A Novel Growth Mode of Mo on Au (111) from a Mo(CO)6 Precursor: An STM Study

Epitaxial Growth of Two-Dimensional Nonlayered AuCrS2 Materials via Au-Assisted Chemical Vapor Deposition

Two-dimensional (2D) nonlayered transition metal dichalcogenide (TMD) materials are emergent platforms for various applications from catalysis to quantum devices. However, their limited availability and nonstraightforward synthesis methods hinder our understanding of these materials. Here, we present a novel technique for synthesizing 2D nonlayered AuCrS2 via Au-assisted chemical vapor deposition (CVD). Our detailed structural analysis reveals the layer-by-layer growth of [AuCrS2] units atop an initial CrS2 monolayer, with Au binding to the adjacent monolayer of CrS2, which is in stark contrast with the well-known metal intercalation mechanism in the synthesis of many other 2D nonlayered materials. Theoretical calculations further back the crucial role of Cr in increasing the mobility of Au species and strengthening the adsorption energy of Au on CrS2, thereby aiding the growth throughout the CVD process. Additionally, the resulting free-standing nanoporous AuCrS2 (NP-AuCrS2) exhibits exceptional electrocatalytic properties for the hydrogen evolution reaction.

The Importance of Decarbonylation Mechanisms in the Atomic Layer Deposition of High-Quality Ru Films by Zero-Oxidation State Ru(DMBD)(CO)3

Achieving facile nucleation of noble metal films through atomic layer deposition (ALD) is extremely challenging. To this end, η⁴ -2,3-dimethylbutadiene ruthenium⁽⁰⁾ tricarbonyl (Ru(DMBD)(CO)₃ ), a zero-valent complex, has recently been reported to achieve good nucleation by ALD at relatively low temperatures and mild reaction conditions. The authors study the growth mechanism of this precursor by in situ quartz-crystal microbalance and quadrupole mass spectrometry during Ru ALD, complemented by ex situ film characterization and kinetic modeling. These studies reveal that Ru(DMBD)(CO)₃ produces high-quality Ru films with excellent nucleation properties. This results in smooth, coalesced films even at low film thicknesses, all important traits for device applications. However, Ru deposition follows a kinetically limited decarbonylation reaction scheme, akin to typical chemical vapor deposition processes, with a strong dependence on both temperature and reaction timescale. The non-self-limiting nature of the kinetically driven mechanism presents both challenges for ALD implementation and opportunities for process tuning. By surveying reports of similar precursors, it is suggested that the findings can be generalized to the broader class of zero-oxidation state carbonyl-based precursors used in thermal ALD, with insight into the design of effective saturation studies.

Boosting the activity of transition metal carbides towards methane activation by nanostructuring

Molybdenum carbide breaks methane by going nano.

In situ synthesis of MoS2 on a polymer based gold electrode platform and its application in electrochemical biosensing

Bulk layers of MoS₂ were synthesized in situ on a polymer substrate at low temperature for electrochemical biosensing.

Patternable large-scale molybdenium disulfide atomic layers grown by gold-assisted chemical vapor deposition

A novel way to grow MoS2 on a large scale with uniformity and in desired patterns is developed. We use Au film as a catalyst on which [Mo(CO)6 ] vapor decomposes to form a Mo-Au surface alloy that is an ideal Mo reservoir for the growth of atomic layers of MoS2 . Upon exposure to H2 S, this surface alloy transforms into a few layers of MoS2 , which can be isolated and transferred on an arbitrary substrate. By simply patterning Au catalyst film by conventional lithographic techniques, MoS2 atomic layers in desired patterns can be fabricated.

Multiple Coordination of CO on Molybdenum Nanoparticles: Evidence for Intermediate Mox(CO)y Species by XPS and UPS

CO chemisorption on the metallic molybdenum nanoparticles supported on the thin alumina film was investigated by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). A binary compound of molybdenum and CO is found to be formed on the surface upon CO dose, accompanied with a positive binding energy shift of the Mo 3d doublet and a localized Mo 4d valence band. A loose packing of the metallic molybdenum favors the formation of this intermediate Mox(CO)y species. The formation of the Mox(CO)y species implies that the property of the metallic molybdenum nanoparticles on the thin alumina film is much different from that of the bulk molybdenum, indicating a significant nanometer size effect.

