1. Introduction
Gas sensors for household and industrial usages have attracted much attention since 1970 [
1]. Toxic gases, such as hydrogen sulfide (H
2S), sulfur dioxide (SO
2), nitrogen dioxide (NO
2), carbon monoxide (CO), and nitric oxide (NO), are harmful to plants, organisms, and people’s health. Therefore, finding a suitable sensor for toxic gases is an emerging and interesting research field. Experimentally, several key problems and challenges persist in the development of sensing components. For example, the sensor’s working or operating temperature may be too high or too low, which restricts its use. The size of the gas-sensing application is also a key factor for a wearable device. Moreover, distinguishing the component and concentration of a specified gas from mixing gases is difficult.
The realization of the sensor response to functional or target gases can significantly enhance applications of the biomimetic response and controlling, as well as autonomously release functional gases [
2,
3]. The conjugated bimetallopolymer with Pt(II)-acetylide and Ru(II)-porphyrin-pyridyl is polymerized with oligo (phenylene ethynylene) (OPE) backbones and insulated by permethylated α-cyclodextrins (PM α-CDs) [
2]. The conjugated bimetallopolymer is depolymerized by Ru complexes, which behave as acceptors with carbon monoxide (CO) reactivity. The depolymerization of the non-radiative conjugated polymer to luminescent monomers of Pt-acetylide complexes is the first self-activation threshold for the activation of the system. The luminescent intensity increase with the increase in CO concentration is an autonomous response above the threshold. Approaching constant amounts of luminescent Pt-monomers is the second self-regulation threshold for the rate-determining step of the Ru complex. The dual self-controlling system of artificial biomimetic materials can be used as sensors, catalytic systems, and machines transporting materials.
The charge-compensating anions HS
− and NO
2− and hydrated water (
mH
2O) in Mg
yAl(OH)
2(y+1) (
y is in the range of 2~4) layered double hydroxides (Mg/Al(
y/1) LDHs) are used as the gas release system for medical applications [
3]. Interlayer HS
− and NO
2− in LDHs are protonated with aerial CO
2 and H
2O, releasing H
2S or HNO
2 under air. NO and NO
2 can be further generated through an automatic disproportionation reaction of HNO
2. LDHs (Mg/Al = 2/1 or 3/1) mixed with NaHS, Na
2S, and NaNO
2, i.e., NaHS-Mg/Al(2/1), NaHS-Mg/Al(3/1), Na
2S-Mg/Al(2/1), Na
2S-Mg/Al(3/1), NaNO
2-Mg/Al(2/1), and NaNO
2-Mg/Al(3/1) LDHs, have been successfully synthesized to release p.p.m.-level H
2S and NO derived from aerial CO
2-stimulated anion exchange, respectively. NaHS-Mg/Al(2/1) and NaNO
2-Mg/Al(3/1) LDHs showed the best release profile for H
2S and NO in a rather flat and elongated gas release, respectively. NaNO
2-Mg/Al(3/1) LDHs have been demonstrated to produce a battery-free respirator for inhaled NO, which is a selective and quick-acting pulmonary vasodilator after inhalation.
Metal oxide semiconductors [
4], such as SnO
2, Ga
2O
3, and WO
3, have been widely studied and used as gas sensors [
5,
6,
7,
8,
9,
10,
11]. The important features of metal oxide semiconductor sensors are reversible interactions between gas and the surface of metal oxide semiconductors [
10]. The SnO
2, Ga
2O
3, WO
3 and other metal oxide semiconductor sensors can be worked stably at a high temperature from 200 °C to 900 °C and can be used for a long time. In recent years, SnO
2, Ga
2O
3, WO
3, and other sensors have been determined as sensors for detecting CO, H
2, CH
4, O
3, NO
2, H
2S, SO
2, and other gases [
11]. The metal oxide semiconductor sensors with good thermal stability and long-term working stability have greatly aroused the interest of researchers and have been extensively studied [
7,
8,
9,
10] in the past decade. The gas adsorption reaction is converted into the sensitivity response of the sensor to detect gas by measuring the work function, capacitance, conductivity [
10,
12,
13,
14,
15], optical characteristics, and other parameters of the sensing material surface. Kolmakov et al. reported that nanowire sensors made by SnO
2 can detect CO at temperatures between 200 °C and 280 °C [
16]. The
β-Ga
2O
3 films can be used to detect several gases, such as CO, hydrogen, methane (CH
4), NO, and ammonia (NH
3) [
17].
