1. Introduction
Environmental sensing is an important input information required by current artificial intelligence. Connecting artificial intelligence through new-generation sensing elements will be applied to human living environments, such as harmful pollution sources in the air, toxic substances generated during processing and production, or biogas in an oxygen-deficient environment. How to improve the sensitivity and the detection gas range is an important research topic of the new generation of sensing elements. The rapid increase in the market demand for artificial intelligence will drive the development of new-generation sensing components. At present, there are still many key technologies for environmental sensing to be overcome. For example, the gas sensors on the market are bulky and need to be installed in a fixed position or hand-held for detection. Recently, we have demonstrated that ZnGa
2O
4(111) films are grown directly on Al
2O
3(0001) using a metalorganic chemical vapor deposition (MOCVD) technique and applied in CO, CO
2, SO
2, NO, and NO
2 gas detection [
1]. In the presence of oxidizing gas on the
n-type ZnGa
2O
4 semiconductor, electrons flow from the
n-type semiconductor to the oxidizing gas until the Fermi level equilibrium is reached, which causes surface energy band bending [
2,
3,
4,
5]. The magnitude of surface band bending is the difference between the work function and electron affinity of target gases on ZnGa
2O
4(111) surfaces. The surface energy band bending makes it difficult for electrons inside the semiconductor to migrate to the surface, thereby increasing the surface resistance of the semiconductor. In contrast, when the reducing gas contacts the
n-type ZnGa
2O
4 semiconductor, electrons flow from the reducing gas to the
n-type semiconductor until the Fermi level is balanced, which results in ohmic contact formation [
2,
3,
4,
5]. Ohmic contact means that the current can flow in both directions at the junction between the reducing gas and the semiconductor, and the ohmic contact make the electrons inside the semiconductor easily migrate to the surface, thereby reducing the surface resistance of the semiconductor.
Recently, we have demonstrated that an NO
2-oxidizing and H
2S-reducing molecule adsorbed on the Ga–Zn–O-terminated ZnGa
2O
4(111) surface exhibit the highest work function change of +0.97 eV and −1.66 eV, respectively [
2]. Pd-decorated ZnGa
2O
4(111) sensors also exhibit high selectivity to NO
2 gas detectors, which have the maximum work function change of +1.37 eV and +2.37 eV for one and two NO
2 molecules to a Pd atom, respectively [
5]. In contrast, the work function change of one H
2S molecule on the Pd-decorated ZnGa
2O
4(111) surface is reduced to −0.90 eV, resulting in the decrease in sensitivity to a palladium adsorbed on the ZnGa
2O
4(111) surface [
5]. In this regard, the work function changes of two H
2S molecules on the Pd-decorated ZnGa
2O
4(111) surface changes significantly to −1.82 eV or shows a trend of increasing sensitivity with increasing gas molecule concentration [
5]. The gas sensing sensitivity is not only dependent on the work function change, but also related to the adsorption energy of gas molecules. The adsorption energy of gas molecules with exothermic reaction can spontaneously react to cause gas molecule adsorption, so that the gas sensing sensitivity can be estimated by the change of the work function [
5]. However, the adsorption energy of an NO
2 (H
2S) molecule adsorbed to the ZnGa
2O
4(111) surface is 0.64 eV (2.32 eV), showing that the positive adsorption energy or the corresponding endothermic process can be supported by an increase in temperature [
5]. High positive adsorption energy means that more external energy is required, suggesting that the film is not easy to attract gas molecules and the sensitivity will be limited even under large work function changes.
