Preparation ofdispersive silver-micro/nanoparticlesby chemicalreductionmethod
Wei Xiea,b, Yaya Zhenga,* Jiacai Kuanga, Zhen Wanga, Shihe Yib,Yingjun Denga
(a.Key Laboratory of Safety Design and Reliability Technology forVehicle Engineering,Hunan Province,Changsha University of Science andTechnology,Changsha 410114, China
e-mail:
b.College of Aerospace Science and Engineering, NationalUniversity of Defense Technology, Changsha 410073, China)
Abstract - Mastery over the microscopicshape and size of a nanoparticleenablesaccurate control of its propertiesfor some strict application.The mechanismof shape-controlled synthesis wasdiscussed byinvestigating the formation of silver nanospheresprepared by chemical reduction method using Ag(NH3)2+ as metal source, ascorbic acid as reducing agent and polyvinylpyrrolidone (K-30) as dispersant.The effects of temperature, PVP/AgNO3 massratio,pH value and the interaction between PVP and silveron the shape and particle sizewere studiedbyXRD and SEM.The results show that themorphology of silver particlescould transform from branched to spherical and the particle sizegradually decrease with the increase of PVP/AgNO3 mass ratio. The particles size can also be significantly influencedby pH value and temperature. The keypoint for preparinghighdispersitysphericalsilver powderis thatthe growth rate of each planeof the particle must be uniform and synchronous.Silver powderswith sphericalparticles withmean size of 0.2μmwere synthesized under theoptimumconditions(PVP/AgNO3 mass ratio 0.6, pH 7, reaction temperature of 40oC).
Key words: silver powder,dispersant,shape-controlledmechanisms,particle size
Introduction
The number of applications of metalmicro/nanoparticlesiscontinuously growingwith development of the methods of their preparation providing controlled shapes and sizesof the particles and regulating their properties[1-2]. It has been confirmed that theperformance of metal micro- and nanostructuresrely not only on the sizeand size distribution,but also on the shape of powderparticles [3-4].Silverpowderhas always been one of the most widelystudied materials for many decades, havingapplicationsincatalysis, information storage,optoelectronics,photonics, andelectronicsindustry because of its special physicochemical properties and the characteristics resulting from different shape and size effect of micro/nanoparticles[5-10].
Up to now,a few methods have been used for preparing silver micro/nanoparticles such aselectrolysis[11], spray pyrolysis[12], chemical synthesis[13] andbiologicalmethods[14].Each processcan providesilver powders with certain characteristics like shape,size and functional properties[15-16]. Liquid-phase chemical reductionmethod is apreferableand widely usedmethod because of its low cost, simple equipment, and possibility tocontrol powder size and morphology.Recently, there are many reports on synthesizing silver micro- and nanostructures with different size and shape.These include reduction of AgNO3 by glucose, ascorbic acid,formaldehyde,and zinc.Various dispersants, such aspolyethyleneglycol,gelatin, arabic gum and PVP have been used in itspreparation. However, many of these methods give undesirable morphology of the particles and serious agglomeration. Precise control of the morphology of the silver powderis a problem almostwithoutsatisfied answer[17-18].It becomes a momentous challenge to launch a core shape-controlled mechanismto direct micro/nanoparticlespreparation with different shape.Therefore, this papermainly focuses onpreparation ofdisperse silver micro/nanoparticlesvia chemical reduction method, and a fundamental shape-controlledmechanism according to the experimental conditions was elaborated toprepareanddesign more complicatedshape of Ag micro- and nanostructuresby demonstrating thereasons of formation of powders with differentcharacteristics of sphericity, dispersibility and size.
Experimental
Silver nitrate, sodium citrate, ascorbic acid (C6H8O6),polyvinylpyrrolidone (K-30),ammonia(28-30%) and concentrated nitric acid were ofanalytical gradeandwere used without further purification. Distilled water was used in all experiments. The effects of PVP, pH value and temperature on the silver particle size were studied.The experimental parameters are shown in Table.1. The pH of the solutions was adjusted by adding dilute ammonia or dilute nitric acid. The solutioncontaining ascorbic acid and PVP was added into a specific amount ofsilver-ammoniasolution with agitationfor 35min by astirrer (260rpm) in awaterbath at a specifiedtemperature. The reaction wascompleted under sonication for 30min.Thesilver powders werefiltered and washed with deionized water and alcohol until pH of filtrate was 7, then the powder was dried at 100oC for 3 h.
The phase structure of the obtained sampleswas examinedbyX-ray diffraction (XRD) analysis using a D/MAX-2200 X-ray diffractometerwith Ni-filtered CuKαradiation (λ=1.5406 Å )at roomtemperature. The morphology, size and dispersibility of the prepared samples werestudiedby scanning electron microscopy (FEI Quanta 200) operated at 20kV.The particles size was measured with PMLS601 laser size distribution analyzer.
