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We describe an experimental method to probe the adsorption of water at the surface of isolated, substrate-free TiO2 nanoparticles (NPs) based on soft X-ray spectroscopy in the gas phase using synchrotron radiation. To understand the interfacial properties between water and TiO2 surface, a water shell was adsorbed at the surface of TiO2 NPs. We used two different ways to control the hydration level of the NPs: in the first scheme, initially solvated NPs were dried and in the second one, dry NPs generated thanks to a commercial aerosol generator were exposed to water vapor. XPS was used to identify the signature of the water layer shell on the surface of the free TiO2 NPs and made it possible to follow the evolution of their hydration state. The results obtained allow the establishment of a qualitative determination of isolated NPs’ surface states, as well as to unravel water adsorption mechanisms. This method appears to be a unique approach to investigate the interface between an isolated nano-object and a solvent over-layer, paving the way towards new investigation methods in heterogeneous catalysis on nanomaterials.

Progress in spin electronics and data storage strongly relies on properties of highly integrated nanostructures with characteristic sizes of only a few atoms. In this respect, a particularly promising approach consists of exploiting advances in coordination chemistry which make it possible to synthesize small clusters with a well-defined number of transition metal ions that behave like identical tiny magnets. [1,2] Various forms of such single-molecule magnets (SMMs) exhibit magnetic bistability at low temperature, which makes them very attractive as elementary units for high-density data-storage applications and in the design of quantum computers.[3] SMMs also exhibit fascinating features related to their borderline behavior between classical and quantum physics: the magnetic hysteresis behavior is superposed by field-dependent features due to quantum relaxation of the magnetization between opposite spin states. These systems are characterized by their blocking temperature, the temperature below which the magnetization does not fluctuate in direction anymore but is blocked on the time scale of the experiment. The highest blocking temperature (T B ) reported so far belongs to the Mn 12 family, whose canonical structure is represented schematically in Figure 1. [4] In these systems, T B » 3 K as measured in SQUID (superconducting quantum interference device)experiments. Besides their comparatively slow relaxation times of magnetization, SMMs also possess distinct advantages over nanoscale metal clusters, such as perfect size calibration and good solubility in organic solvents. A tremendous challenge then consists of assembling monolayers of periodically arranged SMMs on a substrate where they can be addressed easily by near-field techniques. Although a few remarkable strategies aimed at the deposition of Mn 12 -based molecules on surfaces have already been developed, [8±11] detailed topographic information on molecular order is missing. Furthermore, while the magnetic properties of Mn 12 SMMs are reasonably well understood in their crystallized form, only one example has addressed the issue of magnetism in reduced dimensionality, namely by means of a Langmuir±Blodgett stacking of SMMs, [12] and there is virtually no magnetic study of monolayers on surfaces.The surface-science approach adopted in our work encompasses scanning tunneling microscopy (STM) studies in ultrahigh vacuum (UHV) on well-defined crystallographic surfaces. This route is very rewarding since it is meant to provide fundamental information on the organization of atomic [13] and supramolecular [14] clusters on surfaces. A considerable advantage of this approach is to make entities accessible to a detailed study by local, near-field probes, such as STM or atomic force microscopy (AFM) and related spectroscopy, [15] leading to innovating experiences. [16]

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The identification of the active sites in heterogeneous catalysis is important for a mechanistic understanding of the structure–reactivity relationship. Among others, the oxide/metal boundaries are expected to contain the active sites in various catalytic reactions. To reveal their nature and their chemical evolution under reaction conditions, the catalytic role of an oxide/metal system consisting of well-ordered ZnO nanoislands grown on Pt(111) in low-temperature CO oxidation was studied by near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) in operando conditions, and additionally by ultra-high vacuum scanning tunneling microscopy. To illustrate the special role played by the oxide/metal boundaries, a systematic comparative study of ZnO/Pt(111) with the pristine Pt(111) surface was undertaken. The regimes where mass transfer limitation starts to occur were identified using NAP-XPS and mass spectrometry measurements in combination, allowing a sound discussion on the relation between steady-state molar fractions of reactants/product and surface reactivity. Via the measurement of the steady-state CO2 molar fraction, we observed that the CO oxidation reaction rate over the ZnO/Pt(111) system is superior to that over Pt(111) in a temperature range extending to 410 K. The pivotal, albeit unexpected, role of ZnO-bound hydroxyls was clearly highlighted by the observation of the chemical signature of the CO + OH associative reaction at the ZnO/Pt boundaries. The carboxyl formed at low temperature (410 K) can be the intermediate species in the CO oxidation reaction, the OHs at the Pt/ZnO boundary being the cocatalyst, which explains the synergistic effect of ZnO and Pt. However, the species formed at higher temperature (from 445 K) are formates that would essentially be spectators.

