Z-Scheme Photocatalytic CO2 Reduction on a Heterostructure of

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 550−562

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Z‑Scheme Photocatalytic CO2 Reduction on a Heterostructure of Oxygen-Defective ZnO/Reduced Graphene Oxide/UiO-66-NH2 under Visible Light Jingchai Meng, Qian Chen, Jiaqian Lu, and Hong Liu*

ACS Appl. Mater. Interfaces 2019.11:550-562. Downloaded from pubs.acs.org by WESTERN SYDNEY UNIV on 01/10/19. For personal use only.

Department of Chemical Engineering, School of Environmental and Chemical Engineering, Shanghai University, 99 Shangda Road, Shanghai 200444, P. R. China ABSTRACT: The construction of a Z-scheme heterojunction is an effective way to isolate photogenerated electron−holes and enhance the activity of the semiconductor photocatalysts. However, the Z-scheme heterojunctions based on metal−organic frameworks were rarely reported. Herein, a novel oxygen-defective ZnO (O-ZnO)/reduced graphene oxide (rGO)/UiO-66-NH2 Z-scheme heterojunction has been prepared by a facile solvothermal route. The morphologies, structures, and photoelectric characteristics of the acquired materials were characterized in detail. The photocatalytic activity of the O-ZnO/rGO/UiO-66-NH2 heterostructure was assessed by photocatalytic CO2 reduction. The results indicated that the O-ZnO/rGO/UiO-66-NH2 heterostructure could efficiently reduce CO2 to CH3OH and HCOOH, and its activity was significantly superior to that of O-ZnO/UiO-66-NH2 and ZnO/rGO/UiO-66-NH2. Under illumination of visible light, the yield of CH3OH and HCOOH over the O-ZnO/rGO/UiO-66-NH2 heterostructure reached 34.83 and 6.41 μmol g−1 h−1, respectively. The high photoactivity of the O-ZnO/rGO/UiO-66-NH2 heterostructure should be caused by the effective spatial separation of photogenerated electrons and holes via a Z-scheme charge transfer. This research may well present an insight into the design and fabrication of novel Z-scheme photocatalytic systems for environmental remediation and energy conversion. KEYWORDS: metal−organic framework, Z-scheme, CO2 reduction, photocatalytic, oxygen-defective

1. INTRODUCTION Photocatalytic conversion of CO2 into valuable fuels (including CH4, CO, CH3OH HCHO, or HCOOH) has drawn more and more attention because this strategy can not only decrease the concentration of greenhouse gas, but also alleviate the energy shortage. Since Inoue et al. first demonstrated the photoelectrocatalytic reduction of carbon dioxide into hydrocarbon fuels,1 many research studies have been performed to develop highly efficient photocatalysts.2,3 A high-efficiency photocatalytic system for CO2 photoreduction requires photocatalysts with a wide absorption spectrum, high charge separation efficiency, strong redox capability, and long-term stability. However, single-component photocatalysts are usually hard to meet all the above requirements simultaneously. Therefore, composited photocatalytic systems, primarily classified as type-II and Z-scheme in accordance with the transfer pathway of photogenerated charges, are widely utilized to promote charge separation.4,5 The redox capability of photoinduced electrons and holes in the type-II hybrid will be weakened because of the migration of the photoinduced electrons to a more positive conduction band (CB) and the transfer of the photoinduced holes to a more negative valence band (VB), respectively.5,6 In contrast, the Z-scheme composite can realize spatial separation of photoinduced charges and ensure strong redox capability simultaneously.6−16 © 2018 American Chemical Society

Recently, some visible-light-responsive Z-scheme photocatalysts for the reduction of CO2, like Pt@CdS/TiO2,6 CdS/ WO 3, 7 Fe 2V4 O 13/rGO/CdS,8 g-C 3 N4 /ZnO,9 Au@CdS/ TiO 2,6,10 g-C3N4/Bi2WO6,11 BiOI/g-C3N4,12 Ag3PO4/gC3N4,13 and α-Fe2O3/Cu2O,14 have been reported. Unfortunately, a practically viable photocatalyst which possesses satisfactory activity and stability has still not been discovered until now. Therefore, developing novel highly efficient and stable Z-scheme photocatalysts is highly desirable. Metal−organic frameworks (MOFs), composed of organic ligands and inorganic clusters, are a new class of porous inorganic−organic hybrid materials.17−20 Owing to their unique properties, such as high specific surface area, tailorable pore size, designed frame structure, and easy functional group, MOFs have been proved to have promising applications in various fields.21−23 Recently, MOFs have been exploited as photocatalysts for pollutant degradation,18,19,24 water splitting,25 CO2 reduction,26 and organic transformation.20,27 Just as all single-component photocatalysts, MOFs suffer from the fast electron−hole recombination after light excitation.15,26 Combining with other semiconductors to form heterojunctions Received: August 19, 2018 Accepted: December 12, 2018 Published: December 12, 2018 550

DOI: 10.1021/acsami.8b14282 ACS Appl. Mater. Interfaces 2019, 11, 550−562

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ACS Applied Materials & Interfaces

2.5. Preparation of the O-ZnO/rGO/UiO-66-NH2 (OZ/R/U) Composite. The OZ/R/U composite was fabricated via a solvothermal method. 0.1096 g of ATA and 0.1404 g of ZrCl4 were added to 50 mL of DMF to obtain solution A. Meanwhile, 0.250 g of the as-prepared O-ZnO sample and 0.005 g of GO were dispersed in 10 mL of DMF via sonication for 0.5 h to obtain solution B. Then solution A was gradually added to solution B and stirred for 1 h at ambient temperature. Subsequently, the mixture was transferred to a 100 mL Teflon-lined autoclave and crystallized at 120 °C for 48 h. The resulting product was centrifuged, washed with absolute ethanol and deionized water, and dried under vacuum. The mass ratio of OZnO to UiO-66-NH2 was 1:1, and the content of rGO was about 1.5 wt % in the obtained OZ/R/U composite. 2.6. Characterization. The microstructure and morphology of the photocatalysts were analyzed by scanning electron microscopy (SEM, JSM-6700F), transmission electron microscopy (TEM, JEOL 200CX), and high-resolution transmission electron microscopy (HRTEM, JEM-2010F) with an energy-dispersive X-ray (EDX) spectrometer. Crystal phase identification was conducted via a powder X-ray diffraction (XRD, D/MAX-2550) using Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) tests were performed on a PHI ESCA-5000C electron spectrometer. Electron-spin-resonance (ESR) tests were carried out on a JEOL-TE300 spectrometer. The Raman spectra were analyzed by a Renshaw inVia Raman spectrometer with a laser excitation at a wavelength of 785 nm. The UV−vis diffuse reflectance spectra (DRS) of the photocatalysts were recorded on a Hitachi U-3010 spectrophotometer. N2 and CO2 adsorptions were measured on a Micromeritics ASAP 2460 adsorption apparatus. A Hitachi F-7000 fluorescence spectrophotometer was used to measure the photoluminescence (PL) spectra of the samples. Transient timeresolved PL decay measurements were performed on a FLS920 fluorescence spectrometer. 2.7. CO2 Photoreduction. The CO2 photoreduction experiment was undertaken at room temperature in a continuous-flow reaction system using a 300 W Xe lamp combined with a UV cut-off filter (λ > 420 nm) as a light source. In brief, 0.1 g of photocatalyst was dispersed in 100 mL of NaHCO3 aqueous solution (0.1 M) under magnetic stirring. Prior to illumination, CO2 (99.995%) gas was bubbled through the suspension for at least 0.5 h to saturate the solution. Thereafter, the suspension was subjected to irradiation of visible light. The liquid products were monitored by using a GC7900 gas chromatograph with a TM-PLOTU capillary column (30 m × 0.53 mm × 20 μm) and an FID detector every hour. 2.8. Electrochemical Measurements. Photocurrents were measured by a CHI-660 electrochemical workstation (Shanghai Chenhua Instrument, China) with a standard three electrode system. An Ag/AgCl electrode and a Pt foil were applied as the reference electrode and counter electrode, respectively. The photocatalyst was deposited on an indium-tin oxide glass as the working electrode. A 300 W Xe lamp with a UV cut-off filter was used as a light source and a solution of 0.5 M Na2SO4 was employed as the electrolyte. Electrochemical impedance spectroscopy (EIS) was measured under the frequency from 10−2 to 106 Hz in 0.1 M Na2SO4. Mott−Schottky plots were undertaken in the range of −1.0 to +0.5 V (vs Ag/AgCl) at a frequency of 1000 Hz in 0.2 M Na2SO4. 2.9. Investigation of Hydroxyl Radical Formation. The production of hydroxyl radicals (•OH) on the photocatalysts were determined by a PL technique. Coumarin was used as a probe molecule because it can react with •OH to generate a highly fluorescent product, 7-hydroxycoumarin. The PL intensity of 7hydroxycoumarin corresponds to the amount of the hydroxyl radicals produced during the reaction. The detailed experimental procedure was described as below: 100 mg of photocatalysts were dispersed in 100 mL of 0.001 M coumarin aqueous solution. The suspension was allowed to reach an adsorption−desorption equilibrium before irradiation was administered. Next, the mixture was subjected to irradiation of visible light and the intermittent sampling was carried out for analysis per 15 min. A Hitachi F-7000 fluorescence spectrophotometer with an excitation wavelength of 325 nm was used to measure the PL spectra of the produced 7-hydroxycoumarin.

has been demonstrated to be a viable method to address this issue,26,28−30 consequently improving photocatalytic activity. However, almost all of the semiconductor/MOF composites developed to date are traditional type-II heterostructures. The problem is that the reduction ability of electrons and the oxidation ability of the holes are all lower than that of the original. The Z-scheme photocatalytic system can promote the effective separation of the photogenerated charges while maintaining the strong redox capability. Nevertheless, up to now, few reports are related to the MOF-based Z-scheme photocatalytic systems.31 On the other hand, ZnO has been extensively studied because of its good stability, nontoxicity, low-cost, and high photocatalytic activity. However, because of its wide band gap (3.37 eV), ZnO cannot effectively utilize visible light, which limits its practical application. The introduction of oxygen defects in ZnO can extend its optical absorption edge toward visible light.32,33 However, the oxygen-defective ZnO (OZnO) still has the problem of rapid recombination of photoelectron−holes during the photocatalytic process. Therefore, it is very urgent to enhance the separation efficiency of photogenerated charges and boost the photocatalytic activity of O-ZnO. In this paper, for the first time, we construct a novel threecomponent Z-scheme system which utilizes UiO-66-NH2 and O-ZnO as the photocatalysts, and reduced graphene oxide (rGO) as the electron mediator. UiO-66-NH2 is a Zrcontaining MOF, which is characterized by a crystalline structure consisting of hexameric Zr6O32 units and 2-aminoterephthalate linkers.18 UiO-66-NH2 was selected in this study due to its visible-light responsiveness, good stability,18,19 and suitable band edges which can match well with O-ZnO. In this Z-scheme photocatalytic system, the photoinduced electrons in the CB of O-ZnO transferred to rGO, and then recombined with the holes in the VB of UiO-66-NH2, leading to enhanced charge separation efficiency. This three-component composite exhibited excellent photocatalytic activity for CO2 reduction to CH3OH and HCOOH under visible light illumination. Moreover, the ternary Z-scheme photocatalyst revealed good stability. This research may present a new idea for the design and fabrication of novel Z-scheme photocatalysts with superior activity and stability.

2. EXPERIMENTAL SECTION 2.1. Chemicals. 2-Aminoterephthalic acid (ATA), N,N-dimethyl formamide (DMF), Zirconium tetrachloride (ZrCl4), zinc acetate dihydrate (Zn(CH3COO)2·2H2O), sodium hydroxide (NaOH), methanol, graphite powder, sulfuric acid, potassium permanganate, and hydrochloric acid were obtained from Sinopharm Chemical Reagent Co., Ltd. The reagents employed in this study were all of analytical grade. 2.2. Preparation of GO. The fabrication of graphene oxide (GO) was done by the oxidation of natural graphite powder using a modified Hummers’ method.34 2.3. Synthesis of ZnO. Four grams of Zn(CH3COO)2·2H2O was added to 60 mL of anhydrous ethanol. After 60 min of ultrasonic dispersion, the obtained transparent solution was transferred to a 100 mL Teflon-lined autoclave and heated at 180 °C for 4 h. After the reaction, the products were centrifugally separated and washed several times using absolute ethanol and deionized water, then dried at 60 °C for 24 h in a vacuum oven. 2.4. Synthesis of Oxygen-Defective ZnO. The synthesis of OZnO was conducted through a solution conversion method using εZn (OH)2 as a precursor according to the previous literature.32 551


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Figure 1. (a) XRD patterns, (b) ESR spectra, (c) O 1s XPS spectra, and (d) UV−vis diffuse reflectance spectra of O-ZnO and ZnO.

planes of rGO in the pure rGO sample,38 suggesting that the solvothermal method can effectively reduce GO to rGO. The XRD diffraction pattern of the OZ/R/U composite shows the characteristic peaks of O-ZnO and UiO-66-NH2, which indicate that the composite consists of O-ZnO and UiO-66NH2. Nevertheless, the loss of the characteristic peak of rGO may be due to the relatively low diffraction intensity and the low amount of rGO. Besides, there is no characteristic diffraction peak (∼10.6°) of GO,38 demonstrating that GO is successfully reduced to rGO during the solvothermal procedure. To verify the existence of rGO, the Raman tests of OZ/R/U and O-ZnO/UiO-66-NH 2 (OZ/U) were implemented (Figure 2b). In the Raman spectrum of OZnO/rGO/UiO-66-NH2, there are two obvious Raman peaks in 1350 and 1600 cm−1, indicating the D band of the sp3 hybrid structure in rGO and the G band of the ordered sp2 hybrid carbon bond structure, respectively,39,40 which proves the existence of rGO in the O-ZnO/rGO/UiO-66-NH2 material. Figure 2c demonstrates that the O-defects still exist in the O-ZnO/rGO/UiO-66-NH2 hybrid, implying that the pre-existing O-defects were not eliminated during the solvothermal process in preparing O-ZnO/rGO/UiO-66-NH2. As illustrated in Figure 3a, bare UiO-66-NH2 is a cuboid-like morphology with its size being around 100−120 nm. The pristine O-ZnO sample displays nanoplate-like morphology with its length being 100−150 nm (Figure 3b). The SEM (Figure 3c) and TEM (Figure 3d) images of OZ/R/U show that UiO-66-NH2 and O-ZnO are evenly dispersed on rGO nanosheets. Further observations also show that UiO-66-NH2, O-ZnO, and rGO are in close contact. This structure is conducive to the rapid transfer of electrons between UiO-66-

3. RESULTS AND DISCUSSION 3.1. Morphologies and Phase Structures. The XRD patterns of the as-synthesized ZnO and O-ZnO samples are displayed in Figure 1a. Both the XRD peaks can be indexed to the hexagonal würtzite ZnO (JCPDS No. 36-1451).32,35 All samples are well crystallized with no change in the crystal phase of oxygen-defective ZnO. However, the decrease and broadening of the peak intensity of O-ZnO are observed, which may be due to the existence of oxygen deficiencies.35 To verify the presence of oxygen defects, ESR measurements were performed. Figure 1b shows a new signal peak in the ESR specturm of O-ZnO at g = 2.001. The peak at a g-factor of 2.001 can be indexed to the surface oxygen defects in OZnO,32,33,35 which is not observed for ZnO. The surface oxygen vacancies can also be analyzed by XPS. As shown in Figure 1c, the two strong peaks at 529.9 and 531.7 eV appear in the O 1s spectrum of O-ZnO. The peak at 529.9 eV is due to the oxygen bond of Zn−O−Zn, and the peak at 531.7 eV can be assigned to the oxygen ions in the vicinity of the oxygen vacancy.32,33 In the O 1s spectrum of ZnO, the peak at 531.7 eV is very weak. All the results demonstrated above clearly evidence the presence of a high concentration of oxygen defects in O-ZnO. In addition, the UV−vis absorption spectrum of O-ZnO exhibits an obvious red-shift compared to that of ZnO as a result of the rich oxygen vacancy in O-ZnO (Figure 1d).32,33 The crystal phases of the obtained O-ZnO, UiO-66-NH2, rGO, and OZ/R/U were identified by XRD as displayed in Figure 2a. The XRD pattern of pure UiO-66-NH2 is in good agreement with those reported previously.26,36,37 An obvious diffraction peak is present at 25.8° corresponding to (002) 552


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Figure 2. (a) XRD patterns of rGO, O-ZnO, UiO-66-NH2, and OZ/R/U, (b) Raman spectra of OZ/U and OZ/R/U, and (c) ESR spectra of Z/ R/U and OZ/R/U.

Figure 3. SEM images of (a) UiO-66-NH2, (b) O-ZnO, and (c) OZ/R/U; (d) TEM and (e) HRTEM images and (f) EDX spectrum of OZ/R/U.

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Figure 4. XPS spectra of O-ZnO, UiO-66-NH2, and OZ/R/U: (a) survey spectrum of OZ/R/U, (b) Zn 2p, (c) O 1s, (d) Zr 3d, (e) C 1s, and (f) N 1s.

O 1s high-resolution XPS spectrum of OZ/R/U can be deconvoluted into three peaks centered at 531.8, 532.6, and 533.4 eV (Figure 4c), which can be indexed to the lattice oxygen in hexagonal ZnO, oxygen ions in the oxygen-deficient regions and the Zr−O bonds in MOF, respectively.18,26 The Zr 3d peaks of OZ/R/U (Figure 4d) locate at 182.3 and 184.8 eV, indicating the existence of Zr4+.18,26 The C 1s spectrum (Figure 4e) of OZ/R/U can be divided into three peaks centered at 284.8, 285.7, and 288.9 eV, which are originated from graphite carbon, C−NH2, and ATA carboxylic acid (−OOC) group, respectively.18,26,28 The N 1s XPS spectrum (Figure 4f) of OZ/R/U locates at 399.4 eV, which is assigned to the −NH2 group of UiO-66-NH2.18,26,28 In comparison with the bare UiO-66-NH2, the positive shift of C 1s peaks and the negative shifts of Zr 3d and N 1s peaks are observed for the

NH2, O-ZnO, and rGO and the increase in the photogenerated charge separation rate, which in turn enhances its photocatalytic activity (Figure 3c,d). As shown in Figure 3e, the lattice fringe is able to be observed clearly in the HRTEM image of the OZ/R/U sample with a lattice spacing of 0.261 nm, corresponding to the (001) plane of ZnO.32 The EDX spectrum confirms that the OZ/R/U composite contains elements including Zr, Zn, O, C, and N (Figure 3f). Figure 4 shows the XPS spectra of O-ZnO, UiO-66-NH2, and OZ/R/U. The survey spectrum (Figure 4a) of OZ/R/U suggests that the composite mainly consists of Zn, Zr, O, C, and N, which is consistent with the EDX results. The Zn 2p diagram (Figure 4b) of OZ/R/U shows that the sample has obvious absorption peaks at 1022.2 and 1045.3 eV, corresponding to the typical values of Zn2+ in ZnO.41,42 The 554


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Figure 5. (a) Nitrogen adsorption−desorption isotherms, (b) Horvath−Kawazoe pore-size distribution curves, (c) Barrett−Joyner−Halenda poresize distribution curves of UiO-66-NH2, O-ZnO, and OZ/R/U, and (d) CO2 adsorption isotherms of UiO-66-NH2, O-ZnO, OZ/R/U, and Z/R/ U.

Figure 6. (a) UV−vis DRS and (b) band gap energies of O-ZnO, UiO-66-NH2, and OZ/R/U.

the gaps and voids between the O-ZnO nanoplates. The adsorption isotherm type of the OZ/R/U sample is a mixed isotherm type of types I and IV, indicating that the composite material has both micropores and mesopores, which can be further proved by the pore-size distributions as illustrated in Figure 5b,c. The Brunauer−Emmett−Teller specific surface area of UiO-66-NH2 and O-ZnO is 959.3 and 21.3 m2 g−1, respectively. As for the OZ/R/U composite, its specific surface area is between O-ZnO and UiO-66-NH2, which is about 877.3 m2 g−1. Figure 5d shows the CO2-adsorption behavior of UiO-66NH2, O-ZnO, and OZ/R/U at ambient temperature. It is found that the maximum CO2 uptake for UiO-66-NH2, OZ/ R/U, and O-ZnO are 10.7, 6.6, and 1.3 cm3 g−1, respectively.

OZ/R/U sample. In addition, the Zn 2p peaks and O 1s peaks of OZ/R/U shift slightly to higher binding energies in comparison to those of O-ZnO. The shifts in binding energy suggest the presence of electronic interaction among the composite components, rather than a simple physical mixture. The N2 adsorption−desorption method was further used to evaluate the porosity and specific surface area of UiO-66-NH2, O-ZnO, and OZ/R/U. As illustrated in Figure 5a, UiO-66NH2 exhibits a typical type I isotherm indicative of the microporous structure.26,43 Besides, three main pores at 0.54, 0.92, and 1.3 nm are revealed in the pore-size distribution of UiO-66-NH2 (Figure 5b). The prepared O-ZnO shows a type IV isotherm with a distinct H3 hysteresis loop that implies the existence of mesopores.26,44 These mesopores are derived from 555


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Figure 7. Mott−Schottky plots of (a) O-ZnO and (b) UiO-66-NH2.

Figure 8. (a) Original chromatograms for the as-synthesized samples after 6 h of irradiation, (b) CH3OH evolution amount vs illumination time, (c) HCOOH evolution amount vs illumination time, and (d) CO2 reduction rate of the as-prepared samples.

possibly caused by the electronic interaction between the two semiconductors.45,46 Similar observations can also be observed for other MOF-based catalysts.28,30 The improved adsorption of the ternary OZ/R/U composite in the visible light range can result in the production of more electron−hole pairs and thereby favorably enhances the activity of photocatalysts. Here, the band gap of the as-prepared UiO-66-NH2 and O-ZnO can be calculated by using the following equation: αhν = A(hν − Eg)1/2,26,32,33 where α, h, ν, and Eg represent the absorption coefficient, Planck’s constant, the optical frequency, and the band gap energy, respectively. A is a constant. From the plot of (αhν)2 versus (hν) displayed in Figure 6b, the band gap value of UiO-66-NH2 is estimated to be 2.82 eV, close to the reported values.26,36 Furthermore, the band gaps of O-ZnO is calculated to be 2.98 eV.

The order of the CO2-adsorption capacity of these three samples is nearly in accordance with that of their surface area. To explore the effect of oxygen vacancies, the CO2-adsorption behavior of the Z/R/U composite was also investigated. As shown in Figure 5d, Z/R/U exhibits a CO2 adsorption capacity of 4.7 cm3 g−1, which is lower than that of OZ/R/U. This indicates that the existence of oxygen defects is conducive to the CO2 adsorption. The optical absorption of O-ZnO, UiO-66-NH2, OZ/R/U, and the physical mixture (O-ZnO + rGO + UiO-66-NH2) were tested by diffuse reflectance UV−vis spectroscopy. As presented in Figure 6a, there is a strong absorption in the range of 200−440 nm observed in pristine UiO-66-NH2. The absorption edge observed for O-ZnO is at 425 nm. Compared with pure O-ZnO, UiO-66-NH2, and the physical mixture, OZ/R/U exhibits enhanced visible-light absorption, which is 556


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photoinduced electrons, thus hindering the recombination of photogenerated charge carriers in ZnO and improving the photocatalytic efficiency.52,53 The effect of the rGO content on the photoactivity of OZnO/rGO/UiO-66-NH2 was also studied. It can be found from Figure 9 that the rate of the photocatalytic CO2 reduction

The conduction bands of O-ZnO and UiO-66-NH2 were also determined by the Mott−Schottky method, as demonstrated in Figure 7. The positive slopes suggest that both samples are n-type semiconductors. From the Mott−Schottky plots, it is observed that the flat band potential of O-ZnO and UiO-66-NH2 are about −0.39 V vs Ag/AgCl (i.e., −0.19 V vs normal hydrogen electrode (NHE)) and −0.75 V vs Ag/AgCl (i.e., −0.55 V vs NHE), respectively. Because the flat band potential is about 0.10 V below the bottom of the conduction band for n-type semiconductors,28,47 the CB positions of OZnO and UiO-66-NH2 can be determined to be −0.29 and −0.65 V (vs NHE), respectively. According to the band gap values of O-ZnO and UiO-66-NH2 gained from DRS analysis (Figure 6b), the VB positions of O-ZnO and UiO-66-NH2 can be calculated to be +2.69 and 2.17 V (vs NHE), respectively. 3.2. Photocatalytic Activity. With the aim of assessing the photocatalytic activities of the as-prepared photocatalysts, we perform the photocatalytic reduction of CO2 under visible light irradiation in an aqueous system. Under the experimental conditions, only CH3OH and HCOOH are detected as liquidphase products (Figure 8a). Control experiment indicates that no products can be detected when high purity of N2 gas instead of CO2 was injected into the reaction system, confirming that CO2 should be the source of CH3OH and HCOOH. Figure 8b−d show the comparison of photocatalytic activities of different photocatalysts. It can be seen that O-ZnO is not active for the photocatalytic reduction of CO2 because the conduction band position (−0.29 eV) of O-ZnO is lower than the potential of CO2 reduction to the corresponding hydrocarbon.48,49 The previous literature studies have reported that UiO-66-NH2 had a certain ability to reduce CO2.50,51 However, no detectable reduction products are found in this experiment when UiO-66-NH2 is utilized as a photocatalyst alone. The possible reason is that the yield of reduction is too low to be detected by the instrument. After coupling with OZnO to form an O-ZnO/UiO-66-NH2 (OZ/U) heterostructure, the photocatalytic activity is significantly enhanced. The yield of CH3OH and HCOOH on the OZ/U catalyst is 19.67 and 4.94 μmol g−1 h−1, respectively. This increase is caused by the forming of a direct Z-scheme heterojunction between OZnO and UiO-66-NH2 (this can be confirmed by the hydroxyl radical experiments as shown in Figure 13), thus promoting the migration and separation of photoinduced electron−hole pairs and further boosting the photocatalytic activity. Meanwhile, the photocatalytic activity of O-ZnO/rGO/UiO-66NH2 is obviously superior to O-ZnO/UiO-66-NH2, in which the yields of CH3OH and HCOOH reach 34.83 and 6.41 μmol g−1 h−1, respectively. This is owing to the rGO as a kind of effective electronic media, which can not only increase the contact area and tightness of the two semiconductors, but also create a new electron transfer bridge, which is beneficial to the transmission and separation of photoinduced charges, thus significantly increasing the photocatalytic activity. In addition, to study the effect of oxygen vacancies, we also tested the photocatalytic performance of ZnO/rGO/UiO-66-NH2 (Z/R/ U) for comparison. It is found that the yield of CH3OH and HCOOH on the Z/R/U catalyst is 14.86 and 4.20 μmol g−1 h−1, only 42.7 and 65.5% of OZ/R/U, indicating the existence of oxygen vacancies is beneficial to the enhancement of photocatalytic activity. Possible reasons are as follows: (1) the presence of oxygen vacancies can favor the CO2 adsorption on the surfaces of the photocatalyst as demonstrated in Figure 5d. (2) The oxygen vacancies may act as the capture centers of the

