Synthesis, Crystal and Electronic Structure, and Thermal Conductivity Investigation of the Hollandite-like CsxCr5Te8 Phases (0.73 < x < 1)

This article presents a comprehensive study on the synthesis and structural and thermal conductivity properties of cesium-inserted chromium tellurides of formula CsxCr5Te8. Single crystals of three different compositions (x = 0.73, 0.91, and 0.97) were successfully synthesized and suggested the existence of a solid solution in the range 0.73 < x < 1. Through a detailed single-crystal characterization, the complete structure of these compounds is determined, revealing a distinct B-type hollandite-like structural form derived from the hollandite structure, in contrast to the more commonly observed A-type pseudo-hollandite in AM5X8-type chalcogenides (A = cation, M = transition metal, and X = chalcogen). Periodic density functional theory calculations predict the Cs0.73Cr5Te8 composition as the most stable, with a metallic conductive behavior. The thermal conductivity of bulk CsxCr5Te8 samples is measured to be 1.4 W m-1 K-1 at 300 K and increases with temperature up to 2 W m-1 K-1 at 673 K.

Transition Metal Dichalcogenides [MX2] in Photocatalytic Water Splitting

The quest for a clean, renewable and sustainable energy future has been highly sought for by the scientific community over the last four decades. Photocatalytic water splitting is a very promising technology to proffer a solution to present day environmental pollution and energy crises by generating hydrogen fuel through a “green route” without environmental pollution. Transition metal dichalcogenides (TMDCs) have outstanding properties which make them show great potential as effective co-catalysts with photocatalytic materials such as TiO2, ZnO and CdS for photocatalytic water splitting. Integration of TMDCs with a photocatalyst such as TiO2 provides novel nanohybrid composite materials with outstanding characteristics. In this review, we present the current state of research in the application of TMDCs in photocatalytic water splitting. Three main aspects which consider their properties, advances in the synthesis routes of layered TMDCs and their composites as well as their photocatalytic performances in the water splitting reaction are discussed. Finally, we raise some challenges and perspectives in their future application as materials for water-splitting photocatalysts.

Carbon Nanohorn-Based Electrocatalysts for Energy Conversion

In the context of even more growing energy demands, the investigation of alternative environmentally friendly solutions, like fuel cells, is essential. Given their outstanding properties, carbon nanohorns (CNHs) have come forth as promising electrocatalysts within the nanocarbon family. Carbon nanohorns are conical nanostructures made of sp2 carbon sheets that form aggregated superstructures during their synthesis. They require no metal catalyst during their preparation and they are inexpensively produced in industrial quantities, affording a favorable candidate for electrocatalytic reactions. The aim of this article is to provide a comprehensive overview regarding CNHs in the field of electrocatalysis and especially, in oxygen reduction, methanol oxidation, and hydrogen evolution, as well as oxygen evolution from water splitting, underlining the progress made so far, and pointing out the areas where significant improvement can be achieved.

Bottom-Up Synthesized MoS2 Interfacing Polymer Carbon Nanodots with Electrocatalytic Activity for Hydrogen Evolution

The preparation of an MoS₂ -polymer carbon nanodot (MoS₂ -PCND) hybrid material was accomplished by employing an easy and fast bottom-up synthetic approach. Specifically, MoS₂ -PCND was realized by the thermal decomposition of ammonium tetrathiomolybdate and the in situ complexation of Mo with carboxylic acid units present on the surface of PCNDs. The newly prepared hybrid material was comprehensively characterized by spectroscopy, thermal means, and electron microscopy. The electrocatalytic activity of MoS₂ -PCND was examined in the hydrogen evolution reaction (HER) and compared with that of the corresponding hybrid material prepared by a top-down approach, namely MoS₂ -PCND(exf-fun), in which MoS₂ was firstly exfoliated and then covalently functionalized with PCNDs. The MoS₂ -PCND hybrid material showed superior electrocatalytic activity toward the HER with low Tafel slope, excellent electrocatalytic stability, and an onset potential of -0.16 V versus RHE. The superior catalytic performance of MoS₂ -PCND was rationalized by considering the catalytically active sites of MoS₂ , the effective charge/energy-transfer phenomena from PCNDs to MoS₂ , and the synergetic effect between MoS₂ and PCNDs in the hybrid material.

