Polar surface structure of oxide nanocrystals revealed with solid-state NMR spectroscopy

Solid-State Nuclear Magnetic Resonance Spectroscopy for Surface Characterization of Metal Oxide Nanoparticles: State of the Art and Perspectives

Metal oxide materials have found wide applications across diverse fields; in most cases, their functionalities are dictated by their surface structures and properties. A comprehensive understanding of the intricate surface features is critical for their further design, optimization, and applications, necessitating multi-faceted characterizations. Recent advances in solid-state nuclear magnetic resonance (ssNMR) spectroscopy have significantly extended its applications in the detailed analysis of multiple metal oxide nanoparticles, offering unparalleled atomic-level information on the surface structures, properties, and chemistries. Herein, we present an overview of the current state of the art from an NMR perspective. We begin with a brief introduction to contemporary ssNMR methodologies. Subsequently, we introduce and provide critical reviews on the applications of different ssNMR techniques in the detailed characterizations of the surface local structures, disorders, defects, active sites, and acidity on metal oxide nanoparticles, as well as the revelation of mechanisms behind some intriguing chemistries that occur on the surfaces, referencing representative recent studies. Finally, we address the challenges beyond the current status and provide perspectives on the future development and application of advanced ssNMR methodologies in this emerging field.

Tracking the Facet Transformation of CeO2 by 17O Solid-State Nuclear Magnetic Resonance

CeO2 nanomaterials expose various crystal facets with distinct surface geometry, resulting in different surface reactivities and material behaviors that ultimately determine their performances and suitability for a wide range of applications. Here, we apply 17O solid-state nuclear magnetic resonance (NMR) to follow the facet transformation of CeO2 at increased temperatures, observing a transition from (100) to (110) and finally to the more stable (111), based on the characteristic NMR shifts associated with the unique surface structure of each facet. In addition, we explore the effects of Pt ions on the conversion of different facets, which are found to promote the formation of the thermally stable (111) facet. Furthermore, 17O solid-state NMR provides a semiquantitative method for measuring the fractions of exposed facets. This work offers new insights and a more comprehensive understanding of crystal facet structures, and the new approach can be readily extended to study the facets of other oxide-based materials.

Surface Structure and Chemistry of CeO2 Powder Catalysts Determined by Surface-Ligand Infrared Spectroscopy (SLIR)

ConspectusCerium is the most abundant rare earth element in the Earth's crust. Its most stable oxide, cerium dioxide (CeO2, ceria), is increasingly utilized in the field of catalysis. It can catalyze redox and acid-base reactions, and serve as a component of electrocatalysts and even photocatalysts. As one of the most commonly used in situ/operando characterization methods in catalysis, infrared (IR) spectroscopy is routinely employed to monitor reaction intermediates on the surface of solid catalysts, offering profound insight into reaction mechanisms. Additionally, IR vibrational frequencies of probe molecules adsorbed on solid catalysts provide detailed information about their structure and chemical states. Numerous studies in the literature have utilized carbon monoxide and methanol as IR probe molecules on ceria particles. However, assigning their vibrational frequencies is often highly controversial due to the great complexity of the actual surface of ceria particles, which include differently oriented crystal facets, reconstructions, defects, and other structural variations. In our laboratory, taking bulk ceria single crystals with distinct orientations as model systems, we employed a highly sensitive ultrahigh vacuum (UHV) infrared spectroscopy system (THEO) to study the adsorption of CO and methanol. It turns out that the theoretical calculations adopting hybrid functionals (HSE06) can bring the theoretical predictions into agreement with the experimental results for the CO frequencies on ceria single crystal surfaces. The obtained frequencies serve as reliable references to resolve the long-standing controversial assignments for the IR bands of CO and methanol adsorbed on ceria particles. Furthermore, these characteristic frequencies allow for the determination of orientations, oxidation states and restructuring of exposed crystal facets of ceria nanoparticles, which is applicable from UHV conditions to industrially relevant pressures of up to 1 bar, and from low temperatures (∼65 K) to high temperatures (∼1000 K). We also used molecular oxygen as a probe molecule to investigate its interaction with the ceria surface, crucial for understanding ceria's redox properties. Our findings reveal that the localization of oxygen vacancies and the mechanism of dioxygen activation are highly sensitive to surface orientations. We provided the first spectroscopic evidence showing that the oxygen vacancies on ceria (111) surfaces tend to localize in deep layers. In addition, we employed N2O as a probe molecule to elucidate the origin of the photocatalytic activity of ceria and showed that the photocatalytic activity is highly sensitive to the surface orientation (i.e., surface coordination structure). This Account shows that probe-molecule infrared spectroscopy serves as a powerful in situ/operando tool for studying the surface structure and chemistry of solid catalysts, and the knowledge gained through the "Surface Science" approach is essential as a crucial benchmark.

