TiO2 Nanotube Arrays: Fabricated by Soft–Hard Template and the Grain Size Dependence of Field Emission Performance
© The Author(s). 2017
Received: 16 August 2017
Accepted: 2 November 2017
Published: 13 November 2017
Highly ordered TiO2 nanotube (TNT) arrays were successfully synthesized by the combination of soft and hard templates. In the fabrication of them, anodic aluminum oxide membranes act as the hard template while the self-assembly of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) complexed with titanium-tetraisopropoxide (TTIP, the precursor of TiO2) provides the soft template to control the grain size of TiO2 nanotubes. Our results indicate that the field emission (FE) performance depends crucially on the grain size of the calcinated TiO2 which is dominated by the PS-b-PEO and its blending ratio with TTIP. The optimized sample (with the TTIP/PEO ratio of 3.87) exhibits excellent FE performances involving both a low turn-on field of 3.3 V/um and a high current density of 7.6 mA/cm2 at 12.7 V/μm. The enhanced FE properties can be attributed to the low effective work function (1.2 eV) resulted from the smaller grain size of TiO2.
One-dimensional nanomaterials have attracted great interest due to their potential for numerous applications, e.g., electron field emitter [1–5]. TiO2 nanotubes (TNTs) are promising candidate for the emitter due to the high aspect ratio, low work function (4.5 eV), and high oxidation resistance . The nanotube diameters, height, wall thickness, and density as well as the regularity of the nanoarray dependences of the field emission (FE) performance have been investigated in detail [6, 7]. A significant number of nanotube arrays are available by the aid of the development of the synthetic approaches [8, 9]. Especially, the template strategies have been widely employed to fabricate nanotube array. For instance, Tsai et al. prepared diamond nanotip arrays with various sizes and periods by anodic aluminum oxide (AAO) . During the preparation, the micro-channels in AAO membrane can act as an excellent hard template to induce the formation of highly ordered nanoarrays. In the synthesis of porous TiO2 nanofibers in our previous work, the self-assembly of block copolymer has been proved as an effective template for the selective distribution and the grain size manipulation of TiO2 . The highly ordered TNT arrays with tunable grain sizes can be expected by the combination of the soft and hard templates. For one thing, it is facile to tailor the diameter, center-to-center distance, and the length of the TiO2 arrays by means of various AAO membranes; for another thing, the wall thickness, grain size, and the density of the TiO2 nanotubes are under the control of the block copolymer and the precursor of TiO2. Most importantly, the structure control in TNT array and tube levels can be performed separately. In this work, therefore, the TiO2 arrays with various grain sizes have been fabricated in the blend of titanium-tetraisopropoxide (TTIP)/block copolymer. In addition to the hard template (AAO) for the formation of highly ordered arrays, the PS-b-PEO is employed as the soft template to control the grain size of TiO2. The field emission performances of the resultant TNT arrays exhibit obvious grain size dependence, which has been attributed to the variation of the effective work function.
Samples with various molecular weights of PS-b-PEO and its blending ratio with TTIP
Grain size (nm)b
A Hitachi S-4800 FESEM was used for morphology measurement at an accelerating voltage of 5.0 kV. The X-ray diffraction (smartlab3, Rigaku Japan) data were collected at a scanning speed of 2°/min with a step interval of 0.02°. The electron field emission measurements were carried out using a diode configuration, a cathode (sample), and a parallel anode plate at a distance of 150 μm in a vacuum chamber (2 × 10−6 Torr).
Results and Discussion
The FE behavior of TNTs can be modeled following the well-known Fowler–Nordheim (FN) equation, as shown in Fig. 4b. The good linear fit in the curves indicates that the field emission current originates only from barrier tunneling electrons extracted by the electric field. Based on the slope of the FN plot (k), it is facile to calculate the effective work functions using the following equation:
k = − (6.83 × 103)φ 3/2/β.
They are 1.2, 1.5, and 2.1 eV for S1, S4, and S5, respectively, by assuming the field enhancement factor (pristine TNT arrays) is 445 . The reduction in the turn-on electric field of the TNTs is caused by the decrease of the effective potential barrier height resulted from the smaller TiO2 grains. Therefore, it is reasonable to attribute the enhanced field performance to the grain boundary effect and resultant up-shift of Fermi level which can be interpreted as follows [4, 19]. Polycrystalline materials are composed of small nanocrystalline grains separated by grain boundaries, which lead to a large number of grain boundary defects. These defects are benefit for both electron trapping and electron supply due to the effective conducting pathway. This is the reason for the increase of carrier concentration and subsequent up-shift of Fermi level . This rising Fermi level can reduce the work function (Fig. 4b) and the effective potential barrier height of TNTs, corresponding to easy electron emission, accounting for the enhanced field emission performance.
The TNT arrays were synthesized by the combination of soft and hard templates. On one hand, the AAO membranes induce the vertically aligned nanotubes. On the other hand, both the block copolymer and its blend ratio with TTIP produce remarkable influence on the grain size of the TiO2. The relationship between the grain size and the FE performance has been clarified for the first time. Our results indicate that the decrease of grain size accounts for the stronger grain boundary conduction, leading to the lift of the Fermi level. This is the reason for the lower work function, the smaller effective potential barrier, and the resultant-enhanced FE performance.
This work was financially supported by the National Natural Science Foundation of China (21234007 and 61504035).
YXX and MP performed the experiments, analyzed the data, and drafted the manuscript. QH and ZJX participated in the sample preparation and characterization. WQ participated in the measurement of field emission performance. The whole project was under the direction of YJC and LYJ, who designed the experiments and revised the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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