Structural characterization of krypton physisorption on (0001) graphite pre-plated with cyclohexane

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Structural characterization of krypton physisorption on

(0001) graphite pre-plated with cyclohexane

A. Razafitianamaharavo, P. Convert, J.P. Coulomb, B. Croset, N.

Dupont-Pavlovsky

To cite this version:

Structural

characterization

of

_krypton

_{physisorption}

on

(0001)

graphite

pre-plated

with

_cyclohexane

A. Razafitianamaharavo

_(1),

P. Convert

_(2),

J. P. Coulomb

_(3),

B. Croset

₍₄₎

and N.

_{Dupont-Pavlovsky}

₍₁₎

(1)

CNRS, Laboratoire Maurice Letort, 54600 Villers les

Nancy,

France

(2)

Institut Laue

_Langevin,

156 Centre de Tri, 38042 Grenoble Cedex, France

(3)

CRMC2,

Département

de

Physique,

Faculté des Sciences de

_Luminy,

Case 901, 13288 Marseille Cedex 09, France

(4)

Groupe

de

Physique

des Solides, Paris VII, 75251 Paris Cedex 05, France

(Received

on March _{12, 1990,}

_accepted

in

_{final form}

on

_May

_18,

₁₉₉₀₎

Résumé. 2014 Le film mixte formé lors de la

_{physisorption}

de

_krypton

sur une surface de

_graphite

(0001)

pré-recouverte

d’une monocouche de

cyclohexane

a été étudié par diffraction de neutrons et de rayons X.

L’organisation

de la couche mixte

prévue

à

partir

des résultats obtenus par volumétrie

_{d’adsorption}

sur une surface très uniforme

_{(graphite exfolié)}

est dans son ensemble

conservée sur les échantillons de

_graphite

exfolié

_{recomprimé (Papyex)}

utilisés pour les

expériences

de diffraction,

malgré

les difficultés d’atteinte de

l’équilibre d’adsorption

liées à la densité du

_{Papyex. L’adsorption}

de

_krypton

à 77 K provoque la

compression

de la couche de

cyclohexane pré-adsorbé jusqu’à

son état le

plus

dense,

puis

son

_{déplacement partiel}

et la formation de cristallites tridimensionnels. La fraction de

_cyclohexane

restant à l’état adsorbé est

impliquée

dans une solution bidimensionnelle avec le

krypton,

de structure commensurable

~3

x

~3.

Au-delà de la

_{première couche,}

le

_krypton

s’adsorbe sur lui-même.

Abstract. 2014 The structural characterization of the

_composite

film formed

_{by physisorption}

of

krypton

on

_{(0001) graphite pre-plated}

with a

_monolayer

of

cyclohexane

has been

_performed

by

neutron and

_X-ray

diffraction. The

_organization

of the

_composite

film

_{predicted by}

means of

volumetric measurements on a very uniform surface

_{(exfoliated graphite)}

is

_{mostly preserved}

on

compressed

exfoliated

_{graphite (Papyex)}

used for diffraction

_experiments

in

_spite

of enhanced difficulties in

_reaching

the

_{adsorption equilibrium, resulting}

from the

_{Papyex density. Krypton}

adsorption

at 77 K results in the

_compression

of the

_{pre-adsorbed cyclohexane layer}

to its densest state, followed

by

its

_{partial displacement}

and the formation of three-dimensional

_{crystallites.}

The fraction of

_{cyclohexane remaining}

in the adsorbed state is involved in a two-dimensional solution with

_krypton,

of

~3

x

~3

commensurate structure.

_Beyond

the first

_{layer, krypton}

adsorbs on

itself. Classification

Physics

Abstracts 61.10 2013 61.12 - 68.45

Introduction.

The

_study

of

_{two-component}

_physisorbed

films is one of the natural extensions of the

analysis

of pure two-dimensional

_(2D)

film

_properties

on uniform surfaces.

_{Dimensionality}

reduction

and substrate influence _may lead to an

_original

behaviour of these 2D

_composite

films,

1962

compared

to that of bulk

_{binary alloys.}

Within the last few years, structural characterizations

of these films have been carried out and in some cases 2D solid solutions have been evidenced

[1-7].