Surface Structure of Lecithin-Capped Cesium Lead Halide Perovskite Nanocrystals Using Solid-State and Dynamic Nuclear Polarization NMR Spectroscopy

Inorganic colloidal cesium lead halide perovskite nanocrystals (NCs) encapsulated by surface capping ligands exhibit tremendous potential in optoelectronic applications, with their surface structure playing a pivotal role in enhancing their photophysical properties. Soy lecithin, a tightly binding zwitterionic surface-capping ligand, has recently facilitated the high-yield synthesis of stable ultraconcentrated and ultradilute colloids of CsPbX3 NCs, unlocking a myriad of potential device applications. However, the atomic-level understanding of the ligand-terminated surface structure remains uncertain. Herein, we use a versatile solid-state nuclear magnetic resonance (NMR) spectroscopic approach, in combination with dynamic nuclear polarization (DNP) and atomistic molecular dynamics (MD) simulations, to explore the effect of lecithin on the core-to-surface structures of CsPbX3 (X = Cl or Br) perovskites, sized from micron to nanoscale. Surface-selective (cross-polarization, CP) solid-state and DNP NMR (133Cs and 207Pb) methods were used to differentiate the unique surface and core chemical environments, while the head-groups {trimethylammonium [-N(CH3)3+] and phosphate (-PO4-)} of lecithin were assigned via 1H, 13C, and 31P NMR spectroscopy. A direct approach to determining the surface structure by capitalizing on the unique heteronuclear dipolar couplings between the lecithin ligand (1H and 31P) and the surface of the CsPbCl3 NCs (133Cs and 207Pb) is demonstrated. The 1H-133Cs heteronuclear correlation (HETCOR) DNP NMR indicates an abundance of Cs on the NC surface and an intimate proximity of the -N(CH3)3+ groups to the surface and subsurface 133Cs atoms, supported by 1H{133Cs} rotational-echo double-resonance (REDOR) NMR spectroscopy. Moreover, the 1H-31P{207Pb} CP REDOR dephasing curve provides average internuclear distance information that allows assessment of -PO4- groups binding to the subsurface Pb atoms. Atomistic MD simulations of ligand-capped CsPbCl3 surfaces aid in the interpretation of this information and suggest that ligand -N(CH3)3+ and -PO4- head-groups substitute Cs+ and Cl- ions, respectively, at the CsCl-terminated surface of the NCs. These detailed atomistic insights into surface structures can further guide the engineering of various relevant surface-capping zwitterionic ligands for diverse metal halide perovskite NCs.

Can Subsurface Oxygen Species in Oxides Participate in Catalytic Reactions? An 17O Solid-State Nuclear Magnetic Resonance Study

The impacts of subsurface species of catalysts on reaction processes are still under debate, largely due to a lack of characterization methods for distinguishing these species from the surface species and the bulk. By using 17O solid-state nuclear magnetic resonance (NMR) spectroscopy, which can distinguish subsurface oxygen ions in CeO2 (111) nanorods, we explore the effects of subsurface species of oxides in CO oxidation reactions. The intensities of the 17O NMR signals due to surface and subsurface oxygen ions decrease after the introduction of CO into CeO2 nanorods, with a more significant decrease observed for the latter, confirming the participation of subsurface oxygen species. Density functional theory calculations show that the reaction involves subsurface oxygen ions filling the surface oxygen vacancies created by the direct contact of surface oxygen with CO. This new approach can be extended to the study of the role of oxygen species in other catalytic reactions.

