Enhanced C₃H₆O and CO₂ Sensory Properties of Nickel Oxide-Functionalized/Carbon Nanotube Composite: A Comprehensive Theoretical Study

Abstract

Carbon nanotubes (CNTs) functionalized with metal oxides exhibit synergistic properties that enhance their performance across various applications, particularly in electrochemistry. Recent advancements have highlighted the potential of CNT–metal oxide heterostructures, with a specific focus on their electrochemical properties, which are pivotal for applications in sensors, supercapacitors, batteries, and catalytic systems. Among these, nickel oxide (NiO)-modified CNTs have garnered significant attention due to their cost-effectiveness, facile synthesis, and promising gas-sensing capabilities. This study employs quantum-chemical calculations within the framework of density functional theory (DFT) to elucidate the interaction mechanisms between CNTs and NiO. The results demonstrate that the adsorption process leads to the formation of stable CNT-NiO complexes, with detailed analysis of adsorption energies, equilibrium distances, and electronic structure modifications. The single-electron spectra and density of states (DOS) of the optimized complexes reveal significant alterations in the electronic properties, particularly the modulation of the energy gap induced by surface and edge functionalization. Furthermore, the interaction of CNT-NiO composites with acetone (C₃H₆O) and carbon dioxide (CO₂) is modeled, revealing a physisorption-dominated mechanism. The adsorption of these gases induces notable changes in the electronic properties and charge distribution within the system, underscoring the potential of CNT-NiO composites for gas-sensing applications. This investigation provides a foundational understanding of the role of metal oxide modifications in tailoring the sensory activity of CNTs toward trace amounts of diverse substances, including metal atoms, inorganic molecules, and organic compounds. The findings suggest that CNT-NiO systems can serve as highly sensitive and selective sensing elements, with potential applications in medical diagnostics and environmental monitoring, thereby advancing the development of next-generation sensor technologies.

1. Introductions

CNTs are currently considered very promising materials for various applications due to their unique physico-chemical properties: adsorption activity and various types of electrical conductivity depending on the geometric structure [1,2,3,4]. They have excellent mechanical properties, such as a high modulus of elasticity, strength [5,6,7], and flexibility [8,9,10,11]. These properties make it possible to use them in various fields of science and technology [12,13,14], including as components of biosensors [15,16,17,18,19,20,21] and gas sensors [22,23,24]. However, CNTs are insufficiently soluble in aqueous and organic solvents [25]. In addition, they have such long-range Van der Waals gravitational forces that they tend to gather together, and it is very difficult to disperse them [26,27]. Modification of this class of materials by intercalation, surface and edge joining of atoms, molecules, functional groups, etc., makes it possible to overcome the above difficulties and obtain new optical, magnetic, and electrical properties [28,29]. Metal oxides have attractive properties such as high chemical stability, solubility, and adhesion [30]. In addition, they have a high modulus of elasticity and strength at much higher temperatures than many polymers, but at the same time have disadvantages such as brittleness and low fracture toughness [31]. The creation of composite materials based on the combination of carbon nanotubes and metal oxides makes it possible to overcome the disadvantages and combine the advantages of both groups of these materials.

Carbon nanotubes (CNTs) functionalized with metal oxides have emerged as a highly promising class of materials for gas-sensing applications, owing to their unique structural, electronic, and chemical properties [32,33,34]. Among these, nickel oxide (NiO) stands out as a particularly effective modifier due to its exceptional characteristics. First, nickel acts as a catalytic agent, enhancing the stability of carbon nanostructures and facilitating their integration into functional devices. Second, NiO exhibits high electrical conductivity, which significantly improves the sensitivity of the composite material to trace amounts of target gases. Third, nickel possesses the unique ability to form robust bonds with both carbon atoms in CNTs and various functional groups on their surfaces, enabling precise tuning of the material’s properties and enhancing its selectivity toward specific analytes. Since the late 20th century, extensive experimental research has been conducted on nanocomposite materials composed of nickel oxide and carbon nanotubes. These studies have demonstrated their successful application in diverse fields, including the capture of hydroxide ions [35], superconductors [36,37], gas sensors [38], electrochemical catalysts [39], and biosensors [40,41,42]. The versatility and adaptability of CNT-NiO composites highlight their potential as advanced materials for next-generation sensing technologies. This manuscript builds on these foundational studies, employing theoretical approaches to further elucidate the mechanisms underlying the gas-sensing properties of CNT-NiO composites and to explore their potential for practical applications in environmental monitoring and medical diagnostics.

