Surface Termination and CO2

Article pubs.acs.org/cm

Surface Termination and CO2 Adsorption onto Bismuth Pyrochlore Oxides Robert J. Walker,† Anna Pougin,‡ Freddy E. Oropeza,*,† Ignacio J. Villar-Garcia,† Mary P. Ryan,† Jennifer Strunk,§ and David J. Payne† †

Department of Materials, Imperial College London, Exhibition Road, London, SW7 2BP U.K. Laboratory of Industrial Chemistry, Ruhr-University Bochum, Universitätsstr. 150, 44801 Bochum, Germany § Max-Planck-Institute for Chemical Energy Conversion, Stiftstr. 34-36, 45470 Mülheim an der Ruhr, Germany ‡

S Supporting Information *

ABSTRACT: The catalytic activity and gas-sensing properties of a solid are dominated by the chemistry of the surface atomic layer. This study is concerned with the characterization of the outer atomic surfaces of a series of cubic ternary oxides containing Bi(III): Bi2M2O7 (M = Ti, Zr, Hf), using lowenergy ion scattering spectroscopy. A preferential termination in Bi and O is observed in pyrochlore Bi2Ti2O7 and related cubic compounds Bi2Zr2O7 and Bi2Hf2O7, whereas all three components of the ternary oxide are present on the surface of a Bi-free pyrochlore oxide, Y2Ti2O7. This observation can be explained based on the revised lone-pair model for posttransition-metal oxides. We propose that the stereochemically active lone pair resulting from O 2p-assisted Bi 6s−6p hybridization is more energetically favored at the surface than within a distorted bulk site. This leads to reduction of the surface energy of the Bi2M2O7 compounds and, therefore, offers a thermodynamic driving force for the preferential termination in BiOx-like structures. CO2 adsorption experiments in situ monitored by diffuse reflectance IR spectroscopy show a high CO2 chemisorption capacity for this series of cubic bismuth ternary oxides, indicating a high surface basicity. This can be associated with O 2p−Bi 6s−6p hybridized electronic states, which are more able to donate electronic density to adsorbed species than surface lattice oxygen ions, normally considered as the basic sites in metal oxides. The enhanced CO2 adsorption of these types of oxides is particularly relevant to the current growing interest in the development of technologies for CO2 reduction.

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INTRODUCTION The bulk structural and electronic properties of bismuth-based pyrochlore materials have been extensively studied mainly due to their general high dielectric constant and low dielectric loss, which are desirable properties in the field of electronics devising.1,2 There is, however, a series of novel potential applications of this type of material, such as oxygen-evolution catalysis and visible-light photocatalysis, whose development largely depends on the knowledge of the surface chemistry. Pyrochlore bismuth iridates have been shown to have good activity and high stability for the oxygen evolution reaction, which is mediated by surface redox processes.3 Pyrochlore bismuth titanate4,5 and bismuth niobate6 photocatalysts have been used for the photocatalytic reforming of methanol and the photocatalytic oxidation of organic pollutants. Pyrochlore compounds have nominal composition A2B2O7, and their structure consists of two interpenetrating B2O6 octahedral and A2O′ tetrahedral networks in an array that can be derived from a fluorite cell with a vacancy in the 8a Wyckoff position (A2B2O7 vs 4CaF2), and therefore, these two structures are closely related. 7 The stoichiometry of pyrochlore © XXXX American Chemical Society

compounds is often expressed as A2B2O6O′, denoting the existence of two distinguishable oxygen sites. Within the pyrochlore structure, there is only a weak interaction between the two networks, and the A2O′ tetrahedral substructure has a minor contribution to the Madelung energy, which enables the synthesis of nonstoichiometric pyrochlore compounds.7,8 In fact, stable phase-pure pyrochlore bismuth titanate, Bi2Ti2O7, can only be synthesized with a certain amount of bismuth vacancies, which range from 20% to 50%.4,9 We recently showed, however, that phase-pure Bi2Ti2O7 can be stabilized when grown as an epitaxial layer on yttria-stabilized zirconia (YSZ), which allows the preparation of stoichiometric bismuth titanate thin films.10 Disorder in the A2O′ network is often seen when the A cation of a pyrochlore compound has an active lone pair of electrons,11 since this lone pair cannot be easily accommodated in the D3d centrosymmetric local environment of the A site within the pyrochlore structure. The disorder of A Received: August 20, 2015 Revised: October 28, 2015

