Met. Mater. Int., Vol. 17, No. 6 (2011), pp. 943~947doi: 10.1007/s12540-011-6012-x Published 27 December 2011

Effects of the DC Pulse Duty Ratio on the Characteristics of Nanoparticlesin Gold Nanofluids

Yong Kang Heo and Sang Yul Lee*

Center for Surface Technology and ApplicationsDepartment of Material Engineering, Korea Aerospace University,

100 Hangkongdae-Gil, Goyang-si, Gyunggi-Do, 412-791, South Korea

(received date: 13 December 2010 / accepted date: 26 May 2011)

Gold nanoparticles were synthesized using various unipolar DC pulse duty ratios using solution plasma pro-cessing (SPP), and their effects on the size distribution and the shape of gold nanoparticles were investigated.The results demonstrated that the power characteristics used for the synthesis of the gold nanoparticles havea strong effect on the fabrication of Au nanoparticles. Generally spherical shaped nanoparticles were fabricatedby SPP and as the unipolar DC pulse power increased from 55-40 % duty ratio to 100-0 % duty ratio, thesize as well as size distribution of the fabricated nanoparticles decreased, resulting in an average particlesize of approximately 18.1±5.0 nm in diameter at 100-0 % duty ratio. Also, the zeta-potential of the goldnanoparticles synthesized at 100-0 % duty ratio was the highest and was measured at −50.8±1.5 mV, whichsuggested the dispersion of nanoparticles fabricated at 100-0 % duty ratio was more stable than under otherconditions. This confirmed the previously reported results, in that increased electrical energy provided fromthe solution plasma not only made the nanoparticles even smaller in size, but also facilitated good dispersionstability by charging up the nanoparticles negatively.

Keywords: solution plasma processing, transmission electron microscopy (TEM), interfaces, nanostructuredmaterials, powder processing

1. INTRODUCTION

Nanofluids, defined as fluids with suspended nanometer-sized particles, are considered to be the next generation heattransfer fluids as they have greatly enhanced thermal con-ductivity to improve their heat transfer performance com-pared to pure liquids [1]. Therefore since nanofuids werefirst introduced by SUS Choi [1] a great deal of effort to pro-duce nanofluids with high thermal conductivity has beenmade [2].

A wide variety of synthesis methods to produce nanofluidshave been introduced. In two-step methods the nanoparticlesare synthesized by various powder process such as atomiza-tion, mechanical alloying, and so on [3], and are then mixedto be dispersed into the base liquid [4] and in one-step meth-ods, the synthesis of nanofluids is made in a single step. Forexample the direct evaporation of the substrate materials andthe condensation of the nanoparticles in the base liquids [5]occur at the same time in a confined environment.

More recently, a new processing technique for the synthe-

sis of nanoparticles in an aqueous solution called SolutionPlasma Processing (SPP) was also introduced [6,7]. Sincein-situ synthesis of nanoparticles in the base fluids becamepossible, much interest in the use of SPP to make nanofluidshas been generated, and a series of investigations to makegold nanofluids and study the effects of processing variableson the nanoparticles in the nanofluids were made [7-9].

In this work, the fabrication and characterization of goldnanoparticles through SPP were investigated and specialattention was given to the effect of the unipolar DC pulseduty ratio on the Au nanoparticles in nanofluids.

2. EXPERIMENTAL PROCEDURES

Gold nanoparticles were synthesized with various unipolarDC pulse power of 100-0 % ~ 55-40 % duty ratio using SPPat 1000V pulse DC power at 25 kHz. A schematic of thesynthesis device utilized in SPP is shown in Fig. 1. Theexperiment used a stock solution consisting of 0.5 mM ofhydrogen tetracchlorpairate (III) trihydrate (HAuCl4 · 3H2O,99.9 %, Kojima) and 2.0 mM of potassium hydroxide(KOH, 99.9 % purity, Fluka) in 200 ml of distilled water. Adischarge was made for 600 seconds using tungsten wire

*Corresponding author: sylee@kau.ac.kr©KIM and Springer

944 Yong Kang Heo and Sang Yul Lee

electrodes (diameter : 2 mm) and gap distances betweentungsten wire electrodes were 1 mm.

The spectrum of solution plasma was recorded using aUSB4000 spectrometer (Ocean Optics, USA). The I-V curvewas measured using a DLM 2022 oscilloscope (Yokogawa,Japan). Also, a UV–vis spectra graph of gold nanofluids wascollected with a visible ray ranging from 400 to 700 nm on aUV3600 UV–vis nir spectrophotometer (Shimadzu, Japan).Gold nanoparticles were characterized by HR-TEM using aJEM 3010 (JEOL, Japan) operating at 300 kV. The disper-sion stability of nanoparticles was measured by utilizing zetapotential analysis (Brookhaven Instruments Corporation,USA).

