Gold Nanowire Networks: Synthesis ... - American Chemical Society

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Gold Nanowire Networks: Synthesis, Characterization, and Catalytic Activity Mariana Chirea,*,† Andreia Freitas,† Bogdan S. Vasile,‡ Cristina Ghitulica,‡ Carlos M. Pereira,† and Fernando Silva† † ‡

CIQ-UP L4, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal Department of Science and Engineering of Oxide Materials and Nanomaterials, Faculty of Applied Chemistry and Materials, University Politehnica of Bucharest, Gh.Polizu street 1-7, 011061 Bucharest, Romania

bS Supporting Information ABSTRACT: Gold nanowire networks (AuNWNs) with average widths of 17.74 nm (AuNWN1) or 23.54 nm (AuNWN2) were synthesized by direct reduction of HAuCl4 with sodium borohydride powder in deep eutectic solvents, such as ethaline or reline, at 40 °C. Their width and length were dependent on the type of solvent and the NaBH4/ HAuCl4 molar ratio (32 in ethaline and 5.2 in reline). High resolution transmission electron microscopy (HR-TEM) analysis of the gold nanowire networks showed clear lattice fringes of polycrystalline nanopowder of d = 2.36, 2.04, 1.44, and 1.23 Å corresponding to the (111), (200), (220), or (311) crystallographic planes of face centered cubic gold. The purified AuNWNs were used as catalysts for the chemical reduction of p-nitroaniline to diaminophenylene with sodium borohydride in aqueous solution. The reaction was monitored in real time by UV-vis spectroscopy. The results show that the reduction process is six times faster in the presence of gold nanowire networks stabilized by urea from the reline (AuNWN2) than in the presence of gold nanowire networks stabilized by ethylene glycol from ethaline (AuNWN1). This is due to a higher number of corners and edges on the gold nanowires synthesized in reline than on those synthesized in ethaline as proven by X-ray diffraction (XRD) patterns recorded for both types of gold nanowire networks. Nevertheless, both types of nanomaterials determined short times of reaction and high conversion of p-nitroaniline to diaminophenylene. These gold nanomaterials represent a new addition to a new generation of catalysts: gold based catalysts.

1. INTRODUCTION Nowadays, the synthesis of nanomaterials in ionic liquids (ILs) or deep eutectic solvents (DESs) represents an intense research interest. Various shaped nanomaterials have been synthesized in ILs or DESs such as spherical nanoparticles, nanorods, nanostars, nanoplates, or nanosheets.1-5 The synthesis of nanomaterials in these type of solvents can be performed in the absence of a stabilizing ligand. Due to their high ionic charge, high polarity, high dielectric constant, and supramolecular structure,6-8 the ILs and DESs can stabilize metallic nanoparticles in situ. Recent studies show that the presence of a ligand on the nanomaterials surface can affect the catalytic properties of the nanomaterials.9,10 A major problem to overcome during nanoparticle synthesis in ILs or deep eutectic solvents is the presence of water as well as the presence of impurities which can affect the shape of the resulting nanomaterials, the monodispersity of the sample, and the time of reaction.3 Deep eutectic solvents were first reported by Abbott and co-workers.11-13 They have found that substituted quaternary ammonium salts mixed with hydrogen-bond donors such as amides or diols can form liquids at ambient temperature. DESs form extended r 2011 American Chemical Society

hydrogen-bond systems in the liquid state, and they are highly structured supramolecular solvents that can be used for shapecontrolled synthesis of nanoparticles.11-13 These solvents can be easily prepared at low cost and with high purity. Anisotropic nanomaterials have very interesting physicochemical and optoelectronic properties being widely used in catalysis, biosensing, or optics.14-17 Gold-based nanoparticles represent a new class of catalysts, starting with Haruta’s discovery that small supported gold nanoparticles can be effective catalysts for CO oxidation at low temperatures.18 Since then, reports about the size and shape effect on the catalytic properties of nanomaterials, in general, and gold nanomaterials, in particular, have been published.19-21 Kundu and co-workers have demonstrated that the chemical reduction of p-nitroaniline with sodium borohydride was fastest in the presence of smaller gold nanoparticles (6.7 nm average diameters), intermediate in the presence of medium sized Au NPs (13.8 nm average diameters), and slowest in the presence of larger AuNPs (22 nm, average diameters).19 These authors have Received: October 11, 2010 Revised: January 20, 2011 Published: February 24, 2011 3906

