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gold nanohybrid material: Preparation, properties and application in catalysis
Accepted Manuscript Title: New Type of Organic/Gold Nanohybrid Material: Preparation, Properties and Application in Catalysis Author: Alexander G. Majouga Elena K. Beloglazkina Evgeniy A. Manzheliy Dmitriy A. Denisov Evgeniy G. Evtushenko Konstantin I. Maslakov Elena V. Golubina Nikolay V. Zyk PII: DOI: Reference:
S0169-4332(14)02575-6 http://dx.doi.org/doi:10.1016/j.apsusc.2014.11.094 APSUSC 29139
To appear in:
APSUSC
Received date: Revised date: Accepted date:
15-7-2014 16-11-2014 18-11-2014
Please cite this article as: A.G. Majouga, E.K. Beloglazkina, E.A. Manzheliy, D.A. Denisov, E.G. Evtushenko, K.I. Maslakov, E.V. Golubina, N.V. Zyk, New Type of Organic/Gold Nanohybrid Material: Preparation, Properties and Application in Catalysis, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.11.094 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Graphical abstract
NaBH4 H2O
NO2
3,5 300s 120s 0s
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2,5 2 1,5
Au-BTB
1 280
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Wavelength/ nm
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NH2
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Absorbance/ a.u.
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Au-BTB
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Highlights
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Stable nanocomposites of organic compound and gold nanoparticles are synthesized.
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Nanocomposites show good catalytic activity in 4-nitrophenol reduction by NaBH4.
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Organic matrix can be removed with deposition of nanoparticles onto mica surface.
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New Type of Organic/Gold Nanohybrid Material:
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Preparation, Properties and Application in Catalysis
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Alexander G. Majouga a,b, Elena K. Beloglazkina a,b *, Evgeniy A. Manzheliy a, Dmitriy A. Denisov a, Evgeniy G. Evtushenko a, Konstantin I. Maslakov a, Elena V. Golubina a, and Nikolay
a
M
V. Zyk a
Department of Chemistry, Moscow State University, Leninskie gory 1-3,119992, Moscow,
National University of Science and Technology "MISIS" (MISiS), Leninskiy prospekt 4,
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119049, Moscow, Russia
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b
d
Russia
* Corresponding author. Tel/Fax: +7 495 939 4652. E-mail address: [email protected]. KEYWORDS: gold nanoparticles; terpyridine; organic supported catalyst; 4-nitrophenol reduction.
ABSTRACT:
Microcrystals
of
1,4-bis(terpyridine-4'-yl)benzene
(BTB)
adsorb
gold
nanoparticles (AuNPs) with an average size of 15 nm or 25 nm from the solution forming an organic-gold composite with ~20 weight % of Au. The composite with 15 nm nanoparticles was characterized by thermal gravimetric analysis (TGA), transition electron microscopy (TEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Gold nanoparticles are uniformly distributed on the organic surface and retain the same size and shape distribution even after heating at 600oC which removes the organic matrix of 1,4-bis(terpyridine4'-yl)benzene and effectively deposits nanoparticles onto solid substrate (mica). The catalytic
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4 activity of composites was demonstrated using a model reaction of the reduction of 4-nitrophenol to 4-aminophenol. The values of the apparent rate constants at 295K, ca. 1,9×10-3 s-1 for the Au(15nm)-BTB and =7,2×10-4 s-1 for the Au(25 nm)-BTB, obtained using UV-vis spectroscopy, are comparable to those reported for other catalytic systems based on supported Au
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nanoparticles. The catalytic activity is found to be size-dependent; the bigger nanoparticles show lower activity and longer reaction induction period. Au-BTB catalyst can be recovered from the
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reaction mixture and used again.
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1. Introduction
Nanostructures based on gold particles owing to their unique chemical, optical, and electronic
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properties, distinct from the properties of bulk metals, find applications in different fields of chemistry, technology, and materials science leading to functional optical [1-3], electronic [4, 5], photonic [6,7] and memory [8] devices, as well as chemical and biological sensors [9-11] and
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catalysts [12-15]. A big drawback of metal colloid solutions is their instability leading to possible aggregation.
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Anchoring gold nanoparticles (AuNPs) on the different surfaces may affect and enhance their
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functional properties that can be exploited in different applications listed above. Attaching AuNPs to solid surfaces would improve their stability, retaining its useful properties;
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additionally, surface-bound NPs can be readily isolated from the synthesis medium or further derivatized by simple physical transfer of the supporting substrate. Control of the morphology, size and inter-particle separation in composites of AuNPs has recently been at the focus of intense research [16, 17] and their vast potential applications have already been demonstrated [18]. The common approach for functionalizing surfaces with NPs involves either first synthesizing colloidal particles in solution and subsequently binding them to the surface of interest or in-situ reduction of gold ions adsorbed onto a surface [19-21 and references therein]. The latter approach offers some advantages, including the possibility for generating smaller particles and stabilization of the NPs through surface attachment, but in comparison to the conventional approach of separately synthesizing NPs then binding them to a surface, in-situ synthesis of Au NPs at solid surface remains far less explored.
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5 Polymers are usually used as matrix materials for nanoparticles [13-15, 21-24]. However, in most cases, the authors of cited publications describe the formation of nanoparticles by the reduction tetrachloroaurate in the presence of the polymer, which results in a material, the . nanoparticles in which are mostly inside the cores, polymer networks, and brush layers but rarely
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on outside, whereas only on nanoparticles on the surface are able to participate in further chemical processes.. Thus, UV irradiation of polymeric polymethylmethacrylate films containing
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HAuCl4 followed by annealing at 60–80 oC forms gold nanoparticles directly within the bulk material [23, 24]. Because of the weak interaction between polymer supports and Au NPs, direct
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deposition of Au NPs onto polymers such as polystyrene from Au(III) complex compounds was considered to be difficult to achieve. Accordingly, it has been thought that well-designed polymer
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structures are necessary to obtain small Au NPs [see 22 and references herein]. Crystals of small organic molecules can offer greater flexibility and better control of the nanoparticle assembly process than polymers, and therefore their interactions with nanoparticles
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should be further explored, but matrices based on low-weight organic compounds remain largely unexplored [24]. It is known that interactions of nanoparticles with polydentate nitrogen-
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containing ligands most often result in rapid aggregation due to rapid bridging and bonding
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adjacent nanoparticles together [25]. However, we have recently shown [24] that a suspension of microcrystals of 1,4-bis(terpyridine-4'-yl)benzene (BTB) adsorb gold nanoparticles from the
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solution with the formation a 2D lattice of NPs on the surface of organic fiber-like crystals. In this work we report a simple two-step procedure for the synthesis of the 1,4-bis(terpyridine-4'yl)benzene - gold nanohybrid materials (Au-BTB), containing gold nanoparticles with an average size of 15 or 25 nm, and demonstrate their application in catalysis and deposition of gold nanoparticles on mica.
