Role of crystallinity on the nanomechanical and electrochemical properties of TiO2 nanotubes

https://doi.org/10.1016/j.jelechem.2016.03.032Get rights and content

Highlights

  • TiO2 nanotubes were annealed at different temperatures to induce crystallization.

  • Mechanical response of the nanotubes was correlated with their crystallinity.

  • Evolution of rutile phase reduced the donor concentration drastically.

  • Preferential growth of rutile phase enhanced the passivity.

Abstract

In order to study the effect of annealing on the nanomechanical and electrochemical properties, TiO2 nanotubes synthesized via anodic oxidation were subjected to thermal treatment at various temperatures. Structural studies revealed that as-anodized nanotubes are amorphous, but they become crystalline when subjected to thermal treatment in air. TiO2 nanotubes annealed up to 450 °C crystallizes in pure anatase phase, whereas a mixture of anatase and rutile phase was observed for sample annealed at 600 °C. Interestingly, the evolution of rutile phase at 600 °C leads to decrease in crystallite size of the anatase phase. Nanoindentation studies showed that crystallite size plays a strong role in hardness, whereas porosity has significant influence on the elastic modulus of the nanotubes. Presence of two time constants in electrochemical impedance spectroscopy measurement presumably implies the formation of bilayer passive film. Mott Schottky analysis showed n-type semiconducting behavior with decrease in donor concentration (1019 cm 3) upon increasing the annealing temperature. However, the evolution of rutile phase decreases the donor concentration drastically by an order of a magnitude (1018 cm 3). Potentiodynamic polarization studies revealed that rutile phase favors enhanced passivation behavior exhibiting lower passive current density (10 7 A cm 2) compared to the pure anatase nanotubes (10 6 A cm 2). The enhanced passivation behavior of rutile phase was confirmed by Raman spectroscopy.

Introduction

Self-ordering titanium oxide nanotubes synthesized by anodization process have been of a great significance in the field of solar cells, gas sensors, photo-catalysis, biomedical implants, water splitting and batteries [1], [2], [3], [4]. A porous structure with high effective surface area, semiconducting nature, superior electrochemical and corrosion behavior of the oxide nanotubes makes it suitable for these applications. The arrays of discrete nanotubes can be fine-tuned to desired dimension by controlling the potential, the electrolyte composition, the pH, the anodization time and temperature [5], [6], [7].

Several studies [8], [9] show that crystallization of nanotubes enhances the oxide properties. As-synthesized TiO2 nanotubes are generally amorphous in nature and annealing at high temperature (> 300 °C) leads to crystallization of various phases (anatase, rutile or combination of both). Each polymorph of TiO2 nanotubes has its own advantages and disadvantages in the areas related to bio-implants, gas sensors and dye sensitized solar cells.

Passivation behavior of the crystalline nanotubes in the body fluid influences the rate of active dissolution. The kinetics of corrosion process can be reduced by the passive film formed on the nanotubes containing crystalline phases [9], [10]. Furthermore, thermal treatment of TiO2 nanotubes enhances the cell adhesion and proliferation owing to the high surface reactivity, which in turn depends on the crystallinity. Enhancing the crystallinity of nanotubes in solar cells is of particular importance because it affects the electron transfer efficiency [11], [12]. As a consequence of crystalline nature of nanotubes, donor concentration of the oxide film is expected to have a marked influence on their performance. A complete understanding of the fundamental properties of the oxide layer would be essential in improving the efficiency of these materials in applications. Studies devoted in quantifying the electrochemical and electronic properties of crystalline titania nanotubes have been reported by many researchers [13], [14], [15]. However, systematic analysis and correlation of crystallinity and crystallite size of titania nanotubes with their electrochemical and electronic properties of TiO2 nanotubes are not well established. Also, the crystallinity and barrier layer properties of anodic passive film grown on nanotubes are yet to be understood.

Besides electrochemical and electronic properties, there is need for investigating the mechanical stability of the nanotubes, as their physical characteristics determine the performance in sensor, photo-catalytic and implant applications. Owing to the discrete nature of tubes, the mechanical properties such as elastic modulus and hardness would vary from bulk titania films. Although, the nanomechanical characterization of TiO2 nanotubes have been widely studied [16], [17], [18], still there is lack in understanding the effect of crystallinity on the mechanical properties of the crystallized nanotubes.

Hence, the current investigation aims at exploring the influence of crystallinity on the nanomechanical, electrochemical and electronic properties of thermally annealed titania nanotubes. Nanomechanical characterization correlates the mechanical response of the nanotubes with their crystallite size, porosity and phases formed. The capacitance measurements highlighted the n-type semiconducting behavior of nanotubes with decrease in donor density and shift in flat band potential towards more positive potential when annealed at higher temperatures. Passivity of annealed nanotubes was enhanced significantly due to the epitaxial and preferential growth of passive film as confirmed by the change in intensity of Raman bands observed for anatase and rutile phases.

Section snippets

Synthesis of TiO2 nanotubes

Titania nanotubes were synthesized by anodic oxidation in 0.1 M citric acid containing 0.5 wt% of NaF as electrolyte [19]. Prior to anodization, the polished titanium surfaces were etched in a solution containing 1:4:5 ratio of HF: HNO3:H2O. A two-electrode electrochemical cell, with titanium (99.6% pure, Alfa Aesar) as working electrode and graphite rod as counter electrode was employed to carry out the anodization process. The anodization was carried out at room temperature for an hour at a

Surface structure of TiO2 nanotubes

In order to study the morphology of as-anodized and annealed titania nanotubes, SEM micrographs were recorded. As-anodized and annealed TiO2 nanotubes are discretely seen throughout the surface as shown in Fig. 1a–d and the corresponding side view of the nanotubes are shown in Fig. 2a–d. The presence of regularly spaced bumps can be manifested from the bottom of the nanotubes. The sidewall of the tubes exhibits wavy/ripple structure, which is due to the presence of water content in the

Conclusions

Here, we reported on the nanomechanical and electrochemical properties of thermally annealed TiO2 nanotubes. The following results can be drawn from the observations:

  • 1.

    SEM investigation shows minimal change in the diameter (89 to 95 nm) and wall thickness (9 to 11 nm) of nanotubes for the samples annealed at 325 and 450 °C; while annealing at 600 °C causes shrinkage of nanotubes such that wall thickness (21 nm) increases with decrease in inner diameter (76 nm) of the nanotubes.

  • 2.

    Crystallite size of

Acknowledgements

BM thanks Dr. J. Magesh and Mr. Ram Subramanian for their assistance during micro-Raman spectroscopy measurements.

References (40)

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    Citation Excerpt :

    The inner barrier layer resistance, R1, corresponds to the film polarization resistance, while the outer layer resistance, R2, corresponds to the film charge transfer resistance. The higher inner barrier layer resistance when compared to the resistance of outer layer containing nanotubes is attributed to a greater corrosion resistance because the inner barrier layer inhibits penetration of ions of the electrolyte on the boundary between TiO2 nanotubes and substrate (titanium alloy) [28]. The TiO2 NTs crystallized in a mixture of anatase and rutile could be represented by a model shown in Fig. 5c with three time constants, as also reported by Corboba-Torrez et al. [29].

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