As a consequence of economic development, population growth, urbanization, and infrastructural development, the ecological environment is undergoing great stress due to the discharge of wastewater containing heavy metals and organic pollutants [1]. Among various heavy metal ions, hexavalent chromium Cr(Ⅵ), as a mutagenic and carcinogenic pollutant, was widely spreading surface water and groundwater [2-4]. Up to now, some methods like ion exchange [5, 6], adsorption [7], membrane separation [8], electrocoagulation [9], and photocatalytic reduction [10] have been developed to remove Cr(Ⅵ) from wastewater. In all the above methods, photocatalytic reduction transformation from Cr(Ⅵ) to Cr(Ⅲ) is an effective method to remove Cr(Ⅵ) from wastewater, considering that Cr(Ⅲ) is less toxic and was removed as the form of Cr(OH)₃ in neutral or alkaline conditions [11]. Compared with traditional reduction methods like chemical reduction and electrochemical reduction [12], photocatalytic Cr(Ⅵ) reduction into Cr(Ⅲ) exhibited the advantages like energy saving, high efficiency, and free toxic by-product. Organic pollutants like organic dyes are not only toxic but also carcinogenic. Their chemical stability and hard degradation in nature will pose serious threats to human health and ecosystem sustainability. Numerous studies have shown that organic dye decomposition via a photocatalytic process is an efficient and practical solution [13-16]. In recent years, semiconductor photocatalysts have attracted wide attentions in solving global energy problems and environmental pollution [17-19]. Among various semiconductor photocatalysts, titanium dioxide (TiO₂) is widely used for pollutant removal with the aid of its advantages like toxicity free, high stability, and low cost [20-25]. However, the main disadvantage of TiO₂ is its only UV light response due to its large band gap, which greatly limits the use of sunlight (only 2%–3% of sunlight) [26]. At present, Ag₂CO₃ with a narrow band gap has been found to demonstrate remarkably photocatalytic activity in the visible light region [16, 26-30]. Previous research focused on fabrication of Ag₂CO₃ onto different substrates as composite photocatalysts for water treatment, such as Ag₂CO₃/TiO₂ [26], Ag₂O/Ag₂CO₃ [27], and Ag₂CO₃/UiO-66(Zr) [16] along with GO-Ag₂CO₃ [31], which displayed the superior photocatalytic activity to Ag₂CO₃ but still suffered from serious photo-corrosion. Therefore, the improvements in both stability and photocatalytic activity of Ag₂CO₃ are the two main objectives to be achieved in future.

In the last two decades, metal-organic frameworks (MOFs) have been widely utilized as efficient photocatalysts to conduct CO₂ reduction [31-34], H₂ evolution [34-36], Cr(Ⅵ) reduction [2, 13, 17, 37-41], and organic pollutant decomposition [10, 15, 16, 42-44] because of the ultrahigh adsorption capacity and efficient light utilization ability [13, 38, 42, 45]. However, the pristine MOFs as photocatalysts exhibit a large band gap and can only be degraded by ultraviolet light, which will greatly reduce the utilization of solar energy. Up to now, numerous studies have focused on immobilizing other photocatalytically active semiconductors on MOFs substrates to extend the photo-responsive region to the visible region. For example, Ag₂CO₃ was attached to UiO-66 to construct a binary composite for visible-light-driven degradation of organic dyes [16]. Metal free g-C₃N₄ was encapsulated on BUC-21 for photocatalytic reduction of Cr(Ⅵ) under simulated sunlight irradiation [39]. As well, MIL-53(Al) was decorated with g-C₃N₄ for enhanced photocatalytic rhodamine B (RhB) decomposition under visible-light irradiation [46]. These studies demonstrated that MOFs/semiconductors composites exhibited increasing light utilization efficiency than the individual MOFs. Hence, MOFs can act as a perfect substrate to load a variety of semiconductor materials to form heterojunction for potential applications in water treatment. Among all MOF materials, UiO-66 as a zirconium-based three-dimensional MOF not only has the advantages of other MOFs, but also has excellent thermal stability and chemical stability [47, 48]. In 2015, Wu et al. [16] reported that UiO-66 (Zr) and Ag₂CO₃ are combined to enhance the visible light catalytic degradation of RhB. Zhang et al. [49] constructed the Ag₃PO₄/UiO-66 heterojunction for efficient visible light degradation of RhB. Our research group found that UiO-66-NH₂/Ag₃PO₄ and UiO-66-NH₂/Ag₂CO₃ composites exhibited effective capture and visible-triggered release toward sulfonamides due to the formation and split between Ag⁺ and –NH₂ group [50, 51]. Up to now, there has been no report on the efficiently photocatalytic Cr(Ⅵ) reduction over UiO-66-NH₂/Ag₂CO₃ composites under visible light illumination. With this paper, the UiO-66-NH₂/Ag₂CO₃ Z-scheme heterojunction (UAC-X Z-scheme heterojunction) is used to achieve photocatalytic Cr(Ⅵ) reduction under visible light irradiation. In addition, the stability of the UiO-66-NH₂/Ag₂CO₃ heterojunction was investigated, and the photocatalytic mechanism was proposed and tested.

