Polar ZnO Surfaces Research Articles

Activity of ZnO polar surfaces: an insight from surface energies.

The calculation of the accurate surface energies for (0001) surfaces of wurtzite ZnO is difficult because it is impossible to decouple the two inequivalent (0001)-Zn and (0001¯)-O surfaces. By using a heterojunction model we have transformed the uncertainty of the surface energies into that of interface energies which is much smaller than the former and hence estimated the surface energies to a high degree of accuracy. It is found that the oxygen terminated (0001¯)-O face of the wurtzite phase and (1¯1¯1¯O of the zinc blende phase are more stable than their Zn-terminated counterparts within the major temperature and oxygen partial pressure range accessible to experiment. The instability of Zn-terminated polar surfaces explains the experimentally observed high activity of these surfaces. The effects of native surface vacancies on the surface energies have also been discussed. These results provide insights into the modification of the surface stability and activity of ZnO nanoparticles.

Revealing Al evaporation-assisted functions in solution-processed ZnO thin film transistors

Impregnated Al atoms inside the ZnO layer form Al–O bonds with oxygen ions and may reduce the concentration of weakly bonded oxygen as well as the space charge region, significantly reducing the polar ZnO surface.

Stability of polar ZnO surfaces studied by pair potential method and local energy density method

The polar ZnO surfaces have received wide interests due to their higher activity than the nonpolar facets in catalysis, photo-catalysis and gas sensitivity. However, the theoretical study on the relative stability of the polar ZnO surfaces is still limited. In this work, two different methods were used to calculate the surface energy of the polar ZnO(0001)–Zn and Zn(000-1)–O surfaces. The empirical pair potential method shows that the ZnO(000-1)–O terminal is more stable than the ZnO(0001)–Zn terminal because the polarizability of surface O2− is higher than that of surface Zn2+, which is in good agreement with the experimental results. However, the classic local energy density method predicts a higher stability of the ZnO(0001)–Zn terminal. The overestimation of the stability of the ZnO(0001)–Zn terminal originates from more distribution of the transferred charge to the ZnO(0001)–Zn terminal as the electron acceptor. We propose a hybrid method to fairly redistribute the contribution of the transferred charge to electron donor and electron acceptor and make the same stability trend with the experimental studies.

Near-surface structure of polar ZnO surfaces prepared by pulsed laser deposition

Polarity-controlled homoepitaxial ZnO films were prepared in order to investigate the near-surface and the electronic structures of ZnO films with Zn-polar (0001) and O-polar 0001¯ surfaces. Neutral atom scattering spectroscopy revealed that the near surface atomic structure and the polarity of the single crystal substrate with a step and terrace structure was maintained even at the surface of the epitaxial films through substrate-mediated growth without a defect structure. X-ray photoelectron spectroscopy study also revealed that the surfaces of ZnO films and single crystal substrates showed similar band bending regardless of the surface polarity. The state of surface band bending is discussed and compared with other reports regarding ZnO and GaN.

A first-principles study of ZnO polar surface growth: Adsorption of ZnxOy clusters

Adsorption of Zn(x)O(y) (x + y = 1-6) clusters on ZnO (000 ± 1) polar surfaces is studied systematically via density function theory (DFT) calculations. Different adsorption behaviors are predicted for these two surfaces. On the (0001)-Zn surface, O atoms adsorb on hollow sites at the initial stage. Then Zn atoms come in, and the stable structure becomes bulk-like for some specific clusters. On the (0001)-O surface, Zn cluster adsorption leads to stable cage structures formed by pulling substrate O out. In clusters with both Zn and O, O atoms avoid directly bonding with the surface, and no energetically favorable bulk-like structure is found. On the basis of the prediction of these surface adsorption behaviors, experimentally observed growth rate and surface roughness differences on these two polar surfaces can be understood.

