SCANNING-TUNNELING-MICROSCOPY OBSERVATION OF SULFUR ELECTRODEPOSITS ON GRAPHITE SINGLE-CRYSTALS

The early stages of sulfur deposit growth on highly oriented pyrolytic graphite (HOPG) caused by HS- electrooxidation in a neutral buffered solution have been investigated using electrochemical techniques and e x situ scanning tunneling microscopy (STM). In this system sulfur depo sition has been observed at -0.80 V vs SCE, i.e. a potential more nega tive than tbe reversible potential for the HS-/S reaction. The charge density was equivalent to an average surface coverage by sulfur atoms theta congruent to 1/3 monolayer (ML). Ex situ atomic resolution STM i mages of the layer electrodeposited at -0.8 V show sulfur submonolayer s and large uncovered HOPG domains. Sulfur electroadsorption layers ap pear as a diluted (root 3x root 3) surface phase with S atoms atop C a toms of the graphite hexagons and the S-S interatomic distance d(S-S) = 0.42 nm. Further addition of S atoms to a diluted sulfur phase resul ted in the formation of sulfur trimers with three S atoms placed atop the three C atoms constituting the graphite hexagons. In this case d(S -S) = 0.24 nm. Neighbor trimers originate a filled hexagonal lattice. Ex situ STM images of overpotential deposited sulfur also show submono layer sulfur domains with a second hexagonal (root 3x root 3)R30 degre es sulfur lattice with d(S-S) = 0.42 nm. A further increase of theta p roduces either a new honeycomb lattice with d(S-S) = 0.24 nm or a rect angular lattice formed by rows of S atoms with d(S-S) = 0.21 nm and ro w separation d(S-S) = 0.37 nm.