Abstract
The photoelectrocatalytic performance of benzoic acid on TiO2 nanotube array electrodes was investigated. A thin-cell was used to discuss the effect of the bias voltage, illumination intensity, and electrolyte concentration on the photoelectrocatalytic degradation efficiency of benzoic acid. The photogenerated current-time (I-t) profiles were found to be related to the adsorption and the degradation process. The relationship between the initial concentration and the photocurrent peaks () fits the Langmuir-type adsorption model, thus confirming that the adsorption of benzoic acid on TiO2 nanotube arrays (TNAs) was single monolayer adsorption. At low concentrations, the I-t profiles simply decay after the initial transient peak due to the sufficient holes on the TNAs which would oxidize the benzoic acid quickly. However, the I-t profiles varied with increasing benzoic acid concentrations because the rate of diffusion in the bulk solution and the degradation of the intermediate products affect the photoelectrocatalysis on the electrode surface.
1. Introduction
For nearly three decades, the photocatalytic and photoelectrocatalytic degradation of organic substances on wide-bandgap semiconductive TiO2 nanomaterials have drawn considerable attention because of their high photocatalytic efficiency, nontoxicity, nonphotocorrosiveness, favorable biological and chemical inertness, and low cost [1–5]. However, conventional TiO2 nanoparticles also exhibit defects such as a high electron-hole recombination rate, low photocatalytic degradation efficiency, and difficult follow-up process [6].
To overcome these defects, considerable effort has been devoted to improving the photocatalytic ability of TiO2 catalysts, including the synthesis of new TiO2 structures [7–11]. TiO2 nanotube arrays (TNAs) have gained attention and have been successfully applied to the photoelectrocatalytic degradation of a variety of organic pollutants because of their peculiar architecture, large surface area, high adsorption capacity, easy teleportation, and highly efficient degradation of organics [12–15].
Traditional studies on the photoelectrocatalytic oxidation of organic compounds were mainly performed in bulk reactors, wherein the pollutant concentration was repeatedly determined over a certain period via absorption spectrophotometry [16–18]. However, this method limits the in-depth investigation of the reaction mechanism because of the relatively large body solution and long diffusion channel [19]. The use of a thin-cell with a small-volume cavity facilitated the study of the adsorption characteristics of organic substances and their interactions with the catalyst because of the subsequent increase in the ratio of the photoanode area to the organic solution volume. These studies led to the rapid completion of the exhaustive redox reaction [19, 20].
Benzoic acid was typical aromatic ring acid compound. It was the most commonly used food preservative and antibacterial agent [21]. It was also extensively used in the printing and dyeing industry (as a mordant of dyeing and printing) as well as in the pharmaceutical (as the intermediate) [22], steel (as an antirust agent) [23], and other industries [24]. In addition, benzoic acid is the intermediate metabolic product of various aromatic compounds and also the degradation intermediate product of many complex refractory organics [25]. However, few reports on the photoelectrocatalytic performance of benzoic acid compounds have been released.
In this paper, the photoelectrocatalytic performance of benzoic acid on TNA electrodes was investigated using a thin-cell reactor. The adsorption properties, microcosmic degradation process, and reaction characterization of benzoic acid on TNAs were investigated by analyzing the change in the photogenerated current-time (-) profiles. The result of this study provides a solid foundation for future research into the reaction mechanism and reaction kinetics of aromatic organic compounds on TNA electrode surfaces.
2. Materials and Methods
2.1. Materials
Titanium sheets (0.25 mm thick, 99.9% purity) were purchased from Sumitomo Chemical (Japan). Unless otherwise indicated, reagents were obtained from Sinopharm Chemical Reagent Company and were used as received. High-purity deionised water was used in the preparation of all solutions.
2.2. Preparation of TiO2 Nanotube Array Electrode
The preparing method of TiO2 nanotube array electrodes has been published in our previous work [18]. Hence, only key points of the fabrication process were summarized here. The electrolyte contain 0.1 mol/L NaF, 1 mol/L NaHSO4 and 0.2 mol/L trisodium citrate, also with NaOH added to adjust the pH. The TiO2 nanotube array electrodes are prepared under constant stirring for 4 h. After anodization, the as-prepared sample was washed with high-purity deionized water and dried at room temperature. Then it was annealed in a laboratory muffle furnace at 500°C to crystallize in the air ambience. X-ray diffraction (Bruker, D8 ADVANCE, Germany) analysis showed that the crystal structure of TNA is anatase [18].
