Characterization of defect energy levels due to As, Co, and Ni impurities in pyrite: A step toward understanding charge transfer kinetics at the semiconductor/electrolyte interface

KayeEAR-0617396了解黄铁矿氧化的分子尺度机制是环境地球化学、材料科学和选矿等多个领域的基础。在一项新的电化学研究中，我们利用人工生长的掺As、Co和Ni的黄铁矿单晶，证明了这些电活性杂质对体电子结构的影响，影响了氧化速度。我们观察到的氧化速率从高到低的顺序是：黄铁矿中含有As、Co、Ni和可以忽略不计的杂质。这表明，电活性杂质从根本上影响了黄铁矿的氧化速度，提出了这样的问题：观察到的氧化速度的差异与电荷转移电阻、载流子浓度和类型以及表面态的浓度有何关系？不同的杂质对表面态的浓度和反应活性有什么影响？解释观察到的行为的主要假设是：。由杂质引起的带隙中的体缺陷态在电极-溶液界面上变成了反应表面态，并作为电荷转移的管道。P型As掺杂黄铁矿的氧化速度更快，因为从价带到达表面的空穴接受来自Fe2+和S2-表面态的电子，导致断键。用EPS表征和测量了未掺杂黄铁矿和掺As、Co、Ni黄铁矿的体缺陷能级和界面表面态。表征Fe、As、Co、Ni的表面原子环境和价态；为了提出电荷转移机制来解释观察到的氧化速率，我们提出了一系列利用电化学阻抗谱(EIS)、电化学光电容谱(EPS)和交流伏安法来表征掺杂和非掺杂合成黄铁矿的动力学行为和表面电子结构的实验。平行的X射线吸收光谱实验，包括掠入射XAS，将提供关于块体和表面杂质的局部原子环境和电子结构的信息。智力价值我们收集了表征良好的合成掺杂和非掺杂黄铁矿，这使我们处于一个独特的位置，可以进一步探索黄铁矿氧化的动力学和机理。EPS将揭示带隙内的能级和黄铁矿表面活性缺陷态的性质(施主或受主)。不同杂质(掺杂：P型、Co和Nidoped：分别具有浅施主能级和深施主能级的N型)黄铁矿表面态的表征及其与氧化速率的关系将提示电子转移途径。交流伏安法将测量电荷转移动力学，并暗示费米能级钉扎是否控制表面电化学行为。EIS将建议电荷转移机制的等效电路模型，并提供有关电荷转移动力学的附加信息。将体缺陷和表面态的能量与溶液中氧化还原对的受主能级和施主能级联系起来，将使我们能够提出与As、Co和Ni杂质存在相关的特定电荷转移机制。本研究的结果将有助于建立更准确的反应模型，改进矿区和拟建矿区的生态风险分析。了解黄铁矿氧化对选矿工业至关重要，特别是在使用浮选方法进行矿石浓缩时。此外，对用于光伏和液态结太阳能应用的黄铁矿以及电化学存储设备感兴趣的材料科学家将从这方面的知识中受益。这项研究将继续促进范德比尔特大学地球与环境科学和化学系之间的合作，并支持一名新的研究人员完成他的博士论文。

SAVAGE, KayeEAR-0617396Understanding the molecular scale mechanisms of pyrite oxidation is fundamental to a variety of fields including environmental geochemistry, materials science, and mineral processing. In a new electrochemical study using synthetically grown single pyrite crystals doped with As, Co, and Ni, we show that bulk electronic structure, as influenced by these electroactive impurities, influences the rate of oxidation. We observe, in order of highest to lowest oxidation rate: pyrite with As, Co, Ni and negligible impurities. This suggests that electroactive impurities fundamentally affect the rate of pyrite oxidation, raising the questions: How are differences in observed oxidation rates related to charge transfer resistance, carrier concentration and type, and concentration of surface states? What is the effect of different impurities on the concentration and reactivity of surface states? The principle hypotheses to explain the observed behavior are:. Bulk defect states in the band gap arising from impurities become reactive surface states at the electrode-solution interface and act as a conduit for charge transfer. P-type As-doped pyrite oxidizes faster because holes reaching the surface from the valence band accept electrons from Fe2+ and S2 2- surface states, resulting in broken bonds.The research objectives are:. to characterize and measure the energy levels of bulk defect and interface surface states in undoped pyrite and pyrite doped with As, Co, and Ni using EPS;. to characterize the surface atomic environment and valence state of Fe, As, Co, and Ni;. to propose charge transfer mechanisms explaining the observed oxidation ratesWe propose a set of experiments using electrochemical impedance spectroscopy (EIS), electrochemical photocapacitance spectroscopy (EPS) and AC voltammetry aimed at characterizing the kinetic behavior and surface electronic structure of doped and undoped synthetic pyrite. Parallel X-ray absorption spectroscopy experiments, including grazing-incidence XAS, will provide information on the local atomic environment and electronic structure of the impurities in the bulk and at the surface.Intellectual MeritOur collection of well-characterized synthetic doped and undoped pyrite puts us in a unique position to further explore the kinetics and mechanisms of pyrite oxidation. EPS will reveal the energy levels within the bandgap and the nature (donor or acceptor) of active defect states at the pyrite surface. The characterization of surface states in pyrite with different impurities (As-doped: p-type, Co and Nidoped: n-type with shallow and deep donor levels respectively) and its relation to oxidation rates will suggest electron transfer pathways. AC voltammetry will measure charge transfer kinetics, and suggest whether Fermi level pinning is controlling the surface electrochemical behavior. EIS will suggest equivalent circuit models for the charge transfer mechanism and provide additional information about charge transfer kinetics. Relating the energy of the bulk defect and surface states to the acceptor and donor levels of the redox couple in solution will allow us to propose specific charge transfer mechanisms related to the presence of As, Co, and Ni impurities.Broader ImpactsPyrite oxidation in mine tailings and waste rock piles leads to acid drainage posing serious environmental risk. The results of this study will lead to more accurate reaction models and improved ecological risk analysis of mining and proposed mining sites. Understanding pyrite oxidation is critical for the mineral processing industry particularly in the use of flotation methods for ore enrichment. Additionally, materials scientists interested in pyrite for photovoltaic and liquid junction solar applications, and electrochemical storage devices will benefit from this knowledge. This study will continue to foster collaboration between the departments of Earth & Environmental Sciences and Chemistry at Vanderbilt University, and support a new researcher as he completes his doctoral dissertation.