This work reports for the first time electrosynthesis of hexanethiol capped silver nanotriangles cores (Ag@C6SH NCs) by a rapid, clean, and simple Double Pulse Chronopotentiometric (DCP) method in nonaqueous media, using a Taguchi orthogonal array
Over the last two decades, an increasing attention has been focused on nanoclusters, as part of nanostructural materials as almost monodispersed isolable nanoparticles that are usually of sizes <2.0 nm to 100 nm in diameter [
In order to prevent agglomeration, these nanosized entities have to be stabilized by ligand molecules or a whole plethora of “protecting shells.” The resulting metal colloids can be redispersed in water (“hydrosols”) or organic solvents (“organosols”) [
The present work is dealing with the latest experimental challenge for the electrosynthesis and characterization of silver nanocluster cores capped with hexanethiol (C6SH) shell through the electrochemical dissolution of silver rod in the presence of hexanethiol as a suitable stabilizing agent because of the strong affinity of thiol molecules for coordination in Ag atom within
A silver wire and a platinum rod with 99.9% purity, 0.5 mm diameter, and exposed length of about 5 mm were used as sacrificial anode and cathode (counter electrode), respectively. The electrodes were initially cleaned in piranha solution, including mixture of 30% H2O2 and concentrated H2SO4 with the ratio of 1 : 3. Before the use of silver wire as anode, it was subjected to potential cycling between −0.4 and 1.5 V at a scan rate of 50 mVs−1 in 0.1 M H2SO4 solution for about 20 cycles for the sake of electrochemical cleaning. The electrodes were kept parallel to each other in a distance of about 5 to 20 mm (the diameter of electrochemical cell) and then electrolysis was carried out in the solution under nitrogen purge and a constant magnetic stirring. Aqueous solution of 1 M KCl (Merck) in 18.3 MΩ Millipore deionized water (Younglin Korean Instruments from RIPPI) was mixed with 20 mM hexanethiol (C6SH, Aldrich) dissolved in absolute ethanol (Merck) in equal volumes and the resulting solution was used as the electrolyte.
The nanoparticles were electrosynthesized by electrochemical dissolution of the silver electrode in either presence or absence of 1% (w/v) NaBH4 (Aldrich) which is converted rapidly to nanoclusters in presence of hexanethiol. Electrochemical dissolution of the silver wire electrode as a sacrificial anode was carried out in a 25 mL electrochemical cell at ambient temperature (about 25°C), while a temperature range of 55–75°C was used for the two-electrode configuration. The experiments were conducted at different controlled current densities of 0.5 and 0.9 A·cm−2 by using an AUTOLAB PGSTAT 30-Potentiostat/Galvanostat TYPE III (Metrohm) in the chronopotentiometry mode.
During the electrolysis process and dissolution of the anodic metal, continuous evolution of gases, mainly O2, at the surface of cathode was observed. After about 5 to 50 s from starting the electrolysis at the constant current density of 0.5 A·cm−2, the dark brownish particles that floated on the surface of electrolyte solution appeared. The electrolysis was continued for about 15 to 30 min for each experiment. At the end of process, a microdrop of 30% (v/v) H2O2 was added to the solution in order to reduce the possibly formed silver oxide, as a byproduct. Then, the floating particles on the surface of ethanol-water mixture (electrolyte) were extracted into the toluene to obtain a clear dark brown to black suspension. Thereafter, the nanoparticles were removed from the solvent under vacuum, purified by washing with ethanol repeatedly to remove excess C6SH, and finally dried under vacuum. Before SEM imaging, the nanoparticles were redispersed in the organic solvent and the obtained suspension was passed through a 0.2
The process uses a silver electrode together with a platinum rod located in deionized water with a small distance apart. The sacrificial silver anode serves as the metal (ions) source for silver nanoparticle formation in the water/ethanol/potassium chloride electrolyte solution. A low DC voltage (5 to 50 mV) is applied to the electrodes. In this electrochemical process, some of the silver cations in close proximity to the anode accept the electrons from the current passing through and are reduced to metallic atoms as silver zerovalent species (
The electrochemical oxidation process by the galvanostatic method was carried out in a single compartment electrochemical cell at the constant currents of 0.5 A. The cell consisted of 5 mm separated silver (sacrificial anode) and platinum (cathode) plates. Figure
Constant current chronopotentiogram (
During electrolysis, the electrode surface changes seriously as a function of applied potential or current pulse. During the first cycle (green curve), it could be seen that, at the beginning of electrolysis (i.e., about first 10 s), the 0.5 A·cm−2 current pulse causes the oxidization of silver atoms at the surface of sacrificial anode and, consequently, a drastic increase in the potential occurred, due to the corrosion of silver anode. About 10–15 s after starting the electrolysis, an immediate decrease in the voltage demonstrates the reduction of released silver cations in the solution and generation of zerovalent free atoms of silver (
It should be noted that the presence of NaBH4 may accelerate the electroreduction stage in the electrosynthesis and the potential may change rapidly in the presence of this reactive reducing agent. The oxidation-reduction loop was repeated until the electrolysis progresses reached 25 s and then the potential remained more or less constant at 10 V, which obviously shows that no additional redox reaction occurred and the surface of electrode is temporarily passivated, most probably due to formation of silver oxide layers.
