Eric
Grulke
*a,
Kenneth
Reed
b,
Matthew
Beck
a,
Xing
Huang
a,
Alastair
Cormack
c and
Sudipta
Seal
d
aChemical and Materials Engineering, University of Kentucky, Lexington, KY, USA. E-mail: eric.grulke@uky.edu
bCerion Enterprises, LLC, 1 Blossom Road, Rochester, NY, USA
cNY State College of Ceramics, Alfred University, Alfred, NY, USA
dAdvanced Materials Processing and Analysis Centre, University of Central Florida, Orlando, FL, USA
First published on 18th August 2014
Nanoceria redox properties are affected by particle size, particle shape, surface chemistry, and other factors, such as additives that coat the surface, local pH, and ligands that can participate in redox reactions. Each CeO2 crystal facet has a different chemistry, surface energy, and surface reactivity. Unlike nanoceria's industrial catalytic applications, biological and environment exposures are characterized by high water activity values and relatively high oxygen activity values. Electrochemical data show that oxygen levels, pH, and redox species affect its phase equilibria for solution and dissolution. However, not much is known about how the many and varied redox ligands in environmental and biological systems might affect nanoceria's redox behaviour, the effects of coated surfaces on redox rates and mechanisms, and whether the ceria solid phase undergoes dissolution at physiologically relevant pH and oxygen levels. Research that could answer these questions would improve our understanding of the links between nanoceria's redox performance and its morphology and environmental conditions in the local milieu.
Nano impactNanoceria is a well-known redox catalyst, which has been linked to redox effects in biological and environmental systems. The perspective shows connections between nanoceria morphology, including crystallite growth and surface structure, electrochemical phase equilibria, surface coatings (intentional and unintentional), and surface reactivity (both experimental plus computational modeling) to its apparent redox properties. These connections, or lack of them, are used to identify what is known about nanoceria's redox properties and to generate research gaps that might be addressed by future work. |
Some compounds act as both pro- and anti-oxidants. Vitamin C is an example of a monosaccharide oxidation–reduction (redox) catalyst. It can reduce reactive oxygen species such as hydrogen peroxide and can act as a substrate for ascorbate peroxidase, a redox enzyme. In redox reactions, atoms have their oxidation states changed: loss of an electron leads to an increase in the oxidation state of an atom, ion or molecule, whilst reduction occurs when an atom, ion, or molecule gains electrons. Redox reactions are matched sets, i.e., one compound is being oxidized while the other is being reduced. Redox reactions often, but not always, involve an electron transfer between the species.
Other nanoceria applications involve a liquid water phase, such as the polishing aids, anti-corrosion systems (the paint or coating would be in contact with water or sea water), oxidants for electrode sensors,5 and redox agents for treatment of disease (ref. 109 has more detail on such issues). Environmental problems might be associated with the release of nanoceria or nanoceria products into soil and water milieu. Human health applications can include intentional injection or exposure for diseases, or the effects of long-term exposure in the workplace (ref. 110 has more detail on such issues). Ceria nanoparticles have been proposed for a wide variety of oxidative stress conditions, including: radiation,6 acting as ROS scavengers,7–9 cancer,10 neurological oxidative stress diseases,11 ischemic stroke,12 and cosmoceutical applications.13 In these milieux, the water activity at the nanoceria surface is high, and the oxygen activity may be high or low. For such cases, there is less experimental and computational data on nanoceria's surface reactivity, redox reaction mechanisms, and ligand adsorption. There are a number of sources for nanoceria release, including paints, batteries, non-battery metals, catalytic converters, polishing slurries, glass additives, fuel additives and phosphors.
With respect to biological and environmental system, nanoceria will act as a colloid in aqueous, body fluid, and soil environments. The dispersion stability of colloidal material is affected by temperature, surface atomic arrangements, inorganic or organic ligands adsorbed on its surface, ions in solution and their levels, and pH among other factors.
The term, coating, is used broadly in this perspective. Metal oxides (MOx) have a variety of surface chemistries; the metal–oxygen composition of the near-surface layer can often be quite different from that in the bulk material. On ‘neat’ samples, in which there are no surface adducts, there can be a variety of surface-terminated oxygens, such as hydroxyls, M–O–M, and MO moieties. In environmental systems, there can be a number of species adsorbed to the nanoparticle surface, including organic acids and bases, organic ligands, herbicides, pesticides, surfactants, dispersants, proteins and phosphates, to name a few. Adsorbed species are assumed to be in dynamic equilibrium with their levels in aqueous solution, i.e., adsorption and desorption are constantly underway. Therefore, adsorbed species might block nanoparticle reactive surface sites sterically, but only during their adsorption phase. Additional coatings types include ligands covalently bound to the nanoparticle surface (such as coupling agents) or materials form a shell covering the surface. Covalently bound agents may not interact with all reactive sites on the nanoparticle surface. Shell-type coatings also may not react with surface sites, but could block larger molecules from diffusing to the metal oxide surface.
