Pd Islands Research Articles

Palladium islands on iron oxide nanoparticles for hydrodesulfurization catalysis

Deposition of thin Pd islands on iron oxide nanoparticles results in a 4-fold activity enhancement in HDS and suppresses cracking.

Self-organization of palladium nanoislands on GaN and AlxGa1−xN/GaN heterostructures

We report on systematic study on the dewetting process of thin Pd layer and self-organized Pd nano-islands on SiO2, GaN and AlxGa1−xN/GaN heterostructures with various Al content. The influence of factors such as the thickness of metal layer, type of top layer of AlGaN/GaN heterostructures, temperature and time of annealing process on the dimensions, shapes and density of Pd islands was analyzed. Comparing the behavior of self-organization of Pd islands on Al0.25Ga0.75N/GaN and SiO2 we can conclude that solid-state dewetting process on SiO2 occures much faster than on Al0.25Ga0.75N. For substrates with SiO2 this process requires less energy and can arise for thicker layer. On the Al0.25Ga0.75N surface the islands take more crystalline shape which is probably due to surface reconstruction of Pd-Ga alloy thin layer on interface. For thin metal layer the coalescence of islands into larger islands similar to Ostwald ripening mechanism was observed. Greater surface roughness of AlxGa1−xN/GaN heterostructures with higher Al content causes an increase of surface density of islands and the reduction of their sizes which improves the roundness. In case of GaN and AlxGa1−xN layers with Al content lower than 20%, the surface degradation caused by annealing process was observed. Probably, this is due to the decomposition of layers with gallium droplet formation on catalytic metal islands.

Nucleation kinetics of heterogeneous catalyst: Pd/thin layer of MgO (100)

The kinetics describing the formation of the system Pd/thin layer of MgO (100) surface was investigated by applying programs developed on the basis of Fortran Software. The simulation is based on studies related to nucleation, crystallite growth, coalescence and diffusion of clusters on the surface of thin films. The density of Pd islands is obtained by simulating the deposition of 1 × 1013 atoms cm–2 s–1 in 150 s. The density of clusters first increases to reach a plateau. Then, it either remains some time at a nearly constant value or decreases slowly after the saturation regime gives way to the coalescence stage. This phenomenon is explained via island migration process at the surface. The coalescence time strongly depends on the deposition temperature, the coalescence occurs when the substrate temperature is high. Also, the surface coverage decreases when the substrate temperature increases. The coalescence is pronounced at a low surface coverage. It involves the migration on the surface, defined as the dynamic coalescence and precedes the process of the formation of immobile islands that predominates at high extents of surface coverage.

Catalysis of oxygen reduction on electrochemically activated polycrystalline gold by Pd nanoislands in alkaline solution

Catalytic properties of electrochemically activated polycrystalline gold, Au(poly), modified by spontaneously deposited palladium nanoislands with the coverage below a monolayer were examined for the oxygen reduction reaction (ORR) in alkaline solution using the rotating disc electrode (RDE) technique. Atomic force microscopy (AFM) images have shown that the coverage of randomly distributed nanosized Pd islands ranged from 18% to 77%. Cyclic voltammetry characterization involved hydrogen adsorption/desorption on the deposited Pd islands, as well as their oxidation/reduction at more positive potentials. RDE measurements have shown that, although ORR on Au(poly) proceeds partly through 4e-reduction pathway, in the presence of Pd islands it turns completely to 4e-pathway, exceeding the activity of bare Pd(poly). Among different Pd/Au(poly) nanostructures, the ones with Pd coverage of 18% and 30% have shown the highest activity for ORR in alkaline solution.

Single Molecule Vibrational Spectroscopy: CO Bonding to Edge and Terrace Positions on Ag, Au, and Pd Islands on NiAl(110).

The vibrational properties of single CO molecules adsorbed on nanosized Ag, Au, and Pd islands on a NiAl(110) surface were studied with a scanning tunneling microscope. The sensitivity of single molecule vibrational spectroscopy to aspects of the local environment is demonstrated by comparative studies of CO-metal bond vibrations at island terrace and island edge sites. Vibrational spectra of single CO molecules adsorbed on Ag, Au, and Pd island terraces showed peaks at 27, 32, and 44 meV, respectively, which are assigned to the hindered rotational mode. CO molecules on Au and Pd island edges, on the other hand, showed blue-shifted hindered rotational modes at 34 and 46 meV, respectively. On Au islands, CO molecules showed a strong preference for adsorption on edges, while no such preference was observed on Pd.