All materials and reagents were commercially available and used directly without further purification. The characterization methods are listed in the Supporting Information.

The UiO-66-NH₂ was hydrothermally synthesized as described in a previous report with slight modification [35]. Briefly, 2-aminoterephthalic acid (NH₂-BDC)(4.5 mmol, 0.81 g) was completely dissolved in N, N′-dimethylformamide (DMF, 40 mL) by ultrasound for ca. 10 min. Then, ZrCl₄ (4.5 mmol, 1.05 g) and acetic acid (HAc, 17 mL) were added to this solution. All the above processes were carried out in a 100 mL Teflon-lined stainless-steel autoclave. HAc was added to tune the morphology of UiO-66-NH₂. The autoclave was placed in a drying oven and heated at 120 ℃ for 24 h. After cooling to room temperature, the product was centrifuged and cleaned several times with ultra-pure water. The prepared UiO-66-NH₂ was dried at 60 ℃ in a drying oven before use.

UAC-X composites (UAC-20, 50, 100, 150, 200) were fabricated by a simple ion-exchange-solution method with a small modification [16, 50, 51], as illustrated in Scheme 1. Taking UAC-100 (the weight ratio of UiO-66-NH₂ and Ag₂CO₃ being 1:1) for an example, the Na₂CO₃ aqueous solution (10 mL, 0.030 mol/L) was added dropwise into the suspension (100 mL) of UiO-66-NH₂ (1000 mg/L) and AgNO₃ (0.006 mol/L). The mixture was stirred vigorously with a magnetic stirrer for 3.5 h. The precipitates were separated from the solution via filtration and washed with ultra-pure water and EtOH several times. The other UiO-66-NH₂/Ag₂CO₃ (UAC-X) composite samples were fabricated similarly to UAC-100. The weight proportions of UiO-66-NH₂ and AgNO₃ in UAC-20, UAC-50, UAC-150 and UAC-200 were 0.2:1, 0.5:1, 1.5:1, and 2:1, respectively. For the sake of comparison, Ag₂CO₃ particles were also synthesized following the same method as UAC-100 except without adding UiO-66-NH₂.

The photocatalytic performance of UAC-X composites toward Cr(Ⅵ) reduction and organic pollutant degradation was investigated under visible light irradiation at ambient temperature. The photocatalytic Cr(Ⅵ) reduction experiment was carried out in a 50 mL quartz glass vessel containing 10 mg UAC-X composites (or individual UiO-66-NH₂ and Ag₂CO₃) and 40 mL Cr(Ⅵ) solution (10 mg/L). It is worth noting that the pH of Cr(Ⅵ) solution was adjusted to 2.0 with H₂SO₄ solution. The suspension was first magnetically stirred in the dark for 45 min to achieve adsorption-desorption equilibrium. Afterwards, the Cr(Ⅵ) solution was irradiated with 5.00 W LED (PCX50A, Beijing perfect light technology Co., LTD) for 60 min, and the spectrum of light source (longer than 420 nm) is shown in Fig. S1. At specific time interval, 1.5 mL liquor was drawn and filtered through 0.45 μm syringe filter for analysis. The residual Cr(Ⅵ) was determined by a diphenylcarbazide (DPC) method via an Auto Analyzer 3 (Seal, Germany). The operation flow and video of Auto Analyzer 3 are shown in Fig. S2. Blank experiments were also carried out under the identical conditions with no photocatalyst addition.

