Direct Spectroscopic Measurement of Interfacial Electric Fields near an Electrode under Polarizing or Current-Carrying Conditions
Direct Spectroscopic Measurement of Interfacial Electric Fields near an Electrode under Polarizing or Current-Carrying Conditions
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DOI：
10.1021/acs.jpcc.7b03134
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发表时间：
2017-06-01
影响因子：
3.7
通讯作者：
Dawlaty, Jahan M.
中科院分区：
文献类型：
--
作者：
Patrow, Joel G.;Sorenson, Shayne A.;Dawlaty, Jahan M.
界面电场和相关的分子极化是控制电极和分子之间电荷转移的中心量。界面场的存在通常通过传输和电容测量间接推断。希望通过它们在分子振动上引起的斯塔克位移直接测量这样的场。我们报告的斯塔克位移的一个众所周知的振动生色团栓系附近的电化学界面测量振动和频率产生光谱。我们有两个重要的发现。首先，我们观察到，所测量的局部场尺度相对于电解质中的离子浓度，根据一个模型，该模型结合了古伊-查普曼理论与分子层的电容响应。这种行为在离子浓度上保持超过3个数量级，因此支持模型的有效性。我们的结果沿着与此模型允许电极附近的电场的电位和离子浓度的变化估计。第二，我们观察到，上述变化的局部场与变化的电位只发生在正电位，电极极化，但可以忽略不计的电流。对于负电位，观察到持续的电化学电流，其可能由于电子转移和随后电解质中质子的还原而产生。有趣的是，我们观察到，在这种条件下，局部场不随施加的电势越来越负而变化，这让人想起漏电容器内的场。这一观察的重要结果是，当持续电流通过时，电化学反应的热力学驱动的增加不一定转化为表面附近的分子极化增加。这项研究将作为一个基线，在化学的所有领域中，了解界面附近的局部场的作用是重要的，并将提供一个新的视角界面电荷转移理论。
Interfacial electric fields and the related molecular polarization are the central quantities that govern charge transfer between an electrode and a molecule. The presence of the interfacial field is often inferred indirectly through transport and capacitance measurements. It is desirable to measure such fields directly via the Stark shift that they induce on molecular vibrations. We report the Stark shift of a well-known vibrational chromophore tethered near an electrochemical interface measured using vibrational sum frequency generation spectroscopy. We have two important findings. First, we observe that the measured local field scales with respect to the ionic concentration in the electrolyte according to a model that combines the Gouy-Chapman theory with the capacitive response of a molecular layer. This behavior holds over 3 orders of magnitude in ionic concentration, therefore lending support to the validity of the model. Our results along with this model allow for estimation of the electric field near the electrode as the potential and ionic concentration are varied. Second, we observe that the mentioned variation of the local field with changing potential only occurs for positive potentials, for which the electrode is polarized but negligible current flows. For negative potentials, a sustained electrochemical current is observed that likely arises due to electron transfer and subsequent reduction of protons in the electrolyte. Interestingly, we observe that, under this condition, the local field does not vary with increasingly negative applied potential, reminiscent of the field within a leaky capacitor. The important consequence of this observation is that an increase in the thermodynamic drive for an electrochemical reaction does not necessarily translate to increased molecular polarization near the surface when a sustained current is passing. This study will serve as a baseline in all areas of chemistry in which understanding the role of local fields near interfaces is important and will provide a new perspective for interfacial charge transfer theories.