Development of SDS-Modified PbO₂ Anode Material Based on Ti³⁺ Self-Doping Black TiO₂NTs Substrate as a Conductive Interlayer for Enhanced Electrocatalytic Oxidation of Methylene Blue

Abstract

Efficient and stable electrode materials are urgently required for wastewater treatment in the electrocatalytic degradation of toxic and refractory organic pollutants. Ti³⁺ self-doping black TiO₂ nanotube arrays (Ti/B-TiO₂-NTs) as an interlayer were used for preparing a novel PbO₂ electrode via an electrochemical reduction technology, and a sodium dodecyl sulfate (SDS)-modified PbO₂ catalytic layer was successfully achieved via an electrochemical deposition technology. The physicochemical characterization tests showed that the Ti/B-TiO₂-NTs/PbO₂-SDS electrodes have a denser surface and finer grain size with the introduction of Ti³⁺ in the interlayer of Ti/TiO₂-NTs and the addition of SDS in the active layer of PbO₂. The electrochemical characterization results showed that the Ti³⁺ self-doping black Ti/TiO₂-NTs/PbO₂-SDS electrode had higher oxygen evolution potential (2.11 V vs. SCE), higher electrode stability, smaller charge-transfer resistance (6.74 Ω cm⁻²), and higher hydroxyl radical production activity, leading to it possessing better electrocatalytic properties. The above results indicated that the physicochemical and electrochemical characterization of the PbO₂ electrode were all enhanced significantly with the introduction of Ti³⁺ and SDS. Furthermore, the Ti/B-TiO₂-NTs/PbO₂-SDS electrodes displayed the best performance on the degradation of methylene blue (MB) in simulated wastewater via bulk electrolysis. The removal efficiency of MB and the chemical oxygen demand (COD) could reach about 99.7% and 80.6% under the optimal conditions after 120 min, respectively. The pseudo-first-order kinetic constant of the Ti/B-TiO₂-NTs/PbO₂-SDS electrode was 0.03956 min⁻¹, which was approximately 3.18 times faster than that of the Ti/TiO₂-NTs/PbO₂ electrode (0.01254 min⁻¹). In addition, the Ti/B-TiO₂-NTs/PbO₂-SDS electrodes showed excellent stability and reusability. The degradation mechanism of MB was explored via the experimental identification of intermediates. In summary, the Ti³⁺ self-doping black Ti/TiO₂-NTs/PbO₂-SDS electrode is a promising electrode in treating wastewater.

1. Introduction

The harmonious coexistence between humans and nature has always been the ultimate aspiration of mankind. However, the excessive production of non-degradable organic pollutants produced by human beings is putting enormous pressure on the environment [1,2]. Textile wastewater containing persistent and toxic carcinogens such as methyl orange, rhodamine B, Congo red, and methylene blue (MB) is persistent and toxic carcinogens pose significant threats to both the aquatic environment and human health [3]. Therefore, implementing effective strategies for the removal of organic dyes from wastewater is related to the relationship between the sustainable development of society and nature.

Recently, several technologies have been developed for the removal of textile-dyeing wastewater. However, the desired effect cannot be achieved due to the poor mineralization effect in the treatment via conventional treatment methods [4]. Therefore, it is imperative to develop a clean, energy-saving, versatile, and efficient new approach for wastewater treatment.

The electrochemical oxidation (EO) process, as a promising method for degrading organic industrial effluents, has attracted growing interest nowadays due to its high efficiency, simple operation, and environmentally friendly technology [4,5]. For the electrochemical method, the anodic material is a crucial factor because it determines the effectiveness of mechanisms and reaction pathways of electro-catalytic oxidation [6]. Accordingly, developing a high-catalytic-efficiency and stable electrode is essential for enhancing the sewage treatment capacity.

Over the last few decades, various electrode materials have been reported for mineralizing refractory pollutants, including RuO₂, IrO₂, boron-doped diamond (BDD), and SnO₂ electrodes [7,8]. Among them, the PbO₂ electrode, known for its moderate price, simple preparation, high electrocatalytic activity, and good corrosion resistance, has been widely used as an anode material in the field of electrochemical water treatment [9]. However, traditional PbO₂-coated anodes still have some disadvantages, such as a small specific surface area, large internal stress, and easy deactivation and brittleness [10]. A PbO₂ electrode is usually easy to peel off from a flat Ti substrate because of the large internal stress, resulting in the leakage of the Pb element into the environment [11].

To improve the service life and electrocatalytic properties of the PbO₂-based anodes, various strategies have been exploited by doping metal or non-metallic ions, performing substrate modification, using a nanostructured composite, and introducing an intermediate layer [12].

Anodically formed vertically oriented TiO₂ nanotube arrays (Ti/TiO₂-NTs) have also been extensively utilized as the catalyst or the middle layer in (photo-) electrochemical advanced oxidation processes for wastewater treatment [13].

