Local structure and assembly of formate adsorbed on Ni(1 1 0): Low temperature scanning tunneling microscopy study

Adsorption behavior of formate (HCOO−) on Ni(1 1 0) surface has been investigated using a scanning tunneling microscope (STM) at cryogenic temperature. Formic acid (HCOOH) is found to dissociatively adsorb onto Ni(1 1 0) to produce formate at 50 K. Atomically resolved STM images revealed that formates can occupy either short-bridged (S-formate) or long-bridged (L-formate) sites at 50 K. At low surface coverages, formates are isolated and disordered when dosed at 50 K. Annealing of the Ni(1 1 0) surface to 120 K induces site conversion of L-formate to S-formate, indicating L-formate is a metastable state formed only at low temperature. Furthermore, L- and S-formates assemble to form dimers, trimers and short chains through thermal diffusion. This characteristic development of the formate chain is caused by anisotropic lateral interaction between neighboring formate molecules.