The FTIR spectra of Ag₂CO₃, UiO-66-NH₂, and UAC-X composites (Fig. 1a) revealed that the adsorption peaks at 1581.92 and 1382.09 cm^–1 were assigned to the carboxylic functional groups in the BDC-NH₂ ligands, and the peaks between 600–800 cm^–1 were ascribed to Zr–O₂ as vertical and horizontal mode scaling [52, 53]{Abid, 2013 #22;Yang, 2015 #3}. The peaks at 1374.62 and 803.06 cm^–1 were attributed to the characteristic peaks for CO₃^2– [46]. The characteristic adsorption peaks of UiO-66-NH₂ at 1581.92, 1382.09, and 600–800 cm^–1 decreased with the decrease of UiO-66-NH₂ contents. On the contrary, with the decrease of UiO-66-NH₂ contents, the characteristic peaks of Ag₂CO₃ at 1374.62 and 803.06 cm^–1 increased obviously [51].

The powder XRD patterns (Fig. 1b) of the as-synthesized UiO-66-NH₂ are in consistent with the reported ones [46], indicating that UiO-66-NH₂ was successfully prepared. All the diffraction peaks of pristine Ag₂CO₃ matched well with the standard pattern (JCPDS card No. 97-000-8011) (Fig. S3) of Ag₂CO₃ with monoclinic structure [50]. It is worth noting that the characteristic peaks of both UiO-66-NH₂ and Ag₂CO₃ could be detected in UAC-100. The peaks of Ag₂CO₃ vanished gradually as the small Ag₂CO₃ content decreased in UAC-150 and UAC-200. Similarly, the characteristic peaks of UiO-66-NH₂ could be detectable in both UAC-20 and UAC-50 [51].

The UV-vis diffuse reflectance spectra (UV-vis DRS) of Ag₂CO₃, UiO-66-NH₂, and UAC-X composites are displayed in Fig. 1c. The peak of pristine UiO-66-NH₂ is a wide peak between 200 and 430 nm, which is consistent with the peak previously reported [39, 40]. The light absorption region of UAC-X is almost the same as that of Ag₂CO₃, and their E_g values are in the range of 2.25–2.95 (inset in Fig. 1c), indicating that both UiO-66-NH₂ and UAC-X composites can be excited by visible light [40].

The successful fabrication of UAC-X composites was further confirmed by XPS determination (Fig. 1d). The XPS spectrum revealed that the Ag 3d in UAC-X displayed a slight shift to higher binding energy compared to the pristine Ag₂CO₃, which may be due to the synergy between UiO-66-NH₂ substrate and Ag₂CO₃[50, 51].

The SEM and TEM images could be used to specifically understand the morphology of Ag₂CO₃, UiO-66-NH₂, and UAC-X composites. It was observed by SEM (Fig. 2a) that the individual UiO-66-NH₂ particles were standard regular octahedrons of different sizes ranging from 200 nm to 3500 nm, in which the UiO-66-NH₂ with small particle size exhibited regular octahedrons shape (as shown in Fig. S4). Ag₂CO₃ presents irregular cubic structure [27] with particle size distribution between 100 and 400 nm. Both SEM (Fig. 2) and TEM (Fig. S5) images illustrated that Ag₂CO₃ nanoparticles are adhered to the surface of UiO-66-NH₂. The HRTEM image of UAC-100 (Fig. S6) reveals that the lattice spacing of 0.261 nm corresponds to the (130) facet of Ag₂CO₃, further indicating the intimate contact between UiO-66-NH₂ and Ag₂CO₃ [51]. The content of octahedral UiO-66-NH₂ in UAC-X composites affected the adhesion state of Ag₂CO₃ on UiO-66-NH₂. The element distribution results of UAC-100 are depicted in Fig. S7, in which the Ag, Zr, C, N, and O elements are evenly distributed over the composite, further indicating that Ag₂CO₃ is successfully loaded onto the UiO-66-NH₂ material. In the XPS spectrum of UAC-100 (Fig. 3), the combination between Ag₂CO₃ and UiO-66-NH₂ was affirmed by the shift of binding energies of Zr, Ag, C, N, and O, which was deeply discussed in our previous reports [50, 51].

