Synthesis and characterization of NiO nanoparticles 2 using Manihot esculenta aqueous extracts

Synthesis of nickel oxide nanoparticles (NiO NPs) is a low cost and ecofriendly route that brings great benefits over chemical and physical methods of synthesis. Our approach consisted of using aqueous extracts from treated waste of Manihot Esculenta (Cassava) as reducing, stabilizing and capping agents for the synthesis of NiO NPs. The results proved this approach might be a viable alternative. Extracts were mixed during 30 minutes with Ni(NO3)2.6H2O leading to NPs formation. NiO NPs were characterized through FTIR, XRD, TEM and Raman spectroscopy. NiO NPs showed different shapes and sizes around 5-10 nm in agreement with the particle size calculated by XRD Scherrer equation.


Introduction
Cassava (Manihot esculenta Crantz) is a versatile plant used as food, in pharmaceutical industry and fuel, due to its properties such as high purity, low cost, bio-compatibility and stability to form clear viscous pastes 1.Cassava waste from regional markets contains about 40% starch and 11% cellulose 2, its poor management represents a problem because of the increasing of organic waste.
Another use of cassava waste consists of feeding farm animals, which is a sustainable option in the short-term while new and different applications are found.Recently, green synthesis of nanoparticles (NPs) has risen as an ecofriendly alternative way to prepare NPs leading to the development of creative synthesis routes.Among the common approaches, the use of extracts from several kind of plants have shown positive results in the formation of NPs.Generally, the structure plant extracts is constituted by different metabolites like terpenoids, phenols, proteins or carbohydrates 3, these compounds are the direct responsible of the capability of the extract to carry out reduction/oxidation of NPs.Currently several types of metallic NPs have been synthesized from different solvents, used for the extraction of active compounds.It has been reported that the solvent polarity differences allow to obtain extracts with different chemical composition 4.The NiO NPs have received considerable attention because of their applications as catalytic and magnetic materials 5 and medical applications 6.According to the literature, the particle size, shape, and synthesis route of NiO NPs are key parameters to determine the catalytic behavior.The synthesis of NiO NPs using a cassava extract has been little explored, which awakened our interest in the current investigation.In this work, we prepared three different extracts using cassava waste for synthesis of NiO NPs.

Materials and Methods
Cassava waste was cut into pieces of one centimeter edge and dried at 80 C overnight in an oven.
The first extract named ExDCM included the use of HPLC grade dichloromethane as eluent.
Technical grade ethanol was used as solvent in a 2:1 ratio with respect to the waste.Extracts were macerated during two days at room temperature.The solvent was filtered and eliminated with a rotary evaporator, subsequently solid-phase extraction was done.The second extract was obtained from solvothermal treatment (ST) of waste from cassava in the presence of Nejayote.The previous mixture was poured into an autoclave at 200 °C during 4 hours.When the process finished, we waited for the temperature to drop to 120 °C, the obtained solid was immersed in an ice bath.Then a centrifugation process was carried out at 6500 rpm for 10 minutes (ExNEJ).The third extract (ExAS) was obtained by the ST with the same conditions as ExNEJ, using H2SO4 a 0.2 M.
Three different materials of NiO NPs were synthesized, each one was prepared using 1 g-Ni(NO3)2.6H2O(nickel nitrate hexahydrate, 99.99% purity), mixed with 10 mL of cassava extracts during 30 min at 70° C until complete evaporation.Synthesized materials were placed inside a ceramic tube at 400°C during 4 hours in a nitrogen atmosphere.The temperature inside the furnace was monitored.The nomenclatures of the three samples obtained according to the extract used for the green synthesis of NiO NPs are NiDCM (dichloromethane), NiNEJ (nejayote) and NiAS (sulfuric acid).
The organic functional groups attached to the NPs chemical surface were determined by FTIR Spectroscopy (Tensor II, Bruker, 0.05 cm −1 resolution).Their structural analysis (size crystal and crystal planes) were characterized by X-Ray diffraction (XRD, D2 Phaser Bruker) in the area 20°-100° 2θ and a 0.02° step.The Raman spectra were collected on a Thermo Scientific DXR spectrometer using a 633 nm laser.NPs formation was confirmed by Transmission Electron Microscopy (TEM).

Fourier transform infrared (FTIR)
Functional groups present in cassava extracts were analyzed by FTIR studies in the spectral range of 500-4000 cm -1 .Fig. 1a

Raman analysis
Fig. 2b) shows Raman spectra for the three synthesized samples at 400 ℃ for 4 h, characteristic Raman bands at 496 cm −1 were identified.The Raman peaks at around 500 cm -1 belong to NiO according to the literature, and the main peak at 496 cm -1 shows NiO NPs dominant composition 13, confirming NPs formation from the three cassava extracts according to XRD studies discussed previously.

Transmission Electron Microscopy (TEM)
The morphology and structure of the Ni and NiO NPs were investigated by TEM.Different particle shapes and sizes were observed, as well as, the agglomeration during the particle formation was confirmed.NiDCM sample has an irregular formation with a particle size of around 9 nm.Whereas NiAS sample, shows a hexagonal shape confirmed in all three different synthesized NPs, about 7 nm.Finally, the NiNEJ sample presents a more sphere like formation with a particle size of around 9 nm.These results are in agreement with the crystalline sizes calculated though Scherrer formula, as seen above.

Conclusions
A novel green approach was proposed to synthesize Ni and NiO NPs using cassava extracts as reducing and capping agents.NPs formation was completed with heat treatment at 400 °C for 4 h.XRD and TEM results showed NPs formation with particle sizes smaller than 10 nm.Raman analysis showed a band at 496 cm −1 validating NiO NPs formation.This work proposes a specific methodology for the synthesis of Ni and NiO NPs particles based on cassava extracts.However, other synthesis pathways can be expanded with different biomass extracts.Furthermore, NiO NPs obtained can be used as a catalytic converter for ORR in alkaline fuel cells.
Fig. 1 FTIR spectra of a) Cassava extracts and b) NiO NPs prepared using Cassava extract.

Fig. 2a )
Fig.2a) shows XRD patterns obtained after the samples are calcined, this process enables to obtain Ni and NiO NPs, according to the extract used.NiO NPs samples have peaks at 37.2°, 43.2°, 62.8°, 75.4°, and 79.4° which can be attributed to the main characteristic of the (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) crystal planes of face-centered cubic (fcc) nickel, respectively, and they were indexed in accordance with pattern diffraction file of NiO (PDF 04-01601090).Due to the characteristics of the process itself, extracts present available oxygen in the bulk, which would allow the oxidation process and NiO NPs formation.This phenomenon is also confirmed by FTIR analysis results, where the presence of carbon and oxygen promotes the formation of CO to CO2 during the pyrolysis process 12.NiDCM sample clearly shows Ni NPs formation, according to PDF 04-010-6148, and the results indicate a well-crystallized sample.The main NPS sizes were calculated by using Debye-Scherrer equation, resulting in 8.9, 7.9 and 8.2 nm for NiDCM, NiAS and NiNEJ, respectively.

Figure 2 a
Figure 2 a) XRD patterns NiO NPs and b) typical Raman spectrums of the annealed NiO NPs at 400 C for 4 h.