Full Length ArticleCopper nanoparticles synthesis in hybrid mesoporous thin films: Controlling oxidation state and catalytic performance through pore chemistry
Introduction
Metallic nanoparticles (NPs) have interesting size-dependent optical and catalytic properties and a high surface to volume ratio that make them appealing for many different applications, such as sensing, catalysis, energy conversion and storage, biomedicine and environmental technology, to cite a few [1], [2]. Among the various metal NPs, Au and Ag NPs are the most frequently used for optical applications and Pt and Pd NPs are the most appealing for catalysis.
In recent years, copper NPs have also received increasing attention for their excellent performance in electronics, sensing and catalysis [3], [4]. Cu has multiple advantages: it is abundant, it can catalyze a great variety of reactions [5] and it is an inexpensive metal [6]. However, the susceptibility of Cu (0) to oxidation upon exposure to air represents a big challenge for the synthesis of metallic Cu NPs Therefore, literature on Cu NPs is rather limited compared with other metallic NPs [7].
Cu displays multiple oxidation states and usually forms two stable oxides: Cu2O and CuO [8]. In particular, Cu2O is an environmentally friendly p-type semiconductor with a band gap of 2 eV and high optical absorption coefficient, which makes it an excellent candidate for solar-energy-conversion applications [8]. This range of accessible oxidation states results in a reactivity involving the exchange of one or two electron. Because of this versatility, Cu oxides can promote a variety of reactions [1]. For example both Cu (I) and Cu (II) oxides have been reported to have catalytic activity for azide alkyne cycloaddition (CuAAC) reactions [9], [10], 4-nitrophenol (4-NIP) reduction [11] and tryazoles synthesis [12].
Both Cu and CuOx NPs synthesized in solution require stabilization in order to avoid nanoparticle coarsening and/or aggregation. The use of porous templates has become one of the most promising strategies for NPs support and stabilization [1], [13]. In particular, ordered mesoporous oxides are highly appealing supports due to their high specific area and, in the case of transition metal based oxides, for their good stability in reaction media. The use of mesoporous oxides as supports for metallic or Cu NPs can improve the selectivity, conversion and yield of Cu(CuOx) catalyzed reactions and, most important, the mesoporous oxide will facilitate the recovery of the catalysts [1], [13], [14]. Several syntheses of powdered mesoporous materials modified with Cu(CuOx) NPs have been reported in the literature [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. In most of those cases, Cu (II) is first adsorbed to the mesoporous surface through surface functionalization, and then a thermal treatment follows that results in the NPs formation. The Cu(CuOx) mesoporous composite powders have been mainly applied on catalysis of organic reactions. The use of mesoporous thin films (MTF) instead of powders as supports for Cu based catalysts will result in more efficient recovery and reuse than in the case of powdered supports and can also facilitate the integration of the catalyst in devices [29], [30], [31]. However, up to now only Ag, Au and Pt loaded MTF have been synthesized and used as catalysts and no examples can be found in the literature about Cu loaded MTF synthesis and/or applications.
In this work, we have explored the room temperature synthesis of Cu based NPs supported within SiO2 MTF. This silica based MTF present the typical properties of mesoporous oxides, including: high surface area, ordered porosity and versatility for organic functionalization during or after the materials’ synthesis [31], [32]. The pores in silica MTF can be easily modified including silanes during its synthesis. This allows good control of the chemistry of the pore and the functional groups present within. Profiting from this advantage pores were modified with either COOH or NH2 functional groups. Both COOH and NH2 can act as adsorption sites for Cu (II) [33]. However, it will be shown that the choice of one or the other group will finally determine the oxidation state of the resulting NPs. The MTF-Cu(CuOx) composite materials were tested as catalysts for the reduction of 4-nitrophenol in the presence of NaBH4. Catalytic activity depends on the Cu NPs oxidation state, which was in turn determined by the functional group included within the oxide used as support.
To resume, here we will show that it is possible to control the formation of metallic or metal oxide Cu NPs in mesoporous materials by varying the chemistry of the pore. This opens the route for the design of hybrid SiO2 MTF including in situ synthesized metal or metal oxide NPs with controlled chemistry and properties.
Section snippets
Materials
Tetraethyl orthosilicate (TEOS, 98%), vinyltrimethoxysilane (VTMS, 98%), mercaptoacetic acid (MAA, 97%), benzophenone (Ph2CO, 99%), Pluronic F127, hydrochloric acid (37%), methanol, CuSO4·5H2O, NaOH, aminopropyltriethoxysilane (APTES), sodium borohydride, ascorbic acid and 4-nitrophenol (4-NIP) were obtained from Merck. Methanol, pure grade ethanol and Milli-Q water were used as solvents. Methanol was dried over activated MS-3 Å before use.
Synthesis of carboxylic trialkoxysilane precursor by click reaction
2-((2-(trimethoxysilyl)ethyl)thio)acetic acid ((MeO)3
Synthesis and structural characterization
Copper NPs were synthesized inside SiO2 MTF functionalized with carboxylic acid (SiO2–COOH) or amine groups (SiO2–NH2) through Cu (II) adsorption followed by a chemical reduction at room temperature. MTFs were synthesized according to previously reported procedures [34], [35] that result in thin films of around 200 nm thickness, and 30% of porosity. In both cases, Pluronic F127 was used as template for the pore formation and, as a consequence, both kinds of films present a well-ordered
Conclusions
Cu NPs were grown inside accessible porous SiO2 MTF functionalized with COOH and NH2 groups at room temperature, using an adsorption-reduction procedure based on easily available reagents. The oxidation state of the Cu NPs depended both on the functional group present in the pores and the number of adsorption-reduction steps applied, as demonstrated by UV–visible spectra, XPS and EELS analysis. Metallic Cu (0) NPs were obtained for MTF displaying pores functionalized with COOH groups and
Acknowledgements
This work has been funded by CONICET (PIP 00044CO) and ANPCyT (PICT 2013-1303 and PICT 2015-0351). R.C.R. acknowledges CONICET for a postdoctoral scholarship. Dr. Alberto E. Regazzoni is gratefully acknowledged for his comments during manuscript elaboration.
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