Dual MOF and CuInS2-Constructed Dual Z-Scheme Heterojunctions for Enhanced Photocatalytic Hydrogen Production and Methylene Blue Degradation

Abstract

A novel dual Z-scheme heterojunction photocatalyst was constructed by introducing the narrow-bandgap semiconductor CuInS₂ (CIS) into the dual metal-organic framework (MOF) system of UiO-66(Zr) and NH₂-MIL-101(Fe). This structure effectively overcomes the limitations of conventional photocatalysts in terms of light absorption range and the separation efficiency of photogenerated charge carriers. The prepared ternary catalyst, (UiO-66(Zr))-(NH₂-MIL-101(Fe))/CuInS₂, exhibited excellent photocatalytic performance under visible light irradiation, achieving a hydrogen production rate of 888 μmol g⁻¹ h⁻¹ and a methylene blue (MB) degradation efficiency of up to 95.03%. The significant enhancement in performance is attributed to the material’s porous structure, extended light absorption range, and optimized electron transfer pathways. Additionally, the construction of the dual Z-scheme heterojunction further promotes the separation and migration of photogenerated charge carriers, suppressing electron–hole recombination. This study demonstrates the great potential of dual Z-scheme heterojunctions in improving photocatalytic efficiency and provides an important theoretical foundation and design strategy for the development of efficient photocatalysts.

1. Introduction

Currently, due to the increases in energy consumption and human activities, environmental pollution and the energy crisis have become pressing issues [1,2,3,4]. Consequently, utilizing inexhaustible and renewable solar energy presents an effective solution to address energy shortages [5,6,7]. Furthermore, photocatalytic semiconductor technology has garnered widespread attention for its exceptional capabilities, attributed to its economic viability, cleanliness, renewability, and safety [8,9,10]. Therefore, the development of photocatalysts with high efficiency in hydrogen production and organic pollutant degradation has enormous potential.

Metal–organic frameworks (MOFs) are a class of porous materials with periodic network structures constructed through the self-assembly of organic ligands and metal coordination centers [11,12,13]. These materials have attracted extensive attention due to their unique structural characteristics. Their high specific surface areas and well-ordered porous structures provide abundant active sites, facilitating catalytic reactions. Moreover, the excellent electron–hole separation capability of MOFs significantly reduces the recombination of photogenerated charge carriers by shortening the electron transport path, thereby enhancing photocatalytic efficiency. By loading co-catalysts or photosensitizers, MOFs can further optimize photocatalytic performance, significantly improving the hydrogen production rate [14]. Saleh et al. constructed Pt-doped heterojunctions using bimetallic MOFs (e.g., ZnCo-ZIF) as precursors [15]. Through methods such as oxidation, sulfidation, and phosphidation, they synthesized porous structured photocatalysts, which successfully achieved efficient hydrogen production via water splitting. Among them, NH₂-MIL-101(Fe)(NM101) is a representative MOF synthesized by combining Fe(III) and NH₂-BDC ligands, which exhibits excellent absorption properties in both the ultraviolet and visible light regions [16,17]. Owing to its distinctive molecular structure, large specific surface area, excellent thermal stability, and environmentally friendly nature, NH₂-MIL-101(Fe) has been widely applied in the field of photocatalysis, demonstrating great potential. Nevertheless, the practical application of MOFs in photocatalysis is frequently restricted by their poor electrical conductivity, rapid recombination of charge carriers, and insufficient energy conversion efficiency. Previous studies have demonstrated that assembling composite materials by integrating multiple functional materials can overcome these limitations. Specifically, MOF-on-MOF heterojunction catalysts not only retain the distinct properties of each MOF component but also exhibit excellent performance compared to individual MOFs [18,19]. Yuan et al. developed a novel S-shaped MOF-on-MOF heterojunction (MIL-125-NH₂@CoFe PBA) featuring a sandwich-like morphology and Ti–O–Co interfacial chemical bonds [20]. This design significantly enhances the photocatalytic efficiency for pollutant degradation by optimizing charge transfer and redox performance. UIO-66(Zr) (U66), constructed by linking Zr⁴⁺ ions with terephthalic acid ligands, also exhibits UV responsiveness and excellent stability under various conditions [21]. The interfacial interactions between different MOF units with sufficient surface-exposed metals and ligands in MOF-on-MOF heterojunctions provide atomic-level charge transfer pathways, facilitating directional charge transfer and enhancing photocatalytic performance. Hassan et al. constructed an MOF-on-MOF structure using UIO-66 and NH₂-MIL-125, followed by surface modification with g-C₃N₄ nanosheets [22]. This design aims to enhance light absorption and optimize charge separation efficiency, thereby significantly improving the photocatalytic degradation performance for pollutants such as ofloxacin (OFL).

