- Email: [email protected]
Diffusion of H2O on surfaces of clean and Au covered tungsten field emitters
Vacuum/volume
Pergamon PII: SOO42-207X(96)00264-9
4BInumber 3/4/pages 333 to 33511997 0 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207x/97 $17.00+.00
Diffusion of H,O on surfaces of clean and Au covered tungsten field emitters R Bryl, Institute of Experimental
Physics, University
of Wrocfaw, pi. Maxa Borna 9, 50-204 Wrocfaw, Poland
The results of field emission microscopy measurements of the diffusion of water deposited on a thermally clean and a gold covered tungsten emitter are presented. The diffusion of water from a multilayer to a submonolayer H,O was observed on clean Win the temperature range 126147 K and on Au/W (0 2 2) in the range 113-123 K, with activation energies for diffusion of 27f 2 kJ/mol in both cases. It is suggested that water in the low coverage range can diffuse on clean Wand Au surfaces at temperatures as low as 78 K. 0 1997 Elsevier Science Ltd. All rights reserved
Introduction Investigation of the diffusion properties of water seems to be one of the most important tasks as the kinetics of Hz0 diffusion on a surface determines the rate of a water clusters’ nucleation and growth. So far, not many data concerning this problem are available. The energy barrier for surface diffusion was predicted to range between 3 and 27 kJ/mol.’ The Hz0 cluster formation was reported to occur at 20 K on Pd( 100) and Cu(lOO), although the process was very slow. It became relatively fast at about 100 K.’ Diffusion of a thick water layer was observed on Pt field emitters at 115-135 K with activation energies ranging from 19 to 27 kJ/mol depending on the crystallographic direction.2.3 In this paper, which is a continuation of a parallel work,4 results concerning the diffusion of Hz0 over clean and gold precovered tungsten field emitters are presented. Such substrates were chosen as it was known that Hz0 would adsorb on them in completely different ways. Tungsten surfaces promote water dissociation’.4*5 whereas the adsorption on Au covered W emitter was found to be nondissociative when the Au coverage was sufficiently high.4 It was interesting to elucidate whether the diffusion of water was different in these two cases. Experimental The FE microscope, emitter, Au and H,O sources, experimental conditions and procedure were identical as those described in Ref. 4. The experiments were carried out in ultra high vacuum (p< 1 x IO-‘” Torr). The shadowing method was applied to observe the diffusion of the H20 layer.6 In order to provide a lateral deposition of water, the emitter assembly was surrounded by a molybdenium cylinder kept at 78 K during experiments with H20. To reduce the influence of the FE field on the water adlayer, the strength of the electric field applied to the emitter during
experiments with water usually varied from 34 to 39MVicm (estimated in the same way as in Ref. 4) depending on the water and Au coverage and the emission current drawn from the emitter. For these values of F the emission current (obtained for constant voltage applied) was time independent.
Results and discussion The W emitters were thermally cleaned before the deposition of Au or H,O. Gold was deposited onto the emitter in the same way as in Ref. 4. Water was deposited laterally onto an emitter held at 78 K in the absence of an external electric field. A high voltage was applied to the tube about IOmin after closure of the leak valve to avoid any additional adsorption of water during the measurement. The voltage U necessary to maintain a constant emission current 1, rose with increasing amount of deposited water, both for adsorption on clean W and on Au/W, as described in Ref. 4. When water was condensed on the clean or Au precovered (0 2 2) W emitter, part of the emission pattern corresponding to the tip region exposed directly to water vapour became dark, whilst the remaining part also looked unclean. As the diffusion of H20 on metal surfaces was evident at temperatures even as low as 20K,’ it is suggested that water did not adsorb onto the shadowed part of the emitter, but diffused there from the exposed region. Although the entire emitter surface was covered with H,O at 78 K, the coverage varied significantly from the region exposed to water vapour to the region in the shadow. Water coverage was not precisely measured but it was assumed that the first region was covered with a relatively thick layer (several monolayers), whereas the second one was covered with a submonolayer. Under these conditions further diffusion of water could occur. The emitter was heated in the absence of an external electric field in the 333
R Bryl: Diffusion of Hz0 on tungsten field emitters range
113-123K
for
H,O/Au/W
and
126-147K
for
increase
of voltage
9 (a)
H,O/W.
