ZrO2 Surface Research Articles

Interactions of moisture and organic contaminants with SiO2 and ZrO2 gate dielectric films

Zirconium oxide (ZrO2) is a promising candidate for future high-k gate dielectric applications. Atmospheric molecular contaminants may impact the quality and performance of zirconium oxide thin films. The interaction of moisture and organics (in particular IPA) as common interfacial contaminants with a 5nm ZrO2 film deposited by ALCVD™ is investigated using atmospheric pressure ionization mass spectrometry (APIMS); the kinetics and mechanism are compared to that of silicon oxide (SiO2). The ZrO2 surface was found to have a higher affinity for moisture and IPA than SiO2. Under similar conditions, the amounts of moisture and IPA adsorbed on ZrO2 were comparatively greater and more temperature sensitive than on SiO2. ZrO2 films also retained a significant amount of moisture that could prove detrimental during subsequent thermal processing. At high temperatures, ZrO2 was found to catalyze decomposition of IPA. IPA adsorption on SiO2 was increased by pre-adsorbed moisture and it led to the formation of alkoxy groups at elevated temperatures. On the other hand, adsorption on ZrO2 was site limited; consequently, less IPA was adsorbed in presence of pre-adsorbed moisture.

Spectroscopic Study of V2O5 Supported on Zirconia and Modified with WO3

Vanadium oxide−tungsten oxide supported on zirconia was prepared by adding Zr(OH)4 powder into a mixed aqueous solution of ammonium metavanadate and ammonium metatungstate followed by drying and calcining at high temperatures. The characterization of prepared catalysts was performed using Fourier transform infrared (FTIR) and Raman spectroscopies, solid-state 51V NMR, X-ray diffraction (XRD), and differential scanning calorimetry (DSC). In the case of a calcination temperature of 773 K, for samples containing a low loading of V2O5, below 18 wt %, vanadium oxide was in a highly dispersed state, while for samples containing a high loading of V2O5, equal to or above 18 wt %, vanadium oxide was well crystallized because the V2O5 loading exceeded the formation of a monolayer on the surface of ZrO2. The experimental results indicate that the presence of WO3 and V2O5 retards the crystallization of the zirconia and stabilizes the tetragonal ZrO2 phase. The ZrV2O7 compound was formed through the reaction of V2O5 and ZrO2 at 873 K and the compound decomposed into V2O5 and ZrO2 at 1073 K; these results were confirmed by FTIR spectroscopy, solid-state 51V NMR, and XRD. The catalytic tests for 2-propanol dehydration have shown that the addition of WO3 to V2O5/ZrO2 enhanced both catalytic activity and acidity of V2O5-WO3/ZrO2 catalysts.

In Situ UV–vis–NIR Diffuse Reflectance and Raman Spectroscopic Studies of Propane Oxidation over ZrO2-Supported Vanadium Oxide Catalysts

The molecular structures and oxidation states of supported 1–5% V2O5/ZrO2 catalysts during propane oxidative dehydrogenation (ODH), with varying propane/O2 ratios, were examined by in situ UV–vis–NIR diffuse reflectance and in situ Raman spectroscopic studies. The results indicate that the reduction extent of surface V5+ cations to V3+/V4+ cations under steady-state reaction conditions increases with the propane/O2 ratio. At the same propane/O2 ratio, the relative extent of reduction of the supported V2O5/ZrO2 catalysts generally increases with the surface vanadia loading, and the polymerized surface VO4 species are more extensively reduced than the isolated surface VO4 species during steady-state propane oxidation. The reactivity studies reveal that at the same reaction conditions, both polymerized and isolated surface V cations are active sites for propane oxidation and that the specific catalytic reactivity (as measured by turnover frequency; TOF) is independent of the surface density of the two-dimensional vanadia overlayer on the ZrO2 support. Furthermore, the relatively constant TOF with surface vanadia coverage demonstrates that propane ODH to propylene requires only one surface VO4 site. However, the propylene selectivity increases with increasing surface vanadia loading due to the removal of nonselective surface sites, possibly terminal Zr–OH groups, on the ZrO2 surface by the deposition of surface vanadia species. The propane/O2 ratio greatly affects the selectivity of these catalysts. Highly oxygen-rich environments (e.g., propane/O2 ratio = 1/10) give rise to the highest propylene selectivity, revealing that propylene production is favored on highly oxidized surface vanadia (+5) sites. Small V2O5 crystallites above monolayer surface vanadia coverage do not contribute to propane ODH because of their low dispersion and low number of active surface sites (spectator vanadia species).

