Krishnan Rajeshwar

Bactericidal activity of TiO2 photocatalyst in aqueous media : toward a solar-assisted water disinfection system

Irradiation of suspensions of Escherichia coli ([approximately] 10[sup 6] cells/mL) and TiO[sub 2] (anatase) with UV-visible light of wave-lengths longer than 380 nm resulted in the killing of the bacteria within minutes. Oxygen was found to be a prerequisite for the bactericidal properties of the photocatalyst. Bacterial killing was found to adhere to first-order kinetics. The rate constant was proportional to the square root of the concentration of TiO[sub 2] and proportional to the incident light intensity in the range [approximately] 180- [approximately] 1660 [mu]E s[sup [minus]1] m[sup [minus]2]. The trends in these simulated laboratory experiments were mimicked by outdoor tests conducted under the summer noonday sun in Texas. The implications of these results as well as those of previous investigations in terms of practical applicability to solar-assisted water treatment and disinfection at remote sites are discussed relative to water technologies currently considered as viable as alternatives to chlorination. 24 refs., 8 figs.

Electrochemistry and the environment

Advances in electrochemical methods for pollutant remediation, recycling and sensing are reviewed. Additionally, applications of these methods in the drinking water industry, and for disinfection scenarios are discussed. Lastly, new electrode materials for environmental applications are described. In a companion review, photoelectrochemical methods will be discussed.

Solution combustion synthesis of oxide semiconductors for solar energy conversion and environmental remediation

In this tutorial review, we summarize recent research on the solution combustion synthesis of oxide semiconductors for applications related to photovoltaic solar energy conversion, photoelectrochemical hydrogen generation, and heterogeneous photocatalytic remediation of environmental pollutants. First, the advantages of combustion synthesis relative to other strategies for preparing oxide semiconductors are discussed followed by a summary of process variants in combustion synthesis. The possibility of in situ chemical modification of the oxide during its formation in the combustion environment is addressed. Morphological and crystal structure aspects of the combustion-synthesized products are discussed followed by a summary of trends in their photocatalytic activity relative to benchmark samples prepared by other methods.

Photoelectrochemistry and the environment

This article builds upon a companion review on electrochemical techniques [1] and discusses photocatalytic methods for the treatment and analysis of pollutants in water and air. Organic, inorganic and microbiological pollutants are considered. Areas wherein further research and development are needed are identified. Finally, perspectives on commercialization of this technology are presented.

Energy conversion in photoelectrochemical systems — a review

The different approaches to the conversion of solar energy into chemical and/or electrical energy employing photoelectrochemical (PEC) systems are outlined. Current work and the state-of-the-art as regards the use of various semiconductors as photoelectrodes in PEC systems and the stabilization of low band-gap semiconductors by the use of redox couples are reviewed. A critical comparison of the various approaches to the problems associated with optimal photoresponse in PEC systems indicates that electrode stabilization is the most promising amongst the alternatives currently available for efficient conversion of visible light.

Solar hydrogen generation : toward a renewable energy future

Renewable Energy and the Hydrogen Economy.- The Solar Resource.- Electrolysis of Water.- A Solar Concentrator Pathway to Low-Cost Electrolytic Hydrogen.- Thermochemical and Thermal/Photo Hybrid Solar Water Splitting.- Molecular Approaches to Photochemical Splitting of Water.- Hydrogen Generation from Irradiated Semiconductor-Liquid Interfaces.- Photobiological Methods of Renewable Hydrogen Production.- Centralized Production of Hydrogen using a Coupled Water Electrolyzer-Solar Photovoltaic System.

