Effect of anions on the phase stability of γ-FeOOH nanoparticles and the magnetic properties of gamma-ferric oxide derived from lepidocrocite
Effect of anions on the phase stability of γ-FeOOH nanoparticles and the magnetic properties of gamma-ferric oxide derived from lepidocrocite
复制标题
DOI：
10.1016/s0022-3697(03)00270-1
复制
发表时间：
2003-12
影响因子：
4
通讯作者：
C. Sudakar;G. Subbanna;T. Kutty
中科院分区：
文献类型：
--
作者：
C. Sudakar;G. Subbanna;T. Kutty
研究了Cl-、SO 42-和HPO 42-等阴离子对沉淀过程中FeOOH（α或γ）相稳定性的影响。在高Cl-浓度（[Cl-]/[Fe]=RCl≥8）的FeCl 2溶液或[HPO 42-]/[Fe]=RP≥0.02的（NH 4）2Fe（SO 4）2（FAS）溶液中氧化Fe（OH）2·xH 2 O可得到相纯的γ-FeOOH。在中等的RCl和RP范围内，得到α-FeOOH和γ-FeOOH的混合相。用中间相基质中的桥连阳离子或强键合的HPO 42 −离子取代Cl−的OH−（Fex 2 + Fey 3+（OH）2x+2 y −nz·xH 2 O（A）zn−，其中A为Cl−、SO 42 −、HPO 42 −等阴离子），促进了低密度γ-FeOOH的形成。然而，颗粒不太发达，并有差的结晶度，证明从透射电子显微镜和热重-差热分析的沉淀物。而在FeCl_2（RCl≥8）溶液中加入Pt ~（4+）、Pd ~（2+）或Rh ~（3+）等形貌控制剂，可得到长径比>10的双相、尺寸均匀的纳米板条状颗粒。添加剂在（0 k 0）和（h 00）平面上的优先吸附限制了垂直方向上的生长，导致高纵横比。这些添加剂的作用被磷酸根离子（一种强络合配体）抑制，从而产生单个颗粒长度小至10-30 nm的纤维状聚集体。虽然大部分Cl−离子在洗涤时从最终沉淀物中除去，但磷酸盐仍以HPO 42 −形式保留，这从IR吸收光谱中得到证明。通过γ-FeOOH脱羟基化得到的磁赤铁矿含有随机分布的微孔，如穆斯堡尔谱所解释的，其带来了表面原子上的自旋弛豫效应。这导致γ-Fe 2 O3 −δ颗粒的σs（44- 48 emu/g）和Hc（120- 130 Oe）较低。而还原再氧化法得到的几乎无孔的单晶颗粒具有较高的σs（73 emu/g）和Hc（320 Oe），而磷酸盐稳定的纤铁矿得到的纳米颗粒的σ s和Hc分别降至30 emu/g和75 Oe。在γ-FeOOH到磁铁矿到γ-Fe 2 O3 −δ的拓扑转变过程中，铁离子的迁移率和空位的反迁移率使颗粒无孔。
The effect of anions such as Cl−, SO42−, and HPO42−on the phase stability of FeOOH (α or γ) during precipitation is investigated. Oxidation of Fe(OH)2·xH2O from FeCl2solution with high Cl−concentration ([Cl−]/[Fe]=RCl≥8) or (NH4)2Fe(SO4)2(FAS) with [HPO42−]/[Fe]=RP≥0.02 yields phase-pure γ-FeOOH. In the medium ranges of RCland RP, mixed phases of α-FeOOH and γ-FeOOH are obtained. Replacement of OH−by Cl−with the bridging cations or strongly bonded HPO42−ions in the matrix of the intermediate phase (Fex2+Fey3+(OH)2x+2y−nz·xH2O(A)zn−, where A is anions such as Cl−, SO42−, HPO42−, etc.), promoted the lower density γ-FeOOH. However, the particles are less developed and have poor crystallinity as evidenced from transmission electron microscope and thermogravimetry–differential thermal analysis of the precipitates. Whereas, monophasic, uniformly sized, nano-lath shaped particles with high aspect ratio >10 are obtained when morphology-controlling cation additives such as Pt4+, Pd2+or Rh3+are present in FeCl2(RCl≥8) solution. Preferential adsorption of additives on (0k0) and (h00) planes limits the growth in the perpendicular directions leading to high aspect ratios. The effect of these additives are suppressed by the phosphate ion, a strong complexing ligand, giving rise to fibrous aggregate with the length of individual particles as small as 10–30 nm. While most of the Cl−ion is removed from the final precipitates on washing, phosphate remained as HPO42−as evidenced from IR absorption spectra. Maghemite obtained by dehydroxylating γ-FeOOH contains randomly distributed micropores bringing in the relaxation effects of spins on the surface atoms as deciphered from Mössbauer spectroscopy. This leads to the low σs(44–48emu/g) and Hc(120–130Oe) for γ-Fe2O3−δparticles. Whereas nearly pore-free single crystalline particles obtained by reduction followed by reoxidation has high value of σs(73emu/g) and Hc(320Oe), which decreases to 30emu/g and 75Oe, respectively, for nanoparticles obtained from phosphate stabilized lepidocrocite. The mobility of iron ions and counter mobility of vacancies during the topotactic transformation of γ-FeOOH to magnetite to γ-Fe2O3−δrenders the particles pore-free.