Determination of Band Structure of Naturally Occurring Goethite with Al Substitution: A Case Study of Zhushan Iron Zone.

Yan Shao,Guofeng Hu,Cong Ding,Mengqi Zhang,Zihao Liu,Yan Li,Xiaoming Xu

doi:10.3390/ma15041465

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.

Highlights

Fe is the most abundant transition metal element on the Earth’s surface and can form a diversity of oxide minerals in different weathering environments
The natural goethite sample used in this study was collected from the surface Fe coatings in the Zhushan Iron Zone, Hubei Province, China
The band gaps of both natural goethite and synthetic goethite samples were obtained by measuring the oxygen K-edge absorption and emission spectra on a 10ID-2 beamline at the Canadian Light Source

Summary

Introduction

Fe is the most abundant transition metal element on the Earth’s surface and can form a diversity of oxide minerals in different weathering environments. The band gap of iron oxide minerals is 2.0–2.5 eV [12] This means these widespread natural Fe oxide minerals can form active photo-generated hole–electron pairs to promote redox reactions on the Earth’s surface under irradiation with a wavelength shorter than 615 nm, playing important roles in shaping terrestrial environments. Many types of organic matter, including bisphenol, methylene blue, rhodamine, and acetaldehyde, can be photo-catalytically degraded in the presence of Fe oxide minerals [17,18,19]. All these lines of evidence show the potential impact of Fe oxide minerals involved in photochemical reactions on the surface geochemical process. The mineralogical tests, photochemical experiments, together with theoretical simulation can provide some insights on the band structure of natural goethite and deepen the understanding of goethite-involved photochemical reactions in terrestrial environments

Sample Preparation

Density Functional Calculations

Electronic Structure of Different Models

Findings

Conclusions