Trimethylphosphine-Assisted Surface Fingerprinting of Metal Oxide Nanoparticle by 31P Solid-State NMR: A Zinc Oxide Case Study

Spinel Nanostructures for the Hydrogenation of CO2 to Methanol and Hydrocarbon Chemicals

Composite oxides have been widely applied in the hydrogenation of CO/CO2 to methanol or as the component of bifunctional oxide-zeolite for the synthesis of hydrocarbon chemicals. However, it is still challenging to disentangle the stepwise formation mechanism of CH3OH at working conditions and selectively convert CO2 to hydrocarbon chemicals with narrow distribution. Here, we investigate the reaction network of the hydrogenation of CO2 to methanol over a series of spinel oxides (AB2O4), among which the Zn-based nanostructures offer superior performance in methanol synthesis. Through a series of (quasi) in situ spectroscopic characterizations, we evidence that the dissociation of H2 tends to follow a heterolytic pathway and that hydrogenation ability can be regulated by the combination of Zn with Ga or Al. The coordinatively unsaturated metal sites over ZnAl2Ox and ZnGa2Ox originating from oxygen vacancies (OVs) are evidenced to be responsible for the dissociative adsorption and activation of CO2. The evolution of the reaction intermediates, including both carbonaceous and hydrogen species at high temperatures and pressures over the spinel oxides, has been experimentally elaborated at the atomic level. With the integration of a series of zeolites or zeotypes, high selectivities of hydrocarbon chemicals with narrow distributions can be directly produced from CO2 and H2, offering a promising route for CO2 utilization.

Oxygen dynamic exchange and diffusion characteristics of ZnO nanorods from 17O MAS NMR

Interactions of ZnO nanorods with water and the dynamic migration characteristic of surface oxygen species are important in controlling its structure and catalytic properties. Here, we apply 17O solid-state NMR spectroscopy to investigate the interactions, as well as oxygen ion diffusion properties of ZnO nanorods under different conditions.

17O solid-state NMR study on exposed facets of ZnO nanorods with different aspect ratios

The physical and chemical properties of metal oxide nanocrystals are closely related to their exposed facets, so the study on facet structures is helpful to develop facet/morphology-property relationships and rationally design nanostructures with desired properties. In this study, wurtzite ZnO nanorods with different aspect ratios were prepared by controlling the Zn2+/OH- ratio, temperature and time in hydrothermal processes. An 17O solid-state NMR study was performed on these nanorods, after surface 17O labeling, to explore the relationship of the 17O NMR signals with the local surface structure of different exposed facets, i.e., nonpolar (101̄0) and polar (0002) facets. It is observed that, one of the signals, the sharp component of a peak at -18.8 ppm, comprises the contribution from the oxygen ions on the polar (0002) facets, in addition to that from nonpolar (101̄0) facets, which is confirmed by 17O NMR spectra of ZnO nanorods with controlled aspect ratios and different thermal treatment conditions. This is important for accurately interpreting the 17O NMR signal of ZnO-containing materials, especially when studying the facet-related mechanisms. The method applied here can also be extended to study the facet-dependent properties of other faceted oxide nanocrystals.

HVHC-ESD-Induced Oxygen Vacancies: An Insight into the Phenomena of Interfacial Interactions of Nanostructure Oxygen Vacancy Sites with Oxygen Ion-Containing Organic Compounds

The challenging environmental chemical and microbial pollution has always caused issues for human life. This article investigates the detailed mechanism of photodegradation and antimicrobial activity of oxide semiconductors and realizes the interface phenomena of nanostructures with toxins and bacteria. We demonstrate how oxygen vacancies in nanostructures affect photodegradation and antimicrobial behavior. Additionally, a novel method with a simple, tunable, and cost-effective synthesis of nanostructures for such applications is introduced to resolve environmental issues. The high-voltage, high-current electrical switching discharge (HVHC-ESD) system is a novel method that allows on-the-spot sub-second synthesis of nanostructures on top and in the water for wastewater decontamination. Experiments are done on rhodamine B as a common dye in wastewater to understand its photocatalytic degradation mechanism. Moreover, the antimicrobial mechanism of oxide semiconductors synthesized by the HVHC-ESD method with oxygen vacancies is realized on methicillin- and vancomycin-resistant Staphylococcus aureus strains. The results yield new insights into how oxygen ions in dyes and bacterial walls interact with the surface of ZnO with high oxygen vacancy, which results in breaking of the chemical structure of dyes and bacterial walls. This interaction leads to degradation of organic dyes and bacterial inactivation.

