BaO Films Research Articles

Hybrid Molecular Beam Epitaxy for Single-Crystalline Oxide Membranes with Binary Oxide Sacrificial Layers.

The advancement in thin-film exfoliation for synthesizing oxide membranes has led to possibilities for creating artificially assembled heterostructures with structurally and chemically incompatible materials. The sacrificial layer method is a promising approach to exfoliate as-grown films from a compatible material system, allowing for their integration with dissimilar materials. Nonetheless, the conventional sacrificial layers often possess an intricate stoichiometry, thereby constraining their practicality and adaptability, particularly when considering techniques such as molecular beam epitaxy (MBE). This is where easy-to-grow binary alkaline-earth-metal oxides with a rock salt crystal structure are useful. These oxides, which include (Mg, Ca, Sr, Ba)O, can be used as a sacrificial layer covering a much broader range of lattice parameters compared to conventional sacrificial layers and are easily dissolvable in deionized water. In this study, we show the epitaxial growth of the single-crystalline perovskite SrTiO3 (STO) on sacrificial layers consisting of crystalline SrO, BaO, and Ba1-xCaxO films, employing a hybrid MBE method. Our results highlight the rapid (≤5 min) dissolution of the sacrificial layer when immersed in deionized water, facilitating the fabrication of millimeter-sized STO membranes. Using high-resolution X-ray diffraction, atomic-force microscopy, scanning transmission electron microscopy, impedance spectroscopy, and scattering-type near-field optical microscopy (SNOM), we demonstrate single-crystalline STO membranes with bulk-like intrinsic dielectric properties. The employment of alkaline earth metal oxides as sacrificial layers is likely to simplify membrane synthesis, particularly with MBE, thus expanding the research and application possibilities.

Novel ferroelectric phase in bulk BaO obtained by application of anisotropic strain

Ferroelectricity is expected to be observed in alkaline earth metal oxides (BaO, MgO, CaO, and SrO) with the application of adequate strain, according to theoretical predictions by Bousquet et al. The resultant polarization, dielectric constant, and piezoelectric response would be comparable to those of typical ferroelectric materials, like perovskites. In yet another recent work, Nascimento et al. explored these theoretical predictions by simulating the behaviour of ultra-thin films of BaO under applied in-plane strain to mimic the effect of epitaxial strain induced by an adequate substrate. Effects of both in-plane compressive strain as well as temperature, varied in the ranges of 0 to − 10% and 100–1500 K, respectively, were explored in the simulations. An antipolar phase was observed for in-plane compressive strain values from 3 to 10% and a temperature range of 100 to 1500 K. However, only a compressive strain value around 9%, typically too high to be obtained by epitaxial strain, was able to produce an ordered single domain. Polarization and ferroelectricity are well known as hard to obtain in ultra-thin films due to their limited thickness. The observed existence of an antipolar phase in ultra-thin films would in principle avoid the problems associated with uncompensated surface charges created by the discontinuity of the normal polarization component at the surface. However, can ferroelectricity be obtained in thicker films or even in bulk-like BaO systems? To answer this question, we will here explore the effects of in-plane compressive strain in BaO bulk systems. Compressive in-plane strain, along both x and y directions, will be varied in 0–10% range as well as the c/a (out-of-plane/in-plane) lattice ratio for values of 1.0–1.3. The observed structural phases and associated physical properties will be presented and discussed.

Strained ultra-thin films of BaO: a molecular dynamics investigation

A recent theoretical work by Bousquet and collaborators have predicted that ferroelectric ordering could be induced in the rocksalt oxides of alkaline earth metals (BaO, MgO, CaO and SrO) by strain. The expected functional properties present in these strained binary oxides, like polarization, dielectric constant and piezoelectric response, would be comparable to those of typical ferroelectric perovskites. Consequently, the strained binary oxides would be promising materials for fabrication of devices like ferroelectric memories and sensors. One possible way to explore the potential underlying these theoretical predictions is to grow thin and ultra-thin films of these binary oxides under epitaxial strain by choosing an adequate substrate. In such systems the interplay between epitaxial strain and the lack of translational symmetry (limited film thickness) may lead to the formation of interesting (anti)ferroelectric phases. Our goal in this work is to explore the potential structural and functional phase diagram of BaO ultra-thin films (thickness of only 8 BaO layers, ≈ 20 nm) obtained for different values of compressive epitaxial strain and temperature by performing a molecular dynamics investigation. A polarized phase (antipolar) is observed at a compressive strain of -9.0%, wich resists till a melting temperature around 1500 K, which is indeed high for such an ultra-thin film.

