Role of interstitial voids in oxides on formation and stabilization of reactive radicals: Interstitial HO2 radicals in F2-laser- irradiated amorphous SiO2

Article abstract of DOI:10.1021/ja0571390

A procedure to produce stable hydroperoxy radicals (HO₂ ^.) in bulk amorphous SiO₂ (a-SiO₂) has been developed. Oxygen molecules incorporated in the interstitial voids in a-SiO ₂ react with mobile hydrogen atoms (H⁰) generated by the photolysis of silanol (SiOH) groups with F₂-laser light (λ = 157 nm, hν = 7.9 eV), resulting in the efficient creation of interstitial HO₂^.. The high yield of HO₂^. suggests that the collisions of the reaction intermediate with the void wall play an important role in dissipating the excess energy of the intermediate instead of the triple collision observed in the gas phase reaction. The resultant HO₂^. is thermally stable up to 100 °C.

Full text of DOI:10.1021/ja0571390

Published on Web 03/30/2006
Role of Interstitial Voids in Oxides on Formation and
Stabilization of Reactive Radicals: Interstitial HO
2
Radicals in
F
-Laser-Irradiated Amorphous SiO
2 2
†
‡
Koichi Kajihara,* Masahiro Hirano, Linards Skuja, and Hideo Hosono
Contribution from the Transparent Electro-ActiVe Materials Project, ERATO-SORST, Japan
Science and Technology Agency, Frontier CollaboratiVe Research Center, Mail Box S2-13,
Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
Received October 20, 2005; E-mail: kaji2@lucid.msl.titech.ac.jp
•
2
Abstract: A procedure to produce stable hydroperoxy radicals (HO ) in bulk amorphous SiO
2
(a-SiO
2
) has
been developed. Oxygen molecules incorporated in the interstitial voids in a-SiO
2
react with mobile hydrogen
0
atoms (H ) generated by the photolysis of silanol (SiOH) groups with F
2
-laser light (λ ) 157 nm, hν ) 7.9
•
2
•
eV), resulting in the efficient creation of interstitial HO . The high yield of HO suggests that the collisions
2
of the reaction intermediate with the void wall play an important role in dissipating the excess energy of the
•
2
intermediate instead of the triple collision observed in the gas phase reaction. The resultant HO is
thermally stable up to 100 °C.
-
3
1
. Introduction
O-Si bond network because of its low density (2.2 g cm ) in
comparison to the related crystalline form of SiO2 (R-quartz,
•
-
•-
Reactive oxygen species such as O , O2 (superoxide ion
-3
2
.62 g cm ) and other light metal oxides of analogous formular
weight, such as MgO (3.6 g cm ) and Al2O3 (3.97 g cm ).
•
-
radical), and O3 (ozonide ion radical) are important oxidants
in the catalytic oxidation of various organic compounds
-
3
-3
•
1,2
•
The interstitial voids in a-SiO2 often encapsulate small
molecules and radical fragments. For example, the hydrogen
atom (H ), which is one of the most reactive radicals, is
Various molecular
radicals such as HCO , NO , and ClO (x ) 0, 2, 3)
have been formed in a-SiO2 exposed to energetic radiation, such
as X-rays, γ-rays, or ultraviolet laser light. Such small chemical
species can migrate through the interstitial voids in a-SiO2:
hydrogen molecules (H2) diffuse rapidly even at room temper-
ature (average diffusion length ∼10-100 nm in 1 s). Thus,
interstitial voids having entrances to the outer surfaces may be
accessible for small reactive species to influence reactions
catalyzed by a-SiO2.
(
(
denotes an unpaired electron). Their conjugate acids, HO
•
2
•
3
hydroxyl radical), HO (hydroperoxy radical), and HO (hy-
0
drogen trioxide radical), also act as key intermediates in
reactions in acidic solutions or protonic solvents. In actual
9
-13
3
immobilized in a-SiO2 below ∼100 K.
