Vito Cristino

Efficient Photoelectrochemical Water Splitting by Anodically Grown WO3 Electrodes

The potentiostatic anodization of metallic tungsten has been investigated in different solvent/electrolyte compositions with the aim of improving the water oxidation ability of the tungsten oxide layer. In the NMF/H(2)O/NH(4)F solvent mixture, the anodization leads to highly efficient WO(3) photoanodes, which, combining spectral sensitivity, an electrochemically active surface, and improved charge-transfer kinetics, outperform, under simulated solar illumination, most of the reported nanocrystalline substrates produced by anodization in aqueous electrolytes and by sol-gel methods. The use of such electrodes results in high water electrolysis yields of between 70 and 90% in 1 M H(2)SO(4) under a potential bias of 1 V versus SCE and close to 100% in the presence of methanol.

Photoanodes Based on Nanostructured WO3 for Water Splitting

Anodically grown WO(3) photoelectrodes prepared in an N-methylformamide (NMF) electrolyte have been investigated with the aim of exploring the effects induced by anodization time and water concentration in the electrochemical bath on the properties of the resulting photoanodes. An n-type WO(3) semiconductor is one of the most promising photoanodes for hydrogen production from water splitting and the electrochemical anodization of tungsten allows very good photoelectrodes, which are characterized by a low charge-transfer resistance and an increased spectral response in the visible region, to be obtained. These photoanodes were investigated by a combination of steady state and transient photoelectrochemical techniques and a correlation between photocurrent produced, morphology, and charge transport has been evaluated.

New Components for Dye-Sensitized Solar Cells

Dye-Sensitized Solar Cells (DSSCs) are among the most promising solar energy conversion devices of new generation, since coupling ease of fabrication and low cost offer the possibility of building integration in photovoltaic windows and facades. Although in their earliest configuration these systems are close to commercialization, fundamental studies are still required for developing new molecules and materials with more desirable properties as well as improving our understanding of the fundamental processes at the basis of the functioning of photoactive heterogeneous interfaces. In this contribution, some recent advances, made in the effort of improving DSSC devices by finding alternative materials and configurations, are reviewed.

Photoelectrochemical Behavior of Sensitized TiO2 Photoanodes in an Aqueous Environment: Application to Hydrogen Production

The use of TiO(2) photoanodes sensitized with ruthenium(II) polypyridine complexes bearing phosphonic acid anchoring groups has been investigated in the context of photoinduced hydrogen generation. The photoanodes sustained 240 h of irradiation without undergoing appreciable hydrolysis and decomposition in an aqueous environment at pH 3. While the use of organic sacrificial donors, like ascorbic acid, considerably enhanced the photoanodic response, the exploitation of iodide was more problematic because the adsorption of photogenerated I(3)(-) from aqueous media favored charge recombination with conduction band electrons, thus limiting the efficiency of the photoelectrosynthetic device. However, experiments performed in a three-compartment cell, where the photolectrode was in contact with an organic solvent, showed a remarkable photocurrent, with an electrolysis yield close to 87%.

Hematite photoanodes modified with an Fe(III) water oxidation catalyst.

Hematite photoelectrodes prepared via a hydrothermal route are functionalized with a water oxidation catalyst consisting of amorphous Fe(III) oxide, obtained by successive ionic layer adsorption and reaction. The performances of the catalyst-modified photoanodes are considerably higher than those of the parent electrodes, resulting in a nearly doubled photoanodic current in all the basic aqueous electrolytes explored in this study. The combination of electrochemical impedance spectroscopy and laser flash photolysis indicates that the presence of the catalyst results in enhanced hole trapping in surface reactive states exposed to the electrolyte, allowing for a more successful competition between charge transfer and recombination.

Some aspects of the charge transfer dynamics in nanostructured WO3 films

The photoanodic response of two different types of nanocrystalline WO3 electrodes prepared by following either the sol gel approach or the accelerated anodization route was explored in sulfate containing electrolytes with the aim of exploring the mechanism of charge separation at WO3/electrolyte interfaces. Combined evidence by electrochemical impedance spectroscopy and transient absorption spectroscopy indicates that hole transfer occurs through the valence band and that, under applied bias, the voltage drop involves predominantly the space charge layer of the semiconductor, controlling the photocurrent via potential-induced variations of hole density at the surface of WO3. OH radicals were found among the primary water oxidation intermediates, and are partly responsible for mediated back recombination. The generation of hydroxyl radicals suggests, however, that WO3 based materials can find promising applications in environmental photoremediation under visible light, promoting ˙OH mediated oxidation of impervious contaminants. In principle, the removal of ˙OH by organic scavengers will also optimize the photocurrent generation in photoelectrochemical cells where the generation of hydrogen can be coupled to environmental decontamination.

A hybrid molecular photoanode for efficient light-induced water oxidation

A hybrid photoanode comprising a multilayered heterostructured WO3/BiVO4 semiconductor and a molecular water oxidation catalyst Ru(tda)(py-pyr)2 (Ru-WOC, where tda is [2,2′:6′,2′′-terpyridine]-6,6′′-dicarboxylato and py-pyr is 4-(pyren-1-yl)-N-(pyridin-4-ylmethyl)butanamide) is described. Both elements are linked by a highly conductive carbon nanotube fibre film (CNTf), which acts as charge transfer and anchoring platform, to which the catalyst is attached through π–π stacking interactions. Photoelectrochemical characterization of the resulting electrodes shows that the full photoanode WO3/BiVO4/CNTf/Ru-WOC outperforms the bare WO3/BiVO4 electrode in the potential range 0.3–0.8 V vs. NHE at pH 7, with current densities enhanced by 0.05–0.29 mA cm−2. Bulk electrolysis experiments and oxygen gas measurements show that the enhanced photocurrent is due to the catalytic water oxidation reaction. Detailed electrochemical impedance spectroscopy (EIS) analysis is used to investigate the roles of the multiple layers involved in the process. The CNTf/Ru–WOC interface is responsible for increasing charge accumulation and reducing recombination phenomena. The CNTf is able to hold the charge produced from light absorbed by the WO3/BiVO4 semiconductor, as shown by the high capacitive values observed for a WO3/BiVO4/CNTf electrode in the whole range of studied potentials (0.15–0.85 V vs. NHE). Furthermore, Ru-WOC transfers the charge to the solution through fast water oxidation catalysis. This is supported by the low resistivity shown by the full WO3/BiVO4/CNTf/Ru-WOC electrode at low potentials (E < 0.5 V vs. NHE). The robustness and high catalytic rate of Ru-WOC ensures the proper performance of the hybrid photoelectrode device. The latter is particularly important, as it provides opportunities to improve the performance of photoanodes for the water oxidation reaction based on the easy modification of ligands in the molecular catalyst to tune its structural, electronic, and catalytic properties. This is a unique advantage compared with commonly used catalysts based on metal oxides or oxy(hydroxides), which have limited tunability.

Emerging Contaminants Mineralization by a Photo-Electrochemical Method Based on WO 3

WO3 absorbs light up to 470 nm and when illuminated in the presence of water generates OH• radicals, which promote oxidation of organic pollutants, such as drugs. In the case of WO3 photoanodes, a considerable acceleration (4–5 times) of degradation kinetics is obtained through the application of a 1.5 V potential bias, which is instrumental to optimize the charge separation within the films and to maximize holes transfer rate to the electrolyte. Moreover, after sufficiently long irradiation, complete mineralization of the organics is achieved. Photoelectrocatalysis is observed even in diluted supporting electrolyte conditions, representing the average salinity of natural freshwater samples, demonstrating the practical feasibility of this approach.