Organization of Pd Nanoparticles into Fractal Patterns for Highly

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Organization of Pd Nanoparticles into Fractal Patterns for Highly Enhanced Catalytic Activity and Anode Material for Direct Borohydride Fuel Cells Applications Nainsi Saxena, Neeli V. S. Praneeth, K. Jagajjanani Rao, and Santanu Paria ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00211 • Publication Date (Web): 26 Apr 2018 Downloaded from http://pubs.acs.org on April 26, 2018

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ACS Applied Energy Materials

Organization of Pd Nanoparticles into Fractal Patterns for Highly Enhanced Catalytic Activity and Anode Material for Direct Borohydride Fuel Cells Applications Nainsi Saxena, N. V. S. Praneeth, K. Jagajjanani Rao, and Santanu Paria* Interfaces and Nanomaterials Laboratory, Department of Chemical Engineering, National Institute of Technology, Rourkela-769008, Orissa, India

*

To

whom correspondence

should be addressed. E–mail: [email protected] or

[email protected], Tel: +91 661 246 2262

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Abstract Synthesis of highly active and recyclable nanocatalyst is important from the application perspective. This study reports a novel strategy for the organization of as synthesized Pd nanoparticles via green route using natural surfactant Acacia (Acacia auriculiformis) and clove bud (Syzygium aromaticum) extracts into fractal patterns from evaporating sessile drops on flat surfaces (glass and silicon wafer). The studies based on the effects of individual components used during the synthesis process show that the flavonoids present in clove bud extract play a major role in the organization of Pd nanoparticles. The fractal patterns form on glass surface are comparatively denser than that of the silicon wafer surface, which is supported by the fractal dimension calculations. The organization of Pd nanoparticles depends on several factors such as evaporating flux, Marangoni flow, capillary flow, capillary and van der Waals forces. The fractal patterned Pd nanostructures on glass surface were tested for the catalytic reduction of p-Nitrophenol (4-NPh) to p- Aminophenol (4-APh) in the presence of sodium borohydride (NaBH4). These organized nanostructures showed much better catalytic activity compared to that of unorganized Pd nanospheres, with the advantage of easy separation from the reaction mixture. The developed pattern was reproduced on fluorine doped tin oxide (FTO) glass surface to utilize as anode material in NaBH4 electro-oxidation reaction used in direct borohydride type fuel cells. The fractal patterned anode showed much higher efficiency in terms of higher current density (65 mA/cm2) and lower charge transfer resistance (13 times lower) compared to that of nonpatterned one. Keywords: Fractal pattern, evaporation induced assembly, fuel cell, anodic material, Electro-

oxidation.


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Introduction The self-assembled or organized structures of nanoparticles with specific functionalities have much better efficiencies in several applications such as electronics, biotechnology,1 optics,2 water treatment,3 sensors,4 wetting,5 catalysis,6 surface-enhanced Raman scattering (SERS)7 and so on, compared to that of unorganized one. However, fabrication of organized/patterned nanostructures with desired morphology through an easy, simple, and cheap process is a challenging task. In general, the fabrication of these micro or nanostructures by the selfassembly process with desired functionalities is comparatively easier and inexpensive than the many other available methods.8 Self-assembly refers to a process by which colloidal/nanoparticles or macromolecules are organized into an ordered structures in the presence of favorable environmental conditions or driven interaction(s).9,10 These forces include hydrogen bond, van der Waals interaction, hydrophobic force, π-π interaction, and so forth.11,12 The nano or colloidal particles self-assembly process can be broadly classified into ‘dry’ and ‘wet’ phase methods.13 The dry methods include chemical vapor deposition (CVD),14 lithography,15 laser ablation,16 etc. The wet phase methods are evaporation-induced selfassembly (EISA),11 Langmuir–Blodgett (LB),17 bulk phase assembly18 and external field assisted assembly.19 Among all these methods, EISA is the economic and facile method for assembling nanoparticles on the solid surface because of the simplicity of the process. In this method, various metallic/non-metallic NPs suspensions along with structure assisting inorganic/organic compounds are allowed to evaporate in the form of a droplet or a thin film from the solid surfaces and the suspended particles are organized during the evaporation process.20 Till now, mostly organization of various metallic NPs such as Au,21, Ag,4,22 Pd23, and non-metallic NPs such as Sulfur,13 TiO2,5, SiO2,5,24 Proteins25 etc., were reported by EISA 3 ACS Paragon Plus Environment


