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Communication

Phenomenal Insight into Electrochemically Induced Photocatalytic Degradation of Nitrobenzene on Variant Au-Modified TiO2 Nanotubes

by
Meng Wang
1,
Chaoying Li
1,2,
Yingdong Wang
2,
Di Gu
2,* and
Baohui Wang
1,*
1
Institute of New Energy Chemistry and Environmental Science, College of Chemistry and Chemical Engineering, Northeast Petroleum University, Daqing 163318, China
2
National Key Laboratory of Continental Shale Oil, Northeast Petroleum University, Daqing 163318, China
*
Authors to whom correspondence should be addressed.
Catalysts 2023, 13(11), 1445; https://doi.org/10.3390/catal13111445
Submission received: 23 September 2023 / Revised: 27 October 2023 / Accepted: 7 November 2023 / Published: 16 November 2023
(This article belongs to the Section Photocatalysis)
Browse Figures
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Abstract

:
TiO2 nanotubes are a prominent type of TiO2-based nanostructure compared to nanorod arrays. A promising way to improve photocatalytic performance is modifying TiO2 nanotubes with metals, either on the surface or inside the tubes. There is a substantial demand for enhancing the conductivity and charge separation of TiO2 nanotubes, with a major focus on gold (Au) modification. Gold (Au) coatings have significantly improved the photocatalytic activity of TiO2 nanotubes, particularly in pollutant oxidation. However, the mechanism underlying the action of Au-modified TiO2 nanotubes in photocatalytic nitrobenzene oxidation under electrochemical induction remains unclear. Therefore, we conducted related experiments to explore the optimal Au concentration under various conditions. Under electric field induction, the maximum removal rate achieved was 54.9%. Lastly, we analyzed the relevant photocatalytic mechanism to elucidate the responses of electrons and holes to a simulated contaminant under a photo-electrochemical field.

