Size-Controlled Gold Nanoparticles Synthesized in Solution Plasma

ARTICLE pubs.acs.org/JPCC

Size-Controlled Gold Nanoparticles Synthesized in Solution Plasma Maria Antoaneta Bratescu* EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan

Sung-Pyo Cho Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan

Osamu Takai Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan

Nagahiro Saito EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan

bS Supporting Information ABSTRACT: Size-controlled gold nanoparticles (NPs) have been synthesized using an electrical discharge in a liquid environment, termed solution plasma processing (SPP). The gold NPs exhibit sizes from 1 to 2 to 10 nm when the solution pH was adjusted in the range from 12 to 3, respectively. The chemical environment surrounding the gold NPs depends on the preparation conditions and determines the electrostatic interaction among the nanoparticles, which alters their final size. Information obtained from XPS analysis, ToF-SIMS mass spectra, and UV
vis absorption spectroscopy were consistent and demonstrate that the gold NPs are partially oxidized on the surface, when synthesized in a pH 12 solution, and remain surrounded by gold chloride compounds when synthesized in a pH 3 solution. Plasma diagnostics shows that a high electron density contributes to generating a larger number of hydrogen radicals, which represent the main component in the reduction process of the gold ion into the neutral form.

1. INTRODUCTION Gold nanoparticles (NPs) were known in ancient times related to numerous applications from paintings for decoration, jewelry technology, to curative medicine.1
3 In the last 30 years, research has been focused on the synthesis and characterization of gold NPs with sizes ranging from 2 nm to several hundreds of nanometers with different shapes, as well as nanoclusters (NCs) that are formed by several tens of gold atoms surrounded by different ligands. The great diversity of sizes and shapes of gold NPs and NCs has great potential for applications in catalysts, medical imaging and drug delivery systems, optics, paints, and electronics.4
6 This has led to a huge expansion of theoretical and experimental work, taking into account that the knowledge and the control of the surface chemical surroundings of the gold NPs or NCs, and of the washing and purification methods necessary in the context of an effective use in these application fields.1,3
5 The plasma in solution, or solution plasma process (SPP), is a new useful and simple method for the metal NPs synthesis because this nonequilibrium plasma can provide extremely rapid reactions due to the reactive chemical species, radicals, and UV r 2011 American Chemical Society

radiation produced in an atmospheric pressure plasma.7
9 The most important merits of the SPP for the NPs synthesis, as compared with chemical methods, consist in the short processing time (in the range from few minutes to several tens of minutes), preparation in room temperature and pressure conditions, and low energy of plasma. The novelty of the SPP method used in our laboratory consists in the fact that plasma operates in glow discharge limits, offering a suitable medium to control the chemical reactions inside the solutions.8,9 This is possible because plasma offers a new reaction medium, where hydrogen, hydroxyl, and oxygen radicals are produced, where the hydrogen radical is the most responsible for the reduction reaction of the gold ion to the neutral atom, and therefore a reduction agent is not necessary. The SPP method seems well-suited for the NPs synthesis offering the possibility to control the size by controlling the surrounding chemistry of the gold NPs, adding thus another level of utility of this procedure to material science. Received: August 4, 2011 Revised: October 19, 2011 Published: November 14, 2011 24569

