Norma R. de Tacconi

Solution combustion synthesis of oxide semiconductors for solar energy conversion and environmental remediation

In this tutorial review, we summarize recent research on the solution combustion synthesis of oxide semiconductors for applications related to photovoltaic solar energy conversion, photoelectrochemical hydrogen generation, and heterogeneous photocatalytic remediation of environmental pollutants. First, the advantages of combustion synthesis relative to other strategies for preparing oxide semiconductors are discussed followed by a summary of process variants in combustion synthesis. The possibility of in situ chemical modification of the oxide during its formation in the combustion environment is addressed. Morphological and crystal structure aspects of the combustion-synthesized products are discussed followed by a summary of trends in their photocatalytic activity relative to benchmark samples prepared by other methods.

Formation and characterization of self-organized TiO2 nanotube arrays by pulse anodization.

This paper describes TiO2 nanotube arrays prepared by anodic oxidation of Ti substrates using pulse voltage waveforms. Voltages were pulsed between 20 and -4 V or between 20 and 0 V with varying durations from 2 to 16 s at the lower limit of the pulse waveform. Ammonium fluoride or sodium fluoride (and mixtures of both) was used as the electrolyte with or without added medium modifier (glycerol, ethylene glycol, or poly (ethylene glycol) (PEG 400)) in these experiments. The pulse waveform was optimized to electrochemically grow TiO2 nanotubes and chemically etch their walls during its cathodic current flow regime. The resultant TiO2 nanotube arrays showed a higher quality of nanotube array morphology and photoresponse than samples grown via the conventional continuous anodization method. Films grown with a 20 V/-4 V pulse sequence and pulse duration of 2 s at its negative voltage limit afforded a superior photoresponse compared to other pulse durations. Specifically, the negative voltage limit of the pulse (-4 V) and its duration promote the adsorption of NH4+ species that in turn inhibits chemical attack of the growing oxide nanoarchitecture by the electrolyte F- species. The longer the period of the pulse at the negative voltage limit, the thicker the nanotube walls and the shorter the nanotube length. At variance, with 0 V as the low voltage limit, the longer the pulse duration, the thinner the oxide nanotube wall, suggesting that chemical attack by fluoride ions is not counterbalanced by NH3/NH4+ species adsorption, unlike the interfacial situation prevailing at -4 V. Finally, the results from this study provide useful evidence in support of existing mechanistic models for anodic growth and self-assembly of oxide nanotube arrays on the parent metal surface.

Pulsed electrodeposition of WO3-TiO2 composite films

Cathodic electrodeposition was used to prepare thin films of WO3 and TiO2 on F-doped SnO2 glass substrates. A new pulsed deposition technique was developed to prepare WO3–TiO2 composite films over a wide compositional range. Such composite films containing comparable amounts of WO3 and TiO2 showed superior photoelectrochemical performance in 0.1 M Na2SO4 relative to the component oxides themselves.

Tailoring Copper Oxide Semiconductor Nanorod Arrays for Photoelectrochemical Reduction of Carbon Dioxide to Methanol

Solar photoelectrochemical reduction of carbon dioxide to methanol in aqueous media was driven on hybrid CuO/Cu2O semiconductor nanorod arrays for the first time. A two-step synthesis was designed and demonstrated for the preparation of these hybrid copper oxide one-dimensional nanostructures on copper substrates. The first step consisted in the growth of CuO nanorods by thermal oxidation of a copper foil at 400 °C. In the second step, controlled electrodeposition of p-type Cu2O crystallites on the CuO walls was performed. The resulting nanorod morphology with controllable wall thickness by adjusting the Cu2O electrodeposition time as well as their surface/bulk chemical composition were probed by scanning electron microscopy, X-ray diffraction and Raman spectroscopy. Photoelectrosynthesis of methanol from carbon dioxide was demonstrated at -0.2 V vs SHE under simulated AM1.5 solar irradiation on optimized hybrid CuO/Cu2O nanorod electrodes and without assistance of any homogeneous catalyst (such as pyridine or imidazole) in the electrolyte. The hybrid composition, ensuring double pathway for photoelectron injection to CO2, along with high surface area were found to be crucial for efficient performance in methanol generation under solar illumination. Methanol formation, tracked by gas chromatography/mass spectrometry, indicated Faradaic efficiencies of ~95%.

Preparation, photoelectrochemical characterization, and photoelectrochromic behavior of metal hexacyanoferrate–titanium dioxide composite films☆

Abstract Composite films comprising of particulate semiconductors and molecular redox systems present interesting frameworks for exploring interfacial photoinduced electron transfer. Metal hexacyanoferrates as candidates for the molecular redox component have the virtue that they are both redox active and electrochromic. On the other hand TiO2 is an inorganic semiconductor that is extremely stable toward photocorrosion. Therefore we describe in this paper an approach to mate the two components. Specifically we describe the preparation and characterization of metal hexacyanoferrate (MHCF)–TiO2 composite films. The variant routes to the electrosynthesis of these films and the corresponding photoelectrochemical/photoelectrochromic behavior are also described for CuHCF–TiO2 and NiHCF–TiO2 composites. Finally, preliminary data on InHCF films are presented.

Reversibility of Photoelectrochromism at the TiO2/Methylene Blue Interface

This paper describes the photoelectrochromic bleaching and recoloration of methylene blue in aqueous or methanolic slurry suspensions of TiO{sub 2}. Using simple modification of a UV-visible diode-array spectrometer setup, the spectral changes undergone by the dye in these solutions were monitored in situ during irradiation of the TiO{sub 2} particles and in the dark. A particular focus of this study was a systematic investigation of factors underpinning the chemical and electrochemical (kinetic) reversibility of the dye bleaching-recoloration sequence. Thus, choice of solution pH and hole capture agents is shown to be critical to the suppression of parasitic chemical and photochemical reactions involving the dye. On the other hand, dye adsorption on the TiO{sub 2} surface, the solvent medium itself, and its O{sub 2} content are all important factors in dictating the kinetic reversibility of the photoelectrochromic process. Finally, the practical implications of the findings are presented.

Electrochemical grafting of poly(3,4-ethylenedioxythiophene) into a titanium dioxide nanotube host network.

This study focuses on electrodeposition for infiltrating in situ a conducting polymer such as poly(3,4-ethylenedioxythiophene) (PEDOT) into a host titanium dioxide (TiO(2)) nanotube array (NTA) framework. The TiO(2) NTA was electrosynthesized on titanium foil in turn by anodization in a fluoride-containing medium. The PEDOT layer was electrografted into the TiO(2) NTA framework using a two-step potentiostatic growth protocol in acetonitrile containing supporting electrolyte. The nanoscopic features of oligomer/polymer infiltration and deposition in the NTA interstitial voids were monitored by field-emission scanning electron microscopy. Systematic changes in the nanotube inner diameter and the wall thickness afforded insights into the evolution of the TiO(2)NTA/PEDOT hybrid assembly. This assembly was subsequently characterized by UV-visible diffuse reflectance, cyclic voltammetry, and photoelectrochemical measurements. These data serve as a prelude to further use of these hybrids in heterojunction solar cells.

Electrosynthesis of cadmium selenide films on a selenium-modified gold surface

A two-step method for the growth of CdSe films is described that is based on the initial chemical modification of a polycrystalline gold surface with a selenium overlayer. In the second step, this overlayer is cathodically stripped as Se2− in a Se(IV)-free electrolyte medium (0.1 M Na2SO4) that is dosed with the requisite amount of Cd2+ ions. Unlike the classical cathodic route, this new approach does not suffer from problems with excess Se admixed with CdSe. The two-step approach is validated using a combination of voltammetry, microgravimetry, and photoelectrochemical experiments.

Photoelectrochemistry and Raman spectroelectrochemistry of cuprous thiocyanate films on copper electrodes in acidic media

Abstract The formation of cuprous thiocyanate films on polycrystalline copper electrodes in acidic KSCN solutions was studied at controlled potentials and at open circuit using a combination of voltammetry, chronoamperometry, Raman spectroelectrochemistry and photoelectrochemistry techniques. A restricted potential domain was used to avoid incipient oxide formation at the copper surface. Cyclic photovoltammetry, coupling white light illumination and voltammetric scanning between −0.80 V and −0.20 V (vs. Ag/ AgCl) of the working Cu electrode, showed the formation of a p-type semiconductor film at the Cu surface. Film formation was accompanied by the evolution of a Raman band at 2172 cm−1 in the cyanide stretching region. These observations, coupled with other Raman and IR spectral data, enabled the chemical identification of the photoactive film as α-CuSCN. However, the films formed at open circuit (−0.41 V) manifested another Raman band at 2120 cm−1. This band also appeared and “peaked” in intensity during the electroreduction of α-CuSCN. A mechanism involving (CuSCN)x aggregates is proposed. The evolution of the interphasial chemistry at the Cu electrode surface from adsorbed SCN− to aggregate formation, finally culminating in the α-CuSCN phase, is reminiscent of the “sliding scale” observed by previous authors for the Ag + SCN system. However, this study further shows that this scale is dependent on the film growth history.

Driving Multi-electron Reactions with Photons: Dinuclear Ruthenium Complexes Capable of Stepwise and Concerted Multi-electron Reduction

Using biological precedents, it is expected that concerted, multi-electron reduction processes will play a significant role in the development of efficient artificial photosynthetic systems. We have found that the dinuclear ruthenium complexes [(phen)2Ru(tatpp)Ru(phen)2]4+ (P) and [(phen)2Ru(tatpq) Ru(phen)2]4+ (Q) undergo photodriven 2- and 4-electron reductions, respectively, in the presence of a sacrificial reductant. Importantly, these processes are completely reversible upon exposure to air, and consequently, these complexes have the potential to be used catalytically in multi-electron transfer reactions. A localized molecular orbital description of the ligands and complexes is used to explain both the function and spectroscopy of these complexes. In both complexes, the reducing equivalents are stored in the π* orbitals of the bridging ligands and depending on the solution pH, various protonation states of the reduced species of P and Q are obtained. Under basic conditions, the photochemical pathway favors sequential single-electron reductions, while neutral or slightly acidic conditions give rise to proton-coupled multi-electron transfer. In fact, at sufficiently acidic pH, only a coupled two-electron, 2-proton process is seen. Few molecular photocatalysts are capable of proton-coupled multi-electron transfer, which is believed to be a fundamental component of light-activated energy storage in nature.