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Cubic Gold Nanorattles with a Solid Octahedral Core and Porous Shell as Efficient Catalyst: Immobilization and Kinetic Analysis Prem Singh, Shounak Roy, and Amit Jaiswal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07748 • Publication Date (Web): 21 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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The Journal of Physical Chemistry

Cubic Gold Nanorattles with a Solid Octahedral Core and Porous Shell as Efficient Catalyst: Immobilization and Kinetic Analysis Prem Singh §, Shounak Roy §, Amit Jaiswal * BioX Centre and School of Basic Sciences, Indian Institute of Technology Mandi, Kamand, Mandi-175005, Himachal Pradesh, India *Corresponding Author E-mail: [email protected] §

Equal Contribution

Abstract Plasmonic nanostructures having porous morphology have attracted a great deal of attention in catalysis because of high surface-to-volume ratio, better surface reactivity and availability of various structural features. Herein, we report the synthesis, immobilization and kinetic analysis of cubic gold nanorattles (AuNRT) comprising of a solid octahedral core surrounded by a thin porous cube shaped gold shell towards the reduction of environmental pollutants, p-nitrophenol and degradation of organic dyes (Congo red and Methylene blue) as model systems. Kinetic investigation of our study showed that AuNRTs are excellent catalyst compared to solid AuOCT@Ag nanocubes and gold nanospheres (AuNS) which could be attributed to the porous structure of nanorattles with three available surface- outer and inner wall and inner core for catalysis. A detailed analysis of the different kinetic and thermodynamic parameters revealed that AuNRTs showed the highest reaction rate constant, lowest activation energy, pre-

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exponential factors and entropy of activation. Further, the immobilized AuNRT into calcium alginate could retain their catalytic efficiency upto 15 cycles demonstrating high stability and reproducibility. The present system shows the ability to efficiently degrade pollutants and thus can be used for potential environmental remediation application.

1. Introduction Design and synthesis of novel metal nanostructures has always been in lime light because of the unique physico-chemical properties achieved simply by tuning the size, shape, composition and porosity of metal nanoparticles. The ease of synthesis of noble-metal nanoparticles and the ability to attain a significantly different property just by tweaking its structure has made it a protagonist in a variety of applications including, electronics, photonics, plasmonics, theranostics and catalysis. Gold nanoparticles (AuNPs) have been widely exploited for catalysis because of their stability and higher catalytic activity for many chemical and electrochemical reactions. The high catalytic activity of these metal nanoparticles is mainly attributed to the presence of surface active atoms that are usually dependent on multiple surface functionalities such as specific surface area, catalyst surface shape, sharp edges and corners for solid nanostructures, cage and porous effects in hollow and porous nanostructures.1-4 Recent reports clearly suggest that the catalytic performance of metal nanostructures can be improved by controlling the shape, edges and corners of the solid nanostructures and porosity and the ratio of core-shell thickness of the porous and hollow nanostructures respectively.4-7 Different nanostructures such as nanocages, nanoboxes, partially hollow nanoboxes, solid spherical nanoparticles,1 bimetallic nanorattles,5 multi-branched nanoflowers,8, 9 nanorods10 and 2 ACS Paragon Plus Environment

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core/porous shell structures7 have been exploited for the catalytic degradation of nitro-aromatic pollutants and organic dyes and have emerged as good environmental remediation agents. It is generally observed that smaller sized AuNPs shows better catalytic performance due to higher surface to volume ratio; however, these NPs may show diminished activity for redox reactions where the oxidation and reduction half reactions have to occur on different particles due to smaller size and thus lack of electrical conductivity between these two-half reactions may significantly lower the catalytic performance. Thus, switching from solid nanoparticles to porous nanostructures like nanocage has been shown to possess better catalysis due to the presence of porous surface walls having high electrical conductivity.1 However, the synthesis of nanocage involves a complex reaction for the synthesis of parent silver nanocube which can then be converted to nanocages through galvanic replacement reaction. Additionally, a large section of the interior of the nanocage remains hollow and thus no site is available for catalysis at the core of these nanocages. Khalavka et al., have reported the synthesis of rod shaped gold nanorattles and have shown catalytic reduction of p-nitrophenol using them.11 However, detailed investigation regarding the kinetics, thermodynamic parameters and catalytic efficiency was not performed by the authors. Decrease in rate of the reaction even in the presence of metal nanoparticles as catalyst is often reported which occurs due to the aggregation of these metal nanoparticles in solution. This aggregation results from the presence of highly active surface atoms that are distributed on the catalytic surface of the nanoparticles. Upon aggregation, the total number of surface active atoms available for catalysis gets reduced, ultimately leading to a decrease or even complete loss of the catalytic activity of metal nanoparticles.12 Apart from this, the colloidal solution has no reusability or recyclability in catalytic reactions which restrict metal nanoparticles from commercialization. To overcome the problems associated with aggregation



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and cost, immobilized metal nanoparticles have attracted great deal of attention in the field of catalysis. Many substrates such as dendrimers, oleylamines, PVP, sodium alginate, chitosan and inorganic compounds have been exploited for immobilizing the metal nanoparticles and have showed recyclability or reusability for many cycles.12-23

In the present work, we report the synthesis of gold nanorattles (AuNRT) comprising of an octahedral core surrounded by a thin porous gold shell through galvanic replacement of a silver nanocube containing an octahedral gold core (AuOCT@Ag). We demonstrated the efficient catalytic activity of AuNRT in comparison with AuOCT@Ag nanocubes and gold nanospheres (AuNS) of similar size for the reduction of environmental pollutants like pnitrophenol and organic dyes (Congo red and Methylene blue) as model systems (scheme 1). Kinetic data of our study indicates that AuNRTs are catalytically more active than AuOCT@Ag nanocubes and gold nanospheres. Further, we immobilized the AuNRT into calcium alginate beads and observed that the catalytic activity of the nanorattles was retained even after 15 cycles. The present system shows the ability to degrade pollutants efficiently within minutes, can be recycled and thus can suitably be used for environmental remediation.


