Transport and kinetic effects on the morphology of silver nanoparticles

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Kinetics, Catalysis, and Reaction Engineering

Transport and kinetic effects on the morphology of silver nanoparticles in a millifluidic system Krishna V Kinhal, Nirav P Bhatt, and Pushpavanam Subramaniam Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04156 • Publication Date (Web): 21 Dec 2018 Downloaded from http://pubs.acs.org on December 21, 2018

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Industrial & Engineering Chemistry Research

Transport and kinetic effects on the morphology of silver nanoparticles in a millifluidic system Krishna V Kinhal1, Nirav Bhatt2*, S Pushpavanam1* 1

Department of Chemical Engineering 2

Department of Biotechnology

Indian Institute of Technology Madras, Chennai 600036 India

Abstract This work investigates the synthesis of silver nanoparticles of different shapes using silver nitrate as a precursor and ascorbic acid as a reducing agent in different reactors. The main focus of this work is to study the effect of capping agent concentrations and ratios of reducing agent to precursor concentrations (reactant ratios) on the shape and size of the nanoparticles synthesized. Three reactor configurations, namely, batch, straight millichannel, and spiral millichannel, are considered to study transport effect on the morphology of silver nanoparticles synthesized. The kinetic effect on the particle morphology is studied by changing the capping agent concentrations and reactant ratios. It is shown that these variables play an important role in determining the nanoparticle shape and size. In this work, spherical nanoparticles (30-60 nm), triangular plates (80-200 nm), nanorods (130-150 nm) and bent wires (130-150 nm) morphologies are synthesized by controlling thermodynamic and kinetic parameters. Keywords: Nanoparticles, Morphology, Transport effect, Kinetics, Millifluidic reactors 1 ACS Paragon Plus Environment

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1. INTRODUCTION Silver nanoparticles find extensive applications in several fields. In medicine, they are used as drug delivery agents,1 in wastewater treatment as an antibacterial agent,2 in chemical process industries as a catalyst.3 Several methods exist for nanoparticle syntheses, such as thermal reduction,4 sonochemical reduction,5 microemulsions,6 metal vapor synthesis,7 laser ablation,8 or aqueous reduction.9 Among these methods, the aqueous reduction method is preferred for its simplicity. It is economical, as it requires neither an intricate experimental setup nor expensive equipment. Here, the reduction of a metal salt is carried out in presence of a reducing agent. Furthermore, experimental parameters such as pH, temperature etc., can be easily optimized and controlled. Several authors have studied the aqueous reduction method using different reducing agents. Sodium borohydride and hydrazine are typical strong reducing agents which instantaneously reduce metal salts.10,11 Whereas, reagents like ascorbic acid, glucose, ethylene glycol are weak reducing agents which allow control of the reduction kinetics.12–14 Capping agents like polyvinylpyrrolidone (PVP), cetyltriammonium bromide (CTAB) and polysorbates are commonly used to control the aggregation of the particles formed. The combination of reducing agents, which determine reduction kinetics, and capping agents along with several other factors like temperature, pH, and mixing determine the shape and size of the nanoparticles synthesized. Nanoparticle synthesis through chemical reduction method is usually carried out in a batch reactor and in microchannel as a continuous reactor system. Several studies have been carried out to synthesize nanoparticles with different shapes and size by controlling temperature, pH, reactant concentration in the batch reactor.14–19 The control of multiple parameters and mixing in batch reactors poses a challenge at industrial scales. Efficient mixing due to the low diffusional path lengths and better control of several parameters in microchannels render them preferable over the 2 ACS Paragon Plus Environment

