To ensure the validity of recent results obtained in our laboratory, I was asked to attempt to reproduce those results based only on the information provided in the paper we published on the topic:
A. M. Fajardo, N. S. Lewis, Journal Of Physical Chemistry B 101, 11136-11151 (1997).
My own research is in silicon photoelectrochemistry, but does not involve
the use of viologens nor impedance measurements. Thus my approach to these
aspects of the experiments described in the paper above were essentially
those of a novice, and I let the text of the paper guide me where specific
details were given but made my own choices with regard to everything else.
As a result, the cell configuration, calculation methodologies, and even
the impedance equipment I used differed from those used by the paper's
author. My results, however, are in very reasonable agreement with those
presented in the paper, at least for the one redox couple I utilized in
my experiments. Those results are presented here in appreciable detail.
|Exposed Area (cm2)||1.538||1.022||1.556||1.563|
The four electrodes described above were each etched for 30 seconds
in fresh 48%wt HF(aq) (Mallinckrodt AR grade) and
rinsed with 17.8 Mohm·cm Nanopure water. They were then dried under
filtered, rapidly-flowing nitrogen and transferred immediately into the
antechamber of the nitrogen flush box where the experiment was being carried
out. After the 30 minutes required to cycle the antechamber, the electrodes
were brought into the flushbox and the following steps performed on each
of them individually:
1) The electrode was submerged for a minimum of two minutes in a 0.2 M solution of ferrocenium tetrafluoroborate in methanol.
2) The electrode was placed into the cell and cycled at 50 mV/sec between +500 and -100 mV vs. the cell potential (as measured with a Pt wire electrode) under ambient illumination for a minimum of 20 minutes. The current-voltage characteristics of the electrodes were recorded between +500 and -200 mV vs. the cell potential at 50 mV/sec at the beginning and end of the cycle time.
3) Limiting anodic and cathodic current densities were measured at a 0.755 cm2 Pt electrode placed in the cell and potted with epoxy in a manned similar to that used for the Si electrode.
4) The room was darkened completely, and the current-voltage characteristics of the dark junction were measured between +500 and -300 mV vs. the cell potential, scanning at 50 mV/sec.
5) The electrode capacitance was measured using a 10 mV AC bias superimposed on a DC bias incremented from 0 to +800 mV vs. the cell potential at 50 mV intervals. At each DC bias, the electrode impedance was measured at 20 frequencies logarithmically distributed between 1 and 100 kHz. The integration time for each impedance measurement was 10 seconds. Other impedance analyzer settings were left at the manufacturer's defaults as set in the CorrWare (Win 95 32-bit ver. 2) software package used to control the instrument.
6) Step 4 was repeated following the impedance analyses.
7) The lights were turned back on and the electrode's current-voltage characteristics again recorded between +500 and -200 mV vs. cell at 50 mV/sec.
The potentiostat used was a Solartron 1287; the impedance analyzer
a Solartron 1260. The counterelectrode was a Pt ribbon, 6mm wide and 26
mm long, the in-cell reference a 4 mm Pt wire. The solution was stirred
with a large stirbar fast enough that a slight vortex formed in the cell
but not so fast that this vortex was unstable.
|Experimental Electron Transfer Rate Constants
as Measured by Impedance Analysis
at a methyl viologen (++/+•; 10mM : 10 mM in MeOH) / n-Si Junction at 25°C
[V vs. cell]
|ket (Float A)
|ket (Mixed A)
|ket (A = 1)
|RCRPS3||-0.29||1.41||2 x 10-18||6 x 10-17||9.5 x 10-18|
|RCRPS4||-0.27||1.32||3 x 10-18||3.5 x 10-17||8 x 10-18|
|RCRPS5||-0.30||1.21||1 x 10-17||1 x 10-16||3.4 x 10-17|
|RCRPS6||-0.34||1.16||3 x 10-17||2 x 10-16||7 x 10-17|
Dark JV Data: