Surface‐enhanced Raman Spectroscopy for Versatile in situ Measurements of Local pH near Electrode Surface

Abstract

We designed an in situ surface-enhanced Raman spectroscopy (SERS) system to detect pH change near the electrode surface. An optically transparent substrate covered with Ag nanoparticles (NPs) that induce SERS was positioned vertically to the electrode. A probe molecule that protonates/deprotonates according to the change in pH of surroundings were distributed nearby the NPs, so that the SERS spectrum of the probe indicated the pH changes in the vertical direction to the electrode due to hydrogen consumption. Flexible choices of probe molecules allow the system to be versatile. The system operated effectively in Ni electrodeposition as a case example.

Surface pH Effects on Catalytic Behavior of Pyridinic Nitrogen on Nitrogen-doped Carbon Nanotube in CO₂ Electrochemical Reduction

“…The in situ local pH measurement was performed by modifying our original system described previously. 21 In the homemade measurement cell shown in Fig. 1b, a 2 cm 2 NCNT electrode is installed as a working electrode, vertically to the pH sensor.…”

“…The SERS-active pH sensor was fabricated by the electroless deposition of Ag NPs and the modification of p-mercaptobenzoic acid ( p-MBA) as described in the previous paper. 21 The substrate of the sensor was a flat cover glass (18 mm © 18 mm with a thickness of ³170 µm) purchased from Matsunami Glass Ind., Ltd. The back surface of the pH sensor was irradiated with the 532 nm line from He-Ne laser to obtain Raman spectra of the modified p-MBA molecules on the Ag NPs.…”

“…To investigate the effect of pH at the electrode surface in each electrolyte, the in situ local pH measurements were conducted by using our original SERS system described previously. 21 This system detects pH change from the intensities of peaks corresponding to the deprotonated form of the carboxylic group in p-MBA modified to the surface of Ag NPs on flat glass. For a more precise quantification of the local pH, a calibration curve was constructed by evaluating the response toward Britton-Robinson buffer solutions as pH standards in the setup shown in Fig.…”

“…As for the local pH analysis, we have previously developed the in situ measurement system utilizing surface-enhanced Raman spectroscopy (SERS). 21 Employing this technique allows us to obtain information on pH at the NCNT electrode surface which is different from the bulk pH, in an environment quite close to the real condition. Combined with the evaluation of the electrochemical performance and the chemical state analyses, the catalytic mechan- ism such as the active site, and the determining factor of the CO 2 ER activity of NCNT is estimated.…”

Insight into the Change in Local pH near the Electrode Surface Using Phosphate Species as the Probe

“…Employing pH-sensitive materials or reactions as probes is a useful strategy. The probes can be either independent of the reaction, , such as pH-sensitive fluorescent dyes, − or species participating in reactions. For example, in CO 2 reduction, both the oxidation of CO as the product of the CO 2 RR and the buffer pair of CO 3 2– /HCO 3 – in the electrolyte can serve as pH probes. − Although diverse probes have been proposed, most of them are suitable only for a specific reaction, leaving the origin of local pH changes unclear.…”