Synthesis and Catalytic Activities of Metal Shells (monolayer, bilayer

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Synthesis and Catalytic Activities of Metal Shells (monolayer, bilayer and alloy layer) Coated Gold Octahedra Towards Catalytic Reduction of Nitroaromatics Manickam Sundarapandi, Sivakumar Shanmugam, and Ramasamy Ramaraj J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b06298 • Publication Date (Web): 05 Aug 2019 Downloaded from pubs.acs.org on August 11, 2019

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The Journal of Physical Chemistry

Synthesis and Catalytic Activities of Metal Shells (monolayer, bilayer and alloy layer) Coated Gold Octahedra Towards Catalytic Reduction of Nitroaromatics Manickam Sundarapandi, Sivakumar Shanmugam* and Ramasamy Ramaraj* School of Chemistry, Madurai Kamaraj University, Madurai - 625 021, INDIA. * [email protected] & [email protected] ABSTRACT Different types of metal shells such as monolayer, bilayer and alloy layer of gold (Au) and/or silver (Ag) on gold octahedra (AuOh) nanoparticles were synthesized using N-[3 (trimethoxysilyl)propyl]diethylenetriamine (TPDT) without employing any external reducing agents. TPDT, an alternative to sodium borohydride, was used to prepare fifteen different coreshell metal nanostructures. The direct colloidal synthesis and stabilization of silver octahedra nanoparticles is still challenging task. The proposed methodology is an alternative synthetic route for Ag or Au nanoshell coated onto the AuOh core. UV-vis absorption spectroscopy, highresolution electron transmission microscopy analyses with selected-area electron diffraction, energy dispersive X-ray spectroscopy, inductively coupled plasma mass spectrometer, and X-ray diffraction techniques were used to characterize the prepared core-shell octahedra nanostructures. High-angle annular dark field scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (HAADF-STEM-EDS) mapping images and scanning electron microscopy coupled with lines scan EDS (SEM-EDS) were recorded to understand the alloy layer shell and bilayer shell in the core-shell Oh nanostructures. Catalytic reduction of toxic nitroaromatic compounds were chosen to investigate the catalytic activity of the core-shell octahedra nanostructures. The results reveal that the bimetal (AgAu) alloy shell layer coated over AuOh showed best catalytic activity, among the core-shell catalysts investigated for the reduction of nitroaromatics. 1 ACS Paragon Plus Environment

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1. INTRODUCTION Bi-metallic core-shell nanostructures have received significant interest in recent years due to their unique optical properties1, anti-oxidant characteristics,2 applications in catalysis,3 sensors,4 and surface-enhanced Raman scattering.5 Metal nanoparticles (MNPs) with different shapes in colloidal form have been investigated for tuning their size and shape dependent activity in catalysis.6–9 The MNPs with sharp edges exhibit interesting optical properties10 and thus promises application in catalysis. The octahedra morphology, especially, is becoming important in catalysis as it shows much lower surface free energy.11 Although synthesis of silver octahedra using silver nanocubes have been reported11, the direct colloidal synthesis of silver octahedra is still a challenging task.10 Thus an alternative synthesis of AuOh as core for subsequent metal (Ag or Au) shell formation was attempted. The formation of core-shell and alloy structures of bi-metallic NPs mainly depend on the preparation conditions.12 In the earlier reports, the catalytic activity, SERS and nuclear medicine imaging of Au@Au core-shell NPs were studied.13–15 Coating of Au core with Au shell (from Au core to Au@Au core-shell NPs) showed that the activity of the core Au NPs is influenced by the same Au shell. Different shapes of gold@silver (Au@Ag) core-shell nanostructures in solution-phase have been reported.9,12,16–19 The performance of these metal nanostructures can further be improved by preparing hollow metal nanostructures with higher surface area and lower density than their solid counterparts.20 Galvanic replacement reaction takes place immediately when HAuCl4 is added to Ag NPs in aqueous condition even at 0°C.21 Galvanic replacement reaction for various MNPs with different shapes have been reported.22–25 Both Au and Ag NPs show unique and well separated surface plasmon resonance (SPR) absorption bands in the visible region.26 The studies on optical properties of alloy shell on core (Au@AgAu), multi-shells on core (Au@Ag@Au@Ag) have been reported.27–29 However, a systematic investigation on the 2 ACS Paragon Plus Environment

