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: Design, synthesis, catalytic application, and strategic redispersion of plasmonic silver nanoparticles in ionic liquid media
Abhinandan Banerjee1, Robin Theron1#, Robert W. J. Scott1*
1 Department of Chemistry, University of Saskatchewan, 110 Science Place,
Saskatoon, SK, S7N 5C9, Canada.
*Corresponding author
E-mail: robert.scott@usask.ca, voice: 306-966-2017
#Present address: Department of Chemistry, University of Victoria, Victoria, BC V8W 3V6, Canada.
ABSTRACT
Silver nanoparticles synthesized in tetraalkylphosphonium ionic liquids are found to be excellent catalysts for borohydride-induced reductive degeneration of Eosin-Y, a dye that has been classified as a Class 3 carcinogen by the International Agency for Research on Cancer. TEM images indicated that the size of the Ag nanoparticles was significantly influenced by heat-induced sintering. A strategy was devised to redisperse smaller Ag nanoparticles from their aggregated/sintered counterparts via a two-step protocol that involved oxidative etching of Ag nanoparticles, followed by a re-reduction step. This protocol led to a reduction in the sintered Ag nanoparticle size from 15.7 6.1 nm to 3.7 0.8 nm, which was consistent with the size of the as-synthesized nanoparticles. The as-synthesized and the redispersed Ag nanoparticles were found to catalyze the bleaching of Eosin-Y with comparable efficiencies; first order rate constants for Eosin Y reduction were ~8 times higher for smaller Ag nanoparticles compared to their sintered counterparts. An examination of the kinetics of Ag nanoparticle etching was performed via temperature-controlled UV-Visible spectroscopy. Changes in the oxidation state of Ag during this sequence of events were also followed by in situ X-ray Absorption Spectroscopy of Ag nanoparticles in the ionic liquid.
KEYWORDS: Ionic liquids, silver nanoparticles, catalysis, X-ray Absorption Near Edge Structure.
1. Introduction
Nanoparticles (NPs) dispersed in ionic liquids (ILs) or water are of interest as catalysts if they can be stabilized towards aggregation and particle sintering ADDIN EN.CITE ADDIN EN.CITE.DATA [ HYPERLINK \l "_ENREF_1" \o "Astruc, 2008 #1" 1-5]. One of the several intriguing properties of ionic liquids is their well-documented capability to act both as solvents and as stabilizers when it comes to stable NP dispersions ADDIN EN.CITE ADDIN EN.CITE.DATA [ HYPERLINK \l "_ENREF_6" \o "Banerjee, 2012 #2" 6-8]. In imidazolium ILs, this is often due to functionalities deliberately appended to the substituents on the imidazolium cations, but there have been other examples where stabilization stems directly from the nature of the anions ADDIN EN.CITE ADDIN EN.CITE.DATA [ HYPERLINK \l "_ENREF_9" \o "Chen, 2005 #15" 9-12]. Tetraalkylphosphonium halides (PR4X; X=Cl, Br) represent a class of ionic liquids with intrinsic NP stabilizing abilities ADDIN EN.CITE ADDIN EN.CITE.DATA [ HYPERLINK \l "_ENREF_6" \o "Banerjee, 2012 #2" 6, HYPERLINK \l "_ENREF_13" \o "Kalviri, 2011 #7" 13-15]. While these ILs remain less studied for their applications in quasi-homogeneous nanocatalysis than their imidazolium counterparts, recent research indicates that NPs stabilized by these ILs are similar to traditional metal-surfactant combinations that have been used for nanoparticle stabilization for many years, such as CTAB-stabilized metal nanoparticles, which rely on strong halide absorption to the NP surface, along with steric stabilization by the charge balancing cation ADDIN EN.CITE ADDIN EN.CITE.DATA [ HYPERLINK \l "_ENREF_16" \o "Pal, 1997 #18" 16-19]. The exact nature of the forces that stabilize small NPs in tetraalkylphosphonium halide ILs