MOF-derived C-doped ZnO composites for enhanced photocatalytic performance under visible light

Accepted Manuscript MOF-derived C-doped ZnO composites for enhanced photocatalytic performance under visible light Ying Zhang, Jiabin Zhou, Xin Chen, Qinqin Feng, Weiquan Cai PII:

S0925-8388(18)34086-6

DOI:

https://doi.org/10.1016/j.jallcom.2018.10.383

Reference:

JALCOM 48201

To appear in:

Journal of Alloys and Compounds

Received Date: 22 June 2018 Revised Date:

28 October 2018

Accepted Date: 29 October 2018

Please cite this article as: Y. Zhang, J. Zhou, X. Chen, Q. Feng, W. Cai, MOF-derived C-doped ZnO composites for enhanced photocatalytic performance under visible light, Journal of Alloys and Compounds (2018), doi: https://doi.org/10.1016/j.jallcom.2018.10.383. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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MOF-derived C-doped ZnO composites for enhanced photocatalytic

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performance under visible light

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School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, China

School of Resources and Environmental Engineering, Wuhan University of

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Technology, Wuhan 430070, China

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Ying Zhang a, b, Jiabin Zhou a,b,*, Xin Chen b, Qinqin Feng b, Weiquan Cai c, *

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School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, PR China

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*Corresponding author. Tel: +86-28-83037306

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E-mail: [email protected] (J. Zhou); [email protected] (W. Cai)

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ACCEPTED MANUSCRIPT ABSTRACT

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Carbon-doped zinc oxide (ZnO) with porous structure was synthesized by pyrolysis of

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a zinc-based metal organic framework (MOFs). Photocatalytic effect was measured

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by the photodegradation of Rhodamine B (RhB) under visible light irradiation. The

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morphology, structure, and porous properties of the as-synthesized composites were

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characterized by using field emission scanning electron microscopy (SEM), X-ray

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diffraction (XRD), X-ray photoelectron spectroscopy (XPS), the thermo gravimetric

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and differential scanning calorimetry analysis (TG-DSC), diffuse reflectance UV-vis

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spectroscopy (UV-vis DRS), photoluminescence (PL) and N2 sorption-desorption

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isotherms (BET). Compared with other conventional C-doping methods, MOF

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sacrificial template method not only retains the porous structure with interconnected

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ZnO nanoparticles but also introduces carbon doping evenly in ZnO lattice which

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reduces the band gap of ZnO and thus improves the charge-separation efficiency. The

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trapping experiment results showed that superoxide radicals (•O2-) and photoexcited

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hole (h+) are the main and minor oxidative species in the photodegradation of RhB

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respectively. And the enhanced photocatalytic mechanism was also proposed.

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Keywords: Photocatalysis; ZnO; MOF-5; Carbon doping; RhB

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1. Introduction With the increase of serious water pollution issues around worldwide over the

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past decades; photocatalytic technology based on semiconductor has attracted more

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and more attention because of its high efficiency and low toxicity [1-3]. Up to date, a

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large variety of semiconductor photocatalysts have been reported for solar energy

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conversion and a treatment for organic pollutants (e.g., TiO2 [4], g-C3N4 [5], ZnO [6]

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and BiOCl [7]). Among these semiconductors, ZnO have received considerable

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scientific interest as an alternative to TiO2 due to its environmentally friendly, stability

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and high catalytic efficiency [8]. However, ZnO still has many shortcomings in

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practical applications, such as low visible light utilization (due to the wide band gap

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3.27 eV), rapid recombination of photogenerated electron-holes and low specific

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surface area. Many efforts have been made to overcome these weaknesses, such as

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semiconductor coupling [9], metal or non-metal doping [10, 11], self-assembly [12]

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and template method [13]. Among these methods, the template method of metal

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organic frameworks (MOFs) derived C-doping ZnO can not only solve the problem of

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low visible light utilization and rapid charge recombination but also the problem of

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low specific surface area. C doping can introduce oxygen vacancy into the band gap

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and increase the electron density of Fermi level leading to the efficient separation and

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transportation of charges [12]. The porous structure with high specific surface area

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can not only facilitate more pollutant molecules absorbed on the active sites of

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photocatalysts and increase the light transmittance, but also can enable rapid transfer

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of photogenerated charge carriers onto the surface of photocatalysts, promoting bulk

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charge separation [14]. Metal organic frameworks (MOFs) as hybrid organic-inorganic compounds, are

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high porous materials synthesized through the coordination of metallic ions and

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organic ligands [15]. Due to the remarkable characteristics of porous structure and

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tunable pore size and shape, MOFs have received extensive attention in recent years

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as a new type of high surface area and porous materials in catalysis [16], sensing [17],

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storage/separation [18] and supercapacitor [19]. As built from metal ions and organic

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ligands, MOFs have been used as templates for synthesis of carbon and metal oxides

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materials [20, 21]. Using MOFs as precursors has many advantages over traditional

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C-doping methods [22]. The MOFs sacrifice themselves to form the uniform elements

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distribution and the porous structure of MOFs can be remained under certain

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conditions. As one of the most robust porosity MOF structure, MOF-5 which uses

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terephthalic acid and Zn ions as the organic ligands and the metallic sites can be

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employed as carbon and zinc sources to synthesize C doped ZnO without extra

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functional precursors or post-synthesis treatment. Song, et. al. fabricated hollow

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porous ZnO/C nanocages by a one-step pyrolysis of hollow MOF-5 at 500 °C in N2

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atmosphere to increase lithium ion batteries’ storage performance and rate capability

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[13]. However, to the best of our knowledge, there is few report about the C doped

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ZnO by pyrolysis of MOF-5 for visible light photodegradation of organic pollutant in

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water.

