J. Phys. Chem. C 2009, 113, 6577–6583
6577
Probing Porosity and Pore Interconnectivity in Crystalline Mesoporous TiO2 Using
Hyperpolarized 129Xe NMR
Li-Qiong Wang,* Donghai Wang, Jun Liu, and Gregory J. Exarhos
Fundamental Science DiVision, Pacific Northwest National Laboratory, Richland, Washington 99354
Shane Pawsey and Igor Moudrakovski
Steacie Institute for Molecular Sciences, National Research Council Canada, 100 Sussex DriVe, Ottawa,
Ontario, Canada K1A 0R6
ReceiVed: NoVember 4, 2008; ReVised Manuscript ReceiVed: February 24, 2009
Hyperpolarized (HP) 129Xe NMR was used to probe the porosity and interconnectivity of pores in crystalline
mesoporous TiO2. We have demonstrated that HP 129Xe NMR can be used to differentiate between similar
sized pores within different crystalline phases. Pores of 4 nm size resident in mixed anatase and rutile
mesoporous TiO2 phases were identified. Complementary to other pore characterization techniques, HP 129Xe
NMR is able to probe the interconnectivity between pores present in these different phases. The cross peaks
in 2D exchange (EXSY) NMR spectra between the signals of xenon in two types of pores are visible on
millisecond timescale, indicating substantial pore interconnectivity. The obtained information on porosity
and interconnectivity is important for the understanding of ion transport mechanisms in mesoporous TiO2
anode materials.
Introduction
The extent of interconnectivity between nano- or mesopore
domains markedly influences transport properties of porous
materials. In mesoporous materials, both pore size and interconnectivity greatly affect the molecular and ionic transport
properties of materials. These include molecular access and
selectivity, separation and reaction kinetics in catalysis, ion
diffusion for energy storage in batteries and supercapacitors,
adsorption capacity and kinetics of release in hydrogen storage
materials, and proton and oxygen diffusion across fuel cell
membranes. The pore geometry in most porous materials, even
in ordered mesoporous silica, is complex and characterized by
interconnected cages, channels, and micropores.1 As a result of
these complex topographies, characterization of interconnectivity
of the pores in such nano- or mesoporous materials is often
challenging and mandates several methods. The most common
techniques such as small-angle X-ray or neutron scattering, and
gas absorption, however, do not provide direct information on
how channels and cages are connected.
Over the years 129Xe NMR has developed into a powerful
and robust method for studying porous solids.2 The large
chemical shift range of 129Xe is strongly dependent on both local
environmental and chemical factors such as the composition of
the matrix, nature and concentration of coadsorbed molecules,
and the shape and size of resident void spaces.2-5 The use of
optical pumping approaches for the production of hyperpolarized
(HP) xenon5 allows for a dramatic increase in the sensitivity of
129
Xe NMR up to a factor of 104, enabling these pore
characterization studies. Using HP xenon produced under
continuous flow (CF) conditions, measurements are possible at
very low concentrations of xenon, which minimizes the
contribution from Xe-Xe interactions to the observed chemical
shift. As such, the observed 129Xe chemical shifts can be
* Corresponding author. E-mail: [email protected].
assigned principally to interactions between the xenon atoms
and the porous surfaces. Since spin-polarized Xe gas percolates
through the interconnected pores and samples the local pore
environments, HP 129Xe NMR has the unique advantage of
directly probing not only these buried interfaces but also the
interconnectivity between the pores. In this study, temperature
dependent chemical shift spectra and 2D exchange spectroscopy
(EXSY) CF (continuous flow) HP 129Xe NMR were used to
obtain a better understanding of the pore structure, the uniformity
of the adsorption sites, and the pore network interconnectivity
between different adsorption regions.
Crystalline mesoporous TiO2 is an important electronic,
optical, catalytic, and photocatalytic material because of its large
dielectric constant, high surface area, and the presence of ordered
nanosized channels. For example, high-surface-area nanosized
rutile TiO2 is a good candidate for a catalyst support,6 and
recently it has been shown to be a promising anode material in
Li-ion batteries.7,8 However, it is difficult to make highly
crystalline and stable mesoporous TiO2. Our most recent effort
has produced highly crystalline and stable mesoporous TiO2 with
a surface area up to 300 m2/g and tunable pore sizes from 2 to
4 nm.9 Such tunable, high-surface-area mesoporous rutile can
be obtained using a surfactant-directed processing approach.10-13
Unlike the more traditional mesoporous TiO2 materials having
semicrystalline or nanocrystalline walls,14-21 the new materials
are composed of extended, locally aligned nanorod-like building
blocks.