•
14
• 15
•
x
16,17
catalytic reactions, their formation and reactivity are controlled
by various catalysts such as metals, metal oxides, zeolites, metal
complexes, and their hybrids. Among these, catalytic activity
of amorphous SiO2 (a-SiO2), which is often regarded as inert,
has attracted particular interest due to its thermal and chemical
stabilities. It is considered that the main origin of the catalytic
activity of a-SiO2 is nano- and mesopores, which are formed
intentionally and are abundantly present in mesoporous silicas
2
18
4-8
4
-6
and silica gels.
However, besides these “extrinsic” pores,
In this study, we report a novel process for the formation of
a-SiO2 contains a lot of “intrinsic” interstitial voids in the Si-
interstitial HO• in a-SiO2, which contains both interstitial
2
†
Current address: Institute of Solid State Physics, University of Latvia,
oxygen molecules (O2) and silanol (SiOH) groups. Bulk a-SiO2
was used to selectively observe reactions among interstitial
Kengaraga iela 8, LV1063 Riga, Latvia.
‡
Current address: Materials and Structures Laboratory & Frontier
Collaborative Research Center, Tokyo Institute of Technology, 4259
Nagatsuta, Midori-ku, Yokohama 226-8503, Japan.
(9) Brower, K. L.; Lenahan, P. M.; Dressendorfer, P. V. Appl. Phys. Lett. 1982,
(
(
(
1) Lunsford, J. H. Catal. ReV. 1973, 8, 135-157.
41, 251-253.
2) Che, M.; Tench, A. J. AdV. Catal. 1983, 32, 1-148.
3) B u¨ hler, R. E.; Staehelin, J.; Hoign e´ , H. J. Phys. Chem. 1984, 88, 2560-
(10) Griscom, D. L. J. Non-Cryst. Solids 1984, 68, 301-325.
(11) Miyazaki, T.; Azuma, N.; Fueki, K. J. Am. Ceram. Soc. 1984, 67, 99-
2
564.
102.
(
(
(
(
4) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn.
(12) Tsai, T. E.; Griscom, D. L.; Friebele, E. J. Phys. ReV. B 1989, 40, 6374-
1
990, 63, 988-992.
6380.
5) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S.
Nature 1992, 359, 710-712.
6) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242-
(13) Kajihara, K.; Skuja, L.; Hirano, M.; Hosono, H. Phys. ReV. Lett. 2002, 89,
135507.
(14) Griscom, D. L.; Stapelbroek, M.; Friebele, E. J. J. Chem. Phys. 1983, 78,
1638-1651.
1
244.
7) Radzig, V. A. In Defects in SiO
2
and Related Dielectrics: Science and
(15) Hosono, H.; Abe, Y.; Oyoshi, K.; Tanaka, S. Phys. ReV. B 1991, 43, 11966-
11970.
Technology; Pacchioni, G., Skuja, L., Griscom, D. L., Eds.; NATO Science
Series; Kluwer Academic Publishers: Dordrecht, Netherlands, 2000; pp
(16) Griscom, D. L.; Friebele, E. J. Phys. ReV. B 1986, 34, 7524-7533.
(17) Nishikawa, H.; Nakamura, R.; Ohki, Y.; Nagasawa, K.; Hama, Y. Phys.
ReV. B 1992, 46, 8073-8079.
3
39-370.
(
8) Inaki, Y.; Yoshida, H.; Yoshida, T.; Hattori, T. J. Phys. Chem. B 2002,
1
06, 9098-9106.
(18) Shelby, J. E. J. Appl. Phys. 1977, 48, 3387-3394.
10.1021/ja0571390 CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 5371-5374
9
5371

A R T I C L E S
Kajihara et al.
species while excluding the contributions of surface adsorbates.
0
Interstitial O2 traps mobile H generated by F2-laser (λ ) 157
13,19,20
nm, hν ) 7.9 eV) photolysis of the SiO-H bonds
to create
•
HO ,
2
hν)7.9 eV
•
0
≡SiOH
8 ≡SiO + H
(1)
(2)
0
•
O + H f HO
2
2
In this work, we show that the interstitial voids in a-SiO2 play
•
an important role in forming HO in the above mechanism and
2
•
2
in stabilizing the resultant HO up to 100 °C.
