Effect of fractional graphite alloying on the properties of aloe-vera mediated green-synthesized NiO_x-C composite

Abstract

This study emphasizes the synthesis of nickel oxide (NiO_x+ C) nanoparticles with varied graphite (C) proportions (1%, 2.5%, 5%, 7.5%, and 10%) using Aloe-Vera natural extract as a green reducing agent. Prepared NiO_x+ C nanostructures exhibited consistent X-ray diffraction patterns with no additional C peak, signifying enhanced interlayer spacing. The crystalline size (D_p) decreased from 27 nm to 20 nm with an increase in the amount of carbon (C). The determined bandgap (E_g) was 2 eV higher than bulk material due to formation extra energy level at defect sites. The shoulder peak at 1620 cm⁻¹ agrees with the disorder-induced band from C and second-order bands of C at about 2700 cm⁻¹ for NiO_x+ C. FESEM images confirm the hexagonal morphology. Not much particle aggregation was observed except for 10% C content. The selected area electron diffraction (SAED) confirms the existence of NiO_x crystalline orientations including planes (111), (200), (220), (311), and (222).

KEYWORDS:

• Nickel oxide
• graphite alloying
• green synthesis
• natural complexing agent

1. Introduction

Metal oxides (MOs) of nanosized have comprehensively been engaged intended for numerous key scientific as well as industrial research for getting access towards novel types of efficient resources with unparalleled characteristics and striking uses. In the modern era, MOs semiconductors having extensive optical bandgap (E_g) are being immensely considered to be cast off for numerous applications such as gas or chemical detectors, and magnetic elements by the virtue of their outstanding photosensitivity, mechanical robustness, enhanced electrical, magnetic and opto-catalyst attributes [1–6]. Transitional metal-based oxides are very important in optoelectronic devices [7,8]. Among many MOs, NiO_x is a semiconductor with p-type electrical conductivity and a comprehensive optical band gap (E_g of 3.65–4.0 eV) following FCC configuration [9,10]. The NiO_x nanoparticle (NP) in conjunction with transparent conducting oxides (TCOs) exhibits excellent electrical properties, making it ideal for photo-responsive devices [11,12]. NiO_x nanostructures have been engaged as hole conductors in hetero junction (p-n) devices of oxide (e.g. rectifiers [13], light emitting diodes [14], gas sensing [15]). Earth-abundant NiO_x has been reasonably used in further remarkable technological engagement in recent electronic devices like energy storage devices [16] with high energy density, Li-ion batteries [17], and electrochromic phenomena [18]. NiO_x NPs hold prospective usages for their remarkable individualities [19]. It has its footprint in surface coating technology for [20] environments, transistors based on thin film [21], arrays in extremely high density for data collection usages [22], temperature sensors, tuned circuits, nickel cermet, etc. [23]. It is mandatory to synthesize pure and excellence crystallite NiOx nanomaterials to get the required features regarding their optoelectronic and morphology for previously mentioned uses. These features seemed to be diligently associated towards the synthesis methods. Governing the crystallographic coordination besides surface smoothness remains very significant while using NiO_x nanomaterials or films. Both solution route such as spin coating [24], spray pyrolysis [25], electrochemical deposition [20], sol–gel [26], chemical co-precipitation [27], micro-emulsion [28], solvothermal [29] and vapour deposition techniques like sputtering [30], electron-beam [31], chemical vapour [32], microwave-assisted synthesis [33] and thermal evaporation [34] are available to synthesis high-grade NiO_x. Nevertheless, selected approaches involve high vacuum and processing temperatures, severe growth environments, costly investigational arrangements, and long elimination times of solid and lenient residual substances or biological additives. Consequently, emerging an easy method aimed at cost-effective, low heat for processing, commercial-scale manufacturing, and precise growth naturally to formulate NiO_x nanoparticles remains a prodigious contest for material technologists. Among these methods, the sol–gel process is a widely adopted technique for nanoparticle synthesis, offering inherent advantages such as minimal requirement for processing heat, ensuring high purity, and consistent compound properties [35–38]. These attributes of sol–gel method permits stress-free incapacitating of other elements on solution phase, suggest an enhanced elemental arrangement mechanism in a less costly manner [39]. That's why the sol–gel process is preferred here for preparing the precursor for NiO_x nanoparticle synthesis, facilitating the investigation of alloying with carbon (C), and enabling further exploration of the material's properties[40–43]. Meanwhile, the inferior actual specific capacitance originates comparatively deprived electronic conductivity in addition of poor effective surface sites in NiO_x nanoparticles. Many attempts are considered by this time to improve the optoelectronic plus morphological factors of different NiO_x forms [44]. On the contrary, insertion of hetero-atom in their different concentrations into intrinsic MO matrix along with different synthesis protocol/parameter has been proven as a suitable alternative to tune optoelectronic and morphological properties of pristine one. Lin et al. showed that N doping could offer better crystalline structure and greater superficial irregularity for NiO_x which had been agreed by other research groups as well [45]. Here we proposed insertion of graphite (C) into NiO_x nanoparticle to formulate NiO_x+ C matrix by sol–gel technique with different concentrations of C element to perform a brief study on the effect of C content into the mentioned matrix. Therefore, the central point of the present analysis remains examining the significant part of C in green synthesized NiO_x nanoparticles referring to photo-physical and morphological attributes.

