Diameter control of gold nanoparticles synthesized in gas phase using atmospheric pressure H2/Ar plasma jet and gold wire as the nanoparticle source Control by varying the H2/Ar mixture ratio Diameter[.]
Diameter control of gold nanoparticles synthesized in gas phase using atmosphericpressure H2/Ar plasma jet and gold wire as the nanoparticle source: Control by varying the H2/Ar mixture ratio Yoshiki Shimizu Citation: AIP Advances 7, 015316 (2017); doi: 10.1063/1.4975636 View online: http://dx.doi.org/10.1063/1.4975636 View Table of Contents: http://aip.scitation.org/toc/adv/7/1 Published by the American Institute of Physics AIP ADVANCES 7, 015316 (2017) Diameter control of gold nanoparticles synthesized in gas phase using atmospheric-pressure H2 /Ar plasma jet and gold wire as the nanoparticle source: Control by varying the H2 /Ar mixture ratio Yoshiki Shimizua Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan (Received 18 September 2016; accepted 24 January 2017; published online 31 January 2017) This report describes diameter control of gold nanoparticles (AuNPs) during synthesis using an atmospheric-pressure H2 /Ar plasma jet drive with pulse-modulated ultrahigh frequency, employing Au wire as the NP source material During this process, where most of the AuNPs are regarded as formed through condensation from Au vapor derived by the Au wire etching, the mean diameter varied in the approximate range of 2–12 nm with H2 volume ratios up to 3.9% In plasma diagnostics, results showed that the H2 volume ratio influences the plasma discharge behaviour, which affects the heat flux density flowed into the Au wire, and the atomic hydrogen concentration in the plasma Both seemed to influence the etching rate of the Au wire per unit area, which is directly related to the concentration of Au vapor in the plasma The concentration is one factor affecting the particle size evolution because of the collisions among vapor species in reaction field Therefore, the AuNP size variation with the H2 volume ratio was discussed from the perspective of the etching rate of the Au wire at each H2 volume ratio © 2017 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4975636] I INTRODUCTION The use of an atmospheric pressure plasma jet (APPJ) facilitates handling of plasma in open air because of its easy-to-use apparatus, resembling a pencil Consequently, APPJ has been tried for simple use for plasma-enhanced material processing in open air Regarding application to nanoparticle (NP) synthesis in open air, several studies have used chemical vapor deposition (CVD).1–8 Gas or mist of a solution as an NP source is supplied from the upstream region of the APPJ Most of the NPs are formed in the gaseous plasma stream via CVD This technique facilitates NP synthesis and their subsequent deposition on a supported material Consequently, applications such as easy coating with functional NPs can be anticipated However, it is often difficult to synthesize NPs of a desired material via CVD, because toxic and hazardous raw materials, which are often used in CVD with reactor vessels equipped with exhaust equipment, cannot be used in open air To resolve this difficulty and to enable synthesis of widely various NPs using APPJ, there are some methods using solid metal target as NP source.9–14 The author and co-workers have been investigating method using metal wire as the NP source.9–13 The metal wire, placed in the discharge tube, is etched using plasma driven with ultra-high-frequency (UHF) The resultant metal vapor is condensed to form NPs in the plasma flow In this method, the wire etching process plays a key role in the synthesis of NPs efficiently Accordingly, optimization of the plasma working conditions to enhance etching is necessary for each kind of source wire a Corresponding author: E-mail: shimizu.yoshiki@aist.go.jp, Tel.: +81-29-861-9364, FAX: +81-29-861-6355 2158-3226/2017/7(1)/015316/8 7, 015316-1 © Author(s) 2017 015316-2 Yoshiki Shimizu AIP Advances 7, 015316 (2017) In synthesis of gold (Au) NPs from Au wire using the method described above, two conditions are indispensable:13 (1) plasma discharge using a hydrogen (H2 ) – argon (Ar) mixed gas, and (2) pulsing of the plasma discharge The former seems to involve an “atomic hydrogen – metal reaction model”,15 in which an exothermic reaction of the recombination