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Herein, a well-defined octahedral carbon doped ZnO hybrids with high specific

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surface areas (>800 m2/g) using MOF-5 as the template precursor were synthesized 4

ACCEPTED MANUSCRIPT and exhibited high photocatalytic degradation of Rhodamine B (RhB) under visible

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light irradiation. Subsequently, the C@ZnO shows excellent porous structure, optical

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absorption, charge separation and mass transfer, and thus significantly high activity in

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photocatalytic degradation of RhB under visible light irradiation.

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2. Experimental

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2.1 Chemicals

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Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), terephthalic acid (H2BDC), N, N

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dimethyl-formamide (DMF), ethylene glycol, ethanol, methanol, isopropanol,

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benzoquinone, rhodamine B (RhB), commercial ZnO were purchased from

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Sinopharm Co. Ltd. All chemicals and reagents used were in analytical grade without

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any further purification.

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2.2 Sample preparation

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2.2.1 Preparation of MOF-5

MOF-5 were fabricated by a solvothermal method. The preparation processes

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were as followed: 0.4 g Zn(NO3)2·6H2O and 0.2 g terephthalic acid (H2BDC) were

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dissolved in 20 mL ethylene glycol and then 32 mL N, N-dimethyl formamide (DMF)

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were added into the mixture. After that the mixture were stirred for 1 h, transferred to

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a Teflon-lined stainless steel reactor and heated at 150 °C for 6 h. The products were

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washed with DMF and methanol for several times respectively. Finally, the products

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were dried in oven at 80 °C overnight. The resulted sample is MOF-5.

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2.2.2 Preparation of C@ZnO hybrids 5

ACCEPTED MANUSCRIPT The above-prepared MOF-5 was taken into a muffle furnace and was heated at

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350 to 500 °C with a heating rate of 5 °C/min and held for 1 to 4 h under air. The

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C@ZnO hybrids prepared by aforesaid method were named as 350-3h, 400-3h,

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450-1h, 450-2h, 450-3h, 450-4h, and 500-3h. The numbers of 350, 400, 450, 500 and

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1h, 2h, 3h, 4h were denoted the heating temperature and heating time respectively.

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named as ZnO.

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2.3 Characterization

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For comparison with C-doped ZnO, commercial ZnO without C doping was

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X-ray diffraction patterns (XRD) characterizations were carried out using a

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Bruker powder X-ray diffraction D 8 Advance diffractometer with Cu-Kα irradiation.

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The surface composition and chemical environment were analysed with a VG

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ESCA-LAB-210 X-ray photoelectron spectroscopy measurement (XPS). The

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morphologies of the samples were investigated by Hitachi S-4800 field emission

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scanning electron microscopy (SEM). The specific surface area (SBET) and pore size

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distribution were calculated based on N2 adsorption/desorption isotherms recorded on

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a Micromeritics ASAP 2020 nitrogen adsorption apparatus. The pore-size distribution

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was

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thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were

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determined by SDT Q600 V5.0 Build 63 (TGA-DSC). The UV-vis absorbance and

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diffuse-reflectance spectra (UV-vis DRS) were performed by PE Lambda 750 S with

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an integrating sphere diffuse reflectance attachment. Photoluminescence (PL) spectra

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were tested by a Gangdong F-380 fluorescence spectrometer with the excitation light

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calculated

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Barret-Joyner-Halender

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ACCEPTED MANUSCRIPT at 325 nm. Electron spin resonance (ESR) signals were measured on a Bruker A 300

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spectrometer (USA) under visible light using 5,5-dimethyl-l-pyrroline N-oxide

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(DMPO) as spin-trapped paramagnetic species.

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2.4 Adsorption and photocatalytic degradation experiments

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The adsorption and photocatalytic performance of MOF-5 and C@ZnO hybrids

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were investigated by the photodegradation of RhB and phenol. The RhB and phenol

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solution with a concentration of 1 mg/L was prepared by dissolving the dye in

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distilled water. For reaction, 100 mg as-prepared samples were added into 100 mL

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RhB aqueous solution in a 250 mL beaker with a water jacket to keep the temperature

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of the beaker maintain at 25°C. The solution was kept in dark for 60 min under

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magnetic stirring to reach the adsorption/desorption equilibrium. Afterwards, a 350 W

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Xenon lamp with a 420 nm UV-cutoff filter was used (420 nm-780 nm). During the

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reaction 3 mL of samples were withdrawn at every 30 min.

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3. Results and discussion

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3.1 Structure characterization

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The X-ray diffraction patterns of all the above-prepared samples showed the

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changes in the phase structure from MOF-5 to C@ZnO under different calcination

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temperature and time in Fig. 1. X-ray diffraction (XRD) analysis shows that the

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diffraction of pristine MOF-5 sample fits well with the diffraction pattern in

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references [23, 24] which confirms that the sample is successfully synthesized.

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Interestingly, for the calcined composites, when the annealing temperature is below 7

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temperature and time is above 450°C and 1 h, most peaks from the component of

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MOF are disappeared except that only some intensity peaks from MOF-5 are detected

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at approximately 8.9 , 9.9 , 14.3 , 15.8 ºand 17.7 ºin C@ZnO 450-2h and 450-3h

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hybrids. For 450-4h and 500-3h the characteristic diffraction peaks at 31.8 , 34.4 ,

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36.3 , 47.5 , 56.6 , 62.8 , 66.4 , 67.9 , 69.1 , 72.6

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corresponding to the (100), (002), (101), (102), (110), (103), (200), (112), (201), (004)

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and (202) crystalline planes of ZnO (JCPDS No. 36-1451), respectively. Interestingly,

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the above characteristic diffraction peaks also can be found in the C@ZnO 450-2h

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and 450-3h hybrids but the peaks are less sharp than the peaks in 450-4h and 500-3h.

