Matildite Contact with Media: First-Principles Study of AgBiS2 Surfaces and Nanoparticle Morphology

Motivated by the interest in AgBiS2 material for solar light harvesting applications, a detailed bulk first-principles quantum mechanical study of its surface properties is presented. Density functional theory based calculations with the Perdew–Burke–Ernzerhof functional have been carried out for different surface orientations and terminations of the matildite polymorph. From the results, two particularly stable facets are predicted to dominate Wulff shaped AgBiS2 nanoparticles. These are the (001) type nonpolar surface and the (111) polar terminations where facets are exposed containing solely Ag or S atoms. The Wulff equilibrium shape is predicted to be a cube with only two edges capped. This particular shape explains a previously reported surface enrichment of Ag with respect to Bi of ∼1.5. The (001) surfaces display an ionic character similar to bulk AgBiS2, with a low work function of 4.31 eV, although the inspection of the density of states (DOS) reveals a bandgap increased by 0.3 eV compared to bulk. This surface effect could explain the bulk wavelength overestimation of the absorption coefficient decay, as previously determined. Last but not least, the DOS of the (111) polar termination reveals a metallic character, where Fermi level is located below that on the (001) surfaces. Possible implications of the different electronic structure of these surfaces in the reported photoactivity are discussed.Peer ReviewedPostprint (author's final draft

Transition Metal Carbides as Novel Materials for CO2 Capture, Storage, and Activation

The capture and activation of the greenhouse gas carbon dioxide (CO2) is a prerequisite to its catalytic reforming or breakdown. Here we report, by means of density functional theory calculations including dispersive forces, that transition metal carbides (TMC; TM = Ti, Zr, Hf, Nb, Ta, Mo) are able to uptake and activate CO2 on their most-stable (001) surfaces with considerable adsorption strength. Estimations of adsorption and desorption rates predict a capture of CO2 at ambient temperature and even low partial pressures, suggesting TMCs as potential materials for CO2 abatemen

Double-well potential energy surface in the interaction between h-BN and Ni(111)

Density functional theory calculations with non-local correlation functionals, properly accounting for dispersion forces, predict the presence of two minima in the interaction energy between h-BN and Ni(111). These can be described as a physisorbed state with no corrugation of the h-BN structure, and a chemisorbed state exhibiting noticeable corrugation and shorter distance of h-BN to the metallic support. The latter corresponds indeed to the one reported in most experiments. The relative stability of the two minima depends on the specific density functional employed: of those investigated here only the optB86b-vdW yields the correct order of stability. We also demonstrate that the effect of the metal support on the Raman frequency of the chemisorbed boron nitride monolayer cannot be reduced to the associated strain. This is important because the Raman frequency has been proposed as a signature to identify h-BN monolayers from multilayered samples. Our analysis shows that such signatures would be strongly dependent on the nature of the support – h-BN interaction

Matildite Contact with Media: First-Principles Study of AgBiS2 Surfaces and Nanoparticle Morphology

Motivated by the interest in AgBiS2 material for solar light harvesting applications, a detailed bulk first-principles quantum mechanical study of its surface properties is presented. Density functional theory based calculations with the Perdew-Burke-Ernzerhof functional have been carried out for different surface orientations and terminations of the matildite polymorph. From the results, two particularly stable facets are predicted to dominate Wulff shaped AgBiS2 nanoparticles. These are the (001) type nonpolar surface and the (111) polar terminations where facets are exposed containing solely Ag or S atoms. The Wulff equilibrium shape is predicted to be a cube with only two edges capped. This particular shape explains a previously reported surface enrichment of Ag with respect to Bi of ∼1.5. The (001) surfaces display an ionic character similar to bulk AgBiS2, with a low work function of 4.31 eV, although the inspection of the density of states (DOS) reveals a bandgap increased by 0.3 eV compared to bulk. This surface effect could explain the bulk wavelength overestimation of the absorption coefficient decay, as previously determined. Last but not least, the DOS of the (111) polar termination reveals a metallic character, where Fermi level is located below that on the (001) surfaces. Possible implications of the different electronic structure of these surfaces in the reported photoactivity are discussed

Surface activity of early transition-metal oxycarbides: CO2 adsorption case study

