Protective overlayers for light absorbers in photoelectrochemical water-splitting devices have gained considerable attention in recent years. They stabilize light absorbers which would normally be prone to chemical side reactions leading to degradation of the absorber. Atomic layer deposition (ALD) enables conformal and reproducible ultrathin protective layer growth even on highly structured substrates. One of the most widely investigated protective layers is amorphous TiO, deposited by ALD at a relatively low temperature (120-150 °C). We have deposited protective layers from tetrakis(dimethylamido)titanium(IV) at two different temperatures and investigated their chemical composition as well as optical and electrochemical properties. Our main findings reveal a change in the flat band potential with thickness, reaching a stable value of about -50 to -100 mV versus reversible hydrogen electrode for films >30 nm, with doping densities of ∼10 cm. Practical thicknesses to achieve pinhole-free films are evaluated and discussed.

The ground-and excited-state properties of six rhenium(I) 2N-tricarbonyl complexes with 4-(4-substituted-phenyl)terpyridine ligands bearing substituents of different electron-donating abilities were evaluated. Significant modulation of the electrochemical potentials and a nearly 4-fold variation of the triplet metal-to-ligand charge-transfer (3MLCT) lifetimes were observed upon going from CN to OMe. With the more electron-donating NMe2 group, we observed in the 2N complex the appearance of a very strong absorption band, red-shifted by ca. 100 nm with respect to the other complexes. This was accompanied by a dramatic enhancement of the excited-state lifetime (380 vs 1.5 ns), and a character change from 3MLCT to intraligand charge transfer (3ILCT), despite the remote location of the substituent. The dynamics and character of the excited states of all complexes were assigned by combining transient IR spectroscopy, IR spectroelectrochemistry, and (time-dependent) density functional theory calculations. Selected complexes were evaluated as photosensitizers for hydrogen production, with the 2N-NMe2 complex resulting in a stable and efficient photocatalytic system reaching TONRe values of over 2100, representing the first application of the 3ILCT state of a rhenium(I) carbonyl complex in a stable photocatalytic system.

The strategy of anchoring molecular catalysts on electrode surfaces combines the high selectivity and activity of molecular systems with the practicality of heterogeneous systems. The stability of molecular catalysts is, however, far less than that of traditional heterogeneous electrocatalysts, and therefore a method to easily replace anchored molecular catalysts that have degraded could make such electrosynthetic systems more attractive. Here, we apply a non-covalent "click" chemistry approach to reversibly bind molecular electrocatalysts to electrode surfaces via host-guest complexation with surface-anchored cyclodextrins. The host-guest interaction is remarkably strong and allows the flow of electrons between the electrode and the guest catalyst. Electrosynthesis in both organic and aqueous media was demonstrated on metal oxide electrodes, with stability on the order of hours. The catalytic surfaces can be recycled by controlled release of the guest from the host cavities and readsorption of fresh guest. This strategy represents a new approach to practical molecular-based catalytic systems. 3Molecular electrocatalysts can exhibit surprisingly high activities and selectivities that are unmatched by most heterogeneous catalysts. 1,2 Therefore, the development of robust immobilization strategies for these molecular species on electrode surfaces is of great interest if these molecular catalytic activities and selectivities are to be transferred to more practical heterogeneous electrosynthetic systems, 3 which have already shown promising results for CO 2 reduction, water reduction and water oxidation in the context of storage of renewable energy. [4][5][6] Over the past decades, many immobilization strategies have been developed, 7 which can be categorized into covalent binding (achieved by adding anchoring groups such as carboxylate to the catalysts), 8 non-covalent binding (pi-stacking on carbon-based electrodes) 9,10 and polymerization-based binding. [11][12][13] Here, we report a new strategy for surface immobilization of molecular electrocatalysts, which relies on a non-covalent "click" chemistry approach to bind molecular species in welldefined sites on electrode surfaces. 14 The binding of electroactive molecular guests into molecular pockets by means of host-guest complex (HGC) formation has previously been studied by several groups, including those of Stoddart and Kaifer, 15 Reinhoudt 16,17 and Huskens. 18 Liu et al. reported the HGC-formation on gold surfaces with the C 60 monoanion as guest, which was found to be electrochemically stable over prolonged durations. 19 Light-induced electron transfer to and from dye molecules bound via the HGC approach was also demonstrated by Freitag and Galoppini. 20,21 The group of Sun demonstrated the use of HGC to improve electron transfer between a molecular catalyst and a dye molecule bound onto TiO 2 . 22 Among the diverse class of host molecules, cyclodextrins, cucurbiturils and calixarenes are the most studied for HGC formation on different surfaces. 23,24 For cy...

Phosphonic acid multi-layers are used to tune the band alignment in heterojunction devices used for photoelectrochemistry and photovoltaics.