Surface, Subsurface, and Bulk Oxygen Vacancies Quantified by

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Surface, Subsurface, and Bulk Oxygen Vacancies Quantified by Decoupling and Deconvolution of the Defect Structure of RedoxActive Nanoceria Rashid Mehmood,* Sajjad S. Mofarah, Wen-Fan Chen, Pramod Koshy, and Charles C. Sorrell School of Materials Science and Engineering, Faculty of Science, UNSW Sydney, Sydney, NSW 2052, Australia

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S Supporting Information *

ABSTRACT: Oxygen vacancy concentrations are critical to the redox/ photocatalytic performance of nanoceria, but their direct analysis is problematic under controlled atmospheres but essentially impossible under aqueous conditions. The present work provides three novel approaches to analyze these data from XPS data for the three main morphologies of nanoceria synthesized under aqueous conditions and tested using in vacuo analytical conditions. First, the total oxygen vacancy concentrations are decoupled quantitatively into surface-filled, subsurface-unfilled, and bulk values. Second, the relative surface areas are calculated for all exposed crystallographic planes. Third, XPS and redox performance data are deconvoluted according to the relative surface areas of these planes. Correlations based on two independent empirical results from volumetric surface XPS, combined with sequential deep XPS and independent EELS data, confirm that these approaches provide quantitative determinations of the different oxygen vacancy concentrations. Critically, the redox/photocatalytic performance depends not on the total oxygen vacancy concentration but on the concentration of the active sites on each plane in the form of subsurface-unfilled oxygen vacancies. This is verified by the pHdependent performance, which can be increased significantly by exposing these vacancies to the surroundings. These approaches have significance to the design and engineering of semiconducting materials exposed to the environment. potential of the Ce3+ ↔ Ce4+ switching10 establishes a dynamic equilibrium of transient charge compensation between the two valences, where the VÖ provides the driving force for the oxidation of adsorbed molecules, thereby establishing a weak electrostatic bond; the low reduction potential for Ce3+ ↔ Ce4+ switching provides the driving force for the reverse reduction. In effect, there is an equilibrium constant that effectively fixes the Ce3+ concentration ([Ce3+]), even when the VÖ is occupied transiently by adsorbed molecules. The physicochemical, structural, and microstructural features of nanoceria play an important role in determining the locations and concentration of these VÖ . Consequently, the morphologies, exposed surface planes, surface areas, and grain sizes are critical, and these depend principally on the synthesis method.11−20 Synthesis of nanoparticles by methods involving high temperatures not only reduces the concentrations of surface vacancies but also produces hard agglomerates, which cause discrepancies in the measurement of surface areas.21,22 On the other hand, precipitation and hydrothermal methods tend to produce monodisperse particles of higher [VÖ ]20 and soft agglomerates, which potentially can be dispersed by agitation.23 These methods generally synthesize cubes, truncated octahedra, and square rods, all of which expose one or more of the following

1. INTRODUCTION Nanoceria (cerium oxide nanoparticles, CeO2‑x) recently has attracted attention owing to its redox/photocatalytic, catalytic, and acid−base properties for energy, environmental, and biomedical applications. The potential performance of nanoceria is considered widely to be associated with the intrinsic defects Ce3+ and charge-compensating oxygen vacancies (VÖ ) present at the surface or subsurface of the nanoparticles, where continuous reversible Ce3+ ↔ Ce4+ switching and associated changes in oxygen vacancy concentration ([VÖ ]) occur; changes in the pH in aqueous media are known to be one of the means of initiating this switching.1−9 The insuperable challenge to researchers is that defect equilibria for semiconductors such as CeO2‑x typically are described for anhydrous and hightemperature conditions. Furthermore, there does not appear to be any analytical instrumentation capable of directly assessing defect equilibria under hydrated (water vapor) or aqueous (liquid water) conditions. Consequently, the present work presents an alternative method to assess these equilibria through indirect approaches of characterization of nanoceria synthesized under aqueous conditions, the defect structure of which is retained during testing using in vacuo analytical conditions in air and under vacuum. Hypothetically, permanent annihilation (i.e., filling) of the VÖ would require the formation of a covalent bond and the associated electron sharing. However, the low reduction © XXXX American Chemical Society

