Energy transfer on the MgO surface, monitored by UV-induced H2 chemisorption

Reconstruction of Highly-Defective MgO and Exceptional Photochemical Activity on CO2 Upgrade in Aqueous Solution

Defects on metal oxide have attracted extensive attention in photo-/electrocatalytic CO2 reduction. Herein, porous MgO nanosheets with abundant oxygen vacancies (Vo s) and three-coordinated oxygen atoms (O3c ) at corners are reported, which reconstruct into defective MgCO3 ·3H2 O exposing rich surface unsaturated -OH groups and vacancies to initiate photocatalytic CO2 reduction to CO and CH4 . In consecutive 7-cycle tests (each run for 6 h) in pure water, CO2 conversion keeps stable. The total production of CH4 and CO attains ≈367 µmol gcata -1 h-1 . The selectivity of CH4 gradually increases from ≈3.1% (1st run) to ≈24.5% (4th run) and then remains unchanged under UV-light irradiation. With triethanolamine (3.3 vol.%) as the sacrificial agent, the total production of CO and CH4 production rapidly increases to ≈28 000 µmol gcata -1 in 2 h reaction. Photoluminescence spectra reveal that Vo s induces the formation of donor bands to promote charge carrier seperation. A series of trace spectra and theoretical analysis indicate Mg-Vo sites in the derived MgCO3 ·3H2 O are active centers, which play a crucial role in modulating CO2 adsorption and triggering photoreduction reactions. These intriguing results on defective alkaline earth oxides as potential photocatalysts in CO2 conversion may spur some exciting and novel findings in this field.

Sorption of Fluoride and Bacterial Disinfection Property of Biosynthesized Nanofibrous Cellulose Decorated Ag-MgO-Nanohydroxyapatite Composite for Household Water Treatment

An innovative and sustainable approach to integrating modified Ag–MgO–nanohydroxyapatite on a nanofibrous cellulose template (CNF-AgMgOnHaP) as a multifunctional adsorbent via a hydrothermal bioreduction route using Citrus paradisi peel extract was developed and examined. The surface morphology and mineralogical properties of CNF-AgMgOnHaP by UV–vis spectroscopy, SEM-EDS, XRD, FTIR, TEM, and BET techniques are reported. Batch fluoride sorption studies and its disinfection potential against common bacteria in surface water were evaluated. The results showed the successful synthesis of a modified multistructural CNF-AgMgOnHaP composite with an improved BET surface area of 160.17 m²/g. The sorption of fluoride by the adsorbent was found to strongly depend on the different sorption conditions with a maximum F⁻ sorption capacity of 8.71 mg/g at 303 K, and pH of 5 with 0.25 g dosage at 10 min contact time (25 ± 3 °C). Equilibrium fluoride sorption onto the CNF-AgMgOnHaP was best described by the Freundlich isotherm model across all the operating temperatures. The overall kinetic results showed that the adsorption mechanisms not only depend on using the pseudo-second-order process but are also governed by the mass transfer of the adsorbate molecules from the external surface onto the pores of the adsorbent. The thermodynamic parameters revealed that the adsorption process of F⁻ onto CNF-AgMgOnHaP was endothermic and spontaneous at the sorbent/solution interface. The synthesized composite also provides some antibacterial activity against common infectious microbes from contaminated drinking water. The overall results suggested that the CNF-AgMgOnHaP nanocomposite possesses the potential for the simultaneous decontamination of pollutants and microbes in drinking water.

