• Valden, Mika
• Ali-Löytty, Harri
• Palmolahti, Lauri Johannes
• Hannula, Markku
• Lahtonen, Kimmo
• Grönbeck, Henrik
• Tukiainen, Antti
• Kauppinen, Minttu M.
• Saari, Jesse

Abstract

Titanium dioxide (TiO₂) is an ideal material of choice for protective photoelectrode coatings thanks to its intrinsic chemical stability, transparency to visible light and defect-mediated charge transfer properties. Both amorphous and crystalline TiO₂ can serve as a protection layer for semiconductor materials that are inherently unstable under photoelectrochemical (PEC) conditions. [1] Ti³⁺ defects within amorphous TiO₂ (am-TiO₂) can enable polaron hopping-mediated charge carrier transport through a protective am-TiO₂ photoelectrode coating [2]. Crystalline TiO₂ (c-TiO₂) can also exhibit sufficient charge carrier transport properties in case of a suitable band alignment with the photoelectrode [3]. Post-deposition annealing (PDA) treatments that are required for optimal coating performance should be performed at low enough temperatures to prevent growth of interfacial oxides that are detrimental to the charge transfer [4]. The choices of atomic layer deposition (ALD) process parameters are interrelated with the required PDA treatments and photoelectrode coating performance.<br/><br/>Our most recent work [5] examines Ti³⁺-rich am.-TiO₂ thin films grown by ALD at growth temperature of 100–200 °C using tetrakis(dimethylamido)titanium(IV) (TDMAT) and H₂O as the precursors. X-ray photoelectron spectroscopy (XPS) analysis and density functional theory (DFT) calculations allowed us to identify structural disorder-induced penta- and heptacoordinated Ti⁴⁺ ions (Ti_5/7c⁴⁺), which are interrelated to the formation of Ti³⁺ defects in am.-TiO₂. Furthermore, experimental and computational results support the formation of Ti³⁺ defects in am.-TiO₂ structure without releasing oxygen, i.e., simultaneous formation of oxygen vacancies and interstitial peroxo species leading to defective but stoichiometric am.-TiO₂. Upon PDA in air, Ti³⁺-rich am.-TiO₂ thin film crystallizes directly into rutile (grain size &lt;1 µm) at unprecedentedly low temperature of 250 °C. In addition to benefits as photoelectrode coating, the low-temperature synthesis enables photocatalytic applications involving temperature sensitive materials.<br/>1. D. Bae, B. Seger, P. C. K. Vesborg, O. Hansen, I. Chorkendorff, “Strategies for Stable Water Splitting via Protected Photoelectrodes,” Chem. Soc. Rev. 46, pp. 1933–1954, 2017<br/>2. P. Nunez, M. H. Richter, B. D. Piercy, C. W. Roske, M. Cabán-Acevedo, M. D. Losego, S. J. Konezny, D. J. Fermin, S. Hu, B. S. Brunschwig, N. S. Lewis, “Characterization of Electronic Transport through Amorphous TiO₂ Produced by Atomic Layer Deposition,” J. Phys. Chem. C 123, pp. 20116–20129, 2019<br/>3. B. Mei, T. Pedersen, P. Malacrida, D. Bae, R. Frydendal, O. Hansen, P. C. K. Vesborg, B. Seger, I. Chorkendorff, “Crystalline TiO₂: A Generic and Effective Electron-Conducting Protection Layer for Photoanodes and -cathodes,” J. Phys. Chem. C 119, pp. 15019–15027, 2015<br/>4. J. Saari, H. Ali-Löytty, M. Honkanen, A. Tukiainen, K. Lahtonen, M. Valden, “Interface Engineering of TiO₂ Photoelectrode Coatings Grown by Atomic Layer Deposition on Silicon,” ACS Omega 6, pp. 27501–27509, 2021<br/>5. J. Saari, H. Ali-Löytty, M. M. Kauppinen, M. Hannula, R. Khan, K. Lahtonen, L. Palmolahti, A. Tukiainen, H. Grönbeck, N. V. Tkachenko, M. Valden, “Tunable Ti³⁺-Mediated Charge Carrier Dynamics of Atomic Layer Deposition-Grown Amorphous TiO₂,” J. Phys. Chem. C 126, pp. 4542–4554, 2022<br/>

Topics

• density
• impedance spectroscopy
• amorphous
• grain
• grain size
• theory
• thin film
• x-ray photoelectron spectroscopy
• Oxygen
• semiconductor
• chemical stability
• Silicon
• density functional theory
• titanium
• annealing
• interfacial
• interstitial
• additive manufacturing
• atomic layer deposition