Subhash Thota

Bio: Subhash Thota is an academic researcher from Indian Institute of Technology Guwahati. The author has contributed to research in topics: Magnetization & Antiferromagnetism. The author has an hindex of 20, co-authored 83 publications receiving 1467 citations. Previous affiliations of Subhash Thota include Indian Institutes of Technology & Indian Institute of Technology Kanpur.

A comparative study of the magnetic properties of bulk and nanocrystalline Co3O4

Abstract: A comparative study of the magnetic and electron paramagnetic resonance (EPR) parameters of bulk and Co3O4 nanoparticles (NP), synthesized by a sol–gel process, is presented. Both samples possess the cubic phase with a slightly lower (by 0.34%) lattice parameter for the Co3O4 NP. The average crystallite size D = 17 nm determined by x-ray diffraction (XRD) for the Co3O4 NP is quite consistent with the electron microscopic observations. The bulk Co3O4 has particle size in the 1–2 µm range. A Neel temperature of TN = 30 K (lower than the 40 K usually quoted in the literature) is determined from the analysis of the magnetic susceptibility versus temperature data for bulk Co3O4. This TN = 30 K is in excellent agreement with the TN = 29.92 K reported from specific heat measurements. The Co3O4 NP powder exhibits a still lower TN = 26 K, possibly due to the associated finite size effects. The values of coercivity, Hc = 250 Oe, and exchange bias, He = −350 Oe, together with the training effect have been observed in the Co3O4 NP sample (cooled in 20 kOe). Both Hc and He approach zero as . For T>TN, the χ versus T data for both samples fit the modified Curie–Weiss law (χ = χ0+C/(T+θ)). The magnitudes of C, θ and TN are used to determine the following: exchange constants J1ex = 11.7 K, J2ex = 2.3 K, and magnetic moment per Co2+ ion μ = 4.27 μB for bulk Co3O4; and J1ex = 11.5 K, J2ex = 2.3 K and μ = 4.09 μB for Co3O4 NP. EPR yields a single Lorentzian line near g = 2.18 in both samples but with a linewidth ΔH that is larger for the Co3O4 NP. Details of the temperature dependence of ΔH, line intensity I0, and disappearance of the EPR on approach to TN are different for the two samples. These effects are discussed in terms of spin–phonon interaction and additional surface anisotropy present in Co3O4 NP.

On the sol–gel synthesis and thermal, structural, and magnetic studies of transition metal (Ni, Co, Mn) containing ZnO powders

Abstract: Transition metal containing ZnO powders (Zn1−xMxO, 0≤x≤0.30; M = Ni, Mn, Co) have been synthesized by a sol–gel process using zinc acetate dihydrate, respective acetate and oxalic acid as precursors with ethanol as a solvent. The process essentially involves gel formation, drying at 80 °C for 24 h to provide the oxalate, and calcination at 500 °C for 2 h to undergo an exothermic reaction and yield Zn1−xMxO powder. Their XRD patterns correspond to a wurtzite hcp structure similar to that of pure ZnO, but with the lattice parameters varying slightly with type and extent of doping. It is shown that the dissolution of nickel and cobalt in ZnO is less than 10 at.%, whereas that of manganese lies between 10 and 15 at.%. Other phases that emerge include NiO (hexagonal, a = 2.954 A, c = 7.236 A), ZnCo2O4 (cubic, a = 8.094 A) and ZnMnO3 (cubic, a = 8.35 A) in the Ni, Co and Mn containing ZnO systems, respectively. Observations of hysteresis loops both at 10 and 320 K and the nature of ESR spectra provide evidence for the ferromagnetic state in nickel containing ZnO powder. Besides, the deviation occurs in the magnetization versus temperature curves in zero field cooled (ZFC) and field cooled (FC) conditions (blocking temperature TB being 32 K for 5 at.% Ni). The magnetic behaviour of manganese and cobalt doped zinc oxide is, however, different, namely, (i) no hysteresis loops, (ii) decrease in magnetization with increase of Mn or Co content, and (iii) identical M–T curves under ZFC and FC conditions. The inverse susceptibility versus temperature curves of Zn1−xMnxO compounds reveal ferrimagnetism with Neel temperature TN of 4 K for x = 0.02, but antiferromagnetism for x = 0.15 and 0.25 with Curie–Weiss temperature of −43 and −30 K, respectively.

