Crystallographic shear of polymorphic TiO<Subscript>2</Subscript> nanocondensates with enhanced Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> dissolution via pulsed laser ablation

TiO₂ nanoparticles with enhanced solid solution of Cr up to 16 wt% in polymorphs of rutile, anatase, brookite, α-PbO₂-type, and occasionally baddeleyite-type were synthesized via pulse laser ablation on ceramic TiO₂ target dissolved with Cr₂O₃ or clamped Cr/Ti plates in air. Analytical electron microscopic observations indicated these nanocondensates have prevalent crystallographic shear (CS) along specific planes to form superstructures. The rutile type typically shows (100) and (010) CS besides the conventional ones rotating about the [111] zone axis as reported for ambient samples. The CS planes are parallel to (001) for anatase, (001) and ([`1] \overline{1} 10) for brookite, whereas (001) and {1[`3] \overline{3} 1} for the α-PbO₂-type TiO₂ with varied extent of Cr dissolution. Surface modification, as a result of Cr dissolution and/or internal stress, was observed for all the polymorphs.     

铂修饰的{001}面暴露的二氧化钛纳米片阵列的制备和光催化性能

采用有机溶剂热法在FTO衬底上制备{001}面暴露的单晶锐钛矿相TiO₂纳米片阵列,通过FESEM和XRD研究样品的形貌和晶体结构. 与水热法制备的纳米片阵列相比,有机溶剂热法制备的样品取向性更好. 采用光沉积方法在纳米片阵列上沉积Pt,所得到的Pt纳米颗粒粒径更为均匀,并且更容易沉积在{001}面上. 所负载的Pt 纳米颗粒增强了TiO₂纳米片的光吸收性能,同时大大减弱了光致发光强度. 在光催化性能测试中,具有最优负载量的样品催化性能提高了一倍. 与传统的Pt负载相比,{001}面的最优负载量显得相当小,这可能源于高活性{001}面的原子结构.     

Orientation dependence of structural transition in fcc Al driven under uniaxial compression by atomistic simulations

By molecular dynamics simulations employing an embedded atom method potential,we have investigated structural transformations in single crystal Al caused by uniaxial strain loading along the [001],[011] and [111] directions. We find that the structural transition is strongly dependent on the crystal orientations. The entire structure phase transition only occurs when loading along the [001] direction,and the increased amplitude of temperature for [001] loading is evidently lower than that for other orientations. The morphology evolutions of the structural transition for [011] and [111] loadings are analysed in detail. The results indicate that only 20% of atoms transit to the hcp phase for [011] and [111] loadings,and the appearance of the hcp phase is due to the partial dislocation moving forward on {111} fcc family. For [011] loading,the hcp phase grows to form laminar morphology in four planes,which belong to the {111} fcc family; while for [111] loading,the hcp phase grows into a laminar structure in three planes,which belong to the {111} fcc family except for the (111) plane. In addition,the phase transition is evaluated by using the radial distribution functions.     

The stability and structure of high miller index platinum crystal surfaces in vacuum and in the presence of adsorbed carbon and oxygen

The surface structures of twenty-two high Miller Index crystal faces of platinum were studied in ultrahigh vacuum (uhv) when clean and in the presence of a monolayer of chemisorbed oxygen or carbon by low-energy electron diffraction (LEED). Besides the low Miller index planes [the (001), (011), and (111)], only a few of the surfaces [the (112), (113), (133), (122) and (012)] were stable under all conditions of the experiments. The stable surfaces are characterized by a very high density of periodic steps of one atom in height or a complete lack of steps. The other platinum crystal faces restructure as the surface composition is changed. Some of the surface structures that are stable in uhv and in oxygen reconstruct in the presence of carbon while others are stable when clean and when carbon covered but restructure when covered with oxygen. In addition to the one atom high step-terrace configuration there are atomically clean surface structures with multiple height steps and structures in “hill and valley” configuration consisting of large facet planes detectable by LEED. The implications to heterogeneous catalysis of the observed stability and restructuring of the various crystal planes in changing reaction conditions are discussed.     

The atomic structure of alloy surfaces: Ni3Al{001}

The atomic structure of the {001} surface of Ni₃Al has been determined by LEED (low-energy electron diffraction) intensity analysis to correspond to simple truncation of the bulk structure with the Ni-Al mixed layer on top rather than the pure Ni layer. The first interlayer spacing is essentially equal to the bulk interlayer spacing between {001} planes. First-principles calculations of the cohesive energies of slabs terminating in the two types of layers also indicate that the mixed layer termination is more stable.     

