Pd Film Thickness Research Articles

Relatively Low Temperature Growth of Carbon Nanotubes by Thermal Chemical Vapor Deposition using Novel Catalysts

AbstractWe found out the promising catalyst materials(NiPd). The NiPd not only has the low melting point but also has the Pd enhancing the surface diffusion at low temperatures(<500'c ). With the Pd film thickness increasing, we could control the CNT density and synthesize more aligned and uniform CNTs. We also obtained the better electrical properties including lower turn-on field (3.4 V/um) and higher current density (34.3 mA/cm2) for NiPd as catalyst. For the advantages described above, we believe that the difficulty of low temperature on FED can be overcome. Further, the large area field emission display might be fabricated in the future.

Pd bonded on Nb(001): Dependence of noble metal and ferromagnetic characteristics on film thickness

Experimental observations confirmed by density functional theory (DFT) calculations show that strong epitaxial bonds between Pd atoms and the substrate can induce two competing properties: nobleness and ferromagnetic order in the same material, i.e., Pd bonded on Nb(100). Angle-resolved ultraviolet photoelectron spectroscopy measurements, confirmed by first principles, self-consistent DFT calculations show that the strong, direct bonds between a Pd monolayer and Nb(001) push the $d$-band center of the monolayer toward lower binding energies, which results in the Pd reactivity comparable to that of the noble metal Ag. The strong epitaxial constraint of the Nb(001) substrate induces a $(11\overline{2}0)$-oriented hexagonal close-packed structure in thicker Pd films. First principles, self-consistent DFT calculations with spin-orbit coupling included performed at 0 K show that Pd in this structure is ferromagnetically ordered at the optimum lattice constant. Its bands at the Fermi level are flatter in comparison to those of Pd in its natural, nonmagnetic, face-centered-cubic structure, leading to the density of states (DOS) at the Fermi level which fulfills the Stoner criterion for ferromagnetism. We identify these bands in the bulk band structure and probe them with angle-resolved ultraviolet photoelectron spectroscopy in $(11\overline{2}0)$-oriented hexagonal close-packed Pd films.

Alloying of Pd thin films with Nb(0 0 1)

Annealing at elevated temperatures (1000–1600 K) of at least 10 ML thick Pd films deposited on Nb(0 0 1) has been found to result in a substrate capped by a pseudomorphic monolayer of Pd. This 1 ML thick Pd cap layer was characterised with a combination of UPS and DFT-calculations. UPS, RHEED and AES show that this cap layer protects the Nb(0 0 1) surface against (oxygen) contamination, which is a well known problem of Nb substrates. AES sputter profiling indicates that a major part of the Pd material in excess of the pseudomorphic monolayer is dissolved in the Nb lattice just below the surface. XPD shows that these dissolved Pd atoms occupy substitutional sites in the substrate. The analysis of the XPS-anisotropy also provides some information about the concentration and positions of the Pd and Nb atoms in the alloyed samples.

Trends in the adsorption and decomposition of hydrogen and ethylene on monolayer metal films: A combined DFT and experimental study

The combination of density functional theory (DFT) modeling with surface science experiments offers the potential to design catalysts with improved activity and selectivity. In the current paper we report theoretical and experimental studies of the adsorption and decomposition of hydrogen and ethylene on several bimetallic surfaces. DFT calculations predicted that Pd/Mo(1 1 0) and Pt/W(1 1 0) bimetallic surfaces would bind hydrogen and ethylene more weakly than do the corresponding surfaces of the parent metals. The adsorption and decomposition of hydrogen and ethylene on Pd/Mo(1 1 0) and Pt/W(1 1 0) bimetallic systems were investigated using temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), and high-resolution electron energy loss spectroscopy (HREELS) to test the DFT predictions. On the 0.5 and 1.0 ML Pd/Mo(1 1 0) surfaces, the binding strength and the degree of dissociation of hydrogen and ethylene were significantly reduced as compared to Mo(1 1 0) and thick Pd films. Similarly, on 0.5 and 1.0 ML Pt/W(1 1 0) surfaces, TPD results indicated that the degree of dissociation of hydrogen and ethylene was substantially reduced as compared to clean W(1 1 0). HREELS studies of C 2H 4 and C 2D 4 revealed that the adsorption of ethylene on Pt/W(1 1 0) occurred via the weakly π-bonded configuration, which was different from the di-σ bonding on clean W(1 1 0) or Pt(1 1 1).