on flat surfaces. Over the last decade, tremendous progress has been made to generate different micro or nanostructures such as spherulites, dendritic,26 heterodimers,27 superstructure,21 strips,28 and fractal patterns5,29 involving with metallic, metal oxides, nonmetallic elements, polymers, etc. Among them, fractal patterns of noble metal nanoparticles are having huge scope over the simple unorganized nanoparticles in different applications because of higher surface active sites, distinct dimensions, and chemical functionality. The organization of various noble metals such as Au,6,7,30 Ag,4 and Pd6,31 NPs through the wet process were studied mostly for the various applications purpose such as catalysis, fuel cells, sensors, and SERS. Among different noble metals, the organization of Au NPs were studied more for mostly SERS application on flat surfaces. In general, Pd NPs are also extremely useful for catalysis, sensors, fuel cells, etc. applications compared to the other noble metals NPs. So, self-assembly of Pd NPs into different nanostructures are also important from the application perspectives. While considering the wet chemical methods of synthesis and organization of Pd NPs, it has been found that the chain like organization Pd nanocubes in aqueous media showed better performance for the oxidation of formic acid, methanol, which in turn can be further used for the fuel cells applications.31 The mesoporous leaf type organization of Pd NPs at the liquidliquid interface showed very good catalytic activity for the reduction of 4-nitrophenol to 4aminophenol.6 However, the main disadvantages of this method are required more synthesis time (12-14h), consumption of solvent, requirement of UV light source, separation catalyst from the reaction mixture after completion of reaction. Finally, to overcome the above-mentioned issues, in this study we report a novel one-step green route for synthesis and self-assembly of as-synthesized Pd NPs on the solid surface by evaporation induced method. The organization of Pd NPs into fractal patterns on solid surface without any additional agents makes our study unique and cost-effective. To the best of our


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knowledge, the study of natural surfactant (Acacia) and clove bud extract mediated green synthesis of Pd NPs and organization of as synthesized NPs on the solid surfaces into fractal patterns has been reported for the first time. The patterned nanoparticles on the glass surface were used for the catalytic reduction of 4-NPh to 4-APh in the presence of NaBH4 and found to be highly efficient compared to that of simple Pd NPs. The present study shows that the organized Pd catalyst on a flat surface is promising in terms of much higher catalytic activity and easy separability (for the re-use purpose) from the reaction mixture. The literatures on applications of nanocatalysts in energy generation indicate continuous efforts have been made on replacement of expensive materials by inexpensive one or the efficiency enhancement of the catalysts. In this context, the researchers are trying to replace expensive Pt electrocatalyst with less expensive Pd in fuel cell applications. The enhancements of catalytic activity and stability of Pd as an electrocatalyst are of considerable interest among the researcher community because of additional advantage of its higher hydrogen storage/adsorption capacity.32–34 In this study, the role of self-assembled Pd NPs for anodic electro-oxidation of Sodium Borohydride (NaBH4) in the alkaline medium was also studied for a half-cell reaction of direct borohydride fuel cell (DBFC) application because of their enhanced capacity than the similar type of direct alcohol fuel cells.35,36 The organized Pd NPs into fractal patterns on FTO surface served as an active catalyst with enhanced electrochemical performance for less amount of catalyst loading because of its unique structure (open textured) with a high degree of exposed surface atoms. The developed catalyst may also be useful for other types of catalytic reactions and fuel cells applications as well. They were organised on FTO glass surface, as some reports suggest that tin oxide support electrodes are more stable and active than commonly used carbon electrodes.37 Among different doped tin oxides like indium tin oxide (ITO), antimony tin oxide (ATO), FTO was most stable and active for electrochemical studies.37,38 5 ACS Paragon Plus Environment


Experimental Section Materials Chemicals, Palladium chloride (PdCl2), Sodium hydrogen carbonate (NaHCO3), Sulfuric acid (98%) were purchased from Merck, India. PdCl2 was solubilized in NaCl solution to form soluble Na2PdCl4 complex prior to experimentation. The 4-NPh and (3-Aminopropyle) Triethoxy Silane (APTES), from Himedia (India); Sodium borohydride (NaBH4) (98%), and FTO glass were from Sigma-Aldrich; Plant surfactant (Acacia) was prepared as per our previously reported study.39 The clove bud extract (CE) was prepared by modifying the previously reported method.40 Initially, visibly good quality dried clove buds were purchased from the market, washed with deionized water, and dried. After drying, these clove buds were crushed into fine powder. A 2 g of clove powder was taken in a clean conical flask, a 200 mL of water was added, boiled for 10 min, and then kept it for 8 h at room temperature. The aqueous clove bud solution was collected by filtering through Whatman filter paper (No. 40). The obtained solution was centrifuged to remove suspended particles if any, and then freezedried (using Labconco, FreeZone 2.5) to get the dry powder. A 5% stock solution of CE was prepared in deionized water and used for the experimentation. The ultrapure water of 18.2 mΩ.cm resistivity (Millipore) was used throughout the study. Methods The Pd nanoparticles were synthesized as per the protocol of our previously reported study.41 The specific experimental conditions used for the synthesis are as follows: pH 9, acacia (0.428 mM), temperature (55°C), white light (day light), and metal salt/CE ratio (1 mM/0.3 %). The pH of the reaction mixture (CE and acacia extract (AE) in water) was adjusted to 9 using NaHCO3. Then Na2PdCl4 was quickly injected into the reaction mixture under continuous stirring (800 rpm) after attainment of 55 °C. The glass slides and silicon wafers 6 ACS Paragon Plus Environment

