ARTICLE pubs.acs.org/JPCC
Role of Cu Doping in SnO2 Sensing Properties Toward H2S Wei Wei, Ying Dai,* and Baibiao Huang School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, People’s Republic of China ABSTRACT: Adsorption properties of H2S on SnO2 (110) surface and effects of Cu-doping on SnO2 sensitivity toward H2S have been studied by means of the first-principles calculations based on the density functional theory. It has been found that H2S is dissociatively adsorbed on the SnO2 (110) surface with one H atom converged to a bridging oxygen atom while the complex HS is bonded to a five-fold coordinated Sn atom. H2S adsorption has no change on the electrical conductivity of SnO2, which indicates that SnO2 has weak sensitivity toward H2S. Cu tends to be doped close to the surface region and prefers the five-fold coordinated Sn site. Cu-doping can directly improve the sensitivity of SnO2 toward H2S because of the increase in surface electrical conductivity. In addition, Cu-doping can greatly improve the formation of surface oxygen vacancies in SnO2, and the formation energy is in relation to Cu depth from surface. Molecular O2 can be exothermically adsorbed on the reduced SnO2 surface, which also underlies the mechanism of enhanced SnO2 sensitivity toward H2S due to Cu-doping.
1. INTRODUCTION Tin oxide (SnO2) has been receiving extensive investigations because of its wide range of usage in solar cells, catalysis, transparent electrodes, as well as spintronics.13 In particular, SnO2 is the most employed metal oxide semiconductor as gas sensors for the detection of a wide variety of pollutant gases because of its attractive features of low cost, nontoxicity, low power consumption, simple device structure, and easy operation.48 Nevertheless, wide application of SnO2-based gas sensors is limited by the low sensitivity, slow response, lack of selectivity, and effect of aging. To meet the increasing demands for making it work in more complicated systems and under more harsh conditions, many efforts have been made toward modifying the structure, chemistry, or both of SnO2. Because of the modification, sensing performance of SnO2 toward pollutant gases of low concentration at lower temperature can be improved. Doping in semiconductor gas sensors has been a point of interest because of the ability to tailor the electrical and microstructure properties, which has been proven to be pivotal. Doping transition metals in SnO2 also can promote the sensing performance. For instance, Pt, Pd, and Co have been known to promote the sensitivity, selectivity, and response speed of SnO2 to specific gases.911 In addition, Cu is usually used as a dopant in SnO2 to improve the sensitivity toward the flammable and highly toxic H2S.1216 However, the mechanism that Cu-doping promotes the sensitivity of SnO2 toward H2S remains far from being understood. To shed some light on the H2S adsorption properties on SnO2 and the role played by Cu doping in enhancing the sensing performance of SnO2 toward H2S, in the present work, we have studied modification of Cu on the structure and electrical properties of SnO2 (110) surface through the first-principles r 2011 American Chemical Society
calculations based on the density functional theory (DFT). The adsorption properties of molecules H2S and O2 on the SnO2 (110) surface in the absence and presence of Cu-doping have also been investigated. The motivation of using the (110) surface as a template is due to the fact that it is the most thermodynamically stable surface among the low-index surfaces of SnO2 and has received the most attention in previous experimental and theoretical studies.1720 It has been found that H2S is dissociatively adsorbed on the (110) surface of SnO2 and the adsorption has no changes on the electrical conductivity. Because of the increase in electrical conductivity (due to the charge transfer from H2S to SnO2), Cu-doping can directly improve the sensitivity of SnO2 toward H2S. Furthermore, Cu-doping can evidently improve the formation of surface oxygen vacancy, which is vital for the O2 adsorption and improving sensitivity of SnO2 toward H2S.
2. MODEL AND COMPUTATIONAL DETAILS The periodic slab model was adopted to simulate the SnO2 (110) surface, and the slab employed in our present work is composed of five O(Sn2O2)O trilayers. It has been identified that five trilayers are sufficient for reproducing the relaxation of atoms on the outermost layer and for correct estimation of molecule adsorption energies.2124 There are four kinds of surface atoms on the outermost atomic layer: five- and six-fold coordinated Sn, two-fold coordinated bridging O, and three-fold coordinated inplane O, corresponding to the Sn5c, Sn6c, O2c, and O3c, respectively. Received: May 5, 2011 Revised: August 18, 2011 Published: August 22, 2011 18597
dx.doi.org/10.1021/jp204170j | J. Phys. Chem. C 2011, 115, 18597–18602
The Journal of Physical Chemistry C
ARTICLE
molecule adsorption. Geometry relaxation was performed until the residual forces experienced by each ion converged to be