Received: February 4, 2019

A

DOI: 10.1021/acs.inorgchem.9b00330 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry facets: {100}, {110}, and {111},24 which play a key role in the generation of reactive oxygen species (ROS) and the associated Ce3+ ↔ Ce4+ redox chemistry through the presence of VÖ on these planes.25−28 DFT calculations of the oxygen vacancy formation energies of these low-index crystallographic planes10 highlight the role of the exposed facet, suggesting that each plane has a specific Ce3+ ↔ Ce4+ equilibrium constant. Since VÖ are critical to the redox/photocatalytic performance of nanoceria, the concept of oxygen vacancies at the surface and subsurface and in the bulk have been canvassed through theoretical and experimental approaches,1−7,29−31 such as density functional theory (DFT),2 X-ray photoelectron spectroscopy (XPS),3 synchrotron XPS,4 secondary ion mass spectrometry (SIMS),5 and electron energy loss spectroscopy (EELS).29−31 Although EELS is capable of analyzing the oxygen concentration ([O]) and the inferred [VÖ ] at subnanometer resolution, such an analysis cannot be done under aqueous conditions.29,30 Furthermore, although EELS data can be used to obtain lateral concentration profiles for single grains,31 data for the depth concentration profiles remain unknown. Consequently, the present work is designed to elucidate these unknown issues by (1) decoupling the total [VÖ ] into its component f illed (and thus deactivated for redox/photocatalysis) and residual unf illed (and thus available for redox/ photocatalysis) and (2) deconvoluting the total surface analytical data (XPS and surface area) into their component contributions from individual exposed crystallographic planes of nanoceria synthesized under aqueous conditions.

point, NH4OH was added using the same method until the pH increased to ∼10.0. The suspension was aged by magnetically stirring for 36 h at 45 °C, after which it was transferred to a 50 mL plastic centrifuge tube and subjected to procedures identical to those applied to the nanocubes and nanorods. For all three types of nanoparticles, which are photocatalytic, storage between testing was done in the dark in an opaque closed container in order to avoid light-induced reactions.13,14 2.3. Characterization of Nanoceria. 2.3.1. X-ray Diffraction (XRD). The samples were prepared for mineralogical characterization by hand grinding in an agate mortar and pestle and loading into aluminum sample holders. The XRD powder diffraction was done using a Philips X’Pert multipurpose X-ray diffractometer (MPD, Almelo, Netherlands), with Cu Kα radiation (0.15405 nm) at 20°−80° 2θ, with step size 0.02° 2θ and scanning speed 5.5° 2θ/min. The peaks were analyzed using X’Pert High Score Plus software. The crystallite sizes and lattice strains were calculated from line broadening using Scherrer’s formula and the Williamson−Hall formula, respectively).33 2.3.2. Laser Raman Microspectroscopy. The nanoceria also were characterized mineralogically by laser Raman microspectroscopy by using a Renishaw inVia Raman microscope (Raman; Gloucestershire, U.K., beam diameter 1.5 μm), which was equipped with a 35 mW helium−neon green laser (514 nm), at 200−800 cm−1. The spectra were fitted and calibrated using Renishaw WiRE 4.3 software. Relative lattice strains were considered by peak shift.34 2.3.3. Surface Area and Particle Size. The specific surface areas of the nanoparticles were determined by the Brunauer−Emmett−Teller (BET) method using a Micromeritics Tristar-3000 (Norcross, GA). Transmission electron microscopy (TEM) was used to image the nanoparticles directly owing to agglomeration (Supporting Information section 1, Figure S1). Fifty individual grains of each type of nanoparticles were selected at random for determination of the average particle size by image analysis using ImageJ (National Institutes of Health, Bethesda, MD). The NC particles were of highly consistent size. Approximately two-thirds of the NO particles were of consistent size, with only small variation. The NR particles exhibited significantly varying aspect ratios, so the length and width were determined as averages, albeit with larger variations than those of the NC and NO particles. 2.3.4. Transmission Electron Microscopy (TEM). Morphological and structural characterization of the nanoceria particles was done by TEM using a Philips CM200 (Eindhoven, Netherlands) operating at 200 kV. The samples were prepared by dispersion in ethanol and dropping on a copper grid. TEM imaging was done using Gatan Digital Micrograph 3.9 software. Selected area electron diffraction patterns (SAED) of the nanoparticles were done at varying magnifications in order to assess the lattice characteristics of the nanoceria particles. The crystallographic planes were identified by fast Fourier transform analysis using Gatan Microscopy Suite 3 (GMS3) software. The corresponding interplanar spacings were confirmed using the reported X-ray diffraction data for CeO2 (JCPDS 34−0394). 2.3.5. X-ray Photoelectron Spectroscopy (XPS). Surface chemical analysis was done by XPS using a Thermo Fisher Scientific, ESCALAB 250Xi spectrometer (Loughborough, Leicestershire, U.K.; 20 °C, 10−7 Pa, 13.8 kV, 8.7 mA, monochromated Al Kα X-rays at 1487 eV, beam diameter 500 μm, beam exposure time 30 min). Variations in [Ce3+] and [Ce4+] were ±0.5 at. %. The data were analyzed using Thermo Scientific Avantage software and the peak fitting was done by using the standard Gaussian−Lorentzian shape function. Iterations were performed using the Marquardt method. The standard deviations between peak-fitting iterations always were 82 wt %) were obtained from Sigma-Aldrich, Australia. Hydrogen peroxide (H2O2, 30 vol % in H2O, Ajex Finechem) was obtained from Thermo Fischer Scientific, Australia. Sodium hydroxide (NaOH, 98 wt %) was obtained from Chem-Supply, Australia. 2.2. Synthesis of Nanoparticles. Nanoparticles were synthesized by combination of wet-chemicals methods including precipitation and hydrothermal reaction.15,16 2.2.1. Nanocubes (NC) and Nanorods (NR). Nanocubes were synthesized by a precipitation−hydrothermal method, where a solution of 30 mL of 0.9 M Ce(NO3)3·6H2O was magnetically stirred for 10 min in a 250 mL Pyrex beaker, followed by the dropwise addition of 30 mL of a 25 M NaOH solution using a 50 mL glass buret. The mixture was magnetically stirred for 50 min at room temperature. The mixture was centrifuged at 3000 rpm for 5 min, the supernatant decanted, and the residual solid thick slurry was redispersed magnetically in a new rinsed 30 mL of 25 M NaOH solution. The mixture then was transferred to a 50 mL Teflon-lined autoclave reactor, placed in an oven, heated at 5 °C/min to 200 °C, soaked for 24 h, and cooled naturally. The resultant suspension was transferred to a 50 mL plastic centrifuge tube, and any residual solid was transferred following rinsing with deionized (DI) water. This suspension was centrifuged at 2500 rpm for 10 min, the supernatant decanted, the residual solid rinsed, and the suspension pH measured. These four steps were undertaken a total of 10 times, which was sufficient to stabilize the pH at ∼7. The final product was transferred to a 250 mL Pyrex beaker, the residual solid was rinsed, and the suspension was dried at 70 °C for 36 h. For nanorods, the procedure was identical to the above except that the autoclaving was done at 130 °C. 2.2.2. Nanooctahedra (NO). In order to synthesize truncated nanooctahedra (NO), solid Ce(NO3)3·6H2O was dissolved in DI water in a 250 mL Pyrex beaker and magnetically stirred for 30 min to yield a concentration of 0.9 M. H2O2 was added dropwise using a glass dropper while monitoring the pH until it decreased to a value of ∼3.5. At this B


