Latesh K. Nikam

An eco-friendly, highly stable and efficient nanostructured p-type N-doped ZnO photocatalyst for environmentally benign solar hydrogen production

We have investigated an economical green route for the synthesis of a p-type N-doped ZnO photocatalyst by a wet chemical method. Significantly, hazardous H2S waste was converted into eco-friendly hydrogen energy using the p-type N-doped ZnO photocatalyst under solar light, which has previously been unattempted. The as-synthesized p-type N-doped ZnO shows a hexagonal wurtzite structure. The optical study shows a drastic shift in the band gap of the doped ZnO in the visible region (3.19–2.3 eV). The doping of nitrogen into the ZnO lattice is conclusively proved from X-ray photoelectron spectroscopy analysis and Raman scattering. The morphological features of the N-doped ZnO are studied from FESEM, TEM and reveal particle sizes to be in the range of ∼4–5 nm. The N-doped ZnO exhibits enhanced photocatalytic hydrogen generation (∼3957 μmol h−1) by photodecomposition of hydrogen sulfide under visible light irradiation, which is much higher as compared to semiconductor metal oxides reported so far. It is noteworthy that a green catalyst is investigated to curtail H2S pollution along with production of hydrogen (green fuel) using solar light, i.e., a renewable energy source. The green process investigated will have the potential to synthesize other N-doped metal oxides.

Ecofriendly hydrogen production from abundant hydrogen sulfide using solar light-driven hierarchical nanostructured ZnIn2S4 photocatalyst

It is quite well-known that refineries are producing huge amount of H2S which has been used to produce sulphur and water using the well-known Claus process. This process is not an economically viable process, due to the high-cost chemical process and creates further acute environmental problems. Therefore, we have demonstrated the conversion of poisonous H2S into H2 using an ecofriendly phocatalysis process which is a green unconventional energy source. We have investigated ecofriendly nanostructured ZnIn2S4 photocatalyst to produce hydrogen from H2S using solar light. We also demonstrate the controlled synthesis of hierarchical nanostructured ZnIn2S4 using a facile hydrothermal method. The morphologies obtained have been greatly influenced by the presence of triethylamine (TEA) with various concentrations during the reaction. Surprisingly, a highly crystalline hexagonal layer structured ZnIn2S4 was obtained instead of cubic spinel. The hierarchical nanostructure, i.e. marigold flower-like morphology, was obtained without any surfactant. The thin and transparent petals self-assembled to form the unique nanostructured marigold flower. The highly crystalline puffy marigold flowers and nanoplates/nanostrips were obtained using TEA-assisted hydrothermal synthesis. Optical study shows the band gap in the range of 2.34–2.48 eV. Considering the band gap in the visible region, ZnIn2S4 is used as photocatalyst for hydrogen production from hydrogen sulphide under solar light which is hitherto unattempted. The constant photocatalytic activity of hydrogen evolution, i.e. 5287 μmol h−1, was obtained using such hierarchical nanostructured ZnIn2S4 under visible light irradiation. It is noteworthy that the H2 evolution rate obtained is much higher compared to earlier reported photocatalysts. Considering the significance of morphologies for photocatalytic application, the formation mechanism has also been furnished. The unique hierarchical nanostructured ZnIn2S4 ternary semiconductor having hexagonal layer will have potential applications in solar cells, LEDs, charge storage, electrochemical recording, thermoelectricity and other prospective electronic and optical devices.

A green process for efficient lignin (biomass) degradation and hydrogen production via water splitting using nanostructured C, N, S-doped ZnO under solar light

Herein, we have reported the simultaneous water splitting and lignin (biomass) degradation using C, N and S-doped ZnO nanostructured materials. The synthesis of C, N and S-doped ZnO was achieved via calcination of bis-thiourea zinc acetate (BTZA) complex. Calcination of the complex at 500 °C results in the formation of C, N, and S doping in a mixed phase of ZnO/ZnS, whereas calcination at 600 °C gives a single phase of ZnO with N and S-doping, which is confirmed by XRD, XPS and Raman spectroscopy. The band gap of the calcined samples was observed to be in the range of 2.83–3.08 eV. Simultaneous lignin (waste of paper and pulp mills) degradation and hydrogen (H2) production via water splitting under solar light has been investigated, which is hitherto unattempted. The highest degradation of lignin was observed with the sample calcined at 500 °C, i.e., C, N, S-doped ZnO/ZnS when compared to the sample calcined at 600 °C, i.e., N and S doped ZnO. The degradation of lignin confers the formation of a useful fine chemical as a by-product, i.e., 1-phenyl-3-buten-1-ol. However, excellent H2 production, i.e., 580, 584 and 643 μmol h−1 per 0.1 g, was obtained for the sample calcined at 500, 550 and 600 °C, respectively. The photocatalytic activity obtained is considerably higher as compared to earlier reported visible light active oxide and sulfide photocatalysts. The reusability study shows a good stability of the photocatalyst. The prima facie observations show that lignin degradation and water splitting is possible with the same multifunctional photocatalyst without any scarifying agent.

