Construction of MIL-125-NH2@BiVO4 Composites for Efficient Photocatalytic Dye Degradation.

The design and construction of a photocatalyst with a heterostructure are a feasible and effective way to enhance the catalytic performance. Herein, a specially designed composite based on MIL-125-NH2 and BiVO4 was prepared and used for wastewater treatment. In the hybrid MIL-125-NH2@BiVO4, MIL-125-NH2 was uniformly dispersed on the BiVO4 surface. There is a high affinity between MIL-125-NH2 and BiVO4 due to the lattice defects. Under visible light irradiation, the catalytic activity of the as-prepared composite was evaluated by the degradation of various dyes such as malachite green, crystal violet, methylene blue, and Congo red. Nearly 98.7, 99.1, and 41.0% of the initial MG, MB and Cr(VI) were respectively removed over the optical sample of BVTN-5, demonstrating that the hybrid holds great promise for practical applications. Moreover, the composites can be recycled and reused with good stability after five consecutive cycles. The mechanism was proposed and discussed in detail. This work will shed light on the construction of MOF-based composites for efficient photocatalysis.

Introduction

Worldwide, the wastewater produced by increasing industrial activities has become a serious threat for the environment and human health.[1,2] Among the contaminants, the toxic Cr(VI) and dye-containing substances have emerged as a huge global challenge[3,4] and attracted extensive interests.[5] Most of the chromium from the groundwater originates from natural sources and anthropogenic activities. Cr(III) is the dominant species in natural water, while Cr(VI) is believed to be a solely anthropogenic pollutant. The United States Environmental Protection Agency (US EPA) Guidelines for Carcinogen Risk Assessment have classified hexavalent chromium as one of the 17 chemicals posing the greatest threat to human beings. In addition, a concentration of Cr(VI) in drinking water exceeding 0.1 mg·L–1 is considered to be carcinogenic through a mutagenic mode action.[6,7] Therefore, various methods have been proposed for wastewater remediation, including absorption, membrane-based separation, and catalytic reduction.[8,9] Restricted by low removal efficiencies, high energy consumption, and high cost, it is difficult to meet the requirements of practical application. Very recently, semiconductor photocatalysis has been considered as a promising and environmentally benign technique that can convert the organic pollutants into nonhazardous components under visible light.[10,11] Many efforts have been dedicated to the design and preparation of high-performance photocatalysts toward environmental remediation.[12,13] To date, numerous photocatalysts such as metal oxides, inorganics, noble metals, and polymers have been applied for light-driven pollutant elimination.[14,15] Nevertheless, the rapid recombination of photogenerated carriers and the suboptimal utilization of solar energy still bring a huge hindrance to the application of photocatalysts.[16,17]

To improve the photocatalytic efficiency, some researchers developed several bismuth-based metal oxide-based photocatalysts, including BiOX (X = Cl, Br, I), Bi2WO6, BiVO4, Bi4Ti3O12, BiFeO3, Bi2S3, Bi5FeTi3O15, CaBi2O4, etc. All of them exhibit excellent capability in light of their outstanding efficiency and the enhanced charge transfer in photocatalysis.[18,19] As bismuth vanadate (BiVO4) possesses the characteristics of low cost, suitable band gap, good stability, and nontoxicity, it has attracted great interest since the pioneering work conducted by Kudo et al.[20] Restricted by the limited surface area, poor adsorption capacity, and low ability to separate photogenerated electron–hole pairs, pure BiVO4 shows an undesirable photocatalysis performance. Various types of BiVO4-based composites such as n-BiVO4@p-MoS2 (76.5% Cr(VI), 69.2% CV, 60 min),[21] Cu2O/BiVO4 (100% CV, 91% RhB, 73% MO, 120 min),[22] β-Bi2O3/BiVO4 (70% o-DCB, 6 h),[23] Au-(BiOCl/BiVO4) (67% MO, 240 min),[24] and V2O5/BiVO4 (92% MB, 180 min)[25] are synthesized to overcome such defects. Besides, metal–organic frameworks (MOFs), composed of metal units and organic ligands with large pore sizes, high surface areas, structure adaptability, and flexibility,[26] are attractive porous crystal materials and have been found to have a huge potential in the photocatalytic field.[27,28] Prior studies demonstrated that MOFs possess a semiconductor-like property and can be directly used as photocatalysts. Taking MIL-125-NH2 as an example,[29,30] it could be stimulated by ultraviolet light with a small band gap energy at about 2.6 eV in which metal clusters serve as the conduction band (CB) and organic linkers play the role of the valence band (VB). Unfortunately, MIL-125-NH2 suffers from weak visible light response and low efficiency.

