Physicochemical and in Situ Photoluminescence Study of the Reversible Transformation of Oxide Ions of Low Coordination into Hydroxyl Groups upon Interaction of Water and Methanol with MgO†,‖

Transducer-Aware Hydroxy-Rich-Surface Indium Oxide Gas Sensor for Low-Power and High-Sensitivity NO2 Gas Sensing

Low-power metal oxide (MOX)-based gas sensors are widely applied in edge devices. To reduce power consumption, nanostructured MOX-based sensors that detect gas at low temperatures have been reported. However, the fabrication process of these sensors is difficult for mass production, and these sensors are lack uniformity and reliability. On the other hand, MOX film-based gas sensors have been commercialized but operate at high temperatures and exhibit low sensitivity. Herein, commercially advantageous highly sensitive, film-based indium oxide sensors operating at low temperatures are reported. Ar and O₂ gases are simultaneously injected during the sputtering process to form a hydroxy-rich-surface In₂O₃ film. Conventional indium oxide (In₂O₃) films (A0) and hydroxy-rich indium oxide films (A1) are compared using several analytical techniques. A1 exhibits a work function of 4.92 eV, larger than that of A0 (4.42 eV). A1 exhibits a Debye length 3.7 times longer than that of A0. A1 is advantageous for gas sensing when using field effect transistors (FETs) and resistors as transducers. Because of the hydroxy groups present on the surface of A1, A1 can react with NO₂ gas at a lower temperature (∼100 °C) than A0 (180 °C). Operando diffuse reflectance infrared Fourier transform spectrometry (DRIFTS) shows that NO₂ gas is adsorbed to A1 as nitrite (NO₂^-) at 100 °C and nitrite and nitrate (NO₃^-) at 200 °C. After NO₂ is adsorbed as nitrate, the sensitivity of the A1 sensor decreases and its low-temperature operability is compromised. On the other hand, when NO₂ is adsorbed only as nitrite, the performance of the sensor is maintained. The reliable hydroxy-rich FET-type gas sensor shows the best performance compared to that of the existing film-based NO₂ gas sensors, with a 2460% response to 500 ppb NO₂ gas at a power consumption of 1.03 mW.

Approaches to synthesize MgO nanostructures for diverse applications

Magnesium oxide remained interesting from long time for several important phenomena like; defect induced magnetism, spin electron reflectivity, broad laser emission etc. Moreover, nanostructures of this material exhibited suitability for different kinds of applications ranging from wastewater treatment to spintronics depending upon their shape and size. In this way, researchers had grown nanostructures in the form of nanoparticles, thin films, nanotubes, nanowalls, nanobelts. Though nanoparticles and thin films are well known form of nanostructures and wide variety of synthesis approaches are available, however, limited methodology for other nanostructures are available. In order to grow these nanostructures in an optimized way an understanding of these methods is essential. Thus, this review article depicts an overview of various approaches for design of different kinds of nanostructures.

Thin water films and particle morphology evolution in nanocrystalline MgO

A key question in the field of ceramics and catalysis is how and to what extent residual water in the reactive environment of a metal oxide particle powder affects particle coarsening and morphology. With X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM), we investigated annealing-induced morphology changes on powders of MgO nanocubes in different gaseous H2O environments. The use of such a model system for particle powders enabled us to describe how adsorbed water that originates from short exposure to air determines the evolution of MgO grain size, morphology, and microstructure. While cubic nanoparticles with a predominant abundance of (100) surface planes retain their shape after annealing to T = 1173 K under continuous pumping with a base pressure of water p(H2O) = 10-5 mbar, higher water partial pressures promote mass transport on the surfaces and across interfaces of such particle systems. This leads to substantial growth and intergrowth of particles and simultaneously favors the formation of step edges and shallow protrusions on terraces. The mass transfer is promoted by thin films of water providing a two-dimensional solvent for Mg2+ ion hydration. In addition, we obtained direct evidence for hydroxylation-induced stabilization of (110) faces and step edges of the grain surfaces.

Fewer Defects in the Surface Slows the Hydrolysis Rate, Decreases the ROS Generation Potential, and Improves the Non-ROS Antimicrobial Activity of MgO

Magnesium oxide (MgO) is recognised as exhibiting a contact-based antibacterial activity. However, a comprehensive study of the impact of atomic-scale surface features on MgO's antibacterial activity is lacking. In this study, the nature and abundance of the native surface defects on different MgO powders are thoroughly investigated. Their impacts on the hydrolysis kinetics, antibacterial activity against Escherichia coli (ATCC 47076), Staphylococcus epidermidis and Pseudomonas aeruginosa and the reactive oxygen species (ROS) generation potential are determined and explained. It is shown that a reduction in the abundance of low-coordinated oxygen atoms on the surface of the MgO improves its resistance to both hydrolysis and antibacterial activity. The ROS generation potential, determined in-situ using a fluorescence microplate assay and electron paramagnetic resonance spectroscopy, is not an inherent property of the studied MgO, rather it is a side product of hydrolysis (only for the most highly defected MgO particles) and/or a consequence of the MgO/bacteria interaction. The evaluation of the mutual correlations of the hydrolysis, the antibacterial activity and the ROS generation, with their origin in the surface defects' peculiarities, led to the conclusion that the acid/base reaction between the MgO surface and the bacterial wall contributes considerably to the MgO's antibacterial activity.

