Effect of oxidation of catalytically active silicon-based electrodes on water decomposition
In this work, we continue to study the revealed phenomenon of current creation in the electrochemical system with distilled water during its decomposition without any applied external voltage. Investigated are catalytically active (to decompose water) electrodes based on silicon with modified sur...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2007
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| Zitieren: | Effect of oxidation of catalytically active silicon-based electrodes on water decomposition / V.E. Primachenko, O.A. Serba, V.A. Chernobai, E.F. Venger // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2007. — Т. 10, № 1. — С. 88-96. — Бібліогр.: 10 назв. — англ. |
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irk-123456789-1177812017-05-27T03:06:07Z Effect of oxidation of catalytically active silicon-based electrodes on water decomposition Primachenko, V.E. Serba, O.A. Chernobai, V.A. Venger, E.F. In this work, we continue to study the revealed phenomenon of current creation in the electrochemical system with distilled water during its decomposition without any applied external voltage. Investigated are catalytically active (to decompose water) electrodes based on silicon with modified surface (due to lapping, texturing, doping with palladium, creation of silicides) before and after thermooxidizing for 1 hour in dry oxygen at 850 ºC. Also performed are investigations of time dependences for the current between silicon electrodes and counter-electrodes made of Al, Pt, Yb when using the circuit with the external voltage V₀ = ± 9.7 V. Analyzed are the results obtained and prospects for a further study. 2007 Article Effect of oxidation of catalytically active silicon-based electrodes on water decomposition / V.E. Primachenko, O.A. Serba, V.A. Chernobai, E.F. Venger // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2007. — Т. 10, № 1. — С. 88-96. — Бібліогр.: 10 назв. — англ. 1560-8034 PACS 82.30.Lp, 82.45.Fk http://dspace.nbuv.gov.ua/handle/123456789/117781 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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In this work, we continue to study the revealed phenomenon of current
creation in the electrochemical system with distilled water during its decomposition
without any applied external voltage. Investigated are catalytically active (to decompose
water) electrodes based on silicon with modified surface (due to lapping, texturing,
doping with palladium, creation of silicides) before and after thermooxidizing for 1 hour
in dry oxygen at 850 ºC. Also performed are investigations of time dependences for the
current between silicon electrodes and counter-electrodes made of Al, Pt, Yb when using
the circuit with the external voltage V₀ = ± 9.7 V. Analyzed are the results obtained and
prospects for a further study. |
| format |
Article |
| author |
Primachenko, V.E. Serba, O.A. Chernobai, V.A. Venger, E.F. |
| spellingShingle |
Primachenko, V.E. Serba, O.A. Chernobai, V.A. Venger, E.F. Effect of oxidation of catalytically active silicon-based electrodes on water decomposition Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Primachenko, V.E. Serba, O.A. Chernobai, V.A. Venger, E.F. |
| author_sort |
Primachenko, V.E. |
| title |
Effect of oxidation of catalytically active silicon-based electrodes on water decomposition |
| title_short |
Effect of oxidation of catalytically active silicon-based electrodes on water decomposition |
| title_full |
Effect of oxidation of catalytically active silicon-based electrodes on water decomposition |
| title_fullStr |
Effect of oxidation of catalytically active silicon-based electrodes on water decomposition |
| title_full_unstemmed |
Effect of oxidation of catalytically active silicon-based electrodes on water decomposition |
| title_sort |
effect of oxidation of catalytically active silicon-based electrodes on water decomposition |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2007 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/117781 |
| citation_txt |
Effect of oxidation of catalytically active silicon-based electrodes on water decomposition / V.E. Primachenko, O.A. Serba, V.A. Chernobai, E.F. Venger // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2007. — Т. 10, № 1. — С. 88-96. — Бібліогр.: 10 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
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| first_indexed |
2025-07-08T12:47:23Z |
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2025-07-08T12:47:23Z |
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| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2007. V. 10, N 1. P. 88-96.
© 2007, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
88
PACS 82.30.Lp, 82.45.Fk
Effect of oxidation of catalytically active
silicon-based electrodes on water decomposition
V.E. Primachenko, O.A. Serba, V.A. Chernobai, E.F. Venger
V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine
41, prospect Nauky, 03028 Kyiv, Ukraine
E-mail: pve18@isp.kiev.ua
Abstract. In this work, we continue to study the revealed phenomenon of current
creation in the electrochemical system with distilled water during its decomposition
without any applied external voltage. Investigated are catalytically active (to decompose
water) electrodes based on silicon with modified surface (due to lapping, texturing,
doping with palladium, creation of silicides) before and after thermooxidizing for 1 hour
in dry oxygen at 850 ºC. Also performed are investigations of time dependences for the
current between silicon electrodes and counter-electrodes made of Al, Pt, Yb when using
the circuit with the external voltage V0 = ± 9.7 V. Analyzed are the results obtained and
prospects for a further study.
Keywords: current creation, external voltage, water decomposition, catalytically active
electrodes, oxidation.
Manuscript received 20.11.06; accepted for publication 26.03.07; published online 01.06.07.
1. Introduction
In the works [1, 2], we realized decomposition of water
by hydrogen and oxygen by using various catalytically
active (as to this decomposition) electrodes inside
distilled water without applying any external voltage to
them. This decomposition is realized due to the reaction
H2O→HO− + H+ taking place on one or both cataly-
tically active electrodes (cathode and anode) with the
following separation of anions (OH−) and cations (H+)
by the electric voltage arising due to different
electrochemical potentials (electron work functions) for
these two materials used as anode and cathode.