Nevertheless, the light response of synthesized binary photocatalysts remains limited across the full solar spectrum. To enhance photoelectron migration and achieve broad-spectrum responsiveness, the construction of ternary heterojunctions or the introduction of additional photosensitizers has become a common strategy [23,24]. CuInS₂(CIS), with a narrow bandgap (1.50–2.0 eV) and broad light absorption range, can simultaneously fulfill the requirements for hydrogen production through water splitting and pollutant degradation [25,26]. During heterojunction construction, an internal built-in electric field can be formed, effectively separating photogenerated charge carriers. Furthermore, the dense interface within the heterostructure facilitates carrier transfer, extends carrier lifetime, and consequently enhances catalytic activity [27,28].

In this study, (UIO-66(Zr))-(NH₂-MIL-101(Fe))(UNM) was first synthesized via a solvothermal method, followed by the successful loading of CuInS₂ onto the UNM surface through simple heating to produce (UIO-66(Zr))-(NH₂-MIL-101(Fe))/CuInS₂ (UNMC). The photocatalytic activity of the prepared samples was evaluated through the degradation of organic pollutant methylene blue (MB) and hydrogen production from water splitting. The results showed that after 4 h of light irradiation, the hydrogen production rate of the UNMC photocatalyst reached 888 μmol g⁻¹ h⁻¹, and the degradation efficiency for MB reached 95.03% after 60 min of irradiation. UNMC inherits the abundant micro- and mesoporous structures of the individual MOFs while maintaining a large specific surface area, providing more active sites. Moreover, the MOF scaffold effectively prevents the aggregation of CIS, ensuring its successful dispersion on the UNM surface, thereby enhancing the photocatalytic performance of CIS. By skillfully matching the band structures, a dual Z-scheme heterojunction was formed, significantly promoting charge separation and suppressing charge recombination, resulting in excellent photocatalytic performance. Additionally, the suitable conduction band position of the heterojunction further facilitates hydrogen production and dye degradation. This work demonstrates a novel pathway for the synthesis of MOF-based photocatalysts through the construction of ternary heterojunctions.

2. Results and Discussion

2.1. Morphology and Structure

Through the analysis of the X-ray diffraction (XRD) patterns (Figure 1a) of the samples, significant structural features and changes in the crystalline structure of each component and the composites were clearly observed. The single-component CIS exhibited intense and sharp diffraction peaks, primarily distributed in the high-angle region. Peaks located at 27.7°, 46.2°, and 54.8° corresponded to the characteristic diffraction planes (112), (204), and (312) of the chalcopyrite structure of CuInS₂, respectively, indicating high crystallinity and a stable crystal structure [29]. By contrast, NM101 and U66 displayed typical MOF characteristic diffraction peaks in the low-angle region (5°–15°), with strong and distinct peaks reflecting their well-ordered pore structures and good crystallinity [30,31]. For the binary composite UNM, the XRD pattern retained the characteristic diffraction peaks of both NM101 and U66. However, the intensity of the peaks in the low-angle region slightly decreased, and some peak positions shifted slightly, possibly due to interfacial interactions during the composite process, which may have induced adjustments in the crystal structure. This indicated that there was a certain degree of synergistic interaction and microstructural changes between the two MOF materials during the formation of the composite. In the ternary composite UNMC, the XRD pattern showed not only the characteristic high-angle diffraction peaks of CuInS₂ but also retained the low-angle MOF characteristic peaks of NM101 and U66. Additionally, new diffraction peaks or significant changes in peak intensity were observed in the mid-angle range (10°–30°). This suggested that there may have been interactions between the CIS and MOF components, which could lead to the formation of new crystalline phases or ordered structures. These changes enhanced the photocatalytic performance of the composite material.

Figure 1b shows the Fourier-transform infrared spectroscopy (FTIR) results. For NM101, the broad absorption peak around 3397 cm⁻¹ was characteristic of the N-H stretching vibration of amino groups (-NH₂). The absorption peaks in the range of 1661–1434 cm⁻¹ were typically attributed to the stretching vibrations of carbonyl groups (C=O) and the benzene ring framework. In the low wavenumber region (500–700 cm⁻¹), the Fe–O stretching vibration peak was observed, reflecting the key chemical features of its structure [32]. The spectrum of U66 exhibited the asymmetric and symmetric stretching vibration peaks of carboxylate groups (-COO⁻) at 1620 cm⁻¹ and 1427 cm⁻¹, respectively. Additionally, the absorption peaks in the 500–800 cm⁻¹ range corresponded to Zr-O bond vibrations, underscoring its characteristic metal–organic framework structure [33]. As a binary composite material of NM101 and U66, the FTIR spectrum of UNM exhibited the absorption peaks characteristic of both individual materials. Slight peak shifts and intensity variations were observed in specific regions, such as 3300–3500 cm⁻¹ and 1651 cm⁻¹, which could be attributed to interactions between the two materials, such as weak van der Waals forces. By contrast, the spectrum of the ternary composite material UNMC showed more complex changes, reflecting enhanced interactions and structural modifications. The FTIR spectrum retained the key characteristic absorption peaks of the two MOF frameworks, while additional absorption peaks emerged in the 500–800 cm⁻¹ region. These new peaks were likely associated with the vibrational modes of sulfide bonds (M–S, where M represents a metal), which may have resulted from the incorporation of the CIS component. Furthermore, the intensity variations and localized shifts of certain absorption peaks suggested the presence of synergistic interactions among the three photocatalysts. These interactions were likely related to interfacial effects, such as enhanced electron transfer or improved surface adsorption capacity.