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Scheme 1. Schematic showing the catalytic reduction of nitroaromatic pollutant (p-NP) and organic dyes (Methylene blue and Congo red) using gold nanorattles as catalyst

2. Experimental Section: 2.1 Materials Chloroauric acid (HAuCl4), silver nitrate (AgNO3), hexadecyltrimethylammonium chloride (CTAC), hexadecyltrimethylammonium bromide (CTAB), sodium borohydride (NaBH4), Lascorbic acid, sodium chloride (NaCl), poly (allyl amine) hydrochloride (PAH) (Mw: 17,500 g/mol), poly (vinyl pyrrolidone) (Mw = 29 000 g/mol), p-nitrophenol (p-NP), methylene blue hydrate, Congo red, sodium alginate and calcium chloride dihydrate. Calcium chloride dihydrate was purchased from Merck. Methylene blue hydrate, Congo red and CTAC were procured from



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TCI and rest of the chemicals procured from Sigma-Aldrich. Nanopure water (18.2 MΩ cm) was used for all synthesis and experiments.

2.2 Instrumentation The UV-Vis spectra of the synthesized nanoparticles along with the time-dependent kinetic spectra during catalysis were recorded with a UV-1800 spectrophotometer (Shimadzu, Japan). The morphology of the synthesized nanoparticles was characterized using transmission electron microscopy (TEM, FP 5022/22-Tecnai G2 20 S-TWIN, FEI) and scanning electron microscopy (SEM, Nova Nano SEM-450, FEI). The samples for TEM characterization were prepared by drop-casting around 5 µL of the samples on a carbon coated copper grid. For SEM, samples were drop casted on pre-cleaned silicon wafers.

2.3 Synthesis of gold octahedron (AuOCT) Synthesis procedure for gold octahedron was adapted from literature with some modifications.24 Seed was prepared by mixing of 0.514 mL of HAuCl4 (4.86 mM) in 7.5 mL of 0.1 M aqueous CTAB solution followed by addition of 1.86 ml of nanopure water (18.2 MΩ cm). To the above prepared mixture, 0.6 ml of ice-cold NaBH4 (10 mM) was injected rapidly under magnetic stirring. Seed solution was stored at a room temperature for 3 hours and then diluted 100 times for the further growth of AuOCT. Under magnetic stirring, 0.6 mL of Ascorbic acid (0.1 M) was added to the mixture of 7.59 mL of nanopure water, 82.3 µL of HAuCl4 (4.86 mM) and 1.6 ml CTAB (0.1 M). When the above prepared mixture turned colourless, 120 µL of the diluted seed


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was added in the growth solution followed by vigorous stirring for 30 s and the final solution was then kept at room temperature undisturbed for about 12 hrs.

2.4 Synthesis of gold octahedron @ silver nanocubes (AuOCT@Ag) To the 5 mL of as prepared AuOCT growth solution, 0.5 mL of AgNO3 (10 mM) and 2 mL of ascorbic acid (0.1 M) were added. Solution was stirred for ~30 s and kept in an oil bath at 60 0C for 20 hours. 5 mL of as synthesized AuOCT@Ag nanocubes were centrifuged at 12,500 rpm for 30 minutes and the pellet was resuspended in 5 mL of PAH solution (6 mg/mL in 6 mM NaCl). Above formed suspension was sonicated for 1 h followed by centrifugation at 10,000 rpm for 15 minutes. After centrifugation, the pellet was resuspended in PVP solution (90mM) and used for the further synthesis of AuNRTs.

2.5 Synthesis of gold nanorattles (AuNRT) Galvanic replacement reaction was used for the synthesis of AuNRTs. For galvanic replacement, aqueous solution of HAuCl4 (0.5 mM) was added at a rate of 0.25 mL/min to the above processed AuOCT@Ag nanoparticles at a moderate boiling condition. Appearance of dark blue colour of the solution indicated the formation of AuNRTs. The as synthesized AuNRT solution was left undisturbed for 2-3 hours for the precipitation of AgCl. The solution was then processed by centrifuging at 4000 rpm for 1 hour and then redispersed in equal amount of nanopure water for further characterization and catalysis experiment.



