An overview is given of the advance of fabrication of two-dimensional (2D) ternary oxide layers supported on single crystal metal surfaces and the atomic level understanding of their novel structural properties. Specifically, new oxide preparation approaches for metal tungstates (MWOx), utilizing the on-surface solid-state chemical reaction between well-defined 2D binary metal oxide and tungsten oxide phases, are presented. The successful implementation of these fabrication methods is demonstrated for prototypical 2D ternary oxide layers, with different W-O coordination spheres. Structure models of 2D MWOx phases are discussed in the light of new building architecture concepts with no analogues in the bulk.

The reduction of MoO3 profoundly influences its physical and chemical properties, making it a material of central importance across a wide range of applications. While bulk reduction processes and the resulting substoichiometric Magnéli phases─composed of extended shear planes─have been thoroughly investigated, the reduction chemistry of MoO3 nanoscale objects remains largely unexplored and nanoscale-specific mechanisms of oxygen deficiency accommodation are poorly understood. In this study, we employ a combination of atomic-resolution scanning tunneling microscopy (STM), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) modeling to systematically investigate the reduction of an ultrathin MoO3 bilayer supported on a Pd(100) surface. Our results reveal that the bilayer decomposes upon reduction into reduced monolayer and trilayer phases, with the trilayers consistently exhibiting ordered surface defect structures characterized by (2 × 3) and (2 × 4) periodicities. Through DFT modeling, we assign these defects to a peculiar type of surface oxygen vacancy, formed by a transformation of MoO3 octahedra configurations from corner-sharing to edge-sharing. Ordered assemblies of such defects form lattices of substoichiometric shear lines, which are the two-dimensional analogs of the bulk Magnéli crystallographic shear planes. Their genuinely nanoscale-specific character is proved by the instability of alike phases on surfaces of bulk MoO3. Their detection on MoO3 films suggests that the formation of similar two-dimensional (2D) Magnéli-type phases may be a more general phenomenon and occur also in other reducible oxide nanolayers, where enhanced structural flexibility enables the formation of phases not accessible in bulk crystals.

The atomic structure of MoOx films formed upon a gradual thermal reduction of an ordered MoO3 monolayer on the Pd(100) substrate was explored via surface science characterization techniques and density functional theory (DFT) calculations. Two main reduction stages were identified. First, the initial oxygen excess was gradually eliminated by altering the domain boundary length, orientation, and atomic structure. The films nevertheless remained O-rich, with numerous terminal oxygen atoms (formation of Mo--O groups), and an elevated work function. Second, multiple ordered O-lean phases were formed, characterized by either very few or no terminal oxygen atoms, and a much smaller surface work function. According to calculations, the positive charging of the Pd substrate stabilizes the oxygen excess during the first stage, but during the second reduction stage, the substrate becomes negatively charged, stabilizing enhanced cation oxidation states. On their basis, the mechanisms underlying the oxygen release from the initial c(2 x 2) domains were disclosed. The experiments showed that the film reduction is perfectly reversible, which highlights the very promising properties of the MoO3/Pd system for heterogeneous catalysis.

We explore the structural evolution of two-dimensional (2D) MoO3 films beyond the monolayer (ML), which have been prepared by physical vapor deposition and post-oxidation onto a Pd(100) surface, and characterized by the tools of surface science and density functional theory (DFT) calculations. According to DFT, the most stable oxide layers are stoichiometric, and derive their energetic stability from the low cost of creating 2D freestanding layers from the orthorhombic bulk phase, good matching to Pd, and the particularly strong adhesion to the substrate. The observed 2D MoO3 layers are distinguished by well-ordered linear defects, such as domain boundaries in the ML, and misfit dislocations in the bilayer (BL). Applying reactive oxidation preparation conditions results in the formation of ordered arrays of nanostructures, nanowires and nanoclusters, in the MoO3 BL. The formation of such linear structures is accounted for in the DFT by models of missing row defects of various orientations and stoichiometries. Their relative stability is rationalized in terms of the number of broken Mo-O bonds, the polar character of the nanostructure edges and the interaction strength with the Pd substrate. Comparison with similar WO3 layers on Pd(100) is provided.

