When Voltammetry Reaches Nanoseconds To characterize reactive intermediates and gain insight into complex mechanisms, researchers must overcome the experimental difficulties of entering the submicrosecond time scale.
O
btaining faster and faster kinetics has always been a goal in physical chemistry because it will allow the characterization of increasingly reactive intermediates and lead to deeper insights into complex mechanisms. In addition, controlling information transfer to and storage on electronic devices of nanometer dimensions and at gigahertz frequencies is crucial for a wide range of practical applications. However, little understanding exists about phenomena that occur at these intermediate dimensions because our physical laws are based on average macroscopic beChristian Amatore havior. In addition, obtaining direct experEmmanuel Maisonhaute imental data is difficult in this time range École Normale Supérieure unless the system can be observed through a cascade of photochemical reactions. (France)
Experimental voltammogram
U
10
5 Current (µA)
Potential
Potential
Current (µA)
Capacitive current
0 –5 –2 –1.5 –1 Potential (V/ Pt ref)
U + R S i S
5 0 –5 –10 –2
–1.5
–1
Potential (V/Pt ref) Time Time
iDL(t )
CDL Output
Input iS(t)
RS
iF (t ) ZF
Faradaic current Current (µA)
RSiS
Time Feedback loop
2 0 –2 –2 –1.5 –1 Potential (V/Pt ref)
FIGURE 1. Simplified electrical equivalent circuit of an electrochemical cell that has compensation for the ohmic drop. The reduction of 14.3 mM anthracene in CH3CN plus 0.9 M tetraethylammonium tetrafluoroborate is measured. RS, solution resistance; CDL, double-layer capacitance; ZF, faradaic impedance containing the electrochemical information. The input potential U provided by the generator (black), the feedback potential RSiS (violet), and the sum U + RSiS (solid violet line) are shown on the left. The capacitive current (green) due to the double-layer charging, the faradaic current (red) due to the electrochemical reaction, the simulated faradaic current (dashed black), and the total current (violet) are shown on the lower and upper right.
Electrochemical analysis has typically been restricted to the 100-µs domain, and it was not until the introduction of ultramicroelectrodes in the 1980s that submicrosecond measurements became possible. Nevertheless, ultramicroelectrodes are not sufficient for performing analyses in 1 mV at 1 MV/s, compensation should be applied within 1 ns. This requires gigahertz bandwidth in the feedback loop even if the overall signal is in the megahertz range. Moreover, the signal propagation time lag in the complete electronic loop must also be 1, so that at each instant, the whole population of osmium centers in the SAM layer is in redox thermodynamic equilibrium with the electrode potential. (R is the perfect gas constant, T is temperature, and F is the Faraday constant.) When v is increased, becomes smaller and smaller in relation to the electron-transfer rate constant. Therefore, one may ultimately obtain kET
