Formation of polyhedral ceria nanoparticles with enhanced catalytic CO oxidation activity in thermal plasma via a hydrogen mediated shape control mechanism

Ceria nanoparticles with well defined facets are prepared in argon–hydrogen thermal plasma followed by controlled oxidation. With increasing hydrogen fraction in the plasma, a clear sphere-to-polyhedron shape transition is observed. The heat released during the hydrogenation of cerium, which significantly enhances the species mobility on the surface, favors the growth of well defined facets. The polyhedron ceria nanoparticles, though lower in specific surface area, exhibit superior catalytic performance for CO oxidation over the round particles, which is attributed to the higher density of the reactive {200} and {220} facets on the surface. The hydrogen mediated shape control mechanism provides new insights into the shape control of nanoparticles during thermal plasma processing.     

Synthesis and atomic-scale characterization of CeO2 nano-octahedrons

We report the synthesis of ceria nano-octahedrons using a simple yet efficient hydrothermal technique free of surfactant and template. A comprehensive microscopic characterization reveals that the nano-octahedrons exposed with eight {1 1 1} planes have shape corners and well-defined edges at the nanoscale. Atomic-resolution imaging uncovers that a couple of atomic layers are disappeared at corner, the atomic density of oxygen is reduced at surface, and there occur stacking faults inside the nano-octahedrons, which should have implications for our understanding of catalytic activity of ceria. We also propose a growth mechanism for the nano-octahedrons and provide an explanation to the easy formation of nano-octahedrons with the eight {1 1 1} facets.     

Solid-on-solid model of overlayer-induced faceting

A solid-on-solid model is proposed to describe faceting of bcc(1 1 1) metal surface induced by a metal overlayer. It is shown that the first-order phase transition occurs between faceted {2 1 1} or {1 1 0} and disordered phases. The ordered phases consist of large 3-sided pyramids with {2 1 1} facets or {1 1 0} facets. It is shown that the high-temperature disordered phase has not planar bcc(1 1 1) structure but faceted disordered structure. Hysteresis effects were observed when the system was warmed above the transition temperature and then cooled down. Temperature dependence of LEED patterns for faceted and disordered phase are calculated in kinematic approximation.     

Structural and morphological evolution of Co on faceted Pt/W(1 1 1) surface upon thermal annealing

The structural and morphological changes of a 1.1 monolayer (ML) Pt deposit on W(1 1 1) have been investigated in situ, in ultra-high vacuum, as a function of the annealing temperature from 700 to 1340 K, by a combination of grazing incidence X-ray diffraction and grazing incidence small-angle X-ray scattering. Before annealing, the thin Pt layer is two-dimensional and lattice-matched to the W(1 1 1) surface. The faceting of Pt/W(1 1 1) towards nanoscale three-sided pyramids with {2 1 1} facets has been detected from 715 K. At this stage, the pyramids, which have a 5-nm average lateral size, cover nearly perfectly the surface. At higher temperatures, they increase in size. The role of the edge energy in the nanofaceting process is discussed. In addition, 4 MLs Co are deposited at room temperature on the smallest Pt/W pyramids. The obtained three-dimensional Co islands are correlated with the Pt/W nanopyramids and Co is relaxed on Pt/W. At approximately 800 K, a CoPt alloy is formed and becomes better ordered as the annealing temperature increases. At 1100 K, both defaceting and phase separation begin; the CoPt alloy segregates on the W(1 1 1) flat surface, while Co forms an epitaxial layer on the {2 1 1} facets. In addition, in the temperature range of 1100-1200 K, a great majority of {2 1 1} large facets coexist with some {1 1 0} small facets. Finally, the surface becomes flat again at 1250 K.     