W- and Mo-oxides form an interesting class of materials, featuring structural complexities, stoichiometric flexibility, and versatile physical and chemical properties that render them attractive for many applications in diverse fields of nanotechnologies. In nanostructured form, novel properties and functionalities emerge as a result of quantum size and confinement effects. In this topical review, W- and Mo-oxide nanosystems are examined with particular emphasis on two-dimensional (2D) layers and small molecular-type clusters. We focus on the epitaxial growth of 2D layers on metal single crystal surfaces and investigate their novel geometries and structures by a surface science approach. The coupling between the oxide overlayer and the metal substrate surface is a decisive element in the formation of the oxide structures and interfacial strain and charge transfer are shown to determine the lowest energy structures. Atomic structure models as determined by density functional theory (DFT) simulations are reported and discussed for various interface situations, with strong and weak coupling. Free-standing (quasi-)2D oxide layers, so-called oxide nanosheets, are attracting a growing interest recently in the applied research community because of their easy synthesis via wet-chemical routes. Although they consist typically of several atomic layers thick-not always homogeneous-platelet systems, their quasi-2D character induces a number of features that make them attractive for optoelectronic, sensor or biotechnological device applications. A brief account of recently published preparation procedures of W- and Mo-oxide nanosheets and some prototypical examples of proof of concept applications are reported here. (MO3)3(M = W, Mo) clusters can be generated in the gas phase in nearly monodisperse form by a simple vacuum sublimation technique. These clusters, interesting molecular-type structures by their own account, can be deposited on a solid surface in a controlled way and be condensed into 2D W- and Mo-oxide layers; solid-state chemical reactions with pre-deposited surface oxide layers to form 2D ternary oxide compounds (tungstates, molybdates) have also been reported. The clusters have been proposed as model systems for molecular studies of reactive centres in catalytic reactions. Studies of the catalysis of (MO3)3clusters in unsupported and supported forms, using the conversion of alcohols as model reactions, are discussed. Finally, we close with a brief outlook of future perspectives.

Doping of tungsten trioxide (WO3) and molybdenum trioxide (MoO3) materials with alkali atoms, leading to the formation of the so-called sodium bronzes, is a viable approach to achieve a precise control of their electronic, optical, and magnetic properties via electron band structure engineering. Driven by the ongoing trend for thickness reduction and the resulting new functionalities at the nanoscale, using a combination of state-of-the-art experimental and computational techniques, we investigate here the interaction of two isostructural two-dimensional (2D) WO3 and MoO3 layers, grown epitaxially onto a Pd(100) surface, with Na dopants. We identify two interaction regimes as a function of the Na coverage: a low-coverage regime up to 0.3 ML, which we describe in terms of doping interactions, and a reaction regime, where at higher Na coverages, the 2D WO3/MoO3 lattices become destroyed and several ordered 2D bronze-type phases form upon thermal activation. In the doping regime, Na initially decorates the oxide domain boundaries and later adsorbs in a (2 × 2) superstructure, filling the regular adsorption sites within the oxide domains. Further Na accommodation in the 2D oxide lattice is unfavorable due to the poor lateral electrostatic screening and elastic strain increase. In the reaction regime, the most prominent and energetically stable phase is the hexagonal 2D bronze-like layer, whose atomic details are resolved in a density functional theory (DFT) analysis and compared with the structure of the bulk counterpart.

