Effect of annealing temperature on wettability of TiO2 nanotube array films
© Yang et al.; licensee Springer. 2014
Received: 20 May 2014
Accepted: 10 October 2014
Published: 18 November 2014
Highly ordered TiO2 nanotube array (TN) films were prepared by anodization of titanium foil in a mixed electrolyte solution of glycerin and NH4F and then annealed at 200°C, 400°C, 600°C, and 800°C, respectively. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), water contact angle (WCA), and photoluminescence (PL). It was found that low temperature (below 600°C) has no significant influence on surface morphology, but the diameter of the nanotube increases from 40 to 50 nm with increasing temperature. At 800°C, the nanotube arrays are completely destroyed and only dense rutile film is observed. Samples unannealed and annealed at 200°C are amorphous. At 400°C, anatase phase appears. At 600°C, rutile phase appears. At 800°C, anatase phase changes into rutile phase completely. The wettability of the TN films shows that the WCAs for all samples freshly annealed at different temperatures are about 0°. After the annealed samples have been stored in air for 1 month, the WCAs increase to 130°, 133°, 135°, 141°, and 77°, respectively. Upon ultraviolet (UV) irradiation, they exhibit a significant transition from hydrophobicity to hydrophilicity. Especially, samples unannealed and annealed at 400°C show high photoinduced hydrophilicity.
In 1997, Wang et al.  reported that ultraviolet (UV) illumination of TiO2 surfaces could produce a highly hydrophilic surface which was named as super-hydrophilicity. Since then, hydrophilic TiO2 materials have attracted attention for many practical applications such as self-cleaning and antifogging materials . It is well known that the wettability of TiO2 thin films strongly depends on the preparation methods and annealing conditions, which have a decisive influence on the physical and chemical properties of TiO2 thin films [3, 4]. Therefore, it is necessary to investigate the effects of the preparation process and annealing conditions on the wettability of the films.
TiO2 thin films can be prepared by techniques such as sol-gel method , template method , sputtering method [7, 8], vapor and liquid phase deposition , and electrochemical deposition [10, 11]. In 1999, Zwilling and co-workers grew the first highly ordered TiO2 nanotube array (TN) films by anodization of titanium foil in a HF-containing electrolyte, which has attracted wide interest . Thermal treatment of TiO2 and TiO2 nanotubes provides a facile route to control grain size, particle morphology, microstructures, phase composition, and surface photoelectrochemical properties via adjusting experimental parameters such as temperature, time, and atmospheres [13–17]. For the wettability of TN films, previous works mainly focus on tubular geometry modified with low surface energy materials. In this work, by changing the annealing temperature, the effects of phase transition combined with nanotubular morphology on the wettability of TN films were investigated and discussed.
Preparation of samples
Titanium foils (0.2 mm thick, 99.6% purity), glycerin (analytical reagent (A.R.), 99.0%), and NH4F (A.R., 96.0%) were used in this experiment. Titanium foils were degreased by sonicating in acetone and deionized (DI) water, followed by rinsing with DI water and drying in air. Anodization of titanium foils was carried out at room temperature using a two-electrode system (2-cm separation) with a direct current power supply. The samples were anodized in solutions containing 0.175 M NH4F consisting of mixtures of DI water and glycerol (volume ratio 1:20) at 30 V for 3 h, similar to the method described by Schmuki and co-worker . After fabrication, the samples were rinsed in DI water and dried in air. Thermal annealing was performed in ambient air for 2 h at 200°C, 400°C, 600°C, and 800°C, respectively.
Characterization of samples
where L and H are the diameter and height, respectively, of the spherical crown of the droplet dropped on the surface of the thin films. The experimental error of the measurements is ±1°.
Results and discussion
Microstructure and morphology
To characterize the wettability, both WCA and sliding angle (SA) are necessary. However, in our experiment, the films can hold water droplets through adhesive forces, even when the layer is tilted vertically or turned upside down; thus, surface wettability is evaluated by WCA measurement. Herein, the WCAs for all samples freshly annealed at different temperatures are about 0°. That is, the water droplet entirely spreads. This finding implies that the intrinsic state of a titania film is hydrophilic [22, 23] and this is in line with the classical Wenzel model . In this model, it is assumed that a droplet fills up a rough surface, therefore, forming a fully wetted contact, which depends on the roughness factor and the surface free energy. In other words, the Wenzel model represents a classical situation of a droplet on a ‘flat’ surface just modified by a roughness factor. Herein, when a high surface energy material combines with micro- and nanoscale roughness, a super-hydrophilic surface is obtained.
where fv is the area fraction of vapor on the surface and θ is the contact angle measured on the flat surface. As fv is always lower than unity, this model always predicts enhancement of hydrophobicity only .
As shown in Figure 2a,b,c, the average pore size and the space among the tubes increase with increasing annealing temperature up to 400°C. Meanwhile, the WCA increases from 130° to 135°. This can be explained naturally by the Cassie and Baxter model, that the trapped air among tubes would increase the liquid-air contact area, thereby forming bigger WCA. From Figure 2d, at 600°C, the WCA is up to 141° and the sample surface roughness may be enhanced because of the breaking of nanotubes. Therefore, the increase of WCA may be attributed to the increase of area fraction of vapor on the surface fv and the enhanced roughness. When the sample was annealed at 800°C, the WCA decreases to 77°, caused by collapsing of the nanotubular structure.
WCA reduction rate of the samples annealed at different temperatures
Annealing temperature (°C)
Before UV irradiation WCA (o)
After UV irradiation WCA (o)
Contact angle reduction rate (%)
Furthermore, from Table 1 and Figure 3, it is noteworthy that the unannealed sample possesses a WCA of 23° after UV irradiation, which is much smaller than that annealed at 200°C. Combining XRD analysis with the SEM image, it can be seen that they are both amorphous and almost have the same surface morphology. Sun et al. proposed a mechanism of the photoinduced hydrophilicity of TiO2, which originates from an increase of the hydroxyl groups on the TiO2 surface. Some of the photogenerated holes diffuse to the surface of TiO2 and react with lattice oxygen, leading to the formation of surface oxygen vacancies. Water molecules are feasible to occupy those oxygen vacancies to produce surface-adsorbed hydroxyl groups, which tend to make the surface hydrophilic . Therefore, variation of the WCA should be primarily attributed to the difference in their surface defective sites.
In summary, TN films have been prepared by anodization on a pure titanium foil. The crystal structure of TN films transforms from amorphous to anatase and rutile with annealing temperature increasing from room temperature to 800°C. The study of the wettability of the films shows that the WCAs for all samples freshly annealed at different temperatures are about 0°. After the annealed samples have been stored in air for 1 month, the WCA has a significant increase. Upon UV irradiation, it exhibits a significant transition from hydrophobicity to hydrophilicity. The study of the wettability of the films shows that the wettability of the TN films relates to the superficial cleanliness, crystal structure, oxygen vacancy defects, and surface morphology.
This work is supported by the National Natural Science Foundation of China (Nos. 51472003 and 51272001), the National Key Basic Research Program (2013CB632705), and the National Science Research Foundation for Scholars Return from Overseas, Ministry of Education, China. The authors would like to thank Yonglong Zhuang and Zhongqing Lin of the Experimental Technology Center of Anhui University for the electron microscope test and discussion.
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