Role of Interfaces in Two-Dimensional Photocatalyst for Water Splitting

Low Bandgap NiCo2S4 Nanoparticles Decorated 2D-BiOBr Nano Pallets: Atomic Level Insight into the Active Sites for Photocatalytic H2O2 Production

2D-BiOBr (BOB) has recently gained great potential for harvesting solar light and its applications for photocatalysis. However, the lack of active sites with adequate reduction potential has become one of the major challenges in this regard. In this work, a unique heterojunction hybrid has been designed by decorating low band gap NiCo2S4 Nanoparticles (NCS) on BOB nanopallets (NCS@BOB) for efficient photocatalytic H2O2 production. Detailed characterizations suggest that the composite material possesses better surface properties with higher oxygen defects (OVs), enhance visible-light absorption capability, the ability for effective photogenerated charge separation through intimate heterojunction, etc. Periodic PBE-D calculations have been carried out to complement the experimental findings in detail. The optimized photocatalyst displayed a maximum H2O2 production rate of 9.67 mmgcat. -1 in 2 hr, which is 3.3 times higher than the pristine BOB. Further experiments unveil the mechanism of photocatalytic H2O2 production. Results confirm that it follows a dual-step two-electron transfer pathway for oxygen reduction reaction (ORR), with obvious superoxide formation as an intermediate step. Computational calculations critically explain the synergistic role of both "Ni" and "Co" centers as active sites for overall photocatalysis.

Theoretical investigation of an arsenene/g-C6N6 van der Waals heterojunction: a direct Z-scheme system with high photocatalytic efficiency

With advancements in algorithms and computational power, theoretical calculations have become increasingly feasible for designing and constructing functional materials. In this study, we utilized density functional theory (DFT) to investigate the new arsenene/g-C6N6 van der Waals heterojunction, which forms a direct Z-scheme system with an indirect bandgap of 1.41 eV and a minimal lattice mismatch of just 1.4%. The heterojunction's band edge positions are favorable for overall water splitting across a wide strain range (-6% to +6%) and varying pH conditions. Photocatalytic analysis reveals that the oxygen evolution reaction (OER) proceeds spontaneously under light irradiation, while the hydrogen evolution reaction (HER) requires an energy barrier of 0.47 eV, which can be further reduced to 0.2 eV under -6% compressive strain. The heterojunction also demonstrates enhanced visible light absorption, with a redshift in the absorption spectrum under biaxial strain, significantly boosting solar energy utilization. Remarkably, the heterojunction achieves a solar-to-hydrogen (STH) conversion efficiency of 47.84%, outperforming many previously reported photocatalytic materials. With a strong interfacial binding energy of -27.54 meV Å-2, confirmed by molecular dynamics simulations, its exceptional structural stability positions it as a promising candidate for experimental realization. These findings underscore the potential of the arsenene/g-C6N6 heterojunction as a high-performance platform for advanced photocatalytic applications.

Strain Engineering the Optoelectronic and HER Behavior of MoS2/ZnO Heterojunction: A DFT Investigation

The rational design of heterojunctions by coupling two or more two-dimensional (2D) materials is regarded as a feasible strategy to efficiently enhance photocatalytic-hydrogen performance by capturing solar energy to address the increasing global energy crisis. In this work, a functional MoS2/ZnO heterojunction is proposed based on first-principles simulation. Our results reveal that the photogenerated electrons and holes in the MoS2/ZnO heterojunction follow a specific Z-scheme pathway, highly facilitating redox reactions and optimizing optical properties in the visible-light region. Under external strain, the MoS2/ZnO heterojunction demonstrates improved HER performance and remarkable optical-harvesting capabilities. Interestingly, the HER free energy for the heterojunction is only -0.04 eV under -5% compressive strain, highlighting its promising potential for photocatalytic hydrogen production. We observe that the absorption edge of the spectrum shifts gradually to the infrared region with increasing tensile biaxial strains, whereas compressive biaxial strains result in a blue-shift absorption spectrum. Additionally, all heterojunctions achieve excellent solar-to-hydrogen (STH) efficiencies exceeding 10%, demonstrating their capability to store sufficient solar energy. Our work offers a novel strategy for exploring highly efficient photocatalysts in the field of hydrogen energy with the ability to modulate their activity through external strain.

Fluid Mechanical and Visible-Light-Driven Piezophotocatalysis in MoS2/Carbon-Rich Carbon Nitride Heterostructures for Enhanced Green Energy Production and Environmental Remediation

Molybdenum disulfide (MoS2)/carbon-rich carbon nitride (TCN) heterostructure, a piezophotocatalyst sensitive to fluid mechanical energy and visible light, has been developed for green energy production and environmental remediation. The optimized MoS2/TCN heterostructure exhibits an absorption edge at 520 nm, identical to that of TCN but significantly red-shifted compared with conventional carbon nitride. Piezopotential measurements via piezoelectric force microscopy demonstrate that the MoS2/TCN heterostructure generates a much higher piezopotential response than TCN under the same applied voltage. This heterostructure exhibits substantial improvements in photocatalytic performance for both the hydrogen evolution reaction (HER) and the degradation of tetracycline (TC) under visible light. Additionally, its photocatalytic activity is further enhanced by vortex-induced fluid motion. Compared to TCN, the piezophotocatalytic activity of the optimized MoS2/TCN heterostructure increases the HER rate from 1.8 to 3.62 mmol g-1 h-1 and the TC degradation rates from 57.8 to 85.1% and 73.2 to 98.8% in 15 and 60 min, respectively. MoS2 nanosheets act as piezoelectric generators, triggered by fluid flow, to induce a macroscopic piezopotential, aiding in the collection of visible-light-generated electrons and holes on the TCN surface to enhance catalytic activity. This work highlights that the shearing forces from fluid flow, essential for wastewater discharge, piezoelectrically amplify the photocatalytic efficiency of the MoS2/TCN heterostructure.

Constructing a Graphene-like Layered Carbocatalyst by the Dual Templating Effect for an Efficient Fenton-like Reaction

Two-dimensional (2D) carbon materials are receiving increasing attention due to their partly groundbreaking performance in catalysis and electrochemistry based on distinct physiochemical and textural properties. We focus on the challenge to directly achieve a well-developed layered morphology with a high doping level of heteroatoms as the active sites, a standard conflict of interests of high-temperature synthesis. Here, we report a dual-templating strategy to yield graphene-like layered carbon (GLC) by direct carbonization of a texturally prealigned zeolitic imidazolate framework-8 (ZIF-8). The recrystallization of ZIF-8 in an aqueous NaCl solution discloses a 2D packing mode that was retained after freeze-drying with recrystallized NaCl as an exotemplate and a space-confining nanoreactor. Further promoted by the chemical interaction of NaCl in promoting and stabilizing the carbonization process, the final product came with a well-separated layered morphology and high amounts of heteroatoms (16.6 wt % N and 7.5 wt % O). The structurally and catalytically special GLC functioned well in activating peroxymonosulfate-based Fenton-like reactions. It was shown that the reaction proceeded via nonfree-radical-mediated pathways, as reflected in significantly enhanced electron-transfer processes and ultrafast kinetics for pollutant removal. The proposed strategy is expected to afford a broader applicability for the bottom-up design of 2D carbon materials.

Colloidal Design and Preparation of an Internal Electric Modulated Z-Scheme BiOI-CdS Heteronanostructure with Oxygen-Rich Vacancies

Photoelectrochemical (PEC) water splitting offers an ideal strategy for the development of clean and renewable energy. However, its practical implementation is often inhibited by the high recombination rate of photogenerated charge carriers and the instability of photoanodes. Introducing defect engineering (such as oxygen vacancies) and constructing internal electric field-modulated Z-scheme heteronanostructures (HNs) can be considered as effective approaches to overcome these obstacles. Herein, a flexible method is developed for synthesizing Z-scheme BiOI-CdS HNs with oxygen vacancies, which induce an internal electric field between ultrathin BiOI nanosheets and a CdS semiconductor. This Z-scheme mechanism significantly promotes the separation of photogenerated electron-hole pairs, thereby enhancing the PEC performance. The BiOI-CdS photoanode achieves a photocurrent density of 4.22 mA cm-2 at 1.6 V vs RHE under AM 1.5G illumination (100 mW cm-2), outperforming bare BiOI and CdS. Moreover, the photoanode exhibits exceptional stability with only a slight decrease of approximately in its initial photocurrent after a rigorous 4 h test, surpassing other counterparts in terms of durability. This work affords a better understanding of oxygen vacancies and the construction of highly efficient and stable Z-scheme photoanodes for feasible PEC application.

