Influences of FeMn ratio on the photocatalytic performance of wolframite (Fe_xMn_1-xWO₄)

Abstract

Natural semiconducting minerals are widely distributed in supergene environments and are famous for their solar-driven redox activities, in which wolframite (Fe_xMn_1-xWO₄) as a fascinating member got less attention. In this work, the semiconducting photocatalytic activities of Fe_xMn_1-xWO₄ series samples (x = 1, 0.74, 0.48, 0.24, 0) influenced by their crystal chemistry were investigated. The bandgaps of MnWO₄, Fe_0.24Mn_0.76WO₄, Fe_0.48Mn_0.52WO₄, Fe_0.74Mn_0.26WO₄ and FeWO₄ as measured by UV–vis diffuse reflection (DRS), were 2.7, 2.4, 2.3, 2.2, and 2 eV, respectively, which were linearly decreased (R² = 0.971) when x increased from 0 to 1. The density functional theory (DFT) calculations further indicated with the increasing content of Fe, the contribution of valence-band maximum (VBM) was stepwise occupied by Fe 3d orbits and the bandgap was thus gradually decreased. The photocatalytic activities of Fe_xMn_1-xWO₄ samples were examined on the degradation of methylene blue (MB, 5 mg/L). The MB removal rate was the highest in Fe_0.74Mn_0.26WO₄ system, which was 3.2, 1.9, 1.2, and 1.5 times faster than that of MnWO₄, Fe_0.24Mn_0.76WO₄, Fe_0.48Mn_0.52WO₄, and FeWO₄, respectively. The concentration of produced hydroxyl radical (•OH) by Fe_xMn_1-xWO₄, detected in electron paramagnetic resonance (EPR) measurement, had a positive correlation with the degradation rate of MB. The degradation rate slowed down when •OH was removed, demonstrating that •OH was the major reactive oxygen species in the photocatalytic oxidative degradation of MB. The best-performing Fe_0.74Mn_0.26WO₄ photocatalytically produced the most •OH, which was closely linked with its most abundant oxygen-vacancy defects, revealed by X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectroscopy. It thus can be concluded that narrow bandgap and appropriate oxygen vacancies can give rise to synergistic effect on the improvement of photocatalytic performance. This study helps get insight into the role of crystal chemistry in semiconducting properties and photocatalytic activities of wolframite, which also puts forward a new strategy to control environmental pollution by using natural minerals.

Introduction

Semiconducting minerals, such as hematite (α-Fe₂O₃), birnessite (MnO₂), rutile (TiO₂), anatase (TiO₂), can exert solar-driven photocatalysis on Earth's surface (Lu et al., 2004, Lu et al., 2007, Lu et al., 2012; Li et al., 2008, Li et al., 2018, Li et al., 2019c, Li et al., 2019d; Chen et al., 2011), thus have been considered to play important roles in driving the biogeochemical cycles of elements in nature (Xu and Schoonen, 2000; Luck et al., 2008; Doane, 2017; Lu et al., 2019). As compared to those binary metal oxide minerals, some multi-metal oxide minerals such as ilmenite (FeTiO₃), niobite ((Fe, Mn)Nb₂O₆), tantalite ((Fe, Mn)Ta₂O₆)) and spinel ferrites (ZnFe₂O₄) (Zhou et al., 2009; Wildner, 1992; Doane, 2017; Li et al., 2018) are more stable, which result from the stronger covalent nature of the basic coordination polyhedrons. By virtue of this structural stability, these minerals have recently received more attention in the field of geochemistry, materials, and environmental sciences.

Global tungsten resources are geographically widespread (U.S. Geological Survey, 2021) and wolframite, as the most common tungsten ore resource, is usually formed due to the geological background of medium and high-temperature hydrothermal mineralization. Wolframite (AWO₄, A = Fe, Mn, Ni, Zn, Cu, Co), which has a complex chain structure, is such a kind of ternary tungsten oxide (He et al., 2015; Wang et al., 2012; Zhang et al., 2010). The two octahedrons of [WO₆] and [AO₆] are arranged alternately in layers along the a- axis and distributed in a zigzag chain along the c-axis direction, which we can see from the Fig. 1. With different A ions, different degrees of adjustment and distortion occur on the [AO₆] and [WO₆] octahedrons. Natural wolframite often contains Fe²⁺ and Mn²⁺ at A site, and its general crystal formula can be denoted as (Fe, Mn)WO₄. So there are generally three octahedrons in natural wolframite crystals, centering at Fe²⁺, Mn²⁺, and W⁶⁺ ions with different radius of 0.061 nm, 0.067 nm, and 0.060 nm, respectively. To sustain the chain structure, three octahedrons thus display different degrees of deformation, which endows a unique sawtooth chain configuration. The mass percentage of Fe/Mn is varied with different occurring environments, ranging from 3.7 to 14.7% and 3.6 to 14.5%, respectively (Thongtem et al., 2009). It is worth pointing out that the concentration variation of Fe and Mn generally give rise to oxygen vacancies in binary oxides (e.g. TiO₂) and ternary perovskite-type oxide (Wei et al., 2013; Lin et al., 2014), which remarkably dominates their crystal chemistry and semiconducting property. However, there are relatively few reports on the relationship between the substitution of FeMn in wolframite and its photocatalytic performance.

The photocatalytic activity of semiconductors is regulated by many factors. For example, trace elements doped in natural minerals produce impurity energy level and narrow the bandgap, thereby broadening its photoresponse range (Lu et al., 2004, Lu et al., 2007; Li et al., 2008, Li et al., 2018, Li et al., 2019a, Li et al., 2019b, Li et al., 2019c, Li et al., 2019d; Chen et al., 2011; Lunk and Hartl, 2019); structural defects such as isomorphic substitution often cause oxygen vacancies, create electron or hole trapping centers, thereby lengthening the lifetime of carriers and improving the photocatalytic performance (Das et al., 2017; Gan et al., 2013; Pathak et al., 2016; Wang et al., 2010). In the real environment, chemical compositions, crystal structure, and lattice defects are all crucial factors and can interrelate and interact, jointly determining the photocatalytic performance of natural semiconducting minerals (Ertl et al., 2002; Bera et al., 2014; Baur, 1974; Robinson et al., 1971).

This paper aimed to investigate the effect of FeMn content on the photocatalytic performance of Fe_xMn_1-xWO₄. Different Fe/Mn molar ratio of Fe_xMn_1-xWO₄ were synthesized and characterized through XRD, XRF, and IR, and their bandgaps were studied by DRS and DFT. The PL and X-ray photoelectron spectroscopy (XPS) were dedicated to proving the existence of oxygen vacancy. The photocatalytic activity were conducted by the MB degradation experiment and the photocatalytic degradation mechanism was discussed by measuring •OH through EPR. The higher Fe/Mn molar ratio of Fe_xMn_1-xWO₄ with better photocatalytic performance was proposed and highlighted, which provided a basis for screening natural wolframite resources to treat environmental pollution by photocatalysis and expanding the potential application of solar catalysis in wolframite.