Sensing Mechanism of SnO2 (110) Surface to CO: Density Functional

Article pubs.acs.org/JPCC

Sensing Mechanism of SnO2 (110) Surface to CO: Density Functional Theory Calculations Xiaofeng Wang,†,‡ Hongwei Qin,*,† Yanping Chen,† and Jifan Hu*,† †

School of Physics, State Key Laboratory for Crystal Materials, Shandong University, Jinan 250100, China School of Science, Dalian University of Technology at Panjin, Panjin 124221, Liaoning China

‡

ABSTRACT: We investigated the CO sensing mechanism of SnO2 (110) surface by density functional theory calculation. The CO sensing mechanism of SnO2 surface strongly depends upon the concentration of oxygen in the ambient atmosphere. For very high oxygen concentration where oxygen species O2− or O− are not adsorbed on the stoichiometric SnO2 (110) surface, there is the direct interaction between CO and the stoichiometric surface through the CO adsorption on Sn site or formation of CO2, accompanying the release of electrons to the surface. For the considerable high oxygen concentration, the oxygen species O2− and O− adsorbed on the oxygen-deficient SnO2 (110) surface grab electrons mainly from Sn atoms of SnO2 (110) surface. When SnO2 (110) surface is exposed to CO reducing gas, the interactions between CO and preadsorbed oxygen species (O2−, O−) as well as some lattice atoms at certain sites on SnO2 surface lead to the releasing of electrons back to semiconductor SnO2. At very low oxygen concentration, the structural reconstruction is induced by the direct interaction between CO and SnO2 subreduced surface with the removing of 2-fold-coordinated bridging oxygen rows, accompanying the electron transfer from CO to the surface without the formation of CO2 in the sensing response process.

1. INTRODUCTION Carbon monoxide (CO) is a kind of colorless, tasteless, widespread, and toxic gas whose highest tolerance level is just 50 ppm for a time weighted average limit (TWA) in air. It attacks hemoglobin from the blood, thus preventing the supply of needed oxygen to different parts of the body, leading to many physiological problems. Therefore, it is very significant to search certain gas-sensing material with high sensitivity and selectivity to CO. Semiconducting sensing offers an inexpensive and simple method for monitoring gases. The change of the electrical conductivity upon exposure to reducing gases of semiconducting materials has been used for gas detection. The metal oxide semiconductors such as SnO2 and ZnO have been studied for long time for detecting CO gas.1,2 The sensing mechanism of SnO2 sensors has attracted more and more attention.3−37 In general, SnO2 sensors are exposed to air atmosphere before the introduction of reducing gas. Oxygen species such as O2− or O− adsorbed on the surface of SnO2 grab electrons from SnO2, resulting in an increase of the surface resistance.22−25 The existence of oxygen species such as O2− or O− has been found by several experimental works.26−30 The widely used model for the oxygen adsorption process is described as O2(gas) ↔ O2(ads) (1) O2(ads) + e− ↔ O2(ads)−

(2)

O2(ads)− + e− ↔ 2O(ads)−

(3)

O(ads)− + e− ↔ O(ads)2 −

(4) © XXXX American Chemical Society

Usually, the sensing mechanism for reducing gases is the reaction with ionosorbed oxygen species in air or large oxygen concentration in the ambient atmosphere.31 When CO is introduced, there is an interaction between CO and adsorbed oxygen species with the formation of CO2, releasing the trapped electrons back to the SnO2 surface, decreasing the interface barrier as well as resistance of n-type SnO2.32−34 In the condition of very low oxygen concentration, it was suggested that CO may be adsorbed on the lattice surface of SnO2.35 It is interesting to note that a very large sensing response for SnO2 under CO exposure in N2 or absence of oxygen can be obtained.36 It is strange that for this case, rather than CO2, only CO was detected in the response process.36 The origin of this phenomenon is valuable to study. The density functional theory (DFT) calculation has been successfully used to study the SnO2 surface geometric structures, the electronic and chemical properties of SnO2 bulk and surface systems.3−12,14,38−43 The interaction processes of adsorbates (such as O2,13−15 H2,16 H2O,44 CH3OH,17 C2H5OH,45 NOx,18 and CO19−21) on the SnO2 (110) surface have been also investigated. So far, the first-principles calculations on CO sensing of SnO2 have concentrated upon the direct adsorption of CO on stoichiometric SnO2 surface. For the case with oxygen species preadsorbed on the surface of SnO2, the interactions between CO molecule and preadsorbed oxygen species on SnO2 surface should be considered. Received: February 23, 2014 Revised: October 20, 2014

A

dx.doi.org/10.1021/jp501880r | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

However, the research works by first-principles calculations on this side are still absent. It is also important to investigate the CO sensing mechanism at very low oxygen concentration with DFT calculations. In this article, we investigate the CO sensing mechanisms of the SnO2 (110) surface by density functional theory (DFT) calculation under different oxygen concentrations. At first, we investigate the gas sensing processes under considerable high oxygen concentrations, including (i) the oxygen adsorption processes on an oxygen-deficient SnO2 (110) surface and (ii) the followed interaction between CO molecule and preadsorbed oxygen species on SnO2 surface after introducing CO gas. Second, we investigate direct interactions between CO molecule and SnO2 stoichiometric surface, subreduced surface (with the removal of 2-fold-coordinated bridging oxygen rows40−43) and deep-reduced surface (with the removal of the bridging oxygen atoms and some rows of inplane oxygen atoms40−43), respectively. The former case corresponds to the sensing process of SnO2 at very high oxygen concentration, where even oxygen species O2− or O− are not adsorbed on the surface. The two latter cases correspond to CO sensing processes for very low oxygen concentration and absence of oxygen.

Figure 1. Optimized structure of SnO2 (1 1 0) surface.

on the stoichiometric SnO2 (110) surface have been simulated, and optimized results are shown in Figure 2. The sign of adsorption energy can be used to clarify the possibility of the adsorption mode. The adsorption energy can be expressed as the following equation: Eads = Esubstrate + Eadsorbate − Esubstrate − adsorbate

2. THEORETICAL METHODS AND MODELS All calculations in the present work were carried out with the density functional theory (DFT) provided by the program package DMol3.46,47 Spin-polarized calculations were employed using the generalized gradient approximation (GGA) with the Perdew−Burke−Ernzerhof (PBE) method to describe the exchange and correlation energy. Double numerical basis sets with polarization functions (DNP) were utilized. For calculations of geometry optimization, a 3 × 1 × 1 Monkhorst−Pack k-point mesh for the Brillouin zone sampling was used. The convergence criterion of optimal geometry for the energy, force, and displacement were 2 × 10−5 Ha, 4 × 10−3 Ha, and 5 × 10−3 Å, respectively. For calculations of total energy and density of the state (DOS), we used a 3 × 1 × 1 Monkhorst−Pack grid. Charge transfer was calculated on the basis of the Milliken population analysis (MPA).48 The SnO2 crystal with a tetragonal structure (Pmnm (42)) was calculated with 48 atoms in the super cell. The lattice constants were a = 4.73735 Å, b = 4.73735 Å, and c = 3.18640 Å, and the atomic fractional coordinates were Sn (0.0000, 0.0000, 0.0000), O (0.30562, 0.30562, 0). The (110) surface was cleaved from the optimized SnO2 bulk and a 10 Å vacuum was added to the layers. The optimized SnO2 (110) surface is shown in Figure 1. We choose the (110) surface of SnO2, since this surface is the most thermodynamically stable surface and has received much attention in the previous experimental and theoretical studies.7,8,43−45,49−51

