Chemical modification of a porous silicon surface induced by nitrogen dioxide adsorption

Article abstract of DOI:10.1021/jp0450383

The effect of gaseous and liquid nitrogen dioxide on the composition and electronic properties of porous silicon (PS) is investigated by means of optical spectroscopy and electron paramagnetic resonance. It is detected that the interaction process is weak and strong forms of chemisorption on the PS surface, and the process may be regarded as an actual chemical reaction between PS and NO₂. It is found that NO₂ adsorption consists in forming different surface nitrogen-containing molecular groups and dangling bonds of Si atoms (P_b-centers) as well as in oxidizing and hydrating the PS surface. Also observed are the formation of ionic complexes of P _b-centers with NO₂ molecules and the generation of free charge carriers (holes) in the volume of silicon nanocrystals forming PS. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp0450383

4684
J. Phys. Chem. B 2005, 109, 4684-4693
Chemical Modification of a Porous Silicon Surface Induced by Nitrogen Dioxide Adsorption
Constantine S. Sharov,* Elizaveta A. Konstantinova, Lyubov A. Osminkina,
Victor Yu. Timoshenko, and Pavel K. Kashkarov
M. V. LomonosoV Moscow State UniVersity, Physics Faculty, Leninskie Gory, Moscow, 119992 Russia
ReceiVed: October 30, 2004
The effect of gaseous and liquid nitrogen dioxide on the composition and electronic properties of porous
silicon (PS) is investigated by means of optical spectroscopy and electron paramagnetic resonance. It is detected
that the interaction process is weak and strong forms of chemisorption on the PS surface, and the process
may be regarded as an actual chemical reaction between PS and NO2. It is found that NO2 adsorption consists
in forming different surface nitrogen-containing molecular groups and dangling bonds of Si atoms (Pb-centers)
as well as in oxidizing and hydrating the PS surface. Also observed are the formation of ionic complexes of
Pb-centers with NO2 molecules and the generation of free charge carriers (holes) in the volume of silicon
nanocrystals forming PS.
1. Introduction
So far as nitrogen dioxide is concerned, in addition to a
fundamental interest, there is an applied one which is of great
practical importance. As a matter of fact, NO2 is a very
poisonous substance that is always present in the environment
of large cities, in industry districts, and near car highways and
railways because of plant furnace emissions and motor gases.
So, the invention of a sensor for NO2 would be an excellent
challenge to the unique PS properties, taking into account the
urgency of the problem.
In our work, we touched upon the essence of the modification
of the chemical structure of the PS surface caused by NO2
adsorption, a problem which unfortunately has not attracted
scientists’ attention thus far. It was a complex of FTIR, EPR,
and UV-vis techniques that provided us an opportunity to
clarify the processes occurring in silicon nanocrystals volume
and on their surface under NO2 adsorption.
Porous silicon (PS) consisting of silicon nanocrystals is a very
fascinating and a good prospect for investigation. It was initially
obtained in 1956 but still remains under researchers’ close
attention.1,2 The rapid development of the hi-tech industry during
the past decade caused some novel requirements for semicon-
ductor chips, the main one being to decrease the structure size
down to hundreds, even dozens, of nanometers. PS does possess
excellent properties to meet these requirements.3 The principal
ability for PS to be used for microelectronic devices has been
repeatedly outlined.2-5 But the low stability of PS in the air
and the consequent rapid degradation of its physicochemical
properties prevent it from being used as a construction material
for optoelectronic devices.
Another useful feature of PS lies in the fact that it is a unique
porous adsorbent due to its huge and nonhomogeneous surface
equal to ∼1000 m2/g.6 Equilibrium charge carriers in great
amounts (1017-1019 cm-3) were recently found in PS. Their
concentration is sensitive to surface coverage of PS and
molecular ambience of silicon nanocrystals.2 This property arises
from the fact that a large part of silicon atoms in a silicon
nanocrystal (up to a third) is situated on its surface.2 Therefore
PS can serve as a material for a wide spectrum of sensors
including ecological ones and model devices for investigating
fundamental properties of adsorption processes.7
The enormous PS surface area results in a very large affinity
for a great number of comparatively active substances and, in
turn, opens up wide prospects for constructing sensitive and
selective sensors of a new generation for those substances. There
are compounds whose adsorption on PS from the gaseous phase
is actively investigated, for example, oxygen, water, ethanol,
tetracyanoethylene,8 methanol,9 ammonia,10 and O,N-containing
amino acid derivatives.11 Surely, nitrogen dioxide is much less
studied,12 for the most part thanks to the complexity of processes
in silicon nanocrystals occurring on its adsorption. If one
suggests a sensor for a definite compound, close attention should
be paid to the development of a surface coating for PS stable
both in air and in the atmosphere of the compound in question.
2. Experimental Details
The standard means that most investigators usually follow
was used to obtain PS samples, that is, the electrochemical
anodic etching of crystal silicon wafers in hydrofluoric acid (HF)
ethanolic solution (see, for instance, ref 13 and references
therein). Initial crystal silicon substrates of p+-type (doped with
boron) with surface orientation (1 0 0), doping level p ) 5 ×
1018 cm-3, and electrical resistivity F ) 15 mΩ cm commonly
employed for fabricating mesoporous silicon consisting of
silicon nanocrystals of up to 50 nm in dimension14 were used.
