LDH/MXene Synergistic Carrier Separation Effects to Improve the Photoelectric Catalytic Activities of Bi₂WO₆ Nanosheet Arrays

Abstract

Photoelectric catalysis is a green and efficient way to degrade pollutants, which has been paid more and more attention by researchers. Among them, Bi₂WO₃ has been proved to have excellent photocatalytic oxidation activity on its {001} facets. In this study, {001}-oriented facets with high exposure were successfully integrated into Bi₂WO₆ nanoplate arrays (Bi₂WO₆ NAs) to create a photoelectrode. This structure was grown in situ on an indium tin oxide (ITO) substrate. To promote photogenerated carrier separation efficiency and reduce agglomeration of Bi₂WO₆ photocatalysts, the electrochemical deposition of NiFe–layered double hydroxide (NiFe-LDH) and Ti₃C₂ (MXene) were introduced in this research to synergistically catalyze pollutant degradation. Morphology, spectral characterization, and electrochemical analysis jointly confirmed that the outstanding performance of hole capture behavior with LDH and electron conduction properties with MXene were the main reasons for the improvement in catalytic activity of the photoelectrode. Taking bisphenol A (BPA) as the model pollutant, the rate constant k of the NiFe-LDH/Ti₃C₂/Bi₂WO₆ NAs photoelectrode reaches 0.00196 min⁻¹ under photoelectrocatalytic (PEC) conditions, which is 4.5 times that of the pure Bi₂WO₆ NAs photoelectrode. This work provides a new way to improve the reaction kinetics of the PEC degradation of pollutants.

1. Introduction

Photoelectrocatalysis, as a green way to solve water pollution, has the advantages of electrocatalytic (EC) and photocatalytic (PC) oxidation [1,2]. During the photoelectrocatalytic (PEC) process, photogenerated electrons are driven towards the counter electrode by an external potential. This mechanism facilitates the efficient separation of photogenerated electrons and holes, thereby significantly enhancing photocatalytic activity [3,4,5,6].

As a significant semiconductor photocatalytic material, Bi₂WO₆ is the simplest compound in the Aurivillius family, which is alternately composed of [Bi₂O₂]²⁺ layers and perovskite-like (WO₄)²⁻ octahedral layers, and is widely used in PEC oxidation [3,7]. As a narrow band gap semiconductor (2.75 eV), Bi₂WO₆ has the advantages of a non-toxic, high-stability, and visible-light response [8,9,10]. Nevertheless, this material exhibits certain drawbacks, such as a high rate of electron and hole recombination, a low specific surface area, and a tendency towards easy agglomeration [4,11,12]. In order to address these issues, a variety of strategies have been employed to improve the photocatalytic performance of Bi₂WO₆. These methods include doping, the construction of heterojunctions, and surface hybridization [8,13,14,15]. In 2005, Zhu and co-workers synthesized Bi₂WO₆ nanostructures for the first time through simple hydrothermal treatment, which set a precedent for the preparation of Bi₂WO₆ nanostructures [16]. Cao and co-workers successfully prepared the 2D/2D heterojunction of ultra-thin Ti₃C₂/Bi₂WO₆ nanosheets, which have a short charge transfer distance and large interface contact area. Under simulated solar light irradiation, the combined yield of CH₄ and CH₃OH achieved by the Ti₃C₂/Bi₂WO₆ composite is 4.6 times higher than that of pure Bi₂WO₆ in the CO₂ reduction reaction [17].

