Silver nanoparticles were produced using the pin-hole discharge generated by dc non-pulsing high voltage directly in silver nitrate solutions. Sodium nitrate was alternatively added to increase solution conductivity and decrease input energy for the discharge breakdown. Argon or oxygen was bubbled through the discharge region. Comparative experiments were evaluated by UV-VIS spectrometry. Formation of silver nanoparticles with the average size of 100 nm was confirmed by SEM/EDS analysis

Doskočil, L.

Kozáková, Z.

Krčma, F.

Simic, S.

Čechová, L.

English

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

doi:10.14311/ppt.2019.2.180Plasma Physics and Technology 6(2):180–183, 2019 © Department of Physics, FEE CTU in Prague, 2019GENERATION OF SILVER NANOPARTICLES BY THE PIN-HOLE DCPLASMA SOURCE WITH AND WITHOUT GAS BUBBLINGZ. Kozáková∗, F. Krčma, L. Čechová, S. Simić, L. DoskočilFaculty of Chemistry, Brno University of Technology, Purkyňova 118, 612 00 Brno, Czech Republic∗ kozakova@fch.vut.czAbstract. Silver nanoparticles were produced using the pin-hole discharge generated by dc non-pulsing high voltage directly in silver nitrate solutions. Sodium nitrate was alternatively added toincrease solution conductivity and decrease input energy for the discharge breakdown. Argon or oxygenwas bubbled through the discharge region. Comparative experiments were evaluated by UV-VISspectrometry. Formation of silver nanoparticles with the average size of 100 nm was confirmed bySEM/EDS analysis.Keywords: plasma in liquids, pin-hole discharges, silver nanoparticles, silver nitrate, gas bubbling,SEM/EDS analysis.1. IntroductionParticles with specific size in the range between 10−9to 10−7m in diameter are called nanoparticles (NPs)[1, 2]. Materials in these dimensions have differentproperties than the bulk material and this is one ofmany reasons why the contemporary worldwide re-search is focused just on nanoparticles. Properties ofNPs are dependent on their composition, size, shape,and other parameters like internal structure. Themetallic nanomaterials (including the alloyed ones)[3, 4] can be synthetized as well as semiconductors [5]or organics (like graphene [6] or polymers [7]). Be-cause of NPs unique properties, these materials areused in many application fields such as photovoltaics[8], electronics [9], diagnostics sensors [10], conductiveinks [11], nanocomposites [12, 13], surface treatment[14], etc. A specific application field is in the medicaltherapy [15, 16] and health care. Thanks to their an-tibacterial effects, NPs are incorporated in biomedicaldevices, textiles, footwear or wound protections.One of the means how to prepare NPs is the directapplication of electrical discharge in liquids. Formedplasma channels are usually propagating through theliquid phase by the discharge itself with simultaneousdischarge radiation. The discharge usually propagatesfrom one electrode into the liquid and thus, the secondelectrode is formed by the gas (plasma) liquid surface.An advantage of plasma systems generated inside theliquid is the possibility to control the gaseous phase(even if some gas is bubbling into the system). Further,the whole system is efficiently self-mixed which helpsto uniform the size of formed NPs. The precursors canbe dissolved in the liquid or can be dissolved/sputteredfrom the electrode materials.This paper deals with synthesis of silver nanoparti-cles by a special pin-hole electrode system using DChigh voltage up to 1 kV which gave the mean powerup to 150W. Silver nitrate solutions were used as theprecursor. Additionally, two gases (argon and oxygen)were bubbled through the main electrode into theplasma region in order to study effects of their flowrate on the NPs production.2. Experimental setupThe electrode system of plasma reactor (Figure 1)consisted of two electrodes [17]. The main jet elec-trode was constructed by a tungsten wire (diameter of1.0mm) which was inserted into a dielectric cylindri-cal rod made of Macor ceramics with outer diameterof 10mm and with one conical end. An orifice withthe diameter of 1.2mm was made in the ceramicsalong its longitudinal axis. A small gap of 1mmwas kept between the end of the wire electrode andthe end of the dielectric rod. This configuration sub-stantially increased electric field intensity and thusit allowed plasma ignition at relatively low appliedvoltage. An outer glass tube served as a holder and asa gas inlet. The second electrode was a planar platemade of platinum with dimensions of 