Heterometallic antenna−reactor complexes for photocatalysis

Metallic nanoparticles with strong optically resonant properties behave as nanoscale optical antennas, and have recently shown extraordinary promise as light-driven catalysts. Traditionally, however, heterogeneous catalysis has relied upon weakly light-absorbing metals such as Pd, Pt, Ru, or Rh to lower the activation energy for chemical reactions. Here we show that coupling a plasmonic nanoantenna directly to catalytic nanoparticles enables the light-induced generation of hot carriers within the catalyst nanoparticles, transforming the entire complex into an efficient light-controlled reactive catalyst. In Pd-decorated Al nanocrystals, photocatalytic hydrogen desorption closely follows the antenna-induced local absorption cross-section of the Pd islands, and a supralinear power dependence strongly suggests that hot-carrier-induced desorption occurs at the Pd island surface. When acetylene is present along with hydrogen, the selectivity for photocatalytic ethylene production relative to ethane is strongly enhanced, approaching 40:1. These observations indicate that antenna-reactor complexes may greatly expand possibilities for developing designer photocatalytic substrates.

Structure and roughness analysis of thin epitaxial Pd films grown on Cu/Si(111) surface

We investigated growth processes of epitaxial Pd(111) films on Cu(10ML)/Si(111) substrate. We found three stages of growth. In the first stage, up to Pd thickness of 2.6ML, the Pd film is the most disordered. Most of the strains in the Pd film relax in this stage. In the second stage, in the thickness interval from 2.6 to 13ML, the roughness of the Pd does not change significantly and the Pd grows in a layer-by-layer like mode. During the second stage, the lattice parameter of the Pd film gradually increases to volume value. With increasing coverage after 13ML, in the third stage, the lattice parameter does not change, but the roughness of the Pd films increases rapidly. Pd islands grow in height and in lateral size. Epitaxial growth of Pd on Cu(10ML)/Si(111) substrate is compared with the growth of Pd on Cu(111) single crystal; the results obtained are discussed.

Vibrational Spectroscopic Observation of Atomic-Scale Local Surface Sites Using Site-Selective Signal Enhancement.

Molecule-substrate interactions are sensitively affected by atomic-scale surface structures. Unique activity in heterogeneous catalysts or electrocatalysts is often related with local surface sites with specific structures. We demonstrate that adsorption geometry of a model molecule with an isocyanide anchor is drastically varied among one-fold atop, two-fold bridge, and three-fold hollow configurations with increasing the size of atomic-scale local surface sites of Pd islands on an Au(111) model surface. The vibrational spectroscopic observation of such local information is realized by site-selective and self-assembled formation of hotspots, where Raman scattering intensity is significantly enhanced via excitation of localized surface plasmons.

Fabrication of highly sensitive and selective H2 gas sensor based on SnO2 thin film sensitized with microsized Pd islands

Ultrasensitive and selective hydrogen gas sensor is vital component in safe use of hydrogen that requires a detection and alarm of leakage. Herein, we fabricated a H2 sensing devices by adopting a simple design of planar-type structure sensor in which the heater, electrode, and sensing layer were patterned on the front side of a silicon wafer. The SnO2 thin film-based sensors that were sensitized with microsized Pd islands were fabricated at a wafer-scale by using a sputtering system combined with micro-electronic techniques. The thicknesses of SnO2 thin film and microsized Pd islands were optimized to maximize the sensing performance of the devices. The optimized sensor could be used for monitoring hydrogen gas at low concentrations of 25–250ppm, with a linear dependence to H2 concentration and a fast response and recovery time. The sensor also showed excellent selectivity for monitoring H2 among other gases, such as CO, NH3, and LPG, and satisfactory characteristics for ensuring safety in handling hydrogen. The hydrogen sensing characteristics of the sensors sensitized with Pt and Au islands were also studied to clarify the sensing mechanisms.

Effects of gamma irradiation on hydrogen gas-sensing characteristics of Pd–SnO2 thin film sensors

Monitoring hydrogen levels in radioactive environments is important in nuclear energy safety and space study because leakage of this gas can cause destructive detonation. Herein, hydrogen gas sensing devices were fabricated by using a simple design of a planar-type structure sensor containing a SnO2 thin film sensitized with microsized Pd islands. In addition, the effects of gamma irradiation on sensor performance were investigated and results revealed that low doses of gamma irradiation had ignorable effect on the sensing performance of the device. However, a relatively high dose of gamma irradiation improved the sensitivity of the device because of oxygen defect generation. The enhancement of hydrogen gas-sensing characteristics was correlated with microstructure and optical characterization. Results show that gamma irradiation induced defects in the SnO2 thin film, controlling the doping level, and thus enhancing the gas-sensing characteristic of the device. The sensor can be used for monitoring hydrogen gas at low concentrations of 50 ppm–500 ppm, with fast response and recovery time, making it suitable for potential safety applications in monitoring hydrogen levels in radioactive environments.

Sophisticated construction of Au islands on Pt-Ni: an ideal trimetallic nanoframe catalyst.