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catalytic effect of different types of metal shells (mono-, bi- and alloy-layers of Au, Ag and AuAg alloy on AuOh) will shed light on choice of best catalysts to be designed. Amine groups have high affinity towards noble metals and they are generally used to anchor metallic colloids onto different substrates which are precoated with aminosilanes.30 The MNPs embedded in amine functionalized silicate matrixes have found applications in the fields of catalysis and sensors and hence, the scheming of different methods to prepare MNPs embedded in silicate matrixes have been reported.31–35 While the nitroaromatics are of environmental pollutants, mutagenic, carcinogenic and their corresponding aromatic amines have found applications in medicine, dyes, pesticides, and other industrially relevant processes. So the synthesis of aromatic amines from toxic nitroaromatics using catalysts is beneficial for both reducing the pollutant and improving the economy.8 Nanomaterial catalysts attracted significant attention due to their large surface area to volume ratio, potential active sites and high reactivity and selectivity.36 The Fermi potential of MNPs is more negative than their corresponding bulk metals and this peculiar property permits the use of MNPs as catalyst.37 Herein, we report the preparation of various types of core-shell NPs i.e., mono-layer shell (MLS: AuOh@Au and AuOh@Ag), bi-layer shell (BLS: AuOh@Au@Ag and AuOh@Ag@Au) and bi-metal alloy shell (AS: AuOh@AgAu) on preformed AuOh core in the presence of amine functionalized TPDT silane without using any external hazardous reducing agent. Results revealed that galvanic replacement reactions were observed in BLS (AuOh@Ag@Au) and AS (AuOh@AgAu) NPs core-shell nanostructures. Interestingly, the galvanic replacement reaction was not observed in the case of BLS (AuOh@Au@Ag), where the inner shell layer was Au. The pristine AuOh showed good catalytic activity and the activity was further improved by tuning the 3 ACS Paragon Plus Environment


thickness of core-shell NPs in the presence of amine functionalized silane. Our group have previously reported a new class of MLS, BLS and AS on gold nanocube core using conventional reducing agent such as sodium borohydride (NaBH4).38 The TPDT acts as reducing agent for the preparation of metal shell around the core and as stabilizing agent for the core-shell NPs. The catalytic activities of different core-shell Oh NPs were studied towards the reduction of 4nitrophenol (4-NP) to 4-aminophenol (4-AP). The results indicate that AS (AgAu) on the pristine AuOh core (AuOh@AgAu) showed excellent catalytic activity for the reduction of nitroaromatic compounds when compared to other nanostructured catalysts such as core-shell MLS, BLS and pristine AuOh core. 2. EXPERIMENTAL SECTION Materials. Gold(III) chloride hydrate, silver nitrate, cetyltrimethylammonium bromide (CTAB), ascorbic acid (AA) and N-[3-(trimethoxysilyl)propyl]diethylenetriamine (TPDT) were purchased from Sigma-Aldrich. 4-Nitrophenol (4-NP), 2- chloro-4-nitrophenol (2-Cl-4-NP), 2,6dichloro-4-nitrophenol (2,6-diCl-4-NP), 4-nitroaniline (4-NA) and 4-aminophenol (4-AP) were purchased from Alfa Aesar. Sodium borohydride (NaBH4) was purchased from Merck. All chemicals were used without further purification and double distilled water was used to prepare all the solutions. Synthesis of MLS. The pristine AuOh were synthesized by following previously reported method.39 The final concentration of Au (0.04 mM) was calculated from the prepared AuOh solution and it was taken as AuOh1 for convenience. To synthesis the MLS, 8 µL of 10 mM Au3+ (0.02 mM) was added to 4 mL of AuOh1 solution in a beaker which was half to the concentration of AuOh1 core and stirred for 10 min. To this solution 40 µL of 0.5 M TPDT was added and then stirred for 3 h to form [email protected] MLS. Using the same protocol, different Au MLS such as, 4 ACS Paragon Plus Environment

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AuOh1@Au1, AuOh1@Au2, AuOh1@Au3, AuOh1@Au4 and AuOh1@Au5 MLS were prepared. The subscript numbers indicate the molar ratio between the AuOh1 core and Au shell. Similarly, six different concentrations of Ag MLS on AuOh1 core were also prepared and represented as [email protected], AuOh1@Ag1, AuOh1@Ag2, AuOh1@Ag3, AuOh1@Ag4 and AuOh1@Ag5. The subscript numbers indicate the molar ratio between the AuOh1 core and Ag shell. Synthesis of BLS. For BLS synthesis, 16 µL of 10 mM Ag+ solution was added to 4 mL of AuOh1 in a beaker and stirred for 10 min. To this solution 20 µL of 0.5 M TPDT was added and stirred for 3 h. Then, 16 µL of 10 mM Au3+ solution was added to the solution and the stirring was continued for another 10 min. To this 20 µL of 0.5 M TPDT was added and stirred again for 3 h to form AuOh1@Ag1@Au1 BLS. Following the procedure, AuOh1@Au1@Ag1 BLS was also prepared. Synthesis of AS. For AS synthesis, 16 µL of 10 mM Ag+ solution and 16 µL of 10 mM Au3+ solution was added to 4 mL of AuOh1 and stirred 10 min. To this solution 40 µL of 0.5 M TPDT was added and stirred for 3 h to form the AuOh1@Ag1Au1 AS. Catalysis Experiments. For a typical catalytic reaction, 100 µL of 4-NP (2 mM) was added to 1.4 mL distilled water. To this, 0.5 mL of ice-cold 0.05 M NaBH4 was added to the reaction mixture followed by the addition of 50 µL of catalyst (AuOh1 or MLS or BLS or AS). All the core-shell Oh and pristine AuOh1 were used as prepared. The absorption spectral changes were recorded at different time intervals depending on the reaction rate to analyze the catalytic reaction. After the completion of reduction reaction, the light yellowish 4-NP solution turned to colorless due to the formation of 4-aminophenol (4-AP). Characterization. UV-vis absorption spectra were recorded using an Agilent Technologies 8453 spectrophotometer using a 1 cm quartz cell. Transmission electron microscopy

