Platinum Atom-Functionalized Carbon Nanotubes as Efficient Sensors for CO and CO₂: A Theoretical Investigation

Abstract

This study presents a theoretical investigation of platinum-modified single-wall carbon nanotubes (SWCNTs) of types (6.0) and (6.6) for their potential application as gas sensor materials. Quantum chemical calculations using density functional theory (DFT) were performed to evaluate the interaction mechanisms with carbon monoxide (CO) and carbon dioxide (CO₂) molecules. The results revealed that pristine SWCNTs exhibit weak and unstable interactions with CO and CO₂, indicating limited sensing capabilities. However, the modification with platinum atoms significantly enhanced their adsorption properties. The most energetically favorable configuration was found when the platinum atom was located at the center of a C–C bond on the SWCNT surface, ensuring the stability of the metal-functionalized system. The Pt-modified SWCNTs exhibited stable sorption interactions with CO and CO₂, characterized by weak van der Waals forces, enabling the reusability of the sensor without contamination. Additionally, the adsorption of these gas molecules induced changes in the band gap of the nanocomposite system, indicating a variation in conductivity upon gas exposure. The distinct band gap changes for the CO and CO₂ adsorption suggest the selectivity of the sensor towards each gas. Overall, the results demonstrate that platinum modification effectively enhances the sensing performance of SWCNTs, paving the way for the development of highly sensitive and selective nanosensors for CO and CO₂ detection based on changes in electronic properties upon gas adsorption.

1. Introduction

The active development of various fields of industry provides humanity with the opportunity to continue on the path of progress, improving the quality of life of the population through the mass production of diverse products and the creation of new materials and technologies. However, this fact is only one side of the coin, and we should not forget about the accompanying problems that follow it inextricably. The main factor of these problems is the deterioration of the global environmental situation and dangerous working conditions in certain professions related to the extraction of raw materials and the operation of installations and equipment deserve special mention. The expanding needs of the industry, increased attention to safety technology, and the growing requirements of standards devoted to this issue lead to an increasing demand for gas sensors [1,2,3]. A gas sensor is a device designed to track the presence of various gases by measuring changes in physical, chemical, electrical, and magnetic characteristics [4,5,6]. Since their introduction, gas sensors have secured the status of irreplaceable and ubiquitous devices in the lives of a huge number of people. However, many of their modern analogs have disadvantages, such as high operating temperatures [7,8] and an inefficient detection of gases in low concentrations [9]. The use of carbon nanotubes (CNTs) as the main element of a sensor device is selected to solve many of the existing problems, since they have such unique properties, such as a large specific surface area, providing a wide range of possible adsorption centers, and high reactivity. These and other properties make carbon nanotubes an excellent material for gas sensors [8,10,11,12,13].

However, despite most of the positive aspects and possible prospects for use, chemical sensors based on various carbon nanotubulenes may often lack selectivity [14]. Therefore, attempts have been made to improve the basic characteristics of carbon nanotubes (such as sensitivity and selectivity) in order to use them as part of a sensor device. One of the possible methods is to modify the surface of nanotubes with noble metals, metal oxides, and polymers. From studies [14,15,16,17,18], it becomes obvious that such CNT modifications can increase the sensitivity, adsorption capacity, and selectivity, which is extremely important for sensors whose purpose is to detect harmful gases. Modifications of carbon nanotubes are aimed at increasing the number of adsorption centers and creating channels for the passage of electrons from or to CNTs [13,14,15]. Such modified carbon nanotubes are playing an increasingly important role in the research, development, and application of nanomaterials and systems based on them [19].

As an example, in [20], the ability of multi-walled carbon nanotubes (MWCNTs) impregnated with polyethylenimine (PEI) to adsorb carbon dioxide was studied. The MWCNT was obtained using a two-step process: the pretreatment of the MNT with sulfuric and nitric acids, followed by wet impregnation with PEI for functionalization with amines. As a result, a good indicator of carbon dioxide adsorption was established and a rapid recovery of the sensory system after interaction based on the results of cyclic tests was demonstrated. Reference [21] was devoted to the consideration of a formaldehyde film acoustic sensor with PEI-modified single-walled carbon nanotubes (SWCNTs) as a sensor. In [22], the adsorption of gas molecules and atoms onto carbon nanotubes modified with platinum is investigated. The authors carry out computer modeling using the Lanl2dz basis. And in a paper by [23], the authors propose using platinum-modified nanofilms to detect greenhouse gases and household safety gases, including CO and CO₂. An article [24] describes gas sensors, which are films based on platinum and palladium with functionalized carbon nanotubes. These nanomachines are capable of detecting gases in extremely low concentrations, and the operating temperature of these nanomachines is 200 degrees Celsius. This is a significant disadvantage. Further similar experimental studies on this topic have not been conducted, despite the fact that the topic is undoubtedly relevant. The experimental results presented in this paper indicate a high sensitivity of the sensor at room temperature to indoor air pollution. It has been established that the respiratory monitoring of carbon dioxide (CO₂) is an important tool for medical diagnostics [24,25,26,27]. Although optical CO₂ gas monitors are well designed for bedside monitoring, there is still a need for portable gas monitors in many emergency scenarios [28] where it is impractical to use large sampling tubes. Such small portable respiratory CO₂ sensors would find immediate use in emergency diagnostics, home monitoring kits, sleep apnea monitoring, etc. In addition to these specialized applications, such sensors could also be a cheap alternative to existing respiratory monitors used in anesthesia and other clinical applications [29].

