Freestanding palladium nanosheets with plasmonic and ... · Freestanding palladium nanosheets with...

SUPPLEMENTARY INFORMATIONdoi: 10.1038/nnano.2010.235

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Supplementary information for

Freestanding palladium nanosheets with plasmonic and catalytic properties

Xiaoqing Huang, Shaoheng Tang, Xiaoliang Mu, Yan Dai, Guangxu Chen, Zhiyou

Zhou, Fangxiong Ruan, Zhilin Yang, & Nanfeng Zheng*

* Email: [email protected]

Experimental Details

Reagents: Pd(acac)2 (99%) was purchased from Alfa Aesar, PVP (MW=30000), N,N-

dimethylformamide (DMF), N,N-dimethylpropionamide (DMP), benzyl alcohol , 2-

phenylethanol, Cetyltrimethylammonium bromide (CTAB), tetrabutylammonium

bromide (TBAB), NaCl, NaBr and NaI were purchased from Sinopharm Chemical

Reagent Co. Ltd. (Shanghai, China). Carbon nanotubes were purchased from Shenzhen

Nanotech Port Co. Ltd. Human hepatoma cells (QGY-7703) and human hepatocytes

(QSG-7701) were purchased from cell storeroom of Chinese Academy of Science. The

water used in all experiments was ultrapure (18.2 MΩ). All reagents were used as

received without further purification.

Synthesis of 35 nm hexagonal Pd nanoplates: Pd(II) acetylacetonate (Pd(acac)2, 50.0

mg), poly(vinylpyrrolidone) (PVP, MW=30000, 160.0 mg) and CTAB (185 mg) were

mixed together with N,N-dimethylpropionamide (10 mL) and water (2 mL). The

resulting homogeneous yellow solution was transferred to a glass pressure vessel. The

vessel was then charged with CO to 1 bar and heated at 100 °C for 3.0 h before it was

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SUPPLEMENTARY INFORMATION doi: 10.1038/nnano.2010.235

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cooled to room temperature. The dark blue products were precipitated by acetone,

separated via centrifugation and further purified by an ethanol-acetone mixture.



mixed together with N,N-dimethylformamide (10 mL) and water (2 mL). The resulting

homogeneous yellow solution was transferred to a glass pressure vessel. The vessel was

then charged with CO to 1 bar and heated at 100 °C for 3.0 h before it was cooled to

room temperature. The dark blue products were precipitated by acetone, separated via

centrifugation and further purified by an ethanol-acetone mixture.



mixed together with 2-phenylethanol (10 mL). The resulting homogeneous yellow

solution was transferred to a glass pressure vessel. The vessel was then charged with

CO to 1 bar and heated at 100 °C for 3.0 h before it was cooled to room temperature.

The dark blue products were precipitated by acetone, separated via centrifugation and

further purified by an ethanol-acetone mixture.



mixed together with benzyl alcohol (10 mL). The resulting homogeneous yellow

solution was transferred to a glass pressure vessel. The vessel was then charged with

CO to 1 bar and heated at 100 °C for 3.0 h before it was cooled to room temperature.

The dark blue products were precipitated by acetone, separated via centrifugation and

further purified by an ethanol-acetone mixture.


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mg), poly(vinylpyrrolidone) (PVP, MW=30000, 160.0 mg) and NaBr (51.5 mg) were

mixed together with N,N-dimethylpropionamide (10 mL) and water (2 mL). The

resulting homogeneous yellow solution was transferred to a glass pressure vessel. The

vessel was then charged with CO to 1 bar and heated at 100 °C for 3.0 h before it was

cooled to room temperature. The dark blue products were precipitated by acetone,

separated via centrifugation and further purified by an ethanol-acetone mixture.


mg), poly(vinylpyrrolidone) (PVP, MW=30000, 160.0 mg) and NaBr (51.5 mg) were







mg), poly(vinylpyrrolidone) (PVP, MW=30000, 160.0 mg) and TBAB (161 mg) were






Synthesis of 51 nm hexagonal Pd nanoplates: 51 nm hexagonal Pd nanoplates are

obtained by using the Pd nanosheets (41 nm) as the seed. Typically, Pd(II)




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acetylacetonate (Pd(acac)2, 25.0 mg) were directly mixed together with the 41 nm Pd

nanosheets obtained above. The resulting homogeneous solution was transferred to a

glass pressure vessel. The vessel was then charged with CO to 1 bar and heated at 60 °C

for 3.0 h before it was cooled to room temperature. The dark blue products were

precipitated by acetone, separated via centrifugation and further purified by an ethanol-

acetone mixture.