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were washed (first with alcohol and then with deionized water) and dried in a hot air oven. Then the particles suspensions were dropped on both surfaces by using a microsyringe (Hamilton) to get the desired drop volumes (2 to 5 µL). Then the droplets were dried inside a dry block heater at 50 ± 1 °C for 8- 10 min. The resulted structures after drying of drops were observed under optical (Leica DM 2500M) and electron microscopes (SEM, JEOLJSM6480LV and FE-SEM, FEI NOVA). The UV visible spectra of Pd NPs were examined by a spectrophotometer (UV-3600, Shimadzu). The structure and the composition of the NPs were analysed by high-resolution transmission electron microscope (HR-TEM, FEI, Tecnai F 30G2 S twin) coupled with energy dispersive X-ray spectroscopy (EDS), The powder XRD analysis was done by using X-ray diffractometer (Rigaku Japan/Ultima-IV) with Cu Kα radiation (1.5406 A°) operating at 40 kV and 30 mA, within the range of 20-80° (2θ) at 0.05°/s scanning speed. The zeta (ζ) potentials and particles size were measured by dynamic light scattering (DLS, Malvern Zeta size analyser, Nano ZS). The fractal dimensions of selfassembled patterns were calculated by the box-counting method using box value of 2 to 64 from the Image-J software. Reduction of 4-NPh by self-assembled Pd NPs The catalytic reduction of 4-NPh was performed in a 10 ml beaker at ambient condition. A final volume of 6 ml was maintained after addition of 4-NPh (0.2 mM) and NaBH4 (10 mM) solutions. After mixing the solution was changed to yellowish colour (with the red shifting of UV-Vis. absorbance peak from 318 to 400 nm. For the catalytic activity test, the Pd NPs were organized into the fractal patterns on 25 × 25 mm cleaned glass slides as per the previously mentioned method. A volume of 60 µl Pd NPs suspension was spread on the glass surface, dried at 50 ± 1 °C, and heated at 250 °C for 1 h, then the Pd NPs deposited glass surface was dipped into the reaction mixture to test the catalytic activity. Some control tests were also



performed after deposition the NPs under different conditions to compare the activity of organized Pd NPs The concentration of 4-NPh was measured spectrophotometrically using a UV-Vis spectrophotometer with the help of a calibration plot. Electrocatalytic oxidation of sodium borohydride on self-assembled Pd NPs Sodium borohydride (NaBH4) electroreduction by Pd on FTO glass surface were studied using cyclic voltammetry. The self-assembled structures of Pd NPs (green synthesized) were compared with chemically synthesised unorganized Pd NPs (average particles size 70 nm) on FTO glass. The Pd deposited FTO glasses were prepared by dropping six droplets of 5 µL each of both samples and then subsequent heating at 400 ºC for 1 h. The reaction was carried out into the nitrogen environment in the presence of 0.05M NaBH4 and 1M NaOH. The electrochemical measurements were performed using a Metrohm, Autolab N series electrochemical workstation with a Pt wire counter electrode, Ag/AgCl in 3 M KCl reference electrode, and Pd modified FTO glass surface as a working electrode at 25 °C. Results and discussion Synthesis and characterization of Pd NPs The Pd NPs were formed when the Na2PdCl4 solution was added to the premixed solutions of clove and acacia extracts, the instantaneous visible colour change from transparent yellow to dark brown was because of the formation of Pd nanoparticles42. After the formation of Pd nanoparticles, UV-Vis spectrophotometric characterization was done to get an indirect confirmation of the particles formed, from the light absorbing property. It can be observed from the UV-Vis spectrum that the absorption is continuous in the UV region and there is a broad surface plasmon peak between 260-320 nm wavelengths because of the formation of Pd NPs as shown in Fig. 1a. The spectral pattern is also consistent with the literature for Pd


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nanoparticles43,44. Furthermore, as synthesised particles were characterized by XRD, to elucidate Pd NPs formation and to know the structural information as shown in Fig. 1b. The XRD diffraction pattern of synthesized Pd NPs clearly shows that the main intense diffraction peaks of Pd NPs at 2θ= 40.40°, 45.45°, 66.44°, 75.46° corresponds to {111}, {200}, {220}, and {311} planes, respectively, for the face-centered cubic (fcc) structure of the particles (JCPDS File no.-01-1201). The intense peaks at 32°, 56.60° (* marked) are mainly because of the presence of crystalline organic molecules in the reaction media and adsorbed on the particles surface.44,45 Same peaks were also confirmed through the XRD analysis of pure extracts as shown in Fig. S1, SI. Further, the electron microscopic characterization of methanol washed nanoparticles was done to get the information about size and shape of the particles. The TEM images (Figs. 1c, d) show the palladium nanoparticles are nearly spherical shape of size ~10 ± 1.42 nm with characteristic 2D lattice fringes of 0.23 and 0.20 nm. The energy dispersive X-Ray analysis (EDX) of a selected area also confirms the presence of Pd in the NPs as shown in Fig. S2, SI.



