Article pubs.acs.org/cm
Determining the Oxidation State of Small, Hydroxylated Metal-Oxide Nanoparticles with Infrared Absorption Spectroscopy Xing Huang*,† and Matthew J. Beck*,†,‡ †
Department of Chemical and Materials Engineering, University of Kentucky, Lexington, Kentucky 40506, United States Center for Computational Sciences, University of Kentucky, Lexington, Kentucky 40506, United States
‡
S Supporting Information *
ABSTRACT: Metal-oxide nanoparticles (MONPs) are of broad interest as catalysts and catalyst supports in automotive, industrial, and biomedical applications. In water-containing environments, MONPs adsorb hydroxyl groups at nanoparticle corners, edges, and faces. While adsorbed surface groups will influence MONP properties, including oxidation state and the availability and accessibility of catalytically active sites, determination of the density, distribution, and effects of such groups is experimentally challenging. Here, focusing on CeO2 nanoparticles (CNPs) as a representative MONP system, we report computational and experimental results showing that, for small, hydroxylated CNPs, the presence or absence of strongly infrared (IR) absorbing O−H stretching modes associated with O−H−OH complexes present on fully oxidized CNPs can be used to probe the configuration of surface-adsorbed hydroxyl groups and the CNP net oxidation state. As the vibrational modes of O−H−OH complexes are distinct from those of molecular water (3240 vs 3404 cm−1), these results demonstrate that IR absorption spectroscopy can be used to determine the oxidation state of small, hydroxylated CNPs, as well as other MONPs, more generally, in water-containing environments.
■
tional details,10 are primarily applicable in vacuum conditions or model environments. Here, we report results of quantum mechanical calculations along with direct experimental validation showing that infrared (IR) absorption spectroscopy provides a fingerprint of the atomistic configuration and density of surface hydroxyl groups adsorbed on representative MONPs and that this fingerprint can be used to reveal the effective oxidation state of small, hydroxylated MONPs. To demonstrate the potential of IR spectroscopy to reveal the oxidation state and surface structure of MONPs, we focus here on CeO2 nanoparticles as a representative MONP system. Cerium is a multivalent rare-earth metal that takes +4 and +3 charge states. CeO2 (ceria) nanoparticles (CNPs) have been extensively investigated as catalysts and catalyst supports with broad application in CO oxidation,11 the water−gas shift reaction,12 solid oxide fuel cells,13 photocatalytic water splitting,14 and •OH radical scavenging.15,16 In addition, the passivation of CNP surfaces with hydroxyl groups in the presence of water or water vapor has been previously studied.17,18 In the following, we show that characteristic features in the IR absorption spectra of small, hydroxylated CNPs clearly distinguish between fully oxidized CNPs with dense configurations of adsorbed hydroxyl groups and partially reduced
INTRODUCTION Determining the chemical and structural state of catalysts’ surfaces in complex environments, for instance, immediately postsynthesis or under reaction conditions, is a key hurdle for efforts to understand, control, and optimize these important materials for industrial, automotive, and biomedical applications. This is particularly relevant in the context of metal-oxide nanoparticle (MONP) catalysts and catalyst supports. These materials readily incorporate chemical species from their local environments, particularly ionic species, into complex and widely varying surface structures.1−7 In the presence of water vapor, for example, MONPs are passivated by the chemisorption of hydroxyl groups whose density and configuration are controlled by environmental parameters.8 The chemical and structural details of surface groups on MONPs will directly influence both the oxidation state of near surface ions and the availability and accessibility of catalytically active sites, ultimately playing an important role in determining effective catalytic properties. Despite the ubiquitous presence of water vapor and other reactive species during catalyst synthesis and application, the toolkit for determining atomic-scale details of the chemical and structural state of catalyst surfaces in complex environments is limited. In general, light scattering, spectroscopic, and diffraction techniques, which, in some cases, can be deployed in reaction-relevant environments,9 do not reveal the atomistic configurations or density of surface groups, and electron microscopy techniques, which can reveal atomistic configura© XXXX American Chemical Society
