Surface Science最新文献

Surface modification has been established to control chemical, mechanical, and electronic properties of oxide surfaces. Surface chemistry of β-diketones on ZnO nanomaterials presents an opportunity to investigate the dependence of the adsorbate structure on the type of diketone and, specifically, on the presence of electron-donating and electron-withdrawing functional groups. This work compares the adsorption of 1,1,1-trifluoro-2,4-pentane-dione (trifluoroacetylacetone, tfacH) and 1,1,1,5,5,5-hexafluoro-2,4-pentane-dione (hexafluoroacetylacetone, hfacH) on ZnO nanopowder by interrogating the molecular structure of adsorbates with spectroscopic and computational methods. Despite the fact that in the gas phase the enol structure dominates for hfacH and the diketone has substantial presence for tfacH, once these compounds are adsorbed on ZnO, the diketonate is the majority of surface species for hfacH and dissociated enolate is dominant for tfacH. Moreover, given the amphoteric nature of ZnO, it is proposed that on a surface of basic oxide, the O-H dissociation of the enol form could be driven to completion for hfacH, and this proposal is confirmed by comparing chemistry of hfacH on ZnO and MgO surfaces.

Metal nanoparticles supported on different oxidic supports are the most common materials in heterogeneous (photo-)catalysis. This work presents a systematic investigation of copper clusters deposited onto slightly and highly reduced rutile TiO₂(110) single crystals and silicon wafers with native oxide films. The focus is on the electronic properties of the copper clusters and possible metal-support interactions as these can change the catalytic behavior of the catalyst. Specifically, we examine coverage-dependent core-level binding energy shifts and kinetic energy Auger signal shifts of the Cu2p_3/2 and CuLMM signals in X-ray photoelectron spectroscopy as well as a Wagner plot analysis, Auger parameter analysis, and analyze the main support signals. The final-state-induced binding energy shifts dominant at lower coverages are related to the imperfect core-hole shielding of the positive charge remaining after photoemission. At higher copper coverages the more metallic character of the clusters, apparent from dominating initial-state effects, is more prominent. The shift in binding energy, kinetic energy, and Auger parameter are larger for copper on silica than for copper on reduced titania. The formation of Ti³⁺ or Si³⁺ indicates a charge transfer from the metal clusters to the support. For the first nominal monolayer of copper on titania a constant number of Ti³⁺ interstitials of 6% to 8% were observed regardless of the initial reduction degree of the titania. At the highest copper coverage, the local Ti³⁺ density at the (sub)surface increases to 11.0% and 11.7%. For the SiO_x surface the same could be observed as the Si³⁺/Si⁴⁺ ratio increased from 4% at the lowest copper coverage to 73% at the highest. For the inert SiO_x surface, we suggest an interaction of the copper with defects in the amorphous thin film.

The electrolysis of a water for hydrogen production is a promising way to produce clean energy, but the sluggish oxygen evolution reaction (OER) limits the overall efficiency of water electrolysis. In this work, we investigated the water oxidation pathways on the perfect and defect Co₃O₄(111) surfaces by using density functional theory (DFT) calculations. We found that for the perfect surface the free energy barrier of the potential determining step (PDS) in the adsorbate evolution mechanism (AEM) of water is lower than that in the lattice oxygen mechanism (LOM). For the defect surfaces, cobalt vacancies are more easily formed than oxygen vacancies. The Co vacancy promotes the formation of *OH, changes the PDS of the LOM and AEM, and reduces the free energy barrier of both PDS. The PDS of the LOM pathway on the V_Co2Co₃O₄(111) surface is the coupling step of the O adatom and lattice oxygen, which promotes the LOM process. Different from the OER mechanism on the perfect surface and the defect surface with Co vacancy, the LOM is perferred to occur on the defect surface with O vacancy. This work may provide new insight into the relationship between the surface structure and OER activity surface of the Co₃O₄ catalyst and help to design the efficient OER catalysts by surface and vacancy engineering.

