Carbothermal activation synthesis of 3D porous g-C3N4/carbon nanosheets composite with superior performance for CO2 photoreduction
Graphical abstract
3D porous g-C3N4/C nanosheets composite has been successfully prepared for the first time by a simple pyrolysis and subsequent carbothermal activation method using a mixture of melamine and natural soybean oil as precursor. The key feature of this 3D porous architecture is that the formed ultrathin graphene-like g-C3N4/C nanosheets have crumpled morphology and hierarchical mesostructure, leading to a high surface area and large pore volume. Photocatalytic measurements reveal that the obtained 3D porous g-C3N4/C nanosheets composite exhibits an exceptionally higher activity than bulk g-C3N4 in the photocatalytic reduction of CO2 with H2O vapor.
Introduction
Since the discovery of CO2 photoreduction in a semiconductor aqueous suspension [1], searching and optimizing high-efficiency photocatalysts for converting CO2 into value-added resources that can relieve energy crisis and environmental problems has attracted considerable attention worldwide. Up to now, relevant research is mostly focused on metal oxide semiconductor photocatalysts. However, economic cost concern of these metal-containing photocatalysts have become a huge barrier against their widespread applications. Given the requirements of low cost, scalability and nontoxicity or environmental issues, the metal-free materials have manifested as fascinating green alternatives with promising effectiveness in multifaceted applications, such as energy conversion and storage [2,3], and environmental remediation [[4], [5], [6]]. Recently, great efforts have been devoted to preparation of metal-free carbon nitride (C3N4) materials and their use as photocatalysts in solar fuel conversion, pollutant degradation and CO2 reduction due to their novel structures and unique properties [[7], [8], [9], [10], [11], [12]]. Graphitic C3N4 (g-C3N4) is indeed a novel metal-free visible-light photocatalyst characterized by extraordinary properties, an appealing electronic structure and a medium band gap (2.7 eV) [13]. Nevertheless, the activity of bulk g-C3N4 in CO2 photoreduction to value-added fuels is still very low on account of its small specific surface area (SSA), high recombination of photo-generated electron–hole pairs and low electron conductivity [[14], [15], [16], [17], [18], [19], [20], [21], [22]]. Undoubtedly, these properties of g-C3N4 is closely related to the morphology and structure. Therefore, research on structural design and morphologic optimization of bulk g-C3N4 is highly demanded so as to weaken the adverse effects resulting from the small SSA and low electron conductivity [23].
Ultrathin nanosheets, involving van der Waals interaction between adjacent sheets and characterized by unique optical and electronic properties, have attracted extensive attention in the fields of sensors, electronics, catalysis, and energy conversion [24]. As for the photocatalytic application, this characteristic structure can increase the SSA with abundant reactive sites and shorten bulk diffusion length to accelerate the transfer and separation of charge carriers. Theoretically, g-C3N4 has a layered structure that consists of highly-ordered tri-s-triazine moiety sheets connected through planar tertiary amino groups [25]. Since layers are connected by weak van der Waals force analogous to graphite, academically it is feasible to obtain g-C3N4 nanosheets from bulk g-C3N4, thus further expanding its applications [26]. Generally, the properties and photocatalytic activity of g-C3N4 nanosheets are more competitive with multilayer stacked bulk g-C3N4 mainly in two aspects: 1) The electron and hole mobility in nanosheets, on account of the quantum confinement effect, is significantly greater than that in the bulk, as proved in the study on graphene [27]; 2) The surface area of nanosheets is much larger than that of bulk, which largely contributes to light harvest and charge carrier transport. To date, tremendous endeavors have been made to preparation of g-C3N4 nanosheets using various methods, including thermal oxidation “etching” and liquid exfoliation [28]. However, most methods are limited by low yield and technical challenges [29,30]. On the other hand, like many other photocatalysts, g-C3N4 alone exhibits very low electrical conductivity which is one of suppression factors for photocatalytic efficiency. Generally, the integration of well-conductive materials into g-C3N4 can further lower the redox potential of reaction at the corresponding active sites and effectively accelerate electron–hole pair separation and transport. Among various materials, carbonaceous materials (e.g. carbon nanotubes [31], fullerene [32], graphene [33], carbon black [34,35], and carbon quantum dots [36]) have been increasingly used lately to enhance charge carriers transfer. Our recent result also showed that introduction of hybrid carbon in g-C3N4 can remarkably increase its photocatalytic performance such as in the reduction of CO2 with H2O [37].
Herein, novel 3D porous graphitic carbon nitride/carbon (g-C3N4/C) nanosheets composite (denoted as 3D g-C3N4/C-NS) was synthesized for the first time by using a simple “carbothermal activation” method based on our previous hybrid graphitic carbon nitride and carbon composite (H-g-C3N4/C) which was prepared by one-step pyrolysis of a mixture of melamine and natural soybean oil at 600 °C under N2 atmosphere (Fig. 1). It is found that the hybrid carbon in the composite is critical to the formation of 3D porous structure during the activation process. In comparison to H-g-C3N4/C and bulk g-C3N4, the obtained 3D g-C3N4/C-NS has the following advantages: (1) 3D porous architecture with hierarchical mesostructure allows effective gas reactants or products diffusion/transfer; (2) high specific surface area with plentiful exposed functional groups provides a high surface gas reactant (e.g. CO2) concentration and more active sites; (3) graphene-like nanosheets with 3D interconnecting structure can serve as efficient light management to enhance light trapping/utilization; (4) a lower recombination rate of the photogenerated electron-hole pairs is acquired because of the improved surface properties and the quantum confinement effect of graphene-like nanosheets. Consequently, the resulted 3D g-C3N4/C-NS demonstrated an exceptionally high performance for the photoreduction of CO2 with H2O to produce value-added fuels (such as CH4 and CO) under simulated solar irradiation.
Section snippets
Materials preparation
The typical synthesis process of 3D porous g-C3N4/C nanosheets composite is very simple. Specifically, melamine powder (purchased from Sinopharm Chemical Reagent Co.) and soybean oil at the weight ratio of 6:1 were ground together for 10 min in an agate mortar to form a homogenous mixture. Then the mixture was transferred into a temperature-controlled tube furnace and annealed at 600 °C for 2 h with a heating ramp of 2 °C min−1 under N2 flow. After cooling to room temperature, the obtained
Characteristics of materials
Fig. 2a gives the photographs of the synthesized 3D g-C3N4/C-NS, H-g-C3N4/C, and bulk g-C3N4. It is reasonable that the bright yellow color of g-C3N4 becomes black after introducing the hybrid carbon. Compared with the bulk g-C3N4 and H-g-C3N4/C, the volume of 3D g-C3N4/C-NS is much larger under the same weight, indicating the formation of fluffy state nanosheets after activation. The same characteristic is observed for g-C3N4 nanosheets prepared from liquid exfoliation [38,39]. Such structural
Conclusion
In summary, we have synthesized 3D porous g-C3N4/C nanosheets composite for the first time through a simple pyrolysis and subsequent carbothermal activation method using a mixture of melamine and natural soybean oil as precursor. The key feature of this 3D porous architecture is that the formed ultrathin graphene-like g-C3N4/C nanosheets have crumpled morphology and hierarchical mesostructure, leading to a high surface area and large pore volume. Photocatalytic measurements reveal that the
Acknowledgements
This work was carried out with financial supports from National Natural Science Foundation of China (Grant No. 21103024 and No. 61171008), Yancheng Huanbo Energy Technonogy Limited Company, Longyuan Youth Innovative Talents Program, and Technology Development Project of Jiaxing University.
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