Fig. 1. (a) UV-Vis spectra of as synthesize reaction solution and palladium salt. Inset image shows the reaction solution after completion of green synthesis, (b) XRD patterns of Pd NPs (c) TEM image of methanol washed Pd particles, inset shows particle size distribution histogram, (d) HR-TEM image of single Pd NP after methanol wash, with lattice fringes d = 0.23 nm and 0.20 nm. Self-assembly of Pd nanoparticles on glass and Silicon wafer surfaces In this section, we have made an attempt to elucidate the Pd nanoparticles organization behavior during evaporation of droplet under the influence of reducing/capping agents (CE and Acacia) on glass (hydrophilic) and silicon wafer (hydrophobic) surfaces. In this study, a 5 µL drop of as-synthesized Pd NPs suspension was gently placed on glass and silicon wafer surfaces and kept for drying at 50 ± 1 °C inside a dry block heater for 10 min. After drying


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the droplet, the surface was examined microscopically. Fig. 2 shows that palladium particles are organized into fractal type branched patterns with many sub-branches on both glass and Si wafer surfaces. The low magnification full drop view images depict in Figs. 2(a) and (b) are on glass and silicon wafer surfaces respectively, indicate that the NPs are deposited throughout the drop area, but the peripheral region of the droplet is comparatively thick and dense on both surfaces. The live observation of evaporation process through the microscope also supports that the formation of structure starts from the edge of the drop and later covers the inner region of the droplet. This type of sessile droplet drying behaviour can be explained with the help of three important phenomena which are frequently observed, (i) evaporative flux of the liquid,41, 13 (ii) the capillary flow of liquid which exists towards the edge of the droplet because of the presence of deposited particles, to replenish the evaporation loss at the edge46,47 and (iii) Marangoni flow, an inward flow from edge to towards the center to surpass the capillary flow, generated by the uneven distribution of surface tension along the liquid vapor interface because of the presence of surfactant in the system.48 49 When a nanofluid droplet evaporates from a flat surface, the evaporation rate is much faster at the edge than the center of the drop. Due to the higher evaporation rate at the edge, an evaporation flux J(r) at distance r, is generated from the center of the drop towards the perimeter. The evaporating flux can be represented as:

∝ −

(1)

Where, R is the droplet radius, λ=(π-2θ)/(2π-2θ), and θ is the contact angle of the drop on the surface. Because of the higher evaporating flux at the drop edge, an outward flow is generated towards the edge from the center to replenish the evaporation loss at the periphery; as a result, the nanoparticles also move towards drop edge and deposited at the three-phase contact line. During the deposition of particles, capillary and van der Walls forces are mainly responsible.46 Once the particles are deposited at the contact line, the liquid film is pinned 11 ACS Paragon Plus Environment


and the height of the droplet continues to decrease because of the evaporation. During the initial period of drying, as the liquid height is more, larger particles can move towards the edge and deposit, also there is a possibility of multi-layer deposition, which in turn makes a thicker deposit at the edge. After some time, when the liquid film is dried at the edge and moves towards the central region, water evaporation continues from the deposited particles layer because of radially outward capillary flow of liquid through the porous deposited layer. In addition, when the surfactant is present in the system, the Marangoni flow is also important. This Marangoni flow is driven by the surface tension gradient, where liquid flows from lower surface tension to higher surface tension along the liquid-vapor interface. At the edge of the droplet, water surface tension is lower than that of the central region because of the higher surfactant concentration, as the evaporation rate is faster at the edge. This uneven surface tension gradient causes the diffusive flow of nanoparticles from edge to the center. Finally, due to the combination of evaporative flux and Marangoni effect a convective loop occurs and the particles organize gradually from the edge to the central region. In this process, at the later stage of evaporation, the liquid height gradually decreases and less dense structures are formed in the central region because of the presence of comparatively smaller sized particles, lower particle density, and less chance of multilayer deposition. Furthermore, to know the morphology of organized structure of Pd NPs after evaporation of the droplet, the electron microscopic analysis was done on both surfaces. The low magnification microscopic images presented in Figs. 2 (c) and (d) show the fractal type self-similar structures are formed throughout the drop area, along with a ring formation at the edge of the droplet. The ring type structure formation is because of first pinning of droplet as mentioned before. Figs. 2(e) and (f) show magnified view of a single fractal structure on glass and silicon wafer surfaces respectively. The formed self-assembled patterns on glass surface originate from a central point called nucleus and looks like a bouquet which consists


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of many flower-like pattern and each pattern consist of multiple petals originate from the center because of aggregation and organization of the particles into multilayer. However, on silicon wafer surface main branches of an individual structure originate from a central point then the sub-branches are formed because of the linking of particle-particle interaction. Further magnified views of the structures on both surfaces are depicted in Figs. 2(g), (h) indicate multilayer deposition on glass surface and that of mostly single layer on the silicon wafer surface. The schematic presentation of pattern formation on both surfaces are shown in Fig. S3, SI. The noticeable feature of the NPs assembly is that they formed distinct fractals with mostly mono and multi-layer packing on silicon and glass surfaces respectively under the same experimental conditions. The reasons of differences between the pattern formation on both surfaces are explained in detail in section 4 of SI (Fig.S4 and S5).



