Adsorption Of Oxygen Molecules Research Articles

Effect of alloying on the catalytic properties of Pt–Ni bimetallic subnanoclusters: a theoretical investigation

Density functional theory has been applied to study the geometrical and electronic structures of Pt13−n Ni n (n = 0–6) clusters, and the catalytic properties for the oxygen reduction reaction (ORR). The calculated results suggest that the doping of Ni atoms improved the stability of the cluster. Correspondingly, Pt10Ni3 and Pt8Ni5 clusters with local maximum Δ2 E are relatively stable compared to their neighbors, owning to the strong interaction between Pt and Ni atoms in the two alloy clusters. The O2 molecule adsorption properties on PtNi clusters suggest that the doping of Ni atoms enhances the ability of metal clusters to activate oxygen molecules, with the larger adsorption energy and bond length of O2 molecule. This is maybe due to the strong hybridization between O 2p and Pt/Ni d orbitals near the Fermi level and the enhanced polarization of adsorbed oxygen molecules. The interaction between O2 and Pt10Ni3 and Pt8Ni5 clusters are stronger than the others, due to the more positive d band center than the others. For the ORR, the energy barriers on Pt10Ni3 are lower than those on pure Pt13 and Pt8Ni5 clusters, suggesting a high catalytic activity of Pt10Ni3 cluster for the ORR. Our study provides atomic-scale insights into the nature of the interfacial effects that determines O2 reduction reaction on PtNi cluster catalysts.

Structural and Electrochemical Study on Thin Film Cathodes Fabricated By Spray Pyrolysis

Cathode ORR is the most complicated process in solid oxide fuel cells (SOFC), since it involves oxygen molecule adsorption and dissociation, electron transfer and oxygen ion transport. Most of the cathodes used in SOFC contain mixed ionic electronic conducting (MIEC) oxides [1]. Either they are used as single phase, if the material exhibits sufficient ionic conductivity in addition to electronic conductivity, or as a composite with an oxygen ion conductor such as yttria-stabilized zirconia (YSZ) or rare earth oxide doped ceria. It is well known that microstructure plays a crucial role in determining the polarization resistance [2,3]. Typically, the finer the cathode microstructure, the lower is the polarization resistance [2,3]. We have recently reported on the deposition of cathodes from an aqueous solution directly on electrolyte surfaces by spray-pyrolysis followed by thermal treatment at 700oC [4]. In this work, we have investigated cathode polarization resistance of many cathodes prepared by the spray pyrolysis process. This would then allow one to determine similarities and differences across many cathodes prepared by a single process. A total of eight different cathodes containing transition metal ions were selected for this study. Two types of cathodes were synthesized: Single phase MIEC, and MIEC with gadolinia-doped ceria (GDC) composite cathodes. These were: La0.6Sr0.4CoO3-x (LSC), La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF), LaNi0.6Fe0.4O3-x (LNF), PrNi0.6Co0.4O3-x (PNC), Sm0.5Sr0.5CoO3-x (SSC), Ba0.5Sr0.5Co0.8Fe0.2O3-x (BSCF), Pr0.6Sr0.4Co0.8Fe0.2O3-x (PSCF), and PrBa0.5Sr0.5Co1.5Fe0.5O5+y (PBSCF). Water soluble nitrates of the respective elements were used. Thin films of about 100 nm were spray-coated onto 1mm thick 8YSZ discs to fabricate symmetric cells for EIS measurement. Thick films of about 1 um were spray-coated for XRD and EDS measurement. Cathode particle size was calculated from XRD peak widening using Scherrer formula. The particle sizes of all sixteen cathodes were plotted in figure 1. All composite cathodes have smaller particle size than their corresponding single phase cathodes. This was caused by diffusion blocking during co-sintering of composite cathodes. Cathode composition was characterized by EDS. Stoichiometry of these cathodes were very close to target value. Electrochemical impedance spectroscopy was measured over a temperature range between 400oC and 700oC. Figure 2(a) shows the Arrhenius plots of the polarization resistance over a temperature range from 400oC to 700oC for all eight single phase cathodes. Figure 2(b) shows the Arrhenius plots of the polarization resistance of all composite cathodes. As seen in the figures, all plots are linear consistent with Arrhenius behavior over the temperature range. Also, all lines are nearly parallel to each other, indicating essentially the same activation energy. The measured activation energy ranged between ~1.14 eV and ~1.35 eV. The polarization resistance for composite cathodes is given by [2] (1) Therefore, the thermal activation part is contributed by charge transfer resistance and ionic resistance. As a result, the activation energy can be expressed by (2) While, the polarization resistance for single phase cathodes is given by [3] (3) Therefore, the thermal activation part is contributed by oxygen diffusion and surface exchange reaction. As a result, the activation energy can be expressed by (4) Activation energy of the polarization resistance is similar for these materials, which is reasonable as all of them contain one or more transition metals, Co, Fe and Ni. The large difference in polarization resistance is attributed to the pre-exponential factor. One of the factors in the pre-exponential part is the geometrical part. Possible differences in porosity, agglomeration, degree of mixing (uniformity) of the two phases in composite electrodes, may substantially alter the pre-exponential part. Acknowledgements: This work was supported by the National Science Foundation under grant Number NSF-CBET-1604008 and by the US Department of Energy under grant Number DE-FG02-06ER46086.

