Adsorption and gas-sensing characteristics of a stoichiometric α-Fe₂O₃ (0 0 1) nano thin film for carbon dioxide and carbon monoxide with and without pre-adsorbed O₂

Abstract

Herein, for the first time, the adsorption and gas-sensing characteristics of the CO₂ and CO molecules on a stoichiometric α-Fe₂O₃ (0 0 1) nano thin film with and without pre-adsorbed O₂ molecules have been studied using the density functional theory (DFT) method. Without pre-adsorbed O₂ molecules, the CO₂ molecule plays the role of an acceptor and obtains electrons from the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film. For the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film system, the CO₂ molecule also plays the role of an acceptor. However, less number of electrons are transferred to the CO₂ molecule as compared to the pre-adsorbed O₂ molecules. Different from the CO₂ molecule, the CO molecule always plays the role of a donor for the α-Fe₂O₃ (0 0 1) nano thin film system with and without pre-adsorbed O₂. The theoretical results verify that the CO molecule can react with the lattice oxygen or adsorbed oxygen of the α-Fe₂O₃ (0 0 1) nano thin film. The electrons transferred to the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film from the CO molecule or newly formed CO₂ molecule are more than that transferred to the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film. For the stoichiometric or O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film, the CO₂ and CO molecules exhibited opposite behaviors of charge transformation. In addition, pre-adsorbed O₂ molecules displayed competitive adsorption with the CO₂ or CO molecule. The pre-adsorbed O₂ molecules hinder electron transfer to the CO₂ molecules from the α-Fe₂O₃ (0 0 1) nano thin film or hinder electron transfer to the α-Fe₂O₃ (0 0 1) nano thin film from the CO molecule. Theoretical results demonstrate that the surface of α-Fe₂O₃ materials (0 0 1) could be prepared for use as adsorbents or gas sensors for CO₂ and CO molecules. Their structures are stable after CO₂ molecules are adsorbed or after the reaction of CO molecules with the lattice oxygen or adsorbed oxygen of the α-Fe₂O₃ (0 0 1) nano thin film.

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1. Introduction

The ecosystem demands carbon dioxide (CO₂) gas in order to maintain ecological equilibrium. However, the increasing concentration of CO₂ gas in the atmosphere has led to the serious greenhouse effect. Glacier melting and global warming is attributed to this problem. The main component of coal gas is carbon monoxide (CO), which is a widespread, tasteless, colorless, and toxic gas. This highly toxic gas could attach to hemoglobin, which damages the human body by causing a reduction in cellular respiration.¹ Therefore, it has become an urgent subject to detect CO₂ and CO gas in the atmospheric environment accurately and rapidly. CO₂ and CO gases have attracted a great deal of attention in recent years. Numerous experimental and theoretical reports have indicated that various materials or sensors could be prepared for the adsorption, detection, transformation and storage of CO₂ gas,^2–18 and some materials or sensors could be used for the adsorption and detection of CO gas.^19–25

The transition metal oxide, α-Fe₂O₃, is an n-type semiconductor, which has an accepted experimental band gap of approximately 2.0 eV.^26–28 G-type anti-ferromagnetic (AFM) ordering has been confirmed as the ground state of α-Fe₂O₃.²⁹ Due to its advantages of low cost, high stability, pollution-free and multiple functions,³⁰ the transition metal oxide, α-Fe₂O₃, has been extensively studied for potential applications. It has been investigated in many fields, such as pigments, magnetic devices, catalysts, lithium batteries,^31–35 and especially in gas sensors.^36–43 It is known that one of the important indicators for gas sensors is their gas-sensing performance. Materials gas-sensing performance has a strong dependence with their surface areas. Nanostructure materials have a much higher gas-sensing performance than that of microcrystalline powders because nanostructure materials could offer larger surface areas for the adsorption of a test gas.^7,8,44 In fact, the gas-sensing performance is a process of testing the gas adsorbed on a material's surface. More test gas adsorbed on a nano-material's surface, the better its gas-sensing performance. Therefore, metal oxide α-Fe₂O₃ materials prepared in the nano-scale certainly have better gas-sensing performances or adsorption performances when they are used as gas sensors or adsorbents.

For the first time, the density function theory (DFT) method has been employed to investigate the adsorption characteristics and mechanism of CO₂ and CO molecules on a stoichiometric α-Fe₂O₃ (0 0 1) nano thin film. The types of CO₂ and CO molecule adsorption structures are provided in this study. The relative stability between different CO₂ or CO molecules adsorption structures is investigated according to their adsorption energies. The theoretical results reveal that the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film could be applied as a gas sensor or absorbent for CO₂/CO gas. A comparative study on the CO₂/CO adsorption characteristics of the α-Fe₂O₃ (0 0 1) nano thin film with and without pre-adsorbed O₂ molecules is performed to demonstrate the effects of pre-adsorbed O₂. The gas sensing mechanisms and adsorption mechanisms are also investigated in detail in this study.