dx.doi.org/10.1021/la104092b | Langmuir 2011, 27, 3906–3913

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Scheme 1. Chemical Synthesis of Gold Nanowire Networks in Ethaline (A) and Reline (B)

also studied the shape-controlled catalysis of nitro compounds, comparing the catalytic properties of gold nanospheres, gold nanorods, and gold nanoprisms.20 Their results indicate that the reduction of nitro compounds was fastest in the presence of gold nanospheres, intermediate with nanoprisms, and slowest with the nanorods when the numbers of particles were fixed.20 Bai and co-workers have used gold nanorods (AuNRs) as catalysts for the chemical reduction of nitro compounds such as p-nitrophenol and p-nitroaniline, demonstrating that the short nanorods have better catalytic activity than the longer rods.21 Other authors have studied the catalytic activity of platinum-based nanomaterials.22-24 Qin and co-workers have studied the catalytic properties of spherical platinum nanoparticles (PtNPs) and spherical platinum nanoparticles cross-linked into nanowires toward the reduction of p-nitrophenol or potassium ferricyanide. They have demonstrated that PtNPs cross-linked into nanowires have a higher catalytic activity than the spherical PtNPs of similar sizes.22 Tian and co-workers have demonstrated that tetrahexahedral platinum nanocrystals exhibit high catalytic activity for electro-oxidation of formic acid and ethanol.24 Herein we report a novel method for the synthesis of gold nanowires organized into networks (AuNWNs). The solvents used for the nanomaterials synthesis were two different deep eutectic solvents: ethaline and reline. No surfactant was used for the stabilization of the AuNWNs obtained. The catalytic activity of the resulted nanowire networks was tested for the chemical reduction of p-nitroaniline (4-NA) with sodium borohydride in aqueous solution. It has been demonstrated that the shorter and wider gold nanowires have higher catalytic activity than their thinner and longer counterparts.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Choline chloride ((CH3)3N(Cl)CH2CH2OH,99%), urea (NH2CONH2, 99%), ethylene glycol anhydrous (HOCH2CH2OH, 99.8%), p-nitroaniline (O2NC6H4NH2,g99%), hydrogen tetrachloroaurate(III) trihydrate (HAuCl4 3 3H2O, 99.999%,), and sodium borohydride (NaBH4, 96%) were purchased from Sigma Aldrich and used without further purification unless otherwise specified.

2.2. Preparation of Ionic Liquids. The deep eutectic solvents used for the synthesis of gold nanowire networks were ethaline and reline. These deep eutectic solvents were prepared as follows: choline chloride (ChCl) was recrystallized from absolute ethanol, filtered and dried under vacuum. Ethylene glycol (EG) was used as received. Urea was dried under vacuum before use. The eutectic mixtures were formed by stirring choline chloride and ethylene glycol (ethaline, molar ratio (ChCl)/(EG) = 1:2) or choline chloride and urea (reline, molar ratio: (ChCl/U = 1:2) at a temperature of 80 °C until homogeneous and colorless liquids were formed. 2.3. Synthesis of Gold Nanowire Networks in Ethaline. The synthesis of gold nanowire networks was performed as follows: 7 mg of hydrogen tetrachloroaurate(III) trihydrate was dissolved in 10 mL of freshly prepared ethaline at 80 °C solution temperature in a 50 mL round-bottom flask. When the temperature of the solution mixture decreased to 40 °C, 21 mg of sodium borohydride powder was added directly into the HAuCl4/ethaline mixture under strong stirring. The color of the solutions changed immediately from yellow to dark blue (Scheme 1A). After 15 min time of reaction, the chemical reduction of HAuCl4 was complete and the stirring was stopped. The resulting nanomaterials were first precipitated with ethanol and then consecutively purified by washing with ethanol through repeated sonication and decantation of the solvent (five times), filtered, and redispersed in water. Their size, shape, and crystallization were analyzed by transmission electron microscopy (TEM), high resolution TEM (HR-TEM), and X-ray diffraction (XRD). Their optical properties were evaluated by UV-vis spectroscopy, whereas the presence of a ligand on their surface was verified by Fourier transform infrared (FT-IR) spectroscopy. 2.4. Synthesis of Gold Nanowire Networks in Reline. The synthesis of gold nanowire networks in reline was performed by reducing 6.7 mg of hydrogen tetrachloroaurate(III) trihydrate dissolved in 10 mL of reline with 3.5 mg of NaBH4 powder at a 40 °C solution temperature. The color of the solutions changed immediately from yellow to dark gray blue (Scheme 1B). Similarly, the resulting nanomaterials were purified and characterized by TEM, HR-TEM, XRD, UV-vis spectroscopy, and FT-IR spectroscopy.

