Interfacial Electric Fields and Chemical Bonds in Ti₃C₂O-Crafted AgI/MoS₂ Direct Z-Scheme Heterojunction Synergistically Expedite Photocatalytic Performance

Abstract

The photocatalytic performance of heterojunctions is often restricted by inferior contact interface and low charge transfer efficiency. In this work, Ti₃C₂O MXene was crafted with AgI/MoS₂ to produce a Z-scheme heterojunction (AgI/MoS₂/Ti₃C₂O). Interfacial electric fields and chemical bonds were proven to exist in the heterojunction. The interfacial electric fields supplied a powerful driving force, and the interfacial Ti-O-Mo bonds served as an atomic-level channel for synergistically expediting the vectorial transfer of photogenerated carriers. As a result, AgI/MoS₂/Ti₃C₂O exhibited significantly improved photocatalytic activity, demonstrating a high H₂O₂ production rate of 700 μmol·g⁻¹·h⁻¹ and a rapid degradation of organic pollutants.

1. Introduction

The energy crisis and environmental pollution have emerged as two major challenges for humanity, intensifying alongside rapid urbanization and industrialization [1,2,3]. Various cutting-edge technologies have been sought to dissolve the two issues. Among numerous methods, solar energy utilization has emerged as the most effective candidate for addressing energy shortages and environmental contaminants [4,5,6]. On one hand, artificial photosynthesis mimics natural photosynthesis processes and produces a variety of valuable chemicals [7,8,9]. On the other hand, photocatalytic degradation employs renewable solar energy to create active free radicals that oxidize and break down organic pollutants [10,11,12]. Hydrogen peroxide (H₂O₂) plays a vital role in both photosynthesis and photocatalytic degradation. As a significant product of photosynthesis, H₂O₂ is regarded as a green and clean energy source with high energy density that is suitable for using in one-compartment fuel cells [13,14]. Additionally, H₂O₂ is recognized as a clean and versatile oxidant utilized in advanced oxidation processes. It can be converted into hydroxyl radicals (•OH), which efficiently degrade or even completely mineralize organic pollutants [15,16]. Beyond these applications, H₂O₂ serves as a potent disinfectant in water treatment [17] and an efficient oxidant in fuel desulfurization processes [18], demonstrating its versatility in environmental remediation. Therefore, the development of a photocatalyst for efficient H₂O₂ production is a “kill two birds with one stone” strategy for addressing energy needs and environmental remediation.

Various approaches have been investigated to develop photocatalysts with high efficiency in producing H₂O₂ [19,20]. Among the strategies to enhance interfacial charge transfer and spatial carrier separation, constructing a catalyst with a heterojunction structure is a trendy tactic [21,22]. A charge transport channel must be established within the heterojunctions to facilitate the directional transfer of photogenerated carriers at the interfaces. The interfacial chemical bonds can act as efficient atomic-level charge transport channels, significantly accelerating the migration of photoinduced electrons and holes along opposite orientations, leading to a dramatic enhancement in photocatalytic activity. For instance, Zhang et al. [23] combined Bi₂Sn₂O₇ with BiOBr to create an S-scheme heterojunction featuring interfacial Bi-O bonds. This design significantly enhanced the photocatalytic nitrogen reduction, yielding 459.04 μmol·g⁻¹·h⁻¹ of ammonia in pure water. Zhao et al. [24] developed an In-O-Cd bond-modulated S-scheme heterojunction to boost photoinduced electron transfer and reduce the electron transfer distance, thereby enhancing the photocatalytic CO₂ reduction process.

Furthermore, the interfacial electric field has been identified as another driving force in heterojunctions for efficiently separating photogenerated electrons and holes, as well as speeding up their directional transfer [25,26,27]. Zhu et al. [28] integrated Mo-modified ZnIn₂S₄ nanosheets with NiTiO₃ microrods to synthesize an S-scheme heterojunction with an enhanced interfacial electric field. The heterojunction exhibited a high H₂ evolution rate of 14.06 mmol·g⁻¹·h⁻¹ and an apparent quantum efficiency of 44.10% at 420 nm, attributed to the interfacial electric field’s capability to efficiently separate and regulate photogenerated carriers. Jiang et al. [29] described an S-scheme heterojunction composed of NH₂-MIL-125-Ti/WO_3-x with a robust interfacial electric field. This heterojunction efficiently enabled the simultaneous production of CO (12.57 μmol·g⁻¹·h⁻¹) and H₂O₂ (8.41 μmol·g⁻¹·h⁻¹) without requiring a sacrificial agent due to the carrier separation effect enhanced by the interfacial electric field.