20× 100mm2.Both electrodes were immersed in a vessel containing500ml of silver nitrate solution. AgNO3 concentra-tion varied from 25 to 150mg l−1 (initial conductivityof 20–100 µS cm−1). In some experiments, sodiumnitrate (100 or 200mg l−1) was added in order to in-crease initial conductivity up to 300µS cm−1 and todecrease the input energy for the discharge breakdown.Influence of gas bubbling through the discharge regionon the particle formation was studied in two gases(argon and oxygen) with flow rates in the range of10–50mlmin−1.The system was supplied by dc non-pulsing highvoltage up to 1 kV with the negative polarity on themain electrode. The discharge itself showed self-pulsing operation [17] giving the mean input powerup to 150W.Treated solutions were sampled every 30 secondsduring the discharge operation for UV-VIS spectrom-180vol. 6 no. 2/2019 Generation of silver nanoparticlesFigure 1. Simplified scheme of plasma reactor: 1 –jet electrode, 2 – counter electrode, 3 – gas inlet, 4 –glass vessel with AgNO3 solution, 5 – active plasmaregion.etry. Absorption spectra of sample solutions wererecorded by the Helios Omega spectrometer in therange of 300–600 nm. Final solutions after the to-tal two minute treatment were prepared for SEMand EDS analyses. These samples were obtained byevaporation of 20 drops of the final solutions on spe-cial aluminium holders. Detailed photos of formednanoparticles were taken by the scanning electronmicroscope JEOL JSM-7600F with magnification of3 000x and 50 000x, respectively. Material composi-tion of nanoparticles was confirmed by the electrondisperse spectrometer with the accelerating voltage of10 kV.3. ResultsFormation of silver nanoparticles by the pin-hole dis-charge in silver nitrate solution was confirmed bySEM/EDS analysis. Figure 2(a) presents photographsin the smaller magnification of 3 000x where Ag NPsare visible as tiny dots spread among larger aggregates.These larger structures might have been formed bothduring the discharge treatment as well as the samplepreparation (evaporation) for the SEM analysis. Inthe higher magnification of 50 000x (Figure 2(b)), sep-arate Ag NPs with dimensions around 100 nm are wellvisible. Material of formed particles detected by EDSanalysis was composed mostly of silver with sometraces of aluminium (from the holder) and carbon(human contamination).Typical absorption spectra of AgNO3 solutionrecorded during the discharge treatment without anygas bubbling are shown in Figure 3. The initial sampleof pure AgNO3 solution does not absorb in the studiedwavelength range around 400 nm which is the typicalwavelength of silver nanoparticles absorption reportedin the literature [18]. Due to the dc discharge oper-ation, silver nanoparticles are formed in the plasmaregion surrounding the main jet electrode (cathode).Further, they are spread into the bulk solution by thedischarge self-mixing properties. Thus, the absorbanceof colloidal silver solution is increased linearly duringthe discharge treatment with the maximal absorptionat 410 nm.Influence of initial AgNO3 concentration in the so-lution on formation of Ag NPs was studied in the con-centration range of 25–150 mg l−1. No gas was addedin this series of experiments. According to the finalabsorbance of silver particles at 410 nm (Figure 4), itis obvious that formation of Ag NPs after two minutedischarge treatment is increased up to the initial con-centration of 100mg l−1. However, it is substantiallydecreased by higher concentration. We assume thatwith the increasing silver nitrate concentration, higheramount of silver particles is formed but they easilyaggregate into larger structures. These heavier struc-tures come faster under sedimentation and thus ab-sorption of the final solution is decreased. Therefore,it is important to adjust the optimal initial concen-tration of the precursor AgNO3 (until 100mg l−1) inorder to produce homogeneous solution with silvernanoparticles.As the initial conductivity of the primary silvernitrate solution reached only few tens of µS cm−1 de-pending on the initial concentration, sodium nitrate oftwo concentrations was added in order to increase theinitial conductivity. It is already known [19] that ig-nition as well as operation of dc discharges is stronglydependent on initial conductivity. Therefore, by theaddition of sodium nitrate (concentration of 100 or200mg l−1) we have reached a stable discharge oper-ation even at 40W instead of 100W. However, finalproduction of Ag NPs measured by the