(hrhon, Vol. 31. No. 8, pp. 1333-1336. 1993 Printed in Great Bntain.

CARBON MOLECULAR SIEVE FILMS PRODUCED BY DC SPUTTERING

Y. YIN and R. E. COLLINS School of Physics, University of Sydney, Sydney, NSW 2006, Australia

(Received 23 March 1993; uccepted in reuised,form 9 June 1993)

Abstract-This paper reports adsorption measurements that show molecular sieve effects in amorphous hydrogenated carbon (a-C : H) films deposited by d.c. magnetron discharge decomposition of acetylene. Adsorption of organic gases on the films is studied by using a quartz crystal microbalance technique. The sieve effect in this material depends on both deposition and annealing conditions. Films having significant molecular sieve effects are found to be typically microporous, and to have a very small characteristic micropore dimension. It is suggested that the d.c. sputtered a-C : H film may be useful as a molecular sieve material in selective adsorption and gas permeation studies.

Key Words-Carbon molecular sieve, adsorption. thin film. d.c. sputtering, quartz crystal micro- balance.

1. INTRODUCTION 2. EXPERIMENTAL

Hydrogenated amorphous carbon (a-C : H) films have received considerable attention due to their

unique physical properties[ l-31. One of the interest- ing properties of a-C : H is its morphology. The mor- phology of a-C : H films can vary from highly dense to rather porous, depending on deposition condi- tions and subsequent treatment[4,5]. Many tech- niques, such as scanning electron microscopy and small-angle X-ray scattering, have been used to in- vestigate the morphology of this material, but little attention has been paid to using gas adsorption methods for the study of this material. It is recog- nized, however, that the gas adsorption method can provide rich information about the morphology properties of porous materials. The a-C : H material, studied in this work, is prepared by using the d.c. sputtering technique. The deposition conditions and general morphology properties have been presented elsewhere[6,7]. In these papers, we have shown that baking the d.c.-sputtered a-C : H material in vacuum can significantly increase its porosity.

Though molecular sieve effects have been stud- ied widely in bulk carbon materials, as far as we know, little work has been done on the sieve effect in thin carbon films obtained from sputtering. In this study, the quartz crystal microbalance (QCM) technique is used to measure such sieve effects in a-C : H films. It was shown recently[7-91 that the QCM technique is capable of generating useful infor- mation about porous or rough surfaces of thin films. We show evidence in this paper on the existence of molecular sieve effects in the material and the dependence of the sieve effect on deposition and heat-treatment conditions. The origins of the sieve effect in the material are also discussed.

In this study, amorphous hydrogenated carbon

(a-C : H) films are deposited by d.c. magnetron sput- tering decomposition of acetylene in a cylindrical sputtering apparatus. The deposition apparatus used in this study has been described in detail else- where[lO,ll]. In the process, carbon is deposited onto all surfaces in the chamber, including the cath- ode. Some of the carbon reaching the substrate is therefore deposited directly from the discharge, and some is sputtered from the cathode. If the concen- tration of reactive gas is sufficiently high, the deposi- tion rate onto the cathode is greater than the rate of removal by sputtering. Under these conditions, the cathode becomes completely covered with car- bon, and its condition is called “poisoned.” The material deposited when the cathode is poisoned contains no metal, and is called “pure” hydroge- nated carbon. The films studied in this work are about 0.3 pm thick, and are deposited under condi- tions when the cathode is poisoned. Two groups of the films are studied. One material (denoted by a- C: H-R in this paper) is deposited under identical conditions to those discussed above by reactive sputtering in a mixture of argon with acetylene; the second material (called a-C : H-S here) is deposited by sputtering the carbon from the poisoned cathode in pure argon without acetylene.

The QCM adsorption apparatus used in this work has been described previously[6,7]. The mass of a deposited film, the outgrassing quantity due to bak- ing the film in vacuum, and the gas adsorption quan- tity onto the film can be determined by measuring the change of frequency of a quartz crystal. The relationship between the mass change Am and fre- quency change Afof the quartz crystal is[12]

1333

1334 Y. YIN and R. E. COLLINS

Aflf, = -A&n,, (1) q batuane

where mq is the mass of the quartz crystal and fq is the frequency of the quartz crystal, respectively.

(a) 350'% baked

Following deposition, the coated quartz crystal is placed in an ion pumped UHV system and evacu- ated to a pressure of below 10es torr at room temper- ature. All measurements and heat treatment are per- formed in this system. The amount of outgassing is determined from the change of frequency after baking. Baking the films in vacuum is usually per- formed at a constant temperature for one hour. A point-by-point measurement method is used for ob- taining adsorption isotherms to ensure that equilib- rium is reached under all conditions, especially for low-porosity films, because the adsorption can be very slow at low coverage for these films.

