Systematic Shape Evolution of Gold Nanocrystals Achieved through

Subscriber access provided by Kaohsiung Medical University

C: Physical Processes in Nanomaterials and Nanostructures

Systematic Shape Evolution of Gold Nanocrystals Achieved through Adjustment in the Amount of HAuCl4 Solution Used Bing-Hong Kuo, Chi-Fu Hsia, Tzu-Ning Chen, and Michael H. Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08479 • Publication Date (Web): 15 Oct 2018 Downloaded from http://pubs.acs.org on October 15, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34 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

The Journal of Physical Chemistry

Systematic Shape Evolution of Gold Nanocrystals Achieved through Adjustment in the Amount of HAuCl4 Solution Used Bing-Hong Kuo, Chi-Fu Hsia, Tzu-Ning Chen, and Michael H. Huang* Department of Chemistry and Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing Hua University, Hsinchu 30013, Taiwan

ABSTRACT:

Past work on the synthesis of gold nanocrystals has revealed that

changing the reaction cell potential enables particle shape evolution. Although various parameters in the Nernst equation has been used to achieve this, adjusting the metal precursor concentration has not been demonstrated before.

Using the reported synthetic

conditions with cetyltrimethylammonium chloride (CTAC) surfactant to grow gold nanocubes, rhombic dodecahedra, and octahedra as the starting points and fixing the ascorbic acid volume, tuning the volume of HAuCl4 solution introduced allows the formation of the same series of crystal shape evolution from rhombic dodecahedral to trisoctahedral and cubic structures in the presence of a tiny amount of NaBr.

In another

series, by gradually increasing the HAuCl4 solution volume but fixing KI and ascorbic acid volumes, gold nanocrystals with tunable morphologies from octahedra to corner-truncated octahedra, edge- and corner-truncated octahedra, and rhombic 1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

dodecahedra can be synthesized.

Page 2 of 34

These results demonstrate that adjustment of metal

precursor concentration is also effective in metal particle shape control.

Using spectral

analysis, the formation of CTA-AuCl4 and CTA-AuCl2 complexes have been confirmed, so CTAC acts as a coordinating species to the metal source and provides halide ions to influence the overall cell potential.

Thus, all species in the reaction mixture are

non-innocent and are involved in the redox chemistry.

The strategy has been applied to

the synthesis of small Cu nanocubes with tunable sizes and nanowires.

INTRODUCTION Synthesis of polyhedral metal nanocrystals with tunable shapes and controlled sizes enables investigation of their facet-dependent catalytic properties in diverse organic reactions.1‒5

They are also useful as surface-enhanced Raman substrates,6–8 and are

excellent building blocks for the fabrication of supercrystals.9–13

To grow Au, Pd and

other noble metal nanocrystals with shape evolution, different approaches have been adopted, including the variation in the amount of reducing agent added,14–17 metal precursor selection,18 introduction of tunable amounts of bromide or iodide ions,17,19–21 surfactant choice such as cetyltrimethylammonium bromide (CTAB) or CTAC and its amount,7,22 adjustment in the reaction temperature,20 and HCl oxidative etching.23 2 ACS Paragon Plus Environment

All

Page 3 of 34 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


these strategies are actually related, if we understand that Au and Pd particle shapes can be tuned by changing the redox potential of the reaction involved.

These parameters

directly or indirectly tunes the overall reaction potential either by changing the reduction potential of the metal precursor through ligand exchange with different halide ions, or tunes the reaction quotient Q through adjustment in the amount of reducing agent introduced.

Thus, CTAC and CTAB surfactants are not innocent acting only as capping

agents because of the halides they carry.

Basically these parameters tune one or more

variables in the Nernst equation to change the reaction potential and thus the free energy of reaction (∆G = ‒nFE).

Hence varying the reaction temperature can also be effective,

but this is more difficult to control precisely.

Tuning ∆G means tuning the driving force

for the reaction, and this in turn gives different rates of solution color changes or tunable reduction rates in the formation of various particle morphologies.

For Ag and Cu

nanocrystals, systematic shape evolution in aqueous solution is much more difficult to achieve, possibly because of the low reduction potentials of AgCl and AgBr which are hard to avoid, and the favorable formation of CuCl and CuBr through the reduction of CuCl2/CuBr2 or in the presence of CTAB/CTAC surfactant, so cubes are generally obtained.24,25

Although these known strategies are successful in tuning particle shapes,

examination of the Nernst equation suggests that varying the metal precursor 3 ACS Paragon Plus Environment


Page 4 of 34

concentration should also be effective, since both the reducing agent and the metal precursor concentrations appear in the reaction quotient Q.

However, such simple and

obvious way to adjust the overall reaction potential has never been demonstrated before, although adjustment of both the reducing agent and precursor amounts was successful in making bimetallic core–shell nanocrystals with systematic shape evolution and tunable sizes.26,27

Yet there was no good understanding of why such approach was effective.

It is understandable why no one has tried to mainly adjust the metal source amount to tune particle shape, because intuitively doing so should only change the particle size and/or yield.

Thus, the idea that particle shape is linked to the metal source

concentration sounds strange to most people. This work explores the growth of gold nanocrystals with systematic shape evolution using the previously developed synthetic conditions as the starting point, and varies the volume of HAuCl4 solution over a wide range to achieve similar shape tunability.15,19 The same shape transformation from rhombic dodecahedral to cubic structures in the presence of NaBr, and from octahedral to rhombic dodecahedral structures in another series with KI has been achieved.

