Green Biosynthesis of Rhodium Nanoparticles Via

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Jan 6, 2017 - Rhodium Nanoparticles (Rh NPs); Green synthesis; Aspalathus lin- earis ... in catalytic processes, especially soluble metal nanoparticles as.
Ismail et al., J Nanomater Mol Nanotechnol 2017, 6:2 DOI: 10.4172/2324-8777.1000212

Journal of Nanomaterials & Molecular Nanotechnology

Research Article

Green Biosynthesis of Rhodium Nanoparticles Via Aspalathus Linearis Natural Extract Ismail E1-3*, Kenfouch M1,2, Dhlamini M3, Dube S4 and Maaza M1,2

Abstract This contribution report on the bio-synthesis of Rhodium metallic nanoparticles (Rh NPs) synthesized for the 1st time by a completely green process using Aspalathus linearis natural plant extract as an effective bio-oxidizing/bio-reducing agent as well as a capping compound. Their morphological, structural and optical properties were investigated using various complementary surface/interface characterization techniques such as HR-TEM, HR-SEM, EDS, XRD, XPS, UV and ATR-FTIR spectroscopy. The results confirm the formation of quais - monodisperse spherical Rh NPs in the range of 0.8-1.6 nm. Keywords Rhodium Nanoparticles (Rh NPs); Green synthesis; Aspalathus linearis; Natural extract

Introduction Rhodium is one of the rarest and most precious metal of the platinum group (PGMs) [1]. It exhibits remarkable and unique catalytic properties in comparison with other noble metals [2-7]. In addition to that, it has a great potential for many catalytic applications [8-10]. Catalysts based on platinum group metals are the most active in catalytic processes, especially soluble metal nanoparticles as heterogeneous systems [11,12]. Furthermore, Rhodium (0) NPs are considered as one of the most stable metallic nanoparticles as they do not oxidize easily to any other oxide faces even when heated [13]. They have face centered cubic (fcc) unit cell, high melting and boiling points, high thermal and electrical conductivity [14] and extreme resistivity towards acids and bases which implies they can withstand harsh reaction environments [15]. Various synthesis methods for metal nanoparticles such as metal salt reduction, metal vapor condensation, thermal decomposition of metal complexes and electrochemical synthesis were reported [16-18]. The most traditional and widely used standard method is the metal salt reduction in the presence of reducing agent as well as stabilizing agents [16]. For the synthesis of rhodium NPs; different reducing agents such as alcohols, hydrogen, vanadocene and sodium orohydride NaBH4 [19-21] in the presence of different stabilizing polymers, such as polyvinylalcohol (PVA) [22] or polyvinylpyrrolidone (PVP)[23,24] were evaluated. The chemical route to synthesize Rhodium NPs has *Corresponding author: Ismail E, UNESCO-UNISA Africa Chair in Nanosciences/Nanotechnology, College of Graduate Studies, University of South Africa,Pretoria-South Africa, E-mail: [email protected] Received: June 21, 2016 Accepted: December 31, 2016 Published: January 06, 2017

International Publisher of Science, Technology and Medicine

a SciTechnol journal the disadvantage of using such chemical compounds/organic solvents which can be toxic as well as not easy in treatment of the waste end product. Within the green chemistry current trends [25-27], the synthesis and assembly of such Rh nanoparticles would benefit from the development of clean, nontoxic and environmentally friendly biosynthesis procedures. More accurately, green synthesis processes whereby the precursor of the nano-material to be synthesized is reduced effectively via a biochemical interaction with active compounds of the natural extracts, is gaining momentum [28]. Aspalathus Linearis is a plant commonly known as Rooibos, of the family Fabaceae. A. Lin. and grows naturally in the Cederberg area situated in the western parts of the Western Cape Province of South Africa [29]. The chemical composition of its extract is well known [30]. It has a good anti-oxidative activity which may be attributed to the presence of flavonoids and phenolic component [31]. These bioactive components are believed to be potential chemical reduction agents [32]. The basic flavonoid structure is the flavan nucleus, which consists of 15 carbon atoms arranged in three rings (C6-C3-C6), which are labeled A, B, and C [33]. The major phenolic compounds present include flavones, flavanones and flavonols [34-36]. It is established that Aspalathin, which is a rare C-C dihydrochalcone glycoside, and the two structurally related chalcones (nothofagin and aspalalinin) are the most bioactive compounds. These bioactive components are believed to play effecively as potential chemical reduction agents [32]. This is in addition to its constituents of different minerals. So, in this study, a green synthesis of nano-scaled single phase pure Rh particles bio-engineered for the 1st time by green chemistry process using Aspalathus linearis leaves extract as an effective chelating agent without consuming any acid or base standard components is reported.