The significance of interfacial chemical bonds and electric fields in enhancing the photocatalytic process has been clearly demonstrated above. However, there are virtually no examples of both effects being utilized simultaneously in photocatalytic H₂O₂ production. Therefore, the concept of creating a heterojunction with an interfacial electric field and chemical bonds to synergistically enhance its photocatalytic performance is promising and warrants further exploration. In this work, a Ti₃C₂O MXene-crafted AgI/MoS₂ (AgI/MoS₂/Ti₃C₂O) direct Z-scheme heterojunction was prepared and utilized as a photocatalyst for both H₂O₂ production and the degradation of organic contaminants. In addition to the traditional semiconductors AgI and MoS₂, we also incorporated Ti₃C₂O MXene, which is a novel two-dimensional metallic nanomaterial with a large specific surface area and a high abundance of surface-exposed metal atoms. These advantages confer it with excellent light absorption and efficient carrier transport properties. The resulting AgI/MoS₂/Ti₃C₂O Z-scheme heterojunction was demonstrated to possess a strong interfacial electric field and a Ti-O-Mo chemical bond, facilitating high rates of H₂O₂ production and a rapid degradation of organic pollutants.

2. Results and Discussion

2.1. Characterization of AgI/MoS₂/Ti₃C₂O Z-Scheme Heterojunction

As described in Figure 1a, the AgI/MoS₂/Ti₃C₂O Z-scheme heterojunction was prepared by a three-step procedure. First, Ti₃AlC₂ was etched to produce multi-layer Ti₃C₂T_x, which presented the characteristic accordion-like MXene morphology (Figure S1a) but with limited interlaminar space for semiconductor growth [30,31]. Multi-layer Ti₃C₂T_x was exfoliated to be few-layer to provide a larger surface area for semiconductor growth (Figure S1b). Then, MoS₂ grew on the surface of Ti₃C₂T_x through a hydrothermal process to produce MoS₂/Ti₃C₂O compound. Pure MoS₂ presented a spheric flower-like shape (Figure S1c). After composition, MoS₂ grew evenly on the Ti₃C₂O surface, forming a planar structure due to the growth foundation provided by Ti₃C₂O and preventing MoS₂ agglomeration (Figure S1d). Finally, the AgI/MoS₂/Ti₃C₂O heterojunction was obtained through self-assembly. In Figure 1b, AgI/MoS₂/Ti₃C₂O heterojunction displays a lamellar shape. The lattice fringes with interlayer spacing of 0.65 nm were attributed to the few layers of MoS₂ in the (002) crystal plane with vertically aligned edges, which was significantly larger than the interlayer distance of bulk crystals (about 0.62 nm) [32]. A similar lattice expansion had been reported for the formed metallic 1T′-MoS₂ [33]. These vertically aligned MoS₂ crystals on the Ti₃C₂O surface were beneficial for mass diffusion and charge transfer processes during photocatalytic reactions, given the significant three orders of magnitude higher electron mobility within the MoS₂ plane compared to the plane perpendicular to the base plane [34,35,36]. In addition, distinct lattice fringes were observed in the HRTEM images. The spacing of d = 0.354 nm was assigned to the (006) plane of Ti₃C₂O, and spherical lattice fringes of d = 0.248 nm belonged to the (220) plane of AgI, confirming the successful synthesis of the AgI/MoS₂/Ti₃C₂O heterojunction. Furthermore, EDS mapping in Figure 1c–i shows the well-dispersed presence of Ag, I, Ti, C, Mo, and S elements, further confirming the successful construction of the AgI/MoS₂/Ti₃C₂O heterojunction.

