It is known that salicylic acid (SA) has an ototoxic effect and can induce transient hearing loss in zinc-deficient individuals. It can also cause fetal abnormalities and central nervous system depression if swallowed, inhaled or absorbed through skin. Salicylic acid has been identified as a water pollutant which originates from the manufacturing activities of paper milling, cosmetic industries and as a landfill leachate. Thus, there is great interest in developing advance oxidation techniques to effectively treat the salicylic acid pollutant for the purification of wastewater and groundwater . A variety of techniques have been proposed for wastewater treatment. Heterogeneous photocatalysis is a promising alternative technique for eliminating organic pollutants from wastewater. Among various oxide semiconductor photocatalysts, TiO2 is one of the most promising photocatalysts because of its biological and chemical inertness, cost effectiveness, and the strong oxidizing power of its photo-generated holes [2,3]. TiO2 has been intensively investigated as a photocatalytic material for solar energy conversion and environmental purification since Fujishima and Honda discovered the photo-catalytic splitting of water on TiO2 electrodes in 1972 [4-8]. It is well known that TiO2 is an excellent and high efficient photocatalyst. However, it only absorbs the ultraviolet light with a wavelength no longer than 387 nm due to its large band, which means the low utilization ratio of solar light for applications in photo-catalysis. Generally, TiO2 with a large specific surface area can improve the photo-catalytic performance. Therefore, many efforts have been made to obtain nano-structured TiO2-based materials with a large specific surface area . Nano-structured titanium dioxide has been attracting attention from both fundamental and applied perspectives, driven by its unique photo-activity in a broad range of applications, extending from photocatalysis of hazardous chemicals, self-cleaning surfaces, to solar energy conversion, gas sensors, and hydrogen storage [7-17].
One-dimensional TiO2 nanostructures, particularly TiO2 nanotubes, have received great attention because of their superior photocatalytic and photo-electronic performance over TiO2 nanopaticles. TiO2 nanotubes have been synthesized via various approaches, including using a template of nanoporous alumina, sol-gel processes, seeded growth method, hydrothermal techniques and the anodizing of titanium [18-20]. Among these, anodizing of titanium is a relatively simple process for the fabrication of TiO2 nanotubes. Since Zwilling et al. reported the growth of TiO2nanotubes through the electrochemical anodization of titanium, many results on TiO2 nanotubes have been reported to control the length, morphology, orientation, and the pore size of the TiO2 nanotubes by tailoring the electrochemical conditions during anodizing process.
In the present work, we report a facile and effective approach for the enhancement of the photocatalytic activity of TiO2. The TiO2 nanotubes prepared in the two-step anodization process were treated in a methanol solution under UV light irradiation. The treated TiO2 nanotubes showed considerable enhancements for photocatalytic oxidation of salicylic acid.
Chemicals and Solutions
Ethylene glycol (EG), ammonium fluoride (NH4F), Salicylic acid (SA) and methanol of analytical grade were obtained from Aldrich or Merck and used as received. All other chemicals were of analytical grade and were used as received without further purification. Distilled water was used throughout. Stock solutions were made by dissolving the salicylic acid in 0.5 M H2SO4 solution. Subsequent concentrations were obtained by diluting the stock solution with 0.5 M H2SO4 solution.