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batch reactor systems.20 The conditions i.e., flow-field, here are well controlled and hence the process is deterministic. As these units are modular, the production capacity can be increased by scale-out (numbering up) instead of scale-up. Millifluidic channels have characteristic dimensions (diameter) in the millimeter scale and offer advantages of microfluidic devices. For instance, they are easy to fabricate, their millimetric dimension permits their use at higher pressures and flow rates. Hence, millifluidic continuous reactors offer a viable method for large-scale nanoparticle synthesis.21 Mixing in these systems can be greatly enhanced by the use of curved channels.22 Various studies have analyzed the effect of Dean vortices induced by the curvature of a channel in milli-reactors.23–25 Dean vortices enhance the mixing of fluids in the transverse direction. Particles in such flow are subjected to transverse drag force due to Dean vortices. This has been exploited for particle focussing and separation in continuous flows.24,26,27However, the effect of these flows on the particle synthesis is less explored. Several earlier studies synthesized nanoparticles continuously in a straight channel with micromixers.28–35 Comparatively, less attention has been given to particle synthesis in spiral or curved millifluidic reactors.22,36–38 Kumar et al.22 synthesized silver nanoparticles in a spiral microreactor in a two-phase flow. They studied the role of the continuous and dispersed phases on the size of nanoparticles. Wu et al.37 synthesized silver nanoparticles in a helical reactor and studied the effect of the radius of curvature on mixing and nanoparticle size. They concluded that the mixing improves as the radius of curvature decreases, and this results in a narrower particle size distribution. Further, Wu and Torrente-Murciano 38 synthesized silver nanoparticles in coiled flow inverter reactors and studied the effect of sudden changes in centrifugal forces on the mixing patterns and particle size distribution.

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The focus of earlier studies in the existing literature has been on controlling the size of the nanoparticles by changing different parameters. There has been no study on shape control of the nanoparticles in continuous flow reactors. The literature also lacks an effective comparison of different reactor configurations in synthesizing different morphology of nanoparticles. This stems from the fact that the mechanisms of the nanoparticle synthesis is not well understood. This work provides an insight on how the size and morphology of nanoparticles can be controlled by reactor geometry and varying the concentrations of reactants and capping agents. Towards this, the synthesis of silver nanoparticles in a spiral and straight millifluidic channels and a batch reactor was analyzed. Shape tuning of nanoparticles is achieved by control of capping agent concentration, the reducing agent to precursor concentration ratio and the reactor configuration. Effect of these parameters on particle size is also analyzed. The application of these parameters to the continuous synthesis of silver nanoparticles in millifluidic reactors is the focus of this work. This helps in modularization of the unit, process intensification and industrial scale synthesis through scale out.


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2. EXPERIMENTAL PROCEDURE 2.1. Materials used. Analytical grade silver nitrate (AgNO3), L-ascorbic acid (C6H8O6), cetyltrimethylammonium bromide (CTAB), sodium hydroxide (NaOH) were purchased from Merck Life Science Pvt Ltd, Mumbai, India. Deionized water from Evoqua Millipore system was used in all experiments. 2.2. Preparation of stock solutions. Stock solutions were prepared as described in Table 1. Solutions 1- 4 contain ascorbic acid and CTAB, while solutions a and b contain silver nitrate and CTAB. pH of the solution plays an important role in the stability of nanoparticles. High pH enhances the reducing power of ascorbic acid.39 Hence, the pH of the stock solution containing ascorbic acid and CTAB was monitored at 11 prior to the addition of silver nitrate. The pH was adjusted by dropwise addition of 1M sodium hydroxide during an experiment.

Solution

Solution 1 Solution 2 Solution 3 Solution 4 Solution a Solution b

CTAB concentration (mM) 25 25 1 1 25 1

Ascorbic acid concentration (mM) 1000 10 1000 10 -

Silver nitrate concentration (mM) 10 10

pH

11 11 11 11 6.9 6.9

Table 1. Range of concentrations used to understand the effect of CTAB concentration and a reducing agent to precursor concentration ratio in the synthesis of silver nanoparticles.



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2.3. Synthesis of Nanoparticles. Figure 1 summarises the different reactor types, capping agent (CTAB concentrations, and ascorbic acid to silver nitrate ratio (reactant ratio) used in this work for the synthesis of nanoparticles.

Figure 1. Summary of the reagent concentrations and operating conditions analysed in synthesis of silver nanoparticles.