Rubbing Powders: Direct Spectroscopic Observation of Triboinduced Oxygen Radical Formation in MgO Nanocube Ensembles

Powder compaction-induced surface chemistry in metal oxide nanocrystal ensembles is important for very diverse fields such as triboelectrics, tribocatalysts, surface abrasion, and cold sintering of ceramics. Using a range of spectroscopic techniques, we show that MgO nanocube powder compaction with uniaxial pressures that can be achieved by gentle manual rubbing or pressing (p ≥ 5 MPa) excites energetic electron-hole pairs and generates oxygen radicals at interfacial defect structures. While the identification of paramagnetic O^- radicals and their adsorption complexes with O₂ point to the emergence of hole centers, triboemitted electrons become scavenged by molecular oxygen to convert into adsorbed superoxide anions O₂ ^- as measured by electron paramagnetic resonance (EPR). By means of complementary UV-photoexcitation experiments, we found that photon energies in the range between 3 and 6 eV produce essentially the same EPR spectroscopic fingerprints and optical absorption features. To provide insights into this effect, we performed density functional theory calculations to explore the energetics of charge separation involving the ionization of low-coordinated anions and surface-adsorbed O₂ ^- radicals at points of contact. For all selected configurations, charge transfer is not spontaneous but requires an additional driving force. We propose that a plausible mechanism for oxygen radical formation is the generation of significant surface potential differences at points of contact under loading as a result of the highly inhomogeneous elastic deformations coupled with the flexoelectric effect.

A low-temperature oxyl transfer to carbon monoxide from the ZnII–oxyl site in a zeolite catalyst

We demonstrated that the Zn^II–oxyl bond specifically formed by the zeolite lattice ligation has the capability of transferring the oxyl to CO even at 150 K with the generation of a single Zn^I˙ species.

Ultrathin Single Crystalline MgO(111) Nanosheets*

Synthesizing high-quality two-dimensional nanomaterials of nonlayered metal oxide is a challenge, especially when long-range single-crystallinity and clean high-energy surfaces are required. Reported here is the synthesis of single-crystalline MgO(111) nanosheets by a two-step process involving the formation of ultrathin Mg(OH)₂ nanosheets as a precursor, and their selective topotactic conversion upon heating under dynamic vacuum. The defect-rich surface displays terminal -OH groups, three-coordinated O^2- sites and low-coordinated Mg²⁺ sites, as well as single electrons trapped at oxygen vacancies, which render the MgO nanosheets highly reactive, as evidenced by the activation of CO molecules at low temperatures and pressures with formation of strongly adsorbed red-shifted CO and coupling of CO molecules into C₂ species.

How to Constrain Metal-Oxyl Bonds on a Solid Surface? Lesson from Isovalent Zn(II)-Oxyl and Ga(III)-Oxyl Bonds Isolated in Zeolite Matrix

Isolation of the atomic O radical anion bound to a metal ion (metal-oxyl) on solid surfaces is highly desirable for an understanding of how we should design the surface structure for using oxyl as the reactive site. Owing to the analytical difficulty of oxyl, however, even identification of oxyl remains scarce. Herein, we report isovalent Zn^II-oxyl and Ga^III-oxyl bonds isolated in the zeolite matrix. Close similarities in reactivity, spectroscopic property, and bonding nature were observed between them, but their site requirements were entirely different; the former is generated at the monovalent ion-exchangeable site, whereas the latter at the divalent ion-exchangeable site. This study strongly suggests that tuning the polarization of the metal-oxygen bond using the charge-controlled lattice oxygens is a useful way to constrain surface metal-oxyl bonds.

Enhancing the Thermal Performance of a Stearate Phase Change Material with Graphene Nanoplatelets and MgO Nanoparticles

