Nanoporous SnO(2)-ZnO heterojunction nanocatalyst was prepared by a straightforward two-step procedure involving, first, the synthesis of nanosized SnO(2) particles by homogeneous precipitation combined with a hydrothermal treatment and, second, the reaction of the as-prepared SnO(2) particles with zinc acetate followed by calcination at 500 °C. The resulting nanocatalysts were characterized by X-ray diffraction (XRD), FTIR, Raman, X-ray photoelectron spectroscopy (XPS), nitrogen adsorption-desorption analyses, transmission electron microscopy (TEM), and UV-vis diffuse reflectance spectroscopy. The SnO(2)-ZnO photocatalyst was made of a mesoporous network of aggregated wurtzite ZnO and cassiterite SnO(2) nanocrystallites, the size of which was estimated to be 27 and 4.5 nm, respectively, after calcination. According to UV-visible diffuse reflectance spectroscopy, the evident energy band gap value of the SnO(2)-ZnO photocatalyst was estimated to be 3.23 eV to be compared with those of pure SnO(2), that is, 3.7 eV, and ZnO, that is, 3.2 eV, analogues. The energy band diagram of the SnO(2)-ZnO heterostructure was directly determined by combining XPS and the energy band gap values. The valence band and conduction band offsets were calculated to be 0.70 ± 0.05 eV and 0.20 ± 0.05 eV, respectively, which revealed a type-II band alignment. Moreover, the heterostructure SnO(2)-ZnO photocatalyst showed much higher photocatalytic activities for the degradation of methylene blue than those of individual SnO(2) and ZnO nanomaterials. This behavior was rationalized in terms of better charge separation and the suppression of charge recombination in the SnO(2)-ZnO photocatalyst because of the energy difference between the conduction band edges of SnO(2) and ZnO as evidenced by the band alignment determination. Finally, this mesoporous SnO(2)-ZnO heterojunction nanocatalyst was stable and could be easily recycled several times opening new avenues for potential industrial applications.

Ultrasmall, crystalline, and dispersible NiO nanoparticles are prepared for the first time, and it is shown that they are promising candidates as catalysts for electrochemical water oxidation. Using a solvothermal reaction in tert‐butanol, very small nickel oxide nanocrystals can be made with sizes tunable from 2.5 to 5 nm and a narrow particle size distribution. The crystals are perfectly dispersible in ethanol even after drying, giving stable transparent colloidal dispersions. The structure of the nanocrystals corresponds to phase‐pure stoichiometric nickel(ii) oxide with a partially oxidized surface exhibiting Ni(iii) states. The 3.3 nm nanoparticles demonstrate a remarkably high turn‐over frequency of 0.29 s–1 at an overpotential of g = 300 mV for electrochemical water oxidation, outperforming even expensive rare earth iridium oxide catalysts. The unique features of these NiO nanocrystals provide great potential for the preparation of novel composite materials with applications in the field of (photo)electrochemical water splitting. The dispersed colloidal solutions may also find other applications, such as the preparation of uniform hole‐conducting layers for organic solar cells.

9We report on the development of high performance triple and quadruple junction solar cells 10 made of amorphous (a-Si:H) and microcrystalline silicon (µc-Si:H) for the application as 11 photocathodes in integrated photovoltaic-electrosynthetic devices for solar water splitting. We 12 show that the electronic properties of the individual sub cells can be adjusted such that the 13 photovoltages of multijunction devices cover a wide range of photovoltages from 2.0 V up to 14 2.8 V with photovoltaic efficiencies of 13.6 % for triple and 13.2 % for quadruple cells. The 15 ability to provide self-contained solar water splitting is demonstrated in a PV-biased 16 electrosynthetic (PV-EC) cell. With the developed triple junction photocathode in the a-17 Si:H/a-Si:H/µc-Si:H configuration we achieved an operation photocurrent density of 7.7 18 mA/cm 2 at 0 V applied bias using a Ag/Pt layer stack as photocathode/electrolyte contact and 19 ruthenium oxide as counter electrode. Assuming a faradic efficiency of 100 %, this 20 corresponds to a solar-to-hydrogen efficiency of 9.5 %. The quadruple junction device 21 provides enough excess voltage to substitute precious metal catalyst, such as Pt by more Graphical AbstractBias-free solar water splitting is demonstrated using thin film silicon based triple and quadruple junction solar cells with solar-to-hydrogen efficiencies up to 9.5 %.