During the second cycle (blue curve), the first shaking of the electrode which was followed by 3 min ultrasonication caused the release of the electrogenerated clusters from the surface and their falling into the solution. Thereafter, the heavy particles formed tended to settle down in the solution and the lighter particles begin to move to the surface which resulted in the formation of a nanoemulsion (or nanomicelle) of the nanoclusters. In the second cycle of electrolysis, during the first 25 s, the potential again decreased mildly and then jumped to more than 8 V which elucidated that an oxidation step started again and the electrode surface is once more activated. This rapid increase in the potential was then followed by a mild decrease (in the range of 5.5 to 3.0 V) for the next 100 s (usually called transition time
Finally, in the last cycle (red curve), no more significant redox reaction was observed which perceived that the electrode surface cannot be regenerated and refreshed again. In this case, only two mild potential jumps in about 25 and 150 s are observed.
Thus, based on the above-mentioned experiences, it can be concluded that, in the course of electrolysis, the surface of sacrificial anode electrode surface not only undergoes a simple electrocorrosion but also witnesses irreversible structural changes. Briefly, in the first cycle, the surface possesses the highest resistance to the redox reaction (very little
In order to check the possibility of any electrochemical reaction occurring in the electrolyte solution (i.e., C6SH + NaBH4 + KCl + EtOH), the cyclic voltammetric experiments were carried out at an Ag rod, a glassy carbon electrode (GCE), and a Pt electrode, as working electrodes, and the results are shown in Figures
Cyclic voltammograms of electrolyte solution (C6SH + NaBH4 + KCl + EtOH) at the Ag rod (a), GCE (b), and Pt electrode (c) surfaces under nitrogen purge before electrochemical synthesis under optimized electrosynthesis protocol at different scan rates: green: 25 mVs−1; blue: 50 mVs−1; red: 100 mVs−1.
The use of fractional factorial experiments, that is, the Taguchi robust design, leads to the effective reduction of trial numbers required for parameters optimization [
In this work, the Taguchi robust design was used to optimize the electrosynthesis process and identify the effects of different parameters on the size (and possibly shape) of the silver nanoclusters. The studied parameters (i.e., current density, electrolysis time, electrode distance, hexanethiol concentration, solvent ratio, %NaBH4, and temperature) and their levels used for the electrosynthesis of Ag@C6SH NCs as 8 different experiments designed are given in Table
TC1: trial condition 1 (randomly selected order 6) | ||
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Current density (A·cm−2) | 0.5 | 1 |
Time (Min) | 15 | 1 |
Distance (mm) | Minimum | 1 |
Hexanethiol concentration (mM) | 10 | 1 |
Solvent ratio = |
50 : 50 | 1 |
Temperature | 55 | 1 |
NaBH4 (w/v %) | 0% | 1 |
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TC2: trial condition 2 (randomly selected order 4) | ||
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Current density (A·cm−2) | 0.5 | 1 |
