Low Miller Index Surfaces Research Articles

Analysis of surface stability of ferrocolumbite from combined DFT – Wulff construction simulations

Ferrocolumbite, a variegated oxide mineral bearing the strategic element niobium, exhibits an array of surface chemical compositions across various cleavage orientations. The complexity of Nb-bearing ores hinders the efficiency of separation processes, necessitating fundamental research to bridge the knowledge gap regarding the surface dominance and properties of the mineral. In this regard, density functional theory (DFT) simulations were performed to determine the stability of different low Miller index surfaces of ferrocolumbite. The thermodynamic Wulff construction theory was then employed to predict the morphology of a ferrocolumbite nanoparticle, taking into account the DFT-calculated surface energies and the symmetries in the ferrocolumbite crystal. Several descriptors, such as ionic atom density and bond length distribution, were partially successful in correlating with surface energies; however, no global correlation could be captured for this complex system. Our findings suggested oxygen-terminated surfaces generally exhibit greater stability than metal or mixed-metal–oxygen-terminated surfaces within a given facet. Based on the Wulff shape prediction, the {110}, {001}, {111}, and {021} facets contribute to >85% of the surface area of the ferrocolumbite particles after comminution. Therefore, a sound conclusion about the adsorption behavior of reagents on ferrocolumbite should include all of these facets. This fundamental research highlights the complex and diverse nature of the ferrocolumbite surface and paves the way for future investigations.

Stoichiometric and Nonstoichiometric Surface Structures of Pyrochlore Y2Zr2O7 and Their Relative Stabilities: A First-Principles Investigation

First-principle total energy calculations were performed to investigate the atomic structures and relative stabilities of two low miller-index surfaces of pyrochlore Y2Zr2O7. The stoichiometric Y2Zr2O7 (110) and (100) surfaces were predicted, with lowest formation energies of 1.20 and 1.47 J/m2, respectively. Based on a thermodynamic defect model, non-stoichiometric Y2Zr2O7 surface energies were further evaluated as a function of environmental oxygen partial pressure (pO2) and temperature (T). With all of the results, we were able to construct the surface phase diagrams for T = 300 and 1400 K. The strong correlation between the structural stabilities and the surface stoichiometry was revealed as varying T and pO2. At a given T, the most stable termination of the (110) surfaces would change from a (Y,Zr)−rich (ns−2Y2Zr6O) to O−rich ones (ns−4O_2 and ns−4O_1) as increasing pO2, while that of the (100) surfaces would change from the stoichiometric (stoi−1Y1Zr_1) to the O−rich one (ns−5O). The critical pO2 value for termination transition moves to its higher end as increasing T.

Electronic structure, work function and band alignment of low Miller-index β-TaON surfaces; a DFT + U study

First principles calculations using GGA + U method have been carried out to study the stability, electronic structure, work function, and band alignment of low Miller-index surfaces of β-TaON. Fifteen possible surface terminations are included in this study. We found that, among all these surfaces, the O-terminated (111) has the lowest surface energy under all growth conditions followed by the O-terminated (100), the (110) and the (101) surfaces, respectively. The calculated band gap values for these surfaces are; 2.30, 2.20, 2.39 and 2.35 eV, respectively. Based on the determined band edges positions and taking into account the effect of surface states, we found that the (111)_O and (011)_O surfaces are appropriate for photo-catalytic splitting of water while the (100)_O and (101) surfaces are only appropriate for oxygen evolution reaction (OER). Furthermore, our results show that the (110) and (011)_TaN surfaces are suitable for hydrogen evolution reaction only. Our results show also that the electron affinity and work function of TaON depend greatly on the nature of surface termination.

Temperature and hydrogen partial pressure dependence of zirconium hydride surface structures and stabilities

The surface structures and relative stabilities of low miller-index surfaces of ε-ZrH2 and γ-ZrH have been investigated as a direct function of environmental conditions (hydrogen partial pressure pH2 and temperature T) from the first-principles thermodynamic calculations. The results were used to construct their surface phase diagrams for a wide pH2 range at T = 300 and 900K. The γ-ZrH surfaces tend to be dominated by the Zr-rich (101) with the 101-ns-2Zr1H termination. The surface energy would depend on T and pH2, with a value of 1.2–1.3 J/m2. For pH2 > e−53 at 300K (or pH2 > e−5.3 at 900K), γ-ZrH can no longer be stable but have the tendency to transform into ε-ZrH2, by consistently absorbing H from the gaseous environment. The ε-ZrH2 surfaces tend to be dominated by the stoichiometric (111) facet with the 111-stoi-2H termination and the surface energy of 1.29 J/m2.