Probing the nature of Lewis acid sites on oxide surfaces with 31P(CH3)3 NMR: a theoretical analysis

The characterization of catalytic oxide surfaces is often done by studying the properties of adsorbed probe molecules. The ³¹P NMR chemical shift of adsorbed trimethylphosphine, P(CH₃)₃ or TMP, has been used to identify the presence of different facets in oxide nanocrystals and to study the acid-base properties of the adsorption sites. The NMR studies are often complemented by DFT calculations to provide additional information on TMP adsorption mode, bond strength, etc. So far, however, no systematic study has been undertaken in order to compare on the same footing the chemical shifts and the adsorption properties of TMP on different oxide surfaces. In this work we report the results of DFT+D (D = dispersion) calculations on the adsorption of TMP on the following oxide surfaces: anatase TiO₂(101) and (001), rutile TiO₂(110), tetragonal ZrO₂(101), stepped ZrO₂(134) and (145) surfaces, rutile SnO₂(110), (101) and (100), wurtzite ZnO(101̄0), and cubic CeO₂(111) and (110). Beside the stoichiometric surfaces, also reduced oxides have been considered creating O vacancies in various sites. TMP has been adsorbed on top of variously coordinated Lewis acid cation sites, with the aim to identify, also with the support of machine learning algorithms, trends or patterns that can help to correlate the ³¹P chemical shift with physico-chemical properties of the oxide surfaces such as adsorption energy, Bader charges, cation-P distance, work function, etc. Some simple correlation can be found within the same oxide between the ³¹P chemical shift and the adsorption energy, while when the full set of data is considered the only correlation found is with the net charge on the TMP molecule, a descriptor of the acid strength of the adsorption site.

Antagonistic Skin Toxicity of Co-Exposure to Physical Sunscreen Ingredients Zinc Oxide and Titanium Dioxide Nanoparticles

Being the main components of physical sunscreens, zinc oxide nanoparticles (ZnO NPs) and titanium dioxide nanoparticles (TiO₂ NPs) are often used together in different brands of sunscreen products with different proportions. With the broad use of cosmetics containing these nanoparticles (NPs), concerns regarding their joint skin toxicity are becoming more and more prominent. In this study, the co-exposure of these two NPs in human-derived keratinocytes (HaCaT) and the in vitro reconstructed human epidermis (RHE) model EpiSkin was performed to verify their joint skin effect. The results showed that ZnO NPs significantly inhibited cell proliferation and caused deoxyribonucleic acid (DNA) damage in a dose-dependent manner to HaCaT cells, which could be rescued with co-exposure to TiO₂ NPs. Further mechanism studies revealed that TiO₂ NPs restricted the cellular uptake of both aggregated ZnO NPs and non-aggregated ZnO NPs and meanwhile decreased the dissociation of Zn²⁺ from ZnO NPs. The reduced intracellular Zn²⁺ ultimately made TiO₂ NPs perform an antagonistic effect on the cytotoxicity caused by ZnO NPs. Furthermore, these joint skin effects induced by NP mixtures were validated on the epidermal model EpiSkin. Taken together, the results of the current research contribute new insights for understanding the dermal toxicity produced by co-exposure of different NPs and provide a valuable reference for the development of formulas for the secure application of ZnO NPs and TiO₂ NPs in sunscreen products.

Facet-Dependent Activity of CeO2 Nanozymes Regulate the Fate of Human Neural Progenitor Cell via Redox Homeostasis

Neural progenitor cells (NPCs) therapy, a promising therapeutic strategy for neurodegenerative diseases, has a huge challenge to ensure high survival rate and neuronal differentiation rate. Cerium oxide (CeO₂) nanoparticles exhibit multienzyme mimetic activities and have shown the capability of regulating reactive oxygen species (ROS), which is a pivotal mediator for intracellular redox homeostasis in NPCs, regulating biological processes including differentiation, proliferation, and apoptosis. In the present study, the role of facet-dependent CeO₂-mediated redox homeostasis in regulating self-renewal and differentiation of NPCs is reported for the first time. The cube-, rod-, and octahedron-shaped CeO₂ nanozymes with different facets are prepared. Among the mentioned nanozymes, the cube enclosed by the (100) facet exhibits the highest CAT-like activity, causing it to provide superior protection to NPCs from oxidative stress induced by H₂O₂; meanwhile, the octahedron enclosed by the (111) facet with the lowest CAT-like activity induces the most ROS production in ReNcell CX cells, which promotes neuronal differentiation by activated AKT/GSK-3β/β-catenin pathways. A further mechanistic study indicated that the electron density of the surface Ce atoms changed continuously with different crystal facets, which led to their different CAT-like activity and modulation of redox homeostasis in NPCs. Altogether, the different surface chemistry and atomic architecture of active sites on CeO₂ exert modulation of redox homeostasis and the fate of NPCs.

