Contexto de Investigación

Scanning Electrochemical Cell Microscopy (SECCM) emerged as a novel technique for

high-resolution electrochemical imaging in 2010.1 It is a successor of a number of probe-

and pipette-based electrochemical techniques developed in the past. The path to high-

resolution imaging based on electrochemical principles was paved in late 1980 with the

advent of SECM2 and SICM3. Later on, a number of pipette-based methods, in which

electrochemical interrogation of the surface under study was confined within a small liquid

meniscus formed at the tip, emerged and were utilized e.g. for corrosion studies4 in the micrometer range and for probing electrochemical activity of basal plane of HOPG in

submicrometer range.5 A methodology utilizing double barrel probes with positional

34 deposition of extremely small droplets7 was perhaps a step away from turning into SECCM

as it is known today.

The technique surpasses previously developed imaging methods in a number of

aspects.8,9 Thus, in contrast to SECM and SICM, the sample does not have to be immersed

in an electrolyte solution, which is significant for prevention of any possible changes in the

surface properties due to prolonged contact with the solution. Maintenance of constant tip-

to-substrate distance (positional feedback) was not part of conventional version of SECM

and still remains problematic in spite of modernizations of this technique. In this respect,

SECCM, has straightforward way of controlling tip-to-substrate distance, not

involving/interfering with the current due to interfacial ET. Also, SECCM allows for

simultaneous acquisition of topographical information and ionic transfer. Finally, the

measurements are confined within a small meniscus with the size determined by the size of

the tip in use and in this sense the measurements are direct or, put differently, are carried

out in an extremely small electrochemical cell. In SECM, the measurements are said to

have remote character because they originate from a rather complex relation between the

tip and the substrate through diffusion of electroactive species and this relation strongly

depends on the tip-to-substrate distance.

SECCM is still a very young technique and its use has been limited to essentially one

research group so far, though there are other groups using similar techniques and HEKA

(Harvard Bioscience Inc.)* produced the first commercial version of SECCM setup. The

new technique has already shown substantial potential in tackling many challenging topics

of modern electrochemistry that is moving into smaller and smaller scale. Thus, it was

successfully applied to probe electrochemical activity of carbon nanotubes,10–12 imaging

35 surface of HOPG,13,14 mechanically exfoliated graphene,14 polycrystalline Pt,15,16 activity

of single Pt nanoparticles,17 heterogeneity of ET rate at CVD graphene18 and was adapted

for the studies of crystal dissolution kinetics.19

2.1.1 Principles of SECCM8,9

A schematic of the SECCM setup is presented in Fig 2.1. A double barrel pipette pulled

on a laser puller to a fine tapered tip of desired size (1) is filled with an electrolyte solution and attached to a z-piezo positioner (not shown) that provides uniform motion and oscillations of the pipette with frequency in the z-direction. Two quasi-reference counter electrodes (QRCEs) (2) are immersed in each barrel of the pipette and connect it to a voltage source EC that drives conductance current iC. EC is always kept constant during a

scan. Another voltage source E2 connects to the specimen (3) through the ground and

provides potential difference for interfacial ET at the specimen surface that is in contact

with the meniscus; the current in this circuit is referred to as the surface current, iS. The

specimen is firmly attached to a pedestal (4) surrounded by water pool (5) to reduce evaporation from the meniscus by providing increased humidity around it. Two pairs of

piezo positioners (not shown) move the stage (7) in the xy-plane, enabling the pipette to scan the surface laterally, whereas a z-piezo moves the pipette perpendicularly to the specimen surface. The setup described is enclosed in a grounded metallic box (Faraday

cage) to reduce electromagnetic interferences. Lastly, the box and electronics are placed on

a dedicated vibration isolation table to reduce the influence of building vibrations.

One of the key features of SECCM is the superposition of an oscillatory motion in z of the pipette on its uniform motion. Oscillations cause the meniscus at the tip to contract and

enlarge periodically, which results in oscillating conductance current, denoted iAC, due to

36 be very sensitive to the tip-to-substrate distance and positional feedback mechanism of

SECCM is built upon keeping this current constant (at a predefined value) during imaging.

The potential difference, ES, that drives interfacial ET (surface current) is given by

formula (2.1) for a perfectly symmetrical pipette.

ES = E2 + ½ EC (2.1)

Figure 2.1. Schematic of the SECCM setup. 1 – theta pipette filled with an electrolyte solution, 2 – a pair of quasi-reference counter electrode, 3 – surface of a material under study, 4 – Teflon support, 5 –water pool surrounding the specimen to reduce evaporation from the meniscus; 6 – symbolic presentation of piezo- positioners that move sample in xy-plane and the pipette in z.

So far, several modes of SECCM imaging have been developed. The basic one is raster

scan imaging at a fixed potential (ES = const) with the pipette moving laterally (quasi-

continuously along x for a given y then changing y in stepwise manner) and maintaining a preset value of iAC.1,13,18 A recent modification of SECCM in which the pipette scans the

surface laterally (stepwise in both x and y) and potential ES is swept linearly at each pixel,

37 potential mode. The pipette position is also controlled through constancy of iAC. This

modification proved to be of high significance in elucidating of observation of enhanced

surface current along the edges of graphene and HOPG in fixed potential images.13,12

Compared to the basic fixed potential mode, the huge amount of generated data can be

technically challenging to acquire and store. Also such scan takes much longer time and

would not be suitable if the sample properties may change within this time. Two other

variations of imaging with SECCM are based on hopping mode where the pipette (actually

meniscus) is made contact and retracted at each pixel of the image with ES being constant20

or swept linearly.21