This review examines characterization challenges inherently associated with understanding nanomaterials and the roles surface and interface characterization methods can play in meeting some of the challenges. In parts of the research community, there is growing recognition that studies and published reports on the properties and behaviors of nanomaterials often have reported inadequate or incomplete characterization. As a consequence, the true value of the data in these reports is, at best, uncertain. With the increasing importance of nanomaterials in fundamental research and technological applications, it is desirable that researchers from the wide variety of disciplines involved recognize the nature of these often unexpected challenges associated with reproducible synthesis and characterization of nanomaterials, including the difficulties of maintaining desired materials properties during handling and processing due to their dynamic nature. It is equally valuable for researchers to understand how characterization approaches (surface and otherwise) can help to minimize synthesis surprises and to determine how (and how quickly) materials and properties change in different environments. Appropriate application of traditional surface sensitive analysis methods (including x-ray photoelectron and Auger electron spectroscopies, scanning probe microscopy, and secondary ion mass spectroscopy) can provide information that helps address several of the analysis needs. In many circumstances, extensions of traditional data analysis can provide considerably more information than normally obtained from the data collected. Less common or evolving methods with surface selectivity (e.g., some variations of nuclear magnetic resonance, sum frequency generation, and low and medium energy ion scattering) can provide information about surfaces or interfaces in working environments ( or ) or information not provided by more traditional methods. Although these methods may require instrumentation or expertise not generally available, they can be particularly useful in addressing specific questions, and examples of their use in nanomaterial research are presented.

Employing guided-ion-beam mass spectrometry, we identified a series of positively charged stoichiometric zirconium oxide clusters that exhibit enhanced activity and selectivity for three oxidation reactions of widespread chemical importance. Density functional theory calculations reveal that these clusters all contain the same active site consisting of a radical oxygen center with an elongated zirconium-oxygen bond. Calculated energy profiles demonstrate that each oxidation reaction is highly favorable energetically and involves easily surmountable barriers. Furthermore, the active stoichiometric clusters may be regenerated by reacting oxygen-deficient clusters with a strong oxidizer. This indicates that these species may promote multiple cycles of oxidation reactions and, therefore, exhibit true catalytic behavior. The stoichiometric clusters, having structures that resemble specific sites in bulk zirconia, are promising candidates for potential incorporation into a cluster assembled catalyst material.

Studies involving small iron oxide cationic clusters, Fe m O n + (m = 1, 2, n = 1−5), and their reactivity in the presence of CO at near thermal energies were conducted by employing a guided ion beam mass spectrometer. Observed reaction channels included CO oxidation and oxygen replacement by CO which are shown to be dependent on cluster size and stoichiometry. First principles electronic structure studies within the density functional theory framework were carried out to address the structures and energetics of small cationic clusters and to probe the molecular level pathways for CO oxidation. The theoretical calculations, in conjunction with the experimental findings, are used to propose a coherent picture about the trends in the dissociation energies, which are largely dependent upon the metal to oxygen ratio, and the nature of the structure−reactivity relationships for the formation of the reaction products.

Evidence obtained by guided-ion-beam mass spectrometry experiments and density functional theory calculations indicates that by adding one oxygen atom with a full octet of valence electrons (O(2-)) to stoichiometric cationic zirconium oxide clusters (ZrO(2))(x)(+) (x = 1-4), a series of anionic clusters (Zr(x)O(2x+1))(-) (x = 1-4) are formed which contain radical oxygen centers with elongated (elongation approximately 0.24 +/- 0.02 A) metal-oxygen bonds. These anionic clusters oxidize carbon monoxide, strongly associate acetylene, and weakly associate ethylene, in contrast to the cationic species which were found previously to be highly active toward the oxidation of all three molecules. Theoretical investigations indicate that a critical hydrogen transfer step necessary for the oxidation of ethylene and acetylene at metal oxide clusters containing radical oxygen centers is energetically favorable for cationic clusters but unfavorable for the corresponding anionic species. The calculated electrostatic potential of the cluster reveals that in the case of cations, a favorable interaction with nucleophilic molecules takes place over the whole surface of the (ZrO(2))(x)(+) (x = 1-4) clusters, compared to a restricted interaction of ethylene and acetylene with the less coordinated zirconium atom in the case of the anionic (Zr(x)O(2x+1))(-) (x = 1-4) species. Therefore, in spite of the common presence of a radical oxygen center in specific anionic and cationic stoichiometries, the extent to which various classes of reactions are promoted is influenced by charge state. Moreover, the (Zr(x)O(2x+1))(-) (x = 1-4) series of anionic clusters may be regenerated by reacting oxygen deficient clusters with a strong oxidizer. This indicates that not only cationic species, as shown previously, but also anionic clusters may promote multiple cycles of carbon monoxide oxidation.