Finally, nanoceria dispersions in water are often unstable, which can lead to agglomeration and can affect its transport and biodistribution. The local activities of oxygen and water directly affect nanoceria's surface chemistry. Table 1 categorizes the oxygen and water activities of specific nanoceria applications. The activity of a pure liquid is unity (ai = 1), and the activity of a gas component near 1 atmosphere pressure is its partial pressure divided by its saturated vapour pressure at the temperature of interest (ai = pi/pisat). The activity of a species characterizes its chemical availability. Differences in the activity of, e.g., water or oxygen can alter the stable structure and/or oxidation state of ceria surfaces. For example, the water activity of biological fluids at physiological conditions is often between 0.95 and 1. The water activity of air is similar to its relative humidity (on a fractional basis, not a percent). The oxygen activity in air at room temperature is ~0.20.
Application area | aoxygen | awater | Ligands in the application environment |
---|---|---|---|
a Oxygen activity can be high if the water is near saturation with oxygen. However, the local oxygen ‘capacity’ may be low as it is sparingly soluble. | |||
Gas phase catalysis | Moderate | Very low | Combustion products |
Solid oxide fuel cells | Moderate | Very low | Fuels |
Dispersed diesel fuel additives | Moderate | Very low | Diesel fuels |
Polishing aids | Higha | High | Chemical reaction with Si–O species |
Oxygen sensor | Moderate | Varied | Varied |
Polymer or ceramic nanocomposites | Low | Low | |
Anti-corrosion coatings | Higha (salt water) | High | Ions in aqueous solution |
Environmental systems: soil and water | Higha | High | Salts in aqueous dispersions; adsorbing organic acids/polymers; oxidizing/reducing agents and systems |
Biological systems: redox agents for treatment of disease | Higha | High | Salts in aqueous solution; metabolic organic acids; adsorbing proteins; oxidizing/reducing agents and systems |
An objective of this perspective is to illustrate how nanoceria's structure, composition, surface groups, and its local environment affect oxidation/reduction reactions, particularly in aqueous systems. Nanoceria morphology established, in part, by crystallite growth during synthesis. Morphology characterization is relevant to the surface structure of nanoceria at dosing conditions, its dissolution and re-precipitation in biological and environmental systems, and specific surface groups on different crystal facets that might participate in the redox reactions. Electrochemical reactions of soluble cerium salts and cerium solid phases are reviewed; these are relevant both to synthesis conditions and morphologies, as well as dissolution/reprecipitation conditions. Nanoceria particle surfaces may be coated during the manufacturing process for commercial materials, or may be coated by organic or inorganic ligands in local environments. Material on nanoceria surface reactivity is divided into four sections: experimental systems with low water activities, experimental systems with high water activities, computational models for nanoceria reactivity, and acellular reactions of nanoceria. After each section, there is a summary of what is known and research gaps relevant to nanoceria surface reactivity and redox properties in biological and environmental milieu.
Organic acids can be used to complex with ceria nanoparticles during synthesis or with cerium ions in solution, thereby improving their solubility. For example, cerium(III) ions (Ce3+) are generated during ore digestion using strong acids. Oxalic acid, a multidentate carboxylic acid, is added at this stage to form a soluble salt that can be recovered from the mixture. Fig. 1 shows the structure of cerium oxalate; this is typical of a bidentate organic ion complexing with metal atoms. In the case of ceria dissolution in environmental or biological milieu, organic acid complexes with cerium ions could generate soluble salts.
Carboxylic acids are commonly used to control crystallite growth. Taguchi and coworkers15 reported the effects of a series of dicarboxylic acids with varied chain lengths for this purpose. The synthesis was carried out in supercritical water systems (400 °C, 38 MPa) with Ce(OH)4 as the ceria precursor. Assuming that the nanoceria surface consists only of metal and oxygen atoms (no hydroxyls), the carboxylic acid groups could form bidentate, bridging or unidentate configurations on the nanoparticle surfaces (Fig. 2). In this case, all dicarboxylic acids formed bidentate structures, based on FTIR data.
Fig. 2 Three binding states for carboxylate anions and metal atoms on metal oxide surfaces.15 |
Table 2 shows the product morphologies as a function of the –CH2– chain length for the dicarboxylic additives. All of the dicarboxylic acid additives caused changes in the nanoparticle morphologies. For ceria crystallites, the surface energies of the lowest index crystal facets are in the order, γ(111) < γ(100) < γ(110). That is, the (111) crystal face is the most stable.