Raman spectra provided relevant microstructure information (Figure 2a). The characteristic peaks at around 379 and 402 cm⁻¹ corresponded to the in-plane ($E_{2\mathrm{g}}^1$) and out-of-plane ($A_{1\mathrm{g}}$) Mo-S phonon modes of 2H-MoS₂, respectively [35]. The relatively high intensity of the $A_{1\mathrm{g}}$ mode in the AgI/MoS₂/Ti₃C₂O heterojunction indicated that the vertically aligned MoS₂ in the heterojunction had an edge termination structure [36]. Moreover, the characteristic peaks of J₁, J₂, and J₃ were not obvious in MoS₂ owing to the low content of 1T′ phase MoS₂, which gradually increased following the formation of the heterojunction [37]. In addition, compared to pure Ti₃C₂O, the peak of MoS₂/Ti₃C₂O and AgI/MoS₂/Ti₃C₂O at 203 cm⁻¹ decreased because the Ti₃C₂O surface in MoS₂/Ti₃C₂O and AgI/MoS₂/Ti₃C₂O was covered by 2H-MoS₂ [38]. The X-ray powder diffraction (XRD) pattern of the heterojunction showed the characteristic peaks of AgI, MoS₂, and Ti₃C₂O, confirming their successful combination (Figure 2b). The X-ray photoelectron spectra (XPS) procedure was performed to analyze the surface chemical composition of the heterojunction. Carbon C 1s with a binding energy of 284.8 eV was used as the standard reference. Ag, I, Mo, S, Ti, C, and O were found in the survey spectrum of the heterojunction (Figure S2). The C-Ti bond disappeared in the C 1s XPS profile of the heterojunction, indicating that the C-Ti bond broke during synthesis (Figure 2c). Ag⁺ ions easily adsorbed onto the surface of Ti₃C₂O, which had abundant surface-active O-containing groups and grew to be AgI, resulting in the rupture of the C-Ti bond [39]. In the XPS spectra of O 1s (Figure 2d), the peaks at 530.3 and 530.8 eV were ascribed to Ti-O-Ti and Ti-O-Mo bonds, respectively, demonstrating the interaction between Ti₃C₂O and MoS₂ and the successful formation of interfacial chemical bonds [40]. The peak at 532.8 eV belonged to the Ti-O-H bond [41]. The Ti 2p core level of Ti₃C₂T_x was fitted with three doublets (Ti 2p_3/2-Ti 2p_1/2) (Figure 2e) [42]. The signals at 458.85 and 464.85 eV belonged to Ti(IV) 2p_3/2 and Ti(IV) 2p_1/2 of the oxygen-containing group’s Ti-O bond on the Ti₃C₂T_x surface [43], respectively, while the signals at 455.01, 456.25, 461.01, and 462.53 eV were ascribed to Ti(II) 2p_3/2, Ti (III) 2p_3/2, Ti(II) 2p_1/2 and Ti (III) 2p_1/2, respectively. The Ti-O doublet peaks of the heterojunction were intense due to the large number of oxygen-containing functional groups generated during the hydrothermal process of Ti₃C₂O synthesis [44,45]. In the high-resolution XPS spectra of Mo, the two characteristic peaks at 229.46 and 232.61 eV were ascribed to Mo 3d_5/2 and Mo 3d_3/2, respectively (Figure 2f). It is worth noting that the characteristic peak of 1T′ phase MoS₂ appeared in MoS₂/Ti₃C₂O and was enhanced in AgI/MoS₂/Ti₃C₂O, which was consistent with Raman results. In addition, the peak at 226.54 eV belonged to S 2s [43,45]. In the XPS spectrum of S 2p (Figure 2g), 162.04 and 163.25 eV corresponded to S 2p_3/2 and 2p_1/2, respectively. The peaks attributed to Ag 3d_5/2 and Ag 3d_3/2 were located at 368.10 and 374.10 eV (Figure 2h), respectively, while the peaks attributed to I 3d_5/2 and I 3d_3/2 were located at 619.30 and 630.80 eV (Figure 2i), respectively. Peak shifts were observed in the XPS spectra of Ti 2p, Mo 3d, S 2p, Ag 3d, and I 3d. The Ti(IV) 2p_3/2 of MoS₂/Ti₃C₂O positively shifted 0.53 eV due to the oxygen-containing functional groups generated during the synthetic process. Compared with pure MoS₂, the Mo 3d and S 2p of MoS₂/Ti₃C₂O positively shifted 0.06 eV and 0.11 eV, respectively, indicating that electrons transferred from MoS₂ to Ti₃C₂O. After the formation of AgI/MoS₂/Ti₃C₂O, Ti(IV) 2p_3/2, Mo 3d, and S 2p negatively shifted 0.22 eV, 0.3 eV, and 0.34 eV, respectively, indicating that electrons transferred from AgI to MoS₂/Ti₃C₂O, explaing why Ag 3d and I 3d positively shifted 0.20 eV compared to pure AgI. The above results demonstrate the interfacial chemical bond and charge redistribution between components of the heterojunction which were favorable for interfacial electric field formation.

