primitive Bi2O2CO3 and BiOCl. Moreover, the synthesized catalysts showed good stability. The band structures of BiOCl and Bi2O2CO3 were determined, confirming the formation of BiOCl/Bi2O2CO3 S-scheme heterojunction The BiOCl/Bi2O2CO3, which obviously improved the separation efficiency of photoinduced carriers and effectively enhanced the redox ability of the photocatalyst. In addition, density functional theory (DFT) calculations were utilized to analyze the electron transfer properties and the constitution of the built-in electric field at the interface of BiOCl and Bi2O2CO3. The photocatalytic reaction process was further researched by electron paramagnetic resonance (EPR), indicating the active species in the photocatalytic production of hydrogen peroxide. Eventually, a feasible S-scheme electron transfer mechanism on the BiOCl/Bi2O2CO3 heterojunction during the photocatalytic H2O2 production process was proposed and discussed. This work provides a reliable strategy for the fine design of the S-scheme heterojunction.

Direct Z-scheme system of UiO-66 cubes wrapped with Zn0.5Cd0.5S nanoparticles for photocatalytic hydrogen generation synchronized with organic pollutant degradation

Optimized fabrication of Z-scheme photocatalyst based on MOF materials offers sustainable energy generation and environmental improvement due to their attractive properties. The Z-scheme heterojunctions consisting of UiO-66 cubes covered with Zn0.5Cd0.5S nanoparticles were fabricated by a facile solvothermal method. Thanks to the Z-scheme carrier transport under simulated sunlight irradiation, UiO-66@Zn0.5Cd0.5S exhibited enhanced photocatalytic performance of H2 generation synchronized with organic pollutant degradation in fluoroquinolone antibiotic wastewater. Synergistically, the highest comprehensive performance was obtained in ciprofloxacin solution. The H2 yield reached 224 μmol∙ g-1∙ h-1 and simultaneously the removal efficiency was up to 83.6 %. The degradation pathways revealed that the process of piperazine ring cleavage and decarboxylation also generates H protons, further promoting the production of H2. Therefore, the effective spatial separation and transfer of the photoinduced carriers are attributed to the good band structure, large specific surface area, and cooperative reduction and oxidation reactions of UiO-66@Zn0.5Cd0.5S, resulting in significant photocatalytic activity. The toxicity assessment of antibiotics and intermediate products during the photocatalytic reaction also verifies the reduction of environmental risk. This study highlights a promising way to expand the application of the MOFs-based photocatalyst in clean energy conversion coupling with water remediation.

A H2O2 Oxidation Approach to Ti3C2/TiO2 for Efficient Photocatalytic Removal of Distinct Organic Pollutants in Water

To develop versatile photocatalysts for efficient degradation of distinct organic pollutants in water is a continuous pursuit in environment remediation. Herein, we directly oxidize Ti3C2 MXene with hydrogen peroxide to produce C-doped anatase TiO2 nanowires with aggregates maintaining a layered architecture of the MXene. The Ti3C2 MXene provides a titanium source for TiO2, a carbon source for in situ C-doping, and templates for nanowire aggregates. Under UV light illumination, the optimized Ti3C2/TiO2 exhibits a reaction rate constant 1.5 times that of the benchmark P25 TiO2 nanoparticles, toward photocatalytic degradations of trace phenol in water. The mechanism study suggests that photogenerated holes play key roles on the phenol degradation, either directly oxidizing phenol molecules or in an indirect way through oxidizing first the surface hydroxyl groups. The unreacted Ti3C2 MXene, although with trace amounts, is supposed to facilitate electron transfer, which inhibits charge recombination. The unique nanostructure of layered aggregates of nanowires, abundant surface oxygen vacancies arising from the carbon doping, and probably the Ti3C2/TiO2 heterojunction guarantee the high photocatalytic efficiency toward removals of organic pollutants in water. The photocatalyst also exhibits an activity superior to, or at least comparable to, the benchmark P25 TiO2 toward photodegradations for typical persistent organic pollutants of phenol, dye molecule of rhodamine B, antibiotic of tetracycline, pharmaceutical wastewater of ofloxacin, and pesticide of N,N-dimethylformamide, when evaluated in total organic carbon removal.

Augmented photocatalysis induced by 1T-MoS2 bridged 2D/2D MgIn2S4@1T/2H-MoS2 Z-scheme heterojunction: mechanistic insights into H2O2 and H2 evolution

