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.

Sn-based materials in photocatalysis: A review

Development and the application of Sn-based materials have become more prevalent in recent years due to concerns regarding the energy crisis, environmental pollution, and the urgent need of constructing inexpensive and highly effective photocatalysis. The recent advancement in Sn-based materials for efficient photocatalysts, such as Sn alloys, Sn oxides, Sn sulfides, Sn selenides, Sn niobates, Sn tantalites, and Sn tungstates, is summarized in this study. Several design ideas for increasing the photoactivity of Sn-based materials in various photocatalytic applications are emphasized. In addition, we considered their present applications in energy generation (H2 evolution, CO2 reduction, and N2 fixation) and environmental remediation (air purification and wastewater treatment). As a result, the current review will deepen the reader's understanding of the properties and potential uses of Sn-based materials in photocatalysis. Hence, this paper will serve as a guide in promoting the domain of Sn-based materials for future photocatalytic technologies.

Facile synthesis of SnNb2O6@C composite with ultrathin carbon layer as anode materials for high-performance sodium-ion batteries

Niobium-based oxides have attracted a lot of attention as anode materials for sodium-ion batteries (SIBs) due to their high theoretical specific capacity, excellent rate capability and exceptional safety. However, their poor intrinsic electronic conductivity and sluggish sodium ions diffusion kinetics severely hinder their practical applicability. Here, SnNb 2 O 6 @C was successfully prepared by a simple solid-state reaction technique coupled with carbon coating. HRTEM images show that the SnNb 2 O 6 @C particles are covered by a uniformly ultrathin amorphous carbon layer of about 1.8 nm, thus improving the electronic conductivity and diffusion coefficient of sodium ions. As anode for SIBs, the as-obtained SnNb 2 O 6 @C material exhibits excellent specific capacity (369 mAh g -1 at a current density of 50 mA g -1 ) and remarkable rate performance (177 mAh g -1 at 1000 mA g -1 ), which indicates its good prospect in practical application.

Interior and Surface Synergistic Modifications Modulate the SnNb2O6/Ni-Doped ZnIn2S4 S-Scheme Heterojunction for Efficient Photocatalytic H2 Evolution

Interior and surface synergistic modifications can endow the photocatalytic reaction with tuned photogenerated carrier flow at the atomic level. Herein, a new class of 2D/2D SnNb₂O₆/Ni-doped ZnIn₂S₄ (SNO/Ni-ZIS) S-scheme heterojunctions is synthesized by a simple hydrothermal strategy, which was used to evaluate the synergy between interior and surface modifications. Theoretical calculations show that the S-scheme heterojunction boosts the desorption of H atoms for rapid H₂ evolution. As a result, 25% SNO/Ni_0.4-ZIS exhibits significantly improved PHE activity under visible light, roughly 4.49 and 2.00 times stronger than that of single ZIS and Ni_0.4-ZIS, respectively. In addition, 25% SNO/Ni_0.4-ZIS also shows superior structural stability. This work provides advanced insight for developing high-performance S-scheme systems from photocatalyst design to mechanistic insight.

Understanding the morphology evolution of 1D BiVO4 nanoarrays from nanorods to nanocones with enhanced photocatalytic performance

The controllable synthesis of BiVO4 nanocone and nanorod arrays is reported. The morphology transformation of BiVO4 nanoarrays is accompanied with growth mechanism transformation and the tip structure results in superior photocatalytic performance.

Atomically Thin 2D Photocatalysts for Boosted H2 Production from the perspective of Transient Absorption Spectroscopy

Excited state photophysical processes play the most important role in deciding the efficiency of any photonic applications like solar light driven H2 evolution, which is considered to be the next...

Enhanced photocatalytic driven hydroxylation of phenylboric acid to phenol over pyrenetetrasulfonic acid intercalated ZnAl-LDHs

Layered double hydroxides (LDHs) own admirable potential due to their controllable composition and exchangeable interlayer anions. Herein, pyrenetetrasulfonic acid (PTS) intercalated ZnAl-LDHs (denoted as ZnAl-xPTS, x represents the amount of NaPTS in the starting material) are synthesized by a co-precipitation method, which display enhanced photocatalytic activity towards the hydroxylation of phenylboric acid to phenol. Various characterizations suggest that PTS plays significant roles in improving the photocatalytic activity: (1) PTS extends the light absorption from ultraviolet to visible light region; (2) the introduction of PTS upshifts the conduction band, which is feasible for the formation of O₂^∙-; (3) ZnAl-xPTS produces more free electrons under light irradiation, which leads to greatly improved activity. This study develops an alternative LDHs based photocatalyst for the production of phenol, which also provides an efficient strategy to improve the photocatalytic activity of LDHs.