2.2. Photocatalytic H₂O₂ Production

Since H₂O₂ is a significant chemical and the main source of reactive oxygen species (ROS) for photodegradation, the photocatalytic performance of the AgI/MoS₂/Ti₃C₂O heterojunction for H₂O₂ production was evaluated under simulated sunlight using ethanol as a sacrificial reagent. Figure 3a illustrates that the heterojunction exhibited a H₂O₂ yield of 700 μmol g⁻¹ h⁻¹, while the H₂O₂ production rates of pure semiconductors were negligible. The photocatalytic H₂O₂ production of MoS₂/Ti₃C₂O also was not substantial, demonstrating the vital role of AgI in the heterojunction. Various mass ratios of AgI within the heterojunction resulted in different photocatalytic H₂O₂ production rates (Figure S3a). 2-AgI/MoS₂/Ti₃C₂O displayed the highest photocatalytic H₂O₂ yield. Compared to other photocatalysts (Table S1), the AgI/MoS₂/Ti₃C₂O heterojunction demonstrated notable advantages in H₂O₂ yield efficiency. The formation and decomposition rates of H₂O₂ were quantitatively analyzed (Equation (S1)) since the two processes occurred simultaneously during the photocatalytic H₂O₂ production. Among the heterojunction with different mass ratios of AgI, 2-AgI/MoS₂/Ti₃C₂O exhibited the largest difference between the formation constant (K_f) and decomposition constant (K_d), confirming that it had the best H₂O₂ yield (Figure S3b). Furthermore, the stability of the heterojunction was evaluated. After three reaction cycles, the H₂O₂ yield remained at 90% of the first run (Figure 3b). Furthermore, the XRD analysis of AgI/MoS₂/Ti₃C₂O showed that its structure remained stable after three cycles of reaction (Figure S4), and AgI did not undergo photoreduction. In addition, the yield of H₂O₂ in pure water reached about 430 μmol g⁻¹ h⁻¹ (Figure 3c). To investigate the role that O₂ played in the photocatalytic production of H₂O₂, dissolved O₂ was removed by bubbling with N₂, which then resulted in an 87% decrease in H₂O₂ production. Subsequently, upon introducing the electron trapping agent AgNO₃, no H₂O₂ was detected in the N₂ atmosphere. The dependence of H₂O₂ yield on wavelength was measured under various light sources equipped with different filters. Figure 3d outlines that the AQYs of the heterojunction at 350 nm, 375 nm, 400 nm, 450 nm, and 500 nm were 30.85%, 13.80%, 9.23%, 6.53%, and 2.63%, respectively.

2.3. Photodegradation Activity

One common dye, Rhodamine B (RHB), and three kinds of antibiotics—tetracycline hydrochloride (TC), ranitidine (RAN), and sulfamethazine (SMZ)—were selected as typical organic pollutants to evaluate the photodegradation performance of the as-prepared heterojunction. These pollutants are usually difficult to degrade and pose a great threat to human health and the environment. As shown in Figure 4a–d, the prepared heterojunction 2-AgI/MoS₂/Ti₃C₂O exhibited the highest photodegradation efficiency for the four kinds of pollutants compared with AgI, MoS₂, and MoS₂/Ti₃C₂O. Only 20% and 50% of RHB were degraded within half an hour by the pure semiconductors AgI and MoS₂, respectively (Figure 4a). MoS₂/Ti₃C₂O displayed 50% degradation efficiency for RHB after half an hour of dark reaction due to the excellent absorbability of Ti₃C₂O. Among the heterojunction with different mass ratios of AgI, 2-AgI/MoS₂/Ti₃C₂O displayed the highest photodegradation efficiency (Figure S6). Similar to the photocatalytic production of H₂O₂, the degradation efficiency of 2-AgI/MoS₂/Ti₃C₂O was found to decrease slightly after three repeated degradation tests (Figure 4e). Compared to the adsorption-based removal of RHB using Tabebuia rosea peel (yielding 90% efficiency in 5 h) [46], our AgI/MoS₂/Ti₃C₂O Z-scheme heterojunction achieved better degradation within a shorter time, demonstrating superior photocatalytic performance. In order to further understand the role of ROS played during the degradation process, RHB degradation was taken as an example to conduct radical quenching tests. EDTA-2Na, p-benzoquinone (BQ), and thiourea were used as scavengers of hole, superoxide anions (O₂⁻), and hydroxyl radicals (OH), respectively. As shown in Figure 4f, degradation efficiency was greatly reduced in the presence of BQ and thiourea, demonstrating that O₂⁻ and OH played vital roles in the degradation process. EDTA-2Na slightly decreased the degradation efficiency, indicating that hole did not directly act on the pollutants.

