Solely σ-Atop Site Bonding of Phenyl Isocyanide on Au(111)? Comparison with on Cu(111)

The interfacial bonding and electronic structure of 4-methylphenyl isocyanide (CH3C6H4NC, mPNC) on Au(111) dosed at 160 K have been studied using temperature-programmed desorption (TPD) time-of-flight (TOF) mass spectrometry and X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), in comparison with that on Cu(111). The TPD profiles for Au(111) show a multilayer peak at 200 K and two comparable sub-monolayer peaks at 305 and 375 K, direct evidence of two different chemisorbed binding structures on Au. No evidence of dissociation was observed from TPD, XPS, and UPS. For Cu(111), a multilayer and a broad major sub-monolayer peaks were observed at 210 and 360 K, respectively. A weak minor peak was also observed at 290 K. On the basis of the N(1s) XPS binding energies, 400 eV for Au(111) and 398 eV for Cu(111), we conclude that the σ-bonded linear N⋮C form is dominant for Au(111), and the σ/π-bonded bent NC form is dominant for Cu(111). For 1.0 monolayer mPNC on Au(111) and on Cu(111), the work functions were measured to be 4.0 and 2.85 eV, respectively. For multilayer coverages, the work function for Au(111) is 0.8 eV lower while that for Cu(111) is 0.08 eV higher. For the 1.0 monolayer mPNC on Au(111), the HOMO level is located at 2.8 eV below the Fermi level. Upon annealing this to 325 K, the coverage is reduced, and the UPS peak at 3.4 eV becomes stronger while the peak at 2.8 eV becomes weaker, due to a change in binding geometry

Electrochemical Pd Nanodeposits on a Au Nanoisland Template Supported on Si(100): Formation of Pd−Au Alloy and Interfacial Electronic Structures

Palladium nanoparticles have uniformly been electrodeposited on a Au nanoisland template (NIT) supported on a Si(100) substrate, which exhibits Au-rich, Pd-rich, and/or polycrystalline mixed structures upon annealing to 700 °C. Glancing-incidence X-ray diffraction (GIXRD) and energy-dispersive X-ray (EDX) elemental analysis of the as-deposited sample both show metallic Pd, while depth-profiling X-ray photoelectron spectroscopy (XPS) further reveals the presence of Pd−Au (and PdxSi) at the interfaces of the Pd nanodeposits on the Au NIT. Upon the sample being annealed to 700 °C, both Pd 3d3/2 and Au 4f7/2 XPS peaks are found to shift to lower binding energies, which further confirms Pd−Au alloy formation. The convergence of respective GIXRD features of metallic Au and Pd toward intermediate peak positions supports the formation of alloy and their crystalline nature. Depth-profiling XPS analysis of the annealed sample further shows that the Pd nanoparticles are found to consist of an ultrathin shell of PdO2, and a PdO-rich (i.e., Pd-poor) inner-core, which is consistent with the observed GIXRD patterns of PdO and Pd−Au alloy but indiscernible PdO2. We compare the above results with the experimental results for electrodeposited Pd on a bare Si(100) substrate. Our study provides new insight into the formation of Pd−Au alloy composite on Si by electrochemistry. The easy control of the Pd, Au, and Pd−Au composition in the nanodeposits as illustrated in the present method offers new flexibility for developing hybrid nanocatalysts and other applications

Synergy of Low-Energy {101} and High-Energy {001} TiO₂ Crystal Facets for Enhanced Photocatalysis

Controlled crystal growth determines the shape, size, and exposed facets of a crystal, which usually has different surface physicochemical properties. Herein we report the size and facet control synthesis of anatase TiO₂ nanocrystals (NCs). The exposed facets are found to play a crucial role in the photocatalytic activity of TiO₂ NCs. This is due to the known preferential flow of photogenerated carriers to the specific facets. Although, in recent years, the main focus has been on increasing the surface area of high-energy exposed facets such as {001} and {100} to improve the photocatalytic activity, here we demonstrate that the presence of both the high-energy {001} oxidative and low-energy {101} reductive facets in an optimum ratio is necessary to reduce the charge recombination and thereby enhance photocatalytic activity of TiO₂ NCs

Effect of Etching on Electron–Hole Recombination in Sr-Doped NaTaO₃ Photocatalysts

Sodium tantalate (NaTaO₃) photocatalysts doped with Sr²⁺ produce core–shell-structured NaTaO₃–SrSr_1/3Ta_2/3O₃ solid solutions able to split water efficiently, when prepared via the solid-state method. In this study, the photocatalysts were chemically etched to examine the different roles of the core and shell with respect to the recombination of electrons and holes. Under excitation by Hg–Xe lamp irradiation, the steady-state population of electrons in the core–shell-structured photocatalyst with a bulk Sr concentration of 5 mol % increased by 130 times relative to that of the undoped photocatalyst. During etching for the first 10 min, the shell detached from the top of the core, and the electron population in the uncovered core further increased by 40%. This population enhancement indicates that electrons are excited in the core and recombined in the shell. Etching up to 480 min resulted in the reduction of the electron population. To interpret the population reduction in this stage of etching, a Sr concentration gradient that controls the electron population in the core is proposed

Engineered Electronic States of Transition Metal Doped TiO₂ Nanocrystals for Low Overpotential Oxygen Evolution Reaction

