Plasmonics DOI 10.1007/s11468-014-9700-9
Photoreduction of Silver Salts Using Au Nanoparticles to Form a Core-Shell-Type Nanostructure: Insight into the Reaction Mechanism Xiaojuan Chen & Rui Wen & Lisheng Zhang & Abhishek Lahiri & Peijie Wang & Yan Fang
Received: 6 January 2014 / Accepted: 26 February 2014 # Springer Science+Business Media New York 2014
Abstract In this paper, we highlight the formation of Ag/Au core-shell nanoparticles at room temperature by using a lowpower laser. We have investigated the plasmon-induced reduction of Ag+ ions on bare Au nanoparticles synthesized by laser ablation technique, and citrate-capped Au nanoparticles synthesized by chemical method. It is demonstrated that citrate plays an important role for the reduction of silver ions. The citrate gets oxidized by the ‘hot’ holes produced due to the surface plasmon resonance (SPR) of the Au nanoparticles which then reduces the Ag+ ions to Ag. The importance of excitation laser wavelength is also demonstrated to facilitate the reduction process. Keywords Photoreduction . Surface plasmon resonance (SPR) . Gold nanoparticles . Core-shell . Metal reduction
Background Nanoparticles such as Au, Ag, Cu etc. have shown surface plasmon resonance (SPR) on excitation with a particular wavelength of light [1–3]. The plasmon resonance has shown to depend on size, shape and dielectric constant of the material and its environment [4]. The plasmon-induced physical and chemical processes are increasingly being researched X. Chen : R. Wen : L. Zhang : P. Wang (*) : Y. Fang The Beijing Key Laboratory for Nano-Photonics and Nano-Structure, Department of Physics, Capital Normal University, 100048 Beijing, China e-mail:
[email protected] A. Lahiri (*) Institute of Electrochemistry, Clausthal University of Technology, Arnold-Sommerfeld-Str. 6, 38678 Clausthal-Zellerfeld, Germany e-mail:
[email protected]
especially in the field of energy [3]. It has been used for water-splitting [5], generation of hydrogen from alcohol [6] and photocatalytic reactions [7]. It is known that hot electrons are generated during plasmon-excitation. The presence of an electron accepter can then be used to collect the hot electrons to trigger a reaction. The utilization of hot electrons has been shown in DNA melting [8], catalysis [9] and even to dissociate H2 on Au nanoparticles [10]. In the last decade, lots of studies have been performed on the photoconversion of silver nanoparticles into single crystal prism structure. The influence of laser intensity and excitation wavelength, pH of the solution and ligands has been studied to evaluate the differences in the morphology of the silver nanoparticles and the prism structures [11–16]. However, the mechanism into the formation of prism structure is in debate. One of the mechanisms discuss about the photoconversion occurring by an oxidative etching of silver nanoparticles, followed by photoreduction of the silver ions [17]. Another mechanism suggests that the hot holes produced due to the surface plasmon resonance oxidize the citrate-producing electrons which then reduce the silver ions [11, 18]. A generation of photovoltage was also demonstrated due to the oxidation of citrate ions [18]. However, Lee et al. has reported that Ag+ ions could be directly reduced by a high-intensity laser and showed the generation of photovoltage [19]. Therefore, the question remains whether the Ag+ ions can be directly reduced by hot electrons, or citrate oxidation is required to facilitate the reduction process. In this paper, we have investigated the reduction of Ag+ ions by laser exciting the bare Au nanoparticles synthesized by laser ablation and citrate-capped Au nanoparticles synthesized by chemical method. The reaction was followed using UV-visible spectroscopy and transmission electron
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microscopy (TEM) analysis. We also investigated the influence of laser wavelength on the reduction process of Ag+ ions.
Materials and Methods Preparing of Gold Nanoparticles Chemical Route Gold nanoparticles of 30 nm were synthesized using the citrate route according to the literature [22]. To synthesize 30 nm of Au nanoparticles (AuNPs), 95 ml of 0.53 mM HAuCl4 was added to a 250-ml round bottle flask, and then the solution was boiled to 80 °C for 30 min. Under rapid stirring, 5 ml of 0.03 M sodium citrate was added and further boiled for 15 min. The solution was then gradually cooled to room temperature. The gold nanoparticles were washed five times with Milli-Q water in a centrifuge at 7,000 rpm before use.