2. Theoretical methods and models

Density functional theory (DFT) calculations were employed using the projector augmented wave (PAW) pseudopotential method in the Vienna ab initio simulation package (VASP).^45,46 For transition metal or lanthanide compounds with localized d or f electrons, the electron–electron exchange and correlation energies are described incorrectly by the generalized gradient approximation (GGA) pseudopotential with Perdew–Burke–Ernzerhof (PBE) formulation. One way to improve the description of localized d or f electrons is the use of the GGA + U_eff method, which adds an on-site coulomb repulsion.⁴⁷ For α-Fe₂O₃, its calculated band gap is 2.02 eV when the on-site effective U_eff = U − J = 4.5 eV, and this is consistent with the experimental value.^26–28 Throughout the calculations of the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film, special k points were generated with a 5 × 5 × 2 grid based on the Monkhorst–Pack scheme and the energy cutoff was set to 400 eV. Geometry structures were optimized with the conjugate-gradient algorithm.⁴⁸ Their convergence criterion in the process of geometry optimization was 1.0 × 10⁻⁵ eV per atom for energy. The Bader charge analysis program was employed to analyze the charge distribution.^48–51

The hexagonal α-Fe₂O₃ structure with space group R3̄c was selected as the object for research. The primitive cell of α-Fe₂O₃ with G-type anti-ferromagnetic (AFM) ordering was simulated, and its crystal structure parameters originated from a previous experiment.⁵² Its lattice constants were a = b = 5.0351 Å, c = 13.7581 Å, α = β = 90°, and γ = 120° and its atomic fractional coordinates were Fe (0.0000, 0.0000, and 0.3553) and O (0.3062, 0.0000, and 0.2500). After optimization, the (0 0 1) surface was cleaved. There were three different terminated surfaces: (1) the Fe–Fe–O₃ layer, (2) Fe–O₃–Fe layer, and (3) O₃–Fe–Fe layer. The cleaved (0 0 1) nano thin film had five repeated units (Fe–Fe–O₃ or Fe–O₃–Fe or O₃–Fe–Fe) with 15 Fe atoms and 10 O atoms. Subsequently, a 2 × 2 super-cell was built and a 10 Å vacuum layer was added to the super-cell,^53–56 which contained 100 atoms in total. The stoichiometric α-Fe₂O₃ (0 0 1) nano thin films with three different terminated surfaces are shown in Fig. 1.

Fig. 1 Stoichiometric α-Fe₂O₃ (0 0 1) nano thin film with (a) Fe–Fe–O₃-terminated, (b) Fe–O₃–Fe-terminated, and (c) O₃–Fe–Fe-terminated surfaces. The blue and red balls represent Fe and O atoms, respectively.

3. Results and discussion

After optimization, the relative energies (ΔE = E_(a/b/c) − E_lowest) between the three different terminated nano thin films were calculated and shown in Table 1. The values of relative energies are 20.478 eV, 0.000 eV, and 20.479 eV for the stoichiometric α-Fe₂O₃ (0 0 1) nano thin films with Fe–Fe–O₃-terminated, Fe–O₃–Fe-terminated, and O₃–Fe–Fe-terminated surfaces, respectively. The Fe–O₃–Fe-terminated stoichiometric α-Fe₂O₃ (0 0 1) nano thin film had the lowest energy state, which implies that this surface is the easiest to synthesize. In the following calculations, this most stable nano thin film is employed as the substrate.

Table 1 The values of relative energies (eV) between the three different terminated surfaces (ΔE = E_(a/b/c) − E_lowest)

Termination	Fe–Fe–O3	Fe–O3–Fe	O3–Fe–Fe
ΔE (eV)	20.478	0	20.479

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3.1. CO₂, CO and O₂ molecule adsorption on the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film

3.1.1 CO₂ molecule adsorption on the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film. A free CO₂ molecule was introduced into the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film system to simulate the adsorption process. Then, four initial adsorption configurations were considered: (N¹) CO₂ perpendicular to the α-Fe₂O₃ (0 0 1) nano thin film on top of the Fe atom; (N²) CO₂ parallel to the surface Fe–O bond of the α-Fe₂O₃ (0 0 1) nano thin film with the C atom on top of the surface O atom; (N³) CO₂ parallel to the α-Fe₂O₃ (0 0 1) nano thin film with the C atom on top of the Fe atom; and (N⁴) CO₂ parallel to the α-Fe₂O₃ (0 0 1) nano thin film with the C atom on top of the surface O atom and perpendicular to one of the Fe–O bonds. The four different initial adsorption configurations and their optimized structures are shown in Fig. 2(a). After CO₂ molecule adsorption, the adsorption energy (E_ads) was calculated to determine the most stable adsorption structure. The formulation of the adsorption energy (E_ads) is described as follows:⁵⁷

E_ads = E_substrate + E_adsorbate − E_{substrate–adsorbate}

where E_{substrate–adsorbate} is the total energy of the adsorbate–substrate system in the equilibrium state, and E_substrate and E_adsorbate are the total energy of the substrate and adsorbate, respectively. A positive value reveals that the adsorption process is an exothermic process, which implies a favorable and stable adsorption structure. Table 2 summarizes the calculated results of the adsorption configurations, N¹, N², N³, and N⁴, such as the nearest distance, d, between the adsorbed CO₂ molecule and stoichiometric α-Fe₂O₃ (0 0 1) nano thin film, the angle, β_O–C–O, of the adsorbed CO₂ molecule, and their corresponding adsorption energy E_ads.

Fig. 2 The four different initial adsorption structures (left) and corresponding optimized structures (right) of (a) CO₂ molecule and (b) CO molecule. The blue, red and black balls represent Fe, O and C atoms, respectively.