However, despite the rather intensive experimental studies of the sensory sensitivity of carbon nanocomposites, theoretical studies of the mechanisms of modification of carbon nanotubes, including the mechanisms of interactions of such composites with various gases, are still very limited. In this work, a theoretical study of the mechanism of interaction of carbon nanotubes of various types with a platinum atom modifying the CNT surface was carried out. Then, the sorption activity of the CNT + platinum nanosystem against carbon monoxide and carbon dioxide molecules was studied. To prove the necessity and effectiveness of modifying the surface of carbon nanotubes with platinum atoms, studies were conducted on the interaction of gas molecules with the so-called “pure” carbon nanotube (that is, a nanotube without any modifications).

2. Methodology

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) according to the Equation (1):

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

(1)

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.

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 (2):

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

(2)

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 (3):

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

(3)

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 [29]. For the exchange-correlation potential Vxc[ρ] the exact representation is unknown and there are a large number of models to describe it [30].

B3LYP is a hybrid method, with a three-parameter functional (the exact Hartree–Fock exchange operator, the Becke functional, and the Slater functional); the correlation part is a combination of the LYP and VWN functionals. Its uniqueness lies in the fact that the three exchange components are taken with weighting coefficients. This method has semi-empirical features, as a result of which high-accuracy calculations can be achieved. LanL2DZ is a basis based on a combination of pseudo-orbitals with Gaussian functions. It is used for DFT calculations in the study of compounds or clusters containing heavy elements [31,32,33].

3. Results

To determine the effect of the modification of the SWCNT surface by platinum atoms on the sorption efficiency of the nanotube with respect to carbon monoxide and carbon dioxide molecules, the possibility of the interaction of a pure carbon nanotube with selected molecules was studied at the first stage of the research. A molecular cluster of nanotubes (6.0) consisting of eight layers of carbon hexagons along the longitudinal axis of the tube was considered as an SWCNT model. The broken bonds at the cluster boundary were closed by pseudoatoms, which were chosen as hydrogen atoms. The interaction process was modeled by a step-by-step approach of a CO or CO₂ molecule to a carbon atom at the surface located approximately in the middle of the molecular cluster in order to avoid the influence of marginal hydrogen pseudoatoms (Figure 1). The molecules were oriented by an oxygen atom perpendicular to the surface of the nanotube. The molecule was approached step by step, with a step of 0.01 nm. The calculations were performed using the density functional theory (DFT) method within the framework of the B3LYP functional using the LanL2DZ basis.

Based on the results of this study, energy interaction curves were constructed (Figure 2). The analysis of the results showed that the energy values lie in the positive range, which indicates the instability of the obtained complexes and the impossibility of using pure carbon nanotubes as possible sensor elements of sensor devices for identifying carbon monoxide or carbon dioxide gases. Based on this, it can be concluded that it is necessary to modify the SWCNT surface for effective gas adsorption.

3.1. The Theoretical Study of the Possibile Mechanism of Interaction of SWCNTs of Types (6.0) and (6.6) with the Platinum Atom (Pt)

To establish the possibility and mechanism of modifying the surface of carbon nanotubes with a platinum atom, the mechanism of interaction of zig-zag (6.0) and armchair (6.6) SWCNTs with a Pt atom was investigated. To determine the most likely location of the platinum atom attachment to the surface of the SWCNT (the so-called adsorption center) and to establish the dependence of the electronic properties on this position, three possible options for the location of the metal above the surface of the nanotube were considered.: 1—above the C atom; 2—above the center of the bond between carbon atoms; and 3—above the center of the hexagon (Figure 3). A carbon nanotube cluster consisting of eight layers of hexagons along the longitudinal axis of the nanotube for a (6.0) type nanotube and nine layers of hexagons for a (6.6) type nanotube is considered. The cluster lengths were 0.17 nm for the zig-zag type nanotube and 0.22 nm for the armchair type nanotube. The uncompensated valences at the cluster boundaries were closed by hydrogen pseudoatoms.