This paper presents a study of porosity and interconnectivity
in highly crystalline mesoporous TiO2 using HP 129Xe NMR.
Our HP 129Xe NMR measurements allow differentiation between
similar sized pores within different crystalline phases. Complementing other pore characterization methods, the technique
provides insight into interconnectivity between pores constrained
10.1021/jp809740e CCC: $40.75  2009 American Chemical Society
Published on Web 03/30/2009
6578 J. Phys. Chem. C, Vol. 113, No. 16, 2009
Wang et al.
Figure 2. Schematic drawing of mesoporous channels made of rutile
TiO2 nanorods based on the high resolution TEM data. The anatase
particles aggregated at the ends of the rods are not shown for clarity.
Figure 1. (a) TEM image of aggregated spherical anatase particles
outside of nanorod-based mesoporous rutile. (b) SAED pattern from
the oriented nanorod-based mesoporous rutile. (c) SAED pattern from
the spherical nanoparticle-based mesoporous anatase.
to different phases. Such information is critical for the understanding of ion transport mechanisms in mesoporous TiO2 anode
materials.
Experimental Section
Materials and Physical Properties. Three types of TiO2
materials were used in 129Xe NMR measurements. Two commercially available TiO2 powders, anatase TiO2 (10 nm particle
diameter) and rutile TiO2 (0.9-1.6 µm particle diameter), were
purchased from Alfa Aesar. Mesoporous crystalline TiO2 was
prepared according to previous report9 by one-step lowtemperature crystallization. In a typical synthesis, 1.2 mL of
0.5 M sodium dodecyl sulfate (C12H25SO4Na) solution (0.6
mmol) was added into 10 mL of 0.12 M TiCl3 solution (1.2
mmol) under vigorous stirring. Subsequently 0.8 mL of 1.0 wt%
H2O2 solution (0.26 mmol) was added dropwise under vigorous
stirring. The mixture was then diluted to a total volume of 55
mL by adding deionized water and further stirred in a polypropylene flask at 60 °C for 15 h. The precipitates were separated
by centrifuge followed by washing with deionized water and
ethanol, and the product was then dried in a vacuum oven at
60 °C overnight and subsequently calcined in static air at 400
°C for 2 h.
Physical properties of the mesoporous TiO2 including crystalline phase, nanostructure, surface area, and pore size were
investigated and summarized below. The low-angle X-ray
diffraction pattern of the mesoporous TiO2 displays a reflection
peak at d-spacing of 7.4 nm, indicative of short-range mesoscale
ordering. High angle XRD patterns confirm that as-synthesized
materials contain crystalline rutile TiO2 with a small amount
of anatase TiO2. Results from nitrogen sorption measurements
show type-IV isotherms with a rather narrow size distribution
ofmesopores.TheadsorptiondataindicateBrunauer-Emmett-Teller
(BET) surface areas of 300 m2/g for the mesoporous TiO2 after
calcination. The average pore size in the calcined mesoporous
TiO2 calculated using the Barrett-Joynes-Halenda (BJH)
model is 3.1 nm. The transmission electron microscopy (TEM)
image given in Figure 1 displays the rod-like rutile nanocrystals
oriented in parallel and interspaced by mesoporous channels
along with the aggregates of spherical anatase nanoparticles
outside the nanorod-based mesoporous rutile. Selected area
electron diffraction (SAED) patterns (Figure 1b and 1c) further
confirm the rutile crystal structures for the oriented rod-like
nanocrystals and anatase structures for the spherical nanoparticle
aggregates. In agreement with the high angle XRD patterns, a
series of high resolution TEM data show that the oriented rutile
TiO2 nanorods are the dominant structures in mesoporous TiO2
along with a smaller amount of spherical anatase nanoparticles.
A schematic drawing of the mesoporous channels composed
of rutile TiO2 nanorods based on the high resolution TEM data
is given in Figure 2. Here, clearly some nanocrystals become
fused together, and the space between the rods can be observed
as cylindrical channels.