2.5. Low Resolution Transmission Electron Microscopy Measurements. The TEM images were recorded with a transmission electron microscope Hitachi 8100 equipped with a Rontec Standard 3907

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Figure 1. TEM images (a, b), selected area diffraction pattern (c), and size distribution histogram (d) of gold nanowire networks (AuNWN1) synthesized in ethaline at 40 °C. Scale bars are 1000 nm (a) and 200 nm (b). EDS detector and digital images acquisition, operating at 200 kV and having a point resolution of 1.6 nm.

2.6. High Resolution Transmission Electron Microscopy Measurements. The high resolution images were recorded using a TecnaiTM G2 F30 S-TWIN transmission electron microscope (FEI, the Nederlands), equipped with a STEM/HAADF detector, EDS (energy dispersive X-ray analysis, and EFTEM-EELS (electron energy loss spectroscopy). The microscope operates at an acceleration voltage of 300 KV (Shottky field emitter) with a TEM point resolution of 0.2 nm and a TEM line resolution of 0.1 nm. The crystallographic planes were determined based on 02-1095 diffraction data file. 2.7. X-ray Diffraction Measurements. The XRD measurements were performed with a θ-2θ X-ray diffraction Siemens D5000 apparatus, at room temperature, using dry powder of gold nanowire networks (5 g). θ-2θ = 30-90U; Δθ = 0.01U, t = 18 s. 2.8. UV-Vis Spectroscopy Measurements. The optical properties of freshly prepared gold nanowire networks in DES were evaluated using a Hitachi U-3000 spectrophotometer and quartz cuvettes with 1 cm light path. Their catalytic properties were evaluated using the same spectrophotometer, and the experimental conditions are explained in section 2.10. 2.9. FT-IR Spectroscopy Measurements. The existence of urea or ethylene glycol on the gold nanowire surfaces was verified by FT-IR spectroscopy measurements using a Jasco FT-IR 4100 spectrophotometer and dry powders of purified gold nanowire, choline chloride, urea (5 mg), and 3 μL of ethylene glycol (Figure 5).

2.10. Chemical Reduction of p-Nitroaniline Using Gold Nanowire Networks as Catalysts. The gold nanowire networks synthesized in ethaline (AuNWN1) or reline (AuNWN2) were used as catalysts for the chemical reduction of p-nitroaniline. In a typical catalysis reaction, 300 μL of 10-3 M p-nitroaniline (pNA) stock solution was mixed with 2.4 mL of Millipore water and stirred for homogenization.

Consecutively, 0.02 mg of purified gold nanowires dispersed in water by sonication (20 μL aqueous solutions, concentration 1 mg/1 mL) was added to the pNA aqueous solution. Finally, 300 μL of 0.1 M ice-cold solution of NaBH4 was added to the reaction mixture. The reduction of p-nitroaniline was monitored with a Hitachi U-3000 spectrophotometer in the range 190-550 nm, using quartz cuvettes with 1 cm light path and freshly prepared solutions.

3. RESULTS AND DISCUSSION 3.1. Characterization of Gold Nanowire Networks. Figures 1 and 2 represent the transmission electron microscopy images, diffraction patterns, and size distribution histograms of gold nanowire networks synthesized in ethaline (AuNWN1) and reline (AuNWN2), respectively. The lengths and average widths of AuNWNs were dependent on the type of solvent and NaBH4/ HAuCl4 molar ratios. The gold nanowires synthesized in ethaline were thinner and longer (>1000 nm) with average widths of 17.74 ( 0.53 nm (Figure 1, NaBH4/HAuCl4 = 32) than the gold nanowires synthesized in reline with average widths of 23.54 ( 0.42 nm (Figure 2, NaBH4/HAuCl4 = 5.2) and shorter ramifications (∼200 nm). From the selected area diffraction patterns presented in Figures 1c and 2c, we can state that, for both types of AuNWNs, the only phase identified is the polycrystalline face centered cubic form of Au. HR-TEM images (Figure 3) show clear lattice fringes of polycrystalline nanopowder of d = 2.36, 2.04, 1.44, and 1.23 Å corresponding to the (111), (200), (220), and (311) crystallographic planes of face centered cubic gold. This is in agreement with the diffraction standard of Au (JCPDS 02-1095). Interestingly, XRD measurements revealed the presence of the same crystallographic planes for both types of gold 3908


