Journal of Spectroscopy

Volume 2014 (2014), Article ID 732163, 6 pages

http://dx.doi.org/10.1155/2014/732163

Research Article

Influence of Zn/Fe Molar Ratio on Optical and Magnetic Properties of ZnO and ZnFe₂O₄ Nanocrystal as Calcined Products of Layered Double Hydroxides

Abdullah Ahmed Ali Ahmed,¹ Zainal Abidin Talib,² Mohd Zobir Hussein,³ Moayad Husein Flaifel,⁴ and Naif Mohammed Al-Hada²

¹Department of Physics, Faculty of Applied Science, Thamar University, P.O. Box 87246, Dhamar, Yemen

²Department of Physics, Faculty of Science, Universiti Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia

³Advanced Materials and Nanotechnology Laboratory, Institute of Advanced Technology (ITMA), Universiti Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia

⁴School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia

Received 25 April 2014; Revised 2 June 2014; Accepted 23 June 2014; Published 10 July 2014

Academic Editor: Jie-Fang Zhu

Copyright © 2014 Abdullah Ahmed Ali Ahmed et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

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.

1. Introduction

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 [1]. LDHs, also known as hydrotalcite-like structure and anionic clay compounds, belong to a special class of synthetic two-dimensional inorganic compounds with lamellar structures. The chemical composition of LDHs can be described by the general formula  , where , is trivalent metals, and is an anion with charge . Molar fraction number mol of water in LDH interlayer per formula weight of compounds and molar fraction (normally, it is between 0.2 and 0.33) [2].

An important use of LDH precursors is to get mixed nanometal oxides by calcination at temperature above 600°C [3, 4]. This method has been used to address the problem of the fast recombination of photogenerated electron-hole pairs in the mixed semiconductors to enhance the photocatalytic efficiency of semiconductor. Nano-zinc oxide (ZnO) with its excellent features has been used as photocatalysts [5]. At the same time, ZnFe₂O₄ spinel, which is a semiconductor with narrow band gap, exhibits characteristics of excellent visible-light response, as well as favorable magnetism and good photochemical stability [6].

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.

2. Experimental

2.1. Synthesis of Zn-Fe-LDH and Its Calcination Products

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 , where represents the Zn²⁺/Fe³⁺ molar ratio. Heating was performed in an electric tubular furnace at atmospheric pressure in a ceramic boat holder for 3 h at a rate of 3°C/min (heating and cooling rate). The annealing process was performed in the ambient air. After the heat treatment was completed, the sample was left to cool to room temperature and stored in a sample bottle for further characterization.

2.2. Characterizations

Powder X-ray diffraction (PXRD) patterns of the samples were recorded on an X-ray diffractometer (X’PERT-PRO PANALYTICAL) using CuKα () at 40 kV and 40 mA. The slit configuration includes the fixing type of the divergence slit with slit size of 0.25°. The goniometer radius is 240 mm and distance of the focus-divergence slit is 100 mm. The scan configuration includes the continuous scan type with step size of 0.0330 [°] and scan step time of 19.685 s. The optical properties were studied using a UV-VIS-NIR diffuse reflectance spectrophotometer (Shimadzu, UV-3600). This spectrophotometer is equipped with an integrating-sphere detector. The magnetic properties of calcined samples were measured using a vibrating sample magnetometer (VSM Model Lakeshore 7404).