Fig. 2. Microscopic images (from low to high magnification) of self-assembled structures of as-synthesized Pd particles, on glass (a, c, e, g) and silicon wafer (b, d, f, h) surfaces. Images (g), (h) show the magnified view of the marked region of images (e), (f) respectively. Scale bars are 500 µm for (a, b), 100 µm for (c, d), 10 µm for (e, f) and 500 nm for (g, h). 14 ACS Paragon Plus Environment

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As the fractal patterns were formed successfully on both surfaces from the evaporating small droplets, the same protocol was also applied to organize the particles on the large surface for better practical applications. In this process, a glass slide was vertically dipped it in the NPs suspension for 5 min then the slide was dried at 50 ± 1 °C for 15 min., and finally the surface was heated at 250 °C for 1 h for sintering the particles on glass surface. After this process, the surface was also microscopically analyzed and presented Fig. 3, the image shows that the similar type of pattern is formed on the glass surface. To check the stability of the formed patterns, this surface was sonicated for 1 min in an ultra-sonicator bath and again analysed microscopically. The existence of same pattern after sonication indicates the stability of the pattern on the glass surface.

Fig. 3. The self-assembled Pd NPs on large area of glass surface via dip coating and after heating at 250 °C, (a, b) FE-SEM images of patterns formation after dip coating, (c) after 1 min sonication of dip-coated surface. Scale bars are 50 µm for (a, c) and 10 µm for (b). Effect of reactants on self-assembly process To know the responsible component(s) present in the reaction media for the organization of nanoparticles into fractal patterns, some control tests were performed. Firstly, to see the effect of capped molecules on the particles surface, as-synthesized Pd nanoparticles were washed with water and methanol successively to remove capped molecules, then the re-dispersed 15 ACS Paragon Plus Environment


particles suspensions in water were dropped on glass and silicon wafer surfaces. Figs. 4 (a, c) depict that fractal patterns are sparse in case of water washed particles on both surfaces, and there is no organization of particles when washed with methanol (Figs. 4e, g). These observations clearly indicate that the adsorbed capping molecules are playing a crucial role in the formation of organized fractal structure. When the particles are washed with water, some free molecules are washed out and strongly capped molecules remain as it is, that reason less dense fractal structure was formed on both surfaces after washing. However, after methanol wash most of the capped organic molecules are expected to be removed, as a result, only particles were observed on both surfaces without forming any pattern (Figs. 4 e, g).

Fig. 4. FE-SEM images after water wash of Pd nanoparticles on glass (a, b) and on the silicon wafer (c, d) surfaces after drying. Images (b, d) depict the magnified view of the marked region of images (a) and (c) respectively. Images of methanol washed Pd nanoparticles on glass (e, f) and silicon wafer (g, h) surfaces after drying. Images (f, h) depict the magnified view of images (e) and (g) respectively. Scale bars are 10 µm for image (a, c, e, g) and 1 µm for image (b, d, f, h).


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Further, to check the effect of biomolecules and other components on the self-assembly process, few supporting experiments were carried out. A droplet of the solution of each component (same concentration involved in the synthesis process during the reaction) was dropped and dried individually on the glass and silicon wafer surfaces and analyzed microscopically (detailed description is given in Supporting Information Figs. S6 and S7). After observing the drop drying pattern morphologies of CE, NaHCO3, Acacia, and Pd salt, it can be concluded that the pattern obtained from CE is very close to that of the final structure obtained with the Pd NPs. So, the CE plays a primary role in assembling the Pd NPs into fractal patterns and NaHCO3 also assists in this process. It is well known that the main component in CE is the flavonoids (mainly eugenol), because of the presence of this polyphenolic compound CE has the reducing property of metal salt as well as self-organizing ability. Further, to get the additional information about the role of key components in the NPs formation process, the metal precursor was mixed individually by eliminating each ingredient in the reaction media and confirmed the particles formation via UV-Vis spectra, Fig. S7 (a). Fractal dimension calculation As the patterns formed on both surfaces are difficult to quantify for comparison, so the fractal dimension was calculated using Image- J software to get some more information (detailed description about the method is given in Supporting Information). The images of formed patterns on both surfaces were used for the calculation of fractal dimension. The linear plot of log N(l) vs. log(l) corresponding to respective image are shown in Figs.5 (a, b), the values of fractal dimension are ~1.73 and 1.69 on glass and silicon wafer surfaces respectively, suggests that the formation of these fractal patterns are because of the diffusion-limited cluster aggregation (DLA).

50,51

When the Df value is smaller (< 1.5) other patterns such as

Sierpinski triangle, Koch curve are formed. Widely reported the simulated value of Df for



DLA is 1.71 but in the case of a real system, this value may slightly deviate because of the change in surface property or addition of some charged molecules.25,52. The DLA based organization mechanism is described in section 6 of SI.

Fig. 5. Image (a) fractal dimension on glass surface ~1.73 and image (b) fractal dimension on silicon wafer ~1.69 through Image-J after binary conversion. Catalytic activity of organized NPs 4-NPh Hydrogenation: The nitro-aromatic compounds (nitrophenol, nitrobenzene, nitroaniline etc.) have an adverse effect on the environment because of their toxicity.53 However, their reduced hydrogenated compounds are nontoxic and can be used for several applications. Here, the catalytic activity of green synthesized organized fractal patterned Pd NPs on the glass surface was studied for the 4-NPh reduction. Simultaneously, some control tests were also performed for the 18 ACS Paragon Plus Environment

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comparison of the catalytic activity of green synthesized fractal patterned Pd NPs (GF-Pd) on glass surface. Five different forms of Pd catalyst were tested for comparison in this reaction and those are denoted as, (i) GF-Pd, (ii) methanol wash Pd NPs (MWG-Pd) as shown in Fig. 4e,