A

DOI: 10.1021/acs.chemmater.5b03232 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

(Aldrich, 99.999%) was added in a 1:1 mol ratio to prepare 5 cm3 of a solution with 1 M total metal concentration, which was diluted with 2.5 cm3 2-methoxy-ethanol. The resulting solution was used to spincoat (001)-oriented YSZ substrates at 3000 rpm. Before coating, the substrates were cleaned by rinsing in ethanol. After coating, substrates were dried in a convection oven at 200 °C. This led to amorphous films that were then calcined for 2 h at 800 °C to form epitaxial films of pyrochlore Y2Ti2O7 and Bi2Ti2O7. The crystallinity and epitaxial relationship of both films were confirmed by X-ray diffraction (XRD) (see the Supporting Information, Figure S1). Polycrystalline yttrium titanate, bismuth titanate, bismuth zirconate, and bismuth hafnate powders were synthesized via a coprecipitation method. To synthesize the bismuth compounds, bismuth nitrate pentahydrate and the respective B cation source either titanium isopropoxide (Aldrich, 99.999%), zirconium propoxide (70 wt % in 1propanol), or hafnium n-butoxide (Aldrich, 99%) were dissolved in 3 M nitric acid solution in a 1:1 stoichiometric ratio with the exception of bismuth titanate, where a 20% excess of titanium isopropoxide was used to prepare a compound with nominal composition Bi1.67Ti2O6.5. Once the constituents were dissolved, ammonium hydroxide was rapidly added to adjust the pH to basic (pH ∼ 9), causing the formation of a white precipitate. The precipitate was filtrated, washed, then dried at 80 °C for 16 h, and subsequently calcined at 600 °C for 8 h. Y2Ti2O7 was prepared using the same procedure, using a solution of yttrium nitrate hexahydrate (Aldrich, 99.8%) and titanium(IV) isopropoxide (Aldrich, 99.999%) in a 1:1 metal ratio. However, the calcination temperature needed for the crystallization of this sample was 800 °C. Figure S2 shows XRD patterns of all samples. A clean set of diffraction peaks characteristic of a single cubic pyrochlore phase with space group Fd3̅m is observed for Y2Ti2O7 and Bi1.67Ti2O6.5, whereas, for Bi2Zr2O7, the XRD pattern is consistent with a cubic fluorite phase with space group Fd3m ̅ , which is consistent with previous structural characterization of these materials.9,20 The structure of Bi2Hf2O7 cannot be fully resolved based on lab-based XRD analysis as the fluorite and pyrochlore phases are indistinguishable due to the increased structure factor of Hf and the nanocrystalline nature of the material. Sorokina and Sleight first prepared this phase and identified it as a defect fluorite based on XRD analysis.21 However, more recently, Henderson et al. determined Bi2Hf2O7 to have a pyrochlore structure with a space group of Fd3̅m based on neutron diffraction data.11 LEIS Spectroscopy. Low-energy ion scattering spectra were measured in a Qtac 100 instrument (ION-TOF GmbH) fitted with a double toroidal energy analyzer (DTA). The samples were analyzed using a He+ and Ar+ primary ion beam directed perpendicularly to the target surface in a 1 μm2 spot size with 3 keV energy and an analyzer pass energy of 3 keV. The cleaning process was completed by an in situ atomic-oxygen-plasma treatment, which was performed until the last spectrum was the same (within error) to the previous one. Oxygenplasma treatment cycles were approximately 20 min long, and typically the cleaning process was composed of two cycles. All spectra were obtained with doses below 2 × 1014 ions per mm2, which is below the dose that can cause appreciable surface damage.22 Low-energy sputtering was performed by 1 keV Ar+ bombardment at an angle of 59°. The sputtered area was 2 μm2, and it was ensured that the analysis spot size sits well within this area. In Situ IR Spectroscopy. IR spectra were collected in a Protègè 460 spectrometer by Nicolet. All sample spectra were measured against backgrounds of KBr recorded previously. All powder samples were measured without dilution in a Harrick environmental cell (HVC-DRP-2) to allow the adjustment of gas atmosphere and temperature. The default temperature is regulated to be above room temperature at 35 °C. In order to remove adsorbed contaminants, like H2O, from the catalyst surface, a heat treatment was performed to 250 °C with a heat ramp of 10 °C/min in a mixture of 20% O2 in He. The temperature of 250 °C was maintained for 30 min and a cumulative spectrum of 500 measurements. The sample was then cooled back to 35 °C, and again, a spectrum was recorded with 500 scans. This spectrum will later be used as the control spectrum for