We have developed a priority-related chemical etching method to transfer the starting Pt-Ni polyhedron to a nanoframe. Utilizing the lower electronegativity of Ni in comparison to Au atoms, in conjunction with the galvanic replacement of catalytically active Au to Ni tops, a unique Au island on a Pt-Ni trimetallic nanoframe is achieved. The design strategy is based on the structural priority mechanism of multimetallic nanocrystals during the synthesis and thus can be generalized to other analogous metal-bimetallic nanocrystal combinations (such as Pd and Cu islands on Pt-Ni nanoframes), which is expected to pave the way for the future development of efficient catalysts.

Ni electrochemical epitaxy on unreconstructed Au(111): An in-situ STM study

Abstract Gold (111) is a widely used substrate in vacuum and in electrolytes for studying the initial stages of metal growth. Its surface reconstruction is unavoidable using common preparation procedures in vacuum and this affects the growth mechanism of metal overlayers. In the electrochemical environment, the same surface reconstruction is obtained at potentials negative of the Au(111) potential of zero charge, whereas above this potential the surface structure is (1 × 1). In this paper we present a preparation procedure which allows blocking the Au step edges by Pd islands, inhibiting the formation of the Au(111) reconstruction and yielding the unreconstructed Au(111) surface (1 × 1) at any electrochemical potential. Using in-situ scanning tunnelling microscopy, we compare the Ni/Au(111) electrochemical epitaxy on reconstructed and on unreconstructed Au(111). Observations show that performing Ni growth on the (1 × 1) surface drastically improves the long-range order of the (8 × 8) moire pattern created by the lattice mismatch between the Au substrate and Ni adlayer. Detailed characterizations of the moire structure and of its electrochemical dependence provide new insights into the relative energy landscape of the multiple adsorption sites of the Ni atoms within the (8 × 8) unit cell. The possibility of stabilizing a Au(111)-1 × 1 surface opens up new perspectives for studying metal growth and molecular organisation.

The effect of the size of surface Pd island ensembles on electron transfer of adsorbed perchlorate ions on Au(111)

A progressive increase in the size of Pd ensembles on a mica-supported Au(111) single crystal surface can facilitate electron transfer of perchlorate ions at lower anodic potential in CV curves than pure Au(111) due to a strong ligand effect and Pd-Au neighbouring pairs at edge sites render a higher degree of electron transfer.

Effect of Embedded Pd Microstructures on the Flat-Band-Voltage Operation of Room Temperature ZnO-Based Liquid Petroleum Gas Sensors

Three methods were used to fabricate ZnO-based room temperature liquid petroleum gas (LPG) sensors having interdigitated metal-semiconductor-metal (MSM) structures. Specifically, devices with Pd Schottky contacts were fabricated with: (1) un-doped ZnO active layers; (2) Pd-doped ZnO active layers; and (3) un-doped ZnO layers on top of Pd microstructure arrays. All ZnO films were grown on p-type Si(111) substrates by the sol-gel method. For devices incorporating a microstructure array, Pd islands were first grown on the substrate by thermal evaporation using a 100 μm mesh shadow mask. We have estimated the sensitivity of the sensors for applied voltage from –5 to 5 V in air ambient, as well as with exposure to LPG in concentrations from 500 to 3,500 ppm at room temperature (300 K). The current-voltage characteristics were studied and parameters such as leakage current, barrier height, reach-through voltage, and flat-band voltage were extracted. We include contributions due to the barrier height dependence on the electric field and tunneling through the barrier for the studied MSM devices. The Pd-enhanced devices demonstrated a maximum gas response at flat-band voltages. The study also revealed that active layers consisting of Pd microstructure embedded ZnO films resulted in devices exhibiting greater gas-response as compared to those using Pd-doped ZnO thin films or un-doped active layers.

Revealing the Details of the Surface Composition of Electrochemically Prepared Au@Pd Core@Shell Nanoparticles with in Situ EXAFS

Carbon supported Au@Pd core@shell nanoparticles were prepared using two methods based on displacement of a Cu-under potential deposited (upd) layer; the standard method, in which a single Cu-upd layer is formed and then displaced, and a Cu-upd-mediated deposition method, wherein the Pd displaces the Cu-upd layer during the Cu deposition. The resulting materials were characterized using in situ extended X-ray absorption fine structure as a function of the applied potential in an electrochemical cell at both the Au L3 and Pd K absorption edges. This detailed structural analysis shows that the standard, single Cu-upd layer method results in the formation of Pd clusters or islands on the Au core rather than a complete monolayer shell, while the Cu-upd-mediated method produces a mixed/alloyed PdAu shell.