Materials and Methods Preparation of the plant extract 0.03 g washed dried Aspalathus linearis leaves were added to 300 mL of deionized water at room temperature for 30 min yielding a deep orange coloured extract of pH=5. The extraction duration was considered to ensure the extraction of the bioactive compounds from the Aspalathus Linearis leaves powder. The extract was then filtered 3 times to obtain an aqueous leaf extract without any residual solids. One has to mention that the used Aspalathus Linearis leaves’ extracts were slightly fermented to minimize and even preclude the oxidation of the green leaves’ polyphenols. The chemical composition is nearly similar to that of fresh leaves [37].

Bio-Synthesis of Rhodium nanoparticles As mentioned previously, this report is targeting the green chelation of RhCl3.H2O as initial precursor for the bio-synthesis of single stable phase of Rh NPs. Analytical grade reagent RhCl3.H2O from “BDH Chemicals LTD, product no. 30671, England" was used. A 3.0 g of Rh precursor was added to the prepared A. Lin. natural extract at room temperature, while swirling. The Rh salt was observed to dissolve completely in the aqueous extract. The resultant solution was allowed to react over a period of 30 min to ensure a complete reduction process of the precursor. Then the solution was dried at 90°C for 1h to remove bio-compounds in excess.

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Citation: Ismail E, Kenfouch M, Dhlamini M, et al. (2017) Green Biosynthesis of Rhodium Nanoparticles Via Aspalathus Linearis Natural Extract. J Nanomater Mol Nanotechnol 6:2.

doi: 10.4172/2324-8777.1000212

Charachterization techniques Various characterizations were carried out to investigate the physical and chemical properties of the bio synthesized Rhodium nanoparticles such as UV-vis Spectroscopy, X ray diffractometer, X-rays photo-emission spectroscopy, High Resolution Transmission Electron Microscopy, High Resolution Scanning Electron Microscopy, Energy dispersive X-ray spectroscopy and Attenuated Total Reflection- Fourier Transform Infrared spectroscopy

Results and Discussion Optical properties UV-vis Spectrum is an effective method to investigate whether the metal precursors are completely reduced or not [38,39]. The UV-Vis measurement shows the UV-VIS-NIR absorbance in the optical range of 225 - 600 nm. Figure 1 presents the UV-VIS spectrum of A. lin. natural extract and the biosynthesized Rh (0) NPs solution. The initial solution of A. Lin. extract has one sharp peak centered approximately at 284.5 nm which may be attributed to aspalathin and a broad peak centered approximately at 348.5 nm is attributed to other flavonoids identified [29]. On the other hand, Rh precursor color was changed after mixing with the natural extract from deep orange to lit orange. The final biosynthesized Rh NPs solution exhibits an absorption continuum, characteristic for rhodium(0) nanoparticles because of the surface plasmon resonance [40,41]. The Peaks of A. Lin. extract almost disappeared due to the reduction process of Rh3+ to Rh (0) NPs. These result is in agreement with the reported data of Rh NPs [42,43].

Crystallographic structure

proves the amorphous nature of single phase prepared product i.e. pure amorphous Rh NPs. Further study investigating the effect of annealing temperature on the phase crystallinity is underway.

Chemical valence states and XPS X-rays photo-emission spectroscopy (XPS) studies were carried out to confirm the formation of the bio synthesized Rh NPs via A. Lin natural extract at room temp. The XPS studies were acquired using a VG Scientific LAB MK-II spectrometer with an Mg-Ka X-ray source, a constant 50 eV pass energy mode in 0.1 eV increments at 50 ms dwell time with the signal averaged for several regular scans. The carbon C (1s) electron binding energy corresponding to graphitic carbon was used for calibration of Rh(3d) core level peaks. Figure 3 shows two prominent bands at 307.6 eV and 312.0 eV which are assigned to rhodium (0) 3d5/2 and 3d3/2, respectively [49-51]. This 4.4 eV separation distance between both peaks correspond well to the expected value of 4.4 eV of Rh (0) NPs [52]. XPS investigation revealed an evidence on the formation of pure Rh NPs using A. Lin natural extract.