The effectiveness of dispersed nanomaterials to improve the thermal performance of phase change materials (PCMs) is well-proven in the literature. The proposal of new engineered nanoenhanced phase change materials (NePCMs) with customized characteristics may lead to more efficient thermal energy storage (TES) systems. This work is focused on the development of new NePCMs based on the dispersions of graphene nanoplatelets (GnPs) or MgO nanoparticles in a stearate PCM. The new proposed materials were synthesized using a two-step method, and acetic acid was selected as a surfactant to improve the stability of the dispersions. An extensive characterization of the constitutive materials and the developed dispersions using different spectroscopy techniques is reported. Also, the GnP nanopowder was explored by using the XPS technique with the aim to characterize the used carbon nanomaterial. The obtained spectra were investigated in terms of the chemical bonds related to the observed peaks. The thermophysical profile (density, thermal conductivity, isobaric heat capacity, and thermal diffusivity) was experimentally determined once the main components of the NePCMs were characterized and dispersions were designed and developed. This discussion focuses on the differentiated and distinguished effects of the dispersed GnPs and MgO on the properties of the NePCMs. A comprehensive analysis of the measurements to elucidate the mechanism that promoted higher improvements using GnPs instead of MgO was performed.

Electron magnetic resonance in heterogeneous photocatalysis research

The contribution of electron magnetic resonance techniques, and in particular of CW-EPR, to the experimental research on photocatalytic phenomena is illustrated in this paper with selected examples. In the first part of the paper the role of EPR in unravelling the nature and the features of extrinsic point defects in semiconducting oxides is epitomized using the important example of the photoactive nitrogen center in various semiconducting oxides. In the second part we describe how EPR can monitor the processes that follow the initial photoinduced charge separation in photocatalysis, namely the stabilisation, migration and surface reactivity of electrons and holes. Finally, we will discuss how the role of EPR in photocatalysis is not limited to monitor phenomena occurring in the solid or at its surface but it can be extended to the investigation of the liquid phase by employing the spin trapping techniques to monitor the nature and the concentration of the reactive free radicals formed along the photocatalytic process.

Photoactivity and Stability Co-Enhancement: When Localized Plasmons Meet Oxygen Vacancies in MgO

Durability is still one of the key obstacles for the further development of photocatalytic energy-conversion systems, especially low-dimensional ones. Encouragingly, recent studies show that nanoinsulators such as SiO₂ and MgO exhibit substantially enhanced photocatalytic durability than the typical semiconductor p25 TiO₂ . Extending this knowledge, MgO-Au plasmonic defect nanosystems are developed that combine the stable photoactivity from MgO surface defects with energy-focusing plasmonics from Au nanoparticles (NPs), where Au NPs are anchored onto monodispersed MgO nanotemplates. Theoretical calculations reveal that the midgap defect (MGD) states in MgO are generated by oxygen vacancies, which provide the main avenues for upward electron transitions under photoexcitation. These electrons drive stable proton photoreduction to H₂ gas via water splitting. A synergistic interaction between Au's localized plasmons and MgO's oxygen vacancies is observed here, which enhances MgO's photoactivity and stability simultaneously. Such co-enhancement is attributed to the stable longitudinal-plasmon-free Au NPs, which provide robust hot electrons capable of overcoming the interband transition barrier (≈1.8 eV) to reach proton reduction potential for H₂ generation. The demonstrated plasmonic defect nanosystems are expected to open a new avenue for developing highly endurable photoredox systems for the integration of multifunctionalities in energy conversion, environmental decontamination, and climate change mitigation.

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.

An nMgO containing scaffold: Antibacterial activity, degradation properties and cell responses

Bone repair failure caused by implant-related infections is a common and troublesome problem. In this study, an antibacterial scaffold was developed via selective laser sintering with incorporating nano magnesium oxide (nMgO) to poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). The results indicated the scaffold exerted high antibacterial activity. The antibacterial mechanism was that nMgO could cause oxidative damage and mechanical damage to bacteria through the production of reactive oxygen species (ROS) and direct contact action, respectively, which resulted in the damage of their structures and functions. Besides, nMgO significantly increased the compressive properties of the scaffold including strength and modulus, due to its excellent mechanical properties and uniform dispersion in the PHBV matrix. Moreover, the degradation tests indicated nMgO neutralized the acid degradation products of PHBV and benefited the degradation of the scaffold. The cell culture demonstrated that nMgO promoted the cellular adhesion and proliferation, as well as osteogenic differentiation. The present work may open the door to exploring nMgO as a promising antibacterial material for tissue engineering.