In order to explore the photocatalytic performance of Ag₂CO₃, UiO-66-NH₂ and UAC-X composites, the Cr(Ⅵ) reduction efficiency at pH = 2 was investigated. As shown in Fig. 4a, Cr(Ⅵ) can not be reduced to Cr(Ⅲ) without adding photocatalyst both in dark and under the irradiation of visible light. In the experiment, Cr(Ⅵ) was first adsorbed in the dark for 45 min, and the adsorptive removal efficiencies of Ag₂CO₃, UiO-66-NH₂, and UAC-X composites toward Cr(Ⅵ) ranged from 3% to 14%. As shown in Fig. 4a and Table 1, under the identical conditions, the photocatalytic performance of the UAC-X composites for Cr(Ⅵ) reduction is better than those of pristine UiO-66-NH₂ or Ag₂CO₃. UAC-100, UAC-150, and UAC-200 can achieve 100% Cr(Ⅵ) reduction within 50 min, which are higher than the reduction efficiencies of UAC-20 (68%), UAC-50 (86%), UiO-66-NH₂ (19%), and Ag₂CO₃ (8%). Specifically, the pseudo–first–order model of UAC-X for Cr(Ⅵ) reduction further proves that UAC-100 achieved faster efficient reduction rate (k value), as shown in Fig. 4b. Comparing with the UAC-20 and UAC-50, UAC-100 exhibited more outstanding photocatalytic performance toward Cr(Ⅵ) reduction under the identical conditions, which may be related to the larger specific surface area of UAC-100 than both UAC-20 and UAC-50 (Table S1). Both the positive zeta potential (Fig. S8c) and larger surface are facilitate the adsorption toward Cr₂O₇²⁻ for further photocatalysis [29, 37]. As well, the appropriate introduction of Ag₂CO₃ into UiO-66-NH₂ will improve the charge transfer over the composite interfaces under visible light irradiation [54]. However, excessive UiO-66-NH₂ in the UAC-X may suppress the valid heterogeneous interfaces, which is not conducive to the transfer of charge carriers [55]. In view of economic and cost savings, UAC-100 can achieve the desired effect under the same conditions using less UiO-66-NH₂ compared to UAC-150 and UAC-200. Therefore, the UAC-100 was selected to conduct follow-up experiments for deep understand toward its performance.

The initial pH value of Cr(Ⅵ) has a great influence on its reduction effect [56]. In this study, the effects of different pH values (pH = 2, 3, 4, 6, 8) on the Cr(Ⅵ) reduction were investigated using UAC-100 as photocatalyst. As illustrated in Fig. 5a, the lower pH resulted in a better photocatalytic efficiency and rate. Especially, the highest photocatalytic Cr(Ⅵ) reduction efficiency (99.0% within 40 min and 100.0% within 50 min) was accomplished at pH = 2.0. It is well known that hexavalent chromium exists in the form of Cr₂O₇²⁻ at low pH [37]. At this lower pH, the higher degree of protonation on the catalyst surface facilitates the adsorptive interactions with Cr₂O₇^2– ions, and the abundant H⁺ also promotes the transformation from Cr(Ⅵ) to Cr(Ⅲ), as listed in Eq. (1) [56]. Under alkaline conditions, the composite photocatalysts displayed negative zeta potentials (as illustrated in Fig. S8), which would decline the adsorptive interactions toward anionic CrO₄^2–. As well, the Cr(Ⅵ) reduction reaction under alkaline surroundings follows Eq. (2) [17]. However, the formed Cr(OH)₃ can cover the active sites over the catalyst surface, resulting into declined photocatalytic efficiency.

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In order to explore the influences of foreign ions on the photocatalytic Cr(Ⅵ) reduction activity, UAC-100 catalyst was used to treat a Cr(Ⅵ) solution (pH = 2) prepared with real lake water collected from the Ming Lake and tap water of Daxing campus, BUCEA (the quality parameters of lake water and tap water are shown in Table S2). As shown in Fig. 5b, it was found that the Cr(Ⅵ) reduction efficiency declined from 100% to 46% in 60 min, indicating that the Cr(Ⅵ) reduction over UAC-100 was inhibited by the inorganic ions in tap water [44]. It is worth noting that the Cr(Ⅵ) reduction efficiency in lake water was higher (76%, 60 min) than that in tap water because the organic matters in the lake water could consume the holes to facilitate the separation of photo-induced electrons and holes [57]. It can be concluded that the organic matters in the reaction system can enhance the photocatalytic Cr(Ⅵ) reduction performance.