–(CH2) – chain length | Carboxyl state | Carboxylate coverage of surface, % [TGA data] | Crystallite morphology | Plane that limits growth |
---|---|---|---|---|
None | NA | NA | Truncated octahedral | (111) |
4 | No free carboxyl | 54.8 | Cubo-octahedral | (111) ~ (100) |
5 | No free carboxyl | 33.8 | Cubo-octahedral | (111) ~ (100) |
8 | Free carboxyl | 92.4 | Cubic | (100) |
10 | Free carboxyl | 89 | Cubic | (100) |
Crystallite faces, edges, and corners all have different reactivities (see the review of Reed). Adsorption of additives to a crystallite face should hinder its growth. The additives in Table 2 have different effects on the morphologies of nanoceria (Fig. 3). The carboxylic acid groups appear to bind solely to the (100) crystallite faces. When no additive was used, this synthesis method produced truncated octahedrons with large (111) faces (pathway a). Short chain dicarboxylic acids gave moderate protection of the (100) surface and growth was relatively balanced between the two faces, resulting in cubo-octahedrons (pathway b). When the additive provided dense packing on the (100) face, this facet became the surface that limited the rate of crystal growth (pathway c). Both carboxylic acids of the additives with shorter methylene chain segments bound to the nanoceria surface, resulting in moderate surface coverages that left no free carboxylate groups. Long chain acids aligned themselves perpendicular (normal) to the surface, resulting in higher surface coverage and free carboxylates on the exterior of the nanoparticles. Cerium atoms on edges and corners have different local coordinations, and will have different reactivities. As the nanoparticle size decreases, the fractions of surface atoms on edges or corners increases. One of the challenges in understanding the surface reactivity of nanoceria is describing the surface reactivity of all surface atoms for a particular nanoparticle size and morphology.
Fig. 3 Structural models of truncated octahedral, cubo-octahedral, and cubic morphologies.15 |
Nanocrystal growth mechanisms have particular relevance for nanoparticles that may undergo chemical transformations in environmental or biological milieu. Should nanoparticles be exposed to conditions that change their redox environment, i.e., new combinations of temperature, pH, ionic species and/or ionic species concentrations, oxygen, other oxidizing ligands, or reducing ligands, it is possible for them to re-equilibrate their structure (that is, dissolve and/or precipitate).
This conundrum has been identified in a recent review on adsorption, diffusion and reaction, and structural sensitivity of heterogeneous catalysts. Sterre and Freund41 point out that adsorption structures and reaction pathways observed under ultrahigh vacuum conditions may not be the same as those under realistic pressures and fluid phase compositions. One of the more important modifications, in practice, can include hydroxylation of the metal oxide surface. One of the remaining challenges is the characterization and modeling of reaction mechanisms for surface hydroxyls at relevant water activities. For several metal oxides, dissociative adsorption of water is thermodynamically unfavorable; however, this process occurs readily on defect sites such as step edges or oxygen vacancies. Ref. 108 describes mechanisms in greater detail.
Table 3 links specific nanoceria morphologies to their typical crystal facets and the surface energy of that facet. The two references are based on gas phase42 and liquid phase15 data. In general, a crystal facet with high surface energy has a higher reactivity.
Ceria abrasive, with aggregate sizes of ~125 nm and constituent particles of ~50 nm, have a ‘shell’ near their surfaces that is enriched in Ce3+.43 Doping impurities, such as lanthanum which is added to aid redox catalysis, also tend to accumulate near the outer surface of nanoceria particles.43 Ceria nanoparticles synthesized by vapour phase condensation have increasing fractions of Ce3+ ions in the particles as the particle diameter decreases.44 For example, Ce3+ levels on ceria surfaces increase as the particle size drops below 11 nm.44 The amount of CeO1.5 seems to depend on the synthesis method and the crystallite product; nanoparticles with the more stable (111) morphology are less likely to have reduce stoichiometry (CeO1.5).44 Specific analysis of (111) and (100) facets by STEM-EELs showed that, for (111) facets and surface islands, the reduction shell exists over the surface plane and extends 1–2 mixed valence planes below the surface.45 For (100) facets, the reduction shells extends to 5–6 levels of oxygen-valency planes below the surface.45
• Each crystal facet of CeO2 will have a different growth rate, surface energy, and surface reactivity. For very small nanoparticles, the different reactivities of cerium atoms on edges and corners can be important.
• At environmentally-relevant conditions (high oxygen and water activities), some metal oxide surfaces are terminated by an oxygen layer and become fully hydroxylated in the presence of water. Therefore, it would be expected that the chemical properties of these hydroxyls and their surface reactivity would be strong functions of the environmental conditions.
