- Email: [email protected]
Properties of arsenic in molten potassium tetrachlorogallate at 300°C
J: a’+ecrd Chem,. 139 (1982) 329-333 e - E’rinwdin l-he Nelhe&nds Eke+& SequoiySA. La-
329
.., ._ 2.
,.:
a.,-_
.&&Eiiti OF AR&N&N MOLTEN T-EIIZA~~ROG~TE AT 300~~. -. iOIiMATI&Oti
IBRA GUEYE Labomroire Chimie
-GALLIUM
DIOUM.
JACQUES
d’Elec#rochimie
de +nk.
ARSENIDE
VEDEL and BERNARD
Anolyrique
II. rue Pierre
POTASSWM
ef AppliqGe
er Marie
Curie,
Asso&
75005. Paris
TREMILLON au CNRS,
EC&
Nationale
Sup&ieure
de
(France)
(Received 8th February 1982; in revised form 24th March 1982)
Redox properties of As(III) species have been studied in molten KGaCI,. potential of As vs acidity is intcrpretcd in terms of reaction AsCI, +3 e =As+4
The plot of equilibrium CT. X-Ray analysis of
electrodeposits shows that only As is formed under potenliostadc conditions. Conversely, gallium arsenide is observed under galvanosraric conditions. for which rhe elccrrode porenrial becomes very negative.
INTRODUCTION
Some binary semiconductor compounds can be prepared by electrodeposition. The synthesis of II-VI temperature,
compounds
in an aqueous
is rekively
electrolyte
[l-5].
e.zsy and can be achieved
The synthesis
of III-V
at room
compounds,
which have a more marked covalent character, is more difficult and needs higher temperatures and, consequently, the use of a molten salt as electrolyte:gallium phosphide
has been electrodeposited
metaphosphate,
sodium
fluoride
at 800-9OWC.
and gallium
in a melt containing
oxide [6]; gallium
arsenide
sodium has been
synthesized at 720-760°C in molten sodium fluoride and boric oxide, containing sodium arsenite and gallium oxide 171.The high temperatures used are partly due to the necessity potassium
of dissolving
tetrachlorogallate
is also a Cl-
iori donor
a non-volatile
gallium
allows the preparation
[8], permitting
donor
(gallium
of gallium
the dissoluti&,
oxide).
chloride
in an anionic
Molten
solutions_ It form,
of a
conipound donor of the element one wishes to combine tiith gallium, which is in this case arsenic. The present article is devoted to the descriptiqn of what we have done
to electrodeposit gallium arsenide in molten KGaCl, temperature (300°C).
in order
at relatively low
EXPERIMENTAL Gakun trichloride synthesis, KGaCl, formation, the electrochemical cell and the reference ele&ode (Ag/AgCl) have been described + a previous communication [8]_ 0022~72E/8~/ooaooooO/~~~~
6 1982 Hsevier Sequoia SA.
.
330
Arsenic (III) was introduced as arsenic triodide (AsI,)_ Addition of arsenic frichb ride does not change the current voltage curves. This is due to the AsCl, vol&ility (Th = 130.2OC), which evaporates before being solvated by the melt. The higher boiling point of AsI, (403°C) allows the introduction of greater amounts~of &se&. After addition, one cathodic and two anodic waves are observed. The latter correspond to the oxidation of the I- anion [9], which can be elimirated by a stream of oxygen. After this process, only the cathodic wave remains, the position of which did not change as a result of chemical oxidation_ Current-potential curves were obtained using either gold or vitreous carbon electrodes. Electrodeposits were achieved: either potentiostatically or intentiostatically, on gold electroplated nickel electrodes. Deposited materials were separated from the substrate. reduced to powder, washed with water and acetone, and then analyzed by X-ray diffractometry. Silver powder was used as internal standard_ RESULTS
AND
DISCUSSION
The cm-rem-voltage curves obtained with As(lI1) solutions depend on the solute concentration. At low concentration (less than IO-’ mol kg-‘), only one cathodic wave is observed, the height of which is proportional to the AsIj concentration (Fig. 1). The position of the curve depends on the solvent acidity. In btiic medium, the amplitude of the wave remains practically constant over time, and in acidic medium, it gradually decreases_ The shift of thp_As(II1) wave with acidity is less pronounced than that of the solvent reduction curve. In Fig.2 is plotted the variation of the equilibrium potential of an arsenic electrode. in equilibrium with a lo-* mol kg-’ As(II1) solution, vs. the melt acidity. The slope of the straight line is equal to 0.16 V. This variation corresponds to the following equilibrium: AsCl,+3e-As-t4Cl-
-0.4
(1)
0
CJ potential
/V
Fig I. Voltetric curve for dilute solution (IO-* sanuawd); (2) acidic medium (1=300T).
mol kg-’ ) of As(llI).
