Effect of CO Pressure Gradients on the Photochemistry and Thermal

Sep 20, 2011 - Department of Chemistry, City University of New York, Queens ... within the nanoporous matrix that promote Fe(CO)5 diffusion within the...
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Effect of CO Pressure Gradients on the Photochemistry and Thermal Chemistry of Fe(CO)5 Physisorbed into Porous Vycor Glass D. Shantha Amarasinghe,† Dehipalawage Sunil,‡ and Harry D. Gafney*,† † ‡

Department of Chemistry, City University of New York, Queens College, Flushing, New York 11367, United States Department of Physics, Queensborough Community College, Bayside, New York 11364, United States ABSTRACT: Photolysis of Fe(CO)5 physisorbed into porous Vycor glass creates CO pressure gradients within the nanoporous matrix that promote Fe(CO)5 diffusion within the matrix and change the distribution of the complex within the matrix. Occurring on a time scale of the lifetimes of the sub-carbonyl photoproducts, pressure induced diffusion influences both the chemistry of the sub-carbonyl photoproducts and the subsequent oxidation of the elemental iron. High-resolution X-ray absorption near-edge structure (XANES) and Mossbauer spectra reveal photolysis creates an asymmetric distribution of Fe(III) and elemental iron, Fe0. The distribution of oxidation states within the matrix and the effect of temperature on the distribution indicate two oxidation pathways. Both pathways are complementary with respect to Fe(II), yet in contrast to that found in other silica matrices, neither process leads to the net formation of Fe(II) within this silica matrix.

’ INTRODUCTION Photodegradation of Fe(CO)5 physisorbed onto Corning’s code 7930 porous Vycor glass (PVG) or dried (200 °C), base-catalyzed tetramethoxysilane methanol water (TMOS/MeOH/H2O) xerogels followed by thermal consolidation of the silica matrix yields refractive index patterns capable of guiding, focusing, and diffracting light.1 Refractive index, a complex function of density and polarizability, increases with the amount of iron deposited.1 The initial increase correlates with the increase in the density due to the deposition of iron, whereas subsequent increases occur with little change in sample density implying the change in refractive index derives principally from the change in polarizability of the doped volume relative to that of the bulk consolidated silica.1 In consolidated PVG, the iron exists as individual 10 ( 1 nm diameter particles composed of Fe0 dispersed in Fe2O3 spaced 22 ( 1 nm apart,2 whereas the iron dopant in the xerogel exists as particles of Fe2O3 e1 nm in diameter with no indication of Fe0.3 Since the particle spacing is less than the wavelength of UV and visible light, the light traversing the doped volume in either matrix is thought to experience an average polarizability of that of the iron dopant and the immediate surrounding silica matrix which differs from that of the bulk silica matrix. Consolidation of both matrices at 1200 °C, necessary to minimize scattering by the initially porous silica matrix, produces essentially identical declines in transparency with the dopant absorption (50% T) shifting from the 300 to 400 nm region after photolysis to the 550 to 650 nm region in the consolidated silica matrix.1 The changes in transparency occur with little or no measurable change in the composition of the dopant in PVG, while ca. 20% the Fe3+ in the xerogel changes from octahedral to r 2011 American Chemical Society

tetrahedral coordination.3 In addition, particle size increases in PVG, but not in the xerogel,3 yet both show essentially equivalent losses in transparency.1 Improving the optical performance of these structures, particularly their transparency and transmissivity rests on a better understanding of the factors that influence the spectroscopic properties of the dopants that create the refractive index gradient in the consolidated glass.1 One avenue of investigation in this laboratory focuses on identifying the physical and chemical characteristics of these amorphous, nanoporous silicas that influence the photoinduced decomposition of the adsorbed carbonyl precursor and the subsequent thermal oxidation of the metal containing photoproducts since it is the latter species that actually create the refractive index gradient in the consolidated glass.1 A comparison of the photodegradation of Fe(CO)5 in PVG and structurally and chemically similar base (NH3) catalyzed, dried (200 °C) tetramethoxysilane/methanol/water (TMOS/MeOH/H2O) xerogels points to a chemistry determined by a competition between aggregation and oxidation.3 If the rate of aggregation of the metal containing photoproduct exceeds the rate of oxidation, the product is a core shell aggregate composed of the zero valent metal surrounded by its oxide which curtails further oxidation of the core. Conversely, a rate of oxidation that exceeds the rate of aggregation leads to the formation of the fully oxidized metal. In a nanoporous silica possessing different lengths of structural order, the relevant parameter with respect to the chemistry of the adsorbate appears to Received: April 7, 2011 Revised: September 11, 2011 Published: September 20, 2011 20824