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2.6 Synthesis of gold nanospheres (AuNS) For the synthesis of gold nanospheres, a seed mediated growth method reported in literature was followed with some modifications.25 Initially CTAC stabilized 10 nm spheres was synthesized, which were further used for the synthesis of higher sized AuNS of around 60 nm. Seed solution was prepared by mixing 5 mL of HAuCl4 (0.5 mM) in 5 mL of 0.1 M aqueous CTAB solution followed by rapid injection of 0.6 mL of ice-cold NaBH4 (10 mM) under vigorous stirring. This resulted in the formation of brown coloured seed solution which was kept at a room temperature for 3 hours and then used as such for the further synthesis of 10 nm spheres. Under vigorous stirring, 1.5 mL of ascorbic acid (0.1 M) was added in to the solution containing 2 mL of CTAC (0.2 M) and 2 mL of HAuCL4 (0.5 mM) solution followed by the addition of 50 µL of seed solution. The reaction mixture was kept at room temperature for 15 minutes. After 15 minutes, 1 mL of 10 nm AuNS was washed with water once and then redispersed in 1 mL of 20 mM of CTAC solution which was then used for the synthesis of 60 nm sized gold nanospheres. 15 µL of 10 nm AuNS seed solution was introduced in to the solution containing 2 mL of CTAC (0.2 M) and 130 µL of Ascorbic acid (10 mM) followed by the dropwise addition of 2 ml HAuCl4 (0.5 mM) using a syringe pump at a flow rate of 2 mL/hr. After complete addition of aqueous gold solution, reaction mixture was kept at room temperature for 10 minutes and then centrifuged at 4000 rpm for 1 h followed by redispersing in nanopure water for further characterization and catalytic experiments.

2.7 Preparation of AuNRT immobilized calcium alginate beads


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Calcium chloride solution 3 % (w/v) and sodium alginate 2.5 % (w/v) solution containing AuNRTs (1.56 x 1010 particles/mL) were used for the preparation of AuNRT immobilized calcium alginate beads. 5 mL of plastic syringe with a needle of 0.45 x 13 mm size was used for the dropwise addition of 5 mL AuNRTs-sodium alginate solution into 10 mL of calcium chloride solution under magnetic stirring for the formation of beads. Before further use of the beads in catalytic reactions, these were stored in the calcium chloride solution for 3 hours for hardening. In a same manner, bare calcium alginate beads were also prepared as a control for the catalytic reaction.

2.8 Catalytic reduction study of p-nitrophenol (p-NP) To demonstrate the catalytic efficiency of AuNRTs, AuOCT@Ag nanocubes and AuNS of similar size, we used hydrogenated reduction of p-nitrophenol by NaBH4 as a model system. Nanopure water was mixed with 1 mL aqueous solution of p-nitrophenol (0.42 mM) in a 3 mL quartz cuvette followed by the addition of gold based nanocatalysts. After 2 min incubation at 250C, 1.26 mL of freshly prepared ice-cold NaBH4 (100 mM) was added to the reaction mixture. In this process, the final concentration of p-nitrophenol and NaBH4 was 0.14 mM and 42 mM respectively. The nanocatalyst concentration was maintained at 3.8 ×109 particles/ mL for all the three nanocatalysts. Nanopure water was used to adjust the volume of reaction mixture up to 3 mL. The reduction process was monitored using a UV-Vis spectrophotometer by measuring the decrease in extinction intensity of p-NP at λ = 400 nm with time. Rate of the catalytic reduction of p-nitrophenol to p-aminophenol was measured by plotting curves of extinction spectra versus time at λ = 400 nm. This same procedure was followed at 300C, 400C and 450C to calculate the different kinetic parameters. 9 ACS Paragon Plus Environment


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2.9 Dye degradation study Same procedure as explained in the case of p-nitrophenol was also followed for Congo red and Methylene blue degradation except that the final concentration of dyes and NaBH4 were maintained at 0.05 mM and 5 mM respectively. Decrease of extinction intensity with time was measured at λ = 498 nm and 667 nm for CR and MB respectively.

2.10 Recyclability or reusability of AuNRT immobilized calcium alginate beads To demonstrate the recyclability of the AuNRT immobilized calcium alginate beads, 1.8 g of calcium alginate beads containing 1 × 1010 particles/mL of AuNRTs was added in a column containing a mixture of 3 mL of p-nitrophenol (0.28 mM) and 3 mL of NaBH4 (100 mM). The final concentration of p-nitrophenol and NaBH4 was maintained at 0.14 mM and 50 mM respectively. Time dependent catalytic reduction was measured by taking the extinction spectra of the reaction mixture at an interval of 2 minutes from 350 nm to 550 nm by using UV-vis spectrophotometer. After completion of one catalytic cycle, the colourless solution was decanted off from the reaction column followed by washing of the beads with nanopure water. Then the same procedure was followed for the successive cycles with a new portion of NaBH4 and pnitrophenol. For methylene blue degradation, same procedure as p-NP reduction was followed except that the final concentration of methylene blue was maintained at 0.05 mM and the extinction spectra was measured from 500 nm to 800 nm. 10 ACS Paragon Plus Environment

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3. Results and Discussion 3.1 Synthesis and characterization of different shaped gold nanocatalysts A seed-mediated growth technique was adopted for the synthesis of Au nanorattles through a three-step process. The first step involved the synthesis of Au nano-octahedra (AuOCT), which was then used as a core in the second step to grow a thin silver shell on its surface thereby forming bimetallic AuOCT@Ag nanocubes. In the final step, the thin silver shell of these AuOCT@Ag nanocubes were replaced by a porous gold layer through galvanic replacement reaction resulting in the formation of Au nanorattles (AuNRT). Figure 1 shows the TEM and SEM images of the synthesized nanostructures. The synthesized AuOCT@Ag nanocubes were found to be monodispersed having an edge length of 47.48 ± 5.1 nm (n>100) as measured from the TEM images (Figure 1 A) and Figure S1 (Supplementary information). A prominent solid core/shell structure depicting an octahedral core with a cubic shell could be clearly seen in the TEM images. Figure 1E shows the SEM image of AuOCT@Ag nanocubes exhibiting monodisperse uniform structures with high yield. The TEM images of AuNRT (Figure 1 B and C) clearly show the presence of a thin porous wall surrounding an intact octahedral core, with pores clearly visible at the four corners of the cube. The edge length of the synthesized AuNRTs were measured to be 55.35 ± 5.2 nm (n>100) along with a wall thickness of around 7.30 ± 1.7 nm as estimated from the TEM images (Figure 1 B). The size distribution measured from the TEM image of different nanostructures are shown in Figure S1 (Supplementary information). The porous structure of AuNRTs is also clearly visible from the SEM images which can be 11 ACS Paragon Plus Environment


