Metal AI produced by H2 plasma reduction of AlCl3: A thermodynamic and kinetic study on the plasma chemistry

Article abstract of DOI:10.1021/jp8035204

In this paper we reported that low temperature plasma may reverse the direction of a chemical reaction. The thermodynamically forbidden reaction between H₂ and AlCl₃ was able to take place with the assistance of low temperature plasma, yielding metal Al. The plasma chemistry of the reaction was investigated by optical emission spectroscopy, which suggested that the dissociation of H₂ and AlCl₃ molecules by plasma led the reaction to a thermodynamically favorable one by creating reaction channels with low Gibbs free energy change. The addition of Ar promoted the reaction kinetics dramatically, which was attributed to the enhanced dissociation of AlCl₃ molecules by excited Ar species.

Full text of DOI:10.1021/jp8035204

1
2748
J. Phys. Chem. B 2008, 112, 12748–12752
Metal Al Produced by H2 Plasma Reduction of AlCl3: A Thermodynamic and Kinetic Study
on the Plasma Chemistry
Jie Zheng, Bo Sun, Rong Yang, Xubo Song, Xingguo Li,* and Yikang Pu§
†
†
†
†
,†,‡
Beijing National Laboratory for Molecular Sciences (BNLMS) (The State Key Laboratory of Rare Earth
Materials Chemistry and Applications), College of Chemistry and Molecular Engineering, Peking UniVersity,
Beijing, P.R. China, 100871, College of Engineering, Peking UniVersity, Beijing, P.R. China, 100871,
Department of Engineering Physics, Tsinghua UniVersity, Beijing, P.R. China, 100084
ReceiVed: April 23, 2008; ReVised Manuscript ReceiVed: July 1, 2008
In this paper we reported that low temperature plasma may reverse the direction of a chemical reaction. The
thermodynamically forbidden reaction between H
low temperature plasma, yielding metal Al. The plasma chemistry of the reaction was investigated by optical
emission spectroscopy, which suggested that the dissociation of H and AlCl molecules by plasma led the
2 3
and AlCl was able to take place with the assistance of
2
3
reaction to a thermodynamically favorable one by creating reaction channels with low Gibbs free energy
change. The addition of Ar promoted the reaction kinetics dramatically, which was attributed to the enhanced
dissociation of AlCl
3
molecules by excited Ar species.
Introduction
of plasma on the thermodynamics and kinetics were discussed.
In addition to a demonstration of the substantial thermodynamic
effect of plasma, the plasma reaction presented in this paper
also has technological importance. Reduction of metal chlorides
Application of plasma in chemical reactions has intrigued long
lasting research interest because of its wide application,
especially its particular importance in semiconductor industries.
Often referred as “plasma enhanced” chemical reactions, plasma
is capable of accelerating chemical reactions owing to its high
chemical reactivity. Plasma enhanced chemical vapor deposition
1-3
by hydrogen has been used to prepare a large variety of metals
16-18
and alloys.
However, the preparation of Al and its alloy is
1
4,19
difficult owing to the thermodynamic limitations.
The H2
plasma enhanced reduction reaction presented in this paper may
provide a new method to prepare elements and alloys of reactive
metals.
(PECVD) technique has been applied to produce a large variety
of materials with high deposition rate, including silicon (both
4
5
6
amorphous and crystalline ), carbon (diamond, diamond-like
7
8
9
10
carbon, and carbon nanotubes ), oxides, nitrides and car-
Experimental Section
1
1
bides. In contrast to the pronounced kinetic effects of plasma
exhibited in a large number of chemical reactions, the insights
into the thermodynamic aspect of plasma chemistries are still
rare. Whether plasma is able to make a thermodynamically
unfavorable chemical reaction happen, as it does for kinetically
unfavorable ones is still not well studied. Especially, there is lack
of examples showing the substantial thermodynamic effects of
plasma up to now. Plasma does show some thermodynamic effects
on some reactions. For example, in preparation of carbon based
materials, plasma causes hydrocarbon molecules to crack at much
lower temperature than thermodynamics predicts. However, in these
cases, plasma serves mainly as an alternative energy source to the
The experimental setup is schematically illustrated in Figure
1
, which consisted of a horizontal quartz tube (2.8 cm in
diameter and 100 cm in length) and an inductively coupled coil
10 cm in length) surrounding the tube. The coil was driven by
a 13.56 MHz, 500 W radio frequency power source. 0.6 g AlCl3
anhydrous (98%, Beijing Yili Chemical Factory) was loaded
in a small one-end sealed glass tube (5 cm in length and 1 cm
in diameter). The glass tube was inserted horizontally into the
quartz tube 4 cm upstream the coil, with its sealed end pointing
downstream. A mercury thermometer was inserted into the AlCl3
powder to monitor the exact temperature. There are six pinholes
(
12,13
at the sealed end to regulate the evaporation rate of AlCl3. One
conventional heating
and is not a determining factor for the
2
reaction thermodynamics.
