Operando Surface Characterization on Catalytic and Energy Materials from Single Crystals to Nanoparticles.

Modern surface science faces two major challenges, a materials gap and a pressure gap. While studies on single crystal surface in ultrahigh vacuum have uncovered the atomic and electronic structures of the surface, the materials and environmental conditions of commercial catalysis are much more complicated, both in the structure of the materials and in the accessible pressure range of analysis instruments. Model systems and operando surface techniques have been developed to bridge these gaps. In this Review, we highlight the current trends in the development of the surface characterization techniques and methodologies in more realistic environments, with emphasis on recent research efforts at the Korea Advanced Institute of Science and Technology. We show principles and applications of the microscopic and spectroscopic surface techniques at ambient pressure that were used for the characterization of atomic structure, electronic structure, charge transport, and the mechanical properties of catalytic and energy materials. Ambient pressure scanning tunneling microscopy and X-ray photoelectron spectroscopy allow us to observe the surface restructuring that occurs during oxidation, reduction, and catalytic processes. In addition, we introduce the ambient pressure atomic force microscopy that revealed the morphological, mechanical, and charge transport properties that occur during the catalytic and energy conversion processes. Hot electron detection enables the monitoring of catalytic reactions and electronic excitations on the surface. Overall, the information on the nature of catalytic reactions obtained with operando spectroscopic and microscopic techniques may bring breakthroughs in some of the global energy and environmental problems the world is facing.

Nanoscale investigation of improved triboelectric properties of UV-irradiated ultrananocrystalline diamond films.

We report improved the triboelectric properties of ultraviolet (UV)-irradiated ultrananocrystalline diamond (UNCD) films that were measured using atomic force microscopy (AFM). Fabricated using the chemical vapor deposition (CVD) method, UNCD is an artificial diamond film with mechanical properties similar to single-crystal diamond. Surface modification by means of UV irradiation is a simple method to modify the surface properties of carbon-based and oxide materials. While the physical properties (e.g., roughness, adhesion, and friction) of these UNCD films did not exhibit any significant change following the UV treatment, we found that the UV-irradiated UNCD surface was oxidized and became graphitic, as confirmed using X-ray photoelectron spectroscopy, work function measurements using Kelvin probe force microscopy, and ultraviolet photoelectron spectroscopy. The work function of the samples increased with increasing UV exposure time, which is associated with the reduction of carbon atoms on the surface and oxygen-rich surfaces. Tribocharges were generated by scratching the surface of the UNCD films with a diamond-coated AFM tip. The duration of the tribocharges increased because of reactive radicals and the insulating property resulting from the UV/ozone treatment. The radicals were responsible for trapping charges; the UV-irradiated UNCD films preserved the charges for more than 5 h, which is five times longer than that on bare UNCD. This study demonstrated that UNCD is a promising material for generating triboelectricity and that UNCD can be used as a charge-trapping layer in charge-trap flash memory devices.

Ambient-pressure atomic force microscope with variable pressure from ultra-high vacuum up to one bar.

We present the design and performance of an ambient-pressure atomic force microscope (AP-AFM) that allows AFM measurements using the laser deflection technique in a highly controlled environment from ultra-high vacuum (UHV) up to 1 bar with various gases. While the UHV of the AP-AFM system is obtained by a combination of turbo-molecular and ion pumps, for the higher-pressure studies, the ambient-pressure chamber is isolated from the pumps and high-purity gases are dosed via a leak valve from a gas manifold. The AP-AFM system, therefore, provides versatile AFM techniques, including the measurement of topography, friction and local conductance mapping, and force spectroscopy in a highly controlled environment with pressures ranging from UHV up to atmospheric pressure. Atomically resolved stick-slip images and force spectroscopy of highly ordered pyrolytic graphite (HOPG) at variable pressure conditions are presented to demonstrate the performance of the AP-AFM system. Force spectroscopy results of vacuum-cleaved HOPG, followed by exposure to lab air, oxygen, and methane show that adhesion between the AFM tip and the HOPG depends significantly on the exposed gas and pressure. Our results show that the deposition of airborne hydrocarbon impurities at ambient conditions leads to a significant change in adhesion force, implying that the wettability of the HOPG surface depends on the environment and the pressure.

Construction and evaluation of an ultrahigh-vacuum-compatible sputter deposition source.