• Crystallite morphology and specific crystal facets should be part of the nanoceria's characterization for the best understanding of its redox properties and its function as a pro- or anti-oxidant. For very small nanoparticles, it will be important to include atoms on edges and corners in the reactivity ‘audit’.
• Reaction mechanisms for specific crystal facets are not always known.
The original Pourbaix diagram for aqueous cerium46 was revised by Hayes et al.;47 (see Fig. 4) by considering the precipitation of Ce4+ species and the potential reaction between soluble Ce3+ species and insoluble Ce4+ precipitates. In general, this is based on the aqueous solubility of the compounds, Ce(OH)3 and Ce(OH)4. Other cerium compounds, such as those used for nanoceria synthesis or that might form due to redox reactions with surface cerium atoms, will have different solubilities. Experimental data were obtained via a series of titration studies covering a range of pHs under inert (argon) and oxygen atmospheres. For Ce3+ solutions, exposure of aqueous systems to air slowed the pH stabilization during potassium hydroxide titrations, while an argon purge resulted in rapid pH stabilization. Solutions under an air atmosphere developed yellow precipitates, which were consistent with the formation of Ce4+ species. In argon, only white precipitates were noted, which were consistent with no Ce4+ formation. Thus, the presence or absence of dissolved oxygen in aqueous solutions makes a difference in the phase equilibria achieved. Ce3+ and Ce4+ ions in solution tended to hydrolyze and complex. Both Ce(OH)3 and Ce(OH)4/CeO2·2H2O species were soluble to some extent, but were the major components in precipitates. Furthermore, they found that the morphology, agglomeration, and surface chemistry of the solids would be expected to change with the experimental methodology.
Fig. 4 Updated E-pH (Pourbaix) diagram for cerium in aqueous perchlorate solutions.47 Reproduced with permission from J. Electrochem. Soc., 2002, 149, C623. |
Yu and coworkers48 developed a model for the cerium–water–hydrogen peroxide system (Fig. 5), varying these species plus pH and oxygen as well. Typical half-cell reactions for the cerium–water system alone included:
Ce3+ + 4H2O → Ce(OH)4 + 4H+ + e− | (1) |
O2 + 4H+ + 4e− → 2H2O | (2) |
4Ce3+ + O2 + 14H2O → 4Ce(OH)4 + 12H+ | (3) |
Fig. 5 Simplified E-pH diagram of Ce–H2O–H2O2 system showing when the Ce4+ precipitate is CeO2(precip).48 Reproduced with permission from J. Electrochem. Soc., 2006, 153, C74. |
Different half-cell reactions apply when hydrogen peroxide is added to the system. Peroxide acts as a reducing agent for Ce4+ at low pH (<4) and as an oxidizing agent for Ce3+ at higher pH (>4). However, the actual states of the cerium precipitates were not assessed. These diagrams should be considered as computations only and based on kinetics that were readily observed during titrations.
Fig. 5 shows that Ce3+ is oxidized to Ce4+ by hydrogen peroxide, i.e., H2O2 is a stronger oxidizing agent than Ce4+. A typical overall reaction for a soluble Ce3+ ion would be:48
2Ce3+ + H2O2 + xH2O → 2Ce(OH)3−x/21+x/2 + xH+ | (4) |
The half-cell reaction for hydrogen peroxide is:
H2O2 + 2H+ + 2e− → 2H2O | (5) |
In the case that a Ce3+ salt is oxidized to a solid precipitate, the overall reaction would be:
2Ce3+ + 2H2O2 + 2H2O → 2CeO2(s) + 6H+ | (6) |
In addition, it is possible that Ce4+ could form a complex with hydrogen peroxide. In general, Ce3+ is relatively soluble at pH up to 11 while Ce4+ is soluble at pH less than 4 (shown as a series of species in the upper left hand corner of the E(V) vs. pH plot). Eqn (6) is typical of cerium redox systems in solution. If nanoceria is dissolved in the local milieu, it would be most likely to be as a Ce3+ salt. A local oxidizing agent or system might then oxidize the Ce3+ salt to CeO2, which would precipitate and, presumably, recrystallize to minimize surface energy. Similar chemistries occur in aqueous phase synthesis of nanoceria. For example, even bulk CeCl3 can be transformed to CeO2 under oxygen excess.49 However, CeO2 in biological systems could be reduced to form Ce3+ salts if appropriate reducing agents and conditions are present. For example, lysosomal fluid is acidic (~pH 4.5 due to acid hydrolases).50 The presence of different redox systems (such as ligands H2O2, O2, H+, e−) can cause shifts in the phase equilibria between Ce3+ and CeO2 (Ce4+), as shown in Fig. 5.