and O′ ions has been suggested to be due to static displacements in pyrochlore compounds in which the A site is a post-transition-metal cation with a stereochemically active lone pair.12 For the novel applications of Bi-ternary oxides in catalysis, the chemistry of the immediate surface is of paramount importance as it determines the nature of the interaction between the solid and the reacting species and, hence, both the activity and selectivity of the catalytic material. Surface chemistry is also important in other processes such as deposition, sintering, or annealing. Clearly, the surface composition of these materials has an important impact on their surface chemistry; however, there are only few reports specifically addressing the chemical characterization of the surface of bismuth-based oxides, in particular, the series of oxides studied here: Bi2M2O7. X-ray photoelectron spectroscopy (XPS) has been used in order to study the near surface composition of various bismuth-based oxides, finding that, often (but not always), there is a surface enrichment of Bi.13,14 Although this technique is extremely useful to investigate the composition in the near surface region, information about the very outer surface is difficult to deconvolute as it simultaneously probes over several atomic layers, typically 3−20. Here, we report a study of the surface composition of a series of Bi cubic ternary oxides Bi2M2O7, where M is Ti, Zr, or Hf, based on lowenergy ion scattering (LEIS). Unlike XPS, LEIS probes exclusively the outmost atomic monolayer, which is the very surface involved in reactions with adsorbed species. We show that a BiOx preferential surface termination is found in all cubic Bi ternary oxides studied. However, pyrochlore Y2Ti2O7 does not exhibit preferential termination. We present an argument based on the revised lone-pair model for post-transition-metal oxides in order to explain how the O 2p-mediated Bi 6s−6p hybridization at the surface may reduce the surface energy of the material and, therefore, provides a driving force for a surface reconstruction that preferentially allocates BiOx at the surface. We further study the characteristic basicity of these solids based on CO2 adsorption/TPD monitored by in situ diffuse reflectance IR spectroscopy (DRIFTS). Results show that the Bi cubic oxides possess high CO2 chemisorption capacity, which is indicative of a high surface basicity for these materials. High surface basicity can be associated with O 2p−Bi 6s6p hybridized electronic states, which are more able to donate electronic density to adsorbed species than surface lattice oxygen ions, normally associated with basic sites in metal oxides.15,16 Additionally, preferential BiOx-termination observed in these oxides may contribute to increase the density of basic surface sites. Given the high CO2 chemisorption capacity of Bi-based ternary oxides, it would be interesting to explore the use of these types of materials in the development of catalytic, photocatalytic, and electrochemical technologies for the reduction of CO2. In fact, Bi-based electrodes and catalytic oxides have been found to have enhanced performance toward CO2 reduction reactions,17−19 which may be related to a higher CO2 chemisorption.