(I)
Basic medium (KC1
..~ 331
> \
0.6
2
Fig. 2. variation As(III)
solution
of
potential
pCI = -:Og m(CI-)
vs. acidity of an ancnic
electmdc.
in equilibrium
with a -IO-’
mol kg-’
(I =3ClO”C).
for which the equilibrium potential is related to pC1 by equation: ~5~ = EzS + 4(2.3 RT/F)/3pCl=
RzS + 0.152 pC1
(2)
In acidic medium, the tetrachloroarsenate(II1) ion (A&l, neutrabzcd, giving AsCl, which slowly distills: AsCI,
+ Ga,Cl,
- AsCl,(g)
) is presumably partly
+ 2 GaCl,
(3)
At high As(W) concentrations (greater than O-2 mol kg-‘), several reduction waves are observed, both in acidic and basic solution, irrespective of the electrode material (Fig_ 3). This modification indicates the formation of different species. Electrodeposits-made potentiostadcally at -0.05, -0-2, -0.5 and -0.65V US Ag/AgCl, in basic solution, and at O-0 V in acidic solution-were analyzed by X-ray diffractomeuy_ The diffractograms contain only the lines of arsenic, of one of its allotropic varieties, arsenolamprite, and those of some unidentified species, presuma-
“: E
0
a
T-f&6 ,”
c" -1.2 s tE E-l.8 5 "
-2.0 -0.4
Fig 3. Vollammetric
-0.2
0
0.2 potential
curve for rnUoentrsld
0.4 /V dulion
of
As(w
(0.2M). Basic medium, (I
= 3ooT).
bly
different
arsenide
allotropic
forms
of
arsenic.
In
potentiostatic
conditions,
galliuk-
lines were never observed.
In galvanostatic conditions, deposition was carried out with a current. density to -3 V-and. equal to 0.1 A cmm2. In basic medium the electrode potential deem in acidic medium, to - 1 V (Fig. 4)_ This time, X-ray diffr&iomet& shows -the presence
of GaAs,
in addition
to the two arsenic forms
pr&iou~ly
obse&d,
and
other lines, some of which are present in both kinds of electrodeposits. _ Induced electrcxleposits are those for which there is~ codeposition af two (or several) elements forming a defined compound, the formation energy of which permits the deposit of the less noble constituent at a potential higher than that at which it forms when alone [IO]. This fact is illustrated by the emf qf some galvanic cells, estimated using the free energy of formation of pure substances (the formation energies used were interpolated
In KGaCl, melt, Ga(IiL) emf of the galvanic cell:
from tables in references [ 1 I] and [ 121).
is not reduced
to Ga(O), but Ga(I)
[8]. Effstively,
the
Ga/GaCI/GaCl, to which corresponds 2 Ga + GaCl,
-
the reaction:
3 GaCl
--__ -_ E
is equal to +O.G86 V. The emf of the cell:
Ga/GaCl.
AsCl ,/GaAs
corresponding
to reaction:
4 Ga + AsCI,
= GaAs + 3 GaCl
is equal to 0.348 V. Then AsGa formation can be achieved at a higher potential than the Ga(ZI1) to Ga(I) reduction, allowing its electrodeposition in molten KGaCl..
O ._
>
\
m
-.
L
5-l
--_
i
___
I
-z
G
-3
-5
c 1 /
-2
0
-r-
1
q
1UC
200
300 .
Fig. 4. Variation of potential vs. time of an clestrodc submitted to constant Basic medium;
(2) acidic medium.
Current
denshy:
[email protected] cm-*
current whodic (I =XWC).
reduction
(I)
.:
-.
.-~ ~..
.._
333
-.