The electrolyte content was HF (48 vol %)/C2H5OH
(96 vol %) ) 1:1. Current density was maintained at 50 and
100 mA cm-2 and etching time was 20 min, with the
experimental conditions resulting in obtaining silicon nano-
crystals of 7-9 (for 50 mA cm-2), and 3-5 nm (for 100 mA
cm-2) in dimension.15,16 The former is denoted by PS-1, and
the latter is represented by PS-2. Free-standing PS films were
lifted from the crystal silicon substrate by a short (1-3 s) current
pulse of 800 mA cm-2. We avoided rinsing the samples in water
after the synthesis lest they should be drastically hydrated and
oxidized and hence should complicate treating the spectra
obtained. Porosity of free-standing films measured gravimetri-
cally was approximately 50 and 72%, thickness determined by
* Corresponding author. Phone/fax: +7 (095) 304-2522. E-mail: const@
proxima.idb.ac.ru.
10.1021/jp0450383 CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/17/2005

Chemical Modification of a Porous Si Surface
J. Phys. Chem. B, Vol. 109, No. 10, 2005 4685
see the changes of the surface modification both on enhancing
NO2 pressure (Figure 1a) and in time (Figure 1b). The
logarithmic scale is effective for characteristic spectral bands,
and Drude absorption to be apparent. The absorption coefficient
was calculated in accordance with the following well-known
formula:
R(ν) ) -d-1 ln Tν
(1)
where R is the absorption coefficient, T is the transmittance, ν
is the wavenumber, and d is the thickness of the film under
investigation.
First, let us have a look at the spectra of the as-prepared
samples, both vacuumed and nonvacuumed. They indicate the
presence of distinctly visible bands of Si-Hx wagging modes
(620-750 cm-1), Si-H2 symmetric scissors mode (910 cm-1),
Si-Hx and OySi-Hx stretching modes (2090-2260 cm-1), less
discerned bands of SiOy stretching modes (1040-1120 cm-1),
C-Hz stretching modes (2860-2960 cm-1) chancing to come
into being owing to chemisorbed alcohol, O-H stretching modes
in adsorbed H2O molecules (3215-3450 cm-1) and in an
Si-OH group (3610 cm-1), and very small bands of Si-F
stretching modes (870-980 cm-1). The fact that the two spectra
are, in general, alike has two consequences: PS-1 and PS-2 do
not differ at the stage of formation and pumping does not
radically change the as-prepared PS surface.
Figure 1. IR absorption spectra of PS: (a) at different NO2 pressures,
and (b) at different times of the exposure. (a) As-prepared PS-1 in a
vacuum (1), in the ambience of NO2: 0.1 Torr (2) and 10 Torr (3). (b)
As-prepared PS-2 in the air (1), in the ambience of NO2 after 10 s (2),
1 min (3), and 10 min (4) of the exposure. Inset: The first derivative
of the spectrum (4) to distinguish between two peaks within the range
The next step should be to ascribe very apparent bands within
the spectral region 1200-1700 cm-1 to definite vibrations. The
bands have a direct bearing on NO2 adsorption. At low NO2
pressure, the spectrum of PS-1 shows a narrow and sharp band
at 1685 cm-1 with an additional absorption maximum on its
1600-1700 cm-1
.
an optical microscope equalled 60 and 83 µm for PS-1 and PS-
2, respectively.
shoulder near 1730 cm-1 17
The band corresponds to trans-
.
isomers of covalent silicon nitrites, and the additional maximum
arises from one of several N-O stretching modes in adsorbed
N2O4 dimer molecules. Though nitrites are preferably presented
by trans-forms, they can be seen in cis-forms at 1620 cm-1 as
well, but not very clearly because of the slight size of the band
and therefore the inconsiderable concentration of cis-nitrites.
Both forms are just chemical resonance structures and not virtual
optical isomers, hence once we find one we are to seek the other.
The amount of the trans-form is essential, while there are only
traces of the cis-form in PS. The concentration of nitrites
evidently grows with the increase in NO2 pressure, and at 10
Torr it becomes about 15 times more than at 0.1 Torr (calculated
by peak areas).
Turning to PS-2, its spectra reveal the presence of silicon
nitrites principally in trans-forms by a band at 1680 cm-1 as
well particularly at little time of subjection (10 s). This band in
the spectrum of a sample exposed for 10 min should not be
overlooked. To make it more pronounced to differentiate the
spectrum is likely useful. Figure 1b, inset, shows the result of
such differentiation with conspicuous peaks which aid in the
distinction. One can observe a minimum of the differentiated
spectrum at 1673 cm-1 which coincides with the peak in
question in the initial spectrum.
Nitrogen dioxide was synthesized by the following reaction:
Cu(shavings) + 4HNO3(concentrated) f 2NO2(gas.) + Cu(NO3)2(solution)
+ 2H2O, followed by water removal over phosphoric anhydride
placed in a glass tube. Gaseous NO2 adsorption by PS layers
was effected in two different ways: (1) in the atmosphere with
NO2 pressure about 1 atm, and (2) in a vacuumed cell in situ
within the pressure range of NO2 from 0.01 to 10 Torr using
membrane and turbomolecular Varian pumps.
Liquid NO2 was obtained on cooling the gaseous substance
in a U-shaped manifold immersed in a water/sodium chloride/
ammonia eutectic mixture characterized by a melting point near
-25 °C. Adsorption was carried out at 0 °C.
FTIR spectroscopy studies of free-standing PS films were
carried out by means of a PERKIN ELMER XR I FT-IR
spectrometer in the spectral range from 6500 down to 400 cm-1
with resolution 0.5-1 cm-1
.
UV-vis spectra of the solutions were measured with the help
of a registering spectrophotometer Shimadzu UV-2201 in quartz
cells (l ) 1 cm) vs water on adding diphenylamine and FeSO4
as spectrophotometric reagents.
EPR spectroscopy analyses were made by a PS-100.X device
operating at frequency 9.45 GHz, with the sensitivity 5 × 1010
spin G-1. The calculation of g-factors and spin center concentra-
tions was performed by an external standard method by means
of MgO doped with Mn2+ ions and CuCl2‚2H2O as the etalons,
respectively.