piece of 2 × 2 cm glass were placed at 3 cm downstream of
the AlCl3 powder as the substrate. The whole quartz tube was
evacuated to 0.6 Pa by a rotary pump and was flushed with Ar
In this paper we demonstrate the significance of plasma on
reaction thermodynamics using the reaction between AlCl3 and
H2, which is thermodynamically forbidden even at temperature
(
99.99%) several times to remove oxygen and moisture. The
1
4,15
AlCl3 powder was heated by a heating tape under a mixture
gas flow of Ar and H2 (99.99%). The AlCl3 evaporation
temperature was controlled at 90 °C to ensure stable AlCl3
evaporation. The H2 flow was 30 sccm and the chamber pressure
was regulated so that the H2 partial pressure was kept at 20 Pa.
After the temperature reached 90 °C as read from the mercury
thermometer, plasma was ignited, showing a bright pale blue
glow (Figure 2b). The rf power was kept at 100 W. The
temperature of the substrate was about 100 °C due to combined
effects of the heating tape and the rf coil, as measured by a
mercury thermometer without the AlCl3 source. After 20 min,
exceeding 1000 °C.
Using radio frequency plasma, metal
Al could be produced at high rate around 100 °C. Addition of
Ar into the reaction system significantly enhanced the reaction
kinetics. The plasma chemistry of the reaction was studied by
optical emission spectroscopy (OES), based on which the effects
*
Corresponding author. E-mail: xgli@pku.edu.cn.
Beijing National Laboratory for Molecular Sciences (BNLMS) (The
†
State Key Laboratory of Rare Earth Materials Chemistry and Applications),
College of Chemistry and Molecular Engineering, Peking University.
‡
§
College of Engineering, Peking University.
Department of Engineering Physics, Tsinghua University.
1
0.1021/jp8035204 CCC: $40.75  2008 American Chemical Society
Published on Web 09/12/2008

H2 Plasma Reduction of AlCl3
J. Phys. Chem. B, Vol. 112, No. 40, 2008 12749
SEM image (Figure 2e). The conversion of AlCl3 should be
close to 100% since no solid AlCl3 condensed in the low
temperature area was observed, though the exact quantity was
difficult to estimate. Without plasma, nothing happened except
that the unreacted AlCl3 condensed at the low temperature part
of the quartz tube.
Apparently, with the assistance of plasma, the follow reaction
took place at about 100 °C
AlCl3 g
( )
+ 3/2H2 g
( )
f Al s
( )
( )
+ 3HCl g
This reaction is thermodynamically forbidden because the
Gibbs free energy change is a very large positive value (284.7
Figure 1. Schematic illustration of the ICP reaction setup.
-
1
kJ·mol , The formation free energy ∆fG° at 298 K of AlCl3
-
1
and HCl in the gas state is -570.6 and -95.30 kJ ·mol ,
1
5,20
respectively
1
). Even raising the reaction temperature up to
14
000 °C does not change the thermodynamic much. The effect
of plasma on the thermodynamics is significant.
Formation of metal Al in this reaction requires that the Al
concentration in the gas phase exceeds certain critical value (at
least the saturated Al vapor pressure at the reaction temperature).
Once the solid phase Al was formed and removed from the gas
phase, it can not be converted to AlCl3 and back to the gas
phase again, because the product HCl was continuously pumped
away.
3
Consider the reaction: AlCl3 + /2H2 f Al + 3HCl in which
all the matters involved are in the gas phase. The Al concentra-
tion is determined by the equilibrium equation
3
n n
Al HCl
∆G
)
K ) exp -
(1)
1.5
(
)
k T
nH2 nAlCl3
B
Figure 2. (a-c) Photographs of the reaction setup before, during and
where n indicates the equilibrium gas phase concentration of
the species denoted by the corresponding subscript, K is the
equilibrium constant, ∆G is the Gibbs free energy change, kB
is the Boltzmann constant, and T is the reaction temperature.