In this study, we pursued the gas-sensing mechanism of the adsorption of CO-reducing molecules on a clean ZnGa2O
4(111) and Pd-decorated ZnGa
2O
4(111) surfaces. Human inhalation of carbon monoxide will combine with hemoglobin in the human body and diminish the hemoglobin’s oxygen-carrying capacity, resulting in tissue hypoxia [
6]. The lack of oxygen in the human body will cause dizziness, headache, nausea, weakness, and other symptoms, and in severe cases, coma, convulsions, and even death. The development of CO sensors has been an important and popular research topic for a long time [
1,
2,
5,
7,
8,
9,
10,
11]. For example, Yu et al. reported the sensitivity of CO gas detection of the CuO- and ZnO-doped SnO
2 gas sensor [
7]. The maximum sensitivity of SnO
2 to 200 ppm CO is 7 at 350 °C, while the addition of 1 mol% CuO and 3 mol% ZnO increases the sensitivity of SnO
2 sensor to 8 at 200 °C. Gong et al. prepared Cu-doped ZnO (CZO) thin films on glass substrates by cosputtering using ZnO and Cu targets [
8]. The CZO films with the columnar structures consisted of small crystals with an average grain size of about 5 nm. The CO sensing properties of the CZO films exhibit the highest sensitivity to 40 ppm CO at 350 °C, while the resistance values of the CZO films are also observed when the sensor is exposed to 6 ppm CO at 150 °C. Paliwal et al. reported that ZnO sensing films deposited on gold-coated prisms exhibit high sensitivity with very fast response to CO gas in a wide concentration range (0.5–100 ppm) at room temperature [
9]. Belmonte et al. proposed a micromachined twin sensor, which consists of two sensing layers, a sensing layer made of Cr
xTi
yO
2, and a SnO
2 metal oxide gas sensing layer [
10]. When the reducing gas CO interacts with the sensing surface, the
n-type SnO
2 gas sensor materials can obtain more electrons, thereby reducing the resistance value of the SnO
2 material. Chang et al. studied a palladium-doped ZnO gas sensor to detect CO gas [
11]. The sensitivity of the palladium-doped ZnO (Pd/ZnO) was 4.5 times higher than that of ZnO when the CO concentration was 100 ppm. The Pd/ZnO injected with CO concentrations of 10–600 ppm shows a decrease in the resistance responses, whereas ZnO shows no significant change under the CO concentrations of 10–600 ppm. The response time of Pd/ZnO at a CO concentration of 100 ppm is 200 s, compared with 750 s for ZnO, which indicates that the change of the response time of Pd/ZnO is three times shorter than that of ZnO. The experimental results show that doping palladium helps to improve the sensitivity and response time of the ZnO sensor.
Gallium(III) oxide is another material of interest due to its large bandgap and physical and chemical stability. Tin-doped Ga
2O
3 improves material conductivity, which makes the higher Lewis acidity of Sn
4+ cations than Ga
3+ ones, which leads to a significant sharp increase in sensor signal for detecting CO and NH
3 at high temperature of 500 °C [
12]. On the other hand, carbon dioxide adsorption on the nonpolar (10
0) surface of ZnO shows the tridentate binding mode to be the most energetically favorable, which the internal C=O bonds of CO
2 lengthened upon adsorption from the initial value from 1.16 Å to 1.26–1.24 Å [
13]. If defective surface with one oxygen vacancy on the nonpolar (10
0) surface of ZnO exists, the adsorption of carbon dioxide on the surface causes the oxygen atom in CO
2 to be trapped, acting as a CO molecule remaining on the surface. For deep ultraviolet applications integrated on
c-plane (002) sapphire substrates, the ZnGa
2O
4 thin films were prepared using the diethylzinc (DEZn), triethylgallium (TEGa), and oxygen (99.999%), and were transformed from β-Ga
2O
3 to ZnGa
2O
4 with increasing DEZn flow rate [
14]. This is because the sufficient tensile strain in Zn-doped β-Ga
2O
3 provides a driving force for ZnGa
2O
4 formation, obtaining the carrier concentrations up to 6.72 × 10
16 cm
−3 and the resistivity down to 67.9 Ω-cm. For applications of green storage and long persistent phosphors, ZnGa
2O
4 spinel ceramics doped with Mn
2+ ions were prepared by a solid-state reaction at 1200 °C in air and demonstrated a relatively long and strong green afterglow due to the holes released from shallow traps of zinc vacancies [
15]. Although the related studies on CO gas sensors are diverse and have made major breakthroughs, much less attention has been focused on the theoretical calculation of CO gas sensors. In addition, the ZnGa
2O
4 layers are suitable as gas sensors, which can successfully detect CO, CO
2, SO
2, NO, and NO
2, and have wide bandgap of 5.2 eV, excellent optical characteristics, such as transparency in the near ultraviolet region, good conductivity, high resistance, as well as high thermal and chemical stability [
1,
14]. Accordingly, we have performed sufficient models to accurately predict CO gas adsorption on clean ZnGa
2O
4(111) and Pd-decorated ZnGa
2O
4(111) surfaces. Our analysis will focus on the work functions variation, adsorption energy, and the catalytic properties of palladium.