Nevertheless, the semiconductive properties of Ti/TiO₂-NTs impeded their further use in electrochemistry. To solve this problem, electrochemical self-doping via electrochemical reduction under mild conditions has been employed to improve the semiconducting behavior of Ti/TiO₂-NTs. In this method, electrochemical reduction generates Ti³⁺, which increases the electrical conductivity of Ti/TiO₂-NTs and creates more active sites [14].

Therefore, doping the interlayer with Ti³⁺ is supposed to significantly improve the electron-transfer efficiency between the PbO₂ coating layer and the Ti substrate, hence improving the electrocatalytic activity and stability of the PbO₂ anode for the electrochemical degradation of refractory organic pollutants.

To enhance both the electrocatalytic performance and stability of the PbO₂ electrode, a novel Ti³⁺ self-doping black Ti/TiO₂-NTs (Ti/B-TiO₂-NTs) layer was fabricated as an interlayer for making a more stable PbO₂ anode. Additionally, sodium dodecyl sulfate (SDS) was adopted to modify the PbO₂ active layer via electrochemical deposition. The physicochemical and electrochemical properties of this novel PbO₂ anode were characterized. The electrochemical oxidation behavior of MB on the different types of PbO₂ anodes was investigated fully. The degradation efficiencies of MB were also evaluated under various experimental parameters. Radical trapping experiments were performed to further explore the role of ·OH and ·SO₄⁻¹ in the degradation of MB.

2. Results and Discussion

2.1. Morphology and Phase Analysis

The morphologies of the different electrodes were observed using SEM, and the results are shown in Figure 1 and Figure S1. From Figure S1, it can be seen that the high-ordered TiO₂-NTs without Ti³⁺ self-doping possess a large specific volume with an average diameter of approximately 140 nm. Figure 1a shows the Ti³⁺ self-doping TiO₂-NTs after electrochemical reduction treatment, where the nanostructure of TiO₂NTs has not been destroyed, similar to that shown in Figure S1. Figure 1b–d display SEM images of three types of electrodes: Ti/TiO₂-NTs/PbO₂ without Ti³⁺ self-doping, Ti³⁺ self-doping Ti/B-TiO₂-NTs/PbO₂, and Ti/B-TiO₂-NTs/PbO₂-SDS electrodes. From Figure 1b, we can see that the surface of the coated PbO₂ layer appears coarse with noticeable cracks on the surface of Ti/TiO₂-NTs without Ti³⁺ self-doping, which could result in the permeation of electrolytes and reduce electrode stability. Figure 1c shows the morphology of deposited PbO₂ on Ti³⁺ self-doping TiO₂-NTs. Compared with the Ti/TiO₂-NTs/PbO₂ electrode, the surface of the Ti/B-TiO₂-NTs/PbO₂ electrode still exhibits the pyramid structures, but it becomes smoother and the cracks have disappeared. The surface structure of PbO₂ after SDS addition on Ti³⁺ self-doping TiO₂-NTs is the smoothest, most uniform, and extraordinarily compact among these three electrodes (Figure 1d); the pyramid structures disappeared, and the grain size significantly decreased. This result indicates that adding SDS can help obtain a surface coating that is uniform, compact, and smooth. This may be because of SDS’s ability to disperse the Pb²⁺ evenly in the solution [15]. Furthermore, this result also suggests that the surface morphology of the Ti/TiO₂-NTs/PbO₂ electrode is simultaneously affected by both Ti³⁺ self-doping and SDS. The elemental mapping of the Ti/B-TiO₂-NTs/PbO₂-SDS electrode further confirmed the homogeneous distribution of Pb, O, Ti, and C elements in the Ti/B-TiO₂-NTs/PbO₂-SDS electrode (Figure 1e).

The typical XRD patterns of three self-made electrodes are shown in Figure S2 and Figure 2. As shown in Figure S2, all the diffraction peaks of Ti³⁺ self-doping Ti/B-TiO₂-NTs were identically in agreement with those of the no-doping Ti/TiO₂-NTs, indicating no change in the anatase phase of the Ti³⁺ self-doping TiO₂NTs within their crystal structure during electrochemical reduction process.