The intensive band characterized by the equality of the first
derivative of the spectrum to zero is clearly observed both in
PS-1 and in PS-2 spectra at 1631-1638 cm-1 and should be
assigned to covalent silicon nitrates in which the oxidation state
of nitrogen atom is +5 in comparison with +3 in nitrites.17
Silicon nitrates may be also identified by one more pronounced
spectral maximum observed at 1288 cm-1 in PS-1 and at 1302
cm-1 in PS-2. These peaks conform to two types of stretching
vibrational modes in the NO3 group.
3. Results and Discussion
3.1. Interaction between PS and Gaseous Nitrogen
Dioxide. IR absorption spectra of as-prepared PS samples and
those with adsorbed NO2 are presented in Figure 1. We can

4686 J. Phys. Chem. B, Vol. 109, No. 10, 2005
Sharov et al.
As far as N2O4 is concerned, we see not only the band
described above (Figure 1a, curve 2), but also another N-O
stretching mode exhibiting its characteristic maximum near 1270
cm-1. Although wavenumbers of the modes in N2O4 involved
are a little bit different from those supposed typical for N-O
vibration in the nitrogen dioxide dimer, namely 1748 and 1262
cm-1 17,18
it could be explained by adsorption surface effects
,
already known for a long time19,20 that may shift bands by up
to several decades of reverse centimeters.
PS-2 samples also adsorb nitrogen dioxide in the form of
N2O4 that is pointed out by apparent slopes at 1750-1770 cm-1
.
Still, the concentration of N2O4 is much lower than that of other
nitrogen-containing compounds on the PS surface, even if we
ignore the spectral sensitivity to the compounds. Also N2O4 is
completely desorbed on pumping and in the atmosphere in about
1 day after NO2 adsorption, so it does not play a substantial
long-range role in the processes that take place in silicon
nanocrystal ensembles. In summary, we should emphasize that
monomeric NO2 is not adsorbed on the PS surface. Therefore,
one should conclude that the heat of either N2O4 or NO2
adsorption on PS does not exceed the energy of a N-N bond
in a dimeric N2O4 molecule equaling 57.43 ( 0.04 kJ mol-1 21,22
and believed low as compared to a many other gaseous
substances (commonly hundreds and thousands of the units
concerned). We can scarcely accept quite dissimilar values of
adsorption enthalpy suggested by other authors23 only on the
basis of theoretical calculations performed within the limits of
cluster analysis. Thus, the heat of nitrogen dioxide adsorption
on PS in either form is relatively low, and this fact will prevent
it from physiosorbing on PS in large amounts and will aid it in
causing different surface transformations which are of chemical
origin.
Figure 2. Typical IR absorption spectra of PS-2 samples subjected to
gaseous (1) and liquid (2) nitrogen dioxide adsorption. The time of
exposure 100 min.
oxidized and hydrated, as the spectra show. The degrees of PS
oxidation and hydration under interaction with NO2 in the air
exceed that in a vacuum by several times. As for oxidation, it
should be accounted for by the combined action of NO2 and
atmospheric oxygen, the latter aiding in oxidizing the surface
to a great extent. Nitrogen dioxide destroys many Si-Si bonds
and creates the doubled number of surface defects, which is to
be described in detail a bit later. Those defects are formed and
oxygen immediately reacts with them, resulting in the formation
of silicon oxide. Regarding water, there should be different
mechanisms of its adsorption in a vacuum and in the atmosphere.
Discussing experiments in a vacuum on PS-1 samples, as NO2
is dried over phosphoric anhydride it does not contain water
vapor, so what is the source of water? The answer will be the
following reaction affirmed by the change in area of Si-Hx
and H2O bands:
Finally, unlike PS-1 samples, those of PS-2 demonstrate the
presence of two other surface substances. First, one can see a
wide band at 1418 cm-1 and another detected not so well at
1340 cm-1 on the slope of silicon nitrate peak which are
characteristic for N-O stretching modes in covalent cis-nitroso
silicon. Most of the nitroso compounds tend to dimerize
extensively, especially when solidified,18 and PS is not an
exception. The bands ascribed precisely to cis-forms of nitroso
dimers are not accompanied by any spectral lines related to
trans-forms, so it is a characteristic feature of PS as a substrate.
Second, for a longer time of the NO2 exposition, a narrow
maximum appears at 1255 cm-1 assigned to silicon N-oxide.17
On the whole, this compound is similar to the nitroso dimer in
structure but contains only one N-O bond, while the other is
reduced.
2(tSisH) + 2NO2 f tSisOsSit + H2O + 2NO (2)
This is an oversimplified version of the actual process, because
the real equation would include the formation of nonstoichio-
metric substances on the PS surface. The amount of atmospheric
water vapor which hydrates silicon oxide formed on the surface
is obviously much more than that if water is a product of
hydrogenated covering oxidation according to eq 2. The increase
in the O-H absorption band at 3400-3800 cm-1 was also
observed in the adsorption experiments with the evacuated
samples.
3.2. Interaction between PS and Liquid Nitrogen Dioxide.
Figure 2 demonstrates the resemblance and the difference
between the influence of gaseous and liquid NO2 upon the PS
surface.