of atomic hydrogen, originated from the H2 , enhances metal etching In the latter condition, break of the Au wire, which occurs during the continuous discharge of the plasma because of excessive heating, is preventable by lowering the gas temperature on a time-averaged basis Both conditions enable the Au wire etching to be sustained, thereby leading to continuous synthesis of AuNPs over a long period In earlier report describing the synthesis of AuNPs,13 the author and co-workers reported the synthesis of crystallized AuNPs of about nm mean diameter with good monodispersity In subsequent experiments that the author had performed, the diameter was found to be almost independent of the pulse frequency and flow rate of the H2 -Ar mixture gas: the H2 volume ratio was fixed to 3.9% The AuNPs, with diameter of about 6–8 nm, were formed constantly Recently, the author found that composition ratio of H2 -Ar mixture gas influences the diameter of synthesized AuNPs in the method described above The composition ratio is closely related to the plasma dynamic behaviour around the Au wire and the concentration of the atomic hydrogen in the plasma They apparently affect the variation of the diameter of synthesized AuNPs In this paper, the author will present the diameter variation of the AuNPs synthesized by the atmospheric pressure H2 /Ar plasma jet driven with pulse-modulated UHF and discuss diameter variation mechanisms based on plasma diagnostics and observations of the consumed wire II EXPERIMENTAL A Plasma generator for AuNP synthesis Figure presents a schematic drawing of the plasma generator used for AuNP synthesis The alumina discharge tube, which had 0.8 mm inner diameter, was folded with a metal tube The metal FIG Schematic drawing of the plasma generator used for AuNP synthesis 015316-3 Yoshiki Shimizu AIP Advances 7, 015316 (2017) tube was connected to a plasma gas feed line via an insulation tube The electrode was a five-turn brass wire coil and was looped around the 10 mm zone at the tube outlet side One end of the coil was connected to a UHF (450 MHz) generator via a matching circuit, whereas the other end was floated A 100 µm diameter Au wire, the NP source, was inserted inside the discharge tube One end (tip) of the Au wire was located in the zone between the first and second turns of the coiled electrode, counted from the outlet side The other end of the wire was affixed to the inner wall of the metal tube and was grounded B Synthesis of AuNPs For AuNP synthesis, an atmospheric pressure H2 /Ar plasma jet was discharged under the following conditions, which had been found for stable and sustainable processing First, Ar gas mixed with H2 was fed into the discharge tube In this study, the H2 volume ratio in the H2 /Ar mixture gas (hereinafter, H2 %) was a parameter It was varied up to 3.9 H2 %, which is slightly lower than the explosion limit concentration of H2 in air (4%) The mixture gas feed rate was fixed to 200 standard cubic centimetres per minute (SCCM) The UHF voltage, which was pulse-modulated using a signal of constant duty cycle 5% [100 × (on-time)/(on-time + off-time)] from a function generator, was applied to the coiled electrode The pulse repetition frequency and the maximum UHF voltage were fixed, respectively, to 100 Hz and 250 Vp-p Under these conditions, the plasma was pulsingly generated according to the signal from the function generator in the discharge tube The AuNPs derived from Au wire were dispersed directly on a thin carbon film for transmission electron microscopic (TEM, JEM-2010; JEOL Ltd.) observation, placed about mm downstream from the discharge tube outlet The AuNP diameter was determined by direct measurement from the TEM images The Au wire consumed during processing was observed later using scanning electron microscopy (SEM, S-4800; Hitachi Hi-tech Co.) C Plasma diagnostics Optical emission spectroscopy (OES) and high-speed photography were used to elucidate the variation of the plasma characteristics with the H2 volume ratio in the mixture gas For these diagnostics, a quartz capillary was used for the discharge tube The plasma near the Au wire tip, where the wire surface etching had well progressed, was diagnosed through the gap of the coiled electrode A high-resolution fibre optic spectrometer (HR-2000CG-UV-NIR; Ocean Optics Inc.) was used for