Carbon Nitride Supramolecular Hybrid Material Enabled High

Sep 12, 2016 - i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, People's Republic of China ... The s...
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Carbon Nitride Supramolecular Hybrid Material Enabled HighEfficiency Photocatalytic Water Treatments Jinghai Liu,*,†,‡ Shuyuan Xie,† Zhibin Geng,‡ Keke Huang,‡ Long Fan,† Weilei Zhou,† Lixin Qiu,† Denglei Gao,† Lei Ji,† Limei Duan,*,† Luhua Lu,∥ Wanfei Li,§ Suozhu Bai,† Zongrui Liu,† Wei Chen,§ Shouhua Feng,*,‡ and Yuegang Zhang*,§,⊥ †

Inner Mongolia Key Lab of Chemistry of Natural Products and Synthesis of Functional Molecules, College of Chemistry and Chemical Engineering, Inner Mongolia University for Nationalities (IMUN), Tongliao 028000, People’s Republic of China ‡ State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012,People’s Republic of China § i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, People’s Republic of China ∥ Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan, 388 Lumo Road, Wuhan 430074, People’s Republic of China ⊥ Department of Physics, Tsinghua University, Beijing 100084, People’s Republic of China S Supporting Information *

ABSTRACT: Surface defects in relation to surface compositions, morphology, and active sites play crucial roles in photocatalytic activity of graphitic carbon nitride (g-C3N4) material for highly reactive oxygen radicals production. Here, we report a high-efficiency carbon nitride supramolecular hybrid material prepared by patching the surface defects with inorganic clusters. Fe (III) {PO4[WO(O2)2]4} clusters have been noncovalently integrated on surface of g-C3N4, where the surface defects provide accommodation sites for these clusters and driving forces for self-assembly. During photocatalytic process, the activity of supramolecular hybrid is 1.53 times than pure g-C3N4 for the degradation of Rhodamine B (RhB) and 2.26 times for Methyl Orange (MO) under the simulated solar light. Under the mediation of H2O2 (50 mmol L−1), the activity increases to 6.52 times for RhB and 28.3 times for MO. The solid cluster active sites with high specific surface area (SSA) defect surface promoting the kinetics of hydroxide radicals production give rise to the extremely high photocatalytic activity. It exhibits recyclable capability and works in large-scale demonstration under the natural sunlight as well and interestingly the environmental temperature has little effects on the photocatalytic activity. KEYWORDS: supramolecular hybrid, graphitic carbon nitride, surface defects, peroxo polyoxometalate clusters, photocatalysis

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have been employed to design supramolecular self-healing materials relied on the noncovalently reversible and dynamic networks generated between polymer chains.5−7 Stepwise assembly driven by π-stacking and metal-coordination in alternative solvents has successfully fabricated one-dimensional nanoscale supramolecular heterojunction.8 In addition to multifunctional organic materials, hybrid systems coupling functional organic and inorganic structures have also been explored.9,10 Supramolecular π-stacking of organic surfactants forming lamellar structure was employed to build vertically oriented ZnO nanosheets/1-pyrenebutyric acid (PyBA) hybrid photoconductor.11 This material design strategy has also been extended to engineer efficient and robust hybrid photocatalysts. Molecular catalytic sites have been assembled on the interface of semiconductors to construct hybrid photocatalysts

he availability of clean and affordable water is one of the biggest challenges for human health worldwide due to the depletion in natural water reservoirs and environmental pollution. Photocatalytic water treatments as a green and sustainable advanced oxidation technology has shown great potential for removing organic pollutants and microorganisms to obtain fresh water.1−3 A low-cost, high-efficiency, recyclable, and environmental friendly photocatalytic material is the key to advance this technology into scalable industrial applications, where both the surface structures and catalytic active sites are crucial structural factors to affect the photocatalytic activity and quantum efficiency. Bottom-up supramolecular assembly provides a potential tool to integrate individual functional components through noncovalent interactions. With the advancement of supramolecular science, the molecular recognition and self-assembly have surpassed the soluble molecular architectures and bridged the nanoscale solid materials interfaces.4 Hydrogen bonding, metal−ligand coordination and π−π stacking interactions © XXXX American Chemical Society