Effects of deposition and synthesis parameters on size, density, structure, and field emission properties of Pd-catalyzed carbon nanotubes synthesized by thermal chemical vapor deposition

The effects of deposition parameters and NH3 pretreatment on the size and distribution of Pd catalytic particles and subsequently their effects on the characteristics of the synthesized carbon nanotubes (CNTs) were systematically investigated. It was found that the size of Pd particles decreases and the particle density (total number of Pd particles per unit area) increases as the Pd film thickness decreases. Moreover, pretreatment of Pd film in NH3 gas promotes smaller Pd particles and higher particle density which is beneficial for CNT growth. The CNTs were synthesized by thermal chemical vapor deposition at 750 °C using methane (CH4) as the carbon source, and a mixture of Ar/H2 (80 vol %: 20 vol %) as a carrier gas with NH3 serving as a processing reagent. The incorporation of NH3 in CNT synthesis, per the specific pretreatment of catalytic film, has a distinct effect on the size and morphology of CNTs produced. The interrelation between processing, structure and emission behavior of CNTs produced with different synthesis conditions was examined by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and field emission measurements.

Quantum well states of [formula omitted] and d xz, yz orbital character in Pd nanofilms

Quantum well states of quantised d-bands have been measured in the binding energy range from −4.8 to 0 eV with polarised and non-polarised ultraviolet photoelectron spectroscopy in 1–10 monolayer thick Pd films deposited on Nb(100) and on W(100). Using polarised ultraviolet photoelectron spectroscopy we could distinguish between quantum well states with d 3 z 2– r 2 and d xz, yz orbital character. Pd nanofilms grown on the two substrates have the (100)-oriented body centred tetragonal (i.e. pseudomorphic) structure up to the thickness of 2.5 monolayers. For larger thicknesses they have the ( 11 2 ¯ 0 ) -oriented hexagonal close packed structure, which is unusual for Pd. A partial coupling of Pd states to substrate ones and/or ferromagnetic order in the hexagonal-close packed Pd films can be responsible for the broad spectral shapes.

Precise measurement of hydrogen content in thin films using multilayer spin splitter

The results of neutron reflectometry on measuring hydrogen content in a Pd thin film are presented and a new method of detecting hydrogen content in a thin film of hydrogen absorbing material is proposed. By neutron reflectometry, hydrogen content H/Pd in a 40 nm -thick Pd film is obtained as 0.42 at the surface and as 0.69 at the substrate under 0.02 MPa hydrogen pressure. In addition to the above results, we propose a new method for measuring hydrogen content in a thin film using multilayer spin splitters, which enables us to apply neutron phase-spin echo technique. In the new method, precision of hydrogen content can be improved.

Two-dimensional growth of Pd on a thin FeO(1 1 1) film: a physical manifestation of strong metal–support interaction

Nucleation and growth of palladium vapor deposited on a thin FeO(1 1 1) film grown on a Pt(1 1 1) have been studied by scanning tunneling microscopy (STM). STM data shows that Pd randomly nucleates on the oxide surface and forms two-dimensional islands at sub-monolayer coverages. Annealing to 600 K results in strong metal sintering, thus forming extended Pd(1 1 1) monolayer islands at low coverage, and a thick Pd(1 1 1) film wetting the FeO substrate, at higher Pd coverage. Our data is in agreement with theoretical predictions which indicates this wetting behavior arises from a strong metal–support interaction between Pd and the FeO(1 1 1) film.

A Study of the Hydrogen Transport Properties of Palladium/Alumina Composite Membranes

The permeation of hydrogen through palladium composite membranes was investigated at temperatures 523-669 K and hydrogen pressures 80-220 kPa. Two pieces of Pd/α-alumina composite membranes prepared by electroless deposition were used in this study. The permeation behavior of hydrogen through the palladium composite membranes was analyzed by a resistance model in order to obtain the permeation parameters of the support and palladium layer. The results demonstrated that the thickness of the Pd layer played an important role in the permeation properties of hydrogen. The hydrogen flux through a Pd/alumina composite membrane, with a palladium film of 7 μm thickness, deviated from Sievert's law. This was due to decreasing the thickness of the Pd film, resulting in a significant surface limiting process contribution. In addition, Sievert's law was in good agreement with the data obtained for thick Pd film (15 μm), and it provided accurate prediction of permeability coefficient, activation energy, and the values of Δ H H 0 and Δ S H 0 . These obtained parameters were in excellent agreement with the literature.