2. Experimental details

2.1 Materials

Materials: Graphite (C) (powder <20 µm, synthetic), Nickel nitrate hexahydrate [Ni(NO₃)₂.6H₂O] (Purum p.a., crystalized ≥ 98.5%), isopropyl alcohol (IPA) (natural, ≥ 98%, FG) were purchased from Sigma Aldrich Lt., and Aloe-Vera (locally collected), it's water extract (AE) was prepared as the following description.

2.2 Methodology

2.2.1 Collection of aloe-Vera (AE) extraction

As shown in Figure 1, collected Aloe-Vera stems were washed several times by DI (Distilled water) water, dried in normal conditions, and made into slices. Each 15 g Aloe-Vera slice was added to 100 mL DI water in a glass beaker and was stirred at 60°C for 60 min on a hot plate. After the extraction, the mixture was filtered, and the filtrate was stored in the refrigerator to use as a chelating or complexing agent for the synthesis process.

Figure 1. Green synthesis of NiO_x and collection of Aloe-Vera water extract (AE).

2.2.2 Preparation of NiO_x nanoparticles

In order to prepare NiO_x using the green approaching sol–gel method, 14.54 g of Ni(NO₃)₂. 6H₂O (0.05 mol) and 100 mL of AE were taken in a 500 mL beaker. The beaker was set on a hotplate and continuously stirred at a temperature 120°C. When the mixture finally formed a gel and got denser, it dried at 150°C for 12 h before being ground into the final NiO_x.

2.3 Preparation of NiO_x+ C nanoparticles

Different proportions of graphite and green synthesized NiO_x were used to develop the composite materials. Figure 2 illustrates the preparation of NiO_x+ C nanoparticles. First, a 500 mL beaker containing 1 g of NiO_x, 0.1 g (10%) of commercial C, and 50 mL of Isopropyl alcohol (IPA) were filled. The mixture was gently agitated before being dried at 100°C in a vacuum oven, followed by calcination at 500°C (6 h). Following a similar process, five more composites were prepared, using C amounts of 0.01 g (1%), 0.025 g (2.5%), 0.05 g (5.0%), 0.075 g (7.5%) and 0.1 g (10%). All calcined samples were subsequently collected for characterisation.

Figure 2. Synthesis of NiO_x+ C composite.

