A. Sadananda Chary

Synthesis of a hydrogen producing nanocrystalline ZnFe2O4 visible light photocatalyst using a rapid microwave irradiation method

A rapid microwave solid-state synthesis method is systematically investigated to achieve a H2 producing visible light active spinel photocatalyst. ZnFe2O4 nanocrystallites were obtained by microwave irradiation of precursor compacts under optimized conditions. This investigation led to a uniform sized nanocrystalline photocatalyst that yielded a quantum-yield of H2 evolution ∼3.8 times higher than that of conventionally synthesized ZnFe2O4. The synthesis parameters – microwave power, synthesis temperature, and time, were found to control the physico-chemical properties viz phase formation kinetics, phase purity, crystallinity, specific surface area and photochemical efficiency, of the synthesized photocatalyst. The study reveals that the threshold microwave power of ≥3 kW was necessary to obtain a spinel phase structure, while lower power (<3 kW) could not induce the crystallization even after prolonged low-power irradiation of 180 min. At the threshold power, a minimum of 10 min. synthesis time was enough to obtain uniform sized nanocrystallites, indicating that the synthesis method is ∼24 times faster than the solid state reaction method, which needs nearly 4 h. The particle morphology evolution with irradiation time from 10–150 min. exhibited de-crystallization phenomena. Longer irradiation displayed a morphological crystallization probably induced due to the simultaneous area and volumetric heating effect. The possible “formation mechanism” of these uniform nanocrystallites has been presented here for qualitative understanding. Thus synthesized photocatalysts generated hydrogen from a water–methanol mixture even without the co-catalyst loading. The ferrite photocatalyst was found to decolorize methylene blue dye with a maximum decay constant of 0.232 h−1, thereby demonstrating its capability in the pollutant decomposition applications, all under visible light photons.

Enhancement of dc ionic conductivity in dispersed solid electrolyte system - Sr(NO3)2:γ-Al2O3

Abstract dc ionic conductivity in dispersed solid electrolyte system (DSES) - Sr (NO 3 ) 2 :γ-Al 2 O 3 was studied from 100°C to about 570°C. Considerable enhancement was noticed in the conductivity in the extrinsic region. The enhancement was found to be maximum for 29.3 m o of γ-alumina. X-ray studies ruled out any formation of a solid solution between the host matrix and the dispersoid even at the melting point of the host matrix.

Structural phase transition in CsNO3 : dielectric studies

Abstract Dielectric properties - dielectric constant ( K ), dielectric loss (tan δ) and ac conductivity (σ) in the solution grown single crystals of CsNO 3 are presented from room temperature to about 230°C covering the frequency range from 100 Hz to 100 KHz. Anomalous behaviour of K , tan δ and ac σ at about 154°C is attributed to the structural phase transition from trigonal to CsCl cubic structure.

Role of particle size and mole percent of the dispersoid in composite solid electrolyte system Sr(NO3)2 : Al2O3

Abstract The role played by the particle size and mol% of alumina particles when dispersed in strontium nitrate system was studied in the range of temperatures from about 100°C to 560°C. Enhancement of conductivity was noticed to increase with mol% (m/o) of the dispersoid in the extrinsic temperature region. For a given m/o the enhancement was noticed to increase with decrease of particle size. Different particle sizes namely 36.9, 0.3, and 0.06 μm were used with varying mol%. The maximum enhancement observed was between 10 2 -10 3 times with respect to the host material. Analysis of the results clearly indicated the dependence of enhancement on the total surface area of the dispersoid available for a given volume of the host material. The enhancement increased with total surface area upto a limit, started decreasing with further increase in the total surface area and finally dropped to a value below that for the pure material. This phenomenon is expleined on the basis of formation of space charge layers between the host ionic conductor and the inculating particles.