The structure of ZnGa
2O
4 is spinel (space group:
). Both ZnGa
2O
4 and
n-type semiconductor ZnGa
2O
4 thin films have been successfully developed as gas sensors. Satyanarayana et al. [
13] reported that ZnGa
2O
4 films can be used to sense liquid petroleum gas (LPG). When the ZnGa
2O
4 sensor is further doped with palladium at approximately 320 °C, the sensitivity of detecting LPG increases, but the sensitivity to CH
4 and CO [
13] gas becomes poor. Despite the introduction of new approaches for developing gas sensors, the fundamental interaction between gas molecules and metal oxide compounds remains unclear. In recent decades, the density functional theory (DFT) has been widely used in the fields of the gas-sensing mechanism, adsorption of atoms and molecules on surfaces, and surface reconstruction [
6,
7]. For example, the gas-sensing mechanism of ZnO applied to H
2, NH
3, CO, and ethanol (C
2H
5OH) was studied by first-principles DFT calculations [
7]. Due to the adsorption of surface molecules, the ZnO surface is reconstructed. The charge is transferred from the gas to the ZnO surface and vice versa, which affects the conductivity of the ZnO in gas detection. Vorobyeva et al. reported that the resistance of ZnO increases (decreases) in the presence of NO
2 (H
2S) [
8] adsorption. Recently, we demonstrated that the use of the metal organic chemical vapor deposition (MOCVD) technique to grow epitaxial ZnGa
2O
4 films on sapphire substrates can produce highly selective gas sensors [
9].
Reactions of NO
2 and H
2S molecules on Ga-Zn-O terminated ZnGa
2O
4(111) surfaces were modeled and carried out using a first-principles density functional theory method [
18]. Based on our previous theoretical study, we report on the palladium adsorbed on the ZnGa
2O
4(111) surfaces and the work function changes with and without NO
2 (H
2S) molecules. In principle, the sensitivity response of the sensor to detect the target gas can be determined by the following work function changes
according to
where ΔX is the change in electron affinity, and
kT is the product of the Boltzmann’s constant
k and the temperature
T.
Rg is the resistance in the presence of the target gas.
Ra is the resistance in the reference gas, which is usually air. We hope that this work can help experimental researchers to effectively develop gas-sensing devices.
2. Computational Details
The work functions of NO
2 and H
2S adsorbed on Pd-decorated ZnGa
2O
4(111) (Pd-ZGO) surfaces were systematically studied. All calculations were performed by using the density functional theory as implemented in the Vienna ab initio simulation package (VASP) [
19,
20] with the exchange correlation function described by generalized gradient approximation (GGA) and the Perdew–Wang (PW91) correction [
21,
22]. The crystal structure of ZnGa
2O
4 is displayed in
Figure 1a. We used a conventional crystal structure containing 56 atoms, i.e., 8 Zn, 16 Ga, and 32 O atoms. The cutoff energy was set to 500 eV, and the self-consistent total energy criterion was
eV/unit cell.
To understand the work function changes when NO
2 (H
2S) is adsorbed on Pd-ZGO surfaces, we built a supercell containing 112 atoms to simulate ZnGa
2O
4(111) surfaces with a vacuum of 20 Å, which should be wide enough to decouple the nearest surface interaction. This preferred Ga-Zn-O-terminated surface was selected from the previous study on ZnGa
2O
4, and its surface energy was 0.01 eV/Å
2 [
23]. In order to describe the effect of changes in the work function of the NO
2 and H
2S on the surface of Pd-ZGO, we firstly calculated the work function
using the following formula [
24]:
where
is the work function of ZnGa
2O
4(111) surface.
and
are the energy of the vacuum and the Fermi energy, respectively.