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The Journal of Physical Chemistry C be the correlation length of the matrix, i.e., the average length of uniform density, or conversely the average distance between structural discontinuities within these amorphous matrices.3 In other words, regardless of the method of impregnation, the initial loading achieved, or the mechanism by which the precursor adsorbs, its chemistry appears to occur within the uniform region defined by the correlation length of these amorphous, nanoporous matrices. The latter provides a rationale for the differences in the chemistry in these structurally and chemically similar amorphous silicas but provides little insight into the details of how aggregation and oxidation occur within nanoporous silicas. Aggregation is related to initial precursor loading and surface coverage. Fe(CO)5 uniformly impregnates PVG to a depth of g1600 μm, but photolysis changes the distribution within the matrix, rapidly increasing the amount of iron in the outer volumes of the sample.1,3 Most metals derived from the photodegradation of physisorbed metal carbonyls, where the metal is formally zero valent, undergo oxidation within these silica matrices,1 yet little information exists regarding the oxidation mechanism or the factors that influence the extent of oxidation. Wilke and co-workers report that the ferric ferrous ratio in silica melts depends on the O2 fugacity.4 Previous experiments in this laboratory reveal the presence of Fe0 and Fe3+ in PVG1,3 but only Fe3+ in dried TMOS/ MeOH/H2O xerogels.5 Ubiquitous in porous silicas, the difference was originally attributed to different amounts of adsorbed water in the two matrices. Subsequent experiments, however, reveal the amount of coadsorbed water evolved from PVG, on a mole basis, exceeds the initial amount of Fe(CO)5 adsorbed implying that water, although present in a variety of forms in porous silica,6 13 is not the limiting variable.3 Furthermore, more recent studies report the presence of Fe2+ in other silica and borosilicate matrices.4,14 18 Formation of Fe2+, in particular, and its incorporation into the aggregates that create the refractive index gradients in the consolidated glasses offer the opportunity for low energy Fe2+ Fe3+ intervalence charge transfer (IVCT) or polaron absorptions which could account for the shifts in the absorption to longer wavelengths and the declines in UV and visible transparency during consolidation of the glass matrix. These experiments were undertaken to gain further insights into the factors that influence the distribution of iron within these nanoporous silicas and the thermal oxidation of the iron containing photoproduct. Data presented here show that both aggregation and oxidation of the iron containing photoproducts in these nanoporous silicates are strongly influenced by CO pressure gradients that develop during photolysis. These pressure gradients rapidly redistribute the physisorbed precursor and, occurring on the time scale of the photoproduct lifetimes, differentiate the photochemical behavior of Fe(CO)5 in nanoporous silicas and in powdered, nonporous silica from that in fluid solution. X-ray absorption near-edge structure (XANES) and Mossbauer spectra, recorded as functions of cross-sectional distribution and temperature, reveal photolysis creates an asymmetric distribution of elemental and oxidized iron within PVG. Heating reduces the asymmetry but fails to completely oxidize all the iron present in the sample. The temperature dependence of the asymmetry points to two oxidation pathways in PVG, and both are influenced by the CO pressure gradients. Nonetheless, in contrast to the oxidation chemistry of iron in other silica matrices,4,14 18 under the conditions examined, oxidation in PVG is limited to the formation of Fe(III) and its aggregation into small Fe2O3 clusters. The absence of Fe(II) in PVG under all the conditions examined suggests the extent of oxidation of iron in silica

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matrices may be a consequence of the pH of the silica surface on which it occurs.

’ EXPERIMENTAL SECTION Materials. Fe(CO)5 (Alfa Aesar) was used as received since electronic spectra of the vapor agreed with published spectra.19 Polished 25.4  25.4  2 mm pieces of Corning’s code 7930 porous Vycor glass were cleaned as previously described. 19 All samples were impregnated by previously described vapor deposition techniques, and comparisons with that data place the loadings at ca. 10 4 mol of Fe(CO)5/g of PVG.19 Photolysis and Heating Procedures. The impregnated samples were irradiated in air with unfiltered (g300 nm) light from an ThermoOriel 1000 W Xe lamp. All samples were examined after photolysis and after heating in a Thermolyne model 46100 programmable high temperature oven. To minimize differences inherent in comparing doped amorphous materials, a number of the photolyzed samples were cut into four parts, the individual parts were subjected to different thermal treatments, and the individual parts were examined after the different post-treatments. Samples photolyzed at room temperature were subsequently heated at a rate of 2 °C/min to 650 °C and maintained at 650 °C for 4 h. At this point, the oven shut off and the samples were left in the oven to cool to room temperature over a period of 24 h. Physical Measurements. Differences in distribution were determined by grinding off the exposed surface of the samples and measuring the remaining thickness at several locations to obtain the average depth of the matrix removed. In these experiments, ca. 30 μm of the outermost, photolyzed surface was removed. XANES and Mossbauer spectra of the remaining sample were then recorded and compared to spectra recorded prior to removal of the outermost 30 μm. Room temperature Mossbauer spectra were recorded on a previously described spectrometer.5 All spectra were recorded with a 10 mC 57Co source and calibrated with a 25 μm thick α-Fe standard (g99%, Amersham). All Mossbauer absorptions were fit with Lorenzian functions using the χ2 test to determine the quality of the fit. Cross-sectional distributions of Fe(CO)5ads prior to photolysis and the iron photoproducts after photolysis were measured with the X-ray fluorescence microprobe facility at the X27A beamline of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. The microfocusing system at the X27A facility reduces the probe beam to achieve a resolution of 10 μm in the vertical direction and 15 μm in the horizontal direction. PVG samples doped with Fe(CO)5 were photolyzed with an opaque mask covering half of the sample to allow a comparison of the iron distribution within the same sample before and after photolysis and after heating the sample. The samples were then broken perpendicular to the edge of the opaque mask, and the iron distributions in the exposed and unexposed regions were measured across the thickness of the samples. XANES spectra were acquired on the X10C beamline at the Brookhaven NSLS. All spectra were recorded in the energyscanning mode with an Ar-filled Stern-Heard-Lytle fluorescence detector and normalized to the incident beam intensity. Samples 10 mm  10 mm of the impregnated glass were mounted at 45° to the axis of the incoming X-ray beam. XANES spectra were collected from 100 to 400 eV above Fe K-edge in 0.2 eV steps in the 7108 7120 eV pre-edge region. Energies were calibrated periodically during data collection with the iron foil (Fe(0)) 20825