Enrichment and catalysis effect of 2D/2D g-C3N4/Ti3C2 for promoting organic matter degradation and heavy metal reduction in plasma systems: Unveiling the promotion and redox mechanism

This work proposes a novel plasma-assisted 2D/2D g-C3N4/Ti3C2 system for treatment of organics-heavy metals composite wastewater. Unlike traditional materials in plasma system, 2D/2D g-C3N4/Ti3C2 not only improved the mass transfer efficiency of plasma by gathering both reactive species and pollutants onto the surface, but also induced photocatalytic reactions. Besides, the higher specific surface area and faster carrier separation rate can enhance the oxidation and reduction activity, and then promoted organic matter degradation and heavy metal reduction. Remarkably, the removal efficiency of sulfamethoxazole (SMX) and Cr(VI) increased by 16.5 % and 73.1 % respectively when introducing 2D/2D g-C3N4/Ti3C2. Roles of·OH,·H,·O2-, 1O2, e-, and h+ in SMX oxidation and Cr(VI) reduction are clarified. The primary aggregated·OH and 1O2 dominate the degradation of SMX. The influencing factors, synergistic mechanism between plasma and catalyst, and redox mechanism were clarified. This work provides a breakthrough idea for treatment of organics-heavy metals composite wastewater.

Unlocking the potential: machine learning applications in electrocatalyst design for electrochemical hydrogen energy transformation

Machine learning (ML) is rapidly emerging as a pivotal tool in the hydrogen energy industry for the creation and optimization of electrocatalysts, which enhance key electrochemical reactions like the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), the hydrogen oxidation reaction (HOR), and the oxygen reduction reaction (ORR). This comprehensive review demonstrates how cutting-edge ML techniques are being leveraged in electrocatalyst design to overcome the time-consuming limitations of traditional approaches. ML methods, using experimental data from high-throughput experiments and computational data from simulations such as density functional theory (DFT), readily identify complex correlations between electrocatalyst performance and key material descriptors. Leveraging its unparalleled speed and accuracy, ML has facilitated the discovery of novel candidates and the improvement of known products through its pattern recognition capabilities. This review aims to provide a tailored breakdown of ML applications in a format that is readily accessible to materials scientists. Hence, we comprehensively organize ML-driven research by commonly studied material types for different electrochemical reactions to illustrate how ML adeptly navigates the complex landscape of descriptors for these scenarios. We further highlight ML's critical role in the future discovery and development of electrocatalysts for hydrogen energy transformation. Potential challenges and gaps to fill within this focused domain are also discussed. As a practical guide, we hope this work will bridge the gap between communities and encourage novel paradigms in electrocatalysis research, aiming for more effective and sustainable energy solutions.

Comparative study of the photocatalytic activity of g-C3N4/MN4 (M = Mn, Fe, Co) for water splitting reaction: A theoretical study

In this study, nanocomposites of g-C3N4/MN4 (where M is Mn, Fe and Co) have been designed using advanced density functional theory (DFT) calculations. A comprehensive analysis was conducted on the geometry, electronic, optical properties, work function, charge transfer interaction and adhesion energy of the g-C3N4/MN4 heterostructures and concluded that g-C3N4/FeN4 and g-C3N4/CoN4 heterojunctions exhibit higher photocatalytic performance than individual units. The better photocatalytic activity can be attributed mainly by two facts; (i) the visible light absorption of both g-C3N4/FeN4 and g-C3N4/CoN4 interfaces are higher compared to its isolated analogs and (ii) a significant enhancement of band gap energy in g-C3N4/FeN4 and g-C3N4/CoN4 heterostructures limited the electron-hole recombination significantly. The potential of the g-C3N4/MN4 heterojunctions as a photocatalyst for the water splitting reaction was assessed by examining its band alignment for water splitting reaction. Importantly, while the electronic and magnetic properties of MN4 systems were studied, this is the first example of inclusion of MN4 on graphene-based material (g-C3N4) for studying the photocatalytic activity. The state of the art DFT calculations emphasis that g-C3N4/FeN4 and g-C3N4/CoN4 heterojunctions are half metallic photocatalysts, which is limited till date.

Exploring Pt-Impregnated CdS/TiO2 Heterostructures for CO2 Photoreduction

This work focuses on the production of methane through the photocatalytic reduction of carbon dioxide using Pt-doped CdS/TiO2 heterostructures. The photocatalysts were prepared using P25 commercial titania and CdS synthesized through a solvothermal methodology, followed by the impregnation of Pt onto the surface to enhance the physicochemical properties of the resulting photocatalysts. The pure and heterostructure-based materials were characterized using X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible spectroscopy (UV-Vis), ultraviolet photoelectron spectroscopy (UPS), and photoluminescence spectroscopy (PL). The obtained results show the successful synthesis of the heterostructure impregnated with Pt. Moreover, the observed key role of CdS and Pt nanoparticles in the final semiconductor is to reduce the electron-hole pair recombination rate by acting as an electron sink, which slows down the recombination process and increases the photocatalyst efficiency. Thus, Pt-doped CdS/TiO2 heterostructures with the best observed composition presents better catalytic activity than P25 titania with methane production values being 460 and 397 µmol CH4/g·h, respectively.

A compendium of all-in-one solar-driven water splitting using ZnIn2S4-based photocatalysts: guiding the path from the past to the limitless future

Photocatalytic water splitting represents a leading approach to harness the abundant solar energy, producing hydrogen as a clean and sustainable energy carrier. Zinc indium sulfide (ZIS) emerges as one of the most captivating candidates attributed to its unique physicochemical and photophysical properties, attracting much interest and holding significant promise in this domain. To develop a highly efficient ZIS-based photocatalytic system for green energy production, it is paramount to comprehensively understand the strengths and limitations of ZIS, particularly within the framework of solar-driven water splitting. This review elucidates the three sequential steps that govern the overall efficiency of ZIS with a sharp focus on the mechanisms and inherent drawbacks associated with each phase, including commonly overlooked aspects such as the jeopardising photocorrosion issue, the neglected oxidative counter surface reaction kinetics in overall water splitting, the sluggish photocarrier dynamics and the undesired side redox reactions. Multifarious material design strategies are discussed to specifically mitigate the formidable limitations and bottleneck issues. This review concludes with the current state of ZIS-based photocatalytic water splitting systems, followed by personal perspectives aimed at elevating the field to practical consideration for future endeavours towards sustainable hydrogen production through solar-driven water splitting.

Hydrogen Bonds and In situ Photoinduced Metallic Bi0/Ni0 Accelerating Z-Scheme Charge Transfer of BiOBr@NiFe-LDH for Highly Efficient Photocatalysis

For heterojunction system, the lack of stable interfacial driving force and definite charge transfer channel makes the charge separation and transfer efficiency unsatisfactory. The photoreaction mechanism occurring at the interface also receives less attention. Herein, a 2D/2D Z-scheme junction BiOBr@NiFe-LDH with large-area contact featured by short interface hydrogen bonds and strong interfacial electric field (IEF) is synthesized, and in situ photoinduced metallic species assisting charge transfer mechanism is demonstrated. The hydrogen bonds between O atoms from BiOBr and H atoms from NiFe-LDH induce a significant interfacial charge redistribution, establishing a robust IEF. Notably, during photocatalytic reaction, Bi0 and Ni0 are in situ performed in heterojunction, which separately act as electron transport mediator and electron trap to further accelerate charge transfer efficiency up to 71.2 %. Theoretical calculations further demonstrate that the existence of Bi0 strengthens the IEF. Therefore, high-speed spatial charge separation is realized in Bi0/BiOBr@Ni0/NiFe-LDH, leading to a prominent photocatalytic activity with a tetracycline removal ratio of 88.3 % within 7 min under visible-light irradiation and the presence of persulfate, far exceeding majority of photocatalysts reported previously. This study provides valid insights for designing hydrogen bonding heterojunction systems, and advances mechanistic understanding on in situ photoreaction at interfaces.