Ultra-thin film growth of titanium dioxide on W(1 0 0)

We have employed low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy to follow the epitaxial growth of thin films of TiO₂ on W(1 0 0). The films were grown both by metal vapour deposition of titanium onto the substrate in UHV with subsequent annealing in a low partial pressure of oxygen, and by metal vapour deposition in a low partial pressure of oxygen. LEED patterns showed the characteristic patterns of (1 1 0) oriented rutile. A systematic spot splitting was observed and attributed to a stepped surface. The calculated step height was found to be in good agreement with that expected for rutile TiO₂(1 1 0), 3.3 Å. Titanium core level shifts were used to identify oxidation states as a function of film thickness allowing the interpretation in terms of a slightly sub-stoichiometric interface layer in contact with the substrate. In combination with the LEED patterns, the film structure is therefore determined to be (1 1 0) oriented rutile with a comparable level of stoichiometry to UHV prepared bulk crystals. The ordered step structure indicates considerable structural complexity of the surface.     

The growth and structure of titanium dioxide films on a Re(1 0 −1 0) surface: Rutile(0 1 1)-(2 × 1)

Titanium dioxide films were grown on Re(1 0 −1 0) by Ti vapor deposition in oxygen at T = 830 K and studied by means of low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS) and X-ray diffraction (XRD). The Ti oxide stoichiometry was determined by XPS as Ti:O = 1:2, with the Ti oxidation state (4+). The TiO₂ growth was monitored by means of LEED as a function of film thickness. Extending the coverage from the submonolayer into the multilayer regime gives rise to a p(2 × 2) pattern, a (poorly ordered) (1 × 1), and, finally, a stable (2 × 2) structure, the latter being associated with a homogeneous TiO₂ phase. For normal electron incidence, the (2 × 2) LEED pattern exhibits systematically extinguished beams at (n ± 1/2, 0) positions, indicating a glide mirror plane. The pg(2 × 2) structure could be explained by both a rutile(0 1 1)-(2 × 1) reconstructed surface and a bulk truncated brookite(0 0 1) surface. Faceting phenomena, i.e. running LEED spots, observed with thin TiO₂ films point to the formation of a rutile(0 1 1)-(2 × 1) surface with two domains and {0 1 1}-(2 × 1) facets and rule out the brookite alternative. Confirmation of this assignment was obtained by an XRD analysis performed at the Berlin synchrotron facility BESSY.     

Twin Boundaries in Zinc Oxide with Additions of Gallium Oxide

Twin boundaries (TBs) in ZnO sintered with small additions of Ga₂O₃ have been characterized with advanced methods of transmission electron microscopy (TEM). The TBs and accompanying inversion domain boundaries are on {011¯3} planes of ZnO. The Ga content of the TB corresponds to an effectively half occupied {011¯3} plane determined from compositional maps calculated from electron spectroscopic images using electron filtering TEM. The structure of the TBs were investigated by high-resolution TEM, and images of focus series were used to reconstruct the complex electron wave. Simulated electron waves based on structure models of the TB were quantitatively compared with the reconstructed wave to identify and to refine atom positions. The twins can be considered to be created by a mirror operation on a {011¯3} plane of ZnO, and two alternating closed-packed polyhedral clusters of oxygen ions can be identified as building units of the TB structure. Unit 1 is occupied with Zn²⁺ by simply continuing ZnO₄ tetrahedra of the same type from both crystals to the TB. Using arguments of local charge balance unit 2 can only be occupied with the trivalent Ga³⁺ ion. The Ga³⁺ position was refined with high precision (±5 pm), and the resulting polyhedron is a GaO₅ square pyramid. The pyramids form densely occupied columns parallel to the twin axis [21¯1¯0]. The analysis of the TB structure yields a fractional occupancy of the boundary plane by Ga of 0.5, which is in good agreement with the result of the chemical composition measurement with energy filtered TEM.     

