The self-organized 1D TiO₂ nanotubular layers have attracted considerable scientific and technological interest over the past two decades, all motivated by a great performance in the range of applications including photo-catalysis, solar cells, hydrogen generation and biomedical uses [1,2]. The synthesis of these nanotubular layers has been carried out by a conventional electrochemical anodization of Ti sheets. Except the 1D character, these nanotubes possess unique features such as tunable dimensionality, structural flexibility, unidirectional electron transport through nanotube walls, chemical and mechanical stability and biocompatibility.

One of the major application targets of TiO₂ nanotubes has been their utilization as scaffolds or templates for deposition of secondary materials towards new applications [3]. In particular, for the photoelectrochemical applications (e.g. photovoltaics) ordered nanostructures, such as self-organized TiO₂ nanotubes, offer the advantage of directed charge transport and controlled phase separation between the donor and acceptor materials, unlike within randomly ordered mesoporous TiO₂ supports.

Numerous techniques were utilized for this purpose, such as hydrothermal routes, electrodeposition or Atomic Layer Deposition (ALD) techniques. Electrodeposition is very powerful technique to deposit metals [4], oxides [5] or other rather complex materials (such as CuInSe₂ [6]) within the nanotubes various by means of complete filling or decoration with. On the other hand, ALD can deposit the widest range of secondary materials within TiO₂ nanotubes by means of homogenous coatings [7-10].

The presentation will focus on the use of TiO₂ nanotube layers as high aspect ratio substrates, to be decorated by cuprous oxide Cu₂O as a p-type semiconductor using electrodeposition. The as-deposited Cu₂O nanoparticles were subsequently passivated by ultrathin barrier ALD layers with the aim to protect them against electrochemical dissolution in alkaline medium. Experimental details and some very recent photo-electrochemical and structural characterizations of a new type of heterostructured photo-chemical cell will be presented and discussed.

References

[1] M. Macak et al., Curr. Opin. Solid State Mater. Sci., 2007, 1-2, 3-17.

[2] Lee, A. Mazare, P. Schmuki, Chem. Rev., 2014, 114, 9385-9454.

[3] M. Macak, Chapter 3 in: D. Losic and A. Santos, Electrochemically Engineered Nanoporous Structures, Springer International Publishing, Switzerland, 2015.

[4] M. Macak, B.G. Gong, M. Hueppe, P. Schmuki, Adv. Mater., 2007, 19, 3027.

[5] Wu & G. Zangari, J. Phys. Chem. C, 2010, 114, 11551

[6] Das & J. M. Macak et al., ChemElectroChem, 2017, 4, 495

[7] Zazpe et al, Langmuir, 2016, 32, 10551

[8] Ng et al., Adv. Eng. Mater., 2018, 20, 1700589.

[9] Zazpe et al., Nanoscale, 2018, 10, 16601-16612.

[10] Dvorak et al., Appl. Mater. Today, 2019, 14, 1-20.

[11] B. Bawab et al., Ms in preparation.