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This phenomenon indicates that when the annealing temperature and time are 450°C

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and 2h the ZnO structure could be just produced. Therefore the crystallinity of ZnO

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are relatively low. While with the increasing annealing temperature and time (450-3h,

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450-4h and 500-3h), the characteristic peaks of ZnO become sharp and intense,

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suggesting the increase of ZnO crystallinity. The crystallite sizes of pure ZnO and

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C@ZnO 450-2h were calculated to be 27.6 and 34.2 nm by application of the Scherrer

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equation. The calculated crystallite size of C@ZnO was larger than pure ZnO due to

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the C-doping as the carbon anion radius (69-76 pm) is greater than oxygen (57-66 pm)

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[25].

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and 76.9 ºare observed,

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The elemental composition and electronic structure of MOF-5 and C@ZnO

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450-2h hybrid were further analyzed by X-ray photoelectron spectroscopy (XPS).

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Figure 2 shows the XPS survey spectrum and high-resolution XPS spectra of MOF-5 8

ACCEPTED MANUSCRIPT and C@ZnO 450-2h hybrids. It can be seen from Fig. 2a the C@ZnO 450-2h hybrid

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mainly consists of Zn, C and O elements. The Zn 2p is shown in Fig. 2b. The Zn 2p3/2

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and Zn 2p1/2 peaks of MOF-5 are located at 1022.4 eV and 1045.4 eV respectively.

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The peak located at 1022.4 eV corresponding to Zn-O bonds. The binding energy (BE)

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distance of the two peaks is 23 eV, which is within the standard reference value of

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ZnO and indicates that the Zn ions in the composites are +2 states [26]. However, the

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Zn 2p3/2 and Zn 2p1/2 peaks of C@ZnO 450-2h hybrid are located at 1020.8 eV and

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1043.8 eV respectively. Obviously, there is a negative shift (1.6 eV) in the binding

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energy of Zn 2p in C@ZnO hybrid compared to MOF-5. Theoretically, the shifts of

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binding energy in XPS spectra might be caused by the strong interaction (electron

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transfer) between nanocrystals [27] and in our experiment, this phenomenon is due to

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the formation of ZnO particles. The peak located at 1020.8 eV can be identified as

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Zn-C bonds (C-doping) [28, 29]. The C1s shows two carbon species for MOF-5 and

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three carbon species for C@ZnO in Fig. 2c. The two peaks are observed in the C1s

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XPS spectra for MOF-5 with the BE distance of 4 eV. The major C1s XPS spectra for

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MOF-5 is located at 284.6 eV and the satellite peak at higher BE region is located at

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288.7 eV. C 1s spectra of C@ZnO divided into three peaks at 284.6 eV, 285.4 eV and

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288.5 eV. The peak at 284.6 eV is attributed to adventitious carbon contamination [25,

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30, 31]. The peak at 285.4 eV is attributed to the band of C-O [32]. The peak at 288.5

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eV is due to surface loosely bound carbonate species such as C=O [25, 29] and the

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carbon may be incorporating into the interstitial positions of the ZnO lattice [31, 32].

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Obviously, the C1s peaks of C@ZnO 450-2h have a red-shift relative to that in

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there are two peaks located at 531.7 eV and 532.4 eV which are attributed to the

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oxygen atoms on the carboxylate groups of the H2BDC linkers and Zn-O bonds of

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MOF-5 respectively. However, there are three peaks for C@ZnO 450-2h located at

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529.8 eV, 531.2 eV and 531.9 eV respectively. The latter two peaks are at the same

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BE as MOF-5 and the former one at 529.8 eV is assigned to O2- ions of Zn-O bonds in

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Wurtzite structure with Zn2+ in hexagonal coordination [25, 29] indicating the

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existence of ZnO structure.

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3.2 Morphologic characterization

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The morphology and structure of MOF-5 and C@ZnO hybrids (350-3h, 450-2h

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and 500-3h) are revealed by the SEM (Fig. 3a-b, 3c, 3d-e and 3f). As shown in Fig. 3a

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the as-synthesized MOF-5 have a well-defined octahedral morphology. The

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high-magnification SEM (Fig. 3b) shows that there are many textures on the surface

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of the octahedron. All the products of thermal decomposition of MOF-5 under air

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condition maintained the octahedral morphology (Fig. 3c-f). However, the calcined

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temperature has influence on the morphology and structure of C@ZnO. When the

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annealing temperature was 350°C, the surface of the C@ZnO was a little rougher than

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MOF-5 and the size of the particles had almost no change (Fig. 3c). But when the

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annealing temperature was risen to 450°C, the surface of the C@ZnO turned to

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porous particles and the enlarged views (Fig. 3e) clearly show that the surface of the

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octahedral is reorganized into aggregates of ZnO nanoparticles while the 3D

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octahedral structure is still retained with a little shrunken. And with the annealing

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and the octahedral structure began to have large cracks leading to the collapse of the

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octahedral structure. Furthermore the electronic image (Fig. 4b), EDS data (Fig. 4f)

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and element mapping images of Zn, O, and C (Fig. 4c-d) for C@ZnO 450-2h hybrids

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can clearly reveal that C, Zn and O elements are homogeneously distributed in

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octahedron and prove successful preparation of C-doping ZnO.

