Synthesis and Characterization of n-Alkylamine-Stabilized Palladium

J. Phys. Chem. C 2010, 114, 723–733

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Synthesis and Characterization of n-Alkylamine-Stabilized Palladium Nanoparticles for Electrochemical Oxidation of Methane Zhongping Li,† Jie Gao,† Xiaoting Xing,‡ Suozhu Wu,† Shaomin Shuang,*,† Chuan Dong,*,† Man Chin Paau,§ and Martin M. F. Choi*,§ Research Center of EnVironmental Science and Engineering, School of Chemistry and Chemical Engineering, and School of Life Science and Technology, Shanxi UniVersity, Taiyuan 030006, China, and Department of Chemistry, Hong Kong Baptist UniVersity, Kowloon Tong, Hong Kong SAR, China ReceiVed: January 29, 2009; ReVised Manuscript ReceiVed: NoVember 26, 2009

Palladium nanoparticles (PdNPs) have been synthesized using n-alkylamines (Cn-NH2) as stabilizing ligands. The NP size and distribution were controlled by varying the initial mole ratio of PdCl2/Cn-NH2 and carbon chain lengths of Cn-NH2 including hexylamine (C6-NH2), dodecylamine (C12-NH2), and octadecylamine (C18-NH2). The average PdNP sizes were 20 ( 2.0, 6.0 ( 0.8, 5.6 ( 0.8, 6.5 ( 0.9, and 5.2 ( 0.8 nm prepared with 1:7 PdCl2/C6-NH2, 1:7 PdCl2/C12-NH2, 1:7 PdCl2/C18-NH2, 1:5 PdCl2/C18-NH2, and 1:9 PdCl2/C18-NH2, respectively. The particle size decreased with the increase in the carbon chain length of Cn-NH2. The as-synthesized n-alkylamine stabilized PdNPs (Cn-NH2-PdNPs) were fully characterized by transmission electron microscopy, X-ray powder diffraction, UV-visible absorption spectroscopy, infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), proton nuclear magnetic resonance (1H NMR) spectroscopy, thermogravimetric analysis, graphite furnace atomic absorption spectrometry, and mass spectrometry. The interaction of C18-NH2 with PdNPs was verified by IR, XPS, and 1H NMR spectra, demonstrating that the amine functionalities were successfully linked to the Pd core surfaces. The PdNPs are soluble and stable in apolar solvents such as benzene, chloroform, n-hexane, and toluene. The electrochemical reactions between CH4 and Cn-NH2-PdNPs on Pd electrodes were studied by cyclic voltammetry and chronoamperometry. These PdNPs reacted readily and produced good response to CH4 at ambient conditions. The sensitivity to CH4 depends on the PdNPs prepared from various n-alkyl chain lengths of Cn-NH2 and also the mole ratio of PdCl2/Cn-NH2. It was determined that PdNPs synthesized from 1:7 PdCl2/C18-NH2 displayed the best electrocatalytic oxidization of CH4. The C18-NH2-PdNP (5.6 nm) modified Pd electrode could be used repeatedly and had a stable and reproducible response to CH4. Introduction Metal nanoparticles (NPs) continue to attract immense attention because of their unusual optical, magnetic, and electrical properties and also their potential applications in sensing and imaging.1,2 In particular, the high surface area-tovolume ratio of metal NPs makes them highly attractive for catalysis. Currently the hottest research areas include preparation, structural determination, study of the properties, and exploration for diverse applications of NPs. Among these, research efforts have been directed to the synthesis of palladium nanocrystals (PdNCs). So far a variety of techniques were introduced to prepare palladium nanoparticles (PdNPs) including sonochemical reduction,3 chemical liquid deposition,4 refluxing alcohol reduction,5 decomposition of organometallic precursors,6 hydrogen reduction,7 and electrochemical deposition,8-10 where, in general, the particles were formed by the reduction of metal ions in the presence of some stabilizers. Typical ligands for stabilizing PdNPs include dendrimers,11 phosphines,12-14 thiols,15 and polymers.16,17 The choice of stabilizing ligands is extremely important when preparing * Corresponding authors. E-mail: [email protected] (S.S.); dc@ sxu.edu.cn (C.D.); [email protected] (M.M.F.C.). † Research Center of Environmental Science and Engineering, Shanxi University. ‡ School of Life Science and Technology, Shanxi University. § Hong Kong Baptist University.

transition metal NPs as they can introduce dramatic effect on the catalytic properties of these NPs. Thus, a balance is needed between the ability of ligands and stability of NPs such that reactant molecules are able to reach the NP surface for catalytic reactions. If ligands are too bulky, reactant molecule access to the NP core will be restricted due to the steric hindrance of the surface-attached ligands.12,18 It has been reported that nalkylamines (Cn-NH2) can be strongly and chemically adsorbed on metal surfaces by sharing electrons between nitrogen and metal atoms. Thus, they can be effective protecting ligands to prevent NPs from conglomeration and corrosion in acidic conditions.19 They can also be used as stabilizing ligands for transition metal NPs because of the NPs potential catalytic properties.20 Although Rao and Lakshminarayanan21 and Yang et al.22 have reported the synthesis of dodecylamine stabilized PdNPs using a phase-transfer procedure, their PdNPs were 3-4 nm diameter and there is no information on their PdNPs for catalytic oxidation of methane (CH4). As such, the search for metal NPs that can effectively oxidize CH4 under ambient conditions is still a challenge. CH4 is the major constituent of natural gas. It can be formed in the walls of coal mines and is often released in coal mining. Unmonitored CH4 can accumulate and then cause explosions. CH4 is also one of the important greenhouse gases, and its release to the atmosphere contributes to global warming.23 So CH4 is one of the most important target gases to be monitored

10.1021/jp907745v  2010 American Chemical Society Published on Web 12/29/2009

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J. Phys. Chem. C, Vol. 114, No. 2, 2010