Time (Min) | 15 | 1 |
Distance (mm) | Minimum | 1 |
Hexanethiol concentration (mM) | 20 | 2 |
Solvent ratio | 70 : 30 | 2 |
Temperature | 75 | 2 |
NaBH4 (w/v %) | 1% | 2 |
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TC3: trial condition 3 (randomly selected order 5) | ||
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Current density (A·cm−2) | 0.5 | 1 |
Time (Min) | 30 | 2 |
Distance (mm) | Maximum | 2 |
Hexanethiol concentration (mM) | 10 | 1 |
Solvent ratio = |
50 : 50 | 1 |
Temperature (°C) | 75 | 2 |
NaBH4 (w/v %) | 1% | 2 |
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TC4: trial condition 4 (randomly selected order 2) | ||
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Current density (A·cm−2) | 0.5 | 1 |
Time (Min) | 30 | 2 |
Distance (mm) | Maximum | 2 |
Hexanethiol concentration (mM) | 20 | 2 |
Solvent ratio = |
70 : 30 | 2 |
Temperature (°C) | 55 | 1 |
NaBH4 (w/v %) | 0% | 1 |
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TC5: trial condition 5 (randomly selected order 7) | ||
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Current density (A·cm−2) | 0.9 | 2 |
Time (Min) | 15 | 1 |
Distance (mm) | Maximum | 2 |
Hexanethiol concentration (mM) | 10 | 1 |
Solvent ratio = |
70 : 30 | 2 |
Temperature (°C) | 55 | 1 |
NaBH4 (w/v %) | 1% | 2 |
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TC6: trial condition 6 (randomly selected order 3) | ||
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Current density (A·cm−2) | 0.9 | 2 |
Time (Min) | 15 | 1 |
Distance (mm) | Maximum | 2 |
Hexanethiol concentration (mM) | 20 | 2 |
Solvent ratio = |
50 : 50 | 1 |
Temperature (°C) | 75 | 2 |
NaBH4 (w/v %) | 0% | 1 |
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TC7: trial condition 6 (randomly selected order 4) | ||
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Current density (A·cm−2) | 0.9 | 2 |
Time (Min) | 30 | 2 |
Distance (mm) | Minimum | 1 |
Hexanethiol concentration (mM) | 10 | 1 |
Solvent ratio = |
70 : 30 | 2 |
Temperature (°C) | 75 | 2 |
NaBH4 (w/v %) | 0% | 1 |
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TC8: trial condition 6 (randomly selected order 1) | ||
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Current density (A·cm−2) | 0.9 | 2 |
Time (Min) | 30 | 2 |
Distance (mm) | Minimum | 1 |
Hexanethiol concentration (mM) | 20 | 2 |
Solvent ratio = |
50 : 50 | 1 |
Temperature (°C) | 75 | 1 |
NaBH4 (w/v %) | 1% | 2 |
Ag-Pt chronopotentiogram obtained during 1st cycle of electrosynthesis of Ag@C6SH NCs in trial TC8.
SEM images of the electrosynthesized Ag@C6SH NCs based on Taguchi-DOE under optimized electrosynthesis conditions.
Qualitative analyses of SEM images TC1 to TC8 based on different descriptive criteria including nanometric porosity density (NMPD), agglomeration and aggregation (AA), approximate average agglomeration number (AAAN), approximate average particle size (AAPS), electrochemical response (ECR), electrochemical reaction kinetics (ECRK), and surface roughness of as-prepared samples were carried out and the results are summarized in Table
Descriptive explanations for electrosynthesized Ag@C6SH NCs based on their SEM image analysis.