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Inorganic Chemistry

Figure 1. TEM images of nanoceria: rows 1−3 TEM, high resolution TEM, and SAED patterns of (a−c) nanocubes (NC), (d−f) nanorods (NR), (g− i) and nanooctahedra (NO). a rate of 0.14 nm/s and sampling interval of 5 s. Tantalum pentoxide (Ta2O5) was used as a calibration reference for the depth of etching. The expected partial of the surface by argon ion beam is explained in results and discussion. 2.4. Photocatalytic Activity. The photocatalytic performances of all three morphologies of nanoceria were assessed in terms of photobleaching of methylene blue (MB) under UV irradiation separately at pH 6.0 and pH 9.0. Methylene blue dye solutions were prepared by dissolving MB (10−5 M) in deionized water. Nanoceria particles then were dispersed in the dye solution at a concentration of 2 mg/mL in a beaker, which then was placed in a light-obstructing container. Irradiation was done using a UV lamp (UVP, 3UV-38, 8 W, 350 V, ∼50 Hz, 0.16 A, light intensity 0.39 mW/cm2 with the lamp placed 9 cm above the solution) at illuminating intensity 1.6 × 10−17 J at 365 nm for 1 h and 7.9 × 10−17 J at 365 nm for 3 h. The pH values were modified by addition of 1 M NaOH and 1 M HCl. Tests were done in triplicate for both time points. The MB degradation from UV irradiation was analyzed in terms of the change in the absorbance peak height of MB (665 nm) using a PerkinElmer UV−visible spectrometer.

that the residual lattice strain (tensile) was in the converse order NO > NR > NC, which confirms the degrees of crystallinity. As the TEM data, discussed subsequently, reveal that the nanoceria grains are single crystal, then the crystallite sizes (Supporting Information Section 2, Table S1) and grain sizes (Supporting Information Section 2, Table S2) should be approximately the same. This is the case for the NR, where the former (based on a spherical grain) is 11 nm and the latter (based on the rod diameter) is 17 ± 9 nm. It also is the case for the NO, where the former is 5 nm and the latter (based on the cross section) is 7 ± 2 nm. However, there is a significant difference between the two values for the NC, which are 20 nm for the former and 58 ± 5 nm for the latter (based on the width). If the size differences between the spherical and cubic morphologies are calculated on the basis of the TEM data, the crystallite sizes should be in the range 39−46 nm, which represents a range of 26−27% disagreement, which is within the ranges of variation for the other two morphologies. However, there also is a crystallite size effect on the Raman peak shift, where increasing crystallite size (or grain size for these single-crystal nanoparticles) is inversely proportional to the red shift.36 Since the observed red shift is in the order NO > NR > NC and the grain size (using the smaller, more easily vibrated, width of the NR) also shows the converse relationship, then the red shift also may result from a particle size effect, which generally is considered to be a result of phonon confinement7 and which results in the observed trend in asymmetric line broadening. The longitudinal optical vibration at ∼600 cm−1 is well established as being indicative of the presence of oxygen vacancies (VÖ ), albeit invariably at low intensities.37,8 Supporting Information section 1, Figure S2d shows that the