Layered double hydroxide (LDH) is a type of layered inorganic functional material, characterized by a positively charged layer and an anion that is exchangeable in the interlayer space [18,19,20]. LDH has attracted much attention due to its unique layered structure and exchangeable anion between layers [21,22]. In recent years, Zhu et al. reported three-dimensional BiVO₄/NiFe-LDH heterostructures. The combination of BiVO₄ and NiFe-LDH can accelerate photogenerated charge separation and photoelectrocatalytic activity [23]. Zhang and colleagues synthesized a ZnCr–CO₃LDH/ruptured tubular g-C₃N₄ photocatalyst and observed that its capability to separate electron–hole pairs was enhanced by approximately fivefold and twofold, respectively, compared to bulk g-C3N4 and ruptured tubular g-C₃N₄ [24]. MXenes, a novel class of two-dimensional (2D) layered materials, are composed of transition metal carbides or carbonitrides. The general formula for MXenes is M_n+1X_n (where n = 1, 2, or 3), with ‘M’ representing a transition metal (such as Sc, Ti, Zr, Hf, V, Nb, Ta, or Mo), and ‘X’ denoting carbon (C) and/or nitrogen (N). Notably, MXenes (Ti₃C₂), synthesized by selectively etching out the ‘A’ layers from Ti₃AC₂ (where ‘A’ can be Al, Zn, Si, or Ga), have become a focal point in research on the synthesis of MXene-based photocatalysts [25,26]. It is mainly attributed to the following excellent properties of Ti₃C₂: (i) The Ti₃C₂ surface of MXenes, adorned with various terminal functional groups such as -OH, -O, and -F, can establish a strong chemical interface with semiconductors [27]. This interaction significantly contributes to inhibiting the recombination of electrons and holes [17]. (ii) The notable metallic conductivity of Ti₃C₂ ensures rapid carrier migration, thereby facilitating efficient separation of electrons and holes [28,29]. (iii) The exposed terminal metal sites on Ti₃C₂ are likely to result in higher reactivity compared to that of carbon-based materials [26,30]. Given the aforementioned characteristics, Ti₃C₂ has emerged as a favored co-catalyst in the synthesis of photocatalysts [31]. Li et al. reported that mesoporous TiO₂ nanoparticles were successfully anchored onto a highly conductive Ti₃C₂ MXene co-catalyst using an electrostatic self-assembly strategy. The composite material, TiO₂/Ti₃C₂, has demonstrated superior photocatalytic performance. This is particularly evident in its ability to degrade methyl orange, where it achieved an exceptionally high efficiency of 99.6%. Additionally, the material exhibits a significant hydrogen production rate, quantified at 218.85 μmol·g⁻¹·h⁻¹. These findings underscore the composite’s potential efficacy in photocatalytic applications [32]. Ran et al. synthesized Ti₃C₂ nanoparticles with CdS to induce a photocatalytic hydrogen production activity of 14,342 μmol·g⁻¹·h⁻¹. The enhanced performance observed can be attributed to the advantageous characteristics of the Ti₃C₂ nanoparticles. Specifically, favorable positioning of the Fermi level, coupled with the nanoparticles’ excellent electrical conductivity, play critical roles in facilitating the high efficiency of the composite. These properties significantly contribute to the overall superior photocatalytic performance of the material [28].

In this work, we synthesized a Bi₂WO₆ photoelectrode with a large proportional exposure of {001} facets on an ITO substrate in situ (Bi₂WO₆ NAs). Subsequently, the NiFe-LDH/Ti₃C₂/Bi₂WO₆ NAs photoelectrode was synthesized via electrochemical deposition, layering NiFe-LDH and delaminating Ti₃C₂ onto Bi₂WO₆ NAs. Delaminated MXenes were utilized due to their superior electrical conductivity and robust mechanical properties, which are essential for the photocatalytic applications investigated. The layered structure facilitates efficient electron transport and provides numerous active sites for photocatalytic reactions, thereby enhancing the overall performance of the photocatalysts. This approach ensured precise material integration for the composite electrode. Taking the degradation of bisphenol A (BPA) as an example, the effect of the combination of Ti₃C₂ and NiFe-LDH on the catalytic performance of the Bi₂WO₆ NAs photoelectrode was investigated. Under PC, EC, and PEC conditions, the degradation activity of the NiFe-LDH/Ti₃C₂/Bi₂WO₆ NAs photoelectrode is better than that of the Bi₂WO₆ NAs photoelectrode, and the order of activity from high to low is NiFe-LDH/Ti₃C₂/Bi₂WO₆ NAs > NiFe-LDH/Bi₂WO₆ NAs > Ti₃C₂/Bi₂WO₆ NAs > Bi₂WO₆ NAs. The extensive exposure of {001} facets in the Bi₂WO₆ nanoplate arrays (NAs) photoelectrode is advantageous for photocatalytic oxidation reactions. Moreover, the synergistic interaction between NiFe–layered double hydroxide (LDH) and Ti₃C₂ significantly enhances the separation efficiency of photogenerated carriers in Bi₂WO₆. Performance of the NiFe-LDH/Bi₂WO₆ photoelectrode and the mechanism of enhancing its PEC activity have been systematically studied.