(5)

where Esubstrate−adsorbate is the total energy of the adsorbate− substrate system in the equilibrium state, Esubstrate and Eadsorbate are the total energy of substrate and adsorbate, respectively. Our calculated adsorption energies for modes a, b, d, e, f, g, and h are −1.49, −1.20, −0.78, −0.18, −0.32, −0.28, and −0.67 eV, respectively. We did not find the stable configuration for the mode c. All calculated adsorption energies are negative for the considered adsorptions of oxygen molecule O2 and dissociated oxygen atom O on the stoichiometric SnO2 (110) surface. Such negative adsorption energies mean that both oxygen molecule O2 and dissociated oxygen atom O can not be adsorbed on the stoichiometric SnO2 (110) surface. Our calculation results based on DMol3 with GGA are similar to the conclusions based on VASP with GGA.13 The experimental results show that SnO2 has good gas sensing performance. Oxygen adsorption on the surface of SnO2 should be related to the surface with oxygen vacancy of the surface. The TPD experiments showed that the SnO2 surfaces were believed to be lightly reduced.27 Then we studied the interaction of oxygen with the surface created by removing one oxygen atom from the stoichiometric surface. Optimizations of SnO2 (110) surfaces with O defect were conducted, and four sites of oxygen vacancy (Figure 3) were constructed: (A1) inner layer bridging oxygen, (A2) skin layer plane oxygen, (A3) skin layer bridging oxygen, and (A4) inner layer plane oxygen. The vacancy formation energies of O defect modes A1, A2, A3, and A4 were calculated as follows: 1 Eform(Vo) = Etot (def ) − Etot (perfect ) + μ(O2 ) (6) 2 where μ(O2)is the energy of one oxygen molecular, Etot(def) is the total energy of the SnO2 (110) surface with O defect, and Etot(perfect) refers to the total energy of the perfect SnO2 (110) surface. The oxygen vacancy formation energies of A1, A2, A3 and A4 modes are 3.16 eV, 2.11 eV, 2.03 and 3.39 eV respectively. The oxygen vacancy formation energies in the skin layer (mode A2, A3) are much lower than the inner layer (mode A1, A4), what is to say, the oxygen vacancies are formed in the skin layer more probably than the inner layer. It is obvious that the vacancy formation energy of skin layer

3. RESULTS AND DISCUSSION Experimental results22−25 suggest that before introduction of CO reducing gas, oxygen species such as O2− or O− adsorbed on the surface of SnO2 grab electrons from SnO2, resulting in an increase of the surface resistance. In the following, the possible adsorption modes and charge transfers for oxygen species O2− and O− on SnO2 (110) surface are investigated. At first, we show that oxygen species O2− and O−can be adsorbed on the SnO2 (110) surface with oxygen vacancy, rather than the stoichiometric surface. Different adsorption modes for oxygen molecule O2 and dissociated oxygen atom O B



Article

Figure 2. Adsorption models of O2 molecule and O atom on stoichiometry SnO2 (110) surface.

modes are negative, what is to say, the total energy is always higher than the sum energy of the isolated O2 molecule and the relaxed A3 slab. The oxygen gas molecule cannot be exothermically adsorbed on the surface with the modes of B3 and B4. The oxygen gas molecule O2 may be exothermically adsorbed on the defective surface SnO2 with B1 and B2 modes, as they have positive adsorption energies. Also, the adsorbed O2 gain 0.803e for B1 mode and 0.800e for B2 mode, respectively. For B1 mode, the oxygen atoms with the marked numbers 72# and 73# (Figure 5a) gain 0.456e and 0.347e. Meanwhile, Sn atoms with marked numbers 63#, 64#, 65#, 70# and 71# lose 0.18e, 0.14e, 0.184e, 0.118e, and 0.118e, respectively. For B2 mode, oxygen atoms with the marked numbers 72# and 73# gain 0.454e and 0.346e. At the same time, the Sn atoms with marked numbers 63#, 64#, 65#, 70# and 71#, lose 0.169e, 0.169e, 0.169e, 0.117e and 0.117e, respectively. It indicates clearly that the O2 adsorbed on the surface of SnO2 grab electrons mainly from Sn atoms, which is consistent with the experiments results.22−25 Oxygen adsorption is temperature-dependent with O− dominating in the range of 250−400 °C.32 Four geometries for dissociative oxygen atom on SnO2 (110) surface with oxygen vacancy (A3 mode) were considered: (C1) O atom adsorbed at the plane O; (C2) O atom adsorbed at the plane Sn; (C3) O atom adsorbed at the bridging O; (C4) insertion of an O molecule into the vacancy. The optimized geometries of stable adsorption (C1−C4) are shown in Figure 6. The adsorption energies are −1.55, 2.32, −1.76, and 2.27 eV for C1, C2, C3, and C4 modes, respectively. In C1 and C3 modes, the O atom cannot be exothermically adsorbed on the surface as the adsorption energies are negative. The oxygen atom may be exothermically adsorbed on the defective SnO2 (110) surface with C2 and C4 modes with positive adsorption energies. The

Figure 3. SnO2 (110) surfaces with various O defect modes: (A1) inner layer bridging oxygen; (A2) skin layer plane oxygen; (A3) skin layer bridging oxygen; (A4) inner layer plane oxygen.

bridging oxygen (A3 mode) is the lowest. Then, the A3 mode was considered in the following adsorption process of oxygen species. Four geometries for oxygen molecules O2 on SnO2 (110) surface with oxygen vacancy (A3 mode) were considered: (B1) O2 molecular adsorption at the Sn site adjacent to the bridgingoxygen vacancy; (B2) insertion of an O2 molecule into the vacancy; (B3) straddled adsorption on the vacancy along the B axis; (B4) straddled adsorption on the vacancy along A axis. The optimized geometries of stable adsorption (B1−B4) are shown in Figure 4. Our calculated adsorption energies for B1, B2, B3, and B4 modes are 2.07, 2.08, −19.23, and −24.87 eV, respectively. We note that the adsorption energies of B3 and B4 C



Article

Figure 6. Adsorption models of O atom on SnO2 (110) surfaces with O defect: (C1) O atom adsorbed at the plane O; (C2) O atom adsorbed at the plane Sn; (C3) O atom adsorbed at the bridging O; (C4) insertion of an O molecule into the vacancy.

