Catalytic activities of noble metal atoms on WO3 (001): nitric oxide adsorption
© Ren et al.; licensee Springer. 2015
Received: 18 October 2014
Accepted: 23 December 2014
Published: 11 February 2015
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 (E ads) 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 E ads = −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 E ads = −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.
NOx gases such as NO and NO2 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 [1-4]. For the detection of NOx, a number of gas sensors using semiconducting metal oxides such as ZnO [5-7], MoO3 [8,9], In2O3 , SnO2 [11,12], TiO2 , and WO3  have been reported theoretically and experimentally.
Tungsten oxide (WO3) has many unusual properties which make it suitable for various applications, e.g., high sensitivity of reducing and oxidizing gases , excellent electron transport and photosensitivity, and high stability-resisting photocorrosion in aqueous solvent [15-20]. Especially, WO3 sensors have been widely applied for the detection of NOx gases [21-23]. In order to enhance the performance for NO2 detection, WO3 sensors have utilized the addition of metal atoms [24,25] as catalysts. However, there have been relatively few reports for the WO3 sensor detecting NO molecule [26,27], and furthermore, theoretical studies for the adsorption of NO on WO3 surfaces are still lacking. In this sense, an accurate first-principles density functional theory (DFT) calculation for the NO adsorption on WO3 surfaces is highly desirable for the application of WO3 sensors to NO detection.
Our DFT calculations were performed using Vienna ab initio simulation package (VASP) with the projector augmented wave method [29-32]. For the exchange-correlation energy, we employed the generalized gradient approximation functional of Perdew-Burke-Ernzerhof . The electronic wave functions were expanded in a plane wave basis with an energy cutoff of 400 eV. The WO3(001) surface was modeled by a periodic four-atomic-layer slab composing two alternate WO2 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  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 WO3(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.
Results and discussion
NO adsorption on a clean WO3 (001) surface
Charge analysis and bond length of NO molecule
N + O (e)
d N-O (Å)
It is noteworthy that the abovementioned charge transfer from the WO3(001) surface to NO molecule leads to a reduction of conduction electrons in WO3(001), thereby forming the electron-depleted layer at the surface. This change of electrical character at the WO3(001) surface can be utilized to the WO3 gas sensor where the contact resistance can be affected by the exposure of NO gas.
NO adsorption on Cu- or Ag-deposited WO3 (001) surface
We begin to optimize the adsorption structure of Cu or Ag on WO3(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 WO3(001) substrate by 0.7 and 0.6 e, respectively. Using the most stable adsorption configuration of Cu or Ag on WO3(001), we continue to study the adsorption of NO on such noble metal atom-deposited WO3(001) substrates. We consider three different adsorption configurations of NO, where N atom is attached to W (denoted as M1), O (M2), and Cu or Ag (M3) atoms. In addition, we also consider another adsorption configuration of NO, where O atom in NO molecule is attached to Cu or Ag atom (denoted as M4). The calculated adsorption energy of NO for each adsorption configuration on Cu-WO3(001) and Ag-WO3(001) is given in Figure 2b,c, respectively. We find that the M1 configuration is the most stable with E ads = −1.69 and −1.41 eV for NO/Cu-WO3(001) and NO/Ag-WO3(001), respectively, which are much larger in magnitude than E ads = −0.48 eV of the S1 configuration at a clean WO3(001) surface. This indicates that Cu and Ag increases the strength of NO binding on WO3(001), thereby serving as catalysts. In the M1 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-WO3(001) and NO/Ag-WO3(001), respectively. These values become longer than d N-W = 2.07 Å in the S1 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-WO3(001) and Ag-WO3(001) substrates.
In Table 1, we find that for the M1 configuration of NO/Cu-WO3(001), the electrons in the N (O) atom increase (decrease) from 4.44 (6.56) to 5.00 (6.43) e, giving rise to an increase of 0.43 e in adsorbed NO molecule. On the other hand, for the M1 configuration of NO/Ag-WO3(001), the electrons in the N (O) atom are found to increase (decrease) from 4.44 (6.56) to 4.91 (6.42) e, giving rise to an increase of 0.33 e in adsorbed NO molecule. These results indicate that adsorbed NO molecule on Cu-WO3(001) and Ag-WO3(001) captures more electrons from the substrates compared to the case of NO adsorption at a clean WO3(001) surface, where only 0.19 e is transferred from WO3(001) to NO. As shown in Figure 3b,c, the calculated charge density difference ∆ρ shows charge transfer from the O (in NO molecule) and Cu-WO3(001) or Ag-WO3(001) substrate to the N atom, leading to the polar NO molecule with a negatively charged N atom. We note that, as a consequence of the presence of excess electrons in the polar NO molecule, the bond length d N-O of NO molecule increases to 1.212 and 1.203 Å for NO/Cu-WO3(001) and NO/Ag-WO3(001), respectively. These values of d N-O are longer than d N-O = 1.181 Å for NO/WO3(001) as well as d N-O = 1.170 Å of a free NO molecule.
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 WO3(001), therefore increasing the sensitivity of WO3 sensor for NO detection. As a matter of fact, a recent experimental study showed that the deposition of Ag atoms in WO3 sensor improves its sensitivity for NO detection . We note that, even though NO adsorption induces more electron transfer from the Cu-WO3(001) substrate compared to Ag-WO3(001), Cu atoms would be easily oxidized at a usual operation temperature (above 150°C) of WO3 sensor. This oxidizing effect in noble metal atoms should be cautioned for the gas-sensing performance of WO3 sensor.
NO adsorption on Au-deposited WO3 (001) surface
We first optimize the adsorption structure of Au on WO3(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 WO3(001) substrate by 0.34 e because of a high electronegativity of Au atom. For the adsorption of NO on Au-WO3(001), we consider several adsorption configurations of NO, where N atom is attached to Au (denoted as P1), top W (P2), top O (P3), and hollow (P4) sites. In addition, we also consider another adsorption configuration of NO, where O atom in NO molecule is attached to Au atom (P5). The calculated adsorption energy of NO for each adsorption configuration on Au-WO3(001) is displayed in Figure 2d. We find that the P1 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 M1 configurations of NO/Cu-WO3(001) and NO/Ag-WO3(001), respectively. In the P1 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-WO3(001) and NO/Ag-WO3(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-WO3(001), as shown in Table 1. These features of NO/Au-WO3(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.
We have performed first-principles DFT calculations within the generalized gradient approximation for the adsorption of NO molecule on a clean WO3(001) surface as well as on the Cu-deposited, Ag-deposited, and Au-deposited WO3(001) surfaces. We found that the NO molecule prefers to adsorb on the top of W atom at a clean WO3(001) surface, where a charge transfer from WO3(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 WO3(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 WO3(001) surface. On the other hand, Au atoms on WO3(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-WO3(001) system compared to NO/Cu-WO3(001) and NO/Ag-WO3(001) because of a large electronegativity of Au atom. The present results demonstrated that the sensitivity of WO3 sensors for NO detection can be improved by employing the noble metal catalysts such as Cu and Ag atoms.
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