Electronically Nonadiabatic H Atom Scattering from Low Miller Index Surfaces of Silver.

The reactivity of a surface depends strongly on the surface structure. To study the influence of surface structure on H atom adsorption, we performed inelastic scattering experiments and complementary electronically nonadiabatic molecular dynamics (MD) simulations for H atoms colliding with the three low Miller index surface facets of silver. Experiment reveals very similar energy loss distributions for all three investigated facets. However, for the (100) facet a dependence on the surface orientation is observed that is absent for the other two facets. The nonadiabatic MD simulations manage to describe the experiments well. Despite the observed insignificant influence of the surface geometry on the energy loss distributions, our simulations predict that the capability of the H atoms to penetrate the surface critically depends on the surface structure. The observed crystal orientation dependence of the energy loss distributions in the experiment for Ag(100) cannot be explained with our simulations, and we provide a discussion for a better theoretical description of this system to stimulate future computational investigations.

Influence of structural disorder on the photocatalytic properties of ZnS nanocrystals prepared by the one-pot solvothermal approach

This study focuses on the impact of the sulfur vacancies on the photocatalytic response of the ZnS nanocrystals synthesized by solvothermal method varying the concentration of zinc acetate/thiourea precursors. XRD patterns show that these samples have a hexagonal structure with different degrees of crystallinity, varying the crystallite size from 2.48 to 2.85 nm. The UV-Vis data reveals an absorption peak (at about 320 nm) characteristic of ZnS nanocrystals. As a result, a decrease in the bandgap value of these materials was observed from 3.78 to 3.62 eV. In principle, a comparison of these results and theoretical calculations reveals the formation of intermediate levels inside the bandgap due to structural polarization. These findings also corroborate the zeta potential measured for these samples, evidenced by an increase of positive charge of ZnS surfaces. Also, the low Miller-index surfaces, such as (100), (110) and (0001), were investigated by periodic density functional theory calculations, in nice agreement with the experimental data. A photocatalysis mechanism was investigated and confirmed the formation of reactive oxygen species.

Density functional theory study of CH4 dissociation and C[sbnd]C coupling on W-terminated WC(0001) surface

To explore the catalytic activity of tungsten monocarbide (WC), the methane (CH4) dehydrogenation and further CC coupling process on WC surface have been studied systematically by performing density functional theory (DFT) calculations. The scaling relationship of the adsorption energies of CHx intermediates on W-terminated WC(0001) surface has been summarized together with those cases on pure metal surfaces. By the way, we also studied the thermal stabilities of low Miller index surfaces of WC to obtain the Wulff construction of WC nanocrystal. The W-terminated WC(0001), W-terminated WC(112̄1), and WC-terminated WC(112̄0) facets constitute more than 96% of the total exposed surface in the WC Wulff construction. The van der Waals (vdW) interaction between adsorbate and W-terminated WC(0001) surface is corrected by the BEEF-vdW functional. Our calculations show that W-terminated WC(0001) surface is the most favorably exposed surface with a lower surface energy. Based on the data collected from literature, the linear relationship between the CHx adsorption energies and the x in CHx on metal (e.g., Fe, Co, Ni, Cu, Pt, Ir, Rh, Ru, Pd, Ag, and Au) surfaces has been analyzed. More interestingly, we found that the Co(111), Ni(111), and W-terminated WC(0001) surfaces possess very close slope factors in such a linear relationship, suggesting the similar catalytic property of methane dehydrogenation on these surfaces. More importantly, the rate-limiting step of CH4 dissociation on W-terminated WC(0001) surface corresponds to CH∗→ C∗+H∗. The energy barrier [i.e., 1.46 eV (1.63 eV) with (without) zero point energy (ZPE) correction] for this reaction step is slightly larger than the one on Ni(111) surface and smaller than the one on Cu(111) surface. Furthermore, we have studied the CC coupling through the CH∗ intermediates on W-terminated WC(0001) surface and found that the formation of C2H2 is kinetically favorable. Our results provides useful information for the development of tungsten-carbide-based catalyst materials.