This electrochemical system operates as a current
source (unfortunately, with a low power of the order of
0.025 mW/cm2, up to date). In so doing, in dependence
on electrode material one can observe both a decrease
and increase (up to 50 %) of the current value with time
(experiments were carried out for 30...180 min) [1, 2].
Besides, in future one shall be able to use hydrogen that
is released from the cathode or accumulated in it.
Our analysis of the experimental conditions showed
that obtaining the energy (in the form of the current
source and hydrogen) is realized due to a natural
difference of electrochemical potentials between
different materials used as electrodes as well as due to
electrochemical reactions with liberation of ОH− and H+
ions at these electrodes, which promotes keeping the
potential difference between the electrodes within some
limits. In principle, there is a possibility to absorb some
amount of the heat energy of the electrochemical system
medium. At the same time, some reference experiments
showed that possible photochemical processes related to
absorption of sun light quanta do not play any essential
role in our conditions.
As it follows from [1, 2], the efficiency of energy
output in these electrochemical systems depends on
many factors, in particular, on the degree of oxidation of
the electrodes in use. For instance, the Yb-Pt system
operates more effectively under some definite degree of
oxidation inherent to the Yb electrode, although the
potential difference between Yb and Pt electrodes is
decreased. It is related with the higher catalytic activity
of the Yb electrode, when an inhomogeneous oxide film
with the composition of the Yb2O3 type is created on it.
The high catalytic activity of metal oxides in reduction
reactions, to which the water decomposition can be
related, is well known [3].
In this work, we performed investigations of the
efficiency of energy generation by using the above
electrochemical water system with modified silicon and
its compounds obtained after oxidizing them.
2. Experimental
In this work, as an anode we used electrodes made of Yb
or Al that possess a lower electron work function as
compared with silicon or its compounds (cathodes). On
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2007. V. 10, N 1. P. 88-96.
© 2007, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
89
the other hand, as it was revealed in investigations,
silicon electrodes can play the role both of the anode and
cathode relative to the comparatively stable Pt electrode.
Therefore, we performed vast investigations of the
efficiency of energy generation by using the above water
system with Al, Pt, Yb electrodes in combination with
silicon electrodes, the state of which was changed
(lapping, texturing, doping with Pd, oxidation, creation
of silicides Cr3Si, Ni3Si).
When carrying out the experiments, the couple of
corresponding electrodes mounted on the same holder
was dipped into distilled water. The space between
electrodes was within limits L = 1...2 cm, their area S
was between 0.5 to 2 cm2. After 1...2 min from the
moment of dipping the electrodes, using an electron
voltmeter we measured the voltage ∆Vc between
electrodes, and then, after connection with the external
electric circuit, using a microamperemeter we measured
time dependences for the current provided by the
movement of ОH− and H+ ions in water.
As in the case of various couples of electrodes, the
values L, S, ∆Vc were different, to compare the effect of
electrodes on the rate of water decomposition, we
plotted the time dependences for the specific
conductivity of the electrochemical system:
σeff = J(t)(L / ∆Vc·S) = AJ(t), (1)
where the coefficient A = L / (∆Vc·S) V−1cm−1. In
addition to measurements of the current J(t) caused by
the difference ∆Vc, we also measured the currents when
the external circuit was added by an additive power
source with V0 = 9.7 V (battery) and the respective sign
“+” or “−“ on the silicon electrode. It enables us to make
some conclusions about the role of the potential
difference between the electrodes in the value of the
conductivity AJ(t) for this electrochemical system.
In these cases, in the formula (1) we used the value
±V0 + ∆Vc.
As a rule, Al, Pt and Yb electrodes were plain
metal plates. Silicon electrodes, in most cases, were
plain plates made of n-type silicon (2 Ohm⋅cm, (100)
face), but their surface was modified. Namely, we used:
silicon surfaces grinded with diamond paste (grain
diameter of which was close to 1 µm); surfaces
preliminary textured with the etchant [4], when
tetrahonal pyramids of approximately 2-µm height with
side faces (111) were created on them; grinded or
textured surfaces doped with Pd by using water solutions
of PdCl2 (10−3−10−2 M); surfaces of specially grown
silicides (Cr3Si, Ni3Si); all the abovementioned surfaces
after thermal oxidizing them at 850 °С for 1 hour in
dry O2. Work functions for electron escaping from
electrodes φ were measured by us directly in
experiments relative to Pt electrode, the work function of
which is well known (φPt = 5.32 eV [5]). We took into
account that Pt is the most chemically stable material,
therefore, it possesses the relatively stable value of the
work function as compared with other materials. To
determine φ-value for other electrodes, we applied the
voltage (V0 = 9.7 V) between them and Pt electrode first
in one and then in the opposite direction.
Currents J1 and J2 were measured with a
microamperemeter after their fast (less than 5 s)
stabilization in the electric circuit, when equilibrium of
the electron subsystem had been already reached, while
that for the ionic subsystem will be reached considerably
later, which will cause an essential change of electrode
polarization due to chemical reactions on them. Using
the following equations:
J1 = (V0 − ∆φ/q) / R (2)
and
J2 = (V0 + ∆φ/q) / R, (3)
where ∆φ is the difference of electron work functions
between the electrodes, q is the electron charge and R is
the resistance of the electrochemical system, we
determined the value:
∆φ/q = V(J2 − J1) / (J2 + J1), (4)
and then the values φel for the investigated electrode
φel = (5.32 – ∆φ) eV. (5)
It is noteworthy that the values φel obtained in this
manner, for instance, for Al and Ni electrodes were in
good agreement with their values determined by other
methods [5, 6].
