Facile fabrication of BUC 21/g C 3 N 4 composites and their enhanced photocatalytic Cr(VI) reduction performances under simulated sunlight

Received: 21 August 2018 Revised: 7 September 2018 Accepted: 10 September 2018 DOI: 10.1002/aoc.4621 FULL PAPER Facile fabrication of BUC 21/g C 3 N 4 composites and their enhanced photocatalytic Cr(VI) reduction performances under simulated sunlight Xiao Hong Yi 1,2 Fu Xue Wang 1 Xue Dong Du 1 Peng Wang 1,2 Chong Chen Wang 1,2 1 Beijing Key Laboratory of Functional Materials for Building Structure and Environment Remediation, Beijing University of Civil Engineering and Architecture, Beijing 100044, China 2 Beijing Engineering Research Center of Sustainable Urban Sewage System Construction and Risk Control, Beijing University of Civil Engineering and Architecture, Beijing 100044, China Correspondence Chong Chen Wang, Beijing Key Laboratory of Functional Materials for Building Structure and Environment Remediation, Beijing University of Civil Engineering and Architecture, Beijing 100044, China. Email: wangchongchen@bucea.edu.cn Funding information Scientific Research Foundation of Beijing University of Civil Engineering and Architecture, Grant/Award Numbers: KYJJ2017008 and KYJJ2017033; Fundamental Research Funds for Beijing Universities, Grant/Award Numbers: X18276, X18125, X18124, X18076 and X18075; Beijing Talent Project, Grant/Award Number: 2017A38; Project of Construction of Innovation Teams and Teacher Career Development for Universities and Colleges Under Beijing Municipality, Grant/ Award Number: IDHT20170508; Great Wall Scholars Training Program Project of Beijing Municipality Universities, Grant/ Award Number: CIT&TCD20180323; National Natural Science Foundation of China, Grant/Award Numbers: 51878023 and 51578034 A series of BUC 21/g C 3 N 4 composites were facilely fabricated from twodimensional metal organic framework BUC 21 and two dimensional metal free polymer semiconductor graphitic carbon nitride (g C 3 N 4 ) through ball milling, and characterized via powder X ray diffraction, Fourier transform infrared spectrometry, thermogravimetric analysis, transmission electron microscopy, and UV visible diffuse reflectance, X ray photoelectron and photoluminescence emission spectrometry. The photocatalytic activities of B100G100 (weight ratio of BUC 21 to g C 3 N 4 being 1:1) towards Cr(VI) reduction were investigated upon the irradiation of simulated sunlight and real sunlight, in which the influences of various organic compounds (tartaric acid, citric acid and oxalic acid) as hole scavengers, ph values (2, 3, 4, 5, 6, 7 and 8) and foreign ions (ions in tap water and real surface water) were also investigated. The results revealed that B100G100 exhibited more outstanding photocatalytic Cr(VI) reduction than individual BUC 21 and g C 3 N 4, resulting from enhanced separation of photogenerated electrons and holes, which were confirmed via both photoluminescence emission and electrochemical determination. The B100G100 composite exhibited good reusability and stability after several experimental runs. Also, the corresponding mechanism of photocatalytic reaction was proposed. KEYWORDS g C 3 N 4, hexavalent chromium, mechanism, metal organic framework, photo reduction 1 INTRODUCTION Metal organic frameworks (MOFs) assembled by metal template and organic linker have attracted increasing attention, due to their potential applications in catalysis, [1 3] energy gas storage, [4 6] gas separation, [7 9] adsorption of pollutants [10 16] and so on. [17 22] Especially, a range of photoactive MOFs have been investigated by Appl Organometal Chem. 2019;33:e4621. https://doi.org/10.1002/aoc.4621 wileyonlinelibrary.com/journal/aoc 2018 John Wiley & Sons, Ltd. 1of11

