Amorphous titanium dioxide (a-TiO2) combined with an electrocatalyst has shown to be a promising coating for stabilizing traditional semiconductor materials used in artificial photosynthesis for efficient photoelectrochemical solar-to-fuel energy conversion. In this study we report a detailed analysis of two methods of modifying an undoped thin film of atomic layer deposited (ALD) a-TiO2 without an electrocatalyst to affect its performance in water splitting reaction as a protective photoelectrode coating. The methods are high-temperature annealing in ultrahigh vacuum and atomic hydrogen exposure. A key feature in both methods is that they preserve the amorphous structure of the film. Special attention is paid to the changes in the molecular and electronic structure of a-TiO2 induced by these treatments. On the basis of the photoelectrochemical results, the a-TiO2 is susceptible to photocorrosion but significant improvement in stability is achieved after heat treatment in vacuum at temperatures above 500 °C. On the other hand, the hydrogen treatment does not increase the stability despite the ostensibly similar reduction of a-TiO2. The surface analysis allows us to interpret the improved stability to the thermally induced formation of O– species within a-TiO2 that are essentially electronic defects in the anionic framework.

Visually black, electrically leaky, amorphous titania (am-TiO 2 ) thin films were grown by atomic layer deposition (ALD) for photocatalytic applications. Broad spectral absorbance in the visible range and exceptional conductivity are attributed to trapped Ti 3+ in the film. Oxidation of Ti 3+ upon heat treatment leads to a drop in conductivity, a color change from black to white, and crystallization of am-TiO 2 . ALD-grown black TiO 2 , without any heat treatment, is subject to dissolution in alkaline photoelectrochemical conditions. The best photocatalytic activity for solar water splitting is obtained for completely crystalline white TiO 2 .

Amorphous titania (am.-TiO2) has gained wide interest in the field of photocatalysis, thanks to exceptional disorder-mediated optical and electrical properties compared to crystalline TiO2. Here, we study the effects of intrinsic Ti3+ and nitrogen defects in am.-TiO2 thin films via the atomic layer deposition (ALD) chemistry of tetrakis­(dimethylamido)­titanium­(IV) (TDMAT) and H2O precursors at growth temperatures of 100–200 °C. X-ray photoelectron spectroscopy (XPS) and computational analysis allow us to identify structural disorder-induced penta- and heptacoordinated Ti4+ ions (Ti5/7c 4+), which are related to the formation of Ti3+ defects in am.-TiO2. The Ti3+-rich ALD-grown am.-TiO2 has stoichiometric composition, which is explained by the formation of interstitial peroxo species with oxygen vacancies. The occupation of Ti3+ 3d in-gap states increases with the ALD growth temperature, inducing both visible-light absorption and electrical conductivity via the polaron hopping mechanism. At 200 °C, the in-gap states become fully occupied extending the lifetime of photoexcited charge carriers from the picosecond to the nanosecond time domain. Nitrogen traces from the TDMAT precursor had no effect on optical properties and only little on charge transfer properties. These results provide insights into the charge transfer properties of ALD-grown am.-TiO2 that are essential to the performance of protective photoelectrode coatings in photoelectrochemical solar fuel reactors.

Dissolution of a platinum catalyst is a major degradation mechanism of fuel cells, but the exact reaction mechanism has remained unclear. Here, electrochemical ambient pressure X-ray photoelectron spectroscopy (EC-APXPS) was utilized to provide direct information on chemical species on a single-crystal Pt(111) electrode under extremely low pH conditions. Measurements were conducted using a novel condensed electrolyte film electrochemical cell applying work function measurement as a loss-free probe for electrochemical potential. We show that platinum can dissolve chemically as Pt 2+ ion during potential cycling and redeposit as Pt 2+ at the onset potential for cathodic reactions. The dissolution of Pt does not require electrochemical oxidation via oxide place exchange. In contrast, the adsorption of oxygenated species (OH* or O*) at the onset potential for anodic reactions is a sufficient prerequisite to the dissolution. These results provide new insight into the degradation mechanism of Pt under extremely low pH conditions, predicted by the Pourbaix diagram, having practical applications to the durability of Pt-based catalysts in electrochemical energy conversion devices.

A straightforward solution-based method to modify the biofunctionality of stainless steel (SS) using heterobifunctional silane-polyethylene glycol (silane-PEG) overlayers is reported. Reduced nonspecific biofouling of both proteins and bacteria onto SS and further selective biofunctionalization of the modified surface were achieved. According to photoelectron spectroscopy analyses, the silane-PEGs formed less than 10 Å thick overlayers with close to 90% surface coverage and reproducible chemical compositions. Consequently, the surfaces also became more hydrophilic, and the observed non-specific biofouling of proteins was reduced by approximately 70%. In addition, the attachment of E. coli was reduced by more than 65%. Moreover, the potential of the overlayer to be further modified was demonstrated by successfully coupling biotinylated alkaline phosphatase (bAP) to a silane-PEG-biotin overlayer via avidin-biotin bridges. The activity of the immobilized enzyme was shown to be well preserved without compromising the achieved antifouling properties. Overall, the simple solution-based approach enables the tailoring of SS to enhance its activity for biomedical and biotechnological applications.