When the

_respective

_temperatures

of condensation of the two adsorbates are _very

different,

classical volumetric measurements can be

_applied

to the

_{thermodynamic}

characteri-zation of the

_composite

film

_by

means of the

preadsorbed layer technique,

which was

introduced

_{by Halsey}

and

_{Singleton [8],}

and extended later on

_[9-13].

The most condensable adsorbate is introduced first at

_{relatively high}

_temperature,

in order to control the

_adsorption

equilibrium ;

the

_sample

is then cooled down to the

_temperature

of the second

_component

adsorption.

At the

temperature

of the

_composite

film

_study,

the

_partial

pressure of the most

condensable adsorbate can be

_neglected

in the

_gas-phase,

and information obtained from the

comparison

of the less condensable

_component

_properties

on bare and

_{pre-plated graphite.}

This

_technique

was used for the last two

_systems

_studied,

_{Kr-SF6/graphite}

_[12]

and

Kr-C-,6H12/graphite

[13].

In both cases,

krypton adsorption

was shown to result in the

displacement

of the

_pre-adsorbed

film. A recent theoretical work has clarified these

phenomena [14].

We

_present

here a structural

_{investigation}

of the

_{Kr-C6H12/graphite}

_system

by

neutron and

X-ray

diffraction. Our results

_point

out clear evidence of the

_{C6HI2 layer}

displacement by krypton adsorption

and simultaneous formation of bulk

_{(3D) cyclohexane}

crystallites.

Before a detailed

_description

of the

_{experimental procedure}

and diffraction

results,

some information on the

_{adsorptive properties}

of pure

_krypton

and

_cyclohexane

and on the results of the

thermodynamic

characterization of the

Kr-C6H12/graphite

film is

presented.

1.

_{Adsorptive properties}

of _pure

_krypton

and

_cyclohexane

on

_graphite.

1.1 KRYPTON. -

_{Thermodynamical}

and structural

_{properties of pure krypton}

adsorbed on

(0001) graphite

have been

_extensively

studied;

(see,

for

instance,

[15-21]).

Discrete

_layers

may form on the

substrate,

whatever the film thickness.

The

_{krypton monolayer phase diagram}

is characterized

_by

a two-dimensional

(2D)

critical

temperature

estimated to be 86 K

_[16-19]

and

_by

a _very narrow

_[16]

or even inexistent

[18]

temperature

range for the coexistence of 2D gas and 2D

liquid.

At the

temperature

of the

composite

film

_{study (77 K), krypton}

condensation leads to a 2D commensurate solid with a

J3

x

J3

R30° structure, whose

X-ray

diffraction

_profile

exhibits a

_{(10) Bragg}

_peak

located at

Q

= 1.703

 -1.

Near the

monolayer completion,

this solid

changes

into an incommensurate

hexagonal

structure of

_{higher density [19-21].}

1.2 CYCLOHEXANE. -

_{Cyclohexane adsorption}

has been

_{investigated by}

isotherm

measure-ments

_[22-23],

and more

_recently

diffraction

_{techniques [24-25].}

Isotherms measured between 203 and 293 K exhibit two

_steps,

_{corresponding}

to the

_adsorption

of two

_monolayers

before

reaching

the

_saturating

_{vapour pressure. From the}

_temperature

_dependence

of the second

step

pressure, the second

layer

was estimated to _appear at 145 K. Below 145

K,

only

one

monolayer

is adsorbed before 3D condensation.

Depending

on _coverage, three solids may be observed at 77

_K,

whose structures are in

order of

_{increasing density,}

_hexagonal

commensurate

J7

x

J7

(a

= 6.51

Á),

hexagonal

incommensurate

_(a

= 6.35

Á),

and centered

rectangular

incommensurate

(a

= 5.63

Â,

b = 6.06

Â,

_{y =}

117.65’.

2.

_{Thermodynamic}

characterization of the

_composite

film.

The

_{thermodynamic}

characterization of

_{krypton adsorption}

on

(0001) graphite pre-plated

pre-adsorbed

_{layer technique}

mentioned in the introduction

_{[13, 25]. Cyclohexane}

was adsorbed

at 233

_K,

in order to enable the

_equilibrium

pressure control. The

sample

was then cooled

down to the

_temperature

of

_{krypton introduction,}

and isotherm measurements

_performed

at

77 K. At this

_temperature,

the

_partial

_pressure of

_C6Hi2

in the gas

phase

is very low

(about

10-15

_torr)

and can be

_neglected.