Ion dynamic studies through AC conductivity spectra on Pb(NO3)2:Al2O3 composite solid electrolytes

Solid electrolytes with a matrix of Pb(NO3)2 as host and Al2O3 (0.3 µm) as dispersoid is prepared by conventional milling mechanism. Structural and morphological studies were carried out through XRD and SEM respectively which confirm that these composites are two phase systems. AC conductivity studies were performed by employing impedance measurement technique between the temperatures 30 and 300°C and in the frequency range 100Hz to 5MHz. Non-overlapping small polaron tunneling mechanism (NSPT) model suggested for ion hopping in the conduction processes.Solid electrolytes with a matrix of Pb(NO3)2 as host and Al2O3 (0.3 µm) as dispersoid is prepared by conventional milling mechanism. Structural and morphological studies were carried out through XRD and SEM respectively which confirm that these composites are two phase systems. AC conductivity studies were performed by employing impedance measurement technique between the temperatures 30 and 300°C and in the frequency range 100Hz to 5MHz. Non-overlapping small polaron tunneling mechanism (NSPT) model suggested for ion hopping in the conduction processes.

ac impedance analysis of single crystals of Sr(NO3)2 and Sr(NO3)2:Na

Abstract Complex impedance analysis studies on single crystals of Sr(NO 3 ) 2 and Sr(NO 3 ) 2 :Na are presented in the temperature range 300–500°C. Small signal ac response in the frequency range of 5 Hz-1 MHz has yielded the values of dc conductivity, K , tan δ and ac conductivity for both the pure and doped crystals. dc conductivity values deduced from the Cole-Cole plots agree very well with our earlier work in the temperature region of overlap. Increase in the values of all the parameters for the doped case is attributed to the additional anion vacancies created. A small anomaly noticed between 480–520°C is thought to be due to the nitrate ion orientation disorder. Variation of ac conductivity, K and tan δ with temperature are explained to be due to space-charge polarization and the conduction processes.

Study of dielectric properties of RbNO3 single crystals

Dielectric properties, dielectric constant (k), dielectric loss (tan δ) and a.c. conductivity (σ) in the solution-grown single crystals of RbNO3 are presented from room temperature to about 200°C covering the frequency range 102 to 105 Hz. A broad peak observed in tan δ-frequency data between 103 and 105Hz is thought to be due to impurity-vacancy dipoles. The sudden rise of three parameters near 160°C is attributed to the known phase transition from trigonal to CsCl structure.

Study of D.C.Ionic Conductivity in BaCl2:SiO2 Composite Solid Electrolyte System

Study of d.c. ionic conductivity in pure BaCl2 and BaCl2 dispersed with the insulating material SiO2 is presented in this work.Variation of dc ionic conductivity with temperature has shown a gradual enhancement of conductivity with m/o of dispersoid reaching to a maximum of nearly two orders of magnitude at 6.6m/o. The fall of enhancement was observed with further increase in m/o of dispersoid. The enhancement of conductivity in this system is interpreted due to increased concentration of charge carriers in the space charge region formed at the interface between the host BaCl2 and insulating dispersoid particles SiO2. XRD and FTIR studies have ruled out the formation of new compound or solid solution, between the host and the dispersoid, as there are no new peaks observed in all these systems.

Design and Development of Ferrite Composite Film Electrode for Photoelectrochemical Energy Application

Development of efficient photoanodes for water splitting under solar light is desirable to surmount the possible fuel crisis in future. Ferrite systems, with their excellent visible light absorption capability, stability, non-toxicity, cost-effectiveness and abundance, are being preferred to titanates, niobates and sulfides. The present work briefly reviews the modified form of ferrites. Additionally, ZnFe2O4 an n-type semiconductor with the low band gap (~1.9eV) has been considered as special case of visible light PEC application. The work further emphasizes on the utilization of solution processed techniques to develop the ferrite photoanodes. The tuning of photoanode properties by virtue of electrode fabrication parameters say deposition parameters viz., precursor concentration, pH, stoichiometry has been reviewed and discussed.

Complex impedance analysis of Sr(NO3)2 and Sr(NO3)2:?-Al2O3 dispersed solid electrolyte systems

Complex impedance spectroscopic studies were carried out on Sr(NO3)2 and Sr(NO3)2:γ-Al2O3 dispersed solid electrolyte systems (DSES) in the temperature range 300 to 560° C and the frequency range 100 Hz to 1 MHz. Mole percentage of the dispersoid γ-Al2O3 was varied from 17.2 to 34.2. The d.c. conductivity estimated from the Cole-Cole plots was found to increase with the mole percentage of the dispersoid. Dissipation was found to vary in a manner similar to the variation of conductivity with temperature. A.c. conductivity was found to be frequency dependent in the extrinsic region, but frequency independent in the intrinsic region. The enhanced conductivity in DSES was attributed to the formation of a space-charge layer between the host material and the dispersoid.