Enhanced Magnetic Properties of Iron Oxide Nanoparticles Encapsulated in Cobalt Nanopowder Derived from Sunur-Pariaman Beach Sand

Cobalt encapsulated iron oxide nanoparticles based on Sunur-Pariaman beach sand were prepared using ball milling method. Cobalt nanoparticles with concentration of 0, 5,10 and 15 wt.% were added to the iron oxide nanoparticles. Un-encapsulated iron oxide nanoparticles revealed as pure cubic structure of magnetite phase well matched to JCPDS No. 03-0863 with crystallite size in the range of 30.86 -34.91 nm confirmed by X-Ray diffractometer (XRD). Saturation, remanance magnetization and coercivity increase with an increase of cobalt concentration for Co-encapsulated iron oxide nanopartcles showed good magnetic properties. These findings display excellence performance of cobalt encapsulated iron oxide nanparticles compared to un-encapsulated iron oxide nanoparticles and has been considered to be a potential candidate to be used as catalyst for degradation of methylene blue. The concentration of iron (Fe) and titanium (Ti) increase after being milled 100h. However, non iron oxide nanoparticles decrease.


Introduction
Magnetic nanoparticles with desired size currently have attracted great attention for research due to their wide range applications started from drug delivery to environmental remedy [1][2].These applications require suitable magnetic, optical, structural and morphological properties [3].More attention has been paid to magnetite (Fe 3 O 4 ) nanoparticles due to their unique property called superparamagnetic [4].However, magnetic parameters such as saturation magnetization and coecivity have to be improved in order to achieve these requirements.In recent years, the magnetic properties of magnetite nanoparticles prepared from natural beach sand could be enhanced by doping them with transition metals for examples Mn and Zn, [5,6].Introducing these metals could modify the saturation magnetization of iron oxide nanoparticles.Magnetite (Fe 3 O 4 ) nanoparticles may be prepared using various methods starting from hydrothermal to ball milling [7][8][9][10].However, iron oxide nanoparticles from beach sand prepared using ball milling method [10] has some advantages including simple, efficient and low cost.It is commonly known that pure magnetite (Fe 3 O 4 ) nanoparticles have excellent magnetic properties.Previous researcher [11]showed the degradation efficiency of TiO 2 doped magnetite nanoparticles is high.In order to enhance the degradation efficiency, some researchers [12] used transition metals as doping elements into magnetite nanoparticles.Manganese element has been used by other researchers [13] as a doping element to improve the degradation efficiency of magnetite nanoparticles.Furthermore, high degradation efficiency of methylene blue has been observed by other researcher [14] using cobalt doped magnetite nanoparticles that significantly increase the OH radicals.In this study, We aimed to prepare un-encapsulated and cobalt encapsulated iron oxide nonoparticles derived from Sunur beach sand using ball milling method.The effect of cobalt concentration on magnetic properties was investigated.The concentration of cobalt as an encapsulated nanoparticles was varied, and vibration sample magnetometer (VSM) was conducted to confirm the encapsulated nanopartcles.

Methodology 2.1. Preparationofironoxidenanoparticles.
The preparation of un-encapsulated and cobalt encapsulated iron oxide nanoparticles based on Sunur beach sand was performed by ball milling method.The first step was to employ iron sand separator (ISS) for separating between iron oxide and non iron oxide particles.Second step is to mill the ISS product for 100 h.The ball milling product was processed using strong NdFeB magnet for separating between iron oxide and non iron oxide particles.Finally, this product was divided into 4 parts with the same weight, each part was encapsulated with pure cobalt nanoparticles with composition of 0, 5, 10 and 15 wt.% using ball milling for20 h each part of the samples.

Characterization
X-Ray Diffractometer (XRD) was used to determine the structure and crystallite size of the iron oxide nanoparticle without cobalt encapsulate in the range 10 o -75 o .The morphology of materials was observed by scanning electron microscopy (SEM).The magnetization measurement (emu/g) as function of external field (Oe) of the samples was performed using vibrating sample magnetometer (VSM).Chemical elemental identification of the samples was measured using x-ray fluorescence (XRF) spectroscopy.

