Mesoporous TiO2 anatase films for enhanced photocatalytic activity under UV and visible light

Abstract

Mesoporous TiO₂ films with enhanced photocatalytic activity in both UV and visible wavelength ranges were developed through a non-conventional atomic layer deposition (ALD) process at room temperature. Deposition at such a low temperature promotes the accumulation of by-products in the amorphous TiO₂ films, caused by the incomplete hydrolysis of the TiCl₄ precursor. The additional thermal annealing induces the fast recrystallisation of amorphous films, as well as an in situ acidic treatment of TiO₂. The interplay between the deposition parameters, such as purge time, the amount of structural defects introduced and the enhancement of the photocatalytic properties from different mesoporous films clearly shows that our easily upscalable non-conventional ALD process is of great industrial interest for environmental remediation and other photocatalytic applications, such as hydrogen production.

Fig. 1. In situ QCM monitoring on the ALD mass increment recorded for depositions at RT for both precursors with purge time of 5 s and 30 s (A) (inset: full time range of the same experiments) and 1800 s (B).

Fig. 2. SEM and AFM topography images of annealed TiO₂ films deposited with purge time 5 s (A and C) and 30 s (B and D).

Fig. 3. XPS O 1s (a), Ti 2p (b) and Cl 2p (c) spectra on surface of amorphous and annealed samples, grown in short (5 s) and long (30 s) purge time regimes.

Fig. 4. In depth XPS Ti 2p and Cl 2p spectra on amorphous and annealed samples, grown in short (5 s) and long (30 s) purge time regimes at RT after different sputtering times.

Scheme 1. Schematic illustration of the evolution of films composition grown in two purge time regimes, amorphous and after annealing.

Fig. 5. PL spectra of mesoporous TiO₂ films (A) and their deconvolution (B).

Fig. 6. Models for photoluminescence from the electronic transition of trap states (A) for anatase reproduced from Jin et al. and (B) for hydrogenated TiO₂ reproduced from Wang et al.

Fig. 7. Photocatalytic degradation of MB in UV range on TiO₂ films grown on Si(100) with native silicon oxide (a); associated kinetics of MB degradation (b).

Fig. 8. (a) Photocatalytic degradation rate of MB in visible range (400–700 nm) on TiO₂ films grown on Si(100) native silicon oxide and blank test; inset table of the photocatalytic degradation constant; (b) associated kinetics of MB degradation.

Fig. 9. Photocatalytic degradation of MB, RhB and SA on RT TiO₂ films deposited on Si/SiO₂ substrate in the UV and visible ranges.