and determined. Noble metal (Au, Pd, and Pt) electrodes have been used for electrochemical oxidation of CH4 at room temperatures. Unfortunately the reactions took long hours to complete.24 To speed up the reactions, NPs are exploited to act as electrocatalysts since they possess very high specific surface areas and that their surface chemical properties can be tuned as required by using particular ligands. Thus, the development of an electrochemical method based on noble metallic nanomaterials to determine CH4 is plausible. In the present study, we explore the use of Cn-NH2 including hexylamine (C6-NH2), dodecylamine (C12-NH2), and octadecylamine (C18-NH2) as protecting ligands for the synthesis of Cn-NH2 stabilized PdNPs (Cn-NH2-PdNPs) since these ligands bind easily to and stabilize the Pd cores. To our knowledge, this is the first report on studying the effect of carbon chain length of Cn-NH2 on the particle size of the assynthesized Cn-NH2-PdNPs. In addition, the electrochemical behavior of these PdNPs on Pd modified electrodes in 0.50 M H2SO4 to CH4 is also largely dependent on the carbon chain length of Cn-NH2. It was found that the longer the carbon chain length, the higher the sensitivity to CH4 detection. It is anticipated that the proposed Cn-NH2-PdNP modified Pd electrode can offer potential for the future development of portable CH4 sensor. Experimental Section Chemicals. Palladium(II) chloride (PdCl2, 99.9%), hexylamine, dodecylamine, octadecylamine, sodium borohydride (NaBH4), potassium ferrocyanide (K4Fe(CN)6), tetramethylsilane (TMS), absolute ethanol, hydrochloric acid, and sulfuric acid were obtained from Aldrich (Milwaukee, WI). 2,5-Dihydroxbenzoic acid (DHB, 98%) was from Sigma (St Louis, MO). Highest purity concentrated nitric acid (70%) was purchased from Merck (Hoddesdon, Herts, U.K.). All chemicals were of analytical purity and used as received without further purification. Purified water from a Milli-Q-RO4 water purification system (Millipore, Bedford, MA) with a resistivity higher than 18 MΩ · cm was used to prepare all solutions. CH4 (99.99%, v/v) and N2 (99.99%, v/v) gases were purchased from Fujiang Special Gas Co. (Taiyuan, China). Synthesis of n-Alkylamine-Stabilized Palladium Nanoparticles. Various Cn-NH2-PdNP samples were synthesized using a biphasic mixture of water and toluene method.15e In a typical synthesis, 17.7 mg (0.10 mM) of PdCl2 was dissolved in 20 mL of aqueous solution under vigorous stirring. An appropriate concentration, determined by the desired PdCl2/ Cn-NH2 mole ratio, of 50 mL of C6-NH2 (or C12-NH2 or C18-NH2) in toluene was added into the PdCl2 solution to obtain a white turbid dispersion mixture. After about an hour of stirring, a 5.0 mL aqueous solution of NaBH4 (37 mg, 1.0 mM) was dropwisely added and stirred until the mixture turned black, indicating the formation of PdNPs. The black toluene layer was separated from the aqueous phase. The toluene solvent was then removed with a rotary evaporator. The resulting PdNP products were thoroughly rinsed with doubly distilled water and acetone successively three times to remove excess ligands and other reaction byproducts. The PdNP products are very stable and soluble in apolar solvents such as benzene, chloroform, nhexane, and toluene, but are insoluble in polar solvents, e.g., alcohols and water. Totally five different Cn-NH2-PdNP samples were synthesized. Samples at PdCl2/C6-NH2, PdCl2/ C12-NH2, and PdCl2/C18-NH2 of 1:7 were synthesized. Two other samples with PdCl2/C18-NH2 of 1:5 and 1:9 were also

Li et al. done. All these PdNPs are black viscous powders and soluble in apolar solvents to form black solutions. Characterization. Cn-NH2-PdNP samples were prepared by casting and evaporating a droplet of toluene solution of PdNPs onto carbon-coated copper grids for transmission electron microscopy (TEM). TEM images were performed on a JEOL JSM-1010 TEM (Tokyo, Japan). X-ray powder diffraction (XRD) data were collected on a Rigaku D-Max-2500 powder X-ray diffractometer (Tokyo, Japan) with Cu KR radiation (λ ) 1.54056 Å). UV-visible absorption spectra were recorded on a TU-1901 UV-visible absorption spectrophotometer (Beijing, China). Fourier transform infrared spectra (FTIR) were acquired on a Perkins Elmer Paragon 500 FTIR spectrometer (Waltham, MA) in the wavenumber range of 500-4000 cm-1 at a resolution of 4.0 cm-1. Since the as-synthesized PdNPs are very viscous, it is difficult to mix them homogenously with KBr powder to obtain transparent KBr pellet. Thus, the samples were prepared in the form of films by drop-casting 1.0 mL of CHCl3 solutions of PdNPs (i.e., 0.10 mg of PdNPs in 1.0 mL of CHCl3) onto KBr salt plates and left to complete evaporation of CHCl3 at room temperature before IR measurements. Proton nuclear magnetic resonance (1H NMR) spectroscopy was done at 300 MHz on a Bruker Avance DRX-300 NMR spectrometer (Fallanden, Switzerland) in CDCl3 solutions. Chemical shifts were reported relative to TMS. Matrix-assisted laser desorption ionization-time-of-flight (MALDI-TOF) mass spectra (MS) of the C18-NH2-PdNP samples were acquired on a Bruker Autoflex MALDI-TOF mass spectrometer (Bremen, Germany). A sample of PdNPs (∼1 mg) was dissolved in 1.0 mL of toluene. An aliquot of 2 µL of sample toluene solution was mixed with a 2 µL aliquot of 1.0 M DHB in methanol/water (1:1), deposited on a MALDI target plate and air-dried. The sample was irradiated by a pulsed N2 laser working at 337 nm. In general, 30 laser shots were averaged for each spectrum. Thermogravimetric analyses (TGA) of the C18-NH2-PdNP samples synthesized from various mole ratios of PdCl2/C18-NH2 and the free ligand C18-NH2 were determined by a Perkins Elmer TGA 6 thermogravimetric analyzer (Waltham, MA). About 3.5 mg of PdNP or C18-NH2 sample was put into a ceramic pan, and the temperature was ramped from room temperature to 500 °C at a heating rate of 10 °C/min. Each TGA sample was conducted in triplicate, and the TGA curves were obtained from the average values of the TGA data. X-ray photoelectron spectra (XPS) were performed on a Leybold Heraeus SKL-12 X-ray photoelectron spectrometer (Shenyang, China) equipped with a VG CLAM 4 MCD hemispherical electron energy analyzer using a Mg KR excitation source of 1253.6 eV and operating at 10 kV and 20 mA. The base pressure in the chamber during measurements was 5 × 10-10 Torr. Survey spectra were collected at a constant pass energy of 160 eV from a 0.37 × 1.0 mm2 area of the sample. High-resolution spectra of the N 1s and Pd 3d core levels were collected at a pass energy of 20 eV with the same spot size. The binding energies (BEs) were corrected by referencing the C 1s BE to 284.6 eV. The spectra were processed by the Casa XPS v.2.3.12 software using a peak-fitting routine with symmetrical Gaussian-Lorentzian functions. The Pd core compositions of the Cn-NH2-PdNP samples were analyzed by a Varian Spectra AA 240FS graphite furnace atomic absorption spectrometer (GFAAS). A 10 mg amount of dry sample was digested by 1.8 mL of concentrated HNO3 in a CEM MDS-2000 microwave oven (Matthews, NC). After digestion, the sample was washed and transferred to a 10 mL volumetric flask and diluted to the mark with doubly distilled

PdNPs with Stabilizing Ligands Cn-NH2 water. The sample was further diluted (1 + 2) with doubly distilled water prior to analysis. A 40 µL aliquot of the diluted sample was injected into a pyrolytically coated graphite tube. Calibration was done using the standard addition method. The concentration of each sample was determined by fitting the linear regression line to the point defined by the spiked concentration value and the corresponding integrated peak area. Preparation of Palladium Nanoparticles Modified Palladium Electrode. A polycrystalline Pd electrode (3 mm diameter; Chenghua, Shanghai, China) was polished with an aqueous slurry solution of finer R-Al2O3 powder (down to 0.05 mm) and then sonicated in water. The Pd electrode was carefully washed with distilled water and dried under a stream of N2. An 11 µL aliquot of 5.0 mg/mL Cn-NH2-PdNP toluene solution was dropped on the clean Pd electrode surface and kept at room temperature to evaporate the solvent. The Cn-NH2-PdNP modified Pd electrode was ready for electrochemical measurements. Electrochemical Measurement. All electrochemical studies were conducted on a Chenhua CHI660 electrochemical workstation (Shanghai, China) using a standard three-electrode configuration. A Pt wire (1 mm diameter, Chenhua) was employed as the counter electrode and an Ag/AgCl electrode was the reference electrode. The working electrode was the Cn-NH2-PdNP modified Pd electrode. High-purity CH4 (99.99%) and N2 were introduced into 0.50 M H2SO4 solution at a flow rate of 100 mL/min for 15-30 min. Electrochemical methods including cyclic voltammetry (CV) at a scan rate of 20 mV/s unless stated otherwise, and chronoamperometry were applied to study the electrochemical behavior of the Cn-NH2-PdNP modified Pd electrode in 0.50 M H2SO4 in the absence and presence of CH4 at ambient conditions.

