MoS2 as a co-catalyst for photocatalytic hydrogen production from water

Solar-to-hydrogen conversion based on photocatalytic water splitting is a promising pathway for sustainable hydrogen production. The photocatalytic process requires highly active, inexpensive, and earth-abundant materials as photocatalysts. As a presentative layer-structured transition metal dichalcogenides, molybdenum disulfide (MoS2) is attracting intensive attention due to its unique electro and photo properties. In this article, we comprehensively review the recent research efforts of exploring MoS2 as a co-catalyst for photocatalytic hydrogen production from water, with emphasis on its combination with CdS, CdSe, graphene, carbon nitride, TiO2, and others. It is shown that MoS2–semiconductor composites are promising photocatalysts for hydrogen evolution from water under visible light irradiation

MA2_2Z4_4 Family Heteorstructures: Promises and Prospects

Recent experimental synthesis of ambient-stable MoSi2N4 monolayer have garnered enormous research interests. The intercalation morphology of MoSi2N4 - composed of a transition metal nitride (Mo-N) inner sub-monolayer sandwiched by two silicon nitride (Si-N) outer sub-monolayers - have motivated the computational discovery of an expansive family of synthetic MA2Z4 monolayers with no bulk (3D) material counterpart (where M = transition metals or alkaline earth metals; A = Si, Ge; and N = N, P, As). MA2Z4 monolayers exhibit interesting electronic, magnetic, optical, spintronic, valleytronic and topological properties, making them a compelling material platform for next-generation device technologies. Furthermore, heterostructure engineering enormously expands the opportunities of MA2Z4. In this review, we summarize the recent rapid progress in the computational design of MA2Z4-based heterostructures based on first-principle density functional theory (DFT) simulations - a central \emph{work horse} widely used to understand the physics, chemistry and general design rules for specific targeted functions. We systematically classify the MA2Z4-based heterostructures based on their contact types, and review their physical properties, with a focus on their performances in electronics, optoelectronics and energy conversion applications. We review the performance and promises of MA2Z4-based heterostructures for device applications that include electrical contacts, transistors, spintronic devices, photodetectors, solar cells, and photocatalytic water splitting. This review unveils the vast device application potential of MA2Z4-based heterostructures, and paves a roadmap for the future experimental and theoretical development of MA2Z4-based functional heterostructures and devices.Comment: 32 pages, 15 figure

Recent Progress in WS2 -Based Nanomaterials Employed for Photocatalytic Water Treatment

Water pollution is one of the most serious environmental issues globally due to its harmful consequences on the ecosystem and public health. Various technologies have been developed for water treatment such as photocatalysis, which has recently drawn scientists’ attention. Photocatalytic techniques using semiconductors have shown an efficient removal of various water contaminants during water treatment as well as cost effectivity and low energy consumption. Tungsten disulfide (WS2) is among the promising Transition Metal Dichalcogenides (TMDs) photocatalysts, as it has an exceptional nanostructure and special properties including high surface area and high carrier mobility. It is usually synthesized via hydrothermal technique, chemical vapor deposition (CVD), and liquid-phase exfoliation (LPE) to obtain a wide variety of nanostructures such as nanosheets and nanorods. Most common examples of water pollutants that can be removed efficiently by WS2-based nanomaterials through semiconductor photocatalytic techniques are organic contaminants, pharmaceuticals, heavy metals, and infectious microorganisms. This review summarizes the most recent work on employing WS2-based nanomaterials for different photocatalytic water treatment processes.Open Access funding provided by Qatar National Library

Emerging versatile two-dimensional MoSi2_2N4_4 family

The discovery of two-dimensional (2D) layered MoSi$_2$N$_4$ and WSi$_2$N$_4$ without knowing their 3D parents by chemical vapor deposition in 2020 has stimulated extensive studies of 2D MA$_2$Z$_4$ system due to its structural complexity and diversity as well as versatile and intriguing properties. Here, a comprehensive overview on the state-of-the-art progress of this 2D MA$_2$Z$_4$ family is presented. Starting by describing the unique sandwich structural characteristics of the emerging monolayer MA$_2$Z$_4$, we summarize and anatomize their versatile properties including mechanics, piezoelectricity, thermal transport, electronics, optics/optoelectronics, and magnetism. The property tunability via strain engineering, surface functionalization and layered strategy is also elaborated. Theoretical and experimental attempts or advances in applying 2D MA$_2$Z$_4$ to transistors, photocatalysts, batteries and gas sensors are then reviewed to show its prospective applications over a vast territory. We further discuss new opportunities and suggest prospects for this emerging 2D family. The overview is anticipated to guide the further understanding and exploration on 2D MA$_2$Z$_4$.Comment: 29 pages, 21 figure