For the XRD pattern of the PbO₂ electrode in Figure 2, the diffraction peaks for all samples located at 2θ = 25.56°, 32.28°, 36.23°, 49.11°, 58.96°, 60.76°, and 62.53° can be attributed to the (110), (101), (200), (211), (310), (211), and (301) planes of β-PbO₂, respectively, which matched with the standard diffraction peaks of the JCPDS card (No. 76-0564). By comparison with three PbO₂ electrodes, it can be clearly seen that the diffraction intensities of the (101), (211), and (301) planes of Ti/B-TiO₂-NTs/PbO₂ and Ti/B-TiO₂-NTs/PbO₂-SDS electrodes increased, while the diffraction intensities of the (110), (200), and (211) planes decreased. This means that Ti³⁺ self-doping and the addition of SDS do not alter the crystal structure but would affect the crystal plane orientation of β-PbO₂ [16]. It can be argued that Ti³⁺ self-doping or SDS addition will change the electric double-layer structure, altering the growth mechanism of the PbO₂ phase and electrodeposition kinetic process on the anode surface, which could inhibit the growth of (110) and (200). The average grain sizes of three different PbO₂ electrodes were calculated using the Debye–Scherrer formula, as shown in Table S1. The results indicate that the average particle sizes of the β-PbO₂ electrode significantly decreased upon doping with Ti³⁺ and adding the anionic SDS surfactant. The small crystal particle size is favorable for forming a large specific area, which is conducive to improving the electrochemical activity of the electrode. Thus, the results from XRD are consistent with the above-mentioned result of SEM.

For the purpose of analyzing the chemical composition and surface elements on the prepared PbO₂ electrode materials, the XPS measurements were used to investigate the chemical states and elementary composition of PbO₂ electrodes. The results are shown in Figure 3a–d. The binding energy was corrected using the C 1s peak at 284.6 eV. From the survey spectrum shown in Figure S3, characteristic binding energy peaks of Pb, Ti, C, and O elements were found on the surface of the Ti/B-TiO₂-NTs/PbO₂-SDS electrode. The detailed spectrum of Ti 2p is shown in Figure 3a,b. The high-resolution XPS spectra of the Ti 2p peak of Ti/B-TiO₂ can be deconvoluted into four peaks (Figure 3a). Two binding energies of 459.14 and 465.13 eV can be assigned to Ti⁴⁺, while the binding energies of 457.68 and 464.79 eV are attributed to Ti³⁺ [17]. Compared to Ti/TiO₂-NTs (Figure 3b), the peaks of Ti/B-TiO₂-NTs show a slight red-shift to higher binding energy (Figure S4). The results indicate that Ti³⁺ and oxygen vacancies exist in the Ti/B-TiO₂-NTs, demonstrating that the Ti³⁺ reduction is irreversible. Therefore, the conductivity of Ti/B-TiO₂-NTs can be markedly improved. In the detailed spectrum of Pb 4f in Figure 3c, the two peaks of Pb4f corresponding to Pb⁴⁺ 4f_5/2 and Pb⁴⁺ 4f_7/2 are shown at around 137.08 eV and 141.97 eV, which were identified as the spectral values of PbO₂ [18]. The deconvolution of the O 1s spectrum could also be fitted into two characteristic peaks at 528.5 and 529.3 eV (Figure 3d), which belong to lattice oxygen (O_L) and chemisorbed oxygen (O_ads, OH) [19].

2.2. Electrochemical Characterization

Figure S5a shows the CV data of the TiO₂NTs electrodes before and after electrochemical reduction.

The pristine TiO₂NTs exhibited a low response current within the potential window of 0~2 V, indicating the poor conductivity of TiO₂NTs caused by its semiconductive nature. In contrast, the Ti³⁺ self-doping Ti/B-TiO₂-NTs presented a much higher response current in the same potential window, implying that the electrical conductivity was markedly improved. EIS was adopted to probe the conductivity of the Ti/B-TiO₂-NTs. Figure S5b shows the Nyquist plot of Ti/TiO₂NTs and Ti/B-TiO₂-NTs electrodes. It can be clearly observed that the Ti/B-TiO₂-NTs show a smaller semicircle radius on Nyquist plots than the pristine Ti/TiO₂NTs, indicating that the resistance of the Ti/B-TiO₂-NTs electrode was significantly decreased. This is further proof that the introduction of Ti³⁺ ions into TiO₂NTs can improve electrical conductivity [17]. It demonstrated that Ti³⁺ self-doping can significantly improve the electrochemical performance of the TiO₂NTs. The CV curves were obtained for the different PbO₂ electrodes under a 0.2 M Na₂SO₄ solution at a scanning rate of 50 mV s⁻¹. As observed from CV curves of the various PbO₂ electrodes (Figure 4a), all PbO₂ electrodes exhibited similar shapes with a pair of redox peaks, which corresponded to the reduction–oxidation reactions between Pb (II) and Pb (IV) at the electrode interface [20]. However, the peak oxidation current density of the Ti/B-TiO₂-NTs/PbO₂-SDS electrode was higher than that of the other two PbO₂ electrodes, which implies that the introduction of Ti³⁺ in the interlayer of Ti/TiO₂-NTs and the addition of SDS in the active layer of PbO₂ played a role in improving the electrode activity. It also means that the Ti/B-TiO₂-NTs/PbO₂-SDS electrode has a greater number of active sites. Therefore, the Ti/B-TiO₂-NTs/PbO₂-SDS electrode is much more suitable for the electrocatalytic degradation of MB than other electrodes.