3. RESULTS AND DISCUSSION Cerium oxide nanocubes (NC), nanorods (NR), and nanoctahedra (NO) were produced by precipitation and hydrothermal synthesis in the absence of surfactant or capping agent. All three morphologies occurred as essentially single phases and not as mixtures. The X-ray diffraction (XRD) data for the three morphologies (Supporting Information section 1, Figure S2a− c) show that the degree of crystallinity is in the order NC > NR > NO. A decrease in the degree of crystallinity is reflected in the extent of residual lattice strain, which can be assessed by both Xray line broadening34 and laser Raman microspectroscopy peak shift35 (Supporting Information section 1, Figure S2d). Both sets of data (Supporting Information Section 2, Table S1) show C


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Inorganic Chemistry

Figure 2. XPS spectra and relevant correlations of nanoceria: (a−c) morphology−valence correlation; (d−f) crystallographic deconvolution of the relevant O 1s peaks.

Table 1. Correlation of [VÖ ] of Nanoceria with Redox/Photocatalytic Effectsa concentrations (at. %) O bonded to Ce (XPS) total [VÖ ]A

Ce3+

Ce4+

total O sitesB

filled VÖ C

total surface-adsorbed H2O (XPS)

residual unfilled VÖ D

adsorbed H2O on surface Ce3+ E

morphology

[VÖ ]Tot

[Ce3+]O

[Ce4+]O

[OSites]

[VÖ ]Fill

[H2O]Ads/Tot

[VÖ ]Unfill

[H2O]Ads/Ce3+

RedoxF (MB)

NC NR NO

11.3 13.8 13.8

27.0 15.0 30.0

54.9 70.4 54.4

93.2 99.2 98.2

6.8 0.8 1.8

18.1 14.6 15.6

4.5 13.0 12.0

11.3 13.8 13.8

22 ± 7 68 ± 4 59 ± 2

a

Methods of determination of values (superscripts A−F) in table are given in the Supporting Information section 3, note 2

the {100} and {110} planes is another {110} plane, this confirms that the terminating pinacoid is a {110} plane, also as indicated by others.27 The low-resolution TEM images demonstrate that all three morphologies give the appearance of being agglomerated (Supporting Information section 1, Figure S1). However, the NC show corner-to-face contact; the TEM images demonstrate the occasional presence of isolated NC, NR, or NO grains; and there is no evidence of nanoparticle intergrowths; all of which suggest that the nanoparticles did not form hard agglomerates.22 Hence, the face-to-face contact of the NR and NO indicates that soft agglomerates22,23 formed as transient artifacts of the synthesis processes as well as the dispersion process (viz., suspension in ethanol) used for TEM analysis. ImageJ39 was used to process the TEM images in order to calculate the dimensions of the nanoparticles, where 50 grains each of the NC and NO revealed uniform sizes while the NR were of variable aspect ratios, so average values for the dimensions of 50 NR were determined (Supporting Information section 2, Table S2). Finally, relative surface areas (RSA) for each surface-exposed facet for all three morphologies are contrasted with the total specific surface areas (SSA) (Supporting Information section 2, Table S3). The RSA are calculated on the basis of perfect planar

peak for NC is lost in the spectrum noise and the peak for NR is obscured by the noise, but the peak for NO is unambiguous. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) data are presented in Figure 1. In row 1, the particle morphologies, sizes, and the natures of agglomeration can be seen. It may be noted that the commonly observed corner-truncated and corner- and edge-truncated13 cubic morphologies were not observed. Although conversion of the (100) facets of a perfect cube are known to reconstruct to the (111) corner and (110) edge truncations under strongly oxidizing conditions,12 other work16,17 shows that the morphology is strongly dependent on the temperature and NaOH concentration (i.e., pH). In row 2, the crystallographic orientations of the morphologies are confirmed by correlations between measured and reported interplanar spacings.38 In row 3, the diffraction spots of the SAED pattern of the NC confirm the high degree of crystallinity. The XRD crystallinity data for the NR and NO are confirmed by the nature of the rings, which are sharper for the NR and more diffuse for the NO. The crystallographic relations of the NR confirm (Supporting Information section 1, Figure S3) that it exhibits {100} and {110} pinacoids that form what are effectively the prism faces and these are capped by {111} and {100} domes and a {110} pinacoid. Since the only plane that is mutually perpendicular to D