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EXPERIMENTAL SECTION

Synthesis of Samples. Single crystalline films used as well-defined model samples of Bi2Ti2O7 and Y2Ti2O7 were prepared by a method based on the spin-coating technique based on our recent report.10 Briefly: bismuth nitrate pentahydrate (Aldrich, ≥99.99%) or yttrium nitrate hexahydrate (Aldrich, 99.8%) was dissolved in acetic acid (Fischer Scientific, analytical grade). Then, titanium(IV) isopropoxide B


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Chemistry of Materials background subtraction spectra, because it reflects the sample with the lowest level of contaminants. CO2 Adsorption/Temperature-Programmed Desorption (TPD). For the CO2 TPD measurement, the cell was purged with a mixture of 30% CO2 in Ar with a total flow of 20 mL/min for 10 min at 35 °C. After 10 min, the CO2 was purged out of the gas atmosphere by switching to pure Ar with a flow of 20 mL/min. The purging was maintained for 20 min. A spectrum is recorded at the end of the CO2 adsorption and purging process. A heating ramp with 5 °C/min to 250 °C was performed to remove the remaining adsorbates. After the temperature has been held for 30 min, again, a spectrum with 500 scans is recorded. Finally, the sample was cooled to 35 °C and a final spectrum is recorded. During all adsorption, purging, and heating procedures, a series of spectra consisting of one spectrum per minute are recorded with 100 scans each.

cation observed in X-ray photoelectron spectroscopy (XPS) studies of Bi2Ti2O710 and other cubic Bi pyrochlore oxides such as Bi2Ir2O7−y. In these cases, the larger sampling depth of XPS, which could go up to 3 nm with an Al Kα source, is more than enough to observe a substantial amount of the “B” cation. The provided pyrochlore oxides do not have a layered structure of alternative cation planes, like that of perovskite materials; all possible terminations should present all bulk components, as it is indeed the case of Y2Ti2O7. The surface of Bi2Ti2O7 must, therefore, undergo some sort of reconstruction that places Bi3+ cations at the outermost atomic layer. For pyrochlore Bi2Ti2O7, bismuth-terminated surfaces could certainly lead to lower surface energy, and this can be rationalized in terms of the local electronic structure of the Bi3+ cation. As a free ion, Bi3+ has a configuration 6s26p0. It has been shown that, for many post-transitional-metal oxides, the 6s electrons hybridize strongly with O 2p states to give antibonding states of mixed-metal 6s−O 2p character at the top of the valence band. These states can further interact with nominally empty metal 6p states to lower the electronic energy and give a directional electron lone pair, provided that the cation occupies a site that lacks inversion symmetry.24 This O 2p-assisted Bi 6s−6p hybridization has been suggested to be energetically favorable in theoretical studies of Bi2Ti2O7, and related to a displacement of Bi atoms from its centrosymmetric D3d local environment within the pyrochlore structure.25 Provided that the directional lone pair is stereochemically active, it may cause local stress in the bulk of the material and phase instability. In fact, the need to accommodate the active bismuth lone pair in a space that is being compressed by the “B” cation has been identified as the driving force for the phase transition from pyrochlore to lower symmetry structures in Bi2Sn2O7 and Bi2Hf2O7,11 and it is probably related to the wellknown low stability of pyrochlore Bi2Ti2O7 relative to other bismuth titanate compounds. The reduced coordination at the surface, however, provides nonsymmetric sites, which allows the Bi 6s−6p hybridization and, therefore, lowers the internal electronic energy of Bi3+ cations at the surface. The stabilization of surface atoms leads to lower surface energy and, therefore, may offer a thermodynamic driving force for the segregation of Bi3+ cations and a surface reconstruction that preferentially allocates Bi3+ and O2− ions at the outmost atomic layer. We additionally studied the surface composition of polycrystalline free-standing samples of a series of cubic Bi3+ ternary oxides: Bi1.67Ti2O6.5, Bi2Zr2O7, and Bi2Hf2O7. The upper panel of Figure 2 shows the LEIS spectra of polycrystalline free-standing samples of this series of complex oxides after oxygen-plasma surface cleaning. Since the Hf and Bi peaks overlap in the He+ scattering spectrum, an Ar+ scattering spectrum was recorded instead for Bi2Hf2O7. The Ar+ scattering spectrum is not sensitive to oxygen, and therefore, the LEIS spectrum of Bi2Hf2O7 only includes the regions for Bi and Hf (the He+ spectrum for Bi2Hf2O7, where the spectral peaks associated with oxygen can be seen, is reported in Figure S3 of the Supporting Information). Similar to the single crystalline Bi2Ti2O7 samples, characteristic peaks of bismuth and oxygen largely dominate the LEIS spectra of all samples, indicating that the surfaces are mostly composed on the BiOx structure regardless of the nature of the small “B” cation. However, in the LEIS spectrum of these polycrystalline samples, a small peak feature can be seen at the region of the “B” cation indicative of a small fraction of Ti, Zr, and Hf at the surface of Bi1.67Ti2O6.5, Bi2Zr2O7, and Bi2Hf2O7, respectively.