Figure 9. Effect of the rGO content on the photoactivity of the OZ/ R/U photocatalyst.

initially increases with an increase in the rGO amount in the OZ/R/U composite, and then decreases with the further increase in the rGO content. The decreased activity of the photocatalyst with a weight ratio higher than 1.5 wt % is probably attributed to the fact that excessive rGO may have absorbed more visible light and competed with O-ZnO/UiO66-NH2 at absorbing light.54 Thus, the optimum rGO content is 1.5 wt %. In addition to photocatalytic activity, the catalyst lifetime is also of great significance. So, the recycling experiments for the O-ZnO/rGO/UiO-66-NH2 photocatalyst were performed. After six cycles, no noticeable decrease in the photocatalytic activity is observed (Figure 10a). Moreover, there is no obvious change in the crystal structure (Figure 10b) and morphology (Figure 10c) of the O-ZnO/rGO/UiO-66-NH2 composite after the reaction. The aforementioned results clearly illustrate that the as-prepared OZ/R/U composite has relatively high stability during the photocatalytic CO2 reduction. 3.3. Enhancement Mechanism of Photocatalytic Activity. To fully understand the outstanding photocatalytic ability of the O-ZnO/rGO/UiO-66-NH2 heterostructure, the steady-state and time-resolved transient PL, photocurrent, and EIS tests were performed. Figure 11a reveals the steady-state PL emission spectra of O-ZnO, UiO-66-NH2, and the OZ/R/ U composite excited at 380 nm. It is observed that the pristine O-ZnO exhibits a strong emission near 510 nm. The emitting peak at around 465 nm is observed for bare UiO-66-NH2. Compared to pure UiO-66-NH2 and O-ZnO, the OZ/R/U composite shows the weak PL intensity, indicating the efficient interfacial electron separation between UiO-66-NH2 and OZnO. The time-resolved transient PL spectra of O-ZnO, UiO66-NH2, and OZ/R/U are shown in Figure 11b. Multiexponential decays are observed from the curvatures of the plots. Two radiative lifetimes with different percentages are obtained by fitting the decay spectra using bi-exponential models (inset of Figure 11b). The two lifetimes (τ1 and τ2) of OZ/R/U are 1.03 and 6.31 ns, respectively, which are longer than the corresponding lifetimes of O-ZnO or UiO-66-NH2. Besides, OZ/R/U has a higher average fluorescence lifetime 557


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Figure 10. (a) Cycle runs of CO2 reduction on the OZ/R/U photocatalyst, (b) XRD patterns of OZ/R/U before and after the photocatalytic reaction, and (c) SEM image of OZ/R/U after the photocatalytic reaction.

Figure 11. (a) PL spectra, (b) time-resolved transient PL decay, (c) photocurrent responses, and (d) EIS of UiO-66-NH2, O-ZnO, and OZ/R/U.

(τA) of 5.99 ns than that of O-ZnO (1.25 ns) and UiO-66-NH2 (1.71 ns). The increased fluorescent lifetime further indicates

the suppressed recombination of photogenerated carriers in the OZ/R/U composite. 558


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Figure 12. Schematic illustration of the charge transfer and separation in O-ZnO/rGO/UiO-66-NH2: (a) type-II heterojunction and (b) Z-scheme mechanism.

Figure 13. (a) PL spectral evolution of the OZ/R/U sample against illumination time and (b) comparison of the PL peak intensity around 430 nm for O-ZnO, UiO-66-NH2, and OZ/R/U.

O-ZnO and then to the surface of rGO, and the holes will migrate from the VB of O-ZnO to that of UiO-66-NH2. Nevertheless, the photogenerated electrons on the CB of OZnO can hardly reduce CO2 to CH3OH and HCOOH owing to the fact that the CB potential of O-ZnO (−0.29 eV) is more positive compared to the CO2 reduction potential (ECO2/CH3OH = −0.38 eV and ECO2/HCOOH = −0.61 eV).48,49 In contrast, if followed by the Z-scheme photocatalytic mechanism (Figure 12b), the photoinduced electrons on the CB of O-ZnO will transfer through rGO to the VB of UiO-66-NH2 and combine with the photoinduced holes, resulting in the accumulation of the electrons on the CB of UiO-66-NH2 and the accumulation of the holes on the VB of O-ZnO. Because the CB level of UiO-66-NH2 is more negative compared to the CO2 reduction potential, the electrons accumulated on the CB of UiO-66NH2 can easily reduce CO2 to CH3OH and HCOOH. Therefore, based on the above analysis, the electron transport between the two semiconductors obeys the Z-scheme mechanism rather than the traditional type-II heterojunction mechanism. It is worth noting that this Z-scheme charge transfer process can not only decrease the recombination rate of photogenerated charge carriers, but also maintain the high reduction capacity of the photogenerated electrons on the more negative CB of UiO-66-NH2 and the strong oxidation capacity of the photoinduced holes on the more positive VB of O-ZnO. To confirm the Z-scheme mechanism proposed above, hydroxyl radicals generated on the surfaces of pure O-ZnO,

The efficiency of interface charge separation can be further characterized by means of the photoelectrochemical technique. The transient photocurrent responses of O-ZnO, UiO-66NH2, and OZ/R/U composites are shown in Figure 11c. It can be found that the pure O-ZnO and UiO-66-NH2 have low photocurrent intensity as a result of the rapid recombination of their photogenerated charges. The photocurrent intensity of the OZ/R/U sample is significantly higher than those of OZnO and UiO-66-NH2. The result shows that the OZ/R/U sample has high charge transfer and separation capability. Figure 11d shows that the Nyquist semicircle diameter of the OZ/R/U sample is much smaller than that of O-ZnO and UiO-66-NH2. This phenomenon indicates that the electrochemical impedance of the OZ/R/U sample is far less than OZnO and UiO-66-NH2. Therefore, all the results shown above clearly demonstrate that the OZ/R/U hybrid has more efficient transportation and separation of photoinduced carries, which should be the reason for the enhancement of photocatalytic activity. On the basis of the band gap structure of O-ZnO and UiO66-NH2, there are two possible photocatalytic mechanisms in the O-ZnO/rGO/UiO-66-NH2 composite: (I) the conventional type-II heterojunction mechanism and (II) the Zscheme mechanism. As displayed in Figure 12, O-ZnO and UiO-66-NH2 can be excited by visible light to generate electrons and holes. If the charge transfer in the interface phase of the OZ/R/U composites follows the conventional type-II heterojunction mechanism (Figure 12a), the photoinduced electrons on the CB of UiO-66-NH2 will transfer to the CB of 559



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ACKNOWLEDGMENTS This research was funded by the National Natural Science Foundation of China (11472164) and Innovative Research Team Project (IRT13078).