Graphical Abstract

1. Introduction

TiO2, a well-known catalyst, offers several advantages, such as non-toxicity, exceptional chemical durability, and favorable photocatalytic activity, especially in the degradation of organic contaminants in industrial effluents. It has been extensively employed in environmental protection. However, its wide bandgap of 3.2 eV and rapid carrier recombination behavior significantly limit its practical application in the field of solar energy. To effectively reduce the bandgap and enhance visible light adsorption, various strategies have been employed during the synthesis process, including defect introduction, specific element doping, and semiconductor incorporation. For instance, incorporating g-C3N4 into TiO2 can lead to the formation of a heterojunction structure, including the traditional Type II [1] and Z scheme [2], effectively altering the movement of charge carriers. Introducing defects to reduce the bandgap can enhance visible light absorption [3]. In general, the most commonly employed approaches to augmenting photocatalytic activity entail the application of noble metals or metal doping. This is primarily due to their facile integration into the titanium dioxide lattice via photo-reduction or electrochemical deposition. These doping techniques are frequently characterized by their simplicity and high efficiency. Additionally, the utilization of nanoscale particles has proven to be highly effective in enhancing photocatalytic activity, despite the fact that the precise mechanism underlying this nanoscale effect remains to be fully comprehended. For instance, noble metals such as Pt, Ag, or Au have been dispersed into nano-sized particles to optimize titanium dioxide, addressing the previously mentioned drawbacks [4,5,6]. Due to their strong light absorption capabilities, Au nanoparticles induce electrons to transition from the 5d level to the 6s level, leading to a significant characteristic absorption band in the visible light spectrum, centered around 560 nm. Based on the plasma resonance (SPR) effect [7] between the excited electrons and the Au from the surface of the nanotubes, Au nanoparticles capture photo-induced electrons excited by light, restraining the complex of the electrons and holes, increasing the photocatalytic activity of the composites.
Numerous studies have explored the modification of TiO2 with Au using various methods. Tran [8] utilized pulsed laser ablation to reduce Au+ to Au and carried out a series of processes to deposit Au nanoparticles onto a TiO2 substrate. Furthermore, the preparation method, which was both time consuming and complex, significantly reduced the catalytic effects during the degradation reaction. A suspension was prepared by immersing powdered TiO2 in a solution containing Au [9] and coating it onto sol–gel produced vitreous quartz microscope slides. Despite the material having a higher specific surface area and better adherence, its practical application was still hindered by lower repeatability and recoverability to some extent. The synthesis of TiO2 nanotubes through the hydrothermal method involved immersing them in solutions with varying concentrations of HAuCl4∙4H2O, followed by preparing the mixture through calcination [9]. Although this method improved the degradation efficiency of the catalyst, it consumed a significant amount of time and resulted in uneven scattering of Au. Li [10] deposited Au particles on fluorine-doped tin oxide (FTO) containing TiO2 nanorods using an electrochemical method, expanding the visible region and improving utilization efficiency. Doping Co on TiO2 nanotubes as the anode [11] resulted in the degradation of MB into inorganic substances when an electrochemical method was used. This approach significantly enhanced degradation efficiency and reduced carrier recombination. However, in many studies concerning the surface modification of Au [12,13] on TiO2, there has been limited research on the optimal loading amount and the degradation of organic substances under a photo-electro field [14,15,16].
The advancement of electrochemical techniques has led to the emergence of the electro-photo coupling field as a promising tool for enhancing the efficiency of degradation processes. Photoelectrochemical (PEC) processes, known for their simplicity, high efficiency, and low pollution, have gained increasing attention from scientific researchers. These processes primarily focus on minimizing electron–hole recombination, maximizing utilization efficiency, and amplifying catalytic activity. Lsy and his colleagues [17] fabricated Ti/black TiO2/PbO2 micro/nanostructure photoelectrodes that exhibited good degradation efficiency for anthraquinone dye under an external electric field and a supplied light source. Yang et al. [18] used hydrothermal methods to synthesize MoS2/TiO2 materials for the treatment of chromium-containing wastewater. At a concentration of 300 mg/L, the degradation efficiency for the model pollutant reached 90% through the electro-photo effect. Gao et al. [19] constructed CdS-TiO2 nanocomposite-based sensors capable of combining a photoelectrochemical to detect nitrite. These materials, with a photo-electro function, exhibited characteristics of good stability, reusability, and excellent sensing capabilities. Gong et al. [20] synthesized highly ordered Cr-doped titania nanotube arrays that displayed good performance for the degradation of methyl orange under Xe lamp illumination. Lin and her research group [21] utilized highly dispersed TiO2 with graphene oxide sheets for the oxidation of ethanol, integrating photo-electro processes to reduce recombination between TiO2 and graphene oxide and enhance favorable properties. Compared to physical or chemical processes, photo-electron catalysis can be considered an effective approach [22].
To explore the photocatalytic and photoelectrocatalytic properties of Au/TiO2 nanotubes, nitrobenzene, a common component in organic wastewater, was selected to evaluate the catalytic efficacy of Au/TiO2 nanotubes with varying Au loadings.
Both the photocatalytic (PC) and photoelectrocatalytic (PEC) performance of the Au/TiO2 nanotubes were investigated with respect to the removal efficiency of nitrobenzene in the wastewater systems. By varying the Au loading, Au/TiO2 nanotubes exhibited improved catalytic activity under photoelectrocatalytic conditions. The results indicated a significant improvement in catalytic activity and catalyst lifespan for Au-modified Ti-based Au/TiO2 nanotubes in both PC and PEC conditions. Taking into account relevant references and previous studies, a probable mechanism based on the active substances in the reaction is presented, providing further insights into the nature of PC and PEC.