3. Experimental results and discussion
It is known that under the room temperature water
contains ions ОН– and Н+ in concentrations
1.2·1014 cm−3, Н+ ions being in complexes Н5О2
+ [6].
After dipping the electrode into water, there arises
electron-ion exchange between them, which causes a
potential hop (the so-called Galvani-potential [7]). The
latter comprises at least the following components: a
potential drop in a diffusion (for ions) water layer near
the electrode; potential drop in the thin (~10−8 cm)
Helmholtz layer created by ions close to electrode
surface; potential drop in the very electrode (it is
essential for electrodes made of semiconductors that
have a subsurface space charge layer [8]). Besides, when
the thickness and resistance of the oxide layer covering
the electrode are considerable, an essential part of the
Galvani-potential can drop in this layer.
If a pair of chosen electrodes is dipped into water
simultaneously, there arises electron-ion equilibrium
with a respective potential difference ∆Vc. It is realized
due to different values of their electrochemical potentials
(by another words, owing to different electron work
functions φ of these electrodes). One can observe a
partial transition of electrons from the electrode with a
higher electrochemical potential (and a lower φ-value) to
the electrode with a lower electrochemical potential (and
a higher φ-value). As a result, the former electrode
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2007. V. 10, N 1. P. 88-96.
© 2007, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
90
becomes positively charged (i.e., becomes anode), while
the latter – negatively charged (i.e., becomes cathode). It
is noteworthy that, in accord with experimental data, the
initial potential difference between the electrodes ∆Vс
before current going between them was always lower
than the difference ∆φ/q for electrodes. Then, if one
makes an external electric circuit to close, there arises a
current, electrons being in movement from anode to
cathode, as in water ОН– ions move to anode, while Н+
(Н5О2
+) ions do to cathode.
In the course of water decomposition when ОН–
ions discharge on the anode and Н+ ions on the cathode,
the ∆Vc value, in principle, should decrease and vanish
with time. But it did not observe in the variety of used
electrodes, in the first place, owing to electrochemical
reactions on the electrodes.
In accordance with [3, 9], electrolysis of water
(with рН ≈ 7) on the anode can be described by the
following reactions:
4OH– = O2 + 2H2O + 4q–
2H2O = O2 + 4H+ + 4q-
as well as on the cathode:
2H5O2
+ + 2q– = H2 + 4H2O
2H2O + 2q– = H2 + 2OH–.
However, the above reactions do not take into
account a possible catalytic action of electrodes,
reactions of bond creation of oxygen and hydrogen with
electrode matter (including diffusion into them),
mechanisms of oxygen and hydrogen desorption from
electrodes. As it was shown for example in [1, 2], in the
pair Yb-Pt catalytic phenomena of water decomposition
on the Yb electrode play an essential role, while in the
case of a plain Pt electrode, chemically stable, covered
with a super-thin oxide layer, these phenomena can be
ignored in the first approximation.
It was noted in Introduction that the main purpose
of this work is to study the role of oxidation of
electrodes based on silicon in the process of current
creation when water is decomposed in electrochemical
systems. First, we shall consider these phenomena for
the case of thermal oxidation of polished and textured
silicon electrodes, then after oxidation of these
electrodes preliminary doped with Pd, and in fine after
oxidation of metal silicides (Cr3Si and Ni3Si).
Ι. Time dependences AJ(t) for the current through
silicon electrodes were measured using plates of Pt, Yb,
Al as counter-electrodes. The results for the Pt electrode
are shown in Figs 1 and 2 as AJ(t) dependences for
polished and textured silicon surfaces, respectively, before
(curves 1, 1(+), 1(−)) and after (curves 2, 2(+), 2(−)) their
thermal oxidation for 1 hour at Т = 850 °С in dry О2. The
thickness of the SiO2 oxide film on silicon reaches up to
40 nm in this regime of growing. The curves 1 and 2 were
obtained when external voltage was V0 = 0, curves marked
with the sign (+) correspond to V0 = +9.7 V on silicon
electrodes, while with the sign (–) to V0= –9.7 V.
AJ, 10–6 Ohm–1cm–1
10 20 30
0
5
10
t,min
2
2(+)
2(-)
1
1(+)
1(-)
Fig. 1. Time dependences of the effective conductivity AJ(t)
obtained for the electrochemical water system with the pair of
electrodes Pt-Sip (1, 1(+), 1(−) – before and 2, 2(+), 2(−) – after
thermal oxidation of Sip; 1, 2 – V0 = 0 ; (+) – V0 = +9.7 V on
Sip; (–) – V0 = –9.7 V on Sip ).
AJ, 10–6 Ohm–1cm–1
10 20 30
0
5
10
1(-)
1(+)
2(-)
2(+)
t,min
1
2
Fig. 2. Time dependences of AJ(t) for the electrode pair Pt-Sit
(1, 1(+), 1(–) – before and 2, 2(+), 2(–) – after thermal oxidation of
Sit ; 1, 2 – V0 = 0; (+) – V0 = +9.7 V, (–) – V0 = –9.7 V on Sit ).