TEM Analysis: TEM (including high-resolution transmission electron microscopy,

HRTEM) studies were performed on a TECNAI F-30 high-resolution transmission

electron microscopy operating at 300 kV. The samples were prepared by dropping

ethanol dispersion of samples onto 300-mesh carbon-coated copper grids and

immediately evaporating the solvent. For the direct measurements of the thicknesses,

carbon nanotubes were added into ethanol dispersions of the Pd nanosheets to allow the

attachment of Pd nanosheets on the outer surface of the nanotubes.

Quantitative Elemental Analysis of Pd nanosheets: The Pd and Br contents in the Pd

nanosheets (edge length 41 nm) were quantitatively analyzed by inductively coupled

plasma mass spectrometry (ICP) and ion chromatography, respectively. In a typical 12-

mL reaction of the Pd nanosheets (edge length 41 nm), 50.0 mg of Pd(acac)2 were used.

At the end of reaction, 0.35 mL solution was extracted and purified for analysis. To

prepare solution for ICP measurements, the purified Pd nanosheets were digested by

1mL aqua regia and then diluted by ultrapure water into a 1000 mL solution for the ICP

analysis. Based on the amount of Pd(acac)2 used, the Pd content in the solution was

calculated as 50.8 ppb. The actual concentration by ICP was 45.2 ppb. Therefore, the

recovery yield in a typical synthesis of the Pd nanosheets (edge length 41 nm) was 89%.




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UV-Vis- near IR absorption spectra measurements: All the UV-Vis- near IR

absorption spectra of Pd nanoplates were taken on a CARY 5000 Scan UV-Vis-near IR

spectrophotometer (Varian) using ethanol as the solvent. The measured solutions

contained 30 ppm Pd.

Electrochemical Measurements:

Pd-nanosheet-modified working electrodes were fabricated by depositing ethanolic

dispersion of purified nanosheets onto a glassy carbon electrode followed by drying

under an IR lamp. A saturated calomel electrode (SCE) and a platinum foil were used as

the reference and counter electrode, respectively. To measure the electrochemically

active surface area of Pd nanosheets, cyclic voltammetry (CV) measurements were

carried out in 0.5 M H2SO4 solutions at a sweep rate of 50 mV/s. The electrochemically

active surface area of the nanosheets was calculated, from the hydrogen adsorption /

desorption charge in the CV curve, by assuming a charge of 210 μC/cm2 for hydrogen

adsorption on Pd (111). For the electrooxidation of formic acid, the cyclic

voltammgrams were recorded at a sweep rate of 50 mV/s in 0.5 M H2SO4 + 0.25 M

formic acid. Before cyclic voltammetry measurements, two cycles of potential sweeps

between -0.2 V and 1.2 V at a sweep rate of 250 mV/s were applied in order to clean the

Pd surface in-situ.

For the CO stripping voltammetry measurements, CO gas (99.99%) was bubbled for 15

minutes through a 0.1 M H2SO4 or 0.1M HClO4 solution in which the electrode was

immersed. The electrode was quickly moved to a fresh solution and the CO stripping

voltammetry was recorded at a sweep rate of 2 mV/s. To investigate the CO adsorption

on the freshly-prepared Pd nanosheets, the CO stripping voltammogram was recorded in

0.1 M H2SO4 at a sweep rate of 2 mV/s without introducing any additional CO.




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Photothermal Effect Measurement:

To study the photothermal effect induced by the near IR SPR absorption, 1mL aqueous

solutions containing different concentration ( 0, 7, 14 and 27 ppm) of the Pd nanosheets

(side length 41nm) were irradiated by a near IR laser (808nm, 1W). The temperatures of

the solutions were monitored by a thermocouple microprobe (φ = 0.5 mm) submerged in

the solution in a 1-cm square cuvette. The probe was placed at such a position that the

direct irradiation of the laser on the probe was avoided. The tip of the thermocouple was

~5 mm above the bottom of the cuvette. The photothermal stability of the Pd

nanosheets was studied by irradiating the Pd solution with a 2 W 808 nm laser for 30

min.