In this study, a comprehensive theoretical investigation was conducted to explore the interaction between carbon nanotubes (CNTs) and nickel oxide (NiO), with a focus on the mechanisms of NiO attachment to the CNT surface and the subsequent impact on the electronic properties and conductivity of the composite material. The findings reveal that the functionalization of CNTs with NiO significantly alters their electronic structure, leading to enhanced conductivity and modified energy levels, which are critical for advanced material applications. Furthermore, the interaction of CNT-NiO composites with acetone (C₃H₆O) and carbon dioxide (CO₂) molecules was systematically examined, demonstrating the potential of these composites for gas-sensing applications. The results suggest that CNT-NiO systems exhibit selective sensory activity toward specific gases, making them promising candidates for the development of high-performance sensor devices. Such devices could be utilized in medical diagnostics, particularly for non-invasive monitoring of diabetes mellitus through the detection of acetone in exhaled air, as well as in environmental monitoring for the detection of harmful industrial emissions. This work provides a theoretical foundation for the design and optimization of CNT-based composite materials with tailored electronic and sensing properties, paving the way for their integration into next-generation sensor technologies for healthcare and environmental protection.

2. Materials and Methods

In this study, a semiconductor non-chiral (6,0) zigzag-type carbon nanotube (CNT) was investigated using density functional theory (DFT) in conjunction with a molecular cluster model. To simulate the finite-length CNT, the dangling bonds at both ends of the nanotubule cluster were passivated using hydrogen pseudoatoms, ensuring a realistic representation of the electronic structure. To enhance the sensing capabilities of the CNT, its surface was functionalized with nickel oxide (NiO). The functionalization process was systematically modeled by incrementally approaching the NiO molecule (in steps of 0.1 Å) toward three distinct adsorption sites on the CNT surface: (1) a surface carbon atom (C), (2) the center of a C–C bond, and (3) the center of a hexagonal ring (Figure 1). Throughout the simulations, the NiO molecule was oriented such that the nickel (Ni) atom was positioned perpendicular to the longitudinal axis of the CNT, ensuring a consistent geometric configuration for comparative analysis. This approach provides a detailed understanding of the interaction mechanisms between NiO and the CNT surface, offering insights into the structural and electronic modifications induced by functionalization, which are critical for optimizing the material’s sensory properties.

The functional group (nickel oxide) was located approximately in the center of the carbon nanotube cluster to exclude the effect of the influence of edge atoms. An example of a CNT model functionalized with nickel oxide for the three considered orientation options is shown in Figure 2.

All calculations were performed within the framework of density functional theory (DFT) using the B3LYP functional and the 6-31G basis set. According to density functional theory (DFT), the properties of a multi-electron system, including energy, can be determined using the electron density functional. The system is described by the electron density as ρ(r):

$$\rho \left(\mathrm{r}\right)=\int \dots \int {\left|{\mathsf{\Phi }}_{\mathrm{e}}\right|}^2{\mathrm{d}\mathsf{\sigma }}_1{\mathrm{d}\mathsf{\sigma }}_2 . . . {\mathrm{d}\mathsf{\sigma }}_{\mathrm{N}}$$

where Φ_e is the many-electron wave function of the system, σ_i is the set of spin and spatial coordinates of the electrons, and N is the number of electrons.

Thus, ρ(r) is a function of only three spatial coordinates r of the point at which ρ(r) gives the probability of detecting any of the electrons in the molecule [11].