Quasi-"In Situ" Analysis of the Reactive Liquid-Solid Interface during Magnesium Corrosion Using Cryo-Atom Probe Tomography

The early stages of corrosion occurring at liquid-solid interfaces control the evolution of the material's degradation process, yet due to their transient state, their analysis remains a formidable challenge. Here corrosion tests are performed on a MgCa alloy, a candidate material for biodegradable implants using pure water as a model system. The corrosion reaction is suspended by plunge freezing into liquid nitrogen. The evolution of the early-stage corrosion process on the nanoscale by correlating cryo-atom probe tomography (APT) with transmission-electron microscopy (TEM) and spectroscopy, is studied. The outward growth of Mg hydroxide Mg(OH)2 and the inward growth of an intermediate corrosion layer consisting of hydrloxides of different compositions, mostly monohydroxide Mg(OH) instead of the expected MgO layer, are observed. In addition, Ca partitions to these newly formed hydroxides and oxides. Density-functional theory calculations suggest a domain of stability for this previously experimental unreported Mg(OH) phase. This new approach and these new findings advance the understanding of the early stages of magnesium corrosion, and in general reactions and processes at liquid-solid interfaces, which can further facilitate the development of corrosion-resistant materials or better control of the biodegradation rate of future implants.

Wide-direct-band-gap monolayer carbon nitride CN2: a potential metal-free photocatalyst for overall water splitting

Two dimensional metal-free semiconductors with high work function have attracted extensive research interest in the field of photocatalytic water splitting. Herein, we have proposed a kind of highly stable monolayer carbon nitride CN2 with an anisotropic structure based on first principles density functional theory. The calculations of electronic structure properties, performed using the HSE06 functional, indicate that monolayer CN2 has a wide direct band gap of 2.836 eV and a high work function of 6.54 eV. And the suitable band edge alignment, high electron mobility (∼103 cm2 V-1 s-1) and visible-light optical absorption suggest that monolayer CN2 has potential on visible-light photocatalytic water splitting at pH ranging from 0 to 14. Moreover, we have observed that uniaxial strain can effectively control the electronic structure properties and optical absorption of monolayer CN2, which can further improve its solar to hydrogen efficiency from 9.6% to 16.02% under 5% uniaxial tension strain along the Y direction. Our calculations have not only proposed a new type of potential metal-free photocatalyst for water splitting but also provided a functional part with high work function for type-I and scheme-Z heterojunction applied in photocatalytic water splitting.

First-Principles Computational Screening of Two-Dimensional Polar Materials for Photocatalytic Water Splitting

The band gap constraint of the photocatalyst for overall water splitting limits the utilization of solar energy. A strategy to broaden the range of light absorption is employing a two-dimensional (2D) polar material as photocatalyst, benefiting from the deflection of the energy level due to their intrinsic internal electric field. Here, by using first-principles computational screening, we search for 2D polar semiconductors for photocatalytic water splitting from both ground- and excited-state perspectives. Applying a unique electronic structure model of polar materials, there are 13 photocatalyst candidates for the hydrogen evolution reaction (HER) and 8 candidates for the oxygen evolution reaction (OER) without barrier energies from the perspective of the ground-state free energy variation calculation. In particular, Cu2As4Cl2S3 and Cu2As4Br2S3 can catalyze HER and OER simultaneously, becoming promising photocatalysts for overall water splitting. Furthermore, by combining ground-state band structure calculations with excited-state charge distribution and transfer calculated by linear-response time-dependent density functional theory (LR-TDDFT) and time-dependent ab initio nonadiabatic molecular dynamics (NAMD), respectively, the rationality of the 2D polar material model has been manifested. The intrinsic built-in electric field promotes the separation of charge carriers while suppressing their recombination. Therefore, our computational work provides a high-throughput method to design high-performance photocatalysts for water splitting.

First-Principles Investigation on the Tunable Electronic Structures and Photocatalytic Properties of AlN/Sc2CF2 and GaN/Sc2CF2 Heterostructures

Heterostructure catalysts are highly anticipated in the field of photocatalytic water splitting. AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are proposed in this work, and the electronic structures were revealed with the first-principles method to explore their photocatalytic properties for water splitting. The results found that the thermodynamically stable AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are indirect semiconductors with reduced band gaps of 1.75 eV and 1.84 eV, respectively. These two heterostructures have been confirmed to have type-Ⅰ band alignments, with both VBM and CBM contributed to by the Sc2CF2 layer. AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures exhibit the potential for photocatalytic water splitting as their VBM and CBM stride over the redox potential of water. Gibbs free energy changes in HER occurring on AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are as low as -0.31 eV and -0.59 eV, respectively. The Gibbs free energy change in HER on the AlN (GaN) layer is much lower than that on the Sc2CF2 surface, owing to the stronger adsorption of H on AlN (GaN). The AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures possess significant improvements in absorption range and intensity compared to monolayered AlN, GaN, and Sc2CF2. In addition, the band gaps, edge positions, and absorption properties of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures can be effectively tuned with strains. All the results indicate that AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are suitable catalysts for photocatalytic water splitting.

CdS/ZnS core-shell nanorod heterostructures co-deposited with ultrathin MoS2 cocatalyst for competent hydrogen evolution under visible-light irradiation

Hydrogen generation via semiconductor photocatalysts has gained significant attention as a sustainable fuel generation process. To demonstrate the performance of nanoscale core-shell heterostructure in photocatalytic hydrogen production, we have fabricated CdS nanorods coated with ZnS photocatalyst via wet-chemical reaction followed by deposition of ultrathin MoS2 nanosheets by photo reduction process. The effect of ZnS content and suitable amount of MoS2 loading over the visible-light induced photocatalytic hydrogen evolution was examined in Na2S and Na2SO3 aqueous solutions. Interestingly, it is apparent that a close connection (or heterojunction) between CdS and ZnS is believed to easily tunnel the charge carriers to the surplus surface states, making its electrons and holes energetically favourable to transfer from ZnS to MoS2 for photocatalytic reactions and subsequently, enhances the H2 evolution activity in CdS/ZnS type I core-shell heterostructures. The optimal MoS2 concentration is resolved to be 7 mol% and the subsequent visible-light induced H2 generation rate was 13589 μmol h-1g-1, which is 19 and 158 fold higher than pristine CdS and ZnS respectively. The probable photocatalytic mechanism of CdS/ZnS type I core-shell heterostructure with MoS2 cocatalyst is proposed. Our inexpensive and convenient preparation strategy may offer novel prospects in the engineering of desirable nanoheterostructures with better performance.

Recent advances in metal halide perovskite based photocatalysts for artificial photosynthesis and organic transformations

Metal halide perovskites (MHP) emerged as highly promising materials for photocatalysis, offering significant advancements in the degradation of soluble and airborne pollutants, as well as the transformation of functional organic compounds. This comprehensive review focuses on recent developments in MHP-based photocatalysts, specifically examining two major categories: lead-based (such as CsPbBr3) and lead-free variants (e.g. Cs2AgBiX6, Cs3Bi2Br9 and others). While the review briefly discusses the contributions of MHPs to hydrogen (H2) production and carbon dioxide (CO2) reduction, the main emphasis is on the design principles that determine the effectiveness of perovskites in facilitating organic reactions and degrading hazardous chemicals through oxidative transformations. Furthermore, the review addresses the key factors that influence the catalytic efficiency of perovskites, including charge recombination, reaction mechanisms involving free radicals, hydroxyl ions, and other ions, as well as phase transformation and solvent compatibility. By offering a comprehensive overview, this review aims to serve as a guide for the design of MHP-based photocatalysis and shed light on the common challenges faced by the scientific community in the domain of organic transformations.