OES A convex lens was used to collect emissions from the minute analysis zone and focus them on an optical fibre The spectra were recorded with integration time of 100 ms, which corresponds to a 10 pulse cycle of the plasma discharge High-speed photograph for characterizing the plasma dynamic behaviour in one pulse cycle was recorded using a fast intensified charge-coupled device (ICCD) camera (PI-MAX2, PI-Acton; Princeton Instrument, Inc.) The exposure time was set to 10 ms, corresponding to one cycle of the plasma discharge All photographs in this paper were obtained by single shot III RESULTS AND DISCUSSION A Diameter variation of AuNPs AuNPs were synthesized using the following H2 %: 0.0, 0.5, 1.0, 2.0, 3.0, and 3.9 H2 % No AuNPs were formed at 0.0 H2 % Crystallized AuNPs were formed in 0.5–3.9 H2 % and their diameter variation with H2 % was found Figure shows TEM observations of the AuNPs and the diameter distributions in each H2 % At 0.5 H2 % (figure 2(a)), monodispersed AuNPs were formed The mean diameter (m.d.) ± standard deviation (s.d.) was 2.6 ± 0.47 nm At 1.0 H2 % (figure 2(b)), not only AuNPs similar to those in 0.5 H2 %, but also apparently larger AuNPs were formed simultaneously In these observations, the diameter variation seems not to be in one series Actually, the diameter histogram exhibits a bimodal distribution composed of two non-consecutive distributions The m.d ± s.d in the respective distributions were 2.62 ± 0.48 and 12.2 ± 1.71 nm At 2.0 H2 % (figure 2(c)), the AuNPs were formed, similar to the larger ones in 1.0 H2 % The m.d ± s.d was 11.4 ± 1.45 nm 015316-4 Yoshiki Shimizu AIP Advances 7, 015316 (2017) FIG TEM observation of the synthesized AuNPs (left) and the diameter distribution (right) of (a) 0.5, (b) 1.0, (c) 2.0, (d) 3.0, and (e) 3.9 H2 volume% At 3.0 H2 % (figure 2(d)) and 3.9 H2 % (figure 2(e)), monodispersed AuNPs were formed Their m.d ± s.d were, respectively, 6.68 ± 0.90 and 6.76 ± 0.83 nm These results are summarized in graph (figure 3) In this graph, the diameter variation trend seems to change at 1.0 H2 % 015316-5 Yoshiki Shimizu AIP Advances 7, 015316 (2017) FIG Diameter of AuNPs as a function of the H2 volume% Error bars correspond to the distribution range Dots reveal mean diameters in each consecutive distribution range B Observation of the consumed wire surface The SEM observations of the Au wire after processing provided information supporting the discussion of the diameter variation with H2 % Figure presents representative observations of the outer shape of the wire tip and the surface morphology produced with variation of H2 % At 0.5 H2 % (figure 4(a)), the outer shape remained almost unchanged in comparison with that before processing, but the surface morphology was changed to a gentle corrugated shape over a few millimetres of length from the tip However, in 2.0–3.9 H2 % (figure 4(c), 2.0 H2 %), the outer shape was changed drastically The surface form showed rough corrugated morphology in a few hundreds of micrometers from the tip, which is shorter than that at 0.5 H2 % At 1.0 H2 % (figure 4(b)), which is the intermediate range of both above, the gentle and rough corrugated morphologies were mixed on the surface The corrugated morphology that is apparent after processing might be the trace of the etching of the wire The difference of the corrugation manner among H2 % values might reflect the grade of etching, as will be discussed later C Optical emission spectroscopy The plasma in the local area, where morphology of the inserted wire surface changes as shown in figures 4, was characterized using OES to elucidate the diameter variation of the AuNPs from the perspective of the plasma characteristics FIG SEM observations of the outer shape of the Au wire (upper) and surface morphology (lower) after processing at the H2 volume% of (a) 0.5, (b) 1.0, and (c) 2.0% as the representative in range of 2.0–3.9% 015316-6 Yoshiki Shimizu AIP Advances 7, 015316 (2017) FIG (a) Normalized emission intensity of Ar (763 nm) and Hα (656 nm) as a function of the H2 volume% (b) Relative intensity of Hα (656 nm) to Ar (763 nm) as a function of the H2 volume% Figure 5(a) presents variations of the emission intensities of Ar 763 nm and Hα 656 nm lines, with H2 % The emission intensity of Ar decreases concomitantly with increasing H2 %, whereas the intensity