ChemInform Abstract: Methanol Synthesis on ZnO from Molecular Dynamics

This paper reviews our efforts to simulate methanol synthesis from CO and H2 on defective ZnO surfaces using advanced molecular dynamics techniques. This apparently simple chemical reaction occurring on a seemingly well-defined surface appears to be astonishingly complex. First of all, the preferred oxidation state of F centers at the polar oxygen terminated surface is found to be dictated by the chemical composition and the thermodynamic properties of the gas phase in contact with . Secondly, reaction intermediates and pathways along the catalytic cycle taking place at or close to these defects are found to depend in a sensitive way on their oxidation state. Thirdly, it is seen that the gas phase close to the catalytic surface might be transiently involved in some of the reaction steps in a non-trivial manner. Last but not least, the scenario is found to be greatly enriched upon involving copper clusters on polar ZnO surfaces in view of utmost strong metal-support interactions (SMSIs), which are directly related to the polar nature of . Taken together, an unexpectedly rich picture is unveiled by the molecular dynamics approach to computational heterogeneous catalysis when applied to methanol synthesis on bare ZnO.

Structural and electronic properties of graphene–ZnO interfaces: dispersion-corrected density functional theory investigations

Detailed first-principles computations were performed on the geometric and electronic properties of the interfaces between graphene and ZnO polar surfaces. A notable van der Waals force exists at the interface, and charge transfer occurs between graphene and ZnO as a result of the difference in their work functions. The Dirac point of graphene remains intact despite its adsorption on ZnO, implying that its interaction with ZnO does not affect the superior conductivity of graphene. Excited electrons within the energy range of 0–3 eV (versus Fermi energy) in the hybrid systems are mainly accumulated on graphene. The computations provide a theoretical explanation for the good performance of graphene/ZnO hybrid materials in photocatalysts and solar cells.

Density functional theory (DFT) study of Zn, O2 and O adsorption on polar ZnO(0001) and ZnO (0001) surfaces

Ab initio DFT simulations of Zn and O atoms and O2 molecules adsorption on polar 2H–ZnO(0001) and 2H–ZnO(0001) surfaces elucidated the principal processes, important for growth of zinc oxide from the vapour. It was shown that a zinc atom is adsorbed at both ZnO surfaces without any energy barrier but with ultimately different adsorption energies: 0.34eV for the metallic and 3.37 for the non-metallic surface. Oxygen atoms are attached very strongly at both polar surfaces, with energies equal to 5.47eV and 2.47eV. The difference between both polar surfaces is the largest for adsorption of molecular oxygen, the O2 molecule is adsorbed on the Zn-face with the energy 2.45eV while it is not adsorbed at the oxygen face of zinc oxide.

Methanol synthesis on ZnO from molecular dynamics

AbstractThis paper reviews our efforts to simulate methanol synthesis from CO and H2 on defective ZnO surfaces using advanced molecular dynamics techniques. This apparently simple chemical reaction occurring on a seemingly well‐defined surface appears to be astonishingly complex. First of all, the preferred oxidation state of F centers at the polar oxygen terminated surface is found to be dictated by the chemical composition and the thermodynamic properties of the gas phase in contact with ${\rm ZnO}(000\overline {1} )$. Secondly, reaction intermediates and pathways along the catalytic cycle taking place at or close to these defects are found to depend in a sensitive way on their oxidation state. Thirdly, it is seen that the gas phase close to the catalytic surface might be transiently involved in some of the reaction steps in a non‐trivial manner. Last but not least, the scenario is found to be greatly enriched upon involving copper clusters on polar ZnO surfaces in view of utmost strong metal‐support interactions (SMSIs), which are directly related to the polar nature of ${\rm ZnO}(000\overline {1} )$. Taken together, an unexpectedly rich picture is unveiled by the molecular dynamics approach to computational heterogeneous catalysis when applied to methanol synthesis on bare ZnO.