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The Journal of Physical Chemistry C As already mentioned, for applications in medicine and electronic devices, the knowledge of gold NPs and NCs chemical surroundings is crucial because organic molecules or contamination, other than those desired, can promote damage instead of curing as in medical usage or unexplained results as in electronic systems. The surface chemical analysis methods play a significant role in the characterization of NPs and give information about surface composition, chemical state (X-ray photoelectron spectroscopy, XPS), and presence of surface coatings or contaminants (time of flight secondary ion mass spectroscopy, ToF-SIMS). These methods are performed in ultrahigh vacuum chambers, as well as during transmission electron microscopy (TEM) characterization, and can affect the chemical, electrical, and optical properties, in addition to the shape and the size of the NPs.10 In this article, we use the SPP method to synthesize controllably sized gold NPs with different surrounding chemistry on the surface. Solutions containing the same quantity of the gold precursor and the surfactant, but different pH values were processed by plasma, analyzed by optical absorption spectroscopy, with the resulting gold NPs characterized by shape, size, and surface chemistry using appropriate methods. These data were interpreted in connection with the plasma discharge parameters to understand the mechanism of the reduction of gold due to plasma and the formation of NPs. To elucidate the mechanism of NP synthesis by SPP, the electron number density and the relative quantities of the excited species were determined in the plasma gas phase, the appearance of the gold complexes was observed in liquid phase by UV
vis absorption spectroscopy, and the surface surroundings of the gold NPs were measured by XPS and ToF-SIMS methods. The morphology of the gold NPs was analyzed from TEM images, the validation of gold composition was obtained from energy dispersive spectroscopy (EDS) measurements, and the size of the gold NPs was confirmed by small-angle X-ray scattering (SAXS) diffractometry.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Gold NPs in SPP. The necessary materials for the gold NPs synthesis were hydrogen tetrachloroaurate trihydrate (HAuCl4 3 3H2O, Sigma), hexadecyltrimethylammonium chloride (CTAC, Sigma), and NaOH (Kanto Chemical Co., Inc.), used as received without further purification. The water used was deionized (DI) water. NaCl (Kanto Chemical Co., Inc.) was used for particles characterization. The gold NPs were synthesized in an aqueous solution of 1 mM HAuCl4 3 3 H2O used as precursor, 1 mM CTAC used as surfactant, and small amounts of 1 M NaOH solution used to adjust the solution pH (6.5 and 11.2). The 3.2 pH solution was made only of 1 mM HAuCl4 3 3H2O and 1 mM CTAC, where CTAC initially forms precipitates though the solution which becomes turbid after plasma processing. Solutions with pH higher than 6 were transparent before and after SPP. A photo with the solutions before and after SPP is shown in Figure S1 in the Supporting Information. The conductivity and the pH values were measured for the prepared solutions before and after SPP with a Hanna instrument (Table S1 in the Supporting Information). The experimental setup of the SPP was described in detail in a previous publication.11 A pulsed plasma in an aqueous gold solution is generated in a reaction vessel made from a glass Petri dish (Iwaki Co., Japan), with an inner diameter of 35 mm and a height of 12 mm, where two opposing holes with conical silicon stoppers are used for insertion of Pt rod electrodes (1 mm

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Figure 1. (a) Reaction vessel of SPP, made from a glass Petri dish (Iwaki Co., Japan), with an inner diameter of 35 mm and a height of 12 mm. Two opposing holes with conical silicon stoppers are used for insertion of Pt rod electrodes (1 mm diameter), which are then covered by tight ceramic tubes. (b) The voltage (left graph) and the current (right graph) waveforms of the plasma in a solution with 130 μS cm
1 conductivity and 0.2 mm interelectrode gap.

diameter), which are then covered by tight ceramic tubes (part a of Figure 1). The plasma was produced by using a pulsed high voltage (HV) power supply, which was assembled in the laboratory from a high voltage switch (HTS 81
09, Behlke, Germany) and a dc HV power supply (WX Glassman, Inc., USA). An external signal generator controls the HTS switch at a frequency of 10 kHz and a pulse width of 250 ns. At different solution conductivities, the HV pulse width varied from 500 to 900 ns depending on the external working resistance and load capacitance. The electrical characteristics of the plasma in 130 μS cm
1 conductivity of the solution with an interelectrode gap of 0.2 mm are shown in part b of Figure 1. The electric field of the plasma in the interelectrode space was about 3  107 V m
1 and the energy per pulse was approximately 10 mJ per pulse, as can be calculated from the data shown in part b of Figure 1. The synthesis of the gold NPs in SPP was performed during 30 min, applying the same HV to all the processed solutions. The optical emission spectra of the plasma were collected during SPP with a high resolution spectrograph, using a 600 mm
1 or 2400 mm
1 diffraction grating coupled with a back-illuminated CCD detector (1028  128 pixels) in the spectral region from 190 to 1100 nm or 190 to 750 nm, respectively, and a spectral resolution of 0.08 and 0.02 nm respectively, at 500 nm, using an entrance slit of 0.05 mm (Horiba Jobin Yvon, TRIAX 550). 2.2. Methods Used to Characterize the Solutions and the Gold NPs. The solutions, before and after SPP, were measured by UV
vis spectroscopy using a UV
3600 Spectrograph, (Shimadzu, Japan) in the spectral range from 187 to 800 nm, with a spectral resolution of 0.5 nm and an optical absorption path of 1 mm. The stability and the influence of Na+ ion on the gold NPs 24570