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Figure 1. Cross-sectional distribution of iron in PVG before (a) and after (b) photolysis.

absorption edge at 7112.0 eV. All XANES spectra were also compared to an α-Fe2O3 standard (Alfa Aesar). Transmission electron microscopy (TEM) images of the doped glasses were recorded as previously described.2 The TEM images were photographed and the photographs scanned into a computer. Diameters of the resultant particles were obtained from the digitized images, and the images were tested against calculated spheres to determine the sphericity of the resulting dopant particle. The relative intensities of the beam transmitted through the individual particles were converted to a gray scale, and compared to that expected through a spherical particle of uniform density to garner additional structural detail of the particles.

’ RESULT AND DISCUSSION Absorption spectra recorded at different locations on the samples and X-ray microprobe analyses show that Fe(CO)5 uniformly impregnates the 2 mm thick PVG samples to a depth of g1600 mμ (Figure 1). Changes in relative intensities of the carbonyl bands and the appearance of a band at 2114 cm 1, forbidden under D3h symmetry, indicate adsorption causes some distortion of Fe(CO)5.19 Nonetheless, the similarities of the frequencies of the principal CO bands and the electronic spectrum of the adsorbed complex with vapor and solution spectra of the complex point to physisorption and a relatively weak interaction with the glass surfaces.1,3,19 Consistent with a weak interaction with the glass surface, X-ray microprobe analyses (Figure 1) show a slightly smaller amount of Fe adjacent to outer surfaces of the glass due to desorption of Fe(CO)5 from the samples’ outer surfaces at room temperature. Placing a translucent piece of paper over part of the glass during photolysis does not produce a uniform distribution of the desorbed complex on the paper. Instead, a random distribution of spots corresponding to higher densities of desorbed Fe(CO)5 appears superimposed on a background of lower density. The spot pattern varies from sample to sample, but all samples examined in these experiments exhibit spots of higher density superimposed on a background of lower density. In the absence of light, desorption at room temperature yields a much more uniform coloration, although slight differences are evident, but the differences in coloration are significantly smaller than that produced by photolysis. Since Fe(CO)5 uniformly distributes

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within the matrix (Figure 1), the appearance of spots implies more Fe(CO)5ads desorbs at certain locations than that occurring generally across the entire sample surface. And the larger difference in coloration during photolysis establishes that the asymmetric desorption of the complex is facilitated by the photochemical decomposition of Fe(CO)5ads. N2 adsorption/desorption isotherms (IUPAC classification type H2), as well as TEM and atomic force microscopy analyses indicate PVG is a nodular material composed of 3 5 μm diameter silica nodules that are themselves aggregates of smaller silica nodules ranging from 300 to 600 nm, which are covered by 40 ( 20 nm high and 40 ( 20 nm wide stalagmite-like features.1,3 Porosity within the matrix is attributed to the spaces between the nodules, which N2 adsorption/desorption isotherms indicate is composed of 7 ( 3 nm pores randomly dispersed throughout the matrix and interconnected through narrower regions called necks.3,20 Structurally, asymmetric desorption suggest regions of higher porosity are dispersed within the porosity attributed to interconnected 7 ( 3 nm diameter pores within the glass matrix.1 3,20 23 The distribution of iron in the matrix after photolysis (Figure 1), however, suggests these regions of higher porosity extend no more than 100 μm into the PVG matrix. Since the PVG samples examined in these experiments are polished, however, it is not clear whether these regions of higher porosity are inherent to the pore structure or are a consequence of polishing. Enhanced asymmetric desorption during photolysis implies that the rapid rate of Fe(CO)5 diffusion is facilitated by the photoreleased CO. Although initially uniformly distributed within the matrix to a depth of g1600 μm (Figure 1), the absorption of light by Fe(CO)5ads and the high efficiency of photoinduced decarbonylation (Φ = 0.96 ( 0.04 for loss of the first carbonyl and the loss of subsequent carbonyls appear to occur with similar efficiencies19) creates a CO pressure gradient within the sample. CO at the outer surfaces readily desorbs, whereas that further into the sample desorbs more slowly. If the rate of the photochemical reaction exceeds the rate of CO loss from the interior volumes of the glass, CO pressure increases within the glass matrix. Photoinduced loss of one CO from Fe(CO)5ads in a 7 ( 3 nm diameter pore at room temperature, for example, increases the pressure within the pore by ca. 0.2 atm. Fe(CO)5 physisorbs weakly interacting with the silica surface, whereas the subcarbonyl photoproducts, Fe(CO)n (n e 4), coordinate to the surface silanols and chemisorbed water.19 Since time-resolved experiments show the rate of reappearance of Fe(CO)5 exceeds the rates of disappearance of the subcarbonyl photoproducts,19 and since Fe(CO)5 is more volatile than the surface coordinated subcarbonyl photoproducts and much more volatile than the Fe0 and Fe2O3 photoproducts, the photoinduced increase in CO pressure is thought to “sweep” unreacted, physisorbed Fe(CO)5 ads from the interior to the outer volumes where it either desorbs or photochemically reacts. The result is a pressure driven rate of Fe(CO)5ads diffusion within the matrix19 that is extraordinarily rapid considering it occurs at room temperature and through the tortuous, irregular pathways defined by 3 5 μm and 300 600 nm diameter SiO2 nodules covered by 40 ( 20 nm high and 40 ( 20 nm wide stalagmite-like features.3,20 23 Despite the structural impediments and irregularity of the pathways, timeresolved experiments show that Fe(CO)5 diffusion within these amorphous matrices, even at room temperature, occurs on a microsecond time scale.19 In fact, the rate of Fe(CO)5 diffusion within the matrix exceeds the rates of disappearance of the initial 20826