Effect of surface reconstruction on the electronic structure of ZnO(0001)

Density functional calculations are presented for a (4$\ifmmode\times\else\texttimes\fi{}$4) reconstruction of the polar ZnO(0001) surface which is, in agreement with previous knowledge, characterized by one ZnO double-layer high triangular islands or pits. Its structure and electronic properties are compared to those of other reconstructions, the (1$\ifmmode\times\else\texttimes\fi{}$1) and the (2$\ifmmode\times\else\texttimes\fi{}$2) reconstructions with a Zn vacancy, and a (4$\ifmmode\times\else\texttimes\fi{}$4) reconstruction with triangular islands and pits that satisfies the electron counting rule. Among the four calculated reconstructions, the model surface is verified to be energetically preferable for a large range of oxygen chemical potential. This justifies its further exploration. Its layer and wave-vector-resolved projected density of states (PDOS) of the O atoms reveals highly surface-localized valence states well below the Fermi level. The PDOS around the Fermi level, however, is very small, which is compatible with the states not being observed by photoemission. Qualitatively experimental surface core-level shifts by Lahiri et al. [Phys. Rev. B 78, 155414 (2008)] compare favorably with the calculated electrostatic potential variations close to the ZnO(0001) surface. An ionic model with everywhere equally charged Zn and O ions turns out not to explain the potential variation obtained from density functional theory. This points towards an additional charge transfer at the surface not covered by the ionic model.

Vibrational spectroscopic studies on pure and metal‐covered metal oxide surfaces

AbstractMetal oxides and metal nanoparticles dispersed on oxide substrates have gained increasing interest in surface science because of their widespread applications, especially in heterogeneous catalysis. In this review we summarize our recent vibrational spectroscopic studies on pure and metal‐covered oxide surfaces (ZnO, TiO2, Au/ZnO and Cu/ZnO) using a number of small molecules (H2, CO, CO2, NO and HCOOH) as probes and/or reactants. High‐resolution electron energy loss spectroscopy (HREELS) turned out to be a powerful tool to investigate well‐defined oxide and metal/oxide model systems. The application of a novel ultrahigh vacuum IR spectroscopy (UHV‐FTIRS) apparatus allowed us to record high‐quality IR data on oxide surfaces of both single crystals and polycrystalline powder particles. We will particularly focus on following important issues: (i) the interaction of hydrogen with ZnO; (ii) structure and reactivity of polar and nonpolar ZnO surfaces; (iii) the role of defects in surface chemistry of oxides; (iv) the origin of significant difference in photocatalytic activity between anatase and rutile TiO2 and (v) the interaction between metal nanoparticles and oxide supports. We will demonstrate that the data from HREELS and UHV‐FTIRS provide detailed insight into structural, electronic and chemical properties of the studied systems.

First-principles investigation of the size-dependent structural stability and electronic properties of O-vacancies at the ZnO polar and non-polar surfaces

In this paper, all electron full-potential linearized augmented plane wave plus local orbitals method has been used to investigate the structural and electronic properties of polar (0001) and non-polar (101¯0) surfaces of ZnO in terms of the defect formation energy (DFE), charge density, and electronic band structure with the supercell-slab (SS) models. Our calculations support the size-dependent structural phase transformation of wurzite lattice to graphite-like structure which is a result of the termination of hexagonal ZnO at the (0001) basal plane, when the stacking of ZnO primitive cell along the hexagonal principle c-axis is less than 16 atomic layers of Zn and O atoms. This structural phase transformation has been studied in terms of Coulomb energy, nature of the bond, energy due to macroscopic electric field in the [0001] direction, and the surface to volume ratio for the smaller SS. We show that the size-dependent phase transformation is completely absent for surfaces with a non-basal plane termination, and the resulting structure is less stable. Similarly, elimination of this size-dependent graphite-like structural phase transformation also occurs on the creation of O-vacancy which is investigated in terms of Coulomb attraction at the surface. Furthermore, the DFE at the (101¯0)/(1¯010) and (0001)/(0001¯) surfaces is correlated with the slab-like structures elongation in the hexagonal a- and c-axis. Electronic structure of the neutral O-vacancy at the (0001)/(0001¯) surfaces has been calculated and the effect of charge transfer between the two sides of the polar surfaces (0001)/(0001¯) on the mixing of conduction band through the 4s orbitals of the surface Zn atoms is elaborated. An insulating band structure profile for the non-polar (101¯0)/(1¯010) surfaces and for the smaller polar (0001)/(0001¯) SS without O-vacancy is also discussed. The results in this paper will be useful for the tuning of the structural and electronic properties of the (0001) and (101¯0) ZnO nanosheets by varying their size.