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RESULTS AND DISCUSSION Low-Energy Ion Scattering Study of Surface Composition. The surface composition of model single crystalline samples and polycrystalline powders was measured by means of low-energy ion scattering (LEIS). As detailed in comprehensive review articles by Brongersma et al.,22,23 the inelastic interaction of a well-defined low-energy ion beam (noble gas ions) with atoms at the target surface produces a spectrum of backscattered ions with Gaussian peaks at characteristic energies directly related to the atomic number of the interacting atom at the target. Thus, measuring the spectrum of the backscattered ions allows the determination of atoms at the outermost atomic layer of a given sample. Only ions interacting with this outermost atomic layer generate the characteristic Gaussian peaks observed in a spectrum. Ions that penetrate the surface undergo multiple inelastic collisions with atoms at the second and further atomic layers, but may also reach the detector to form a continuous background that extends across a wide range of energies. Figure 1 shows LEIS spectra of a (001)-oriented pyrochlore Bi2Ti2O7 and Y2Ti2O7 single crystalline films after oxygen-

Figure 1. LEIS spectra of Y2Ti2O7 and Bi2Ti2O7. Samples are both (001)-oriented single crystalline films calcined at 800 °C.

plasma cleaning. In the He+ scattering spectrum of Y2Ti2O7, the expected characteristic peaks for yttrium, titanium, and oxygen can be clearly seen at 2500, 2125, and 1200 eV, respectively, which indicates that all three components of the ternary oxide are present on the surface. In contrast, the LEIS spectrum of Bi2Ti2O7 consists of only characteristic peaks for bismuth (at 2700 eV) and oxygen. Although no spectral peak can be detected in the titanium region, there is a clear background step increase in this area. This spectrum suggests that the surface of the Bi2Ti2O7 films terminates in a BiOx-like structure, but titanium atoms are present in the second and further atomic layers underneath this surface termination.22,23 This may be related to reports of appreciable surface enrichment of the “A” C


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Figure 2. LEIS spectra of bismuth complex oxides Bi2M2O7 (where M is Ti, Zr, and Hf). The Bi2Hf2O7 sample is an Ar+ beam measurement, which is not sensitive to oxygen (hence, there is no O signal).

accommodation of stereochemically active lone pairs by O 2p-assisted 6s−6p hybridization within the bulk of the material,26,29 without compromising the crystal energy of the system, and thus diminishing the driving force for BiOx surface termination. CO2 Adsorption. The remarkable difference in composition clearly observed between the bulk and the surface of bismuth pyrochlore and related cubic materials has never been considered in the numerous theoretical and experimental studies of this type of materials. This characteristic may, however, have strong implications in their surface behavior and is, therefore, an important factor to take into account when considering the interaction of these materials with other chemical species. For instance, Bi3+ is generally considered a weakly basic cation and the interaction of the Bi3+ cation with surface O2− anions may favor the overall surface basicity of the Bi compounds and their interaction with acidic species. In order to address this issue, we carried out a study of the basicity of the surface of these compounds by means of CO2 adsorption monitored by in situ IR spectroscopy. Figure 3 shows IR spectra in the hydroxyl groups stretching (νOH) region of pretreated polycrystalline samples of Y2Ti2O7,