In-situ scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) measurements have been performed to investigate the formation and thermal stability of mono-and bimetallic Au x Fe 1-x (x ) 1, 0.8, 0.5, 0.2, 0) nanoparticles (NPs) supported on TiO 2 (110). Nearly hexagonal arrangements of sizeselected Au, Fe, and Au-Fe NPs with well-defined interparticle distances have been achieved by diblockcopolymer encapsulation. Upon stepwise annealing from 300 to 1060 °C, a remarkable thermal stability of the Au-Fe NPs was observed, maintaining their original spatial arrangement on the TiO 2 surface up to 900 °C. A majority phase of a gold-iron alloy (solid solution) was achieved for our Au 0.5 Fe 0.5 NPs in the temperature range of 700 °C -800 °C, and for Au 0.2 Fe 0.8 NPs at 800 °C, while a phase mixture of bcc Fe and Au-Fe alloy was observed for the Au 0.8 Fe 0.2 system at 800 °C-900 °C. For all samples the segregation of Au atoms toward the NP surface was detected upon high temperature annealing (800 °C) in vacuum. Nearly complete Au desorption was observed by XPS at 900 °C for Au 0.2 Fe 0.8 NPs, at 1000 °C for Au 0.5 Fe 0.5 NPs, and at 1060 °C for Au 0.8 Fe 0.2 NPs. The enhanced thermal stability of Au in the Au 0.8 Fe 0.2 NPs is believed to be related to the formation of core(Fe)/shell(Au) structures. Furthermore, contrary to the case of pure Fe or Fe-rich NPs where nearly complete Fe desorption or Fe diffusion into TiO 2 was observed at 1000 °C, an Fe signal was detected at this temperature for the Au-rich samples (Au 0.8 Fe 0.2 and Au 0.5 Fe 0.5 ).

We provide a comprehensive investigation of the adsorption of a tertiary amine molecule, triethylamine (TEA, N(CH2CH3)3), on the Si(001)-(2 × 1) surface at room temperature (RT), using real-time synchrotron radiation X-ray photoemission (XPS) and scanning tunneling microscopy (STM), in combination with density functional theory (DFT) calculations. Real-time XPS measurements point to two consecutive reactions, the first one leading to the formation of a dative adduct, the second one to the conversion of the latter into a dissociated adduct via N–C bond cleavage (to give Si–N(CH2CH3)2 and Si-CH2CH3 moieties). The kinetic model, that fits the experimental data, distinguishes unreactive dative bond molecules (forming a reservoir) from reactive ones, exchanged in a two-way reaction. At low coverage, the STM images show that the adsorption of a dative-bond TEA molecule induces a static buckling of the Si dimers, leading to the c(4 × 2) templating of the silicon surface. Approaching saturation coverage, dative bonded TEA molecules self-organize into c(4 × 2) domains, with an occupancy of one molecule per two dimers, and exhibit a ternary symmetry. While DFT points to two possible molecular conformations (staggered and eclipsed) with almost the same adsorption energy, only the eclipsed conformer is observed at RT. STM helps also in identifying sites and experimental conditions (pressure) that make the dissociation of TEA possible. At high dilution, dissociated molecules are systematically sitting as pairs on two adjacent dimers in a row. Remarkably, dissociated adducts do not induce static buckling, in stark contrast with the case of dative bonding. Close to surface saturation, the dative-to-dissociated conversion starts from the boundaries of dative-bond self-assembled c(4 × 2) domains, emphasizing the role of these domains as reservoirs of stable molecular adducts. Our data show also that the conversion reaction needs the presence of the gas phase and is accelerated by an increase of pressure.

Local investigation of the electronic properties of size-selected Au nanoparticles by scanning tunneling spectroscopy Applied Physics Letters 89, 043101 (2006);