Spectroscopic observation of oxygen dissociation on nitrogen-doped graphene

Carbon nanomaterials’ reactivity towards oxygen is very poor, limiting their potential applications. However, nitrogen doping is an established way to introduce active sites that facilitate interaction with gases. This boosts the materials’ reactivity for bio-/gas sensing and enhances their catalytic performance for the oxygen reduction reaction. Despite this interest, the role of differently bonded nitrogen dopants in the interaction with oxygen is obscured by experimental challenges and has so far resisted clear conclusions. We study the interaction of molecular oxygen with graphene doped via nitrogen plasma by in situ high-resolution synchrotron techniques, supported by density functional theory core level simulations. The interaction leads to oxygen dissociation and the formation of carbon-oxygen single bonds on graphene, along with a band gap opening and a rounding of the Dirac cone. The change of the N 1 s core level signal indicates that graphitic nitrogen is involved in the observed mechanism: the adsorbed oxygen molecule is dissociated and the two O atoms chemisorb with epoxy bonds to the nearest carbon neighbours of the graphitic nitrogen. Our findings help resolve existing controversies and offer compelling new evidence of the ORR pathway.

Oxygen adsorption on palladium monolayer as a surface catalyst

In the recent work, we study on the structural and electronic properties of the graphene like Pd monolayer with the adsorption of oxygen adatoms by using first-principles calculations. The electronic band structure and projected density of states investigate that Pd-surface with oxygen molecule adsorption gives metallic behaviour. We found that the behaviour changed at M-point in the electronic band structure as adding oxygen atoms. The oxygen adsorption was dissociative until the Pd surface immersed with oxygen atoms. The electron charge density increases as the number of oxygen atoms on Pd-surface increases. The noticeable observation is that by adding 7th oxygen atom, they started to ripple from fixed Pd-surface without making a bond due to oxygen coverage increases. The results show that Pd monolayer has different applications as a oxygen catalyst and it can be utilized as the pellet, surface, and film materials to safeguard sustenance from oxidation.

Tungsten diselenide/porous carbon with sufficient active edge-sites as a co-catalyst/Pt-support favoring excellent tolerance to methanol-crossover for oxygen reduction reaction in acidic medium

Developing an acid-stable, highly active and methanol-tolerant electrocatalyst towards the oxygen reduction reaction (ORR) is crucial for commercialization of direct methanol fuel cells (DMFCs). In this study, via a simultaneous synthesis method, tungsten diselenide/porous carbon (WSe2/C) composites are prepared as the supports/ORR co-catalysts to support Pt with a low loading of 5wt.%. Varied WSe2/C supports are obtained by tuning the carbonization temperature (600–1000°C) to investigate the relationships between structural characteristics and ORR performance. Pt-WSe2/C (800°C) exhibits a considerably higher specific activity (4.57mAcm−2) for ORR than those of WSe2/C (2.45mAcm−2) and commercial Pt/C (10wt.%, 2.69mAcm−2), owing to the high ORR co-catalytic activity of WSe2/C for Pt. The robust contacts among Pt, WSe2 and porous carbon with high surface area can significantly improve the exposure of Pt active sites, which correspondingly promote the charge transfer efficiency and the fast adsorption, activation and reduction of oxygen molecules. Moreover, Pt-WSe2/C (800°C) catalyst exclusively exhibits a four-electron pathway for ORR. With the intimate cooperation among Pt, WSe2 and porous carbon skeleton, more available Pt active sites are exposed to improve the ORR kinetics and durability. The tolerance to methanol-crossover on Pt-WSe2/C are remarkably enhanced, which should be attributed to the synergistic effects between the exposed edge sites of embedded WSe2 and the porous carbon skeleton with abundant oxygen-containing functional groups. The use of WSe2/C supports with low-cost, high co-catalytic/catalytic activity and strong methanol-tolerance provides a promising way to enhance ORR activity in DMFCs.