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(TEM) and selected area electron diffraction (SAED) images were recorded with a FEI TECKNAI T20 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 coreshell Oh nanostructures. HAADF-STEM-EDS images were recorded using FEI Tecnai F20. SEMEDS lines scan spectra was obtained on VEGA3 TESCAN, Czech Republic. X-ray diffraction (XRD) patterns were recorded with a XPERT-PRO diffractometer (λ = 1.54060 Å). 3. RESULTS AND DISCUSSION The core AuOh1 was synthesized as reported earlier39 and different core-shell AuOh were synthesized by chemical reduction method by using TPDT silane as reducing agent and as stabilizer. The formation of core-shell Oh NPs was primarily confirmed by absorption spectra. The absorption spectrum of AuOh1 solution revealed a characteristic sharp SPR band at 547 nm (Inset: Figure 1A).


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Figure 1. (A) Normalized absorption spectra (normalized at 480 nm) of various concentrations of Au MLS on AuOh1 core. (B) UV-vis absorption spectra of various concentrations of Ag MLS on AuOh1 core. (Inset: UV-vis absorption spectra of (A) AuOh1). 7 ACS Paragon Plus Environment


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To start with, mono-metal (Au or Ag) shell was coated on the AuOh1 core. A blue-shift of 12 nm from 547 nm to 535 nm was observed for [email protected] MLS. Interestingly a gradual red-shift was observed when the Au shell concentration was increased from Au0.5 to Au5 (Figure 1A). However, an overall blue-shift was observed for all the Au shell concentrations on AuOh1 when compared to pristine AuOh1. The Au shell coated on AuOh1, AuOh1@Au1, AuOh1@Au2, AuOh1@Au3, AuOh1@Au4 and AuOh1@Au5 MLS showed a single SPR band at 537, 538, 539, 541 and 544 nm, respectively since both core and shell metals are made up of the same metal, Au. The unusual blueshift rather than a red-shift (in Au0.5 shell formation) is due to the increased core-shell size while increasing the concentration of shell.17 The formation of Ag shell coated on AuOh (AuOh@Ag MLS core-shell) was confirmed by the appearance of three SPR bands17 at 346, 426 and 510 nm. Increasing the concentration of Ag shell from Ag0.5 to Ag5 lead to a red shift in the Ag SPR band from 405 nm to 426 nm (Figure 1B). The spectral data shown in Figure 1 confirm the formation of Au or Ag shell onto the pristine AuOh1 core. To understand the effect of size and thickness of Au or Ag shells coreshell Oh MLS, the TEM images were recorded and the same are shown in Figure 2. The TEM images confirm the formation AuOh1 (Figure 2i), Au shells (AuOh1@Au2 (Figure 2ii)) and Ag shells (AuOh1@Ag2 (Figure 2iii)) on pristine AuOh1. The observed increase in average core-shell NPs size confirms the formation of shell layers onto the AuOh1 (Figure S1). This is further confirmed from the observation of contrast difference between the core and shell due to difference in the atomic mass of Au and Ag in the TEM image.40 The TEM images of Au shell (other than AuOh1@Au2 MLS) and Ag shell (other than AuOh1@Ag2 MLS) coated AuOh1 core are shown in Figure S1 for comparison. The formation of Au shell coated on pristine AuOh1 confirmed from the increase in the average size of AuOh@Au MLS core-shell Oh when compared to the pristine AuOh1 (Figures 2ii and S1i-v). The Ag MLS coated AuOh was identified from the contrast difference in the TEM images between the


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pristine AuOh1 and Ag shell (Figures 2iii and S1vi-x). The TEM images of AuOh@Mx MLS (M: Au or Ag; x: 0.5 to 5) revealed that the AuOh1 core was highly visible and spheroidal like Au or Ag shell formation was observed on AuOh1 core. From Figures 2 and S1, the average sizes were calculated as 30, 31.8, 33.1, 36.4, 38.2, 40.4, 45.5, 34.4, 40.6, 43.8, 46.9, 52.6 and 55.5 nm for pristine AuOh1, [email protected],