2. Experimental Section
2
Commercial synthetic “wet” a-SiO pieces containing network-bound
2
0
-3
18
-3
SiOH (∼1 × 10 cm ) and SiCl (∼5 × 10 cm ) groups, 10 × 4
3
×
2 mm in size, were used. Heating of the pristine pieces in O
atm) for 720 h at 900 °C yielded O -loaded hydrogenated samples that
2
2
(∼3
1
7
-3
21,22
contain ∼10 cm of interstitial O
of SiOH groups and O
at 7.9 eV in this sample were ∼35 and ∼2
respectively. A separate set of pristine pieces was first treated
(∼3 atm) for 96 h at 900 °C prior to the O loading to obtain
-loaded deuterated samples, whose SiOH groups were completely
∼99%) replaced by SiOD groups. The third set of pristine pieces
was treated in a vacuum for 240 h at 900 °C to prepare O -free
hydrogenated samples for use as references for the O -loaded samples.
The samples were exposed to F -laser pulses (LPF-210, Lambda Physik,
pulse duration ∼20 ns) of ∼2-10 mJ cm pulse either at room
temperature or at 77 K. To avoid F -laser photolysis of interstitial O
into a pair of oxygen atoms (O ), the irradiation was terminated at a
2
.
The absorption coefficients
23
2
-
1 24,25
cm
in D
,
2
2
O
(
2
2
5
2
2
2
-
2
-1
2
2
0
-2
small cumulative fluence (e1 J cm ). The laser-induced paramagnetic
species were measured by an X-band electron paramagnetic resonance
(EPR) spectrometer (Bruker, Model E580) at 77 K.
3. Results
Figure 1a shows the EPR spectra of the O2-free and O2-loaded
hydrogenated samples exposed to F2-laser light at room tem-
-
2
Figure 1. EPR spectra of (a) hydrogenated and (b) deuterated samples
perature with a cumulative fluence of ∼1 J cm . The EPR
-
2
exposed to F2-laser light of ∼1 mJ cm cumulative fluence at room
temperature (recorded at 77 K; microwave power, 20 mW; modulation
amplitude, 0.1 mT; modulation frequency, 100 kHz). A new paramagnetic
species, also referred to as Z in the text, is formed along with NBOHC (0)
and POR (4) in the O2-loaded samples (traces A). In contrast, the only
paramagnetic species detected in the O2-free sample is a small amount of
NBOHC (trace B, magnified by 5 times). Subtraction of the NBOHC and
POR components from trace A yields the signal due to Z, which is assigned
spectrum of the O2-loaded deuterated sample is shown in Figure
1
b. In the O2-free sample, only a very weak signal assigned to
the oxygen dangling bond (nonbridging oxygen hole center,
•
26
NBOHC, ≡SiO ) was observed. In contrast, signals for the
O2-loaded hydrogenated and deuterated samples were much
more intense and complicated. The dominant part of the signals
was due to one or several unidentified paramagnetic species
to interstitial HO• or DO•
(traces C). For the hydrogenated sample, the
2 2
subtracted NBOHC and POR components are shown in the lower part of
panel (a). A simulated spectrum of the hydrogenated sample [trace D in
panel (a)] was calculated using the g and A values listed in Table 1, and
that of the deuterated sample [trace D in panel (b)] was obtained by the
same procedure but employing the nuclear parameters of D instead of H.
(
hereafter denoted as “Z”) overlapping the well-known signals
•
26,27
of NBOHC and the peroxy radical (POR, ≡SiOO ).
To isolate the signal specific to Z, the components of NBOHC
g1 ) 2.0014, g2 ) 2.0103, and g3 ≈ 2.08) and POR (g1 )
(
2
.0017, g2 ) 2.0073, and g3 ) 2.070), whose spectral shapes
were recorded using the techniques shown in refs 13, 20 and
2
8, 29, respectively, were subtracted. Characteristic signals of
(
(
(
(
(
(
(
(
(
19) Kajihara, K.; Skuja, L.; Hirano, M.; Hosono, H. Appl. Phys. Lett. 2001,
7
9, 1757-1759.
