Hierarchical Porous Nickel Oxide Nanoparticles with High Specific Surface Area by Green Synthesis ^†

Abstract

Porous nickel oxide nanoparticles with a hierarchical structure and high specific surface area were obtained by green synthesis followed by thermal annealing. The influence of the choice of precursor plant extract (Fumaria officinalis L. and Origanum vulgare L.) and the extractants in aqueous solutions on the parameters of the synthesized particles was studied. Characterization of the NiO morphology and composition, as well as the specific surface area, was performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and the BET method of nitrogen thermal desorption. Resulting particles have a spherical shape and a size from 30 to 50 nm. According to the data obtained, it can be seen that when the precursor is changed from Fumaria officinalis L. to Origanum vulgare L., the size of the synthesized particles increases, while the structure becomes more friable. It has been revealed that certain parameters and the nature of the assembly of porous particles lead to an increase in the surface area: the highest value of the SSA of 130.0 m²/g is observed in NiO nanoparticles obtained using Fumaria officinalis L. extract based on isopropyl alcohol. Also, a relatively high SSA value of 73.5 m²/g is observed in nanoparticles obtained using the same extractant for Origanum vulgare L. extract, while the use of an ethyl alcohol-based extractant for Fumaria officinalis L. resulted in the lowest value of 40.2 m²/g. The developed semiconductor particles are promising for use in catalysis, sensors, and as part of supercapacitor electrodes and functional layers in device structures for solar cells.

1. Introduction

Currently, one of the key directions of modern scientific and technological progress is the transition from extensive to sustainable development, while the main focus is on maximizing the conservation of natural resources and human health [1,2,3,4].

In this regard, green methods for the production of nanoparticles are actively developing, according to the principles of “green chemistry” and “green technologies”, whose task is to make chemical products, as well as the production process itself, safe and waste-free, in order to eliminate harmful effects on the environment or minimize them. The active development of the green synthesis method begins with the introduction of the idea of green chemistry in response to the Pollution Prevention Act of 1990 [5]. Over the past twenty years, the concepts and practice of green chemistry have moved in separate directions, striving to achieve the triple result—the sustainability of economic, social, and environmental indicators (the three directions of sustainable development or ESG—Environmental, Social, Governance [6,7,8]).

Green synthesis, which refers to a bottom-up technological approach in which metal atoms are assembled into clusters and eventually into nanoparticles, is similar to chemical reduction, in which an expensive chemical reducing agent is replaced with an extract of natural origin to produce metal nanoparticles or their oxides.

The use of plant extracts in green synthesis to produce metal nanoparticles and their oxides is the most widespread method due to their accessibility, simplicity, and fast reaction time, in addition to their ability to reduce metal ions to metallic nanoparticles and to produce nanoparticles on a large scale [9,10,11].

Plant extracts make it possible to control the synthesis of nanoparticles to achieve well-defined sizes and morphology during single-stage synthesis with high yield [12,13,14,15,16]. Many published studies on the synthesis of biological objects have demonstrated that it is possible to achieve a more specific size and morphology than using certain physical and chemical methods [17].

Porous metal oxide nanoparticles are a class of nanomaterials of great scientific and technological importance [18,19]. Thus, nickel oxide nanoparticles can be used in catalysis [20,21] and in the design of sensors [22] and batteries [23]. The increased surface area makes it possible to achieve greater dispersion of the catalyst, ensuring more efficient use of catalytic materials. In addition, the sharp curvature of the nanoparticle surface leads to an increase in surface energy. This may lead to an increase in the reactivity or catalytic activity of the carrier [24]. Nickel oxide nanoparticles are actively used in sensorics as sensitive elements since they have high chemical stability [25,26].

One of the main factors that has a primary impact is the choice of phytochemicals, and, accordingly, the type of the plant itself. Different plants contain different sets of metabolites involved in the synthesis of nanoparticles as reducing agents and stabilizers. It also provides the choice of extractant in the preparation of the extract [27]. The choice of synthesis conditions, such as the plant extract, also affects the value of the specific surface area of nickel oxide particles. Examples of the values of the specific surface area of NiO, depending on the precursor plants used are shown in

Table 1. Examples of specific surface area values of nickel oxide nanoparticles obtained by green synthesis using various plants.

Plant	SSA, m2/g	Source
Croton macrostachyus	22.4	[28]
Parkia biglobosa	88.5	[29]
Aloe vera	58.4	[30]
Tamarix serotina	126.1	[31]
Spirogyra sp.	16.7	[32]

.