absorption at410 nm seems much lower with the NaNO3 addition(Figure 5).Influence of additional gas bubbled through the dis-charge region on Ag NPs formation was studied for twogasses. Argon was selected as an inert representativeand oxygen as a reactive gas. We had supposed that bythe ionisation of argon, the transfer of plasma energyfor reduction of silver ions to nanoparticles should havebeen more effective. However, the experimental effectwas totally opposite (Figure 6). Maximal absorbanceat 410 nm recorded for final AgNO3 solutions aftertwo minute discharge operation significantly decreaseswith the increasing flow rate of argon. The problemis probably in the formation of huge bubbles whichinterrupt the reduction process. The same trend wasobserved also for oxygen addition (Figure 7) wherethis suppressing effect was even stronger. The lower181Z. Kozáková, F. Krčma, L. Čechová et al. Plasma Physics and TechnologyFigure 2. SEM photographs of silver nanoparticles formed after two minute discharge treatment (100W) in AgNO3solution (100mg l−1); magnification: a) 3 000x, b) 50 000x.Figure 3. Absorption spectrum of AgNO3 solution(100mg l−1) during the dc discharge treatment (100W)without the gas bubbling.Figure 4. Comparison of NPs absorbance (at 410 nm)after two minute discharge treatment (100W) inAgNO3 solutions with different initial concentrationand without the gas bubbling.production of Ag NPs in presence of oxygen could beprobably explained by higher variety of chemical re-actions combining oxygen and water molecules whichleads to the dissipation of energy to these mechanismsinstead of the charge transfer needed for the silverreduction.Figure 5. Comparison of NPs absorbance (at 410 nm)after two minute discharge treatment (40 or 100W) inAgNO3 solutions (100mg l−1) with different NaNO3addition and without the gas bubbling.Figure 6. Absorption spectrum of AgNO3 solution(100mg l−1) after two minute discharge treatment(100W) with different flow rate of argon.4. ConclusionsSilver nanoparticles were produced using the pin-holedischarge generated by dc non-pulsing high voltagedirectly in water solutions containing silver nitrateas the nanoparticle precursor. Formation of silver182vol. 6 no. 2/2019 Generation of silver nanoparticlesFigure 7. Comparison of NPs absorbance (at 410 nm)after two minute discharge treatment (100W) inAgNO3 solutions (100mg l−1) with different flow rateof argon and oxygen.nanoparticles (mean size of 100 nm) was confirmed bySEM/EDS analysis. Further, UV-VIS spectrometry(absorption maximum at 410 nm) was involved forcomparative experiments. Maximal production of AgNPs was achieved at optimal AgNO3 concentrationof 100mg l−1 and applied power of 100W. Additionof sodium nitrate, which increased the initial solutionconductivity as well as decreased the minimal appliedpower, negatively influenced the Ag NPs formation.Also additional gas bubbling through the dischargeregion has a negative effect on the nanoparticle forma-tion, both for argon and oxygen. Especially in caseof oxygen, dissipation of applied energy into variablechemical reactions instead of to the reduction processwas assumed.References[1] M. Naito et al. Nanoparticle technology handbook.Second printing. Elsevier, 2012.[2] Z. Abdullaeva. Synthesis of nanoparticles andnanomaterials: Biological approaches. SpringerInternational Publishing, 2017.[3] D. L. Fedlheim and C. Foss. Metal nanoparticles:Synthesis, characterization, and applications. CRCPress, 2001.[4] G. Sharma et al. Revolution from monometallic totrimetallic nanoparticle composites, various synthesismethods and their applications: A review. Mat. Sci.Eng. C Mater., 71:1216–1230, 2017.doi:10.1016/j.msec.2016.11.002.[5] A. Rahnama and M. Gharagozlou. Preparation andproperties of semiconductor CuO nanoparticles via asimple precipitation method at different reactiontemperatures. Opt. Quant. Electronics, 44:313–322,2012. doi:10.1007/s11082-011-9540-1.[6] W. Choi and J. Lee. Graphene: Synthesis andapplications. CRC Press, 2011.[7] P. R. Sharma et al. Spherical shaped nanoparticles ofcellulose and its derivatives: A short review. TrendsCarbohydrate Res., 7:1–5, 2015.[8] B. 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Generation of Silver Nanoparticles by the Pin-Hole DC Plasma Source with and without Gas Bubbling

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Generation of Silver Nanoparticles by the Pin-Hole DC Plasma Source with and without Gas Bubbling

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