Figure 1c and Figure S1 presents the thermogravimetric analysis (TGA) curve, which shows the mass loss of the sample as the temperature increases. The observed mass loss in the temperature range of 30–111 °C is generally attributed to the evaporation of physically adsorbed water and any residual solvents present in the material. This indicates that the sample contains moisture or volatile components that are released when heated within this range [34]. As MOF materials, NM101 and U66 possess large specific surface areas and porosity, which facilitate the adsorption of water or solvents. Within the temperature range of 111 °C to 321 °C, the weight loss likely included the removal of chemically bound water, such as water molecules associated with carboxyl or amino groups in the MOFs. During this stage, the organic ligands in the MOF structures may also have begun to undergo mild decomposition, primarily involving the partial pyrolysis of functional groups like carboxyl and amino groups. Between 321 °C and 518 °C, the weight loss was predominantly caused by the extensive thermal decomposition of the MOFs’ organic ligands. As the temperature increased, the organic ligands (2-aminoterephthalic acid and terephthalic acid) in the MOF structures underwent large-scale cleavage, particularly the breaking of C-C, C-H, and C-N bonds. During this stage, the framework of NM101 disintegrated, exposing the metal centers (Fe nodes), which could further catalyze the oxidation and volatilization of residual carbonaceous materials. Above 518 °C, the weight loss was primarily due to the oxidation of residual organic carbonaceous materials from the MOFs and potential interfacial chemical reactions. At this stage, the organic components of the MOFs were nearly fully decomposed, and metal oxides (Fe₂O₃ and ZrO₂) began to form. The minimal weight loss observed beyond 518 °C corresponded to the complete removal of trace organic residues and the reorganization of inorganic components. In this temperature range, CIS remained thermally stable, which played a crucial role in maintaining the residual composition. Consequently, the weight loss above 518 °C was relatively minor, indicating the final elimination of residual organic materials and the structural stabilization of the inorganic components.

Figure 2 illustrates the morphology and microstructure of the photocatalysts. Figure 2a shows U66, which exhibited a small octahedral structure with particle sizes ranging from 100 to 200 nm. NM101 displayed a well-defined octahedral shape, characterized by a rough surface, with a particle size around 500 nm (Figure 2b). UNM displayed a spindle-like structure with some aggregation (Figure 2c). Figure 2d shows the UNMC image, where CIS is encapsulated on the surface of UNM. Figure 2g presents the energy-dispersive X-ray spectroscopy (EDS) data for UNMC, revealing the presence of eight elements on its surface, including C, O, N, S, Fe, Zr, Cu, and In. Figure 2e depicts the TEM image of UNMC, showing a spindle-like shape with CIS encapsulated on its surface. Figure 2f shows a high-resolution TEM image, which further reveals a lattice spacing of 0.323 nm for the (112) crystal plane of CIS [35], confirming the successful formation of a composite material involving the three components.

Figure 3 shows the surface element compositions and chemical states of UIO-66 (Zr), NH2-MIL-101 (Fe), CuInS₂, and UNMC, as determined by X-ray photoelectron spectroscopy (XPS). The full survey spectrum of UNMC revealed the presence of Cu, In, S, Fe, Zr, N, O, and C, confirming the successful integration of U66, NM101, and CIS in the composite. The Fe 2p spectrum exhibited broad peaks at 710.55 eV and 723.93 eV, corresponding to Fe 2p_3/2 and Fe 2p_1/2, respectively [36]. The Fe 2p core-level spectrum was analyzed, revealing the presence of both Fe³⁺ and Fe²⁺ species, which confirmed the coexistence of multiple oxidation states of Fe ions. The presence of satellite peaks further confirmed the redox activity of Fe, consistent with the structure of NM101. The distribution of Fe oxidation states and ligand interactions likely modulated the interfacial electronic density, enhancing the catalytic or photoelectronic properties of the composite. For Zr, the Zr 3d_5/2 and Zr 3d_3/2 peaks were observed at 182.59 eV and 184.96 eV, respectively, characteristic of Zr⁴⁺ [37]. The positive shift in the Zr 3d binding energy in UNMC indicated a reduction in the electron density around Zr, which was likely attributed to the transfer of electrons from U66 to NM101 or CIS. This electron redistribution could be driven by interfacial interactions or changes in coordination environments, such as the formation of weak chemical bonds between Zr and other components.

