Electrochemical preparation of photosensitive porous n-type Si electrodes, modified with Pt and Ru nanoparticles

Article abstract of DOI:10.1016/j.electacta.2010.02.008

A novel electrochemical procedure for preparation of the very stable, thin modifying layer onto the n-type Si surface was elaborated. The modification consisted of platinum or/and ruthenium ultrafine particles etched into the porous Si film. A unique sequence of modifications was applied: at first the metal particles were evenly electrodeposited onto a flat silicon surface, and in the next electrochemical step the porous structure was produced. The platinum coverage and mean particle diameter were well controlled by the electrochemical programs. All the attempts and progress in modifications were monitored by scanning electron microscope (SEM) observations. Furthermore, the materials obtained were compared with the non-porous, Pt or/and Ru modified electrodes by testing them as anodes in the photoelectrochemical (PEC) cell with organic Br₂/2Br^- solution. In general, the porous photo-anodes gave higher output powers and the light-to-electricity conversion efficiencies. The best performance was observed for the PEC cell employing the porous anode with sequentially electrodeposited Ru and Pt particles, respectively (PS-Si/Ru/Pt).¹1 PS-Si means the porous silicon film; Si/Pt/Ru describes the sequence of metal depositions onto Si, in this case the Pt deposition is followed by the Ru deposition. This cell maintained good electrical parameter values during the 2-week tests, having a maximum output power equal to 0.23 mW/cm² and a cell conversion efficiency of 8.5%. The PS-Si/Pt photo-anode gained 0.21 mW/cm² and 7.8%, respectively.

Full text of DOI:10.1016/j.electacta.2010.02.008

Electrochimica Acta 55 (2010) 4395–4401
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrochemical preparation of photosensitive porous n-type Si electrodes,
modified with Pt and Ru nanoparticles
M. Macherzynskia,b,∗, G. Milczareka,c, S. Mamykina,d, V. Romanyuka,d, A. Kasuyaa
a
Center for Interdisciplinary Research, Tohoku University, Sendai, Japan
Department of Environmental Sciences in Energy Research, AGH University of Science and Technology, Krakow, Poland
Institute of Chemistry and Technical Electrochemistry, Poznan University of Technology, Poznan, Poland
Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, Kiev, Ukraine
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel electrochemical procedure for preparation of the very stable, thin modifying layer onto the n-type
Si surface was elaborated. The modification consisted of platinum or/and ruthenium ultrafine particles
etched into the porous Si film. A unique sequence of modifications was applied: at first the metal particles
were evenly electrodeposited onto a flat silicon surface, and in the next electrochemical step the porous
structure was produced. The platinum coverage and mean particle diameter were well controlled by the
electrochemical programs. All the attempts and progress in modifications were monitored by scanning
electron microscope (SEM) observations. Furthermore, the materials obtained were compared with the
Received 20 January 2010
Accepted 2 February 2010
Available online 10 February 2010
Keywords:
Electrochemical deposition
Platinum nanoparticles
Photo-etching
Porous silicon
PEC cell
non-porous, Pt or/and Ru modified electrodes by testing them as anodes in the photoelectrochemical
−
(
PEC) cell with organic Br2/2Br solution.
In general, the porous photo-anodes gave higher output powers and the light-to-electricity conversion
efficiencies. The best performance was observed for the PEC cell employing the porous anode with sequen-
1
tially electrodeposited Ru and Pt particles, respectively (PS-Si/Ru/Pt). This cell maintained good electrical
2
parameter values during the 2-week tests, having a maximum output power equal to 0.23 mW/cm and a
2
cell conversion efficiency of 8.5%. The PS-Si/Pt photo-anode gained 0.21 mW/cm and 7.8%, respectively.
©
2010 Elsevier Ltd. All rights reserved.
1
. Introduction
been used as photosensitive electrodes, because of the antireflec-
tive properties of porous silicon in dry [12,13] and wet [14] cells.
