Controllable Synthesis of Silver Dendrites via an Interplay of Chemical

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Controllable Synthesis of Silver Dendrites via an Interplay of Chemical Diffusion and Reaction Wei Liu, Tao Yang, Jianmei Liu, Ping Che, and Yongsheng Han Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01227 • Publication Date (Web): 12 Jul 2016 Downloaded from http://pubs.acs.org on July 18, 2016

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Industrial & Engineering Chemistry Research

Controllable Synthesis of Silver Dendrites via an Interplay of Chemical Diffusion and Reaction Wei Liu1,2, Tao Yang,1,3, Jianmei liu1,4, Ping Che2, Yongsheng Han1, * 1. State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China

2. School of Chemistry and Biological Engineering, University of Science & Technology Beijing, Beijing, 100083, China

3. University of Chinese Academy of Sciences, Beijing, 100049, China

4. Institute of Materials Science and Engineering, Ocean University of China, Qingdao, 266100, China

[email protected]

Abstract Here we report a study on the role of chemical diffusion and reaction on the morphology evolution of silver particles. Electrochemical reaction is employed to synthesize silver particles. The reaction rate is regulated by current density while the diffusion rate of silver ions is regulated by the viscosity of solution. Silver dendritic particles are largely synthesized at various diffusion and reaction conditions. At a high reaction rate, silver dendrites are formed at the medium diffusion rates of silver ions while the dendritic structures are only formed at an extremely low diffusion rate of silver ions when the reaction becomes slowly. Therefore, the interplay of diffusion and reaction plays a key role on the structure development of dendrites, which is because the interplay determine the chemical distribution on the growth front of crystals. Once a chemical concentration gradient is established at the growth front, the 1

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dendritic structures is triggered.

Keywords ： Materials, Particle technology, Crystallization, Reaction kinetics, Diffusion

Introduction Silver dendrites, as one kind of the most amazing hierarchical structures, demonstrate unique properties in left-handed metamaterials surface-enhanced Raman scattering (SERS)

1

and as substrates for

2-7

. Various approaches have been

developed to synthesize these structures, including solution-based chemical reduction 8-13

, surface deposition (direct replacement, electrolysis and galvanic replacement)

14-18

, and external-field-assisted approaches (using magnetic and ultrasonic fields) 19-24.

Although silver dendrites have been largely synthesized, their formation mechanism is still under debate. It is normally accepted that dendritic structures are not thermodynamically favored. They are the products dominated by kinetics. The Mullins–Sekerka (MS) instability model

25

has been widely used to explain the

formation of dendritic structures, in which the chemical concentration gradient around the growth front of crystals is believed to play a key role on the formation of dendritic structures. The gradient induces a preferential growth into one direction forming the main trunk of a dendrite followed by secondary nucleation on the sides of the trunk forming side branches. Theory and simulation on this model have made considerable progress recently 26-28. However, the experimental investigation in this direction is not 2


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sufficient, which is partly because of the limitation of techniques to regulate the concentration gradient around the crystal surface. In the study of chemical diffusion and reaction in shaping particles

29-32

, we

gradually realized that chemical diffusion and reaction influence the chemical distribution in macro- and nano-scales, which in turn affects the structure development of materials. By changing the diffusion and reaction processes, calcium carbonate particles with diverse morphologies were synthesized

33, 34

. Besides, we

have extended the diffusion and reaction method to synthesize other material systems 35-39

. The results obtained suggested that the control on chemical diffusion and

reaction play a significant role on the structure development of materials. But why the diffusion and reaction affect material structures is still an open question, which is needed to disclose, and we provide some clues in this study. Since the chemical concentration gradient is determined by the rates of chemical transport and consumption, it is possible to establish a concentration gradient around the growth surface of crystals via regulating either the transport of chemicals or their reduction rate. With this assumption, we synthesize silver particles in a designed electrolytic cell under various diffusion and reaction. The reduction rate is regulated by currents applied while the diffusion rate is changed by the viscosity of solvents. By changing the reduction and diffusion rates of chemicals, diverse morphologies of silver particles are synthesized. The dendritic structures are largely formed in various diffusion and reaction conditions. The dependence of silver dendrites on the diffusion and reaction is investigated, and the dominant role of the interplay between chemical 3



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diffusion and reaction in shaping particles is discussed in this paper.