Note nHCl ) 3nAl, which gives
3
after reaction, showing a pale blue glow of AlCl plasma and metallic
coating after reaction. (d) XRD pattern of the deposition. (e) SEM image
of the deposition.
the heating tape was removed from the AlCl3 area. The plasma
was switched off when the AlCl3 temperature cooled down to
1.5
1⁄4
nH2 nAlCl3
∆G
4kBT
4
0 °C (in about 10 min). The heating procedure used in the
n )
(
)
exp -
(2)
Al
(
)
2
7
experiment resulted in an average AlCl3 evaporation rate of 0.1
-
1
mmol min , as measured from the AlCl3 mass loss after
reactions. A shining metallic layer was coated on the inner wall
of the quartz tube. The product collected on the substrate was
examined by X-ray diffraction (XRD, Rigaku Dmax 200, Cu
Ka) and scanning electron microscopy (SEM, Hitachi S4800,
Since the concentrations of H2 and AlCl3 are fixed by H2 gas
flow and AlCl3 evaporation temperature, the gas phase Al
concentration is solely determined by ∆G. The large positive
∆
G without plasma leads to very small gas phase Al concentra-
-2
-3
15
-3
tion (10 cm , estimated by using nH ) 10 cm and nAlCl3
)
2
10kV). The amount of deposited Al was calculated by dissolving
13
-3
10 cm , according to their flow rate and the partial pressure
the deposition in 1 M HCl and subsequent measurement of the
Al concentration in the solution by inductively coupled plasma-
atomic emission spectroscopy (ICP-AES, Leeman Profile).
In situ plasma optical emission spectra were collected using
a fiber spectrometer (Avantes 2048) at the substrate position.
Spectra could only be collected in the first 30 s because the
deposition of Al on the tube wall no longer allowed the access
of the plasma emission from the side. The OES spectra of
Ar-H2 mixture and Ar-AlCl3 with the same configuration as
the deposition experiments were also collected for thermody-
namic and kinetic study.
of H2), below the threshold of solid Al formation. Thus, no solid
phase Al was formed.
Almost all the species are in the electronic ground-state at
1
00 °C in the nonplasma reaction. When plasma is introduced,
excited states created by plasma have to be taken into account.
Optical emission spectroscopy (OES) is a useful technique to
qualitatively identify the excited species. As shown in Figure
3
, three Balmor lines of H atoms (HR, 656.6 nm; Hꢀ, 486.0
nm; Hγ, 343 nm) could be clearly identified in the spectrum.
21-23
Spikes originated from the emission of AlCl radicals
(261
24
nm) and Al atoms (308 and 396 nm) were observed in the
short wavelength region. No discernible emission from Cl atoms
was observed. Therefore, the excited species created by plasma
are mainly AlCl radicals, Al atoms and H atoms through the
dissociation of AlCl3 and H2. This dissociation is rather
Results and Discussion
24
The formation of metal Al was evident, as confirmed by both
the metallic shining of the product (Figure 2c) and the XRD
pattern (Figure 2d). According to the XRD pattern, Al was also
the only product. The deposited Al was in the form of thin films
composed of fine particles around 100 nm, as seen from the
2
3
inefficient by heating. Despite of the intense emission, the
-
4
-2
dissociation degree was quite low, typically 10 to 10 for
2
5,26
H2.

1
2750 J. Phys. Chem. B, Vol. 112, No. 40, 2008
Zheng et al.
collection of the AlCl3-H2-Ar mixture spectra impossible, we
studied the effects of Ar addition on H2 and AlCl3 separately
by looking into Ar-H2 and Ar-AlCl3 spectra taken at different
Ar flows (Figure 4b,c). The intensity of the H atomic lines
decreased when more Ar was introduced (Figure 4b), indicating
the decrease of H atom density caused by Ar addition. This
can be easily understood as the power dilution effect, since the
power imposed on H2 molecules became lower for lower H2
fraction when the input power was fixed. The electron temper-
ature could be estimated based on the relative intensity of the
atomic hydrogen lines. Higher Ηꢀ/HR ratio means higher
population at higher excited levels, which also indicated higher
electron energy if assuming that the excitation of H atoms is
mainly caused by electron impact. More accurately, the emission
2
7
intensity could be written as
Figure 3. A typical OES spectrum of the AlCl
3
-H
2
-Ar system.
