Influence of Speciation of Aqueous HAuCl4 on the Synthesis

Mar 26, 2009 - Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of Materials for Energy Conversion and Department...
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J. Phys. Chem. C 2009, 113, 6505–6510

6505

Influence of Speciation of Aqueous HAuCl4 on the Synthesis, Structure, and Property of Au Colloids Shu Wang,†,‡ Kun Qian,† XingZhen Bi,† and Weixin Huang*,† Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of Materials for Energy ConVersion and Department of Chemical Physics, UniVersity of Science and Technology of China, Jinzhai Road 96, Hefei 230026, China, and Department of Chemistry, Huangshan UniVersity, Jilingshan Road 9, Huangshan 245041, China ReceiVed: December 21, 2008; ReVised Manuscript ReceiVed: February 26, 2009

We have studied the pH-dependent speciation of aqueous HAuCl4 and its influences on the synthesis, structure, and property of Au colloids. Aqueous HAuCl4 consists of [AuClx(OH)4-x]- (x g 2) at low pH but [AuClx(OH)4-x]- (x < 2) at high pH. By employment of ascorbic acid as the reducing agent and sodium benzenesulfonate (SDBS) as the protecting agent, reduction of aqueous HAuCl4 at low pH leads to the synthesis of well-dispersed and uniform fine Au colloids, whereas that at high pH forms large Au colloids and ensembles of fine Au colloids. These large Au colloids and ensembles of fine Au colloids exhibit strong surface plasmon resonance in the near-infrared region. The SDBS molecules bind to the surface of Au colloids through the S element, and the charge transfer from Au atoms to S elements occurs. The charge is localized around Au atoms directly interacting with SDBS for fine Au colloids but delocalized to the entire Au colloid for large Au colloids and ensembles of fine Au colloids. 1. Introduction Gold colloids have attracted great interest because of their unique physical and chemical properties, which render them ideal candidates in the fields of photonics, optoelectronics, sensor, information storage, catalyst, biolabeling, magnetic ferrofluids, and medicine.1 Very interestingly, some properties of gold colloids, such as surface plasmon resonances (SPR)2 and catalytic activity3 depend sensitively on their size and shape; therefore, great efforts have been devoted to the controllable synthesis of Au colloids, in which aqueous HAuCl4 is the most commonly used Au precursor. The pH of aqueous HAuCl4 usually varies in different approaches for the synthesis of Au colloids; however, the speciation of aqueous HAuCl4 affected by the pH has seldom been considered as a factor influencing the nucleation and growth of Au colloids. Randa et al. studied the synthesis of Au colloids by H2O2 reduction of aqueous HAuCl4 at pH ranging from 2.9 to 12 and obtained Au nanoaprticles with different morphologies, which was attributed to the pH-dependent reduction ability of H2O2.4 Very recently Brin˜as et al. developed a method in preparing size-controllable Au colloids capped with glutathione by varying the pH before reduction, and they proposed a mechanism based on the formation of polymeric nanoparticle precursor, Au(I) glutathione polymers, which change size and density depending on the pH.5 The speciation of aqueous HAuCl4 as a function of pH was previously studied by means of ultraviolet/visible adsorption and Raman/resonance Raman spectroscopies aiming to elucidate the mechanisms of Au transport and deposition related to the formation of Au deposits in Geology.6,7 Recently, the pHdependent speciation of aqueous HAuCl4 have been frequently discussed during the course of preparing supported Au catalysts. * To whom correspondence should be addressed. Fax: +86-551-3600437. E-mail: [email protected]. † University of Science and Technology of China. ‡ Huangshan University.

