A careful choice of the surface coverage of iron phthalocyanine (FePc) on Ag (110) around the single monolayer allows us to drive with high precision both the long-range supramolecular arrangement ...and the local adsorption geometry of FePc molecules on the given surface. We show that this opens up the possibility of sharply switching the catalytic activity of FePc in the oxygen reduction reaction and contextual surface oxidation in a reproducible way. A comprehensive and detailed picture built on diverse experimental evidence from scanning tunnelling microscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy, coupled with density functional theory calculations, sheds new light on the nature of the catalytically active molecule-surface coordination and on the boundary conditions for its occurrence. The results are of relevance for the improvement of the catalytic efficiency of metallo-macrocycles as viable substitutes for platinum in the cathodic compartment of low-temperature fuel cells.
Iron phthalocyanines (FePc) adsorbed onto a Ag(110) substrate self-assemble into different monolayer phases going from rectangular to different oblique phases, with increasing molecular density. We ...have investigated the oxygen uptake capability of the different phases and their associated magneto-structural changes. Our study combines scanning tunneling microscopy and spectroscopy (STM/STS), X-ray magnetic circular dichroism (XMCD), and density functional theory (DFT) calculations. STM measurements reveal that the oxygenation reaction of the FePc/Ag(110) generally involves a displacement and a rotation of the molecules, which affects the electronic state of the Fe centers. The oxygen intercalation between FePc and the substrate is greatly obstructed by the steric hindrance in the high-density phases, to the point that a fraction of oblique phase molecules cannot change their position after oxidizing. Depending on the oxidation state and adsoption geometry, the STS spectra show clear differences in the Fe local density of states, which are mirrored in the XAS and XMCD experiments. Particularly, XMCD spectra of the oxidized phases reflect the distribution of FePc species (nonoxygenated, oxygenated-rotated, and oxygenated-unrotated) in the different cases. Sum rule analysis yields the effective spin (m s eff) and orbital (m L) magnetic moments of Fe in the different FePc species. Upon oxygenation, the magnetic moment of FePc molecules increases about an order of magnitude, reaching m TOT ∼ 2.2 μB per Fe atom.
High surface‐to‐volume ratio Co3O4/TiO2 heterojunctions were fabricated by combining different methods. Atomic layer deposition (ALD) and a photochemical method were used to coat polystyrene (PS) ...3D‐Direct Opal (3D‐DO) structures on conductive ITO substrates. Firstly, 3D‐DO of PS were crystallized on ITO substrates to form the high surface‐to‐volume ratio template via a self‐assembly method. A low‐temperature ALD TiO2 film was infiltrated onto the PS opal structure. Then, the PS template was removed by a thermal treatment in air at 450 °C for 5 h. Hollow anatase phase nanospheres were obtained, crystallized in a face centered cubic (FCC) lattice with the (111) plane oriented parallel to the substrate surface. Finally, the hollow TiO2 nanospheres were coated with Co3O4 via a photochemical method. This ordered 3D nanostructure with designed morphology may find applications as surface‐enhanced materials for photovoltaic devices.
TiO2 hollow nanospheres obtained via low‐pressure ALD (a) and Co3O4/TiO2 heterostructure (scanned region: 1.5 μm) (b).
Multicomponent supramolecular self-assemblies of exceptional long-range order and low defectivity are obtained if C60 and 5-(4-aminophenyl)-10,15,20-triphenylporphyrin (TPP-NH2) are assembled on ...Ag(110) by sequential evaporation in the submonolayer range of TPP-NH2 and fullerene on the substrate surface and subsequent annealing. A (±2 −3, 6 ± 3) array consisting of supramolecular stripes of a 1:1 C60/TPP-NH2 2D adduct develops at 410 K (the low temperature, LT, phase). If the LT phase is annealed at 470 K, then a 3:1 fullerene/TPP-NH2 (±3 −5, 5 ± 5) nanoporous array (the HT phase) forms, with each pore containing a single porphyrin molecule. Phase separation occurs by annealing the HT phase at 520 K. Structural models are proposed and discussed on the basis of the experimental scanning tunneling microscopy results.
Thermal self-assembly of C60 on (1 × 2)-Pt(110) following room-temperature deposition has been studied by means of scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and ...density functional theory (DFT) calculations. Two distinct C60 chemisorption phases have been identified and characterized as a function of the annealing temperature. After a thermal treatment at 700 K, islands of a C60 quasi-hexagonal lattice form. These islands are characterized by the highest surface density so far reported for a two-dimensional surface-supported fullerene phase (the surface area per molecule is 87.0 Å2), with intermolecular nearest-neighbor distances equal to 9.6 ± 0.1 Å. Embedded nanowires of fullerene dumbbell dimers (with an intermolecular distance equal to 9.2 ± 0.1 Å) occasionally nucleate within this high-density phase following a “molecular zip” mechanism. Highly site selective chemisorption driven by the particularly strong overlayer−substrate bonding is proposed to be responsible for the first reported example of surface-templated chemical bond formation between fullerene molecules. After annealing at 850 K, an oblique C60 chemisorption phase forms. This is characterized by a tetramolecular basis associated to each lattice point of the two-dimensional superstructure and by a peculiar bright−dim contrast in STM images acquired at positive sample bias values. The combination of high-resolution STM images with LEED data and DFT calculations leads to the conclusion that the (1 × 2) substrate reconstruction is lifted at the fullerene−platinum interface and also provides a detailed description of the molecular bonding sites and orientations found within the phase. It is proposed that the main factor ruling the interconversion of chemisorption phases is the variation of substrate atom mobility as a function of temperature.