2. Results and Discussion

2.1. Catalyst Characterization

The crystallographic structure and compositional analysis of the synthesized samples were examined utilizing X-ray diffraction (XRD) techniques. (Figure 1a). The XRD pattern shows three diffraction peaks at 8.9°, 18.3°, and 60.8°, corresponding to the (002), (004), and (110) planes of Ti₃C₂, respectively [29]. In addition, all of the diffraction peaks match well with the Bi₂WO₆ NAs according to JCPDS 39-0256 [33]. The observed intensity ratio of the {002} and {131} peaks in Bi₂WO₆ nanoplate arrays (NAs) is approximately 0.5, which is notably higher than the standard value of 0.19. This indicates that the as-prepared Bi₂WO₆ NAs exhibit anisotropic growth along the {001} planes. The extensive exposure of {001}-oriented Bi₂WO₆ NAs is advantageous, as it facilitates photocatalytic oxidation reactions [34]. After adding Ti₃C₂ (MXene) in the formation of Bi₂WO₆ NAs, the area integral ratio of diffraction peak between {002} and {131} crystal planes increases to 0.54, indicating that the existence of MXene expanded the {001}-orientated crystal faces exposure of Bi₂WO₆ NAs. The observed negativity of the Ti₃C₂ surface can be attributed to the adsorption of a substantial quantity of -F and -OH groups. This characteristic is particularly relevant during the preparation of MXene, where Bi³⁺ ions are adsorbed onto the Ti₃C₂ surface, further influencing its chemical behavior and interactions. Therefore, Bi₂WO₆ NAs expose more [Bi₂O₂]²⁺ layers parallel to the {001} crystal plane. In addition, the diffraction peak of Bi₂WO₆ NAs becomes wider, which is due to the growth-limiting effect of MXene on Bi₂WO₆ NAs [17]. UV-Vis Diffuse Reflectance Spectroscopy (DRS) spectra provide insights into the optical properties of the as-prepared samples. As shown in Figure 1b, Bi₂WO₆ NAs, NiFe-LDH/Bi₂WO₆ NAs, Ti₃C₂/Bi₂WO₆ NAs, and NiFe-LDH/Ti₃C₂/Bi₂WO₆ NAs photoelectrodes have obvious absorption responses in the visible light region. Moreover, the absorption edge of Ti₃C₂/Bi₂WO₆ NAs and NiFe-LDH/Ti₃C₂/Bi₂WO₆ NAs photoelectrodes is near 450 nm, indicating that the combination of Ti₃C₂ broadens the response of Bi₂WO₆ NAs to visible light.

Scanning electron microscopy (SEM) images (Figure 2a and Figure S1) exhibit that Bi₂WO₆ nanosheets are vertically grown on the ITO substrate with a size of about 200 nm. The nanoplates are interconnected, forming a framework array structure. Following compounding with Ti₃C₂, the size of Bi₂WO₆ nanosheets is observed to be approximately 100 nm. This dimension is attributed to the growth restriction imposed by Ti₃C₂ on Bi₂WO₆. (Figure 2b,c) [17]. Figure 2d displays the structural morphology of the NiFe–layered double hydroxide (LDH)/Ti₃C₂/Bi₂WO₆ nanoplate arrays (NAs) photoelectrode. NiFe-LDH is observed to be uniformly coated on the Ti₃C₂/Bi₂WO₆ surface, presenting a flocculent morphology that is evenly dispersed. As shown in Supplementary Figure S2, the uniform detection of elements such as tungsten (W), bismuth (Bi), oxygen (O), nickel (Ni), iron (Fe), titanium (Ti), and carbon (C) confirms the even distribution of NiFe-LDH and Ti₃C₂ across the Bi₂WO₆ NAs.