A g
jk
j
Ej - Ek
Ijfk
)
exp -
(
)
λjk
k T
B
e
Including the excited species, additional reaction channels
with lower ∆G were available. The most important one was
AlCl + H f Al + HCl, since AlCl and H were the dominant
excited species according to the OES results. The ∆G of this
reaction was roughly the difference of the bond energy of Al-Cl
where Ijfk is the line intensity corresponding to the transition
from higher energy level j to lower one k, Ajk is the correspond-
ing transition probability, λjk is the wavelength of the transition,
Ej - Ek is the energy difference between level j and level k, gj
is the statistic weight (or degeneracy) of the higher energy level,
kB is the Boltamann constant, and Te is the electron temperature.
Using well established transition probability values of H
atoms and plotting Ijfkλjk/Ajkgj against Ej - Ek, Te can be
obtained. The statistic weight of the higher energy levels of
H , H , and H are 18, 32, and 50, corresponding to the principle
quantum number 3, 4, and 5, respectively. The results were
plotted in Figure 4b, showing a slight decrease with increasing
Ar. This could be attributed to the suppression of lower energy
electron loss rather than consumption of high energy electrons.
The electron density in Ar-H2 plasma was known to increase
with Ar fraction, because the most important electron loss
-
1
-1 20
(
494 kJ ·mol ) and H-Cl (432 kJ ·mol ), which is a much
-1
smaller positive value (62 kJ ·mol ) compared to the nonplasma
process. The equilibrium Al atom concentration is given by a
similar exponential expression
2
8
2
9
1
⁄2
n n
R
ꢀ
γ
H
AlCl
∆G
2kBT
n )
exp -
(3)
Al
(
)
(
)
4
The much smaller ∆G value gives higher equilibrium Al atom
concentration. Assuming 0.1% dissociation ratio of both H2 and
AlCl3 which is typical for the plasma operating in the experi-
mental conditions, the equilibrium concentration of Al atoms
is estimated to be 10 cm , 8 orders of magnitude higher than
that of the nonplasma case. In a macroscopic energy point of
view, the plasma helped to break two Al-Cl bonds and one
H-H bond for each reaction, resulting in a large energy gain
toward the solid Al formation.
3
0
6
-3
+
channel ArH + e f Ar + H is enhanced at higher H2 fraction,
2
5
resulting in the loss of some low energy electrons. In addition,
the presence of Ar could also generate low energy electrons
through the ionization of H atoms by metastable Ar species
+
(Ar*): Ar* + H f Ar + H + e. The decrease of H atom
density and electron temperature with Ar addition is in agree-
ment with the results obtained in capacitively couple rf plasma
and direction current glow discharge. Obviously, Ar addition
was not expected to enhance the reaction kinetic from the H2
side, since neither the density of reactive H atoms was raised
nor the electrons became more powerful.
As a direct deduction, the overall kinetic enhancement must be
attributed to the positive effects on the AlCl3 side caused by Ar
Addition. The change of the AlCl3 OES spectra due to Ar addition
is shown in Figure 4c. Both the AlCl radical emission at 261 nm
and the Al atom emission at 396 nm became more intense with
raising Ar flow, while their ratio remained the same. The influence
of Ar flow on the AlCl3 evaporation was negligible in our
experiment. The effect of Ar was mainly owing to the change of
the plasma processes. The OES results suggested that the dissocia-
tion of AlCl3 were enhanced at higher Ar flow.
Ar addition resulted in higher density of electrons, as
discussed above, as well as Ar and Ar*, all of which were
favorable for the dissociation of AlCl3 molecules, through either
direct electron impact or energy transfer from Ar ions and
metastables. The dissociation enhanced by direct electron impact
was not expected to be significant, since the extra electrons
Another reaction AlCl f Al + Cl also yielded Al atoms in the
gas phase and was readily observed by OES. However, it could
not be the main reason for the substantial equilibrium Al increase
because the ∆G of this reaction is still large, in agreement with
the lack of metal Al formation in AlCl3-Ar plasma.
30
3
1
In summary, due to the excited states created by plasma, the
reaction could proceed through some channels with lower ∆G. The
overall effect showed a decrease of free energy change, which gave
higher equilibrium Al atom concentration in the gas phase.