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clearly distinguished from the smooth appearance of the AuOCT@Ag nanoparticles (Figure 1 E and F). In addition to these nanostructures, solid gold nanospheres (AuNS) were also synthesized using a seed-mediated method as mentioned in the experimental section. Figure 1 D and 1 G shows the TEM and SEM images of the synthesized AuNS respectively, having a mean size of around 61.07 ± 4.5 nm (n>100) as measured from the TEM images (Figure 1 D).

Figure 1.TEM images of (A) AuOCT@Ag nanocubes, (B & C) Au nanorattles (AuNRT), (D) Au nanospheres (AuNS). SEM images of (E) AuOCT@Ag nanocubes, (F) Au nanorattles (AuNRT), (G) Au nanospheres (AuNS). Scale bars correspond to (A) 50 nm, (B) 100 nm, (C) 20 nm, (D) 50 nm, (E) 500 nm, (F) 400 nm and (G) 500 nm.

Figure 2 shows the normalized extinction spectra of the synthesized AuOCT, AuOCT@Ag nanocubes, AuNRTs. AuOCT shows a single localized surface plasmon resonance (LSPR) peak at 556 nm. Whereas, three LSPR peaks at 404 nm, 486 nm and 586 nm were observed for AuOCT@Ag nanocubes. The peaks at 404 nm and 486 nm corresponds to the outer 12 ACS Paragon Plus Environment

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cubic Ag shell and the peak at 586 nm is attributed to the inner Au octahedral core, which shows a red shift in comparison to the native Au octahedron NPs due to plasmonic coupling between the outer Ag and inner Au layers. With Galvanic replacement, the LSPR peaks at 404 nm and 486 nm corresponding to the Ag shell disappeared with the appearance of a new single peak at a higher wavelength (698 nm), thereby validating the replacement of silver shell by gold. The AuNRTs display a major broad peak at 698 nm along with a shoulder at around 540 nm which corresponds to the inner octahedral core. The broadness of the peak at 698 nm depicts the porosity of the structure. The appearance of the colloidal solution of AuOCT, AuOCT@Ag nanocubes and AuNRTs in white light is shown in the inset of Figure 2. The extinction spectra of the AuNSs showed a single prominent peak at 542 nm (Figure S2, Supporting information).

Figure 2. Normalized extinction spectra of the synthesized AuOCT, AuOCT@Ag and AuNRTs. The insets depict the color of the colloidal solutions of AuOCT, AuOCT@Ag and AuNRTs.



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3.2 Evaluation of catalytic performance of synthesized gold nanorattles towards pnitrophenol reduction In order to evaluate the catalytic performance of the synthesized AuNRT, we used the reduction of p-nitrophenol to p-aminophenol with NaBH4 as a model system. The rationale behind choosing this reaction as model system is (a) 4-Nitrophenol or p-nitrophenol (p-NP) is a major environmental pollutant which causes serious health hazards due to its mutagenic and carcinogenic potential in humans and thus listed on the United States Environmental Protection Agency’s (EPA) list of 126 priority pollutants26 and (b) the kinetics of this reaction can be easily monitored using a UV-Visible spectrophotometer. p-NP solution shows strong absorption at 317 nm, which gets shifted to 400 nm upon addition of NaBH4 due to the immediate formation of pnitrophenolate ions.1,7,10 In the presence of a catalyst, the intensity of the absorption peak at 400 nm decreases gradually with time showing the reduction of p-NP, along with the appearance of a new peak at 315 nm corresponding to the formation of p-aminophenol (p-AP).1,7,10 We monitored the decrease in the intensity of absorption peak at 400 nm with time as a parameter to evaluate the catalytic performance of the synthesized Au nanocatalysts. The amount of p-NP and NaBH4 used in each of the reaction was held constant at 0.14 mM and 42 mM respectively.


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Figure 3. Time dependent absorption spectra showing the reduction of p-NP to p-AP using AuNRTs as catalysts.



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Figure 4. (a-d) Normalized absorbance (A/A0) at 400 nm showing the reduction of p-NP to p-AP with time using AuNRT, AuOCT@Ag and AuNS as catalysts at four different temperatures: (b) 250C, (c) 300C, (d) 400C and (e) 450C. For all experiments, the final concentration of p-NP, NaBH4 and nanocatalysts were maintained at 0.14 mM, 42 mM and 3.8 × 109 particles/ml respectively.

