Electrochemical nitriding of Sn in LiCl-KCl-Li3N systems

Article abstract of DOI:10.1016/j.jpcs.2004.06.088

Electrochemical nitriding of a liquid phase tin metal has experimentally been confirmed by using the oxidation of nitride ions in molten LiCl-KCl-Li ₃N melts according to the following reactions:N3-=Nads+3e-Nads+Sn= SnNx From the XPS analysis, N 1s signal and Sn 3d signals are observed, which corresponds to the formation of SnN_x, after conducting argon ion sputtering for 1000 s. This showed that a thick and stable nitride film was formed by electrochemical nitriding.

Full text of DOI:10.1016/j.jpcs.2004.06.088

Journal of Physics and Chemistry of Solids 66 (2005) 418–421
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Electrochemical nitriding of Sn in LiCl–KCl–Li3N systems
*
Takuya Goto , Yasuhiko Ito
Department of Fundamental Energy Science, Graduate School of Energy Science, Kyoto University, Kyoto 606-8501, Japan
Accepted 4 June 2004
Abstract
Electrochemical nitriding of a liquid phase tin metal has experimentally been confirmed by using the oxidation of nitride ions in molten
LiCl–KCl–Li3N melts according to the following reactions:
N
3K Z Nads C3eK
Nads CSn Z SnNx
From the XPS analysis, N 1s signal and Sn 3d signals are observed, which corresponds to the formation of SnNx, after conducting argon ion
sputtering for 1000 s. This showed that a thick and stable nitride film was formed by electrochemical nitriding.
q 2004 Elsevier Ltd. All rights reserved.
Keywords: D. Optical properties
1. Introduction
However, some metal elements exist as liquid in molten
LiCl–KCl and it is important to examine the possibility of
the nitriding of liquid metal by the electrochemical process.
Gallium nitrides, indium nitrides and tin nitrides, involve
elements that exist as liquid in molten LiCl–KCl.
Especially, tin nitrides are anticipated to become useful
materials for solar cell, microbattery electrode, optical
recording media, etc. [3–5]. Therefore, synthetic processes
of tin nitrides have been proposed and experimentally
confirmed to form films or powders of tin nitrides [3–5].
However, in the case of the sputtering processes, it is
difficult to control the composition of tin nitrides due to their
instability under sputtering conditions (low pressure and
high temperature). Also, some CVD processes use toxic
chemicals as precursors, such as organic tin compounds,
that are difficult to prepare. Thus, a new process is necessary
for producing tin nitrides reproducibly with ease.
We have already proposed an electrochemical nitriding
process using an anodic oxidation of nitride ions (N3K) and
experimentally confirmed the formation of metal nitrides
according to the following manner [1].
Molten chloride melts (w723 K) containing Li3N are
used as electrolytes and Li3N is decomposed in chloride
melts into LiC and N3K. Metal electrode (M) is used as an
anode and is anodically polarized to promote electroche-
mical nitriding according to the following reaction steps:
N
3K Z N C3e ðoxidation of N3K on the anodeÞ;
N CM Z M KN ðformation of metal nitrideÞ:
The key feature of this process is that the compositions of
the obtained nitrides can be controlled by the electrolytic
conditions. In addition, the operation temperature is
relatively low and harmful chemicals are not needed [1].
We have already conducted electrochemical surface nitrid-
ing for solid metal electrodes such as Ti, Zr, Al, Fe, etc. [2].
Against this background, electrochemical nitriding of tin
has been investigated in order to clarify the possibility of the
electrochemical formation of tin nitrides on a nickel
substrate in molten LiCl–KCl containing Li3N at 700 K.
First, the electrochemical formation of tin in molten LiCl–
KCl was carried out to prepare a tin film that stably
maintains a liquid film state with wide area in molten
* Corresponding author. Tel.: C81 75 753 5060; fax: C81 75 753 5906.
E-mail address: goto@energy.kyoto-u.ac.jp (T. Goto).
0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2004.06.088

