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3.
Faraday Discuss ; 2022 Nov 09.
Artigo em Inglês | MEDLINE | ID: mdl-36349781

RESUMO

The Ag/Co nanoalloy system is a model system situated energetically at the limit of stability of the core-shell chemical ordering with respect to a simple phase separation behavior. This makes the system highly susceptible to effects of the environment, such as interaction with a substrate. However, kinetic effects may also be exploited by careful atom-by-atom particle growth that allows to lock in certain out-of-equilibrium configurations, such as off-center, quasi-Janus and even Janus type particles. In this contribution, we explore to what extent out-of-equilibrium structures are due to kinetic effects and the influence of the interaction of the particles with an amorphous carbon substrate by a joint experimental and molecular dynamics study. The simulation set up performed at 300 K and 600 K mimicks the experimental growth process. The substrate deforms the particles, but has also an ordering effect on particle orientation and particle structure. In the case of growth of Ag on Co seeds, particles assume close to equilibrium quasi-Janus structures, while for the deposition of Co on Ag seeds, highly out-of-equilibrium structures with several subsurface Co clusters are obtained.

4.
Faraday Discuss ; 2022 Oct 06.
Artigo em Inglês | MEDLINE | ID: mdl-36200810

RESUMO

In this study, the atomic and chemical structure and the optical response of AxB1-x bimetallic nanoparticles (BNPs) combining gold or silver (A) with aluminium or indium (B) were investigated at various stoichiometries in order to examine if stable alloyed phases could exist and promote the emergence of localized surface plasmon resonance (LSPR) in the UV range. The structure and morphology of BNPs of a few nanometres, produced by laser vaporization, were analysed by transmission electron microscopy (TEM) and optical absorption measurements were performed on matrix-embedded BNPs. Information about the oxidation state of the BNPs can be inferred from a comparison between experimental optical spectra and Mie calculations in the dipolar approximation. The BNPs' internal structures were further investigated by additional characterization techniques. Firstly, in situ X-ray photoelectron spectroscopy provided information about the chemical state of the constituent elements and their evolution with time. Secondly, synchrotron-based X-ray scattering techniques were performed on Ag-Al BNPs in a wide-angle configuration under grazing incidence, giving complementary information about structural and morphological heterogeneities in the BNPs. Finally, the restructuring of the partially oxidized Au0.33Al0.67 BNPs annealed in a reducing atmosphere was also attempted by environmental TEM. The complementary techniques of characterization show that silver-based Ag-In and Ag-Al BNPs form metallic silver-rich alloyed cores surrounded by an indium or aluminium oxide shell. The initial LSPR is in the UV range for both systems, but the difference in the kinetics of oxidation between indium and aluminium involves less blue-shifted LSPR for Ag-Al BNPs. In the case of gold-based BNPs, we show evidence of ordered nanoalloys just after air exposure and the appearance of gold and indium (or aluminium) demixing during oxidation. The initial LSPR of Au-In BNPs is the one the most in the UV range among the four systems, with an LSPR peak centred at 254 nm, which may be a sign of the formation of the Au0.33In0.67 alloy. Nevertheless, strategies to preserve BNPs from oxidation have to be developed.

5.
Faraday Discuss ; 2022 Sep 30.
Artigo em Inglês | MEDLINE | ID: mdl-36178069

RESUMO

The methanol oxidation reaction (MOR) is crucial in many energy-conversion devices. Although intensive efforts have been devoted to improving the MOR catalytic activity of Pt-based catalysts by treatment or alloying, enhancing the MOR catalyst performance utilizing solar energy has been less investigated. PtAg nanoalloys, combining the intrinsic catalytic activity of Pt toward the MOR with the visible spectrum plasmonic response of Ag, are expected to be a good MOR catalyst for solar energy, however, it remains challenging to incorporate these immiscible elements into a nanoalloy in a controlled way using conventional synthetic techniques. Herein, we proposed a general strategy for alloying silver and platinum elements into single-phase solid-solution nanoparticles with arbitrarily desired composition by bonding pure Pt targets with pure Ag strips in an improved pulsed laser deposition. The as-prepared PtAg nanoalloys show two crystalline phases and an average particle size of about 4 nm. To prove utility, we use the PtAg nanoalloys as support-free MOR catalysts anchored on the surface of a glassy carbon electrode solidly and uniformly. The PtAg nanoalloys exhibit a mass catalytic activity of 3.6 A mg-1, which is 4.5 times higher than that of the commercial Pt/C catalyst. Besides, the PtAg nanoalloys exhibit a promising regenerability after reactivation by cyclic voltammetry. Furthermore, the MOR catalytic activity of PtAg nanoalloys increased by 16% under irradiation by simulated sunlight, which is attributed to the surface plasmon resonance as ascertained from the UV-vis absorption spectra and photocurrent response experiments. These studies are believed to provide a new strategy for the enhancement of MOR catalytic activity with visible light as the driving force.

6.
Nanoscale ; 13(25): 11289-11297, 2021 Jul 07.
Artigo em Inglês | MEDLINE | ID: mdl-34156049

RESUMO

Understanding of cobalt nanorods' (Co NRs) formation still remains challenging when it comes to enhancing their anisotropic properties applicable in magnetic or catalytic areas. Herein, we propose a mechanism for the morphological transition from spherical cobalt nanoparticles (NPs) to Co NRs over time (9 h) in a mixture of [CoCl(PPh3)3] and oleylamine (OAm). In the literature, we described how spherical Co NPs are synthesized via a disproportionation process. Based on in situ and pseudo in situ observations, two steps of this unique mechanism are characterized first by the dissolution of the spheres and then the regrowth in rods' shape in the presence of an OAm template. Furthermore, ex situ experiments show that these steps are the result of interdependent reactions occurring between Co NPs, cobalt(ii) and OAm. The latter plays numerous roles in this synthesis: as a surfactant, a disproportionation promoter, and a hydrogen source allowing the reduction of cobalt(ii) complexes; its ammonium salt derivative is involved in oxidative etching of Co NPs and it promotes the anisotropic growth in NRs. These coupling actions of reduction and etching generate two cobalt reservoirs of nuclei under thermodynamic conditions.