Received: August 2, 2016

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DOI: 10.1021/acs.nanolett.6b03229 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. Schematic illustration of supramolecular hybrid (FePW−g-C3N4) formation process by noncovalent integration of Fe (III) {PO4[WO(O2)2]4} (FePW4) clusters onto the surface of g-C3N4.

active sites for mediating photocatalytic oxidation reaction. But, anionic POMs cluster is susceptible to pH and will change its existing form and structure. A soluble and pH tolerant Fe (III) [PW12O40] complex was considered as the active cluster due to its activity for catalyzing decomposition of H2O2 into strong oxidant radicals even at neutral pH in aqueous solution.38 The H2O2 catalytic active POMs anion in water has been demonstrated to be the peroxo phosphotunsgtate {PO4[WO(O2)2]4}3− (PW4).39−41 And, this peroxo POM anion has also been assembled on dendritic cation by ionic bonding to obtain recoverable heterogeneous catalyst for oxidation.42,43 Here, we present strategies to integrate Fe (III) {PO4[WO(O2)2]4} (FePW4) clusters on the surface of g-C3N4 to form a carbon nitride supramolecular hybrid (FePW−g-C3N4). The surface defects at g-C3N4 provide both the metal−ligand coordination forces and accommodation sites for FePW4 clusters during the assembly process in aqueous solution. We used scanning transmission electron microscope (STEM), EDS elemental mapping, high-resolution transmission electron microscope (HRTEM), X-ray photoelectron spectroscopy (XPS), and pore size measurement to analyze the cluster entities, their locations and PW4 chemical structure in clusters. It presents highly photocatlaytic activity, recyclable capability and works in large-scale demonstration for the degradation of organic dyes pollutants in water under natural sunlight. The defect-cluster coupling between the defect surface and solid clusters at supramolecular hybrids significantly promote the photocatalytic activity for active oxygen radical production. Figure 1 schematically shows the carbon nitride supramolecular hybrid (FePW−g-C3N4) formation process accomplished by the supramolecular approach with building blocks of two-dimensional (2D) layered g-C3N4 and FePW4 clusters as molecular active sites. The formation of this peroxo polyoxometalate salt cluster under acidic condition and corresponding pH tolerance were observed under UV−vis spectra. The absorption peak at 258 nm appears for FePW4 clusters in aqueous solution in contrast to ferric cations (Fe3+) and phosphotungstic acid anions ([PW12O40]3−), respectively. Also, this peak sustains the same absorption intensity even at higher pH (pH at 7) provided that the concentrations keep constant (Figure S1). This phenomenon clearly indicates the hydrolysis-resistance capability of the cluster in water even at high pH, in comparison with the susceptible hydrolysis of the ferric cations and the phosphotungstic acid anions. To further demonstrate the successful integration of FePW4 clusters on the surface of g-C3N4, the morphology, elemental

with more functionalities than the single semiconductor component. The natural hydrogenases12 and water-soluble hydrogenase mimic13 were attached on the photoactive semiconductors (TiO2 nanocrystals and CdTe quantum dots) to endow the hybrids with hydrogen (H2) evolution activity from water-splitting. Assemblies of metalloporphyrin at the interface of TiO2 nanocrystals realized the photocatalytic twoelectron (2e) reactions by coupling single-electron transfers and sequential multielectron catalytic reactions under UV and visible light irradiation.14 Integrating cobalt pentapyridine complex15 and Mo3S4 clusters16 on semiconductor nanowires solid-state photosensitizers brought about photoelectrocatalytic hybrid devices with improved activity for solar-driven watersplitting to generate H2. Bounding Ni-DHLA complex on the surface of photosensitive quantum dots (QD) in water achieved coupling effects of photo-driven charge-transfer dynamics and catalytic activity for H2 evolution.17 Recently, supramolecular hybrids from Ru dinuclear complex as CO2 reduction unit and Ag/TaON semiconductor achieved visible light-driven photocatalytic CO2 reduction to produce formic acid (HCOOH).18 Despite this research progress,19 little attention has been paid to exploring rationally designed supramolecular hybrid architectures as the photocatalysts to boost highly reactive oxygen radical production for advanced oxidation techniques in water treatment. To this end, a metal-free graphitic carbon nitride (g-C3N4) has been shown to be a promising material due to its environmental-friendly and earth-abundant chemical composition, as well as good photoelectrocatalytic activity,20−25 and the simple large-scale preparation method by the pyrolysis of carbon (C)- and nitrogen (N)-containing molecular precursors.26−30 However, structural defects brings about poor electrical conductivity and localized excitons, which result in low activity of interfacial electron transfer reaction and hindering the transformation of adsorbed oxygen toward active oxygen radicals, such as hydroxide radical (·OH).31 To fully utilize the defects and the functional N-groups at the twodimensional (2D) sheet, introducing catalytically active inorganic clusters would be a reasonable approach. A anionic polyoxometalate (POM) cluster with charge-balancing cations has been explored as inorganic cluster for homogeneous molecular active sites32,33 and heterogeneous solid ones34,35 toward catalyzing sun/electricity-driven water oxidation to oxygen (O 2 ). Keggin-type phosphotunsgtate anions ([PW 12 O 40 ] 3− ) 36 and substituted ones ([PW 11 O 39 M (H2O)]3−, M = Fe (III), Mn (II))37 have been also used as B

