Sensing local pH and ion concentration at graphene electrode surfaces using in situ Raman spectroscopy

• Published in: Nanoscale 2018-02, Vol.10 (5), p.2398-2403
• Main Authors: Shi, Haotian, Poudel, Nirakar, Hou, Bingya, Shen, Lang, Chen, Jihan, Benderskii, Alexander V, Cronin, Stephen B
• Format: Article
• Language: English
• Subjects: Capacitance-voltage characteristics; Carrier density; Charge density; Electrochemical potential; Electrodes; Graphene; Graphical user interface; Ion concentration; Ions; Raman spectroscopy; Spectrum analysis
• ISSN: 2040-3364; 2040-3372
• DOI: 10.1039/c7nr08294k

We report a novel approach to probe the local ion concentration at graphene/water interfaces using in situ Raman spectroscopy. Here, the upshifts observed in the G band Raman mode under applied electrochemical potentials are used to determine the charge density in the graphene sheet. For voltages up to ±0.8 V vs. NHE, we observe substantial upshifts in the G band Raman mode by as much as 19 cm , which corresponds to electron and hole carrier densities of 1.4 × 10 cm and Fermi energy shifts of ±430 meV. The charge density in the graphene electrode is also measured independently using the capacitance-voltage characteristics (i.e., Q = CV), and is found to be consistent with those measured by Raman spectroscopy. From charge neutrality requirements, the ion concentration in solution per unit area must be equal and opposite to the charge density in the graphene electrode. Based on these charge densities, we estimate the local ion concentration as a function of electrochemical potential in both pure DI water and 1 M KCl solutions, which span a pH range from 3.8 to 10.4 for pure DI water and net ion concentrations of ±0.7 mol L for KCl under these applied voltages.