2. Materials and methods
General. All reagents and solvents were of analytical grade and were purchased from SigmaAldrich. 1,4-Bis(terpyridine-4'-yl)benzene was synthesized using a literature technique [26]. Distilled deionized water prepared in the Milli-Q-RO4 (Millipore) system was used for the preparation of all solutions. Centrifugation was performed in a LMC-3000 centrifuge (centrifugation velocity 3000 revolutions per second). Electronic UV-vis spectra were recorded
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6 on a Hitachi U-2900 instrument. Thermogravimetry analysis (TGA) was carried out on a differential scanning calorimeter DSC 204 F1 Phoenix. TEM microphotographs of samples were recorded on a LEO 912 AB OMEGA transmission
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electron microscope (Carl Zeiss, Germany) operated at the accelerating voltage of 100 kV. TEM samples were prepared by depositing a solution of material (1–2 μl) onto a copper mesh coated
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with formvar (d = 3.05 mm), which was then air dried.
X-ray photoelectron spectroscopy (XPS) took place in a Kratos Axis Ultra DLD, using a
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nonmonochromated AlKa X-ray source with neutralizer at 160 and 40 eV. The energy was calibrated by setting the major C1s HOPG peak to 284.8 eV.
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Study of the surface topography of mica samples by AFM was carried out in tapping mode on a N'tegra Prima atomic force microscope (NT-MDT, Russia). Several AFM images were
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obtained for different regions of each sample. The SPM mica (NT-MDT, Russia) was cleaved to obtain a flat surface. One drop (10 μl) of the composite suspension in 1% aqueous acetonitrile was deposited on the mica surface of the sample and dried for 2 hours in air. The annealing was
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carried out for 3 hours in air atmosphere at 6000C. Study of the surface topography of samples
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was carried out using NSG11 cantilevers with conical silicon nitride tips. The force constant of the beam was 5.0-6.0 N/m, resonance frequency, 120-180 kHz, tip curvature radius, <5 nm, and
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the cone angle, 22°. The scanning parameters were chosen to minimize the impact of the probe on the sample.
Synthesis of Gold Nanoparticles. The stabilized gold nanoparticles with the average particle size of 15 nm were prepared according to the method [27]; gold nanoparticles with the average particle size of 25 nm - according to the method [28].
Synthesis of 1,4-Bis(terpyridine-4'-yl)benzene/Gold Nanoparticle Composite (Au-BTB). To the 5 ml of distilled acetonitrile 27 mg of 1,4-bis (terpyridine-4'-yl) benzene was added, the resulting mixture was carefully suspended in an ultrasonic bath and added to 45 ml of deionized water. 63 ml of a AuNPs solution was added to the resulting solution and it was kept in the freezer at -18 oC for 30 minutes. The resulting suspension was centrifuged for 15 minutes at a rotation speed of 3000 revolutions per minute, and the solution of unabsorbed nanoparticles was
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7 decanted. The precipitate was suspended in a small amount of deionized water, centrifuged for 15 minutes and decanted. Resulting nanomaterial was then suspended in deionized water and was stored as suspension at room temperature. The Au-BTB composite precipitate can also be dried
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under low pressure at room temperature for 6 hours and kept as a powder. Reduction of 4-nitrophenol (4NP) by NaBH4 catalyzed by 1,4-Bis(terpyridine-4'yl)benzene/Gold Nanoparticle Composite. Aqueous solution of NaBH4 (0.1 ml, 0.3 M) and
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aqueous solution of 4NP (0.1 ml, 3 mM) were mixed with 2.8 mL of water in the quartz cell (1 cm path length), leading to a color change from light yellow to dark yellow-green. Then, Au-
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BTB composite (5.00, 0.50, 0.25 and 0.05 mg) was added to the mixture and it was quickly placed in the cell holder of the spectrophotometer. The progress of the conversion of 4NP to 4-
an
aminophenol (4AP) was then monitored by UV-vis spectroscopy recording the time-dependent adsorption spectra of the reaction mixture with time intervals of 2.5 or 10 min in a spectral range of 200-700 nm at ambient temperature. The rate constants of the reduction process were
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determined through measuring the change in absorbance at 400 nm as a function of time. The
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3. Results and discussion
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product was identified by 1H NMR (D2O) by comparison with an authentic sample.
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3.1. Formation and characterization of the Au-BTB composites. The procedure of Au-BTB nanocomposites preparation was developed in our group [24] and carried out in two steps. First step is the preparation of gold nanoparticles solution in deionized water accordingly to literature procedure [27, 28]. After preparation of gold nanopartcles they were mixed with the freshly prepared suspension of 1,4-bis(terpyridine-4'-yl)benzene in water/CH3CN mixture. Quickly forming light purple precipitate was then separated by centrifugation from non-adsorbed nanoparticles, washed thoroughly with water, and then it may be stored as an aqueous solution, and as a dried powder. The dried sample of Au-BTB composite obtained from Au-NP with average particle size of 15 nm (Au(15nm)-BTB) was characterized by TEM, XPS and TGA. Transition Electron Microscopy. Figure 1 shows the TEM image of Au(15nm)-BTB composite. Transmission electron microscopy investigation revealed that nanoparticles adsorbed
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8 onto organic crystalline fibers in a uniform way without any changes in nanoparticles sizes. Such type of composite material appears to be very useful for sequestering nanoparticles from suspension and storing them in their intact form for a long time. Stability of the adsorbed nanoparticles in this state was confirmed by TEM spectroscopy. Even after 1 year of storage of
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nanocomposite TEM showed no measurable changes in sizes or shapes of adsorbed
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nanoparticles.