emission current was observed at tixed temperatures both for clean and Au covered W emitters. The related isotherms (T = const.) are presented in Figure l(a) and (b), respectively. The straight horizontal lines in Figure 1(a) and (b) determine the diffusion start- and end-points and thus the time period AI necessary to obtain the Arrhenius plots based on the relation Atzexp(E,,/kT), which are shown in Figure 2(a) and (b). The activation energies for HI0 diffusion E,,, as calculated on the basis of the slopes are 27 f 2 kJjmol in both cases. This seems to be in good agreement with the E.iR values obtained so far (19-27 kJ/mol for H,0/Pt2,3) and predicted (3-27 kJ/mol for water on metal surfaces’). It is interesting that Edlff is the same for such different substrates as tungsten (on which water dissociates) and gold (which does not promote water dissociation). As was mentioned above, the water diffused not on clean surfaces but on the ones covered with a submonolayer of H20 or maybe its dissociation products. Moreover, the obtained value of E,,, is close to the strength of the hydrogen bond in ice which ranges typically from 15 to 25 kJjmo1 1. This can suggest that in that case the interactions between water molecules play a main role and the hydrogen bonds must have been broken first for the diffusion to occur. The apparent diffusion of water at 78 K on both clean substrates, which might indicate that no The
U for a constant
HZOIW
l-
52 V
=‘
z 5-
31’ 6.6
1.4
7.0
7.8
8.2
103/T (l/K) 6) 6.5
6.0 1.10
(4 I
_ c z? Z -
5.5
5.0
4.5
4.0 8.0 1’1’1’1 1000
0
4 2000
3000
Diffusion
4000
II
5000
8.4
8.6
8.8
9.0
103/T (l/K)
6000
time (s)
Figure 2. Arrhenius Figure l(b).
0) 1.11
8.2
plots obtained
on the basis of (a) Figure
1(a) and (b)
(
water dissociation temperature.
on the tungsten
emitter
took
place at this
Conclusions
2000
3000
Diffusion
4000
time (s)
Figure 1. The isotherms of H,O diffusion on: (a) a surface emitter and (b) Au precovered W field emitter (@x2.5). 334
of W field
The activation energy for the diffusion of a thick water layer on a water submonolayer present on clean and Au precovered W field emitters has been estimated to be 27 f 2 kJ/mol in both cases. Diffusion was measured in the temperature range 113-123 K in the case of Au/W and in the 127-147 K range for clean W. It is also supposed that Hz0 in the low coverage range diffuses on surfaces of clean and Au covered W emitter at temperatures as low as 78 K.
R Bryl: Diffusion of HZ0 on tungsten
field emitters
Acknowledgements
References
This research is a continuation of previous work2.3.7 inspired by Professor T E Madey and supported by US-Poland M Sklodowska-Curie Joint Found II, within the grant MEN/DOE-9157. The author would like to thank Professor R Blaszczyszyn for his continual interest and help during the measurements, and many discussions. Thanks are also due to E Galewska for her assistance during the course of this work and to Dr S Surma for helpful comments. The work was supported by the University of Wrodaw, Project No. 2016/W;IFD/95, which is gratefully acknowledged.
1. Thiel, P. A. and Madey, T. E., Surf. Sci. Rep., 1987,7, 1987, 211 and references therein. 2. Bryl, R., Wysocki, T., Blaszczyszyn, R., Appl. Surf. Sci., 1995,87/M, 69. 3. Btaszczyszynowa, M., Blaszczyszyn, R. and Bryl, R., Acta. Phys. Polonica, 1995, A88, 5 11. 4. Bryl, R., Btaszczyszyn, R. and Galcwska, E., Vacuum: 1997, 48,
329. 5. Heras, J. M. and Viscido, L., Cafal. Rev. Sci. Eng., 1988,30, 281. 6. Gomcr, R., Rep. Pro. and Phys., 1990, 53, 917. 7. Blaszczyszyn, R., Ciszewski, A., Blaszczyszynowa, M. and Bryl, R., Zuber, S., Appi. Surj. Sci., 1993, 67, 21 I.