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Table 1. Intense Emission Lines and Bands in OES during SPP and Significant Emission Lines of Au and Pt Atoms in the Region from 260 to 290 nm λ (nm)

species

transition

Hα

656.3

3d f 2p

Hβ

486.1

4d f 2p

Hγ

434.0

5d f 2p

O

777

5

H2

844.6 560 - 630

3

CH

431.4

A2Δ f X2Π

AuCl

524.0

AfX

Au

267.6; 274.8.

Pt

263.8; 271.9; 273.4; 283.0

P f 5S0

P f 3S0 d3Π f a3Σ

3. EXPERIMENTAL RESULTS AND DISCUSSION 3.1. Plasma Characterization. During NP synthesis, plasma was investigated by optical emission spectroscopy (OES), which provides information about the presence of radicals such as H, OH, O, O2, and H2 produced from water, CH radical produced from surfactant decomposition, Au atoms produced from the decomposition of AuCl4
ions, and Pt atoms produced from electrode sputtering (Figure 2). The presence of different excited species of hydrogen, oxygen, and hydroxyl, observed by the atomic lines and molecular bands in OES demonstrates the chemical reactions which take place in SPP (Table 1):12,13

H2 O f H 3 þ OH 3 2H2 O f O2 þ 2H2 O2 f 2O 3 H2 f 2H 3 H þ e f H þ e O þ e f O þ e

Figure 2. Typical optical emission spectra of the SPP generated in solutions with (a) pH 3 and (b) pH 12. The spectra have been normalized to the highest intensity of the Hα line. The insert in the figure represents a detailed spectrum in the region from 260 to 290 nm, where atomic Au and Pt lines dominate the spectrum.

size were determined by adding NaCl to the obtained solutions after SPP. For this purpose 1 M NaCl stock solution was prepared. The UV
vis spectra of the gold NPs synthesized in the pH 6 and 12 solutions were recorded after adding NaCl solution and after the mixture stabilized. The NaCl final concentration in the gold NPs solution was changed from 10 to 80 mM. As prepared the gold colloid solutions were analyzed by SAXS method in transmission geometry, using the SmartLab X-ray diffractometer (Rigaku Corporation), equipped with Cu Kα X-rays source at 0.154186 nm. The particle size distribution was calculated using the NANO-solver software (Rigaku Corporation). On a substrate and in high vacuum environment, the Au NPs were characterized using TEM and EDS (JEM
2500SE, Jeol), ToF-SIMS (ION-TOF, GmbH), and XPS (ESCA, Omicron Nanotechnology). TEM was performed at 200 kV and the samples were prepared on carbon coated Cu grids. For ToF-SIMS and XPS measurements the samples were made by dropping 300 μL of the gold colloidal solution on clean silicon substrates, which were then dried and kept in a vacuum desiccator. The ToF-SIMS measurements were performed using a Bi1 ion source with the energy of 25 kV, a target current of 0.95 pA, and 16 ns pulse width. For mass calibration, for positive and negative modes, CH3+, C3H5+, and C4H7+ peaks, and C
, CH
, F
, and Cl
peaks were used, respectively. The XPS analysis was carried out with a Mg Kα source at 1253.6 eV in a vacuum chamber at 10
8 Pa.