Deposition of Conformal Catalyst Coatings on Porous Electrodes Using a Layer-By-Layer Surface Sol-Gel Process

The state-of-the-art cathode of solid oxide fuel cells suffers from accelerated degradation and slow reaction kinetics for the oxygen reduction reaction (ORR). Surface modification of the cathode has been widely investigated and shown to greatly improve catalytic activity and prevent the formation of insulating phases. However, there is a need to improve the quality of the surface modification coatings while reducing the cost. In this presentation, we report a novel surface modification technique, coined layer-by-layer surface sol-gel (LbL SSG), for deposition of BaO and Pr2O3 on a porous La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) cathode, greatly increasing the catalytic activity for ORR and tolerance against contaminants. This low-cost, liquid phase deposition technique produces conformal coatings with precise thickness control by utilizing alternating self-limiting reactions between chemisorbed metal alkoxides and water on hydroxyl-rich surfaces. The availability of many different metal alkoxide precursors previously proved in traditional sol-gel processes, capability to deposit at room temperature, and lack of expensive vacuum equipment make this technique extremely promising over other conformal deposition techniques such as atomic layer deposition. LbL SSG is shown to produce much more conformal coatings throughout the entire porous cathode and further reduce degradation over similar coatings produced via solution infiltration. Electrochemical impedance spectroscopy analysis shows a great reduction in polarization resistance of symmetrical cells with electrodes modified by catalyst coatings. Utilizing the precise thickness control of the layer by layer process via deposition cycle count, the optimum thickness is determined to be about 8 nm (derived from 30 cycles). Thinner films are shown to be instable, breaking down and degrading over the first 30 hours, while thicker films increase the polarization resistance due to the lower ionic conductivity of the catalyst coatings. The process also demonstrates excellent scalability and versatility by coating larger cathodes (one inch in diameter) and depositing various oxides using different metal alkoxide precursors. Figure 1. SEM images of bare and coated porous electrodes showing the conformal BaO film deposited via LbL SSG. The polarization resistance versus time for various deposition cycle counts showing greatly improved stability for coated cells. Figure 1

Growth and lattice dynamics of ultrathin BaO films on Pt(0 0 1)

We report on the surface phonons of long-range ordered BaO thin films grown on Pt(0 0 1). In the thickness range between 4 and 28 ML, we find unstrained BaO(0 0 1)-() bulk-like terminated films which coincide with a Pt(0 0 1)-c() lattice periodicity. The dipole-active lattice vibrations were determined using high-resolution electron energy loss spectroscopy. For all BaO film thicknesses, a single Fuchs–Kliewer phonon-polariton is observed. Its intensity increases and its frequency softens with increasing film thickness. These thickness-dependent properties and the spectral shape are quantitatively discussed on the basis of dielectric theory, where a proper modeling requires three components: the dielectric response of the BaO film itself, the plasmonic response of the metallic substrate, and a weak damping due to a defect-induced doping within the oxide film. For a full description, also the quantization of the phonon wavevector due to the confinement within the film of finite thickness has to be taken into account.

ChemInform Abstract: Understanding Automotive Exhaust Catalysts Using a Surface Science Approach: Model NOx Storage Materials

AbstractReview: 56 refs.

Understanding Automotive Exhaust Catalysts Using a Surface Science Approach: Model NOx Storage Materials