2. Results and Discussion

Figure 1a shows an SEM image of pristine TiO2 nanotubes, revealing the inner TiO2 nanotubes’ highly ordered array structures. An inerratic hexagonal structure with a diameter of 150–2000 nm was observed on top of the material. Figure 1b–h are Au/TiO2 nanotubes of different loading concentrations. Compared with the pristine TiO2 nanotubes in Figure 1a, the top of the tube became thick when increasing the loading concentration of Au, allowing the active surface of the catalyst to significantly enhance the catalytic activity [23]. In Figure 1f, the morphology of the nanotubes and the loadings of Au nano-particles inside the tubes can be clearly observed, proving that the electrochemical deposition method was successfully used to load nanometallic particles onto the inner walls of the tubes, which can improve the photocatalytic effect. When the concentration reached 0.7 g/L (Figure 1g), a large number of Au particles block the pores on the top layer and induced the shielding of the active area of the TiO2 nanotubes [24]. From this, it can be inferred that an appropriate gold loading can positively promote the photocatalytic effect of TiO2 nanotubes. However, when the loading amount continues to increase, excessive gold on the surface can block the excitation of photons on the surfaces of TiO2 nanotubes, thereby failing to promote the photocatalytic effect and possibly even weakening the original photocatalytic effect.
The elemental content was detected by EDS. As shown in Figure 2, the TiO2 nanotubes consisted of Ti, O, and C. The appearance of the C peak was attributed to the reduction of CO2 from the air [25] during the calcination process. Moreover, part of the EG was reduced by carrying out anodization. In the sample of the Au/TiO2 nanotubes, successful loading of gold was (illustrated in Figure 2) noted by the characteristic peak of Au. In contrast with the different samples, the peak intensity of Au enhanced with a changing concentration.Since EDS can only be used to qualitatively and quantitatively analyze the microelements on a sample’s surface, ICP-AES was employed to determine the overall elemental composition of the samples. Table 1 displays the mass fractions of each element, revealing that both Au/TiO2 NTs are composed of Ti, Au, and O, and this composition aligns with our expectations. The mass fraction of Ti was slightly higher than that of Au. This can be attributed to the nanotubes on the titanium substrate and the deposited Au being primarily concentrated on the nanotubes, causing the titanium substrate to completely dissolve in the solution during ICP-AES measurement. The presence of Au in the Au/TiO2 NTs confirms the successful introduction of Au into TiO2 NTs. To further confirm the successful preparation of the material, other characterization methods were also applied.
Figure 3 displays a series of XRD spectra obtained for the Au-doped and pristine TiO2 nanotubes. It is known that TiO2 has different crystalline phases, including anatase, rutile, and brookite. Among these phases, anatase exhibits the most excellent photocatalytic activity. The XRD pattern displayed common peaks, namely, (101), (004), (200), and (105), characteristic of anatase. Additional diffraction peaks, namely, (100), (110), and (103), corresponded to the Ti metal phase [26], indicating that the TiO2 nanotubes consisted of a mixture of the anatase phase and Ti phase.
In the case of the Au/TiO2 nanotubes, they corresponded to the mixture phase and exhibited no significant differences from pristine TiO2 nanotubes. The peaks at 2 theta values of 38.21°, 44.32°, and 63.80° match the (111), (200), and (220) planes [27] of the Au phase in the patterns (JCPDS 65-2870), confirming the presence of metallic Au. Further analyses, in combination with SEM and EDS, can demonstrate the successful preparation of Au/TiO2 nanotubes.
To investigate the impact of ultraviolet (UV) light and gold concentrations on nitrobenzene degradation, cyclic voltammetry curves of the nitrobenzene solution were examined before and after UV light exposure. A conventional three-electrode system was employed in the experiment, with Au/TiO2 nanotubes serving as the photoanode, Pt serving as the photocathode, and Ag/AgCl acting as the reference electrode. To emphasize the nitrobenzene characteristic peak, a high electrolyte concentration (1 g/L nitrobenzene) and 15 g/L of Na2SO4 serving as the supporting electrolyte were employed. In the experiment, a UV lamp was used as a light source, and the scanning rate was 50 mV/s during the whole experiment. The volt–ampere characteristic curve of the nitrobenzene solution was analyzed under the condition of avoiding light, and the detection results are shown in Figure 4a. The cyclic voltammetry curve of the nitrobenzene solution was analyzed under ultraviolet light, and the detection results are shown in Figure 4b. As can be seen from the two figures, the cyclic voltammetry curve of nitrobenzene is regular under the two conditions; only one obvious oxidation peak was detected in the positive direction of the scanning of nitrobenzene on each oxidation curve, and likewise, only one reduction peak was detected in a negative direction. This means that in this mode, nitrobenzene was oxidized in only one step. Upon comparing the two figures, it is evident that the oxidation and reduction peaks obtained with the Au/TiO2 nanotubes loaded with Au were significantly higher than those of the two-step TiO2 nanotubes. In comparison to the TiO2 nanotubes, the chemical reaction was more pronounced under Au/TiO2 nanotube conditions, making nitrobenzene more susceptible to oxidation. A comparison of the two figures reveals that the area under the cyclic voltammetry curve of nitrobenzene exposed to UV light was significantly larger than that under dark conditions, indicating a more intense chemical reaction under UV light, facilitating nitrobenzene oxidation.
UV-Vis spectroscopy was utilized to analyze the absorption properties of TiO2 and Au/TiO2 nanotubes. Figure 5 shows that the absorption intensity of the TiO2 nanotubes and Au/TiO2 nanotubes in the visible light region was lower than that in the UV light region. In comparison to the TiO2 nanotubes, the Au/TiO2 nanotubes exhibited superior absorption in the UV light region. Additionally, Au/TiO2 nanotubes displayed notable absorption at 520 nm, which can be attributed to the surface plasmon resonance (SPR) effect of the Au nanoparticles. TiO2 exhibited a negative conduction band at -0.5 V, while the Fermi level of the Au nanoparticles was 0.45 V. Therefore, when Au was incorporated into the TiO2 nanotubes, the Au and TiO2 nanotubes counteracted the negative and positive potentials, respectively, ultimately achieving an equilibrium [28]. Due to the SPR effect, electrons from Au nanoparticles can transfer to the conduction band of TiO2, thereby enhancing catalytic performance [29].