Uncovering Structure-Activity Relationships in Pt/CeO2 Catalysts for Hydrogen-Borrowing Amination

The hydrogen-borrowing amination of alcohols is a promising route to produce amines. In this study, experimental parameters involved in the preparation of Pt/CeO₂ catalysts were varied to assess how physicochemical properties influence their performance in such reactions. An amination reaction between cyclopentanol and cyclopentylamine was used as the model reaction for this study. The Pt precursor used in the catalyst synthesis and the properties of the CeO₂ support were both found to strongly influence catalytic performance. Aberration corrected scanning transmission electron microscopy revealed that the most active catalyst comprised linearly structured Pt species. The formation of these features, a function result of epitaxial Pt deposition along the CeO₂ [100] plane, appeared to be dependent on the properties of the CeO₂ support and the Pt precursor used. Density functional theory calculations subsequently confirmed that these sites were more effective for cyclopentanol dehydrogenation-considered to be the rate-determining step of the process-than Pt clusters and nanoparticles. This study provides insights into the desirable catalytic properties required for hydrogen-borrowing amination but has relevance to other related fields. We consider that this study will provide a foundation for further study in this atom-efficient area of chemistry.

Unveiling the Surface Structure of ZnO Nanorods and H2 Activation Mechanisms with 17O NMR Spectroscopy

ZnO plays a very important role in many catalytic processes involving H₂, yet the details on their interactions and H₂ activation mechanism are still missing, owing to the lack of a characterization method that provides resolution at the atomic scale and follows the fate of oxide surface species. Here, we apply ¹⁷O solid-state NMR spectroscopy in combination with DFT calculations to unravel the surface structure of ZnO nanorods and explore the H₂ activation process. We show that six different types of oxygen ions in the surface and subsurface of ZnO can be distinguished. H₂ undergoes heterolytic dissociation on three-coordinated surface zinc and oxygen ions, while the formed hydride species migrate to nearby oxygen species, generating a second hydroxyl site. When oxygen vacancies are present, homolytic dissociation of H₂ occurs and zinc hydride species form from the vacancies. Reaction mechanisms on oxide surfaces can be explored in a similar manner.

Surface differences of oxide nanocrystals determined by geometry and exogenously coordinated water molecules

Determining the different surfaces of oxide nanocrystals is key in developing structure-property relations. In many cases, only surface geometry is considered while ignoring the influence of surroundings, such as ubiquitous water on the surface. Here we apply 17O solid-state NMR spectroscopy to explore the facet differences of morphology-controlled ceria nanocrystals considering both geometry and water adsorption. Tri-coordinated oxygen ions at the 1st layer of ceria (111), (110), and (100) facets exhibit distinct 17O NMR shifts at dry surfaces while these 17O NMR parameters vary in the presence of water, indicating its non-negligible effects on the oxide surface. Thus, the interaction between water and oxide surfaces and its impact on the chemical environment should be considered in future studies, and solid-state NMR spectroscopy is a sensitive approach for obtaining such information. The work provides new insights into elucidating the surface chemistry of oxide nanomaterials.

Enhancing ionic conductivity in solid electrolyte by relocating diffusion ions to under-coordination sites

Solid electrolytes are highly important materials for improving safety, energy density, and reversibility of electrochemical energy storage batteries. However, it is a challenge to modulate the coordination structure of conducting ions, which limits the improvement of ionic conductivity and hampers further development of practical solid electrolytes. Here, we present a skeleton-retained cationic exchange approach to produce a high-performance solid electrolyte of Li₃Zr₂Si₂PO₁₂ stemming from the NASICON-type superionic conductor of Na₃Zr₂Si₂PO₁₂. The introduced lithium ions stabilized in under-coordination structures are facilitated to pass through relatively large conduction bottlenecks inherited from the Na₃Zr₂Si₂PO₁₂ precursor. The synthesized Li₃Zr₂Si₂PO₁₂ achieves a low activation energy of 0.21 eV and a high ionic conductivity of 3.59 mS cm^-1 at room temperature. Li₃Zr₂Si₂PO₁₂ not only inherits the satisfactory air survivability from Na₃Zr₂Si₂PO₁₂ but also exhibits excellent cyclic stability and rate capability when applied to solid-state batteries. The present study opens an innovative avenue to regulate cationic occupancy and make new materials.