In addition to forming nitrogen-containing surface com-
pounds, nitrogen dioxide is shown to oxidize PS heavily and
that is indicated by a strongly dominating wide band at 1050-
1160 cm-1 coming into being owing to different SiOy stretching
modes (Figure 1a,b) and by shifting Si-Hx stretching vibration
bands from 2090-2140 cm-1 to 2130-2260 cm-1 resulting
from oxidizing back-bonds in Si-Hx groups (Figure 1). Silicon
oxide is hydrophilic, consequently a noticeable amount of water
is being gradually adsorbed on the PS surface during NO2
adsorption confirmed by an incredible growth of O-H stretching
modes bands both in a H2O molecule and in a Si-OH group
within the range 3215-3700 cm-1. Thus, high values of
wavenumbers may be referred to the effect of the hydrogen
bonds21 terminating the PS surface.
We see that liquid nitrogen dioxide adsorption involves a few
effects other than those arising from gaseous substance adsorp-
tion. The most important of them is the absolute absence of
hydrogen-containing groups on the surface of silicon nano-
crystals despite the fact that they are present in the initial PS-2
sample. The explanation consists of kinetic and thermodynamic
reasons. The first is that the concentration of NO2 in the liquid
phase is beyond the comparison with that in gaseous phase;
consequently, the process described by eq 2 proceeds with much
higher rates. The second is that liquid NO2 possesses a very
high affinity for water, thanks to indefinite water solubility in
liquid NO218 which leads to instantaneous and complete removal
of water from the scope of the reaction (eq 2). As a result of
this, the reaction equilibrium constant is raised according to Le
Chatelier’s principle.
The overall quantity of the oxide and water increases with
an increase in NO2 pressure and in the time of adsorption.
Nitrogen dioxide interacted with PS, and it becomes PS surface
The amount of oxide and water adsorbed on it is ap-
proximately 3 times more in the samples subjected to liquid

Chemical Modification of a Porous Si Surface
J. Phys. Chem. B, Vol. 109, No. 10, 2005 4687
TABLE 1: Nitrogen Containing Compounds Appearing on Nitrogen Dioxide Adsorption Upon PS Surface
NO2 adsorption than that in the samples exposed to gaseous
substance. The sets of nitrogen-containing molecular groups
arising from liquid and gaseous NO2 adsorption are for the most
part alike. To mention but one exception, nitrogen dioxide in
the gaseous phase does not always give rise to N-oxide
appearance, though it is the case rather often; however, if it is
present in liquid phase, NO2 does not lead to synthesizing
N-oxide ever.
3.3. Generalization of the Interaction Processes. In sum-
mary, on the whole, liquid NO2 adsorption on PS does not result
in any principally different mechanisms of PS surface modifica-
tion dissimilar with the gas adsorption. Even nitrogen dioxide
dimer N2O4 is not adsorbed on the surface from the liquid phase
in larger amounts than from the gaseous phase, although the
liquid particularly consists of the dimer (99.95 mol % of N2O4
at 0 °C21,22). We suggest that this fact should be accounted for
by the disregarding affinity of the PS surface for liquid nitrogen
dioxide. PS is relatively unpolar, with dielectric constant ꢀ
assumed to equal that of monocrystalline silicon, namely, 11.9.24
Therefore, very polar liquid NO2 with ꢀ > 30017 does not tend
toward adsorption on the unpolar surface.
amounts, may it be possible that NO2 reacts with it, giving nitric
acid as a result of the reaction
2NO2 + H2O f HNO3 + HNO2
(3)
and, in turn, that nitric acid oxidizes silicon atoms as an actual
oxidizing agent? PS samples were treated by a mixture of NO2
and water vapor in stoichiometric amounts as shown in eq 3,
and we found that the oxidation degree does not depend on
water content in the adsorbate. It is the direct oxidation of PS
by NO2 rather than by HNO3 that occurs on the surface, because
the degrees of oxidation by the mixture involved and by pure
NO2 are equal.
3.3.2. Silicon Nitrite. It seems strange from the first glance
that covalent nitrites are present in PS principally in trans-forms.
Whatever substrate R in R-ONO compound to which nitrite
group is bonded may be chosen, this is generally not the case
and the quantities of the isomers are comparable. It is probable
that the reason for this lies in the following. Although nitrites
are represented in Table as though they contained charged
oxygen atoms, one is to comprehend that there is a resonance
equilibrium:
If we summarize all pieces of information in Table 1, we
will be in a position to consider the point on the whole.
To comment on the processes taking place in PS upon NO2
adsorption, let us divide the discussion into several subchapters
in accordance with the types of the compounds synthesized.
3.3.1. What Substance Oxidizes a PS Surface? The question
arises: if adsorbed water is present on the surface in large

4688 J. Phys. Chem. B, Vol. 109, No. 10, 2005
Sharov et al.
One quasioptical isomer containing NdO+ group undergoes
the transformation into the other within a femtosecond scale,17
hence the equilibric concentrations of the isomers are equal if
no factors shifting the equilibrium are present. Regarding PS,
there is a tendency to trans-isomer in eq 4 since silicon dangling
bonds repel O- ion from the surface by local Coulomb fields.
3.3.3. Silicon Nitrate. Silicon nitrate is present in either type
PS samples in large amounts. Nitrogen atom is oxidized to
oxidation number +5. It is a consideration to admit the
possibility for NO2 to be oxidated near the surface with the
help of the catalytic activity of PS. The reaction of direct NO2
oxidation by oxygen is prohibited thermodynamically.25 Even
though it were enabled, it would never lead to covalent silicon
nitrate but only to nitrate ion, so the question is: what is the
source of oxygen atom taking an electron from NO2 and making
the oxidation state of nitrogen equal to +5? The answer will
be evident from the mechanism of the interacting NO2 with PS
about to be discussed.