AuOh1@Au1,

AuOh1@Au2,

AuOh1@Au3,

AuOh1@Au4,

AuOh1@Au5,

[email protected], AuOh1@Ag1, AuOh1@Ag2, AuOh1@Ag3, AuOh1@Ag4 and AuOh1@Ag5, respectively. Hence, the average thickness of Au shell or Ag shell on pristine AuOh1 was calculated from the average diameter of the AuOh1@Mx MLS (M: Au or Ag and x: 0.5 to 5). The plot of SPR absorption band maximum observed for Au shell in AuOh1@Au MLS and Ag shell in AuOh1@Ag MLS against Au and Ag shell concentrations and also against shell thicknesses are shown in Figure S2i-iv. It was found that among the MLS, the catalytic behavior was higher for AuOh1@Au2 and AuOh1@Ag2 and the catalytic activity remained constant with further increase in the shell concentration (AuOh1@Au2-5 and AuOh1@Ag2-5) (vide infra). This may be due to the increase in the metal shell thickness.41 The metal shell concentration was optimized to AuOh1@Au2 and AuOh1@Ag2 for preparing BLS and AS coated AuOh. Figure 3 shows the absorption spectra obtained for BLS and AS coated AuOh. The AuOh1@Ag1@Au1 BLS and AuOh1@Au1@Ag1 BLS showed strong SPR absorption band for Au at 526 and 522 nm, respectively, whereas the AuOh1@Ag1Au1 AS showed the SPR absorption band for Au at 519 nm.



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Figure 2. TEM images of (i) pristine AuOh1, (ii) AuOh1@Au2 MLS, (iii) AuOh1@Ag2 MLS and (ivvi) their corresponding SAED patterns.


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The AuOh1@Ag1@Au1 BLS showed a red shift of 7 nm while AuOh1@Au1@Ag1 BLS showed a red shift of 3 nm when compared to the AuOh1@Ag1Au1 AS. It is also interesting to note that the intensity of the Au absorbance was higher for AuOh1@Ag1@Au1 BLS (Figure 3) and lower for AuOh1@Au1@Ag1 BLS while the intensity of Ag absorbance was higher for AuOh1@Au1@Ag1 BLS and lower for AuOh1@Ag1@Au1 BLS (Figure 3). This is attributed to the intensified SPR activity of the outer metal shell layer.42 In AuOh1@Ag1Au1 AS, the SPR bands for both Ag and Au appeared in-between the SPR band region of Au and Ag in BLS. A weak intensity due to Ag absorption was observed for AuOh1@Ag1@Au1 BLS and AuOh1@Ag1Au1 AS which indicates that the Ag metal shell was partially dissolved due to galvanic replacement reaction in solution.43 Consequently a void formation was observed around AuOh1 core (Figure 4ii-iii and Figure S3i).

Figure 3. UV-vis absorption spectra of AuOh1@Ag1Au1 AS, AuOh1@Ag1@Au1 BLS and AuOh1@Au1@Ag1 BLS.



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The SPR band shift is attributed to both the increased size and surface composition of AuOh1@Ag1Au1 AS when compared to both the BLS (AuOh1@Ag1@Au1 and AuOh1@Au1@Ag1). From the absorption spectral studies, the size of AuOh1@Ag1@Au1 BLS was found to be larger than the AuOh1@Au1@Ag1 BLS and the AuOh1@Ag1Au1 AS was the smallest in size among three. TEM analysis was performed to understand the sizes of BLS (AuOh1@Au1@Ag1 or AuOh1@Ag1@Au1) and AS (AuOh1@Ag1Au1) (Figure 4).


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Figure 4. TEM images of (i) AuOh1@Au1@Ag1 BLS, (ii) AuOh1@Ag1@Au1 BLS, (iii) AuOh1@Ag1Au1 AS and (iv-vi) their corresponding SAED patterns. 13 ACS Paragon Plus Environment