2.2. Enhanced PEC Degradation Activity

Catalytic performance of as-prepared catalysts is estimated by the degradation of BPA in aqueous solution under PC, EC, and PEC systems. As can be seen from Figure S3, BPA does not undergo self-degradation under visible light irradiation. The EC activity of the Bi₂WO₆ NAs photoelectrode also shows a low degradation rate of BPA (Figure 3a and Figure 4a). Generally speaking, Ti₃C₂ and NiFe-LDH are excellent electrocatalytic materials with high conductivity, so the EC activity is significantly improved after compounding with Ti₃C₂ and NiFe-LDH. Upon exposure exclusively to visible light, the degradation rate constant, denoted as k, of the NiFe-LDH/Ti₃C₂/Bi₂WO₆ nanocomposite photoelectrode exhibits an enhancement by a factor of approximately 10 compared to the degradation rate constant of the pristine Bi₂WO₆ nanocomposite photoelectrode (Figure 3b and Figure 4b) due to the excellent electron transport properties of Ti₃C₂ and the fast trapping of photogenerated holes by NiFe-LDH. In the PEC system, the catalytic performance of Bi₂WO₆ NAs, Ti₃C₂/Bi₂WO₆ NAs, NiFe-LDH/Bi₂WO₆ NAs, and NiFe-LDH/Ti₃C₂/Bi₂WO₆ NAs photoelectrodes is further improved, all of which are higher than the PC or EC system alone (Figure 3c and Figure 4c). The external potential can quickly transfer the photogenerated electrons generated by Bi₂WO₆ NAs to the counter electrode through the external circuit. Consequently, PEC activity escalates with the augmentation of external potential. At external potentials below the water decomposition threshold, such potential facilitates the migration of electrons towards the counter electrode, thereby enhancing the segregation of photogenerated electrons and holes. Conversely, when the external potential surpasses the threshold for water decomposition, it instigates complex oxidation processes. Figure S4 shows the PEC performance of the NiFe-LDH/Ti₃C₂/Bi₂WO₆ NAs photoelectrode under different external potentials. Under the irradiation of visible light (λ ≥ 420 nm), the PEC degradation rate of BPA gradually increases with increasing external potential. However, when the external potential reaches 3.0 V, the surface of the Bi₂WO₆ NAs photoelectrode blackens rapidly, leading to deactivation of the catalyst and a sharp decrease in degradation activity.

2.3. Enhancement Mechanism of Photoelectrochemical (PEC) Activity

To elucidate the underlying mechanism behind the augmented PEC activity of NiFe-LDH/Ti₃C₂/Bi₂WO₆ nanocomposites, an array of photoelectrical characterizations has been conducted. As can be seen from Figure 5a, the photocurrent density of the NiFe-LDH/Ti₃C₂/Bi₂WO₆ NAs photoelectrode reaches about 3.0 μA/cm², which represents an enhancement of approximately 3.8-fold compared to the Ti₃C₂/Bi₂WO₆ nanocomposite photoelectrode and a 6.0-fold increase over the pristine Bi₂WO₆ nanocomposite photoelectrode. The increase of photocurrent density of the NiFe-LDH/Ti₃C₂/Bi₂WO₆ NAs photoelectrode is attributed to the synergistic effect of NiFe-LDH and Ti₃C₂, which greatly improves the separation efficiency of Bi₂WO₆ NAs photogenerated charge. Electrochemical impedance spectroscopy is utilized to investigate the impedance characteristics of the synthesized samples, as depicted in Figure 5b. The curve radius of the pure Bi₂WO₆ NAs photoelectrode is the largest, and the impedance value is also the largest. After compounding with Ti₃C₂ and NiFe-LDH, the curve radius of the NiFe-LDH/Ti₃C₂/Bi₂WO₆ NAs photoelectrode decreases and the impedance value decreases accordingly, which is consistent with the above transient photocurrent response results. The catalytic performance of the as-prepared sample is investigated by linear sweep voltammetry (LSV) (Figure 5c). The order of current density under different external potentials is NiFe-LDH/Ti₃C₂/Bi₂WO₆ NAs > NiFe-LDH/Bi₂WO₆ NAs > Ti₃C₂/Bi₂WO₆ NAs > Bi₂WO₆ NAs. Furthermore, the starting potential of the NiFe-LDH/Ti₃C₂/Bi₂WO₆ NAs photoelectrode has an obvious negative shift, indicating that the NiFe-LDH/Ti₃C₂/Bi₂WO₆ NAs photoelectrode has a stronger redox ability. From Figure 5d, the curve slopes of the Bi₂WO₆ NAs, NiFe-LDH/Bi₂WO₆ NAs, Ti₃C₂/Bi₂WO₆ NAs, and NiFe-LDH/Ti₃C₂/Bi₂WO₆ NAs photoelectrodes are all positive, which demonstrates that Bi₂WO₆ is an N-type semiconductor. At the same time, after compounding with Ti₃C₂ and NiFe-LDH, the flat band potential shifts significantly, illustrating that the band edge bending of the NiFe-LDH/Ti₃C₂/Bi₂WO₆ NAs photoelectrode decreases. It is beneficial to promote charge transfer at the electrode/electrolyte interface, accelerate the surface reaction kinetics process, and then enhance catalytic performance. To investigate the correlation between the photoelectric properties and light absorption capabilities of the synthesized samples, the photoelectric conversion efficiency of Bi₂WO₆ NAs, NiFe-LDH/Bi₂WO₆ NAs, Ti₃C₂/Bi₂WO₆ NAs, and NiFe-LDH/Ti₃C₂/Bi₂WO₆ NAs photoelectrodes are calculated (Figure S5). All samples have a photoelectric conversion ability in the ultraviolet region, and the NiFe-LDH/Ti₃C₂/Bi₂WO₆ NAs photoelectrode has the strongest photoelectric conversion ability.