Figure 3 shows the UV-Vis absorption spectra for the catalytic reduction reaction of p-NP to pAP at different reaction times in the presence of AuNRTs as catalyst. The disappearance of the p-NP peak at 400 nm and appearance of p-AP peak at 315 nm with time can be clearly observed in the spectrum. Figure 4 (a) shows a plot of normalized extinction intensity (A/A0) at 400 nm versus time (t) for the reduction reaction carried out at 250C using AuNRT, AuOCT@Ag and AuNS as catalysts. In the absence of any of the catalysts, the reduction did not occur and no decrease in the extinction intensity at 400 nm was observed with time as can be seen from the figure. However, in the presence of any of three AuNPs used in the present study as catalysts, significant decrease in the extinction intensity at 400 nm was observed with time. This result demonstrated that all the three nanostructures were capable of catalysing the reduction of p-NP to p-AP in the presence of NaBH4. As can be seen from the plot (Figure 4 a), AuNRTs were the fastest in catalysing the reaction as compared to solid Au@Ag nanocubes and Au nanospheres (AuNS) of similar sizes and concentration. In addition to this, it was also observed from the plot that the reduction was not initiated immediately after addition of catalysts in the reaction solution, but was characterized by the presence of an induction period (tind) before the beginning of the actual reduction in case of all the nanocatalysts, irrespective of the morphology or structure. This indicated that a certain period of time was required for the reactant species (p-NP) to get adsorbed on the surface of the nanocatalysts before the reduction reaction could be initiated. This time is referred to as the induction period (tind) and was found to be shorter for the 17 ACS Paragon Plus Environment


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porous AuNRTs as compared to solid Au@Ag nanocubes (Table1). Solid Au nanospheres showed almost similar induction time as that of AuNRT. A similar pattern with respect to the induction (tind) was observed at different temperatures of 30 0C, 40 0C and 45 0C for all the three Au based nanocatalysts as shown in Figure 4 (b, c, d). At all temperatures, AuNRT exhibited the fastest catalytic reduction of p-NP to p-AP as compared to Au@Ag nanocubes and AuNS. Additionally, the tind for each catalyst decreased with increase in temperature thereby demonstrating the temperature dependent increase in catalytic activity of the porous and solid nanoparticles which can be attributed to the increase in collision frequency between the reactat species and nanocatalysts with rise in temperature.

3.3 Determination of kinetic parameters of nanorattle catalysed p-nitrophenol reduction Next, the different kinetic parameters such as average reaction rate constant (k), activation energy (Ea), pre-exponential factors (A) and entropy of activation (∆S) for p-NP reduction catalysed by each of the three catalysts were determined from the respective plots. The concentration of NaBH4 used in the reaction was 300 times more than that of p-NP concentration. Hence, we considered NaBH4 concentration as constant during the entire course of the reaction and assumed that the reaction followed pseudo-first order kinetics with respect to the concentration of p-NP. The average reaction rate constant (k) for each of the three catalysts at different temperatures (25 0C, 30 0C, 40 0C and 45 0C) were calculated from the slope of the plot between logarithm of normalized extinction intensity (-ln (A/A0)) at 400 nm versus reaction time as shown in Figure 5. Data sets from three independent experiments were used to calculate k. The approximate linear relationship between (-ln (A/A0)) versus time (excluding the tind region) as observed from our experimental data validates the assumption that the catalytic reduction of p18 ACS Paragon Plus Environment

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NP under the present experimental conditions follow pseudo-first order kinetics. The obtained values of k (min-1) and tind (min) for the three catalysts at four different temperatures is summarized in Table 1. The data clearly demonstrates that the rate of the reaction catalysed by porous AuNRTs is highest at each temperature studied as compared to the solid Au@Ag nanocubes and AuNS. The k values at different temperatures for each of the catalyst were then used to calculate the activation energy (Ea) and pre-exponential factors (A) according to the Arrhenius equation ln k = ln A – Ea/RT (where R is the universal gas constant). Figure 6 (a) shows the plot between ln k versus 1000/T. Activation energy (Ea) and pre-exponential factors (A) for each of the catalysts were calculated from the slope (-Ea/R) of the linear fit and the intercept ln A, respectively. As seen in Figure 6 (a), AuNRTs showed the lowest activation energy (29.87 ± 0.28 kJ/mol) and pre-exponential factor (5.44 × 103 min-1) as compared to Au@Ag nanocubes (38.66 ± 0.19 kJ/mol; 1.02 × 105 min-1) and AuNS (41.39 ± 0.40 kJ/mol; 1.9 × 105 min-1), thus further corroborating a higher catalytic activity for the nanorattles. In addition to activation energy and pre-exponential factors, entropy of activation (∆S) for the catalysed reaction was also determined using the equation ln A = ∆S/R. AuNRT again showed the lowest ∆S value (71.52 ± 7.80 J/mol.K) with nanospheres showing the highest ∆S value (101.09 ± 10.89 J/mol.K) and Au@Ag nanocubes having ∆S values (95.94 ± 5.21 J/mol.K) in between of nanorattles and nanospheres. The obtained values of Ea (kJ/mol), A (min-1) and ∆S (J/mol.K) for the three catalysts is summarized in Table 2.



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Figure 5. Plots of –ln (A/A0) versus time for the catalytic reduction of p-NP using AuNRT, AuOCT@Ag and AuNS as catalysts at four different temperatures: (a) 250C, (b) 300C, (c) 400C and (d) 450C. The reaction rate constant (k) and induction period (tind) were calculated from these plots.