The In 3d spectrum exhibited peaks at 444.65eV (In 3d_5/2) and 452.2eV (In 3d_3/2), corresponding to the typical In³⁺ signals, confirming that CIS retained its original chemical state within UNMC [38]. The Cu 2p spectrum showed peaks at 931.94 eV (Cu 2p_3/2) and 951.76 eV (Cu 2p_1/2), characteristic of Cu⁺. Importantly, no satellite peaks were detected in the 938–944 eV range, which confirmed the absence of Cu²⁺. This result indicated that Cu predominantly existed in the Cu⁺ oxidation state, consistent with the chemical state of CIS. These findings demonstrated that CIS was stably incorporated into the UNMC composite, serving as the primary light-absorbing material [39]. The S 2p spectrum displayed peaks at 161.6 eV and 162.85 eV, attributed to S 2p_3/2 and S 2p_1/2, respectively, indicative of sulfur in a metal sulfide environment. The splitting of the S signal further supported the presence of CIS and the stability of the sulfide chemical state. The N 1s peak at 399.8 eV was assigned to the NH₂ group (C–NH), consistent with the amino ligands in NM101. The C 1s spectrum exhibited a main peak at 284.8 eV, corresponding to C–C bonds, with additional peaks at 286.49 eV and 288.58 eV assigned to C–N and C=O bonds, respectively. In the high-resolution O 1s spectrum, the low-binding-energy peak at 531.8 eV was attributed to metal–oxide bonds (e.g., Zr–O in UIO-66 (Zr)), indicating the stability of oxygen coordination around the metal centers. The high-binding-energy peak at 533.62 eV corresponded to carbonyl oxygen (C=O), such as the carboxylate ligands in NM101 or oxygen in other organic functional groups. The distribution of these peaks reflected the synergistic interaction between the inorganic framework and organic ligands, as well as the complex interfacial chemical environment within the composite material.

2.2. Photocatalytic Performance

To evaluate the photocatalytic activity of the catalysts, their hydrogen evolution rates were measured. Using 2 wt% Pt as a co-catalyst and lactic acid as a sacrificial agent, the photocatalytic hydrogen evolution performance of various samples was tested under simulated visible light (300 W Xe lamp, λ > 420 nm). As shown in Figure 4a, the H₂ evolution amounts for all photocatalysts increased linearly with time. Figure 4b compares the hydrogen evolution rates of different samples, with the composite photocatalyst UNMC exhibiting the highest performance, achieving an optimal hydrogen evolution rate of approximately 888 μmol g⁻¹ h⁻¹. The hydrogen evolution rates of U66, NM101, and CIS were 241.5, 535, and 418.5 μmol g⁻¹ h⁻¹, respectively. The hydrogen evolution rate of UNMC was 3.67 times that of pure U66, 1.66 times that of NM101, and 2.12 times that of CIS. This demonstrated that the formation of heterojunctions in UNMC facilitated the separation and transfer of photogenerated electrons, significantly enhancing the photocatalytic hydrogen evolution rate [40]. As shown in Figure 4c, the cyclic stability test for photocatalytic hydrogen production indicated no significant decrease in the hydrogen evolution rate after four cycles, confirming that the prepared composite material possessed high stability.

Figure 5a shows the photocatalytic degradation of MB dye evaluated using U66, NM101, CIS, UNM, and UNMC composite materials. Figure S2 shows the degradation of MB dye in composite samples with different proportions. Under dark conditions, the MB solution and catalysts were stirred for 30 min to achieve adsorption–desorption equilibrium. The results show a gradual decrease in MB concentration over time. The UNMC composite demonstrated the highest degradation efficiency, removing 95.03% of MB within 60 min. It can be seen from the dynamic curve fitting in Figure 5b that UNMC had the best performance. This performance surpassed that of the binary composite UNM composed of U66 and NM101, as well as that of the individual catalysts. The degradation efficiency of UNMC was 3 times that of U66, 1.5 times that of NM101, and 2.3 times that of CIS. The inferior performance of U66 and CIS could be attributed to their higher carrier recombination rates and limited charge transfer capabilities. The significant enhancement in the photocatalytic performance of UNMC could be ascribed to the formation of its dual Z-scheme heterojunction structure. This efficient structure not only reduced the carrier recombination rate and improved charge separation and transfer efficiency but also provided sufficient electrons and holes for the degradation reaction, thereby enhancing the degradation effect on MB dye. As shown in Figure 5c, the apparent degradation rate constants for U66, NM101, CIS, UNM, and UNMC were 0.00486, 0.01193, 0.00657, 0.0375, and 0.1453 min⁻¹, respectively. In addition, the stability of the catalyst is crucial for its commercial application. Figure S3 shows the mass spectrum of MB before and after degradation. To evaluate its reusability and photostability, the UNMC composite was subjected to five consecutive degradation cycles (Figure 5d). Notably, UNMC retained its stability even after multiple uses, confirming its excellent photodegradation efficiency and potential for practical applications. Meanwhile, Figure S4 shows the degradation effects of different dyes by UNMC.