(iii)

NaBH4

reduced

chemically

synthesized

Pd

nanoparticles

and

4-

Aminopropoxytetraethoxysilane (APTES) functionalised for binding on to the glass surface which does not shows any self-assembled pattern on glass after drying, (C-Pd), (iv) C-Pd NPs without APTES, mixed with CE and deposited on glass surface which helps in self-assembly and binding of Pd NPs ( discussed in previous section ) (CE + C-Pd), and (v) Clove extract mediated green synthesized particles suspension directly mixed into the reaction mixture (GPd). A 60 µL each Pd NPs suspension was spread on the glass surface (for i to iv), dried at 50 ± 1 °C, heated at 250 °C, and then the Pd NPs deposited glass surface was dipped into the reaction mixture to test the catalytic activity. Further to test the activity of uncoated NPs (for v) a 60 µL Pd NPs suspension was directly added to the reaction mixture. For the catalytic activity of GF-Pd, the change in absorbance of UV-Vis spectra at 400 nm was recorded into a time interval and presented in Fig. 6a. From the UV-Vis spectra it is clear that the peak intensity of 4-NPh gradually decreases and a new peak at 298 nm corresponds to paminophenol (4-APh) increases. Similarly, other control experiments were also performed. The rate of change in absorbance at 400 nm wavelength is the measure of catalytic activity and is plotted on a logarithmic scale as shown in Fig. 6b. From this plot between ln(A/A0) Vs. t (time), it is clearly understood that the reaction is following the first-order kinetics with a linear fitting. The individual catalytic activity in terms of rate constant is shown in Table 1, along with the regression coefficient of the linear fitting. From the Table 1, it is clearly observed that the organized particles show much better catalytic activity than that of the unorganized one. Among them GF-Pd has the higher catalytic activity in terms of the rate constant, the values are ~86% higher than that of the G-Pd and 194 % higher than C-Pd.



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Similarly, the activities of CE + C-Pd has ~43% higher than G-Pd and 126 % higher than CPd. The reduction of 4-NPh to 4-APh follows the Langmuir-Hinshelwood mechanism, 54,55

the rate of reaction is dependent on the rate of adsorption of both the reactant and the

reducing agent on the catalyst surface. The higher availability of surface sites plays an important role in the reaction rate. The 4-NPh adsorbs on the surface of the catalyst through the phenolic group and subsequently, the hydrogen atoms from borohydride also adsorb on it. This close and unstable presence of two reactants allows the reduction of the potential energy barrier of the reaction in favour of a faster reaction, which otherwise required the high kinetic energy of collision to proceed. The reduction reaction can be presented as follows: NaBH4 + 4H2O →NaB(OH)4 + 8H+ + 8e‾

(2)

2H+ + 2e‾→ H2

(3)

(4)

It has been found that GF-Pd has the highest rate of reaction compared to G-Pd and C-Pd. The higher catalytic activity can be attributed as follows. When the NPs are organized immediately after formation, small particles in the range of 10-20 nm are assembled without agglomeration exhibits higher activity compared to that in the suspension, as there is a change of agglomeration with time to minimize the surface energy. The crystallographic and microscopic analyses infer that the particles in fractal pattern arrangement are exposed of majorly higher energy {100} facet compared to that of lower energy {111} facet as shown in Fig. 1b.56,57 In case of colloidal suspension, the small particles grow into larger size because of Ostwald ripening to minimise the surface energy and are generally bound by the stable 20 ACS Paragon Plus Environment

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{111} facet in the absence of any passivating agents.58 The dominance of higher energy facets plays an important role in enhanced catalytic activity in the assembled structures. Additionally, when the particles are organized by connecting to each other, the better electron transfer ability through the patterned structure may also help in enhanced catalytic activity.

Fig. 6. (a) UV- vis spectra of 4-NPh reduction by NaBH4 in the presence of green synthesized Pd NPs assembled into fractal patterns on the glass surface, (b) Plot of ln (A/A0) at 400 nm vs. time with various control experiments. Table 1. Catalytic Activity of various Catalysts Sample

GF-Pd C-Pd CE+ C-Pd MWG-Pd G-Pd

Rate constant, k (min-1)

0.156 0.053 0.120 0.012 0.084

Regression coefficient, R2 0.99 0.99 0.99 0.97 0.98

NaBH4 Electro-oxidation: Electrochemical Active surface area and Cyclic voltammetry studies:



The electrochemical oxidation reaction of NaBH4 was studied using cyclic voltammetry in the presence of organized fractal Pd NPs as a catalyst on FTO surface. To check the role of assembly on the oxidation of NaBH4, both GF-Pd and C-Pd deposited on FTO were studied and compared. Noteworthy to mention that the GF-Pd also forms organized fractal patterns on FTO surface and they are stable after the electro-oxidation reaction, as supported by the SEM images shown in Fig. S8 (Supporting Information). We have also characterized C-Pd deposited surface before and after reaction and observed that these particles are not forming any organized patterns on FTO surface and deposited randomly on it (Fig. S9, Supporting Information). Firstly, the electrochemically active catalyst surface area (EACSA) was determined using PdO reduction region of the CV obtained in 0.1 M NaOH with a scan rate of 50 mV/s as shown in Fig. 7a. The integrated PdO reduction zone and theoretical charge density of PdO reduction of 424 µC/cm2 were used to obtain the value of ESCSA. The ESCSA for GF-Pd and C-Pd on FTO surface were found to be 13.4 and 4.9 cm2 respectively, which is much higher than the geometrical area 0.075 cm2 of only FTO which used to deposit 30 µL of Pd NPs. The higher surface area value of GF-Pd compared to the C-Pd on FTO surface is attributed to the high surface exposure because of the organized fractal pattern formed via self-assembly of Pd NPs than that of the random arrangement of C-Pd on FTO. Further, the DBFC half-cell reaction was studied using Voltammogram obtained with cyclic scans for 0.05 M NaBH4 in 1 M NaOH for GF-Pd and C-Pd on FTO. Both the obtained structures show a low onset potential of around -0.2 V but current density for GF-Pd is higher than the C-Pd shown in Fig. 7b indicates the higher catalytic activity/faster borohydride oxidation by fractal pattern Pd NPs. This increased oxidation current is because of the fact that selfassembled particles are of 3D multilayered fractal nature exposing higher particle surface for oxidation and charge transfer with minimal resistance, whereas, the chemically synthesized


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structures are larger sized aggregated form with no regular pattern, low surface exposure, and hinder faster charge transfer. We have also compared the performance of commercially available Pd/C (10 wt% Pd) catalyst with GF-Pd and C-Pd for electro-oxidation of NaBH4 on FTO, as shown in Fig. S10 (Supporting Information). The result shows better performance of self-assembled Pd NPs than the Pd/C catalyst. While comparing with the reported study, a 35.4 µg/cm2 equivalent amount of Pd in Pd/C catalyst achieved a current density of 30 mA/cm2 at 400 rpm electrode rotation for 0.01M NaBH4 and 1M NaOH at 5 mV/s scan rate.59 Whereas, our single layer deposited electrode with 3.2 µg/cm2 equivalent amount of organized Pd catalyst achieved current density of 65 mA/cm2 in static condition for 0.05M NaBH4 - 1M NaOH at 50 mV/s scan rate, which clearly shows an improved performance. In another study Pd cubes with exposed {200} facets on carbon support (Pd-NC/C) were compared with commercial Pd/C (10 wt.% Pd) on glassy carbon electrode and found to have current densities of 600 and 400 mA/cm2 with very high concentration of NaBH4 (3.26 M) and 3.08 M NaOH.60 It is a well-known fact that the current responses are higher for higher concentration of the fuel cell and for higher rotation of electrodes due to enhanced diffusion. Thus, from the comparison with the reported results it can be concluded that fractal patterned Pd NPs on FTO is having fairly better performance as anode material for DBFC application. The ideal electrochemical reaction at the electrode surface can be presented as in equation (5): NaBH4 + 8OH− → NaBO2 + 6H2O + 8e−

(5)

Practically, there are other hindrances to get the ideal performance of 8e- generation, mainly the formation of gaseous H2 which bubbles out from the electrode surface as represented in equation (6): BH4−+ 2H2O → BO2− + 4H2

(6)



Page 24 of 37

If the actual number of electrons generated is ‘x’ and the reaction can be presented as in equation (7): BH4− + xOH− → BO2− + (x-2)H2O + (4-x/2)H2 + xe−

(7)

Hads + OH− → H2O + e−

(8)

Most of the modifications to the catalyst material, structure, texture etc. are focused on the achieving highest possible value of ‘x’ and simultaneously utilization of the generated H2 as shown in equation (8). Since Pd has very good activity and selectivity towards H2, therefore, the catalyst performance can be improved by minimizing the escape of H2 from the catalyst surface. The obtained results in the previous section also support the higher catalytic reduction rate of 4-NPh to 4-APh in the presence of GF-Pd as a catalyst.

v

Fig. 7. Cyclic voltammograms obtained with GF-Pd and C-Pd on FTO surface (a) in 0.1M

NaOH at 50 mV/s scan and (b) for oxidation of NaBH4 (0.05 M) in the presence of 1 M 24 ACS Paragon Plus Environment

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NaOH at 50 mV/s scan rate, (c) Chronoamperometric curves of GF-Pd and C-Pd in 1 M NaOH and 0.05 M NaBH4 solution in Nitrogen environment at 50 mV constant potential, (d) Impedance spectra for NaBH4 oxidation on GF-Pd and C-Pd catalyst at 50 mV in a solution containing 1 M NaOH and 0.05 M NaBH4, the frequency was varied from 0.1 Hz to 1,00,000 Hz. The inset shows the respective equivalent circuits. Chronoamperometry study In order to check the electrocatalytic activity and the stability of both GF-Pd and C-Pd NPs for BH4− electro-oxidation, Chronoamperometry (CA) test was performed. Fig. 7c shows the CA response for both the electrode in 1 M NaOH + 0.05 M NaBH4 solution in a nitrogen environment. The current density for both electrocatalysts reduces with time, signifies the progress of hydrolysis and oxidation of borohydride reactions on the electrode surface.33,61 The lower rate of current decay and higher current density for GF-Pd electrode than that of non-assembled C-Pd electrode signifies that the GF-Pd has better electrocatalytic activity and stability towards borohydride oxidation than that of C-Pd. The higher activity of GF-Pd is attributed to the higher surface area because of the fractal type arrangement which helps in the better utilization/conversion of adsorbed BH4− species on the electrode surface, higher adsorption of H on the electrode surface, subsequent oxidation in the presence of OH−, and release of electron (equation 8). In addition, during the process of electro-oxidation by GFPd, H2 bubbles were not observed on the electrode material but such bubbles were hindering the process when C-Pd was used. This also indicates the better faradaic efficiency of the GFPd electrode. Electrochemical Impedance Spectroscopic (EIS) Study EIS is also an important characterization in the electrochemical study to test the electrocatalytic activity of an electrode. This technique analyses the impedance parameters



