Probing Porosity and Pore Interconnectivity in Crystalline Mesoporous

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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

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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

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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

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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

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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. References and Notes (1) Moudrakovski, I. L.; Terskikh, V. V.; Ratcliffe, C. I.; Ripmeester, J. A.; Wang, L.-Q.,; Shin, Y.; Exarhos, G. J. J. Phys. Chem. B. 2002, 106, 5938. (2) Ripmeester, J. A. J. Am. Chem. Soc. 1982, 104, 289. (3) Ito, T.; Fraissard, J. J. Chem. Phys. 1982, 76, 5225. (4) Ratcliffe, C. I. Annu. Rep. NMR Spectrosc. 1998, 36, 124. (5) (a) Grover, B. C. Phys. ReV. Lett. 1978, 40, 391. (b) Happer, W.; Miron, E.; Schaefer, S.; Schreiber, D.; Vn Wingaarden, W. A.; Zeng, X. Phys. ReV. A 1984, 29, 3092. (c) Driehuys, B.; Cates, G. D.; Miron, E.; Sauer, K.; Walter, D. K.; Happer, W. Appl. Phys. Lett. 1996, 69, 1668. (6) Bosc, F.; Ayral, A.; Keller, N.; Keller, V. Appl. Catal., B 2007, 69, 133. (7) Hu, Y. S.; Kienle, L.; Guo, Y. G.; Maier, J. AdV. Mater. 2006, 18, 1421. (8) Jiang, C. H.; Honma, I.; Kudo, T.; Zhou, H. S. Electrochem. Solid State Lett. 2007, 10, A127. (9) Wang, D. H.; Choi, D. W.; Yang, Z. G.; Viswanathan, V. V.; Nie, Z. M.; Wang, C. M.; Song, Y. J.; Zhang, J. G.; Liu, J. Chem. Mater. 2008, 20, 3435. (10) Li, Y. Z.; Lee, N. H.; Lee, E. G.; Song, J. S.; Kim, S. J. Chem. Phys. Lett. 2004, 389, 124. (11) Luo, H. M.; Wang, C.; Yan, Y. S. Chem. Mater. 2003, 15, 3841.

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