3.3.4. Nitroso Silicon. Nitroso silicon is dimerized with the
simultaneous transformation of the double NdO bonds to
donor-acceptor N+sO- bonds when it reacts with SisSi
surface bonds:
Figure 3. (a) The dependences of hole (1) and Pb-center (2)
concentrations in PS-1 upon NO2 pressure. Adsorption effected in a
vacuum. (b) The dependences of hole (1) and Pb-center (2) concentra-
tions in PS-1 upon the time of NO2 adsorption. Adsorption effected in
the air.
We may expect that the formation of such a substance
proceeds in a few stages and the reaction shown in eq 5 follows
the interaction of PS with nitrogen dioxide. Otherwise we have
a process of the third order which is almost impossible. As
contrasted to silicon nitrate, discussed in the previous subsection
in which the nitrogen atom is oxidized, in nitroso silicon the
nitrogen is reduced to oxidation number +2. Thus, the fact that
contraversal processes take place on the PS surface can be stated
even not considering the details.
Trans-isomers of nitroso silicon dimers are absent on the
surface, and it is not unexplainable. NO2 molecules attack
surface silicon atoms situated on one side of a pore. The
morphology of pores is not suitable for trans-modifications
about PS-1 and PS-2 holes (Drude absorption). It is enhanced
on NO2 adsorption. Undeniable arguments in favor of existing
pronounced Drude absorption in PS are described in ref 26.
Once NO2 is adsorbed from vacuum, hole concentration is
dependent upon NO2 pressure nonmonotonically with a maxi-
mum at 0.1 Torr27 which is shown in Figure 3a. But if it is
adsorbed from the air, hole concentration growth does possess
monotonic character vs the time of adsorption (Figure 3b). The
dependences are somewhat contrary concerning NO2 pressure
and time of exposure. The decrease in hole concentration toward
high NO2 pressures demands that surface defects should be taken
into consideration.
3.3.7. Paramagnetic Defects in PS. There is a large amount
of paramagnetic defects of crystal lattice in PS resulting from
uncompensated bonds of silicon atoms which are generally
referred to as Pb-centers. Different types of Pb-centers are
described in detail in refs 28-32, and they are dissimilar
primarily in the number of back-bonded oxygen atoms, sym-
metry, and charge. As for our research, the main components
values of the g-tensor (orthorhombically symmetric g1 ) 2.0098
( 0.0005, g2 ) 2.0061 ( 0.0005, and g3 ) 2.0018 ( 0.0005)
in EPR spectra lead these defects to be regarded as dangling
bonds of silicon atoms situated at silicon/silicon oxide interfaces,
or Pb-centers.30,31 EPR spectra of PS-2 samples subjected to
different times of NO2 adsorption are presented in Figure 4.
The concentration of Pb-centers can be calculated by double
integrating the spectra and comparing the result with the etalon.
The dependence of Pb-center concentration on the time of NO2
adsorption is demonstrated in Figure 3.
synthesized on pore borders, wherever the attack may take place,
perhaps with only one exceptionsthe pore bottom. But even if
they were formed there, the overall amount of trans-dimers can
be ignored.
3.3.5. Silicon N-Oxide. In this compound, the oxidation state
of the nitrogen atom is +1, and N-oxide is shown to represent
an extreme degree of reducing nitrogen in nitrogen-containing
molecular groups found on the PS surface. Here we should
search for a reducing agent removing one oxygen atom from
the nitroso silicon dimer.
Nitroso silicon is synthesized only on the PS-2 surface, so is
N-oxide. The presence of Si-Si bonds on the surface in PS-2
and its absence in PS-1 play the crucial role in it.
3.3.6. Free Charge Carriers. One observes not only spectral
absorption maxima in Figures 1 and 2, but also a strong increase
in absorbance with a decrease in wavenumbers. It is attributed
to absorption of IR radiation by free charge carriers, theorizing
We have noticed27 that the Pb-center concentration is en-
hanced with an increase in NO2 pressure monotonically as
contrasted to nonmonotonic dependence of hole concentration
and suggested that it should be accounted for by capturing holes

Chemical Modification of a Porous Si Surface
J. Phys. Chem. B, Vol. 109, No. 10, 2005 4689
Figure 4. EPR spectra of PS-2. An as-prepared sample (1), samples
subjected to NO2 adsorption with the time of adsorption 10 s (2), 1
min (3), and 10 min (4).
Figure 5. The diagram of NO2 and NO molecular orbitals. Loosening
orbitals are denoted by an asterisk. In triple centered NO2 orbital system,
connecting orbitals are designated by letter c, in double centered NO
orbital system all nonloosening orbitals are connecting ones.
to Pb-centers, passivating Pb-centers by NO2, and consequent
releasing holes by such passivation. In the presence of atmo-
spheric oxygen, this is not the case and another oxidation
mechanism is present. A decrease of Pb-center concentration is
detected at short times of PS exposure in NO2, small as it is. It
is trapping electrons from Pb-centers by NO2 molecules rather
than forming new Pb-centers that prevails at this stage.
With an increase in adsorption time up to 1 min, trapping
electrons and synthesizing new Pb-centers by NO2 takes place
which result in an increase both in hole and in Pb-center
concentrations. Then the interaction of atmospheric oxygen with
Pb-centers that gives rise to silicon oxide commences to
dominate over the second process concerned, and the amount
of Pb-centers reduces. Oxygen is less active than nitrogen
dioxide, and the oxidation of surface defects does not start at
once but only after a stage of induction.
an sp3-hybridized Pb-center.17 If NO2 were bound with a
π-orbital one, for example benzene, the binding energy would
be much greater thanks to a stabilizing -NO2 group with π-bond
structure by π-conjugation. Finally, the third consideration
involves the free rotation of a nitro group around an R-N bond,
where R is a substrate. The nitro group exhibiting C2V symmetry
generally possesses a very high rate of changing its conforma-
tions.34 PS is characterized by considerable lack of surface
uniformity and hence low symmetry of its fragments, and this
free rotation is prevented or even prohibited by different sources
of nonuniformity such as Pb-centers, silicon atoms terminated
by hydrogen or hydroxyl radicals, oxygen atoms in tSisOs
Sit molecular groups, adsorbed water molecules, etc. All these
reasons combined make the formation of a nitro group on the
PS surface not very possible.
