van der Waals layered materials have large crystal anisotropy and crystallize spontaneously into two-dimensional (2D) morphologies. Two-dimensional materials with hexagonal lattices are emerging 2D confined electronic systems at the limit of one or three atom thickness. Often these 2D lattices also form orthorhombic symmetries, but these materials have not been extensively investigated, mainly due to thermodynamic instability during crystal growth. Here, we show controlled polymorphic growth of 2D tin-sulfide crystals of either hexagonal SnS2 or orthorhombic SnS. Addition of H2 during the growth reaction enables selective determination of either n-type SnS2 or p-type SnS 2D crystal of dissimilar energy band gap of 2.77 eV (SnS2) or 1.26 eV (SnS) as a final product. Based on this synthetic 2D polymorphism of p–n crystals, we also demonstrate p–n heterojunctions for rectifiers and photovoltaic cells, and complementary inverters.

Deterministic Two-Dimensional Polymorphism Growth of Hexagonal n-Type SnS2 and Orthorhombic p-Type SnS Crystals

A review of Sb2Se3 photovoltaic absorber materials and thin-film solar cells

Nano sized semiconductor photocatalysts have a great scope for removal of large organic molecules like dyes and pesticides in an eco-friendly and sustainable manner. The photocatalytic decomposition of dyeing industrial wastewater produces negligible amount of solid by products. The problem associated with the existing catalysts are, their high band gap. Doping one catalyst with other suitable metals and metal oxides will enhance the efficiency of the photocatalyst and also makes the catalyst to be active in the visible region. The photocatalytic activity of the composite catalysts were highly improved through the modification in the crystallinity, micro structures, band gap, morphology, particle size and the surface area of the catalyst. This paper reviews the recent developments in the synthesis and application of composite photocatalysts. The role of synthetic pathways on the structure and activity of composite catalyst derived from TiO2, CdS, WO3, SnS and ZnO were analyzed. This paper will be more helpful for the scientists working in the field of nano sized photocatalysis.

Review on the photocatalytic activity of various composite catalysts

TiO2 nanoribbons (TiO2 NRs) loaded with FeCo-Al2O3 catalyst were synthesized and used as a precursor in the synthesis of TiO2 nanoribbons/carbon nanotubes (TiO2 NRs/CNTs) composite by a chemical vapor deposition (CVD) method. TiO2 NRs and TiO2 NRs/CNTs composite were characterized by XRD, FT-IR, TEM, SEM, EDX and UV-Vis spectrophotometer. The results revealed the formation of TiO2-B and hydrogen titanate nanoribbon like structures by the hydrothermal treatment. After loading TiO2 NRs by FeCo-Al2O3 catalyst and the CVD growth of carbon nanotubes, the synthetic TiO2 nanoribbons converted entirely to TiO2-B nanoribbons with nanopits structure. The composite composed of tube-like nanostructures forming an interlocked network from CNTs and TiO2-B NRs. The composite shows a relatively red-shifted band gap (3.09 eV), broader and stronger UV absorption band relative to TiO2 NRs. The photocatalytic properties of TiO2 NRs and TiO2 NRs/CNTs composite were studied under sunlight irradiation. The photocatalytic degradation of methylene blue (MB) dye was investigated as a function of contact time, dye concentration, and catalyst dose. The kinetics and mechanisms of degradation were discussed. TiO2 NRs/CNTs composite showed higher stability after six runs and 50% shorter irradiation time than TiO2 NRs photocatalyst.

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TiO2 Nanoribbons/Carbon Nanotubes Composite with Enhanced Photocatalytic Activity; Fabrication, Characterization, and Application.

New types of thin film solar cells made from earth-abundant, non-toxic materials and with adequate physical properties such as band-gap energy, large absorption coefficient and p-type conductivity are needed in order to replace the current technology based on CuInGaSe2 and CdTe absorber materials, which contain scarce and toxic elements. One promising candidate absorber material is tin monosulfide (SnS). The constituent elements of the SnS film are abundant in the earth’s crust, and non-toxic. If this compound is used as the absorber layer in solar cells, high efficient devices should be fabricated with relative low cost technologies. Despite these properties, low efficiency SnS-based solar cells have been reported up to now. In this work, we present a review about the state of the art of SnS films and devices. Finally, an analysis about different factors that are limiting high efficiency solar cells is presented.

SnS-based thin film solar cells: perspectives over the last 25 years

Tin sulfide (SnS) is a material of interest for use as an absorber in low cost solar cells. Single crystals of SnS were grown by the physical vapor deposition technique. The grown crystals were characterized to evaluate the composition, structure, morphology, electrical and optical properties using appropriate techniques. The composition analysis indicated that the crystals were nearly stoichiometric with Sn-to-S atomic percent ratio of 1.02. Study of their morphology revealed the layered type growth mechanism with low surface roughness. The grown crystals had orthorhombic structure with (0 4 0) orientation. They exhibited an indirect optical band gap of 1.06 eV and direct band gap of 1.21 eV with high absorption coefficient (up to 10(3) cm(-1)) above the fundamental absorption edge. The grown crystals were of p-type with an electrical resistivity of 120 Omega cm and carrier concentration 1.52 x 10(15) cm(-3). Analysis of optical absorption and diffuse reflectance spectra showed the presence of a wide absorption band in the wavelength range 300-1200 nm, which closely matches with a significant part of solar radiation spectrum. The obtained results were discussed to assess the suitability of the SnS crystal for the fabrication of optoelectronic devices. (C) 2011 Elsevier B.V. All rights reserved.

Optical and electrical properties of SnS semiconductor crystals grown by physical vapor deposition technique

Vacuum annealed tin sulfide (SnS) thin films for solar cell applications

Photovoltaic structures using thermally evaporated SnS and CdS thin films

Demineralization of coal carried out using EDTA for different time and HF of varying concentration. The residual coal from each treatment was characterized together with virgin coal using Scanning Electron Microscopy and energy dispersive X-ray analysis (EDAX). Micrograph and EDAX analysis of Virgin coal reveal the features lithophiles like aluminium, silicates and calcium. The absence of the features corresponding inorganic elements in residual samples is an indication of demineralization. An elemental analyzer was adopted to analyze CHNS on virgin and residual coal sample. The current research compares the leaching efficiency of a mild leachant and a strong leachant. The final analysis showed that the coal under study was sub bituminous coal and leaching could improve the amount of carbonaceous material.

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Chemical Leaching of Low Rank Coal and its Characterization using SEM/EDAX and FTIR

Single crystal whiskers of gallium telluride (GaTe) have been grown by the physical vapour deposition (PVD) method. Microindentation studies were carried out on the prism faces of the needles to understand their mechanical behaviour. The variation in the microhardnessof GaTe crystals with applied load has been determined at room temperature using Vickers microhardness indenter. The work- hardening exponent has also been computed for different load regions. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A. G. Kunjomana

Papers

Optical and electrical properties of SnS semiconductor crystals grown by physical vapor deposition technique

Vacuum annealed tin sulfide (SnS) thin films for solar cell applications

Photovoltaic structures using thermally evaporated SnS and CdS thin films

Chemical Leaching of Low Rank Coal and its Characterization using SEM/EDAX and FTIR

Microhardness studies of GaTe whiskers