Sawanta S. Mali

Cation distribution, structural, morphological and magnetic properties of Co1−xZnxFe2O4 (x = 0–1) nanoparticles

Co1−xZnxFe2O4 (x = 0.0, 0.2, 0.5, 0.6, 0.8, 1.0) nanoparticles (NPs) are prepared by a wet chemical co-precipitation method. The presence of zinc ions causes appreciable changes in the structural and magnetic properties of the Zn-substituted CoFe2O4. The structural, morphological and magnetic properties of the samples are determined and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and vibrating sample magnetometry (VSM). The particle size measured from TEM and XRD patterns confirms the nanosized dimension of the NPs in the size range of 9.0–15 nm. The saturation magnetization and the experimental magnetic moment are observed to initially increase (up to x = 0.2), which is explained by Neels collinear two-sublattice model, and then continuously decrease with further increase in Zn content x. This decrease obeys the three-sublattice model suggested by Yafet–Kittel (Y–K). The Y–K angle is zero for the CoFe2O4 NPs, it increases gradually with increasing Zn concentrations and can be extrapolated to 79.71 for ZnFe2O4 NPs.

Development of nanocoral-like Cd(SSe) thin films using an arrested precipitation technique and their application

Nanocrystalline cadmium sulfoselenide thin films have been synthesized using a self-organized arrested precipitation technique with different deposition times using triethanolamine as a complexing agent. Optical, structural, morphological and photoelectrochemical solar cell properties were investigated as a function of deposition time. A UV-Vis-NIR absorption study suggested a direct allowed transition type and the band gap energy decreased from 2.01 to 1.86 eV with the increase in deposition time. X-ray diffraction studies revealed that the thin films are nanocrystalline by nature with a pure hexagonal crystal structure and a calculated crystallite size of 51–68 nm. Field emission scanning electron microscopy demonstrated that the surface morphology was altered from nanoflakes to assorted nanoflakes–nanospheres and finally to a nanocoral-like morphology. X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy showed that the composition of the Cd(SSe) thin films was of good stoichiometry. Electrical conductivity and thermoelectric power measurements confirmed that the deposited films were n-type semiconductors. From J–V measurements, a highest photo-conversion efficiency of 0.57% was achieved. The significant boost in the PEC performance might be due to the improved crystallinity along with lower values of the grain boundary resistance, dislocation density and the microstrain of the Cd(SSe) thin films.

Single-step synthesis of 3D nanostructured TiO2 as a scattering layer for vertically aligned 1D nanorod photoanodes and their dye-sensitized solar cell properties

In the present investigation we have successfully synthesized 1D vertically aligned rutile TiO2 nanorods (TNR) and 3D TiO2 nanostars (TNS) as a scattering layer by a single step hydrothermal route. The synthesized nanostructures were characterized by X-ray diffraction, scanning-and transmission electron microscopy and X-ray photoelectron spectroscopy. The 1D TiO2 nanorod and 3D TiO2 nanostar samples were further used for N-719 dye sensitized solar cells (DSSC) application. Compared to a nanorod based cell, the photovoltaic performance of the nanostars/nanorods TiO2 cell exhibits excellent DSSC performance, including superior light scattering, rapid electron transport and lower electron recombination rate. The 3D/1D TNS/TNR based DSSC cell exhibits 5.39% power conversion efficiency, which is remarkably higher than that of the bare 1D nanorod based (3.74%) photoelectrode. The detailed interface and transient properties of these nanorod and nanostar based photoanodes in DSSCs were analyzed by electrochemical impedance spectroscopy measurements and open circuit voltage decay measurements in order to understand the critical factors contributing to such high power conversion efficiency. The enhancement of the efficiency for the 3D/1D TNS/TNR photoanode based cell compared to the 1D TNR is mainly attributed to better light scattering capability, faster electron transport and lower electron recombination.

Single step hydrothermal synthesis of hierarchical TiO2 microflowers with radially assembled nanorods for enhanced photovoltaic performance

Herein, 3D hierarchical TiO2 microflowers with a well faceted profile and high crystallinity were successfully obtained via a surfactant directed single step facile hydrothermal technique. TiO2 thin films were subjected to different characterization techniques such as UV-Vis-NIR spectrometry, X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) for their optical, structural, morphological and compositional analysis. The morphological characterization indicated that the microflowers are made from numerous nanorods growing homocentrically. The length, diameter and degree of aggregation of the nanorods increase rapidly and become aggregated with increase in concentration of CTAB. The effect of CTAB concentration on the microstructure and photoelectric properties of solar cells i.e. open circuit voltage (Voc), short circuit current density (Jsc) and photoelectric conversion efficiency (η%) were investigated under UV illumination. The synthesized 3D hierarchical microflowers can act as a scattering overlayer and 1D nanorod underlayer. 1D nanorods can accelerate the movement of electrons in one direction, while microflowers can scatter light and can enhance the cell performance by light harvesting. An effective improvement in the photoconversion efficiency was observed and lies in the range 0.23% to 3.72%.