ð1Þ

where H 3 , H* and O 3 , O* means the hydrogen and oxygen radical and excited atom respectively, OH 3 represents the hydroxyl radical, and e is the electron from plasma. To better characterize the plasma produced in solution, the electron density (ne) was estimated from the Stark broadening of the Hβ line, which is typically used for this calculation, even if the values of ne are of the order of 1020 m
3.14 In SPP, it must be taken into consideration that plasma operates at atmospheric pressure and in room temperature. The hydrogen lines are broadened not only due to the electric field of the surrounding electrons but also due to collisions with other atoms and molecules (van der Waals broadening), due to gas temperature (Doppler effect) and to instrumental broadening. The natural line broadening has been neglected and the gas temperature was considered to be 1600 K.15 To calculate ne, a spectral deconvolution procedure has been applied to the Hβ line considering the van der Waals, Doppler and instrumental broadenings to be 0.03, 0.02, and 0.02 nm, respectively (formulas are given in the Supporting Information). In Figure 3, the dependence of ne on solution conductivity is shown together with an example of the Hβ line (black curve) fitted with a Voigt line profile (red dashed curve), from which the full width at half-maximum (fwhm) 24571

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Figure 3. Dependence of ne on solution conductivity. The insert in the figure represents an example of the Hβ line (black curve) fitted with a Voigt line profile (red dashed curve), from where the full width at half-maximum (fwhm) was extracted and the electron density was calculated.

was extracted and the electron density was calculated from the equation:14   ne 0:68116 fwhm ¼ 4:800  ð2Þ 1023 By comparing the emitted spectrum obtained from a discharge in a solution at pH 3 (part a of Figure 2) to that obtained from one at pH 12 (part b of Figure 2), and then evaluating the resulting ne (Figure 3), we note the following: (i) the sputtered material from the Pt electrodes detected in all the solutions was in small quantity and not detected as NPs in TEM-EDS analysis, (ii) the high conductivity of the pH 12 solution produces a high electron density due to an increased number of discharge channels inside the plasma gas phase and consequently, by impact electron dissociation, the number of the resulted excited H atoms was high.15,16 3.2. Solution Analysis. The solutions were investigated before and after SPP by UV
vis spectroscopy (Figure 4). Before SPP, in the pH 3 solution, the absorption bands at 223 and 307 nm corresponding to pπ 5d x2‑y2 and pσ 5d x2‑y2 ligand
metal transitions indicate the presence of the square planar AuCl4
configuration.17
19 These absorption bands can be seen in the solution with pH 6 at 212 and 276 nm. As the insertion of the OH
ions in the inner coordination sphere proceeds, both bands shift toward the lower wavelength. The band initially in the pH 3 solution at 307 nm decreases in intensity and finally disappears in the pH 12 solution because of the progressive increases and equalization of the energy level of pπ and pσ orbitals.17,18 After SPP, in solution with low pH value, two bands appear at 267 and 357 nm attributed to the AuCl4
- CTA+ ligand
metal charge transfer (LMCT) complex, formed by electrostatic interaction between these two ions. The bands due to the charge transfer from metal to the cationic surfactant are barely observed in the solution at pH 6 and were not detected at pH 12.20 In all of the processed solutions, the surface plasmon resonance (SPR) band was detected, with the maximum band wavelength at different values depending on the gold NPs size and the surrounding molecules.3 Usually, the maximum wavelength of the SPR is redshifted when the gold NPs diameter of ten-nanometer size increases. This behavior is not longer valid for smaller size than 10 nm of the NPs, where there is a high difference in the electron

Figure 4. UV
vis spectroscopy of different solutions, before and after SPP: (a) pH 3, (b) pH 6, and (c) pH 12. The inserts in the figures represent an enlarged part of the absorption spectrum in the region of SPR.

conductivity of the gold atoms inside the NP and that on the NP surface because they interact with the outer ligand.21 In this case, the wavelength of the maximum SPR band decreases with the increase of the NPs diameter, which has also been observed in the present experiment. The maximum wavelength of the SPR band is blue-shifted as the solution pH decreases, because of the increasing of the gold NPs size, as will be shown by TEM analysis and confirmed by SAXS results. We measure after four months the UV
vis spectra of the gold colloidal solutions and we observed that the spectra had no significant changes, indicating the stability of the gold NPs in solutions. The effect of Na+ ion on the nanoparticles size was analyzed through the SPR absorption band of the gold NPs during the 24572

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Figure 5. Gold NPs size distributions measured by small-angle X-ray scattering diffractometry corresponding to different initial pH.