There is at least one report of nanoceria dissolving and reprecipitating in biological studies.51 In this case, ~30 nm cubic nanoceria, in which the (100) face predominated, were intravenously administered to rats. Samples of organ tissues were gather for analysis up to 90 days after the dose, which was high and well above the previously determined therapeutic window for ceria in vivo.52 Many of the cubic nanoceria particles had become fragmented and rounded along their edges. There were clouds of small nanoceria, 1–3 nm in size, with dominant faces of (111), (200), and (311), which were not seen in the original dosed nanoceria (primarily (100) faces). In addition, polycrystalline large grains of nanoceria were observed that could have been the result of Ostwald-type ripening. Therefore, these nanoceria appeared to undergo in vivo bioprocessing after prolonged exposure in the liver. In addition to this one report of processing, there are a number of reports showing that stored nanoceria can change its structure and composition. Some of these occurrences may be related to the local redox conditions during storage.
• It seems likely that a number of oxidizing and reducing agents could play roles in ceria dissolution and/or precipitation at storage, in vitro, or in vivo conditions.
• When nanoceria are in aqueous biological or environmental milieu, the following mechanisms may alter their surface morphologies: Ostwald ripening, in which less stable crystal facets lose surface atoms to more stable crystal facets; adsorption of inorganic and organic ligands, such as proteins leading to coronas; and reactions with liquid-phase ligands, which can include redox-type reactions.
• Demonstration of systems in which nanoceria's redox cycles are ‘turned on’ and ‘turned off’. This could lead to better understanding of how nanoceria's redox capabilities, i.e., pro- or anti-oxidant, can be controlled in environmental and biological systems.
Rezwan and coworkers published a series of papers illustrating the general principles of protein adsorption on nanoparticles. In the first study,61 adsorption isotherms of bovine serum albumin (negatively charged) and lysozyme (positively charged) were determined for hydrophilic (alumina, silica, and titania) and hydrophobic (zirconia) nanoparticles. In all cases, the adsorbed proteins changed the zeta potentials curves of the metal oxide dispersion. For neat, hydrophilic nanoparticles, the amounts of protein adsorbed correlated with zeta potential. Hydrophobic zirconia nanoparticles adsorbed high amounts of protein even under electrostatically repulsive conditions, suggesting that multiple adsorption sites and mechanisms are possible.
In a second study,62 protein mixtures were adsorbed on hydrophilic nanoparticles (alumina and titania). The amount of protein in solution was sufficient to develop coronas that completely ‘masked’ the nanoparticle surfaces. The researchers were able to model the shift in the isoelectric point of the complex nanoparticle. This included analysis of the amino acid compositions and pK values of the two proteins, estimating their charges, using a weighted average to estimate the isoelectric point of the adsorbed mixture, and using a factor for the accessible surface area to fit the data. In this case, bovine serum albumin had the most effect on the isoelectric point; its accessible surface area was 2.6 times that of lysozyme. Additional discussion of the zeta potential for protein-coated nanoparticles is given by the Das et al. perspective plus other recent references.63–66
Wang and coworkers67 studied the effects of surface charge (positive, neutral, and negative) of various ceria nanoparticles on the adsorption of one protein, lysozyme (net positive charge). The data were modeled using the Toth and Sip isotherms, and compared to monolayer coverage estimates for both ‘side-on’ and ‘end-on’ packing of the lysozyme molecules under the assumption of random packing on the nanoparticle surface. Adsorption of a protein with a net positive charge on negatively charged nanoparticles had a very broad adsorption site energy distribution, which controlled the adsorption process. On the other hand, adsorption levels of this protein onto a positively charged surface were lower, but were influenced by lateral effects from adsorbed protein species.
Flocculation and agglomeration of ceria dispersions were studied by Kong and Leong.69 Below the isoelectric point (~pH 7), high concentration (30 wt%) ceria dispersions flocculated but were homogeneous. Above this pH, dispersions phase separated rapidly, forming large agglomerates, consistent with the work of Nabavi,70 which was based on nanoceria dispersions stabilized with nitrate anions. Below the isoelectric point, nitrate anions adsorbed and were bound covalently to the nanoceria, forming a steric layer that prevented particle aggregation. Aggregation was shown to occur when the nitrate groups were displaced by hydroxyls, followed by condensation reactions between contacting nanoparticles (forming Ce–O–Ce bonding bridges). At high pH, agglomeration was prevented by adding a pyrophosphate. However, phosphate ligands may complex with nanoceria particles, changing their stability in water dispersions.32,53
A number of coating types are preferred for drug delivery by nanoparticles, including poly(ethylene glycol), sugar solutions for controlling osmotic pressure, organic acids (mono- to multidentate), polymers, and oligomers. These are intended to improve the dispersion stability at dosing conditions and during the transport and biodistribution of the nanoparticles to their intended target.
