X-ray photoelectron spectroscopy studies of sodium modified carbon films suitable for use in humidity sensors

Article abstract of DOI:10.1023/A:1018687716524

Carbon films obtained using the spray pyrolysis technique are used in the construction of humidity sensitive resistors and capacitors. Both the electronic and surface properties of these films are dependent on several factors such as the carbonization temperature and duration. An addition of sodium containing salts (sodium acetate) to the basic precursor i.e., furfuryl alcohol, produces carbon films that have very specific surface properties. X-ray photoelectron spectroscopy studies revealed that the sodium atoms are located in the mother carbon network. These sodium atoms are associated with oxygen forming structures corresponding to Na₂O. These spots act as centres of durable CO₂ adsorption. This results in an intensive oxidation of the surface leading to a state where the surface layer contains more than 45% oxygen. Such an intensively oxidized surface is highly hydrophilic which results in an intensive water vapour adsorption that follows an I-type adsorption isotherm. The electronic conduction in the sodium modified carbon films is extremely sensitive to water adsorption and it increases by thousands of times after an exposure to humid air.

Full text of DOI:10.1023/A:1018687716524

JOURNAL OF MATERIALS SCIENCE 32 (1997) 6063—6068
X-ray photoelectron spectroscopy studies
of sodium modified carbon films suitable
for use in humidity sensors
/
J . P . LU KAS ZEW ICZ
Faculty of Chem istry, Nicholas Copernicus University, 87-100 Torun, Poland
Carbon film s obtained using the spray pyrolysis technique are used in the construction
of hum idity sensitive resistors and capacitors. Both the electronic and surface properties
of these film s are dependent on several factors such as the carbonization tem perature
and duration. An addition of sodium containing salts (sodium acetate) to the basic
precursor i.e., furfuryl alcohol, produces carbon film s that have very specific surface
properties. X-ray photoelectron spectroscopy studies revealed that the sodium atom s
are located in the m other carbon network. These sodium atom s are associated with
oxygen form ing structures corresponding to Na2O. These spots act as centres of durable
CO2 adsorption. This results in an intensive oxidation of the surface leading to a state
where the surface layer contains m ore than 45% oxygen. Such an intensively oxidized
surface is highly hydrophilic which results in an intensive water vapour adsorption that
follows an I-type adsorption isotherm . The electronic conduction in the sodium m odified
carbon film s is extrem ely sensitive to water adsorption and it increases by thousands of
tim es after an exposure to hum id air.
1. Introduction
because of their high electric conductivity, have been
utilized in the manufacture of chemically sensitive
electrodes [3].
Despite numerous papers in the literature that deal
with the sensing properties of LT and HT carbons the
surface state of pure and Na-modified carbon films
has not been rigorously studied.
The spray pyrolysis technique has found application
in the growth of thin carbon films on inert substrates
such as glass, silica and silicon. The application of
these films in the construction of humidity sensitive
resistors has been reported in the literature [1, 2].
It has been noted that the modification of the stan-
dard precursor by the addition of sodium containing
water soluble salts such as acetate or carbonate
yielded carbon films with electrical properties that
were highly sensitive to the humidity content in the
surrounding atmosphere. As an example of such be-
haviour, one may highlight the dramatic decrease
in the electrical resistance of some Na-modified car-
bon films from 100—500 MOhm to the level of sev-
eral kohms on an increase in the relative humidity
from 5 to 75% at room temperature [2]. This
high sensitivity to the adsorption of water molecules
may be achieved by carbonization of the films at the
relatively low temperature of 873—923 K. Films of this
type are generally denoted as low temperature (LT)
carbons.
2. Experimental procedures
The carbon films were obtained using the spray pyro-
lysis technique that has been described in detail else-
where [4]. Furfuryl alcohol is used as the main com-
ponent of the liquid precursor sprayed onto a hot
substrate (glass or silicon plate). The thus obtained
polymer films are subjected to a vacuum carboniz-
ation at either 873—923 K or 1073—1123 K that lasts
for 60—90 s. The LT and HT carbon films after cooling
in vacuum, are stored in the open air.