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Figure 1. The procedure of formation and TEM images of Au-BTB nanocomposite. Microphotography for the material with 15 nm AuNP are shown. According to the TEM data, the size distribution diagrams for the initial 15 nm and 25 nm nanoparticles and the corresponding Au-BTB nanocomposites were constructed. Distribution diagrams are shown in Figure 2. Note that NPs size distribution for the nanomaterials remains essentially the same as for the initial nanoparticles.
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Figure 2. Size distribution of Au NPs: а) initial 15 nm nanoparticles, b) initial 25 nm nanoparticles, c) Au-BTB (15 nm) nanocomposite, d) Au-BTB (25 nm) nanocomposite. Proportion of nanoparticles sorbed onto 1,4-bis(terpyridine-4'-yl)benzene from the
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original NP solution, was calculated using the ratio of absorbance at 519 nm (plasmon band of
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the gold nanoparticles) for the starting NP solution and a solution of the nanoparticles remaining after centrifugation of the powdered nanocomposite. The obtained calculation results for the
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percentage of the adsorbed gold NP and Au concentration in the resulting suspension are given in Table 1. We have not performed specific investigations into the possibility to remove NP from carrier surface, however, both obtained nanomaterials are quite stable as suspensions in distilled water and after storage in suspension throughout the year any coloration of the solutions which might indicate that nanoparticles washing off from the material surface was not observed. Table 1. The percentage gold NP adsorbed from a solution of gold nanoparticles in the processes of Au(15nm)-BTB composites synthesis and gold concentration in the resulting Au(15nm)-BTB suspension. The NP size in the AuBTB, nm.
Adsorbed NP, %
Concentration of gold in Au-BTB suspension, mg/ml
15
74,9
0,387
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10
25
68,5
0,396
We used in our work 15 and 25 nm NP obtained by Turkevich method that allows to obtain stable citrate anions particle size from 15 nm to 150 nm [27]. Smaller nanoparticles can be
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prepared by an alternative technique [29] using tannic acid. We attempted to obtain nanomaterial with tannic acid-stabilized 5 nm nanoparticles, however, the percentage of sorbed from the
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solution nanoparticles was very low (1.5%), and the further research of this material wasn’t carried out. Poor adsorption of nanoparticles, presumably, is due to steric hindrance created by
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the bulky molecules of tannic acid for interaction of the surface gold atoms with 1,4-bis (terpyridine-4'-yl) benzene.
X-ray Photoelectron Spectra. The XPS spectra of the Au(15nm)-BTB show the presence of
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both the Au core and the organic molecules (See Supplementary materials, Fig. 1S). Figure 3 shows the high-resolution spectrum of the Au4f core level spectra. The Au 4f7/2 and 4f5/2 bands
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occur at 84.5 and 88.2 eV, respectively. Au4f7/2-Electron binding energy in this spectrum (84.5 eV) corresponds to the binding energy, which is observed for nanoparticles of gold in different media [30]. The binding energies of this line pair are slightly larger than the binding energy of
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bulk samples of metallic gold (84.0 eV and 87.3 eV).
Intensity, c/s
40000
30000
20000
10000
Ac ce p
50000
96
92
Au4f7/2
Au4f5/2
88
84
80
Binding energy, eV
Figure 3. Au 4f region of the XPS spectrum of Au(15nm)-BTB composite.
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11 Thermogravimetric Study of Au-BTB. To determine the amount of gold nanoparticles in the dry composite thermogravimetry was employed. For the thermogravimetric analysis the suspension of the Au(15nm)-BTB was centrifuged to form solid precipitate which was dried in vacuum. The onset of mass loss was found to be at 480 0C which showed an acceleration of mass
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loss at 600 0C (Figure 2S). The fraction of the sample remained after 750oC was 20 % which was assigned to the percentage of Au in the dry nanomaterial.
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3.2. Catalytic Reduction of 4-Nitrophenol.
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Catalytic activity of 1,4-bis(terpyridine-4'-yl)benzene/gold composite was examined in a wellknown catalytic reaction of reduction of 4-nitrophenol (4NP) to 4-aminophenol (4AP) by NaBH4.
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In the absence of catalysts the mixtures of 4NP and NaBH4 show an absorption band at λmax = 400 nm, corresponding to the 4NP anion in alkaline conditions. This peak remains unaltered with the time, indicating that the reduction did not take place in the absence of a catalyst [12, 13].
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However, the addition of a small amount of gold-containing material to the above reaction mixture caused bleaching of the yellow color of 4NP anion. Time-dependent absorption spectra
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of this reaction mixture confirm the disappearance of the band at 400 nm which is accompanied by gradual development of a new band at 300 nm corresponding to 4AP formed in this reaction.
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To investigate the effects of the amount of catalyst on the rate of the reaction we added different quantities of the composite material into the reaction mixture. Briefly, 5.00, 0.50, 0.25 and 0.05
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mg of the Au-BTB nanocomposite (containing 1.00 mg, 0.10 mg, 0.05 mg and 0.01 mg of gold respectively) were added into 3 ml of the reaction mixture. The progress of the reaction could be monitored by UV–vis spectroscopy (Figure 4 and 3S). The addition of 5 mg of nanocomposite leads to the completion of the reduction reaction in 15 minutes, which is in good agreement with the results obtained for the similar systems earlier [14]. The reduction of 4-nitrophenol by sodium borohydride is pseudo first order reaction [31]. Kinetic curves of the 4NP reduction in some cases consist of two regions: an induction period and a linear part corresponding to the first-order reaction. Various models have been proposed to explain the induction period [see 32 and references therein], the most rational explanation is provided the surface restructuring of the nanoparticles. Figure 5 shows the correlations between the absorbance at 400 nm and the reaction time as well as the correlation between ln(C/C0) and the reaction time at the linear part of kinetic curves
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12 for the Au(15nm)-BTB and Au(25 nm)-BTB. The kinetic curve is linear in logarithmic coordinates, and the reaction rate constants were found from the slope of this curves. The apparent rate constants (k) at 295K are found to be k =1.9×10-3 s-1 for the Au(15nm)-BTB and k =7.2×10-4 s-1 for the Au(25 nm)-BTB. This constants comparable to previous findings in the
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literature for the supported Au nanoparticles (5.1×10-1 – 7.4×10-3 s-1; see [22] and references therein). The reduction of 4-nitrophenol was catalyzed by Au-BTB more quickly and with less
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induction period in the case of a composite with a less size of the nanoparticles which is consistent with previously reported data [22, 33]. When the size of gold nanoparticles decreases,
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it means the increasing in the number of low-coordinated Au atoms which promote the adsorption of the reactants (4-nitrophenolate ions and BH4-) on the catalyst surface and facilitates the reduction. The larger particles consist of relatively high-coordinated Au atoms which are
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unfavorable for the adsorption of reactants and do not facilitate reduction.
