Regular Article
Ultrathin Z-scheme 2D/2D N-doped HTiNbO5 nanosheets/g-C3N4 porous composites for efficient photocatalytic degradation and H2 generation under visible light

https://doi.org/10.1016/j.jcis.2020.09.018Get rights and content

Highlights

  • Z-scheme N-doped HTiNbO5 nanosheets/g-C3N4 heterojunctions were constructed.

  • 2D/2D heterojunction of composites promoted the charge separation.

  • The synergistic effects improved photocatalytic efficiency.

  • The active species of h+, •OH and •O2 contributed to RhB photodegradation.

  • A direct Z-scheme photocatalytic mechanism was proposed.

Abstract

To realize highly efficient utilization of solar energy for solving problems of environmental pollution and energy shortage has attracted increasing attention. Herein, a two-step exfoliation-restacking process was employed to construct ultrathin Z-scheme two-dimensional (2D)/2D N-doped HTiNbO5 nanosheets/g-C3N4 (RTCN) heterojunction composites with the increased specific surface areas, showing the enhanced photocatalytic performance for rhodamine B (RhB) degradation and hydrogen (H2) generation under visible light irradiation. A 2D/2D heterojunction structure was formed between N-doped H+-restacked HTiNbO5 nanosheets (N-RTNS) and g-C3N4, which was beneficial for the effectively spatial separation of photogenerated charge carriers. The improved photocatalytic activities may be attributed to the synergistic effects of the increased specific surface area, N-doping and 2D/2D heterostructure. The active species of holes (h+), hydroxyl (•OH) and superoxide (•O2) radicals contributed to RhB photodegradation. A Z-scheme photocatalytic mechanism was proposed over RTCN-2 composite, showing dual advantages of the highly redox ability and efficient charge carrier separation.

Graphical abstract

Ultrathin Z-scheme 2D/2D N-doped HTiNbO5 nanosheets/g-C3N4 composites were constructed by a two-step exfoliation-restacking process, showing the enhanced photocatalytic performance for RhB degradation and H2 generation under visible light due to the synergistic effects of the increased specific surface area, N-doping and 2D/2D heterostructure.

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Introduction

Photocatalytic technology has been regarded as one of the most promising approaches to address problems of energy shortage and environmental pollution using solar energy, such as photocatalytic H2 generation and pollutants photodegradation [1], [2], [3], [4], [5], [6], [7], [8]. However, the single semiconductor photocatalysts show the relatively low photocatalytic efficiency owing to their high charge-carrier recombination probability and limited redox potential [9]. Constructing suitable heterojunction structure is considered as an effective route and widely employed to solve the above mentioned issues [10], [11], [12], [13]. However, two major challenges of the heterojunction system are the suitable band staggered arrangement and the closely interfacial contact for charge transfer/separation between two semiconductors. Given that, the unique 2D/2D heterojunction photocatalysts can profoundly increase the interfacial area and reduce the barriers for electron transport between two components, leading to the efficiently interfacial charge transfer by the electron tunneling effect and thus highly photocatalytic efficiency [14], [15].

Since graphitic carbon nitride (g-C3N4) was firstly applied in visible-light-driven photocatalytic H2 generation by Wang’s group in 2009 [16], it has attracted increasing attention in photocatalytic fields due to its typical layered structure, easy synthesis, suitable electronic band structure, environmental friendliness, highly thermal stability and favorable chemical stability [17], [18], [19], [20], [21]. Generally, g-C3N4 could be directly synthesized by a thermal polymerization process using the nitrogen-rich precursors of urea and melamine as the starting materials, which also showed the potential for the simultaneous N doping [22]. However, the relatively low photocatalytic activity of bulk g-C3N4 is still far from the practical applications.