Device Performance Improvement of Transparent Thin-Film Transistors With a Ti-Doped GaZnO/InGaZnO/Ti-Doped GaZnO Sandwich Composite-Channel Structure

In this paper, two transparent thin-film transistors (TFTs) with distinct channel designs were fabricated. The first was a single-channel TFT (SC-TFT) with a typical 50-nm-thick amorphous indium–gallium–zinc oxide (a-IGZO) layer, and the second was a sandwich composite-channel TFT (CC-TFT) comprising three layers of 10-/30-/10-nm-thick Ti-doped GaZnO (GTZO)/a-IGZO/GTZO. In the CC-TFT, the bottom GTZO thin film in the composite channel exhibited a highly smooth surface, serving as a buffer template for superior IGZO channel layer thin-film growth. The top GTZO thin film had a high carrier concentration and served as both a carrier supplement and passivation layer for reducing the adsorption of moisture and oxygen molecules. The studies of material quality indicated that the CC-TFT exhibited improved a-IGZO thin-film quality; relative to the SC-TFT, the oxygen vacancy concentration of the composite structure was reduced from 25.1% to 18.2%. The CC-TFT demonstrated a high level of device performance, exhibiting an excellent field-effect mobility of 14.1 cm2/ $\text{V}\,\cdot \, \text{s}$ , subthreshold swing of 0.33 V/decade, off current of $2.92 \times 10^{-12}$ A, threshold voltage of 1.7 V, and on–off current ratio of $3.95 \times 10^{7}$ . Stable device operation with nearly unaltered device characteristics was observed following a bias-stress test.

Mechanism of Light-Soaking Effect in Inverted Polymer Solar Cells with Open-Circuit Voltage Increase.

In this study, we present novel insights into the light-soaking effect of inverted polymer solar cells (PSCs), where the open-circuit voltage (Voc) of the cells improves over time under light irradiation. The effect was investigated by electron spin resonance (ESR) studies of bare indium tin oxide (ITO) and piperazine derivative-modified ITO/regioregular poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61 butyric acid methyl ester (PCBM) substrates. These results were combined with alternating current impedance spectroscopy (IS) measurements of inverted PSCs based on the above substrates. In ESR experiments with the substrates under white light irradiation, with a UV light component, many P3HT•+ radical cations were observed in the bare-ITO/P3HT:PCBM substrate. The number of radical cations was considerably suppressed in the ITO/P3HT:PCBM substrates with ITO modified by piperazine derivatives. This is because adsorbed oxygen molecules on the ITO acted as acceptor dopants for photoexcited P3HT, and the amount of adsorbed oxygen was decreased by modifying the ITO with piperazine derivatives. In IS measurements of the inverted PSCs under white light irradiation, a decrease in the electric capacitance (CPE2) of an electric double layer formed at the ITO/P3HT:PCBM interface was observed. A strong correlation was observed between the decrease of CPE2 and the increase of Voc. From these results, the light-soaking behavior was attributed to the removal of an electron injection barrier formed between ITO and PCBM, under white light irradiation.

The synergy effect and reaction pathway in the oxygen reduction reaction on the sulfur and nitrogen dual doped graphene catalyst

The first-principle calculations are performed to explore the synergy effects between two dopants (N, S) and the detailed reaction pathway of oxygen reduction reaction (ORR) on the graphene catalysts. The co-doping N and S induces the significant spin density and has a strong chemical bonding with oxygen molecule which is not observed on the mono-doped cases. Three different reaction pathways are revealed from the calculations. Due to the large barrier of the OO breaking, the hydrogenation of the adsorbed oxygen molecule is kinetically more favorable. The free energy change of reaction under different electrode potential is also evaluated.