3. Results and Discussion

3.1. Powder X-Ray Diffraction (PXRD) Study

Figure 1 shows XRD patterns of Zn-Fe-CO₃-LDH prepared at different Zn²⁺/Fe³⁺ molar ratios of 1, 2, 3, and 4. XRD patterns exhibit two characteristic intense peaks of basal reflection of Zn-Fe-CO₃-LDH which were located near 2θ of 11.9° and 23.1° corresponding to diffraction by (003) and (006) planes, respectively. As seen in Figure 1, Zn-Fe-CO₃-LDH phase is observed with ZnO phase which was formed during the aging of the coprecipitated products or during the titration process [7, 8]. At Zn²⁺/Fe³⁺ molar ratio of 1, the Fe metal also was clearly observed. This may be attributed to the presence of a residual of the Fe ions which could not precipitate into prepared Zn-Fe-CO₃-LDH layer. At Zn²⁺/Fe³⁺ molar ratio values of 2, 3, and 4, the high crystallinity of (003) and (006) peaks was observed which indicated the presence of a well-ordered layered structure [7].

Figure 1: 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 2. X’Pert-High-Score Plus software has been used to analyze the XRD data. The peaks of hexagonal wurtzite structure of ZnO were observed near 2θ of 31.8, 34.5, 36.3, 47.7, 56.6, 62.9, 66.5, 68.1, 69.1, and 77° which correspond to diffraction by (100), (002), (101), (102), (110), (103), (200), (112), (201), and (202) planes, respectively, (reference code: 01-079-0206). For ZnFe₂O₄ nanocrystal, the peaks were located at 2θ of 18.2, 29.9, 35.2, 42.8, 53.2, 56.6, 62.1, and 66.4° which corresponds to diffraction by (111), (220), (311), (400), (422), (511), (440), and (442) planes, respectively (reference code: 01-089-4926). This is identified as face-centered-cubic (FCC) franklinite ZnFe₂O₄ with space group Fd-3m and lattice parameters of .

Figure 2: 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 [7]. The average crystallite size of ZnFe₂O₄ in the (111) crystal plane is 18.1 nm [9] and for ZnO in the (002) crystal plane it is 43.3 nm [3, 10, 11] as estimated using Scherrer’s equation.

3.2. UV-Visible Spectra

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 (). In this study, the optical band-gap energy can be calculated using the Kubelka-Munk equation in terms of reflectance ():

where is the diffuse reflectance of the examined samples and is called the remission or Kubelka-Munk function [12, 13]. Thus, the band gap is obtained using the following equation:

The variation of versus is plotted and the straight line range of these plots is extended on axis to obtain the values of optical band gap .

Figure 3(a) shows the recorded reflectance of calcined Zn-Fe-LDH samples and Figure 3(b) exhibits the band-gap energies of ZnO phase which is found at around 3.19 eV. The average band gap of ZnO/ZnFe₂O₄ nanocomposite annealed at 600°C is found at ~2.30 eV as shown in Figure 3(c). The band-gap value of pure ZnFe₂O₄ is 1.9 eV [14], while the band gap for whole nanocomposite (ZnO/ZnFe₂O₄) increased to 2.32 eV as seen in the recent study [15]. This is in good agreement with our results. On the other hand, the ZnO band-gap energy decreases from ~3.37 eV (for pure ZnO phase) [16] to around 3.19 eV in this work. This phenomenon occurs due to the presence of ZnO with another metal oxide (ZnFe₂O₄) which indicates the photocatalytic activity [17]. As seen in Figure 3(c), another band gap also is observed at ~2.70 eV which may be attributed to the presence of ZnO/ZnFe₂O₄ nanocomposite.

(a)

(b)

(c)

(d)

Figure 3: 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 1 exhibits the effect of Zn²⁺/Fe³⁺ molar ratio on the crystallite size of mixed metal oxide (ZnO and ZnFe₂O₄) and ZnO band-gap energy. As Zn²⁺/Fe³⁺ molar ratio value increases from 1 to 4, the crystallite size of ZnO phase decreases from 51.2 to 36.6 nm; thus, the ZnO band gap increases from 3.188 to 3.198 eV. The fluctuation in band-gap value of ZnO in Table 1 may be attributed to the additional ZnO phase which formed during the preparation of Zn-Fe-LDH as shown in Figure 1.

Table 1: Crystallite size and optical band-gap energy () for Z1F, Z2F, Z3F and Z4F samples.