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CTAB concentration and reactant ratio were denoted as C and R, respectively. The experiments under investigation are classified into four main sets based on the values of C and R: a) High C (25mM) – High R (100:1) b) High C (25mM) –Low R (1:1) c) Low C (1mM) – High R (100:1) d) Low C (1mM) – Low R (1:1). The effect of reactor configuration was studied for each combination of C and R by conducting experiments in a batch reactor, straight millifluidic reactor and spiral millifluidic reactor. The experimental protocols are described for each reactor configuration. 2.3.1. Batch reactor: Here, the reaction was carried out in a 2 ml centrifuge tube. Initially, 1ml of a stock solution containing ascorbic acid and CTAB was added to the centrifuge tube. To this, 1ml of silver nitrate and CTAB stock solution was added instantaneously. The tube was shaken vigorously to ensure proper mixing. The solution was then transferred to a glass cuvette where the progress of the reaction was monitored. The solution was kept unstirred in order to ensure transport limitation in the reactor. The evolution of the reaction was monitored by performing time-resolved absorption measurement in a UV-vis spectrophotometer. The absorbance was measured at max, the wavelength at which the peak in the absorption spectra is located (See Figure S5 of the Supporting Information). The peak wavelength was predetermined from initial trial under similar conditions. In a time-based measurement, the absorbance measured at maximum wavelength (max) increases until the system reaches a steady state (See Figure 2). The constant absorbance at long times indicates completion of the reaction



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and particle growth. The time taken for the reaction and particle growth completion is referred as the total synthesis time in this work. Figure 2 indicates the time-based absorption data obtained from the UV-vis spectrophotometer for two cases: (a) High C - High R and (b) Low C - High R. Time course measurements were not performed for the other two experiment sets as they did not exhibit any Localised Surface Plasmon Resonance (LSPR) peak in the absorption spectra due to their large size. The observed values for total synthesis time and max for different experimental conditions are tabulated in Table 2. The total synthesis time determined in the batch reactor helped establish the residence time required in the millifluidic systems.

a

b

Figure 2. Time based spectra measurements for a) High C-High R, b) Low C-High R to determine the total synthesis time for silver nanoparticle synthesis in a batch reactor


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Experiment set

max

Total synthesis time (Seconds)

High C-High R

450

817

High C-Low R

Not observed

N.A

Low C-High R

400

670

Low C-Low R

Not observed

N.A

Table 2. Total synthesis time for batch reactors obtained from time course measurement in UVvis spectrophotometer for different experimental conditions. 2.3.2. Straight and Spiral millifluidic reactors: For the synthesis of nanoparticles in a continuous reactors, straight and spiral millichannels were constructed using a 0.001 m inner diameter silicone tube. The total synthesis time for the nanoparticle synthesis in the batch reactor was 11-13 minutes (see Table 2 for the batch reactor). To ensure complete synthesis of the nanoparticles, the residence time for both the reactors were fixed approximately at 19 minutes (more than total synthesis time in batch). The fluid flow rate and length of the reactor were fixed to achieve this residence time. The reactor length was 2.4 m and the total fluid flow rate was 100 μl/min. A dual extruder syringe pump (Harvard Apparatus Elite 11) was used to pump the reactants to millifluidic reactors connected by a T junction. The flow rate of both the individual streams (ascorbic acid and silver nitrate) was kept constant at 50 μl/min. For these parameters,



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the residence time of 18.87 minutes was achieved theoretically. The Reynolds number (Re) is defined as the ratio of inertial force and viscous force as follows:

𝑅𝑒 =

𝐷𝑣𝜌 𝜇

where D is the diameter of the channel, v is the fluid velocity,  and  denote the fluid density and viscosity, respectively. The calculated Reynolds number for the system is 2.5.

The Dean number (De) is calculated from

𝐷𝑒 = 𝑅𝑒√

𝐷 2𝑅𝑐

where Rc is the radius of curvature of the channel. The radius of curvature in spiral channel varies from 4.5 cm at the inlet to 1 cm at the outlet. The Dean number was calculated to be 0.25 at the inlet and 0.55 at the outlet of the spiral channel. This is zero for the straight channel. The reactors were operated for 30 minutes to attain steady state. Thereafter, the solutions were collected from the exit of the reactors. These solutions were further analysed using different characterization techniques. Figure 3 depicts the experimental setups for the synthesis of the particles in straight and spiral reactors, respectively.


