Catalytic activities of noble metal atoms on WO3 (001): nitric oxide adsorption

Using first-principles density functional theory calculations within the generalized gradient approximation, we investigate the adsorption of NO molecule on a clean WO3(001) surface as well as on the noble metal atom (Cu, Ag, and Au)-deposited WO3(001) surfaces. We find that on a clean WO3 (001) surface, the NO molecule binds to the W atom with an adsorption energy (Eads) of −0.48 eV. On the Cu- and Ag-deposited WO3(001) surface where such noble metal atoms prefer to adsorb on the hollow site, the NO molecule also binds to the W atom with Eads = −1.69 and −1.41 eV, respectively. This relatively stronger bonding of NO to the W atom is found to be associated with the larger charge transfer of 0.43 e (Cu) and 0.33 e (Ag) from the surface to adsorbed NO. However, unlike the cases of Cu-WO3(001) and Ag-WO3(001), Au atoms prefer to adsorb on the top of W atom. On such an Au-WO3(001) complex, the NO molecule is found to form a bond to the Au atom with Eads = −1.32 eV. Because of a large electronegativity of Au atom, the adsorbed NO molecule captures the less electrons (0.04 e) from the surface compared to the Cu and Ag catalysts. Our findings not only provide useful information about the NO adsorption on a clean WO3(001) surface as well as on the noble metal atoms deposited WO3(001) surfaces but also shed light on a higher sensitive WO3 sensor for NO detection employing noble metal catalysts.


Background
NOx gases such as NO and NO 2 which are produced from the reaction of nitrogen and oxygen gases in the air during combustion damage not only our environment including air pollution and land contamination but also human health. Therefore, it has attracted much attention in recent years to develop a high-performance NOx-sensing equipment [1][2][3][4]. For the detection of NOx, a number of gas sensors using semiconducting metal oxides such as ZnO [5][6][7], MoO 3 [8,9], In 2 O 3 [10], SnO 2 [11,12], TiO 2 [13], and WO 3 [14] have been reported theoretically and experimentally.
Tungsten oxide (WO 3 ) has many unusual properties which make it suitable for various applications, e.g., high sensitivity of reducing and oxidizing gases [14], excellent electron transport and photosensitivity, and high stability-resisting photocorrosion in aqueous solvent [15][16][17][18][19][20]. Especially, WO 3 sensors have been widely applied for the detection of NOx gases [21][22][23]. In order to enhance the performance for NO 2 detection, WO 3 sensors have utilized the addition of metal atoms [24,25] as catalysts. However, there have been relatively few reports for the WO 3 sensor detecting NO molecule [26,27], and furthermore, theoretical studies for the adsorption of NO on WO 3 surfaces are still lacking. In this sense, an accurate first-principles density functional theory (DFT) calculation for the NO adsorption on WO 3 surfaces is highly desirable for the application of WO 3 sensors to NO detection.
In this work, we perform a first-principles DFT calculation to investigate the adsorption of NO molecule on a clean WO 3 (001) surface as well as on the noble metal atom (Cu, Ag, and Au) deposited WO 3 (001) surface. Here, the (001) surface (see Figure 1a,b) of γ-monoclinic WO 3 is taken into account because it is the most stable at room temperature [28]. We demonstrate that the Cu-, Ag-, and Au-deposited WO 3 (001) surfaces exhibit different catalytic behaviors for NO adsorption, that is, the magnitude of adsorption energy (E ads ) is in the order of Cu > Ag > Au. This different binding behavior of NO on WO 3 (001) depending on the noble metal species can be traced to the difference in charge transfer from the substrate to adsorbed NO molecule. Based on our DFT results, we will discuss the enhanced sensitivity of WO 3 sensors for NO detection by employing the noble metal catalysts.

Methods
Our DFT calculations were performed using Vienna ab initio simulation package (VASP) with the projector augmented wave method [29][30][31][32]. For the exchangecorrelation energy, we employed the generalized gradient approximation functional of Perdew-Burke-Ernzerhof [33]. The electronic wave functions were expanded in a plane wave basis with an energy cutoff of 400 eV. The WO 3 (001) surface was modeled by a periodic fouratomic-layer slab composing two alternate WO 2 plus O layers with approximately 16 Å of vacuum in between the slabs. The k-space integration was carried out using a Monhkorst-Pack grid [34] of 4 × 4 × 1 k points in the surface Brillouin zone of the monoclinic (1 × 1) unit cell whose size is as large as the cubic (2 × 2) unit cell. We relaxed all atoms except the bottom layer along the calculated forces until all the residual force components were less than 0.01 eV/Å. For the interaction of the NO molecule with the clean and metal-deposited WO 3 (001) surfaces, we initially placed the NO molecule about 3.5 Å away from the surfaces and obtained the adsorption structure by fully structural optimization.

