Surface localization of CdZnS quantum dots onto 2D g-C3N4 ultrathin microribbons: Highly efficient visible light-induced H2-generation
Graphical abstract
Introduction
During last few decades, high dependence and overuse of fossil fuels have led to serious energy crisis and environment concern. Thus, the development of clean, low-cost and efficient alternative to fossil fuels is a matter of utmost urgency [1], [2]. Hydrogen (H2) is considered as an effective clean energy resource and idealized substitution for fossil fuels, because of its high specific enthalpy and environmentally-friendly combustion products. Recently, water splitting with the aid of semiconductor photocatalysts has been perceived as one of the most promising technologies to obtain H2 [3], [4]. Moreover, the design and construction of high-efficiency noble-metal-free photocatalysts are quite appealing with the aim of providing sustainable and cost-competitive H2 [5].
In this sense, graphitic carbon nitride (g-C3N4), as an intriguing earth-abundant photocatalysts has attracted dramatically increasing interest in visible-light-driven H2-generation due to the unique two-dimensional (2D) layered structure, high chemical stability and facile preparation [6], [7]. However, the pure g-C3N4 is usually restricted by low photocatalytic efficiency because of the rapid recombination of photo-induced electron-hole pairs, small specific surface area and low visible light utilization [8], [9]. To solve these problems, many approaches have been devoted to the enhancement of its photocatalytic activity (such as exfoliated g-C3N4 nanosheets [10], [11], formation of mesoporous and O-doping g-C3N4 [12], [13], hybridization with ZnO and CdS [14], [15], loading of Pt particles [16], fabrication of nanojunctions with MoS2 [17], and cooperation with carbon quantum dots [18], [19]), the H2-generation rate was, however, not satisfactory to date. Furthermore, to enhance the visible-light absorption and to obtain high active sites of g-C3N4, optimizing appropriate band structure and integrating multi-functional composites are reasonable ways towards efficient visible-light utilization and high photocatalytic activity in the process of splitting water [20], [21].
Among various semiconductor photocatalysts, the CdZnS solid solutions have been proven to be efficient photocatalysts due to the relatively narrow band gap, which can serve as a light harvesting antenna for effective absorption of the solar energy [22], [23], [24]. However, similar to CdS, pure CdZnS tends to form large particle aggregation and photocorrosion layer, resulting in a higher recombination rate of photo-induced electron-hole pairs and unstable structures, which have limited the further enhancement of photocatalytic efficiency [25]. To achieve the deaggregation of CdZnS and to develop high-efficiency noble-mental-free photocatalysts, herein, we have put forward the strategy that localizing CdZnS QDs onto the 2D layered g-C3N4 micro/nanostructures (Scheme 1). Firstly, the visible-light-absorption efficiency can be improved by the high dispersion of the QDs in the 2D host interface. Secondly, the electronic structures of CdZnS QDs and g-C3N4 are strongly coupled and the matched band structure between two components could promote the photogenerated charge separation and transfer, which lowers charge recombination possibility. Thirdly, the nano-sized CdZnS QDs present a sufficient contact with g-C3N4, which leads to efficient electronic mobile channel and short migration distance, and thus resulting in increased reactive sites. Therefore, it is expected that the combination of CdZnS and g-C3N4 can be an ideal model to overcome the disadvantages that exist in both g-C3N4 and CdZn.S simultaneously.
In this work, 2D g-C3N4 microribbons have been fabricated firstly via a combination of thermal exfoliation and liquid exfoliation, which were then used as a host supporter for anchoring Cd0.5Zn0.5S QDs (particle size: ca. 5 nm). The Cd0.5Zn0.5S QDs are highly dispersed and localized onto the surface of the g-C3N4 microribbons as confirmed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), and the aggregation of the Cd0.5Zn0.5S is successfully avoided during both hydrothermal process and photocatalytic test. At an optimal Cd0.5Zn0.5S content of 32 wt%, highly efficient visible-light-induced H2-generation can be achieved with a rate of 33.41 mmol h−1 g−1, which is 27.39 and 9.18 times higher than the pure g-C3N4 microribbons and Cd0.5Zn0.5S respectively. Therefore, the Cd0.5Zn0.5S@g-C3N4 can serve as an effective system to integrate enhanced visible-light harvest, charge separation and decreased photocorrosion.
Section snippets
Preparation of the g-C3N4 microribbons
All chemicals (analytical grade purity) used in this research were purchased from Aladdin (Shanghai) Chemistry Co., Ltd and used without further purification. The bulk g-C3N4 powders were prepared via pyrolysis of urea in a tube furnace. In a typical synthesis, 4 g of urea was placed into a porcelain boat with cover and heated to 550 °C for 2 h at 2 °C/min heating rate. After cooling naturally to room temperature, the resultant powder was then direct thermal exfoliation heated to 550 °C for 2 h at 2
Synthesis of g-C3N4 microribbons
As shown in Scheme 1, bulk g-C3N4 was prepared by heating urea at 550 °C for 2 h followed by thermal exfoliation for another 2 h at 550 °C to obtain g-C3N4 [10], [26]. Then, pale-yellow g-C3N4 were suspended in water (concentration: 0.5 mg/ml), and then sonicated at room temperature for 6 h to exfoliate and get uniformly dispersed g-C3N4 product. The white homogenous dispersion of g-C3N4 was highly stable, and showed no precipitation even after being stored for 1 month under ambient conditions (Fig.
Conclusions
In summary, 2D g-C3N4 ultrathin microribbons were prepared by a thermal exfoliation and liquid exfoliation process, and a new type Cd0.5Zn0.5S QDs sensitized g-C3N4 microribbons photocatalysts can be further obtained via an in-situ hydrothermal method. The Cd0.5Zn0.5S QDs@C3N4 micro/nanostructures present a high-efficiency photocatalytic H2-generation rate during water splitting process under visible-light irradiation. The Cd0.5Zn0.5S QDs in the hybrids have largely enhanced visible-light
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (21173021, 21231002, 21276026, 21301016 and 21473013), 973 Program (2014CB932103), Institute of Chemical Materials, China Academy of Engineering Physics (20121941006), the Beijing Municipal Natural Science Foundation (2152016), and the 111 Project (B07012).
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