ML-Aided Computational Screening of 2D Materials for Photocatalytic Water Splitting

The exploration of two-dimensional (2D) materials with exceptional physical and chemical properties is essential for the advancement of solar water splitting technologies. However, the discovery of 2D materials is currently heavily reliant on fragmented studies with limited opportunities for fine-tuning the chemical composition and electronic features of compounds. Starting from the V2DB digital library as a resource of 2D materials, we set up and execute a funnel approach that incorporates multiple screening steps to uncover potential candidates for photocatalytic water splitting. The initial screening step is based upon machine learning (ML) predicted properties, and subsequent steps involve first-principles modeling of increasing complexity, going from density functional theory (DFT) to hybrid-DFT to GW calculations. Ensuring that at each stage more complex calculations are only applied to the most promising candidates, our study introduces an effective screening methodology that may serve as a model for accelerating 2D materials discovery within a large chemical space. Our screening process yields a selection of 11 promising 2D photocatalysts.

2D Monolayer Catalysts: Towards Efficient Water Splitting and Green Hydrogen Production

A viable alternative to non-renewable hydrocarbon fuels is hydrogen gas, created using a safe, environmentally friendly process like water splitting. An important role in water-splitting applications is played by the development of two-dimensional (2D) layered transition metal chalcogenides (TMDCs), transition metal carbides (MXenes), graphene-derived 2D layered nanomaterials, phosphorene, and hexagonal boron nitride. Advanced synthesis methods and characterization instruments enabled an effective application for improved electrocatalytic water splitting and sustainable hydrogen production. Enhancing active sites, modifying the phase and electronic structure, adding conductive elements like transition metals, forming heterostructures, altering the defect state, etc., can improve the catalytic activity of 2D stacked hybrid monolayer nanomaterials. The majority of global research and development is focused on finding safer substitutes for petrochemical fuels, and this review summarizes recent advancements in the field of 2D monolayer nanomaterials in water splitting for industrial-scale green hydrogen production and fuel cell applications.

Research progress of dual-atom site catalysts for photocatalysis

Dual-atom site catalysts (DASCs) have sparked considerable interest in heterogeneous photocatalysis as they possess the advantages of excellent photoelectronic activity, photostability, and high carrier separation efficiency and mobility. The DASCs involved in these important photocatalytic processes, especially in the photocatalytic hydrogen evolution reaction (HER), CO2 reduction reaction (CO2RR), N2/nitrate reduction, etc., have been extensively investigated in the past few years. In this review, we highlight the recent progress in DASCs that provides fundamental insights into the photocatalytic conversion of small molecules. The controllable preparation and characterization methods of various DASCs are discussed. Subsequently, the reaction mechanisms of the formation of several important molecules (hydrogen, hydrocarbons and ammonia) on DASCs are introduced in detail, in order to probe the relationship between DASCs's structure and photocatalytic activity. Finally, some challenges and outlooks of DASCs in the photocatalytic conversion of small molecules are summarized and prospected. We hope that this review can provide guidance for in-depth understanding and aid in the design of efficient DASCs for photocatalysis.

Designing a superhydrophobic cotton fiber coating exploiting TiO2@g-C3N4 layered structure for augmented photocatalysis and efficient water-oil separation

This study introduces a novel approach to develop a multifunctional coating on cotton fabric, emphasizing the utilization of cotton fiber as a biological macromolecule, by integrating a TiO2@g-C3N4 layered structure to confer superhydrophobic properties and multiple functionalities. The engineered structure not only enhances fabric roughness but also incorporates non-fluoro hydrophobic agents, thereby imparting diverse capabilities such as photocatalysis, oil-water separation, and self-cleaning to the cotton substrate. Fabrication of the TiO2@g-C3N4 layered structure involved ultrasonic dispersion of TiO2 and g-C3N4, subsequently deposited onto cotton fabric. Sequential hydrophobic treatment with polydimethylsiloxane (PDMS) and isophorone diisocyanate (IPDI) achieved superhydrophobicity, exhibiting an exceptional water contact angle (WCA) of 157.9°. Comprehensive characterization via scanning electron microscopy (SEM), X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FT-IR), and thermogravimetric validated the composite's structural and chemical properties. The introduced TiO2@g-C3N4 structure significantly enhanced fabric roughness, while PDMS treatment lowered surface energy and IPDI hydrolysis facilitated cross-linking, ensuring durability. The resultant TiO2@g-C3N4/PDMS cotton exhibited outstanding self-cleaning properties and demonstrated oil adsorption capacity, accommodating both heavy and light oils. Notably, this superhydrophobic cotton efficiently separated water-oil mixtures, achieving 96.8 % efficiency even after 10 cycles. Moreover, under simulated light, it displayed outstanding photocatalytic degradation (93.2 %) of methylene blue while maintaining a WCA of 150° post-degradation, highlighting sustained functionality. This innovation holds promise for sustainable applications, offering robust physical and chemical durability within the realm of biological macromolecules. The amalgamation of TiO2@g-C3N4 layered structure and PDMS treatment on cotton fabric underscores a sustainable approach to address water-oil separation challenges and enable efficient self-cleaning. This research demonstrates a significant step towards sustainable material applications and addresses pertinent real-world challenges in diverse technological domains.

Cyanuric Acid-Assisted Synthesis of Hierarchical Amorphous Carbon Nitride Assembled by Ultrathin Oxygen-Doped Nanosheets for Excellent Photocatalytic Hydrogen Generation

Amorphous carbon nitride with typical short-range order arrangement as an effective photocatalyst is worth exploring but remains a great challenge because its disordered structure induces severe recombination of photogenerated charge carriers. Herein, for the first time, we demonstrate that a hierarchical amorphous carbon nitride (HACN) with structural oxygen incorporation can be synthesized via a cyanuric acid-assisted melem hydrothermal process, accompanied by freeze-drying and pyrolysis. The complex composed of melem and cyanuric acid exhibiting a unique 3D self-supporting skeleton and significant phase transformation is responsible for the formation of an interconnected hierarchical framework and amorphous structure for HACN. These features are beneficial to enhance its visible light harvesting by the multiple-reflection effect within the architecture consisting of more exposed porous nanosheets and introducing a long band tail absorption. The well-designed morphology, band tail state, and oxygen doping effectively inhibit rapid band-to-band recombination of the photogenerated electrons and holes and facilitate subsequent separation. Accordingly, the HACN catalyst exhibits exceptional visible light (λ > 420 nm)-driven photoreduction for hydrogen production with a rate of 82.4 μmol h-1, which is 21.7 and 9.5 times higher than those of melem-derived carbon nitride and crystalline nanotube carbon nitride counterparts, respectively, and significantly surpasses those of most reported amorphous carbon nitrides. Our controlling of rearrangement of the in situ supramolecular self-assembly of melem oligomer using cyanuric acid directly instructs the development of highly efficient amorphous photocatalysts for converting solar energy into hydrogen fuel.

Understanding the Behaviors of Plasmon-Induced Hot Carriers and Their Applications in Photocatalysis

Photocatalysis driven by plasmon-induced hot carriers has been gaining increasing attention. Recent studies have demonstrated that plasmon-induced hot carriers can directly participate in photocatalytic reactions, leading to great enhancement in solar energy conversion efficiency, by improving the catalytic activity or changing selectivity. Nevertheless, the utilization efficiency of hot carriers remains unsatisfactory. Therefore, how to correctly understand the generation and transfer process of hot carriers, as well as accurately differentiate between the possible mechanisms, have become a key point of attention. In this review, we overview the fundamental processes and mechanisms underlying hot carrier generation and transport, followed by highlighting the importance of hot carrier monitoring methods and related photocatalytic reactions. Furthermore, possible strategies for the further characterization of plasmon-induced hot carriers and boosting their utilization efficiency have been proposed. We hope that a comprehensive understanding of the fundamental behaviors of hot carriers can aid in designing more efficient photocatalysts for plasmon-induced photocatalytic reactions.