First of all, let us analyze the AJ(t) dependence
when V0 = 0 before oxidation of silicon electrodes
(curves 1). For both polished Sip and textured Sit
electrodes, initial values of the efficient conductivity are
practically the same at t = 0. Then, one can see a
relatively fast drop of AJ(t) (in the case of Sip for 2 min,
and Sit for 5 min), which can be associated with
separation of Н+ і ОН– ions that were created near the
electrodes before t = 0 due to catalytic action of the latter
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2007. V. 10, N 1. P. 88-96.
© 2007, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
91
on water decomposition. For the times t ≥ 5 min, AJ(t)
values for the Sip electrode are quasi-stationary, while
for Sit electrode these are decreased to some extent,
being more than 2 times less as compared with those for
Sip electrode. Some clarity in understanding the AJ(t)
dependences for Sip and Sit can be reached using
measurements of ∆Vc before the current starts (at t = 0)
and after its switching off (t ≥ 30 min), i.e., ∆Vc
(1) and
∆Vc
(2), respectively. In the case of Sip electrode, these
values were ∆Vc
(1) = +0.50 V and ∆Vc
(2) = +0.44 V, while
for Sit electrode, ∆Vc
(1) = +0.38 V and ∆Vc
(2) = +0.18 V.
The higher values of Vc
(1) and ∆Vc
(2) for Sip electrode
define the distinction between AJ(t) dependences for Sip
and Sit electrodes as a consequence of stronger local
electric fields arising near non-homogeneities of the
polished surface, which enhances its catalytic activity in
water decomposition. This conclusion is confirmed by
AJ(t) dependences obtained for Sip and Sit electrodes
when applying the voltage V0 = ±9.7 V to them (curves
1(+) and 1(−) in Figs 1 and 2). The considerably higher
AJ(t) values for these curves (at t ≥ 5 min) as compared
with the curves 1 are unambiguously indicative of the
fact that the efficient conductivity AJ(t) of these
electrochemical systems essentially grows with
increasing the voltage between electrodes, which results
in a higher catalytic activity of the electrodes in water
decomposition.
Determined work functions for electrons before
thermal treatments of electrodes for Sip and Sit were φp =
= 4.47 eV and φt = 4.85 eV, respectively. In our opinion,
the difference between φp and φt is caused by the higher
density of surface electron states (SES) on the polished
surface as compared to the textured one, which provides
a larger electron charge in SES that bends the energy
bands down and results in lowered φp as compared with
φt. However, both electrodes behave as anodes relatively
to the Pt electrode.
Quite another situation is realized after thermal
treatments (TT) of Sip and Sit electrodes. In the case of
SiTT
p electrode, we found φTT
p = 10.5 eV, but for SiTT
t
φTT
t = 11.4 eV, that is relatively to Pt electrode (φPt =
5.32 eV) thermally treated silicon electrodes behave as
cathodes. High φ-values in this case are indicative of the
fact that we measure the work function not for silicon
but for an oxide film SiO2 that covers silicon. It is
known [10] that the width of the SiO2 forbidden gap is
equal to ∆Eg = 9 eV, while the energy of electron affinity
χ = 0.9 eV, i.e., the energy distance from the top of filled
with electrons SiO2 valence band to the vacuum level is
∆Eg + χ = 9.9 eV. It should be the value of the work
function for SiO2. The values obtained by us for the
thermooxidized Sip and Sit electrodes exceed that value
by 0.6 and 1.5 eV, respectively. In our case, this φ
excess can be caused both by a positive charge built-in
inside the oxide films and by the layer of Н+ ions
adsorbed on the SiO2 surface (Helmholtz’s layer). We
prefer the second reason, as the work function measured
by us depended to some extent on the conditions of its
measurements (for example, before or after measuring
the AJ(t) dependence at V0 = 0).
Although the difference ∆φ between Pt and
thermally oxidized silicon electrodes is approximately 5
to 6 eV, the measured values ∆Vc for these pairs do not
exceed tens of microvolts. These are corresponded by
low currents J(t) (no more than 5·10−8 А). It is caused by
the fact that the main part of voltage drop ∆φ/q is related
to the SiO2 layer. The curves 2 in Figs 1 and 2 obtained
at V0 = 0 after the thermal treatment reflect these facts.
Despite the fact that the conductivity value AJ(t) is
always positive, we presented these data as negative to
emphasize the opposite directions of currents before and
after the thermal treatment (curves 1 and 2,
respectively). As seen from Figs. 1 and 2, after the
thermal treatment the AJ(t) magnitudes are also lower
when applying the voltage V0 = +9.7 V to silicon
electrodes, while application of the voltage V0 = −9.7 V
to the same electrodes do not change these magnitudes
considerably. Thus, thermal oxidation of polished and
textured surfaces when using them in the pair with Pt
electrode does not yield positive results.