To investigate the roles of reactive species in photocatalysis, a series of trapping experiments was conducted using isopropanol (IPA, 1 mmol), p-benzoquinone (BQ, 1 mmol), and disodium ethylenediaminetetraacetate (EDTA-2Na, 1 mmol) as scavengers for hydroxyl radicals (·OH), superoxide radicals (·O₂⁻), and holes (h⁺), respectively, with the results presented in Figure 5e [41]. Each of these scavengers markedly reduced the photocatalytic performance. Upon the addition of BQ and EDTA-2Na, the degradation efficiency of MB by UNMC decreased from 95.03% to 23.54% and 45.5%, respectively. On the other hand, introducing IPA resulted in a comparatively minor impact, decreasing the degradation efficiency from 95.03% to 68.42%. In Figure 5f, electron spin resonance (ESR) measurements were used to detect the relative presence of ·O₂⁻ and ·OH in the composite photocatalyst UNMC [42]. When DMPO was reacted with ·O₂⁻ to form the stable DMPO-·O₂⁻ adduct, six distinct peaks were observed under light irradiation, confirming the generation of ·O₂⁻ in the system. Similarly, the DMPO-·OH adduct, formed by the reaction of DMPO with ·OH, exhibited four characteristic peaks with an intensity ratio of 1:2:2:1 under light irradiation, indicating the generation of ·OH.

2.3. Photoelectric Property

For the given photocatalysts, achieving efficient charge carrier separation is essential for superior photocatalytic performance. Photoluminescence (PL) spectra were recorded to evaluate the charge separation ability [43]. As shown in Figure 6a, under 365 nm excitation, all photocatalysts exhibited emission peaks around 550–570 nm. U66 and NM101 showed higher fluorescence emission intensities compared to UNMC. Among the materials, CIS demonstrated the lowest emission intensity, indicating more effective transfer of photogenerated electrons. Upon the addition of CIS, the PL emission intensity of the UNMC composite decreased significantly, indicating that the recombination of electron–hole pairs was effectively suppressed. Furthermore, compared to pure MOF photocatalysts, the lower PL intensity of the UNMC composite confirmed its ability to transfer more photogenerated charge carriers, making them available for pollutant degradation. This suggested that the incorporation of CIS played a crucial role in enhancing the photocatalytic performance of the composite. As shown in Figure 6b, the transient photocurrent response was studied using I–t curves to investigate the charge carrier transport capability in the UNMC photocatalyst. Under UV–visible light irradiation, the photocurrent density of UNMC was significantly higher than that of U66, NM101, and CIS. These results indicated that UNMC exhibited the highest efficiency in the transport and separation of photogenerated charge carriers. This enhanced performance further supported the superior photocatalytic activity of the UNMC composite material. In addition, electrochemical impedance spectroscopy (EIS) was used to further validate the photoelectrochemical performance. As shown in Figure 6c, the UNMC composite exhibited the smallest Nyquist radius, indicating the lowest interfacial resistance among all materials [44]. This characteristic facilitated the migration of photogenerated charge carriers. Based on the above analysis, it can be concluded that the UNMC composite material possessed stronger light absorption capability, better photogenerated charge carrier separation efficiency, and faster migration rates. Therefore, the UNMC composite demonstrated superior photocatalytic activity compared to single photocatalysts.

Figure 6d shows the UV–Vis absorption spectrum. The binary composite material U66-NM101 combined the optical properties of both U66 and NM101, achieving efficient absorption in the UV region (200–400 nm) for U66 and in the visible light region (400–600 nm) for NM101. This successfully enabled wide-spectrum absorption covering both the UV and visible light ranges. The composite material not only enhanced light absorption but also improved the separation efficiency of photogenerated charge carriers through the synergistic effects between the two MOFs [45]. Building on this, the narrow-bandgap semiconductor CIS was further incorporated to form the ternary composite material UNMC, effectively extending the light absorption range into the near-infrared region (600–800 nm). With its excellent light absorption properties and narrow bandgap, CIS enabled the ternary composite to exhibit strong absorption in the near-infrared region. Moreover, CIS provided additional excitation energy levels, offering more efficient channels for the separation and transfer of photogenerated charge carriers. UNMC exhibited high-intensity light absorption in the ultraviolet, visible, and near-infrared regions. This broad-spectrum absorption capability demonstrated excellent photocatalytic activity and photoelectric conversion efficiency. Additionally, as shown in Figure 6e, the bandgap of the sample was calculated using the following formula:

(1)$\begin{array}{c}\mathrm{a}\left(\mathrm{hv}\right)=\mathrm{A}{\left(\mathrm{hv}-{\mathrm{E}}_{\mathrm{g}}\right)}^{\left(\mathrm{n}/2\right)}\end{array}$

