Pati S S, Philip John
J Nanosci Nanotechnol. 2014 Jun;14(6):4114-23. doi: 10.1166/jnn.2014.8074.
We investigate the effect of sodium trapping on thermal stability of magnetite (Fe3O4) nanoparticles. The pure magnetite nanoparticles incubated in sodium hydroxide solutions and subsequently washed with water to remove the excess sodium. The amount of sodium in magnetite is measured using atomic absorption spectroscopy. The size distribution obtained from Small angle X-ray scattering measurements show that particles are fairly monodisperse. The FTIR spectra of nanoparticles show transmission bands at 441 and 611 cm(-1) are due to the symmetric stretching vibrations (v) of Fe-O in octahedral and tetrahedral sites respectively. With 500 ppm of sodium ions (Na+) in magnetite, the cubic ferrite structure of maghemite (gamma-Fe2O3) to hexagonal hematite (alpha-Fe2O3) phase transition is enhanced by -150 degrees C in air. The Rietveld analysis of sodium doped magnetite nanoparticles show that above 99% of metastable gamma-Fe2O3 is converted to a thermodynamically stable alpha-Fe2O3 after air annealing at 700 degrees C. A decrease in enthalpy observed in doped magnetite unambiguously confirms that the activation energy for maghemite to hematite transition is increased due to the presence of trapped sodium ions. These results suggest that the trapped cations in ferrite nanoparticles can stabilize them by increasing the activation energy.
我们研究了钠捕获对磁铁矿（Fe3O4）纳米颗粒热稳定性的影响。将纯磁铁矿纳米颗粒在氢氧化钠溶液中孵育，随后用水洗涤以去除过量的钠。使用原子吸收光谱法测量磁铁矿中的钠含量。从小角X射线散射测量获得的尺寸分布表明颗粒相当单分散。纳米颗粒的FTIR光谱显示，在441和611 cm(-1)处的透射带分别归因于八面体和四面体位置中Fe-O的对称伸缩振动（v）。在磁铁矿中含有500 ppm的钠离子（Na+）时，在空气中，磁赤铁矿（γ-Fe2O3）到六方赤铁矿（α-Fe2O3）的立方铁氧体结构相变温度提高了150℃。对钠掺杂磁铁矿纳米颗粒的Rietveld分析表明，在700℃空气退火后，超过99%的亚稳γ-Fe2O3转变为热力学稳定的α-Fe2O3。在掺杂磁铁矿中观察到的焓降低明确证实，由于捕获的钠离子的存在，磁赤铁矿到赤铁矿转变的活化能增加。这些结果表明，铁氧体纳米颗粒中捕获的阳离子可以通过增加活化能来使其稳定。
