Large-scale preparation of nanoporous TiO2 film on titanium substrate with improved photoelectrochemical performance
© Tan et al.; licensee Springer. 2014
Received: 13 March 2014
Accepted: 12 April 2014
Published: 24 April 2014
Fabrication of three-dimensional TiO2 films on Ti substrates is one important strategy to obtain efficient electrodes for energy conversion and environmental applications. In this work, we found that hierarchical porous TiO2 film can be prepared by treating H2O2 pre-oxidized Ti substrate in TiCl3 solution followed by calcinations. The formation process is a combination of the corrosion of Ti substrate and the oxidation hydrolysis of TiCl3. According to the characterizations by scanning electron microscopy (SEM), X-ray diffraction (XRD), and diffuse reflectance spectroscopy (DRS), the anatase phase TiO2 films show porous morphology with the smallest diameter of 20 nm and possess enhanced optical absorption properties. Using the porous film as a working electrode, we found that it displays efficient activity for photoelectrocatalytic decolorization of rhodamine B (RhB) and photocurrent generation, with a photocurrent density as high as 1.2 mA/cm2. It represents a potential method to fabricate large-area nanoporous TiO2 film on Ti substrate due to the scalability of such chemical oxidation process.
KeywordsNanoporous TiO2 film Titanium substrate Photocurrent Photoelectrocatalysis
In recent years, TiO2 has been widely studied and applied in diverse fields, such as photocatalysis, dye-sensitized solar cell, self-cleaning surface, sensor, and biomedicine [1–6]. It is well known that TiO2 nanoparticles have the potential to remove recalcitrant organic pollutants in wastewater. However, it is prerequisite to produce immobilized TiO2 photocatalysts with highly efficient activity by scale-up methods. Recently, considerable efforts have been taken to use metallic titanium as the precursor to develop three-dimensional TiO2 films with controllable ordered morphologies, such as nanotubes , nanorods , nanowires , and nanopores . The in situ-generated TiO2 films over titanium substrates possess such advantages as stable with low carbon residual, excellent mechanical strength, and well electron conductivity, which make them suitable to be used as electrodes for photoelectrochemical-related applications [6, 11]. Although a well-defined structural nanotube or nanoporous TiO2 film on metallic Ti can be synthesized by an anodic method [6, 7, 10–13], it is still a big challenge to scale up the production of such TiO2 film due to the limitation of electrochemical reactor and the high energy consumption. Chemical oxidation methods by treating titanium substrates in oxidation solutions are more scalable for various applications. By soaking titanium substrates in H2O2 solution followed with calcinations, titania nanorod or nanoflower films can be obtained [8, 14]. However, the film always displays discontinuous structure with many cracks, and its thickness is less than 1 μm [8, 15]. Both of these would result in a low photoelectrochemical performance. With the addition of concentrated NaOH in the H2O2 solution, a porous nanowire TiO2 film can be achieved after an ionic exchange with protons and subsequent calcinations . Employing NaOH and organic solvent as the oxidation solution and elevating the treating temperature, Ti substrate would completely transform into free-standing TiO2 nanowire membranes . However, the disappearance of Ti substrate makes this membrane impossible to serve as an electrode.
Compared to titanium alkoxides or TiCl4, there are much fewer reports on the synthesis of TiO2 nanostructure with the precursor of TiCl3. Normally, anatase TiO2 film can be fabricated via the anodic oxidation hydrolysis of TiCl3 solution [17, 18]. Recently, Hosono et al. synthesized rectangular parallelepiped rutile TiO2 films by hydrothermally treating TiCl3 solution with the addition of a high concentration of NaCl , and Feng et al. developed TiO2 nanorod films with switchable superhydrophobicity/superhydrophilicity transition properties via a similar method . Moreover, a hierarchically branched TiO2 nanorod film with efficient photon-to-current conversion efficiency can be achieved by treating the nanorod TiO2 film in TiCl3 solution . However, all of these nanostructural TiO2 films from TiCl3 solution were grown over glass or alumina substrates. Fabricating nanostructral TiO2 films over metallic Ti substrates is a promising way to providing high-performance photoresponsible electrodes for photoelectrochemical applications. The obstacle for starting from Ti substrates and TiCl3 solution must be the corrosion of metallic Ti at high temperatures in the HCl solution, which is one of the components in TiCl3 solution. However, the corrosion could also be controlled and utilized for the formation of porous structures. According to reports, the general method to prepare nanoporous TiO2 film on Ti substrate is through anodic oxidation and post-sonication [10, 12]. In this contribution, we proposed a facile way to fabricate nanoporous TiO2 films by post-treating the H2O2-oxidized TiO2 film in a TiCl3 solution. The as-prepared nanoporous TiO2 film display homogeneous porous structure with enhanced optical adsorption property and photoelectrocatalytic performance, which indicates that the film is promising in the applications of water purification and photoelectrochemical devices.
