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/Tisub). The barrier layer between the Tisub 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 Tisub 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/Tisub. Additionally, FSL-TiO2NTs exhibited fourfold higher electron transport rates and better thermal stability compared to TiO2NTs/Tisub.
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 Tisub were also evaluated.
TiO2NTs/Tisub 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/Tisub 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/Tisub synthesized in electrolytes were aged for 12, 8, 4, and 0 hours. The final TiO2NTs/Tisub samples were calcinated at 500 oC in the air for two hours to improve their crystallinity. The calcinated TiO2NTs/Tisub 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/Tisub and synthesized FSL-TiO2NTs.
FSL-TiO2NTs with a thickness of 9-17 µm were successfully synthesized by detaching TiO2NTs from Tisub. 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/Tisub 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.
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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