Results and Discussions Diverse

morphologies

of

silver

electro-deposition in our previous study

particles

have

been

synthesized

by

39

. It was found that the morphologies of

silver particles are very sensitive to the reduction rates which were regulated by the currents applied to the reaction. At a low current density (from 12.5 µA•cm-2 to 100µA•cm-2), polyhedral silver aggregates were formed while small silver crystals were generated at a high current density (above 750 µA•cm-2). The silver dendrites are largely formed at the medium currents from 250 to 500 µA•cm-2. Based on the model of the MS instability

25, 40

, the formation of dendritic structures is caused by the

chemical concentration gradient around the growth front of the crystals. Therefore, we supposed a chemical concentration gradient was established near the growth front of particles in the formation of silver dendrites. Since the chemical concentration gradient is determined by the rates of transport and consumption of chemicals, the regulation by means of current densities changes the rate of chemical consumption, which results in an establishment of a concentration gradient. If this assumption is right, a regulation on the rate of chemical transport should also be able to induce the establishment of concentration gradients around crystals. With this assumption, we attempt to regulate the diffusion of chemicals in solution to synthesize silver dendrites. According to the Einstein-Stokes equation

40

, the diffusion coefficient of solute is

inversely proportional to the viscosity of the solvent. The viscosity of the solvent 4


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composed of glycerol and water is regulated by changing the volume ratio of glycerol. A

B

0%

C

10%

10µm

30%

10µm

50

48.2

169.7

10µm

D

50%

40

Viscosity / cP

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30

F 24.5

G

H 10µm

20

10

0

B

A 0.89

1.24

00

10 10

D 5.93

C

E

E 9.78

60%

2.43 20 30

40 60

30 50

50 70

60 80

90 70 10µm

Glycerin volume / % H

G

90%

1µm

F

80%

1µm

70%

1µm

Figure 1 Viscosities (Bar graph) of the solvents at different volume ratios of glycerol in water and SEM images (A, B, C, D, E, F,H) of silver particles synthesized in each solvent under the current density of 50 μA•cm-2.

Figure 1 shows the viscosities of solvents with different glycerol volume ratios and the morphologies of silver particles formed in each solvent. In the bar graph of Figure 1, the viscosities of solvents increase remarkably with the volume ratio of glycerol. At the low volume ratios, polyhedral silver aggregates are formed at the 5



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current density of 50 µA•cm-2, as shown in Figure 1A-1C. With the increase of volume ratio to 50% and 60%, dendritic structures are gradually formed. At the volume ratio of 70%, silver dendrites are largely formed, as shown in Figure 1F. A further increase of the volume ratio to 80% and 90% leads to the formation of short silver fibers with protuberances on their sides. Here we are interested in the formation of silver dendrites. Previous study

39

showed that there are no dendritic structures

formed at the current densities below 100 µA•cm-2, since the low reaction rates cannot establish a concentration gradient around the growth front of crystals. But in this study, by reducing the diffusion of chemicals, a concentration gradient seems to be established at a low reaction rate, which partly confirms our assumption, that the concentration gradient is determined by diffusion and reaction of chemicals and a change on each of these two factors could induce a concentration gradient. To further evaluate the role of chemical diffusion on the formation of dendritic structures, we synthesize silver dendrites at a high current density but different solvents. At the current density of 500 µA•cm-2, silver dendrites are largely formed in solution without glycerol, as shown in Figure 2A. With the increase of glycerol volume, dendritic structures gradually disappear. At the volume ratio of 50%, the silver dendrites are almost replaced by long silver fibers, as shown in Figure 2D. Further increasing the volume ratios above 60%, till 90% leads to the formation of irregular short fibers, and these fibers become shorter with the increase of glycerol fraction. Both the XRD and EDX results indicates that the dendrites and fibers particles are the products of silver, as shown in Figure 3. These results indicate that a 6


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reduction of chemical diffusion could destroy the concentration gradient established around the growth front of the crystals, resulting in the disappearance of dendritic structures, which again confirms that the chemical concentration gradient is determined by an interplay of diffusion and reaction of chemicals.

