Construction of CdS@UIO-66-NH2 core-shell nanorods for enhanced photocatalytic activity with excellent photostability

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Abstract

A novel class of CdS@UIO-66-NH2 core shell heterojunction was fabricated by the facile in-situ solvothermal method. Characterizations show that porous UIO-66-NH2 shell not only allows the visible light to be absorbed on CdS nanorod core, but also provides abundant catalytic active sites as well as an intimate heterojunction interface between UIO-66-NH2 shell and CdS nanorod core. By taking advantage of this property, the core-shell composite presents highly solar-driven photocatalytic performance compared with pristine UIO-66-NH2 and CdS nanorod for the degradation of organic dyes including malachite green (MG) and methyl orange (MO), and displays superior photostability after four recycles. Furthermore, the photoelectrochemical performance of CdS@UIO-66-NH2 can be measured by the UV–vis spectra, Mott-Schottky plots and photocurrent. The remarkably enhanced photocatalytic activity of CdS@UIO-66-NH2 can be ascribed to high surface areas, intimate interaction on molecular scale and the formation of one-dimensional heterojunction with n-n type. What’s more, the core-shell heterostructural CdS@UIO-66-NH2 can facilitate the effective separation and transfer of the photoinduced interfacial electron-hole pairs and protect CdS nanorod core from photocorrosion.

Introduction

Semiconductor-based photocatalysis with its green and sustainable advantages can be seen as a promising approach for environment sustainability [1], [2], [3]. Among the visible-light-driven semiconductors, CdS is attracting great attention for photocatalysis due to its well-suited band gap (2.4 eV) and excellent electronic and optical properties which benefit for the rapid excitation of charge carriers by effective visible light absorption [4], [5], [6]. However, several factors limit the utilization of bare CdS such as its fast recombination rate of photogenerated electron-hole pairs and photocorrosion [7], [8], [9]. One-dimensional CdS nanorod can be used to overcome these difficulties to some extent, demonstrating a unidirectional flow of photogenerated charge carriers as well as a short surface transfer distance, although the photocatalytic activity of CdS nanorod is relatively low [10], [11], [12]. Therefore, CdS nanorod combined with other components, such as noble metals (Pt, Au) [13], carbon materials (carbon nitride, graphene) [14] and sulfide semiconductors (MoS2) [15] has been proved to be effective.

Metal-organic frameworks (MOFs), due to the high surface areas, tunable pore size, exposed active sites and structural flexibility, have attracted considerable attention for photocatalysis [16], [17], [18]. MOFs with high surface area not only can avoid the aggregation of semiconductor nanoparticle, but provide more reaction centre as well as catalytic active sites, which can improve the photocatalytic performance [19], [20], [21]. Recently, the encapsulation of active species like nanoparticles into MOFs to form core-shell structures is proven to be effective for the enhanced photocatalytic activity [22], [23]. For example, Zeng et al. prepared core-shell CdS@ZIF-8 structure by polyvinylpyrrolidone-stabilized CdS, promoting the photocatalytic activity of pure ZIF-8 [24]. Since the sulfide ions of CdS are easy to oxidize to sulfurs by the effect of holes, it is efficient to stabilize sulfide ions on the surface of CdS and transfer holes via core-shell structure [25]. Therefore, suitable band gaps between MOFs and CdS as well as core-shell structure can offer the strong driving force to separate the photogenerated charge carriers. Combining the MOFs and CdS nanorod to fabricate the effective and stable photocatalyst is beneficial to obtain the intimate interfacial contact and protect unstable compound. Nevertheless, there are few reports on the construction of core-shell structures between MOFs and CdS nanorod.

Herein, the novel CdS@UIO-66-NH2 heterojunction with different amounts of UIO-66-NH2 shell on the CdS core nanorod is fabricated using a facile one-pot in situ solvothermal approach as shown in Scheme 1. Malachite green (MG) and methyl orange (MO) are chosen as the model pollutant to measure the degradation efficiency and stability of synthesized catalysts under visible-light irradiation. In comparison with pristine CdS nanorod and UIO-66-NH2, the constructed n-n type core-shell photocatalyst of CdS@UIO-66-NH2 presents high surface area, intimate heterojunction interface, extended visible-light absorption ability, remarkably improved photocatalytic degradation efficiencies of MG (99.5%) in 25 min and MO (95.7%) in 45 min, and superior photostability under optimized experimental conditions. Besides, the photoelectrochemical behaviour of as-prepared samples can be provided by combing the UV–vis spectra, Mott-Schottky plots and photocurrent, and the underlying mechanism can be explained by the trapping experiments. Furthermore, the charge transfer process between CdS core and UIO-66-NH2 shell is systematically investigated.

Section snippets

Synthesis of CdS-NR

CdS-NR was synthesized by a modified one-step solvothermal method. Typically, CdCl2·2.5H2O (0.4621 g, 2.0 mmol) and CH4N2S (0.4621 g, 6.0 mmol) were dissolved into ethylenediamine (50 mL). The mixture was transferred to a 100 mL Teflon-lined autoclave and maintained at 160 °C for 48 h. After natural cooling, the resulting yellow solid products were washed several times with distilled water and ethanol. The final CdS nanorod was then dried at 60 °C overnight.

Synthesis of UIO-66-NH2

ZrCl4 (0.2332 g, 1.0 mmol) and 2-NH2

Characterization of morphology and structure

Fig. 1 displays the XRD patterns of the CdS-NR, UIO-66-NH2, and CdS@UIO-66-NH2 core-shell composites. The characteristic diffraction peaks of CdS-NR can be clearly indexed to the hexagonal phase (JCPDS, No. 41-1049) [26], with the major (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2) planes clearly observed. For pristine UIO-66-NH2, all the diffraction peaks are in good agreement with the previous reports [27], [28], showing the excellent crystallinity. Although the composites

Conclusion

In summary, a novel CdS@UIO-66-NH2 core-shell hybrid was successfully prepared by using a facile in-situ solvothermal method, and the UIO-66-NH2 shell was coated on the surface of CdS nanorod core homogeneously to form one-dimensional heterostructures. The organic dyes MG and MO are chosen as model pollutant to evaluate the photocatalytic performance upon visible-light irradiation, and the obtained CdS@UIO-66-NH2 shows highest degradation efficiency of MG (99.5%) in 25 min and MO (95.7%) in

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21703019, 51702025), Natural Science Foundation of Jiangsu Province (BK20150259, BK20160277).

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