Photoelectrochemistry of two-dimensional and layered materials: a brief review

Two-dimensional (2D) materials have unique band structure and show a great promise for optoelectronic and solar energy harvesting applications. Photoelectrochemical (PEC) processes are intensively studied employing these materials, due to their high specific surface area, and the possibility of surface modification by defect engineering/catalyst deposition. The PEC activity of different 2D and layered materials was scrutinized for water oxidation/reduction and for inorganic ion oxidation by a statistical analysis to reveal any specific trends. Furthermore, some frequently studied performance improvement strategies (i.e., heterojunctions, tunnelling, and co-catalysts) are also discussed. Overall, exploring novel materials of 2D family, and new directions are both needed to initiate further discussions and additional research activity, which might enable to harness the full potential of these exciting materials

Developing High-Performance 2D Heterostructured Electrocatalysts and Photocatalysts for Hydrogen Production and Utilizationsts and Photocatalysts for Hydrogen Production and Utilization

H2 is a pivotal chemical in modern society, not only as a clean energy carrier but also as a versatile chemical reactant. However, traditional hydrogen production and utilization heavily rely on thermocatalysis, which is highly energy-intensive and can result in heavy carbon emission and severe environmental problems. Photocatalysis and electrocatalysis are greener alternatives to thermocatalysis that can capitalize on the renewable sunlight and electricity and thus dramatically reduce energy requirements. However, heterogeneous electro/photocatalysts are still far from application to hydrogen economy due to the lack of design principles that can lead to sufficient efficiency. To address this challenge, the dissertation primarily focuses on developing high-performance electrocatalysts and photocatalysts by understanding the impact of surface defects and interactions between different phases on catalytic performance. With the obtained understanding, electro/photocatalysts with high efficiency in H2 production and utilization (herein, transfer hydrogenation) can be facilely fabricated. To better achieve an in-depth understanding of fabricating electro/photocatalysts used for the hydrogen economy, my thesis work starts with the research on H2 evolution reaction (HER) via electrocatalysis, and then moves to the HER using the more challenging photocatalytic approach and finally proceeds to the most challenging part, photocatalytic transfer hydrogenation. For electrocatalytic HER, MoS2 nanosheets are in situ grown on carbon fiber paper for the fabrication of the proton exchange membrane (PEM) cell electrode. Impressively, this integrated electrode with an ultralow MoS2 loading of 0.14 mg/cm2 can achieve small cell voltages of 1.96 and 2.25 V under 1 and 2 A/cm2, respectively, in a practical PEM cell, which is superior to most cells using noble-metal-free HER electrocatalysts even with extremely high catalyst loadings of 3~6 mg/cm2 under the similar cell operation conditions. The ultrahigh activity of the as-synthesized electrode is attributed to the intimate contact between MoS2 and CFP, vertical alignment of MoS2 nanosheets on CFP, the coexistence of 1T and 2H multiphase MoS2 and the existence of various defects on MoS2. For photocatalytic HER, an Au nanocage/MoS2 system is investigated to understand the effect of localized surface plasmon resonance (LSPR) on photocatalysis. The match between the LSPR wavelength of Au nanocages and the optical absorption edge of MoS2 is found to be critical to the activity of the composite. When the match is achieved, a 40-fold activity increase over the bare MoS2 is observed, while the other unmatched counterparts show much less activity enhancement (~15-fold). The near field enhancement (NFE) is proposed to govern the LSPR process with the energy of surface plasmon transferred from Au to MoS2 to promote electron excitation in MoS2, the efficiency of which maximized when the LSPR wavelength of Au matches the MoS2 absorption edge. In the photocatalytic transfer hydrogenation case, phenylacetylene (PA) semi-hydrogenation is selected as a model reaction to understand how vacancies in 2D semiconductors may be utilized to manipulate photocatalytic efficiency. 2D g-C3N4 nanosheets loaded with Ni single-atoms (SAs) are used as the catalyst for this reaction. By controlling both the Ni loading and the density of surface vacancies on g-C3N4, it is found that the numbers of vacancies and Ni SAs had a synergistic impact on the activity of the catalyst. Therefore, a fine tuning of both factors should be important to achieve an optimal hydrogenation activity. Overall, all research examples highlight the important role played by surface defects and metal-semiconductor interactions, and the findings from the research can be potentially used to guide the design of high-performance photocatalysts for hydrogen evolution and hydrogenation reactions