To completely understand the roles of all species

added in the growth of these nanocrystals, the possibility of CTA+ acting as a coordinating ligand to form CTA-AuCl4 and CTA-AuCl2 complexes was examined 4 ACS Paragon Plus Environment

Page 5 of 34 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


spectroscopically.

The complex formation has been confirmed.

Thus, all species

added to the reaction mixture actually take part in the redox reaction, including halide ions and CTAC surfactant.

Using this synthetic approach, the amount of copper acetate

was varied to achieve the formation of small Cu nanocubes with tunable sizes and nanowires.

EXPERIMENTAL METHODS Chemicals.

Hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O, Aldrich,

99.9%), cetyltrimethylammonium chloride (CTAC, TCI, 95%), sodium borohydride (NaBH4, Sigma‒Aldrich, 98%), ascorbic acid (AA, Riedel-de Haën, 99.7%), sodium bromide (NaBr, UCW), potassium iodide (KI, J. T. Baker), cupric acetate monohydrate (Cu(CH3COO)2·H2O, 98%, J. T. Baker), and L(+)-sodium ascorbate (SA, 98%, Sigma-Aldrich) were used without further purification. Ultrapure distilled and deionized water (18.3 MΩ) was used for all preparations. Preparation of Gold Seed Solution.

A volume of 10 mL aqueous solution

containing 0.10 M CTAC and 2.5 × 10‒4 M HAuCl4 was prepared. mL of 0.02 M NaBH4 was made in ice-cold solution. added 0.45 mL of the NaBH4 solution with stirring.

Concurrently, 10

To the HAuCl4 solution was The resulting solution immediately

5 ACS Paragon Plus Environment


turned brown, indicating the formation of gold particles.

Page 6 of 34

The seed solution was aged for

1 h at 30 ºC to decompose excess borohydride by water. Synthesis of Gold Nanocrystals with Shape Evolution from Rhombic Dodecahedral to Cubic Structures.

The procedure is based on our reported method.15

In the first set

of reaction conditions, a growth solution was prepared in two vials. First, 0.32 g of CTAC surfactant was added.

The concentration of CTAC in the final solution is 0.1 M.

Depending on the gold particle shape to be synthesized, slightly different volumes of deionized water were added to each vial (9.625, 9.655, and 9.715 mL for cubes, trisoctahedra, and rhombic dodecahedra, respectively) to reach 10 mL for the total solution volume.

The vials were placed in a water bath set at 25 ºC.

Next, 160, 220,

and 250 µL of 0.01 M HAuCl4 solution were added for the syntheses of rhombic dodecahedra, trisoctahedra, and cubes, respectively, and 10 µL of 0.01 M NaBr was introduced. solution.

Finally, 90 µL of 0.04 M ascorbic acid was added to form the growth

The solution color turned colorless after the addition of ascorbic acid,

indicating the reduction of Au3+ to Au+ species.

Then 25 µL of the seed solution was

added to one growth solution with shaking until the solution color turned light pink (~5 sec).

Next, 25 µL of this solution was added to the second growth solution with

thorough mixing for 5‒10 sec.

The solution was aged for 15 min and centrifuged at 6 ACS Paragon Plus Environment

Page 7 of 34 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


3000 rpm for 10 min to collect the particles using a Hermle Z323 centrifuge.

After

removing the top solution, 40 mL of deionized water was added to the precipitate and the solution was centrifuged again using the same condition. In another set of experiments, the reaction conditions are the same as described above with the exception of the volumes of HAuCl4 and ascorbic acid solution used. Here the volumes of HAuCl4 solution added are 250, 300, and 420 L for the formation of Au rhombic dodecahedra, trisoctahedra, and cubes, respectively.

And 150 L of

ascorbic acid was introduced. Synthesis of Gold Nanocrystals with Shape Evolution from Octahedral to Rhombic Dodecahedral Structures.

The procedure is based on our reported method.19

first set of reaction conditions, a growth solution was prepared in two vials.

In the

First, 0.32 g

of CTAC surfactant was added, and then 9.635, 9.595, 9.565, 9.525, and 9.485 mL of deionized water were added to each vial for the growth of Au octahedra, corner-truncated octahedra, edge- and corner-truncated octahedra (type I and type II), and rhombic dodecahedra, respectively).

The vials were kept in a water bath set at 20‒25 ºC.

To

the first growth solution was added 100, 140, 170, 210, and 250 µL of 0.01 M HAuCl4 solution to make Au octahedra, corner-truncated octahedra, edge- and corner-truncated octahedra (I), edge- and corner-truncated octahedra (II), and rhombic dodecahedra, 7 ACS Paragon Plus Environment


respectively.

Page 8 of 34

Finally, 20 µL of 0.001 M KI and 220 µL of 0.04 M ascorbic acid were

introduced to the growth solution.

Again the solution color turned colorless after the

addition of ascorbic acid, signifying the formation of Au+ species.

Next, 25 µL of the

seed solution was added to the first growth solution with shaking until the solution color turned light pink (~5 sec).

Then 25 µL of this solution was transferred to the second

growth solution with thorough mixing for 5‒10 sec.

