La₂CuO₄-CuO複合体の表面組成と伝導度に与えるH₂S処理効果

抄録

A composite material containing La₂CuO₄ and CuO was directly synthesized by thermal decomposition of a heteronuclear CuLa (dhbaen) (NO₃)⋅nH₂O complex, where dhbaen is N, N′-bis (3-hydroxysalicylidene)-ethylenediamine. The mean particle size was of the order of 200nm for a sample decomposing at 800°C. The exposure to 7.8ppm H₂S at 50°C for 6d (the exposure gas was alternately changed from air to 7.8ppm H₂S at 6h intervals for 6d) resulted in the formation of copper sulfide and lanthanum sulfate on the surface. At 100°C, the surfaces were mostly covered with both sulfide and sulfate. Lanthanum sulfate and copper sulfide, without copper sulfate, were formed at temperatures below 200°C. Copper sulfate was detected when the sample was exposed at 250°C. For samples exposed at 100°C, the atomic ratio determined by X-ray photoelectron spectroscopy (XPS) for [S (sulfate)]/[La] was 1.6 on the surface and decreased to 0.01 with increasing depth of Ar sputtering on the surface. The ratio of [S (sulfide)]/[Cu] was 0.99 on the surface and decreased to about 0.45 upon Ar sputtering. However, [S (sulfate)]/[La] and [S (sulfide)]/[Cu] ratios on the surface were 0.03 and 0.39, respectively, for single-phase La₂CuO₄ prepared by the solid state reaction method after the exposure. It is concluded that the reactions of Cu and La in La₂CuO₄ with hydrogen sulfide are accelerated by the presence of well-dispersed CuO.