After recording the LEIS spectrum of the surface, all three samples were sputtered with a high flux Ar+ beam until the area of the characteristic peaks of each element remained constant, at which point the preferential sputtering effects are minimized, and therefore, the spectrum fairly represents the composition of the bulk of the material. The LEIS spectrum of each sample at this point is shown in the lower panel of Figure 2. A remarkable difference in composition between the surface and bulk can be clearly observed by comparing the LEIS spectra before and after Ar+-sputtering. Given the tendency of this series of cubic Bi complex oxides to have BiOx surface termination, it is reasonable to think that this termination is largely driven by a lowering of the internal energy of the Bi3+ cation via O 2p-assisted 6s−6p hybridization at the surface. However, it is important to notice that the limited stability of bulk Bi3+ cations and low energy of the Biterminated surface can both contribute to the surface bismuth segregation observed in the Bi2M2O7 materials studied in this work. In general, the surface energy of a solid arises from the high energy of atoms at the surface relative to those in the bulk (due to the lower coordination). However, the noncentrosymmetric coordination around the Bi3+ cation usually found in bismuth oxides, which is associated with the O 2p-assisted 6s− 6p hybridization and the relaxation of the internal electronic energy of the cation, fits well the intrinsic broken symmetry of the surface. This provides stability Bi3+ cations at the surface described above. On the other hand, for cubic oxides, the limited distortion that can occur in centrosymmetric cation sites may prevent the stabilization of bulk Bi3+ cations and decrease the crystal energy, which would further favor the relative stabilization of surface Bi3+ cations. The segregation of Bi3+ to the surface and near surface may both relax the bulk lattice and maximize the Bi3+ surface exposure. In this sense, it is interesting to note that the related oxide α-Bi2Sn2O7, which possesses a distorted pyrochlore structure, presents a stoichiometric near-surface composition, as indicated by an XPS study.26 Additionally, in a LEIS study we performed on this material (reported in Figure S4 in the Supporting Information), we observed a significant amount of Sn at the surface, which contrasts the results for Bi cubic oxides studies in this work. Similarly, the near-surface composition of the monoclinic scheelite phase of BiVO4 has been also shown to have near stoichiometry by means of XPS ESCA.27,28 In these cases, the low symmetry of the crystals allows the

Figure 3. Infrared spectra of Y2Ti2O7, Bi1.67Ti2O6.5, Bi2Zr2O7, and Bi2Hf2O7 in the hydroxyl groups stretching (νOH) region of pretreated polycrystalline samples at 35 °C.

Bi1.67Ti2O6.5, Bi2Zr2O7, and Bi2Hf2O7 at 35 °C. In general, all spectra consist of a region of sharp peaks from 3800 to 3500 cm−1 overlapping a broad feature from 3600 to 3000 cm−1. This is a common observation in IR spectroscopic characterization of hydroxyl groups on oxide surfaces. The sharp peaks of absorption have been assigned to stretching modes of unperturbed or weakly perturbed OH groups, whereas the D


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Chemistry of Materials broad feature has been related to stretching modes of interacting OH groups via hydrogen bonds.30−32 Although broad features in the 3600−3000 cm−1 region may also be associated with strongly adsorbed molecular water, the absence of the H−O−H bending vibration mode at 1630 cm−1 and the stretching/bending combination vibration at 5230 cm−1 rules out the presence of molecular water adsorbed after the heat treatment. The relative intensity of the two components of the spectra (broad and sharp peaks) is different in each sample. The intensity of the sharp features is higher for Y2Ti2O7, Bi1.67Ti2O6.5 than those in Bi2Zr2O7 and Bi2Hf2O7, where the broad feature seems to be more predominant. This, therefore, suggests that the surface hydroxyl groups of both Y2Ti2O7 and Bi1.67Ti2O6.5 are predominantly isolated, while those of Bi2Zr2O7 and Bi2Hf2O7 are largely interacting with each other. Although the M cations in the Bi2M2O7 samples studied in this work seem not to be present at the surface of the material, LEIS analysis suggests that they may be present already in the second atomic layer and can, therefore, influence the chemistry of the surface. The influence of the M cation on the −OH groups configuration seems to be significant in the samples studied in this work. Figure 4 shows the infrared spectra of CO2 adsorbed on pretreated polycrystalline samples of Y2Ti2O7 and Bi1.67Ti2O6.5