UiO-66-NH2, and OZ/R/U upon visible light illumination were detected by PL spectroscopy using coumarin as a probe molecule. Figure 13a shows the spectral evolution of the OZ/ R/U sample against the illumination time, and Figure 13b illustrates the comparison of the PL intensity around 430 nm of O-ZnO, UiO-66-NH2, and OZ/R/U. It can be observed from Figure 13b that the PL intensity of O-ZnO and OZ/R/U gradually increases with the increased illumination time, demonstrating the formation of •OH radicals under irradiation. For pure UiO-66-NH2 sample, no PL signal is detected under the same conditions, manifesting that none of the hydroxyl radicals are formed. This is because the VB potential (2.17 V vs NHE) of UiO-66-NH2 is more negative than that of OH−/•OH (2.3 V vs NHE).7 The photoinduced holes on the VB of UiO-66-NH2 are unable to react with OH−/H2O to generate hydroxyl radicals. Compared with pure O-ZnO, the OZ/R/U sample displays higher fluorescence intensity. The enhanced PL intensity can be attributed to the Z-scheme charge transfer over the OZ/R/U photocatalyst (Figure 12b), which results in the enrichment of holes on the VB of O-ZnO to generate more •OH radicals. If the obtained OZ/R/U composite complies with the type-II heterojunction mechanism as illustrated in Figure 12a, the photogenerated electrons are likely transferred from the CB of UiO-66-NH2 to that of OZnO, and the holes are transferred from the VB of O-ZnO to that of UiO-66-NH2. The holes on the VB of UiO-66-NH2 cannot react with OH−/H2O to produce •OH radicals. Consequently, the traditional OZ/R/U heterojunction should have no PL signals. Nevertheless, this hypothesis contradicts the results of the •OH experiments. Hence, the photogenerated electron−hole transfers on the OZ/R/U composite follow the Z-scheme mechanism instead of the classical heterojunction mechanism.

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REFERENCES

(1) Inoue, T.; Fujishima, A.; Konishi, S.; Honda, K. Photoelectrocatalytic Reduction of Carbon Dioxide in Aqueous Suspensions of Semiconductor Powders. Nature 1979, 277, 637−638. (2) Mao, J.; Li, K.; Peng, T. Y. Recent Advances in the Photocatalytic CO2 Reduction over Semiconductors. Catal. Sci. Technol. 2013, 3, 2481−2498. (3) Marszewski, M.; Cao, S. W.; Yu, J. G.; Jaroniec, M. Semiconductor-Based Photocatalytic CO2 Conversion. Mater. Horiz. 2016, 1, 261−278. (4) Sarkar, D.; Ghosh, C. K.; Mukherjee, S.; Chattopadhyay, K. K. Three Dimensional Ag2O/TiO2 Type-II (p-n) Nanoheterojunctions for Superior Photocatalytic Activity. ACS Appl. Mater. Interfaces 2013, 5, 331−337. (5) Tada, H.; Mitsui, T.; Kiyonaga, T.; Akita, T.; Tanaka, K. AllSolid-State Z-Scheme in CdS-Au-TiO2 Three-Component Nanojunction System. Nat. Mater. 2006, 5, 782−786. (6) Wei, Y. C.; Jiao, J. Q.; Zhao, Z.; Zhong, W. J.; Li, J. M.; Liu, J.; Jiang, G. Y.; Duan, A. J. 3D Ordered Macroporous TiO2-Supported Pt@CdS Core-Shell Nanoparticles: Design, Synthesis and Efficient Photocatalytic Conversion of CO2 with Water to Methane. J. Mater. Chem. A 2015, 3, 11074−11085. (7) Jin, J.; Yu, J. G.; Guo, D. P.; Cui, C.; Ho, W. A Hierarchical ZScheme CdS-WO3 Photocatalyst with Enhanced CO2 Reduction Activity. Small 2015, 11, 5262−5271. (8) Li, P.; Zhou, Y.; Li, H. J.; Xu, Q. F.; Meng, X. G.; Wang, X. Y.; Xiao, M.; Zou, Z. G. All-Solid-State Z-Scheme System Arrays of Fe2V4O13/RGO/CdS for Visible Light-Driving Photocatalytic CO2 Reduction into Renewable Hydrocarbon Fuel. Chem. Commun. 2015, 51, 800−803. (9) Yu, W. L.; Xu, D. F.; Peng, T. Y. Enhanced Photocatalytic Activity of g-C3N4 for Selective CO2 Reduction to CH3OH via Facile Coupling of ZnO: A Direct Z-Scheme Mechanism. J. Mater. Chem. A 2015, 3, 19936−19947. (10) Wei, Y. C.; Jiao, J. Q.; Zhao, Z.; Liu, J.; Li, J. M.; Jiang, G. Y.; Wang, Y. J.; Duan, A. J. Fabrication of Inverse Opal TiO2-Supported Au@CdS Core-Shell Nanoparticles for Efficient Photocatalytic CO2 Conversion. Appl. Catal., B 2015, 179, 422−432. (11) Li, M. L.; Zhang, L. X.; Fan, X. Q.; Zhou, Y. J.; Wu, M. Y.; Shi, J. L. Highly Selective CO2 Photoreduction to CO over g-C3N4/ Bi2WO6 Composites under Visible Light. J. Mater. Chem. A 2015, 3, 5189−5196. (12) Wang, J. C.; Yao, H. C.; Fan, Z. Y.; Zhang, L.; Wang, J. S.; Zang, S. Q.; Li, Z. J. Indirect Z-Scheme BiOI/g-C3N4 Photocatalysts with Enhanced Photoreduction CO2 Activity under Visible Light Irradiation. ACS Appl. Mater. Interfaces 2016, 8, 3765−3775. (13) He, Y.; Zhang, L. H.; Teng, B. T.; Fan, M. H. New Application of Z-Scheme Ag3PO4/g-C3N4 Composite in Converting CO2 to Fuel. Environ. Sci. Technol. 2015, 49, 649−656. (14) Wang, J. C.; Zhang, L.; Fang, W. X.; Ren, J.; Li, Y. Y.; Yao, H. C.; Wang, J. S.; Li, Z. J. Enhanced Photoreduction CO2 Activity over Direct Z-Scheme α-Fe2O3/Cu2O Heterostructures under Visible Light Irradiation. ACS Appl. Mater. Interfaces 2015, 7, 8631−8639. (15) Cai, T.; Liu, Y. T.; Wang, L. L.; Zhang, S. Q.; Zeng, Y. X.; Yuan, J. L.; Ma, J. H.; Dong, W. Y.; Liu, C. B.; Luo, S. L. Silver PhosphateBased Z-Scheme Photocatalytic System with Superior Sunlight Photocatalytic Activities and Anti-Photocorrosion Performance. Appl. Catal., B 2017, 208, 1−13. (16) Cai, T.; Liu, Y. T.; Wang, L. L.; Zhang, S. Q.; Dong, W. Y.; Chen, H.; Ma, J. H.; Liu, C. B.; Luo, S. L. Ultrafine Ag@AgI Nanoparticles on Cube Single-Crystal Ag3PO4(100): An All-DayActive Z-Scheme Photocatalyst for Environmental Purification. J. Colloid Interface Sci. 2019, 533, 95−105.