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Figure 1. Transmission electron micrographs of Cn-NH2-PdNPs prepared with various synthetic conditions: (A) 1:7 PdCl2/C6-NH2, (B) 1:7 PdCl2/C12-NH2, (C) 1:5 PdCl2/C18-NH2, (D) 1:7 PdCl2/ C18-NH2, and (E) 1:9 PdCl2/C18-NH2.

Results and Discussion Transmission Electron Microscopy. The use of capping ligands with metal NPs play an important role in governing the particle size and distribution and, hence, the catalytic activity of the NPs. In general, a capping ligand that interacts strongly with the surface of metal NPs can effectively stabilize the NPs and form smaller NPs. The particle sizes of Cn-NH2-PdNPs prepared with different types of Cn-NH2 were studied by TEM and are shown in Figure 1A-E. Particle analysis revealed that the average sizes of PdNPs prepared with 1:7 PdCl2/C6-NH2, 1:7 PdCl2/C12-NH2, and 1:7 PdCl2/C18-NH2 were 20 ( 2.0, 6.0 ( 0.8, and 5.6 ( 0.8 nm, respectively. The PdNPs synthesized from 1:7 PdCl2/C6-NH2 were heavily agglomerated and were the largest, indicating that the Pd atoms were not well protected by C6-NH2 from aggregation. The particle size decreased with the increase in the carbon chain length of Cn-NH2 when the same mole ratio of PdCl2/Cn-NH2 was used. The longer carbon chain of Cn-NH2 can reduce the chance of collision between and aggregation of Pd atoms, thus help minimizing the growth of Pd nuclei. Among the Cn-NH2 used, C18-NH2 produced the smallest PdNPs, inferring that it is a better ligand to prevent Pd atoms from aggregation. As such, C18-NH2 was chosen as the ideal Cn-NH2 ligand for the synthesis of PdNPs. PdNPs synthesized with various mole ratios of PdCl2/C18-NH2 were then studied. The TEM results (Figures 1C-E) show that the average sizes of the C18-NH2-PdNPs prepared with 1:5, 1:7, and 1:9 PdCl2/C18-NH2 were 6.5 ( 0.9, 5.6 ( 0.8, and 5.2 ( 0.8 nm, respectively, inferring that the initial PdCl2/C18-NH2 mole ratio can indeed determine the size of C18-NH2-PdNPs. Although increasing the PdCl2/C18-NH2 mole ratio could produce smaller PdNPs, the size did not differ much when the mole ratio was between 1:7 and 1:9. This is

Figure 2. X-ray diffraction patterns of Cn-NH2-PdNPs prepared with various synthetic conditions: (a) 1:7 PdCl2/C6-NH2, (b) 1:7 PdCl2/ C12-NH2, (c) 1:5 PdCl2/C18-NH2, (d) 1:7 PdCl2/C18-NH2, and (e) 1:9 PdCl2/C18-NH2.

possibly attributed to the fact that a monolayer of ligand had already been packed densely on the Pd core surface and further increase in ligands would not cause any significant effect on the Pd core size; instead, a multilayer of ligands would cap the PdNPs (vide infra). X-ray Diffraction Analysis. XRD is a useful technique to determine the crystalline structure of metal NPs. Figure 2 displays the XRD patterns of the Cn-NH2-PdNP samples synthesized with various conditions. The XRD patterns show diffraction peaks at 40.0, 46.7, 68.3, and 82.2° which can be assigned to the (111), (200), (220), and (311) planes of Pd from the Fm3m space group of the face-centered cubic (fcc) structure, respectively.17a The XRD patterns reveal that all our Cn-NH2-PdNP samples belong to the fcc geometry of crystal-

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Figure 3. UV-visible absorption spectra of Cn-NH2-PdNPs synthesized with (a) 1:7 PdCl2/C6-NH2, (b) 1:7 PdCl2/C12-NH2, (c) 1:5 PdCl2/C18-NH2, (d) 1:7 PdCl2/C18-NH2, and (e) 1:9 PdCl2/C18-NH2. All absorption spectra were normalized at 300 nm for ease of comparison.

line Pd (JCPDS card no. 05-0681). The (111) peak broadens with the decrease in core size which is consistent with the nanoscale structural features of PdNCs. The average core size of the PdNPs can be calculated by the Scherrer formula using the diffraction (111) peak of full width at half-maxima (fwhm).10 The calculated values are 17.3, 5.0, 5.5, 4.5, and 4.1 nm for PdNPs prepared with 1:7 PdCl2/C6-NH2, 1:7 PdCl2/C12-NH2, 1:5 PdCl2/C18-NH2, 1:7 PdCl2/C18-NH2, and 1:9 PdCl2/ C18-NH2 respectively, corroborating the size trend determined by the TEM images. However, the average core size determined by XRD is smaller than that of TEM, attributing to the fact that XRD reflects only the size of crystalline grains, whereas the measurements from TEM images may also include some aggregated NPs. In addition, XRD employed much more PdNPs for the measurement while the TEM images were just obtained from limited areas even though they were selected as representative areas. UV-Visible Absorption Spectroscopy. Figure 3 displays the UV-visible absorption spectra of various types of Cn-NH2-PdNPs in toluene (0.5 mg/mL). The spectra were normalized at 300 nm for ease of comparison and also to neglect the concentration effect of the PdNP samples. All spectra exhibit a broad and monotonic increase in absorption from the visible to UV region, being consistent with the spectrum of PdNPs in the literature.24a The small-sized PdNP displays a sharper decrease in absorbance from the UV to visible region. No distinctive surface plasmon resonance peaks are observed for all spectra which are similar to other transition metals including Pd.25 In fact, the presence of residual or excess stabilizing agents, metal cations, and various core sizes of NPs in a NP sample solution can possibly give rise to overlapping and broadening absorption bands, thus making the interpretation of UV-visible data very difficult.26 Infrared Spectroscopy. To confirm the attachment of the C18-NH2 ligand to the PdNPs, FTIR spectra of the free C18-NH2, C18-NH2-PdNP, and other Cn-NH2-PdNP samples have been studied and are depicted in Figure 4. The IR spectrum of the NPs reveals more information about the local molecular environment of the ligands. The free C18-NH2 ligand (curve A) and C18-NH2-PdNPs (curves B-D) show some similar spectral characteristics in the wavenumber range of 500-4000 cm-1, indicating that C18-NH2 has successfully attached to the Pd core. Typical absorption bands of free C18-NH2, C18-NH2-PdNPs, and other Cn-NH2-PdNPs (curves E and F) at 2850-3000 cm-1 are observed with the methylene

Li et al.