When the

_graphite

surface is

_pre-covered

with a

monolayer

of

_cyclohexane,

it was shown that

_{krypton adsorption}

becomes

important

under pressures

definitely higher

than on bare

_graphite.

From the

_comparison

of

krypton

adsorption

isotherms on bare

_graphite

and on

_{cyclohexane pre-plated graphite, krypton adsorption}

was

suggested

to result in a

partial displacement

of the

_cyclohexane

film,

followed

by krypton

condensation on itself.

_{Displaced cyclohexane}

was assumed to form 3D

_{crystallites.}

The fraction of 2D

_{cyclohexane remaining}

on the surface was

_suggested

to be involved in a 2D

krypton-cyclohexane

solution.

3.

_{Expérimental}

details.

3.1 APPARATUS. - The structural characterization was

_{performed by}

neutron and

_X-ray

diffraction. The neutron diffraction

_experiments

were carried out at the Laue

_Langevin

Institute on the D 1 B

_{spectrometer.}

The

_X-ray

diffraction

experiments

were carried out at

the G.P.S.

_laboratory.

Both diffraction

_experiments

have been described elsewhere

_[25-26].

3.2 ADSORBATES. -

_Krypton

of 99.94 %

_purity

was

_{supplied by}

« l’Air

_{liquide ».}

Cyc-lohexane was a Merck

_product

of 99.7 %

_purity.

The adsorbates were

purified

before each

experiment by pumping

on the condensed

_phase

inside the introduction

system

at 77 K for

krypton

and 193 K

_(dry

ice

_temperature)

for

_cyclohexane.

3.3 SUBSTRATES. - Whereas the

thermodynamic

characterization was

_performed

on

exfoliated

_graphite,

which exhibits a _{very uniform}

_surface,

diffraction

_experiments

were

performed

on

_{Papyex, supplied by}

the firm « Le Carbone Lorraine ». It is a

_compressed

exfoliated

_graphite

with a

_preferential

orientation of the six-fold axis. Two

_samples

were used.

One was

_compressed

to 1.1

_g.cm-3,

_having

a

_specific

surface area estimated to be 20

_m2.g-l

and a mosaic

_spread

of 38°

_(full

width at half

_maximum).

The other was

_compressed

to

0.1

_g.cm-3,

with a 40

_m2.g-1

_specific

surface area and mosaic

_spread

of 74°

_[27].

Compression

of exfoliated

_graphite

results in an increase of the surface area _{per volume}

unit and a

_preferential

orientation of the six fold

_axis,

which makes it more suitable than

uncompressed

exfoliated

_graphite

for diffraction

_experiments,

but it also results in a decrease

of the surface

_{uniformity [27-28].}

The main modifications in the

_composite

film

_formation,

compared

to that observed on exfoliated

_graphite,

are then extra

_capillary

condensation and

enhanced

_difficulty

in

_reaching

the

_equilibrium

after

_krypton

introduction.

_Krypton

adsorp-tion isotherms at 77 K on

_{cyclohexane pre-plated graphite presented}

in

_figure

1 are measured

on the same surface area for each of the three substrates

considered,

and under the same

conditions of film formation. Their main différence lies in the

_height

of the first

_step,

which is

lower on

_Papyex,

and decreases with

_{increasing density}

of the

_sample.

But on the

_whole,

the

properties

of the

_{(cyclohexane-krypton)}

films formed on exfoliated

_graphite

and on

_Papyex

respectively

appear close

enough

to enable the

_comparison

of results obtained on both kinds

of

_samples.

1964

Fig.

1. -

Adsorption

isotherms of

cyclohexane

on

_graphite,

and

_krypton

on

_{cyclohexane-pre-plated}

graphite. a) Adsorption

of

_cyclohexane

on

_graphite

at 233 K. Schematic

_{drawing presented}

in order to

define the amount of

_{cyclohexane corresponding}

to a

_{monolayer. (J}

=

cyclohexane

fractional coverage.