The LSV of the different PbO₂ electrodes was examined in 0.2 M Na₂SO₄ at a scan rate of 50 mV s⁻¹ to investigate the anodic oxygen evolution potential (OEP). As presented in Figure 4b, the Ti/B-TiO₂-NTs/PbO₂-SDS electrode achieved the highest OEP of 2.11 V vs. SCE. This result might be due to the small crystal structure and the synergistic effect of SDS and Ti³⁺. As is known, the oxygen evolution reaction is the main competitive side reaction on the anode surface (Equation (1)), which can lead to increased energy consumption and decreased oxidation efficiency.

∙OH_ad +H₂O→O₂ +3H⁺ +3 e⁻¹

(1)

Therefore, the highest OEP would favor the Ti/B-TiO₂-NTs/PbO₂-SDS electrode and exhibit a stronger oxidation ability for the electrochemical oxidation of organics [21]. A higher OEP value means a substantial amount of ·OH could be generated and accelerate the electrochemical degradation of contaminants, leading to a stronger electrocatalytic ability with higher current efficiency.

To further evaluate the electrochemical performance of all PbO₂ electrodes, EIS was carried out to investigate the electrical conductivity and charge-transfer resistance through the electrolyte of different electrodes. The Nyquist plots for these electrodes obtained in 0.5 M H₂SO₄ solution with a frequency range of 10,000 to 0.01 Hz are displayed in Figure 3c. As shown in Figure 4c, the semicircle arc of the Ti/B-TiO₂-NTs/PbO₂-SDS electrode in the high-to-medium frequency range of Nyquist plots was smaller than that of the other PbO₂ electrodes, suggesting that the charge-transfer capacity of modified Ti/B-TiO₂-NTs/PbO₂ electrode is dramatically enhanced. The suitable equivalent circuit model shown in the inner part of Figure 4c and the EIS simulation results were obtained by fitting experimental data as shown in Table S2, in which R_s is used to present the solution resistance and R_ct and W correspond to charge-transfer resistance and Warburg resistance, respectively. The constant phase elements (CPE) refer to the real double-layer capacitors. It can be seen that the R_ct of the Ti/B-TiO₂-NTs/PbO₂-SDS electrode was smaller than that of the other PbO₂ electrodes, indicating its better electrical conductivity and high charge-transfer rate [22]. The significantly promoted charge-transfer performance of the Ti/B-TiO₂-NTs/PbO₂-SDS electrode may be attributed to its compact β-PbO₂ crystal structure and the good conductivity of the Ti³⁺ self-doping Ti/B-TiO₂-NTs interlayer. So, the electrochemical degradation reaction can proceed more easily on this electrode. In addition, the improvement in electron-transfer efficiency of the undoped Ti/TiO₂-NTs, the Ti³⁺ self-doping Ti/B-TiO₂-NTs and Ti/B-TiO₂-NTs/PbO₂-SDS was further confirmed by the Mott–Schottky measurements at a frequency of 500 Hz in the 0.5 M H₂SO₄ solution. Figure 4d presents the Mott–Schottky plots of the pristine TiO₂NTs and Ti³⁺ self-doping Ti/B-TiO₂-NTs. It was intriguing to note that the Ti³⁺ self-doping TiO₂NTs material displayed a substantially smaller slope of Mott–Schottky plots than the other pristine TiO₂NTs, suggesting an increase in donor density due to the presence of Ti³⁺ (oxygen vacancies) under the electrochemical reduction, which effectively promotes the electrical conductivity and charge transport. Therefore, compared with the traditional Ti/TiO₂-NTs-PbO₂ electrode, the Ti³⁺ self-doping Ti/B-TiO₂-NTs-PbO₂ electrode with its improved electron-transfer efficiency possessed an enhanced electrochemical activity in the degradation process.

The electrochemical activity of electrode materials is strongly associated with the quantity of active sites on the surface of electrodes [23]. The voltammetric charge quantity (q*) is considered to be related to the electrochemical activity and the number of active sites of PbO₂ electrodes. The higher q* means more accessible active sites on the anode surface. The q* value can be calculated with the following equation [24]:

q* = q_o* + kν^−1/2

(2)

The outer charge quantity (q_o*) is the total quantity of theoretically active sites on the electrode surface, where ν is the scan rate of voltage and k is a constant. The linear relationship between q* and v^−1/2 by fitting with linear correlations is shown in Figure 4e. It was noticed that the Ti/B-TiO₂-NTs/PbO₂-SDS electrode obtained the largest q* among the three PbO₂ electrodes. This result indicates that the Ti/B-TiO₂-NTs/PbO₂-SDS electrode would exhibit good electrocatalytic performance for the electrochemical degradation of contaminants, which can be attributed to its larger specific surface area and a large number of active sites.