The photocatalytic and photoelectrocatalytic performance bestowed by the Au loading on the TiO2 nanotubes were estimated via the degradation of nitrobenzene solution, a refractory organic wastewater, under room-temperature. The degradation rate curve of nitrobenzene fundamentally corresponded to the first-order kinetic equation (lnC0/Ct = kat) in photocatalysis and photoelectrocatalysis, as shown in Figure 6a,b. Significantly, ka increased gradually with the increase in the Au loading, and ka decreased as the Au loading continued to increase. The degradation rates of the nitrobenzene solution with different concentrations are summarized in Figure 6c.
To determine the optimal Au loading, we prepared samples with varying amounts of Au and induced the photocatalytic degradation of nitrobenzene from wastewater. After a two-hour degradation process, the degradation rates obtained were as follows (shown in Figure 6c (the black bar chart)): 30.50%, 31.09%, 31.92%, 39.71%, 42.24%, 39.70%, and 36.17%. Compared to the 18.35% degradation rate of pristine TiO2 nanotubes, the efficiency of the Au-loaded TiO2 nanotubes was higher. However, when the doping concentration reached 0.6 g/L, the degradation effect was optimal among the six different loadings. The results of SEM analysis suggested that excessive Au could obstruct the tube orifices, reducing the transmittance of available light and the number of active sites, ultimately leading to lower degradation efficiency [30].
In Figure 6b, the curve concerns the test of photoelectrocatalytic performance. The data on the degradation of nitrobenzene in Figure 6c (red bar chart) are as follows: 48.21%, 50.35%, 54.90%, 44.07%, 43.79%, 42.70%, and 40.19%. From the above results, it can be gleaned that all the samples of loaded Au were superior to the original TiO2 nanotubes (29.2%). When the doping concentration reached 0.4 g/L, the degradation efficiency was 54.90%. This can contribute to accelerating the transfer of electrons, restrain the combination of holes with electrons, and extend the life of the hole in the electron–hole pair under applied voltage conditions. However, with a concentration over 0.4 g/L, the efficiency of degradation started to drop. This can be attributed to the mass of Au that will form the Au layer, which is insufficient for the promotion of electrons transfer and the recombination of electron–hole pairs [31]. So, when the loading concentration was 0.4 g/L, the efficiency of photoelectrocatalysis was the best. To improve the efficiency of refractory organic wastewater treatment, photoelectrocatalysis was proposed to enhance the degradation rate. Under the combined action of photocatalysis and bias voltage, the separation efficiency of the photogenerated charge was improved obviously. At the same time, it can also be seen that the combination of photocatalysis and electrocatalysis allows for better degradation of refractory organic wastewater under lower Au loading conditions.
By analyzing the degradation trend depicted in Figure 7, the rate constant can be determined. As shown in Figure 7a, the degradation rate for the 0.6 g/L Au/TiO2 nanotubes was approximately three times higher than that of the TiO2 nanotubes. Adjusting the Au concentration could enhance the chemical reaction, leading to more effective separation of holes and electrons. In comparison to photocatalysis, the reaction rate was significantly higher with photo-electro coupling. This can be attributed to the presence of more carriers, which, in turn, stimulated more reactions with the target degradation substance.
Maintaining stability in production is crucial for applications. Consequently, the stability of various Au loadings was tested under both PC and PEC conditions, as illustrated in Figure 8. The performance of the modified TiO2 nanotubes was assessed by subjecting the sample to five degradation cycles. The graphs clearly demonstrate that the degradation rate of the sample remained nearly constant, indicating the excellent reusability and stability of the photocatalyst.
The potential mechanism behind the enhanced optical performance achieved can be described as follows: Scheme 1a illustrates the photocatalytic degradation mechanism of nitrobenzene. When Au/TiO2 nanotubes are exposed to ultraviolet light, electrons in the valence band are excited, jumping to the conduction band, while leaving holes in the valence band, ultimately forming electron–hole pairs. Due to the Schottky barrier between the TiO2 nanotubes and Au particles, electrons are induced to move from Au to the conduction band of TiO2, resulting in an increased difference in Fermi energy and ultimately achieving an energy level balance. Furthermore, the oxygen absorbed on the catalyst’s surface is restricted by the transferred electrons, leading to the formation of superoxide anions (·O2−) [32,33]. Holes in the valence band participate in an oxidation reaction with H2O, forming hydroxyl radicals (·OH) [34]. Additionally, due to the SPR effect of the noble metal on the surface, the Au particles become the center of electron capture, further enhancing the separation of the photoinduced electrons and holes and improving the degradation efficiency of nitrobenzene.
Scheme 1b illustrates the photoelectrocatalytic degradation mechanism of nitrobenzene. Under an applied bias of 1.2 V, the Au/TiO2 nanotubes, serving as a photoanode, undergo an oxidation–reduction reaction. Firstly, the Au/TiO2 nanotubes accelerate the directional movement of photoelectric charge. Secondly, with the application of additional bias voltage, electrons transfer to the Ti substrate along the TiO2 nanotubes, ultimately reaching the counter electrode. This process prolongs the lifetime of the electrons and accelerates the formation of electron–hole pairs, resulting in an increased degradation rate of nitrobenzene. Additionally, since Au/TiO2 nanotubes have a wide absorption band in the visible light region and special optical properties in photocatalysis, it is easier to achieve higher degradation efficiency under natural light conditions, and this is more conducive to their industrial expansion and application in the actual organic wastewater degradation process. Therefore, Au modified TiO2 nanotubes received widespread attention in wastewater degradation.