Assessment of Lewis-Acidic Surface Sites Using Tetrahydrofuran as a Suitable and Smart Probe Molecule

Measuring the Lewis‐acidic surface sites in catalysis is problematic when the material‘s surface area is very low (S_BET ≤1 m² ⋅ g⁻¹). For the first time, a quantitative assessment of total acidic surface sites of very small surface area catalysts (MoO₃ as pure and mixed with 5–30 % CdO (wt/wt), as well as CdO for comparison) was performed using a smart new probe molecule, tetrahydrofuran (THF). The results were nearly identical compared to using another commonly used probe molecule, pyridine. This audition is based on the limited values of the surface area of these samples that likely require a relatively moderate basic molecule as THF with pK_b=16.08, rather than strong basic molecules such as NH₃ (pK_b=4.75) or pyridine (pK_b=8.77). We propose mechanisms for the interaction of vapour phase molecules of THF with the Lewis‐cationic Mo and Cd atoms of these catalysts. Besides, dehydration of isopropyl alcohol was used as a probe reaction to investigate the catalytic activity of these catalysts to further support our findings in the case of THF in a temperature range of 175–300 °C. A good agreement between the obtained data of sample MoO₃‐10 % CdO, which is characterised by the highest surface area value, the population of Lewis‐acidic sites and % selectivity of propylene at all the applied reaction temperatures was found.

Identification of CO2 adsorption sites on MgO nanosheets by solid-state nuclear magnetic resonance spectroscopy

The detailed information on the surface structure and binding sites of oxide nanomaterials is crucial to understand the adsorption and catalytic processes and thus the key to develop better materials for related applications. However, experimental methods to reveal this information remain scarce. Here we show that ¹⁷O solid-state nuclear magnetic resonance (NMR) spectroscopy can be used to identify specific surface sites active for CO₂ adsorption on MgO nanosheets. Two 3-coordinated bare surface oxygen sites, resonating at 39 and 42 ppm, are observed, but only the latter is involved in CO₂ adsorption. Double resonance NMR and density functional theory (DFT) calculations results prove that the difference between the two species is the close proximity to H, and CO₂ does not bind to the oxygen ions with a shorter O···H distance of approx. 3.0 Å. Extensions of this approach to explore adsorption processes on other oxide materials can be readily envisaged.

The characterization of the surface structure and binding sites of materials is crucial for designing advanced materials for adsorption processes. Here, the authors use ¹⁷O solid-state nuclear magnetic resonance spectroscopy to identify specific CO₂ adsorption sites on MgO nanosheets.

Cluster Nanozymes with Optimized Reactivity and Utilization of Active Sites for Effective Peroxidase (and Oxidase) Mimicking

Single-atom catalysts have attracted attention in the past decade since they maximize the utilization of active sites and facilitate the understanding of product distribution in some catalytic reactions. Recently, this idea has been extended to single-atom nanozymes (SAzymes) for the mimicking of natural enzymes such as horseradish peroxidase (HRP) often used in bioanalytical applications. Herein, it is demonstrated that those SAzymes without constructing the reaction pocket of HRP still undergo the OH radical-mediated pathway like most of the reported nanozymes. Their positively charged single-atom centers resulting from support electronegative oxygen/nitrogen hinder the reductive conversion of H₂ O₂ to OH radicals and hence display low activity per site. In contrast, it is found that this step can be facilitated over their metallic counterparts on cluster nanozymes with much higher site activity and atom efficiency (cf. SAzymes with 100% atom utilization). Besides the mimicking of HRP in glucose detection, cluster nanozymes are also demonstrated as a better oxidase mimetic for glutathione detection.

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

Both atomic geometry and the influence of surroundings (e.g., exogenously coordinated water) are key issues for determining the chemical environment of oxide surfaces, whereas the latter is usually ignored and should be considered in future studies.

Investigation of Adsorption Behavior of Anticancer Drug on Zinc Oxide Nanoparticles: A Solid State NMR and Cyclic Voltammetry (CV) Analysis

The present study aims to comprehend the adsorption behavior of a set of anticancer drugs namely 5-fluorouracil (5-FU), doxorubicin and daunorubicin on ZnO nanoparticles (ZnO NPs) proposed as drug delivery systems employing solid state (ss) NMR, FTIR and Cyclic Voltammetry (CV) analysis. FTIR and ¹H MAS ssNMR data recorded for bare ZnO nanoparticle confirmed the presence of adsorbed -OH groups on the surface. ¹³C CP-MAS NMR spectra recorded for free and ZnO surface adsorbed drug samples exhibited considerable line broadening and chemical shift changes that complemented our earlier report on UV-DRS and XRD data of surface adsorption in case of 5-FU. Moreover, a remarkable enhancement of ¹³C signal intensity in case of loaded 5-FU was observed. This clearly indicated rigid nature of the drug on the surface allowing efficient transfer of ¹H polarization from the hetero nitrogen of 5-FU to ZnO to form surface hydroxyl (-OH) groups and the same has been observed in the quantum chemical calculations. To further analyze the motional dynamics of the surface adsorbed 5-FU, longitudinal relaxation times (T₁) were quantified employing Torchia method that revealed significant enhancement of ¹³C relaxation rate of adsorbed 5-FU. The enhanced rate suggested an effective role of quadrupolar contribution from ⁶⁷Zn to the ¹³C relaxation mechanism of ZnO_5-FU. The heterogeneous rate constant (k_het), average free energy of activation (∆G^≠) and point of zero charge (PZC) measured for free and drug loaded ZnO NPs samples using CV further support the SS-NMR results.