Figure 5. Carbonate species identified based on the assignment of IR bands observed upon chemisorption of CO2 on metal oxides.

monodentate carbonates with symmetric stretching vibration (νs) at 1389 cm−1 and asymmetric stretching vibration (νas) at 1522 cm−1, (II) bidentate carbonates with νs at 1312 cm−1 and νas at 1572 cm−1, (III) “free” carbonates with νas at 1462 cm−1, and (IV) bicarbonate species with νs at 1462 cm−1, νas at 1620 cm−1, and −OH bending vibration (γOH) at 1224 cm−1. The depletion of the IR spectral signal at 3659 cm−1 (marked with a star “∗” in Figure 4) can be related to a decrease of surface hydroxyl groups, which react with CO2 to form bicarbonate species. The signals assigned to bicarbonate species disappear upon CO2 purging, which indicates that they are stable only under high CO2 concentration. Other carbonate species remain stable after the CO2 purge and can only be completely desorbed at temperatures above 200 °C, as confirmed by an in situ TPD measurement (data not shown). Similarly, the infrared spectra of adsorbed CO2 species on Bi2Zr2O7 and Bi2Hf2O7 (Figure 6) show that there is a series of

Figure 4. Infrared spectra of CO2 adsorbed on pretreated polycrystalline samples of Y2Ti2O7 and Bi1.67Ti2O6.5 during and after CO2 exposure at 35 °C. Figure 6. Infrared spectra of CO2 adsorbed on pretreated polycrystalline samples of Bi2Zr2O7 and Bi2Hf2O7 during and after CO2 exposure at 35 °C.

during and after CO2 exposure at 35 °C. In order to minimize the interference from the characteristic IR absorption of the samples, in each case, the IR spectrum of the bare sample (before CO2 exposure) was subtracted from the IR spectrum with adsorbed CO2 surface species. For the yttrium titanate sample, the characteristic IR absorption of gas phase and physisorbed CO2 can be seen at 2350 cm−1 and overtones at 3620 and 3720 cm−1. However, no vibrational bands assignable to chemisorbed species of CO2 can be found in the spectra. As soon as the CO2 is purged out, all IR signals disappear. In contrast, aside from the gas phase and physisorbed CO2, a series of chemisorbed carbonate species with characteristic IR absorption in the 1000−2000 cm−1 region are formed on the bismuth titanate sample. On the basis of the assignment of IR bands observed upon chemisorption of CO2 on metal oxides,33 these species include the carbonates displayed in Figure 5: (I)

stable carbonate species formed on these solids as well. However, the analysis of the band assignment indicates that no bicarbonate is formed upon CO2 chemisorption. The most intense bands at about 1550 and 1330 cm−1 can be associated with the symmetric and asymmetric stretching vibration of both (I) monodentate and (II) bidentate carbonate species. The absence of bicarbonate species on Bi2Zr2O7 and Bi2Hf2O7 suggests a low reactivity of surface hydroxyl groups of these samples, which may be related to the large interaction with each other observed in the IR study of the hydroxyl groups. These carbonate species that remain stable after CO2 purge out can only be completely removed by a purging procedure at temperatures above 200 °C. E


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O 2p-assisted Bi 6s−6p hybridization can easily occur. All bismuth ternary oxides studied here present outstanding CO2 chemisorption capacity, which indicates a high surface basicity. This can be associated with O 2p−Bi 6s−6p hybridized electronic states at the VBM, which are more able to donate electronic density to adsorbed species than surface lattice oxygen ions normally associated with basic sites in metal oxides. The preferential BiOx surface termination may also contribute to the surface basicity by substantially increasing the number of basic sites.