Hydrogen adsorption on the tetragonal ZrO2(101) surface: a theoretical study of an important catalytic reactantElectronic supplementary information (ESI) available: data for geometrical and charge differences in detail. See http://www.rsc.org/suppdata/cp/b2/b202330j/

In order to understand the fundamental properties of the activation of zirconia by hydrogen in relation to the dehydrogenation of hexane, we have performed first-principles calculations using the density functional formalism (PW91 functional) and a plane wave basis set to describe the valence electronic wavefunctions. The interaction of hydrogen atoms and molecules with the thermodynamically most stable, stoichiometric (101) surface has been examined in detail. Three main stages of the hydrogen–ZrO2(101) interaction can be found: a weak interaction corresponding to molecular adsorption of H2 on the top of one surface Zr atom (Ead = −7.1 kJ mol−1), dissociative adsorption, which leaves H atoms on top of one Zr and one O atom (−17.8 kJ mol−1), and, finally, a repulsive interaction (+81.0 kJ mol−1) as precursor of water formation when hydrogen atoms are located above oxygen positions. Desorption of water (+179.9 kJ mol−1) forms a defect at the surface and creates therefore a zirconia suboxide. To separate these effects, atomic hydrogen adsorption has also been considered. Changes in the geometry and charge are discussed as well as the band structures of the adsorbates relative to the vacuum energy. The results are discussed and compared with the available experimental data.

Hydrogenation of Nitrate in Water to Nitrogen over Pd–Cu Supported on Active Carbon

Catalytic removal of nitrate in water by hydrogenation was investigated by using supported Pd–Cu catalysts at 333 K in a flow system. Active carbon (AC) was superior in conversion, selectivity, and insolubility to other supports. Pd and Cu were not dissolved when supported on AC, even at low pH reaction conditions, while they were considerably released from the surface of γ-Al2O3, SiO2, and ZrO2. The contact time dependence of the reactant and products suggested that the hydrogenation of NO3− is a consecutive reaction with NO2− as an intermediate product and that the hydrogenation of NO2− is a key step in determining selectivity to N2 and NH3. The conversion of NO3− was enhanced greatly by the addition of a small amount of Cu to 5 wt% Pd/AC or of Pd to 3 wt% Cu/AC, indicating that the Pd–Cu site was indispensable to activate the nitrate ion. The yield of N2 went through a maximum at 0.6 wt% Cu in the 5 wt% Pd–Cu/AC. A similar dependence of the N2 yield on Cu content for NO2− hydrogenation strongly supports the proposed reaction pathways. The enhancement of N2 selectivity by the addition of small amounts of Cu can be explained by the selective poisoning of the edge or corner sites of the Pd particles by Cu atoms.

Acetonitrile adsorption as an IR spectroscopic probe for surface acidity/basicity of pure and modified zirconias

The ambient temperature adsorption/desorption of acetonitrile (ACN), in its deuterated form, was used for the surface characterisation of several ZrO2-based systems. On pure m-ZrO2, ACN uptake originates four types of chemisorptive interaction: σ-coordination to coordinatively unsaturated (cus) surface cations (Lewis acid sites), formation of weak H-bonding with bridging surface hydroxyls, and two surface reactions. The latter are a partial hydrolytic reaction, leading to (at least) three acetamide-like surface anionic species, and the extraction by basic surface sites of one H (D) atom, leading to monomeric and/or polymeric surface carbanionic species. The proportion among the four types of interaction is mainly determined by the samples activation conditions. Comparison of m-ZrO2 data with some preliminary data of ACN adsorption on t-ZrO2 suggest that the two surface reactions of ACN depend on the presence at the ZrO2 surface of terminal (single-coordinated) hydroxyl groups, whose IR vibrational frequency is at ∼ 3775 cm−1. This hypothesis has been confirmed by ACN adsorption/desorption on some surface-modified m-ZrO2 systems: (i) m-ZrO2 pre-treated with chloroform, which selectively interacts with terminal hydroxyls; (ii) m-ZrO2 pre-treated with a weak acid (CO2), which interacts weakly with surface basic sites; (iii) m-ZrO2 with the surface partly covered by strong acid groups (sulfates). The latter system (sulfated m-ZrO2) presents, when highly hydrated, a medium-weak Bronsted acidity, otherwise absent on all other m-ZrO2 systems considered.