Self assembled CdLa2S4 hexagon flowers, nanoprisms and nanowires: novel photocatalysts for solar hydrogen production

We report here a new ternary chalcogenide material, cadmium lanthanum sulfide (CdLa2S4) produced using a facile hydrothermal method at 433 K. The effect of the solvent on the morphology of the CdLa2S4 was demonstrated for the first time. The prima facie observations revealed the formation of highly crystalline hexagonal structures in the form of flowers in aqueous medium. The flowers comprise hexagonal columns ∼300 nm in diameter and 1–1.2 μm in length. All the hexagonal structures have a sharp tip with a cavity of 10 nm and are almost equal in size. The nanoprisms have an average base size of 35 nm with 35 nm edges, and the nanowires have a diameter of 10–15 nm; both were obtained in methanol. Crystal and electronic structure calculations were performed using the Vienna ab initio simulation package (VASP) based on density functional theory (DFT). Considering the band gap of pristine CdLa2S4 in the visible region (2.3 eV), we have demonstrated CdLa2S4 as a photocatalyst for the production of H2 under solar light. Nanostructured CdLa2S4 prisms gave the maximum hydrogen production, i.e. 2552 μmol h−1. Being a stable ternary nanostructured metal sulfide (with nanohexagons, nanoprisms, nanowires), CdLa2S4 may have other potential prospective applications in solar cells and optoelectronic devices.

Self-assembled hierarchical nanostructures of Bi2WO6 for hydrogen production and dye degradation under solar light

Three dimensional (3D) hierarchical nanostructures of orthorhombic Bi2WO6 with unique morphologies were successfully synthesized by a solvothermal method. The precursor concentration plays a key role in the architecture of the hierarchical nanostructures. A peony flower-like morphology was obtained at higher precursor concentrations, and a red blood cell (RBC)-like morphology with average diameter of 1.5 μm was obtained at lower concentrations. These hierarchical nanostructures were assembled by self-alignment of 20 nm nanoplates. As their band gap is in the visible region, the photocatalytic activity of the Bi2WO6 hierarchical nanostructures for the production of hydrogen from glycerol, and the degradation of rhodamine B (RhB) and methylene blue (MB) under ambient conditions in the presence of solar light was investigated. The Bi2WO6 with peony flower morphology was observed to be the most efficient photocatalyst (H2: 7.40 mmol h−1 g−1, kRhB: 0.240 and kMB: 0.100) of the reported nanostructures. The higher activity of the peony flowers was due to their porous nature, high surface area and lower band gap. Such unique 3D nanostructures of Bi2WO6 have been fabricated for the first time, and their use as photocatalysts in the production of hydrogen from glycerol has hitherto not been attempted. These nanostructures may have potential in ferroelectric, piezoelectric, pyroelectric and nonlinear dielectric applications.

Nanostructured 2D MoS2 honeycomb and hierarchical 3D CdMoS4 marigold nanoflowers for hydrogen production under solar light

Unique two dimensional (2D) honeycomb layered MoS2 nanostructures and hierarchical 3D marigold nanoflowers of CdMoS4 were designed using a template free and facile solvothermal method. The MoS2 structure is depicted with a sheet like morphology with lateral dimensions of 5–10 μm and a thickness of ∼200 nm and a honeycomb nanostructure architecture produced via the self-assembling of vertically grown thin hexagonal nanosheets with a thickness of 2–3 nm. The 3D CdMoS4 marigold nanoflower architecture comprised thin nanopetals with lateral dimensions of 1–2 μm and a thickness of a few nm. The CdMoS4 and MoS2 structures displayed hydrogen (H2) production rates of 25 445 and 12 555 μmol h−1 g−1, respectively. The apparent quantum yields of hydrogen production were observed to be 35.34% and 17.18% for CdMoS4 and MoS2, respectively. The 3D nanostructured marigold flowers of CdMoS4 and honeycomb like 2D nanostructure of MoS2 were responsible for higher photocatalytic activity due to inhibition of the charge carrier recombination. The prima facie observation of H2 production showed that the ternary semiconductor confers enhanced photocatalytic activity for H2 generation due to its unique structure. Such structures can be designed and implemented in other transition metal dichalcogenide based ternary materials for enhanced photocatalytic and other applications.

