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Electrochemical Scanning Probe
Microscopy for Catalyst Characterisation
Andrew J. Wain
National Physical Laboratory, Teddington, TW11 0LW, United Kingdom
[email protected]
Introduction
Structure-function relationships in heterogeneous catalysts have long been a topic of profound
interest and intense research. An integral part of designing and developing new catalyst
materials is understanding the correlation between surface morphology and catalytic activity
on different length scales. In particular spatially resolved characterization of catalysts under
reaction conditions remains a fundamental challenge.
Scanning electrochemical microscopy offers a unique means with which to map the chemical
and electrochemical activity of surfaces in the liquid phase. In this work we demonstrate the
use of such scanning probe microscopy in electrocatalyst characterisation and explore its
potential application to the measurement of heterogeneous catalyst activity in-situ.
Scanning Electrochemical Microscopy (SECM)
Imaging Topography
and Activity
SECM at the nanoscale requires short and
constant working distances (~10 nm), and
thus the probe must respond in real-time to
surface topography.
Detector
Laser
AFM
Cantilever
An elegant method of achieving this
is to combine SECM with atomic force
microscopy (AFM). This enables simultaneous
measurement of the surface terrain and
electro(chemical) activity.
Nano-disk
Electrode
A
B
In SECM a microelectrode probe is scanned laterally across the substrate, and is used to monitor
processes locally in solution and map corresponding surface reactivity.
Probe Development:
We have developed dual-function needle probes for
this purpose.
Probe
Reference
Probes consist of a high aspect
ratio nanoelectrode integrated
onto the tip of a conventional
AFM cantilever.
Auxillary
Well-behaved electrochemistry
demonstrated with measured
electrochemical radius close to
150 nm.
Substrate
Imaging:
Test substrate consists of 2 µm
gold strips on silicon.
How it Works
Amperometric Mode:
Potentiometric Mode:
• Apply voltage to probe,
measure current
• “Reactant” electrochemically
generated at probe tip
• Current sensitive to fate of
reactant at substrate surface
• Measure probe potential at
open circuit
• Probe coated with pHsensitive layer
• Use pH changes near
surface to indicate reaction
Topography (left image) show
well resolved gold features.
Electrochemistry (right
image) in amperometric
mode demonstrates current
increase over electrocatalytic
gold regions.
A. J. Wain, D. Cox, S. Zhou, A. Turnbull, Electrochem. Commun., 13 (2011) , 78.
Future Applications: High-Throughput Screening
Microarrays of catalyst libraries deposited on flat substrate
(spot size ~200 µm). Parameters such as catalyst composition
and nanoparticle size can be varied within a single array plate.
SECM may be used as a screening tool to determine optimum
catalyst properties within a single experiment. We are
developing this approach in our laboratory and we present
here some preliminary findings in amperometric mode.
Substrate Imaging:
• Scan probe at fixed height above substrate (~10 µm)
• Measure current/potential as a function of position
Applications have thus far been limited to screening of
electrocatalysts; it is our goal to extend application to a range
of heterogeneously catalysed liquid phase reactions e.g.
glucose oxidation.
e.g. H electroreduction at 200 mm Pt disk substrate
+
• Substrate and probe biased to give
electrochemical reduction of dissolved O2.
• Competitive reduction of O2 over the gold
spots results in decrease in local current.
Amperometric Mode:
Potentiometric Mode:
Increased current at disk due to H2
generation at the substrate
Increase in local pH at disk due to H+
consumption
Micron Scale Electrocatalytic
Activity Mapping
Hydrogen oxidation reaction mapping at Pt/C electrocatalyst
film on HOPG in amperometric mode
• Normalised current shows activity variations
• Non-uniform catalyst distribution reflected in response
P. G. Nicholson, S. Zhou, G. Hinds, A. J. Wain, A. Turnbull,
Electrochim. Acta, 54 (2009) 4525.
Conclusions
• SECM is a powerful tool for measuring electrocatalytic activity on a local scale
• SECM-AFM enables simultaneous topographical and chemical imaging
• There is notable potential for the application of SECM to high throughput catalyst screening
Acknowledgements
This work was funded by the Innovation R&D Program of the National Measurement System in
the Department for Business Innovation and Skills.