Table 2 The calculated results of adsorption configurations N¹, N², N³ and N⁴: d is the nearest distance between the adsorbed CO₂ molecule and stoichiometric α-Fe₂O₃ (0 0 1) nano thin film; β_O–C–O is the angle of the adsorbed CO₂ molecule and E_ads is the adsorption energy

Configuration	d (Å)	β (°)	Eads (eV)
N^1	2.274	179.947	0.194
N^2	1.375	126.392	1.253
N^3	3.720	178.697	0.130
N^4	3.032	177.555	0.156

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For the N¹ adsorption configuration, the optimized nearest distance, d, was 2.274 Å, which is short enough for the O atom in the CO₂ molecule to chemically bond with the surface Fe atom of the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film. The angle of the adsorbed CO₂ molecule changed to 179.947°. For the N² adsorption configuration, the optimized nearest distance, d, was 1.375 Å. After optimization, the CO₂ molecule had a large deformation. The C atom in the CO₂ molecule chemically bonded with the surface O atom and the two O atoms in the CO₂ molecule chemically bonded with the adjacent surface Fe atoms. The optimized angle of the CO₂ molecule in the adsorbed state changed to 126.392°. For the N³ and N⁴ adsorption configurations, the theoretical results show that the CO₂ molecule kept far away from the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film after optimization. The optimized nearest distances were 3.720 Å and 3.032 Å for the N³ and N⁴ adsorption configurations, respectively. The larger distance, d, revealed that the stoichiometric α-Fe₂O₃ (0 0 1) nano thin films with the N³ and N⁴ adsorption configurations were inadaptable for CO₂ molecules to be chemically absorbed. The adsorption energies were 0.194 eV, 1.253 eV, 0.130 eV, and 0.156 eV for the adsorption configurations N¹, N², N³, and N⁴, respectively. The adsorption configuration N² had the largest adsorption energy, which corresponds to the most stable adsorption structure. Therefore, the most stable adsorption configuration, N², was considered as the final adsorption structure after CO₂ molecule adsorption in the following analysis and discussion. These theoretical results demonstrate that the stoichiometric α-Fe₂O₃ material (0 0 1) surface could be applied as an adsorbent or gas sensor for CO₂ gas.

3.1.2 CO molecule adsorption on the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film. Furthermore, a free CO molecule was also introduced into the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film system to simulate the adsorption process. Four initial adsorption configurations were considered: (E¹) CO perpendicular to the α-Fe₂O₃ (0 0 1) nano thin film on top of the Fe atom with an O-down configuration; (E²) CO perpendicular to the α-Fe₂O₃ (0 0 1) nano thin film on top of the Fe atom with a C-down configuration; (E³) CO perpendicular to the α-Fe₂O₃ (0 0 1) nano thin film on top of the O atom with the C-down configuration; (E⁴) CO parallel to the α-Fe₂O₃ (0 0 1) nano thin film with the O atom of the CO molecule close to the Fe atom and the C atom of the CO molecule close to the O atom. The four different initial adsorption configurations and corresponding optimized structures are shown in Fig. 2(b). Table 3 summarizes the calculated results of the adsorption configurations E¹, E², E³, and E⁴, such as the nearest distance, d, between the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film and the adsorbed CO/newly formed CO₂ molecule, the C–O bond length, d₀, in the adsorption state, and their corresponding adsorption energies, E_ads.

Table 3 The calculated results of adsorption configurations E¹, E², E³ and E⁴: d is the nearest distance between the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film and the adsorbed CO/newly formed CO₂ molecule, d₀ is the C–O bond length in the adsorption state and E_ads is the adsorption energy

Configuration	d (Å)	d0 (Å)	Eads (eV)
E^1	3.165	1.142	0.081
E^2	2.200	1.140	0.499
E^3	2.471	1.173/1.179	0.931
E^4	2.006	1.242/1.247	0.581

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For the E¹ adsorption configuration, the optimized nearest distance was 3.165 Å. The larger distance, d, reveals that the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film is inadaptable for CO molecules to be chemically absorbed in the O-down adsorption configuration. For the E² adsorption configuration, the optimized nearest distance, d, was 2.200 Å, which is short enough for the C atom in the CO molecule to chemically bond with the surface Fe atom of the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film. The bond length of the adsorbed CO molecule was 1.140 Å. For the E³ adsorption configuration, the CO molecule reacted with the lattice oxygen on the surface of the α-Fe₂O₃ (0 0 1) nano thin film after CO gas was introduced into the system. This reaction could be described as follows:

CO_(gas) + O_lattⁿ⁻ = CO_2(ads) + V_Oⁿ⁻ (a)

where V_Oⁿ⁻ and O_lattⁿ⁻ represent the oxygen vacancy and lattice oxygen of the α-Fe₂O₃ (0 0 1) nano thin film, respectively. After the reaction, there was one new CO₂ molecule formed and an oxygen vacancy was introduced into the α-Fe₂O₃ (0 0 1) nano thin film. Subsequently, the new formed CO₂ molecule adsorbed onto the α-Fe₂O₃ (0 0 1) nano thin film with an oxygen vacancy. The optimized nearest distance, d, between the newly formed CO₂ molecule and α-Fe₂O₃ (0 0 1) nano thin film with an oxygen vacancy was 2.471 Å. The bond lengths for the newly formed CO₂ molecule in the adsorption state were 1.173 Å and 1.179 Å, respectively. One O atom in the formed CO₂ molecule chemically bonded with an adjacent surface Fe atom. The optimized angle of the newly formed CO₂ molecule in the adsorbed state was 178.544°. For the E⁴ adsorption configuration, the CO molecule also reacted with the lattice oxygen on the surface of the α-Fe₂O₃ (0 0 1) nano thin film, as described in Formula (a). After optimization, the optimized nearest distance, d, was 2.006 Å. The two O atoms of the newly formed CO₂ molecule chemically bonded with adjacent surface Fe atoms, with the C–O bond lengths of 1.242 Å and 1.247 Å, respectively. The adsorption energies were 0.081 eV, 0.499 eV, 0.931 eV, and 0.581 eV for adsorption configurations E¹, E², E³, and E⁴, respectively. The adsorption configuration E³ had the largest adsorption energy, which corresponds to the most stable adsorption structures. Therefore, the most stable adsorption configuration, E³, is considered as the final adsorption structure. These theoretical results demonstrate that the CO molecule can react with the lattice oxygen on the surface of the α-Fe₂O₃ (0 0 1) nano thin film and successfully introduce one oxygen vacancy into the α-Fe₂O₃ (0 0 1) nano thin film.