Herein, by combining the advantages of BiVO4 and MIL-125-NH2, a MIL-125-NH2@BiVO4 composite was prepared via a facile hydrothermal method. In the hybrid, BiVO4 serves as the support/matrix for MIL-125-NH2 coating. The element substitution in lattice substantially promoted the stability of composites. Effects of Ti/Bi molar ratio on the structure and property of composites were investigated. Under visible light irradiation, photocatalytic removal of various pollutants was conducted, and results showed that the as-prepared composites exhibited high efficiency.[31,32] This work provides a facile approach for the design and preparation of high-performance photocatalysts for water treatment.

Experimental Section

Materials

Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O, 99%), ammonium metavanadate (NH4VO3, 99%), nitric acid (HNO3, Sinopharm Chemical Reagent Co., Ltd.), 2-aminoterephthalic acid (H2ATA, 98%), titanium isopropoxide (TTIP, 95%), methanol (MeOH), and N,N-dimethylformamide (DMF, 99.8%) were obtained from Shanghai Macklin Biochemical Co., Ltd., and utilized as received without further treatment.

Synthesis of MIL-125-NH2@BiVO4 Composites

The BiVO4 particles were synthesized via a hydrothermal method modified from a previous study and nominated as BV.[33] For the synthesis of MIL-125-NH2@BiVO4 composites with varying molar ratios, a solvothermal method was used and is shown in Figure a.[34] At first, 1 mmol of the BV was ultrasonically dispersed into the 40 mL mixture of DMF and MeOH (4:1, v/v). Subsequently, 2 mmol of H2ATA was added slowly under violent agitation until it dissolved completely, and then 1 mmol of TTIP was added dropwise to the suspension. After vigorous stirring for 30 min, the resultant solution was transferred into a Teflon-lined autoclave and kept under 423 K for 16 h. After that, the precipitates were collected by centrifugation and washed several times with DMF and MeOH. Finally, the obtained dark yellow sample was dried under a vacuum oven at 333 K overnight. The final product was referred to as BVTN-5. Based on the procedure mention above, BVTN-n (n = 1, 2, 3, 4, 5, 6) composites were synthesized, while the numerical suffixes represent the molar ratio of Ti/Bi (1:19, 1:5, 1:4, 1:2, 1:1, 2:1). For comparison, pure MIL-125-NH2 was collected under the same conditions in the absence of BiVO4 and denoted as TN.

Figure 1

(a) Schematic illustration of the construction of MIL-125-NH2@BiVO4 composites. (b) XRD patterns (c) and FTIR spectra of BV, TN, and BVTN-n (n = 1, 2, 3, 4, 5, 6) samples.

Characterization

The X-ray diffraction (XRD) patterns were recorded by an Ultima IV equipped with Cu Kα radiation at a scanning angle (2θ) range of 5 to 80° and scanning rate of 10°·min–1, while AXIS UltraDLD was employed to achieve X-ray photoelectron spectroscopy (XPS) to determine the elemental composition and the chemical states on the surface of the prepared samples. The Fourier transform infrared spectroscopy (FT-IR) was documented using a VERTEX 80v by means of a KBr disk as the reference in the scanning range of 4000–400 cm–1; Raman spectra were gained from DXR532. Morphology and structure were observed by a scanning electron microscope (SEM) spectrophotometer (JSM-7600F), while the energy-dispersive X-ray spectroscopy (EDS) of the samples was also performed during the SEM measurement (INCA X-Act, Oxford Instruments). Moreover, the Brunauer–Emmett–Teller (BET) (BSD-PM2) method was used to determine gas adsorption isotherms and the specific surface area, whereas the average pore diameter and pore size distribution were analyzed by the Barrett–Joyner–Halenda (BJH) method, based on the foundation of N2 adsorption–desorption isotherms. UV-2802 and UV-2600 were used to acquire the UV–vis absorption spectra as well as the UV–visible diffuse reflectance spectra (UV-DRS) of the specimens by taking BaSO4 as reference. The photoluminescence (PL) spectra of the samples were excited at 365 nm and performed on a HORIBA Scientific fluorescence spectrophotometer (FluoroMax-4). Photocurrent transient was studied using an electrochemical workstation (CHI-660E, Shanghai Chenhua, China) with the standard three-electrode system.