HP129Xe NMR Measurements. 129Xe NMR measurements
were performed on a Bruker DSX-400 instrument operating at
110.6 MHz (magnetic field of 9.4 T) using a continuous flow
(CF) of HP xenon. A Bruker wide-line probe modified to
accommodate the CF system was employed for HP flow
experiments. The continuous flow polarizer for the production
of HP 129Xe was of a design similar to that previously
reported.22,23 An 80 W continuous wave diode laser from
Coherent operating at a wavelength of 795 nm was used as the
excitation source. A xenon-helium-nitrogen mixture with a
volume composition of 1%-96%-3% was used in all CF HP
experiments. The flow rate was monitored with a Vacuum
General flow controller (Model 80-4) and was kept constant in
the range of 200-250 scc/min (gas flow normalized to standard
conditions). In the CF HP experiments, the HP xenon flow was
delivered directly from the polarizer to the coil region of the
NMR probe through 1.5 mm ID plastic tubing. Variable
temperature NMR experiments in the 170-400 K range were
performed using a BVT3000 temperature controller. The temperature inside the NMR coil of the CF probe was calibrated
using the 207Pb resonance in Pb(NO3)2.24 2D-EXSY spectra were
obtained using the standard sequence described elsewhere.25-30
All 129Xe NMR chemical shifts were referenced to xenon gas
extrapolated to zero pressure (0 ppm).
Results and Discussion
Hyperpolarized 129Xe NMR Spectra. Variable temperature
continuous flow HP 129Xe spectra for mesoporous TiO2 are given
in Figure 3 where the top spectrum was recorded at 173 K; the
temperature was raised in 20 K increments up to 353 K. Because
of increased adsorption at lower temperature, a larger signal
for Xe adsorbed on the pore surfaces was observed as compared
Crystalline Mesoporous TiO2
J. Phys. Chem. C, Vol. 113, No. 16, 2009 6579
Figure 4. Variable temperature continuous flow HP 129Xe spectra for
rutile TiO2 micron particles. The top spectrum was recorded at 173 K,
and the temperature was raised 20 K for each subsequent spectrum up
to 353 K.
Figure 3. Variable temperature continuous flow HP 129Xe spectra for
mesoporous TiO2. The top spectrum was recorded at 173 K, and the
temperature was raised 20 K for each subsequent spectrum up to 353
K.
to the signal associated with the void space. The slower
exchange between the gas phase and adsorbed Xe at reduced
temperatures results in larger observed chemical shifts. Figure
3 shows two resonance peaks for adsorbed xenon at temperatures
from 233 to 353 K. As the temperature decreases, the chemical
shifts of both resonances increase but at different rates. The
signals collapse into one peak at temperatures below 233 K and
finally appear at 173 K as a single asymmetric peak near 110
ppm. At 293 K, the resonance peak with a larger area is found
at 45 ppm whereas a comparatively sharper peak but with a
smaller peak area is observed at 60 ppm. The coexistence of
two distinct peaks at temperatures above 233 K suggests there
are at least two different pore environments in mesoporous TiO2
in which Xe can adsorb.
For comparison, variable temperature CF HP 129Xe NMR
measurements were also taken for two commercially available
TiO2 samples, and their spectra are shown in Figures 4 and 5
for TiO2 rutile micron-sized particles and anatase nanoparticles,
respectively. The CF HP 129Xe spectrum taken at RT for TiO2
rutile micron size (0.9-1.6 µm) particles shows a peak for
adsorbed xenon at near 10 ppm and a free gas peak at 0 ppm.
TiO2 rutile micron-sized particles are known to be a nonporous
form of TiO2, which is confirmed in the 293 K spectrum where
no signal is observed at high chemical shift. The relatively low
chemical shift resonance at 10 ppm is most likely associated
with the interparticle spacing or the shallow pores present in
these micron-sized TiO2 rutile particles. At temperatures below
233 K, multiple peaks, although the intensity is very small, are
observed. Clearly the exchange between adsorbed xenon and
the free gas becomes slower as the chemical shift associated
with adsorbed Xe moves to a higher ppm value. As well, the
exchange between adsorption sites also slows to the point that
xenon does not sample all available sites on the time scale of
the NMR experiment. Different types of large diameter void
spaces such as small surface indentations or shallow largemouthed cave structures in micron-sized rutile TiO2 for xenon
to adsorb may be responsible for the observation of multiple
resonances at low temperature.
Figure 5. Variable temperature continuous flow HP 129Xe spectra for
anatase TiO2 nanoparticles. The top spectrum was recorded at 173 K,
and the temperature was raised 20 K for each subsequent spectrum up
to 353 K.