The interaction process was modeled as follows. The platinum atom approached stepwise (in 0.01 nm increments) to the selected adsorption center on the SWCNT surface (carbon atom, C-C bond center, or hexagon center (Figure 4)) along the normal passing through it to the longitudinal axis of the nanotube. Figure 4a shows clusters of the considered nanotubes with an indication of the attachment points of the platinum atom to the (6.0) type nanotube. Figure 4b shows clusters of the considered nanotubes with an indication of the attachment points of the platinum atom to the (6.6) type nanotube. At each step, the potential energy of the system was calculated. The calculations made it possible to construct profiles of the surface of the potential energy of the interaction of carbon nanotubes of both types with the Pt atom (Figure 5). The analysis of the curves established the optimal interaction distances and energies (corresponding to the minima on the energy curves), which determines the most probable positions of the selected atom relative to the surface of the SWCNT. The analysis of the electron energy state of such systems made it possible to determine the values of the band gap. The results of the analysis are presented in

Table 1. Comparative table of electron energy characteristics of the nanosystems under consideration.

Type of Structure	Interaction Distance, nm	Energy of Interaction (eV)	The Width of the Forbidden Zone (eV)
«Clean» nanotube type (6.0)	-	-	0.69
SWCNT (6.0)—Pt on the C atom	0.21	−1.57	0.62
SWCNT (6.0)—Pt on the C-C center	0.20	−1.82	0.62
SWCNT (6.0)—Pt on the center of the hexagon	0.20	−0. 71	0.59
«Clean» nanotube type (6.6)			0.92
SWCNT (6.6)—Pt on the C atom	0.21	−1.16	0.90
SWCNT (6.6)—Pt on the C-C center	0.22	−1.18	0.94
SWCNT (6.6)—Pt on the center of the hexagon	0.20	−0.52	0.91

. Based on the conducted model experiments, it was found that from an energy point of view, the most effective interaction is observed when a single platinum atom is attached to the bonding center between carbon atoms of the nanotube surface (Figure 3b) (the value of the interaction energy in this case is the maximum in absolute terms among all the considered options, Table 1).

The theoretical calculations performed made it possible to construct the densities of states (Figure 6) of the nanosystems considered. The values of the energy gap width ΔEg for each variant of the arrangement of the platinum atom relative to the surface of the nanotubes, defined as the energy difference between the upper occupied and lower vacant molecular orbitals, are shown in Table 1.

An analysis of the electron energy structure of the obtained SWCNT-based systems with platinum atoms attached to the surface for all the positions and types of nanotubes considered revealed that the molecular orbitals are grouped into zones, which, according to the generally accepted terminology, can be called the valence band and the conduction band, separated by a band gap.

The magnitude of the band gap ΔE_g for a (6.0) type nanotube was 0.69 eV (Figure 6a), and for a (6.6) type nanotube it was 0.92 eV (Figure 6a) and was calculated as the energy difference between the upper filled E_HOMO and lower vacant E_LUMO orbitals.:

ΔE_g = E_LUMO − E_HOMO

(4)

This parameter determines the conductive properties of the composite nanostructures studied. An analysis of the obtained ΔE_g values showed that when a platinum atom is attached to a (6.0) type nanotube, the band gap decreases compared to this parameter for a pure carbon nanotube (Figure 6b–d). In the case of the attachment of a platinum atom to the surface of a (6.6) type nanotubulene, the width of the forbidden gap decreased slightly for the adsorption variants above the carbon atom and above the hexagon center (Figure 6e,h). When the Pt was oriented above the C-C bond center, the ΔE_g increased slightly (Figure 6g).

An analysis of the charge distribution in the SWCNT + platinum atom systems revealed that in all the cases considered, the electron density is transferred from the platinum atom to the carbon atoms of the nanotube (

Table 2. The charge distribution at the unit approximation.

