Synthesis of TiO2/Al2O3 Double-Layer Inverse Opal by Thermal and Plasma-Assisted Atomic Layer Deposition for Photocatalytic Applications

Abstract

In comparison to conventional nano-infiltration approaches, the atomic layer deposition (ALD) technology exhibits greater potential in the fabrication of inverse opals (IOs) for photocatalysts. In this study, TiO₂ IO and ultra-thin films of Al₂O₃ on IO were successfully deposited using thermal or plasma-assisted ALD and vertical layer deposition from a polystyrene (PS) opal template. SEM/EDX, XRD, Raman, TG/DTG/DTA-MS, PL spectroscopy, and UV Vis spectroscopy were used for the characterization of the nanocomposites. The results showed that the highly ordered opal crystal microstructure had a face-centered cubic (FCC) orientation. The proposed annealing temperature efficiently removed the template, leaving the anatase phase IO, which provided a small contraction in the spheres. In comparison to TiO₂/Al₂O₃ plasma ALD, TiO₂/Al₂O₃ thermal ALD has a better interfacial charge interaction of photoexcited electron-hole pairs in the valence band hole to restrain recombination, resulting in a broad spectrum with a peak in the green region. This was demonstrated by PL. Strong absorption bands were also found in the UV regions, including increased absorption due to slow photons and a narrow optical band gap in the visible region. The results from the photocatalytic activity of the samples show decolorization rates of 35.4%, 24.7%, and 14.8%, for TiO₂, TiO₂/Al₂O₃ thermal, and TiO₂/Al₂O₃ plasma IO ALD samples, respectively. Our results showed that ultra-thin amorphous ALD-grown Al₂O₃ layers have considerable photocatalytic activity. The Al₂O₃ thin film grown by thermal ALD has a more ordered structure compared to the one prepared by plasma ALD, which explains its higher photocatalytic activity. The declined photocatalytic activity of the combined layers was observed due to the reduced electron tunneling effect resulting from the thinness of Al₂O₃.

Keywords: Al2O3; TiO2; inverse opal; nanocomposites; photocatalysis; plasma ALD; thermal ALD.

Figure 1

Experimental route for synthesis of the TiO₂ IO and Al₂O₃/TiO₂ composites by ALD and vertical layer deposition.

Figure 2

Illustrates the thermal analysis curve for PS opal template in air.

Figure 3

SEM images of PS opal: (a,b); pure TiO₂ IO: (c); Al₂O₃ thermal ALD on IO: (d); and Al₂O₃ plasma ALD on IO: (e).

Figure 4

(a) XRD; (b) Raman Spectroscopy.

Figure 5

A linear fit to the straight part of the hν vs (αhν)^1/2 plot (Tauc plot) intercepts the energy axis at the optical band gap energy: the indirect energy band gap for TiO₂ IO (a), Al₂O₃/TiO₂ thermal ALD, and (b) Al₂O₃/TiO₂ plasma ALD (c).

Figure 6

PL spectra of the samples.

Figure 7

Photocatalytic decolorization of the MB dye in the presence of TiO₂ (a), TiO₂/Al₂O₃ thermal ALD (b), TiO₂/Al₂O₃ plasma ALD (c), and MB dye (d).

Figure 8

Photocatalytic performance—relative absorbance in % vs. irradiation time in minutes (a) and pseudo-first-order linear plots of ln (C₀/C_t) vs. irradiation time for the kinetics of the sample materials (b).