A simple plasma reduction for synthesis of Au and Pd nanoparticles at room temperature☆

Zhao Wang*,Yu Zhu

State Key Laboratory of Chemical Engineering,School of Chemical Engineering,Tianjin University,Tianjin 300072,China

Keywords:Glow discharge plasma Nanoparticles Gold Palladium Reduction Preparation

ABSTRACT A simple and fast plasma reduction method is developed for synthesis of Au and Pd metal nanoparticles.The scanning electron microscopy(SEM)analysis indicates a formation of aggregates of Au and Pd nanoparticles with branched structure.The transmission electron microscopy(TEM)image shows that the inclusive nanoparticles are all about 5 nm in size.Compared to conventional hydrogen reduction method,plasma method inhibits the agglomeration of metal particles.The room temperature operation is very helpful to limit the nanoparticle size.Most interestingly,plasma reduction produces moreflattened metal particles.This plasma reduction does not require the use of any hazardous reducing chemicals,showing the great potential for the fabrication of noble metal nanoparticles.

1.Introduction

Various noble metal nanoparticles have attracted a great attention because of their specific properties[1–4].Their performance is closely related to the morphology and size of the metalnanoparticles.In general,smalland flatnanoparticles are more effective because there willbe a higher fraction of the metal atoms exposed at the surface where they are accessible to reactant molecules and available for reactions.Reduction of noble metal ions is a basic operation for the synthesis of metal nanoparticles[1,5–9]and also for the recovery of noble metals from metal containing waste water[10,11].The properties of metal nanoparticles are determined by the preparation method in which the reduction is a crucial factor.Metal ions can be reduced using hydrogen at elevated temperature,but the resulting particles are heterogeneously dispersed with a large size distribution due to aggregations.Many techniques have been developed to prepare metal nanoparticles with controllable particle size and shape[4,12–15].Chemical reduction uses reducing agents(like NaBH4)to produce homogenously distributed metal particles.However,the utilization of chemicals leads to some environmental problems.Alternative reduction technologies recently attract much attention.The developing technologies include reduction with alternative reducing agents[1,12],reduction using electrochemical reduction[13]and photocatalytic reduction using UV irradiation[14].Most of these reduction technologies still require a chemical reducing agentto reduce metal ions into the metallic state.Recently,hydrogen plasmas have been applied for the fabrication of nanometal particles[16].There exist a plentiful of hydrogen radicals and atoms within hydrogen plasmas,which are excellent reducing agents.Although reduction using hydrogen plasmas is clean and dry,it still requires the use of hydrogen as reducing agent.

Previously,we utilized argon glow discharge plasma to treat supported noble metal catalysts in order to improve their catalytic performance[17–22].Some metal nanoparticles on the porous support are formed after plasma treatment,which suggests that these noble metal ions are reduced to their metallic states.This is very attractive because no chemical reducing reagents would be needed.The interest of Au and Pd nanoparticles in the field of nanocatalysis and nanosensors is growing[23,24].In this work,it is attempted to extend this new glow discharge plasma reduction for the syntheses of Au and Pd nanometal particles directly from metal precursors with no supports.