The CF HP 129Xe spectrum at 293 K for anatase nanoparticles
(Figure 5) shows a strong peak near 67 ppm with a pronounced
shoulder on the lower chemical shift side unlike the two distinct
resonances observed in Figure 3 for mesoporous TiO2. As
compared with the 10 ppm peak observed for micron-sized TiO2
rutile particles at 293 K (Figure 4), a much higher chemical
shift peak of 67 ppm (Figure 5) is detected for TiO2 anatase
nanoparticles, indicating the existence of nano or meso void
spaces in TiO2 nanoparticle samples. Since each individual
nanoparticle is made of nonporous, dense, solid crystalline
anatase, the nanoporous spaces most likely come from the
internanoparticle spacing because of the agglomeration of these
small (and dense) TiO2 nanocrystallites. The pore size is
estimated to be about 3.3 nm based on the diameter of 10 nm
for spherical TiO2 anatase nanoparticles, assuming an ideal tight
packing. The breadth of the resonance signal is an indication
of nonuniformity in pore size. Previous work by the Pines group
did not observe such nanoporosity most likely due to the much
larger anatase particles used in their study.27 The spectrum taken
at 173 K in Figure 5 displays a very large and broad peak
centered at 115 ppm. At reduced temperatures, Xe exchange
between different pore sizes is slow, resulting in broad resonance
peaks that are associated with nonuniformity of the pores formed
6580 J. Phys. Chem. C, Vol. 113, No. 16, 2009
Wang et al.
Figure 6. Plots of continuous flow HP 129Xe chemical shift for the
adsorbed xenon resonances as a function of temperature for meso-1
and meso-2 in mesoporous TiO2, anatase TiO2 nanoparticles, and rutile
TiO2 micron particles. The dashed lines are a visual guide only.
from nanoparticles of difference sizes. As the temperature
increases, the peak moves to lower chemical shifts and becomes
narrower because of the faster exchange with free Xe gas.
Heats of Adsorption and Pore Sizes. Figure 6 displays the
temperature dependence of the adsorbed 129Xe chemical shifts
for all three TiO2 samples. The temperature dependent adsorbed
129
Xe chemical shifts for anatase nanoparticles and mesoporous
TiO2 are similar but different from those for the rutile phase. It
is understandable that the nanometer scale void spaces in both
anatase nanoparticles and meso TiO2 samples give rise to much
larger chemical shifts for adsorbed xenon than for what is
observed for the nonporous rutile micron sized particles.
Although the chemical shift range for anatase nanoparticles and
mesoporous TiO2 are similar, the temperature dependence of
their chemical shifts at temperatures greater than 300 K is
different.
Temperature-dependent chemical shift data can be used to
obtain physical parameters related to the adsorption properties
of materials. Variations in the 129Xe chemical shifts with
temperature can be fit to extract parameters related to the
adsorption properties using a model based upon Henry’s Law,
as described previously.31 In the fast exchange approximation
with weak adsorption, the temperature dependence of the
observed chemical shifts, δobs for arbitrary pore sizes can be
expressed as:
(
δ ) δs 1 +
B - ∆Hads
e RT
RT
)
-1
,
B)
( )
Vg
SK0
(1)
where Vg is the free volume inside TiO2, T is the temperature,
S is a specific surface area, K0 is the pre-exponential term of
Henry’s constant, R is the universal gas constant, ∆Hads is the
heat of adsorption and δs is the component of the observed 129Xe
chemical shift characteristic of the interaction between xenon
and the surface. Although eq 1 contains three variables, in reality
only two parameters, ∆H and Vg/S, have a pronounced variation
and effect on the fit. The situation is further simplified as the
possible range of δs is very well-defined from the low temperature measurements. Thus a strongly constrained δs effectively
reduces the situation to a two-parameter fit. We also note that
the ranges over which the other variables in the equation can
change are in fact also well constrained. The preliminary
estimates for ∆H have been obtained from adsorption measure-
Figure 7. Plots of 129Xe chemical shift for adsorbed xenon as a function
of temperature at 173 K to 298 K for anatase TiO2 nanoparticles, meso-1
and meso-2 in mesoporous TiO2. The solid dots are experimental data
obtained from CF HP 129Xe NMR spectra recorded at 20 K intervals,
and the solid line is the fit using eq 1.