The controlled electrochemical deposition of Pt ultrafine particles
onto n-Si electrodes used in PEC cells was described in [9,15], and
The theoretical and practical aspects of electrochemical nucle-
ation and growth of metals on semiconductor surfaces has attracted
attention for the last 20 years [1–7]. Metal/crystalline silicon
materials have many potential applications in solar-energy con-
version devices, photocatalysis, photoelectrolysis, electrocatalysis,
sensor production and electronics. For the Pt/silicon junction to
be considered as a satisfactory photoelectrochemical (PEC) cell
electrode-material, a few requirements have to be fulfilled: (i)
deposition of ultrafine and strongly adherent metal structures,
2
in [16], where the very efficient PEC solar cell (13%; 13.3 mW/cm )
with the Pt/porous silicon/n-type Si electrode was introduced.
When exposed to light, n-type Si (n-Si) is able to produce
electron–hole pairs, and in the next step, the holes can oxidize the
reduced form of the redox couple from the electrolyte, being in
contact with the Si electrode. It contributes to the photocurrent
recorded in the external circuit [17,18]. On the other hand, silicon
(especially porous silicon) surfaces are very sensitive to the super-
ficial corrosion in wet cells (decomposition or creation of the thick
superficial SiO2 layer) and can lose their photoelectrical properties
quickly.
(
ii) a moderate coverage, (iii) a small direct contact area between
metal and semiconductor. These requirements have been defined
and intensively studied for ultrafine Pt particles/silicon elec-
trodes [3,8,9]. The metal/porous silicon/silicon junctions are firstly
promising electroluminescent devices [10,11], but they have also
In the present work, we have employed and developed several
electrochemical procedures to obtain reproducible porous n-type
Si (PS-Si) electrodes, modified with metal (M) nanoparticles [M = Pt,
Ru or Pt/Ru (Pt-deposited first, Ru-second) and Ru/Pt]. Metal depo-
1
“PS-Si” means the porous silicon film; “Si/Pt/Ru” describes the sequence of metal
sition from acidic PtCl62 and Ru(NO)Cl solutions, followed by the
−
depositions onto Si, in this case the Pt deposition is followed by the Ru deposition.
3
∗
Corresponding author at: Department of Environmental Sciences in Energy
anodic photo-etching in the HF/ethanol solution, was performed.
This is a novel sequence of modifications. So far, the porous Si has
been formed on the n-Si surface, modified with Pt particles, by
Research, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059
Krakow, Poland. Tel.: +48 12 617 22 93; fax: +48 12 617 23 99.
E-mail address: macherzy@agh.edu.pl (M. Macherzynski).
0
013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.02.008

4
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M. Macherzynski et al. / Electrochimica Acta 55 (2010) 4395–4401
immersing the wafers in a HF solution, without a bias [16] or the
particles have either been deposited onto the porous silicon [19],
or the Pt deposition plus silicon-etching modification have been
carried out simultaneously [20].
The materials obtained were compared with the non-porous
electrodes modified in the same way with Pt or/and Ru, by testing
all of them as photo-anodes in the PEC cell.
2
. Experimental
Reagent-grade chemicals and deionized water were used in
all experiments. Single crystal n-Si wafers (ꢀ100ꢁ, ca.7 ꢀ cm) were
cut into 1 m × 1 m squares, sequentially washed with acetone and
ethanol in ultrasonic bath for 15 min, rinsed with water, and etched
with 10% HF solution for 60 s. Finally the pieces were rinsed out with
water, dried and rear side ohmic contacts were formed with the
In–Ga alloy. These electrodes and their modifications were used in
all electrochemical and PEC cell experiments. The cylindrical Teflon
electrochemical cell with the O-ring electrode holder (d = 0.4 cm;
2
the working area 0.125 cm ) placed on the bottom, turned out to
be a convenient setup to keep solutions away from the rear side of
silicon electrodes and to carry out reproducible experiments.
A Pt-mesh counter electrode was used in HF-containing solu-
tions and during the PEC cell tests. The three-electrode setup with
Pt-mesh auxiliary, and Ag/AgCl (ca. +0.23 V vs. SHE) reference elec-
trodes, was used in the HF-free plating solutions. The electrodes
were linked to an Electrochemical Analyzer (CH Instruments, model
4
3
40). Electrochemical metal deposition details are given in Section
.1. The surface imaging was performed with a scanning electron
microscope (SEM) (HITACHI S-430E).
