Nanostructured hematite photoelectrodes for photoelectrochemical organic reactions and all-oxide solar cells

Abstract

This thesis is concerned with the development of materials and devices in the field of renewable energies. Particular attention is devoted to the optimization of the semiconductor nanostructures and the possibility to build heterojunctions between different semiconductors for photocatalysis and solar cell applications. The activity comes within a worthwhile collaboration between the Experimental Physics research group of Luleå University of Technology and the Department of Molecular Sciences and Nanosystems of Ca’ Foscari University of Venice. The project focuses on two different semiconductors whose synthesis and characterization were performed in parallel. Different nanostructures were prepared to maximize the functional properties of the semiconductors, due to the enhancement of the optical and catalytic properties often observed in nanoscale confined materials, among enhanced light absorption, rapid light response, and elevated surface area. In the first part of the thesis is discussed hematite (α‐Fe2O3), which is a cheap and widely available n‐type semiconductor, extensively used in the last few years for water splitting and solar cell applications due to the low band‐gap and the elevated oxidation potential of the photoinduced holes. This material was studied as an oxidative photo‐catalyst and prepared on fluorine-doped tin oxide (FTO) substrates by a hydrothermal method in order to get nanorods. Hematite nanorods were characterized by means of a wide spectrum of techniques: scanning electron microscopy, UV-Vis spectroscopy, X-ray diffraction, cyclic voltammetry, Mott–Schottky plot and impedance spectroscopy. The photo‐electrochemical activities of the prepared electrodes were investigated towards the oxidation of benzylamine to n-benzylidene benzylamine, as a useful replacement of the oxygen evolution reaction, usually limiting the efficiency of the water splitting processes. The products were studied using gas chromatography and nuclear magnetic resonance spectroscopy in order to get an insight into the correlation between the structure of the prepared photoelectrodes and the photocatalytic activity. Under solar simulator irradiation, the hematite photoanodes allow for a drop of the onset potential for the oxidation process equal to 0.8V, with respect to the pure electrochemical process. At the same time, the hydrogen evolution reaction has been observed at the counterelectrode, applying an external bias as low as 0.3 V, under constant illumination. The second part of the thesis is concerned cuprous oxide (Cu2O), which is one of the few p‐type oxide semiconductors showing elevated reduction potential of photoinduced electrons and wide absorption in the visible spectrum. To date, the applications of Cu2O have mainly been in the field of environment and energy conversion, particularly catalysts, sensors and chemical templates. Some examples are degradation of pollutants, reduction of CO2 and detection of in flammable or toxic gases. In this project, cuprous oxide was studied as a reductive photo‐catalyst and produced by an electrodeposition process. The characterization was the same as hematite nanorods. The performances of Cu2O films in photoelectrochemical and photovoltaic devices were evaluated using an FTO/α‐Fe2O3/Cu2O electrode structure. This design provided a working heterojunction all-oxide solar cell, with limited photocurrent at 0V due to the poor electrical properties of the two oxide semiconductors. Nevertheless, this class of all-oxide solar cells is characterized by extremely cheap realization process and the lack of any encapsulation need due to the optimal stability, therefore we expect it to be the subject of further interest among the new generation photovoltaic devices.