Two-dimensional materials as catalysts, interfaces, and electrodes for an efficient hydrogen evolution reaction

Two-dimensional (2D) materials have been significantly investigated as electrocatalysts for the hydrogen evolution reaction (HER) over the past few decades due to their excellent electrocatalytic properties and their structural uniqueness including the atomically thin structure and abundant active sites. Recently, 2D materials with various electronic properties have not only been used as active catalytic materials, but also employed in other components of the HER electrodes including a conductive electrode layer and an interfacial layer to maximize the HER efficiency or utilized as templates for catalytic nanostructure growth. This review provides the recent progress and future perspectives of 2D materials as key components in electrocatalytic systems with an emphasis on the HER applications. We categorized the use of 2D materials into three types: a catalytic layer, an electrode for catalyst support, and an interlayer for enhancing charge transfer between the catalytic layer and the electrode. We first introduce various scalable synthesis methods of electrocatalytic-grade 2D materials, and we discuss the role of 2D materials as HER catalysts, an interface for efficient charge transfer, and an electrode and/or a growth template of nanostructured noble metals.

Z-scheme Al2SeTe/GaSe and Al2SeTe/InS van der Waals heterostructures for photocatalytic water splitting

Designing Z-scheme van der Waals (vdW) heterostructured photocatalysts is a promising strategy for developing highly efficient overall water splitting. Herein, by employing density functional theory calculations, we systematically investigated the stability, electronic structures, photocatalytic and optical properties of Al2SeTe, GaSe, and InS monolayers and their corresponding vdW heterostructures. Interestingly, electronic structures show that all vdW heterostructures have direct band gaps, which is conducive to the transition of electrons from the valence band to the conduction band. Notably, Al2TeSe/GaSe and Al2TeSe/InS vdW heterostructures possess large overpotentials for Z-scheme photocatalytic water splitting, as proved by the results of band edge positions and band structure bending. Moreover, these vdW heterostructures exhibit good optical absorption in ultraviolet and visible light regions. We believe that our findings will open a new avenue for the modulation and development of Al2TeSe/GaSe and Al2TeSe/InS vdW heterostructures for photocatalytic water splitting.

Overall Spontaneous Water Splitting for Calcium Bismuthate Ca(BiO2)2: Flexible-Electronic-Controlled Band Edge Position and Adsorption-Site-Modulated Bond Strength

Eco-friendly photocatalysts for water splitting, highly efficient in oxygen/hydrogen evolution reactions, hold great promise for the storage of inexhaustible solar energy and address environmental challenges. However, current common photocatalysts rarely exhibit both H2 and O2 production performances unless some regulatory measures, such as strain engineering, are implemented. Additionally, the extensive utilization of flexible electronics remains constrained by their high Young's modulus. Herein, on the basis of density functional theory calculations, we identify a novel spontaneous oxygen-producing two-dimensional Ca(BiO2)2 material, which can efficiently regulate the electronic structures of the surface active sites, optimize the reaction pathways, reduce the reaction energy barriers, and boost the overall water-splitting activity through biaxial strain modulation. In detail, an unstrained Ca(BiO2)2 monolayer not only possesses a suitable band gap value (2.02 eV) to fulfill the photocatalytic water-splitting band edge relationships but also holds favorable transport properties, excellent optical absorption across the visible light spectrum, and spontaneous oxygen production under neutral conditions. More excitingly, under application of a 7% biaxial tensile strain modulation with an ideal biaxial strength of 32.35 GPa nm, the Ca(BiO2)2 monolayer not only maintains its structural integrity but also exhibits a completely spontaneous reaction for photocatalytic hydrogen precipitation with superior optical absorption. This can primarily be attributed to the substantial reduction of the potential barrier through strain engineering as well as the weakening of bond energy resulting from changes of the adsorption site as calculated by crystal orbital Hamiltonian population analysis. This flexible stretchable electronic modulated the photocatalyst behavior and bond energy of O-Bi and O-Ca interactions, offering outstanding potential for photocatalytic water spontaneous oxygen and hydrogen evolution among all of the reported metal oxides, and is more likely to become a promising candidate for future flexible electronic devices.

Activating two-dimensional semiconductors for photocatalysis: a cross-dimensional strategy

The emerging two-dimensional (2D) semiconductors substantially extend materials bases for versatile applications such as semiconductor photocatalysis demanding semiconductive matrices and large surface areas. The dimensionality, while endowing 2D semiconductors the unique properties to host photocatalytic functionality of pollutant removal and hydrogen evolution, hurdles the activation paths to form heterogenous photocatalysts where the photochemical processes are normally superior over these on the mono-compositional counterparts. In this perspective, we present a cross-dimensional strategy to employ thenD (n= 0-2) clusters or nanomaterials as activation partners to boost the photocatalytic activities of the 2D semiconductors. The formation principles of heterogenous photocatalysts are illustrated specifically for the 2D matrices, followed by selection criteria of them among the vast 2D database. The computer investigations are illustrated in the density functional theory route and machine learning benefitted from the vast samples in the 2D library. Synthetic realizations and characterizations of the 2D heterogenous systems are introduced with an emphasis on chemical methods and advanced techniques to understand materials and mechanistic studies. The perspective outlooks cross-dimensional activation strategies of the 2D materials for other applications such as CO2removal, and materials matrices in other dimensions which may inspire incoming research within these fields.

Metalloporphyrin based MOF-545 coupled with solid solution ZnxCd1-xS for efficient photocatalytic hydrogen production

The growing concern over the rapid consumption of fossil fuels has spurred scientists to seek environment-friendly and sustainable energy alternatives. The utilization of photocatalytic water splitting shows great potential in generating environmentally friendly hydrogen energy. Zirconium-based porphyrin MOF-545 has drawn extensive attention in photocatalysis since it has strong light absorption and fast electron transfer capability. A series of Zr-based metalloporphyrins containing different metals, MOF-545M (M = Co, Ni, Cu, Zn), were synthesized and assembled with solid solution ZnxCd1-xS (ZCS) nanoparticle to develop the corresponding of hybrid photocatalysts, ZCS/MOF-545M. Various characterizations show that introducing the porphyrin MOF-545Co not only improve the visible light response range of Zn0.5Cd0.5S (ZCS-0.5) but also enhance the carrier separation efficiency of ZCS-0.5. The carrier transfer direction during the reaction was confirmed by surface photovoltage spectroscopy and shows the formation of S-scheme. Under visible light irradiation, the hybrid photocatalyst ZCS-0.5/MOF-545Co exhibits a high H2 evolution rate of 148 μmol h-1, with increases of 52.8, 22.9 and 6.5 times than those of bare ZnS, CdS and ZCS-0.5, respectively. The present work gives a strategy for erecting S-scheme heterojunction to inhibit the photocorrosion of metal sulfides and enhancement the carrier separation efficiency.

Unraveling the Emerging Photocatalytic, Thermoelectric, and Topological Properties of Intercalated Architecture MZX (M = Ga and In; Z = Si, Ge and Sn; X = S, Se, and Te) Monolayers

The continuous advancements in studying two-dimensional (2D) materials pave the way for groundbreaking innovations across various industries. In this study, by employing density functional theory calculations, we comprehensively elucidate the electronic structures of MZX (M = Ga and In; Z = Si, Ge, and Sn; X = S, Se, and Te) monolayers for their applications in photocatalytic, thermoelectric, and spintronic fields. Interestingly, GaSiS, GaSiSe, InSiS, and InSiSe monolayers are identified to be efficient photocatalysts for overall water splitting with band gaps close to 2.0 eV, suitable band edge positions, and excellent optical harvest ability. In addition, the InSiTe monolayer exhibits a ZT value of 1.87 at 700 K, making it highly appealing for applications in thermoelectric devices. It is further highlighted that GaSnTe, InSnS, and InSnSe monolayers are predicted to be 2D topological insulators (TIs) with bulk band gaps of 115, 54, and 152 meV, respectively. Current research expands the family of 2D GaGeTe materials and establishes a path toward the practical utilization of MZX monolayers in energy conversion and spintronic devices.

