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Basic insight into the structural evolution of electrocatalysts under operating conditions is of substantial importance for designing water oxidation catalysts. The first‐row transition metal‐based catalysts present state‐of‐the‐art oxygen evolution reaction (OER) performance under alkaline conditions. Apparently, confinement has become an exciting strategy to boost the performance of these catalysts. The van der Waals (vdW) gaps of transition metal dichalcogenides are acknowledged to serve as a suitable platform to confine the first‐row transition metal catalysts. This study focuses on confining Ni(OH)2 nanoparticle in the vdW gaps of 2D exfoliated SnS2 (Ex‐SnS2) to accelerate water oxidation and to guarantee long term durability in alkaline solutions. The trends in oxidation states of Ni are probed during OER catalysis. The in situ studies confirm that the confined system produces a favorable environment for accelerated oxygen gas evolution, whereas the un‐confined system proceeds with a relatively slower kinetics. The outstanding OER activity and excellent stability, with an overpotential of 300 mV at 100 mA cm−2 and Tafel slope as low as 93 mV dec−1 results from the confinement effect. This study sheds light on the OER mechanism of confined catalysis and opens up a way to develop efficient and low‐cost electrocatalysts. [ABSTRACT FROM AUTHOR]

Abstract: Transition metal phosphorus trichalcogenides (MPX3, X = S, Se) are layered materials possessing high chemical diversity and wide range of applications in a broad wave length spectrum. Theoretical studies reveal that auspicious activity of photocatalytic water splitting can be realized from them. However, experimental efforts have so far been challenged with the synthesis bottleneck. Described herein is the general chemical vapor deposition (CVD) growth method and photocatalytic activity of these materials. A novel route to systematically grow MnPX3 nanosheets on flexible carbon fiber substrate is reported. The temperature profile of the CVD process is carefully optimized that confer a facile and successful conversion of oxide precursor to phospho‐trichalcogenide with high crystallinity. Moreover, the obtained manganese‐based phosphorus trichalcogenide nanosheets demonstrate promising activity in sacrificial agent‐free photocatalytic water splitting under simulated solar light (AM 1.5G). This study provides a significant stepping stone in exploring the fascinating world of functional 2D materials and pursuing performance enhancement. [ABSTRACT FROM AUTHOR]

[Display omitted] • We interfaced CrOx with CuS on carbon fiber substrate whose junction deliver intriguing structural/electronic property. • The theoretical calculation proved that charges are mainly distributed between the CuS layer and the Cr 2 O 3. • The co-existence of CrOx and CuS triggered charge transfer at the interface. • The heterojunction gave rise to the advent of defects/low-coordinated central metal atom. • The presented methodology would guide subsequent works focused on altering the intrinsic properties of oxide-sulfide interface. The sluggish kinetics associated with the oxygen evolution reaction (OER) limits the sustainability of fuel production and chemical synthesis. Developing catalysts based on Earth abundant elements with a reasonable strategy could solve the challenge. Here, we present a heterostructure built from CrOx and CuS whose interface gives rise to the advent of new functionalities in catalytic activity. Using X-ray photoelectron and absorption spectroscopies, we identified the multiple oxidation states and low coordination number of Cr metal in CrOx-CuS heterostructure. Benefitting from these features, CrOx-CuS generates oxygen gas through water splitting with a low over potential of 190 mV vs RHE at a current density of 10 mA cm−2. The catalyst shows no evident deactivation after a 36-hours operation in alkaline medium. The high catalytic activity, inspired by first principles calculations, and long-time durability make it one of the most effective OER electrocatalysts. [ABSTRACT FROM AUTHOR]

Keywords: CO2 reduction; confined catalysis; pore filling; van der Waals gaps; water splitting In article number 1902307 Tofik Ahmed Shifa and Alberto Vomiero review the recent developments in confined catalysis with emphasis on energy conversion reactions. CO2 reduction, confined catalysis, pore filling, van der Waals gaps, water splitting. [Extracted from the article]