合成氨钌催化剂的晶体和形貌敏感性

哈伯-博施法合成氨反应是高温高压的耗能过程,因此降低该过程的能量消耗及开发温和条件下合成氨反应催化剂具有重要意义. 金属钌是合成氨反应中最有前途的催化剂之一,一直备受广泛关注. 确定金属钌催化剂的结构敏感性并提高其比质量活性是多相催化中亟待解决的重要问题. 氮气(N₂)活化是合成氨反应中的关键步骤. 本文通过第一性原理理论计算和微观动力学模拟方法系统研究了具有六方密排和面心立方晶体结构的钌催化剂上N₂活化过程和N₂解离反应速率. 理论计算研究表明,在六方密排Ru形貌中,{2130}晶面具有最高的N₂解离活性,其次是{0001}台阶面,它们比六方密排Ru其他表面上N₂解离反应速率高3个数量级以上;在面心立方Ru形貌中,{211}和{311}表面上N₂解离活性最高. 这些结果都表明台阶面/台阶位对氮气活化至关重要. 虽然六方密排Ru {2130}晶面具有最低的N₂解离能垒,然而由于面心立方Ru上可以暴露更高密度的活性位点,使得面心立方Ru比六方密排Ru具有更高的N₂转化速率. 本研究深入理解了N₂解离过程中,金属Ru 催化剂形貌和晶相结构敏感性,这为设计和优化高活性的合成氨Ru催化剂提供了理论基础.     

The initial oxidation of molybdenum: IV. Epitaxial growth of oxide on molybdenum

The oxide which grows in low oxygen pressure and at temperatures between 700 and 1000 K on molybdenum is shown to be MoO₂. The epitaxial relationships between the oxide and the metal (100), (110) and (111) surfaces are given. The epitaxial relationships of oxide on the molybdenum (100) and (110) surfaces are geometrically equivalent. The oxide grows on the (111) molybdenum surface with no major oxide plane parallel to the substrate. It is suggested that the epitaxy of MoO₂ on the (111) surface is a consequence of growth on {211} molybdenum facets. The atomic positions in the pairs of interfacial planes found are given. There is little agreement between the positions of ions in the oxide and substrate lattice sites. Only in the postulated case of MoO₂ on {211} Mo facets is a small misfit found.     

Field emission and field ion microscope study of Ga,In and Sn on W: Structure,work function,diffusion and binding energy

Gallium, indium and tin were deposited on a tungsten tip by making a contact between the tip and these metals in the liquid state. The activation energies of diffusion of the adsorbates on tungsten were found to be 0.29 eV for Ga, 0.35 eV for In and 0.71 eV for Sn. The adsorbates were field-evaporated by gradually increasing a positive tip voltage by a small increment each time and the variation of the work function with the decreasing coverage was examined for each evaporation stage. The result indicates that the adatoms assume one of two different adsorption states. The adatoms bound as strongly as in a bulk crystal were field-evaporated at a low evaporation field. The remaining adatoms form a more strongly bound covering layer which maximizes the average work function of the covered surfaces, 4.75 eV for Ga, 4.63 eV for In and 5.10 eV for Sn, and are field-evaporated at a significantly higher field. The covering layer of the strongly bound adatoms were observed on the areas from the {001} to {114} planes and were hardly noticed on the {011} and 112 areas. The arrangement of the strongly bound adatoms, particularly on the {114} planes, is found to be a precise replica of the substrate arrangement. Thus, the surface density of the adatoms is exactly the surface density of the substrate atoms. The observed results suggest that an adatom occupying a tungsten lattice site and contacting four substrate tungsten atoms can establish unusually strong bonding with the substrate.     

The influence of the oxygen exposure on the thermal faceting of W[1 1 1] tip

The oxygen induced faceting of the macroscopic W[1 1 1] tip has been studied for oxygen exposures in the range 0.5-31 L and annealing temperatures 800-1800 K using the field ion microscopy (FIM) technique. After annealing at temperatures lower than 800 K, higher than 1850 K or for exposures lower than 0.5 L faceting was not observed. For exposures 0.5-1.9 L and annealing temperatures 800-1600 K well developed {1 1 2} facets with sharp edges formed. For exposures higher than 2.0 L edges of the {1 1 2} facets were broadening and disappearing, what has been attributed to the formation of three-dimensional tungsten oxides. The oxides could be easily removed by annealing the tip at 1700 K, what leads to formation of sharp facet edges. On the basis of these results a modified procedure of the ultrasharp tip fabrication has been proposed.     