increase of Na+ ion concentration. We observed that SPR wavelength, intensity and line width were not influenced by the increased quantity of Na+ ion in solution, demonstrating that the amount of the surfactant necessary to prevent salt-aggregation and to stabilize the solutions was enough to coat the surface of the gold NPs (Figure S2 in the Supporting Information).22 In the processed solutions, the gold NPs size as measured by SAXS decreases with increasing pH of the solution (Figure 5). The average NPs diameter is 2.1, 4.2, and 20.0 nm, with a maximum probability at 2.02, 3.89, and 15.00 nm, for the initial pH solution at 12, 6, and 3, respectively. 3.3. Size, morphology and local environment of gold NPs. In high vacuum environment the gold NPs were analyzed by TEM, ToF-SIMS, and XPS. The size and morphology of the gold NPs were determined using TEM and HRTEM as is shown in parts a, b, and c of Figure 6. The gold NPs synthesized in SPP have various sizes depending on the initial solution pH and most of them show a single crystal structure. The HRTEM analysis shows that in the pH 12 solution, after SPP, the average particle diameter is around 1
2 nm, whereas in the pH 3 solution the gold NPs size is around 10 nm (part d of Figure 6). The elemental composition of NPs is gold as measured by the EDS analysis, although Pt atoms were detected in the OES spectra in the plasma gas phase due to the electrode sputtering (Figure S3 in the Supporting Information). If we compare the SAXS results with the estimated diameter from the TEM images, a good agreement is observed between the gold NPs size determined by TEM and SAXS measurements, but less so in the case of the pH 3 solution (Figures 5 and 6). We propose that the turbidity of the pH 3 solution influenced the SAXS results. The UV
vis absorption spectroscopy data show the presence of the LMCT complex in the pH 3 solution after SPP. This complex forms an apparent fine crystal-like dispersion which increases the scattering X-ray light.20 The local environment of the gold NPs was measured by XPS (Figure 7). The XPS spectra of the gold NPs were compared with the XPS spectra of a thick gold film sputtered on a silicon substrate. The spectra of the Au 4f core level of the elemental gold is characterized by the peaks at 84.0 and 87.8 eV corresponding to Au 4f7/2 and Au 4f5/2 levels, the first peak being used for XPS energy scale calibration. For the gold NPs synthesized in different pH solutions, the XPS spectra were shifted comparing

Figure 6. Size and morphology of the gold NPs synthesized in SPP analyzed by TEM (upper figures) and HRTEM (bottom figures). The processed solution pH was (a) 3, (b) 6, and (c) 12. (d) Dependence of the gold NPs diameter on solution pH.

to the XPS spectra of an Au film. The gold NPs fabricated in a low pH solution have mostly gold chloride compounds, being characterized by the XPS peaks at 84.2 and 85.1 eV, attributed to Au 4f7/2 in [(C4H9)4N][AuCl2] and [N(C2H5)4][AuCl2], respectively. The XPS peaks of Au 4f5/2 with a binding energy of 88.3 and 89.3 eV correspond to AuCl and AuO, respectively. However, the gold NPs fabricated in a high pH solution are partially oxidized confirmed by the characteristic peaks at 85.6 and 89.3 eV, attributed to Au 4f7/2 and Au 4f5/2 of AuO, respectively (Table 2).23,24 The XPS peaks assignment and references are listed in Table 2. From the XPS data, the amount of AuO surrounding the gold NPs was evaluated to be about 21% and 55% in case of the gold NPs synthesis in pH 3 and 12 solution, respectively. The molecular information, the elemental composition and the chemical states near the NPs surface was determined using ToF-SIMS. The results of the gold NPs analyzed by ToF-SIMS 24573

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Table 2. Identification of the XPS Peaks of the Au 4f Core Level Figure 7/peak no.

binding energy/eV

XPS line and ref

84.2

Au 4f7/2 [(C4H9)4N][AuCl2]23

2. 3.

85.1 88.3

Au 4f7/2 [N(C2H5)4] [AuCl2]23 Au 4f5/2 AuCl23

4.

89.3

Au 4f5/2 AuO24

84.6

Au 4f7/2 AuCl23

2.

85.6

Au 4f7/2 AuO24

3.

87.8

Au 4f5/2 Au23

4.

89.3

Au 4f5/2 AuO24

5.