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The thermo gravimetric and differential scanning calorimetry (TG-DSC) curves

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are shown in Fig. 5. There are two main weight loss events observed in TG-DSC

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curves in Fig. 5. The continuous weight loss under 450 °C were mainly attributed to

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the solvent liberation or the loss of guest molecules, and the second weight loss

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occurred from 450 to 510°C and a mass loss of 48.34% is observed which can be

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ascribed to the decomposition of organic components in air. Obviously the direct

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annealing MOF-5 in air can thermal decompose the organic components and remain

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the exclusively ZnO with wurtzite structure (JCPDS 36-1451) as shown in XRD data

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(Fig. 1).

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The adsorption-desorption N2 isotherms of MOF-5 and C@ZnO 450-2h hybrid

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are shown in Fig. 6 (a) and (b). The parameters of MOF-5, C@ZnO 450-2h hybrid

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and ZnO were calculated from the isotherms showing in Table 1. The calculated BET

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surface area of MOF-5, C@ZnO (450-2h) and ZnO are 1101 m2/g, 833 m2/g and 7.82

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m2/g, respectively. And the total pore volumes of MOF-5, C@ZnO (450-2h) and ZnO

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are 0.40 cm3/g; 0.32 cm3/g; 0.06 cm3/g, respectively. From the above data, it can be

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seen that the specific surface area and pore volumes of carbon doped ZnO hybrids

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Fig. 6 (a) and (b), MOF-5 and C@ZnO 450-2h mainly follow the typical type- º

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isotherm, according to the classification of Brunauer Deming, Deming and Teller

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(BDDT). An obvious H2 type hysteresis loop is observed at P/P0 between 0.45 and

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1.0 for MOF-5 according to IUPAC, which are associated with capillary condensation

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taking place in mesopores. Correspondingly, the pore size distribution curve of the

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inset image of Fig. 6a are revealed that the presence of a combination of micropores

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(<2 nm) and mesopores (2-5 nm) in the samples. In contrast, a H3 type hysteresis loop

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is observed at P/P0 between 0.5 and 1.0 for C@ZnO 450-2h hybrid indicating the

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existence of narrow slit-shaped pores and the mesopores (3-5 nm) with a combination

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of micropores (<2 nm). This phenomenon is consistent with the SEM results (Fig. 3).

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The surface of MOF-5 is uniform porous structure with a H2 type hysteresis loop.

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When the annealing temperature risen to 450°C, the organic framework of MOF-5

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release the volatile gases such as CO2 and H2O, resulting in the uniform porous

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structure. Meanwhile the MOF-5 was reorganized into aggregates of ZnO

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nanoparticles leading to the uniform pore turned to mesoporous or macroporous with

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slits and the type of hysteresis loop changed from H2 to H3.

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3.3 The optical properties

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UV-vis diffuse reflectance spectroscopy (DRS) of MOF-5, ZnO and C@ZnO

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hybrids were carried out and shown in Fig. 7 (a). Consistent with the reference,

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MOF-5 and ZnO have an absorption edge at about 330 nm and 400 nm out of the

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visible light absorption region. In comparison with MOF-5 and ZnO, C-doped ZnO 12

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(C@ZnO) displays significant red shift of optical bandgap absorption edge into

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visible-light region due to the electronic transitions from the valence to the conduction

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band (O2p

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The color of the samples also changed from white (MOF-5) to beige (C@ZnO)

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suggesting that visible light can be used by C@ZnO. The visible-light absorption is

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important for C doping ZnO as a photocatalyst and in order to reveal more

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information of the optical band gap of C@ZnO hybrids, the optical band gap energy

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of the samples can be calculated by following calculation formula (Eq.1).

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Zn3d) [33] thus enhanced light absorption in the whole UV-visible band.

αhυ = A(hυ − Eg)/

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(1)

α is the diffuse absorption coefficient, h is the Planck constant, and ν is the light

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frequency. The plots of (αhν)2 and (αhν)1/2 vs. hν were plotted for calculation of

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direct and indirect transitions band gaps, respectively. As shown in Fig. 7 (b), the

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calculation of indirect energies of MOF-5, ZnO and C@ZnO hybrids of 350-3h,

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400-3h, 450-1h, 450-2h, 450-3h, 450-4h and 500-3h estimated from a plot of (αhν)1/2

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vs. photo energy (hν) according to the K-M model were 3.82 eV, 3.20 eV, 3.73 eV,

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3.00 eV, 2.95 eV, 2.92 eV, 3.02 eV 3.06 eV and 3.07 eV respectively. Consistent with

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the report in the reference, the band gap of ZnO is approximately 3.2 eV [3] and the C

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doping ZnO (450-2h) in our experiment is 2.92 eV revealing the C-doping can shorten

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the band gap of ZnO to enhance light harvesting and utilize visible light. In addition,

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the band gap of C doping of ZnO can be observed clearly revealing the C was doped

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into the crystal structure of ZnO than just covered on the surface of ZnO. Moreover,

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introducing impurity carbon into the lattice of ZnO could revise the band structure of

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ACCEPTED MANUSCRIPT ZnO thus tune its bandgap. Besides the changed band gap of ZnO, we can see clearly

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in Fig. 7a the visible light absorption of C@ZnO is extent to 800 nm which is a

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typical red shift induced by impurity adsorption (C doping) or oxygen-vacancies (Ovac)

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[25]. This result is also in line with the XPS data analysis.

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The recombination rate of photogenerated electrons and holes is an important

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factor affecting the photocatalytic efficiency and PL spectra can be used to reveal it.

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The PL spectra of MOF-5, pure ZnO and C@ZnO 450-2h are shown in Fig. 8.

