ZnO@CdS Core-Shell Heterostructures: Fabrication, Enhanced Photocatalytic, and Photoelectrochemical Performance
© Ding et al. 2016
Received: 14 March 2016
Accepted: 13 April 2016
Published: 18 April 2016
ZnO nanorods and ZnO@CdS heterostructures have been fabricated on carbon fiber cloth substrates via hydrothermal and electrochemical deposition. Their photocatalytic properties were investigated by measuring the degradation of methylene blue under ultraviolet light irradiation. The result illustrated that the photodegradation efficiency of ZnO@CdS heterostructures was better than that of pure ZnO nanorods, in which the rate constants were about 0.04629 and 0.02617 min−1. Furthermore, the photocurrent of ZnO@CdS heterostructures achieved 102 times enhancement than pure ZnO nanorods, indicating that more free carriers could be generated and transferred in ZnO@CdS heterostructures, which could be responsible for the increased photocatalytic performance.
Recently, semiconductor-based photocatalysts, as a kind of “green technology”, have attracted much more attention, in which varieties of metal oxide semiconductors, such as TiO2 [1–4], ZnO , SnO2 [6, 7], Cu2O [8, 9], CdS , and ZnS , have been fabricated as photocatalysts to decompose environmental pollutants. Among them, ZnO has been systemically investigated as photocatalysts due to its high electron mobility, flexible morphologies, easy synthesis, low cost, and nontoxicity [12, 13]. However, the inherent drawbacks of ZnO, including larger bandgap and fast internal recombination of photogenerated electron–hole pairs, result in low photodegradation efficiency [14, 15]. In addition, serious photocorrosion in the photocatalytic process also influences the degredation effect for organic pollutants. All the abovementioned have greatly hindered the potential applications. Many efforts have been put to improve the efficiency of photogenerated carriers and extend the spectral response range, such as doping , loading noble metals [17, 18], and combining with other semiconductors [19–30], and so on. Combining ZnO with other narrow bandgap semiconductors (for example ZnSe [19, 20], Cu2O , CdSe , CdS [21, 30] etc.) has proved to be feasible for promoting photocatalytic performance. Among these materials, CdS attracts much interest because of the similar lattice structures between CdS and ZnO. Moreover, ZnO/CdS heterojunction can induce a type-II band structure, the conduction band of ZnO is located between the valence band and the conduction band of CdS, which can hinder the recombination of photogenerated electron and hole. Xu et al. prepared the ZnO sheet-based hierarchical microspheres incorporated with CdS nanoparticles by hydrothermal method followed by ultrasonication treatment. The ZnO/CdS heterostructures exhibit higher photocatalytic activity than pure ZnO under sunlight . Kundu reported a simple wet chemical route to obtain nanoscale heterostructures of ZnO/CdS without using any molecular linker, the heterostructures with the CdS loading exhibit high activity for the degradation of methylene blue (MB) under solar irradiation conditions, and also the photoactivity of the material could be tuned by manipulating the interface of the heterostructure . Though some reports about ZnO@CdS heterostructure with enhanced photocatalytic activity have been reported, however, the efficiency of photocatalytic degradation needs further improvement. Furthermore, the photoelectrochemical performance of ZnO@CdS heterostructure grown on carbon fiber cloth with high photocurrent response is rarely reported.
Herein, ZnO@CdS heterostructures have been fabricated on carbon fiber cloth substrate by a two-step method including electrochemical deposition and hydrothermal method. The morphologies, structures, photocatalytic, and photoelectrochemical properties of as-grown ZnO and ZnO@CdS heterostructures were carefully investigated.
ZnO nanorods were grown on carbon fiber cloth by electrochemical deposition method. Firstly, carbon fiber cloth was cleaned by sonication in acetone, ethanol, and deionized water. Then, the mixed aqueous solution of 5 mM zinc nitrate hexahydrate (Zn(NO3)2 · 6H2O) and 5 mM hexamethylenetetramine (HMT) were used as the electrolyte, in which the carbon fiber cloth substrate, a 2 × 2 cm platinum plate, and Ag/AgCl in a saturated KCl solution were used as working electrode, counter electrode, and reference electrode, respectively. Finally, the electrolytic cell was placed in a water bath to keep constant temperature of 90 °C. The reaction was carried out for 1 h at a constant potential of −0.9 V versus the reference electrode. After reaction, samples were washed by deionized water several times, and dried in an oven at 60 °C for several hours.
