Density functional theory calculations of surface properties and H2 adsorption on the Cu2O (1 1 1) surface

First-principles calculation on the basis of the density functional theory (DFT) and generalized gradient approximation have been applied to study the adsorption of H₂ on the stoichiometric O-terminated Cu₂O (1 1 1), Cu₂O (1 1 1)-Cu_CUS and Cu-terminated Cu₂O (1 1 1) surfaces. The optimal adsorption position and orientation of H₂ on the stoichiometric O-terminated Cu₂O (1 1 1) surface and Cu-terminated Cu₂O (1 1 1) surface were determined and electronic structural changes upon adsorption were investigated by calculating the Local Density of States (LDOS) of the Cu_CUS 3d and Cu_CUS 4s of stoichiometric O-terminated Cu₂O (1 1 1) surface. These results showed that H₂ molecule adsorption on Cu_CUS site parallel to stoichiometric O-terminated Cu₂O (1 1 1) surface and H₂ molecule adsorption on Cu₂ site parallel to Cu-terminated Cu₂O (1 1 1) surface were the most favored, respectively. The presence of surface copper vacancy has a little influence on the structures when H₂ molecule adsorbs on Cu_CSA, O_CUS and O_CSA atoms and the H₂ molecule is only very weakly bound to the Cu₂O (1 1 1)-Cu_CUS surface. From the analysis of stoichiometric O-terminated Cu₂O (1 1 1) Local Density of States, it is observed that Cu_CUS 3d orbital has moved to a lower energy and the sharp band of Cu_CUS 4s is delocalized when compared to that before H₂ molecule adsorption, and overlapped substantially with bands due to adsorbed H₂ molecule. The Mulliken charges of H₂ adsorption on Cu_CUS site showed that H₂ molecule obtained electron from Cu_CUS which was consistent with the calculated electronic structural changes upon H₂ adsorption.     

Adsorption and dissociation of O2 on the Cu2O(1 1 1) surface: Thermochemistry, reaction barrier

The adsorption and dissociation of O₂ on the perfect and oxygen-deficient Cu₂O(1 1 1) surface have been systematically studied using periodic density functional calculations. Different kinds of possible modes of atomic O and molecular O₂ adsorbed on the Cu₂O(1 1 1) surface are identified: atomic O is found to prefer threefold 3Cu site on the perfect surface and O_vacancy site on the deficient surface, respectively. Cu_CUS is the most advantageous site with molecularly adsorbed O₂ lying flatly over singly coordinate Cu_CUS-Cu_CSA bridge on the perfect surface. O₂ adsorbed dissociatively on the deficient surface, which is the main dissociation pathway of O₂, and a small quantity of molecularly adsorbed O₂ has been obtained. Further, possible dissociation pathways of molecularly adsorbed O₂ on the Cu₂O(1 1 1) surface are explored, the reaction energies and relevant barriers show that a small quantity of molecularly adsorbed O₂ dissociation into two O atoms on the deficient surface is favorable both thermodynamically and kinetically in comparison with the dissociation of O₂ on the perfect surface. The calculated results suggest that the presence of oxygen vacancy exhibits a strong chemical reactivity towards the dissociation of O₂ and can obviously improve the catalytic activity of Cu₂O, which is in agreement with the experimental observation.     

A density functional theory study on the adsorption and dissociation of N2O on Cu2O(1 1 1) surface

Density functional theory has been employed to investigate the adsorption and the dissociation of an N₂O at different sites on perfect and defective Cu₂O(1 1 1) surfaces. The calculations are performed on periodic systems using slab model. The Lewis acid site, Cu_CUS, and Lewis base site, O_SUF are considered for adsorption. Adsorption energies and the energies of the dissociation reaction N₂O → N₂ + O(s) at different sites are calculated. The calculations show that adsorption of N₂O is more favorable on Cu_CUS adsorption site energetically. Cu_CUS site exhibits a very high activity. The Cu_CUS-N₂O reaction is exothermic with a reaction energy of 77.45 kJ mol⁻¹ and an activation energy of 88.82 kJ mol⁻¹, whereas the O_SUF-N₂O reaction is endothermic with a reaction energy of 205.21 kJ mol⁻¹ and an activation energy of 256.19 kJ mol⁻¹. The calculations for defective surface indicate that O vacancy cannot obviously improve the catalytic activity of Cu₂O.     