The structure–reactivity relationships of model BaO-based NOx storage/reduction catalysts were investigated under well controlled experimental conditions using surface science analysis techniques. The reactivity of BaO toward NO2, CO2, and H2O was studied as a function of BaO layer thickness [0 < θBaO < 30 monolayer (ML)], sample temperature, reactant partial pressure, and the nature of the substrate the NOx storage material was deposited onto. Most of the efforts focused on understanding the mechanism of NO2 storage either on pure BaO, or on BaO exposed to CO2 or H2O prior to NO2 exposure. The interaction of NO2 with a pure BaO film results in the initial formation of nitrite/nitrate ion pairs by a cooperative adsorption mechanism predicted by prior theoretical calculations. The nitrites are then further oxidized to nitrates to produce a fully nitrated surface. The mechanism of NO2 uptake on thin BaO films (<4 ML), BaO clusters (<1 ML) and mixed BaO/Al2O3 layers are fundamentally different: in these systems initially nitrites are formed only, and then converted to nitrates at longer NO2 exposure times. These results clarify the contradicting mechanisms presented in prior studies in the literature. After the formation of a nitrate layer the further conversion of the underlying BaO is slow, and strongly depends on both the sample temperature and the NO2 partial pressure. At 300 K sample temperature amorphous Ba(NO3)2 forms that then can be converted to crystalline nitrates at elevated temperatures. The reaction between BaO and H2O is facile, a series of Ba(OH)2 phases form under the temperature and H2O partial pressure regimes studied. Both amorphous and crystalline Ba(OH)2 phases react with NO2, and initially form nitrites only that can be converted to nitrates. The NO2 adsorption capacities of BaO and Ba(OH)2 are identical, i.e., both of these phases can completely be converted to Ba(NO3)2. In contrast, the interaction of CO2 with pure BaO results in the formation of a BaCO3 layer that prevents to complete carbonation of the entire BaO film under the experimental conditions applied in these studies. However, these “carbonated” BaO layers readily react with NO2, and at elevated sample temperature even the carbonate layer is converted to nitrates. The importance of the metal oxide/metal interface in the chemistry on NOx storage-reduction catalysts was studied on BaO(<1 ML)/Pt(111) reverse model catalysts. In comparison to the clean Pt(111), new oxygen adsorption phases were identified on the BaO/Pt(111) surface that can be associated with oxygen atoms strongly adsorbed on Pt atoms at the peripheries of BaO particles. A simple kinetic model developed helped explain the observed thermal desorption results. The role of the oxide/metal interface in the reduction of Ba(NO3)2 was also substantiated in experiments where Ba(NO3)2/O/Pt(111) samples were exposed to CO at elevated sample temperature. The catalytic decomposition of the nitrate phase occurred as soon as metal sites opened up by the removal of interfacial oxygen via CO oxidation from the O/Pt(111) surface. The temperature for catalytic nitrate reduction was found to be significantly lower than the onset temperature of thermal nitrate decomposition.

Synthesis of barium oxide nanorod by chemical bath deposition

We report on the control of nanocrystal sizes in barium oxide nanorods prepared by chemical bath deposition technique. Barium chloride and potassium hydroxide are used as source materials to obtain barium oxide films. Different chemical baths are used to study changes in the structures of the BaO films The effect of hydrazine hydrate on growth of these films has been studied. The results reveal that the additive played a very important role on crystalline size and shape.

Reactions of NO2 with BaO/Pt(111) model catalysts: the effects of BaO film thickness and NO2 pressure on the formation of Ba(NOx)2 species

The adsorption and reaction of NO(2) on BaO (<1, ∼3, and >20 monolayer equivalent (MLE))/Pt(111) model systems were studied with temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and infrared reflection absorption spectroscopy (IRAS) under ultra-high vacuum (UHV) as well as elevated pressure conditions. NO(2) reacts with sub-monolayer BaO (<1 MLE) to form nitrites only, whereas the reaction of NO(2) with BaO (∼3 MLE)/Pt(111) produces mainly nitrites and a small amount of nitrates under UHV conditions (P(NO(2))≈ 1.0 × 10(-9) Torr) at 300 K. In contrast, a thick BaO (>20 MLE) layer on Pt(111) reacts with NO(2) to form nitrite-nitrate ion pairs under the same conditions. At elevated NO(2) pressures (≥1.0 × 10(-5) Torr), however, BaO layers at all these three coverages convert to amorphous barium nitrates at 300 K. Upon annealing to 500 K, these amorphous barium nitrate layers transform into crystalline phases. The thermal decomposition of the thus-formed Ba(NO(x))(2) species is also influenced by the coverage of BaO on the Pt(111) substrate: at low BaO coverages, these species decompose at significantly lower temperatures in comparison with those formed on thick BaO films due to the presence of a Ba(NO(x))(2)/Pt interface where the decomposition can proceed at lower temperatures. However, the thermal decomposition of the thick Ba(NO(3))(2) films follows that of bulk nitrates. Results obtained from these BaO/Pt(111) model systems under UHV and elevated pressure conditions clearly demonstrate that both the BaO film thickness and the applied NO(2) pressure are critical in the Ba(NO(x))(2) formation and subsequent thermal decomposition processes.