Surface acidity of tin dioxide nanomaterials revealed with 31P solid-state NMR spectroscopy and DFT calculations

Tin dioxide (SnO₂) nanomaterials are important acid catalysts. It is therefore crucial to obtain details about the surface acidic properties in order to develop structure–property relationships. Herein, we apply ³¹P solid-state NMR spectroscopy combined with a trimethylphosphine (TMP) probe molecule, to study the facet-dependent acidity of SnO₂ nanosheets and nanoshuttles. With the help of density functional theory calculations, we show that the tin cations exposed on the surfaces are Lewis acid sites and their acid strengths rely on surface geometries. As a result, the (001), (101), (110), and (100) facets can be differentiated by the ³¹P NMR shifts of adsorbed TMP molecules, and their fractions in different nanomaterials can be extracted according to deconvoluted ³¹P NMR resonances. The results provide new insights on nanosized oxide acid catalysts.

Facet-dependent acidity of SnO₂ nanosheets and nanoshuttles is revealed with TMP-assisted ³¹P solid-state NMR spectroscopy and DFT calculations.

Electronic-State Manipulation of Surface Titanium Activates Dephosphorylation Over TiO2 Near Room Temperature

Dephosphorylation that removes a phosphate group from substrates is an important reaction for living organisms and environmental protection. Although CeO₂ has been shown to catalyze this reaction, cerium is low in natural abundance and has a narrow global distribution (>90 % of these reserves are located within six countries). It is thus imperative to find another element/material with high worldwide abundance that can also efficiently extract the phosphate out of agricultural waste for phosphorus recycle. Using para-nitrophenyl phosphate (p-NPP) as a model compound, we demonstrate that TiO₂ with a F-modified (001) surface can activate p-NPP dephosphorylation at temperatures as low as 40 °C. By probe-assisted nuclear magnetic resonance (NMR), it was revealed that the strong electron-withdrawing effect of fluorine makes Ti atoms (the active sites) on the (001) surface very acidic. The bidentate adsorption of p-NPP on this surface further promotes its subsequent activation with a barrier ≈20 kJ mol^-1 lower than that of the pristine (001) and (101) surfaces, allowing the activation of this reaction near room temperature (from >80 °C).

Unusual Catalytic Properties of High-Energetic-Facet Polar Metal Oxides

ConspectusHeterogeneous catalysis is an area of great importance not only in chemical industries but also in energy conversion and environmental technologies. It is well-established that the specific surface morphology and structure of solid catalysts exert remarkable effects on catalytic performances, since most physical and chemical processes take place on the surface during catalytic reactions. Different from the widely studied faceted metallic nanoparticles, metal oxides give more complicated structures and surface features. Great progress has been achieved in controlling the shape and exposed facets of transition metal oxides during nanocrystal growth, usually by using surface-directing agents (SDAs). However, the effects of exposed facets remain controversial among researchers. It should be noted that high-energetic facets, especially polar facets, tend to lower their surface energy via different relaxation processes, such as surface reconstruction, redox change, adsorption of countercharged species, etc. These processes can subsequently lead to surface defect formation and break the surface stoichiometry, and the resulting changes in electronic configurations and charge migration properties all play important roles in heterogeneous catalysis. Because different materials prefer different relaxation methods, various surface features are created, and different techniques are required to investigate the different features from facet to facet. Conventional characterization techniques such as X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, etc. appear to be insufficient to elucidate the underlying principles of the facet effects. Consequently, an increasing number of novel techniques have been developed to differentiate the surface features, enabling greater understanding of the effects of facets on heterogeneous catalysis.In this Account, on the basis of previous studies by our own group, we will focus on the effects of tailored facets on heterogeneous catalysis introduced by engineered simple binary metal oxide nanomaterials primarily with exposed polar facets, in combination with detailed surface studies using a range of new characterization techniques. As a result, fundamental principles of the effects of facets are elucidated, and the structure-activity correlations are demonstrated. The surface features introduced by different relaxation processes are also investigated using a range of characterization techniques. For example, electron paramagnetic resonance spectroscopy is used to detect the oxygen vacancies, while probe-assisted solid-state NMR spectroscopy is shown to be facet-sensitive and able to evaluate the surface acidity. It is also shown that such different features influence the heterogeneous catalytic performances in different ways. With the help of first-principles density functional theory calculations, unique properties of the faceted metal oxides are discussed and unraveled. Besides, other materials such as transition metal chalcogenides and layered double hydroxides are also briefly discussed with regard to their application in facet-dependent catalysis studies.