90.5

Au 4f5/2 [(C4H9)4N][AuCl4]23

(b) 1.

(c) 1.

3.4. Mechanism of Gold NPs Formation in SPP. The main role of SPP during the synthesis of the gold NPs consists in the production of the H radicals in the plasma gas phase, which are necessary to reduce the gold ion Au3+ to atomic Au0, in the liquid phase.9 Before plasma, in solution, the hydrolysis of HAuCl4 occurs:


AuCl
4 þ jOH T AuCl4
j ðOHÞj þ jCl

ð3Þ

where 0 < j < 4 and the replacement of Cl
by OH
depends on solution pH, as was found in the UV
vis results (Figure 4).19,25,26 In the liquid phase of plasma, the reduction of gold ions occurs in different ways: Figure 7. Local environment of the gold determined by XPS analysis: (a) a gold film deposited on silicon substrate and the gold NPs synthesized in (b) pH 3 solution, and (c) pH 12 solution. XPS peak identification is given in Table 2.

are shown in Figure 8 and Figure S4 in the Supporting Information. Only the most intense signals, higher than 2  103 counts, were considered from the ToF-SIMS spectra. The negative peaks of the ion fragments reveal the presence of Au
(196.9664 amu), AuOH
(213.9703 amu), CNAu
(222.9734 amu), Au2OH
(410.9375 amu), and Au2Cl3
(498.8588 amu). Though the ToF-SIMS method does not produce an accurate quantitative analysis of the fragments on the surface, the signal intensities that differ by an order of magnitude can give an evaluation of the surface compound amount.10 The gold atom was found to be almost in the same amount in both samples provided from the gold NPs synthesized in pH 3 and pH 12 solutions (part a of Figure 8). Among the sputtered fragments, the Au2Cl3 fragment was found in a higher amount surrounding the gold NPs synthesized from a pH 3 solution than those produced in the pH 12 solution (part d of Figure 8), whereas the gold NPs synthesized in a low acidity solution are surrounded by the oxidized forms of Au (parts b and c of Figure 8). It is interesting to mention that in high pH solutions high amounts of CNAu fragments can be found on the gold NPs surface, suggesting the formation of the gold compounds, where Au is bonded to the CN group supplied from the surfactant molecule. The available information obtained from XPS analysis, ToFSIMS mass spectra, and UV
vis absorption spectroscopy are consistent and demonstrate that the environment surrounding the gold NPs has various compositions, as the NPs provided from the SPP in a pH 12 solution are oxidized, whereas the NPs synthesized in a pH 3 solution remain enclosed by gold chloride compounds.

0
AuCl
4 þ 3H 3 f 3HCl þ Au þ Cl

ð4Þ

0
AuðOHÞ
4 þ 3H 3 f 3H2 O þ Au þ OH

ð5Þ

where the hydrogen radicals (H 3 ) are provided by the gas phase plasma and reactions 4 and 5 take place in the pH 3 and pH 12 solution, respectively. The pH 6 solution is an intermediate case to the other two solutions, where the hydroxo complexes as AuCl3(OH)
and AuCl2(OH)2
represent a molar fraction of about 0.6 in the solution.17
19,25,27 The hydrochloric acid released in the reaction 4, in an amount depending on the number of Cl
ligands in the gold complex lowers the acidity of the gold solutions. It is also known that in plasma in liquids, the solution pH decreases after SPP due to the formation of H radicals in the gas phase plasma, which are transferred in the solution.9,28 After SPP, all of the used solutions in this experiment had lower pH values and higher conductivities (Table S1 in the Supporting Information), except the conductivity of the pH 12 solution, which slightly decreased. Even in the pH 12 solution, the reduction reaction leads to OH radical formation, the pH decreases after SPP. To explain this, we have to take into account the differences in plasma characteristics between the solution with different acidities, where in the pH 12 solution a higher electron density was measured compared to that at pH 3, and consequently the number of the H radicals was higher because the radicals are mainly produced by electron impact dissociation of water molecules.15,16 The amount of H radicals is enough to contribute to gold ion reduction and to recombination with OH radicals compensating for a drastic reduction of solution pH value. The reduction of gold ions and the recombination process at pH 12 might explain the lower solution conductivity after SPP than in the beginning. By changing the solution pH in the preparation condition, the size of the gold NPs can be controlled. The connection between 24574