Figure 4. Adsorption models of O2 molecule on SnO2 (110) surfaces with O defect: (B1) molecular adsorption at the Sn site adjacent to the bridging-oxygen vacancy; (B2) insertion of an O2 molecule into the vacancy; (B3) straddled adsorption on the vacancy, with one O of the molecule coordinated to a Sn site; (B4) straddled adsorption on the vacancy, with one O of the molecule coordinated to a bridge-O site.

and 0.778e in C4 (insertion of an O molecule into the vacancy) mode, respectively. The transfer electrons mainly come from the Sn atoms in a line (63#, 64#, 65, 70#, 71#) of SnO2. For C2 mode, Sn atoms with marked numbers 63#, 64#, 65#, 70# and 71# lose 0.145e, 0.145e, 0.145e, 0.122e, and 0.122e, respectively. For C4 mode, the Sn atoms with marked numbers 63#, 64#, 65#, 70# and 71# lose 0.138e, 0.138e, 0.138e, 0.113e, and 0.113e, respectively. It means that the O atom (with the marked number 72#) may be adsorbed on the defective surface, trapping electrons from the SnO2 (110) surface, which is consistent with the experimental results.22−25 Experimental results suggest that after exposing SnO2 sensor to CO reducing gas, there is an interaction between CO and adsorbed oxygen species O2− or O− with the formation of CO2, releasing the trapped electrons back to the SnO2 surface and resulting in the reduction of the interface barrier as well as resistance of n-type SnO2.32−34 In the following, the possible interaction modes between CO and preadsorbed oxygen species O2− and O− on SnO2 (110) surface, as well as charges transferring from the CO molecule to the material surface are investigated. The initial four configurations of CO on the SnO2 (110) surfaces with preadsorbed oxygen species O2− (Figure 7) were constructed: (M1) with C-down upon preadsorbed O 73# (see Figure 5(b)); (M2) with C-down upon preadsorbed O 72#; (M3) with C-down upon plane O 43#; (M4) with Cdown upon plane O 27#.

Figure 5. Mark numbers of atoms in modes (a) B1, B2, (b) M1, M2, M3, M4, (c) C2, C4 and (d) N1, N3.

dissociative O with the marked number 72# (see Figure 5c) gains 0.795e in C2 (O atom adsorption at the plane Sn) mode D



Article

semiconductor SnO2, which is consistent with experimental conclusion.32−34 As for M2 mode, the lost electrons of CO mainly transfer to two oxygen atoms of preadsorbed oxygen species O2−. This mode is an intermediate state, which may vary after the thermal annealing. Figure 8 shows the plots of the deformation electron

Figure 8. Plots of the deformation electron density for M1 and M2 modes. The color blue represents the trapping of electrons, and the color yellow corresponds to the electron-release.

density for M1 and M2 modes. The color of blue represents to the trapping of electrons, and the color of yellow responds to the electron- release. M1 and M2 modes are the direct interactions between CO molecule and per-adsorbed O2−. The CO molecule can react with the oxygen atom (O 73#: which is bonded with one Sn atom) of preadsorbed O2− to form CO2 (see M1 mode). There is another possibility (M2 mode) that CO molecule may be adsorbed on the oxygen atom of preadsorbed O2−, which is inserted into the initial oxygen vacancy and bonds with two Sn atoms, to form an intermediate state. This oxygen atom of preadsorbed O2− is closely bonded with the lattice, which is more difficult to be grabbed by CO than another oxygen atom of preadsorbed O2−, which is bonded with only one Sn atom. Further insight into the bonding mechanism for CO adsorption on the SnO2 (110) surface with O2 preadsorbed can be obtained by analysis of the DOS of CO and preadsorbed O2 before and after CO adsorption. As shown in Figure 9a, the peaks of the adsorbed CO shift toward lower energy, compared to those of free CO. The density of states (DOS) of CO adsorbed in M1 and M2 modes change greatly, reflecting that the CO adsorption belong to chemical adsorption. For M1 mode, after CO adsorption, the CO molecule grabs one oxygen atom from the preadsorbed O2 of SnO2 (110) surface to form a structure of CO2. Compared with free CO2 molecule in Figure 9b, we found that this CO2 was not a free CO2 molecule and it still has a strong interaction with the surface. The DOS of the four atoms (C and O in CO; two O atoms in O2−) in M2 mode are shown in Figure 9c. The overlap of DOS peaks of the four atoms suggesting that there are orbital hybridizations between their orbits. One generally agrees is that the ionsorbed oxygen species are CO reaction partners, different carbonates and carboxylates are reaction intermediates and the final reaction product is CO2.52−60 At low O2 concentration, only a small number of possible oxygen adsorption sites are occupied, CO still interacts with SnO2 (mode M3 and mode M4). CO reacts with the lattice oxygen atom and form CO2, when CO is closed to the lattice oxygen atom O 43# (M3 mode), since bond strength of Sn−O for this lattice oxygen atom O 43# becomes weak after the oxygen O2 adsorbed on the SnO2 (110) surface. Before oxygen

Figure 7. Calculations of configurations of CO on the SnO2 (110) surfaces with preadsorbed oxygen species: (M1) with C-down upon preadsorbed O 73#; (M2) with C-down upon preadsorbed O 72#; (M3) with C-down upon plane O 43#;(M4) with C-down upon plane O 27#.

During these interaction processes between CO and preadsorbed oxygen species O2− on SnO2 (110) surface, our ab initio calculations show that charges transfer from the CO molecule to the surface. The CO molecule (with the marked numbers 74# and 75#) loses 0.377e, 0.460e, 0.333e, and 0.277e for M1, M2, M3, and M4 modes, respectively. It clearly shows that in the interaction of CO and preadsorbed oxygen species O2−on the SnO2 (110) surface, there are electron transferring from CO to the surface. For M1 mode, the oxygen atom of preadsorbed oxygen species O2− (with the marked number 73# in Figure 5b) combined with CO also loses 0.05e, another oxygen atom (with the marked number 72#) of preadsorbed oxygen species O2− gains 0.329e. As a result, besides of the releasing electrons of CO, the lattice atoms of SnO2 totally gain 0.098e. Oxygen atoms with the marked numbers 17#, 19#, 21#, 22#, and 23# totally gain 0.114e. Meanwhile, the Sn atoms with the marked numbers 64# and 65# of SnO2 lattice gain 0.043e and 0.047e, respectively. However, one Sn atom with the marked number 49# in lattice loses 0.112e, and the rest of the atoms of SnO2 (110) totally gain 0.006e. Such calculated results show that the interaction between CO and preadsorbed species O2− on SnO2 (110) surface leads to the releasing of electrons back to E



Article

0.034e. As a result, the lattice atoms of SnO2 in total gain 0.243e. The 51# Sn atom bonding with CO gains 0.143e, the O atoms with the marked numbers 17#, 27#, 29#, 43#, and 45# of SnO2 lattice around the 51# Sn atom bonding with CO gain 0.025e, 0.009e, 0.014e, 0.016e and 0.019e, respectively. However, O marked 23# and Sn atoms marked 63#, 65# lose 0.014e, 0.01e, and 0.01e. The rest of the atoms of SnO2 (110) in total lose 0.051e. Four configurations of CO on the SnO2 (110) surfaces with preadsorbed oxygen species O− (Figure 10) were constructed:

Figure 9. DOS of (a) free CO and CO in model M1, M2, (b) free CO2 and CO2 in model M1, and (c) C, O atoms in CO and O atoms in O2− in M2 mode.

molecule adsorption, the bond lengths of O43−Sn49, O43−Sn51 and O43−Sn67 are 2.018, 2.042, and 2.112 Å which are changed into 2.081, 2.034, and 2.163 Å respectively after adsorption. There is another possibility that CO is adsorbed on Sn position when CO is closed to the lattice oxygen atom O 27# (mode M4), as the O 27# has stronger bonds with the surrounding Sn atoms. The bond lengths of O27−Sn49, O27−Sn51 and O27−Sn63 are 2.088, 2.024, and 2.047 Å respectively. For the M3 mode, besides the releasing electrons of CO, preadsorbed O2− loses 0.011e. The lattice oxygen atom (with the marked number 43# in Figure 5b) combined with CO also loses 0.538e. As a result, the lattice atoms of SnO2 except the oxygen atom combined with CO totally gain 0.882e. The Sn atoms with the marked numbers 49#, 51#, and 67# of SnO2 lattice around the 43# oxygen atom combined with CO gain 0.232e, 0.554e, and 0.121e, respectively, and the rest of the atoms of SnO2 (110) in total lose 0.025e. For M4 mode, besides the releasing electrons of CO, preadsorbed O2− gains

Figure 10. Calculations of CO adsorption on the SnO2 (110) surfaces with O preadsorbed (adsorption on preadsorbed O (N1), on plane Sn (N2), on bridging Sn (N3), and on plane O (N4), respectively).