for the charge transfer during the reaction across the electrode-anolyte interface. The EIS spectra were taken at 1 M NaOH and 0.05 M NaBH4 anolyte concentration at the potential of 50 mV in the frequency range of 1 kHz to 0.1 Hz and shown in Fig.7d. These semi-circular part of the Nyquist plots are proportional to charge transfer resistance represented as (Rct) for the respective electrocatalysts obtained from the equivalent circuit derived by curve fitting using NOVA 2.1 software. The calculated Rct for C-Pd and GF-Pd are 18.72 and 1.39 Ω cm-2 respectively. The EIS spectra of fractal assembled GF-Pd and non-assembled C-Pd electrocatalyst indicate that the charge transfer resistance between the anolyte and electrode for GF-Pd is less than the C-Pd electrode. This also implies that GF-Pd has a higher active surface area as seen earlier. Because of its unique fractal texture, we expect GF-Pd shows faster electron transport as the energy barrier is reduced and it is evident from Rct values thus responsible for higher activity for borohydride oxidation and minimization of H2(g) escape, as evident from the other electroanalytical analysis already presented above. Durability Test: To check for the stability of the electrode’s performance for borohydride oxidation the durability was checked up to 100 cycles using cyclic voltammetry (Fig. S11, Supporting Information), a plot of peak current density for each cycle is shown in Fig. 8. It can be seen that the GF-Pd electrode has a small drop of initial peak current density after a few cycles before reaching to the optimum value of ~66 mA/ cm2. However, for the C-Pd electrode, an initial peak current density of ~60 mA/ cm2 decreases continuously with the number of cycles and finally reaches to an equilibrium value of ~30 mA/ cm2. This lower current density of C-Pd can be attributed to the adsorption of hydrogen on C-Pd electrode which was not oxidized readily.


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Fig. 8. The durability of GF-Pd and C-Pd electrodes up to 100 cycles. Conclusions The present study shows that CE and AE mediated green route synthesized Pd nanoparticles are organized into fractal patterns after evaporation of sessile drops/films on glass and silicon wafer surfaces without any additional agents for organization. The fractal dimensions obtained from the box-counting method are 1.73 and 1.69 on glass and silicon wafer surfaces respectively, indicate that the DLA mechanism is important for the fractal formation. The formed fractal patterns on glass surface are denser than that of silicon wafer. The organization of Pd NPs is mainly driven by the evaporating flux, Marangoni flow, capillary flow, capillary and van der Waals forces during the droplet evaporation; and the flavonoids present in CE play a key role to organize the NPs into fractal pattern. In general, such pattern structures have several applications, here the catalytic activity for the reduction of 4-NPh to 27 ACS Paragon Plus Environment


4-APh was tested and found GF-Pd has 86 and 194 % higher activities compared to that of GPd and C-Pd respectively. The higher catalytic activity of GF-Pd is attributed to the presence of higher energy [100] facet of Pd NPs, smaller size, higher surface area, and better electron transfer through the pattern structure. The electrocatalytic activity towards the anodic electrooxidation of NaBH4 were investigated using CV, CA, and EIS tests. The CV result shows that GF-Pd on FTO has nearly 100% enhancement in current density compared to C-Pd. Similar results were obtained by other characterizations like CA, EIS which support that these organized Pd NPs act as an excellent catalyst and can be used as an anode in DBFCs because of higher EACSA, faster charge collection and transport. This developed catalyst can be used for other catalytic applications too, with the advantages of higher activity and easy separation from the reaction mixture for the reuse, including other types of fuel cells. ASSOCIATED CONTENT Supporting Information Available: The XRD patterns of pure clove and acacia extract, EDX analysis of as-synthesized NPs, Schematic presentation of pattern formation on glass and silicon surfaces, average contact angle values of water and Pd suspension, FT-IR spectra of organic molecules, UV-Vis spectra, and microscopic images of formed patterned by the reactants, fractal dimension calculation method, SEM images of GF-Pd and C-Pd on FTO surface, CV study of Pd/C catalyst for NaBH4 electro-oxidation, CV for various cycles. ACKNOWLEDGMENT The ﬁnancial support from the DST-SERB (Ref. No. EMR/2016/000810) for this project is gratefully acknowledged.


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Organization of Pd Nanoparticles into Fractal Patterns for Highly

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