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Inorganic Chemistry

Figure 3. Structure−activity correlations and deep XPS of nanoceria: (a) redox/photocatalytic performance correlation with time at pH 6.0; (b) redox/photocatalytic performance correlation with time at pH 9.0; (c) redox/photocatalytic performance correlations with [VÖ ]Tot and [VÖ ]Unfill; (d) Microstructural and structural correlations with [VÖ ]Unfill; (e) depth profiles for [O] from deep XPS O 1s peak (depths from calibrated data based on Ta2O5 depth profiles31) and EELS (positive locations [purple] inverted relative to negative locations [black] (Ce vacancies at greater depths not shown); (f) depth profiles for [Ce3+] from deep XPS Ce 3d (depths from calibrated data based on Ta2O5 depth profiles31).

of the Ce3+−O peaks in the O 1s spectra reflect the concentrations of the oxygen vacancies. Since, in principle, there is one oxygen vacancy per two Ce3+ ions, then the [Ce 3+ ] and the total oxygen vacancy concentrations ([VÖ ]Tot) for all three morphologies can be determined (Supporting Information section 3, note 1). These calculations show that the order of [VÖ ]Tot (and hence 1 /2[Ce3+]) is NO = NR > NC, which suggests the expected order of redox/photocatalytic performance. The quantitative XPS data have been decoupled in order to differentiate the total oxygen vacancy concentration ([VÖ ]Tot)

facets, but some degree of surface irregularity is inevitable in all systems. The surface XPS spectra for the Ce 3d region, which are represented in Figure 2a−c, demonstrate the presence of both valences of cerium.40 The identification of Ce3+ confirms the presence of charge-compensating oxygen vacancies at the surfaces and subsurfaces of the nanoparticles.32,40−42 Furthermore, the O 1s spectra are represented in Figure 2d−f, where the binding energies of Ce 4+ −O, Ce 3+ −O, and Ce−H 2 O (adsorbed) are given.11,41 The intensities and areas under the peaks of the Ce3+-O and Ce−H2O regions have been deconvoluted, which is discussed subsequently. The intensities E


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Inorganic Chemistry into filled ([VÖ ]Fill) and unfilled ([VÖ ]Unfill) oxygen vacancy concentrations, which are given in Table 1. Since it is required that the total number of O sites must equate to [VÖ ]Tot + [Ce3+]O + [Ce4+]O = 100%, then the fact that [OSites] are less than theoretical (100%) indicates that the difference arises from the transient filling of the VÖ with OH− of the H2O from the environment43 (the data have been normalized to eliminate adventitious carbon and adventitious oxygen associated with carbon; these data are given in Supporting Information section 3, note 1). Consequently, these inequalities allow the determination of the [VÖ ]Fill, which have been transiently filled by the adsorbed OH−, and the [VÖ ]Unfill, which are determined by the Ce3+ ↔ Ce4+ equilibrium constants. This decoupling of the two different types of VÖ is critical because the former are deactivated for redox/photocatalysis while the latter remain active, the concentrations of which correlate with average decrease in methylene blue (MB) dye degradation. The rationale for this decoupling is supported by recent DFT calculations,2 which indicate that the formation of the subsurface oxygen vacancies is energetically more favorable than that at the surface. The relevant experimental and redox/photocatalytic performance data are shown in Figure 3. Parts a−d of Figure 3 clearly link the morphology to the [VÖ ]Unfill and corresponding redox/ photocatalytic performance. This is highlighted by the matched data for the latter two figures (Figure 3a,b), which reveal NC ≪ NO < NR. Furthermore, the similarity in the data sets (1 h vs 5 h) shown in the former two figures indicates the rapidity of the redox/photocatalytic reactions. Also, while the effect of pH also is significant, the published data are for irregularly shaped particles, which probably explains their contradictory trends.42−46 The present work clarifies the relevant features. At pH 6.0, the reducing conditions favor the presence of Ce3+, which should enhance the performance. However, it is pH 9.0 and the oxidizing conditions favoring Ce4+ that are superior. This surprising result is explained clearly by the speciation and Pourbaix diagrams.47 That is, at pH 6.0, the surface of the CeO2‑x experiences solubility of both Ce4+ (reductive) and Ce3+, so the active sites are degraded continuously by partial removal of Ce3+ and associated surface and subsurface oxygen vacancies. In contrast, at pH 9.0, a Ce(OH)4 passivating layer is established on the surface of the CeO2‑x. When the UV radiation decomposes it,48 this results in the conversion to the soluble species Ce(OH)2+ (at all pH) and Ce(OH)3 (up to pH ∼ 9.7).47 This results in removal of the surface-filled oxygen vacancies (incorporated in the gel layer) and exposure of the subsurface oxygen vacancies directly to the solution. This explains the significant increase in the redox/photocatalytic performance, as shown in Figure 3c,d. As will be discussed subsequently, the origin of the morphological correlation lies in the low-index exposed crystallographic planes. Furthermore, it is well-known that there are converse correlations between the particle size and other structural parameters,42 as shown in Figure 3d (also see Supporting Information section 2, Table S4). In light of recent reports of the role of the [VÖ ]Tot (and hence 1/2[Ce3+]),10,34 the redox/ photocatalytic performance should exhibit a converse correlation with the particle size. In contrast, it is significant that parts c and d of Figure 3 show that the present work does not support the conventionally assumed direct relation between [Ce3+] and the redox/photocatalytic performance. The data supporting this apparent anomaly are given in Table 1, where it can be seen that the [VÖ ]Unfill correlates with the redox/photocatalytic perform-