3.1.3 O₂ molecule adsorption on the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film. In the atmospheric environment, the proportion of O₂ gas is about 21%. This high concentration of O₂ gas results in the fact that O₂ molecules must be captured by gas-sensing material surfaces when exposed to atmospheric environment. The majority of gas sensors for the detection of harmful gas in the atmospheric environment are influenced by the adsorbed O₂ molecules on their surface. The oxygen adsorbed on the surface of nano-materials has a huge effect on their gas-sensing performance. Therefore, it is necessary and significant to investigate the influence of pre-adsorbed O₂ gas. The adsorption of oxygen on the surface of materials has been verified by several experimental reports.^58–62 In order to have a clear understanding about what happens after O₂ gas is adsorbed on the surface of gas-sensing materials, the widely used model for the oxygen adsorption process is excerpted and shown as the following:^11,14

O_2(gas) ↔ O_2(ads) (1)

O_2(ads) + e⁻ ↔ O_2(ads)⁻ (2)

O_2(ads)⁻ + e⁻ ↔ 2O_(ads)⁻ (3)

O_(ads)⁻ + e⁻ ↔ O_(ads)²⁻ (4)

where “gas” and “ads” represent oxygen in the gas state and adsorption state, respectively. Oxygen adsorbed on the surface of materials has great effects on their gas-sensing performance.

Moreover, the probable adsorption structures of the O₂ molecule on the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film were investigated and are shown in Fig. 3. The O₂ molecule can only be adsorbed on Fe atoms with two adsorption configurations: (M¹) O₂ molecule perpendicular to the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film on top of the Fe atom; and (M²) O₂ molecule parallel to the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film on top of the Fe atom. Table 4 summarizes the calculated results of the adsorption configurations M¹ and M², such as the nearest distance, d, between the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film and the adsorbed O₂ molecule, and the O–O bond lengths, d₀, in the adsorption state and their corresponding adsorption energy E_ads. For adsorption configuration M¹, the O₂ molecule chemically adsorbs on the Fe atom site of the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film after optimization. For adsorption configuration M², its optimized adsorption structure is very similar to the adsorption structure M¹. The total energy of the optimal adsorption structure, M², is lower than that of the optimal adsorption structure, M¹, by about 0.001 eV, which can be neglected. In adsorption structure M², the O–O bond length in the adsorbed O₂ molecule is 1.269 Å with the nearest distance, d, of 2.055 Å. Therefore, the optimized adsorption structure, M², was selected to represent the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film. The adsorption energies of the most stable adsorption configurations for the CO₂ molecule (N²), CO molecule (E³) and O₂ molecule (M²) are 1.253 eV, 0.931 eV and 0.338 eV, respectively. The theoretical results indicate that the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film prefers to capture the CO₂ molecule than the CO molecule and O₂ molecule.

Fig. 3 The two different adsorption structures of the O₂ molecule on the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film. The blue and red balls represent Fe and O atoms, respectively.

Table 4 The calculated results of adsorption configurations M¹ and M²: d is the nearest distance between the adsorbed O₂ molecule and stoichiometric α-Fe₂O₃ (0 0 1) nano thin film, d₀ is the bond length of the O₂ molecule in the adsorption state and E_ads is the adsorption energy

Model	d (Å)	d0 (Å)	Eads (eV)
M^1	2.057	1.268	0.337
M^2	2.055	1.269	0.338

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3.2. Influence of pre-adsorbed O₂ on adsorption characteristics of CO₂ and CO on an α-Fe₂O₃ (0 0 1) nano thin film

3.2.1 Influence of pre-adsorbed O₂ on adsorption characteristics of CO₂ on an α-Fe₂O₃ (0 0 1) nano thin film. In order to understand the influence of pre-adsorbed O₂, one free CO₂ molecule was introduced into the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film system with the adsorption structure M². Five initial adsorption configurations were considered: (C¹) CO₂ perpendicular to the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film on top of the Fe atom; (C²) CO₂ parallel to the surface Fe–O bond of the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film with the C atom on top of the surface O atom; (C³) CO₂ parallel to the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film with the C atom on top of the Fe atom; (C⁴) CO₂ parallel to the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film with the C atom on top of surface O atom and perpendicular to one of the Fe–O bonds; and (C⁵) CO₂ parallel to the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film with the C atom on top of the pre-adsorbed O₂. The five different initial adsorption configurations and their corresponding optimized structures are shown in Fig. 4(a).