In this paper, porous nickel oxide nanoparticles obtained by green synthesis using extracts of Fumaria officinalis L. and Origanum vulgare L. [33] as sources of biological reducing agents are investigated, as well as the effect of choosing an extractant based on isopropyl alcohol or ethyl alcohol to produce a plant extract.

This paper also describes a model of hierarchical assembly of the porous structure of the obtained nickel oxide particles, and explores the possibility of using the template synthesis method to obtain compositions based on porous nickel oxide and porous silicon particles. Such multimodal porous compositions are of great interest for use in various fields where an increase in specific surface area or the presence of pores of different sizes is required [34,35,36,37].

2. Materials and Methods

2.1. NiO Nanoparticles Synthesis

The plants Fumaria officinalis L. [38] and Origanum vulgare L. were selected for a comparative analysis of the effect of precursor selection on particle formation. Extracts from plant raw materials were obtained by ultrasonic extraction. To obtain plant extracts for all samples, 25 g of the plant was used in dried form per 100 mL of water–alcohol solution. A 1:1 solution of isopropyl or ethyl alcohol and water was used as an extractant. The description of the obtained samples is presented in

Table 2. Conditions for obtaining plant samples and extracts for the synthesis of nanoparticles.

Sample Number	Synthesis Conditions
	Plant Extract	Extractant	Annealing Temperature
Sample 1	Fumaria officinalis L.	Isopropyl alcohol: distilled water (1:1)	500 °C
Sample 2	Fumaria officinalis L.	Isopropyl alcohol: distilled water (1:1)	400 °C
Sample 3	Fumaria officinalis L.	Isopropyl alcohol: distilled water (1:1)	300 °C
Sample 4	Fumaria officinalis L.	Ethyl alcohol: distilled water (1:1)	500 °C
Sample 5	Origanum vulgare L.	Isopropyl alcohol: distilled water (1:1)	500 °C

.

In the synthesis, a solution containing 15 mmol of NiSO₄ (LenReactiv JSC, Saint-Petersburg, Russia) was mixed with a plant extract with the addition of a 2% NaOH (LenReactiv JSC, Saint-Petersburg, Russia) solution. The volume of 2% NaOH added does not exceed 1 mL per 15 mL of nickel salt, and NaOH is only an activator of the reaction. After adding a small amount of NaOH, a small number of nuclei appeared in a cloudy dark green solution of NiSO₄ and a plant extract, which indicates the start of a reaction between all three components. At the same time, after half an hour, the solution became transparent, and the formed particle nuclei were in the sediment; this indicates a reaction with a plant extract, which confirms the operation of the green synthesis method based on the use of plant extract as a reducing agent. An average optimal concentration of 2% was taken based on published papers in order to avoid a reaction between NaOH and nickel salt. It can be noted that there are works in the literature devoted to the study of the effect of the amount of NaOH added in the process of green synthesis using plant extracts, and works where a small amount of NaOH is also used to activate the reaction [39,40].

The resulting mixture was washed and centrifuged. To form nickel oxide nanoparticles, annealing was performed at 300–500 °C for 30 min.

To analyze the effect of the choice of extractant, nickel oxide nanoparticles were obtained using Fumaria officinalis L. plant, and aqueous solutions of isopropyl and ethyl alcohols were used to obtain the plant extract.

The scheme of the green synthesis of nickel oxide nanoparticles is shown in Figure 1. The results of the study of particles obtained after drying and before annealing were published earlier [25,41]. The atomic composition of the obtained nanoparticles was determined by X-ray Photoelectron Spectroscopy; as a result of the research, Ni, Si, and O atoms, C and S contamination atoms, and insignificant amounts of N and Na were detected. The energy position of Ni2p maximum before purification was 856 eV, and after purification it was 853.5 eV. The position of Ni2p3/2 (856 eV) corresponds to compounds of Ni or nickel oxide (Ni₂O₃); it may also be an unreacted part of the nickel salt. The position Ni2p3/2 (853.5 eV) corresponds to metallic nickel or slightly oxidized nickel.

2.2. XRD

The phase composition of the samples was studied by X-ray phase analysis using a powder X-ray diffractometer DRON-8N (Bourevestnik JSC, Saint-Petersburg, Russia). This instrument features a Mythen2 R 1K linear position-sensitive detector (PSD) (Dectris Ltd., Baden, Switzerland) and a parabolic Goebel mirror. Powder X-ray diffraction was used with X-rays emitted from a Cu anode as the source of diffraction patterns. The X-rays were scanned at an angle of 10–70° two-theta with a 0.01° step and an exposure time of 10 s.