2.Material and Methods

Details of the plasma equipment and the treatment procedure have been described elsewhere[17].To perform the reduction with argon glow discharge plasma,drops of HAuCl4or PdCl2solution(0.001 mol·L-1)were added on the small glass boat(1 cm ×2 cm×0.2 cm).To prepare the sample for atomic force microscope(AFM)characterization,HAuCl4solution wasadded on the Si(111)substrate(iNano,Aarhus University).The boatorsubstrate was then placed in the glow discharge chamber,which was a quartz tube(i.d.35 mm)with two stainless steel electrodes(o.d.30 mm).The gap between the electrodes was 170 mm.When the pressure in the discharge tube was about 100 Pa,the glow discharge plasma was generated by applying around 900 V to the electrode using a high voltage amplifier(Trek,20/20B).The signal input for the high voltage amplifier was supplied by a function/arbitrary waveform generator(HP 33120A)with a 100 Hz square wave.The current was in the range of 1–2 mA.Argon(99.9%)or oxygen(99.9%)was introduced and served as the plasmaforming gas.The time of each plasma treatment was 10 min.The discharge voltage and current were measured using an oscilloscope(Tektronix TDS210)equipped with a high-voltage probe(Tektronix P6015A)and a currentprobe(Pearson Electronics 411).The gas temperature of the plasma was measured by infrared imaging(Ircon,100PHT).The measurementconfirmed thatthe glow discharge reduction was conducted at room temperature.X-ray diffraction(XRD)characterization was conducted using a Rigaku D/Max-2500 V/PC diffractometer with Cu Kαradiation(λ =0.154178 nm).Scanning electron microscopy(SEM)analysis was performed with a Philips-XL30 ESEM system.Transmission electron microscopy(TEM)analysis was carried out using a Philips TecnaiG2 F20 system.AFMtests were performed by a Veeco Multimode Microscope V(Veeco Instruments Inc.,Plainview,NY,USA)operating in contact mode.

3.Results and Discussion

With the operation of glow discharge plasma,a significant change in color of metal salts loaded can be observed.It indicates that the metal ions are easily and effectively reduced by the glow discharge and metallic particles formed by this way.Fig.1 shows the XPS spectra.It is analyzed to determine the chemical state of the plasma-reduced metal.As shown in Fig.1(a)and(b),each sample has two peaks centered at 340.2 eV and 334.9 eV for the Pd sample,and at 86.7 eV and 83.1 eV for the Au sample.According to reference data,these peaks can be assigned to the electron transitions of metallic Pd3d3/2and Pd3d5/2,and Au4f5/2and Au4f7/2,respectively[25].The results clearly confirm that the metal ions have been reduced to their metallic states by the glow discharge plasma.

Fig.1(c)and d also shows the XPS spectrum for the O2plasma treated samples.The peaks for them are same as those obtained for the Arplasma reduced samples,which proves that the O2plasmas can also effectively reduce noble metal ions to their metal states and the plasma reduction is independent of the type of plasma-forming gas.

Fig.2.XRD patterns of the plasma reduced samples.(a)Argon plasma reduced Pd,(b)oxygen plasma reduced Pd,(c)argon plasma reduced Au,and(d)oxygen plasma reduced Au.

Fig.1.XPS spectra of plasma reduced samples.(a)Argon plasma reduced Pd,(b)argon plasma reduced Au,(c)oxygen plasma reduced Pd,and(d)oxygen plasma reduced Au.

Fig.3.SEM image of reduced samples.(a)Argon plasma reduced Au,(b)hydrogen reduced Au at 500 °C,(c)argon plasma reduced Pd,and(d)Hydrogen reduced Pd at 500 °C.

Fig.2 shows the XRD patterns of plasma-reduced Au and Pd which further confirm the metal ion has been totally reduced by argon glow discharge at room temperature.Four peaks are observed for the Ar plasma-reduced Au,corresponding to Au(111)at 38.1°,Au(200)at 44.3°,Au(220)at 64.7°and Au(311)at 77.6°.For the Ar plasmareduced Pd,the three peaks at 2θ=40.1°,46.6°and 68.0°can be attributed to the metallic crystals Pd(111),Pd(200)and Pd(220),respectively.It also shows the existence of stable metal particles.In the XRD spectra,no diffraction peak of the metal precursors is observed,confirming that the plasma-reduced metals are completely in the metallic states without detectable impurities.The solution by soaking the plasma-treated samples is characterized by opticalemission spectroscopy with ICP(VISTA-MPX).The metal content of the metal is zero.It means no metal ions dissolved in the water.Based on these characterizations,the yield for the metal nanoparticles is 100%.XRD patterns of oxygen glow discharge reduced sample are also shown in Fig.2.No oxide peaks can be observed.Thus,the plasma reduction is not dependent on the plasma-forming gas.