Variant of Pt Atom Adsorption	The Value of the Charge on the Metal Atom Before Attachment	The Value of the Charge on the Metal Atom After Attachment	The Average Value of the Charge of Carbon Atoms on the Surface of the Nanotube Before the Addition of the Pt Atom	The Average Value of the Charge of the Nearest Atomic Neighbors on the Surface of a Nanotube After the Addition of a Pt Atom
SWCNT (6.0)—Pt on the C atom	0	0.015	−0.010	−0.077
SWCNT (6.0)—Pt on the C-C center	0	0.014	0.009	−0.382
SWCNT (6.0)—Pt on the center of the hexagon	0	0.017	−0.011	−0.085
SWCNT (6.6)—Pt on the C atom	0	0.049	−0.130	−0.033
SWCNT (6.6)—Pt on the C-C center	0	0.048	−0.020	−0.860
SWCNT (6.6)—Pt on the center of the hexagon	0	0.049	0.005	−0.128

). The charge carrier, which in this nanostructure is an electron, passes from the metal atom, which is a donor, to the surface of the nanotube. Accordingly, most of the charge carriers are formed there, and conduction is realized over its surface.

The analysis of the data obtained made it possible to establish that the most stable adsorption complex is formed when a metal atom is located above the center of the carbon bond (position 2), (Table 1). The radius of a platinum atom is 139 Å, which is perhaps what makes its position relative to the CNT surface more advantageous, namely above the C-C bond center, and not above the carbon atom, as is the case with atoms of other metals of smaller diameters (for example, copper, nickel, etc.). The adsorption distance and energy correspond to the bond formation, which makes the simulated nanosensor a stable structure. This condition corresponds to a smaller band gap compared to a pure nanotube (except for the location of the platinum atom above the C-C bond center, where the band gap increases slightly). The charge redistribution has a positive effect on the process of the sensory interaction.

3.2. The Investigation of the SWCNT Interaction Modified by a Platinum Atom with a Carbon Monoxide Molecule

The simulation of the interaction of the carbon monoxide molecule with the SWCNT +platinum system was carried out as follows. The carbon monoxide molecule was positioned perpendicular to the surface of the carbon nanotube and approached the platinum atom step by step, guided by the oxygen atom (Figure 7). As a result of the calculations, energy curves were constructed (Figure 8), and energy minima were identified, which indicates the stability of the resulting complexes. These results confirm it. It is shown that the modification of SWCNTs with a platinum atom improves the sorption properties of nanotubes with respect to CO, creating prerequisites for a more efficient adsorption process.

The analysis of the electron energy structure of the obtained nanostructures was carried out using the constructed density of electronic states. A conclusion was formed about the electronic structure of the initial nanotubes, which was the nanocomplex “SWCNT + platinum”. The effect of the adsorbed CO molecule on it was also evaluated. The analysis of the changes in the band gap showed that for the nanosystem “SWCNT + Pt”, type (6.0), the ΔE_g = 0.62 eV, and when a carbon monoxide molecule is attached, this value increases slightly to ΔE_g = 0.63 eV (Figure 9a). During the sensory interaction of carbon monoxide with a nanotube of the “armchair”-type modified platinum, there is no change in the width of the forbidden zone (Figure 9b). The analysis of the charge distribution showed that when the modified system interacts with a CO molecule, the electron density shifts from the molecule to the platinum atom. The main parameters of this interaction are given in

Table 3. The main energy characteristics of the interaction mechanisms of carbon nanotubes of types 6.0 and 6.6 during interactions with carbon monoxide molecules.

	SWCNT Type (6.0)	SWCNT Type (6.6)
Ead, eV	−0.60	−0.85
Rad, nm	0.22	0.21
ΔEg, eV SWCNT + CO systems	0.69	-
ΔEg, eV SWCNT + Pt systems	0.62	0.94
ΔEg, eV SWCNT +Pt + CO systems	0.63	0.94

.

3.3. The Investigation of the SWCNT Interaction Modified by a Platinum Atom with a Molecule of Carbon Dioxide

The interaction of the SWCNT + platinum nanosystem with a carbon dioxide molecule was studied. The carbon dioxide molecule had two orientation options relative to the tubulene surface: (a) parallel to the SWCNT surface and (b) perpendicular to the SWCNT surface (Figure 10).

According to the results of this study, it was found that when a carbon dioxide molecule is attached parallel to the surface, the system is metastable (energy values lie in the positive region). In this regard, the results of studies were further considered and analyzed when the carbon dioxide molecule is positioned only when it is attached perpendicularly (Figure 10b) and when the oxygen atom is oriented to the platinum atom, modifying the surface of a carbon nanotube.

