Sudakar Chandran

A novel composite photocatalyst of indium doped cadmium sulfide dispersed on zirconium oxide has been synthesized, which shows enhanced photocatalytic activity for hydrogen generation from water. In this system, cadmium sulfide exists as a separate dispersed phase on the zirconia support. Optical absorption spectra indicate a blue shift of absorption edge for CdS and In doped CdS dispersed on ZrO2 compared to pure CdS and indium doped CdS. Among the supported CdS, In doped CdS exhibits better optical absorption property. Photocatalytic studies for hydrogen generation from water show an enhanced activity for CdS dispersed on ZrO2 and indium doping in CdS enhances the activity further. Fluorescence lifetime studies indicate that, in the supported CdS, the charge carriers have higher lifetime than that in the unsupported CdS. Photocurrent response experiments show a relatively higher current output for the In doped CdS dispersed on ZrO2 support. The enhanced photocatalytic activity of this composite sample is attributed to a combination of factors like enhanced lifetime of the photogenerated charge carriers, increased photoresponse and improved surface area. The present study leads to a new observation that the photocurrent response and photocatalytic activity of CdS and indium doped CdS are enhanced when they are dispersed on a support like ZrO2. These composites with Pd as co-catalyst exhibit a large increase in the photocatalytic activity due to the increased availability of electrons on the metal surface by the interfacial transfer of electrons from CdS to Pd, when irradiated. © 2011 The Royal Society of Chemistry.

In this paper solution mixing and casting of Cd(NO3) 2·4H2O and poly(ethylene oxide) (PEO) at different molar ratios (1:100-1:600) followed by hydrogen sulfide treatment were employed to fabricate solid films of cadmium sulfide (CdS)/polyethylene oxide (PEO) nanocomposites. The nanocomposites were found to exhibit uniform distribution of CdS nanoparticles in the polymer matrix without any additional capping agent. Systematic investigations on the role of PEO on the optical properties of CdS are presented. The optical properties of the composites examined by UV-vis absorption spectroscopy show that the band gap of CdS nanoparticles increases from 2.45 eV to 2.54 eV with decreasing concentration of CdS in PEO films. X-ray diffraction pattern shows the broadening in shape of the PEO peaks which is induced by the CdS particles in PEO matrix. The CdS particle sizes ranging from 10 to 20 nm are clearly seen in a transmission electron microscope (TEM). The X-ray photoelectron spectroscopic studies (XPS) also confirm the presence of CdS in PEO. Fourier transform infrared spectroscopy studies using attenuated total reflectance (FTIR-ATR) indicate the influence of Cd2+ ion on C-O-C stretching in PEO and confirm the presence of CdS nanoparticles within PEO. Photoluminescence spectroscopy (PL) shows the broad emission due to the presence of surface trapped carrier states. © 2013 Elsevier Ltd.

A synergetic approach of employing smooth mesoporous TiO2 microsphere (SμS-TiO2)-nanoparticulate TiO2 (np-TiO2) composite photoanode, and size and defect controlled CdSe quantum dots (QD) to achieve high efficiency (η) in a modified Grätzel solar cell, quantum dot sensitized whisperonic solar cells (QDSWSC), is reported. SμS-TiO2 exhibits whispering gallery modes (WGM) and assists in enhancing the light scattering. SμS-TiO2 and np-TiO2 provide conductive path for efficient photocurrent charge transport and sensitizer loading. The sensitizer strongly couples with the WGM and significantly enhances the photon absorption to electron conversion. The efficiency of QDSWSC is shown to strongly depend on the size and defect characteristics of CdSe QD. Detailed structural, optical, microstructural and Raman spectral studies on CdSe QD suggest that surface defects are prominent for size ~2.5 nm, while the QD with size > 4.5 nm are well crystalline with lower surface defects. QDSWSC devices exhibit an increase in η from ≈0.46% to η ≈ 2.74% with increasing CdSe QD size. The reported efficiency (2.74%) is the highest compared to other CdSe based QDSSC made using TiO2 photoanode and I-/I3- liquid electrolyte. The concept of using whispering gallery for enhanced scattering is very promising for sensitized whisperonic solar cells.

Based on experimental and density functional studies, we show that tailoring of oxygen vacancies (OV) leads to large scale enhancement of photoconductivity in BiFeO3 (BFO). The OV concentration is increased by substituting an aliovalent cation Ca2+ at Bi3+ sites in the BFO structure. Furthermore, the OV concentration at the disordered grain boundaries can be increased by reducing the particle size. Photoconductivity studies carried out on spark plasma sintered Bi1-xCaxFeO3-δ ceramics show four orders of enhancement for x = 0.1. Temperature dependent Nyquist plots depict a clear decrease in impedance with increasing Ca2+ concentration which signifies the role of OV. A significant reduction in photoconductivity by 2 to 4 orders and a large increase in impedance of the air-annealed (AA) nanocrystalline ceramics suggest that OV at the grain boundaries primarily control the photocurrent. In fact, activation energy for AA samples (0.5 to 1.4 eV) is larger than the as-prepared (AP) samples (0.1 to 0.5 eV). Therefore, the room temperature J-V characteristics under 1 sun illumination show 2-4 orders more current density for AP samples. Density-functional calculations reveal that, while the defect states due to bulk OV are nearly flat, degenerate, and discrete, the defect states due to surface OV are non-degenerate and interact with the surface dangling states to become dispersive. With large vacancy concentration, they form a defect band that enables a continuous transition of charge carriers leading to significant enhancement in the photoconductivity. These studies reveal the importance of tailoring the microstructural features as well as the composition-tailored properties to achieve large short circuit current in perovskite oxide based solar cells.

