Carbon 40 (2002) 145–149

Surface oxides on carbon and their analysis: a criticalassessment

*H.P. Boehm¨ ¨ ¨Department Chemie, Universitat Munchen, Butenandtstr. 5 –13, 81377 Munchen, Germany

Received 15 March 2001; accepted 15 June 2001

Abstract

The methods for the determination of various types of oxygen surface functions on carbon materials are briefly described,and their relative advantages and problems that may arise are discussed. Acidimetric titration techniques, IR spectroscopy,XPS, thermal desorption spectroscopy, and electrokinetic measurements are described. 2002 Elsevier Science Ltd. Allrights reserved.

Keywords: C. Infrared spectroscopy; Temperature-programmed desorption; X-ray photoelectron spectroscopy; D. Surface oxygen complexes

1. Introduction ides can be created by treatment with liquid oxidants, e.g.,aqueous solutions of H O , NaOCl, (NH ) S O , AgNO ,2 2 4 2 2 8 3

Many properties of carbon materials, in particular their H PtCl , etc., at 20–1008C. Oxidation with HNO is often2 6 3

wetting and adsorption behavior, are decisively influenced used because its oxidizing properties can be controlled byby chemisorbed oxygen. Oxygen in the surface oxides can concentration and temperature.be bound in the form of various functional groups which The various aspects of the surface chemistry of carbonare similar to those known from organic chemistry. This materials have been described in detail [1,7–9]. Thearticle deals mainly with high surface area carbon materi- present review discusses the most frequently used methods

2als which consist predominantly of sp -hybridized carbon for the characterization of surface oxides.atoms (non-graphitized, turbostratic carbons). The surfaceof such carbons is heterogeneous, it consists of the faces ofgraphene sheets and of edges of such layers. The edge sites 2. Titration methodsare much more reactive than the atoms in the interior ofthe graphene sheets, and chemisorbed foreign elements, in 2.1. Titration of acidic surface functionsparticular oxygen, are predominantly located on the edges.

The surface oxides decompose to CO and CO on2 The surface oxides on a carbon can have acidic as wellheating to high temperatures. Highly reactive sites remain as basic properties and can be conveniently determined byon the carbon surface which have free-radical character to titration methods. Basic surface character and anion ex-some relatively small extent [1,2]. After cooling to room change properties are found when a carbon surface,temperature, they can react with oxygen (air) or water cleaned at ca. 9508C or higher in vacuo or under an inertvapor, giving new surface oxides. A continuous, very slow gas is exposed to air and aqueous acid (or water) afteroxidation of the surface occurs after a first, rapid cooling to room temperature. Oxygen and acid (or water)chemisorption of oxygen. The presence of water is essen- are chemisorbed at the same time [9–11]. Surface oxidestial for this ‘aging’ of the carbons [3,4]. Aging is reduced created with oxygen at elevated temperatures (or by aging)after prior chenmisorption of hydrogen, e.g., at 9508C or with liquid oxidants are acidic in character and cause[5,6]. Much more oxygen is chemisorbed at elevated cation exchange properties. Acidic and basic surface sitestemperatures, e.g., 300–4208C. Alternatively, surface ox- coexist usually, but the concentration of basic sites de-

creases with increasing acidic character of the surface.The acidic surface properties are caused by the presence*Fax: 149-89-2180-7492.

E-mail address: hpb@cup.uni-muenchen.de (H.P. Boehm). of carboxyl groups (also in the form of their cyclic

0008-6223/02/$ – see front matter 2002 Elsevier Science Ltd. All rights reserved.PI I : S0008-6223( 01 )00165-8

146 H.P. Boehm / Carbon 40 (2002) 145 –149

Fig. 1. A few possible surface groups.

anhydrides), lactones or lactols (see Fig. 1), and hydroxyl constants (Fig. 2). They agree quite well with the titrationgroups of phenolic character. These groups differ in their data [13,14]. Such determinations are very time-consumingacidities and can be differentiated by neutralization with since equilibration in direct titration is very slow [13–15].0.05 N solutions of NaHCO , Na CO and NaOH, respec- This may be caused by slow diffusion in narrow pores, if3 2 3

tively. The groups have been identified by other chemical present, and by slow hydrolytic ring opening of carboxylicmethods used in organic group analysis [9,11,12]. Still anhydrides and lactones. The method is limited to a rangehigher base uptake than with NaOH is observed with of pK (or pH) values between 3.5 and 10.5 because of thea

alcoholic 0.1 N sodium ethoxide; this can be explained by buffering effect of water at very high or very low pHthe presence of reactive carbonyl groups which form the values. Differences in the number and positions of peaks in

2 1sodium salt of a hemiacetal, =C(OEt)(O Na ). In a the distribution curve have been observed for differentlysimple way, the carbons are agitated with an excess of the pretreated carbons.bases, and the excess is determined by back titration afterequilibration (see Refs. [11,12] for experimental details). 2.2. Titration of basic surface sites