Investigation of the Photocatalytic Activity of Electrospun and Surface-Modified PAN/α-FeOOH Nanofibers for the Degradation of Hazardous Azo Dyes

Decoration of α-FeOOH nanorods over PAN nanofibers was performed using the electrospinning technique. The as-designed decorated nanofibers were characterized using various techniques such as wide-angle powder X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FTIR) spectroscopy, UV-vis spectrophotometry (UV-vis), thermogravimetry analysis (TGA), N2 adsorption-desorption isotherm (BET), and X-ray photoelectron spectrometry (XPS). α-FeOOH NRs were decorated uniformly over PAN fibers, as observed from its morphological investigation, which shows novelty. 1D α-FeOOH nanorods with PAN nanofibers have not been studied for photocatalytic characteristics. No literature mentions that α-FeOOH nanorods coated in PAN NFs act as photocatalysts to degrade hazardous azo dyes. α-FeOOH nanorods on PAN NFs inhibit aggregation and increase dye binding, boosting photocatalytic performance. PAN/α-FeOOH NFs have a maximal specific surface area with a reduced bandgap than α-FeOOH NRs. PAN/α-FeOOH nanofibers showed excellent photocatalytic activity for the degradation of Trypan blue (TB) (120 min, 99.7%) and Eriochrome black T (EBT) dyes (160 min, 97.6%), respectively, under solar light irradiation. PAN/α-FeOOH NFs have the potential to be used in the degradation of azo dyes and the treatment of wastewater due to their low energy requirements and versatility.

Comprehensive Insights into the Family of Atomically Thin 2D-Materials for Diverse Photocatalytic Applications

2D materials with their fascinating physiochemical, structural, and electronic properties have attracted researchers and have been used for a variety of applications such as electrocatalysis, photocatalysis, energy storage, magnetoresistance, and sensing. In recent times, 2D materials have gained great momentum in the spectrum of photocatalytic applications such as pollutant degradation, water splitting, CO2 reduction, NH3 production, microbial disinfection, and heavy metal reduction, thanks to their superior properties including visible light responsive band gap, improved charge separation and electron mobility, suppressed charge recombination and high surface reactive sites, and thus enhance the photocatalytic properties rationally as compared to 3D and other low-dimensional materials. In this context, this review spot-lights the family of various 2D materials, their properties and their 2D structure-induced photocatalytic mechanisms while giving an overview on their synthesis methods along with a detailed discussion on their diverse photocatalytic applications. Furthermore, the challenges and the future opportunities are also presented related to the future developments and advancements of 2D materials for the large-scale real-time photocatalytic applications.

Intriguing type-II g-GeC/AlN bilayer heterostructure for photocatalytic water decomposition and hydrogen production

Adapting two-dimensional (2D) van der Walls bilayer heterostructure is an efficient technique for realizing fascinating properties and playing a key role in solar energy-driven water decomposition schemes. By means of first-principles calculations, this study reveals the intriguing potential of a novel 2D van der Walls hetero-bilayer consisting of GeC and AlN layer in the photocatalytic water splitting method to generate hydrogen. The GeC/AlN heterostructure has an appropriate band gap of 2.05 eV, wherein the band edges are in proper energetic positions to provoke the water redox reaction to generate hydrogen and oxygen. The type-II band alignment of the bilayer facilitates the real-space spontaneous separation of the photogenerated electrons and holes in the different layers, improving the photocatalytic activity significantly. Analysis of the electrostatic potential and the charge density difference unravels the build-up of an inherent electric field at the interface, preventing electron-hole recombination. The ample absorption spectrum of the bilayer from the ultra-violet to the near-infrared region, reaching up to 8.71 × 105/cm, combined with the resiliency to the biaxial strain, points out the excellent photocatalytic performance of the bilayer heterostructure. On top of rendering useful information on the key features of the GeC/AlN hetero-bilayer, the study offers informative details on the experimental design of the van der Walls bilayer heterostructure for solar-to-hydrogen conversion applications.

Efficient photocatalytic hydrogen production by bacteriochlorophyll-a derivatives with different esterifying side chains

Ti3C2Tx MXene, a new family of 2D materials, has been successfully used as an ideal co-catalyst for enhancing photocatalytic activity due to its excellent physicochemical properties. Bacteriochlorophyll (BChl), one of the most abundant photosynthetically active pigments, has been studied in the field of artificial photosynthesis due to its constant and efficient absorption of solar energy and conversion of the excitation energy into chemical energy. In this work, five BChl-a derivatives with a range of different side chain groups in the C17 substituent were prepared and deposited on the Ti3C2Tx MXene surface by a solvent evaporation process to form BChl-n@Ti3C2Tx (n = 1∼5) composite photocatalysts, which were used for the visible light-driven photocatalytic hydrogen evolution reaction (HER) in an aqueous solution. Among the five BChls investigated, BChl-5 possessing a quaternary ammonium terminal showed the best performance and BChl-5@Ti3C2Tx composite exhibited an excellent hydrogen production activity of as high as 51 μmol/h/gcat. This is primarily attributed to the excellent photophysical and photochemical properties provided by the self-aggregation of BChl-5 in the BChl-5@Ti3C2Tx composite and the outstanding charge transport capability of the 2D Ti3C2Tx MXene. At the same time, the fast carrier separation and transfer capability between BChl-5 and Ti3C2Tx provides reliable charge transport for photocatalytic HER.

Insights into Photogenerated Carrier Dynamics and Overall Water Splitting of the CrS3/GeSe Heterostructure

Based on the nonadiabatic molecular dynamics (NAMD) simulations and the first-principles calculations, we explore the overall water-splitting schemes and the photogenerated carrier dynamics for two configurations (CG and CyG) of the CrS3/GeSe van der Waals heterostructures. The photocatalytic direct Z-schemes and carrier migration pathways for hydrogen and oxygen evolution reactions (HER/OER) are constructed based on the electronic properties. The solar-to-hydrogen efficiency (η'STH values) of the schemes can reach 10.60% and 10.17% and further rise under tensile strain. The NAMD results demonstrate similar transfer times of the electron/hole for HER/OER and more rapid electron-hole recombination in CG enables it to be superior to CyG in photocatalytic performance. Moreover, the Gibbs free energy indicates that both the HERs and OERs turn to spontaneously proceed with CG and CyG at pH = 0-12.37 and pH = 2.55-11.01, respectively. These facts reveal that the CrS3/GeSe heterostructure is promising in photocatalytic overall water splitting.

Constructing a noble-metal-free 0D/2D CdS/SnS2heterojunction for efficient visible-light-driven photocatalytic pollutant degradation and hydrogen generation

Semiconductor photocatalysis has attracted the attention of a wide audience for its outstanding capabilities in water purification and energy conversion. Herein, a noble-metal-free nanoheterojunction is created by planting zero-dimensional (0D) CdS nanograins, of 10-20 nm in size, on the surface of 2D SnS2nanosheets (NSs) using anin situchemical bathing deposition process, where SnS2NSs have an average diameter of 400 nm and thicknesses of less than 20 nm. The possible formation mechanism of the CdS/SnS2(CS/SS) heterogeneous nanostructure is elaborated upon. The catalytic activities over CS/SS nanocomposites for the photodegradation of organic dye and hydrogen evolution from photolysis water splitting are examined under visible light irradiation. The apparent rate constant (k) of the optimal CS/SS-3 composite in the decontamination of methylene blue (MB) is up to 3.34 and 1.87 times as high as that of pristine SnS2and pure CdS counterparts, respectively. The optimized CS/SS-3 sample consistently achieves the highest photocatalytic hydrogen production rate, at 10.3 and 5.7 folds higher than that of solo SnS2and CdS panels, respectively. The boosted photocatalytic capacities of CdS/SnS2heterostructures are essentially attributed to the formation of the closely interfacial incorporation of CdS and SnS2semiconductors, resulting in the effective charge transportation and spatial separation of the photoinduced electron-hole pairs. Furthermore, the traditional type-II charge transfer pathway is proposed based on the perfect band structure and the free radical experiment results.