0

10-2 1

Molecular sieve effects in porous materials are usually studied by using gases of different molecular sizes. In this study, the organic gases benzene (C,H,) and 2,2-dimethylbutane (C,H,,) are used as adsorbates. These two gases have similar molecular masses but different mean molecular sizes (5.2A and 6.OA, respectively)[l3]. A freeze-thaw method was used to purify the adsorbates. All isotherms were measured at room temperature.

3. RESULTS AND DISCUSSION

Isotherms in Fig. 1 show effects due to molecular size of the adsorbates for the a-C : H-R films (baked at 350, 450, and 500°C). The relative adsorption (r) is defined as the ratio of the adsorbed mass on a film to the mass of the film after baking. Isotherms for the 350°C baked films exhibit a more pronounced molecular size dependence than for the 500°C baked films. The adsorption quantity of benzene at mono- layer coverage, corresponding to relative pressures approximately 0.2, is about three and two times that of the adsorption quantity of 2,2_dimethylbutane for the 350°C and 500°C baked films, respectively. How- ever, the isotherms for the 450°C baked a-C : H-R films show no evidence of a sieve effect.

t i

Relative Pressure

Fig. 1. Isotherm data for benzene and 2,2-dimethylbutane on the a-C : H-R films baked at (a) 350°C; (b) 450°C; and (c) 500°C. The relative adsorption (r) is defined as the ratio

of mass adsorbed to the mass of the baked films.

Significant molecular sieve effects were observed in isotherms for the a-C : H-S films as shown in Fig. 2, corresponding to the 350, 450, and 500°C baking conditions. The adsorption capacity of 2,2-dimeth- ylbutane adsorbate on the a-C : H-S films is about 10 times less than that of benzene in all cases.

In order to discuss the origins of the sieve effect, the isotherm data of benzene are replotted in Fig. 3 and Fig. 4 on a nonlogarithmic scale for the a-C : H- R and a-C : H-S films, respectively. The isotherm of benzene on the 350°C baked a-C : H-R films can be classified as Type I. The adsorption of benzene oc- curs mainly in the range of relative pressures below 0.2 (i.e., in the micropore adsorption range). This indicates that the micropore structure dominates the porosity of the 350°C baked films. However, the isotherm for the 450°C baked films should be classi- fied as pseudo-type II with a significant contribution from mesoporosity. The open knee (near a relative

pressure of 0.1) and the smooth mesopore adsorp- tion (in the range of relative pressures above 0.2) imply a wide range of pore sizes in the 450°C baked films. Following baking of the a-C : H-R films at 5OO”C, the mesoporosity is found to be less signifi- cant than in the 450°C baked films, whereas the microporosity increases.

A high level of mesoporosity will substantially reduce molecular sieve effects in the film as meso- pores can be filled by both the adsorbates. In addi- tion, it is possible that mesopores can enhance the accessibility of micropores in the film to both ad- sorbates. The 350°C baked a-C: H-R films, being predominantly microporous, would therefore be ex- pected to have obvious molecular sieve effects. The 450°C baked a-C :-H-R films have very broad pore size distribution and large mesopore volume; thus the sieve effect should not be significant. The de-

l 2,Pdimethylbutane

Carbon molecular sieve films produced by DC sputtering 1335

0 banzens l 2.2dimethylbutane

100

(c) 500°C baked

Relative Pressure

Fig. 2. Isotherm data for benzene and 2,2_dimethylbutane on the a-C : H-S films baked at (a) 350°C; (b) 450°C; and

(c) 500°C.

crease of the adsorption capacity of 2,2_dimethylbu- tane relative to that of benzene on the 500°C baked a-C : H-R films is probably due to the decrease of mesoporosity.

In contrast to the isotherms for benzene on the a-C : H-R films, Fig. 4 shows that the benzene iso- therms on the a-C : H-S films are all approximately Type I, indicating a typically microporous behav- iour for this material. Baking the films results in an increasing micropore content, but mesopores are not evident.

In addition to the microporous behaviour, an- other important factor influencing the sieve effect in this material is the size distribution of micropores. One of the approaches to the analysis of micropore structure was proposed by Dubinin and Radushkev- ich (DR)[14]. These workers showed empirically that the adsorption data was linearized by a plot of 1nN vs. A2, where N is the adsorption quantity, and A = RT ln(P,IP) is the adsorption potential. The

5 - o.4(o g l ._ 0.3 0

Gl

E m

z

0.2 4 0

.

? 0.1 q .I

c8

2 0.0 0.0

350°C baked ’ 450% baked

500°C baked

0 .

.

.