2.3. Apparatus and Methods
All experiments were performed at room temperature in a three-electrode electrochemical cell with a quartz window (0.82 cm2). The thickness of the spacer was 0.1 mm, which defined the cell volume of 8.2 μL. The structure of thin-cell reactor has been reported in our previous paper [18]. The TiO2 nanotube array electrode was used as a working electrode; a saturated Ag/AgCl electrode and a platinum foil were used as the reference and the counter electrodes, respectively. Also 2 mol/L NaNO3 was chosen as the supporting electrolyte in the experiment. The supply bias and work current are controlled by a CHI (CH Instrument, Inc., CHI601d, USA) electrochemical analyzer; 2W LED UV lamp (365 nm) was chosen as UV light source. The light intensity in experiment was measured by UV-A ultraviolet irradiator.
3. Results and Discussion
3.1. The Photocurrent-Time Profile Property and Net Charge Determined in Thin-Cell
In the photoelectrochemical reaction, the oxidation of organic compounds at the electrodes can be represented by the following electrode reaction equation: where X represents the halogen atom. The numbers of carbon, hydrogen, oxygen, nitrogen, and halogen atoms in the organic compound are represented by , , , , and .
Faraday’s law can be used to quantify the concentration by measuring the charge passed if the organics is completely oxidized. That is, where is the photocurrent from the oxidation of the organics and is the time elapsed upon illumination; is the direct measure of net charge of the organics; is the number of electrons transferred and is the concentration of the organics, while the volume (V) and the Faraday constant (F) are known constants.
Figure 1 shows a set of typical photocurrent-time profiles in the exhaustive mode in thin-cell at the fixed applied bias potential of 2.5 V. Figures 1(c) and 1(d) show the photocurrent response of a blank solution of 2.0 mol/L NaNO3 and a 2.0 mol/L NaNO3 solution containing 0.4 mmol/L benzoic acid in a thin-cell in the absence of illumination (electrocatalytic performances). The results show that there is no photocurrent under dark conditions which indicate that the benzoic acid in thin-cell cannot be oxidized. Figure 1(a) shows the typical photocurrent response of a blank solution of 2.0 mol/L NaNO3 under the illumination conditions (7.9 mW/cm2). The even photocurrent () observed for the blank solution originated from the stable oxidation of water. Figure 1(b) shows the photocurrent response of a 2.0 mol/L NaNO3 solution containing 0.4 mmol/L benzoic acid in a thin-cell under the same illumination (7.9 mW/cm2). The total photocurrent () observed for the benzoic acid solution was the total of two different components: one originates from the photoelectrocatalytic oxidation of benzoic acid, whereas the other was from water oxidation, which was the same as (Figure 1(a)). When the decreases to the level of , the oxidation of organics was finished. Hence, the processes of the photoelectrochemical reaction in thin-cell were described by the change of I-t profile.
The net charge which originated from the oxidation of organics was defined as , which can be obtained by subtracting from . The net charge which obtained by (2) was defined as . Because the change of net charge is directly related to concentration of organics during the photoelectrochemical reaction. The degradation efficiency (DE) of the photoelectrochemical reaction can be defined as the ratio of to , which can be used in describing the property of thin-cell.
According to (1), the stoichiometric oxidation of benzoic acid can be represented as That is, if the complete mineralisation of benzoic acid has been achieved, 1 mol of benzoic acid requires 30 mol of electrons. The relationship between the net charge and the concentration should be written as follows with the cell volume, μL, being used: Hence, the of 0.2 mmol/L, 0.4 mmol/L, and 0.8 mmol/L were 0.00474 mC, and 0.00948 mC, 0.01896 mC.
