Vishal K. Mahajan

Self-organized TiO2 nanotubular arrays for photoelectrochemical hydrogen generation: effect of crystallization and defect structures

The effect of crystallization and surface chemistry of nanotubular titanium dioxide (TiO2) in connection with the photoelectrochemical process is reported in this investigation. TiO2 nanotubular arrays were synthesized by a simple anodization process in an acidified fluoride electrolyte at room temperature. The TiO2 nanotubes were amorphous in as-anodized condition; their transformation to crystalline phases was a function of annealing temperature and gaseous environment. The anatase phase was observed predominantly after annealing in non-oxidizing atmospheres, whereas annealing in an oxygen environment showed a mixture of anatase and rutile phases. X-ray photoelectron spectroscopy was used to determine the chemical environment of the surface, which revealed the presence of phosphate, oxygen vacancies and pentacoordinated Ti in hydrogen annealed samples. Diffuse reflectance photospectrometry of non-oxygen annealed samples showed long absorption tails extending in the visible region. The photoelectrochemical response of the TiO2 nanotubes annealed in different conditions was investigated. Photoelectrochemical performance under simulated solar light was improved by annealing the nanotubular TiO2 samples in non-oxidizing environment.

Enhancement of the photoelectrochemical conversion efficiency of nanotubular TiO2 photoanodes using nitrogen plasma assisted surface modification.

A synergistic combination of nanostructure synthesis and plasma surface modification was used to enhance the photoelectrochemical activity of titania (TiO(2)) anodes. Titania nanotubular photoanodes were synthesized by electrochemical anodization of Ti thin foils. Nitrogen plasma was used to dope N at the surface of the photoanodes while removing chemisorbed species. X-ray photoelectron spectroscopy analysis showed an increase in the surface concentration of nitrogen. The photocurrent density of plasma treated samples was approximately 80% higher than that of the control. The open circuit potential of the plasma treated samples was more negative compared to that of the control, implying a favorable energetics for water splitting. This increase in photoactivity could be ascribed to: (1) increased absorption of visible light due to bandgap reduction, (2) minimization of charge carrier traps, (3) optimal oxygen vacancies, and (4) increased surface area for enhanced optical absorption and improved charge carrier generation.

Evaluation of Atmospheric-Pressure Plasma for Improving Photoelectrochemical Response of Titania Photoanodes

A synergistic combination of nanostructure synthesis and surface engineering was used to enhance the photoelectrochemical activity of titanium dioxide (TiO2) photoanodes. Titania nanotubular arrays were synthesized by electrochemical anodization of Ti thin foils. An atmospheric-pressure helium plasma followed by exposure to nitrogen was used to modify the surface properties of TiO2 nanotubes. The photocurrent from plasma-treated samples was approximately 25% higher than that from untreated samples. This increase in photoactivity could be ascribed to the following: 1) increased absorption of visible light due to bandgap reduction; 2) efficient charge separation; 3) production of optimal oxygen vacancies; and 4) increased surface area and, hence, enhanced electrode-electrolyte area to provide maximum optical adsorption and efficient charge transfer. The diffused reflectance Ultraviolet-visible (DR-UV-Vis) absorption spectra indicated a marginal increase in absorbance for the plasma-treated samples in the visible region, suggesting a change in surface electronic structure, although bulk electronic properties remain unchanged during plasma treatment.

Application of Atmospheric-Pressure Plasma for Enhancing Photoelectrochemical Properties of TiO2 Electrodes

A synergistic combination of nanostructure synthesis and surface engineering was used to enhance photoelectrochemical activity of TiO2 photoanodes. Titania (TiO2) nanotubular arrays were synthesized by electrochemical anodization of Ti thin foils. A combination of atmospheric-pressure helium plasma and nitrogen exposure was used to modify the surface properties of TiO2 nanotubes. The photocurrent from plasma treated samples was approximately 25% higher than untreated samples. This increase in photoactivity could be ascribed to (1) increased absorption of visible light due to bandgap reduction, (2) efficient charge separation, (3) production of optimal oxygen vacancies, and (4) increased surface area and hence enhanced electrode-electrolyte area to provide maximum optical adsorption and efficient charge transfer. The diffused reflectance ultraviolet-visible (DR-UV-Vis) absorption spectra indicated a marginal increase in absorbance for the plasma treated samples in the visible region suggesting a change in surface electronic structure even though bulk electronic properties remain unchanged during plasma treatment.

Photo-electrochemical generation of hydrogen using hybrid titanium dioxide nanotubular arrays

Anodization of Ti in acidified fluoride solution resulted in a vertically oriented and an ordered nanotubular titanium oxide surface. Annealing of the TiO2 nanotubular arrays in a carbonaceous or nitrogen containing atmosphere presumably resulted in band-gap states, which enhanced the photo-activity. Composite electrode of nanotubular TiO2 + carbon doping resulted in a photocurrent density of more than 2.75 mA/cm2 at 0.2 V(Ag/AgCl) under simulated solar light illumination. The enhanced photo-activity of the carbon-modified nanotubular TiO2 is highly reproducible and sustainable for longer duration. The charge carrier densities, calculated based on the Mott-Schottky analyses, were in the range of 1-3 x 1019 cm-3 for both the carbon modified and the nitrogen-annealed nanotubular TiO2 samples. The asanodized and oxygen-annealed samples showed a charge carrier density of 5 x 1017 and 1.2 x1015 cm-3 respectively. In this study, the measured photo current density was not directly related to the charge carrier densities of the nanotubes. Presence of different phases, such as amorphous, anatase and rutile, influenced the photo activity more than the charge carrier density.