(N1) with C-down upon the preadsorbed O; (N2) with Cdown upon plane Sn; (N3) with C-down upon bridging Sn; (N4) with C-down upon plane O. When CO is introduced, the CO molecule can react with preadsorbed O− to form CO2. CO may also react with the bridging oxygen atom of lattice neighbored with preadsorbed O− to form CO2. But the CO molecule cannot be adsorbed on the plane O and plane Sn. As shown in Figure 10, for the interaction processes between CO and oxygen species O− on SnO2 (110) surface, electrons mainly transfer from the CO molecule to the Sn atoms (49#, 51#, 57#, 59#, 63#, 65#) of the surface layer (see Figure 5d) in F



Article

calculation. At first, we choose two configurations of SnO2 (110) surface with five layers (see Figure 13) to study the

modes N1 and N3. The numbers of the transferred charges for N1 and N3 mode are 0.356e and 0.364e, respectively. For N1 mode, the Sn atoms with the marked numbers 49#, 51#, 57#, 59#, 63# and 65# gain 0.079e, 0.127e, 0.079e, 0.129e, 0.083e, and 0.085e. For N3 mode, the lattice Sn atoms with the marked numbers 49#, 51#, 57#, 59#, 63# and 65# gain 0.108e, 0.067e, 0.139e, 0.084e, 0.119e, and 0.114e. Such calculated results demonstrate that the interaction of CO and oxygen species O− on SnO2 surface brings about the releasing of electrons back to semiconductor SnO2, consistent with experimental conclusion.22−25,32−34 Figure 11 shows the plots of the deformation

Figure 13. SnO2 (110) surfaces with (F1) only one bridging oxygen defect on the outermost layer and with (F2) two defects: one bridging oxygen defect on the outermost layer and one oxygen defect in the inner layer.

influence of oxygen vacancy located in the inner layer. As shown in Figure 14, P1 represents the adsorption model of O2 Figure 11. Plots of the deformation electron density for N1 and N3 modes. The color of blue represents the trapping of electrons, and the color of yellow corresponds to the electron-release.

electron density for N1 and N3 modes. The color blue represents the trapping of electrons, and the color yellow corresponds to the electron-release. The density of states of SnO2 (110) surface with preadsorbed O before and after CO adsorption of Model N1 and N3 are shown in Figure 12. It can

Figure 12. Density of states (DOS) of SnO2 (110) surface with O preadsorbed before and after CO adsorption of models N1 and N3. Figure 14. Adsorption models of O2 molecule when an O2 molecule is placed above the oxygen defect on SnO2 (110) surfaces (P1) with only one bridging oxygen defect on the outermost layer and (P2) with two defects: one bridging oxygen defect on the outermost layer and one oxygen defect in the inner layer.

be observed that the CO adsorptions have remarkable change on the electronic structure of the surface. The most prominent effect of CO adsorptions is that the DOS shifts toward the lowenergy direction. The DOS of N1 and N3 modes almost coincide with each other. In the discussion above, we investigated the CO sensing response process of SnO2 (110) surface under considerable high oxygen concentrations. After removing CO, the SnO2 (110) surface is exposed to the environment containing high oxygen concentration, and oxygen adsorption process would occur and oxygen species traps the electrons from the surface again, which corresponds to recovery process. We also investigated the effects of oxygen vacancies on the CO gas sensing processes for SnO2 (110) surface with DFT

molecule when an O2 molecule is placed above the oxygen defect on SnO2 (110) surfaces with only one bridging oxygen defect on the outermost layer, and P2 denotes the adsorption model of O2 molecule on SnO2 (110) surfaces with two defects: one bridging oxygen defect on the outermost layer and one oxygen defect in the inner layer. The bond orientations (O−O or O−Sn) of these two optimized adsorption (P1 and P2) modes are different. The adsorption energies of P1 and P2 oxygen adsorption modes are 1.80 and 1.99 eV, respectively. G



Article

Also, we investigated the Mulliken charges of adsorbed O2 in P1 and P2 modes, which were −0.787e and −0.865e, respectively. For P1 mode (see Figure 15), the oxygen atoms

Figure 16. Calculations of configurations of CO with C-down upon the SnO2 (110) surfaces with preadsorbed oxygen species: (Q1) with C on O marked 121#, the surface having only one bridging oxygen defect on the outermost layer; (Q2) with C on marked 119#, the surface having two defects, one bridging oxygen defect on the outermost-layer and one oxygen defect in the inner layer.

carbonation. For Q2 mode, oxygen atoms with the marked numbers 119#, 120#, and 122# gain 0.326e, 0.19e, and 0.219e, respectively. At the same time, the Sn atom (81#) loses 0.672e for C atom (121#), respectively. In this process, CO molecule loses 0.453e. However, except adsorbed oxygen atoms, the lattice atoms of SnO2 (110) surface loss 0.969e. From the point of this process, it seems that the inner oxygen vacancy may reduce CO sensing response. Of course, the intermediate state of carbonation may develop into CO2, after an appropriate heat-treatment is applied. In the decomposition processes, the situation of charge transfer is beyond capability of our calculation. In order to provide an insight of the influence of nonoutmost surface oxygen vacancy on the CO sensing performance, we reinvestigated the SnO2 (110) surface with three atomic layers. Calculated results indicate that the oxygen molecule cannot be adsorbed on the SnO2 (110) surface with the oxygen vacancy located in the second layer of SnO2 alone. It seems that the defective outermost layer is essential for the oxygen chemisorption on SnO2 (110) surface. Figure 17 shows the adsorption model of O2 molecule when an O2 molecule is placed above the oxygen defect on SnO2 (110) surfaces (S1) with two defects: one bridging oxygen defect on the outermost layer and one oxygen defect in the second layer. The adsorption energy is 2.04 eV. O2 molecule gains 0.77e. For mode S1, two adsorbed oxygen atoms with the marked number 71# and 72# (see Figure 18) gain 0.425e and 0.345e, respectively. As shown in Figure 19, when CO is introduced, CO could directly interact with an oxygen atom of adsorbed oxygen molecule to

Figure 15. Mark numbers of atoms in modes (a) P1, (b) Q1, (c) P2, and (d) Q2.