Dynamics of Octadecylphosphonate Monolayers Self-Assembled on Zirconium Oxide: A Deuterium NMR Study

Deuterium NMR spectroscopy has been used to probe the dynamics of deuterated octadecylphosphonate (−1,1−d2) (ODPA−d2) monolayers on nonporous ZrO2 powder (surface area ≈ 40 m2/g) over the temperature range of 200−340 K. At 200 K, a broadened Pake doublet with distinct nonrigid characteristics and a horn splitting of approximately 120 kHz was observed. With increasing temperature, the 2H spectrum gradually transforms into a relatively narrow and featureless peak. Spectral simulations are performed with the help of plausible motional models, and the results show that, over the whole temperature range studied, the C1−D bonds have substantial motional freedom with respect to the characteristic 2H NMR time scale. In addition, at each temperature, a weighted superposition of several simulated line shapes with different rates and site populations is required to account for the observed spectral features, indicating the presence of considerable motional heterogeneity within the ODPA monolayers. The results are further discussed in terms of the known characteristics of the ZrO2 surface and of the conformational transitions in the octadecyl chain.

Fractal surfaces of ZrO2, WO3, and CeO2 powders

The surface fractal properties of ZrO2 , WO3 , and CeO2 powders prepared by the thermal decomposition of ZrO(NO3)2 , (NH4)4W5O17 , and (NH4)2Ce(NO3)6 , respectively, were studied by mercury porosimetry. The results demonstrate that these oxides may, in principle, have fractal surfaces owing to topochemical processes of the type A(s) → B(s) + C(g). The surface fractal dimension of individual crystallites and their aggregates are determined.

Effects of Zirconia Phase on the Synthesis of Methanol over Zirconia-Supported Copper

A study has been conducted to identify the influence of zirconia phase and copper to zirconia surface area on the activity of Cu/ZrO2 catalysts for the synthesis of methanol from either CO/H2 or CO2/H2. To determine the effects of zirconia phase, a pair of Cu/ZrO2 catalysts was prepared on tetragonal (t-) and monoclinic (m-) zirconia. The zirconia surface area and the Cu dispersion were essentially identical for these two catalysts. At 548 K, 0.65 MPa, and H2/COx= 3 (x = 1, 2), the catalyst prepared on m-ZrO2 was 4.5 times more active for methanol synthesis from CO2/H2 than that prepared on t-ZrO2, and 7.5 times more active when CO/H2 was used as the feed. Increasing the surface area of m-ZrO2 and the ratio of Cu to ZrO2 surface areas further increased the methanol synthesis activity. In situ infrared spectroscopy and transient-response experiments indicate that the higher rate of methanol synthesis from CO2/H2 over Cu/m-ZrO2 is due solely to the higher concentration of active intermediates. By contrast, the higher rate of methanol synthesis from CO/H2 is due to both a higher concentration of surface intermediates and the more rapid dynamics of their transformation over Cu/ZrO2.

Improvement of the Catalytic Performance of Pd/WO3/ZrO2 in the Selective NO–CH4–O2 Reaction by the Addition of Water Vapor

Abstract The Pd/WO3/ZrO2 catalyst exhibited activity for the NO–CH4–O2 reaction in the presence of water vapor when WO3 monolayer covered the ZrO2 surface. The selectivity of methane consumption for NO reduction was considerably improved by the addition of 10% of water vapor.