Solid-state 31P NMR mapping of active centers and relevant spatial correlations in solid acid catalysts

Solid acid catalysts are used extensively in various advanced chemical and petrochemical processes. Their catalytic performance (namely, activity, selectivity, and reaction pathway) mostly depends on their acid properties, such as type (Brønsted versus Lewis), location, concentration, and strength, as well as the spatial correlations of their acid sites. Among the diverse methods available for acidity characterization, solid-state nuclear magnetic resonance (SSNMR) techniques have been recognized as the most valuable and reliable tool, especially in conjunction with suitable probe molecules that possess observable nuclei with desirable properties. Taking ³¹P probe molecules as an example, both trimethylphosphine (TMP) and trimethylphosphine oxide (TMPO) adsorb preferentially to the acid sites on solid catalysts and thus are capable of providing qualitative and quantitative information for both Brønsted and Lewis acid sites. This protocol describes procedures for (i) the pretreatment of typical solid acid catalysts, (ii) adoption and adsorption of various ³¹P probe molecules, (iii) considerations for one- and two-dimensional (1D and 2D, respectively) NMR acquisition, (iv) relevant data analysis and spectral assignment, and (v) methodology for NMR mapping with the assistance of theoretical calculations. Users familiar with SSNMR experiments can complete ³¹P-¹H heteronuclear correlation (HETCOR), ³¹P-³¹P proton-driven spin diffusion (PDSD), and double-quantum (DQ) homonuclear correlation with this protocol within 2-3 d, depending on the complexity and the accessible acid sites of the solid acid samples.

Nanoisozymes: The Origin behind Pristine CeO2 as Enzyme Mimetics

It is known that the interplay between molecules and active sites on the topmost surface of a solid catalyst determines its activity in heterogeneous catalysis. The electron density of the active site is believed to affect both adsorption and activation of reactant molecules at the surface. Unfortunately, commercial X-ray photoelectron spectroscopy, which is often adopted for such characterization, is not sensitive enough to analyze the topmost surface of a catalyst. Most researchers fail to acknowledge this point during their catalytic correlation, leading to different interpretations in the literature in recent decades. Recent studies on pristine Cu₂ O [Nat. Catal. 2019, 2, 889; Nat. Energy 2019, 4, 957] have clearly suggested that the electron density of surface Cu is facet dependent and plays a key role in CO₂ reduction. Herein, it is shown that pristine CeO₂ can reach 2506/1133 % increase in phosphatase-/peroxidase-like activity if the exposed surface is wisely selected. By using NMR spectroscopy with a surface probe, the electron density of the surface Ce (i.e., the active site) is found to be facet dependent and the key factor dictating their enzyme-mimicking activities. Most importantly, the surface area of the CeO₂ morphologies is demonstrated to become a factor only if surface Ce can activate the adsorbed reactant molecules.

Unravelling the Role of Structural Geometry and Chemical State of Well-Defined Oxygen Vacancies on Pristine CeO2 for H2O2 Activation

Although H₂O₂ has been often employed as a green oxidant for many CeO₂-catalyzed reactions, the underlying principle of its activation by surface oxygen vacancy (V_o) is still elusive due to the irreversible removal of postgenerated V_o by water (or H₂O₂). The metastable V_o (ms-V_o) naturally preserved on pristine CeO₂ surfaces was adopted herein for an in-depth study of their interplay with H₂O₂. Their well-defined local structures and chemical states were found facet-dependent affecting both the adsorption and subsequent activation of H₂O₂. It is concluded that a strong adsorption of H₂O₂ on ms-V_o may not guarantee its subsequent activation. The ms-V_o can be only free for the next catalytic cycle when the electron density of surface Ce is high enough to reduce/break the O-O bond of adsorbed H₂O₂. This explains the 211.8 and 35.8 times enhancement in H₂O₂ reactivity when the CeO₂ surface is changed from (111) and (110) to (100).

Effects of catalyst surfaces on adsorption revealed by atomic force microscope force spectroscopy: photocatalytic degradation of diuron over zinc oxide

Controlling adsorption of a heterogeneous catalyst requires a detailed understanding of the interactions between reactant molecules and the catalyst surface. Various characteristics relevant to adsorption have been theoretically predicted but have yet to be experimentally quantified. Here, we explore a model reaction based on diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] photo-degradation over a ZnO particle catalyst. We used atomic force microscope (AFM)-based force spectroscopy under ambient conditions to investigate interactions between individual functional groups of diuron (NH2, Cl, and CH3) and surfaces of ZnO particles (polar Zn and O-terminated, and nonpolar Zn-O terminated). We were able to distinguish and identify the two polar surfaces of conventional ZnO particles and the nonpolar surface of ZnO nanorods based on force-distance curves of functionalized probe/surface pairs. We posit that the reaction involved physisorption and could be described in terms of Hamaker constants. These constants had an order-of-magnitude difference among the probe/surface interacting pairs based on polarity. Hence, we confirmed that van der Waals interactions determined the adsorption behavior. We interpreted the electronic distribution models of the probe-modifying molecules. The functional group configurations inferred the diuron adsorption configurations during contact with each ZnO facet. The adsorption affected characteristics of the reaction intermediates and the rate of degradation.