2of11 researchers with great enthusiasm, [23 28] since the historic work of García and co workers on phenol photodegradation using MOF 5 as a semiconductor under UV light irradiation in 2006. [29] Unfortunately, most MOFs exhibited efficiently photocatalytic performances only under UV light illumination, which limited their wide potential applications. Two strategies, namely the introduction of NH 2 group and the construction of heterostructures with the aid of narrow gap semiconductors, were generally adopted to allow MOFs to utilize visible light, even sunlight. [23,24] However, it is difficult for some ligands to adopt NH 2 groups at ideal sites, or the introduced NH 2 group would change the original coordination mode of the ligands. Therefore, it is not always feasible to modify functional groups on organic ligands. The other strategy of constructing heterostructures between narrow gap semiconductors and MOFs was preferred for promoting photocatalytic activity under visible light or sunlight. [23,24,30] In recent years, metal free polymer semiconductor photocatalysts such as fullerenes, carbon nanotubes, graphene, carbon nanofibers, carbon quantum dots and especially graphitic carbon nitride (g C 3 N 4 ) have attracted wide attention with the merits of easy preparation, good chemical stability, narrow band gap and low cost. [31 37] However, the photogenerated electrons and holes of g C 3 N 4 are easily combined, which seriously affects its photocatalytic efficiency. [6,38,39] Therefore, some researches have focused on the combination of g C 3 N 4 and MOF materials to improve the photocatalytic efficiency considering MOFs can transfer photoelectrons rapidly as a result of their metal nodes. Some composites like MIL 53(Al)/g C 3 N 4, [40] g C 3 N 4 /MIL 53(Fe), [41] MIL 125(Ti)/g C 3 N [42] 4 and ZIF 8(Zn)/g C 3 N [43] 4 were prepared for the degradation of organics and effective reduction of Cr(VI). BUC 21 is a chemically stable two dimensional MOF synthesized by our group, which exhibits superior photocatalytic performance under UV light irradiation towards Cr(VI) reduction and organic decomposition. [44] To extend its light absorption from the UV to visible region, g C 3 N 4 was encapsulated on the surface of BUC 21 to achieve rapid photoinduced charge separation and mobility enhancement, which finally improved the efficiency of the photoreduction of Cr(VI). diffractometer with Cu Kα radiation. Fourier transform infrared (FT IR) spectra in the range 4000 to 400 cm 1 were recorded with a Nicolet 6700 infrared spectrophotometer with KBr pellets. Thermogravimetric analysis (TGA) at 70 800 C was executed using a DTU 3c thermal analyzer in air atmosphere with α Al 2 O 3 as reference. UV visible diffuse reflectance spectra (UV vis DRS) of solid samples were obtained with a PerkinElmer Lambda 650S spectrophotometer, in which BaSO 4 was used as the reference with 100% reflectance. Determination of the surface area of composites via nitrogen adsorption desorption isotherms was conducted with a BELSORPmini II surface area analyzer at 77 K using the BET (Brunauer Emmett Teller) nitrogen absorption method. The morphologies of the samples were observed using a JEM 1200EX transmission electron microscopy (TEM) instrument and a Tecnai G2 F20 S TWIN (HRTEM). X ray photoelectron spectroscopy (XPS) was carried out with a Thermo ESCALAB 250XI. 2.2 Preparation of BUC 21/g C 3 N 4 composites BUC 21 was hydrothermally synthesized as described in a previous report. [13] Briefly, a mixture of cis 1,3 dibenzyl 2 imidazolidone 4,5 dicarboxylic acid (0.1063 g), 4,4 bipyridine (0.0469 g) and ZnCl 2 (0.0409 g) was sealed in a 25 ml Teflon lined stainless steel pressure vessel containing ultrapure water (18 ml), then heated at 160 C for 72 h. The g C 3 N 4 was prepared by urea cracking at high temperature without the aid of templates, in which 10 g of urea pellets was heated to 500 C in a covered crucible using a muffle furnace for 2 h. Light yellow products were obtained, which were further treated by being ultrasonically dispersed in deionized water for 6 h and being separated via centrifugation. [45] The g C 3 N 4 and BUC 21 powder were mixed by ballmilling at 30 Hz for 20 min. The composite materials with various ratios of g C 3 N 4 to BUC 21 are detailed in Table 1. 