C.R. Chenthamarakshan

Titania-based heterogeneous photocatalysis. Materials, mechanistic issues, and implications for environmental remediation

Using hexavalent chromium [Cr(VI)] and methylene blue (MB) as model substrates, we discuss three aspects of TiO2-based heterogeneous photocatalysis. We show first that a given TiO2 sample may not be simultaneously optimal for photocatalytically driving the reduction of Cr(VI) and the oxidation of MB. We further show that a TiO2 sample that strongly adsorbs either of these substrates in the dark is not optimal as a photocatalyst. The other two aspects concern circumventing the rather poor surface catalytic properties and visible light photoresponse of TiO2, respectively. Strategies revolving around the visible light photoexcitation of the substrate itself and metal-modification of the TiO2 surface, are described as possible solutions.

Pulsed electrodeposition of WO3-TiO2 composite films

Cathodic electrodeposition was used to prepare thin films of WO3 and TiO2 on F-doped SnO2 glass substrates. A new pulsed deposition technique was developed to prepare WO3–TiO2 composite films over a wide compositional range. Such composite films containing comparable amounts of WO3 and TiO2 showed superior photoelectrochemical performance in 0.1 M Na2SO4 relative to the component oxides themselves.

Cathodic electrodeposition of mixed oxide thin films

Abstract Cathodic electrodeposition is a rather unexplored route for the synthesis of mixed oxides. Two different mechanisms are generally reported which lead to oxide thin film formation: (i) the direct reduction of the oxidation state of the metallic element, (ii) an interfacial pH increase and local supersaturation followed by oxide precipitation. We emphasize the major requirements for mixed thin film formation via electrosynthesis and present results obtained with two different systems of special interest. The first, ZnO/Eu(OH) x , is based on the second mechanism. The second, TiO 2 /WO 3 , is a mixed mechanism process. We describe the conditions under which europium oxide can be deposited cathodically. This oxide is obtained by co-deposition with zinc oxide. After a heat treatment at 400 °C, X-ray diffraction shows that the films are mainly made of monoclinic Eu 2 O 3 . The successful WO 3 –TiO 2 mixed film electrodeposition in a wide compositional range is also reported.

Photocatalytic reduction of divalent zinc and cadmium ions in aqueous TiO2 suspensions: an interfacial induced adsorption–reduction pathway mediated by formate ions

A new pathway is described for the photocatalytic reduction of metal ions in organic-loaded aqueous TiO2 suspensions. Thus, formate ions act as interfacial anchors to initially sequester Zn(II) and Cd(II) ions onto the TiO2 particle surfaces. Subsequent UV illumination of the suspensions results in the facile photoreduction of these metal ions. In the absence of formate, the metal ions show little proclivity either to adsorb on the TiO2 particle surfaces or to undergo photoreduction.

Photocatalytic reactivity of zinc and cadmium ions in UV-irradiated titania suspensions

Abstract The reduction of Zn(II) and Cd(II) species to their corresponding metallic form is characterized by negative reduction potentials in aqueous media. Thus the direct reduction of these species by photogenerated electrons in UV-irradiated titania (Degussa, P 25) suspensions is difficult unless co-additives are present. This paper presents such scenarios involving nitrate and organic additives and describes conditions where facile conversion of Zn(II) and Cd(II) species can be secured. The photocatalysis reaction products depend on the conditions extant in the TiO 2 dispersions and can be either the corresponding oxy-hydroxy compound or the metal. Interesting interfacial changes occur when formate ions are added to a TiO 2 suspension containing Zn(II) species. Such changes were probed by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy.