Nonreciprocal Transport in a Bilayer of MnBi2Te4 and Pt

MnBi₂Te₄ (MBT) is the first intrinsic magnetic topological insulator with the interaction of spin-momentum locked surface electrons and intrinsic magnetism, and it exhibits novel magnetic and topological phenomena. Recent studies suggested that the interaction of electrons and magnetism can be affected by the Mn-doped Bi₂Te₃ phase at the surface due to inevitable structural defects. Here, we report an observation of nonreciprocal transport, that is, current-direction-dependent resistance, in a bilayer composed of antiferromagnetic MBT and nonmagnetic Pt. The emergence of the nonreciprocal response below the Néel temperature confirms a correlation between nonreciprocity and intrinsic magnetism in the surface state of MBT. The angular dependence of the nonreciprocal transport indicates that nonreciprocal response originates from the asymmetry scattering of electrons at the surface of MBT mediated by magnon. Our work provides an insight into nonreciprocity arising from the correlation between magnetism and Dirac surface electrons in intrinsic magnetic topological insulators.

Unraveling the Surface Hydroxyl Network on In2O3 Nanoparticles with High-Field Ultrafast Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy

Hydroxyl groups are among the major active surface sites over metal oxides. However, their spectroscopic characterizations have been challenging due to limited resolutions, especially on hydroxyl-rich surfaces where strong hydroxyl networks are present. Here, using nanostructured In₂O₃ as an example, we show significantly enhanced discrimination of the surface hydroxyl groups, owing to the high-resolution ¹H NMR spectra performed at a high magnetic field (18.8 T) and a fast magic angle spinning (MAS) of up to 60 kHz. A total of nine kinds of hydroxyl groups were distinguished and their assignments (μ₁, μ₂, and μ₃) were further identified with the assistance of ¹⁷O NMR. The spatial distribution of these hydroxyl groups was further explored via two-dimensional (2D) ¹H-¹H homonuclear correlation experiments with which the complex surface hydroxyl network was unraveled at the atomic level. Moreover, the quantitative analysis of these hydroxyl groups with such high resolution enables further investigations into the physicochemical property and catalytic performance characterizations (in CO₂ reduction) of these hydroxyl groups. This work provides insightful understanding on the surface structure/property of the In₂O₃ nanoparticles and, importantly, may prompt general applications of high-field ultrafast MAS NMR techniques in the study of hydroxyl-rich surfaces on other metal oxide materials.

Facet-dependent stability of near-surface oxygen vacancies and excess charge localization at CeO2surfaces

To study the dependence of the relative stability of surface (V^A) and subsurface (V^B) oxygen vacancies with the crystal facet of CeO₂, the reduced (100), (110) and (111) surfaces, with two different concentrations of vacancies, were investigated by means of density functional theory (DFT + U) calculations. The results show that the trend in the near-surface vacancy formation energies for comparable vacancy spacings, i.e. (110) < (100) < (111), does not follow the one in the surface stability of the facets, i.e. (111) < (110) < (100). The results also reveal that the preference of vacancies for surface or subsurface sites, as well as the preferred location of the associated Ce³⁺polarons, are facet- and concentration-dependent. At the higher vacancy concentration, theV^Ais more stable than theV^Bat the (110) facet whereas at the (111), it is the other way around, and at the (100) facet, both theV^Aand theV^Bhave similar stability. The stability of theV^Avacancies, compared to that of theV^B, is accentuated as the concentration decreases. Nearest neighbor polarons to the vacant sites are only observed for the less densely packed (110) and (100) facets. These findings are rationalized in terms of the packing density of the facets, the lattice relaxation effects induced by vacancy formation and the localization of the excess charge, as well as the repulsive Ce³⁺-Ce³⁺interactions.