Figure 6 presents the outcomes of X-ray photoelectron spectroscopy (XPS) analyses performed on the catalysts. It was found that Bi, W, O, Ni, Fe, Ti, and C elements are present in the NiFe-LDH/Ti₃C₂/Bi₂WO₆ NAs photoelectrode (Figure 6a). Figure 6b illustrates the predominance of Ti-C bonds within the Ti2p peaks, alongside a minor presence of Ti-O structures. This suggests that while there may be slight oxidation of the Ti₃C₂ material, it does not detract from the dominant influence of the Ti₃C₂ structure in the ternary catalytic system. Figure 6c,d show the chemical states of Bi 4f and W 4f of as-prepared samples. Compared with that of Bi₂WO₆ NAs, the binding energy of Ti₃C₂/Bi₂WO₆ NAs in Bi 4f and W 4f spectra show positive shifts. However, the peak shifts of Bi 4f and Bi 4f_5/2 in the NiFe-LDH/Bi₂WO₆ NAs photoelectrode are opposite [13,35]. Moreover, the O 1s spectra of the as-prepared sample in Figure 6e consists of Bi-O, W-O, and surface adsorbed oxygen [36]. The peak positions of Bi-O and W-O in the Ti₃C₂/Bi₂WO₆ NAs photoelectrode show positive shifts, and the position of adsorbed oxygen shifts to 531.78 eV. On the contrary, the peak positions of Bi-O and W-O in the NiFe-LDH/Bi₂WO₆ NAs photoelectrode shift negatively to 529.22 eV and 530.04 eV, and the peaks at 530.55 eV, 531.72 eV, and 532.30 eV belong to surface adsorbed oxygen, interlayer anion (OH⁻), and H₂O of NiFe-LDH, respectively. The above proves that Bi₂WO₆ NAs interact with Ti₃C₂ and NiFe-LDH, respectively, inducing the migration of photogenerated electrons from Bi₂WO₆ NAs to Ti₃C₂ and photogenerated holes from Bi₂WO₆ NAs to NiFe-LDH. Furthermore, an upward shift in the peak positions of Ni 2p and Fe 2p observed in NiFe-LDH/Bi₂WO₆ nanocomposite photoelectrodes and NiFe-LDH/Ti₃C₂/Bi₂WO₆ nanocomposite photoelectrodes (as shown in Figure 6f,g) corroborates the interaction between the surface energy of Bi₂WO₆ nanocomposites and NiFe-LDH. This charge transfer mechanism aids in the efficient separation of electrons and holes, thereby improving photoelectrochemical performance.

Building on the preceding discussion, a more detailed mechanism is proposed for the photoelectrochemical (PEC) degradation of organic pollutants using the NiFe-LDH/Ti₃C₂/Bi₂WO₆ nanocomposite photoelectrode, as illustrated in Figure 7. The ternary composite system comprising Ti₃C₂, Bi₂WO₄, and layered double hydroxide (LDH) forms a unique and sophisticated heterojunction that exhibits characteristics of both Type-II and Z-scheme heterojunctions. This innovative configuration leverages the distinct electronic properties of each component to facilitate efficient charge separation and transfer, thereby enhancing photocatalytic performance under light irradiation. The Ti₃C₂/Bi₂WO₄ interface forms a Type-II heterojunction, where alignment of their conduction and valence bands allows for the spatial separation of photogenerated electrons and holes. Electrons tend to migrate towards Ti₃C₂, while holes accumulate in Bi₂WO₄, thus reducing the recombination rate and enhancing photocatalytic efficiency. The addition of LDH into the Ti₃C₂/Bi₂WO₄ system introduces a Z-scheme mechanism, particularly when LDH acts as a bridge for electron transfer between Ti₃C₂ and Bi₂WO₄. This configuration preserves the high reduction potential of Ti₃C₂’s electrons and the high oxidation potential of Bi₂WO₄’s holes, making the composite highly effective for redox reactions.