ResultsandDiscussion
The diffraction pattern of prepared iron oxide derived from Sunur beach sand measured by x-ray diffractometer (XRD) is shown in Fig. 1.The diffraction pattern of sample exhibited peaks at 2θ 30.38°,35.40°,37.02°,43.04°,53.41°,56.89°and62.38°.These peaks are related to hkl planes of (200), (311), ( 222), (400), (422), ( 511) and (440), respectively [15] and identified as pure cubic structure [16] of magnetite phase (JCPDS No. 03-0863).There is no extra diffraction peaks other than magnetite (Fe 3 O 4 ) phase in the diffraction pattern confirmed that the ball milling product has been dominated by magnetite phase.Moreover, appearance of sharp and high intensity diffraction peaks confirmed the formation of small crystallites size.The crystallite size of this sample was estimated using Scherrer's equation, D=0.9λ/β cos (θ), where λ is X-ray wavelength, β is the full width at half maximum intensity of the XRD peaks and θ is the angle of x-ray diffraction.The crystal size of iron oxide nanoparticles calculated for all observed diffraction peaks was found to be in the range of 30.86-34.91 nm.The morphology of the sample was inserted in Fig. 1.The micrograph of scanning electron microscopy (SEM) of the sample shows irregular shape, randomly organized and agglomerated form nanoparticles which is believed due to surface energies of iron oxide nanoparticles [17].The average particle size estimated from this image is roughly 258 nm.In line with XRD results, which revealed the average particles size is much bigger than that compared to determined by XRD and this is due to XRD analysis only studied single crystallite size.The magnetic parameters such as saturation, remanance magnetization and coercivity of unencapsulated and cobalt encapsulated iron oxide nanoparticles were studied using vibration sample magnetometer (VSM).To obtain the saturation condition of the sample, then the external magnetic field (-20000Oe to +20000Oe ) was employed in the measurement.The saturation, remanent magnetization and coercivity values were calculated from a plot of external magnetic field (Oe) and magnetization (emu/g) which is call hysteresis loop.The hysteretic phenomenon is observed when magnetization of the samples is plotted as a function of external magnetic field as displayed in Fig. 2. As the cobalt concentration increased, the saturation, remanent magnetization and coercivity increased.Upon cobalt encapsulated iron oxide nanoparticles, the formation of hard magnetic material was confirmed by larger values of coercivities that is 301.64 -520.15Oe as indicated in Fig. 3.It is clear that when the cobalt is encapsulated in iron oxide nanoparticles, the saturation magnetization increases from 37.51 -48.05 emu/g with amount of cobalt.This value is similar to that of iron oxide nanoparticles synthesized by method of co-precipitation observed by previous researchers [1], which is smaller than the saturation magnetization of magnetite nanoparticles.The saturation magnetization data of pure magnetite nanoparticles is 92emu/g [18].The presence of non-magnetic elements showed by x-ray fluorescence (XRF) results in Fig. 4 surrounding the iron oxide nanoparticles is believed to cause this phenomena.Moreover, as a comparison, magnetic properties especially saturation magnetization of the samples is comparable with this of pure magnetite (Fe 3 O 4 ) nanoparticles prepared using electro-synthesized [19] that is Ms = 72.96emu/g.Comparison of elemental composition between Sunur beach sand and sample after being milled for 100 h/cobalt 5 wt.% encapsulate iron oxide nanoparticles identified using x-ray fluorescence (XRF) is presented in Fig 4 .The amount of iron and titanium increases after being milled 100h.The increased value of titanium concentration is thought to be due to the existence of Fe-Ti compound which cannot be broken down by ball milling process.However, non iron oxide nanoparticles such as Al, Si, Ca and others decrease.Hence, we can conclude that cobalt encapsulated iron oxide nanoparticles samples were prepared successfully.

Conclusion
Using Sunur beach sand we have prepared magnetite (Fe 3 O 4 ) nanoparticles with diameters of around 258 nm by ball milling.Iron oxide nanoparticles encapsulated with cobalt.Cobalt encapsulated iron oxide nanoparticles are confirmed to have a ferromagnetic behavior.The XRD result revealed the presence of crystalline magnetite (Fe 3 O 4 ) phase with the crystallite size in the range of 30.86 -34.91 nm.Moreover, XRD analysis revealed cobalt encapsulated iron oxide nanoparticles does not affect their crystal structure.SEM result shows its morphological surface of un-encapsulated iron oxide nanoparticles and confirmed irregular shape, non-homogenous distribution and agglomerated form.Enhancement of ferromagnetism in the samples is largely affected by the cobalt concentration of encapsulated iron oxide nanoparticles.

Acknowledgement
Authors are grateful to the Directorate General of Higher Education, Indonesian Ministry of Education and Culture 2023 for providing financial support to perform this work.The authors also gratefully acknowledge Indonesian Sciences Institute (LIPI), Department of Physics UNP Padang for providing characterization facilities.

Figure1.
Figure1.XRD Pattern of prepared nanoparticles.The inset picture shows the corresponding morphology of the sample.

Figure 4 .
Figure 4.The XRF result for sample before and after being milled for 100h/ 5 wt.% cobalt encapsulated sample.