This work intends to simulate the interaction of metal single-atom(s) supported on surfaces of H-magadiite (H₄Si₁₄O₃₀) and Al substituted H-[Al]-magadiites (H₅AlSi₁₃O₃₀), hereafter called M/H-magadiite and M/H-[Al]-magadiite (M = Ag, Au, Pt, Pd), using DFT calculations (PBE and PBE-D3 functionals). Three distinct positions were defined in all surfaces to optimize each simulated model: “hydroxyl”, “edge” and “cavity”. The Au/H-magadiite and Ag/H-magadiite models were more stable at the “hydroxyl” sites. Meanwhile, in the aluminated surfaces, the presence of an extra hydrogen atom (here called H_extra, located in the “edge” region) was responsible for a more stable situation of these metal atoms. On the other hand, the Pd and Pt single-atoms present in H-magadiite and H-[Al]-magadiites showed greater interaction with all the sites, compared to the Au- and Ag- models. Based on the binding energies and other electronic calculations, the aluminol site at H-[Al]-magadiites has the best capacity to support metal species. For example, the Pt/H-[Al]-magadiite showed the lowest binding energy (-2.64 eV for PBE and -2.93 eV for PBE-D3), the strongest charge interaction and the smallest Pt – H_extra distance (1.55 Å). The migration barriers (PBE) in Ag/H-magadiite, Au/H-magadiite, and Pd/H-magadiite were lower than 21.50 kJ·mol⁻¹, suggesting the high possibility of metal sintering. For all the cases, the PBE-D3 overestimated the barriers. Contrarily, the Pt/H-magadiite structures stabilized in the “cavity” region, inside the silicon rings of the silicate, and presented a migration barrier greater than 200 kJ·mol⁻¹. These calculations offered the first indications of the behavior of single-atoms, which will serve as the basis for a broader description, in future works, of the migration of metal species in the Al-models simulated here, as well as for modeling single-atom catalysts that can be used in stable conditions.

Texturing the surface of crystalline silicon wafers is a very important step in the production of high-efficiency solar cells. Alkaline texturing creates pyramids on the silicon surface, lowering surface reflectivity and improving light trapping in solar cells. This article provides a comparative evaluation of various wet texturing methods using alkaline solutions with or without additives commonly known as surfactants. One method uses sodium hydroxide (NaOH) and isopropyl alcohol (IPA) to create a surface with a height of about 4.5 μm by texturing for about 30 min, while the other method uses potassium hydroxide (KOH) and other additions known as additives. Texturing was performed using chemicals for only 15 min to create a surface shape with a height of approximately 3.5 μm. Additionally, the two solutions showed reflectance of 8.01 % or 12.1 % in 400–1100 nm, respectively. Both processes used alkaline etching at 80 °C for saw damage removal (SDR) before texturing. These processes have also been investigated in terms of removing potential organic contaminants from surfaces. Characterization techniques used throughout the investigation included optical microscopy, surface reflectance measurements, scanning electron microscopy (SEM), and electron dispersive spectroscopy (EDS). The purpose of this study is to confirm through experiments which texturing techniques are more suitable for mass production and to develop time- and cost-effective texturing techniques for industrial production of high-throughput, high-efficiency solar cells.

The great moment of fame for germanium was in December 1947. In that year the first transistor was made by a research team of Bell Laboratories. Owing to some problems with germanium, it was soon supplanted by silicon. Currently, germanium is still used in the microelectronic industry for opto-electronic and solar electric applications, but its role is very minor compared to its big brother silicon. After the rise of graphene, germanium received renewed interest because of the predicted stability of the graphene-like allotrope of germanium. Germanene, the germanium analogue of graphene, shares many properties with graphene, but there are also a few interesting differences that makes this material very appealing for device applications. In this contribution, I will give a brief historical overview of germanene, discuss the pros and cons of germanene and elaborate on its potential for future device applications.

The unsatisfactory reactive activity and structural stability in acidic oxygen evolution reaction (OER) have been the main bottleneck in exploiting hydrogen energy from water splitting. Herein, we suggest a halogen (H)-doping strategy in 1T phase iridium dioxide (IrO₂) monolayer to optimize its electronic structure for accelerating the reaction kinetics process, in which the bonding interaction difference between Ir-H and Ir-O bonds causes an electronic reconfiguration through asymmetric orbital hybridization. The doped F elements with a lower valence state make more valence electrons revert to the Ir-5d orbitals to reduce the activation energy, leading to a higher catalytic activity. In addition, a stronger bonding interaction at Ir-F bonds also can lead to a higher structural stability. However, this advantage cannot occur at Cl-doped or Br-doped IrO₂ monolayer. This work provides a new insight into designing new-type catalysts for acidic OER.