Computational study of bulk and surface properties on ruthenium oxide (RuO2)

Metal oxides are widely used in lithium-air batteries to improve the formation of stable discharge products and improve lifespan and electrochemical performance. Despite the intense studies on metal oxides catalysts, ruthenium oxide attracted the most attention since it doesn’t only catalyse the redox processes but reduces the over-potential and stabilizes the Li cyclability. Hence, in this work we discuss the bulk and low Miler index surfaces of RuO2 using the first principle density functional theory calculations. It was found that the lattice parameters are in good agreement with the reported results, with less than 1.4% difference. Furthermore, RuO2 was also found to be mechanically stable with all positive independent elastic constants (Cij) obeying the mechanical stability criteria and a positive tetragonal shear modulus (C’> 0). The bulk to shear ratio indicates that the structure is ductile. The density of states shows a slight pseudo gap for RuO2 at the Fermi energy, which suggests that the structure is stable. Finally, low Miller index surfaces (i.e. (110), (010), (001), (111), and (101)) were modelled using METADISE code, and the most stable facet was in agreement with the reported literature.

Complete three-dimensional structure of Bi-adsorbed Si(110) surface: Discovery of heavily reconstructed Si(110) substrate

Silicon is one of the most well-studied crystals in terms of surface structure. In particular, the three low Miller index surfaces---(100), (110), and (111)---which are located at the three apexes of the triangle of the irreducible orientation of the cubic crystal, are the most fundamental and have been extensively studied. However, the surface structure on Si(110) substrates is not as well understood as on other substrates. Here, we have investigated a surface structure induced by submonolayer Bi deposition on Si(110). The complete atomic arrangement from the surface to the fifth layer was directly determined by solving the Patterson function, which was obtained using a very large number of electron diffraction patterns. In contrast to the case of the Si(111) substrate, the Si(110) substrate showed significant reconstructions under a Bi overlayer. The obtained structure shows how dangling bonds can be reduced at the Si(110) substrate. The present result proves the high capability of the surface structure determination of the present method.

Dramatic Change in the Step Edges of the Cu(100) Electrocatalyst upon Exposure to CO: Operando Observations by Electrochemical STM and Explanation Using Quantum Mechanical Calculations

Systematic structure–activity correlations in the electrochemical surface science of CO2 reduction (CO2R) are typically anchored on low Miller-index (hkl) surfaces with characterization directed at the dominant well-defined basal planes. The present investigation focused on the visualization of the step edges of unreconstructed Cu(100), with and without CO dissolved in 0.1 M KOH, at the early onset-potential region of CO2R. Operando electrochemical scanning tunneling microscopy revealed that the step-edge direction changed dramatically upon the adsorption–desorption of CO at potentials of −1.0 to −0.8 V. Quantum mechanical calculations corroborated the favorable transformation of the step-edge direction from ⟨110⟩ to ⟨001⟩ as the pristine surface was decorated by CO.

Reference plane for the electronic states in thin films on stepped surfaces

During the photoemission process the parallel component of the photoelectron momentum is conserved across the modulation plane of the wavefunction, which is the so-called reference plane. The identification of this plane is straightforward for low Miller-index surfaces, but questionable for surfaces of lower symmetry. In this work, the authors show experimentally the coexistence of two different reference planes for surface and bulklike quantum-well states of thin nanostructured films grown on a regularly stepped substrate.

Need for Rationally Designed SnWO4 Photo(electro)catalysts to Overcome the Performance Limitations for O2 and H2 Evolution Reactions

Although the α-SnWO4 material has recently been considered as a new good candidate for visible-light-driven photo(electro)chemical water splitting, the performance is still low and requires further improvement. Here, we present a deep fundamental work on the influence of the various possible facets exposed on this material for oxygen and hydrogen evolution reactions using hybrid density functional theory. The energetic, electronic, water redox, and charge carrier transport features of the four possible (100), (010), (001), and (110) facets (low-Miller index surfaces) are investigated, and significant anisotropic nature is revealed. The relevant properties of each facet to the water oxidation/reduction reactions are correlated with the surface W coordination number. Taking into account the stability and combining optoelectronic and water redox features together of each surface, our work demonstrates that the (110) facet is photocatalytically the best candidate for the OER, while the (100) facet is the best candidate for the HER. Their transport characteristics are found to be much better than those obtained for the three major (121), (210), and (111) facets of synthesized α-SnWO4 samples. Substitutional Ge at the Sn site and Mo at the W site on the two (110) and (100) facets are expected to increase the rates of the water oxidation/reduction reactions. An analysis of the reaction mechanism for the OER in (110)-oriented α-SnWO4 reveals a promising performance of this facet for electrocatalytic water oxidation. These outcomes will greatly motivate experimentalists for carefully designing (110)- and (100)-oriented α-SnWO4 samples to enhance the photo(electro)catalytic OER and photocatalytic HER performances.

