Surface-Enhanced Infrared Spectra of Nicotinic Acid and Pyridoxine on Copper Substrates: What Is the Effect of Temperature and Deposition Conditions?

Nicotinic
acid and pyridoxine, adsorbed on electrochemically prepared copper
substrates, were studied by Surface-Enhanced Infrared Absorption (SEIRA)
spectroscopy. Assignment of the bands in SEIRA spectra to appropriate
vibrational modes was based on results of DFT calculations. The role
of dissolved gas in deposition solution was elucidated for adsorption
of nicotinic acid. Temperature dependence of SEIRA spectral features
of both nicotinic acid and pyridoxine was studied in the temperature
range 15–50 °C. In order to evaluate the temperature-induced
changes of SEIRA spectra of both two adsorbates, the bands were fitted
with Voigt profiles and their heights and widths were compared. Principal
Component Analysis (PCA) and Partial Least Squares (PLS) regression
were applied. It was found out that heating even to 50 °C does
not induce any observable changes in molecular structure of adsorbates.
However, noticeable changes of band intensities and widths were detected
and assigned to the partial reorientation of adsorbed molecules, to
the changes of intermolecular interactions within the adsorbed layers,
and to a minor extent, to the other changes of metal–adsorbate
systems properties, for example, to the changes of surface moisture.
The changes of metal reflectivity are considered to be the minor source
of spectral variation.

本研究采用表面增强红外吸收光谱（Surface-Enhanced Infrared Absorption，SEIRA）技术，对吸附于电化学制备铜基底上的烟酸与吡哆醇展开了系统研究。本研究依据密度泛函理论（DFT）计算结果，将SEIRA光谱中的各谱带归属至恰当的振动模式；针对烟酸的吸附过程，阐明了沉积液中溶解气体所发挥的作用。本研究在15~50 ℃的温度区间内，考察了烟酸与吡哆醇的SEIRA光谱特征随温度的变化规律。为评估两种吸附物的SEIRA光谱经温度诱导产生的变化，本研究采用沃伊特（Voigt）轮廓对各谱带进行拟合，并对比了其峰高与峰宽；同时应用了主成分分析（PCA）与偏最小二乘（PLS）回归方法。研究发现，即便升温至50 ℃，也不会对吸附物的分子结构造成可观测的变化。但研究检测到了谱带强度与峰宽的显著变化，这些变化可归因于吸附分子的部分取向重排、吸附层内分子间相互作用的改变，以及在较小程度上源于金属-吸附物体系的其他属性变化——例如表面吸附水的变化。金属反射率的变化被认为是光谱变化的次要诱因。