2.3. Scanning Electron Microscopy and Transmission Electron Microscopy Methods

The morphology of the obtained particles was studied using scanning electron microscopy (TESCAN MIRA3 electron microscope, TESCAN, Brno, Czech Republic). For SEM studies, nickel oxide particles were dispersed in isopropanol and applied to the cleaned polished monocrystalline silicon substrates, then dried. Cleavages of a series of composite porous silicon and nickel oxide samples were also examined. The SEM studies were conducted in secondary electron mode at typical accelerating voltages of 3–20 kV.

TEM imaging was performed on a Hitachi HT7700 electron microscope (Hitachi High-Technologies Corp., Tokyo, Japan) with thermionic electron source optimized for material characterization at relatively low accelerating voltages up to 120 kV.

2.4. The BET Method for Studying the Specific Surface Area

The 4-point BET method was used to study the parameters of the resulting porous structure, such as the specific surface area [42,43].

The determination of the specific surface area is based on measuring the amount of adsorbate gas sorbed on the surface of the test sample at a temperature of liquid nitrogen and various relative partial pressures P/P₀ (P is the partial pressure of the adsorbate, P₀ is the pressure of saturated steam of the adsorbate at a temperature of liquid nitrogen T = −196 °C).

The specific surface area was studied using a Sorbi device (META CJSC, Novosibirsk, Russia).

2.5. Capillary Condensation Method

According to the IUPAC definition, bodies with a porous structure are divided into microporous (pore diameter is less than 2 nm), mesoporous (diameter ranging from 2 to 50 nm), and macroporous (more than 50 nm) [38]. As is known, larger pores, macropores, are not filled in bulk in the sorption process. A distinctive feature of adsorption in the mesopores of the material is the presence of a capillary condensation process.

The process of capillary condensation can proceed both reversibly and irreversibly. With irreversible condensation, there is hysteresis on the adsorption isotherm, whereas, with reversible condensation, the adsorption and desorption branches coincide.

2.6. Template Synthesis Method for the Formation of Compositions Based on Nickel Oxide and Porous Silicon

To obtain a composition of porous nickel oxide and porous silicon (porSi) by template synthesis, at the moment of mixing the plant extract and a solution containing nickel sulfate, as described in Section 2.1, plates of porous silicon KEF-4.5 (111) obtained under various etching conditions were preliminarily immersed in the nickel sulfate solution (the samples are described in

Table 3. Description of porous silicon samples for use as templates in the template synthesis of compositions.

Sample Number	Conditions for Obtaining Porous Silicon
	Anodizing Current	Etching Time	Comments
porSi 1	$j_a=30 {\displaystyle \frac{\mathrm{m}\mathrm{A}}{{\mathrm{c}\mathrm{m}}^2}}$	20 min	–
porSi 2	$j_a=180 {\displaystyle \frac{\mathrm{m}\mathrm{A}}{{\mathrm{c}\mathrm{m}}^2}}$	20 min	Nanorods

).

In this experiment, an extract of the Fumaria officinalis L. plant was used, obtained using a solution of isopropyl alcohol and distilled water (1:1). The further experiment differed only in that, in order to remove excess moisture from their pores before annealing, the obtained porous silicon plates with nanoparticles were dried in a dry place for several days to completely evaporate moisture inside the channels of the pores of the porous silicon plates. The annealing temperature of the dried samples was also 500 °C.

$j_a=30 {\displaystyle \frac{\mathrm{m}\mathrm{A}}{{\mathrm{c}\mathrm{m}}^2}}$$j_a=180 {\displaystyle \frac{\mathrm{m}\mathrm{A}}{{\mathrm{c}\mathrm{m}}^2}}$

2.7. EDX Method

The qualitative composition of samples obtained by template synthesis (Section 2.6) was provided by X-ray spectral microanalysis using the EDS OXFORD instruments X-MaxN80 detector (Oxford Instruments, High Wycombe, UK).

3. Results and Discussion

3.1. The Structural Characterization of NiO Samples

The XRD patterns of the NiO samples obtained under various conditions are shown in Figure 2.

XRD patterns show the diffraction peaks of the (111), (200), (220), (311), and (222) crystal planes, corresponding to the face-centered-cubic (fcc) structure of the NiO.