• Adsorption of ligands, such as proteins, is dynamic; protein composition within the coating can change over time.
• Does the coating prevent redox reactions or merely retard redox reactions of the nanoceria, i.e., is the coating porous to ligands participating in redox reactions?
• Does the coating help reduce or control dispersion agglomeration or aggregation at environmental or biological exposures?
• Does the coating affect the transport and biodistribution of the nanoparticle?
In a highly cited review, Mogensen and coworkers72 evaluated the physical, chemical and electrochemical properties of pure and doped ceria; the data available at that time (2000) was mostly focused on high temperature, low water conditions relevant to applications such as solid oxide fuel cells. The process of ceria reduction was modeled as generating a defect in the form of Ce3+, rather than linked to oxygen vacancies.
In 1993, Nabavi and coworkers evaluated the acid–base behavior of surface hydroxyls in aqueous dispersions70 using a model (the “MUSIC” model) developed by Van Riemsdijk.86,87 In bulk ceria, Ce4+ coordinates with eight oxygen atoms while each oxygen atom coordinates with four cerium atoms. The formal valence bond, the number of shared electrons, is v = 0.5. In the bulk ionic crystal, the charges balance. Assuming that a ceria surface is isolated from the bulk, a ceria surface in water would have the coordination of metal atoms completed by the oxygen(s) of adsorbed water. Thus, oxygens on the surface would have a charge. The model was used to evaluate the equilibria for various hydroxyl sites, which could be coordinated with different numbers of cerium atoms. The general equilibria equations were:
Mn − Onv−2 + H+ ↔ Mn − OHnv−1 + H+ ↔ Mn − OHnv2 |
Ce2 − OH2+1 ↔ Ce2 − OH0 + H+ ↔ Ce2 − O−1 + 2H+ |
Ce1 − OH2+0.5 ↔ Ce1 − OH−0.5 + H+ ↔ Ce1 − O−1.5 + 2H+ |
Ce3 − OH2+1.5 ↔ Ce3 − OH0.5 + H+ ↔ Ce3 − O−0.5 + 2H+ |
Quantum mechanical calculations of, e.g. Mulliken population analyses on ceria nano-particles (A.N. Cormack, personal communication, and see the discussion of Fig. 7, below) suggest that while the surface atoms do have a different effective charge from those in the bulk, they do not carry a net charge, as supposed by the “MUSIC” model: the particle is, overall, electrostatically neutral. Thus, the bond valence analysis must differ from that used by Nabavi et al., and the parameters used in calculating the equilibrium constants are, therefore, suspect.
Fig. 7 Three adsorption sites for a water molecule on reduced (111) surfaces.100 Left = top view; right = side view. Cerium = beige; oxygen = red; hydrogen = blue. The lowest binding energy configuration is c–d. In the side views (b, d, and f), the transecting plane passes through the oxygen on the water molecule; therefore, the second hydrogen is outside the plane and does not appear in d and f. |
Nabavi and coworkers explored the surface chemistry of ceria nanoparticles in aqueous dispersions through a series of titrations.70 Their nanoceria was synthesized using Ce3+ nitrate, by which nitrate ions are associated with the surface. The agglomeration and aggregation properties of colloidal dispersions are usually related to their surface chemistry plus the presence of other ions at their surfaces. This dispersion exhibited three distinct states: 1) between pH 2 to pH 6, nitrates are released and the dispersion aggregated, 2) between pH 6 to pH 10, there were no more transformations, and 3) above pH 10, the surfaces underwent transformation(s) that suppressed the sites for nitrate binding. In particular, when long times occurred prior to using the nanoceria, Ce3+ could be detected in solutions, presumably from the dissolution of Ce4+ ions and their reduction. The rates of these processes will be affected by the nature and size of the nanoparticles.
The monodentate and tridentate sites appear to take part in binding equilibria for protons and hydroxyls, while bidentate sites appear to be inactive over the range of pHs studied. When nitrates begin to be released during titration (~pH 6), covalent nitrates are released and the nanoparticles aggregate. Reactions of active monodentate and tridentate sites were thought to cause this change in surface energy. When the dispersion has additional hydroxyls (titration above pH 10), the ceria surfaces restructure by transforming bidentate sites into tridentate sites. These surfaces have lost the ability to bind nitrates with partly covalent bonds. The links between surface chemistry and colloidal stability for nanoceria appear to be typical of other metal oxide nanoparticle dispersions. Dispersions are typically stable in narrow pH ranges; washing or titrating these systems results in a loss of surface functional groups and unstable performance.