Received: January 20, 2015 Revised: April 7, 2015
A
DOI: 10.1021/acs.chemmater.5b00259 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
matrix was constructed using the differences of the second derivatives of the total energy with respect to the positions of each in the system by 0.005 Å in all Cartesian directions from the relaxed (ground state) geometry. The IR absorption intensities were obtained from the Born effective charge (BEC) matrices.33 During calculation of IR absorption properties, no constraints were applied, and full coupling of motion among atoms in the system is allowed. In addition, IR properties were computed for entire CNPs (including adsorbed hydroxyls), and therefore, any applicable surface selection rules would arise organically. To directly validate computed results, ∼5 nm cubic CNPs were synthesized using a previously reported method.34 By way of overview, 50 mL of 16.7 mmol/L cerium(III) nitrate aqueous solution was transferred to a 125 mL Teflon-lined stainless-steel autoclave. 50 mL of toluene, 5 mL of oleic acid, and 0.5 mL of tert-butylamine were added without stirring. The autoclave was then sealed and transferred to a furnace, held at 180 °C for 24 h, and then naturally cooled to room temperature. The resultant solution was centrifuged to remove solid impurities, and then, the upper brown supernatant containing the CNPs was precipitated with ethanol and further isolated by centrifugation without any additional size sorting process. The resultant CNPs were washed with ethanol three times and dried at 80 °C in air overnight. To completely remove any physisorbed molecular water and/or organic adsorbates remaining from synthesis, CNPs were heated from room temperature to 900 °C at 10 °C min−1 in pure Ar and held at 900 °C for 5 h before being naturally cooled to room temperature. To controllably yield fully oxidized and partially reduced CNP samples, as-synthesized CNPs were subjected to one of two heat treatments. Annealing conditions were chosen according to a previously computed phase diagram of stable CNP configurations in O2- and H2O-containing environments.8 To yield fully oxidized, hydroxyl-saturated CNPs, as-synthesized CNPs were held at 180 °C for 5 h in saturated water vapor. To yield partially reduced, hydroxylterminated CNPs, as-synthesized CNPs were held at 700 °C for 5 h in a pure Ar atmosphere. CNPs were characterized with transmission electron microscopy (TEM) (JEOL-2010 operating at 200 kV) and IR absorption spectroscopy (Varian 7000e FT-IR spectrometer). IR spectra for fully oxidized and partially reduced samples were collected immediately after their respective final heat treatments.
CNPs with less dense configurations of adsorbed hydroxyl groups. Dense coverage results in pairs of surface hydroxyl groups separated by distances of less than ∼2 Å. These hydroxyl group (O−H−OH) complexes exhibit stretching mode vibrational states with strong IR absorption at characteristic frequencies that can be distinguished from both weakly absorbing modes associated with isolated adsorbed hydroxyl groups as well as the background stretching mode vibrational states of molecular water. The absence of stretching modes with strong IR absorption arising from such O−H−OH complexes is characteristic of the lower density of surface hydroxyl groups present on partially reduced CNPs. The density-dependent formation of surface O−H−OH complexes (shown here on CNPs) is expected to occur on a range of MONP systems and has likely already been observed in the case of titania nanoparticles.19 Therefore, IR spectroscopy combined with calculations of vibrational frequencies of IR-active O−H−OH complexes can be used to probe the effective oxidation state of MONP systems, generally. In addition, as IR spectroscopy can be applied to MONPs in a wide range of environments, including in air or aqueous systems, the present results show that IR spectroscopy can be a powerful tool for interrogating the oxidation state of MONP catalyst and catalyst supports in application-relevant environments.