Nanostructured oxide materials ultra-thin films, nanoparticles and other nanometer-scale objects play prominent roles in many aspects of our every-day life, in nature and in technological applications, among which is the all-oxide electronics of tomorrow. Due to their reduced dimensions and dimensionality, they strongly interact with their environment gaseous atmosphere, water or support. Their novel physical and chemical properties are the subject of this book from both a fundamental and an applied perspective. It reviews and illustrates the various methodologies for their growth, fabrication, experimental and theoretical characterization. The role of key parameters such as film thickness, nanoparticle size and support interactions in driving their fundamental properties is underlined. At the ultimate thickness limit, two-dimensional oxide materials are generated, whose functionalities and potential applications are described. The emerging field of cation mixing is mentioned, which opens new avenues for engineering many oxide properties, as witnessed by natural oxide nanomaterials such as clay minerals, which, beyond their role at the Earth surface, are now widely used in a whole range of human activities. Oxide nanomaterials are involved in many interdisciplinary fields of advanced nanotechnologies: catalysis, photocatalysis, solar energy materials, fuel cells, corrosion protection, and biotechnological applications are amongst the areas where they are making an impact; prototypical examples are outlined. A cautious glimpse into future developments of scientific activity is finally ventured to round off the treatise.

The novel physical and chemical properties and functionalities of two-dimensional (2-D) oxide materials are assessed. The synthesis of one unit-cell thick 2-D oxides poses particular challenges, since in contrast to other 2-D materials, which can be fabricated by exfoliation of layered bulk compounds, the majority of oxides do not occur in layered bulk structures. Most 2-D oxides are therefore prepared by thin-film deposition methods on substrates. However the fabrication of free-standing quasi-2-D oxide nanosheets, with less restrictive several monolayer thickness, has been successfully achieved by wet chemical procedures. New geometry concepts and electronic properties are observed in 2-D oxides, due to quantum confinement and interface proximity effects. Atomic geometries, electronic structure, ferroic properties and catalytic behaviour of 2-D oxides are discussed, together with promising prototypical proof-of-concept experiments for prospective applications. The edge states in oxide nanoribbons, 2-D objects of limited width, and their polarity aspects are discussed.

This chapter outlines the fabrication methods of oxide thin films, from the oxidation of the outer layers of bulk elemental solids to thin film deposition methodologies. The classical theories treating the thermal oxidation of metals and silicon are reviewed. A particular focus is put on the oxidation of alloy single crystal surfaces to generate ultrathin oxide films and the formation of surface oxides, the latter as precursor layers for thicker bulk-type oxide phases. The diverse deposition techniques to grow epitaxial thin oxide films are introduced, with a classification into physical and chemical methods for the ease of presentation; the benefits and disadvantages of the different methods are pointed out. The synthesis of oxide nanoparticles is discussed in the gas phase and in liquid phase environments. The fundamental concepts of nucleation and growth of thin films and nanoparticles are introduced, including the classical capillary approach and atomistic descriptions.

Oxide nanomaterials have an impact in many interdisciplinary fields of the emerging nanotechnologies as reported here. They are components of heterogeneous catalyst systems with specific features that are outlined. In photocatalysis, chemical reactions are catalytically enabled by photon-energy conversion: oxide photocatalysts are prominent and discussed in relation to the photochemical water splitting reaction. Solid oxide fuel cells are promising energy sources, in which oxide nanomaterials are expected to boost further progress. Solar energy materials are elements of the “green chemistry” revolution for energy saving: the chromogenic functionality of oxide nanolayers with use in advanced fenestration is introduced. The formation and structure of corrosion protective nanolayer oxides, which is vital for the everyday use of metallic components, are examined. Biotechnology applications of oxide nanostructures comprise their biocompatibility, antibacterial properties, theranostic systems as well as biosensor platforms. An interesting bioapplication of ferroelectric oxide thin films is reported.

This chapter provides an introduction to the properties of naturally occurring oxide ultra-thin films and nanoparticles of complex composition, namely clay minerals. Clays represent a wide family of nanomaterials, formed from the weathering of primary rocks at the Earth surface. Their layered structure is responsible for their extreme anisotropy and their extremely high specific surface area which is at the origin of their unique properties and of their strong interaction with their surrounding. The focus of this chapter is on their structural and composition characteristics, the conditions of their formation, their interaction with water and their chemical properties. A mention is given of their uses in human cultural activities, in dwelling constructions and statuary, in porcelain fabrication, in retention of metal contaminants in the environment, in the pharmaceutical and cosmetic industries, and as lubricants, catalysts, therapeuthic agents, among others.