Fig. 4 The five different adsorption structures of (a) CO₂ molecule and (b) CO molecule on the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film. The blue, red and black balls represent Fe, O and C atoms, respectively.

After optimization, the nearest distance, d, between the adsorbed CO₂ molecule and O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film, the angle, β_O–C–O, of the adsorbed CO₂ molecule, and the adsorption energy, E_ads, are listed in Table 5. For C¹ adsorption configuration, one O atom of CO₂ molecule chemically adsorbed on the surface Fe atom of O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film, similarly to the optimized adsorption structure N¹. In this adsorption structure, the optimized nearest distance d and optimized angle β_O–C–O were 2.272 Å and 179.861°, respectively. For the C² adsorption configuration, the CO₂ molecule had a similar deformation as the optimized adsorption structure N². The C atom in the CO₂ molecule also chemically bonded with the surface O atom, and the two O atoms in the CO₂ molecule chemically bonded with the adjacent surface Fe atoms. The optimized nearest distance, d, and optimized angle, β_O–C–O, were 1.373 Å and 126.103°, which are both less than that of the optimized adsorption structure N². For the C³, C⁴ and C⁵ adsorption configurations, the CO₂ molecule moved far away from the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film. Their optimized nearest distances, d, were 3.655 Å, 3.082 Å and 3.020 Å. Under the CO₂ atmosphere, the two atoms of the O₂ molecule were simultaneously adsorbed on the Fe atom of the α-Fe₂O₃ (0 0 1) nano thin film in adsorption configuration C⁵. The adsorption energies for adsorption configurations C¹, C², C³, C⁴, and C⁵ were 0.224 eV, 0.990 eV, 0.130 eV, 0.175 eV, and −0.137 eV, respectively. The adsorption structure C² had the largest adsorption energy, which corresponds to the most stable adsorption structures. The final most stable adsorption structure was C² for CO₂ on the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film.

Table 5 The calculated results of adsorption configurations C¹, C², C³, C⁴ and C⁵: d is the nearest distance between the adsorbed CO₂ molecule and O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film, β_O–C–O is the angle of the adsorbed CO₂ molecule and E_ads is the adsorption energy

Model	d (Å)	β (°)	Eads
C^1	2.272	179.861	0.224
C^2	1.373	126.103	0.990
C^3	3.655	178.849	0.130
C^4	3.082	177.957	0.175
C^5	3.020	179.065	−0.137

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3.2.2 Influence of pre-adsorbed O₂ upon adsorption characteristics of CO on an α-Fe₂O₃ (0 0 1) nano thin film. One free CO molecule was also introduced into the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film system with the adsorption structure M² to investigate the influence of pre-adsorbed O₂. Five initial adsorption configurations were considered: (P¹) CO perpendicular to the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film on top of the Fe atom in the O-down configuration; (P²) CO perpendicular to the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film on top of the Fe atom in the C-down configuration; (P³) CO perpendicular to the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film on top of the O atom in the C-down configuration; (P⁴) CO parallel to the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film with the C atom of the CO molecule close to one of the surface lattice oxygens, and O atom of the CO molecule close to the adjacent Fe atom; and (P⁵) CO perpendicular to the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film with the C atom of the CO molecule on the top of the pre-adsorbed O₂ molecule. The five different initial adsorption configurations and their corresponding optimized structures are shown in Fig. 4(b).

After optimization, the nearest distance, d, between the adsorbed CO/newly formed CO₂ molecule and O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film, the C–O bond lengths d₀ in adsorption state and the adsorption energy E_ads are listed in Table 6. For the P¹ adsorption configuration, the CO molecule moved far away from the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film. The optimized nearest distance, d, was 3.211 Å. For the P² adsorption configuration, the C atom of the CO molecule chemically adsorbed on the surface Fe atom of the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film, similarly to the optimized adsorption structure E². In this adsorption structure, the optimized nearest distance, d, and optimized bond length, d₀, were 2.203 Å and 1.139 Å, respectively. For the P³ adsorption configuration, the Fe–O bond between the CO molecule and pre-adsorbed O₂ broke after the CO molecule was introduced into the system. Then, the CO molecule chemically bonded with this disconnected oxygen atom. For the pre-adsorbed O₂ molecule, its two O atoms were simultaneously adsorbed on the disconnected Fe atom of the α-Fe₂O₃ (0 0 1) nano thin film, as shown in Fig. 4(b). The optimized nearest distance, d, and optimized bond length, d₀, were 1.256 Å and 1.202 Å, respectively. For the P⁴ adsorption configuration, the CO molecule reacted with the lattice oxygen of the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film after CO gas was introduced into the system, which is described as follows:

CO_(gas) + O_lattⁿ⁻ = CO_2(gas) + V_Oⁿ⁻ (b)

where “gas” and “latt” represent oxygen in the gas state and lattice state, respectively. As a result of the reaction, the Fe–O bond between the CO molecule and pre-adsorbed O₂ broke, which was most likely in the P³ adsorption configuration. Subsequently, the disconnected O atom was captured by the CO molecule to form a new CO₂ molecule and moved far away from the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film system, as shown in Fig. 4(b). Under the CO atmosphere, the two atoms of the pre-adsorbed O₂ molecule were simultaneously adsorbed on the disconnected Fe atom of the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film. After optimization, the optimized nearest distance between the newly formed CO₂ molecule and new O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film system with an oxygen vacancy was 2.734 Å, and the optimized bond lengths, d₀, of the newly formed CO₂ molecule were 1.173 Å and 1.179 Å, respectively. Different from the P⁴ adsorption configuration, the CO molecule reacted with the pre-adsorbed O₂ molecule directly in the P⁵ adsorption configuration. The process could be described as follows:

CO_(gas) + O_2(ads)ⁿ⁻ = CO_2(gas) + O_(ads)ⁿ⁻ (c)

where “gas” and “ads” represent oxygen in the gas state and adsorption state, respectively. After optimization, the newly formed CO₂ molecule also moved far away from the O pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film system, similar to the adsorption configuration P⁴. The optimized nearest distance between the newly formed CO₂ molecule and O pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film was 3.185 Å, and the optimized bond lengths, d₀, of the newly formed CO₂ molecule were 1.177 Å and 1.175 Å, respectively. The adsorption energies for adsorption configurations P¹, P², P³, P⁴, and P⁵ were 0.099 eV, 0.209 eV, 0.749 eV, 1.908 eV, and 2.426 eV, respectively. The adsorption structure P⁵ had the largest adsorption energy, which corresponds to the most stable adsorption structure. The final most stable adsorption structure was P⁵ for CO on the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film.

Table 6 The calculated results of adsorption configurations P¹, P², P³, P⁴ and P⁵: d is the nearest distance between the adsorbed CO/formed CO₂ molecule and O₂ pre-adsorbed stoichiometric α-Fe₂O₃ (0 0 1) nano thin film, d₀ is the C–O bond length in the adsorption state and E_ads is the adsorption energy

Model	d (Å)	d0 (Å)	Eads (eV)
P^1	3.211	1.141	0.099
P^2	2.203	1.139	0.509
P^3	1.256	1.202	0.749
P^4	2.734	1.173/1.179	1.908
P^5	3.185	1.177/1.175	2.426

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3.3. Analysis and discussion

The net charge-transfer (ΔQ) between the gas micro-molecule and stoichiometric/O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film is defined as follows:

ΔQ = Q_ads − Q_free (8)

where Q_ads and Q_free are the charge of the gas micro-molecule in the adsorbed state and isolated state, respectively. By this definition, a positive value indicates that electrons were transferred to the stoichiometric/O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film from the gas micro-molecule. The pre-adsorbed O₂ molecule captured approximately 0.2265 e valence electrons from the α-Fe₂O₃ material surface, according to the Bader charge analysis in adsorption structure M². These results led to a decrease in electron-carrier concentration in the n-type α-Fe₂O₃ material and an increase in resistance of the gas sensor/adsorbent based on the n-type α-Fe₂O₃ material.

3.3.1 Comparative study of CO₂ molecule on an α-Fe₂O₃ (0 0 1) nano thin film with and without pre-adsorbed O₂. For a clear understanding of the adsorption characteristics of CO₂ on the α-Fe₂O₃ (0 0 1) nano thin film and the influence of pre-adsorbed O₂, the two most stable adsorption structures, N² and C², were employed for investigation. Fig. 5(a) and (b) show detailed structures of the adsorption structures N² and C². The Bader program was used to analyze the charge distribution of adsorption structures N², M², and C². The results of charge distribution about the CO₂ molecule, O₂ molecule and part atoms of the α-Fe₂O₃ (0 0 1) nano thin film labeled in Fig. 5(b) are listed in Table 7.

Fig. 5 Detailed structures of adsorption configurations (a) N²; (b) C²; (c) E³ and (d) P⁵. The blue, red and black balls represent Fe, O and C atoms, respectively.

Table 7 The results of the Bader charge distribution about the CO₂ molecule, O₂ molecule and part atoms of the α-Fe₂O₃ (0 0 1) nano thin film in the free state and adsorption configurations N², M² and C²

	Atom	Free (e)	Adsorption configuration (e)
			N^2	M^2	C^2
CO2	C	3.1810	1.8921	—	1.8653
	O1	6.4095	7.1607	—	7.1490
	O2	6.4095	7.1300	—	7.1280
O2	O3	6.0000	—	6.2058	6.1206
	O4	6.0000	—	6.0207	6.0099
Fe2O3	Fe1	6.4033	6.3261	6.3638	6.3082
	Fe2	6.4033	6.3407	—	6.3423
	O5	7.0958	7.1452	—	7.1628

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On comparing the adsorption structures N² and C², the appearance of pre-adsorbed O₂ resulted in an adsorption energy decrease from 1.253 eV (in adsorption structure N²) to 0.990 eV (in adsorption structure C²), which implies that the pre-adsorbed O₂ opposed the CO₂ molecule adsorption on the α-Fe₂O₃ (0 0 1) nano thin film. The theoretical results imply that the CO₂ molecule always acts as an acceptor and obtains electrons in the adsorption structures N² and C², as shown in Table 7. The CO₂ molecule captured 0.1828 e valence electrons from the α-Fe₂O₃ (0 0 1) nano thin film in adsorption structure N². These results demonstrate the decrease in the electron-carrier concentration and increase of resistance for gas sensors/adsorbents based on n-type α-Fe₂O₃ materials. The CO₂ molecule captured less electrons in adsorption structure C² (0.1428 e) than in adsorption structure N² (0.1828 e). The appearance of pre-adsorbed O₂ hindered the electron transfer to the CO₂ molecule from the α-Fe₂O₃ (0 0 1) nano thin film, which implies a smaller change in electrical properties for the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film after the CO₂ molecule was adsorbed. These results led to a smaller decrease in electron-carrier concentration and smaller increase in resistance, which indicates a poor gas sensing performance or a poor adsorption performance for gas sensors or adsorbents based on n-type α-Fe₂O₃ materials. However, Gurlo et al. proved that n-type α-Fe₂O₃ materials could transform to p-type α-Fe₂O₃ materials with a high oxygen concentration atmosphere.⁶³ The hole-carrier concentration on the surface α-Fe₂O₃ materials exceeded the electron-carrier concentration with an inversion layer. Their report pointed out that O₂ pre-adsorption α-Fe₂O₃ materials have p-type conducting behavior. The aforementioned theoretical results lead to an increase in hole-carrier concentration and decreasing in resistance, after the CO₂ is molecule adsorbed, for gas sensors/adsorbents based on p-type O₂ pre-adsorption α-Fe₂O₃ materials. These different n-type and p-type gas sensing phenomena would be simultaneously observed in α-Fe₂O₃ materials for CO₂ gas sensing measurement, which is similar in SnO₂ nanowire materials for NO₂ gas sensing experiments.⁶⁴

