DESIGN AND CHARACTERIZATION OF CU2O PHOTOCATHODES FOR PHOTOELECTROCHEMICAL WATER SPLITTING.

The exploitation of renewable energy sources is one of the most addressed aspects for the sustainable development of human activities. Molecular hydrogen can be considered one of the most interesting energy vectors to fulfill humankind’s needs. In this context, photoelectrochemical water splitting (PEC-WS) using solar energy to produce H2 at semiconducting materials is considered one of the most interesting technologies. In the present study, Cu2O was selected as cathode material for its correct band position for hydrogen evolution, together with non–toxic and low-cost starting material. The research activity was devoted to the development of photocathodes for the PEC-WS and their characterization under working conditions with different techniques. The work was divided in: I) The role of the underlayer in Cu2O photocathodes for PEC-WS; II) Characterization of copper oxide based materials under PEC-WS by X-Ray Absorption Spectroscopy (XAS) III) Characterization of Photoactive Semiconductor Materials by Cavity Micro-Electrodes (C_ME) & Scanning ElectroChemical Microscopy (SECM) IV) Development of new protective layers for the PEC_WS systems: FeOOH and CuO/Cu2O core-shell systems V) Application of the Density Function Theory (DFT) to study doping materials and vacancy formation in Cu2O 1) The complete electrode scheme is constituted by (i) a transparent conductive support (usually FTO), (ii) a conductive underlayer of electrodeposited Cu, that is assumed to enhance the electron-hole separation (iii) the semiconductor, (iv) a thin transparent protective overlayer of about 80 nanometers. In the present study, the underlayer of Cu replaces the expensive and toxic Cr/Au underlayer. Therefore, a specific protocol for the preparation of the Cu2O photoconverter was developed and validated, obtaining good results in terms of generated photocurrent (Figure 1). Appropriate modifications of underlayer deposition conditions also lead to large increase in the Cu2O photocurrents, thus denoting that the underlayer has a marked influence on the system performances. In the final deposition protocol, several parameters were defined and controlled: deposition potential and the relevant current intensity; temperature, pH and stirring conditions; loading (C/cm2) of Cu(0) underlayer and Cu2O active layer. In particular, the thickness of the Cu underlayer was controlled in order to obtain a transparent layer while maintaining high electric conductivity. Transparency of the entire support is an important feature, allowing the use of the electrode in both front and back illumination configurations. Moreover, a complete study of the copper lactate bath was performed with electrochemical methods and X-ray Absorption Spectroscopy (XAS). The experiments show that the best fit is obtained by a model where four lactate ions act as monodentate ligands, 1:4 (Cu:L), in a distorted tetrahedral geometry. Notwithstanding the good results in photocurrent, the actual Cu2O photoconverter is able to work at the highest potential only for a few minutes and for this reason the development of a protective overlayer was mandatory as explained later. 2) The short life of Cu2O photoconverter is due to photodegradation. In order to investigate this phenomenon, XAS measurements at Cu-K edge were performed to define photodegradation products, individuate stability potential window and elucidate the possible combined effects of light and potential. In-situ and operando techniques like X-Ray Absorption Near Edge Structure (XANES), Extended X-Ray Absorption Fine Structure (EXAFS) and Fixed Energy X-Ray Absorption Voltammetry (FEXRAV) allow us to better understand material behavior. Changes in copper oxidation states upon light and/or applied potential were observed and with XANES was possible to evaluate the amount of photo-generated Cu(0) responsible of the loss of activity (Figure 2A). FEXRAV measurements allow following the material (photo)degradation and defining the stability windows (Figure 2B). With difference light and dark XANES spectra the local changes in electronic structure upon spectro-electrochemical conditions were also investigated. 