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Figure 3. Images of experimental setup for synthesis of nanoparticles in a straight millifluidic reactor and spiral millifluidic reactor.

2.4. Particle Characterization. The metal nanoparticle interacts with light and exhibits LSPR behaviour. This is caused by the interaction of electrons present on the surface of metals and the electromagnetic field of photons. LSPR causes the formation of a peak in the UV-vis spectrum. This peak provides qualitative information about average particle shape and size. The formation of nanoparticles can be monitored by studying the spectra using a UV-visible Spectrophotometer. Silver nanoparticles exhibit LSPR in the range of 350-500 nm depending on the shape and size of the particles. The nanoparticles solution obtained from the three different reactors were characterized by UV-visible absorption spectrometer (JASCO V-630) in the spectrum range of 300-900 nm. The particle morphology was analyzed by scanning electron microscopy (SEM) (Hitachi S-4800). Samples for SEM were prepared by centrifuging the solution at 12500 RPM 11 ACS Paragon Plus Environment


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for 15 minutes. The bottom residue was collected and washed in deionized water to remove excess CTAB and ascorbic acid. This residue was dried under ambient conditions before performing microscopy. The presence of silver was confirmed by performing Energy Dispersive Spectroscopy (EDS) (EDAX-Ametek®). Quantitative size analysis was performed on the SEM image obtained. 100 particles in the SEM image were considered and the Ferret’s diameter was measured for each particle using Image J software provided by the National Institute of Health, USA. Based on the particle size and the corresponding LSPR, the particles with a size in the range of 30-200 nm are referred as nanoparticles in this work and the particles larger than 200 nm which do not exhibit LSPR are not referred as nanoparticles. 3. RESULTS This section discusses the morphology of nanoparticles and size obtained after particle characterization under different experimental conditions. The results are described for each operating condition in different reactor configurations. a) High C - High R. I.

Batch reactor: The particles formed in this reactor are spherical in shape under this condition, with an average size of 55 nm, as observed by the SEM image in Figure 4a. A LSPR peak at 447 nm corresponding to the average particle size can be seen in the UV-vis absorption data as shown Figure 4d. The EDS data in Figure 4f confirms the element Ag in spherical nanoparticles. The secondary peaks in the EDS denote Aluminum (1.5 keV) and Oxygen (0.5 keV) as the sample analysis was performed on the Aluminum foil.

II.

Straight millifluidic reactor: The particles synthesized this reactor are multifaceted polygons with an average size of 130 nm (Figure 4b). These particles 12 ACS Paragon Plus Environment

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exhibit two peaks in a UV-vis absorption spectra. A primary peak at 489 nm and a secondary peak at 430 nm are observed in Figure 4e. Peaks contributing red shift due to large size are considered as primary peaks. Similar peaks were obtained by simulations performed by Noguez et al.,40 who attributed these secondary peaks to the truncation of facets of the particles. The EDS data in Figure S8a of the Supporting Information confirm Ag polygonal nanoparticles. III.

Spiral millifluidic reactor: The morphology of particles obtained in the spiral millifluidic reactor is similar to that of the particles synthesized in the straight millifluidic reactor. The SEM image in Figure 4c indicates the formation of multifaceted polygonal particles with an average size of 150 nm. These particles have a primary LSPR peak at 485 nm and a secondary LSPR peak at 433 nm as seen in the UV-vis absorption spectrum in Figure 4e.

a

c

b

d



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e

f

Figure 4. The SEM images of the silver nanoparticles synthesized at High C – High R in a) a batch reactor, b) straight reactor, c) spiral reactor; UV-vis absorption data for nanoparticle synthesized in d) a batch reactor, e) straight and spiral millifluidic reactors with primary peaks in the inset, f) EDS spectrum for spherical nanoparticles confirming the presence of Ag.


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b) High C - Low R. I.

