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Synthesis of Au Nanoparticles with Benzoic Acid as Reductant and Surface Stabilizer Promoted Solely by UV Light Yasuhiro Shiraishi, Haruki Tanaka, Hirokatsu Sakamoto, Naoto Hayashi, Yusuke Kofuji, Satoshi Ichikawa, and Takayuki Hirai Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03192 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 8, 2017
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Synthesis of Au Nanoparticles with Benzoic Acid as Reductant and Surface Stabilizer Promoted Solely by UV Light Yasuhiro Shiraishi,*,†,‡ Haruki Tanaka,† Hirokatsu Sakamoto,† Naoto Hayashi,† Yusuke Kofuji,† Satoshi Ichikawa§ and Takayuki Hirai† †
Research Center for Solar Energy Chemistry, and Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan ‡
Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), Saitama 332-0012, Japan § Institute for NanoScience Design, Osaka University, Toyonaka 560-8531, Japan ABSTRACT: Photoreductive synthesis of colloidal gold nanoparticles (AuNPs) from Au 3+ is one important process for nanoprocessing. Several methods have been proposed; however, there is no report of a method capable of producing AuNPs with inexpensive reagents acting as both reductant and surface stabilizer, promoted solely under photoirradiation. We found that UV irradiation of water with Au3+ and benzoic acid successfully produces monodispersed AuNPs, where thermal reduction does not occur in the dark condition even at elevated temperatures. Photoexcitation of a benzoate–Au3+ complex reduces Au3+ while oxidizing benzoic acid. The benzoic acid molecules are adsorbed on the AuNPs and act as surface stabilizers. Change in light intensity and benzoic acid amount successfully creates AuNPs with controllable sizes. The obtained AuNPs can easily be redispersed in an organic solvent or loaded onto a solid support by simple treatments. KEYWORDS: Gold nanoparticle · Colloid · Preparation · Photoreduction · Photocatalysis
INTRODUCTION Gold nanoparticles (AuNPs) exhibit optical,1 electronic,2 and chemical properties,3 which are significantly different from those of bulk Au. Its application to cancer therapy and bioimaging has attracted much attention due to the localized surface plasmon resonance (LSPR) based light absorption and scattering properties.4,5 Recently, catalytic,6 photocatalytic,7–10 and electronic properties11 of AuNPs have also been attracted a great deal of attention based on the high surface energy and strong LSPR absorption in visible region. These properties depend on the size of AuNPs.12 Synthesis of size-controlled and monodispersed AuNPs are therefore necessary. In particular, creation of AuNPs in “targeted position” is crucial for memory devises, photonic materials, and surface-enhanced Raman scattering (SERS) because interparticle distance and location of AuNPs strongly affect the optical properties.13,14 A preparation method for size-controlled and monodispersed AuNPs on a targeted position by an external stimulus is therefore desired. Thermal reduction of Au3+ with reductants such as trisodium citrate1517 and citric acid18 is a popular approach for the synthesis of AuNPs. These reagents act as a reductant for Au3+ and a surface stabilizer for the formed AuNPs. Simple stirring of water containing HAuCl4 and the reagent under heating successfully creates AuNPs. This method is, however, unable to create AuNPs in a targeted position because local heating of the designated position is difficult. Photoreduction of Au3+ is one
potential approach because light is non-invasive and can be delivered instantaneously to a targeted position. Several photoreduction methods have been proposed; however, many of them need both reductants and surface stabilizers.19–25 A few systems are able to use single reagents acting as both reductant and stabilizer such as ionic liquids,26 Irgacure 2959 (I-2959),27 polymers,28–30 and dendrimers,31 but they are relatively expensive and difficult for further processing due to the use of polymeric stabilizers. Among the reported methods, use of trisodium citrate32 or citric acid33,34 is the simplest way because they are inexpensive, easy to handle, and act as both reductant and stabilizer. UV irradiation of water with HAuCl4 and the reagent successfully reduces Au3+ and produces monodispersed AuNPs. These reagents, however, have strong reducing capability and inevitably promote thermal reduction of Au 3+ even at room temperature in the dark condition. Creating a new photoreduction method, which produces AuNPs solely under photoirradiation with inexpensive reagents behaving as both reductant and surface stabilizer, is therefore necessary. Here we report that the answer may lie in the use of “benzoic acid,” which behaves as both reductant and surface stabilizer. UV irradiation of water with HAuCl4 and benzoic acid produces monodispersed AuNPs at room temperature, while the dark condition does not produce AuNPs even at elevated temperature. We also report that varying the light intensity and the benzoic acid amount successfully creates AuNPs with tunable sizes.
