silver) nanoshells

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Catalytic activities of mono- and bi-metal (gold/silver) nanoshells coated gold nanocubes towards catalytic reduction of nitroaromatics Manickam Sundarapandi, Perumal Viswanathan, Shanmugam Sivakumar, and Ramasamy Ramaraj Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02096 • Publication Date (Web): 26 Oct 2018 Downloaded from http://pubs.acs.org on October 28, 2018

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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.

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Catalytic activities of mono- and bi-metal (gold/silver) nanoshells coated gold nanocubes towards catalytic reduction of nitroaromatics Manickam Sundarapandi, Perumal Viswanathan, Shanmugam Sivakumar* and Ramasamy Ramaraj* School of Chemistry, Madurai Kamaraj University, Madurai - 625 021, INDIA. * [email protected] & [email protected]

ABSTRACT A new class of core-shell metallic nanostructures with tunable near-surface composition and surface morphology with excellent catalytic activity is reported. Very thin shell of metal nanoassemblies such as mono-layer (Ag and Au), bi-layer of Ag or Au and AgAu alloy layer with controlled size and morphology were deposited onto gold nanocubes (AuNC) core. UV-vis absorption spectroscopy and HR-TEM analyses along with SAED, EDX, ICP-MS and XRD techniques were used to characterize the prepared core-shell nanocubes. HAADF-STEM-EDS mapping images were recorded for the bi-layer shell and alloy layer shell in the core-shell nanostructures. Reduction of 4-nitroaniline in the presence of sodium borohydride was chosen to validate the catalytic activity of the prepared core-shell metal nanocubes. Interestingly, AgAu alloy shell layer over the AuNC ([email protected]) showed excellent catalytic activity compared to the pristine AuNC and mono-layer, bi-layer core-shell nanostructures.

Keywords: gold nanocubes, core-shell nanostructures, mono-layer shells, bi-layer shells, alloy shells, nitroaromatic reduction

1 ACS Paragon Plus Environment

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INTRODUCTION In recent years, bi-metallic nanoparticles (BMNPs) have attracted significant attention due to their unique optical, electronic, magnetic and catalytic properties which are different from their respective mono-metallic NPs.1,2 Synthesis of BMNPs with controlled size, shape, composition, and surface properties is a key challenge as the properties of these BMNPs vary largely with shape, composition and size. In particular, gold-silver (Au-Ag) core-shell nanostructures with different shapes have been examined intensely for their surface plasmon resonance (SPR) properties in the visible region.3 If the shell is consists of more than one metal i.e.,

core@innerlayer@outerlayershell

(For

eg.,

Au@Ag@Au

or

Au@Au@Ag)

and

core@alloyshell (Au@AgAu), the arrangement of metals in the nanostructures is of challenging for analyzing the optical and catalytic properties. Gold and silver in the form of core-shell structures and in alloy format have been studied4-11 and the optical properties can easily be tuned by changing the composition of the materials.12,13 The SERS, nuclear medicine imaging agent studies and catalytic activities of Au@Au core-shell NPs have been reported.14-16 In these reports, higher activity for the Au NPs is observed after coating the Au shell on the Au NPs core. Hence, the activities of core Au NPs is influenced by the same Au metal shell coated on the Au core i.e., Au core to Au@Au core-shell. Synthesis of Au@Ag core-shell NPs with different morphologies has been reported. For example, Huang and coworkers successfully synthesized Au@Ag core-shell NPs with different geometries such as nanocubes, truncated cubes, octahedra, truncated octahedra and cuboctahedra.17 The morphology of AuNC is more attractive because the Au nanostructures with different shapes can be prepared from the Au nanocube intermediates.18 To date, the optical properties of core-alloy shell (Au@AgAu), multi-shells


