Density Functional Theory Study of Ni Clusters Supported on the ZrO2(111) Surface

The nickel/zirconia (Ni/ZrO2) interface plays a key role in the performance of the anode of solid oxide fuel cells (SOFC) and it is therefore important to understand the interaction between nickel nanoparticles and the ZrO2 surface. Here, we have described the interaction of five Nin (n = 1–5) clusters with the (111) surface of cubic zirconia, c‐ZrO2(111), using spin polarized density functional theory (DFT) calculations with inclusion of long‐range dispersion forces. We have systematically evaluated the geometric and electronic structure of different cluster configurations and sizes and shown how the clusters interact with the oxygen and zirconium surface atoms. The cluster‐surface interaction is characterized by a charge transfer from the Ni clusters to the surface. From calculations of the hopping rate and clustering energies, we have demonstrated that Ni atoms prefer to aggregate rather than wet the surface and we would therefore suggest that modifications in the synthesis could be needed to modify the coalescence of the supported metal particles of this catalytic system

Density functional theory study of the interaction of H2O, CO2 and CO with the ZrO2 (111), Ni/ZrO2 (111), YSZ (111) and Ni/YSZ (111) surfaces

The triple phase boundary (TPB), where the gas phase, Ni particles and the yttria-stabilised zirconia (YSZ) surface meet, plays a significant role in the performance of solid oxide fuel cells (SOFC). Indeed, the key reactions take place at the TPB, where molecules such as H2O, CO2 and CO interact and react. We have systematically studied the interaction of H2O, CO2 and CO with the dominant surfaces of four materials that are relevant to SOFC, i.e. ZrO2(111), Ni/ZrO2(111), YSZ(111) and Ni/YSZ(111) of cubic ZrO2 stabilized with 9% of yttria (Y2O3). The study employed spin polarized density functional theory (DFT), taking into account the long-range dispersion forces. We have investigated up to five initial adsorption sites for the three molecules and have identified the geometries and electronic structures of the most stable adsorption configurations. We have also analysed the vibrational modes of the three molecules in the gas phase and compared them with the adsorbed molecules. A decrease of the wavenumbers of the vibrational modes for the three adsorbed molecules was observed, confirming the influence of the surface on the molecules' intra-molecular bonds. These results are in line with the important role of Ni in this system, in particular for the CO adsorption and activation

A DFT+U study of the oxidation of cobalt nanoparticles: Implications for biomedical applications

Nanomaterials – magnetic nanoparticles in particular have been shown to have significant potential in cancer theranostics, where iron oxides are commonly the materials of choice. While biocompatibility presents an advantage, the low magnetisation is a barrier to their widespread use. As a result, highly magnetic cobalt nanoparticles are attracting increasing attention as a promising alternative. Precise control of the physiochemical properties of such magnetic systems used in biomedicine is crucial, however, it is difficult to test their behaviour in vivo. In the present work, density functional theory calculations with the Dudarev approach (DFT+U) have been used to model the adsorption of oxygen on low Miller index surfaces of the hexagonal phase of cobalt. In vivo conditions of temperature and oxygen partial pressure in the blood have been considered, and the effects of oxidation on the overall properties of cobalt nanoparticles are described. It is shown that oxygen adsorbs spontaneously on all surfaces with the formation of non-magnetic cobalt tetroxide, Co3O4, at body temperature, confirming that, despite their promising magnetic properties, bare cobalt nanoparticles would not be suitable for biomedical applications. Surface modifications could be designed to preserve their favourable characteristics for future utilisation

Interaction of SO2 with the Platinum (001), (011), and (111) Surfaces: A DFT Study

Given the importance of SO2 as a pollutant species in the environment and its role in the hybrid sulphur (HyS) cycle for hydrogen production, we carried out a density functional theory study of its interaction with the Pt (001), (011), and (111) surfaces. First, we investigated the adsorption of a single SO2 molecule on the three Pt surfaces. On both the (001) and (111) surfaces, the SO2 had a S,O-bonded geometry, while on the (011) surface, it had a co-pyramidal and bridge geometry. The largest adsorption energy was obtained on the (001) surface (Eads = −2.47 eV), followed by the (011) surface (Eads = −2.39 and −2.28 eV for co-pyramidal and bridge geometries, respectively) and the (111) surface (Eads = −1.85 eV). When the surface coverage was increased up to a monolayer, we noted an increase of Eads/SO2 for all the surfaces, but the (001) surface remained the most favourable overall for SO2 adsorption. On the (111) surface, we found that when the surface coverage was θ > 0.78, two neighbouring SO2 molecules reacted to form SO and SO3. Considering the experimental conditions, we observed that the highest coverage in terms of the number of SO2 molecules per metal surface area was (111) > (001) > (011). As expected, when the temperature increased, the surface coverage decreased on all the surfaces, and gradual desorption of SO2 would occur above 500 K. Total desorption occurred at temperatures higher than 700 K for the (011) and (111) surfaces. It was seen that at 0 and 800 K, only the (001) and (111) surfaces were expressed in the morphology, but at 298 and 400 K, the (011) surface was present as well. Taking into account these data and those from a previous paper on water adsorption on Pt, it was evident that at temperatures between 400 and 450 K, where the HyS cycle operates, most of the water would desorb from the surface, thereby increasing the SO2 concentration, which in turn may lead to sulphur poisoning of the catalyst

Interaction of H2O with the Platinum Pt (001), (011), and (111) Surfaces: A Density Functional Theory Study with Long-Range Dispersion Corrections

Platinum is a noble metal that is widely used for the electrocatalytic production of hydrogen, but the surface reactivity of platinum toward water is not yet fully understood, even though the effect of water adsorption on the surface free energy of Pt is important in the interpretation of the morphology and catalytic properties of this metal. In this study, we have carried out density functional theory calculations with long-range dispersion corrections [DFT-D3-(BJ)] to investigate the interaction of H2O with the Pt (001), (011), and (111) surfaces. During the adsorption of a single H2O molecule on various Pt surfaces, it was found that the lowest adsorption energy (Eads) was obtained for the dissociative adsorption of H2O on the (001) surface, followed by the (011) and (111) surfaces. When the surface coverage was increased up to a monolayer, we noted an increase in Eads/H2O with increasing coverage for the (001) surface, while for the (011) and (111) surfaces, Eads/H2O decreased. Considering experimental conditions, we observed that the highest coverage was obtained on the (011) surface, followed by the (111) and (001) surfaces. However, with an increase in temperature, the surface coverage decreased on all the surfaces. Total desorption occurred at temperatures higher than 400 K for the (011) and (111) surfaces, but above 850 K for the (001) surface. From the morphology analysis of the Pt nanoparticle, we noted that, when the temperature increased, only the electrocatalytically active (111) surface remained

DFT plus U Study of the Electronic, Magnetic and Mechanical Properties of Co, CoO, and Co3O4

Cobalt nanoparticles play an important role as a catalyst in the Fischer-Tropsch synthesis. During the reaction process, cobalt nanoparticles can become oxidized leading to the formation of two phases: CoO rock-salt and Co3O4 cubic spinel. Experimentally, it is possible to evaluate the phase change and follow the catalyst degradation by measuring the magnetic moment, as each material presents a different magnetic structure. It is therefore important to develop a fundamental description, at the atomic scale, of cobalt and its oxide phases which we have done here using density functional theory with the Dudarev approach to account for the on-site Coulomb interactions (DFT+U). We have explored different Ueff values, ranging from 0 to 5 eV, and found that Ueff = 3.0 eV describes most appropriately the mechanical properties, as well as the electronic and magnetic structures of Co, CoO and Co3O4. We have considered a ferromagnetic ordering for the metallic phase and the antiferromagnetic structure for the oxide phases. Our results support the interpretation of the catalytic performance of metallic cobalt as it transforms into its oxidized phases under experimental conditions

Tuning ZnO Sensors Reactivity toward Volatile Organic Compounds via Ag Doping and Nanoparticle Functionalization

Nanomaterials for highly selective and sensitive sensors toward specific gas molecules of volatile organic compounds (VOCs) are most important in developing new-generation of detector devices, for example, for biomarkers of diseases as well as for continuous air quality monitoring. Here, we present an innovative preparation approach for engineering sensors, which allow for full control of the dopant concentrations and the nanoparticles functionalization of columnar material surfaces. The main outcome of this powerful design concept lies in fine-tuning the reactivity of the sensor surfaces toward the VOCs of interest. First, nanocolumnar and well-distributed Ag-doped zinc oxide (ZnO:Ag) thin films are synthesized from chemical solution, and, at a second stage, noble nanoparticles of the required size are deposited using a gas aggregation source, ensuring that no percolating paths are formed between them. Typical samples that were investigated are Ag-doped and Ag nanoparticle-functionalized ZnO:Ag nanocolumnar films. The highest responses to VOCs, in particular to (CH3)2CHOH, were obtained at a low operating temperature (250 °C) for the samples synergistically enhanced with dopants and nanoparticles simultaneously. In addition, the response times, particularly the recovery times, are greatly reduced for the fully modified nanocolumnar thin films for a wide range of operating temperatures. The adsorption of propanol, acetone, methane, and hydrogen at various surface sites of the Ag-doped Ag8/ZnO(0001) surface has been examined with the density functional theory (DFT) calculations to understand the preference for organic compounds and to confirm experimental results. The response of the synergistically enhanced sensors to gas molecules containing certain functional groups is in excellent agreement with density functional theory calculations performed in this work too. This new fabrication strategy can underpin the next generation of advanced materials for gas sensing applications and prevent VOC levels that are hazardous to human health and can cause environmental damages