Room temperature deposition of nanostructured Bi2Se3 thin films for photoelectrochemical application: effect of chelating agents

Nanostructured bismuth selenide (Bi2Se3) thin films have been deposited by chemical bath deposition method at room temperature using three different chelating agents, trisodium citrate, triethanolamine and ethylenedimminetetraacetic acid. The structural, morphological, optical and photoelectrochemical properties of Bi2Se3 thin films have been investigated as a function of different chelating agents. X-Ray diffraction studies revealed that the films were nanocrystalline in nature with a rhombohedral crystal structure. Trisodium citrate chelate resulted in Bi2Se3 thin films with poor crystallinity; further improvement in the crystallinity of the films was observed with triethanolamine and ethylenedimminetetraacetic acid chelates. From scanning electron microscopy, a uniform sphere-like morphology having an average sphere diameter of 90 nm was observed with trisodium citrate chelate. In the case of triethanolamine, a fibrous morphology with an average fiber thickness of 60 nm was observed, whereas for ethylenedimminetetraacetic acid chelate, a vertically arrayed petal-like morphology having petal thickness of 50–70 nm was observed. The UV-Vis absorption studies revealed that the band gap energy of the Bi2Se3 thin films with trisodium citrate, triethanolamine and ethylenedimminetetraacetic acid chelates was 1.55, 1.48 and 1.40 eV, respectively. The maximum short circuit current densities (Jsc) of 0.158, 0.214 and 0.284 mA cm−2 and the corresponding open circuit voltages (Voc) of 196, 206 and 217 mV were obtained with trisodium citrate, triethanolamine and ethylenedimminetetraacetic acid chelates, respectively. The Bi2Se3 thin films deposited using triethanolamine and ethylenedimminetetraacetic acid chelates show better photoelectrochemical performance as compared with trisodium citrate chelate.

Reduced graphene oxide (rGO) grafted zinc stannate (Zn2SnO4) nanofiber scaffolds for highly efficient mixed-halide perovskite solar cells

Electron transporting materials based on ternary metal oxides (TMOs) are the best electron transport layers (ETLs) for perovskite solar cells (PSCs). In the present investigation, reduced graphene oxide (rGO) grafted highly porous zinc stannate (Zn2SnO4) (ZSO) nanofiber scaffolds have been synthesized by a single step electrospinning technique and successfully used as ETLs for mixed halide PSCs whose perovskite material is composed of MAPb(I1−xBrx)3 and (FAPbI3)0.85(MAPbBr3)0.15 (MA: methyl ammonium and FA: formamidinium). The fabricated optimized perovskite solar cells having FTO/Bl-ZSO/rGO–ZSO0.7–MAPb(I1−xBrx)3/PTAA/Au devices exhibited a 13.41% power conversion efficiency (PCE) with an open circuit voltage (VOC) of 1.036 V, a current density (JSC) of 19.62 mA cm−2 and a fill factor (FF) of 0.66 under AM 1.5G sunlight (100 mW cm−2) which is higher than that of bare Zn2SnO4 nanofiber (η = 7.38%) based PSCs. The optimized conditions were further used for formamidinium lead halide (FAPbI3)1−x(MAPbBr3)x (x = 0.15) perovskite and our optimized results show η = 17.89% PCE (JSC = 22.50 mA cm−2, VOC = 1.046 V, FF = 0.76) for the FTO/Bl-ZSO/rGO–ZSO0.7–(FAPbI3)0.85(MAPbBr3)0.15/PTAA/Au device configuration. The role of rGO grafting and electron transfer mechanisms are investigated with complementary characterization, including photoluminescence (PL) and time-resolved photoluminescence (TRPL) decay measurements. The TRPL results revealed that the grafting of rGO in ZSO scaffolds reduces the slow decay lifetime which facilitates efficient electron injection from the perovskite conduction band (CB) to the rGO Fermi level to the CB of ZSO compared to the bare ZSO ETL. Furthermore, the stability of these devices based on various configurations has been discussed. This improvement is achieved due to the high conductivity of rGO and grafting with high porosity Zn2SnO4 nanofibers which make them promising new ETLs for the fabrication of highly efficient PSCs.