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Figure 2. TEM images (a, b), selected area diffraction pattern (c), and size distribution histogram (d) of gold nanowire networks (AuNWN2) synthesized in reline at 40 °C. Scale bars are 500 nm (a) and 200 nm (b).

nanowires networks (Figure 4). The difference is that the AuNWN1 featured lower peaks in the XRD patterns for (311) lattice fringes than AuNWN2, meaning that the number of (311) lattice fringes is higher on the AuNWN2 surface than on the AuNWN1 surface. The intensity ratios (311)/(111) were 0.24 for AuNW1 and 0.50 for AuNW2. This explains the different catalytic behavior observed for the two types of gold nanowire networks, as it will be further presented. Also the regular succession of the atomic planes in the HR-TEM images indicates that the nanowires are structurally uniform and crystalline, with no amorphous phase present. The AuNWNs show optical properties with surface plasmon bands at 515 nm (AuNWN1, Figure 5a) or 518 nm (AuNWN2, Figure 5b). To the best of our knowledge, this is the first time synthesis of gold nanowire networks in deep eutectic solvents. As seen in the literature, Sun et al. have synthesized similar nanostructures, namely, spherical platinum nanoparticles cross-linked into nanowires but in aqueous solution using glucose as the stabilizing ligand and sodium borohydride as the reducing agent.22 Three NaBH4/HAuCl4 molar ratios for each DES were tested in order to optimize the synthesis of gold nanowire networks: 9.6, 22, and 32 (in ethaline, Supporting Information Figure S1) and 1.2, 3, and 5.2 (in reline, Supporting Information Figure S2). For NaBH4/HAuCl4 molar ratios of 9.6 (ethaline, Figure S1a) and 1.2 (reline, Supporting Information Figure S2a) large plates and spherical nanoparticles were obtained. At a NaBH4/HAuCl4 molar ratio of 22 (ethaline, Supporting Information Figure S1b) and 3 (reline, Supporting Information Figure S2b) tripods, tetrapods, and spherical nanoparticles were synthesized, whereas for the highest molar ratios of 32 (ethaline, Figure 1, Supporting Information Figure S1c) and 5.2 (reline,

Figure 2, Supporting Information Figure S2c) gold nanowires networks were obtained. The growth mechanism of the AuNWNs seems to be dependent on the type of deep eutectic solvent and the amount of sodium borohydride used to reduce the tetrachloroauric acid. The reline has a freezing point of 12 °C, which is considerably lower than that of either of the constituents (mp choline chloride =302 °C and urea = 133 °C), allowing its use as solvent at room temperature.12 This significant decrease of the freezing point arises from an interaction of urea molecules and the chloride ions of choline chloride described by eq 1:12 Catþ Cl- þ 2urea T Catþ þ Cl- 3 2urea

ð1Þ

Abbot et al. have used FAB-mass spectrometry (MS) measurements in order to determine the eutectic structure of reline. They have demonstrated both the coordination of Cl- with two urea molecules (M- = 155) and in a lower ratio Cl- with one urea (M- = 95).12 Additionally, they have confirmed the existence of hydrogen bonds established through an intense cross-correlation between the chloride ion and the NH2 protons on the urea molecule, using NMR spectroscopy. The formation of an adduct with the structure [HO-CH2-CH2Nþ(CH3)]Cl- 3 2(NH2)2CO was confirmed.12 In our work, the HAuCl4 dissolved in reline can form hydrogen bond networks in a similar manner. Upon dissolution in reline, the AuCl4- anion may replace the chloride anion stabilizing the choline chloride and may form hydrogen bonds with urea (Scheme 1b). This interaction is favored by the increased solution temperature and can generate an adduct with a similar structure: [HO-CH2-CH2-Nþ(CH3)3]AuCl4- 3 2(NH2)2CO. This is consistent with the work of Lecocq et al.25 who have used 13C and 35Cl NMR spectroscopy to show that at 110 °C 1BMMICl:1ZnCl2 varies its structure with time from 3909


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Figure 3. HR-TEM images of AuNWN1 (a-c) and AuNWN2 (d-f). Scale bars are 2 nm.