2.3 Electrochemical tests A Metrohm Autolab PGSTAT204 electrochemical station in a typical three electrode mode was used to accomplish 2 EXPERIMENTAL 2.1 Materials, instrumentation and methods All chemicals were of reagent grade, which were used without further treatment. Powder X ray diffraction (PXRD) patterns were obtained using a DX 2700B X ray TABLE 1 BUC 21/g C 3 N 4 with various dosages of g C 3 N 4 Sample no. BUC 21 (mg) g C 3 N 4 (mg) Sample name 1 100 50 B100G50 2 100 100 B100G100 3 100 150 B100G150 4 100 200 B100G200

3of11 the electrochemical tests. FTO glass was used as the working electrode. After ultrasonic cleaning with ethanol for 15 min, the electrode was dried in air. An amount of 5.0 mg of BUC 21, g C 3 N 4 or B100G100 powder sample was mixed with 400.0 μl of ethanol Nafion (19/1 v/v), which was further dispersed under sonication for 30 min. An amount of 10.0 μl of prepared slurry was drop cast onto the surface of FTO substrate (1.0 cm 1.0 cm), then dried in air for 30 min. The reference electrode and the counter electrode were a Pt electrode and a saturated Ag/AgCl electrode, respectively. The band positions of BUC 21 and g C 3 N 4 were measured by an impedance potential method of Mott Schottky tests. Electrochemical impedance spectroscopy (EIS) measurements were carried out in darkness, with frequencies of 500, 1000 and 1500 Hz. All of the tests were conducted in a typical three electrode mode with 0.2 M Na 2 SO 4 solution (ph = 7.3) as the electrolyte. 2.4 Photocatalytic activity A 200 ml K 2 Cr 2 O 7 aqueous solution (10 mg l 1 ) was selected as Cr(VI) model to test the photocatalytic performances of the composites (50.0 mg) under simulated sunlight irradiation with a 500 W xenon lamp (Beijing Aulight Co. Ltd), and light source spectrum (>400 nm) is shown in Figure S1. During photocatalytic processes, 2 ml of solution was extracted using a 0.45 μm filter at 20 min intervals for further analysis. The Cr(VI) concentration was measured at 540 nm using the DPC method [46] with a Laspec Alpha 1860 spectrometer. 3 RESULTS AND DISCUSSION 3.1 Characterization All characteristic peaks of g C 3 N 4 and BUC 21 were detected in the FT IR spectra of the series BUC 21/g C 3 N 4 composites, as demonstrated in Figure 1a. The spectrum of the original g C 3 N 4 showed a peak at 810 cm 1, which is a typical peak for heptazine rings (C 6 N 7 ). [47] The peak at 2861 cm 1 corresponds to the stretching vibration of ν( CH 2 ) of BUC 21. [13] There are two obvious peaks at 13.1 and 27.2 in the PXRD pattern of pure g C 3 N 4 as shown in Figure 1b, which were assigned to the typical interplanar laminated graphite tiered structure. [48] As for all the composites, their peaks at 10.2, 13.6, 14.8, 15.9, 20.5, 24.5 and 25.5 originated from the pure BUC 21. The TGA results (Figure 1c) demonstrated that with BUC 21 content increasing in BUC 21/g C 3 N 4 FIGURE 1 (a) FT IR spectra, (b) PXRD patterns, (c) TGA curves and (d) UV vis DRS and E g plot (inset) of BUC 21, g C 3 N 4 and the series of composites

4of11 composites, the loss of organic components (H 2 L and 4,4 bpy ligands) in BUC 21 results in larger weight loss, and the residual weight matched well with the ones of the different BUC 21 content in the composites. The thermal stability of the composites is consistent with that of BUC 21, indicating that BUC 21 was first lost in the weight loss process. The residual quantity is BUC 21 > B100G50 > B100G100 > B100G150 = B100G200 > g C 3 N 4 when the temperature rises to 800 C. The g C 3 N 4,as an organic polymer, will be decomposed completely into volatile compounds in air under elevated temperature. Hence, ZnO is the residual from BUC 21 in the composites. The UV vis DRS spectra (Figure 1d) indicated that the original BUC 21 showed selective absorption in the UV region (250 300 nm) with E g value being 3.4 ev, implying that BUC 21 could demonstrate photocatalytic activity only upon the irradiation of UV light. The addition of g C 3 N 4 onto BUC 21 resulted into the absorption region being shifted from UV light (E g = 3.4 ev) to visible light (E g = 2.95 ev), which indicated that the series of composites could exhibit photocatalytic performances. The successful fabrication of BUC 21/g C 3 N 4 composites was further verified using TEM and HRTEM. As shown in Figure 2 and Figure S2, the edge of original BUC 21 was smooth and the particle sizes were distributed in the range 100 500 nm. The edge of BUC 21 in the composites became rough after the introduction of g C 3 N 4, which resulted from the interaction between g C 3 N 4 and BUC 21. But the HRTEM image (Figure S2) was not attainable as g C 3 N 4 and BUC 21 were solids with low crystalline quality, which was similar to g C 3 N 4 /MIL 53(Fe), [41] g C 3 N 4 /NH 2 MIL 88B(Fe) [49] and g C 3 N 4 /UiO 66. [50] And the nitrogen adsorption experiments demonstrated that the BET surface area increased from 1.06 m 2 g 1 for BUC 21 to 11.65 m 2 g 1 for B100G100 as presented in Table 2, which further confirmed the successful combination between g C 3 N 4 and BUC 21. As illustrated in Figure 3, the chemical compositions and states of B100G100 were further investigated using XPS. The C 1s spectrum of B100G100 can be divided into five peaks centered at 284.8, 285.7, 286.2, 288.1 and 288.8 ev (Figure 3b), in which the peaks at 284.8 and 285.7 ev can be assigned to C C and C H bonds, [51,52] the peaks at 286.2 and 288.1 ev can be attributed to C N C andc (N) 3 of g C 3 N 4, [42] and the peak at 288.8 ev can be assigned to O C O speciesofbuc 21. [42,53] The N 1s spectrum for B100G100 is also presented in Figure 3c, in which the peaks at 398.5 and 399.6 ev revealed the presence of C N C andn (C) 3 or H N (C) 2, respectively. [54] The oxygen atom in Zn OH and hydroxyl group forms canbeevidencedbythepeakswithbindingenergies of 531.4 ev [55] and 532.0 ev, [42] as illustrated in Figure 3d. The presence of Zn in the composites was confirmed by the peaks of Zn 2p 3/2 at 1022.6 ev (between 1022 and 1024 ev) and Zn 2p 1/2 at 1045.7 ev (between 1045 and 1047 ev). [56] TABLE 2 Surface area of BUC 21, B100G100 and g C 3 N 4 Material a s,bet (m 2 g 1 ) a s,lang (m 2 g 1 ) BUC 21 1.06 1.69 B100G100 11.65 21.40 g C 3 N 4 34.02 52.58 FIGURE 2 TEM images of (a) BUC 21, (b) B100G50, (c) B100G100, (d) B100G150, (e) B100G200 and (f) g C 3 N 4

5of11 FIGURE 3 composite (a) XPS survey spectrum; (b) C 1s spectrum, (c) N 1s spectrum, (d) O 1s spectrum and (e) Zn 2p spectrum of B100G100 3.2 Photocatalytic performance 3.2.1 Photocatalytic Cr(VI) reduction The photocatalytic activities of the prepared samples towards Cr(VI) reduction were evaluated under simulated sunlight illumination. As depicted in Figure 4, the series of BUC 21/g C 3 N 4 composites demonstrated better photocatalytic activities towards Cr(VI) reduction than BUC 21 or pure g C 3 N 4 under identical conditions, for which only 13 and 18% Cr(VI) reduction was accomplished after 120 min in the presence of BUC 21 and g C 3 N 4, respectively. However, the photocatalytic Cr(VI) reduction activities were markedly increased for the series of BUC 21/g C 3 N 4 composites, evidenced by the photocatalytic rate in the following order: B100G100 > B100G50 > B100G150 > B100G200 > g C 3 N 4 > BUC 21. As can be seen in Figure 4 and evident from Table 3, Cr(VI) was completely deoxidized within 80 min with B100G100 as photocatalyst, indicating that B100G100 possesses the longest carrier lifetime because of the formation of heterostructures between g C 3 N 4 and BUC 21. [41] 3.2.2 Influence of initial ph value Initial ph value of the solution can exert a great effect on Cr(VI) photoreduction. [57] In the present work, the photocatalytic Cr(VI) reduction efficiencies were determined

