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STM examination of O2 etching on graphite surfaces in air
Short
Communications
STM
examination
of O2 etching
on graphite
Dong-ke Zhang”, Ting Lit, Guy F. Cotterill?,
surfaces
in air
D. J. O’Connort
and Terry F. Wall
Department of Chemicai Engineering and TDepartment of Physics, The University of Newcastle, NS W 2308, Australia (Received 3 February 1993)
Carbon reactions with gases, such as 02, CO*, HzO, NO,, etc. have been an important subject in combustion science, coal gasification, pollutant control and many other practical applications’*‘. However, fundamental understanding of these reactions is still relatively undeveloped due to the complicated chemicwphysical processes associated with the reactions and the variable properties of the carbon material?. Graphite may be used as a model for these processes since graphite has a well-defined structure. Therefore, in a recently commenced project, we have chosen a highly oriented pyrolytic graphite (Union Carbide grade ZYA) to study O2 etching on its surface. The scanning tunnelling microscopy (STM)3-6 technique has been used to examine the graphite surface changes caused by O2 etching and to obtain further insights into the graphite-O, reaction. This brief communication reports and discusses the preliminary results observed. (Keywords: STM; graphite; oxygen etching)
The STM may be operated in either a constant-current mode or a constantheight mode3-6. In an STM, a sharp metal tip and the sample surface are used as electrodes. This confines the tunnelling to a very small area and the current is, therefore, only sensitive to local surface features. In STM operation with the constant-current mode, the tunnelling current is maintained at a set level by feeding it to a servo system which regulates the tip-sample distance. When the tip scans over the surface, it will follow a constant tunnelling current path above the surface and the variation of the tip-sample distance gives a measure of the surface structure of the sample. When a STM is operated with the constant-height mode, the system records the changes in the current as the tip scans over the sample surface and the measured current is then converted to height (depth) values. Chu and Schmid@ have used the constant-height mode and stated that they calibrated their images regularly, and found that their images were consistent with a monolayer height. Nevertheless, the constant-current mode gives more accurate height (depth) measurements than the constant-height mode. We have therefore, used the constantcurrent mode in our STM work. Freshly cleaved graphite surfaces were repeatedly observed using STM and it was generally found that there are surface defects’. These defects may be classified into point defects (also called basal plane defects6), line defects and minor step defects. The reaction of a gas with graphite usually commences at such defects5,‘j. * Present address: Chemical Engineering Department, The University of Adelaide, GPO Box 498, Adelaide, South Australia 5005 00162361/93/10/1454-02 0 1993 Butterworth-Heinemann
1454
Fuel 1993
Volume
For graphite reactions with 02, freshly cleaved samples were treated in air at atmospheric pressure in an electrically heated furnace set at specific temperatures for various times. After the treatments, the samples were quenched with N, to prevent any significant reactions during cooling. When cooled to room temperature, the samples were transfered to the STM for surface examination in air using the constant-current mode, which gives accurate depth or height measurements. Tens of images were taken at different areas on the same sample surface and a number of round pits of different sizes, crevices and some irregular hollows created by O2 etching on the surface were recorded. It was observed that the type and size of the etched areas depend on the reaction conditions. The typical results observed are reported in Figures 1 to 3. Figure 1 shows a high magnification image of a sample which has been heated in air at 500°C for 36min, creating
Figure 1 A 150 dm x 150 nmimage of graphite treated in air at SOO”C for 36min. A typical single monolayer pit of 60 nm in diameter is shown
Ltd
72 Number
10
Figure 2 Images of graphite treated in air at 550°C for 20 min: (a) a single double-layer pit in a 150nm x 150 nm image; (b) two doublelayer pits in a 1280mm x 1280nm image
circular or hexagonal pits. These pits have diameters of 60 to 70nm, and a depth of 0.34 nm, indicating the monolayer etching (for graphite, the monolayer step is about 0.34nm3*4). Figure 2 shows a micrograph of a pit for O2 etching in a sample treated at 550°C for 20min. The pits created under this condition have fairly uniform diameters in the range from 110 to 120 nm and their depths are exclusively in the range from 0.65 to 0.7nm, indicating double-layer etching. It is believed that these round pits originate from point defects where C atoms are loosely bonded and are more susceptible
Short Communications
Figure 3 A wide and deep crevice (multilayer etching) observed on the surface of graphite treated in air at 550°Cfor 80 min (over-reacted)