The mixing of the furfuryl alcohol with a 2M
aqueous solutions of sodium acetate in the ratio of
3 : 1 before the spray pyrolysis experiments results
in Na-modified carbon films. The films obtained with-
out sodium acetate are designated as ‘‘pure’’ carbon
films.
All the films underwent surface examination by
means of X-ray photoemission spectroscopy (XPS)
using an ESCALAB 210, (VG Scientific). The thin-
ness of the films and their moderate porosity makes
vacuum outgasing easy to perform inside the spec-
trometer and one may begin measurements after a
The enhanced sensing properties of LT carbons
towards water vapour are in contrast to those of HT
carbons, i.e., carbons that undergo carbonization at
elevated temperatures in the range of 1073—1173 K.
Generally, the electronic properties of HT carbons
show little sensitivity to water adsorption or to ca-
talysis induced alterations. Therefore such films
have not been used in the construction of chemically
sensitive resistors or diodes. However the HT carbons,
0022—2461 ( 1997 Chapman & Hall
6063

20—30 min evacuation of the vacuum chamber (pres-
sure ca. 5;10\ꢂ Pa).
The XPS spectra were recorded using a K Al
?
(34 mA, 15 kV) lamp and the data were analysed using
both standard manufacturers software and a program
written at the Institut Chemii Fizycznej PAN, War-
saw, Poland.
3. Results and discussion
3.1. C1s spectra of LT and HT carbon film s
Figs 1 and 2 present C1s spectra of pure-LT and
Na-modified-LT carbon films, respectively. The C1s
spectrum of the pure-LT carbon is closely related to
the C1s spectra of active carbons described in the
literature [5—7]. The spectrum of Fig. 1 and those
published in the literature are characterized by a con-
tinuously decreasing intensity of the sub peaks that
can be attributed to highly oxidized forms of carbon.
The larger the energy shift of a peak representing an
oxidized carbon atom, the lower is the height of that
peak. This situation usually leads to the conclusion
that the content of carbon atoms present as carbonate
or carboxylic type surface functional groups is smaller
than the number of carbon atoms bonded in the
carbonyl groups. Thus, the number of carbon atoms in
Figs 1 and 2 with the specified chemical state changes
in the following order: carbon (graphite)'alcoholic
carbon'carbonyl carbon'carboxylic carbon'
Figure 1 Typical C1s spectrum of pure LT carbon film.
carbonate carbon(carbon in CO .
ꢀ
Figure 2 Typical C1s spectrum of Na-modified LT carbon film.
The most intensively shifted sub peak, near 292 eV,
can be attributed to chemically adsorbed CO . This
ꢀ
sub peak’s intensity is relatively larger than the one
TABLE II Elemental composition of the surface layer of pure and
Na-modified carbon films of the LT and HT types determined by
XPS
corresponding to a carbonate carbon.