Howkvei, t& ~‘dk-iiation ok the -emf. ta@$ into socount neither the mixing or soltia_tioti.eqergir of the %pecis with KCI, ,nor.the-kinetics of-the deposition rextic+. Tk@Itintrary to the rough estimate, only keni? is.dep+ted tit electrode potentials inch&d in the +)Jvent electroatitivity range. Ch~the other hand, GaAs formation taJ&@a& at.vew negative values of .electrode potent@. This suggests thai a high a&$atjon &ergy is. necessary to form the compound. Although the increase of acidity actifavorably ori the kinetics of the formation reaction. it is not sufficient for the preparation. of gallium arsenide of useful quality, particularly free of arsenic. REFERENCES 1 E!. Gobrccht, 2 J. Mana~xn, 3 4 5 6 7 8 9 IO II
H.-D. Licsr and A. Tauund_ l&r_ Bunscngcs. Phys. Chem.. 67 (1963) G. Hodcs and D. Cahaq J. Watrochem. Sot.. 124 (1977) 532.
930.
i.M. P&r, Watrochim. Act& 23 (1978) 165. M-P+ Panicker, M, Knauer and FA. Krager. J. Wcctrochcm. Sot., 125 (1978) 556. L.M. Pcter,~ J. Wectroanal. Cbcm.. 98 (1979) 49. JJ. Cuomo and RI. Gambino, J. Elcctrochcrn. Sot.. I IS (1968) 765. RC. de Mattci. D. Wwell and RS. Fcigclson. J. Ctyst. Growth, 43 (1978) 643. LG. Dioum, J. Vedcl and B. Tr&nilloo. I. Wectroanal. Chcm., 137 (1982) 219. LG. Dioum, J. Vcdcl and B. Ttimillon. J. Wcctroanal. Chem.. I39 (1982) 323. FA_ KrBgcr. J. Elcctrochcm. Sot.. I25 jl978) 2028. I. Barin and 0. Knackc, Thermodynamical Propcriics of Inorganic Substances, Springer Verlag. Berlin. 1973. I2 I. Barin. O_ Knacke and 0. Kubaschewski. Tbermodynamical Properties ol Inorganic Substances. Supplement. Springer Verlag_ Ekrlin. 1978.
329
.., ._ 2.
,.:
a.,-_
.&&Eiiti OF AR&N&N MOLTEN T-EIIZA~~ROG~TE AT 300~~. -. iOIiMATI&Oti
IBRA GUEYE Labomroire Chimie
-GALLIUM
DIOUM.
JACQUES
d’Elec#rochimie
de +nk.
ARSENIDE
VEDEL and BERNARD
Anolyrique
II. rue Pierre
POTASSWM
ef AppliqGe
er Marie
Curie,
Asso&
75005. Paris
TREMILLON au CNRS,
EC&
Nationale
Sup&ieure
de
(France)
(Received 8th February 1982; in revised form 24th March 1982)
Redox properties of As(III) species have been studied in molten KGaCI,. potential of As vs acidity is intcrpretcd in terms of reaction AsCI, +3 e =As+4
The plot of equilibrium CT. X-Ray analysis of
electrodeposits shows that only As is formed under potenliostadc conditions. Conversely, gallium arsenide is observed under galvanosraric conditions. for which rhe elccrrode porenrial becomes very negative.
INTRODUCTION
Some binary semiconductor compounds can be prepared by electrodeposition. The synthesis of II-VI temperature,
compounds
in an aqueous
is rekively
electrolyte
[l-5].
e.zsy and can be achieved
The synthesis
of III-V
at room
compounds,
which have a more marked covalent character, is more difficult and needs higher temperatures and, consequently, the use of a molten salt as electrolyte:gallium phosphide
has been electrodeposited
metaphosphate,
sodium
fluoride
at 800-9OWC.
and gallium
in a melt containing
oxide [6]; gallium
arsenide
sodium has been
synthesized at 720-760°C in molten sodium fluoride and boric oxide, containing sodium arsenite and gallium oxide 171.The high temperatures used are partly due to the necessity potassium
of dissolving
tetrachlorogallate
is also a Cl-
iori donor
a non-volatile
gallium
allows the preparation
[8], permitting
donor
(gallium
of gallium
the dissoluti&,
oxide).