0

0

0

.

0

.

0.2 0.4 0.6 0.8 1 .o

0 0 l

.

1 0 0

Relative Pressure

Fig. 3. Replotted isotherms of benzene on the a-C : H-R films baked at different temperatures.

linearization occurs over a wide range of pressures at sub-monolayer coverages. The DR linearization can be interpreted as resulting from adsorption in micropores, having a gaussian distribution of pore

sizes. The form of the DR equation can be expressed as

N = N,exp(-&A’). (2)

where m is a characteristic parameter associated with adsorbent and adsorbate. and N, is the mono- layer capacity. Dubinin et rd. proposed[ 15,161 a rela- tion between the characteristic dimension of micro- pores, x, and the characteristic parameter. tn. for adsorption of benzene on active carbons:

x = Km(nm), (3)

where K = 13 (nm kJimo1) and m is in units of moli kJ[13].

Applying eqns (2) and (3) to our data, values for the characteristic parameter m and the characteris- tic size of micropores s obtained for a-C : H-S and a-C : H-R films are shown in Table I.

These estimates of the characteristic micropore size of the films provide further insight into the sieve

s 0.3 I ’ I ’ 0 350°C baked

0 0 0 0 0 0

. ’ . ’ . ’ * ’ . 0.2 0.4 0.6 0.8 1.0

Relative Pressure

Fig. 4. Replotted isotherms of benzene on the a-C : H-S films baked at different temperatures.

1336 Y. YIN and R. E. COLLINS

Table 1. Characteristic parameter m (in units of mol/kJ) and characteristic dimension of micropores x (in units of nm) of the DR equation for various isotherms on the a-C : H-R and a-C : H-S films

Sample

Baking temperature 350°C

a-C : H-R

450°C 500°C 350°C

a-C : H-S

450°C 500°C

m (mol/kJ) 0.063 0.06 0.051 0.049 0.045 0.045 x (nm) 0.82 0.78 0.65 0.63 0.58 0.58

effect in the a-C : H films. The characteristic micro- pore size of the a-C : H-S films is very close to the mean molecular size of 2,2_dimethylbutane (6.0 A), but significantly larger than the mean molecular size of benzene (5.2 b). Therefore, a large proportion of the micropores in the a-C : H-S films are not ac- cessible to 2,2_dimethylbutane molecules, resulting in the significant molecular sieve effect in the films. In contrast, the characteristic micropore size of the a-C : H-R films is always larger than the mean molec- ular size of both 2,2-dimethylbutane and benzene. Thus, it is not surprising that the a-C : Ii-R fifms display much smaller molecular sieve effects than the a-C : H-S films using these two adsorbates.

The mechanism of the porosity formation in the films produced by d.c. sputtering has also been stud- ied, and will be reported in detail in another paper. The as-deposited films are initially nonporous or slightly mesoporous. Heating the material in vac- uum, however, produces a highly porous material. The amount of porosity and the pore size distribu- tion are directly related to the quantity of material outgassed during heating. The outgassing material consists of hydrocarbons, as is found for conven- tional active carbons[17,18], and also, predomi- nately, carbon-oxygen compounds. Many of the properties of the a-C: H films are very similar to those exhibited by conventional activated carbons, and it is likely that the porous baked a-C : H film is a form of activated carbon.

This initial study on the molecular sieve effects in a-C : H films produced by d.c. sputtering indicates that this material could be useful for studies on selec- tive adsorption and gas permeation in carbon materi- als. The pore size distribution and porosity can be adjusted by varying the deposition and baking condi- tions. It appears to be possible, by using this sput- tering technique, to incorporate other elements, such as metals and oxygen, into the carbon film, resulting in changes of the adsorption properties and outgassing behaviour.

4. CONCLUSIONS

In summary, this paper has described some initial experimental results showing molecular sieve ef- fects in two types of a-C : H films deposited by using d.c. sputtering. We have shown that the sieve effect depends on both the deposition and heat treatment conditions, and have correlated this effect with the porosity and pore size distribution. Significant mo-

lecular sieve effects have been found in films sput- tered from a poisoned cathode. We conclude that these sieve effects arise from low mesoporosity in the material, a small characteristic micropore di- mension, and its relatively low porosity. We suggest that a-C : H films produced using d.c. sputtering may be useful for studies on selective adsorption and gas permeation of porous carbons.

Acknon,Iedgements-This project was supported by the Energy Research and Development Corporation, and by His Royal Highness Prince Nawaf Bin Abdul Aziz of the Kingdom of Saudi Arabia through the Science Foundation for Physics within the University of Sydney. Y. Yin ac- knowledges financial support from the Australia Interna- tional Development Abroad Bureau.

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