This definition of the

_monolayer

was

_adopted

in the course _{of volumetric measurements, before the}

structural

_{investigation.}

It does not

_correspond

to the densest state of the solid

_monolayer,

as was shown

afterwards,

by

the diffraction

_{experiments [24]. b) Adsorption}

of

_krypton

at 77 K on

_graphite

_pre-plated

with a

_monolayer

of

_{cyclohexane. Dependence}

of the isotherm

shape

on the substrate :

1) adsorption

on

exfoliated

_{graphite ; 2) adsorption}

on 0.1

_{density Papyex ; 3) adsorption}

on 1.1

_{density Papyex ;}

P : pressure ; 0 k, : krypton fractional _{coverage ; 0} = 1

corresponds

_{to a}

monolayer

_on bare

graphite.

In the

following figures, describing

the

composite

film, the

_{krypton monolayer}

will be defined as the _top of the first step of the

_composite

film isotherm for each kind of

_sample.

3.4 ADSORPTION PROCEDURE. - The

_experimental

conditions for the

_composite

film

formation are

_exactly

the same as those of the

_{thermodynamic}

characterization

_[13].

A

cyclohexane

amount

_{corresponding}

to the

_adsorption

of a

_monolayer,

as defined in

_figure

_la,

is introduced at 233 K. The

_sample

is then

_slowly

cooled

_{(0.5 degree}

per

minute),

in order to avoid any distillation on the cell

_walls,

down to 150

K,

which is the

temperature

of

_krypton

introduction.

_Spectra

are measured at 77

K,

but the

_sample

is annealed at 150 K for each

krypton

introduction,

in order to increase the

_mobility

of the adsorbates

_during

the

_composite

film

_{organization.}

The

_krypton

_{coverage which is assumed} to

_correspond

to a

_monolayer

in the

_composite

film

is defined as the

_top

of the first

_step

of the

_composite

film isotherm for each kind of

_sample.

4. Results.

Neutron and

_X-ray

diffraction are two

_{complementary techniques}

for the

_study

of the

(krypton-cyclohexane)

films adsorbed on

_graphite,

since neutrons are

_strongly

scattered

_by

cyclohexane

at least in its deuterated form

_C6D12,

and

_{weakly by krypton,}

whereas for

modification after

krypton adsorption

were then examined

by

neutron diffraction.

Krypton

patterns

were obtained

by X-ray

diffraction.

4.1 NEUTRON DIFFRACTION EXPERIMENTS -

_C6DI2

PROFILES. - The

spectra

in

_figure

2

are

_presented

in order to show the

_{reversibility}

of the

_composite

film formation. The evolution of a

_{monolayer C6D12 profile}

in the _presence of a

_{monolayer krypton}

amount is examined as a function of

_temperature,

which is varied in the course of time. Each line

represents

a different

_spectrum.

In the wave vector _range

_considered,

_{the pure}

_C6D12

_profile

is characterized

_by

one

_peak,

observed at

_{150 K,}

no

_{krypton adsorption occurring}

at this

temperature.

If the

_system

is cooled down to 77

K,

krypton adsorbs,

and the

_spectrum

is

strongly modified,

with two new

peaks arising.

The _pure

C6D12

profile

is restored

by heating

again

the

_sample

up to 150 K and

_{desorbing krypton.}

Fig.

2. - Neutron diffraction

_profiles

of a

_{cyclohexane monolayer}

adsorbed on 0.1

_{density Papyex}

in the presence of

krypton. Dependence

of the

profile shape

on the _temperature, i.e. on

_krypton

adsorption.

Int. :

_intensity,

in

_arbitrary

_{units ;}

_{Q :}

wave vector. Each line _represents a different

spectrum. The time between two _spectra is 30 min.

Spectra

measured at 77 K for a

_larger

wave vector _range than in

_figure

_2,

and for different

krypton

coverages are shown in

_figure

_3,

as well as pure

C6D12 profiles

for the

_hexagonal

incommensurate and centered

_rectangular

structures

_{(the profile}

of the centered

_rectangular

structure is obtained

_{by subtracting}

the 3D contribution to the

_spectrum

obtained with a 1.3

layer

film adsorbed at 77

_K).

