Electrochemical impedance spectroscopy, photoelectron spectroscopy, and Kelvin probe measurements on various TiO 2 single-crystal surfaces show that the position of the Fermi level and the conduction band minimum depend significantly on the sample environment (vacuum, air, water vapor, and aqueous or aprotic electrolyte solutions). In most cases, the conduction-band minimum increases in the series: anatase < rutile < brookite. The offset between anatase and rutile indicates a type II staggered alignment if the environment is vacuum, air, water vapor, or aprotic electrolyte solution, but a type IV aligned configuration is found in an aqueous electrolyte solution. The photoelectron spectra in water vapor reveal a strong upshift of the conduction band, which is nearly reversible in the early stages of water/titania interactions. Our results rationalize various earlier contradictions and highlight the need for proper analytical techniques and experimental conditions for investigation of the band energetics, which is relevant to practical applications of titania materials.

Titanium dioxide (anatase, rutile) and quasi-amorphous tin dioxide are prepared on F-doped SnO2 in the form of dense thin films, which can serve as electron-selective layers in perovskite solar cells and dye-sensitized solar cells (DSSCs). The present study brings new data about electronic and electrochemical properties of these films at the authentic conditions occurring in a dye-sensitized solar cell (DSSC). Hydrolysis of TiCl4 provides pure rutile TiO2 at low temperatures, but TiO2 (anatase) grows in these layers upon calcination. In acetonitrile medium, the flat band potential of TiO2 (rutile) is more negative than that of TiO2 (anatase). This is opposite ordering to that observed in aqueous media. The energy of conduction band minimum of TiO2 (anatase) equals −4.15 ± 0.07 eV at the conditions mimicking the DSSC’s environment. Electrochemical reductive doping of SnO2 provides a material with the most negative flat band potential and the largest overpotential for the reduction of I3 –, Co­(bpy)3 3+, and Cu­(tmby)2 2+. Voltammetric screening of all the electrode materials in six different electrolyte solutions, relevant to DSSC applications, gives salient information about the mediator type and effects of calcination and the addition of 4-tert-butylpyridine. These data provide novel inputs for optimization of DSSCs and for perovskite photovoltaics, too.

The effect of particle size and support on the catalytic performance of supported subnanometer copper clusters was investigated in the oxidative dehydrogenation of cyclohexene. From among the investigated seven size-selected subnanometer copper particles between a single atom and clusters containing 2 to 7 atoms, the highest activity was observed for the titania-supported copper tetramer with 100 % selectivity towards benzene production, and being about an order of magnitude more active than all the other investigated cluster sizes on the same support, but also the same tetramer on the other supports Al2O3, SiO2, and SnO2. In addition to the profound effect of clusters size on activity and with Cu4 outstanding from the studied series, Cu4 clusters supported on SiO2 provide an example of tuning selectivity through support effects when this particular catalyst produces also cyclohexadiene with about 30% selectivity. Titania-supported Cu5 and Cu7 clusters supported on TiO2 produce a high fraction of cyclohexadiene in contrary to their neighbors, while Cu4 and Cu6 produce solely benzene without any combustion, thus representing odd-even oscillation of selectivity with the number of atoms in the cluster.

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The growth of two-dimensional (2D) materials directly on the substrates that are relevant to device fabrication is crucial for their large-area production and application. This is because their production via transfer processes not only increases the costs but, more importantly, induces contamination and mechanical defects in the transferred material. The presence of a dielectric interface layer and the control of its thickness in transistors and p−n heterojunctions are essential aspects in the semiconductor industry. In the present work, MoS 2 flakes and films with thicknesses down to the monolayer limit were grown using chemical vapor deposition (CVD) on Si substrates covered with a native oxide layer. The high quality of the as-grown MoS 2 resting on a flat SiO 2 surface was documented by a combination of atomic force microscopy, optical spectroscopy, including tip-enhanced photoluminescence spectroscopy, and photoelectron microspectroscopy methods. The changes of the interfacial oxide were then interrogated using spectroscopic imaging ellipsometry and X-ray photoelectron spectroscopy, both with micrometer scale resolution, to show the increase of the oxide layer thickness by several nanometers during the heating and MoS 2 growth processes. Our results evidence the possibility of growing high-quality MoS 2 directly on thin dielectrics. However, at the same time, if this type of MoS 2 deposition is to be used for device fabrication, the simultaneous increase of the SiO 2 thickness makes it important to have proper knowledge and control of the growth process. For the applications in energy harvesting where only a thin (or none) insulating layer is required, alternative growth protocols, surface passivation, or a different dielectric material (e.g., Al 2 O 3 ) are suggested.

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Electrochemical impedance spectroscopy is used to study novel cathode materials for lithium-sulfur batteries, based on commercial carbon and titania. Coin cells with Li anode are investigated at various stages of galvanostatic cycling. For comparison, also symmetrical coin cells with a pair of positive (S/C/TiO2) or negative (Li) electrodes are studied. In addition to the application of titania as a barrier material impeding the polysulfide diffusion in the electrolyte solution, the inherent Li-insertion activity of TiO2 (anatase) and its contribution to the sulfur redox reactions is discussed.