2. Computational Details
Systematic ab initio theoretical calculations were performed to study the equilibrium bond lengths, adsorption reactions, and work functions of carbon monoxide on the surfaces of the clean and Pd-decorated ZnGa
2O
4(111). The Vienna ab initio simulation package [
16,
17] was applied at the generalized gradient approximation (GGA) with the Perdew–Wang (PW91) correction [
18,
19] in all cases. The ground state structure of bulk ZnGa
2O
4 is cubic
Fd-3m and performed using 8 Zn, 16 Ga, and 32 O atoms per unit cell, as shown in
Figure 1. The Zn
2+ and Ga
3+ cations are distributed in tetrahedral and octahedral lattice sites, respectively. The cutoff energy and the self-consistent total energy criterion were set to 500 eV and 10
−5 eV/unit cell, respectively, and the equilibrium lattice parameter of bulk ZnGa
2O
4 can be obtained as 8.334 Å. To simulate the work function of the clean and Pd-decorated ZnGa
2O
4(111) with and without adsorbed CO molecules, we have employed a repeated slab geometry with a 112-atom ZnGa
2O
4 with an in-plane lattice constants of 11.85 Å × 11.85 Å, which is sufficient to decouple the interactions between CO molecules, separated by a vacuum region equivalent to 20 Å vacuum, which decouples top and bottom interactions. Our slabs are terminated by the Ga-Zn-O surface with a low surface energy of 0.10 eV/Å
2, proposed earlier study by Jia et al. [
20]. The top and side views for the selected adsorption sites of Ga-Zn-O-terminated ZnGa
2O
4(111) are displayed in
Figure 1. A gamma-centered 3 × 3 × 1 Monkhorst–Pack grid was used for the density of state integration. This supercell was fully relaxed until the force acting on each atom was less than 0.001 eV/Å.
To determine the preferred adsorption sites of the CO molecules on the ZnGa
2O
4(111) surface, we labeled the surface atoms as Ga
3c, Zn
3c, O
3c, and O
4c in the top and side views of
Figure 1. To gain further insight into the CO adsorption on the ZnGa
2O
4(111) surface, we consider three following types. One type plotted in the upper panel of
Figure 2 is that a carbon atom from a CO molecule perpendicular to the surface is adsorbed on Ga
3c, Zn
3c, O
3c, and O
4c sites, labeled as CO-C1, CO-C2, CO-C3, and CO-C4, respectively, or CO-C
i denoted by model number
i (where
i = 1–4). The second type, plotted in the middle panel of
Figure 2, is an oxygen atom from a CO molecule perpendicular to the surface is adsorbed on Ga
3c, Zn
3c, O
3c, and O
4c sites, labeled as CO-O1, CO-O2, CO-O3, and CO-O4, respectively, or CO-O
i denoted by model number
i (where
i = 1–4). Thirdly, four adsorption models, CO-CO
i, denoted by model number
i (where
i = 1–4), with both the O and C atoms from a CO molecule parallel to the surface interacting with the ZnGa
2O
4(111) surface were constructed as shown in the bottom panel of
Figure 2. In model CO-CO1 (CO-CO2), a carbon (an oxygen) atom from a CO molecule is adsorbed on the Ga
3c site. In model CO-CO3 (CO-CO4), a carbon (an oxygen) atom from a CO molecule is adsorbed on the Zn
3c site. On the other hand, we also constructed four adsorption models of Pd-decorated ZnGa
2O
4(111), denoted as Pd-ZGO
i (where
i = 1–4), as shown in the upper panel of
Figure 3. Here, the subscripts 1, 2, 3, and 4 in Pd-ZGO, respectively, indicate the positions of the initial adsorbed sites Ga
3c, Zn
3c, O
3c, and O
4c on the ZnGa
2O
4(111) surface. An oxygen atom from a CO molecule perpendicular to the surface is adsorbed on Pd-ZGO
i models, labeled as CO-Pd-ZGO
i (where
i = 1–4), as shown in the bottom panel of
Figure 3. The initial distance from the adsorbed atom to the surface atom was set as the sum of the van der Waals radii of each of atom.