NBOHC at ∼345-347 mT, and of POR at ∼334 mT (the g3
peak, not shown in Figure 1) and ∼345 mT, were used to
determine the amplitudes of the subtracted components, which
are shown in Figure 1a for the case of the hydrogenated sample.
The spectrum obtained by the subtraction is referred to as the
“extracted spectrum” (Figure 1, traces C). The positions of the
peaks remaining in the extracted spectrum were insensitive to
small variations in the amplitudes of the subtracted components.
20) Hosono, H.; Kajihara, K.; Suzuki, T.; Ikuta, Y.; Skuja, L.; Hirano, M. Solid
State Commun. 2002, 122, 117-120.
21) Kajihara, K.; Miura, T.; Kamioka, H.; Hirano, M.; Skuja, L.; Hosono, H.
J. Ceram. Soc. Jpn. 2004, 112, 559-562.
22) Kajihara, K.; Kamioka, H.; Hirano, M.; Miura, T.; Skuja, L.; Hosono, H.
J. Appl. Phys. 2005, 98, 013529.
23) Kajihara, K.; Hirano, M.; Uramoto, M.; Morimoto, Y.; Skuja, L.; Hosono,
H. J. Appl. Phys. 2005, 98, 013527.
24) Morimoto, Y.; Nozawa, S.; Hosono, H. Phys. ReV. B 1999, 59, 4066-
4
073.
25) Kajihara, K.; Hirano, M.; Skuja, L.; Hosono, H. Phys. ReV. B 2005, 72,
2
14112.
26) Stapelbroek, M.; Griscom, D. L.; Friebele, E. J.; Sigel, G. H., Jr. J. Non-
Cryst. Solids 1979, 32, 313-326.
(28) Kajihara, K.; Skuja, L.; Hirano, M.; Hosono, H. Phys. ReV. Lett. 2004, 92,
015504.
27) Friebele, E. J.; Griscom, D. L.; Stapelbroek, M.; Weeks, R. A. Phys. ReV.
Lett. 1979, 42, 1346-1349.
(29) Kajihara, K.; Skuja, L.; Hirano, M.; Hosono, H. J. Non-Cryst. Solids 2004,
345 & 346, 219-223.
5372 J. AM. CHEM. SOC.
9
VOL. 128, NO. 16, 2006

Role of Interstitial Voids in Oxides
A R T I C L E S
Table 1. Principal Values of g and Hyperfine (A) Tensors
•
2
a
Determined for HO in a-SiO2
Direction cosines
Direction cosines
g
1
2
3
A (mT)
1
2
3
xx 2.0033 ( 0.001
yy 2.0081 ( 0.001
zz 2.0368 ( 0.001
1
0
0
0
1
0
0
0
1
-1.34 ( 0.2 -1
0
0
-1.44 ( 0.2
-1.26 ( 0.2
0
0
0.105
0.995 -0.105
0.995
a
It was assumed that the z axis of A is along the O-H bond and has an
angular separation of 96° relative to the z axis of g, which is in the molecular
30
plane and lies nearly along the O-O bond. The x axes of g and A were
both taken to be perpendicular to the molecular plane. The signs of the
•
2
principal values of A were determined following the data of gaseous HO ,
reported in refs 37 and 38.
•
0
•
Figure 2. Variations in concentrations of HO , NBOHC, POR, and H
(
those of HO reported in ref 30. Considering that the para-
2
2
inset) in the O2-free (open symbols) and O2-loaded (filled symbols)
hydrogenated samples with isochronal annealing for 10 min at each
temperature. The concentrations of H are normalized to the signal intensity
at 77 K because of the difficulty in evaluating the absolute value due to the
microwave-saturation effect. The paramagnetic centers were created by
magnetic species is randomly oriented in a-SiO2, the extracted
spectrum of the hydrogenated sample (Figure 1a) was simulated
via the least-squares method by optimizing the principal values
of g, A, and the line width tensors, and the Gaussian-Lorentzian
ratio of the pseudo-Voigt peak convolution function. The
optimized principal values of g and A (Table 1), which were
obtained by adopting an almost pure (∼95%) Lorentzian
convolution function, agree well with those determined for
0
-
2
exposure to F2-laser light at 77 K (cumulative fluence, ∼0.2 mJ cm ).