Supported Tantalum Oxide and Supported Vanadia-tantala Mixed Oxides: Structural Characterization and Surface Properties

Silica-, alumina-, and zirconia-supported tantalum oxide and supported vanadia-tantala mixed oxide catalysts were investigated by XRD, UV−vis-DR, FTIR, and FT-Raman spectroscopy to determine the nature of the surface tantala species. Two types of surface TaOx species were identified. On the SiO2 support, at low coverages tetrahedral TaO4 species exist, with a TaO bond and three bridging Ta−O-support bonds. At higher coverages, octahedral TaO6 species occur. On Al2O3 and ZrO2 both surface species exist already at considerably low Ta loading. The acidic properties of the catalysts were investigated by an FTIR study of adsorbed pyridine. Supported tantala is clearly more Lewis acidic in comparison to vanadia. The oxidation of methanol was used to probe the catalytic performance of the supported oxide species and from the activity and product distribution it is concluded that supported tantala predominantly shows acidic over redox behavior.

Study of the Adsorption Mechanism and the Structure of Adsorbed Layers of Polyelectrolytes at the Metal Oxide/Solution Interface

The influence of the molecular weight of polyacrylic acid (PAA) and polyacrylamide (PAM) as well as of inorganic contaminants on the ZrO2 surface on the adsorption and electrokinetic properties of the metal oxide/polyelectrolyte solution interface were studied for ZrO2 and Fe2O3 solid particles. The calculated concentrations of the various surface groups on ZrO2 and Fe2O3 enabled an investigation of the possible mechanism for the bonding of the studied polyelectrolytes with the surfaces of both oxides. PAA and PAM macromolecules bond with the solid surface mainly via the –OH groups of the oxides, which may interact with the carboxy groups of polyelectrolytes through hydrogen bridging. A comparison of the change in values of the diffuse layer charge with the surface charge enabled the principal factors responsible for the changes in the zeta potential of the oxides, e.g. pH, polymer molecular weight and concentration of the polymer solutions, to be evaluated. From such zeta potential values, it was possible to determine the free energies of adsorption of PAA and PAM on the surfaces of both oxides. The thicknesses of adsorbed polymer layers on Fe2O3 and ZrO2 were calculated on the basis of measurements of their suspension viscosities in the absence and presence of adsorbed polymer. It was shown that the thickness of the adsorption layer increased with increasing polymer molecular weight, pH and concentration of the polymer solution. Because the experimental determination of the number and the length of trains, loops and tails in such polyelectrolytes was not possible, the participation of such segments of polymer structure at the interface was computed using the Scheutjens–Fleer model of polymer adsorption. The polymer adsorption expressed as the number of equivalent monolayers was calculated and compared with the experimental data.

Development of Surface Compressive Stresses in Zirconia–Alumina Composites by an Ion‐Exchange Process

A two‐step ion‐exchange technique was developed for introducing compressive stresses on the surface of ZrO2–Al2O3 composites. In the first step, a thin layer (∼250 μm) of Na‐β″‐Al2O3 was formed on the surface of the composite by a vapor‐phase process at ∼1400°C. In the second step, Na+ ions were replaced by K+ ions by a heat treatment at ∼385°C for 2 h in a molten KNO3 bath. Replacement of sodium by potassium led to the creation of surface compressive stresses. The flexural strength and Weibull modulus of ZrO2–Al2O3 composite were ∼915 MPa and 10, respectively, for the as‐sintered samples. By contrast, the flexural strength and Weibull modulus were ∼1140 MPa and 26, respectively, for the ion‐exchanged samples. A residual surface compressive stress of ∼480 MPa was measured by a strain‐gauge technique in K+‐ion‐exchanged samples. The presence of surface compressive stresses also was confirmed using an indentation technique. The technique developed here can be used to introduce compressive stresses on components of virtually any shape.