Batch reactor: Here, large plate-like structures are obtained with an average size of 2600 nm (Figure 5a). These big particles do not exhibit any LSPR peak as evident from the UV-vis absorption spectra in Figure 5d. The EDS spectra in Figure S8b of the Supporting Information confirms the formation of Ag particles.

II.

Straight millifluidic reactor: Triangular plates and irregularly shaped particles are formed in this reactor under this condition. The SEM image in Figure 5b shows particles of average size 100 nm. The UV-vis spectrum in Figure 5d shows two weak LSPR peaks at 350 and 490 nm. This spectrum corresponds to different shapes of particles obtained in this reactor.

III.

Spiral millifluidic reactor: The particle morphology obtained in this reactor is similar to the case of straight millifluidic reactor indicating no effect of the curvature. The SEM image in Figure 5c shows triangular plates and irregularly shaped particles with an average size of 85 nm. Here, the LSPR peaks are weak and occur at 356 nm and 493 nm.



a

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b

d

c

Figure 5. The SEM images of the silver nanoparticles synthesized at High C- Low R in a) a batch reactor, b) straight reactor, c) spiral reactor; d) UV-vis absorption data for nanoparticle synthesized in the three different reactors. c) Low C - High R. I.

Batch reactor: Particles formed under this condition are spherical in shape with an average size of 29 nm (Figure 6a). These particles show a strong LSPR peak at 406 nm in the UV-vis absorption spectra (Figure 6d).

II.

Straight millifluidic reactor: Rod-like nanoparticles with an average length of 126 nm were formed in this case. This could be observed in the SEM image in Figure 6b. These particles exhibit an LSPR peak at 389 nm in the UV-visible spectrum. The absorption curve for these particles is shown in Figure 6e. The EDS spectrum in Figure S8c of the Supporting Information confirms the nanorods made of silver.


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III.

Spiral millifluidic reactor: In the case of a spiral millifluidic reactor, bent wires are formed with an average length of 133 nm. The wires obtained are entangled as seen in Figure 6c. Size measurement was performed only for the wires which were not entangled. It can be observed in Figure 6e, that these particles exhibit an LSPR peak at 384 nm. Presence of silver in wires is also confirmed by the EDS spectrum in Figure S8d of the Supporting Information.

IV. a

c

b

d



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e

Figure 6. The SEM images of the silver nanoparticles synthesized at Low C- High R in a) batch reactor, b) straight reactor, c) spiral reactor; UV-vis absorption data for nanoparticle synthesized in d) a batch reactor, e) straight and spiral millifluidic reactors.

d) Low C - Low R. I.

Batch reactor: Here, a mixture of particles with large and small plate-like structures viz. triangular and hexagonal are observed. These particles have an average size of 427 nm as seen in the SEM image in Figure 7a. These large particles do not show any LSPR in the UV-vis absorption spectra (Figure 7d).

II.

Straight millifluidic reactor: Under this condition, 172 nm large triangular plates are formed as seen in the SEM image in Figure 7b. The particles do not show a strong LSPR in the visible region (Figure 7d). This can be attributed to the formation of a fewer number of particles. Weak LSPR peaks can be observed at


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282 nm and 317 nm, respectively. The presence of Ag in these particles is confirmed by EDS spectrum (See Figure S8e of the Supporting Information). III.

Spiral millifluidic reactor: Here, triangular nanoparticles similar to particles obtained in the straight millifluidic reactor were produced (Figure 7c). These particles have a well-defined structure and have an average size of 138 nm. They exhibit weak LSPR at 279 and 318 nm in the UV-vis absorption spectrum as depicted in Figure 7d.

a

b

c

d

Figure 7. SEM images of the silver nanoparticles synthesized at Low C – Low R in a) a batch reactor, b) straight reactor, c) spiral reactor; d) UV-vis absorption data for nanoparticle synthesized in the three different reactors.