Fabrication of TiO2 Nanotube Film Electrode
A two-step electrochemical anodizing method was used to fabricate the TiO2 nanotube. Prior to anodizing, a piece of titanium sheet (99.99% purity, 1-mm thick) was cut into small rectangular pieces 10 × 40 mm and the titanium electrodes were first mechanically polished with different emery type abrasive papers, rinsed in a bath of distilled water, and then chemically etched by immersing in a mixture of HF and HNO3 acids for 1 min. The ratio of components HF/HNO3/H2O in the mixture was 1:4:5 in volume. The last step of pretreatment was rinsing with acetone and distilled water. The anodizing experiments were carried out using a conventional two-electrode system with titanium foil as anode and a platinum foil with about 20 cm2 geometric area as cathode, respectively. All electrolytes consisted of 0.3 wt% NH4F in an aqueous EG solution (2 vol% water in EG). All the anodizing experiments were carried out at room temperature. In the first-step anodizing, the titanium foil was anodized at 50 V for 1 h, and the nanotube layer was grown on the Ti surface. The nanotube layer was removed by sonicating in distilled water, and a mirror surface of titanium was exposed. Then, the pretreated titanium was used as anode again for the second-step anodizing at 50 V for 30 min. After the anodizing processes, the sample was cleaned with distilled water. To crystallize the as-anodized TiO2 nanotubes, all samples were annealed at 450 °C for 3 h in air with heating and cooling rate of 30 °C min-1. For the UV pre-treatment, a TiO2 sample was placed in a quartz tube containing 5 ml of 50% methanol-H2O (v/v) and it was irradiated under UV light for 30 min.
Characterization of TiO2 Nanotube Film
The morphology and composition of the titanium oxide nanotube films were characterized by a scanning electron microscope (Philips, Model XL30) and an energy dispersive X-ray spectroscopy (EDX). XRD analyses were performed with a XRD using Simens 5000 diffractometer with a Cu-Kα radiation source in scan mode.
At first, we designed a reactor system constructed from a quartz reactor, a TiO2 nanotube film electrode as photo-anode (working electrode) and a platinum foil electrode as cathode (counter electrode). A 400 W Hg lamp was used as the UV light source. The distance between the light source and the electrode surface was 2 cm. The lamp was positioned vertically outside the reactor. The anode and cathode were placed in parallel and a saturated calomel electrode (SCE) served as the reference electrode. The solution in the cell was continuously stirred with a magnetic stirrer bar. An EG&G 2273 potentiostat/galvanostat was used to apply an anodic potential bias during the photocatalytic degradation of salicylic acid.
RESULTS AND DISCUSSION
Characterization of the Electrode Morphology
Scanning electron microscopy (SEM) image of TiO2 nanotubes prepared in the two-step anodizing process is shown in Fig. 1. The titanium oxide nanotubes are well aligned and organized into a highly oriented array. The tops of the tubes are open, similar to that of porous alumina obtained by anodic oxidation on aluminum. The average diameters of these tubes are approximately 20-70 nm. Figure 2 shows images of the titanium oxide nanotubes pretreated in a methanol solution under UV light irradiation. Figure 3 shows the XRD patterns of the prepared titanium oxide nanotubes. Except for the peaks (marked with stars) derived from Ti substrate, all of the diffraction peaks are attributed to those of the anatase TiO2, revealing that the formed TiO2 nanotubes exist in the anatase phase.
The photocatalytic activity of the samples was evaluated by photocatalytic degradation of salicylic acid aqueous solution under UV light irradiation. However, under dark conditions without light illumination, the salicylic acid concentration does not change for every measurement using TiO2 nanotube samples. Illumination in the absence of TiO2 nanotube sample does not result in the degradation of salicylic acid. Therefore, the presence of both illumination and TiO2 nanotube sample is necessary for the efficient degradation. These results also suggest that the degradation of salicylic acid aqueous solution is caused by photocatalytic reactions on TiO2 nanotube sample surface under the UV illumination.
Figure 4 shows time dependence of the spectral absorbance of 100 ppm salicylic acid in 0.1 M H2SO4 taken at 5-min intervals during the photo-degradation process. The absorbance of salicylic acid on TiO2 nanotubes prepared in one-step anodization process (curve a) at 205 nm decreased with time and after 20 min of photocatalytic degradation, 18% of the salicylic acid was removed from the solution. For comparison, the photocatalytic oxidation of salicylic acid was also carried out using TiO2 nanotubes prepared in 2-step anodization process (curve b) and TiO2 nanotubes prepared in 2-step anodizing process with the UV pretreatment in methanol (curve c); ca. 36% and 88% removal of the salicylic acid from solution was observed, respectively.