The adsorption of gas molecules on the surface of the ZnGa
2O
4(111) surface causes a change in the work function
, resulting in a change between the resistance in the presence of the investigated gas (
Rg) and the resistance in the reference gas (
Ra) to measure the gas sensitivity. The gas sensitivity could be determined from the ratio of
Rg/
Ra, and the work function change
is given by [
3]
where
denotes the change in electron affinity and
kT denotes the product of the Boltzmann constant
k and the temperature
T. Note that the work function
is given by the following equation [
21]:
where
and
are the energies of a vacuum level and a Fermi level, respectively. Electrons in solids and molecules obey the Fermi–Dirac distribution. Electrons with the same quantum properties are forbidden to occupy the same energy state, while up to two electrons with opposite spins are allowed to occupy one energy state. The energy of the highest occupied state is known as the Fermi energy. Additionally, the limitations of the first-principles calculations do not allow the calculation of the absolute vacuum level because it depends on the periodic boundary conditions and surface terminations of the materials. The vacuum level here refers to the energy estimated from the planar average of the potential of the periodic slabs in the vacuum region along the direction perpendicular to the surface of the ZnGa
2O
4(111) surface. The gas adsorption on the ZnGa
2O
4(111) surface that enables exothermic or endothermic reaction is also one of the serious issues affecting the sensitivity of the sensor. The adsorption energy ∆E can be calculated through the equation:
where
is the total energy of a target molecule or a CO molecule adsorbed on the ZnGa
2O
4(111) surface, and
and
are the total energies of a slab of ZnGa
2O
4(111) surface model and a free (isolated) CO molecule, respectively. Whether the adsorption energy is related to the work function is also an issue to be discussed in this study.
3. Results and Discussion
Calculated equilibrium bond lengths (Å) for the CO-C
i, CO-O
i, CO-CO
i, Pd-ZGO
i, and CO-Pd-ZGO
i are shown in
Figure 2 and
Figure 3. In models CO-C
i and CO-O
i, the equilibrium bond lengths are ranged from 2.16 Å to 3.53 Å. The lowest bond length occurs in the CO-C2, where the carbon atom of a CO molecule perpendicular to the surface is bonded to the zinc atom. The bond length of the CO-C
i is smaller than the CO-O
i for each model number
i. The smallest bond length of model CO-O
i is 2.51 Å. In models CO-CO
i, the bond lengths are ranged from 2.18 Å to 2.65 Å. The equilibrium bond lengths for models CO-CO
i are remarkably similar to those of the CO-C
i and CO-O
i. For example, the equilibrium bond length for the CO-C1 is 2.17 Å, where the carbon atom of a CO molecule is bonded to the Ga atom of the ZnGa
2O
4(111) surface. Similarly, the bond length for the CO-CO1 is 2.18 Å, showing the adsorption of CO molecule from being parallel to the surface to being perpendicular to the surface. In the case of the CO-CO4, our calculations show that the corresponding bond length for C-Ga and O-Zn bonds are 2.65 Å and 2.60 Å, respectively, suggesting that an increased bond length means less attraction between atoms. In our preliminary work [
5], a Pd atom on the ZnGa
2O
4(111) surface shows that the calculated Pd-Ga, Pd-Zn, and Pd-O equilibrium bond lengths are 2.32 Å, 2.57 Å, and 2.04 Å for Pd-ZGO1, Pd-ZGO2, and Pd-ZGO3, respectively. In contrast with the Pd atoms adsorbed on the ZnGa
2O
4(111) surface, the calculated equilibrium O-Pd bond lengths for models CO-Pd-ZGO
i are between 2.08 Å and 2.50 Å.
