Robust Nanotubes Fabricated with Promising Photoelectronic Properties

By Samudrapom DamReviewed by Susha Cheriyedath, M.Sc.Mar 24 2022

A research article available as a pre-proof in the journal Materials Research Bulletin demonstrates a new method to synthesize robust free-standing titanium dioxide nanotube layers (FSL-TiO₂NTs) with excellent photoelectronic properties and complete structures.

Study: Efficient Fabrication of Robust and Highly Ordered Free-Standing TiO2 Nanotube Layers. Image Credit: ustas7777777/Shutterstock.com

Titanium Dioxide Nanotube Layers

TiO₂NTs have gained significant attention since their introduction owing to their exceptional properties such as efficient charge mobility, higher physical strength, closely packed and highly ordered tubular structures, and larger specific surface areas.

The electrochemical anodization of Ti foil is the most commonly used method to synthesize TiO₂ nanotubular arrays as it is a simple, reproducible, and controllable process. However, several factors, such as the arrangement of electrodes, electrolytes, and the number of anodization steps, can influence the anodization process. These factors were previously investigated to obtain different architectures and morphologies of TiO₂NT arrays.

Although the tubular structure TiO₂NT arrays can be enhanced through modifications in anodization processes, such modifications can adversely impact their electrochemical properties by reducing the electrical conductivity, hindering the electron mobility, and generating charge separation rate.

Additionally, the TiO₂NT arrays obtained from different anodization processes possess closed bottoms and are often attached to the underlying Ti substrate (TiO₂NTs/Ti_sub). The barrier layer between the Ti_sub and the tubular bottom leads to certain disadvantages, such as poor light-to-current conversion, which can limit the applications of the NTs.

The detachment of the TiO₂NT arrays from the Ti_sub to obtain FSL-TiO₂NTs can help in overcoming these disadvantages. Previous studies demonstrated that the photocatalytic activity of FSL-TiO₂NTs is significantly higher than TiO₂NTs/Ti_sub. Additionally, FSL-TiO₂NTs exhibited fourfold higher electron transport rates and better thermal stability compared to TiO₂NTs/Ti_sub.

FSL-TiO₂NTs are often detached through chemical dissolution, voltage application, ultrasonication, and methanol evaporation. However, these approaches can lead to the formation of incomplete structures, cracks, and curling.

Hydrogen peroxide (H₂O₂), an environmental-friendly agent, can be used to detach TiO₂NTs as it dissolves TiO₂ to form complexes. However, the detachment process generally requires a long anodization time for thick FSL-TiO₂NTs. Additionally, fabricating thin, flat FSL-TiO₂NTs with complete structures and large areas has remained extremely challenging until now.

In order to fabricate FSL-TiO₂NTs with fewer defects and high strength, the physicochemical stability of the NTs should be enhanced through conventional anodization, which can be achieved by using the calcination process and an aged electrolyte, along with a proper detachment process. 

New Method to Synthesize Free-Standing TiO₂ Nanotubes

In this study, researchers developed an eco-friendly and simple procedure to synthesize complete, robust, thin, and transparent FSL-TiO₂NTs with a highly ordered morphology.  Additionally, the methods used for growing NTs using aged electrolyte and detaching FSL-TiO₂NTs from Ti_sub were also evaluated.

TiO₂NTs/Ti_sub were synthesized through a two-step process by electrochemical anodization at room temperature using a DC power supply. The first anodization step was performed at 60 Volts for 2 hours in a fresh electrolyte containing 3.0 vol.% water and 0.5 wt.% ammonium fluoride (NH₄F) in ethylene glycol. A Ti foil measuring 1.0 cm × 3.0 cm was utilized as an anode, and a part of the foil was immersed in the electrolyte. A platinum (Pt) plate with the same dimensions as Ti foil was employed as a cathode.

The resultant TiO₂NTs/Ti_sub obtained after first anodization was ultrasonicated in water to eliminate irregular TiO₂NT arrays and obtain Ti foil with a concave surface. The aging treatment using the electrolyte was performed in a similar manner as the first anodization. The Ti foil was employed as an electrode to condition the electrolyte under different reaction/aging times. After the aging treatment, the electrolyte was utilized in the second anodization of the concave Ti foils at 60 Volts and a reaction time of 10 hours.

The TiO₂NTs/Ti_sub synthesized in electrolytes were aged for 12, 8, 4, and 0 hours. The final TiO₂NTs/Ti_sub samples were calcinated at 500 ^oC in the air for two hours to improve their crystallinity. The calcinated TiO₂NTs/Ti_sub was then subjected to a third anodization step in a similar manner as the first anodization for 15 minutes to synthesize an amorphous NTs layer under the crystallized layer and fabricate FSL-TiO₂NTs. Subsequently, the NTs layer was gradually detached by performing a chemical dissolution method for 30 minutes.

Field emission scanning electron microscope (FE-SEM), ultraviolet-visible (UV-Vis) spectrometer, transmission electron microscope (TEM), electrochemical analyzer, and Orion Star A215 pH/conductivity meter were used to characterize the physicochemical properties of the TiO₂NTs/Ti_sub and synthesized FSL-TiO₂NTs.    

Observations

FSL-TiO₂NTs with a thickness of 9-17 µm were successfully synthesized by detaching TiO₂NTs from Ti_sub. The NTs possessed uniform, robust, and aligned nanostructures. A suitably aged electrolyte for growing TiO₂NTs, proper calcination conditions to obtain anatase-phase TiO₂NTs, a moderate concentration of H₂O₂, and the orientation of TiO₂NTs/Ti_sub in H₂O₂ solution were acted as the key parameters to synthesize robust and well-ordered FSL-TiO₂NTs.

The FSL-TiO₂NTs demonstrated robust photo-response to the incident irradiation. Robust and highly ordered TiO₂NTs/Ti was found to be necessary to synthesize large-sized FSL-TiO₂NTs without bending or cracking. The crystallinity of the FSL-TiO₂NTs did not influence by the thickness of the film. The detachment procedure did not change the crystallinity of TiO₂NTs.

H₂O₂ accelerated the chemical dissolution of the amorphous TiO₂ and helped in the successful detachment of FSL-TiO₂NTs. TiO₂NTs fabricated in electrolytes aged 12 and 8 hours possessed sufficient mechanical hardness to resist the detachment drag and highly ordered structure, leading to the synthesis of FSL-TiO₂NTs with a complete nanotube layer structure.

Nanoribs functioned as connectors that kept the nanotubes close together and reinforced the arrays, which prevented the deconstruction of arrays during the detachment process.  

Taken together, the findings of this study demonstrated the exceptional potential of FSL-TiO₂NTs for different applications in the future. 

Reference

Luan, N.H., Chang, C.-F. (2022) Efficient Fabrication of Robust and Highly Ordered Free-Standing TiO₂ Nanotube Layers. Materials Research Bulletin https://www.sciencedirect.com/science/article/pii/S0025540822001027?via%3Dihub

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