Optimizing TiO₂ Nanotube Arrays via Soft–Hard Template for Superior Field Emission Performance

Background

One‑dimensional nanomaterials, particularly TiO₂ nanotubes (TNTs), are attractive for electron field‑emitter applications due to their high aspect ratio, low intrinsic work function (≈ 4.5 eV) and excellent oxidation resistance. The field‑emission performance of TNT arrays is known to depend on geometric factors—diameter, height, wall thickness, density—and on the regularity of the array. Recent advances in template‑based synthesis, especially the use of AAO membranes as hard templates, have enabled the creation of highly ordered nanotube arrays with controllable spacing and length. Complementary soft‑template strategies using block copolymers allow fine tuning of grain size and wall thickness. By integrating both approaches, we can independently tailor macro‑scale array geometry and nano‑scale tube properties, which is critical for optimizing FE performance.

Methods

AAO membranes (Whatman, Germany) with ~200 nm pores and 60 µm thickness served as the hard template. PS‑b‑PEO (Sigma‑Aldrich) with MW 58,500–37,000, 58,600–71,000, and 60,000–14,500 g mol^–1 and TTIP (Sigma‑Aldrich) were dissolved in chloroform at varying blend ratios (see Table 1). After 5 h stirring at room temperature, the solutions filled the AAO channels via capillary action. Samples were dried at 120 °C for 12 h under vacuum, calcined at 450 °C for 2 h in air, and the alumina scaffold was removed by 3 M NaOH for 1 h. Final rinsing and drying at 40 °C for 24 h completed the process (Scheme 1). Morphology was examined by Hitachi S‑4800 FESEM (5 kV). X‑ray diffraction (Rigaku smartlab3) collected at 2° min^–1 provided grain size estimates via the Debye–Scherrer equation. FE measurements employed a diode configuration with a 150 µm anode gap in a 2 × 10^–6 Torr vacuum chamber.

Preparation of TNT arrays using soft–hard template integration

Results and Discussion

SEM images (Fig. 1) confirm vertically aligned, ~200 nm diameter TNTs with consistent ordering across all samples. XRD patterns (Fig. 2) show sharp anatase TiO₂ peaks at 25°, 38°, and 48°, indicating strong (101) orientation. Grain sizes derived from the (101) peak broadenings (Table 1) increase with higher TTIP content (S1 → S3). This trend directly correlates with FE behavior: S1 (grain size 10.7 nm) exhibits the lowest turn‑on field (3.3 V µm^–1) and the highest current density (7.6 mA cm^–2 at 12.7 V µm^–1), whereas S2 and S3 (12.5 nm and 14.9 nm) require >10 V µm^–1 to initiate emission and show no clear threshold within the studied range.

SEM images of TNTs: (a) side view; (b) top view

XRD profiles of TNT arrays

Fowler–Nordheim analysis (Fig. 3a) confirms that emission follows quantum tunneling. Turn‑on and threshold fields are defined at 0.01 and 1.0 mA cm^–2, respectively. Sample S1 shows exceptional stability over 180 min at 10 V µm^–1 (Fig. 3b). To isolate grain‑size effects, additional samples (S4, S5) were fabricated with similar tube thickness but varying grain sizes. Their average turn‑on fields were 4.2 V µm^–1 (S4) and 8.7 V µm^–1 (S5), reinforcing the conclusion that finer grains reduce the effective work function and thus lower the required field.

Current density–electric field (J–E) plot (a) and long‑term stability at 10 V µm^–1 (b)

(a) J–E plot; (b) Fowler–Nordheim linear fits

From the slope of the FN plots, effective work functions were extracted: 1.2 eV (S1), 1.5 eV (S4), and 2.1 eV (S5), assuming a field‑enhancement factor of 445. The dramatic reduction in work function for the smallest grains is attributed to increased grain‑boundary density, which elevates the Fermi level and lowers the potential barrier for electron tunneling. This grain‑boundary‑driven mechanism explains the superior FE performance of the finely grained TNT arrays.

Conclusions

By integrating hard AAO and soft PS‑b‑PEO templates, we achieved highly ordered TiO₂ nanotube arrays with controllable grain sizes. The study establishes a clear, quantitative link between grain size and field‑emission performance: smaller grains enhance grain‑boundary conduction, elevate the Fermi level, reduce the effective work function, and ultimately deliver lower turn‑on fields and higher current densities. These insights provide a practical pathway for designing next‑generation nano‑emitters with tailored electronic properties.