Morphology and size distribution Figure 4A presents the high resolution transmission electron microscopy (HR- TEM) of the biosynthesized Rh NPs before the annealing phase. The High Resolution Transmission Electron Microscopy were carried out on a Jeol JEM 4000EX electron microscopy unit with a resolution limit of about 0.12 nm, equipped with a Gatan digital camera. As one can notice, the as prepared Rhodium particles are non-agglomerated and nano-scaled in size within the range of 0.8-1.6 nm. The mean size of the nanoparticles

To identify the crystallographic phase of the Rh NPs, room temperature XRD analysis was carried out using a Smart Lab Diffractometer with a monochromatic CuKα1=1.5406 Å, operating at a voltage of 45 kV and a current of 200 mA in the Bragg-Brentano geometry. The XRD pattern of the Rhodium NPs displays clearly an amorphous signature, Figure 2. It does not exhibit any Bragg peaks from the crystallographic reflections of Rh NPs (111), (200), (220) and (311) planes [43-46]. Also, there were no peaks from any rhodium oxide phases found in the XRD spectrum [47,48]. This

Figure 2: Typical XRD profile of the biosynthesized Rh NPs.

25

∆IIµA

20 15 10 5 0

0.005

0.006

0.007

0.008

0.009

0.010

0.011

0.012

IIP (g)

Figure 1: UV-VIS-NIR spectrum of the biosynthesized Rh NPs.

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Figure 3: Rh3d X-ray photoemission spectroscopy spectrum of the biosynthesized Rh NPs.

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Citation: Ismail E, Kenfouch M, Dhlamini M, et al. (2017) Green Biosynthesis of Rhodium Nanoparticles Via Aspalathus Linearis Natural Extract. J Nanomater Mol Nanotechnol 6:2.

doi: 10.4172/2324-8777.1000212 estimated by the histogram data statistics was 〈φparticle〉 ∼ 1.2 ± 0.3 nm (Figure 4C). To conclude on the degree of crystallinity of Rh NPs, Selective Area Electron Diffraction (SAED) analyses were carried out (Figure 4B). From such observations, it was noticed that, the synthesized particles are amorphous in nature [53]. Figure 5 reports the High Resolution Scanning Electron Microscopy (HR- SEM) of the biosynthesized Rh NPs. The High Resolution Scanning Electron Microscopy were carried out on an Auriga high resolution scanning electron microscope Silicon drift detector operating at 5 keV to identify the surface morphology of the particles. The as prepared Rh NPs displays an amorphous signature which in agreement with SAED analysis. These HR-TEM and HR- SEM results are in agreement with the XRD investigation. The formation of spherical Rh nanoparticles is a strong evidance for the effectiveness of A. Lin. natural extract to act as a reducing agent as well as capping egent.

(EDS) spectrum of the nanopowder deposited onto a Silicon substrate and coated with a thin layer of sputtered Au-Pd. The latter was used to minimize the electron charging during the EDS investigations. The EDS spectra were collected with an Auriga high resolution scanning electron microscope Silicon drift detector operating at 20 keV. The spectra correspond to the EDS profiles of the biosynthesized Rh nanoparticles via A. Lin. natural extract. In addition to the Si , Au and Pd peaks which are related to the substrate and to the conducting metallic surface coating, one can distinguish expected Rh peaks (2.5 - 3 KeV) matching with other reports [54,55]. Additional Na, Mg, O and C contribution peaks are believed to originate from the organic compounds of the natural extract. The washing process of the prepared Rh NPs could help to remove some of these elements. However, ultra high centrifuge will be used and this will affect on the monodispersity of the particles and increase the agglomoration.

Elemental composition

Vibrational bands

Energy dispersive X-ray spectroscopy (EDS) analysis proved to be an efficient chemical tool for qualitative and quantitative elemental analysis. Figure 6 displays the Energy Dispersive X-Ray Spectroscopy

ATR-FTIR spectra were recorded using a Perkin Elmer Spectrum 2000 FTIR spectrometer, employing a single-reflection diamond MIRTGS detector (PerkinElmer Spectrum100, Llantrisant, Wales,

Figure 4: HR-TEM of Rhodium nanoparticles (a); their size distribution (b) and a typical electron diffraction pattern (c).