have remained unidentified so far, although it has been suggested by us that a surfactant-like double layer, aided by the high viscosity coefficients of these systems, might contribute towards preventing NP coalescence in PR4Cl ILs ADDIN EN.CITE ADDIN EN.CITE.DATA [ HYPERLINK \l "_ENREF_6" \o "Banerjee, 2012 #2" 6, HYPERLINK \l "_ENREF_20" \o "Reinaudi, 2013 #21" 20, HYPERLINK \l "_ENREF_21" \o "Fritz, 2002 #29" 21]. Others have suggested that this interaction originates in the well-ordered three-dimensional network structure of ionic liquids, which leads to the formation of well-defined hydrophobic and hydrophilic zones, with pockets where the NPs might find accommodation ADDIN EN.CITE Luska20128[14]8817Luska, Kylie L.Moores, AudreyRuthenium nanoparticle catalysts stabilized in phosphonium and imidazolium ionic liquids: dependence of catalyst stability and activity on the ionicity of the ionic liquidGreen ChemistryGreen ChemistryGreen Chem.1736-17421462012The Royal Society of Chemistry1463-9262http://dx.doi.org/10.1039/C2GC35241A[ HYPERLINK \l "_ENREF_14" \o "Luska, 2012 #8" 14]. Presumably, the catalytic behavior of the NPs formed within these ILs would be influenced by the nature of both the metal used and the ILs ADDIN EN.CITE Zhao200910[22]101017Zhao, YangCui, GuirongWang, JianjiFan, MaohongEffects of Ionic Liquids on the Characteristics of Synthesized Nano Fe(0) ParticlesInorganic ChemistryInorganic ChemistryInorg. Chem.10435-10441482120092009/11/02American Chemical Society0020-1669http://dx.doi.org/10.1021/ic901559y10.1021/ic901559y2012/08/28[ HYPERLINK \l "_ENREF_22" \o "Zhao, 2009 #10" 22]. It is essential, therefore, to examine NP/PR4Cl IL systems in greater detail, which would give us valuable information not only about the individual systems under scrutiny, but also on the chemical and catalytic behaviors of this class of composite materials as a whole.
While Au, Pt, Pd, Ru and Rh NPs in tetraalkylphosphonium ILs have been studied by several groups in the recent past from the point of view of their catalytic behavior, Ag NPs in PR4Cl ILs are yet to be explored as catalytic systems ADDIN EN.CITE ADDIN EN.CITE.DATA [ HYPERLINK \l "_ENREF_13" \o "Kalviri, 2011 #7" 13, HYPERLINK \l "_ENREF_14" \o "Luska, 2012 #8" 14, HYPERLINK \l "_ENREF_23" \o "Prechtl, 2013 #22" 23, HYPERLINK \l "_ENREF_24" \o "Maclennan, 2013 #23" 24]. This is surprising, since Ag is one of the cheaper noble metals; also, it also has various applications in areas as diverse as photography, anti-bacterial coatings and surface-enhanced Raman spectroscopy ADDIN EN.CITE ADDIN EN.CITE.DATA [ HYPERLINK \l "_ENREF_25" \o "Ahamed, 2010 #24" 25-27]. The presence of a well-characterized, intense surface plasmon resonance (SPR) band for silver nanoparticles in the visible region also makes them attractive candidates for morphological studies, since it is known that the size, shape and intensity of these bands depend upon the size and shape of the NPs being studied ADDIN EN.CITE ADDIN EN.CITE.DATA [ HYPERLINK \l "_ENREF_28" \o "Moores, 2006 #27" 28-30]. It has been recently been shown by us that Ag NPs can be synthesized in trihexyl(tetradecyl)phosphonium halides via a conventional borohydride reduction protocol. It has also been demonstrated that these NPs etch upon exposure to oxygen at high temperatures, presumably aided by the presence of a large excess of halide ions in the system ADDIN EN.CITE Banerjee201331[31]313117Banerjee, AbhinandanTheron, RobinScott, R W JRedispersion of transition metal nanoparticle catalysts in tetraalkylphosphonium ionic liquidsChemical CommunicationsChemical CommunicationsChem. Commun.3227-322949312013[ HYPERLINK \l "_ENREF_31" \o "Banerjee, 2013 #31" 31].
