Transition Metal Substrates Research Articles

Theoretical investigations on hydroxyl carbon precursor fueled growth of graphene on transition metal substrates

Theoretical investigations on hydroxyl carbon precursor fueled growth of graphene on transition metal substrates

Theoretical study on the electrochemical CO2 reduction performance of MoS2-supported Ni single atoms with transition metal substrate doping

In the context of increasing energy issues and climate change, the development of electrochemical CO2 reduction strategies using single atom catalysts (SACs) presents a viable solution by converting CO2 into high-value-added products. Transition metal dichalcogenides (TMDs) have emerged as the main candidates for SACs in electrocatalytic CO2 reduction reaction (CO2RR) catalysts. The regulation of the second coordination sphere of SACs is known to modulate their electronic structure and catalytic behavior. This study delves into the effect of transition metal-doped substrates on SACs. We employed spin-polarization density functional theory (DFT) to design a series of Ni/TM-MoS2 monolayer MoS2 materials, with transition metals (including Cr, Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, W, Re, Os, Ir, Pt and Au) substituting the Mo atoms and anchoring single atom Ni. Among them, six Ni/TM-MoS2 catalysts doped with Mn, Fe, Co, Ru, Rh, and Ir demonstrate enhanced CO2RR activity and selectivity when compared to unmodified Ni/MoS2. The Ni/Ru-MoS2 SAC exhibits the best catalytic activity and selectivity for formic acid production, with a limiting potential (UL) of -0.06 V. The incorporation of the metal-doped substrate lowers the work function of the catalyst, facilitating electron transfer and reaction progression. Furthermore, a volcanic relationship was observed between the work function and UL. This study provides strategic insights into the design and development of SACs, highlighting the potential for future modifications and applications.

Atmospheric-pressure CVD-grown h-BN for the detector with deep ultraviolet response

Hexagonal boron nitride (h-BN) has a wide range of applications, especially as a protective coating, dielectric layer/substrate, transparent film, or deep ultraviolet detectors. High-quality h-BN thick films are indispensable for practical deep-ultraviolet (DUV) photodetector applications. However, the controlled synthesis of h-BN films in terms of thicknesses and crystallinity often requires high growth temperatures, low pressures, and catalytic transition metal substrates, which will ultimately hinder their applicability. In this work, we conducted a detailed study of h-BN films with thickness ranging from 50 nm to 160 nm directly synthesized by chemical vapor deposition (CVD) on SiO2/Si substrate under atmospheric pressure. The synthesized h-BN is clean, uniform, and exhibits excellent optical and photoelectrical properties for ultraviolet light in the range of 210 nm-280 nm. This sensitive h-BN photodetector has a high repulsion rate (R220nm/R280nm > 102 and R220nm/ R290nm > 103), a large detection rate (2.8 × 1010 Jones). This work presented here demonstrates the great potential of this h-BN thick film for the development of DUV photodetectors.

On-Surface Synthesis of Graphene Nanoribbons with Atomically Precise Structural Heterogeneities and On-Site Characterizations.

Graphene nanoribbons (GNRs) are strips of graphene, with widths of a few nanometers, that are promising candidates for future applications in nanodevices and quantum information processing due to their highly tunable structure-dependent electronic, spintronic, topological, and optical properties. Implantation of periodic structural heterogeneities, such as heteroatoms, nanopores, and non-hexagonal rings, has become a powerful manner for tailoring the designer properties of GNRs. The bottom-up synthesis approach, by combining on-surface chemical reactions based on rationally designed molecular precursors and in situ tip-based microscopic and spectroscopic techniques, promotes the construction of atomically precise GNRs with periodic structural modulations. However, there are still obstacles and challenges lying on the way toward the understanding of the intrinsic structure-property relations, such as the strong screening and Fermi level pinning effect of the normally used transition metal substrates and the lack of collective tip-based techniques that can cover multi-internal degrees of freedom of the GNRs. In this Perspective, we briefly review the recent progress in the on-surface synthesis of GNRs with diverse structural heterogeneities and highlight the structure-property relations as characterized by the noncontact atomic force microscopy and scanning tunneling microscopy/spectroscopy. We furthermore motivate to deliver the need for developing strategies to achieve quasi-freestanding GNRs and for exploiting multifunctional tip-based techniques to collectively probe the intrinsic properties.