Preparation and electrical characterization of amorphous BaO, SrO and Ba0.7Sr0.3O as high‐k gate dielectrics

AbstractWe report on the measurement of band offsets and electrical characterizations of amorphous BaO, SrO and Ba0.7Sr0.3O as alternative gate oxides grown on n‐Si(001) at room temperature without further treatments. These materials provide relative dielectric constants close to those expected from bulk values even for ultra‐thin films (equivalent oxide thicknesses below 1 nm) and posess very low rechargeable trap densities. Interface defect densities are comparable to other high‐k materials for BaO and SrO films, but an order of magnitude lower for Ba0.7Sr0.3O. This demonstrates the importance of both chemical and structural interface effects (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Interaction of D2O with a Thick BaO Film: Formation of and Phase Transitions in Barium Hydroxides

The interaction of D2O with a thick BaO film (≥20 monolayer equivalent (MLE)) on ultrathin Al2O3/NiAl(110) was investigated with temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS). Upon D2O exposure of a thick BaO film, amorphous barium hydroxide formed at room temperature that readily converted to crystalline Ba(OD)2 phases during annealing in ultrahigh vacuum (UHV). The formation of crystalline hydroxide phases depends on the initial D2O exposure at 300 K. Following low D2O exposure at room temperature that results in the formation of amorphous barium hydroxide with no hydrating water, only the α-Ba(OD)2 phase was observed after 400 K annealing. The sample that was exposed to D2O extensively (i.e., hydrated amorphous barium hydroxide formed) showed a series of phase transformations as the sample was annealed to increasingly higher temperatures: amorphous-Ba(OD)2·xD2O (x > 1) → β-Ba(OD)2·D2O → β-Ba(OD)2 → α-Ba(OD)2. The results of TPD experiments completely agr...

Reactivity of a Thick BaO Film Supported on Pt(111): Adsorption and Reaction of NO2, H2O, and CO2

Reactions of NO2, H2O, and CO2 with a thick (>20 monolayer equivalent (MLE)) BaO film supported on Pt(111) were studied with temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). NO2 reacts with a thick BaO layer to form surface nitrite-nitrate ion pairs at 300 K, while only nitrates form at 600 K. In the thermal decomposition process of nitrite-nitrate ion pairs, first nitrites decompose and desorb as NO. Then nitrates decompose in two steps: at lower temperature with the release of NO2 and at higher temperature, nitrates dissociate to NO+O2. The thick BaO layer converts completely to Ba(OH)2 following the adsorption of H2O at 300 K. Dehydration/dehydroxylation of this hydroxide layer can be fully achieved by annealing to 550 K. CO2 also reacts with BaO to form BaCO3 that completely decomposes to regenerate BaO upon annealing to 825 K. However, the thick BaO film cannot be converted completely to Ba(NOx)2 or BaCO3 under the experimental conditions employed in this study.

Synthesis of superconducting (Cu, C)-Ba-O films on SrCuO2 buffer by pulsed laser deposition

(Cu, C)-1201 films have been grown on SrCuO2 buffer layer over SrTiO3 (100) by pulse laser deposition. Effects of the insertion of SrCuO2 buffer on growth mode of the 1201 phase has been investigated by comparing in-situ RHEED pattern of the films growth on SrCuO2 / SrTiO3, SrTiO3 and NdGaO3. (Cu, Q-1201 films directly grown on SrTiO3 and NdGaO3 exhibit diffusive streak patterns in the beginning of the film growth. On the other hand, the films on the buffer show sharp streaks in the beginning from the film growth on buffer, a-axis parameters of the films on SrTiO3 and NdGaO3 estimated by RHEED patterns drastically expanded from 3.90 ± 0.06 Å and 3.93 ± 0.10 Å at 10 nm thick to 4.07 ± 0.05 Å and 3.99 ± 0.03 Å at 40 nm thick, respectively. The films on 10 nm thick buffer gradually expanded from 3.95 ± 0.05 Å at 10 nm thick to 4.01 ± 0.05 Å at 40 nm thick. The films on 10 nm and 20 nm thick buffer layers showed the rises of superconducting onset temperature Tc-onset to 72 K and 62 K, respectively, whereas Tc-onset of the films directly grown on SrTiO3 was about 50 K. Due to the growth mode without misfit dislocations in the beginning of the film growth, the SrCuO2 buffer could strain the 1201 phase. These results indicated that the interfacial strain should be important Tc for (Cu, Q-1201 films, and superlattice based on it.

