Effect of Graphite Aging on Its Wetting Properties and Surface Blocking by Gaseous Nanodomains

Early works considered basal planes of highly ordered pyrolytic graphite (HOPG) as hydrophobic, relatively inert materials with low electrocatalytic activity due to nonpolar sp2 carbon. On the contrary, a freshly prepared HOPG surface exhibits intrinsically mildly hydrophilic properties, with a low contact angle of water, which increases after exposure to an ambient atmosphere. This process, called aging, ascribed to adsorption of airborne hydrocarbons, is reportedly accompanied by strong decay of electron transfer kinetics, the mechanism of which is not yet fully understood. Examining both freshly prepared and aged basal plane HOPG immersed in water by PeakForce quantitative nanomechanical imaging, we have found that aged HOPG is occupied by ambient gaseous nanodomains, the existence of which is explained by incomplete wetting. They cover up to 60% of the immersed surface and their incidence is in direct relation with graphite aging time. In contrast with aged graphite, gaseous nanodomains were absent on the freshly stripped HOPG surface. It can be concluded that ambient gaseous nanodomains can prevent aged basal plane HOPG from contact with aqueous media and may thus affect processes at the solid–liquid interface.


Nanomechanical properties of gaseous nanodomains
Surface gaseous nanodomains are identified by both nanomechanical and surface properties, indicated by negative phase shift, low Y M (DMT), high deformation and low adhesion to hydrophilic tip, which significantly differ from the rest of (wetted) surface, as presented in Fig. S1 and Fig. 6.As follows from Fig. S1F, the highest contrast in tapping phase images corresponds to areas identified by PFQNM analyses (DMT and deformation) as "soft" (Fig. S1B, C), assigned to gaseous nanodomains, where also the adhesion to hydrophilized tip is significantly lower, compared to rest of (wetted) solid surface (Fig. S1D).Though all values of nanomechanical S4 parameters just illustrate their difference between graphite and gaseous surface and do not represent absolute values, YM fits well to the range known for HOPG surfaces 1,2.

S5
Different forces (setpoints 0.9 nN and 1.5 nN) applied on the nanobubbles by the AFM tip, caused nanobubble symmetrical compression leading to the change of its dimensions -the decrease of apex height, as shown in Fig. S2.The dimension change depends, besides applied pressure, also on nanobubble pinning to the solid surface.were obtained by in-situ AFM scanning (PFQNM mode) at peak force setpoints 0.9 nN (profile 1) and 1.5 nN (profile 2).

Gaseous nanodomains in pre-degassed water
AFM (PFQNM) images of basal plane HOPG aged for 25 h, immersed in pre-degassed deionized water (DIW) show both nanobubbles and micropancakes -appearing just upon immersion (Fig. S4).Their incidence in pre-degassed water, where gas concentration was lowered, is similar to water equilibrated with air, which indicates, that partial degassing does not significantly affect incidence of surface gaseous nanodomains formed upon immersion.
This finding thus supports participation of incomplete wetting in surface nanodomain formation upon immersion.

Identification of incomplete wetting by Total Internal Reflection
To further clarify the participation of incomplete wetting on the formation of surface gaseous domains, modified experiment utilizing total internal reflection (TIR) of incident light is presented for identification of gaseous microlayers formed on hydrophobic surfaces upon immersion in water: Silicon wafers with flame-deposited carbon (FDC) mimicking S9 hydrophobic surface of aged HOPG were utilized as model samples with carbonaceous hydrophobic surface allowing direct optical visualization of surface gaseous layers by TIR.
Samples were immersed in water equilibrated with air at 20ºC and in water pre-degassed by boiling, which was kept heated to 87ºC to minimize re-gassing.In both cases gaseous microlayers formed on FDC surface are clearly visible by manifesting TIR on mirror-like gasliquid interface 3 , as shown in Fig. S5.This experiment is thus proving feasibility of ambient gas dragging (incomplete wetting) to create surface gaseous domains just upon immersion, independently on gas concentration in water.It should be noted, that visualization of TIR effect is possible if thickness of gaseous layer exceeds the penetration depth of evanescent wave, which is in orders of 10 2 nm.Therefore, the direct visual detection of gaseous nanodomains by TIR is not possible, due to their thickness typically below 10 nm.

Figure S1 :
Figure S1: The in-situ PFQNM (A-D) and TM (G, H) images of identical location of basal plane

S6 3 .
Deformation of nanobubblesWhen surface nanobubbles are scanned in deformation (PFQNM) mode, the steadily increasing force at rising part of the force curve imposed on nanobubbles causes their symmetrical deformation without any anomality, as shown by two orthogonal profiles drawn across center of each nanobubble deformation image (Fig.S3).

Figure S5 :
Figure S5: Silicon wafer coated with flame-deposited carbon (FDC, insert in left image) is