4. CONCLUSIONS In conclusion, a novel O-ZnO/rGO/UiO-66-NH2 Z-scheme heterojunction was achieved by coupling UiO-66-NH2 and rGO with oxygen-defective ZnO. This Z-scheme photocatalyst demonstrates excellent photocatalytic activity for the conversion of CO2 to CH3OH and HCOOH under visible light illumination, in which the yields of CH3OH and HCOOH reaches 34.85 and 6.40 μmol g−1 h−1, respectively. In addition, the composite photocatalyst shows a good stability. The boosted activity of the O-ZnO/rGO/UiO-66-NH2 composite photocatalyst is ascribed to the formation of the Z-scheme photocatalytic system, which can not only inhibit the recombination of photogenerated charge carriers, but also maintain the high reduction ability of UiO-66-NH2 and strong oxidizability of O-ZnO. This study demonstrates an insight into the design and fabrication of novel MOF-based Z-scheme photocatalytic systems.

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Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-21-66137487. Fax: +86-21-66137725. ORCID

Hong Liu: 0000-0002-4926-1960 Notes

The authors declare no competing financial interest. 560


Research Article


Heterostructure for Photocatalytic Hydrogen Generation. J. Colloid Interface Sci. 2017, 503, 198−204. (34) Cote, L. J.; Kim, F.; Huang, J. X. Langmuir-Blodgett Assembly of Graphite Oxide Single Layers. J. Am. Chem. Soc. 2009, 131, 1043− 1049. (35) Wang, J.; Hou, S.; Chen, H. Y.; Xiang, L. Defects-Induced Room Temperature Ferromagnetism in ZnO Nanorods Grown from ε-Zn(OH)2. J. Phys. Chem. C 2014, 118, 19469−19476. (36) Shen, L. J.; Liang, S. J.; Wu, W. M.; Liang, R. W.; Wu, L. CdSDecorated UiO-66(NH2) Nanocomposites Fabricated by a Facile Photodeposition Process: An Efficient and Stable Visible-LightDriven Photocatalyst for Selective Oxidation of Alcohols. J. Mater. Chem. A 2013, 1, 11473−11482. (37) Liang, Q.; Cui, S. N.; Liu, C. H.; Xu, S.; Yao, C.; Li, Z. Y. Construction of CdS@UiO-66-NH 2 Core-Shell Nanorods for Enhanced Photocatalytic Activity with Excellent Photostability. J. Colloid Interface Sci. 2018, 524, 379−387. (38) Ma, D.; Wu, J.; Gao, M. C.; Xin, Y. J.; Ma, T. J.; Sun, Y. Y. Fabrication of Z-Scheme g-C3N4/RGO/Bi2WO6 Photocatalyst with Enhanced Visible-Light Photocatalytic Activity. Chem. Eng. J. 2016, 290, 136−146. (39) Zhu, C.; Guo, S. J.; Fang, Y. X.; Dong, S. J. Reducing Sugar: New Functional Molecules for the Green Synthesis of Graphene Nanosheets. ACS Nano 2010, 4, 2429−2437. (40) Liu, H.; Su, Y.; Chen, Z.; Jin, Z. T.; Wang, Y. Graphene Sheets Grafted Three-Dimensional BiOBr0.2I0.8 Microspheres with Excellent Photocatalytic Activity under Visible Light. J. Hazard. Mater. 2014, 266, 75−83. (41) Vaiano, V.; Matarangolo, M.; Murcia, J. J.; Rojas, H.; Navío, J. A.; Hidalgo, M. C. Enhanced Photocatalytic Removal of Phenol from Aqueous Solutions using ZnO Modified with Ag. Appl. Catal., B 2018, 225, 197−206. (42) Kumar, S.; Baruah, A.; Tonda, S.; Kumar, B.; Shanker, V.; Sreedhar, B. Cost-Effective and Eco-friendly Synthesis of Novel and Stable N-Doped ZnO/g-C3N4 Core-Shell Nanoplates with Excellent Visible-Light Responsive Photocatalysis. Nanoscale 2014, 6, 4830− 4842. (43) Hu, Z.; Peng, Y. W.; Kang, Z. X.; Qian, Y. H.; Zhao, D. A Modulated Hydrothermal (MHT) Approach for the Facile Synthesis of UiO-66-Type MOFs. Inorg. Chem. 2015, 54, 4862−4868. (44) He, F.; Chen, G.; Zhou, Y. S.; Yu, Y. G.; Zheng, Y.; Hao, S. The Facile Synthesis of Mesoporous g-C3N4 with Highly Enhanced Photocatalytic H2 Evolution Performance. Chem. Commun. 2015, 51, 16244−16246. (45) Dong, F.; Zhao, Z. W.; Xiong, T.; Ni, Z. L.; Zhang, W. D.; Sun, Y. J.; Ho, W. K. In Situ Construction of g-C3N4/g-C3N4 Metal-Free Heterojunction for Enhanced Visible-Light Photocatalysis. ACS Appl. Mater. Interfaces 2013, 5, 11392−11401. (46) Ge, L.; Zuo, F.; Liu, J. K.; Ma, Q.; Wang, C.; Sun, D. Z.; Bartels, L.; Feng, P. Y. Synthesis and Efficient Visible Light Photocatalytic Hydrogen Evolution of Polymeric g-C3N4 Coupled with CdS Quantum dots. J. Phys. Chem. C 2012, 116, 13708−13714. (47) Saha, S.; Das, G.; Thote, J.; Banerjee, R. Photocatalytic MetalOrganic Framework from CdS Quantum Dot Incubated Luminescent Metallohydrogel. J. Am. Chem. Soc. 2014, 136, 14845−14851. (48) Habisreutinger, S. N.; Schmidt-Mende, L.; Stolarczyk, J. K. Photocatalytic Reduction of CO2 on TiO2 and Other Semiconductors. Angew. Chem., Int. Ed. 2013, 52, 7372−7408. (49) Kondratenko, E. V.; Mul, G.; Baltrusaitis, J.; Larrazábal, G. O.; Pérez-Ramírez, J. Status and Perspectives of CO2 Conversion into Fuels and Chemicals by Catalytic, Photocatalytic and Electrocatalytic Processes. Energy Environ. Sci. 2013, 6, 3112−3135. (50) Wang, X. J.; Zhao, X. L.; Zhang, D. Q.; Li, G. S.; Li, H. X. Microwave Irradiation Induced UiO-66-NH2 Anchored on Graphene with High Activity for Photocatalytic Reduction of CO2. Appl. Catal., B 2018, 228, 47−53. (51) Sun, D. R.; Fu, Y. H.; Liu, W. J.; Ye, L.; Wang, D. K.; Yang, L.; Fu, X. Z.; Li, Z. H. Studies on Photocatalytic CO2 Reduction over NH2-UiO-66(Zr) and Its Derivatives: Towards a Better Under-