The acidity of a given functional group depends on itschemical environment, i.e., the size and shape of the While the nature of the acidic surface sites is quite wellpolyaromatic layers, the presence and position of other understood, the origin of surface basicity is still undersubstituents, and the charge of neighboring dissociated discussion. Continuous titration showed the existence ofgroups. However, the differences in acidity of the various three peaks in the pK distribution curves [16]. One reasontypes of functional groups seem to be sufficiently large to for a basic behavior of carbon surfaces may be the p

allow differentiation by the simple titration method, e.g., basicity of the exposed graphene layers [17–19]. However,the difference between NaOH and Na CO consumption this basicity is relatively weak. The chemisorption of2 3

corresponds to the weakly acidic phenolic groups. It has oxygen together with acid suggests that the basicity maybeen shown by careful, continuous titration with alkali that be due to oxygen functional groups, and the existence ofseveral peaks appear in the distribution curve of acidity pyrone-type structures on the edges of the polyaromatic

Fig. 2. Distribution of acidity constants for an activated carbon oxidized with nitric acid (curve taken from Ref. [13], Fig. 8).

H.P. Boehm / Carbon 40 (2002) 145 –149 147

C=O vibrations of quinones or isolated carbonyl groups21[24–28]. Peaks at 1000–1300 cm are ascribed to C–O

single bonds [24,25]. Most carbons contain hydrogen, andband of C–H stretch and wagging vibrations are observed.

4. X-ray photoelectron spectroscopy

In X-ray photoelectron spectroscopy (XPS, also ESCA),core electrons are excited by X-ray irradiation to leave theFig. 3. Pyrone-type structure.atoms. Their binding energy (b.e.) is derived from themeasured kinetic energies. XPS is surface-sensitive since

layers has been suggested [20]. However, it has been the escape depth of the photoelectrons amounts to only aobjected that g-pyrone is a much too weak base to account few atomic layers. The b.e. depends on the atomic speciesfor the observed strong peak at a pK of |8.5 of the but is also affected by the shielding of the nuclear chargea

conjugate Brønsted acid [16]. Support for the hypothesis of which is lowered or raised by bonding of the atom to morepyrone-type structures came from theoretical calculations electronegative or electropositive atoms, respectively. Thewhich showed that the base strength increases strongly differences in b.e. for various binding states are quite smallwhen the carbonyl group and the ring oxygen of a pyrone- compared to the line width, especially with electronegativetype structure are distributed on polycyclic aromatic elements such as oxygen. A deconvolution of overlappingcompounds, e.g., phenanthrene, as shown in Fig. 3 [21,22]. peaks is necessary, therefore. However, the results of theSuch structures may have basicities even stronger than that curve fitting are influenced to some extent by the some-of pyridine [21]. what arbitrary inputs for the number, shape and width of

Most carbons contain more oxygen than can be ac- the peaks. Although modern instruments provide a suffi-counted for by the observed functional groups. The cient resolution also for O1s electrons [15,29], it is moredifference may be due to ether-type oxygen or to carbonyl convenient to measure the C1s signal. Carbon atoms differgroups that do not react with NaOC H . in their b.e. depending on whether they are linked to one O2 5

atom by a single bond (phenols and ethers), a double bond(carbonyl groups), or two oxygen atoms (carboxyl groups,

3. Infrared spectroscopy lactones). The corresponding signals appear as satellites onthe high-b.e. side of the main C1s peak of the carbons

Infrared spectroscopy became very useful in analysis of (Fig. 4). The observed ranges of b.e. are listed e.g., inthe functional groups when the limitations due to the high Refs. [15,27,29].absorbance of carbon were overcome by the development XPS requires calibration since charging of the sampleof FTIR spectroscopy. It is often used in combination with will influence the kinetic energies. Usually, the main C1sdiffuse reflectance spectroscopy (DRIFTS) or with total peak of the carbon samples is taken with a b.e. of 284.6 eVreflection spectroscopy [23–28]. assigned to it. However, values differing by a few tenths of

The assignment of the absorption bands is based on an eV have been also used (see Ref. [29]). It has beenexperience with molecular organic compounds. However, suggested to list rather the shifts relative to the mainthere is often disagreement in their assignments. For signals instead of the absolute binding energies [29,31].

21instance, an intensive band near 1600 cm has been Alternatively, gold has been sputtered onto the samplesexplained by stretching vibrations of aromatic C=C bonds (84.0 eV b.e. for Au 4f ). It is an advantage of XPS that7 / 2

which are polarized by oxygen atoms bound near one of the relative surface concentrations of the various speciesthe C atoms. An alternative assignment to hydrogen- can be estimated from the peak sizes. The results can bebonded, highly conjugated carbonyl groups was only misleading, however, with porous samples when therecently refuted [24]. exterior surface is more strongly oxidized by aging.