Photocatalytic Experiment

The catalytic activities of the samples were investigated in detail through the catalytic decomposition of various target organic pollutants including one typical dye, malachite green (MG), and Cr(VI) under visible light illumination. In addition, MB, CR, and CV were also selected to evaluate the performance of the prepared samples. The photocatalytic reaction was carried out under room temperature, and a 300 W Xe lamp (λ > 420 nm) (CEL-HXF300, light intensity: 100 mW·cm–2) was the light source. Experimental details are as follows: 40 mg of the as-prepared sample was dispersed into the 40 mL MG solution (75 mg·L–1). To achieve an adsorption–desorption equilibrium, the suspension was magnetically stirred and maintained in the dark for 1 h before being irradiated with visible light. At given intervals of time, 3 mL of the reaction suspension was filtered with a 0.22 μm syringe filter to eliminate the residue particles, and the concentrations of filtrate were collected at the maximum absorption wavelength of the specific position (MG: 617 nm, CV: 590 nm, MB: 664 nm, CR: 496 nm, Cr(VI): 540 nm) by a UV–vis spectrophotometer. The concentrations of Cr(VI) were tested according to the 1,5-diphenylcarbazide (DPC) colorimetric method.[35] Triple testing was performed for each sample for averaging the results to ensure the reproducibility of the results. To determine the stability of the prepared composite, the used material was collected and the same degradation experiment was performed for five cycles. The photocatalytic oxidation (PCO) efficiency of dyes and the photocatalytic reduction (PCR) efficiency of Cr(VI) were estimated by η = (C0 – Ct)/C0 × 100%, where η represents the photocatalytic removal efficiency and C0 and Ct refer to the concentration of the dye or Cr(VI) before and after the reaction.

It is essential to gain more information on the band structures of the composite to explore the separation of photogenerated carriers over the heterostructures. The positions of the semiconductor’s VB and CB were calculated using the following empirical equations:[36,37]where EVB and ECB are the VB and CB edge potential, respectively, while Eg and Ee correspond to the band-gap energy of the semiconductor and the energy of free electrons vs hydrogen (4.5 eV), with X being the electronegativity of the semiconductor.[22] The VB and CB values of BiVO4 were calculated by the above formula (eqs , 2, and 3), and the photocatalytic mechanism of the MIL-125-NH2@BiVO4 composite to efficiently remove the dye is proposed in Figure based on the band gaps of BiVO4 and MIL-125-NH2.

Figure 6

Proposed mechanism for the simultaneous removal of dyes/Cr(VI) over the MIL-125-NH2@BiVO4 composite.

Results and Discussion

As illustrated in Figure a, MIL-125-NH2@BiVO4 composites were obtained through the hydrothermal method. Pre-prepared BiVO4 was used as substrate for the deposition and in situ growth of MOF particles. The obtained powder is dark yellow in color.

To find out the crystal structure of the as-prepared samples, XRD patterns were explored and summarized in Figure b. The characteristic peaks of BV centered at 19.0, 28.8, 30.6, and 53.3° are matched well with the (011), (121), (040), and (161) faces of BiVO4, confirming the successful synthesis of BiVO4 (JCPDS Card No. 83-1699). Moreover, the narrow line widths indicate a high degree of crystallinity for the as-prepared BV. For the TN product, the sharp peaks are located at 6.8, 9.7, 11.7, 15.0, 16.6, 17.9, 19.0, and 22.5°, evidencing the effective preparation of MIL-125-NH2.[28,36] No peaks for any other phases or impurities are detected, and the diffraction peaks of BVTN-5 are consistent (those of BVTN-6 match well) with those of BV and TN.[22] However, the signals attributed to MIL-125-NH2 over BVTN-n (n = 1, 2, 3) composites are almost undetectable probably due to the low content of TN and the good distribution of TN in composites. The above data show a high degree of consistency with the previous literature.[38]

FTIR spectra (Figure c) were determined to explore the functional groups in composites. The stretching vibration of the O–H bond near 3446 cm–1 can be associated with the presence of water absorbed over the surface. Predictably, the absorption band at 700–900 cm–1 is assigned to the asymmetric stretching and symmetric stretching vibration of the V–O bond.[39] In the spectrum of BV, the broad and strong peak of 731 cm–1 is assigned to the asymmetric stretching of VO43–, revealing the successful synthesis of monoclinic sheelite BiVO4 powder. This observation is consistent with the conclusion obtained from XRD analysis. Seen from the spectrum of the TN sample,[40,41] the bands at 1300–1600 cm–1 are attributed to the stretching vibrations of the carboxylate group. C–N and N–H bonds at 1259 and 1626 cm–1 are correlated with the linkers in the framework of MIL-125-NH2. Moreover, the characteristic absorption of 400–800 cm–1 belongs to O–Ti–O. BVTN-n shows the same adsorption bands as BV and TN, further proving the presence of BiVO4 and MIL-125-NH2 in the composite.

The Raman spectra (Figure S1) provide further information on the local structure and bonding states. The Raman signal of BV exhibited a monoclinic phase.[42] Besides, the V–O bond length was calculated to be 1.69 Å according to the empirical expression υ = 21,349e–1.9176, wherein υ and R are the peak positions of the group’s stretching mode and the bond length, respectively.[43] The peak of the composite was consistent with two monomer materials, supporting the successful complexation of BV and TN.

Figure displays the XPS analysis of the prepared BV, TN, and BVTN-5 samples. The relevant peaks were calibrated with the C 1s signal of contaminant carbon.[22] The typical survey spectrum depicted in Figure a confirms the presence of Bi, Ti, O, V, N, and C elements in BVTN-5, which is in agreement with the EDS analysis results. As presented in Figure b, the binding energies at 533.2, 531.8, 530.4, and 529.6 eV were show in the high resolution of O 1s. These peaks can be ascribed to the signals of O from the adsorbed H2O or surface hydroxyl group, C=O, lattice oxygen for BiVO4, and MIL-125-NH2, respectively.[44] Additionally, two signal peaks at 401.8 and 399.2 eV exhibited in Figure S2 are associated with the −N=+ or −NH–+ and −NH2 groups of the organic linkers.[45,46] In the high-resolution spectrum of the C 1s orbital (Figure S2), the peaks located at 284.8, 285.3, 286.4, and 288.8 eV could be ascribed to C–C, C–N, C–O, and C=O bonds, which were mainly derived from H2ATA linkers and benzoic rings, confirming that MIL-125-NH2 was formed instead of TiO2.[41,47] For bismuth and titanium, they are in good accordance with those in previous researches.[33,34] The typical peaks centered at 164.5 and 159.3 eV in the Bi 4f spectrum (Figure c) correspond to Bi 4f5/2 and Bi 4f7/2, and the splits at 464.4 and 458.5 eV are assigned to Ti 2p1/2 and Ti 2p3/2 (Figure d), manifesting that the Bi and Ti species in BVTN-5 exist in the form of Bi3+ and Ti4+ oxidation state. Meanwhile, the Ti 2p1/2 peak has a broad bump because of the overlap between Bi 4d3/2 at 466.4 eV and Ti 2p1/2 at 464.4 eV.[24] Compared with the pristine BV, the presence of TN on the BV caused the band of Bi3+ to shift toward low energy, suggesting that Bi3+ has been doped into the lattice of TN.[48,49] Besides, BV can work as an electron producer; thus, electrons can transfer to TN, thereby enhancing the charge density around Ti4+. A similar situation has been reported by Zhao et al.[44] XPS results provide sufficient evidence for the formation of MIL-125-NH2@BiVO4 composites via the hydrothermal method.

Figure 2

XPS spectra of BV, TN, and BVTN-5 samples: (a) survey scan, (b) O 1s, (c) Bi 4f, and (d) Ti 2p spectra.