Samples
ZnO sizea (nm)
ZnFe₂O₄ sizea (nm)
of ZnO phase (eV)

Z1F
51.2
10.3

Z2F
42.7
20.6

Z3F
42.7
20.6

Z4F
36.6
20.7

Crystallite size: calculated using Scherrer’s equation.

As seen in Figure 3(d), the absorbance of LDH calcination products has shown an edge at 360 nm which indicates the free exciton absorption of the lower particle size of ZnO [18]. The broad band at around 295 nm may suggest charge transfer transition from O²⁻ to Fe³⁺ in ZnFe₂O₄ spinel [19]. The threshold of the absorption band of ZnFe₂O₄ spinel occurs at around 685 nm. The blue shift absorption of ZnFe₂O₄ spinel and ZnO compared with the ZnFe₂O₄ (700 nm) may be attributed to the quantum-size effect.

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 4. At the same time, the number of holes with positive charge left (VB) of ZnAl₂O₄ to (VB) of ZnO. The difference in the energy levels of ZnO and ZnFe₂O₄ semiconductors is important in achieving such charge separation which gives rise to the improvement of the photocatalytic activity of ZnO [17].

Figure 4: Scheme of the coupling action of different energy bands between ZnO and ZnFe₂O₄ [17].

3.3. Magnetic Spectroscopy

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 5. The typical curves for paramagnetic systems can be defined as

where is the magnetization, is the applied magnetic field, and is the magnetic susceptibility.

Figure 5: 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 [9] is superparamagnetic behavior, while this behavior changed to paramagnetic when ZnFe₂O₄ nanocrystal was calcined at temperature of 650°C [9]. This is due to the oxygen vacancies of ZnFe₂O₄ fully occupied during the heat treatment in air, which is responsible for the reduction in magnetic moment. In this study, the room temperature magnetic hysteresis loop of the calcined products of LDH (ZnO and ZnFe₂O₄) shows a paramagnetic behavior as shown in Figure 5. The final magnetization value (there is no saturation magnetization) decreases as Zn content (Zn²⁺/Fe³⁺ molar ratio) increases due to the presence of high crystalline ZnO. Figure 6 shows the magnetic susceptibility as function of the Zn²⁺/Fe³⁺ molar ratio in the samples. The value of paramagnetic response () decreases with the increase in Zn²⁺/Fe³⁺ molar ratio according to the observed formation of ZnO [7].

Figure 6: Susceptibility as function of Zn²⁺/Fe³⁺ molar ratio for calcined samples.

4. Conclusions

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.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors would like to thank Universiti Putra Malaysia (UPM) for supporting this work. A. A. A. Ahmed thanks RMC-UPM for the support.

References

F. Kovanda, E. Jindová, K. Lang, P. Kubát, and Z. Sedláková, “Preparation of layered double hydroxides intercalated with organic anions and their application in LDH/poly (butyl methacrylate) nanocomposites,” Applied Clay Science, vol. 48, no. 1, pp. 260–270, 2010.

F. Cavani, F. Trifirò, and A. Vaccari, “Hydrotalcite-type anionic clays: preparation, properties and applications,” Catalysis Today, vol. 11, no. 2, pp. 173–301, 1991.

A. A. A. Ahmed, Z. A. Talib, M. Z. B. Hussein, and A. Zakaria, “Improvement of the crystallinity and photocatalytic property of zinc oxide as calcination product of ZnAl layered double hydroxide,” Journal of Alloys and Compounds, vol. 539, pp. 154–160, 2012.

M.-Y. Guan, D.-M. Xu, Y.-F. Song, and Y. Guo, “ZnO/ZnAl₂O₄ prepared by calcination of ZnAl layered double hydroxides for ethanol sensing,” Sensors and Actuators B: Chemical, vol. 188, pp. 1148–1154, 2013.