The spectra of apparently similarly pretreated samplesshow often differences, especially in the C=O bands. Free

21carboxylic acids absorb near 1700 cm [25], while cyclic 5. Thermal desorption spectroscopy21anhydrides give rise to peaks at 1780 and 1840 cm (sh)

[26]. The anhydrides can be hydrolyzed to the free acids; While carboxyl groups evolve CO at relatively low2

the reaction can be reversible on heating. The evidence for temperatures, other groups are thermally more stable.cyclic lactones is not as conclusive. They are described to Therefore, thermal desorption spectroscopy (TDS), also

21 21give a single peak at 1740 cm [27] or 1760 cm [28], called temperature-programmed desorption (TPD), is usedand a clear distinction from cyclic anhydrides is not for the study of surface oxides. The samples are heated in apossible. Problems arise also in the assignment of bands to vacuum or in a helium stream with a constant heating rate

148 H.P. Boehm / Carbon 40 (2002) 145 –149

Fig. 4. Typical C1s XPS spectrum of oxidized carbon fibers: (I) phenols, (II) carbonyl groups, (III) carboxyl groups, (IV) plasmon peak(after Ref. [30], Fig. 3).

(often 10 K/min), and the evolved gases, H O, CO , CO provide a good overview of the sample’s surface prop-2 2

and H , are determined, mostly by use of a quadrupole erties.2

mass spectrometer. The peaks shift to higher temperatureswith increasing heating rate. They are usually very broad,often with pronounced tailing, and there is considerable 6. Electrokinetic measurementsoverlapping. A deconvolution of the spectra into separatepeaks has been described [32]. Techniques to achieve A carbon surface of acidic or basic character is sur-better resolution and to determine the heat of activation for rounded in aqueous suspension by a diffuse cloud of

1 2the decomposition reactions have been described recently dissociated H or OH ions, respectively, and pH values[33]. of ,7 or .7, respectively, are measured in such suspen-

It is generally assumed that each type of surface group sions. The pH returns to near neutral, however, afterdecomposes to a defined product, e.g., that CO derives sedimentation of the carbon particles when the carbon2

from carboxyl groups and CO from carbonyl and hydroxyl sample is electrolyte-free and purified water is used [17].groups and ether-type oxygen. A good correlation has been In the presence of an electrolyte, ion-exchange results in afound of the NaOH consumption with the CO -forming permanent pH change. The surface charge depends on the2

complexes on the surface of an activated carbon [34]. The pH of the surrounding electrolyte. There is a pH value,results of TDS are not always unambiguous, however. Two called the ‘point of zero charge’ (PZC) at which the netadjacent carboxyl groups may be first dehydroxylated to surface charge is zero. The ZPC can be easily determinedthe cyclic anhydride which, in turn, decomposes to CO by a method called ‘mass titration’ [36]. It is based on theplus CO . This decomposition occurs at higher tempera- fact that the pH of an electrolyte changes in the direction2

ture than that of free carboxyl groups [32,35]. The of the ZPC on contact with a solid powder.degradation of lactols will probably also produce both The charged particles move in an applied electric fieldgases. A cyclic lactone can either give one CO molecule (electrophoresis). A thin water layer, containing a part of2

1 2or two CO molecules, with both reactions possibly running the diffuse cloud of dissociated H or OH ions, adheresconcurrently. Furthermore, secondary reactions cannot be to the particles and moves with it. The charge and potentialexcluded. Diffusion of the evolved gases is rather slow in at its boundary determine the electrokinetic phenomena.narrow pores, and CO molecules may react to CO with The electrokinetic potential (or z-potential) can be calcu-2

surface-bound oxygen, or CO molecules hitting the pore lated from the measured electrophoretic mobilities. The pH2

walls may form two CO molecules. Freshly created active of zero z-potential is the ‘isoelectric point’ (IEP). It is notsurface sites or free radical sites can facilitate such identical with the ZPC, but usually not very far from itreactions. with non-porous carbon materials. The observed ZPC

Carbons oxidized with liquid oxidants usually contain range from pH |2 to pH |10.5. With porous carbons,relatively more carboxyl groups than O -oxidized samples. however, the IEP values are often considerably lower than2

Part of these carboxyl groups decompose already at the ZPC because the electrokinetic behavior is determinedtemperatures significantly below 3008C. by the charge on the external surface of the carbon

It is not surprising that the TPD spectra published in the particles which is usually oxidized by aging [37]. The PZCliterature often differ in details, since they are influenced is determined, in contrast, by the much larger internalby the pore structure of the carbons and experimental surface of the pore walls which are oxidized much moreparameters such as the heating rate. Nevertheless, they can slowly since diffusion of oxygen in narrow pores is very

H.P. Boehm / Carbon 40 (2002) 145 –149 149

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