For testing the catalytic behaviour of these systems, we selected a well-studied reaction whose progress could be followed spectrophotometrically. The reaction chosen was the borohydride-induced degradation of Eosin-Y (EY), an organic dye used extensively in staining of histological tissue samples, in photoelectrochemical cells, and as a fluorescent pigment ADDIN EN.CITE ADDIN EN.CITE.DATA [ HYPERLINK \l "_ENREF_32" \o "Kim, 2003 #32" 32-34]. The addition of Ag NPs to this system leads to complete degradation of Eosin-Y. This is in accordance with previous reports, which indicate that coinage metal NPs serve as electron-transfer relays in borohydride-induced reductive degradation of dyes, with the nucleophilic borohydride ions transferring electron density onto the NP surface, which in turn injects those electrons into the dye molecule ADDIN EN.CITE ADDIN EN.CITE.DATA [ HYPERLINK \l "_ENREF_35" \o "Jiang, 2005 #38" 35-39]. Most studies carried out in this context focus on the degradation of EY in water phase. Since ILs as solvents show behaviors drastically different from those of water as well as conventional organic solvents, it is worthwhile to examine such electron-transfer processes in these novel media ADDIN EN.CITE Freemantle20095[40]556Freemantle, M.An Introduction to ionic liquids2009Cambridge, U.K.RSC[ HYPERLINK \l "_ENREF_40" \o "Freemantle, 2009 #5" 40].
In this paper, we show the synthesis of Ag NPs in tetraalkylphosphonium halide ILs via reduction of Ag(I) salts with lithium borohydride. The Ag NPs formed in tetraalkylphosphonium halide ILs remained stable for months. These Ag NPs were also able to catalyze the borohydride-induced degeneration of EY in non-aqueous media. It was found that over a large number of catalytic cycles and/or higher temperatures, Ag NPs sintered and aggregated in the IL media. We show that the resulting particles could then be etched via exposure to air or oxygen, which, followed by chemical reduction, can redisperse smaller Ag NPs. The kinetics of the oxidative etching of Ag NPs in P[6,6,6,14]Cl and P[6,6,6,14]Br were examined in this study by in situ UV-Vis spectroscopy and X-ray absorption near edge spectroscopy. The catalytic activity of the redispersed Ag NPs was determined to be similar to freshly synthesized NPs.
2. Experimental
2.1 Materials
Unless otherwise mentioned, all chemicals were used as received. Silver nitrate (>99.7%) was purchased from Fisher Scientific. Eosin-Y and 2(M) LiBH4 (in THF) were purchased from Sigma Aldrich. THF was purchased from EMD and used as received. The tri(hexyl)tetradecylphosphonium halide (P[6,6,6, 1 4 ] X ; X = C l o r B r ) I L s w e r e p r o v i d e d b y C y t e c I n d u s t r i e s L t d . , a n d w e r e d r i e d u n d e r v a c u u m a t 7 0 o C f o r 1 0 - 1 2 h o u r s w i t h s t i r r i n g b e f o r e u s e . 1 8 M &!c m M i l l i - Q w a t e r ( M i l l i p o r e , B e d f o r d , M A ) w a s u s e d t h r o u g h o u t .
2 . 2 P r o c e d u r e f o r s y n t h e s i s o f A g N P s i n I L u s ing lithium borohydride.
In a representative synthesis of 5.0 mM Ag NPs in P[6,6,6,14]Cl, 8.5 mg of AgNO3 (0.050 mmol) was added under nitrogen to a 10 mL sample of the IL at 80oC (all the ILs studied are liquids at this temperature) in a Schlenk flask, and vigorously stirred. The solution was cooled to 50oC, and a stoichiometric excess of LiBH4 reagent (1.5 mL, 2.0 M in THF) was injected drop-wise into it over a period of 2-3 minutes. A brisk effervescence followed, and the entire solution turned deep yellowish-brown, indicating nanoparticle formation. After the addition of LiBH4, volatile impurities were removed by vacuum-stripping the system at 80oC. The Ag NP-IL composites thus obtained was stored under nitrogen in capped vials wrapped with foil until use.