The LDH/Ti₃C₂/Bi₂WO₆ ternary composite system leverages a unique combination of Type-II and Z-scheme heterojunctions, offering significant advantages for photocatalytic applications. This sophisticated heterostructure ensures enhanced charge separation and transfer, effectively minimizing recombination and maximizing the availability of reactive charge carriers for photocatalysis. Additionally, the incorporation of these materials broadens the light absorption range, enabling the composite to utilize a larger portion of the solar spectrum, particularly under visible light, which significantly improves its photocatalytic activity. The Z-scheme configuration within this ternary system preserves the high reduction and oxidation potentials of electrons and holes, respectively, enabling the composite to efficiently participate in a wide range of redox reactions, including the degradation of pollutants and water splitting. Moreover, the synergistic effects among Ti₃C₂, Bi₂WO₄, and LDH not only enhance the composite’s stability and durability under photocatalytic conditions but also ensure sustained activity over extended periods, making the LDH/Ti₃C₂/Bi₂WO₆ system a highly efficient and versatile option for environmental remediation and energy conversion technologies.

The enhancement of PEC activity in this nanocomposite photoelectrode is the result of multiple synergistic effects, notably the following: (i) The enhanced exposure of {001} facets in Bi₂WO₆ nanocomposites within the NiFe-LDH/Ti₃C₂/Bi₂WO₆ framework significantly contributes to an increased generation of holes. This feature not only facilitates more active participation in the degradation process but also substantially strengthens photocatalytic oxidation reactions. The {001} facets of Bi₂WO₆ are known for their high photocatalytic activity, and their prominent exposure within the composite structure maximizes the photocatalytic sites available for reaction; (ii) The strategic integration of NiFe-LDH with Ti₃C₂ creates a highly effective conduit for the swift separation and transfer of photogenerated charges. Under illumination, the electrons excited from the conduction band of Bi₂WO₆ are quickly transferred to Ti₃C₂. This transition is facilitated by the close interfacing and strong electronic interaction between Bi₂WO₆ and Ti₃C₂, which serves as an effective conduit for the rapid transportation of electrons to the counter electrode via the external circuit. This bridging significantly curtails the recombination of photogenerated electron–hole pairs, thereby enhancing the overall separation efficiency of charge carriers; (iii) The directed migration of photogenerated holes towards the NiFe-LDH surface plays a crucial role in the oxidative degradation of organic pollutants. NiFe-LDH acts as a catalyst that accelerates the oxidation process, converting organic contaminants into carbon dioxide (CO₂) and water (H₂O). This process is facilitated by the high affinity of NiFe-LDH for photogenerated holes, which promotes their accumulation on the LDH surface, thereby increasing local hole concentration and enhancing the oxidation reaction rate. As a result, the NiFe-LDH/Ti₃C₂/Bi₂WO₆ nanocomposite photoelectrode represents a sophisticated integration of materials that leverages the unique properties of each component to maximize PEC degradation efficiency. The optimized exposure of photocatalytically active facets, coupled with the strategic facilitation of charge separation and targeted reaction pathways, underpin the superior performance of this ternary composite in the PEC degradation of organic pollutants. This comprehensive understanding of the degradation mechanism provides valuable insights into designing more efficient PEC systems for environmental remediation.

3. Conclusions

In summary, the NiFe-LDH/Ti₃C₂/Bi₂WO₆ nanocomposite photoelectrode, featuring a significant proportion of {001}-oriented facets, was successfully synthesized through a solvothermal reaction followed by electrochemical deposition. Comparative analysis under electrochemical (EC), photocatalytic (PC), and photoelectrochemical (PEC) conditions revealed that the NiFe-LDH/Ti₃C₂/Bi₂WO₆ nanocomposite photoelectrode exhibits superior degradation performance relative to the pristine Bi₂WO₆ nanocomposite photoelectrode. The activity ranking, in descending order, is as follows: NiFe-LDH/Ti₃C₂/Bi₂WO₆ NAs > NiFe-LDH/Bi₂WO₆ NAs > Ti₃C₂/Bi₂WO₆ NAs > Bi₂WO₆ NAs. The augmented photocatalytic efficacy of the NiFe-LDH/Ti₃C₂/Bi₂WO₆ nanocomposite photoelectrode can be ascribed to the synergistic interplay between Ti₃C₂ and NiFe-LDH, which significantly enhances the charge separation efficiency and reaction kinetics of Bi₂WO₆ NAs. This study elucidates a novel approach for enhancing reaction kinetics in the PEC degradation of pollutants.

