Heterostructured two dimensional materials of MXene and graphene by hydrothermal method for efficient hydrogen production and HER activities

Recent development on two-dimensional (2D) heterostructured graphene and MXene materials were explored for electrochemical water splitting hydrogen evolution reaction (HER) activity. The hybrid MXene/reduced graphene oxides as two-dimensional (2D) hybrid structures were prepared by facile hydrothermal techniques at 150 °C with MXene and RG hybrid layered composites. As-prepared electrocatalytic active materials have been confirmed through structural and surface morphological studies such as XRD, RAMAN, FT-IR and SEM analysis. The prepared 2D materials were carried out for HER activities due to attractive conductivity and mass transfer process. HER performance were tested from linear sweep voltammetry (LSV) cures. The prepared MX, RG and MX@RG hybrid electrocatalyst exhibited overpotential values as observed as 220 mV, 193 mV, 121 mV respectively at 10 mAcm^?2 cathodic on set. MX@RG hybrid heterostructure exhibited enhanced HER action with lowest overpotential (η = 121 mV) and good H₂ productions as an active future electrocatalyst for energy storage and conversion applications.     

g-C3N4/CoAl-LDH 2D/2D hybrid heterojunction for boosting photocatalytic hydrogen evolution

In this work, a 2D/2D heterojunction composed of CoAl layered double hydroxide (LDH) and graphitic carbon nitride nanosheets (CNNS) was designed and fabricated for boosting photocatalytic hydrogen generation. The as-prepared 20 mol% CoAl-LDH/CNNS exhibited a remarkable photocatalytic hydrogen evolution rate of 680.13 μmol h⁻¹ g⁻¹, which was 21 times higher than that of pure CoAl-LDH (32.91 μmol h⁻¹ g⁻¹). The enhanced activity could be mainly attributed to its unique structure and high surface area. Distinct from ordinary heterojunction photocatalysts, two-dimensional (2D) heterojunctions with abundant 2D coupling interfaces and strong interfacial interaction could efficiently suppress the recombination of photo-induced charge carriers and shorten charge transmission distance. Particularly, compared with other concentrations, the increased surface area (138.70 m² g⁻¹) of 20 mol% CoAl-LDH/CNNS, which is 3.94 times of pure CNNS (35.48 m² g⁻¹), is more favorable for enhanced photocatalytic activity. Increasing the surface area of sheet-on-sheet heterostructure is an effective and novel strategy to facilitate the photocatalytic hydrogen evolution from water splitting.     

0D, 1D and 2D nanomaterials for visible photoelectrochemical water splitting. A Review

Global energy problems of the 21st century have led to the search for alternative energy sources, among which is hydrogen produced via photoelectrochemical solar water splitting. Photo-electrochemical water splitting using semiconductor nanostructured materials is a progressive method for producing hydrogen. The unique electronic, mechanical, surface and optical properties of nanomaterials make it possible to create photocatalysts with complex structures of energy zones, allowing the use of a wide range of sunlight and exerting a positive effect on absorption and scattering of sunlight. This review contains a detailed analysis of current studies aimed at improving the efficiency of photocatalytic systems by using 0D, 1D and 2D nanostructures. Special attention is paid to the mechanisms of photocatalytic water splitting to produce hydrogen with the help of various nanostructures.     

Synergistically modulating the electronic structure of Cr-doped FeNi LDH nanoarrays by O-vacancy and coupling of MXene for enhanced oxygen evolution reaction

Water splitting is an environmentally friendly method of hydrogen generation. However, it is severely limited by the slow anodic oxygen evolution reaction (OER). Iron-nickel layered double hydroxides (FeNi LDH) are promising electrocatalysts for OER, but their intrinsically low electrical conductivity and activity limit the practical applications. Herein, chromium-doped FeNi LDH nanoarrays in situ vertically grown on the surface of the Ti₃C₂T_x MXene (Cr-FeNi LDH/MXene) are successfully synthesized. Remarkably, the robust interaction and electrical coupling between Cr–FeNi LDH and MXene, as well as conspicuous charge transfer and the oxygen vacancies optimizing the adsorption free energy of intermediates, equip the nanocomposites with brilliant catalytic activity and stability toward OER. Thus, the optimized Cr–FeNi LDH/MXene shows a considerable boost in the OER, which affords low overpotential (232 mV at 10 mA cm^?2) and excellent durability. This work offers a new path to designing highly efficient and earth-abundant catalysts for water splitting and beyond.     