of Hα increases to 1.0 H2 %, above which it decreases The variation trend in Ar seems reasonable because, in a mixture of H2 /Ar, the electron kinetic energy in plasma can be consumed not only for excitation and ionization of Ar atoms but also for dissociation of H2 molecules and the subsequent reaction, such as the excitation of the resultant hydrogen atoms and their ionization Consequently, the author presumes that the electron kinetic energy in the H2 /Ar plasma becomes lower with H2 % Then the emission intensity of Ar and the Ar ion density decrease Based on the consideration presented above, Hα emissions are expected to increase in inverse relation to those of Ar, but the trend at 1.0 H2 % and higher ratios reveals a decline Regarding this decline trend, the author presents the following discussion In the H2 /Ar plasma, atomic hydrogen can be produced not only via the dissociation of H2 by electron impaction, but also other reactions such as the following: Ar+ + H2 → ArH+ + H.16 Above 1.0 H2 %, the decrease of Ar ion density with H2 % strongly affects the decline of this reaction Then the production rate of atomic hydrogen decreases, thereby leading to decreasing the Hα emissions intensity Based on the results presented in figure 5(a), the relative intensity of Hα to Ar is shown against the H2 % in figure 5(b) for rough estimation the atomic hydrogen density in the plasma It increases gradually until 2.0 H2 %, above which it decreases slightly but remains higher D ICCD analysis Plasma dynamic behaviour in the area, corresponding to the OES measurement, was characterized using high shutter speed photography using an ICCD camera Figure 6(a) depicts a photograph showing the apparatus configuration within the analysis area, and figures 6(b–g) show ICCD images of the plasma for respective values of H2 % Figure 6(b) is the discharge with pure Ar The bright emission can be confirmed over a wide range in the discharge tube This emission decreases drastically at 0.5 H2 % (figure 6(c)) At 1.0 H2 %, the plasma is constricted along the inserted Au wire and the 015316-7 Yoshiki Shimizu AIP Advances 7, 015316 (2017) FIG (a) Photograph portraying the apparatus configuration in the following ICCD analysis area, and (b–g) high-shutterspeed ICCD images of plasmas generated at respective H2 volume ratios up to 3.9% The exposure time for each image was 10 ms, corresponding to one pulse cycle time of the plasma discharge extended line toward downstream (figure 6(d)) This constriction is attributable to a thermal pinch effect, induced by H2 introduction In 2.0–3.9 H2 %, the plasma is much more constricted The strong emission in the vicinity of the tip of the Au wire can be confirmed In the 2.0–3.9 H2 % range, the emissions become weaker with H2 %, which corresponds to the trend above 1.0% in the OES, as shown in figure 5(a) E Mechanism of the diameter variation of the AuNPs with H2 % As indicated in the results presented above, introducing H2 is necessary to synthesize AuNPs in this process Accordingly, the author assumes that the AuNPs are derived via the following “atomic hydrogen – metal reaction model”15 in this method, as described in the Introduction: (1) Hydrogen atoms dissolve into molten Au wire (2) Exothermic reactions of their recombination enhance the Au wire etching (3) The AuNPs are formed through condensation from the resultant Au vapor In this condensation, the concentration of the Au vapor in the plasma can affect the resultant diameter of the AuNPs because the concentration is one factor that influences the collision frequency among the vapor species Generally, higher concentration engenders higher collision frequency, thereby enhancing the particle growth In this method, the concentration is expected to be reflected by the etching rate per unit area on the Au wire Based on this consideration and results described in sections III B–III D above, the diameter variation of the AuNPs will be discussed below from the perspective of the etching rate The trend of the diameter variation can be classified roughly into three groups as the following H2 %: 0.5, 1.0, and 2.0–3.9 H2 % Large difference of the diameter between the 0.5 H2 % and the 2.0–3.9 H2 % is probably attributed to the heat flux density and the atomic hydrogen concentration of the plasma, which are flowed into the Au wire