Role of B-doping in g-C3N4 nanosheets for enhanced photocatalytic NO removal and H2 generation
Graphic abstract
Introduction
With the development of society and economy, the demand for energy is gradually increasing. While focusing on development, there are two issues that require attention. One problem is environmental pollution. The combustion of fuel produces a large amount of polluted gas while providing energy. For example, NO produced by fuel combustion is one of the prime reasons of air pollution [1]. The formation of acid rain and acid mist and the destruction of ozone layer are attributed to the existence of NO [2]. It is necessary to develop suitable methods to purify the polluted gases in the air. Selective Catalytic Reduction is an effective method to remove NO. This method converts NO into nitrogen and water under the combined effect of a suitable temperature and catalyst. The large-scale application of this method is limited due to high reaction temperature, complicated process and high cost.
Photocatalytic technology using solar energy reasonably is an ideal way to remove NO. Because of request of environmental protection and economy life, it has attracted a large number of researchers to study the removal of NO under visible light [3], [4], [5], [6]. Excellent photocatalytic NO removal performance is closely related to the high efficiency of electron-hole separation and adsorption capacity of the catalyst. A lot of works focused on improving the separation efficiency of photo-generated carriers. For example, constructing heterogeneous photocatalyst is an effective method. The heterostructure composed of two semiconductors with appropriate energy band structures provides a channel for the transfer of electrons and holes [7], [8], [9]. Therefore, the heterogeneous photocatalyst has stronger redox ability. In addition, photocatalytic reaction is a simultaneous process of adsorption and reaction. The adsorption of catalyst affects the photocatalytic performance. In the photocatalytic process in which the photo-generated electrons or holes play a major role, the adsorbate is adsorbed on the active sites on the catalyst surface and reacts with the photo-generated electrons or holes to be removed [10]. In the photocatalytic process in which the active groups (superoxide radical or hydroxyl radical) play a major role, oxygen and water are adsorbed on the surface of the catalyst and react with photogenerated electrons and holes to generate active groups with oxidation capacity [11]. The adsorbate reacts with that active group. Therefore, adsorption also plays an important role in the photocatalytic reaction. In addition, another problem is the shortage of energy. The amount of fossil fuels stored in nature is limited. Hydrogen energy, as a renewable energy source, has attracted people's attention because of the high energy and harmless combustion products produced in the process of hydrogen combustion [12]. Therefore, it is an ideal method to solve the energy problem by using photocatalyst to split water to produce hydrogen under visible light. A great variety of photocatalysts for NO removal and hydrogen production have been developed and have shown excellent photocatalytic performance in applications [13], [14], [15], [16], [17], [18], [19], [20].
As an organic semiconductor, graphitic carbon nitride (g-C3N4) has attracted wide attention because of its visible light response, excellent thermal and chemical stability and low cost [21], [22], [23]. However, the g-C3N4 prepared by the traditional method is a bulk structure composed of a stack of multilayer sheets. Low specific surface area and rapid recombination of photo-generated carriers seriously limit the photocatalytic performance of bulk g-C3N4. In order to improve the photocatalytic performance of g-C3N4, a large number of modification methods have been reported, including morphological adjustment, elemental doping, and heterostructures [24], [25], [26], [27], [28]. Among them, the thermal polymerization temperature has a great influence on the morphology and photocatalytic properties of g-C3N4. Zhang et al. investigated the effects of different thermal polymerization temperatures on the morphology, structure and photocatalytic performance of g-C3N4 [29]. In the thermal polymerization process, the precursor was incompletely polymerized at lower polymerization temperature, which lead to structural defects in g-C3N4. These defects capture photo-generated carriers as trap sites, thus reducing the number of effective electrons and holes. At high polymerization temperature the bulk g-C3N4 was exfoliated into thin sheets with better crystallinity and higher specific surface area. Because of the compact crystal structure and less internal defects, electrons could be effectively transferred to the surface to participate in photocatalytic reaction [30]. In addition, element doping has been proved to be an effective modification strategy to improve the photocatalytic performance of g-C3N4. Chen et al. synthesized B-doped g-C3N4 by a one-step microwave heating method [31]. Compared with pure g-C3N4, the obtained B-doped g-C3N4 have excellent photocatalytic hydrogen production ability. The introduction of foreign atoms changed the distribution of electrons in the original g-C3N4 structure, and promote the effective migration of electrons, thus achieving the purpose of inhibiting the recombination of photogenerated electrons and holes. However, because of the lower polymerization temperature of the one-step microwave heating method, the structure of the obtained samples was still a layer-stacked bulk material, which had not yet achieved the desired photocatalytic performance. Therefore, the combination of both high polymerization temperature and elemental doping as an effective strategy modifying the photocatalytic performance of g-C3N4 is worthy of investigating.