2.4. Photocatalytic Mechanism

2.4.1. Band Structure

The light absorption and band structure of the prepared samples were surveyed by UV/Vis diffuse reflection spectroscopy (DRS), ultraviolet photoelectron spectroscopy (UPS), and in-situ XPS spectra. As demonstrated in Figure 5a, AgI displayed weak absorption in the interval of the visible light window. Obviously, the light absorption intensity and boundary of AgI/MoS₂/Ti₃C₂O were effectively enhanced and extended compared to AgI. The reason was primarily attributed to the strong light-harvesting abilities of Ti₃C₂O and MoS₂. The Kubelka–Munk transform (Equation (1)) is a standard method for estimating the optical bandgap from DRS data [47,48]. This approach is widely adopted for semiconductors like AgI [49] and MoS₂ [50], as evidenced by recent studies. According to the Kubelka–Munk equation, the band gaps (E_g) of AgI and MoS₂ were established to be 1.8 eV and 2.7 eV, respectively (Figure 5b).

$${\left(\alpha h\nu \right)}^n=B(h\nu -E_g)$$

(1)

Here, B is constant, h is Planck constant (4.13  × 10⁻¹⁵ eV⋅s), ν (Hz) is light frequency, E_g (eV,) and α (cm⁻¹) is the band gap energy and light absorption coefficient, respectively. AgI and MoS₂ are direct semiconductors (n  =  2).

The UPS technique was employed to acquire energy level information on the semiconductor interface. As shown in Figure 5c, the work functions (Φ) of AgI and MoS₂ were detected to be 4.24 eV and 5.24 eV, respectively. The Φ value of AgI was proven to be smaller than that of MoS₂. Thus, the interfacial electric field easily formed when AgI and MoS₂ contacted. Free electrons in AgI with a low work function spontaneously diffused to MoS₂ with a high work function to equilibrate the fermi energy level (E_f) of the two components. For AgI and MoS₂, the energy gap between the valence band (VB) and Fermi level were calculated to be 2.64 eV and 0.22 eV, respectively. Mott–Schottky curves (Figure S6) revealed that both AgI and MoS₂ exhibited a positive slope, displaying n-type semiconductor traits. The flat band potential (E_fb) of MoS₂ and AgI were determined to be −1.00 V and −0.60 V (vs Ag/AgCl), respectively. The conduction band (CB) potential of AgI and MoS₂ were calculated to be −0.40 V and −0.80 V (vs normal hydrogen electrode), respectively, using the formulas (Equations (S2) and (S3)), and they were approximately equal to E_fb. Moreover, in-situ XPS was coupled with a xenon lamp to record the binding energy shift direction, which was aimed at predicting the photoinduced charge separation and transfer direction in the heterojunction. It can be seen in Figure 5d–f that, under illumination, both Ag 3d and Mo 3d peaks shifted to the positive direction, while low-valence Ti (II) 2p peaks appeared at more negative positions. Ti₃C₂ with O terminal had large work function and easily accepted photoinduced electrons as electron traps [45]. As an electron-trapping trap, Ti₃C₂O promoted rapid electron migration between AgI and MoS₂, and at the same time, the electrons of AgI continuously transferred to MoS₂. The above results unveil that the interfacial electric field constructed in the Z-type heterojunction acted as a powerful driving force to facilitate charge separation and guided charge migration, and the Ti-O-Mo interface chemical bond served as an atomic-level charge transfer channel, accelerating the rapid charge migration. Their band structure is depicted in Figure 5g. The UPS-estimated band location supplied further support to the staggered alignment of AgI and MoS₂ band structures, thereby favoring the formation of a Z-type heterojunction.

2.4.2. Carrier Transfer Performance

The separation and transportation behavior of photogenerated charges were systematically inspected in order to investigate the origin of the significantly improved photocatalytic activity of the AgI/MoS₂/Ti₃C₂O heterojunction. The recombination efficiency of photogenerated carriers was studied by steady state photoluminescent (PL) spectra (Figure 6a). In general, electric potential energy is converted into PL emissions after the recombination of electrons and holes in the excited state. The sharply decreased PL emission of the AgI/MoS₂/Ti₃C₂O heterojunction indicated that the recombination of photogenerated carriers was inhibited by the interfacial electric field and chemical bond. Transient photocurrent detection also displayed that AgI/MoS₂/Ti₃C₂O exhibited the strongest photocurrent response (Figure 6b). The photoinduced carrier separation capacity of the heterojunction was further evaluated by the OCVD curves shown in Figure 6c, which demonstrate that AgI/MoS₂/Ti₃C₂O had the strongest open-circuit voltage response. The V_oc of AgI/MoS₂/Ti₃C₂O declined slowly when the light was turned off due to the charge separation effect resulting in a longer carrier lifetime and lower carrier recombination rate. EIS spectra further uncovered that AgI/MoS₂/Ti₃C₂O possessed excellent electrical conductivity (Figure 6d). The detection of ROS generated in the photocatalytic process was of guiding significance for the transfer direction of photoexcited carriers. Here, O₂⁻ and OH were measured by in-situ EPR signals under light excitation (Figure 6e,f). A ROS signal was apparent in the EPR spectra of AgI/MoS₂/Ti₃C₂O under illumination, demonstrating that efficient carrier separation was responsible for the generation of abundant active species, which significantly enhanced photocatalytic activity and actively participated in pollutant degradation. The above results illustrates the vital roles that interfacial electric fields and chemical bonds played in inhibiting the rapid recombination of photoinduced excited electrons with holes and in promoting interfacial charge separation.