[BMMI]þ-[ZnCl3]- to [BMMI 3 3 3 Cl 3 3 3 ZnCl2]. Similarly, in our work, by increasing the solution temperature, the structure of the deep eutectic solvent can be changed. During the dissolution of HAuCl4 in reline, initially a precipitation of the HAuCl4 was observed. Upon sonication and stirring, the yellow precipitate disappeared and a clear darker yellow solution was obtained, proving a complete dissolution of the acid in reline and an adduct formation (Scheme 1b). FT-IR spectroscopy measurements show the presence of urea on the AuNWN2 surface (Figure 6b) which seems to confirm this hypothesis. During the fast reduction of AuCl4- by NaBH4 in reline, the urea molecules seem to stabilize the resulting gold nanowires network (AuNWN2) which can be due to the fact that AuCl4- is part of an adduct structure previously formed between the choline chloride, AuCl4-, and urea molecules. This is consistent with the crystallographic data for a solid adduct [(CH3)NþCH2CH2OH]2C2O4- 3 2(NH2)2CS which showed extensive hydrogen bonding established between the thiourea molecule and the oxalate anion.26

Analysis of the FT-IR spectrum recorded for urea powder shows multiple νN-H symmetric and asymmetric stretching vibration bands at 3423, 3333, 3006, and 2926 cm-1 due to the fact that the amide group can bond to produce dimers with a cis conformation or polymers with a trans conformation (Figure 6b, black curve). The stretching vibration band of the νCdO group was identified at 1669 cm-1 (between 1695 and 1650 cm-1), δN-H bending vibration band was identified at 1588 cm-1 (between 1650 and 1515 cm-1), whereas νC-N symmetric and asymmetric stretching vibrations were observed at 1455 and 1313 cm-1.27 The disappearance of three νN-H stretching bands in the FT-IR spectrum of AuNWN2 proves a coordination of the urea molecules to the gold nanowire surface through Clions, whereas the presence of νN-H stretching bands at 3006 and 2926 cm-1, diminished peaks for νCdO stretching vibration band at 1669 cm-1, δN-H bending vibration band at 1588 cm-1, and νC-N stretching vibration at 1455 cm-1 confirms the presence of urea as a stabilizing ligand on the AuNWN2 surface 3910


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Figure 5. UV-vis spectra of gold nanowire networks of 17.74 nm average widths, synthesized in ethaline (a) and gold nanowire networks of 23.54 nm average widths, synthesized in reline (b).

Figure 4. XRD diffraction patterns of gold nanowire networks synthesized in ethaline (AuNWN1, a) and reline (AuNWN2, b).

(Figure 6b, red curve). In both spectra, N-H wagging vibrations were identified at 998 cm-1. Additionally, these FT-IR spectroscopy measurements show no choline chloride on the AuNWN2 surface. In the case of ethaline, it is thought that comparatively weaker interactions take place between ethylene glycol and choline chloride, meaning that some free “glycol” is able to move, decreasing the viscosity of the solvent (36 mPa 3 s for ethaline as compared to 120 mPa 3 s for reline) and increasing its molar conductivity.13,28 This is because the ethylene glycol forms linear aggregates of hydrogen-bonded molecules with choline chloride (Scheme 1a). FAB-MS showed the presence of chloride coordinated to both one and two glycol molecules.13 The charge transport in ethaline will be also dependent upon the number of charged carriers, but the dominant mode of charge transport in ethaline is via the mobility of holes (voids in the solvent) as demonstrated by Abbot at al.13 In consequence, a weak bonded adduct such as [HO-CH2-CH2-Nþ(CH3)3]AuCl4- 3 2(HO-CH2-CH2-OH) could be formed but also tetrachloroauric acid can be simply dissolved in the ethaline without coordination. This explains the lighter yellow color of the mixture 7 mg HAuCl4/10 mL ethaline (Scheme 1a) as compared to the stronger and darker yellow color of the mixture 6.7 mg HAuCl4/ 10 mL reline (Scheme 1b), proving a stronger coordination of the AuCl4- in reline than in ethaline, for a similar amount of acid dissolved. As presented in the Experimental Section, the synthesis of gold nanowires in ethaline requires a higher amount of sodium borohydride (21 mg) yielding longer nanowire networks than in reline for which only 3.5 mg of reducing agent was enough to yield nanowires. This is consistent with a greater mass transport and better dissolution of HAuCl4 in ethaline than in