Second, we constructed four adsorption models of Pd atoms, denoted as Pd-ZGO
i (
i = 1,2,3,4). Here, the superscripts 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, as shown in
Figure 1b. The initial distance from the Pd atom to the ZnGa
2O
4(111) surface was set as the sum of the van der Waals radii of the Pd atom and Ga (Zn or O) surface atoms, as shown in
Figure 2. Third, we considered one NO
2 (H
2S) molecule and two NO
2 (H
2S) molecules adsorbed on the surface Pd atoms and compared the corresponding results of the calculated work functions in the second step, denoted as
nNO
2-Pd-ZGO
i and
nH
2S-Pd-ZGO
i (
n = 1,2;
i = 1,2,3,4). Here,
n represents the number of molecules. All supercells were fully relaxed until the force attraction on each atom was less than 0.001 eV/Å, and a 3 × 3 × 1 Monkhorst–Pack grid was used. The initial distance from NO
2 (H
2S) molecules to Pd surface atoms was also set to the sum of the van der Waals radii of N (S) from the NO
2 (H
2S) molecules and the Pd surface atoms. The van der Waals distances of H, N, O, S, Zn, Ga, and Pd atom were 1.20 Å, 1.55 Å, 1.52 Å, 1.80 Å 1.39 Å, 1.87 Å, and 1.63 Å, respectively.
When the NO
2 (H
2S) molecules were adsorbed on the ZnGa
2O
4(111) and Pd-ZGO(111) surface, we calculated the work function
, which in turn determined the work function differences
(eV) between
and
. The difference in these obtained work functions is the key factor of gas sensor performance [
25]. Finally, the adsorption response of the sensor is closely related to the energy lost or gained by the corresponding gas adsorption. The lost or gained energy of the adsorbed gas can be determined by the adsorption energy. The adsorption energy is the energy released when NO
2 (H
2S) molecules are adsorbed on the surface of Pd-ZGO
i. The adsorption energy Δ
E was calculated as follows:
where
Egas+Pd-ZGO represents the energy of the Pd-ZGO
i surface with the adsorbed NO
2 (H
2S) molecules.
Egas represents the energy of free NO
2 (H
2S) molecules, and
EPd-ZGO represents the energy of the Pd-ZGO
i surface. Whether the adsorption energy is related to the work function is also a question to be discussed in this study.
3. Results and Discussion
3.1. Structures
The calculated equilibrium bond lengths of Pd-ZGOi, 1NO
2-Pd-ZGOi, 2NO
2-Pd-ZGOi, 1H
2S-Pd-ZGOi, and 2H
2S-Pd-ZGOi (
i = 1, 2, 3, 4) are listed in
Table 1. The calculated Pd-Ga equilibrium bond lengths are 2.32 Å and 2.37 Å, respectively, for Pd-ZGO1 and Pd-ZGO4 structures; the calculated Pd-Zn equilibrium bond lengths are 2.57 Å and 2.66 Å, respectively, for Pd-ZGO2 and Pd-ZGO4 structures; the calculated Pd-O equilibrium bond length is 2.04 Å for the Pd-ZGO3 structure. The equilibrium bond length of Pd-Ga or Pd-Zn is significantly larger than that of Pd-O (see
Figure 1 and
Table 1). This is because the valence electron for the O atom is negative, while it is positive for Pd, Ga, and Zn atoms. The Wigner–Seitz radii of Pd, Zn, Ga, and O atoms are 1.434 Å, 1.217 Å, 1.402 Å, and 0.820 Å, respectively.
The bond length of Pd-Zn is larger than that of Pd-Ga because it is dominated by the Coulomb interaction. In other words, the electronic configurations of Zn and Ga are [Ar] 3d104p2 and [Ar] 3d104s24p1, respectively. When the NO2 molecule is adsorbed on the Pd-ZGO structure, the Pd atom is a bridge connecting the NO2 molecule and the ZGO(111) surface. The calculated equilibrium bond length is completely different from Pd-ZGOi. In the 1NO2-Pd-ZGO1 structure, the bond length of Pd-Ga is 2.53 Å, where it is 2.32 Å in the absence of the NO2 molecule without Pd atoms. Note that in the initial structure of 1NO2-Pd-ZGO1, the distance between the NO2 molecule and the surface Pd atom is 3.18 Å. The electronic configuration of the Pd atom is [Kr]4d95s1. This implies the attraction between Pd and N atoms. As a result, the bond length of N-Pd is reduced to 2.01 Å, while the bond length for Pd-Ga is increased. Generally, the N-Pd bond lengths in nNO2-Pd-ZGOi are about 2 Å.