According to the formula, the bandgaps of U66, NM101, and CIS were calculated to be 3.8 eV, 2.48 eV, and 1.68 eV, respectively. The synergistic effects of U66, NM101, and CIS during the photocatalytic process were studied using Mott–Schottky plots, as shown in Figure 6f. The Mott–Schottky tests were conducted in a 0.1 mol·L⁻¹ Na₂SO₄ electrolyte. From the plots, the conduction band potentials (ECB) of U66, NM101, and CIS were determined to be −0.73 V, −0.77 V, and −0.91 V (vs. Ag/AgCl), respectively. The potential with respect to the normal hydrogen electrode (NHE) was determined using the equation below:

(2)$\begin{array}{c}{\mathrm{E}}_{\mathrm{NHE}}={\mathrm{E}}_{\mathrm{SCE}}+0.241\mathrm{V}\end{array}$

Relative to the NHE, the E_CB values of U66, NM101, and CIS were −0.49 V, −0.52 V, and −0.67 V, respectively. The valence band (VB) potentials can be calculated using the following equation:

(3)$\begin{array}{c}{\mathrm{E}}_{\mathrm{VB}}={\mathrm{E}}_{\mathrm{CB}}+{\mathrm{E}}_{\mathrm{g}}\end{array}$

The valence band (VB) potentials (E_VB) of U66, NM101, and CIS were calculated to be +3.31 V, +1.96 V, and +1.01 V, respectively.

2.4. Photocatalytic Mechanism

The possible photocatalytic routes include the following (Figure 7):

(4)$\begin{array}{c}\left(\mathrm{UIO}-66\left(\mathrm{Zr}\right)\right)-\left({\mathrm{NH}}_2-\mathrm{MIL}-101\left(\mathrm{Fe}\right)\right)/\mathrm{C}\mathrm{u}\mathrm{I}\mathrm{n}{\mathrm{S}}_2+\mathrm{h}\mathrm{v}\to {\mathrm{e}}^{-}+{\mathrm{h}}^{+}\end{array}$

(5)$\begin{array}{c}{\mathrm{e}}^{-}+·{\mathrm{O}}_2\to ·{{\mathrm{O}}_2}^{-}\end{array}$

(6)$\begin{array}{c}{\mathrm{H}}_2\mathrm{O}+{\mathrm{h}}^{+}\to ·\mathrm{O}\mathrm{H}+{\mathrm{H}}^{+}\end{array}$

(7)$\begin{array}{c}2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2\end{array}$

(8)$\begin{array}{c}·\mathrm{OH}/·{{\mathrm{O}}_2}^{-}+\mathrm{M}\mathrm{B}\to \mathrm{D}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{e}\mathrm{d} \mathrm{P}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{s}\end{array}$

The electron transfer mechanism of these three materials is likely governed by Type II and Z-scheme mechanisms. In the binary system, when U66 and NM101 formed a composite, electrons transferred from NM101 to U66 until the Fermi levels reached equilibrium. In the case of the heterojunction between CIS and U66-NM101, following the Type II mechanism, electrons in the conduction band of NM101 and CIS tended to migrate to the conduction band of U66, while holes accumulated in the valence band of CIS. However, the valence band of CIS was significantly lower than the standard oxidation–reduction potential of OH⁻/·OH (1.99 eV), which hindered the generation of hydroxyl radicals (·OH) during the oxidation process and thus impeded the degradation of pollutants [46,47]. If the heterojunction between U66-NM101 and CIS followed the Z-scheme mechanism, under visible light irradiation, U66 continuously injected electrons into the valence band of NM101, while the valence band of U66 reacted with water to oxidize OH⁻/H₂O to ·OH, accompanied by the generation of H⁺. The narrow bandgap of CIS facilitated enhanced light utilization for photocatalysis, and electrons eventually transferred to the VB of CIS. Since the VB potentials of CIS and NM101 were lower than that of OH⁻/·OH (1.99 eV vs. NHE), the conventional Type II heterojunction electron transfer pathway could not enable the catalysts to exhibit photocatalytic activity. This analysis aligned with the results of the ESR measurements. Therefore, the double Z-scheme pathway was more suitable for this heterostructure. Various experiments and characterizations confirmed that the U66-NM101/CIS ternary catalyst exhibited excellent photocatalytic hydrogen production activity and organic degradation performance.

3. Materials and Methods

3.1. Materials

Zirconium chloride (ZrCl₄), ferric chloride hexahydrate (FeCl₃·6H₂O), copper chloride (CuCl₂), indium chloride tetrahydrate (InCl₃·4H₂O), and thiourea were purchased from Shanghai Maclin Biochemical Company, Shanghai, China. Terephthalic acid (H₂-DBC) and 2-amino-terephthalic acid (NH₂-DBC) were purchased from Shanghai Aladdin Biochemical Reagent Company, Shanghai, China. N, N-dimethylformamide (DMF) was obtained from from Quanray Reagents, Liaoning, China. All chemicals were pure and could be used directly without further purification.