We present results from our joint experimental and theoretical study of the reactivity of anionic and cationic gold oxide clusters toward CO, focusing on the role of atomic oxygen, different charge states, and mechanisms for oxidation. We show that anionic clusters react by an Eley-Rideal-like mechanism involving the preferential attack of CO on oxygen rather than gold. In contrast, the oxidation of CO on cationic gold oxide clusters can occur by both an Eley-Rideal-like and a Langmuir-Hinshelwood-like mechanism at multiple collision conditions as a result of the high adsorption energy of two CO molecules. This large energy of CO adsorption on cationic gold oxide clusters is the driving force for the CO oxidation. Therefore, in the presence of cationic gold species at high pressures of CO, the oxidation reaction is self-promoting (i.e., the oxidation of one CO molecule is promoted by the binding of a second CO). Our findings provide new insight into the role of charge state in gold-cluster-based nanocatalysis.

Monodisperse gold clusters have been prepared on surfaces in different charge states through soft landing of mass-selected ions. Ligand-stabilized gold clusters were prepared in methanol solution by reduction of chloro(triphenylphosphine)gold(I) with borane tert-butylamine complex in the presence of 1,3-bis(diphenylphosphino)propane. Electrospray ionization was used to introduce the clusters into the gas phase, and mass selection was employed to isolate a single ionic cluster species (Au(11)L(5)(3+), L = 1,3-bis(diphenylphosphino)propane), which was delivered to surfaces at well-controlled kinetic energies. Using in situ time-of-flight secondary ion mass spectrometry (TOF-SIMS), it is demonstrated that the Au(11)L(5)(3+) cluster retains its 3+ charge state when soft landed onto the surface of a 1H,1H,2H,2H-perfluorodecanethiol self-assembled monolayer (FSAM) on gold. In contrast, when deposited onto 16-mercaptohexadecanoic acid (COOH-SAM) and 1-dodecanethiol (HSAM) surfaces on gold, the clusters exhibit larger relative abundances of the 2+ and 1+ charge states, respectively. The kinetics of charge reduction on the FSAM and HSAM surfaces are investigated using in situ Fourier transform ion cyclotron resonance (FT-ICR) SIMS. It is shown that an extremely slow interfacial charge reduction occurs on the FSAM surface while an almost instantaneous neutralization takes place on the surface of the HSAM. Our results demonstrate that the size and charge state of small gold clusters on surfaces, both of which exert a dramatic influence on their chemical and physical properties, may be tuned through soft landing of mass-selected ions onto carefully selected substrates.

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The rational design of improved electrode–electrolyte interfaces (EEI) for energy storage is critically dependent on a molecular-level understanding of ionic interactions and nanoscale phenomena. The presence of non-redox active species at EEI has been shown to strongly influence Faradaic efficiency and long-term operational stability during energy storage processes. Herein, we achieve substantially higher performance and long-term stability of EEI prepared with highly dispersed discrete redox-active cluster anions (50 ng of pure ∼0.75 nm size molybdenum polyoxometalate (POM) anions on 25 μg (∼0.2 wt%) carbon nanotube (CNT) electrodes) by complete elimination of strongly coordinating non-redox species through ion soft landing (SL). Electron microscopy provides atomically resolved images of a uniform distribution of individual POM species soft landed directly on complex technologically relevant CNT electrodes. In this context, SL is established as a versatile approach for the controlled design of novel surfaces for both fundamental and applied research in energy storage.