- Nano Express
- Open Access
Anatase TiO2 nanotube powder film with high crystallinity for enhanced photocatalytic performance
© Chen et al.; licensee Springer. 2015
- Received: 3 January 2015
- Accepted: 12 February 2015
- Published: 4 March 2015
We report on the synthesis of TiO2 nanotube (NT) powders using anodic oxidation and ultrasonication. Compared to free-standing NT array films, the powder-type NTs can be easily fabricated in a cost-effective way. Particularly, without the substrate effect arising from underlying Ti metals, highly crystallized NT powders with intact tube structures and pure anatase phase can be obtained using high-temperature heat treatment. The application of NTs with different crystallinity for the photocatalytic decomposition of methylene blue (MB) was then demonstrated. The results showed that with increasing annealing temperature, the photocatalytic decomposition rate was gradually enhanced, and the NT powder electrode annealed at 650°C showed the highest photoactivity. Compared to typical NTs annealed at 450°C, the rate constant increased by 2.7-fold, although the surface area was 21% lower. These findings indicate that the better photocatalytic activity was due to the significantly improved crystallinity of anatase anodic NTs in powder form, resulting in a low density of crystalline defects. This simple and efficient approach is applicable for scaled-up water purification and other light utilization applications.
- Titanium dioxide
In the past three decades, titanium dioxide (TiO2) and its nanocomposites have been widely investigated as promising photocatalysts [1-4]. Among various TiO2 nanostructures, TiO2 nanotube (NT) arrays synthesized by a simple electrochemical anodization provide the advantages of adjustable structures, high surface area, and excellent charge transport properties. Various applications such as photocatalytic decomposition, water splitting, photovoltaics, batteries, and sensors have been explored. TiO2 NTs have been found to have better photocatalytic properties of organic pollutants compared to commonly used TiO2 nanoparticles, although the NT surface area is much smaller [5-7]. This is due to the excellent light trapping, enhanced electron-hole separation, and much slower deactivation of NTs during the photocatalytic reaction . To further enhance the photocatalytic activity of NTs, various strategies have been used such as the improvement of morphology, crystal structure, and surface area [9,10], introduction of heterogeneous structures by decoration [11,12], and metal or non-metal doping [13,14]. Among these, the enhancement of NT crystallinity has been considered as an important and widely investigated approach to achieve better photocatalytic performances.
NT crystallinity can be simply enhanced by increasing the annealing temperature. More importantly, the stability of structures and geometries has to be considered during annealing. As to aligned NT array, the retention of its porous structure during annealing is essential to preserve its large specific surface area, which is desired for photocatalytic reaction. However, for NT arrays fabricated on metallic Ti foils, the substrate effect would result in crystallite growth in the tube walls during the high-temperature annealing process, leading to thicker tube walls and a significant decrease of the surface area. Therefore, the discussion of optimal annealing temperature and crystallinity is difficult. The highest photodegradation efficiency has been reported using NT films annealed at different temperatures such as 450°C , 500°C , 550°C , and 600°C . These earlier studies have concluded that the tubular structure was stable at or below the optimal temperatures. On the other hand, at higher temperatures, the NT structure collapses and undergoes deformation, dramatically decreasing the surface area and thus photocatalytic activity. Another disadvantage caused by the substrate effect is the phase instability of NTs under high-temperature treatment. TiO2 NTs with a stable anatase phase have excellent performance in photocatalytic and photoelectrochemical reactions [17,19-21]. However, after annealing of the as-formed NTs on their metallic substrate above approximately 550°C, a crystal phase transformation from anatase to rutile developed [15,22], which would play a role in lowering the photocatalytic performance.
The synthesis of free-standing NT films is a promising way to generate NTs without the presence of a Ti metal substrate, resulting in both structure and phase stability up to a high temperature of approximately 600°C  or 700°C , although the fabrication process is tedious and inefficient. By rapid breakdown anodization [25,26], NT bundles in powder form can be prepared. However, these bundles featured broken tubes and also contained impurities of remnants from the anodization electrolyte. Therefore, the bundles maintained stable phase and structure specifically at temperature <450°C. Therefore, the development of alternative methods is warranted. In the present study, we evaluated TiO2 NTs in powder form by dispersing as-anodized NT arrays. The morphology, structure, and crystal phase composition of the synthesized NT powders were characterized. The photocatalytic property of the powders annealed at different temperatures was also investigated to achieve optimized photocatalytic activity.