Strain Anisotropy Driven Spontaneous Formation of Nanoscrolls from Two-Dimensional Janus Layers

Two-dimensional Janus transition metal dichalcogenides (TMDs) have attracted attention due to their emergent properties arising from broken mirror symmetry and self-driven polarisation fields. While it has been proposed that their vdW superlattices hold the key to achieving superior properties in piezoelectricity and photovoltiacs, available synthesis has ultimately limited their realisation. Here, we report the first packed vdW nanoscrolls made from Janus TMDs through a simple one-drop solution technique. Our results, including ab-initio simulations, show that the Bohr radius difference between the top sulphur and the bottom selenium atoms within Janus M_Se^S (M=Mo, W) results in a permanent compressive surface strain that acts as a nanoscroll formation catalyst after small liquid interaction. Unlike classical 2D layers, the surface strain in Janus TMDs can be engineered from compressive to tensile by placing larger Bohr radius atoms on top (M_S^Se) to yield inverted C scrolls. Detailed microscopy studies offer the first insights into their morphology and readily formed Moir\'e lattices. In contrast, spectroscopy and FETs studies establish their excitonic and device properties and highlight significant differences compared to 2D flat Janus TMDs. These results introduce the first polar Janus TMD nanoscrolls and introduce inherent strain-driven scrolling dynamics as a catalyst to create superlattices

Photocatalytic performance of nanocatalysts

This study centered on the strategies for photocatalytic towards increasing photocatalytic performance

New Insights into the Surfactant-Assisted Liquid-Phase Exfoliation of Bi2S3 for Electrocatalytic Applications

During water electrolysis, adding an electrocatalyst for the hydrogen evolution reaction (HER) is necessary to reduce the activation barrier and thus enhance the reaction rate. Metal chalcogenide-based 2D nanomaterials have been studied as an alternative to noble metal electrocatalysts because of their interesting electrocatalytic properties and low costs of production. However, the difficulty in improving the catalytic efficiency and industrializing the synthetic methods have become a problem in the potential application of these species in electrocatalysis. Liquid-phase exfoliation (LPE) is a low-cost and scalable technique for lab- and industrial-scale synthesis of 2D-material colloidal inks. In this work, we present, to the best of our knowledge, for the first time a systematic study on the surfactant-assisted LPE of bulk Bi2S3 crystalline powder to produce nanosheets (NSs). Different dispersing agents and LPE conditions have been tested in order to obtain colloidal low-dimensional Bi2S3 NSs in H2O at optimized concentrations. Eventually, colloidally stable layered nano-sized Bi2S3 suspensions can be produced with yields of up to ~12.5%. The thus obtained low-dimensional Bi2S3 is proven to be more active for HER than the bulk starting material, showing an overpotential of only 235 mV and an optimized Tafel slope of 125 mV/dec. Our results provide a facile top-down method to produce nano-sized Bi2S3 through a green approach and demonstrate that this material can have a good potential as electrocatalyst for HER

Two-dimensional metal halide perovskites and their heterostructures: from synthesis to applications

Size- and shape- dependent unique properties of the metal halide perovskite nanocrystals make them promising building blocks for constructing various electronic and optoelectronic devices. These unique properties together with their easy colloidal synthesis render them efficient nanoscale functional components for multiple applications ranging from light emission devices to energy conversion and storage devices. Recently, two-dimensional (2D) metal halide perovskites in the form of nanosheets (NSs) or nanoplatelets (NPls) are being intensively studied due to their promising 2D geometry which is more compatible with the conventional electronic and optoelectronic device structures where film-like components are employed. In particular, 2D perovskites exhibit unique thickness-dependent properties due to the strong quantum confinement effect, while enabling the bandgap tuning in a wide spectral range. In this review the synthesis procedures of 2D perovskite nanostructures will be summarized, while the application-related properties together with the corresponding applications will be extensively discussed. In addition, perovskite nanocrystals/2D material heterostructures will be reviewed in detail. Finally, the wide application range of the 2D perovskite-based structures developed to date, including pure perovskites and their heterostructures, will be presented while the improved synergetic properties of the multifunctional materials will be discussed in a comprehensive way.Comment: 83 pages, 38 Figure