Figure 4. FTIR spectra of (A) free C18-NH2 ligand and Cn-NH2-PdNPs synthesized with (B) 1:9 PdCl2/C18-NH2, (C) 1:7 PdCl2/C18-NH2, (D) 1:5 PdCl2/C18-NH2, (E) 1:7 PdCl2/C12-NH2, and (F) 1:7 PdCl2/C6-NH2. The IR spectra are offset for clarity and ease of comparison.

asymmetric and symmetric vibration modes at 2920 and 2850 cm-1, respectively, as well as the methyl stretching mode at 2951 cm-1.27 The position of the peak of methylene groups relative to the methyl stretching peak for the C18-NH2-PdNPs has not changed, indicating that the structural integrity of the octadecyl (C18) hydrocarbon chain is maintained during the formation of the NPs.27-30 Both free C18-NH2 and C18-NH2-PdNPs have absorption bands at 1636 cm-1 corresponding to the N-H bending vibration of amino groups. Careful examination of the IR spectra reveals that the absorption bands at 3350 cm-1 corresponding to the N-H stretch modes of vibration are very different. The peak in free C18-NH2 is very sharp, while the one in Cn-NH2-PdNPs is relatively broader and some of them (i.e., C6-NH2-PdNPs and C12-NH2-PdNPs) are almost unobservable. Thus, it seems reasonable to conclude that the N-H stretch is heavily perturbed by the Pd atoms since the amino functionality is in close proximity to the Pd core surface. This explanation is consistent with the results of 1H NMR data (vide infra). Finally, most of the IR absorption bands for the C6-NH2-PdNP sample (curve F) are much broader as compared to the other Cn-NH2-PdNPs, probably attributing to the much stronger electronic perturbation effect of the larger core (20 nm) PdNP on the vibrational/stretching bands of its protecting ligands. X-ray Photoelectron Spectroscopy. To obtain further structural features of the C18-NH2-PdNPs, XPS analysis of PdNP samples synthesized with 1:5, 1:7, and 1:9 PdCl2/C18-NH2 were performed. Figure 5A displays the Pd 3d core level spectra of these PdNP samples. The core level BE and fwhm were analyzed with particular attention to the Pd 3d spin-orbit components, which are of major interest for assessing the particle size effect and oxidation state of Pd atom on PdNPs. Figure 5A depicts the Pd 3d spectra of our PdNPs which have a main and a minor peak at the lower and higher BEs, respectively. It has been reported that the Pd0 3d spectra of PdNPs comprise two spin-orbit components of Pd 3d5/2 and Pd 3d3/2 at 335.0 and 340.6 eV, respectively.31 The BEs of Pd 3d electrons slightly increase with the decrease in Pd core diameter, inferring that the surface Pd atoms of the core are relatively more positive when the Pd core size gets smaller, which is in agreement with most metal NPs. All the BE data are displayed in the right panel of Figure 5A. When inspected

PdNPs with Stabilizing Ligands Cn-NH2


Figure 5. XPS spectra of (A) Pd 3d and (B) N 1s of various core sizes of C18-NH2-PdNPs: (1) 6.5, (2) 5.6, and (3) 5.2 nm and (4) free C18-NH2 ligand. The right panels display the binding energies of Pd 3d and N 1s.

carefully, it is observed that each Pd 3d peak is quite broad, implying that they possibly are composed of two overlapping peaks. As such, the experimental data (dotted lines) were deconvoluted by a curve-fitting program to two components (solid lines). The minor component has higher BE than the major one (right panel of Figure 5A). It is plausible that some surface Pd of the NPs exist as Pd0 and Pd2+ species. The BEs of the minor component are 342.5 and 337.0 eV, which match well with the BE for Pd2+ 3d3/2 in PdO.24 Indeed, strong O 1s signals at about 529.3 eV corresponding to O2- species were identified in the XPS (not shown). The N 1s spectra of the free C18-NH2 and C18-NH2-PdNPs were studied and displayed in Figure 5B. In general, the N 1s peaks of C18-NH2-PdNPs (curves 1-3 of Figure 5B) have higher BEs than that of the free ligand (curve 4), inferring that the N atoms of the C18-NH2-PdNPs are relatively more positive than that in its free ligand. For N to be more electropositive and higher BE, the electrons shift from the N atom to the Pd atoms, and this is opposite compared to the case of thiols on metals. The N 1 s peak BE for the C18-NH2-PdNPs increases with the increase in the core size of the PdNPs, demonstrating that the electronic effect of the larger core size PdNPs on the N atom of the ligand is more prominent. Moreover, a very obvious shoulder peak was observed for the 5.2 nm C18-NH2-PdNP sample (dotted line of curve 3). The spectrum was deconvoluted to two components by the curve-fitting program. The minor

component has a lower BE (right panel of Figure 5) which matches with the free ligand (curve 4) and the literature.32 Again, these results indicate that this C18-NH2-PdNP sample (5.2 nm) comprises two types of C18-NH2 ligands: the Pd surface-bound and the free ligands. In other words, the PdNPs are capped by a bilayer of ligand (vide supra). Proton Nuclear Magnetic Resonance Spectroscopy. The 1 H NMR spectra of the (A) free C18-NH2 ligand and (B) C18-NH2-PdNPs (5.2 nm) in CDCl3 are displayed in Figure 6. Four major types of proton peaks a, b, c, and d of the free ligand and a, b, c′, and d′ of the Pd-bound ligand were identified corresponding to the -CH3, -CH2-, -CH2-N, and N-H2 protons, respectively. Compared to the free ligand, the chemical shift (δ) of the -CH3 (a) and -CH2- (b) protons remain more or less the same as they are further away from the Pd core, i.e., immunity from the Pd core effect. When protons are furthermost from the Pd core, they will experience freedom of motion and their spin relaxations are similar to those of free ligands. However, the -CH2-N (c′) protons which are in close proximity to the amino/Pd interface are downfield-shifted. In addition, the N-H proton is upfield-shifted from 2.67 to 2.33 ppm because the free surface electrons of Pd atoms adjacent to the amino functionality shield the amino protons by increasing the electron cloud density. Interestingly, the C18-NH2-PdNPs (5.2 nm) produced both proton peaks c, c′ and d, d′ (spectrum B), inferring that a bilayer of Pd-bound and free ligands exists in this sample. The ratio of the peak area for C18-NH2 is peaks

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Figure 6. 1H NMR spectra of (A) free C18-NH2 ligand and (B) C18-NH2-PdNPs (5.2 nm). The ratios of the integrated peak area for C18-NH2 are peaks a:b:c:d ) 3:32:2:2 and C18-NH2-PdNPs a:b:c: c′:d:d′ ) 3:32:1.5:2:1.5:1.