Apoptosis assay:

Human healthy liver cells (QSG-7701) were cultured in RPMI 1640 medium in 24-well

plates. The cell density was 1×105 cells/well. After being seeded for 18 h, the media

were replaced by culture media containing different concentration of the Pd nanosheets

(side length 41nm). The incubations were carried out at 37 °C in 5% CO2 atmosphere

for 48 h. After incubation for 48 h, cell viabilities were measured by standard MTT

assay, a colorimetric assay based on the ability of viable cells to reduce 3-[4,5-

dimethylthiazol-2-yl]-2, 5-diphenyltetrazolium bromide.

In Vitro Photothermal Therapy:

Surface modification of Pd nanosheets with polyethyleneimine (PEI). 0.5 mL

solution of purified Pd nanosheets (200μg/mL, side length 41nm) was mixed with 0.5

mL PEI (5 mg/mL). The mixture was kept at 4°C overnight before centrifugation. The




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particles were then washed with water at least 5 times and then dispersed in 5 mL RPMI

1640 culture medium.

Cancer cells culture and their incubation with Pd nanosheets. Human hepatoma

cells (QGY-7703) were cultured in RPMI 1640 medium in 24-well plates. The density

was 1×105 cells/well. Before incubation with Pd nanosheets, the cells were seeded for

18 h. 0.5 mL of PEI-modified Pd nanosheets in water were added to each well at a

concentration of 20 μg/mL. The incubations were carried out at 37 °C in 5% CO2

atmosphere for 12 h. After incubation, the cell medium was removed, and the cells were

washed before PBS buffer solution was added.

Photothermal killing of cancer cells incubated with Pd nanosheets. After incubation

with Pd nanosheets, human hepatoma cells were exposed to a 2 W 808-nm laser for

various periods to induce photothermal cell damage. To identify the cell viability, the

dead cells were stained with Trypan Blue. Cell viabilities were also measured by the

standard MTT assay.




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30 40 50 60 70 80

(222)

(311)

(220)(200)

Inte

nsity

(a.u

.)

2θ (degree)

(111)

JCPDS:05-0681

Figure S1. The XRD pattern of the obtained Pd nanosheets produced from 3-hr

reactions in the presence of both PVP and CTAB.




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Figure S2. CO stripping voltammetry of (a) the as-prepared uniform hexagonal Pd

nanosheets (edge length 60 nm) and (b) the irregular Pd nanosheets (prepared in the

presence of PVP but in the absence of halide) in 0.1 M H2SO4 solution at a scan rate of

2 mV/s. CO was dosed for 15 minutes before measurements. The peaks observed in CO

stripping curve (a) can be assigned to the CO stripping on Pd (100) and Pd (111) facets.

In comparison, the only CO stripping peak for the irregular nanosheets shown in

Fig.S2b is attributed to Pd (111). (Hara, M.; Linke, U.; Wandlowski, T. Electrochim.

Acta. 2007, 52, 5733–5748.)




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Figure S3. Comparison studies of the reactions in the presence and in the absence of CO. The reactions were carried out in the presence

of PVP and CTAB using DMF as the solvent.




S11

0.4 0.6 0.8 1.0 1.2

0.2

0.4

0.6

0.8

Cur

rent

/(m

A/m

g)

Potential/ V(RHE)

1.035 V

2100 2050 2000 1950 1900

Inte

nsity

/ a.

u.

Wavelength / cm-1

1960cm-1 bridge-bonded CO

2046cm-1 linear-bonded CO

(a)

(b)

Figure S4. Characterization of CO adsorption on the as-prepared Pd nanosheets

without introducing additional CO after the synthesis: (a) CO stripping voltammetry of

freshly-prepared Pd nanosheets in 0.1 M H2SO4 solution at a scan rate of 2 mV/s. The

major peak observed in CO stripping curve is attributed to the CO adsorption on the

(111) facets of the as-prepared Pd nanosheets; (b) FTIR spectrum of the as-prepared Pd

nanosheets dispersed in DMF.




S12

Figure S5. Representative TEM of stacked Pd nanosheets collected from different reaction times using DMF as the solvent and CTAB as

the Br- source (left) and the thickness distribution of the nanosheets (right). (a) 0.5 h, (b) 1.5 h, (c) 2.5 h, and (d) 3.0 h.




S13

Figure S6. Representative TEM images of Pd nanosheets obtained from the reactions

under different conditions: (a-b) in absence of both PVP and CTAB; (c-d) in the

presence of CTAB but the absence of PVP; (e-f) in the presence of PVP but the absence

of CTAB .




