Nilima S. Chaudhari

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.

Organization of cubic CeO2 nanoparticles on the edges of self assembled tapered ZnO nanorods via a template free one-pot synthesis: significant cathodoluminescence and field emission properties

The present investigation explores the controlled architecture of a CeO2–ZnO nanocomposite via a template-free, low temperature, facile single step solvothermal approach. This complex architecture depicts cubic single crystalline CeO2 nanoparticles (size ∼15 nm) grown on the edges of tapered ZnO nanorods with definite orientations and alignments. The formation of wurtzite ZnO, cubic CeO2 and the coexistence of Ce3+ and Ce4+ on the surface of the CeO2–ZnO nanocomposites are confirmed using various characterization tools. The finding of such unique nanostructures by a facile method is exemplified by a plausible growth mechanism. Surprisingly, the aqueous mediated ultrasonication reaction conferred the formation of crystalline ZnO nanotubes of diameter ∼50 nm. Spatially resolved cathodoluminescence spectra are obtained by linearly scanning an individual CeO2–ZnO nanorod along its length, which reveals the size-dependent surface effects. Interestingly, such hybrid CeO2–ZnO nanoarchitecture is observed to exhibit enhanced field emission properties, demonstrating better current stability as compared to other ZnO nanostructures. This is attributed mainly to strong surface interactions between the Ce-ionic species and the ZnO nanorods. Herein, a soft-chemical approach is used for the first time to architect a binary oxide nanostructure, which is otherwise accomplished using high temperature techniques, as reported elsewhere. Also, the present work not only gives insight into understanding the hierarchical growth behaviour of the CeO2–ZnO nanocomposite in a solution phase synthetic system, but also provides an efficient route to enhance the field emission performance of ZnO nanostructures, which could be extended to other potential applications, such as chemical sensors, optoelectronic devices and photocatalysts.

Novel sonochemical assisted hydrothermal approach towards the controllable synthesis of ZnO nanorods, nanocups and nanoneedles and their photocatalytic study

ZnO nanovariants possessing different morphologies have been synthesized via a sonochemical as well as a sonochemical assisted hydrothermal (SAH) method. The synthesis process variables such as concentration of the precursors, ultrasonic irradiation period and duty cycle are observed to influence the resultant morphology of the ZnO nanostructures. Novel and significant morphologies of the ZnO nanostructures such as nanoneedles, tetrapods, nanowires, nanopetals, self assembled hexagonal rods and nanocups have been successfully obtained by controlling the process parameters. The surface morphology of the ZnO nanostructures was investigated using a scanning electron microscope (SEM). The SEM investigations showed that the nanoneedles originate from the hexagonal tube. Transmission electron microscope (TEM) analysis clearly demonstrates the nanocrystalline nature of the ZnO structures with unique morphologies like hexagonal nanocups. The ZnO nanostructures were characterized by UV-visible and photoluminescence spectrometers and a possible growth mechanism of the ZnO nanostructures is proposed. The photocatalytic activity of these nanostructures has also been presented. The ZnO nanostructures like nanocups and nanoneedles exhibit an excellent photocatalytic activity. Being a wide band gap semiconductor, these unique nanostructures will have a prospective application in ZnO based dye sensitized solar cells.

Hierarchical Nanostructured ZnO with Nanorods Engendered to Nanopencils and Pin-Cushion Cactus with Its Field Emission Study

In the present investigation, we report the synthesis of highly crystalline ZnO nanorods engendered to pin-cushion cactus and 1D nanopencil like nanoforms on zinc (Zn) foil via a simple sonochemical assisted hydrothermal route. The work reported herewith is attractive for two reasons: (i) the facile one step solution approach assisted by prior ultrasonication converts nanorods/nanobelts into nanopencils, and (ii) the sharp and quasi-aligned ZnO nanopencils are potential field electron emitters. In addition, the controlled growth of pinhole like ZnO nanopencils and aligned hexagonal ZnO nanodisc was obtained. The changes in the growth rate, diameter, density, and surface area of highly oriented ZnO nanorods are examined. Considering the significances of such novel morphologies, technically detailed formation mechanism has been proposed. The field emission study of pin-cushion cactus like ZnO nanopencils was performed. Field emission measurements demonstrate remarkably low turn-on field which is explained on the basis of a sequential enhancement mechanism involving the consecutive stem and tip contribution. The Folwer-Nordheim (F-N) plot showed nonlinear behavior indicating the semiconducting nature of the emitter. Significantly, emission current is stable at the preset value of 3 μA over the period of 3 h. The simplicity of the synthesis route coupled with the promising emission properties is envisaged to be an important candidate for potential nanoelectronic devices. These unique imperative ZnO nanostructures may have potential for sensors, solar cell, photocatalysis, varisters, etc.

Self-Powered UV-vis Photodetector Based on ZnIn2S4/Hydrogel Interface

The photosensing properties of vertically aligned ZnIn2S4 nanopetal films grown hydrothermally on FTO coated glass are examined without and with surface dispensed agarose gel. For the ZnIn2S4 nanopetals photodetector (without gel) there is no photoresponse for zero bias. Most interestingly, with surface dispensation of agarose gel, the hybrid electronic-iontronic interface system shows a strong photoresponse even under zero bias, a highly efficient self-powered UV-visible light photodetector. Indeed, the zero bias ZnIn2S4/gel hybrid photoresponse is a factor of 100 stronger as compared to the response of the only ZnIn2S4 device (at -1.5 V bias) and that too without any significant degradation in response time. The possible operating mechanisms are proposed.