■
METHODOLOGY
In the present study, quantum mechanical calculations were used to compute the stable atomic structure, total energy, electronic density of states and vibrational modes of cubic, {001}-terminated CNPs with varying numbers of chemically bound surface hydroxyl groups. The results of these calculations were then directly validated by the experimental synthesis and IR characterization of 3000 cm−1 in the computed spectra (see Figure 3a,b) arise from O−H stretching mode vibrations, the disappearance of the intense IR absorbing modes between 3000 and 3500 cm−1 might, at first, be considered to C
DOI: 10.1021/acs.chemmater.5b00259 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
atom in a neighboring surface hydroxyl group, as highlighted in Figure 4. Stretching of the IR active O−H bond in these cases is effectively an oscillation of the H atom within an O−H−OH complex. The intense IR absorption peaks can be explained on the basis of the large effective charge redistribution which, in the limit of complete transfer of the H atom (H+ ion), consists of two OH− groups becoming an O2− and a neutral H2O. Fifteen individual O−H stretching modes with/without relatively high IR absorption intensity are enumerated in Figure 3a, the details of which are summarized in Table 1. Three examples of these O−H−OH complexes with different (O−)H−O(H) separations are shown in Figure 4 and are numbered using the same schema. The absence of strong IR absorption peaks between 3000 and 3500 cm−1 in the calculated IR spectrum of partially reduced Ce32O81H54 CNPs is therefore not due to the removal of the IR-active surface hydroxyl groups themselves (which are typically hydroxyl groups at CNP faces) but rather to the removal or reconfiguration (see Table 1) of the neighboring surface hydroxyl group with which the oscillating H atom interacts (which are typically corner or edge hydroxyl groups). That is, on fully oxidized CNP surfaces, where a large number of corner and edge sites are occupied by adsorbed hydroxyl groups, a subset of hydroxyl groups are sufficiently close to form O−H−OH complexes, resulting in strong IR absorption between 3000 and 3500 cm−1. In contrast, on partially reduced CNPs, which will lack corner and edge hydroxyl groups,8 such pairings are rare or not possible, and no strong IR absorption peaks are present above 3000 cm−1. It is instructive to note that intense IR absorption peaks associated with O−H−OH complexes depend only on the relative position of two neighboring hydroxyl groups. That is, the presence of strongly absorbing peaks between 3000 and 3500 cm−1 is not dependent on a specific net arrangement of hydroxyl groups, for example, the specific arrangements considered here. Any arrangement of hydroxyl groups permitting the close association of neighboring groups can yield O−H−OH complex behavior and, therefore, intense IR absorbing peaks between 3000 and 3500 cm−1. That is, the present results can be be summarized as a definite prediction for the appearance (or absence) of experimentally measured IR absorption peaks for small, fully oxidized (versus partially reduced) CNPs. Calculations show that two distinct peaks should be experimentally observed for O−H stretching modes arising from surface adsorbed hydroxyls on small CNPs. One peak will arise from O−H−OH complexes (when present on CNPs with dense hydroxyl coverages) and will be centered at ∼3240 cm−1 with a width (approximate full-width halfmaximum) of at least 600 cm−1 (see Figure 3a). Another will arise from isolated surface-adsorbed hydroxyl groups, and will be centered at ∼3700 cm−1 with a width of at least 100 cm−1 (see Figure 3a,b). As processing conditions expected to yield fully oxidized and partially reduced hydroxyl-terminated CNPs have been previously predicted,8 the present computational results can be directly validated by synthesizing and characterizing small CNPs. Using a previously described synthesis procedure, welldispersed, sub-10 nm, crystalline, cubic {100}-terminated CNPs were synthesized. Figure 5 shows TEM images of representative CNPs (postsynthesis) and confirms that the synthesized CNPs are indeed sub-10 nm, cubic CNPs with lattice spacings consistent with cubic fluorite ceria (a = 0.27 nm). It should be noted that, while the synthesized CNPs are larger than the
Figure 2. Calculated IR spectra for (a) a fully oxidized Ce32O101H74 CNP and (b) a partially reduced Ce32O81H54 CNP and (c) experimentally measured IR absorption spectra for both fully oxidized and partially reduced ∼5 nm cubic CNPs. See the discussion in the text.