Photo-induced electron-hole pairs can be produced upon visible light irradiation. The consumption of holes (h⁺) will accelerate the charge separation of photo-induced electron-hole pairs, resulting in excellent Cr(Ⅵ) reduction efficiency. In order to test the effect of small organic acids as hole scavengers on the Cr(Ⅵ) reduction efficiency, some organic compounds such as citric acid, tartaric acid, and oxalic acid were selected as hole scavengers at pH = 2.0. As shown in Fig. 5c and 5d, the participation of these small organic acids can improve the photocatalytic Cr(Ⅵ) reduction activity, because the organic matters consume the holes generated over the UAC-100 photocatalyst upon light irradiation. It is worth noting that the presence of small organic compounds can improve the photocatalytic reaction rate of Cr(Ⅵ) reduction, which may be related to the number of α-hydroxyl carboxylate functional groups (zero, one, and two α-hydroxyl groups in oxalic acid, citric acid, and tartaric acid, respectively) [12, 39].

Different organic dyes like rhodamine B (RhB), methyl orange (MO), congo red (CR), and methylene blue (MB) were selected to investigate the degradation performance of UAC-100. UAC-100 displayed negative zeta potential when pH was higher than 3.3 (pH_PZC) (Fig. S8), which favored the adsorption toward cationic organic dyes. During the adsorption process in dark, UAC-100 displayed different adsorption performance toward cationic RhB and MB along with anionic MO and CR. It can be found that UAC-100 exhibits the best adsorption ability toward MB due to electrostatic interaction. However, the size of RhB (1.56 nm × 1.35 nm × 0.42 nm) is slightly larger than MB (1.38 nm × 0.64 nm × 0.21 nm), and the pore size of UAC-100 is ca. 0.6 nm (Fig. S9), which leads to the poorer adsorption performance toward RhB than that toward MB. With the aid of weak interaction between Ag⁺ in UAC-100 and the –NH₂ group [50, 51, 58], UAC-100 displays noticeable adsorption toward CR with the adsorption efficiency of 28%. And, the electrostatic repulsion between UAC-100 and MO leads to nearly no adsorptive interactions. Upon the light irradiation, 96% MB and 81% CR can be decomposed within 60 min, while 90% MO and 85% RhB can be degraded up to 90 min. It can be concluded that UAC-100 can achieve the photocatalytic degradation toward organic pollutants like stable organic dyes.

In order to further understand the intrinsic reactions, some measures were taken to explore the active species that play a role in the process of photocatalytic. Specifically, EDTA-2Na (0.2 mmol/L), benzoquinone (BQ, 0.2 mmol/L), and isopropyl alcohol (IPA, 0.6 mmol/L) were added to the solution to capture h⁺, ·O₂^–, and ·HO, respectively. It has been pointed out that ·HO and ·O₂^– are important photoactive substances that can degrade pollutants [16, 27, 31, 44, 59]. As shown in Fig. 7a, the degradation of MB by UAC-100 was significantly inhibited after the addition of different sacrificial agents, indicating that all the h⁺, ·HO, and ·O₂^– can achieve the oxidative degradation toward the organic dyes under visible light. The electron spin resonance (ESR) determination can also prove the existence of ·O₂^– and ·HO, as shown in Fig. 7b and 7c.

It is well known that heavy metals like Cr(Ⅵ) often existed with various organic pollutants in industrial wastewater. In this experiment, the photocatalytic activity of UAC-100 for simultaneous Cr(Ⅵ) reduction and organic dye degradation at pH = 2 was investigated. As shown in Fig. 8, in the mono-system of Cr(Ⅵ) and MB, UAC-100 could remove 100% Cr(Ⅵ) and 96% MB after 60 min illumination, respectively. However, in the Cr(Ⅵ) and MB matrix, only 79% Cr(Ⅵ) and 78% MB were removed under visible light for 60 min. The removal efficiency of both Cr(Ⅵ) and MB over UAC-100 in the mixed system became slower than that of the mono-component, possibly due to the competition of ·O₂^– between Cr(Ⅵ) and MB (detailed discussion can be found in Section 3.2.5).

It is important to explore the reusability and stability of the UAC-X composites for practical application, which was performed by accomplishing repeated cycles under determinate reaction conditions. As illustrated in Fig. 9a, UAC-100 shows the superior photocatalytic Cr(Ⅵ) reduction activity of more than 99% after four cycles. The PXRD patterns (Fig. 9b) of the recovered UAC-100 after four cycles matched well with the original ones, implying that UAC-100 has excellent reusability and stability. In addition, there is no obvious change in the octahedron shape of UiO-66-NH₂ after four runs (Fig. S10), indicating that the composite material can be reused without obvious erosion and deterioration. The XPS spectrum of UAC-100 after the photocatalytic reaction further demonstrates the integrity of UAC-100 (as shown in Fig. S11), which has only a small amount of Cr(Ⅲ) formed by Cr(Ⅵ) reduction (the binding energy of 573.3 eV corresponds to Cr 2p_3/2 orbital of Cr(Ⅲ)) [60].