6of11 is dominant. [23,60] Also, the formation of Cr(OH) 3 precipitate would mask the active sites of B100G100 at ph > 6, resulting in the decline of its photocatalytic activity. [61] 14H þ þ Cr 2 O 7 2 þ 6e 2Cr 3þ þ 7H 2 O (1) 2H 2 O þ 2h þ H 2 O 2 þ 2H þ (2) CrO 2 4 þ 4H 2 O þ 3e CrðOHÞ 3 þ 5OH (3) 3.2.3 Influence of diverse hole scavengers FIGURE 4 Photoreduction of Cr(VI) over samples under simulated sunlight irradiation at ph = 2, 3, 4, 5, 6, 7 and 8. In Figure 5a, with an increase of ph, the efficiencies of Cr(VI) reduction over B100G100 decreased. In Figure S3, the well matched PXRD patterns of fresh B100G100 and that immersed in H 2 SO 4 or NaOH (ph = 6 or 8) for 6 h confirmed that the composite was stable. The photocatalytic Cr(VI) reaction occurs following Equations (1) and (2) under acidic conditions, and the plentiful H + further promotes the reaction. [58,59] While in alkaline medium, the reaction is carried out according to Equations (2) and (3) as CrO 4 2 In order to obtain the best conditions for photocatalytic reaction, various organic compounds, namely citric acid, tartaric acid and oxalic acid, were selected to investigate their influences on Cr(VI) reduction at ph = 2.0. The rate of reduction of Cr(VI) can be improved by adding the above stated organic compounds following the order of tartaric acid > citric acid > oxalic acid, as shown in Figure 5b. It was characterized by pseudo first order kinetics as illustrated in Equation (4) to further study and understand the kinetics of Cr(VI) reduction by adding hole scavengers: lnðc 0 =C t Þ ¼ kt (4) where C 0 is the initial Cr(VI) concentration after adsorption equilibrium under dark condition, C t is the Cr(VI) TABLE 3 Photocatalytic Cr(VI) reduction efficiencies (%) of various composite catalysts BUC 21 g C 3 N 4 B100G50 B100G100 B100G150 B100G200 80 min 7 12 93 100 74 68 120 min 13 18 100 100 97 89 FIGURE 5 (a) Influence of initial ph on Cr(VI) reduction. (b) Photoreduction of Cr(VI) with various hole scavengers. Conditions: B100G100 = 50 mg, Cr(VI) = 10 mg l 1, 200 ml, ph = 2.0

7of11 concentration at any time t during the photocatalytic reaction process, k is the apparent rate constant and t is the photocatalytic reaction time (min). [62,63] In Figure 5 b (inset), the k values decreased following the order tartaric acid (0.099 min 1 ) > citric acid (0.097 min 1 ) > oxalic acid (0.084 min 1 ) > no hole scavenger (0.032 min 1 ), which can be ascribed to the number of functional groups of α hydroxyl carboxylate (two, one and zero α hydroxyl groups in tartaric acid, citric acid and oxalic acid, respectively). [64,65] 3.2.4 Influence of foreign ions According to the literature, [66 69] the presence of foreign ions including inorganic salts and organic matter will influence the photocatalytic efficiency. In order to explore the practical application of BUC 21/g C 3 N 4 composites, Cr(VI) aqueous solutions prepared with tap water and lake water were photocatalytically treated with B100G100 as photocatalyst under simulated sunlight. The water quality parameters of lake water and tap water are presented in the Table S1 (supporting information). As illustrated in Figure 6a, B100G100 exhibited outstanding photocatalytic activity in the simulated actual water samples under simulated sunlight irradiation. In detail, the Cr(VI) reduction efficiencies in tap water and lake water were still more than 96% within 120 min, indicating that the foreign ions did not inhibit strongly the reduction of Cr(VI). 