ACS Appl Mater Interfaces. 2018 Oct 3;10(39):33678-33683. doi: 10.1021/acsami.8b11961. Epub 2018 Sep 18.
We report the use of surface-enhanced Raman scattering (SERS) to measure the vibrational Stark shifts of surface-bound thiolated-benzonitrile molecules bound to an electrode surface during hydrogen evolution reactions (HERs). Here, the electrode surface consists of Au nanoislands deposited both with and without an underlying layer of monolayer graphene on a glass substrate. The Stark shifts observed in the nitrile (C-N) stretch frequency (around 2225 cm) are used to report the local electric field strength at the electrode surface under electrochemical working conditions. Under positive (i.e., oxidative) applied potentials [vs normal hydrogen electrode (NHE)], we observe blue shifts of up to 7.6 cm, which correspond to local electric fields of 22 mV/cm. Under negative applied potentials (vs NHE), the C-N stretch frequency is red-shifted by only about 1 cm. This corresponds to a regime in which the electrochemical current increases exponentially in the hydrogen evolution process. Under these finite electrochemical currents, we estimate the voltage drop across the solution ( V = IR). Correcting for this voltage drop results in a highly linear electric field versus applied electrochemical voltage relation. Here, the onset potential for the HER lies around 0.2 V versus NHE and the point of zero charge (PZC) occurs at 0.04 V versus NHE, based on the capacitance-voltage ( C- V) profile. The solution field is obtained by comparing the C-N stretch frequency in solution with that obtained in air. By evaluating the local electric field strength at the PZC and the onset potential, we can separate the solution field from the reaction field (i.e., electrode field), respectively. At the onset of HER, the solution field is -0.8 mV/cm and the electrode field is -1.2 mV/cm. At higher ion concentrations, we observe similar electric field strengths and more linear E-field versus applied potential behavior because of the relatively low resistance of the solution, which results in negligible voltage drops ( V = IR).
我们报告了使用表面增强拉曼散射（SERS）来测量在析氢反应（HER）期间结合到电极表面的表面结合的巯基苯甲腈分子的振动斯塔克位移。在这里，电极表面由沉积在玻璃基底上的具有和没有单层石墨烯底层的 Au 纳米岛组成。在腈（C-N）伸缩频率（约 2225cm）中观察到的斯塔克位移用于报告在电化学工作条件下电极表面的局部电场强度。在正（即氧化）施加电位[相对于标准氢电极（NHE）]下，我们观察到高达 7.6cm 的蓝移，这对应于 22mV/cm 的局部电场。在负施加电位（相对于 NHE）下，C-N 伸缩频率仅红移约 1cm。这对应于电化学电流在析氢过程中呈指数增长的范围。在这些有限的电化学电流下，我们估计溶液中的电压降（V=IR）。对该电压降进行校正会导致电场与施加的电化学电压之间呈高度线性关系。在这里，HER 的起始电位约为 0.2V 相对于 NHE，零电荷点（PZC）发生在 0.04V 相对于 NHE，基于电容-电压（C-V）曲线。通过将溶液中的 C-N 伸缩频率与空气中的伸缩频率进行比较，可以获得溶液中的电场。通过评估 PZC 和起始电位处的局部电场强度，我们可以分别将溶液场与反应场（即电极场）分离。在 HER 开始时，溶液场为-0.8mV/cm，电极场为-1.2mV/cm。在较高的离子浓度下，由于溶液的电阻相对较低，我们观察到相似的电场强度和更线性的 E 场与施加的电势行为，因为这导致可忽略的电压降（V=IR）。
