Synthesis of hollow CdS micro-/nanospheres by CoSP technique and their visible light photocatalytic activities

Highlights

• The CdS hollow micro-/nanospheres with high yield, purity and catalytic property have been successfully synthesized by a continuous spray pyrolysis technique.

• The experimental conditions like temperature and nature of solvent are responsible for the creation of hollow spheres.

• The electric field quenches the hollow morphology and photocatalytic property.

• The product prepared by this technique show high photodegradation efficiency (∼90%) of methylene blue dye under visible light.

Abstract

CdS hollow micro-/nanospheres with high yield, purity, and photocatalytic property have been successfully synthesized by a continuous spray pyrolysis (CoSP) technique. The experimental conditions mainly the temperature and the nature of solvent are found to influence the creation of hollow spheres. The synthesized products have been characterized by X-ray powder diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), UV–vis and photoluminescence spectra. The XRD analysis confirmed the hexagonal crystal structure of the CdS microspheres supported by the measured lattice spacing from HR-TEM. The effects of synthesis temperature and the electric field applied during the spray on the formation of hollow spheres and their visible light photocatalytic properties are reported. Photodegradation of methylene blue (MB) dye under visible light was used to evaluate the performance of these CoSP made CdS micro-/nanospheres. The MB photodegradation efficiency was maximum, ∼90% in 2 h, for CdS microspheres prepared at 500 °C. The importance of hollow morphology over solid morphology is also proven; the hollow sphere sample (CdS₅) is about twice efficient than solid sphere sample (CdS₇). The results show that the CoSP method is a simple and inexpensive technique for preparing highly photo-active CdS hollow spheres which can be extended to prepare other sulphide and oxide hollow spheres at a large scale. Typically 100 mg of the powder can be prepared from ∼100 ml of spray solution.

Introduction

Hollow micro-/nanostructures are characterized by very unique properties such as high surface to volume ratio, good surface permeability, low coefficient of thermal expansion, and increased light-harvesting capacity. These properties make them fascinating for various scientific and technological applications in multidisciplinary fields like medical, energy, environmental, etc. Some examples of their applications are catalytic support [1], [2], [3], antireflection coatings [4], scattering layers [5], [6], [7], [8], rechargeable batteries [9], gas sensing [10], and solar cells [5], [6], [7], [8]. Some medical applications of hollow spheres include encapsulation of the sensitive materials for therapeutic [11], fluorescent marking [12], drug delivery [11], [12], and biomedical imaging applications [12]. Various types of hollow structures and their synthesis techniques were well described in the review article by Lou et al. [13]. Among the various semiconductor materials, cadmium sulfide (CdS, E_g = 2.4 eV) is an attractive visible light photocatalyst because it absorbs energy up to 517 nm (green, blue, violet wavelengths) of solar spectrum. The CdS hollow spheres have been used in various applications like photocatalysis [14], light-emitting diodes [15], organic–inorganic hybrid solar cells [16], and smart optoelectronic devices [17]. CdS nanostructures with enhanced properties have been prepared and applied in lasers and optical waveguide [18], [19]. CdS hollow microstructures have been also applied in some technological areas, for example, hydrogen evolution under visible light and bio-sensors [20], [21]. The preparation of CdS hollow microspheres using diverse methods have been reported including hydrothermal process [22], surfactant assisted method [23], microwave assisted synthesis [24], template-free [25], and using template synthesis [26], [27], as well as Kirkendal effect [28].

The spray pyrolysis technique has been used by several groups to synthesize the nano-/micro structured ceramic/oxide and semiconductor particles. Okuyama's and Messing's group have reported synthesized nano- and microparticles/powders by spray pyrolysis [29], [30], [31], [32]. Suslik's group has used ultrasonic pyrolysis to produce semiconductor and oxide nanocrystals [33], [34], [35], [36], [37]. To the best of our knowledge, the synthesis of CdS hollow spheres by spray pyrolysis technique is not reported in the literature.

In the present work, we report a single step process to prepare high quality CdS micro-/nanospheres using a simple technique termed continuous spray pyrolysis (CoSP) technique. The CoSP technique has been used for preparing CdS micro-/nanospheres for the first time with distinct optical and photocatalytic properties. The size and morphology of the product can be controlled by using different spray parameters, e.g., furnace temperatures, solvent, and electric field (the voltage applied during the spray). The creation of hollow spheres happens only under proper furnace temperature and the solvent used in spray solution. Our results prove that the CoSP synthesis process has potential to synthesize CdS and other semiconductor hollow structures in large quantity.