Synthesis of {111} Facet-Exposed MgO with Surface Oxygen Vacancies for Reactive Oxygen Species Generation in the Dark

Seeking a simple and moderate route to generate reactive oxygen species (ROS) for antibiosis is of great interest and challenge. This work demonstrates that molecule transition and electron rearrangement processes can directly occur only through chemisorption interaction between the adsorbed O₂ and high-energy {111} facet-exposed MgO with abundant surface oxygen vacancies (SOVs), hence producing singlet oxygen and superoxide anion radicals without light irradiation. These ROS were confirmed by electron paramagnetic resonance, in situ Raman, and scavenger experiments. Furthermore, heat plays a crucial role for the electron transfer process to accelerate the formation of ·O₂^-, which is verified by temperature kinetic experiments of nitro blue tetrazolium reduction in the dark. Therefore, the presence of oxygen vacancy can be considered as an intensification of the activation process. The designed MgO is acquired in one step via constructing a reduction atmosphere during the combustion reaction process, which has an ability similar to that of noble metal Pd to activate molecular oxygen and can be used as an effective bacteriocide in the dark.

Room temperature stable CO x -free H2 production from methanol with magnesium oxide nanophotocatalysts

Methanol, which contains 12.6 weight percent hydrogen, is a good hydrogen storage medium because it is a liquid at room temperature. However, by releasing the hydrogen, undesirable CO and/or CO₂ byproducts are formed during catalytic fuel reforming. We show that alkaline earth metal oxides, in our case MgO nanocrystals, exhibit stable photocatalytic activity for CO/CO₂-free H₂ production from liquid methanol at room temperature. The performance of MgO nanocrystals toward methanol dehydrogenation increases with time and approaches ~320 μmol g^-1 hour^-1 after a 2-day photocatalytic reaction. The CO _x -free H₂ production is attributed to methanol photodecomposition to formaldehyde, photocatalyzed by surface electronic states of unique monodispersed, porous MgO nanocrystals, which were synthesized with a novel facile colloidal chemical strategy. An oxygen plasma treatment allows for the removal of organic surfactants, producing MgO nanocrystals that are well dispersible in methanol.

Polyethylene Nanocomposites for the Next Generation of Ultralow-Transmission-Loss HVDC Cables: Insulation Containing Moisture-Resistant MgO Nanoparticles

The use of MgO nanoparticles in polyethylene for cable insulation has attracted considerable interest, although in humid media the surface regions of the nanoparticles undergo a conversion to a hydroxide phase. A facile method to obtain MgO nanoparticles with a large surface area and remarkable inertness to humidity is presented. The method involves (a) low temperature (400 °C) thermal decomposition of Mg(OH)2, (b) a silicone oxide coating to conceal the nanoparticles and prevent interparticle sintering upon exposure to high temperatures, and (c) heat treatment at 1000 °C. The formation of the hydroxide phase on these silicone oxide-coated MgO nanoparticles after extended exposure to humid air was assessed by thermogravimetry, infrared spectroscopy, and X-ray diffraction. The nanoparticles showed essentially no sign of any hydroxide phase compared to particles prepared by the conventional single-step thermal decomposition of Mg(OH)2. The moisture-resistant MgO nanoparticles showed improved dispersion and interfacial adhesion in the LDPE matrix with smaller nanosized particle clusters compared with conventionally prepared MgO. The addition of 1 wt % moisture-resistant MgO nanoparticles was sufficient to decrease the conductivity of polyethylene 30 times. The reduction in conductivity is discussed in terms of defect concentration on the surface of the moisture-resistant MgO nanoparticles at the polymer/nanoparticle interface.

Defects induced changes in the electronic structures of MgO and their correlation with the optical properties: a special case of electron–hole recombination from the conduction band

Defect induced tunable emission in MgO is investigated using photoluminescence and DFT calculations.