Reversible Stacking of 2D ZnIn2 S4 Atomic Layers for Enhanced Photocatalytic Hydrogen Evolution

It is technically challenging to reversibly tune the layer number of 2D materials in the solution. Herein, a facile concentration modulation strategy is demonstrated to reversibly tailor the aggregation state of 2D ZnIn2 S4 (ZIS) atomic layers, and they are implemented for effective photocatalytic hydrogen (H2 ) evolution. By adjusting the colloidal concentration of ZIS (ZIS-X, X = 0.09, 0.25, or 3.0 mg mL-1 ), ZIS atomic layers exhibit the significant aggregation of (006) facet stacking in the solution, leading to the bandgap shift from 3.21 to 2.66 eV. The colloidal stacked layers are further assembled into hollow microsphere after freeze-drying the solution into solid powders, which can be redispersed into colloidal solution with reversibility. The photocatalytic hydrogen evolution of ZIS-X colloids is evaluated, and the slightly aggregated ZIS-0.25 displays the enhanced photocatalytic H2 evolution rates (1.11 µmol m-2 h-1 ). The charge-transfer/recombination dynamics are characterized by time-resolved photoluminescence (TRPL) spectroscopy, and ZIS-0.25 displays the longest lifetime (5.55 µs), consistent with the best photocatalytic performance. This work provides a facile, consecutive, and reversible strategy for regulating the photo-electrochemical properties of 2D ZIS, which is beneficial for efficient solar energy conversion.

Photoreforming for Lignin Upgrading: A Critical Review

Photoreforming of lignocellulosic biomass to simultaneously produce gas fuels and value-added chemicals has gradually emerged as a promising strategy to alleviate the fossil fuels crisis. Compared to cellulose and hemicellulose, the exploitation and utilization of lignin via photoreforming are still at the early and more exciting stages. This Review systematically summarizes the latest progress on the photoreforming of lignin-derived model components and "real" lignin, aiming to provide insights for lignin photocatalytic valorization from fundamental to industrial applications. Considering the complexity of lignin physicochemical properties, related analytic methods are also introduced to characterize lignin photocatalytic conversion and product distribution. We finally put forward the challenges and perspective of lignin photoreforming, hoping to provide some guidance to valorize biomass into value-added chemicals and fuels via a mild photoreforming process in the future.

Magnetically Stimulable Graphene Oxide/Polypropylene Nanocomposites

Core-shell magnetic air-stable nanoparticles have attracted increasing interest in recent years. Attaining a satisfactory distribution of magnetic nanoparticles (MNPs) in polymeric matrices is difficult due to magnetically induced aggregation, and supporting the MNPs on a nonmagnetic core-shell is a well-established strategy. In order to obtain magnetically active polypropylene (PP) nanocomposites by melt mixing, the thermal reduction of graphene oxides (TrGO) at two different temperatures (600 and 1000 °C) was carried out, and, subsequently, metallic nanoparticles (Co or Ni) were dispersed on them. The XRD patterns of the nanoparticles show the characteristic peaks of the graphene, Co, and Ni nanoparticles, where the estimated sizes of Ni and Co were 3.59 and 4.25 nm, respectively. The Raman spectroscopy presents typical D and G bands of graphene materials as well as the corresponding peaks of Ni and Co nanoparticles. Elemental and surface area studies show that the carbon content and surface area increase with thermal reduction, as expected, following a reduction in the surface area by the support of MNPs. Atomic absorption spectroscopy demonstrates about 9-12 wt % metallic nanoparticles supported on the TrGO surface, showing that the reduction of GO at two different temperatures has no significant effect on the support of metallic nanoparticles. Fourier transform infrared (FT-IR) spectroscopy shows that the addition of a filler does not alter the chemical structure of the polymer. Scanning electron microscopy of the fracture interface of the samples demonstrates consistent dispersion of the filler in the polymer. The TGA analysis shows that, with the incorporation of the filler, the initial (T_onset) and maximum (T_max) degradation temperatures of the PP nanocomposites increase up to 34 and 19 °C, respectively. The DSC results present an improvement in the crystallization temperature and percent crystallinity. The filler addition slightly enhances the elastic modulus of the nanocomposites. The results of the water contact angle confirm that the prepared nanocomposites are hydrophilic. Importantly, the diamagnetic matrix is transformed into a ferromagnetic one with the addition of the magnetic filler.

Hydrogen-Bond Regulation of the Microenvironment of Ni(II)-Porphyrin Bifunctional Electrocatalysts for Efficient Overall Water Splitting

Accurately regulating the microenvironment around active sites is an important approach for boosting the overall water splitting performance of bifunctional electrocatalysts, which can drive both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) in the same electrolyte. Herein, pseudo-pyridine-substituted Ni(II)-porphyrins (o-NiTPyP, m-NiTPyP, and p-NiTPyP) with pseudo-pyridine N-atoms located at the ortho-, meta-, or para-position are prepared and used as model catalysts for alkaline water splitting. Experimental and theoretical results reveal that the pseudo-pyridine N-atom positions can regulate the microenvironment around the active sites and the adsorption free energy of H-donating substances by affecting the H-bonding interaction and the NNiN bond angles of active sites, and thus those pseudo-pyridine-substituted Ni(II)-porphyrins deliver better electrocatalytic activity than the Ni(II)-tetraphenylporphyrin (NiTPP) without pseudo-pyridine N-atoms. Among them, m-NiTPyP on carbon nanotubes delivers the lowest overpotentials of 267 and 138 mV at 10 mA cm^-2 for the OER and HER, respectively. Specifically, m-NiTPyP as bifunctional electrocatalyst in an alkaline electrolyzer requires only 1.62 V to drive efficient overall water splitting at 10 mA cm^-2 while remaining durable. This work proposes a new H-bond-regulating approach of the microenvironment of electrocatalysts for effectively boosting the overall water splitting activity and deeply understanding its related mechanism.

Alkali functionalized carbon nitride with internal van der Waals heterostructures: Directional charge flow to enhance photocatalytic hydrogen production

Improving the charge separation and migration in graphitic carbon nitride (CN) is the critical issue to enhance its photocatalytic performance, but still remains very challenging. Herein, the alkali metals were introduced into the interlayer and intralayer of CN to tackle this challenge. The lithium sodium-modifying carbon nitride layer (LiNaCN2) and the adjacent CN layer formed a van der Waals heterostructures (VDWHs), while the potassium-intercalating served as interlayer charge transfer channels to induce the directional charge flow. Experiments and theoretical calculations indicated that such unique construction provided intrinsic driving force to obtain the electrons from LiNaCN2 to CN via directional potassium channels. In accordance with the theoretical prediction, a dramatically red-shift of the light absorption feature was achieved for interlayer potassium-intercalating and intralayer lithium sodium-modifying co-functionalized carbon nitride (LiNaCN-K-CN2) to show narrowed bandgap energy of 2.15 eV. This directional charge flow in CN resulted in the rapid transfer of charge carriers in both interlayer as well as intralayer of CN, which reduced the electronic localization as well as extended the π conjugative effect. Consequently, the LiNaCN-K-CN2 displayed stable and remarkable hydrogen production rate of about 2.46 mmol g^-1 h^-1 with apparent quantum yield (AQY) of about 13.68% at 435 nm, which was 22 folds higher than that of the pristine CN. This finding provides the feasible strategy to precisely tune the directions of charge transfer for high-performance CN-based photocatalysts.

Ti3C2 MXene supporting platinum nanoparticles as rapid electrons transfer channel and active sites for boosted photocatalytic water splitting over g-C3N4

Two-dimension (2D) MXene materials have increasingly attracted attentions in improving the photocatalytic conversion of solar-to-chemical energy over graphitic carbon nitride (g-C₃N₄). In this work, Pt nanoparticles modified few-layer Ti₃C₂ MXene sheet (MXene@Pt) was successfully prepared by chemical reduction, which was used as efficient co-catalysts to enhance the photocatalytic hydrogen evolution over porous g-C₃N₄ (PCN). The high work function of MXene@Pt and the tight 2D/2D interfacial contact between MXene@Pt and PCN significantly promoted the transfer and separation of photogenerated electron-hole. Besides, the MXene@Pt could enhance the light-harvesting of PCN and provide plentiful active sites for hydrogen evolution reaction. The hydrogen evolution activity of optimum 2D/2D MXene@Pt modified PCN (PCN/MPt-5) composite was dramatically enhanced, even higher than that of equal Pt mass modified PCN. Besides, overall water splitting was realized via a two-electron pathway with H₂O₂ and H₂ generation. This work may provide the fabrication strategy for developing MXene-based co-catalyst in photocatalysis.