4. Materials and Methods

4.1. Preparation of Bi₂WO₆ NAs Photoelectrode

Initially, 0.9 g of polyvinylpyrrolidone (PVP) was dissolved in 30 mL of ethylene glycol at a temperature of 95 °C in two separate beakers to achieve a homogeneous, transparent solution. Subsequently, 0.3299 g of sodium tungstate dihydrate (Na₂WO₄·2H₂O) was dissolved in the first beaker (hereafter referred to as solution 1), while 0.9701 g of bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O) was dissolved in the second beaker (referred to as solution 2). Solution 1 was then gradually introduced into solution 2 with constant stirring to ensure thorough mixing. The resultant mixture was subjected to a pre-reaction phase at 98 °C for 1.5 h with sustained stirring. Following this, the mixture was transferred into a Teflon-lined autoclave, in which cleaned indium tin oxide (ITO) substrates had been vertically positioned, and subsequently heated at 178 °C for 24 h. Upon cooling to ambient temperature, the ITO substrates were carefully removed, and washed multiple times with ethanol and deionized water to obtain the Bi₂WO₆ nanoplate arrays (NAs) photoelectrode.

4.2. Preparation of Ti₃C₂

A mass of 1.0 g of MAX phase powder (Ti₃AlC₂) was dispersed in a Teflon container. To this, 10 mL of 40% HF solution was added per gram of MAX phase, ensuring the mixture was stirred gently to achieve uniform etching. The reaction was allowed to proceed for 24 h at room temperature under constant stirring to facilitate the complete removal of the A layer. Following etching, the mixture was diluted with deionized water and allowed to settle, enabling the separation of undissolved solids. The supernatant, containing excess HF and reaction byproducts, was carefully decanted. This washing process was repeated several times until the pH of the supernatant reached neutrality, indicating the effective removal of residual HF. The resulting MXene was then collected by centrifugation at 3500 rpm for 5 min, washed with deionized water, and spread out for drying at room temperature in a fume hood or under vacuum at 60 °C. The dried MXene was stored in an airtight container under N₂ atmosphere to prevent oxidation.

4.3. Preparation of NiFe-LDH

A volume of 70 mL of deionized water was introduced into an electrolytic cell, followed by the addition of 0.15 mol of nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O) and 0.15 mol of iron(II) sulfate heptahydrate (FeSO₄·7H₂O). An indium tin oxide (ITO) substrate, a platinum wire, and an Ag/AgCl electrode were utilized as the working electrode, counter electrode, and reference electrode, respectively. The electrochemical deposition was conducted at a voltage of −1.0 V for a duration of 300 s. Subsequent to the electrochemical deposition, the ITO substrate was removed and exposed to ambient air to facilitate the natural oxidation of Fe²⁺ ions to Fe³⁺ ions. This process culminated in the formation of the NiFe–layered double hydroxide (LDH) electrode.

4.4. Preparation of Ti₃C₂/Bi₂WO₆ NAs Photoelectrode

Initially, 20 mL of ethylene glycol was introduced into two beakers, and subsequently placed in a 95 °C water bath. Nitrogen gas was infused into each beaker to establish an inert environment. To these, 0.6 g of polyvinylpyrrolidone (PVP) was added as a dispersant. Specifically, beaker 2 received 0.0093 g of Ti₃C₂ powder, with nitrogen flow maintained to protect the sensitive material. The mixture was sealed and magnetically stirred for 20 min, ensuring uniform dispersion of Ti₃C₂.