Theoretical studies and experiments have suggested that transition metal carbides (TMCs) can be useful materials for carbon capture and storage or use technologies from air sources. However, TMCs are known to become easily oxidized in the presence of molecular oxygen, and their properties jeopardized while being transformed into transition metal oxycarbides (TMOCs), which can affect the TMCs chemical activity, e.g. towards CO2. Here, by means of density functional theory (DFT) based calculations including dispersion we address the possible effect of oxycarbide formation in the CO2 capture course. A careful analysis of different models show that for group 4 TMCs (TM = Ti, Zr, Hf), their oxidation into TMOCs involves a negligible structural distortion of the outermost oxide surface layer, whereas severe rumplings are predicted for group 5 and 6 TMOCs (TM = V, Nb, Ta, Mo). The large surface distortion in the latter TMOCs results in a weak interaction with CO2 with adsorption energies below -0.27 eV. On the contrary, on group 4 TMOCs surfaces CO2 adsorption becomes stronger, with the adsorption values strengthening by 0.44-1.2 eV, a fact that, according to adsorption/desorption rates estimates, increments the air CO2 capture temperature window by 175-400 K. The present DFT results point to group 4 TMCs, TiC in particular, as promising materials for air CO2 capture and storage/conversion, even in the presence of oxygen and the possible formation of transition metal oxycarbides

Bandgap engineering by cationic disorder: case study on AgBiS2

The influence of cationic disorder on the electronic structure of ternary compounds, here exemplified on AgBiS2 material, is studied by means of accurate first principles periodic density functional theory based calculations. For AgBiS2 cationic disorder in going from semiconducting matildite to a metallic arrangement crystal structure is found to induce a significant decrease in the band gap, as a result of cation-disorder conduction band tail states penetrating into the matildite bandgap. Properly aligned conduction band minimum and valence band maximum show that cationic disorders lead to a noticeable drop of the former and a slight increase of the latter. The present results indicate that temperature effects triggering cationic disorder will have a beneficial effect on the photoactivity of AgBiS2 samples provided that the metallic limit is not reached

Biogas upgrading by transition metal carbides

The separation of carbon dioxide (CO2) from methane (CH4) is critical in biogas upgrading, requiring materials with high selectivity toward one of the two gas components. Hereby we show, by means of density functional theory based calculations including dispersive forces description, the distinct interaction of CO2 and CH4 with the most stable (001) surfaces of seven transition metal carbides (TMC; TM = Ti, Zr, Hf, V, Nb, Ta, and Mo). Transition state theory derived ad-/desorption rates suggest a very high CO2 uptake and selectivity over CH4 even at ambient temperature and low partial gas pressures

Transition metal adatoms on graphene: A systematic density functional study

Transition Metal (TM) atoms adsorption on graphene results in a tuning of their electronic, magnetic, storage, sensing, and catalytic properties. Herein we provide a thorough density functional theory study, including dispersion, of the structural, energetic, diffusivity, magnetic, and doping properties for all 3d, 4d, and 5d TM atoms adsorbed on graphene. TMs prefer to sit on hollow sites when chemisorbed, but on bridge or top sites when physisorbed; which is the case of atoms with d5 and d10 configurations. Diffusion energy barriers follow the adsorption energy trends. Dispersive forces simply increase the adsorption strength by ∼0.35 eV. Adatom height seems to be governed by the bond strength. All TMs are found to n-dope graphene, except Au, which p-dopes. The electron transfer decays along the d series due to the electronegativity increase. Early TMs infer noticeable magnetism to graphene, yet for elements with more than five electrons in the d shell the local magnetic moments abruptly decay to low or zero values. Experimental observations on adatom position, height, temperature clustering and Ostwald ripening, p- or n-doping, or the electronic configuration can be rationalized by present calculations, which deliver a solid theoretical ground from which experimental features can be interpreted and discussed

Performance of the TPSS functional on predicting core level binding energies of main group elements containing molecules: a good choice for molecules adsorbed on metal surfaces

Here we explored the performance of Hartree-Fock (HF), Perdew-Burke-Ernzerhof (PBE), and Tao-Perdew-Staroverov-Scuseria (TPSS) functionals in predicting core level 1s Binding Energies (BEs) and BE shifts (ΔBEs) for a large set of 68 molecules containing a wide variety of functional groups for main group elements B→F and considering up to 185 core levels. A statistical analysis comparing with X-Ray Photoelectron Spectroscopy (XPS) experiments shows that BEs estimations are very accurate, TPSS exhibiting the best performance. Considering ΔBEs, the three methods yield very similar and excellent results, with mean absolute deviations of ~0.25 eV. When considering relativistic effects, BEs deviations drop approaching experimental values. So, the largest mean percentage deviation is of 0.25% only. Linear trends among experimental and estimated values have been found, gaining offsets with respect ideality. By adding relativistic effects to offsets, HF and TPSS methods underestimate experimental values by solely 0.11 and 0.05 eV, respectively, well within XPS chemical precision. TPSS is posed as an excellent choice for the characterization, by XPS, of molecules on metal solid substrates, given its suitability in describing metal substrates bonds and atomic and/or molecular orbitals