7.
ACS Macro Lett ; 7(7): 789-794, 2018 Jul 17.
Artigo em Inglês | MEDLINE | ID: mdl-35650769

RESUMO

A binary blend of poly(isoprene-block-styrene-block-(2-vinylpyridine)) (ISP) triblock terpolymers, having the same chain length but different compositions, was used to achieve an ordered lattice with 4-fold symmetry of rectangular-shaped rods of poly(isoprene) (I) and poly(2-vinylpyridine) (P). In given conditions, the I and P domains were oriented perpendicularly to the substrate, providing an appealing type of templates for nanopatterning. Thin films were prepared by spin coating, exposed to solvent vapor (providing morphological reorganization), and then characterized by atomic force microscopy, transmission electron microscopy, and grazing-incidence small-angle X-ray scattering. Selective I and P identifications were carried out by AFM and TEM on a model ISP, as well as development of a technique of electronic contrast enhancement to better assign the self-assembly structure in GISAXS.

8.
ACS Appl Mater Interfaces ; 8(12): 8295-304, 2016 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-26950246

RESUMO

Solvothermal vapor annealing (STVA) was employed to induce microphase separation in a lamellar forming block copolymer (BCP) thin film containing a readily degradable block. Directed self-assembly of poly(styrene)-block-poly(d,l-lactide) (PS-b-PLA) BCP films using topographically patterned silicon nitride was demonstrated with alignment over macroscopic areas. Interestingly, we observed lamellar patterns aligned parallel as well as perpendicular (perpendicular microdomains to substrate in both cases) to the topography of the graphoepitaxial guiding patterns. PS-b-PLA BCP microphase separated with a high degree of order in an atmosphere of tetrahydrofuran (THF) at an elevated vapor pressure (at approximately 40-60 °C). Grazing incidence small-angle X-ray scattering (GISAXS) measurements of PS-b-PLA films reveal the through-film uniformity of perpendicular microdomains after STVA. Perpendicular lamellar orientation was observed on both hydrophilic and relatively hydrophobic surfaces with a domain spacing (L0) of ∼32.5 nm. The rapid removal of the PLA microdomains is demonstrated using a mild basic solution for the development of a well-defined PS mask template. GISAXS data reveal the through-film uniformity is retained following wet etching. The experimental results in this article demonstrate highly oriented PS-b-PLA microdomains after a short annealing period and facile PLA removal to form porous on-chip etch masks for nanolithography application.

9.
Faraday Discuss ; 181: 19-36, 2015.
Artigo em Inglês | MEDLINE | ID: mdl-25925553

RESUMO

Platinum nanocrystals and their derivatives with palladium and cobalt are of fundamental interest due to their wide field of application in chemistry and physics. Their properties are strongly dependent on their shape and composition. However the chemical route is far from allowing control of both shape and composition. In this paper, we show both experimentally and theoretically the important role of the interaction of small adsorbed molecules on the shape but also on the composition. This has been studied by comparing the case of pure palladium and platinum nanocrystals and the case of PtPd and PtCo nanoalloys synthesized by the liquid-liquid phase transfer method.


Assuntos
Ligas , Platina/química , Cinética , Microscopia Eletrônica de Transmissão , Nanopartículas
10.
ACS Appl Mater Interfaces ; 6(23): 20700-8, 2014 Dec 10.
Artigo em Inglês | MEDLINE | ID: mdl-25408428

RESUMO

This paper reports the preparation and properties of color-switchable fluorescent carbon nanodots (C-dots). C-dots that emit dark turquoise and green-yellow fluorescence under 365 nm UV illumination were obtained from the hydrothermal decomposition of citric acid. Dark green fluorescent C-dots were obtained by conjugating prepared C-dots to form C-dot@C-dot nanoparticles. After successful conjugation of the C-dots, the fluorescence emission undergoes a blue-shift of nearly 20 nm (∼0.15 eV) under UV excitation at 370 nm. The C-dots emit goldenrod, green-yellow, and gold light under excitation at 455 nm, which shows that the prepared C-dots are color-switchable. Furthermore, conjugation of the C-dots results in enhanced, red-shifted absorption of the π-π* transition of the aromatic sp(2) domains due to the conjugated π-electron system. N incorporation in the carbon structure leads to a degree of dipoles for all the aromatic sp(2) bonds. The enhanced absorption in a wide range from 226 to 601 nm indicates extended conjugation in the C-dot@C-dot structure. The time-resolved average lifetimes for the three different types of C-dots prepared in this study are 7.10, 7.65, and 4.07 ns. The radiative rate (reduced decay lifetime) increases when the C-dots are conjugated in the C-dot@C-dot nanoparticles, leading to the enhanced fluorescence emission. The fluorescence emission of the C-dot@C-dot nanoparticles can be used in applications such as flow cytometry and cell imaging.


Assuntos
Carbono/química , Fluorescência , Nanopartículas/química , Ácido Cítrico/química , Cor , Citometria de Fluxo , Luz
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