Jun
Geng
ab,
Xiang-Dong
Jia
ac and
Jun-Jie
Zhu
*a
aKey Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210093, P. R. China. E-mail: jjzhu@nju.edu.cn; Fax: +86-25-83594976; Tel: +86-25-83594976
bDepartment of Chemistry, Jiangsu Institute of Education, Nanjing, 210013, P. R. China
cCollege of Science, Nanjing Forestry University, Nanjing, 210037, P. R. China
First published on 27th August 2010
The core/shell-type ZnO nanosphere/CdS nanorod and ZnO nanosphere/CdS nanoparticle composites have been selectively prepared through a simple ultrasound-assisted solution phase conversion process using monodispersed ZnO nanospheres as a starting reactant and in situ template. The formation mechanism of the products is closely connected with the sonochemical effect of ultrasonic irradiation. The photoluminescence and electrogenerated chemiluminescence properties of the as-prepared core/shell structures were investigated.
Surface coating or modification has been recognized as one of the most advanced and intriguing methods to build tailored nanomaterials.1,2 Materials are coated for a number of reasons: Coatings can alter the charge, functionality, and reactivity of the surface, and enhance their thermal, mechanical, or chemical stability.1,2 Much effort has recently been invested to create new class of nanomaterials through surface coating, and different kinds of core/shell nanostructures have been fabricated by various synthetic methods, including the CdSe/ZnS nanocrystals obtained by a biomimetic method using a bi-functional peptide,3 FePt/ZnO core/shell nanoparticles synthesized by a seed-mediated growth,4 SnO2/TiO2 hollow nanostructures prepared via a colloid seeded deposition process,5 Pd@CeO2 core-shell nanostructures by self-assembly,6 ZnO@Cd(OH)2 core-shell nanoparticles by a sol–gel method,7 fluorescent core/shell CdTe@SiO2 particles via a water-in-oil reverse microemulsion method,8 CdTe/CdS/ZnS core/shell/shell quantum dots synthesized in the aqueous phase assisted by microwave irradiation,9 branched core/shell bismuth telluride/bismuth sulfide nanorods prepared by using a biomolecular surfactant,10 and so on.
Among the various non-linear optical (NLO) materials investigated, the direct band gap semiconductors, especially ZnO and CdS, have attractive non-linear properties that make them as the ideal candidates for NLO based devices. ZnO is a transparent oxide semiconductor that possesses piezoelectric properties, which has been widely used for solar cells,11 nanolasers,12 optoelectronic devices,13 and electromechanical coupled sensors and transducers.14 Moreover, ZnO is biocompatible and can be directly used for biomedical applications.14 CdS nanocrystals are one of the most studied systems among all the semiconducting nanocrystals.15 The bulk CdS has a direct band gap of 2.4 eV at 300 K, and the typical Bohr exciton diameter of CdS is around 5.8 nm; consequently, CdS nanocrystals in the size range of 1–6 nm show sizable quantum confinement effects with remarkably different optical properties.
The surface coating of ZnO nanomaterials with a preferred CdS can promote the construction of ZnO-based core/shell-type composites with novel optical and electrical properties. A few approaches have been reported for the surface coating of ZnO nanomaterials with CdS. For example, ZnO nanowire/CdS nanoparticle heterostructure arrays were fabricated by a two-step chemical solution deposition method,16 core/shell-type ZnO nanorod/CdS nanoparticle composites were prepared via a sonochemical route,17 ZnO nanowire arrays coated with CdS quantum dots were successfully fabricated with a chemical bath deposition process,18 CdS quantum dots were deposited on the vertically aligned ZnO nanorods electrode by chemical bath deposition,19 10 nm sized hexagonal CdS nanoparticles were decorated on the surface of well-aligned ZnO nanowall through a facile hydrothermal approach,20 ZnO/CdS nanocomposites were obtained by colloidal chemical synthesis,21 and so on. However, all these reports involved coating core structure with nanoparticles, no other coating status with different morphology was reported.
In this paper, core/shell-type ZnO nanosphere/CdS nanorod and ZnO nanosphere/CdS nanoparticle composites are selectively achieved by a simple ultrasonic irradiation of a mixture of monodispersed ZnO nanospheres, cadmium chloride, and thiourea or TAA in an aqueous medium. The use of different sulfur sources resulted in different surface coating statuses. The ultrasonic irradiation played an important role in the controllable synthesis of the core/shell nanostructures. The photoluminescence and electrogenerated chemiluminescence of the obtained ZnO/CdS composites have been investigated to show the interaction between the ZnO cores and the external CdS nanorod or nanoparticle shells.