A

B

0%

C

10%

30%

1µm

1µm

1µm

D

50%

Increase of solvent viscosity

1µm

E

60%

1µm

H

G

90%

1µm

F

80%

1µm

70%

1µm

Figure 2. SEM images (A, B, C, D, E, F, G,H) of silver particles synthesized at different volume ratio of glycerol in water under the current density of 500 μA•cm-2.

7



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Figure 3. X-ray diffraction (XRD) and energy dispersive X-ray spectrometer (EDX, inset of Figure 3) of the synthesized silver dendrites.

To have detailed information on the silver dendrites and silver fibers formed at different diffusion conditions, these two structures are characterized by TEM, as shown in Figure 4. The silver dendrite shown in Figure 4A1 has a main trunk and side branches sitting on the trunk parallel. The selected area electron diffraction (SAED) pattern shown in Figure 4A2 indicates that both the trunk and branches grow along direction, which agree well with the XRD data in Figure 3. The high magnification image of Figure 4A3 shows that the dendrite is well crystallized with a lattice spacing of 0.236 nm which is close to the distance between {111}planes. The silver fiber shown in Figure 4B1 has a long axis with small protuberances on its side. The SAED image of Figure 4B2 shows, that the long axis grows along direction. The high magnification of Figure 4B3 indicates, that the fiber is also well crystallized with a lattice spacing of 0.235 nm, which corresponds to the spacing of {111}. The comparison on these two structures indicates that the fiber is a partly-developed dendrite with a preferential growth along the trunk and a restrained 8


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growth on side branches. This change of silver structures is presumably a result of the disturbance of the concentration gradient around crystals. A2

A1

{111}

B1

{111}

B2 {111}

A3

500nm

B3

2n

200

2n

Figure 4 TEM images of silver dendrites (A1, A2, A3) and silver fibers (B1, B2, B3) formed at glycerol volume ratios of 30% and 60% under the current density of 500 μA•cm-2, respectively. A1 and B1 are the low magnification images and A2 and B2 are the selected area electron diffraction while A3 and B3 are the high magnification images of silver dendrite and silver fiber.

Figure 5. A schematic illustration on the change of chemical concentration gradient around growing crystals at different diffusion and reaction conditions and the morphologies of products formed at each condition.

9



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Based on the above results and discussions, we propose a mechanism on the interplay of diffusion and reaction in shaping particles, as shown in Figure 5. The major role of diffusion and reaction is to regulate the chemical distribution around the growth front of crystals. In the case of reaction limitation, sufficient chemicals are delivered to the surfaces of crystals, which leads to the surface having nearly the same concentration as the bulk solution. Because the reaction is limited, the production of atoms or other building blocks is slowly, which allows each building block having sufficient time to find its favor position on the surface of crystals. Hence, the growth of crystals follows a minimization of surface energy, yielding a thermodynamically favored shape. If the reaction becomes fast while keeping the diffusion of chemicals unchanged, a concentration gradient forms around the growth front of crystals as a result of insufficient delivery of chemicals, which induces an instability of the crystal surface as depicted in the MS model, leading to the formation of dendritic structures. Further increasing the reaction rate leads to the dilution of the concentration gradient, which results in the formation of fiber-shaped particles. In a dilute concentration gradient, the accumulation of chemicals on the side of the growing trunk is limited, which attenuates the second nucleation and growth of side branches. Therefore, the chemical concentration gradient plays a significant role in the shape development of particles. This gradient is determined by the interplay of chemical diffusion and reaction rates.. Hence, by regulating diffusion and reaction relatively, a concentration gradient around the surface of crystals could be established, which leads to the 10


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formation of dendritic structures.