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Notably, C@ZnO 450-2h exhibit lower PL intensity than MOF-5 and pure ZnO,

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suggesting the C@ZnO has higher efficiency of charge transmission and the C doping

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can effectively inhibit the electrons/holes recombination thus to improve

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photocatalytic degradation activity.

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3.4 Photocatalytic activity

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To investigate the advantages of C doping ZnO with 3D porous structure for

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photocatalytic degradation, we further investigated its photodegradation performance

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using Rhodamine B (RhB) as a target pollutant. Fig. 9 depicts the photocatalytic

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activities for the RhB degradation on MOF-5, ZnO and C@ZnO hybrids under visible

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light. To achieve adsorption-desorption equilibration of the photocatalyst and

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pollutants, the system was stirred for 60 min in the dark before visible light irradiation

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and the concentration of the RhB at beginning was used as the initial concentration C0.

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The Y axis is set up as C/C0, where C is the actual concentration of RhB. After 210

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min of irradiation it was found the photodegradation data was well fitted with a

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fist-order kinetic. The rate constants (k) were gotten by fitting the data with the

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following Eq. (2): 

ln (  ) = κt

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(2)

κ is the apparent first-order rate constant (min-1), t is the irradiation time for

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photodegradation (min), C0 is the concentration of the RhB at beginning, C is the

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actual concentration of RhB at the indicated reaction time t. As can be clearly seen in

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Fig. 9b, the linearity in the kinetic plots was verified that the photodegradation of RhB

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followed first-order kinetics.

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It can be seen in the Fig. 9 that in the absence of any photocatalyst there was

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little degradation of RhB, indicating that the RhB was stable under visible light

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irradiation. From Fig. 9a and b it can clearly see that the MOF-5 and pure ZnO

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microspheres played only a minor role in degradation of RhB as the band gap energies

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of them are 3.82 and 3.2 eV respectively, therefore, the visible light utilization was

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negligible. In contrast, all the C@ZnO hybrids performed the ability to

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photodegradation of RhB under visible light and C@ZnO 450-2h exhibited the best

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degradation ability. The rate constant of RhB photodegradation was 0.01570 min-1 for

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450-2h, which was at least six times higher than that for MOF-5 (0.00075 min-1), ZnO

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(0.00076 min-1), 500-3h (0.00161 min-1) and 350-3h (0.00242 min-1). The different

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photocatalytic effects of C@ZnO hybrids are due to the crystalline, the amount of

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carbon doping and the porous structure. With the increase of annealing temperature,

318

the C@ZnO 450-2h exhibits enhanced crystallization. When the annealing

319

temperature increases to 500°C, the C@ZnO 500-3h exhibits the best crystal

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ACCEPTED MANUSCRIPT structure. Theoretically, the crystallization is an important factor influencing the

321

photocatalytic activity [33]. The highly crystalline structure means there are fewer

322

defects in the crystal lattice that facilitates the transfer/separation of photogenerated

323

electrons and holes and thus more electrons migrate to the surface of the

324

photocatalysts to generate more radicals. However, the content of carbon in the

325

C@ZnO lattice gradually decreased with the increasing annealing temperature leading

326

to a reduction of visible light response region (Fig. 7). In addition, the unique MOFs

327

structure and porous structure can provide more active sites and photocatalytic

328

reaction centres for the reactant molecules. Furthermore, the 3D porous structure also

329

can enhance the absorption of visible light and increase the utilization of visible light

330

[34]. However with the increased annealing temperature the MOFs structure gradually

331

collapse and the mesopores decreased resulting in a decrease of the specific surface

332

area. Therefore, in the balance of the above three factors, the C@ZnO 450-2h showed

333

the best photocatalytic activity. An overview of works published on C@ZnO

334

photocatalyst by other preparation methods as well as the present work were listed in

335

Table S1. As shown in Table S1, the C@ZnO 450-2h shows the biggest BET SSA

336

and excellent photodegradation ability.

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The degradation of azo dye in wastewater is still a difficult problem at present,

338

but due to the sensitization, the photocatalytic effect of the azo dye may be affected.

339

Phenol is known as a stable organic pollutant with even more toxic. To avoid the

340

sensitization effect of azo dye and further verify the true photocatalytic capacity of

16

ACCEPTED MANUSCRIPT 341

our catalyst, phenol was used as pollutant, and the results are shown in Fig. S1.

342

Consistent with expectations, C@ZnO still shows excellent photodegradation activity.

343

3.5 Photocatalytic degradation mechanism To get insights into the photocatalytic mechanism and to identify the contribution

345

of the main active species, several different scavengers such as potassium iodide (KI)

346

[35], p-benzoquinone (BQ) [36], and isopropanol (IPA) [37] were used as species

347

scavengers for photoexcited hole (h+) , superoxide radicals (•O2-) and hydroxyl radical

348

(•OH) formed during the degradation of RhB in our experiment. As shown in Fig. 10,

349

the photocatalytic performance of RhB was obviously inhibited when BQ was added,

350

indicating the •O2- radical dominated the degradation of RhB and the degradation rate

351

dropped greatly. When KI scavenger was added in the system, the degradation

352

efficiency decreased a little, suggesting that h+ played a minor role in RhB

353

photodegradation. However, the photocatalytic performance of RhB was affected

354

slightly when IPA was added, indicating the •OH specie played little role in the

355

mechanism for RhB degradation.