Boosting the Catalytic Performance of CeO2 in Toluene Combustion via the Ce-Ce Homogeneous Interface

Catalytic combustion is an advanced technology to eliminate industrial volatile organic compounds such as toluene. In order to replace the expensive noble metal catalysts and avoid the aggregation phenomenon occurring in traditional heterogeneous interfaces, designing homogeneous interfaces can become an emerging methodology to enhance the catalytic combustion performance of metal oxide catalysts. A mesocrystalline CeO₂ catalyst with abundant Ce-Ce homogeneous interfaces is synthesized via a self-flaming method which exhibits boosted catalytic performance for toluene combustion compared with traditional CeO₂, leading to a ∼40 °C lower T₉₀. The abundant Ce-Ce homogeneous interfaces formed by both highly ordered stacking and small grain size endow the CeO₂ mesocrystal with superior redox property and oxygen storage capacity via forming various oxygen vacancies. Surface and bulk oxygen vacancies generate and activate crucial oxygen species, while interfacial oxygen vacancies further promote the reaction behavior of oxygen species (i.e., activation, regeneration, and migration), causing the splitting of redox property toward lower temperature. These properties facilitate aromatic ring decomposition, the important rate-determining step, thus contributing to toluene catalytic degradation to CO₂. This work may shed insights into the catalytic effects of homogeneous interfaces in pollutant removal and provide a strategy of interfacial defect engineering for catalyst development.

Labeling and Probing the Silica Surface Using Mechanochemistry and 17 O NMR Spectroscopy*

In recent years, there has been increasing interest in developing cost‐efficient, fast, and user‐friendly ¹⁷O enrichment protocols to help to understand the structure and reactivity of materials by using ¹⁷O NMR spectroscopy. Here, we show for the first time how ball milling (BM) can be used to selectively and efficiently enrich the surface of fumed silica, which is widely used at industrial scale. Short milling times (up to 15 min) allowed modulation of the enrichment level (up to ca. 5 %) without significantly changing the nature of the material. High‐precision ¹⁷O compositions were measured at different milling times by using large‐geometry secondary‐ion mass spectrometry (LG‐SIMS). High‐resolution ¹⁷O NMR analyses (including at 35.2 T) allowed clear identification of the signals from siloxane (Si−O−Si) and silanols (Si−OH), while DNP analyses, performed by using direct ¹⁷O polarization and indirect ¹⁷O{¹H} CP excitation, agreed with selective labeling of the surface. Information on the distribution of Si−OH environments at the surface was obtained from 2D ¹H−¹⁷O D‐HMQC correlations. Finally, the surface‐labeled silica was reacted with titania and using ¹⁷O DNP, their common interface was probed and Si−O−Ti bonds identified.

Improved NMR transfer of magnetization from protons to half-integer spin quadrupolar nuclei at moderate and high magic-angle spinning frequencies

