Facet-Dependent Optical and Photothermal Properties of Au@Ag

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Facet-Dependent Optical and Photothermal Properties of Au@Ag–Cu2O Core–Shell Nanocrystals Kung-Hsun Yang, Shih-Chen Hsu, and Michael H. Huang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02187 • Publication Date (Web): 04 Jul 2016 Downloaded from http://pubs.acs.org on July 10, 2016

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Chemistry of Materials

Facet-Dependent Optical and Photothermal Properties of Au@Ag–Cu2O Core–Shell Nanocrystals Kung-Hsun Yang, Shih-Chen Hsu, and Michael H. Huang* Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan

ABSTRACT:

This study examines the facet-dependent optical properties of

size-tunable Ag–Cu2O core–shell nanocrystals with 38, 42, and 50 nm cubic Ag cores. The Ag cores were prepared from octahedral Au seeds. single-crystalline.

The Cu2O shells are

In the case of Au@Ag–Cu2O nanocrystals with 42 nm Ag cores,

the Ag surface plasmon resonance (SPR) absorption band at 485 nm has been widely red-shifted to 730, 755, and 775 nm for rhombic dodecahedra, truncated octahedra, and cuboctahedra, respectively, after forming the Cu2O shells.

The Ag SPR band

positions are mostly fixed despite large changes in the shell thickness, showing the presence of facet-dependent optical properties.

Due to the strong Ag SPR band

absorption, all samples exhibit a better photothermal activity than that of Au–Cu2O nanocrystals.

Facet-dependent heat transmission may be present for particles with

Ag SPR band much deviated from the laser wavelength, but this phenomenon is lost for particles with SPR band approaching the excitation wavelength as the particles become highly photothermally efficient to give solution temperatures of 80–95 ºC 1

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within 3 min of laser irradiation.

INTRODUCTION The remarkable ability of cuprous oxide to form single-crystalline shells over various metal particle cores has enabled the observation of facet-dependent optical properties.1–5 The position of localized surface plasmon absorption (SPR) band of the polyhedral Au or Pd nanocrystal cores is fixed despite large changes in the Cu2O shell thickness, but is greatly tunable depending on the shape, or more precisely the exposed facets, of the Cu2O shells.

Due to the large refractive index and dielectric

constant of Cu2O (ε = 7.2), the metal SPR absorption band can be significantly red-shifted by more than 300 nm after forming the Cu2O shell.6–8

Hence, the Au

SPR band in Au–Cu2O nanocrystals can cover the entire near-infrared region if short Au nanorod cores are used.5

Earlier studies did not recognize possible presence of a

fixed SPR band position.9,10

The Cu2O absorption band position was found to be

similarly facet-dependent, and smaller {100}-bound cubes give a more red-shifted absorption band than larger {111}-bound octahedra.

{110}-bound rhombic

dodecahedra show the most blue-shifted band for both the core and shell components. Recently, it has been demonstrated that the observed facet-dependent optical effects originate from Cu2O by examining the absorption band locations of ultrasmall Cu2O 2


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cubes and octahedra with tunable sizes.11

Ag3PO4 sub-microcrystals and possibly

ultrathin Si nanowires also exhibit the same optical phenomenon.12,13

If

single-crystalline Cu2O shells are not formed, or the metal cores are too big beyond 70 nm, the facet-dependent optical effects may not be observed.7,8,14–18

In this case,

the plasmonic band red-shifts continuously with increasing shell thickness, but the extent of spectral shift diminishes.

Because the facet-dependent optical effect cannot

be observed in particles without well-defined shapes, and thus not recorded in many studies, there is a sense that the scientific community is still not fully convinced of its existence.

More demonstrations seem necessary.

Since silver particles absorb at

around 400 nm, small Ag nanocrystals (40 nm or less) may be used to fabricate Ag–Cu2O heterostructures, thereby keeping the Ag plasmonic band within the visible light region.14

However, small Ag nanocubes and octahedra are difficult to

synthesize, so we employed ultrasmall Au–Ag cubes as cores to produce Au@Ag–Cu2O nanocrystals for optical property examination.19,20

Another

interesting aspect of these particles is their potential to heat up a solution to very high temperatures upon laser excitation because of the plasmonic cores.5,19–22 Previously we have proposed that even heat transmission is facet-dependent because the thin surface layer responsible for the facet effects can have different refractive indices and therefore different thermal conductivity efficiencies.5,23

It is necessary to evaluate

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possible facet-dependent photothermal properties in Ag–Cu2O nanocrystals. Here we have synthesized Au@Ag–Cu2O cuboctahedra and truncated octahedra with tunable sizes from 38 and 50 nm cubic Ag cores.

The cubic Ag cores were

formed by growing a shell of Ag over an octahedral Au core.

Size-tunable

cuboctahedra, truncated octahedra, and rhombic dodecahedra were also prepared from 42 nm Ag cores.

Their structures have been carefully characterized.

Facet-dependent optical properties are clearly observable in these nanocrystals. Ag cores produce much larger spectral red-shifts than the Au cores do.

The

They also

show a stronger SPR absorption band compared to that seen in Au–Cu2O nanocrystals, and this gives the Au@Ag–Cu2O particles exceptional photothermal efficiency. Facet-dependent heat transmission effects can also be observed in nanocrystals with their Ag SPR band much deviated from the laser wavelength.

EXPERIMENTAL SECTION Chemicals.

Cetyltrimethylammonium bromide (CTAB, 98%, Alfa Aesar),

cetyltrimethylammonium chloride (CTAC, 95%, TCI), hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O, 99%, Aldrich), trisodium citrate (Na3C6H5O7·H2O, 99.9%, Avantor), silver nitrate (AgNO3, 99.8%, Showa), ascorbic acid (AA, 99.7%, Sigma-Aldrich), sodium dodecyl sulfate (SDS, 99.6%, J.T. Baker), anhydrous 4


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copper(II) chloride (CuCl2, 97%, Sigma-Aldrich), sodium hydroxide (NaOH, 99%, Sigma-Aldrich), and hydroxylamine hydrochloride (NH2OH·HCl, 99%, Sigma-Aldrich) were used as received without further purification.

Ultrapure

deionized water (18.2 MΩ) was used for all solution preparations. Synthesis of Au@Ag–Cu2O Core–Shell Nanocrystals. was dissolved in deionized water.

First, 0.087 g of SDS

Next, 0.1 M CuCl2 solution and different volumes

of pre-synthesized Au@Ag nanocube solution (0.075–0.175 mL for cuboctahedra and 0.1–0.6 mL for truncated octahedra) were added. solution was introduced.

Subsequently, 1.0 M NaOH

After 20 s, NH2OH·HCl solution was added and the

solution was stirred for 20 s.

Please see Tables S1 in the Supporting Information for

the exact reagent amounts used for the growth of Au@Ag nanocubes with tunable sizes.

Tables S2 to S4 give exact reagent amounts used for the formation of

Au@Ag–Cu2O cuboctahedra, truncated octahedra, and rhombic dodecahedra from 38, 42, and 50 nm Au@Ag nanocubes.

By gradually reducing the amount of the metal

cores used, the Au@Ag–Cu2O nanocrystal size can be increased progressively. After 2 h, the particles were centrifuged at 5000 rpm for 3 min and washed with 1:1 volume ratio of deionized water and ethanol two times using the same centrifugation condition.

The particles were dispersed in absolute ethanol for characterization.

Photothermal Measurements.

To evaluate photothermal efficiency, the 5



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Au@Ag–Cu2O nanocrystals were centrifuged in water and redispersed in deionized water.

The volume of water was adjusted so that the final particle concentration is

same for all samples.

Particle number is determined by the amount of Au@Ag

nanocubes used in the synthesis of Au@Ag–Cu2O nanocrystals.

For instance, if 0.2

mL Au@Ag core solution was used, the resulting Au@Ag–Cu2O particles would be dispersed in 2 mL of water. water was added. tube.

If 0.1 mL of Au@Ag core solution was used, 1 mL of

0.1 mL of the particle solution was placed in a sealed Eppendorf

An 808 nm laser with a power of 1.4 W was used to irradiate the particles.

An infrared imaging camera was used to monitor the solution temperature rise under laser illumination.

The laser and infrared camera were perpendicularly placed 15 cm

away from the Eppendorf tube. Instrumentation.

Scanning electron microscopy (SEM) images of the samples

were obtained using a JEOL JSM-7000F electron microscope.

Transmission

electron microscopy (TEM) images were acquired using a JEOL JEM-2100 electron microscope operating at 200 kV.

Powder X-ray diffraction (PXRD) patterns were

collected with the use of a Shimadzu XRD-6000 diffractometer with Cu Kα radiation. UV–vis spectra were recorded using a JASCO V-670 spectrometer.

NEC

TH3102MR and Avio G100EX/G120EX infrared imaging cameras were used to detect temperature changes. 6


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RESULTS AND DISCUSSION Synthesis of Au@Ag‒Cu2O Core–Shell Nanocrystals and Their Structural Characterization.

The ultrasmall Au–Ag core–shell nanocubes with octahedral gold

cores were prepared following our reported procedure.19,24 By tuning the amount of Au particles added in the growth of Ag shell, Au–Ag core–shell nanocubes with average edge lengths of 38, 42, and 50 nm have been synthesized.

These bimetallic

particles are sufficiently small for observation of their facet-dependent optical properties after forming the Cu2O shells.

Figure 1 shows SEM images of the

octahedral gold cores with an average opposite corner distance of 35 nm and Au–Ag nanocubes with sizes of 38, 42, and 50 nm (see Table S5 for the particle size distribution).

The Au–Ag nanocubes are highly uniform in size and shape that they

spontaneously self-assemble into an ordered packing arrangement. Au cores give a SPR absorption band at 543 nm.

The octahedral

The 38, 42, and 50 nm Au–Ag

nanocubes shift the major SPR band to 473, 488, and 509 nm, respectively.

A large

blue-shift of 70 nm has been measured after forming 38 nm Au–Ag nanocubes.

A

standard procedure was adopted to fabricate Au@Ag–Cu2O nanocrystals.

Size

control was achieved by adjusting the volume of Au–Ag nanocubes used.

Shape

evolution was possible by mainly varying the volumes of CuCl2, NaOH, and 7



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NH2OH·HCl solutions introduced.

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Since several variables are needed to tune the

Cu2O shell morphology, the development of synthetic conditions is not trivial.

The

great difficulty to obtain perfect Au@Ag–Cu2O cubes prevents them from being used for the facet-dependent optical property characterization. Figure 2 presents SEM images of Au@Ag–Cu2O core–shell cuboctahedra with average sizes of 121, 137, 143, 156, and 174 nm synthesized from 38 nm Au–Ag nanocube cores.

SEM images of Au@Ag–Cu2O truncated octahedra with average

sizes of 125, 135, 150, 163, and 190 nm are also shown.

The nanocrystals are

sufficiently uniform in size and shape, which is necessary for facet-dependent optical property examination. Table S6 provides standard deviations for the Au@Ag–Cu2O nanocrystals synthesized from 38 nm cubic Au–Ag cores.

The truncated octahedra

were prepared using twice the volume of 0.2 M NH2OH·HCl solution than that needed to make cuboctahedra, so some particles show slightly etched faces due to a higher concentration of HCl in the solution.

For a more complete demonstration of

facet effects on optical and photothermal properties, Au@Ag–Cu2O cuboctahedra, truncated octahedra, and rhombic dodecahedra with tunable sizes have been synthesized from 42 nm cubic Au–Ag cores (see Figure S1).

Some rhombic

dodecahedra also show etched faces due to greater amounts of CuCl2 and NH2OH·HCl solutions added to lower the solution pH.25

Rhombic dodecahedra

8


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have only been obtained using 42 nm Au–Ag cores.

Similarly, Au@Ag–Cu2O

cuboctahedra and truncated octahedra with tunable sizes were also prepared from 50 nm cubic Au–Ag cores (see Figure S2).

Tables S7 and S8 give standard deviations

of the Au@Ag–Cu2O nanocrystals synthesized from 42 and 50 nm Au–Ag cores. Similar to the color changes observed in Au–Cu2O nanocrystals, the solution color goes from brown for 257 nm Au@Ag–Cu2O truncated octahedra to light green for 192 nm truncated octahedra and then dark green for 159 nm truncated octahedra (see Figure S3).1 Structural characterization of these nanocrystals has been performed.

Figure S4

displays XRD patterns of Au@Ag–Cu2O cuboctahedra, truncated octahedra, and rhombic dodecahedra with 42 nm Au–Ag cores, showing reflection peaks from both Au/Ag and Cu2O.

The metal peaks are lower in intensity because they are located

inside the particles. is shown in Figure 3.

TEM characterization on a single Au@Ag–Cu2O cuboctahedron The cubic core is located in the center of the cuboctahedron

and has the same orientation as that of Cu2O shell.

Selected-area electron diffraction

(SAED) pattern indicates single-crystalline Cu2O shell.

High-resolution TEM image

gives parallel Ag and Cu2O (200) lattice planes, confirming exact lattice orientation relationship between the core and shell components.

Energy-dispersive X-ray

spectroscopy (EDS) line scan shows the core is made of interior Au and external Ag. 9



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TEM analysis on Au@Ag–Cu2O truncated octahedra is available in Figure S5. High-angle annular dark-field scanning TEM (HAADF-STEM) image shows a cubic core located in the center of an octahedron. planes have been observed.

Parallel Ag and Cu2O (111) lattice

The TEM image for EDS line scan reveals identifiable

octahedral Au core and cubic Ag shell in the particle center.

Clearly no crystal

defects have been found. Optical Property Characterization.

Figure 4 presents UV–vis absorption

spectra of Au@Ag–Cu2O truncated octahedra and cuboctahedra having 38 nm Au–Ag cores.

The particles were dispersed in ethanol for spectral measurements. When

the particles are around 250 nm or less, strong light scattering interference can be avoided to allow clear observation of the Ag SPR absorption band.

The Cu2O

absorption band is progressively red-shifted with increasing particle size as expected. Interestingly, cuboctahedra are consistently more red-shifted than truncated octahedra of comparable size by 10–12 nm (for example, 480 nm band for 121 nm cuboctahedra vs. 470 nm band for 125 nm truncated octahedra).

This is understandable because

cuboctahedra expose a greater fraction of {100} faces to {111} faces than that for truncated octahedra, and the {100} faces of Cu2O produce the most red-shifted absorption band.1–5

The Ag SPR band is exceptionally strong, compared to that seen

in Au–Cu2O nanocrystals.1,2 This feature is quite important for improved 10


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photothermal efficiency and possibly useful in plasmon-enhanced catalysis.

The Ag

SPR absorption band is greatly shifted to 707 nm after forming the Cu2O shell and fixed in position for truncated octahedra with sizes varying from 125 to 163 nm. cuboctahedra, the Ag SPR band is largely fixed at 715 nm.

For

This is a huge spectral

red shift of 234–242 nm from the SPR band of 38 nm Au–Ag cubes at 473 nm.

This

magnitude of SPR band shift is much larger than that seen in Au–Cu2O truncated octahedra and cubes with 47 nm Au cores producing a SPR band red-shift of 165–195 nm, showing that Ag is more effective than Au in producing an ultra-large magnitude of spectral shift.1

Thus, the use of Ag cores does not shift the SPR band to the

visible light region, unless even smaller cores are employed.

However, this outcome

cannot be predicted without synthesizing these nanocrystals.

Again the SPR band

from the Ag core is more red-shifted for cuboctahedra exposing a great fraction of {100} faces, so the same facet-dependent optical properties are observed regardless of which plasmonic metal is used as the core. Figure 5 displays UV–vis absorption spectra of Au@Ag–Cu2O rhombic dodecahedra, cuboctahedra, and truncated octahedra with tunable sizes having 42 nm Au–Ag cubic cores.

Interestingly the Cu2O absorption band position in

Au@Ag–Cu2O rhombic dodecahedra is more red-shifted than that of cuboctahedra. Previously Au–Cu2O rhombic dodecahedra showed the most blue-shifted Cu2O band.2 11



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The broader bandwidth and larger band shift with increasing particle dimension should be related to a wider particle size distribution.

The Cu2O rhombic

dodecahedra bound by the {110} faces may indeed give a more red-shifted absorption band compared to cubes and octahedra of similar sizes.

Significant etching in

Au–Cu2O rhombic dodecahedra may give rise to a more blue-shifted Cu2O absorption band.

The Ag SPR absorption band is located at 730 nm for rhombic dodecahedra,

and largely fixed at 755 nm for truncated octahedra and 775 nm for cuboctahedra. Consistent with previous observation made on the Au–Cu2O rhombic dodecahedra, the SPR band from the Ag core is most blue-shifted in Au@Ag–Cu2O rhombic dodecahedra.

The SPR band positions differ by as much as 45 nm between rhombic

dodecahedra and cuboctahedra.

If Au@Ag–Cu2O cubes can be successfully

synthesized, the band separation is expected to be over 55 nm.

This estimation is

based on the observation made for Au–Cu2O cuboctahedra and cubes with a SPR band separation of 10 nm.1

The gold SPR band positions differ by 47 nm from

rhombic dodecahedra to cubes.

Thus, silver cores can produce an even larger degree

of SPR band shift than gold cores can, showing the power of facet effects on SPR band tunability.

Comparing to the SPR band location of the cubic Au–Ag cores at

485 nm, this magnitude of spectral shift at 245–290 nm is significantly larger than that seen for 38 nm cubic Au–Ag cores, showing that metal cores with a more 12


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red-shifted SPR band position can produce a larger spectral shift after forming the Cu2O shell.5 Figure 6 offers UV–vis absorption spectra of Au@Ag–Cu2O truncated octahedra and cuboctahedra with tunable sizes and 50 nm cubic Au–Ag cores.

The Cu2O

absorption band for larger 159 nm truncated octahedra at 480 nm, as compared to that of smaller 125 nm cuboctahedra at 502 nm again shows the facet-dependent optical properties of Cu2O shells.

Larger particles also display the same optical effect.

Two Cu2O absorption bands appear for the truncated octahedra presumably due to their larger particle sizes.

The Ag SPR band red-shifts from 509 nm to roughly 770

nm for truncated octahedra and 790 nm for cuboctahedra. position is also fixed for most samples. nm after forming the Cu2O shell.

The plasmonic band

This is a very large spectral shift of 260–280

Again the Ag SPR band difference between

truncated octahedra and cuboctahedra is 20 nm. Photothermal Effect.

Photothermal efficiency of the various Au@Ag–Cu2O

nanocrystals was examined by irradiating a solution in an Eppendorf tube containing the same concentration of particles with an 808 nm laser.

Previously Au–Cu2O

nanocubes were found to display superior photothermal activity than truncated octahedra, and facet-dependent heat transmission was used to explain the results.5 The idea is that the thin surface layer responsible for the observation of various facet 13



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effects can have dissimilar refractive indices due to their different band structures and degrees of band bending, so thermal conductivity efficiencies within this ultrathin surface layer can be different.23

Figure 7 gives plots of solution temperature changes

as a function of laser irradiation time on aqueous solutions of Au@Ag–Cu2O nanocrystals with 38 nm, 42 nm, and 50 nm metal cores.

UV–vis spectra of the

nanocrystals are also shown to assist analysis of the photothermal performance of these nanocrystals.

For Au@Ag–Cu2O cuboctahedra and truncated octahedra with

38 nm metal cores, the Ag plasmonic band at 707–715 nm is far from the excitation wavelength.

However, the nanocrystals still exhibit excellent photothermal activity

with temperatures rising to 80–87 ºC for cuboctahedra and 55–63 ºC for truncated octahedra in 3 min.

This exceptionally good photothermal activity should be related

to the strong SPR band from the Ag cores to absorb light more efficiently, showing the great promise of using these Ag-based nanocrystals for light energy conversion. Previously Au–Cu2O cubes with the best photothermal performance only reached 61 ºC after 3 min of 808 nm laser irradiation and 65 ºC after 5 min of laser irradiation, despite an excellent match of the Au SPR band to the laser wavelength.5 Since the irradiated solution should contain approximately the same number of particles, cuboctahedra with a greater fraction of {100} faces appear to be much more efficient at converting light to heat.

Facet-dependent heat transmission effect is also observed 14


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and the result agrees with that found for Au–Cu2O nanocrystals. water gave no temperature change.

Laser irradiation of

To evaluate the stability of these particles, five

cycles of photothermal experiments on cuboctahedra samples have been performed with an interval of 30 min between each run (see Figure S6). performance can still be obtained after 5 cycles.

Good photothermal

The decrease in the highest

temperature reached after the second cycle may be related to some particles sticking to the vial wall by the generated steam pushing the particles upward. For Au@Ag–Cu2O nanocrystals with 42 nm metal cores, the rhombic dodecahedra produce the least temperature increase to just 55 ºC.

This is

understandable considering their Ag SPR band at 730 nm, which is appreciably farther away from the excitation wavelength than that for other samples.

Thus

plasmonic band position with respect to the excitation wavelength is the most important factor in predicting photothermal efficiency.

In addition, particles with a

thicker shell seem to perform better in generating higher solution temperatures. Since heat transmission through the Cu2O shell involves lattice vibrations or phonons, thicker shells may support more lattice vibration modes to facilitate heat transfer. The same trend has been observed in Au–Cu2O nanocrystals.5 The 196 nm cuboctahedra clearly outperform the 216 nm truncated octahedra in the time needed to reach 90 ºC, but this may be linked to their SPR band being much closer to the 15



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excitation wavelength.

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For nanocrystals with thinner shells, there is no difference in

photothermal efficiency for the two particle shapes, suggesting that facet effect is not so important for the strong Ag plasmonic band with its position sufficiently close to the excitation wavelength.

For Au@Ag–Cu2O nanocrystals with 50 nm metal cores,

all samples can reach beyond 80 ºC, showing greatly enhanced photothermal efficiency with their plasmonic band matching with the excitation wavelength. particular, larger cuboctahedra and truncated octahedra can reach 93–95 ºC.

In

A large

amount of steam has been generated, and the steam may be converted into mechanical energy.

Again facet effect is not important when the SPR band is close to the

excitation wavelength, because efficient light absorption dominates the photothermal activity.

To summarize, Ag cores are better than Au cores in producing a stronger

photothermal effect.

Next, a good match of the plasmonic band position with the

excitation wavelength yields the best photothermal efficiency. Cu2O shells generally show better photothermal activity.

Particles with thicker

Finally, facet-dependent

heat transmission effect is best observable when the SPR band sufficiently deviates from the excitation wavelength.

CONCLUSIONS To investigate the facet-dependent optical properties of Ag–Cu2O core–shell 16


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nanocrystals, Cu2O cuboctahedra, truncated octahedra, and rhombic dodecahedra with tunable sizes have been synthesized from Ag nanocubes.

The cubic Ag cores with

sizes of 38, 42, and 50 nm were prepared by growing Ag over octahedral Au nanocrystals.

Facet-dependent optical properties have been observed from both the

Cu2O shells and the Ag cores.

The Ag SPR absorption band remains fixed despite

large changes in the shell thickness, but it is tunable depending on the shape, or more exactly the exposed facets, of the Cu2O shells.

Interestingly, the Ag SPR band

exhibits a greater extent of spectral red-shift than that seen in Au–Cu2O nanocrystals to move the band position to the near-infrared region.

The Ag SPR band is much

stronger than that of Au cores, a feature that is beneficial for enhanced photothermal activity.

Regardless of the Ag SPR band locations, the samples generally showed

better photothermal activity than that of Au–Cu2O nanocrystals.

Facet-dependent

heat transmission effect appears to be present for nanocrystals with Ag SPR band much deviated from the laser wavelength at 808 nm.

As the Ag SPR band

approaches that of the excitation wavelength, the facet effect is lost because all samples give excellent photothermal efficiency with temperatures reaching 80–95 ºC within 3 min of laser irradiation.

These nanocrystals may be used to turn light to

other forms of energy.

17



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ASSOCIATED CONTENT Supporting Information Exact reaction conditions used to synthesize Au@Ag nanocubes and Au@Ag–Cu2O nanocrystals, SEM images and standard deviations of the nanocrystals, photograph of nanocrystal solutions, XRD patterns, TEM analysis on Au@Ag–Cu2O truncated octahedra, and results of repeated photothermal experiments.

This material is

available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding author *E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank the Ministry of Science and Technology of Taiwan for support of this research (MOST 104-2119-M-007-013-MY3 and MOST 104-2633-M-007-001).

REFERENCES 18


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(1) Yang, Y.-C.; Wang, H.-J.; Lin, F.-C.; Huang, J.-S.; Huang, M. H. Facet-Dependent Optical Properties of Polyhedral Au–Cu2O Core–Shell Nanocrystals. Nanoscale 2014, 6, 4316‒4324. (2) Hsu, S.-C.; Liu, S.-Y.; Wang, H.-J.; Huang, M. H. Facet-Dependent Surface Plasmon Resonance Properties of Au–Cu2O Core–Shell Nanocubes, Octahedra, and Rhombic Dodecahedra. Small 2015, 11, 195‒201. (3) Huang, M. H.; Rej, S.; Chiu, C.-Y. Facet-Dependent Optical Properties Revealed through Investigation of Polyhedral Au–Cu2O and Bimetallic Core–Shell Nanocrystals. Small 2015, 11, 2716‒2726. (4) Rej, S.; Wang, H.-J.; Huang, M.-X.; Hsu, S.-C.; Tan, C.-S.; Lin, F.-C.; Huang, J.-S.; Huang, M. H. Facet-Dependent Optical Properties of Pd–Cu2O Core–Shell Nanocubes and Octahedra. Nanoscale 2015, 7, 11135‒11141. (5) Wang, H.-J.; Yang, K.-H.; Hsu, S.-C.; Huang, M. H. Photothermal Effects from Au–Cu2O Core–Shell Nanocubes, Octahedra, and Nanobars with Broad Near-Infrared Absorption Tunability. Nanoscale 2016, 8, 965–972. (6) R. Bradhan, N. K. Grady, T. Ali and N. J. Halas, Metallic Nanoshells with Semiconductor Cores: Optical Characteristics Modified by Core Medium Properties. ACS Nano, 2010, 4, 6169–6179. (7) Zhang, L.; Jing, H.; Boisvert, G.; He, J. Z.; Wang, H. Geometry Control and 19



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Optical Tunability of Metal–Cuprous Oxide Core–Shell Nanoparticles. ACS Nano 2012, 6, 3514‒3527. (8) Zhang, L.; Blom, D. A.; Wang, H. Au–Cu2O Core–Shell Nanoparticles: A Hybrid Metal-Semiconductor Heterostructure with Geometrically Tunable Optical Properties. Chem. Mater. 2011, 23, 4587‒4598. (9) Meir, N.; Jen-La Plante, I.; Flomin, K.; Chockler, E.; Moshofsky, B.; Diab, M.; Volokh, M.; Mokari, T. Studying the Chemical, Optical and Catalytic Properties of Noble Metal (Pt, Pd, Ag, Au)–Cu2O Core–Shell Nanostructures Grown via a General Approach. J. Mater. Chem. A 2013, 1, 1763‒1769. (10) Kong, L.; Chen, W.; Ma, D.; Yang, Y.; Liu, S.; Huang, S. Size Control of Au@Cu2O Octahedra for Excellent Photocatalytic Performance. J. Mater. Chem. 2012, 22, 719–724. (11) Ke, W.-H.; Hsia, C.-F.; Chen, Y.-J.; Huang, M. H. Synthesis of Ultrasmall Cu2O Nanocubes and Octhaedra with Tunable Sizes for Facet-Dependent Optical Property Examination. Small 2016, DOI:10.1002/smll.201600064. (12) Bi, Y.; Ouyang, S.; Umezawa, N.; Cao, J.; Ye, J. Facet Effect of Single-Crystalline Ag3PO4 Sub-microcrystals on Photocatalytic Properties. J. Am. Chem. Soc. 2011, 133, 6490–6492. (13) Holmes. J. D.; Johnston, K. P.; Doty, R. C.; Korgel, B. A. Control of Thickness 20


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and Orientation of Solution-Grown Silicon Nanowires. Science 2000, 287, 1471–1473. (14) Jing, H.; Large, N.; Zhang, Q.; Wang, H. Epitaxial Growth of Cu2O on Ag Allows for Fine Control Over Particle Geometries and Optical Properties of Ag–Cu2O Core–Shell Nanoparticles. J. Phys. Chem. C 2014, 118, 19948‒19963. (15) Liu, D.-Y.; Ding, S.-Y.; Lin, H.-X.; Liu, B.-J.; Ye, Z.-Z.; Fan, F.-R.; Ren, B.; Tian, Z.-Q. Distinctive Enhanced and Tunable Plasmon Resonant Absorption from Controllable Au@Cu2O Nanoparticles: Experimental and Theoretical Modeling. J. Phys. Chem. C 2012, 116, 4477‒4483. (16) Li, J.; Cushing, S. K.; Bright, J.; Meng, F.; Senty, T. R.; Zheng, P.; Bristow, A. D.; Wu, N. Ag@Cu2O Core–Shell Nanoparticles as Visible-Light Plasmonic Photocatalysts. ACS Catal. 2013, 3, 47‒51. (17) Lu, B.; Liu, A.; Wu, H.; Shen, Q.; Zhao, T.; Wang, J. Hollow Au–Cu2O Core–Shell Nanoparticles with Geometry-Dependent Optical Properties as Efficient Plasmonic Photocatalysts under Visible Light. Langmuir 2016, 32, 3085–3094. (18) Shi, X.; Ji, Y.; Hou, S.; Liu, W.; Zhang, H.; Wen, T.; Yan, J.; Song, M.; Hu, Z.; Wu, X. Plasmon Enhancement Effect in Gold Nanorods@Cu2O Core–Shell 21



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Nanostructures and Their Use in Probing Defect States. Langmuir 2015, 31, 1537‒1546. (19) Chiang, C.; Huang, M. H. Synthesis of Small Au–Ag Core–Shell Cubes, Cuboctahedra, and Octahedra with Size Tunability and Their Optical and Photothermal Properties. Small 2015, 11, 6018–6025. (20) 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. (21) Chen, H.; Shao, L.; Ming, T.; Sun, Z.; Zhao, C.; Yang, B.; Wang, J. Understanding the Photothermal Conversion Efficiency of Gold Nanocrystals. Small 2010, 6, 2272–2280. (22) Neumann, O.; Neumann, A. D.; Silva, E.; Ayala-Orozco, C.; Tian, S.; Nordlander, P.; Halas, N. J. Nanoparticle-Mediated, Light-Induced Phase Separations. Nano Lett. 2015, 15, 7880–7885. (23) Tan, C.-S.; Hsu, S.-C.; Ke, W.-H.; Chen, L.-J.; Huang, M. H. Facet-Dependent Electrical Conductivity Properties of Cu2O Crystals. Nano Lett. 2015, 15, 2155–2160. (24) Chang, C.-C.; Wu, H.-L.; Kuo, C.-H.; Huang, M. H. Hydrothermal Synthesis of 22


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Monodispersed Octahedral Gold Nanocrystals with Five Different Size Ranges and Their Self-Assembled Structures. Chem. Mater. 2008, 20, 7570–7574. (25) Huang, W.-C.; Lyu, L.-M.; Yang, Y.-C.; Huang, M. H. Synthesis of Cu2O Nanocrystals from Cubic to Rhombic Dodecahedral Structures and Their Comparative Photocatalytic Activity. J. Am. Chem. Soc. 2012, 134, 1261–1267.

23



a

Au 35 nm

c

Au@Ag 42 nm

b

Au@Ag 38 nm

d

Au@Ag 50 nm

e

473 485 509

543

35 nm 38 nm 42 nm 50 nm

Extinction (a. u.)

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

Page 24 of 29

250 300 350 400 450 500 550 600 650 700 750 800 Wavelength (nm)

Figure 1.

(a–d) SEM images of 35 nm Au octahedra and Au@Ag core–shell

nanocubes with edge lengths of 38, 42, and 50 nm. these particles.

(e) UV–vis absorption spectra of

All scale bars are equal to 100 nm.

a

b

c

d

e

f

g

h

i

j

Figure 2.

(a–e) SEM images of Au@Ag–Cu2O core–shell cuboctahedra with 24


Page 25 of 29

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


average sizes of (a) 121, (b) 137, (c) 143, 156, and (e) 174 nm.

(f–j) SEM images of

Au@Ag–Cu2O truncated octahedra with average sizes of (f) 125, (g) 135, (h) 150, (i) 163, and (j) 190 nm. equal to 100 nm.

38 nm Au@Ag cubic cores were used.

All scale bars are

Drawings of single Au@Ag–Cu2O cuboctahedron, truncated

octahedron, and rhombic dodecahedron are also provided with arrows indicating the measured particle sizes.

a

b

Zone axis [

c

]

d Cu2O (200) 2.13 Å ― Cu ―O ― Ag ― Au

Ag (200) 2.03 Å Figure 3.

(a) TEM image of an Au@Ag–Cu2O cuboctahedron with 38 nm Au–Ag 25



core.

(b) SAED pattern viewed along the [100] direction.

Drawing of a

Au@Ag–Cu2O cuboctahedron with an orientation matching the SAED pattern is provided.

(c) High-resolution TEM image of the red square region in panel a.

EDS line scan of a Au@Ag‒Cu2O cuboctahedron.

(d)

All scale bars are equal to 100

nm.

a

707 470

Extinction (a. u.)

478 485

125 nm 135 nm 150 nm 163 nm 190 nm

494 547 480

715

300 400 500 600 700 800 900 1000 1100 1200 1300

Wavelength (nm)

b

480

707

487 715

Extinction (a. u.)

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

Page 26 of 29

496

121 nm 137 nm 143 nm 156 nm 174 nm

506 527 720

300 400 500 600 700 800 900 1000 1100 1200 1300

Wavelength (nm)

Figure 4.

UV–vis absorption spectra of Au@Ag–Cu2O (a) truncated octahedra and

(b) cuboctahedra with tunable sizes synthesized from 38 nm Au@Ag nanocubes.

26


Page 27 of 29

a

144 nm 154 nm 163 nm

500 730

144 nm 154 nm 166 nm 178 nm 199 nm 216 nm

483

b

743 491 747

528

Extinction (a. u.)

500 755 507

542

c

490 761 502 775 503

142 nm 152 nm 166 nm 172 nm 196 nm

521 514 502 493

300

400

500

600

700

800

Figure 5.

575

565

783

900 1000 1100 1200 1300 300 400 500 600 700 800 900 1000 1100 1200 1300 300 400 500 600 700 800 900 1000 1100 1200 1300

Wavelength (nm)

Wavelength (nm)

Wavelength (nm)

UV–vis absorption spectra of Au@Ag–Cu2O (a) rhombic dodecahedra, (b)

truncated octahedra, and (c) cuboctahedra with tunable sizes synthesized from 42 nm Au@Ag nanocubes.

a

480

760

485

770 565

Extinction (a. u.)

489 570

159 nm 178 nm 192 nm 221 nm 257 nm

495 576 521 784

300 400 500 600 700 800 900 1000 1100 1200 1300

Wavelength (nm)

b

502

783

508

Extinction (a. u.)

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


790

518

125 nm 137 nm 153 nm 165 nm 184 nm 202 nm

532 567 485 498

801 590 808

300 400 500 600 700 800 900 1000 1100 1200 1300

Wavelength (nm)

Figure 6.

UV–vis absorption spectra of Au@Ag–Cu2O (a) truncated octahedra and

(b) cuboctahedra with tunable sizes synthesized from 50 nm Au@Ag nanocubes. 27



100

CO-38-143 CO-38-121 TO-38-150 TO-38-125 Water

90

o

Temperature ( C)

80

715

b

70 60 50 40

CO-38-143 CO-38-121 TO-38-150 TO-38-125

707

Extinction (a. u.)

a

707 707

30 20 0

15

30

45

60

75

90 105 120 135 150 165 180

400

500

600

Time (sec)

700

800

900

1000

1100

Wavelength (nm)

100

c

d

90

60

CO-42-196

50

TO-42-216 CO-42-142 TO-42-144 RD-42-154 Water

40 30

755 761

Extinction (a. u.)

o

Temperature ( C)

70

CO-42-196 TO-42-216 CO-42-142 TO-42-144 RD-42-154

783

80

743 730

20 0

15

30

45

60

e

75

90 105 120 135 150 165 180

400

500

600

Time (sec)

100

700

f

90

60

TO-50-257 CO-50-202 TO-50-192 CO-50-153 Water

40 30

900

1000

1100

TO-50-257 CO-50-202 TO-50-192 CO-50-153

784 808

Extinction (a. u.)

o

70

50

800

Wavelength (nm)

80

Temperature ( C)

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

Page 28 of 29

770 790

20 0

15

30

45

60

75

90 105 120 135 150 165 180

400

500

600

Time (sec)

Figure 7.

700

800

900

1000

1100

Wavelength (nm)

(a, c, e) Plots of solution temperature changes as a function of laser

irradiation time on aqueous solutions of Au@Ag–Cu2O nanocrystals with (a) 38 nm, (c) 42 nm, and (e) 50 nm metal cores. nanocrystals.

(d, e, f) Corresponding UV–vis spectra of the

CO refers to cuboctahedra, TO to truncated octahedra, and RD to

rhombic dodecahedra.

Core and shell sizes are indicated.

laser wavelength at 808 nm. 28


The orange line gives the

Page 29 of 29

144nm 154nm 163nm

500 730

144nm 154nm 166nm 178nm 199nm 216nm

483 743 491 747

528

Extinction (a. u.)

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


500 755 507

542

490 761 502 775 503

142nm 152nm 166nm 172nm 196nm

521 514 502 493

400

500

600

700

800

900

1000 400

500

575

565

600

783

700

800

900

1000 400

500

600

Wavelength (nm)

TOC Graphic

29


700

800

900

1000

Facet-Dependent Optical and Photothermal Properties of Au@Ag

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