In the realm of composite photocatalysts, the fusion of the co-catalyst effect with interfacial engineering is recognized as a potent strategy for facilitating the segregation and migration of photo-induced charge carriers. Herein, an innovative mediator-based Z-scheme hybrid, i.e. MIS@1T/2H-MoS2, has been well designed by pairing MIS with 1T/2H-MoS2via a facile hydrothermal strategy as a competent photocatalyst for H2O2 and H2 generation. The co-catalyst, i.e. metallic 1T-phase bridging between semiconducting 2H-MoS2 and MIS, serves as a solid state electron mediator in the heterostructure. Morphological findings revealed the growth of 1T/2H-MoS2 nanoflowers over MIS microflowers, verifying the close interaction between MIS and 1T/2H-MoS2. By virtue of accelerated e-/h+ pair separation and migration efficiency along with a proliferated density of active sites, the MMoS2-30 photocatalyst yields an optimum H2O2 of 35 μmol h-1 and H2 of 370 μmol h-1 (ACE of 5.9%), which is 3 and 2.7 fold higher than pristine MIS. This obvious enhancement can be attributed to photoluminescence and electrochemical aspects that substantiate the diminished charge transfer resistance along with improved charge carrier separation, representing a good example of a noble metal-free photocatalyst. The proposed Z-scheme charge transfer mechanism is aided by time-resolved photoluminescence (TRPL), XPS, radical trapping experiments, and EPR analysis. Overall, this endeavour provides advanced insights into the architecture of noble metal-free Z-scheme heterostructures, offering promising prospects in photocatalytic applications.

Carbonized Polymer Dots/Bi/β-Bi2O3 for Efficient Photosynthesis of H2O2 via Redox Dual Pathways

A novel heterojunction photocatalyst of carbonized polymer dots (CPDs)/Bi/β-Bi2O3 is successfully synthesized via a one-pot solvothermal method by adjusting the reaction temperature and time. As a solvent and carbon source, ethylene glycol not only supports the conversion of Bi3+ to β-Bi2O3 but also undergoes its polymerization, cross-linking, and carbonization to produce CPDs. In addition, partial Bi3+ is reduced to Bi by ethylene glycol. As a result, the CPDs and Bi are deposited in situ on the surface of β-Bi2O3 microspheres. There are four built-in electric fields in the CPDs/Bi/β-Bi2O3 system, namely, the n-type semiconductor β-Bi2O3/H2O interface, the p-type CPDs/H2O interface, the ohmic contact between Bi and β-Bi2O3, and the Schottky junction between Bi and CPDs. Under the action of four built-in electric fields, the Z-type charge separation mechanism is formed. It promotes the effective separation of the photogenerated electron-hole and greatly improves the yield of H2O2. Under irradiation for 2 h, the H2O2 production is 1590 μmol g-1 h-1. The solar energy to H2O2 conversion efficiency is 0.11%.

Compositionally engineered Cd-Mo-Se alloyed QDs toward photocatalytic H2O2 production and Cr(VI) reduction with a detailed mechanism and influencing parameters

With the exceptional advantages of safety, greenness, and low cost, photocatalytic H2O2 generation has kindled a wonderful spark, although being severely hampered by the terrible photoinduced exciton recombination, migration, and surface decomposition. Here, employing reflux method, the Cd-Mo-Se quantum dots of varying molar ratios of Cd and Mo were synthesized using thioglycolic acid as the capping ligand to regulate their growth. This type of metal alloying promotes rapid charge migration, improves light harvesting, and reduces the rate of charge recombination. The improved optoelectronic properties and boosted activity of Cd-rich ternary CMSe-1 QDs led to the observed exceptional photocatalytic H2O2 yield of 1403.5 μmol g-1 h-1 (solar to chemical conversion efficiency, 0.27%) under visible light, outperforming the other ternary and Se-based QD photocatalysts. Additionally, CMSe-1 shows 93.6% (2 h) hazardous Cr(VI) photoreduction. The enhanced catalytic performance of CMSe-1 corresponds to effective charge carrier separation and transfer efficiency, well supported by PL, TRPL, and electrochemical measurements. Photocatalytic H2O2 production was also studied under varying experimental conditions and the scavenger test suggests a superoxide radical intermediate 2-step single electron reduction pathway. The catalyst-assisted Cr(VI) reduction is substantiated by the zero-order kinetics as well as the determination of the pHPZC value. The catalyst can be employed for a maximum of four times while retaining its activity, according to the photostability and reusability test outcomes. This research presents interesting approaches for producing ternary QDs and modified systems for efficient photocatalytic H2O2 production and Cr(VI) reduction.

A novel SnIn4S8/ZnFe2O4 S-scheme heterojunction with excellent magnetic properties and photocatalytic degradation activity for tetracycline

The development of efficient and economical photocatalysts is considered a promising strategy for pollution remediation. Magnetically separable SnIn4S8/ZnFe2O4 composites (SIS/ZFO) were prepared by combining SIS with ZFO. The composite with a 30% ZFO mass ratio (SIS/ZFO-30) was the most effective and achieved 60% removal of tetracycline (TC) in 120 min. It has a rate constant of 7.94 × 10-3 min-1, which is 6.3 and 27.2 times higher than those of pure SIS and pure ZFO, respectively. The improved photocatalytic performance can be attributed to the formation of S-scheme heterojunctions between SIS and ZFO, which results in the strong absorption of visible light, the enhanced separation of electron-hole pairs, and the higher redox ability of photoinduced charges. Additionally, SIS/ZFO composites have excellent magnetic properties and high stability, and the recovered samples still retained good photocatalytic degradation performances after four cycles of experiments. Thus, the coupling of SIS with ZFO provides a valuable strategy for enhancing photocatalytic potential and offers a promising pathway for water remediation.