3. Experimental Procedures

3.1. Chemicals and Instruments

Ti sheet (0.2 mm thick, Strem Chemicals, 99.6%) was cut into a piece that was 20 mm long and 10 mm wide. Ethylene glycol (AR, 99.0%), ammonium fluoride (NH4F, AR), chloroauric acid (HAuCl4·3H2O, AR), acetone (C3H6O, AR), hydrochloric acid (HCl, AR), p-benzoquinone (C6H4O2, AR, 99%), and nitrobenzene (C6H5NO2, AR, 99.0%) were used. Ultrapure water (resistivity > 18 MΩ·cm) was used in the reaction of nitrobenzene degradation, and the preparation of TiO2 nanotubes and Au/TiO2 nanotubes as shown in Scheme 2; the photocatalyst was put into the quartz beaker with circulating cooling water. Utilizing a 500 W high-pressure mercury lamp as a UV light, the nitrobenzene solution was kept at room temperature and stirred during the photocatalytic reaction process.

3.2. Preparation of Photocatalyst

TiO2 nanotubes were prepared using a two-step anodization method [35]. The titanium sheets were placed in acetone, ethyl alcohol, hydrochloric acid, and deionized water for sonicating 30 min; and then dried under a pure nitrogen stream. The electrolyte consisted of NH4F (0.5 wt%) in EG with (2 vol%) water. The first-step anodization was conducted for 30 min at 60 V of power. The as-prepared sample was placed in deionized water via sonication treatment, which mainly stripped the thin membrane. Subsequently, the Ti induced the second anodization as above, forming a cellular nanotube structure in the base, and a hexagonal structure on the top layer. Through this two-step anodization, the sample was cleaned and dried with deionized water and then nitrogen stream, respectively. Then, the prepared sample was calcined for 1h in a muffle furnace, maintained at a temperature of 450 °C, with heating rate at 5 °C/min, and cooled at room temperature. Using chloroauric acid as the gold source, different concentrations of electrolyte were prepared, including 0.2–0.8 g/L of HAuCl4∙4H2O. Au/TiO2 nanotubes were prepared as follows: TiO2 was used as the cathode, and Pt sheet was used as the anode. The Au/TiO2 nanotubes were prepared using an electrochemical reduction method at a fixed electrode potential of 1.2 V in solutions with different concentrations for 10 min. Then, the Au/TiO2 nanotubes were removed and dried at room temperature.