Table I summarizes the data in numerical form. The
distribution of oxidized carbon states shown on Fig. 1
is characteristic for non-modified carbons, i.e., car-
bons obtained from an organic precursor containing
mainly carbon, oxygen and hydrogen without any
subsequent chemical or physical treatment. The pres-
ence of significant amounts of other elements such as
nitrogen or sulphur could complicate the direct attri-
bution of the C1s peaks exclusively to carbon—oxygen
structures. Therefore the elemental composition of
each carbon film was also determined, see Table II. It
can be seen that the carbon, oxygen (and sodium in
the Na-modified films) exceeds 95 at%. The remaining
Carbon type
Element at%
C
O
Na
3.8
LT
virgin
LT
Na-modified
HT
virgin
HT
68.8
29.8
71.2
34.3
24.4
43.4
18.1
37.7
21.8
3.3
26.1
Na-modified
TABLE I Deconvolution of C1s peaks recorded for pure and Na-modified carbon films of the LT and HT types
Carbon type
C1s sub peaks
C (graphite)
C—O
C"O
COO—
COꢀ\
ads. CO
ꢁ
ꢀ
Maximum (eV)/contribution to the total C1s peak (at%)
LT
virgin
LT
Na-modified
HT
virgin
HT
284.6/56
284.6/43
284.6/47
284.6/48
286.5/16
286.5/14
286.1/15
286.5/14
287.5/6
288.6/7
288.3/12
289.3/5
288.3/10
290.5/4
290.6/8
291.0/3
290.6/14
—
—
292.4/8
293.0/9
—
287.8/8
—
292.2/14
Na-modified
6064

5% is shared by 4 to 5 other elements. Therefore, the
small impurity content causes only minor side
effects as compared to the very high oxygen content
in the surface layer and its major influence on the C1s
binding energy (BE). In addition, the BEs of C—N
and C—S species are much closer to the basic C1s
284.6 eV maximum than are the BE maxima recorded
for highly oxidized carbon atoms. As a matter of fact,
the binding energies of 293 eV and higher are due to
functional groups contribute less intensively to the
whole C1s peak. Such a carbon surface, after the Na-
modification, is covered by a relatively high number
of carbon atoms oxidized so intensively that their
corresponding energy shift exceeds 290 eV. In some
papers [8—10] this intensive shift in the binding en-
ergy is ascribed to carboxylic or carbonate structures.
With regarding to the method of carbon film produc-
tion and particularly the Na-modification process
one has to discuss whether the 290.7—292.5 eV region
corresponds to: (i) residual carboxylic groups from
*
pPp electronic transitions resulting from the
‘‘aromatic nature’’ of the solid. However the heights
of the corresponding peaks do not exceed 5—10% of
the height of the basic (‘‘graphite’’) C1s sub peak at
CH COONa introduced to the precursor, (ii) newly
ꢁ
formed carbonate groups that may appear in the
*
284.6 eV [5—10]. The intensity of the pPp sub
carbon after the thermal transformation of sodium
acetate into sodium carbonate during carbonization,
and (iii) carbon dioxide adsorbed onto the carbon
surface during the storage of the samples in the open
air.
peak is usually lower than the intensity of ante-
cedent sub peaks ascribed to carbon—oxygen surface
groups. Any departure from these two normal features
*
may be treated as a superposition of a pPp sub
peak and another sub peak resulting from the pres-
ence of a carbon derivative (probably carbon—
oxygen groups). The exclusive attribution of the sub
peaks on Figs 1—4 to carbon—oxygen structures is
discussed in the next section of this paper that ana-
lyses the O1s spectra.
The C1s spectrum of a Na-modified LT carbon
shown in Fig. 2 is dramatically different from the C1s
spectrum of the pure carbon film. In this case two
forms of carbon are dominating the spectrum; namely
graphitic carbon (284.6 eV) and heavily oxidized car-
bon (290.7—292.5 eV). Moderately oxidized forms
of carbon atoms entrapped in alcohol or carbonyl
If the residual sodium acetate is present in large
quantities in the Na-modified carbons then one
should observe intensive sub peaks in the range
288—289 eV [10]. However, this region in Figs 2 and
4 contains only minor sub peaks suggesting a low
contribution from sodium acetate. Therefore one may
assume that carboxylic groups do not dominate the
chemical map of the Na-modified carbon surface. Of
particular interest to this discussion is that thermo-
gravimetric examinations confirm that sodium acetate
added to furfuryl alcohol decomposes to Na CO
ꢀ
ꢁ
during carbonization [11].
