Abstract

We report on the novel green biosynthesis of Hydrozincite (Zn₅(CO₃)₂(OH)₆) from peanut shell water exacted dye and its conversion to Zincite (ZnO). The structural, morphology and thermogravimetric properties of the as synthesized nanopowders were analyzed using X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy(FTIR), Raman spectroscopy, Scanning Electron Microscopy (SEM) and TG/DTA thermal analysis. The XRD pattern confirmed the formation of Hydrozincite. The correlations of the results from different techniques confirm the formation of hydrozincite and its conversion to Zincite after heat treatment. The obtained ZnO powders are composed of nanoparticles which are well crystalline with a grain size of 31.11 nm.

1. Introduction

Nanomaterials represent an important part of the technological advancements of the 21^st century as they lead to a set of new interesting physicochemical properties ^{1, 2}. Their economic potentialities are considerable and visible in several applications fields such as oil and gas ^{3, 4}, solar energy ^{5, 6}, electronics ⁷, electro-mechanics ^{8, 9, 10}, the biomedical domain ^{11, 12} etc. This development of nanotechnologies has expanded the sources of nanoparticles with the increased production of oxides nanomaterials with exceptional properties such as ZnO, TiO₂ and Vanadium oxides family etc.

ZnO belongs to the class of metal oxides, which present a special interest due to the fact that they are not only stable under difficult conditions but also generally considered to be safe for humans and animals ^{13, 14, 15}. Thus zinc oxide is a semiconductor with attractive properties: the highest piezoelectric effect (e₃₃ = 1.2 C/m²); the highest exciton binding energy (Ei = 60 meV at 300 K); a large band of gap (3.37 eV at 300 K); a shearing modulus (~ 45.5 GPa) higher than ZnSe (18.35 GPa) and GaAs (32.6 GPa) ¹⁶. Its high chemical, thermal and mechanical stability and its aforementioned properties make it attractive for use in solar thermal applications, etc. ¹⁷. It has a very high thermal conductivity (σ= 0.54 W.cm^-1.K^-1) hence its use in nanofluid technology for heat transfer applications where it competes strongly with metal nanoparticles such as gold, copper, aluminum etc. ^{18, 19, 20, 21}.

However, access to its nanostructures requires the development of adapted methods of production. The main production methods often used are physical (laser ablation, Sputtering; spray pyrolysis, etc.) and chemical (electrochemical reduction, hydrothermal synthesis, etc.) ^{22, 23, 24, 25, 26, 27}. The major disadvantage in the physical method is the low yield and in the chemical method is the use of toxic solvents. These physicochemical methods are also very costly and potentially dangerous for the environment. As a result, biosynthesis has received increasing attention due to the growing need to develop environmentally and ecofriendly technology in nanoparticles synthesis. The major advantage of this biological approach is its relative simplicity and its speed in the synthesis of nanoparticles ^{28, 29, 30}. It provides high yields, low toxicity, low cost and biocompatibility ³¹. Another advantage is that the size of the nanoparticles can also be easily controlled by various parameters such as pH and temperature ^{32, 33, 34}. Due to the rich biodiversity of plants, this green synthesis has become a subject of interest throughout the world with different plant species explored recently and evaluated for the synthesis of nanoparticles of zinc oxide. The focus is then placed on an environmentally-friendly procedure involving the bio-reduction/oxidation of zinc salts by extracts from plants such as: Punica Granatum ³⁵, Ocimum Tenuiflorum ³⁶, Aloe Barbadensis Miller ³⁷, Imperata Cylindrica ³⁸, Moringa Oleifera ³⁹, Aspalathus Linearis ⁴⁰, Nyctanthes Arbor-Tristis ⁴¹; Cassia Fistula ⁴², Trachyspermum Ammi ⁴³; Abrus Precatorius ⁴⁴. This green synthesis presents enormous environmental and health benefits including the use of invasive ³⁸ and toxic ⁴⁴ plants and has enabled the production of zinc oxide nanoparticles with excellent physicochemical properties.