The potentials of zero charge of Pd(1 1 1) and thin Pd overlayers on Au(1 1 1)

The potential of zero charge (pzc) of Pd(1 1 1) has been determined in dilute NaF solutions by measuring the Gouy–Chapman minimum of the double-layer capacity. For a massive Pd(1 1 1) single crystal electrode a pzc of −0.12 V vs. SCE has been found. The corresponding values for thin Pd(1 1 1) overlayers on Au(1 1 1) have also been determined. While the pzc of the first, pseudomorphic Pd layer on Au(1 1 1) is −0.09 V vs. SCE, the pzc of a five monolayers thick Pd film on Au(1 1 1) is practically identical to the pzc of the massive Pd(1 1 1) electrode. By comparing pzc's and work functions for Au(1 1 1) and Pd(1 1 1), the dipole contribution to the potential drop across the Pd(1 1 1)/water interface is estimated.

PERMEATION OF HYDROGEN THROUGH PALLADIUM/ALUMINA COMPOSITE MEMBRANES

In this study, the Pd/alumina composite membranes for hydrogen separation were prepared on a disk-type of α-alumina porous support and supported γ-alumina membrane by electroless-plating technique, respectively. The composite membranes with thickness of palladium layer ranging from 7–15 μm are used in the gas permeation experiment under 600–669 K and 50–220 kPa. The experimental results demonstrated that the hydrogen transport through these composite membranes is mainly controlled by solution-diffusion mechanism. It also showed that the selectivity coefficient of H2/N2 in the composite membranes reaches to infinite. The exponential dependency of the hydrogen partial pressure on permeation rate is in the range of 0.627 to 0.688, which is somewhat above that predicted by Sievert's law. Furthermore, the apparent activation energies of hydrogen permeation from the experimental results are within 8.8–9.3 kJ/mol. Additionally, the transport behavior of hydrogen through the bulk Pd layer of Pd/alumina composite membranes were analyzed based on the resistance model. Consequently, the hydrogen flux through a Pd/alumina composite membrane, with a palladium film of 7 μm thick, deviated from Sievert's law. In addition, the hydrogen flux through thick Pd film (15 μm) was observed to be proportional to the difference in the square root of the hydrogen pressure on either side of the film, in accordance with Sievert's law.

Tuning the chemistry of metal surfaces: II. Acetylene cyclotrimerization on ultrathin Pd films on Ta(1 1 0)

Chemisorption of acetylene (C 2H 2) and benzene (C 6H 6) on ultrathin Pd films (monolayer, bilayer, trilayer, etc.) deposited on Ta(1 1 0) is weaker than on bulk-terminated Pd surfaces, but it is not as strongly perturbed as was seen for H 2, CO, NO, and C 2H 4. Thus, the well-known alterations of the chemical properties of ultrathin metal films deposited on refractory metal substrates can be modified by strongly adsorbed species. C 2H 2 is predominately reversibly adsorbed on the Pd monolayer ( θ Pd=1) at 91 K, with thermal desorption peaks at 180 and 265 K. C 6H 6 is formed via cyclotrimerization of C 2H 2 and desorbs in a single peak at 407 K, but the yield is small (1% of an adsorbed benzene monolayer on that surface). Bilayer and trilayer Pd films ( θ Pd=2–3) have similar C 2H 2 chemistry to that from thicker films. On a thick film ( θ Pd=5) at 175 K, C 2H 2 adsorption leads to C 2H 2 desorption in a very broad peak near 330 K, and to benzene desorption at 250 and 500 K, with a yield of twice that on the Pd monolayer. Benzene chemisorption on the Pd monolayer is only slightly weaker than that on a thick Pd film ( θ Pd=5) or on Pd(1 1 1), (a reduction of 6 kcal/mol) but is completely reversibly adsorbed.

Tuning the chemistry of metal surfaces: I. Adsorption and reaction of NO and N 2O on ultrathin Pd films on Ta(1 1 0)

