Preparation of core/shell ZnS/CdS nanoparticles and their ...

16. - 18. 10. 2013, Brno, Czech Republic, EU

PREPARATION OF CORE/SHELL ZnS/CdS NANOPARTICLES AND THEIR PHOTOCATALYTIC PROPERTIES Ladislav SVOBODA1, Petr PRAUS2, Jonáš TOKARSKÝ1, Volker KLEMM2 1VŠB-

Technical University of Ostrava, Ostrava, Czech Republic, EU, [email protected] 2TU Bergakademie Freiberg, Freiberg, Germany, EU

Abstract Semiconductor ZnS/CdS core/shell nanoparticles with 1, 2 and 3 shells were prepared by a two step precipitation procedure at room temperature. In the first step ZnS core nanoparticles were synthesized by precipitation of zinc acetate with sodium sulphide in aqueous solutions stabilized by a cationic surfactant cetyltrimethylammonium bromide (CTAB). In the second step CdS shells were created by adding drops of cadmium acetate into the previous solutions. The Materials Studio software was used to determine theoretically required amounts of CdS to create a specific number of shells. Energy gaps of ZnS/CdS nanoparticles were determined from their UV-Vis spectra by the Tauc method [1]. The molecular modelling results were verified by high resolution transmission electron microscopy (HRTEM) and a photocatalytic decomposition of methylene blue (MB) in aqueous solutions under UV irradiation. Keywords: CdS, ZnS, nanoparticles, core/shell, photocatalysis, 1.

INTRODUCTION

Semiconductor nanoparticles (NPs) with diameters smaller than sizes of their exciton Bohr radii are well known for their unique physical and chemical properties towards bulk materials, such as quantum confinement effect, high reactivity and adsorption capacity [2]. One of the most important properties of semiconductor NPs is their capability to generate electrons and holes after absorbing photon with energy greater or equal to their gap energy. Semiconductor NPs can be used as photocatalysts for oxidation and reduction reactions, such as hydrogenation [3], reduction of carbon dioxide [4] or decomposition of organic compounds [5]. The aims of this work were 1) to model and prepare core/shell ZnS/CdS NPs in the presence of CTAB and 2) to characterize the core/shell nanoparticles by experimental methods in order to verify correctness of molecular models. Molecular modelling was performed in the Materials Studio environment. Besides HRTEM, a decomposition of methylene blue catalysed by ZnS/CdS nanoparticles were used for the characterization. 2. 2.1

EXPERIMENTAL Material and chemicals

All the chemicals used for preparation of ZnS, CdS and ZnS/CdS nanoparticles were analytical grade reagents: zinc acetate, cadmium acetate, sodium sulphide (all from Lachema, Czech Republic), cetyltrimethylammonium bromide (Sigma chemical Co., USA), methylene blue (Merck, Germany). Water deionised by reverse osmosis (Aqua Osmotic, Czech Republic) was used for preparation of all solutions. 2.2

Preparation of ZnS/CdS SNPs

In the first step, ZnS core nanoparticles were synthesized by precipitation of zinc acetate with sodium sulphide in an aqueous solution in the presence of CTAB. In the second step, shells of CdS were created by adding drops of cadmium acetate (0.79 mmol·l-1) into the previous solution. The mole ratio of Zn2+ : S2- :


CTAB : Cd2+ was about 1 : 2.1 : 2.5 : 0.395 for one layer of CdS, 1: 2.83 : 3.5 : 0.885 for two layers of CdS and 1 : 3.75 : 4.5 : 1.495 for three layers of CdS. The initial concentration of zinc acetate was kept at 2 mmol·l-1 for all three prepared types of core/shell NPs. Amounts of required ZnS and CdS molecules necessary for the ZnS/CdS nanoparticles preparation (Tab. 1) were estimated from ZnS/CdS models built in the Material Studio software. 2.3

UV-Vis spectroscopy

UV-Vis spectra of the ZnS/CdS colloidal dispersions were recorded by an UV-Vis spectrometer Lambda 25 (Perkin Elmer, USA) with wavelength range of 200-800 nm. All spectra were recorded using 1 cm quartz cuvettes. 2.4

Transmission electron miscroscopy

Transmission electron microscopy with high resolution images of ZnS/CdS NPs deposited on a clay mineral montmorillonite (MMT) were taken by a JEM 220FS microscope (Jeol, Japan) operating at 200 kV. The HRTEM microscope was also equipped with an energy-dispersive X-ray spectra analyser (EDX) to prove local chemical composition of ZnS/CdS NPs deposited on MMT. The analysed particles were dispersed in ethanol and with ultrasonic sprayer deposited on a TEM grid with carbon holey support film. 2.5

Photocatalytic activity

Photocatalytic activity of the colloidal dispersions of ZnS/CdS NPs was evaluated by decomposition of methylene blue in aqueous solutions under irradiation of an UV lamp with a maximum emission wavelength at 365 nm (Hg lamp HSC-1L Pen-Ray, UVP, Germany) in a stirred batch reactor opened to the air. In a typical experiment, the colloidal dispersion of ZnS/CdS was added into the aqueous solution of MB, a total volume of the solution was 100 ml. The initial concentrations of MB and ZnS/CdS were set at 8 x 10-3 mmol·l-1 and 2 mmol·l-1, respectively. Before photocatalytic experiments the MB solution was stirred in contact with photocatalysts for 10 minutes in dark to reach adsorption equilibrium and provide good homogenization of dispersions. After that the UV lamp was switched on. The temperature in the reactor was kept at 18 °C. 3. 3.1

RESULTS AND DISCUSSION Modelling of ZnS/CdS nanoparticles

ZnS/CdS nanoparticles were modelled in the Material studio environment. A shape of the ZnS core was not a perfect sphere but a shape of rhombicuboctahedron (Fig. 1a). An average radius of the modeled ZnS core was calculated at 2.55 nm and 2076 ZnS molecules was used for its building. Amounts of CdS molecules used for building of shells are summarized in Table 1.