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4. DISCUSSION Based on the mechanisms described in Section 1 of the Supporting Information, different morphologies of the particles obtained at different experimental conditions are now discussed: a) High C - High R. The concentration of CTAB (25mM) in the solution is higher than the critical micelle concentration (1mM). In the case of a batch reactor, the absence of fluid flow due unstirred reactor ensures the transport limitations. This allows CTAB micelles to form interconnected networks. These networks hinder the movement of the metal ions and ascorbic acid molecules. This causes supersaturation zones inside micelles as well as around it. The nucleation occurs in the inter-micellar and intra-micellar spaces. The growth of the particles is dominated by isotropic diffusion. This results in the formation of spherical nanoparticles (Figure 4 a). The micellar networks, in this case, do not direct the shape of a particle but they control the aggregation and transport of a particle. Hence, the nanoparticle synthesis is governed by diffusion. In the case of millifluidic reactors, the micellar networks rupture due to shear induced by fluid flow. CTAB structures are present in the micellar form without any network between them. The fluid flow around the micelle prevents the supersaturation of monomers outside the micelles. This results in nucleation inside the micelles. These micelles also direct the growth of the particles by forming a soft template. The high rate of metal ion reduction causes the formation of thermodynamically stable structures. These conditions lead to the formation of multi-faceted polygonal shaped nanoparticles (Figures 4 b and c). Hence, in millifluidic reactors under this condition, the growth of nanoparticles is predominantly governed by thermodynamics. 20 ACS Paragon Plus Environment

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b) High C - Low R. Here, the low reactant ratio results in a slow reduction rate of metal ions and hence the particle formation is kinetically controlled. The seeds, in this case, have stacking faults as depicted in Figure S4 of the Supporting Information, which leads to the formation of plate-like structures. In case of the batch reactor, the diffusion directed growth induces the formation of large plates without any definite shape (Figure 5 a). In spiral and straight millifluidic reactors, the particle growth is directed by slow kinetics and micellar soft template. The slow kinetics result in the triangular plated structure as discussed.41,42 Micelles, on the other hand, formed due to high CTAB concentration, also control the growth of these particles by acting as a soft template. This soft template lead to the formation of irregularly shaped particles. This results in a mixture of particles of triangular plates and irregular shapes seen in Figures 5 b and c. The particle formation is kinetic controlled and growth is due to micellar soft template.

c) Low C - High R. Under this condition, since the rate of reduction of metal ions is high, a large number of seeds are formed. This increases the number of collisions between them and leading to their aggregation. This phenomenon is predominant in the milli fluidic reactors. In the case of a batch reactor, the particle transport is primarily via diffusion due to absence of stirring. This reduces the probability of collisions of the seeds and thereby reducing aggregation and the seeds in the batch reactor eventually grow into spherical nanoparticles (Figure 6 a).



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When the synthesis is carried out in a straight millifluidic reactor, the aggregation of the seeds occurs in the direction of the flow. This directs the particle growth based on the aggregation, which leads to the formation of nanorods (Figure 6 b). In a spiral reactor, the Dean vortices create secondary flows transverse to the primary fluid flow.43 Although the Dean number is low (0.25 at inlet and 0.55 at the outlet), the transverse flow in the channel is present due to wall confinement. These Dean vortices contribute to the drag forces on the particles.27The morphology of the particles is affected by low Dean number as they are subjected to the transverse flow for a longer period of time in the millifluidic reactor.This drag in the transverse direction on the aggregated structures result in bent wires (Figure 6 c). To understand the effect of Dean number on the particle morphology, two experiments were performed at higher Dean number (Refer Section 3 of the Supporting Information for the details). At Dean number 0.5(inlet)-1.1(outlet), the particles

synthesized were

nanowires (See Figure S6 of the Supporting Information) with a smaller size (85 nm) compared to the experiment performed at low Dean number (0.25-0.55). This is attributed to enhanced transverse flow at higher Dean number. When the Dean number was further increased to 25 at the inlet and 55 at the outlet, the particles obtained were spherical (See Figure S7 of the Supporting Information) with an average size of 46 nm. Here, high Dean number results in good transverse mixing, no axial dispersion and plug flow like behaviour. Thus, at Low C and High R, the particle morphology is influenced by mixing (Dean number) and fluid flow.

d) Low C - Low R. In this case, the particle growth is kinetically controlled and the effect of CTAB concentration is minimal. The stacking faults direct plate-like structure formation. Thus mixing has a minimal effect on the particle morphology. Triangular plates could be 22 ACS Paragon Plus Environment

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seen in all the three reactor configurations. In the case of a batch reactor, large particles are also formed apart from the triangular plates (Figure 7a). This is attributed to the slow growth controlled by diffusion the batch reactor.