Decoration of CdS nanoparticles on 3D self-assembled ZnO nanorods: a single-step process with enhanced field emission behaviour

A well-defined CdS–ZnO heteroarchitecture has been synthesized via a facile single-step hydrothermal approach. The morphological and structural studies reveal the formation of 3D nano-architectures, in which self-assembled ZnO nanorods (diameter ~50 nm) are well decorated with single crystalline CdS nanoparticles (size ~10 nm). The CdS–ZnO heteroarchitecture exhibits a remarkable change in the optical absorption due to the surface modification of ZnO nanorods by CdS. Surprisingly, under identical reaction conditions, the global ZnS nanoparticles are selectively grown at the apex of ZnO nanorods on the Zn substrate. Furthermore, a plausible growth mechanism has been presented on the basis of experimental results. Interestingly, the CdS–ZnO heteroarchitecture shows enhanced field emission properties such as low turn-on field, high emission current density and better current stability in comparison to other ZnO-based nanostructures. The present CdS–ZnO heteroarchitecture could be extended to other potential applications, such as chemical sensors, photodetectors, optoelectronic devices, and photocatalysts.

Single step hydrothermal approach for devising hierarchical Ag–ZnO heterostructures with significant enhancement in field emission performance

Hierarchical Ag–ZnO heterostructures have been synthesized via a template free single step hydrothermal method. Structural and morphological studies reveal the formation of heterostructures comprised of Ag nanoparticles (∼20 nm) organized on tapered ZnO nanorods under the prevailing experimental conditions. A plausible reaction and growth mechanism has been discussed. Furthermore, the hierarchical creation of ZnO with Ag as a relatively low work function material offers an effective approach to tailor its field emission properties. The field emission studies reveal a remarkable low turn-on field of ∼ 1 V μm−1, corresponding to an emission current density of ∼10 μA cm−2, and an emission current density of ∼400 μA cm−2 has been drawn at an applied field of 2.24 V μm−1. In addition, the Ag–ZnO heterostructures exhibit a good emission current stability at the pre-set value of ∼1 and 4 μA over a duration of 3 h. The enhancement of the field emission characteristics resulting from Ag nanoparticles decorating the tapered ZnO nanorods is discussed on the basis of band structure modifications. The ease of the synthesis route and the remarkable field emission properties offer Ag–ZnO heterostructures as a promising electron source for high current density applications.

Controlled synthesis of aligned Bi2S3 nanowires, sharp apex nanowires and nanobelts with its morphology dependent field emission investigations

Well-aligned ultra-long Bi2S3 nanowire arrays with three kinds of apex morphology – abruptly sharpened apex, thin belt and flat – have been systematically fabricated on tungsten (W) foil by a facile hydrothermal method. The structural and morphological studies reveal formation of distinct tip morphologies, possessing high aspect ratio, single crystalline nature and growth along the orthorhombic [001] axis. A plausible growth mechanism has been proposed on the basis of observed experimental results. The field emission properties of Bi2S3 nanowires and sharp apex Bi2S3 nanowires are investigated. From the field emission studies, the values of the turn-on field, required to draw emission current density of ∼0.1 μA cm−2, are observed to be ∼2.01 and 1.21 V μm−1 for nanowires and sharp apex nanowires, respectively. Furthermore, ultra-long Bi2S3 nanowires are also grown on the W microtip (brush-like) from which very high emission current density ∼11 mA cm−2 has been drawn. These results are helpful for the design, fabrication and optimization of integrated field emitters using 1D nanostructures as cold cathode material.

Architecture of rose and hollow marigold-like ZnIn2S4 flowers: structural, optical and photocatalytic study

In the present investigation, a surfactant-assisted hydrothermal route has been employed to design self-assembled rose and hollow marigold-like ZnIn2S4 flowers. In the absence of the surfactant, uniform (∼3–5 μm) marigold-like flowers are observed. The self-alignment of the transparent petals (∼3–5 nm thick with a length of ∼25–100 nm) leads to the formation of hollow marigold-like flowers, for which a plausible growth mechanism has also been proposed. Moreover, DEA assisted ZnIn2S4 demonstrates a rose flower-like via self assembly of hexagonal nanoplates. Structural and optical characterization shows the existence of hexagonal structures with a band gap in the range of ∼2.4–2.6 eV. Considering the ideal band gap in the visible region, we have used such unique nanostructured self assemblies of ZnIn2S4 as photocatalysts and demonstrated visible light-driven photocatalytic production of clean hydrogen by toxic hydrogen sulphide, which is abundantly available as a waste gas from oil refineries (15–20%). We believe that continuous efforts in this direction may open up new insights into the design of controllable nanostructures and their potential applications in advanced fields.

Dramatic Enhancement in Photoresponse of β-In2S3 through Suppression of Dark Conductivity by Synthetic Control of Defect-Induced Carrier Compensation

We report on the synthesis of dense and faceted indium sulfide (β-In2S3) nano-octahedron films on fluorine-doped tin oxide-coated glass by the hydrothermal method and their photoresponse properties in a flip chip device configuration. We have examined the temporal evolution of the phase constitution, morphology, and optoelectronic properties for films obtained after growth interruption at specific intervals. It is noted that, initially, an In(OH)3 film forms, which is gradually transformed to the β-In2S3 phase over time. In the case of the film wherein most, but not all, of In(OH)3 is consumed, an exceptionally large photoresponse (light to dark current ratio) of ∼10(4) and response time(s) (rise/fall) of ∼88/280 ms are realized. This superior performance is attributed to nearly complete carrier compensation achievable in the system under high pressure growth leading to dramatic reduction of dark conductivity. It is argued that the temporally growth-controlled equilibrium between quasi-In interstitials and cation vacancies dictates the optoelectronic properties.