3.2. Preparation of Composite Photocatalysts

3.2.1. Synthesis of UiO-66(Zr)(U66)

The synthesis of UiO-66(Zr) followed a procedure adapted from the literature, with slight modifications [31]. Typically, 0.46 g of ZrCl₄ and 0.33 g of H₂-DBC were dissolved in 40 mL of DMF, followed by the addition of 5 mL of acetic acid, and the mixture was stirred for 30 min. The resulting mixture was transferred to a high-pressure reactor lined with PTFE and heated at 120 °C under static conditions for 30 h. After the reaction, the mixture was cooled to room temperature, and the white UiO-66 suspension was collected by centrifugation. The product was thoroughly washed several times with DMF, ethanol, and deionized water, and finally vacuum-dried at 60 °C overnight.

3.2.2. Synthesis of (NH₂-MIL-101) (NM101) and (UiO-66)-(NH₂-MIL-101) (UNM)

The synthesis method of UNM was primarily designed in this study, though certain steps were adapted from the synthesis procedures described in the relevant literature [48,49]. Solution A was prepared by dissolving 0.54 g of FeCl₃·6H₂O in 30 mL of DMF and stirred for 10 min. In another container, 30/60/90 mg of U66 and 30/60/90 mg of PVP were dispersed in 10 mL of DMF and stirred for 10 min, after which 0.182 g of NH₂-DBC was added to form solution B. Solution B was then poured into solution A, and the mixture was stirred for 10 min. The combined solution was transferred to a reaction vessel and heated at 110 °C for 20 h. After the reaction, the product was washed with DMF and ethanol and dried in an oven at 60 °C to obtain the final product, UMN (U_xMN, x = 30/60/90). In a separate experiment, without adding U66 and PVP, the same procedure was followed to obtain NM101.

3.2.3. Synthesis of (UiO-66)-(NH₂-MIL-101)/CuInS₂ (UNMC)

CuCl₂ (0.5 mmol), InCl₃·4H₂O (0.5 mmol), and thiourea (1 mmol) were added to ethylene glycol and stirred by ultrasound to form a uniform solution. The solution was added to the UMN sample, stirred continuously for 30 min, evenly dispersed, and then the mixed solution was heat-treated at 160 °C for 20 h. Subsequently, the resulting product was dried in an oven at 60 °C to obtain the product UMNC (UNMCx, x is the mass number of CIS in UNMC, x = 10, 20, 30, 40).

3.3. Characterization of Materials

Powder X-ray diffraction (XRD) patterns were recorded using a Bruker D8 Advance diffractometer (Billerica, MA, USA) with Cu Kα radiation (λ = 1.5406 Å). Scanning electron microscopy (SEM) images were captured using a Philips XL-30-ESEM-FEG instrument (Amsterdam, The Netherlands), operated at 20 kV. Additionally, transmission electron microscopy (TEM) was performed with a JEOL JEM-2010 microscope (JEOL, Tokyo, Japan) at an accelerating voltage of 200 kV to obtain electron micrographs of the samples. X-ray photoelectron spectroscopy (XPS) was conducted using a PHI-5700 ESCA instrument (Chanhassen, MN, USA) with an Al-Kα X-ray source. UV–Vis diffuse reflection spectra (DRS) were obtained using a UV–Vis spectrophotometer (UV-2550, Shimadzu, Jiangsu, China), equipped with an integrating sphere attachment, and BaSO₄ was used as the reference material. Fourier-transform infrared (FT-IR) spectra were recorded using a PerkinElmer Spectrum One system (Waltham, MA, USA). Electron spin resonance (ESR) was performed using an EPR/ESR spectrometer (MEX-nano, Bruker, Germany).

3.4. Photocatalytic Experiment

3.4.1. Photocatalytic Hydrogen Evolution

The experiment of photocatalytic hydrogen evolution was carried out in the Beijing Perfectlight (Labsolar-6A) at room temperature (Figure 8a). A typical process was to suspend 100 mg of photocatalyst in a 100 mL closed-phase gas cycle reaction tank with 80 mL of deionized water and 20 mL of triethanolamine as sacrifice reagents, and 2 wt% Pt was added. After that, the suspension was purified several times with nitrogen to remove O₂ and CO₂. After that, the suspension was irradiated with a 300 W xenon (Beijing Perfectlight, Beijing, China) lamp, and the hydrogen was analyzed periodically every 1 h.

