Synthesis, Optical Properties, and Photocatalytic Activity of One-Dimensional CdS@ZnS Core-Shell Nanocomposites
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One-dimensional (1D) CdS@ZnS core-shell nanocomposites were successfully synthesized via a two-step solvothermal method. Preformed CdS nanowires with a diameter of ca. 45 nm and a length up to several tens of micrometers were coated with a layer of ZnS shell by the reaction of zinc acetate and thiourea at 180 °C for 10 h. It was found that uniform ZnS shell was composed of ZnS nanoparticles with a diameter of ca. 4 nm, which anchored on the nanowires without any surface pretreatment. The 1D CdS@ZnS core-shell nanocomposites were confirmed by XRD, SEM, TEM, HR-TEM, ED, and EDS techniques. The optical properties and photocatalytic activities of the 1D CdS@ZnS core-shell nanocomposites towards methylene blue (MB) and 4-chlorophenol (4CP) under visible light (λ > 420 nm) were separately investigated. The results show that the ZnS shell can effectively passivate the surface electronic states of the CdS cores, which accounts for the enhanced photocatalytic activities of the 1D CdS@ZnS core-shell nanocomposites compared to that of the uncoated CdS nanowires.
KeywordsNanocomposites Nanowires Semiconductor Photocatalysis Cadmium sulfide
One-dimensional (1D) nanocomposites consisting of two important functional materials have attracted significant attention with respect to their fascinating properties and potential applications in the field of nanodevice fabrication [1–3]. Superior or new properties and diverse functions have been realized by assembling different types of constituents into nanocomposites with controlled structure and interface interactions. Recently, considerable research efforts have been directed on the shape and compositional control of 1D semiconductor-included nanocomposites, such as nanowires with superlattice structures [4, 5], core-shell coaxial nanowires [6–13], biaxial or sandwich-like triaxial nanowires [14–18], and anisotropic (e.g., dimer-type and hierarchical composite materials) heterostructures [19–23]. In particular, core-shell nanostructures are reported most often because various mechanisms can be involved in shell growth that do not necessarily relate to epitaxy between the inorganic components, and consequently enhanced or modified properties are resulted from the particular dimensionality. For example, Lieber et al.  have reported on the synthesis of Ge/Si core-shell nanowires (NW) and high-performance as field-effect transistors due to the reduced interface scattering. Xu et al.  demonstrated that Ni nanowires encapsulated within fullerene cables exhibited enhanced and anisotropic ferromagnetic behavior along the nanowire axes.
The development of new heterostructures is still a challenging subject, for the critical step of this work remains how to modulate the properties by tailoring the nucleation of one phase on the surface of the other. Wurtzite CdS, a direct band gap semiconductor with a gap energy of 2.42 eV at 300 K, is one of the first discovered semiconductors which has promising applications in photochemical catalysis, gas sensor, detectors for laser and infrared, solar cell, nonlinear optical materials, various luminescence devices, and optoelectronic devices [26–28]. On the account of this, various 1D CdS nanostructural materials have been generated through various routes [29–32]. ZnS has a wider band gap (Eg = 3.7 eV) than CdS. The surface modification of a wide band gap semiconducting shell around a narrow band gap core can alter the charge, functionality, and reactivity of the materials and consequently enhance the functional properties due to localization of the electron-hole pairs [33–35]. Up to now, CdS@ZnS core-shell nanostructures with stronger luminescence and electrical properties have been successfully prepared by metal-organic CVD (MOCVD) process or wet–chemical approach [10, 36–38].
In this paper, we try to use preformed CdS nanowires as 1D nanoscale substrates for the growth of ZnS shell by a two-step solution method. No surface pretreatments were needed to introduce new surface functional groups, or additional covalent and/or noncovalent interconnectivity for the growth of ZnS onto CdS nanowires in our experiments. The optical properties and photocatalytic activities of the 1D CdS@ZnS core-shell nanocomposites under visible light (λ > 420 nm) were investigated. The results show that the ZnS shell can effectively passivate the surface electronic states of the CdS cores, which helps to enhance the photocatalytic activities of the 1D CdS@ZnS core-shell nanocomposites.