Fig. 1. Surface morphology of the TiO2 nanotubes prepared in the two-step anodization process.
Fig. 2. Surface morphology of the TiO2 nanotubes prepared in the two-step anodization process and treated in a methanol solution under UV light irradiation.
Fig. 3. XRD patterns of TiO2 nanotubes prepared in 2-step anodization process.
Figure 5 depicts the photocatalytic degradation profiles of salicylic acid as a function of time with (a) TiO2 nanotubes prepared in one-step anodizing process (b) TiO2 nanotubes prepared in 2-step anodizing process and (c) TiO2 nanotubes prepared in 2-step anodizing process with the UV pretreatment in methanol. The experimental data of Fig. 5 were found to fit approximately a pseudo-first-order kinetic model by the linear transforms ln(C0/C) = kt (k is rate constant). The reaction rate of photocatalytic oxidation process of salicylic acid on TiO2 nanotubes prepared in 2-step anodizing process with the UV pretreatment in methanol (0.0615 min-1) was more than 2.9 times higher than that of photocatalytic oxidation process of salicylic acid on TiO2 nanotubes prepared in 2-step anodizing process (0.0213 min-1), indicating a significant enhancement of photocatalytic activity by the pretreatment in methanol solution under UV irradiation. This significant enhancement of the photocatalytic performance may be attributed to the inhibition of charge recombination of photo-induced electrons and holes, creation of oxygen vacancies, improvement in conductivity, and the promotion of interfacial charge-transfer kinetics at TiO2 nanotubes .
Photo-electrochemical Properties of the TiO2 Nanotubes
To further study, the transient photocurrent responses of the TiO2 nanotube samples prepared at different conditions are measured by switching the UV light on and off. Figure 6 shows a comparison of I-t curves of different samples. It can be seen that the photocurrent value rapidly decreases to zero as soon as the irradiation of light turns off, and the photocurrent comes back to a constant value when the light is on. This indicates that under UV light irradiation, most of the photo-generated electrons are transported to the walls of TiO2 nanotubes and then transferred to titanium substrate to produce photocurrent. As depicted in Fig. 6, the response of the photocurrent under UV irradiation is very rapid, and the photocurrents for TiO2 nanotubes prepared in one-step anodizing process (a), TiO2 nanotubes prepared in 2-step anodizing process (b), and TiO2 nanotubes prepared in 2-step anodizing process with the UV pretreatment in methanol (c) are 266 (curve a), 802 (curve b) and 3580 (curve c) µA cm-2, respectively. The steady-state photocurrent of the TiO2 nanotubes with the pretreatment is ca. 4.5 times higher than that of the un-pretreated TiO2 nanotubes.
We further tested the stability of the pretreated TiO2 nanotubes in a 0.1 M H2SO4 solution containing 100 ppm salicylic acid. The photo-current transients were recorded at an applied electrode potential of 0.56 V vs. SCE by several on-off cycles of intermittent irradiation. The duration of light pulses was set at 100 s, followed by dark current measurements for 50 s. As seen in Fig. 7, for the UV pretreated TiO2 nanotubes, there is almost no change in photocurrent, and the relative standard error for the photocurrent is ca. 1%, indicating the remarkable stability of the UV pretreated TiO2 electrode. A comparison of the photocatalytic activity of the present study with similar studies is presented in Table 1.
Fig. 4. UV-Vis absorbance spectra during the photo-catalytic degradation of salicylic acid using: (a) TiO2 nanotubes prepared in one-step anodization process, (b) TiO2 nanotubes prepared in 2-step anodization process and (c) TiO2 nanotubes prepared in 2-step anodization process with the UV pretreatment in methanol.
Fig. 5. Comparison of photo-catalytic activity of the samples prepared at various conditions for the photo- catalytic degradation process of salicylic acid.