SEM images were used to illustrate the morphology of the materials. As revealed in Figure a, pure BiVO4 exhibited a structure of aggregated branch-like dendrite shape with a length size in the 50–300 nm range and a unique backbone about 1–2 μm long, while bare MIL-125-NH2 (Figure b) had a tablet-like morphology. However, compared with the smooth surface of pristine BiVO4, stick structures wrapped with the smaller size MIL-125-NH2 were observed for the BVTN-5 composite in Figure c,d. Elemental mapping analysis was performed on the BVTN-5 composite for further confirmation. Figure S3 unambiguously confirms the presence of Ti and Bi elements in the sample. In addition, the actual molar ratio of Ti/Bi in the prepared BVTN-5 was obtained by ICP-OES. The actual amount of Ti/Bi in the position is quite similar to the nominal values (0.98). The actual contents of BiVO4 (33.1%) and MIL-125-NH2 (66.9%) in BVTN-5 were evaluated by TG analysis (Figure S4). Thermogravimetric analysis revealed two regions of significant weight losses in BVTN-5. The surface adsorbed water was lost to evaporation at the first stage of loss below 120 °C. And the second weight loss between 300 and 400 °C belongs to structural disintegration.[50,51] It is believed that BVTN-5 almost shows chemical stability up to 300 °C.

Figure 3

SEM images of (a) BV, (b) TN, and (c, d) BVTN-5 at different resolutions.

Photocatalytic Results

The photocatalytic performance of the sample was tested with MG as the target pollutant. A dark adsorption was operated for 1 h before the light was turned on to achieve the adsorption desorption equilibrium. Besides, the blank examination was set for verifying the direct photolysis of MG. As shown in Figure a, the total removal efficiency generally has a positive correlation with the content of TN in BVTN-n composites. BVTN-5 presented an efficiency of MG (10 mg·L–1) removal (93.6%) close to TN (95.1%). It is also clear that the BVTN-5 composite exhibited a higher photocatalytic performance than that of the pristine BV (40 mg, 68.1%) and BV/TN mixture (BV: 13.24 mg, TN: 26.76 mg, 85.9%), which is indicative of the formation of the desired heterostructure (Figure b), matching well with the conclusion of DRS. From XPS, ICP, BET, SEM, and photocatalytic reaction test results, the specific surface area of the sample and the content of TN on the surface play a key role in the photocatalytic performance. The photocatalytic efficiency of BVTN-6 is arguably unsatisfactory as a result of the excessive TN prompting greater adsorption performance than photocatalytic performance. It is inferred that excess loadings possibly hinder photogenerated electron transfer and reduce the accessibility of active sites, which is detrimental to the photocatalytic performance. To survey the influence of coexisting ions, MG is also dissolved in tap water and conducted under the same experimental conditions. BVTN-5 shows a striking removal efficiency without distilled water (92.6%, 3 h) (Figure b). BVTN-5 is hence chosen for the follow-up research.

Figure 4

(a, b) Photocatalytic oxidation of 10 mg·L–1 MG solution via the as-prepared samples. (c) Effect of MG concentration on the reduction over BVTN-5. (d) Simultaneous photocatalytic removal ability of dye mixture via the as-prepared BVTN-5 samples (75 mg·L–1 MG, 75 mg·L–1 CV, 75 mg·L–1 MB). Reaction conditions: 40 mg catalysts, 40 mL solution, λ > 420 nm, pH ∼6, room temperature.

To evaluate the effect of initial MG concentration in the photocatalytic activity, the concentration versus removal efficiency is plotted (Figure c). It is obvious that 75 mg·L–1 reaches the maximum movement ability (95.5%) of BVTN-5 in 3 h. To some extent, the removal efficiency gradually increased with concentration until the concentration of 75 mg·L–1, which is driven by the concentration difference. Contrarily, a too high concentration causes the saturation of dye accumulated on the photocatalyst surface; there is not enough time to degrade them into small molecules.[52,53] The catalytic performance of this work is comparable with other MOF-based photocatalysts (Table S1).