A sputter deposition source for the use in ultrahigh vacuum (UHV) is described, and some properties of the source are analyzed. The operating principle is based on the design developed by Mayr et al. [Rev. Sci. Instrum. 84, 094103 (2013)], where electrons emitted from a filament ionize argon gas and the Ar+ ions are accelerated to the target. In contrast to the original design, two grids are used to direct a large fraction of the Ar+ ions to the target, and the source has a housing cooled by liquid nitrogen to reduce contaminations. The source has been used for the deposition of zirconium, a material that is difficult to evaporate in standard UHV evaporators. At an Ar pressure of 9×10-6 mbar in the UHV chamber and moderate emission current, a highly reproducible deposition rate of ≈1 ML in 250 s was achieved at the substrate (at a distance of ≈50 mm from the target). Higher deposition rates are easily possible. X-ray photoelectron spectroscopy shows a high purity of the deposited films. Depending on the grid voltages, the substrate gets mildly sputtered by Ar+ ions; in addition, the substrate is also reached by electrons from the negatively biased sputter target.

Metal Adatoms and Clusters on Ultrathin Zirconia Films.

Nucleation and growth of transition metals on zirconia has been studied by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. Since STM requires electrical conductivity, ultrathin ZrO2 films grown by oxidation of Pt3Zr(0001) and Pd3Zr(0001) were used as model systems. DFT studies were performed for single metal adatoms on supported ZrO2 films as well as the (1̅11) surface of monoclinic ZrO2. STM shows decreasing cluster size, indicative of increasing metal-oxide interaction, in the sequence Ag < Pd ≈ Au < Ni ≈ Fe. Ag and Pd nucleate mostly at steps and domain boundaries of ZrO2/Pt3Zr(0001) and form three-dimensional clusters. Deposition of low coverages of Ni and Fe at room temperature leads to a high density of few-atom clusters on the oxide terraces. Weak bonding of Ag to the oxide is demonstrated by removing Ag clusters with the STM tip. DFT calculations for single adatoms show that the metal-oxide interaction strength increases in the sequence Ag < Au < Pd < Ni on monoclinic ZrO2, and Ag ≈ Au < Pd < Ni on the supported ultrathin ZrO2 film. With the exception of Au, metal nucleation and growth on ultrathin zirconia films follow the usual rules: More reactive (more electropositive) metals result in a higher cluster density and wet the surface more strongly than more noble metals. These bind mainly to the oxygen anions of the oxide. Au is an exception because it can bind strongly to the Zr cations. Au diffusion may be impeded by changing its charge state between -1 and +1. We discuss differences between the supported ultrathin zirconia films and the surfaces of bulk ZrO2, such as the possibility of charge transfer to the substrate of the films. Due to their large in-plane lattice constant and the variety of adsorption sites, ZrO2{111} surfaces are more reactive than many other oxygen-terminated oxide surfaces.

Growth of an Ultrathin Zirconia Film on Pt3Zr Examined by High-Resolution X-ray Photoelectron Spectroscopy, Temperature-Programmed Desorption, Scanning Tunneling Microscopy, and Density Functional Theory.

Ultrathin (∼3 Å) zirconium oxide films were grown on a single-crystalline Pt3Zr(0001) substrate by oxidation in 1 × 10-7 mbar of O2 at 673 K, followed by annealing at temperatures up to 1023 K. The ZrO2 films are intended to serve as model supports for reforming catalysts and fuel cell anodes. The atomic and electronic structure and composition of the ZrO2 films were determined by synchrotron-based high-resolution X-ray photoelectron spectroscopy (HR-XPS) (including depth profiling), low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. Oxidation mainly leads to ultrathin trilayer (O-Zr-O) films on the alloy; only a small area fraction (10-15%) is covered by ZrO2 clusters (thickness ∼0.5-10 nm). The amount of clusters decreases with increasing annealing temperature. Temperature-programmed desorption (TPD) of CO was utilized to confirm complete coverage of the Pt3Zr substrate by ZrO2, that is, formation of a closed oxide overlayer. Experiments and DFT calculations show that the core level shifts of Zr in the trilayer ZrO2 films are between those of metallic Zr and thick (bulklike) ZrO2. Therefore, the assignment of such XPS core level shifts to substoichiometric ZrO x is not necessarily correct, because these XPS signals may equally well arise from ultrathin ZrO2 films or metal/ZrO2 interfaces. Furthermore, our results indicate that the common approach of calculating core level shifts by DFT including final-state effects should be taken with care for thicker insulating films, clusters, and bulk insulators.