Adsorption and dissociation of H2 on the Cu2O(1 1 1) surface: A density functional theory study

Interactions of atomic and molecular hydrogen with perfect and deficient Cu₂O(1 1 1) surfaces have been investigated by density functional theory. Different kinds of possible modes of H and H₂ adsorbed on the Cu₂O(1 1 1) surface and possible dissociation pathways were examined. The calculated results indicate that O_SUF, Cu_CUS and O_vacancy sites are the adsorption active centers for H adsorbed on the Cu₂O(1 1 1) surface, and for H₂ adsorption over perfect surface, Cu_CUS site is the most advantageous position with the side-on type of H₂. For H₂ adsorption over deficient surface, two adsorption models of H₂, H₂ adsorbing perpendicularly over O_vacancy site and H₂ lying flatly over singly-coordinate Cu-Cu short bridge, are typical of non-energy-barrier dissociative adsorption leading to one atomic H completely inserted into the crystal lattice and the other bounded to Cu_CUS atom, suggesting that the dissociative adsorption of H₂ is the main dissociation pathway of H₂ on the Cu₂O(1 1 1) surface. Our calculation result is consistent with that of the experimental observation. Therefore, Cu₂O(1 1 1) surface with oxygen vacancy exhibits a strong chemical reactivity towards the dissociation of H₂.     

Adsorption of chlorophenol on the Cu(1 1 1) surface: A first-principles density functional theory study

The interaction between a 2-chlorophenol (C₆H₄OHCl) molecule and the Cu(1 1 1) surface has been investigated using density functional theory as an initial step in gaining a better understanding of the catalyzed formation of dioxin compounds on a clean copper surface. The 2-chlorophenol molecule is found to form several weakly bonded, horizontally and vertically oriented configurations. Dissociative modes have also been investigated. For the latter, the formation of phenyl and benzyne fragments is found to be more energetically favourable than the formation of 2-chlorophenoxy radicals.     

Formation and characterization of the Cu2O overlayer on Zn-terminated ZnO(0 0 0 1)

Low-energy electron diffraction, X-ray photoelectron spectroscopy and synchrotron-radiation-excited angle-resolved photoelectron spectroscopy have been used to characterize Cu-oxide overlayers on the Zn-terminated ZnO(0 0 0 1) surface. Deposition of Cu on the ZnO(0 0 0 1)-Zn surface results in the formation of Cu clusters with (1 1 1) top terraces. Oxidation of these clusters by annealing at 650 K in O₂ atmosphere (1.3 × 10⁻⁴ Pa) leads to an ordered Cu₂O overlayer with (1 1 1) orientation. Good crystallinity of the Cu₂O(1 1 1) overlayer is proved by energy dispersion of one of Cu₂O valence bands. The Cu₂O(1 1 1) film exhibits a strong p-type semiconducting nature with the valence band maximum (VBM) of 0.1 eV below the Fermi level. The VBM of ZnO at the Cu₂O(1 1 1)/ZnO(0 0 0 1)-Zn interface is estimated to be 2.4 eV, yielding the valence-band offset of 2.3 eV.     

Atomic structure of Cu2O(1 1 1)

Low-energy electron diffraction and scanning tunneling microscopy have been used to probe the surface atomic structure of Cu₂O(1 1 1) after various sample preparations. Annealing in oxygen gives a stoichiometric (1 × 1) oxygen terminated surface and further annealing in ultra-high vacuum results in a clear reconstruction and surface faceting. Tunneling from filled states in the reconstructed surface reveals a hexagonal pattern of large protrusions, which show an internal structure. The reconstruction is believed to be due to one-third of a monolayer of ordered oxygen vacancies. At areas on the surface where the large features are missing, another smaller type of protrusions is visible, which is associated with the ideal (1 × 1) surface. The relative position of the two types of features gives two possible models of the (1 1 1) surface. In the first model, the (1 × 1) surface is the ideal bulk terminated surface and coordinatively unsaturated oxygen ions are missing in the reconstructed surface. The second model agrees with the first model with the exception that coordinatively unsaturated copper ions in the outmost copper layer are missing in both the (1 × 1) and the reconstructed surface. The latter model is supported by previous surface free energy calculations. Since the undercoordinated copper ions have been suggested to be the catalytic active sites of Cu₂O(1 1 1), the presence or absence of these cations could be of great importance for the fundamental understanding of the surface reactivity of Cu₂O and of copper-based catalysts.     