Chemical strategies in molybdenum based chalcogenides nanostructures for photocatalysis

Since the last two decades, plenty of environmental issues have risen up due to the damage which humans have caused to the planet for the sake of development. The continual ignorance of global climate change and the stalemate approach of major oil producing industries led to the catastrophic melting of glaciers in Arctic and Antarctica and very recently the highest mountain peak of Sweden have become 24 m shorter which is the evident outcome of climatic disturbance. The chaotic unbalancing in the reservoirs of natural resources is leading us to the several crisis which has a potential to affect the livelihood. Among the various techniques used for the development of sustainable energy, photocatalysis is regarded as one of the simplest technique which can yield enormous amount of energy by the utilization of solar energy for meeting the world's demand of an energy requirement and which can be exploited in the degradation of toxic pollutants i.e. organic as well as inorganic pollutants for environment remediation. Transition metal chalcogenides (TMCs) have a potential to get adsorb easily and be utilized for solving the energy-related problems. Large number of photocatalysts has been fabricated, among them Molybdenum (Mo) chalcogenides nanostructures, which also belong to the class of TMCs exhibit exceptional properties such as non-toxicity, low cost and structural flexibility which give them edge over the other materials. Furthermore, the tunable band gap of Mo chalcogenides nanostructures makes them the promising candidates for efficient hydrogen evolution via photocatalytic water splitting in the visible light illumination. This review deals with the photocatalytic applications of Mo based chalcogenides nanostructures in efficient hydrogen production via water splitting and degradation of dyes. It also discusses the recent developments in fabricating Molybdenum chalcogenides nanostructures, their role in the photocatalytic water splitting and discusses the efforts which have been made to improve their photocatalytic activity for extending their applications to the scalable point.     

Robust hydrogen generation over layered crystalline silicon materials via integrated H2 evolution routes

Hydrogen generation is the initial challenge in utilization of hydrogen energy. In this work, robust hydrogen generation with a high yield of 53,930 μmol g⁻¹ is demonstrated over layered crystalline silicon material derived from topochemical reaction from CaSi₂. The physicochemical properties of the resultant layered crystalline Si material before and after H₂ generation are investigated in detail to illustrate the H₂ generation mechanism. Integrated H₂ evolution routes, including destruction of Si–H bonds, oxidation of Si–Si bonds (hydrolysis of Si) and photocatalytic splitting water, are revealed to be responsible for the robust H₂ generation. This work delivers a facile route to synthesize layered crystalline Si material with promising H₂ generation performance and gives a deeply insight into the H₂ evolution mechanisms of Si-based materials.     

Recent advances in covalent organic framework (COF) nanotextures with band engineering for stimulating solar hydrogen production: A comprehensive review

Due to the continuous consumption of fossil fuels, natural reserves are depleting and it has been earnest need for developing new sources of energy. Among the several solar energy conversion techniques, photocatalytic hydrogen (H₂) generation is regarded as one of the most promising routes. Till date, several metal-based semiconductor materials have been investigated, however, H₂ generation is not substantial with the notion of sustainable development. Current research trends show the growing interest in advanced and metal free photocatalyst materials such as covalent organic frameworks (COFs) due to their several benefits such as crystalline porous polymers with pre-designed architectures, large surface area, exceptional stability, and ease of molecular functionalization. By combining COFs with other functional materials, composites may be created that display unique characteristics that exceed those of the separate components. This work provides a comprehensive development on COFs as a photocatalysts and their composites/hybrids for photocatalytic hydrogen generation with a focus on visible-light irradiation. To reduce the dependency on novel metals and overcome the drawbacks of individual material, the creation of composite materials based on covalent-organic frameworks (COFs) are systematically discussed. In addition, advantages in terms of performance, stability, durability of composites/hybrids COFs for photocatalytic hydrogen production in reference to traditional catalysts are investigated. Different composites such as metals loading, morphological development, band engineering, and heterojunctions are systematically discussed. Finally, challenges and opportunities associated with constructing COF-based catalysts as future research prospective for chemistry and materials science are highlighted.     

