M.P. Suryawanshi

Highly selective and sensitive response of 30.5 % of sprayed molybdenum trioxide (MoO3) nanobelts for nitrogen dioxide (NO2) gas detection.

The molybdenum trioxide (MoO3) thin films have been successfully deposited onto the glass substrates using chemical spray pyrolysis (CSP) deposition technique at various substrate temperatures ranging from 300°C to 450°C with an interval of 50°C. The effect of substrate temperature on the structural, morphological, optical and gas sensing properties of MoO3 thin films has been thoroughly investigated. X-ray diffraction analysis reveals that all the films have an orthorhombic crystal structure and are polycrystalline in nature. FE-SEM micrographs depict the formation of nanobelts-like morphology. AFM study reveals that the RMS surface roughness of MoO3 thin films increases from 8.6nm to 12nm with increase in substrate temperature from 300°C to 400°C and then decreases to 11.5nm for substrate temperature of 450°C. Optical results show that the band gap of MoO3 thin films decreases from 3.92eV to 3.44eV. The selectivity studies show that the gas response of various gases varies as NH3<SO2<CO2<CO<H2S<NO2. Moreover, typical MoO3 film deposited at substrate temperature of 400°C is highly selective and sensitive for detection of NO2 gas in comparison with other gases. The maximum response of 30.5 % is obtained towards 100ppm NO2 gas concentration at an operating temperature of 200°C with response and recovery times of 20s and 160s, respectively. Finally, NO2 gas sensing mechanism model based on the chemisorption process is discussed.

A Simple Aqueous Precursor Solution Processing of Earth-Abundant Cu2SnS3 Absorbers for Thin-Film Solar Cells

A simple and eco-friendly method of solution processing of Cu2SnS3 (CTS) absorbers using an aqueous precursor solution is presented. The precursor solution was prepared by mixing metal salts into a mixture of water and ethanol (5:1) with monoethanolamine as an additive at room temperature. Nearly carbon-free CTS films were formed by multispin coating the precursor solution and heat treating in air followed by rapid thermal annealing in S vapor atmosphere at various temperatures. Exploring the role of the annealing temperature in the phase, composition, and morphological evolution is essential for obtaining highly efficient CTS-based thin film solar cells (TFSCs). Investigations of CTS absorber layers annealed at various temperatures revealed that the annealing temperature plays an important role in further improving device properties and efficiency. A substantial improvement in device efficiency occurred only at the critical annealing temperature, which produces a compact and void-free microstructure with large grains and high crystallinity as a pure-phase absorber layer. Finally, at an annealing temperature of 600 °C, the CTS thin film exhibited structural, compositional, and microstructural isotropy by yielding a reproducible power conversion efficiency of 1.80%. Interestingly, CTS TFSCs exhibited good stability when stored in an air atmosphere without encapsulation at room temperature for 3 months, whereas the performance degraded slightly when subjected to accelerated aging at 80 °C for 100 h under normal laboratory conditions.

A facile and green synthesis of colloidal Cu2ZnSnS4 nanocrystals and their application in highly efficient solar water splitting

The development of solution-processable routes as well as compounds consisting of earth abundant elements is highly desirable to reduce the fabrication cost. Recently, kesterite Cu2ZnSnS4 (CZTS) nanocrystals (NCs) have attracted great attention for photoelectrochemical (PEC) water splitting owing to their suitable low-cost, earth-abundancy and suitable band gap energy. However, the environmentally benign synthesis of high-quality CZTS NCs without toxic solvents remains elusive. Here, a green chemistry approach employing vegetable oil as a non-toxic solvent for the synthesis of monodisperse and size-tunable CZTS NCs is introduced for the first time. Additionally, the relationship between the abnormal size behavior of the CZTS NCs and the degree of decomposition in the vegetable oil using electrospray ionization mass spectrometry (ESI-MS) measurements is elucidated for the first time. As a conceptual strategy, a ternary abundant compound based heterojunction nanostructure for efficient solar water splitting by introducing CZTS NCs onto 5 nm Zn(O,S) passivated layer/hydrothermally grown TiO2 nanorod arrays (TNRs) is designed and developed. Remarkably, this ternary CZTS NCs/Zn(O,S)/TNR photoelectrode shows a photocurrent density as high as 15.05 mA cm−2 at 1.23 V (vs. the NHE), which is the highest ever for previously reported CZTS NC-based photoelectrodes. The reasons for the enhanced PEC performance are discussed in detail based on different PEC characterizations. More importantly, this work reflects the sophistication of eco-friendly solution phase synthesized CZTS NCs without using any toxic chemicals as an earth abundant sensitizer and constitute a new paradigm towards the enhanced PEC performance with quantum dot based hetero-nanostructures.