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(Au@Ag@Au@Ag) have been studied.19-21 Similarly, Maye and coworkers have reported a layer-by-layer alloy shell growth. 22,23 Nitroaromatics are important organic compounds, have attracted in relation to pollutants, toxic and carcinogenic. Aromatic amines are important intermediates and starting materials for many chemical industrial products. Hence, the conversion of hazardous nitroaromatics into industrially important aromatic amines has attracted significant attention. Reduction of nitroaromatic compounds to their corresponding aromatic amine is one of the conventional method for the preparation of aromatic amines.24 Various materials are available for the catalytic reduction of nitroaromatics and among these, noble metals, especially, nano-sized Au and Ag were identified to be the potential candidates.25 The Au core nanostructures coated with Ag shell architectures (Au@Ag) act as admirable catalyst towards nitrocompounds reduction and the catalytic properties of such materials are widely influenced by their size and shape.17,24 The Fermi potential of NPs becomes more negative compared to bulk material and this particular property is essential in the electron transfer reaction of catalyst in catalysis reaction.26 Bi-metal alloy NPs catalyst becoming important candidates as they offer greater reaction-specific catalytic tenability and higher performance than their monometallic counterparts.27 In the present investigation, a detailed investigation was carried out on the catalytic activity of the (i) Au(shell) or Ag(shell) on AuNC(core) (mono-layer shell (MLS): AuNC@Au or AuNC@Ag), (ii) bi-layer Au(inner shell)-Ag(outer shell) on AuNC(core) (bi-layer shell (BLS): AuNC@Au@Ag), (iii) bi-layer Ag(inner shell)-Au(outer shell) on AuNC(core) (BLS: AuNC@Ag@Au) and (iv) alloy AgAu(shell) on AuNC(core) (alloy shell (AS): AuNC@AgAu). Synthesizing Au shell layers onto the Ag nanostructures (AuNP@Ag@Au) is a challenging task due to the galvanic replacement reaction between Ag and Au3+.28,29 In the earlier studies, the


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galvanic replacement reaction between Ag and Au3+ did not take place when dilute concentration of Au3+ was used and also by adding the solution at slow rate. Addition of very dilute concentration of metal ions solution also helped in controlling the thickness of the shell layers at nm scale range. However, in the case of Ag shell on Au NPs (AuNP@Au@Ag), the outer layer is Ag and the galvanic replacement reaction will not take place between Au0 and Ag+ and the bilayer shell around Au NPs will form without galvanic replacement reaction. The catalytic property of the core-shell nanostructures towards the conversion of 4-nitroaniline (4-NA) to 1,4diaminobenzene (1,4-DAB) was demonstrated. All the catalysts were prepared by a simple method at room temperature. Results showed that the AS thinner coating (~3 nm) on the AuNC core exhibited better catalytic activity than the MLS, BLS and pristine AuNC core. EXPERIMENTAL SECTION Materials. Gold(III) chloride hydrate, silver nitrate, cetyltrimethylammonium bromide (CTAB), ascorbic acid (AA) was purchased from Sigma-Aldrich. 4-nitroaniline (4-NA), 4-nitrophenol (4NP), 2-chloro-4-nitrophenol (2-Cl-4-NP), 2,6-dichloro-4-nitrophenol (2,6-Cl-4-NP) were purchased from Alfa Aesar and sodium borohydride was purchased from Merck. All chemicals are used without further purification and double distilled water was used to prepare all the solutions. Synthesis of MLS. The core AuNC was synthesized by following the previously reported procedure.30 From the prepared AuNC solution, the final concentration of Au (0.19 mM) was calculated and it was taken as AuNC1 for convenient. Five different types of Au MLS were prepared by keeping AuNC1 as core. For synthesis of MLS, 5 mL of AuNC1 was taken in a beaker and to that 95 µL of 10 mM Au3+ (0.19 mM) solution was added which was equal to the core AuNC1 concentration and stirred for 10 min. To this 50 µL of 0.05 M ice-cold NaBH4 was 4 ACS Paragon Plus Environment