Two-dimensional Sc2C: A reversible and high-capacity hydrogen storage material predicted by first-principles calculations

Recently, a new family of two-dimensional (2D) MXene materials was prepared by exfoliating the MAX phases (ACS Nano 2012, 6, 1322). Among all possible MXene phases, theoretically 2D Sc₂C possesses the highest surface area per weight and thus is expected to have the highest gravimetric hydrogen storage capacities. In this work, using first-principles total energy pseudopotential calculations, we systematically investigated the hydrogen storage properties of 2D Sc₂C phase. Depending on different adsorption sites, the hydrogens are bound by three modes: chemisorption, physisorption and Kubas-type interactions with the binding energies of 4.703, 0.087 and 0.164 eV respectively. The maximum hydrogen storage capacity was calculated to be 9.0 wt.%, which meets the gravimetric storage capacity target (5.5 wt.% by 2015) set by the U.S. DOE. Ab-initio molecular dynamic simulations confirmed that 3.6 wt.% hydrogen molecules storaged by Kubas-type interactions can be adsorbed and released reversibly at ambient conditions.     

Pristine and Se-/In-doped TlAsS2 enhance the solar energy-driven water splitting for hydrogen generation

The enhanced photocatalytic performance of Se-/In-doped TlAsS₂ to generate hydrogen from water splitting is investigated based on the first-principle density functional theory calculation with meta-GGA + TPSS. Three structures, namely, pristine TlAsS₂ and substitutions of S with Se and Tl with In, are considered. Their geometrical lattices are fully optimized and their electronic and optical properties are calculated to evaluate the photocatalytic efficiency for hydrogen generation. Results show that the three structures can be used for solar energy photocatalysis to generate hydrogen from water splitting. Moreover, the Se- and In-doped atoms can strengthen the absorption coefficient within the visible light range. Therefore, these structures are promising catalysts for generating hydrogen from water splitting through solar energy photocatalysis.     

Investigating hydrogen evolution reaction properties of a new honeycomb 2D AlC

The hydrogen due to its high mass energy density is a new renewable, economically viable and clean resource. The most eco-friendly and economical approaches for the generation of hydrogen through hydrogen evolution is electrochemical water splitting. The two-dimensional (2D) nanomaterials have been recently found as potential candidates as non-noble metal catalyst for hydrogen evolution. In this work, we have systematically studied the structural and electronic properties of the newly predicted hexagonal-aluminium carbide monolayer (h-AlC ML) under the framework of dispersion-corrected density functional theory (DFT) calculations. The calculated electronic total density of states (TDOS) of h-AlC ML predict its metallic nature in contrast to other polar honeycomb 2D materials which are either semiconducting or semimetallic. The metallic behavior of h-AlC monolayer which motivates us to investigate its HER activity results due to the presence of delocalized charge density near Fermi level. Thus, we have investigated the HER activity of h-AlC ML by calculating hydrogen (H) adsorption energy (ΔE_H) and Gibbs free energy (ΔG_H) at three different sites of the 3 × 3 and 4 × 4 supercells of h-AlC ML; top of carbon atom (E_H-C), top of aluminium atom (E_H-Al) and hollow site (E_H-Hollow). Our results show that the hollow site is most catalytically active site in both supercells of h-AlC ML. We believe that our results will inspire experimentalists to fabricate this new 2D material for achieving the desired range of HER activity.     