ance. More importantly, these data must be considered in light of the effects of the morphology and the surrounding environment. A key striking observation for Table 1 (see Supporting Information section 3, note 2, footnote D) is that the two independently obtained sets of experimental data, which are the [Ce3+] and the [H2O]Ads/Tot, yield equal values for [VÖ ]Tot and [H2O]Ads/Ce3+ for all three morphologies with no variation between the numerical values, viz., 100% precision. This triple set of identities shows that the [H2O]Ads/Ce3+ is associated solely with the Ce3+ ions and not Ce4+ ions. This association of the adsorbed H2O and Ce3+ ions is expected since the Pourbaix diagram47 shows that Ce4+ is not compatible with H2O, resulting in reductive solubility. The equality between these two variables actually is as expected because each oxygen vacancy in CeO2‑x is integrated with an unpaired electron associated with each of the two Ce3+ ions. These two electrons thus are available to interact with the valence electrons of the OH− ions in a single adsorbed H2O molecule. This is why there is a 1:1 ratio between the total number of oxygen vacancies and the number of H2O molecules adsorbed on the free surface (viz., Ce3+ ions) of the solid. Furthermore, for redox/photocatalytic performance, it has been established both theoretically and experimentally that molecular oxygen dissolved in aqueous solutions can be adsorbed at oxygen vacancy sites on nanoceria surfaces, thus potentially generating a superoxide radical (••O2−) by accepting one of the two unpaired electrons during continuous reversible redox Ce3+ ↔ Ce4+ switching18 These superoxide radicals are effectively ROS and so can enhance the redox/photocatalytic performance of nanoceria. The concept of f illed and unfilled oxygen vacancies suggests that the former are accessed readily by environmental H2O because they are located in the outermost surface monolayer while the latter are blocked partially from access by H2O because they are located at the adjacent subsurface monolayers. This model has been explored in the present work by depth profiling using deep XPS. However, this technique is contentious owing to the established partial reduction of CeO2‑x by both the X-ray beam49−51 and the Ar ion beam52,53 used in XPS. Examination of the relevant literature for pure ceria (a summary of the key studies is given in the Supporting Information, Table S5) reveals that this reduction is a function of the beam power,50−53 exposure time,50,52 and particle size.4,8,31 While differentiation between the surface altered by the Ar and X-ray beams and the unaltered bulk has been demonstrated,50 the depth of the surface alteration has been determined to be 1.5−2.052 or 2.0−3.0 nm,3 depending on X-ray beam power and time. It appears that the only data reporting a cross sectional compositional profile for pure ceria are those of Stroppa et al.,31 who used EELS to determine the [O] across single coprecipitated octahedral (described as spherical) grains of ceria (Figure 3e). These data reveal that the entire surface is hyperstoichiometric in oxygen, which is surprising in light of both the expectation of the presence of oxygen vacancies and the partial reduction from the X-ray beam. More importantly, these data indicate that there is a steep downward oxygen gradient over the first monolayer (surface) and the adjacent one or two monolayers (subsurface), below which the data trend toward a constant oxygen concentration (bulk). The scatter in the bulk data is likely to have resulted from variations in beam penetration volume across each rounded grain, grain roughness, the presence of Ce3+/Ce4+ clusters3, and/or the presence of oxygen vacancy clusters.54 However, the spike to higher [O] at a F


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Inorganic Chemistry

Table 2. Exposed Surface Planes, Relative Defect Concentrations, And Redox/Photocatalytic Performance As a Function of Morphologya relative deconvoluted surface data for exposed facets and their redox performances morphology

SSA (m /g)

[VÖ ]Tot

exposed facets

RSAA (%)