3. Results and discussions

3.1. XRD Exploration

Figure 3 displays XRD analysis of NiO_x+ C complex containing 1%, 2.5%, 5%, 7.5% and 10% C in the configuration. Altogether, the deflection peaks seemed at angles about 37.25°, 43.29°, 62.90°, 75.65° and 79.72° which stayed indexed to (111), (200), (220), (311) and (222) orientations, respectively (JCPDS 00-044-1159, Rhombohedral, R-3m) [46,47] of NiO_x. The non-stoichiometric 200 crowning is the leading main peck overhead of all aimed at individual portions of C content. Non-stoichiometric elemental composition in a semiconductor, for example, oxygen vacancies due to carbon insertion, may be responsible for influencing p-type semiconductor attributes of NiO_x [48]. The formation of such defect states might initiate a quality decrease in the NiO_x films’ crystalline at higher C content, which could be validated in crystallite size reduction from Table 1. Calculated lattice constants (a) of all prepared different NiO_x+ C matrix expressed well treaty JCPDS No. 00-047-1049. Ahmed et al. [49] and Jassim et al. [50] reported such phenomena, which are acknowledged by the current work. The film crystalline parameters, such as lattice constant (a), crystallite size (D_p), dislocation density (δ) and full-width half maximum (FWHM), were determined from the XRD profile data. Brag's law and Scherrer equation were engaged to determine the lattice parameter “a” and particle size of the corresponding plane (200) for the prepared NiO_x and NiO_x+ C nanoparticles using Equations 1, 2 and 3 [31]. Bragg's Law for X-ray diffraction is: (1) $\text{n}\lambda =2\text{d sin}\left(\theta \right)$(1) The equation for calculating the lattice parameter “a” for a crystal lattice based on the Miller indices (hkl) is given by: (2) $\begin{array}{rl}{\text{d}}_{\left(\text{hkl}\right)}& =\left(\frac{\lambda }{2\text{sin}\left(\theta \right)}\right)\end{array}$(2) (3) $\begin{array}{rl}\text{ a}& ={\text{d}}_{\left(\text{hkl}\right)}\sqrt{{\text{h}}^2+k^2+{\text{l}}^2}\end{array}$(3) The Scherrer equation estimates the average size of crystallites from the following equation: (4) ${\text{D}}_{\text{p}}=\frac{0.9\lambda }{\text{β}\cos \theta }$(4)

Figure 3. XRD pattern (a–e) and shift (f) of different percentages of C in NiO_x+ C composite materials.

Table 1. XRD data analysis of NiO_x+ C composite containing different C content considering 200 as primary crystal orientation.

Sample % of C	FWHM(rad) ×10^−3	dhkl (nm)	a (Å)	Dp (nm)	ε (×10^−3)	δ (×10^11)/cm^2
1%	6.10	0.2087	4.17	20.85	4.05	2.99
2.5%	5.57	0.2089	4.18	25.90	3.63	1.90
5%	5.58	0.2085	4.17	26.72	3.51	1.40
7.5%	6.97	0.2088	4.17	21.39	4.39	2.18
10%	7.23	0.2092	4.18	20.35	4.60	2.41

The microstrain (ε) can be calculated using the formula: (5) $ϵ=\frac{\mathrm{K}\lambda }{\text{D}\cos \theta }$(5) The equation for estimating dislocation density (δ) based on XRD data is: (6) $\delta =\frac{1}{\text{D}\cos \theta }$(6) Where “λ” signifies the X-ray wavelength (0.15406 nm), “d” is atomic interplanar space, K is the shape factor, “θ” is Brag’s angle between approaching X-ray and scattering planes, “a” refers to lattice constant, and β is FWHM at 2θ of diffraction peak. Highly pure NiO_x+ C composite realization might be endorsed since any supplementary diffractogram was absent. The comparatively overextended peaks represented the formation of a small D_p [51].

The XRD configurations, as illustrated in Figure 3f, depict a noticeable change in the diffractogram at the (200) peak position. The C insertion into NiO_x might initiate internal mechanical stress, causing the peak position to be shifted marginally [52]. The 200 crystal orientation maxima negatively shifted towards lower angles (from 43.3° to 43.15°), proposing stretched interplanar spaces amongst highly crystallized NiO_x+ C parallel crystallographic planes [53]. A feeble and wide C diffractogram at 26.5° is witnessed in NiO_x+ C examples, as illustrated in Figure 3. This endorses the presence of unstructured C within a specific matrix [9]. The analysis indicates that the lattice parameters remain nearly constant despite increasing carbon content. However, the average particle size demonstrates sensitivity to the variation in carbon content. Specifically, as the carbon content increases, the particle size initially increases up to 5%, followed by a decrease. It can be observed that initially, FWHM gets reduced to 5% C content, providing better crystallite values.

Further increment of the C content (7.5% and 10%) produces increased FWHM values, leading to the decay of the crystallite size of nanoparticles. The highest and lowest D_p values, 27 and 20.35 nm were achieved for 5% and 10% C, respectively. Reduced D_p might be due to excess C in the lattice. This trend suggests that higher carbon content contributes to the formation of smaller particle sizes within the NiO_x. The δ and ϵ values likewise reached the lowest 1.4 × 10¹¹ cm⁻² and 3.51 × 10¹¹ cm⁻², respectively, for high D_p values (5% C), indicating that insertion of higher C in NiO_x matrix caused D_p reduction also increased the flaws in the nanoparticle atmosphere [54].

