Chernyshova I V, Hochella M F, Madden A S
The Center for NanoBioEarth, Department of Geosciences, Virginia Tech, Blacksburg, VA 24061, USA.
Phys Chem Chem Phys. 2007 Apr 14;9(14):1736-50. doi: 10.1039/b618790k. Epub 2007 Mar 16.
Using Fourier Transform InfraRed (FTIR) spectroscopy, Raman spectroscopy, X-ray diffraction (XRD), and Transmission Electron Microscopy (TEM), we characterize the structure and/or morphology of hematite (alpha-Fe(2)O(3)) particles with sizes of 7, 18, 39 and 120 nm. It is found that these nanoparticles possess maghemite (gamma-Fe(2)O(3))-like defects in the near surface regions, to which a vibrational mode at 690 cm(-1), active both in FTIR and Raman spectra, is assigned. The fraction of the maghemite-like defects and the net lattice disorder are inversely related to the particle size. However, the effect is opposite for nanoparticles grown by sintering of smaller hematite precursors under conditions when the formation of a uniform hematite-like structure throughout the aggregate is restricted by kinetic issues. This means that not only particle size but also the growth kinetics determines the structure of the nanoparticles. The observed structural changes are interpreted as size-induced alpha-Fe(2)O(3)<-->gamma-Fe(2)O(3) phase transitions. We develop a general model that considers spinel defects and absorbed/adsorbed species (in our case, hydroxyls) as dominant controls on structural changes with particle size in hematite nanoparticles, including solid-state phase transitions. These changes are represented by trajectories in a phase diagram built in three phase coordinates-concentrations of spinel defects, absorbed impurities, and adsorbed species. The critical size for the onset of the alpha-->gamma phase transition depends on the particle environment, and for the dry particles used in this study is about 40 nm. The model supports the existence of intermediate phases (protohematite and hydrohematite) during dehydration of goethite. We also demonstrate that the hematite structure is significantly less defective when the nanoparticles are immersed in water or KBr matrix, which is explained by the effects of the electrochemical double layer and increased rigidity of the particle environment. Finally, we revise the problem of applicability of IR spectroscopy to the lattice vibrations of hematite nanoparticles, demonstrating that structural comparison of different samples is much more reliable if it is based on the E(u) band at about 460 cm(-1) and the spinel band at 690 cm(-1), instead of the A(2u)/E(u) band at about 550 cm(-1) used in previous work. The new methodology is applied to analysis of the reported IR spectra of Martian hematite.
我们使用傅里叶变换红外（FTIR）光谱、拉曼光谱、X射线衍射（XRD）和透射电子显微镜（TEM），对尺寸为7、18、39和120纳米的赤铁矿（α-Fe₂O₃）颗粒的结构和/或形态进行了表征。结果发现，这些纳米颗粒在近表面区域存在类似磁赤铁矿（γ-Fe₂O₃）的缺陷，在FTIR和拉曼光谱中均有活性的690 cm⁻¹处的振动模式即归因于此。类似磁赤铁矿的缺陷比例和净晶格无序与颗粒尺寸成反比。然而，对于在动力学问题限制整个聚集体形成均匀赤铁矿样结构的条件下，由较小赤铁矿前驱体烧结生长的纳米颗粒，情况则相反。这意味着不仅颗粒尺寸，而且生长动力学也决定了纳米颗粒的结构。观察到的结构变化被解释为尺寸诱导的α-Fe₂O₃⇄γ-Fe₂O₃相变。我们开发了一个通用模型，该模型将尖晶石缺陷和吸附/吸收的物种（在我们的案例中为羟基）视为赤铁矿纳米颗粒中随颗粒尺寸变化的结构变化（包括固态相变）的主要控制因素。这些变化由在由尖晶石缺陷、吸收杂质和吸附物种的浓度这三个相坐标构建的相图中的轨迹表示。α→γ相变开始的临界尺寸取决于颗粒环境，对于本研究中使用的干燥颗粒，约为40纳米。该模型支持针铁矿脱水过程中存在中间相（原赤铁矿和水赤铁矿）。我们还证明，当纳米颗粒浸入水中或KBr基质中时，赤铁矿结构的缺陷明显减少，这可由电化学双层的作用和颗粒环境刚性增加来解释。最后，我们修正了红外光谱对赤铁矿纳米颗粒晶格振动适用性的问题，表明如果基于约460 cm⁻¹处的E(u)带和690 cm⁻¹处的尖晶石带，而不是先前工作中使用的约550 cm⁻¹处的A(2u)/E(u)带，不同样品的结构比较会更可靠。这种新方法被应用于分析报道的火星赤铁矿的红外光谱。