First-Principles Investigation of CO Adsorption on h-Fe7C3 Catalyst

h-Fe7C3 is considered as the main active phase of medium-temperature Fe-based Fischer–Tropsch catalysts. Basic theoretical guidance for the design and preparation of Fe-based Fischer–Tropsch catalysts can be obtained by studying the adsorption and activation behavior of CO on h-Fe7C3. In this paper, the first-principles method based on density functional theory is used to study the crystal structure properties of h-Fe7C3 and the adsorption and activation CO on its low Miller index surfaces ( 1 1 ¯ 0 ) , ( 1 1 ¯ 1 ) , ( 101 ) , ( 1 1 ¯ 1 ¯ ) and ( 001 ) . It was found that the low Miller index crystal plane of h-Fe7C3 crystal has multiple equivalent crystal planes and that the maximum adsorption energy of CO at the 3F2 point of the ( 1 1 ¯ 1 ) plane is −2.50 eV, indicating that h-Fe7C3 has a better CO adsorption performance. In addition, the defects generated at the truncated position of the h-Fe7C3 crystal plane have a great impact on the adsorption energy of CO on its surface, that is, the adsorption energy of CO on Fe atoms with C vacancies is higher. The activity of CO after adsorption is greatly affected by the adsorption configuration and less affected by the adsorption energy. The higher the coordination number of Fe atoms after adsorption, the higher the CO activity. At the same time, it was found that the bonding of O and Fe atoms is conducive to the activation of CO.

Hydrogen sorption capacity of crystal lattice defects and low Miller index surfaces of copper

The effect of hydrogen on the physical–chemical properties of copper is directly dependent on the types of chemical bonding between H and lattice defects in Cu. In this work, we performed a systematic study of the bonding of H-atoms with crystal lattice defects of copper. This included three types of symmetric tilt grain boundaries (GBs), Σ3, Σ5 and Σ11, and the low Miller index surfaces, (111), (110) and (100). A comparison with literature data for the bonding of H-atoms with point defects such as vacancies was done. From the defects investigated and analyzed, we conclude that the bond strength with H-atoms varies in the decreasing order: surfaces [(111), (110) and (100)] > vacancy > Σ5 GB > Σ11 GB > bulk ≈ Σ3 GB. A study on the effects of the fcc lattice expansion on the binding energies of H-atoms shows that the main driving force behind the segregation of H-atoms at some GBs is the larger volume at those interstitial GB sites when compared to the interstitial bulk sites.

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.

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.

Study on the surface activity of t-YSZ nanomaterials by first-principles calculation

The internal mechanism underlying the high surface activity of t-YSZ nanomaterials composed of low Miller index facets is not completely understood at present. Using first-principles calculations, the surface energies, as well as geometrical and electronic properties of t-YSZ morphology are simulated and analyzed by conventional bulk surface and new microfacet models. The results show that the surface energies follow the trend Esurface [(0 1 0 × 0 1 0)] > Esurface [(1 1 1 × 0 1 0)] > Esurface [(1 0 1 × 0 1 0)] > Esurface [(1 1 1 × 1 0 1)] > Esurface [(1 0 1 × 1 0 1)] > (1 1 1) bulk surface > (1 0 1) bulk surface > (0 1 0) bulk surface, where the surface energies of the microfacet models are several times greater than those of the bulk surfaces. The surface activity of the microfacet is therefore much more vigorous than that of the bulk surface. The very high chemical activity of the microfacets is derived from their large Fermi energy, pseudo energy gap, and change in Mulliken population. Although the low Miller index surfaces exhibit weak activity, their interface, such as [(0 1 0 × 0 1 0)], have high chemical activity, because of which they are easily and quickly corroded by CaO-MgO-Al2O3-SiO2 (CMAS), as confirmed by experimental reports. Hence, our findings can be considered a first and vital step toward understanding the unusual properties of nano-YSZ.