Adhesion between ceria nanoparticles and other surfaces has been used to study the complexation of various ligands on nanoceria.88 In the presence of complexing agents, such as nitrates, ceria surfaces are protected and adhesion is prevented (mica as an adsorbate in this case). The efficiency of the ligand increases in the order, nitrate < acetate < acetyl acetone. In this study, controlling adhesion at nanoparticle contact appeared more important than achieving long-range electrostatic repulsion. In the case of nitrate ions, they also found that nitrate was released at pH > 3, with the dispersion becoming unstable. Some nitrate ions appeared to be more strongly bound to the nanoceria than others. In general, adhesion at contact might be eliminated by the addition of complexing molecules, i.e., by applying coatings that provide steric or electrostatic stabilization.
• Titration experiments of nanoceria demonstrate that specific surface hydroxyls, which have different numbers of coordinating cerium atoms, have different apparent charges. This should lead to differences in adsorption, complexation, and reaction for specific sites.
Condition | Crystal facets | Method | Ref. |
---|---|---|---|
Low water activity | |||
CO adsorption | (110), slab | 96 | |
Reduction of ceria by hydrogen | (111), (110) bulk | Ultra-accelerated QCMD, DFT | 81 |
Oxygen vacancies | (111) – nanoparticles | DFT | 97 |
Oxygen defect formation | (110), (111) | QCMD, others | 98 |
(111) effects of oxidizing environment | DFT | 99 | |
High water activity | |||
Chemical polishing | SiO2 surface with hydroxyls, bulk CeO2 | QCMD | 92 |
Adsorption of water | (111) – CeO2; CeO2−x | DFT, SRPES | 91 |
(111) – CeO2; CeO2−x | DFT | 100 and 89 | |
Preferred adsorption on (100), (110), edges, corners | DFT | 93 | |
Oxygen vacancies increase adsorption energy of water on (111) | DFT | 90 | |
Configuration of low energy surfaces | 94 | ||
Stability of surface hydroxyls | Nanoceria supercell | DFT | 95 |
Since water is ubiquitously present in environmental or biological milieu, the role of water on the surface structure and redox chemistry of nanoceria cannot be ignored. To address this issue, Traversa's group100 have investigated the variation of surface structures for (111) ceria faces and Huang et al.95 have focused on the expected surface structural evolution for ceria nanoparticles in response to changing conditions in high water activity environments.
Water adsorption has been used as a probe to evaluate the effects of ceria surface on binding energy, among other factors.89,100 Fronzi and coworkers100 evaluated binding energies for water molecules adsorbing and/or dissociating on CeO2 (111) surfaces. Two different surfaces were considered: clean stoichiometric (111) surfaces and reduced (111) surfaces with oxygen vacancies. An example is shown in Fig. 7, based on 2 × 2 unit cells and considering one molecule adsorbing from the gas phase on different available surface sites of a reduced (111) surface. The most energetically favorable adsorption site for a single water molecule was a surface Ce atom above which the water was in a planar geometry parallel to the ceria surface with its own oxygen atom positioned, and each hydrogen atom bound to a nearest surface oxygen [c & d in Fig. 7]. This site had a lowest binding energy of 0.49 eV, about the same as those reported for similar experimental conditions.89 All three of the dissimilar water adsorption configurations on reduced (111) surfaces had higher binding energies than that of water adsorption configurations on the clean (111) surface. When oxygen vacancies were present at the ceria surface, the binding energy of water molecules to ceria surface (above oxygen vacancy sites) became stronger, suggesting an attraction between the water molecule and the oxygen vacancy. The presence of water appeared to lower the energy cost of creating oxygen vacancies on (111) surfaces. Some simulations and experiments suggest that the reactivity of ceria nanoparticles is environmentally dependent and is influenced by its history in reaching a particular milieu.93
Different assumptions regarding nanoparticle surface energies can lead to different results. For example, it is possible to construct nanoparticles assuming they conserve stoichiometry or mass ratios, or that they conserve net charge and oxidation state. Recent quantum mechanical calculations of the stability of cubic nanoceria examined this issue in detail, considering the density and distribution of bound surface groups, and deriving scaling relationships for the stable surface concentration of these groups.95 While the analysis of acid–base behaviour of surface hydroxyls by Nabavi70 assumed that the surface atoms are separated from the bulk, quantum mechanical calculations permit charge to be distributed throughout the nanocrystal. Conservation of stoichiometry did not lead to minimum energy nanoceria structures, but non-stoichiometric configurations that conserved oxidation state did. These calculations reveal the stable concentration and configuration of species of cubic nanoceria as a function of oxygen and hydrogen chemical potentials, which are dictated by its environment. Nanoceria might be considered as consisting of a bulk “core” decorated with combinations of anionic surface groups (e.g., –Ox and –OH groups). Hence stable nanoceria configurations balance the energy costs of anions from the environment against energy variations within the nanoceria itself, including effects of changes in oxidation state of Ce cations.101
For cubic nanoceria structures considered in ref. 74, DFT calculations predict stable configurations similar to those shown in Fig. 8. Analysis of the local arrangement of various surface groups at nanoceria corners, edges and facets allows the development of analytic scaling relationships that can be used to predict the relative concentration of various surface groups on similarly-shaped nanoceria of arbitrary size. Based on these results, cubic 30 nm nanoceria terminated with only hydroxyl groups is predicted to exhibit hydroxyl coverages that are 33% higher than reported experimentally. Nanoceria structures with multiple surface groups, i.e., –Ox and –OH, have predicted surface areal densities of hydroxyl groups on 30 nm nanoceria of 12.5 per nm2, quite close to the measured surface density of 12.9 per nm2. As environmental conditions control the chemical potential of available surface groups along with their concentration and configuration, these results suggest that changes to nanoceria synthesis conditions can tune nanoceria surface structures, i.e., the type, distribution and density of surface functional groups. This provides an approach to tuning the surface structure, and thus an avenue to manipulate the redox properties of nanoceria.