Competitive effects of metal dissolution and passivation modulated by surface structure: An AFM and EBSD study of the corrosion of alloy 22

Electron backscatter diffraction (EBSD) and atomic force microscopy (AFM) are used to correlate crystallographic grain orientation with corrosion rates of polycrystalline alloy 22 following immersion in 1 and 3 molar (M) hydrochloric acid. For each acid concentration, relative corrosion rates are simultaneously characterized for approximately 50 unique grain orientations. The results demonstrate that the corrosion rate anisotropies are markedly different in the two acid concentrations. In very aggressive acidic environments (3M HCl), where electrochemical impedance spectroscopy and spectroscopic ellipsometry data demonstrate that the passive oxide film of alloy 22 is completely dissolved, alloy dissolution rates scale inversely with the average coordination number of surface atoms for a given grain orientation, where highly correlated surfaces dissolve the slowest. Thus, similar to simple metallic systems, the corrosion rates scale with the surface plane-normal crystallographic orientations as {1 1 1} < {1 0 0} < {1 1 0}. Less intuitively, in milder corrosive environments (1M HCl), where the passive film of the alloy is still intact, the dissolution does not scale inversely with surface atomic density. Rather, corrosion rates scale with crystallographic orientations as {1 1 1} < {1 1 0} < {1 0 0}. This is attributed to the fact that facets most susceptible to corrosion (least coordinated) are also the most able to form protective oxides, so that the dissolution anisotropy is a result of the delicate balance between metal dissolution and oxide growth.     

Oxidation enthalpies for reduction of ceria surfaces

The thermodynamic properties of surface ceria were investigated through equilibrium isotherms determined by flow titration and coulometric titration measurements on high-surface-area ceria and ceria supported on La-modified alumina (LA). While the surface area of pure ceria was found to be unstable under redox conditions, the extent of reduction at 873 K and a P(O₂) of 1.6 × 10⁻²⁶ atm increased with surface area. Because ceria/LA samples were stable, equilibrium isotherms were determined between 873 and 973 K on a 30-wt% ceria sample. Oxidation enthalpies on ceria/LA were found to vary with the extent of reduction, ranging from −500 kJ/mol O₂ at low extents of reduction to near the bulk value of −760 kJ/mol O₂ at higher extents. To determine whether +3 dopants could affect the oxidation enthalpies for ceria, isotherms were measured for Sm⁺³-doped ceria (SDC) and Y⁺³-doped ceria. These dopants were found to remove the phase transition observed in pure ceria below 973 K but appeared to have minimal effect on the oxidation enthalpies. Implications of these results for catalytic applications of ceria are discussed.     

Dopant control over the crystal morphology of ceramic materials

Doping is a common way to activate the behavior of ceramics. Its effect is not limited to the bulk: segregation of dopants to the surfaces also yields a way to modify, and ultimately control the crystal morphology. We propose a model that allows us to calculate the surface energy beyond the Langmuir isotherm for doped and defective surfaces from atomic-level simulations. The model also allows us to account for different compositions between the bulk and surface. Computational materials design can thus be applied to optimize simultaneously the crystal behavior at the atomic (surface structure and composition) and mesoscopic (crystal size and shape) length scales. We exemplify the model with orthorhombic CaTiO₃ perovskite doped with Mg²⁺, Fe²⁺, Ni²⁺, Sr²⁺, Ba²⁺ and Cd²⁺ ions, by predicting the effect that different dopants and dopant concentrations have on the crystal morphology. We find that a higher proportion of reactive {0 2 1} and {1 1 1} surfaces are exposed with the presence of divalent Mg²⁺, Fe²⁺ and Ni²⁺ ions than in the undoped material and in perovskite doped with Ba²⁺ and Sr²⁺. Cd²⁺ has only minor effects on crystal morphologies. These findings have important implications for predicting the reactivity of crystals doped with different ions and we show how this can be related to a simple parameter such as the ionic radius. We have tested our newly derived model by comparison with laboratory flux grown single crystals of CaTiO₃, (Ni, Ca)TiO₃ and (Ba, Ca)TiO₃ and find excellent agreement between theory and experiment.     

Facetting and (n × 1) reconstructions of SrTiO3(1 1 0) surfaces

(n × 1) reconstructions and facetting of the (1 1 0) polar surface of SrTiO₃ are studied by means of a combination of shell model and density functional calculations. The polarity compensation can be achieved through the formation of {1 0 0} nano-facets, which play a crucial role in the reconstruction process. The behaviors of various possible terminations (Sr, Ti, and O) are analyzed, as well as their atomic structure and energetics. Their stability in different chemical environments is discussed, with respect to previous formulations and experimental results. The Sr-terminated surface tends to expose large facets, while the TiO and O terminations are marginally stabilized or even destabilized by (n × 1) reconstructions, respectively. Trend to facetting results from a subtle competition between the thermodynamic stability of the ideal non stoichiometric (n × 1) surfaces, and huge atomic relaxations that contribute to the lowering of the surface energy differently for each termination.     