Hole concentration in PS on NO2 adsorption does not exceed
that in the initial crystal silicon. Thus, there is a good reason to
believe that in as-prepared PS most holes are captured by Pb-
centers. Only when NO2 interacts with Pb-centers do these
centers finish trapping holes and instead release them.
It is the oxidation number of the nitrogen atom +1 that is
stated to represent the extreme degree of nitrogen reduction in
nitrogen-containing molecular groups on the PS surface. Still,
there is a slight possibility for the group with 0-oxidized nitrogen
present to be present on the surface, namely, tSisNdNsSit.
But even if it were present, this group could scarcely be detected
by FTIR spectroscopy in PS as sNdNs stretching modes
bands are within far IR range; to be more precise, their
3.3.8. On the Question of Other Compounds Which Are
Expected to Be Present on the PS Surface. Let us consider the
supposed possibility of some other compounds present on the
PS surface. First of all, it is difficult to understand a reason
-
wavenumbers are less than 250 cm-1 17
, and in the presence of
why NO2 anions bands are not distinctly seen in IR spectra,
and this obstacle has already attracted investigators’ attention.26
The response of silicon nanocrystals electronic properties (Figure
3) is essential, and nitrite anions are unlikely to be present on
the surface, and even if present, they are present in such small
amounts that apparently they cannot be detected by the rather
sensitive technique of IR spectroscopy. We can attribute this
fact to the effect of suppressing molecular vibrations by strong
Coulomb fields.19,20 When near the surface, an NO2 molecule
undergoes weakening oscillators’ strength and the bands in IR
spectra become less pronounced.
Further, it may be quite unclear why we do not observe any
amount of nitro groups on PS surface. The NO2 molecule is a
free radical containing an unpaired electron at the1b1u molecular
orbital that becomes clear from Figure 5. From the beginning,
it seems probable that direct combination of the two electrons
involved should take place. The first reason underlying the fact
that it does not take place is the very large affinity of NO2 for
an electron. Quantum mechanical calculations show that for the
NO2 molecule to seize an electron is more preferable than to
form a covalent bond with such a rather nonelectronegative atom
such as silicon.33 The second consists of the low binding energy
of the nitro group when bonded to a σ-orbital substrate such as
very strong Drude absorption in this range they cannot be
observed distinctly.
3.4. Mechanism of the Interaction. It is desirable that the
discussion of interacting NO2 with PS should be divided into
several parts, according to the nature of bonds formed between
the substances in question.
3.4.1. Van der Waals Interaction. For every substance in
question approaching the PS surface, this type of binding always
makes a definite contribution to adsorption, but this nonspecific
interaction is the most essential for the N2O4 dimer adsorbed
on the surface. As we have supposed above, the heat of N2O4
adsorption on PS is low and not enough for an N-N bond to
be broken. There is an undisputable explanation for it. With
N2O4 being on the whole not a dipole molecule (both of the
‚NO2 dipoles compensate each other), its dipole interaction with
the surface (regarding half a molecule ‚NO2 bonded to silicon
nanocrystals) is very weak. van der Waals interaction is one of
the main factors resulting in physiosorbing N2O4 on PS, while
the hydrogen-bond effect is not realized for N2O4 nonexcited
molecules.
3.4.2. Hydrogen Bond Interaction. For example, water is
adsorbed on silicon oxide primarily by strong hydrogen bonds

4690 J. Phys. Chem. B, Vol. 109, No. 10, 2005
Sharov et al.
formed between an oxygen atom either in a tSisOsSit or
in a tSisOH group and protons in water molecules. The energy
of such a hydrogen bond existing at a nanocrystal/gas interface
is believed to be extremely strong and to equal about 100-150
kJ mol-1, while the energy of van der Waals binding water with
a solid substrate commonly do not exceed 15 kJ mol-1 33,34
Also,
.
hydrogen-bond formation can be confirmed experimentally by
shifting bands of vibrations taking part in it toward lower
wavenumbers. It arises from diminishing energy constants of
bonds. As to most of the substances considered, their bonding
to the surface by a hydrogen bond is physiosorption, but
concerning water it is weak chemisorption taking into account
high values of binding energy.
3.4.3. Ionic Interaction. Both ionic and covalent NO2
interacting with PS are preceded by breaking the N-N bond in
a N2O4 molecule which leads to two NO2 molecules being
virtually free radicals. In general, any exothermic physicochem-
ical process giving an energetical profit which exceeds the N-N
binding energy in the nitrogen dioxide dimer may occur.
Nitrogen dioxide contains one unpaired electron at the odd
1b1u triple centered orbital (Figure 5) which is neither a
connecting nor a loosening one. In other words, the energy of
this orbital is halfway between that of beneficial low states and
that of disadvantageous high ones. On the one hand, this orbital
is not loosening and it gives rise to capturing another electron
from a Pb-center and coupling it with its own electron, or ionic
interaction. On the other hand, this orbital is not connecting,
and it results in high chemical activity of NO2, or covalent
interaction.
Figure 6. UV-vis spectra of diphenylamine solution without impuri-
ties (1), diphenylamine solution on adding 1.250 × 10-8 M NaNO2
(2), and the dispersion of 4.3 mg of PS in 4 mL of diphenylamine
solution (3). Solvent: concentrated sulfuric acid. Diphenylamine
concentration 5.882 × 10-2 M. The detection limit of the procedure
employed is approximately 2.5 × 10-9 M, or 1.5 × 1012 ions mL-1
.
