Efficient Photocatalytic Hydrogen Evolution via Band Alignment Tailoring: Controllable Transition from Type-I to Type-II

Considering the sizable band gap and wide spectrum response of tin disulfide (SnS2), ultrathin SnS2 nanosheets are utilized as solar-driven photocatalyst for water splitting. Designing a heterostructure based on SnS2 is believed to boost their catalytic performance. Unfortunately, it has been quite challenging to explore a material with suitable band alignment using SnS2 nanomaterials for photocatalytic hydrogen generation. Herein, a new strategy is used to systematically tailor the band alignment in SnS2 based heterostructure to realize efficient H2 production under sunlight. A Type-I to Type-II band alignment transition is demonstrated via introducing an interlayer of Ce2S3, a potential photocatalyst for H2 evolution, between SnS2 and CeO2. Subsequently, this heterostructure demonstrates tunability in light absorption, charge transfer kinetics, and material stability. The optimized heterostructure (SnS2–Ce2S3–CeO2) exhibits an incredibly strong light absorption ranging from deep UV to infrared light. Significantly, it also shows superior hydrogen generation with the rate of 240 µmol g−1 h−1 under the illumination of simulated sunlight with a very good stability

Carbon dots decorated vertical SnS2 nanosheets for efficient photocatalytic oxygen evolution

Metal sulfides are highly desirable materials for photocatalytic water splitting because of their appropriate energy bands. However, the poor stability under light illumination in water hinders their wide applications. Here, two-dimensional SnS2 nanosheets, along with carbon dots of the size around 10 nm, are uniformly grown on fluorine doped tin oxide glasses with a layer of nickel nanoparticles. Significantly, strong light absorption and enhanced photocurrent density are achieved after integration of SnS2 nanosheets with carbon dots. Notably, the rate of oxygen evolution reached up to 1.1 mmol g-1 h-1 under simulated sunlight irradiation featuring a good stability

Recent advances in transition-metal dichalcogenide based nanomaterials for water splitting

The desire for sustainable and clean energy future continues to be the concern of the scientific community. Researchers are incessantly targeting the development of scalable and abundant electro- or photo-catalysts for water splitting. Owing to their suitable band-gap and excellent stability, an enormous amount of transition-metal dichalcogenides (TMDs) with hierarchical nanostructures have been extensively explored. Herein, we present an overview of the recent research progresses in the design, characterization and applications of the TMD-based electro- or photo-catalysts for hydrogen and oxygen evolution. Emphasis is given to the layered and pyrite-phase structured TMDs encompassing semiconducting and metallic nanomaterials. Illustrative results and the future prospects are pointed out. This review will provide the readers with insight into the state-of-the-art research progresses in TMD based nanomaterials for water splitting

High Crystal Quality 2D Manganese Phosphorus Trichalcogenide Nanosheets and their Photocatalytic Activity

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

High-Yield Production of Monolayer FePS3 Quantum Sheets via Chemical Exfoliation for Efficient Photocatalytic Hydrogen Evolution

2D layered transition metal phosphorus trichalcogenides (MPX3) possess higher in-plane stiffness and lower cleavage energies than graphite. This allows them to be exfoliated down to the atomic thickness. However, a rational exfoliation route has to be sought to achieve surface-active and uniform individual layers. Herein, monolayered FePS3 quantum sheets (QSs) are systematically obtained, whose diameters range from 4–8 nm, through exfoliation of the bulk in hydrazine solution. These QSs exhibit a widened bandgap of 2.18 eV as compared to the bulk (1.60 eV) FePS3. Benefitting from the monolayer feature, FePS3 QSs demonstrate a substantially accelerated photocatalytic H2 generation rate, which is up to three times higher than the bulk counterpart. This study presents a facile way, for the first time, of producing uniform monolayer FePS3 QSs and opens up new avenues for designing other low-dimensional materials based on MPX3

