Synthesis and Characterization of Iron-Oxide (Hematite) Nanocrystals. Z.H. Lee

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1 ABSTRACT Synthesis and Characterization of Iron-Oxide (Hematite) Nanocrystals Z.H. Lee Engineering Science Programme, National University of Singapore Kent Ridge, Singapore Monodispersed iron oxide based nanocrystals of uniform size and shape were prepared using the LSS (liquid-solid-solid) synthetic route. In particular, the shape control of iron (III) oxide nanocrystals can be achieved by varying the growth parameters (concentration of solvent and precursors etc.). The morphology and structures of the iron oxide nanocrystals were examined using scanning electron (SEM) and transmission electron microscope (TEM). Detailed chemical analysis on the nanocrystals was carried out using X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray (EDX). INTRODUCTION Magnetic ferrite nanocrystals have gained much popularity during recent years and are increasingly being studied. This is because of their properties which are largely different compared to large compounds natural magnetic properties and especially due to their magnetic characteristics. Table 1 shows the different reagents, surfactant and solvents used to fabricate α-fe 2 O 3 monodispersed nanocrystals i. The surfactants are denoted as OLEA as oleic acid, TOA as trioctylamine, OAm as oleylamine and HDD as hexacanediol while solvents are denoted OE as octyl ether, ODE as 1-octadecene and PE as phenyl ether. The methods often used in the synthesis of α-fe 2 O 3 are thermal decomposition (T) and reduction (R). The difficulty encountered in synthesizing iron oxide nanocrystals lie in the quantity and quality as the nanocrystals tend to agglomerate together. In addition, high heat and toxic chemicals are often used in the synthesis. Thus, it would be pertinent to find a synthesis route that will be environmentally friendly and cost efficient. For this project, the synthesis route used is the LSS ii (liquid-solid-solid). The LSS is a new method to fabricate nanoparticles and might revolutionise iii the process of nanoparticle synthesis. In general, the LSS synthetic route makes use of a general phase transfer and separation mechanism occurring at the interfaces of the liquid, solid and solution phases present during the synthesis. Importantly, the process is simple in procedure and requires only standard chemicals, yet it successfully produces nanoparticles. To elucidate the process, the synthesis of noble metal nanoparticles is explained. Firstly, the aqueous solution of the noble metal salt, followed by sodium linoleate or sodium stearate, the fatty acid and finally the ethanol are added in the order. From figure 1, when the mixture is heated, reduction will occur due to the movement of the metal ions. Finally, nanoparticles are formed. They have a hydrophobic surface, and they will fall to the bottom of the container.

2 Table 1. Synthesis of monodispersed nanocrystals and their oxides. The advantage of the LSS synthetic route is its ability to effectively generate nanocrystals of differing properties such as semiconducting and magnetic properties. In addition, the approach can be used to generate rare earth fluorescent nanocrystals and doping materials to create special properties. Further studies can be made to explore further use of the LSS method, including turning the reaction conditions and chemicals. The wide range of application for mono-dispersed nanocrystals makes the LSS synthetic route applicable for the synthesis of mono-dispersed α- Fe 2 O 3. EXPERIMENTAL Iron(III) chloride hexahydrate, sodium hydroxide, oleic acid, were obtained from Aldrich. All chemicals were used as received and de-ionised water was used throughout the experiments. 8ml of distilled water were mixed with 5 ml of ethanol and 5ml of oleic acid at room temperature respectively. The solution was mixed evenly. The precursor solution (0.002 mol Fe 3+ ) was obtained by mixing 2ml of distilled water with g of FeCl 3.6H 2 O. The iron precursor was added drop wise into the solution. A red precipitate was observed appearing immediately. The solution was stirred for 10 minutes before being transferred to an autoclave. The mixed reactants were heated to 180 C for 10 hours. The autoclave was then allowed to cool. The final products were washed with 10ml of cyclohexane followed by 15ml of ethanol. The non polar solvent and polar solvent would help to disperse and deposit the iron oxide product respectively. The solution was centrifuged and the washings were repeated three times. The iron oxide powder samples were obtained by removing the deposited product at the bottom of the solution and then drying them in a drying oven at temperature of 50 C. RESULTS AND DISCUSSION In order to explore the synthesis of α-fe 2 O 3 nanoparticles, the synthesis conditions were adjusted. The results are tabulated in table 2. The amount of NaOH and ethanol solvent were adjusted downwards and upwards to determine their influence on the end-product. In the case of varying ethanol content, it is important to note that the water/solvent ratio (2:1) remains the same. The resulting nanoparticles were characterized and analysed using SEM, TEM, XRD, EDX and XPS equipment.