ZnO/CdS core/shell structures. ZnO nanospheres (0.001 mol) and CdCl2 (0.001 mol) were well dispersed in EtOH (50 mL), and then thiourea (0.001 mol) or TAA (0.001 mol) was introduced. The mixture was exposed to high-intensity ultrasound irradiation for 30 min to prepare ZnO/CdS core/shell structures.
Cyclic voltammetry (CV) and electrogenerated chemiluminescence (ECL) curves were obtained simultaneously using a model MPI-A electrochemiluminescence analyzer systems (Xi'An Remax Electronic Science & Technology Co. Ltd., Xi'An, China). The photomultiplier tube (PMT) used in the above analyzer was held at 800 V during the whole process of detection. Carbon-paste electrodes with a diameter of 4 mm were used as the working electrode.23 A conventional three-electrode electrochemical cell that was used for all measurements consisted of a carbon-paste working electrode (0.5 cm2), a Pt wire counter electrode, and an Ag/AgCl reference electrode.
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Fig. 1 The XRD patterns of (a) ZnO spheres, (b) ZnO nanosphere/CdS nanorod core/shell structure using thiourea as a sulfur source, (c) ZnO nanosphere/CdS nanoparticle core/shell structure using TAA as a sulfur source. |
The as-prepared ZnO sample appeared as uniform solid nanospheres with diameter of ca. 400 nm, as shown in Fig. 2a, b and c. With this nanoscale ZnO spheres as template, ZnO/CdS core/shell structures were obtained. When Cd2+ reacted with thiourea under ultrasonic irradiation in the existence of ZnO template, CdS nanorods with diameter of ca. 40–50 nm and length of about 300 nm were found on the shell of ZnO nanospheres (Fig. 2d, e, f) to form a special core/shell structure with size of about 600 nm. While TAA substituted thiourea as sulfur source, an ordinary core/shell structure with CdS nanoparticles as shell coating on the ZnO nanospheres (Fig. 2g, h, i) could be obtained. The size of this core/shell structure is about 500 nm.
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Fig. 2 The low-magnification (left column), high-magnification (middle column) SEM images and TEM images (right column) of (a, b, c) ZnO spheres, (d, e, f) ZnO nanosphere/CdS nanorod core/shell structure using thiourea as sulfur source, and (g, h, i) ZnO nanosphere/CdS nanoparticle core/shell structure using TAA as sulfur source. |
The HRTEM image recorded on the tip of CdS nanorods coated on the ZnO nanospheres (Fig. 3a) shows lattice fringes with interplanar spacing of 0.33 nm for the (002) faces of hexagonal CdS, indicating the rod grew preferentially along the c axis. The HRTEM image recorded on the shell of ZnO nanosphere/CdS nanoparticle structure (Fig. 3b) shows that the shells are polycrystalline nature. The interplanar spacings of about 0.33 nm and 0.36 nm, which correspond to the lattice spacing for the (002) and (100) faces of hexagonal CdS, respectively, could be detected on the shell of ZnO nanosphere/CdS nanoparticle structure.
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Fig. 3 The HRTEM images recorded (a) on the tip of CdS nanorods coated on the ZnO nanospheres, (b) on the shell of ZnO nanosphere/CdS nanoparticle structure. |
On account of the enhanced surface effects of the shock waves generated from ultrasound, a two-steps sonochemical route was developed to assemble ZnO/CdS core/shell nanostructures via a simple template method: Firstly, the ZnO nanospheres were prepared through a sonochemical route as templates; then, 1D CdS nanorods or 0D CdS nanoparticles were generated and attached on the ZnO templates after the surface nucleation and crystal growth processes.