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added and stirred for 2 h to form AuNC1@Au1. Using this protocol, [email protected], [email protected], [email protected] and [email protected] MLS were prepared. The subscript numbers indicate the molar ratio between the AuNC1 core and Au shell. Similarly, five different types of Ag MLS were prepared and represented as AuNC1@Ag1, [email protected], [email protected], [email protected] and [email protected] MLS. The subscript numbers indicate the molar ratio between the AuNC1 core and Ag shell. The ten different MLS core-shell NC and pristine AuNC were used for catalytic studies. Synthesis of BLS. For BLS synthesis, 5 mL of AuNC1 was taken in a beaker and to that 23.75 µL of 10 mM of Ag+ solution was added and stirred for 10 min. To this 50 µL of 0.05 M ice-cold NaBH4 was added and stirred again for another 2 h. Then 23.75 µL of 10 mM Au3+ solution was added to the resultant solution and the stirring was continued for additional 10 min. To this 50 µL of 0.05 M ice-cold NaBH4 was added and stirred again for 2 h to form [email protected]@Au0.25 BLS. Following similar protocol, [email protected]@Ag0.25 BLS was also prepared. Synthesis of AS. For AS synthesis, 23.75 µL of 10 mM Ag+ solution and 23.75 µL of 10 mM of Au3+ solution were added to a 5 mL solution of AuNC1 and allowed to stir for10 min. To this, 50 µL of 0.05 M ice-cold NaBH4 was added and stirring was continued for 2 h to form the [email protected] AS. Catalysis Experiments. For a typical catalytic reaction studies, 0.1 mL of 4-NA (2 mM) was first added to a cuvette containing 1.4 mL double distilled water. To this, 0.5 mL of 0.05 M icecold NaBH4 was added followed by the addition of 0.05 mL of catalyst (pristine AuNC/MLS/BLS/AS). All the core-shell NC and pristine AuNC are used as prepared. The absorption spectra were recorded at different time intervals to study the catalytic reaction. Once


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the reduction reaction is complete, the light yellowish 4-nitroaniline (4-NA) solution will turn to colorless due to the formation of 1,4-diaminobenzene (1,4-DAB). Characterization. UV-vis absorption spectra were recorded using an Agilent Technologies 8453 spectrophotometer using 1 cm quartz cell. High resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) images were recorded with a FEI TECKNAI–G2 20 Twin instrument operated at 200 kV. Energy dispersive X-ray spectroscopy (EDX) was recorded using Bruker instrument. Inductively coupled plasma mass spectrometer (ICP-MS, Element XR, Thermo Fisher Scientific, Germany) was used to analyze the elemental composition of the core-shell NC nanostructures. HAADF-STEM-EDS mapping of the NC (bilayer and alloy shells) was obtained on FEI Tecnai F20. X-ray diffraction (XRD) patterns were recorded using a XPERT-PRO diffractometer (λ = 1.54060 Å). RESULTS AND DISCUSSION The AuNC and different core-shell AuNC were successfully synthesized and the formations were confirmed by monitoring the changes in absorption spectra. As a first step, Au or Ag mono-metal nano shells were coated over the AuNC following the procedure mentioned in the experimental section. A gradual increase in the SPR band intensity along with a 16 nm red shift from 538 nm to 554 nm was observed for AuNC@Au core-shell nanostructures when the Au shell concentrations were increased from Au0.1 to Au1 (Figure 1A). The red shift in SPR band infers the formation of Au shells over the AuNC core.


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Figure 1. (A) UV-vis absorption spectra and normalised absorption spectra for various concentrations of AuNC@Au (a: AuNC1@Au1; b: [email protected]; c: [email protected]; d: [email protected] and e: [email protected]) and (B) AuNC@Ag (a: AuNC1@Ag1; b: [email protected]; c: [email protected]; d: [email protected] and e: [email protected]) core-shell NC, respectively (Inset: UV-vis absorption spectra of (A) AuNC and (B) Enlarge spectra of Ag shell).


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As seen from Figure 1A, only a single absorption band was observed for the AuNC@Au. This is because both the core and shell were made up of the same metal, Au. As expected, two characteristic absorption bands were observed for AuNC@Ag core-shell nanostructures corresponding to the Au core and Ag shell (Figure 1B).31 Coating of Ag shell over AuNC showed a new absorption band at 410 nm due to the formation of Ag shell and this did not alter the absorption band position of AuNC core at 538 nm. However, increasing concentration of Ag shell from Ag0.1 to Ag1 caused a red shift in the Ag shell band position around 410 nm (inset: Figure 1B) similar to the case of Au shell formation on AuNC core. This confirms the formation of Ag shells onto the AuNC core. However, the increase in Ag shell concentration decreases the intensity of Au absorbance and this is due to the screening effect of Ag shell.31


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Figure 2. HR-TEM images of (i) AuNC, (ii) [email protected], (iii) [email protected] and (iv)-(vi) their corresponding SAED pattern. The shell thickness marked in the TEM images.