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The average diameters of AuOh1@Au1@Ag1 BLS, AuOh1@Ag1@Au1 BLS and AuOh1@Ag1Au1 AS were calculated as 38, 40 and 36 nm from the TEM images (Figure 4i-iii) and the average shell thicknesses were calculated as 8, 10 and 6 nm, respectively. Results from the SPR absorption spectral data and TEM analysis confirmed the lowest shell thickness for AuOh1@Ag1Au1 AS when compared to BLS (AuOh1@Au1@Ag1 and AuOh1@Ag1@Au1). Among the two BLS, the AuOh1@Ag1@Au1 BLS showed higher shell thickness than that of AuOh1@Au1@Ag1 BLS. The galvanic replacement reaction took place during the formation of Au outer shell in AuOh1@Ag1@Au1 BLS (Figure 4ii) and AuOh1@Ag1Au1 AS (Figure 4iii). During galvanic replacement reaction, the metal in the MNPs reacts with the metal salt precursor which possesses higher electrochemical potential, leading to the formation of hollow nanostructures.22 The previous report shows that the galvanic replacement between Ag NPs and Au3+ was avoided by adding a dilute concentration of Au3+ at a slow rate.21 In the present work, dilute concentrations of Au3+ were used for the formation of shell. However, the pH of the solution also plays the role to obtain hollow shell21 around the AuOh1 core. In the core-shell oh (MLS, BLS and AS) synthesis, the solution pH was found to be pH 9.5. Due to the presence of triamine in the TPDT silane the pH was high enough to form the hollow nanostructure. The Ag shell was dissolved due to galvanic replacement reaction upon the addition of Au precursor to create hollow space around AuOh1 core due to the presence of TPDT silane (Figures 4ii and 4iii). The TEM images (Figure S3i) of AuOh1@Ag1@Au1 BLS clearly confirmed the galvanic replacement reaction in AuOh1@Ag1@Au1 BLS with the creation of hollow space between AuOh1 core and Au outer shell. But in the case of AuOh1@Au1@Ag1 BLS, the galvanic replacement reaction will not occur44 between Au0 and Ag+. In the absence of galvanic replacement reaction, the bi-layer shell around AuOh1 core is highly visible in AuOh1@Au1@Ag1 BLS (Figure 4i and Figure S3ii). The EDX spectra supported the presence of Ag and Au in the pristine AuOh1 and core-shell Oh


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(Figure S4). The ring patterns with intense spot obtained in the SAED patterns show the presence of (111) and (200) planes of Ag and Au (Figure 2iv-vi and Figure 4iv-vi). The formation of AS and BLS on AuOh1 core was further confirmed by recording HAADF-STEM-EDS mapping images (Figure 5) and SEM-EDS lines scan spectra (Figure S5) for AuOh1@Ag1Au1 AS and AuOh1@Au1@Ag1 BLS. The HAADF-STEM-EDS mapping images of AuOh1@Ag1Au1 AS (Figure 5i-v) and AuOh1@Au1@Ag1 BLS (Figure 5vi-x) confirm the core and shell arrangement. The arrangement of Au (cyan) and Ag (red) is shown in Figure 5iii-iv for AS and in Figure 5viii-ix for BLS. The overlay of both the Au and Ag present in the core-shell nanoparticles (AuOh1@Au1Ag1 AS and AuOh1@Au1@Ag1 BLS) is shown in Figure 5v,x. In addition, SEM-EDS lines scan spectra of the AuOh1@Ag1Au1 AS (Figure S5i-ii) and AuOh1@Au1@Ag1 BLS (Figure S5iii-iv) confirms the composition of Au and Ag as core and shell. The composition of metals (Au and Ag) in MLS, BLS, AS and pristine AuOh1 measured using the ICP-MS analysis are summarized in Table S1.

Figure 5. HAADF-STEM-EDS mapping images of (i-v) AuOh1@Ag1Au1 AS and (vi-x) AuOh1@Au1@Ag1 BLS.



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Figure 6. XRD patterns of pristine AuOh1 and various shells containing Au and Ag in MLS, BLS and AS. The XRD patterns recorded for pristine AuOh1, MLS (AuOh1@Au1, AuOh1@Ag1), BLS (AuOh1@Au1@Ag1, AuOh1@Ag1@Au1) and AuOh1@Ag1Au1 AS are shown in Figure 6. The diffraction peak observed at 37.8° is attributed to the (111) facet of the face-centred cubic (fcc) metal gold. The observed other peak at 45.1° ascribed to the (200) facet (JCPDS 04-0784) and the XRD 16 ACS Paragon Plus Environment

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patterns match with the fcc of gold and silver. The XRD results confirm the formation of highly crystalline nanocrystals. Catalytic Studies. Present study is aimed at investigating the catalytic activity of various metal shell layers over the core AuOh1. The catalytic reduction of 4-NP to 4-AP using NaBH4 as reducing agent was chosen to investigate the catalytic activity of the prepared sixteen different AuOh1, MLS, BLS and AS. It is well known that the catalytic reduction of 4-NP to 4-AP will not occur in the presence of only NaBH4. The presence of suitable catalyst is essential to drive this chemical reduction reaction. Initially thirteen different AuOh catalysts (pristine AuOh1, [email protected],