Pb-centers cease capturing holes and the hole concentration that
is fixed experimentally grows.
The transfer of an electron is complete and the complex
formed is exactly of ionic nature and exists only because of
Coulomb attraction force. There may not be any other type bond
-
between a Pb-center and an NO2 anion because (1) the Pb-
center has an empty sp3-hybridized orbital and no electrons
which may be liberated without breaking back-bonds with Si
(O) atoms, and (2) the nitrite ion possesses no vacant non-
loosening orbital.
It is essential that the values of ionic binding energy are
between those of hydrogen-bond energy and the covalent bond
Considering the former case, there is an electron transfer e-
(sp3 in Si) f 1b1u in NO2. The mechanism of the process
involved consists of several steps:
energy, approximately within the interval 50-200 kJ mol-1 22,33
.
Ionic interaction does possess properties of chemisorption, that
is, withstanding strong chemical attacks. If subjected to the
action of strong concentrated nonoxidative acids, namely,
sulfuric, phosphoric, perchloric, and hydrobromic acids, an ionic
bond between PS and a nitrite anion is not broken and nitrite
anions remain chemically nonactive. Figure 6 demonstrates a
piece of experimental evidence for this statement by showing
UV-vis spectra of the dispersion of PS with adsorbed NO2
molecules in sulfuric acid with the addition of diphenylamine
and etalon diphenylamine solutions. Colorless diphenylamine
dissolved in concentrated sulfuric acid is oxidized by free nitrite
anions to benzidinic violet dye whose even dilute solution
possesses high values of optical density, thanks to the tremen-
dous molar absorption coefficient of the dye R > 4 × 108 M-1
Equation 6a describes the stage occurring before NO2
adsorption, namely, the capture of a hole at the Pb-center, which
is equivalent to injecting an electron into silicon nanocrystals.
Still, this step is not indispensable, and there is a certain amount
of noncharged Pb-centers in as-prepared PS. Their presence
justifies the slight decrease in Pb-center concentration at low
NO2 pressures as well as at short time spans of adsorption (see
section 3.3.7). When NO2 approaches a Pb-center, the reverse
process in eq 6a, namely, releasing a hole from a Pb-center,
commences to prevail. The liberation of a hole will occur as a
result of acceptor properties of NO2 at the adsorption on a
semiconductor surface and inductive effect of shifting electron
density (see eq 6b).33 The shift concerned is caused by NO2
attracting electron density to the Pb-center through back-bonds
of a silicon atom. The second step, summed up by eq 6b, is the
virtual interaction of NO2 with a Pb-center, enabled by partial
overlap of molecular orbitals of a silicon atom and a NO2
molecule. It is to emphasize that as a result of the three processes
discussed, a free hole is immersed into silicon nanocrystals
volume. This process is not direct, owing to the seizing of an
electron from a Pb-center by NO2 in an indirect way (by
passivating a Pb-center by NO2). If carrying a positive charge,
cm-1 21
In Figure 6, one can see that the spectrum of PS
.
dispersion is similar to that of pure diphenylamine and differs
drastically from that of benzidinic violet. Consequently, the
amount of free nitrite anions in the dispersion is lower than the
detection limit. They are firmly bonded to Pb-centers and not
removed by sulfuric acid, and this is also the case for phosphoric,
perchloric, and hydrobromic concentrated acids. This fact states
that ionic interaction of PS with NO2 is a form of chemisorption.
When PS samples are placed in a vacuum, nitrite anions return
captured electrons to silicon nanocrystals and desorb from the
surface on transforming back to NO2 and, in turn, to N2O4
molecules, which is affirmed by a partial decrease in hole
-
concentration. Pumping ensures an energetical benefit for NO2
to liberate an electron, and the process of desorption in the
noncharged form (that was earlier forbidden) becomes possible.
Thus, ionic interaction between NO2 and PS should be
considered as a form of weak chemisorption.
3.4.4. CoValent Interaction. This type of interaction of PS
with NO2 commences with the formation of an intermediate

Chemical Modification of a Porous Si Surface
J. Phys. Chem. B, Vol. 109, No. 10, 2005 4691
SCHEME 1: Formation of the Intermediate Complex
Two silicon atoms near DSF bonded to it before adsorption
by broken bonds transform to Pb-centers. Complex D gives rise
to the different number of Pb-centers which is dependent on
final products. These Pb-centers may react with oxygen to
become silicon oxide, or with water to transform to silicon
hydroxide, or remain as Pb-centers. Further destiny of IC
prodigious in reactivity depends on a substance interacting with
it which becomes evident from Scheme 2.
The processes involved should be explained. Most of them
are made possible by heat-internal molecular energy transfor-
mation arising from the fact that IC is the target of free molecule
attacks.
A. The formation of covalent silicon nitrite will happen if
excessive energy is delocalized in a N-Si bond in IC. After
breaking the N-Si bond, the oxygen atom terminating IC returns
an electron ensured by the nitrogen atom in NO2 to nitrogen
and a double covalent bond between the atoms concerned is
formed.
B1. Nitroso silicon may be synthesized on the condition that
the oxygen atom terminating IC is captured by strong reducing
agents on the PS surface -Si-Hx groups situated near IC. The
interaction between hydrogen-terminated silicon atoms and IC
probably proceeds along two different lines:
state. Generally speaking, all nonprimitive chemical processes
consisting of several stages pass through an intermediate state.