Grain-boundary faceting at a = 3, [110]/{112} grain boundary in a cubic zirconia bicrystal

The atomic structure of a = 3, [110]/{112} grain boundary in a yttria-stabilized cubic zirconia bicrystal has been investigated by high-resolution transmission electron microscopy (HRTEM). It was found that the grain boundary migrated to form periodic facets, although the bicrystal was initially joined so as to have the symmetric boundary plane of {112}. The faceted boundary planes were indexed as {111}/{115}. The structure of the {111}/{115} grain boundary was composed of an alternate array of two types of structure unit: {112}- and {111}-type structure units. HRTEM observations combined with lattice statics calculations verified that both crystals were relatively shifted by (α/4)[110] along the rotation axis to form a stable grain-boundary structure. A weak-beam dark-field image revealed that there was a periodic array of dislocations along the grain boundary. The grain-boundary dislocations were considered to be introduced by the slight misorientation from the perfect = 3 orientation. The fact that the periodicity of the facets corresponded to that of the grain-boundary dislocations must indicate that the introduction of the grain-boundary dislocations is closely related to the periodicity of the facets. An atomic flipping model has been proposed for the facet growth from the initial = 3, {112} grain boundary.     

High-resolution identification of ½⟨110⟩ stacking faults in epitaxial Ba0.3Sr0.7TiO3 thin films

The near-interface region of an epitaxial Ba_0.3Sr_0.7TiO₃ thin film grown on LaAlO₃ (001) was found to consist of a high density of ½?110? stacking faults bounded by partial dislocations. The stacking faults can extend over large distances (greater than 50 nm). Various possible atomic configurations of the faults were considered. The atomic structures of the faults were identified using high-resolution electron microscopy and simulation as well as energy-filtered imaging. The ½[101] and faults (where [001] is normal to the film plane) were found to lie predominately on the {100} and {110} planes. The ½lsqb;101] faults on (010), (110) or (1&1tilde;0) have never been observed before in perovskites. The stacking faults on [100] have a structure consisting of a double layer of edge-sharing TiO₆ octahedra. The excess of Ti was detected by energy-filtered imaging. The formation of the extended stacking faults is probably related to a small amount of excess Ti during the film deposition, which may originate from the non-stoichiometry of the ceramic targets BaTiO₃ and SrTiO₃. It is also enhanced by the misfit-induced compressive strain in the early stages of the film growth.     

CHaracterization of CO binding sites on Rh{111} and Rh{331} surfaces by XPS and LEED: Comparison to EELS results

Changes in the nature of the binding site of chemisorbed CO on the Rh{111} and Rh{331} single crystal surfaces during adsorption and desorption have been monitored by X-ray Photoelectron Spectroscopy (XPS) and Low Energy Electron Diffraction (LEED). Two bonding states of molecular CO have been identified from the O 1s photoemission line. These states are assigned as atop and bridge-bonded species and are observed to be coverage and temperature dependent. On both surfaces atop sites are populated first and at higher CO coverages bridge sites are filled. On Rh{111} the bridge sites are filled at a CO coverage of θ_CO ～ 0.50 and their presence is correlated with a change in the LEED pattern. The presence of the step atoms on the Rh{331} surface markedly influenced the sequential filling of binding sites in comparison to that observed on the Rh{111} surface. A comparison of our data to previous Electron Energy Loss Spectroscopy (EELS) work on Rh{111} is in remarkable quantitative agreement with EELS peak heights.     

The influence of sodium lauryl sulfate on the crystal phases of titania by hydrothermal method

In this paper, we prepared TiO₂ nanostructures by a hydrothermal method and investigated the influence of the $\mathrm{SO}_{4}^{2-}$ ion and the effect of long alkyl chains of sodium dodecyl sulfate on the crystal phases of TiO₂ by experiments and theoretical calculations. The results indicate that the absorption of the H+HSO₄ fragment on rutile (110) is more stable than that of the 2H+SO₄ fragment and more favorable to the formation of anatase. The absorption and steric effects of sodium dodecyl sulfate on the surfaces of TiO₂ grains also have an important influence on the formation of mixed crystals by changing the speed and the way of octahedral TiO₆ units combining. Based on the above facts, we revised the original reaction scheme for crystalline titania formation by previous authors.     