Comparative investigation of the c(2 × 2)-Si/Cu(0 1 1) and (√3 × √3)R30°-Cu2Si/Cu(1 1 1) surface alloys using DFT

The electronic structure of the c(2 × 2)-Si/Cu(0 1 1) surface alloy has been investigated and compared to the structures seen in the three phases of the (√3 × √3)R30°Cu₂Si/Cu(1 1 1) system, using LCAO-DFT. The weighted surface energy increase between the alloyed Cu(0 1 1) and Cu(1 1 1) surfaces is 126.7 meV/Si atom. This increase in energy for the (0 1 1) system when compared to the (1 1 1) system is assigned to the transition from a hexagonal to a rectangular local bonding environment for the Si ion cores, with the hexagonal environment being energetically more favorable. The Si 3s state is shown to interact covalently with the Cu 4s and 4p states whereas the Si 3p state, and to a lesser extent the Si 3d state, forms a mixture of covalent and metallic bonds with the Cu states. The Cu 4s and 4p states are shown to be altered by approximately the same amount by both the removal of Cu ion cores and the inclusion of Si ion cores during the alloying of the Cu(0 1 1) surface. However, the Cu 3d states in the surface and second layers of the alloy are shown to be more significantly altered during the alloying process by the removal of Cu ion cores from the surface layer rather than by the addition of Si ion cores. This is compared to the behavior of the Cu 3d states in the surface and second layers of the each phase of the (√3 × √3)R30°-Cu₂Si/Cu(1 1 1) alloy and consequently the loss of Cu-Cu periodicity during alloying of the Cu(0 1 1) surface is conjectured as the driving force for changes to the Cu 3d states. The accompanying changes to the Cu 4s and 4p states in both the c(2 × 2)-Si/Cu(0 1 1) and (√3 × √3)R30°-Cu₂Si/Cu(1 1 1) alloys are quantified and compared. The study concludes with a brief quantitative study of changes in the bond order of the Cu-Cu bonds during alloying of both Cu(0 1 1) and Cu(1 1 1) surfaces.     

Solvent effects for CO and H2 adsorption on Cu2O (1 1 1) surface: A density functional theory study

To investigate solvent effects, CO and H₂ adsorption on Cu₂O (1 1 1) surface in vacuum, liquid paraffin, methanol and water are studied by using density functional theory (DFT) combined with the conductor-like solvent model (COSMO). When H₂ and CO adsorb on Cu_cus of Cu₂O (1 1 1) surface, solvent effects can improve CO and H₂ activation. The H-H bond increases with dielectric constant increasing as H₂ adsorption on O_suf of Cu₂O (1 1 1) surface, and the H-H bond breaks in methanol and water. It is also found that both the structural parameters and Mulliken charges are very sensitive to the COSMO solvent model. In summary, the solvent effects have obvious influence on the clean surface of Cu₂O (1 1 1) and the adsorptive behavior.     

Active oxidation of Cu3Au(1 1 0) using hyperthermal O2 molecular beam

Oxidation of Cu₃Au(1 1 0) using a hyperthermal O₂ molecular beam (HOMB) was investigated by X-ray photoemission spectroscopy in conjunction with a synchrotron light source. From the incident energy dependence of the O-uptake curve, the precursor-mediated dissociative adsorption occurs, where the trapped O₂ molecule can migrate and dissociate at the lower activation-barrier sites, dominantly at thermal O₂ exposures. Dissociative adsorption of O₂ on Cu₃Au(1 1 0) is as effective at the thermal O₂ exposure as on Cu(1 1 0). On the other hand, at the incident energies of HOMB where the direct dissociative adsorption is dominant, it was determined that the dissociative adsorption of O₂ implies a higher activation barrier and therefore less reactivity due to the Au alloying in comparison with the HOMB oxidation of Cu(1 1 0). The dissociative adsorption progresses with the Cu segregation on Cu₃Au(1 1 0) similarly as on Cu₃Au(1 0 0). The growth of Cu₂O for 2 eV HOMB suggests that the diffusion of Cu atoms also contribute to the oxidation process through the open face, which makes the difference from Cu₃Au(1 0 0).     