(Invited) Toward Rigorous Modeling of Metal-Metal, Metal-Oxide and Oxide-Oxide Interfaces for the Electrochemical Oxygen Cycle

To achieve Net-Zero Emissions by 2050, hydrogen and hydrogen fuel cells will play a significant role in powering vehicles. In this presentation, I will show, through first-principles-based modeling, how to achieve rigorous modeling of solid-solid interface for the hydrogen and oxygen evolution reactions in electrolysis and the oxygen reduction reaction in fuel cells; and how the fundamental understanding paves the way toward developing both model electrocatalysts and industrial electrocatalysts with significantly improved performance. Specifically, I will introduce the following three topics.[1] metal-metal interfaces: strain evolution and strain tuning of epitaxial, stepped and free-standing platinum group metals for the oxygen reduction reaction in fuel cells.(1)[2] metal-oxide interfaces: stability and activity of monolayer oxide/Pt interface toward improving the hydrogen evolution reaction in alkaline conditions.(2)[3] oxide-oxide interfaces: self-consistent modeling of active phases, reaction centers, and catalytic mechanisms of Ni- and Co-based layered double hydroxides for the oxygen evolution reaction.(3, 4) L. Wang, Z. Zeng, W. Gao, T. Maxson, D. Raciti, M. Giroux, X. Pan, C. Wang, J. Greeley, Tunable intrinsic strain in two-dimensional transition metal electrocatalysts. Science 363, 870-874 (2019).Z. Zeng, K.-C. Chang, J. Kubal, N. M. Markovic, J. Greeley, Stabilization of ultrathin (hydroxy)oxide films on transition metal substrates for electrochemical energy conversion. Nature Energy 2, 17070 (2017).F. Dionigi, Z. Zeng, I. Sinev, T. Merzdorf, S. Deshpande, M. B. Lopez, S. Kunze, I. Zegkinoglou, H. Sarodnik, D. Fan, A. Bergmann, J. Drnec, J. F. d. Araujo, M. Gliech, D. Teschner, J. Zhu, W.-X. Li, J. Greeley, B. R. Cuenya, P. Strasser, In-situ structure and catalytic mechanism of NiFe and CoFe layered double hydroxides during oxygen evolution. Nature Communications 11, 2522 (2020).F. Dionigi, J. Zhu, Z. Zeng, T. Merzdorf, H. Sarodnik, M. Gliech, L. Pan, W.-X. Li, J. Greeley, P. Strasser, Intrinsic Electrocatalytic Activity for Oxygen Evolution of Crystalline 3d-Transition Metal Layered Double Hydroxides. Angew. Chem., Int. Ed. 60, 14446-14457 (2021). Figure 1

The Mechanism of Hydrogen Evolution Reaction at the Buried Interface of Silica-Coated Electrocatalysts

Semipermeable oxide coatings can protect electrocatalysts in harsh environments without reducing the catalytic performance (Labrador, Esposito et al. ACS Catal. 8, 2018, 1767–1778), making them attractive for direct seawater electrolysis. We recently showed that the buried SiO2/Pt interface of silica-coated platinum electrocatalysts is environment-dependent and changes with the pH value of the electrolyte and the electrode potential (Qu and Urban, ACS Appl. Mater. Interfaces 12, 2020, 52125–52135). Here, we discuss the impact of silica membrane coatings on the hydrogen evolution reaction (HER) mechanism at the interface with different transition-metal surfaces. Stable configurations of the buried SiO2/TM interface at HER conditions were determined using density-functional theory (DFT) calculations. Computed Pourbaix diagrams for different transition-metal substrates show the pH and potential dependence of reaction intermediates and the hydrogen coverage on the metal surface. Our results indicate that the HER mechanism at the buried SiO2/catalyst interfaces may involve the silica membrane. Hence, besides the protective quality of silica membranes, this also points to the possibility of designing synergistic membrane-coated electrocatalysts that surpass the bare surfaces of earth-abundant transition metals in terms of catalytic performance (stability, activity, and/or selectivity).