2.4.3. First-Principles Calculations

First-principles calculations based on density functional theory (DFT) were carried out to comprehensively investigate the formation of internal electric fields and the mechanism of interfacial charge transfer. Similar to the methodology employed in defect studies of Rb₂O [51], our DFT approach systematically analyzed interfacial charge redistribution through the orbital-resolved density of states and work function calculations. The electronic state of Ti₃C₂O, which was primarily composed of Ti 3d and C 2p orbitals, spanned the Fermi level, indicating its excellent conductivity (Figure 7a). However, the electronic states of MoS₂ and AgI did not distribute on the Fermi level (Figure 7b,c), indicating their limited conductivity and carrier separation efficiency. Notably, while previous studies primarily focused on point defects in bulk oxides [51], our work extends this DFT framework to interfacial chemical bonds (Ti-O-Mo), revealing their unique role as atomic-level charge transfer channels. Depending on the density of states (DOS) diagram, it could be further concluded that the VB of AgI was mainly composed of Ag 3d and I 5p orbitals, while the CB was mainly composed of Ag 5s orbitals. On the other hand, both the VB and CB of MoS₂ were primarily composed of Mo 4d and S 3p orbitals. The surface Φ value was utilized to investigate the separation characteristics of interfacial charges and free electron transfer. As depicted in Figure 7d–f, Ti₃C₂O exhibited the largest Φ value of 6.40 eV, giving promise for the migration of space charges when it was combined with MoS₂ and AgI as an electron trap. The Φ value of AgI was calculated to be 4.70 eV, distinctly smaller than that of MoS₂, which was 5.70 eV. These Φ values were consistent with those derived from the UPS analysis. Φ values of Ti₃C₂T_x terminated with other functional groups were also calculated and are shown in Figure S7. Φ values of Ti₃C₂T_x with -F terminal and without functional groups were calculated to be 3.70 eV and 4.20 eV, respectively, which were lower than that of Ti₃C₂O synthesized in this work. Based on the calculation results, it can be seen that the Ti-O-Mo bond behaved as an atomic-level charge flow highway and accelerated the electron flux between MoS₂ and Ti₃C₂O. Simultaneously, electrons in AgI transferred to MoS₂ and Ti₃C₂O. The charge density difference further revealed the surface electron distribution state of the heterojunction. Significant electron transfers could be observed at the component interfaces. As shown in Figure 7g–i, after AgI was combined with MoS₂ and Ti₃C₂O, electrons accumulated from AgI to MoS₂ or Ti₃C₂O while holes were left on the surface. At the interface between MoS₂ and Ti₃C₂O, electrons converged to the surface of Ti₃C₂O, leading to an electron accumulation layer with negative charge near the Ti₃C₂O side and an electron depletion layer with positive charge near the MoS₂ side. The drifting of free electrons rendered the charge redistribution at the interface and resulted in the formation of an internal electric field that facilitated the separation of photogenerated electron–hole pairs.

Based on the preceding results and analysis, Scheme 1 illustrates the mechanism of charge transfer in the Z-scheme heterojunction. When AgI with a low E_f contacted MoS₂ with a high E_f, free electrons migrated from AgI to MoS₂ until their E_f levels were balanced, resulting in the upward bending of the band edge of AgI and the downward bending of MoS₂. Simultaneously, Ti₃C₂O with the lowest E_f contacted tightly with MoS₂ via an interfacial chemical bond and acted as an electron trap. Upon illumination, both AgI and MoS₂ were excited to produce photoelectrons and holes. Interfacial electric fields and chemical bonds synergistically accelerated the transfer of photogenerated electrons from the CB of AgI to the VB of MoS₂, thereby significantly enhancing the effective spatial separation of carriers and maintaining the strong redox ability of the Z-scheme heterojunction to generate abundant ROS, which greatly improved the photocatalytic performance.

3. Experimental Section

The detailed materials and apparatus are described in Supplementary Materials Sections S1 and S2.