2.2 Procedure for EY degradation.
In a typical experiment, 0.2 mL of a 0.2 mM EY solution in THF was added to a 7 mL (5:2) mixture of IL:THF in a foil-wrapped vial under nitrogen. A stoichiometric excess of LiBH4 (0.1 mL, 2.0 M in THF) was then added to t h e s y s t e m , a n d s t i r r i n g w a s c o m m e n c e d . A f t e r t h e d e s i r e d t i m e - i n t e r v a l , 2 0 0 L o f 5 . 0 m M A g N P s i n P [ 6 , 6 , 6 , 1 4 ] C l w a s a d d e d t o t h e s y s t e m . U V - V i s s p e c t r a o f t h e b l a n k s a m p l e ( i . e . , w i t h o u t a d d e d A g N P s ) w a s a l s o r e c o r d e d . A q u a r t z c u v e t t e w a s t h e n f i l l e d with the aliquot, and recording of spectra was initiated. Between successive readings, the cuvette was taken out of the spectrophotometer, wrapped in tinfoil to minimize exposure to ambient light, and manually shaken to ensure homogeneity of analyte. It has been noted by others that effervescence owing to the presence of borohydride in the reaction mixture also promotes thorough mixing, even in absence of a magnetic stir-bar ADDIN EN.CITE Weng201236[41]363617Weng, GuojunMahmoud, Mahmoud A.El-Sayed, Mostafa A.Nanocatalysts Can Change the Number of Electrons Involved in OxidationReduction Reaction with the Nanocages Being the Most EfficientThe Journal of Physical Chemistry CThe Journal of Physical Chemistry CJ. Phys. Chem. C24171-241761164520122012/11/15American Chemical Society1932-7447http://dx.doi.org/10.1021/jp308869m10.1021/jp308869m2014/03/24[ HYPERLINK \l "_ENREF_41" \o "Weng, 2012 #36" 41]. The recording of spectra was continued at suitable intervals of time, until the pink color of the solution faded to straw-yellow.
2.2 Procedure for Ag NP sintering
To bring about heat-induced growth in particle size, nitrogen was bubbled through Ag NPs in P[6,6,6,14]Cl at 1350C for an hour, followed by overnight heating under a n i t r o g e n a t m o s p h e r e a t t h e s a m e t e m p e r a t u r e . I n a d d i t i o n , t h e A g N P s i z e w a s m o n i t o r e d a f t e r r e p e a t e d c a t a l y t i c c y c l e s ; f i v e p o r t i o n s o f 0 . 2 m L , 0 . 2 m M E Y s o l u t i o n s i n T H F w e r e a d d e d t o a s i n g l e 7 m L ( 5 : 2 ) m i x t u r e o f I L : T H F , c o n t a i n i n g 5 0 0 L o f 5 . 0 m M A g NP, in a foil-wrapped vial under nitrogen, with a gap of one hour between each successive addition. 0.1 mL portions of LiBH4 (2.0 M in THF) were also added to the system after each dye addition, and stirring was commenced. After five such cycles of EY degradation, a TEM sample was prepared from the reaction mixture, and Ag NP sizes were studied. We note that our reaction of choice is conducted at room temperature, rather than at elevated temperatures, where greater particle sintering might be expected after repeated reaction cycles as compared to a reaction that occurs under mild, ambient conditions.
2.3 Procedure for Ag NP oxidative etching and redispersion.
The following procedure was adopted to redisperse the agglomerated Ag NPs back to ca. their initial sizes: oxygen was flushed through the Ag NP/ P[6,6,6,14]Cl system at 650C until the characteristic yellow color of the NPs disappeared, followed by re-reduction of the redispersed precursor by drop-wise addition of 1.5mL LiBH4 solution in THF, followed by quenching of excess reductant and low-pressure removal of volatiles from the medium. The progress of the oxidative etching of Ag NPs in various tetraalkylphosphonium halide ILs was monitored spectrophotometrically by UV-Vis spectroscopy. The kinetic studies were conducted in a Cary 6000i spectrophotometer. Ag NP/ P[6,6,6,14]Cl samples were taken in quartz cuvettes and small Teflon-coated magnetic stir-bars were immersed in them; they were then placed in a constant-temperature bath with a magnetic stirring base inside the spectrophotometer. Oxygen from a compressed gas cylinder was passed directly into the contents of the cuvettes at regular intervals using a gas regulator connected to a system of hoses, syringes and needles.