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330 335 340 345

Cou

nts

Binding Energy / eV

Pd(0)

340.1 eV(Pd 3d3/2)

(Pd 3d5/2)335.1 eV

60 65 70 75

Cou

nts

Binding Energy / eV

Br-

(Br 3d5/2)67.1 eV

(Br 3d3/2)68.1 eV

0 1 2 3 4 5 6 7

-1

0

1

2

3

4

5

Inte

nsity

/ μS

Retention Time / min

reference Purified Pd nanosheets

Br-

-150 -100 -50 0 50 100 150

-0.5 mV

Inte

nsity

/ a.

u.Zeta potential / mV

without Br-

with Br-

-25 mV

(a) (b)

(c) (d)

Figure S7. XPS, ion chromatography and zeta potential analyses on the as-prepared Pd nanosheets: (a) Pd3d XPS spectrum ; (b) Br3d

XPS spectrum; (c) Br- ion chromatograph; (d) Zeta potential of the as-prepared Pd nanosheets compared with the products obtained under

the same reaction conditions as the Pd nanosheets but without using Br-.




S15

(a)

(c)

(b)

(d)

(e) (f)

Figure S8. Representative TEM images of Pd nanosheets collected from the reactions

where different halide anions were used. (a-b) NaCl (29.3 mg), (c-d) NaBr (51.5 mg),

and (e-f) NaI (75.0 mg). All the other reaction conditions were the same: Pd(acac)2 (50

mg), PVP (160 mg), DMF (10 mL) and water (2 mL). As revealed by the TEM

observations, halide anion play important role in the preferential growth of the

hexagonal nanosheets.




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Figure S9. Representative TEM (a-d) and AFM images (e-h) of different-sized Pd

nanosheets for thickness measurements. (a,e) 21 nm, (b,f) 27 nm, (c,g) 41 nm, and (d,h)

51 nm. The respective thickness distribution of the nanosheets is illustrated at the right

of each image. The average thickness of all nanosheets measured by TEM was 1.8 nm.

The nanosheets were deposited on SiO2-coated Si wafers for AFM analysis. As

measured by AFM, all different-sized Pd nanosheets had a same average height of 3.8

nm. The AFM height was constantly larger than the thickness analyzed by TEM, which

might be attributed to the presence of capping agent on the nanosheets.




S17

Figure S10. Representative TEM images of Pd nanosheets prepared from the reactions

using different solvents: (a) dimethylpropionamide, (b) dimethylformamide, (c) 2-

phenylethanol, and (d) benzyl alcohol. All the other reaction conditions were the same:

the solvent (10 mL), Pd(acac)2 (50 mg), PVP (160 mg), and water (2 mL). The average

edge lengths are 35, 60, 125 and 160 nm, respectively.




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400 600 800 1000 1200 1400

Abs

orba

nce

/ a.u

.

Wavelength / nm

fresh solution stocked solution

0.4 0.6 0.8 1.0 1.2

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Cur

rent

/(m

A/m

g)

Potential / V(RHE)

fresh solution stocked solution

1.035 V

(a)

(b)

Figure S11. Absorption spectra (a) and CO stripping voltammetric curves (b) of the

freshly prepared Pd nanosheets and the Pd nanosheets stocked in ethanol at 0°C for 2

months. The voltammograms were recorded at a scan rate of 2 mV/s without

introducing any external CO.




S19

400 600 800 1000 1200 1400 16000.0

0.5

1.0

1.5

2.0

Edge length, Thickness 21nm, 6nm 21nm, 4nm 21nm, 3nm 21nm, 2nm 25nm, 2nm

Extin

ctio

n Ef

ficie

ncy

(a.u

.)

Wavelength (nm)

Figure S12. Calculated extinction spectra of hexagonal Pd nanosheets with various

diameters. The orientation averaged extinction efficiency spectra of Pd nanosheets in

ethanol was numerically simulated by discrete-dipole approximation (DDA) method.

DDA is a flexible and powerful technique for computing scattering and absorption by

targets of arbitrary geometry. For the application of the DDA method, we thank Draine

and Flatau for their DDSCAT free source code (Draine, B. T.; Flatau, P. J. User guide

for the discrete dipole approximation code DDSCAT 6.1, 2004;

http://arxiv.org/abs/astro-ph/030969). In the calculations, the dielectric function data

were taken from Ref. (Johnson P B, Christy R W. Optical constants of transition metals:

Ti,V, Cr, Mn, Fe, Co, Ni, and Pd. Phys. Rev. B, 1974, 9, 5056–5070), and more than

20000 dipoles are used in the calculations.