3.1. Synthesis of AgI/MoS₂/Ti₃C₂O Heterojunction

The synthetic process of AgI/MoS₂/Ti₃C₂O heterojunction consisted of three steps. The first step was synthesizing Ti₃C₂T_x, which was prepared according to a previously reported method [52] with some modifications. The chemical etchant was prepared by slowly adding 1 g LiF into 15 mL HCl solution (9 M). Then, 5 mL distilled (DI) water was added and stirred for a while. Subsequently, 1 g Ti₃AlC₂ was slowly added into the prepared chemical etchant in an ice-water bath to avoid local overheating. After heating and stirring at 35 °C for 48 h, the obtained acidic solution was repeatedly washed with DI water via centrifugation at 3500 rpm for 5 min until the pH of the supernatant reached approximately neutral. Finally, multilayered Ti₃C₂T_x was collected via lyophilization and exfoliated by ultrasound to be few-layer.

The second step was synthesizing MoS₂/Ti₃C₂O, which was prepared by a traditional hydrothermal method. In brief, 15 mL Ti₃C₂T_x (7 mg·mL⁻¹) dispersion was evenly mixed with 242 mg Na₂MoO₄·2H₂O and 380 mg NH₂CSNH₂ and then transferred into a polytetrafluoroethylene high-pressure reactor and heated at 210 °C for 24 h. The resultant product was washed with ethanol and DI water several times and lyophilized to obtain the MoS₂/Ti₃C₂O compound. Pure MoS₂ was prepared with the same method but without using Ti₃C₂T_x.

The last step was synthesizing AgI/MoS₂/Ti₃C₂O Z-scheme heterojunction, which was prepared by a self-assembly method. Firstly, a certain mass of MoS₂/Ti₃C₂O was ultrasonically dispersed into DI water. Then, 83 mg KI was slowly dropped into the dispersion solution. The mixture was treated with ultrasound for 1 h to make I⁻ be evenly distributed on the surface of MoS₂/Ti₃C₂O. Finally, 85 mg AgNO₃ was added dropwise under continuous stirring. The AgI nanoparticles (NPs) grew in situ on the surface of MoS₂/Ti₃C₂O. The resultant product was washed with DI water and lyophilized. AgI/MoS₂/Ti₃C₂O Z-scheme heterojunctions with different mass ratios of AgI were obtained by using different masses of MoS₂/Ti₃C₂O (13, 30, 50, and 78 mg) and were denoted as 1-AgI/MoS₂/Ti₃C₂O, 2-AgI/MoS₂/Ti₃C₂O, 3-AgI/MoS₂/Ti₃C₂O, and 4-AgI/MoS₂/Ti₃C₂O.

3.2. Photocatalytic H₂O₂ Production

H₂O₂ production was measured by ultrasonically dispersing 10 mg photocatalyst in 100 mL ethanol solution (10 vol%). Prior to the photocatalytic reaction, the suspension was stirred in a dark room for 30 min to achieve adsorption and desorption equilibrium. The photocatalytic reaction was carried out under the irradiation of a 300 W xenon lamp configured with a simulated sunlight filter (AM1.5G). Oxygen bubbling was involved in making the suspension saturated with O₂. The reactor was kept at about 10 °C using condensed water. During the photocatalytic process, 2.00 mL solution was sampled at a given time interval. After the catalyst was removed by filtrating with a 0.22 μm filter, the solution was mixed with 1 mM Ce(SO₄)₂ solution, which was prepared by dissolving 1 mol Ce(SO₄)₂ into 1 L H₂SO₄ solution (0.50 M). The concentration of H₂O₂ was obtained by indirectly measuring Ce(SO₄)₂ concentration depending on UV-vis spectrophotometry [53]. The absorbance of the solution was measured at 318 nm by a UV-vis spectrometer. The concentration of H₂O₂ was calculated from the consumption of Ce(SO₄)₂ in the reaction (Equations (S4) and (S5)). The linear relationship between Ce(SO₄)₂ concentration and UV absorption intensity is shown in Figure S8.

The apparent quantum yield (AQY) of the photocatalytic reaction was measured under the irradiation of a 300 W xenon lamp equipped with bandpass filters of 350 nm, 375 nm, 400 nm, 450 nm, and 500 nm. Typically, 10 mg AgI/MoS₂/Ti₃C₂O Z-scheme heterojunction was dispersed into 100 mL ethanol solution (10 vol%) and then sonicated for 5 min. After 60 min of light irradiation reaction, the absorption curve of the suspension was detected by UV-vis spectrometer. The AQY value was calculated according to the following Equation (Equation (2)):

$$\begin{array}{c}{\eta }_{AQY}={\displaystyle \frac{N_e}{N_p}}\times 100\%={\displaystyle \frac{2\times n\times N_A}{\frac{E_{total}}{E_{photon}}}}\times 100\%={\displaystyle \frac{2\times n\times N_A}{\frac{S\times P\times t}{h\times \frac{c}{\lambda }}}}\times 100\%={\displaystyle \frac{2\times n\times N_A\times h\times c}{S\times P\times t\times \lambda }}\\ \end{array}$$