Coherent hexagonal platinum skin on nickel nanocrystals for enhanced hydrogen evolution activity

Metastable noble metal nanocrystals may exhibit distinctive catalytic properties to address the sluggish kinetics of many important processes, including the hydrogen evolution reaction under alkaline conditions for water-electrolysis hydrogen production. However, the exploration of metastable noble metal nanocrystals is still in its infancy and suffers from a lack of sufficient synthesis and electronic engineering strategies to fully stimulate their potential in catalysis. In this paper, we report a synthesis of metastable hexagonal Pt nanostructures by coherent growth on 3d transition metal nanocrystals such as Ni without involving galvanic replacement reaction, which expands the frontier of the phase-replication synthesis. Unlike noble metal substrates, the 3d transition metal substrate owns more crystal phases and lower cost and endows the hexagonal Pt skin with substantial compressive strains and programmable charge density, making the electronic properties particularly preferred for the alkaline hydrogen evolution reaction. The energy barriers are greatly reduced, pushing the activity to 133 mA cmgeo–2 and 17.4 mA μgPt–1 at –70 mV with 1.5 µg of Pt in 1 M KOH. Our strategy paves the way for metastable noble metal catalysts with tailored electronic properties for highly efficient and cost-effective energy conversion.

Microstructural Characteristics of ECR Plasma Etched and Heat Treated Commercial Copper Foil

AbstractThe structural quality of the catalytic transition metal substrate plays a critical role in pristine graphene synthesis by chemical vapor deposition. In this work, the microstructural parameters of commercial copper foils are investigated in lieu of conventional laboratory grade crystalline copper foils. Initially, the commercial copper foils are simultaneously plasma etched and heat treated for a period of 30, 45 min, and 1 h respectively at two elevated temperature levels of 823 and 923 K under the exposure of hydrogen plasma. The experiments are carried out in a 2.45 GHz Electron Cyclotron Resonance (ECR) Plasma Enhanced Chemical Vapor Deposition (PE‐CVD) reactor, at an operating pressure of 5 × 10−2 mbar. X‐ray diffraction (XRD) results are plotted and analyzed by Williamson–Hall (W–H) method to determine microstructural parameters, e.g., micro‐strain, stress, average domain size, and energy density associated to the processed copper foils.

Universal properties of metal-supported oxide films from linear scaling relationships: elucidation of mechanistic origins of strong metal-support interactions.

The properties of ultrathin (1-2 monolayer) (hydroxy)oxide films on transition metal substrates have been extensively studied as models of the celebrated Strong Metal-Support Interaction (SMSI) and related phenomena. However, results from these analyses have been largely system specific, and limited insights into the general principles that govern film/substrate interactions exist. Here, using Density Functional Theory (DFT) calculations, we analyze the stability of ZnO x H y films on transition metal surfaces and show that the formation energies of these films are related to the binding energies of isolated Zn and O atoms via linear scaling relationships (SRs). Such relationships have previously been identified for adsorbates on metal surfaces and have been rationalized in terms of bond order conservation (BOC) principles. However, for thin (hydroxy)oxide films, SRs are not governed by standard BOC relationships, and a generalized bonding model is required to explain the slopes of these SRs. We introduce such a model for the ZnO x H y films and confirm that it also describes the behavior of reducible transition metal oxide films, such as TiO x H y , on metal substrates. We demonstrate how the SRs may be combined with grand canonical phase diagrams to predict film stability under conditions relevant to heterogeneous catalytic reactions, and we apply these insights to estimate which transition metals are likely to exhibit SMSI behavior under realistic environmental conditions. Finally, we discuss how SMSI overlayer formation for irreducible oxides, such as ZnO, is linked to hydroxylation and is mechanistically distinct from the overlayer formation for reducible oxides such as TiO2.