Facile one-step hydrothermal synthesis of single-crystalline SnNb2O6 nanosheets with greatly extended visible-light response for enhanced photocatalytic performance and mechanism insight

The conventional preparation of SnNb₂O₆ invariably involves complex and laborious steps, which unavoidably introduces defect into the host lattice and also increases the reaction period and costs, resulting in undesired recombination of photo-generated electron-hole pairs. For the first time in this work, we manage to synthesize single-crystalline two-dimensional (2D) SnNb₂O₆ nanosheets with ultrathin structure through a facile one-step hydrothermal method. Comparative studies were explored to analyze the structure and phase evolution during the preparation course. The synthesized 2D structure demonstrated a narrower band gap of 2.09 eV and specific surface area of 76.1 m² g^-1, which exhibited significantly extended visible-light-responsive range and larger surface area by contrast with the state-of-the-art reports, resulting in excellent visible-light-driven photoactivity towards H₂ production and water purification as well. Additionally, further enhanced photocatalytic performance was achieved by the incorporation of Pt as co-catalyst to indirectly indicate the advantage of the SnNb₂O₆ nanosheets in this method over other reported counterparts. It was found that, a very small amount of Pt loaded on the surface of SnNb₂O₆ nanosheets would contribute to remarkably higher activity than pure SnNb₂O₆ nanosheets and exhibit superior stability as well. Moreover, a deep insight into the underlying photocatalytic mechanism was proposed. This work sheds light on a new facile way to fabricate high-performance photocatalytic materials and provided new opportunities for solar-energy conversion.

SnNb2O6/NiCo-LDH Z-scheme heterojunction with regulated oxygen vacancies obtained by engineering the crystallinity for efficient and renewable photocatalytic H2 evolution

SnNb2O6/NiCo-LDH Z-scheme heterojunction with abundant oxygen vacancies exhibited highly activity and stability toward photocatalytic H2 evolution, ascribed to the regeneration of oxygen vacancy by engineering the crystallinity.

Eco-friendly synthesis of self-regenerative low-cost biosorbent by the incorporation of CuO: a photocatalyst sensitive to visible light irradiation for azo dye removal

Acid pretreated biomass Lemna minor (BM-H3PO4) was used as support for CuO nanoparticles loading, to investigate the dye biosorption capacity and the photocatalytic performance under artificial visible light. The surface morphology, crystal structure, elemental composition, and the bandgap of modified biomass have been determined using FE-SEM, XRD, EDX, XPS, FTIR, and UV-DR analysis. The results showed that NH2 and P-O functional groups of (BM-H3PO4) can attract the copper ions (Cu2+), which can facilitate the loading of CuO nanoparticles hence, smaller nanoparticles with an average diameter of 21 nm was obtained. It was also found that when the CuO was incorporated in BM-H3PO4 in a proper mass ratio of 0.4, the biosorption efficiency was enhanced to 3 times compared with BM-H3PO4 and reached a maximum of 91%, at a dye concentration of 20 mg/L, solution pH equal to 5, and an ambient temperature of 25 °C. Furthermore, CuO-modified BM-H3PO4 exhibits a better photocatalytic activity than pure CuO in the presence of H2O2 and visible light irradiation, where the dye was completely removed and mineralized after 240 min, evidenced by COD measurement. The photocatalytic regeneration also shows that the biosorption efficiency was maintained at 91% over 3 cycles, indicating the significant self-regenerative capacity of the biosorbent.

Two-Dimensional Materials and Composites as Potential Water Splitting Photocatalysts: A Review

Hydrogen production via water dissociation under exposure to sunlight has emanated as an environmentally friendly, highly productive and expedient process to overcome the energy production and consumption gap, while evading the challenges of fossil fuel depletion and ecological contamination. Various classes of materials are being explored as viable photocatalysts to achieve this purpose, among which, the two-dimensional materials have emerged as prominent candidates, having the intrinsic advantages of visible light sensitivity; structural and chemical tuneability; extensively exposed surface area; and flexibility to form composites and heterostructures. In an abridged manner, the common types of 2D photocatalysts, their position as potential contenders in photocatalytic processes, their derivatives and their modifications are described herein, as it all applies to achieving the coveted chemical and physical properties by fine-tuning the synthesis techniques, precursor ingredients and nano-structural alterations.