Fig. 6(a) shows the DOSs of a free CO₂ molecule and adsorbed CO₂ molecule in adsorption structures N² and C². The energy states of the adsorbed CO₂ move to lower energy states. The DOSs of the CO₂ molecule in adsorption structures N² and C² changed greatly and exhibited spin-splitting around the Fermi energy (E_f). Their DOSs revealed that the chemisorption introduced a weak magnetic moment on the CO₂ molecule in the adsorbed states of about 0.183 μ_B and 0.156 μ_B for adsorption structures N² and C², respectively. The magnetic moments were caused by the asymmetry of the structures and electrons. It can be seen from Fig. 7(a) and (c) that there was an overlap of the PDOSs peaks for C-2p (CO₂) and O-2p (surface O) in adsorption structures N² and C². A similar overlap phenomenon is also observed from the PDOSs peaks of O-2p (CO₂) and Fe-3d (surface Fe), as shown in Fig. 7(b) and (d). These results imply that there was strong chemisorption between the CO₂ molecule and the stoichiometric/O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film. The interaction mainly came from (1) the hybridization of C-2p (CO₂) and O-2p (surface O); and (2) the hybridization of O-2p (CO₂) and Fe-3d (surface Fe).

Fig. 6 Total density of states (TDOSs) of (c) free CO₂ molecule; (d) adsorbed CO₂ in the configuration N²; (e) adsorbed CO₂ in the configuration C²; (f) free CO molecule; (g) adsorbed CO + O₂ molecule in the configuration E³ and (h) adsorbed CO + O₂ molecule in the configuration P⁵. The vertical dotted line indicates the Fermi energy level.

Fig. 7 Partial density of states (PDOSs) of C-2p/O-2p (from CO₂) and O-2p/Fe-3d (from surface O/Fe) in adsorption structures N² and C², respectively.

3.3.2 Comparative study of CO molecule on an α-Fe₂O₃ (0 0 1) nano thin film with and without pre-adsorbed O₂. For a clear understanding of the adsorption characteristics of CO on the α-Fe₂O₃ (0 0 1) nano thin film and influence of pre-adsorbed O₂, the most two stable adsorption structures E³ and P⁵ were employed for investigation. Fig. 5(c) and (d) show detailed structures of the adsorption structures E³ and P⁵. The Bader program was also used to analyze the charge distribution of adsorption structures E³, M², and P⁵. The results of charge distribution about the CO molecule, O₂ molecule and part atoms of the α-Fe₂O₃ (0 0 1) nano thin film labeled in Fig. 5(c) and (d) are listed in Table 8.

Table 8 The results of the Bader charge distribution about the new CO₂ molecule, O₂ molecule and part atoms of the α-Fe₂O₃ (0 0 1) nano thin film in the free state and adsorption configurations E³, M² and P⁵

	Atom	Free (e)	Adsorption configuration (e)
			E^3	M^2	P^5
CO	C	2.9293	1.8771	—	1.8823
	O1	7.0707	7.0419	—	7.0646
O2	O3	6.0000	—	6.0207	7.0612
	O4	6.0000	—	6.2058	6.6031
Fe2O3	Fe1	6.4033	6.6915	6.3638	6.3542
	O2	7.0958	7.0954	7.0818	7.0069