Enhanced hydrogen production under a visible light source and dye degradation under natural sunlight using nanostructured doped zinc orthotitanates

The nanostructured Ag and Co doped zinc orthotitanates (ZOT) were synthesized using a combustion method. The structural and optical analysis shows the existence of cubic and tetragonal phases. Morphological study by FESEM reveals the formation of a web like structure along with pot holes by the self-assembly of spherical nanoparticles of ∼50 nm size. Further, TEM investigations reveal diffused and uneven shaped nanoparticles in the range of 10–25 nm. BET surface area measurements show a decrease in surface area due to doping. These ZOTs were employed for photocatalytic dye degradation (Acid Orange-8 and Rhodamine-B) under natural sunlight. The prima facie observations showed Ag@Zn2TiO4 to be an excellent photocatalyst for dye degradation. The kinetics study shows the order of the reaction to be in the range of 1.1–1.41. The ZOTs synthesized have also been used for photocatalytic hydrogen production from H2S under visible light irradiation. It is noteworthy that utmost H2 production (2784 μmol h−1/100 mg) was observed for Ag@Zn2TiO4 which is much higher than that achieved with visible light active photocatalysts reported so far. The dye degradation and hydrogen production from H2S using ZOT are hitherto unattempted. The nanostructured Zn2TiO4 will be a potential visible light active photocatalyst for waste degradation and water splitting.

Self assembly of nanostructured hexagonal cobalt dendrites: an efficient anti-coliform agent

Highly crystalline self assemblies of three dimensional cobalt nanostructures are successfully synthesized by an electrochemical method without any template and surfactants. The cobalt nanostructures obtained by using two precursors, cobalt chloride (CoCl2) and cobalt acetate [(CH3–COO)2Co], shows similar dendritic structure, but with different hierarchical architecture. The architecture of cobalt dendrites (Co-DNDs) prepared by using CoCl2 consists of a long central trunk with hierarchical nanostructures of well aligned dendrites of length in the range 15–20 μm and sub branch is in the range 100–200 nm, while Co-DNDs prepared by using (CH3–COO)2Co additionally shows unique feature of hexagonal nanopoles orthogonal to main trunk. Well defined and highly crystalline Co-DNDs were obtained within thirty seconds at 15 V. Architecture of such well aligned and highly crystalline Co-DNDs with hexagonal fixtures within 30 seconds reaction is hitherto unattempted. The as-synthesized Co-DNDs showed an efficient antibacterial activity against model organisms, Bacillus subtilis NCIM 2063, Escherichia coli NCIM 2931, and fecal coliforms in a sewage waste. The inactivation of bacterial growth is due to the generation of reactive oxygen species (ROS) mediated rupture of cell membrane. An inactivation of fecal coliforms in the sewage wastewater is significant in eradicating water-borne diseases. This is an economical approach as compared to conventional and expensive metal nanoparticles like silver and gold.

Novel nanocrystalline zinc silver antimonate (ZnAg3SbO4): an efficient & ecofriendly visible light photocatalyst with enhanced hydrogen generation

Herein, we report zinc silver antimonate (ZnAg3SbO4)/ZAS, a novel visible light active photocatalyst for hydrogen generation. The XRD pattern confirmed the formation of a highly crystalline single phase orthorhombic ZAS. The FESEM and TEM micrographs exhibited that the size of the nanoparticles are in the range ∼20–30 nm. An optical study showed a broad absorption edge from 400 to 1000 nm, with an estimated band gap of about ∼1.48 eV. Considering this ideal band gap, ZAS was used as a photocatalyst for the photodecomposition of H2S under visible light irradiation to produce hydrogen for the first time. We obtained the utmost hydrogen evolution i.e., ∼10 200 μmol h−1 g−1 for the naked ZAS (without a co-catalyst) catalyst under visible light, which is much higher than the earlier reported photocatalysts. Generally, in complex oxides of p-block metals, the bottom of the conduction band (CB) consists of the merely localized s and/or p orbitals which are largely dispersed. This large dispersion is responsible for a high electron mobility and extremely high photocatalytic activity. Therefore, a complex oxide (ZAS) of Ag, Zn and the p-block metal (Sb) is found to be a promising visible light active photocatalyst. This is the most stable, efficient and eco-friendly novel visible light active oxide photocatalyst for hydrogen production.