chloride
in an anionic
Molten
solutions_ It form,
of a
conipound donor of the element one wishes to combine tiith gallium, which is in this case arsenic. The present article is devoted to the descriptiqn of what we have done
to electrodeposit gallium arsenide in molten KGaCl, temperature (300°C).
in order
at relatively low
EXPERIMENTAL Gakun trichloride synthesis, KGaCl, formation, the electrochemical cell and the reference ele&ode (Ag/AgCl) have been described + a previous communication [8]_ 0022~72E/8~/ooaooooO/~~~~
6 1982 Hsevier Sequoia SA.
.
330
Arsenic (III) was introduced as arsenic triodide (AsI,)_ Addition of arsenic frichb ride does not change the current voltage curves. This is due to the AsCl, vol&ility (Th = 130.2OC), which evaporates before being solvated by the melt. The higher boiling point of AsI, (403°C) allows the introduction of greater amounts~of &se&. After addition, one cathodic and two anodic waves are observed. The latter correspond to the oxidation of the I- anion [9], which can be elimirated by a stream of oxygen. After this process, only the cathodic wave remains, the position of which did not change as a result of chemical oxidation_ Current-potential curves were obtained using either gold or vitreous carbon electrodes. Electrodeposits were achieved: either potentiostatically or intentiostatically, on gold electroplated nickel electrodes. Deposited materials were separated from the substrate. reduced to powder, washed with water and acetone, and then analyzed by X-ray diffractometry. Silver powder was used as internal standard_ RESULTS
AND
DISCUSSION
The cm-rem-voltage curves obtained with As(lI1) solutions depend on the solute concentration. At low concentration (less than IO-’ mol kg-‘), only one cathodic wave is observed, the height of which is proportional to the AsIj concentration (Fig. 1). The position of the curve depends on the solvent acidity. In btiic medium, the amplitude of the wave remains practically constant over time, and in acidic medium, it gradually decreases_ The shift of thp_As(II1) wave with acidity is less pronounced than that of the solvent reduction curve. In Fig.2 is plotted the variation of the equilibrium potential of an arsenic electrode. in equilibrium with a lo-* mol kg-’ As(II1) solution, vs. the melt acidity. The slope of the straight line is equal to 0.16 V. This variation corresponds to the following equilibrium: AsCl,+3e-As-t4Cl-
-0.4
(1)
0
CJ potential
/V
Fig I. Voltetric curve for dilute solution (IO-* sanuawd); (2) acidic medium (1=300T).
mol kg-’ ) of As(llI).
(I)
Basic medium (KC1
..~ 331
> \
0.6
2
Fig. 2. variation As(III)
solution
of
potential
pCI = -:Og m(CI-)
vs. acidity of an ancnic
electmdc.
in equilibrium
with a -IO-’
mol kg-’
(I =3ClO”C).
for which the equilibrium potential is related to pC1 by equation: ~5~ = EzS + 4(2.3 RT/F)/3pCl=
RzS + 0.152 pC1
(2)
In acidic medium, the tetrachloroarsenate(II1) ion (A&l, neutrabzcd, giving AsCl, which slowly distills: AsCI,
+ Ga,Cl,
- AsCl,(g)
) is presumably partly
+ 2 GaCl,
(3)
At high As(W) concentrations (greater than O-2 mol kg-‘), several reduction waves are observed, both in acidic and basic solution, irrespective of the electrode material (Fig_ 3). This modification indicates the formation of different species. Electrodeposits-made potentiostadcally at -0.05, -0-2, -0.5 and -0.65V US Ag/AgCl, in basic solution, and at O-0 V in acidic solution-were analyzed by X-ray diffractomeuy_ The diffractograms contain only the lines of arsenic, of one of its allotropic varieties, arsenolamprite, and those of some unidentified species, presuma-
“: E
0
a
T-f&6 ,”
c" -1.2 s tE E-l.8 5 "
-2.0 -0.4
Fig 3. Vollammetric
-0.2
0
0.2 potential
curve for rnUoentrsld
0.4 /V dulion
of
As(w
(0.2M). Basic medium, (I
= 3ooT).
bly
different
arsenide
allotropic
forms
of
arsenic.