Space confined catalysis has emerged as viable strategy for achieving potent and efficient catalysts in various important reactions. It offers a means of creating unique nanoscale chemical environments partitioned from the surrounding bulk space. This gives rise to the phenomena of nanoconfinement, where the energetics and kinetics of catalytic reactions can be modulated upon confining the catalysts in a particular site. Various scaffolds have been reported so far for confinement. Among these, void spaces under the cover of 2D materials, van der Waals (vdW) gaps of layered 2D materials, nanotubes, and porous surfaces have recently won copious attention. In this review, the concept of space confinement with respect to its effect on the electronic and structural properties of a catalyst is discussed. Emphasis is devoted to the catalysis of water splitting and CO2 reduction reactions. The progress in the design and applications of space confined catalysts is then traced. Finally, a discussion of emerging issues yet to be explored for this strategy to achieve a high efficiency, and future directions with the potential to become a new hotspots are presented. [ABSTRACT FROM AUTHOR]

Holding novel physical properties, high flexibility and strong integration ability with Si-based electronic devices, two-dimensional (2D) non-layered materials have received considerable attentions in recent years. To achieve the 2D anisotropic growth, various strategies have been developed. And a variety of applications have been demonstrated. This review provides an overview on the recent progress of this material family. The scope will cover the preparation strategies, including dry methods and wet chemical approaches, as well as the applications in catalysis, energy conversion and storage, optoelectronic devices and topological crystalline insulators. Conclusion and future perspectives are also given. [ABSTRACT FROM AUTHOR]

To address the urgent need for clean and sustainable energy, the rapid development of hydrogen-based technologies has started to revolutionize the use of earth-abundant noble-metal-free catalysts for the hydrogen evolution reaction (HER). Like the active sites of hydrogenases, the cation sites of pyrite-type transition-metal dichalcogenides have been suggested to be active in the HER. Herein, we synthesized electrodes based on a Se-enriched NiSe2 nanosheet array and explored the relationship between the anion sites and the improved hydrogen evolution activity through theoretical and experimental studies. The free energy for atomic hydrogen adsorption is much lower on the Se sites (0.13 eV) than on the Ni sites (0.87 eV). Notably, this electrode benefits from remarkable kinetic properties, with a small overpotential of 117 mV at 10 mA cm−2, a low Tafel slope of 32 mV per decade, and excellent stability. Control experiments showed that the efficient conversion of H+ into H2 is due to the presence of an excess of selenium in the NiSe2 nanosheet surface. [ABSTRACT FROM AUTHOR]

To address the urgent need for clean and sustainable energy, the rapid development of hydrogen-based technologies has started to revolutionize the use of earth-abundant noble-metal-free catalysts for the hydrogen evolution reaction (HER). Like the active sites of hydrogenases, the cation sites of pyrite-type transition-metal dichalcogenides have been suggested to be active in the HER. Herein, we synthesized electrodes based on a Se-enriched NiSe2 nanosheet array and explored the relationship between the anion sites and the improved hydrogen evolution activity through theoretical and experimental studies. The free energy for atomic hydrogen adsorption is much lower on the Se sites (0.13 eV) than on the Ni sites (0.87 eV). Notably, this electrode benefits from remarkable kinetic properties, with a small overpotential of 117 mV at 10 mA cm−2, a low Tafel slope of 32 mV per decade, and excellent stability. Control experiments showed that the efficient conversion of H+ into H2 is due to the presence of an excess of selenium in the NiSe2 nanosheet surface. [ABSTRACT FROM AUTHOR]

Promoting the activities of photocatalysts is still the critical challenge in H2generation area. Here, a Au plasmon enhanced photocatalyst of WS2-Au-CuInS2 is developed by inserting Aunanoparticles between WS2nanotubes and CuInS2 (CIS) nanoparticles. Due to the localized surface plasmonic resonance properties from Aunanoparticles, WS2-Au-CIS shows the best performance as compared to Au-CIS, CIS, WS2-CIS, CIS-Au, WS2-Au, and WS2-CIS-Au. The surface plasmonic resonance effects dramatically intensify the absorption of visible light and help to inject hot electrons into the semiconductors. Our findings open up an efficient method to optimize the type-II structures for photocatalytic water splitting. [ABSTRACT FROM AUTHOR]