to attack from oxygen. It is also noted that the growth rate of these pits is clearly dependent on the temperature at which the reaction takes place. As evident in the two typical runs discussed above, at the higher temperature (550°C) the 0, reaction created larger and deeper pits than at lower temperature (500°C). Qualitatively, the removal of carbon is likely to occur by increased monolayer etching at lower temperatures, but both
width and depth etching occur at higher temperatures. Deep crevices were also observed under various conditions. Figure 3 shows a crevice, 27.3 nm deep (N 81 times the graphite interlayer spacing), suggesting multilayer etching of a sample treated at 550°C for 80 min. The high magnification STM image of this feature shows that its edges are rather rough and there are steps of different depths at the bottom. This irregular crevice probably originated from a line or step defect which was present on the original sample, and its very large width and depth are due to the length of the reaction time, i.e. over reacted. This preliminary study demonstrates that the STM technique can be used to study graphite gasification processes by examining the graphite surface modification. The constant-current mode employed in the present study is believed to give more accurate depth (or height) measurements than the constant-height mode used by Chu and Schmidt5,6. Of particular interest is that the size of the pits originating at point defects due to O2 etching may be quantitatively measured by STM images, which yields the number of C atoms reacted under given
conditions. Such data can be used to estimate the intrinsic reactivity of graphite gasification processes which may be compared with those obtained using other combustion techniques*. To achieve such a quantitative analysis, well-controlled reaction conditions are required and the over-reaction (Figure 3) should be avoided. Application of the STM technique to fossil fuel combustion research is another potential direction in which further work is warranted. For this purpose, petroleum coke and anthracite are the appropriate fuels for such a study. REFERENCES
4 5 6
Smoot, L. D. and Pratt, D. T. ‘Pulverized Coal Combustion and Gasification’, Plenum Press, New York, 1979 Smith, 1. W. 19th Symposium (International) on Combustion, The Combustion Institute. Pittsburgh, 1982, pp. 1041065 Kuk, Y. and Silverman, P. J. Rev. Sci. Instrum. 1989, 60(2), 165 Shen, J., Pritchard, R. G. and Thurstans, R. E. Contemp. Phys. 1991, 32(l), 11 Chu, X. and Schmidt, L. D. Carbon 1991, 29, 1251 Chu, X. and Schmidt, L. D. Su$ace Sri. 1992, 268, 325
Fuel 1993 Volume 72 Number 10
1455
Communications
STM
examination
of O2 etching
on graphite
Dong-ke Zhang”, Ting Lit, Guy F. Cotterill?,
surfaces
in air
D. J. O’Connort
and Terry F. Wall
Department of Chemicai Engineering and TDepartment of Physics, The University of Newcastle, NS W 2308, Australia (Received 3 February 1993)
Carbon reactions with gases, such as 02, CO*, HzO, NO,, etc. have been an important subject in combustion science, coal gasification, pollutant control and many other practical applications’*‘. However, fundamental understanding of these reactions is still relatively undeveloped due to the complicated chemicwphysical processes associated with the reactions and the variable properties of the carbon material?. Graphite may be used as a model for these processes since graphite has a well-defined structure. Therefore, in a recently commenced project, we have chosen a highly oriented pyrolytic graphite (Union Carbide grade ZYA) to study O2 etching on its surface. The scanning tunnelling microscopy (STM)3-6 technique has been used to examine the graphite surface changes caused by O2 etching and to obtain further insights into the graphite-O, reaction. This brief communication reports and discusses the preliminary results observed. (Keywords: STM; graphite; oxygen etching)
The STM may be operated in either a constant-current mode or a constantheight mode3-6. In an STM, a sharp metal tip and the sample surface are used as electrodes. This confines the tunnelling to a very small area and the current is, therefore, only sensitive to local surface features. In STM operation with the constant-current mode, the tunnelling current is maintained at a set level by feeding it to a servo system which regulates the tip-sample distance. When the tip scans over the surface, it will follow a constant tunnelling current path above the surface and the variation of the tip-sample distance gives a measure of the surface structure of the sample. When a STM is operated with the constant-height mode, the system records the changes in the current as the tip scans over the sample surface and the measured current is then converted to height (depth) values. Chu and Schmid@ have used the constant-height mode and stated that they calibrated their images regularly, and found that their images were consistent with a monolayer height. Nevertheless, the constant-current mode gives more accurate height (depth) measurements than the constant-height mode. We have therefore, used the constantcurrent mode in our STM work. Freshly cleaved graphite surfaces were repeatedly observed using STM and it was generally found that there are surface defects’. These defects may be classified into point defects (also called basal plane defects6), line defects and minor step defects. The reaction of a gas with graphite usually commences at such defects5,‘j. * Present address: Chemical Engineering Department, The University of Adelaide, GPO Box 498, Adelaide, South Australia 5005 00162361/93/10/1454-02 0 1993 Butterworth-Heinemann