Sulfur-doped graphene/transition metal dichalcogenide heterostructured hybrids with electrocatalytic activity toward the hydrogen evolution reaction

A facile route for the preparation of molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂), uniformly deposited onto sulfur-doped graphene (SG), is reported. The realization of the SG/MoS₂ and SG/WS₂ heterostructured hybrids was accomplished by employing microwave irradiation for the thermal decomposition of ammonium tetrathiomolybdate and tetrathiotungstate, respectively, in the presence of SG. Two different weight ratios between SG and the inorganic species were used, namely 3 : 1 and 1 : 1, yielding SG/MoS₂ (3 : 1), SG/MoS₂ (1 : 1), SG/WS₂ (3 : 1) and SG/WS₂ (1 : 1). SG and all newly developed hybrid materials were characterized by ATR-IR and Raman spectroscopy, TGA, HR-TEM and EELS. The electrocatalytic activity of the SG/MoS₂ and SG/WS₂ heterostructured hybrids was examined against the hydrogen evolution reaction (HER) and it was found that the presence of SG not only significantly improved the catalytic activity of MoS₂ and WS₂ but also made it comparable to that of commercial Pt/C. Specifically, hybrids containing higher amounts of SG, namely SG/MoS₂ (3 : 1) and SG/WS₂ (3 : 1), exhibited extremely low onset overpotentials of 26 and 140 mV vs. RHE, respectively. The latter results highlighted the beneficial role of SG as a substrate for immobilizing MoS₂ and WS₂ and stressed its significance for achieving optimum electrocatalytic performance toward the HER. Finally, examination of the Tafel slopes as extracted from the electrocatalytic polarization curves, manifested the adsorption of hydrogen as the rate-limiting step for SG/MoS₂ (3 : 1), while for SG/WS₂ (3 : 1) the electrochemical desorption of adsorbed hydrogen atoms to generate hydrogen was revealed to be the rate-limiting step.

Photocatalytic Hydrogen Production: Role of Sacrificial Reagents on the Activity of Oxide, Carbon, and Sulfide Catalysts

Photocatalytic water splitting is a sustainable technology for the production of clean fuel in terms of hydrogen (H2). In the present study, hydrogen (H2) production efficiency of three promising photocatalysts (titania (TiO2-P25), graphitic carbon nitride (g-C3N4), and cadmium sulfide (CdS)) was evaluated in detail using various sacrificial agents. The effect of most commonly used sacrificial agents in the recent years, such as methanol, ethanol, isopropanol, ethylene glycol, glycerol, lactic acid, glucose, sodium sulfide, sodium sulfite, sodium sulfide/sodium sulfite mixture, and triethanolamine, were evaluated on TiO2-P25, g-C3N4, and CdS. H2 production experiments were carried out under simulated solar light irradiation in an immersion type photo-reactor. All the experiments were performed without any noble metal co-catalyst. Moreover, photolysis experiments were executed to study the H2 generation in the absence of a catalyst. The results were discussed specifically in terms of chemical reactions, pH of the reaction medium, hydroxyl groups, alpha hydrogen, and carbon chain length of sacrificial agents. The results revealed that glucose and glycerol are the most suitable sacrificial agents for an oxide photocatalyst. Triethanolamine is the ideal sacrificial agent for carbon and sulfide photocatalyst. A remarkable amount of H2 was produced from the photolysis of sodium sulfide and sodium sulfide/sodium sulfite mixture without any photocatalyst. The findings of this study would be highly beneficial for the selection of sacrificial agents for a particular photocatalyst.