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To further prove the existence of •O2- and •OH, the production of •O2- and •OH

357

radicals in the reaction system were detected by the ESR technique using DMPO as a

358

trapping reagent. The result is shown in Fig. 11. It is clear that at 0 min in dark there

359

are no peaks, however when the visible light irradiated the four characteristic peaks of

360

DMPO- •O2- adducts for C@ZnO are observed. It is obvious that •O2- radicals were

361

generated on the surface of C@ZnO after irradiation. And the •OH radicals were also

362

found in the reaction system after 10 min irradiation.

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ACCEPTED MANUSCRIPT In order to further verify the formation of above-mentioned active species in the

364

photocatalytic degradation process, Mott-Schottky measurement was carried out to

365

determine the band structure of C@ZnO. Mott-Schottky measurement of the C@ZnO

366

catalyst is performed in 0.5 M Na2SO4 solution under dark condition at a frequency of

367

100 Hz. As shown in Fig. 12, the position slope of the plot reveals that C@ZnO is an

368

n-type semiconductor characteristic which is the same type as ZnO [3]. The flat-band

369

potential of C@ZnO derived from Mott-Schottky plot is about -0.50 V versus SCE

370

corresponding to -0.26 V versus normal hydrogen electrode (NHE). For n-type

371

semiconductors, the flat-band potential was 0-0.1 V higher than the conduction-band

372

potential [38]. Therefore the conduction band potential (ECB) of the C@ZnO is -0.36

373

V vs. NHE. And the valence band (VB) and conduction band (CB) potentials of

374

C@ZnO at the point of zero charge can be calculated by the following Eq. (3):

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375

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E = E − E

(3)

Combining with the band gap energy estimated from UV-vis DRS spectra, the

377

calculated optical bandgap of C@ZnO is 3.09 eV. Accordingly to the Eq. (3), the

378

valence band position (EVB) of C@ZnO is determined to be 2.73 V vs. NHE.

379

According to reports in the literature and our experiment, the band gap of ZnO is 3.2

380

eV and the valence band and conduction band position is 2.89 eV and -0.31 eV

381

respectively. Obviously, the bandgap of C@ZnO is narrow than pure ZnO due to the

382

defect produced in the crystal structure of ZnO after C doping which lifts up VB of

383

ZnO and pushes the CB of ZnO down to lower energy level [25]. Because of the wide

384

bandgap, the electrons of ZnO on VB cannot be excited by visible light therefore RhB

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18

ACCEPTED MANUSCRIPT photodegradation on ZnO is attributed to dye sensitized photocatalysis resulting in the

386

low degradation rate. In contrast, with a narrowed bandgap, C-doped ZnO can be

387

excited by visible light and the electrons on the VB of C@ZnO can be excited by

388

visible light to the CB leaving the holes on the VB. Moreover the ECB position of

389

C@ZnO (-0.36 V vs. NHE) is more negative than the oxidation potential of O2/•O2-

390

(-0.33 V vs. NHE), the molecular oxygen can be photoexcited to •O2- by

391

photoelectron on the CB of C@ZnO. At same time, the H2O and OH- in the system

392

can be taken by h+ to generate •OH radical species as the EVB position of C@ZnO

393

(2.73 V vs. NHE) is more positive than the oxidation potential of •OH/OH- (2.40 V vs.

394

NHE). In addition the holes leaving on the VB of C@ZnO also can oxidize RhB

395

directly according to the scavenger’s experiments. The proposed photocatalytic

396

mechanism of C@ZnO under visible light scheme is shown in scheme 1. In addition,

397

as the C@ZnO was derived by annealing MOF-5 leaving the unique MOF porous

398

structure

399

adsorption-diffusion-exchange of reactant and the use of visible light leading to the

400

increase of the degradation rate.

401

3.6 Stability

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especially

large

surface

area

which

promotes

the

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and

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Regeneration and reusability are also very important for photocatalyst. The

403

results of reusability for C@ZnO hybrid is shown in Fig. 13. After 5 cycles of

404

degradation, the degradation rate of RhB retained well and the little loss might from

405

the loss of photocatalyst during recycling.

406 19

ACCEPTED MANUSCRIPT 407

4. Conclusions Carbon (C) doped octahedral porous ZnO have been synthesized annealing

409

MOF-5 at different temperature and time in air. The results show that the 450-2h

410

C@ZnO hybrid retains the octahedral porous morphology with aggregates of ZnO

411

nanoparticles and porous structure. Moreover, carbon was introduced in the ZnO

412

lattice effectively and evenly. There is no doubt that C@ZnO 450-2h exhibited best

413

photocatalytic performance on RhB degradation under visible light irradiation. The

414

enhanced photocatalytic performance was ascribed to the unique porous structure

415

from MOFs and a narrow band gap from C doping. The former can facilitate more

416

dye molecules absorbed on the active sites of photocatalysts and increase the light

417

transmittance. And the latter can improve visible light absorption. The active species

418

trapping experiments results proved that superoxide radicals (•O2-) was the main

419

active species in photodegradation of RhB, photoexcited holes (h+) played the minor

420

effect on it, however, hydroxyl radical (•OH) almost have little contribution on it.

421

This work provides a new insight into the design and synthesis of highly efficient

422

photocatalysts for organic dyes containing wastewater treatment in environmental

423

applications.

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425

Acknowledgements

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This work was supported by the National Natural Science Foundation of China (No.

427

21277108; 21476179), one hundred talents project of Guangzhou University and 2016

428

Wuhan Yellow Crane Talents (Science) Program. 20

ACCEPTED MANUSCRIPT 429 430

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Solvothermal synthesis of graphene/BiOCl0.75Br0.25 microspheres with excellent visible-light [37] J. Zhuang, W. Dai, Q. Tian, Z. Li, L. Xie, J. Wang, P. Liu, X. Shi, D. Wang, Photocatalytic Degradation of RhB over TiO2 Bilayer Films: Effect of Defects and Their Location, Langmuir 26 (2010) 9686-9694.