The solution was aged for 15 min

and centrifuged at 2500 rpm for 10 min to collect the particles.

After removal of the top

solution, 40 mL of deionized water was added to the precipitate and the solution was centrifuged again using the same condition. In another set of experiments, the synthesis conditions are the same as described above with the exception of the volumes of HAuCl4 and KI solutions used.

Here 250,

300, 380, 460, and 520 L of HAuCl4 solution were added to make Au octahedra, corner-truncated octahedra, edge- and corner-truncated octahedra (I), edge- and corner-truncated octahedra (II), and rhombic dodecahedra, respectively.

And 50 L of

0.001 M KI solution was introduced. Synthesis of Copper Nanocubes with Tunable Sizes and Nanowires. procedure was adopted from our reported method.25

The

To make copper nanocrystals from

cubes to wires, 0.048 g of CTAC and different volumes of deionized water were added to 8 ACS Paragon Plus Environment

Page 9 of 34 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


each vial (see Table S1 in the Supporting Information for detail).

Then 50, 250, 270,

and 300 µL of 0.1 M copper acetate solution were added and mixed thoroughly. 100 µL of 0.5 M sodium ascorbate solution was introduced and mixed well. were placed in an oven and heated to 100 ºC for 40 min.

Next,

The vials

After cooling the vials to room

temperature, the solutions were centrifuged at 6500 rpm for 5 min.

The precipitate was

centrifuged one more time with 5 mL of deionized water at 6500 rpm for 5 min. Finally, the synthesized copper nanocrystals were dispersed in deionized water. Instrumentation.

TEM characterization was performed on a JEOL JEM-2100

microscope with an operating voltage of 200 kV.

SEM images of the samples were

obtained using a JEOL JSM-7000F electron microscope.

Powder XRD patterns were

recorded on a Shimadzu XRD-6000 diffractometer with Cu Kα radiation.

UV‒vis

absorption spectra were acquired with the use of a JASCO V-570 spectrophotometer.

RESULTS AND DISCUSSION To make gold nanocrystals, HAuCl4 is typically reduced by ascorbic acid through the following two reactions. (DAA).

Ascorbic acid (AA) is oxidized to dehydroascorbic acid

AuCl2‒ species is formed as an intermediate species because the gold solution

initially turns colorless before being further reduced to metallic gold. 9 ACS Paragon Plus Environment


Page 10 of 34

AuCl4‒ + AA (C6H8O6) → AuCl2‒ + DAA (C6H6O6) + 2Cl‒ +2H+ 2AuCl2‒ + AA (C6H8O6) →2Au + DAA (C6H6O6) + 4Cl‒ + 2H+

(1) (2)

The Nernst equation gives the cell potential under non-standard states with reagent concentrations differing from 1 M. 𝑅𝑇 𝑙𝑛𝑄 𝑛𝐹

𝐸 = 𝐸º –

Combining equations (1) and (2), we can write the reaction quotient as:

Q=

8 6 [𝐷𝐴𝐴]3[𝐶𝑙 ― ] [𝐻 + ] 2

[𝐴𝑢𝐶𝑙4 ― ] [𝐴𝐴]3

Previously we have synthesized Au nanocubes, trisoctahedra, and rhombic dodecahedra by only varying the volume of ascorbic acid added in an aqueous mixture of CTAC surfactant, HAuCl4, NaBr, and ascorbic acid.15 According to the reaction quotient, it should be possible to tune the cell potential by changing the HAuCl4 concentration.

To test whether varying the volume of HAuCl4 can also give the same

crystal shape evolution, the same reaction conditions used to synthesize Au cubes (adding 90 L of AA) and rhombic dodecahedra (150 L of AA) were adopted as the starting points.

This strategy makes perfect sense because there is no need to search for new

reaction conditions.

For the complete reagent amounts and experimental procedure

used, see Scheme 1.

Scanning electron microscopy (SEM) images in Figure 1a‒c show 10 ACS Paragon Plus Environment

Page 11 of 34 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


that keeping the volume of 0.04 M ascorbic acid added at 90 L, but decreasing the volume of 0.01 M HAuCl4 solution from 250 µL originally used to grow gold nanocubes to 220 and 160 L, trisoctahedra and rhombic dodecahedra were respectively synthesized.

If using the condition of 150 L of ascorbic acid and 250 L of HAuCl4

solution for making rhombic dodecahedra, but increasing the HAuCl4 volume to 300 and 420 L, trisoctahedra and cubes can be respectively synthesized (Figure 1d‒f).

Figure

S1 summarizes the HAuCl4 volumes used and the resulting nanocrystal structures.

The

particles are highly uniform in size and shape that the cubes and rhombic dodecahedra spontaneously self-assemble into ordered packing arrangements.

The results clearly

demonstrate that simply tuning the amount of metal source is also effective in crystal shape evolution.

Figure S2 gives the size measurements of these particles.

Average

sizes for the synthesized Au rhombic dodecahedra, trisoctahedra, and cubes are respectively 82, 96, and 87 nm using 90 L of AA, and 90, 104, and 94 nm using 150 L of AA.

Figure S3 provides their size distribution histograms.