Facile synthesis of defect-induced highly-luminescent pristine MgO nanostructures for promising solid-state lighting applications

Successful demonstration of the facile synthesis of defect-induced highly-luminescent pristine MgO nanostructures for white-light generation and proposed WLED application.

Kinetically stabilized aliovalent europium-doped magnesium oxide as a UV sensitized phosphor

Doping of size mismatched aliovalent ions is challenging due to the associated elastic and electronic stress making the thermodynamics unfavorable.

Effect of different fuels on structural, photo and thermo luminescence properties of solution combustion prepared Y(2)SiO(5) nanopowders

Y(2)SiO(5) nanopowders are prepared by solution combustion method using DFH, sugar and urea as fuels. The final product was well characterized by powder X-ray diffraction, Scanning Electron Microscopy and UV-Vis spectroscopy. The average crystallite size was estimated using Debye-Scherer's formula and Williamson-Hall plots and are found to be in the range 34-40nm for DFH, 45-50nm for urea and 35-42nm for sugar respectively. X1-X2 type YSO phase was obtained for all the samples calcined from 1200 to 1400°C. The optical energy band gaps (Eg) of the samples were estimated from Tauc relation and varies from 5.58 to 5.60eV. SEM micrographs of sugar and urea used Y(2)SiO(5) show agglomerated particles with porous morphology. However, for the sample prepared using DFH fuel observed to be almost spherical in shape. Thermoluminescence (TL) properties of γ-irradiated (1-5kGy) and UV irradiated (1-30min) Y(2)SiO(5) nanopowder at a heating rate of 2.5°Cs(-1) was studied. The samples prepared by using urea and sugar fuels show a broad TL glow peak at 189°C. However, DFH used Y(2)SiO(5) show a well resolved peak at 196°C with shouldered peak at 189°C. Among the fuels, DFH used Y(2)SiO(5) show simple glow peak structure which perhaps useful in radiation dosimetry. This may be due to fuel and particle size effect. The kinetic parameters such as activation energy (E), frequency factor (s) and order of kinetics are estimated by Chens glow peak shape method.

Synthesis, structural and luminescence studies of magnesium oxide nanopowder

Nanoparticles of magnesium oxide (MgO) have been prepared by low temperature solution combustion and hydrothermal method respectively. Powder X-ray diffraction (PXRD) patterns of MgO samples prepared by both the methods show cubic phase. The Scanning Electron Microscopy (SEM) studies reveal, the combustion derived product show highly porous, foamy and fluffy in nature than hydrothermally derived sample. The optical absorption studies of MgO show surface defects in the range 250-300 nm. The absorption peak at ∼290 nm might be due to F-centre. Photoluminescence (PL) studies were carried upon exciting at 290 nm. The sample prepared via combustion method show broad emission peak centred at ∼395 nm in the bluish-violet (3.14 eV) region. However, in hydrothermal prepared sample show the emission peaks at 395 and 475 nm. These emission peaks were due to surface defects present in the sample since nanoparticles exhibits large surface to volume ratio and quantum confinement effect.

MgO:Eu3+ red nanophosphor: low temperature synthesis and photoluminescence properties

Nanoparticles of Eu(3+) doped (0-9 mol%) MgO were prepared using low temperature (400°C) solution combustion technique with metal nitrate as precursor and glycine as fuel. The powder X-ray diffraction (PXRD) patterns of the as-formed products show single cubic phase and no further calcination was required. The crystallite size was obtained using Scherer's formula and was found to be 5-6 nm. The effect of Eu(3+) ions on luminescence characteristics of MgO was studied and the results were discussed in detail. These phosphors exhibit bright red emission upon 395 nm excitation. The characteristic photoluminescence (PL) emission peaks at ∼580, 596, 616, 653, 696 and 706 nm ((5)D0→(7)Fj=0, 1, 2, 3, 4) were recorded due to Eu(3+) ions. The electronic transition corresponding to (5)D0→(7)F2 of Eu(3+) ions (616 nm) was stronger than the magnetic dipole transition corresponding to (5)D0→(7)F1 of Eu(3+) ions (596 nm). The international commission on illumination (CIE) chromaticity co-ordinates were calculated from emission spectra, the values (x, y) were very close to national television system committee (NTSC) standard value of red emission. Therefore the present phosphor was highly useful for display applications.