In the 1NO
2-Pd-ZGO2 and 1NO
2-Pd-ZGO4 structures, Pd atoms do not remain on the top of the Zn
3C and O
4C sites. Therefore, the calculated bond lengths change significantly, especially for the O
4C site (see
Figure 3). It seems that Pd atoms are attracted by Ga atoms on the ZGO(111) surface. In one and two NO
2 molecules attracted to Pd atoms, the results of 1NO
2-Pd-ZGO1 (1NO
2-Pd-ZGO3) and 2NO
2-Pd-ZGO1 (2NO
2-Pd-ZGO3) are similar, while Pd atoms are retained on the top of the Ga
3C and O
3C sites. When two NO
2 molecules are adsorbed into the Pd atom, the calculated bond length of N-Pd is larger. In the 2NO
2-Pd-ZGO4 structure, the interaction between the Pd and surface Zn (O) atom disappears. Generally, when NO
2 molecules are adsorbed on Pd atoms, the bond length of Pd-Ga, Pd-Zn, and Pd-O will increase.
When H
2S molecules were adsorbed on the surface of Pd-ZGO, as shown in
Figure 4, we first noticed an increase in the bond lengths of Pd-Ga, Pd-Zn, and Pd-O. We also found that in the 1H
2S-Pd-ZGO1 and 1H
2S-Pd-ZGO4 structures, the Pd atom is approximately at the top of Ga
3C site, and the distances for the S-Pd and Pd-Ga are equal. The bond length of S-Pd in nH
2S-Pd-ZGOi (i = 1,2,3,4) is larger than that of N-Pd in nNO
2-Pd-ZGOi. This difference is mainly due to the van der Waals radii.
3.2. Work Functions
Table 2 shows that the work function decreases when Pd atoms are adsorbed on the ZGO surface. The reduction in work function ranges from −0.44 eV to −0.15 eV. The larger magnitude of
is better. Therefore, the Pd-ZGO1 structure, i.e., Pd on the top of the Ga
3C site, is the most sensitive.
When a NO2 molecule was adsorbed on the surface of Pd-ZGOi, we found that 1NO2-Pd-ZGO1 had the largest (+1.37 eV). In the 1NO2-Pd-ZGO1 structure, N-Pd and Pd-Ga have the largest bond lengths. The magnitude of ranges from +1.22 eV to +1.37 eV in 1NO2-Pd-ZGOi. This increase in is further increased as the number of NO2 molecules becomes two. Note that when H2S is adsorbed on the ZGO surface, the work function difference changes significantly to −1.69 eV. When H2S molecules are adsorbed on the surface of Pd-ZGOi, the magnitude of is about half that of H2S-ZGO, ranging from −0.49 eV to −0.90 eV in 1H2S-Pd-ZGOi. The 1H2S-Pd-ZGO3 structure has the largest (−0.90 eV). When two H2S molecules are adsorbed on the surface of Pd-ZGOi, the magnitudes of of 2H2S-Pd-ZGO1 (−1.77 eV) and 2H2S-Pd-ZGO4 (−1.82 eV) are larger than that of H2S-ZGO without the Pd atom (−1.69 eV), showing that the sensitivity of the ZGO surface doped with palladium becomes higher.
