7272
J. Phys. Chem. 1993,97, 7272-7276
Atomic Force Microscopy for Local Characterization of Surface Acid-Base Properties Xue-Yun Lin,' Franqois Creuzet, and Hew6 Arribart Laboratoire CNRSlSaint-Gobain "Surface du Verre et Interfaces", B.P. 135, 39 Quai Lucien Ltfranc, 93303 Aubervilliers, France Received: December 10, 1992; In Final Form: March 25, 1992
In this work, we describe a new method for characterizing surface acid-base properties by surface force measurements using an atomic force microscope (AFM) in a liquid electrolyte. The potential of A F M is to make possible the local study of surface acid-base properties of rough and inhomogeneous substrates. We demonstrate the technique by measuring the force profile between a Si02 substrate and the Si3N4 A F M tip over a wide range of p H in an aqueous electrolyte. These first results are satisfactorily compared to the DLVO theory; we can infer that the surface isoelectric point (IEP) of the Si3N4 A F M tip has a value of 6.0 f 0.4. Consequently, we propose that A F M is capable of imaging surface inhomogeneities which modify the IEP by more than 0.4.
Introduction In the field of surface science and interfaces, it is well-known that the acid-base interaction plays an important role for a large number of phenomena, such as adhesion of polymers,' polishing,* etc. As oxide surfaces in an aqueous solution are generally electrically charged due to the amphoteric dissociation of surface M-OH groups, the BriSnsted acidity or basicity of an oxide surface can be characterized by the point of zero charge (PZC).3p4 This value corresponds to the pH value required to achieve zero net surface charge. An oxide can also be characterized by the isoelectric point (IEP) corresponding to the pH at which the zeta potential is zero. If there is no specific adsorption of ions other than H+or OH-, the IEP is simply identical to the PZC. Electrophoretic measurements and potentiometric titration have been used to measure the JEP or PZC of oxide particles in s u s p e n ~ i o n .Most ~ ~ ~ ~of~the experimental data concerning the IEP or the PZC of oxide surfaces found in the literature are obtained in this way. Only a small amount of data related to the IEP or PZC of plane oxide surfaces has been obtained by streaming potential measurements.7.8 Surface force measurements provide information on the charge distribution at a solid-aqueous electrolyte i n t e r f a ~ e .Thesurface ~ potential and charge can be evaluated from the interaction between twodouble layers. From thevariation of thesevalues asa function of pH and ionic strength, it is possible to investigate the mechanism of surface charging (with or without specific ion adsorption). Using a surface force apparatus, Horn and Smithiohave measured the evolution of thedouble layer interaction between two identical surfaces (Alz03or SiOz) in aqueous solution as a function of pH and ionic strength. The surface potentials inferred from these experiments are in good agreement with the results obtained by electrophoretic measurements on the same oxides. Also, it has been shown that the atomic force microscope (AFM), primarily used to image surfaces of insulating materials, could be turned into a local surface force apparatus.llJ2 Hoh et aLi3have shown that the adhesion force between the Si3N4 AFM tip and a glass substrate depends on the electrolyte pH. Ducker et al.14J5have shown the feasibility of using AFM for the determination of the double layer interaction between a silicate microsphere and a silicate plane substrate in aqueous solution. Recently, Meagherl6 has applied this method to the study of cation specific adsorption on silica. As far as the acid-base surface properties of oxides are concerned, force measurements, using a surface force apparatus, have proved useful in evaluating the average interaction between two surfaces as a function of pH.93'0 This method cannot be 0022-3654/93/2097-7272%04.00/0
extended to any surface because it suffers from two severe limitations: (i) from the experimental point of view, only very smooth surfaces (root-mean-square roughness < l nm). can be studied if the zero distance is to be well defined; (ii) moreover, due to the small curvature of the samples, physical information is averaged over an area of about (10 pm)2. Real surfaces often present heterogeneities in texture or chemical composition at a much smaller scale. The new approach presented in this paper shows that the use of AFM overcomes these limitations and opens the possibility to locally resolve the acid-base propertiesof rough and heterogeneous surfaces.
Principle The electrostatic surface potential C#J of oxide-like materials in an aqueous electrolyte is governed by the amphoteric character of the surface sites. For example, the Si-OH groups of silica may exchange protons with water to give Si-OH2+ and Si+. Therefore, two p C s must be defined: pK+ for the site acting as a proton donor, pK- for the site acting as a proton acceptor. The point of zerocharge (PZC) is equal to thequantity (pK+ + pK-)/2 and it is useful to introduce the quantity ApK = pK+ - pK-. The surface potential C#J is positive (negative) when the pH of the solution is smaller (larger) than the PZC. More precisely, 141is an increasing function of pH, a detailed study of which can be found in ref 17. Within the framework of the DLVO theory? the electrostatic forces between two surfaces (of similar or dissimilar materials) in an aqueous electrolyte can be related to the corresponding surface potentials. For instance, the double layer interaction Fdl between a sphere of radius R and a plane can be written as a function of the plane-sphere distance d by18
Fd,/R = 4rrcoC#Jl$2e4ix/X
(1)
I = 0.5ZCiZi2
(3)
with
where 41and 42 are the surface potentials of the sphere and the plane, respectively, Iand X are respectively the ionic strength and the Debye screening length of the electrolyte, c, and Z,are respectively the molar concentration (in mol/L) and the valency 0 1993 American Chemical Society
Surface Acid-Base Properties
The Journal of Physical Chemistry, Vol. 97,No. 28, I993 7273
To h
m
i
-?
v
40.0
.-
h=
8 = (A-B) / /cantilever
1
e
8
20.0
z
0.0
3I base
acid
pH scale
-
:
0) .a
* c
zo
Figure 1. Expected behavior of the double-layer interaction between two surfaces of different IEP's as a function of the electrolyte pH.
of ionic species i, d is the shortest distance between the sphere and the plane, and e, €0,kg, T,e, and NAhave their usual meaning. Expression 1 is obtained by integrating the interaction over the sphere surface; if an exponential decay is assumed for the elementary interaction, the force profile is found to be the same as in the plane-plane case, provided that an effective area of interaction (-2rRX) is defined. It is important to note that the validity of expression 1 requires the following conditions: 1411< kBT/e, 1421 < keT/e, d > A, and d
