Fabrication And Characterization Of CdS Hollow Nanospheres And Colloidal Spheres

Inorganic materials with hollow structures have recieved considerable attention in recent years, owing to their unique chemicophysical properties including high specific surface area, good permeability, and special optical/electrical/magnetic properties. Many applications could be realized using hollow materials, such as catalysis, nanoscale chemical reactors, encapsulation and controlled release of bioactive agents, photonic devices, lightweight fillers and acoustic insulators. Among the methods employed for the preparation of hollow nanostructures, hard or soft template-assisted synthesis has been most widely investigated. Compared with template-assisted methods involving multistep procedures, a one pot template-free method for controlled preparation of hollow nanostructures with rationally designed parameters is highly attractive. On the other hand, monodispersed colloidal spheres have been extensively exploited as building blocks to fabricate photonic crystals by self-assembly. Of the various colloidal nanocrystals, semiconductor types, especially metal sulfides, have been the most studied due to their high refractive index and excellent optical, magnetic, and optoelectronic properties that can be further exploited to produce photonic crystals with tunable or switchable band gaps. As a semiconductor with important wide band gap (2.4 eV at room temperature), CdS is useful for many important applications such as light emitting diodes, flat panel displays, solar cells, photo-catalysts, and thin film transistors. Since the properties of the material greatly depend on their morphological features, nanostructured CdS with different sizes and morphologies has been fabricated and characterized, among which the hollow structure and colloidal spheres have received particular attention. Herein, firstly, we present an ionic liquid assisted hydrothermal method for the preparation of CdS hollow nanospheres, and investigate the effect of the reaction parameters on the formation of CdS hollow nanospheres, and a hollowing growth mechanism involve Ostwald ripening is proposed. Secondly, we demonstrate that CdS colloidal spheres could be prepared by a facile hydrothermal method.1. Template-free synthesis of CdS hollow nanospheres based on an ionic liquid assisted hydrothermal process and their application in photocatalysis.Until now, the synthesis of hollow CdS has been largely based on the use of templates. It is the first time for the preparation of CdS hollow nanospheres in high yield with a template-free ionic liquid (IL) assisted hydrothermal route in the presence of polyvinylpyrrolidone (PVP) and hexamethylenetetramine (HMT). Structural characterization by TEM, SEM, SAED, and XRD indicates that the CdS hollow nanospheres are polycrystalline consisting of some smaller nanoparticles. The formation of such nanospheres is attributed to the preferential adsorption of PVP molecules on the CdS crystal nuclei. The small crystal nuclei aggregated to form solid nanoparticles that are protected by the adsorption of HMT molecules on certain faces of nanoparticles. The solid nanoparticles experienced a localized Ostwald ripening process and finally the CdS hollow nanospheres are obtained. The reaction temperature, the molar ratios of Cd/S, the concentration of capping agents (PVP and HMT) and IL [bmim][PF6] were found to be crucial for the formation of CdS hollow nanospheres. These hollow structured materials can be used as efficient photocatalysts for the degradation of MB. The CdS hollow nanospheres with near band emission, may find promising applications in microelectronic and photovoltaic devices. Importantly, the ionic liquid (IL) assisted hydrothermal route may be a versatile method to prepare hollow nanomaterials.2. Facile synthesis and properties of uniformed CdS colloidal spheresDue to high refractive index and excellent properties of luminescence and photochemistry, CdS could be used as building blocks for the fabrication of wide band photonic crystals. In our study, we developed a facile one-pot hydrothermal approach for the synthesis of uniform CdS colloidal spheres with tunable sizes, by changing the concentrations of capping agents PVP and HMT. To the best of our knowledge, this is the first report that CdS colloidal spheres could be prepared with such a rapid method. Only by changing the concentrations of PVP and HMT, the sizes of the product can be tuned from 70 um to approximately 200 nm. Comparative experiments reveal that synergistic effect of HMT and PVP are crucial for the formation of colloidal spheres. Meanwhile, the nearly uniformed CdS colloidal spheres could serve as an ideal building block for the fabrication of wide band gap photonic crystals.