The Preparation of 1D CdS@ZnS Core-Shell Nanocomposites was Achieved via a Two-Step Solvothermal Process. All Reagents were Analytical Grade and were Used Without Further Purification
Preparation of CdS Nanowire
In a typical process, Cd(S2CNEt2)2(1.124 g, 0.1 mmol) prepared by precipitation from a stoichiometric mixture of NaS2CNEt2and CdCl2in water, was added to a Teflon-lined stainless steel autoclave with a capacity of 55 mL. Then the autoclave was filled with 40 mL ethylenediamine up to about 70% of the total volume. The autoclave was maintained at 180 °C for 24 h and then allowed to cool to room temperature. A yellowish precipitate was collected and washed with absolute ethanol and distilled water to remove residue of organic solvents. The final products were dried in vacuum at 70 °C for 6 h.
Preparation of 1D CdS@ZnS Core-Shell Nanocomposites
As a general procedure, CdS nanowires (0.03 g, 0.2 mmol) were well-dispersed in 45 mL absolute ethanol under sonication, then Zn(CH3COO)2 · 2H2O (0.022 g, 0.1 mmol) and (NH2)2CS (0.015 g, 0.2 mmol) were added in sequence. The resulting mixture was loaded into a 55 mL-Telfon-lined autoclave and maintained at 180 °C for 12 h. After the reaction was completed, the autoclave was cooled to room temperature naturally, and the resulting solid products were collected, washed with absolute ethanol and distilled water for twice, and then dried in vacuum at 70 °C for 6 h.
The crystal structure of the product was determined from the X-ray diffractometer (Bruker D8) with a graphite monochromator and CuK α radiation (λ = 1.5418 Å) in the range of 15–80° at room temperature while the tube voltage and electric current were held at 40 kV and 20 mA. The morphology and microstructure of the products were determined by FESEM (Hitachi S-4800), TEM (JEM-100CXII) with an accelerating voltage of 80 kV, and high-resolution TEM (HR-TEM, JEOL-2100) with an accelerating voltage of 200 kV equipped with an energy-dispersive X-ray spectrometer (EDS). The UV-vis spectra and room photoluminescence (PL) were performed on a TU-1901 UV-vis spectrophotometer and WGY-10 spectrofluorimeter.
Photocatalytic Decomposition of MB and 4CP
To evaluate the photocatalytic activity of the synthesized 1D CdS@ZnS core-shell nanocomposites, the degradation of MB and 4CP were carried out in a jacketed quartz reactor filled with 50 mL of the test solution in the presence of the catalyst (50 mg) by using a 300-W Xe lamp with a cutoff filter (λ > 420 nm) as light source. Prior to illumination, the suspension was stirred for 20 min in the dark to favor the pollutant’s adsorption onto the catalyst surface, followed by determination of the concentration of the pollutants as the initial concentrationC0. The remaining concentration of pollutants in the suspension at given intervals of irradiation was measured on a TU-1901 UV-vis spectrophotometer.
Results and Discussion
Characterizations of the Final Products
The existence of ZnS layer on the surface of CdS nanowires was further confirmed by EDS data (Fig. 3e). EDS analysis conducted on the central region of a CdS@ZnS core-shell nanowire indicates that the nanowire is mainly composed of Cd, Zn and S with Cd/Zn/S ratio of 0.62:0.36:1 (a stoichiometry close to Cd x Zn1−xS). The Cu peaks were detected from the grid for TEM observation.
Optical Properties and Photocatalytic Activity of 1D CdS@ZnS Core-Shell Nanocomposites
In summary, 1D CdS@ZnS core-shell nanocomposites were successfully synthesized via a two-step mild solution method. It has been demonstrated that preformed CdS nanowires with a diameter of ca. 45 nm and a length up to several tens of micrometers were coated with a uniform layer of ZnS shell. This shell was composed of ZnS nanoparticles with a diameter of ca. 4 nm, anchoring on the surface of CdS nanowires without any surface pretreatment or functionalization. The optical properties and photocatalytic activities of the 1D nanocomposites under visible light were separately investigated. Compared to the neat CdS nanowires, the as-obtained 1D CdS@ZnS core-shell nanocomposites showed significantly enhanced photocatalytic activities owning to the effective passivation of the surface electronic states by the ZnS shells.