Theoretical studies on the adsorption and decomposition of H2O on Pd(1 1 1) surface

To provide information about the chemistry of water on Pd surfaces, we performed density functional slab model studies on water adsorption and decomposition at Pd(1 1 1) surface. We located transition states of a series of elementary steps and calculated activation energies and rate constants with and without quantum tunneling effect included. Water was found to weakly bind to the Pd surface. Co-adsorbed species OH and O that are derivable from H₂O stabilize the adsorbed water molecules via formation of hydrogen bonds. On the clean surface, the favorable sites are top and bridge for H₂O and OH, respectively. Calculated kinetic parameters indicate that dehydrogenation of water is unlikely on the clean regular Pd(1 1 1) surface. The barrier for the hydrogen abstraction of H₂O at the OH covered surface is approximately 0.2-0.3 eV higher than the value at the clean surface. Similar trend is computed for the hydroxyl group dissociation at H₂O or O covered surfaces. In contrast, the O-H bond breaking of water on oxygen covered Pd surfaces, H₂O_ad + O_ad → 2OH_ad, is predicted to be likely with a barrier of ∼0.3 eV. The reverse reaction, 2OH_ad → H₂O_ad + O_ad, is also found to be very feasible with a barrier of ∼0.1 eV. These results show that on oxygen-covered surfaces production of hydroxyl species is highly likely, supporting previous experimental findings.     

Formamide reactions on rutile TiO2(0 1 1) surface

The reaction of formamide over the (0 1 1) faceted TiO₂(0 0 1) surface has been studied by Temperature Programmed Desorption (TPD) and X-ray Photoelectron Spectroscopy (XPS). Two main reactions were observed: dehydration to HCN and H₂O and decomposition to NH₃ and CO. The dehydration reaction was found to be three to four times larger than the decomposition at all coverages. Each of these reactions is found to occur in two temperature domains which are dependent upon surface coverage. The low temperature pathway (at about 400 K) is largely insensitive to surface coverage while the high temperature pathway (at about 500 K) shifts to lower temperatures with increasing surface coverage. These two temperature pathways may indicate two adsorption modes of formamide: molecular (via an η¹(O) mode of adsorption) and dissociative (via an η²(O,N) mode of adsorption). C1s and N1s XPS scans indicated the presence of multiple species after formamide absorption at 300 K. These occurred at ca. 288.5 eV (-CONH-) and 285 eV (sp³/sp² C) for the C1s and 400 eV-(NH₂), 398 eV (-NH) and 396 eV (N) for the N1s and result from further reaction of formamide with the surface.     

Initial stages of oxidation of Cu(1 1 1)

Several surface analysis techniques were combined to study the initial stages of oxidation of Cu(1 1 1) surfaces exposed to O₂ at low pressure (<5 × 10⁻⁶ mbar) and room temperature. Scanning tunneling microscopy (STM) results show that the reactivity is governed by the restructuring of the Cu(1 1 1) surface. On the terraces, oxygen dissociative adsorption leads to the formation of isolated O adatoms and clusters weakly bound to the surface. The O adatoms are located in the fcc threefold hollow sites of the unrestructured terraces. Friedel oscillations with an amplitude lower than 5 pm have been measured around the adatoms. At step edges, surface restructuring is initiated and leads to the nucleation and growth of a two-dimensional disordered layer of oxide precursor. The electronic structure of this oxide layer is characterised by a band gap measured by scanning tunneling spectroscopy to be ∼1.5 eV wide. The growth of the oxide islands progresses by consumption of the upper metal terraces to form triangular indents. The extraction of the Cu atoms at this interface generates a preferential orientation of the interface along the close-packed directions of the metal. A second growth front corresponds to the step edges of the oxide islands and progresses above the lower metal terraces. This is where the excess Cu atoms extracted at the first growth front are incorporated. STM shows that the growing disordered oxide layer consists of units of hexagonal structure with a first nearest neighbour distance characteristic of a relaxed Cu-Cu distance (∼0.3 nm), consistent with local Cu₂O(1 1 1)-like elements. Exposure at 300 °C is necessary to form an ordered two-dimensional layer of oxide precursor. It forms the so-called “29” superstructure assigned to a periodic distorted Cu₂O(1 1 1)-like structure.     