The thermal stability of paramagnetic species was examined
by isochronal pulse annealing of the O2-loaded hydrogenated
sample irradiated with F2-laser light (cumulative fluence, ∼0.2
HO• trapped in ice
3
0-34
35,36
and inert gas matrixes.
The
-
2
2
J cm ) at 77 K, for 10 min up to 673 K (Figure 2). A similar
sequence was applied to the O2-free hydrogenated sample. EPR
spectra, recorded at 77 K after each irradiation or annealing
step, were deconvoluted following the procedure shown in
Figure 1a, to evaluate the concentrations of NBOHC, POR, Z,
resultant parameters were then used to generate the signal of
the deuterated species by merely accounting for the differences
between H and D (nuclear spin, IH ) 1/2, ID ) 1; AH ) 6.5144
AD), resulting in a good agreement between the extracted and
simulated spectra (Figure 1b). Thus, we conclude that the
unidentified species is interstitial HO .
0
and H . The temperature dependences of these are summarized
in Figure 2. Because of a distinct microwave saturation effect,
•
2
This assignment was further verified by the magnitudes of
the isotropic (aiso) and anisotropic (T) parts of A,39 calculated
0
the concentration of H (sharp doublets at ∼318 and 368 mT)
was determined in relative units only, normalized to the initial
signal intensity at 77 K.
as
In both the O2-free and O2-loaded samples, the main
paramagnetic centers created at 77 K were NBOHC and H ,
0
.01
0
A ) a 1 + T ) -1.35 × 1 +
-0.09
(3)
iso
|
|
and they were progressively annihilated with an increase in the
annealing temperature. However, in the O2-loaded sample,
NBOHC disappeared at a much higher temperature (∼600 K),
0
.09
where 1 is the 3 × 3 unit matrix. aiso has a negative sign, and
its absolute value is ∼3% of the isotropic hyperfine constant
of a free H atom (50.7 mT), but it is larger than the components
of the anisotropic part T. These results accord well with the
characteristic features of HO , namely that the unpaired elec-
tron is located on the π* orbital of the O-O bond and that spin
polarization is the dominant origin of the hyperfine interaction.
The EPR line width of HO in a-SiO2 is narrower than that
0
and the new signal Z appeared along with the decay of H . In
addition, POR was found to be formed above 200 K, suggesting
that interstitial O2 is partially photolyzed at 77 K, and a part of
0
•
2
the resultant O becomes mobile above 200 K, turning NBOHC
2
8
into POR. Thermal decomposition of paramagnetic species
in the O2-loaded sample started above 400 K and was almost
complete at 700 K.
39
•
2
3
0-34
in H2O ice,
allowing the hyperfine splitting resulting from
the H nucleus to be well resolved. This is due to the absence of
4
. Discussion
The shape of the “extracted spectrum” remains the same
during the change in the intensity in the isochronal annealing
experiment, indicating that the extracted spectrum most likely
(
30) Bednarek, J.; Plonka, A. J. Chem. Soc., Faraday Trans. 1 1987, 83, 3737-
3747.
(
(
31) Smith, R. C.; Wyard, S. J. Nature 1960, 186, 226-228.
32) Wyard, S. J.; Smith, R. C.; Adrian, F. J. J. Chem. Phys. 1968, 49, 2780-
2783.
belongs to a single type of paramagnetic species Z. There is
•
2
(33) Bednarek, J.; Plonka, A.; Hallbrucker, A.; Mayer, E.; Symons, M. C. R. J.
Am. Chem. Soc. 1996, 118, 9387-9390.