(2)

where n is the amount of H₂O₂ molecules (mol), N_A is the Avogadro constant (6.022 × 10²³ mol⁻¹), h is the Planck constant (6.626 × 10⁻³⁴ J·S), c is the speed of light (3 × 10⁸ m·s⁻¹), S is the irradiation area (cm²), P is the intensity of irradiation light (W·cm⁻²), t is the photoreaction time (s), and λ is the wavelength of the monochromatic light (m).

3.3. Photocatalytic Degradation of Organic Pollutants

The degradation of organic pollutants was conducted by adding 10 mg catalyst into 100 mL 10 mg·L⁻¹ organic pollutant solution. Rhodamine B (RHB), tetracycline hydrochloride (TC), ranitidine (RAN), and sulfamethazine (SMZ) were selected as four typical organic pollutants studied in this work. The mixtures of catalyst and organic pollutant were stirred in a dark room for 30 min to achieve adsorption and desorption equilibrium. The in-situ Fenton-like degradation reaction was initiated by turning on the xenon lamp. During the degradation process, 2.00 mL solution was sampled at a given time interval. After the catalyst was removed by centrifugation, RHB, TC, and RAN concentrations were detected by UV-visible spectrophotometer, and SMZ concentrations were measured by high-performance liquid chromatography (HPLC).

3.4. Photoelectrochemical (PEC) Performance

The Mott–Schottky, electrochemical impedance spectroscopy (EIS), photoelectric response, and transient open-circuit voltage decay (OCVD) curves were performed on an electrochemical workstation (CHI 760e) coupled with a three-electrode system, which consisted of a platinum (Pt) sheet electrode (reference electrode), an Ag/AgCl electrode (counter electrode), and a piece of modified indium tin oxide (ITO) glass (working electrode). A total of 4 mg AgI/MoS₂/Ti₃C₂O heterojunction was evenly dispersed into 1 mL ethanol by ultrasound. Then, the conductive surface of the ITO glass was modified by the suspension (25 μL) with an area of 1 cm × 1 cm for natural drying and used as the working electrode for the PEC tests. The electrolyte was 0.10 M Na₂SO₄ solution. EIS was performed in a solution containing 2.50 mM K₃[Fe(CN)₆], 2.50 mM K₄[Fe(CN)₆], and 0.10 M KCl.

3.5. Theoretical Calculation

In order to further study the charge-transfer characteristics of the AgI/MoS₂/Ti₃C₂O Z-scheme heterojunction, theoretical calculations were conducted by using the Vienna abinitio simulation package (VASP) based on the density functional theory (DFT). The projector augmented wave method (PAW) was adopted together with the generalized gradient approximation (GGA) for the exchange–correlation energy functional constructed by Perdew–Burke–Ernzerhof (PBE). During the process of structure optimization, the atomic coordinates converged when the energy convergence limit was set to be 10⁻⁵ eV, and the maximum force imposed on each atom was 0.02 eV·A⁻¹. The cutoff energy was 400 eV, and the k-point meshes of Ti₃C₂O and AgI were generated using VASPKIT.

4. Conclusions

In summary, a Ti₃C₂O-crafted AgI/MoS₂ direct Z-scheme heterojunction with interfacial electric fields and chemical bonds was easily synthesized through a hydrothermal and self-assembly method. Ti₃C₂O MXene has a huge specific surface area and rich surface-exposed metal atoms, thereby leading to excellent light absorption and efficient carrier transport properties. The Z-type heterojunction exhibited significantly enhanced photocatalytic activity for H₂O₂ production and pollutant degradation compared with AgI, MoS₂ and MoS₂/Ti₃C₂O. A strong interfacial electric field was proven to be constructed, and it facilitated the vectorial transfer of photogenerated carriers from the CB of AgI to the VB of MoS₂. Additionally, the interfacial Ti-O-Mo bond acted as an atomic-level channel for promoting charge migration, thereby expediting effective charge transfer between interfaces. These advantages synergistically contributed to the superb activity for high H₂O₂ production and efficient pollutant degradation. The synergy between electric fields and chemical bonds offers a generalizable strategy for designing high-performance heterojunction photocatalysts. Potential applications of this approach include industrial wastewater treatment and solar-driven H₂O₂ synthesis, addressing energy and environmental challenges.