Selectivity of multi-pathway surface reactions depends on subtle differences in the activation barriers of competing reactive processes, which is difficult to control. One of the most promising strategies to overcome this problem is to introduce a specific selective interaction between the reactant and the catalytically active site, directing the chemical transformations towards the desired route. This interaction can be imposed via functionalization of a solid catalyst with organic ligands, promoting the desired pathway via steric constrain and/or electronic effects. The microscopic-level understanding of the underlying surface processes is an important prerequisite for rational design of such new class of ligand-functionalized catalytic materials. In this perspective, we present an overview over our recent mechanistic studies on heterogeneous Pd(111) catalysts functionalized with different types of organic ligands for chemoselective hydrogenation of a,b-unsaturated aldehyde acrolein. Employing a combination of real space microscopic (STM) and in operando spectroscopic (IRAS) surface sensitive techniques, we show that self-ordered active ligand layers are formed under operational conditions and identify their chemical nature and the geometric arrangement on the surface turning over. Deposition of a ligand layer renders Pd highly active and nearly 100 % selective toward propenol formation by promoting acrolein adsorption in a specific adsorption configuration via the O atom of the C = O bond. In this adsorption configuration, acrolein can be hydrogenated first to the desired reaction intermediate propenoxy species followed by formation of the target product propenol. Both the reaction intermediate and the final product propenol as well as their time evolution were identified by IRAS and gas phase analysis via quadrupole mass spectrometry (QMS). Particular focus of these studies was on the role of geometric and electronic effects imposed by specific functional groups purposefully introduced in the ligand layer. Obtained atomistic-level insights into the formation and dynamic evolution of the active ligand layer under operational conditions as well as into the role of geometric vs. electronic effects imposed by the ligand provide important input required for controlling chemoselectivity by purposeful surface functionalization.

The surface structure and atomic composition can affect the adsorption characteristics of water on metal surfaces. In this study, we investigated the adsorption of water on Cu(211) stepped surfaces decorated with Pt by a combination of infrared reflection absorption spectroscopy (IRRAS) and temperature-programmed desorption (TPD) studies. We have observed that step sites of Cu can increase the strength of the binding of water molecules to the surface and facilitate water partial dissociation and the formation of OH groups on the surface. Step decoration by Pt can change the water adsorption characteristics and eliminate the water dissociation. Water adsorbs molecularly on the fully Pt-decorated steps of the Cu(211) surface. Molecular water and OH adsorbed on Cu(211), which can make a chain structure, are disrupted with Pt atoms.

Traditional methods to study electrochemical (EC) processes, although successful, are based on current/voltage measurements, providing information about performances rather than offering a direct observation of chemical and structural changes occurring at the electrode surface. These processes are localized at the electrode-electrolyte interface, the structure of which is the main determinant of their behavior, but most surface sensitive experimental techniques are limited to ex situ conditions, owing to the need for an ultra-high vacuum environment. In this contribution, we report operando X-ray absorption spectroscopy in total external reflection geometry (Refle-XAFS) at P64 beamline (DESY, Hamburg), using a simple and versatile EC flow cell designed for multimodal surface sensitive studies with hard X-ray scattering and spectroscopy techniques. We show that the Refle-XAFS method can be used to study chemical surface changes of industrial alloys and model electrodes in harsh electrochemical environments, without being limited to thin film samples. The surface passive film development and breakdown of a corrosion-resistant Ni-Cr-Mo alloy and the electro-oxidation of polycrystalline gold (poly-Au), relevant for fundamental studies on water electrolysis, were investigated. Despite the strong attenuation of the beam by the electrolyte and the PEEK walls of the EC cell, nanoscale surface oxide films were detected using beam energies down to 8 keV. The passivity breakdown region of Ni alloy 59 in 1 M NaCl at pH 7 and pH 12 was identified, showing differences in the composition of the surface oxides during anodic polarization. The electro-oxidation of poly-Au in 0.05 M H₂SO₄ was observed, showing a progression from two-dimensional Au^1+/3+ to three-dimensional thick Au³⁺ surface oxide/hydroxide during OER.