Layered NiFe-LDH/MXene nanocomposite electrode for high-performance supercapacitor

Layered double hydroxide (LDH) is potentially excellent supercapacitor (SC) materials, but the low conductivity and easy agglomeration limit the further improvement of their electrochemical properties. Therefore, LDHs are requisite to grow on some conductive substrates to produce high-performance SC. In this paper, the conductive two-dimensional (2D) transition metal carbides, nitrides and carbonitrides (called MXene) were explored as the substrate to directly deposit NiFe-LDH nanosheets by a one-step hydrothermal method, then a three-dimensional (3D) porous NiFe-LDH/MXene electrode was obtained. The morphology and electrochemical performance of the composite electrodes were analyzed and investigated. The results show that the NiFe-LDH/MXene electrode has larger specific capacitance (720.2 F/g) than NiFe-LDH (465 F/g), and the capacitance of the composite electrode retained 86% after 1000 cycles (only 24% for NiFe-LDH), showing excellent cycle stability. The improved electrochemical performance of the composites is caused by the stable sheet-like structure of NiFe-LDH during charge-discharge time and the conductive network formed by the MXene, which can accelerates electron transport. In addition, the asymmetric SC based on NiFe-LDH/MXene positive electrode display a power density of 758.27 W/kg at an energy density of 42.4 Wh/Kg. These results indicate the NiFe-LDH/MXene composites can be applied as the novel candidate of high-performance SC electrodes.     

Structural,elastic, phononic,optical and electronic properties investigation of two-dimensional XIS (X=Al,Ga, In) for photocatalytic water splitting

Two-dimensional (2D) materials have been widely developed due to their attractive properties. Here, by using density functional theory (DFT) calculations, for the first time, we explore potential applications of the novel XIS (X = Al, Ga, In) monolayer 2D materials on photocatalytic water splitting. A series of simulations were carried out to predict and study the structural, elastic, phononic, optical and electronic properties of 2D XIS materials. The results show that GaIS and InIS demonstrate low thermal conductivity. For optical properties, AlIS shows strong light absorption coefficients and refractive index only under ultraviolet (UV) light, while GaIS and InIS show stronger performance under both visible light and UV light with the band edge positions spanned the redox potential of water. The reasonable band positions and bandgaps make them promising photocatalysts for water splitting. This work reveals the potential applications of monolayer 2D XIS in thermal, electronic, and photocatalytic water splitting.     

Photophysical and photocatalytic water splitting performance of stibiotantalite type-structure compounds,SbMO4 (M = Nb,Ta)

Metal oxides with ferroelectric properties are considered to be a new family of efficient photocatalysts. Here, we investigate stibiotantalite type-structure compounds, SbMO₄ (M = Nb, Ta), with layered crystal structures, and ferroelectric properties as photocatalysts for hydrogen generation from the splitting of pure water. Both compounds were prepared by a conventional solid-state reaction method, and their optical properties, electronic band structure, and photocatalytic water splitting performance were characterized and evaluated. Diffuse reflectance analysis showed that both compounds have moderate band gaps of 3.7 eV for SbTaO₄ and 3.1 eV for SbNbO₄ (cf. 3.0 eV for TiO₂). Mott–Schottky analysis reveals that their conduction-band edge potentials are higher than the water reduction (hydrogen evolution) potential (0 V vs. RHE), indicating both compounds can generate hydrogen from water splitting. The photocatalytic water splitting performance was conducted by using pure water and UV-light irradiation, and photocatalytic H₂ production was confirmed for both compounds. After loading RuO₂ cocatalyst, the rates of hydrogen evolution of SbNbO₄ and SbTaO₄ were 24 μmol/g h and 58 μmol/g h, respectively. It was concluded that both compounds can be used as photocatalysts for water splitting under UV irradiation. The photocatalytic activity difference in both compounds was discussed with regard to electronic band structure and dipole moment difference, resulting from their crystal structures.     