AuOh1@Au1,

AuOh1@Au2,

AuOh1@Au3,

AuOh1@Au4,

AuOh1@Au5,

[email protected], AuOh1@Ag1, AuOh1@Ag2, AuOh1@Ag3, AuOh1@Ag4 and AuOh1@Ag5) were studied to understand their catalytic activities. Aqueous solutions of pristine AuOh1 and different (Au or Ag) MLS core-shell catalysts were added individually to the solution containing a mixture of 4NP and ice-cold NaBH4 to initiate the catalytic reduction reaction. The 4-NP reduction reaction was monitored by using UV-vis absorption spectroscopy at different time intervals. Upon the addition of 50 µL of pristine AuOh1 or core-shell Oh, to a mixture of 4-NP and NaBH4 a rapid decrease in the absorbance of 4-NP at 400 nm was observed due to the formation of nitrophenolate ion and another two new absorption bands were observed at 230 and 300 nm due to the formation of 4-AP.45 The light yellow solution turned to colourless indicating the formation of 4-AP. The conversion of 4-NP to 4-AP was monitored in situ by recording the time-dependent absorption spectral changes. In the presence of catalyst (pristine AuOh1 and MLS), the catalytic reduction of 4-NP to 4-AP was confirmed from the absorption spectral changes coupled with a decrease in the absorbance at 400 nm and formation of new absorption bands at 230 and 300 nm which indicates the important role played by the present core-shell catalysts in the chemical reduction. This observation confirms that the



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pristine AuOh1 and core-shell Oh MLS act as good catalysts for the conversion of 4-NP to 4-AP. The reduction reaction follows pseudo-first order kinetics due to the excess concentration of NaBH4 and the rate constant (k) values was calculated from the slope of ln (absorbance) versus time.

Figure 7. Time-dependent UV-vis absorption spectral changes recorded for catalytic reduction of 4NP in the presence of (i) Pristine AuOh1, (ii) AuOh1@Au2 MLS, (iii) AuOh1@Ag2 MLS, (iv) AuOh1@Au1@Ag1 BLS, (v) AuOh1@Ag1@Au1 BLS and (vi) AuOh1@Ag1Au1 AS.


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Figure 8. The plots of rate constant (k) values against (i) Au shell concentration, (iii) Au shell thickness in AuOh1@Au MLS, (ii) Ag shell concentration and (iv) Ag shell thickness in AuOh1@Ag MLS.

The time-dependent absorption spectral changes were recorded for the catalytic reduction of 4-NP by using pristine AuOh1 (Figure 7i), AuOh1@Au2 (Figure 7ii) and AuOh1@Ag2 (Figure 7iii) and other Au and Ag shells coated MLS (Figure S6 and S7) respectively. When core-shell MLS catalysts were added the complete reduction of 4-NP to 4-AP was observed at a particular core-shell composition (AuOh1@Au2 and AuOh1@Ag2) and the catalytic activity remained constant on further increasing the concentration of Au or Ag shell (Au2 to Au5 or Ag2 to Ag5). From the rate constant



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values obtained for MLS catalysts (Figure S6vi and S7vi), it was found that the AuOh1@Au2 and AuOh1@Ag2 acted as better catalysts among Au MLS and Ag MLS, respectively. The plot of rate constant values against the concentration Au or Ag shell on AuOh1 MLS and also against average metal shell thickness observed for 4-NP catalytic reduction are shown in Figure 8i-iv, respectively. The increase in the Au or Ag shell thickness on AuOh1 core (Figure 8iii-iv) showed that the AuOh1@Au2 MLS and AuOh1@Ag2 MLS are the best catalysts among the other MLSs for the reduction of 4-NP to 4-AP. Hence, the AuOh1@Au2 and AuOh1@Ag2 MLS were chosen to prepare the BLS (AuOh1@Ag1@Au1 and AuOh1@Au1@Ag1) and AS (AuOh1@Ag1Au1). In BLS and AS, the AuOh1 core was kept constant and the shell was made up of two different metals (Au and Ag). The absorption spectra recorded in aqueous solution for 4-NP (Figure S8A(a)), 4-AP (Figure S8A(b)), 4-NP in the presence of NaBH4 at 0 min (Figure S8A(c)), at 15 min (Figure S8A(d)) and 4-NP in the presence of catalyst (AuOh1) only (at 0 and 15 min) (Figure S8B) are shown in Figure S8A-B. Upon the addition of NaBH4, the absorption band of 4-NP was shifted from 317 to 400 nm due to the formation of 4-nitrophenolate ion (Figure S8A(c)). The absorption spectra were also recorded for 4NP in the presence of catalyst AuOh1 at different time intervals and the absorbance intensity at 317 nm did not change with time (Figure S8B). Similarly, the absorbance intensity observed at 400 nm for 4-NP in the presence of only NaBH4 did not decrease with time (Figure S8A(c,d)). The decrease in absorbance intensity at 400 nm was observed only when both the catalyst and NaBH4 were present due to the catalytic reduction of 4-NP to 4-AP (Figure 7). It was reported that the adsorption and desorption of 4-NP to 4-AP on the nanogold catalyst surface is fast and modeled in terms of Langmuir isotherm.46,47


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Figure 9. Kinetic plots of ln(A) vs. time obtained for AuOh1 (R2: 0.931), AuOh1@Ag1@Au1 (R2: 0.851), AuOh1@Au1@Ag1 (R2: 0.921) and AuOh1@Ag1Au1 (R2: 0.710) from Figure 6(i, iv, v and vi).



