5.1. Nanoalloys of Group 11 (Cu, Ag, Au) 865 5.1.1. Cu−Ag 866 5.1.2. Cu−Au 867 5.1.3. Ag−Au 870 5.1.4. Cu−Ag−Au 872 5.2. Nanoalloys of Group 10 (Ni, Pd, Pt) 872 5.2.1. Ni−Pd 872 * To whom correspondence should be addressed. Phone: +39010 3536214. Fax:+39010 311066. E-mail: ferrando@fisica.unige.it. † Universita di Genova. ‡ Argonne National Laboratory. § University of Birmingham. | As of October 1, 2007, Chemical Sciences and Engineering Division. Volume 108, Number 3

Nanoalloys: From Theory to Applications of Alloy Clusters and Nanoparticles

Atomic layer deposition(ALD), a chemical vapor deposition technique based on sequential self-terminating gas–solid reactions, has for about four decades been applied for manufacturing conformal inorganic material layers with thickness down to the nanometer range. Despite the numerous successful applications of material growth by ALD, many physicochemical processes that control ALD growth are not yet sufficiently understood. To increase understanding of ALD processes, overviews are needed not only of the existing ALD processes and their applications, but also of the knowledge of the surface chemistry of specific ALD processes. This work aims to start the overviews on specific ALD processes by reviewing the experimental information available on the surface chemistry of the trimethylaluminum/water process. This process is generally known as a rather ideal ALD process, and plenty of information is available on its surface chemistry. This in-depth summary of the surface chemistry of one representative ALD process aims also to provide a view on the current status of understanding the surface chemistry of ALD, in general. The review starts by describing the basic characteristics of ALD, discussing the history of ALD—including the question who made the first ALD experiments—and giving an overview of the two-reactant ALD processes investigated to date. Second, the basic concepts related to the surface chemistry of ALD are described from a generic viewpoint applicable to all ALD processes based on compound reactants. This description includes physicochemical requirements for self-terminating reactions,reaction kinetics, typical chemisorption mechanisms, factors causing saturation, reasons for growth of less than a monolayer per cycle, effect of the temperature and number of cycles on the growth per cycle (GPC), and the growth mode. A comparison is made of three models available for estimating the sterically allowed value of GPC in ALD. Third, the experimental information on the surface chemistry in the trimethylaluminum/water ALD process are reviewed using the concepts developed in the second part of this review. The results are reviewed critically, with an aim to combine the information obtained in different types of investigations, such as growth experiments on flat substrates and reaction chemistry investigation on high-surface-area materials. Although the surface chemistry of the trimethylaluminum/water ALD process is rather well understood, systematic investigations of the reaction kinetics and the growth mode on different substrates are still missing. The last part of the review is devoted to discussing issues which may hamper surface chemistry investigations of ALD, such as problematic historical assumptions, nonstandard terminology, and the effect of experimental conditions on the surface chemistry of ALD. I hope that this review can help the newcomer get acquainted with the exciting and challenging field of surface chemistry of ALD and can serve as a useful guide for the specialist towards the fifth decade of ALD research.

Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process

Atomic layer deposition (ALD) is gaining attention as a thin film deposition method, uniquely suitable for depositing uniform and conformal films on complex three-dimensional topographies. The deposition of a film of a given material by ALD relies on the successive, separated, and self-terminating gas–solid reactions of typically two gaseous reactants. Hundreds of ALD chemistries have been found for depositing a variety of materials during the past decades, mostly for inorganic materials but lately also for organic and inorganic–organic hybrid compounds. One factor that often dictates the properties of ALD films in actual applications is the crystallinity of the grown film: Is the material amorphous or, if it is crystalline, which phase(s) is (are) present. In this thematic review, we first describe the basics of ALD, summarize the two-reactant ALD processes to grow inorganic materials developed to-date, updating the information of an earlier review on ALD [R. L. Puurunen, J. Appl. Phys. 97, 121301 (2005)], and give an overview of the status of processing ternary compounds by ALD. We then proceed to analyze the published experimental data for information on the crystallinity and phase of inorganic materials deposited by ALD from different reactants at different temperatures. The data are collected for films in their as-deposited state and tabulated for easy reference. Case studies are presented to illustrate the effect of different process parameters on crystallinity for representative materials: aluminium oxide, zirconium oxide, zinc oxide, titanium nitride, zinc zulfide, and ruthenium. Finally, we discuss the general trends in the development of film crystallinity as function of ALD process parameters. The authors hope that this review will help newcomers to ALD to familiarize themselves with the complex world of crystalline ALD films and, at the same time, serve for the expert as a handbook-type reference source on ALD processes and film crystallinity.

Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends

We synthesize vertically oriented core–shell nanowires with substoichiometric MoO3 cores of ∼20–50 nm and conformal MoS2 shells of ∼2–5 nm. The core–shell architecture, produced by low-temperature sulfidization, is designed to utilize the best properties of each component material while mitigating their deficiencies. The substoichiometric MoO3 core provides a high aspect ratio foundation and enables facile charge transport, while the conformal MoS2 shell provides excellent catalytic activity and protection against corrosion in strong acids.

Core–shell MoO3–MoS2 Nanowires for Hydrogen Evolution: A Functional Design for Electrocatalytic Materials

Steam Reforming and Graphite Formation on Ni Catalysts

We discuss the design of a Ni−Au nanoparticle catalyst system, which is based on the detailed experimental and theoretical understanding of the alloying and the chemical reaction processes on single-crystal surfaces. The alloy formation and structure of Ni−Au catalysts supported on SiO2 and on MgAl2O4 are simulated by Monte Carlo schemes as well as experimentally studied by a combination of in situ X-ray absorption fine structure, transmission electron microscopy, and in situ X-ray powder diffraction. On-line mass spectrometry is used to follow the reactivity of the catalyst and thermogravimetric analysis provided information on the deposition rate of carbon during steam reforming of n-butane. The simulations and the experiments give evidence for the formation of a Ni−Au surface alloy on the Ni particles for both supports. The Ni−Au catalysts exhibiting the surface alloy are active for steam reforming and are more resistant toward carbon formation than the pure Ni catalyst. Blocking of highly reactive Ni ...

Structure and Reactivity of Ni−Au Nanoparticle Catalysts

In this paper we show a new application of the atomic layer epitaxy technique to prepare binary alloy nanoparticle catalysts on various supports. The technique is used to prepare Cu/Pd bimetallic catalysts on SiO2 and on γ-Al2O3. The samples were characterized by means of transmission electron microscopy (TEM) and extended X-ray absorption fine structure (EXAFS) measurements. The EXAFS results show alloying in the Cu/Pd samples with a Cu-rich surface for the alumina-supported sample and random alloying for the silica-supported sample. From TEM and EXAFS measurements the particle sizes are determined to be in the range 20−60 A.

Alloying in Cu/Pd Nanoparticle Catalysts

Imaging MoS2 nanocatalysts with single-atom sensitivity

We present extended X-ray absorption fine structure (EXAFS) and transmission electron microscopy (TEM) measurements on three zeolite-supported bimetallic Pt−Pd catalysts prepared by wet impregnation. Alloy formation and segregation of Pd to the surface of the small bimetallic particles is clearly identified. Ab initio LMTO calculations are used to predict segregation profiles of low-index single-crystal Pt−Pd alloy surfaces. Pd is found to be enriched at the surface. EMT potentials for Pt and Pd are adjusted in order to give the same segregation profiles at low-index single-crystal surfaces as linear muffin tin orbital (LMTO) calculations. Monte Carlo simulations based on the adjusted EMT potentials are then used to calculate the equilibrium structure of small bimetallic Pt−Pd particles. Coordination numbers derived from these equilibrium structures are compared with those derived from EXAFS analysis, and a good agreement is found.

Alloy formation and surface segregation in zeolite-supported pt-pd bimetallic catalysts

In situ XAFS spectroscopic studies have been carried out at 450 °C on the hydrogen reduction of a rhenium-promoted Co3O4/Al2O3 catalyst. The results show that reduction at this temperature yields a...

Alfons M. Molenbroek

Papers

Structure and Reactivity of Ni−Au Nanoparticle Catalysts

Alloying in Cu/Pd Nanoparticle Catalysts

Imaging MoS2 nanocatalysts with single-atom sensitivity

Alloy formation and surface segregation in zeolite-supported pt-pd bimetallic catalysts

X-ray Absorption Spectroscopic Studies at the Cobalt K-Edge on a Reduced Al2O3-Supported Rhenium-Promoted Cobalt Fischer−Tropsch Catalyst