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515 516 517 518 519 520 521 522 523 524 525 526 527 528

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ACCEPTED MANUSCRIPT Scheme Caption

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Scheme 1 The proposed photocatalytic mechanism of C@ZnO under visible light.

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24

ACCEPTED MANUSCRIPT 535

Table 1 Textural parameters of MOF-5, C@ZnO 450-2h hybrid and ZnO.

536

These parameters were derived from the N2 sorption isotherms obtained at -196°Cº

537

SBET (m2/g)

VTotal (cm3/g)

MOF-5

1101

0.40

450-2h

833

0.33

ZnO

7.82

0.06

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Sample

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538

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539

25

ACCEPTED MANUSCRIPT Figure Caption

541

Fig. 1 XRD patterns of MOF-5 and C@ZnO hybrids.

542

Fig. 2 XPS spectra of MOF-5 and C@ZnO 450-2h hybrids: (a) survey, (b) Zn 2p, (c)

543

C 1s, and (d) O 1s.

544

Fig. 3 FE-SEM images of (a) and (b) MOF-5, (c) C@ZnO 350-3h, (d) and (e)

545

C@ZnO 450-2h, (f) C@ZnO 500-3h.

546

Fig. 4 (a) FE-SEM images, (b) Electronic image, (c) mapping of C element, (d)

547

mapping of Zn element, (e) mapping of Zn element and (f) EDS analysis of C@ZnO

548

450-2h hybrid

549

Fig. 5 TG-DSC image of MOF-5.

550

Fig. 6 N2 adsorption/desorption isotherms of (a) MOF-5 and (b) C@ZnO 450-2h. The

551

inset of (a) and (b) shows the pore-size distribution of the samples respectively.

552

Fig. 7 (a) UV-vis DRS spectra, (b) (αhν)1/2 vs. photon energy (hν) of MOF-5 and

553

C@ZnO hybrids and (c) (αhν)2 vs. photon energy (hν) of MOF-5 and C@ZnO

554

hybrids .

555

Fig. 8 PL spectra of MOF-5 and C@ZnO 450-2h hybrids.

556

Fig. 9 Photocatalytic activity of MOF-5 and C@ZnO hybrids for the degradation of

557

RhB under visible light.

558

Fig. 10 Effects of different scavenger addition in the photocatalytic degradation of

559

RhB under visible light.

560

Fig. 11 ESR signals of the (a) DMPO- O2- (b) DMPO- OH- with visible light

561

irradiation for C@ZnO 450-2h.

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26

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Fig. 12 Mott−Schottky plot for the C@ZnO electrodes.

563

Fig. 13 Recycling experiments of visible light photocatalytic degradation of RhB over

564

the C@ZnO 450-2h hybrids.

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27

ACCEPTED MANUSCRIPT 566

ZnO

Intensity (a.u.)

500-3h

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450-4h 450-3h

450-2h

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450-1h 400-3h

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350-3h MOF-5

ZnO PDF 36-1451

10

20

567

571

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570

50

60

70

Fig. 1 XRD patterns of MOF-5 and C@ZnO hybrids.

AC C

569

40

2θ θ (degree)

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568

30

28

ACCEPTED MANUSCRIPT

Zn 2p1/2

MOF-5

200

400

600

800

1000 1200

1020

1030

1040

Bind Energy (eV)

Bind Energy (eV)

579

c

d

O1s

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Counts/s

C 1s

450-2h

583

450-2h

Counts/s

580

MOF-5

280

585

285

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MOF-5

584

290

1050

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0

578

582

Zn 2p1/2

450-2h

577

581

b

Zn 2p3/2

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576

a

Counts/s

Zn 2p3/2

O 1S O 1S

C 1S

MOF-5 C 1S

575

Counts/s

574

450-2h

Zn 2p1/2

573

Zn 2p3/2

572

295 525

Bind Energy (eV)

530

535

Bind Energy (eV)

Fig. 2 XPS spectra of MOF-5 and C@ZnO 450-2h hybrids: (a) survey, (b) Zn 2p, (c)

587

C 1s, and (d) O 1s.

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586

29

540

ACCEPTED MANUSCRIPT 589 590 591

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592 593 594

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595

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596 597 598 599

603 604 605

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602

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601

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600

606

Fig. 3 FE-SEM images of (a) and (b) MOF-5, (c) C@ZnO 350-3h, (d) and (e)

607

C@ZnO 450-2h, (f) C@ZnO 500-3h.

608

30

ACCEPTED MANUSCRIPT 609

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610

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f

C

0

O Zn Zn

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Zn

1

2

3

4

5

6

7

8

KeV

Fig. 4 (a) FE-SEM images, (b) Electronic image, (c) mapping of C element, (d)

612

mapping of Zn element, (e) mapping of Zn element and (f) EDS analysis of C@ZnO

613

450-2h hybrid

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10

ACCEPTED MANUSCRIPT 615

516.3

DSC

10

60

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40 20

5

510.7

0

200

616

400

600

Temperature (°°C)

800

Fig. 5 TG-DSC image of MOF-5.