A

_spectrum

obtained when

_introducing

a

_C6D12

amount

equivalent

to fifteen

_layers

in contact with the

_sample

is

_{representative}

of the 3D structure at

this

_temperature

_{(Fig. 3e).}

Before

_{krypton adsorption,}

the

_{C6D12 profile}

is that of

_figure

3a. A

_significant

modification of the

C6D12

structure occurs even when a 0.5

_{krypton monolayer}

is adsorbed

_{(Fig. 3c).}

Numerous new

_{Bragg peaks}

_{appear, whose} relative

_intensity

1966

Fig.

3. - Neutron diffraction

profiles

of films adsorbed on 0.1

_{density Papyex}

at 77 K : a) pure

C6H 12,

hexagonal

incommensurate structure ;

b) pure

C6H12,

centered

rectangular

structure ;

c) composite

film,

0.5

_{krypton monolayer}

on a

_{C6H12 layer ; d)}

_composite

film, 1.2

_{krypton monolayer}

on a

_C6H12

layer ; e) pure C6H 12,

fifteen

layers,

this

profile

is

_{representative}

of the 3D

C6H12

structure ;

Q :

wave

vector.

_Bragg

_peaks,

which are hatched and marked

_by

an arrow in solid line, increase in

_intensity

with

increasing krypton

amount in the

composite

film

_(Fig.

c and

_{d). They}

all appear in the pure

C6HI2

3D

profile

(Fig. e).

The

_Bragg

_peak,

which is marked

_by

an arrow in dotted _line, decreases in

intensity

with

increasing krypton

amount in the

composite

film

_(Fig.

c and

d).

This

_peak

also appears in the pure

C6H12 profile corresponding

to the centered

_rectangular

structure

_{(Fig. b).}

-

some of

_them,

marked

_by

arrows in

_figures

3c and

_{3d, clearly}

increase with

increasing

krypton

coverage.

They

are located at

_{1.62, 1.66, 1.93,}

2.27

Â-1.

_They

all

_belong

also to the 3D

C6DI2

profile

shown in

_figure

3e.

_Moreover,

with

_{increasing krypton}

coverage, the

spectrum

of

_C6DI2

involved in the

_composite

film becomes closer and closer to that of _pure

3D

_C6DI2

in accordance with the

_assumption

of 3D

_C6D12

formation

_{produced by krypton}

adsorption ;

- _a

Bragg

peak

marked

_by

a dotted arrow, and located at 2.1

Â-1,

can be

_{distinguished}

in

the two

_composite

film

_spectra.

Its

_intensity

decreases with

_{increasing krypton}

coverage. This

corresponding

to the densest state of the

_{monomolecular}

_C6D12

film

_{(Fig. 3b) ;}

it seems then

that

_{krypton adsorption}

results first in a

compression

of the

_C6D12

film,

which is

displaced

in a second

_step

and forms 3D

_{crystallites ;}

- the

remaining Bragg peaks

of the

_composite

film _spectra, located at

_1.17,

1.26 and 2.36

_Â-1,

may

belong

to

_spectra

of both 3D

C6D,2

and 2D

_C6DI2

with centered

_rectangular

structure. Their relative

_intensity

does not

_{change significantly}

with

_krypton

_coverage. This behaviour can be

_assigned

to a balance between the

_displacement

of 2D

_C6D12

with

rectangular

structure and the

_growth

of 3D

_{C6DI2 crystallites}

with an

_increasing

amount of adsorbed

_krypton.

The

_displacement

of

_{pre-adsorbed cyclohexane by}

_{krypton adsorption,}

and the formation of 3D

_{cyclohexane crystallites,}

which were assumed after the volumetric measurements, are

then

_clearly

demonstrated.

_Moreover,

the

_compression

of the 2D

_cyclohexane

film from a

hexagonal

incommensurate to a centered

rectangular

_structure,

occurring

in the initial

_stages

of

_{krypton adsorption,}

is also evidenced. It should be noticed that the 2D

_{compressed C6D12}

structure can still be

_{distinguished}

after admission of 1.2

_{krypton monolayer.}

This observation is

_{quite unexpected,}

since a 2D

_cyclohexane

_{displacement, according}

to a first order transition

process should have resulted in a

_complete

_removing

of the

_preadsorbed

gas after admission of more than one

_{krypton monolayer.}

_But,

as has

already

been

emphazised

above,

it is much more difficult and time

_consuming

to reach the

_equilibrium

state on

_Papyex,

used for diffraction

_experiments,

than on exfoliated

_graphite,

used for the volumetric characterization. 2D

_cyclohexane

with a dense structure which remains on the surface could be a

non-equilibrium

manifestation.

