Uttam Gupta

Highly Effective Visible-Light-Induced H2Generation by Single-Layer 1T-MoS2and a Nanocomposite of Few-Layer 2H-MoS2with Heavily Nitrogenated Graphene

Two sorts of MoS2 : A single-layer, metallic form of MoS2 (1T-MoS2 ) and a nanocomposite of a second form of MoS2 (few-layer 2H-MoS2 ) with heavily nitrogenated reduced graphene oxide (NRGO; N content ca. 15 %) show outstanding performance in the production of H2 under visible-light illumination.

Weyl Semimetals as Hydrogen Evolution Catalysts

The search for highly efficient and low-cost catalysts is one of the main driving forces in catalytic chemistry. Current strategies for the catalyst design focus on increasing the number and activity of local catalytic sites, such as the edge sites of molybdenum disulfides in the hydrogen evolution reaction (HER). Here, the study proposes and demonstrates a different principle that goes beyond local site optimization by utilizing topological electronic states to spur catalytic activity. For HER, excellent catalysts have been found among the transition-metal monopnictides-NbP, TaP, NbAs, and TaAs-which are recently discovered to be topological Weyl semimetals. Here the study shows that the combination of robust topological surface states and large room temperature carrier mobility, both of which originate from bulk Dirac bands of the Weyl semimetal, is a recipe for high activity HER catalysts. This approach has the potential to go beyond graphene based composite photocatalysts where graphene simply provides a high mobility medium without any active catalytic sites that have been found in these topological materials. Thus, the work provides a guiding principle for the discovery of novel catalysts from the emerging field of topological materials.

Characterization of few-layer 1T-MoSe2 and its superior performance in the visible-light induced hydrogen evolution reaction

Based on earlier results on the photocatalytic properties of MoS2, the 1T form of MoSe2, prepared by lithium intercalation and exfoliation of bulk MoSe2, has been employed for the visible-light induced generation of hydrogen. 1T-MoSe2 is found to be superior to both 2H and 1T MoS2 as well as 2H-MoSe2 in producing hydrogen from water, the yield being in the 60–75 mmol h−1 g−1 range with a turn over frequency of 15–19 h−1. First principles calculations reveal that 1T-MoSe2 has a lower work function than 2H-MoSe2 as well as 1T and 2H-MoS2, making it easier to transfer an electron from 1T-MoSe2 for the production of H2.

Visible‐Light‐Induced Generation of H2 by Nanocomposites of Few‐Layer TiS2 and TaS2 with CdS Nanoparticles

Graphene analogues of TaS2 and TiS2 (3-4 layers), prepared by Li intercalation followed by exfoliation in water, were characterized. Nanocomposites of CdS with few-layer TiS2 and TaS2 were employed for the visible-light-induced H2 evolution reaction (HER). Benzyl alcohol was used as the sacrificial electron donor, which was oxidized to benzaldehyde during the reaction. Few-layer TiS2 is a semiconductor with a band gap of 0.7 eV, and its nanocomposite with CdS showed an activity of 1000 μmol h(-1)  g(-1). The nanocomposite of few-layer TaS2, in contrast, gave rise to higher activity of 2320 μmol h(-1)  g(-1), which was attributed to the metallic nature of few-layer TaS2. The amount of hydrogen evolved after 20 and 16 h for the CdS/TiS2 and CdS/TaS2 nanocomposites was 14,833 and 28,132 μmol, respectively, with turnover frequencies of 0.24 and 0.57 h(-1), respectively.

Assemblies of covalently cross-linked nanosheets of MoS2 and of MoS2–RGO: synthesis and novel properties

We report the synthesis, characterization, gas adsorption and hydrogen evolution activity of covalently cross-linked assemblies of MoS2 and MoS2–RGO generated by the Sonogashira coupling reaction. These assemblies show a high surface area and CO2 storage capacity comparable with some of the high surface area MOFs. MoS2 assemblies show enhanced hydrogen evolution activity compared to few-layer MoS2.

2D‐GaS as a Photocatalyst for Water Splitting to Produce H2

Using first-principles local and hybrid density functional theoretical calculations, a thickness-dependent electronic structure of layered GaS is determined, and it is shown that 2D GaS has an electronic structure with valence and conduction bands that straddle the redox potentials of hydrogen evolution reaction and oxygen evolution reaction up to a critical thickness (<5.5 nm). Here, simulations of adsorption of H2O on nanoscale GaS reveal that localized electronic states at its edges appear in the gap and strengthen the interaction with H2O, further activating the surface atomic sites. It is thus predicted that GaS synthesized with a controlled thickness and preferred edges may be an efficient catalyst for photocatalytic splitting of water. Experiments that verify some of the predictions in this study are presented, and it is shown that GaS is effective in absorption of light and evolution of H2 (887 μmol h(-1) g(-1)) in the presence of aqueous solution of hydrazine (1% v/v). This study should open up the use of nanoscale GaS in conversion of solar energy into environment-friendly chemical energy in the form of hydrogen.

Hydrazine as a hydrogen carrier in the photocatalytic generation of H2 using CdS quantum dots

The compelling need for safe storage and transportation of H2 has made liquid-phase materials safer H2-carriers with a high gravimetric and volumetric hydrogen density. Unlike thermal or electrocatalytic decomposition on precious metal catalysts, a photocatalytic route to decomposing these liquid-phase materials can offer triggered onboard production of H2 and help mitigate the safety issues concerned with H2 storage. We have investigated visible-light induced H2 evolution from aqueous hydrazine using CdS quantum dots (QDs) as metal-free photocatalysts. Hydrazine acts as a H2 carrier as well as a donor, giving rise to a visible-light induced H2 evolution activity as high as 33 mmol h-1 g-1 at pH 8. This has been achieved by the use of CdS QDs capped with S2- ligands. The use of larger ligands such as mercaptopropionic acid hinders the adsorption of hydrazine onto CdS QDs and significantly decreases the activity. The effect of pH on the hydrogen yield in aqueous hydrazine has also been examined.

Photochemical water splitting by bismuth chalcogenide topological insulators

As one of the major areas of interest in catalysis revolves around 2D materials based on molybdenum sulfide, we have examined the catalytic properties of bismuth selenides and tellurides, which are among the first chalcogenides to be proven as topological insulators (TIs). We find significant photochemical H2 evolution activity with these TIs as catalysts. H2 evolution increases drastically in nanosheets of Bi2 Te3 compared to single crystals. First-principles calculations show that due to the topology, surface states participate and promote the hydrogen evolution.

Visible-light-induced hydrogen evolution reaction with WS x Se2−x

WS2 is a promising catalyst for the hydrogen evolution reaction. We have explored photocatalytic properties of ternary sulphoselenides of tungsten (WSxSe2−x) by the dye-sensitized hydrogen evolution. WSxSe2−x solid solutions are found to exhibit high activity reaching 2339 μmol h−1 g−1 for WSSe, which is three times higher than that of WS2 alone (866 μmol h−1 g−1). The turnover frequency is also high (0.7 h−1). Such synergistic effect of selenium substitution in WS2 is noteworthy.

Remarkable photochemical HER activity of semiconducting 2H

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