Elsevier

Applied Surface Science

Volume 258, Issue 10, 1 March 2012, Pages 4370-4376
Applied Surface Science

Photocatalytic activity of CdS nanoparticles synthesized by a facile composite molten salt method

https://doi.org/10.1016/j.apsusc.2011.12.116Get rights and content

Abstract

Single-crystalline CdS nanoparticles were synthesized for the first time by the composite-molten-salt (CMS) method, with advantages of one-step, ambient pressure, low temperature, template-free and low cost. The influence of temperature, growth time and amount of salts on the morphology of CdS nanoparticles was systematically investigated. It shows that a smaller size of CdS nanoparticles can be obtained under lower temperature, less growth time and more composite salts. UV–vis reflection spectrum of the nanoparticles reveals that the nanoparticles have a bandgap of 2.34 eV. Photoluminescence spectrum was also carried out to explore its optical property. Photocatalytic degradation of rhodamine B (RhB) and methylene blue (MB) in presence of the CdS nanoparticles was compared with that in presence of the commercial TiO2 nanoparticles under the simulated sunlight.

Highlights

► CdS nanoparticles with clean surface have been obtained for the first time by the CMS approach. ► The influence of temperature, time and amount of salts on the size was systematically investigated. ► High photocatalystic activity of the CdS nanoparticles for degradation of RhB and MB was found.

Introduction

CdS is an important II–VI semiconductor with a direct bandgap of 2.35 eV [1]. In recent years, much attention has been paid to CdS nanostructures for its applications in photodevices [2], [3], [4], [5], logic circuits [6] and photocatalyst [7], [8], [9], [10], [11]. Photocatalysis has been considered as a cost-effective alternative for the treatment of wastewater containing organic dyes. CdS nanocrystals are believed to be a promising photocatalyst for removing organic waste in water due to its suitable bandgap which can effectively absorb solar-light, and due to its higher specific surface area that allows more photon absorption on the surfaces.

CdS nanostructures have been synthesized through various processes including chemical vapor deposition (CVD) [2], [6], physical vapor deposition (PVD) [3], vapor–liquid–solid (VLS) process [4], hydrothermal approach [12], solvothermal method [13] and electrodeposition [14], etc. However, CVD, PVD methods and VLS process normally consist of complicated steps, and rigorous conditions, such as high vacuum, high temperature and sometimes high pressure [2], [3], [4], [12]. Hydrothermal and solvothermal method for the synthesis of nanoparticles usually employs organic solvent or organic capping agent which dramatically contaminates the surface of nanoparticles and reduces its surface function. In this paper, we have adopted the composite-molten-salt (CMS) method for the synthesis of CdS nanoparticles. The CMS method is a new strategy for synthesizing nanostructures, which has advantages of being one-step, easy scale-up, low cost and environmentally friendly [15], [16]. The synthesis of CdS nanoparticles is based on the reaction between a metallic salt and metallic sulfide in a solution of eutectic composite salts at a temperature of above 125 °C and ambient pressure, without using any organic dispersant or capping agent, and the obtained nanocrystals have clean surfaces. The influence of temperature, growth time and amount of salts on the size of CdS nanoparticles were systematically investigated. The band gap (Eg) value of the CdS nanoparticles was evaluated from the optical diffuse reflectance spectrum. Photoluminescence (PL) spectrum was also carried out to explore its optical property. The photocatalytic degradation of rhodamine B (RhB) and methylene blue (MB) in presence of the CdS nanoparticles under irradiation of the simulated sunlight was investigated in order to evaluate the photocatalytic activity of the synthesized CdS sample. The results were compared with the degradation in presence of the commercial TiO2 nanoparticles.

Section snippets

Synthesis of CdS nanoparticles

All chemicals used in this work, such as cadmium nitrate (Cd(NO3)2·4H2O), sodium sulfide (NaS·9H2O), lithium nitrate (LiNO3), potassium nitrate (KNO3), and TiO2 powder are of analytical grade purchased from Chongqing Chemical Company. In a typical synthesis process, mixture of LiNO3 and KNO3 at a mass ratio of 1/2 was placed in a 25 mL Teflon vessel. 0.308 g Cd(NO3)2·4H2O and 0.24 g NaS·9H2O were put in the vessel and shaken to make sure a good mixture of the reactants. The vessel was sealed and

Results and discussion

Fig. 1a and b shows the FE-SEM images of S1 and S2 which were synthesized with 12 g (S1) and 3 g (S2) composite-molten-salt at temperature of 220 °C for 24 h. It can be seen from the images that the S1 and S2 are composed of nanoparticles having diameter about 20 nm and 50–100 nm respectively. It shows that the higher solvent to reactant ratio or the less reactant concentration in high viscosity molten salts can reduce the size of particles. Fig. 1c and d shows the images of the samples synthesized

Conclusions

The systematically studies on size-control synthesis of CdS nanoparticles via the CMS method have been reported. The synthesis process is novel, one-step, template-free, low cost, nontoxic and mass-producible. Compared with hydrothermal and solvothermal method for synthesis of nanoparticles, the CMS method can obtain CdS nanocrystals with clean surfaces as it does not employ any organic solvent or organic capping agent which would dramatically contaminate the surface of nanoparticles and would

Acknowledgments

This work is supported by the NSFC (60976055), and Postgraduates’ Science and Innovation Fund (CDJXS11100006) and Innovative Training Project (S-09109) of the 3rd-211 Project, and the large-scale equipment sharing fund of Chongqing University.

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