Elsevier

Applied Surface Science

Volume 485, 15 August 2019, Pages 361-367
Applied Surface Science

Full length article
Fast photogenerated electron transfer in N-GQDs/PTI/ZnO-QDs ternary heterostructured nanosheets for photocatalytic H2 evolution under visible light

https://doi.org/10.1016/j.apsusc.2019.04.230Get rights and content

Highlights

  • N-GQDs were induced into the heterojunction of polytriazine imide and ZnO QDs.

  • New π-p-π networks were formed in the ternary heterostructured nanosheets.

  • The enhanced H2 evolution under visible light suggests the competitive advantages.

Abstract

The efficiency of photogenerated charge transport is one of the critical factors determining the photocatalytic performance in heterostructured photocatalysts. In this work, a two-dimensional conjugated polytriazine imide (PTI)-based photocatalyst was developed to achieve fast photoexcited electron transfer. These heterostructured nanosheets were synthesized by a one-pot condensation of a precursor mixture of cyanamide and N-doped graphene quantum dots (N-GQDs) in ZnCl2 containing salt melts. The ionothermal synthetic approach results in the N-GQDs/PTI in-plane heterostructured nanosheets, loaded with well-dispersed ZnO quantum dots (ZnO-QDs). Especially, the N-GQDs are coupled with PTI through the nitrogen atoms at the edge of triazine rings, forming new π-p-π conjugated connection units. Therefore, the obtained ternary heterostructured nanosheets can synchronously promote photoelectron transport through the new π-p-π conjugated networks as well as extend the optical response. Importantly, it exhibits approximately 230 and 3.6 times enhancement of H2 evolution over pure PTI and the PTI/ZnO-QDs heterostructure under visible light irradiation, respectively.

Introduction

The photocatalytic production of hydrogen from water splitting is a promising strategy for converting solar energy to chemical energy in the form of clean and renewable fuel. To achieve this goal, semiconductor photocatalysts have been widely studied over past few decades [[1], [2], [3]]. As is well known, the photocatalytic reaction primarily involves three processes: photoabsorption and electron-hole pairs generation; photocarrier separation and migration; and surface reduction/oxidation reactions. Unfortunately, it is difficult to balance all the above terms simultaneously on a single semiconductor photocatalyst [4]. To overcome this issue, heterostructured photocatalysts stacked by various materials such as CdS/TiO2 [5,6] and MoS2/graphene [[7], [8], [9]] have been sought to tune the band structure and restrain the photo-generated charge recombination. However, most of these photocatalysts are van der Waals heterostructures. It is still a great obstacle of sluggish photocarrier transfer because the interfacial transfer resistance can lead to the limited increase of photocarrier separation. Thus, developing novel heterostructures with more efficient interfacial charge transfer is highly imperative.

In recent years, polymeric semiconductor carbon nitride (CN) with favorable band gap has emerged as an effective photocatalyst for water splitting [[10], [11], [12], [13], [14]]. Owing to the two-dimensional conjugated unites and tunable electronic structures, it shows great promise for fast photocarrier transfer. The basic subunit of CN would be either triazine or heptazine or even both, interconnected through NH-bridges, forming π-conjugated networks. A well-defined and crystalline CN, polytriazine imide (PTI) has been synthesized in molten salts [[15], [16], [17]]. Recently, we reported on the one-step synthesis of PTI/ZnO quantum dots (PTI/ZnO-QDs) heterostructure composites in the KCl/ZnCl2 salt melts [18]. This binary heterojunction presents a significantly improved photocatalytic activity for H2 evolution with respect to pure PTI. Even so, the transport efficiency of photogenerated electrons is still not satisfied due to the low electrical conductivity and the obvious interfacial resistance. To accelerate the charge carrier transfer, it is necessary to introduce a new electron mediator between PTI and ZnO QDs.

