Growth of C3N4 nanosheets on carbon-fiber cloth as flexible and macroscale filter-membrane-shaped photocatalyst for degrading the flowing wastewater
Graphical abstract
Introduction
The global water resource situation is getting worse and worse; the total volume of wastewater generated globally was about 1500 km3 in 1995, and it increased to ∼2212 km3 in 2010 [1]. To decontaminate wastewater, many methods have been developed, including coagulation sedimentation technology, adsorption technology, ion exchange technology, membrane separation technology, activated sludge technology, biofilm technology, advanced oxidation technology, biological treatment technology, electrochemical technology and photocatalytic technology [2], [3], [4]. Usually, the combination of these methods has been used for the practical decontamination of wastewater. Among these methods, photocatalytic technology has been demonstrated to be an efficient and cheap method to degrade organic contaminations and disinfect bacteria for the deep purification of water. In the photocatalytic process, the photocatalysts absorb a certain amount of energy of the photon, so that the electrons transfer from the valence band to the conduction band and the positively charged holes are left in the valence band, participating in the redox reaction [5]. Obviously, the prerequisite for photocatalysis method is to develop excellent photocatalysts.
Generally, three kinds of semiconductor photocatalysts have been well developed. One kind is powder-shaped semiconductor nanomaterials, including nanoparticles [6], [7], nanowires [8], [9], [10], nanosheets [11], [12], [13], nanorods [14], [15], nanotubes [16] and so on. These nanopowders usually show excellent photocatalytic activity due to their nanoparticles and large specific surface area. Unfortunately, these nanopowders are difficult to recycle and easy to lose in practical application (such as degrading the flowing wastewater), resulting in high-cost and second pollution. The second kind is semiconductor films on the substrates, such as nanoparticles films on ITO glass [17], nanowires/nanotubes-based film on metal foil [18]. These film-shaped photocatalysts have the tuned photocatalytic activity and can be efficiently recycled, but they still have some limitations (such as high production cost) and will prevent the flowing of wastewater. The last kind is free-standing semiconductor film (such as TiO2 [19]) or nonwoven cloth (such as TiO2 [20] and WO3 [21]) with porous structure. We have also prepared Ta3N5-Pt [22], Ta3N5-Bi2MoO6 [23] and Fe2O3-AgBr [24] nonwoven cloth as efficient and easily recyclable macroscale photocatalysts. However, the mechanical properties of these free-standing film/nonwoven cloths are unsatisfactory, and especially large-area film/cloth are easily fragile in the flowing wastewater. Therefore, it is very necessary to develop novel photocatalysts with large area, excellent flexibility and high visible-light-driven photocatalytic activity for degrading the flowing wastewater.
As we all know, the filtration technology has been widely used in the purification of flowing water, and it can prevent part of pollutants and allow clean water to pass through easily [25], [26]. Many types of filter-membranes have been well developed, such as inorganic (Al2O3 [27], [28] and SiO2 [29]) filter-membrane and polymer (polyethyleneimine [30] and polysulfone [31]) filter-membrane. However, these filter-membranes can only block and concentrate the pollutants, and they can not degrade pollutants. If the filtration technology is combined with photocatalysis process, undoubtedly the pollutants can be blocked, concentrated and then degraded, resulting in better decontamination effect and greater potential for the practical purification of the flowing wastewater. The key for the synergetic filtration-photocatalysis technology is to develop the filter membrane that not only has porous structure for blocking and concentrating the pollutants, but also has high photocatalytic activity. The design and preparation of such filter-membranes still remains a serious challenge.
Recently, C3N4 as a visible-light-driven photocatalyst has attracted increasing interests due to its low production cost, unique electronic structure and excellent photocatalytic activity and stability [32], [33], [34]. In addition, carbon-fiber cloth (CF cloth) is the most popular substrate with excellent flexibility and conductivity, high strength [35], [36], [37], [38]. With C3N4 and carbon-fiber cloth as the model, herein we firstly designed and prepared C3N4 nanosheets on carbon-fiber cloth as filter-membrane-shaped photocatalyst. The resulting CF/C3N4 cloth exhibit excellent flexibility, strong visible-light absorption, and high photocatalytic activity for degrading RhB and 4-CP. More importantly, a new photocatalytic setup was constructed for degrading the flowing wastewater (rate: 1.5 L h−1) with CF/C3N4 cloth as the filter-membrane. The degradation efficiency of RhB in the flowing wastewater reaches up to 92% after 7 times of filtering/degrading process under visible-light irradiation. In addition, the effect of direct contact, photocatalytic mechanism and stability were further investigated.
Section snippets
Growth of C3N4 nanosheets on CF cloth
Materials and chemicals are shown in Supporting Information. The growth of C3N4 nanosheets on CF cloth was realized by a dip-coating and thermal condensation method as follow. To obtained urea-seed layer, a square piece of CF cloth (area: 4 × 4 cm2) was ultrasonically washed in a mixture solution (deionized water, ethanol and acetone; v/v/v, 1:1:1) for 30 min. Then CF cloth was immersed in saturated urea solution (100 mL) for 30 min and dried at 60 °C for 2 h (step 1 in Fig. 1). The as-coated CF cloth
Preparation and characterization of catalysts
With flexible and conductive CF cloth as the substrate, CF/C3N4 cloth was prepared by a thermal condensation method (Fig. 1). For facilitating the subsequent heating in the crucible, the commercial CF cloth was tailored to have the area of 4 × 4 cm2 (Fig. S1a). Obviously, CF cloth is weaved with many CF bunches with the diameter of ∼400 μm, and there are many pores among and inside these CF bunches (Fig. S1b). In fact, each bunch is composed of ∼1000 CFs with the coarse surface and the diameter of
Conclusions
In summary, filter-membrane-shaped CF/C3N4 cloth has been realized by using a dip-coating and thermal condensation two-step method. The photocatalysis characterizations of the resultant CF/C3N4 cloth composites were investigated. Under visible light illumination, in static sewage experiments, it exhibits excellent photocatalytic activities on RhB and 4-CP degradation. Importantly, the degradation efficiency of flowing RhB goes up from 18% to 92% with the increase of filtering/degrading grade
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (Grant No. 21477019 and 51473033), Program for Innovative Research Team in University of Ministry of Education of China (Grant No. IRT_16R13), Science and Technology Commission of Shanghai Municipality (Grant No. 16JC1400700), Innovation Program of Shanghai Municipal Education Commission (Grant No. 2017-01-07-00-03-E00055), the Fundamental Research Funds for the Central Universities, and DHU Distinguished
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