It should be noted that 99% Cr(Ⅵ) reduction can be achieved after four cycles of visible light irradiation for 50 min over UAC-100 without adding any hole scavenger to promote the reaction. Compared to some typical metal-organic frameworks or their composites, UAC-100 exhibits an excellent ability to reduce Cr(Ⅵ) after 50 min irradiation under 5.00 W LED, as illustrated in Table 2. It is found that UAC-100 displayed superior photocatalytic Cr(Ⅵ) performance among the reported photocatalysts in Table 2, considering the photocatalyst dosage, initial Cr(Ⅵ) concentration, solution volume, cycle times, and light source.

The photoluminescence technique (PL) was used to evaluate the separation efficiency of carrier charges in catalytic process of the composites [41, 64, 65]. Generally, the higher fluorescence emission intensity implies the faster recombination of photo-induced electrons and holes, resulting in a decrease of the photocatalytic performance. UiO-66-NH₂, Ag₂CO₃, and UAC-100 are excited to produce the excitation peak centered at 350 nm. However, UAC-100 displayed a slight blue-shift relative to UiO-66-NH₂ and Ag₂CO₃, in which the emission peaks of UiO-66-NH₂, Ag₂CO₃, and UAC-100 are centered at 542, 540, and 540 nm, respectively. It can be seen from Fig. 10a that the fluorescence intensity of UAC-100 is significantly weaker than that of UiO-66-NH₂ and Ag₂CO₃, indicating that photo-induced electrons and holes over UAC-100 are effectively separated. This can also be confirmed by the transient photocurrent response value [64, 65]. The photo-electrode prepared from UAC-100 can generate reversible and stable photocurrent under visible light illumination, and the photocurrent intensity of UAC-100 is obviously higher than that of UiO-66-NH₂ and Ag₂CO₃ (Fig. S12). The transient photocurrent responses further prove that the separation efficiency of photo-induced electron and hole pairs over the composites is improved significantly.

Electrochemical impedance spectroscopy (EIS) of UiO-66-NH₂, Ag₂CO₃, and UAC-100 was performed for analysis of charge transfer and recombination, in which the diameter of the arc on an EIS Nyquist diagram is equal to the charge transfer resistance occurred at the semiconductor-electrolyte interface [66]. The smaller Nyquist radius implied the smaller corresponding charge transfer resistance. As depicted in Fig. 10b, the arc radius of the UAC-100 composite is much smaller than that of UiO-66-NH₂ and Ag₂CO₃, indicating that the photo-induced holes and electrons over the UAC-100 are effectively separated to accomplish the higher photocatalytic activity.

The Mott-Schottky curves of Ag₂CO₃ and UiO-66-NH₂ (Fig. 10c and 10d) illustrate negative and positive correction between C² value and potential value, implying that Ag₂CO₃ and UiO-66-NH₂ displayed the typical behavior of p-type semiconductor and n-type semiconductor, respectively [67]. Compared with the pristine Ag₂CO₃ and UiO-66-NH₂, the enhanced photocatalytic activity of UAC-X composites may be assigned to the p-n junction formed between the n-type UiO-66-NH₂ and the p-type Ag₂CO₃ semiconductor. It can be roughly inferred from the Mott-Schottky spectrogram that the flat band potentials (E_FB) of UiO-66-NH₂ and Ag₂CO₃ are ca –1.29 and 0.51 eV vs Ag/AgCl electrode, respectively. According to the band gap value (E_g (UiO-66-NH₂) = 2.92 eV, E_g (Ag₂CO₃) = 2.64 eV) and E_FB in Mott-Schottky, the maximum occupied molecular orbital (HOMO) of UiO-66-NH₂ and VB of Ag₂CO₃ were 1.83 eV and 3.35 eV vs NHE at pH = 7.0, respectively.

