The coprecipitation method has been used to synthesize layered double hydroxide (Zn-Fe-LDH) nanostructure at different Zn2+/Fe3+ molar ratios. The structural properties of samples were studied using powder X-ray diffraction (PXRD). LDH samples were calcined at 600°C to produce mixed oxides (ZnO and ZnFe2O4). The crystallite size of mixed oxide was found in the nanometer scale (18.1 nm for ZnFe2O4 and 43.3 nm for ZnO). The photocatalytic activity of the calcination products was investigated using ultraviolet-visible-near infrared (UV-VIS-NIR) diffuse reflectance spectroscopy. The magnetic properties of calcined LDHs were investigated using a vibrating sample magnetometer (VSM). The calcined samples showed a paramagnetic behavior for all Zn2+/Fe3+ molar ratios. The effect of molar ratio on magnetic susceptibility of the calcined samples was also studied.
Layered double hydroxides (LDHs) are one of the popular inorganic hosts to form an organic-inorganic hybrid type nanocomposite or so-called nanolayered composite materials [
An important use of LDH precursors is to get mixed nanometal oxides by calcination at temperature above 600°C [
In the current paper, Zn-Fe-CO3-LDH has been synthesized by the coprecipitation method with Zn2+/Fe3+ molar ratios of 1, 2, 3, and 4 at the final pH value of 8. The mixed metal oxides were formed by the calcination process of LDH at 600°C. The structural, optical, and magnetic properties of calcination products were studied.
LDH precursors (Zn-Fe-LDH) were synthesized using coprecipitation method at pH 8 with Zn2+/Fe3+ molar ratios of 1, 2, 3, and 4. Zn2+/Fe3+ molar ratio was changed to evaluate its effect on the properties of the calcination products of LDH.
The synthesis was carried out by a slow addition of two metal nitrates solutions which were Zn(NO3)2·6H2O and Fe(NO3)3·9H2O. The concentrations of Zn(NO3)2·6H2O solution were 0.025, 0.050, 0.075, and 0.1 M for Zn2+/Fe3+ molar ratios of 1, 2, 3, and 4, respectively, while the concentration of Fe(NO3)3·9H2O solution was fixed at 0.025 M for all samples. The solution that contained 0.1 M of Na2CO3 was added slowly (dropwise addition) to the metal nitrates solutions with constant stirring. The pH value for all samples was controlled by addition of aqueous NaOH (0.5 M). The resulting slurry was aged at 70°C for 18 h in an oil bath shaker (50 rpm). The precipitate was washed with deionized water many times with centrifugation. Finally, the precipitate was dried in an oven at 70°C for two days. The resultant Zn-Fe-CO3-LDH was ground into fine powder.
Heat-treated samples of Zn-Fe-CO3-LDH were prepared by heating LDH at 600°C. The samples were labeled
Powder X-ray diffraction (PXRD) patterns of the samples were recorded on an X-ray diffractometer (X’PERT-PRO PANALYTICAL) using CuK
Figure
PXRD patterns of Zn-Fe-LDH samples synthesized at different Zn2+/Fe3+ molar ratios.
XRD patterns exhibit ZnO/ZnFe2O4 nanocomposite as the calcination products of Zn-Fe-CO3-LDH samples at different Zn2+/Fe3+ molar ratios (Z1F, Z2F, Z3F, and Z4F) as shown in Figure
PXRD patterns of calcined Zn-Fe-LDH nanostructure.
The crystallinity of ZnO phase increased with the increase in Zn2+/Fe3+ molar ratio, while ZnFe2O4 nanocrystal was observed with almost the same crystallinity due to the fixing of Fe3+ content during the preparation of Zn-Fe-CO3-LDH samples. This observation is in good agreement with the literature [
The fundamental absorption of light, which corresponds to an electronic excitation from the valance band to the conduction band, can be applied to calculate the optical band-gap energy (
Figure
Kubelka-Munk transformed reflectance spectra of (a) reflectance spectra of ZnO/ZnFe2O4 nanocomposite, (b) band-gap energy of ZnO, (c) Kubelka-Munk plot ZnO/ZnFe2O4 nanocomposite (for sample Z3F as an example), and (d) diffuse reflectance UV-visible absorption spectra of calcined Zn-Fe-LDH nanostructure.
Table
Crystallite size and optical band-gap energy (
Samples | ZnO sizea (nm) | ZnFe2O4 sizea (nm) |
|
---|---|---|---|
Z1F | 51.2 | 10.3 |
|
Z2F | 42.7 | 20.6 |
|
Z3F | 42.7 | 20.6 |
|
Z4F | 36.6 | 20.7 |
|
As seen in Figure
Under visible-light irradiation, photogenerated electrons (e) and holes (h) are produced in mixed oxides (ZnO and ZnFe2O4) via electron excitation from valence band (VB) to conductance band (CB) of ZnFe2O4 nanocrystal. Due to the differences in the positions of band gap in these oxides, the photogenerated electrons transferred from CB of ZnFe2O4 to CB of ZnO as illustrated schematically in Figure
Scheme of the coupling action of different energy bands between ZnO and ZnFe2O4 [
The magnetic behavior of mixed oxides (ZnO and ZnFe2O4) as a function of Zn2+/Fe3+ molar ratio incorporated is quite different. All calcined samples exhibit a paramagnetic behavior as shown in Figure
Paramagnetic contribution of the hysteresis loop after the calcination.
Normally, the magnetic contribution of pure ZnFe2O4 nanocrystal below a critical value of particle size 10 nm for [
Susceptibility as function of Zn2+/Fe3+ molar ratio for calcined samples.
The coprecipitation method was used to synthesize Zn-Fe-LDH at the Zn2+/Fe3+ molar ratios of 1, 2, 3, and 4. XRD patterns of the calcined LDH showed ZnO and ZnFe2O4 phases. XRD confirmed that ZnFe2O4 is composed of franklinite. The crystallite sizes of both phases in nanocrystal range were found as 18.1 nm for ZnFe2O4 and 43.3 nm for ZnO. The optical band-gap value of ZnO was improved to around 3.19 eV due to the photocatalytic activity, while the optical band-gap energy of ZnO/ZnFe2O4 nanocomposite was found at around two values of 2.30 and 2.70 eV. The magnetic behavior of mixed oxides (ZnO and ZnFe2O4) phases showed a paramagnetic behavior due to the oxygen vacancies of ZnFe2O4 fully occupied during the heat treatment in air, which resulted in the reduction in magnetic moment. The magnetic susceptibility decreased as Zn2+/Fe3+ molar ratio increased due to the increase in the crystallinity of formed ZnO.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to thank Universiti Putra Malaysia (UPM) for supporting this work. A. A. A. Ahmed thanks RMC-UPM for the support.