Table 1. Reaction rate constant (k) and induction period (tind) for AuNRT, AuOCT@Ag and AuNS at four different temperatures CATALYST

250 C k (min-1)

300 C tind

400 C

k (min-1)

tind

(min) AuNRT

1.85 ± (1.4 × 10-4 )

1.08 ±

(min) 2.43 ± (8.6 × 10-5 )

0.78 ±

0.04 Au@Ag

1.01 ± (1.5 × 10-4)

3.67 ±

1.37±(8.8 × 10-5)

3.07 ±

0.63 ± (3.8 × 10-5)

1.01 ±

3.26 ± (4.7 × 10-4 )

0.77 ±

0.02

k (min-1)

0.68 ±

2.12 ± (1.7 × 10-4 )

1.76 ±

4.11±(1.9 × 10-4)

0.03

0.44 ± 0.02

0.45 ± 0.02

2.77±(2.1 × 10-4 )

0.08 1.52±(1.8 × 10-4 )

tind (min)

0.15

0.03 0.83 ± (4.6 × 10-5 )

tind (min)

0.03

0.03 AuNS

k (min-1)

450 C

1.46 ± 0.03

1.72±(1.7 × 10-4 )

0.43 ± 0.04

Figure 6 (b) is a plot between ln A and Ea showing a linear relationship between these two kinetic parameters which clearly demonstrates that our catalytic system follows the compensation effect, where the decrease in the activation energy of a catalyst is accompanied by increase in the pre-exponential factor which is related to the turnover frequency of the catalyst. The compensation effect is mostly observed in heterogeneous catalysis and in some of the homogeneous catalysis, however the exact mechanism for this is still an enigma.



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Figure 6. (a) The Arrhenius plot of ln k versus 1000/T for AuNRT, AuOCT@Ag and AuNS catalysed reactions. Activation energy (Ea) and pre-exponential factors (A) for each of the catalysts were calculated from the slope (-Ea/R) and the intercept (ln A) of the plot, respectively. (b) Plot between ln A and Ea for demonstrating the compensation effect of the catalysts.


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Table 2. Activation energy (Ea), pre-exponential factor (A) and entropy of activation (∆S) for AuNRT, AuOCT@Ag and AuNS

CATALYST

Ea (kJ/mol)

A (min-1)

ΔS (J/mol.K)

AuNRT

29.87 ± 0.28

5.44 × 103

71.52 ± 7.80

AuOCT@Ag

38.66 ± 0.19

1.02 × 105

95.94 ± 5.21

AuNS

41.39 ± 0.40

1.9 × 105

101.09 ± 10.89

Thus, from the above observations and considering the values of different kinetic parameters as obtained for the NaBH4 mediated reduction of p-NP to p-AP using AuNRT, Au@Ag nanocubes and AuNS as catalysts, it is quite evident and reasonable to conclude that gold nanorattles (AuNRT) having an octahedral core surrounded by a thin porous shell showed the highest catalytic activity in terms of fastest reduction time, shorter tind, highest k and lowest Ea. Nanocatalysis is a surface dependent process whose efficiency greatly depends on the surface area available for the reacting materials to interact and the availability of chemically unsaturated active atoms.4,5,7 Presence of sharp corners, edges and rough structure increases the number of unsatisfied chemical bonds, leading to increase in the efficiency of the catalytic reaction.4,5,7 Additionally, in contrast to solid nanoparticles, porous hollow nanostructures provide larger surface area for catalysis due to porous structure where the presence of rough pores increases the surface to volume ratio and the interior walls of the hollow structure can also participate in the catalytic reaction.4,6,7 Further, the inner walls of the hollow nanoparticles can show better catalytic activity as it will be poorly passivated with surface capping agent and will have rougher structure. The confinement effect of hollow nanostructures also increases the steady state concentration of the species in the rate-determining step of the reaction and is termed as the cage 23 ACS Paragon Plus Environment


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effect. All these factors add up to contribute towards the better efficacy of nanorattles for catalytic reaction. The cube shaped outer shell with porous structure provides the enhanced surface area and unsaturated chemical bonds and the inner walls of the nanorattles provides additional active surface area for catalysis. On top of this, the solid octahedron core inside the porous cubic cage can further provide additional active surface area for catalysis. Thus, the efficient catalytic performance of the synthesized gold nanorattles can be attributed to these structural features which resulted in lower activation energy and higher rate constant for the reduction of p-NP to p-AP as compared to solid nanostructures.

3.4 Evaluation of catalytic performance of synthesized gold nanorattles towards organic dye degradation In addition to nitroaromatic pollutants, textiles, foods, drugs and cosmetic industries extensively use organic synthetic dyes in their manufacturing processes. Wastewater from these industries contain traces of these dyes which are difficult to eliminate through the conventional effluent treatment plants, thus leading to environmental pollution and potential hazards to living organisms.27 Organic dyes are the major group of the synthetic dyes characterized by the presence of azo (-N=N-) and alkene moieties (-C=C-) along with heterocyclic and aromatic rings in their structure. Therefore, it is necessary to cleave -C=C-, -N=N- bonds and aromatic rings for decolourization or conversion of toxic dyes into non-toxic form.28 The catalytic efficiency of AuNRTs was further investigated by studying the decolorization reaction of two very common organic dye - Methylene blue (MB) and Congo Red (CR) using NaBH4 as a reducing agent at room temperature. MB is a cationic dye which shows a strong absorption peak at 664 nm in aqueous solution.30 The catalytic degradation or decolorization of MB into colourless Leuco MB 24 ACS Paragon Plus Environment

