Cu 2 O has been considered as candidate material for transparent conducting oxides and photocatalytic water splitting. Both applications require suitably tuned band gaps. Here we explore the influence of compressive and tensile strain on the band gap by means of density functional theory (DFT) modeling. Our results indicate, that the band gap decreases under tensile strain while it increases to a maximum under moderate compressive strain and decreases again under extreme compressive strain. This peculiar behavior is rationalized through a detailed analysis of the electronic structure by means of density of states (DOS), density overlap region indicators (DORI) and crystal overlap Hamilton populations (COHP). In contrary to previous studies we do not find any indications that the band gap is determined by d10-d10 interactions. Instead, our analysis clearly shows that both the conduction and valence band edges are determined by Cu−O antibonding states. The band gap decrease under extreme compressive strain is associated with the appearance of Cu 4sp states in the conduction band region.

Three-dimensional printed multi-purpose electrochemical devices for X-ray absorption spectroscopy are presented in this paper. The aim of this work is to show how three-dimensional printing can be a strategy for the creation of electrochemical cells for in situ and in operando experiments by means of synchrotron radiation. As a case study, the description of two cells which have been employed in experiments on photoanodes for photoelectrochemical water splitting are presented. The main advantages of these electrochemical devices are associated with their compactness and with the precision of the three-dimensional printing systems which allows details to be obtained that would otherwise be difficult. Thanks to these systems it was possible to combine synchrotron-based methods with complementary techniques in order to study the mechanism of the photoelectrocatalytic process.

α-FeOOH (goethite) and γ-FeOOH (lepidocrocite) were found to be the main corrosion products of the steel cathode in the sodium chlorate process; the identification of the phases formed under reducing potentials, along with the study of the electrodes during the reoxidation, is fundamental to understanding their role in this process. In this work, FeOOHbased electrodes were investigated through in situ and in operando X-ray absorption spectroscopy (XAS), combined to electrochemical measurements (e.g., voltammetry and chronoamperometry). At sufficiently negative potentials (below −0.4 V vs RHE ca.) and under hydrogen evolution conditions an unknown iron(II)-containing phase is formed. A comprehensive analysis of the whole XAS spectrum allowed proposing a structure bearing a relation with that of green rust (space group P3̅ 1m). This phase occurs independently of the nature of the starting electrode (α-or γ-FeOOH). During electrochemical reoxidation, however, the original phase is restored, meaning that the reduced phase brings some memory of the structure of the starting material. Spontaneous reoxidation in air suppresses the memory effect, producing a mixture of α and γ phases.

Energy-dispersive X-ray absorption spectroscopy was applied, aimed at solving the problem of the structure and stability of a copper(II) lactate complex in alkaline solution, used as a precursor for the electrodeposition of CuO. The application of multiple scattering calculations to the simulation of the X-ray absorption near-edge structure part of the spectra allowed an accurate resolution of the structure: the copper(II) cation is surrounded by four lactate ions in a distorted tetrahedral environment, with the lactate anions acting as monodentate ligands. This results in an atomic arrangement where copper is surrounded by four oxygen atoms located at quite a short distance (ca. 1.87 Å) and four oxygen atoms located quite far apart (ca. 3.1-3.2 Å). The complex was finally found to be stable in a wide range of applied potentials.

This work aims at reviewing the most impactful results obtained on the development of Cu-based photocathodes. The need of a sustainable exploitation of renewable energy sources and the parallel request of reducing pollutant emissions in airborne streams and in waters call for new technologies based on the use of efficient, abundant, low-toxicity and low-cost materials. Photoelectrochemical devices that adopts abundant element-based photoelectrodes might respond to these requests being an enabling technology for the direct use of sunlight to the production of energy fuels form water electrolysis (H2) and CO2 reduction (to alcohols, light hydrocarbons), as well as for the degradation of pollutants. This review analyses the physical chemical properties of Cu2O (and CuO) and the possible strategies to tune them (doping, lattice strain). Combining Cu with other elements in multinary oxides or in composite photoelectrodes is also discussed in detail. Finally, a short overview on the possible applications of these materials is presented.

Cu2O is one of the most studied semiconductors for photocathodes in photoelectrochemical water splitting (PEC-WS). Its low stability is counterbalanced by good activity, provided that a suitable underlayer/support is used. While Cu2O is mostly studied on Au underlayers, this paper proposes Cu(0) as a low-cost, easy to prepare and highly efficient alternative. Cu and Cu2O can be electrodeposited from the same bath, thus allowing in principle to tune the final material's physicochemical properties with high precision with a scalable method.Electrodes and photoelectrodes are studied by means of electrochemical methods (cyclic voltammetry, Pb underpotential deposition) and by ex-situ X-ray absorption spectroscopy (XAS).While the potential applied for the deposition of Cu has no influence on the bulk structure and on the 2 photocurrent displayed by the semiconductor, it plays a role on the dark currents, making this strategy promising for improving the material's stability. Au/Cu2O and Cu/Cu2O show similar performances, the latter having clear advantages in view of future use in practical applications.The influence of Cu underlayer thickness was also evaluated in terms of obtained photocurrent.

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