pH-sensitive Ni(OH)2-based microelectrochemical transistors

Michael J. Natan, Daniel Bélanger, Michael K. Carpenter, Mark S. Wrighton

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57 Scopus citations

Abstract

Properties of arrays of closely spaced (1.2 μm) Au or Pt microelectrodes (∼2 μm wide × 50 μm long × 0.1 μm high) coated with cathodically grown films of Ni(OH)2 are reported. Electrical connection of microelectrodes by Ni(OH)2 was verified by cyclic voltammetry. The ratio of anodic charge to cathodic charge in cyclic voltammograms for the Ni(OH)2 ⇌ NiO(OH) interconversion exceeds one. However, it is shown that excess charge in the anodic cyclic voltammetric wave for oxidation of Ni(OH)2 does not affect the conductivity of Ni(OH)2 films. The steady-state resistance of Ni(OH)2 connecting two microelectrodes has been measured as a function of potential from 0 to 0.7 V vs. SCE and was typically found to vary from ∼107 to ∼104 ohms. The measured resistance corresponds to a resistivity of approximately 30 ohm·cm in the oxidized state. The decrease in resistance is caused by electrochemical oxidation of insulating Ni(OH)2 to "conducting" NiO(OH). At fixed drain voltage, VD, the gate current, IG, and the drain current, ID, can be measured simultaneously as the gate voltage, VG, is varied at a given frequency. The frequency response is limited by the slow electrochemistry of Ni(OH)2 films. At a frequency of 3.8 × 10-2 Hz, Ni(OH)2-based microelectrochemical transistors can amplify electrical power by a factor of 20. The temperature dependence of ID indicates an activation energy for conductivity in NiO(OH) of 23 ± 2 kJ/mol at VG = 0.45 V vs. SCE. A pair of microelectrodes connected by Ni(OH)2 functions as a pH-sensitive microelectrochemical transistor, because there is a pH dependence in the potential associated with the oxidation of Ni(OH)2. The pH dependence of the transistor behavior is illustrated under dynamic and steady-state conditions; as the pH of a basic solution is increased, VG for device turn on moves negative, in accord with the known pH dependence of the redox chemistry of Ni(OH)2. Detection of a change in pH from 12 to 13 in a flowing stream was demonstrated with Ni(OH)2-based microelectrochemical transistors.

Original languageEnglish
Pages (from-to)1834-1842
Number of pages9
JournalJournal of Physical Chemistry
Volume91
Issue number7
DOIs
StatePublished - 1987
Externally publishedYes

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