This state is desynthesized almost instantaneously after coming
into being; to be more exact, from atto- up to picoseconds,
depending on the type of a process.33 There is no reason to
believe that the interaction between PS and NO2 has another
character. If we made the thorough analysis of all possible
intermediate states, the four-centered N,O,Si,Si-containing
intermediate complex (IC) would uncontroversially be the most
justifiable. The double silicon fragment tSisSit (DSF) is
crucial for the interaction process to commence because
hydrogen-terminated silicon atoms are comparatively inert for
direct attacks. Scheme 1 aims at clarifying the formation of the
intermediate state discussed.
Step A is the approach of an NO2 molecule to DSF, step B
is the coordination of a NO2 molecule by DSF, step C is the
formation of a covalent bond between N, O, and the two silicon
atoms in DSF, and step D is the closing of the four-centered
cycle. The final step occurs for silicon atoms in DSF to have
nearly tetrahedral surround. Complex C does not exist because
of a structure that is too strained, resulting from unnatural valent
angles for the silicon atom.33
IC + (3 - x)SisHx f SisNdO + (4 - x)Pb0 + H2O (7a)
IC + (3 - x)SisHx f NO + (5 - x)Pb0 + H2O (7b)
The latter will take place, if the excess of energy is too large
for stable nitroso silicon to exist. The former will dominate, if
the amount of energy to break the bonds is still not sufficient
to destroy the whole IC. Also it highlights that eq 7b virtually
reflects process G. Nitroso silicon can be synthesized by NO
directly attacking the surface too. It is the case, because a NO
molecule can break an ordinary covalent Si-Si bond. It is
explained primarily by the fact that it possesses an unpaired
electron situated at a high-energy πx* loosening orbital (Figure
5).
SCHEME 2: Transforming the Intermediate Complex to Final Products

4692 J. Phys. Chem. B, Vol. 109, No. 10, 2005
Sharov et al.
B2. Dimeric cis-nitroso silicon is formed when two nitroso
silicon monomers are formed near each other.
physiosorption and weak and strong chemisorption. The adsorp-
tion consists of the following: (1) physiosorbing nitrogen
dioxide dimer on the surface, (2) synthesizing different nitrogen-
containing surface molecular groups, (3) strong oxidation and
hydration at the surface, (4) forming Pb-centers, (5) causing the
appearance of ionic complexes of nitrite anions with Pb+-centers
accompanied by increasing the free hole concentration in PS.
The mechanism of the processes considered is proposed in
accordance with which they are related to each other.
B3. Silicon N-oxide appears if a nitroso dimer is reduced by
NO which gives its unpaired electron and seizes one oxygen
atom from a nitroso dimer. To provide an electron rather than
to capture one more is beneficial for nitrogen monoxide because
a πx* orbital is energetically disadvantageous. Therefore, NO
exhibits reducing and not oxidizing properties.
B4. Further supposed reduction of N-oxide proceeds owing
to NO action too.
The chemical modification is not very stable in the atmo-
sphere and the surface covering is gradually degrading. It leads
to silicon oxide formation. The interaction of PS with liquid
nitrogen dioxide is shown to be a process of greater interest
than that of the gaseous substance. It enables us to eliminate
all hydrogen-terminating silicon atoms on PS surface, while
changes in free hole concentration are similar to those resulting
from gaseous NO2 adsorption. If one constructed a sensor for
NO2 on the basis of PS, it would be worthy inventing a liquid-
containing sensor somewhat like a gas-liquid chromatography
device where a thin liquid layer is bonded to a solid substrate.
If the liquid phase in this sensor is relatively polar, gaseous
NO2 will dissolve in it and could be detected more easily.
The knowledge about the mechanism of interacting PS with
NO2 could aid in avoiding undesirable effects of surface
transformation. If we think about the fact that DSF is crucial
for covalent interaction, we will be able to prevent PS from
containing them in large amounts by changes in preparation
procedure and further surface treatment. Thus, if we are aware
of the detailed interaction mechanism, we will be in a position
to control the overall adsorption process. Also this will provide
us with an opportunity to advance new sensor technology based
on PS.
C. IC leads to silicon nitrate, if the N-Si bond is not just
broken but attacked by an oxidizing agent. There is only one
candidate to meet all requirements such as energetical, elec-
tronic, structural, etc., and we mean free nitrogen dioxide:
IC + NO2 f Si-ONO2 + Pb0 + NO
(8)
and this process provides the most considerable quantity of NO.
D. IC may also be behind forming the conventional oxide
group tSisOsSit not containing nitrogen:
IC (+ heat) f IC* f tSisOsSit + 2Pb0 + NO (9)
where an exceptionally excited state of IC is denoted by an
asterisk. This is the source of free NO as well.
Nitrogen monoxide liberated as a result of the oxidation
reaction can be detected spectrophotometrically by means of a
ligand exchange transformation:
NO + [Fe(H2O)6]SO4 f [Fe(H2O)5NO]SO4 + H2O
in the case in which the PS sample is placed in a 0.05-0.1 M
iron(II) sulfate solution. The dark-brown color of the triple
complex compound in the right side of this equation is
represented in an UV-vis spectrum by a series of strong
absorption bands within the interval 405-480 nm.
Acknowledgment. One of the authors (E.A.K.) is grateful
for the financial support by a Grant of the President of the
Russian Federation (MK-2036.2003.02). The work was partially
supported by the Ministry of Industry, Science and Technology,
the Russian Federation, and the MSU Center of Collective Use.
E. The process of interacting IC with water causing two
hydroxyl silicon groups is related to the previous one:
IC + H2O f 2tSisOH + 2Pb0 + NO
(10)
References and Notes
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4. Conclusions
The interaction between NO2 and PS is a complex and
entangled physicochemical process including van der Waals,
hydrogen-bond type, ionic, and covalent binding. The PS surface
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