Preparation and photoelectric effect of Zn²⁺-TiO₂ nanotube arrays

ZHOU Yi(周 艺)^{1, 2}, SHI De-hui(石德晖)², LI Hong(李 宏)², DANG Ming-ming(党铭铭)²,

L? Cai-xia(吕彩霞)² , HUANG Ke-long(黄可龙)¹

1. School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China;

2. School of Chemical and Biological Engineering, Changsha University of Science and Technology,

Changsha 410114, China

Received 27 November 2009; accepted 26 April 2010

Abstract: Zn²⁺- TiO₂ nanotube arrays were prepared by anodic oxidation method. The current—time curves were used to investigate their growth mechanism. Scanning electron microscopy and X-ray diffractometry were applied to characterizing their structures and properties. The photoelectrochemical properties were studied by electrochemical impedance spectrum (EIS). The optimised working conditions for TiO₂ nanotube arrays were found to be pH 1, 0.5% HF (mass fraction), 20 V oxidation voltage and for 2 h. The produced sample was in anatase form, with length of 70-100 nm, thickness of 10 nm, uniform diameter and structure that does not collapse under the preparation conditions. The EIS results show that TiO₂ nanotube arrays prepared with 0.5% HF (mass fraction) present a low impedance and TiO₂ nanotube arrays loaded by Zn²⁺ could have a decreased resistance. This decrease could likely accelerate the transfer of carriers and even increase photoelectric conversion.

Key words: Zn²⁺-TiO₂ nanotube array; anodic oxidation method; photoelectric effect; growth mechanism

1 Introduction

TiO₂ is an important inorganic functional material with good photoelectronic, photosensitive, gas sensing, and pressure-sensitive characteristics. It has recently attracted much attention in terms of its promising application prospects, such as in the fields of photoelectricity (for solar energy), electronics (types of sensors), and biology (bone growth)[1-3]. Highly ordered TiO₂ nanotube arrays fabricated by anodization constitute a material architecture that offers large internal surface area without a concomitant decrease in geometric and structural order. And the precisely oriented nature  of the nanotube arrays makes them excellent electron percolation pathways for vectorial charge       transfer between interfaces[4-5]. ZWILLING et al[6]   prepared porous TiO₂ films by the anode oxidation method using a metal titanium sheet. This provided   the research idea for preparing TiO₂ nanotube arrays. GONG et al[7] prepared TiO₂ nanotube arrays with   the electrochemical anode oxidation method. The TiO₂

nanotube arrays with uniform distribution were previously prepared by the anode oxidation method, but the conversion efficiency still needed improvement[8-9]. During the procedure for improving the photoelectronic activity of TiO₂, various methods such as element doping[10-11], noble metal deposition[12], surface modification[13] and semiconductor composition[14], have been carried out. Among them, doping element into TiO₂ lattice is an effective way. And at present, nanotubes treated by loading modification have little report in photoelectric material fields. This study is based on the nanotube arrays treated by Zn²⁺ loading modification. Zn(NO₃)₂ was added during the oxidation process to load a certain amount of Zn²⁺ to the nanotubes. The influence of the oxidation voltage, temperature, time, electrolyte concentration, and content of Zn, as well as the photoelectric effect, on the structure and morphologies of the TiO₂ nanotubes was investigated. And the goal of this study is to prepare Zn²⁺-TiO₂ nanotube arrays on Ti sheet and further investigate their photoelectric properties.

2 Experimental

2.1 Materials

The materials used for this experiment were hydrofluoric acid (HF, AR), titanium foils (Ti), sodium sulfate (Na₂SO₄, AR), zinc nitrate [Zn(NO₃)₂, AR], isopropanol (C₃H₃O, AR), acetone (CH₃COCH₃, AR), and ethanol (CH₃CH₂OH, AR).

2.2 Synthesis

2.2.1 Synthesis of TiO₂ nanotube arrays

1) The titanium foils were anodized constantly at pH 4 with 1% HF (mass fraction, the same below if not mentioned) under different oxidation voltages. A potentiostat-galvanostat in a two-electrode setup was used to apply constant anodizing voltages of 15, 20, and 30 V typically for 2 h at 20 °C.

2) The titanium foils were anodized at a constant oxidation voltage of 20 V with 0.3%, 0.5%, 0.7%, and 1% HF at 20 °C. The increase rate of voltage was 0.1 V/s.

3) The titanium foils were anodized at a constant HF mass fraction of 0.5% in electrolyte systems with pH values of 1, 4, 7, and 9 at 20 °C.

Finally, the anodized samples were washed with de-ionised water, dried in air, and then calcined at 450 °C for 2 h.

2.2.2 Synthesis of Zn²⁺-TiO₂ nanotube arrays

The titanium foils were anodized under conditions of 0.5% HF, pH 4, and an oxidation voltage of 20 V. The electrolytes contained 1.0% Zn(NO₃)₂. The anodized samples were washed with de-ionised water, dried in air after reacting for 2 h, and then calcined at 450 °C for 2 h.

