Template-Assisted Growth of Open-Ended TiO₂ Nanotubes with Hexagonal Shape Using Atomic Layer Deposition

Abstract

TiO 2 nanotubes have emerged as potential building blocks in nanostructured solar cells and photocatalytic devices, but the improvement of their crystalline quality and morphological control still requires further development. We report an original fabrication process to form open-ended TiO 2 nanotubes in the framework of the positive template-assisted growth using ZnO nanowires. The process involves the combination of atomic layer deposition with post-deposition thermal treatment in air at moderate temperature to crystallize an amorphous TiO x layer into a soluble, cubic-Zn 2 TiO 4 shell on the top face of ZnO nanowires and an insoluble, anatase-TiO 2 shell with large-sized domains on their sidewalls. The differential crystallization process originates from the differential reactivity on the polar c-and nonpolar m-plane surfaces of ZnO nanowires, further enhanced by the different thickness of the amorphous TiO x layer. Following chemical etching, open-ended TiO 2 nanotubes exhibit hexagonal shape and high crystalline quality with sharp, well-oriented walls. These findings open the way forintegrating high quality open-ended TiO 2 nanotubes with tunable length and wall thickness on a wide range of substrates. The present approach can further be developed to form other types of nanotubes and original nanowire heterostructures with highly localized, distinct phases for the shell using one simple thermal treatment.

Electrochemical and Mechanical Properties of Hexagonal Titanium Dioxide Nanotubes Formed by Sonoelectrochemical Anodization

“…To date, research into hexagonal TiO 2 nanotubes has taken two directions: (1) the hexagonal arrangement of circular TiO 2 nanotubes [20][21][22][23][24], and (2) the formation of right hexagonal TNTs with a hexagonal base and six rectangular sides, referred to as hexagonal TiO 2 nanotubes (hTNTs) [25][26][27][28][29][30][31][32][33][34][35][36][37][38].…”

“…To date, hTNTs have been formed by sonochemical-assisted chelating agent-driven anodization [28], conventional anodization/one-step anodization [32][33][34], two-step anodization [25,26,29,30,36], three-step anodization [27], pulse anodization with both positive upper and lower limits [31], anodization in a sol electrolyte [38], optimized anodization preceded by atomic layer deposition (ALD), pre-texturing by focused ion beam milling [35], and template-assisted growth using ALD [37].…”

“…The most complicated anodizing method consists of three anodization processes, whereas the second method aims to remove the oxide layer formed during the first anodization [27]. Other anodization methods include anodization in a sol electrolyte [38] and template methods [35,37]. The formation mechanism of sunken nanotube clusters in a sol electrolyte (nitric acid solution with ethyl alcohol) involves a captivating blend of chemical and physical processes.…”

“…Among the described methods, the most homogenous and ideally hexagonal shape of hTNTs was obtained by one-step anodization [25,33,34] and template methods [35,37]. Ideal open-ended hTNTs were formed using the framework of positive template-assisted growth with ZnO nanowires and ALD [37].…”

“…Among the described methods, the most homogenous and ideally hexagonal shape of hTNTs was obtained by one-step anodization [25,33,34] and template methods [35,37]. Ideal open-ended hTNTs were formed using the framework of positive template-assisted growth with ZnO nanowires and ALD [37]. However, the orientation of the hTNT and the crystallinity of the wurtzite-ZnO, anatase-TiO 2 , and cubic-Zn 2 TiO 4 phases were not uniform, negatively affecting the mechanical and corrosion properties.…”