Fig. 8 Relaxed, ground-state atomic structures of three ceria nanoparticles terminated with mixed –OH and –Ox surface groups. Atomic representations: ceria = yellow, hydrogen = green, O = purple (either O atoms or –OH groups), O atoms in O2− molecules = blue, and O atoms in O3− molecules = pink.95 |
• There have been a few other studies that address possible mechanisms occurring at high water activities and temperatures relevant to environmental and biological system exposures. The surface structures of nanoceria at these conditions are still a matter of debate.
• This work could include studying relevant redox couples and ligands, as well as common ions that are known to form complexes with cerium. One example of the latter is phosphate ions, which complex with cerium. For more information, see ref. 110.
Oxidant or chemical | Liquid phase analysis | Other analyses | Reference |
---|---|---|---|
Hydrogen peroxide | UV-Vis; difficult to quantify | EPR, XPS | 102 |
Hydrogen peroxide | UV-Vis, 500 nm | FTIR, XPS | 103 |
Ferricytochrome C; competitive inhibition by superoxide | UV-Vis | 102 and 104 | |
Antioxidants: ascorbic acid, gallic acid | UV-Vis | 105 (Fig. 3) | |
Dopamine (citric acid solution); proposed mechanism is surface attachment | UV-Vis | FTIR, TGA, XPS | 106 |
Serotonin (5-HT) | UV-Vis; supernatant | FTIR, TGA, DLS | 107 |
Some of these acellular redox reactions could be used to characterize the surface reactivity of nanoceria prior to in vitro or in vivo tests. Surface activity assays would then be part of the characterization of nanoparticle properties, and should provide a better basis for conclusions regarding the effects of specific environmental or biological conditions.
A wide variety of ligands, from organic acids, to silanes, to proteins, can adsorb onto nanoceria surfaces. Coverage levels range from sub-monolayer to multilayer. However, surface coatings may only slow, but may not completely prevent, redox reactions.
In environmental and biological milieu, there are a plethora of redox ligand and cycles. Nanoceria may participate in these via two mechanisms: 1) direct activity as an oxidant or reductant for the specific molecule, or 2) indirectly, by provide or receiving oxygen to an existing cycle. Acellular redox reactions (e.g., the nanoceria test kit for glucose in which glucose oxidase provides the catalytic activity and nanoceria provides oxygen storage) may be a way to study these redox cycles more fully.
Finally, a wide variety of ligands could provide half-reactions to drive the dissolution or re-precipitation of nanoceria. Local pH, temperature, and oxygen levels will affect these phase equilibria processes. While initial reaction products will be driven by kinetics (for example, capping molecules can permit a less stable facet to grow quickly), the final crystallite structure will be the more thermodynamically stable. Typically, the most stable crystal face is (111), which has the lowest surface energy, and, generally, the lowest redox activity. However, since adsorption of ligands on the surface of nanoceria can alter the relative surface energies, one needs to consider the equilibrium of the whole system, nanoceria plus adsorbents plus solution conditions (pH, ions, etc.). In addition, there is now evidence for dissolution and re-precipitation of nanoceria for long-term in vivo exposures. The different surface structures of crystal facets, and the wide variety of electrochemical reactions that are possible, seems to be a relatively unexplored area of research with high relevance to the performance of nanoceria in environmental and biological milieu.
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