Quantitative Comparison of the Photocatalytic Efficiency of TiO₂ Nanotube Film and TiO₂ Powder

Abstract

We compared the plausible reaction mechanism and quantitative efficiency of highly self-organized TiO₂ nanotube (ntTiO₂) film with TiO₂ powder. Film was fabricated by electrochemical potentiostatic anodization of titanium thin film in an ethylene-glycol electrolyte solution containing 0.3 wt% NH₄F and 2 vol% deionized water. Nanotubes with a pore size of 80-100 nm were formed by anodization at 60 V for 3 h. Humic acid (HA) was degraded through photocatalytic degradation using the ntTiO₂ film. Pseudo first-order rate constants for 0.3 g of ntTiO₂, 0.3 g TiO₂ powder, and 1 g TiO₂ powder were 0.081 min⁻¹, 0.003 min⁻¹, and 0.044 min⁻¹, respectively. HA adsorption on the ntTiO₂ film was minimal while adsorption on the TiO₂ powder was about 20% based on thermogravimetric analysis. Approximately five-fold more normalized OH radicals were generated by the ntTiO₂ film than the TiO₂ powder. These quantitative findings explain why ntTiO₂ film showed superior photocatalytic performance to TiO₂ powder.

1. Introduction

Semiconducting catalysts can be used to mineralize organic contaminants, such as aromatics, halohydrocarbons, HA and so on, in natural groundwater (Haga and Yosomiya, 1997, Wold, 1993). Titanium dioxide (TiO2), especially in anatase form, is one of the most significant catalysts in environmental applications (Fujishima et al., 2000, Fujishima and Zhang, 2006, Kim et al., 2014). It has been applied mostly in the form of glass or glass fibers, but more practical forms of this semiconductor photocatalyst are in films or on ceramic substrates (Choi, 2006, Jang and Park, 2011). A sol-gel process with heat treatment of titanium sol material is most commonly utilized to produce TiO2 in film form (Carotta et al., 2007, Haga et al., 1997, Quan et al., 2009). Aerosol pyrolysis and hydrolysis with different alkoxides or titanium-containing compounds are also used (Aromaa et al., 2007, Luyo et al., 2007, Subba Ramaiah and Sundara Raja, 2006). It is, however, difficult to produce pure TiO2 film because of residual impurities. Crystal defects in TiO2 due to these impurities are the cause of reduced catalytic activity. Porous oxide growth in alumina has motivated electrochemical synthesis of oxide nanostructures with various metals (Jang et al., 2009, Karlinsey, 2005, Lee and Smyrl, 2005, Mukherjee et al., 2003, Tsuchiya and Schmuki, 2005). Certain metal oxide nanostructures possess dramatically improved charge transport properties, enabling a variety of advanced applications. Self-organized TiO2 nanotube (ntTiO2) film has received considerable attention in recent years because it shows remarkable performance in a variety of applications. This is due to the high surface area, stability, and excellent charge transfer properties of this type of film (Liu et al., 2008, Mor et al., 2006, Nischk et al., 2014, Yang et al., 2006, Yun et al., 2013, Zhang et al., 2007).

Our objective in this research was to quantitatively compare the photocatalytic efficiencies of ntTiO2 thin film and TiO2 powder. Although ntTiO2 film has been suggested as a replacement for TiO2 powder, the photocatalytic activity of ntTiO2 film versus TiO2 powder has not been compared directly. ntTiO2 thin film evaluated in this research was produced by electrostatic anodization. Photoreactivity of the ntTiO2 film was demonstrated by measuring OH radical formation rate under UV irradiation. ntTiO2 films were characterized by Field-emission scanning election microscopy (FE-SEM), X-ray diffraction (XRD), and energy-dispersive X-ray (EDX) spectroscopy.

 

2. Material and Methods

2.1. Fabrication of ntTiO2 film via electrostatic anodization

Titanium (Ti) film (99.5% pure and 25 μm thick) was purchased from Alfa Aesar and was cut into 20 mm × 60 mm sections. Ti film was then placed in a mixture of HF, HNO3, and H2O at a volume ratio of 1:4:5 for 30 to 60 sec to remove impurities on the surface. This was followed by chemical etching, washing with deionized water for 20 min, and drying for anodization. Ethylene glycol (EG) electrolyte solution containing 0.3 wt% NH4F and 2 vol% deionized water was used. TiO2 array films were synthesized by electrostatic anodization of titanium foil as an anode in a two-electrode system (Paulose et al., 2007). Platinum (99.9% pure) was used as cathode. Distance between the two poles was fixed at 5 cm. Anodization was performed at a constant voltage of 60 V for 3 h using a regulated DC power supply with a maximum current of 5 A and maximum voltage of 160 V (EP1605, PNCYS). Temperature of the electrolyte solution was adjusted to 15℃ using coolant. The thin-film sample was stored in a vacuum desiccator after surface cleaning with distilled water. Heat treatment was carried out in electric furnace at 400-450℃ to convert the TiO2 into anatase form, which is the most photoactive among the three TiO2 crystal forms of brookite, anatase, and rutile. All chemical reagents were from Aldrich.