Synthesis of TiO2 nanoparticles on the Au(111) surface

The growth of titanium oxide nanoparticles on reconstructed Au(111) was investigated by scanning tunneling microscopy and x-ray photoelectron spectroscopy. Ti was deposited by physical-vapor deposition at 300 K. Regular arrays of titanium nanoparticles form by preferential nucleation of Ti at the elbow sites of the herringbone reconstruction. The titanium oxide nanoclusters were synthesized by subsequent exposure to O(2) at 300 K. Two-and three-dimensional titanium oxide nanocrystallites form during annealing in the temperature range from 600 to 900 K. At the same time, the Au(111) surface assumes a serrated 110-oriented step-edge morphology suggesting step-edge pinning by titanium oxide nanoparticles. The oxidation state of the titanium oxide nanoparticles varies with annealing temperature. Specifically, annealing to 900 K results in the formation of stoichiometric TiO(2) nanocrystals as judged by the Ti(2p) binding energies measured in the x-ray photoelectron data. The nanodispersed TiO(2) on Au(111) is an ideal system to test the various models proposed for the enhanced catalytic reactivity of supported Au nanoparticles.

Formation of TiO2 nanoparticles by reactive-layer-assisted deposition and characterization by XPS and STM

Stoichiometric TiO2 nanoparticles (1-5 nm) were prepared by reactive-layer-assisted deposition (RLAD), in which Ti was initially deposited on a multilayer of H2O (or NO2) on a Au(111) substrate at approximately 90 K. The composition and atom-resolved structure of the nanoparticles were studied by XPS and STM. The approximately 5 nm TiO2 particles had either a rutile or anatase phase with various crystal facets. STS of the nanoparticles suggests size-dependent electronic structure. These well-defined nanoparticles can be used in molecular-level studies of the reactions and mechanisms of photocatalytic processes on TiO2 nanoparticle surfaces.

Characterization of Molybdenum Carbide Nanoparticles Formed on Au(111) Using Reactive-Layer Assisted Deposition

Temperature programmed desorption (TPD), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM) have been used to characterize molybdenum carbide nanoparticles prepared on a Au(111) substrate. The MoC(x) nanoparticles were formed by Mo metal deposition onto a reactive multilayer of ethylene, which was physisorbed on a Au(111) substrate at low temperatures (<100 K). The resulting clusters have an average diameter of approximately 1.5 nm and aggregate in the fcc troughs located on either side of the elbows of the reconstructed Au(111) surface. Core level XPS shows that the electronic environment of the Mo and C atoms in the nanoparticles is similar to that found in Mo(2)C(0001) single crystals and carburized Mo metal surfaces. Peak intensities in XPS and AES spectra were used to estimate an average Mo/C atomic ratio of 1.2 +/- 0.3 for nanoparticles annealed above 600 K.

Structure and reactivity of Ru nanoparticles supported on modified graphite surfaces: a study of the model catalysts for ammonia synthesis

Supported ruthenium metal catalysts have higher activity than traditional iron-based catalysts used industrially for ammonia synthesis. A study of a model Ru/C catalyst was carried out to advance the understanding of structure and reactivity correlations in this structure-sensitive reaction where dinitrogen dissociation is the rate-limiting step. Ru particles were grown by chemical vapor deposition (CVD) via a Ru(3)(CO)(12) precursor on a highly oriented pyrolytic graphite (HOPG) surface modified with one-atomic-layer-deep holes mimicking activated carbon support. Scanning tunneling microscopy (STM) has been used to investigate the growth, structure, and morphology of the Ru particles. Thermal desorption of dissociatively adsorbed nitrogen has been used to evaluate the reactivity of the Ru/HOPG model catalysts. Two different Ru particle structures with different reactivities to N(2) dissociation have been identified. The initial sticking coefficient for N(2) dissociative adsorption on the Ru/HOPG model catalysts is approximately 10(-6), 4 orders larger compared to that of Ru single-crystal surfaces.