Influence of acid–base properties of Mg-based catalysts on transesterification: role of magnesium silicate hydrate formation

Catalysts with the strongest basic properties are not always the most efficient ones for transesterification, a series of magnesium-based materials, exhibiting a large range of acido–basic properties, was investigated.

Magnesium-based systems for carbon dioxide capture, storage and recycling: from leaves to synthetic nanostructured materials

Magnesium is used as leitmotif in this review in order to explore the systems involved in natural and artificial CO₂ cycles.

On the relationship between the basicity of a surface and its ability to catalyze transesterification in liquid and gas phases: the case of MgO

The influence of the basic properties of MgO is not the same for liquid and for gas phase transesterification.

Surface Decoration of MgO Nanocubes with Sulfur Oxides: Experiment and Theory

We investigated the effect of surface sulfate formation on the structure and spectroscopic properties of MgO nanocubes using X-ray diffraction, electron microscopy, several spectroscopic techniques, and ab initio calculations. After CS₂ adsorption and oxidative treatment at elevated temperatures the MgO particles remain cubic and retain their average size of ∼6 nm. Their low coordinated surface elements (corners and edges) were found to bind sulfite and sulfate groups even after annealing up to 1173 K. The absence of MgO corner specific photoluminescence emission bands at 3.4 and 3.2 eV substantiates that sulfur modifies the electronic properties of characteristic surface structures, which we attribute to the formation of (SO₃)^2- and (SO₄)^2- groups at corners and edges. Ab initio calculations support these conclusions and provide insight into the local atomic structures and spectroscopic properties of these groups.

Surface exciton separation in photoexcited MgO nanocube powders

In MgO nanocube powders surface excitons can separate and the resulting charge carriers provide reactive adsorption sites at well-defined surface elements. We employed photoluminescence (PL) emission bands originating from the photoexcitation of nanocube corners and edges as quantitative probes to explore their chemical reactivity towards molecular hydrogen. Surface excitons which form at corners and edges exhibit similar cross-sections for separation in vacuum. The separation of edge excitons, however, is significantly enhanced in hydrogen atmosphere when hydrogen adsorption occurs as a simultaneous surface process. The electronic structure of MgO nanocube edges which split hydrogen heterolytically upon generation of surface hydroxyls and hydrides is unaffected by the photoexcitation of corners. Respective edges, however, are efficient absorption sites for UV photons. Transfer of exciton energy to oxygen ions in corners is followed by exciton separation which transforms corner ions into surface radicals leading to a well-defined starting point for the site selective functionalization of metal oxide nanostructures.

Tuning surface basic properties of nanocrystalline MgO by controlling the preparation conditions

Different nanocrystalline magnesias were synthesized by precipitation and hydrothermal treatments of aqueous salt solutions in an attempt to tune their surface basicity. CO(2) was chosen as an acidic molecule to probe the basic sites by both temperature-programmed desorption and infrared spectroscopy. All samples were shown to be crystalline, and except that obtained by nitrate decomposition, they all possessed high surface areas. The oxides presented different basic site distributions, evidencing the significant role of the preparation conditions on tuning the surface basicity: while medium-strength basic centers are dominant in the samples prepared by precipitation aging or hydrothermal treatment, the one obtained by precipitation features a roughly equal concentration of medium-strength and strong centers. Infrared spectra revealed that hydrogen carbonate and monodentate and bidentate carbonates were formed in distinct proportion on all oxides. However, the bidentate complexes were shown to have different thermal stabilities; the more stable species are thought to be formed on acid-base pair centers associated with an anionic vacancy. Distinct morphological and structural characteristics were also observed by high-resolution transmission electron microscopy. It was consistently found that the high-surface area samples are formed by aggregates of nanoparticles (2-5 nm) randomly oriented and with a high concentration of structural defects. These findings allowed us to conclude that the surface heterogeneity promoted during synthesis increases the concentration of basic sites and plays an important role in tuning the basicity of the solids.