Exp. | TC | NMPD | Morphology and geometry | AA | AANN | AAPS (nm) | Dispersity (size distribution) | Surface particle density | ECR |
ECRK | Surface roughness | Overall |
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1 | TC1 | Highly porous | Spherical | High >85% | 0.6788 | 40 | Low | Very low | Highly gradient | Fast <100 s | Relatively high | OK |
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2 | TC2 | High | Spherical | Very high >99% | 0.7906 | 43 | Low | Medium | Gradient | Fast | High >90% | OK |
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4 | TC4 | Highly porous | Spherical | High >95% | 0.7586 | 35 | Medium | Very low | Constant | Fast <50 s | High >90% | OK |
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5 | TC5 | Highly irregular | Spherical | High >80% | 0.6530 | 39 | Low | Very low | Constant | Fast | Irregular-high >80% | OK |
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6 | TC6 | High | Spherical | High >65% | 0.5324 | 44 | Low | Very low | Constant | Fast | Low <50% | OK |
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7 | TC7 | Medium-irregular | Spherical | High >90% | 0.7187 | 41 | Low | Low | Constant | Fast | Irregular-medium | OK |
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8 | TC8 | Very low | Spherical-hemispherical | Low | 0.1900 | 37 | Low | High | Gradient | Fast | High >90% | OK |
TC: trial condition; NMPD: nanometric porosity density; AA: agglomeration and aggregation; AAAN: approximate average agglomeration number calculated based on the following equation: (1.1)3 × (1 − 0.4) = 0.799; AAPS: approximate average particle size; ECR: electrochemical response; Surface roughness: (real surface area/geometric surface area); Fast: <125 s; ECRK: electrochemical reaction kinetics.
Approximate estimated size distribution of TC1 to TC8 for 20 estimations only based on the feature size of SEM images.
Number | TC1 | TC2 | TC3 | TC4 | TC5 | TC6 | TC7 | TC8 |
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1 | 57 | 48 | 33 | 51 | 40 | 47 | 45 | 39 |
2 | 51 | 49 | 38 | 34 | 41 | 41 | 40 | 38 |
3 | 50 | 45 | 32 | 39 | 30 | 40 | 34 | 40 |
4 | 30 | 49 | 36 | 45 | 49 | 40 | 40 | 45 |
5 | 35 | 38 | 29 | 38 | 40 | 45 | 42 | 38 |
6 | 51 | 46 | 38 | 34 | 41 | 51 | 45 | 39 |
7 | 45 | 48 | 31 | 36 | 38 | 45 | 45 | 36 |
8 | 51 | 45 | 28 | 34 | 30 | 41 | 44 | 30 |
9 | 21 | 45 | 26 | 29 | 32 | 50 | 38 | 34 |
10 | 39 | 43 | 35 | 44 | 32 | 37 | 46 | 30 |
11 | 27 | 37 | 31 | 29 | 40 | 49 | 33 | 36 |
12 | 55 | 50 | 31 | 33 | 48 | 45 | 43 | 42 |
13 | 30 | 40 | 23 | 29 | 47 | 40 | 41 | 43 |
14 | 37 | 42 | 27 | 35 | 45 | 47 | 40 | 43 |
15 | 32 | 35 | 24 | 31 | 30 | 32 | 32 | 30 |
16 | 28 | 39 | 24 | 30 | 32 | 49 | 43 | 30 |
17 | 39 | 39 | 30 | 29 | 38 | 43 | 40 | 32 |
18 | 33 | 38 | 27 | 30 | 28 | 49 | 45 | 30 |
19 | 52 | 45 | 28 | 40 | 53 | 42 | 40 | 41 |
20 | 37 | 39 | 29 | 30 | 46 | 47 | 44 | 44 |
Average size (nm) | 40 | 43 | 30 | 35 | 39 | 44 | 41 | 37 |
SD | 10 | 5 | 4 | 6 | 7 | 5 | 4 | 5 |
CV (%) | 26 | 11 | 15 | 18 | 19 | 11 | 10 | 14 |
The orthogonal
Orthogonal
Trial | Current density (A·cm−2) | Time (Min) | Distance (mm) | Thiol conc. (mM) | Solvent ratio | Temp. (°C) | NaBH4 (w/v%) | Average size (nm) |
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1 | 0.5 | 15 | Min. | 10 | 50 : 50 | 55 | 0 | 40 (10) |
2 | 0.5 | 15 | Min. | 20 | 70 : 30 | 75 | 1 | 43 (4) |
3 | 0.5 | 30 | Max. | 10 | 50 : 50 | 75 | 1 | 30 (4) |
4 | 0.5 | 30 | Max. | 20 | 70 : 30 | 55 | 0 | 35 (6) |
5 | 0.9 | 15 | Max. | 10 | 70 : 30 | 55 | 1 | 39 (7) |
6 | 0.9 | 15 | Max. | 20 | 50 : 50 | 75 | 0 | 44 (5) |
7 | 0.9 | 30 | Min. | 10 | 70 : 30 | 75 | 0 | 41 (4) |
8 | 0.9 | 30 | Min. | 20 | 50 : 50 | 55 | 1 | 37 (5) |
Average values of particle size corresponding to the effect of factors at studied levels.