During the sonochemical process in aqueous solution, the elevated temperatures and pressures inside the collapsing bubbles caused water to vaporize and further pyrolyze into H˙ and OH˙ radicals.30 In the present case, the sonochemical formation processes of the as-prepared nanostructures are probably related to the radical species generated from water molecules by the absorption of ultrasonic energy. The ZnO nanospheres are formed through the sonohydrolysis mechanism which have been formulated.31,22 The likely reaction step is shown as follow:
Zn2+(aq) + H2O(l) → ZnO(s) + 2H+(aq) | (1) |
The formation of CdS nanorods or nanoparticles using different sulfur sources may be related with the different formation process and different release speed of S2−. In the system using thiourea as a sulfur source, thiourea may act as not only the sulfur source but also a bidentate ligand to form relatively stable Cd-thiourea complexes. The complex ions lead to a high remaining monomer concentration after the nucleation stage. Thus, a non-equilibrium growth for the elongated crystals is facilitated.32
So the possible growth mechanism of ZnO nanosphere/CdS nanorod may be described as follows. Firstly, the complex action between Cd2+ and thiourea leads to the formation of Cd–thiourea complexes, which prevent the production of a large number of free S2− in the solution, and will be favorable for the formation of the nanorods.33 Secondly, the Cd–thiourea complexes undergo a decomposition process under ultrasonic irradiation for a certain period of time to produce CdS nuclei. Owing to the slow release of reaction ions, the elongated growth along the [001] direction of rodlike crystals is favored.32 Thirdly, when there exists a supporter such as ZnO nanospheres, the sonochemically generated CdS clusters would be attached on its surface to form a composite nanomaterial with core/shell-type geometry.34 The formation process can be expressed as the following:
Cd2+ + thiourea → [Cd(thiourea)2] 2+ → CdS(nanorod) | (2) |
n(CdS) + ZnO → ZnO nanosphere/CdS nanorod | (3) |
While using TAA as a sulfur source, the reaction process follows a different mechanism. The whole process can be described as below:
H2O ))) H˙ + OH˙ | (4) |
H˙ + OH˙ + CH3CSNH2 → CH3C(NH2)(OH)-SH | (5) |
H˙ + OH˙ + CH3C(NH2)(OH)-SH → CH3C(NH2)(OH)2 + H2S | (6) |
CH3C(NH2)(OH)2 → CH3CO(NH2) + H2O | (7) |
Cd2+ + H2S → CdS (nanoparticle) + H2O | (8) |
n(CdS) + ZnO → ZnO nanosphere/CdS nanoparticle | (9) |
The quicker release of the intermediate gas leads to nucleation occurring at an outburst speed right after the reaction solution was irradiated, resulting in a large quantity of CdS nuclei and extremely low monomer concentration in the solution, so small dot-shaped product was obtained under this situation.35
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Fig. 4 PL spectra of (a) ZnO nanospheres, (b) ZnO nanosphere/CdS nanorod composite and (c) ZnO nanosphere/CdS nanoparticle composite. |
S2O82− + e− → SO42− + SO4−˙ | (10) |
CdS−˙ + SO4−˙ → CdS* + SO42− | (11) |
CdS* → CdS + hv | (12) |
Fig. 5 shows the ECL emission of ZnO nanospheres, ZnO nanosphere/CdS nanorod and ZnO nanosphere/CdS nanoparticle in 0.1 M KOH and 0.1 M K2S2O8 aqueous solution containing 0.1 M KCl as the supporting electrolyte. The electrode potential was cycled between 0.1 and −1.5 V at a scan rate of 100 mV s−1. The ECL light emissions of the crystals all show quite good stability and enhanced intensity. As shown in Fig. 5, the ECL intensities of the two ZnO/CdS composites are both higher than that of pure ZnO nanospheres, which imply that surface coating could enhance the ECL intensity of ZnO nanospheres. It is interesting that the ECL intensity of ZnO nanosphere/CdS nanorod composite is higher than that of ZnO nanosphere/CdS nanoparticle composite. As proved previously, the ECL emission is much more sensitive to their surface electronic structure than the PL, and is dependent on the surface properties and the presence of surface defects.39,40 We assume that the ZnO nanosphere/CdS nanorod structure would possess more defects due to the faster crystal formation which might result in higher ECL intensity. These results could indicate that ECL properties are closely related to the structure of these nanocrystals. Therefore, more detailed investigation on ECL of these prepared core/shell semiconductor nanocrystals to find the relationship between structure and ECL emission is urgently needed and still in progress. The optical measurement conditions need to be optimized in further work to improve the stability of ECL signals.
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Fig. 5 ECL emission from (a) pure ZnO nanospheres, (b) ZnO nanosphere/CdS nanorod and (c) ZnO nanosphere/CdS nanoparticle in 0.1 M K2S2O8 + 0.1 M KOH + 0.1 M KCl aqueous solution with potential cycles between 0.1 V and −1.5 V (scan rate: 100 mV s−1). |
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