Preparation of the NT powders
The preparation details of anodic NT arrays were similar to our previous reports . Self-organized TiO2 NTs were grown on a Ti substrate (10 × 10 cm, 0.89 mm thickness, 99.7% purity, Alfa Aesar, Ward Hill, MA, USA). Then, the as-anodized irregular surface oxide layer was stripped off the Ti substrate by ultrasonication in deionized (DI) water. The cleaned Ti foil was anodized again for 2 h to grow aligned NT arrays in the same electrolyte. After the two-step anodization, ultrasonication in ethanol or DI water was carried out for 5 to 10 min to disperse the obtained oxide tube layer. Then, the Ti foil was reused for anodization formation of NTs. The above process was repeated until the Ti foil was completely consumed. The NT powder was collected by centrifugation and dried in a vacuum drying oven at 120°C overnight. The powder product was heat-treated at 450°C, 550°C, 650°C, and 750°C for 2 h in air to crystallize the NTs.
Characterization of the NT powders
The morphology of the resulting NT powders was characterized by a field emission scanning electron microscope (FE-SEM; Sirion 200, FEI, Hillsboro, OR, USA). The relative Brunauer-Emmett-Teller (BET) surface area was evaluated by adsorption-desorption isotherms using nitrogen gas at 77 K (ASAP 2010, Micromeritics, Norcross, GA, USA). Samples were degassed at 200°C for 4 h under high vacuum prior to measurement. The pore size distribution was analyzed by Barrett-Joyner-Halenda (BJH) adsorption differential pore volume. TiO2 paste was prepared by adding of the powder (20 wt%) to a mixture of isopropanol:n-butyl alcohol = 1:4 (v/v) solution. Thereafter, the paste was mixed under magnetic stirring for 24 h. The paste was deposited on fluorine-doped tin oxide (FTO) glasses by using a doctor blade technique and then air-dried, forming a porous NT film. The film was reinforced by annealing again at 450°C. The thickness of the NT layer was approximately 10 μm, which was controlled by tapes (Scotch Magic Tape, 3 M, St. Paul, MN, USA). The crystal structures were verified by X-ray diffraction (XRD; Cu Kα radiation, Rigaku 9KW SmartLab, Rigaku, Tokyo, Japan) patterns.
The photocatalytic properties of the samples annealed at four different temperatures were evaluated by the photodecomposition of methylene blue (MB). An NT-coated substrate with an active area of 9 cm2 was immersed in 100 mL of MB aqueous solution (initial concentration: 10 mg/L) for 1 h in the dark to reach the adsorption-desorption equilibrium at the TiO2 surface. Ultraviolet (UV) light (λ = 365 nm) was provided by a UV lamp assembly (8 W × 3) with the distance between lamps and the photocatalyst film of approximately 15 cm (irradiation intensity of 5 mW/cm2). All the samples were tested under the same condition. The absorbance of the MB solution was measured using a UV-vis/NIR spectrophotometer (UV-3600, Shimadzu, Kyoto, Japan) at a wavelength of 664 nm to determine variations in MB concentration with the UV irradiation time.
For the 550°C and 650°C samples, after photocatalytic reaction, the aqueous solutions were almost completely decolorized, showing complete degradation of MB. Although the 650°C sample presented a relatively small surface area, its photocatalytic activity was the highest, with the rate constant of 0.414/h. During the photocatalytic oxidation, the photogenerated electrons and holes either recombined inside TiO2 nanocrystallites or reacted with adsorbed species on the TiO2 surface. The crystallinity of NT powders gradually improved with increasing annealing temperatures, resulting in (a) decreased density of residual elements and thus diminished crystalline defects in anodic tubes , which in turn led to lower electron-hole recombination probability in bulk, and (b) effective diffusion of electrons and holes to adsorbed reactants on the TiO2 surface to participate in the decomposition reaction. Both factors lead to a more efficient electron-hole separation and thus enhanced photocatalytic activity. On the other hand, for the 750°C sample, a significantly smaller surface area and, in turn, a lower photocatalytic activity were observed.
We present a simple and cost-effective approach for the growth, dispersion, annealing, and immobilization of TiO2 NT powders. The structure and crystal phase of the NT powders at low and high temperatures were investigated. The NTs in powder form showed improved structure and crystal phase stability under high-temperature treatment compared to the conventional NTs on a Ti substrate. By increasing the temperature from 450°C to 650°C, the highly crystallized NT photocatalyst showed a significantly enhanced photocatalytic activity, mainly due to the low density of defects. Furthermore, an increase in temperature from 650°C to 750°C led to worse photocatalytic activity due to the aggregation of NTs and thus a largely diminished surface. Further utilization of high-crystallized NTs in photoelectrocatalytic degradation is currently in progress.
The work was supported by the National Natural Science Foundation of China (Grant Nos. 61125503, 61404081, and 11374204), the Shanghai Municipal Natural Science Foundation (Grant No. 14ZR1417700), the “Shu Guang” project of the Shanghai Municipal Education Commission and Shanghai Education Development Foundation (No. 13SG52), and the Science and Technology Commission of Shanghai Municipality (Nos. 12JC1404400 and 14520501000).
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