Cathodic photoprocesses on titania films and in aqueous suspensions

Abstract The interaction and photocatalytic reactivity of 12 metal (or metalloid) ions: Mn 2+ , Zn 2+ , Cd 2+ , Tl + , Ni 2+ , Pb 2+ , Cu 2+ , Se 4+ , Ag + , Hg 2+ , Cr 6+ , and Au 3+ , with UV-irradiated TiO 2 surfaces were compared. Two distinct groups were discerned, the first six ionic species showing essentially no or a very weak tendency to accept photogenerated electrons from TiO 2 , and the last six, showing a strong tendency to undergo photocatalytic reduction. The selenium species were abnormal, both in their tendency toward strong adsorption on the TiO 2 surface in the dark, and in their very high photocatalytic reactivity. Chronopotentiometry measurements on TiO 2 films provided complementary data to those obtained from UV-irradiated titania suspensions. Collectively, the photocatalytic reactivity of the 12 species of interest here, could be correlated with the energy location of the corresponding redox couples in solution with respect to the TiO 2 conduction band-edge.

Synergistic photocatalysis mediated by TiO2: mutual rate enhancement in the photoreduction of Cr(VI) and Cu(II) in aqueous media

Copper (II) ions were found to accelerate the heterogeneous photocatalytic reduction of Cr(VI) in N2-purged UV-irradiated TiO2 suspensions. Conversely, Cr(VI) enhanced the rate of reduction of Cu(II) to metallic form on the TiO2 particle surfaces. Thus 800μM Cr(VI) was not completely converted to Cr(III) even after ∼4 h irradiation, in the absence of Cu(II), while the presence of 800μM Cu(II) resulted in complete Cr(VI) conversion within ∼20 min. Similarly, the half-life for the photocatalytic reduction of 600μM Cu(II) decreased from ∼1 h with no Cr(VI) to ∼5 min in the presence of 1.6 mM Cr(VI).

Redox characterization of furnace carbon black surfaces

Abstract The redox properties of furnace-type carbon blacks were studied by UV–visible spectrophotometry and potentiometry. The carbon black surface was exposed to redox reagents such as Cr(VI), Mn(VII), Fe(III), and sulfite. The ability of the carbon black surface to undergo redox reactions with these reagents was correlated with the morphological attributes of the black such as crystallite size and surface area. The effect of high temperature treatment was also explored. These data show that the ability of carbon black to accept or donate electrons is a sensitive function of the defect site density of the carbon black surface. Thus an initially inert graphitic surface could be rendered redox active by mechanically damaging it. Conversely, high temperature treatment ‘graphitized’ the carbon black and weakened its redox activity. The ‘structure’ of carbon black (as probed by dibutylphthalate absorption) did not appear to influence its redox activity. Laser Raman spectroscopy and cyclic voltammetry provided further insights into the redox activity of the furnace carbon black surface.

Self-organized TiO2 nanotube arrays by anodization of Ti substrate: Effect of anodization time, voltage and medium composition on oxide morphology and photoelectrochemical response

This study probes the relationship between the morphology of anodic titania (TiO 2 ) layers grown on Ti foil substrates and their subsequent photoelectrochemical response in 0.5 M Na 2 SO 4 supporting electrolyte. The effects of anodization variables (voltage and time) and anodization medium composition [water, glycerol, poly(ethylene glycol), ethylene glycol] along with fluoride ion concentration on the oxide layer of morphology and photoresponse are described. The degree of order of the self organized TiO 2 nanotube arrays and the extent to which these arrays organize themselves over the entire substrate surface are key variables dictating the corresponding quality of the resultant photoresponse.

Cathodic Electrodeposition of CdO Thin Films from Oxygenated Aqueous Solutions

Cadmium oxide semiconductor films were cathodically electrodeposited on F-doped SnO 2 substrates from an aqueous 0.1 M KCl medium containing dissolved Cd 2 + species and O 2 , The mechanistic aspects of the electrodeposition were briefly explored using cyclic voltammetry in the supporting electrolyte where either Cd 2 + or O 2 or both were intentionally omitted. The CdO films were potentiostatically deposited at 80°C followed by a postdeposition thermal anneal at 450°C. These samples were then analyzed by X-ray diffraction, scanning electron microscopy, Auger electron spectroscopy, and UV-visible diffuse reflectance spectrophotometry. The photoelectrochemical behavior of these films is then presented.