Elsevier

Chemosphere

Volume 212, December 2018, Pages 523-532
Chemosphere

Visible light photocatalytic degradation of MB using UiO-66/g-C3N4 heterojunction nanocatalyst

https://doi.org/10.1016/j.chemosphere.2018.08.117Get rights and content

Highlights

  • UiO-66/g-C3N4 nanocatalyst was synthesized by annealing method.

  • It exhibits excellent photocatalytic degradation of MB in visible light.

  • Superoxide radicals were the dominant reactive oxidative species.

Abstract

A unique hybrid of Zr-based metal-organic framework (UiO-66) with graphitic carbon nitride (g-C3N4) nanosheets was synthesized by a facile annealing method. Photocatalytic effect was measured by the photodegradation of methylene blue (MB) under visible light irradiation. The morphology, structure, and porous properties of the as-synthesized composites were characterized by using transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), the thermogravimetric and differential scanning calorimetry analysis (TG-DSC), diffuse reflectance UV–vis spectroscopy (UV–vis DRS), photoluminescence (PL), and N2 sorption-desorption isotherms (BET). The results showed that about 100% of MB (200 mL of 10 mg L−1) photodegradation was achieved by the UiO-66/g-C3N4 hybrids (UC 10:10) in 240 min under visible light. The enhanced photocatalytic activity could be attributed to the heterojunction between UiO-66 and g-C3N4 therefore the photoelectron transfers efficiently from the conduction band (CB) of g-C3N4 to the CB of UiO-66 through the inner electric field generated by the heterojunction resulting the decreasing of recombination of electron/hole and the porous structures which enhance adsorption of the dye molecules on the catalyst surface thereby facilitates the electron/hole transfer within the framework. The trapping experiment and electron spin resonance (ESR) results showed that superoxide radicals (•O2) was the main oxidative species in the photodegradation of MB and the enhanced photocatalytic mechanism of UiO-66/g-C3N4 heterojunction hybrids was also proposed.

Introduction

Semiconductor photocatalysis is an efficient method for environmental detoxification, which is able of converting toxic and non-biodegradable organic compounds into carbon dioxide, water and inorganic salts (Li and Li, 2002; Hu et al., 2018; Zhang et al., 2018b, 2018c). As a good semiconductor photocatalyst, graphitic carbon nitride (g-C3N4) has attracted an increasing attention of scientists recently (Hu et al., 2016; Chen et al., 2017). It is composed of carbon and nitrogen only and has a graphite-like layered structure. In addition the optical band gap of g-C3N4 is approximately 2.7 eV that exhibits a strong response for the visible-light region (Thomas et al., 2008). Owing to its advantages of visible light response, low cost, chemical-stable and non-toxicity (Thomas et al., 2008; Ge and Han, 2012; Niu et al., 2012), g-C3N4 has been used as photocatalyst for water or air purification (Yu et al., 2013, 2014; Tong et al., 2015; Hong et al., 2016; Ye et al., 2016; Giannakopoulou et al., 2017). Even though, the photocatalytic activity of g-C3N4 is somewhat unsatisfactory by the low specific surface area and the rapid recombination of photogenerated electrons and holes because of its narrow band gap. Strategies such as different precursor (e.g. urea (Liu et al., 2011; Zhang et al., 2012; Zhang et al., 2014)) preparation methods, hard template method (e.g. SBA-11 (Zhang et al., 2017), SBA-15 (Chen et al., 2009), SiO2 (Bu et al., 2014; Zhu et al., 2015, 2016)), soft template method (e.g. Pluronic P123 (Zhang et al., 2017)) and exfoliating bulk g-C3N4 to g-C3N4 sheet method (Yang et al., 2013; Wang et al., 2016) were applied to increase the specific surface area. In addition strategies such as coupled with other compounds (e.g. Fe2O3 (Christoforidis et al., 2016), SnO2 (Zang et al., 2014), TiO2 (Chen et al., 2017), Bi2WO6 (Ma et al., 2016), BiPO4 (Pan et al., 2012), Ag3PO4 (Zhang et al., 2013), CdS (Fu et al., 2013)), doping with metal or nonmetal species (e.g. S (Liu et al., 2010), B (Yan et al., 2010), carbon nanotubes (Ge and Han, 2012), Au/Ag nanoparticles (Cheng et al., 2013; Bai et al., 2014)) and coupled with graphene (Ma et al., 2016) were used to ameliorate the rapid recombination of photogenerated electrons and holes to improve the photocatalytic efficiency. However, very few of methods can simultaneously solve the problem of low specific surface area and rapid recombination of electron and holes.