Table1.Experimental parameters
Sample No / pH / Temperature, C / m(PVP)/m(AgNO3)1 / 7 / 40 / 0.0
2 / 7 / 40 / 0.2
3 / 7 / 40 / 0.4
4 / 7 / 40 / 0.6
5 / 6 / 40 / 0.6
6 / 7 / 40 / 0.6
7 / 8 / 40 / 0.6
8 / 9 / 40 / 0.6
9 / 7 / 30 / 0.6
10 / 7 / 50 / 0.6
11 / 7 / 60 / 0.6
Notes:Other experimental parameters are as follows: AgNO3 concentration is 0.2 mol/L, C6H8O6 concentration is 0.2 mol/L.
Results and discussion
The formation of silver powder
It is important to slow down and stabilize the reduction to achievecontrolled synthesis of thehigh dispersity silver-micro/nanoparticleswith desired and uniform sizeand morphology, becauseslowreduction process is beneficial to maintain reduction parameters and generatehighly anisotropic nanostructures.For this purpose, Ag(NH3)2+was selected as the precursor instead ofAg+, and ascorbic acid was used as reducing agent.Reactionsare shownas follows：
Ag++2NH3→Ag(NH3)2+ (1)
2Ag(NH3)2++C6H6O4(OH)2→2Ag+2NH4+ +C6H6O6 (2)
According to the Nernst equation, the followinghalf-reaction potentials are found :
Ag++e-=Ag,E00=0.7996V(3)
Ag(NH3)2++e-=2NH3+Ag,E10=0.373V(4)
According to these potentials Ag+is a stronger oxidizer than Ag(NH3)2+.
Theprocess of formation of silver powderby the reduction reaction (2) includes three stages. At the first stage the concentration of Ag atoms increaseswith time steadily. ThenAgatoms start to aggregateintonucleiand formprimary particles when the concentration of atoms reaches a point of supersaturation, and this process won’t stop until the concentration of atoms turns belowthe level of minimum supersaturation, finally, with a continuous supply of atoms viareducingprecursor saltsteadily the primary particles grow intothe secondaryparticles with lager size by absorbingor agglomeratingsurrounding particles,theschematic diagramcan be confirmed by the Lanmer’smodelof silver powder formationshown in Fig.1[19].
In order to obtainsilver micro/nanoparticleswith uniform size, it is necessary tothe nucleation process to be in accordance with the "outbreak of nucleation" modeto make the silver nucleigrowsynchronouslyby shorteningthe time for second stage of the process,so at high concentration of primary particlesthey rapidlyagglomeratetogether.In addition,primary particles are generated more spontaneously, so the size and morphology of the silver particlesaremainly affected during the thirdstage. With the purpose ofpreparingsilver particles withdesired size and morphology, a suitable dispersant is needed to protect the primary particles. In the present work we selectedPVPas a dispersant, because it canpreferentially cover theparticles surface to hindertheirgrowth and agglomeration[20].Moreover, its effect on initial particle size and crystallinestructure is not studied well. The SEM images of silver powders obtained at different PVP/AgNO3 mass ratios (Nos.1, 2, 3, 4) are shown in Fig.2.
Fig.2SEM images of silver powders obtained at different PVP/AgNO3mass ratio
(1#)m(PVP)/m(AgNO3)=0 (2#)m(PVP)/m(AgNO3)=0.2
(3#)m(PVP)/m(AgNO3)=0.4 (4#)m(PVP)/m(AgNO3)=0.6
The XRD patterns of silver powders prepared at different PVP/AgNO3 mass ratios are shown in Fig.3.There are four remarkablepeaksattributable tothe crystalline silver (JCPDS cardNo. 04-0783) which provethat the prepared powder is indeed silver with face-centered cubic structurenot affected by PVP. At the same time, sharpdiffraction peaksindicate that the crystallinity ofsilver powders is very high.The calculation of the Ag primary particle sizebyScherrer formula (for the Ag {111} crystal planereflection peak) gives the value of ~30nm, soPVPaffects the formation of primary particles only slightly, becauseitjust influences the stability of primary particles and the formation environment of the secondary particles.
Fig.3XRD patterns of silver powders prepared at different PVP/AgNO3ratios. The numbers near the curves correspond to the sample numbers.
Table.2 Thecrystallite sizeof sliver powders prepared at different PVP/AgNO3ratios
Sample No / 2θ (°) / Lambda(Å) / Peak FWHM / D(nm)1 / 38.201 / 1．5406 / 0.279 / 29.1
2 / 38.198 / 1．5406 / 0.269 / 30.2
3 / 38.209 / 1．5406 / 0.270 / 30.0
4 / 38.197 / 1．5406 / 0.273 / 29.7
Effect of PVP
A series of experiments was carried out to investigate the effect of PVP on characteristics of silver particle size and morphology.The SEM images and the average size of silver samples (Nos.1 -4) obtainedatdifferentPVP/AgNO3 mass ratio are shown in Fig.2 and Fig.4, respectively.It can be seen that the sizes of the particles significantly depend on PVP/AgNO3 mass ratio. The size of the particles decreases linearly from 9.1to 0.2μm with increase of this ratio, and their shape transforms from branched to spherical. When the mass ratio is 0.6, the powders have the better shape and dispersibility than others.Because enough dosage of PVP dispersant can fully isolatethe powder particlesfrom each otherto hinder further agglomeration of immediate particles, and it is also able to balance the growth rate of each face of the crystallites by preventing the preferential growth of high surface energy planes. This condition could cause the particlesshapetoturn near to spherical with the lowest surface energy.The particles shape turns into branch when the mass ratiois 0.4. As the quantity of PVP is only justenoughtocove the {100} planes, thepreferential addition of silver atoms to the {111} planes takes place，so the particles eventually grow in branched morphology through the overgrowth on their {111} corners.With the continuously decreasing of this mass ratio, the silver particles become larger insize and irregular in shape throughrandom growth and collision agglomerationwithout adequate PVP protective coating.