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Figure 1. Absorption spectra of water with HAuCl4·4H2O (0.2 mM) and benzoic acid (1.0 mM) during stirring (a) at 90 C in the dark and (b) at 25 C under 254 nm photoirradiation (intensity: 60 mW m–2). The red is the spectrum of a solution containing HAuCl4·4H2O, and the blue is the spectrum for the solution after the addition of benzoic acid (2 min). (c) DLS analysis of the solutions after different photoirradiation time. (d) TEM images of AuNPs after 200 min photoirradiation.
RESULTS AND DISCUSSION Effect of benzoic acid. Benzoic acid does not promote thermal reduction of Au3+. To clarify this, water (2 mL) containing HAuCl4·4H2O (0.2 mM) and benzoic acid (1.0 mM) was stirred at 90 C in the dark condition. Figure 1a shows the change in absorption spectra of the solution with time. Stirring the solution for 200 min does not create LSPR absorption of AuNPs at ca. 500–600 nm, indicating that thermal reduction does not occur. In contrast, water containing HAuCl4·4H2O and benzoic acid, when stirred at 25 C under irradiation of 254 nm light (intensity: 60 mW m2), produces AuNPs. As shown in Figure 1b, typical LSPR band appears at 538 nm. In that, longer wavelength absorption at >600 nm assigned to interparticle plasmon excitons of aggregated AuNPs35 scarcely appear, suggesting that subsequent aggregation of the formed AuNPs does not occur. The obtained AuNPs solution, when left at room temperature for 7 days, maintains its transparency without formation of precipitate. These findings suggest that benzoic acid behave as reductant and surface stabilizer, and selectively produces AuNPs under photoirradiation. Figure 1c shows the hydrodynamic diameter of the solution containing HAuCl4 and benzoic acid during UV irradiation, measured by dynamic light scattering (DLS) apparatus. The solutions show unimodal distribution of AuNPs, and 200 min irradiation creates AuNPs with average diameter ca. 55 nm. As shown in Figure 1d, transmission electron microscopy (TEM) observations of the AuNPs obtained after 200 min irradiation
confirm the formation of ca. 55 nm AuNPs. As shown in Figure S1 (Supporting Information), X-ray photoelectron spectroscopy (XPS) of the AuNPs shows Au 4f5/2 and 4f7/2 peaks at 84.2 and 87.9 eV, respectively, assigned to Au0.36 In that, Au3+ or Au+ component is scarcely observed at 8391 eV, indicating the AuNPs consist of Au0 species solely.37 Mechanism for AuNPs formation. The AuNPs formation mechanism in the photoprocess with benzoic acid can be explained by the nucleation/growth processes (Scheme 1). Coordination of benzoic acid with Au3+Cl4 produces a benzoateAu3+Cl3 complex (eq. 1). This is confirmed by absorption spectra. As shown by the red line in Figure 1b, water containing HAuCl4 shows a ligand-to-metal charge transfer (LMCT) band of Au3+Cl4 at 309 nm.38 As shown by the blue line, addition of benzoic acid rapidly leads to blue shift of this band to 273 nm (2 min) due to the benzoateAu3+Cl3 complex formation, as is the case for citrateAu3+Cl3 complex.15 UV irradiation of the benzoateAu3+Cl3 complex leads to decrease of this LMCT band, along with a formation of LSPR band of AuNPs. This indicates that, as shown by eq. 2 (Scheme 1), photoexcitation of the benzoate–Au3+Cl3– complex produces a reduced Au+Cl2– species, together with a formation of CO2, benzene radical, and Cl radical as the oxidation products of benzoic acid. The benzene and Cl radicals promote addition and/or coupling reactions, producing phenol, chlorobenzene, chlorophenols, and biphenyl (eqs. 3–6). Subsequent reaction of Au+Cl2– produces Au0 (eq. 7),39 and their coalescence creates
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Au0 nuclei. Further adsorption of Au0 onto the nuclei leads to a growth of AuNPs,34 where benzoate anions are adsorbed onto the surface and act as a surface stabilizer.40 Negative charge of the anions suppresses aggregation of AuNPs due to electrostatic repulsion.41,42 Based on the above reactions, as expressed by eq. 8, the reduction of Au3+Cl3 to Au0 need 1.5 equiv of benzoic acid, along with the formation of benzene derivatives and 1.5 equiv of CO2.