Figure 7. Thermogravimetric curves of various core sizes of C18-NH2-PdNPs: (a) 6.5, (b) 5.6, and (c) 5.2 nm. The weight losses are 44.9, 55.1, and 82.9%, respectively, and the onset desorption temperatures are 203, 214, and 163 °C, respectively The inset displays the thermogravimetric curve of pure C18-NH2.

a:b:c:d ) 3:32:2:2 which is in agreement with the chemical structure of the free ligand. The ratio of the peak area for C18-NH2-PdNPs (5.2 nm) is peaks a:b:c:c′:d:d′ ) 3:32:1.5: 2:1.5:1, indicating that the quantity of the first layer C18-NH2 on PdNPs is larger than the second layer C18-NH2. The surfaceattached ligand C18-NH2 (the first layer) seems to contain only one amino proton. In summary, the 1H NMR data further confirm the successful attachment of C18-NH2 to the Pd core. Thermogravimetric Analysis and Atomic Absorption Spectrometry. TGA is a common analytical technique to determine the organic weight fraction of ligand stabilized metal NPs and to study the thermostability of metal NPs at higher temperatures. TGA of PdNPs can cause thermal desorption of the Cn-NH2 ligand as volatile Cn-NH2, leaving Pd as the residual mass. Thus, the attached Cn-NH2 ligand component can be measured in weight loss and residual Pd core mass. In general, smaller metal NPs have larger organic mass fractions.33 Figure 7 displays the TGA of C18-NH2-PdNP samples synthesized with 1:5, 1:7, and 1:9 initial PdCl2/C18-NH2 mole ratios, respectively. The inset of Figure 7 shows the TGA curve of pure free ligand C18-NH2 for comparison. It is obvious that the TGA curves of C18-NH2-PdNPs are broader while the free ligand C18-NH2 is steeper. Qualitatively, one expects that the weight fraction of C18-NH2 in the PdNPs should parallel the NP core size, and this expectation is realized in the TGA results. The weight losses are 44.9, 55.1, and 82.9% (comprising the first 67.7% and second layers 15.2% of ligands; vide infra) for the 6.5, 5.6, and 5.2 nm size PdNPs, respectively, indicating

Li et al. that larger PdNPs have smaller C18-NH2 ligand weight fractions. Our TGA results are in agreement with the literature that smaller NPs have larger ligand mass fractions.34 On the other hand, the very large weight fraction for PdNPs synthesized from 1:9 PdCl2/C18-NH2 is unexpected, possibly attributed to the formation of multilayer ligands on the Pd metal cores. The ElSayed35 and Murphy36 groups have also reported that a bilayer of surfactants (ligands) can cap the surface of the AuNPs. Here, the C18-NH2 molecule possibly forms a compact bilayer on the PdNP surface in a head-tail-tail-head arrangement: the amine head groups of the first layer C18-NH2 molecules are strongly attached to the PdNPs with the tail C18 groups extended far from the surface which interact with the tail C18 groups of the second layer C18-NH2 molecules by the van der Waals forces. The quantity of the first layer C18-NH2 on PdNPs is larger than the second layer C18-NH2 (vide supra). When the mole ratio of PdCl2/C18-NH2 for synthesizing the PdNPs is too small, excess ligands will then deposit on the monolayerprotected PdNPs, resulting in formation of a bilayer ligand and a very large organic weight fraction. All C18-NH2 ligands of the PdNP samples are completely desorbed at approximately 420-450 °C, which is reasonable and in agreement with the literature27 as the boiling point of C18-NH2 is 346.8 °C.37 The corresponding onset desorption temperatures of the C18-NH2 ligands for the 6.5, 5.6, and 5.2 nm PdNPs are 203, 214, and 163 °C, respectively. The desorption temperature increases with the decrease in particle size, inferring that the interaction of C18-NH2-PdNP depends on the Pd core size, which is in good agreement with the thiolate-protected AuNPs that smaller AuNPs possess higher thermal desorption temperatures and stronger RS-Au bonds.38 The abnormal low desorption temperature for 5.2 nm PdNPs is possibly attributed to the bilayer ligand effect. Unfortunately no distinct multistep ligand desorption of the TGA curve was observed for the 5.2 nm PdNPs (Figure 7c). However, if the calculation of the ligand percent for the 5.2 nm PdNP is approximately based on the desorption temperature of the 5.6 nm PdNP, the weight losses for the first and second and/or further layers will be around 67.7 and 15.2%, respectively. These results are more consistent with the change in the weight percent of the monolayer ligand on the PdNPs, i.e., 44.9% for 6.5 nm, 55.1% for 5.6 nm, and 67.7% for 5.2 nm PdNPs. Collectively, the TGA data support the concept that higher initial PdCl2/ C18-NH2 mole ratio will harvest a larger core size of PdNP products.39 Finally, the weight percents of Pd in C18-NH2-PdNP samples were determined by GFAAS (graphite furnace atomic absorption spectroscopy) as 55, 45, and 17% for the 6.5, 5.6, and 5.2 nm PdNPs, respectively. Our AAS data are in complete agreement with the TGA data. Mass Spectrometry. Mass spectrometry is a valuable tool in evaluating the mass of NPs. Figure 8 displays the MS of C18-NH2-PdNPs synthesized with the 1:5, 1:7, and 1:9 PdCl2/ C18-NH2 mole ratios. Broad mass peaks are obtained at ∼36.9, ∼28.1, and ∼23.5 kDa for PdNPs synthesized with 1:5, 1:7, and 1:9 PdCl2/C18-NH2 mole ratios, respectively. Unfortunately the exact mass of PdNPs cannot be determined since the mass peaks are determined by the mass to charge ratio (m/z) of PdNPs. The charges (z) of PdNPs are not known for our PdNP samples via MALDI. However, the empirical formula for C18NH2PdNPs can be represented as [Pdx(C18-NH2)y]z, where z is an integer and x and y are the numbers of Pd atom and surfaceattached C18NH2 ligand, respectively. Combining the major mass peaks and the TGA results of the PdNP samples, we can deduce that the average compositions of our 6.5, 5.6, and 5.2 nm


Figure 8. (A) MALDI-TOF mass spectra of C18-NH2-PdNP samples synthesized with (a) 1:5, (b) 1:7, and (c) 1:9 PdCl2/C18-NH2. The m/z ratios are 36.9, 28.1, and 23.5 kDa, respectively.

C18NH2-PdNPs are approximately [Pd191(C18NH2)62]z, [Pd119(C18NH2)57]z, and [Pd84(C18NH2)54]z, respectively. Detailed calculation of the empirical formula is shown in the Supporting Information. Electrochemical Study of PdNPs and Response to Methane. Metal NPs and in particular PdNPs have found applications in catalysis due to their high specific surfaces. Since CH4 is one of the most important analyte gases to be monitored and determined in coal mines and our environment, it will be important to study the electrochemical oxidation of CH4 on PdNP modified Pd electrode at room temperature. Since most of the previous research11-17 does not have any information on the use of PdNPs for electrocatalytic oxidation of CH4, the effect of PdNPs capped with various Cn-NH2 on the electrocatalytic oxidation of CH4 is investigated in this work. Before that, the electrochemical behavior of the redox couple Fe(CN)63-/ Fe(CN)64- on the PdNP modified Pd electrodes was also studied (Figure S1 of the Supporting Information). It was found that the PdNP modified Pd electrodes are electroactive to the redox couple Fe(CN)64-/Fe(CN)63- but the charge-transfer rate is lower than that of unmodified Pd electrode. It seems that the Cn-NH2 attached on PdNPs has a blocking effect on the charge transfer of Fe(CN)64-/Fe(CN)63- anions.40 However, this is not the case for a more hydrophobic molecule such as CH4. Figure 9 depicts the CVs of the (A) bare and (B-F) Cn-NH2-PdNP modified Pd electrodes in 0.50 M H2SO4 saturated with either N2 or CH4 gases. All the CV scans were performed from -0.2 to +1.40 V and then back to -0.2 V. When the electrolytes were deaerated by a stream of N2 gas, a very broad anodic peak at +0.02 V (vs Ag/AgCl) and a small anodic peak at +1.00 V for the bare Pd electrode were observed (Figure 9A(a)) corresponding to the hydrogen (H2) desorption/ electrochemical oxidation at -0.1 V and Pd oxidation at about 0.85 V on the Pd electrode surface, respectively, which are consistent with the literature results.24b By contrast, the anodic H2 peak is not profound or even not observed at Cn-NH2-PdNP modified Pd electrodes, implying that adsorption and desorption of H2 on Cn-NH2-PdNPs are inhibited by the presence of CnNH2 ligands on the PdNPs. The small Pd oxidation peaks (PdO and/or PdO2) were observable between +0.60 and +1.15 V for these Cn-NH2-PdNP modified Pd electrodes. When the potentials were switched from +1.40 to -0.20 V in the reverse scan, all the Cn-NH2-PdNP modified Pd electrodes displayed