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Figure 2. Morphology and compositions of FePW−g-C3N4. (a) TEM image and (b) corresponding STEM image. Scale bar 400 nm. (c−h) EDS element mappings for C, N, W, Fe, P, and O and overlapped images of C, N, O, and W. (i) High-resolution TEM image and (j) High-resolution STEM image. The yellow circles localize the individual cluster with size of 2.2 nm. Scale bar 5 nm.

(C) and nitrogen (N) with the oxygen (O) and tungsten (W) elements from clusters (Figures S3c and S4). The phosphorus (P) in the core of PW4 and iron (Fe) were barely visible. The EDS elemental mappings clearly exhibit the clusters distribution over the surface of g-C3N4. The dispersed green-colored W elements in Figure 2e were observed with respect to the continuous C and N elements-based surface (Figure 2c,d). The yellow-colored Fe elements with close distribution along with the P elements and W elements (Figures 2f and S3h) provides evidence for the existence of individual cluster entities with components of Fe3+ and PW4 electrostatically attracted together. The overlapped C, N, W, and O elemental mapping

compositions and locations of individual cluster were investigated using a scanning transmission electron microscope (STEM) and high-resolution TEM (HRTEM). Figure 2a shows a typical corrugated morphology of 2D layered g-C3N4, which is resulted from the unconstrained growth during selfsupporting pyrolysis of urea26,27,44 and buckling distortions from the repulsion between lone pairs on nitrogen atoms of adjacent monomer units.45−49 The STEM image exhibits a rough defect-rich surface with curvatures, distortions, protuberances, and crumpled edges (Figures 2b and S2). The energy dispersive X-ray spectroscopy (EDS) analysis shows the compositions of FePW−g-C3N4 comprising of rich carbon C

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Figure 3. Surface chemical structures of FePW−g-C3N4. (a) High-resolution C1s spectrum. (b) N1s spectrum. (c) O1s spectrum for pure g-C3N4. (d) Schematic chemical structure of defected surface. (e) XPS survey for FePW−g-C3N4. (f) High-resolution W4f spectrum. (g) O1s spectrum. (h) Molecular structure of anionic peroxo POM {PO4[WO(O2)2]4}3−.

in Figure 2h indicates the dispersed states of clusters on defect surface. To observe the locations of individual cluster, HRTEM and STEM was examined. The EDS elemental mapping analysis in Figure S3 confirm that the brighter spots in STEM images was coincident with distributions of W elements, which indicates the localization and size of cluster. The cluster aggregation tended to mainly localize along the defect-rich surface area, for example, crumpled edges and wrinkles. The individual solid clusters with size about 2.2 nm for the four W atoms were observed on the surface (Figures 2j and S3b), which is in

accordance with observations of atomic structures of cluster polymers.50 The location of FePW4 clusters on the surface was also deduced from the pore size distributions of FePW−gC3N4. The specific surface area exhibits a small decrease for FePW−g-C3N4 in comparison with g-C3N4 due to the presence of the clusters on the surface (Figure S5a,c). Interestingly, the pore size distribution analysis indicates that the pores in g-C3N4 have changed after introducing the clusters, especially that the pores with the size of 3.8 nm disappear for FePW−g-C3N4 (Figure S5b,d). These results imply that these clusters tend to D