with the marked numbers 7#, 120#, and 121# gain 0.048e, 0.333e, and 0.454e, respectively. The Sn atoms with the marked numbers 96#, 112#, 113#, 114#, and 115# gain 0.088e, 0.057e, 0.057e, 0.057e, and 0.057e, respectively. Meanwhile, the Sn atoms with marked numbers 106#, 107#, 108#, 109#, 116#, 117#, 118#, and 119# lose 0.139e, 0.139e, 0.139e, 0.139e, 0.075e, 0.069e, 0.069e, and 0.069e, respectively. The O atoms with marked numbers 16# and 35# lose 0.004e and 0.107e, respectively. For P2 mode, oxygen atoms with the marked numbers 7#, and 120# gain 0.047e, 0.456e, and 0.349e. The Sn atoms with marked numbers 106#, 107#, 108#, 109#, 116#, 117#, 118# and 119# lose 0.139e, 0.139e, 0.139e, 0.139e, 0.075e, 0.069e, 0.069e and 0.069e, respectively. Sn atoms with marked number 81# gains 0.065e. At the same time, the Sn atoms with marked numbers 105# and 108# lose 0.249e and 0.320e, respectively. Meanwhile, the O atom with marked number 37# loses 0.041e. In mode Q1 (see Figure 16), CO runs away from the SnO2 (110) surface. Compared with the case of three layers, it seems that the CO sensing performance becomes bad for SnO2 (110) surface having five atomic layers, which may be connected with the relative low density of O defects. For Q2 mode, CO molecule interacted with two oxygen atoms preadsorbed on SnO2 (110) surface, forming an intermediate state of H



Article

(110) for the corresponding mode of M2 with only one bridging oxygen defect on the outermost layer. Meanwhile, the charge value lost from CO for the lattice atoms of SnO2 (110) is smaller with the case of one bridging oxygen defect on the outermost layer and one oxygen defect in the second layer (0.375e) than with the case of only one bridging oxygen defect on the outermost layer (0.377e). It seems that the inner oxygen vacancy may reduce the CO sensing response. In the following, we investigate direct interaction between CO molecule and SnO2 stoichiometric surface, subreduced surface (with the removing of 2-fold-coordinated bridging oxygen rows40−43), and deep-reduced surface (with the removing of bridging oxygen atoms and some rows of inplane oxygen atoms40−43), respectively. It has been found that the surface configuration of SnO2 strongly depends upon the oxygen concentration. Only at very high oxygen chemical potentials, the stoichiometric surface with the bulk termination is the energetically favored surface. The reduced surface state should correspond to the case of low oxygen chemical potentials,40−43 which is consistent with LEIS experiments that presented compositional transitions of the surface upon vacuum annealing.61 At first, we investigate the direct interaction between CO molecule and stoichiometric SnO2 (110) surface. The stoichiometric SnO2 surface with the bulk termination occurs at high oxygen concentrations, where even oxygen species O2− or O− are not adsorbed on the surface. As shown in Figure 20, six initial adsorption configurations of CO on SnO2 (110) surfaces are considered: (E1) adsorption on five-coordinated Sn with C-down; (E2) adsorption on the surface O ion with Cdown; (E3) adsorption on the bridge O with C-down; (E4) adsorption on the bridge O with CO parallel to the surface along the bridge O row; (E5) adsorption on the bridge O with CO perpendicular to the surface along the bridge O row; (E6) adsorption on five-coordinated surface Sn with O-down. For E1, E2, and E5 modes, the optimized results show that the CO molecule is adsorbed on Sn ion at the surface of SnO2 via the C down mode. The adsorption energy is 0.909 eV for E1, 0.929 eV for E2 and 0.928 eV for E5. Meanwhile the charges of 0.268e, 0.261e and 0.261e transfer from CO to semiconductor surface for E1, E2 and E5 respectively, consistent with the experiment that the resistance of SnO2 decreases when exposed to CO in the high concentration oxygen background.36 For mode E1 (as the typical example), the O atom and C atom of CO molecular releases 0.054e and 0.214e, respectively from the surface of SnO2 (1 1 0), while the Sn atoms marked 1#, 3#, 19#, 37#, 39#, 55#, and 57# and O atom marked 66# gain 0.038e, 0.037e, 0.037e, 0.065e, 0.052e, 0.040e, 0.202e, and 0.031e, respectively. It should be noted that in E1, E2 and E5 modes, CO molecule is directly adsorbed on five-coordinated Sn site of SnO2 (110) accompanying the electrons transfer without the formation of CO2. For modes E3 and E4, after optimization the CO molecule reacts with bridge O of SnO2 (1 1 0) surface, accompanying the formation of CO2. The CO molecule releases 0.248e and 0.345e in modes E3 and E4. Similar case has been found in CO adsorption on p-type stoichiometric and Ca-doped LaFeO3 (010) surface with the Fe-CO configuration, where CO acts the donor in the adsorption process and loss charges to the LaFeO3 (010) surface.62 For the present case, in mode E4 after optimization, the O atom (73#) of the CO molecular gain 0.145e but the C atom (74#) loses 0.490e. The O marked 30# loses 0.017e. The remaining atoms of lattice SnO2 (except O atom marked 30#)

Figure 17. Adsorption model of O2 molecule when an O2 molecule is placed above the oxygen defect on SnO2 (110) surfaces (S1) with two defects: one bridging oxygen defect on the outermost layer and one oxygen defect in the second layer.

Figure 18. Mark numbers of atoms in modes (a) S1and (b) T1.

Figure 19. Calculation of the configuration of CO with C-down upon the SnO2 (110) surfaces with preadsorbed oxygen species: (T1) the surface having two defects: one bridging oxygen defect on the outermost-layer and one oxygen defect in the second layer.

form CO2. The CO losses 0.375e, O2 gains 0.095e, and the lattice atoms of SnO2 (110) with two defects (one bridging oxygen defect on the outermost layer and one oxygen defect in the second layer) totally gain 0.095e, which is smaller than the charge value (0.098e) gained by the lattice atoms of SnO2 I



Article

Figure 20. Six modes of initial adsorption of CO molecular on stoichiometric SnO2(1 1 0) surface. The left drawings are the configurations of initial adsorption and the right ones are those of optimized adsorption.

gain 0.362e. In this process, the Sn atoms marked with number 3#, 6#, 21#, 24#, and 39# gain 0.118e, 0.104e, 0.087e, 0.103e, and 0.114e, respectively. The O atom marked number with 27# loses 0.050e. The distance between C and O atoms with the marked numbers 74# and 30# (see Figure 21) is 1.180 Å, which is similar to the C−O bond length of 1.176 Å in the isolated CO2 molecule, and the angle of O−C−O30# is 179.943° close to the bond angle 180° of CO2 molecule. It means that a linear CO2 molecule occurs. As for modes E6, the optimized results show that the CO molecular leaves far away from the SnO2 (1 1 0) surface with O down mode. After removing CO, the SnO2 (110) surface is exposed to the environment containing very high oxygen concentration. In these recovery processes, the desorption of CO from the surface in E1, E2, and E5 modes may occur, accompanying the release of electrons from SnO2 back to CO. meanwhile, the oxygen in environment inserts back to the bridge O site in E3 and E4, accompanying the trapping of electrons by the inserted O from the surface. We have also investigated the CO adsorption on the subreduced SnO2 (110) surface obtained by the removing of 2-fold-coordinated bridging oxygen rows. Such subreduced SnO2 (110) surface may occur at low oxygen concentration. As shown in Figure 22, only for mode R1, CO can be adsorbed on 5-fold-coordinated Sn of SnO2 (110) sub reduced surface with C-down configuration. The adsorption energy for R1 mode is 0.72 eV. However, CO gains 0.04 e from the SnO2 reduced surface. The O atom of CO molecular gains 0.134e from the surface of SnO2 (110). Meanwhile, the C atom of CO molecular releases 0.094e to the surface. The Sn atoms marked 1#, 18#, 35#, 52#, and 57# (see Figure 23) of SnO2 lattice gain 0.065e, 0.066e, 0.066e, 0.063e, 0.086e, respectively. The Sn atoms marked 23# and 56# lose 0.036e and 0.018 e. The O atom marked 8#, 9#, 25# and 68# release 0.027e, 0.036e, 0.026e and 0.025e, respectively. By DFT calculation, the backdonation from the oxide to the CO molecule was also found when CO molecule is adsorbed on the oxygen vacancy site of TiO2 (110)63 and LaFeO3 (101) surfaces with the defect-CO

Figure 21. Mark numbers of atoms in E1 and E4 modes.