Page 22 of 35

Table 1: Rate constant values obtained for the catalytic reduction of 4-NP to 4-AP in the presence of NaBH4 for different pristine AuOh1 and core-shell Oh catalysts.

Nanocatalysts

Average shell

Reduction Time

Rate Constant (k)

(core and core-shell catalysts)

thickness (nm)

(s)

(×10-3 s-1)

30

1000

2.30

[email protected]

1.8

700

0.83

AuOh1@Au1

3.1

700

1.44

AuOh1@Au2

6.4

550

8.04

AuOh1@Au3

8.2

550

7.13

AuOh1@Au4

10.4

550

5.32

AuOh1@Au5

15.5

550

7.56

[email protected]

4.4

700

5.03

AuOh1@Ag1

10.6

550

6.19

AuOh1@Ag2

13.8

450

8.75

AuOh1@Ag3

16.9

450

8.64

AuOh1@Ag4

22.6

450

7.65

AuOh1@Ag5

25.5

450

7.77

AuOh1@Ag1@Au1

10

650

4.29

AuOh1@Au1@Ag1

8

1000

2.96

6

150

10.59

Core Pristine AuOh1 MLS

BLS

AS AuOh1@Ag1Au1

In addition to the MLS, the catalytic activities of AuOh1@Au1@Ag1 BLS (Figure 7iv), AuOh1@Ag1@Ag1 BLS (Figure 7v) and AuOh1@Ag1Au1 AS (Figure 7vi) towards the reduction of 4-NP were studied. The complete reduction of 4-NP was observed at the core-shell catalysts 22 ACS Paragon Plus Environment

Page 23 of 35 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


AuOh1@Au1@Ag1 BLS, AuOh1@Ag1@Au1 BLS and AuOh1@Ag1Au1 AS in 1000 s, 650 s, and 150 s, respectively. The rate constant (k) values obtained from the plots of ln(A) versus time (Figure 9) are 2.30×10-3 s-1, 4.29×10-3 s-1, 2.96×10-3 s-1 and 10.59×10-3 s-1 for pristine AuOh1 core, AuOh1@Ag1@Au1 BLS, AuOh1@Au1@Ag1 BLS, AuOh1@Ag1Au1 AS, respectively. The time taken for complete reduction of 4-NP for different catalysts along with Au and Ag shell thickness and the corresponding rate constant values are summarized in Table 1. Among all the pristine AuOh1, MLS, BLS and AS catalysts, the Ag1Au1 AS (AuOh1@Ag1Au1 AS) formed over AuOh1 showed the highest catalytic activity towards 4-NP reduction to 4-AP. The improved catalytic activity of Ag1Au1 AS over the MLS and BLS is mainly attributed to the composition of shell, size and synergistic catalytic effect.48,49 Bi-metal alloy NPs catalysts are interesting in catalysis, because they offer specific catalytic tenability and better performance than the bi-metal core-shell.48 The catalytic activity not only depends on the composition of shell and size but also depends on the electronegativity of Au. The improved catalytic activity of bi-metal alloy (AgAu) NPs can be assigned to the higher electronegativity of Au when compared to Ag, which permits electrons to be shifted from Ag to Au, leading to the electron density of the surface when compared to that of monometal Au or Ag shell on Au core50 and thus enhances the catalytic reduction of 4-NP. The catalytic activity mechanism for such bi-metal NPs is still in argument and the improvement in the catalytic activity is expected due to the composition of metal shell, size, electronegativity of Au and synergistic effect.13,48,50,51 The Ag shell showed significantly better catalytic activity than the Au shell in MLS. However, the addition of Au layer between the core and Ag shell (AuOh1@Au1@Ag1 BLS) considerably increased the activity compared to the pristine AuOh1 core and decreased in the activity compared to other bi-layer AuOh1@Ag1@Au1. The activity of AuOh1@Ag1Au1 AS (150 s) was found to be ~7 times higher than that of the unmodified core



Page 24 of 35

AuOh1 (1000 s). The deposition of bi-metal alloy Ag1Au1 AS on AuOh1 showed best catalytic activity when compared to BLS and MLS deposited AuOh1 and pristine AuOh1. This observation clearly reveals that the dependence of catalytic activity is mainly based on the composition of metal shell, size and composition. Comparison of catalytic performances of AuOh1@Ag1Au1 AS catalyst with already reported MNPs catalysts towards the reduction of 4-NP are summarized in Table 2. Table 2. Comparison of catalytic activity of present AuOh1@Ag1Au1 AS with previously reported MNPs towards the reduction of 4-NP to 4-AP. Nanocrystals