Nitric oxide (NO) chemisorption is a sensitive chemical probe of the electronic structure and reactivity of metal surfaces. We have used NO, in conjunction with temperature programmed desorption and high resolution electron energy loss spectroscopy, to explore the altered reactivity of ultrathin (monolayer, bilayer, trilayer) Pd films deposited on Ta(1 1 0). The reactivity of the Pd-monolayer film is strongly altered from that of bulk-terminated Pd surfaces. NO is molecularly adsorbed on the Pd monolayer at 95 K, but the desorption activation energy is decreased to only 8 kcal/mol, and N 2O is the primary desorption product. At low initial NO coverages, N 2O desorbs in a reaction rate-limited peak at 129 K, which grows and shifts up in temperature, with increasing coverage, to 159 K at saturation. NO desorption occurs at 135 K, in addition to N 2O, at high coverages. Separate N 2O adsorption experiments show that N 2O is weakly and reversibly bound to the Pd monolayer film, desorbing by 115 K. NO chemisorption and reaction was independent of the initial geometric structure of the Pd monolayer, i.e., nearly identical results were obtained when using a pseudomorphic-bcc(1 1 0) or incommensurate-fcc(1 1 1) Pd monolayer. However, the chemical properties and reactivity of the Pd films rapidly returned to that of bulk Pd(1 1 1) surfaces as the Pd film thickness was increased above one monolayer. “Tuning” of NO chemistry on these Pd films was possible for initial thicknesses of 2–3 layers. The interaction of NO with a Pd monolayer on Ta(1 1 0) closely resembles that of NO with a Ag(1 1 1) surface, supporting the interpretation that Pd–Ta bonding interactions lead to a filled Pd d-band resulting in more noble metal-like properties.

Spontaneous ordering of oxide nanostructures

We report the spontaneous formation of uniformly distributed arrays of "tips" (tall conical hillocks) upon oxidation of palladium (Pd) thin films. The formation of the palladium oxide tips depended on the thickness and granularity of the Pd film and on annealing and oxidation conditions. As the Pd film thickness increased from 40 to 200 nanometers, the average height of the tips increased from 0.5 to 1.2 micrometers, their height distribution became broader, and their density decreased from 55 x 10(6) to 12 x 10(6) per square centimeter. Enhanced photoelectron emission from locations corresponding to the tips suggests their possible use in field emission applications.

Identification of a laterally mobile state during CO adsorption

We have investigated CO adsorption at 300 K on ~1.5 atomic layer thick Pd films on a Mo(110) surface by high-resolution core level photoemission. We describe how high-resolution core level spectroscopy may be utilized to study the influence of laterally mobile states on the sticking probability of molecules on such a laterally heterogeneous surface. The present Pd films are laterally heterogeneous in the sense that the additional ~0.5 atomic Pd layer forms mesoscopic one-layer thick islands on top of the first Pd layer. At 300 K, CO chemisorbs on these two-layer thick islands but not on the one-layer parts of the film. The rate at which these two-layer islands are filled by CO molecules as the surface is exposed to CO is found to be consistent with a picture where CO molecules that initially impinge on the one-layer parts of the surface enter a laterally mobile state and diffuse to the two-layer islands and adsorb there. This mobile state is in many respects similar to a classical precursor state.

Structural effects of supported Pd particles in D2 and CO oxidation experiments

In the temperature range 100–300°C, isothermic D2 and CO titration experiments have been performed on oxygen pre-exposed 1.5, 4, 20 and 200nm thick Pd films, epitaxially cube-on-cube grown on MgO(100). The oxygen pre-exposures varied from 0.2 to 150L. The three thinnest films consisted of faceted particles, mainly bounded by Pd(111) and Pd(100) surfaces, while the 200nm sample was a continuous, flat Pd(100) film with atomic steps. Atomic force microscopy demonstrated that the titration experiment did not affect the structure of the (large) Pd particles in the 20nm film. The reaction behavior of the 1.5nm film can be understood by assuming that the film consists of (111)-bounded Pd particles, where O2 and D2 dissociate and spill over to the oxide support, whereas CO adsorbs and reacts both on the Pd and on the oxide. The CO oxidation thus occurs in parallel on Pd and on MgO, while the D2 oxidation is sequential, first occurring on the Pd and then on the MgO. To explain the D2O desorption curves for the 4 and 20nm films, spillover to the MgO support as well as reactions on the Pd(111) and Pd(100) facets have to be taken into account. The greater the oxygen pre-exposure, the higher the probability that the D2 molecules dissociate primarily on the Pd(111) surfaces.

SO 2 adsorption on thin Pd/Ni(111) films studied by X-ray absorption fine structure spectroscopy

Surface structures and the electronic properties of submonolayer sulfur dioxide SO 2 molecularly absorbed on ultrathin Pd films grown on Ni(111) have been investigated by means of S K-edge surface extended X-ray absorption fine structure and near-edge X-ray absorption fine structure (NEXAFS). On the 1.2 ML Pd film, SO 2 was found by polar-angle dependent NEXAFS to be adsorbed with the molecular plane nearly parallel to the surface. This configuration is quite similar to that on Ni(111), but shows a clear contrast to the SO 2/Pd(111) case where the molecule was adsorbed with its molecular plane normal to the surface. As the Pd-film thickness increases, SO 2 gradually stands up on the surface. On the 6 ML Pd film, the surface structure of SO 2 is essentially the same as that on Pd(111). These results indicate drastic perturbations in the electronic and chemical properties of the thin Pd film compared to that of the bulk.