3.2. Effect of Photoelectrocatalytic Degradation on Benzoic Acid
3.2.1. Effect of the Bias Voltage
The cyclic voltammograms of a 2.0 mol/L NaNO3 solution is shown in Figure 2. The photocurrent increases with the bias voltage, indicating that the bias voltage can improve the electronic transmission performance of TNAs in the thin-cell. This increase may affect the photoelectrocatalytic degradation efficiency of benzoic acid. In the low-bias voltage range (<2 V), the photocurrent rapidly increases with the bias voltage, thus demonstrating that the photogenerated electrons cannot be completely exported to the external circuit. When the bias voltage is between 2 and 4 V, the photocurrent appears in a saturated platform, indicating that the photogenerated electrons have been completely exported and that the recombination of electrons and holes has been restricted. After the platform becomes saturated (>4 V), the photocurrent again increases with the bias voltage and can cause the water molecules to decompose when the bias voltage reaches the oxygen evolution potential of the TNA electrode. The results in Figure 3 support this view.
Figure 3 shows the photocurrent-time profile of benzoic acid (0.4 mmol/L) degradation at different bias voltages and under 7.9 mW/cm2 ultraviolet (UV) illuminations. As show in Figure 3, when the bias voltage is below 3 V, the degradation time of benzoic acid decreased as the bias voltage increased. This was indicated that the increase in the bias voltage can also improve the degradation rate of benzoic acid on TNAs. However, the photocurrent curve continuously increases when the bias voltage is 4 V, which leads to failure in identifying the reaction termination using a computer. This result indicates that the degradation of benzoic acid is significantly perturbed by the water decomposition when the bias voltage is too high.
Table 1 shows the value and degradation efficiency (DE) of benzoic acid under different bias voltages. When the bias voltage is below 2 V, the DE of benzoic acid increases with the bias voltage, indicating that the application of a bias voltage can enhance the photoelectrocatalytic capability of TNAs for benzoic acid. When the bias voltage is set between 2 and 3 V, the degradation efficiency of benzoic acid exceeds 99.5%, indicating that the benzoic acid is completely degraded. The value remains stable in this condition, indicating that the value of is related only to the transferred electronics during the photoelectrocatalytic oxidation. Therefore, the bias voltage should be set within the electrochemical window (2 V to 3 V).
3.2.2. Effect of Light Intensity
Figure 4 shows the photocurrent responses of a 2 mol/L NaNO3 blank solution containing benzoic acid (0.4 mmol/L) under different light intensities. The more intense the light, the shorter the time for complete degradation, indicating that an increase in the intensity of light is conducive to promoting the photoelectrocatalytic degradation rate.
Figure 4 shows that the photocurrent increases with the light intensity, indicating that the increases of light intensity can improve the electronic transmission performance of TNAs. The photocurrent-time curve shapes vary with the light intensity, demonstrating that the light intensity affects the photoelectrocatalysis of benzoic acid on TNAs.
Table 2 shows that the and degradation efficiency of 0.8 mmol/L benzoic acid increase with the light intensity when the light intensity is below 5.53 mW/cm2. This result indicates that benzoic acid is not completely mineralized because of the insufficient light intensity not inducing enough electron-hole pairs of TNAs in the thin-cell.
However, the of benzoic acid at low concentrations (0.2 and 0.4 mmol/L) and at 0.8 mmol/L under sufficient light intensity (above 5.53 mW/cm2) remain comparatively constant, confirming that is not related to the light intensity but is associated with the transferred electrons in the photoelectric catalytic oxidation when benzoic acid is completely mineralized.
3.2.3. Effect of Electrolyte Concentration
The transfer of electrons in the thin-cell reactor was affected by the electrolyte solution. The voltammograms of 0.4 mmol/L benzoic acid with different NaNO3 concentrations under light (7.9 mW/cm2) are shown in Figure 5. The photocurrent rapidly increases with the voltage when the NaNO3 concentration is below 1 mol/L, indicating that the photoinduced electrons are not completely exported to the external circuit. When the concentration exceeds 1.5 mol/L, the photocurrent exhibits a saturated platform, indicating that the photogenerated electrons are completely exported and that the recombination of electrons and holes is restricted. The higher the electrolyte concentration, the stronger the conductivity.
Table 3 shows that gradually increases with the electrolyte concentration and is lower than when the NaNO3 electrolyte concentration is below 1 mol/L. These results indicate that benzoic acid cannot be completely degraded because of the insufficient number of hole on TNAs. When the NaNO3 electrolyte concentration exceeds 1.5 mol/L, remains stable and close to , indicating that benzoic acid has been completely degraded because of the sufficient number of holes in this condition.