Simultaneously, 0.3299 g of Na₂WO₄·2H₂O and 0.9701 g of Bi(NO₃)3·5H₂O were added to beakers 1 and 2, respectively, under nitrogen to prevent any oxidative reactions. The contents of beaker 1 were slowly added to beaker 2, and the combined solution was pre-reacted for one hour at 95 °C, still under nitrogen. This mixture was then transferred to a 50 mL autoclave containing a pre-treated indium tin oxide (ITO) substrate, with the entire assembly purged with nitrogen. The autoclave underwent hydrothermal treatment at 180 °C for 24 h, facilitating formation of the composite on the ITO, all while under nitrogen to minimize degradation. Upon cooling to room temperature under nitrogen, the ITO substrate, coated with the Ti₃C₂/Bi₂WO₆ composite, was meticulously rinsed with deionized water and anhydrous ethanol, and dried in a vacuum oven at 60 °C.

4.5. Preparation of NiFe-LDH/Bi₂WO₆ NAs Photoelectrode

A volume of 70 mL of deionized water was added to an electrolytic cell. Then, under a nitrogen atmosphere and with continuous stirring, 0.15 M of Ni(NO₃)2·6H₂O and 0.15 M of FeSO₄·7H₂O were added, respectively. The flow of N₂ gas was to prevent the oxidation of Fe²⁺. The electrochemical deposition of NiFe-LDH was carried out using a three-electrode system. A Bi₂WO₆ photoelectrode, a platinum wire, and an Ag/AgCl electrode were used as the working electrode, counter electrode, and reference electrode, respectively. The deposition voltage was set to −1.0 V, and the deposition time was 30 s. After the electrochemical deposition finished, the ITO was removed, rinsed with deionized water, and then placed in air to allow Fe²⁺ to naturally oxidize to Fe³⁺. Finally, a NiFe-LDH/Bi₂WO₆ or NiFe-LDH/Ti₃C₂/Bi₂WO₆ composite photoelectrode was obtained.

4.6. Preparation of NiFe-LDH/Ti₃C₂/Bi₂WO₆ NAs Photoelectrode

A Ti₃C₂/Bi₂WO₆ NAs photoelectrode was used as the working electrode, and the experimental steps were repeated as in Section 4.5.

4.7. Characterization Techniques

X-ray diffraction (XRD) analyses were conducted using a Bruker D8-Focus diffractometer equipped with Cu Kα radiation. The morphology of the synthesized sample was examined via a scanning electron microscope (SEM, SU8010, Hitachi Ltd., Chiyoda, Japan) across an acceleration voltage range of 200 V to 30 kV, supplemented by energy dispersive X-ray (EDX) spectroscopy for elemental analysis. The interplanar spacing of the lattice fringes was determined using a high-resolution transmission electron microscope (HRTEM, Tecnai F20, FEI Company, Hillsboro, OR, USA). Ultraviolet-visible diffuse reflectance spectra (DRS) were acquired on a Hitachi 4100 spectrophotometer, employing BaSO₄ as the reference standard. X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI Quantera ULVAC XPS system (ULVAC, Inc., Chigasaki, Japan). Photoelectrocatalytic (PEC) characterizations were conducted in a quartz electrochemical cell, incorporating a three-electrode setup, with an electrochemical workstation (CHI 660D, CH Instruments, Inc., Shanghai, China). A 0.1 M solution of Na₂SO₄ served as the electrolyte. Electrochemical impedance spectroscopy (EIS) was executed over a frequency range from 0.005 Hz to 105 Hz, applying a sinusoidal AC disturbance signal of 5 mV to probe the electrochemical properties of the materials.

4.8. Degradation Activity Test

The PC and PEC activities of the synthesized samples were assessed through the degradation of bisphenol A (BPA) at a concentration of 10 mol·L⁻¹ in a 50 mL solution of Na₂SO₄ (0.1 mol·L⁻¹). Prior to initiating the PC and PEC reactions, the solution was stirred in darkness for 30 min to establish adsorption–desorption equilibrium. For the PEC activity evaluation, a photoelectrode, a Pt wire, and a saturated calomel electrode (SCE) were employed as the working electrode, counter electrode, and reference electrode, respectively. The photoelectrode was exposed to either simulated sunlight or visible light (λ > 420 nm), generated by a 300 W Xe lamp (PLS-SXE300C/300CUV, Perfect Light Ltd, Beijing, China), with an average intensity of 100 mW cm⁻². For comparative analysis, the samples were also subjected to electrochemical (EC) degradation of BPA under identical conditions but without light irradiation. The degradation process was conducted over a duration of 4 h, with aliquots of 2.5 mL sampled every 30 min. The concentration of BPA was determined using a high-performance liquid chromatography (HPLC) system (Shimadzu LC-20AT, Shimadzu Corporation, Kyoto, Japan).