In
potentiostatic
conditions,
galliuk-
lines were never observed.
In galvanostatic conditions, deposition was carried out with a current. density to -3 V-and. equal to 0.1 A cmm2. In basic medium the electrode potential deem in acidic medium, to - 1 V (Fig. 4)_ This time, X-ray diffr&iomet& shows -the presence
of GaAs,
in addition
to the two arsenic forms
pr&iou~ly
obse&d,
and
other lines, some of which are present in both kinds of electrodeposits. _ Induced electrcxleposits are those for which there is~ codeposition af two (or several) elements forming a defined compound, the formation energy of which permits the deposit of the less noble constituent at a potential higher than that at which it forms when alone [IO]. This fact is illustrated by the emf qf some galvanic cells, estimated using the free energy of formation of pure substances (the formation energies used were interpolated
In KGaCl, melt, Ga(IiL) emf of the galvanic cell:
from tables in references [ 1 I] and [ 121).
is not reduced
to Ga(O), but Ga(I)
[8]. Effstively,
the
Ga/GaCI/GaCl, to which corresponds 2 Ga + GaCl,
-
the reaction:
3 GaCl
--__ -_ E
is equal to +O.G86 V. The emf of the cell:
Ga/GaCl.
AsCl ,/GaAs
corresponding
to reaction:
4 Ga + AsCI,
= GaAs + 3 GaCl
is equal to 0.348 V. Then AsGa formation can be achieved at a higher potential than the Ga(ZI1) to Ga(I) reduction, allowing its electrodeposition in molten KGaCl..
O ._
>
\
m
-.
L
5-l
--_
i
___
I
-z
G
-3
-5
c 1 /
-2
0
-r-
1
q
1UC
200
300 .
Fig. 4. Variation of potential vs. time of an clestrodc submitted to constant Basic medium;
(2) acidic medium.
Current
denshy:
[email protected] cm-*
current whodic (I =XWC).
reduction
(I)
.:
-.
.-~ ~..
.._
333
-.
Howkvei, t& ~‘dk-iiation ok the -emf. ta@$ into socount neither the mixing or soltia_tioti.eqergir of the %pecis with KCI, ,nor.the-kinetics of-the deposition rextic+. Tk@Itintrary to the rough estimate, only keni? is.dep+ted tit electrode potentials inch&d in the +)Jvent electroatitivity range. Ch~the other hand, GaAs formation taJ&@a& at.vew negative values of .electrode potent@. This suggests thai a high a&$atjon &ergy is. necessary to form the compound. Although the increase of acidity actifavorably ori the kinetics of the formation reaction. it is not sufficient for the preparation. of gallium arsenide of useful quality, particularly free of arsenic. REFERENCES 1 E!. Gobrccht, 2 J. Mana~xn, 3 4 5 6 7 8 9 IO II
H.-D. Licsr and A. Tauund_ l&r_ Bunscngcs. Phys. Chem.. 67 (1963) G. Hodcs and D. Cahaq J. Watrochem. Sot.. 124 (1977) 532.
930.
i.M. P&r, Watrochim. Act& 23 (1978) 165. M-P+ Panicker, M, Knauer and FA. Krager. J. Wcctrochcm. Sot., 125 (1978) 556. L.M. Pcter,~ J. Wectroanal. Cbcm.. 98 (1979) 49. JJ. Cuomo and RI. Gambino, J. Elcctrochcrn. Sot.. I IS (1968) 765. RC. de Mattci. D. Wwell and RS. Fcigclson. J. Ctyst. Growth, 43 (1978) 643. LG. Dioum, J. Vedcl and B. Tr&nilloo. I. Wectroanal. Chcm., 137 (1982) 219. LG. Dioum, J. Vcdcl and B. Ttimillon. J. Wcctroanal. Chem.. I39 (1982) 323. FA_ KrBgcr. J. Elcctrochcm. Sot.. I25 jl978) 2028. I. Barin and 0. Knackc, Thermodynamical Propcriics of Inorganic Substances, Springer Verlag. Berlin. 1973. I2 I. Barin. O_ Knacke and 0. Kubaschewski. Tbermodynamical Properties ol Inorganic Substances. Supplement. Springer Verlag_ Ekrlin. 1978.