SrTiO3 Epitaxially Protected Si-Photocathode for Photoelectrochemical Water Splitting
Authors: Ho Hsin-Chia , Smiljanic Milutin , Jovanović Zoran , Jaklič Blaž , Kovač Janez , Čekada Miha , Hlinka Jiří , Hodnik Nejc , Spreitzer Matjaž 
Published: February 2024

Although silicon has been demonstrated to be one of the best photoelectrochemical (PEC) substrate materials thanks to its low cost, mature manufacturing technique, and high theoretical photovoltage and saturation current density [1], the development of robust Si-based PEC device has remained an existing challenge due to the fact that it is very easily susceptible to (photo)corrosion under PEC operational conditions [2]. Therefore, a suitable protective layer over the Si substrate which is able to simultaneously provide the corrosion resistance and retain a highly efficient charge transfer across the photoelectrode device is highly desired [3].
To this end, we prepared high-quality SrTiO3 (STO) layer to passivate p-Si substrate via the mediation of an epitaxial enabler reduced graphene oxide (rGO) using the pulsed laser deposition (PLD) technique. STO layers with different thicknesses were prepared on both rGO-buffered and bare Si substrates to investigate the effects of epitaxy and charge carrier migration distance on the PEC performance. It was realized that very thin (3.9 nm ~ 10 unit cells) STO layer epitaxially overgrown on rGO-buffered Si showed the highest onset potential (0.326 V vs RHE, defined as potential at -0.1 mA cm-2) than the counterparts with thicker and/or non-epitaxial STO. The photovoltage and flat-band potential analyses, and the electrochemical impedance spectroscopy measurements showed that the epitaxial protected photocathode was more beneficial for charge separation, charge transfer, and the targeted redox reaction (hydrogen evolution reaction in this case) than the non-epitaxial one. The STO/rGO/Si with a smooth and highly epitaxial STO layer outperforming the direct contacted STO/Si with textured and polycrystalline STO layer underscored the importance of having well-defined passivation layer. In addition to that, numerous pinholes formed in STO/Si attributable to highly energetic bombardment process in the PLD deposition led to a rapid degradation of photocathode during PEC measurements, possibly because the electrolyte could easily penetrate into the film and consequently corrode the substrate. Single-oriented STO realized by rGO-buffered substrate potentially rendered easier path for charge migration, and the thinnest STO layer provided the shortest transportation route while maintained the robust protection ability. Polycrystalline STO in direct contacted STO/Si however presented more recombination sites which eventually impaired the PEC performance. Smooth surface of high-quality epitaxial STO layer with sub-nano roughness demonstrated to be a pivotal factor in remotely protecting underlying p-Si substrate from corrosion, while the presence of pinholes in non-epitaxial sample showed a similar degradation rate as bare Si substrate which was without any protection. We also studied the capability of STO layer in protecting underlying Si substrate from corrosion, and the long-term stability tests demonstrated the robustness of epitaxial STO layer-coated photocathode in comparison to bare Si.
We demonstrated that STO layer overgrown on Si buffered with few-layer epitaxy enabler rGO could promote charge carrier transfer across the photocathode and lead to better PEC performance. Through a series of in-situ and ex-situ characterizations we verified that the addition of rGO layer could significantly alter the STO layer crystallinity and thin film morphology and, most importantly, enhance the stability of Si photocathode with only 10 unit cells layer. This study reported a facile approach for preparing a robust protection layer over photoelectrode substrate in realizing efficient and at the same time durable PEC device.

Acknowledgement: This study was supported by Slovenian Research Agency (Project No. N2-0187) and the Czech Science Foundation (Project No. 21-20110K). We also thank the supports from Mr. Damjan Vengust in electron microscope observations, Mr. Jožko Fišer in Pt thin film preparation, Dr. Vasko Jovanovski in discussion of electrochemical measurements.
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