Factor | Level 1 (nm) | Level 2 (nm) |
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Current density (A·cm−2) | 38.3 | 40.3 |
Time (s) | 42.8 | 35.8 |
Distance (mm) | 41.5 | 37.0 |
Thiol concentration (mM) | 38.8 | 39.8 |
Solvent ratio | 39.0 | 39.5 |
Temperature (°C) | 39.0 | 39.5 |
%NaBH4 | 41.3 | 37.3 |
As it could be seen in Table
Results of ANOVA for Ag@C6SH NCs through electrochemical synthesis using
Factor | DOF |
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Pooled | |||
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DOF |
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Current density ( |
1 | 8.0 | 8.0 | 1 | 8.0 | 8.0 | 3.9 |
Time ( |
1 | 98.0 | 98.0 | 1 | 98.0 | 98.0 | 53.4 |
Distance ( |
1 | 40.5 | 40.5 | 1 | 40.5 | 40.5 | 21.8 |
Hexanethiol C6SH concentration | 1 | 2.0 | 2.0 | — | — | — | — |
Solvent ratio ( |
1 | 0.5 | 0.5 | — | — | — | — |
Temperature (Temp.) | 1 | 0.5 | 0.5 | — | — | — | — |
%NaBH4 ( |
1 | 32.0 | 32.0 | 1 | 32 | 32 | 17.1 |
Error ( |
0 | — | — | 3 | 3 | 1 | 3.8 |
Note: the critical value was for a 90% confidence level; pooled error resulted from pooling of insignificant effect. a is designated for percentage of contribution.
The calculations showed that the estimated size of silver nanoclusters at optimum conditions will be about
The solutions of colloidal nanoparticles of silver have a very distinctive color which arises from the tiny dimension of the silver particles. The resonance of nanoparticles, known as surface plasmon absorbance (SPR), could be used as an analytical tool to reveal the characteristic of some mesoscale surfaces [
UV-Vis absorbance of the electrosynthesized Ag@C6SH NCs dispersed in toluene.
Irreversible and inevitable aggregation and agglomeration competing phenomena will certainly influence the nanostructure of sample and need more purification and fractionation dispersion after treatments. SPR absorbance spectra of the metal nanoparticles are sensitive to the changes in core size, especially in the metal-to-molecule size range. Based on our reported SEM image size (30 nm), it is evident that SPR peaks will be observed here with so much difficulty.
For silver nanoparticles, the intensity and wavelength of SPR band depend on the size being intense and sharp for larger nanoparticles (>2 nm) with
Figure
Scherer equation PXRD-based particle size data.
hkl | 2 |
Height | FWHM |
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Rel. int. |
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111 | 38.158 | 1739 | 0.26 | 2.35655 | 100.00 | 1.540 | 91.63 |
200 | 44.344 | 696 | 0.32 | 2.04112 | 40.03 | 1.540 | 74.16 |
220 | 64.507 | 707 | 0.31 | 1.44342 | 40.66 | 1.540 | 76.56 |
311 | 77.458 | 821 | 0.33 | 1.23123 | 47.25 | 1.540 | 71.92 |
222 | 81.580 | 225 | 0.43 | 1.17914 | 12.96 | 1.540 | 55.19 |
Powder XRD pattern of as-prepared Ag@C6SH NCs (right) and standard XRD (left) [
A facile electrochemical procedure, based on the dissolution of a metallic silver rod anode in a protic solvent (EtOH), was successfully used to obtain silver nanoclusters capped by hexanethiol with average sizes ranging from 30 to 44 nm based on SEM images estimation. The prepared nanoclusters were stabilized by the aid of hexanethiol (C6SH). By precisely tuning the process parameters, it was possible to obtain silver with different particle sizes and shapes. Characterization of the electrosynthesized nanoparticles was carried out by SEM, XRD, and UV-Vis spectroscopy. Meanwhile, as evaluated by Taguchi
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors gratefully acknowledge the support for this work by Research Councils of University of Tehran, K. N. Toosi University of Technology, and Razi University.