Oxidation Activity and 18O-Isotope Exchange Behavior of Cu-Stabilized Cubic Zirconia

A series of Cu–ZrO2 samples with varying concentrations of Cu from 1 to 33 mol% have been prepared by sol–gel technique and calcined at 873 K. XRD characterization of the samples with a copper content of 2 to 20 mol% reveals the stabilization of zirconia into cubic (fluorite) phase. The Cu–ZrO2 samples with higher Cu content (>20 mol%) revealed the presence of bulk CuO. A linear decrease in lattice parameter with increase in Cu content up to 20 mol% indicates the possible incorporation of Cu2+ in the lattice position of Zr4+ ions. XPS data of these samples provide further evidence for the incorporation of Cu2+ ions in the ZrO2 lattice up to 5 mol%. At higher concentrations of Cu, about half of the input of Cu goes into the lattice and the remaining stays as extra lattice Cu, possibly on the surface or subsurface layer of ZrO2. The BET surface area of these samples was in the range of 2 to 8 m2 g−1. The activity of these samples in CH4 and CO oxidation was investigated by 18O-isotope exchange as well as by catalytic reaction studies in complete oxidation of CH4 and CO. The Cu–ZrO2 with 20 mol% Cu was found to be the most active sample in the series, which has the maximum amount of copper in the substitutional position. For comparison, yttrium-stabilized zirconia samples with and without Cu was also prepared following the same procedure. Yttrium-stabilized zirconia without Cu was almost inactive in complete methane oxidation, whereas Cu-containing sample was more active. This confirms that the presence of Cu species in substitutional positions along with oxygen vacancies in zirconia lattice are substantially responsible for the catalytic activity in CH4 and CO oxidation as well as in complete heterogeneous 18O exchange processes. The light-off temperature for 50% conversion of CH4 (T50) decreases with an increase in Cu content up to 20 mol% and matches well with the results of 18O exchange measurements. The shapes of the curves of T50 and Texchange follow a similar trend indicating that both CH4 oxidation and 18O exchange processes occur via a completely heterogeneous mechanism.

Change in the Physicochemical Properties of Nanocrystalline Powder Based on ZrO2 in the Presence of a Mineralizing Agent

The change in physicochemical properties of nanocrystalline powder of the composition ZrO2 ― 3 mole% Y2O3 in the presence of aluminum fluoride is studied. The starting powder is prepared by a complex method including elements of hydrothermal synthesis and sol-gel technology. It is established that these conditions expand the temperature limits for the existence of ZrO2 monoclinic solid solution. Transformation is connected with adsorption of fluorine at the ZrO2 surface, diffusion in the solid phase, and a reduction in anion vacancy concentration.

Infrared Study of ZrO2 Surface Sites Using Adsorbed Probe Molecules. 2. Dimethyl Ether Adsorption

The properties of two types of surface OH species, terminal (t) and bridged (b), as well as corresponding vacant sites on ZrO2 at 573 K have been studied using dimethyl ether (DME) adsorption as an IR molecular probe. On the dehydroxylated ZrO2 surface, the molecularly adsorbed CH3OCH3 species was dissociated moderately. On the other hand, the molecularly adsorbed CH3OCH3 species was not observed on the fully hydroxylated ZrO2 surface because of the rapid dissociation of CH3OCH3. Kinetic analysis revealed that the activity of the dissociative adsorption of DME on surface sites is in the order t-OH sites > vacant sites > b-OH sites on ZrO2, which parallels that of the adsorption of pyridine, CO2, and HCOOH at the same sites, as reported previously. Furthermore, the surface t-18OH species has been found to provide active 18O atoms for the dissociation of CH3OCH3 to OCH3 and 18OCH3 species at 573 K, which shows that the 18OH species is involved in the dissociation of DME. We suppose that the nucleophilic ads...

Spectral analysis of photons emitted during scratching of an insulator surface by a diamond in air

Spectral analysis of photons emitted during scratching of insulator surfaces of Si3N4, Al2O3, ZrO2 and soda-lime glass with a diamond was performed under relatively slight frictional conditions, i.e., with a normal force of less than 1 N and a sliding velocity of less than 16 cm/s in ambient air. All the spectra showed similar profiles with strong sharp peaks in the ultraviolet region and photon energies of 2.8–4 eV. The wavelengths of the photons were perfectly matched to those for the second positive band of N2, demonstrating that an electric discharge of N2 gas occurred at the frictional contact.