The problem of the origin of the 290.7—292.5 eV BE
region has another aspect, i.e., the form of the remain-
ing sodium carbonate in the Na-modified carbon films
provided that such residuals really exist. Particularly,
one has to consider the crystalline form of this com-
pound and the size of the Na CO crystallites. X-ray
ꢀ
ꢁ
diffraction data presented elsewhere [11] suggest that
during the carbonization of Na-modified LT and HT
carbons, sodium acetate undergoes a transformation
with the formation of Na CO microcrystallites. This
ꢀ
ꢁ
transformation only proceeds efficiently under the
specific conditions created in bulk carbon samples. In
the reported case the carbon powder had a grain size
of 0.02—0.12 mm. The possible crystallization of
Na CO in thin carbon films is still not fully under-
ꢀ
ꢁ
stood and it is under intensive examination. The
possible presence of Na CO microcrystallites in the
Figure 3 Typical C1s spectrum of pure HT carbon film.
ꢀ
ꢁ
carbon films cannot be fully excluded but their size
should be comparable to the molecular scale
(1—10 nm). It should be noted that any potential crys-
tallization of Na CO is a 3-dimensional process that
ꢀ
ꢁ
requires a certain space to exist inside the films. Thus,
any Na CO microcrystallites formed in the films
ꢀ
ꢁ
could influence the XPS detectable surface properties
of the Na-modified carbon films provided some
microcrystallites are placed just in the subsurface
region and are partly exposed to the ambient
atmosphere.
The analysis of the Na1s spectrum will be discussed
at a later point of this paper.
C1s sub peaks that have a very large BE shift (above
290 eV) are observed in the spectra of Na-modified
HT carbon films (Fig. 4) while there are only a minor
Figure 4 Typical C1s spectrum of Na-modified HT carbon film.
6065

feature in the case of pure HT carbon films (Fig. 3).
TPD temperature programmed desorption (TPD) and
thermogravimetric measurements revealed that the
Na CO microcrystallites decompose at a temperature
ꢀ
ꢁ
below 873 K [11, 12] due to a chemical reaction with
the carbon matrix. Thus, the significantly shifted (above
290 eV) sub peaks in the C1s spectrum of HT carbon
(Fig. 4) cannot be attributed to Na CO microcrystal-
ꢀ
ꢁ
lites since they should not exist in these samples. There-
fore, it is probable that these peaks result from highly
oxidized carbon atoms of the type of: CO chemisorbed
ꢀ
onto Na O (HT films) and CO chemisorbed on Na O
ꢀ
ꢀ
ꢀ
Figure 5 Typical O1s spectrum of pure LT carbon film.
and/or Na CO microcrystallites (LT).
ꢀ
ꢁ
The contribution of the significantly displaced C1s
sub peaks to the total C1s peak is higher for LT
carbons. This agrees with the concept that the inten-
sively shifted sub peaks are the result of two forms
of heavily oxidized carbon atoms in the LT films (CO
ꢀ
and Na CO ) while in the case of HT carbon films
ꢀ
ꢁ
the sub peak is solely the result of chemisorbed CO .
ꢀ
Data in the literature suggests that the typical position
ꢀ\
of the C1s sub peak of the carbon atom in a CO
ꢁ
group is 290.7 eV [10] which does not explain the
presence of the sub peaks above 292 eV (Fig. 4). In fact
this value is characteristic for CO [13] and it can
ꢀ
be attributed to the chemisorbed CO on the Na O
ꢀ
ꢀ
centres.
The existence of a Na O · CO structure on the
surface of Na-modified HT carbon films and the
Figure 6 Typical O1s spectrum of Na-modified LT carbon film.
ꢀ
ꢀ
TABLE III Deconvolution of the O1s peaks recorded for pure
and Na-modified carbon films of the LT and HT types
ꢀ\
presence of CO ions [2] in LT carbons was con-
ꢁ
firmed by Fourier transform afra-red spectroscopy
[FTIR].