In order to compare the catalytic properties of Au-BTB composite and parent nanoparticles,
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we also proceed the similar study to catalyze the 4NP reduction by the solution of non-supported nanoparticles with an average size of 15 nm. The reaction was performed under similar conditions, using the same gold and reagents concentrations. It has been found that the reaction
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induction period in this case is noticeably lower than in the case of Au-BTB utilization (see
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Supplementary materials, Figure 4S). The calculated rate constant in this case is found to be 1.1×10-3 s-1. We suppose that the higher reaction rate in the case of Au NP colloidal solution as
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compared to Au-BTB composite to be determined by surface area of nanoparticles, available for reagents coordination. However, it should be noted that the composites, unlike free nanoparticles, are resistant to agglomeration and may be stored as powders. Moreover, after completion of the reaction the catalyst may be filtered off, dried and re-introduced into reaction. The catalytic activity of recycled catalyst is almost not changed up to the fifth cycles (the completion reaction times for the reduction in the presence of 0.5 mg of Au(15 nm)-BTB composite are 1700 s at the first use of the catalyst, and 1730, 1730, 1760, 1760 s at the 2-nd, 3-rd, 4-th and 5-th runs respectively); leaching of Au NPs into the reaction solution did not occur. The reaction mechanism can be reasoned by the hydrogen adsorption/desorption on metal surface; thus, the Au nanoparticles shuttle the hydrogen transport between the NaBH4 and 4nitrophenol. Au nanoparticles adsorb hydrogen from the NaBH4 and efficiently release during the reduction reaction and hence Au acts as a hydrogen carrier in this reduction reaction [19, 22].
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13 Apparently it is crucial for catalytic activity, so that the particles were on the surface of the nanocomposite and were available to contact with dissolved reagents. 3,5 300s 120s 0s
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2,5 2 1,5 1 280
300
320
340
360
380
400
420
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Absorbance/ a.u.
3
440
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Wavelength/ nm
Figure 4. UV-vis absorption spectra for the reduction of 4NP by NaBH4 in the presence of 5
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mg of Au-BTB composite (gold concentration about 0.33 mg/ml).
Figure 5. Plot of optical density (A) versus time: (a) for Au(15nm)-BTB composite; (b) for Au(25nm)-BTB composite. Plot of ln(C/C0) versus time for the reduction of 4-NP: (c) for Au(15nm)-BTB composite; (d) for Au(25nm)-BTB composite. Reaction conditions: 295 K, 0.50 mg of the composite powder.
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14 Thus, the supported gold nanoparticles act as heterogeneous catalytic centers enabling to the complete conversion of 4-nitrophenol to the 4-aminophenol. Note, that the composite catalyst can be recovered from the reaction mixture, washed, dried and re-used in the reduction reactions.
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3.3. Nanoparticles transfer from 1,4-bis(terpyridine-4'-yl)benzene to mica. In order to evaluate whether AuNPs can be transferred from the organic crystal onto a solid
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inorganic substrate Au(15nm)-BTB composite was deposited on mica by drop-casting the suspension. The AFM imaging (Figure 6a) confirms the microfibers of 1,4-bis(terpyridine-4'-
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yl)benzene decorated with AuNPs. When the sample was heated at 600 oC (temperature at which 1,4-bis(terpyridine-4'-yl)benzene removed according to the TGA measurements; see below),
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AuNPs become transferred and adhered onto mica, but retain their size and shape (Figure 3b). Surprisingly, only about 7.5 % of the total number of nanoparticles becomes agglomerated after heating at 600 oC, as the rest of AuNPs were found to cover mica surface very uniformly.
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Whereas even addition citrate-stabilized nanoparticles on mica surface brings to agglomeration of nanoparticles [34]. We assume that particles don’t undergo sintering and Ostwald ripening
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b
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a
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under these conditions due to it stabilization in melt of terpyridine ligand.
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15 d 35
60 55 50 45 40 35 30 25 20 15 10 5 0
32,5 30 27,5 25 22,5
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20
%
17,5 15 12,5 10 7,5 5 2,5 0
13
14
15
16
18
22
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Height / nm
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%
c
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15
16
18
22
50
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Height / nm
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Figure 6. AFM images a) before and b) after heating the nanocomposite on mica at 600 oC. Histogram of nanoparticles size distribution c) before and d) after heating at 600 oC.
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4. Conclusions
We have developed a simple procedure for assembly of a new type of the nanocomposite
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where metallic nanoparticles are adsorbed on the surface of organic microcrystals. This
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nanocomposite material can be used for storage of gold nanoparticles for long periods of time. It exhibits catalytic properties in a reaction 4-nitrophenol reduction with NaBH4 and can be readily
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recycled. The organic microcrystals can also serve as vehicles for transfer of nanoparticles onto solid substrates, such as mica, without significant changes in the structure of nanoparticles. Acknowledgements
The authors gratefully acknowledge the financial support of the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST «MISiS»(No К1-2014-022) and financial support of Russian Foundation for Basic Research (Grants ## 12-03-33148, 12-04-00988, 13-03-00399). Supporting Information. Supplementary data relating to the synthesis and characterization of the organic/gold nanoparticle composite materials can be found in the online version.
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[25] A. G. Majouga, E. A. Manzheliy, E. K. Beloglazkina, N. V. Zyk, Nanotechnologies in
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[26] S. H. Toma, J. A. Bonacin, K. Araki, H. E. Toma, Eur. J. Inorg. Chem. 2007 (2007) 3356. [27] A. Winter, D. A. M. Egbe, U. S. Schubert, Org. Lett. 9 (2007) 2345. [28] A. N. Shipway, ChemPhysChem. 1 (2000) 18. [29] S. A. Aromal, D. Philip, Physica E 44 (2012) 1692. [30] W. Hongli, Colloids and Surfaces A: Physicochemical and Engineering Aspects 415 (2012) 174.
