FABRICATION OF TITANIUM DIOXIDE (TiO2) NANOTUBE ARRAYS DOPED WITH PLATIN (Pt) AND THEIR PHOTOELECTROCHEMICAL CHARACTERISTICS

In this work, titanium dioxide nanotube arrays (TiO2 NTAs) doped with Platin (Pt/TiO2 NTAs) was successfully synthesized at low temperature by anodic and chemical reduction methods. The synthesized nanomaterials are characterized by X-ray energy dispersion (EDX), and scanning electron microscope (SEM). The photochemical properties of the catalyst were characterized by cyclic voltammetry method (CV), photocurrent responses (I-t) and UVVis diffuse reflectance spectra. Results showed that Pt/TiO2 NTAs with 0.57 (wt.%) Pt exhibited the highest photoelectrochemical properties. Furthermore, high surface area, small particle size, and enhanced visible-light absorption, as well as improved charge transfer and separation, are believed to be important for the improvement of photocatalytic activity of the doped materials.


INTRODUCTION
TiO 2 has long been an important photocatalytic material, it was discovered by Fujishima and Honda (1972) through its ability to split water into oxygen and hydrogen on TiO 2 electrodes [1].TiO 2 semiconductor photocatalyst has been one of the most promising advanced oxidation processes in the decomposition of environmental pollutants, the detoxification of water and air, etc.TiO 2 has the advantage over other semiconductors with photocatalytic activity because TiO 2 is a low cost, chemically inert, photoactive as well as self-recovery, easy to reuse [2,3]. However, the applications of TiO 2 in the photocatalytic field have not yet brought about high efficiency due to certain limitations: (i) -TiO 2 has a large Eg (3.0 -3.2 eV bandgap energy) corresponds to light energy of short wavelength (λ ≤ 400 nm). Therefore, it can only absorb ultraviolet (UV) radiation, while UV radiation accounts for a minimal proportion (~ 5 %) in the solar radiation spectrum; (ii)-The rapid recombination of photogenerated electron-hole pairs also significantly reduces its photocatalytic efficiency [4,5]. To improve the catalytic activity, various metal nanoparticles like Pt, Au, and Ag, etc [6 -8] have been doped onto TiO 2 nanotubes through deposition precipitation or ion exchange. This strategy is considered a Schottky junction formed by contact of semiconductor with metal. Precious metal particles have the property of concentrating free charges at the Schottky junction when the particles are excited by light, known as surface Plasmon resonance. In recent years, there have been many methods for improving the photocatalytic performance of TiO 2 , including noble metal doping [9 -12], surface modification [13,14], coupling with other semiconductor compounds [15 -17], etc. For noble metal nanoparticles (NPs), the Pt NPs decorate onto the TiO 2 NTAs exhibits convenient electrochemical performance and excellent durability. The photo-excited electrons in the TiO 2 nanotube array transfers from conduction band to Pt NPs due to the Schottky barriers formed at the interface between TiO 2 nanotube array and Pt NPs, which depresses the recombination of photogenerated electron/hole pairs and good enhances the photocatalytic performance [7,18].
In this study, the fabrications of TiO 2 nanotube arrays doped with Pt photoelectrode (Pt/TiO 2 NTAs) by anodic and chemical reduction methods at low temperature were investigated. Their structure and photoelectrochemical properties were also studied by SEM, EDX, cyclic voltammetry (CV) method, photocurrent responses (I-t) and UV-Vis diffuse reflectance spectra.

Methods
Fabrication of TiO 2 /Pt NTAs photoelectrodes: Fist, Piece of titanium (1 × 3 × 0.127 mm) were cleaned in 3 % hydro fluoric acid (20 s) and twice distilled water, then ultrasonic cleaned for 20 min in acetone and ethanol solution. TiO 2 NTAs were fabricated by anodizing cleaned Ti foil using a two-electrode configuration with a platinum cathode in an aqueous electrolyte containing 0.1 M NaF and 0.5 M NaHSO 4 at room temperature for 2.0 h [19,20]. The asanodized were ultrasonic cleaned for 10 min in iso-propanol solution to remove the debris on the surface of NTAs and dried in air, then annealed at 500 o C for 3 h in air with heating and cooling rate of 2 o C min -1 . Pt NPs were deposited into pores and onto surface of the TiO 2 NTAs by successive ionic layer adsorption and reaction (SILAR) technique [7], in which the TiO 2 NTAs photoelectrode was immersed into a 10 g L -1 H 2 PtCl 6 solution for 5 min, and then moved into 0.0022 g L -1 NaBH 4 solution for 5 min, achieving an uniform deposition of Pt NPs within the TiO 2 NTAs internal surface. The two-step dipping procedure is termed as one SILAR cycle and the procedure was repeated until a desired deposition of Pt NPs was achieved. The achievement photoelectrode denoted Pt (x)/TiO 2 NTAs (x: deposition cycles).