3. RESULTS AND DISCUSSION

Optical emission spectrum analysis of the solution plasmagenerated under the condition of 1000V DC pulse power at25 kHz is shown in Fig. 2(a). Various excited species such asoxygen ion, hydrogen ion are present in the solution plasmaand the major peaks detected were Hβ (486.1 nm), Hα (656.3nm), OI (777.1 nm), OV (844.34 nm) with the strongest peakdetected at Hα (656.3 nm). The intensity of the excitedhydrogen atom is dominant and this could be due to the pres-ence of a large concentration of excited hydrogen atomssince the hydrogen required less energy to be excited. Theplasma is generated by the flux of ions and electrons in solu-tion. The applied high voltage results in the movement of H+

to the cathode and OH− to the anode. If the flux of ions andelectrons is accelerated with the addition of electrical energy,the plasma reaction is expected to be enhanced as well. Inother words, an increase of unipolar DC pulse duty ratioincreases the flux of ions and electrons and the current dis-placement in the solution, which in turn enhances the plasmareaction by increasing the total electrical energy to generateplasma in the solution. The Hα (656.3 nm) peak intensitieswith various unipolar DC pulse powers of 100-0 % ~ 55-40% duty ratio are shown in Fig. 2(b). As unipolar DC pulsepower increased from 55-40% duty ratio to 100-0% duty

ratio, optical emission spectrum intensity increased as well.The current displacements (A) as a function of discharge

Fig. 1. A schematic diagram of the SPP system.

Fig. 2. Emission spectrum of solution plasma prepared by a conditionof 1000 V pulse DC power at 25 kHz, (a) wide spectra (400~1000 nm),(b) narrow spectra of Hα (656.3 nm).

Fig. 3. Measured current displacement in solution plasma for variousunipolar DC pulse powers.

Effects of the DC Pulse Duty Ratio on the Characteristics of Nanoparticles in Gold Nanofluids 945

duration with various unipolar DC pulse powers were mea-sured as shown in Fig. 3 and from these measurements theelectrical energy provided to the solution was calculated andsummarized in Table 1. Energy in gold nanofluids increasedwith increasing unipolar DC pulse power duty ratio and thehighest recorded electrical energy of gold nanofluids wasmeasured to be 331.7 kJ at the unipolar DC pulse power of100-0 % duty ratio. Detailed analysis of the I-V curve ofsolution plasma was performed using an oscilloscope andthe results are shown in Fig. 4. As the solution plasma wasgenerated in the solution, breakdown, including a rapidincrease in current and a rapid decrease in voltage at thesame time, occurred. During the breakdown, the currentincreased approximately up to 250 A and the voltagedropped down to approximately 0 V, although the voltagereturned to 1000 V instantaneously. The average current wasfrom 0.4 A to 0.6 A as the solution plasma was generated.

The absorbance spectra of gold nanoparticles with variousunipolar DC pulse powers of 100-0 % ~ 55-40 % duty ratioare shown in Fig. 5. As the unipolar DC pulse power dutyratio increased and approached to 100-0 % duty ratio, themaximum absorbance peak (λmax) of the optical spectra drewcloser to approximately 520 nm. As the unipolar DC pulsepower duty ratio increased from 55-40 % to 100-0 %, theλmax of the optical spectra decreased from 568 nm to 526 nm,which indicates that the diameter of the gold nanoparticlesalso decreased. It has been reported previously that the λmax

of gold nanoparticles is located between 520 nm and 700 nmand the λmax of optical spectra for gold nanoparticles tends toshift toward 520 nm with a decrease in particle size [10]. Inaddition, the smallest full width half maximum peak λFWHM

occurred at a unipolar DC pulse power of 100-0 % dutyratio, suggesting that the particle size distribution was com-paratively narrow.

The formation of gold nanoparticles has been explained bythe role of electrons in aqueous surfactant solutions. Pérez-Juste et al. [11] suggested that the electrons generated in thesolution by the surfactant played a vital role in the produc-tion of the gold nanoparticles. As anions such as AuCl4

− andAuCl2

− dissociated from HAuCl4 in distilled water and wereprovided with electrons, they reduced into Au0 via chemicalreactions in the solution as below:

Table 1. Calculated energy, UV, TEM and zeta potential results with various unipolar DC pulse power duty ratiosA B C D

Unipolar DC pulse power duty ratio (%) 100-0 85-10 70-25 55-40Electrical energy* (kJ) 311.8 257.6 229.4 162.3

UV (nm)λmax 526 547 550 568

λFWHM 61.4 81.2 84.7 101.6Diameter measured from TEM (nm) 18.1±5.0 42.6±16.1 45.3±19.3 70.2±22.4

Zeta potential (mV) -50.8±1.8 -46.1±0.9 -44.9±1.1 -40.9±3.3*Energy (J) = Voltage (V) × Ampere (A) × Time (second) × duty ratio (%)

Fig. 4. I-V curve of solution plasma at 85-10% duty ratio as a mea-sured duration is (a) 0.1 sec. (b) 30 usec.