Nanoporous RuO 2 /TiO 2 heterostructures, in which ruthenium oxide acts as a quasi-metallic contact material enhancing charge separation under illumination, were prepared by impregnation of anatase TiO 2 nanoparticles in a ruthenium-(III) acetylacetonate solution followed by thermal annealing at 400 °C. Regardless of the RuO 2 amount (0.5−5 wt %), the asprepared nanocatalyst was made of a mesoporous network of aggregated 18 nm anatase TiO 2 nanocrystallites modified with RuO 2 according to N 2 sorption, TEM, and XRD analyses. Furthermore, a careful attention has been paid to determine the energy band alignment diagram by XPS and UPS in order to rationalize charge separation at the interface of RuO 2 /TiO 2 heterojunction. At first, a model experiment involving stepwise deposition of RuO 2 on the TiO 2 film and an in situ XPS measurement showed a shift of Ti 2p 3/2 core level spectra toward lower binding energy of 1.22 eV which was ascribed to upward band bending at the interface of RuO 2 /TiO 2 heterojunction. The band bending for the heterostructure RuO 2 /TiO 2 nanocomposites was then found to be 0.2 ± 0.05 eV. Photocatalytic decomposition of methylene blue (MB) in solution under UV light irradiation revealed that the 1 wt % RuO 2 /TiO 2 nanocatalyst led to twice higher activities than pure anatase TiO 2 and reference commercial TiO 2 P25 nanoparticles. This higher photocatalytic activity for the decomposition of organic dyes was related to the higher charge separation resulting from built-in potential developed at the interface of RuO 2 /TiO 2 heterojunction. Finally, these mesoporous RuO 2 −TiO 2 heterojunction nanocatalysts were stable and could be recycled several times without any appreciable change in degradation rate constant that opens new avenues toward potential industrial applications.

N-doped carbon materials are discussed as catalyst supports for the electrochemical oxygen reduction reaction (ORR) in fuel cells. This work deals with the preparation of Pt nanoparticles (NPs) supported on N-doped carbon nanofibers (N-CNF) from a polyaniline nanofiber (PANI NF) precursor, and investigates the ORR activity of the produced materials. Initially, Pt NPs are deposited on PANI NFs. The PANI NF precursors are characterized by near-edge X-ray absorption fine structure (NEXAFS) and transmission electron microscopy (TEM) measurements. It is shown, that in the PANI NF precursor materials electrons from the Pt are being transferred toward the π-conjugated systems of the aromatic ring. This strong interaction of Pt atoms with PANI explains the high dispersion of Pt NPs on the PANI NF. Subsequently, the PANI NF precursors are carbonized at different heattreatment conditions resulting in structurally different N-CNFs which are characterized by NEXAFS, X-ray photoelectron spectroscopy (XPS) ,and TEM measurements. It is shown that an interaction between N-groups and Pt NPs exists in all investigated N-CNFs. However, the N-CNFs differ in the composition of the N-species and the dispersion of the Pt NPs. A small mean Pt NP size with a narrow size distribution is attributed to the presence of pyrdinic N-groups in the N-CNFs, whereas, for the N-CNFs with mainly graphitic and pyrrolic N-groups, an increase in the average Pt NP size with a broad size distribution is found. The ORR activity in alkaline media investigated by Koutecky−Levich analysis of rotating disk electrode measurements showed a largely enhanced ORR activity in comparison to a conventional Pt/C catalyst.