Spectroelectrochemical Photoluminescence of Trap States of Nanocrystalline TiO2 in Aqueous Media

• Published in: Journal of physical chemistry. C 2013-07, Vol.117 (26), p.13654-13662
• Main Authors: Knorr, Fritz J, McHale, Jeanne L
• Format: Article
• Language: English
• Subjects: Applied sciences; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Energy; Exact sciences and technology; Low-dimensional structures (superlattices, quantum well structures, multilayers): structure, and nonelectronic properties; Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals; Natural energy; Optical properties and condensed-matter spectroscopy and other interactions of matter with particles and radiation; Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures; Photovoltaic conversion; Physics; Solar cells. Photoelectrochemical cells; Solar energy; Structure of solids and liquids; crystallography; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties)
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp402264p

Trap state photoluminescence of nanocrystalline TiO2 electrodes is investigated as a function of applied bias and pH in aqueous electrolyte. Films composed of the anatase polymorph reveal an increase in a broad red emission at increasingly negative potentials, with an onset about 200 mV positive of the pH-dependent literature value of the conduction band potential, followed by conversion to the green emission characteristic of hole traps at more negative bias. Green photoluminescence is the only emission seen from mixed-phase (P25, anatase/rutile) films at any applied potential, while red-emitting electron traps in P25 appear to be quenched by electron transfer to rutile hole traps. The influence of surface treatment by TiCl4 is investigated for both anatase and P25 in order to shed light on the mechanism by which this treatment improves the performance of TiO2-based solar cells. Our results reveal the difference between trap state distributions of P25 and anatase nanoparticles and address the molecular basis for red and green emitting traps. The results establish the redox potentials of the traps as a function of pH and reveal the breadth of their energetic distribution.