Photochemical water splitting offers an economic and sustainable approach for solar energy conversion into hydrogen fuel to mitigate the problem of greenhouse gas emissions. To this end, exploring novel semiconductor photocatalysts, which have efficient light absorption and thermodynamically favorable band alignment for water splitting, is crucial. Here, we rationally develop a new photocatalyst of CuInP2S6 nanosheets to generate hydrogen gas under light illumination. The CuInP2S6 nanosheet (with a thickness of around 4–7 nm) photocatalyst exhibits a high hydrogen production rate of 804 μmol g−1 h−1, eight times higher than that of the microsheet counterpart, due to the introduced abundant sulfur vacancies. Experimental characterization and theoretical calculations verify that the prolonged carrier lifetime and optimized band alignment in ultrathin CuInP2S6 nanosheets boost photocatalytic hydrogen evolution. This work opens a new avenue for photocatalysis via using novel layered binary metal phosphorous trichalcogenides. [ABSTRACT FROM AUTHOR]

The interfacial coupling in van der Waals (vdWs) heterostructures can effectively modulate the device performance. In this study, we demonstrate a high performance infrared detector fabricated by direct epitaxial growth of Te nanowires onto a MoS2 monolayer. Such directly grown Te/MoS2 heterostructures show much stronger interfacial coupling than artificially transferred Te/MoS2 hybrids, as evidenced from their Raman and photoluminescence spectra. The strong vdWs interfacial coupling leads to a high performance infrared detector with both ultrahigh photoresponsivity (>103 A/W) and rapid response time (τrising = 15 ms) at the telecommunication wavelength of 1550 nm. The anti-bipolar and rectification behaviors observed in the strongly coupled grown Te-MoS2 heterojunction further confirm the effective interfacial coupling. In contrast, the weakly coupled transferred Te-MoS2 heterojunction that is obtained by artificial transfer exhibits negligible anti-bipolar behavior and slight rectification behavior. These findings indicate that the coupled vdWs hybrid structures have great potential for achieving high performance photodetectors. [ABSTRACT FROM AUTHOR]

Two-dimensional layered materials (2DLMs) have attracted a tremendous amount of attention as photodetectors due to their fascinating features, including high potentials in new-generation electronic devices, wide coverage of bandgaps, ability to construct van der Waals heterostructures, extraordinary light–mass interaction, strong mechanical flexibility, and the capability of enabling synthesis of 2D nonlayered materials. Until now, most attention has been focused on the well-known graphene and transition metal dichalcogenides (TMDs). However, a growing number of functional materials (more than 5619) with novel optoelectronic and electronic properties are being re-discovered, thereby widening the horizon of 2D libraries. In addition to showing common features of 2DLMs, these new 2D members may bring new opportunities to their well-known analogues, like wider bandgap coverage, direct bandgaps independence with thickness, higher mechanical flexibility, and new photoresponse phenomena. The impressive results communicated so far testify that they have shown high potentials with photodetections covering THz, IR, visible, and UV ranges with comparable or even higher performances than well-known TMDs. Here, we give a comprehensive review on the state-of-the-art photodetections of two-dimensional materials beyond graphene and TMDs. The review is organized as follows: fundamentals of photoresponse first are discussed, followed by detailed photodetections of new 2D members including both layered and non-layered ones. After that, photodiodes and hybrid structures based on these new 2D materials are summarized. Then, the integration of these 2D materials with flexible substrates is reviewed. Finally, we conclude with the current research status of this area and offer our perspectives on future developments. We hope that, through reading this manuscript, readers will quickly have a comprehensive view on this research area. [ABSTRACT FROM AUTHOR]

Abstract: Transition metal phosphides (TMPs) have recently been utilized as promising electrocatalysts for oxygen evolution reaction (OER) in alkaline media. The metal oxides or hydroxides formed on their surface during the OER process are hypothesized to play an important role. However, their exact role is yet to be elucidated. Here unambiguous justification regarding the active role of oxo(hydroxo) species on O‐Ni(1−x)FexP2 nanosheet with pyrite structure is shown. These O‐Ni(1−x)FexP2 (x = 0.25) nanosheets demonstrate greatly improved OER performance than their corresponding hydroxide and oxide counterparts do. From density function theory (DFT) calculations, it is found that the introduction of iron into the pyrite‐phased NiP2 alters OER steps occurred on the surface. Notably, the partially oxidized surface of O‐Ni(1−x)FexP2 nanosheets is vital to improve the local environment and accelerate the reaction steps. This study sheds light on the OER mechanism of the 3d TMP electrocatalyst and opens up a way to develop efficient and low‐cost electrocatalysts. [ABSTRACT FROM AUTHOR]