Stable Hexylphosphonate-Capped Blue-Emitting Quantum-Confined CsPbBr3 Nanoplatelets

Quantum-confined CsPbBr₃ nanoplatelets (NPLs) are extremely promising for use in low-cost blue light-emitting diodes, but their tendency to coalesce in both solution and film form, particularly under operating device conditions with injected charge-carriers, is hindering their adoption. We show that employing a short hexyl-phosphonate ligand (C₆H₁₅O₃P) in a heat-up colloidal approach for pure, blue-emitting quantum-confined CsPbBr₃ NPLs significantly suppresses these coalescence phenomena compared to particles capped with the typical oleyammonium ligands. The phosphonate-passivated NPL thin films exhibit photoluminescence quantum yields of ∼40% at 450 nm with exceptional ambient and thermal stability. The color purity is preserved even under continuous photoexcitation of carriers equivalent to LED current densities of ∼3.5 A/cm². ¹³C, ¹³³Cs, and ³¹P solid-state MAS NMR reveal the presence of phosphonate on the surface. Density functional theory calculations suggest that the enhanced stability is due to the stronger binding affinity of the phosphonate ligand compared to the ammonium ligand.

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.

A Stimulus-Responsive Zinc-Iodine Battery with Smart Overcharge Self-Protection Function

Zinc-iodine aqueous batteries (ZIABs) are highly attractive for grid-scale energy storage due to their high theoretical capacities, environmental friendliness, and intrinsic non-flammability. However, because of the close redox potential of Zn stripping/platting and hydrogen evolution, slight overcharge of ZIABs would induce drastic side reactions, serious safety concerns, and battery failure. A novel type of stimulus-responsive zinc-iodine aqueous battery (SR-ZIAB) with fast overcharge self-protection ability is demonstrated by employing a smart pH-responsive electrolyte. Operando spectroelectrochemical characterizations reveal that the battery failure mechanism of ZIABs during overcharge arises from the increase of electrolyte pH induced by hydrogen evolution as well as the consequent irreversible formation of insulating ZnO at anode and soluble Zn(IO₃ )₂ at cathode. Under overcharge conditions, the designed SR-ZIABs can be rapidly switched off with capacity degrading to 6% of the initial capacity, thereby avoiding continuous battery damage. Importantly, SR-ZIABs can be switched on with nearly 100% of capacity recovery by re-adjusting the electrolyte pH. This work will inspire the development of aqueous Zn batteries with smart self-protection ability in the overcharge state.

Chemical state tuning of surface Ce species on pristine CeO2 with 2400% boosting in peroxidase-like activity for glucose detection

2400% increase in CeO₂ peroxidase-like activity can be easily achieved on the (100) surface due to the promoted H₂O₂ adsorption/activation.

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

Compared to nanomaterials exposing nonpolar facets, polar-faceted nanocrystals often exhibit unexpected and interesting properties. The electrostatic instability arising from the intrinsic dipole moments of polar facets, however, leads to different surface configurations in many cases, making it challenging to extract detailed structural information and develop structure-property relations. The widely used electron microscopy techniques are limited because the volumes sampled may not be representative, and they provide little chemical bonding information with low contrast of light elements. With ceria nanocubes exposing (100) facets as an example, here we show that the polar surface structure of oxide nanocrystals can be investigated by applying 17O and 1H solid-state NMR spectroscopy and dynamic nuclear polarization, combined with DFT calculations. Both CeO4-termination reconstructions and hydroxyls are present for surface polarity compensation and their concentrations can be quantified. These results open up new possibilities for investigating the structure and properties of oxide nanostructures with polar facets.

The Effects of Exposed Specific Facets and Sulfation on the Surface Acidity of Cu2 O Solids

Cuprous oxide microcrystals with {111}, {111}/{100}, and {100} exposed facets were synthesized. ³¹ P MAS NMR using trimethylphosphine as the probe molecule was employed to study the acidic properties of samples. It was found that the total acidic density of samples increases evidently after sulfation compared with the pristine cuprous oxide microcrystals. During sulfation, new {100} facets are formed at the expense of {111} facets and lead to the generation of two Lewis acid sites due to the different binding states of SO₄ ^2- on {111} and {100} facets. Moreover, DFT calculation was used to illustrate the binding models of SO₄ ^2- on {111} and {100} facets. Also, a Pechmann condensation reaction was applied to study the acidic catalytic activity of these samples. It was found that the sulfated {111} facet has better activity due to its higher Lewis acid density compared with the sulfated {100} facet.