Photocatalytic H2 evolution of porous silicon derived from magnesiothermic reduction of mesoporous SiO2

In this work, porous silicon (PSi) was synthesized by magnesiothermic reduction of mesoporous SiO₂ (MCM-41) and its photocatalytic hydrogen evolution performance was investigated. The unique mesoporous structure of PSi expands the band gap of silicon and shifts its conduction band to a more negative position. As a result, excellent photocatalytic water splitting efficiency of 604.7 μmol h⁻¹ g⁻¹ under visible-light radiation is recorded for the synthesized PSi photocatalysts without loading noble metal cocatalysts. This study presented a promising visible light response photocatalysts for the generation green renewable hydrogen energy basing on PSi material deriving from simple magnesiothermic reduction of mesoporous SiO₂.     

Exploration of 1D-2D LaFeO3/RGO S-scheme heterojunction for photocatalytic water splitting

Sustainable energy innovation is spearheading the way to achieve decarbonisation through commercially viable and highly competitive renewable technologies for green hydrogen. Photocatalytic water splitting has received global attention, as it promotes the direct conversion of solar energy to chemical energy and hydrogen production. Lanthanum orthoferrite (LaFeO₃) has been selected due to its narrow bandgap perovskite-oxides (ABO₃) type nature, low cost and high chemical stability but it is limited with fast charge recombination. To circumvent its constraint of fast charge recombination, an efficient graphene-based nanocomposite has been prepared by employing reduced graphene oxide (RGO) nanosheets as charge separators for visible light driven photocatalytic water splitting. Here, we present a thorough physical and spectroscopic characterization of the Lanthanum orthoferrite/Reduced Graphene oxide (LaFeO₃/RGO) nanocomposites, and investigate its photocatalytic and photoelectrochemical performance. The photocurrent density of the nanocomposites demonstrated ∼21 times higher in comparison to pure LaFeO₃. The as-prepared nanocomposites have been successfully used as photocatalysts for H₂ generation through water reduction under visible light. A significant enhancement in H₂ generation has been recorded for nanocomposites (∼82 mmol g⁻¹ h⁻¹) as compared to that of bare LaFeO₃ (∼9 mmol g⁻¹ h⁻¹) which is among the highest values obtained using noble-metal-free graphene-based photocatalytic nanocomposites. This work offers a facile approach for fabricating highly efficient 1D-2D heterostructure for photocatalysis application.     

Metal-organic-framework based catalyst for hydrogen production: Progress and perspectives

Fossil fuel shortage and global warming have inspired scientists to search for alternative energy sources which are green, renewable, and sustainable. Hydrogen formed from water splitting has been considered as one of the most promising candidates to replace traditional fuels due to its low production cost and zero-emission. Metal-organic frameworks (MOFs) have been considered as potential catalysts for hydrogen production from water splitting account for their flexible structure, ultra-large surface area, and chemical component diversification. This paper reviews different kinds of MOF-related electrocatalysts, involving metals, metal oxides, single atoms, metal phosphides, metal nitrides, and metal dichalcogenides for hydrogen production. Also, MOF-based photocatalysts consisting of pristine MOFs, MOFs as supporters, and MOF-derived heterojunction architectures are reviewed. The finding of MOF-based catalysts for hydrogen generation is summarized. The pros and cons of different MOF-based materials as catalysts for water splitting are discussed. Finally, current challenges and the potential developments of these unique materials as catalysts are also provided.     