Grisel et al. observed that the pH value of aqueous HAuCl4 strongly affects the loading of Au in Au/Al2O3 catalysts and attributed this to the hydrolysis of [AuCl4]- as pH was raised.8 Yang et al. employed extended X-ray absorption fine structure (EXAFS) to study the deposition-precipitation process for the preparation of Au/Al2O3 catalysts from aqueous HAuCl4 and observed that the Au3+ ion containing 4 Au-Cl bonds decreased with increasing pH of aqueous HAuCl4.9 Moreau et al. calculated the relative equilibrium concentration of gold complex as a function of pH of aqueous HAuCl4 with a constant Clconcentration of 2.5 × 10-3 mol L-1.10 Although it is known that HAuCl4 undergoes pH-dependent hydrolysis, the unambiguous speciation of aqueous HAuCl4 as a function of pH still lacks, particularly at high pH. Meanwhile, the nature of the gold species in solution participating chemical reactions must exert a critical influence on the reaction process. In this paper, we employed ion chromatography to examine the speciation of aqueous HAuCl4 as a function of pH. The concentration of free Cl- in the aqueous solution as the product of HAuCl4 hydrolysis was quantitatively determined by ion chromatography, which thus enabled us to determine the speciation of aqueous HAuCl4 at different pH. We also investigated the reduction of aqueous HAuCl4 at various pH values by ascorbic acid (VC) in the presence of sodium benzenesulfonate (SDBS) and found that the speciation of has great influence on the size and thus properties of Au colloids. Au colloids exhibiting the localized surface plasmon resonance in the near-infrared (NIR) region were facilely synthesized at high pH. The size dependent Au-SDBS interactions were also observed and discussed. 2. Experimental Section A HAuCl4 aqueous solution, whose concentration was measured to be 8.905 × 10-4 mol L-1 by inductively coupled plasmon atomic emission spectrometry (ICP-AES), was prepared by employing HAuCl4 · 4H2O (Au content, 47.8%). The pH of

10.1021/jp811296m CCC: $40.75  2009 American Chemical Society Published on Web 03/26/2009

6506 J. Phys. Chem. C, Vol. 113, No. 16, 2009 HAuCl4 aqueous solution was measured to be 2.91. HAuCl4 aqueous solutions with various pH values were prepared by the addition of certain amounts of NaOH aqueous solution (concentration of 6.166 × 10-3 mol L-1 measured by ICP-AES) into 10 mL of HAuCl4 aqueous solution. The acquired HAuCl4 aqueous solution was stirred and stabilized for 4-8 days until the measured pH value varied within (0.01. A series of NaCl aqueous solutions with different concentrations measured by ICP-AES were prepared and used to plot the working curve of ion chromatography for Cl-. Chemicals with an analytical reagent grade and triply distilled water were used in the experiments. In a typical experiment for the synthesis of Au colloids, 5 mL of 8.905 × 10-4 mol L-1 HAuCl4 aqueous solution was adequately mixed with 12 mL of 3.3 × 10-2 mol L-1 SDBS aqueous solution, and the pH value of the aqueous solution was adjusted to the desired value. Then 8 mL of 3.5 × 10-4 mol L-1 VC aqueous solution was added dropwise into the above solution. The system was then stirred for 24 h. All performances were carried out in the absence of light. The pH of aqueous solutions was measured with a DELTA320 pH meter. The ICP-AES measurements were performed on an Atomscan Advantage inductively coupled plasmon atomic emission spectrometer. The UV-vis absorption spectra of aqueous HAuCl4 solutions at various pH values were obtained on a UV-2450 UV-visible spectrophotometer. The ion chromatography experiments were performed on a DX-120 ion chromatography with an AS14 column employing Na2CO3 (3.5 × 10-3 mol L-1) NaHCO3 (1.0 × 10-3 mol L-1) solution as the mobile phase at a flow rate of 1.2 mL min-1. The AS14 column could separate anions except OH-. Prior to the ion chromatography experiment, the HAuCl4 aqueous solution was filtered by a 0.45 µm micropore filtration membrane. The acquired ion chromatogram data was fitted using the XPSPEAK41 software. The UV-vis absorption spectra of Au colloidal solutions were recorded on a DUV-3700 UV-vis-NIR recording spectrophotometer. XPS measurements were performed on an ESCALAB 250 high performance electron spectrometer using nonmonochromatized Al KR excitation source (hν ) 1486.6 eV). The sample for XPS measurements was prepared by repeated cycles of dripping-evaporation of centrifuged aqueous solution containing Au colloids in a fine and shallow hole on a Cu plate. The binding energies in XPS spectra were referenced with respect to the C 1s binding energy of adventitious carbon at 284.5 eV. TEM experiments were performed on a JEOL-2010 high-resolution transmission electron microscope. Dynamic light scattering (DLS) experiments were conducted with a DynaPro-MS800 laser dynamic light scattering photometer. The cyclic voltammograms were acquired at a scan rate of 100 mv s-1 on a CHI 660C Electrochemical workstation. A three-electrode system was used in the measurements with a glassy carbon electrode (GCE) as the working electrode, a saturated calomel electrode (SCE) as the reference electrode, and a platinum wire as the auxiliary electrode. Aqueous HAuCl4 at various pH was used as the electrolyte. 3. Results and Discussion Aqueous HAuCl4 at various pH values show different UV-vis absorption spectra (Figure S1 of Supporting Information). At pH ) 2.91 the peak maximum occurs at 306 nm, which could be assigned to two unresolved ligand (π)-to-metal (σ*) charge transfer (LMCT) transitions in [AuCl4]-.5 With the pH increased to 3.39, the peak blue-shifts to 293 nm accompanied by the reduction of the absorption intensity. The peak continu-