Study of device instability of bottom-gate ZnO transistors with sol–gel derived channel layers

In this paper, the authors report the device instability of solution based ZnO thin film transistors by studying the time-evolution of electrical characteristics during electrical stressing and subsequent relaxation. A systematic comparison between ambient and vacuum conditions was carried out to investigate the effect of adsorption of oxygen and water molecules, which leads to the creation of defects in the channel layer. The observed subthreshold swing and change in field effect mobility under gate bias stressing have supported the fact that oxygen and moisture directly affect the threshold voltage shift. The authors have presented the comprehensive analysis of device relaxation under both ambient and vacuum conditions to further confirm the defect creation and charge trapping/detrapping process since it has not been reported before. It was hypothesized that chemisorbed molecules form acceptorlike traps and can diffuse into the ZnO thin film through the void on the grain boundary, being relocated even near the semiconductor/dielectric interface. The stretched exponential and power law model fitting reinforce the conclusion of defect creation by oxygen and moisture adsorption on the active layer.

Density functional theory study the effects of oxygen-containing functional groups on oxygen molecules and oxygen atoms adsorbed on carbonaceous materials.

Density functional theory was used to study the effects of different types of oxygen-containing functional groups on the adsorption of oxygen molecules and single active oxygen atoms on carbonaceous materials. During gasification or combustion reactions of carbonaceous materials, oxygen-containing functional groups such as hydroxyl(-OH), carbonyl(-CO), quinone(-O), and carboxyl(-COOH) are often present on the edge of graphite and can affect graphite’s chemical properties. When oxygen-containing functional groups appear on a graphite surface, the oxygen molecules are strongly adsorbed onto the surface to form a four-member ring structure. At the same time, the O-O bond is greatly weakened and easily broken. The adsorption energy value indicates that the adsorption of oxygen molecules changes from physisorption to chemisorption for oxygen-containing functional groups on the edge of a graphite surface. In addition, our results indicate that the adsorption energy depends on the type of oxygen-containing functional group. When a single active oxygen atom is adsorbed on the bridge site of graphite, it gives rise to a stable epoxy structure. Epoxy can cause deformation of the graphite lattice due to the transition of graphite from sp2 to sp3 after the addition of an oxygen atom. For quinone group on the edge of graphite, oxygen atoms react with carbon atoms to form the precursor of CO2. Similarly, the single active oxygen atoms of carbonyl groups can interact with edge carbon atoms to form the precursor of CO2. The results show that oxygen-containing functional groups on graphite surfaces enhance the activity of graphite, which promotes adsorption on the graphite surface.

Enhanced Oxygen Reduction Activity on Ruddlesden-Popper Phase Decorated La0.8Sr0.2FeO3-δ 3D Heterostructured Cathode for Solid Oxide Fuel Cells.

A new heterostructured (La,Sr)2FeO4-δ (LSF214)-La0.8Sr0.2FeO3-δ (LSF113) electrode has been synthesized to improve the oxygen reduction reaction (ORR). This new materials system was fabricated by the deposition of Sr(NO3)2 into the LSF113 framework followed by subsequent heat treatment, resulting in a new three-dimensional (3D) LSF214-LSF113 heterostructured electrode. This material system consists of a with Ruddlesden-Popper (R-P) LSF214 phase formed on the surface of the LSF113 framework. The ORR activity has been enhanced by 1 order of magnitude using the LSF214-LSF113 heterostructured electrode. The ORR enhancement was the result of higher catalytic activity of the LSF214 phase and a mismatch in the lattice parameter between LSF214 and LSF113 regions which results in oxygen molecule adsorption and oxygen vacancy formation become more favered. Impedance spectroscopy measurements revealed that the presence of LSF214 reduced the polarization resistance of the LSF113 electrode on a ceria-based electrolyte. The high frequency resistance (RH) and low frequency resistance (RL) decreased substantially due to the enhanced oxygen transport process and accelerated oxygen incorporation rate in the LSF214-LSF113 heterostructured electrode. The heterostructured LSF214-LSF113 electrode provides a promising new approach to improve the oxygen reduction reaction activity through multiphase materials systems with tailored microstructures.

