Hydrogen Production with Nanostructured and Sensitized Metal Oxides. In: Bignozzi CA, editor. Photocatalysis. Berlin: Springer Berlin Heidelberg; 2011. pp. 39-94. [DOI: 10.1007/128_2011_137]

Sensitization of Nanocrystalline Metal Oxides with a Phosphonate-Functionalized Perylene Diimide for Photoelectrochemical Water Oxidation with a CoOx Catalyst

A planar organic thin film composed of a perylene diimide dye (N,N'-bis(phosphonomethyl)-3,4,9,10-perylenediimide, PMPDI) with photoelectrochemically deposited cobalt oxide (CoO_x) catalyst was previously shown to photoelectrochemically oxidize water (DOI: 10.1021/am405598w). Herein, the same PMPDI dye is studied for the sensitization of different nanostructured metal oxide (nano-MO_x) films in a dye-sensitized photoelectrochemical cell architecture. Dye adsorption kinetics and saturation decreases in the order TiO₂ > SnO₂ ≫ WO₃. Despite highest initial dye loading on TiO₂ films, photocurrent with hydroquinone (H₂Q) sacrificial reductant in pH 7 aqueous solution is much higher on SnO₂ films, likely due to a higher driving force for charge injection into the more positive conduction band energy of SnO₂. Dyeing conditions and SnO₂ film thickness were subsequently optimized to achieve light-harvesting efficiency >99% at the λ_max of the dye, and absorbed photon-to-current efficiency of 13% with H₂Q, a 2-fold improvement over the previous thin-film architecture. A CoO_x water-oxidation catalyst was photoelectrochemically deposited, allowing for photoelectrochemical water oxidation with a faradaic efficiency of 31 ± 7%, thus demonstrating the second example of a water-oxidizing, dye-sensitized photoelectrolysis cell composed entirely of earth-abundant materials. However, deposition of CoO_x always results in lower photocurrent due to enhanced recombination between catalyst and photoinjected electrons in SnO₂, as confirmed by open-circuit photovoltage measurements. Possible future studies to enhance photoanode performance are discussed, including alternative catalyst deposition strategies or structural derivatization of the perylene dye.

Visible-light-driven photocatalytic H2 evolution from aqueous suspensions of perylene diimide dye-sensitized Pt/TiO2 catalysts

Solution-processed composite photocatalysts employing photosensitive perylene diimide assemblies and Pt/TiO₂ nanoparticles demonstrate efficient visible-light-driven H₂ evolution activity in aqueous solutions.

The electrochemistry of nanostructured titanium dioxide electrodes

Several of the multiple applications of titanium dioxide nanomaterials are directly related to the introduction or generation of charge carriers in the oxide. Thus, electrochemistry plays a central role in the understanding of the factors that must be controlled for the optimization of the material for each application. Herein, the main conceptual tools needed to address the study of the electrochemical properties of TiO(2) nanostructured electrodes are reviewed, as well as the electrochemical methods to prepare and modify them. Particular attention is paid to the dark electrochemical response of these nanomaterials and its direct connection with the TiO(2) electronic structure, interfacial area and grain boundary density. The physical bases for the generation of currents under illumination are also presented. Emphasis is placed on the fact that the kinetics of charge-carrier transfer to solution determines the sign and value of the photocurrent. Furthermore, methods for extracting kinetic information from open-circuit potential and photocurrent measurements are briefly presented. Some aspects of the combination of electrochemical and spectroscopic measurements are also dealt with. Finally, some of the applications of TiO(2) nanostructured samples derived from their electrochemical properties are concisely reviewed. Particular attention is paid to photocatalytic processes and, to a lesser extent, to photosynthetic reactions as well as to applications related to energy from the aspects of both saving (electrochromic layers) and accumulation (batteries). The use of TiO(2) nanomaterials in solar cells is not covered, as a number of reviews have been published addressing this issue.