Oxide micro- and nanoparticles are ubiquitous in the natural environment. They have long been used as catalysts, but have found novel applications with the advent of nanotechnologies. From a fundamental point of view, they bridge the gap between the properties of isolated atoms or molecules and those of bulk condensed phases. They present a large spectrum of atomic and electronic structures, due to their finite size in the three dimensions of space and the fact that their atoms are nearly all surface atoms. They represent the first stages of nucleation and growth of larger size oxides, and their stability and properties which depend upon the thermodynamic conditions under which they are formed, largely impact the final product. This chapter reviews their specific properties and analyses their physical origin, both when they are produced in the gas phase, in an aqueous environment, or when cation mixing takes place.

Chapter 4 focuses on the various physico-chemical properties of thin films, and in particular on their thickness dependence. It aims at bridging the gap between the well established knowledge acquired on oxide surfaces and truely two-dimensional oxides. It reviews the consequences of the reduced local environment of their atoms and of the confinement effects due to their finite thickness. It analyzes the various manifestations of structural, electronic, vibrational, ferroelectric and magnetic modifications. A special focus is put on the interaction of thin films with metal or oxide substrates, possibly leading to interfacial dislocations, electron transfer or mixing, with consequences on their chemical reactivity.

The synopsis part of this last chapter gives a brief summary of the book content. The outlook attempts to identify future areas of scientific activity, in which according to the authors´ visions nano-oxide materials may promote new developments. Among them are the controlled synthesis of oxide nanosheets and the experimental realization of oxide nanoribbons. The preparation of well-defined oxide heterostructures may reveal novel emergent states and new topological phases of matter. Mixed nano-oxides will be of interest for band structure engineering and to adjust band edges for photochemical reactivity. Programmable defect chemistry may open up new selective pathways for catalytic reactions. In parallel with experimental progress, advanced theoretical and simulation methods will take advantage of the ever-increasing computer power to tackle highly correlated materials and allow highthroughput computing. The interaction of nano-oxides with biological systems has great potential for opening up new avenues in the biotechnological area.

Two-dimensional (2D) WO3 nanosheets exhibit a range of novel properties and functionalities that render them attractive for advanced nanotechnologies. However, at the ultimate 2D limit of single-layer thickness, the structural properties of WO3 are unclear. Here, we fabricated, using molecular beam epitaxy techniques, a crystalline 2D WO3 overlayer on a Ag(100) surface and unveiled its geometric, electronic, and vibrational structure via a combination of state-of-the-art experimental (microscopic and spectroscopic) and computational techniques. The 2D WO3 phase forms a bilayer with a staggered arrangement of WO6 octahedra, linked together by corner- and edge-sharing, which is significantly different from the cubic and monoclinic WO3 bulk structures, but resembles a bilayer of the alpha-MoO3 layered bulk lattice. Such a 2D WO3 bilayer on Ag(100) is a robust nonpolar structure, which is incommensurate in various rotational orientations, weakly coupled to the metal substrate, and, according to the density functional theory calculations, should survive as a stable freestanding layer, that is, as a nanosheet.

Graphene oxides are promising materials for novel electronic devices or anchoring of the active sites for catalytic applications. Here we focus on understanding the atomic oxygen (AO) binding and mobility on different regions of graphene (Gr) on Ru(0001). Differences in the Gr/Ru lattices result in the superstructure, which offers an array of distinct adsorption sites. We employ scanning tunneling microscopy and density functional theory to map out the chemical identity and stability of prepared AO functionalities in different Gr regions. The AO diffusion is utilized to establish that in the regions that are close to the metal substrate the terminally bonded enolate groups are strongly preferred over bridge-bonded epoxy groups. No oxygen species are observed on the graphene regions that are far from the underlying Ru, indicating their low relative stability. This study provides a clear fundamental basis for understanding the local structural, electronic factors and C-Ru bond strengthening/weakening processes that affect the stability of enolate and epoxy species.