Mg-alloyed ZnO nanocombs for self-gating photodetectors.

Mg-alloyed ZnO nanocombs were synthesized by the chemical vapor deposition method. The morphology and optoelectronic properties of the nanocombs were systematically investigated. The photodetection capability of the Mg-alloyed ZnO nanocomb was demonstrated by fabrication of a two terminal nanocomb device. It was found that the nanocomb with a high surface-to-volume ratio absorbed the photons effectively in the 310-400 nm range and enabled ultra-high photoconductive gain of 1.9 × 106. From experiments and theoretical analysis, the teeth part of the nanocomb served as a negative gate upon accumulation of electrons by adsorption of oxygen molecules at the teeth, which reduced the dark current of the backbone of the nanocomb and led to an increase in the photoconductive gain of the nanocomb detector.

Enhancing the Sensing Properties of TiO2 Nanosheets with Exposed {001} Facets by a Hydrogenation and Sensing Mechanism.

Hydrogenation is successfully employed to improve sensing performances of the gas sensors based on TiO2 nanosheets with exposed {001} facets for the first time. The hydrogenated TiO2 nanosheets show a significantly higher response toward ethanol, acetone, triethylamine, or formaldehyde than the samples without hydrogenation, and the response further increases with an increase of the hydrogenation temperature. The excellent sensing performances are ascribed to an increase of the density of unsaturated Ti5c atoms on the {001} surface resulting from the hydrogenation process. The unsaturated Ti5c atoms are considered to serve as sensing reaction active sites. They can generate noncontributing (free) electrons and adsorb oxygen molecules, and the detailed sensing mechanism is described at atomic and molecule level. The hydrogenated strategy may be employed to enhance the sensing performances of other metal oxide sensors and catalytic reaction activities of catalyst. The concept of the surface unsaturated metal atoms serving as sensing reaction active sites not only deepens the understanding of the sensing reaction and catalytic reaction mechanism but also provides new insights into the design of advanced gas sensing materials, catalysts, and photoelectronic devices.

An antimonate pyrochlore (H1.23Sr0.45SbO3.48) for photocatalytic oxidation of benzene: effective oxygen usage and excellent activity

The effective adsorption of oxygen molecules makes the antimonate pyrochlore sample (H1.23Sr0.45SbO3.48) an excellent photocatalyst.

Synergistic Effects of Water and Oxygen Molecule Co-adsorption on (001) Surfaces of Tetragonal CH3NH3PbI3: A First-Principles Study

The poor environmental stability of organometallic halide perovskite solar cells presents a big challenge for its commercialization, which is mainly due to the degradation of perovskite materials in humid air. The role played by water molecules has been extensively studied in the degradation processes, where strong interactions between water molecules and perovskite surfaces are found. Using first-principles simulations, we find that oxygen molecules also have strong interactions with (001) surfaces of tetragonal CH3NH3PbI3 through the formation of a chemical Pb–O bond on the PbI2-terminated surface and a hydrogen bond on the CH3NH3I-terminated surface. The adsorbed oxygen molecules introduce empty states near the Fermi level of the surfaces, which can facilitate charge transfer between the surface and oxygen molecules. Furthermore, when an oxygen molecule is located atop a Pb atom on PbI2-terminated surface, the calculated adsorption energies indicate that the surface is more attractive to water molecule...

Acetone gas sensing mechanism on zinc oxide surfaces: A first principles calculation

Semiconducting metal oxide gas sensors have attracted growing interest as a result of their outstanding performance in the bio and industrial applications. Nevertheless, the sensing mechanism is yet not totally understood. In this study, we extensively investigate the adsorption mechanism of acetone molecule on ZnO-based thin film sensors by performing ab initio density functional theory calculations and employing quantum molecular dynamic simulations. Since the sensitivity of a metal oxide sensor is exceedingly depends on molecular oxygen exposure and operating temperature, we explore the competitive adsorption of acetone and oxygen molecule on the most stable orientation of ZnO surface (101̅0) at different temperatures. Results indicate that at elevated temperatures acetone gains required thermal energy to remove preadsorbed oxygen molecule from the surface in a competitive process. We will show that this competition is responsible for the resistive switching behavior in the ZnO-based gas sensors.