Carbonaceous candidates such as carbon quantum dots (CQDs) and graphene quantum dots (GQDs) have been used as solid-state electron mediator for the construction of heterostructured photocatalysts [[19], [20], [21], [22], [23], [24]]. However, in most of the reported heterostructures, the intermolecular forces between the carbon dots and the semiconductors are usually relatively weak. They predominantly interact via physisorption, electrostatic attraction or van der Waals forces. Therefore, they may not be beneficial for the fast interfacial charge transfer. To end this, a strong electronic coupling or a covalent bond instead of the weak intermolecular interaction would be considerable.

Compared with GQDs, N-doped graphene quantum dots (N-GQDs) decorated with abundant amino groups [25,26] could be an ideal precursor, which can be stitched with PTI via Nsingle bondC bonds by thermal condensation [27]. Besides, the coupling of N-GQDs may promote the π-electron delocalization [28,29], and provide new transfer paths for light with lower energy and thus facilitate visible light harvesting.

Herein, to achieve more effective interfacial charge transfer, we introduced N-GQDs as the solid-state electron mediator into the PTI/ZnO-QDs heterostructure. New N-GQDs/PTI/ZnO-QDs (NPZ) ternary heterostructure composites were developed by a thermal pyrolysis of pre-synthesized N-GQDs and cyanamide in ZnCl2-based salt melts (As shown in Scheme 1). In the synthesis, ZnCl2 acts not only as the reactive solvent but also as the precursor for the formation of ZnO QDs. During the polymerization process, N-GQDs with abundant amino as the leaving groups could bond to the s-triazine units through the nitrogen atoms with rich lone-pair electrons at the edge of triazine rings, forming new π-p-π conjugated connection units. In addition, the ZnO QDs are dispersed on the N-GQDs−nitrogen−PTI nanosheets very well. It is believed that the enhancement of photocatalytic H2-evolution of NPZ sample is due to the fast photogenerated electron transfer and extended optical response resulting from the construction of the new in-plane π-p-π conjugated networks.

Section snippets

Synthesis of N-GQDs and GQDs

N-GQDs were prepared according to the method reported in the previous literature [26]. Typically, 0.72 g (12 mmol) urea and 0.84 g (4.0 mmol) citric acid were dissolved in 20 mL water. The obtained solution was then transferred into a 50 mL Teflon lined stainless autoclave and heated in an electric oven at 160 °C for 4 h. After cooling to the room temperature, 200 mL ethanol was added to the resulting solution. The precipitant was collected as the product by centrifugation and dried at 80 °C.

Results and discussion

The NPZ samples were synthesized as illustrated in Scheme 1. Similar to the pyrolysis of cyanamide, N-GQDs rich in amino groups could connect to the cyanamide-drived PTI through Nsingle bondC bonds, resulting in the N-GQDs/PTI nanosheets [27]. Simultaneously formed ZnO QDs randomly dispersed on the nanosheets, which can be verified by TEM and HRTEM. As shown in Fig. 1a and S1a, the NPZ0.5 sample is composed of nanosheets, which is similar to the NPZ0 sample synthesized in the absence of N-GQDs. Further,

Conclusions

In this study, N-GQDs have been introduced into PTI/ZnO heterojunction via a one-pot approach to prepare N-GQDs/PTI/ZnO-QDs ternary heterostructure nanosheets in ZnCl2-based salt melts. N-GQDs and PTI are conjugated linked through the nitrogen atoms at the edge of triazine rings, forming a new in-plane π-p-π network, which allows fast and efficient charge transfer. In addition, the in-situ synthesized ZnO QDs are well-dispersed on the PTI-based nanosheets enabling an excellent interfacial

Acknowledgements

This work was financially supported by the Natural Science Foundation of China (No. 21373103 and 51702134), the Natural Science Foundation of Jiangsu Province (No. BK20170310 and BK20170316), and the Australian Research Council Discovery Early Career Researcher Award scheme (DE140100716).

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