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On comparing the adsorption structures E³ and P⁵, the appearance of pre-adsorbed O₂ resulted in an adsorption energy increase from 0.931 eV (in adsorption structure E³) to 2.426 eV (in adsorption structure P⁵), which implies that the pre-adsorbed O₂ was beneficial for the CO molecule to react with the α-Fe₂O₃ (0 0 1) nano thin film. The results imply that the newly formed CO₂ molecule always acts as a donor, and electrons transfer to the α-Fe₂O₃ (0 0 1) nano thin film in adsorption structures E³ and P⁵, as shown in Table 8. The newly formed CO₂ molecule transferred 1.0814 e valence electrons to the α-Fe₂O₃ (0 0 1) nano thin film in adsorption structure E³. This theoretical result demonstrates the increase in the electron-carrier concentration and decrease in resistance for gas sensors/adsorbents based on n-type α-Fe₂O₃ materials. The newly formed CO₂ molecule transferred less electrons in adsorption structure P⁵ (0.0126 e) than in adsorption structure E³ (1.0814 e). The appearance of pre-adsorbed O₂ hindered the electrons transfer to the α-Fe₂O₃ (0 0 1) nano thin film from the newly formed CO₂ molecule, which implies a smaller change in electrical properties for the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film after the CO molecule was introduced. These results led to a smaller increase in electron-carrier concentration and smaller decrease in resistance, which indicate a poor gas sensing performance or a poor adsorption performance for gas sensors or adsorbents based on n-type α-Fe₂O₃ materials. When n-type α-Fe₂O₃ materials transform into p-type α-Fe₂O₃ materials in a high oxygen concentration atmosphere, there is a decrease in hole-carrier concentration. Then, there is an increase in resistance after the CO molecule is introduced for gas sensors/adsorbents based on p-type O₂ pre-adsorption α-Fe₂O₃ materials. Fig. 6(b) shows the DOSs of a free CO molecule and newly formed CO₂ molecule in adsorption structures E³ and P⁵. It can be seen from Fig. 2(b) and 8 that there is an overlap of the PDOSs peaks for O-2p (new formed CO₂) and Fe-3d (surface Fe) in adsorption structure E³. These results imply that there was chemisorption between the newly formed CO₂ molecule and α-Fe₂O₃ (0 0 1) nano thin film with an oxygen vacancy. This interaction mainly came from the hybridization of O-2p (newly formed CO₂) and Fe-3d (surface Fe).

Fig. 8 Partial density of states (PDOSs) of O-2p (from new formed CO₂) and Fe-3d (from surface Fe) in adsorption structure E³.

For the α-Fe₂O₃ (0 0 1) nano thin film with and without pre-adsorbed O₂, CO₂ and CO molecules exhibit opposite behaviors of charge transformation: (1) electrons transferred to the CO₂ molecule from the α-Fe₂O₃ (0 0 1) nano thin film with and without pre-adsorbed O₂ (approximately 0.1828 e and 0.1428 e, respectively); and (2) electrons transferred from the CO molecule to the α-Fe₂O₃ (0 0 1) nano thin film with and without pre-adsorbed O₂ (approximately 1.0814 e and 0.0126 e, respectively). The pre-adsorbed O₂ decreased the adsorption energies from 1.253 eV to 0.990 eV for the CO₂ molecule adsorbed on the α-Fe₂O₃ (0 0 1) nano thin film. However, it increased the adsorption energies from 0.931 eV to 2.426 eV for the CO molecule adsorbed on the α-Fe₂O₃ (0 0 1) nano thin film.

4. Conclusions

Carbon dioxide and carbon monoxide adsorption characteristics, mechanism for the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film and influence of pre-adsorbed O₂ were fully investigated via the density functional theory method in this study. Theoretical results revealed that the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film can be prepared as an adsorbent or gas sensor for CO₂ and CO gas. For the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film, the most stable adsorption structure for the CO₂ molecule is the C atom of the CO₂ molecule adsorbed on top of the surface O atom, and the O atoms of the CO₂ molecule adsorbed on the adjacent surface Fe atom. The CO₂ molecule acts as an acceptor and attracts 0.1828 e electrons from the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film. However, the CO molecule reacts with the lattice oxygen on the surface of the α-Fe₂O₃ (0 0 1) nano thin film to form a new CO₂ molecule and the newly formed CO₂ molecule adsorbs onto the α-Fe₂O₃ (0 0 1) nano thin film with an oxygen vacancy when the CO molecule is introduced. After this reaction, the newly formed CO₂ molecule acts as a donor and transfers 1.0814 e electrons to the α-Fe₂O₃ (0 0 1) nano thin film with an oxygen vacancy. For the α-Fe₂O₃ (0 0 1) nano thin film with pre-adsorbed O₂ molecules, the most stable adsorption structure for the CO₂ molecule is the C atom adsorbed on the top of surface O atom and the O atoms adsorbed on the adjacent surface Fe atom, similarly to the α-Fe₂O₃ (0 0 1) nano thin film without pre-adsorbed O₂ molecules. The CO₂ molecule also acts as an acceptor and attracts 0.1428 e electrons from the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film, which is less than that of the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film. Nevertheless, the CO molecule reacts with the pre-adsorbed oxygen on top of the α-Fe₂O₃ (0 0 1) nano thin film directly to form a new CO₂ molecule and moves far away from the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film when the CO molecule is introduced. After this reaction, the newly formed CO₂ molecule also acts as a donor and transfers 0.0126 e electrons to the O pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film, which is also less than that of the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film. The CO molecule exhibited opposite behavior for charge transformation with the CO₂ molecule for the α-Fe₂O₃ (0 0 1) nano thin film with and without pre-adsorbed O₂ molecules. The theoretical results prove that pre-adsorbed O₂ molecules decrease the adsorption energy of the CO₂ molecule, thus making it more difficult for the CO₂ molecule to be adsorbed on the α-Fe₂O₃ (0 0 1) nano thin film. However, they increase the adsorption energy of the CO molecule, which makes the CO molecule adsorb on the α-Fe₂O₃ (0 0 1) nano thin film more easily. In addition, the pre-adsorbed O₂ hinders electron transfer to the CO₂ molecule from the α-Fe₂O₃ (0 0 1) nano thin film system and hinders electron transfer to the α-Fe₂O₃ (0 0 1) nano thin film system from the CO molecule, which indicates a poor gas sensing performance or a poor adsorption performance.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (No. 51431004, 11274151, 11204120, 11404158, and 51272133) and the Key Disciplines of Condensed Matter Physics of Linyi University.

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