The heat flux density in the 2.0–3.9 H2 % can be much higher than that at the 0.5 H2 % because of constriction of the plasma and the localized flow into the wire through the vicinity of the tip, as indicated by comparison of Figures 6(e–g) with 6(c) Consequently, the vicinity of the wire tip is heated efficiently, raising the temperature Deformation of the wire tip configuration, as portrayed in figure 4(c), can reflect this intense heating near the tip The higher temperature is favourable for the dissolution of atomic hydrogen in Au.17 Additionally, the atomic hydrogen concentration in the plasma can be high in 2.0–3.9 H2 %, as indicated in figure 5(b) These conditions enhance the dissolution of atomic hydrogen in large quantities Then the quantity of exothermic heat becomes higher, thereby leading to a higher etching rate in 2.0–3.9 H2 % in comparison with that at 0.5 H2 % The trend of declining diameter in 2.0–3.9 H2 % might be attributable to the decline of both the plasma intensity and the atomic hydrogen concentration, as shown in Figures 5(a) and 5(b) The etching rate decreases with H2 % Thereby the diameter decreases 015316-8 Yoshiki Shimizu AIP Advances 7, 015316 (2017) Finally, the reason why AuNPs with a bimodal distribution form at 1.0 H2 % will be discussed This result is probably attributed to mixing of the constrictive discharge near the wire tip, as presented in figures 6(e–g), and the widespread discharge surrounding the larger surface area, as presented in figure 6(c) The plasma discharge at the 1.0 H2 % might be in a transient state between these discharges This consideration is supported by the observation in figure 4(b), in which gentle and rough corrugated morphologies as the traces of the etching were mixed In this transient state, the rate of the Au wire etching can fluctuate The AuNPs with the bimodal distribution in a wide diameter range form accordingly IV CONCLUSIONS This report describes diameter control of gold nanoparticles (AuNPs) during synthesis using an atmospheric-pressure H2 /Ar plasma jet drive with pulse-modulated ultrahigh frequency, employing Au wire as the NP source material During this process, where most of the AuNPs are regarded as formed through condensation from Au vapor, the diameter varied in the approximate range of 2–12 nm with H2 volume ratios up to 3.9% Plasma diagnostics results showed that the H2 volume ratio values influence the plasma discharge behaviour, which in turn affects the heat flux density flowed into the Au wire and the concentration of atomic hydrogen in the plasma Both seemed to influence the etching rate of the Au wire per unit area, which is related directly to the concentration of the Au vapor in the plasma Because the concentration is one factor affecting the size evolution of particles caused by collision among vapor species in the reaction field, the author discussed the size variation of the AuNPs with H2 volume ratios from the perspective of the etching rate of the Au wire at respective H2 volume ratio ACKNOWLEDGMENTS The author thanks Prof Naoto Koshizaki (Hokkaido Univ.), Dr Kenji Kawaguchi, and Dr Takeshi Sasaki (AIST), co-authors of an earlier report and collaborators in the development of the fundamental methods described in this report, Prof David Mariotti (Ulster Univ.) for teaching skilled OES measurement techniques, and Dr Yukiya Hakuta for his encouragement T Nozaki, K Sasaki, T Ogino, D Asahi, and K Okazaki, Nanotechnology 18, 235603 (2007) Wittmann-Teneze, K Valle, L Bianchi, P Belleville, and N Caron, Surf Coat Technol 202, 4349 (2008) Y M Su, Y L Kuo, C M Lin, and S F Lee, Powder Technol 267, 74 (2014) S Askari, I Levchenko, K Ostrikov, P 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(2017) Diameter control of gold nanoparticles synthesized in gas phase using atmospheric- pressure H2 /Ar plasma jet and gold wire as the nanoparticle source: Control by varying the H2 /Ar mixture ratio. .. around the Au wire and the concentration of the atomic hydrogen in the plasma They apparently affect the variation of the diameter of synthesized AuNPs In this paper, the author will present the diameter. .. because of constriction of the plasma and the localized flow into the wire through the vicinity of the tip, as indicated by comparison of Figures 6(e–g) with 6(c) Consequently, the vicinity of the wire