The interaction of single C60 molecules with the (1 × 2)-Pt(110) surface has been studied by scanning tunneling microscopy and density functional theory (DFT) calculations on slab models. Molecules ...are observed to be frozen at room temperature and are found to be almost exclusively in the same configuration. Extensive DFT calculations show that this configuration is the global energy minimum, suggesting that adsorbed molecules have enough rototranslational freedom to escape from the numerous local minima. The adsorption energy (3.81 eV) is the strongest ever found for C60, and it is roughly proportional to the number of the Pt and C atoms at contact distance. Analysis of DFT results shows that the surface−adsorbate interaction is covalent in nature. A minority fraction of C60 molecules appear to be adsorbed on surface defects. A careful investigation of their registry and height with respect to the regularly adsorbed units leads to an indirect structural characterization of the nanopits which act as their adsorption sites.
A surface stabilized monolayer phase of nickel oxide, c(4 × 2)-Ni3O4, has been found to grow epitaxially under reactive deposition conditions on Pd(100), in the presence of other adsorbed phases and ...in competition with them. High-quality scanning tunneling microscopy data are reported and discussed, including a detailed analysis of the defects and of the border morphology of this new phase. The data are discussed in the light of ab initio simulations of the electronic, energetic, and geometric properties of such a phase. A hybrid-exchange density functional theory approach has been used, and a slab model is adopted where palladium is simulated by a thin film covered on both sides by regular epilayers. A growth model has been developed that explains both the unusual stoichiometry of the phase and the observed defects.
This paper reports on the optimisation of the growth parameters of NiO ultrathin films on Pd(1
0
0). Growth is performed by means of UHV metal deposition and post-oxidation cycles. Chemical and ...structural characterisation of the deposits is achieved by means of electron spectroscopy (X-ray photoelectron spectroscopy, XPS; angle resolved XPS) and electron diffraction techniques (low energy electron diffraction, LEED; X-ray photoelectron diffraction, XPD).
Three growth procedures have been investigated, which differ for the particular growth parameters adopted in each case. We demonstrate that post-oxidation is effective in order to obtain epitaxial NiO only if the initial dose of Ni evaporated on the clean Pd(1
0
0) substrate exceeds a critical value, corresponding approximately to two equivalent monolayers. However, the overlayer thus obtained is strongly understoichiometric in oxygen close to the metal/oxide interface and poorly ordered on the long range. When a Ni dose below this limiting first value is used, the layer evolves toward polycrystalline NiO, due to substantial oxidation of the Pd substrate promoted by the presence of Ni, very likely through a work function decrease upon direct metal/metal interface formation. On the contrary, epitaxial NiO(1
0
0) layers of good structural quality, with limited oxygen deficiency at the interface, with negligible substrate oxidation and with a good degree of long-range order are obtained if deposition and post-oxidation cycles are initiated on an oxygen pre-saturated Pd surface, characterised by the (√5×√5)-
R27° O/Pd(1
0
0) LEED pattern. We therefore demonstrate that oxygen can act either as an inhibitor or as a promoter of NiO epitaxial growth on Pd(1
0
0), depending on the way it is used.
NiO ultrathin films have been grown on Pd(1
0
0) following a reactive deposition procedure. Ni has been dosed at room temperature on the substrate surface in an oxygen partial pressure of 4
×
10
−6 ...mbar. The electronic and structural evolution of the resulting NiO(1
0
0) ultrathin films has been followed by means of X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD), low energy electron diffraction (LEED), and scanning tunnelling microscopy (STM). XPS, XPD and STM data indicate a 2D growth of the first NiO monolayer, while further growth leads to the nucleation of 3D islands, in a Stranski–Krastanov growth scheme. Combined XPD and LEED data indicate an initially pseudomorphic growth, characterised by in-plane compressive tetragonal strain of the NiO film, with a consequent out-of-plane interlayer expansion. Partial strain relaxation occurs abruptly, very likely between the second and the third atomic layer of the 3D islands, while a completely bulk-like cubic environment is reached only gradually as a function of thickness. NiO(1
0
0) films even ∼50 equivalent monolayers thick can be grown with good long-range order, as shown by (1
×
1) LEED images.
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