Atomically thin two-dimensional ZnSe/ZnSe(ea)x van der Waals nanojunctions for synergistically enhanced visible light photocatalytic H2 evolution

Two-dimensional (2D) photocatalysts have been widely studied due to their short charge carrier migration pathways and tunable electronic structures. Herein, a facile one-pot solvothermal process with ethylamine (ea) constructs a novel 2D nanojunction based on ZnSe. The ea molecules coordinate with Zn2+ to form 2D ZnSe(ea)x, where the consequent 2D ZnSe grows in an epitaxial way resulting in the self-assembled 2D/2D ZnSe/ZnSe(ea)x nanojunctions driven by van der Waals (VDW) force, which largely extend the absorption range. The atomic thickness of the 2D structure offers a short charge migration pathway, low electric resistance and rich active sites for the surface reaction of photocatalysis. All the above favorable factors work synergistically to reach a superior hydrogen evolution of 2875 μmol g-1 h-1 under visible light irradiation (≥420 nm) and a notable quantum yield of 64.5% at 450 nm, which are among the highest recorded values of non-noble metal photocatalysts.

Progress on ternary oxide-based photoanodes for use in photoelectrochemical cells for solar water splitting

Solar water splitting using photoelectrochemical cells (PECs) has emerged as one of the most promising routes to produce hydrogen as a clean and renewable fuel source. Among various semiconductors that have been considered as photoelectrodes for use in PECs, oxide-based photoanodes are particularly attractive because of their stability in aqueous media in addition to inexpensive and facile processing compared to other types of semiconductors. However, they typically suffer from poor charge carrier separation and transport. In the past few years, there has been tremendous progress in developing ternary oxide-based photoelectrodes, specifically, photoanodes. The use of ternary oxides provides more opportunities to tune the composition and electronic structure of the photoelectrode compared to binary oxides, thus providing more freedom to tune the photoelectrochemical properties. In this article, we outline the important characteristics to analyze when evaluating photoanodes and review the major recent progress made on the development of ternary oxide-based photoanodes. For each system, we highlight the favorable and unfavorable features and summarize the strategies utilized to address the challenges associated with each material. Finally, by combining our analyses of all the photoanodes surveyed in this review, we provide possible future research directions for each compound and an outlook for constructing more efficient oxide-based PECs. Overall, this review will provide a critical overview of current ternary oxide-based photoanodes and will serve as a platform for the design of future oxide-based PECs.

Simple Synthesis of K₄Nb₆O17/C Nanosheets for High-Power Lithium-Ion Batteries with Good Stability

In this work, a series of two-dimensional (2D) large-size nanosheets were prepared through one-step exfoliation of the huge K₄Nb₆O17 crystals. The K₄Nb₆O17 nanosheets with the thickness of about 2 nm was used as the templates of dopamine polymerization and was then carbonized to form C-doped K₄Nb₆O17 nanosheets. More importantly, the C-doped K₄Nb₆O17 nanosheets exhibited excellent electrochemical performance with high specific capacity (381 mA h g-1 at 0.05 A g-1, 0.5⁻3.0 V vs. Li/Li⁺) and stable cyclability at high current density (remarkably, preserved a capacity of discharge approximately 90 mA h g-1 at 5 A g-1 after 1000 cycles). The good electrochemical performances of the C-doped K₄Nb₆O17 nanosheets can be attributed to the outstanding 2D structure and large specific surface, which afforded the short transport route for ion and electron. These noteworthy results demonstrated that the new 2D nanomaterials might be potential candidates for the high-performance, environmentally friendly, and low-cost electrochemical energy storage equipment.

Amorphous MoS2 decorated on uniform Cd0.8Zn0.2S microspheres with dramatically improved photocatalytic hydrogen evolution performance

A series of Cd_1−xZn_xS solid solutions and samples of amorphous MoS₂ decorated on Cd_1−xZn_xS microspheres were successfully prepared.