The particles

synthesized with 150 L of ascorbic acid are generally larger than those obtained in the previous report.15

This is understandable because more gold precursor has been added

to achieve shape evolution. X-ray diffraction (XRD) patterns of the synthesized gold cubes, trisoctahedra, and 11 ACS Paragon Plus Environment


Page 12 of 34

rhombic dodecahedra are given in Figure 2, matching with the standard XRD pattern of gold.

Cubes show only the (200) peak, and rhombic dodecahedra display strong (111)

and (220) peaks due to their oriented assembly on the substrate.

Figure S4 presents

transmission electron microscopy (TEM) characterization of the Au cubes, trisoctahedra, and rhombic dodecahedra. surfaces.28

The perfect trisoctahedra should expose high-index {221}

Selected-area electron diffraction (SAED) patterns taken on individual

crystals show they are all single-crystalline. On the basis of this successful demonstration, we have tested another series to grow gold nanocrystals with systematic shape evolution from rhombic dodecahedral to octahedral structures by keeping the amount of 0.04 M ascorbic acid constant at 220 L, but increasing the volume of 0.001 M KI from 20 to 50 L.

Despite the tiny difference

in the amount of KI introduced, shape evolution has been achieved due to partial chloride ligand replacement of AuCl4– by iodide to tune the reduction potential of the metal precursor.19

Beginning with the addition of 20 L of KI solution and 250 L of HAuCl4

solution originally used to make rhombic dodecahedra, continuously decreasing the volume of HAuCl4 solution added to 100 L yielded edge- and corner-truncated octahedra (type II, 210 L of HAuCl4; type I, 170 L of HAuCl4), corner-truncated octahedra, and octahedra.

Figure 3a–e shows SEM images of the synthesized gold 12 ACS Paragon Plus Environment

Page 13 of 34 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


nanocrystals.

Again the particle size and shape are highly uniform that they readily

self-assemble into ordered packing structures.

Please see Figure 4 for the shape

difference between the two types of edge- and corner-truncated octahedra (perfect triangular {111} faces for type II, and truncated triangular {111} faces or octagonal {100} faces for type I).

Alternatively, by adding 50 L of KI solution and 250 L of

HAuCl4 solution to make octahedra, and gradually increasing the volume of HAuCl4 solution added to 520 L, corner-truncated octahedra, edge- and corner-truncated octahedra (type I and type II), and rhombic dodecahedra can be synthesized. particles are also highly monodisperse in size and shape.

The

Again systematic shape

evolution is successful by simply adjusting the gold precursor amount.

Figures S2 and

S3 gives the size measurements of these particles and their size distribution histograms. Average sizes of the octahedra, corner-truncated octahedra, edge- and corner-truncated octahedra (I), edge- and corner-truncated octahedra (II), and rhombic dodecahedra are respectively 80, 69, 92, 90, and 83 nm using 20 L of KI, and 85, 71, 97, 93, and 94 nm using 50 L of KI. XRD patterns of this series of particle shapes are also available in Figure 2. Because {111} and {110} faces are often observed to orient parallel to the substrate surface for corner-truncated octahedra and edge- and corner-truncated octahedra, their 13 ACS Paragon Plus Environment


XRD patterns show strong (111) and (220) reflection peaks.

Page 14 of 34

For octahedra, the

appearance of strong (111) and (222) peaks is expected because their {111} faces are oriented parallel to the substrate surface. shapes are displayed in Figure 4.

UV–vis absorption spectra of all these particle

All samples show only a single surface plasmon

resonance (SPR) absorption band, indicating their good size and shape homogeneity. The narrow range of the recorded band positions between 550 and 580 nm reflects their similar sizes.

The smallest 69 nm corner-truncated octahedra give a SPR band at 550

nm. The next goal of this work is to clarify the role of CTAC surfactant used in the synthesis of gold nanocrystals.

This is important because if CTA+ actually coordinates

to the metal center to tune its reduction potential, then the reaction equations should be modified to reflect this fact.

The notion of CTA+X– (X– = Cl– or Br–) selectively

adsorbing to some faces of crystals promoting the growth of certain particle shapes is questionable, because gold crystals exposing {100}, {110}, and {111} faces can all be synthesized as demonstrated here and has been observed in another study,29 and supercrystal formation from the assembly of cubes, octahedra, rhombic dodecahedra, and other shapes is all possible using the same surfactant molecules packing between adjacent nanocrystals.11

A series of spectral experiments has been conducted to check the 14 ACS Paragon Plus Environment

Page 15 of 34 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


existence of CTA-HAuCl4 and CTA-HAuCl2 complex formation.

Figure S5 presents

SEM images of the gold nanocubes synthesized under normal 0.1 M CTAC concentration and by replacing 0.1 M CTAC with 0.1 M NaCl concentration.

Nanocubes have

completely disappeared in the absence of CTAC, and only rice-shaped particles were produced.

Since this shape deterioration is not caused by chloride ions, it is considered

to be the CTA+effect.

Next, UV–vis absorption spectra of HAuCl4, CTAC, and several

molar ratios of HAuCl4 to CTAC were taken (see Figure 5a–d and Figure 6a).

CTAC

has no absorption band, while the 1.0 × 10–4 M HAuCl4 solution shows a sharp band at 217 nm and a shoulder band at 290 nm.

A solution with 1:1 molar ratio of HAuCl4 and

CTAC shifts the HAuCl4 absorption bands to 238 and 334 nm with a long absorption tail into the visible light region.

This signifies the formation of CTA-AuCl4 complex.

Increasing the molar ratio of HAuCl4:CTAC to 1:10 barely shifts the absorption bands to 233 and 331 nm.

If 0.5 mL of 2 mM ascorbic acid solution is added to the 10–4 M

HAuCl4 solution in the absence of CTAC, there is no change to the absorption bands of HAuCl4.

Increasing the ascorbic acid volume to 1 mL gives a new band at 568 nm due

to the formation of gold particles, yet the HAuCl4 bands remain unchanged. indicates direct reduction of HAuCl4 to Au in the absence of CTAC.

This result

Notice that the

HAuCl4 peak absorbance has decreased substantially because some gold source has been 15 ACS Paragon Plus Environment


converted to metallic gold. If one starts with the solution having a 1:10 molar ratio of HAuCl4:CTAC, adding 325 L of 0.01 M ascorbic acid generates a new peak at 206 nm in addition to the original CTA-AuCl4 bands, which are slightly shifted to 230 and 325 nm (Figure 6). With 400 L of ascorbic acid introduced, only two bands at 205 and 245 nm are observed with significantly higher absorbance values than the bands recorded adding just 325 L of ascorbic acid.

This spectral feature should result from the complete formation of

CTA-AuCl2 complex, since a reducing agent of sufficient amount has been added to CTA-AuCl4.

There is no visible light absorption, further supporting the formation of

colorless CTA-AuCl2 species.

After adding 500 L of ascorbic acid, the 245 nm band

has dropped significantly in absorbance, and the Au SPR band at 600 nm has emerged. The complex formation kinetics has been probed using the reagent concentrations for making a growth solution for the preparation of Au octahedra in the presence of 50 L of 0.001 M KI solution.

This growth solution condition was chosen because the

intermediate dendritic structures observed in the synthesis of Au octahedra has been reported.19

Figure S6 reveals immediate formation of CTA-AuCl4 upon mixing HAuCl4

and CTAC.

Conversion of CTAC-AuCl4 to CTAC-AuCl2 also happens instantly after

the introduction of ascorbic acid.

If the molar ratio of HAuCl4:CTAC is substantially 16

ACS Paragon Plus Environment

Page 16 of 34

Page 17 of 34 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


reduced to just 1:10, formation of CTA-AuCl4 is very slow, but its conversion to CTA-AuCl2 also occurs instantly. Since the spectral analysis suggests that CTA-AuCl4 and CTA-AuCl2 complexes have been formed, equations (1) and (2) should be modified as shown below. CTA-AuCl4+ AA (C6H8O6) → CTA-AuCl2+ DAA (C6H6O6) + 2H+ + 2Cl‒

(3)

2CTA-AuCl2 + AA (C6H8O6) → 2Au + DAA (C6H6O6) + 2H+ + 4Cl–+ 2CTA+ (4) A new reaction quotient can be obtained combining equations (3) and (4).

Again Q is

mainly determined by the metal precursor and ascorbic acid concentrations. 8

𝑄 =

6 2 [𝐷𝐴𝐴]3[𝐻 + ] [𝐶𝑙 ― ] [𝐶𝑇𝐴 + ]

[𝐶𝑇𝐴 ― 𝐴𝑢𝐶𝑙4]2[𝐴𝐴]3

For simplicity of calculation, Q has been replaced with Q’ by removing the [DAA], [Cl‒], and [CTA+] terms.

The obtained Q’ values for cubes and rhombic dodecahedra

synthesized in the presence of NaBr are available in the Supporting Information, including the cell potential differences in the formation of Au cubes and rhombic dodecahedra.

In addition, we have found that for gold cubes and rhombic dodecahedra

synthesized using two different HAuCl4 and ascorbic acid amounts, their AA/HAuCl4 mole ratio serves as a simple guideline for predicting the particle shape (Table S2).

For

AA/HAuCl4 mole ratio in the range of 2.25–2.4, rhombic dodecahedra are synthesized. 17 ACS Paragon Plus Environment


Page 18 of 34

Lowering AA/HAuCl4 mole ratio to 1.43–1.44, cubes are obtained.

Such simple rules

illustrate how gold particle shape evolution is linked to metal precursor quantity. As a test and an application of the synthetic strategy discovered in this work, we have used our reported method for growing Cu nanocubes and nanowires to achieve possible shape evolution of polyhedral Cu nanocrystals.25

Previously Cu nanocubes

with average edge lengths of 82, 95, and 108 nm were synthesized by heating an aqueous mixture of CTAC, copper acetate (Cu(OAc)2), and sodium ascorbate to 100 ºC for 40 min.

Cu nanowires were also obtained the same way, but much more sodium ascorbate

was needed.

CuCl42‒ should form in the presence of CTAC, although initial CuCl2

formation is also possible.

In the new method, the amount of sodium ascorbate used

was fixed, but the volume of 0.1 M Cu(OAc)2 solution was varied from 50 to 250 and 270 L to form Cu nanocubes with average edge lengths of 45, 51, and 72 nm.

Further

increasing the Cu(OAc)2 volume to 300 L led to the production of nanowires, so similar crystal shape change was observed. the synthesized Cu nanocubes.

Figure S7 gives the size distribution histograms of

These cubes are far smaller than achievable using the

original conditions, making small Cu cubes below 50 nm easily obtainable.

The 45, 51,

and 72 nm Cu cubes show a SPR absorption band centered at 602, 605, and 620 nm, respectively (Figure S8).

These SPR band positions match reasonably well with those 18 ACS Paragon Plus Environment

Page 19 of 34 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


of Au‒Cu cubes of similar sizes.30

CONCLUSIONS Recognizing that synthesis of gold particles with systematic shape evolution is achieved by tuning the reaction cell potential, a parameter in the Nernst equation never explored to change particle shape is the metal precursor concentration.

Using our two reported

series to make gold particles with tunable shapes, and starting with the conditions listed to grow rhombic dodecahedra, cubes, and octahedra, increasing the volume of HAuCl4 solution introduced also yields the same series of crystal shape evolution from rhombic dodecahedra to trisoctahedra and cubes, and from octahedra to corner-truncated octahedra, edge-and corner-truncated octahedra, and rhombic dodecahedra. Furthermore, spectral analysis has confirmed the formation of CTA-AuCl4 and CTA-AuCl2 complexes, so CTAC (or CTAB) added to the reaction mixture does not really act as a capping agent, but as a metal-complexing agent and a source of halide. And both roles actively influence the overall reaction potential.

This work illustrates

that practically everything added to the reaction mixture in the synthesis of metal particles are involved in the redox chemistry.

This fact should be recognized in the

design of reaction conditions and in explaining the synthesis results. 19 ACS Paragon Plus Environment


ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on theACS Publications website at DOI: Exact reagent amounts used for Au particle synthesis, particle size measurements and size distribution histograms, TEM characterization, and additional UV–vis spectra.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Michael H. Huang: 0000-0002-5648-4345 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was funded by Ministry of Science and Technology of Taiwan (MOST 20 ACS Paragon Plus Environment

Page 20 of 34

Page 21 of 34 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


104-2119-M-007-013-MY3 and 105-2633-M-007-003).

REFERENCES (1)

Chiu, C.-Y.; Chung, P.-J.; Lao, K.-U.; Liao, C.-W.; Huang, M. H. Facet-Dependent Catalytic Activity of Gold Nanocubes, Octahedra, and Rhombic Dodecahedra toward 4-Nitroaniline Reduction. J. Phys. Chem. C 2012, 116, 23757‒23763.

(2)

Chanda, K.; Rej, S.; Liu, S.-Y.; Huang, M. H. Facet-Dependent Catalytic Activity of Palladium Nanocrystals in Tsuji–Trost Allylic Amination Reactions with Product Selectivity. ChemCatChem 2015, 7, 1813–1817.

(3)

Rej, S.; Chanda, K.; Chiu, C.-Y. Huang, M. H. Control of Regioselectivity over Gold Nanocrystals of Different Surfaces for the Synthesis of 1,4-Disubstituted Triazoles through the Click Reaction. Chem. Eur. J. 2014, 20, 15991–15997.

(4)

Kim, M.; Kim, Y.; Hong, J. W.; Ahn, S.; Kim, W. Y.; Han, S. W. The Facet-Dependent Enhanced Catalytic Activity of Pd Nanocrystals. Chem. Commun. 2014, 50, 9454–9457.

(5)

Madasu, M.; Hsia, C.-F.; Huang, M. H. Au–Cu Core–Shell Nanocube-Catalyzed Click Reactions for Efficient Synthesis of Diverse Triazoles. Nanoscale 2017, 9, 6970–6974. 21 ACS Paragon Plus Environment


(6)

Henzie, J.; Andrews, S. C.; Ling, X. Y.; Li, Z.; Yang, P. Oriented Assembly of Polyhedral Plasmonic Nanoparticle Clusters. Proc. Natl. Acad. Sci. U.S.A. 2013, 110, 6640–6645.

(7)

Liu, Y.; Zhou, J.; Yuan, X.; Jiang, T.; Petti, L.; Zhou, L.; Mormile, P. Hydrothermal Synthesis of Gold Polyhedral Nanocrystals by Varying Surfactant Concentration and Their LSPR and SERS Properties. RSC Adv. 2015, 5, 68668– 68675.

(8)

Wu, H.-L.; Tsai, H.-R.; Hung, Y.-T.; Lao, K.-U.; Liao, C.-W.; Chung, P.-J.; Huang, J.-S.; Chen, I-C.; Huang, M. H. A Comparative Study of Gold Nanocubes, Octahedra, and Rhombic Dodecahedra as Highly Sensitive SERS Substrates. Inorg. Chem. 2011, 50, 8106–8111.

(9)

Reichhelm, A.; Haubold, D.; Eychmüller, A. Ligand Versatility in Supercrystal Formation. Adv. Funct. Mater. 2017, 27, 1700361.

(10) Lee, Y. H.; Shi, W.; Lee, H. K.; Jiang, R.; Phang, I. Y.; Cui, Y.; Isa, L.; Yang, Y.; Wang, J.; Li, S.; Ling, X. Y. Nanoscale Surface Chemistry Directs the Tunable Assembly of Silver Octahedra into Three Two-Dimensional Plasmonic Superlattices. Nat. Commun. 2015, 6, 6990. (11) Huang, M. H.; Thoka, S. Formation of Supercrystals through Self-Assembly of 22 ACS Paragon Plus Environment

Page 22 of 34

Page 23 of 34 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


Polyhedral Nanocrystals. Nano Today 2015, 10, 81–92. (12) Chiu, C.-Y.; Chen, C.-K.; Chang, C.-W.; Jeng, U-S.; Tan, C.-S.; Yang, C.-W.; Chen, L.-J.; Yen, T.-J. Huang, M. H. Surfactant-Directed Fabrication of Supercrystals from the Assembly of Polyhedral Au–Pd Core–Shell Nanocrystals and Their Electrical and Optical Properties. J. Am. Chem. Soc. 2015, 137, 2265– 2275. (13) Yang, C.-W.; Chiu, C.-Y.; Huang, M. H. Formation of Free-Standing Supercrystals from the Assembly of Polyhedral Gold Nanocrystals by Surfactant Diffusion in the Solution. Chem. Mater. 2014, 26, 4882–4888. (14) Huang, M. H. Chiu, C.-Y. Achieving Polyhedral Nanocrystal Growth with Systematic Shape Control. J. Mater. Chem. A 2013, 1, 8081–8092. (15) Wu, H.-L.; Kuo, C.-H.; Huang, M. H. Seed-Mediated Synthesis of Gold Nanocrystals with Systematic Shape Evolution from Cubic to Trisoctahedral and Rhombic Dodecahedral Structures. Langmuir 2010, 26, 12307–12313. (16) Eguchi, M.; Mitsui, D.; Wu, H.-L.; Sato, R.; Teranishi, T. Simple Reductant Concentration-Dependent Control of Polyhedral Gold Nanoparticles and Their Plasmonic Properties. Langmuir 2012, 28, 9021–9026. (17) Langille, M. R.; Personick, M. L.; Zhang, J.; Mirkin, C. A. Defining Rules for the 23 ACS Paragon Plus Environment


Shape Evolution of Gold Nanoparticles. J. Am. Chem. Soc. 2012, 134, 14542– 14554. (18) Wang, Y.; Xie, S.; Liu, J.; Park, J.; Huang, C. Z.; Xia, Y. Shape-Controlled Synthesis of Palladium Nanocrystals: A Mechanistic Understanding of the Evolution from Octahedrons to Tetrahedrons. Nano Lett. 2013, 13, 2276–2281. (19) Chung, P.-J.; Lyu, L.-M.; Huang, M. H. Seed-Mediated and Iodide-Assisted Synthesis of Gold Nanocrystals with Systematic Shape Evolution from Rhombic Dodecahedral to Octahedral Structures. Chem.–Eur. J. 2011, 17, 9746–9752. (20) Niu, W.; Zhang, L.; Xu, G. Shape-Controlled Synthesis of Single-Crystalline Palladium Nanocrystals. ACS Nano 2010, 4, 1987–1996. (21) Liu, S.-Y.; Shen, Y.-T.; Chiu, C.-Y.; Rej, S.; Lin, P.-H.; Tsao, Y.-C.; Huang, M. H. Direct Synthesis of Palladium Nanocrystals in Aqueous Solution with Systematic Shape Evolution. Langmuir 2015, 31, 6538–6545. (22) Stone, A. L.; King, M. E.; McDarby, S. P.; Robertson, D. D.; Personick, M. L. Synthetic Routes to Shaped AuPt Core–Shell Particles with Smooth Surfaces Based on Design Rules for Au Nanoparticle Growth. Part. Part. Syst. Charact. 2018, 1700401. (23) Zhang, J.; Feng, C.; Deng, Y.; Liu, L.; Wu, Y.; Shen, B.; Zhong, C.; Hu, W. 24 ACS Paragon Plus Environment

Page 24 of 34

Page 25 of 34 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


Shape-Controlled Synthesis of Palladium Single-Crystalline Nanoparticles: The Effect of HCl Oxidative Etching and Facet-Dependent Catalytic Properties. Chem. Mater. 2014, 26, 1213–1218. (24) Lin, Z.-W.; Tsao, Y.-C.; Yang, M.-Y.; Huang, M. H. Seed-Mediated Growth of Silver Nanocubes in Aqueous Solution with Tunable Size and Their Conversion to Au Nanocages with Efficient Photothermal Property. Chem.–Eur. J. 2016, 22, 2326–2332. (25) Thoka, S.; Madasu, M.; Hsia, C.-F.; Liu, S.-Y.; Huang, M. H. Aqueous Phase Synthesis of Size-Tunable Copper Nanocubes for Efficient Aryl Alkyne Hydroboration. Chem. Asian J. 2017, 12, 2318–2322. (26) Chiu, C.-Y.; Yang, M.-Y.; Lin, F.-C.; Huang, J.-S.; Huang, M. H. Facile Synthesis of Au–Pd Core–Shell Nanocrystals with Systematic Shape Evolution and Tunable Size for Plasmonic Property Examination. Nanoscale 2014, 6, 7656–7665. (27) Tsao, Y.-C.; Rej, S.; Chiu, C.-Y.; Huang, M. H. Aqueous Phase Synthesis of Au– Ag Core–Shell Nanocrystals with Tunable Shapes and Their Optical and Catalytic Properties. J. Am. Chem. Soc. 2014, 136, 396–404. (28) Ma, Y.; Kuang, Q.; Jiang, Z.; Xie, Z.; Huang, R.; Zheng, L. Synthesis of Trisoctahedral Gold Nanocrystals with Exposed High-Index Facets by a Facile 25 ACS Paragon Plus Environment


Chemical Method. Angew. Chem. Int. Ed. 2008, 47, 8901–8904. (29) Lin, H.-X.; Lei, Z.-C.; Jiang, Z.-Y.; Hou, C.-P.; Liu, D.-Y.; Xu, M.-M. Tian, Z.-Q.; Xie, Z.-X. Supersaturation-Dependent Surface Structure Evolution: From Ionic, Molecular, to Metallic Micro/Nanocrystals. J. Am. Chem. Soc. 2013, 135, 9311‒9314. (30) Hsia, C.-F.; Madasu, M.; Huang, M. H. Aqueous Phase Synthesis of Au–Cu Core– Shell Nanocubes and Octahedra with Tunable Sizes and Noncentrally Located Cores. Chem. Mater. 2016, 28, 3073‒3079.


Page 26 of 34

Page 27 of 34 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


Growth Solution Formulation

0.1 M

0.01 M

0.01 M

0.04 M

CTAC (g)

HAuCl4(µL)

NaBr (µL)

AA (µL)

H2O (µL)

a. Rhombic dodecahedra

0.32

160

10

90

9715

b. Trisoctahedra

0.32

220

10

90

9655

c. Cubes

0.32

250

10

90

9625

d. Rhombic dodecahedra

0.32

250

10

150

9565

e. Trisoctahedra

0.32

300

10

150

9515

f. Cubes

0.32

420

10

150

9395

Growth Solution Formulation

0.1 M

0.01 M

0.001 M

0.04 M

CTAC (g)

HAuCl4(µL)

KI (µL)

AA (µL)

H2O (µL)

a. Octahedra

0.32

100

20

220

9635

b. Corner-truncated octahedra

0.32

140

20

220

9595

c. Edge- and corner-truncated octahedra (I)

0.32

170

20

220

9565

d. Edge- and corner-truncated octahedra (II)

0.32

210

20

220

9525

e. Rhombic dodecahedra

0.32

250

20

220

9485

f. Octahedra

0.32

250

50

220

9455

g. Corner-truncated octahedra

0.32

300

50

220

9405

h. Edge- and corner-truncated octahedra (I)

0.32

380

50

220

9325

i. Edge- and corner-truncated octahedra (II)

0.32

460

50

220

9245

j. Rhombic dodecahedra

0.32

520

50

220

9185

Scheme 1.

Scheme illustration of the synthetic conditions used for growing gold

nanocrystals with systematic shape evolution.



Figure 1.

SEM images of the synthesized Au (a, d) rhombic dodecahedra, (b, e)

trisoctahedra, and (c, f) cubes by adding (a‒c) 90 L and (d‒f) 150 L of ascorbic acid and increasing the volumes of HAuCl4 solution added.


Page 28 of 34

Page 29 of 34 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


Figure 2.

XRD patterns of the synthesized Au nanocrystals with tunable shapes.

Figure 3.

SEM images of Au (a, f) octahedra, (b, g) corner-truncated octahedra, (c, h)

edge- and corner-truncated octahedra (I), (d, i)edge- and corner-truncated octahedra (II), and (e, j) rhombic dodecahedra synthesized by increasing the volume of HAuCl4 solution introduced and adding(a–e) 20 L and (f–j) 50 L of 0.001 M KI solution. 29 ACS Paragon Plus Environment


Figure 4.

UV–vis absorption spectra of the 94 nm Au nanocubes, 96 nm trisoctahedra,

82 nm rhombic dodecahedra, 90 nm edge- and corner-truncated octahedra (II), 92 nm edge- and corner-truncated octahedra (I), 69 nm corner-truncated octahedra, and 80 nm octahedra. 30 ACS Paragon Plus Environment

Page 30 of 34

Page 31 of 34 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


Figure 5.

UV–vis absorption spectra of (a) 1 × 10–4 M HAuCl4 solution, (b) pure

CTAC solution, (c, d) HAuCl4 and CTAC solution with molar ratios of 1:1 and 1:4, (e, f) 1 × 10–4 M HAuCl4 solution and different volumes of 2 mM ascorbic acid added.



Figure 6.

Page 32 of 34

UV–vis absorption spectra of (a) HAuCl4 and CTAC solution with a molar

ratio of 1:10 and (b–e) different volumes of 0.01 M AA added to this solution. nm peak indicates the formation of Au particles.


The 580

Page 33 of 34 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


Figure 7.

(a) Cu nanowires and Cu nanocubes with average edge lengths of (b) 45, (c)

51, and (d) 72 nm by adjusting the volume of copper acetate solution added.



Rhombic dodecahedra

Trisoctahedra

Page 34 of 34

Cubes

Fixed ascorbic acid amount and increasing HAuCl4 volume

Ocatahedra

Edge- and Corner-truncated corner-truncated ocatahedra (I) ocatahedra

Edge- and corner-truncated ocatahedra (II)

Rhombic dodecahedra

Fixed KI amount and increasing HAuCl4 volume

TOC graphic


Systematic Shape Evolution of Gold Nanocrystals Achieved through

Recommend Documents