Titanium dioxide/graphene composites have recently been demonstrated to improve the photocatalytic activity of TiO2 in visible light. To better understand the interactions of TiO2 with graphene we have investigated the growth of TiO2 nanoclusters on single-layer graphene/Ru(0001) using scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). Deposition of Ti in the O2 background at 300 K resulted in the formation of nanoclusters nucleating on intrinsic defects in the graphene (Gr) layer. The saturation nanocluster density decreased as the substrate temperature was increased from 300 to 650 K, while deposition at 700 K resulted in the significant etching of the Gr layer. We have also prepared nanoclusters with Ti2O3 stoichiometry using lower O2 pressures at 650 K. Thermal stability of the TiO2 nanoclusters prepared at 300 K was evaluated with AES and STM. No change in oxidation state for the TiO2 nanoclusters or etching of the Gr layer was observed up to ∼900 K. Annealing studies revealed that cluster ripening proceeds via a Smoluchowski mechanism below 800 K. Above 800 K, the changes in cluster shapes indicate an onset of diffusion within the clusters. At even higher temperatures, the nanoclusters undergo reduction to TiOx (x ≈ 1-1.5) which is accompanied by oxidation and etching of the Gr. Our studies demonstrate that highly thermally stable TiOx nanoclusters of controlled composition and morphology can be prepared on Gr supports.

Mixed CuO(2 × 1)-CuWO4 layers on a Cu(110) surface have been prepared by the on-surface reaction of the CuO(2 × 1) surface oxide with adsorbed (WO3)3 clusters. The adsorption and decomposition of methanol on these well-defined CuO-CuWO4 surfaces has been followed by high-resolution X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), and temperature-programmed desorption (TPD) to assess the molecular surface species and their concentration, while the state of the surface oxide phases before and after methanol decomposition has been characterized by scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and XPS. Surface methoxy species form the primary methanol decomposition products, which desorb partly by recombination as methanol at 200-300 K or decompose into CHx and possibly CO. The most reactive surfaces are mixed CuO-CuWO4 phase, with CuWO4 coverages 0.5-0.8 monolayer, thus pointing at the importance of oxide phase boundary sites. In a minority reaction channel, a small amount of formaldehyde is detected on the CuWO4 surface. The CuWO4 oxide phase becomes modified as a result of reduction and a morphology transition triggered by the methanol decomposition, but the pristine surface state can be recovered by a postoxidation treatment with oxygen.

The epitaxial growth of two-dimensional oxide layers on metal surfaces is examined in view of the 1949 van der Merwe proposition that epitaxy requires a pseudomorphic monolayer. It is argued that this limitation is relaxed in the 2-D case and that ordered oxide phases can grow out of a variety of interface scenarios, ranging from pseudomorphic to incommensurate. Prototypical examples of binary and ternary oxides supported on noble metal surfaces are presented, and the structural peculiarities of 2-D oxide phases are emphasized. The usually strong coupling at the oxide-metal interface leads to the stabilization of novel structure concepts that are not encountered in the, respective, bulk phases. The structural flexibility of 2-D lattices is discussed, and their ability to accommodate strain in generating novel 2-D oxide phases is emphasized. In the case of weakly coupled systems, it is reported that more subtle interactions at the interface can create periodic nanoscale morphologies and particular growth patterns in subsequent layers.

The structure and properties of ternary oxide materials at the nanoscale are poorly explored both on experimental and theoretical levels. With this work we demonstrate the successful on-surface synthesis of two-dimensional (2D) ternary oxide, MnWOx and FeWOx, nanolayers on a Pd(1 0 0) surface and the understanding of their new structure and phase behaviour with the help of state-of-art surface structure and spectroscopy techniques. We find that the 2D MnWOx and FeWOx phases, prepared under identical thermodynamic conditions, exhibit similar structural properties, reflecting the similarity of the bulk MnWO4 and FeWO4 phases with the wolframite structure. Structure models of prototypical 2D ternary oxide phases are proposed and discussed in the light of new structure architecture concepts which have no analogues in the bulk.