The Al deposition rate in pure H2 was estimated to be 6.5
-
2
-1
mg · cm · s , according to the amount of Al deposited on
the glass substrate measured by ICP-AES. The Al deposition
rate could be significantly enhanced when Ar is introduced into
the system. To quantitatively estimate the effect of Ar on the
kinetics, the H2 flow was kept at 30 sccm and the Ar flow
increase from 0 to 60 sccm. The chamber pressure was regulated
to maintain the H2 partial pressure at 20 Pa so that the amount
of reactive gas was the same for each run. The deposition rate
at different Ar flows is shown in Figure 4a, showing a monotonic
increase with increasing Ar flow. Although Ar is an inert species
in most chemical reactions, it clearly served as a kinetic
promoter here. The error in each data point was about 15%, as
estimated from 3 repeated runs at each Ar flow.
+
3
0
brought by Ar addition were low energy ones. The energy
transfer from the excited Ar species (Ar* and Ar ) were
responsible for the enhancement of AlCl3 dissociation. As shown
+
The kinetic effects of Ar are also studied by OES. Because
the deposition of Al on the quartz tube made the systematic

H2 Plasma Reduction of AlCl3
J. Phys. Chem. B, Vol. 112, No. 40, 2008 12751
Figure 4. (a) Al deposition rate at different Ar flows. (b) Variation of the H
R
line intensity (open circles) and electron temperature (filled squares)
2
with the Ar flow in Ar-H plasma. (c) Variation of the AlCl line intensity (filled squares) and the Al/AlCl intensity ratio (open circles) with the
Ar flow in Ar-AlCl
introduction of AlCl
3
plasma. (d) Comparison of pure Ar and Ar-AlCl
vapor.
3
OES spectra, showing a significant decrease of Ar line intensity after the
3
in Figure 4d, significant intensity drop of Ar 4p lines32 (700 to
000 nm) and the relative intensity change among different Ar
dissociation and reaction kinetics caused by Ar addition
according to the OES results. Here H2 served as the reducing
agent which was indispensible for the reaction. The free
electrons in the plasma were too low in number density
1
lines were observed when AlCl3 vapor was introduced, sug-
gesting efficient energy transfer between Ar* and AlCl3
molecules. However, the OES results are only able to give a
qualitative explanation. To quantitatively estimate the contribu-
tion from each energetic species, their number density, energy
distribution and reaction cross sections need to be known, which
requires combining other diagnostics with OES.
1
1
-3
(typically 10 cm for plasma operating at the experimental
2
6
conditions ) to serve as the reducing agent, as confirm by the
fact that the discharge of AlCl3 in pure Ar plasma did not lead
any detectable metal Al formation.
Conclusion
The addition of Ar caused a decrease of H atoms density but
an increase of AlCl and Al density, together with an increase
of low energy electron density. The overall enhancement of Ar
on the kinetics suggested that the positive effects on the AlCl3
dissociation overweighed the negative effects on the H2 dis-
sociation. A possible reason is that the dissociation of AlCl3
molecules was the rate-limit step of this reaction. In the gas
phase at the experimental temperature, AlCl3 exists as dimmers,
in which the Al atoms are 4-fold coordinated by Cl atoms
without any vacancies for electron transfer to the Al atoms.
Breaking Al-Cl bonds would remove the stereo hindrance of
the electron transfer from the H atoms or H2 molecules to the
Al atoms. Thus, reduction of AlCl radicals was much faster
compared to that of Al2Cl6 dimmers. A possible reaction
mechanism for this plasma reaction is written as follows.
We demonstrated the significant change in reaction thermo-
dynamics caused by plasma in the reduction of AlCl3 by H2
plasma. Due to the species in excited electronic states created
by plasma, reaction channels with lower free energy change
were available, through which the thermodynamically forbidden
reaction in non plasma conditions was able to take place.
Addition of Ar promoted the reaction kinetics substantially,
which is attributed to the enhancement of AlCl3 dissociation
caused by the excited Ar species. The plasma enhanced
reduction discussed in this paper provides an effective new
method to prepare reactive metals and their alloys.
Acknowledgment. This study was supported by the National
Natural Science Foundation of China (Nos. 20221101 and
20671004),MOSTofChina(Nos.2006AA05Z130,2007AA05Z118)
or Ar+
or Ar+
and MOE of China (No. 707002). Dr Li Zhang is highly
appreciated for her kind support in the ICP-AES measurement.
AlCl +Ar*
(
)
f AlCl + 2Cl + Ar
or 1/2H2 f Al + HCl
(
)
(
Slow
)
3
AlCl + H
(
)
(
Fast
)
References and Notes
The assignment of the dissociation of AlCl3 molecules being
the rate-limit step is in agreement with the same trend of AlCl3
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JP8035204