Carbon type
O1s sub peaks
The Na-modified HT carbons are characterized
by the presence of the sub peaks in the range
290.7—292.5 eV, i.e., in the region associated with heav-
ily oxidized forms of carbon atoms. Their summed
contribution to the total C1s peak is slightly smaller
than that for the Na-modified LT carbons despite the
observed decomposition of the Na CO microcrystal-
C—O
C"O
COO—
Maximum (eV)/contribution to the total O1s
peak (at%)
LT
virgin
LT
Na-modified
HT
virgin
HT
531.1/21
531.1/15
531.6/51
532.4/31
532.3/69
534.1/71
533.2/41
533.8/48
535.0/10
538.4/14
ꢀ
ꢁ
lites. The Na CO microcrystallites probably exist in
ꢀ
ꢁ
535.3/5
537.0/2
538.0/21
the LT carbons but their presence is questionable in
the HT carbons. These observations can be explained
by assuming that the carbon atoms from the Na CO
Na-modified
ꢀ
ꢁ
microcrystallites, that were decomposed during the
vacuum heat treatment, are replaced by the chemisor-
ption of CO molecules during storage in the
ꢀ
atmosphere.
of the C1s spectra. Figs 5 and 6 and Table III present
typical O1s spectra of pure LT and Na-modified
LT carbons. The spectra are characterized by the
presence of three major sub peaks at: 531—532,
532.5—534 eV and 536—538 eV. The first two sub
peaks are usually ascribed to C—O and C"O struc-
tures respectively [14]. The interpretation of the
third sub peak is not clear but some authors attribute
the peak to the single bonded oxygen atom in the
carboxylic group or to carbonate entities [15].
No direct information on dealing with the O1s BE
As mentioned, the summed contribution of the
290.7—292.5 sub peaks to the total C1s peak is almost
equal for Na modified HT (28%) and LT (25%) car-
bons but the distribution between the sub peak near
290.7 eV (C atom in carbonate entity) and the sub
peak near 292 eV (C atom in carbon dioxide) is differ-
ent for these carbons (see Table I). The relative
amount of adsorbed CO increased with the change
from LT to HT carbon. This behaviour supports the
idea of the replacement of thermally removed carbon-
ꢀ
ꢀ\
ate CO species by chemisorbed, CO .
shift specific for chemisorbed CO was found in the
literature but it has been suggested [13] that these
ꢁ
ꢀ
ꢀ
BE values will be close to the carbonate functional
O1s BE region. Keeping in mind this assignment,
one can state as follows (see Table III): (i) the num-
ber of O atoms (the first O1s sub peak) bonded to
3.2. O1s spectra of LT and HT carbon film s
The O1s spectra are usually used as complementary
data to confirm the conclusions obtained by analysis
6066

C atoms by a single C—O bond decreases on the
Na-modification but increases on the transition from
LT carbon to HT carbon, (ii) the number of O atoms
(the second O1s sub peak) oxidizing C atoms in
a moderate way (C"O) increases on the Na-modifica-
tion, and (iii) the number of O atoms (the third sub
peak) included in the structures containing highly
oxidized C atoms (carbonate and/or carbon dioxide)
increases on the Na-modification.
These statements are in good qualitative agreement
with the interpretation of the C1s spectra but the
deconvolution of the O1s spectrum does not yield
as precise conclusions as does the C1s spectra. It is
not possible to separate the sub peaks that correspond
to carboxylic entities, carbonate entities and
chemisorbed carbon dioxide. All these species are
probably included in the third sub peak. It is again
shown that the Na-modification leads to the forma-
tion of surface entities that contain highly oxidized
carbon atoms.
Figure 8 TPD spectrum of the Na-modified HT carbon.
of quasi Na CO molecules bonded to the surface
ꢀ
ꢁ
(290.7 eV C1s maximum).
Fig. 8 presents a thermal desorption spectrum of
species with a mass of 44 a.m.u from the surface of
the Na-modified HT carbon. The spectrum contains
three major desorption peaks near the temperatures
of 383, 653 and 883 K. As predicted, based on the
3.3. Na1s spectra of Na-m odified LT
and HT carbon film s
The Na1s spectrum of a Na-modified HT carbon film
is presented in Fig. 7. The spectrum is unique because
of the wide sub peaks located at 1072.8 and 1074.7 eV.