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HR-TEM images also confirm the formation of uniform shell layers of Au and Ag

(Figures 2(ii) and 2(iii) respectively) on the pristine Au core (Figure 2(i). The TEM images of all the AuNC1 MLS coated with different concentrations of Au shell (from 0.1 to 1 (except 0.5)) and Ag shell (from 0.1 to 1 (except 0.5)) were recorded and are shown in Figure S1. The TEM images of NC with Au0.5 and Ag0.5 MLS shells are shown in Figures 2(ii) and 2(iii). The size of Au shell thickness is understood from the increase in the average size of the AuNC@Au MLS core-shell NC when compared to the size of the pristine AuNC which indicated the formation of Au shell on the AuNC. The Ag shell on AuNC (AuNC@Ag) was identified from the contrast in the TEM image between the AuNC core and Ag shell structure due to the difference in the atomic mass of Au and Ag.32 From the TEM images the average Ag shell thickness of the MLS NC was calculated (Figure S4). In both the MLS, the average diameter of the outer shell layers was found to be 1.7 nm and the average diameter of the pristine AuNC was found to be 42.1 nm. It was also found that the catalytic activity of the MLS was higher for [email protected] and [email protected] nanostructures and the activity is decreased with further increase in the shell concentrations. So, the outer shell concentration was optimized to [email protected] (where M= Au or Ag) for preparing both BLS and AS. Figure 3 shows the absorption spectra recorded for the BLS and AS coated AuNC. The [email protected]@Au0.25 and [email protected]@Ag0.25 BLS showed a strong SPR absorption band for Au at 536 and 533 nm, respectively, whereas [email protected] AS showed the Au SPR band at 541 nm. Thus, the [email protected]@Au0.25 BLS showed a blue shift of 5 nm for Au while [email protected]@Ag0.25 BLS showed a blue shift of 8 nm when compared to the Au present in AS. It is also interesting to see that the intensity of the Au absorbance was higher in the case of [email protected]@Au0.25 BLS and lower in the case of [email protected]@Ag0.25 BLS (Figure 3) while the absorbance observed for Ag was higher in 10 ACS Paragon Plus Environment

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[email protected]@Ag0.25 BLS and lower in [email protected]@Au0.25 BLS (Figure 3. inset). This is attributed to the enhanced SPR activity of the outer shell layer. In the AS, the SPR intensity of both Ag and Au are falling between the BLS. This SPR band shift is attributed to the increased size and surface composition of AS compared to the BLS. Blue shift also gives the information on the thickness of the outer shell layer. The spectral data suggests that the thickness of outer shell

layer

is

higher

in

[email protected]@Au0.25

BLS

when

compared

to

the

[email protected]@Ag0.25 BLS and the [email protected] AS has the thickest outer layer among the three. TEM analysis was also performed to compare the results (Figure 4).

Figure

3.

Normalised

absorption

spectra

of

[email protected]@Ag0.25

BLS,

[email protected]@Au0.25 BLS and [email protected] AS (Inset: Enlarged spectra of Ag shell).


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Figure 4. HR-TEM images of (i) [email protected] AS, (ii) [email protected]@Au0.25 BLS and (iii) [email protected]@Ag0.25 BLS and (iv)-(vi) their corresponding SAED pattern. The shell thickness marked in the TEM images.


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The TEM images of the MLS, BLS and AS were recorded to elucidate the outer shell layer

thickness.