[31] M. P. Casaletto, A. Longo, A. Martorana, A. Prestianni, A. M. Venezia, Surface and Interface Analysis 38 (2006) 215. [32] S. Wunder, Y. Lu, M. Albrecht, .M. Ballauff, ACS Catalysis 1 (2011) 908.
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18 [33] S. A. Aromal, D. Philip, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 1.
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d
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an
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cr
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[34] C. George, D. Ricci, E. Zitti, Microstructures 44 (2008) 608.
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S0169-4332(14)02575-6 http://dx.doi.org/doi:10.1016/j.apsusc.2014.11.094 APSUSC 29139
To appear in:
APSUSC
Received date: Revised date: Accepted date:
15-7-2014 16-11-2014 18-11-2014
Please cite this article as: A.G. Majouga, E.K. Beloglazkina, E.A. Manzheliy, D.A. Denisov, E.G. Evtushenko, K.I. Maslakov, E.V. Golubina, N.V. Zyk, New Type of Organic/Gold Nanohybrid Material: Preparation, Properties and Application in Catalysis, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.11.094 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Graphical abstract
NaBH4 H2O
NO2
3,5 300s 120s 0s
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2,5 2 1,5
Au-BTB
1 280
300
320
340
360
380
400
420
440
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Wavelength/ nm
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BTB
NH2
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Absorbance/ a.u.
3
HO
Au-BTB
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HO
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2
Highlights
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Stable nanocomposites of organic compound and gold nanoparticles are synthesized.
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Nanocomposites show good catalytic activity in 4-nitrophenol reduction by NaBH4.
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Organic matrix can be removed with deposition of nanoparticles onto mica surface.
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3
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New Type of Organic/Gold Nanohybrid Material:
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Preparation, Properties and Application in Catalysis
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Alexander G. Majouga a,b, Elena K. Beloglazkina a,b *, Evgeniy A. Manzheliy a, Dmitriy A. Denisov a, Evgeniy G. Evtushenko a, Konstantin I. Maslakov a, Elena V. Golubina a, and Nikolay
a
M
V. Zyk a
Department of Chemistry, Moscow State University, Leninskie gory 1-3,119992, Moscow,
National University of Science and Technology "MISIS" (MISiS), Leninskiy prospekt 4,
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119049, Moscow, Russia
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b
d
Russia
* Corresponding author. Tel/Fax: +7 495 939 4652. E-mail address: [email protected]. KEYWORDS: gold nanoparticles; terpyridine; organic supported catalyst; 4-nitrophenol reduction.
ABSTRACT:
Microcrystals
of
1,4-bis(terpyridine-4'-yl)benzene
(BTB)
adsorb
gold
nanoparticles (AuNPs) with an average size of 15 nm or 25 nm from the solution forming an organic-gold composite with ~20 weight % of Au. The composite with 15 nm nanoparticles was characterized by thermal gravimetric analysis (TGA), transition electron microscopy (TEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Gold nanoparticles are uniformly distributed on the organic surface and retain the same size and shape distribution even after heating at 600oC which removes the organic matrix of 1,4-bis(terpyridine4'-yl)benzene and effectively deposits nanoparticles onto solid substrate (mica). The catalytic
Page 3 of 18
4 activity of composites was demonstrated using a model reaction of the reduction of 4-nitrophenol to 4-aminophenol. The values of the apparent rate constants at 295K, ca. 1,9×10-3 s-1 for the Au(15nm)-BTB and =7,2×10-4 s-1 for the Au(25 nm)-BTB, obtained using UV-vis spectroscopy, are comparable to those reported for other catalytic systems based on supported Au
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nanoparticles. The catalytic activity is found to be size-dependent; the bigger nanoparticles show lower activity and longer reaction induction period. Au-BTB catalyst can be recovered from the
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reaction mixture and used again.
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1. Introduction
Nanostructures based on gold particles owing to their unique chemical, optical, and electronic
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properties, distinct from the properties of bulk metals, find applications in different fields of chemistry, technology, and materials science leading to functional optical [1-3], electronic [4, 5], photonic [6,7] and memory [8] devices, as well as chemical and biological sensors [9-11] and
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catalysts [12-15]. A big drawback of metal colloid solutions is their instability leading to possible aggregation.
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Anchoring gold nanoparticles (AuNPs) on the different surfaces may affect and enhance their
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functional properties that can be exploited in different applications listed above. Attaching AuNPs to solid surfaces would improve their stability, retaining its useful properties;
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additionally, surface-bound NPs can be readily isolated from the synthesis medium or further derivatized by simple physical transfer of the supporting substrate. Control of the morphology, size and inter-particle separation in composites of AuNPs has recently been at the focus of intense research [16, 17] and their vast potential applications have already been demonstrated [18]. The common approach for functionalizing surfaces with NPs involves either first synthesizing colloidal particles in solution and subsequently binding them to the surface of interest or in-situ reduction of gold ions adsorbed onto a surface [19-21 and references therein]. The latter approach offers some advantages, including the possibility for generating smaller particles and stabilization of the NPs through surface attachment, but in comparison to the conventional approach of separately synthesizing NPs then binding them to a surface, in-situ synthesis of Au NPs at solid surface remains far less explored.
