Direct Observations of Electron Hopping in Iron Oxide

使用超速泵探针X射线光谱，科学家团队能够通过氧化还原活性氧化铁氧化物通过自发熨斗到铁啤酒花运输的速率，直接观察它们已经发生在电子之后转移到氧化铁颗粒上。

Rust – iron oxide – is a poor conductor of electricity, which is why an electronic device with a rusted battery usually won’t work. Despite this poor conductivity, an electron transferred to a particle of rust will use thermal energy to continually move or “hop” from oneatomof iron to the next. Electron mobility in iron oxide can hold huge significance for a broad range of environment- and energy-related reactions, including reactions pertaining to uranium in groundwater and reactions pertaining to low-cost solar energy devices. Predicting the impact of electron-hopping on iron oxide reactions has been problematic in the past, but now, for the first time, a multi-institutional team of researchers, led by scientists at the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have directly observed what happens to electrons after they have been transferred to an iron oxide particle.

“We believe this work is the starting point for a new area of time-resolved geochemistry that seeks to understand chemical reaction mechanisms by making various kinds of movies that depict in real time how atoms and electrons move during reactions,” says Benjamin Gilbert, a geochemist with Berkeley Lab’s Earth Sciences Division and a co-founder of the Berkeley Nanogeoscience Center who led this research. “Using ultrafast pump-probe X-ray spectroscopy, we were able to measure the rates at which electrons are transported through spontaneous iron-to-iron hops in redox-active iron oxides. Our results showed that the rates depend on the structure of the iron oxide and confirmed that certain aspects of the current model of electron hopping in iron oxides are correct.”

吉尔伯特是在杂志上描述这项工作的论文的相应作者。本文标题为“Electron small polarons and their mobility in iron (oxyhydr)oxide nanoparticles.” Co-authoring the paper were Jordan Katz, Xiaoyi Zhang, Klaus Attenkofer, Karena Chapman, Cathrine Frandsen, Piotr Zarzycki, Kevin Rosso, Roger Falcone and Glenn Waychunas.

在宏观尺度、岩石和矿物不出现to be very reactive – consider the millions of years it takes for mountains to react with water. At the nanoscale, however, many common minerals are able to undergo redox reactions – exchange one or more electrons – with other molecules in their environment, impacting soil and water, seawater as well as fresh. Among the most critical of these redox reactions is the formation or transformation of iron oxide and oxyhydroxide minerals by charge-transfer processes that cycle iron between its two common oxidation states iron(III) and iron(II).

“因为铁（II）比铁（III）易溶，铁（III）的还原转化，氧化铁和羟基氧化物矿物质可能会显着影响土壤和表面的化学和矿物质，”Gilbert说。国外欧洲杯足球彩票“在铁（III）氧化物的情况下，氧化铁的还原（II）可导致矿物溶解在变化矿物流动途径的非常快速的时间内。还可以将铁的动员转化为能够为生物体提供重要的生物可利用铁来源。“

吉尔伯特还注意到，许多有机和无机环境污染物可以用氧化铁相交换电子。无论是铁（III）还是铁（II）是氧化物是降解或螯合给定污染物的重要因素。此外，某些细菌可以将电子转移到氧化铁作为其代谢的一部分，将铁氧化还原反应与碳循环连接。指导这些关键生物地理化化学结果的机制仍然不清楚，因为矿物氧化还原反应是复杂的，并且涉及在几十亿分钟内发生的多个步骤。直到最近，这些反应无法观察到，但随着同步辐射设施和超快X射线光谱的出现，事情发生了变化。

“就像一个体育摄影师必须使用一个带有非常快速的快门速度的相机，在没有模糊的情况下捕捉运动员的运动员，以便能够观看电子移动，我们需要使用非常短而非常明亮（强大）脉冲的X-“丹尼森大学的科学论文”的牵头作者乔丹卡茨说，“光线说。“对于这项研究，X射线在阿尔冈国家实验室的先进光子源生产。”

In addition to short bright pulses of X-rays, Katz said he and his co-authors also had to design an experimental system in which they could turn on desired reactions with an ultrafast switch.

“The only way to do that on the necessary timescale is with light, in this case an ultrafast laser,” Katz says. “What we needed was a system in which the electron we wanted to study could be immediately injected into the iron oxide in response to absorption of light. This allowed us to effectively synchronize the transfer of many electrons into the iron oxide particles so that we could monitor their aggregate behavior.”

With their time-resolved pump-probe spectroscopy system in combination with ab initio calculations performed by co-author Kevin Rosso of the Pacific Northwest National Laboratory, Gilbert, Katz and their colleagues determined that the rates at which electrons hop from one iron atom to the next in an iron oxide varies from a single hop per nanosecond to five hops per nanosecond, depending on the structure of the iron oxide. Their observations were consistent with the established model for describing electron behavior in materials such as iron oxides. In this model, electrons introduced into an iron oxide couple with phonons (vibrations of the atoms in a crystal lattice) to distort the lattice structure and create small energy wells or divots known as polarons.

“这些电子小极性有效地形成了局部的低价金属位点，并且通过从一个金属位点到下一个金属位点的热激活电子进行导电，”Gilbert说。“通过测量电子跳率，我们能够通过实验证明来自晶体的铁（II）脱离是整体溶解反应中的速率限制。我们还能够表明氧化铁中的电子跳跃不是使用这些矿物作为电子受体的微生物生长的瓶颈。蛋白质与矿物电子传递速率较慢。“

KATZ对这些结果的应用很兴奋，以寻找使用氧化铁用于太阳能收集和转换的方法。

“Iron oxide is a semiconductor that is abundant, stable and environmentally friendly, and its properties are optimal for absorption of sunlight,” he says. “To use iron oxide for solar energy collection and conversion, however, it is critical to understand how electrons are transferred within the material, which when used in a conventional design is not highly conductive. Experiments such as this will help us to design new systems with novel nanostructured architectures that promote desired redox reactions, and suppress deleterious reactions in order to increase the efficiency of our device.”

Adds Gilbert, “Also important is the demonstration that very fast, geochemical reaction steps such as electron hopping can be measured using ultrafast pump-probe methods.”

这项研究受到了美国国务院of Energy’s Office of Science, which also supports the Advanced Photon Source.

Image: Benjamin Gilbert, Berkeley Lab