Fig. 5. Absorbance spectra of gold nanoparticles synthesized withvarious unipolar DC pulse powers.

946 Yong Kang Heo and Sang Yul Lee

AuCl4− + 2e− ↔ AuCl2

− + 2Cl− (1)3AuCl2

− ↔ AuCl4− + 2Au0 + 2Cl− (2)

AuCl2− + e− ↔ 2Au0 + 2Cl− (3)

Possible mechanisms for the formation of nanoparticles inSPP had been suggested in the previous publication [8, 9].According to the continuous charging model [12], increasedelectrical energy continuously provided from the plasma isresponsible for increasing the surface areas of the nanoparti-cles in the solution by making the nanoparticles even smallerin size.

In Fig. 6, the TEM images showed that the synthesizednanoparticles in the nanofluid were close to spherical inshape. The gold nanoparticles synthesized at a unipolar DCpulse power of 100-0 % duty ratio were approximately18.1±5.0 nm in diameter and possessed uniform size distri-bution as shown in Fig. 5(a). A significantly increased parti-cle size of approximately 70.2±3.3 nm was observed fromthose nanoparticles synthesized at a unipolar DC pulse powerof 55-40 % duty ratio, as shown in Fig. 5(b). This suggeststhat the TEM images shown in Fig. 5 correspond well withthe results from the UV-Vis spectrophotometer analysis interms of the unipolar DC pulse power duty ratio effect on thesize of the nanoparticles.

As shown in Table 1 the increased electrical energy pro-vided from the solution plasma using a unipolar DC pulsepower with 100-0 % duty ratio correlates with the decreasein the size of Au nanoparticles, compared with those with55-40 % duty ratio in the nanofluid synthesized using SPP.

To evaluate the dispersion stability of the nanofluids, thezeta potential was measured and the results are summarizedin Table 1. As the unipolar DC pulse power duty ratioincreased, the zeta potential values increased. The highestzeta potential of −50.8±1.8 mV was measured from thenanofluids synthesized at a unipolar DC pulse power of 100-0 % duty ratio. This could also be explained by the increasedelectrical energy provided from the solution plasma using aunipolar DC pulse power with 100-0 % duty ratio. As increasedelectrical energy was provided from the plasma with 100-0 %

duty ratio, the charged nanoparticles in the nanofluid becamecharged further and these unipolar charged nanoparticlespushed each other away, preventing or strongly reducingparticle agglomeration [13]. Consequently, the nanofluidscontaining charged nanoparticles showed excellent disper-sion stability which is revealed in the zeta potential analysis.As shown in Table 1, the zeta potentials of the Au nanofluidsincreased as the unipolar DC pulse power duty ratio increased.As the unipolar DC pulse power duty ratio increased, theplasma intensity in the solution also increased, and this inturn allowed for increases in the electrical energy from theplasma in the solution to produce strongly charged nanopar-ticles in the solution for excellent dispersion stability.

4. CONCLUSIONS

Nanofluids containing gold nanoparticles were success-fully synthesized with various unipolar DC pulse ratios usingSolution Plasma Processing. The average particle diameterof the gold nanoparticles synthesized by SPP decreased withthe increasing unipolar DC pulse power duty ratio. Thesmallest gold nanoparticles, with an average of 18.1±5.0 nm,as well as the most uniform gold nanoparticles in size distri-bution were observed at the unipolar DC pulse ratio of 100-0 %. The highest zeta potential of −50.8±1.8 mV was mea-sured from the gold nanofluids synthesized with a unipolarDC pulse ratio of 100-0 %. Most of the gold nanoparticleswere close to spherical in shape.

The mechanisms for the formation of nanoparticles innanofluid using SPP reported in the previous publicationhave been confirmed in that increased electrical energy pro-vided from the solution plasma plays a vital role in determin-ing the size and dispersion stability of nanoparticles innanofluids.

ACKNOWLEDGMENT

This work was supported by the Korea Research Founda-tion Grant funded by the Korean Government (KRF-No2011-0017930).

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Fig. 6. TEM images of gold nanoparticles synthesized with the vari-ous unipolar DC pulse powers of (a) 100-0 % duty ratio, (b) 55-40 %duty ratio.

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