Atomically Thin 2D Multinary Nanosheets for Energy-Related Photo, Electrocatalysis

The severe energy crisis and environmental issues have led to an increase in research on the development of sustainable energy. Atomically thin 2D multinary nanosheets with tunable components show advantages for producing sustainable energy via photo, electrocatalysis processes. Here, recent progress of atomically thin 2D multinary nanosheets from the design, synthesis, tuning, and sustainable energy production via photo, electrocatalysis processes is summarized. The regulating strategies such as alloying, doping, vacancy engineering, pores construction, surface modification, and heterojunction are summarized, focusing on how to optimize the catalytic performance of atomically thin 2D multinary nanosheets. In addition, advancements in versatile energy-related photo, electrocatalytic applications in the areas of oxygen evolution, oxygen reduction, hydrogen evolution, CO2 reduction, and nitrogen fixation are discussed. Finally, existing challenges and future research directions in this promising field are presented.

Phase Transformation Synthesis of Strontium Tantalum Oxynitride-Based Heterojunction for Improved Visible Light-Driven Hydrogen Evolution

Tantalum oxynitride-based materials, which possess narrow band gaps and sufficient band energy potentials, have been of immense interest for water splitting. However, the efficiency of photocatalytic reactions is still low because of the fast electron-hole recombination. Here, a Sr₂Ta₂O_{7- x}N _x/SrTaO₂N heterostructured photocatalyst with a well-matched band structure was in situ constructed by the nitridation of hydrothermal-prepared Sr₂Ta₂O₇ nanosheets. Compared to Sr₂Ta₂O_{7- x}N _x and pure SrTaO₂N, the Sr₂Ta₂O_{7- x}N _x/SrTaO₂N heterostructured photocatalyst exhibited the highest rate of hydrogen evolution, which is ca. 2.0 and 76.4 times of Sr₂Ta₂O_{7- x}N _x and pure SrTaO₂N, respectively, under the similar reaction condition. The enhanced performance arises from the formation of suitable band-matched heterojunction-accelerated charge separation. This work provides a promising strategy for the construction of tantalum oxynitride-based heterojunction photocatalysts.

Steering Charge Kinetics of Tin Niobate Photocatalysts: Key Roles of Phase Structure and Electronic Structure

Tin niobate photocatalysts with the phase structures of froodite (SnNb₂O₆) and pyrochlore (Sn₂Nb₂O₇) were obtained by a facile solvothermal method in order to explore the impact of phase structure and electronic structure on the charge kinetics and photocatalytic performance. By employing tin niobate as a model compound, the effects of phase structure over electronic structure, photocatalytic activity toward methyl orange solution and hydrogen evolution were systematically investigated. It is found that the variation of phase structure from SnNb₂O₆ to Sn₂Nb₂O₇ accompanied with modulation of particle size and band edge potentials that has great consequences on photocatalytic performance. In combination with the electrochemical impedance spectroscopy (EIS), transient photocurrent responses, transient absorption spectroscopy (TAS), and the analysis of the charge-carrier dynamics suggested that variation of electronic structure has great impacts on the charge separation and transfer rate of tin niobate photocatalysts and the subsequent photocatalytic performance. Moreover, the results of the X-ray photoelectron spectroscopy (XPS) indicated that the existent of Sn⁴⁺ species in Sn₂Nb₂O₇ could result in a decrease in photocatalytic activity. Photocatalytic test demonstrated that the SnNb₂O₆ (froodite) catalyst possesses a higher photocatalytic activity toward MO degradation and H₂ evolution compared with the sample of Sn₂Nb₂O₇ (pyrochlore). On the basis of spin resonance measurement and trapping experiment, it is expected that photogenerated holes, O₂^-•, and OH^• active species dominate the photodegradation of methyl orange.

Preparation of Y2SiO5:Pr3+,Li and Na2NbxTa2−xO6/(Au/RGO) composites and investigation into visible-light driven photocatalytic hydrogen production

A three-component photocatalytic system is constructed by using Na₂Nb_xTa_2−xO₆ as the main catalyst, Y₂SiO₅:Pr³⁺,Li as the up-conversion luminescence agent and Au/RGO as the co-catalyst.

Unique physicochemical properties of two-dimensional light absorbers facilitating photocatalysis

The emergence of two-dimensional (2D) materials with a large lateral size and extremely small thickness has significantly changed the development of many research areas by producing a variety of unusual physicochemical properties.

Enhanced photocatalytic performance and stability of visible-light-driven Z-scheme CdS/Ag/g-C3N4 nanosheets photocatalyst

A novel Z-scheme CdS/Ag/g-C₃N₄ nanosheets photocatalyst was synthesized by combining the hydrothermal and photo-deposition techniques, where Ag nanoparticles as carrier transport channels significantly improve the separation efficiency of electron–hole pairs.

