Abstract
The synthesis of nanoparticles by reaction of liquid aerosol droplets containing precursors in a flow-through, atmospheric-pressure, non-thermal plasma offers a continuous, scalable, substrate- and stabilizer-free approach for direct deposition into liquids or onto soft substrates. However, the combination of multiphase and non-equilibrium chemistry makes the process complicated and poorly understood. Here, we present ion mobility spectrometry measurements of liquid water droplets containing silver nitrate passing through an atmospheric-pressure dielectric barrier discharge reactor that allows us to monitor silver nanoparticle formation online for the first time. Mobility diameter distributions were obtained with the plasma on and off, and exhibited a shift, which was related to the degree of conversion of silver nitrate. The silver nanoparticles were also collected and characterized by UV–visible absorbance spectroscopy and transmission electron microscopy to support the online measurements. Importantly, negligible conversion was found when the water was removed by a diffusion dryer, suggesting that the key reducing species are in the liquid phase, such as solvated electrons. Overall, the study demonstrates how ion mobility spectrometry measurements can be applied to provide insight into this approach to nanoparticle synthesis.
| Original language | English (US) |
|---|---|
| Article number | 105631 |
| Journal | Journal of Aerosol Science |
| Volume | 150 |
| DOIs | |
| State | Published - Dec 2020 |
| Externally published | Yes |
Keywords
- Ion mobility spectrometry
- Liquid droplets
- Nanoparticles
- Plasma
ASJC Scopus subject areas
- Environmental Engineering
- Pollution
- Mechanical Engineering
- Fluid Flow and Transfer Processes
- Atmospheric Science
Fingerprint
Dive into the research topics of 'Online ion mobility spectrometry of nanoparticle formation by non-thermal plasma conversion of metal salts in liquid aerosol droplets'. Together they form a unique fingerprint.Cite this
- APA
- Standard
- Harvard
- Vancouver
- Author
- BIBTEX
- RIS
Online ion mobility spectrometry of nanoparticle formation by non-thermal plasma conversion of metal salts in liquid aerosol droplets. / Gallingani, Tommaso; Abuyazid, Nabiel H.; Colombo, Vittorio et al.
In: Journal of Aerosol Science, Vol. 150, 105631, 12.2020.Research output: Contribution to journal › Article › peer-review
}
TY - JOUR
T1 - Online ion mobility spectrometry of nanoparticle formation by non-thermal plasma conversion of metal salts in liquid aerosol droplets
AU - Gallingani, Tommaso
AU - Abuyazid, Nabiel H.
AU - Colombo, Vittorio
AU - Gherardi, Matteo
AU - Sankaran, R. Mohan
N1 - Funding Information: N. H. A. and R. M. S. acknowledge the support of the Department of Energy under grant no. DE-SC0018202 and the Air Force Office of Scientific Research under grant no. FA9550-19-1-0088 . T. G. acknowledges the financial support of Democenter - Sipe Foundation (Modena, Italy). The authors thank Prof. Alessandro Paglianti from the Department of Civil, Chemical, Environmental, and Materials Engineering of the University of Bologna for his help with the droplet size measurement and valuable discussions. Funding Information: Here, we apply online measurements for the first time to a continuous-flow, plasma-droplet scheme for nanoparticle synthesis using ion mobility spectrometry (IMS). This tool has been similarly applied previously to plasma synthesis of NPs from vapor precursors with the important distinction that those reactions were only in the gas phase (Chiang & Sankaran, 2008; X. Chen, Ghosh, Buckley, Sankaran, & Hogan, 2018). We focused our study on the conversion of AgNO3 dissolved in water to Ag NPs, which is relatively well-established in plasma-liquid experiments (Lee et al., 2013; Shirai, Uchida, & Tochikubo, 2014; Ghosh, Klek, Zorman, & Sankaran, 2017; Vos, Baneton, Witzke, Dille, & Sankaran, 2017), albeit using batch configurations, and provides a straightforward chemistry to benchmark our study. Droplets with a relatively narrow diameter distribution and mean diameter of 1.3 ?m were generated by a commercial nebulizer and introduced into a home-built atmospheric-pressure dielectric barrier discharge reactor with external, parallel ring electrodes. The aerosol effluent leaving the plasma was monitored and mobility diameter