Structural properties of TiO2 nanocrystallites condensed in vapor-phase for photocatalyst applications

We have synthesized titanium dioxide (TiO₂) nanocrystallites by pulsed laser ablation (PLA) in oxygen (O₂) background gas for photocatalyst applications. Varying O₂ background gas pressure \( \left( {P_{{{\text{O}}_{ 2} }} } \right) \) or substrate target distance (D _TS), it was possible to change weight fraction of anatase phase in the anatase/rutile mixture from 0.2 to 1.0. Porosity of the deposited TiO₂ films increased with increasing \( \left( {P_{{{\text{O}}_{ 2} }} } \right) \) and D _TS. Relation between the process parameters and the formed crystal phases was explained from the point of cooling process in vapor-phase. Furthermore, rapid thermal annealing (RTA) was performed as post-annealing, suppressing sintering of the nanocrystallites. Photocatalytic activities of the TiO₂ nanocrystallites depended on the RTA temperature and following crystallinity restoring as well as the crystal phase: anatase or rutile.     

Surface structure of GaAs(211)

The atomic structures of the two inequivalent (211) surfaces of GaAs have been investigated by LEED. Both surfaces, prepared by etching and heat-cleaning or ion-sputtering and annealing, are unstable and develop large (110) facets which exhibit the atomic geometry of the (110) GaAs surface. These facets entirely cover the surface. Three sets of facets, making 30°, 30° and 54° angles with the (211) plane, are detected on one surface. Only two sets, making 30° angles with the (211) plane, are detected on the other. The reasons for this difference are not understood at this time. The LEED study of Si(211) and Ge(211) shows that the Si surface is flat whereas the Ge surface exhibits reconstructed (311) facets. The structural difference between the (211) surfaces of GaAs and Ge and the facetting of the compound are invoked to explain the problems encountered in the MBE growth of GaAs on Ge(211).     

Enhanced piezoelectric-induced catalysis of SrTiO3 nanocrystal with well-defined facets under ultrasonic vibration

Facet engineering of nanocomposite has been confirmed to be an efficient strategy to accelerate their catalytic performances, but to improve their piezoelectric catalytic activities by facet engineering has been seldom reported. Herein, we developed a series of SrTiO₃ nanocrystals with exposed {0 0 1} facet, dominant {1 1 0} facet and co-exposed {0 0 1} and {1 1 0} facets, respectively, and firstly revealed its piezoelectric catalytic performance under ultrasonic vibration. Moreover, the relationship between piezoelectric-induced catalytic activity and facet-dependence of SrTiO₃ nanocrystal was disclosed for the first time. The SrTiO₃ nanocrystal with co-exposed {0 0 1} and {1 1 0} facets exhibited effectively enhanced piezoelectric catalytic activity by degrading Rhodamine B (RhB) under ultrasonic vibration, as compared to that of SrTiO₃ nanocrystals with exposed {0 0 1} facet and dominant {1 1 0} facet, respectively. In addition, trapping experiments and active species quantitative experiments confirmed that the co-exposed {0 0 1} and {1 1 0} facets were beneficial to produce O₂⁻ and OH with the generation rates of 8.3 and 132.2 μmol g⁻¹ h⁻¹, respectively. The OH radical played a dominant role in piezoelectric catalytic process. Finally, the piezoelectric catalysis mechanism of SrTiO₃ surface heterojunction was proposed based on a DFT study. This study presents an in-depth understanding of piezoelectric-induced catalytic of perovskite nanocrystals with exposed well-defined facets.     

Change in the emitter surface during high-current-density field emission from the 〈111〉 trihedral angle of a reconstructed tungsten tip

Changes in the shape and emission characteristics of the atomically sharp trihedral 〈111〉 angle of a tungsten tip reconstructed in an electric field are studied by continuous-mode field desorption microscopy during high-current-density field emission. The main changes in the tip shape and the slope of the Fowler-Nordheim characteristic occur at an emission current of 1–5 μA. At a current of 50–100 μA taken from the angle, the tip shape and emission characteristics stabilize and remain unchanged in the range 0–150 μA. The new tip shape is characterized by the widening of the angle edges; the appearance of {112} and {001} plane steps in them; a decrease in the sizes of the {011} planes forming the angle faces; and the appearance of steplike transition regions between {011}, {001}, and {112} faces. These changes in the tip shape are related to the fact that the emission field is weaker than the electric field used for preliminary tip reconstruction, the weakening of the field by the space charge of emitted electrons, and a nonuniform temperature distribution in the tip.