These locations are quite unusual for sodium contain-
ing compounds. For example the Na1s BE values
recorded for crystalline Na CO , NaHCO and
analysis of the Na1s XPS line, CO is adsorbed in
ꢀ
two forms (the first and second peaks) that are very
different with regard to their thermal stability.
One has to remember that the Na-modified HT car-
bon underwent carbonization at 1073 K and thus
a major amount of the Na CO microcrystallites
ꢀ
ꢁ
ꢁ
CH COONa have maxima in the range 1071.3—
ꢁ
1071.7 eV [16,17]. Thus the currently measured
values are shifted towards the Na1s peak of elemental
sodium. Previously the 290—291 eV C1s sub peaks
were interpreted as resulting from the presence of
ꢀ
ꢁ
had already decayed. The third peak at 883 K is prob-
ably due to the decomposition of the residual Na CO
ꢀ
ꢁ
microcrystallites.
ꢀ\
a CO group probably in Na CO microcrystallites
ꢁ
ꢀ
ꢁ
The Na1s 1074.7 eV maximum cannot be assigned
to any common sodium containing species. This value
is relatively close to the 1078 eV Na1s BE charac-
teristic for elemental sodium. This maximum could
be due to the Na atoms that are built into the
mother carbon matrix and therefore possess a speci-
fic intermediate oxidation level below #1 and
above 0.
however the Na1s data disagree with this conclusion if
we neglect the presence of isolated Na CO micro-
ꢀ
ꢁ
crystallites in the surface region. The 1072.8 eV max-
imum corresponds to the reported Na1s BE for Na O,
i.e., 1072.5 eV [16]. It seems to be true that the sodium
ꢀ
on the surface is present as Na O and is the centre of
ꢀ
two forms of CO adsorption: (i) ‘‘weak’’ adsorption
ꢀ
with a low thermal stability providing an Na O ad-
ꢀ
sorption centre and adsorbate CO (292.5 eV C1s
ꢀ
The assignment of the C1s 290.7 maximum to
ꢀ\
the CO
group of pure Na CO microcrystallites
maximum) to the complex Na O · CO , and
ꢁ
ꢀ
ꢁ
ꢀ
ꢀ
is rather questionable because the analysis of the Na1s
spectra does not confirm the presence of sodium
atoms contained in isolated microcrystallites of pure
Na CO in the surface region. Such microcrys-
(ii) ‘‘strong’’ chemisorption leading to the formation
ꢀ
ꢁ
tallites, if they exist, are perhaps situated inside
the carbon films with a limited access to the surface
region.
3.4. Elem ental com position of the surface
layer of the carbon film s
Table II lists the elemental compositions of all the
investigated carbon films. It is interesting to note that
the Na-modification produces carbon films with
a highly oxidized surface. The degree of oxidation is
high for both the LT and HT carbon films with
a slight decrease in this tendency with an increasing
Figure 7 Typical Na1s spectrum of Na-modified carbon films of LT
and HT type.
6067

Acknowledgements
The investigation was fully supported by a grant from
the Komitet Badan Naukowych (Scientific Research
Committee, Poland). I also thank Dr G. Szymanski
for helpful discussions. I also thank Dr Sobczak for
help with the XPS measurements.
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in the carbonization temperature from 923 to 1073 K.
In some cases, the number of carbon atoms is smaller
than 1/3 of all the atoms in the surface region. Almost
50% of all the surface atoms in Na-modified carbon
films are oxygen atoms whereas the oxygen atom
content for the pure HT and LT carbons is below 25%
of all the surface atoms while the carbon atom content
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carbon films produced in this study are hydrophilic
[18] and their electrical resistance is very sensitive to
the adsorption of water vapour. Fig. 9 shows a dra-
matic decrease of the resistance of such films on the
adsorption of water vapour. This behaviour is typical
for alkali metal modified carbons that find application
as humidity sensitive resistors and capacitors [4]. The
required properties of the films may be optimized by
a judicious choice of the Na concentration in the
precursor and the carbonization conditions (temper-
ature and duration).
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Received 6 February 1996
and accepted 22 May 1997
.
6068