Driving force for the WO3(0 0 1) surface relaxation

The optimized structure of the WO₃(0 0 1) surface with various types of termination ((1 × 1)O, (1 × 1)WO₂, and c(2 × 2)O) has been simulated using density functional theory with the Perdew-Wang 91 gradient corrected exchange-correlation functional. While the energy of bulk WO₃ depends weakly on the distortions and tilting of the WO₆ octahedra, relaxation of the (0 0 1) surface results in a significant decrease of surface energy (from 10.2 × 10⁻² eV/Ų for the cubic ReO₃-like, c(2 × 2)O-terminated surface to 2.2 × 10⁻² eV/Ų for the relaxed surface). This feature illustrates a potential role of surface relaxation in formation of crystalline nano-size clusters of WO₃. The surface relaxation is accompanied by a dramatic redistribution of the density of states near the Fermi level, in particular a transformation of surface electronic states. This redistribution is responsible for the decrease of electronic energy and therefore is suggested to be the driving force for surface relaxation of the WO₃(0 0 1) surface and, presumably, similar surfaces of other transition metal oxides.     

Adsorption of water on TiN (1 0 0), (1 1 0) and (1 1 1) surfaces: A first-principles study

We use first-principles density functional theory-based calculations in the analysis of the interaction of H₂O with (1 0 0), (1 1 0) and (1 1 1) surfaces of TiN, and develop understanding in terms of surface energies, polarity of the surface and chemistry of the cation, through comparison with H₂O adsorption on ZrN. While water molecule physisorbs preferentially at Ti site of (1 0 0) and (1 1 1) surfaces, it adsorbs dissociatively on (1 1 0) surface of TiN with binding stronger than almost 1.32 eV/molecule. Our analysis reveals the following general trends: (a) surfaces with higher energies typically lead to stronger adsorption, (b) dissociative adsorption of H₂O necessarily occurs on a charge neutral high energy surface and (c) lower symmetry of the (1 1 0) plane results in many configurations of comparable stability, as opposed to the higher symmetry (1 0 0) and (1 1 1) surfaces, which also consistently explain the results of H₂O adsorption on MgO available in literature. Finally, weaker adsorption of H₂O on TiN than on ZrN can be rationalized in terms of greater chemical stability of Ti arising from its ability to be in mixed valence.     

In situ XPS study of Pd(1 1 1) oxidation. Part 1: 2D oxide formation in 10 mbar O2

The oxidation of the Pd(1 1 1) surface was studied by in situ XPS during heating and cooling in 3 × 10⁻³ mbar O₂. A number of adsorbed/dissolved oxygen species were identified by in situ XPS, such as the two dimensional surface oxide (Pd₅O₄), the supersaturated O_ads layer, dissolved oxygen and the R 12.2° surface structure.Exposure of the Pd(1 1 1) single crystal to 3 × 10⁻³ mbar O₂ at 425 K led to formation of the 2D oxide phase, which was in equilibrium with a supersaturated O_ads layer. The supersaturated O_ads layer was characterized by the O 1s core level peak at 530.37 eV. The 2D oxide, Pd₅O₄, was characterized by two O 1s components at 528.92 eV and 529.52 eV and by two oxygen-induced Pd 3d_5/2 components at 335.5 eV and 336.24 eV. During heating in 3 × 10⁻³ mbar O₂ the supersaturated O_ads layer disappeared whereas the fraction of the surface covered with the 2D oxide grew. The surface was completely covered with the 2D oxide between 600 K and 655 K. Depth profiling by photon energy variation confirmed the surface nature of the 2D oxide. The 2D oxide decomposed completely above 717 K. Diffusion of oxygen in the palladium bulk occurred at these temperatures. A substantial oxygen signal assigned to the dissolved species was detected even at 923 K. The dissolved oxygen was characterised by the O 1s core level peak at 528.98 eV. The “bulk” nature of the dissolved oxygen species was verified by depth profiling.During cooling in 3 × 10⁻³ mbar O₂, the oxidised Pd²⁺ species appeared at 788 K whereas the 2D oxide decomposed at 717 K during heating. The surface oxidised states exhibited an inverse hysteresis. The oxidised palladium state observed during cooling was assigned to a new oxide phase, probably the R 12.2° structure.     