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Figure 4. Photocatalytic activity of FePW−g-C3N4 mediated by H2O2. Rhodamine B (RhB) (a) and Methyl Orange (MO) (b) under simulated solar light irradiation, Rhodamine B (RhB) (c) and Methyl Orange (MO) (d) under natural sunlight irradiation without pH adjustment at R.T., with the FePW−g-C3N4 of 0.1 g L−1 and H2O2 of 50 mmol L−1. FePW−g-C3N4+H2O2(1) represents the addition of H2O2 after adsorption process. (e) Mechanism of FePW−g-C3N4 with high photocatalytic activity. (f) Large scale application demonstration at 250 L under natural sunlight with FePW−g-C3N4 of 0.02 g L−1 and H2O2 of 40 mmol L−1. Natural Sunlight with location at 42° 15′ North, 119° 15′ East.

spectroscopy (Figure S7a), where the typical vibrations of hepatizine-based molecular units appeared and no newly formed chemical bonds were detected for FePW−g-C3N4. Note that the typical vibrations of phosphotungstic acid at 829.1, 1086.3, and 984.7 cm−1 corresponding to chemical bonds of WOW, WO, and PO in the Keggin structure were not found, which could be due to the cluster formation of ferric cation with the structurally transformed peroxo phosphotungstic acid anion. To further understand the origin of these noncovalent interactions, the surface structures of g-C3N4 were analyzed by XPS and elemental analysis. A defect surface with compositions and chemical bonds different from the bulk was observed, where the atomic compositions is C6N4O0.68 in comparison with that of C2.7N4H1.6O0.07 of bulk (Tables S3 and S4). Distinct from bonding structures in the bulk, the graphitic (284.6 eV), CO/CN bondings (285.7 eV), cyanide/cyanoquione (287.7 eV), and heptazine (288.2 eV) typed carbons (C) construct the surface with nitrogen (N) of heptazine N (398.2 eV), aromatic amide/imide N or cyanoquinone N (399.0 eV), graphitic N (400.6 eV) and oxidic

localize at the 3.8 nm pore to patch the surface defects, which agrees with the HRTEM results of individual cluster. Semiquantitative X-ray fluorescence (XRF) measurement of the FePW−g-C3N4 sample shows that the contents of P, Fe, and W are predominant among the detectable elements (Table S1). The measurable chloride (Cl−) would be the residual from the precursor solution containing FeCl3. The accurate contents of Fe and P in FePW−g-C3N4 were measured by inductively coupled plasma optical emission spectrometer (ICP-OES) with 0.0705 wt % for Fe and 0.0286 wt % for P (Table S2). The molar ratio is 1.37 for Fe and P, indicating the nonstoichiometric feature for these FePW4 clusters. Electron paramagnetic resonance (EPR) spectra provides extra evidence to support the integration of FePW4 cluster on g-C3N4. The FePW−g-C3N4 exhibits a paramagnetic property with single signal at g = 2.0 for the magnetic field from 3200 G to 3500 G (Figure S6). The increased EPR intensity in comparison with that of g-C3N4 is attributed to the ferric ions in the clusters.51,52 The noncovalent interactions between the FePW4 clusters and g-C3N4 in FePW−g-C3N4 were investigated by FT-IR E