reline. FT-IR spectroscopy measurements (Figure 6a) revealed that the ethylene glycol exist in some extent on the gold nanowires surface. The FT-IR spectra of ethylene glycol and AuNWN1 presented in Figure 6a show some similarities. The ethylene glycol molecule generated the following absorption bands in the FTIR spectrum: O-H stretching vibration at 3293 cm-1, νC-H symmetric and asymmetric stretching vibration of the -CH2- groups at 2936 and 2874 cm-1. In the fingerprint region, νC-O symmetric and asymmetric stretching vibration at 1413 cm-1 (weak band between 1500 and 1400 cm-1), at 1082 and 1036 cm-1 (strong band between 1100 and 1000 cm-1), and rocking vibration of the -CH2- groups at 881 cm-1 were observed.27 Comparatively, the AuNWN1 showed no absorption band at 3293 cm-1, meaning that the ethylene glycol is bonded at the surface through the coordination of O-H bond. Moreover, the νC-H symmetric and asymmetric stretching vibration of the -CH2- groups at 2936 and 2874 cm-1, the νC-O stretching vibration at 1082 and 1036 cm-1, and the rocking vibration of the -CH2- groups at 881 cm-1 were also identified in the FT-IR spectrum of AuNWN1, but they are highly diminished (Figure 6a, red curve).27 New absorption bands at 1741 and 1539 cm-1 were observed. These absorption bands were observed also in the FT-IR spectrum of ethaline (results not shown) and can be due to the νC-O stretching vibration inside the adduct structure established between the ethylene glycol, Cl-, and choline chloride. This means that ethaline can also exist on the gold nanowire surface, in some extent. The AuNWNs were further used as catalyst for the chemical reduction of p-nitroaniline with sodium borohydride in aqueous solution. 3.2. Catalytic Activity of Gold Nanowire Networks. The chemical reduction of p-nitroaniline (4-NA) with sodium borohydride is extremely slow, and the use of a catalyst is necessary. The final product resulting from this chemical reaction is the p-phenylenediamine which is widely used in industry as an intermediate in the preparation of polymers, azo dyes, fur dyes, or rubber antioxidant.29 In consequence, efficient catalysts for the chemical reduction of 4-nitroaniline or other nitro compounds have potential industrial applications. We have monitored this chemical reaction by UV-vis spectroscopy for both situations: with and without catalysts (Figures 7 and 8; Supporting Information Figure S3). As represented in Supporting Information Figure S3, there was a very slow decrease of absorbance at 378 nm during the chemical reaction without catalyst. After the addition of AuNWN1 or AuNWN2 into the solution mixture and consecutive reduction of 3911


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Figure 7. (a) Successive UV-vis absorption spectra recorded for the chemical reduction of p-nitroaniline (0.001M) with 0.1 M NaBH4 catalyzed by 0.02 mg of AuNWN1 (average width 17.74 nm) and (b) plot of ln A versus time for the reduction reaction. Figure 6. (a) FT-IR spectra of ethylene glycol (black curve) and gold nanowire networks synthesized in ethaline (AuNWN1, red curve). (b) FT-IR spectra of urea (black curve) and gold nanowire networks synthesized in reline (AuNWN2, red curve).