Fine dispersion and self-assembly of ZnO nanoparticles driven by P3HT-b-PEO diblocks for improvement of hybrid solar cells performance

The rod-coil conjugated diblock copolymers of poly(3-hexylthiophene)-b-poly(ethylene oxide) (P3HT-b-PEO), acting as an electron donor, were blended with ZnO nanoparticles to fabricate the hybrid bulk-heterojunction (BHJ) solar cells. According to the ultraviolet-visible (UV-vis) absorption spectroscopy, the intensity at 610 nm, derived from a strong inter-molecular interaction of π–π stacking and the high crystallizability of the P3HT main chains, was higher in P3HT-b-PEO/ZnO blend films than that in P3HT/ZnO blend films especially after thermal treatment, revealing that PEO segments could make P3HT form more densely stacked and orderly structured. Due to the nature of block copolymers and the interaction between the oxygen atoms of the PEO chains and the ZnO polar surface, the surface defects of ZnO were passivated and the fine dispersion and self-assembly of ZnO in the polymer matrix driven by the P3HT-b-PEO diblock copolymer was obtained, leading to the improvement of device power conversion efficiency and thermal stability. Overall, this work demonstrated that the application of conjugated block copolymers in hybrid BHJ solar cells was a promising approach to improve the device performance and thermal stability.

Structure, Electrical and Optical Properties of the Polar ZnO(0001) Surfaces

Based on the density functional theory, using the first-principles calculations method, the geometrical structure, electronic structure and optical properties of the ZnO(0001) surface were investigated. The calculated results show that a great change appears in the structure after the surface relaxation. The new energy levels were found in the band structure. The conductivity of the ZnO (0001) surface becomes stronger, showing the electrical properties of metal. On the other hand, imaginary part of the dielectric function of the ZnO(0001) surface also changes certainly. A new peak is observed in the low energy region. The results provide a theoretical basis for photoelectric device manufacturing and further development on the ZnO surface.

Self-assembled CdS/Au/ZnO heterostructure induced by surface polar charges for efficient photocatalytic hydrogen evolution

A ternary heterostructure of CdS/Au/ZnO, where a core–shell structure of Au/CdS is selectively deposited on the polar surface of ZnO induced by the surface polar charges of ZnO crystals, is prepared through a two-step self-assembly process. The resultant heterostructure shows an activity 4.5 times higher in photocatalytic hydrogen evolution from water containing Na2S/Na2SO3 electron donors than the binary CdS/ZnO heterostructure as a result of the effective vectorial Z-scheme transfer of photogenerated charge carriers between ZnO and CdS mediated by Au nanoparticles at the interface.

Formation mechanism of homo-epitaxial morphology on ZnO (000 ± 1) polar surfaces

The formation mechanism of experimentally observed epitaxial morphologies on (000 ± 1) polar surfaces of ZnO microwire was studied by density functional theory simulation of atomic step models on both polar surfaces. In situ observations by environmental scanning electron microscopy were employed to control and detect the morphology evolutions during the epitaxial growth process. Edge formation energies for representative step models were calculated as a function of Zn chemical potential. The energetically favorable step structures were determined under different conditions, which could be related to certain surface morphologies regarding the aspects of crystallography and growth kinetics. Our experiments can be well explained by theoretical simulations.