A1

A3

A2

A4

30 % 1µm

10µm

B1

B3

B2

1µm

1µm

B4

50 % 10µm

C1

10µm

D1

1µm

1µm

C3

C2

60 %

Increase of diffusion

C4

1µm

1µm

1µm

D3

D2

1µm

D4

70 %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1µm

50 μA•cm-2

1µm

250 μA•cm-2

1µm

1µm

500 μA•cm-2

750 μA•cm-2

Increase of Reaction Figure 6. An overview on the morphologies of silver particles synthesized at different reaction and diffusion conditions. The transverse images (A1, A2, A3, A4) show the silver particles formed at the currents of 50 μA•cm-2, 250 μA•cm-2, 500 μA•cm-2 and 750 μA•cm-2, respectively. The longitudinal images (A1, B1, C1,D1) show the silver particles synthesized in glycerol aqueous solution with the glycerol volume ratios of 30%, 50%, 60% and 70%, respectively. The images covered by the green shadow are polyhedral silver aggregates while the images covered by the red and blue one are dendritic and fibroid structures, respectively.

To have an overview on how diffusion and reaction influence the shapes of silver particles, more experiments were performed, as shown in Figure 6. The reaction rate is regulated by the current densities at 50 µA•cm-2, 250 µA•cm-2, 500 µA•cm-2 and 750 µA•cm-2 while the diffusion rate is regulated by the volume ratios of glycerol at 11



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30%, 50%, 60% and 70%, respectively. At slow reaction and fast diffusion, polyhedral silver aggregates are formed as shown in Figure 6A1, which is attributed to the homogeneous distribution of chemicals around the growth front of crystals. Increasing the reaction rate, while keeping the diffusion unchanged, silver dendrites are largely formed, as shown in Figure 6A2 and 6A3, which is attributed to the establishment of a chemical concentration gradient around the growth front. Further increasing the reaction rate leads to the formation of fiber-shaped particles. If we keep the reaction constant while change the diffusion of chemicals, similar results are observed, as shown in Figure 6 A1-D1. An increase of glycerol volume to 50% does not remarkably change the shapes of particles, as shown in Figure 6B1. Further increasing the glycerol volume to 60% leads to the formation of dendritic structures. When the glycerol volume ratio is increased to 70%, silver dendrites are largely formed. A further increase of the glycerol volume ratio to 80% and 90% leads to the formation of fiber-shaped particles, as shown in Figure 1. Therefore, the formation of dendritic structures depends on the relative values of diffusion and reaction rates since their interplay determines the chemical distribution around the growth front of the crystals.

12


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A1

A3

A2

A4

0% 10µm

B1

1µm

1µm

B3

B2

1µm

B4

10 % 10µm

50 %

Increase of diffusion

C1

1µm

C3

C2

10µm

D1

1µm

C4

1µm

D3

D2

1µm

1µm

1µm

D4

70 %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10µm

50 μA•cm-2

1µm

1µm

500 μA•cm-2

750 μA•cm-2

1µm

250 μA•cm-2

Increase of Reaction Figure 7. SEM images of silver products synthesized in glycol solvents with various volume fractions and at different current densities. Transverse images (A1, A2, A3,A4) show the silver particles formed at the densities of 50 μA•cm-2, 250 μA•cm-2, 500 μA•cm-2 and 750 μA•cm-2, respectively. Longitudinal images (A1, B1, C1,D1) show the silver particles synthesized in a glycol aqueous solution with glycol volume ratios of 0%, 10%, 50% and 70%, respectively.

To confirm the significance of the interplay of diffusion and reaction in shaping particles, we carry out one more series of experiments to synthesize silver particles in the ethylene glycol and water mixtures, as shown in Figure 7. At the low reaction rate, polyhedral silver aggregates are formed in the solvents containing ethylene glycol fractions from 0% to 70%, as shown in Figure 6A1-6D1. The dendritic structures are 13



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not formed even at 70% volume ratio of ethylene glycol, because of the lower viscosity of ethylene glycol than that of glycerol. The viscosity of the solvent containing 70 vol% ethylene glycol is similar to that of the solvent containing 50 vol% glycerol. In these two conditions, silver aggregates are formed, which indicates, that a similar diffusion and reaction leads to a similar morphology, suggesting that the effect of diffusion and reaction is not dependent on the chemicals of solvents. These results confirm that the interplay of diffusion and reaction is the underlying mechanism for chemical diffusion and reaction to shape particles. Here we should mentions that the addition of glycerol to water not only change the viscosity of solvents, but also alter the conductivity and polarity of solvents, which also influences the structure development of particles 41-43. Since similar results are obtained in the different types of solvents (glycerol and ethylene glycol), we assume that the change of viscosity play a major role in structure development of silver particles. More details are under study.