The electrochemical photo-etching was performed with the
mixture of HF (46–48% water solution) and ethanol (1:1), by the
Fig. 1. Optimization of bromine (a) and bromide (b) concentrations in the PEC cell
employing non-aqueous electrolyte.
2
chronopotentiometric (CP) technique (in most cases 20 mA/cm ,
2
5
1
s), under cold light illumination at ca. 30 mW/cm (Intralux 5000-
VOLPI, equipped with a flexible light guide and focusing lens).
of a chosen cell during a long-term (5 h) period was tested. The
light source used in the PEC cell was an Intralux 5000-1 (VOLPI)
In this case no external filter has been applied between the light
source and samples. Unless Pt deposition was controlled with the
help of SEM, the Pt plating solution was removed from the Teflon
cell, the electrode was rinsed with ethanol, dried with nitrogen,
and the etching solution was added. In order to remove solutions,
which could remain after modifications, all electrodes were rinsed
2
lamp operating at 2.7 W/cm . A daylight conversion filter has been
used to change the shape of light spectrum from the halogen cold
light source to the sun-like form, with a wavelength of maximum
intensity around 550 nm.
◦
◦
with ethanol, dried with nitrogen and finally dried at 150 C (40 C
3. Results and discussion
higher than a boiling point of the H O–HF azeotrope) in a furnace
2
for 1 h, under nitrogen flow. Samples were kept in a furnace until
the temperature dropped to 70 C. A coverage with metal particles
Reproducible electrochemical deposition of metal on electrodes
can be fairly well controlled if a suitable electrochemical technique
and parameters, as well as proper plating electrolyte (composition,
concentrations), are carefully chosen. Another important factor is
the pretreatment of the surface prior to the modification.
◦
(
the visible ones, bigger than ca. 4–6 nm) was calculated from SEM
pictures.
The non-aqueous electrolyte used in PEC cells was a mixture
of bromides from 0.1 mol/L tetrabutylammonium bromide (TBAB),
bromine (Br )−4 mmol/L, and 0.1 mol/L tetrabutylammonium per-
3.1. Electrochemical deposition of Pt nanoparticles and the
anodic photo-etching
2
chlorate (TBAP), all dissolved in acetonitrile. The electrolyte
composition was optimized on a basis of a series of experiments,
the results of which are presented in Fig. 1. As can be seen, the
◦
2
Fig. 2a is a view (tilt 25 ) of a porous silicon film [photo-etching,
−
optimal concentration of Br is in the range of 0.05–0.15 mol/L, but
CP technique −40 mA/cm , 10 s in a HF–ethanol solution (1:1)]. The
fill factors slightly decrease for the higher concentrations, whereas
the optimal concentration of Br2 is between 4 and 8 mmol/L. The
smallest possible concentration (4 mmol/L) was chosen because of
the strong oxidizing properties of bromine. In each cell with a newly
modified anode, the open-circuit potential (OCP), short-circuit pho-
tocurrent (SCPC) and i–U curves were recorded under illumination.
The tests were repeated over a 2-week period. After each series
of measurements the electrodes were rinsed with ethanol, dried
with nitrogen and kept in a desiccator. The maximum output pow-
ers (Pmax) and fill factors were calculated from the recorded i–U
curves, and PEC cell efficiencies (ꢁ) were estimated. The stability
anodic oxidation and formation of porous n-Si in HF solutions [21]
occurs in the reverse biased region, and is possible only when the n-
Si electrode is illuminated. The light produces indispensable holes
[22], which are absent in n-Si.
An attempt to deposit well-distributed fine particles onto
the porous film did not succeed. The primary solution for
platinum deposition consisted of hexachloroplatinic (IV) acid
(H PtCl )—1 mmol/L and perchloric acid (HClO )—0.1 mol/L. The
2
6
4
cyclic voltammetry (CV) technique (scan rate 0.1 V/s, 50 cycles in
the range −0.4 V to +0.2 V vs. Ag/AgCl), was tentatively employed.