Shown in Figs 3 and 4 are the AJ(t) dependences
for polished and textured silicon electrodes, respectively,
before (curves 1, 1(+), 1(−)) and after (curves 2, 2(+), 2(−))
their thermal treatment in pairs with Yb electrodes as
well as with Al electrode (curves 3(+), 3(−)). In all these
measurements, silicon electrodes served as cathodes. Let
us note the most interesting features of these
dependences. First, the thermal treatment of Sip and Sit
electrodes results in a positive effect in their pairs with
the Yb electrode, namely: the AJ(t) value is considerably
increased at V0 = 0 (curves 2) and at V0 = −9.7 V (curves
2(−)) as compared to those AJ(t) values obtained before
the thermal treatment (curves 1 and 1(−)). Besides,
changed is the time character of these dependences: one
can observe only growth of AJ(t) with time. It means
that the character of electrochemical processes on Sip
and Sit electrodes covered with SiO2 is changed after the
thermal treatment, and, as a result, liberation of
hydrogen on these electrodes is improved. The increase
of AJ(t) values with time (curves 2) is caused by
growing ∆Vc value that changed on Sip electrode within
the range +0.30 up to +0.50 V, and on Sit electrode from
+0.22 up to +0.47 V (while before the thermal treatment
∆Vc value on Sip electrode was changed from +0.50
down to +0.40 V, and on Sit electrode – from +0.35
down to +0.32 V). Second, the pairs Yb-SiTT
p(SiTT
t) are
more efficient as compared with pairs where the Al
electrode is used instead of the Yb one. It may be related
both with a larger ∆φ difference in the case of Yb and
with higher catalytic activity of Yb. Third, after the
thermal treatment of silicon electrodes when the voltage
V0 = +9.7 V is applied to them, the AJ(t) values (curves
2(+), 3(+)) are considerably lower than those before the
thermal treatment (curve 1(+)). It means that after
oxidation the electrodes are very difficult to “take” or
“create” hydrogen when their potential V0 is positive.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2007. V. 10, N 1. P. 88-96.
© 2007, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
92
AJ, 10–6 Ohm–1cm–1
10 20 30
0
10
20
30
t,min
1
2
1(-)
2(-)
1(+)
3(-)
2(+)
3(+)
Fig. 3. AJ(t)-dependences inherent to the electrode pairs Yb-Sip
(1, 1(+), 1(−) – before; 2, 2(+), 2(−) – after thermal oxidation of
Sip) and Al-Sip (3(+), 3(−) – after thermal oxidation of Sip); 1, 2 –
V0 = 0; (+) – V0 = +9.7 V, (–) – V0 = –9.7 V on Sip). The
dependence 3 (V0 = 0) is not presented as AJ(t) is close to zero.
AJ, 10–6 Ohm–1cm–1
10 20 30
0
5
10
15
20
t,min
J, 0 O c
1
3(-)
2
1(+)
1(-)
2(-)
2(+) 3(+)
3
Fig. 4. AJ(t)-dependences inherent to the electrode pairs Yb-Sit
(1, 1(+), 1(−) – before; 2, 2(+), 2(−) – after thermal oxidation of
Sit) and Al-Sit (3(+), 3(−) – after thermal oxidation of Sit). 1, 2 –
V0 = 0; (+) – V0 = +9.7 V, (–) – V0 = –9.7 V on Sit. The
dependence 3 (V0 = 0) is not presented as AJ(t) is close to zero.
II. It was shown in our works [1, 2] that
preliminary doping all the structurally modified silicon
electrodes with Pd results in increased AJ(t) values, that
is in an increased rate of water decomposition due to
high catalytic activity of an island-like structure that is
characteristic for palladium deposited on silicon from
PdCl2 water solutions [8]. On the other hand, the
abovementioned means that usage of the Yb counter-
electrode increases the AJ(t) values like to thermal
oxidation of Sip and Sit electrodes does. Therefore, it
was interesting to combine the action of these two
factors – doping and thermal oxidation of Sip and Sit
electrodes, especially predicting that doping can
considerably decrease the resistivity of SiO2 films.
Figs 5 to 8 show AJ(t) dependences obtained for
electrodes Sip〈Pd;10−3M〉, Sit〈Pd;10−2M〉, Sit〈Pd;10−3M〉
before and after their thermal treatment when these were
used in pairs with Pt, Yb, Al electrodes. The notation
〈Pd;10−3M〉 means that the sample of silicon is doped
with palladium from 10−3М PdCl2 water solution, the
sample being kept in this solution for 1 hour.
First of all, it is worth to note that if before the
thermal treatment of electrodes Sip〈Pd;10−3M〉,
Sit〈Pd;10−2M〉 and Sit〈Pd;10−3M〉 the electron work
functions φ determined relatively the Pt electrode were
5.24, 5.12 and 5.53 eV, respectively, then after the
thermal treatment they acquired the values 4.74, 4.64
and 4.93 eV. That is, contrary to oxidation of non-doped
Sip and Sit samples when φ-values were considerably
grown, oxidation of Pd-doped samples causes a decrease
of the electron work function, which is indicative of the
low resistivity of the created SiO2 film “transfused” with
palladium. It is also confirmed by AJ(t) dependences
shown in Figs 5 – 8. It is very probable that thermal
treatments create palladium silicides as well as its
oxides. As to that, some conclusion can be drawn from
the analysis of the work function values for silicon
electrodes doped with Pd. The φ-values higher than 5 eV
indicate that the island-like Pd film is oxidized, which
provides the φ-values higher than that of Pd [5, 6]. Then,
a decrease of φ-values after thermal treatments most
likely indicates creation of palladium silicides on the
electrode surface. When keeping these electrodes in air,
the φ-values increase to some extent, which can be
caused by oxidation of palladium silicides (oxidation in
humid air can be considerably efficient than in dry О2).