3.2.5 Photocatalytic Cr(VI) reduction under real sunlight Photoreduction of Cr(VI) over B100G100 as photocatalyst under real sunlight in a field experiment was also carried out. The real experimental setup and the spectrum of real sunlight are illustrated in Figures S4 and S5. In Figure 6b, the elevated temperature resulting from the irradiation of real sunlight and without circulating condensate water led to enhanced adsorption performance (35% in field experiment versus 5% at room temperature in the laboratory) of B100G100 towards Cr(VI). Also, nearly 100% Cr(VI) reduction was achieved under real sunlight within 60 min, which was more efficient than under simulated sunlight (100 min), possibly due to elevated temperature in the field experiment. 3.2.6 Reusability and stability of B100G100 It was essential to test the reusability and stability of the B100G100 photocatalyst to investigate its potential practical application. The efficiency of photocatalytic Cr(VI) reduction over B100G100 declined slightly after five recycles, as demonstrated in Figure 7a, indicating that B100G100 could be used repeatedly. As illustrated in Figure 7b, the well matched PXRD patterns of B100G100 before and after the photocatalytic reaction confirmed that the composite was stable even after the five reaction cycles. Overall, it can be concluded that B100G100 can be reused repeatedly due to its good reusability and stability. 3.2.7 Photocatalytic mechanism of B100G100 (BUC 21/g C 3 N 4 ) Photoluminescence (PL) analysis was used to study the carrier separation efficiency of the composites. [70 74] The PL spectra of B100G100 and g C 3 N 4 are shown in Figure 8a. There is a strong emission peak at ca 440 nm in the parental g C 3 N 4 PL spectrum, which decreased significantly once it was combined with BUC 21 to fabricate B100G100, implying that B100G100 can maintain the longest lifetime of photon generated electrons and holes. [40,41] It was shown in detail that the formation of BUC 21 and g C 3 N 4 heterostructures could validly improve the separation efficiency of the carriers and thus prolonged the lifetime of the carriers, which is beneficial FIGURE 6 (a) Influence of preparation of Cr(VI) solution with tap/lake water on Cr(VI) reduction. (b) Photoreduction of Cr(VI) under various light sources. Conditions: B100G100 = 50 mg, Cr(VI) = 10 mg l 1, 200 ml, ph = 2.0

8of11 FIGURE 7 (a) Cyclic experiments of B100G100 catalyzed reduction of Cr(VI). (b) PXRD patterns of before and after five recycles for photocatalytic Cr(VI) reduction of B100G100 FIGURE 8 frequencies (a) PL spectra and (b) electrochemical impedance spectra. Mott Schottky curves of (c) g C 3 N 4 and (d) BUC 21 at various for improving the photocatalytic activity of BUC 21/g C 3 N 4 composites. [40,41] EIS measurements of BUC 21, B100G100 and g C 3 N 4 were obtained to further verify the above stated hypothesis. It is well known that the arc diameter in a Nyquist plot is equal to the charge transfer resistance (R ct ) at the semiconductor electrolyte interface, in which a smaller diameter of the Nyquist circle indicates a lower R ct value. [75 77] As seen in Figure 8b, the results showed that R ct of B100G100 is lower than that of BUC 21 and g C 3 N 4, indicating that the photogenerated electrons and holes in BUC 21 can be effectively separated when the interface charge transfer occurs in the electron acceptor to g C 3 N 4. The EIS results were in accordance with the PL