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using AuNRT as catalyst and NaBH4 as reducing agent was monitored using a UV-Vis spectrophotometer in the wavelength range from 450 nm to 800 nm. Fig 7 (a) depicts the UV-Vis absorption spectra of catalytic degradation of MB using AuNRTs as catalyst, showing the timedependent decrease in the absorption intensity at 664 nm finally leading to the disappearance of the peak, thus demonstrating complete degradation. The normalized absorption spectra (A/A0) at 664 nm versus Time (min) for the MB both in presence and in absence of AuNRT is shown in Fig. 7(b). This clearly shows that in the absence of the catalyst, almost no degradation of MB occurred. However, with the addition of AuNRTs in the reaction solution the reaction proceeded very fast and complete degradation of the dye was observed within 4 min. The pseudo-first order rate constant (k) for this reaction was calculated from the slope of the linear plot between – ln (A/A0) vs Time (min) and was found to be 1.071 min-1(Figure 7 c).



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Figure 7. Methylene blue (MB) degradation kinetics. (a) Time dependent absorption spectra showing the degradation of MB using AuNRTs as catalysts. (b) Normalized absorbance (A/A0) at 664 nm showing the degradation of MB with time using AuNRT. (c) Plot of –ln (A/A0) versus time for the catalytic reduction of MB using AuNRT. The reaction rate constant (k) was calculated from the slope of the plot.


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Next, we also demonstrated the ability of the AuNRT to degrade the organic dye, Congo red. CR is an anionic azo dye containing N=N in its structure which makes the degradation of this dye more problematic using conventional techniques. An aqueous solution of CR shows a strong absorption peak at 498 nm and this was used in our study to monitor its degradation.8, 30,31 We observed a time dependent decrease in the absorbance of CR with complete disappearance of peak (498 nm) within 15 minutes (Figure 8 a). The normalized absorption spectra (A/A0) at 498 nm versus Time (min) clearly revealed that in the absence of the catalyst, no degradation of CR occurred (Figure 8 b). However, with the addition of AuNRTs in the reaction solution, the reaction initially proceeded with an induction period (tind) of around 8.2 mins and complete degradation was observed within 15 mins. The appearance of this tind in case of CR degradation can be explained by taking into consideration the overall surface charge of both the dye and the catalyst. The PVP-capped AuNRTs used as catalyst possess a net negative charge on its surface32 which makes the adsorption of the anionic CR molecules on the surface of the catalyst difficult, thereby leading to a delay in the initiation of the actual degradation process and thus an induction period(tind) is observed. The pseudo-first order rate constant (k) for this reaction was calculated from the slope of the linear plot between – ln (A/A0) vs Time (min) (Figure 8 c) and was found to be 0.362 min-1.



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Figure 8. Congo Red (CR) degradation kinetics. (a) Time dependent absorption spectra showing the degradation of CR using AuNRTs as catalysts. (b) Normalized absorbance (A/A0) at 498 nm showing the degradation of CR with time using AuNRT. (c) Plot of –ln (A/A0) versus time for the catalytic reduction of CR using AuNRT. The reaction rate constant (k) was calculated from the slope of the plot.


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The mechanism of organic dye degradation using metal nanoparticles as catalyst in the presence of NaBH4 as a reducing agent have been well discussed in literature.8,30,31,33 The reduction occurs via an electron transfer process from NaBH4 to dye molecules through the help of metal nanoparticles. First, the dye molecules get adsorbed on the surface of the metal nanoparticles followed by the transfer of electrons from the BH4- ions present in the reaction solution to the metal nanoparticles which readily accept the electrons. These electrons then get relayed to the dye molecules adsorbed on the nanoparticle surface which finally cause interruption of the double bonds present in the conjugated system of dyes like MB and cleavage of the azo bond (N=N) in azo dyes like CR, resulting in their degradation into colorless non-toxic products.

3.5 Immobilization of AuNRT into calcium alginate beads and evaluation of its catalytic performance and reusability towards p-nitrophenol reduction Immobilization of catalysts in support matrices such as polymers allow them to be reused or recycled multiple times for catalyzing the same reaction without affecting their overall catalytic performance.17,18,21,23 This is very important and economical from an industrial point of view where recycling of a single batch of immobilized catalysts for catalyzing the same reaction can greatly reduce the overall cost of the production. In the present study, calcium alginate was chosen for the purpose of immobilization of AuNRTs because of its high stability, very low cost, ease of preparation by simple gelation of a sodium alginate solution in the presence of a divalent cation such as calcium, ease of recovery and separation of the catalyst immobilized beads from the reactants and products and environmental friendly properties such as no toxicity and biodegradability.18 Though calcium alginate beads were demonstrated to be used for 29 ACS Paragon Plus Environment