NO adsorption on a clean WO 3 (001) surface
We first investigate the adsorption of a single NO molecule on a clean WO 3 (001) surface. Figure 1a,b shows the top and side views of the optimized WO 3 (001) surface, respectively. For the adsorption of NO on WO 3 (001), we consider the three different adsorption sites such as top W (hereafter denoted as S 1 ), top O (S 2 ), and hollow (S 3 ) sites. We find that the N atom of NO is bonding to the substrate atoms, consistent with a previous theoretical calculation [21]. However, in the hollow site, the O atom of NO can be bound to the substrate atoms (denoted as S 4 ). We calculate the adsorption energy defined as [35] E ads = E(NO/surf) -E(surf) -E(NO), where E(NO/surf) is the total energy of the NO-adsorbed WO 3 (001) system, E(surf) is the energy of a clean WO 3 (001) before NO adsorption, and E(NO) is the energy of a free NO molecule, obtained using a 12 × 12 × 12 Å 3 supercell calculation. As shown in Figure 1f, the S 1 configuration is found to be the most stable with E ads = −0.48 eV, larger in magnitude than E ads = −0.05, −0.05, and −0.03 eV for S 2 , S 3 , and S 4 , respectively; see Figure 2a. We note that, in the S 1 configuration, the bond length d N-W between the N and W atoms is calculated to be 2.07 Å, which is much shorter than the sum (3.7 Å) of van der Waals radius of the two atoms [14,36]. Thus, we can say that NO molecule can form a chemical bond with the WO 3 (001) surface.
To evaluate charge transfer in the S 1 configuration, we perform Bader charge analysis for NO before and after its adsorption on the WO 3 (001) surface [37,38]. The results for a free NO molecule and adsorbed NO on various substrates are given in Table 1. We find that, upon NO adsorption on a clean WO 3 (001) surface, the electrons in the N (O) atom increase (decrease) from 4.44 (6.56) to 4.84 (6.35) e, giving rise to an increase of 0.19 e in adsorbed NO molecule. This fact shows that adsorbed NO molecule captures electrons from the WO 3 (001) surface, indicating that NO behaves as a charge accepter. Indeed, the charge density difference, defined as Δρ = ρ NO/WO3 − (ρ NO + ρ WO3 ), clearly shows a charge transfer from the O (in NO molecule) and W atoms to the N atom; see Figure 3a. As a consequence of the additional electrons in NO in the NO/WO 3 (001) system, the bond length d N-O of NO molecule slightly increases to 1.181 Å, compared to that (1.170 Å) of a free NO molecule; see Table 1.
It is noteworthy that the abovementioned charge transfer from the WO 3 (001) surface to NO molecule leads to a reduction of conduction electrons in WO 3 (001), thereby forming the electron-depleted layer at the surface. This change of electrical character at the WO 3 (001) surface can be utilized to the WO 3 gas sensor where the contact resistance can be affected by the exposure of NO gas.