If any property of the ground state of a molecule can be expressed as a function of ρ, then the electron energy in DFT is of the form:

$$E\left[\rho \right]=T\left[\rho \right]+Ven\left[\rho \right]+Vee\left[\rho \right]$$

where T[ρ] is the kinetic energy, Ven[ρ] is the potential energy of electron–nuclear interactions, and Vee[ρ] is the energy of electron–electron interactions, which can be written in the form:

$$Vee\left[\rho \right]=VCoul\left[\rho \right]+Vxc\left[\rho \right],$$

where VCoul[ρ] is the Coulomb interaction energy of electrons and Vxc[ρ] is the exchange-correlation energy.

The functionals T[ρ], Ven[ρ], and VCoul[ρ] can be precisely determined [11]. For the exchange-correlation potential Vxc[ρ], the exact representation is unknown, and there are a large number of models to describe it.

B3LYP is one of the most popular and widely used functionals in DFT calculations. It combines local density approximation (LDA) and gradient exchange in the form of gradient approximation (GGA) with some contributions from Hartree–Fock (HF) parameters, which makes it suitable for the description of various types of molecules and chemical reactions. The B3LYP functionality was first proposed by Becke in 1993 [12] and quickly became a popular choice for a wide range of applications, including organic, inorganic, and photochemical systems. Functionality is also used to determine energies, molecular geometries, electronic excitation spectra, thermochemical parameters, reaction barriers, and more. It is based on a hybridization method that combines local functionals and the Hartree–Fock functional, with correlation energy applied in some proportion.

The main benefits are as follows:

• Applicability to a wide range of systems, from small organic molecules to biologically active compounds and transition metals.
• Better accuracy than other functionals, while requiring less computational resources than, for example, high-level approximation methods such as CCSD(T).
• Possibility to describe both covalent and ionic bonds.

Some disadvantages of B3LYP, however, must be considered:

• The functional may be inaccurate for some systems, such as those containing transition metals or strongly polarizable atoms.
• It is not always possible to correctly describe systems with very long links using this functional because of insufficient correlational energy.
• B3LYP may give an incorrect description of donor–acceptor bonds, especially in systems with a large charge distribution.

Another feature of this functional is that the three exchange components are fitted with coefficients chosen on the basis of comparison with experimental data. As a result, the functional acquires the features of a semi-empirical method. It turns out that its accuracy in most cases is significantly higher than in the case of methodologically “pure” functionals. Apparently, this is a consequence of the fact that the exchange energy has a nonlocal character and any attempts to reduce it to local functionals lead to errors. The Hartree–Fock exchange allows one to take into account this nonlocality.

In general, B3LYP is a well-balanced functional for most chemical applications. It has sufficient accuracy and versatility to be useful for the description of various molecular systems, including those considered in the present work. Therefore, in the presented theoretical study, the B3LYP functional was used in the framework of density functional theory.

The 6-31G valence-splitting basis is one of the most widely used bases in quantum-chemical calculations. It is developed for use in calculations of organic molecules, which contain elements from the first to the seventh group of the periodic table [13]. The valence–split basis means that the basis functions in this set are divided into two groups: valence and split functions. The valence functions are used to describe the electronic regions of the outer electron shells of the molecule, while the split functions describe the behavior of electrons inside the atomic nuclei. This separation allows for a more accurate account of the electrostatic interaction between electrons and nuclei.

The number 6 in the name of the 6-31G basis indicates the number of basis functions for each atom in the system. In this case, 6 basis functions are used for hydrogen and halogens (the first and seventh groups), and 31 functions are used for all other elements. The number of basis functions in 6-31G provides sufficiently high accuracy of calculations, while possessing an acceptable time of calculation execution. Other existing bases, for example, 3-21G and STO-3G, in turn, contain fewer functions and provide faster calculations, but their accuracy is usually insufficient for the description of complex organic systems. Therefore, we can consider that the choice of the 6-31G basis in quantum-chemical calculations is justified by its ability to describe nanosystems quite accurately, while providing moderate calculation time.