Catalytic Conversion of CO and H₂ into Hydrocarbons on the Cobalt Co(111) Surface: Implications for the Fischer-Tropsch Process

The Fischer-Tropsch (FT) process consists of the reaction of a synthesis gas (syngas) mixture containing carbon monoxide (CO) and hydrogen (H₂), which are polymerised into liquid hydrocarbon chains, often using a cobalt catalyst, although the mechanistic pathway is not yet fully understood. Here, we have employed unrestricted density functional theory calculations with a Hubbard Hamiltonian and long-range dispersion corrections [DFT+U−D3−BJ] to investigate the reaction of syngas and the selectivity toward the hydrocarbons formed on the cobalt Co(111) surface. The single CO and dissociated H₂ molecules prefer to adsorb in the two different types of trigonal surface sites, and we discuss how the interatomic distances, fundamental vibrational modes, charge transfers, surface free energies and work function are modified by the adsorbates. The co-adsorption of the syngas molecules in close proximity provides enough energy for the system to cross the saddle points on the minimum energy pathway (MEP), leading to the catalytic hydrogenolysis of the C−O bond. The adsorbed CO, alongside the intermediates CH and OH, are further stabilised when the ratio of equilibrium coverages (C) is CH:CCO, CH, OH > 6:1 under the temperature conditions required for the FT process. We propose several mechanistic pathways to account for the formation of ethane (C₂H₆), as a model for long-chain hydrocarbons, as well as methane (CH₄) which is an undesirable product. The MEPs for these processes show that the coupling of the C−C bond followed by hydrogenation are the most favourable processes, which take precedence over the production of CH₄. The termination reaction suggests that water (H₂O) remains weakly physisorbed to the surface, allowing the re-utilisation of its catalytic site. The simulated fundamental vibrational frequencies and scanning tunnelling microscopy images of the surface-bound intermediates are in agreement with the available experimental data. Our findings are important in the interpretation of the elementary steps of the FT process on the Co(111) surface

Surface functionalization of ZnO:Ag columnar thin films with AgAu and AgPt bimetallic alloy nanoparticles as an efficient pathway for highly sensitive gas discrimination and early hazard detection in batteries

For a fast and reliable monitoring of hazardous environments, the discrimination and detection of volatile organic compounds (VOCs) in the low ppm range is highly demanded and requires the development of new chemical sensors. We report herein, a novel approach to tailor the selectivity of nanocomposite thin film sensors by investigating systematically the effect of surface decoration of Ag-doped ZnO (ZnO:Ag) columnar thin films. We have used AgPt and AgAu noble bimetallic alloy nanoparticles (NPs) to decorate the surfaces of ZnO:Ag and measured the resulting gas sensing properties towards VOC vapors and hydrogen gas. The gas response of the nanocomposite containing AgAu NPs to 100 ppm of ethanol, acetone, n-butanol, 2-propanol and methanol vapors was increased on average by a factor of 4 compared to the pristine ZnO:Ag columnar films. However, decoration with AgPt NPs led to a considerable reduction of the gas response to all VOC vapors and an increase of the response to H2 by roughly one order of magnitude, indicating a possibile route to tailor the selectivity by surface decoration. For this reason, the reported NPs decorated ZnO:Ag thin film sensors are suitable for the detection of H2 in Li-ion batteries, which is an early indication of the thermal runaway that leads to complete battery failure and possible explosion. To understand the impact of NP surface decoration on the gas sensing properties of ZnO:Ag thin films, we employed density functional theory calculations with on-site Coulomb corrections and long-range dispersion interactions (DFT+U−D3-(BJ)) and investigated the adsorption of various VOC molecules and hydrogen onto the Ag-doped and NP decorated (101 @#x0305;0) surface of zinc oxide ZnO. The calculated surface free energies indicate that Ag5Au5/ZnO(101 @#x0305;0):Ag is the most favourable system for the detection of VOCs, which is also the most reactive towards them based on its work function. Our calculated adsorption energies show that Ag9Pt/ZnO(101 @#x0305;0):Ag has the largest preference for H2 and the lowest preference for the organic asdorbates, which is in line with the high selectivity of AgPt/ZnO:Ag sensors towards the former molecule observed in our experiments