Dissociative adsorption and adsorbate-induced reconstruction on Fe{2 1 1}

The products of CO, NO, O₂ and N₂ dissociation on Fe{2 1 1} have been studied by means of first-principles density functional theory. Preferred adsorption sites for adatoms C, N and O are identified, and trends in charge transfer and surface magnetism described. An experimentally observed (2 × 1) reconstruction induced by O is confirmed to be energetically stable, and a similar reconstruction induced by N is tentatively predicted. It is argued that these reconstructions may be important not only in the context of the catalytic reactivity of the Fe{2 1 1} surface, but also for the initial stages of surface nitridation and oxidation.     

Surface reconstruction of clean bcc-Fe{1 1 0}: A quasi-hexagonal top-layer with periodic height modulation

Hexagonal-pillar shaped pure Fe single crystal whiskers with six {1 1 0} side planes were obtained by means of chemical vapor deposition. Atomically resolved scanning tunneling microscopy images obtained on the {1 1 0} surface showed a quasi-hexagonal atomic array with mesoscopic-range periodic height modulation of about 1/3 of an atomic step. This height modulation was found to be a result of an interference between the quasi-hexagonal top-layer and the sub-surface bcc-Fe{1 1 0} layer. Unit vectors of the mesoscopic-range modulation turned out to be expressed as , where and are the primitive vectors of the two-dimensional atomic array in the top-layer and those in the sub-surface layer, respectively. The two-dimensional density of atoms in the top-layer is slightly higher by 0.46% than that in the sub-surface layer.     

Morphology and defect structure of the CeO2(1 1 1) films grown on Ru(0 0 0 1) as studied by scanning tunneling microscopy

The morphology of ceria films grown on a Ru(0 0 0 1) substrate was studied by scanning tunneling microscopy in combination with low-energy electron diffraction and Auger electron spectroscopy. The preparation conditions were determined for the growth of nm-thick, well-ordered CeO₂(1 1 1) films covering the entire surface. The recipe has been adopted from the one suggested by Mullins et al. [D.R. Mullins, P.V. Radulovic, S.H. Overbury, Surf. Sci. 429 (1999) 186] and modified in that significantly higher oxidation temperatures are required to form atomically flat terraces, up to 500 Å in width, with a low density of the point defects assigned to oxygen vacancies. The terraces often consist of several rotational domains. A circular shape of terraces suggest a large variety of undercoordinated sites at the step edges which preferentially nucleate gold particles deposited onto these films. The results show that reactivity studies over ceria and metal/ceria surfaces should be complemented with STM studies, which provide direct information on the film morphology and surface defects, which are usually considered as active sites for catalysis over ceria.     

TiAlN coatings deposited by triode magnetron sputtering varying the bias voltage

TiAlN films were deposited on AISI O1 tool steel using a triode magnetron sputtering system. The bias voltage effect on the composition, thickness, crystallography, microstructure, hardness and adhesion strength was investigated. The coatings thickness and elemental composition analyses were carried out using scanning electron microscopy (SEM) together with energy dispersive X-ray (EDS). The re-sputtering effect due to the high-energy ions bombardment on the film surface influenced the coatings thickness. The films crystallography was investigated using X-ray diffraction characterization. The X-ray diffraction (XRD) data show that TiAlN coatings were crystallized in the cubic NaCl B1 structure, with orientations in the {1 1 1}, {2 0 0} {2 2 0} and {3 1 1} crystallographic planes. The surface morphology (roughness and grain size) of TiAlN coatings was investigated by atomic force microscopy (AFM). By increasing the substrate bias voltage from −40 to −150 V, hardness decreased from 32 GPa to 19 GPa. Scratch tester was used for measuring the critical loads and for measuring the adhesion.     