Superior selectivity and enhanced response characteristics of palladium sensitized vanadium pentoxide nanorods for detection of nitrogen dioxide gas

Vanadium pentoxide (V2O5) nanorods have been deposited onto the glass substrates by spraying 75ml of 30mM vanadium trichloride (VCl3) solution at optimized substrate temperature of 400°C. The XRD study confirms the formation of orthorhombic crystal structure of V2O5 nanorods. The FE-SEM micrograph shows the nanorods-like morphology of V2O5. The presence of palladium (Pd) in the Pd-sensitized V2O5 nanorods is confirmed using EDAX study. The gas sensing measurements show that the Pd-sensitized V2O5 sensing material is an outstanding candidate for nitrogen dioxide (NO2) gas detection. Obtained results demonstrate that the Pd-sensitized V2O5 nanorods show the superior selectivity for NO2 gas in comparison with other gases such as NH3, H2S, CO, CO2 and SO2 at an operating temperature of 200°C. It shows the 75% response for 100ppm NO2 gas concentration with response and recovery times of 22s and 126s, respectively. Finally, the gas sensing mechanism based on chemisorption process is proposed to illustrate how Pd nanoparticles affect the gas sensing characteristics (response and response-recovery times).

Investigations on Chemo-Mechano Stabilities of the Molybdenum Thin Films Deposited by DC-Sputter Technique

Abstract This paper reports the chemical and mechanical stability of Molybdenum (Mo) thin films deposited by direct current magnetron sputtering technique onto soda lime glass substrates. Mo thin films were deposited at various Ar (working) gas pressures to get optimized structural, morphological, adhesive and electrical properties. Mo thin films were further characterized by field emission scanning electron microscope (FE-SEM), X-ray diffraction, Hall measurements and the cross hatch tape test. To study their chemical stability the prepared Mo thin films were further dipped in acetic acid and ammonia solution for 6 h. Mechanical stability of Mo thin films was tested by high speed ultrasonication for an hour. Both the chemical and mechanical stability studies showed that Mo thin films were highly stable since morphology, adhesion and electrical properties did not alter significantly. FE-SEM results showed that the grain size of the chemo-mechano stability tested Mo thin films remained significantly similar with an unimportant effect on the film thickness. Electrical properties showed that electrical resistivity and hall mobility for as-deposited Mo thin films were 2.7 · 10–5 Ω cm and 5.1 cm2/Vs, respectively and remained nearly stable regardless of chemical and mechanical treatment. All of the films passed the cross hatch tape test and showed an excellent adhesion with glass substrates. The wettability investigations showed that all the Mo thin films were hydrophilic in nature and having contact angles in the range of 35○ to 40○.

Towards environmentally benign approaches for the synthesis of CZTSSe nanocrystals by a hot injection method

With the earths abundance of kesterite, recent progress in chalcogenide based Cu2ZnSn(Sx,Se1-x)4 (CZTSSe) thin films has drawn prime attention in thin film solar cells (TFSCs) research and development. This review is focused on the current developments in the synthesis of CZTS nanocrystals (NCs) using a hot injection (HI) technique and provides comprehensive discussions on the current status of CZTSSe TFSCs. This article begins with a description of the advantages of nanoparticulate based thin films, and then introduces the basics of this technique and the corresponding growth mechanism is also discussed. A brief overview further addresses a series of investigations on the developments in the HI based CZTSSe NCs using different solvents in terms of their high toxicity to environmentally benign materials. A variety of recipes and techniques for the NCs ink formulation and thereby the preparation of absorber layers using NC inks are outlined, respectively. The deposition of precursor thin films, post-deposition processes such as sulfurization or selenization treatments and the fabrication of CZTSSe NCs based solar cells and their performances are discussed. Finally, we discussed concluding remarks and the perspectives for further developments in the existing research on CZTSSe based nanoparticulate (NP) TFSCs towards future green technology.