Half-integer spin quadrupolar nuclei are the only magnetic isotopes for the majority of the chemical elements. Therefore, the transfer of polarization from protons to these isotopes under magic-angle spinning (MAS) can provide precious insights into the interatomic proximities in hydrogen-containing solids, including organic, hybrid, nanostructured and biological solids. This transfer has recently been combined with dynamic nuclear polarization (DNP) in order to enhance the NMR signal of half-integer quadrupolar isotopes. However, the cross-polarization transfer lacks robustness in the case of quadrupolar nuclei, and we have recently introduced as an alternative technique a D -RINEPT (through-space refocused insensitive nuclei enhancement by polarization transfer) scheme combining a heteronuclear dipolar recoupling built from adiabatic pulses and a continuous-wave decoupling. This technique has been demonstrated at 9.4 T with moderate MAS frequencies, ν R ≈ 10 -15 kHz, in order to transfer the DNP-enhanced 1 H polarization to quadrupolar nuclei. Nevertheless, polarization transfers from protons to quadrupolar nuclei are also required at higher MAS frequencies in order to improve the 1 H resolution. We investigate here how this transfer can be achieved at ν R ≈ 20 and 60 kHz. We demonstrate that the D -RINEPT sequence using adiabatic pulses still produces efficient and robust transfers but requires large radio-frequency (rf) fields, which may not be compatible with the specifications of most MAS probes. As an alternative, we introduce robust and efficient variants of the D -RINEPT and PRESTO (phase-shifted recoupling effects a smooth transfer of order) sequences using symmetry-based recoupling schemes built from single and composite π pulses. Their performances are compared using the average Hamiltonian theory and experiments at B 0 = 18.8  T on γ -alumina and isopropylamine-templated microporous aluminophosphate (AlPO4 -14), featuring low and significant 1 H-1 H dipolar interactions, respectively. These experiments demonstrate that the 1 H magnetization can be efficiently transferred to 27 Al nuclei using D -RINEPT with SR 4 1 2 (2700 90180 ) recoupling and using PRESTO with R 22 2 7 (1800 ) or R 16 7 6 (2700 90180 ) schemes at ν R = 20 or 62.5 kHz, respectively. The D -RINEPT and PRESTO recoupling schemes complement each other since the latter is affected by dipolar truncation, whereas the former is not. We also analyze the losses during these recoupling schemes, and we show how these magnetization transfers can be used at ν R = 62.5  kHz to acquire in 72 min 2D HETCOR (heteronuclear correlation) spectra between 1 H and quadrupolar nuclei, with a non-uniform sampling (NUS).

Natural abundance oxygen-17 solid-state NMR of metal organic frameworks enhanced by dynamic nuclear polarization

The 17O resonances of zirconium-oxo clusters that can be found in porous Zr carboxylate metal-organic frameworks (MOFs) have been investigated by magic-angle spinning (MAS) NMR spectroscopy enhanced by dynamic nuclear polarization (DNP). High-resolution 17O spectra at 0.037% natural abundance could be obtained in 48 hours, thanks to DNP enhancement of the 1H polarization by factors ε(1H) = Swith/Swithout = 28, followed by 1H → 17O cross-polarization, allowing a saving in experimental time by a factor of ca. 800. The distinct 17O sites from the oxo-clusters can be resolved at 18.8 T. Their assignment is supported by density functional theory (DFT) calculations of chemical shifts and quadrupolar parameters. Protonation of 17O sites seems to be leading to large characteristic shifts. Hence, natural abundance 17O NMR spectra of diamagnetic MOFs can thus be used to probe and characterize the local environment of different 17O sites on an atomic scale.

Solid-state NMR spectroscopy

Solid-state nuclear magnetic resonance (NMR) spectroscopy is an atomic-level method used to determine the chemical structure, three-dimensional structure, and dynamics of solids and semi-solids. This Primer summarizes the basic principles of NMR as applied to the wide range of solid systems. The fundamental nuclear spin interactions and the effects of magnetic fields and radiofrequency pulses on nuclear spins are the same as in liquid-state NMR. However, because of the anisotropy of the interactions in the solid state, the majority of high-resolution solid-state NMR spectra is measured under magic-angle spinning (MAS), which has profound effects on the types of radiofrequency pulse sequences required to extract structural and dynamical information. We describe the most common MAS NMR experiments and data analysis approaches for investigating biological macromolecules, organic materials, and inorganic solids. Continuing development of sensitivity-enhancement approaches, including 1H-detected fast MAS experiments, dynamic nuclear polarization, and experiments tailored to ultrahigh magnetic fields, is described. We highlight recent applications of solid-state NMR to biological and materials chemistry. The Primer ends with a discussion of current limitations of NMR to study solids, and points to future avenues of development to further enhance the capabilities of this sophisticated spectroscopy for new applications.

Cu oxide deposited on shape-controlled ceria nanocrystals for CO oxidation: influence of interface-driven oxidation states on catalytic activity

The polar CeO2 (100) surface facets contribute considerably to the formation of surface hydroxyl groups, which are necessary for selective, stable Cu1+ state loading and enhancement of CO oxidation activity.