3.4.2. Photocatalytic Degradation

To assess the photocatalytic performance under visible light (using a 300 W xenon lamp), the degradation of the organic dye methylene blue was carried out at room temperature (Figure 8b). In a typical experiment, 30 mg of photocatalyst was introduced into separate solutions of methylene blue (40 mL, 20 mg L⁻¹). The suspensions were then allowed to equilibrate in the dark for 30 min to ensure adsorption equilibrium. Subsequently, the suspensions were exposed to light from a 300 W xenon lamp equipped with a filter (λ ≥ 420 nm). The residual solution concentrations were analyzed using a T6 UV–Vis spectrophotometer. The species trapping experiments were conducted by adding additional trapping agents (1 mM) such as EDTA-Na, BQ, and IBA. The photocatalytic efficiency was calculated from the following equation:

(9)$\begin{array}{c}\mathrm{\eta }={\displaystyle \frac{{\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}}\times 100\%\end{array}$

4. Conclusions

In this study, a novel double Z-scheme heterojunction photocatalyst (UNMC) was designed and synthesized in multiple steps to improve the photocatalytic performance. In the first step, dual MOF composites (UNM) of UIO-66(Zr) and NH₂-MIL-101(Fe) were prepared by the solvothermal method, which combined the UV light responsiveness provided by UIO-66(Zr) with the strong absorption properties of NH₂-MIL-101(Fe) in the visible range. A double MOF composite structure with good stability and optical properties was formed. On this basis, the narrow bandgap semiconductor CuInS₂ was loaded onto the surface of UNM by a simple heating method, and the terpolymer with high photocatalytic activity (UNMC) was successfully prepared. The introduction of CuInS₂ not only significantly expands the light absorption range of the catalyst to the near infrared region but also constructs a unique double Z-scheme heterojunction through interaction with the MOF materials, which effectively optimizes the photogenerated carrier separation and migration path and inhibits the electron–hole recombination phenomenon.

The whole synthesis process has the characteristics of high efficiency, mild conditions, and controllable process, which effectively avoids the problems of structural collapse and property degradation that may occur during the preparation of traditional multicomponent composite materials. The resulting UNMC material integrates the flexible chemical properties of the double MOFs with the strong light response of CuInS₂ and shows excellent catalytic activity in both water decomposition to produce hydrogen and organic dye degradation experiments. The hydrogen yield reached 888 μmol g⁻¹ h⁻¹, and the degradation efficiency of methylene blue reached 95.03% within 60 min. In addition, the material maintained good stability in multiple cycle experiments, indicating its potential for practical applications. This synthesis method not only provides a new idea for the design of photocatalysts but also lays a theoretical and technical foundation for expanding the application of metal–organic framework materials in clean energy development and environmental governance.

Author Contributions

Conceptualization, Y.L. (Yuning Liang), B.W. and T.C.; methodology, Y.L. (Yuning Liang), T.C. and M.B.; validation, T.C., W.C. and Y.L. (Yuxi Liu); formal analysis, Y.L. (Yuxuan Liu) and W.Z.; writing—original draft preparation, Y.L. (Yuning Liang); writing—review and editing, B.W. and T.C.; visualization, Y.L. (Yuning Liang) and T.C.; supervision, B.W.; funding acquisition, B.W. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

All relevant data are provided in the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures

Figure 1. (a) XRD patterns of U66, NM101, CIS, UNM, and UNMC. (b) FTIR spectra of U66, NM101, UNM, and UNMC. (c) TGA curve of UNMC.

Figure 2. SEM images of (a) U66; (b) NM101; (c) UNM; and (d) UNMC. (e) TEM and (f) HRTEM images of UNMC. (g) EDS mapping of UNMC.

Figure 3. High-resolution XPS spectra of (a) survey, (b) Fe 2p, (c) Zr 3d, (d) In 3d, (e) Cu 2p, (f) S 2p, (g) N 1s, (h) C 1s, and (i) O 1s over as-prepared U66, NM101, CIS, and UNMC.

Figure 4. (a,b) Comparison of photocatalytic H2 evolution activity for BOB-OVs and xTB and production rate. (c) Four cycles of 4 h H2 evolution with intervals between cycles.

Figure 5. (a) Photocatalytic degradation of MB; (b) Degradation kinetic curve fitting of MB. (c) Apparent rate constant. (d) Running the cycle 5 times to degrade MB. (e) Effect of different scavengers on performance. (f) DMPO-·O2− and DMPO-·OH ESR signals in the presence of UNMC.

Figure 6. (a) Photoluminescence spectra; (b) Electrochemical impedance spectra; (c) Transient photocurrent responses; (d) UV–Vis absorption spectra of U66, NM101, CIS, UNM, and UNMC. (e) Tauc plots; (f) Mott–Schottky plots of U66, NM101, and CIS.

Figure 7. (a,b) Diagram of possible photocatalytic mechanisms of UNMC.

Figure 8. (a) Photocatalytic hydrogen production reaction. (b) Photocatalytic degradation of MB.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15010069/s1, Figure S1.TGA curves for U66 and NM101; Figure S2. The effect of different proportions of photocatalyst on the catalytic effect; Figure S3. Mass spectrum of MB before and after degradation; Figure S4. Degradation effects of samples on different pollutants; Figure S5. TRPL curve of the sample; Table S1. Comparison of degradation performance with other photocatalysts; Table S2. Comparison of hydrogen production performance with other photocatalysts. Refs. [50,51,52,53,54,55,56,57] were cited in Supplementary Materials.

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