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Electrochemical STM (EC-STM) is a particular type of Scanning probe microscopy (SPM), which allows to combine the classic Scanning tunneling microscopy (STM) technique together with electrochemical measurements. By mean of EC-STM, it is possible to perform in-situ measurements in order to investigate the morphological changes of the surface of an electrode during electrochemical reactions. The solid-liquid interface is thus investigated at atomic or molecular scale. This technique was developed by Itaya and Tomita in 1988, who wanted to construct a STM able to work under electrochemical conditions, using different types of electrolytes. The principal aim of their work was to control, at the same time, both the tunnelling tip and the electrode, under potentiostat conditions.

Electrochemical AFM

Electrochemical AFM (EC-AFM) is a particular type of Scanning probe microscopy (SPM), which combines the classical Atomic force microscopy (AFM) together with electrochemical measurements. EC-AFM allows to perform in-situ AFM measurements in an electrochemical cell, in order to investigate the actual changes in the electrode surface morphology during electrochemical reactions. The solid-liquid interface is thus investigated. This technique was developed for the first time in 1996 by Kouzeki et coworkers, who studied amorphous and policristallines thin films of Naphthalocyanine on Indium titanium oxide in 0.1 M Potassium chloride (KCl). Unlike the Electrochemical scanning tunneling microscope, prevoiusly developed by Itaya and Tomita in 1988 , the tip is non-conductive and it is easily steered in a liquid environment.

Principles and experimental precautions

The tecqnique consists in an AFM apparatus integrated with a three electrode electrochemical cell.

Schematic of the electrochemical cell.

Electrochemical AFM section.
(a) Cantilever and tip
(b) electrochemical cell
(c) Liquid (electrolyte)
(d) sample
(e) Sample holder. The elctrochemical connections are usually placed under the sample holder.

The sample works as working electrode (WE) and must be conductive. The AFM probe is a "passive" element, as it is unbiased and it monitors the surface changes as a function of time, when a potential is applied to the sample. Several electrochemical experiments can be performed on the sample, such as cyclic voltammetry, pulse voltammetry etc. During the potential sweeping, the current flows through the sample and the morphology is monitored.

The electrochemical cell is made of a plastic material resistant to various chemical solvents (e.g sulfuric acid, perchloric acid etc.), with a good mechanical resistance and low fabrication costs. In order to achieve these requests, various materials can be employed, such as polytetrafluoroethylene (PTFE) or teflon. Platinum and AgCl wires are widely employed as reference electrode and platinum wires as counter electrode.

Since the measurement is performed in a liquid environment, some precautions must be taken. The chosen electrolyte must be transparent, in order to allow the laser beam to reach the sample and be deflected. For the right electrolyte opacity, depending on the solute concentration, very diluited solutions should be selected. The choice of a suitable electrolyte for the measurement must be taken considering also possible corrosion effects on the AFM scanner, which can be affected by strong acid solutions. The same problem affects the AFM cantilever. It is preferable to select AFM tip with a specific coating resistant to acids, as gold. The liquid environment adds one more constraint related to the choice of the tip material, as the laser sum registered on the photodiode must be scarcely affected. The change in the refractive index of the solution with respect to air lead to a change in the position of the laser spot, so a repositioning of the photodiode is necessary.

Applications

EC-AFM can be used in various applications, where monitoring the electrode surface during electrochemical reactions lead to interesting results. Among the applications, the studies on battery and electrode corrosion in acid environment are widely spread. Concerning batteries, studies on lead–acid battery pointed out the change in the morphology during the reduction/oxidation cycles in a CV, when a acid electrolyted is used. Different corrosion effects are widely considered for the applications of EC-AFM. Different phenomena are studies, from pitting corrosion of steel, to crystal dissolution. Highly oriented pyrolytic graphite (HOPG) is widely employed as an electrode for EC-AFM. In fact, various surface phenomena are studied, from the application to lithium batteries to anion intercalation leading to blister formation on the electrode surface. A rather interesting application is the EC-AFM Dip pen nanolithography. Recently, SPM based lithography gained attention due to its simplicity and precise control the structure and location. REF A new development of this technique is the dip pen nanolithography (DPN), which uses the AFM technique to deliver organic molecules on different substrates, as gold (Au). Using EC-AFM allows to fabricate metal and semiconductor nanostructures on the WE, gaining high thermal stability and a higher chemical diversity. Finally, it is possible to perform and study the electrodeposition of different materials on electrodes, from metals (i.e. copper) to polymers, such as polyaniline (PANI).

Forces vs tip geometry

The forces between the tip and the sample strongly depend on the geometry of the tip. Variuos studies were exploited in the past years to write the forces as a function of the tip parameters.

Among the different forces between the tip and the sample, the water meniscus forces are highly interesting, both in air and liquid environment. Other forces must be considered, like the Coulomb force, van der Waals forces, double layer interactions, solvatation forces, hydration and hydrophobic forces.

Water Meniscus

Water meniscus forces are highly interesting for AFM measurements in air. In fact, due to humidity, a thin layer of water is formed between the tip and the sample during air measumements. Moreover, this phenomenon is interesting for AFM measuments in liquid environment.

While this layer does not broadly affect the attractive forces, it creates high adhesion forces between the tip and the sample and, as a consequence of the high surface energy, it is difficult to pull the tip away from the surface. whereas it prevents the tip from pulling off from the surface due to its high surface energy.

In order to quantify this force, it is necessary to start from the Laplace equation for pressure:

$P=\gamma _{L}({\frac {1}{r}}_{1}+{\frac {1}{r}}_{0})\simeq {\frac {\gamma _{L}}{r_{eff}}}$

where γ_L is the surface energy and r₀ and r₁ are defined in the figure.

The pressure is applied on an area of

Model for AFM water meniscus.

$A\simeq 2\pi R\simeq [r_{eff}(1+cos\theta )+h]$

where d, θ , and h are defined in the figure.

The force which pulles together the two surfaces is

$F=2\pi R\gamma _{L}(1+cos\theta +{\frac {h}{r_{eff}}})$

The same formula could also be calculated as a function of relative humidity.

Gao calculated fomulas for different tip geometries. As an example, the forse decreases by 20% for a conical tip with respect to a spherical tip.

When these forces are calculated, a difference must be made between the wet on dry situation and the wet on wet situation.

For a spherical tip, the force is:

$f_{m}=-2\pi R\gamma _{L}(cos\theta +cos\phi )(1-{\frac {dh}{dD}})$ for dry on wet

$f_{m}=-2\pi R\gamma _{L}{\frac {dr_{0}}{dD}}$for wet on wet

where θ is the contact angle of the dry sphere and φ is the immersed angle, as shown in the figure Also R,h and D are illustrated in the same figure.

For a conical tip, the formula becomes:

$f_{m}=-2\pi R\gamma _{L}{\frac {tan\delta }{cos\delta }}(cos\theta +sin\delta )(hD)(1-{\frac {dh}{dD}})$ for dry on wet

$f_{m}=-2\pi R\gamma _{L}({\frac {1}{cos\delta }}+sin\delta )(r_{0})({\frac {dr_{0}}{dD}})$ for wet on wet

where δ is the half cone angle and r₀ and h are parameters of the meniscus profile.