Tunable Ti3+-Mediated Charge Carrier Dynamics of Atomic Layer Deposition Grown Amorphous TiO2

Jesse Saari, Harri Ali-Löytty, Minttu M. Kauppinen, Markku Hannula, Ramsha Khan, Kimmo Lahtonen, Lauri Palmolahti, Antti Tukiainen, Henrik Grönbeck, Nikolai Tkachenko, Mika Valden

Tutkimustuotos: PosteriScientific


Amorphous titania (am.-TiO2) has gained broad interest in the field of photocatalysis due to its exceptional disorder-mediated optical and electrical properties compared to crystalline TiO2 [1–3]. For instance, Ti3+ defects within am-TiO2 can enable essential charge carrier transport through a protective am-TiO2 photoelectrode coating in photoelectrochemical (PEC) cells [1], and Ti3+-mediated visible light active amorphous “black” titania is regarded as a potential material for photocatalytic applications [2]. Atomic layer deposition (ALD) allows for tuning the defect composition and structure of am.-TiO2 thin films via precursor choices and process parameters. Recent progress in computational analysis of am.-TiO2 [3] has provided means to accurately correlate experimental insights with theoretical models, which can be utilized to tailor am.-TiO2 coatings with desired properties.

This work examines how intrinsic titanium and nitrogen defects in am.-TiO2 can be tailored in a controlled and elegant manner via tuning the ALD growth temperature between 100–200 °C when using 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 (Ti5/7c4+), 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. When changing the ALD growth temperature from 100 °C to 200 °C, increase in Ti3+ concentration results in “black” TiO2 and electrical conductivity via polaron hopping mechanism. Furthermore, transient absorption spectroscopy (TAS) shows that the high concentration of Ti3+ defects in “black” TiO2 increases the carrier lifetime to the nanosecond time domain comparable to crystalline low-defect TiO2. These insights into the formation of Ti3+ defects in am.-TiO2 and into tuning the charge transfer properties of ALD grown am.-TiO2 are beneficial in wide range of applications, such as protective photoelectrode coatings.

[1] 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, and N. S. Lewis, J. Phys. Chem. C 123 (33), 20116–20129 (2019).
[2] V.-A. Glezakou, and R. Rousseau, Nat. Mater. 17 (10), 856–857 (2018).
[3] D. Mora-Fonz, M. Kaviani, and A. L. Shluger, Phys. Rev. B 102 (5), 054205 (2020).
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