Catalysts

Reaction

Rate Constant

Reference

(mg)

Time

(k)

Au/PMMA

3.5

600 s

7.9×10-3 s-1

52

TAC-Ag-1.0

4.0

415 s

5.19×10-3 s-1

53

Ag-NP/C composite

1.0

25 min

1.69×10-3 s-1

54

nanocomposite

1.0

8 min

7.67×10-3 s-1

55

NCMSS

3

16 min

2.81×10-3 s-1

56

Au/graphene hydrogel

0.024

720 s

3.17×10-3 s-1

57

Spongy AuNCs

6

>780 s

2.1×10-3 s-1

58

AgNP-PG-5K

0.004

∼10 min

5.50×10-3 s-1

59

AuNPs

200

50 min

-

60

PS-NIPA-Ag

6.3

30 min

-

61

AuOh1@Ag1Au1

1

150 s

10.59×10-3 s-1

This work

Fe3O4@SiO2-Ag

Further, the AuOh1@Ag1Au1 AS was tested for the catalytic reduction of 4-NA and derivatives of 4NP. The complete reduction of 2-Cl-4-NP, 2,6-diCl-4-NP and 4-NA was observed in 700 s and the corresponding kinetic plots are shown in Figure S9 and their corresponding rate constant values are summarized in Table S2. Hence, the AuOh1@Ag1Au1 AS acts as the best catalyst for the reduction of 4-NP, 4-NA and derivatives of 4-NP.


Page 25 of 35 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


4. CONCLUSION In conclusion, we have synthesized sixteen different octahedra nanostructures including pristine AuOh1 core, Au and/or Ag shells (MLS, BLS and AS) coated on AuOh1 core using amine functionalized TPDT silane as reducing agent and as stabilizer. All the prepared Oh NPs were characterized using UV-vis absorption spectra, TEM, SAED, EDX, ICP-MS and XRD analyses. The HAADF-STEM-EDS mapping images and SEM-EDS lines scan spectra of AuOh1@Ag1Au1 AS and AuOh1@Au1@Ag1 BLS core-shell nanostructures were performed to confirm the core-shell nanostructure morphology. Moreover, the metal (Au and Ag) shell concentration was optimized for MLS, and using the optimized MLS concentration the BLS and AS nanostructures were prepared. A detailed analysis of morphology, shell thickness and elemental compositions of the pristine AuOh1 and fifteen different core-shell octahedra nanostructures has been performed. Catalytic activities of these core-shell NPs were also studied towards the chemical reduction of toxic aromatic nitro compounds to aromatic amino compounds. The deposition of alloy (Ag1Au1) shell layer over the AuOh1 (AuOh1@Ag1Au1 AS) core showed best catalytic activity when compared to pristine AuOh1 core and other core-shell NPs (MLS and BLS). The AuOh1@Ag1Au1 AS also showed a highest rate constant value among the other core-shell NPs and pristine AuOh1. The present investigation reveals that the catalytic activity of core-shell NPs mainly depends on the composition of metal shell, size and composition of MNPs.

Supporting Information (SI). The SI contains 9 pages with 9 figures and 2 tables. TEM images; plot of Au and Ag SPR in MLS against concentration of metal shells and average shell thickness; EDX spectra; SEM-EDS lines scan spectra; time-dependent absorption spectral changes; absorption spectra; metal composition analysis and rate constant plot. 25 ACS Paragon Plus Environment


ACKNOWLEDGEMENT RR acknowledges the financial support from the Council of Scientific and Industrial ResearchEmeritus Scientist Scheme (No. 21(1006)/15/EMR-II), New Delhi. We acknowledge CIC, UPE, Madurai Kamaraj University for TEM analysis.

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TOC Graphic

4-NP 4-AP

4-NP 4-AP -3

-1

-3

-1

(k = 8.04×10 s ) (k = 8.75×10 s ) AuOh1@Au2

A DT 3 TP u

AuOh1@Ag2 T ATgPD (k = 4.29×10-3 s-1) AuOh 4-NP +

4-NP -3 -1 (k = 2.30×10 4-AP s ) AgT + P&D AT u

1

+

(i)Ag 3+ /TPDT 4-AP (ii)Au /TPDT AuOh1@Ag1@Au1

T PD 3+ /T + / Au (i) Ag (ii) DT TP

3+

+

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 34 of 35

AuOh1@Ag1Au1

AuOh1@Au1@Ag1 -3

-1

-3

-1

(k = 10.59×10 s ) (k = 2.96×10 s ) 4-NP 4-NP 4-AP 4-AP


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217x185mm (150 x 150 DPI)

ACS Paragon Plus Environment

Synthesis and Catalytic Activities of Metal Shells (monolayer, bilayer

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