2.2. Photocatalysis

Humic acid (HA) was the target contaminant in this research as it is a precursor of trihalomethanes (Li et al., 2002, Yang and Lee, 2006). HA used was technical grade from Aldrich. Batch experiments to confirm the photocatalytic activity of the produced ntTiO2 were performed in a circular reactor with a 500 mL reactor column made with quartz, a Sankyo 16 W (UV-C) UV lamp, a magnetic stirrer, and a pump. The concentration of stock HA solution was 1,000 mg/L and it was diluted to 10 mg/L for batch experiments. Anatase-phase TiO2 powder (Showa) was put into HA solution for photocatalytic degradation experiments for comparison with the powder form. At different time intervals, 3-mL aliquots of HA solution were collected and analyzed.

2.3. Characterization and analysis

Surface morphologies of the samples were observed using a scanning electron microscope (SEM) with an acceleration voltage of 15.0 kV (JEOL, JJSM-6340F). Changes in crystal structure before and after calcination treatment were monitored with an X-ray diffractometer (XRD) using 1.5406 Å of CuKα radiation (λ) from 20° to 80° at a scanning speed of 5°min−1 (RIGAKU, D/MAX-2500/PC). An energy-dispersive X-ray (EDX) spectrophotometer was used for elemental analysis of the surface. HA concentration was analyzed using a total organic carbon (TOC) analyzer (Analytic Jena, Multi N/C 3100). Adsorption mass measurement with external light blocked out was done with a thermogravimetric analyzer (Perkin Elmer, TGA7) under N2 from 30℃ to 700℃ at a rate of increase of 5℃/min. Fluorescence spectra and OH radicals were measured using a fluorescence spectrophotometer (Scinco, FluoroMate FS-2) excited by a 150 W Xe lamp. Terephthalic acid (TA) was used as a fluorescence probe because it can react with OH radicals in basic solution to generate 2-hydroxy TA (TAOH), which emits a unique fluorescence signal with a spectrum peak around 426 nm (Hirakawa and Nosaka, 2002, Yang et al., 2009). Most TiO2 particles were precipitated by adding KCl, after which light scattering became negligible. Because TAOH molecules were not adsorbed on the TiO2 powder and ntTiO2 in the presence of KCl at pH 11.5, all TAOH produced was assumed to be in the supernatant. The mass of TAOH produced was determined by fluorescence intensity.

 

3. Results and Discussion

3.1. Characterization

Fig. 1 shows the surface morphology of ntTiO2 produced by electrostatic anodization. Nanotubes with a pore size of 80-100 nm formed after anodization at 60 V for 3 h. Nanotubes were about 25 μm-long at both 20 V and 40 V, consistent with a previous study that showed that nanotube growth rate is proportional to anodization time rather than voltage.

Fig. 1.FE-SEM image of the ntTiO2 film in this research via electrostatic anodization: (a) top, (b) bottom, (c) cross-section, and (d) side.

Fig. 2(a) shows ntTiO2 exposed to visible light. Hand-written letters, “Ti”, were clearly visible at the ntTiO2 part while the letters, including part of “O”, under ntTiO2/Ti could not be readily seen. The body of ntTiO2 was physically very weak; a support was therefore required for the film form and thermal treatment was used to enhance the structural strength of ntTiO2. ntTiO2 on Ti films was used after electrostatic anodization. Results of EDX analysis of the ntTiO2/Ti pattern of TiO2 are shown in Fig. 2(b). The chemical composition of ntTiO2 was 69% Ti, 18% O, and 3.12% C, similar to typical TiO2.

Fig. 2.(a) A transparent ntTiO2 film and (b) its chemical composition.

Fig. 3(a) shows the XRD pattern of ntTiO2 immediately after anodization and (b) the crystal structure of the film surface after annealing. ntTiO2 film after annealing treatment could assume one of three different forms: brookite, anatase, or rutile. Rutile is stable across a broad range of temperatures, while the other two phases are semi-stable thermodynamically. In particular, brookite is created at high temperatures and pressures. It is very unstable and it is difficult to obtain pure crystals. Anatase crystal was found on non-crystal TiO2 film with some rutile characteristics after heat treatment at 450℃.

Fig. 3.XRD patterns of thin film: (a) after anodization and (b) with annealing treatment at 450℃ for 2 hours at heating and cooling rate of 30℃/min.

