The sensitivity of a semiconductor sensor using SnO2 to H2S in air could be enormously promoted by loading SnO2 with a small amount of CuO. However, response kinetics began to deteriorate rather sharply as the H2S concentration decreased below a certain limit, which depended on CuO loadings. XPS measurements on a series of CuO-SnO2 samples calcined in air revealed that the binding energies (BEs) for O1s and Sn3d5/2 levels shifted downward from those of pure SnO2, while Cu2p3/2 level showed an

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... ward shift from that of pure CuO. With increasing CuO loading, the shifts in BE increased or decreased linearly for O1s and Sn3d5/2 levels or Cu2p3/2 level, respectively, indicating continuous shifts in Fermi levels of SnO2 and CuO. For a fixed CuO loading, the magnitudes of the BE shifts were dependent on the methods of CuO loading, reflecting differences in the dispersion of CuO particles on SnO2 grains. These phenomena were well consistent with the formation of p-n contacts between the finely dispersed CuO (p) and the underlying SnO2 (n) grains. The sensitivity to H2S was shown to be well correlated with the magnitudes of the BE shifts, indicating that the formation of the p-n contacts in air and the rupture of them upon exposure to H2S are the origin of the high H2S sensitivity.