TABLE 1: The Heats of Adsorption and Characteristic
Chemical Shift for Xenon with the TiO2 Samples along with
the Estimated Pore Diameters Based upon Spherical (Ds)
and Cylindrical (Dc) Models
TiO2 samples
rutile micron-particles
anatase nanoparticles
meso-1
meso-2
a
∆H (kJ/mol) δs (ppm) Ds (nm) Dc (nm)
NA
9.1
9.8
14.2
NA
125
122
112
NA
3.3a
3.8a
5.7
NA
2.2
2.6
3.8a
The best suited models for meso-1 and meso-2.
ments, while the limits on the Vg/S ratio are imposed by the
TEM data. Considering the rather broad temperature range
studied, we believe the results of the fit are sufficiently reliable.
The applicability and limitations of eq 1 for analysis of the
temperature dependence of the 129Xe chemical shift has been
discussed previously.31-33
Since mirco-sized rutile particles are bulk-like crystals, we
only fit the data for mesoporous and nanopaticle TiO2 samples
using the above equation. It would be ideal to have data for
crystalline rutile nanoparticle for comparison; however, they
were not commercially available. Figure 7 displays the fitted
temperature dependent chemical shift curves using eq 1 plotted
along with the original data taken at temperatures from 160 K
to 300 K. Since the high temperature portion of the original
data could not be fit to this model of Henry’s Law, only the
data from 160 K to 300 K was used to estimate heats of
adsorption. The slope of the chemical shift versus temperature
curve for adsorbed xenon in anatase nanoparticle TiO2 at
temperatures from 160 K to 300 K is similar to that for meso-1
but larger than that for meso-2, which may indicate the similar
binding sites for Xe adsorption on meso-1 and anatase nanoparticle TiO2 surfaces (meso-1 and meso-2 represent the pores
in mesoporous TiO2 associated with the 129Xe NMR peak at
higher and lower chemical shifts shown in Figure 3, respectively).
Table 1 lists the ∆Hads and δs values obtained from the fits
of these variable temperature chemical shift curves. The δs
values range from 112 ppm to 125 ppm for anatase nanoparticles
and mesoporous TiO2 samples which are comparable to the 109
ppm previously reported for nonporous anatase TiO2 obtained
using thermally polarized Xe.27 It is not surprising to observe
Crystalline Mesoporous TiO2
J. Phys. Chem. C, Vol. 113, No. 16, 2009 6581
Figure 8. 2D EXSY spectra of HP 129Xe in mesoporous TiO2 under continuous flow conditions at 313 K and mixing times of 5 ms (A) and 15
ms (B). Positions of 1D cross sections of the spectra shown in insets are indicated by the dashed lines.
similar characteristic chemical shifts because δs is characteristic
of the surface chemical composition, and Ti-O chemical
bonding predominates in these samples. However, the small
variance in δs for all samples is due to other subtle differences
such as the level of hydration, reduced oxidation state, or
different TiO2 phases. The estimated ∆Hads values for these
samples are in the range from 9 to16 kJ/mol, which is consistent
with what is generally seen for the physical adsorption of Xe
on solid surfaces and have been observed before in numerous
micro- and mesoporous materials.31,33 Table 1 also shows a
larger heat of adsorption of 14.2 kJ/mol for meso-2 than that
of 9.8 kJ/mol for meso-1. Observation of different slopes in
temperature dependent chemical shifts for Xe adsorbed on
meso-1 and meso-2 surfaces together with the difference in
∆Hads values suggest the existence of two types of binding sites
for Xe. Since both meso-1 and meso-2 are made of TiO2, the
binding sites are largely determined by the geometries of the
pore surfaces. The different geometries of pore surfaces may
arise from the coexistence of different crystalline phases such
as anatase and rutile TiO2. Hence, meso-1 and meso-2 are most
likely associated with the anatase and rutile phases identified
in mesoporous TiO2 samples from the XRD and TEM data.10
The previous results of Monte Carlo simulations and isotherm
analysis demonstrated that the rows of coordinatively unsaturated Ti4+ ions on (110) rutile plane are the places of preferential
xenon adsorption, especially at low xenon loadings,34 indicating
that the surface geometry influences the binding strength of Xe
on surfaces.