T. Goto, Y. Ito / Journal of Physics and Chemistry of Solids 66 (2005) 418–421
419
chlorides. Then, thus prepared liquid tin film was used as
an anode and potentiostatic electrolysis of the anode was
conducted to obtain tin nitride film. After the electrolysis,
the products were analyzed by means of X-ray photo-
electron spectroscopy (XPS) and photoluminescence
measurement (PL). Through the experimental results, we
clarified the relationships between tin nitrides and electro-
lytic conditions.
2. Experimental
All experiments were conducted in an argon atmosphere
using a glove box with a continuous gas-refining instrument
(Japan Pionics Co., Ltd: MBN-R-07). The experimental
setup used was the same as previously reported [6,7]. The
solvent was a binary eutectic mixture of LiCl–KCl prepared
from vacuum dried reagent grade chemicals (Wako Pure
Chemical Co., Ltd). Li3N (Wako Pure Chemical Co., Ltd)
was added directly into the melt as a nitride ion source.
SnCl2 was added directly into the melt as bivalent tin cation
source. An electrochemically polished nickel plate (The
Nilaco Co., Ltd: 99.99%) was used as the working electrode.
Aluminum was used as the counter electrode. The reference
electrode was a Li–Al alloy in the state coexisting (aCb)
phase, and prepared electrochemically from an aluminum
wire (20 mm!: 1 mm, The Nilaco Co., Ltd: 99.99%). The
potential of this electrode shows the equilibrium potential of
the following reversible reaction [8]:
Fig. 1. A cyclic voltammogram of Ni in LiCl–KCl–SnCl2 at 673 K.
prepare a tin film that stably maintains a liquid film state
with wide area in molten chlorides. For this purpose,
the electrochemical formation of tin in molten LiCl–KCl
was carried out.
Before conducting electrochemical formation of tin, the
electrochemical behavior of tin ions on a nickel electrode in
LiCl–KCl was studied by means of cyclic voltammetry.
Fig. 1 shows a cyclic voltammogram of nickel in LiCl–KCl–
SnCl2 (0.1 mol%) at 673 K. One cathodic wave is observed
at 2.2 V, which corresponds to the formation of metallic tin
according to the following reaction:
SnðIIÞ C2eK Z Sn
(1)
The corresponding anodic wave is attributable to the
dissolution of tin. Further electrochemical measurement
was carried out by means of chronopotentiometry. Fig. 2
shows a chronopotentiogram of Ni in LiCl–KCl–SnCl2 at
673 K. While galvanostatic electrolysis was conducted at
K10 mA, the plateau potential of 2.12 V corresponds to
the formation of a liquid tin. The following potential
plateau of 2.16 V corresponds to the coexistence of nickel–
tin alloy phase [10] during conducting galvanostatic
electrolysis at 0.5 mA. Based on the results, potentiostatic
electrolysis was conducted at 1.9 V, which potential was
negative enough to form metallic tin on a nickel substrate.
AlðaÞ CLiC CeK Z 2Li0:5Al0:5ðbÞ
A dynamic reference electrode using electrochemically
deposited lithium was used to correct the potential
measured by the Li–Al alloy reference electrode. All
potentials in this paper are referred to this potential and
shown as (V vs. LiC/Li) [9]. Electrochemical measure-
ments were conducted using a potentio/galvanostat
combined with a function generator (Hokuto Denko Co.,
Ltd: HZ-3000). Measurements of surface chemical states
and depth profiles were performed by an X-ray photo-
electron spectroscopy (XPS) (Shimadzu Co., Ltd: ESCA-
3200-01) using Mg Ka line as the X-ray source. The
measurement of the nitrogen concentration profiles was
also conducted by an electron probe microanalyser
(EPMA) (Horiba Co., Ltd: EMAX-2200). Photolumines-
cence (PL) measurement was carried out using a He–Cd
laser of 325 nm at room temperature.
3. Results and discussion
3.1. Electrochemical formation of tin from molten LiCl–KCl
In order to obtain a tin nitride film by means of the
electrochemical nitriding of liquid tin, it is necessary to
Fig. 2. A chronopotentiogram of Ni in LiCl–KCl–SnCl2 at 673 K.