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The Journal of Physical Chemistry C photoproducts19 and far exceeds the rate of Fe(CO)5 adsorption into the interior portions of PVG by either gas phase diffusion or adsorption of hexane solutions of the complex, both of which occur on a time span of hours.3,19 Regardless of initial Fe(CO)5 loading, the initially uniform Fe(CO)5ads distribution rapidly converts during photolysis to one (Figure 1) with the largest amount of iron within the first few hundred micrometers of the sample surface. In the samples examined in these experiments, photoenhanced diffusion increases the amount of Fe(CO)5 in the outer volumes of PVG by a factor of ca. 6, which is within experimental error of past measurements of the increase in iron at the samples surface.1 3 Redistribution of Fe(CO)5ads is thought to be the reason for the different photochemical behavior of Fe(CO)5 physisorbed on silica relative to that in fluid solution. Changing the distribution of precursor changes not only its local concentration but that of the subcarbonyl photoproducts as well, and occurring on a time scale of the lifetimes of the sub-carbonyl photoproducts,19 changes the modes of reaction accessible to these species and their kinetics. Since Fe(CO)n (n e 4) sub-carbonyls react with Fe(CO)5 to form polymetallic carbonyls,19 redistribution is thought to be the reason for the proclivity of polymetallic Fe2(CO)n and Fe3(CO)m formation in PVG and in powdered, nonporous silicas.19,24,25 Jackson and Trusheim, for example, report Fe3(CO)12 is the only major product during 337 and 355 nm photolyses of Fe(CO)5 physisorbed onto 400 600 m2/g, 20 60 mesh silica particles.24 While the individual particles do not appear to be porous, packed into the photolysis cell, the collection of nonporous particles can be viewed as a porous solid composed of a random array of pores interconnected by narrower necks. On the basis of the size of the silica particles, 0.25 0.85 mm,24 the average size of the pores between the silica particles will be larger and therefore the photogenerated pressure gradient smaller than that in PVG, where the largest silica nodules are 3 5 μm in diameter and the average pore diameter 7 ( 3 nm.3 Nonetheless, photolysis of Fe(CO)5ads in a collection of packed silica particles could create pressure gradients that redistribute the physisorbed precursor. With respect to the kinetics of polymetallic formation, a photoinduced pressure gradient removes, or at least significantly modifies, the expected dependence on initial loading and/or surface coverage. Consequently, Fe2(CO)n and Fe3(CO)m formation in PVG during photolysis of Fe(CO)5ads, even at initial Fe(CO)5ads loadings corresponding to surface coverages of e10%,19 and as noted by Jackson and Thrusheim, at less than monolayer coverage within the packed silica particles,24,25 is attributed to the rapid redistribution of the Fe(CO)5 precursor promoted by pressure gradients created by the photoreleased CO. Redistribution also influences the oxidation of the iron containing photoproduct and the distribution of oxidation states within the silica matrix. Photolabilization of the coordinated CO produces zero valent iron, Fe0, or a coordinatively unsaturated species containing elemental iron.3,26 In either case, the chemistry of these low oxidation state species is defined by a competition between oxidation and aggregation,3 where the extent and most likely the rate of aggregation are defined by the number of Fe0 species within an area defined by the correlation length of the matrix, 22 ( 1 nm in PVG.27,28 The pressure gradient created by the photoreleased CO affects the rates of both aggregation and oxidation. By rapid increase of the Fe(CO)5 surface coverage in the outer volumes, the pressure gradient increases the number of irons within the correlation length of the matrix thereby increasing the rate of aggregation of the zero valent photoproducts. In addition, the photoinduced CO

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Figure 2. X-ray absorption spectrum of iron doped PVG. Circled area corresponds to pre-edge absorptions and dotted line represents an extrapolation of the signal prior to adsorption. ha is the difference between the extrapolation and the intensity following the XFAS absorptions (see text).