Optical, electrical and magnetic properties of Co3O4 nanocrystallites obtained by thermal decomposition of sol–gel derived oxalates

Abstract: Nanocrystallites of tricobalt tetraoxide (Co 3 O 4 ) have been synthesized by sol–gel process using cobalt acetate tetrahydrate, oxalic acid as precursors and ethanol as a solvent. The process comprises of gel formation, drying at 80 °C for 24 h to obtain cobalt oxalate dihydrate (α-CoC 2 O 4 ·2H 2 O) followed by calcination at or above 400 °C for 2 h in air. These results combined with thermal analysis have been used to determine the scheme of oxide formation. The room temperature optical absorption spectra exhibits blue shift in both (i) ligand to metal (p(O 2− ) → e g (Co 3+ ), 3.12 eV), and (ii) metal to metal charge transfer transitions (a) t 2g (Co 3+ ) → t 2 (Co 2+ ), 1.77 eV, (b) t 2 (Co 2+ ) → e g (Co 3+ ), 0.95 eV together with the d–d transitions (0.853 and 0.56 eV) within the Co 2+ tetrahedra. The temperature dependent ac electrical and dielectric properties of these nanocrystals have been studied in the frequency range 100 Hz to 15 MHz. There are two regimes distinguishing different temperature dependences of the conductivity (70–100 K and 200–300 K). The ac conductivity in both the temperature regions is explained in terms of nearest neighbor hopping (NNH) mechanism of electrons. The carrier concentration measured from the capacitance ( C )–voltage ( V ) measurements is found to be 1.05 × 10 16 m −3 . The temperature dependent dc magnetic susceptibility curves under zero field cooled (ZFC) and field cooled (FC) conditions exhibit irreversibilities whose blocking temperature ( T B ) is centered at 35 K. The observed Neel temperature ( T N ∼ 25 K) is significantly lower than the bulk Co 3 O 4 value ( T N = 40 K) possibly due to the associate finite size effects.

Size-dependent shifts of the Néel temperature and optical band-gap in NiO nanoparticles

Abstract: Bulk NiO is a well-known antiferromagnet with Neel temperature TN(∞) = 524 K and an optical band-gap Eg = 4.3 eV. With decrease in particle size D from 40 nm to 4 nm of NiO, systematic changes of TN and Eg are observed and discussed here. From magnetic measurements, the changes in TN with D are found to fit finite-size scaling equation TN(D) = TN(∞) [1 − (ξo/D)λ] with λ = 3.2 ± 0.5 and ξo = 3.2 ± 0.2 nm, in good agreement with the predictions for a Heisenberg system. The observed blue shifts of Eg with decrease in D reaching Eg = 5.12 eV for D ∼ 4 nm are likely due to quantum confinement and non-stoichiometry.

Photochemical Route for Accessing Amorphous Metal Oxide Materials for Water Oxidation Catalysis

Abstract: Large-scale electrolysis of water for hydrogen generation requires better catalysts to lower the kinetic barriers associated with the oxygen evolution reaction (OER). Although most OER catalysts are based on crystalline mixed-metal oxides, high activities can also be achieved with amorphous phases. Methods for producing amorphous materials, however, are not typically amenable to mixed-metal compositions. We demonstrate that a low-temperature process, photochemical metal-organic deposition, can produce amorphous (mixed) metal oxide films for OER catalysis. The films contain a homogeneous distribution of metals with compositions that can be accurately controlled. The catalytic properties of amorphous iron oxide prepared with this technique are superior to those of hematite, whereas the catalytic properties of a-Fe100-y-zCoyNizOx are comparable to those of noble metal oxide catalysts currently used in commercial electrolyzers.

Caloric materials near ferroic phase transitions

Abstract: A magnetically, electrically or mechanically responsive material can undergo significant thermal changes near a ferroic phase transition when its order parameter is modified by the conjugate applied field. The resulting magnetocaloric, electrocaloric and mechanocaloric (elastocaloric or barocaloric) effects are compared here in terms of history, experimental method, performance and prospective cooling applications.

Electrochemical Water Oxidation with Cobalt-Based Electrocatalysts from pH 0–14: The Thermodynamic Basis for Catalyst Structure, Stability, and Activity

Abstract: Building upon recent study of cobalt-oxide electrocatalysts in fluoride-buffered electrolyte at pH 34, we have undertaken a mechanistic investigation of cobalt-catalyzed water oxidation in aqueous buffering electrolytes from pH 0–14 This work includes electrokinetic studies, cyclic voltammetric analysis, and electron paramagnetic resonance (EPR) spectroscopic studies The results illuminate a set of interrelated mechanisms for electrochemical water oxidation in alkaline, neutral, and acidic media with electrodeposited Co-oxide catalyst films (CoOxcfs) as well as for a homogeneous Co-catalyzed electrochemical water oxidation reaction Analysis of the pH dependence of quasi-reversible features in cyclic voltammograms of the CoOxcfs provides the basis for a Pourbaix diagram that closely resembles a Pourbaix diagram derived from thermodynamic free energies of formation for a family of Co-based layered materials Below pH 3, a shift from heterogeneous catalysis producing O2 to homogeneous catalysis yielding