3.3. Adsorption Energies
Table 2 also shows adsorption energies of the NO
2 and H
2S molecules on the surface of ZGO with and without Pd atoms. It can be clearly seen that a NO
2 (H
2S) molecule is adsorbed to the ZGO surface, which is endothermic by 0.64 eV (2.32 eV). In contrast with Pd atoms adsorbed on the surface of ZGO, i.e., Pd-ZGO
i, the NO
2 and H
2S molecules on the surface of Pd-ZGO
i have exothermic or negative adsorption energy. It should be noted that the spontaneous reaction occurs in the direction where the Gibbs free energy change, Δ
G, decreases. The Gibbs free energy change consists of enthalpy change Δ
H and entropy change Δ
S by the following equation:
The enthalpy change is defined as the sum of the internal energy change Δ
E and the product of the pressure
P and volume change Δ
V, as follows:
Therefore, the Gibbs free energy change can be expressed by the following equation:
In our study, we ignored the contribution of entropy change, and the volume change in the Pd-ZGOi structures was negligible. The Gibbs free energy change can be further simplified as the internal energy change caused by the kinetic energy, potential energy, and chemical energy of the target system. The negative Gibbs free energy change, its negative internal energy change, or its negative adsorption energies, ΔE, provides a means of spontaneous physical and chemical changes to take place without any external help. On the contrary, the positive adsorption energy indicates that the corresponding endothermic process can be supported by the increase in temperature. A higher positive adsorption energy means that more energy is required from the outside world, which also indicates that the endothermic reaction does not occur easily. Our results on the surface of ZGO show that the adsorption of NO2 molecules on the surface is energetically more favorable than the adsorption of H2S molecules on the surface by 1.68 eV.
Our results on NO2 (H2S) molecules adsorbed on the surface of Pd-ZGOi show that the adsorption reaction is spontaneous and exothermic. Most importantly, the structures of Pd atoms remaining at the top of the Zn3C site, namely, 1NO2-Pd-ZGO2, 2NO2-Pd-ZGO2, 1H2S-Pd-ZGO2, and 2H2S-Pd-ZGO2, have the overall lowest adsorption energy for NO2 and H2S molecules, regardless of the number of NO2 and H2S molecules. It is interesting to note that the magnitudes of adsorption energy of 1NO2-Pd-ZGO2 (−2.94 eV) and 2NO2-Pd-ZGO2 (−3.72 eV) are larger than that of 1H2S-Pd-ZGO2 (−2.89 eV) and 2H2S-Pd-ZGO4 (−2.44 eV), showing that the surface of ZGO doped with palladium benefits the adsorption reaction to increase the amount of NO2. However, the adsorption reaction on the surface of Pd-ZGOi is unfavorable for the increase in the amount of H2S. The largest adsorption energy occurs in the structures of the Pd atoms remaining on the top of the Zn3C site, but the largest work function difference occurs in the structures of Pd atoms remaining on the top of the Ga3C site. Overall, the difference in work functions is not closely related to their respective adsorption energies.
4. Conclusions
Using the framework of density functional theory, we studied the adsorption reactions and work functions of NO
2 and H
2S on Pd-ZnGa
2O
4(111) surfaces. Our previous studies showed that the bonding of the nitrogen (sulfur) atom from a single NO
2 (H
2S) molecule to the Ga atom of Ga-Zn-O-terminated ZnGa
2O
4(111) surfaces exhibits the highest work function change of +0.92 eV (−1.69 eV). In this study, we found that the work function change of one NO
2 (H
2S) molecule on the Pd-ZGO(111) surface is enhanced (reduced) to +1.37 eV (−0.90 eV). This difference is mainly due to the decrease in work function when the palladium is adsorbed on the ZGO(111) surface. The work function difference for two NO
2 (H
2S) molecules adsorbed on the surface of Pd-ZGO
i is about twice as large as that for one NO
2 (H
2S) molecule, showing a higher work function difference, i.e., a higher sensitivity. The resistance response experiment of ZnO(Ga) samples to NO
2 (H
2S) molecules shows that the sensitivity response of ZnGa2O4-based thin-film sensors to NO
2 (H
2S) molecules presents the same trend as the positive (negative) work function differences [
8]. Adsorption energies of NO
2 (H
2S) molecules on the surface of ZGO with and without Pd atoms were analyzed to explain the exothermic and endothermic adsorption reactions, but no fully satisfactory explanation related to their respective work function differences can be provided at this time.