Whether BVTN-5 can achieve the selective degradation of cationic or anionic pollutants is illustrated in Figure S5. BVTN-5 performed better in cationic dyes (MG, MB, CV) than in the anionic dye (CR). To discuss the selectivity of different kinds of dyes, the variation in zeta potential of the BVTN-5 photocatalyst in aqueous dispersion has been carried out, and the results are shown in Figure S6. With the increase in the value of pH within the pH range tested, the zeta potential of the composite becomes more negative. The zero-point charge (pHZPC) was determined to be at ca. 2.7. It is clear that the surface of BVTN-5 was negatively charged when the pH value was above 2.7, contributing to an electrostatic attraction effect between BVTN-5 and the cationic pollutant.[54] Thus, the cationic dye can be selectively accumulated onto the surface and rapidly degraded by the migrated photogenerated electron, leading to the superior removal efficiency compared to the anionic dye.[29]

In real industrial activities, multicomponent dyes or heavy metal ions along with dye are often discharged inevitably, so it is of great significance to treat different kinds of pollutant mixture systems simultaneously. As shown in Figure d, a mixture containing MG, MB, and CV (the concentration of both pollutants was controlled as 75 mg·L–1) can be efficiently degraded (88.7/99.2/80.3%, 3 h) after irradiation with the presence of BVTN-5 conducted in neutral media without acid adjustment, giving the persuasive proof that construction of the inherent heterostructure boasted a remarkable photocatalytic ability.

The ability of products to remove other binary system and ternary system pollutants is displayed in Figure S5 (the concentration of Cr(VI) and dyes was fixed at 10 and 75 mg·L–1, respectively). To some extent, it could be seen from the result that the final removal efficiency of the mixed dye solution is slightly lower than that of the individual dye solution; on the contrary, there is a mutual promotion in the photocatalytic removal of Cr(VI) and dye binary system (MG/CV (95.8/97.6%), MB/CV (82.1/89.6%), MG/MB (96.6/96.9%), CV/Cr(VI) (97.6/55.2%), CR/Cr(VI) (97.1/62.5%), MG/MB/Cr(VI) (98.7/99.1/41.0%), 3 h). Meanwhile, the single removal of CV, MB, CR, CV, and Cr(VI) over BVTN-5 is reported to be 97.9, 97.6, 58.2, 97.9, and 15.1%, respectively. It is reckoned that the dye molecules compete for the available active sites of catalysts in the targeted multicomponent system, and hence, the dye removal percentage was reduced. By the way, it is well known that the pH value is a crucial prerequisite in the removal efficiency of Cr(VI).[29,50] BVTN-5 presented better performance under the acidic conditions (Cr(VI) (20.5%), MG/MB/Cr(VI) (71.4/81.7/57.1%), 3 h, pH ∼2).

Additionally, Kyung et al.[55] reported that a complex intermediate can be formed in the mixture of dyes and Cr(VI). No new absorption peak for the intermediate was detected from UV–vis, indicating that dyes and Cr(VI) stably existed in the solution. Furthermore, the product after photocatalytic process was monitored though XPS to analyze the valence state of Cr on the photocatalysts (Figure S7). According to the high-resolution spectrum of Cr 2p, discernible peaks that appeared at 576.7 and 586.2 eV correlate with Cr 2p3/2 and Cr 2p1/2 of Cr(III), respectively. It is disclosed that the BVTN-5 composites efficiently detoxify harmful Cr(VI) into innoxious Cr(III) under visible light.

The stability and recyclability of the catalysts play a decisive role in real applications. The used catalyst was collected by centrifugation (8000 rpm) and regenerated by ethanol (50 mL, 50%, v/v). After drying at 60 °C, the recovered catalysts were used for the next cycle under the same conditions. As illustrated in Figure a, the sample still maintained prominent removal efficiency after five cycles. XRD (Figure b) and FTIR results (Figure S8) of the used BVTN-5 exhibited no obvious deviation, confirming that the structure integrity composition was well preserved. In addition, it was found that BVTN-5 can maintain its original structure after five times of the cycling test (Figure S9). Moreover, the ICP detection of the solution after photocatalytic activity demonstrates that the leaching of the Ti4+ ions can be nearly ignored, suggesting that the sample has excellent stability for the photocatalytic degradation of pollutants.

