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Formation of a Single Gold Nanoparticle on a NanometerSized Electrode and its Electrochemical Behaviors Peng Sun, Fei Li, Cheng Yang, Tong Sun, Ismail O. Kady, Benjamin Hunt, and Jian Zhuang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp308501j • Publication Date (Web): 06 Mar 2013 Downloaded from http://pubs.acs.org on March 9, 2013
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Formation of a Single Gold Nanoparticle on a Nanometer-Sized Electrode and Its Electrochemical Behaviors Peng Sun*†Fei Li,‡ Cheng Yang,† Tong Sun,† Ismail Kady,† Benjamin Hunt,† Jian Zhuang,‖ † ‡ ‖
Department of Chemistry, East Tennessee State University, Box 70267, Johnson City, TN 37614-0054 Department of Chemistry, School of Sciences, Xi’an Jiaotong University, Xi'an 710049, P. R. China
School of Mechanical Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, P. R. China
*E-mail:
[email protected] 1 ACS Paragon Plus Environment
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ABSTRACT A new method to form a single gold nanoparticle (Au NP) is been introduced. In our method, a single Au NP is spontaneously formed on the surface of a nanometer-sized platinum (Pt) electrode under open circuit potential. The single NP has been characterized by using Scanning Electron Microscopy and electrochemical methods. Electrochemical studies reveal that the small Au particle has extraordinary stability compared to that of a bulk Au phase. The extra stability of the Au NP is attributed to the formation of alloy with the Pt substrate.
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INTRODUCTION Nanoparticles (NPs) have been widely used in electronic and optical detection systems, such as therapeutics,1 sensor technology2,3, clean energy and catalysis4-11. The application of nanomaterials requires quantitative information on their properties. Studies of NPs based on large amounts of NPs can only provide statistical information on NPs, while studies of NPs on a single NP level can give unambiguous and microscopic information on the NP. Scanning probe microscopy (SPM) is one of the main methods to study the electrochemical behaviors of single NPs.12-14 In this method, a large amount of NPs are randomly dispersed on a conductive substrate and one particle is singled out to study its properties. Since the electrochemical properties of the particle which is under study by SPM can be affected by other particles on the substrate through the electrochemical Ostwald Ripening, it is not surprising to see conflicting information in the study. For example, results from several groups have shown that the electrochemical stability of NPs is inversely proportional to their size,12,13while others have reported the opposite conclusion.14 Thus, new methods other than SPM for studying the electrochemical behaviors of a single NP should be helpful for better understanding of NPs. Bard’s group was the first to report on the study of the single particle collision event.15 A similar method has also been used in Compton’s group to study the electrochemical behaviors of silver particles.16 Unwin’s group developed a new method named scanning electrochemical cell microscopy to study the relationship between the structural properties of a single particle and its catalytic activity.17 Since Mirkin’s and Schuhmann’s groups reported the method for fabricating polishable nanometer-sized electrodes by using a laser puller,18,19,20 the method has been adopted by many groups and is becoming a routine method to fabricate polishable nanometer-sized Pt or Au electrodes. 10, 21 In the last decade, polishable nanometer-sized electrodes have been mainly used in the study of charge transfer kinetics and high resolution imaging. Since the diameter of a NP is in the range of 1-100 nm, and the radius of a nanometer-sized electrode can also be in the same range, it is possible that only a single NP can be anchored on a nanometer-sized electrode. This strategy opens a new way to study the electrochemical behaviors of a single NP. Chen et al. studied single nucleation and growth processes of a Pt particle on a nanometer-sized carbon electrode.11 Zhang et al studied the electrocatalytic activities of a single gold NP (AuNP) by chemically anchoring the particle on a nanometer-sized electrode.10 Recently, we reported a method for quantitatively studying the electrochemical properties of a single monolayer protected NP which is attached on a small electrode.22 By using this method, we are able to study the relationship between electrochemical behaviors of monolayer protected NPs and its size.22 3 ACS Paragon Plus Environment
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Although monolayer protected NPs have many advantages, the monolayer must be removed in most cases in order to use the NPs as efficient catalysts. Compared with monolayer protected NPs, the “naked” NPs are more difficult to study since they can easily aggregate, thus complicating the study of the relationship between the size of “naked” NPs and their electrochemical properties. This paper is focused on the study of the electrochemical behaviors of a single “naked” AuNP, which is formed on a nanometer-sized electrode.