[VÖ ]RelB

[VÖ ]FillC

[VÖ ]UnfillE

RedoxRelFb

NC NR

14 105

11.3 13.8

NO

199

13.8

{100} {100} {110} {111} {100} {111}

100 48 49 3 43 57

11.3 5.4 8.1 0.3 4.8 9

6.8 0.8 0D 0D 1.8 0D

4.5 4.6 8.1 0.3 3 9

22 10.6 55.5 1.5 9.5 49.5

2

a Methods of determination of values (superscripts A−F) in the table are given in Supporting Information section 3, note 3. bCalculated by rational analysis using the RSA.

depth of 2.1 nm and the reverse trend in the subsurface can be explained by the presence of metal vacancies, which is discussed subsequently. The nature of these EELS data31 was confirmed using deep XPS depth profiling for powder films of all three morphologies, as shown in Figure 3e. This correlation is significant partly because the EELS data report point lateral analyses across single equiaxed grains while the deep XPS data report planar depth analyses into multiple grains of three different morphologies. The largest change in oxygen concentration is over the first monolayer (surface), followed by more gradual decreases over the adjacent five monolayers (subsurface); the constancy of the oxygen vacancy concentration at greater depths is clear (bulk). In light of the expected partial reduction of the surface 1.5− 3.0 nm by an argon ion beam, the reliability of these data must be considered. First, the bulk composition is not affected by the beam,3,8,50,52 so the data at depths greater than ∼3.0 nm are reliable. Second, since surface reduction takes place, this forces the trend from the surface to the bulk to increasing [Ce3+], which means that the overall trends of the curves are correct. Third, the acquisition time of 5 s is substantially less than those used in similar deep XPS studies (Supporting Information, Table S4, 900 s at 1487 eV and 5400 s at 50 eV), so considerably less reduction from the X-ray beam is expected in the present work. Since, as shown in Figure 3f, the bulk [Ce3+] from deep XPS of ∼7% is a maximum but the surface XPS data in Tables 1 and 2 are substantially higher, this apparent anomaly must be explained. The reason for this lies in the two different techniques. With deep XPS, each layer is etched and analyzed sequentially over a period of 5 s at 1487 eV, so these data provide local compositional information. In contrast, surface XPS, which is analyzed over a period of 30 min at 1487 eV, provides volumetric data over a larger region that corresponds approximately to the depth of the surface + subsurface. Although the surface XPS probe depth depends principally on the X-ray beam power and time and the X-ray absorption coefficient of the material, typical beam penetration depths are reported to be two to three monolayers,55 1−3,56 2−5,57 and >{110} = −91.90 kJ/mol > {111} = −63.10 kJ/mol

• Table 2 stresses that H2O adsorption on {100} dominates and effectively excludes those on {110} and {111} because [VÖ ]Fill is in the order: NC{100} = 6.9% > > NO{100} + {111} = 1.8% > NR{100} + {111} + {110} = 0.8%

Consequently, the concentration of unfilled oxygen vacancies ([VÖ ]Unfill) for each plane can be calculated by difference from [VÖ ]Rel. The deconvoluted data for the redox/photocatalytic performance in Table 2 show that the ranking of the effectiveness of the planes in the NR and the corresponding RSA are in the respective orders: {110} > {100} > {111} 49% > 48% > >3%

This relation is identical to evaluations by density functional theory (DFT) calculations of oxygen vacancy formation energies for these exposed crystallographic planes.26,68 Since the {110} planes have the highest [VÖ ]Unfill, they have the highest RSA, and they are unique to NR, then these confirm the superior performance of this morphology. Hence, it is the contribution of the {110} planes that dominates the redox/ photocatalysis results. In contrast, the ranking of the effectiveness of the planes in NO and the corresponding RSA are not in the same respective relative orders for the two other planes: {111} > {100} 57% > 43%

Again, the reason for this is shown in Table 2, where the values for [VÖ ]Unfill and the RSA correspond with this order. These DFT data as well as those differentiating between surface and subsurface oxygen vacancies2,68 emphasize the importance of the deconvolution because both morphologies exhibit the same [VÖ ]Tot but this is not the parameter that determines the redox/photocatalytic performance; it is the [VÖ ]Unfill. Thus, the present work not only supports the view that subsurface oxygen vacancies exist on the three principal low index crystallographic planes, but it also identifies these defects as the active sites for redox/photocatalysis. More specifically, the XPS data and their analyses highlight the following points: I


Article

Inorganic Chemistry

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(18) (19)