L. Xu, Y.-L. Hu, C. Pelligra et al., “ZnO with different morphologies synthesized by solvothermal methods for enhanced photocatalytic activity,” Chemistry of Materials, vol. 21, no. 13, pp. 2875–2885, 2009.

X. Li, Y. Hou, Q. Zhao, W. Teng, X. Hu, and G. Chen, “Capability of novel ZnFe₂O₄ nanotube arrays for visible-light induced degradation of 4-chlorophenol,” Chemosphere, vol. 82, no. 4, pp. 581–586, 2011.

A. l. C. Heredia, M. I. Oliva, C. I. Zandalazini, et al., “Synthesis, characterization, and catalytic behavior of MgAlZnFe mixed oxides from precursors layered double hydroxide,” Industrial & Engineering Chemistry Research, vol. 50, no. 11, pp. 6695–6703, 2011.

A. A. A. Ahmed, Z. A. Talib, and M. Z. bin Hussein, “Thermal, optical and dielectric properties of Zn-Al layered double hydroxide,” Applied Clay Science, vol. 56, pp. 68–76, 2012.

Y. Sun, W. Wang, L. Zhang, S. Sun, and E. Gao, “Magnetic ZnFe₂O₄ octahedra: Synthesis and visible light induced photocatalytic activities,” Materials Letters, vol. 98, pp. 124–127, 2013.

E. M. Seftel, E. Popovici, M. Mertens et al., “Zn-Al layered double hydroxides: synthesis, characterization and photocatalytic application,” Microporous and Mesoporous Materials, vol. 113, no. 1–3, pp. 296–304, 2008.

S. Yuan, Y. Li, Q. Zhang, and H. Wang, “ZnO nanorods decorated calcined Mg-Al layered double hydroxides as photocatalysts with a high adsorptive capacity,” Colloids and Surfaces A: Physicochemical and Engineering Aspects, vol. 348, no. 1–3, pp. 76–81, 2009.

J. Torrent and V. Barron, Encyclopedia of Surface and Colloid Science, Marcel Dekker, New York, NY, USA, 2002.

P. F. Wang, Z. H. Li, and Y. M. Zhu, “Research on the direct doping effect of silicon on cubic boron nitride ceramics by UV-VIS diffuse reflectance,” Materials Chemistry and Physics, vol. 123, no. 2-3, pp. 356–359, 2010.

J. S. Jang, P. H. Borse, J. S. Lee et al., “Synthesis of nanocrystalline ZnFe₂O₄ by polymerized complex method for its visible light photocatalytic application: an efficient photo-oxidant,” Bulletin of the Korean Chemical Society, vol. 30, pp. 1738–1742, 2009.

M. H. Habibi and A. H. Habibi, “Nanostructure composite ZnFe₂O₄FeFe₂O₄ZnO immobilized on glass: photocatalytic activity for degradation of an azo textile dye F3B,” Journal of Industrial and Engineering Chemistry, vol. 20, no. 1, pp. 68–73, 2013.

Z. L. Wang, “Zinc oxide nanostructures: growth, properties and applications,” Journal of Physics: Condensed Matter, vol. 16, pp. R829–R858, 2004.

L. Sun, R. Shao, L. Tang, and Z. Chen, “Synthesis of ZnFe₂O₄/ZnO nanocomposites immobilized on graphene with enhanced photocatalytic activity under solar light irradiation,” Journal of Alloys and Compounds, vol. 564, pp. 55–62, 2013.

D. Sridev and K. Rajendran, “Synthesis and optical characteristics of ZnO nanocrystals,” Bulletin of Materials Science, vol. 32, no. 2, pp. 165–168, 2009.

T. Xie, L. Xu, C. Liu, and Y. Wang, “Magnetic composite ZnFe₂O₄/SrFe₁₂O₁₉: preparation, characterization, and photocatalytic activity under visible light,” Applied Surface Science, vol. 273, pp. 684–691, 2013.