The article focuses on a study on an infrared-memory device using molybdenum disulfide/lead sulfide (MoS2/PbS) van der Waals heterostructures. It mentions that the device indicates its potential for nonvolatile storage devices. The study concludes that optical memory and the electrical erasing phenomenon, respectively, originate from the localization of infrared-induced holes in PbS and gate voltage pulse-enhanced tunneling of electrons from MoS2 to PbS.

The vertically stacked devices based on van der Waals heterostructures (vdWHs) of two-dimensional layered materials (2DLMs) have attracted considerable attention due to their superb properties. As a typical structure, graphene/hexagonal boron nitride (h-BN)/graphene vdWH has been proved possible to make tunneling devices. Compared with graphene, transition metal dichalcogenides possess intrinsic bandgap, leading to high performance of electronic devices. Here, tunneling devices based on graphene/h-BN/MoSe2 vdWHs are designed for multiple functions. On the one hand, the device shows a typical tunneling field-effect transistor behavior. A high on/off ratio of tunneling current (5 x 10³) and an ultrahigh current rectification ratio (7 x 105) are achieved, which are attributed to relatively small electronic affinity of MoSe2 and optimized thickness of h-BN. On the other hand, the same structure also realizes 2D non-volatile memory with a high program/erase current ratio (>105), large memory window (~150V from ±90V), and good retention characteristic. These results could enhance the fundamental understanding of tunneling behavior in vdWHs and contribute to the design of ultrathin rectifiers and memory based on 2DLMs. [ABSTRACT FROM AUTHOR]

For increasing scalability and reducing cost, transition metal dichalcogenides-based electrocatalysts presently have been proposed as substitutes for noble metals to generate hydrogen, but these alternatives usually suffer from inferior performance. Here, a Ravenala leaf-like W xC@WS2 heterostructure is grown via carbonizing WS2 nanotubes, whose outer walls being partially unzipped along with the W x C 'leaf-valves' attached to the inner tubes during the carbonization process. This heterostructure exhibits a catalytic activity for hydrogen evolution reaction with low overpotential of 146 mV at 10 mA cm−2 and Tafel slope of 61 mV per decade, outperforming the performance of WS2 nanotubes and W xC counterparts under the same condition. Density functional theory calculations are performed to unravel the underlying mechanism, revealing that the charge distribution between W xC and WS2 plays a key role for promoting H atom adsorption and desorption kinetics simultaneously. This work not only provides a potential low-cost alternative for hydrogen generation but should be taken as a guide to optimize the catalyst structure and composition. [ABSTRACT FROM AUTHOR]

Two-dimensional layered materials have exhibited great potential in electronic and optoelectronic applications, because of their unique chemical and physical properties. Recently, Hf-based transition metal dichalcogenides are predicted to own high room temperature mobility and are attracting increasing interest. However, only few experimental investigations are reported hitherto. In this paper, we demonstrate ultrafast and ultrasensitive back-gated HfSe2 phototransistors. Au-contacted HfSe2 phototransistors display a high on/off ratio of 106 and modest mobilities in the range of 2.6-6.5 cm² V-1 s-1. Additionally, the phototransistors based on HfSe2 present prominent optoelectronic performance with a high responsivity of 252 A/W and an ultrafast response time of 7.8 ms, implying the sensitive photoswitching behavior. Moreover, the response time can be modulated by gate voltages. The excellent field effect transistor performance coupled to the sensitive and fast photodetection makes HfSe2 have a broad application prospect for electronic and optoelectronic devices in the future. [ABSTRACT FROM AUTHOR]