The hydroxyl radical (·OH) generation capacity of PbO₂ can also be used to evaluate its electrochemical catalytic performance [25]. Hence, the ·OH radical generation ability of different electrodes was tested using fluorescent spectrometry. Terephthalic acid was used as a probe for ·OH radicals, generating a highly fluorescent product of 2-hydroxyterephthalic acid. So, the amounts of the generated ·OH radicals can be reflected by the fluorescence intensity of 2-hydroxyterephthalic acid. A linear relationship between fluorescence intensity and electrolysis time is shown in Figure 4f. According to Figure 4f, the fluorescence intensity on the Ti/B-TiO₂-NTs/PbO₂-SDS electrode is higher than that of the other two PbO₂ electrodes, which implies that it has the strongest ·OH radical generation ability. This demonstrates once again that the Ti/B-TiO₂-NTs/PbO₂-SDS electrode exhibits higher electrocatalytic activity than the pristine PbO₂ electrode for degrading organic pollutants.

2.3. Electrochemical Oxidation Degradation of Methylene Blue (MB)

2.3.1. Effects of Different Anodes

As the core component of the electrochemical oxidation process, anode materials play a key role that strongly depends on the efficiency of organic wastewater degradation. The electrocatalytic oxidation abilities of the three different PbO₂ electrodes were investigated for the degradation of 30 mg L⁻¹ MB with an applied current density of 40 mA cm⁻² at 25 °C. In Figure 5a, it can be found that an MB decolorization rate of up to 99.7% was achieved by the Ti/B-TiO₂-NTs/PbO₂-SDS electrode during 120 min electrolysis, which was higher than that of the Ti/TiO₂-NTs/PbO₂ and Ti/B-TiO₂-NTs/PbO₂ electrodes. From Figure 5b, it is easy to observe that the Ti/B-TiO₂-NTs/PbO₂-SDS electrode exhibited the highest COD removal efficiency among the three prepared electrodes during 120 min, indicating its higher electrocatalytic oxidation capacity compared to the Ti/TiO₂-NTs/PbO₂ and Ti/B-TiO₂-NTs/PbO₂ electrodes.

In addition, the pseudo-first-order kinetics of the degradation of MB concentration with time was analyzed, and the results are shown in Figure 5c. From Figure 5c, the semilogarithmic plot was in good agreement with the pseudo-first-order kinetics, and the highest rate constant (k) was obtained under the optimal conditions by employing the enhancement of Ti/B-TiO₂-NTs/PbO₂-SDS electrode. The pseudo-first-order kinetic constant of the Ti/B-TiO₂-NTs/PbO₂-SDS electrode was 0.03956 min⁻¹, which was approximately 3.18 times faster than that of the Ti/TiO₂-NTs/PbO₂ electrode (0.01254 min⁻¹). The results were attributed to the large specific surface area and more active sites of the Ti/B-TiO₂-NTs/PbO₂-SDS electrode. Furthermore, the average current efficiency (ACE) of the different PbO₂ electrodes was investigated under different degradation time period, and the results are exhibited in Figure 5d. The Ti/B-TiO₂-NTs/PbO₂-SDS electrode exhibits a higher level of ACE than that of the Ti/TiO₂-NTs/PbO₂ and Ti/B-TiO₂-NTs/PbO₂ electrodes during the whole degradation of MB, which indicated its high-efficiency performance and better energy conservation. Table S3 lists the COD removal efficiency of other types of PbO₂ electrodes reported in the literature. By comparison, obviously, the Ti/B-TiO₂-NTs/PbO₂-SDS electrode had better activity.

The above results demonstrate that the Ti/B-TiO₂-NTs/PbO₂-SDS electrode possesses the best electrochemical activity for the degradation of MB compared to the other two electrodes. This can be attributed to its stronger hydroxyl radical generation rate, more electrochemical active sites, and smaller charge-transfer resistance than the other two electrodes.

2.3.2. Effect of Current Density

The current density is an important influencing parameter in the electrochemical oxidation for the degradation of MB [26,27]. Therefore, it is necessary to investigate the influence of current density on the degradation of MB. The effect of current density on the Ti/B-TiO₂-NTs/PbO₂-SDS electrode for the degradation of MB was investigated at pH = 3, the MB concentration of 30 mg L⁻¹, and the electrolyte Na₂SO₄ concentration of 0.2 M. It can be found from Figure 6a that the removal efficiency of MB increased from 87.60% to 99.9% when the current density changed from 20 to 60 mA cm⁻². In addition, the degradation of MB at different current densities was fitted by a pseudo-first-order kinetic model, and the kinetics parameters are shown in the inset of Figure S6a. The corresponding k value also increased with the increase in current density. As displayed in Figure 6b, the COD degradation efficiencies also gradually increased with the increase in current densities. The reason for this phenomenon is that a high yield of ·OH radicals can be formed at a high current density [28,29]. However, according to Figure 5a,b, there was a slight increase in decolorization efficiency and COD removal efficiency when the current density was more than 50 mA cm⁻². This could be attributed to the more serious oxygen evolution side reaction, leading to a decreased current efficiency [30]. Therefore, 40 mA cm⁻² was selected as the optimal current density for the following studies.