pressure gradients reduce the amount of air and oxygen initially present within the matrix thereby reducing the rate of iron oxidation (vide infra). Both are thought to contribute to a rate of Fe0 aggregation that exceeds the rate of oxidation leading to a core shell structure composed of an elemental iron core surrounded by sufficient oxide to prevent or at least curtail further oxidation of the core.3 Because of diffusion and desorption, the initial moles of Fe(CO)5 adsorbed are not necessarily indicative of the amount or distribution of iron within this nanoporous silica matrix. A more realistic measure of the relative amounts of iron in the different samples after photolysis and/or the thermal treatments is calculated from the difference between the extrapolated initial absorption and the absorption beyond the initial XANES and EXAFS regions (Figure 2). Beyond the EXAFS oscillations, ha is directly proportional to the total amount of iron present in the glass. Total iron content was measured after photolysis (samples PVG-01 and PVG-02), and after heating the photolyzed samples to 650 °C for 4 h (samples PVG-03 and PVG-04). The highest temperature accessible prior to collapse of the pore structure and consolidation of PVG is 650 °C.1,3 To probe iron distribution after each treatment, the outermost 30 μm of samples PVG-02 (photolyzed) and PVG-04 (photolyzed and heated) was removed prior to X-ray examination. Comparing the values of ha in PVG-01 and PVG 02 shows that 92% of the iron present in the samples after photolysis is within the first 30 μm of the outermost volumes of the PVG matrix. The result is consistent with the X-ray microprobe results (Figure 1) and past measurements of Fe distribution determined by SIMS using an Ar+ beam to ablate the sample.1 3 The values of ha for PVG-01 and PVG-03 show that heating the samples to 650 °C reduces the amount of iron in the sample to e18% of the amount of iron present after photolysis. Since the elemental iron and iron oxide photoproducts are not volatile at 650 °C, the decline in iron content is attributed to the thermally induced desorption of unreacted Fe(CO)5ads that has diffused into the outermost volumes during photolysis. Comparing samples PVG-03 and PVG-04 indicates that, after heating at 650 °C, 67% of the photodeposited iron exists within the first 30 μm of the PVG samples. Hence, the predominant change in refractive index created by photolysis of Fe(CO)5ads and therefore the depth of a waveguide photopatterned in PVG with Fe(CO)5 is e30 μm.1 20827

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Figure 3. Resolved pre-edge features of Fe2O3 standard (a), PVG-01 (b), PVG-02 (c), PVG-03 (d), and PVG-04 (e) .

X-ray absorption spectra of each PVG sample exhibit absorptions prior to the onset of the major absorption (Figure 2, circle). To facilitate a comparison of the different samples, since different amounts of Fe are present, the spectra of the different samples were normalized with the relative amounts of iron present in each sample taken to be proportional to ha (Figure 2). To extract the pre-edge features, pre- and postedge backgrounds were subtracted from the normalized spectra. The pre-edge background was modeled with a linear function, whereas the postedge background was removed by fitting a Victoreen function to the spectrum and subtracting the function as the background.29 The residues were normalized for atomic absorption using the average absorption coefficient in the 7300 7500 eV spectral region, and the pre-edge feature extracted following the approach used to extract the spectra of iron in silicate glasses.30 The extracted pre-edge absorptions of the iron in each PVG sample are similar consisting of a broad absorption centered at ca. 7115 eV similar to that exhibited by the Fe2O3 standard (Figure 3). The centroids of the pre-edge features of the photolyzed samples (PVG-01 and PVG-02) are 2.69 eV above the Fe K-edge, while those of the photolyzed and heated samples (PVG-03 and PVG-04) are 2.72 eV higher in energy. Since the spectra are recorded in 0.2 eV increments, the difference is taken to be within experimental error. Consequently, the general shape and energies of the pre-edge absorption are independent of whether the photolyzed samples are heated or not (PVG-01 and PVG-03). With the exception of differences in the relative intensities, the pre-edge absorptions are independent of whether the outermost 30 μm are removed (PVG-02 and PVG-04). Since 30 μm is large in comparison to the correlation length of the glass, 22 ( 1 nm,27,28 and any of its structural features,1,3 the spectral similarity implies little difference between the iron formed in the outermost and interior volumes of the glass (vide infra). The pre-edge features of the Fe2O3 standard show four maxima (Figure 3), and fitting the spectrum with four Voigt functions provides excellent agreement with integrated intensity of the absorption envelope. Although Mossbauer indicates the presence of Fe2O3 in PVG,1,3,5,26 attempts to fit the Fe in PVG spectra with four Voigt functions failed to match the integrated intensity of the entire absorption envelope. With the PVG samples, better agreement with the absorption envelope occurs

Figure 4. Normalized 1s f 3d absorbance of Fe2O3 standard (1), PVG samples before (a) and after (b) heating to 650 °C, and before (2)and after (3) removal of the outermost 30 μm.