Facile, Room Temperature, Electroless Deposited (Fe1− x ,Mn x )OOH Nanosheets as Advanced Catalysts: The Role of Mn Incorporation

Herein, bimetallic iron (Fe)-manganese (Mn) oxyhydroxide ((Fe1-x, Mnx )OOH, FeMnOOH) nanosheets on fluorine-doped tin oxide conducting substrates and on semiconductor photoanodes are synthesized by a facile, room temperature, electroless deposition method as catalysts for both electrochemical and photo-electrochemical (PEC) water splitting, respectively. Surprisingly, Mn-doped FeOOH can significantly modulate the nanosheet morphology to increase the active surface area, boost more active sites, and augment the intrinsic activity by tuning the electronic structure of FeOOH. Due to the 2D nanosheet architecture, the optimized FeMnOOH exhibits superior electrochemical activity and outstanding durability for the oxygen evolution reaction with a low overpotential of 246 mV at 10 mA cm-2 and 414 mV at 100 mA cm-2 , and long-term stability for 40 h without decay, which is comparable to the best electrocatalysts for water oxidation reported in the literature. By integrating with semiconductor photoanodes (such as α-Fe2 O3 nanorod (NR) arrays), bimetallic FeMnOOH catalysts achieve solar-driven water splitting with a significantly enhanced PEC performance (3.36 mA cm-2 at 1.23 V vs reversible hydrogen electrode (RHE)) with outstanding long-term stability (≈8 h) compared to that of the bare Fe2 O3 NR (0.92 mA cm-2 at 1.23 V vs RHE).

Thickness dependent photoelectrochemical performance of chemo-synthesized nanostructured CdS thin films

Abstract Thin films of cadmium sulfide (CdS) with different film thicknesses were chemo-synthesized onto soda lime glass and fluorine doped tin oxide (FTO) coated glass substrates. The synthesized CdS films were characterized by using UV-vis spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The XRD patterns revealed the formation of CdS with a cubic crystal structure. The variation in band gap energies was found to be in the range of 2.42–2.85 eV. An increase of 0.43 eV over the bulk band gap energy of CdS was found due to the quantum size effect in CdS thin films. The atomic force microscopy study depicted a novel egg-like morphology of CdS nanoparticles. Further, photoelectrochemical (PEC) performance of as grown CdS thin films was investigated using two electrode configurations in polysulfide electrolyte. The sample with film thickness 1389 Å showed the best PEC performance compared to other samples.

Chemically Deposited CdS Buffer/Kesterite Cu2ZnSnS4 Solar Cells: Relationship between CdS Thickness and Device Performance

Earth-abundant, copper-zinc-tin-sulfide (CZTS), kesterite, is an attractive absorber material for thin-film solar cells (TFSCs). However, the open-circuit voltage deficit (Voc-deficit) resulting from a high recombination rate at the buffer/absorber interface is one of the major challenges that must be overcome to improve the performance of kesterite-based TFSCs. In this paper, we demonstrate the relationship between device parameters and performances for chemically deposited CdS buffer/CZTS-based heterojunction TFSCs as a function of buffer layer thickness, which could change the CdS/CZTS interface conditions such as conduction band or valence band offsets, to gain deeper insight and understanding about the Voc-deficit behavior from a high recombination rate at the CdS buffer/kesterite interface. Experimental results show that device parameters and performances are strongly dependent on the CdS buffer thickness. We postulate two meaningful consequences: (i) Device parameters were improved up to a CdS buffer thickness of 70 nm, whereas they deteriorated at a thicker CdS buffer layer. The Voc-deficit in the solar cells improved up to a CdS buffer thickness of 92 nm and then deteriorated at a thicker CdS buffer layer. (ii) The minimum values of the device parameters were obtained at 70 nm CdS thickness in the CZTS TFSCs. Finally, the highest conversion efficiency of 8.77% (Voc: 494 mV, Jsc: 34.54 mA/cm2, and FF: 51%) is obtained by applying a 70 nm thick CdS buffer to the Cu2ZnSn(S,Se)4 absorber layer.

Size and shape controlled hydrothermal synthesis of kesterite Cu2ZnSnS4 nanocrystals

Quaternary kesterite Cu2ZnSnS4 (CZTS) nanocrystals (NCs) have been synthesized using a simple, size and shape tunable, and low-cost hydrothermal technique without using toxic chemicals. The size and shape of the kesterite CZTS NCs could be controlled by using different complexing agents including non-complexing agent, hydrazine hydrate, tri-sodium citrate (Na3-citrate), and tetra-acetate disodium salt (Na2EDTA). The hydrothermally synthesized CZTS nanocrystals showed a kesterite structure, high optical absorption, and suitable band gap energy characteristics, indicating potential for application to thin film solar cells.