Wang et al.

Figure 1. Ion chromatograms of Cl- with various concentrations: (a) 6.686 × 10-4 mol L-1; (b) 9.839 × 10-4 mol L-1; (c) 1.468 × 10-3 mol L-1; (d) 1.994 × 10-3 mol L-1. The scatter data and solid lines represent experimental and simulation results, respectively. The inset shows the linear dependence of the peak area of Cl- on the concentration of Cl-.

ously blue-shifts and attenuates with the further increasing pH of aqueous HAuCl4. When the pH value reaches 6.16 and higher, no obvious absorption could be observed. The UV-vis results are consistent with previous UV-vis results,5 clearly demonstrating that the speciation of aqueous HAuCl4 varies with pH. The ion chromatography was employed to quantitatively determine the speciation of aqueous HAuCl4 at various pH. Since Cl- is a product of the hydrolysis of [AuCl4]-, a working curve of the ion chromatograms of Cl- was plotted by using aqueous NaCl with various Cl- concentrations (Figure 1). The retention time of Cl- in the ion chromatogram is 3.78 min. A minus water peak always appears at ca. 1.88 min in the ion chromatograms. The peak area of Cl- peak (A) varies linearly with the concentration of Cl- ([Cl-]) following A ) 6.428 ((0.029) × [Cl-] - 0.660 ((0.003). We found that the ion chromatogram data of Cl- with various Cl- concentrations could be satisfactory fitted by a peak with the same peak shape; therefore the acquired peak shape was adopted when simulating ion chromatogram data of aqueous HAuCl4. Figure 2 shows the ion chromatograms of aqueous HAuCl4 at pH ) 2.91, 6.16, and 10.35. The ion chromatograms of aqueous HAuCl4 at other pH values are presented in Figure S2 of Supporting Information. It could be clearly seen that the ion chromatogram of aqueous HAuCl4 varies with the pH, directly proving the pH-dependent speciation of aqueous HAuCl4. All the experimental data could be well fitted by peaks with the same peak shape which was determined during the course of the peak-fitting of the ion chromatogram data of Cl-. Depending on the pH of aqueous HAuCl4, the ion chromatograph totally detected seven peaks with the retention time at 5.41, 4.02, 3.78, 3.63, 3.31, 2.89, and 2.75 min. The detailed simulation results are summarized in Table S1 of Supporting Information. It could be seen that the same peak in different ion chromatograph is with the same retention time and the similar fwhm (full-width at half-maximum), implying that the peak-fitting results are reliable. The peak with a retention time at 3.78 min arises from Cl-, and the remaining six peaks should result from [AuClx(OH)4-x]- (x ) 0-4) species. Previous experimental and theoretical results demonstrate that [AuCl4]- undergoes a pHdependent stepwise hydrolysis;6-10 therefore, it is likely to make an assignment on basis of the pH-dependent appearance and disappearance of six peaks in the ion chromatograms. The peak with a retention time of 5.41 min is present in aqueous HAuCl4 with pH ) 2.91 and attenuates with the increasing pH. This

Influence of Speciation of Aqueous HAuCl4

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6507 TABLE 1: Speciation of Aqueous [AuCl4]- at Various pH [AuClx(OH)4-x]- (x ) 0-4) pH

x)0

x)1

x)2

x)3

x)4

average formula

2.91 3.39 4.01 5.01 6.16 7.52 8.01 10.35

+ + + +

+ + + +

+ + + + + -

+ + + + -

+ + + -

[AuCl2.91(OH)1.09][AuCl2.56(OH)1.44][AuCl2.46(OH)1.54][AuCl2.43(OH)1.57][AuCl1.09(OH)2.91][AuCl0.83(OH)3.17][AuCl0.67(OH)3.33][AuCl0.10(OH)3.90]-

a

Figure 2. Ion chromatograms of aqueous HAuCl4 at indicated pH. The scatter data and solid lines represent experimental and simulation results, respectively.