with two lower energies and one higher energy, broad absorption (Figure 3) instead of the two higher energy absorptions evident in the spectrum of the Fe2O3 standard. The distinction may be a mathematical artifact of the extraction and resolution processes, as opposed to structural and/or compositional differences. However, it is worthwhile to note that the iron species in PVG differ from the Fe2O3 standard. The iron present in PVG contains Fe0, which is not present in Fe2O3 standard, and the iron aggregates formed in PVG, e1 nm diameter after photolysis and e3 4 nm diameter after heating to 650 °C, are significantly smaller than the Fe2O3 standard.3,26 Nonetheless, since the origins of the higher energy transitions in Fe2O3 are not clear,29 and the band envelopes of all PVG samples give no indication of a fourth absorption, the higher energy absorption of the PVG samples is presented as a single broad absorption (Figure 3). Resolution of the lower energy absorptions in the PVG spectra shows that the iron present after photolysis exhibits two absorptions with maxima at 7113.8 and 7115.3 eV. Fe(II) in the minerals pyrite, pyrrhotite, and marcasite exhibit pre-edge X-ray absorption maxima in the 7112.8 ( 0.6 eV range,31 while Fe(II) in a large number of model compounds appear in the 7113.2 ( 0.3 eV range.29 In contrast, the lowest energy pre-edge absorption in the Fe(III) containing minerals ferrihydrite, goethite, lepidocrocite, hematite, and jarosite appear in the 7115.0 ( 0.4 eV range,31 while Fe(III) in the model compounds exhibits an absorption maximum in the 7114.5 ( 0.1 eV range.29 Even with an assumed experimental uncertainty of 0.2 eV, since the spectra were recorded in 0.2 eV steps, the absence of any indication of an absorption maximum of e7113.6 eV (Figure 3) in any of the spectra recorded in these experiments precludes the presence of Fe(II) in these PVG 20828

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The Journal of Physical Chemistry C samples. The result is consistent with the 57Fe Mossbauer spectra which also fail to give any indication of Fe(II) in these PVG samples.1,3,26 Rather, in contrast to that found in other silica matrices,4,14 18 the sole products detected in PVG are Fe(III) and Fe0. The Fe0/ Fe3+ ratio varies from sample to sample, but in all cases examined to date,1 3,5,26 regardless of whether the samples are prepared and photolyzed in air or vacuum or consolidated at 1200 °C, which evolves more water, an oxidant of elemental iron, than the initial amount of Fe(CO)5 adsorbed into the sample,3 Fe0 and Fe3+ in the form of Fe2O3 are detected. Since the pre-edge intensities are normalized to total iron content, differences in the normalized pre-edge areas reflect the Fe0/Fe3+ ratio and, in the different samples, their distribution within the matrix. Comparing the area due to Fe2O3 with the total area in samples PVG-01 (photolyzed) and PVG-02 (photolyzed and ground) (Figure 4) shows that, after photolysis, Fe(III) accounts for 75.0% of the total iron in the samples, but only 56.8% after removal of the front 30 μm of the sample. or 25% of the iron present after photolysis is elemental iron3,26 with larger amounts of elemental iron present in the interior volumes at a depth of g30 μm. Since there is no evidence of the elemental iron reacting with the silica matrix,1,3,5 the larger amount of elemental iron in the interior volumes is attributed to a reduced air or O2 content in the interior volumes due to the photoinduced increased CO pressure displacing air and O2 thereby reducing the probability of oxidation. More than likely, CO pressure gradients also influence the Fe0/Fe3+ ratio within the outermost 30 μm in at least two ways. First by increasing the amount of Fe(CO)5 and, in turn, photogenerated Fe0 within the correlation length of the matrix, CO pressure gradients increase the rate aggregation.3 Second, effusion of the photoreleased CO from the matrix reduces the amount of O2 (air) within the pores and adjacent to the glass air interface thereby decreasing the rate of Fe0 oxidation. Collectively, both effects favor aggregation relative to oxidation and for sufficient aggregation to occur to limit oxidation to the outer periphery of the aggregate.3 The ratio of areas in PVG-01 (photolyzed) corresponds to 25.0% of the iron present in the outermost volumes as Fe0, while the ratio of areas in PVG-03 (photolyzed and heated) corresponds to the 23.8% of the iron present as Fe0. More than likely the values are within experimental error, and thought to reflect an enhanced access to air provided by the regions of higher porosity suggested by the asymmetric Fe(CO)5 desorption (vide supra). The largest difference in the amounts of Feo present occurs in the ground samples, where the outermost 30 μm have been removed. The ratios of the areas in PVG-02 (photolyzed and surface ground) correspond to 45.2% of the iron present as elemental iron, whereas the ratio of areas in PVG-04 (photolyzed, heated and surface ground) corresponds to 26.3% of the iron present in the interior volume and is present as Fe0. Heating leads to oxidation of ca. half of the elemental iron present in the interior volumes. In contrast to the room temperature chemistry and that at the air glass interface, where air (O2) is the principal oxidant and the principal determinant of the Fe0/Fe3+ ratio, an equivalent mechanism is unlikely in the interior volumes of the glass during heating. The heat of adsorption of O2 into porous silica is not known, but in general heating decreases the amount of gas adsorbed. Increasing temperature also increases the CO pressure which would be expected to further reduce the amount of air or O2 present in the interior volumes. Rather, since physi-and chemisorbed water are ubiquitous within PVG1,3 and a product