simply reflect the removal of particular IR active surface hydroxyl groups from the shell of Ce32O101H74 to form Ce32O81H54. Figure 3d−g shows that this is not the case. Figure 3d−g shows ball-and-stick models of the relaxed atomic structures of Ce32O101H74 (Figure 3d,e) and Ce32O81H54 (Figure 3f,g). Bulk Ce and O atoms that are part of the core crystalline CNP are shown in orange and pink, respectively. H atoms in surface hydroxyl groups are gray, while O atoms in surface hydroxyl groups are various colors as detailed below. Figure 3d,e for Ce32O101H74 (and similarly Figure 3f,g for Ce32O81H54) shows identical structures but with different ball sizes for O and H atoms in surface hydroxyl groups. Figure 3d,f (large balls with sizes set according to atomic covalent radii) highlights the extent to which adsorbed surface hydroxyl groups screen the crystalline “core” of considered CNPs. Figure 3e,g (small balls) exposes the binding networks of surface hydroxyl groups with Ce−O(H) bonds drawn for Ce−O distances less than 2.5 Å. The vertical lines indicating individual vibrational modes in the computed spectra (Figure 3a,b) are color coordinated with the O atoms in surface hydroxyl groups whose vibration gives rise to each peak (Figure 3d−g). Comparison of Figure 3a,b with Figure 3d,e and Figure 3f,g shows that the IR active surface hydroxyl groups responsible for the intense IR absorbing peaks on the fully oxidized CNP are not missing from the partially reduced CNP. Instead, the local configuration of surface hydroxyl groups surrounding these IR active surface hydroxyl groups has changed (see Table 1). In all cases, intense IR absorbing peaks for fully oxidized CNPs are associated with surface hydroxyl groups oriented in such a way that the H atom in the IR active surface hydroxyl group is within 2 Å of the O D
DOI: 10.1021/acs.chemmater.5b00259 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Figure 3. Upper panels show the region of IR spectra highlighting O−H stretching modes computed for (a) fully oxidized Ce32O101H74 and (b) partially reduced Ce32O81H54 as well as (c) experimentally measured IR absorption spectra for ∼5 nm cubic CNPs highlighting O−H stretching modes within this region. A double Gaussian fit to the measured experimental data for both fully oxidized and partially reduced CNPs is shown. For the fully oxidized, the fit only includes peaks for O−H−OH complexes (light pink) with positions and full-width half-maximums as calculated here (see text for details). For the partially reduced, the fit combines component peaks for individual hydroxyl groups (peak 1, dashed green) and O−H− OH complexes (peak 2, dashed olive) with positions and full-width half-maximums as calculated here (see text for details). Lower panels show relaxed atomic structures of (d and e) fully oxidized Ce32O101H74 and (f and g) partially reduced Ce32O81H54. Panels (d and f) show space filling balls with radii set according to atomic covalent radii, while panels (e and g) show the same structures as panels (d and f), respectively, but reduce the size of surface hydroxyl groups to expose the connectivity of hydroxyl groups to the CNP core. Colors are coordinated as discussed in detail in the text.
Table 1. Details of 15 O−H−OH Complexes on Ce32O81H74a peak
intensity (a.u.)
νO−H−OH (cm−1)
d[(O−)H−O(H)] (Å)
...neighbor in Ce32O81H54
11 12 10 9 1 2 13 15 14 5 3 8 6 4 7
0.76 0.76 0.70 0.57 0.06 0.06 0.87 0.61 0.47 0.07 0.06 0.33 0.18 0.13 0.17
3313.4 3302.6 3348.6 3378.3 3685.4 3669.7 3273.8 3111.7 3259.8 3640.1 3655.2 3444.1 3578.2 3652.3 3523.8
1.85 1.81 1.88 1.92 2.36 2.26 1.84 1.73 1.80 2.42 2.23 1.95 2.13 2.29 1.98
removed removed removed removed removed removed rearranged rearranged rearranged rearranged rearranged rearranged rearranged rearranged rearranged
Figure 4. Ball-and-stick model of relaxed Ce32O101H74 with three O− H−OH complexes highlighted. Numerical labels are as above in Figure 3a and Table 1 and indicate O−H−OH complexes with strong (11 and 13) and weak (4) IR absorption.