(17) Zeng, L.; Guo, X. Y.; He, C.; Duan, C. Y. Metal-Organic Frameworks: Versatile Materials for Heterogeneous Photocatalysis. ACS Catal. 2016, 6, 7935−7947. (18) Shen, L. J.; Wu, W. M.; Liang, R. W.; Lin, R.; Wu, L. Highly Dispersed Palladium Nanoparticles Anchored on UiO-66(NH2) Metal-Organic Framework as a Reusable and Dual Functional Visible-Light-Driven Photocatalyst. Nanoscale 2013, 5, 9374−9382. (19) Liang, R.; Shen, L. J.; Jing, F. F.; Qin, N.; Wu, L. Preparation of MIL-53(Fe)-Reduced Graphene Oxide Nanocomposites by a Simple Self-Assembly Strategy for Increasing Interfacial Contact: Efficient Visible-Light Photocatalysts. ACS Appl. Mater. Interfaces 2015, 7, 9507−9515. (20) Shen, L.; Liang, S. J.; Wu, W. M.; Liang, R. W.; Wu, L. Multifunctional NH2-Mediated Zirconium Metal-Organic Framework as an Efficient Visible-Light-Driven Photocatalyst for Selective Oxidation of Alcohols and Reduction of Aqueous Cr(VI). Dalton Trans. 2013, 42, 13649−13657. (21) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M.; Yaghi, O. M. Systematic Design of Pore Size and Functionality in Isoreticular MOFs and Their Application in Methane Storage. Science 2002, 295, 469−472. (22) Lammert, M.; Wharmby, M. T.; Smolders, S.; Bueken, B.; Lieb, A.; Lomachenko, K. A.; Vos, D. D.; Stock, N. Cerium-Based Metal Organic Frameworks with UiO-66 Architecture: Synthesis, Properties and Redox Catalytic Activity. Chem. Commun. 2015, 51, 12578− 12581. (23) Huo, J.; Aguilera-Sigalat, J.; El-Hankari, S.; Bradshaw, D. Magnetic MOF Microreactors for Recyclable Size Selective Biocatalysis. Chem. Sci. 2015, 6, 1938−1943. (24) Wang, D.; Jia, F. Y.; Wang, H.; Chen, F.; Fang, Y.; Dong, W. B.; Zeng, G. M.; Li, X. M.; Yang, Q.; Yuan, X. Z. Simultaneously Efficient Adsorption and Photocatalytic Degradation of Tetracycline by FeBased MOFs. J. Colloid Interface Sci. 2018, 519, 273−284. (25) Lan, G.; Zhu, Y. Y.; Veroneau, S. S.; Xu, Z. W.; Micheroni, D.; Lin, W. B. Electron Injection from Photoexcited Metal-Organic Framework Ligands to Ru2 Secondary Building Units for VisibleLight-Driven Hydrogen Evolution. J. Am. Chem. Soc. 2018, 140, 5326−5329. (26) Su, Y.; Zhang, Z.; Liu, H.; Wang, Y. Cd0.2Zn0.8S@UiO-66-NH2 Nanocomposites as Efficient and Stable Visible-Light-Driven Photocatalyst for H2 Evolution and CO2 Reduction. Appl. Catal., B 2017, 200, 448−457. (27) Sun, D. R.; Ye, L.; Li, Z. H. Visible-Light-Assisted Aerobic Photocatalytic Oxidation of Amines to Imines over NH2-MIL125(Ti). Appl. Catal., B 2015, 164, 428−432. (28) Liu, H.; Zhang, J.; Ao, D. Construction of Heterostructured ZnIn2S4@NH2-MIL-125(Ti) Nanocomposites for Visible-Light-Driven H2 Production. Appl. Catal., B 2018, 221, 433−442. (29) Li, X.; Pi, Y. H.; Hou, Q. Q.; Yu, H.; Li, Z.; Li, Y. W.; Xiao, J. Amorphous TiO2@NH2-MIL-125(Ti) Homologous MOF-Encapsulated Heterostructures with Enhanced Photocatalytic Activity. Chem. Commun. 2018, 54, 1917−1920. (30) Liang, Y.; Shang, R.; Lu, J. R.; Liu, L.; Hu, J. S.; Cui, W. Q. Ag3PO4@UMOFNs Core-Shell Structure: Two-Dimensional MOFs Promoted Photoinduced Charge Separation and Photocatalysis. ACS Appl. Mater. Interfaces 2018, 10, 8758−8769. (31) Zhou, G.; Wu, M. F.; Xing, Q. J.; Li, F.; Liu, H.; Luo, X. B.; Zou, J. P.; Luo, J. M.; Zhang, A. Q. Synthesis and Characterizations of Metal-Free Semiconductor/MOFs with Good Stability and High Photocatalytic Activity for H2 Evolution: A Novel Z-Scheme Heterostructured Photocatalyst Formed by Covalent Bonds. Appl. Catal., B 2018, 220, 607−614. (32) Wang, J.; Xia, Y.; Zhao, H. Y.; Wang, G. F.; Xiang, L.; Xu, J. L.; Komarneni, S. Oxygen Defects-Mediated Z-Scheme Charge Separation in g-C3N4/ZnO Photocatalysts for Enhanced Visible-Light Degradation of 4-Chlorophenol and Hydrogen Evolution. Appl. Catal., B 2017, 206, 406−416. (33) Xie, Y. P.; Yang, Y. Q.; Wang, G. S.; Liu, G. Oxygen Vacancies Promoted Interfacial Charge Carrier Transfer of CdS/ZnO 561


Research Article

ACS Applied Materials & Interfaces standing of Photocatalysis on Metal-Organic Frameworks. Chem. Eur. J. 2013, 19, 14279−14285. (52) Pan, Y. X.; Sun, Z. Q.; Cong, H. P.; Men, Y. L.; Xin, S.; Song, J.; Yu, S. H. Photocatalytic CO2 Reduction Highly Enhanced by Oxygen Vacancies on Pt-Nanoparticle-Dispersed Gallium Oxide. Nano Res. 2016, 9, 1689−1700. (53) Wu, J.; Li, X. D.; Shi, W.; Ling, P. Q.; Sun, Y. F.; Jiao, X. C.; Gao, S.; Liang, L.; Xu, J. Q.; Yan, W. S.; Wang, C. M.; Xie, Y. Efficient Visible-Light-Driven CO2 Reduction Mediated by Defect-Engineered BiOBr Atomic Layers. Angew. Chem., Int. Ed. 2018, 57, 8719−8723. (54) Cui, W. Q.; He, J.; Wang, H.; Hu, J. S.; Liu, L.; Liang, Y. H. Polyaniline Hybridization Promotes Photo-Electro-Catalytic Removal of Organic Contaminants over 3D Network Structure of rGH-PANI/ TiO2 Hydrogel. Appl. Catal., B 2018, 232, 232−245.

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Z-Scheme Photocatalytic CO2 Reduction on a Heterostructure of

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