Efficient Catalysis of Hydrogen Evolution Reaction from WS2(1−x)P2x Nanoribbons

The rational design of Earth abundant electrocatalysts for efficiently catalyzing hydrogen evolution reaction (HER) is believed to lead to the generation of carbon neutral energy carrier. Owing to their fascinating chemical and physical properties, transition metal dichalcogenides (TMDs) are widely studied for this purpose. Of particular note is that doping by foreign atom can bring the advent of electronic perturbation, which affects the intrinsic catalytic property. Hence, through doping, the catalytic activity of such materials could be boosted. A rational synthesis approach that enables phosphorous atom to be doped into WS2 without inducing phase impurity to form WS2(1−x)P2x nanoribbon (NRs) is herein reported. It is found that the WS2(1−x)P2x NRs exhibit considerably enhanced HER performance, requiring only −98 mV versus reversible hydrogen electrode to achieve a current density of −10 mA cm−2. Such a high performance can be attributed to the ease of H-atom adsorption and desorption due to intrinsically tuned WS2, and partial formation of NRs, a morphology wherein the exposure of active edges is more pronounced. This finding can provide a fertile ground for subsequent works aiming at tuning intrinsic catalytic activity of TMDs

Hierarchically heterostructured metal hydr(oxy)oxides for efficient overall water splitting

The design of highly efficient electrocatalysts containing non-precious metals is crucial for promoting overall water splitting in alkaline media. In particular, Janus catalysts simultaneously facilitating the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are desirable. Herein, we fabricated a unique hierarchical heterostructure via growing Ni4W6O21(OH)2·4H2O (denoted as Ni-W-O) nanosheets on NiMoO4 rods, which was indispensable for regulating the morphology of the Ni-W-O structure. This heterostructure of Ni-W-O/NiMoO4 could be utilized as an electrocatalyst to realize superior activity for overall water splitting in 1.0 M KOH. It substantially promoted overall water splitting with 1.6 V at 30 mA cm-2, outperforming numerous bifunctional electrocatalysts under the same conditions. Notably, the remarkable stability for continuously splitting water endowed this hierarchical heterostructure with potential applications on a large scale. This work emphasizes the effectively controlled growth of heterostructured non-noble-metal catalysts for energy-conversion reaction

Configuration-Dependent Electrically Tunable Van der Waals Heterostructures Based on MoTe2/MoS2

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

Strong electrically tunable MoTe2/graphene van der Waals heterostructures for high-performance electronic and optoelectronic devices

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 (122.37 meV). Moreover, the G/MoTe2/G phototransistors show high photoresponse performances with on/off ratio, responsivity, and detectivity of ∼105, 87 A/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

Dendritic growth of monolayer ternary WS2(1-: X)Se2 x flakes for enhanced hydrogen evolution reaction

Two-dimensional transition-metal dichalcogenides (TMDs) have attracted much research interest in the hydrogen evolution reaction (HER) due to their superior electrocatalytic properties. Beyond binary TMDs, ternary TMD alloys, as electrocatalysts, were also gradually acknowledged for their remarkable efficiency in HER. Herein, we successfully synthesized monolayer dendritic ternary WS2(1-x)Se2x flakes possessing abundant active edge sites on a single crystalline SrTiO3 (STO(100)). And the obtained dendritic WS2(1-x)Se2x flakes could be transferred intact to arbitrary substrates, for example, SiO2/Si and Au foils. Intriguingly, the transferred dendritic WS2(1-x)Se2x flakes on Au foil demonstrate a significant HER performance, reflected by a rather lower Tafel slope of ∼69 mV dec-1 and a much higher exchange current density of ∼50.1 μA cm-2 outshining other CVD-grown two-dimensional TMD flakes. Furthermore, our new material shows excellent stability in electro-catalyzing the HER, suggestive of its robustness for being an excellent electrocatalyst. We believe that this work broadens the outlook for the synthesis of two-dimensional TMDs toward satisfying the applications in electrocatalysis