The most striking observation in the CO2 adsorption experiments is the very high CO2 chemisorption capacity of the series of cubic Bi3+ ternary oxide: Bi1.67Ti2O6.5, Bi2Zr2O7, and Bi2Hf2O7, in contrast with poor/no chemisorption on Y2Ti2O7. This can be interpreted as an increased basicity of the Bi compounds. The basicity of oxides is often associated with surface lattice oxygen anions or hydroxyl groups. Although LEIS analysis indicates the presence of surface oxygen, and hydroxyl groups are readily observed in the IR spectrum of all samples, only those sites of bismuth ternary oxides seem to act as basic sites for CO2 chemisorption. Ultimately, the basicity of these oxides depends on the ability of the surface of the material to donate electron density with an acceptor, CO2 in this case. While the top of the valence band of Y2Ti2O7 is almost entirely made of O 2p orbitals,34 the valence band maximum (VBM) of the series of Bi3+ oxides has a strong contribution from the Bi 6s and Bi 6p orbitals due to the O 2passisted Bi 6s−6p hybridization described above. The mixed character of the top of the valence band may induce a basic character to the series of cubic Bi3+ ternary oxides studied here, for two reasons. First, the O 2p−Bi 6s−6p hybridized electronic states lie above the O 2p energy levels, which leads to lower ionization energy and higher chemical potential for the bismuth compounds compared with that of Y2Ti2O7 due to a general shift upward of the top of the valence band.35 Second, a VBM with high Bi 6s character is more polarizable and dispersive, which stabilizes the formation of holes in the valence band,36 therefore, making the oxide semiconductor more able to donate electron density to an acceptor. These two factors may contribute to generate strong basicity on the surface of the series of bismuth ternary oxides studied. Additionally, the preferential surface termination in BiOx may substantially increase the number of basic sites available on the surface of all bismuth compounds, increasing, therefore, their CO2 chemisorption capacity. The characteristic surface basicity of Bi2M2O7 (M = Ti, Zr, Hf) may play a major role in novel potential applications in the field of catalysis and photocatalysis, and thus it should be considered further for the development of highly efficient materials. For instance, there is a growing interest in the development of catalytic, photocatalytic, and electrochemical technologies for the reduction of CO2.37,38 Provided the high CO2 chemisorption capacity of bismuth-based ternary oxides, it would be interesting to explore the use of this type of materials in this field. In fact, Bi-based electrodes and catalytic oxides have been found to have enhanced performance toward CO2 reduction reactions,17−19 which may be related to a higher CO2 chemisorption capacity by Bi-containing materials. Additionally, the chemistry behind the surface Bi3+ termination and the increased surface basicity we report here for bismuth-based oxides can be extrapolated to other oxides containing ns2 posttransition-metal cations like Sn2+, Tl+, and Pb2+, and these findings can be applicable to a wide range of materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b03232. Experimental details, X-ray diffractograms of Y2Ti2O7 and Bi2Ti2O7, X-ray diffraction patterns of all polycrystalline samples studied in this work, the He+ LEIS spectrum for Bi2Hf2O7, and the Ne+ LEIS spectrum for α-Bi2Sn2O7 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Sarah Fearn for her technical assistance and advice in the acquisition and interpretation of the LEIS spectra. The research reported in this work was supported by the King Abdullah University of Science and Technology. D.J.P. acknowledges support from the Royal Society (UF100105).

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REFERENCES

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CONCLUSIONS Our results demonstrate that cubic pyrochlore Bi2Ti2O7 and related cubic bismuth ternary oxides, Bi2Zr2O7 and Bi2Hf2O7, present preferential surface termination of a BiOx-like structure, whereas no favored surface termination occurs in cubic pyrochlore Y2Ti2O7. On the basis of the revised lone-pair model, we suggest that the BiOx-termination is driven by a surface energy lowering upon a decrease of the internal energy of Bi3+ cations at (noncentrosymmetric) surface sites, where an F


Article

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Surface Termination and CO2

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