Microwave assisted synthesis, characterization and thermoelectric properties of nanocrystalline copper antimony selenide thin films

In the present work, we have synthesized p-type copper antimony selenide (Cu3SbSe4) thin films in an aqueous alkaline medium using a microwave assisted synthesis technique. The deposited thin films were characterized by UV-Vis-NIR spectroscopy, X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM) and thermoelectric techniques. On the basis of experimental results, a possible reaction mechanism has been discussed in detail. The band gap of the as deposited film is 1.94 eV and after annealing it reaches 1.87 eV for Cu3SbSe4. XRD results indicate that the as deposited thin films of CuSbSe2 have an orthorhombic crystal structure with secondary mixed phases and after annealing this is converted to Cu3SbSe4 having a pure tetragonal crystal structure. FESEM micrographs of Cu3SbSe4 showed a spherically diffused granular morphology having an average grain size of 25 nm. The HRTEM result of Cu3SbSe4 shows good crystallinity with a lattice spacing of 0.327 nm along the (112) plane. The EDS spectrum shows the presence of Cu, Sb and Se elements. The thermoelectric figure of merit (ZT) of the as deposited film is calculated to be 0.059 at 300 K and that of annealed Cu3SbSe4 is found to be 0.141 at 300 K.

Simplistic construction of cadmium sulfoselenide thin films via a hybrid chemical process for enhanced photoelectrochemical performance

We have successfully synthesized cadmium sulfoselenide (Cd(S1−xSex)) thin films via a simplistic and promising self-organized chemical growth process for photoelectrochemical (PEC) application. The effects of bath composition on the optical, structural, morphological, and electrical properties and the photoelectrochemical performance of (Cd(S1−xSex)) thin films have been investigated. Deposited thin films were characterized using UV-Vis spectrophotometry, X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) with a selected area electron diffraction (SAED) pattern, field-emission scanning electron microscopy (FESEM) coupled with energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), electrical conductivity (EC) and thermoelectric power (TEP) measurement techniques. An optical absorption study showed that the maximum light absorption in the 630–720 nm wavelength range and the linear nature of absorption plots indicate that the transition is a direct allowed type. The optical band gap energy decreased from 2.13 to 1.71 eV with varying bath composition. The XRD study illustrated that deposited thin films are in the pure phase with a nanocrystalline nature. HRTEM images highlight the formation of clearly-defined, interconnected particle, which aggregated to form a well-grown custard apple-like morphology over the entire substrate and are in good accordance with FESEM micrographs. The SAED pattern shows a ring pattern indicating the nanocrystalline nature of the deposited thin film. The FESEM study demonstrated that the developed surface morphology is favorable for effectual light absorption in the solar spectrum. The XPS analysis specified the presence of Cd2+, S2− and Se2− elements in the deposited thin film. The EDS spectrum confirmed that thin film deposition occurs in a stoichiometric manner. From the EC measurement study, it was observed that electrical conductivity increases correspondingly for all thin films, indicating semiconducting behavior. TEP measurements established that Cd(S1−xSex) thin films are n-type in nature. Finally, the deposited thin films were tested for photoelectrochemical (PEC) application. The PEC study illustrated that (Cd(S0.2Se0.8)) thin film showed the highest power conversion efficiency (η) of 1.02% among reported values.

Langmuir–Blodgett self organized nanocrystalline tungsten oxide thin films for electrochromic performance

We explore a novel method to synthesize pebble-like nanocrystalline WO3 thin films for the first time by thermal decomposition of a multilayer Langmuir–Blodgett film of an octadecylamine–tungsten complex. The resulting film was thoroughly characterized by various characterization techniques. The electrochromic performance was evaluated in Li+ as a charge-balancing ion. The WO3 thin film displays a state-of-the-art performance with respect to optical modulation of 25.94% at λ630 nm with a very rapid coloration and bleaching time of 3.57 s and 3.14 s, respectively and a high coloration efficiency of 71.26 cm2 C−1. The excellent electrochromic performance can be attributed to the high size uniformity of WO3 nanoparticles, whose crystalline nature offer more active sites for Li+ diffusion and control the diffusion path length. Thus, the Langmuir–Blodgett WO3 film contributes to high energy conversion devices.

A Merrifield resin supported Pd–NHC complex with a spacer(Pd–NHC@SP–PS) for the Sonogashira coupling reaction under copper- and solvent-free conditions

Synthetic applications of a polymer supported air-stable palladium NHC complex with a spacer (catalyst 6, Pd–NHC@SP–PS) and without a spacer (catalyst 7, Pd–NHC@PS) have been studied for the Sonogashira cross-coupling reaction. The catalysts were prepared by immobilizing pre-formed IL on the polymer and reacting the resultant modified polymer with Pd(OAc)2 in THF at 60 °C. Both the catalysts were characterized using IR, SEM, EDS, ICP, XPS and TGA/DTA. Catalyst 6 has been found to be more active than catalyst 7, due to the greater accessibility of active catalytic sites, for a variety of aryl bromides and terminal alkynes in solvent and copper free Sonogashira cross-coupling reactions under aerobic conditions. The effect of the spacer, nature of base, reaction temperature, the catalyst loading as well as reusability was also investigated.