Abstract
We report the synthesis of PVP-capped Au-CdSe hybrid nanostructures synthesized using the UV-irradiation method. The high resolution transmission electron microscopy (HRTEM) and powder X-ray diffraction (XRD) studies confirm the presence of the hybrid gold and CdSe nanoparticles.
1. Introduction
Metals and semiconductor nanomaterials with diverse morphologies such as rods [1, 2], tetrapods [3], prisms [4], cubes [5], and other complex shapes [6, 7] have been regularly reported in nanoscience literature. However, the synthesis of multicomponent materials incorporating a metal and a semiconductor material is not very common. Such systems represent a new class of materials, where catalytic metals are paired with a semiconductor material within the same structure. This combination of a metal-semiconductor material provides new functionalities to the nanostructures. They have been studied as photocatalysts, in photoelectrochemical cells, in the photochemical purification of organic contaminants, and in bacterial detoxification [8]. The excitation of the surface plasmon in metal nanoparticles placed into a semiconductor can be expected to enhance optical properties such as absorption and photoluminescence. Heterostructured materials such as Au-CdSe [9], Au-CdS [9], Au or Ag on ZnO [10], Co and Au on TiO2 and PbS/Au nanowires have been reported [11].
In order to successfully synthesize multi-component nanostructures there must be a match of the interface between materials which may have different crystallographic structures, lattice dimensions, and thermal stability as well as chemical reactivity. New properties may emerge due to the combination of different material systems on the nanoscale. The optical properties of these nanostructures, for example, often exhibit interesting deviations from either their individual components or from a physical mixture of the two components. These optical effects may include a shift in the surface plasmon resonance (SPR) of noble metal nanocrystals when combined or coated with other materials or changes in the photoluminescence intensity of semiconductor nanocrystals [12, 13].
Here we report the synthesis of PVP-capped Au-CdSe hybrid nanoparticles. We have adapted the synthetic methodologies for our recently reported individual PVP-capped Au and cysteine-capped CdSe nanoparticles [14]. The anisotropic, water soluble PVP-capped gold nanoparticles were synthesized using a UV-radiation technique. UV-light was used to reduce Au3+ ions into metal nanoparticles. The concentration of the starting materials, lamp wavelength, and irradiation time were also varied towards determining the size and morphology of the as-synthesized metal nanoparticles. Our group has also reported the synthesis of water-soluble cysteine-capped CdSe nanoparticles [15]. In the current study the above synthetic procedures were adapted to synthesize Au-CdSe hybrid core shell particles. Very briefly, a stock solution of metal nanoparticles was prepared by mixing HAuCl4 and PVP in water. This solution was irradiated to reduce the metal. In a separate reaction, CdSe nanoparticles were prepared by adding selenium, NaBH4 under N2 gas in a three-necked flask. After 3 hours a solution of CdCl2 solution was added with continuous stirring. The resultant CdSe was then added to the PVP-capped Au in the UV reactor to produce Au-CdSe metal hybrid nanoparticles.
2. Materials and Experimental Procedure
2.1. Materials and Instrumentation
Cadmium chloride, chloroauric acid (HAuCl4), sodium borohydride, acetone, and PVP were all purchased from Aldrich and selenium powder from Merck. All the chemicals were of analytical grade and used as purchased. A Perkin Elmer Lambda 20 UV-vis Spectrophotometer was used to carry out optical measurements in the 200–1100 nm wavelength range at room temperature. Samples were placed in quartz cuvettes (1 cm path length). A film CM120 BIOTWIW at 80 K was used for transmission electron microscopy (TEM). Samples for TEM analysis were prepared by placing a drop of Au-CdSe solution on carbon-coated copper TEM grids. The solution on the TEM grids were allowed to stand for few minutes following which, the extra solution was removed with the help of a blotting paper and the grids were allowed for drying prior to measurement. The HRTEM images were taken with a JOEL-2100 model transmission electron microscope with an accelerating voltage of 200 kV.
2.2. Synthesis of PVP-Capped Au-CdSe Hybrid Nanoparticles
Gold nanoparticles were prepared using the UV-irradiation method reported previously [14]. The method involves the reaction of chloroauric acid (0.3146 g, 4.00 10−3 M) with PVP (0.900 g) in distilled water (200.0 mL). This solution was irradiated with UV-light (mercury lamp (450–500 nm)) for an hour in order to reduce the gold to form gold nanoparticles. The CdSe nanoparticles were prepared by dissolving selenium powder (0.0253 g, 0.320 mmol) in distilled water (20.0 mL) in a three-necked flask under N2 atmosphere with continuous stirring. To this solution, NaBH4 (0.03 g, 0.320 mmol) dissolved in distilled water (20.0 mL) was added. The NaBH4 was allowed to completely reduce the selenium for 3 h. Cadmium chloride (0.0644 g, 0.320 mmol) dissolved in distilled water (20.0 mL) was then added to this solution to produce CdSe.
The Au-CdSe hybrid nanoparticles were synthesized by adding the solution of CdSe nanoparticles, to the Au-metal nanoparticles (4.00 10−3 M) solution. This resulted in a colour change to purple-wine-red solution. This solution was allowed to completely react for an hour under N2 atmosphere with continuous stirring. The resultant solution was centrifuged to isolate the Au-CdSe hybrid nanoparticles.
3. Results and Discussion
Heterostructured materials exhibit enhanced electronic and optical properties including a tunable band gap as a result of the strong quantum confinement effect and high band edge absorption [16–19]. Figure 1(a) shows the absorption spectrum of the Au-CdSe hybrid nanoparticles prepared from the 4.00 10−3 M gold nanoparticle solution. The surface plasmon resonance (SPR) peak assigned to gold in the Au-CdSe metal-semiconductor nanoparticles is broader than the SPR peak in the pure AuNPs solution (inset Figure 1(b)). Previous reports of hybrid nanocrystals have demonstrated that the optical properties of the colloids are simply a linear combination of the properties of the individual components [20, 21]. In this work the SPR peak for the sample of AuNPs is located at 530 nm (Figure 1(a)) whereas the SPR peak of the hybrid Au-CdSe particles is at 540 nm, a red shift of 10 nm to the parent AuNPs. This difference could be due to the overlap of the electronic states of the different components of the hybrid particles which modifies the surface plasmon resonance [20, 22]. Alternatively, the shift may reflect that the Au portion of the hybrid nanocrystals is partially covered with CdSe, which possesses the higher index of refraction than the organic capping ligands. The presence of a material with a higher index of reflection is expected to shift the Au plasmon towards longer wavelengths and has been observed experimentally by varying the refractive index of the solvent [23, 24], as well as in hybrid nanocrystals containing Au domains [21].
The structural properties of the hybrid nanocrystals were studied by transmission electron microscopy (TEM) and high resolution TEM. Figures 2 and 3 show the TEM and HRTEM images of Au-CdSe hybrid nanoparticles, respectively. The TEM image shows particles with varying sizes. There are a few particles which are very large (appearing as dark spots) while some particles appear as agglomerates. The average particle size observed was 6.09 1.1 nm. The denser spherical gold particles are distinctly visible in the TEM image. The gold nanoparticles appear as a dark core with the CdSe particles appearing less dark on the surface. The lattice spacing of 0.24 nm corresponds to the (222) planes of cubic gold. The clearly visible lattice fringes of the CdSe shell can be assigned to the (101) plane of hexagonal CdSe. The XRD pattern (Figure 4) shows peaks attributed to both gold and CdSe. The (100), (103), and (112) planes of hexagonal CdSe are distinctly visible in the diffractogram. Cubic gold is represented by the (111), (220), (311), and (222) planes. Our previous study on hybrid Au-CdSe nanoparticles showed the presence of cubic phase of CdSe [25]. In this method the presence of the stable hexagonal phase is similar to CdSe prepared via the high temperature organometallic routes.
4. Conclusions
PVP-capped Au-CdSe hybrid nanostructures have also been synthesized at room temperature using UV-irradiation method. The absorption properties of the hybrid material are very similar to that of the parent gold nanoparticles. The lattice spacing visible in the HRTEM image can be indexed to both gold and CdSe, with the gold appearing as a dark core and the CdSe less contrasted on the surface. The XRD pattern shows peaks which belong to both the cubic gold and hexagonal CdSe.
Acknowledgments
This work was supported by the National Research Foundation (NRF) and Department of Science and Technology (DST) the South African Research Chair Initiative (SARCHi) program. The authors also acknowledge Dr. James Wesley-Smith from the University of KwaZulu-Natal Electron Microscopy Unit for the TEM measurements and the CSIR, Pretoria, for the HRTEM measurements.