As can be seen from Figure 2, for samples using medicinal smoke at temperatures of 400 °C and 500 °C, nickel oxide phases are obtained, while at temperatures of 300 °C an amorphous phase is obtained, which indicates an unformed nickel oxide phase and which is probably influenced by the biological components present in the structure that are not removed at this temperature.

The Debye Scherrer equation,

$$D={\displaystyle \frac{K\lambda }{\beta cos\theta }}$$

is used to calculate the crystalline size of the nanoparticles, where D is the nanoparticle crystalline size, K represents the Scherrer constant (0.98), λ denotes the wavelength (1.54), and β denotes the full width at half maximum (FWHM). The results of the crystallite size calculations are presented in

Table 4. Calculated crystalline size of the nanoparticles.

Sample Number	Size of the Nanoparticles
Sample 1	7.5–13.9 nm
Sample 2	5.5–8.0 nm
Sample 4	5.6–8.3 nm
Sample 5	5.6–7.6 nm

.

As can be seen from Table 3, the particles with the largest size are synthesized using an extract of Fumaria officinalis L. on isopropyl alcohol. The use of ethyl alcohol in the production of plant extract helped to reduce the particle size. The same reduction in the size of nanoparticles is facilitated by the use of an extract of Origanum vulgare L. as a reagent.

3.2. Investigation of the Morphology of NiO Nanoparticles

To compare the NiO nanocrystallites obtained before and after annealing, TEM and SEM studies were performed using the example of Sample 1 (Figure 3).

As can be seen from Figure 3a, the particle sizes before annealing are 3–5 nm, which indicates sintering and particle enlargement during annealing at 500 °C (Table 2). Figure 3b shows a typical appearance of the studied materials after drying and before annealing: the organic component envelops and permeates the aggregates of nickel oxide particles. Subsequently, during high-temperature annealing, the organic component is removed. As will be shown below, this is an important factor: the organic fragments act as a kind of framework element that prevents the nanoparticles from sintering during annealing and maintains the porous structure of the resulting porous hierarchical material. This, in turn, ensures a high specific surface area of the nickel oxide.

Images of the morphology of nanoparticles obtained by scanning electron microscopy are shown in Figure 4 and Figure 5.

At the stage of washing the samples, it was noticed that the number of nanoparticles obtained using Origanum vulgare L. extract was less than the number of nanoparticles obtained using Fumaria officinalis L. extract.

According to the data obtained, the synthesized nanoparticles have a developed porous structure, and when using Origanum vulgare L., the particles become more friable, and the particle size is about 30 nm, and when using Fumaria officinalis L., the nanoparticle size is 30–50 nm.

Images of the morphology of nickel oxide nanoparticles synthesized with extracts of the Fumaria officinalis L., obtained using extractants of different compositions, are shown in Figure 6.

As can be seen in Figure 6c,d, the structure of the resulting sample is very dense. The use of an ethyl alcohol solution as an extractant led to the production of particles of a porous structure with a denser packing compared to particles obtained using an isopropyl alcohol solution. This effect may be related to the difference in viscosity of alcohols.

3.3. Investigation of Porous Structure

A comparison of the specific surface area (SSA) of the obtained samples is shown in

Table 5. Values of the specific surface area of nickel oxide nanoparticles obtained under different conditions.

Sample Number	SSA, m2/g
Sample 1	130.0 ± 6.7
Sample 4	40.2 ± 0.8
Sample 5	75.3 ± 1

.

According to the obtained data, it can be seen that Sample 1, obtained using an extract of Fumaria officinalis L. on isopropyl alcohol, has the largest surface area SSA = 130.0 m²/g. The value of the specific surface area for the sample obtained using the extract of Fumaria officinalis L. with ethyl alcohol is significantly lower and amounted to 40.2 m²/g, which correlates with a more compact structure and fewer visible pores in Figure 5. For the NiO sample obtained using Origanum vulgare L., the specific surface area was 75.3 m²/g, which also correlates with the SEM image in Figure 4.

For the studied Samples 1, 4, and 5, pore size distributions were obtained by the capillary condensation method (Figure 7).

As can be seen from the pore size distribution diagram, Sample 1 has only two pore sizes that differ by about 2 times. In Sample 4, pores of 3–4 nm, 6–8 nm, 14 nm, 33, and 51 nm are recorded in approximately equal volumes. As in Sample 1, the pore levels in Sample 4 increase by about 2 times. The same trend is observed for Sample 5: pore sizes of 3–4 nm, 8 nm, 24 nm, and 51 nm are also approximately equal in volume.

