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This review summarizes the superiorities and utilizations of 2D materials for photoelectrochemical water splitting including transition metal dichalcogenides, graphene, graphdiyne, black phosphorus, layered double hydroxides, g-C3N4, and MXenes.

Highlights MoP nanorod-array catalysts were directly synthesized on graphene passivated silicon photocathodes without secondary phase. Mo-O-C covalent bondings and energy band bending at heterointerfaces facilitate the electron transfer to the reaction sites. Numerous catalytic sites and drastically enhanced anti-reflectance of MoP nanorods contribute to the high solar energy conversion efficiency. Abstract Transition metal phosphides (TMPs) and transition metal dichalcogenides (TMDs) have been widely investigated as photoelectrochemical (PEC) catalysts for hydrogen evolution reaction (HER). Using high-temperature processes to get crystallized compounds with large-area uniformity, it is still challenging to directly synthesize these catalysts on silicon photocathodes due to chemical incompatibility at the heterointerface. Here, a graphene interlayer is applied between p-Si and MoP nanorods to enable fully engineered interfaces without forming a metallic secondary compound that absorbs a parasitic light and provides an inefficient electron path for hydrogen evolution. Furthermore, the graphene facilitates the photogenerated electrons to rapidly transfer by creating Mo-O-C covalent bondings and energetically favorable band bending. With a bridging role of graphene, numerous active sites and anti-reflectance of MoP nanorods lead to significantly improved PEC-HER performance with a high photocurrent density of 21.8 mA cm−2 at 0 V versus RHE and high stability. Besides, low dependence on pH and temperature is observed with MoP nanorods incorporated photocathodes, which is desirable for practical use as a part of PEC cells. These results indicate that the direct synthesis of TMPs and TMDs enabled by graphene interlayer is a new promising way to fabricate Si-based photocathodes with high-quality interfaces and superior HER performance. Graphic Abstract

Hydrogen energy is a promising energy source that is environmentally friendly due to its long-term, large-capacity storage and low greenhouse gas emissions. However, the mass production of hydrogen is still technically difficult due to limitations in efficiency, stability, and cost, even though it can satisfy all of the current energy demands. Water splitting using an electrocatalyst is an efficient method for environmentally friendly hydrogen production, and various catalyst-related studies are being conducted for this purpose. For the last decade, transition metal-based compositions have been at the center of water splitting catalyst research. Despite numerous studies and developments, studies on transition metal-based catalysts so far still have various problems to be solved. Although excellent review papers on transition metal-based catalysts have been reported, the overall scope of transition metal-based catalysts has rarely been covered in the reports. In this review, we present the research about overall transition metal-based electrocatalysts for hydrogen production from four different categories, namely, alloys, transition-metal dichalcogenides (TMDs), layered double hydroxides (LDHs), and single-atom catalysts (SACs). The fundamental roles of metal alloying and unique electrical properties of TMDs, LDHs, and SACs are mainly discussed. Furthermore, we present the recent advances in photovoltaic-electrochemical (PV-EC) systems for sustainable hydrogen production. Finally, perspectives on the issues to be addressed in the research on transition metal-based electrocatalysts are provided.

Stabilizing atomically dispersed single atoms (SAs) on Si photoanodes for photoelectrochemical-oxygen evolution reaction is still challenging due to the scarcity of anchoring sites. Here, we elaborately demonstrate the decoration of Ir SAs on Si photoanodes and assess the role of SAs on the separation and transfer of photogenerated charge carriers. NiO/Ni thin film, an active and highly stable catalyst, is capable of embedding the Ir SAs in its lattices by locally modifying the electronic structure. The isolated Ir SAs enable the effective photogenerated charge transport by suppressing the charge recombination and lower the thermodynamic energy barrier in the potential-determining step. The Ir SAs/NiO/Ni/ZrO2/n-Si photoanode exhibits a benchmarking photoelectrochemical performance with a high photocurrent density of 27.7 mA cm− 2 at 1.23 VRHE and 130 h stability. This study proposes the rational design of SAs on Si photoelectrodes and reveals the potential of the Ir SAs to boost photogenerated charge carrier kinetics.

2-Dimensional semiconductor SnS2 has emerged as one of the most prospective candidates for chemoresistive gas sensor applications due to its outstanding gas sensing performance. Herein, we propose the room temperature nitrogen dioxide (NO2) sensing of SnS2 nanoflowers (NFs) enabled by visible light activation. SnS2 NFs were successfully prepared by solvothermal synthesis with abundant edge sites. The high absorbance in the visible light region triggered the generation of charge carriers resulting in decreased resistance and enhanced gas sensing characteristics. Even under red light and green light with low photon energy, the room temperature NO2 sensing performance was improved. The highest NO2 sensing performance was accomplished under blue light, including the highest response, excellent selectivity towards NO2 and an extremely low detection limit. Moreover, the sensor exhibited reliable gas sensing performance in humid conditions and maintained its properties after long-term relaxation. Taking advantage of surface properties, optical properties and gas sensing properties, a light-activated SnS2 NF based gas sensor is anticipated to further develop SnS2 nanostructures for use in an electronic nose.

Green hydrogen is being considered as a next-generation sustainable energy source. It is created electrochemically by water splitting with renewable electricity such as wind, geothermal, solar, and hydropower. The development of electrocatalysts is crucial for the practical production of green hydrogen in order to achieve highly efficient water-splitting systems. Due to its advantages of being environmentally friendly, economically advantageous, and scalable for practical application, electrodeposition is widely used to prepare electrocatalysts. There are still some restrictions on the ability to create highly effective electrocatalysts using electrodeposition owing to the extremely complicated variables required to deposit uniform and large numbers of catalytic active sites. In this review article, we focus on recent advancements in the field of electrodeposition for water splitting, as well as a number of strategies to address current issues. The highly catalytic electrodeposited catalyst systems, including nanostructured layered double hydroxides (LDHs), single-atom catalysts (SACs), high-entropy alloys (HEAs), and core-shell structures, are intensively discussed. Lastly, we offer solutions to current problems and the potential of electrodeposition in upcoming water-splitting electrocatalysts.

In the pandemic era, the development of high-performance indoor air quality monitoring sensors has become more critical than ever. NO

To construct a highly efficient photoelectrochemical tandem device with silicon photocathode operating in alkaline conditions, it is desirable to develop stable and active catalysts which enable the photocathode to reliably perform under an alkaline environment. With nanostructured passivation layer and edge-exposed transition metal disulfides, silicon photocathode provides new opportunities for achieving unbiased alkaline solar water splitting. Here, the TiO

Transferable 2-dimensional (2D) MoS2 thin films have a versatile potential for constructing highly efficient photoelectrodes when combined with conventional semiconductor light absorbers, taking advantage of its optical transparency and high electrochemical activity. Here, we firstly report fully vertically aligned MoS2 (VMS)/p-Si heterostructure photocathode for photoelectrochemical (PEC) water splitting. Furthermore, 3D iron-nickel sulfide nanoparticles of tailored atomic composition are formed simultaneously during the synthesis of VMS via one-step sulfurization to build 3D/2D transition metal sulfide (TMD) heterostructure thin film catalyst. The spectroscopic results reveal that the Fe-doped Ni3S2 nanoparticles on VMS/p-Si photocathode induce the electrochemically-benign band bending in the overall heterostructure, enabling a significant improvement in PEC performance and long-term stability. Scanning photoelectrochemical microscopy is used to vividly visualize the photocurrent enhancement by the various 3D/2D TMD heterostructures. This work provides promising strategies in developing high performance TMD-based electrocatalysts for practical applications in a wide variety of electrochemical energy conversion processes.