3 Starting material [g Oleic acid [ml] NaOH [g] Ethanol [ml] Water [ml] Phase of Product Morphology of Product FeCl 3.6H 2 O α-fe 2 O 3 Nanoparticles (reference sample) FeCl 3.6H 2 O α-fe 2 O 3 Nanoparticles FeCl 3.6H 2 O α-fe 2 O 3 Nanoparticles FeCl 3.6H 2 O α-fe 2 O 3 Nanoparticles FeCl 3.6H 2 O α-fe 2 O 3 Nanoparticles Effect of NaOH concentration Table 2: Synthesis conditions for the preparation of α-fe 2 O 3 Since NaOH quantity has been found to be a major synthetic parameter in the synthesis of iron nanoparticles of magnetite and goethite, it is not surprising to find that variation of NaOH does influence the size of α-fe 2 O 3 nanoparticles iv. In the experiment, the amount of NaOH was increased and decreased by 0.25g, while the Fe 2+ concentration, ethanol/water ratio, temperature and oleic acid quantity were kept constant. It was found that the size of the product varied. From figure 1 a and b derived from SEM images, the size of the nanoparticles decreases as the amount of NaOH increases. The average sizes of the nanoparticles are estimated to be nm, and nm respectively. According to Liang et al, the influence of NaOH on the synthesis of Fe 3 O 4 nanoparticles is also significant. It is reported that an increase in the alkalinity of the synthetic system will also influence the phase of product. However, it still remains to be investigated if this applies to α-fe 2 O 3 nanoparticles synthesized using FeCl 3.6H 2 O. (a) (b) Figure 1: SEM image of the nanocrystals prepared using 0.5 and 0.75g of NaOH

4 Effect of Ethanol Concentration While keeping the water/ethanol ratio constant, the ethanol quantity was varied. The other parameters of Fe 2+ concentration, reaction temperature, NaOH quantity, and oleic acid quantity were kept constant. Figure 2 a and b shows the SEM images of the end product using 5 and 10 ml ethanol of solvent respectively. At high ethanol content, besides the agglomerated nanoparticles, nanowires are also observed. (a) (b) Figure 2: SEM image for the nanocrystals prepared using (a) 5 and (b) 10 ml of ethanol TEM characterisation was carried out on the reference sample as shown in figure 3. The low resolution image of figure 3a shows the hexagonal shape nanocrystals dispersed on TEM copper grid. High resolution TEM image (figure 3b-c) shows high crystallinity structure of the synthesized nanocrystals. The corresponding lattice-plane spacing is approximately 2.5 Å (figure 3c inset) and can be identified to be α-fe 2 O 3. (a) (b) (c) 2.5 Ǻ 50 nm 5 nm 5 nm Figure 3: TEM images of the nanocrystals at (a) low and (b-c) high resolution. Figure 4a shows the XRD results of the reference sample. The content of our results matches with JCPDS (Joint Committee on Powder Diffraction Standards) card on Annex A. (JCPDS card ), identifying the sample to be α-fe 2 O 3. EDX spectrum was obtained as

5 shown in figure 4b. The elemental composition is confirmed to be Fe and O with no other impurities being detected. (104) Intensity (a.u.) (110) (113) (024) (116) Theta (degree) Figure 4: (a) XRD pattern and (b) EDX spectrum of the as-synthesized nanocrystals X-ray photoelectron spectroscopy was used to confirm the chemical composition of the reference sample. In figure 5 a and b, the Fe2p peaks at 719eV and 726eV correspond to the α-fe 2 O 3. The O1s main peak of ev was attributed to the O1s of α-fe 2 O 3. v.the extra hump observed in the O1s spectrum at 526.7eV is unidentified at the present moment.

6 Intensity (a.u) Fe2p Intensity (a.u) O1s Binding Energy (ev) Binding Energy (ev) Figure 5: XPS spectra of (a) Fe2p and (b) O1s CONCLUSIONS The LSS (liquid-solid-solution) synthetic route is an effective method of synthesizing iron oxide nanocrystals. The synthesized nanocrystals are monodispersed, highly crystalline and have an average diameter of nm. It was found that the variation of the NaOH and ethanol concentration changes the morphology of the iron oxide nanocrystals. Finally, we expect that with a good control of the reaction parameters, the dimension of the iron oxide nanocrystals can be tailored for various applications. REFERENCES i Jongnam Park, Jin Joo, Soon Gu Kwon, Youngjin Jang, and Taeghwan Hyeon Synthesis of Monodisperse Spherical Nanocrystals Angew. Chem. Int. Ed. 2007, 46, (2007). ii Xun Wang, Jing Zhuang, Qing Peng and Yadong Li. A general strategy for nanocrystal synthesis. Nature, Vol 437, , 1 September iii Emlsey John. A Great Leap Forward in Nanotechnology.Science Watch. Jan/Feb Retrieved 16th January of 2008 from iv Xin Liang, Xun Wang, Jing Zhuang, Yongtao Chen, Dingsheng Wang, and Yadong Li. Synthesis of Nearly Monodisperse Iron Oxide and Oxyhyrdoxide Nanocrystals. Advanced Functional Materials. 2006, Vol 16, v A. P. Grosvenor, B. A. Kobe, M. C. Biesinger and N. S. McIntyre. Investigation of multiplet splitting of Fe 2p XPS spectra and bonding in iron compounds. Surf. Interface Anal. 2004; 36: