Article pubs.acs.org/JPCC
Gold−Manganese Oxide Core−Shell Nanoparticles Produced by Pulsed Laser Ablation in Water Teresa Simao,† Daniel M. Chevrier,‡ Jurij Jakobi,§ Andreas Korinek,∥ Gregory Goupil,† Marcus Lau,§ Sébastien Garbarino,† Peng Zhang,‡ Stephan Barcikowski,§ Marc-André Fortin,*,⊥,#,∇ and Daniel Guay*,† Institut National de la Recherche Scientifique, Centre Énergie Matériaux Télécommunications, 1650 Lionel-Boulet Boulevard, Varennes, Quebec J3X 1S2, Canada ‡ Department of Chemistry, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada § Technical Chemistry I, University of Duisburg-Essen and Center for Nanointegration Duisburg-Essen CENIDE, Universitaetsstrasse 7, 45141 Essen, Germany ∥ Canadian Centre for Electron Microscopy, Brockhouse Institute for Materials Research, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4M1, Canada ⊥ Centre de Recherche du Centre Hospitalier Universitaire de Québec (CR-CHUQ), Axe Médecine Régénératrice, Quebec City, Quebec G1L 3L5, Canada # Centre de Recherche Sur Les Matériaux Avancés (CERMA), Université Laval, Quebec City, Quebec G1 V 0A6, Canada ∇ Département de Génie des Mines, de la Métallurgie et des Matériaux, Université Laval, Quebec City, Quebec G1 V 0A6, Canada †
S Supporting Information *
ABSTRACT: A single-step procedure for the preparation of Au−MnOx NPs was achieved through pulsed laser ablation of a gold−manganese metal target made of a pressed metal powder mixture. First, a 248 nm nanosecond laser at 66.7 J cm−2 was used to synthesize Au−MnOx NPs from a gold− manganese metal target immersed in an aqueous solution at pH 11 (NaOH). It is demonstrated that the Au−MnOx NPs are made of a small Au core (around 5 nm in diameter) surrounded by a very thin manganese oxide layer (0.3−1.3 nm) as characterized by TEM, HAADF HR-STEM, and EELS. The superficial MnOx layer has a local structure that bears a close resemblance to that of Mn2O3 and MnO2 as revealed by EXAFS and XANES measurements. Comparative studies were also performed with a 1064 nm nanosecond laser at 1.4 J cm−2. In that case, the resulting colloids are mainly made of a mixture of Au NPs and MnOx NPs, with few Au−MnOx NPs, thereby suggesting the impact of the laser wavelength and fluence on the synthesis process. The mechanisms responsible for the production of Au−MnOx core−shell NPs are discussed.
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used for low-temperature CO oxidation.14−16 In addition, Au NPs supported on MnO2 nanorods were used in the selective liquid-phase oxidation of benzyl alcohol, while Au NPs deposited on mesostructured MnO2 eliminated volatile organic compounds from the air.17,18 Furthermore, Au NPs deposited on a MnO2-modified carbon support were used as a catalyst for glycerol oxidation.19 For catalytic applications, the combination of Au with Mn oxides presents many beneficial aspects: MnO2 can (i) act as an active cocatalyst by means of oxygen spillover onto Au to enhance the oxidation of organic molecules,19 (ii) prevent Au agglomeration, leading to larger active gold surface reaction
INTRODUCTION
The synthesis of bimetallic nanoparticles is a promising strategy for combining and improving the electronic, magnetic, optical, and catalytic properties of a material, which can revolutionize multiple fields of applications ranging from catalysis and sensing to medical diagnostics and therapy.1,2 Combining gold nanoparticles (Au NPs) with other metals to form bimetallic compounds has been a subject of intense research, as these materials can possess unique properties that can be useful in several areas.3−12 Among this new class of bimetallic compounds, Au−MnOx NPs have emerged as a versatile option for numerous applications. For example, core− shell Au@MnO2 NPs have been used to enhance the Raman signal in alkaline solution.13 In catalysis, Au NPs supported on mesoporous Mn3O4 or α-Mn2O3 and Au NPs supported on hybrid manganese carbonate and oxide nanostructures were © 2016 American Chemical Society
Received: June 9, 2016 Revised: September 5, 2016 Published: September 8, 2016 22635
DOI: 10.1021/acs.jpcc.6b05838 J. Phys. Chem. C 2016, 120, 22635−22645
The Journal of Physical Chemistry C
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areas,19 (iii) prevent the formation of inert Au oxidized species as opposed to other oxide supports,14 and (iv) present corrosion-resistant properties as compared to those of carbon-based materials while maintaining high surface area values.18 Among the diverse Mn oxide phases, α-Mn2O3 displays better redox properties and structural stability15 while preserving the formation of activated oxygen species or reactive perimeter sites at Au/Mn oxide interfaces. Au−MnOx NPs also proved useful in the biomedical area. Au@MnO Janus NPs were used for fluorescence spectroscopy and biomarker targeting, while Au@MnO nanoflowers were applied as optical imaging and magnetic resonance imaging contrast agents.20,21 Manganese oxides (MnO and Mn3O4, in particular) are beneficial for biotechnology-related applications due to the magnetic properties provided by the unpaired electrons in Mn 3d orbitals.22,23 Despite some successful applications of Au−MnOx structures, the synthesis methods used to prepare them require several steps involving reducing agents and organic solvents as well as extensive purification procedures for the removal of chemical reactants and byproducts.13−16,20,21 These procedures can be time-consuming, generate chemical waste, and may leave the NP surface contaminated with various molecules, which can hinder the catalytic performance or become a source of adverse reactions when these structures are used in biomedical applications, as demonstrated for different types of materials, including Au, Pt, and Gd oxide NPs.24−28 Therefore, it would be highly desirable to devise a single-step synthesis procedure for Au−MnOx NPs that would not use reducing agents or stabilization chemicals. Such a process would significantly reduce the preparation time while minimizing waste and loss of the final product. The possibility to prepare these Au−MnOx NPs with their surfaces free of any chemical could provide an opportunity to minimize their in vivo toxicity in biomedical applications but also to enhance both the mechanical stability and electrical conductivity of metal oxides used in energy storage devices such as ultrafast supercapacitors.29 Pulsed laser ablation in liquids (PLAL) is an interesting technique for the synthesis of pure NPs30 as well as alloyed, composite, and rare earth metal doped NPs in aqueous solutions.7,9,31−35 In PLAL, a metal target that can easily be prepared through a consolidation of metallic powders is immersed in a liquid and ablated with a laser beam impinging on the target surface.7 The ablation is a consequence of laser light absorption by the target, which leads to the ejection of material, the formation of a plasma plume, and the expansion of a cavitation bubble. Nanoparticles formed during these processes are released into the liquid during the collapse of the cavitation bubble.36−38 In this study, PLAL was used to produce Au−MnOx NPs. These NPs were made of a small Au core surrounded by a very thin Mn oxide layer, as observed by transmission electron microscopy (TEM) and high-angle annular dark field highresolution scanning transmission microscopy (HAADF HRSTEM). The oxidation state of Mn atoms in the MnOx layer was determined by using X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge spectroscopy (XANES), and extended X-ray absorption fine structure spectroscopy (EXAFS). The surface of the Au−MnOx NPs was free of potentially poisoning ligands, and the particles were stable in an aqueous electrolyte at pH 11 for several months without the addition of any stabilizing ligand.
Article
MATERIALS AND METHODS
Preparation of Metal Targets. Au micropowder (0.432 g,