Vibrational Frequencies of Cerium-Oxide-Bound CO: A Challenge for Conventional DFT Methods

In ceria-based catalysis, the shape of the catalyst particle, which determines the exposed crystal facets, profoundly affects its reactivity. The vibrational frequency of adsorbed carbon monoxide (CO) can be used as a sensitive probe to identify the exposed surface facets, provided reference data on well-defined single crystal surfaces together with a definitive theoretical assignment exist. We investigate the adsorption of CO on the CeO_{2}(110) and (111) surfaces and show that the commonly applied DFT(PBE)+U method does not provide reliable CO vibrational frequencies by comparing with state-of-the-art infrared spectroscopy experiments for monocrystalline CeO_{2} surfaces. Good agreement requires the hybrid DFT approach with the HSE06 functional. The failure of conventional density-functional theory (DFT) is explained in terms of its inability to accurately describe the facet- and configuration-specific donation and backdonation effects that control the changes in the C─O bond length upon CO adsorption and the CO force constant. Our findings thus provide a theoretical basis for the detailed interpretation of experiments and open up the path to characterize more complex scenarios, including oxygen vacancies and metal adatoms.

Surface Refaceting Mechanism on Cubic Ceria

Polar surfaces of solid oxides are intrinsically unstable and tend to reconstruct due to the diverging electrostatic energy and thus often exhibit unique physical and chemical properties. However, a quantitative description of the restructuring mechanism of these polar surfaces remains challenging. Here we provide an atomic-level picture of the refaceting process that governs the surface polarity compensation of cubic ceria nanoparticles based on the accurate reference data acquired from the well-defined model systems. The combined results from advanced infrared spectroscopy, atomic-resolved transmission electron microscopy, and density functional theory calculations identify a two-step scenario where an initial O-terminated (2 × 2) reconstruction is followed by a severe refaceting via massive mass transport at elevated temperatures to yield {111}-dominated nanopyramids. This significant surface restructuring promotes the redox properties of ceria nanocubes, which account for the enhanced catalytic activity for CO oxidation.

Interactions of Oxide Surfaces with Water Revealed with Solid-State NMR Spectroscopy

Hydrous materials are ubiquitous in the natural environment and efforts have previously been made to investigate the structures and dynamics of hydrated surfaces for their key roles in various chemical and physical applications, with the help of theoretical modeling and microscopy techniques. However, an overall atomic-scale understanding of the water-solid interface, including the effect of water on surface ions, is still lacking. Herein, we employ ceria nanorods with different amounts of water as an example and demonstrate a new approach to explore the water-surface interactions by using solid-state NMR in combination with density functional theory. NMR shifts and relaxation time analysis provide detailed information on the local structure of oxygen ions and the nature of water motion on the surface: the amount of molecularly adsorbed water decreases rapidly with increasing temperature (from room temperature to 150 °C), whereas hydroxyl groups are stable up to 150 °C, and dynamic water molecules are found to instantaneously coordinate to the surface oxygen ions. The applicability of dynamic nuclear polarization for selective detection of surface oxygen species is also compared to conventional NMR with surface selective isotopic-labeling: the optimal method depends on the feasibility of enrichment and the concentration of protons in the sample. These results provide new insight into the interfacial structure of hydrated oxide nanostructures, which is important to improve performance for various applications.

Solid-state nuclear magnetic resonance studies of nanoparticles

In this trends article, we review seminal and recent studies using static and magic-angle spinning solid-state NMR to study the structure of nanoparticles and ligands attached to nanoparticles. Solid-state NMR techniques including one-dimensional multinuclear NMR, cross-polarization, techniques for measuring dipolar coupling and internuclear distances, and multidimensional NMR have provided insight into the core-shell structure of nanoparticles as well as the structure of ligands on the nanoparticle surface. Hyperpolarization techniques, in particular solid-state dynamic nuclear polarization (DNP), have enabled detailed studies of nanoparticle core-shell structure and surface chemistry, by allowing unprecedented levels of sensitivity to be achieved. The high signal-to-noise afforded by DNP has allowed homonuclear and heteronuclear correlation experiments involving nuclei with low natural abundance to be performed in reasonable experimental times, which previously would not have been possible. The use of DNP to study nanoparticles and their applications will be a fruitful area of study in the coming years as well.