3.2. Theoretical calculation of generated ntTiO2

Diameters and lengths of the ntTiO2 film ranged from 80-100 nm and 2,500 nm, respectively, based on top (Fig. 1(a)) and cross-sectional (Fig. 1(c)) images. The mass of ntTiO2 was measured indirectly using Faraday’s law (Jang and Park, 2014):

where Q is the circulated charge (C·cm−2), A is the electric current, S is the surface area of Ti film, t is the anodization time, M is the molecular weight of the oxide (79.866 g mol−1), F is the Faraday constant (96,487 C·equiv−1), n is the number of electrons involved in the reaction, δ is the density of TiO2 (3.8-4.1 g·cm−3), and L is the theoretical length of the nanotube layers. This theoretical mass (LδS) can be calculated as shown in Eq. (3) using Eqs (1) and (2). The theoretically-calculated mass of the nanotube layers in this research was about 0.102 g per film.

3.3. HA degradation

Photocatalytic experiments to degrade HA in solution were carried out using 0.3 g commercial TiO2 powder and ntTiO2 film prepared by anodization and annealing at 450℃. Kinetics of HA degradation were fitted with a pseudo-first order rate equation as follows:

where C and C0 are the residual and initial HA concentration (mg/L), respectively. The pseudo-first order rate constant is kobs (min−1). A control experiment without any catalyst was performed with 10 mg/L HA solution; less than 2% of total HA was lost under UV illumination for 90 min. A photolysis test of HA was also performed without catalysis and a light source. Typical thermogravimetric (TGA) curves of photocatalytic samples before and after HA adsorption are presented in Fig. 4. The weight change before and after HA adsorption was about 15% and 2% for 0.3 g of TiO2 powder and ntTiO2, respectively. Therefore, HA removal by adsorption on the ntTiO2 surface was negligible in HA photocatalytic degradation. In addition, aqueous TOC was measured to quantify the HA concentration; degraded HA in this research corresponded to completely-degraded HA.

Fig. 4.TGA results from the ntTiO2 and TiO2 powder form (control: without HA adsorption).

Two different samples of TiO2 powder, 0.3 g and 1 g, were evaluated to make a catalytic comparison between ntTiO2 film and TiO2 powder (see Fig. 5). ntTiO2 film exhibited superior photocatalytic efficiency to TiO2 powder. HA degradation by ntTiO2 containing a theoretical mass of 0.3 g TiO2 was completely finished in 60 min, but HA still remained after 90 min when the TiO2 powder sample was used. Table 1 showed the pseudo-first order rate constants for HA degradation in photocatalytic reactor systems with TiO2 powder or ntTiO2 film as photocatalyst.

Fig. 5.Photocatalytic degradation of HA using TiO2 powder form and ntTiO2 film.

Table 1.Pseudo first-order rate constants (k, min−1) for HA degradation on various TiO2

3.4. Comparison of generated OH radicals

Formation of active hydroxyl radicals upon UV irradiation was monitored. OH radicals react with TA and generate TAOH, which emits fluorescence. As shown in Fig. 6(a), significant fluorescence spectra due to TAOH were generated upon irradiation with 300-600 nm light. The capability of forming OH radicals per unit mass of ntTiO2 or TiO2 powder was evaluated. About five-fold more OH radicals were generated from ntTiO2 than TiO2 powder, as shown in Fig. 6(b). The higher normalized production of OH radicals from ntTiO2 films than TiO2 powder indicates that ntTiO2 film has superior catalytic efficiency to TiO2 powder.

Fig. 6.Fluorescence spectra (a) and normalized fluorescence intensity per unit mass (b) of ntTiO2 and TiO2 powder form.

NtTiO2 exhibited much lower HA adsorption on its surface than TiO2 powder, which implies that photogenerated OH radicals were greatly inhibited in ntTiO2. Photogenerated OH radicals of excited ntTiO2 were degraded instantly or blocked HA adsorption from the surface of ntTiO2, resulting in higher photocatalytic activity than the TiO2 powder. Due to lower HA adsorption on the surface, the photocatalytic effect of the ntTiO2 film was better than that of TiO2 powder.

 

4. Conclusions

NtTiO2 film was produced by electrostatic anodization at 60 V in an EG electrolyte solution containing 0.3 wt% NH4F and 2 vol% deionized water. Anatase phase photocatalytic crystals were formed after annealing treatment at 450℃. ntTiO2 film containing 0.3 g of theoretical ntTiO2 mass exhibited better photocatalytic performance than 0.3 g and 1 g of TiO2 powder with respect to HA degradation. HA adsorption on ntTiO2 film was negligible, while adsorption on TiO2 powder was about 20% based on TGA. Normalized concentration of OH radicals, measured by TAOH fluorescence, was about five times higher for ntTiO2 film than TiO2 powder. Greater generation of OH radicals by irradiation of the ntTiO2 film than the powder decreased HA adsorption by the film relative to the powder.