2.5 Characterization Techniques.
Unless otherwise stated, all reactions were performed using standard Schlenk techniques, with nitrogen to provide an inert atmosphere, in oven-dried Schlenk glassware. A Varian Cary 50 Bio UV-Visible spectrophotometer with a scan range o f = 2 0 0 8 0 0 n m a n d q u a r t z c u v e t t e s w i t h o p t i c a l p a t h l e n g t h s o f 0 . 4 c m o r 1 c m w e r e u s e d f o r a m b i e n t t e m p e r a t u r e U V - V i s s p e c t r a a n d s p e c t r o p h o t o m e t r i c s t u d i e s o f E Y d e g r a d a t i o n . A C a r y 6 0 0 0 i s p e c t r o p h o t o m e t e r , e q u i p p e d w i t h a n a u t o - s a m p l e r , a c o n s t a n t t e mperature bath, and magnetic stirring capabilities was used for the etching studies. To avoid effects of oxygen depletion on etching of NPs, the contents of the cuvettes were flushed with oxygen between readings. TEM analyses of the NPs in ILs were conducted by using a Philips 410 microscope operating at 100 kV. The TEM samples were prepared by ultrasonication of ~5% solution of the NP/IL solution in CHCl3 followed by drop-wise addition onto a carbon-coated copper TEM grid (Electron Microscopy Sciences, Hatfield, PA). To determine particle diameters, a minimum of 100 particles from each sample from several TEM images were manually measured by using the ImageJ program.
Ag XANES spectroscopy was carried out on the SXRMB Beamline (06B1-1) at the Canadian Light Source (CLS). The beamline was equipped with InSb(111) and Si(111) crystals, and has an energy range of 1700 - 10000 eV with a resolution of 1 10-4 "E / E . X A N E S s p e c t r a w e r e o b t a i n e d i n f l u o r e s c e n c e m o d e . T h e s e t u p f o r l i q u i d X A N E S w o r k w a s s i m i l a r t o p r e v i o u s i n v e s t i g a t i o n s ; s o l u t i o n s a m p l e s w e r e p l a c e d i n S P E X C e r t i P r e p D i s p o s a b l e X R F X - C e l l s a m p l e c u p s f i t t e d w i t h p o l y p r o p y l e n e i n s e r t s a n d s e a l e d w i t h a n X - r a y t r a n s p a r e n t f i l m ( U l t r a l e n e f i l m , 4 m t h i c k , p u r c h a s e d f r o m F i s h e r S c i e n t i f i c , O t t a w a , O N ) A D D I N E N . C I T E A D D I N E N . C I T E . D A T A [ H Y P E R L I N K \ l " _ E N R E F _ 4 2 " \ o " M a c l e n n a n , 2 0 1 3 # 5 8 " 4 2 , H Y P E R L I N K \ l " _ E N R E F _ 4 3 " \ o " L i u , 2 0 1 3 # 5 9 " 4 3 ] . The sample solution cell was placed on the sample holder that faces the incident beam at 45 angle. The software package IFEFFIT was used for data processing.
3. Results and discussion
3.1 Characterization of Ag NPs.
It has been previously shown by us and others that tetraalkylphosphonium ILs are excellent solvents and stabilizers for NPs. NPs synthesized in these ILs via borohydride reduction tend to remain stable for months or even years depending upon storage conditions ADDIN EN.CITE Banerjee20122[6]2217Banerjee, AbhinandanTheron, RobinScott, Robert W. J.Highly Stable Noble-Metal Nanoparticles in Tetraalkylphosphonium Ionic Liquid s f o r i n s i t u C a t a l y s i s <