In this study, we report on a novel green biological way for the synthesis of ZnO nanoparticles using Arachis Hypogaea shell (peanut shell) dye water extract as a solvent of Zinc nitrate hydrate. Indeed, Peanut is the sixth crop among the most important oil seeds cultivated in the world with an annual output of 37.1 million tons ⁴⁵. Moreover peanut shells represent 20% of dried peanut paste by weight. This means that there is a substantial amount of shell residue left after peanut processing ⁴⁶. However most of these residues are arbitrarily incinerated or throw away and this can add to environmental pollution ⁴⁷ of countries large producers such as China, India, Senegal, etc. It is therefore of great economic and environmental importance to explore the use of peanut shell. Efforts have been made in this direction with its use as a source of fodder for livestock, composting of wet materials, wastewater treatment as insulating board, as well as activated charcoal and ethanol production ⁴⁸. However, it contains many functional components, which are safe for humans and which could be of interest to the green synthesis of nanoparticles. It is in this sense that this study aims to synthesize nanoparticles of zinc oxide from dye of peanut shell and studied their main physical properties.

2. Experimental Details

Peanut shells were collected and dried under sunny conditions. They were then grounded to give a fine powder. An amount of 3 g of this powder was dissolved in 150 ml of deionized water (DI-H₂O) and stirred vigorously using a magnetic stirrer for 3 hours to ensure maximum extraction of the bioactive compounds. The resulting aqueous extract was then filtered to remove residual solids.

In 100 ml of the peanut shell extract, 0.3 g of Zn(NO₃)₂.6H₂O (zinc nitrate hexahydrate) is dissolved therein. This solution after being magnetically stirred for 1 hour, was then placed in laboratory oven at 300°C for 3 hours to ensure the evaporation of water. Then a brown powder was obtained. This brown powder was divided into three parts: P1, P2 and P3. P1 was not subjected to heat and was taken as a reference sample, P2 was heated for 3h and Finally P3 was heated for 4h, all of them at 500°C in an open-air oven.

X-ray diffraction (XRD) spectra of the as-prepared ZnO powders were collected using a diffractometer (Brucker D8 Advance) with theta/2theta geometry, operating with a copper tube at 50 kV and 30 mA and reflection geometry at 2θ values ranging from 10–90° with a step size of 0.01°. Raman spectroscopy measurements were obtained using a T64000 micro-Raman spectrometer (HORIBA Scientific, Jobin Yvon Technology) with a 514 nm laser wavelength and spectral acquisition time of 120 s was used to characterize the as-prepared samples powders. The FTIR was employed to complete the structural investigation using an ALPHA BRUKER, Platinum ATR, the experimental was done in air. The morphology of the as-prepared powders was studied using a high-resolution Zeiss Ultra Plus 55 field emission scanning electron microscope (FE-SEM) operated at a voltage of 2.0 kV. Thermal properties were characterized using thermal gravimetric analysis (TGA, Q50, TA, New Castle, DE, USA).

3. Results and Discussions

The X-ray diffraction patterns (XRD) of the as prepared and annealed powders are shown in Figure 1. Figure 1a shows the X-ray diffraction pattern of the as prepared powders sample P1. The peaks indexed in the spectrum are closely associated with the powder diffraction JCPDS data for hydrozincite [Zn₅(CO₃)₂(OH)₆] (JCPDS number 00-019-1458 and 01-072-1100) as reported in Table 1. In Figure 1b were is shown the diffraction pattern of sample P2, the presence of less but very intense and well defined diffraction peaks is observed compare to sample P1. These peaks were observed at angle of 2θ(^o) ranging from 28.55 to 89.62 (ref Table 2). According to the standard XRD pattern, the annealed powders (P2) can be indexed as the hexagonal wurtzite structure of ZnO as per predicted with crystallographic orientation directed growth (JCPDS number 36-1451) as reported in Table 2. Also it is noted that the intensities of the Bragg peaks of the P2 sample were sharp and narrow compared with the P1. The Debye-Scherrer approximation makes it possible to estimate the average size of the crystalline grains of P2, which was found to be 31.11 nm as compared to 25.15 nm for P1 (see Table 1 and Table 2). It can therefore be said that the annealing of the sample resulted in the formation of ZnO nanoparticles with excellent crystallinity and increased particle size ^{49, 50}. The peak at 2θ(^o) = 23.76 shown in the diffraction profile of P2 belongs to Hydrozincite and confirms that after 3hours annealing in air we still have hydrozincite in the powder. Figure 1c shows the obtained diffractions peaks from P3 which belonging to pure hexagonal Zinc Oxide, which shows the complete conversion of Hydrozincite to Zincite after 4h of annealing in air at 500°C.