Unravelling the key role of surface features behind facet-dependent photocatalysis of anatase TiO2

The high activity of the anatase TiO₂(001) facet in photocatalytic H₂ evolution is due to local electronic effects created by surface F on the facet.

Directed self-assembly of dual metal ions with ligands: towards the synthesis of noble metal/metal oxide composites with controlled facets

The large-scale synthesis of noble metal/metal oxide spheres with controlled facets was for the first time enabled by making use of a bottom-up self-assembly strategy.

Room temperature sintering of polar ZnO nanosheets: II-mechanism

In a previous work by the authors (A. Fernández-Pérez el al., Room temperature sintering of polar ZnO nanosheets: I-evidence, 2017, DOI: 10.1039/C7CP02306E), polar ZnO nanosheets were stored at room temperature under different atmospheres and the evolution of their textural and crystal properties during storage was followed. It was found that the specific surface area of the nanosheets drastically decreased during storage, with a loss of up to 75%. The ZnO crystals increased in size mainly through the partial merging of their polar surfaces at the expense of narrow mesoporosity, in a process triggered by the action of moisture, oxygen and, in their absence, by light. In the present work, a set of spectroscopic techniques (FTIR, Raman and XPS) has been used in an attempt to unravel the mechanism behind this spontaneous sintering process. The mechanism starts with the molecular adsorption of water, which takes place on Zn atoms close to oxygen vacancies on the (100) surface, where H₂O dissociates to form two hydroxyl groups and to heal one oxygen vacancy. This process triggers the room temperature migration of Zn interstitials towards the outer surface of the polar region. What were previously interstitial Zn atoms now gradually occupy the mesopores, with interstitial oxygen being used to build up the O sublattice until total occupancy of the narrow mesoporosity is achieved.

Differentiating surface titanium chemical states of anatase TiO2 functionalized with various groups

The local electronic effects on surface Ti, caused by adsorbates on TiO₂ facets, are probed experimentally (using probe-assisted NMR spectroscopy) and theoretically (using DFT).

As the chemical state of titanium on the surface of TiO₂ can be tuned by varying its host facet and surface adsorbate, improved performance has been achieved in fields such as heterogeneous (photo)catalysis, lithium batteries, dye-sensitized solar cells, etc. However, at present, no acceptable surface technique can provide information about the chemical state and distribution of surface cations among facets, making it difficult to unambiguously correlate facet-dependent properties. Even though X-ray photoelectron spectroscopy (XPS) is regarded as a sensitive surface technique, it collects data from the top few layers of the sample, instead of a specific facet, and hence fails to distinguish small changes in the chemical state of Ti imposed by adsorbates on a facet. Herein, based on experimental (chemical probe-assisted NMR) and theoretical (DFT) studies, the true surface Ti chemical states associated with surface modification using –O–, –F, –OH and –SO₄ functional groups on the (001) and (101) facets of anatase TiO₂ are clearly distinguished. It is also demonstrated, for the first time, that the local electronic effects on surface Ti imposed by adsorbates vary depending on the facet, due to different intrinsic electronic structures.

ZnO-ZnS Heterojunctions: A Potential Candidate for Optoelectronics Applications and Mineralization of Endocrine Disruptors in Direct Sunlight

Simple solution combustion synthesis was adopted to synthesize ZnO-ZnS (ZSx) nanocomposites using zinc nitrate as an oxidant and a mixture of urea and thiourea as a fuel. A large thiourea/urea ratio leads to more ZnS in ZSx with heterojunctions between ZnS and ZnO and throughout the bulk; tunable ZnS crystallite size and textural properties are an added advantage. The amount of ZnS in ZSx can be varied by simply changing the thiourea content. Although ZnO and ZnS are wide band gap semiconductors, ZSx exhibits visible light absorption, at least up to 525 nm. This demonstrates an effective reduction of the optical band gap and substantial changes in its electronic structure. Raman spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and secondary-ion mass spectrometry results show features due to ZnO and ZnS and confirm the composite nature with heterojunctions. The above mentioned observations demonstrate the multifunctional nature of ZSx. Bare ZSx exhibits a promising sunlight-driven photocatalytic activity for complete mineralization of endocrine disruptors such as 2,4-dichlorophenol and endosulphan. ZSx also exhibits photocurrent generation at no applied bias. Dye-sensitized solar cell performance evaluation with ZSx shows up to 4% efficiency and 48% incident photon conversion efficiency. Heterojunctions observed between ZnO and ZnS nanocrystallites in high-resolution transmission electron microscopy suggest the reason for effective separation of electron-hole pairs and their utilization.

