Images of demineralized coal surfaces by scanning tunnelling microscopy

Zonguldak bituminous coal was demineralized with HCl and HF. Mineral removal was followed by classical chemical analysis and FT-i.r. spectroscopy to determine the types of inorganics dissolved. Scanning tunnelling microscopy (STM) images of the surfaces of original and acid-treated Zonguldak coal were compared with those of an original semianthracite and a highly oriented pyrolytic graphite. The scanning tunnelling microscope was operated in variable-current mode with an average tunnelling current of 20 pA and a bias voltage in the range 250-500 mV. It was concluded that STM has great potential for the investigation of the structural changes occurring on coal surfaces due to different treatments.