Figure 2 shows the SEM images at different magnifications of the morphology of the biosynthesized powders. The visual reading of the images clearly shows the formation of agglomerated particles having hexagonal shapes with a high yield (Figure 2 (a,b,c)). These nanodisks-like stractures have polished surfaces devoid of asperities thus promoting their surface properties ^{51, 52}. We observe a multi-structuration during their growth leading to a policrystallization according to different planes as reported on the XRD results and to the different sizes of the grains (Figure 1a and Table 1). There are also nanograins, which tend to coalesce in a given direction whose extremity forms a hexagonal structure thus giving ZnO nanoplates. After 4h of annealing in air at 500°C the morphology of the powders have changed to spherical-like nanoparticles corresponding to the conversion of the nanoplates like structures to nanoparticles (Figure 2d).

Figure 3 reports on the room temperature Raman spectroscopy of the synthesized nanopowders. Figure 3 (a) shows the spectrum of the P1 sample. We observe the appearance of peaks at position around 62, 98, 102, 145, 730, 1050 cm^-1. The observed peak at 102 cm^-1 can be ascribed to the E₂ (Low) of hexagonal crystal structure of ZnO complex, the peak at 145 cm⁻¹ could be related to local vibrational modes associated with intrinsic lattice defects while the peak at 730 cm^-1 is assigned to A₁ (TO) transversal optical mode of ZnO ⁵³. We assign the mode at 1050 cm⁻¹ to A₁ (TO+LO) combinations at the A and H points according to the group theory. Raman microscopy showed vibration modes for the nanopowders for sample P2 and P3 (Figure 3 (b, c)) around: 98, 203, 335, 388 and 437 cm^-1 assigned to E₂ (Low), 2(A₁(TA) + E₂ (low)), 3E₂ (H)–E₂ (L), A₁(TO) and E₂ (high) respectively for ZnO. The peak at 583 cm^-1 corresponding to A₁ (LO) is correlated with vacancies of oxygen in ZnO, and the others two peaks at 1054 and 1144 cm^-1 are due to A₁(TO+LO) combinations at the A and H points and to E₁ (2LO) respectively ^{54, 55, 56}. After annealing at 500°C (Figure 3 b and c), the E₂ (high) mode appears, and the peak at 142 cm^-1 related to local vibrational modes associated with intrinsic lattice defects disappears which evidence a better crystalline quality of the ZnO nanopowders with the heat treatment. These results confirm the formation of highly crystalline ZnO after the annealing from the decomposition of Zn₅(CO₃)₂(OH)₆ as reported by the XRD results.

The functional groups of samples nanoparticles were analyzed through FTIR spectrum. Figure 4 shows the FTIR spectrum of the biosynthesized nanopowders before and after annealing in air. In the FTIR spectra of P1 sample (Figure 4a), a series of absorption peaks in the range of 500 to 4000 cm-¹ can be found, corresponding to the basic features of zinc carbonate and hydroxyl groups. To be more specific, the broad peak at 3215 cm^-1 is assigned to the O-H stretching vibration mode of O-H group. The main peaks observed at 1332, 1420, 1545 and 1655 cm-¹ are assigned to different carbonate species. The peaks located at 1332, 1420, 1545 and 1655 cm^-1 are attributed to the adsorption of free CO₃^2- species or to antisymmetric O-C-O stretching vibrations. These free CO₃^2- species can be associated with vibration bands at 1055, 802 and 730 cm^-1, which correspond to the v₁ symmetry, v₂ out-of-plane mode and v₄ in-plane mode, respectively, which are characteristic of the Zn₅(CO3)₂(OH)₆ as formed ^{57, 58}.