Moreover, the observed strain and dislocation density fluctuations coincide with higher carbon percentages. This tendency implies a potential rise in lattice strain and structural irregularities within the composite material as the carbon concentration increases. Overall, these findings suggest that higher carbon content not only influences particle size but also affects the structural integrity of the NiO_x, leading to potential lattice distortions and increased dislocation density.

3.2. Optical property study

UV-Vis spectra of green synthesized NiO_x+ C complex were logged within 300–900 nm wavelength, as shown in Figure 4. The compounds were homogeneously dispersed in distilled water by ultra-sonication for 15 min before running the spectrophotometer. The green synthesized NiO_x+ C NPs show absorption edge shifting of 256, 248, 254, 246 and 236 nm for 1%, 2.5%, 5%, 7.5% and 10% C content, respectively. This phenomenon could be consigned to the captivation of light at inherent E_g. These peaks confirm electronic transitions occurring in the presence of light spectrum moving from the valence band (VB) of O(p) to conduction band (CB) of Ni d($e_g^2$) [55,56]. The E_g was designed utilizing the Tauc relation: (5) $\alpha h\upsilon =A(hv-Eg{)}^n$(5) Where α denotes the absorption coefficient, “A” stands for constant, and the “n” exponent rests on photosensitive transition. The plot concerning (αhυ)² vs photon energy (hυ) had been designed for NiO_x+ C compounds shown in Figure 4 on account of direct band transition. The extrapolation of the linear portion of the curve in the higher energy zone, intercepting the X-axis, provides the bandgap energy (E_g) of the NiO_x+ C matrix. An increase in absorption towards the higher energy range may suggest the emergence of a direct bandgap. Although NiO_x typically exhibits an indirect bandgap, graphene alloying significantly alters the band structure, resulting in the formation of a direct bandgap. A clear linear extrapolation at higher energies in the Tauc plot also indicates a direct bandgap. This method helps estimate the bandgap energy and infer the nature of the bandgap (direct or indirect) [57,58]. The E_g obtained are 5.35, 5.43, 5.63, 5.4, and 5.33 eV with increasing the C content (Figure 5). The extended E_g of NiO_x+ C is higher than the bulk (3.65-4.00 eV) on account of quantum confinement impact with further E_g of ∼2 eV [59] also from chemical imperfections or vacancies occurring laterally at crystal or grain boundaries of metal oxide matrix creating different energy level to rise E_g [60–64].

Figure 4. (a) Absorption spectra and (b-f) calculated E_g by Touc plot of NiO_x+ C matrix containing different C percentages.

Figure 5. E_g variation for all NiO_x+ C NPs at different C amount.

3.3. Raman analysis

Vibrational spectroscopy has been nominated as a super-efficient measure in categorizing the phase along with detecting C in a complex [65]. The Raman spectroscopy of samples within the 200–2700 cm⁻¹ wavenumber range was conducted. The Raman spectra of NiO_x+ C nanoparticles with various C concentrations are shown in Figure 6. Only the TO (Transverse Optical) and LO (longitudinal-optical) modes in Raman spectra have been detected; this may be because of the deprived foundation of D_p sizes. The presence of nickel defects, i.e. Ni³⁺ ions in the NiO_x+ C complex, is indicated by the 1st order slanting mode TO located on 480 cm⁻¹ [66]. A substantial collaboration amid nickel and oxygen bonding is revealed by the tougher 2nd order transverse mode at 1100 cm⁻¹, represented by a 2LO (two-longitudinal-optical) [67]. In this research, we have seen that there is a slight shift in the spectra towards higher wavelength caused by the existence of strain within NiO_x+ C complex. The strain may be developed due to the insertion of C-atoms in the films. The 1620 cm⁻¹ shoulder peak resembles an added disorder-related band originating from graphite. A 2nd order band of C near 2700 cm⁻¹ was observed for NiO_x+ C [68].

Figure 6. Raman spectra of synthesized NiO_x+ C composite with different C content.

3.4. EDX and FESEM investigation

Figure 7(a) illustrates the EDX analysis depicting individual component mapping and confirming the presence of carbon (C) in the NiO_x+ 5%C composites. The EDX spectrum displayed a carbon (C) peak within the NiOx sample, with the inset depicting their respective elemental compositions. Therefore, Figure 7(b) demonstrates EDX mapping of elements as performed. It expresses regular carbon dissemination in NiO_x+ C complexes, reflecting the findings from XRD. The empirical formula from the EDX analysis is calculated as NiO_0.67C_0.17.

Figure 7. (a,b) EDX spectra and (c) EDX mapping image for 5.0 g CA-NiO_x nanostructure.

The morphology of NiO_x+ C carrying at different C quantities is illustrated in Figure 8(a–e). All concentrations of C in NiO_x+ C were quite able to form the morphology of nanocrystals, but lower (1%) and higher (10%) C produced a bit of nanoparticle aggregation observed in Figure 8(a,e), which disappear with intermediate C content [Figure 8(b–d)]. Again, higher elements seem identical and regular profiles with marginally confined size scattering. The particle size distribution curve shows that most of the nanoparticles range from 30–40 nm (Figure 9). However, the morphology was not affected radically by the increase in the concentration of C from 1% to 10% by restricting the nanoparticle's individuality. This study is directed towards preparing NiO_x + C composite using natural reducing agents for innovative usages such as photocatalysts and air purifiers by withdrawing volatile organic compounds, decontaminating, and separating natural and industrial gases.

Figure 8. FESEM images of NiO_x containing different C content.

Figure 9. Particle size distribution from FESEM image of the prepared nanoparticles.

3.5. Morphological studies by TEM

The morphology of the synthesized NiO_x+ C complex was additionally analyzed through TEM. The nanoparticles seem to be accumulated; nonetheless, every element possesses an external coating (Figure 9a). At the early crystallisation stage, the C element acts as a coating agent to cover the particles, restricting them from taking the place of Ostwald coarsening. As a result, the particle size exhibits minimal deviation for each change in C content. The TEM image determined an average particle size of approximately 23 nm. Selected area electron diffraction (SAED) pattern of NiO_x+ C matrix is displayed in Figure 10b. The occurrence of (111), (200), (220), (311) and (222) crystalline orientation matching to FCC conformation and polycrystalline nature of NiO_x was confirmed harmonizing with mentioned the XRD findings.

Figure 10. (a) TEM images and (b) SAED of NiO_x containing 5% C content.

4. Conclusions

Nickel oxide (NiO) is a notable semiconductor material extensively investigated for its diverse applications in electronics, energy storage, and catalysis due to its unique electronic, and optical properties. Graphite (C) incorporated NiO_x nanocomposite were effectively attained via a simple, cost-effective liquid-based sol–gel route, providing an advantageous approach for scalable production. There were no obvious C individual peaks seen from XRD analysis, except at 26.5° for the highest 10% C content relating to the association of the free C presence in NiO_x+ C structure. The determined D_p values fall within the 21–27 nm range. FTIR spectroscopy exploration similarly established the NiO_x segment and survival of C. Moreover, FESEM and EDX plotting were performed for 5% C content sample, validating the findings related to XRD analysis. FESEM assured morphology to hexagonal nanocrystal structure without much noticeable particle agglomeration with slight variation in particle size distribution. The E_g rested with 5.3–5.65 eV through an added E_g∼1.5 eV elevation from bulk values. While pristine NiO_x possesses an indirect bandgap, modifications through doping or alloying are known to alter its band structure, potentially transitioning it to a direct bandgap semiconductor. Disorder-induced bands from carbon and second-order carbon bands at 1620 and 2700 cm⁻¹ were observed. EDX confirmed the presence of carbon, with FESEM images showing hexagonal morphology and minimal particle aggregation except at 10% C. SAED confirmed NiO_x crystalline orientations. The investigation aims to shed light on the impact of carbon doping on the resulting material's electronic structure and potential applications in advanced electronic devices and energy-related technologies.

Acknowledgements

The authors wish to express gratitude for the financial support provided by The National University of Malaysia via the DIP-2021-026 research grant. The authors would also like to express their sincere appreciation to the Researchers Supporting Project number (RSPD2024R579) at King Saud University, Riyadh, Saudi Arabia. 

Disclosure statement

No potential conflict of interest was reported by the author(s).