Time-resolved investigations on the charge carrier dynamics of CuBi2O4 and Cu2O photocathodes for photoelectrochemical hydrogen evolution reaction

As green hydrogen starts to become a more prominent candidate for being the fuel of the future, research on solar hydrogen production methods is gaining widespread attention. Among these methods, photoelectrochemical (PEC) overall water splitting (OWS) stands out as a compact technology that aims to combine sunlight with electrochemistry by using light-absorber semiconductor materials. A conventional PEC OWS setup consists of a photoanode and a photocathode that can convert the energy of solar light into photocurrent to split water molecules into molecular oxygen and hydrogen, respectively. Although the molecular oxygen production at the photoanode surface, known as oxygen evolution reaction (OER), is the main bottleneck of PEC OWS, the achievement of a fully functional cell equally relies on the optimization of the molecular hydrogen production at the photocathode compartment, also known as hydrogen evolution reaction (HER). Therefore, the investigations carried out within the scope of this thesis focus on photocathode research, which has been in scarce volumes compared to photoanode research. In particular, two promising photocathode candidates, CuBi2O4 (CBO) and Cu2O are studied in detail with the utilization of two different approaches. The PEC activity of a photoelectrode is a vital property that is primarily dependent on the successful utilization of the charge carriers within the system. In this context, a time-resolved investigation of the charge carrier dynamics upon illumination opens new possibilities for comprehending and optimizing photoelectrodes to unlock the full potential of PEC OWS. Such charge carrier events that are influential on the generation of photocurrent in photoelectrodes can occur at timescales as short as a few femtoseconds up to a relatively longer time duration of a second. In the first part of the thesis, ultrafast polaronic charge trapping and recombination dynamics of compositionally manipulated CBO photocathodes are screened via ex situ and in situ ultrafast transient absorption spectroscopy (TAS). In the second part, interfacial charge trapping, accumulation, and utilization dynamics of Cu2O-based single or heterojunction photocathodes are investigated via transient photocurrent (TPC) experiments. Throughout the thesis, time-resolved investigations are combined with advanced structural characterization techniques and complementary PEC performance tests to develop a thorough understanding of the effects of structural differences on ultrafast charge transport and interfacial charge transfer dynamics. Intrinsically, p-type photocathodes such as CBO gain their p-type behavior from the cation vacancies that are found in their crystal lattices as natural dopant sites. Yet, such vacancy sites hold the capacity to polaronically trap valence band (VB) holes, which results in their recombination with conduction band (CB) electrons. There are two common types of cation vacancies (Cu2+ vacancy: "V" _"Cu" ^"''" and Bi3+ vacancy: "V" _"Bi" ^"'''" ) that can be found within the CBO lattice, which can act as polaronic trap sites and hinder the ultrafast charge transport. Therefore, unraveling the differences between the two types of vacancy sites in terms of polaronic charge trapping is of great importance. For this purpose, two different types of cation-deficient CBO photocathodes were fabricated. The structural and PEC properties of these Cu- and Bi-deficient CBO photocathodes were analyzed to understand their possible influence on the charge transfer processes. Later, ultrafast charge carrier dynamics screening of CBO photocathodes was carried out under ex situ and in situ conditions via ultrafast TAS measurements. Through the sequence of such experiments, the transient absorption (TA) spectral features near 460 nm and 560 nm were successfully assigned to dominant hole absorption (HA) and hole polaron absorption (HPA) character, respectively. Later, it was revealed that the "V" _"Bi" ^"'''" sites are more prone to polaronically trap the VB holes and retard the ultrafast hole transport causing poorer photocurrent generation in Bi-deficient CBO photocathodes. Heterojunction photoelectrodes are prominent devices for achieving enhanced interfacial charge transfer dynamics that allow better PEC OWS efficiencies. Therefore, the application of such a concept to one of the most popular photocathode candidates, Cu2O, has been introduced previously. Cu2O photocathodes and their heterojunctions with TiO2 are widely studied topics within the field of PEC HER research. Still, the potential-dependent interfacial charge transfer dynamics of such systems have yet to be understood. In this context, the fabrication of bare Cu2O, bare TiO2, and TiO2/Cu2O systems was carried out, and their structural properties were analyzed via surface- and bulk-sensitive characterization methods. The PEC performances of all three systems were tested under chopped light conditions through consecutive linear sweep voltammetry (LSV) scans. The Cu2O optimization experiments showed that an hour-long electrodeposition of the Cu2O layer results in larger grain sizes, higher crystallinity, and a smaller number of defects compared to shorter durations. Therefore, an hour-long electrodeposition duration was found to increase the PEC stability of photocathodes to a great extent. Later, the interfacial charge carrier dynamics of bare Cu2O, bare TiO2, and TiO2/Cu2O heterojunction systems were investigated through potential-dependent TPC measurements performed at millisecond resolution. The potential-dependent TPC responses of the bare Cu2O and heterojunction photocathodes revealed that the interfacial hole trapping is more dominant below 0.3 V for Cu2O while the heterojunction system mostly suffers from the electron trapping above 0.3 V. As a result of these findings, it was determined that the heterojunction system was subjected to less Cu reduction below 0.3 V compared to bare Cu2O, enabling the heterojunction system to have a more stable PEC performance. More importantly, the periodic oscillations observed along the photocurrent baseline are considered to stem from the side processes that occur in parallel with the PEC OWS reactions. Such periodically occuring anodic and cathodic events are expected to involve the consumption of trapped charge carrier populations and, therefore, influence the working mechanisms of photocathodes. Yet, further in situ time-resolved spectroscopic investigations are essentially required to resolve such concepts. Overall, the thesis sheds light on the charge carrier dynamics of photocathodes during their operation for PEC HER by combining time-resolved investigations with an elaborate understanding of structure and performance. Eventually, a detailed picture of the photocurrent generation mechanism in both CBO and Cu2O photocathodes is provided.