Mapping surface-modified titania nanoparticles with implications for activity and facet control

The use of surface-directing species and surface additives to alter nanoparticle morphology and physicochemical properties of particular exposed facets has recently been attracting significant attention. However, challenges in their chemical analysis, sometimes at trace levels, and understanding their roles to elucidate surface structure–activity relationships in optical (solar cells) or (photo)catalytic performance and their removal are significant issues that remain to be solved. Here, we show a detailed analysis of TiO₂ facets promoted with surface species (OH, O, SO₄, F) with and without post-treatments by ³¹P adsorbate nuclear magnetic resonance, supported by a range of other characterization tools. We demonstrate that quantitative evaluations of the electronic and structural effects imposed by these surface additives and their removal mechanisms can be obtained, which may lead to the rational control of active TiO₂ (001) and (101) facets for a range of applications.

Metal oxide nanocrystals can be grown with different facets exposed to give variations in reactivity, but the chemical state of these surfaces is not clear. Here, the authors make use of a phosphine probe molecule allowing the differences in surface chemistry to be mapped by NMR spectroscopy.

Hydrodeoxygenation of water-insoluble bio-oil to alkanes using a highly dispersed Pd-Mo catalyst

Bio-oil, produced by the destructive distillation of cheap and renewable lignocellulosic biomass, contains high energy density oligomers in the water-insoluble fraction that can be utilized for diesel and valuable fine chemicals productions. Here, we show an efficient hydrodeoxygenation catalyst that combines highly dispersed palladium and ultrafine molybdenum phosphate nanoparticles on silica. Using phenol as a model substrate this catalyst is 100% effective and 97.5% selective for hydrodeoxygenation to cyclohexane under mild conditions in a batch reaction; this catalyst also demonstrates regeneration ability in long-term continuous flow tests. Detailed investigations into the nature of the catalyst show that it combines hydrogenation activity of Pd and high density of both Brønsted and Lewis acid sites; we believe these are key features for efficient catalytic hydrodeoxygenation behavior. Using a wood and bark-derived feedstock, this catalyst performs hydrodeoxygenation of lignin, cellulose, and hemicellulose-derived oligomers into liquid alkanes with high efficiency and yield.

Bio-oil is a potential major source of renewable fuels and chemicals. Here, the authors report a palladium-molybdenum mixed catalyst for the selective hydrodeoxygenation of water-insoluble bio-oil to mixtures of alkanes with high carbon yield.

Distinguishing faceted oxide nanocrystals with 17O solid-state NMR spectroscopy

Facet engineering of oxide nanocrystals represents a powerful method for generating diverse properties for practical and innovative applications. Therefore, it is crucial to determine the nature of the exposed facets of oxides in order to develop the facet/morphology–property relationships and rationally design nanostructures with desired properties. Despite the extensive applications of electron microscopy for visualizing the facet structure of nanocrystals, the volumes sampled by such techniques are very small and may not be representative of the whole sample. Here, we develop a convenient ¹⁷O nuclear magnetic resonance (NMR) strategy to distinguish oxide nanocrystals exposing different facets. In combination with density functional theory calculations, we show that the oxygen ions on the exposed (001) and (101) facets of anatase titania nanocrystals have distinct ¹⁷O NMR shifts, which are sensitive to surface reconstruction and the nature of the steps on the surface. The results presented here open up methods for characterizing faceted nanocrystalline oxides and related materials.

The exposed facets of oxide nanocrystals are key to their properties. Here, the authors use ¹⁷O solid-state NMR spectroscopy to discriminate between oxygen species on different facets of anatase titania nanocrystals, providing compelling evidence for the value of NMR spectroscopy in characterizing faceted oxides.

Room temperature sintering of polar ZnO nanosheets: I-evidence

Unambiguous evidence of the spontaneous loss of surface area at room temperature in polar ZnO.

In Situ Growth and Characterization of Metal Oxide Nanoparticles within Polyelectrolyte Membranes

This study describes a novel approach for the in situ synthesis of metal oxide-polyelectrolyte nanocomposites formed via impregnation of hydrated polyelectrolyte films with binary water/alcohol solutions of metal salts and consecutive reactions that convert metal cations into oxide nanoparticles embedded within the polymer matrix. The method is demonstrated drawing on the example of Nafion membranes and a variety of metal oxides with an emphasis placed on zinc oxide. The in situ formation of nanoparticles is controlled by changing the solvent composition and conditions of synthesis that for the first time allows one to tailor not only the size, but also the nanoparticle shape, giving a preference to growth of a particular crystal facet. The high-resolution TEM, SEM/EDX, UV-vis and XRD studies confirmed the homogeneous distribution of crystalline nanoparticles of circa 4 nm and their aggregates of 10-20 nm. The produced nanocomposite films are flexible, mechanically robust and have a potential to be employed in sensing, optoelectronics and catalysis.