In the beginning, we shall consider the action of
silicon electrodes doped with Pd on water decomposition
if these are coupled with Pt counter-electrodes (Figs.
5, 6). The AJ(t) dependences obtained before and after
TTs of Sip〈Pd;10−3M〉, Sit〈Pd;10−2M〉 and Sit〈Pd;10−3M〉
electrodes essentially differ from those for electrodes Sip
and Sit without Pd (Figs. 1, 2). In the case of doped
electrodes, AJ(t) values are noticeably higher both at
V0 = 0 and ±9.7 V, because the nondoped electrodes
have SiO2 isolated layer. There is no such a layer
on electrodes doped with Pd after the thermal treat-
ment, therefore one can observe considerably higher AJ(t)
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2007. V. 10, N 1. P. 88-96.
© 2007, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
93
AJ, 10–6 Ohm–1cm–1
10 20 30
0
10
20
t,min
,
1
2
1(+)
1(-)
2(+)
2(-)
Fig. 5. AJ(t)-dependences obtained for the electrode pair
Pt-Sip〈Pd,10−3〉 (1, 1(+), 1(−) – before; 2, 2(+), 2(−) – after thermal
oxidation Sip). 1, 2 – V0 = 0; (+) – V0 = +9.7 V, (–) – V0 =
= –9.7 V on Sip〈Pd,10−3〉.
AJ, 10–6 Ohm–1cm–1
10 20 30
0
10
20
,
t,min
4(-)
4(+)
1(-)
1(+)
4
3
2
1
3(+)
3(-)
Fig. 6. AJ(t)-dependences obtained for the electrode pair Pt-
Sit〈Pd,10−2〉 (1, 1(+), 1(−)), Pt-Sit〈Pd,10−3〉 (2) and Al-
Sit〈Pd,10−2〉 (3, 3(+), 3(−)), Al-Sit〈Pd,10−3〉 (4, 4(+), 4(−)) after
thermal oxidation of Sit〈Pd〉. 1, 2, 3, 4 – V0 = 0; V0 = +9.7 V
and (–) – V0 = –9.7 V on Sit〈Pd〉. The curves 2(+), 2(−) are not
presented as they are very close to 1(+), 1(−) curves.
values, especially at Vo = 0 and V0 = +9.7 V (Fig. 5,
curves 2, 2(+), 2(–); Fig. 6, curves 1, 1(+), 1(−) and 2, 2(+)).
AJ(t) dependences (Figs. 5 and 6) were obtained
with Pt electrode when it always plays the role of cathode,
while the doped silicon electrodes act as anodes. To
ascertain the role of “cathode – anode”, we obtained AJ(t)
dependences for doped silicon structures (as cathodes) and
Al electrode used as anode (Figs. 6 and 7). AJ(t)
dependences inherent to doped textured electrodes with
the Al counter-electrode after the thermal treatment
(Fig. 6, curves 3, 3(+), 3(−) and 4, 4(+), 4(−)) have no prin-
ciple differences as compared with those for the Pt
counter-electrode, although AJ(t) magnitudes differ to
some extent. It is seen from Fig. 6 that for the pair Al-
SiTT
t〈Pd,10−3〉 AJ(t) values at V0 = 0, ±9.7 V are
respectively higher (curves 4, 4(+), 4(−)) than for the pair
Pt-SiTT
t〈Pd,10−3〉 (curves 2, 2(+), 2(−)), while for the pair Al-
SiTT
t 〈Pd,10−2〉 (curves 3, 3(+), 3(−)) these are lower than for
the pair Pt-SiTT
t 〈Pd,10−2〉 (curves 1, 1(+), 1(−)). These data
are indicative of the fact that the thermal treatment of
silicon electrodes doped with various Pd concentrations
results in their distinct properties.
If the thermal treatment of silicon electrodes doped
with Pd enhances water decomposition when using the
Pt counter-electrode, then when using Al and Yb
counter-electrodes the rate of water decomposition (AJ(t)
value) can both decrease and increase. The decrease can
be clearly seen in Fig. 7 if comparing AJ(t) dependences
for the pair Al-Sip 〈Pd,10−3〉 at V0 = 0, ±9.7 V before
(curves 3, 3(+), 3(−)) and after (curves 4, 4(+), 4(−)) the
thermal treatment. Moreover, at V0 = 0 even the
character of AJ(t) dependences is distinct: before the
thermal treatment it is ascending (curve 3) and after –
descending (curve 4). In its turn, it is caused by the
growth or drop of ∆Vc with time of exposure, which can
be related with different character of interaction between
Н+ ions and surfaces of the Si〈Pd〉 electrode before and
after its thermal treatment.
When using the Yb electrode (Figs 7 and 8), at
V0 = 0 AJ(t) values are decreased after the thermal
treatment (compare the curves 2 with curves 1). In the
case of the Sit〈Pd,10−2〉 electrode, lower AJ(t) values
after the thermal treatment are also observed if applying
to it (cathode) the voltage V0 = –9.7 V (curve 2(–) as
compared with the curve 1(−), Fig. 8). However, for the
electrode Sip〈Pd,10−3〉 at V0 = ±9.7 V AJ(t) values grow
after the treatment (curves 2(+), 2(−) as compared with the
curves 1(+), 1(−), Fig. 7). This diversity of results for
Sit〈Pd〉 and Sip〈Pd〉 electrodes at V0 = 0 and at V0 =
±9.7 V before and after the thermal treatment indicates
that there changed are the mechanisms of Н2О molecule
decomposition as well as interaction of Н+ ions with
Si〈Pd〉 electrodes after changing the abovementioned
factors. Fig. 8 illustrates also the dependence of AJ(t)
values obtained after treating the Sit〈Pd〉 on the
concentration of Pd doping impurity (curves 2, 2(−)
(10−2М) and curves 3, 3(+), 3(−) (10−3M)).