We report on the suppression of FeLi• antisite defects in LiFe(PO 4)1-xF3x with 0 ≤ x ≤ 0.4 prepared by sol-gel and hydrothermal methods. Evidence for a systematic suppression of antisite defects upon fluorine doping in LiFePO4 is inferred from Fourier transform infrared spectra, wherein a noticeable red shift in the symmetric P-O stretching vibrational mode of (PO4) 3- polyanion from 970 to 957 cm-1 was observed. We report detailed structural and compositional studies of LiFe(PO4) 1-xF3x, and discuss the implications for the performance of LiFePO4 cathodes in lithium ion batteries.© 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

We present a detailed study on the photovoltaic performance of dye and quantum dot (QD) sensitized solar cells fabricated using nanoporous TiO2 photoanodes prepared by a template approach. Nanoporous TiO2 particles (TNPO) are prepared by sol-gel method using dextran (TNPO-D) and glucose (TNPO-G) as templates. The decomposition of templating agents during calcination introduces nanopores (∼10 nm) in TiO2 nanoparticles as evidenced from the HRTEM studies. Oxygen vacancies are also introduced in TiO2 nanoparticles due to prevailing reducing ambient during decomposition. Oxygen vacancy (VO) related defects are evidenced from EPR spectra from a strong absorption signal observed at g-value of 1.994 related to Ti3+-VO-Ti3+ defect sites. The defect concentration estimated from EPR spectra is found to be ∼5 to 15 times larger for nanoporous TiO2 particles compared to Degussa P25-TiO2. TNPO samples also show enhanced defect level photoluminescence emission. The photovoltaic efficiency (η) and the IPCE of DSSC and QDSSC are strongly influenced by the defect concentration and seen to decrease with increasing VO in TiO2 nanoparticles. Twofold lower η is found in both DSSC (∼6.38% to 3.66%) and QDSSC (∼1.69% to 0.46%) devices made using nanoporous TiO2 compared to P25-TiO2 photoanode solar cells. P25-TiO2 also shows significant decrease in η when oxygen vacancies are introduced by hydrogen annealing process, which reveal detrimental effect of VO on the photovoltaic properties. EIS studies suggest increased interfacial resistance in solar cells made from nanoporous TiO2 photoanodes.

Perovskite Solar Cells (PSCs) gained attention by solar photovoltaic community owing to excellent optoelectronic properties, solution processability, low cost and competing efficiencies with established technologies. Methyl ammonium lead iodide (MAPbI3) is the most commonly used perovskite absorber layer in PSCs owing to its optimum bandgap, ease of phase stabilization and high performance. MAPbI3 based PSCs are known to best perform when the perovskite films are prepared by 1-step recipe followed by conventionally heat treating at 100 °C. Here we report a different precursor where such heat treatment is unwanted and is in fact detrimental in MAPbI3 absorber layer-based PSCs. MAPbI3 films are developed by liquefying their single crystal counterparts in methylamine gas. The obtained precursor from the single crystals is suitable for both lab scale as well as large scale depositions. Here we study its crystallization mechanism in lab-scale spin coating process. The precursor crystallized instantaneously during spin-coating itself without any post annealing step. Our results indicate that annealing of the films is detrimental to its morphological, structural and optical properties thus rendering it unsuitable for photovoltaic applications. The films without annealing have resulted in an efficiency greater than 13% while the annealed films have resulted in less than 2%.

Mn-rich nanoscale secondary phases were identified in LiFe1-xMnxPO4, despite the known complete solubility for the LiFePO4-LiMnPO4 system and the observed linear increase in the lattice parameters of LiFe1-xMnxPO4 with increasing Mn concentration. Carbon free LiFe1-xMnxPO4 (x = 0, 0.05, 0.10, 0.25) were prepared by the sequential precipitation of Li3PO4 and (Fe1-xMnx)3(PO4)2, followed by hydrothermal treatment. At low doping concentration (x ≤ 0.05), Li-Mn-O secondary phases were discerned by Raman spectra, which corroborated with the inductively coupled plasma elemental analysis. Though energy dispersive elemental mapping with scanning transmission electron microscopy do not show segregation of Mn at low concentrations, Mn-rich phases were clearly discerned at high doping concentration (x = 0.25). The kinetics of Mn-rich phase formation during hydrothermal synthesis of carbon free LiFe1-xMnxPO4, which was attributed to the difference in the solubility constant of the intermediate products of Li3PO4 and (Fe1-xMnx)3(PO4)2, and its implications on the capacity of LiFe1-xMnxPO4 cathode material were discussed. Our results present how de-convoluted Raman peaks show clear signatures of nanophase impurity segregations and how an increase in the lattice constant with Mn doping concentration can be decisive. This journal is

We have investigated the structural, electrical, and magnetic properties of both sputter deposited indium oxynitride and Cr substituted indium oxynitride films as well as InN films grown by molecular beam epitaxy. The degenerate oxynitride films exhibit n-type carrier concentrations in excess of 10 20 cm -3 and remain conducting to low temperatures, while the InN samples have much lower carrier concentrations and are insulating at low temperatures. At the same time all of these films show a room temperature ferromagnetic signal, with saturation magnetization ranging from 0.05 emu cm -3 for the indium oxynitride and InN films to 0.30 emu cm -3 for the Cr substituted film. Low temperature point contact Andreev reflection measurements find a spin polarizations from 46 ± 2% for indium oxynitride to 50 ± 2% for the Cr substituted films. These results highlight the potential of nitrides for spintronic applications. © 2014 Elsevier B.V. All rights reserved.