It is proposed that the mechanism of plasma reduction may be related to two processes.One is a direct process.The high energy electrons generated by the plasma most likely serve as the reducing agents.When the glow discharge reducible metal ions are immersed in the plasma zone,electrons will move towards the Au or Pd ions due to the electrostatic attraction.Then the surface of metal particle will form an electric shell.The electrostatic repulsion between the metal particles will contribute to limit the nanoparticle size.

The second process is an irradiation process.The primary radicals and molecules produced from water upon electron-beam pulse[26].The solvated electrons eaq-and H˙atoms are indeed the strongest reducing agents that are responsible for reduction of the metal ions to the metallic state through reactions(2)and(3).The Au3+is reduced to Au0by the reducing agents,as indicated below.

The non-precious metals(e.g.Fe,Co,Ni,Cu)are also treated by plasma method.But the non-precious metal cannot be reduced by plasma without H2gas.The standard reduction potential for an ion pair may be a criterion to determine whether a metal ion can be easily reduced or not.For example,the standard reduction potentials of Pd2+/Pd(0.92),and[AuCl4]-/Au(1.00 V)are much higher than that of Ni2+/Ni(-0.25 V).Further investigates are needed to verify this proposal.

Fig.3(a)shows the specific branched structure of Au with the argon glow discharge reduced aggregate.Comparing with the image of hydrogen-reduced sample[Fig.3(b)],the aggregate size of plasmareduced sample is much smaller and the distribution of the particles is more homogeneous.Because the glow discharge reduction isconducted at room temperature,the low temperature operation can effectively avoid the size control problems generated by hydrogen reduction at elevated temperatures.The reduced Au could retain some electric charge and repeleach other thatalso lead to reduce the aggregate.Similarly,for Pd sample[Fig.3(c)and(d)],plasma reduction method is also better to form the branched structure and limit the aggregate.

Fig.4.TEM image of argon plasma reduced samples.(a)Argon plasma reduced Au and(b)argon plasma reduced Pd.

Fig.5.AFM images of Au/Si(111)by plasma.

TEManalysis is performed in orderto observe the single particle.The TEM image(Fig.4)shows that the inclusive nanoparticles of Au and Pd are all about 5 nm in size.A narrow size distribution has also been achieved.

In our previous work,the HAuCl4solution has directly treated withoutsupport by plasma[27].Gold were successfully synthesized atroom temperature using glow discharge plasma within only 5 nm.The size of colloidal Au nanoparticles could be effectively tuned in the nanometer range by easily adjusting the initialconcentration ofaqueous HAuCl4solution.So,the role of support maybe not the key point in this process.

Fig.5 shows the AFM images for the argon plasma reduced samples.With increasing concentration ofprecursor,the particle size increases to 40 nm on the silicon substrate.Most interestingly,plasma reduction produces moreflattened metalparticles.The length along horizontaldirection of gold particles is 40–50 nm about 10 times longer than that along the perpendicular direction(4–5 nm).The proposed mechanism is that the internal and external particles have very strong electric force,changing the shape of particles.Similar modification of particle shape has been observed for the plasma-treated NiO particles,suggesting thatitmay be a universaleffectofplasma treatment[28].A flattened morphology means large coverage and a large metal–support interface when the metal is loaded onto support materials.This is very useful in catalysis because it can provide more active sites on surface.

4.Conclusions

In conclusion,the argon glow discharge plasma can effectively transfer metalions to metalnanoparticles withoutthe addition of any reducing agents.It is very promising to prevent the aggregation for the control of Au and Pd particle size.The plasma reduction also shows a modification of particle shape,producing moreflattened nanoparticles.Compared to the traditional reduction technologies,this novel plasma reduction method can reduce the use of hazardous chemicals.Therefore,the plasma reduction is green and easily manipulated,which will provide a new way to the preparation of noble metal nanoparticles.