Ammonia activation on platinum {1 1 1}: A density functional theory study

By means of density functional theory calculations we have investigated the role of adsorbed atomic oxygen and adsorbed OH in the oxidation of ammonia on Pt{1 1 1}. We have investigated the dissociation of NH_3,ads, NH_2,ads and NH_ads on Pt{1 1 1} and the oxidation of these species by O_ads and OH_ads. We have done normal mode frequency analysis and work function calculations to characterise reactant, product and transition states. We have determined reaction energies, activation entropies, kinetic parameters and corrected total energies with the zero point energy. We have shown that O_ads only activates the dehydrogenation of NH_3,ads and that OH_ads activates the dehydrogenation of all NH_x,ads species and have reasoned this difference in activation by a bond order conservation principle. We have pointed out the importance of a zero point energy correction to the reaction energies and barriers. We have compared the calculated vibrational modes of the adsorbates with corresponding experimental EELS data. This has led to a revise of the frequency assignment of ν(Pt-OH₂), a revise in the identification of a NH₂ species on the Pt{1 1 1} surface after electron bombardment of pre-adsorbed NH₃ and the confirmation of an ammonia dimer binding model at the expense of a hollow site occupation by ammonia on the Pt{1 1 1} surface.     

3D atom probe study of gas adsorption and reaction on alloy catalyst surfaces II: Results on Pt and Pt-Rh

The 3-dimensional atom probe (3DAP) is a unique instrument providing chemical analysis at the atomic scale for a wide range of materials. A dedicated 3DAP has been built specifically for analysing reactions at metal surfaces, called the catalytic atom probe (CAP). This paper presents an overview of results from the CAP on structural and chemical transformations to surface layers of Pt and Pt-17.4 at.%Rh catalysts following exposure to a number of gases typically emitted by vehicle engine exhausts, normally for 15 min at pressures of 10 mbar. Following exposure to the oxidising gases NO on Pt, and NO, O₂ or N₂O on Pt-Rh, both surfaces appear disrupted, while for Pt-Rh, Rh enrichment of the surface atomic layer is noted over the entire specimen apex for exposure temperatures up to 523 K. However, for oxidising exposures at 573-773 K relatively clean, Rh-depleted surfaces are observed on {0 0 1}, {0 1 1} and {0 1 2} crystallographic regions of Pt-Rh. It is suggested that this result is due to surface diffusion of oxide species over the specimen apex, towards the {1 1 1}-orientated areas where the oxides appear to be stabilised. In contrast, CO exposure appears to have little effect on the either the surface structure or composition of the Pt-Rh alloy. Finally, combinations of two gases (NO + CO, O₂ + NO) were also dosed onto Pt-Rh alloys in the same exposure. These revealed that while NO and CO can co-adsorb without interference, CO prevents the build up of oxide layers and reduces the extent of Rh segregation seen under NO exposure alone. On exposing Pt-Rh to NO after an oxygen exposure, heavily oxidised surfaces, Rh segregation and no intact NO molecules were seen, confirming the ability of oxidised Pt-Rh to dissociate nitric oxide.     

Vacancy formation and CO adsorption on gold-doped ceria surfaces

We study the effect of gold doping on oxygen vacancy formation and CO adsorption on the (1 1 0) and (1 0 0) surfaces of ceria by using density functional theory, corrected for on-site Coulomb interactions (DFT + U). The Au dopant substitutes a Ce atom in the surface layer, leading to strong structural distortions. The formation of one oxygen vacancy near a dopant atom is energetically “downhill” while the formation of a second vacancy around the same dopant requires energy. When the surface is in equilibrium with gaseous oxygen at 1 atm and room temperature there is a 0.4 probability that no oxygen atom left the neighborhood of a dopant. This means that the sites where the dopant has not lost oxygen are very active in oxidation reactions. Above 400 K almost all dopants have an oxygen vacancy next to them and an oxidation reaction in such a system takes place by creating a second vacancy. The energy required to form a second vacancy is smaller on (1 1 0) than on (1 0 0). On the (1 1 0) surface, it is much easier to form a second vacancy on the doped surface than the first vacancy on the undoped surface. The energy required to form a second oxygen vacancy on (1 0 0) is comparable to that of forming the first vacancy on the undoped surface. Thus doping makes the (1 1 0) surface a better oxidant but it has a small effect on the oxidative power of the (1 0 0) surface. On the (1 1 0) surface CO adsorption results in formation of a carbonate-like structure, similar to the undoped surface, while on the (1 0 0) surface direct formation of CO₂ is observed, in contrast to the undoped surface. The Au dopant weakens the bond of the surrounding oxygen atoms to the oxide making it a better oxidant, facilitating CO oxidation.     