1454
Fuel 1993
Volume
For graphite reactions with 02, freshly cleaved samples were treated in air at atmospheric pressure in an electrically heated furnace set at specific temperatures for various times. After the treatments, the samples were quenched with N, to prevent any significant reactions during cooling. When cooled to room temperature, the samples were transfered to the STM for surface examination in air using the constant-current mode, which gives accurate depth or height measurements. Tens of images were taken at different areas on the same sample surface and a number of round pits of different sizes, crevices and some irregular hollows created by O2 etching on the surface were recorded. It was observed that the type and size of the etched areas depend on the reaction conditions. The typical results observed are reported in Figures 1 to 3. Figure 1 shows a high magnification image of a sample which has been heated in air at 500°C for 36min, creating
Figure 1 A 150 dm x 150 nmimage of graphite treated in air at SOO”C for 36min. A typical single monolayer pit of 60 nm in diameter is shown
Ltd
72 Number
10
Figure 2 Images of graphite treated in air at 550°C for 20 min: (a) a single double-layer pit in a 150nm x 150 nm image; (b) two doublelayer pits in a 1280mm x 1280nm image
circular or hexagonal pits. These pits have diameters of 60 to 70nm, and a depth of 0.34 nm, indicating the monolayer etching (for graphite, the monolayer step is about 0.34nm3*4). Figure 2 shows a micrograph of a pit for O2 etching in a sample treated at 550°C for 20min. The pits created under this condition have fairly uniform diameters in the range from 110 to 120 nm and their depths are exclusively in the range from 0.65 to 0.7nm, indicating double-layer etching. It is believed that these round pits originate from point defects where C atoms are loosely bonded and are more susceptible
Short Communications
Figure 3 A wide and deep crevice (multilayer etching) observed on the surface of graphite treated in air at 550°Cfor 80 min (over-reacted)
to attack from oxygen. It is also noted that the growth rate of these pits is clearly dependent on the temperature at which the reaction takes place. As evident in the two typical runs discussed above, at the higher temperature (550°C) the 0, reaction created larger and deeper pits than at lower temperature (500°C). Qualitatively, the removal of carbon is likely to occur by increased monolayer etching at lower temperatures, but both
width and depth etching occur at higher temperatures. Deep crevices were also observed under various conditions. Figure 3 shows a crevice, 27.3 nm deep (N 81 times the graphite interlayer spacing), suggesting multilayer etching of a sample treated at 550°C for 80 min. The high magnification STM image of this feature shows that its edges are rather rough and there are steps of different depths at the bottom. This irregular crevice probably originated from a line or step defect which was present on the original sample, and its very large width and depth are due to the length of the reaction time, i.e. over reacted. This preliminary study demonstrates that the STM technique can be used to study graphite gasification processes by examining the graphite surface modification. The constant-current mode employed in the present study is believed to give more accurate depth (or height) measurements than the constant-height mode used by Chu and Schmidt5,6. Of particular interest is that the size of the pits originating at point defects due to O2 etching may be quantitatively measured by STM images, which yields the number of C atoms reacted under given
conditions. Such data can be used to estimate the intrinsic reactivity of graphite gasification processes which may be compared with those obtained using other combustion techniques*. To achieve such a quantitative analysis, well-controlled reaction conditions are required and the over-reaction (Figure 3) should be avoided. Application of the STM technique to fossil fuel combustion research is another potential direction in which further work is warranted. For this purpose, petroleum coke and anthracite are the appropriate fuels for such a study. REFERENCES
4 5 6
Smoot, L. D. and Pratt, D. T. ‘Pulverized Coal Combustion and Gasification’, Plenum Press, New York, 1979 Smith, 1. W. 19th Symposium (International) on Combustion, The Combustion Institute. Pittsburgh, 1982, pp. 1041065 Kuk, Y. and Silverman, P. J. Rev. Sci. Instrum. 1989, 60(2), 165 Shen, J., Pritchard, R. G. and Thurstans, R. E. Contemp. Phys. 1991, 32(l), 11 Chu, X. and Schmidt, L. D. Carbon 1991, 29, 1251 Chu, X. and Schmidt, L. D. Su$ace Sri. 1992, 268, 325
Fuel 1993 Volume 72 Number 10
1455