335
Pergamon PII: SOO42-207X(96)00264-9
4BInumber 3/4/pages 333 to 33511997 0 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207x/97 $17.00+.00
Diffusion of H,O on surfaces of clean and Au covered tungsten field emitters R Bryl, Institute of Experimental
Physics, University
of Wrocfaw, pi. Maxa Borna 9, 50-204 Wrocfaw, Poland
The results of field emission microscopy measurements of the diffusion of water deposited on a thermally clean and a gold covered tungsten emitter are presented. The diffusion of water from a multilayer to a submonolayer H,O was observed on clean Win the temperature range 126147 K and on Au/W (0 2 2) in the range 113-123 K, with activation energies for diffusion of 27f 2 kJ/mol in both cases. It is suggested that water in the low coverage range can diffuse on clean Wand Au surfaces at temperatures as low as 78 K. 0 1997 Elsevier Science Ltd. All rights reserved
Introduction Investigation of the diffusion properties of water seems to be one of the most important tasks as the kinetics of Hz0 diffusion on a surface determines the rate of a water clusters’ nucleation and growth. So far, not many data concerning this problem are available. The energy barrier for surface diffusion was predicted to range between 3 and 27 kJ/mol.’ The Hz0 cluster formation was reported to occur at 20 K on Pd( 100) and Cu(lOO), although the process was very slow. It became relatively fast at about 100 K.’ Diffusion of a thick water layer was observed on Pt field emitters at 115-135 K with activation energies ranging from 19 to 27 kJ/mol depending on the crystallographic direction.2.3 In this paper, which is a continuation of a parallel work,4 results concerning the diffusion of Hz0 over clean and gold precovered tungsten field emitters are presented. Such substrates were chosen as it was known that Hz0 would adsorb on them in completely different ways. Tungsten surfaces promote water dissociation’.4*5 whereas the adsorption on Au covered W emitter was found to be nondissociative when the Au coverage was sufficiently high.4 It was interesting to elucidate whether the diffusion of water was different in these two cases. Experimental The FE microscope, emitter, Au and H,O sources, experimental conditions and procedure were identical as those described in Ref. 4. The experiments were carried out in ultra high vacuum (p< 1 x IO-‘” Torr). The shadowing method was applied to observe the diffusion of the H20 layer.6 In order to provide a lateral deposition of water, the emitter assembly was surrounded by a molybdenium cylinder kept at 78 K during experiments with H20. To reduce the influence of the FE field on the water adlayer, the strength of the electric field applied to the emitter during
experiments with water usually varied from 34 to 39MVicm (estimated in the same way as in Ref. 4) depending on the water and Au coverage and the emission current drawn from the emitter. For these values of F the emission current (obtained for constant voltage applied) was time independent.
Results and discussion The W emitters were thermally cleaned before the deposition of Au or H,O. Gold was deposited onto the emitter in the same way as in Ref. 4. Water was deposited laterally onto an emitter held at 78 K in the absence of an external electric field. A high voltage was applied to the tube about IOmin after closure of the leak valve to avoid any additional adsorption of water during the measurement. The voltage U necessary to maintain a constant emission current 1, rose with increasing amount of deposited water, both for adsorption on clean W and on Au/W, as described in Ref. 4. When water was condensed on the clean or Au precovered (0 2 2) W emitter, part of the emission pattern corresponding to the tip region exposed directly to water vapour became dark, whilst the remaining part also looked unclean. As the diffusion of H20 on metal surfaces was evident at temperatures even as low as 20K,’ it is suggested that water did not adsorb onto the shadowed part of the emitter, but diffused there from the exposed region. Although the entire emitter surface was covered with H,O at 78 K, the coverage varied significantly from the region exposed to water vapour to the region in the shadow. Water coverage was not precisely measured but it was assumed that the first region was covered with a relatively thick layer (several monolayers), whereas the second one was covered with a submonolayer. Under these conditions further diffusion of water could occur. The emitter was heated in the absence of an external electric field in the 333
R Bryl: Diffusion of Hz0 on tungsten field emitters range
113-123K
for
H,O/Au/W
and
126-147K
for
increase
of voltage
9 (a)
H,O/W.