discussions of CNP size scaling effects on the nature and configuration of surface-adsorbed anion groups can be found elsewhere.31 As described above, synthesized CNPs were first annealed in pure Ar at 900 °C to remove any organic absorbates and molecular water remaining from the synthesis process. One 0.5 g sample of CNPs was then held in saturated water vapor at 180 °C for 5 h, while a second 0.5 g sample was held in pure Ar at 700 °C for 5 h. Previous calculations8 show that CNPs annealed in saturated water vapor will be fully oxidized, with corners, edges, and faces saturated with hydroxyl groups, while CNPs annealed at 700 °C in pure Ar will be partially reduced,
a
Peaks are numbered as in Figure 3a. d[(O−)H−O(H)] is the distance in Å between the H in the IR-active hydroxyl group and the O in a neighboring hydroxyl group. The “...neighbor in Ce32O81H54” indicates how each complex differs between fully-oxidized Ce32O101H74 and partially-reduced Ce32O81H54, and refers to how the neighboring hydroxyl group, not the IR active group itself, has changed. In no case is the IR-active hydroxyl group itself removed.
model CNPs studied computationally, the surface functionalization patterns (that is, the nature and configuration of adsorbed surface anions and therefore the nature of their IR absorption “fingerprints”) should be the same. More detailed E
DOI: 10.1021/acs.chemmater.5b00259 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
in the ∼2800−3800 cm−1 range: a broad peak at ∼3240 cm−1 (O−H−OH complexes), a more narrow peak at ∼3700 cm−1 (isolated hydroxyl groups), and a molecular water peak at ∼3450 cm−1. This is highlighted by the direct comparison of experimentally measured peak positions and widths to computationally predicted values in Table 2. These data Table 2. Summary of Computed and ExperimentallyMeasured IR Absorption Peaks for Both Isolated Hydroxyl Groups and O−H−OH Complexes at Surfaces of Representative CNPs Figure 5. TEM images of synthesized cubic shaped CNPs. (a) Mass production of uniformly sized sub-10 nm cubic CNPs and (b) welldefined crystalline structure confirming CNPs of exposed {100} crystal facets with lattice spacing of 0.27 nm.
computed (cm−1)
measured (cm−1)
assigned mode
3240 ± 600
3266 ± 773 3231 ± 986
O−H−OH (oxidized CNP) O−H−OH (reduced CNP) isolated −OH (oxidized CNP) isolated −OH (reduced CNP) H2O (liquid)
3710 ± 100 3670 ± 200 3471
with reduced densities of hydroxyl groups, particularly at CNP edges and corners. That is, the experimentally prepared CNP samples are expected to have oxidation states and hydroxyl configurations similar to the representative Ce32O101H74 and Ce32O81H54 structures considered above. IR absorption spectra of the fully oxidized and partially reduced CNP samples were collected and are presented in Figure 2c. IR absorption peaks were observed below 600 cm−1 and between 2800 and 3800 cm−1. Note that no peaks corresponding to the bending mode of molecular water (∼1600 cm−1) were experimentally observed for either sample, demonstrating that no physisorbed molecular water is present at surfaces of synthesized CNPs. In addition, no IR absorption peaks associated with presence of C−O, C−O, N−N, or N−H bonds were observed. In sum, the experimental spectra correspond directly to the computed spectra (Figure 2a,b). It should be noted that, while computed spectra consist of ideal, discrete vibrational modes, in experimental spectra effects of finite temperature and interactions with the ambient environment result in broadening of ideal vibrational modes and the observed smoothed spectrum. Quantitative Gaussian fitting of the high-wavenumber experimentally observed IR peaks (