2.3.3. Effect of Initial pH

In this work, the influence of initial pH values ranging from 3 to 11 on the degradation of MB is shown in Figure 6c,d. The results revealed that the decolorization ratio of MB and COD in an acidic medium is higher than that in neutral and alkaline environments. The highest removal ratio of MB and COD is achieved at pH= 3. The kinetic rate constant (k) value at pH = 3 is 4.02 times higher than that at pH = 11 (Figure S6b). In an acid medium, H⁺ can inhibit the decomposition reaction of ·OH into oxygen [31,32]. In order to keep a high content of hydroxyl radicals, pH = 3 was selected for further research.

2.3.4. Effect of Initial Concentration

It is meaningful to investigate the effect of the initial concentration of MB in the electrochemical oxidation process because MB exists in industrial wastewater with a wide range of concentrations.

The influence of the initial MB concentration on the degradation rate is shown in Figure 6e,f. As observed in Figure 5e,f, the MB decolorization ratio significantly decreased from 99.98% to 71.45% and the removal efficiency of COD decreased from 88.70% to 49.46% with an increase in initial MB concentration. The kinetic constant (k) increased as the concentration decreased (Figure S6c). The possible reason for this phenomenon was that more intermediates were generated and accumulated in large quantities on the electrode surface with the increase in the initial concentration during the degradation of MB. This hindered the formation of ·OH and indirect oxidation, leading to a decrease in the degradation of MB [33]. The kinetic constant (k) gradually increased from 0.01145 min⁻¹ to 0.05495 min⁻¹ as the concentration decreased (Figure S6c).

2.3.5. Effect of Electrolyte Concentration

The supporting electrolyte concentration promotes the conductivity of the degradation solution and is an essential factor in the electrochemical degradation process for reducing energy consumption. The effect of Na₂SO₄ concentration from 0.05 to 0.25 M on the MB degradation efficiency was investigated at different supporting electrolyte concentrations. As displayed in Figure 7a,b, the MB decolorization and COD removal efficiency initially increase and then decrease. A similar tendency can be observed for the kinetic constant (k) (Figure S6d).

This phenomenon is ascribed to the fact that Na₂SO₄ can improve the conductivity of the solution with an increase in electrolyte concentration. However, when the concentration of Na₂SO₄ is too high, the electrode would be coated with adsorbed SO₄²⁻ and occupy the active site on the electrode surface, thus leading to a reduction in the generation of oxidizing radicals during electrochemical oxidation [34,35].

2.4. Degradation Mechanism of MB Wastewater

2.4.1. The UV-Vis Absorption Spectra Analysis

The UV–visible absorption spectra were measured at different times, and the results are shown in Figure 8. As shown in Figure 8, the original UV-vis absorption peaks of MB had three distinct absorption peaks at 292, 602 and 665 nm, respectively [36,37]. With the increase in degradation time, the characteristic peaks at 610 nm and 664 nm decreased. This may be ascribed to the N-demethylation of MB [37]. More importantly, no obvious characteristic peaks of MB were found after a degradation period of 120 min, illustrating that MB had been completely degraded. Therefore, the as-prepared Ti/B-TiO₂-NTs/PbO₂-SDS electrode had an efficient capacity in the degradation of MB.

2.4.2. Trapping Experiments of Radicals

Electron spin resonance (ESR, Hitachi JES-FA200) was employed to trap the possible free radicals of ·OH and sulfate (·SO₄⁻¹) generated in the electrochemical system using a Ti/B-TiO₂-NTs/PbO₂-SDS electrode. 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) as spin-trapping agent was used to capture possible free radicals generated in the electrochemical system. As shown in Figure 9a, significant signals of DMPO-·OH and DMPO-·SO₄⁻¹ can be observed, suggesting that the reactive groups produced were ·OH and ·SO₄⁻¹ in the electrochemical oxidation process.

To further explore the role of ·OH and ·SO₄⁻¹ in the degradation of MB, radical trapping experiments were performed to track the contribution of ·OH and ·SO₄⁻¹ radicals during the electrocatalytic reaction. Methanol (MA) and Tertbutanol (TBA) were used as free radical scavengers. MA could capture both ·OH and ·SO₄⁻¹ radicals, while TBA was only adopted as the scavenger of ⋅OH radicals [38]. Figure 9b clearly indicates that ·OH played a primary role in MB degradation. From Figure 9b, it can be observed that the efficiency of MB degradation decreased significantly when MA and TBA were added to the wastewater containing MB. The high suppression ratios indicate that the indirect oxidation process is mainly responsible for the degradation of MB.