The

average

diameter

of

the


BLS,

[email protected]@Au0.25 BLS and [email protected] AS were found to be 44.2 nm, 44.4 nm and 45.1 nm and the average shell thickness were found to be 2.1 nm, 2.3 nm and 3 nm respectively. The HR-TEM images clearly confirm the Ag or Au shell formation. The image contrast between the core and shell structure due to difference in the atomic mass of Au and Ag

(Figure 4(i)- (iii)).32 Similarly, HR-TEM image is also used to differentiate the formation of MLS structure from the BLS nanostructures. As observed with optical studies, the HR-TEM images also showed that the AS has highest shell thickness when compared to BLS. It is also observed that among the two BLS the outer shell layer thickness is higher for [email protected]@Au0.25 than the [email protected]@Ag0.25. The other TEM images of core-shell NC and pristine AuNC was recorded and are shown in Figure S2. The formation of BLS and AS on AuNC were further confirmed by recording the HAADF-STEM-EDS mapping images for [email protected] AS (Figure 5(i)-(iv)) and [email protected]@Au0.25 BLS (Figure 5(v)-(viii)). The arrangement of Au (blue) and Ag (red) was seen as random image for AS ([email protected] AS) in Figure 5(ii and iii), respectively. Whereas for BLS ([email protected]@Au0.25 BLS), the Au (blue) and Ag (red) images were observed with core-shell arrangement in Figure 5(vi and vii), respectively. The overlay of both the Au and Ag present in the core-shell is shown in Figure 5 (iv and viii). The EDX spectra confirm the presence of Ag and Au metals in the prepared pristine AuNC and core-shell NC (Figure S3). The ring patterns with intense spot obtained from SAED pattern due to the (111) and (200) planes of Ag and Au. The composition of metals (Au and Ag) in MLS, BLS, AS and pristine AuNC measured using the ICP-MS analysis are shown in Table S1. 13 ACS Paragon Plus Environment

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Figure 5. HAADF-STEM-EDS mapping images of (i-iii) [email protected] AS and (iv-vi) [email protected]@Au0.25 BLS.


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Figure 6. XRD patterns of the AuNC and various core-shell nanostructures. The XRD patterns recorded for the AuNC, MLS ([email protected], [email protected]), BLS

([email protected]@Ag0.25, [email protected]@Au0.25) and AS ([email protected]) are shown in Figure 6. The strong diffraction peak observed at 38.18° is attributed to the {111} facet of the face-centred cubic (fcc) metal gold. The other peak at 44.5° ascribed to the {200} facet (JCPDS 04-0784) and the XRD patterns match with the fcc gold and silver. The XRD results further confirm the formation of highly crystalline nanocrystals.


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Catalytic Studies It is aimed to study the catalytic activity of core-shell metal through depositing a very thin layer of the same or different metal shell over the core metal. The deposition of same metal over the pre-formed core metal i.e., depositing Au shell over pre-formed AuNC was found to improve the catalytic activity greatly. In this regard, the catalytic reduction of 4-NA was chosen to investigate the catalytic activity of the prepared AuNC and the various shells formed over the AuNC. It is well known that the NaBH4 alone will not reduce 4-NA in the absence of any catalyst, which emphasizes the need of a catalyst for the successful reduction of 4-NA. Upon the addition of aqueous suspension of AuNC and core-shell NC, a rapid decrease in the absorbance of 4-NA at 380 nm was observed. This observation confirmed the catalytic reduction of 4-NA to 1,4-DAB in the presence of nanocatalyst. In this catalytic reduction, NaBH4 acts as the hydride ion source and can charge the metal surface. In the initial step, both NaBH4 and 4-NA gets adsorbed on the surface of the nanostructure and charge the metal nanocube thereby inducing the hydrogenation of 4-NA. In subsequent steps, the aromatic nitrocompound is reduced to the nitroso compound and then quickly to the corresponding hydroxylamine compound. The hydroxylamine compound is finally reduced to the aromatic amine.33 As the concentration of NaBH4 employed to reduce the 4-NA is much higher than the concentration of 4-NA, the reduction reaction follows pseudo-first order kinetics and the rate constant (k) is calculated from the slope of ln (absorbance) versus time. All eleven different nanostructures solutions (pristine AuNC, [email protected], [email protected], [email protected], [email protected], AuNC1@Au1, [email protected], [email protected], [email protected], [email protected] and AuNC1@Ag1) were investigated to understand their catalytic properties. To the solution containing 4-NA and ice-cold NaBH4, the aqueous solution


