Green synthesis of NiO nanoparticles using Nigella sativa extract and their enhanced electro-catalytic activity for the 4-nitrophenol degradation

Highlights

• The catalyst was synthesized via green method as function of pH values.

• The NiO nanoparticles exhibit better catalytic and electrocatalytic activities toward 4-Nitrophenol reduction.

• pH values has an influence on the number of active sites.

• The catalyst that has low charge transfer resistance and good ions diffusion has better catalytic activity.

Abstract

Green-synthesized NiO nanoparticles (NiONP_S) have been developed using Nigella Sativa seeds extract as stabilizing agent. The pH of the solution has been modulated, by the addition of NaBH₄. NiONPs have been characterized by XRD, FTIR, SEM and TEM. FTIR measurements and TEM images highlighted that NiONPs were surrounded by some biological compounds coming from the extract. The total reduction of 4-nitrophenol to 4-aminophenol corresponding to the catalytic activity of NiONPs was occurred at pH7, pH9 and pH11 in 60min, 10min and 45min respectively. These results revealed that the best catalytic activity is obtained at pH9 which is confirmed by electrocatalytical activity measurements where the resistance charge transfer corresponding is 53.7 Ω cm² and the Warburg constant 0.0395Ωs^-1/2 related a good ion diffusion, indicated consequently an increase number of electroactive sites. Thus, the carrier charges generated in the NiO mass can easily reach the surface which contribute to the reduction of 4-nitrophenol.

Introduction

It is now obvious that the industry, in general, and its all-round development, has allowed greater comfort of life, including pharmaceuticals that have improved health and longevity, fertilizers that have greatly increased food production, fuels that have made travel easier, and semiconductors that have made computers, electronics and other devices possible [[1], [2], [3], [4]]. However, the fact remains that it has caused, among other things, environmental and ecological damage disastrous, likely to endanger our hard-won settings and even our lives [[5], [6], [7], [8]]. The chemical industry is not an exception to this observations, it is clearly responsible for the deterioration of our ecosystem. It was therefore urgent to remedy this situation. In this context, one of the most relevant and fashionable ways would be to take advantage of the unavoidable nature of the all-round use of nanomaterials in many fields [9,10].

Because of their outstanding properties, these nanostructures have found a wide range of applications, including biotechnology [11], medicine [12], medical imaging [9], electronics [13], optics [13], and catalysis [14].

One of the most innovative methods consists of replacing polluting chemical reagents with living organisms like bacteria [10], fungi [15], plants [16], yeasts [17,18], etc.) during inorganic and organic synthesis, to reduce the quantity of reagents and also save energy. For example, plants seem to be the best candidates to achieve this strategy during the synthesis of nanoparticles on a large enough scale. These nanomaterials produced by plants are rapidly synthesized and distinguished by a better stability [19,20], with a large variety in terms of shape and size [21], compared to those obtained from other living organisms. The advantages of using plant extracts during the green synthesis of nanostructures prompted researchers to examine the reduction mechanisms of metal salts, trying to understand the formation mechanisms of nanoparticles in the presence of plant extracts [22]. Among a lot of metal oxides and due to their specific functional characteristics, as well as their importance in a wide diversity of applications, nickel oxide (NiO) based nanostructures have been largely studied [23,24]. Conventionally, NiO NPs is synthesized though various procedures including: hydrothermal synthesis [25], solvothermal reaction [26], precipitation or co-precipitation [27], electrodeposition [28] and sol–gel process [29]. However, green synthesis methods of NiO NPs, which is cheap easy to achieve and environmentally friendly, are increasingly used. They are realized by simple reflux heating in the presence of a plant extract, that is not only an oxygen donor but also a reducing agent [30,31].

NiO NPs have been successfully synthesized using plant extracts like Moringa oleifera extract [32]. Authors have found that the defect states are responsible for their good antibacterial property and cytotoxic activity. On the other hand, Hydrangea paniculata flower extracts is used in the synthesis of NiO NPs by the green rout, where the NiO nanoparticles are then used in the supercapacitors field [33]. Additionally, NiO NPs was also prepared from Aegle marmelos leafs extract, for the evaluation of in-vitro cytotoxicity, antibacterial and photocatalytic properties [31].

The NiO NPs have been widely studied not only for their various structural and morphological properties [[34], [35], [36]] but also for their wide range of applications, such as therapeutic advances [37], electrodes [38], solar cells [39], or electrochromism [40].

In particular, NiO NPs are attracting increasing attention due to their high electron transfer capacity and chemical stability, as well as of their electrocatalysis and super-capacitance properties [41]. Sabouri et al. synthesized photocatalysts based on NiO NPs characterized by a band gap of about 3.55 eV, and observed that about 80% of methylene blue (MB) degrades, under UV light after 270 min [42]. Moreover, NiO NPs have been elaborated and studied for their many catalytic reaction such as the synthesis of methanol from H₂ and CO₂ [43], the decomposition of ammonium perchlorate [44], or the conversion of toluene to CO₂ [45].

Azhagu Raj and coll, have shown that NiO NPs synthesized in the presence of leaf extract from Coriandrum sativum, with a crystallite size ranging from 15 to 16 nm, reveal good catalytic properties for the oxidation of styrene to benzaldehyde [46]. Indeed, Din and al, showed that the green synthesized of NiO NPs, using Calotropis gigantea extract, presented an excellent catalytic efficiency towards the decomposition of the methylene blue dye, the maximum degradation efficiency being 98.8% after15 min [47]. The reduction of 4-nitrophenol (4-NP) using conventionally synthesized NiO NPs (Hydrothermal [48] and precipitation [49]) has also been widely studied. However, the reports on the use of these green synthesis catalysts are limited and scarce. On the other hand, some authors have shown the ability to elaborate modified electrodes based on NiO NPs. Guofu and al [50] deposited uniform NiO NPs on a variety of substrates. These modified electrodes with NiO NPs have a high capacitance and excellent rate capacity, and can be used as a supercapacitor electrode [[51], [52], [53]]. Besides, the use of the NiO NPs for electrochronic applications [40,50], has been demonstrated to have good cycling stability and high coloration efficiency.

In this study, NiO NPs have been successfully prepared using extract of Nigella Sativa seeds. The synthesis of the nanoparticles have been conducted at various pH by the addition of NaBH₄. It led to the formation of NiO catalysts of different sizes. The physicochemical and structural properties of these nano-objects were investigated using different analytical techniques such as electrochemistry and FTIR, XRD, SEM or TEM respectively. The catalytic activities of the nano-objects were evaluated in the case of the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of NaBH₄ in an aqueous solution. Electrochemical study of 4-NP electroreduction has also been used with the aim of understanding the effect of pH on the active sites density on the NiO NPs, and on the charge transfer resistance of the modified electrode with NiO NPs.