2.5. Electrode Stability and Recyclability

The stability of the electrodes is a significant factor in evaluating the electrochemical oxidation performance of the electrodes. It can be inferred from the data displayed in Figure 10 that the Ti/B-TiO₂-NTs/PbO₂-SDS electrode possessed a longer service life (442 h), which was 4.49 times longer than that of the Ti/TiO₂-NTs/PbO ₂ electrode (99 h). This result indicates that the stability of Ti/TiO₂-NTs/PbO₂ was significantly improved after introducing Ti³⁺ and SDS. Meantime, the electrochemical recycling test was also carried out under the optimal conditions. Figure S7 shows that the MB removal efficiency slightly decreased from 99.2 to 96.1% after 20 consecutive cycles. Such a small reduction in MB removal percentage demonstrates the high chemical stability and recyclability for electrochemical oxidation, which can be attributed to the compact structure of PbO₂ film with the introduction of Ti³⁺ and SDS. These results indicate that the Ti/B-TiO₂-NTs/PbO₂-SDS is a promising electrode anode material for the treatment of textile wastewater.

3. Experimental

3.1. Materials and Reagents

All chemical reagents were analytical-grade and provided by Sinopharm Chemical Regent Co. Ltd., Shanghai, China. The solutions were prepared using ultrapure water.

3.2. Preparation of Ti³⁺ Self-Doping Ti/B-TiO₂-NTs/PbO₂-SDS Electrode

As illustrated in Scheme 1, the Ti³⁺ self-doping Ti/B-TiO₂-NTs/PbO₂-SDS electrode was prepared by using two main steps.

(1)

Fabrication of the Ti³⁺ self-doping Ti/B-TiO₂-NTs middle layer on the Ti substrate.

The method for preparing the Ti³⁺ self-doping Ti/B-TiO₂-NTs material was consistent with our previous work [39]. Firstly, a Ti sheet (1.0 cm × 2.0 cm × 0.8 cm) was mechanically polished using different sizes of the abrasive papers in sequence and then was cleaned ultrasonically. Afterward, the cleaned Ti sheet was chemically etched in a mixture of HF and ethylene glycol for several minutes. The pre-treated Ti sheet was electrochemically oxidized at 60 V for 7 h to obtain as-grown nanotube arrays. For the fabrication of Ti³⁺ self-doping Ti/B-TiO₂-NTs, Ti/TiO₂-NTs were electrochemically reduced in 1 M (NH₄)₂SO₄ solution under a constant current (5 mA cm⁻²) for 10 min with Ti/TiO₂-NTs and platinum foil serving as cathode and anode, respectively. Then, it was calcined at 550 °C in a muffle furnace for 2 h under Ar condition. Ti³⁺ self-doping Ti/B-TiO₂-NTs were prepared to obtain the intermediate layer. The reduced electrode was labeled as Ti/B-TiO₂-NTs.

(2)

Deposition of the PbO₂ coating on the middle layer

Afterward, the Ti³⁺ self-doping Ti/B-TiO₂-NTs/PbO₂-SDS electrode was prepared by the electrodeposition method in an electrolyte containing 0.1 M HNO₃, 0.5 M Pb(NO₃)₂ 10 mg L⁻¹ SDS and 0.02 M NaF under 20 mA cm⁻² for 1 h at 60 °C. For comparison, the Ti/B-TiO₂-NTs/PbO₂ and Ti/TiO₂-NTs/PbO₂ without Ti³⁺ self-doping electrodes were prepared using the same electrodeposition method.

3.3. Characterization

The surface morphologies and crystallographic structures of the as-fabricated electrodes were observed by field emission scanning electron microscopy (SEM, ZEISS, Sigma 300, Sigma Aldrich, St. Louis, MO, USA) and X-ray diffraction (XRD, PHILIPS, Amsterdam, The Netherlands, X’Pert PRO). The chemical composition and valence state of the electrode were determined by X-ray photoelectron spectroscopy (XPS, Thermo, Waltham, MA, USA, ESCALAB 250). In addition, a UV-vis spectrophotometer (UV-vis, Shimadzu, Kyoto, Japan, UV-3600Plus) analyzer was used to determine the various MB concentrations during different degradation processes.

3.4. Electrochemical Measurement

The studies of electrochemical performances were performed on an electrochemical workstation (Metrohm, Autolab302N, Utrecht, The Netherlands) with a three-electrode cell. The as-fabricated PbO₂ electrode, platinum foil and saturated calomel electrode (SCE) were used as the working electrode, auxiliary electrode and reference electrode, respectively. Cyclic voltammetry (CV) in a 0.2 M Na₂SO₄ solution was utilized for analyzing the oxidative properties of all different electrodes. Linear sweep voltammetry (LSV) was performed from 0 to 2.5 V with a scan speed of 10 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) was implemented in a frequency range of 100 kHz to 10 MHz with a potential amplitude of 5 mV. The accelerated service lifetime tests of all different electrodes were performed at a high current density of 1 A·cm⁻² in 2 M H₂SO₄. The electrode was deactivated when the cell voltage reached 10 V higher than the initial voltage.