Plastic recognition and electrogenic uniport translocation of 1st-, 2nd-, and 3rd-row transition and post-transition metals by primary-active transmembrane P1B-2-type ATPase pumps.

Transmembrane P1B-type ATPase pumps catalyze the extrusion of transition metal ions across cellular lipid membranes to maintain essential cellular metal homeostasis and detoxify toxic metals. Zn(ii)-pumps of the P1B-2-type subclass, in addition to Zn2+, select diverse metals (Pb2+, Cd2+ and Hg2+) at their transmembrane binding site and feature promiscuous metal-dependent ATP hydrolysis in the presence of these metals. Yet, a comprehensive understanding of the transport of these metals, their relative translocation rates, and transport mechanism remain elusive. We developed a platform for the characterization of primary-active Zn(ii)-pumps in proteoliposomes to study metal selectivity, translocation events and transport mechanism in real-time, employing a "multi-probe" approach with fluorescent sensors responsive to diverse stimuli (metals, pH and membrane potential). Together with atomic-resolution investigation of cargo selection by X-ray absorption spectroscopy (XAS), we demonstrate that Zn(ii)-pumps are electrogenic uniporters that preserve the transport mechanism with 1st-, 2nd- and 3rd-row transition metal substrates. Promiscuous coordination plasticity, guarantees diverse, yet defined, cargo selectivity coupled to their translocation.

Origin of Tearing Paths in Transferred Graphene by H2 Bubbling Process and Improved Transfer of T ear-Free Graphene Films U sing a H eat Press

Among efforts to improve techniques for the chemical vapor deposition of large-area and high-quality graphene films on transition metal substrates, being able to reliably transfer these atomistic membranes onto the desired substrate is a critical step for various practical uses, such as graphene-based electronic and photonic devices. However, the most used approach, the wet etching transfer process based on the complete etching of metal substrates, remains a great challenge. This is mainly due to the inevitable damage to the graphene, unintentional contamination of the graphene layer, and increased production cost and time. Here, we report the systematic study of an H2 bubbling-assisted transfer technique for graphene films grown on Cu foils, which is nondestructive not only to the graphene film but also to the Cu substrate. Also, we demonstrate the origin of the graphene film tearing phenomenon induced by this H2 bubbling-assisted transfer process. This study reveals that inherent features are produced by rolling Cu foil, which cause a saw-like corrugation in the poly(methyl methacrylate) (PMMA)/graphene stack when it is transferred onto the target substrate after the Cu foil is dissolved. During the PMMA removal stage, the graphene tearing mainly appears at the apexes of the corrugated PMMA/graphene stack, due to weak adhesion to the target substrate. To address this, we have developed a modified heat-press-assisted transfer technique that has much better control of both tearing and the formation of residues in the transferred graphene films.

Carbon-rehybridization-induced templated growth of metal nanoclusters on graphene moiré patterns

Templated growth of monodisperse metal nanoclusters has successfully been realized through graphene moiré patterns on transition metal substrates, but the underlying mechanism is still unclear. Herein, based on first-principles calculations, we systematically study the growth mechanism of Pt and Au nanoclusters on graphene-covered Ir(111) substrate. Our results indicate that the induced sp2-sp3 rehybridization of C atoms plays a key role for the templated growth. From the center of the template to the outside, such rehybridization is gradually suppressed due to the enlarged horizontal distance between C atoms and the adjacent Ir atoms, which causes weaker bonding of Pt atoms to larger clusters and thus the perfect templated growth. Meanwhile, negligible sp2-sp3 rehybridization is observed due to the weak adsorption of Au atoms on graphene, which leads to the failure of the template. This mechanism is then extended to other metal nanoclusters successfully. We believe that it will help to understand the structure-activity relationship of more two-dimensional templates.

Charge Transfer-Mediated Dramatic Enhancement of Raman Scattering upon Molecular Point Contact Formation.