Thus, silicon electrodes Sip and Sit coupled with Pt
counter-electrodes enhance water decomposition
(increase AJ(t)) both after doping them with Pd and after
following thermal oxidation. When using Al and Yb
counter-electrodes, only in separate cases doping and the
following thermal oxidation of Sip and Sit electrodes also
increases AJ(t). The best result was obtained at V0 = 0
using the Sip〈Pd,10−3〉 electrode before its thermal
oxidation (AJmax(t) = 32·10−6 Ohm−1·cm−1).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2007. V. 10, N 1. P. 88-96.
© 2007, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
94
AJ, 10–6 Ohm–1cm–1
10 20 30
0
10
20
30
40
50
60
,
t,min
4
3
2
4(+)
1(+)2(+)1(-)
2(-)
3(+)
3(-)
1
4(-)
Fig. 7. AJ(t)-dependences inherent to the electrode pairs Yb-
Sip〈Pd,10−3〉 (1, 1(+), 1(−) – before; 2, 2(+), 2(−) – after thermal
oxidation of Sip〈Pd,10–3〉) and Al-Sip 〈Pd,10−3〉 (3, 3(+), 3(−) –
before; 4, 4(+), 4(−) – after thermal oxidation of Sip〈Pd,10−3〉). 1,
2, 3, 4 – V0 = 0, (+) − V0 = +9.7 V and (–) – V0 = –9.7 V on
Sip〈Pd〉.
AJ, 10–6 Ohm–1cm–1
10 20 30
0
10
20
,
t,min
3(-)
3
1(-)
3(+)
2(-)
1
2
Fig, 8. AJ(t)-dependences obtained for the electrode pairs Yb-
Sit〈Pd,10−2〉 (1, 1(−) – before; 2, 2(−) – after thermal oxidation of
Sit〈Pd,10−2〉) and Yb-Sit 〈Pd,10−3〉 (3, 3(+), 3(−) – after thermal
oxidation of Sit〈Pd,10−3〉). 1, 2, 3 – V0 = 0, 3(+) – V0 = +9.7 V;
1(−), 2(−), 3(−) – V0 = –9.7 V on Sit〈Pd〉.
At V0 = ±9.7 V, the best results (AJ(t) =
(40…55)·10−6 Ohm−1·cm−1) were obtained using the
thermally oxidized electrode SiTT
p〈Pd,10−3〉 coupled with
the Al counter-electrode as well as using the same
electrode at V0 = –9.7 V on it in pair with Yb counter-
electrode (AJ(t) = (25…56)·10−6 Ohm−1·cm−1).
III. To obtain high values of AJ(t) along with
catalytic activity of electrodes as to water
decomposition, it is necessary to have a higher
difference ∆φ between electrodes, that is a minimal
value of φ on the anode and its maximal value on the
cathode. Among catalytically active metals, the highest
φ values are inherent to Pt (5.32 eV) and Pd (5.00 eV)
that are oxidized very weakly. As Pt and Pd are precious
metals, it is reasonable to find cathodes from other
materials with relatively high φ-values. In the work [2],
we used the cathode from chromium silicide (Cr3Si). In
this work, we investigated the influence of thermal
oxidation of Cr3Si on AJ(t) dependences when using Pt,
Yb, Al as counter-electrodes. We studied nickel silicide
as well. The thermal treatment of Cr3Si and Ni3Si was
performed in the same manner as for silicon electrodes
at 850 °С in dry oxygen for one hour.
Measurements relatively to Pt electrode showed
that before the thermal treatment φ(Cr3Si) = 4.72 eV and
φ(Ni3Si) = 4.85 eV, while after it φ(Cr3Si)TT = 5.45 eV
and φ(Ni3Si)TT = 4.89 eV. Thus, the thermal oxidation
increased considerably the only φ-value for Cr3Sі. The
same was also found when studying AJ(t) dependences
with various counter-electrodes.
Shown in Fig. 9 are the AJ(t) dependences for the
pair Pt-Cr3Si before (1, 1(+), 1(−)) and after (2, 2(+), 2(−))
thermal oxidation. The AJ(t)-values obtained at V0 = 0
(curves 1, 2) indicate an alternating role of Cr3Sі
electrode coupled with the Pt one after the thermal
treatment: instead of “anode” it becomes “cathode”
(which is conventionally shown with the “sign” of the
curve 2), conductivity of the system Pt-Cr3Si being
increased. The AJ(t)-value is also essentially increased
after the thermal treatment when applying the additional
voltage V0 = ±9.7 V to the Cr3Sі electrode (curves 1(+),
1(−) and 2(+), 2(−)).
Positive changes of the AJ(t) dependences after the
thermal treatment of Cr3Si can be also observed in pairs
Al-Cr3Si (Fig. 10) and Yb-Cr3Si (Fig. 11). If the Yb
electrode is used, thermal oxidation of Cr3Si improves
all the AJ(t) dependences (at V0 = 0 and V0 = ±9.7 V).