Table 3 summarizes the size, shape obtained under different conditions and the corresponding mechanism responsible.



CTAB concentration (mM) 25

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Reactant ratio

Reactor Type

Shape

Size (nm)

Dominant mechanism

100:1

Batch

Spherical

55.199.86

Straight

Multifacetedpolygons

129.8619.27

Spiral

Multifacetedpolygons

146.3422.07

Batch

Plate

2646.11627.88

Straight

100.1423.49

Batch Straight

Triangular plates And irregularly shaped particles Triangular plates and irregularly shaped particles Spherical Rods

Micellar network and diffusion Micellar template and Thermodynamic growth Micellar template and Thermodynamic growth Surface capping and stacking faults Surface capping and stacking faults

Spiral

Wires

133.7338.51

Batch Straight

Plates Triangular Plates Triangular Plates

427.35115.43 172.5831.74

(Thermodynamic Controlled )

1:1 (Kinetic Controlled )

Spiral

1

100:1 (Thermodynamic Controlled )

1:1 (Kinetic Controlled)

Spiral

84.8916.74

Surface capping and stacking faults

28.875.78 126.4914.09

Diffusion Aggregation due to flow Aggregation due to Dean vortices Stacking faults Stacking faults and mixing Stacking faults and mixing

138.4829.48

Table 3. Summary of the different morphology of nanoparticles synthesized, their size and dominant mechanism contributing to the shape of the particles under different conditions.


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5. CONCLUSIONS The shapes and size of nanoparticles play an important role in different applications. In this work the effect of precursors, and capping agent concentrations on particle morphology in a batch, straight and spiral millifluidic reactors has been studied. At high capping agent concentration, the particle growth is primarily driven by the capping agent characteristics. Further, the reactant ratio also plays an important role in the reduction reaction rate and growth rate. It has been observed that the effect of reactor configuration is insignificant in the case of high capping agent concentrations. Whereas, at lower capping agent concentration, the effect of reactor configuration plays a dominant role. At higher reactant ratio, mixing and transport phenomena cause the particle to aggregate and grow into rods and wire shaped particles in straight and curved millifluidic reactors. At low reactant ratios, the shape of the particle is either triangular or rectangular in the three reactor configurations. Hence, the reactor configuration plays a less important role as the process is kinetically controlled. This study shows that the desired nanoparticle morphology can be achieved by precise control of the reduction kinetics and mixing. The three reactor geometry have well-defined flow fields. This has enabled us to understand the interaction of various parameters which affect nanoparticle synthesis. For example, for synthesizing triangular nanoplates, the millifluidic reactors can be used in kinetically controlled regime by manipulating the reactant ratio. Similarly, the spherical nanoparticle can be obtained in a thermodynamically controlled regime via fast reduction kinetics. Hence, this study provides a broad guideline for producing differently shaped particles by manipulating reactant ratio and capping agent concentrations in millifluidic reactors.



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ASSOCIATED CONTENT Supporting Information Section 1: Mechanism of nanoparticle formation (Figures S1, S2, S3, and S4) Section 2: Calculation of total synthesis time of the nanoparticles in the batch reactor (Figure S5) Section 3: Effect of Dean number on the morphology of the particles.( Figure S6 and S7) Section 4: EDS analysis of the particles.(Figure S8 a, b, c, d, e ) AUTHOR INFORMATION Email: [email protected], [email protected] Phone: 091-44-22574161, Fax: 091-44-22574152 ACKNOWLEDGMENT The financial support to Dr. Nirav Bhatt from Department of Science & Technology, India through INSPIRE Faculty Fellowship is acknowledged.

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Transport and kinetic effects on the morphology of silver nanoparticles

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