Template free-synthesis of cobalt-iron chalcogenides [Co0.8Fe0.2L2, L = S, Se] and their robust bifunctional electrocatalysis for the water splitting reaction and Cr(vi) reduction.

The ease of production of materials and showing multiple applications are appealing in this modern era of advanced technology. This paper reports the synthesis of a pair of novel cobalt-iron chalcogenides [Co0.8Fe0.2S2 and Co0.8Fe0.2Se2] with enhanced electro catalytic activities. These ternary metal chalcogenides were synthesized by a one-step template-free approach via a hexamethyldisilazane (HMDS)-assisted synthetic method. Transient photocurrent (TPC) studies and electrochemical impedance spectra (EIS) of these materials showed free electron mobility. Their bifunctional activities were verified in both the electrochemical oxygen evolution reaction (OER) and in the electrochemical reduction of toxic inorganic heavy metal ions [Cr(vi)] in polluted water. The materials showed robust catalytic ability in the oxygen evolution reaction with minimum possible over potential (345 and 350 mV @ η10) as determined by linear sweep voltammetry and the lower Tafel values (52.4 and 84.5 mV dec-1) for Co0.8Fe0.2Se2 and Co0.8Fe0.2S2 respectively. Surprisingly, both the materials also showed an excellent activity towards electrochemical Cr(vi) reduction to Cr(iii). Besides the maximum current achieved for Co0.8Fe0.2S2, a minimum value for the Limit of detection (LOD) was obtained for Co0.8Fe0.2S2 (0.159 µg L-1) compared to Co0.8Fe0.2Se2 (0.196 µg L-1). We tested the durability of catalysts, the critical factor for the prolonged use of catalysts, through the recyclability measurements of these materials as catalysts. Both the catalysts presented outstanding durability and balanced electro catalytic activities for up to 1500 CV cycles, and chronoamperometry studies also confirmed exceptional stability. The enhanced catalytic activities of these materials are ascribed to the free electron movement, evidenced by the increased TPC measured and EIS. Therefore, the template-free synthesis of these electro catalysts containing non-noble metal illustrates the practical approach to develop such types of catalysts for multiple functions.

Impact of bandgap tuning on ZnS for degradation of environmental pollutants and disinfection.

The materials showing multiple applications are appealing for their practical use and industrial production. To realize the suitable property for various applications, we have produced ZnS (sf-ZnS) and metal-doped ZnS nanoflakes (sf-m-ZnS; where m = Cu, Ni, Cd, Bi, or Mn) and correlated their activity with bandgap variation. We obtained all these materials via hexamethyldisilazane (HMDS)-assisted synthetic method without using any surfactants, polymers, or template molecules and characterized them thoroughly using various techniques. Photocatalytic, as well as antibacterial, activities of these materials showed their bifunctional utility. We have demonstrated the effect of doping and consequent extension of absorption band to the visible region and resultant improved photocatalytic activity under sunlight. Thus, the change in bandgap influenced their performance as photocatalysts. Among all materials produced, sf-Cd-ZnS provided superior results as a photocatalyst while degrading two organic pollutants-rhodamine B (RhB) and methylene blue (MB) in water. The antibacterial activity of sf-ZnS and sf-m-ZnS against Gram-positive bacteria, i.e., Staphylococcus aureus (S. aureus), was examined by the zone of inhibition method, wherein sf-Ni-ZnS showed maximum activity. The enhanced activity of these ZnS materials can be attributed to the free surface of nanoparticles without any capping by organic molecules, which provided an intimate interaction of inorganic semiconductor material with organic and biomolecules. Thus, we have demonstrated modification of properties both by bandgap tuning of materials and providing the opportunity for intimate interaction of materials with substrates. The photocatalytic activity and antibacterial action of metal-doped ZnS produced by our method exhibited their potential for environmental remediation, specifically water purification.

Importance of Clean Surfaces on the Catalyst: SnS2 Nanorings for Environmental Remediation.

The focus of the work is the synthesis of SnS2 nanomaterials with (peg-SnS2NF) and without (sf-SnS2NR) the involvement of the organic template and the comparative study of their catalytic activities. The synthesis of these materials was achieved in a single-step procedure aided by hexamethyldisilazane (HMDS). These nanoparticles were subjected to X-ray diffraction, transmission electron microscopy, scanning electron microscopy, Raman spectroscopy, and UV-vis spectroscopy analyses to investigate their structural, topographical, surface, and optical properties. The present work suggests that the surfactant-free SnS2 nanoring (sf-SnS2NR) catalyst has lower surface area compared to the poly(ethylene glycol)-stabilized SnS2 nanoflower (peg-SnS2NF) catalyst but shows high activity under visible light for the photoreduction of Cr(VI) and the photocatalytic degradation of organic dyes. The work exposed the importance of the clean surfaces on the catalyst and is expected to have a high impact on the photocatalytic activity of the SnS2 nanomaterial. The study also endorses the utility of the HMDS-assisted synthetic method for the production of multifunctional semiconductor tin disulfide nanomaterials with multiple potential applications.