Recently, various semiconductors, including In2O3 [23], Bi4NbO8Cl [24], CoFe-layered double hydroxide (LDH) [25] and ZnIn2S4 [26], have coupled with g-C3N4 to construct heterojunction photocatalysts, showing the enhanced photocatalytic efficiency. As to these type-II heterojunctions, the photogenerated electrons will transfer from the semiconductor I with a high conduction band (CB) potential to semiconductor II with a less negative CB potential, while holes will transfer from valence band (VB) of semiconductor II to semiconductor I with a less positive VB potential [27]. Thus, the overall redox abilities will be greatly reduced, where charge transfer pathway thermodynamically is not advantageous for the photocatalytic oxidation and reduction reactions. Instead, in a Z-scheme heterojunction system, the photogenerated electrons from one semiconductor with a less negative CB potential will recombine with holes generated from the other semiconductor with a less positive VB potential, resulting in the maximized redox potentials [28], [29], [30]. Thus, the construction of a Z-scheme photocatalytic system, with dual advantages of effective charge-carrier separation and high redox potential, is more advantageous for achieving highly photocatalytic efficiency [31]. Considering the advantages of 2D/2D heterostructure and Z-scheme photocatalytic system, an increasing attention was paid to Z-scheme 2D/2D g-C3N4-based photocatalysts with the highly photocatalytic performance, such as Bi3O4Cl/g-C3N4 [32], MnIn2S4/g-C3N4 [33], BiVO4/g-C3N4 [34], Bi2WO6/g-C3N4 [35], et al.

Layered HTiNbO5 shows some characteristics of layered structure, structural controllability, easy synthesis and favorable charge transfer character [36]. When HTiNbO5 was exfoliated and then combined with thin g-C3N4 nanosheets, the composite would have some intriguing properties such as suitable band staggered arrangement, larger specific surface area, efficient charge-carrier separation, highly redox potential and excellent photocatalytic activity. To the best of our knowledge, the ultrathin Z-scheme 2D/2D N-doped HTiNbO5 nanosheets/g-C3N4 composite for photocatalytic RhB degradation and H2 production has not been reported yet.

Herein, Z-scheme 2D/2D N-doped HTiNbO5 nanosheets/g-C3N4 (RTCN) heterojunction photocatalysts were prepared by a two-step exfoliation-restacking process. The crystal structure, morphology, light harvesting capacity and chemical states were well characterized. The photocatalytic activities were evaluated by RhB degradation and H2 generation under visible light irradiation. The transfer, separation and recombination processes of charge carriers were discussed in detail. Moreover, a possible direct Z-scheme photocatalytic mechanism was proposed based on the ESR and active species trapping experiments.

Section snippets

Catalysts preparation

All chemicals were used directly without further purification. Layered KTiNbO5 was synthesized based on our previous work [37]. The acidic solid HTiNbO5 was obtained by treating KTiNbO5 with HNO3 aqueous solution (2 M) at 60 °C for 3 days with a replaced treatment every day. The obtained HTiNbO5 (5.0 g) was firstly dispersed in the distilled water (1.0 L), and then tetrabutylammonium hydroxide (TBAOH) solution (15 wt%) was added into the above suspension until the pH value reached 9–10. After

Synthetic mechanism of RTCN nanocomposite

A schematic illustration for synthesizing RTCN nanocomposite is shown in Fig. 1. The protonated form HTiNbO5 is formed by an acid-exchange reaction of KTiNbO5 in the presence of HNO3 aqueous solution. After addition of TBAOH aqueous solution, HTiNbO5 can be exfoliated into HTiNbO5 nanosheets via a driving force of the acid–base reaction with the effective neutralization of interlayered protons [38]. Upon the further addition of dilute HNO3 aqueous solution, HTiNbO5 nanosheets agglomerate

Conclusions

In summary, Z-scheme 2D/2D N-doped HTiNbO5 nanosheets/g-C3N4 (RTCN) heterojunction photocatalysts were constructed by heating RTNS with melamine. The surface of RTNS was doped by nitrogen atoms to form N-RTNS and simultaneously covered with g-C3N4 to form a 2D/2D heterostructure between N-RTNS and g-C3N4 for the effectively spatial separation of charge carriers. The increased specific surface area and porous structure of RTCN-2 are responsible for the generation of more reactive sites as well

CRediT authorship contribution statement

Chao Liu: Conceptualization, Funding acquisition, Investigation, Writing - original draft, Writing - review & editing. Zitong Han: Investigation, Software. Yue Feng: Formal analysis. Hailu Dai: Methodology. Yefan Zhao: Visualization. Ni Han: . Qinfang Zhang: Validation, Project administration, Writing - review & editing. Zhigang Zou: Project administration, Supervision.

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

This work was supported by the National Natural Science Foundation of China (No. 51902282), China Postdoctoral Science Foundation (No. 2018M632283), and Joint Project of Industry–University–Research of Jiangsu Province (No. BY2019225).

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