NEW APPROACHES FOR THE DESIGN, SCREENING AND CHARACTERIZATION OF INNOVATIVE SEMICONDUCTOR/OVERLAYER ARCHITECTURES FOR PHOTO-DRIVEN WATER ELECTROLYSIS

New approaches for the design, screening and characterization of innovative semiconductor/overlayer architectures for photo-driven water electrolysis The optimization of environmental protection and remediation and the better exploitation of renewable power sources undoubtedly represent the key issues to lead to the sustainable development of civil and industrial activities. In this context, hydrogen and electricity can be considered as the most promising and as the most adopted energy vector, respectively. The possibility of adopting the former depends on the availability of suitable devices to convert renewable energy into chemical energy. Here, the production of H2 from sunlight, by photoelectrochemical water splitting (PEC-WS), represents one of the most attractive processes giving the possibility of a direct use of sunlight to drive water splitting into molecular hydrogen and oxygen. Notwithstanding the strong interest for PEC‐WS, its development on an industrial scale is hindered by the high costs of core materials and their inadequate efficiency and/or stability. In the last years, a lot of effort has been devoted to the study of different semiconductor/electrocatalyst combinations. In particular, recent studies highlighted the ability of electrocatalyst overlayers of inducing modifications in the semiconductor electron density [1] or of storing the photogenerated holes, thus decreasing the probability of charge recombination [2,3]. This greatly extends the possible candidates for photoelectrocatalysts and requires new efficient screening methods. In this context, scanning electrochemical microscopy (SECM) is an optimal tool for the rapid screening of big libraries of materials [4-8]. In this Ph.D. thesis, innovative approaches for the study of different photoanode architectures are discussed. In the first part, preliminary results of promising OER electrocatalysts deposited onto Ti sheets are shown. The electrocatalytic activity of the different spots has been studied by SECM using a substrate generation/tip collection (SG-TC) approach, according to which the substrate spots generate molecular oxygen that is reduced at the tip. Since the spots generate O2 all together, the attention was also focused on the application of the double pulse method [9] that allows to address and compare each spot’s activity. The most performing electrocatalysts were then deposited onto a photoconverter (n-doped semiconductor), to study the photo-electrochemical behaviour of the so-obtained semiconductor/overlayer architecture. The semiconductor chosen for its low-cost, stability and favourable band position was hematite (α-Fe2O3), in the form of nanowires [10]. Various low-cost metal oxides were then deposited onto the same photoconverter layer and screened via SECM, by the double pulse method and under white light irradiation (by means of an optical fibre probe, able to move over the sample under investigation). Considering that the average spot size is of 500 µm, the spot-spot distance (centre-to-centre) is set at 800 µm and the fibre diameter is 200 µm, we can assume that the local illumination addresses only one spot at time during the screening. The SECM rapid evaluation was aimed to select the best semiconductor/overlayer combinations that were then synthesized on lab-scale photoelectrodes and tested with conventional photoelectrochemical characterization methods (CV, EIS, etc.). As already mentioned, screening of catalysts mostly involve the adoption of generation/collection modes of SECM that use the tip to locally produce one of the reactants or to locally sense the reaction products. Unfortunately, the materials available in the form of microwires useful to produce tips are limited to a short list of metals. For this reason, another part of this thesis was focused on the preparation, characterization and use of cavity-microelectrodes (C-ME) as tips for the SECM [11]. These C-ME tips [12-14] can be filled with a desired finely dispersed material and used as conventional microdisk tip in several SECM configurations. A third part of the thesis is devoted to the study of stability and performances of different semiconductors, appositely synthesised in the form of amorphous nanoparticles through a different and innovative method. This work was performed during a stay at Professor Joaquin Rodriguez-Lopez laboratories, University of Illinois at Urbana-Champaign (UIUC). The semiconductors under investigation were TiO2, WO3 and BiVO4. These materials were tested at the SECM under white light irradiation, and different conditions were applied to enhance the material photoactivity (Nafion® addition, etc.). The last part of this thesis discusses the role of different materials for the water oxidation, both deposited onto inert electrodes or semiconductors, using innovative in-situ and operando X-ray absorption (XAS) techniques. These techniques are particularly interesting when combined with electrochemistry, being able to provide information about the oxidation state and surrounding atoms. The techniques were initially applied to conventional, “model” catalysts, like amorphous iridium oxides [15,16] or Pt nanoparticles, and can be in principle extended towards any desired electrochemical or photoelectrochemical system. [1] M. Barroso, C.A. Mesa, S.R. Pendlebury, A.J. Cowana, T. Hisatomi, K. Sivula, M. Grätzel, D.R. Klug, J.R. Durrant, PNAS, 109, (2012), 15640-15645. [2] L. Badia-Bou, E. Mas-Marza, P. Rodenas, E.M. 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