Application of this modification method did not give reproducible

M. Macherzynski et al. / Electrochimica Acta 55 (2010) 4395–4401
4397
Fig. 2. Electrodeposition of Pt particles onto the porous n-Si film: (a) SEM view of the porous n-Si layer ca. 150 nm; (b) SEM view of the porous n-Si modified with Pt in a
mmol/L H2PtCl6 solution in 0.1 mol/L HClO4—CV: 50 cycles, 0.1 V/s, scan range −0.4 V to +0.2 V vs. Ag/AgCl; (c and d) SEM views of the n-Si surface after repeated etching and
cathodic Pt deposition—CV: 10 cycles, 0.1 V/s, under illumination in plating electrolyte from Table 1, scan ranges: −1.37 to 0.0 V vs. Pt and −1.87 to 0.0 V vs. Pt, respectively.
1
results. The metal nucleation process started only in the specific
probably less porous, less oxidized and more conducting) areas of
trary to our expectations, deposited particles did not penetrate the
porous layer, thus no direct contact of Pt with silicon surface was
possible, even for much thinner porous films than the one shown
in Fig. 2a.
Another approach was the use of wide-scan CV (repeated etch-
ing and cathodic platinum deposition, 10 cycles, 0.1 V/s) under
illumination, in two ranges: (a) −1.37 to 0.0 V or (b) −1.87 to
(
the electrode, often on the rims of samples. Relatively big particle
aggregates grow only around these nuclei, whereas the remaining
parts of porous sample stayed almost uncovered. Even if the sin-
gle Pt particles grew in some parts of porous silicon, they did not
adhere well to the film, and were poorly distributed (Fig. 2b). Con-
◦
Fig. 3. SEM view of the porous n-Si/Pt electrode obtained according to the procedure from Table 1: (a) detailed overview; (b) tilt 45 ; (c and d) overviews, magnification
2
k, and 20 k, respectively.

4
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M. Macherzynski et al. / Electrochimica Acta 55 (2010) 4395–4401
Fig. 4. Current–voltage (a) and current–time (b) curves for CV and CA deposition of Pt and Ru, respectively, on silicon. Curves (c) and (d) represent the same processes
performed on Ru- and Pt-modified silicon, in order to obtain mixed Ru/Pt and Pt/Ru deposits, respectively. Plating procedures are in Table 1 (step CV-1a) and from Section
3
.2 (CA).
0
.0 V vs. Pt electrode—Fig. 2c and d, respectively. The modifica-
Pt deposition experiments and tests of different etching programs.
The preliminary PEC cell experiments (data not presented) were
also helpful.
tion was performed in a solution of H PtCl6 (1 mmol/L) in HClO4
2
(
(
0.1 mol/L), mixed in (1:1) proportion with etching electrolyte [HF
46–48%) and ethanol (1:1)]. The potential of platinum counter
At first, the platinum cathodic deposition (CV-1a) was run in
the HF/ethanol-containing electrolyte, in order to grow a sufficient
number of nuclei. As can be concluded from cyclic voltammograms
shown in Fig. 4a, the deposition process starts in the first cathodic
potential sweep at ca. −1.0 V vs. Pt electrode indicating a high reac-
tion overvoltage. In the subsequent scans the deposition starts at ca.
−1.1 V and is overlapped with hydrogen evolution as can be judged
from characteristic exponential increase of the cathodic current
with the increasing cathodic overpotential. The lack of the distinct
voltammetric peak that could be assigned with the reduction of
electrode in this solution was ca. +0.9 V vs. SHE. Fig. 2c reveals
a much better spatial and size distribution of deposited particles
than Fig. 2d. The coverage was 3.5–4 × 10 particles/cm . The elon-
gated cathodic scan of the CV program (b) resulted in deposition of
many particles bigger than 80 nm (Fig. 2d). In this case the cover-
age was 2.5–3 × 10 particles/cm . The developed porous structure
was clearly seen, but the platinum islands also did not adhere well
to the electrode. No well-covered areas were noticed on the rims of
the samples—a result of the generation of hydrogen bubbles during
the cathodic scans. Without further development, both procedures
could not be applied to get more uniform modifications with a
microstructure satisfactorily controlled by the electrochemical pro-
cedures.
In order to obtain more stable and adherent ultrafine Pt par-
ticles “etched into” porous silicon, which are in intimate contact
with the semiconductor, a two-step modification procedure was
developed (Table 1 and Fig. 3). It was based on the aforementioned
9
2
9
2
H PtCl6 is also indicative of kinetic rather than diffusional control
2
of the Pt deposition. To avoid the influence of hydrogen evolution
on deposition, an intensive stirring of the electrolyte was applied.