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the XPS and redox/photocatalytic performance data accordingly. The key outcome of these procedures is the quantitative determination that the redox/photocatalytic performance depends not on the total oxygen vacancy concentration for each plane assumed from the [Ce3+] from surface XPS but on the concentration of the residual unfilled oxygen vacancies as determined by the present analysis. Furthermore, the performance can be increased significantly through the imposition of basic pH conditions. These result in the formation of a Ce(OH)4 passivating layer, which is decomposed by UV radiation, thereby exposing the unfilled oxygen vacancies directly to the surrounding environment. The data analysis also introduces the importance of the acquisition of sequential deep XPS data, which provide the bulk stoichiometry, identify the relative stoichiometry at the surface, and indicate the trend between these two across the subsurface. Furthermore, deep XPS and corresponding EELS data demonstrate that oxygen hyperstoichiometry requires the formation of cerium vacancies, which also can be expected to contribute to redox/photocatalysis. The approaches of the present work provide guidance to the engineering of different morphologies, sizes, and aspect ratios of the nanoceria grain morphologies that can be synthesized by precipitation and hydrothermal processes. They also have significant practical ramifications to semiconductors as these materials inevitably are exposed to water vapor in the environment.

unfilled. The unfilled subsurface oxygen vacancies remain active sites for redox/photocatalysis. The consistency of the surface XPS, deep XPS, and EELS data indicate that the defect structures established during the aqueous synthesis conditions were retained during the vacuum testing conditions. The effect of pH is critical in that the expected benefit of acidic (reduction to form Ce3+) conditions is contradicted by the superior redox/photocatalytic performance of basic conditions. The preceding phenomenon results from the dissolution of Ce3+ and loss of associated oxygen vacancies from the surface of CeO2‑x under acidic conditions while basic conditions result in the formation of a protective passivating layer of Ce(OH)4, which, upon decomposition from exposure to UV radiation, exposes the unfilled subsurface oxygen vacancies. Ceria contains not only oxygen vacancies but cerium vacancies, the latter of which are imposed by the confirmation of oxygen hyperstoichiometry on the surface and in the bulk. These cerium vacancies also may contribute to the redox/photocatalytic effects owing to the associated unpaired electrons. The bulk of the grain contains a constant level of unfilled oxygen vacancies, of which the contribution, if any, to the redox/photocatalytic activity remains unknown. Since the oxygen vacancy concentration never has been deconvoluted according to the exposed facets, it remains unrecognized that the effect of morphology on the redox/ photocatalytic performance of ceria is a function of the equivalent exposed crystallographic planes. Each of these exposed facets exhibits a Ce3+ ↔ Ce4+ equilibrium constant, which is moderated by the nature of the OH− electrostatic bond, the reduction potential, the oxygen vacancy formation energy, and the planar surface area. The match (NR) and nonmatch (NO) between the DFT calculations for the oxygen vacancy formation energies for the three low-index crystallographic planes and the redox/ photocatalytic performance are a reflection of the roles of the oxygen vacancy formation energy and the planar surface area in the Ce3+ ↔ Ce4+ equilibrium constants for the different exposed crystallographic planes.

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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.inorgchem.9b00330.

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Transmission electron microscopy, X-ray diffraction, Xray photoelectron spectroscopy, laser Raman microspectroscopy, surface areas, and photocatalytic redox performance data (PDF)

AUTHOR INFORMATION

Corresponding Author

*(R.M.) E-mail: [email protected]. ORCID

Rashid Mehmood: 0000-0003-4073-7835 Wen-Fan Chen: 0000-0002-2917-3875

4. CONCLUSIONS The present work reports data for the structural parameters and redox/photocatalytic performance of three morphologies of nanoceria. A critical observation is that the defect structures established during synthesis in aqueous environments are retained during testing under vacuum conditions, which is confirmed by the consistency of three different sets of experimental data. The overall data analysis introduces three novel procedures that enable volumetric surface XPS data to be used to determine quantitative data for surface, subsurface, and bulk oxygen vacancy concentrations. First, the total oxygen vacancy concentration is decoupled in order to differentiate between (a) deactivated transiently f illed oxygen vacancies in the outermost surface monolayer and (b) active sites in the form of unf illed oxygen vacancies at the adjacent subsurface monolayers. Second, the relative surface areas for each lowindex exposed crystallographic plane for all three morphologies, each of which has a Ce3+ ↔ Ce4+ equilibrium constant, are calculated. Third, the combined data allow the deconvolution of

Author Contributions

R.M. designed the project; undertook the syntheses, characterization, TEM imaging, and data analysis; wrote the first draft of the manuscript; and worked on all subsequent drafts. S.S.M. assisted in TEM imaging and data analysis. W.-F.C. assisted in Raman analysis. P.K. assisted with the data analysis. C.C.S. provided the concepts for the data analysis, worked on all subsequent drafts of the manuscript, and supervised the overall project. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to acknowledge grant support from the Australian Research Council (DP170104130), scholarship support (R.M.) through an Australian Government Research Training Program (RTP) Scholarship, and the characterization J


Article

Inorganic Chemistry

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facilities of the Mark Wainwright Analytical Centre at UNSW Sydney.

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Surface, Subsurface, and Bulk Oxygen Vacancies Quantified by

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