Trap States in TiO₂ Films Made of Nanowires, Nanotubes or Nanoparticles: An Electrochemical Study

Abstract

The presence of electronic traps in nanoporous TiO(2) electrodes has been studied by cyclic voltammetry in aqueous media. These simple measurements allow us to map the density of states, providing evidence for the presence of a relatively small number of discrete electron traps at the band gap. We have taken advantage of the variety of TiO(2) synthetic procedures that lead to well-defined morphologies (such as nanowires, nanocolumns, nanotubes, and nanoparticles) of anatase and rutile to investigate the nature of these electron traps. They derive from the structural disorder at the contact between neighboring crystalline nanoparticles. As expected, both their density and energetic location are highly dependent, not only on the crystalline structure (whether it is anatase or rutile), but also on the electrode morphology (i.e. the facets that meet at the grain boundaries). The trap density is also sensitive to pH changes and to the presence of some adsorbates. This variation of the number of traps with the electrolyte indicates that on one hand, an apparent electronic density of states is actually measured. On the other, it indicates that the traps are surface-related in agreement with their particular location at the perimeter of the grain boundaries. The effect of these traps on the observed electrode catalytic reactivity has also been studied. In the dark, it is found that they are directly involved in the electron transfer toward oxygen. In addition, under illumination, the trap states show a deleterious effect, favoring electron recombination.

Electrochemical Doping as a Way to Enhance Water Photooxidation on Nanostructured Nickel Titanate and Anatase Electrodes

“…In agreement with the previous results, [11] the pair of peaks ascribed to grain boundaries [47] appearing at À0.8 V (TiO 2 ) and À0.95 V (NiTiO 3 ) are highly promoted and in the case of anatase, they are clearly displaced toward more positive potential values after the electrochemical pretreatment. In agreement with the previous results, [11] the pair of peaks ascribed to grain boundaries [47] appearing at À0.8 V (TiO 2 ) and À0.95 V (NiTiO 3 ) are highly promoted and in the case of anatase, they are clearly displaced toward more positive potential values after the electrochemical pretreatment.…”

“…This is probably linked to the initial larger recombination existing at NiTiO 3 electrodes. In this regard, there are few publications that have shown that it exists an exponential relation between the charge corresponding to the grain boundary peak and the electrolyte pH [46,47] and these traps have been linked to recombination processes. [11] This result clearly points to the key role of pH in the electrochemical doping process.…”

Electron trapping induced electrostatic adsorption of cations: a general factor leading to photoactivity decay of nanostructured TiO2

“…20,26,27 It is reasonable to think that TiO 2 can be negatively charged if electrons are trapped in band gap states. Moreover, it has long been realized that electron trapping in band gap states can induce proton (or other cation ions) charge compensation uptake (call ETPU).…”

Modification of Charge Trapping at Particle/Particle Interfaces by Electrochemical Hydrogen Doping of Nanocrystalline TiO2

“…10 In addition, these states may act as recombination sites exerting a further deleterious impact on the photocurrent. 32 …”