The adsorption energies ∆E of a CO molecule on the ZnGa
2O
4(111) surface without Pd atoms are shown in
Table 1. The calculated adsorption energies ranged from −0.04 eV to −1.88 eV, indicating that all adsorption reactions occurred spontaneously. Note that spontaneous reactions occur in the direction of decreasing Gibbs free energy change ΔG. The Gibbs free energy change consists of the change in enthalpy, ΔH, and the change in entropy, ΔS, with the following formula: ΔG = ΔH − TΔS. The enthalpy change is defined as the sum of the change in internal energy, ΔE, and the product of the pressure, P, and the change in volume, ΔV, as follows: ΔH = ΔE + PΔV. Therefore, the Gibbs free energy change can be expressed as: ΔG = ΔE + PΔV − TΔS. In our study, we ignored the contribution of entropy changes and the volume changes in the ZnGa
2O
4 structure. The Gibbs free energy change can be further simplified to the internal energy change caused by the kinetic, potential, and chemical energy of the material system. Negative Gibbs free energy change, negative internal energy change, or negative adsorption energy ΔE provides a means by which spontaneous physical and chemical changes can occur without any external help. Conversely, positive adsorption energies suggest that the corresponding endothermic process can be supported by increasing the temperature. It can be clearly seen that the CO-C1 has the lowest adsorption energy of −1.88 eV. Consistently, this adsorption site Ga
3c exists a low C-Ga bond length of 2.17 Å, i.e., a strong interaction, suggesting that that the Ga
3c site is more favored if CO molecules are adsorbed on the ZnGa
2O
4(111) surface. It is therefore also of significant interest to carry out a detailed comparison of the adsorption energies of the CO-O
i with those obtained from the CO-C
i. The CO-O
i have low adsorption energies, and ranged from −0.04 eV to −0.44 eV, showing less attraction between atoms. It is perhaps not surprising that the CO-O
i have longer bond lengths than the CO-C
i. In the cases of the CO-CO
i, the CO-CO2 has the lowest adsorption energy of −1.55 eV and the CO-CO1 exhibits the second lowest adsorption energy of −1.28 eV. The oxygen atoms of CO molecules are more attractive to the Ga surface atoms of the ZnGa
2O
4(111) surface than the carbon atoms of CO molecules.
The gas sensing sensitivity of the various CO-C
i, CO-O
i, CO-CO
i, and CO-Pd-ZGO
i configurations is intimately related to their surface resistances or work function changes. A common approach adopted in many studies is to analyze energies of the Fermi level and the vacuum level to determinate their work functions. We use the clean ZnGa
2O
4(111) surface as a reference state surface to calculate the work function of 3.91 eV, as shown in
Table 1. In
Table 1, it can be clearly seen that the Fermi energies of all models increase when a CO molecule is adsorbed on the ZnGa
2O
4(111) surface. This is because a CO molecule adds extra electrons to the system, causing the increase of the Fermi energy. From
Table 1, it is apparent that the vacuum energies of the only two models, i.e., the CO-C1 and CO-O1, are lower than the reference value (0.41 eV) of the vacuum level of the clean ZnGa
2O
4(111), leading to the small work functions. However, it is interesting to note that the bonding of carbon or oxygen atoms from CO molecules to Ga surface atoms on the ZnGa
2O
4(111) surface decreases the vacuum energy level, which may explain why the Ga
3c sites of adsorbed gas molecules have a great influence on the gas sensitivity. In the CO-C
i, the work function is proportional to the adsorption energy, while the CO-C1 model had the largest reduction with the work function change of −0.53 eV. In the CO-O
i, CO-O1 has the largest reduction with the work function change of −0.55 eV. Surprisingly, the work function change of CO-O1 is slightly higher than that of CO-C1 (−0.53 eV), which means that the Ga
3c sites of adsorbed CO molecules have a great influence on the gas sensitivity. In the CO-CO
i, CO-CO2 has the largest reduction with the work function change of −0.49 eV, showing the oxygen atoms of CO molecules are more sensitive to the Ga surface atoms of the ZnGa
2O
4(111) surface than the carbon atoms of CO molecules, corresponding to the work function change of −0.43 eV.
Based on our previous study [
5], we found that Pd atoms can be used to enhance the performance of ZnGa
2O
4-based gas sensors for detecting NO
2 and H
2S. Here, we calculate the work function change and adsorption energy of the CO-Pd-ZGO
i by following a similar procedure, as listed in
Table 1. The work function changes of the CO-Pd-ZGO
i range from −0.43 to −0.79 eV, making the latter 1.43 times larger than the former. Our results show that an increase in the work function changes can be used to improve the performance of the gas sensor. In addition, the adsorption energies of CO-Pd-ZGO
i drop drastically and range from −1.58 to −3.36 eV, indicating CO molecules are catalyzed by Pd atoms to promote adsorption on the surface of ZnGa
2O
4(111) to enhance the work function change. In the CO-Pd-ZGO
i, CO-Pd-ZGO2 has the lowest adsorption energy of −3.36 eV. This implies that the existence of the Pd atom on the initial adsorbed site Zn
3c of the ZnGa
2O
4(111) surface is particularly attractive for CO molecules.