The electronic structure and reactivity of the oxygen-modified Mo2C(0 0 0 1) surface

Oxygen adsorption on Mo₂C(0 0 0 1) has been investigated with angle-resolved photoemission spectroscopy (ARPES). When the surface is reacted with O₂, the O 2p-induced states are formed at 4.1 and 5.3 eV at the point. The emissions around the Fermi level are also intensified by oxygen adsorption, which is due to the formation of a partially filled state. It is found that the reactivity of the surface toward H₂O adsorption is much enhanced by pre-adsorption of oxygen. The reactivity is found to be maximized at θ_O ∼ 0.2.     

Reaction of O2 with the boron-terminated TaB2(0 0 0 1) surface

X-ray photoelectron spectroscopy has been used to study the clean TaB₂(0 0 0 1) surface and its reaction with O₂. In agreement with previous studies, XPS indicates that the clean surface is boron terminated. The topmost boron layer shows a chemically shifted B 1s peak at 187.1 eV compared to a B 1s peak at 188.6 eV for boron layers below the surface. The 187.1-188.6 eV peak intensity ratio and its variation with angle between the crystal normal and the detector is well described by a simple theoretical model based on an independently calculated electron inelastic mean free path of 15.7 Å for TaB₂. The dissociative sticking probability of O₂ on the boron-terminated TaB₂(0 0 0 1) surface is lower by a factor of 10⁴ than for the metal-terminated HfB₂(0 0 0 1) surface.     

Formation of oxygen vacancies on the TiO2(1 1 0) surfaces

Density functional theory was employed to investigate the formation and properties of the oxygen vacancies on the rutile TiO₂(1 1 0) surface. It is found that the formation of the positively charged bridging-oxygen vacancy (BOV⁺, 4.2 eV) is the most favored one, followed by the positively charged in-plane-oxygen vacancy (POV⁺, 4.5 eV). In contrast, the formation of the neutral bridging-oxygen and in-plane-oxygen vacancies (BOV and POV), and their dication oxygen vacancies (BOV²⁺ and POV²⁺) needs much higher energies (7.9 and 8.3 vs. 8.1 and 8.6 eV), respectively.     

A DFT study on the formation of CH3O on Cu2O(1 1 1) surface by CH3OH decomposition in the absence or presence of oxygen

The formation mechanism of CH₃O by the adsorption and decomposition of CH₃OH on clean and oxygen-precovered Cu₂O(1 1 1) surface has been investigated with density functional theory method together with the periodic slab models. Two possible formation pathways of CH₃O by CH₃OH decomposition on oxygen-precovered (O_pre) Cu₂O(1 1 1) surface were proposed and discussed. One is the O-H bond-cleavage of CH₃OH with H migration to O_pre to form CH₃O; the other is the C-O bond-scission of CH₃OH with CH₃ migration to O_pre leading to CH₃O_pre. The calculated results show that the O-H bond-breaking path has the lowest activation barrier 26.8 kJ mol⁻¹, the presence of oxygen-precovered on Cu₂O(1 1 1) surface exhibits a high surface reactivity toward the formation of CH₃O by the O-H bond-cleavage of CH₃OH, and reduce the activation barrier of O-H bond-cleavage. The C-O bond-breaking path was inhibited by dynamics, suggesting that the O atom of CH₃O is not from the oxygen-precovered, but comes from the O of CH₃OH. Meanwhile, the calculated results give a clear illustration about the formation mechanism of CH₃O in the presence of oxygen and the role of oxygen at the microscopic level.