4-NA with sodium borohydride, time-dependent absorption spectra of this reaction mixture showed a progressive decrease of the absorbance peak at 378 nm, a progressive increase and shifting of the absorbance peak at 223 nm, together with the appearance of a new absorbance peak at 303 nm (Figures 7a and 8a). These results indicate that the AuNWNs can successfully catalyze the chemical reduction of p-nitroaniline (yellow color) to the colorless p-phenylenediamine (4-PDA). In this reduction process, the concentration of NaBH4 exceeds the concentration of p-nitroaniline (see the Experimental Section) which means that the reaction rate follows first order kinetics. The logarithm of absorbance of p-nitroaniline at 378 nm (ln A) will decrease linearly with reaction time. From the linear regression of the slope of ln A versus time, it was estimated the apparent rate constant (k) of the reaction. Figure 7b shows the plot of ln A versus time for AuNWN1 used as catalyst (corresponds to Figure 7a), whereas Figure 8b shows the ln A plot versus time for AuNWN2 used as catalyst in the reduction process (corresponds to Figure 8a). The first-order rate constant k was calculated to be 3.0 10-2 min-1 for the sample in Figure 7a and 18.5 10-2 min-1 for the sample in Figure 8a. These values of k suggest that the gold nanowires synthesized in reline have a better catalytic activity than the gold nanowires synthesized in ethaline. AuNWN2 has determined the complete reduction of p-nitroaniline, whereas AuNWN1 determined 81.6% conversion of p-nitroaniline to p-phenylene diamine. Taking into account the fact that the amount of catalysts was the same, this difference of catalytic activity is due to several factors: the AuNWN2 have shorter ramification implying higher

Figure 8. (a) Successive UV-vis absorption spectra recorded for the chemical reduction of p-nitroaniline (0.001M) with 0.1 M NaBH4 catalyzed by 0.02 mg of AuNWN2 (average width 23.54 nm) and (b) plot of ln A versus time for the reduction reaction.

surface area than AuNWN1. Urea stabilizing the AuNWN2 is a shorter molecule than ethylene glycol stabilizing the AuNWN1, 3912


Langmuir and this implies that the surface of AuNWN2 is less covered than the surface of AuNWN1. It is well-known that the presence of a ligand on the nanomaterial surfaces can affect their catalytic properties.9,10 Additionally, high indexed facets can improve the catalytic properties of nanomaterials as observed by many authors.24 XRD measurements show higher peaks for (311) lattice fringes in the case of the gold nanowires synthesized in reline (Figure 4b) than in the case of gold nanowires synthesized in ethaline (Figure 4a). In consequence, there are a higher number of corners and edges atoms (low coordinated gold atoms) on the AuNWN2 surface than on the AuNWN1 surface. The intensity ratios of (311)/(111) crystallizations were 0.24 for AuNW1 and 0.5 for AuNW2 which is consistent with a higher number of indexed facets for AuNW2. The presence of these sites is a key factor for the catalytic activity observed (Figures 7 and 8). Moreover, it is widely assumed that the oxidation state of all gold atoms in the nanoparticles is 0. This is obviously an oversimplification, and it can be envisioned that even if most of the gold atoms in the clusters have “0” oxidation state, others, particularly at the surface, edges, or corners, may have a formal positive oxidation state, as it was observed by other authors through X-ray photoelectron spectroscopy (XPS) measurements30 and by interaction with probe molecules.31-34 In the case of our gold nanowire, the presence of urea or ethylene glycol as stabilizing ligand (Figure 5) implies the existence of Cl1- ions coordinated further to Au1þ on the nanowire surface. Based on the high reactivity of some molecular gold species in solution, it is reasonable to assume that if these positive gold atoms are present on the nanowires surface, they play a crucial role in the catalysis reaction.30-33 Further studies by XPS measurements are to be made in order to confirm this hypothesis.

4. CONCLUSIONS In summary, we have presented a simple and efficient method for the synthesis of gold nanowire networks in deep eutectic solvents. Furthermore, we have demonstrated that these gold nanowires of small widths, uniform and crystalline structures, with high index facets have a high catalytic activity for the chemical reduction of p-nitroaniline. These new nanomaterials are good candidates for future applications in catalysis. Further studies are currently being performed in our laboratory in order to clearly understand the electron transfer mechanism taking place at the gold nanowire network surfaces. ’ ASSOCIATED CONTENT

bS

Supporting Information. TEM images of nanomaterials synthesized for various NaBH4/HAuCl4 molar ratios and UVvis spectra of the chemical reduction of p-NA without catalysts. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail [email protected]. Telephone: þ351220402634. Fax: þ351220402659.

’ ACKNOWLEDGMENT Financial support from European Union and Fundac-~ao para a Ciência e a Tecnologia (FCT) of Portugal through Fellowship Number SFRH/BPD/39294/2007 is gratefully acknowledged.

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