J



Article

Figure 22. Five modes of initial adsorption of CO molecular on reduced SnO2(1 1 0) surface with the removing of 2-fold-coordinated bridging oxygen rows. The left drawings are the configurations of initial adsorption and the right ones are that of optimized adsorption.

Meanwhile, the C atom of CO molecular releases 0.221e to the surface. The Sn atoms marked 1#, 6#, 18#, 23#, 35# and 52# of SnO2 lattice gain 0.114e, 0.142e, 0.114e, 0.142e, 0.126e and 0.116e, respectively. The Sn atoms marked 5#, 37# and 22# lose 0.018e, 0.036e and 0.018e. The O atom marked 8#, 11#, 17# and 25# release 0.025e, 0.027e, 0.025e and 0.026e, respectively. In mode R2, instead of the adsorption on the Sn site of the surface, CO leaves away from the surface (see Figure 22). However, the structural reconstruction of SnO2 subreduced surface is induced by the presence of CO. It seems that there is a weak interaction between CO and surface in R2 mode. Similar to mode R2, a slight reconstruction of SnO2 subreduced surface can be induced by CO in R3 mode (see Figure 22), accompanying the transfer of 0.092e from CO to the surface. Such results of DFT calculation on R2 and R3 are well consistent with the experimental observation observed by Bârsan et al.36 In addition, as shown in Figure 22, CO leaves away from SnO2 sub reduced surface without the evident reconstruction of the surface in R4 and R5. The deep reduced SnO2 (110) surfaces with the removing of bridging oxygen atoms and some rows of in-plane oxygen atoms may occur for the very low oxygen concentration. As shown in Figure 24, four initial adsorption (Z1, Z2, Z3, and Z4) configurations of CO on SnO2 (110) deep reduced surfaces are considered. For mode Z3, CO can be adsorbed on in-plane oxygen atom of SnO2 (110) reduced-surface with C-down configuration. CO gains 0.025e from the SnO2 reduced surface, belonging to a back-donation process. However, the adsorption energy for Z3 mode is −0.11 eV. It means that CO molecule cannot be exothermically adsorbed on the in-plane oxygen atom of SnO2 (110) deep reduced surface with the mode of Z3. In addition, it is obvious that CO leaves far away from the SnO2 (110) reduced-surface for Z1, Z2, and Z4 modes without evident reconstruction (see Figure 24).

Figure 23. Mark numbers of atoms in modes R1 and R2.

configuration,62 as well as on low- coordinated Cu atoms on Cu2O (111) surface.64 However, no phenomenon of backdonation process was detected experimentally in SnO2. On the contrary, a large sensing response for SnO2 under CO exposure in N2 or very low oxygen concentration can be obtained, accompanying the electron shift from CO to SnO2.36 In fact, for R2 mode (see Figure 23) we find that CO releases 0.138e to the SnO2 (110) subreduced surface. The O atom of CO molecular gains 0.083e from the surface of SnO2 (110). K



Article

concentration, and oxygen adsorption process would occur and oxygen species traps the electrons from the surface again, corresponding to recovery process. At relatively low O2 concentration with few O2−, CO can also react with the lattice oxygen atom to form CO2 when CO is closed to the lattice oxygen atom, since bond strength of Sn−O for this lattice oxygen atom becomes weak after the oxygen adsorption. There is another possibility that CO is adsorbed on the Sn position. For the case of O−, when CO is introduced, the CO molecule can react with preadsorbed O− to form CO2. CO may also react with the bridging oxygen atom of lattice neighbored with the preadsorbed O− to form CO2. Our above results are consistent with the conventional CO sensing mechanism for SnO2 in the considerable large oxygen concentration.22−25 Meanwhile, we also find that when SnO2 (110) surface is exposed to CO reducing gas, the interactions between CO and preadsorbed oxygen species (O2−, O−) as well as some lattice atoms at certain sites on SnO2 surface lead to the releasing of electrons back to semiconductor SnO2. In addition, it seems that the inner oxygen vacancy may reduce the CO sensing response. At low oxygen concentration, subreduced SnO2 (110) surface may occur via the removing of 2-fold-coordinated bridging oxygen rows. Some structural reconstructions of SnO2 subreduced surface can be induced by CO, accompanying the electron transfer from CO to the surface without the formation of CO2 in the CO sensing response process. Such results are consistent with the experimental observation for the CO sensing of SnO2 at very low oxygen concentration.36 The process of backdonation from the oxide to the CO molecule may also exist when CO reacts with the sub reduced SnO2 surface. However, such back-donation phenomenon is very difficult to detect, since simultaneously the amount of electron transfer releasing from CO to other site of lattice is much larger than that of the back-donation. Very little CO2 may be produced in the recovery processes, connected with the desorption of CO preadsorbed on the SnO2 subreduced surface with the backelectron donation process in the response process. Since the amount of CO2 produced in the recovery processes is very small, it is very difficult to observe CO2 in the experiment at very low oxygen concentration. The DFT calculation can provide a good description for CO sensing processes on SnO2 surface.

Figure 24. Four modes of initial adsorption of CO molecular on reduced SnO2(1 1 0) surface with the removing of bridging oxygen atoms and some rows of in-plane oxygen atoms. The left drawings are the configurations of initial adsorption and the right ones are that of optimized adsorption.

4. CONCLUSIONS In the present work, we investigate the CO sensing mechanism of SnO2 (110) surface by DFT calculation. The CO sensing mechanism of SnO2 surface strongly depends upon the concentration of oxygen in the ambient atmosphere. Oxygen species O2− or O− are not adsorbed on the stoichiometric SnO2 (110) surface at very high oxygen concentration. CO may interact with the stoichiometric SnO2 (110) lattice through the adsorption on Sn site or formation of CO2, accompanying the release of electrons to the surface. Oxygen species O2− and O− can be adsorbed on the SnO2 (110) surface with oxygen vacancy. Before the introduction of CO, the oxygen species O2− and O− adsorbed on the oxygen−deficient SnO2 (110) surface grab electrons mainly from Sn atoms of SnO2, when the oxygen concentration is large. For the case of O2−, when CO is introduced the CO molecule can react with the oxygen atom of preadsorbed O2−, which is bonded with one Sn atom, to form CO2. There is another possibility that CO molecule may be adsorbed on the oxygen atom of preadsorbed O2−, which is inserted into the initial oxygen vacancy and is bonded with two Sn atoms, to form an intermediate state which may finally transform into CO2. After removing CO, the SnO2 (110) surface is exposed to the environment containing high oxygen

■

AUTHOR INFORMATION

Corresponding Authors

*(H.Q) E-mail: [email protected]. Telephone: +86-53188377035. *(J.H) E-mail: [email protected]. Telephone: +86-53188361560. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Nos: 51472145, 51272133 and 51472150), Shandong Natural Science Foundation (No. ZR2013MM016) and Foundation of Dalian University of Technology at Panjin (No. 5007-852005).