3.5. Electrocatalytic Degradation

MB was selected as the simulated pollutant because it is widely used in printing, the dyeing industry and coloring paper [13,40]. During the process of experiments, a model dye wastewater of MB was electrochemically oxidized in a 100 mL beaker with a rotor stirring function. In the electrolytic cell, the prepared electrode (2 cm × 3 cm) was used as an anode, and a same-sized Ti sheet was used as the cathode. The liquid samples were drawn from the reactor every 20 min and analyzed, including the removal of MB through a UV–vis spectrophotometer and the variation in chemical oxygen demand (COD) by using a COD analyzer (Lianhua Tech. Co. Ltd., Shenzhen, China). The absorbance of MB at different times was monitored by using the UV-vis spectrophotometer analyzer at 240–800 nm. The decolorization rate (η_MB) was calculated by the following formula [41]:

$${\mathrm{\eta }}_{\mathrm{MB}}=\frac{{\mathrm{A}}_{0 }-{ \mathrm{A}}_{\mathrm{t}}}{{\mathrm{A}}_{\mathrm{t}}}\times 100\%$$

(3)

A₀ and A_t are the absorbance of MB at the initial and different times (t).

The COD removal efficiency (η_COD) was calculated by the following formula [42]:

$${\mathrm{\eta }}_{\mathrm{COD} }=\frac{{\mathrm{COD}}_0 - {\mathrm{COD}}_{\mathrm{t}}}{{\mathrm{COD}}_0} \times 100\%$$

(4)

COD₀ is the COD of the initial concentration, and COD_t is the COD at a given time t.

The average current efficiency of different current densities was calculated using Equation (3) [43]:

$$\mathrm{ACE}=\frac{{(\mathrm{COD}}_0- {\mathrm{COD}}_{\mathrm{t} })·\mathrm{F}·{\mathrm{V}}_{\mathrm{l}}}{8\mathrm{I}·∆\mathrm{t}·1000}\times 100\%$$

(5)

where COD₀ and COD_t are the chemical oxygen demand at time 0(s) and t(s), respectively; the Faraday constant F is equal to 96,487 C·mol⁻¹; V_L is the volume of electrolyte solution (L); △_t is the degradation time (s); and I is the current intensity (A). The electrochemical degradation of MB was found to well fit pseudo-first-order kinetics [44,45]:

$$\ln (\frac{{\mathrm{C}}_0}{{\mathrm{C}}_{\mathrm{t}}})=\mathrm{kt}$$

(6)

where C₀ is the initial MB concentration, C_t is the concentration of MB at the given time t, and k is the kinetic rate constant.

The hydroxyl (·OH) radicals generated during the electrochemical processes were detected using terephthalic acid as a fluorescence probe and then analyzed using a fluorescence spectrophotometer (Cary Elipse, Agilent, Santa Clara, CA, USA) in the range of 370–520 nm (excitation wavelength: 315 nm).

4. Conclusions

The target Ti³⁺ self-doping black Ti/TiO₂-NTs/PbO₂-SDS electrode was successfully prepared through electrochemical reduction and electrodeposition methods in this study. The morphology and crystal structure were confirmed by SEM and XRD, respectively. The experimental results demonstrated that the Ti/B-TiO₂-NTs/PbO₂-SDS electrode possessed a more compact structure and finer grain size than the no-doping TiO₂NTs/PbO₂ electrode. Moreover, the introduction of the Ti³⁺ self-doping middle layer (Ti/B-TiO₂-NTs) and doping of SDS significantly enhanced the electrochemical activity of the resulting Ti/B-TiO₂-NTs/PbO₂-SDS electrode. The enhanced electrocatalytic performance of the Ti/B-TiO₂-NTs/PbO₂-SDS electrode was due to its higher oxygen evolution potential (2.11 V), smaller charge-transfer resistance (6.74 Ω) and stronger ·OH generating ability. After 120 min of electrolysis, the MB decolorization rate and COD removal rate were 99.7% and 80.6%, respectively. Importantly, the value of ACE was significantly higher than that of the TiO₂NTs/PbO₂ electrode, indicating superior current efficiency and energy conservation capabilities. The electrochemical degradation of MB followed pseudo-first-order kinetics, and the kinetic constant of the Ti/B-TiO₂-NTs/PbO₂-SDS electrode reached 0.03958 min⁻¹, which was 3.18 times higher than that of the pristine Ti/TiO₂NTs/PbO₂ electrode (0.01254 min⁻¹). Furthermore, the accelerated lifetime tests confirmed the excellent stability of the prepared Ti/B-TiO₂-NTs/PbO₂-SDS electrode with a lifetime about 4.49 times longer than that of the pristine Ti/TiO₂-NTs/PbO₂ electrode. As a whole, the synthesized Ti/B-TiO₂-NTs/PbO₂-SDS electrode has the potential for application in the electrocatalytic oxidation degradation of refractory pollutants.