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5 Polymers are usually used as matrix materials for nanoparticles [13-15, 21-24]. However, in most cases, the authors of cited publications describe the formation of nanoparticles by the reduction tetrachloroaurate in the presence of the polymer, which results in a material, the . nanoparticles in which are mostly inside the cores, polymer networks, and brush layers but rarely
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on outside, whereas only on nanoparticles on the surface are able to participate in further chemical processes.. Thus, UV irradiation of polymeric polymethylmethacrylate films containing
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HAuCl4 followed by annealing at 60–80 oC forms gold nanoparticles directly within the bulk material [23, 24]. Because of the weak interaction between polymer supports and Au NPs, direct
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deposition of Au NPs onto polymers such as polystyrene from Au(III) complex compounds was considered to be difficult to achieve. Accordingly, it has been thought that well-designed polymer
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structures are necessary to obtain small Au NPs [see 22 and references herein]. Crystals of small organic molecules can offer greater flexibility and better control of the nanoparticle assembly process than polymers, and therefore their interactions with nanoparticles
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should be further explored, but matrices based on low-weight organic compounds remain largely unexplored [24]. It is known that interactions of nanoparticles with polydentate nitrogen-
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containing ligands most often result in rapid aggregation due to rapid bridging and bonding
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adjacent nanoparticles together [25]. However, we have recently shown [24] that a suspension of microcrystals of 1,4-bis(terpyridine-4'-yl)benzene (BTB) adsorb gold nanoparticles from the
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solution with the formation a 2D lattice of NPs on the surface of organic fiber-like crystals. In this work we report a simple two-step procedure for the synthesis of the 1,4-bis(terpyridine-4'yl)benzene - gold nanohybrid materials (Au-BTB), containing gold nanoparticles with an average size of 15 or 25 nm, and demonstrate their application in catalysis and deposition of gold nanoparticles on mica.
2. Materials and methods
General. All reagents and solvents were of analytical grade and were purchased from SigmaAldrich. 1,4-Bis(terpyridine-4'-yl)benzene was synthesized using a literature technique [26]. Distilled deionized water prepared in the Milli-Q-RO4 (Millipore) system was used for the preparation of all solutions. Centrifugation was performed in a LMC-3000 centrifuge (centrifugation velocity 3000 revolutions per second). Electronic UV-vis spectra were recorded
Page 5 of 18
6 on a Hitachi U-2900 instrument. Thermogravimetry analysis (TGA) was carried out on a differential scanning calorimeter DSC 204 F1 Phoenix. TEM microphotographs of samples were recorded on a LEO 912 AB OMEGA transmission
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electron microscope (Carl Zeiss, Germany) operated at the accelerating voltage of 100 kV. TEM samples were prepared by depositing a solution of material (1–2 μl) onto a copper mesh coated
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with formvar (d = 3.05 mm), which was then air dried.
X-ray photoelectron spectroscopy (XPS) took place in a Kratos Axis Ultra DLD, using a
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nonmonochromated AlKa X-ray source with neutralizer at 160 and 40 eV. The energy was calibrated by setting the major C1s HOPG peak to 284.8 eV.
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Study of the surface topography of mica samples by AFM was carried out in tapping mode on a N'tegra Prima atomic force microscope (NT-MDT, Russia). Several AFM images were
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obtained for different regions of each sample. The SPM mica (NT-MDT, Russia) was cleaved to obtain a flat surface. One drop (10 μl) of the composite suspension in 1% aqueous acetonitrile was deposited on the mica surface of the sample and dried for 2 hours in air. The annealing was
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carried out for 3 hours in air atmosphere at 6000C. Study of the surface topography of samples
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was carried out using NSG11 cantilevers with conical silicon nitride tips. The force constant of the beam was 5.0-6.0 N/m, resonance frequency, 120-180 kHz, tip curvature radius, <5 nm, and
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the cone angle, 22°. The scanning parameters were chosen to minimize the impact of the probe on the sample.
Synthesis of Gold Nanoparticles. The stabilized gold nanoparticles with the average particle size of 15 nm were prepared according to the method [27]; gold nanoparticles with the average particle size of 25 nm - according to the method [28].
Synthesis of 1,4-Bis(terpyridine-4'-yl)benzene/Gold Nanoparticle Composite (Au-BTB). To the 5 ml of distilled acetonitrile 27 mg of 1,4-bis (terpyridine-4'-yl) benzene was added, the resulting mixture was carefully suspended in an ultrasonic bath and added to 45 ml of deionized water. 63 ml of a AuNPs solution was added to the resulting solution and it was kept in the freezer at -18 oC for 30 minutes. The resulting suspension was centrifuged for 15 minutes at a rotation speed of 3000 revolutions per minute, and the solution of unabsorbed nanoparticles was
Page 6 of 18
7 decanted. The precipitate was suspended in a small amount of deionized water, centrifuged for 15 minutes and decanted. Resulting nanomaterial was then suspended in deionized water and was stored as suspension at room temperature. The Au-BTB composite precipitate can also be dried
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under low pressure at room temperature for 6 hours and kept as a powder. Reduction of 4-nitrophenol (4NP) by NaBH4 catalyzed by 1,4-Bis(terpyridine-4'yl)benzene/Gold Nanoparticle Composite. Aqueous solution of NaBH4 (0.1 ml, 0.3 M) and
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aqueous solution of 4NP (0.1 ml, 3 mM) were mixed with 2.8 mL of water in the quartz cell (1 cm path length), leading to a color change from light yellow to dark yellow-green. Then, Au-
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BTB composite (5.00, 0.50, 0.25 and 0.05 mg) was added to the mixture and it was quickly placed in the cell holder of the spectrophotometer. The progress of the conversion of 4NP to 4-
an
aminophenol (4AP) was then monitored by UV-vis spectroscopy recording the time-dependent adsorption spectra of the reaction mixture with time intervals of 2.5 or 10 min in a spectral range of 200-700 nm at ambient temperature. The rate constants of the reduction process were
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determined through measuring the change in absorbance at 400 nm as a function of time. The
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3. Results and discussion
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product was identified by 1H NMR (D2O) by comparison with an authentic sample.
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3.1. Formation and characterization of the Au-BTB composites. The procedure of Au-BTB nanocomposites preparation was developed in our group [24] and carried out in two steps. First step is the preparation of gold nanoparticles solution in deionized water accordingly to literature procedure [27, 28]. After preparation of gold nanopartcles they were mixed with the freshly prepared suspension of 1,4-bis(terpyridine-4'-yl)benzene in water/CH3CN mixture. Quickly forming light purple precipitate was then separated by centrifugation from non-adsorbed nanoparticles, washed thoroughly with water, and then it may be stored as an aqueous solution, and as a dried powder. The dried sample of Au-BTB composite obtained from Au-NP with average particle size of 15 nm (Au(15nm)-BTB) was characterized by TEM, XPS and TGA. Transition Electron Microscopy. Figure 1 shows the TEM image of Au(15nm)-BTB composite. Transmission electron microscopy investigation revealed that nanoparticles adsorbed
Page 7 of 18
8 onto organic crystalline fibers in a uniform way without any changes in nanoparticles sizes. Such type of composite material appears to be very useful for sequestering nanoparticles from suspension and storing them in their intact form for a long time. Stability of the adsorbed nanoparticles in this state was confirmed by TEM spectroscopy. Even after 1 year of storage of
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nanocomposite TEM showed no measurable changes in sizes or shapes of adsorbed
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nanoparticles.