Fig. 1 Molecular models of a) ZnS core, b) ZnS/1CdS SNP, c) ZnS/2CdS SNP


Results of our previous study revealed that ZnS nanoparticles (cores) can grow during 24 hours after precipitation to reach radii of 2.45 nm. This value was used for the molecular modelling and well agrees with the final average of modelled ZnS radii calculated at 2.55 nm. Table 1 Number of molecules used for synthesis of ZnS/CdS NPs and thickness of CdS shells Type of NPs Number of CdS molecules Thickness of CdS layers (nm) ZnS/1CdS 820 0.401 ZnS/2CdS 1838 0.765 ZnS/3CdS 3103 1.120 CdS 2076 --Note: #CdS is a number of CdS monolayers deposited on the ZnS core 3.2

Optical absorption properties of nanoparticles

All three types of ZnS/CdS nanoparticles were prepared by precipitation reactions of zinc and/or cadmium ions with sulphide ions in the presence of CTAB. UV-Vis absorption spectra of the resulting dispersions were recorded immediately after the precipitation. The typical UV-Vis absorption spectra of CdS (included in this study for comparison), ZnS and ZnS/CdS nanoparticles are shown in Figure 2. They were used for evaluation of the nanoparticles gap energies by means of the Tauc equation [1] εhν = C(hν – Eg)x

(1)

where  is the molar extinction coefficient, which can be obtained from Beer-Lambert law, h is the energy of incident photons, C is a constant, Eg is the gap energy of the material and the power x depends on the type of transition. For direct semiconductors like ZnS and CdS x = 1/2. The usual method for the determination of Eg involves plotting (hv)1/x against (hv) [6].

Fig. 2 Absorption spectra of ZnS, CdS and ZnS/CdS colloid dispersions. A detail view of CdS absorption edges (inset) The obtained values of ZnS and CdS gap energies were used for estimation of average radii of ZnS, CdS nanoparticles by the Brus equation [7]


Eg (nano)  Eg (bulk) 

h2  1 1  1.8e2   2  8r  me mh  4r 0r

(2)

where Eg(nano) and Eg(bulk) are the gap energies of nano and bulk-semiconductors, respectively, h is Planck's constant, r is the radius of the nanoparticle, me and mh are the effective masses of electron and hole respectively, e is the charge of electron, εr is the dielectric constant and ε0 is the permittivity of vacuum. From the core/shell UV-Vis spectra, a blue shift of the CdS absorption edge with the decreasing amount of CdS molecules was observed. The CdS gap energies are summarized in Table 2. Table 2 Gap energies of CdS and ZnS/CdS nanoparticles Type of nanoparticles CdS Ebg (eV) CdS 2.580 ZnS/1CdS 2.797 ZnS/2CdS 2.692 ZnS/3CdS 2.608

3.3

Transmission electron microscopy

Figure 3 shows a HRTEM micrograph of ZnS/3CdS nanoparticles. Unfortunately, it was not possible to recognize a structure of ZnS/3CdS nanoparticles because of fuzzy boundaries of displayed objects caused by the presence of CTAB. The scanning modus transmission electron microscopy revealed the EDX intensities of sulphur, zinc and cadmium along a red abscissa marked in a left image indicating the presence of ZnS/CdS nanoparticles.

Fig. 3 HRTEM analysis of ZnS/CdS nanoparticles. HRTEM micrograph of ZnS/3CdS SNPs (left), EDX intensities of S, Zn and Cd 3.4

Photocatalytic activity of ZnS/CdS nanoparticles

Besides the HRTEM analysis, ZnS/CdS nanoparticles were also investigated using the photocatalytic decomposition of MB. The heterogeneous reaction rate r of MB and hydroxyl radicals on surface of ZnS/CdS NPs can be described by the Langmuir-Hinshelwood equation [8]


r =-

K OHc OH dcMB KMBcMB =kh dt 1+KMBcMB +Kici 1+K OHc OH

(3)

where kh is a kinetic parametr; KMB, KOH, Ki, and cMB, cOH, ci, are the adsorption constants and concentrations of remaining MB, hydroxyl radicals and intermediates, respectively. In case cOH >> cMB and ΣKici can be neglected and Eq. (3) can be simplified to its mostly used form

r =k app

KMB cMB 1+KMBcMB

(4)

where kapp is an apparent kinetic parameter depending on irradiation intensity, mass and nature of the solid phase (catalyst) and the concentration of OH radicals. It was theoretically approved this model is appropriate for the first order kinetics [9]. Since MB concentrations were very low of 0.01-0.001 mmol·l-1 then we can give KMBcMB