In this work, B-doped g-C3N4 (BCN) nanosheets were synthesized by a two-step thermal polymerization method. In the first step of the thermal polymerization, thick sheets of g-C3N4 were obtained by the polycondensation of the precursors. The boron acid molecules were attached to the surface and interlayer of g-C3N4 by grinding. In the second step of the thermal polymerization, the thick sheets were exfoliated into thin sheets by the combined effect of high temperature and decomposition of the boron acid, and the B atoms were introduced into the structure of the g-C3N4. The obtained BCN shows excellent photocatalytic properties. Among all samples, BCN-0.75 shows the best photocatalytic performance with 54% removal of NO which is 1.9 times higher than that of pure g-C3N4. And hydrogen production rate of BCN-0.75 was 1639.29 μmol/g/h under the irradiation of visible light. The high polymerization temperature converted the thick flakes into thin flakes and enhanced the crystallinity of the g-C3N4 [29]. In addition, the B atom was introduced into the structure of the g-C3N4 by replacing the C atom. The introduction of B accelerated the transfer of electrons, thus inhibiting the recombination of electrons and holes and reducing the band gap. The synergistic effect of these factors enhanced the photocatalytic performance of BCN. This paper provided a convenient and effective method to modify g-C3N4 by doping with nonmetallic elements.
Section snippets
Synthesis of B-doped g-C3N4 (BCN) nanosheets
B-doped g-C3N4 nanosheets were prepared by a two-step thermal polymerization method. Fig. 1 shows the preparation procedure of BCN samples. The melamine was placed in an agate mortar for grinding. The ground melamine powders were transferred to an alumina boat and heated to 600 °C at a rate of 2 °C/min under Ar atmosphere to obtain thick sheets of g-C3N4. 1.5 g of g-C3N4 (600 °C) were added in an agate mortar and grinded by adding the appropriate amount of boric acid (H3BO3). The ground powder
Results and discussion
For the formation of B-doped g-C3N4 as shown in Fig. 1, a two-step polymerization method was developed. Melamine was used as precursors to get think g-C3N4 nanosheets via a thermal condensation at 600 °C. In this step, bulk g-C3N4 was obtained. A mechanical-chemical pre-reaction was used to incorporate B into the layered g-C3N4. Superior thin B-doped g-C3N4 nanosheets were obtained by a further thermal polymerization process at 700 °C.
Fig. 2(a) shows the XRD patterns of samples CN, BCN-0.5,
Conclusion
A two-step thermal polymerization method is proposed to prepare B-doped g-C3N4 (BCN) nanosheets. Superior thin BCN nanosheets exhibited enhanced adsorption and activation for NO. BCN nanosheets revealed a large specific surface area and much more active sites. In addition, B is introduced into the structure of g-C3N4 by a simple method. The transfer and separation of electrons is accelerated by the synergistic effect of defects and delocalized π bonds due to the introduction of B. The
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The work in the paper was supported by the projects from the National Natural Science Foundation of China (no. 51972145 and 51772130) and the Independent Innovation Team from Ji Nan Science & Technology Bureau (grant no. 2019GXRC016).
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