Adsorption of oxygen atom on Zn-terminated (0001) surface of wurtzite ZnO: A density-functional theory investigation

Using first-principles density-functional theory, the structures, stabilities, and electronic structures of the clean and oxygen atom adsorbed Zn-terminated (0001) polar surface of wurtzite ZnO have been systematically investigated. The clean polar surface exhibits n-type conductivity. The calculations of the adsorption energy indicate that the hollow (H) site at the hexagon center of the ZnO surface is the most stable site for the adsorption of oxygen atom. The adsorption of oxygen atom is an exothermic process, and the adsorption is chemisorption. After the adsorption of oxygen atom, the work function of the ZnO surface increases. This indicates that the adsorbed oxygen atom is expected to withdraw electrons from the surface and there is charge transfer from the surface to the adsorbed oxygen atom, which will cause the increase of the resistance. For the ZnO surface with adsorbed oxygen atom at the H site, the work function is the lowest. Furthermore, the modifications of the electronic structures of ZnO surface by the adsorption of oxygen atom have been investigated in terms of the electron density difference and density of states. The results indicate that there are ionic bonds formed between the surface zinc and adsorbed oxygen atoms. After the adsorption of oxygen atom, the Fermi level shifts down and the n-type conduction characteristic is significantly weakened.

CO2 adsorption on polar surfaces of ZnO

Physical and chemical adsorption of CO2 on ZnO surfaces were studied by means of two different implementations of periodic density functional theory. Adsorption energies were computed and compared to values in the literature. In particular, it was found that the calculated equilibrium structure and internuclear distances are in agreement with previous work. CO2 adsorption was analyzed by inspection of the density of states and electron localization function. Valence bands, band gap and final states of adsorbed CO2 were investigated and the effect of atomic displacements analyzed. The partial density of states (PDOS) of chemical adsorption of CO2 on the ZnO(0001) surface show that the p orbitals of CO2 were mixed with the ZnO valence band state appearing at the top of the valence band and in regions of low-energy conduction band.

Surface effects on the piezoelectricity of ZnO nanowires

We utilize classical molecular dynamics to study surface effects on the piezoelectric properties of ZnO nanowires as calculated under uniaxial loading. An important point to our work is that we have utilized two types of surface treatments, those of charge compensation and surface passivation, to eliminate the polarization divergence that otherwise occurs due to the polar (0001) surfaces of ZnO. In doing so, we find that if appropriate surface treatments are utilized, the elastic modulus and the piezoelectric properties for ZnO nanowires having a variety of axial and surface orientations are all reduced as compared to the bulk value as a result of polarization reduction in the polar [0001] direction. The reduction in effective piezoelectric constant is found to be independent of the expansion or contraction of the polar (0001) surface in response to surface stresses. Instead, the surface polarization and thus effective piezoelectric constant is substantially reduced due to a reduction in the bond length of the Zn–O dimer closest to the polar (0001) surface. Furthermore, depending on the nanowire axial orientation, we find in the absence of surface treatment that the piezoelectric properties of ZnO are either effectively lost due to unphysical transformations from the wurtzite to non-piezoelectric d-BCT phases, or also become smaller with decreasing nanowire size. The overall implication of this study is that if enhancement of the piezoelectric properties of ZnO is desired, then continued miniaturization of square or nearly square cross-section ZnO wires to the nanometer scale is not likely to achieve this result.

Naturally asymmetrical double-Schottky barrier model: Based on observation of bicrystal

ZnO bicrystals with specific coincidence site lattice twist boundaries are manufactured for investigation. Asymmetrical I-V behaviors occur with maximum nonlinear coefficient of forward direction tenfold of reverse one. Energy dispersion spectra measurement reveals the nonuniform distributions of elements such as bismuth and oxygen in the grain boundary region. This reflects a higher Schottky barrier formed at Zn-polar face than that at O-polar face, which is confirmed by capacitance-voltage method. Such natural asymmetry may result from the effect of spontaneous polarization of polar surfaces of ZnO. Finally, simulation results prove the validity of the asymmetrical model proposed.