The addition of HF to the plating solution significantly improved the
uniformity of platinum particles and its spatial distribution. A SiO2
layer was directly removed by HF from the Si surface, which prob-
ably activated the electrode and overcame the weak interaction
energy [5] between the Si substrate and Pt during nucleation.
Fig. 5. SEM views of deposited Pt particles onto n-Si. Plating electrolyte from Table 1. CV: scan rate 0.1 V/s, scan range −0.77 to −1.77 V vs. Pt: (a) 10 cycles: mean diameter
1
0
2
10
2
is ca. 20 nm, coverage = 1.7–1.75 × 10 particles/cm ; (b) 50 cycles: mean diameter is ca. 40 nm, coverage = 1.15–1.2 × 10 particles/cm ; (c) 100 cycles: mean diameter is
10
2
ca. 60 nm, coverage = 0.85–0.9 × 10 particles/cm .

M. Macherzynski et al. / Electrochimica Acta 55 (2010) 4395–4401
4399
Table 1
The optimized electrochemical procedure for modification of Si electrodes with Pt nanoparticles (PS-Si/Pt), which were used in further PEC cell experiments.
Step
Technique and parameters
Electrolyte
Remarks
(
1) Pt deposition
(1a) CV, scan rate 0.1 V/s, −0.77 V to
H2PtCl4 (1 mmol/L) in
HClO4 (0.1 mol/L):HF
(46–48%):ethanol (2:1:1)
EP vs. SHE = +0.79 V
Intensive stirring with
magnetic stirrer
−
1.77 V vs. Pt 15 cycles, and
(
−
1b) CV, scan rate 0.1 V/s, −0.87 V to
1.87 V vs. Pt 5 cycles
3
(
2) Etching
CP −20 mA/cm , 5 s
HF (46–48%):ethanol (1:1)
–
Illumination—cold light at
2
3
0 mW/cm , turned on 30 s
prior to etching
Constant N2 flow
◦
◦
C
(
3) Drying/evaporation
150 C-1 h + ca. 1 h to cool down to 70
dure in Table 1, a coverage of 1.2–1.3 × 1010 Pt particles/cm2 was
observed. The maximum diameter of particles was 30–40 nm, but
the majority did not exceed 15 nm.
Fig. 5 shows three electrodes modified with Pt in the potential
window CV-1a, after 10, 50, and 100 cycles. A significant influence
of number of applied cycles on diameter and coverage is seen. The
coverage decreases with the number of CV cycles, which indicates
the agglomeration of growing particles during the prolonged elec-
trodeposition. The step CV-1b was applied in order to get more
intensive particle growth and creation of new nuclei (a significant
number of ultrafine particles) in a short time. Two potential win-
dows CV-1a and CV-1b were matched to obtain the three regions
within each CV segment: (i) 0 A—the rest current at the potentials
down to −1.1 V, (ii) a moderate particle growth within potentials
from −1.1 to ca. −1.55 V, and (iii) an intensive growth accompa-
nied with further nucleation within a potential window of −1.55
to −1.87 V. The maximum current of electrodeposition was equal
In the cases when the maximum deposition current (in the
2
step CV-1b) did not reach 1.25 mA (10 mA/cm ), the minimum CP
etching current of 40 mA/cm2 had to be applied to observe the
distinct “immersion” of particles inside the porous film (data not
2
presented). When the etching current was set to 5 mA/cm , Pt par-
ticles did not penetrate the porous film significantly (Fig. 6).
The time of the etching step mainly determines the thickness
of a porous film, which should not exceed 100 nm if used in PEC
cells [25]. The thickness of porous film should not also significantly
exceed the diameter of metal particles. From this point of view, the
etching time in our procedure was set to 5–10 s. The short etching
time can also reduce the movement and agglomeration of particles.
A PC etching current of 20 mA/cm2 within 5 s was finally chosen.