Two distinct chemical shifts observed for meso-1 and meso-2
in Figure 3 indicate that there are at least two different types of
pores or regions in mesoporous TiO2. The 129Xe chemical shift
is sensitive to its chemical composition, pore geometries, and
sizes inside the porous materials. Since the XRD and TEM show
the same TiO2 composition across the bulk sample of this
material, the difference in chemical shift data for meso-1 and
meso-2 are most likely due to the difference in geometries and/
or sizes of the pores. From the different heat of adsorption data
and the slopes of temperature dependent chemical shift data,
the surface geometries for meso-1 and meso-2 are assumed to
be different as discussed earlier. If this is the case, the large
difference in chemical shift is most likely as a result of the
different geometries of the pore surfaces in meso-1 and meso-2
if the pore sizes for both meso-1 and meso-2 are similar. There
are many possibilities for the arrangement of meso-1 and meso-2
pores. One possibility is a random distribution of pore geom-
etries and/or sizes throughout the mesoporous TiO2. However,
this would lead to only one broad peak characterizing this
distribution. Another possibility as suggested from a previous
129
Xe NMR study on porous silicon35 is the configuration of
having larger pores (conical shaped) and smaller channels
running perpendicular to the larger pores. On the basis of the
difference in chemical shift of 45 ppm and 61 ppm at 293 K
for the mesoporous TiO2, the diameter of the larger pores would
be about twice the size of the smaller channels.32 This geometry
requires xenon gas first to migrate from the gas phase to a pore
and then from the pore to a channel rather than directly
exchanging between the gas phase and the channels. If this is
the case, the majority of mesoporous channels of nanorods
observed in our high resolution TEM data would correspond to
the larger pores and there are smaller channels aligned perpendicular to these larger pores. However, this configuration is not
possible in our case. The large pores would be the mesoporous
channels made of fused highly crystalline TiO2 nanorod bundles
that are well oriented in one direction as shown in the schematic
drawing in Figure 2. Even if these bundles cross over each other
occasionally, these is no reason to expect the formation of the
much smaller channels that are perpendicular to the main
channels made of the same nanorods. In addition, the high
resolution TEM data did not show any evidence to support such
a configuration. Hence, the difference in chemical shift HP 129Xe
NMR data for meso-1 and meso-2 is most likely due to the
different crystalline phases but not the large difference in pore
sizes, in agreement with the temperature dependent chemical
shift data and the heat of adsorption as discussed earlier.
TEM and XRD data10 have provided evidence that the
majority of the mesopores in TiO2 particles are made from the
orientation of crystalline rutile TiO2 nanorods with a small
amount of anatase TiO2 particles aggregated at the ends or on
the surface of a bundle of rutile nanorods (Figure 1). It was
also found from the 129Xe NMR spectra that the peak area for
meso-2 in Figure 3 at 293 K is significantly larger than that of
meso-1 pores. Therefore, based on the 129Xe NMR data
supported by TEM and XRD data it is reasonable to assign the
meso-2 to the mesoporous channels comprised of rutile TiO2
nanorods and the meso-1 to the mesopores formed from the
aggregated anatase nanoparticles.
The non-Henry’s Law behavior in temperature dependent
chemical shift data at high temperature for mesoporous TiO2
(Figure 6) is most likely attributed to the existence of a small
number of strong adsorption centers in both types of pores in
6582 J. Phys. Chem. C, Vol. 113, No. 16, 2009
mesoporous TiO2. These centers could be surface defect sites
such as Ti3+ which are associated with oxygen vacancies,36 and
these sites have a pronounced effect on the Xe adsorption
behavior when the adsorbed Xe concentration is low at
temperatures above 300 K.
Using empirical chemical shift-pore size correlations (eq 9
from ref 31) developed to fit inorganic systems such as MCMs
and zeolites,31,32 pore diameter estimates of the TiO2 samples
were attempted using a geometrical model. The results derived
in the context of spherical and cylindrical models reported in
Table 1 are to be treated as approximations. The pore sizes
should be regarded with caution, as there may be an unaccounted
scaling factor due to differences in the chemical composition.
However, we believe that the result on the comparison of two
types of pores is sufficiently reliable. For a given model, the
resonance with a larger chemical shift often corresponds to a
smaller pore size. Since the interparticle spacing is representative
of more spherical pores than cylindrical pores, it is reasonable
to use the spherical pore model to estimate the pore sizes for
all pores formed from the aggregation of the nanoparticles.