9of11 analyses, implying that the combination of g C 3 N 4 (electron acceptor) and BUC 21 was a useful strategy for improving the efficiency of photocatalysis. The flat band potentials (E FB ) of BUC 21 and g C 3 N 4 were determined via typical Mott Schottky measurements, as illustrated in Figure 8c,d, in which a positive slope of C 2 values versus potential is observed, demonstrating their typical behaviors of n type semiconductors. [78,79] The conduction band (CB) potential (E CB )is close to E FB for n type semiconductors. [79] Therefore, the flat band potentials of g C 3 N 4 and BUC 21 determined from Mott Schottky plots are ca 1.10 and 1.41 ev versus the Ag/AgCl electrode, respectively. Therefore, the CB and lowest unoccupied molecular orbital (LUMO) of g C 3 N 4 and BUC 21 are 0.90 and 1.21 ev versus normal hydrogen electrode (NHE) at ph = 7.3. Taking the band gap values (E g ) into account, the valence band and highest occupied molecular orbital of g C 3 N 4 and BUC 21 are 2.05 and 2.19 ev versus NHE, respectively. As stated above, the Mott Schottky experiments demonstrated that the LUMO and CB of BUC 21 and g C 3 N 4 were 1.21 and 0.9 V at ph = 7.3 (Figure 8c,d), and the band gaps of BUC 21 and g C 3 N 4 were 3.40 and 2.95 ev from the UV vis DRS spectra. According to these values, a possible mechanism of photocatalytic Cr(VI) reduction over B100G100 is proposed as shown in Figure 9. The electron hole pairs can be generated on both BUC 21 and g C 3 N 4 upon irradiation of simulated sunlight. The photoexcited electrons generated from the LUMO of BUC 21 are moved to the CB of g C 3 N 4, which can inhibit the recombination of photogenerated electrons and holes, and provide more free electrons in the LUMO of BUC 21. Consequently, B100G100 demonstrated an enhanced photocatalytic Cr(VI) reduction activity under simulated sunlight irradiation. 4 CONCLUSIONS The facile fabrications of a series of enhanced photocatalytically active BUC 21/g C 3 N 4 composites have been accomplished. The optimal B100G100 demonstrated excellent photocatalytic Cr(VI) reduction activity under simulated sunlight and real sunlight, which is clearly superior to that of pure BUC 21 and g C 3 N 4. The electrochemical measurement results and PL analysis demonstrated that the enhanced Cr (VI) reduction was owing to the effective transference of interfacial charge from photoexcited g C 3 N 4 to BUC 21. And the influences of hole scavengers, ph values and foreign ions on Cr(VI) reduction were investigated. It is worth noting that Cr(VI) can be reduced effectively in the presence of foreign ions (Cr(VI) aqueous solutions being prepared with real tap water or surface water) and under real sunlight. The cycling experiments of the photoreduction of Cr(VI) also indicated the reusability and stability of B100G100 as a photocatalyst. This work further demonstrates that the composites constructed from versatile MOFs and economic g C 3 N 4 will be useful to conduct water treatment in the future. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51578034 and 51878023), 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 (2017A38), the Fundamental Research Funds for Beijing Universities (X18075/X18076/X18124/X18125/X18276) and Scientific Research Foundation of Beijing University of Civil Engineering and Architecture (KYJJ2017033/ KYJJ2017008). ORCID Chong Chen Wang http://orcid.org/0000-0001-6033-7076 REFERENCES [1] J. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen, J. T. Hupp, Chem. Soc. Rev. 2009, 38, 1450. [2] P. Garcíagarcía, M. Müller, A. Corma, Chem. Sci. 2014, 5, 2979. [3] C. J. Doonan, C. J. Sumby, CrystEngComm 2017, 19, 4044. FIGURE 9 Proposed reaction mechanism with the presence of B100G100 photocatalyst [4] O. K. Farha, A. Ö. Yazaydın, I. Eryazici, C. D. Malliakas, B. G. Hauser, M. G. Kanatzidis, S. B. T. Nguyen, R. Q. Snurr, J. T. Hupp, Nat. Chem. 2010, 2, 944.

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