Dioxygen molecule adsorption and oxygen atom diffusion on clean and defective aluminum(111) surface using first principles calculations

First principles calculations are conducted to investigate kinetic behavior of oxygen species at the surface of clean and defective Al(111) substrate. Oxygen island, aluminum vacancy, aluminum sub-vacancy, aluminum ad-atom and aluminum terraces defects are addressed. Adsorption of oxygen molecule is first performed on all these systems resulting in dissociated oxygen atoms in main cases. The obtained adsorbed configurations are then picked to study the behavior of atomic oxygen specie and get a detailed understanding on the effect of the local environment on the ability of the oxygen atom to diffuse on the surface. We pointed out that local environment impacts energetics of oxygen atom diffusion. Close packed oxygen island, sub-vacancy and ad-atoms favor oxygen atom stability and decrease mobility of oxygen atom on the surface, to be seen as surface area for further nucleation of oxygen island.

Oxygen-Molecule Adsorption and Dissociation on BCN Graphene: A First-Principles Study.

Boron and nitrogen co-doped (BCN) graphene is an attractive material for use as a metal-free oxygen reduction reaction electrocatalyst and as other catalysts due to its unique structure and electronic properties. Reported here is the structure, determined by using density functional theory, of the active O2 -dissociation site of BCN graphene containing different types of BN cluster. The results show that the edge termination and shape of substitutional BN clusters are two important factors that determine the catalytic activity of BCN graphene for the dissociation of molecular oxygen. N-Terminated triangular BN (t-BN) cluster doping can reduce the energy barrier more effectively compared to a t-BN with a B edge or quadrangular BN cluster. Interestingly, the B atom neighboring the N edge, only in the case of N-terminated t-BN doping, is determined to be the most active site for O2 dissociation due to the barrier being as low as 0.08 eV. The electronic structure calculations reveal that in addition to the large positive charge densities, the catalytic activity of graphene enhanced by B,N doping is also attributed to the increased density of states of the π* states of the active site around the Fermi level.

Thermodynamic models of the adsorption and desorption of molecular oxygen at the UV and blue emitting centers in mesoporous silica under variable oxygen pressure

The UV excited emission properties of mesoporous silica in the blue and UV range are modified by the adsorption and desorption of oxygen molecules at the two defect sites. To interpret the experimental purging cycles we model the oxygen-silica interaction at thermodynamic level, succeeding in explaining the interaction mechanism and the trend of UV centre to undergo photo-bleaching when excited under vacuum conditions. The analysis called for the presence of two different set of defects for both the centres, pinpointed as defects liable to interact with O2 and non-interacting isolated defects. To support the thermodynamic analysis, we simulated the O2 adsorption and desorption processes within the first principles DFT cluster framework. The calculated atomic configurations and the potential energy curves along selected probable reaction paths allow suggesting the kinetics of molecular oxygen interaction on examined surface silica defects and pointing at an oxygen vacancy E’-like defect as the possible result of the UV centre photo-bleaching.

Insights into the Catalytic Activity of Barium Carbonate for Oxygen Reduction Reaction

Efficient electrocatalyst for oxygen reduction reaction (ORR) is crucial for the performance improvement of fuel cells and metal-air batteries. However, catalyst with high activity, easy fabrication process, and low cost is still a daunting challenge. In this work, low-cost BaCO3 nanorods have been demonstrated as efficient electrocatalysts toward the ORR in alkaline media for the first time. The activity of BaCO3 nanorods can be further enhanced by hybridizing with reduced graphene oxide (BaCO3/rGO). The mechanism of ORR on the surface of BaCO3 catalyst was investigated via in situ electrochemical Raman spectroscopy (in situ EC-Raman). Our findings suggest that the barium ions on the surface of catalyst play a key role in the adsorption of oxygen molecules and the formation/decomposition of intermediates. This work provides an important insight into the catalytic activity of BaCO3 for ORR, which can serve as a guide for the design of alkali-earth metal-carbonate-based catalyst.