(
(
(
strong evidence that Z is HO generated via eq 2. First, Z is
34) Mihelcic, D.; Volz-Thomas, A.; P a¨ tz, H. W.; Kley, D.; Mihelcic, M. J.
observed only in the O2-loaded samples. Second, the distinct
difference in the extracted spectra between the hydrogenated
and deuterated samples indicates the presence of hydrogen atoms
that cause a large hyperfine interaction. Third, the formation of
Atmos. Chem. 1990, 11, 271-297.
35) Adrian, F. J.; Cochran, E. L.; Bowers, V. A. J. Chem. Phys. 1967, 47,
5441-5442.
36) Norizawa, K.; Hirai, M.; Kanosue, K.; Ikeya, M. Jpn. J. Appl. Phys. 2000,
39, 6759-6762.
0
(37) Saito, S. J. Mol. Spectrosc. 1977, 65, 229-238.
(
Z is accompanied by the decay of interstitial H . To confirm
38) Barnes, C. E.; Brown, J. M.; Carrington, A.; Pinkstone, J.; Sears, T. J.;
Thistlethwaite, P. J. J. Mol. Spectrosc. 1978, 72, 86-101.
39) Weil, J. A.; Bolton, J. R.; Wertz, J. E. Electron Paramagnetic Resonance:
Elementary Theory and Practical Applications; John Wiley & Sons: New
York, 1994.
this supposition, line-shape simulation of the extracted spectra
was performed. The mutual orientations of principal axes of g
and hyperfine (A) tensors listed in Table 1 were set identical to
(
J. AM. CHEM. SOC.
9
VOL. 128, NO. 16, 2006 5373

A R T I C L E S
Kajihara et al.
•
2
another triple collision reaction in the gas phase,47,48
superhyperfine interaction between HO and the surrounding
matrix. Analogous spectra with reduced line widths have been
0
0
•
34
36
H + H + M f H
2
+ M
(5)
observed for HO in D2O ice and CO2.
2
In “wet” O2-free a-SiO2 exposed to F2-laser light, a large
should take place by an analogous mechanism in a-SiO2.
Although the numerical simulation generally reproduced the
peak positions of the experimental EPR spectra well, some
discrepancies still remain in the spectral shape and the relative
peak amplitudes (Figure 1). This is probably due to the thermal
amount of NBOHC is transiently formed by efficient (quantum
yield > 0.1) cleavage of SiO-H bonds.1
3,19,20
The resultant
NBOHC quickly decays at room temperature through recom-
0
bination with the photogenerated mobile H and H2, leaving
only a small amount of persistent NBOHC.1
9,13
In contrast, in
2
motion of paramagnetic species, which makes the simulation
the O2-loaded sample, a large part of NBOHC remains unreacted
at room temperature (Figures 1 and 2) as a result of the
using a static spin-Hamiltonian difficult, particularly when the
motion is anisotropic. A reason for such anisotropic motion
4
9
0
competitive trapping of mobile H by O2 (eq 2), preventing
is the steric hindrance. The interstitial voids in a-SiO would
2
0
•
2
recombination of H with NBOHC. The trapping is efficient,
be too small for HO to rotate unhindered, considering that O ,
whose size and shape are similar to those of HO , does not
2
0
•
2
because the cumulative concentration of H induced at 77 K
1
6
-3 13,19
under a similar irradiation fluence (∼2 × 10 cm
) is
rotate freely about the axes perpendicular to the O-O bond in
•
2
5
0
roughly equal to the concentration of HO subsequently formed
a-SiO . Another possible factor restricting the free motion of
2
16
-3
•
2
above 150 K (∼1 × 10 cm ) in the annealing experiment
32,33,45
HO is hydrogen bonding,
which can take place with the
(
Figure 2). A study of the quenching of singlet photolumines-
bridging oxygen atoms and SiOH groups in the a-SiO2 network.
cence of O2 by SiOH groups suggests a homogeneous distribu-
tion of O2 and SiOH groups in O2-loaded samples of present
type.40 The average separation between O2 and SiOH groups
An analogous type of hydrogen bonding is suggested for
25
interstitial H2O in a-SiO2. It is considered that hydrogen
•
32,33
bonding suppresses the motion of HO in ice,
whereas its
2
•
35
(
∼1 nm) is sufficiently smaller than the average diffusion length
absence allows HO to rotate in solid Ar even at 4 K. On the
basis of these considerations, we suggest a rocking motion of
2
0
1
2
of H in a-SiO2 (∼1-100 nm in 1 s at 100 K ), supporting the
51
•
•
2
formation of HO via eq 2.