Cleansed Ti plates (99.5% in purity, Baoji Ronghao Ti Co. Ltd., Shanxi, China) with sizes of 1.5 × 1.5 cm2 were pickled in a 5 wt% oxalic acid solution at 100°C for 2 h, followed by rinsing with deionized water and drying in an air stream. The nanoporous TiO2 film was prepared by a two-step oxidation procedure. Briefly, the pretreated Ti plate was firstly soaked in a 15 mL 20 wt% H2O2 solution in a tightly closed bottle, which was maintained at 80°C for 12 h. The treated Ti plate was rinsed gently with deionized water and dried. Then, it was immersed in a 10 mL TiCl3 solution (0.15 wt%) at 80°C for 2 h. Finally, the film was cleaned, dried, and calcined at 450°C for 2 h. The obtained nanoporous TiO2 film was designed as NP-TiO2. Two control samples were synthesized, including the one designed as TiO2-1, which was obtained by directly calcining the cleansed Ti plate, and the other named as TiO2-2, which was prepared by one-step treatment of the Ti plate in a TiCl3 solution.
The surface morphology of TiO2 films was observed using a field emission scanning electron microscope (SEM; Zeiss Ultra 55, Oberkochen, Germany). The crystal phases were analyzed using a powder X-ray diffractometer (XRD; D8 Advance, Bruker, Ettlingen, Germany) with Cu Kα radiation, operated at 40 kV and 36 mA (λ = 0.154056 nm). UV-vis diffuse reflectance spectra (DRS) were recorded on a Lambda 950 UV/Vis spectrophotometer (PerkinElmer Instrument Co. Ltd., Waltham, MA, USA) and converted from reflection to absorption by the Kubelka-Munk method.
Photoelectrochemical test systems were composed of a CHI 600D electrochemistry potentiostat, a 500-W xenon lamp, and a homemade three-electrode cell using as-prepared TiO2 films, platinum wire, and a Ag/AgCl as the working electrode, counter electrode, and reference electrode, respectively. A 0.5 M Na2SO4 solution purged with nitrogen was used as electrolyte for all of the measurements.
The photocatalytic or photoelectrocatalytic degradation of rhodamine B (RhB) over the NP-TiO2 film was carried out in a quartz glass cuvette containing 20 mL of RhB solution (C28H31ClN2O3, initial concentration 5 mg/L). The pH of the solution was buffered to 7.0 by 0.1 M phosphate. The solution was stirred continuously by a magnetic stirrer. Photoelectrocatalytic reaction was performed in a three-electrode system with a 0.5-V anodic bias. The exposed area of the electrodes under illumination was 1.5 cm2. Concentration of RhB was measured by spectrometer at the wavelength of 554 nm.
Results and discussion
A nanoporous TiO2 film on Ti substrate was synthesized by treating the initially H2O2-oxidized Ti plate in hot TiCl3 solution and followed by calcinations. The pre-oxidation in H2O2 solution is necessary to form such porous structure, indicating that the formation process is a combination of the corrosion of Ti substrate and the oxidation hydrolysis of TiCl3. The film possesses exclusively anatase phase and hierarchical porous morphology, with the diameter of the inside pores as small as 20 nm. The porous TiO2 film displays enhanced optical absorption, photocurrent generation, and efficient photoelectrocatalytic activity for RhB decolorization. The generated photocurrent density can reach as high as 1.2 mA/cm2. The chemical oxidation method for the nanoporous TiO2 film is possible to be scaled up and developed into a strategy to provide efficient TiO2 electrodes for diverse applications.
This work is financially supported by the Natural Science Foundation of China (No. 21377084) and Shanghai Municipal Natural Science Foundation (No. 13ZR1421000). We gratefully acknowledge the support in DRS measurements and valuable suggestions by Ms. Xiaofang Hu of the School of Environmental Science and Engineering, Shanghai Jiao Tong University.
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