AC C

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617

32

0 1000

DSC (mW/mg)

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80

0

618

15

TG

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Weight loss (%)

100

300 a

0.25 0.20 0.15 0.10 0.05 0.00 0

5

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260

10

15

20

Pore width (nm)

240 0.0

0.4

0.6

0.8

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1.0

Relative pressure (P/P0)

260

0.4 0.3 0.2 0.1 0.0

0

5

10

15

20

Pore width (nm)

EP

220

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dV/dD (cm3/g)

240

0.5

200

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Adsorbed Volume (cm3/g )

619

620

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280

dV/dD (cm3/g)

Adsorbed Volume (cm3/g )

ACCEPTED MANUSCRIPT

180 0.0

b 0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

621

Fig. 6 N2 adsorption/desorption isotherms of (a) MOF-5 and (b) C@ZnO 450-2h. The

622

inset of (a) and (b) shows the pore-size distribution of the samples respectively.

623 33

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MOF-5

450-2h

200

300

400

500

600

700

Wavelength (nm)

624

c

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b

800

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Ahv^(1/2) (eV )^1/2

MOF-5 ZnO 350-3h 400-3h 450-1h 450-2h 450-3h 450-4h 500-3h

3.06

2.95

3.07

2.6

2.8

3.73

3.02 3.00

2.92

3.0

3.2

3.4

3.6

3.82

3.8

4.0

hv (eV)

TE D

625

AC C

EP

Ahv^2 (eV)^2

c

2.6

626

MOF-5 ZnO 350-3h 400-3h 450-1h 450-2h 450-3h 450-4h 500-3h

RI PT

Absorbance (a.u.)

a

MOF-5 ZnO 350-3h 400-3h 450-1h 450-2h 450-3h 450-4h 500-3h

3.14 3.13 3.09

2.8

3.0

3.20 3.19 3.18

3.95 3.92

3.16

3.2

3.4

3.6

3.8

4.0

hv (eV)

627

Fig. 7 (a) UV-vis DRS spectra, (b) (αhν)1/2 vs. photon energy (hν) of MOF-5 and

628

C@ZnO hybrids and (c) (αhν)2 vs. photon energy (hν) of MOF-5 and C@ZnO

629

hybrids .

630 34

ACCEPTED MANUSCRIPT

350

400

M AN U

SC

RI PT

Intensity

MOF-5 ZnO 450-2h

450

500

550

Wavelength (nm) 631

Fig. 8 PL spectra of MOF-5 and C@ZnO 450-2h hybrids

633

(excitation wavelength: 325nm).

EP AC C

634

TE D

632

35

600

ACCEPTED MANUSCRIPT

1.0

0.5

0.0

-60 -30

0

RI PT

Self MOF-5 ZnO 350-3h 400-3h 450-1h 450-2h 450-3h 450-4h 500-3h

30 60 90 120 150 180 210

M AN U

Time (min)

635

3.0 Self MOF-5 ZnO 350-3h 400-3h 450-1h 450-2h 450-3h 450-4h 500-3h

TE D

2.0

b

1.5 1.0

EP

ln(C0/C)

2.5

SC

C/C0

a

0.5

AC C

0.0

0

636

20

40

60

80 100 120 140 160

Time (min)

637

Fig. 9 Photocatalytic activity of MOF-5 and C@ZnO hybrids for the degradation of

638

RhB under visible light.

639

36

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1.0

RI PT

C/C0

a

0.5

0.0

-60 -30

0

SC

RhB Self C@ZnO +IPA +KI +BQ

30 60 90 120 150 180 210

M AN U

Time (min)

640

2.5

b

2.0

ln(C0/C)

TE D

RhB Self C@ZnO +IPA +KI +BQ

1.5

EP

1.0 0.5

AC C

0.0

0

641

20

40

60

80 100 120 140 160

Time (min)

642

Fig. 10 Effects of different scavenger addition in the photocatalytic degradation of

643

RhB under visible light. (IPA: 100 mM, KI: 100 mM, BQ: 10 mM, C@ZnO: 100 mg,

644

RhB: 100 mL 1mg/L )

645 37

ACCEPTED MANUSCRIPT 646

a

M AN U

SC

RI PT

Intensity (a.u.)

10 min 0 min

3420 3450 3480 3510 3540 3570 3600

Magnetic Field (Gauss) 647

TE D

10 min 0 min

AC C

EP

Intensity (a.u.)

b

3420 3450 3480 3510 3540 3570 3600

Magnetic Field (Gauss)

648 649

Fig. 11 ESR signals of the (a) DMPO- O2- (b) DMPO- OH- with visible light

650

irradiation for C@ZnO 450-2h.

651 38

ACCEPTED MANUSCRIPT 652

2.0

RI PT

1.5

0.5

SC

1.0

-0.50 V

M AN U

Csc-2(1011cm4F2)

100 Hz

0.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

Potential (V vs SCE) 653

TE D EP

655

Fig. 12 Mott−Schottky plot for the C@ZnO electrodes.

AC C

654

39

ACCEPTED MANUSCRIPT 656 657

1 st

2 nd

4 th

3 rd

0.4

0.2

0.0

5 th

SC

0.6

0

100

200 0

100

M AN U

(C0-C)/C0

0.8

RI PT

1.0

200 0

100

200 0

100

200 0

100

200

Time (min)

TE D

658

Fig. 13 Recycling experiments of visible light photocatalytic degradation of RhB over

660

the C@ZnO 450-2h hybrid.

AC C

EP

659

40

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Research highlights


Carbon-doped octahedral zinc oxide with porous structure was synthesized using metal organic frameworks (MOFs) as a precursor. The band gap of ZnO was shortened from 3.20 eV to 3.09 eV by C doping.




The unique porous structure from MOFs and a narrow band gap from C doping

The superoxide radicals (•O2-) is the main oxidative species for the degradation of

EP

TE D

M AN U

RhB.

AC C




SC

allow C@ZnO octahedral to utilize visible light.

RI PT