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Citation: Ismail E, Kenfouch M, Dhlamini M, et al. (2017) Green Biosynthesis of Rhodium Nanoparticles Via Aspalathus Linearis Natural Extract. J Nanomater Mol Nanotechnol 6:2.

doi: 10.4172/2324-8777.1000212

Figure 5: HR- SEM of the biosynthesized Rh NPs.

Figure 6: Typical EDS profile of the biosynthesized Rh NPs.

UK). The samples were analyzed by a universal ATR polarization accessory for the FTIR spectrum series at a resolution of 4 cm-1. Samples were placed on a diamond crystal running each sample 100 times in order to reduce the signal to noise ratio to a minimum of 10, using a constant pressure of 120 psi. The ATR-FTIR results in Figure 7 reveals the corresponding spectra for A. Lin. extract as well as the prepared Rh NPs. Almost all the peaks corresponding to A. Lin. extract are still present in Rh NPs sample spectrum. However there is a slight shift due to the reduction process of rhodium chloride and the formation of Rh NPs. The result revealed a broad absorption band around 3300 cm-1 due to the O-H stretching vibration and the absorbed water molecules [56]. Changes in the intensity of the OH band in Rh sample was noticed, and may Volume 6 • Issue 2 • 1000212

be attributed to the reduction mechanism. The characteristic bands of A. lin. extract at 1,606 cm-1 are due to the C=C stretching [57], while the bands at 1,255 and 1,076 cm-1 correspond to the C-O stretching and -C-O-C stretching, respectively [57]. In addition, a new band at 2930 cm-1 in Rh NPs sample due to asymmetrical stretch vibration of CH2 [43] may be attributed to the reduction of RhCl3 to Rh NPs. The presence of all the functional groups of the extract in the prepared Rh sample, is an indication of their vital effect as capping agent to prevent the aggregation.

Conclusion A novel unique simple green synthesizing of Rhodium nanoparticles via Aspalathus linearis natural extract was carried out • Page 4 of 7 •

Citation: Ismail E, Kenfouch M, Dhlamini M, et al. (2017) Green Biosynthesis of Rhodium Nanoparticles Via Aspalathus Linearis Natural Extract. J Nanomater Mol Nanotechnol 6:2.

doi: 10.4172/2324-8777.1000212

Figure 7: ATR-FTIR of A. Lin. natural extract and the biosynthesized Rh NPs.

for the 1st time. High purity quais-monodisperse spherical Rhodium nanoparticles (Rh NPs) in the range of 0.8-1.6 nm was revealed. The amorphouse nature of the prepared Rh NPs was confirmed via HRTEM, HR-SEM, and XRD investigations. XPS measurments revealed a strong evidence for the formation of the bio synthesized Rh NPs via A. Lin extract. The vital effect of Aspalathus linearis extract as an effective bio-oxidizing/bio-reducing agent [37] as well as a capping compound was demonstrated and confirmed via different characterisation techniques. The follow up study will focus on investigating the effect of annealing temperature on the bio synthesized nanoparticles as well as the possiblity of preparing Rhodium NPs using different plant extracts. Acknowledgements This research program was generously supported by grants from the National Volume 6 • Issue 2 • 1000212

Research Foundation of South Africa (NRF), iThemba LABS, the UNESCOUNISA Africa Chair in Nanosciences & Nanotechnology and the Nanosciences African Network (NANOAFNET) to whom we are grateful. Ms. E. Ismail is also affiliated to the Physics Dept., Faculty of science, Al Azher University, Egypt.

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Citation: Ismail E, Kenfouch M, Dhlamini M, et al. (2017) Green Biosynthesis of Rhodium Nanoparticles Via Aspalathus Linearis Natural Extract. J Nanomater Mol Nanotechnol 6:2.

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Citation: Ismail E, Kenfouch M, Dhlamini M, et al. (2017) Green Biosynthesis of Rhodium Nanoparticles Via Aspalathus Linearis Natural Extract. J Nanomater Mol Nanotechnol 6:2.

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Author Affiliation

Top

UNESCO-UNISA Africa Chair in Nanosciences/Nanotechnology, College of Graduate Studies, University of South Africa, Pretoria-South Africa

1

Nanosciences African Network (NANOAFNET), iThemba LABS-National Research Foundation, Somerset West, Western Cape Province, South Africa

2

Physics Department, Colleague of Science and Engineering and technology, University of South Africa, Florida campus, Johannesburg, South Africa

3

Chemistry Department, Colleague of Science and Engineering and technology, University of South Africa, Florida campus, Johannesburg, South Africa

4

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