Crystal orientation control of polycrystalline TiN thin films using ZnO under layers deposited by magnetron sputtering

The effect of ZnO under layers on crystal growth of TiN thin films was investigated. TiN single layers and double-layered ZnO/TiN thin films were deposited on soda-lime-silicate glass substrates by magnetron sputtering. XRD analysis indicated that TiN single layers exhibited {1 1 1} preferred orientation on glass substrates; on the other hand, the TiN thin films with {1 0 0} preferred orientation were obtained using ZnO under layers and crystallized better than the TiN single layers. This crystal orientation change of TiN thin films should come from heteroepitaxial-like growth because the TiN{1 0 0} and ZnO{0 0 1} crystal lattice planes have similar atomic arrangements. Besides, the possible mismatch between TiN and ZnO atomic arrangements was estimated to be 7.8%. Furthermore, the resistivity and optical absorbance of TiN thin films decreased when they were deposited on ZnO under layers. It can be considered that electrical and optical properties should be improved due to the well-crystallization of TiN thin films using ZnO under layers.     

Preferred orientations in nano-gold/silica/silicon interfaces

Gold in contact with silicon substrates Si(1 0 0), Si(1 1 1), and SiO₂ is studied by thermal evaporation and annealing in N₂ using the modified sphere-plate technique. The final orientation distribution of crystalline Au films grown on Si substrate systems that incorporate a native amorphous oxide layer of silica and Au on amorphous silica (SiO₂ glass) substrates is influenced by preferred orientations and twinning. Experimental evidence suggests that the orientation of Au{1 1 1} close packed planes (multiply twinned) was found to be of low-energy as the annealing temperature was increased to 530 °C and 920 °C. Additional orientations were observed for Au{1 0 0} on Si(1 0 0) substrates and Au{1 0 0}, {1 1 0}, and {3 1 1} on SiO₂ substrates. After annealing at 920 °C the size distribution of the gold particles was determined to be within the range of 20-800 nm while the morphology of gold surface appears spherical to faceted in character. These results show similarities to recent findings for smaller nano-size 1D particles, islands and thin Au films on silicon annealed over lower temperature ranges.     

Synthesis and characterization of pure and ZrO2-doped nanocrystalline CuO-NiO system

The physicochemical, surface and catalytic properties of pure and doped 0.25CuO-NiO solids prepared by sol-gel method were investigated. The dopant concentration was 2, 4 and 6 mol% ZrO₂. The solids investigated were calcined at 400 and 600 °C. The techniques employed were XRD, EDX, TEM, surface excess oxygen, nitrogen adsorption at −196 °C and catalytic oxidation of CO by O₂ using both static and flow methods. The results revealed that the investigated system dissolved 4 mol% ZrO₂ by heating at 400 °C. This process was accompanied by a significant increase in the S_BET and V_p with subsequent decrease in the (r) values of the doped adsorbent. ZrO₂-doping of the system investigated followed by calcination at 400 and 600 °C led to a considerable increase in its catalytic activity in CO oxidation by O₂ using static and flow methods. The doping process was not accompanied by any change in the activation energy of the catalyzed reaction.     

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.     

Alloy formation in the Au{1 1 1}/Ni system - An investigation with scanning tunnelling microscopy and medium energy ion scattering

Using the techniques of scanning tunnelling microscopy (STM) and medium energy ion scattering (MEIS), we examine the growth and annealing behaviour of ultrathin Ni films on Au{1 1 1} at 300 K. As has been shown previously, submonolayer growth of Ni on Au{1 1 1} is strongly influenced by the presence of the herringbone reconstruction with two-dimensional clusters nucleating at herringbone elbows. Second layer growth commences prior to the completion of the monolayer. After multiple layers have been deposited, the surface morphology retains a similar cluster-like appearance. Annealing produces surfaces exhibiting long range Moiré structures and, at higher temperature, triangular misfit dislocations. We use MEIS to examine the composition and structure of these surface alloy phases and conclude that in each case, they consist of an essentially pure Au surface layer on a bimetallic second layer.