Earlier in [35], a model of the formation of porous nickel oxide particles was presented, taking into account the influence of the centrifugation rate during synthesis (Figure 8).

Figure 9 shows a model of the hierarchical structure of the obtained NiO particles based on quasi-two-dimensional projection of a three-dimensional deterministic fractal Julien aggregate.

Based on the images obtained by the TEM method and the results of the pore size distribution according to the capillary condensation study (Figure 7), as already noted, two lower structural elementary levels were identified, from which larger porous particles are formed, and two types of mesopores, which differ in size by a factor of 2. It is assumed that the structure of nickel oxide is formed due to the sequential connection of quasi-spherical particles. The model of Julien’s fractal aggregate is the closest to the type of particles being formed (Figure 9). In the three-dimensional case the idealized Julien fractal model, which is an assembly of equal-sized initial particles, is complemented by three particles in the upper layer and three particles in the lower.

3.4. Investigation of the Possibility of Using the Template Synthesis Method for the Formation of Compositions Based on Nickel Oxide and Porous Silicon

The samples obtained in Section 2.6 were examined by the EDX method. The maps obtained for the samples in Table 3 are shown in Figure 10 and Figure 11.

As can be seen from Figure 10d,e and Figure 11d,e the EDX images show a fairly significant nickel and oxygen content throughout the entire depth of the porous silicon substrate, which allows us to conclude that the synthesis of NiO using porous silicon templates has been successful.

It was found that the chosen method allows for spatially homogeneous intercalation of nickel oxide, more than 20 μm deep, into the volume of a meso-macroporous matrix of porous silicon. At the same time, the guest material was introduced throughout the entire thickness of the porous layer, despite the complex, hierarchically organized morphology of the porous structure. The results obtained indicate the high efficiency of the template synthesis method for obtaining composite systems based on porous silicon and nickel oxide and confirm the prospects of this approach for creating functional materials with controlled structure and properties.

Thus, the possibility of using the template synthesis method to form composite materials based on nickel oxide, which acts as a guest component, and porous silicon, which is used as a template matrix, was investigated.

4. Conclusions

Nickel oxide particles were obtained by the green synthesis method using extracts of Fumaria officinalis L. and Origanum vulgare L. as precursors, and aqueous solutions of ethanol and isopropanol as extractants.

XRD patterns confirmed the crystalline structure of NiO NPs. Scanning electron microscopy studies have shown that the resulting particles have a spherical shape and a size from 30 to 50 nm, further aggregating while maintaining the porous structure into larger formations. According to the data obtained, it can be seen that when the precursor is changed from Fumaria officinalis L. to Origanum vulgare L., the size of the synthesized particles is larger, while the structure is more friable. When the extractant is changed from isopropyl alcohol to ethyl alcohol, the packing density of the particles increases.

According to the obtained data, it can be seen that NiO obtained using an extract of Fumaria officinalis L. with isopropyl alcohol had the largest surface area SSA = 130.0 m²/g; NiO obtained using the extract of Fumaria officinalis L. with ethyl alcohol was significantly lower and amounted to 40.2 m²/g, which correlates with a more compact structure and fewer visible pores; NiO obtained using Origanum vulgare L. with isopropyl alcohol had SSA = 75.3 m²/g.

The results of particle morphology studies at various scales, the data on meso- and macropores using capillary condensation, and BET data on a specific square surface suggest the “bottom-up” assembling mechanism with Julien fractal type aggregation. This type of particle assembly leads to the formation of a hierarchical porous structure with a developed morphology. Plant-based reducing agents play a key role in this process, not only ensuring the reduction of nickel metal but also acting as a kind of framework element that prevents particles from sticking together during annealing and maintaining the porous structure, resulting in high specific surface areas. The experimental study of template green synthesis of compositions based on nickel oxide (as a guest material) and porous silicon (as a template) demonstrated the possibility of deep (more than 20 μm) and uniform intercalation of the guest material into a porous meso-macroporous matrix of porous silicon with a complex morphology. This is an important result for development of functional elements (sensors, catalytic, etc.) in an integrated design on silicon.

The ability to control the properties and quantity of nanoparticles produced opens up new horizons for their research and application. The porous structure of NiO provides a large surface area, improving its characteristics for use as electrode materials in supercapacitors and batteries, and as a material for catalysis and sensors.