Effect of Adhesion Layer at the Pt/TiO2 Interface on the Degradation Pathways of TiO2-Protected III-V Water-Splitting Photocathodes

Photoelectrochemical (PEC) water splitting has been identified as a promising route towards the production of green hydrogen as a solar fuel to meet the global energy demands [1]. Highly efficient photoelectrodes made of III-V compounds are promising candidates to reach the goal of >15% solar-to-hydrogen efficiency [2]. However, the main bottle neck of these materials is their poor stability in direct contact with acidic electrolytes, which is needed to ensure efficient mass transport and an efficient hydrogen evolution reaction (HER). Atomic layer deposited (ALD) titanium dioxide (TiO2) is commonly utilized as a protective layer to shield the absorber from the electrolyte [3]. To reduce HER overpotentials, the TiO2 film needs to be modified with a catalyst layer. In a demonstration of a TiO2 protected III-V PEC device, Cheng et al. proposed that the detachment of the catalyst layer induced the degradation of the protective TiO2 film in a pH 0 electrolyte which eventually led to the corrosion of the photoabsorber [4]. In this work, GaInP/GaAs/Ge photocathodes for water reduction were protected with ALD TiO2 layers deposited from tetrakis(dimethylamido)titanium (TDMAT) and water precursors with varying thicknesses. A 2 nm catalytic Pt layer was deposited via electron beam evaporation (EBE) to reduce the overpotential of the HER. The conformal coverage of the absorber with TiO2 was confirmed with atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). None of the TiO2 films were able to stabilize the illuminated III-V electrodes for 60 min at 0 V versus the counter electrode (CE) in a pH 0 electrolyte (1 M H2SO4). Characterization of the electrochemical potential at the electrode’s front side revealed an unstable regime for the TiO2 protection layer. ICP-MS of the electrolytes, SEM/EDX and electrochemical analyses revealed the detachment of the catalytic Pt layer that led to the dissolution of the TiO2 and the III-V degradation (Figure 1A). The adhesion of Pt on TiO2 was improved by depositing a 2 nm Ti layer via EBE at the Pt/TiO2 interface (Figure 1B). The adhesion layer acts as a wetting layer for Pt, reducing the nucleation energy barrier and increasing the number of nucleation sites compared to Pt/TiO2. The enhanced wetting is due to the formation of Ti−Pt chemical bonds [5] and the overpotential @ 10 mA/cm2 to perform HER was 60 mV, similar to that of a pristine EBE Pt layer. The Ti layer greatly improves the stability of TiO2/III-V photoelectrodes, confirming the results of Cheng et al. and illustrating the importance of a stable catalyst layer. Figure 1. A) SEM micrograph of degradation of Ge/GaAs/GaInP/TiO2/Pt photocathode after 60 min. of photocurrent measurements at 0 V vs. CE in 1 M H2SO4. B) Accelerated stability test in dark for 2 nm Pt/15 nm TiO2 on FTO with (red curve) and without (black curve) a 2 nm Ti adhesion layer in between the Pt and TiO2 layers. References: [1] Segev, G.; Kibsgaard, J.; Hahn, C.; Xu, Z. J.; Cheng, W.-H.; Deutsch, T. G.; Xiang, C.; Zhang, J. Z.; Hammarström, L.; Nocera, D. G.; Weber, A. Z.; Agbo, P.; Hisatomi, T.; Osterloh, F. E.; Domen, K.; Abdi, F. F.; Haussener, S.; Miller, D. J.; Ardo, S.; McIntyre, P. C.; Hannappel, T.; Hu, S.; Atwater, H.; Gregoire, J. M.; Ertem, M. Z.; Sharp, I. D.; Choi, K.-S.; Lee, J. S.; Ishitani, O.; Ager, J. W.; Prabhakar, R. R.; Bell, A. T.; Boettcher, S. W.; Vincent, K.; Takanabe, K.; Artero, V.; Napier, R.; Cuenya, B. R.; Koper, M. T. M.; Van De Krol, R.; Houle, F., The 2022 solar fuels roadmap. Journal of Physics D: Applied Physics 2022, 55 (32). [2] Tournet, J.; Lee, Y.; Karuturi, S. K.; Tan, H. H.; Jagadish, C., III–V Semiconductor Materials for Solar Hydrogen Production: Status and Prospects. ACS Energy Letters 2020, 5 (2), 611-622. [3] Ros, C.; Andreu, T.; Morante, J. R., Photoelectrochemical water splitting: a road from stable metal oxides to protected thin film solar cells. Journal of Materials Chemistry A 2020, 8 (21), 10625-10669. [4] Cheng, W.-H.; Richter, M. H.; May, M. M.; Ohlmann, J.; Lackner, D.; Dimroth, F.; Hannappel, T.; Atwater, H. A.; Lewerenz, H.-J., Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency. ACS Energy Letters 2018, 3 (8), 1795-1800. [5] Todeschini, M.; Bastos da Silva Fanta, A.; Jensen, F.; Wagner, J. B.; Han, A., Influence of Ti and Cr Adhesion Layers on Ultrathin Au Films. ACS Applied Materials & Interfaces 2017, 9 (42), 37374-37385. Figure 1