J. Phys. Chem. C, Vol. 114, No. 2, 2010 729 reduction peaks at around +0.15-0.40 V. For the C6-NH2PdNP (prepared with 1:7 PdCl2/C6-NH2) and C18-NH2-PdNP (prepared with 1:5 and 1:9 PdCl2/C18-NH2) modified Pd electrodes (Figure 9B, D, and F), the sharp reduction peaks at 0.20 V correspond to the reduction of Pd(II) to Pd. For the C12-NH2- and C18-NH2-PdNP (1:7 PdCl2/ligand mole ratio) modified Pd electrodes (Figures 9C and E), the cathodic peaks were very broad, attributed to the overlap of the H+ and PdO/ PdO2 reduction steps.41 Interestingly, the C18-NH2-PdNP (from 1:7 PdCl2/C18-NH2) modified Pd electrode exhibited large redox peaks (Figure 9E(a)) under N2 bubbling. For instance, two anodic peaks at a1 and a2 were observed corresponding to the formation of PdO and PdO2, and two cathodic peaks at c1 and c2 were the electrochemical reduction of palladium oxides that formed at the previous anodic scan.42 When the electrolytes were saturated by a stream of CH4 gas, there was no electrocatalytic oxidation of CH4 on bare Pd and C6-NH2-PdNP modified Pd electrodes (Figure 9A(b) and B(b)) but some electrocatalytic oxidation of CH4 on the other PdNP modified Pd electrodes were observed (Figure 9C-F(b)). Among the Cn-NH2-PdNP modified Pd electrodes, the C18-NH2-PdNPs (prepared with 1:7 PdCl2/C18-NH2) produced a stronger and broader CH4 oxidation peak in the CV curve at around 0.68 V (vs Ag/AgCl) in the forward sweep and a large broad reduction peak at -0.06 V (Figure 9E(b)). There is no change of this CV curve after 30 cycles of voltammetric scans (Figure S2 of the Supporting Information), indicating that the C18-NH2-PdNP (5.6 nm) modified Pd electrode is very stable. The C18-NH2-PdNPs (55 µg) attached on the surface of the Pd electrode forms a stable film. They do not leach out into the 0.50 M H2SO4 solution since they are very hydrophobic and insoluble in 0.50 M H2SO4 aqueous solution. The electroactive area of this C18-NH2-PdNP modified Pd electrode is 48.07 mm2 which is almost 7 times the area of the bare Pd electrode (7.07 mm2). Detailed calculation of the electroactive area is displayed in the Supporting Information. Several oxidation and reduction peaks were overlapped in the CV curve, implying that the electrochemical oxidation of CH4 by PdNPs went through multiple steps. The broad oxidation and reduction peaks were presumably corresponding to the oxidation/reduction of the adsorbed species (CH4).24b The amperometric response of C18-NH2-PdNP (5.6 nm) modified Pd electrode in 0.50 M H2SO4 to N2 and CH4 gases is reproducible (Figure S3 of the Supporting Information). The CV did not change much in N2, then in CH4, and then back in N2 gases again. Similarly, smaller oxidation and reduction peaks (Figure 9C, D, and F(b)) occurred on C12-NH2-PdNPs (synthesized with 1:7 PdCl2/C12-NH2) and C18-NH2-PdNPs (synthesized with 1:5 and 1:9 PdCl2/ C18-NH2). The slopes of their CV curves are steeper than that of bare Pd electrode, indicating that Cn-NH2-PdNPs can improve the conductivity of the modified electrodes by an electron hopping mechanism through the surface electrode.28 A much smaller CH4 oxidation peak was found on C6-NH2PdNP modified Pd electrode (Figure 9B(b)). Our results demonstrate that a shorter carbon chain length (e.g., C6) of the stabilizing ligand will attenuate CH4 oxidation. When the carbon chain length of Cn-NH2 increases to C12 and C18, a facile CH4 oxidation takes place on the PdNPs. Among the three types of C18-NH2-PdNPs, PdNPs synthesized with 1:7 PdCl2/C18-NH2 generated the highest CH4 oxidation current. It is understandable that PdNPs synthesized with 1:9 PdCl2/C18-NH2 have a bilayer of ligands (vide supra). As such, the access of reactant molecules (CH4) to the NP core will be restricted due to the thicker layers of the surface-attached ligands,18 thus resulting in lower

730


Li et al.

Figure 9. Cyclic voltammograms of (A) bare Pd electrode, (B) C6-NH2-PdNP (20 nm), (C) C12-NH2-PdNP (6.0 nm), (D) C18-NH2-PdNP (6.5 nm), (E) C18-NH2-PdNP (5.6 nm), and (F) C18-NH2-PdNP (5.2 nm) modified Pd electrodes obtained in (a) N2- and (b) CH4-saturated 0.50 M H2SO4. The electrochemical cell was deaerated by N2 or CH4 bubbling for 15-30 min prior to each CV scan, and a blanket of N2 or CH4 was maintained throughout the experiments. The scan rate is 20 mV/s.

sensitivity to CH4 detection. In summary, the carbon chain length and the amount of Cn-NH2 ligands on the surfaces of PdNPs have significant impacts on the Pd catalytic surface activity. If the carbon chain length increases, the hydrophobic reactants

(CH4) will be better adsorbed on the Cn-NH2-PdNPs and thus have high sensitivity. When the amount of Cn-NH2 attached to the PdNPs is excessive, it will limit the accessibility of reactants to the Pd catalytic surface and thereby attenuate the



Figure 10. (A) Effect of scan rate V at (a) 10, (b) 20, (c) 50, (d) 80, and (e) 100 mV/s on C18-NH2-PdNP (5.6 nm) modified Pd electrode in CH4-saturated 0.50 M H2SO4. (B) Plots of Ip against V1/2 and (C) Ep against log V.