Ti3C2 MXene coupled with CdS nanoflowers as 2D/3D heterostructures for enhanced photocatalytic hydrogen production activity

As a novel co-catalyst, Ti₃C₂ MXene has an excellent prospect in the field of photocatalysis. Herein, the 2D/3D Ti₃C₂ MXene@CdS nanoflower (Ti₃C₂@CdS) composite was successfully synthesized by a hydrothermal method. The combination of 2D Ti₃C₂ MXene and 3D CdS nanoflowers can promote carrier transfer and separation, which can improve the performance of CdS. Compared to pure CdS nanoflowers, Ti₃C₂@CdS composite presents lower photoluminescence intensity, longer fluorescence lifetime, higher photocurrent density and smaller electrochemical impedance. The Ti₃C₂@CdS composite with 15 wt% Ti₃C₂ adding amount presents high photocatalytic hydrogen evolution activity (88.162 μmol g^?1 h^?1), 91.57 times of pure CdS. The improved photocatalytic activity of Ti₃C₂@CdS composite is ascribed to the addition of lamellar Ti₃C₂ MXene, which improves the electrical conductivity of the photocatalytic system and effectively accelerates the excited electrons transfer from CdS to Ti₃C₂ MXene.     

A concise review: MXene-based photo catalytic and photo electrochemical water splitting reactions for the production of hydrogen

Since 2015, a number of breakthroughs in the generation of new MAX phases using specified double transition metals have made possible the synthesis of unique MXenes with significant chemical diversity and structural complexity, which are rare in 2D families. MXene and semiconductor hybrids are shown to be effective photocatalysts because to their unique interface features. For photocatalytic purposes, a Schottky heterojunctions may provide faster charge separation and a lower Schottky barrier. When it comes to photocatalytic and photo electrochemical applications, photocatalysts are predicted to be the greatest and most popular new photocatalysts. We discussed some of the semiconductor-based nanocomposites supported by MXenes, including photocatalytic and photo electrochemical water splitting. Next, we discussed some of the difficulties and opportunities that have arisen from working with MXenes to advance semiconductor-based photocatalysts. Scientists in related fields are noticed to prompt the development of novel photocatalysts based on semiconductors.     

Design and research progress of nano materials in cathode catalysts of microbial fuel cells: A review

With the advantages of clean, efficient and energy-saving, microbial fuel cells (MFCs) were characterized with perfect significance in the field of degrading environmental pollutants and generating electricity meanwhile. The cathode materials affected the activity of oxygen reduction reaction (ORR), and affected the power generation performance for MFCs. There were many kinds of nano materials played an important role in the field of cathode catalysis. The advantages of metal and non-metal composites were easy to obtain and low cost; layered double hydroxide (LDH) was easy to control and compound, and could be fully realized functionalization; metal organic frameworks (MOFs) were widely used since their porosity, high specific surface area and high activity; covalent organic frameworks (COFs) were low density and easy to be modified, so as to modify and realize functionalization; MXene was an excellent two-dimensional material, which could provide more channels for the movement of ions. The nano materials formed by the composite of various materials combined the advantages of various materials and played key role in improving ORR performance of MFCs.     

Progress of graphene and loaded transition metals on Mg-based hydrogen storage alloys

Mg-based hydrogen storage alloys have become a research hotspot in recent years owing to their high hydrogen storage capacity, good reversibility of hydrogen absorption/desorption, low cost, and abundant resources. However, its high thermodynamic stability and slow kinetics limit its application, so the modification of Mg-based hydrogen storage alloys has become the development direction of Mg-based alloys. Transition metals can be used as catalysts for the dehydrogenation of hydrogen storage alloys due to their excellent structural, electrical, and magnetic properties. Graphene, because of its unique sp² hybrid structure, excellent chemical stability, and a specific surface area of up to 2600 m²/g, can be used as a support for transition metal catalysts. In this paper, the internal mechanism of graphene as a catalyst for the catalysis of Mg-based hydrogen storage alloys was analyzed, and the hydrogen storage properties of graphene-catalyzed Mg-based hydrogen storage alloys were reviewed. The effects of graphene-supported different catalysts (transition metal, transition metal oxides, and transition metal compounds) on the hydrogen storage properties of Mg-based hydrogen storage alloys were also reviewed. The results showed that graphene played the roles of catalysis, co-catalysis, and inhibition of grain aggregation and growth in Mg-based hydrogen storage materials.