The calculations performed during the step-by-step approximation of the CO₂ molecule to the modified nanotube system allowed us to understand the dependence of the energy of the interaction of the molecule on its distance from the platinum atom of the nanosystem. The analysis of the curves has been established. It is shown that a stable complex is formed for both types of nanotubes considered, (6.0) and (6.6). These results confirm it. It is shown that the modification of SWCNTs with a platinum atom improves the sensory properties of nanotubes in relation to CO₂, creating prerequisites for a more efficient adsorption process. Graphs of the interaction energy (Figure 11) demonstrate the stability of the complexes. This opens up opportunities for the use of such materials.

The analysis of the electron energy structure of the obtained nanostructures was carried out using the constructed density of electronic states. A conclusion was formed about the electronic structure of the initial nanotubes, which were the nanocomplex “ SWCNT + platinum”, as well as the effect of the adsorbed CO₂ molecule on it. The analysis of the changes in the band gap showed that the nanosystem “SWCNT (6.0) + Pt” has a band gap ΔE_g = 0.62 eV, and when a carbon dioxide molecule is attached, it decreases to ΔE_g = 0.61 eV (Figure 12a). For SWCNT type (6.6) modified by a platinum atom, there is an increase in the energy gap with a value of ΔE_g = 0.96 eV (Figure 12b). During adsorption, as shown by the distribution charge, the electron density shifts from the molecule to the surface of the nanosystem. The main parameters of this interaction are given in

Table 4. The main energy characteristics of the interaction mechanisms of carbon nanotubes of types 6.0 and 6.6 during interactions with carbon monoxide molecules.

	SWCNT Type (6.0)	SWCNT Type (6.6)
Ead, eV	−0.43	−0.36
Rad, nm	0.22	0.23
ΔEg, eV SWCNT + CO2 systems	0.69	-
ΔEg, eV SWCNT + Pt systems	0.62	0.94
ΔEg, eV SWCNT + Pt + CO2 systems	0.60	0.96

.

4. Conclusions

The theoretical studies carried out allowed us to conclude that the surface modification of carbon nanotubes of types (6.0) and (6.6) with platinum is effective for the possibility of using such modified nanosystems as sensor materials for sensor devices capable of detecting the presence of carbon monoxide and carbon dioxide. The quantum chemical calculations performed allowed us to establish the following:

(1)

An analysis of the energy curves of the interaction of a “pure” carbon nanotube with CO and CO₂ showed the presence of a minimum interaction energy. However, the energy values are in the positive range; this indicates the instability of the obtained complexes and determines the prospects of work to improve the properties of SWCNTs for the adsorption of selected gases.

(2)

The study of the interaction of a carbon nanotube with a platinum atom has shown that the most optimal position of the metal atom relative to the surface of the nanotubulene from an energy point of view and the stability of the configuration is its location in the center of the C-C bond.

(3)

Studies of the interactions of the SWCNT + Pt nanocomplex with carbon monoxide and carbon dioxide molecules have shown the presence of sorption interactions and the formation of stable complexes. The energy values correspond to the weak Van der Waals interaction. This type of interaction, unlike the chemical one, allows you to reuse the sensor without polluting it.

(4)

The interaction of the SWCNT + Pt nanocomplex with carbon monoxide and carbon dioxide molecules is accompanied by a change in the band gap. Thus, sensor devices manufactured using such modified nanotube materials as sensors, work on the basis of fixing changes in the conductive characteristics of the system when additional charge carriers occur, caused by the redistribution of electron density. In the considered cases, the values of the band gap differ for the cases of the interaction of the systems with each selected gas.

All of the above is reflected in

Table 5. A generalized table for studying the interaction of the molecules of the studied gases with carbon nanotubes of various modifications.

	CO				CO2
	SWCNT		SWCNT + Pt Systems		SWCNT		SWCNT + Pt Systems
	Type (6.0)	Type (6.6)	Type (6.0)	Type (6.6)	Type (6.0)	Type (6.6)	Type (6.0)	Type (6.6)
Rad, nm	0.32	-	0.22	0.21	0.33	-	0.22	0.23
Ead, eV	11.30	-	−0.60	−0.85	11.12	-	−0.43	−0.36
ΔEg, eV	-	-	0.63	0.94	-	-	0.60	0.96

’s summary. This indicates the selectivity of the sensor that can be created based on these systems. Thus, the modification of carbon nanotubes with platinum atoms improves the sensory properties of nanotubes in relation to CO and CO₂ gases, creating prerequisites for the creation of highly efficient nanosensors. Such sensors are capable of detecting even minimal amounts of carbon monoxide and carbon dioxide. The functioning of the sensors is based on the sorption interaction of surface-modified SWCNTs with CO and CO₂ molecules and the subsequent detection. This is possible due to a change in the charge distribution on the surface of nanotubes and a change in the conductive state of nanosystems.