Rather high AJ(t) values in the case of the thermally
oxidized Cr3Si allows to hope that there is a principle
opportunity to solve the problem of the efficient
electrode based on silicides and their oxides. For the pair
Al-Cr3Si, thermal oxidation improves AJ(t) dependences
at V0 = ±9.7 V. The AJ(t) dependences at V0 = 0 also
have a tendency to improve after thermal oxidation at
t ≥ 28 min exposures (curves 1, 2). In addition, Fig. 10
contains the AJ(t) dependence for the Cr3Si electrode
grinded with abrasive (grain diameter is close to 10 µm)
before oxidation (curve 1p). It is seen that grinding
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2007. V. 10, N 1. P. 88-96.
© 2007, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
95
AJ, 10–6 Ohm–1cm–1
10 20 30
-10
0
10
20
30
40
t,min2
1
1(+)
1(-)
2(+)
2(-)
Fig. 9. AJ(t)-dependences measured using the electrode pair
Pt-Cr3Si (1, 1(+), 1(−)–before thermal oxidation; 2, 2(+), 2(−) –
after thermal oxidation Cr3Si). 1, 2 – V0 = 0, (+) – V0 =
= +9.7 V, (–) – V0 = –9.7 V on Cr3Si.
AJ, 10–6 Ohm–1cm–1
10 20 30
0
10
20
t,min
2
1
1(-)
1(+)
2(-)
2(+)
1
10
Fig. 10. AJ(t)-dependences measured using the electrode
pair Al-Cr3Si (1, 1(+), 1(−), 1p – before thermal oxidation, 2,
2(+), 2(−) – after thermal oxidation of Cr3Si). 1, 1p, 2 – V0 = 0;
(+) – V0 = +9.7 V, (–) – V0 = –9.7 V on Cr3Si.
(texturing) the electrode results in its enhanced
catalytical activity when decomposing water before
making the circuit to close.
Nickel silicide Ni3Si was found to be less efficient
cathode even after its thermal oxidation. The AJ(t)-
values both before and after the thermal oxidation of
Ni3Si coupled with other electrodes were, as a rule, less
at V0 = 0 and V0 = ±9.7 V than the respective AJ(t)-values
for the Cr3Si electrode. Possibly, it is caused by some
distinctions in Ni3Si production technology, which
results in less porous structure of Ni3Si than that of
Cr3Si. Our work aimed at production of more efficient
cathodes based on silicides via changing their
composition and structure is going on.
The investigations performed in this work are a
definite stage on the way to obtain a source of current
and hydrogen by using the water decomposition based
on a natural difference of electrochemical potentials
inherent to various materials both existing in nature and
that produced artificially without great energy and
material expenses. To solve this problem, we plan, first
of all, to obtain positive solutions of the following tasks:
i) choice of catalytically active electrodes (both cathode
and anode) capable to decompose water and made of
various materials, studying the structural and chemical
states of their surface, which could provide a higher
potential difference between the electrodes without
losses of their catalytic activity; ii) stability and self-
reproducibility of electrodes in the course of operation;
iii) technical solution for creation of an efficient
electrochemical system capable to yield the energy from
water by using the electric current and hydrogen, etc.
It is noteworthy that this work is promising in
creation of a photoelectrochemical system that could use
not only the abovementioned processes to yield the
energy but conversion the solar energy, too. Really, the
oxide film SiO2 obtained on a silicon surface and
possessing a wide forbidden gap can be doped with
impurities (using, for example, the method of ion
implantation) that could absorb solar quanta and due to
two(or three)-photon electron transitions provide current
creation in the electrochemical system.
AJ, 10–6 Ohm–1cm–1
10 20 30
0
10
20
30
40
,
t,min
1
2
1(-)
1(+)
2(-)
2(+)
Fig. 11. AJ(t)-dependences measured using the electrode pair
Yb-Cr3Si (1, 1(+), 1(−) – before; 2, 2(+), 2(−) – after thermal
oxidation of Cr3Si). 1, 2 – V0 = 0; (+) – V0 = +9.7 V, (–) – V0 =
= –9.7 V on Cr3Si).
1P
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2007. V. 10, N 1. P. 88-96.
© 2007, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
96
4. Conclusions
1. Investigated were the time dependences of the
effective conductivity AJ(t) for electrochemical systems
capable to decompose water due to catalytic processes
on the electrodes and a potential difference between
them. We used electrodes based on modified silicon
(polished, chemically textured, doped with palladium
silicon surface, silicides of metals) and counter-
electrodes from Al, Pt, Yb both in absence of external
voltage in the electric circuit (V0 = 0) and in presence of
it (V0 = ±9.7 V) on the silicon electrode. It was found
that the most efficient water decomposition takes place
on the polished and then doped with Pd silicon surface.
2. Investigated was the influence of thermal
oxidation of silicon electrodes (850 °С, dry О2, 1 hour)
on AJ(t) dependences when using Al, Pt, Yb as counter-
electrodes. Found were both positive and negative role
of thermal oxidation, an essential role of Pd doping
before thermal oxidation, the opportunity to use thermal
oxidation for changing the electron work function φ for
silicon electrodes in a rather wide interval (φ ≈
≈ 4.5…11.5 eV). Thermal oxidation of Cr3Si was also
efficient for enhancing water decomposition.
Acknowledgments
Authors express their sincere gratitude to I.V. Kud’ for
growing chromium and nickel silicides.
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