Fig.4 Effect of PVPquantity on silver particles size
Effect of pH
The effect of pH on characteristics of silver particlesis shownin Figs.5 and 6. The particles size(D10, D50 and D90) decreases initially and laterincreases with the increase of pH value. Low or high pH values results in largerparticles.According to the following equations:
C6H6O6+2H++2e=C6H8O6 (5)
E=E0-0.059pH (6)
The reduction capacity of ascorbic acid will increasewiththe increaseof pH value. Whenthe pH value is less than 7, so the reducing ability of ascorbic acid is rather weak, the formation of nuclei is slower,facilitating theparticlesgrowth.Whenthe pH valueis greater than 7,the chemical stability of ascorbic acid is low.It acceleratesthe nucleation rate and the formationprocess of primary particlesgoes in accordance with the "outbreak of nucleation" mode.However,the newlygenerated primary particles haven’tbeen protectedinstantly by PVP coating,which makesmost of theprimary particles toagglomerate together.Only when pH is 7, the silver powderswith gooddispersibilityand size(D50) of 2.1μm can be prepared.
Fig.5 Effect of pH value on silver particles size
Fig.6.SEM images of silver powders obtained at different pH values
(5#) pH=6, (6#) pH=7, (7#) pH=8,(8#)pH=9
Effect of temperature
The characteristicsof particle size and morphology of the samples(Nos2, 5, 6, 7) obtained at different temperatures are illustrated by Figs.7 and 8.The silver powders prepared at 40oC have the perfect characteristics of relatively highsphericity and small size(D50) of 0.9μm. The silverparticle size primarilydecreases,and later increaseswith theincreaseoftemperature.The main reason for this phenomenon may be explained as the degree of particles agglomeration is significantlyenhancedby Brownianmovementwhentemperature is above 40oC.Another reason for large size is that the reduction rate andthe formation velocityof the silver particles are quick at high temperature, which makes the silverparticles agglomerate without immediate covering with PVP. Forthetemperatureof30oC, the low chemical activity of solutionhindersthe primaryparticle formation and facilitates the growth and agglomeration.
Fig.7 Effect of temperature on silver particles size
Fig.8SEM images of silver powders obtained at different temperatures
(9#)30oC, (6#)40oC, (10#) 50oC, (11#) 60oC
From above experimental results, we can conclude that PVP/AgNO3 mass ratio is the predominant factor inducing the variationof morphologies of micro- and nanostructures. A probably process of shapeevolutionof micro/nanoparticlesis illustrated by followingdetails: we have already mentioned that the growth rate of each direction of particle’s planes can be changed by capping agent (PVP).The highly reactive surface atoms of the growing particle are prone to adsorb free silver atoms forming in the solution phase and agglomerate with adjacent particles together, reducing the surface energy. Therefore, the shape of micro/nanoparticlesbecomes variable. On the other hand, when appropriate capping agent is added to the solution the surface energy of the certain planesof the particlesis also reduced. Preferential physical capping of certain planes,and/orforming coordination bondswith thesesurfaceatomsimpede the agglomeration of particles sothe growth rate of these planes would belower and fixed.Additionally, the further growth will mainly increase the size of other planes not capped by PVP, then thesubmicron structureshowspredictableshape of branch.For thesamereason, the particles would grow into sphere when the quantity of PVP is enough to capall its surfaces. Therefore, here we come to a practicable conclusion that the change of starting silver compound and/orcapping agent quantitycan tune the process of shapeevolution of micro- and nanostructures.The schematic diagram of shape evolutionof micro- and nanostructuresis shown in Fig.9.
4.Conclusions
The dispersive silvermicro/nanoparticleshave been prepared using PVP as a dispersant, ascorbic acid as a reducing agent, and Ag(NH3)2+as the source of silver. Results show that the PVP/AgNO3 mass ratiocan change theshape andsize of particles greatly,and thepH value and the temperature mainly influence the particle size. The probably mechanism of shape-controlled synthesisfor preparinghighly dispersivesilver nanoparticles is that PVPfully covers theparticle’ssurface,making the particle growth at each surface uniform.The average size of silver powder is 0.2μm when the reaction temperature is 40℃, PVP/AgNO3 mass ratiois 0.6and the pH value is 7.
Acknowledgements
This study is supported by National Nature Science Foundation of China No.51201022, Science and technology programs of Land and Resources Bureau of Hunan Province (No.2011-02), China Postdoctoral Science Foundation No.2013M542555, the Science & Technology Program of Hunan Province No.2014FJ6027, and the Science & Technology Program of Changsha No.K1303017-11.
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