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of MO (Figure 4b). In order to demonstrate the roles of the clusters in boosting activity, we measured the activity of pure gC3N4 with H2O2 under the same conditions. The results show that the activity of FePW−g-C3N4 is 1.53 times higher than gC3N4 for RhB and 2.26 times for MO. The pure H2O2 (50 mmol L−1) exhibits almost no activity for RhB and MO. To further demonstrate the real application potential of the supramolecular hybrid photocatalysts, we employed the natural sunlight (location: 42° 15′ North, 119° 15′ min East) as the irradiation source. Under the natural sunlight irradiation, the FePW−g-C3N4 with H2O2 (50 mmol L−1) exhibits a similar activity to that under simulated solar light. As shown in Figure 4c,d, an irradiation time of only 15 min could completely remove RhB or MO from water. The activity is 3.18 times higher than that of pure FePW−g-C3N4 without H2O2 for RhB and 6.20 times for MO. It is 1.63 times higher than pure gC3N4 for RhB and 2.56 times for MO under the mediation of H2O2. The increase amount of adsorption for RhB observed in Figure 4c would be due to the strengthened surface charges of FePW−g-C3N4, which is evidenced by the phenomenon that lowering pH in the solution induces the increase of adsorption capacity (Figure S12). Interestingly, we found that the environmental temperature has nearly no effect on the photocatalytic kinetics. Even in the winter with a temperature at 280 K, the photocatalytic process of FePW−g-C3N4 with H2O2 for RhB still occurs as quickly as that at normal room temperature (Figures 4c, S13, and S14). Large scale application demonstration at 250 L was also performed to show the material’s application potential, where the FePW−g-C3N4 with H2O2 worked strikingly well under environmental sunlight irradiation and it took only 6 h to completely remove the RhB from water (Figure 4f). This result represents an important advance for practical photocatalytic water treatment application in the perspective of significantly reduced energy consumption. The mechanism for FePW−g-C3N4 with high photocatalytic activity was probed in perspective of the cluster and surface structures of g-C3N4. Figure 4e show a significant influence of cluster component on the photocatalytic activity. Ferric chloride (FeCl3) salt was chosen to show the effects of peroxo POMs. With FePW4 cluster as normalized activity, the activity of FeCl3/g-C3N4 is only 61.4% for RhB and 22.9% for MO. Magnesium cation (Mg2+) was usually used as a peroxidestabilizing agent to inhibit the catalytic decomposition of H2O2.53 When the ferric cation (Fe3+) is replaced with the magnesium cation (Mg2+) in the cluster, the activity of MgPW/ g-C3N4 decreases to 68.7% for RhB and 33.5% for MO. For the surface structure of g-C3N4, specific surface area (SSA) and surface composition were considered. As the SSA decreases from 73.42 m2 g−1 to 5.94 m2 g−1, the activity of g-C3N4 becomes only 30.3% for RhB. In comparison to the defect surface with composition of C6N4O0.68, the activity of control (surface with same composition as bulk) is 54.7% for RhB. The origin of the extremely high photocatalytic activity of FePW−gC3N4 with H2O2 was revealed by investigating the kinetics of active oxygen radical (e.g., ·OH) formation in the system. The results show that hydroxide radicals are generated for FePW−gC3N4 under irradiation, and the concentration linearly increases along with the irradiation time (Figure S15). With addition of H2O2 (50 mmol L−1), the generation rate of hydroxide radicals becomes 4.65 times higher than that without H2O2. Thus, the active sites of ferric cation and peroxo POMs anion in cluster with high SAA defect surface promoting the kinetics of

N (403.9 eV), and oxygen (O) of CO typed O (531.9 eV), nitro/nitroso or COC typed O (533.2 eV) and oxazole/ C(O)O* typed O (534.4 eV) (Figure 3a−c). Figure 3d shows a schematic chemical structure of the surface defect. On the basis of these analyses, we propose that the metal−ligand coordination interactions between the N- or O-rich groups at the periphery of the defect and ferric ions in the clusters are the driving forces for this supramolecular hybrid architecture. The X-ray photoelectron spectroscopy (XPS) was also employed to investigate the structure of peroxo POM in cluster. The XPS survey in Figure 3e shows typical elements of tungsten (W) and oxygen (O) from the POM in the clusters. A trace amount of phosphorus (P) is also detectable despite of the weak signal (Figure S8), which gives an atomic ratio of 1:4.2 for P and W, indicating the transformation of Keggin structured [PW12O40]3−. The high-resolution W4f spectrum in Figure 3f shows the characteristic spin−orbital coupling doublet of W 4f7/2 located at 35.7 eV, which represents all the W atoms with the equal +6 valences. These results infer the newly formed POMs have uniform chemical configurations between W and O. The high-resolution O 1s spectrum in Figure 3g shows three deconvoluted peaks with peak area ratio of 1.08:1.87:1. These three peaks are identified as terminal O bonded directly with W (OW), peroxo O bonded to W (WOOW) and O bridge linked W and P atoms (W OP). Combining the composition with structural analysis, a peroxo structure of {PO4[WO(O2)2]4}3− was proposed to illustrate the new chemical structure of POMs in solid FePW4 cluster (Figure 3h). The POMs consists of the central PO4 tetrahedron linked through its oxygen atoms to two pairs of edge-sharing distorted pentagonal bipyramids W(O2)2O3. Each tungsten atom is linked to two peroxo groups, one nonbridging and the other bridging, located in the equatorial plane of the pentagonal bipyramid.39,40 The photocatalytic activity of the FePW−g-C3N4 was evaluated by degrading model organic dyes pollutants (MOP) under simulated solar light and natural sunlight irradiation. Rhodamine B (RhB) and Methyl Orange (MO) were chosen as the representative MOPs in considerations of their weak adsorption effects and difficulty to be degraded (Figure S10). As shown in Figure 4, the FePW−g-C3N4 as a photocatalyst exhibits extremely high activity toward the degradation of the MOP solution through the mediation of H2O2 under irradiation. The photocatalytic process plays a crucial role in the overall decomposition process of RhB and MO in water, which is evidenced by the concentration decrease and blue-shift of absorption peak in UV−vis spectrum (Figure S11b,d). Under the darkness, only adsorption process occurs. After the adsorption reaches saturation, the concentration of RhB and MO in water keeps almost constant in the dark even over an extended time. The addition of H2O2 exhibits an positive effect on the saturated adsorption amount with an increase from 3.67% to 16.6% for RhB (Figure 4a). The Fenton effect is not observed according to the nonshifted UV−vis absorption peak of RhB and MO in solution during adsorption process (Figure S11a,c). Under the condition of simulated solar light irradiation, the FePW−g-C3N4 with H2O2 (50 mmol L−1) exhibits the highest activity toward the degradation of RhB and MO in water. It takes only 20 and 15 min, respectively, to completely remove RhB and MO from water. The activity of FePW−g-C3N4 with H2O2 is 6.52 times higher than pure FePW−g-C3N4 for the degradation of RhB (Figure 4a) and 28.3 times higher for that F