Figure 5

(a) Recycling tests for photocatalytic reaction of 75 mg·L–1 MG and (b) XRD spectra of fresh and used BVTN-5 (reaction conditions: 40 mg catalysts, 40 mL solution, λ > 420 nm, pH ∼6, room temperature). (c) UV–vis diffuse reflectance spectra of all samples (d) with (Ahυ)2 vs hυ curves.

The optical properties of BV, TN, and the BVTN-5 composite were investigated by UV–vis DRS. From the results depicted in Figure c, the addition of MIL-125-NH2 led to a red shift by the BiVO4 based composite. The degradation activity of the composite is substantially boosted because of the increase in visible light response. The band gap energy (Eg, eV) value of different samples can be estimated using the Tauc plot method,[56] described in eq .where α, hυ, and A represent the absorption coefficient, the photon energy, and the constant, respectively, while the n value depends on the characteristics of the transition, i.e., direct or indirect.

According to Figure d, the band gap for the BV, TN, and BVTN-5 is calculated to be 2.3, 2.7, and 2.4 eV, respectively. Collectively, it can be deduced that combining TN with BV to form the intimate heterointerface can narrow the band gap and enhance visible light absorption, endowing the composites with favorable optical properties. The charge separation extents for products are presented to investigate the photocatalytic mechanism (Figure S10). The decreased intensity represents the low recombination of electron–hole pairs. BVTN-5 showed a larger PL quenching effect compared to the bare samples, demonstrating the improved charge separation. Photocurrent-response spectra were investigated and required in Figure S11. It can be observed that BVTN-5 bared outstanding photocurrent density among the three materials, suggesting the higher efficient separation of photogenerated carriers.

Mechanism of the Enhanced Photocatalytic Property

Generally, a plurality of reactive species was expected in photocatalytic degradation contribution. Radical quenching experiments were executed to definitely investigate the oxidizing substance produced in the photocatalytic removal of MG over BVTN-5 (Figure S12). Ethylenediaminetetraacetic acid disodium (EDTA-2Na, 0.2 mmol·L–1), AgNO3 (0.2 mmol·L–1), tertiary butanol (TBA, 0.2 mmol·L–1), and p-benzoquinone (BQ, 0.2 mmol·L–1) were applied for the capture of holes (h+), electronics (e–), hydroxyl radicals (·OH), and superoxide anion radicals (·O2–), respectively.[22,44] The predominant reactive species in the process was relation to h+, for the corresponding removal efficiency decrease strongly.There was a slight increased with the scavengers’ addition of BQ and AgNO3. By consuming·O2–and e– selectively, the separation of photocarriers was promoted and h+ increased.

As discussed above, the compete and synergistic for the available active sites leading a reduce or promote in photocatalytic activity plausibly. We can attribute the removal ability of dyes without irradiation to the adsorption of MG (75 mg·L–1) on the surface of BVTN-5 (Figure S13). Moreover, the photolysis of dyes cannot be neglected under visible light (Figure S13); the involved photolysis steps were presumed in eqs , 6, 7, 8, and 9. During the photolysis process, dye can be excited into dye* and then oxidized by O2 to degraded products. Meanwhile, with the irradiation, Cr(VI) acts as photo-induced electron acceptor, and dyes serve as a hole scavenger, which could hence hinder the photogenerated carriers’ recombination and enormously promote the simultaneous removal of Cr(VI) and dyes.

Conclusions

In summary, a series of novel visible-light driven MIL-125-NH2@BiVO4 composites with different molar ratios of Ti/Bi were successfully synthesized and characterized. They show broad use in strong photocatalytic activity under visible irradiation in the degradation of different dyes (MG (95.5%), MB (97.6%), CV (97.9%), CR (58.2%)) or simultaneous removal of different pollutant (MG/MB/Cr(VI) (98.7/99.1/41.0%)) mixture systems in a neutral medium, and the optimal molar ratio of Ti/Bi is 1:1. It was found that the recombination of photogenerated electrons and holes in BiVO4 is significantly inhibited with the presence of MIL-125-NH2, leading to the enhanced photocatalytic activity. In addition, the products also have excellent stability and recyclability. This is a report on the utilization of the MIL-125-NH2@BiVO4 composite, which can remove organic dyes and Cr(VI) in a neutral media simultaneously. Besides, we hope that this work can bring a new insight into the treatment of wastewater in real practice.