3) The photodegradation reaction showed by Cu2O, is common to many other materials for HER and OER (Oxygen Evolution Reaction,). This instability occurs when the redox potential of the material lies between valence band and conduction band. This implies that the photocurrents recorded in normal pulsed experiments are composed by the sum of water splitting and photodegradation. The novel method here presented allows the evaluation of the intensity of the photo-degradation processes, giving at the same time a rapid screening tool for differently prepared materials. Moreover, this method allows to evaluate the activity of a semiconductor without any influence of the supporting material. In addition, low experimental times and low amount of photo-produced material are required if compared to actual methods (GC and volume displacement). Scanning ElectroChemical Microcopy (SECM), here used in Tip Generation/ Substrate Collection mode (TG/SC), allows to discriminate between water splitting and photo-degradation. The system is so composed: • Tip (working electrode 1) is a micro cavity electrode filled with the semiconducting powder. The electrode potential is varied in the potential window of interest under light steps. • Substrate is a Pt foil (working electrode 2) with potential fixed at the H2 (or O2) oxidation (reduction) value, working as a “probe” for the species of interest. Several materials were studied with good results: CuXO, CuI, NiO and TiO2. Influence of cavity depth and light intensity were tested too. From data analysis we can demonstrate different photocurrent efficiencies for the studied materials. Obviously TiO2 is near to 100% (Figure 3B) as expected from its very high stability while other materials, as CuI, in spite the high photocurrents, shows very low amount of photogenerated H2. 4) Previously, we have seen how Cu2O photodegradation is due to the combination of light and potential. The phenomenon occurs because the photogenerated electrons have enough energy to reduce water but also to reduce the material itself. For this reason, the development of a protective layer that can avoid the photodegradation is of paramount importance. Different materials were considered, and the final choice falls on the two phases of iron oxyhydroxide, α-FeOOH and γ-FeOOH, non-toxic and low-cost materials, deeply studied because they are corrosion products of chlorate process cathodes. These materials were here studied by electrochemical and XAS techniques to gain information about their oxidation state during HER and its reversibility. Then the final interaction of FeOOH and Cu2O was studied to explore the possibility of protecting the Cu(I) oxide by 80 nm of a transparent FeOOH layer. A different approach was then adopted with the preparation of a CuO/Cu2O core-shell material (denoted as CuXO). CuXO is composed by a CuO core and an external shell of Cu2O with the proportion determined via FEXRAV. Cu2O is grown on CuO by cycling the potential as in Figure 4, and eventually a stable and active material for HER is obtained. We were also able to assess that CuO is not an active species for HER, but also that the electrochemically generated Cu2O (the true active material for HER) is protected from photo-degradation by the CuO core. Our explanation for this protective action lies in the relative band position of CuO and Cu2O. CuO has a smaller band gap (1.5eV) that lies inside the larger band gap of Cu2O. FEXRAV (Figure 4B) supports this explanation, since the signal becomes constant in time, to indicate that the material transformation is stopped and that the only reaction occurring at the electrode is hydrogen evolution. These results are in agreement with the high photoefficiency of the CuXO powder determined with SECM. 5) According to literature the p-type character of Cu2O is strictly related to the formation of copper vacancies in its lattice. Doping can enhance the formation of vacancies, improving the number of majority carriers (holes) and their mobility, and modifying the band gap by shifting the Cu2O states or by introducing an intermediate band within the band gap (Figure 5B). On the basis of the electrochemical results obtained with different underlayers, this work firstly aims to study a large array of potentially effective metal underlayers. The role of Density Functional Theory (DFT) is here to rank the underlayers’ aptitude in modifying the properties of Cu2O, and, in particular, the formation of copper or oxygen vacancies in the semiconductor. Then, the doping of the material is studied for a selected number of transition metals. For each material, the valence and conduction band positions were computed as well as the number of vacancies and their formation energies. Eventually the influence of lattice strain on material band gap and energy levels is studied both with expansion and reduction of the Cu2O lattice after the deposition onto different substrates. The use of the alkaline metals as well as atomic hydrogen as dopants allowed to evaluate the influence of the dopant size on the doped material.