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of the silanol condensation during consolidation of PVG,1,3 water is thought to be the principal oxidant of the elemental iron in the interior volumes. On a mole basis, heating and eventually consolidation of PVG evolves far more water than the amount of Fe(CO)5 initially adsorbed onto the glass,3 yet no set of experimental conditions examined to date eliminates Fe0 in PVG.3 As noted the ratios vary, but the continued presence of elemental iron implies the rates of Fe0 aggregation in PVG exceed the rates of oxidation by air or adsorbed water even at elevated temperatures (e650 °C) since both yield essentially equivalent Fe0/Fe3+ ratios throughout the PVG; 26.3% of Fe0 in PVG-04 (photolyzed, heated and surface ground) and 25% in PVG-01 (photolyzed). The similarity of the Fe0/Fe3+ ratios implies the level of aggregation needed to create an aggregate of sufficient number or size to limit oxidation to the outer atoms is not facilitated by heating but is defined by the photolysis conditions, most likely, the excitation intensity. Competitive aggregation and oxidation processes account for the presence of elemental iron and/or oxidized iron, but it is not clear how or whether these processes influence the extent of oxidation of the individual iron atoms. Sub-carbonyl photoproducts bind to the surface silanols,19 but EXAFS and Mossbauer give no indication that Fe0 or Fe3+ bond to the silica surface.1,3 The quadrupole doublet assigned to Fe2O3 exhibits an asymmetry indicative of an asymmetric electron distribution about the iron. However, this is attributed a polarization of the Fe2O3 cladding about the iron core by the electron-rich silicate surface rather than direct bonding to the silicate since EXAFS gives no indication of Si in the first shell of atoms about iron.1,3,5 More detailed EXAFS spectra,32 which will be described elsewhere, also give no indication of Si in the second shell of surrounding atoms. Oxidation of elemental iron by either O2 or water is complementary with respect to Fe2+ formation. Fe2+ is found in other silica and silica borate matrices,4,14 18 and care must be exercised in the interpretation of these pre-edge features, particularly with respect to the presence of Fe(II) and Fe(III).32 Nonetheless, the absence of any indication of Fe(II) in any Mossbauer and EXAFS spectra of the photoproducts generated to date in PVG and dried TMOS/MeOH/H2O xerogels,1 3,5,33 the similarity of the resolved absorptions with those of the Fe2O3 standard, which does not contain Fe(II), and a ligand field splitting within experimental error of those for Fe2O3 (vide infra), collectively point to Fe(III) as the sole oxidation product in these nanoporous silicas under the conditions of these photochemical experiments. Since oxidation of Fe0 by either O2 or adsorbed water depends on [H+],34 the extent of oxidation in silica matrices may reflect the pH of the different silica surfaces on which it occurs.35 To a significant extent, surface pH reflects the method of preparation of the silica matrix and the thermal history of the sample.36 The pH of the PVG surface, for example, is between 4 and 5.35 The absence of diffraction from the any of the Fe0/Fe3+ particles formed in PVG or in the consolidated glass led to the suggestion that the particles are not crystalline but amorphous.1,3 Yet, the TEM and spectroscopic data point to the formation of uniform, well-defined, spherical particles within PVG. The two XANES absorptions with maxima at 7113.8 and 7115.3 eV, for example, are similar to those exhibited by the Fe2O3 standard (Figure 3) and are assigned to transitions between the Fe 1s orbital and the split 3d orbitals.37,38 The difference in energies, 1.5 eV, is equivalent to the ligand field strength measured for the Fe2O3 standard, 1.5 eV and, within an uncertainty of 0.2 eV, is within experimental error of the reported value of the ligand field splitting for Fe2O3, 1.41 eV.39 A ligand field strength equivalent 20829

