One-Step Hydrothermal Synthesis of Cu2ZnSnS4 Nanoparticles as an Efficient Visible Light Photocatalyst for the Degradation of Congo Red Azo Dye

Abstract

A hydrothermal method was successfully employed to synthesize kesterite Cu₂ZnSnS₄ (CZTS) nanoparticles. X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), and optical ultraviolet-visible (UV-vis) spectroscopy were used for characterization of structural, chemical, morphological, and optical properties. XRD results confirmed that a nanocrystalline CZTS phase corresponding to the kesterite structure was formed. Raman analysis confirmed the existence of single pure phase CZTS. XPS results revealed the oxidation states as Cu⁺, Zn²⁺, Sn⁴⁺, and S^2-. FESEM and TEM micrograph images revealed the presence of nanoparticles with average sizes between 7 nm to 60 nm. The synthesized CZTS nanoparticles bandgap was found to be 1.5 eV which is optimal for solar photocatalytic degradation applications. The properties as a semiconductor material were evaluated through the Mott-Schottky analysis. The photocatalytic activity of CZTS has been investigated through photodegradation of Congo red azo dye solution under solar simulation light irradiation, proving to be an excellent photo-catalyst for CR where 90.2% degradation could be achieved in just 60 min. Furthermore, the prepared CZTS was reusable and can be repeatedly used to remove Congo red dye from aqueous solutions.

Keywords: CZTS; Congo red azo dye; hydrothermal; photocatalysis.

Figure 1

(a) X-ray diffraction pattern of a typical nanoparticulate Cu₂ZnSnS₄ sample. Diffraction planes are indicated for the Cu₂ZnSnS₄ phase (CZTS(hkl)). The signal marked with a green symbol (●) corresponds to the cassiterite SnO₂ phase. The tetragonal kesterite JCPDS pattern (#04-023-6315) is also shown for comparison (Cu₂ZnSnS₄ JCPDS: thick blue bars). (b) Raman spectrum of nanoparticulate Cu₂ZnSnS₄ nanopowder sample measured with an excitation wavelength of 785 nm, indicating the characteristic peaks. The cyan line is the fit line while the red lines represent each component. The original spectrum is represented by a blue line.

Figure 2

XPS survey spectrum for the synthesized particulate Cu₂ZnSnS₄ sample.

Figure 3

X-ray photoelectron spectroscopy high-resolution spectra for (a) Cu 2p, (b) Zn 2p, (c) Sn 3d, and (d) S 2p, for the Cu₂ZnSnS₄ sample. The short dashed black lines are the fit lines while the colored lines represent each component. The original spectra are represented as unfilled dots.

Figure 4

(a,b) FE-SEM plane-view micrograph images of a typical hydrothermally synthesized nanoparticulate Cu₂ZnSnS₄ sample at two different magnifications and different observed sample areas. EDX elemental mapping of Cu (violet), Zn (cyan), Sn (brown), and S (green) corresponding to Figure 4b FE-SEM micrograph image is depicted in four panels in the lower part of this (b). (c) Low-magnification TEM micrograph image of a typical hydrothermally synthesized nanoparticulate Cu₂ZnSnS₄ sample. Different CZTS nanocrystal shapes can be visualized: rod-like (indicated by a dashed red circle) and spherical forms (indicated by a dashed yellow circle). (d) High-magnification TEM micrograph of one of the zones highlighted by a dashed yellow circle in (c). (e,f) High-magnification TEM micrographs and different observed sample areas of the zone are highlighted by a dashed red circle in (c).

Figure 5

(a) UV-visible optical absorbance spectrum of a typical particulate Cu₂ZnSnS₄ sample. Inset: (αhν)² vs. hν Tauc plot, for kesterite bandgap gap energy (E_g) determination and corresponding linear fitting (red dashed line). (b) Mott–Schottky plot for a synthesized particulate Cu₂ZnSnS₄ sample. Red dashed line represents the fitted data in the linear range of the curve. The intercept of this line with the x-axis determinates the flat-band potential (E_FB) as indicated. Measurements were carried out in 0.1 M Na₂SO₄ (pH = 6.5), with an AC frequency of 10 kHz.

Figure 6

Photocatalytic degradation of Congo red azo dye in aqueous solution using nanoparticulate Cu₂ZnSnS₄ photocatalyst under AM1.5G simulated sunlight (100 mW cm⁻²) illumination. (a) Absorbance spectra as a function of illumination time, demonstrating the photodegradation of Congo red dye against CZTS (see direction black arrow). (b) Relative concentration (C/C₀) versus time for photodegradation of Congo red azo dye in the presence (square blue symbols) and in the absence (square red symbols) of nanoparticulate Cu₂ZnSnS₄ photocatalyst, under AM1.5G, simulated sunlight (100 mW cm⁻²) illumination. Inset: recyclability of nanoparticulate Cu₂ZnSnS₄ sample as photocatalyst for the photodegradation of Congo red azo dye.