Metal organic frameworks (MOFs), as hybrid organic-inorganic compounds, are high porous materials synthesized through the coordination of metallic ions and organic ligands (Li et al., 2012). Due to their ultrahigh porosity and incredibly high internal surface areas, they have been applied in catalysis (Timofeeva et al., 2014; Arrozi et al., 2015; Kaur et al., 2016; Crake et al., 2017), storage/separation (Huang et al., 2016; Seo et al., 2016; Sarker et al., 2018) and sensing (Hu et al., 2015). In recent years, MOFs are of interest for photocatalysis because of their high surface area, tunable pore size and high light harvesting capacity. Until now, several MOFs including, HKUST-1 (Mosleh et al., 2016), UiO-66 (Xu et al., 2017), MIL-68 (In) (Yang et al., 2017), MOF-235 (Li et al., 2016), MOF-46 (Rad and Dehghanpour, 2016) and MOF-5 (Zhang et al., 2018b), etc. have been studied as photocatalysts. Among many MOFs materials, zirconium-based MOF (UiO-66) attracted widely attention because of its higher chemical stability in water and thermostability compared to other MOFs materials (Shi et al., 2015). It has been reported that due to the synergistic effect arising from the catalytically reactive sites (metal centers/organic linkages) of MOFs, upon its exposure to UV, UiO-66 can act as semiconductors (Shen et al., 2013; He et al., 2014; Sha and Wu, 2015; Ding et al., 2017). Based on the above mentioned superior properties, UiO-66 can be used as a photocatalyst for eliminating organic pollutants from wastewater. Nevertheless, the band gap of UiO-66 is about 3.6 eV which limit its optical adsorption in the visible light region. To achieve this, a narrow band gap semiconductors coupling formed a heterojunction method is an ideal choice. The heterojunction can form an inner electric field to facilitate the transfer/separation of photogenerated electron/hole and inhibit the recombination of electrons and hole so as to enhance the photocatalytic activity (Lin et al., 2014; Shen et al., 2015). Based on the above two types of excellent semiconductors, UiO-66/g-C3N4 heterojunction photocatalyst received great attention. Wang et al. prepared UiO-66/g-C3N4 heterojunction through annealing the mixture of UiO-66 octahedrons and g-C3N4 with various mass ratio in Ar atmosphere for enhanced photocatalytic hydrogen evolution under visible light irradiation (Wang et al., 2015a). Zhang et al. synthesized g-C3N4/UiO-66 nanohybrids via solvothermal method for the oxidation of dye under visible light irradiation (Zhang et al., 2018a). However, the preparation methods mentioned above are complicated which need to be annealing in Ar atmosphere or twice solvothermal reaction. Therefore, based on the excellent properties of UiO-66/g-C3N4 heterojunction (UC) hybrids, it is worthy to further study on the synthesis of UC hybrids to find a more simple and effective method.

Herein, we report the synthesis of UiO-66/g-C3N4 heterojunction photocatalyst through simply annealing the mixture of UiO-66 and g-C3N4 nanosheets in air atmosphere and photocatalytic degradation of the MB in water under visible light irradiation. The hybrids were presented as excellent adsorbents and catalysts because the accessible surface area of g-C3N4 is enlarged and a heterojunction interaction is formed between the two components. The ultrahigh porosity and incredibly high internal surface areas structure of MOFs were retained in the UiO-66/g-C3N4 which could solve the low specific surface area problem of g-C3N4, at the same time, the heterojunction between two semiconductors can facilitate the electron transfer and separation, thus preventing the recombination of electrons and holes. Furthermore, UiO-66/g-C3N4 can utilize the visible light owe to the narrow band gap of g-C3N4. To our knowledge, this kind of synthesis method is rarely reported for UiO-66/g-C3N4 heterojunction photocatalyst to organic photodegradation.

Section snippets

Chemicals

Urea, terephthalic acid (H2BDC), N, N dimethyl-formamide (DMF), zirconium chloride (ZrCl4), ethanol (EtOH), acetic acid, methanol, isopropanol (IPA), and methylene blue dye (MB) were purchased from Sinopharm Co. Ltd. All chemicals and reagents used were in analytical grade.

Synthesis of UiO-66 octahedrons

UiO-66 Octahedrons were synthesized by a solvothermal method. 0.2 g ZrCl4, and 0.14 g terephthalic acid (H2BDC) were dissolved in 40 mL N, N-dimethylformamide (DMF) with a continuous stirring for 1 h and then added 8 mL of

Structure characterization

The X-ray diffraction patterns of pristine g-C3N4, UiO-66 octahedrons and UC x:y hybrids are included in Fig. 1 X-ray diffraction (XRD) analysis shows that the diffraction of UiO-66 fits well with the diffraction pattern in references (Cavka et al., 2008; Valenzano et al., 2011). For the pure g-C3N4, the peaks at 27.5° and 13.5°are found corresponding to the typical interplanar stacked graphitic layered structure and the (002) plane (JCPDS No. 87–1526). In addition, all diffraction patterns in

Conclusion

UiO-66/g-C3N4 hybrids have been synthesized from annealing the mixture of UiO-66 and g-C3N4 nanosheets. The results showed that the UC 10:10 hybrids exhibited best photocatalytic performance on MB degradation under visible light irradiation. The enhanced photocatalytic performance was ascribed to the large specific surface area and unique pore structure from MOFs, a narrow band gap and an excellent heterojunction. The former can facilitate more dye molecules absorbed on the active sites of

Acknowledgements

This work was partially supported by the National Natural Science Foundation of China (21277108).

References (69)

  • R. Hao et al.