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Figure 8. ToF-SIMS mass spectra of the negative ions fragments (a) Au, (b) AuOH and CNAu, (c) Au2OH, and (d) Au2Cl3, corresponding to solutions with pH 3 and 12 after SPP.

the gold NPs size and the solution pH can be understood by: (i) the effect of the redox standard potential in the reactions (4) and (5), (ii) the electrostatic repulsion force between AuO
ions, and (iii) the protective layer of the surfactant. The standard redox potential of the reduction of the AuCl4
ion to Au0 is about 0.95 eV, in a pH 3 solution and the standard redox potential of the reduction of the Au(OH)4
ion to Au0, in pH 12 solution, is about 0.60 eV.17 A higher redox potential leads to supersaturation and consequently a larger number of atoms is formed. In this case, the large size of the gold NPs formed in a pH

3 solution is explained by the formation of a higher amount of Au atoms, which aggregate and generate nanoparticles with sizes larger than 10 nm. The XPS and ToF-SIMS results demonstrate that the gold NPs synthesized in a pH 3 solution are surrounded by Cl
ions from gold chloride compounds, which are not so important in an acidic solution containing a large amount of Cl atoms. However, in the case of the gold NPs fabricated in SPP at pH 12, these NPs are oxidized on the surface to AuO as seen from XPS spectra, or to fragments such as AuO
, AuOH
, and Au2O
, as seen from the 24575

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The Journal of Physical Chemistry C ToF-SIMS mass spectra. The negative charge on the oxidized gold NPs surface determines a repulsive force among the nanoparticles, which prevent a further agglomeration of the gold NPs, thus explaining their small size.2,20,29 In addition, in the case of pH 12 solution, in ToF-SIMS measurements the presence of CNAu fragment resulted from the sputtered material from the NPs surface, suggests that the surfactant molecules are bound to the gold surface, protecting the NPs and preventing agglomeration.20,29

5. CONCLUSIONS We showed that the gold NPs synthesized in SPP can have sizes ranging from 1 to 2 to 10 nm, and are covered by a gold oxide or by gold chloride layers, depending on the initial solution pH. After SPP, synthesized NPs resist agglomeration in different ways depending on the chemical environment. At pH 3, the surfactant formed with tetrachloroaurate ion a ligand
metal complex. At pH 12, the surfactant was chemically bound with oxidized gold atoms on the nanoparticle surfaces. The repulsion force among the gold NPs covered with an oxide layer prevents formation of large particles. In the gold NPs synthesis process, plasma supplies H radicals, which are used in the reduction of the gold ion to the neutral atom, without any other chemical reducing agent. The relative number of the H radical depends on the electron number density because these are mainly produced by electron impact dissociation of water molecules. The relative amount of gold atoms present in the plasma gas phase is not influenced by the solution pH. In high pH solution, part of the H radicals recombines with OH radicals. ’ ASSOCIATED CONTENT

bS

Supporting Information. Photo of the gold solutions prepared at different pH values, before and after SPP, UV
vis spectra for gold NPs solutions with pH 6 and 12 before and after addition of NaCl solution, EDS analysis of gold NPs synthesized in pH 12 solution, ToF-SIMS maps of different solutions dropped on silicon substrates, and formulas for calculation of different broadening effects of hydrogen line. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (M.A.B.), [email protected]. nagoya-u.ac.jp (S.-P.C.), [email protected] (O.T.), [email protected] (N.S.).

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’ ACKNOWLEDGMENT This work was partially supported by Knowledge Cluster Initiative - “Tokai Region Nanotechnology Manufacturing Cluster” sponsored by Ministry of Education, Culture, Sports, Science, and Technology (MEXT) and Core Research for Evolutional Science and Technology (CREST) of Japan Science and Technology (JST) Agency. ’ REFERENCES (1) Jin, R. Nanoscale 2010, 2, 343–362. (2) Zsigmondy, R. Properties of Colloids, Nobel Foundation; December 11, 1926, 45
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