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0.0 0.1 0.2 0.3 0.40.0

0.5

1.0

1.5 4.1 x 109 M-1cm-1

(b)

Abs

orba

nce

(O.D

.)

Pd Nanosheet Concentration (nM)

(a)

400 600 800 1000 12000.0

0.4

0.8

1.2

1.6 0.076 nM 0.152 nM 0.228 nM 0.304 nM 0.380 nM

Abs

orba

nce

(O.D

.)

Wavelength (nm)

Figure S13. Concentration-dependent SPR of the Pd nanosheets (edge length 41nm): (a)

absorption spectra; (b) the linear relationship between the absorbance at 1045 nm and

the concentration of Pd nanosheets. As determined by ICP, the Pd contents of the five

solutions measured were 4.3, 8.6, 12.9, 17.2, and 21.5 ppm. The morphologic

parameters (edge length 41nm, thickness 1.8nm) were then used to calculate the

number-based concentrations for the determination of the extinction coefficient of the

nanosheets. The concentrations shown in (a) are the number-based concentrations of

the Pd nanosheets.




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Figure S14. Representative TEM images, and the distributions of the edge length and

thickness of the Pd nanosheets before (a-d) and after (e-h) 30-min irradition with a

2W 808nm laser. For their direct thickness measurments, the nanosheets were attached

on carbon nanotubes. Before irradiation, the nanosheets had an average edge length of

41 nm and a thickness of 1.8 nm. After irradiation, the edge length and thickness of the

Pd nanosheets changed into 30 nm and 2.3 nm, respectively.




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400 600 800 1000 1200 1400 1600

Before irradiation of laser After 30-min irradiation of a 2W 808nm laser

Abs

orba

nce

/ a.u

.

Wavelength / nm

Figure S15. SPR absorption of Pd nanosheets before and after 30-min irradition with a

2W 808nm laser. Although the total energy input on the Pd nanosheets is as high as 3.6

kJ, the irradiated Pd nanosheets still displayed a good SPR response in the near IR

region.




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Figure S16 The photothermal instability of Ag (a-c) and Au (d-f) nanoprisms upon 30-

min irradition with a 2W 808nm laser. (a) and (d) are the TEM images of the

nanoprisms before the laser irradiation. (b) and (e) are the TEM images of the

nanoprisms after the laser irradiation. (c) and (f) show the significant SPR change of the

nanoprisms after the laser irradiation. The Ag nanoprisms were synthesized by the

method reported by Mirkin et al. (Metraux, G. S.; Mirkin, C. A. Adv. Mater. 2005, 17,

412-415). The Au nanoprisms were prepared by the method reported by Chung et al.

(Ha, T. H.; Koo, H-J; Chung, B. H. J. Phys. Chem. C 2007, 111, 1123-1130).




S24

-0.2 0.0 0.2 0.4 0.6 0.8 1.0

-0.4

-0.2

0.0

0.2

0.4

Cur

rent

/ m

A

Potential / V (SCE)

Figure S17 CV voltammetry of as-prepared Pd nanosheets in 0.5 M H2SO4 solution at

a scan rate of 50 mV/s. The electrochemically active surface area of the nanosheets was

calculated, from the hydrogen adsorption / desorption charge in the CV curve, between

-0.185V and 0.120 V (SCE) to be 67 m2/g after the correction of current background.




S25

Figure S18 A representative TEM image of the Pd black (47m2/g) from Aldrich.




S26

Figure S19 Micrographs of liver cancer cells treated under different conditions: (a)

after 12-hr incubation with Pd nanosheets (20 μg/mL) but before any with a 2W 808-nm

laser (1.4 W / cm2). (b) after 12-hr incubation with Pd(II) (20 μg/mL) followed by 5-

min irradiation of 2W 808-nm laser (1.4 W / cm2). Trypan blue was used to stain the

dead cells before microscopic evaluations.




S27

Figure S20 Structure drawing of a hexagonal Pd nanosheet with CO molecules on the

(111) facets, Br- on the six side (100) facets and NBu4+ as the counter ions.


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