    Template-free preparation of macro/mesoporous g-C3N4/TiO2 heterojunction photocatalysts with enhanced visible light photocatalytic activity

    Appl. Catal. B Environ.

    (2016)
  • Y. Hong et al.

    Efficient and stable Nb2O5 modified g-C3N4 photocatalyst for removal of antibiotic pollutant

    Chem. Eng. J.

    (2016)
  • J.-Y. Hu et al.

    Improvement of phenol photodegradation efficiency by a combined g-C3N4/Fe(III)/persulfate system

    Chemosphere

    (2016)
  • Z. Hu et al.

    The formation of a direct Z-scheme Bi2O3/MoO3 composite nanocatalyst with improved photocatalytic activity under visible light

    Chem. Phys. Lett.

    (2018)
  • H.L. Huang et al.

    Enhancing CO2 adsorption and separation ability of Zr(IV)-based metal-organic frameworks through ligand functionalization under the guidance of the quantitative structure-property relationship model

    Chem. Eng. J.

    (2016)
  • R. Kaur et al.

    Efficient photocatalytic degradation of rhodamine 6G with a quantum dot-metal organic framework nanocomposite

    Chemosphere

    (2016)
  • F.B. Li et al.

    The enhancement of photodegradation efficiency using Pt-TiO2 catalyst

    Chemosphere

    (2002)
  • D. Ma et al.

    Fabrication of Z-scheme g-C3N4/RGO/Bi2WO6 photocatalyst with enhanced visible-light photocatalytic activity

    Chem. Eng. J.

    (2016)
  • M. Sarker et al.

    Carboxylic-acid-functionalized UiO-66-NH2: a promising adsorbent for both aqueous- and non-aqueous-phase adsorptions

    Chem. Eng. J.

    (2018)
  • P.W. Seo et al.

    Adsorptive removal of nitrogen-containing compounds from a model fuel using a metal-organic framework having a free carboxylic acid group

    Chem. Eng. J.

    (2016)
  • B. Shaabani et al.

    Preparation of CuO nanopowders and their catalytic activity in photodegradation of Rhodamine-B

    Adv. Powder Technol.

    (2014)
  • L. Shen et al.

    Noble-metal-free MoS2 co-catalyst decorated UiO-66/CdS hybrids for efficient photocatalytic H-2 production

    Appl. Catal. B Environ.

    (2015)
  • Y. Sun et al.

    Hierarchically porous NiAl-LDH nanoparticles as highly efficient adsorbent for p-nitrophenol from water

    Appl. Surf. Sci.

    (2015)
  • M.N. Timofeeva et al.

    Effects of linker substitution on catalytic properties of porous zirconium terephthalate UiO-66 in acetalization of benzaldehyde with methanol

    Appl. Catal. A Gen.

    (2014)
  • Z. Tong et al.

    Biomimetic fabrication of g-C3N4/TiO2 nanosheets with enhanced photocatalytic activity toward organic pollutant degradation

    Chem. Eng. J.

    (2015)
  • H. Wang et al.

    Facile synthesis of Sb2S3/ultrathin g-C3N4 sheets heterostructures embedded with g-C3N4 quantum dots with enhanced NIR-light photocatalytic performance

    Appl. Catal. B Environ.

    (2016)
  • W. Wang et al.

    Cr(VI) removal from aqueous solutions by hydrothermal synthetic layered double hydroxides: adsorption performance, coexisting anions and regeneration studies

    Colloid. Surface.

    (2014)
  • W. Wang et al.

    Different surfactants-assisted hydrothermal synthesis of hierarchical gamma-Al2O3 and its adsorption performances for parachlorophenol

    Chem. Eng. J.

    (2013)
  • X. Xu et al.

    PANI/FeUiO-66 nanohybrids with enhanced visible-light promoted photocatalytic activity for the selectively aerobic oxidation of aromatic alcohols

    Appl. Catal. B Environ.

    (2017)
  • C. Yang et al.

    A novel visible-light-driven In-based MOF/graphene oxide composite photocatalyst with enhanced photocatalytic activity toward the degradation of amoxicillin

    Appl. Catal. B Environ.

    (2017)
  • L. Ye et al.

    Phosphorylation of g-C3N4 for enhanced photocatalytic CO2 reduction

    Chem. Eng. J.

    (2016)
  • Y. Zang et al.

    Synergistic collaboration of g-C3N4/SnO2 composites for enhanced visible-light photocatalytic activity

    Chem. Eng. J.

    (2014)
  • F.-J. Zhang et al.

    A novel photofunctional g-C3N4/Ag3PO4 bulk heterojunction for decolorization of Rh

    B. Chem. Eng. J.

    (2013)
  • M. Zhang et al.

    Enhancement of visible light photocatalytic activities via porous structure of g-C3N4

    Appl. Catal. B Environ.

    (2014)
  • Cited by (165)

    • State of the art in visible-light photocatalysis of aqueous pollutants using metal-organic frameworks

      2023, Journal of Photochemistry and Photobiology C: Photochemistry Reviews
    View all citing articles on Scopus
    View full text