emission current was observed at tixed temperatures both for clean and Au covered W emitters. The related isotherms (T = const.) are presented in Figure l(a) and (b), respectively. The straight horizontal lines in Figure 1(a) and (b) determine the diffusion start- and end-points and thus the time period AI necessary to obtain the Arrhenius plots based on the relation Atzexp(E,,/kT), which are shown in Figure 2(a) and (b). The activation energies for HI0 diffusion E,,, as calculated on the basis of the slopes are 27 f 2 kJjmol in both cases. This seems to be in good agreement with the E.iR values obtained so far (19-27 kJ/mol for H,0/Pt2,3) and predicted (3-27 kJ/mol for water on metal surfaces’). It is interesting that Edlff is the same for such different substrates as tungsten (on which water dissociates) and gold (which does not promote water dissociation). As was mentioned above, the water diffused not on clean surfaces but on the ones covered with a submonolayer of H20 or maybe its dissociation products. Moreover, the obtained value of E,,, is close to the strength of the hydrogen bond in ice which ranges typically from 15 to 25 kJjmo1 1. This can suggest that in that case the interactions between water molecules play a main role and the hydrogen bonds must have been broken first for the diffusion to occur. The apparent diffusion of water at 78 K on both clean substrates, which might indicate that no The
U for a constant
HZOIW
l-
52 V
=‘
z 5-
31’ 6.6
1.4
7.0
7.8
8.2
103/T (l/K) 6) 6.5
6.0 1.10
(4 I
_ c z? Z -
5.5
5.0
4.5
4.0 8.0 1’1’1’1 1000
0
4 2000
3000
Diffusion
4000
II
5000
8.4
8.6
8.8
9.0
103/T (l/K)
6000
time (s)
Figure 2. Arrhenius Figure l(b).
0) 1.11
8.2
plots obtained
on the basis of (a) Figure
1(a) and (b)
(
water dissociation temperature.
on the tungsten
emitter
took
place at this
Conclusions
2000
3000
Diffusion
4000
time (s)
Figure 1. The isotherms of H,O diffusion on: (a) a surface emitter and (b) Au precovered W field emitter (@x2.5). 334
of W field
The activation energy for the diffusion of a thick water layer on a water submonolayer present on clean and Au precovered W field emitters has been estimated to be 27 f 2 kJ/mol in both cases. Diffusion was measured in the temperature range 113-123 K in the case of Au/W and in the 127-147 K range for clean W. It is also supposed that Hz0 in the low coverage range diffuses on surfaces of clean and Au covered W emitter at temperatures as low as 78 K.
R Bryl: Diffusion of HZ0 on tungsten
field emitters
Acknowledgements
References
This research is a continuation of previous work2.3.7 inspired by Professor T E Madey and supported by US-Poland M Sklodowska-Curie Joint Found II, within the grant MEN/DOE-9157. The author would like to thank Professor R Blaszczyszyn for his continual interest and help during the measurements, and many discussions. Thanks are also due to E Galewska for her assistance during the course of this work and to Dr S Surma for helpful comments. The work was supported by the University of Wrodaw, Project No. 2016/W;IFD/95, which is gratefully acknowledged.
1. Thiel, P. A. and Madey, T. E., Surf. Sci. Rep., 1987,7, 1987, 211 and references therein. 2. Bryl, R., Wysocki, T., Blaszczyszyn, R., Appl. Surf. Sci., 1995,87/M, 69. 3. Btaszczyszynowa, M., Blaszczyszyn, R. and Bryl, R., Acta. Phys. Polonica, 1995, A88, 5 11. 4. Bryl, R., Btaszczyszyn, R. and Galcwska, E., Vacuum: 1997, 48,
329. 5. Heras, J. M. and Viscido, L., Cafal. Rev. Sci. Eng., 1988,30, 281. 6. Gomcr, R., Rep. Pro. and Phys., 1990, 53, 917. 7. Blaszczyszyn, R., Ciszewski, A., Blaszczyszynowa, M. and Bryl, R., Zuber, S., Appi. Surj. Sci., 1993, 67, 21 I.
335