3.3. Photoelectrocatalytic Degradation Property and Mechanism of Benzoic Acid
The photocurrent response profiles of benzoic acid for seven different concentrations in the thin-cell reactor are shown in Figure 6. A comparison between and shows that the different concentrations of benzoic acid have all been completely degraded in a photoelectrocatalytic manner.
The relationship between the initial organic concentrations and was used to analyze the adsorption properties of benzoic acid on TNAs because the value corresponds to the initial degradation rate according to the principle of photoelectric catalysis. Figure 6(a) shows that the peak photocurrent () increases with . At low organic concentrations, is proportional to , possibly because of the photohole capture process during benzoic acid mineralization. However, as the benzoic acid concentration increases to a certain level, photohole generation becomes the rate-limiting step, and the increase in begins to slow down.
The relationship between and was simulated using a computer as follows:
The data (4) fit the Langmuir adsorption isotherm. These results confirm that the adsorption behavior of benzoic acid on the TNAs is that of a monolayer adsorption.
The photocurrent response profiles of benzoic acid at seven different concentrations are shown in Figure 6. The degradation I-t profiles all show decay to a stable value. However, this decay varies with increasing concentrations, thereby illustrating the disparities in the photoelectrocatalytic process as well as the mechanism of benzoic acid degradation at different concentrations.
At low concentrations (0 mmol/L to 0.02 mmol/L), the transient photocurrent rapidly reaches its peak at the initial stage because of the rapid degradation of the benzoic acid originally adsorbed on the TNA surface. The photocurrent then begins to decrease because of benzoic acid mineralization. After a certain period, the photocurrent finally reaches a stable value and overlaps with . This result indicates that the photoelectrocatalytic degradation of benzoic acid in the thin-cell reactor is complete.
When the benzoic acid concentration reaches an intermediate level (0.07 mmol/L to 0.1 mmol/L), the curve continues increasing after the initial peak, possibly because of the degradation of the intermediates. Figure 6(a) shows that the adsorption of the electrode surface has not reached saturation at that point, indicating that the number of photogenerated holes on the electrode surface is sufficient. Thus, the rapid oxidation of intermediate products, which can be more easily oxidized than benzoic acid, may have caused the current to increase.
As the concentration increases to levels above 0.2 mmol/L (0.1 mmol/L, e.g.), the profiles can be divided into five major processes: a rapid increase to the transient photocurrent spikes, a continuous increase, a rapid decrease, a reincrease, and a decrease to a stable value. Meanwhile, Figure 6(a) shows that the adsorption on the TNAs has reached saturation, indicating that the holes on the TNA electrode surface cannot oxidate all benzoic acid on the TNAs simultaneously. This photoelectrocatalytic process can be explained as follows. In the initial stage, the benzoic acid molecule on the surface of the TNA electrode is rapidly oxidized, and the photocurrent rapidly reaches the initial peak. After the first oxidization, the resting benzoic acid absorbed on the TNA surface at elevated concentrations as well as the intermediate products is oxidized, which results in the continuous increase of the curve for benzoic acid after the initial peak. The benzoic acid concentration cannot be supplemented in time because the rate of adsorption may be slower than the speed of degradation. Therefore, the optical current rapidly decreases. After a certain period, the area covered by the benzoic acid absorbents begins to increase, which results in a re-increase in the current. Afterward, the benzoic acid concentration decreases, which results in the decrease in the photocurrent. These findings indicate that when the benzoic acid concentration is high, the diffusion of benzoic acid in the bulk solution of the reactor as well as the degradation of intermediate products from benzoic acid decomposition would affect the photoelectrocatalysis on the electrode surface.
4. Conclusions
The photoelectrocatalytic performance of benzoic acid on the TiO2 nanotube array electrode was investigated using a thin-cell reactor. The adsorption, degradation rate, and reaction characteristics of benzoic acid degradation were discussed by analyzing the changes in the photogenerated I-t profiles. The result of this paper provides a solid foundation for future research into the degradation characteristics, reaction mechanism, and reaction kinetics of aromatic organic compounds on the TNA electrode surface.
Acknowledgments
The project was supported by the National High Technology Research and Development Program of China (863) (2009AA063003), the National Natural Science Foundation of China (no. 20 677039), and R&D Foundation of Shanghai Jiaotong University. The authors are for support of XRD lab in Instrumental Analysis Center of SJTU.