Based on the properties and experimental studies of the UAC-100 composite, UiO-66-NH₂, and Ag₂CO₃, the possible direct Z-scheme photocatalytic mechanism of organic dyes and Cr(Ⅵ) reduction under the irradiation of visible light was proposed (Scheme 2) [68-72]. Under the visible light irradiation, the electrons in the highest occupied molecular orbital (HOMO) of UiO-66-NH₂ will be excited to the lowest occupied molecular orbital (LUMO), and similarly, the electrons in the VB position of Ag₂CO₃ will be excited to the CB position. Considering that the formation of UiO-66-NH₂/Ag₂CO₃ (UAC-100), the photo-induced electrons can be transferred from the CB of Ag₂CO₃ to consume the h⁺ formed over HOMO of UiO-66-NH₂, which effectively shorten the electron transfer distance and facilitate the separation of electron-hole formed on the individual UiO-66-NH₂ and Ag₂CO₃. The ·O₂^– is a key active substance that exhibits direct oxidative degradation of organic dyes, which can be generated as the LUMO value of UiO-66-NH₂ is –1.29 eV vs NHE, much more negative than the standard redox potential E(O₂/·O₂^–) (–0.33 eV vs NHE) [64]. At the same time, O₂^–· will also participate in the photocatalytic Cr(Ⅵ) reduction via Eqs. (3) and (4) [73, 74], which can be demonstrated by the inhibition of Cr(Ⅵ) reduction in nitrogen atmosphere compared to normal indoor environments under the same conditions, as shown in Fig. S13. It is worth noting that the electrons generated on the LUMO of UiO-66-NH₂ will also directly participate in the Cr(Ⅵ) reduction because of the strong ability of Cr(Ⅵ) to capture electrons in the photocatalytic reduction of Cr(Ⅵ) to avoid the photo-corrosion of Ag₂CO₃, which can be proved from the integrity of the material before and after four cycles and XPS spectrum before and after photocatalytic Cr(Ⅵ) reduction (Fig. 9b and Fig. S11). Subsequently, the holes on the VB of Ag₂CO₃ can directly oxidize organic dyes and also oxidize the H₂O or –OH molecules to form ·OH, which then participates in the photocatalytic degradation of organic dyes, because the VB potential (3.35 eV vs NHE) is greater than the H₂O/HO· potential (2.40 eV vs NHE) [64]. The photocatalytic mechanism of the UAC-X composites shows that photo-induced holes are also contributors to the degradation process of organic dyes. In the oxidation-reduction reaction of the UAC-X composites, the effective separation of holes and photo-induced electrons in the formed Z-scheme heterojunction contributes to the improved photocatalytic performance.

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The UAC-X heterojunction with enhanced photocatalytic properties was successfully prepared by a simple ion-exchange-solution method. The morphology and structure of the UAC-X composites were characterized by FTIR, PXRD, SEM, TEM, HRTEM, UV-Vis DRS, and XPS. The photocatalytic activity of the UAC-X composites for Cr(Ⅵ) reduction is better than that of pure Ag₂CO₃ and UiO-66-NH₂ under visible light irradiation. Especially for UAC-100, the ideal effect can be achieved with less UiO-66-NH₂ dosage. PL analysis, electrochemistry measurement, capture of active substances, and ESR spectra demonstrated that the reduction enhancement of Cr(Ⅵ) is due to the faster separation of photo-induced electrons and holes and the reduction of ·O₂^– over the interface of Z-scheme heterostructure. The effects of pH value, foreign ions, and small organic acids on the reduction of Cr(Ⅵ) and the organic dye oxidation properties of UAC-100 were also investigated. The presence of organic matters in the reaction system can enhance the photocatalytic Cr(Ⅵ) reduction performance. In addition, UAC-100 exhibits excellent photocatalytic oxidation properties for RhB, MO, CR, and MB due to the generation of h⁺, ·HO, and·O₂^– with oxidizing properties under visible light. It is worth noting that UAC-100 had excellent reusability and stability in the experiment of photocatalytic reduction of Cr(Ⅵ). This work showed that the combination of UiO-66-NH₂ and Ag₂CO₃ can enhance the photocatalytic efficiency, which might be used in water treatment. We will carry out further work to address the photo-corrosion of Ag₂CO₃, especially during the oxidative degradation toward organic pollutants.

This work was supported by the National Natural Science Foundation of China (51878023, 51578034), Great Wall Scholars Training Program Project of Beijing Municipality Universities (CIT & TCD20180323), Project of Construction of Innovation Teams and Teacher Career Development for Universities and Colleges Under Beijing Municipality (IDHT20170508), Beijing Talent Project (2018A35), and BUCEA Post Graduate Innovation Project (PG2019039).