Study of the Alkylation of Isobutane with n-Butene over WO3/ZrO2 Strong Solid Acid. 1. Effect of the Preparation Method, WO3 Loading, and Calcination Temperature

A series of strong solid acids composed of WO3/ZrO2 were prepared. Their crystal structure, surface state, and acidity were determined by the methods of X-ray diffraction, thermal gravimetric and differential thermal analysis, temperature-programmed reduction, laser Raman, and acidity measurement. The results revealed that ZrO2 in WO3/ZrO2 existed mainly in the tetragonal phase, the addition of WO3 plays an important role in stabilizing the tetragonal phase of ZrO2, and all of the samples possessed large surface areas. WO3 in WO3/ZrO2 is mainly monolayer dispersed, and a small amount crystallized on the ZrO2 surface and partly reacted with ZrO2 to form the bond of Zr-O-W, acting as the strong solid acid center. The catalytic properties of WO3/ZrO2 strong solid;acids for alkylation of isobutane with butene at different conditions were investigated. They had a better reaction performance than other strong solid acids; a parallel relationship could be drawn between the catalytic activity and the acid amounts as well as the acidic strength of the catalysts.

Role of Hydrogen Spillover in Methanol Synthesis over Cu/ZrO2

Recent studies have shown that the synthesis of methanol from both CO and CO2 over Cu/ZrO2 involves the spillover of H atoms formed on Cu to the surface of ZrO2. The atomic H then participates in the hydrogenation of carbon-containing species (i.e., HCOO–Zr and HCO3–Zr) to methanol. The present study examines the dynamics of H/D exchange of HO–Zr, HCOO–Zr, and CH3O–Zr groups present on the surface of ZrO2 and Cu/ZrO2 by means of in situ infrared spectroscopy. The rate of H/D exchange of HO–Zr groups is much more rapid in the presence than in the absence of Cu dispersed on the surface of ZrO2. This effect is attributed to the higher effectiveness of Cu to dissociate H2(D2). While adsorbed water significantly inhibits the rate of H/D exchange on ZrO2, the opposite effect is observed for Cu/ZrO2. The reason is that adsorbed water inhibits the dissociation of H2(D2) on the surface of ZrO2 but not on the surface of Cu. Adsorbed water facilitates the transport of H(D) atoms formed on the surface of Cu across the surface of ZrO2 as a consequence of hydrogen bonding between adsorbed H2O and HO–Zr groups. Formate groups are formed on the surface of ZrO2 primarily via the process CO(g)+HO–Zr→HCOO–Zr. Formate groups can also form on the surface of Cu and spill over onto the surface of ZrO2. The presence of formate groups inhibits the rate of H/D exchange of HO–Zr groups. H/D exchange of HCOO–Zr is also observed but occurs at a slower rate than the isotopic exchange of HO–Zr groups. As in the absence of formate groups, adsorbed water inhibits the rate of H/D exchange for ZrO2 but enhances it for Cu/ZrO2. The dynamics of H/D exchange are compared with the dynamics of methanol formation as measured by the rate of CH3O–Zr formation on Cu/ZrO2. On the basis of this analysis it is concluded that the rate of hydrogen spillover from Cu is more than an order of magnitude faster than the rate of methanol formation, and, hence, not a rate-limiting step in the synthesis of methanol over Cu/ZrO2.

Ultrafast Excited-State Dynamics of Re(CO)3Cl(dcbpy) in Solution and on Nanocrystalline TiO2and ZrO2Thin Films

Femtosecond infrared spectroscopy was used to study the excited-state dynamics of Re(CO)3Cl(dcbpy) in DMF solution and on the surface of ZrO2 and TiO2 nanocrystalline thin films. For Re(CO)3Cl(dcbpy) in DMF solution, we observed a long-lived 3MLCT state with a lifetime of >1 ns. The frequencies for the CO stretching bands were blue-shifted compared to those in the ground state, consistent with the metal-to-ligand charge-transfer nature of the excited state. Rapid spectral evolution of the excited-state CO stretching bands was observed within the first 12 ps. For Re(CO)3Cl(dcbpy) on ZrO2 thin films, a similar 3MLCT state was observed. However, the spectral blue shift was much less pronounced and occurred on a faster time scale. We suggest that vibrational relaxation is the primary contribution to the spectral evolution of Re(CO)3Cl(dcbpy) on the ZrO2 film, whereas both vibrational relaxation and solvation of the MLCT state contribute to the spectral evolution in DMF solution. The excited-state decay rate o...