BaO/Al2O3/NiAl(110) Model NOx Storage Materials: The Effect of BaO Film Thickness on the Amorphous-to-Crystalline Ba(NO3)2 Phase Transition

The reaction of NO2 with BaO (0.15−2 ML and >30 ML)/Al2O3(12 ML)/NiAl(110) model NOx storage materials was studied. A thick (∼12 ML), ordered Al2O3 film was prepared as the support oxide on a NiAl(110) substrate to minimize the effect of the intermixing between the two oxide phases (BaO and Al2O3) on the NOx chemistry of BaO. The growth of a thick alumina film, prepared by atomic oxygen deposition onto NiAl(110), follows a layer-by-layer growth mode, and the resulting film is much more stable when exposed to NO2 than the ultrathin alumina films studied before. The interaction of NO2 with the model NOx storage systems at low coverages of BaO shows behaviors fundamentally different from a thick BaO film, as nitrite species form at low exposures of NO2, followed by nitrate formation at high NO2 exposures. In contrast, on the thick BaO layer, nitrite−nitrate ion pairs form at 300 K under UHV conditions (PNO2 ∼ 1 × 10−9 Torr). However, at elevated NO2 pressures (≥1 × 10−5 Torr), the thick BaO film is gradually converted into amorphous Ba(NO3)2 at 300 K. Raising the temperature of the samples with ΘBaO > 1 ML after NO2 exposure (in the absence of gas phase NO2) leads to the phase transformation of the amorphous Ba(NO3)2 layer into crystalline Ba(NO3)2 particles in the temperature range of 500−600 K. No phase transformation is observed in samples with ΘBaO < 1 ML.

Reaction of NO2 with a Pure, Thick BaO Film: The Effect of Temperature on the Nature of NOx Species Formed

The adsorption and reaction of NO2 on a thick (>30 ML), pure BaO film deposited onto an Al2O3/NiAl(110) substrate were investigated in the temperature range of 300 − 660 K using temperature programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS), and X-ray photoelectron spectroscopy (XPS) techniques. The adsorption of NO2 on BaO at room temperature and the subsequent decomposition of the thus formed Ba(NOx)2 species follow the same mechanisms we have reported previously for NO2 adsorption at cryogenic temperatures. In cyclic experiments when the BaO film was exposed to NO2 at 300 K, followed by annealing to 575 K, a large amount of NOx was stored as nitrates, and no saturation was achieved even after the 10th adsorption/anneal cycle. This suggests the gradual conversion of the BaO film into barium nitrate clusters at elevated temperatures. The rate of nitrate formation increases as the sample temperature during NO2 exposure increases up to 610 K, while at even higher temperatures t...

Interaction of NO2 with BaO: From Cooperative Adsorption to Ba(NO3)2 Formation

The adsorption and reaction of NO2 on a thick (>30 ML), pure BaO film deposited onto an Al2O3/NiAl(110) substrate were investigated using temperature-programmed desorption, infrared reflection absorption spectroscopy, and X-ray photoelectron spectroscopy techniques. For the first time, it was clearly demonstrated that BaO reacts with NO2 to initially form nitrite−nitrate ion pairs by the cooperative adsorption mechanism predicted by theoretical calculation. These NO2-/NO3- ion pairs readily form even at 90 K. In the decomposition process of these ion pairs, first the nitrite species release an NO molecule and then form BaO2. At higher temperatures, nitrate species decompose in two steps: at lower temperature as NO2 only, then, at higher temperature, as NO + O2. The results of NO2 adsorption/reaction on this model system are identical to those we have found on a high surface area 20 wt % BaO/γ-Al2O3 sample with the exception of surface nitrates that were only observed on the high surface area material.