Jacob's Ladder as Sketched by Escher: Assessing the Performance of Broadly Used Density Functionals on Transition Metal Surface Properties.

The present work surveys the performance of various widely used density functional theory exchange-correlation (xc) functionals in describing observable surface properties of a total of 27 transition metals with face-centered cubic (fcc), body-centered cubic (bcc), or hexagonal close-packed (hcp) crystallographic structures. A total of 81 low Miller index surfaces were considered employing slab models. Exemplary xc functionals within the three first rungs of Jacob's ladder were considered, including the Vosko-Wilk-Nusair xc functional within the local density approximation, the Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient approximation (GGA), and the Tao-Perdew-Staroverov-Scuseria functional as a meta-GGA functional. Hybrids were excluded in the survey because they are known to fail in properly describing metallic systems. In addition, two variants of PBE were considered, PBE adapted for solids (PBEsol) and revised PBE (RPBE), aimed at improving adsorption energies. Interlayer atomic distances, surface energies, and surface work functions were chosen as the scrutinized properties. A comparison with available experimental data, including single-crystal and polycrystalline values, shows that no xc functional is best at describing all of the surface properties. However, in statistical mean terms the PBEsol xc functional is advised, while PBE is recommended when considering both bulk and surface properties. On the basis of the present results, a discussion of adapting GGA functionals to the treatment of metallic surfaces in an alternative way to meta-GGA or hybrids is provided.

CO2 activation and dissociation on the low miller index surfaces of pure and Ni-coated iron metal: a DFT study.

We have used spin polarized density functional theory calculations to perform extensive mechanistic studies of CO2 dissociation into CO and O on the clean Fe(100), (110) and (111) surfaces and on the same surfaces coated by a monolayer of nickel. CO2 chemisorbs on all three bare facets and binds more strongly to the stepped (111) surface than on the open flat (100) and close-packed (110) surfaces, with adsorption energies of -88.7 kJ mol-1, -70.8 kJ mol-1 and -116.8 kJ mol-1 on the (100), (110) and (111) facets, respectively. Compared to the bare Fe surfaces, we found weaker binding of the CO2 molecules on the Ni-deposited surfaces, where the adsorption energies are calculated at +47.2 kJ mol-1, -29.5 kJ mol-1 and -65.0 kJ mol-1 on the Ni-deposited (100), (110) and (111) facets respectively. We have also investigated the thermodynamics and activation energies for CO2 dissociation into CO and O on the bare and Ni-deposited surfaces. Generally, we found that the dissociative adsorption states are thermodynamically preferred over molecular adsorption, with the dissociation most favoured thermodynamically on the close-packed (110) facet. The trends in activation energy barriers were observed to follow that of the trends in surface work functions; consequently, the increased surface work functions observed on the Ni-deposited surfaces resulted in increased dissociation barriers and vice versa. These results suggest that measures to lower the surface work function will kinetically promote the dissociation of CO2 into CO and O, although the instability of the activated CO2 on the Ni-covered surfaces will probably result in CO2 desorption from the nickel-doped iron surfaces, as is also seen on the Fe(110) surface.

Surface Morphology and Surface Stability against Oxygen Loss of the Lithium-Excess Li2MnO3 Cathode Material as a Function of Lithium Concentration.

There is a growing appreciation for the role of surface reactivity and subsequent reconstruction affecting the performance of high-voltage, high-capacity Li-ion cathode materials. In particular, the promising Li-excess materials are known to exhibit significant vulnerability toward oxygen release, which can cause surface densification and impede Li intercalation. Here we focus on the end member, Li2MnO3, as a Li-excess, Mn-rich representative of this class of materials and systematically elucidate all possible stoichiometric low Miller index surfaces with various cation ordering on each surface. We apply surface cation reconstruction rules that depend on the local environment, including target Mn-Li site exchanges, and optimize the resulting surface Li configurations using metadynamics. The equilibrium Wulff shape shows dominant (001), (010) surface facets, and almost all facets exhibit favorable Mn reconstruction. Most importantly, we find that while all equilibrium LixMnO3 surfaces become unstable toward oxygen release for x < 1.7, some facets are consistently more resistant than others which may provide a design metric for more stable particle morphologies and enhanced surface oxygen retention.