■

REFERENCES

(1) Giulio, M. D.; Micocci, G.; Serra, A.; Tepore, T. Characteristics of Reactively Sputtered Pt-SnO2 Thin Films for CO Gas Sensors. J. Vac. Sci. Technol., A 1996, 14, 2215−2219. L



Article

(2) Aste, T.; Beruto, D.; Botter, R.; Ciccarelli, C.; Giordani, M.; Pozzolini, P. Microstructural Development During the Oxidation Process in SnO2 Thin Films for Gas Sensors. Sens. Actuators, B 1994, 18, 637−641. (3) Lantto, V.; Rantala, T. T. Atomistic Understanding of Semiconductor Gas Sensors. J. Eur. Ceram. Soc. 2001, 21, 1961−1965. (4) Rantala, T. S.; Rantala, T. T.; Lantto, V. Computational Studies for the Interpretation of Gas Response of SnO2 (110) Surface. Sens. Actuators, B 2000, 65, 375−378. (5) Rantala, T. T.; Rantala, T. S.; Lantto, V. Surface Relaxation of the (110) Face of Rutile SnO2. Surf. Sci. 1999, 420, 103−109. (6) Oviedo, J.; Gillan, M. J. Reconstructions of Strongly Reduced SnO2 (110) Studied by First-principles Methods. Surf. Sci. 2002, 513, 26−36. (7) Oviedo, J.; Gillan, M. J. Energetics and Structure of Stoichiometric SnO2 Surfaces Studied by First -principles Calculations. Surf. Sci. 2000, 463, 93−101. (8) Manassidis, I.; Goniakowski, J.; Kantorovich, L. N.; Gillan, M. J. The Structure of the Stoichiometric and Reduced SnO2 (110) Surface. Surf. Sci. 1995, 339, 258−271. (9) Maki-Jaskari, M. A.; Rantala, T. T. Band Structure and Optical Parameters of the SnO2 (110) Surface. Phys. Rev. B 2001, 64, 075407. (10) Rantala, T. T.; Rantala, T. S.; Lantto, V. Electronic Structure of SnO2 (110) Surface. Mater. Sci. Semicond. Process. 2000, 3, 103−107. (11) Maki-Jaskari, M. A.; Rantala, T. T. Theoretical Study of OxygenDeficient SnO2 (110) Surface. Phys. Rev. B 2002, 65, 245428. (12) Maki-Jaskari, M. A.; Rantala, T. T.; Golovanov, V. V. Computational Study of Charge Accumulation at SnO2 (110) Surface. Surf. Sci. 2005, 577, 127−138. (13) Oviedo, J.; Gillan, M. J. First-principles Study of the Interaction of Oxygen with the SnO2 (110) Surface. Surf. Sci. 2001, 490, 221−236. (14) Sensato, F. R. Periodic Study on the Structural and Electronic Properties of Bulk, Oxidized and Reduced SnO2 (110) Surfaces and the Interaction with O2. Surf. Sci. 2002, 511, 408−420. (15) Xue, Y. B.; Tang, Z. A. Density Functional Study of the Structure of SnO2 (110) Surface and the Property of Oxygen Adsorption. Chin. J. Sens. Actuators 2007, 20, 2364−2368. (16) Martins, J. B. L.; Longo, E.; Andres, J.; Taff, C. A. Theoretical Study of Cluster Models and Molecular Hydrogen Interaction With SnO2 (110) Surfaces. J. Mol. Struct.: THEOCHEM 1995, 335, 167− 174. (17) Calatayud, M.; Andres, J.; Beltran, A. A Theoretical Analysis of Adsorption and Dissociation of CH3OH on the Stoichiometric SnO2 (110) Surface. Surf. Sci. 1999, 430, 213−222. (18) Prades, J. D.; Cirera, A.; Morante, J. R.; Pruneda, J. M.; Ordejon, P. Ab Initio Study of NOx Compounds Adsorption on SnO2 Surface. Sens. Actuators, B 2007, 126, 62−67. (19) Melle-Franco, M.; Pacchioni, G. CO Adsorption on SnO2 (110): Cluster and Periodic Ab Initio Calculations. Surf. Sci. 2000, 461, 54−66. (20) Ciriaco, F.; Cassidei, L.; Cacciatore, M.; Petrella, G. First Principle Study of Processes Modifying the Conductivity of Substoichiometric SnO2 Materials upon Adsorption of CO from Atmosphere. Chem. Phys. 2004, 303, 55−61. (21) Xue, Y. B.; Tanga, Z. A. Density Functional Study of the Interaction of CO with Undoped and Pd Uoped SnO2 (110) Surface. Sens. Actuators, B 2009, 138, 108−112. (22) Bârsan, N.; Schweizer-Berberich, M.; Göpel, W. Fundamentals and Practical Applications to design Nanoscaled SnO2 Gas Sensors: A Status Report. Fresenius J. Anal. Chem. 1999, 365, 287−304. (23) Ihokura, K.; Watson, J. The Stannic Oxide Gas Sensor Principles and Applications; Chemical Rubber Company Press: Boca Raton, FL, 1994. (24) Göpel, W.; Schierbaum, K. D. SnO2 Sensors: Current Status and Future Prospects. Sens. Actuators, B 1995, 26, 1−12. (25) Williams, D. Semiconducting Oxides as Gas-sensitive Resistors. Sens. Actuators, B 1999, 57, 1−16.