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immobilizing enzymes for industrial applications, however, in the field of metal nanoparticle catalysis, there are very few reports available which have exploited these properties of calcium alginate or alginate gels for immobilization and recycling of heterogeneous metal nanocatalysts18,34-36 Thus, to evaluate the recyclability of AuNRTs for catalyzing the reduction of p-NP to p-AP, we immobilized the AuNRTs in calcium alginate beads and monitored the reduction process spectrophotometrically at different time points. The schematic for the use of immobilized AuNRT as recyclable catalysts is demonstrated in Scheme 2. The digital and SEM image of the immobilized beads is shown in Figure S4 and S5 (Supplementary information) We observed that the AuNRT immobilized calcium alginate beads could be recycled upto 15 cycles without any change in the catalytic performance and compromising the stability of the beads. Immobilized catalysts often suffer from the limitation of surface poisoning due to the adsorption of reactant and product species on the surface of the catalysts with each cycle. This leads to a reduction in the net surface area available for catalysis and ultimately results in a decrease in the catalytic rate as well as % conversion efficiency of the catalysts with each successive cycle. However, in this present study, we observed that the immobilized beads could efficiently carry out the reduction process for 15 consecutive cycles without any significant reduction in its catalytic efficiency as can be seen in terms of % catalytic conversion of p-NP to p-AP in each cycle and also the time taken by the immobilized catalysts in each cycle for carrying out complete reduction. Catalytic conversion percent for each of the 15 cycles was calculated using the following equation: % catalytic conversion = (1 – At/A0) x 100 where A0 is the absorbance value at time t = 0 and At is the absorbance value at time t.


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Scheme 2. Schematic showing the recyclability of gold nanorattle immobilized calcium alginate beads for catalytic reduction of p-NP

Figure 9(a) shows that each cycle took around 14 mins on an average to complete the reduction process without any delay in successive cycles, which clearly depicts that there was no decrease in the rate of the reaction with recycling. Even after 15 cycles, the morphology of beads were intact without any sign of swelling and rupture and also the blue color of the AuNRTs were retained in the beads thereby diminishing the chances of catalyst leaching. This showed that the 31 ACS Paragon Plus Environment


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beads could further be reused for many more cycles. Figure 9 (b) is a plot of the % catalytic conversion observed in each cycle which clearly demonstrates that more than 98% catalytic conversion was observed in each successive cycle. As a control experiment, we also performed the p-NP reduction reaction using bare calcium alginate beads (without AuNRT) to check whether calcium alginate as such showed any reducing/catalytic activity. It was observed that with time there was a decrease in the extinction intensity of the nitrophenolate ion peak at 400 nm, but there was no appearance of a new peak at 315 nm thereby showing that the decrease in the intensity observed was not due to the reduction of p-NP to p-AP, but was due to the absorption of the nitrophenolate ions by the porous calcium alginate beads (Figure S3, Supplementary information). A similar observation was reported by Saha et al.18 and Sharma et al.37 in their studies. Thus, the immobilization of AuNRTs in calcium alginate beads was successful in retaining the overall catalytic performance of AuNRTs upto 15 cycles towards the reduction of p-NP, thereby showing the high stability, recyclability and usefulness of these immobilized AuNRT beads for industrial applications.

Figure 9. Recyclability or reusability of AuNRT immobilized calcium alginate beads. (a) The plot shows the time taken in each cycle by the AuNRT immobilized calcium alginate beads for reduction of p32 ACS Paragon Plus Environment

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NP to p-AP. (b)% catalytic conversion of p-NP to p-AP in each cycle using AuNRT immobilized calcium alginate beads.

4. Conclusion In conclusion, we have demonstrated the applicability of gold nanorattles with a cubic porous gold shell and solid octahedron core as novel catalyst for the degradation of environmental pollutants p-nitrophenol and organic dyes. The kinetic analysis of the catalytic reaction revealed that the AuNRT are much efficient than solid Au nanospheres and AuOCT@Ag nanocubes with a high rate constant and low activation energy. The better performance of the AuNRT as catalyst could be attributed to the presence of porous cubic hollow structure with a solid octahedron core where the outer and inner walls of the cubic porous Au shell and pores along with the surface of the inner Au octahedron core can contribute towards catalysis. Compared to other hollow structure like nanocages, AuNRT are easy to synthesize and also have an additional core surface available for catalysis. Further, these nanorattle structure can easily be immobilized into calcium alginate beads and remain stable for at least 15 rounds of catalysis without affecting the efficiency of the reaction. Considering the ease of synthesis of AuNRT with different available active surfaces, relatively higher abundance of Au with respect to other noble metals and stability and recyclability, the present nanorattles based catalyst can find widespread applicability in industry and environmental remediation.

ASSOCIATED CONTENT Supporting Information Description Size distribution of the synthesized nanoparticle. Normalized extinction spectra of gold nanospheres (AuNS). Time dependent absorption spectra of p-NP reduction using bare calcium alginate beads. Digital photographs of bare and AuNRT immobilized calcium alginate beads. 33 ACS Paragon Plus Environment


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SEM image of calcium alginate bead. This information is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author *E-mail ID : [email protected]

Author Contributions §

These authors contributed equally

ACKNOWLEDGEMENT We gratefully acknowledge the financial support from Indian Institute of Technology Mandi, Department of Biotechnology (DBT), Government of India, under project number: BT/PR14749/NNT/28/954/2015 and Department of Atomic Energy – Board of Research in Nuclear Sciences (DAE-BRNS) under the project number 37(2)/20/29/2016-BRNS/37260. We are also thankful to the Advanced Materials Research Centre (AMRC) and BioX Centre, IIT Mandi for laboratory and the characterization facilities.

References: (1) Zeng, J.; Zhang, Q.; Chen, J.; Xia, Y. A Comparison Study of The Catalytic Properties of Au-Based Nanocages, Nanoboxes, and Nanoparticles. Nano Letters 2010, 10, 30-35.


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