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T. Goto, Y. Ito / Journal of Physics and Chemistry of Solids 66 (2005) 418–421
Fig. 3. N 1s and Sn 3d XPS spectra of sample after potentiostatic electrolysis for 0.5 h at 1.5 V in LiCl–KCl–Li3N (0.5 mol%) at 703 K. (After 1000 s Ar
etching.)
After conducting potentiostatic electrolysis and taking out
the electrode from the melt, it was found that the nickel
electrode was completely covered with a tin film. This
result implied that a liquid tin film existed stably on nickel
plate in molten LiCl–KCl at 673 K.
and Sn 3d spectra are clearly observed. This shows that the
obtained nitride film was stable and thick.
Further surface analysis was carried out by means of
photoluminescence (PL) measurement excited with a
He–Cd laser of 325 nm at room temperature. A steep
emission spectrum is observed with a peak at about 430 nm
in Fig. 4. Assuming that this spectrum would result from a
direct-bandgap transition, the bandgap energy was esti-
mated to be about 2.88 eV. Fig. 5 shows the relationship
between substrate temperature and bandgap energy (Eg) of
SnNx obtained by various formation methods. It is clear
from Fig. 5 that there is a correlation between substrate
3.2. Tin nitride formation
Electrochemically prepared tin was used as the anode
and potentiostatic electrolysis of the anode was carried out
at 1.5 V for 0.5 h in LiCl–KCl–Li3N (0.5 mol%). The
nitriding potential of 1.5 V was determined from Fig. 1
together with the results of cyclic voltammetric data of a
nickel electrode in LiCl–KCl–Li3N, as we reported
previously [11]. From the reported data, the oxidation of
nitride ions occurs at potentials more positive than 0.4 V.
This indicates that the electrochemical nitriding proceeds at
potentials more positive than 0.4 V. On the other hand, both
the formation of tin–lithium alloy [10] and the dissolution of
tin do not occur at potentials from 1.4 to 1.8 V, as shown in
Fig. 1. Therefore, 1.5 V was selected as the electrochemical
nitriding potential. After conducting electrochemical nitrid-
ing, the surface color of the sample turned light brown,
which suggested the formation of SnNx [12]. Surface
analysis was carried out by XPS, combined with argon ion
sputtering for depth analysis. Fig. 3 shows XPS spectra of
the sample after conducting argon ion sputtering for 1000 s.
The binding energies at peak positions of N 1s spectra and
Sn 3d spectra, which were coincided with those of SnNx
[13], were observed. These results showed that electro-
chemical nitriding was applicable to the formation of tin
nitrides. In general, due to the instability of tin nitride under
vacuum and/or argon ion sputtering conditions, it is difficult
to perform analysis by means of XPS combined with long-
term argon ion etching. However, as shown in Fig. 3,
although argon ion etching was conducted for 1000 s, N 1s
Fig. 4. PL spectrum of Sn after potentiostatic electrolysis for 0.5 h at 1.5 V
in LiCl–KCl–Li3N (0.5 mol%) at 703 K.

T. Goto, Y. Ito / Journal of Physics and Chemistry of Solids 66 (2005) 418–421
421
scheme:
N
3K Z Nads C3eK
Nads CSn Z SnNx
(2) From the XPS analysis, N 1s signal and Sn 3d signals
are observed, corresponding to the formation of SnNx,
after conducting argon ion sputtering for 1000 s.
A stable and thick nitride film was thus formed by
electrochemical nitriding.
(3) The PL spectrum was measured by excitation of He–Cd
laser at room temperature. A steep emission spectrum is
observed with a peak at about 430 nm.
Fig. 5. Relationship between substrate temperature an band-gap energy of
SnNx. Obtained by various methods: (1) this work; (2, 3, 5) R.G. Gordon,
et al., Chem. Mater. 4 (1992) 68; (4) N. Takahashi, et al., J. Mater. Chem.
10 (2000) 2835; (7) T. Maruyama, et al., J. Electrochem. Soc. 143 (1996)
326; and (8) Y. Inoue, et al., Vacuum 51 (1998) 673.
(4) The bandgap energy of tin nitrides can be controlled by
the substrate temperature or synthetic temperature.
temperature or synthetic temperature and the bandgap
energy. This suggested that the bandgap energy of tin
nitrides can be controlled by the substrate temperature or
synthetic temperature.
References
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Cell 73 (2002) 377.
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4. Conclusions
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Park, OH, 1986.
Electrochemical nitriding of a liquid phase tin metal was
investigated in LiCl–KCl containing Li3N at 703 K. The
results are summarized as follows:
(1) When a liquid phase tin metal was anodically polarized
at 1.5 V, oxidation of nitride ion proceeded to produce
adsorbed nitrogen atoms, which react with the tin
surface to form tin nitride according to the reaction
[11] T. Goto, M. Tada, Y. Ito, J. Electrochem. Soc. 144 (1997) 2271.
[12] N. Takahashi, K. Terada, T. Nakamura, J. Mater. Chem. 10 (2000)
2835.
[13] Y. Inoue, M. Nomiya, O. Takai, Vacuum 51 (1998) 673.