Sol-gel synthesis and solar photocatalytic activity of Ca-alloyed ZnO nanoparticles elaborated using different precursors.

A Rosset,G Plantard,K Djessas,S Grillo,V Goetz

doi:10.1039/c9ra10131d

A Rosset, G Plantard + Show 3 more

Open Access

https://doi.org/10.1039/c9ra10131d

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Abstract

Ca-alloyed ZnO nanoparticles elaborated using different calcium precursors (CaSO4, CaCl2, Ca(NO3)2 and CaCO3) at different [Ca]/[Zn] ratios (0, 1, 5, 10, 15 and 20%) have been prepared by a sol–gel method followed by supercritical drying and annealing at 300 °C. The synthesized samples have been characterized by a number of techniques including Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Energy Dispersive X-ray Spectroscopy (EDS), X-ray Diffraction (XRD), Raman Spectroscopy and Electron Paramagnetic Resonance (EPR). SEM and TEM images reveal that the nanoparticles have a quasi-spherical shape with a grain size between 20 and 40 nm. The EDS results on chemical elementary compositions show that the Ca-alloyed ZnO with a CaCO3 precursor and [Ca]/[Zn] ratios of 5 and 10% are quasi-stoichiometric. The XRD results indicate that all the elaborated nanoparticles have a hexagonal wurtzite structure. Using Raman Spectroscopy a supplementary vibrational mode is detected in the case of CaSO4, CaCO3 and Ca(NO3)2 precursors. The intrinsic defect centers and defect number have been studied using EPR. Two intrinsic defects with different g factors are identified by EPR for which the spectral intensities change with calcium precursors. Furthermore, EPR reveals a correlation between the defect number and photocatalytic efficiency. The photocatalytic efficiency of the nanoparticles elaborated with different precursors and compositions has been studied for the solar photocatalytic degradation of pyrimethanil, using a solar simulator. The results show that the nanoparticles of Ca-alloyed ZnO elaborated with a CaCO3 precursor give promising results and enhance the photocatalytic efficiency in the solar field.

Highlights

Heterogeneous solar photocatalysis belongs to the family of Advanced Oxidation Processes (AOPs), which are respectful of the environment
The morphology and the size of the undoped ZnO and of the Zn1ÀxCaxO nanoparticles have been investigated by Scanning Electron Microscopy (SEM)
It should be noted that the crystallites size obtained with a CaCO3 precursor is lower than the one of ZnO nanoparticles

Summary

Introduction

The main problem of the semiconductors studied in the literature for this process is the photo-sensibility to the ultraviolet or near visible irradiation. The two main catalysts that have been widely reported in the literature are ZnO and TiO2.15–17 ZnO is a promising catalyst for the photocatalytic process due to its photosensitivity, its strong oxidizing capacity, its non-toxicity, its large band gap of 3.37 eV and its excellent chemical and mechanical stability.[18,19] ZnO has several weaknesses, such as the fast recombination rate of the photo-generated electrons and holes and a low efficiency in photocatalytic reactions, which obstruct the photocatalytic degradation mechanism.[2,20] To address these limitations, modi cations of the ZnO structure by doping and/or alloying with different elements have been proposed to improve the photocatalytic activity. This allows the photocatalytic efficiency under solar irradiation (computed as the number of degraded molecules divided by the effective photons absorbed available for the photocatalytic reaction) to be determined

Experimental

Synthesis of Ca-alloyed ZnO

Characterization

Photocatalytic efficiency

Characterization of the photocatalyst

Conclusions