The energy gap ΔE_G was chosen as the value determining the electronic properties of the nanomaterial in question. It was defined as the difference between the energy of the lowest unoccupied molecular orbital E_LUMO and the energy of the highest occupied molecular orbital E_HOMO:

$$\Delta {\mathrm{E}}_{\mathrm{g}}={\mathrm{E}}_{\mathrm{LUMO}}-{\mathrm{E}}_{\mathrm{HOMO}}$$

(1)

The Gauss Sum program was used to construct single-electron spectra, density of states (DOS), and partial density of states (PDOS) [39].

3. Results

During the functionalization of the carbon nanotube (CNT) with nickel oxide (NiO), the interaction parameters were evaluated for three distinct adsorption configurations: (1) NiO oriented toward a surface carbon atom (C), (2) NiO oriented toward the center of a C–C bond, and (3) NiO oriented toward the center of a hexagonal ring. When NiO was oriented toward a surface carbon atom, the bond formation between the CNT and NiO occurred at a distance of 1.8 Å, with an associated interaction energy of −2.58 eV. In the case of NiO orientation toward the C–C bond center, the bond formation distance remained at 1.8 Å, but the interaction energy increased to −3.17 eV, indicating a stronger binding affinity. Notably, the most energetically favorable configuration was observed when NiO was oriented toward the center of the hexagon, where the bond formation distance decreased to 1.4 Å, accompanied by a significantly higher interaction energy of −3.99 eV. These distances and corresponding interaction energies are graphically represented in the potential energy curves depicted in Figure 3. The results demonstrate that the hexagon-centered configuration is the most thermodynamically stable, making it the optimal choice for functionalization. Consequently, this configuration will be the focus of further investigations to explore its implications for enhancing the electronic and sensory properties of the CNT-NiO composite system.

An analysis of the electron-energy structure of the CNT-based systems functionalized with nickel oxide (NiO) molecules revealed that the molecular orbitals are organized into distinct energy zones, consistent with conventional terminology: the valence band and the conduction band, separated by a band gap (ΔEg). The ΔEg parameter is a critical determinant of the conductive properties of nanostructures. For the (6,0) zigzag-type carbon nanotube, the band gap was calculated using Equation (1), yielding a value of 0.69 eV. This indicates the semiconducting nature of the nanotube, with the band gap playing a pivotal role in governing its electronic behavior. The density of states (DOS) for the pristine (6,0) carbon nanotube is illustrated in Figure 4a, providing a detailed representation of the electronic states available within the valence and conduction bands. The functionalization of the CNT with NiO was found to modulate these electronic properties, as evidenced by shifts in the DOS and alterations in the band gap, which are critical for tailoring the material’s performance in sensing and electronic applications. These findings underscore the importance of surface modification in engineering the electronic structure of CNT-based systems for advanced technological applications. In accordance with the methodology for determining the contributions of charge carriers to bond formation proposed in [43,44], it was found that in the case of nickel oxide joining an atom or the center of a hexagon, π electrons participate in bond formation, which leads to the appearance of an impurity level and a decrease in the band gap. In this case, the electron density shifts towards the nanotube, as indicated by the charge values shown in

Table 1. Comparative table of electron-energy characteristics of the nano-systems under consideration.

Structure	Interaction Distance, nm	Interaction Energy, eV	ΔEg After Adsorption, eV
“Clean” nanotube type (6,0)			0.69
CNT (6,0)—NiO over an atom with	0.18	−2.58	0.68
CNT (6,0)—NiO above the C-C communication center	0.18	−3.17	0.74
CNT (6,0)—NiO above the center of the hexagon	0.14	−3.99	0.66

.