Topotaxial formation of titanium-rich barium titanates during solid state reactions on (110) TiO 2 (rutile) and (001) BaTiO 3 single crystals

The orientation relationships of Ti-rich barium titanate phases formed by solid state reactions at high temperatures were studied using (110) TiO 2 (rutile) and (001) BaTiO 3 single-crystal substrates. Well-oriented Ba 6Ti 17O 40 islands were observed after a vapor–solid reaction of a BaO quantity equivalent to a nominal BaO film thickness of 1 nm with the TiO 2 substrate, whereas a thin film consisting of well-oriented BaTiO 3 and Ba 6Ti 17O 40 grains was formed after vapor–solid reaction of a BaO quantity equivalent to a nominal BaO film thickness of 50 nm with the rutile substrate. A topotaxial orientation relationship between Ba 6Ti 17O 40 and TiO 2 was found. Topotaxy is facilitated by a certain similarity in the oxygen sublattices of TiO 2 and Ba 6Ti 17O 40. The mechanism of the reaction occurring between BaO vapor and the TiO 2 surface at high temperature is discussed. On the other hand, several well-oriented Ba 4Ti 13O 30, Ba 6Ti 17O 40 and Ba 2Ti 5O 12 phases were observed to be embedded in the mainly forming Ba 2TiSi 2O 8 phase after a solid–solid reaction of amorphous SiO 2 thin films with (001) BaTiO 3 substrates at temperatures above 1000 °C. They were formed by a topotaxial reaction involving the transformation of (111) planes of BaTiO 3 into (001) planes of the Ti-rich phases by removal of BaO and insertion of TiO 2. Cross-sections of the interfaces between the substrates and the various reaction products are studied by (high-resolution) transmission electron microscopy.

Buffer layers grown by replicating the texture of an original template tape

We propose a fabrication method of the buffer layers, whose biaxial textures are replicated from an original template tape. The purpose of this method is economical texturing process for coated conductors. At first we prepared a biaxially textured metal tape (TM-tape). Then a sacrifice layer (SA), a buffer layer (BU) and a thick metallic layer (SM) were sequentially deposited on the TM-tape. SA-layer and BU-layer were deposited epitaxially to copy the texture of the TM-tape. SA-layer was dissoluble in water. SM-layer with the textured BU-layer was separated and could be used for a supporting tape for the further growth of a superconducting layer. In this way, it is possible to reuse the original textured TM-tape many times. In this paper, we report the results of our experiments, in which we used a biaxially Ni tape, BaO film, STO film, and a thick Ag film for TM-tape, SA-layer, BU-layer, and SM-layer, respectively. The Ag/STO layers were successfully separated form the Ni tape by dissolving the BaO layer in water. The texture quality of the STO layer was well secured after the separation.

Bulk and surface oxygen vacancy formation and diffusion in single crystals, ultrathin films, and metal grown oxide structures

The neutral oxygen vacancy (OV) energy formation for bulk, subsurface sites at different depths from the surface and various surface sites has been estimated for single crystals, unsupported ultrathin films of MgO, CaO, and BaO, and MgO ultrathin films supported on Ag(001). From the calculated energy barriers for diffusion through the surface and from the surface to the bulk it is found that diffusion is a hindered event, especially for MgO. Nevertheless, diffusion from the terrace to step edges is largely favored while diffusion through terrace sites is less likely and surface to bulk has a very low probability. It is argued that this explains recent scanning tunneling microscopy images for MgO thin films supported on Ag(001) showing OV populating preferentially the step edge sites.

Model NSR catalysts: Fabrication and reactivity of barium oxide layers on Cu(1 1 1)

The growth of barium oxide on a Cu(1 1 1) substrate, formed by the deposition of barium and its subsequent oxidation, yields stable BaO films which expose predominantly the BaO(1 0 0) surface. The interaction of the oxide films with common components of motor-vehicle exhaust gases (CO 2, H 2O, NO x ) has been studied using surface analytical techniques, including X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and reflection IR spectroscopy (RAIRS). The spectroscopic identification of Ba(OH) 2, BaCO 3 and Ba(NO 2) 2 phases is discussed, and the relative stabilities and decomposition mechanisms of these materials when supported on Cu(1 1 1) is revealed by a combination of TPD and XPS. BaO is shown to be resistant to reaction with pure NO and NO/O 2 mixtures, but exposure to NO 2 leads to the rapid formation of barium nitrite. The formation of the nitrite is proposed to be the first-step in the production of barium nitrate, which has previously been shown to be the main phase involved in NO x storage and reduction (NSR) catalysis.