Ultrathin 2D Photocatalysts: Electronic-Structure Tailoring, Hybridization, and Applications

As a sustainable technology, semiconductor photocatalysis has attracted considerable interest in the past several decades owing to the potential to relieve or resolve energy and environmental-pollution issues. By virtue of their unique structural and electronic properties, emerging ultrathin 2D materials with appropriate band structure show enormous potential to achieve efficient photocatalytic performance. Here, the state-of-the-art progress on ultrathin 2D photocatalysts is reviewed and a critical appraisal of the classification, controllable synthesis, and formation mechanism of ultrathin 2D photocatalysts is presented. Then, different strategies to tailor the electronic structure of ultrathin 2D photocatalysts are summarized, including component tuning, thickness tuning, doping, and defect engineering. Hybridization with the introduction of a foreign component and maintaining the ultrathin 2D structure is presented to further boost the photocatalytic performance, such as quantum dots/2D materials, single atoms/2D materials, molecular/2D materials, and 2D-2D stacking materials. More importantly, the advancement of versatile photocatalytic applications of ultrathin 2D photocatalysts in the fields of water oxidation, hydrogen evolution, CO₂ reduction, nitrogen fixation, organic syntheses, and removal pollutants is discussed. Finally, the future opportunities and challenges regarding ultrathin 2D photocatalysts to bring about new opportunities for future research in the field of photocatalysis are also presented.

New Insight of Water-Splitting Photocatalyst: H2O2-Resistance Poisoning and Photothermal Deactivation in Sub-micrometer CoO Octahedrons

Hydrogen production by photocatalytic overall water-splitting represents an ideal pathway for clean energy harvesting, for which developing high-efficiency catalysts has been the central scientific topic. Nanosized CoO with high solar-to-hydrogen efficiency (5%) is one of the most promising catalyst candidates. However, poor understanding of this photocatalyst leaves the key issue of rapid deactivation unclear and severely hinders its wide application. Here, we report a sub-micrometer CoO octahedron photocatalyst with high overall-water-splitting activity and outstanding ability of H₂O₂-resistance poisoning. We show that the deactivation of CoO catalyst originates from the unintended thermoinduced oxidation of CoO during photocatalysis, with coexistence of oxygen and water. We then demonstrate that introduction of graphene, as a heat conductor, largely enhanced the photocatalytic activity and stability of the CoO. Our work not only provides a new insight of CoO for photocatalytic water splitting but also demonstrates a new concept for photocatalyst design.

Efficient Solar Light Harvesting CdS/Co9 S8 Hollow Cubes for Z-Scheme Photocatalytic Water Splitting

Hollow structures with an efficient light harvesting and tunable interior component offer great advantages for constructing a Z-scheme system. Controlled design of hollow cobalt sulfide (Co₉ S₈ ) cubes embedded with cadmium sulfide quantum dots (QDs) is described, using hollow Co(OH)₂ as the template and a one-pot hydrothermal strategy. The hollow CdS/Co₉ S₈ cubes utilize multiple reflections of light in the cubic structure to achieve enhanced photocatalytic activity. Importantly, the photoexcited charge carriers can be effectively separated by the construction of a redox-mediator-free Z-scheme system. The hydrogen evolution rate over hollow CdS/Co₉ S₈ is 134 and 9.1 times higher than that of pure hollow Co₉ S₈ and CdS QDs under simulated solar light irradiation, respectively. Moreover, this is the first report describing construction of a hollow Co₉ S₈ based Z-scheme system for photocatalytic water splitting, which gives full play to the advantages of light-harvesting and charges separation.

Transparent Nb-doped TiO2 films with the [001] preferred orientation for efficient photocatalytic oxidation performance

[001]-Oriented Nb-TiO₂ films via topotactic transformation from [100]-oriented Nb-TiN exhibit efficient photoactivity due to highly-reactive-facet exposure and increased surface-reactive sites.

Ultrathin-nanosheet-assembled Bi2MoO6 mesoporous hollow framework for realizing optimized sunlight-driven photocatalytic water oxidation

A Bi₂MoO₆ mesoporous hollow framework composed of atomically-thin nanosheets was fabricated for the first time using a one-step solvothermal route and exhibited improved photocatalytic performance for water oxidation.

State-of-the-art Sn2+-based ternary oxides as photocatalysts for water splitting: electronic structures and optoelectronic properties

Synthesis and electronic structures for Sn²⁺-based oxide materials are reviewed in an attempt to develop visible-light-responsive photocatalysts.