electrocatalytic activity of the PdNPs. The excess ligand C18-NH2 on PdNP surface can possibly slow the electron transfer of NP. Besides the ligand effect, CH4 oxidation can be affected by the size of PdNPs. It was observed that the smaller the PdNPs, the larger the CH4 oxidation current, i.e., the anodic current generated on the PdNPs with size 5.6 > 6.0 > 6.5 > 20 nm. But the smallest PdNPs (5.2 nm) did not fit in the particle trend because the multilayer ligands effect on the PdNPs hindered the accessibility of reactants to the Pd catalytic surface, as explained earlier. In essence, the response of the Cn-NH2PdNPs to CH4 oxidation is mainly governed by the CH4 adsorption onto the carbon chain, diffusion to the PdNP surface, and the surface-to-volume ratio of PdNPs. Since smaller PdNPs stabilized by a monolayer of C18-NH2 exhibited better electrochemical activity, the 5.6 nm C18-NH2-PdNPs was chosen for further study. Various volumes of 5.6 nm sized C18-NH2-PdNPs solution (5.0 mg/mL) were employed to fabricate PdNP modified Pd

electrodes, and the CH4 oxidation currents at 0.68 V on these electrodes are displayed in Figure S4 (Supporting Information). When the amount was small, the current was low. The current increased with the increase in the amount of PdNPs. When the amount was above 11 µL, the oxidation current dropped, indicating that thicker deposited layers on the Pd electrode surface could reduce oxidation current. As such, 11 µL of 5.0 mg/mL C18-NH2-PdNP solution was chosen for fabrication of PdNP modified Pd electrode. Figure 10A displays the CVs of C18-NH2-PdNP (5.6 nm) modified Pd electrode in a CH4 saturated 0.50 M H2SO4 solution at various scan rates V: 10, 20, 50, 80, and 100 mV/s. A linear relationship was found between the CH4 oxidation peak current and V1/2, as shown in Figure 10B, inferring that the oxidation of CH4 could be a diffusion-controlled process. In addition, the peak potential (Ep) increased with the increase in V and also a linear relationship was obtained between Ep and log V, as depicted in Figure 10C. This result shows that the oxidation of CH4 on the PdNP modified Pd electrode is an irreversible

732


Li et al. clearly the good electrocatalytic activity for CH4 oxidation and the stability of the PdNP modified Pd electrode. In essence, PdNPs prepared with 1:7 PdCl2/C18-NH2 mole ratio is an ideal nanomaterial for fabrication of PdNP modified Pd electrode to detect CH4 at ambient conditions. The preparation of this PdNP modified Pd electrode is reproducible since the RSD (relative standard deviation) of the response to CH4 for five separate PdNP modified Pd electrodes is 9.60%. Our proposed C18-NH2-PdNP modified Pd electrode are reusable and can produce stable and reproducible response to CH4 (99.1% of the initial current intensity) even after 6 months of storage. Conclusion

Figure 11. Typical current-time transient curves obtained from (a) bare Pd electrode and (b) C18-NH2-PdNP (5.6 nm) modified Pd electrode in CH4-saturated 0.50 M H2SO4.

process. For an irreversible charge-transfer electrode process, the plot of Ep against log V should be a straight line with the slope ) RT/2RnF, where R and n are the electron-transfer coefficient and number, respectively. R, T, and F are the usual physical constants. Since the slope of the linear curve in Figure 10C is 154, the Rn value is calculated as 0.08. The relationship between the peak current (Ip) and V is43

Ip ) 0.4958 × 10-3nF3/2(RT)-1/2(Rn)1/2ACD1/2V1/2 where A is the electrode area (cm2) and D ) 2.2 × 10-5 cm2 s-1. The electron-transfer number n was determined to be 4.13 ≈ 4. It is reported that CH3OH can be oxidized at the PdNP or Pd nanoflower electrodes.41,44 It is possible that CH4 on the PdNP is oxidized to CH3OH first and then to HCHO by the electrogenerated PdO/PdO2. Therefore, the CH4 oxidation reaction on the C18-NH2-PdNP modified Pd electrode can be summarized as

CH4 + PdO f CH3OH + Pd

(or

CH4 + PdO2 f CH3OH + PdO)

CH3OH + PdO f HCHO + H2O + Pd (or CH3OH + PdO2 f HCHO + H2O + PdO) Because the CH4 oxidation reaction is multistep, a broad anodic peak results, as shown in Figure 9E(b). The formation of unstable HCHO can explain why the cathodic scan wave is multiple and the overall reaction is irreversible. In fact, CH3OH also gave broad anodic and cathodic peaks at similar potentials on the C18-NH2-PdNP modified Pd electrode, as shown in Figure S5 (Supporting Information). It is very obvious that the oxidation of CH4 must produce CH3OH as an intermediate product. Further work to elucidate the detailed oxidation mechanisms will be pursued in the near future. Figure 11 displays the typical current-time transient curves for the bare Pd and C18-NH2-PdNP (5.6 nm) modified Pd electrodes in 0.50 M H2SO4 saturated with CH4. Curve a represents the current-time curve of the bare Pd electrode, while curve b is the C18-NH2-PdNP modified Pd electrode. As expected, the initial and limiting currents of PdNP modified Pd electrode were higher than that of the bare Pd electrode during CH4 oxidation. Curve b exhibits much larger currents, showing

We have demonstrated a facile procedure for controllable synthesis of PdNPs with Cn-NH2 as stabilizing ligands. The main attribute of these small NPs is that they are protected by long n-alkyl chain amines which preserve their electrocatalytic activity. The as-prepared nanomaterial has been well-characterized by TEM, XRD, UV-visible, IR, XPS, 1H NMR, TGA, AAS, and MS techniques. The results show that the response to CH4 is principally related to the carbon chain length of the stabilizing ligand. The electrocatalytic activity to CH4 oxidation is also correlated to the particle size of NP which can be tuned by varying the mole ratio of PdCl2/ligand. However, a balance has to be sought so that the Pd core is only protected by a monolayer of ligands. In our work, the 1:7 PdCl2/C18-NH2 mole ratio produced a smaller size PdNP which exhibits a faster, more active and stable response to CH4 as compared to the bare Pd and other PdNP modified Pd electrodes. Our proposed C18-NH2-PdNPs are armed with the advantages of hydrophobic surface and high activities of NPs, which should be promising for applications in various fields such as electrocatalysts and gas sensors. Acknowledgment. We express our sincere thanks to Mr. Guojin Li (China Research Institute of Daily Chemical Industry) for taking the TEM images, Mr. Qiang Zhao (College of Chemical Engineering, Taiyuan University of Technology) for obtaining the XRD patterns, Mr. Xu Wu (College of Chemical Engineering, Taiyuan University of Technology) for conducting the TGA, Ms. Silvia T. Mo (Department of Chemistry, Hong Kong Baptist University) for obtaining the MALDI-TOF MS of PdNPs, and Ms. Y. K. Wu (Centre for Surface Analysis and Research, Hong Kong Baptist University) for the XPS measurements. The work described in this article was supported by grants from the Key Project of the National Natural Science Foundation of China (50534100) and 2008 Undergraduate Creative Activities Support Fund of Taiyuan City (08122055). Financial support from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. HKBU 2006/06P) is also gratefully acknowledged. Supporting Information Available: Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Badetti, E.; Caminade, A.-M.; Majoral, J.-P.; Moreno-Manãs, M.; Sebastiań, R. M. Langmuir 2008, 24, 2090. (2) Naka, K.; Sato, M.; Chujo, Y. Langmuir 2008, 24, 2719. (3) Mizukoshi, Y.; Okitsu, K.; Maeda, Y.; Yamamoto, T. A.; Oshima, R. J. Phys. Chem. B 1997, 101, 7033. (4) Cardenas, T. G.; Munoz, D. C.; Vera, L V. Bol. Soc. Chil. Quim. 1996, 41, 235. (5) Teranishi, T.; Miyake, M. Chem. Mater. 1998, 10, 594. (6) Giorgio, S.; Chapon, C.; Henry, C. R. Langmuir 1997, 13, 2279.