Laser Ablation A pure Au plate (99.99 %) was placed at the bottom of a quartz cell with 10 ml of distilled deionized water. A Nd:YAG pulsed laser (Quanta Ray QBY-200) with the excitation wavelength at 1,064 nm (the repetition of 5 Hz, pulse duration between 6 and 9 ns, and pulse energy of 300 mJ) was focused on the Au plate at 90° incidence angle for ablation. After 60 min, the bare Au nanoparticles (AuNPs) were obtained.
Preparing of Au-Ag Nanoparticles Core-Shell In a typical experiment, 2 ml of AuNPs solution was taken, and 1 ml of 5 mM AgNO3 solution was added in a 5-ml quartz cell. A 50-mW, 405-nm laser was directly focused into the solution. The distance between the laser and the solution top surface was about 5 cm. We repeated the same procedure by using different laser wavelengths such as 478, 532, 780 and 980 nm. After each experiment, the solution was then analyzed using UV-visible spectroscopic (Shimadzu UV 2401 PC) analysis.
TEM Measurements For TEM analysis, one drop of solution was placed on a carbon-coated copper grid and left to dry in the natural atmosphere. HRTEM was performed using an FEI F20 instrument at 200 kV accelerating voltage.
Results and Discussion Figure 1a shows the transmission electron microscope (TEM) image of the bare Au nanoparticles. The nanoparticles are about 20 nm in diameter. It is evident that some coagulation of the nanoparticles has taken place (marked by a red arrow). Figure 1b shows the changes in the absorption spectra of the bare Au nanoparticles before and after the addition of 5 mM AgNO3. A peak around 525 nm is observed which is due to the surface plasmon resonance (SPR) of the bare Au nanoparticles. On addition of AgNO3 into the Au nanoparticles, a slight decrease in the absorption was noted (red curve, Fig. 1b). When the solution was irradiated with the 532-nm laser, with time, only a decrease in the absorption peak was observed (Fig. 1b). This decrease in absorption is due to the coagulation of the bare gold nanoparticles. Therefore, the Au nanoparticles were first stabilized. To stabilize the bare Au nanoparticles, Polyvinylpyrrolidone (PVP) was added into the solution, and the solution was again irradiated with the 532-nm laser. On performing the experiment for 120 min, no observable changes in the SPR of the gold were observed. From this, we can infer that although the hot electrons are generated in AuNP by laser excitation, the Ag+ ions cannot accept these electrons, maybe due to the difference in the electronic states. Experiments were also performed by increasing the AgNO3 concentration in the solution to 1 M. However, no evidence for Ag+ ion reduction was observed. On the contrary, Lee et al. [19] had directly photoreduced Ag+ ions by using AuNPs modified indium tin oxide (ITO) substrate where they had observed the generation of photovoltage. It might have been that the hot electron could transfer into the ITO conduction band which then helped in reduction of Ag+ ions, or the ITO itself generated a photovoltage. On addition of citrate into the bare gold containing 5 mM AgNO3 and irradiating it with the 532-nm laser, reduction of Ag+ ions were observed. Figure 1c shows the changes in the absorption spectra on addition of AgNO3 to bare gold before and after 60 min of irradiation process. It is evident that after 60 min of the irradiation, a broad peak at 400 nm is seen which corresponds to the plasmon resonance of Ag nanoparticles. TEM analysis of the product revealed the formation of core-shell structure with Au core and Ag forming the shell (Fig. 1d). Therefore, from the above experiments, it appears that photooxidation of citrate is essential for the reduction of Ag+ ions. Further confirmation into the reaction mechanism was evaluated using the citrate-capped Au nanoparticles which have been shown previously [16]. The Au nanoparticles were synthesized using direct reduction process in sodium citrate. Figure 2a shows the absorption curve of the citrate-capped Au nanoparticles showing a plasmon resonance at 520 nm. On addition of 5 mM AgNO3, there is decrease in the absorption. On excitation with the 532-nm laser for 60 min,