4.2 X-RAY DIFFRACTION EXPERIMENTS - KRYPTON PROFILES.

_{- X-ray}

diffraction

_spectra

shown in

_figure

4 are

_{mostly representative}

of

_krypton

involved in the

_composite

film. The unusual

_shape

of these 2D

_peaks

is

_mainly

due to the

_vicinity

of the

_{(002) graphite Bragg}

peak.

More

_surprising

is the

_shape

of the

_low-Q

side of the

_{krypton Bragg peak}

in

_figure

4c. We have no clear

_explanation

for

it,

but it should be noticed that 3D

_cyclohexane

exhibits two

Bragg

peaks

located at

_Q

= 1.6 and

Q

=

1. 68 Â

respectively,

which could alter the

shape

of the

_{krypton Bragg peak.}

The

_Bragg

_peaks

observed for

_krypton

coverages below two

_layers

are located at

Q

=

1. 70 Â -1.

This _wave-vector value is

representative

of a

J3 x J3

structure, whose stabilization

_during

the

_cyclohexane

film

_{displacement,}

which was

_{predicted by}

the volumetric

characterization,

is then confirmed. This stabilization is observed at an

_equilibrium

_pressure

much

_higher

_{than that of the pure film} commensurate-incommensurate transition at the same

temperature,

near the

_{monolayer completion.}

This is a serious indication in favour of the

existence of a 2D

_{binary solution,}

as has

_already

been observed in the case of other

_binary

mixtures adsorbed on

_graphite.

Whenever a commensurate structure exists in the first

_layer,

it is stabilized over a

_larger

_range of

_temperatures

_{and pressures}

_by

dissolution of

_larger

molecules

_[3,

_6,

_11,

_32].

When the coadsorbed _gas is insoluble in the

_{krypton film,}

the commensurate-incommensurate 2D solid transition is

_observed,

as, for

instance,

for the

D2/Kr

system

[33].

D2

was then used as a source of 2D _pressure to

_push

a commensurate

krypton layer

to a denser incommensurate structure. When two

_layers

of

_krypton

are

adsorbed on

_{cyclohexane pre-plated graphite (Fig. 4c),}

the

_{Bragg peak}

shifts towards

larger

wave vectors, and its

_position

is that of the usual _pure

_krypton

_{incommensurate structure, in}

accordance with the

_assumption

of

_{krypton adsorbing}

on itself after

_cyclohexane

1968

- % - 1

Fig.

4. -

X-ray

diffraction

_profiles

of

composite

films adsorbed on 1.1

_{density Papyex}

at 77 K.

Adsorption

on one

_{C6H12 monolayer}

of

_a)

0.8

_{krypton monolayer ; b)}

1

_{krypton monolayer ;}

c)

2

_{krypton monolayers.}

Dashed lines on the abscissa

_correspond

to the

_position

of the most intense substrate

peaks.

Conclusion.

In

_conclusion,

the structural characterization of the

_composite

film formed

_by

the

_adsorption

of

_krypton

on

_{cyclohexane pre-plated graphite}

corroborates the

_{assumptions proposed by}

A. Razafitianamaharavo et al.

_[13]

after their

_{volumetriq studies,}

and

_gives

additional

information.

_{Krypton adsorption}

at 77 K results in the

_compression

of a

_pre-adsorbed

cyclohexane layer

to its densest state, followed

_by

its

_{partial displacement}

and the formation of 3D

_{crystallites.}

The fraction of

_{cyclohexane remaining}

in the adsorbed film is involved in a

2D

_{(krypton-cyclohexane)}

solution with a commensurate

J3

x

à

structure. For coverages

higher

than one

_{monolayer, krypton}

is adsorbed on itself with an incommensurate

_hexagonal

structure.

B

Acknowledgments..

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