distributions were obtained at different process conditions. Importantly, we observe a shift in the mean diameter of the aerosol particles with the plasma off and on, and develop a relationship with the conversion of the precursor to NPs. Moreover, we show that there is negligible shift and thus precursor conversion, when the droplets were dried and the water was removed before the plasma. The online measurements were supported in all cases by standard materials analysis such as ultraviolet?visible (UV?vis) absorbance spectroscopy and transmission electron microscopy (TEM). Based on these measurements, we suggest that successful conversion of the metal salt to metal NPs relies on reactions in liquid water, which supports earlier reports of solvated electron-mediated chemistry (Ghosh, Klek, et al., 2017; Vos et al., 2017). More generally, the study shows how online measurements based on IMS can provide important insight into such a complex system.Direct deposition was carried out by diluting the aerosol flow with 1000 sccm N2 to prevent gas breakdown and passing through an electrostatic precipitator (Nanometer Aerosol Sampler, TSI, Inc.) operating at a voltage of ?9.6 kV and a gap of 1 cm for 3 min. TEM (FEI Tecnai F30) was performed on material deposited onto ultrathin carbon lacey supported copper grids (TedPella, Inc.). Image analysis of the particle diameter (projected area diameter) was performed using Image-J software.IMS measurements of the mobility diameter distributions of untreated AgNO3 particles and treated particles are shown in Fig. 5b. The measured distribution of the untreated AgNO3 particles and in particular, the geometric mean diameter of 38.1 nm, is in relatively good agreement with the calculation. The slight difference between the two distributions, mainly for smaller diameters, may be attributed to the previously described differences between the diffraction technique, which measures volume-weighted diameters, and IMS, which measures number-weighted diameters. Another possible issue is that the AgNO3 formed aggregates made up of smaller particles rather than a single densified spherical particle, which would produce a different mobility diameter (X. Chen et al., 2018). A shift is observed for the distribution of the treated particles with a geometric mean diameter of 26.6 nm which is very close to the prediction. The slightly larger diameter for the experimental measurement may be because the conversion is not complete as we assumed. Assuming that the Ag particles are densified and spherical, we can estimate the true conversion efficiency to be ~90%. There is also a small decrease in the particle number concentration, which we suggest may be related to losses associated with deposition on the reactor wall driven by electrophoretic and thermophoretic effects (Borra, 2006). In support, we performed a material balance by analyzing the particle size distributions from IMS measurements for both the plasma off and on cases (see Supporting Information for details). We estimated that ~8% more material was lost when the plasma was on as compared to off. A control experiment was performed with a diffusion dryer between the plasma reactor and the IMS to examine whether residual liquid water could contribute to the observed differences. We found negligible difference in the measured diameter distributions (Supporting Information, Fig. S3), confirming that the aerosol is indeed dry after the N2 dilution as qualitatively found by laser scattering.The size and morphology of the synthesized AgNPs were further assessed by TEM. The particles were directly deposited onto TEM grids using an electrostatic precipitator to avoid breaking up the aggregates and compare them as close to their state as measured by IMS. Representative images in Fig. 7b show both unaggregated, spherically-shaped and agglomerated particles. Unagglomerated particle diameters were measured to be in the range of 10?35 nm, while aggregates were 60?80 nm composed of primary particles of about 12?20 nm. The formation of aggregates supports the differences previously discussed between calculated and measured diameter distributions. Agglomeration can be ascribed to particle-particle collisions driven by diffusion and electrostatic forces taking place in the spatial afterglow or even further downstream of the plasma (X. Chen et al., 2018). High magnification images reveal that the particles are crystalline and the measured lattice spacing of 0.2 nm can be indexed to the Ag (200) crystalline plane (see Fig. 7b), confirming Ag (R. Chen, Nuhfer, Moussa, Morris, & Whitmore, 2008). The selected area diffraction (SAED) pattern further corroborates the presence of crystalline Ag. From analysis of TEM images and sizing approximately 200 particles, a particle size distribution was obtained. Fig. 7d shows that based on TEM, the Ag NPs have an as-deposited geometric mean diameter of 21.6 ? 