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Academy of Sciences (CAS) for the assistance of testing hydroxide hydricals during irradiation process. Dr. Jinghai Liu thank Prof. Guang Yang from FEI company for test and discussion of HRTEM, STEM and EDS elemental Mapping results.

hydroxide radicals production gives rise to high photocatalytic activity of FePW−g-C3N4. To evaluate the material’s cycling stability, the used FePW− g-C3N4 materials were collected and redispersed it into the RhB and MO solutions. Then, the adsorption and photocatalytic process were repeated following the same procedures as described above. The results show that the irradiation time needs to be increased to 25 min in order to completely remove RhB from water under simulated solar light during the first cycle, and this value remains the same for the subsequent four cycles (Figure S21), indicating the stable photocatalytic activity of the supramolecular structures. Although the photocatalytic activity exhibits a small decrease during the cycles, it is still 1.27 times higher for RhB and 2.27 times higher for MO than that of g-C3N4 at the first use. In summary, we have fabricated a carbon nitride supramolecular hybrid by metal−ligand coordination driven noncovalent assembly of Fe (III) {PO4[WO(O2)2]4} clusters on the defect surface of g-C3N4. It presents excellent photocatalytic activity due to accelerated the production of highly reactive hydroxide radicals under mediation of H2O2. Recycling capability and successful large-scale application demonstration under the natural sunlight would advance the photocatalytic water treatment development. The defect-cluster coupling and supramolecular approach would open an exciting route toward designing highly efficient hybrid photocatalysts.





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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b03229. Experimental details for the material preparation, characterization, and photocatalytic measurements, along with additional supporting data. (PDF)



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[email protected] (J.H.L.). [email protected] (L.M.D.). [email protected] (S.H.F.). [email protected] (Y.G.Z.).

Author Contributions

J. H. L., S.Y.X., and B.Z.G. contributed equally to the work. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS We thank funding support from The National Natural Science Foundation of China (21303080, 21303129, 21461018, 21433013, 21671076, 21661026). Cooperative Project of Tongliao-IMUN (SXYB2012027, SXYB2012072). Supported By Program for Young Talents of Science and Technology in Universities of Inner Mongolia Autonomous Region (NJYT-15B14). Supported by Program for the Top Young Innovative Talents of Inner Mongolia Autonomous Region. Open Project from State Key Laboratory of rare earth resource utilization, Changchun Institute of Applied Chemistry (CIAC), Chinese Academy of Sciences (RERU2016011). Inner Mongolia Autonomous Region Incentive Funding Guided Project for Science & Technology Innovation (2016). Dr. Jinghai Liu thank Prof. Gang Liu from Shenyang Metal Institute of Chinese G

DOI: 10.1021/acs.nanolett.6b03229 Nano Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.nanolett.6b03229 Nano Lett. XXXX, XXX, XXX−XXX