Electrocatalysis of oxygen reduction on electrodeposited Pd coatings on gold

Abstract The electrochemical reduction of oxygen on electrodeposited palladium coatings was studied in acid and alkaline media employing the rotating disc electrode (RDE) method. Gold was used as an electrode substrate and Pd was deposited at a constant potential varying the deposition time. Cyclic voltammetry and CO stripping experiments were used for surface characterisation of Pd coatings. The surface morphology of the as-prepared electrodes was examined by atomic force microscopy (AFM). The AFM images revealed the formation of Pd islands on the substrate surface. It was found that in alkaline solution the palladised electrodes show high specific activity for oxygen reduction and it does not depend on deposition time. In acid solution the dependence on the deposition time was in evidence and the specific activity surpassed that of bulk palladium. The oxygen reduction reaction (ORR) on electrodeposited Pd proceeds mainly via a four-electron pathway in both solutions and the reduction mechanism is similar to that on bulk Pd.

4-Mercaptopyridine adsorbed on pure palladium island films: A combined SERS and DFT investigation

Surface-enhanced Raman scattering (SERS) can provide vibrational information with molecular specificity and potential single-molecule sensitivity. SERS studies on pure transition metals, however, remain challenging due to the weak SERS activity of transition metals compared to coinage metals. Here we fabricated alumina-supported Pd island films by depositing laser-ablated Pd colloids onto an Al substrate. Robust SERS signal employing 4-mercaptopyridine (4-Mpy) as a probe was registered from the as-prepared films. The adsorption information of 4-Mpy molecules such as orientation and coordinating site was discussed in detail based on SERS data. It was inferred that 4-Mpy adsorbs via its sulfur atom to Pd surface with a tilted binding configuration. The Raman wavenumber and intensity of an adsorption model including one 4-Mpy and Pd atom were computed using density functional theory (DFT) at the Beck’s three-parameter Lee–Yang–Parr (B3LYP) level with the LANL2DZ basis set. The simulated Raman spectrum was in good agreement with the experimental one except for the relative intensity. The current investigation could be helpful to gain a comprehensive understanding of SERS.

Selective Alcohol Dehydrogenation and Hydrogenolysis with Semiconductor-Metal Photocatalysts: Toward Solar-to-Chemical Energy Conversion of Biomass-Relevant Substrates

Photocatalytic conversion of biomass is a potentially transformative concept in renewable energy. Dehydrogenation and hydrogenolysis of biomass-derived alcohols can produce renewable fuels such as H2 and hydrocarbons, respectively. We have successfully used semiconductor-metal heterostructures for sunlight-driven dehydrogenation and hydrogenolysis of benzyl alcohol. The heterostructure composition dictates activity, product distribution, and turnovers. A few metal (M = Pt, Pd) islands on the semiconductor (SC) surface significantly enhance activity and selectivity and also greatly stabilize the SC against photoinduced etching and degradation. Under selected conditions, CdS-Pt favors dehydrogenation (H2) over hydrogenolysis (toluene) 8:1, whereas CdS0.4Se0.6-Pd favors hydrogenolysis over dehydrogenation 3:1. Photochemically generated, surface-adsorbed hydrogen is useful in tandem catalysis, for example, via transfer hydrogenation. We expect this work will lead to new paradigms for sunlight-driven conversions of biomass-relevant substrates.

Strain relief and Pd island shape evolution on the palladium and palladium hydride (100) surface

The mesoscopic relaxation of small Pd islands on Pd(100) and PdH(100) surfaces is investigated on the atomic scale by performing molecular statics calculations. A strong strain and stress inhomogeneity in islands and topmost layers of the substrate is revealed. An unusual size dependence of the shape of islands is discovered.

Pd-catalyzed hydrodehalogenation of chlorinated olefins: Theoretical insights to the reaction mechanism

Density functional theory calculations are applied to study energetics of trichloroethene (TCE) hydrodechlorination over pure Pd(111), chlorine-covered Pd(111), and Pd island supported by Au(111). Our results show that in all cases CCl bond breakings take place more readily than CH bond formations and that TCE dechlorinates fully producing CCH precursors for the hydrogenations. The reaction pathway through radical-like species provides a possible explanation to the experimental product distributions that show a nominal amount of lesser chlorinated species in the presence of excess hydrogen. The surface chlorine resulted from the TCE decomposition weakens the binding of the adsorbates and reduces the gap between the dechlorination and hydrogenation barriers but is not expected to affect the overall reaction route. The underlying gold induces stronger adsorption and decreases the activation barriers considerably compared to the pure Pd(111). The change in Gibbs free energy between the gas-phase molecule and the activated surface species explains the experimentally observed reactivity ordering among chlorinated ethenes. Although vinyl chlorine has a stronger CCl bond than TCE, it dechlorinates faster when introduced from the gas phase to the catalyst surface.