Ultrathin TiO2 films received renewed attention in the field of photoelectrochemical water splitting as corrosion protection layers for unstable, small-bandgap semiconductors. Because nucleation on the substrate can differ from steady-state growth of the film itself, it is important to understand the nucleation behavior on a specific surface. In this work, we studied the nucleation mechanism of atomic layer deposition-grown TiO2 from TiCl4 and H2O on as-received silicon by means of in-line X-ray photoelectron spectroscopy. Within a region of ∼0.4 nm of the SiO2/TiO2 interface, the presence of Ti3+ states are detected. In this region, the Ti, O, and Cl species are found to be more strongly bonded. At the initial stages of film growth, prolonged TiCl4 exposure is necessary to reach a saturated surface chemistry, which is in contrast to the outcome of growth per cycle saturation curve analysis. A prolonged water exposure experiment suggests that residual chlorine impurities can be prevented by using a sufficiently long water dose. This is particularly interesting for photoelectrode systems that cannot tolerate high temperatures. When this restriction does not apply, a postdeposition anneal at 400 °C in vacuum is a well-known option to reduce the chlorine content from the surface and the bulk of up to 10 nm thick films without affecting the stoichiometry. These insights will facilitate the optimization of the electronic properties and the materials design of efficient ultrathin protection layers for photoelectrodes for photoelectrochemical water splitting applications.

Although the deposition of alternating layers from poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and polyaniline (PANI) salts has recently provided a breakthrough in the field of conductive polymers, the cause for the conductivity improvement has remained unclear. In this work, we report a cooperative doping effect between alternating PANI base and PEDOT:PSS layers, resulting in electrical conductivities of 50-100 S cm and power factors of up to 3.0 ± 0.5 μW m K, which surpass some of the recent values obtained for protonated PANI/PEDOT:PSS multilayers by a factor of 20. In this case, the simultaneous improvement in the electrical conductivity of both types of layers is caused by the in situ protonation of PANI, which corresponds to the removal of the excess acidic PSS chains from the PEDOT:PSS grains. The interplay between the functional groups' reactivity and the supramolecular chain reorganization leads to an array of preparation-dependent phenomena, including a stepwise increase in the film thickness, an alternation in the electrical conductivity, and the formation of a diverse surface landscape. The latter effect can be traced to a buildup of strain within the layers, which results in either the formation of folds or the shrinkage of the film. These results open new paths for designing nanostructured thin-film thermoelectrics.

Operando attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy is discussed in this paper for water splitting application. The first part of the paper focuses on the discussion of the opportunities and challenges of this method for the characterization of the solid-liquid interface in water splitting. The second part of the paper focuses on recent results and future perspectives. We present stable and robust operando ATR-FTIR measurements using low temperature processing of hematite and a set-up where the functional thin film is integrated on the ATR crystal. We find increased absorbance as a function of applied potential at wavenumber values of 1000 cm−1–900 cm−1 and relate this to changes in the surface species during water oxidation. We argue that this approach has the potential to be developed to a routine method for the characterization of interfaces in water splitting. Such ATR-FTIR data is of crucial importance for the validation of models in microkinetic modeling. We show some recent results of microkinetic modeling of the hematite–electrolyte interface and explain how a combination of operando ATR-FTIR measurements and microkinetic modeling enables the identification of the reaction mechanism in water splitting. We discuss how this combined approach will enable designing of tailored catalysts and accelerating their development in the future.

There is an urgent need for cheap, stable, and abundant catalyst materials for photoelectrochemical water splitting. Manganese oxide is an interesting candidate as an oxygen evolution reaction (OER) catalyst, but the minimum thickness above which MnO x thin films become OER-active has not yet been established. In this work, ultrathin (<10 nm) manganese oxide films are grown on silicon by atomic layer deposition to study the origin of OER activity under alkaline conditions. We found that MnO x films thinner than 1.5 nm are not OER-active. X-ray photoelectron spectroscopy shows that this is due to electrostatic catalyst–support interactions that prevent the electrochemical oxidation of the manganese ions close to the interface with the support, while in thicker films, MnIII and MnIV oxide layers appear as OER-active catalysts after oxidation and electrochemical treatment. From our investigations, it can be concluded that one MnIII,IV–O monolayer is sufficient to establish oxygen evolution under alkaline conditions. The results of this study provide important new design criteria for ultrathin manganese oxide oxygen evolution catalysts.

The application of a pulse shaped biasing method implemented to a capacitive probe is described. This approach delivers an accurate and simple way to determine ion fluxes in diverse plasma mixtures. To prove the reliability of the method, the ion probe was used in a different configuration, namely, a planar Langmuir probe. In this configuration, the ion current was directly determined from the I-V characteristic and compared with the ion current measured with the pulse shaped ion probe. The results from both measurements are in excellent agreement. It is demonstrated that the capacitive probe is able to perform spatially resolved ion flux measurements under high deposition rate conditions (2–20 nm/s) in a remote expanding thermal plasma in Ar/NH3/SiH4 mixture.