2.3 Characterisation

The surface morphology and dimension characterisation of the anodized samples were observed on scanning electron microscope (SEM, JEOL JSM-6700F). The crystallographic structures of the produced nanotube arrays were characterised on X-ray diffractometer (XRD, Siemens D5000).

2.4 Photoelectric analysis

Photoelectric test was carried out with a three- electrode system, including a TiO₂/Zn²⁺-TiO₂ nanotube arrays working electrode, a Pt counter electrode, and a saturated calomel reference electrode. A 0.1 mol/L Na₂SO₄ solution was used as the electrolyte solution. The electrochemical impedance was measured at room temperature on a CHI660C Electrochemical Workstation working under 8 W ultraviolet lamp（λ=365 nm）with 5 cm-distance between the lamp and electrode.

2.5 Analysis of growth mechanism of TiO₂ nanotube arrays

The titanium foils were anodized with 1.0% HF, an oxidation voltage of 5 V, and a duration of 20 min. A CHI660C Electrochemical Workstation with a three- electrode system, including a TiO₂/Zn²⁺-TiO₂ nanotube arrays working electrode, a Pt counter electrode, and a saturated calomel reference electrode, was used to measure the current and voltage in order to analyze the growth mechanism of the TiO₂ nanotube arrays.

3 Results and discussion

3.1 XRD analysis

XRD was used to characterize the crystalline structure of the TiO₂ nanotube arrays. By comparing the two XRD patterns shown in Fig.1, it is easy to see that the crystalline structures of the TiO₂ nanotube arrays prepared in different electrolyte systems are not obviously different. The main diffraction peaks at 25.29° and 38.41° are indexed as the (101) and (004) reflections of the crystalline anatase phase.

Fig.1 XRD patterns of TiO₂ nanotubes prepared with different electrolyte systems

The XRD patterns of pure TiO₂ and Zn²⁺-TiO₂ nanotube arrays are shown in Fig.2. The pattern of the Zn²⁺-TiO₂ nanotube arrays shows a diffraction peak at 56.70°, which is indexed as the (111) reflection of ZnO. The result demonstrates that Zn²⁺ has been loaded on the TiO₂ nanotube arrays, and ZnO is the main form of Zn²⁺ in the Zn²⁺-TiO₂ nanotube arrays. Therefore, the half peak width of the sample increases and the lattice constant decreases. The patterns shown here have obvious diffraction peaks at 25.29° and 38.41°, indicating that those samples have a regular anatase crystal structure and the addition of Zn²⁺ has no effect on the crystalline lattice of the TiO₂ nanotube arrays.

3.2 SEM analysis of TiO₂ nanotube arrays prepared under different conditions

The SEM images of the TiO₂ nanotube arrays prepared on the surface of titanium in electrolyte systems with different PH values are shown in Fig.3. When pH=1, the nanotube arrays with diameter about 70 nm were uniformly distributed on the surface. However, the TiO₂ nanotube arrays have different lengths, creating an irregular arrangement. A TiO₂ nanotube arrays with a sponge structure can be obtained when prepared under pH 4. The reason for the phenomenon is the low mass fraction of H⁺ in the reaction during its progress. And a low mass fraction of H⁺ in the reaction leads to the decrease of the etching rate and creates an incomplete array structure. There are no nanotube arrays, but some corrosion pits appear in the electrolyte system with pH 7 (Fig.3(b)).

Fig.2 XRD patterns of TiO₂ nanotube arrays and Zn²⁺-TiO₂ nanotube arrays

In the 0.5% HF reaction system, although the TiO₂ arrays have different lengths, it is obvious that the TiO₂ nanotube arrays have a better morphology, a regular arrangement, and an average tube diameter of 70 nm (Fig.4). Therefore, the formation rate and the dissolution rate of the TiO₂ nanotube arrays reach equilibrium when the mass fraction of HF is 0.5%.

Fig.5 shows that the nanotube arrays have different morphologies under different anodic oxidation voltages. When the anodic oxidation voltage is 20 V, the nanotube arrays have a regular arrangement, and the average tube diameter is about 80 nm. An unorganised arrangement is obtained when the voltage is 30 V. When it is 15 V, there are spots of nanotube arrays and some corrosion pits on the surface of titanium. The results exhibit that the anodic oxidation voltage has a significant influence on the morphology of the TiO₂ nanotube arrays. When the voltage is too low, there are no TiO₂ nanotube arrays generated, but rather some corrosion pits are formed. When it is too high, the nanotube arrays had an unorganised arrangement and easily collapsed.

Fig.3 SEM images of TiO₂ nanotube arrays prepared in electrolyte systems with different pH values: (a) pH = 1; (b) pH = 7; (c) pH = 4

Fig.4 SEM images of TiO₂ nanotube arrays prepared in electrolyte systems with different mass fractions of HF: (a) 0.3% HF;      (b) 0.5% HF; (c) 1.0% HF

Fig.6 shows the SEM images of TiO₂ and Zn²⁺-TiO₂ nanotube arrays. It is obvious that the two morphologies are similar. The tube diameter is in the range of 70–100 nm, the tube wall is about 10 nm, and the TiO₂ nanotube arrays are homogeneously distributed on the surface of titanium. The results indicate that Zn²⁺ loading has little effect on the morphology of the Zn²⁺-TiO₂ nanotube arrays.