Is it homogeneous or heterogeneous catalysis? Identification of bulk ruthenium metal as the true catalyst in benzene hydrogenations starting with the monometallic precursor, Ru(II)(eta 6-C6Me6)(OAc)2, plus kinetic characterization of the heterogeneous nucleation, then autocatalytic surface-growth mechanism of metal film formation

A reinvestigation of the true catalyst in a benzene hydrogenation system beginning with Ru(II)(eta(6)-C(6)Me(6))(OAc)(2) as the precatalyst is reported. The key observations leading to the conclusion that the true catalyst is bulk ruthenium metal particles, and not a homogeneous metal complex or a soluble nanocluster, are as follows: (i) the catalytic benzene hydrogenation reaction follows the nucleation (A --> B) and then autocatalytic surface-growth (A + B --> 2B) sigmoidal kinetics and mechanism recently elucidated for metal(0) formation from homogeneous precatalysts; (ii) bulk ruthenium metal forms during the hydrogenation; (iii) the bulk ruthenium metal is shown to have sufficient activity to account for all the observed activity; (iv) the filtrate from the product solution is inactive until further bulk metal is formed; (v) the addition of Hg(0), a known heterogeneous catalyst poison, completely inhibits further catalysis; and (vi) transmission electron microscopy fails to detect nanoclusters under conditions where they are otherwise routinely detected. Overall, the studies presented herein call into question any claim of homogeneous benzene hydrogenation with a Ru(arene) precatalyst. An additional, important finding is that the A --> B, then A + B --> 2B kinetic scheme previously elucidated for soluble nanocluster homogeneous nucleation and autocatalytic surface growth (Widegren, J. A.; Aiken, J. D., III; Ozkar, S.; Finke, R. G. Chem. Mater. 2001, 13, 312-324, and ref 8 therein) also quantitatively accounts for the formation of bulk metal via heterogeneous nucleation then autocatalytic surface growth. This is significant for three reasons: (i) quantitative kinetic studies of metal film formation from soluble precursors or chemical vapor deposition are rare; (ii) a clear demonstration of such A --> B, then A + B --> 2B kinetics, in which both the induction period and the autocatalysis are continuously monitored and then quantitatively accounted for, has not been previously demonstrated for metal thin-film formation; yet (iii) all the mechanistic insights from the soluble nanocluster system (op. cit.) should be applicable to metal thin-film formations which exhibit sigmoidal kinetics and, hence, the A --> B, then A + B --> 2B mechanism.

Molecular level study of the formation and the spread of MoO3 on Au(111) by scanning tunneling microscopy and X-ray photoelectron spectroscopy

The formation of MoO(3) and its spontaneous spread over an Au (111) surface have been studied by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Metallic Mo clusters grown by Mo(CO)(6) chemical vapor deposition (CVD) have a constant size independent of the Mo coverage. Molecular oxygen does not react with low coverage of Mo, probably due to the encapsulation of the Mo clusters by Au. At higher coverage, O(2) reacts with Mo, partially transforming the metallic Mo to Mo(4+). NO(2) can oxidize Mo efficiently to Mo(6+) and Mo(5+) species at all coverages investigated. XPS experiments show that the integrated intensity of the Mo 3d peaks increases by a factor of 2 upon the oxidation, suggesting the spread of the MoO(3) over the surface. The STM study confirms this suggestion and provides the mechanistic details of the spreading. Mo oxide forms ramified two-dimensional islands covering a substantially larger fraction of the Au surface than the metallic Mo. We propose that the morphology change starts with the diffusion of oxide clusters (ramified-cluster-diffusion mechanism), followed by their breakdown to highly disordered two-dimensional islands of molecular MoO(3).