Optical surface properties and morphology of MgO and CaO nanocrystals

Optical absorption and photoluminescence emission properties of dehydroxylated MgO and CaO nanocrystals are discussed with respect to particle morphology and size. On MgO nanocubes with pronounced corner and edge features two emission bands at 3.4 and 3.3 eV result from the excitation of 4-coordinated surface O(4C)(2-) anions in edges at 5.4 eV and of regular oxygen-terminated corners at 4.6 eV, respectively. Morphologically ill-defined CaO particles are a factor of 5 larger, do not display regular corner features, and show only one photoluminescence emission band at 3.0 eV. The associated excitation spectrum indicates electronic excitations above the energy required to excite regular oxygen-terminated CaO corners. It is concluded that in the case of morphologically well-defined MgO nanocubes variations in the next coordination of oxygen-terminated corners can effectively be probed by photoluminescence spectroscopy and thus allows for discrimination between 3-coordinated surface O(2-) in regular corner sites and kinks.

Effect of Protons on the Optical Properties of Oxide Nanostructures

Site-specific functionalization of oxide nanostructures gives rise to novel optical and chemical surface properties. In addition, it can provide deeper insights into the electronic surface structure of the associated materials. We applied chemisorption of molecular hydrogen, induced by ultraviolet (UV) light, followed by vacuum annealing to MgO nanocubes to selectively decorate three-coordinated oxygen ions (oxygen corner sites, for simplicity) with protons. Fully dehydroxylated nanocubes exhibit 3.2 +/- 0.1 eV photoluminescence induced by 4.6 eV light, where both emission and absorption are associated with three-coordinated oxygen sites. We find that partially hydroxylated nanocubes show an additional photoluminescence feature at 2.9 +/- 0.1 eV. Interestingly, the excitation spectra of the 2.9 and 3.2 eV emission bands, associated with protonated and nonprotonated oxygen corner sites, respectively, nearly coincide and show well-pronounced maxima at 4.6 eV in spite of a significant difference in their local atomic and electronic structures. These observations are explained with the help of ab initio calculations, which reveal that (i) the absorption band at 4.6 eV involves four-coordinated O and Mg ions in the immediate vicinity of the corner sites and (ii) protonation of the three-coordinated oxygen ions eliminates the optical transitions associated with them and strongly red-shifts other optical transitions associated with neighboring atoms. These results demonstrate that the optical absorption bands assigned to topological surface defects are not simply determined by the ions of lowest coordination number but involve contributions due to the neighboring atoms of higher coordination. Thus, we suggest that the absorption band at 4.6 eV should not be regarded as merely a signature of the three-coordinated O2- ions but ought to be assigned to corners as multiatomic topological features. Our results also suggest that optical absorption signatures of protonated and nonprotonated sites of oxide surfaces can be remarkably similar.

Revisiting Acido-basicity of the MgO Surface by Periodic Density Functional Theory Calculations: Role of Surface Topology and Ion Coordination on Water Dissociation

Low-coordinated (LC) ions at the MgO surface (noted Mg2+LC and O2-LC with L = 1-5), located on monatomic and diatomic steps, corners, step divacancies, and kinks, have been modeled thanks to periodic density functional theory (DFT) calculations (VASP). Ions of lowest coordination induce the strongest surface geometry relaxation and the highest surface energies. The hydration energies of these sites and thermodynamic stabilities of the resulting surfaces were studied. The factors controlling the interaction strength between water and the surface are the possibility for the hydroxyl group to adopt a bridging geometry between two Mg2+ cations in concave areas of the surface, such as the bottom of the monatomic step, and at second order the surface atomic coordination, and especially the presence of three-coordinated ions. The Lewis basicity and acidity of O2-LC and Mg2+LC, respectively, increase as their coordination number decreases, which implies the same trend for the Brønsted basicity of the Mg2+-O2- pair toward water. However, this trend can be changed if pairs leading to the formation of bridging OH groups are involved, typically on monatomic steps or in step divacancies where O2C-H and O3C-H are obtained, respectively, instead of the expected O1C-H. Thanks to thermodynamic calculations, the state of the surface as a function of temperature can be determined at a given pressure, unraveling the roles of surface topology and ions coordination.

Kinetic Model of Energy Transfer Processes Between Low-Coordinated Ions on MgO by Photoluminescence Decay Measurements

Photoluminescence decay studies of emitting species on MgO nanocubes at room temperature provide evidence of three surface species characterized by an excitation and emission wavelength couple {lambda(exc);lambda(em)}. Species A corresponds to {lambda(exc)=240 nm; lambda(em)=380 nm}, whereas the couple {lambda(exc)=280 nm; lambda(em)=470 nm} is assigned to two species: B and B', the former is involved in energy transfer from excited state A* and the latter in direct emission from excited state B'*. A simple model for energy transfer from species A* to B is proposed. The numerical resolution of equations corresponding to this model is in good agreement with experimental data. This method quantifies the kinetics of intrinsic emission and energy transfer processes. Lifetime values indicate that phosphorescence is taking place, and species A, B and B' are identified as edge O(2-) (4 C), corner O(2-) (3 C) and kink O(2-) (3 C) oxide ions respectively.