17O Solid-State NMR Studies of Ta2O5 Nanorods

17O solid-state NMR spectroscopy was used to study the structure of Ta2O5 nanorods for the first time. Although the observations of oxygen ions in the "bulk" part of the Ta2O5 nanorods can be achieved with conventional high-temperature enrichment with 17O2, low-temperature isotopic labeling with H2 17O generated samples whose surfaces are selectively enriched, leading to surface-only detection of oxygen species. By applying 17O-1H double-resonance NMR techniques and 1H NMR spectroscopy, surface hydroxyl species and adsorbed water can also be studied. The results form the basis for further understanding of the structure-property relationship of Ta2O5 nanomaterials.

Direct 17O Isotopic Labeling of Oxides Using Mechanochemistry

While ¹⁷O NMR is increasingly being used for elucidating the structure and reactivity of complex molecular and materials systems, much effort is still required for it to become a routine analytical technique. One of the main difficulties for its development comes from the very low natural abundance of ¹⁷O (0.04%), which implies that isotopic labeling is generally needed prior to NMR analyses. However, ¹⁷O-enrichment protocols are often unattractive in terms of cost, safety, and/or practicality, even for compounds as simple as metal oxides. Here, we demonstrate how mechanochemistry can be used in a highly efficient way for the direct ¹⁷O isotopic labeling of a variety of s-, p-, and d-block oxides, which are of major interest for the preparation of functional ceramics and glasses: Li₂O, CaO, Al₂O₃, SiO₂, TiO₂, and ZrO₂. For each oxide, the enrichment step was performed under ambient conditions in less than 1 h and at low cost, which makes these synthetic approaches highly appealing in comparison to the existing literature. Using high-resolution solid-state ¹⁷O NMR and dynamic nuclear polarization, atomic-level insight into the enrichment process is achieved, especially for titania and alumina. Indeed, it was possible to demonstrate that enriched oxygen sites are present not only at the surface but also within the oxide particles. Moreover, information on the actual reactions occurring during the milling step could be obtained by ¹⁷O NMR, in terms of both their kinetics and the nature of the reactive species. Finally, it was demonstrated how high-resolution ¹⁷O NMR can be used for studying the reactivity at the interfaces between different oxide particles during ball-milling, especially in cases when X-ray diffraction techniques are uninformative. More generally, such investigations will be useful not only for producing ¹⁷O-enriched precursors efficiently but also for understanding better mechanisms of mechanochemical processes themselves.

The direct ¹⁷O enrichment of s-, p-, and d-block metal oxides is achieved with high efficiency using mechanochemistry. Atomic-level insight into the enrichment process is obtained using high-resolution solid-state ¹⁷O NMR and dynamic nuclear polarization analyses, which demonstrate that enriched oxygen sites are present both at the surface and within the oxide particles. Moreover, it is demonstrated how these labeling schemes allow the study of unique aspects of mechanochemical reactions between oxides by ¹⁷O NMR.

Probing Interactions of γ-Alumina with Water via Multinuclear Solid-State NMR Spectroscopy

Interaction of γ-alumina with water are important in controlling its structure and catalytic properties. We apply solid-state multinuclear NMR spectroscopy to investigate this interaction by monitoring ¹H and ¹⁷O spectra in real-time. Surface-selective detection is made possible by adsorbing ¹⁷O-enriched water on γ-alumina nanorods. Structural evolution on the surface was selectively probed by ¹H/¹⁷O double resonance NMR and ²⁷Al NMR at ultrahigh 35.2 T magnetic field. Formation of hydroxyl species on the surface of nanorods is rapid upon the exposure of water, which involves low coordinated aluminum ions with doubly bridging and isolated hydroxyl species being generated first. Fast exchange occurs between oxygen atoms in the water molecules and bare surface sites, indicating high reactivity of these oxygen species. These results provide new insights into the structure and dynamics on the surface of γ-alumina and the methods applied here can be extended to study the interaction of other oxides with water.