Fundamental charge transfer and utilization processes of CdS photoanodes for photoelectrochemical hydrogen production from H2O and H2S splitting

Photoelectrochemical (PEC) green hydrogen production is one of the most important clean energy technologies as an efficient, facile, and promising approach to produce green hydrogen which is a carbon neutral, storable, and transportable alternative to fossil fuels. There are different reactions which are utilized for hydrogen production and the most important one is H2O splitting as a non-toxic and available source. Another very important, yet less studied reaction is H2S splitting which is a thermodynamically less hindered process than H2O. However, its applications remained unconsidered due to experimental difficulties and its high toxicity. H2S is a very critical subject for refinery industry, and it will be even more important as green hydrogen production gains more attention. Over time, serious consequences of global warming and the rising energy demand of the society have initiated extensive research on semiconductor materials that can be utilized as photoelectrodes. Among all, CdS is suitable as a photoanode for solar to hydrogen energy conversion because of its narrow bandgap, suitable band-edge alignment for H2O and H2S splitting half reactions, and facile fabrication. However, it suffers from photocorrosion and instability due to surface charge recombination. Extensive PEC characterization studies showed that improving hole extraction and utilization on the CdS surface is crucial for efficient PEC conversion. So far, the research on CdS was more focused on modifying it with different materials to benefit from its superior qualities rather than understanding the reasons behind activity loss. In this dissertation, the fundamental charge carrier dynamics at photoanode/electrolyte interface and possible pathways for light-induced deactivation of pristine CdS were studied in detail by using transient photocurrent (TPC) measurements. H2O and H2S were used as hydrogen sources. For the H2O splitting, we showed that hexadecyltrimethylammonium bromide (CTAB) coordinated to surface sulfur vacancies facilitates the charge separation and utilization by passivating the electron trapping on the surface. Tuning the surface characteristics of the CdS photoanodes with the CTAB diminished the surface hole accumulation and increased the injection efficiency from 0.61 to 0.99 at 1 V by a surface passivation mechanism where Br– of CTAB occupies surface sulfur vacancies and acts like a hole-extracting layer on the CdS surface, passivates the surface charge recombination, and improves efficiency. Secondly, atomic layer deposition (ALD) was used to deposit a thin layer of TiO2 on CdS surface, which can be used to suppress electron trapping. Although ultrathin passivation overlayers are well-known for their positive effect on PEC activity of photoelectrodes, TiO2 on CdS is an inefficient way to passivate charge trapping according to the findings of this study. In fact, it exhibited negative effect on PEC water splitting activity especially with high overlayer thickness. We used X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX) to monitor the TiO2 overlayer growth and investigated the surface, optical and crystal structure properties by UV-vis spectroscopy and X-ray diffraction (XRD) analysis. Chronoamperometry (CA) measurements at different potentials were used to investigate the change in TPC behavior with different TiO2 overlayer thickness. For the H2S splitting, we tested bare and TiO2 modified CdS photoanodes and analyzed the stability and charge transfer processes by using surface sensitive and PEC characterization methods such as the XPS, CA and TPC in H2S feeded alkaline electrolyte. The stability and PEC activity of CdS were rather promising in the presence of H2S considering its poor stability in H2O splitting. After providing an electron trapping mechanism for CdS where surface sulfur vacancies and light-induced S2− dissolution play an important role in the deactivation process, H2S feeded electrolyte was found to have a recovery effect for CdS photocorrosion. Furthermore, TiO2 overlayers showed a reversed effect in H2S splitting by increasing the photocurrent density of bare CdS by 53% at 0.8 V. This work might be an important contribution for further studies on CdS and other photoelectrodes to enhance the understanding of charge utilization processes at photoanode-electrolyte interfaces by providing a detailed and systematic analysis of the charge carrier dynamics.