The adsorption of chloride on Cu(110) single-crystal surface has been investigated in hydrochloric acid aqueous solution by using in situ scanning tunneling microscopy (STM). A (1 × 1) structure of Cu(110) substrate lattice was visualized at the potentials negative than -450 mV vs SCE, where chloride anions were desorbed. In the potential range from -400 mV to -150 mV vs SCE, two main complex adlayers with 3-fold and 4-fold periodicity along [11 h0] direction of the substrate were observed. These structures with long-range periodicity are attributed to ordered chloride adlayers on unreconstructed Cu( 110)-(1 × 1) structure. The different corrugation heights and periodic modulations along [11 h0] direction observed in STM images indicate that chloride anions are located at nonequivalent binding sites. The average measured lattice constants for the two adlayers have the same size as the substrate lattice along [001] direction and have 4 and 5 times the lattice spacing of the underlying copper substrate along [11 h0] direction. Models are proposed to interpret these two structures to (4 × 1) and (5 × 1) chloride adlayers containing three and four chloride anions, respectively.

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