Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, USA.
J Chem Phys. 2019 Sep 14;151(10):104701. doi: 10.1063/1.5115163.
Small polaron formation is known to limit the photocatalytic charge transport efficiency of hematite via ultrafast carrier self-trapping. While small polaron formation is known to occur in bulk hematite, a complete description of surface polaron formation in this material is not fully understood. Theoretical predictions indicate that the kinetics and thermodynamics of surface polaron formation are different than those in bulk. However, to test these predictions requires the ability to experimentally differentiate polaron formation dynamics at the surface. Near grazing angle extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy is surface sensitive and provides element and oxidation state specific information on a femtosecond time scale. Using XUV-RA, we provide a systematic comparison between surface and bulk polaron formation kinetics and energetics in photoexcited hematite. We find that the rate of surface polaron formation (250 ± 40 fs) is about three times slower than bulk polaron formation (90 ± 5 fs) in photoexcited hematite. Additionally, we show that the surface polaron formation rate can be systematically tuned by surface molecular functionalization. Within the framework of a Marcus type model, the kinetics and energetics of polaron formation are discussed. The slower polaron formation rate observed at the surface is found to result from a greater lattice reorganization relative to bulk hematite, while surface functionalization is shown to tune both the lattice reorganization as well as the polaron stabilization energies. The ability to tune the kinetics and energetics of polaron formation and hopping by molecular functionalization provides the opportunity to synthetically control electron transport in hematite.
小极化子的形成被认为会通过超快载流子自陷限制赤铁矿的光催化电荷输运效率。虽然小极化子的形成在体赤铁矿中是已知的，但对该材料表面极化子形成的完整描述尚未完全理解。理论预测表明，表面极化子形成的动力学和热力学与体相中的不同。然而，要验证这些预测，需要能够在实验上区分表面极化子形成动力学。近掠角极紫外反射吸收（XUV-RA）光谱具有表面敏感性，可在飞秒时间尺度上提供元素和氧化态的特定信息。利用 XUV-RA，我们对光激发赤铁矿中表面和体相极化子形成动力学和能态进行了系统比较。我们发现，表面极化子的形成速率（250±40 fs）比光激发赤铁矿中的体相极化子形成速率（90±5 fs）慢约三倍。此外，我们表明，表面极化子的形成速率可以通过表面分子功能化来系统地调节。在马库斯型模型的框架内，讨论了极化子形成的动力学和能态。在表面上观察到的较慢的极化子形成速率是由于相对于体赤铁矿更大的晶格重排所致，而表面功能化被证明可以调节晶格重排以及极化子稳定能。通过分子功能化调节极化子形成和跃迁的动力学和能态的能力为在赤铁矿中合成控制电子输运提供了机会。