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Figure 1. The procedure of formation and TEM images of Au-BTB nanocomposite. Microphotography for the material with 15 nm AuNP are shown. According to the TEM data, the size distribution diagrams for the initial 15 nm and 25 nm nanoparticles and the corresponding Au-BTB nanocomposites were constructed. Distribution diagrams are shown in Figure 2. Note that NPs size distribution for the nanomaterials remains essentially the same as for the initial nanoparticles.
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9
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Figure 2. Size distribution of Au NPs: а) initial 15 nm nanoparticles, b) initial 25 nm nanoparticles, c) Au-BTB (15 nm) nanocomposite, d) Au-BTB (25 nm) nanocomposite. Proportion of nanoparticles sorbed onto 1,4-bis(terpyridine-4'-yl)benzene from the
d
original NP solution, was calculated using the ratio of absorbance at 519 nm (plasmon band of
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the gold nanoparticles) for the starting NP solution and a solution of the nanoparticles remaining after centrifugation of the powdered nanocomposite. The obtained calculation results for the
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percentage of the adsorbed gold NP and Au concentration in the resulting suspension are given in Table 1. We have not performed specific investigations into the possibility to remove NP from carrier surface, however, both obtained nanomaterials are quite stable as suspensions in distilled water and after storage in suspension throughout the year any coloration of the solutions which might indicate that nanoparticles washing off from the material surface was not observed. Table 1. The percentage gold NP adsorbed from a solution of gold nanoparticles in the processes of Au(15nm)-BTB composites synthesis and gold concentration in the resulting Au(15nm)-BTB suspension. The NP size in the AuBTB, nm.
Adsorbed NP, %
Concentration of gold in Au-BTB suspension, mg/ml
15
74,9
0,387
Page 9 of 18
10
25
68,5
0,396
We used in our work 15 and 25 nm NP obtained by Turkevich method that allows to obtain stable citrate anions particle size from 15 nm to 150 nm [27]. Smaller nanoparticles can be
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prepared by an alternative technique [29] using tannic acid. We attempted to obtain nanomaterial with tannic acid-stabilized 5 nm nanoparticles, however, the percentage of sorbed from the
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solution nanoparticles was very low (1.5%), and the further research of this material wasn’t carried out. Poor adsorption of nanoparticles, presumably, is due to steric hindrance created by
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the bulky molecules of tannic acid for interaction of the surface gold atoms with 1,4-bis (terpyridine-4'-yl) benzene.
X-ray Photoelectron Spectra. The XPS spectra of the Au(15nm)-BTB show the presence of
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both the Au core and the organic molecules (See Supplementary materials, Fig. 1S). Figure 3 shows the high-resolution spectrum of the Au4f core level spectra. The Au 4f7/2 and 4f5/2 bands
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occur at 84.5 and 88.2 eV, respectively. Au4f7/2-Electron binding energy in this spectrum (84.5 eV) corresponds to the binding energy, which is observed for nanoparticles of gold in different media [30]. The binding energies of this line pair are slightly larger than the binding energy of
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bulk samples of metallic gold (84.0 eV and 87.3 eV).
Intensity, c/s
40000
30000
20000
10000
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50000
96
92
Au4f7/2
Au4f5/2
88
84
80
Binding energy, eV
Figure 3. Au 4f region of the XPS spectrum of Au(15nm)-BTB composite.
Page 10 of 18
11 Thermogravimetric Study of Au-BTB. To determine the amount of gold nanoparticles in the dry composite thermogravimetry was employed. For the thermogravimetric analysis the suspension of the Au(15nm)-BTB was centrifuged to form solid precipitate which was dried in vacuum. The onset of mass loss was found to be at 480 0C which showed an acceleration of mass
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loss at 600 0C (Figure 2S). The fraction of the sample remained after 750oC was 20 % which was assigned to the percentage of Au in the dry nanomaterial.
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3.2. Catalytic Reduction of 4-Nitrophenol.
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Catalytic activity of 1,4-bis(terpyridine-4'-yl)benzene/gold composite was examined in a wellknown catalytic reaction of reduction of 4-nitrophenol (4NP) to 4-aminophenol (4AP) by NaBH4.
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In the absence of catalysts the mixtures of 4NP and NaBH4 show an absorption band at λmax = 400 nm, corresponding to the 4NP anion in alkaline conditions. This peak remains unaltered with the time, indicating that the reduction did not take place in the absence of a catalyst [12, 13].
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However, the addition of a small amount of gold-containing material to the above reaction mixture caused bleaching of the yellow color of 4NP anion. Time-dependent absorption spectra
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of this reaction mixture confirm the disappearance of the band at 400 nm which is accompanied by gradual development of a new band at 300 nm corresponding to 4AP formed in this reaction.
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To investigate the effects of the amount of catalyst on the rate of the reaction we added different quantities of the composite material into the reaction mixture. Briefly, 5.00, 0.50, 0.25 and 0.05
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mg of the Au-BTB nanocomposite (containing 1.00 mg, 0.10 mg, 0.05 mg and 0.01 mg of gold respectively) were added into 3 ml of the reaction mixture. The progress of the reaction could be monitored by UV–vis spectroscopy (Figure 4 and 3S). The addition of 5 mg of nanocomposite leads to the completion of the reduction reaction in 15 minutes, which is in good agreement with the results obtained for the similar systems earlier [14]. The reduction of 4-nitrophenol by sodium borohydride is pseudo first order reaction [31]. Kinetic curves of the 4NP reduction in some cases consist of two regions: an induction period and a linear part corresponding to the first-order reaction. Various models have been proposed to explain the induction period [see 32 and references therein], the most rational explanation is provided the surface restructuring of the nanoparticles. Figure 5 shows the correlations between the absorbance at 400 nm and the reaction time as well as the correlation between ln(C/C0) and the reaction time at the linear part of kinetic curves
Page 11 of 18
12 for the Au(15nm)-BTB and Au(25 nm)-BTB. The kinetic curve is linear in logarithmic coordinates, and the reaction rate constants were found from the slope of this curves. The apparent rate constants (k) at 295K are found to be k =1.9×10-3 s-1 for the Au(15nm)-BTB and k =7.2×10-4 s-1 for the Au(25 nm)-BTB. This constants comparable to previous findings in the
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literature for the supported Au nanoparticles (5.1×10-1 – 7.4×10-3 s-1; see [22] and references therein). The reduction of 4-nitrophenol was catalyzed by Au-BTB more quickly and with less
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induction period in the case of a composite with a less size of the nanoparticles which is consistent with previously reported data [22, 33]. When the size of gold nanoparticles decreases,
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it means the increasing in the number of low-coordinated Au atoms which promote the adsorption of the reactants (4-nitrophenolate ions and BH4-) on the catalyst surface and facilitates the reduction. The larger particles consist of relatively high-coordinated Au atoms which are
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unfavorable for the adsorption of reactants and do not facilitate reduction.