Charge-transfer enhancement of Raman scattering plays a crucial role in current-carrying molecular junctions. However, the microscopic mechanism of light scattering in such nonequilibrium systems is still imperfectly understood. Here, using low-temperature tip-enhanced Raman spectroscopy (TERS), we investigate how Raman scattering evolves as a function of the gap distance in the single C60-molecule junction consisting of an Ag tip and various metal surfaces. Precise gap-distance control allows the examination of two distinct transport regimes, namely tunneling regime and molecular point contact (MPC). Simultaneous measurement of TERS and the electric current in scanning tunneling microscopy shows that the MPC formation results in dramatic Raman enhancement that enables one to observe the vibrations undetectable in the tunneling regime. This enhancement is found to commonly occur not only for coinage but also transition metal substrates. We suggest that the characteristic enhancement upon the MPC formation is rationalized by charge-transfer excitation.

Exfoliating silica bilayers via intercalation at the silica/transition metal interface

The growth of the silica (SiO2) bilayer (BL) films on transition metal (TM) surfaces creates a new class of two-dimensional (2D) crystalline, self-contained materials that interact weakly with the TM substrate. The BL-silica/TM heterojunction has shown unique physical and chemical properties that can lead to new chemical reaction mechanisms under the sub-nm confinement and broad potential applications ranging from surface protection, nano transistors, molecular sieves to nuclear waste removal. Novel applications of BL-silica can be further explored as a constituent of van der Waals assembly of 2D materials. Key to these applications is an unmet technical challenge to exfoliate and transfer BL-silica films in a large area from one substrate to another without material damage. In this study, we propose a new exfoliation mechanism based on gas molecule intercalation from density functional theory studies of the BL-silica/TM heterojunction. We found that the intercalation of O atoms and CO molecules at the BL-silica/TM interface weakens the BL-silica—TM hybridization, which results in an exponential decrease of the exfoliation energy against the interface distance as the coverage of interfacial species increases. This new intercalation mechanism opens up the opportunity for non-damaging exfoliation and transfer of large area silica bilayers.

Stabilities of Isomers of Phosphorus on Transition Metal Substrates

Stabilities of Isomers of Phosphorus on Transition Metal Substrates

Mixing properties of Al2O3(0001)-supported M 2O3 and MM′O3 monolayers (M, M′ = Ti, V, Cr, Fe)

Considering the importance of sub-monolayer transition metal oxides supported on another oxide in many industrial processes, with the help of a DFT + U approach, we provide information on the structural and electronic properties of pure M 2O3 and mixed MM′O3 oxide monolayers (M, M′ = Ti, V, Cr, Fe) supported on an α-Al2O3(0001) support. With their structure in the prolongation of the alumina corundum lattice, the monolayers have non-equivalent surface and interface cations, which leads to two different cation configurations in the mixed oxides. In all cases, the interfacial charge transfer is weak, but strong cation–cation electron redistributions may take place as in TiVO3, TiFeO3, VFeO3, and TiCrO3 in which actual redox processes lead to cation oxidation states different from the expected +3 value. We show that the tendency to mixing relies on the interplay between two very different driving forces. Cation–cation redox reactions, in most cases, strongly stabilise mixed configurations, but preference for a given cation position in the monolayer, because of surface energy reasons, may strengthen, weaken or even block the mixing tendency. By comparison with results obtained in bulk ilmenite, in free-standing monolayers and in MLs deposited on transition metal substrates, we evidence the flexibility of their electronic structure as a function of size, dimensionality and nature of support, as a lever to tune their properties for specific applications.

Theoretical calculation boosting the chemical vapor deposition growth of graphene film

Chemical vapor deposition (CVD) is a promising method for the mass production of high-quality graphene films, and great progress has been made over the last decade. Currently, the CVD growth of graphene is being pushed to achieve further advancements, such as super-clean, ultra-flat, and defect-free materials, as well as controlling the layer, stacking order, and doping level during large-scale preparation. The production of high-quality graphene by CVD relies on an in-depth knowledge of the growth mechanisms, in which theoretical calculations play a crucial role in providing valuable insights into the energy-, time-, and scale-dependent processes occurring during high-temperature growth. Here, we focus on the theoretical calculations and discuss the recent progress and challenges that need to be overcome to achieve controllable growth of high-quality graphene films on transition-metal substrates. Furthermore, we present some state-of-the-art graphene-related structures with novel properties, which are expected to enable new applications of graphene-based materials.

