The effect of mixing on silver particle morphology in flow synthesis
Introduction
The synthesis of particles with controlled sizes and shapes is an important topic in the field of materials science and particle technology. However, it is widely known that not only thermodynamic conditions but also kinetic processes have significant and manifold effects on the final products. In various systems, these kinetic processes are adjusted to achieve narrow particle size and shape distributions (Kwon and Hyeon, 2008, Tao et al., 2008, Wang et al., 2015), which are often required in applications.
Two universal kinetic factors, reaction and mass transfer play a key role in particle morphology evolution (Dirksen and Ring, 1991, Tai, 1997, Xiao et al., 1990). Witten et al. proposed two classic models: diffusion-limited aggregation (DLA) (Witten and Sander, 1981) and reaction-limited aggregation (RLA) (Ball et al., 1987). This motivated Grzybowski’s team to use a reaction-diffusion strategy to explain and analyse the formation of various kinds of patterns (Grzybowski et al., 2005). On the basis of these findings, our team proposed the reaction-diffusion methodology as a strategy for controlling particle shapes. Along these lines, interesting shapes were observed, e.g. snowflake-like calcium carbonate particles were synthesized by a compromise of reaction and diffusion (Wang et al., 2013); reversible shape variation between silver dendrites and polyhedrons was realized by designed competition between the complexation and reduction of silver ions (Liu et al., 2015); and the reaction-diffusion conditions during particle growth were quantified experimentally by the Damköhler number (Yang and Han, 2016).
Mixing as the inevitable procedure bringing the reactants into contact is the first step during particle synthesis and thus can greatly affect the course of particle formation (Bałdyga and Bourne, 1999, Gradl and Peukert, 2010, Metzger and Kind, 2016, Schwarzer and Peukert, 2004). Although in our previous works (Yang and Han, 2016, Yang et al., 2015, Yang et al., 2017), the effects of monomers’ microscopic mass transfer (i.e. diffusion) have been considered and discussed under the assumption of ideal mixing, the influence of mixing on the formation and evolution of particles with complex shape is still unknown. However, in particular, in a technical production process, beyond molecular diffusion, convective mass transfer is expected to play a significant role for particle formation. In the case of particle systems with fast formation kinetics, the mixing-dependent distribution of monomer concentration in time and space governs supersaturation, the thermodynamic driving force for phase transition. An incomplete chemical reaction due to poor mixing has a strong influence on both nucleation and growth of particles. Thus, mixing can become a decisive factor for the product quality (Meincke et al., 2017, Peukert et al., 2015), not only in terms of the width of a particle size distribution (PSD) (Fitchett and Tarbell, 1990, Yu. Shekunov et al., 2001, Zauner and Jones, 2000) but also in terms of the final particle shape.
To address this important issue, we used silver nanoparticles as the technically relevant model system with promising applications in biology (Abbasi et al., 2016, Yoon et al., 2008), optics (Klupp Taylor et al., 2010, Sathyavathi et al., 2010), and catalysis (Baruah et al., 2013). To unravel the effects of mixing on particle shape evolution, scanning electron microscopy (SEM) and analytical centrifugation (AC) were used to study the morphologies and sedimentation properties of the particles, respectively. In particular, AC turned out be the method of choice for the characterization of complex particles as it allows taking into account all particles of a sample and not only snapshots of the representative species. First, the influence of mixing modes was tested by batch experiments to get an overview whether or not mixing has an effect on the shape evolution. Then, controlled mixing conditions were realized by continuous experiments in a T-mixer under different flow rates. Particles were characterized by SEM and AC, and the particles’ morphology was linked to the sedimentation coefficient distributions supported by Brownian Dynamics (BD) simulations. For all experimental settings, the mixing quality was quantified by previously determined mixing time using the Villermaux-Dushman protocol (Commenge and Falk, 2011, Guichardon and Falk, 2000, Guichardon et al., 2000). Via this way, correlations between mixing intensity and particle size and shape distributions could be derived. Although many studies on the influence of mixing on spherical particles do exist (Metzger and Kind, 2014, Thajudeen et al., 2017, Walter and Peukert, 2016), the impact of mixing on the evolution of highly complex structures, i.e. in our case from sheet-like to dendritic products, has not yet been demonstrated. On the basis of these findings, the reaction-diffusion mechanism from our previous works was extended for mixing effects.
Section snippets
Chemicals and equipments
Silver nitrate (AgNO3, 99.9999% trace metals basis) and hydroxylamine (NH2OH, 50 wt% aqueous solution) were purchased from Sigma-Aldrich. Ethanol (absolute) was provided by VWR. Sulphuric acid (H2SO4, 96%), sodium hydroxide (NaOH, ≥99%), potassium iodide (KI, ≥99.5%), potassium iodate (KIO3, ≥99.5%), boric acid (H3BO3, ≥99.8%) and ethylene glycol (EG, 99.5%) were bought from Carl Roth. All chemicals were used without further purification. Deionized water from a Millipore (MQ) system had a
Preliminary studies on mixing influence
As described in our previous work (Yang et al., 2015), silver particles were synthesized by reduction of silver nitrate using hydroxylamine. For the mixing type M0 (i.e. addition of a small amount of reducing agent into a large precursor volume) which follows the previously published routine, a representative SEM image of the obtained product is shown in Fig. 2a. However, switching the mixing procedure to MA/MB (i.e. 1:1 mixing) leads to different particle shapes. As shown in Fig. 2b, pouring
Conclusion
Herein, the influence of mixing on the shape evolution of silver particles was investigated. First, in batch experiments following different mixing procedures, the role of mixing modes on particle shapes was qualitatively demonstrated. Then, the influence of mixing intensity was systematically studied by continuous experiments performed in a T-mixer. For quantitative characterization of the highly complex, non-spherical particle structures, sedimentation properties were evaluated by analytical
Notes
The authors declare no competing financial interest.
Acknowledgements
Tao Yang and Yongsheng Han appreciate the financial support from China Scholarship Council. Doris Segets, Thaseem Thajudeen and Wolfgang Peukert acknowledge the financial support by the Deutsche Forschungsgemeinschaft (DFG) within the Cluster of Excellence “Engineering of Advanced Materials”(project EXC 315) (Bridge Funding).
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Current address: School of Mechanical Sciences, Indian Institute of Technology Goa, 403401 Ponda, India.