0.76 nm, which agrees well with IMS measurements. Overall, UV?vis absorbance and TEM results provide additional support that the origin of the shifts in diameter distributions measured by IMS is plasma-driven conversion of AgNO3 in the droplets to Ag NPs.The fundamental processes that underlie the conversion of metal salt precursors in the liquid water droplets via plasma interactions to zero-valent metallic NPs should be similar to what has been reported by previous studies conducted at the interface of a plasma and bath (Q. Chen et al., 2012; Patel et al., 2013; Kondeti, Gangal, Yatom, & Bruggeman, 2017; Ghosh, Hawtof, et al., 2017). Even in those somewhat simpler systems, the nature of the plasma-liquid interactions is complex and one of the outstanding questions remains the identity of the reducing species. Solvated electrons have been detected in the bath systems (Rumbach, Bartels, Sankaran, & Go, 2015) and are known to be one of the strongest chemical reducing species, easily capable of reducing metal cations such as Ag+ to Ag0 based on the reduction potentials. However, in the case of gold (Au), there is evidence that the reducing agent is hydrogen peroxide, which is expected to be formed from water vapor dissociation in the gas (plasma) phase (Q. Chen et al., 2012; Patel et al., 2013). While our experiments do not directly address these mechanistic issues, there are several important points that are relevant. Ag has a less favourable reduction potential than Au, and it has been shown that hydrogen peroxide cannot reduce Ag+ (Vos et al., 2017). The markedly different and somewhat negligible shift observed when liquid water is removed in our process echoes the importance of some dissolved species. A recent study has shown that reduction of AgNO3 thin films requires high-energy ions to initially dissociate the AgNO3 before electrons can subsequently reduce the Ag+ (Sui et al., 2018). We thus infer that removal of the water does not allow the generation of the dissolved reducing species, and the gaseous ions lack sufficient energy in an atmospheric-pressure DBD to dissociate AgNO3. The mostly likely dissolved reducing species based on bath experiments is solvated electrons (Rumbach et al., 2015, 2018), injected from the plasma at the droplet surface. This picture is consistent with one proposed by Maguire et al. (2017) who provided evidence in support of solvated electrons by correlating the very high observed nanoparticle synthesis rate with the electron flux from the plasma to the droplet surface. Water could also have other roles such as allowing the diffusion, or in the presence of electric fields, electric migration of Ag+ (Chiang, Carolyn, & Sankaran, 2010; Ghosh et al., 2014, 2016). The dissolved reducing species are generated at the plasma-liquid interface, which for our system is at the surface of the droplets, and will have a limited penetration depth. In the case of solvated electrons, the penetration depth in water is ~10 nm (Rumbach et al., 2015; Gopalakrishnan, Kawamura, Lichtenberg, Lieberman, & Graves, 2016), which is significantly smaller than the size of the droplets studied here of ~1 ?m. Thus, diffusion/electric migration of Ag+ is critical to their reduction inside the droplets. We assessed the ability of solvated electrons to carry out Ag+ reduction in our system by calculating the faradaic efficiency (Ghosh, Hawtof, et al., 2017). The molar conversion rate for Ag nanoparticles was obtained from the particle size distributions measured by IMS measurements and compared to a theoretical molar production rate using Faraday's law and based on the charge dissipated in the plasma (see Supporting Information for details). The faradaic efficiency was estimated to be ~22.6%, which is lower than that found with a DC plasma jet contacting