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Scheme 1. Proposed mechanism for photoreductive formation of AuNPs with benzoic acid under UV irradiation.
Figure 2 shows the time-dependent change in the amount of CO2 formed during UV irradiation of water (300 mL) containing HAuCl4·4H2O (60 mol, 0.2 mM) and benzoic acid (300 mol, 1.0 mM), determined by GC analysis. The profile for CO2 generation agrees well with that for the increase in LSPR absorbance. The amount of CO2 formed after 90 min is 87 mol, which is close to the theoretical amount of CO2 (90 mol) formed by complete reduction of Au3+Cl4 to Au0 (eq. 8, Scheme 1). The AuNPs formed were removed by centrifugation. The resultant was concentrated by evaporation and dissolved in CDCl3. As shown in Figure S2, 1H NMR chart of the solution shows chemical shifts at 6.57.7 ppm, assigned to phenol, chlorobenzene, chlorophenols, and biphenyl as confirmed by authentic samples, indicating that the benzene and Cl radicals indeed promote addition and/or coupling reactions (eqs. 3–6).
The photoexcitation of the benzoateAu3+Cl3 complex (eq. 2) is confirmed by photoirradiation of the solution with different pH, adjusted with HCl or NaOH (Figure S3). As summarized in Figure 3, increase in LSPR band of the solution at pH 2, 7, or 13 is much slower than that in pure water (pH 4). At pH 2, benzoic acid scarcely undergo deprotonation (pKa = 4.2), 43 and the benzoateAu3+Cl3 complex scarcely form. At pH 7 or 13, AuCl4 is substituted with OH– and exists predominantly as AuCl(4-x)OHx species.33 These data suggest that photoexcitation of the benzoateAu3+Cl3 complex indeed promotes the Au3+ photoreduction; pure water is effective for AuNPs synthesis. This is further confirmed by irradiation of different wavelength light at constant light intensity (60 mW m2; Figure S4). As summarized in Figure 4, increase in LSPR band of AuNPs becomes slower by the irradiation of longer wavelength light in the order of 254 nm> 300 nm> 360 nm, where irradiation of 510 nm light scarcely create LSPR band. This tendency agrees with the absorption spectrum of the benzoateAu3+Cl3 complex, as shown by the blue line in Figure 1b. These data strongly support the mechanism proposed in Scheme 1 (eq. 2): photoexcitation of the benzoateAu3+Cl3 complex promotes Au3+ reduction along with oxidation of benzoic acid. As shown in the bottom of Scheme 1, formation of Au0 nuclei and their growth creates AuNPs. As shown in Figure 1c, DLS
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analysis scarcely detect the formation of AuNPs after 20 min irradiation (detection limit: 3 nm). 30 min irradiation creates small AuNPs (ca. 7 nm). Further photoirradiation creates larger AuNPs, finally producing ca. 55 nm AuNPs. These data support the nucleation/growth mechanism (Scheme 1), as observed for thermal or photoreduction of Au3+.34,44 1 254 nm
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of benzoic acid added (Figure S7). Figure 5a summarizes the effect of light intensity on the increase in LSPR absorbance. The absorbance increases rapidly with increasing the intensity since it enhances photoexcitation of the benzoate–AuCl3 complexes and reduces Au3+ more rapidly. Figure 5b shows the size of AuNPs obtained by 200 min irradiation at different light intensities. Increased intensity indeed creates smaller AuNPs. In that, monodispersed 30–80 nm AuNPs are successfully produced while keeping relatively narrow size distribution with