Fig. 6. The transient photocurrent-time profiles of (a) TiO2 nanotubes prepared in one-step anodization process, (b) TiO2 nanotubes prepared in 2-step anodization process and (c) TiO2 nanotubes prepared in 2-step anodization process with the UV pretreatment in methanol.
Fig. 7. Stability test for TiO2 nanotubes with the UV pretreatment in methanol.
Table 1. Comparison of the Photocatalytic Activity of the Present Study with Similar Studies
Effect of Addition of Persulfate and Periodate on the Degradation of Salicylic Acid
The effect of addition of S2O82- and IO4- on the photocatalytic degradation of salicylic acid has been investigated by varying the amount of (NH4)2S2O8 and NaIO4 from 0-100 mg l-1. The results are shown in Figs. 8A,B. It can be seen from Fig. 8A that increasing of persulfate concentration causes the increasing of degradation of salicylic acid. Addition of persulphate to photocatalytic processes enhances the degradation rate by two ways [22,23]:
Fig. 8. Effect of addition of persulfate and periodate on the degradation of salicylic acid in solution containing 100 ppm salicylic acid in 0.1 M H2SO4.
(I) S2O82- scavenges the conduction band electron and promotes the charge separation and production of other oxidizing species namely sulphate radical anion (Eq. (1)).
S2O82- + e-(CB) → SO4●- + SO42- (1)
(II) S2O82- can generate sulphate radical anion (SO4●-) both thermally and photocatalytically in aqueous solution. This radical anion is a strong oxidant and participates in the degradation processes by the following pathways (Eq. (2-5)):
SO4●- + e-(CB) → SO42- (2)
SO4●- + H2O2 → ●OH + SO42- + H+ (3)
SO4●- + salicylic acid → salicylic acid intermediate + SO42- (4)
SO4●- + salicylic acid intermediate → mineralization (5)
Figure 8B shows the effect of addition of IO4- on the photocatalytic degradation of salicylic acid. It can be seen that increasing of periodate concentrations increases the degradation efficiency of salicylic acid. IO4- increases the degradation of salicylic acid by capturing the electrons ejected from TiO2 so that the probability of recombination of electrons and holes will decrease, i.e. the available number and the survival time of holes will be higher and thus react effectively with adsorbed water molecules to produce more hydroxyl radicals .
IO4- + 8e-(CB) + 8H+ → 4H2O + I- (6)
Also, the photocatalytic degradation of periodate under UV irradiation involves the formation of a number of highly reactive radical and non-radical intermediates as follows:
IO4- + hυ→ IO3● + O●- (7)
O● + H+ ↔ ●OH (8)
●OH + IO4- → -OH + IO4● (9)
H4IO6- + hυ → H3IO5●- + ●OH (10)
H3IO5●- → IO3- + H2O + ●OH (11)
Comparison of UV/TiO2/IO4- and UV/TiO2/S2O82- processes on the photocatalytic degradation of salicylic acid shows that the efficiency of salicylic acid degradation using UV/TiO2/IO4- is higher than UV/TiO2/S2O82- because IO4- is one group of oxidants, which has more than two atoms of oxygen and one atom of halogen (I) as a central atom. Polarizability difference in oxidant atoms makes its central atom extremely electropositive. Therefore, IO4- can capture the electrons ejected from photo-catalyst more than other oxidants .
In summary, we have successfully demonstrated a facile and effective approach for enhancing the activity of TiO2 nanotubes. Compared with TiO2 nanotubes without the UV pretreatment, the TiO2 nanotubes pretreated in a methanol solution under UV light irradiation exhibited substantial enhancements in both photocurrent and activity. The degradation rate of salicylic acid using UV/TiO2/oxidant was higher than that for UV/TiO2 and increased with increasing oxidant concentration. The rate of degradation of salicylic acid using UV/TiO2/IO4- was higher than UV/TiO2/S2O82-. The facile and effective approach proposed in this study opens many opportunities to develop highly active TiO2 photo-catalysts for the pressing energy and environmental applications, ranging from water splitting, solar energy, and air purification to wastewater treatment.