Role of Photoexcitation and Field Ionization in the Measurement of Accurate Oxide Stoichiometry by Laser-Assisted Atom Probe Tomography

The addition of pulsed lasers to atom probe tomography (APT) extends its high spatial and mass resolution capability to nonconducting materials, such as oxides. For a prototypical metal oxide, MgO, the measured stoichiometry depends strongly on the laser pulse energy and applied voltage. Very low laser energies (0.02 pJ) and high electric fields yield optimal stoichiometric accuracy. Correlated APT and aberration-corrected transmission electron microscopy (TEM) are used to establish the high density of corner and terrace sites on MgO sample surfaces before and after APT. For MgO, long-lifetime photoexcited holes localized at oxygen corner sites can assist in the creation of oxygen neutrals that may spontaneously desorb either as atomic O or as molecular O2. The observed trends are best explained by the relative field-dependent ionization of photodesorbed O or O2 neutrals. These results emphasize the importance of considering electronic excitations in APT analysis of oxide materials.

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.

Probing the surfaces of heterogeneous catalysts by in situ IR spectroscopy

This critical review describes the reactivity of heterogeneous catalysts from the point of view of four simple, but essential for Chemistry, molecules (namely dihydrogen, carbon monoxide, nitrogen monoxide and ethylene) that are considered as probes or as reactants in combination with "in situ" controlled temperature and pressure Infrared spectroscopy. The fundamental properties of H(2), CO, NO and C(2)H(4) are shortly described in order to justify their different behaviour in respect of isolated sites in different environments, extended surfaces, clusters, crystalline or amorphous materials. The description is given by considering some "key studies" and trying to evidence similarities and differences among surfaces and probes (572 references).

BaO clusters on MgO nanocubes: a quantitative analysis of optical-powder properties

Uniformly sized and shaped nanoparticles are well suited for the quantitative characterization of optical-powder properties. For the first time, quantum yields related to photoluminescence emissions that originate from the photoexcitation of MgO nanocube corners and edges are measured. In addition, the surfaces of these nanoparticles are doped with submonolayer barium, which oxidizes during adsorption onto the MgO nanocrystal surfaces and transforms in O(2) atmosphere into BaO. UV-Vis diffuse reflectance and photoluminescence spectroscopy is employed to explore whether 10(-3) monolayer equivalents of these dopants affect the MgO specific optical properties. Surface-admixed BaO produces additional absorption and photoluminescence emission features but does not significantly affect those specific to MgO nanocubes. On this basis the number of optically active sites that can be sampled inside a powder of alkaline earth oxide nanoparticles using a standard spectrometer system is estimated.

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.

Partially Hydroxylated Polycrystalline Ionic Oxides: A New Route toward Electron-Rich Surfaces

Charge traps at the surface of oxide materials play a fundamental role in various chemical processes, such as the activation of supported metal clusters. In this study, combining electron paramagnetic resonance with cluster model DFT calculations, we show that excess electrons at the surface of MgO, CaO, and SrO polycrystalline materials can be generated by preparing weakly hydroxylated surfaces followed by deposition of small amounts of alkali metals. The residual OH groups present on specific sites of the partially dehydroxylated surface act as stable traps for electrons donated by the alkali metal (Na in this case) which forms a Na+ ion distant from the trapped electron. This process results in the formation of thermally stable (H+)(e-) color centers at the surface of the oxide. The procedure could be of interest for the stabilization and activation of supported metal nanoparticles with potential use in catalysis.