The surface ligation of metal halide perovskite nanocrystals (NC) influences the size/dimensionality, optoelectronic properties, chemical stability, and use of the NCs.

The size of the band gap and the energy position of the band edges make several oxynitride semiconductors promising candidates for efficient hydrogen and oxygen production under solar light illumination. Intense research efforts dedicated to oxynitride materials have unveiled the majority of their most important properties. However, two crucial aspects have received much less attention: One is the critical issue of compositional/structural surface modifications that occur during operation and how these affect photoelectrochemical performance. The second concerns the relation between electrochemical response and the crystallographic surface orientation of the oxynitride semiconductor. These are indeed topics of fundamental importance, since it is exactly at the surface where the visible-light-driven electrochemical reaction takes place. In contrast to conventional powder samples, thin films represent the best model system for these investigations. This study reviews current state-of-theart oxynitride thin film fabrication and characterization, before focusing on LaTiO 2 N, selected as a representative photocatalyst. An investigation of the initial physicochemical evolution of the surface is reported. Then, it is shown that after stabilization the absorbed photon-to-current conversion efficiency of epitaxial thin films can differ by about 50% for different crystallographic surface orientations, and be up to 5 times larger than for polycrystalline samples.

The solar water splitting process assisted by semiconductor photocatalysts attracts growing research interests worldwide for the production of hydrogen as a clean and sustainable energy carrier. Due to their optical and electrical properties several oxynitride materials show great promise for the fabrication of efficient photocatalysts for solar water splitting. This study reports a comparative investigation of particle-and thin films-based photocatalysts using three different oxynitride materials. The absolute comparison of the photoelectrochemical activities favors the particle-based electrodes due to the better absorption properties and larger electrochemical surface area. However, thin films surpass the particlebased photoelectrodes due to their more suitable morphological features that improve the separation and mobility of the photo-generated charge carriers. Our analysis identifies what specific insights into the properties of materials can be achieved with the two complementary approaches. INTRODUCTIONThe possibility to use and store efficiently the virtually unlimited energy that we receive from the sun is the target of several research efforts since many decades.One possible strategy is to convert solar energy into chemical energy by using visible light to split the water molecule and produce O2 and H2 gas. The latter can then be stored and used when and where is needed as a clean and renewable solar fuel. Even though this research field started back in 1972 with the pioneering work of Honda and Fujishima, 1 the interest in solar water splitting is still growing. This is mainly due to the necessity of clean and renewable energy resources to face the increasing energy demand, the depletion of fossil fuels and their unsustainable environmental impact.Solar water splitting can be achieved by employing a semiconductor photocatalyst that is capable of absorbing the sunlight. When the semiconductor is irradiated, an electron-hole (e --h + ) pair is generated. The excited ein the conduction band (CB) and the h + in the valence band (VB) contribute to the water reduction and oxidation reactions, respectively. Therefore, for efficient water splitting, the CB and VB of the semiconductor must be well positioned with respect to the water redox potentials.Despite the chemical stability of oxide semiconductors and the suitable energy matching of the band edges with the water redox potentials, their performance remains restricted by their light absorption properties. In fact, due to the relatively large band gap, many oxide semiconductors can absorb photons mainly in the UV energy range which represents only a few percent of the solar spectrum. Exceptions are hematite (α-Fe2O3) and WO3 which have appeared to be promising visible-light absorbing photocatalysts. However, even though these materials have been extensively studied, the reported efficiencies are still too low to be used for practical applications. Both photocatalysts have CB edges lower than the H + /H2 redox potential which makes them only active for O2 ev...

We performed a comparative study on the photoelectrochemical performance of LaTaON loaded with NiO, NiFeO, CoO and IrO as cocatalysts. Ni-based oxides lead to the highest improvement on the photoelectrochemical performance, while CoO and IrO also enhance the performance though to a lower extent, but they simultaneously introduce more pseudocapacitive current thus resulting in an inefficient utilization of the photo-generated holes. Repetitive voltage cycling between 1.0 V and 1.6 V transforms the NiO and NiFeO into oxyhydroxides known to possess higher catalytic activities. However, these oxyhydroxides lead to lower photoelectrochemical performance compared to the as-loaded oxides, most probably due to the decay of the passivation centers at the photoelectrode-cocatalyst interface. High catalytic activities cannot be achieved without sufficient passivation of surface recombination states. Despite that the photoelectrochemical performance of LaTaON can be improved by cocatalysts, the maximum achievable photocurrent density is still not comparable to that reported for other oxynitride compounds. Our study suggests that poor electronic conductivity or severe bulk recombination of the photo-generated electron-hole pairs are the main limiting factors for the photon-to-current conversion efficiency in LaTaON photoanodes.

This study investigates the ability of Se2– redox electrolyte to separate the photoholes and stabilize Q-CdTe quantum dot solar cell with a liquid junction. We examined the photophysical and photoelectrochemical behaviors of Q-CdTe in two sizes, green-emitting dots of 2.3–2.7 nm diameter and red-emitting dots of 4 nm diameter, in the presence of alkaline Se2– electrolyte prepared under inert atmosphere. Photoelectrochemical, absorbance, emission and emission quenching measurements revealed the presence of size dependence in Se2– surface binding to Q-CdTe, growth of type II Q-CdTe/CdSe, and stability in the photoelectrochemical cell. Emission quenching measurements show that Se2– scavenges the Q-CdTe photohole, with mechanisms that depended on size and quencher concentration. Binding of Se2– to green-emitting Q-CdTe occurred with a greater binding constant compared to the red-emitting dots, resulting in formation of type II Q-CdTe/CdSe at the smaller core indicated in red-shifted absorbance and emission spectra with incremental Se2– addition at room temperature. Photoelectrochemical measurements acquired at Q-CdTe sensitized nc-TiO2 and TiO2 inverse opal with a stop band at 600 nm, 600-i-TiO2-o, in Se2– electrolyte confirmed this redox species ability to scavenge the photohole and to protect Q-CdTe against fast photoanodic dissolution, with greater stability observed for the larger dots. Gains in the photon-to-current conversion efficiency attributed to light trapping were measured at Q-CdTe sensitized 600-i-TiO2-o relative to nc-TiO2.