When the Hydrozincite is annealed at 500°C for 3h (Figure 4b), the content of the carboxylate (COO-) and hydroxyl (-OH) groups in the samples decreased because their broad peaks became small and shifted towards lower wavenumbers. This indicating the possible dissociation of zinc carboxylate and conversion to ZnO during annealing.

Annealing at 500°C for 4 hours, significantly reduces the carboxylate and remove hydroxyl groups as shown in Figure 4c. Whereas the stretching of ZnO NPs are expected at 730, 875 and 940 cm^-1, the FT-IR spectra reveal peak at 1400 and 1055 cm^-1 which may be assigned respectively to the symmetric stretching of the carboxyl side groups in the amino acid residues of the protein molecules and The band to C–N stretching vibration of amine ⁵⁹. No significant absorption peaks at higher wavenumbers are revealed, indicating the nature of the formed ZnO nanoparticles.

The TG-DTG/DTA curves of the biosynthesized samples-NPs are presented in Figure 5. In Figure 5a, corresponding to the P1 sample, the profile of the TGA curve (red line) shows a continuous weight loss with 5 near-sharp changes occurring at 154, 267, 288, 343 and 421°C followed by an almost constant plateau. The peak around 154°C can be attributed to the removal of water, crystallization or evaporation of surface-active molecules adsorbed on the surface of zinc-based complexes during phase solubilization ^{60, 61}. The mass losses accruing around 267 and 288°C can be attributed respectively to the sublimation of the Zinc complex and its conversion to Zinc oxide ⁶². The DTA curve has large exothermic peaks at these temperatures, which can be attributed to the fusion and decomposition process of Hydrozincite and the formation of ZnO, which confirm the Raman, FTIR and XRD results. The loss of mass at the temperature range between 343 and 421°C is very high and the corresponding DTA curve shows two important exothermic peaks. This could be attributed to the formation of zinc oxide nanoparticles and the decomposition of carbonates ⁶³. When the temperature reached above 421°C, no other weight loss was observed. This indicates that the entire reaction process ends with the formation of ZnO nanocrystalline. Hence, annealing above 421°C appears to ensure the formation of stable ZnO nanoparticles. Figure 5b shows the thermal behavior of the ZnO nanoparticles obtained after annealing at 500°C for 3h. It can be seen that the weight losses throughout the temperature range are relatively small in contrast to Figure 5a. Those observed in the temperature ranges of 200 to 400°C are due to the volatilization of the combustible carbonates species present in the sample. The endothermic peak of the DTA curve at 750°C is due to the increase of crystallization rate with increasing temperature ^{64, 65}. This shows the high stability of the ZnO nanoparticles in the measured temperature range.

4. Conclusion

The novel green biosynthesis method was successfully used to synthesis a well crystalline ZnO nanoparticles from the peanut shell water exacted dye. The structural analysis results confirm that indeed the synthesized nanoparticles are of a high quality ZnO. These nanoparticles have the disk-like morphology as seen from the SEM results. The correlations of the results from different techniques confirm the formation of hydrozincite and its conversion to Zincite after heat treatment leads to the ZnO nanoparticles, which are well crystalline with a grain size of 31.11 nm.

Acknowledgments

This work was supported by the TWAS-UNESCO Associate Scheme through the University of South Africa (UNISA) as a host institution. The TWAS-UNESCO Associate holder Prof BD Ngom is very thankful to the TWAS-UNESCO as well as the host institution. We are also thankful to iThemba LABS-National Research Foundation of South Africa and the University of Pretoria for the use of their facilities.

References