a liquid water bath (Ghosh, Hawtof, et al., 2017). Particularly in light of mass transport limitations being overcome in a flow system, the analysis suggests that the DBD system may not be as efficient in injecting charge (electrons) from the plasma into the droplet as in the case of the DC plasma-liquid system where an electric field is created between the plasma and liquid water surface. Future studies should be aimed at more directly providing evidence for the conversion mechanism, revealing the identity of key species involved in plasma-droplet reactions, and developing plasma configurations with more efficient charge injection into droplets.N. H. A. and R. M. S. acknowledge the support of the Department of Energy under grant no. DE-SC0018202 and the Air Force Office of Scientific Research under grant no. FA9550-19-1-0088. T. G. acknowledges the financial support of Democenter - Sipe Foundation (Modena, Italy). The authors thank Prof. Alessandro Paglianti from the Department of Civil, Chemical, Environmental, and Materials Engineering of the University of Bologna for his help with the droplet size measurement and valuable discussions. Publisher Copyright: © 2020 Elsevier Ltd
PY - 2020/12
Y1 - 2020/12
N2 - The synthesis of nanoparticles by reaction of liquid aerosol droplets containing precursors in a flow-through, atmospheric-pressure, non-thermal plasma offers a continuous, scalable, substrate- and stabilizer-free approach for direct deposition into liquids or onto soft substrates. However, the combination of multiphase and non-equilibrium chemistry makes the process complicated and poorly understood. Here, we present ion mobility spectrometry measurements of liquid water droplets containing silver nitrate passing through an atmospheric-pressure dielectric barrier discharge reactor that allows us to monitor silver nanoparticle formation online for the first time. Mobility diameter distributions were obtained with the plasma on and off, and exhibited a shift, which was related to the degree of conversion of silver nitrate. The silver nanoparticles were also collected and characterized by UV–visible absorbance spectroscopy and transmission electron microscopy to support the online measurements. Importantly, negligible conversion was found when the water was removed by a diffusion dryer, suggesting that the key reducing species are in the liquid phase, such as solvated electrons. Overall, the study demonstrates how ion mobility spectrometry measurements can be applied to provide insight into this approach to nanoparticle synthesis.
AB - The synthesis of nanoparticles by reaction of liquid aerosol droplets containing precursors in a flow-through, atmospheric-pressure, non-thermal plasma offers a continuous, scalable, substrate- and stabilizer-free approach for direct deposition into liquids or onto soft substrates. However, the combination of multiphase and non-equilibrium chemistry makes the process complicated and poorly understood. Here, we present ion mobility spectrometry measurements of liquid water droplets containing silver nitrate passing through an atmospheric-pressure dielectric barrier discharge reactor that allows us to monitor silver nanoparticle formation online for the first time. Mobility diameter distributions were obtained with the plasma on and off, and exhibited a shift, which was related to the degree of conversion of silver nitrate. The silver nanoparticles were also collected and characterized by UV–visible absorbance spectroscopy and transmission electron microscopy to support the online measurements. Importantly, negligible conversion was found when the water was removed by a diffusion dryer, suggesting that the key reducing species are in the liquid phase, such as solvated electrons. Overall, the study demonstrates how ion mobility spectrometry measurements can be applied to provide insight into this approach to nanoparticle synthesis.
KW - Ion mobility spectrometry
KW - Liquid droplets
KW - Nanoparticles
KW - Plasma
UR - http://www.scopus.com/inward/record.url?scp=85090719254&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85090719254&partnerID=8YFLogxK
U2 - 10.1016/j.jaerosci.2020.105631
DO - 10.1016/j.jaerosci.2020.105631
M3 - Article
AN - SCOPUS:85090719254
SN - 0021-8502
VL - 150
JO - Journal of Aerosol Science
JF - Journal of Aerosol Science
M1 - 105631
ER -