Efficient electrocatalyst assemblies for proton and oxygen reduction: the electrosynthesis and characterization of polypyrrole films containing nanodispersed platinum particles

Polypyrrole films containing nanometer-sized platinum particles (ppy/Pt) were electrosynthesized at glassy carbon and gold electrode surfaces. This was done either by voltammetric cycling between +0.95 and −0.80 V (vs. Ag/AgCl/sat. KCl) or via a potential step technique in solutions containing colloidal platinum particles. A chloroplatinate medium with a citrate reducing/protection agent was employed for generating the colloids. An “electrotrapping” mechanism is proposed wherein the (negatively charged) platinum colloidal particles form a three-dimensional array within a growing polypyrrole matrix. The growth of the ppy/Pt films was studied by combined voltammetry—electrochemical quartz crystal microgravimetry. Quantitative analyses of these data afforded estimates of the platinum loading within the ppy matrix and the polymerization efficiency. The former are compared with assays via X-ray photoelectron spectroscopy. The ppy/Pt films exhibited high catalytic activity towards the hydrogen evolution reaction (HER) and O2 reduction. The enhanced activity is attributed to the fine (nanometer-size range) dispersion of the platinum catalyst particles within the ppy matrix. Comparisons with bulk platinum and platinum electrodeposited on the surface of ppy films are provided. Thus, for the HER at a potential of −400 mV (vs. a saturated calomel electrode (SCE)), the ppy/Pt samples show comparable catalytic activity with bulk platinum and an order of magnitude rate enhancement relative to the surface platinum(0)/ppy case. Correspondingly, the rate enhancement does not saturate with increasing film thickness for our ppy/Pt samples. This behavior contrasts with that characteristic of surface-confined catalyst situations. That the platinum particles within the ppy matrix truly are in the catalytically useful nanometer range ( ⩽ 10 nm) is demonstrated by transmission electron microscopy data.

Photocatalytic Reduction and Immobilization of Hexavalent Chromium at Titanium Dioxide in Aqueous Basic Media

The thermodynamic aspects of the photocatalytic reduction of Cr(VI) at TiO[sub 2] are first discussed and contrasted for pH 0 and pH 10 aqueous media. The photocorrosion of TiO[sub 2] is shown to be thermodynamically less favorable in basic media. A scheme for the one-step removal of Cr(VI) at TiO[sub 2] via its reduction and subsequent immobilization [as Cr(OH)[sub 3]] is demonstrated for pH 10 chromate solutions. Dissolved oxygen is shown to inhibit the reduction of Cr(VI). A preliminary kinetics analysis reveals adherence of the initial rate of Cr(VI) reduction to the simple Langmuir model.

A re-examination of the mechanisms of electrodeposition of CdX and ZnX (X = Se, Te) semiconductors by the cyclic photovoltammetric technique

Abstract The electrodeposition mechanisms in two families of compound semiconductors, namely CdX and ZnX (X = Se, Te), were re-examined by cyclic voltammetry in the dark and under illumination of the substrate electrode/electrolyte interphase. In situ diagnostic probes of the mechanistic pathway and the electrodeposition chemistry were furnished by the photoresponse of the forming semiconductor films under band-gap illumination. Thus, a new mechanism involving the metal ion catalyzed deposition of the chalcogen species, is proposed for CdSe and ZnSe. In the case of CdTe and ZnTe, Teo nucleation preceded subsequent semiconductor formation via the underpotential deposition of the metal ions. Two types of photoresponse, namely photoconductive and photoelectrochemical, were distinguished on the basis of their varying dependence on the applied potential. These cyclic photovoltammetric data are finally discussed within the framework of existing methods combining light excitation (or emission) and voltammetric scanning of the electrode/electrolyte interphase.

Homogeneous and heterogeneous photocatalytic reactions involving As(III) and As(V) species in aqueous media

Thermodynamic considerations show that photogenerated holes in UV-irradiated TiO2 can oxidize As(III) to As(V). Similarly, the photogenerated electrons can reduce As(V) to the elemental state. Experimental data are presented in verification of these expectations. An electron acceptor (e.g., O2) is needed for the hole transfer pathway whereas the heterogeneous photocatalytic reduction of As(V) is facilitated by a hole scavenger such as methanol. The oxidation of As(III) to As(V) also proceeds in the dark when a large excess of H2O2 is present. However, this process is very facile with simultaneous UV irradiation of the peroxide. These data point toward the feasibility of a two-step pollution abatement scheme for the initial oxidation and subsequent immobilization of the toxic As(III) species.