Characterization methods
The morphologies of the as-prepared materials were studied using scanning electron microscope (SEM, Model Jeol 6510LV). Energy dispersive X-ray spectrometers (EDX) fitted to the electron microscope was used for elemental analysis.
All electrochemical measurements were performed with a CHI electrochemical analyzer (CHI660B, Shanghai Chenhua Instrument Co. Ltd.), using a conventional three-electrode system including a Ag/AgCl reference electrode, a Pt sheet counter electrode, and the Pt/TiO2 NTAs sample as the working electrode. A 500 W Xe lamp (CHF-XQ-500 W, Beijing Changtuo Co., Ltd.) was used as the light source, filtered to 100 mW cm −2 AM1.5G as determined by a radiometer (NOVA Oriel 70260). Photoelectrochemical properties were measured in a 0.24 M Na 2 S and 0.35 M Na 2 SO 3 aqueous solution. The method of cyclic potential scanning (CV) was measured by the device IM6 electrochemical equipment firm Zahner Elektrik -Germany. UVvis absorption spectra were recorded using a UV-Visible Cary 300 spectrophotometer equipped with an integrating sphere 150 mm in diameter.   (Figs. 1(A-D)). The loaded Pt NPs onto the TiO 2 NTAs surface increased with increasing the cycle deposition (Figs 1. A-D). While with too much loading, the Pt NPs block the nanotube array pores (Fig. 1D). The deposition of the sensitizers significantly increased the surface area, benefitting the photoelectrochemical properties due to the enhanced adsorption of targets. Figure 2 shows the cyclic voltammetry (CV) curves of the TiO 2 NTAs and Pt(5)/TiO 2 NTAs under light and black conditions. Figure 2A shows a comparative cyclic voltammetry (CV) study of the TiO 2 NTAs light on and TiO 2 light off at a scan rate of 10 mV/s. It clearly reflects the significant increase in the area under the CV curve that was achieved for the active TiO 2 NTAs electrode material under fixed light intensity. Photocurrent density increased with an increase in the applied potential under fixed light intensity. Notably, the Pt NPs link with the TiO 2 NTAs can act as electron sinks, store and shuttle photo-generated electrons, thereby it can facilitate the charge carrier separation and reduce the charge carrier recombination rate [21]. Compared with the TiO 2 electrodes, Pt/TiO 2 NTAs electrodes (Fig. 2B) show larger voltammetric currents; therefore, supporting superior charge storage property. The difference in potential of anodic (E A ) and cathodic peaks (E C ) is a measure of reversibility and internal resistance of the material [22]. The smaller value of (E C -E A ) of Pt/TiO 2 NTAs electrode indicates that the electrode material is highly reversible; the origin of which is described as lower internal resistance due to improved electrical conductivity upon Pt NPs doping. This could be attributed the increased carrier mobility and thus higher electron diffusion coefficient.  Figure 3 shows the photocurrent response diagram of TiO 2 and TiO 2 /Pt NTAs with different Pt content under AM1.5G illumination. The photocurrent density of the modified TiO 2 NTAs photoelectrodes is higher than the unmodified TiO 2 NTAs. This result implies that the formation of the Schottky barrier at the interface of Pt and TiO 2 suppresses the recombination of photogenerated charge carriers [7,23]. The photocurrent density increased with the deposition cycles as shown in Fig. 3(b-e). The highest photocurrent was achieved with the 5 cycles deposition, i.e. the Pt(5)/TiO 2 NTAs (curve d) on which as high as 0.538 mA.cm -2 is obtained. Compared with pure TiO 2 NTAs, Pt/TiO 2 NTAs with 5 deposition cycles of Pt (Pt(5)/TiO 2 ) are higher photocurrent response of about 3 times. It recognized that there is an optimum number of cycles deposition: low Pt NPs loading results in minimal sensitization, while the deposition of too many Pt NPs block the nanotube array pores which in turn decreases overall photoactivity.  The EDX analysis shown in Fig. 4 reveals that the Pt(5)/TiO 2 NTAs are composed of Ti, O, Pt, with Pt content of 0.57 wt.%. Figure 5 shows the UV-Vis diffuse reflectance spectra of a pure TiO 2 NTAs sample (curve a) and Pt(5)/TiO 2 NTA sample (curve b). As can be seen, the absorption band for pure TiO 2 is observed in the UV region (at ~ 400 nm), whereas it was shifted to the visible region for Pt/TiO 2 NTAs due to the contribution of the Pt NPs localized surface Plasmon resonance (LSPR) absorption at 416 nm. The deposition of Pt NPs (curve b) has extended and enhanced the absorption spectrum into the visible light region, with a red shift of the absorption peaks from 571 nm for TiO 2 NTAs to 704 nm for Pt(5)/TiO 2 NTAs.

CONCLUSIONS
A Pt/TiO 2 nanotube array photoelectrode was successfully prepared and characterized. The TiO 2 nanotube arrays were achieved by anodization of Ti foil. After that Pt NPs with an average size of approximately 25 nm were deposited onto the TiO 2 nanotube arrays using the SILAR technique. With five cycles deposition of Pt onto TiO 2 NTAs surface, the highest obtained photoelectric current density is 0.538 mA.cm -2 . The UV-Vis measurement results of the material showed that the light absorption spectrum was widened and shifted to the visible region. As expected, the material promises good results in photoelectric sensing applications, decomposition of organic pollutants and other photocatalytic applications, etc.