Interface Engineering of TiO₂ Photoelectrode Coatings Grown by Atomic Layer Deposition on Silicon

Abstract

Titanium dioxide (TiO2) can protect photoelectrochemical (PEC) devices from corrosion, but the fabrication of high-quality TiO2 coatings providing long-term stability has remained challenging. Here, we compare the influence of Si wafer cleaning and postdeposition annealing temperature on the performance of TiO2/n+-Si photoanodes grown by atomic layer deposition (ALD) using tetrakis­(dimethylamido)titanium (TDMAT) and H2O as precursors at a growth temperature of 100 °C. We show that removal of native Si oxide before ALD does not improve the TiO2 coating performance under alkaline PEC water splitting conditions if excessive postdeposition annealing is needed to induce crystallization. The as-deposited TiO2 coatings were amorphous and subject to photocorrosion. However, the TiO2 coatings were found to be stable over a time period of 10 h after heat treatment at 400 °C that induced crystallization of amorphous TiO2 into anatase TiO2. No interfacial Si oxide formed during the ALD growth, but during the heat treatment, the thickness of interfacial Si oxide increased to 1.8 nm for all of the samples. Increasing the ALD growth temperature to 150 °C enabled crystallization at 300 °C, which resulted in reduced growth of interfacial Si oxide followed by a 70 mV improvement in the photocurrent onset potential.

Transformation kinetics for low temperature post-deposition crystallization of TiO 2 thin films prepared via atomic layer deposition (ALD) from tetrakis(dimethylamino)titanium(IV) (TDMAT) and water

“…From the present study, an increased nucleation frequency factor indicates that there is a greater frequency of attempts to form anatase nuclei of critical size, therefore aligning with increased vibrational motion at higher deposition temperatures. This result could also inform a recent literature report that describes how increasing the ALD growth temperature from 100 to 150 °C enables the formation of high-quality, protective anatase TiO 2 coatings at decreased annealing temperatures [33]. The amorphous TiO 2 coating deposited at 150 °C may have a higher degree of vibrational motion causing a greater number of critical nuclei formed per attempt frequency, enabling phase transformation to occur at lower temperature.…”

“…However, TiO 2 ALD films grown from TiCl 4 /H 2 O chemistry have been shown to exhibit anatase crystals greater in diameter than the film thickness [30,31,32]. Large-grained anatase can have improved photoelectrochemical performance [16,33] and photocatalytic activity due to its small grain boundary volume [3,34]. Highly defected grain boundary regions do not contribute to the functional performance of the crystalline film, and so maximizing grain size and minimizing grain boundary volume is essential to optimizing material performance.…”

“…Other reports have probed resultant ALD-TiO 2 film structure with post-deposition annealing (PDA) [5,14,33,37]. PDA temperatures are typically higher than those used for the ALD growth process.…”

Light transfer through bubble-filled electrolyte for solar water splitting

“…6 However, many other photoelectrodesmade of TiO 2 or WO 3 , for exampleusually show higher photocurrent when front-illuminated. 7,8 For biasfree water splitting, most congurations use a photoanodephotocathode tandem or a photoelectrode-PV tandem cell. In both congurations, the incident light inevitably interacts with either hydrogen or oxygen bubbles.…”

Low-Temperature Route to Direct Amorphous to Rutile Crystallization of TiO2 Thin Films Grown by Atomic Layer Deposition

“…Titanium dioxide (TiO 2 ) is one of the most widely applied and studied photocatalyst materials, being earth-abundant, non-toxic, and stable in various environments. − Generally, TiO 2 appears in four phases: amorphous, anatase, rutile, and brookite. The first three are the most commonly used having their own advantages, whereas brookite has remained mainly inapplicable due to the challenges in fabrication of its pure form. , Amorphous titania (am.-TiO 2 ) thin films, typically grown at low temperatures, can provide exceptional optical properties and charge carrier dynamics due to the disordered structure and intrinsic Ti 3+ defects. − However, concerning photocatalytic applications, defect-induced gap states may increase the possibility to detrimental electron–hole recombination, and the chemical instability of am.-TiO 2 without additional electrocatalysts limits the operating conditions. − Crystalline defect-free TiO 2 , instead, is chemically stable and exhibits reduced charge carrier recombination. ,, …”