Direct Growth of Hexagonal Boron Nitride Thick Films on Dielectric Substrates by Ion Beam Assisted Deposition for Deep‐UV Photodetectors

AbstractHigh quality hexagonal boron nitride (h‐BN) thick films are indispensable for practical deep‐ultraviolet (DUV) photodetector applications. However, the controllable synthesis of h‐BN films in terms of thickness and crystallinity often requires high growth temperatures, tedious chemical precursors and, catalytic transition metal substrates, that will eventually hinder their applicability. In this work, the direct growth of h‐BN films with thickness of 50–500 nm on silicon(100), sapphire, and quartz substrates by ion beam assisted deposition at a lower temperature (≈500 °C) without employing extra catalysis is reported. The as‐synthesized h‐BN thick films are stoichiometric, smooth, and continuous without obvious pinholes and cracks, consisting of nanocrystalline domains on the dielectric substrates, distinct from those grown on metallic substrates. More intriguingly, the DUV photodetector fabricated from a 500 nm thick h‐BN film on quartz substrate exhibits high peak responsivity (0.5 A W−1) with a ultrahigh on/off ratio of >103 at 204 nm, a high rejection ratio (R204nm/R250nm > 103), a large specific detectivity (6.92 × 109 Jones), and a sharp cutoff wavelength at 218 nm. The work herein demonstrates the great potential of this form of h‐BN thick films toward the development of DUV photodetectors.

The Mechanism of Graphene Vapor-Solid Growth on Insulating Substrates.

Wafer-scale single-crystal graphene film directly grown on insulating substrates via the chemical vapor deposition (CVD) method is desired for building high-performance graphene-based devices. In comparison with the well-studied mechanism of graphene growth on transition metal substrates, the lack of understanding on the mechanism of graphene growth on insulating surfaces greatly hinders the progress. Here, by using first-principles calculation, we systematically explored the absorption of various carbon species CHx (x = 0, 1, 2, 3, 4) on three typical insulating substrates [h-BN, sapphire, and quartz] and reveal that graphene growth on an insulating surface is dominated by the reaction of active carbon species with the hydrogen-passivated graphene edges and thus is less sensitive to the type of the substrate. The dominating gas phase precursor, CH3, plays two key roles in graphene CVD growth on an insulating substrate: (i) to feed the graphene growth and (ii) to remove excessive hydrogen atoms from the edge of graphene. The threshold reaction barriers for the growth of graphene armchair (AC) and zigzag (ZZ) edges were calculated as 3.00 and 1.94 eV, respectively; thus the ZZ edge grows faster than the AC one. Our theory successfully explained why the circumference of a graphene island grown on insulating substrates is generally dominated by AC edges, which is a long-standing puzzle of graphene growth. In addition, the very slow graphene growth rate on an insulating substrate is calculated and agrees well with existing experimental observations. The comprehensive insights on the graphene growth on insulating surfaces at the atomic scale provide guidance on the experimental design for high-quality graphene growth on insulating substrates.

Modeling and Performance Assessment of a Millimeter-Wave CVD Film-Graphene Based Antenna

The transfer of graphene onto dielectric substrates to form an antenna needs to be performed with care, as tears and cracks may appear on the samples transferred improperly. This paper studies a detailed method in estimating the performance of a millimeter-wave (MMW) antenna built using commercially-available graphene layers. In contrast to the widely studied monolayer graphene, the proposed antenna is produced based on the chemical vapor deposition (CVD) technique, which is to be secured onto the antenna substrate via adhesive material without removal of the transition metal substrate from the graphene layer sample. The antenna performance modeled using this technique indicated quite similar results to a model using ideal monolayer graphene in terms of gain, radiated power and radiation efficiency. Its performance benchmarked against a copper antenna operating at the same MMW frequency also indicated similar performance improvements as with another antenna designed using monolayer graphene.