Abstract：
The photocatalytic property of Fe oxide minerals has long been considered to play an important role in shaping modern terrestrial environments. However, due to the complexity of natural settings, a precise determination of the band structure of natural goethite has not been achieved. In this work, the mineralogical characteristics of natural goethite samples obtained from Zhushan, China, were systematically studied through X-ray diffraction, transmission electron microscopy, X-ray energy dispersive spectroscopy, and X-ray fluorescence spectroscopy. Afterward, the band structure for both natural and synthetic goethite samples was determined by synchrotron-based X-ray absorption and emission spectra and photoelectron spectroscopy. The band gap of natural goethite (2.25 eV) was narrower than that of its synthetic counterpart (2.55 eV), and the valence band position of natural goethite was slightly lifted (-5.06 eV) compared to that of synthetic goethite (-5.38 eV). Al doping in natural goethite crystal, as revealed by the mineralogical tests, was the main reason that contributed to this difference. The theoretical calculation showed the narrowed band gap was caused by the contribution of Al-2p orbits at the top of the valence band. Therefore, free electrons can be created under light irradiation with a shorter wavelength. The experiments showed that natural goethite can photo-catalytically degrade methyl orange, and the degradation efficiency was better (47.5%) than that of the synthetic goethite group (31.5%). This study, for the first time, revealed the band structure and confirmed the photocatalytic properties of natural goethite, which should play an important role in surface substance evolution and elemental cycling.
摘要：
长期以来，人们一直认为氧化铁矿物的光催化性能在塑造现代陆地环境中起着重要作用。然而,由于自然环境的复杂性，尚未实现对天然针铁矿能带结构的精确测定。在这项工作中,竹山天然针铁矿样品的矿物学特征，中国,通过X射线衍射系统研究，透射电子显微镜,X射线能量色散谱,和X射线荧光光谱法。之后,通过基于同步加速器的X射线吸收和发射光谱以及光电子能谱确定了天然和合成针铁矿样品的能带结构。天然针铁矿的带隙(2.25eV)比合成针铁矿的带隙(2.55eV)窄,与合成针铁矿（-5.38eV）相比，天然针铁矿的价带位置略有提升（-5.06eV）。天然针铁矿晶体中的Al掺杂，正如矿物学测试所揭示的，是造成这种差异的主要原因。理论计算表明，带隙变窄是由价带顶部的Al-2p轨道的贡献引起的。因此,自由电子可以在较短波长的光照射下产生。实验表明,天然针铁矿可以光催化降解甲基橙,降解效率（47.5％）优于合成针铁矿组（31.5％）。这项研究,第一次,揭示了天然针铁矿的能带结构，证实了其光催化性能，应该在表面物质演化和元素循环中起重要作用。