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of pristine AuNC and MLS were introduced to initiate the reduction reaction. Changes in the UV-vis absorption spectra were recorded at different time intervals to monitor the progress of the reduction reaction. Figure 7 shows the time-dependent absorption spectra recorded for the catalytic reduction of 4-NA by AuNC (Figure 7A), [email protected] (Figure 7B) and [email protected] (Figure 7C), respectively. The time dependent spectral studies of the other MLS are shown in Figure S4. The time-dependent absorption spectral changes obtained for the catalytic reduction of 4-NA shows a rapid decrease in the absorbance of 4-NA with simultaneous increase in the absorbance of 1,4-DAB as the reduction reaction proceeds. Time taken by the eleven NC for the complete reduction of 4-NA were compared in Table 1. The [email protected] and [email protected] exhibit better catalytic activity than the pristine AuNC. This indicates that the deposition of a small shell layer of metal over the core metal AuNC improves the catalytic performance of the core metal catalyst. When the size and shape of the Au NPs change, the electric field density on the surface of Au NPs changes with an increase in the SPR absorption intensity due to the change in the surface geometry which helps in enhancing the catalytic activity of Au NPs.34 Here, the change in the AuNC nanostructures by the deposition of Au shell / Ag shell influences the catalytic activity of the pristine AuNC. To assess the relative reaction rates of AuNC, AuNC@Au and AuNC@Ag MLS nanostructures, the plot of ln(A) against time were plotted for the reduction of 4-NA as shown in Figures 7D and 7E, respectively. From these plots, the [email protected] and [email protected] MLS showed higher k values when compared to the other core-shell nanostructures and these two MLS NC have been chosen to prepare the BLS and AS. The plots of rate constant (k) against the shell thickness of MLS obtained for both Au (Figure S5A) and Ag (Figure S5B) shells on AuNC1 showed that the [email protected] and [email protected] exhibited best catalytic activity among the other MLS.


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Figure 7. Time-dependent UV-vis absorption spectra recorded for the catalytic reduction of 4NA in the presence of (A) AuNC, (B) [email protected] MLS, (C) [email protected] MLS and (D,E) the corresponding kinetic plots. In

addition

to

the

Au

and

Ag

MLS

coatings,

([email protected]@Ag0.25,

[email protected]@Au0.25) BLS and ([email protected]) AS were deposited on AuNC to 19 ACS Paragon Plus Environment

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compare the catalytic activities. For the complete reduction of 4-NA, the time taken by the [email protected]@Au0.25, [email protected]@Ag0.25 and [email protected] NC were 200 s, 500 s, and 110 s, respectively (Figure 8(A-C)). Hence, the AS deposited over AuNC exhibited higher catalytic activity than the BLS deposited AuNC. Consequently, [email protected] AS showed higher k values than the BLS deposited AuNC and pristine AuNC (Figure 8D). Hence, the deposition of a thin metal alloy-shell layer over the core NC increased the catalytic activity of the resulting core-shell NC drastically. Table S2 summarized the average diameter and rate constants obtained for the pristine AuNC and various core-shell NC.

Figure 8. Time-dependent UV-vis absorption spectra recorded for the catalytic reduction of 4NA in the presence of (A) [email protected]@Au0.25 BLS, (B) [email protected]@Ag0.25 BLS and (C) [email protected] AS, and (D) the corresponding kinetic plots.


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The mechanism of the catalytic activities of BMNPs is still in debate and the improvement in the catalytic activity perhaps attributed to the composition of shell, size synergistic effect.35,36 In MLS, the Ag shell showed better catalytic activity than the Au shell. However, addition of another layer between the core and Ag shell decreased the activity of BLS. The activity of [email protected] AS (110 s) was found to be ~10 times higher than the unmodified core AuNC (1000 s). This result indicates a clear dependence of catalytic activity on the composition of the shell, size and synergistic effect of alloy shell (AS) and AuNC present in the [email protected]. Moreover, depositing Au/Ag AS on AuNC showed better catalytic activity than the BLS and MLS deposited AuNC and pristine AuNC. The catalytic reduction time and rate constant for all core-shell NC and pristine AuNC are summarized in Table 1 and the performance comparison of catalytic reduction of 4-NA by previously reported MNPs with the present AS NC are also given in Table 2.