However, the cylindrical pore model is best suited for the
cylindrical mesoporous channels observed in rutile TiO2 by
TEM. Using appropriate models, both rutile and anatase pores
in meso TiO2 have similar sizes as shown in Table 1. Although
the chemical shift for meso-1 is larger than that for meso-2, we
obtained similar pore sizes because of the difference in pore
geometries. The pore diameter of 3-4 nm for meso-TiO2
determined from the 129Xe NMR data is in agreement with the
TEM and BET results.10 Unlike BET, 129Xe NMR allows us to
identify pores of similar size in different phases of TiO2.
Interconnectivity of the Pores Evaluated by 2D EXSY
129
Xe NMR. 2D exchange spectroscopy (EXSY) probes exchange and provides insight into the interconnectivity between
different adsorption sites and pores. The 2D EXSY experiments
indicate the presence of exchange, possible exchange pathways,
and the relative time scale of the process. Exchange between
regions with different chemical shifts manifests itself in the
appearance of cross-peaks between the signals from the sites
in exchange. For the sites without exchange on the time scale
of the experiment, signal intensity will only be observed on the
main diagonal of the spectrum. When performed in equilibrium
conditions, the 2D EXSY is a quantitative experiment, allowing
estimation the exchange rates. This was previously demonstrated
with thermally polarized Xe in zeolites and other materials.27-30
The main practical drawback of performing the experiment
under equilibrium conditions is the long acquisition time due
to low concentrations and long relaxation time. Performing the
2D EXSY with HP Xe alleviates the problem of sensitivity.
However, since the experiment is now done in nonequilibrium
CF conditions, its quantification becomes substantially more
difficult. It was demonstrated recently that although the quantification of the exchange rates using 2D continuous-flow HP
129
Xe NMR is feasible, it requires developing an accurate kinetic
model to account for flow conditions of the polarized gas.37
Quantification of the experiment would normally require accurate control of the flow and high stability in the level of
polarization throughout the experiments. Although in our
experiments those parameters were routinely monitored, no
special control has been performed, and the obtained results
can be interpreted only in qualitative terms.
2D EXSY spectra for the meso TiO2 sample have been
obtained at 313 K where the two adsorbed peaks are well
separated. Figure 8 displays the 2D EXSY spectra taken with
mixing times of 5 and 15 ms. The cross peaks in the spectra
Wang et al.
show that the exchange takes place between both adsorption
sites and the free gas. As one can see, the exchange between
different phases through the free gas is prominent only at the
longer mixing time. At the shorter mixing time, however, the
exchange between the phases of mesoporous TiO2 is more likely
to proceed directly. The spectrum obtained at 5 ms mixing time
shows clearly that Xe from the gas goes first into the phase
with the smaller pores (channels), exchanging further with the
larger sized pores. The extensive exchange between the two
types of pores at mixing times as short as 5 ms indicates that
these two types of pores are well connected, similar to what
was observed previously in RFA aerogels with open, well
connected pores.38
Conclusions
Hyperpolarized (HP) 129Xe NMR was used for the first time
to obtain information on porosity and interconnectivity of pores
for highly crystalline mesoporous TiO2 networks. In this work
it has been shown that HP 129Xe NMR measurements allow
differentiation between similar sized pores within different
crystalline phases as seen in Table 1. Pores of 4 nm size were
identified in mesoporous TiO2. The values of the heat of
adsorption for anatase and rutile pores are 9.8 and 14.2 kJ/mol,
respectively. Complementary to other pore characterization
methods (such as BET), we are able to probe interconnectivity
between pores constrained to different phases. The cross peaks
in 2D exchange (EXSY) NMR spectra show exchange takes
place between both types of pores and the free gas with a short
mixing time of 5 ms, indicating that these two types of pores
are well connected. Such information on porosity and interconnectivity is critical for understanding ion transport mechanisms
in mesoporous TiO2 anode materials.
Acknowledgment. HP 129Xe NMR work was supported by
Materials Sciences and Engineering Division, Office of Basic
Energy Sciences, U.S. Department of Energy (US DOE). The
synthesis effort was conducted under the Laboratory Directed
Research and Development Program (LDRD) at Pacific Northwest National Laboratory (PNNL). TEM investigation was
performed in the EMSL, a national scientific user facility
sponsored by the Department of Energy’s Office of Biological
and Environmental Research and located at Pacific Northwest
National Laboratory. PNNL is a multiprogram national laboratory operated for the USDOE by Battelle Memorial Institute
under Contract DE-AC06-76RL0 1830.
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