• N GOPALAKRISHNAN

Articles written in Bulletin of Materials Science

• Synthesis of ZrO$_2$ nanostructure for gas sensing application

ZrO$_2$ nanopowder has been synthesized by the conventional precipitation method for gas-sensing application. The synthesized powder was dropcast and subsequently annealed at 100$^{\circ}$C. The drop-casted film has been subjected to X-ray diffraction analysis, scanning electron microscopy, ultraviolet–visible diffuse reflectance spectroscopy, photoluminescence study and $I –V$ measurement in order to observe its structural, morphological, optical and electrical properties. The gas sensing measurement has been performed for the thus prepared ZrO$_2$ film by an exposure to different reducing gases(ammonia, ethanol, formaldehyde, acetone and xylene) at different temperatures for various gas concentrations. It has been observed that the film shows a better response towards ammonia (about 18%) compared to all other gases due to faster diffusion. Also, a quick response and recovery time have been found as 70 and 70 s, respectively, for ammonia.

• Effect of heat treatment on the optical properties of layered muscovite single crystal sheets

Naturally, thin sheets of layered, dioctahedral muscovite single crystals are transparent to visible light. Upon heat treatments performed up to 900$^{\circ}$C, the single crystal sheets of thickness $\sim$200 $\mu$m exhibited changes in its optical transparent character. The muscovite sheets exhibited either translucent or nontransparent, dark-brown, gold or copper like luster, varying optical band gap (from $\sim$3.7 to 2.7 eV) and photoluminescence (PL) properties, which depend on the temperature of the heat treatment. A comprehensive investigation has been made using X-ray diffraction, thermal analyses, energy dispersive X-ray and Raman spectroscopies to understand how/why the optical properties of the muscovite sheets changed up on heat treatments. Besides, no phase changes were found in the layered, lamellar crystal structure, evidences for the known dehydroxylation of muscovite were clearly seen from the thermal analyses data. Further, the Raman spectrum of the muscovite sheet heat treated at 900$^{\circ}$C showed no evidence for the presence of hydroxyl group (OH$^−$), which confirmed the completion of the dehydroxylation. The dihydroxylation-induced changes in the structural-elements (for example, Al-(O,OH)$_6$ partly or fully changing to Al-O$_5$/O$_6$ and its effects on the lattice constants, and also the defect levels introduced by the heat treatments are responsible for the changes in the optical transparency, colour of appearance, band gap and PL of the muscovite natural single crystal sheets.

• Ferromagnetism in Gd-doped ZnO thin films mediated by defects

Defects play an inevitable role in controlling the optical and magnetic properties of ZnO. In this study, defects were introduced in gadolinium (Gd)-doped ZnO films by depositing in pure argon atmosphere. The pristine- and Gddoped (0.05, 0.1 and 1 at%) films were deposited on Si(111) substrate by radio frequency magnetron sputtering at a substrate temperature of 450°C under Ar pressure of 0.02 mbar. Structural, morphological, chemical, optical and magnetic properties of the deposited films were studied by X-ray diffraction and Raman spectroscopy, atomic force microscopy, X-ray photoelectron spectroscopy, photoluminescence and vibrating sample magnetometer, respectively. It is confirmed that oxygen-deficient growth condition leads to the formation of oxygen vacancy (V$_O$$^+) and zinc interstitial (Zn_i$$^+$) defects in the films. It is shown that a critical amount of Zn$_i$$^+ and V_O$$^+$ along with the appropriate amount of Gd$^{3+}$ ions are required to induce room temperature ferromagnetism in Gd-doped ZnO thin film deposited on Si(111) substrate. A possible mechanism has been proposed based on bound magnetic polaron model to explain the observed ferromagnetism.

• Effect of Cr on ZrO$_2$ nanostructures for gas sensing investigation

This study deals with the synthesis, characterization and gas sensing interrogations of pure and Cr (5 and 15%) doped ZrO$_2$. The conventional precipitation route is committed for the synthesis of the pure and Cr-doped ZrO$_2$ powder and the films were formulated by drop casting mode by exploiting ZrOCl$_2$.8H$_2$O as precursor, CrO$_3$ as doping agent and NH$_4$OH as stabilizing agent. The structural properties, optical properties and electrical properties of the handledsamples were anatomized by X-ray diffraction (XRD), scanning electron microscopy, ultraviolet–differential reflectance spectroscopy (UV-DRS) spectroscopy, photoluminescence (PL) studies and current–voltage (I–V) measurement. The structure and size of the crystal were resolved by XRD. The uniform spherical-like morphology of the particles was favoured by SEM investigation. The bandgap of the materials was reckoned by UV-DRS report. The occupancy of oxygen vacancies in the samples was conceded by PL studies. The ohmic contact of the samples with the electrodes has been professed by I–V measurements. The gas sensing competency of the drop casted films were monitored by the exposure of the different concentrations of reducing gases like ammonia, acetone, ethanol, formaldehyde and xylene. The output datarender that all the processed samples exhibits a peak response towards ammonia gas, exclusively 15% of Cr-doped ZrO$_2$ sensor shows an utmost gas response of about 48%, excellent response time and recovery time of about 68 and 72s,respectively, among other samples. Thus, the fusion of Cr stimulates the gas sensing proficiency of ZrO$_2$.

• # Bulletin of Materials Science

Volume 45, 2022
All articles
Continuous Article Publishing mode

• # Dr Shanti Swarup Bhatnagar for Science and Technology

Posted on October 12, 2020

Prof. Subi Jacob George — Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bengaluru
Chemical Sciences 2020