peak disappears when pH reaches 5.01 and above. Therefore, this peak could be reasonably assigned to [AuCl4]-. Another peak with a retention time of 4.02 min present in aqueous HAuCl4 with pH ) 2.91 also weakens with the increasing pH and eventually disappears when pH reaches 6.16 and above; thus this peak arises from [AuCl3(OH)]-. The peak with a retention time of 2.87 min and the peak with a retention time of 2.74 min only appear in aqueous HAuCl4 with pH of 6.16 and above, and the peak area ratio between these two peaks (Table S1 of Supporting Information) decreases from 1.5 to 0.5 when pH increases from 6.16 to 10.35. Therefore we assign these two peaks with the retention time at 2.87 and 2.74 min to [AuCl(OH)3]- and [Au(OH)4]-, respectively. The remaining two peaks with the retention time at 3.66 and 3.32 min varies simultaneously. With the increasing pH value, both grow until pH reaches 6.16 and then disappear, but the peak area ratio between these two peaks in aqueous HAuCl4 with various pH values keeps quite constant, varying between 20.0 and 21.5. This indicates that these two peaks arise from highly correlated species. [AuCl2(OH)2]- has trans and cis isomers, which have been identified in aqueous HAuCl4 by Raman/resonance Raman spectroscopy.6 We thus tentatively assigned these two peaks with the retention time at 3.66 and 3.32 min to [AuCl2(OH)2]- (I) and [AuCl2(OH)2]- (II), respectively. On basis of above assignments, it could be seen that the retention time of [AuClx(OH)4-x]- (x ) 0-4) species decreases with the decreas-

a

b

Absence. b Presence.

ing value of x in the ion chromatography with an AS14 column. The concentration of free Cl- in aqueous HAuCl4 at different pH could be calculated from the Cl- peak area in the ion chromatography on basis of its working curve. Knowing the total concentration of Cl- and the concentration of free Cl-, we could calculate the average formula ([AuClx(OH)4-x]- (x ) 0-4)) of aqueous HAuCl4 at different pH. Table 1 summarizes the speciation of aqueous HAuCl4 at various pH values and the corresponding average formula of [AuClx(OH)4-x]-. There are two novel findings: first, [AuCl4]- easily undergoes the hydrolysis reaction as soon as dissolved in water; second, even in the aqueous solution with pH ) 10.35, [AuCl(OH)3]- still coexists with [Au(OH)4]-. These results demonstrate that the Au species in aqueous HAuCl4 is always a mixture of several [AuClx(OH)4-x]- (x ) 0-4) under the investigated range of pH. Different Au species will definitely exhibit different chemical reactivity; therefore, when employing the reduction of HAuCl4 to synthesize Au colloids, the pH of aqueous HAuCl4 should have influences on the nucleation and growth of Au clusters. Recently Au colloids with various sizes/shapes have been reported to be synthesized by the reduction of HAuCl4 in aqueous solutions with varied pH, but the influence of pHdependent speciation of aqueous HAuCl4 has not been considered in the proposed mechanisms.4,5 Therefore, we investigated the reduction of aqueous HAuCl4 at various pH by VC in the presence of SDBS to manifest the influence of speciation of aqueous HAuCl4 on the synthesis of Au colloids. VC was chosen as the reducing agent because its reduction ability strengthens with the increasing pH, and SDBS was chosen because its structure in the aqueous solution does not change with pH. Figure 3 shows UV-visible absorption spectra of Au colloid solutions prepared by the reduction of aqueous HAuCl4 by VC at various pH. At pH ) 2.91, the acquired Au colloids exhibit a symmetric SPR peak at 549 nm. With the pH increasing to 5.11, the SPR peak position of acquired Au colloids does not change much, but absorption at the long wavelength region obviously gains intensities. The SPR feature of Au colloids changes abruptly when the pH of aqueous HAuCl4 is 6.16, showing a broad feature centering at 690 nm. The SPR peak further red-shifts to 787 nm with the pH value increased to 10.35. It is interesting that the Au colloids prepared by the reduction of aqueous HAuCl4 by VC at high pH exhibit quite strong SPR peak in the NIR region. It is well-known that Au nanostructures responding to NIR have potential important applications in optical imaging in early stage tumor detection and as a therapeutic agent for photothermal cancer treatment;11 therefore, much effort has been devoted to developing synthesis strategies for NIR-responsive Au nanostructures. Successful examples include the self-assembly of gold colloids into ordered structures,12 Au nanocages,13 core/shell nanostructures with Au