Comparison of the CO and D 2 oxidation reactions on Pd supported on MgO(100), MgO(110) and MgO(111)

Oxidation of D 2 and CO on oxygen pre-exposed 200 nm thick Pd films, epitaxially grown on MgO(100), MgO(110) and MgO(111), has been investigated in the temperature range 100–300°C. Oxygen initial sticking coefficients have been determined to be close to 1 for the 100 and 110 films, and around 0.8 for the 111 film. The sticking coefficient and reactive sticking coefficient for CO oxidation on Pd/MgO(100) is also close to 1, and the maximum reactive sticking coefficient for hydrogen oxidation is determined to be around 0.9 at temperatures above 200°C. It is shown that the reactivities for the different surfaces vary strongly with surface and oxygen coverage, and the consequence of this for supported particle catalysts is pointed out.

Current oscillations in thin metal–oxide–semiconductor structures observed by ballistic electron emission microscopy

Quantum interference oscillations of electrons in a thin SiO2 layer were observed by ballistic electron emission microscopy (BEEM). With BEEM, electrons are injected across the gate of a metal–oxide–semiconductor (MOS) structure and directly into the conduction band of the SiO2. The MOS capacitor consisted of a 5 nm thick Pd film deposited on a 2.8±0.2 nm oxide thermally grown on Si(100). Oscillations with up to four peaks in an energy range of 0–3 eV above the injection threshold were noted. Their magnitude is of the order of 30% of the underlying BEEM current. The oscillations were most salient and their energy location repeatable at points of the sample that were previously not exposed to the electron beam. Even modest exposures caused a buildup of positive charge. This charge resulted in energy shifts, as well as a weakening of the oscillations, both of which are a consequence of the added scattering and local field inhomogeneities associated with the random distribution of the positive charge. Solutions of the Schrödinger equation that included a built-in oxide potential of 0.20 V and image force effects at both interfaces gave excellent fits to the experimental data for an effective electron mass in the oxide mox=0.63±0.09mo. The uncertainty in mox arises from an uncertainty of ±0.2 nm in the determination of the oxide thickness by ellipsometric methods. Nevertheless, the obtained value is well above the generally accepted value of 0.5mo.

Effects of acoustic waves on activation of thin film Pd and Ni catalysts for ethanol and CO oxidation

The effects of acoustic wave excitation of surface acoustic waves (SAWs) and resonance oscillations (ROs) on catalytic and adsorptive properties of metal thin films have been studied. A 5–200 nm thick Pd thin film was deposited as a catalyst on the Rayleigh SAW propagation path of a 128°-rotated y-cut LiNbO3 single crystal. For ethanol oxidation, the introduction of a 10 MHz SAW increased acetaldehyde production. The activity enhancement varied, depending not only on the surface states (reduced and oxidized Pd surfaces), but also on the thickness of the Pd films: larger activity enhancement occurred for a thick Pd film with an oxidized surface. For CO oxidation, the oxidized Pd had slightly greater effect than reduced Pd, but catalyst activation with SAW-on was small over a whole range of film thickness for both the reduced and the oxidized Pd. The high-resolution IR reflection absorption spectra of CO adsorbed on a 40 nm Pd thin film deposited on the propagation path of a shear horizontal leaky SAW (36°-rotated y-cut LiTaO3) exhibited a large peak, assigned to bridging CO, at 1982 cm-1 and a small on-top CO peak at 2073 cm-1. With SAW-on, the intensity and position of both the peaks remained nearly unchanged, but a shoulder appeared on the bridging CO peak at ca. 1930 cm-1. A laser Doppler measurement showed that a vertical lattice displacement due to the Rayleigh SAW increased with increasing Pd film thickness, up to 50 nm, and was nearly constant over the range 50–200 nm, in which the magnitude of lattice displacement was larger for reduced than for oxidized Pd. SAW-induced catalyst activation is discussed on the basis of reaction mechanisms and the adsorbed states of the reactants. From the dependence of lattice displacement on the thickness and surface states of the Pd films, a model in which the SAW interacts with the charged carriers in the thin films and affects the adsorbed states of the reactants is proposed. Furthermore, it was demonstrated that RO generated on a Pd thin film deposited on z-cut LiNbO3 had a remarkable effect on its catalytic activity for ethanol oxidation.