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Fig. 1 a TEM of the bare gold nanoparticle synthesized by laser ablation technique. b Changes in the absorption spectra on irradiating bare gold nanoparticles+5 mM AgNO3 solution with the 532-nm laser. c Changes
in the absorption spectra in irradiating bare Au nanoparticles in presence of AgNO3 and citrate with the 532-nm laser. d TEM image of the coreshell structure formation due to plasmon-induced reduction of Ag
another peak at 400 nm is formed which corresponds to the formation of Ag nanoparticle. Also, there is an overall increase in the absorbance which suggests the formation of the core-shell structure [20]. A high-resolution TEM image is shown in Fig. 2b which confirms the formation of the coreshell structure. The EDX of the particle (Fig. 2c) shows the presence of both Au and Ag, thus proving that indeed reduction of Ag took place on irradiating the citrate-capped Au nanoparticles. To further understand the reaction mechanism, the reduction process was evaluated by using different wavelengths of laser. Figure 3 shows the UV-visible spectra after 60 min of illumination of the citrate-capped gold nanoparticles in the presence of 5 mM AgNO3 illuminated with different laser wavelengths. It is evident from the absorption spectra that the peaks around 400 nm, corresponding to the SPR of Ag nanoparticles, occur on irradiating the solution at lower wavelengths of 405, 478 and 532 nm. However, on irradiating with higher wavelengths of the 780- and 980-nm lasers, no clear peak is formed at 400 nm in the absorption spectra. This indicates that reduction of Ag+ takes place at lower wavelengths between
400 and 540 nm which suggests that when the laser frequency resonates with similar frequency of the SPR of Au nanoparticles, hot hole-induced oxidation of citrate can take place according to Eq. 1. citrate→actone−1; 3−dicarboxylate þ CO2 þ 2e−
ð1Þ
This release of electrons then results in the reduction of Ag+ ions showing various colours (Fig. 3b). Figure 3c shows the various TEM images of Au nanoparticles exposed at different wavelengths. It is evident that at lower wavelengths of 405, 478 and 532 nm, Au-Ag core-shell structure has formed, whereas at higher wavelengths, only Au nanoparticles are seen. Furthermore, some particles on excitation with the 478-nm laser showed the tendency to form Au-Ag core-shell prism structure. Such Au prism structure has been shown to arise only by the underpotential deposition (UPD) of Ag on Au [21]. Thus, it appears that not only the presence of citrate is important for the reduction of silver ions, but also the laser wavelength plays an important role in the SPR-induced reduction of Ag+ ions.
Plasmonics Fig. 2 a Changes in the absorption spectra on irradiating the citrate-capped gold nanoparticles+5 mM AgNO3 solution with the 532-nm laser. b High-resolution TEM image of the core-shell structure. Scale bar 5 nm. c EDX of the reduced product showing the presence of both Au and Ag
Fig. 3 a Absorption spectra of solution containing Au nanoparticle and 5 mM AgNO3 after 60 min of irradiation with different wavelengths of laser. b Optical photograph of the solution after 60 min. c TEM showing Au nanoparticles after 60 min exposure to laser. The scale bar is 5 nm
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Conclusions In conclusion, we have shown that SPR-induced citrate oxidation is needed for the reduction of Ag+ ions to form an AuAg core-shell structure. On using bare Au nanoparticles, no reduction of Ag+ ions was observed. On exciting the Au nanoparticles with a lower wavelength laser, prominent reduction of Ag+ ions took place, whereas on exciting with a higher wavelength laser, no reduction of Ag+ ions occurred. This indicates that when the laser wavelength is similar to the SPR of Au nanoparticles, hot hole-induced oxidation of citrate can take place which releases electrons. These electrons assist in the reduction of Ag+ ions forming core-shell-type structure. Acknowledgments This work was supported by the National Natural Science Foundation of China under grant no. 21073124.
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