In order to compare the catalytic properties of Au-BTB composite and parent nanoparticles,
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we also proceed the similar study to catalyze the 4NP reduction by the solution of non-supported nanoparticles with an average size of 15 nm. The reaction was performed under similar conditions, using the same gold and reagents concentrations. It has been found that the reaction
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induction period in this case is noticeably lower than in the case of Au-BTB utilization (see
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Supplementary materials, Figure 4S). The calculated rate constant in this case is found to be 1.1×10-3 s-1. We suppose that the higher reaction rate in the case of Au NP colloidal solution as
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compared to Au-BTB composite to be determined by surface area of nanoparticles, available for reagents coordination. However, it should be noted that the composites, unlike free nanoparticles, are resistant to agglomeration and may be stored as powders. Moreover, after completion of the reaction the catalyst may be filtered off, dried and re-introduced into reaction. The catalytic activity of recycled catalyst is almost not changed up to the fifth cycles (the completion reaction times for the reduction in the presence of 0.5 mg of Au(15 nm)-BTB composite are 1700 s at the first use of the catalyst, and 1730, 1730, 1760, 1760 s at the 2-nd, 3-rd, 4-th and 5-th runs respectively); leaching of Au NPs into the reaction solution did not occur. The reaction mechanism can be reasoned by the hydrogen adsorption/desorption on metal surface; thus, the Au nanoparticles shuttle the hydrogen transport between the NaBH4 and 4nitrophenol. Au nanoparticles adsorb hydrogen from the NaBH4 and efficiently release during the reduction reaction and hence Au acts as a hydrogen carrier in this reduction reaction [19, 22].
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13 Apparently it is crucial for catalytic activity, so that the particles were on the surface of the nanocomposite and were available to contact with dissolved reagents. 3,5 300s 120s 0s
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2,5 2 1,5 1 280
300
320
340
360
380
400
420
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Absorbance/ a.u.
3
440
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Wavelength/ nm
Figure 4. UV-vis absorption spectra for the reduction of 4NP by NaBH4 in the presence of 5
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mg of Au-BTB composite (gold concentration about 0.33 mg/ml).
Figure 5. Plot of optical density (A) versus time: (a) for Au(15nm)-BTB composite; (b) for Au(25nm)-BTB composite. Plot of ln(C/C0) versus time for the reduction of 4-NP: (c) for Au(15nm)-BTB composite; (d) for Au(25nm)-BTB composite. Reaction conditions: 295 K, 0.50 mg of the composite powder.
Page 13 of 18
14 Thus, the supported gold nanoparticles act as heterogeneous catalytic centers enabling to the complete conversion of 4-nitrophenol to the 4-aminophenol. Note, that the composite catalyst can be recovered from the reaction mixture, washed, dried and re-used in the reduction reactions.
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3.3. Nanoparticles transfer from 1,4-bis(terpyridine-4'-yl)benzene to mica. In order to evaluate whether AuNPs can be transferred from the organic crystal onto a solid
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inorganic substrate Au(15nm)-BTB composite was deposited on mica by drop-casting the suspension. The AFM imaging (Figure 6a) confirms the microfibers of 1,4-bis(terpyridine-4'-
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yl)benzene decorated with AuNPs. When the sample was heated at 600 oC (temperature at which 1,4-bis(terpyridine-4'-yl)benzene removed according to the TGA measurements; see below),
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AuNPs become transferred and adhered onto mica, but retain their size and shape (Figure 3b). Surprisingly, only about 7.5 % of the total number of nanoparticles becomes agglomerated after heating at 600 oC, as the rest of AuNPs were found to cover mica surface very uniformly.
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Whereas even addition citrate-stabilized nanoparticles on mica surface brings to agglomeration of nanoparticles [34]. We assume that particles don’t undergo sintering and Ostwald ripening
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b
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a
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under these conditions due to it stabilization in melt of terpyridine ligand.
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15 d 35
60 55 50 45 40 35 30 25 20 15 10 5 0
32,5 30 27,5 25 22,5
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20
%
17,5 15 12,5 10 7,5 5 2,5 0
13
14
15
16
18
22
13
Height / nm
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%
c
14
15
16
18
22
50
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Height / nm
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Figure 6. AFM images a) before and b) after heating the nanocomposite on mica at 600 oC. Histogram of nanoparticles size distribution c) before and d) after heating at 600 oC.
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4. Conclusions
We have developed a simple procedure for assembly of a new type of the nanocomposite
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where metallic nanoparticles are adsorbed on the surface of organic microcrystals. This
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nanocomposite material can be used for storage of gold nanoparticles for long periods of time. It exhibits catalytic properties in a reaction 4-nitrophenol reduction with NaBH4 and can be readily
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recycled. The organic microcrystals can also serve as vehicles for transfer of nanoparticles onto solid substrates, such as mica, without significant changes in the structure of nanoparticles. Acknowledgements
The authors gratefully acknowledge the financial support of the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST «MISiS»(No К1-2014-022) and financial support of Russian Foundation for Basic Research (Grants ## 12-03-33148, 12-04-00988, 13-03-00399). Supporting Information. Supplementary data relating to the synthesis and characterization of the organic/gold nanoparticle composite materials can be found in the online version.
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