6508 J. Phys. Chem. C, Vol. 113, No. 16, 2009

Figure 3. UV-vis absorption spectra of Au colloid solutions synthesizes at various pH. The inset shows the corresponding photographs.

Figure 4. Representative TEM images of Au colloids synthesized at pH ) 2.98 (A), 6.11 (B-1 and B-2), and 10.35 (C-1 and C-2). The inset shows the corresponding selected-area electron diffraction patterns.

shell,14 Au nanorods,15 and Au hollow nanoparticle chains.16 By comparison with these synthesis strategies, ours is quite simple. Figure 4 presents the TEM images of Au colloids prepared with pH of aqueous HAuCl4 at 2.91, 6.16, and 10.35. We found that Au nanospheres were formed in all cases, but their size and shape distributions differ with the pH of aqueous HAuCl4. With pH ) 2.91, acquired Au colloids are well dispersed and uniform with sizes of 3-4 nm. When the pH increases to 6.16, the well-dispersed fine Au colloids with sizes of 3-4 nm still dominates, but ensembles formed by the self-assembly of fine Au colloids (3-4 nm) are clearly visible. Well-dispersed fine Au colloids are not visible with the further increasing pH to 10.35; instead, ensembles consisting of fine Au colloids (3-5 nm) and large Au colloids dominates. Figure S3 of Supporting Information shows the DLS spectra of Au colloids synthesized at various pH, whose results are summarized in Table 2. The results demonstrate that the average size of acquired Au colloids increase and their homogeneity decreases with the increasing pH of aqueous HAuCl4, consistent with TEM results. The optical properties of small particles in the nanometer size regime are mainly determined by two contributions: the properties of individual nanoparticle and the collective properties of the whole ensemble.17 On the basis of the TEM results, we attribute the SPR feature in the NIR region of Au colloids prepared at high

Wang et al.

Figure 5. UV-vis absorption spectra of Au colloid solution synthesized at pH ) 2.91 (black line), Au colloid solution synthesized at pH ) 10.35 (red line), and Au colloid solution synthesized at pH ) 2.91 but with the same Na+ concentration as in Au colloid solution synthesized at pH ) 10.35 (green line).

TABLE 2: DLS Results of Au Colloid Solutions Synthesized at Various pH pH

average particle size (nm)

polydispersity

2.91 6.16 10.35

17.5 26.0 54.5

18.2% 20.0% 26.1%

pH to the presence of large ensembles formed by the selfassembly of fine Au colloids. It has been well established that strong surface-plasmon coupling could occur between neighboring fine Au colloids within self-assemble nanostructures of fine Au colloids, leading to the red-shift of the SPR peak.18 The above results clearly show that the pH of aqueous HAuCl4 will exert great influence on the structure of acquired Au colloids. Low pH value facilitates the formation of well dispersed fine Au colloids whereas high pH value leads to the formation of large ensembles and large Au aggregates. NaOH aqueous solution was employed to adjust the pH value of aqueous HAuCl4. Keating et al. reported that Na+ could induce the aggregation of fine Au colloids.19 We found that the UV-vis spectrum of Au colloid solution prepared by aqueous HAuCl4 with pH ) 2.91 did not change after the addition of NaOH aqueous solution to obtain the same [Na+] as in Au colloid solution prepared by aqueous HAuCl4 with pH ) 10.35 (Figure 5). This result clearly rules out the likely role of Na+ in the formation of large ensembles and large Au aggregates at high pH. Meanwhile, the reduction ability of VC aqueous solution strengthens with the increasing pH and the structure of SDBS in the aqueous solution does not change with pH. Therefore, it should be the pH-dependent speciation of aqueous HAuCl4 that exerts great influence the structure of acquired Au colloids. We measured the UV-vis absorption spectra of HAuCl4, SDBS, and HAuCl4 + SDBS aqueous solutions at various pH (Figure S4 of Supporting Information). The spectrum of SDBS aqueous solution does not change with the pH. It could be seen that the spectrum of HAuCl4 + SDBS aqueous solution could be approximately considered as the addition of the spectrum of the corresponding individual counterpart. These results indicate that the existence of SDBS in HAuCl4 aqueous solutions does not have much influence on the speciation of aqueous HAuCl4. The reduction potential of aqueous HAuCl4 at pH ) 2.91, 6.16, and 8.01 were measured to be 0.66, 0.59, and 0.53 V, respectively (Figure 6), and no reduction peak was observed in the cyclic voltammogram of aqueous HAuCl4 at pH ) 10.35 (Figure S5 of Supporting Information). This indicates that [AuCl(OH)3]- and [Au(OH)4]- are more difficult to be reduced