PdNPs with Stabilizing Ligands Cn-NH2 (7) Schmid, G.; Harms, M.; Malm, J.-O.; Bovin, J.-O.; van Ruitenbeck, J.; Zandbergen, H. W.; Fu, W. T. J. Am. Chem. Soc. 1993, 115, 2046. (8) Reetz, M. T.; Quaiser, S. A. Angew. Chem., Int. Ed. 1995, 34, 2240. (9) Li, F.; Zhang, B.; Dong, S.; Wang, E. Electrochim. Acta 1997, 42, 2563. (10) Bradley, J.-C.; Ma, Z. Angew. Chem., Int. Ed. 1999, 38, 1663. (11) Son, S. U.; Jang, Y.; Yoon, K. Y.; Kang, E.; Hyeon, T. Nano Lett. 2004, 4, 1147. (12) Oh, S.-K.; Niu, Y.; Crooks, R. M. Langmuir 2005, 21, 10209. (13) Ye, H.; Scott, R. W. J.; Crooks, R. M. Langmuir 2004, 20, 2915. (14) Oh, S.-K.; Kim, Y.-G.; Ye, H.; Crooks, R. M. Langmuir 2003, 19, 10420. (15) (a) Chen, S.; Huang, K.; Stearns, J. A. Chem. Mater. 2000, 12, 540. (b) Zamborini, F. P.; Gross, S. M.; Murray, R. W. Langmuir 2001, 17, 481. (c) Shen, C. M.; Su, Y. K.; Yang, H. T.; Yang, T. Z.; Gao, H. J. Chem. Phys. Lett. 2003, 373, 39. (d) Zelakiewicz, B. S.; Lica, G. C.; Deacon, M. L.; Tong, Y. Y. J. Am. Chem. Soc. 2004, 126, 10053. (e) Mei, Y.; Lu, Y.; Polzer, F.; Ballauff, M. Chem. Mater. 2007, 19, 1062. (f) Iqbal, M.; McLachlan, J.; Jia, W.; Braidy, N.; Botton, G.; Eichhorn, S. H. J. Therm. Anal. Calorim. 2009, 96, 15. (16) (a) Narayanan, R.; El-Sayed, M. A. J. Am. Chem. Soc. 2003, 125, 8340. (b) Yang, Z.; Klabunde, K. J. J. Organomet. Chem. 2009, 694, 1016. (17) (a) Liu, J.; He, F.; Durham, E.; Zhao, D.; Roberts, C. B. Langmuir 2008, 24, 328. (b) Karousis, N.; Tsotsou, G.-E.; Evangelista, F.; Rudolf, P.; Ragoussis, N.; Tagmatarchis, N. J. Phys. Chem. C 2008, 112, 13463. (18) Shen, C.; Hui, C.; Yang, T.; Xiao, C.; Tian, J.; Bao, L.; Chen, S.; Ding, H.; Gao, H. Chem. Mater. 2008, 20, 6939. (19) Tsuji, N.; Nozawa, K.; Aramaki, K. Corros. Sci. 2000, 42, 1523. (20) (a) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723. (b) Meulenberg, R. W.; Bryan, S.; Yun, C. S.; Strouse, G. F. J. Phys. Chem. B 2002, 106, 7774. (c) Li, Y.; Li, X. H.; Yang, C.; Li, Y. F. J. Phys. Chem. B 2004, 108, 16002. (d) Feng, Y. Y.; Chen, S.; You, J.; Guo, W. Electrochim. Acta 2007, 53, 1743. (e) Brink, M. V.; Peck, M. A.; More, K. L.; Hoefelmeyer, J. D. J. Phys. Chem. C 2008, 112, 12122. (21) Rao, C. R. K.; Lakshminarayanan, V. Mater. Lett. 2006, 60, 3165. (22) Yang, J.; Lee, J. Y.; Too, H.-P. Anal. Chim. Acta 2007, 588, 34. (23) Yu, L.; Jin, X.; Zeng, X. Langmuir 2008, 24, 11631. (24) (a) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881. (b) Hahn, F.; Melendres, C. A. Electrochim. Acta 2001, 46, 3525. (25) Henglein, A.; Giersig, M. J. Phys. Chem. B 2000, 104, 6767. (26) Vasilyeva, S. V.; Vorotyntsev, M. A.; Bezverkhyy, I.; Lesniewska, E.; Heintz, O.; Chassagnon, R. J. Phys. Chem. C 2008, 112, 20269.

J. Phys. Chem. C, Vol. 114, No. 2, 2010 733 (27) Coronado, E.; Ribera, A.; Garcıá-Martıńez, J.; Linares, N.; LizMarzań, L. M. J. Mater. Chem. 2008, 18, 5682. (28) Iban˜ez, F. J.; Zamborini, F. P. J. Am. Chem. Soc. 2008, 130, 622. (29) Cioffi, N.; Torsi, L.; Sabbatini, L.; Zambonin, P. G.; Bleve-Zacheo, T. J. Electroanal. Chem. 2000, 488, 42. (30) Campell, M. L. Chem. Phys. Lett. 2002, 365, 361. (31) Liu, J.; Cheng, L.; Song, Y.; Liu, B.; Dong, S. Langmuir 2001, 17, 6747. (32) Aslam, M.; Schultz, E. A.; Sun, T.; Meade, T.; Dravid, V. P. Cryst. Growth Des. 2007, 7, 471. (33) (a) Cha, S.-H.; Kim, J.-U.; Kim, K.-H.; Lee, J.-C. Chem. Mater. 2007, 19, 6297. (b) Ganesan, M.; Freemantle, R. G. Chem. Mater. 2007, 19, 3464. (34) (a) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17. (b) Naka, K.; Itoh, H.; Tampo, Y.; Chujo, Y. Langmuir 2003, 19, 5546. (35) Nikoobakht, B.; El-Sayed, M. A. Langmuir 2001, 17, 6368. (36) (a) Sau, T. K.; Murphy, C. J. Langmuir 2005, 21, 2923–2929. (b) Alkilany, A. M.; Murphy, C. J. Langmuir 2009, 25, 13874. (37) Lide, D. R., Frederiks, H. P. R., Eds. CRC Handbook of Chemistry and Physics, 76th ed.; CRC Press: Boca Raton, FL, 1995; pp 3-226. (38) Zhang, Y.; Shuang, S.; Dong, C.; Lo, C. K.; Paau, M. C.; Choi, M. M. F. Anal. Chem. 2009, 81, 1676. (39) (a) Liu, J.; Ong, W.; Romań, E.; Lynn, M. J.; Kaifer, A. E. Langmuir 2000, 16, 3000. (b) Liu, J.; Alvarez, J.; Ong, W.; Romań, E.; Kaifer, A. E. J. Am. Chem. Soc. 2001, 123, 11148. (c) Huang, T.; Murray, R. W. J. Phys. Chem. B 2001, 105, 12498. (40) Corthey, G.; Rubert, A. A.; Benitez, G. A.; Fonticelli, M. H.; Salvarezza, R. C. J. Phys. Chem. C 2009, 113, 6735. (41) Pan, W.; Zhang, X.; Ma, H.; Zhang, J. J. Phys. Chem. C 2008, 112, 2456. (42) Jia, F.; Wong, K.-W.; Zhang, L. J. Phys. Chem. C 2009, 113, 7200. (43) (a) Bard, A. J., Faulker, L. R., Eds. Electrochemical Methods; Chemical Industry Press: Beijing, 1984; pp 195-258. (b) Wang, S. Enchiridion of Petrochemistry; Chemical Industry Press: Beijing, 2002; p 873. (44) Yin, Z.; Zheng, H.; Ma, D.; Bao, X. J. Phys. Chem. C 2009, 113, 1001.

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Synthesis and Characterization of n-Alkylamine-Stabilized Palladium

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