3.3. Experiment Ways

In the photocatalytic degradation experiment, a high-pressure mercury lamp with a radiation distance of 10 cm was used as the light source to simulate the ultraviolet part of the sunlight and irritate the organic wastewater (30 mL, 200 mg/L of nitrobenzene; 0.5 g/L of Na2SO4). For photoelectrocatalysis, the bias voltage was kept constant at 1.2 V, with Au/TiO2 nanotubes serving as the photoanode and Pt serving as the photocathode. Each experiment was repeated 3 times, and the average and standard errors were calculated.

3.4. Characterization of Catalyst

The surface morphology and distribution of the nanotubes loaded with Au nanoparticles were analyzed utilizing a scanning electron microscope (SEM, Zeiss, Oberkochen, Germany, Sigma HV). The relevant chemical components of TiO2 nanotubes and Au-loaded nanotubes were detected using an energy dispersive spectrometer (EDS). The crystal form of catalysis was examined via X-ray diffraction (XRD, Rigaku, Tokyo, Japan, D/MAX2200) with Cu Ka resource. The bulk elemental composition was determined using Inductively coupled plasma atomic emission spectrometer (ICP-AES, Agilent 730, Agilent, Santa Clara, CA, USA). For analysis, the samples were dissolved in aquaregia and subsequently diluted to a fixed volume of 50 mL prior to detection.
Utilizing the degradation of nitrobenzene, we were able to estimate the photocatalytic activity and stability of the sample at room temperature. The absorbance of nitrobenzene was measured using a UV spectrophotometer after degradation each 30 min for 2 h. The concentration of nitrobenzene with the corresponding time was calculated using the standard graph. Nitrobenzene has a characteristic absorption peak in the ultraviolet region. The absorbance of nitrobenzene solutions at 25, 50, 100, 150, and 200 mg/L was measured using a stepwise dilution method. The measurement results are shown in Figure 9a. As shown in the figure, the nitrobenzene solution exhibited a relatively obvious absorption peak at around 265 nm. Using the measured absorbance as the vertical axis and the corresponding nitrobenzene concentration as the horizontal axis, the relationship between nitrobenzene concentration and absorbance was obtained. The standard curve was plotted, as shown in Figure 9b:

4. Conclusions

In summary, a catalyst was synthesized using the electrochemical deposition method, resulting in the formation of Au/TiO2 nanotubes with varying doping concentrations. These nanotubes exhibited favorable photocatalytic (PC) and enhanced photoelectrocatalytic (PEC) properties, surpassing those of pure TiO2, toward the degradation of nitrobenzene. The successful deposition of Au onto the TiO2 surface was confirmed by the collected data. The catalytic activity was assessed by measuring the degradation of nitrobenzene, and the optimal PC and PEC performances were observed at concentrations of 0.6 g/L and 0.4 g/L, respectively. The improved PC/PEC performance can be attributed to the presence of Au nanoparticles, which expanded the response range to visible light, enhanced the efficiency of photoelectric hole separation, and suppressed charge recombination. Notably, the degradation efficiency surpassed that of photocatalysis at the same doping concentration under a photoelectric field. Based on its superior photoelectro performance, this catalyst holds promising potential for practical applications in environmental protection.

Author Contributions

Conceptualization, D.G. and B.W.; methodology, D.G.; validation, M.W., C.L. and Y.W.; investigation, M.W.; data curation, C.L.; writing—original draft preparation, M.W. and C.L.; writing—review and editing, M.W.; supervision, B.W.; funding acquisition, D.G. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (No. 21808030), the Natural Science Foundation of Heilongjiang Province (No. LH2022B006), the Postdoctoral scientific Research developmental fund of Heilongjiang Province (No. LBH-Q21082), and the Foundation of Northeast Petroleum University (2021YDL-06).