Influence of Speciation of Aqueous HAuCl4

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TABLE 3: Peak-Fitting Results of Au 4f and S 2p XPS Spectra for Au Colloids Synthesized at Various pH Au 4f7/2 (eV) component 1 pH 2.91 6.16 10.35 a

BEa 85.7 85.7 85.7

fwhm 0.70 0.70 0.70

S 2p3/2 (eV)

component 2 BE 86.1 86.1

fwhm 0.70 0.70

Au-SDBS peak area (86.1)/peak area (85.7) 1.40 1.14 0

BE 167.9 168.1 168.1

fwhm 1.20 1.38 1.40

free SDBS BE 169.5 169.5 169.5

fwhm 1.20 1.40 1.30

Binding energy.

Figure 6. The cyclic voltammograms of aqueous HAuCl4 at various pH. The scan rate is 100 mv s-1.

Figure 7. Au 4f and S 2p XPS spectra of Au colloids synthesized at various pH. The scatter data and solid lines represent experimental and peak-fitting results, respectively.

than [AuCl4]- and [AuCl3(OH)]-. During the course of synthesizing metal colloids, the nucleation process and the growth of metal nuclei are two critical factors determining the colloid size: the faster the nucleation process and the slower the growth process, the finer the synthesized metal colloids. Therefore, when [AuCl4]- and [AuCl3(OH)]- that could be facilely reduced dominate the speciation of aqueous HAuCl4, the nucleation process proceeds fast, facilitating the formation of well-dispersed fine Au colloids; however, when [AuCl(OH)3]- and [Au(OH)4]-, which are very difficult to be reduced, dominate the speciation of aqueous HAuCl4, the nucleation process proceeds slowly and the growth of Au nuclei adequately occurs, leading to the formation of large Au colloids. Our results clearly show that the pH-dependent speciation of aqueous HAuCl4 must be considered as an important factor to affect the structure of synthesized Au colloids. The electronic structure and the Au-SDBS interaction of Au colloids were also investigated by means of XPS. Figure 7 shows the Au 4f and S 2p XPS spectra of Au colloids prepared at different pH and Table 3 summarizes the peak-fitting results.