Electrochemical oxygen evolution reaction (OER) involves high overpotential at the oxygen evolving electrode and thereby suffers significant energy loss in the proton exchange membrane water electrolyzer. To reduce the OER overpotential, precious ruthenium and iridium oxides are most commonly used as anode electrocatalyst. Here we report marked reduction in overpotential for the OER using transition metal (TM) doped TiO₂ nanocrystals (NCs). This reduction in overpotential is attributed to d-orbitals splitting of the doped TMs in the TM-doped TiO₂ NCs and their interactions with the oxyradicals (intermediates of OER) facilitating the OER. The d-orbital spitting of TMs in TM-doped TiO₂ NCs is evident from the change in original pearl white color of undoped TiO₂ NCs and UV–vis absorption spectra

Bimetallic Au@M (M = Ag, Pd, Fe, and Cu) Nanoarchitectures Mediated by 1,4-Phenylene Diisocyanide Functionalization

Hybridization with gold has attracted a lot of attention in many application areas such as energy, nanomedicine, and catalysts. Here, we demonstrate electrochemical hybridization of two different metals by using bare and 1,4-phenylene diisocyanide (PDI) functionalized gold nanoislands (GNIs) supported on a Si substrate. As pristine GNIs are not tightly locked on the Si surface, bimetallic Au@M (M = Ag, Pd, Fe, and Cu) core–shell type nanostructures are produced by an electric-field-induced clustering of GNIs and metal deposition. On the other hand, upon functionalization of GNIs by PDI, 3D island growth on the functionalized GNI template is observed as PDI acts as a protector against the electric-field-induced clustering. Depth-profiling X-ray photoelectron spectroscopy reveals no discernible difference in the interfacial electronic structures of hybrid metals prepared by using pristine and PDI-functionalized GNI templates. This work demonstrates a new approach to produce a secured template and to manipulate growth of hybrid nanoparticles on this template supported on a Si substrate by using electrodeposition and organic functionalization

Green Synthesis of Anatase TiO₂ Nanocrystals with Diverse Shapes and their Exposed Facets-Dependent Photoredox Activity

The exposed facets of a crystal are known to be one of the key factors to its physical, chemical and electronic properties. Herein, we demonstrate the role of amines on the controlled synthesis of TiO₂ nanocrystals (NCs) with diverse shapes and different exposed facets. The chemical, physical and electronic properties of the as-synthesized TiO₂ NCs were evaluated and their photoredox activity was tested. It was found that the intrinsic photoredox activity of TiO₂ NCs can be enhanced by controlling the chemical environment of the surface, i.e.; through morphology evolution. In particular, the rod shape TiO₂ NCs with ∼25% of {101} and ∼75% of {100}/{010} exposed facets show 3.7 and 3.1 times higher photocatalytic activity than that of commercial Degussa P25 TiO₂ toward the degradation of methyl orange and methylene blue, respectively. The higher activity of the rod shape TiO₂ NCs is ascribed to the facetsphilic nature of the photogenerated carriers within the NCs. The photocatalytic activity of TiO₂ NCs are found to be in the order of {101}+{100}/{010} (nanorods) > {101}+{001}+{100}/{010} (nanocuboids and nanocapsules) > {101} (nanoellipsoids) > {001} (nanosheets) providing the direct evidence of exposed facets-depended photocatalytic activity

Synthesis and Photophysical Properties of an Eu(II)-Complex/PS Blend: Role of Ag Nanoparticles in Surface-Enhanced Luminescence

A novel Eu­(II) complex with 2-ethylhexyl hydrogen 2-ethylhexyl phosphonate (EHHEHP or PC88A) was synthesized and blended with polystyrene polymer (PS). Both an independent complex and the Eu­(II)/PS blend excited by near-UV light produced blue luminescence, arising from the 5d→ 4f transitions of Eu­(II). Time-dependent density functional theory (TD-DFT) calculations on electronic structures of the complex molecule indicated that the absorbing and emitting center was associated with the ²A­(d_<i>z</i>²) state under the C₂ crystal field. We also synthesized silver nanoparticles (Ag NPs) with an average particle size of 4.48 nm (σ = 0.91 nm) using EHHEHP as a stabilizer. The effects of Ag NPs as a colloidal suspension and an interfacial layer on the luminescence intensity of the blend were investigated as functions of the concentration of Ag NPs and the thickness of the Ag NP layer, respectively. The critical concentration of the colloidal Ag NPs and the critical thickness of the interfacial Ag NP layer were ∼355 ppm and ∼0.16 μm, respectively. Under critical conditions, the Ag NPs increased the luminescence intensity by 4.4 times as a colloidal suspension in CH₂Cl₂ and 2.2 times as an interfacial-layer state

ZnO-TiO₂ Core–Shell Nanowires: A Sustainable Photoanode for Enhanced Photoelectrochemical Water Splitting

We present the synthesis of a unique vertically aligned ZnO-TiO₂ core–shell nanowires (NWs) heterostructure on an Si-wafer using a chemical vapor deposition method. The structural study shows the well-developed ZnO-TiO₂ core–shell NWs heterostructure. This unique ZnO-TiO₂ core–shell NWs heterostructure displays a photocurrent density of 1.23 mA cm^–2, which is 2.41 times higher than pristine ZnO NWs. A cathodic shift in the flat band potential and a lower onset potential of a ZnO-TiO₂ core–shell NWs heterostructure over ZnO NWs indicates more favorable properties for photoelectrochemical water splitting with a photoconversion efficiencey of 0.53%. A higher photocurrent density/photoconversion efficiency is due to the effective addition of photogenerated electron–hole separation originating from the ZnO NWs core and the conformal covering of a amorphous TiO₂ passivation shell. Therefore, these results suggest that the vertically aligned one-dimentional ZnO-TiO₂ core–shell NWs heterostructure is a promising photoanode for solar energy conversion devices