NO adsorption on Cu-or Ag-deposited WO 3 (001) surface
We begin to optimize the adsorption structure of Cu or Ag on WO 3 (001). We find that the adsorption of Cu (Ag) on the hollow site is more stable than the other adsorption sites such as top W and top O sites by 0.66 (0.14) and 0.75 (0.14) eV, respectively. Using Bader charge analysis, we find that the adsorption of Cu and Ag on the hollow site loses electrons to the WO 3 (001) substrate by 0.7 and 0.6 e, respectively. Using the most stable adsorption configuration of Cu or Ag on WO 3 (001), we continue to study the adsorption of NO on such noble metal atomdeposited WO 3 (001) substrates. We consider three different adsorption configurations of NO, where N atom is   Figure 2b,c, respectively. We find that the M 1 configuration is the most stable with E ads = −1.69 and −1.41 eV for NO/Cu-WO 3 (001) and NO/Ag-WO 3 (001), respectively, which are much larger in magnitude than E ads = −0.48 eV of the S 1 configuration at a clean WO 3 (001) surface. This indicates that Cu and Ag increases the strength of NO binding on WO 3 (001), thereby serving as catalysts. In the M 1 configuration, the N atom is also bonding to the W atom with d N-Cu/Ag (bond length between N and Cu or Ag atoms) = 1.86 or 2.19 Å because of a Coulomb interaction between the negatively charged N atom and the positively charged Cu or Ag atom (see Figure 3b,c), as discussed below. We note that the values of d N-W amount to 2.36 and 2.37 Å for NO/Cu-WO 3 (001) and NO/Ag-WO 3 (001), respectively. These values become longer than d N-W = 2.07 Å in the S 1 configuration but are still much shorter than the sum (3.7 Å) of van der Waals radius of N and W atoms [14,36], therefore concluding that NO molecule adsorbs chemically on the Cu-WO 3 (001) and Ag-WO 3 (001) substrates.
In Table 1, we find that for the M 1 configuration of NO/Cu-WO 3  Since more electrons transfer from the substrate to adsorbed NO molecule by the deposition of Cu or Ag atoms, one expects an enhanced reduction of conduction electrons in WO 3 (001), therefore increasing the sensitivity of WO 3 sensor for NO detection. As a matter of fact, a recent experimental study showed that the deposition of Ag atoms in WO 3 sensor improves its sensitivity for NO detection [27]. We note that, even though NO adsorption induces more electron transfer from the Cu-WO 3 (001) substrate compared to Ag-WO 3 (001), Cu atoms would be easily oxidized at a usual operation temperature (above 150°C) of WO 3 sensor. This oxidizing effect in noble metal atoms should be cautioned for the gas-sensing performance of WO 3 sensor.

NO adsorption on Au-deposited WO 3 (001) surface
We first optimize the adsorption structure of Au on WO 3 (001). Unlike the cases of Cu and Au catalysts, Au atom adsorbs only on top of the W atom, as shown in Figure 1e. Here, the adsorption of Au captures electrons from the WO 3 (001) substrate by 0.34 e because of a high electronegativity of Au atom. For the adsorption of NO on Au-WO 3 (001), we consider several adsorption configurations of NO, where N atom is attached to Au (denoted as P 1 ), top W (P 2 ), top O (P 3 ), and hollow (P 4 ) sites. In addition, we also consider another adsorption configuration of NO, where O atom in NO molecule is attached to Au atom (P 5 ). The calculated adsorption energy of NO for each adsorption configuration on Au-WO 3 (001) is displayed in Figure 2d. We find that the P 1 configuration is the most stable with E ads = −1.32 eV, which is relatively smaller in magnitude than E ads = −1.69 and −1.41 eV for the M 1 configurations of NO/Cu-WO 3 (001) and NO/ Ag-WO 3 (001), respectively. In the P 1 configuration, the calculated bond length of adsorbed NO is d N-O = 1.182 Å (see Table 1), which is shorter than 1.212 and 1.203 Å for NO/Cu-WO 3 (001) and NO/Ag-WO 3 (001), respectively. This shortest value of d N-O is due to the fact that NO captures the least electrons (0.04 e) from Au-WO 3 (001), as shown in Table 1. These features of NO/Au-WO 3 (001) such as the smaller adsorption energy, the shorter bond length, and the less electron capture of adsorbed NO is traced to a large electronegativity of Au.

Conclusions
We have performed first-principles DFT calculations within the generalized gradient approximation for the adsorption of NO molecule on a clean WO 3 (001) surface as well as on the Cu-deposited, Ag-deposited, and Au-deposited WO 3 (001) surfaces. We found that the NO molecule prefers to adsorb on the top of W atom at a clean WO 3 (001) surface, where a charge transfer from WO 3 (001) to NO occurs by 0.19 e and E ads is calculated to be −0.48 eV. We also found that, on the Cu-and Ag-deposited WO 3 (001) surface, the NO molecule also binds to the W atom with E ads = −1.69 and −1.41 eV, respectively, accompanying the relatively larger charge transfer of 0.43 e (Cu) and 0.33 e (Ag) to adsorbed NO compared to the clean WO 3 (001) surface. On the other hand, Au atoms on WO 3 (001) prefer to adsorb on the top of W atom, and the NO molecule forms a bond to the Au atom with a small electron transfer of 0.04 e to adsorbed NO. We obtained a relatively smaller adsorption energy of E ads = −1.32 eV for the NO/Au-WO 3 (001) system compared to NO/Cu-WO 3 (001) and NO/Ag-WO 3 (001) because of a large electronegativity of Au atom. The present results demonstrated that the sensitivity of WO 3 sensors for NO detection can be improved by employing the noble metal catalysts such as Cu and Ag atoms.