3.3 Complex impedance analysis of TiO₂ nanotube arrays

The Nyquist diagrams (Fig.7) show that TiO₂ nanotube arrays prepared with 0.5% HF exhibit low impedance, which might have been influenced by the structure of the TiO₂ photoelectrodes. The SEM images of the TiO₂ nanotube arrays show that the nanotubes are distributed in a disordered fashion when the mass fractions of HF are 0.3% and 1.0%. A collapse even takes place at the mass fraction of 1.0%. All of these could increase the resistance of the array. When the mass fraction of HF is 0.5%, an orderly arrangement of the TiO₂ nanotube arrays is obtained. The order can reduce the interface resistance between the electrode and solution and be beneficial to the interface charge transfer. The general behaviour of the impedance spectra exhibited by the Zn²⁺-TiO₂ nanotube arrays is similar to that of the TiO₂ nanotube arrays with 0.5% HF. TiO₂ nanotube arrays loaded with Zn²⁺ would not result in an increase of resistance. Some correspondences at low and high frequencies are revealed in Fig.8(a) and 8(b), respectively. It can be inferred that the response at high frequency is related to the interface between the electrode and the electrolyte, and the response at low frequency can be associated with the diffusion process of the carrier. Furthermore, TiO₂ nanotube arrays loaded by Zn²⁺ could decrease the resistance, which is likely to accelerate the transfer of the carrier and even increase the photoelectric conversion.

3.4 Current—time curve of TiO₂ nanotube array anodizing process

In the first few seconds of the anodic process, an exponential decay of the current occurs (Fig.9). Then the current increases slowly to a quasisteady state. The causes of the current drop are analyzed as follows. Initially, Ti⁴⁺ is produced by the reaction of Ti and HF. Then, Ti⁴⁺ is oxidized on the surface of the Ti foils. As the dense membranes of TiO₂ are formed on the surface of the Ti foils, a decrease in the anodic current takes place[15-16]. The oxide thickness increases with the increase of electric field intensity in the next stage. With the solubility of the TiO₂ in HF-containing electrolytes and the electric field, TiO₂ nanotubes with homogenous distribution could be formed on titanium oxide barrier with the disordered pore sites etched by HF. It is favorable for the transfer of Ti⁴⁺ from the oxide barrier to the solution during the process of the pitting cores to form titanium oxide nanopores, and Zn²⁺ is loaded on the TiO₂ nanotubes simultaneously. Due to all of these,   the current starts to increase in the stage, leading to the random growth of TiO₂ nanopores. At the last stage, a more stable and regular self-aligned TiO₂ nanopore growth and dissolution equilibrium of Zn²⁺ were established; therefore, the current begins to decrease slowly again.

Fig.5 SEM images of TiO₂ nanotube arrays prepared at different voltages: (a) 30 V; (b) 20 V; (c) 15 V

Fig.6 SEM images of TiO₂ (a) and Zn²⁺-TiO₂ (b) nanotube arrays

Fig.7 Nyquist diagrams of TiO₂ nanotubes

Fig.8 Frequency-phase of AC impedance diagram: (a) TiO₂ nanotubes; (b ) Zn²⁺-TiO₂ nanotubes

Fig.9 Anodic oxidation process of TiO₂ nanotube arrays

4 Conclusions

1) TiO₂ nanotube arrays with regular arrangements are obtained when the mass fraction of HF is 0.5%-1.0%, the pH value is 2-4, and the anodizing voltage is 20 V. The length of these tubes is 70-100 nm, and the thickness of these tubes is about 10 nm. Moreover, the adulterate of zinc does not affect obviously the array structure of the TiO₂ nanotubes.

2) It is known that the growth of the Zn²⁺-TiO₂ nanotube arrays could be divided into three obvious phases in the preparation process. Firstly, a TiO₂ film is generated on the titanium substrate by HF acid etching. Secondly, the pitting and etching that occur on the oxide film result in microporous dent and microholes, which are evenly distributed in the basal surface gradually over time. Thirdly, nanotubes no longer elongate when the loading of Zn²⁺ and the current reach equilibrium. Finally, TiO₂ regions are dissolved through the pores among the holes and formed tubes.

3) According to the analysis on the complex impedance of nanotube arrays, it is obviously that TiO₂ nanotube arrays loaded by Zn²⁺ could decrease the resistance, which is likely to accelerate the transfer of the carrier and even increase the photoelectric conversion.

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(Edited by LI Xiang-qun)

Foundation item: Project(20976016) supported by the National Natural Science Foundation of China; Project(09JJ606) supported by the Natural Science Foundation of Hunan Province, China; Project(08FJ1002) supported by Key Science Research Project of the Hunan Provincial Natural Science, China

Corresponding author: HUANG Ke-long; Tel: +86-731-88879850; E-mail: huangkelong@163.com

DOI: 10.1016/S1003-6326(10)60648-6