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Table 1: Rate constant values obtained for the catalytic reduction of 4-NA to 1,4-DAB in the presence of NaBH4 and different NC catalysts. Nanocrystals

Reduction

Rate Constant (k)

Time (s)

(s-1)

1000

2.49×10-3

[email protected]

400

1.076×10-2

[email protected]

300

1.429×10-2

[email protected]

250

1.638×10-2

[email protected]

300

1.276×10-2

AuNC1@Au1

350

9.58×10-3

[email protected]

250

1.023×10-2

[email protected]

250

9.66×10-3

[email protected]

200

1.426×10-2

[email protected]

250

1.029×10-2

AuNC1@Ag1

250

1.255×10-2

[email protected]@Au0.25

200

1.709×10-2


500

5.11×10-3

110

3.07×10-2

Core AuNC MLS

BLS

AS [email protected]


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Table 2. Comparison of catalytic activity of present AS NC with previously reported MNPs towards the reduction of 4-NA. Catalyst Au Rhombic Dodecahedra NPs

Time 60 min

Rate Constant Reference 7.57 × 10-2 min[21] 1

Fe3O4/SiO2/Ag Nanocubes Au Nanowire networks

200 s 17 min

0.18 min-1

[37] [38]

Ag NPs

27 min

–

[39]

MnFe2O4@SiO2@Ag

18 min

min-1

[40]

PVP-AuNPs

20 min

-

[41]

Ag@AuNPs (1:1)

3.83 min

5.61×10-3 s-1

[42]

GO-bare-surfaced AuNPs

8 min

9.76×10-4 min-1

[43]

AgNPs/Citrus aurantifolia peel extract

6 min

4.69×10-3 s-1

[44]

pillar[5]arene stabilized AuNPs

6 min

8.34×10-3 s-1

[45]

Au nanosphere

64 min

2.76 × 10-2

[46]

110 s

3.07×10-2 s-1

This work

[email protected]

0.08

The [email protected] AS was checked for the catalytic reduction towards nitroaromatics such as 4-nitrophenol (4-NP), 2-chloro-4-nitrophenol (2-Cl-4-NP) and 2,6 dichloro-4-nitrophenol (2,6-Cl-4-NP). The observed complete reduction of all three nitroaromatics in 200 s and the corresponding kinetic plots are shown in Figure S6. The AS NC acts as the best catalyst for 4-NA and 4-NP and their derivatives. CONCLUSIONS In conclusion, we have synthesized various core-shell nanostructures (mono, bi and alloy metal) using Ag and Au deposited over the AuNC by simple method using NaBH4 as a reducing agent. The formation of pristine AuNC, MLS NC, BLS NC and AS NC were characterized by using HR-TEM and ICP-MS analysis. The HAADF-STEM-EDS mapping analysis of 23 ACS Paragon Plus Environment

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[email protected] AS (highest catalytic activity) and [email protected]@Au0.25 BLS (highest catalytic activity among the two BLS) core-shell nanostructures was also performed to confirm the nanostructures morphology. Catalytic activity of these core-shell nanomaterials were studied for the reduction of 4-nitroaniline to 1,4-diaminobenzene. A small deposition of alloy shell layer (around 3 nm) over the AuNC ([email protected]) shows larger increase in the catalytic activity than the pristine AuNC and other core-shell nanomaterials. The AS NC layer showed highest rate constant (k = 3.07×10-2 s-1) than other MLS and BLS NC.

ACKNOWLEDGEMENT RR acknowledges the financial support from the Council of Scientific and Industrial ResearchEmeritus Scientist Scheme (No. 21(1006)/15/EMR-II), New Delhi.


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TOC Graphic


silver) nanoshells

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