(26) Che, M.; Tench, A. J. Characterization and Reactivity of Mononuclear Oxygen Species on Oxide Surfaces. Adv. Catal. 1982, 31, 77−133. (27) Joly, J. P.; Gonzalez-Cruz, L.; Arnaud, Y. Désorption à Température Programmée de L’oxygène Labile de SnO2. Bull. Soc. Chim. Fr. 1986, 1, 11−17. (28) Shen, G. L.; Casanova, R.; Thornton, G. Interaction of O2 with SnO2 (110)1 × 1 and 4 × 1. Vacuum 1992, 43, 1129−1131. (29) Nagasawa, Y.; Choso, T.; Karasuda, T.; Shimomura, S.; Ouyang, F.; Tabata, K.; Yamaguchi, Y. Photoemission Study of the Interaction of a Reduced Thin Film SnO2 with Oxygen. Surf. Sci. 1999, 433, 226− 229. (30) Chang, S. C. Oxygen Chemisorption on Tin Oxide: Correlation between Electrical Conductivity and EPR Measurements. J. Vac. Sci. Technol. 1980, 17, 366−369. (31) Heiland, G.; Kohl, D. In Chemical Sensor Technology 1; Seiyama, T., Ed.; Kodansha: Tokyo, 1988; Chapter 2, p 15. (32) Bârsan, N.; Weimar, U. Understanding the Fundamental Principles of Metal Oxide Based Gas Sensors; the Example of CO Sensing with SnO2 Sensors in the Presence of Humidity. J. Phys.: Condens. Matter 2003, 15, R813−R839. (33) Huang, J.; Matsunaga, N.; Shimanoe, K.; Yamazoe, N.; Kunitake, T. Nanotubular SnO2 Templated by Cellulose Fibers: Synthesis and Gas Sensing. Chem. Mater. 2005, 17, 3513−3518. (34) Morrison, S. R. Selectivity in Semiconductor Gas Sensors. Sens. Actuators, B 1987, 12, 425−440. (35) Hahn, S. H.; Bârsan, N.; Weimar, U.; Ejakov, S. G.; Visser, J. H.; Soltis, R. E. CO Sensing with SnO2 Thick Film Sensors: Role of Oxygen and Water Vapour. Thin Solid Films 2003, 436, 17−24. (36) Bârsan, N.; Hübner, M.; Weimar, U. Conduction Mechanisms in SnO2 Based Polycrystalline Thick Film Gas Sensors Exposed to CO and H2 in Different Oxygen Backgrounds. Sens. Actuators, B 2011, 157, 510−517. (37) Bârsan, N.; Hübner, M.; Weimar, U. Conduction Model of Metal Oxide Gas Sensors. J. Electroceram. 2001, 7, 143−167. (38) Duan, Y. H. Electronic Properties and Stabilities of Bulk and Low-index Surfaces of SnO in Comparison with SnO2: A Firstprinciples Sensity Functional Approach with an Empirical Correction of Van Der Waals Interactions. Phys. Rev. B 2008, 77, 045332. (39) Garcia, J. C.; Assali, L. V. C.; Justo, J. F. The Structural and Electronic Properties of Tin Oxide Nanowires: An Ab Initio Investigation. J. Phys. Chem. C 2012, 116, 13382−13387. (40) Batzill, M.; Diebold, U. The Surface and Materials Science of Tin Oxide. Prog. Surf. Sci. 2005, 79, 47−154. (41) Batzill, M.; Katsiev, K.; Burst, J. M.; Losovyj, Y.; Bergermayer, W.; Tanaka, I.; Diebold, U. Tuning Surface Properties of SnO2 (101) by Reduction. J. Phys. Chem. Solids. 2006, 67, 1923−1929. (42) Bergermayer, W.; Tanka, I. Reduced SnO2 Surfaces by FirstPrinciples Calculations. Appl. Phys. Lett. 2004, 84, 909−911. (43) Batzill, M.; Katsiev, K.; Burst, J. M.; Diebold, U.; Chaka, A. M.; Delley, B. Gas-phase-dependent Properties of SnO2 (1 1 0),(1 0 0), and (1 0 1) Single-crystal Surfaces: Structure, Composition, and Electronic Properties. Phys. Rev. B 2005, 72, 165414. (44) Bandura, A. V.; Kubicki, J. D.; Sofo, J. O. Comparisons of Multilayer H2O Adsorption onto the (110) Surfaces of α-TiO2 and SnO2 as Calculated with Density Functional Theory. J. Phys. Chem. B 2008, 112, 11616−11624. (45) Yue, J.; Jiang, X. C.; Yu, A. B. Adsorption of the OH Group on SnO2 (110) Oxygen Bridges: A Molecular Dynamics and Density Functional Theory Study. J. Phys. Chem. C 2013, 117, 9962−9969. (46) Delley, B. An All-electron Numerical Method for Solving the Local Density Functional for Polyatomic Molecules. J. Chem. Phys. 1990, 92, 508−517. (47) Delley, B. From Molecules to Solids with the DMol Approach. J. Chem. Phys. 2000, 113, 7756−7764. (48) Mulliken, R. S. Electronic Population Analysis on LCAO-MO Molecular Wave Functions. I. J. Chem. Phys. 1955, 23, 1833−1840. (49) Yamaguchi, Y.; Nagasawa, Y.; Shimomura, S.; Tabata, K.; Suzuki, E. A Density Functional Theory Study of the Interaction of M



Article

Oxygen with a Reduced SnO2 (110) Surface. Chem. Phys. Lett. 2000, 316, 477−482. (50) Yamaguchi, Y.; Tabata, K.; Suzuki, E. Density Functional Theory Calculations for the Interaction of Oxygen with Reduced M/ SnO2 (1 1 0) (M = Pd, Pt) Surfaces. Surf. Sci. 2003, 526, 149−158. (51) Koziej, D.; Thomas, K.; Bârsan, N.; Thibault-Starzyk, F.; Weimar, U. Influence of Annealing Temperature on the CO Sensing Mechanism for Tin Dioxide Based Sensors−Operando Studies. Catal. Today 2007, 126, 211−218. (52) Baraton, M. I.; Merhari, L. Influence of the Particle Size on the Surface Reactivity and Gas Sensing Properties of SnO2 Nanopowders. Mater. Trans. 2001, 42, 1616−1622. (53) Amalric-Popescu, D.; Bozon-Verduraz, F. Infrared Studies on SnO2 and Pd/SnO2. Catal. Today 2001, 70, 139−154. (54) Sahm, T.; Gurlo, A.; Bârsan, N.; Weimar, U.; Mädler, L. Fundamental Studies on SnO2 by Means of Simultaneous Work Function Change and Conduction Measurements. Thin Solid Films 2005, 490, 43−47. (55) Chiorino, A.; Ghiotti, G.; Carotta, M. C.; Martinelli, G. Electrical and Spectroscopic Characterization of SnO2 and Pd-SnO2 Thick Films Studied as CO Gas Sensors. Sens. Actuators, B 1998, 47, 205−212. (56) Saukko, S.; Lassi, U.; Lantto, V.; Kroneld, M.; Novikov, S.; Kuivalainen, P.; Rantala, T. T.; Mizsei, J. Experimental Studies of O2− SnO2 Surface Interaction Using Powder, Thick Films and Monocrystalline Thin Films. Thin Solid Films 2005, 490, 48−53. (57) Baraton, M. I.; Merhari, L.; Ferkel, H.; Castagnet, J. F. Comparison of the Gas Sensing Properties of Tin, Indium and Tungsten Oxides Nanopowders: Carbon Monoxide and Oxygen Detection. Mater. Sci. Eng., C 2002, 19, 315−321. (58) Lenaerts, S.; Roggen, J.; Maes, G. FT-IR Characterization of Tin Dioxide Gas Sensor Materials under Working Conditions. Spectrochim. Acta, Part A 1995, 51, 883−894. (59) Baraton, M. I.; Merhari, L. Electrical Behavior of Semiconducting Nanopowders versus Environment. Rev. Adv. Mater. Sci. 2003, 4, 15−24. (60) Xu, G.; Zhang, Y. W.; Sun, X.; Xu, C. L.; Yan, C. H. Synthesis, Structure, Texture, and CO Sensing Behavior of Nanocrystalline Tin Oxide Doped with Scandia. J. Phys. Chem. B 2005, 109, 3269−3278. (61) Cox, D. F.; Fryberger, T. B.; Semancik, S. Oxygen Vacancies and Defect Electronic States on the SnO2 (110)-1 × 1 Surface. Phys. Rev. B 38, 2072−2083. (62) Sun, L.; Hu, J.; Qin, H.; Zhao, M.; Fan, K. Influences of Ca Doping and Oxygen Vacancy upon Adsorption of CO on the LaFeO3 (010) Surface: A First-Principles Study. J. Phys. Chem. C 2011, 115, 5593−5598. (63) Menetrey, M.; Markovits, A.; Minot, C. Reactivity of a Reduced Metal Oxide Surface: Hydrogen, Water and Carbon Monoxide Adsorption on Oxygen Defective Rutile TiO2(110). Surf. Sci. 2003, 524, 49−62. (64) Bredow, T.; Pacchioni, G. Comparative Periodic and Cluster Ab Initio Study on Cu2O (111)/CO. Surf. Sci. 1997, 373, 21−32.

N


Sensing Mechanism of SnO2 (110) Surface to CO: Density Functional

Recommend Documents