Data Availability Statement

The data that support this study are available in the article.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. SEM: (a) TiO2 nanotubes produced via two-step oxidation; (b–h) Au/TiO2 nanotubes in doping amounts of 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, and 0.8 g/L Au.
Figure 1. SEM: (a) TiO2 nanotubes produced via two-step oxidation; (b–h) Au/TiO2 nanotubes in doping amounts of 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, and 0.8 g/L Au.
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Figure 2. EDS spectra of TiO2 nanotubes and Au/TiO2 nanotubes with different doping concentrations.
Figure 2. EDS spectra of TiO2 nanotubes and Au/TiO2 nanotubes with different doping concentrations.
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Figure 3. XRD patterns of (a) TiO2 NTs; (b) 0.2 g/L Au@TiO2 NTs; (c) 0.3 g/L Au@TiO2 NTs; (d) 0.4 g/L Au@TiO2 NTs; (e) 0.5 g/L Au@TiO2 NTs; (f) 0.6 g/L Au@TiO2 NTs; (g) 0.7 g/L Au@TiO2 NTs.
Figure 3. XRD patterns of (a) TiO2 NTs; (b) 0.2 g/L Au@TiO2 NTs; (c) 0.3 g/L Au@TiO2 NTs; (d) 0.4 g/L Au@TiO2 NTs; (e) 0.5 g/L Au@TiO2 NTs; (f) 0.6 g/L Au@TiO2 NTs; (g) 0.7 g/L Au@TiO2 NTs.
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Figure 4. Cyclic voltammetry curves of nitrobenzene in the dark (a) and (b) under UV light.
Figure 4. Cyclic voltammetry curves of nitrobenzene in the dark (a) and (b) under UV light.
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Figure 5. UV-vis DRS spectra of Au/TiO2 and TiO2 nanotubes.
Figure 5. UV-vis DRS spectra of Au/TiO2 and TiO2 nanotubes.
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Figure 6. The degradation of nitrobenzene with the prepared samples under the effect of (a) photocatalysis and (b) photoelectrocatalysis; (c) comparison of degradation efficiency between photocatalysis and photoelectrocatalysis.
Figure 6. The degradation of nitrobenzene with the prepared samples under the effect of (a) photocatalysis and (b) photoelectrocatalysis; (c) comparison of degradation efficiency between photocatalysis and photoelectrocatalysis.
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Figure 7. The reaction rate constant of the prepared samples under the effect of (a) photocatalysis and (b) photoelectrocatalysis.
Figure 7. The reaction rate constant of the prepared samples under the effect of (a) photocatalysis and (b) photoelectrocatalysis.
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Figure 8. The repeated experiments concerning the use of (a) 0.6 g/L Au/TiO2 nanotubes and (b) 0.4 g/L Au/TiO2 nanotubes for nitrobenzene degradation.
Figure 8. The repeated experiments concerning the use of (a) 0.6 g/L Au/TiO2 nanotubes and (b) 0.4 g/L Au/TiO2 nanotubes for nitrobenzene degradation.
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Scheme 1. The mechanism of Au/TiO2 nanotubes under (a) photo and (b) photo-electro co-induction.
Scheme 1. The mechanism of Au/TiO2 nanotubes under (a) photo and (b) photo-electro co-induction.
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Scheme 2. Reaction equipment used for the degradation of nitrobenzene.
Scheme 2. Reaction equipment used for the degradation of nitrobenzene.
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Figure 9. UV absorption curve (a) and absorbance–concentration standard curve (b) of nitrobenzene standard solution.
Figure 9. UV absorption curve (a) and absorbance–concentration standard curve (b) of nitrobenzene standard solution.
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Table 1. Mass fractions (%) of Au/TiO2 NT samples.
Table 1. Mass fractions (%) of Au/TiO2 NT samples.
SamplesTiOAu
0.4 g/L Au/TiO2 NTs99.720.28320.0071
0.6 g/L Au/TiO2 NTs99.560.34240.0137
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Wang, M.; Li, C.; Wang, Y.; Gu, D.; Wang, B. Phenomenal Insight into Electrochemically Induced Photocatalytic Degradation of Nitrobenzene on Variant Au-Modified TiO2 Nanotubes. Catalysts 2023, 13, 1445. https://doi.org/10.3390/catal13111445

AMA Style

Wang M, Li C, Wang Y, Gu D, Wang B. Phenomenal Insight into Electrochemically Induced Photocatalytic Degradation of Nitrobenzene on Variant Au-Modified TiO2 Nanotubes. Catalysts. 2023; 13(11):1445. https://doi.org/10.3390/catal13111445

Chicago/Turabian Style

Wang, Meng, Chaoying Li, Yingdong Wang, Di Gu, and Baohui Wang. 2023. "Phenomenal Insight into Electrochemically Induced Photocatalytic Degradation of Nitrobenzene on Variant Au-Modified TiO2 Nanotubes" Catalysts 13, no. 11: 1445. https://doi.org/10.3390/catal13111445

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