Conclusions Diverse morphologies of silver particles were synthesized in an electrochemical reaction by regulating the diffusion and reaction of chemicals. The diffusion was regulated by changing the viscosity of the solvent, while the reaction was regulated by altering the currents applied to the reaction. Previous studies have shown that silver dendrites were largely formed at the medium values of reaction rate with the current 14


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densities from 250 μA•cm-2 to 500 μA•cm-2. In this paper, by slowing down the diffusion of chemicals, silver dendrites were also formed at the current density of 50

μA•cm-2 while disappeared at the current density of 500 μA•cm-2, which indicated the formation of dendritic structures is influenced by both reaction and diffusion of chemicals. Once diffusion and reaction reach a dynamic balance, a concentration gradient is established around the growth front of crystals, which triggers and dominates the structure development of dendrites. Therefore, a smart control on diffusion and reaction promises a general and green approach for a rational synthesis of materials without the additions of directing agents and surfactants.

Acknowledgements This study was supported by the Hundreds Talent Program from the Chinese Academy of Sciences and the project from the State Key Laboratory of Multiphase Complex Systems (MPCS-2014-D-05). The financial support from National Natural Science Foundation of China (U1462130，91534123) is warmly appreciated. We thank Prof. Helmuth Moehwald from Max Planck Institute of Colloids and Interfaces for the fruitful discussions and revisions.

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Experiments Materials Silver nitrate and potassium nitrate were purchased from Sigma-Aldrich. Ethylene glycol and glycerol are bought from Xilong Chemical Co., Ltd. All chemicals were used as received. Al and zinc wires (>99.9% in purity) were obtained from Beijing Jiaming Non-ferrous Metals Industry Co. Ltd. (Beijing, China). Deionized water with a resistivity higher than 18.2 MΩ generated by a Milli-Q system (Millipore, USA) was used throughout the experiments.

Synthesis of Silver Particles by Electrochemical Reaction The silver particles were synthesized in a designed electrochemical reactor

39

which includes two cells connected by an electric wire and a saturated potassium nitrate salt bridge. An aluminum rod and a zinc rod were connected by a wire, the former one serving as the cathode immersed in 6mM silver nitrate solution, and the latter serving as the anode placed in 1M potassium nitrate solution. A power stabilizer supplying 20-200V voltage and a digital ammeter were placed in the circuit. The current density was regulated by changing resistances. Electrons were transported into the silver nitrate solution through the cathode, which led to the formation of silver particles. The electrodes are metal rods with a diameter of 1 mm. They were washed 16


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by ethanol and deionized water to have a clean surface before immersion into the solution. The lengths of cathode and anode immersed in solution were 3cm and 5cm, respectively. The electrochemical reaction was carried out in the designed reactor for half an hour at 25℃ in a water bath. The electrochemical deposition was performed at various applied current densities and in different viscosities of silver nitrate solutions. The viscosity was regulated by the volume ratio of alcohols (ethylene glycol and glycerol) and water. After reaction, the cathode coated with silver product was gently taken out from the solution and cut into two segments. One was washed by pure water and dried in air for scanning electron microscopy (SEM) characterization. The other was sonicated in pure water, and the silver samples collected from the water were used for transmission electron microscopy (TEM) and other characterizations.

Characterizations of Silver Products A viscometer (DMA5000-AMVn, Anton Paar, Austria) was used to measure the viscosity of solvents. The morphology of silver products was characterized by a field-emission scanning electron microscope (JSM-7001F SEM) equipped with energy-dispersive X-ray spectroscopy (EDX) and a JEM-2100F (UHR) high resolution transmission electron microscope (JEOL, Japan) at an accelerating voltage of 200 kV. The phase and composition of the products were determined by X-ray Diffractometry (PANalytical B.V., Netherlands), using Cu Kα radiation, and the data were collected over the range of 2θ from 5° to 90° with a scanning step of 0.1°. 17



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For Table of Contents Only

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Controllable Synthesis of Silver Dendrites via an Interplay of Chemical

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