The S 2p XPS peak could be adequately fitted by two components with the S 2p3/2 binding energy at 169.5 and ca. 168.0 eV. The former could be assigned to S in free SDBS molecules,20 and the latter to S in SDBS molecules interacting with Au colloids. The S 2p XPS results clearly show that SDBS interacts and stabilizes Au colloids by the S element. Moreover, the lower S 2p3/2 binding energy of S in SDBS molecules interacting with Au colloids than that in free SDBS molecules indicates the occurrence of charge transfer from Au colloids to the interacting S element. We calculated that the peak area ratios between the S 2p XPS peak at 168.0 eV and the Au 4f XPS peak are 12.5, 1.9, and 0.4 for the Au colloids synthesized at pH ) 2.91, 6.16, and 10.35, respectively. This implies that the average number of SDBS molecules adsorbed on one Au colloid decreases dramatically with the increasing pH value of employed HAuCl4 aqueous solution. Since the stabilization of fine Au colloids is mainly accomplished by the repulsive interaction of negatively charged SDBS molecules adsorbed on one fine Au colloid with those on other fine Au colloids, the decreasing number of SDBS molecules adsorbed on one Au colloid will destabilize the fine Au colloids. This result well explains the appearance of ensembles of fine Au colloids in Au colloids synthesized at pH ) 6.16 and 10.35. The Au 4f XPS feature of Au colloids is size dependent. Au colloids synthesized at pH ) 10.35 exhibit the single feature with the Au 4f7/2 binding energy at 85.7 eV that, on basis of TEM results, could be assigned to Au ensembles and large Au colloids. The much higher Au 4f7/2 binding energy of Au ensembles and large Au colloids than that of bulk Au (84.0 eV) indicates that Au ensembles and large Au colloids are positively charged, consistent with the occurrence of charge transfer from Au to S element in SDBS. The Au 4f XPS peak of Au colloids prepared at pH ) 2.91 consists of two components with the Au 4f7/2 binding energy at 85.7 and 86.1 eV. TEM results show that well-dispersed and uniform fine Au colloids with sizes of 3-4 nm are prepared under this condition, from which both components could only arise. A reasonable assignment could be that the component with the Au 4f7/2 binding energy at 86.1 eV arises from Au atoms on fine Au colloids directly interacting with S element in SDBS and the other component with the Au 4f7/2 binding energy at 85.7 eV from Au atoms on fine Au colloids not interacting with S element. It is well established that fine Au nanoparticles exhibit a much higher Au 4f binding energy than bulk Au metals, whose value depends on the size of Au nanoparticles.21 Because of the charge transfer from Au to S element in SDBS, Au atoms on fine Au colloids directly interacting with S element in SDBS are positively charged and thus exhibit a higher Au 4f binding energy than those not interacting with S element. From these observations, we could draw a very important conclusion that the charge are localized around the Au atoms directly participating the charge transfer process when charge transfer occurs between fine Au colloids (3-4 nm) and the stabilizing agent (SDBS), indicating that fine Au colloids are more or less with

6510 J. Phys. Chem. C, Vol. 113, No. 16, 2009 the characteristic of semiconductors. However, the charge will be delocalized to the entire Au colloids with large sizes, characteristic of metals, such as in the case at pH ) 10.35, Au ensembles and large Au colloids only exhibit a single XPS peak although charge transfer also occurs. The Au 4f XPS peak of Au colloids prepared at pH ) 6.16 also shows two components with the Au 4f7/2 binding energy at 85.7 and 86.1 eV, but the peak area (86.1 eV)/peak area (85.7 eV) ratio is smaller than that of Au colloids prepared at pH ) 2.91. It is understandable because Au colloids prepared at pH ) 6.16 comprise both welldispersed and uniform fine Au colloids with sizes of 3-4 nm and Au ensembles. Therefore, the XPS component at 86.1 eV arises from Au atoms on fine Au colloids directly interacting with S element in SDBS, whereas that at 85.7 eV is contributed from both Au atoms on fine Au colloids not interacting with S element in SDBS and from Au ensembles. 4. Conclusion In summary, we have successfully elucidated the pHdependent speciation of aqueous HAuCl4 and its influences on the synthesis of Au colloids. Aqueous HAuCl4 consists of [AuClx(OH)4-x]- (x g 2) at low pH but [AuClx(OH)4-x]- (x < 2) at high pH. By employment of VC as the reducing agent and SDBS as the protecting agent, reduction of aqueous HAuCl4 at low pH leads to the synthesis of well-dispersed and uniform and fine Au colloids, whereas that at high pH leads to the formation of Au ensembles and large Au colloids. These Au ensembles and large Au colloids exhibit strong surface plasmon resonance in the NIR region. The SDBS molecules bind to the surface of Au colloids through the S element and charge transfer from Au atoms to S elements occurs. The charge is localized around Au atoms directly interacting with SDBS for fine Au colloids but delocalized to the entire Au atoms for Au ensembles and large Au colloids. Acknowledgment. This work is financially supported by National Science Foundation of China (Grant 20773113), the “Hundred Talent Program” of CAS, the MOE program for PCSIRT (IRT0756), and the MPG-CAS partnergroup program. Supporting Information Available: UV-vis absorption spectra of aqueous HAuCl4 at various pH values, ion chromatograms of aqueous HAuCl4 at other pH values, DLS spectra of Au colloids synthesized at various pH, UV-vis absorption spectra of HAuCl4, SDBS, and HAuCl4 + SDBS aqueous solutions at various pH, and cyclic voltammogram of aqueous HAuCl4 at pH ) 10.35. This material is available free of charge via the Internet at http://pubs.acs.org.

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