High temperature stability of surfactant capped CoFe₂O₄ nanoparticles

Abstract

We investigate the effect of adsorbed surfactant on the structural stability of CoFe₂O₄ nanoparticles during vacuum thermal annealing. In-situ high temperature X-ray diffraction studies show a reduction of oleic acid coated CoFe₂O₄ nanoparticles into α-Fe and CoO under annealing at 800 °C. On the contrary, the uncoated CoFe₂O₄ nanoparticles remains stable, with its cubic phase intact, even at 1000 °C. Thermo-gravimetric analysis coupled mass spectra reveals that the evolved carbon from the surfactant aids the removal of oxygen atom from CoFe₂O₄ lattice thereby reducing it to α-Fe and CoO phases. These results are important in tailoring stable CoFe₂O₄ nanostructures for various applications.

Introduction

Surfactant molecules are used to control the particle size and to protect the surface of the nanoparticles from reactive atmospheres. Transition metal oxide ferrite nanoparticles, MFe₂O₄ (M = Mn, Fe, Co, Ni and Zn) have been a subject of intense research during the last few decades due to their interesting physical properties and technological applications [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. CoFe₂O₄ nanoparticles (CF np's) exhibit several interesting magnetic properties [6]. They are tunable coercivity, large anisotropy, moderate saturation magnetization, site specific and strong binding to the serum albumin proteins etc. [11], [12], [13], [14]. By exploiting the unique magnetic properties of ferrite nanoparticles, many innovative applications have been developed [15], [16], [17], e.g. DNA-based magnetic nanoparticle assembly based on nanoswitch for screening of DNA-cleaving agents [18], novel drug delivery systems [19], microwave absorption [20], [21], humidity sensors [22], magnetic recording media, photo detector, and spintronics.

In recent years, several synthesis protocols have been established to produce monodisperse nanoparticles of size ranging from single domain to superparamagnetic limits [23], [24], [25], [26], [27], [28], [29], [30]. This include solvothermal [31], hydrothermal [32], micellar method [33], mechanical milling [34], [35], sol–gel method [36], [37], combustion method, miniemulsion [38], bacterial synthesis [39], and co-precipitation method [40], [41], [42]. Further, CoFe₂O₄ phase with various morphologies including thin films, nanoribbons, nanowires, nanorods, nanoring, nanocube, etc. are obtained using advanced synthesis methods [10], [43], [44], [45], [46], [47], [48], [49]. Among these synthesis methods, we follow a simple co-precipitation method to synthesis oleic acid coated (OC-CF) and uncoated CoFe₂O₄ (U-CF) using NaOH as an alkali [50].

High temperature stability and proper understanding of the reduction kinetics of surface functionalized ferrite nanoparticles during annealing process are important to assess their suitability for high temperature applications [32], [51], [52], [53], [54], [55], [56]. The mechanism and kinetics of the reduction of powdered iron oxide samples have been investigated under nonisothermal conditions [57]. They have shown that the mechanism of pre-reduction step of Fe₂O₃ to Fe₃O₄ follow an nth order expression where nucleation or diffusion was not the rate-controlling factor while the main reduction to metallic phase was described by a model involving the random formation and growth of nuclei. A novel route of phase transformation induced by reduction of γ-Fe₂O₃ nanocubes in solution is reported recently [51]. Magnetization studies of nanosized cobalt ferrite above room temperature show that the magnetization decreases with increasing temperature approaching to zero at 830 K [58]. High temperature stable monodisperse superparamagnetic SnO₂-coated iron-oxide nanoparticles have been synthesized via thermal decomposition [55]. It was proposed that the SnO₂ shell acts as a barrier to the growth of α-Fe₂O₃ and consequently, avoiding the γ-Fe₂O₃-to-α-Fe₂O₃ phase transition at high temperatures [55]. Attempts have been made to understand the reduction kinetics of metal oxide nanoparticles during annealing under air, inert, vacuum, H₂, and CO. Factors influencing the phase transition of ferrite nanoparticles during annealing are preparation methods, size, metal ion doping, reaction media, heating rate, etc. However, there are only a few systematic high temperature studies on CF np's. Jeppson et al. [59] prepared CoFe₂O₄ by normal micelle precipitation method using SDS as a surfactant and compared the structural changes during annealing under inert (N₂) and oxidative (O₂) atmospheres. They have found that the surfactant causes the reduction of cobalt ferrite into metallic Co–Fe alloys, FeS, and α-Fe phases at 600 °C. It exhibits superparamagnetic behavior under inert annealing condition, whereas nonmagnetic hematite and CoSO₄·6H₂O together with CoFe₂O₄ are observed as end products under O₂ atmosphere. Wu et al. studied pressure induced electrical properties of CoFe₂O₄ nanoparticles, where they found size dependent phase transition from spinel to tetragonal structure at 7.5 and 12.5 GPa for 6 and 80 nm size particles [60]. Further, the reduction mechanism of carbon nanotube (CNT) encapsulated magnetic materials such as Fe, Ni, Co, Fe₃O₄, and CoFe₂O₄ have been successfully demonstrated [61], [62], [63]. Our earlier studies show that the phase transformation of Fe₃O₄ involves dehydration followed by immediate oxidative decomposition to hematite during annealing under O₂ or air [50]. However, studies show that CoFe₂O₄ is stable up to 1000 °C and preserve the cubic phase under different annealing atmospheres [46], [64], [65], [66]. In this paper, we provide the first experimental evidence for surfactant assisted reduction of CoFe₂O₄ nanoparticles under vacuum.