Analysis of the band gap (ΔEg) values revealed that the attachment of nickel oxide (NiO) to the (6,0) zigzag-type carbon nanotube (CNT) significantly modulates its electronic properties. Specifically, the band gap decreases when NiO is positioned above a surface carbon atom or the center of a hexagonal ring, as illustrated in Figure 4b,d, respectively. This reduction in ΔEg suggests enhanced electronic conductivity due to the introduction of additional states near the Fermi level. In contrast, when NiO is attached to the center of a C–C bond, the band gap exhibits a slight increase compared to that of the pristine CNT (Figure 4c), indicating a more localized perturbation of the electronic structure.

Further investigation of the charge distribution within the CNT + NiO systems demonstrated that, in all configurations, electron density is transferred from the nickel (Ni) atoms to the carbon atoms of the nanotube, as detailed in Table 1 and

Table 2. Charge distribution when attaching the NiO molecule to the CNT surface for three location options.

The Variant of Adsorption of the Pt atom	The Value of the Charge on the Nickel Atom Before Joining	The Value of the Charge on the Nickel Atom After Attachment	The Average Value of the Charge of Carbon Atoms on the Surface of the Nanotube Before the Addition of the NiO Atom	The Average Value of the Charge of the Nearest Atomic Neighbors on the Surface of the Nanotube After the Addition of the NiO Atom
CNT (6,0)—NiO over an atom with	0	0.838	0.01	−0.117
CNT (6,0)—NiO above the C-C communication center	0	0.896	0.009	−0.271
CNT (6,0)—NiO above the center of the hexagon	0	0.727	0.011	−0.179

. This charge transfer mechanism designates the Ni atom as an electron donor, while the CNT surface acts as an acceptor. Notably, the oxygen atom in the NiO molecule retains a significant negative charge of −0.47, highlighting its role in stabilizing the system through electrostatic interactions. These findings underscore the critical influence of NiO functionalization on the electronic properties of CNTs, particularly in terms of charge redistribution and band gap modulation, which are essential for optimizing their performance in electronic and sensing applications.

To investigate the sensory interaction capabilities of the CNT-NiO composite system, computational simulations were conducted to model the interaction processes between the composite and gas molecules, specifically carbon dioxide (CO₂) and acetone (C₃H₆O).

The simulation involved systematically approaching the oxygen atom of the nickel oxide (NiO) component with CO₂ or acetone molecules in incremental steps of 0.1 Å, as illustrated in Figure 5. This approach enabled the construction of potential energy profiles for the interaction processes, allowing for the determination of equilibrium distances and corresponding interaction energies, as depicted in Figure 6. Given that the most energetically favorable configuration of NiO on the CNT surface was determined to be position 3 (oriented above the center of a carbon hexagon), the adsorption of CO₂ molecules was simulated specifically for this optimized nanocomplex. This configuration was chosen to ensure the most stable and representative interaction dynamics. The results of these simulations provide critical insights into the binding mechanisms and energetics of gas adsorption on the CNT-NiO composite, highlighting its potential as a highly sensitive material for gas-sensing applications. The systematic approach employed in this study offers a robust framework for understanding and optimizing the sensory properties of CNT-based nanocomposites for use in environmental monitoring and medical diagnostics.

The analysis of the interaction between the CNT-NiO composite nanosystem and gas molecules revealed distinct adsorption characteristics for carbon dioxide (CO₂) and acetone (C₃H₆O). The interaction with CO₂ occurs at a distance of 3.4 Å, corresponding to an interaction energy of −3.13 eV, while the interaction with acetone occurs at a slightly larger distance of 4 Å, with an interaction energy of −2.64 eV. These findings indicate that both gases undergo physisorption on the CNT-NiO surface, with CO₂ exhibiting a stronger binding affinity compared to acetone. Notably, the band gap (ΔEg) of the CNT-NiO system remains largely unchanged upon the adsorption of these carbon-containing molecules, as illustrated in Figure 7. This suggests that the semiconducting properties of the composite are preserved during gas adsorption. However, significant charge redistribution occurs within the “CNT-NiO + gas molecule” systems, particularly on the metal oxide component. Prior to interaction with acetone, the charge on the nickel (Ni) atom was +0.72, and on the oxygen (O) atom, it was −0.47. Following adsorption, the charge on the Ni atom increased to +0.93, while the charge on the O atom shifted to −0.43 (

Table 3. Comparative table of the electron energy characteristics of the CNT + NiO nanosystem in interaction with carbon dioxide and acetone molecules.

Structure	Interaction Distance, nm	Interaction Energy, eV	ΔEg After Adsorption, eV
CNT-NiO			0.66
CNT-NiO + CO2	0.34	−3.13	0.66
CNT-NiO + C3H6O	0.40	−2.63	0.65

). This redistribution of charge indicates a modification in the electronic environment of the system upon gas capture. A similar trend was observed for the adsorption of CO₂, further confirming the sensitivity of the CNT-NiO composite to changes in its chemical environment. These results highlight the potential of CNT-NiO composites as highly responsive materials for gas-sensing applications. The ability to detect and quantify trace amounts of gases such as acetone and CO₂, coupled with the observed charge redistribution, underscores the suitability of these composites for use in medical diagnostics (e.g., diabetes monitoring via exhaled breath analysis) and environmental monitoring (e.g., detection of industrial emissions). The findings provide a foundation for the development of advanced gas sensors with enhanced sensitivity and selectivity.

4. Conclusions

In summary, density functional theory (DFT) was employed to investigate the modification mechanisms of a single-layer (6,0) zigzag-type carbon nanotube (CNT) through the attachment of nickel oxide (NiO) to its surface. The most probable adsorption mechanisms were identified, and the charge distribution, as well as the electronic properties of the resulting CNT-NiO complexes, was systematically analyzed. The results demonstrate that NiO attaches to the CNT surface via chemical adsorption, with three distinct configurations considered for the perpendicular orientation of NiO: (1) above a surface carbon atom, (2) above the midpoint of a C–C bond, and (3) above the center of a carbon hexagon. Among these configurations, the orientation of NiO above the center of the hexagon was found to be the most energetically favorable, exhibiting the highest binding affinity and stability. The interaction between NiO and the CNT surface induces a redistribution of charge, with electron density transferring from the nickel oxide to the carbon atoms of the nanotube. This charge transfer mechanism enhances the electronic coupling between the components, while the band gap (ΔEg) of the CNT remains relatively unchanged, indicating that the semiconducting nature of the nanotube is preserved. These findings highlight the potential of NiO-functionalized CNTs for applications in nanoelectronics and sensing, where controlled charge transfer and tailored electronic properties are critical. The study provides a fundamental understanding of the adsorption mechanisms and electronic modifications induced by metal oxide functionalization, offering valuable insights for the design of advanced CNT-based hybrid materials with optimized performance.

The gas-sensitive properties of the CNT-NiO composite were further investigated with respect to acetone (C₃H₆O) and carbon dioxide (CO₂). The results indicate that both gases undergo physisorption on the composite surface, leading to measurable changes in the electronic and charge distribution properties of the material. Specifically, the adsorption of acetone and CO₂ induces modifications in the density of states (DOS) and charge transfer characteristics, which are critical for gas-sensing applications. These findings suggest that CNT-NiO composites exhibit significant potential as highly sensitive and selective gas sensors. The ability of CNT-NiO composites to detect acetone is particularly relevant for medical diagnostics, such as the non-invasive monitoring of diabetes mellitus through the analysis of acetone concentrations in exhaled breath. Additionally, the sensitivity of these composites to CO₂ makes them promising candidates for environmental monitoring applications, particularly in detecting and quantifying harmful industrial emissions. The observed changes in electronic properties upon gas adsorption underscore the utility of CNT-NiO composites as advanced sensing materials, offering a versatile platform for the development of next-generation gas sensors with applications in both healthcare and environmental protection. This study highlights the potential of CNT-NiO systems as efficient, cost-effective, and scalable solutions for gas detection in diverse fields.