EXPERIMENTAL SECTION Chemicals. Tetrabutylammonium Perchlorate (TBAP) was obtained from Aldrich (Milwaukee, WI). Dichloroethane (DCE) is freshly distilled. Solutions were prepared with deionized water (Milli-Q, Millipore Co.). Preparation of a nanometer-sized Pt electrode and formation of “naked” AuNP on the electrode A puller made nanometer-sized Pt electrode18,19 is polished on a rotating disk of a micropipette beveller (Sutter instrument Co, Novato, CA). The radius of the polished electrode is evaluated from the steady-state limiting current of the cyclic voltammogram in 1mM Ru(NH3)6Cl3 and 0.2 M KNO3 degassed aqueous solution.19 The electrode surface is then cleaned in degassed 0.5M sulfuric acid solution by scanning the electrode potential from 0.0 V to 1.2 V. Then, a Au NP is spontaneously formed by dipping the electrode in 1% HAuCl4 solution for less than 0.5 seconds. After deposition, the electrode is aged in sulfuric acid solution for almost 5 minutes while its potential is cycled from 0.0 V (vs. Ag/AgCl) to 1.2 V (for experiments in organic solvent, this step is omitted). Characterization of “naked” AuNP A Quanta_FEG450 (for Figure 2, 4, S1 and S2) scanning electron microscope is used to observe the surface of electrodes and AuNPs. No coating materials, like carbon or Au, is used to increase the conductance of the electrode. Electrochemical characterization of an Au NP is performed in a twoelectrode mode using an Epsilon with a low current module (BSAi, West Lafayette, IN). For experiments in 0.5 M sulfuric acid solution, a 0.25 millimeter in diameter Ag/AgCl wire is inserted into a glass pipette containing 0.1M KCl solution saturated with AgCl to serve as a reference electrode. For experiments in organic solution, a 0.25 millimeter diameter Ag wire is used as quasi reference electrode.
RESULTS AND DISCUSSION 4 ACS Paragon Plus Environment
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Formation of “naked” AuNPs on a nanometer-sized Pt electrode It is well known that Au can be spontaneously deposited on a macro Pt electrode surface under open circuit conditions.23-25 Our results show that this is also true for a nanometer-sized Pt electrode. According to reference 25, the formation mechanism is as such: Au is formed via reaction 1, and the oxidation half reaction is reaction 3. However, the evidence from our group shows that the aforementioned formation mechanism is not valid. We found that significant amount of Au is formed if a polished nanometer-sized electrode was soaked in 1% HAuCl4 solution for up to two hours, and the formation of Au stops when the entire Pt surface is covered with Au. If the formation of Au is a result of the corrosion of Pt substrate, we should see significant etching of the nanometer-sized Pt electrode after the Au crystals are removed. However, we did not see any significant etching of the Pt electrode after the Au crystals were dissolved in aqua regia (see Figure 1). This means the electrons transferred in the reduction of AuCl4- are not from reaction 3. Actually, recent research shows that an Au layer on Pt can stabilize the Pt substrate.26 Since Pt was involved in the spontaneous formation of Au, we believe that reaction 2 should be the reaction which provides an electron for reaction 1.27,28 However, according to reference 24, Au cannot be formed on the oxygen covered Pt surface, meaning that the formation of Au will be automatically stopped before Au can cover the entire surface of the Pt electrode, which seems to contradict our observation. Possibly, the adsorbed hydroxyl group formed in reaction 2 can associate to form an oxygen molecule (reaction 4),29 or vanish through a way which is still under study in our group. Since the electron transferred in reaction 2 can be transferred onto Au particle surface, three dimensions of Au particle can be formed. It is unfortunate that it is very hard to precisely measure the open circuit potential at a nanometer-sized Pt electrode in 1% HAuCl4 solution since the resistance at the electrode is huge,30 with values varying from 0.3 V to 0.7 V (vs. Ag/AgCl) observed on the same electrode. However, based on the rough open circuit potential value, it is possible that both reaction 1 and 2 can occur. Many metallic cations, like Ag+, can be deposited on Pt under open circuit condition,31,32 and the formation mechanism may be similar to Au. AuCl4- +3e- Au + 4Cl-
(E0=1.002 V vs. SHG)
reaction1
Pt+ H2O Pt-OHad + H+ + e- (0.85 V
