Haryani Permana

Investigation by scanning tunneling microscopy of the effect of preparative variables on the degree of aggregation of platinum on highly oriented pyrolytic graphite

The scanning tunnel microscope (STM) technique was used to investigate the effect of preparative variables on the degree of aggregation of platinum clusters on a highly oriented pyrolytic graphite (HOPG). Two methods of depositing Pt were investigated: impregnation and vapor deposition. The diameters of the Pt clusters formed on an oxidized HOPG sample prepared by impregnation range from 50 to 500 A. The clusters were apparently concentrated along the steps and rough patches on the surface. The sizes of the Pt clusters formed by vapor deposition range from 20 to 50 A. However, the heights of the Pt clusters formed by these two methods are similar and range from about 10 to 40 A. Distribution of Pt cluster size was formed to be depended on impregnated platinum loading on an oxidized HOPG surface prepared by impregnation. The platinum clusters observed were not mobile on an oxidized HOPG surface during scanning, and STM images were very reproducible.

Effects of annealing and gas treatment on the morphology of platinum cluster size on highly oriented pyrolytic graphite by scanning tunneling microscopy

We have used STM to examine the effects of annealing time in the presence of different gaseous species (Ar, H2 and N2) on the morphological transformation of vacuum-vapor deposited Pt clusters on highly oriented pyrolytic graphite (HOPG). This study shows that the morphological transformation of the Pt clusters on HOPG is a function of both gaseous species and annealing time. Without annealing, the as-deposited platinum consists of an aggregation of smaller clusters. These clusters become more uniform and spherical after annealing for 4 h in the presence of Ar and H2. Further annealing shows a morphological transformation from spherical to elongated shape. Small amounts of hydrocarbon impurities in Ar or hydrogen might possibly explain the shape transformation of the Pt clusters. However, the morphological transformation of the Pt clusters is totally different when the sample is annealed in low sticking coefficient gas, n2, suggesting adsorbed gases (hydrocarbon or hydrogen) have profound effects on the shape transformation. These results indicate that pretreatment conditions are important factors in the shape transformation, which may have significant influence on catalytic activity and selectivity.

Observation of protrusions and ring structures on MoS2 by scanning tunneling microscopy

Natural MoS2 was investigated by scanning tunneling microscopy operated in air. The atomic resolution image yields the hexagonal symmetry of the surface of the crystal with lattice constant of 3.1 A, in agreement with the reported x‐ray crystallographic value. Corrugations along the [110] cell diagonal show major and secondary peaks which have been reported to correspond to molybdenum atoms and sulfur atoms, respectively. A large‐area image of MoS2 reveals protrusions and ring‐type structures of similar sizes on the surface of the sample. The islands and ring‐type structures may be due to vanadium and fluoride ions, which are found in low concentrations (0.03 and 0.5 at. %, respectively) in the MoS2 sample when investigated by secondary ion mass spectroscopy. The protrusions and ring‐type structures were observed only at a higher bias voltage (−0.084 V), suggesting that the impurities are found in the molybdenum layer only. Low bias voltage (−0.003 V) does not show any indications of islands or ring structures, which suggests that there are no impurities on the sulfur layer.

Scanning tunneling microscopy investigations of cluster sizes of molybdenum-based catalysts on graphite

Surface morphologies of molybdenum-based catalysts supported on graphite were investigated with a scanning tunneling microscope. The oxide phases of Mo, Co, and Mo-Co were observed to form distinct types of clusters on graphite. MoOx clusters appeared to be elliptical, with sizes ranging from 100 to 200 Å in length and 60 Å in width, while CoOx formed round and needle-shaped clusters, about 200 Å in size. Oxidized Co-Mo was observed to form ringtype structures which were uniformly 100 Å in diameter. This ring-type structure was attributed to the CoMoOx structure with Co decorating the edges of Mo. Sulfided phases of these catalysts were observed to form large slabs on graphite. Sulfided MoOx formed large islands (100 Å × 150 Å). These may be formed from the aggregation of small MoS2 crystallites. Besides forming large patches, CoS also formed needle-type clusters similar to CoOx, but smaller (120 Å) in length. These clusters and patches are believed to be bulk Co9S8. Sulfided CoMoOx formed CoMoS islands similar to the needle-type clusters observed in CoS, but much larger in size, from 300 to 600 Å. No MoS2 and CoS types of clusters were observed on this sample. No apparent movement of the clusters was observed during repeated scanning, suggesting that there is an interaction between the cluster and the graphite support.

Observation by scanning tunneling microscopy of the morphology change of Pt/HOPG after NO-CO reaction

Abstract We performed STM experiments to investigate the effects of NO-CO reaction the morphology change of Pt deposited on a HOPG surface. The gasification reaction on the Pt/HOPG resulted in straight channels, multidirection channels, and circular monoatomic step pits on the basal plane. We have observed more channel formation than pit formation in the presence of platinum particles, suggesting that catalyzed gasification is much faster than noncatalyzed gasification reaction. The observed channeling structure was found to be 3 to 8 monolayers deep and is due to NO-CO reaction at higher temperature at which Pt particles attack the edges and steps of the basal plane of graphite. The unique ability of STM technique was used to resolve atomic structure of 3 to 8 monolayers deep inside channel. The corrugation width of adjacent hexagons was found to be 2.5 A, which is in good agreement with the C-C interatomic distance in the literature.

Study of the Effects of Annealing on the Morphology of Platinum Cluster size on Highly Oriented Pyrolytic Graphite by Scanning Tunneling Microscopy

Abstract The effects of annealing on the morphological transformation of platinum clusters supported on graphite was investigated by scanning tunneling microscopy. Without annealing, the as-deposited platinum consists of an aggregation of smaller clusters. These clusters become more uniform and spherical after annealed for 4 hours. Further annealing shows a transformation from spherical to elongated shape.