The CdS layer was synthesized on ZnO nanorods using hydrothermal method. Further, 114.2 mg cadmium chloride (CdCl2 · 2H2O), 114 mg thiourea (CH4N2S), and 49 mg polyethylene glycol (PEG) were dissolved into a given amount (40 mL) of deionized water. The mixture solution was transferred into a Teflon-lined stainless autoclave. Then, the as-grown ZnO nanorods on carbon fiber cloth substrate were put into autoclave. After that, the autoclave was sealed and maintained at 140 °C for 9 h, and cooled to room temperature naturally. The samples were taken out of the solution and washed with ethanol and deionized water several times, followed by drying at 60 °C in an oven for several hours.
The morphologies and structures of as-grown ZnO nanorods and ZnO@CdS heterostructures were characterized by the field emission scanning electron microscopy (FESEM) (model: NoVaTM Nano SEM 250, FEI Company), X-ray diffraction (XRD) (model: Bruker D8 Advance) and transmission electron microscopy (TEM) (model: Tecnai G2 F20, FEI Company). The surface chemical composition and states of the ultimate ZnO@CdS heterostructure was analyzed using an X-ray photoelectron spectrometer equipped with a monochromatic Al Ka source (1486.6 eV) (model: Thermo ESCALAB 250XI).
Photocatalytic activities were tested by the photodegradation of methylene blue (MB) with photocatalytic reaction apparatus (XPA series-7, Nanjing) equipped with a 500-W mercury lamp as the light source. Typically, the sample (ZnO nanorods and ZnO@CdS nanomaterials) grown on carbon fiber cloth substrate (2 × 1.5 cm) as photocatalyst was placed into a quartz tube filled with 5 mL of MB (5 mg/L) aqueous solution. The solution was kept for 60 min in the dark to ensure the adsorption–desorption equilibrium between photocatalyst and methylene blue, and then irradiated with UV irradiation. The photocatalytic degradation of MB dye was analyzed by measuring the absorbance at 664 nm in the presence of photocatalyst exposed at different irradiation time intervals with a UV-Vis spectrophotometer (TU-1900/1901, Beijing). The photocatalytic measurements of the three photocatalysts (ZnO and ZnO@CdS) were performed in three independent experiments. All the experiments were performed at room temperature.
All electrochemical measurements were performed using a typical three-electrode system, which the sample grown on carbon fiber cloth substrate, a 2 × 2 cm platinum plate, and Ag/AgCl in a saturated KCl solution were used as working electrode, counter electrode, and reference electrode, respectively. Further, 0.5 M Na2SO4 aqueous solution (with pH buffered to ∼7.0) was used as the electrolyte. A 300-W Xe lamp was used as the light source for photocurrent test.
Results and Discussion
In summary, ZnO@CdS heterostructure was successfully fabricated on carbon fiber cloth substrate by a simple two-step method including electrochemical deposition and hydrothermal method. The photoelectrochemical measurement proved that more free carriers could be generated and transferred in ZnO@CdS heterostructure leading to high separation efficiency than that of ZnO nanorods. The type-II band structure of ZnO@CdS could improve the efficiency of carrier separation and transport; thus, the photocatalytic activities and photoelectrochemical could be significantly enhanced by the introduction of CdS layer on ZnO nanorods.
This work is supported by the National Natural Science Foundation of China (Grant No. 61504048, 21505050, 11304120, 11304121), the Encouragement Foundation for Excellent Middle-aged and Young Scientist of Shandong Province (Grant No. BS2013CL020, BS2014CL012), Shandong Provincial Natural Science Foundation (Grant No. ZR2013AM008), and a Project of Shandong Province Higher Educational Science and Technology Program (Grant No. J15LJ06, J14LJ03).
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