2
HO in a-SiO2 at 77 K.
•
2
0
The formation of HO through the reaction of O2 with H
41-43
5. Conclusions
has been intensively studied in the gas phase.
The reaction
is very slow when the participation of an inert gas molecule M
that takes the excess energy away from the reaction intermediate
•
The hydroperoxy radical (HO ) was synthesized in bulk
2
amorphous SiO2 (a-SiO2), and its properties were studied by
electron paramagnetic resonance (EPR). Interstitial O2 in a-SiO2
is efficiently converted into HO through reaction with a
mobile hydrogen atom (H ), which is formed by F2-laser
(triple collision),
•
0
2
O + H + M f HO + M
(4)
2
2
0
•
photolysis of SiOH groups. It is likely that the formation of
is absent. However, HO is efficiently formed in a-SiO2. Thus,
2
•
HO is promoted by collisions of the reaction intermediate
2
it seems likely that the dissipation of the excess energy of the
reaction intermediate through its collisions with the a-SiO2
network plays a vital role in the creation of HO in interstitial
voids in a-SiO2. The presence of such energy redistribution
processes is well-known for reactions in solid gas matrixes,
including the formation of HO .
that the same reaction occurs near room temperature in a-SiO2,
with the a-SiO2 network that dissipates the excess energy of
•
2
•
2
the intermediate. The resultant HO is stable against thermal
annealing up to 100 °C, due to its encapsulation in the
chemically inert a-SiO2 network. It was demonstrated that bulk
SiO2 provides nanospaces appropriate for photomanipulation of
small reactive compounds.
4
4
•
2
45,46
However, it is interesting
a covalent macromolecule with a three-dimensional tetrahedral
Acknowledgment. We thank Dr. S. Matsuishi of Tokyo
Institute of Technology for useful discussions.
0
10,12,13
network. The dimerization of H in a-SiO2,
which is
JA0571390
(
40) Kajihara, K.; Kamioka, H.; Hirano, M.; Miura, T.; Skuja, L.; Hosono, H.
J. Appl. Phys. 2005, 98, 013528.
(
(
(
41) Kurylo, M. J. J. Phys. Chem. 1972, 76, 3518-3526.
42) Lee, L. C. J. Chem. Phys. 1982, 76, 4909-4915.
43) Schwab, J. J.; Brune, W. H.; Anderson, J. G. J. Phys. Chem. 1989, 93,
(47) Larkin, F. S. Can. J. Chem. 1968, 46, 1005.
(48) Trainor, D. W.; Ham, D. O.; Kaufman, F. J. Chem. Phys. 1973, 58, 4599-
4609.
1
030-1035.
44) Himmel, H.-J.; Downs, A. J.; Greene, T. M. Chem. ReV. 2002, 102, 4191-
241.
(49) Budil, D. E.; Lee, S.; Saxena, S.; Freed, J. H. J. Magn. Reson. 1996, A120,
155-189.
(
4
(50) Skuja, L.; G u¨ ttler, B.; Schiel, D.; Silin, A. R. J. Appl. Phys. 1998, 83,
6106-6110.
(
45) Nelander, B. J. Phys. Chem. A 1997, 101, 9092-9096.
46) Khriachtchev, L.; Pettersson, M.; Lignell, A.; Rasanen, M. J. Am. Chem.
Soc. 2001, 123, 8610-8611.
(
(51) Matsuishi, S.; Hayashi, K.; Hirano, M.; Tanaka, I.; Hosono, H. J. Phys.
Chem. B 2004, 108, 18557-18568.
5374 J. AM. CHEM. SOC.
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