MoTe2 is an emerging two-dimensional layered material showing ambipolar/p-type conductivity, which makes it an important supplement to n-type two-dimensional layered material like MoS2. However, the properties based on its van der Waals heterostructures have been rarely studied. Here, taking advantage of the strong Fermi level tunability of monolayer graphene (G) and the feature of van der Waals interfaces that is free from Fermi level pinning effect, we fabricate G/MoTe2/G van der Waals heterostructures and systematically study the electronic and optoelectronic properties. We demonstrate the G/MoTe2/G FETs with low Schottky barriers for both holes (55.09 meV) and electrons (122.37meV). Moreover, the G/MoTe2/G phototransistors show high photoresponse performances with on/off ratio, responsivity, and detectivity of ~105, 87A/W, and 1012 Jones, respectively. Finally, we find the response time of the phototransistors is effectively tunable and a mechanism therein is proposed to explain our observation. This work provides an alternative choice of contact for high-performance devices based on p-type and ambipolar two-dimensional layered materials. [ABSTRACT FROM AUTHOR]

Molybdenum disulfide (MoS2) has attracted a great deal of attention in optoelectronic applications due to its high mobility, low off-state current and high on/off ratio. However, its intrinsic large bandgap limits its application in infrared detection. Here, we have developed a high-performance infrared photodetector by integrating nonlayered PbS and layered MoS2 nanostructures via van der Waals epitaxy. Density functional theory (DFT) calculations indicate that PbS nanoplates are in contact with MoS2 edges through strong chemical hybridization, which is expected to offer a fast transmission path for carriers that enhances the response speed. The phototransistor exhibits a fast response (τrising = τdecay = 7.8 ms) as well as high photoresponsivity (4.5 × 104 A·W-1) and Ilight/Idark (1.3 × 10²) in the near-infrared spectral region at room temperature. In particular, the detectivity (D*) is as high as 3 × 1013 Jones, which is even better than that of commercial Si and InGaAs photodetectors. Furthermore, by controlling the growth and microfabrication patterning, periodic device arrays of PbS-MoS2 that are capable of infrared detection are achieved on Si/SiO2 substrates. Our work provides a possible method for the integration of photodetector arrays on Si-based electronic devices and lays a solid foundation for the practical applications of MoS2-based devices in the future. [ABSTRACT FROM AUTHOR]

Van der Waals heterostructures (vdWHs), obtained by artificially stacking 2D layered material (2DLM) plains upon each other, are brand new structures that have exhibited novel electronic and optoelectronic properties and attracted a great deal of attention. So far, the results are only based on devices with symmetrical configurations: devices predominated by vdWH parts, or cross-like configurations combined with both vdWHs and extra individual 2DLM layers. Quite different gate tunable phenomena have been observed for these two configurations even though 2DLMs with similar band alignments were used, which may be due to the different device configurations utilized. For a deeper understanding, rational investigation on configuration-dependent properties of vdWHs is needed. Here, using MoTe2/MoS2 as an example, vdWH device is artificially designed with two asymmetrical configurations. Through comparing the respective results, it is found that the properties that stem only from the vdWH, i.e., the rectification behavior and open voltage in photovoltaic effect, are independent of the device structures. However, other properties, i.e., drain currents, short circuit currents, and photoreponse performances, strongly depend on the configuration used. These results give a guideline on studying the intrinsic properties of vdWHs and optimizing the device structures for better performances. [ABSTRACT FROM AUTHOR]

As an effective alternative to noble platinum electrocatalyst, earth abundant and inexpensive layered transition metal dichalcogenides (TMDs) are investigated for the hydrogen evolution reaction (HER). Compared with binary TMDs, the tunably composed ternary TMDs have hitherto received relatively little attention. Here, few-layered ternary WS2(1− x)Se2 x nanoribbons (NRs) with metallic 1T phases, much more catalytically active in HER, are prepared for the first time. The favorable Δ GHo introduced by the tensile region on the surface, along with the presence of local lattice distortions of the WS2(1− x)Se2 x nanoribbons with metallic 1T phases, greatly promotes the HER process. These ternary NRs achieve the lowest overpotential of ≈0.17 V at 10 mA cm−2 and a Tafel slope of ≈68 mV dec−1 at a low catalyst loading (≈0.30 ± 0.02 mg cm−2). Notably, the long-term durability suggests the potential of practical applications in acid electrolytes. The results here suggest that the ternary WS2(1− x)Se2 x NRs with 1T phases are prominent alternatives to platinum-based HER electrocatalysts. [ABSTRACT FROM AUTHOR]