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The Journal of Physical Chemistry C to that of Fe2O3 standard implies that, even in the initially formed, small, e1 nm diameter, Fe0/Fe2O3 photoproducts, where the insensitivity of the Fe0/Fe(III) ratio to air suggests a core shell structure,3 the Fe2O3 cladding, at least that portion that influences the oxide ion field strength, achieves a degree of structural order similar to that in the bulk Fe2O3.37 39 Heating to 650 °C increases particle size with the e1 nm diameter Fe0/Fe2O3 core shell photoproducts aggregating to form individual 3 4 nm diameter particles.2 However, aggregation does not produce a random shaped particle or a random array of different shapes. Assuming a random orientation of the particles on the TEM grid, analysis of the digitized TEM images of these particles indicates that the majority of the particles, g99%, are spherical. Furthermore, TEM images recorded in the “bright field” mode, where transmission through the particle is a function of sample thickness and composition,40 are consistent with a core shell structure. Electron transmission along a plane dissecting a spherical particle of uniform composition exhibits a parabolic distribution of intensities with the highest transmitted intensity at the outer edges and the minimum at the point corresponding to the diameter of the particle. Electron transmission also declines in proportion to number of iron atoms, or the density, F, of the iron encountered passing through the sample. Consequently, a core shell particle composed of an iron core, F = 7.8 g/cm3, surrounded by Fe2O3 cladding, F = 5.24 g/cm3, will exhibit an additional decline in transmitted intensity beyond the parabolic distribution through a spherical particle of uniform composition. Indeed, gray scale analyses of the relative intensity transmitted through the 3 4 nm particles formed in PVG show additional reductions in intensity near the center of the particle consistent with a particle composed of an iron core surrounded by Fe2O3 cladding. Further work is needed to quantitate this approach but, at least at this qualitative level, aggregation leads to uniformly spherical core shell particles, which retain the ligand field splitting of the e1 nm precursors. In addition, the halfwidths of the lower energy 1s 3d transitions in PVG (Figure 3), 1.6 eV, are equivalent to those of the Fe2O3 standard. Formation of a 3 4 nm diameter core shell particle from a collection of e1 nm diameter core shell particles requires disruption of the Fe2O3 shell of the aggregating smaller particles, which in turn provides an opportunity for further oxidation. The decline in the Fe0/Fe3+ ratio on heating, which increases particle size, is consistent with some disruption of surrounding Fe2O3 cladding of the aggregating initial photoproducts. Occurring with e20% change in the Fe0/Fe3+ ratio, however, the disruption of the cladding and subsequent oxidation of the core appear to be limited. Aggregation is random in the sense of which particles aggregate and appears to involve disruption of the outer shells that curtail the oxidation of the elemental iron. Nonetheless, collectively, the data indicate these random processes lead to the formation of well-defined 3 4 nm diameter, spherical, core shell particles that, retaining the ligand field strength of bulk Fe2O3, achieve an Fe2O3 structure very similar to that of the bulk oxide. It is not clear that inability to detect diffraction from the individual particles is necessarily indicative of the formation of an amorphous, irregular structure.

’ CONCLUSION The photochemistry of Fe(CO)2 physisorbed into PVG and the thermal chemistry of the iron containing photoproducts are influenced by pressure gradients arising from photodissociated

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CO. These gradients account for the rapid rate of Fe(CO)5 diffusion within PVG and, occurring on a time scale of the lifetimes of the subcarbonyl photoproducts, influence the photochemistry and thermal chemistry within the matrix. Pressure induced redistribution of Fe(CO)5 increases the probability of forming polymetallic products in PVG19 and Fe3(CO)12 in nonporous, packed silica even at low initial Fe(CO)5 surface coverages.24,25 By flushing air from the interior pores and, to a lesser extent, limiting the amount of air at or near the glass/air interface at room temperature, the CO pressure gradients create an asymmetric distribution of elemental and oxidized iron within the matrix, and by retarding the rate of oxidation relative to rate of aggregation allow sufficient Fe0 aggregation so that oxidation is limited to the outer periphery of the aggregate. At higher temperatures, adsorbed water appears to be the principal oxidant of iron within these nanoporous silicas. Although oxidation of Fe0 by O2 or H2O is complementary with respect to Fe(II) formation, unlike that found in other silicas, neither oxidant leads to Fe2+ in PVG suggesting that the extent of oxidation within a silica matrix may be influenced by the pH of the silica surface on which it occurs. The absence of Fe(II) in PVG implies the spectroscopy of the iron dopants, and therefore the transparency of photopatterned refractive index gradients formed in PVG are not biased by charge transfer transitions between Fe2+ and tetrahedral- and octahedral-Fe3+, or Fe2+ O2 Fe3+ interactions reported to occur in other silica matrices.41 44 Although thermally induced aggregation in PVG appears to be a random process, the sphericity of 3 4 nm diameter Fe0 core Fe2O3 shell aggregates formed, and the similarity of the ligand field splitting in these aggregates with that in bulk Fe2O3 suggests a degree of structural order similar to that in the bulk oxide is achieved.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Support of this research by the National Science Foundation (CHE-0514458 and 0079040), the Air Force Office of Scientific Research, the Petroleum Research Foundation, administered by the American Chemical Society, and the City University of New York Center of Advanced Technology in Photonics, funded by the New York State Science and Technology Foundation and Corning, Inc., is gratefully acknowledged. ’ REFERENCES (1) Sunil, D.; Devi, P. S.; Dong, J.; McQuade, A. W; Mendoza, E. A.; Gafney, H. D. Comments Inorg. Chem. 2003, 24 (3 4), 69–136. (2) Sunil, D.; Rafailovich, M. H.; Sokolov, J.; Gambino, R. J.; Halada, G.; Huang, D. M.; Gafney, H. D. J. Non-Cryst. Solid 2003, 319 (1 2), 154–162. (3) Sunil, D.; Dong, J.; Gafney, H. D. J. Am. Chem. Soc. 2009, 131, 14768–14777and references therein. (4) Wilke, M.; Behrens, H.; Burkhard, D. J. M.; Rossano, S. Chem. Geol. 2002, 189 (1 2), 55–67. (5) Mendoza, E. A.; Wolkow, E.; Sunil, D.; Sokolov, J.; Rafailovich, M. H.; Gafney, H. D. Langmuir 1991, 7, 3046–3051. (6) Mahadevan, T. S.; Gaofalini, S. H. J. Phys. Chem. C 2008, 112, 1507. (7) Demarquay, J.; Fraissard, J.; Imelik, B. C. R. Seances Acad. Sci., Ser. C 1971, 273 (21), 1405–1408. 20830

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