Anisotropic Heavy-Metal-Free Semiconductor Nanocrystals: Synthesis, Properties, and Applications

Heavy-metal (Cd, Hg, and Pb)-containing semiconductor nanocrystals (NCs) have been explored widely due to their unique optical and electrical properties. However, the toxicity risks of heavy metals can be a drawback of heavy-metal-containing NCs in some applications. Anisotropic heavy-metal-free semiconductor NCs are desirable replacements and can be realized following the establishment of anisotropic growth mechanisms. These anisotropic heavy-metal-free semiconductor NCs can possess lower toxicity risks, while still exhibiting unique optical and electrical properties originating from both the morphological and compositional anisotropy. As a result, they are promising light-emitting materials in use various applications. In this review, we provide an overview on the syntheses, properties, and applications of anisotropic heavy-metal-free semiconductor NCs. In the first section, we discuss hazards of heavy metals and introduce the typical heavy-metal-containing and heavy-metal-free NCs. In the next section, we discuss anisotropic growth mechanisms, including solution-liquid-solid (SLS), oriented attachment, ripening, templated-assisted growth, and others. We discuss mechanisms leading both to morphological anisotropy and to compositional anisotropy. Examples of morphological anisotropy include growth of nanorods (NRs)/nanowires (NWs), nanotubes, nanoplatelets (NPLs)/nanosheets, nanocubes, and branched structures. Examples of compositional anisotropy, including heterostructures and core/shell structures, are summarized. Third, we provide insights into the properties of anisotropic heavy-metal-free NCs including optical polarization, fast electron transfer, localized surface plasmon resonances (LSPR), and so on, which originate from the NCs' anisotropic morphologies and compositions. Finally, we summarize some applications of anisotropic heavy-metal-free NCs including catalysis, solar cells, photodetectors, lighting-emitting diodes (LEDs), and biological applications. Despite the huge progress on the syntheses and applications of anisotropic heavy-metal-free NCs, some issues still exist in the novel anisotropic heavy-metal-free NCs and the corresponding energy conversion applications. Therefore, we also discuss the challenges of this field and provide possible solutions to tackle these challenges in the future.

The regulating effect of boron doping and its concentration on the photocatalytic overall water splitting of a polarized g-C3N5 material

Photocatalytic overall water splitting with two-dimensional materials is a promising strategy to solve the problems of environmental pollution and energy shortage. However, conventional photocatalysts are often limited to a narrow visible photo-absorption range, low catalytic activity, and poor charge separation. Herein, given the intrinsic polarization facilitating the improvement of photogenerated carrier separation, we adopt a polarized g-C₃N₅ material combining the doping strategy to alleviate the abovementioned problems. Boron (B), as a Lewis acid, has a great chance to improve the capture and catalytic activity of water. By doping B into g-C₃N₅, the overpotential for the complicated four-electron process of the oxygen reduction reaction is only 0.50 V. Simultaneously, the B doping-induced impurity state effectively reduces the band gap and broadens the photo-absorption range. Moreover, with the increase of B doping concentration, the photo-absorption range and catalytic activity can be gradually improved. Whereas when the concentration exceeds 33.3%, the reduction potential of the conduction band edge will not meet the demand for hydrogen evolution. Therefore, excessive doping is not recommended in experiments. Our work affords not only a promising photocatalyst but also a practical design scheme by combining polarizing materials and the doping strategy for overall water splitting.

2D Transition Metal Dichalcogenides for Photocatalysis

Two-dimensional (2D) transition metal dichalcogenides (TMDs), a rising star in the post-graphene era, are fundamentally and technologically intriguing for photocatalysis. Their extraordinary electronic, optical, and chemical properties endow them as promising materials for effectively harvesting light and catalyzing the redox reaction in photocatalysis. Here, we present a tutorial-style review of the field of 2D TMDs for photocatalysis to educate researchers (especially the new-comers), which begins with a brief introduction of the fundamentals of 2D TMDs and photocatalysis along with the synthesis of this type of material, then look deeply into the merits of 2D TMDs as co-catalysts and active photocatalysts, followed by an overview of the challenges and corresponding strategies of 2D TMDs for photocatalysis, and finally look ahead this topic.

Biogenic Preparation of ZnO Nanostructures Using Leafy Spinach Extract for High-Performance Photodegradation of Methylene Blue under the Illumination of Natural Sunlight

To cope with environmental pollution caused by toxic emissions into water streams, high-performance photocatalysts based on ZnO semiconductor materials are urgently needed. In this study, ZnO nanostructures are synthesized using leafy spinach extract using a biogenic approach. By using phytochemicals contained in spinach, ZnO nanorods are transformed into large clusters assembled with nanosheets with visible porous structures. Through X-ray diffraction, it has been demonstrated that leafy spinach extract prepared with ZnO is hexagonal in structure. Surface properties of ZnO were altered by using 10 mL, 20 mL, 30 mL, and 40 mL quantities of leafy spinach extract. The size of ZnO crystallites is typically 14 nanometers. In the presence of sunlight, ZnO nanostructures mineralized methylene blue. Studies investigated photocatalyst doses, dye concentrations, pH effects on dye solutions, and scavengers. The ZnO nanostructures prepared with 40 mL of leafy spinach extract outperformed the degradation efficiency of 99.9% for the MB since hydroxyl radicals were primarily responsible for degradation. During degradation, first-order kinetics were observed. Leafy spinach extract could be used to develop novel photocatalysts for the production of solar hydrogen and environmental hydrogen.

InN/XS2 (X = Zr, Hf) vdW heterojunctions: promising Z-scheme systems with high hydrogen evolution activity for photocatalytic water splitting

Z-scheme van der Waals heterojunctions are very attractive photocatalysts attributed to their excellent reduction and oxidation abilities. In this paper, we designed InN/XS₂ (X = Zr, Hf) heterojunctions and explored their electronic structure properties, photocatalytic performance, and light absorption systematically using first-principles calculations. We found that the valence-band maximum (VBM) and conduction-band minimum (CBM) of the InN/XS₂ (X = Zr, Hf) heterojunctions are contributed by InN and XS₂, respectively. Photo-generated carriers transferring along the Z-path can accelerate the recombination of interlayer electron-hole pairs. Therefore, the photogenerated electrons in the CBM of the InN layer can be maintained making the hydrogen evolution reaction occur continuously, while photogenerated holes in the VBM of the Ti₂CO₂ layer make the oxygen evolution reaction occur continuously. The band edge positions of heterojunctions can straddle the required water redox potentials, while pristine InN and XS₂ (X = Zr, Hf) can only be used for photocatalytic hydrogen evolution or oxygen evolution, respectively. Furthermore, the HER barriers can be tuned by transition metal doping. With Cr doping, the hydrogen evolution reaction (HER) barriers decrease to -0.12 for InN/ZrS₂ and -0.05 eV for InN/HfS₂, very close to the optimal value (0 eV). In addition, the optical absorption coefficient is as high as 10⁵ cm^-1 in the visible and ultraviolet regions. Therefore, the InN/XS₂ (X = Zr, Hf) heterojunctions are expected to be excellent photocatalysts for water splitting.

Surface-assisted synthesis of biomass carbon-decorated polymer carbon nitride for efficient visible light photocatalytic hydrogen evolution

Template is frequently studied as a structure-directing agent to tune the nanomorphology of photocatalysts. However, the influences of template on the polymerization of precursors and compositions of the resulting samples are rarely considered. Herein, a biomass carbon-modified graphitic carbon nitride (CCNx) with a thin-layer morphology is synthesized via one-pot surface-assisted polymerization of melamine precursor on organic yeast. The formation of the hydrogen bond between melamine and yeast induces a strong interfacial confinement, giving rise to small-sized CCNx. In addition, the carbon materials derived from yeast dramatically broaden n → π* visible light harvesting, improve electron delocalization, and greatly enhance charge carrier separation. The optimized CCNx presents a much higher photocatalytic hydrogen production rate of 2704 μmol g^-1h^-1 under visible light irradiation (λ ≥ 420 nm), which is nearly 11-fold that of its pristine counterpart. This work realizes the synergistic effect between morphology tunning and composition tailoring by using biomass template, which shows a great potential in developing efficient metal-free photocatalysts.