2
to 1.25–1.63 mA (10–13 mA/cm ). The level of this current proba-
bly indicates the quality of the Pt–silicon contact, but also a level
of hydrogen evolution on growing platinum islands. The quality of
metal–silicon contact (area, adhesion, possible presence and thick-
ness of silica interlayers) influences the second etching step, and
a way how the Pt particles interact with the creating porous film.
We revealed that in most cases, during the electrochemical etch-
ing step, the Pt particles penetrated (were etched into) the porous
film (Fig. 3). A similar effect was observed for p-type Si modified
with Ag particles [23]. Some particles were still observed near the
surface, but the rest must have got deeper inside the porous struc-
ture. During the etching process, particles also moved along the
surface and often created aggregates or short chains, which was
observed previously in [15] for the aggregating Pt atoms. Appar-
ently, the particles strongly catalyze reductive dissolution of silicon
3.2. Electrochemical deposition of Ru, Pt/Ru and Ru/Pt
nanoparticles
Ruthenium electrodeposition was performed in a solution of
RuCl , HCl and HNO , which is generally regarded as Ru(NO)Cl
3
3
3
(ruthenium nitrosyl trichloride) [26]. The stock solution contained
Ru—15 mmol/L in HCl—2 mol/L mixed (1:1) with 60% HNO . Prior
3
to electrodeposition, it was ten times diluted (1:8:1) in 1 M HCl
and ethanol. We applied chronoamperometry (CA) with con-
stant potential pulses: 30× 10 s, imposed convertibly at −300 and
−
900 mV vs. Ag/AgCl. The CA program was preceded with the CP
5 mA/cm , 2 s) pretreatment step. As shown in Fig. 4b, nearly equal
2
(
[
15,16,24], creating meso- and macropores, whereas the flat, sur-
currents are associated with each potential pulse indicating con-
stant increase of new nucleation centers rather than a growth of
preformed particles.
face micropore areas exist between these pores (Fig. 3a and b).
The uniformity of modification is well seen in Fig. 3c. Some struc-
ture details and distribution of particles and macropores can be
observed in Fig. 3d. For samples prepared according to the proce-
In sequential Pt/Ru (Pt-first, Ru-second) depositions, platinum
was electrodeposited according to programs (CV-1a and CV-1b) in
Table 1. The sequence of plating was important. The charge den-
sity was up to five times higher when Ru was deposited directly
onto Si, comparing to its depositions onto Si/Pt electrodes (com-
pare Fig. 4b and d). Moreover, in this case the increasing currents
were associated with each potential pulse indicating a growth of
preformed particles. On the other hand Pt deposition was not sig-
nificantly influenced by the presence of Ru on silicon substrate with
the exception that Pt deposition started at ca. 0.05 V less negative
overvoltage (Fig. 4c).
To obtain a porous structure, metal modifications were followed
by anodic photo-etching, according to the program in Table 1. A
more detailed study about optimization of controlled electrodepo-
sition of Ru and Ru/Pt ultrafine particles onto n-Si is in preparation.
3.3. PEC cell characteristics
Table 2 shows the comparison of eight electrodes modified
in different ways, used as photo-anodes in the PEC cell, under a
lamp operating at 2.7 mW/cm . In order to prolong the stability
of the photosensitive electrodes prepared in our laboratory, the
Fig. 6. SEM view of Pt nanoparticles onto n-Si. Electrodeposition—CV: 0.1 V/s, 10
cycles −0.77 to −1.77 V plus 10 cycles −0.87 to −1.87 V vs. Pt, in plating electrolyte
2
2
from Table 1, was followed by the slight etching program—CP: 5 mA/cm , 80 s.

4
400
M. Macherzynski et al. / Electrochimica Acta 55 (2010) 4395–4401
Table 2
Electrical parameters in PEC cells with anodes prepared according to the procedures
from Table 1 and Section 3.2. Measurements were repeated over a 2-week period, in
the 1st, 2nd, 7th, and 14th day after preparation. The ranges of OCP and SCPC from
these 4 measurements are shown. The maximum output powers, the day they were
recorded, as well as estimated maximum conversion efficiencies, are also presented.
Anode
OCP [mV]
SCPC [mA/cm2]
Pmax
ꢁ [%]
[
mW/cm2]
(day)
PS-Si/Rn/Pt
PS-Si/Pt/Ru
Si/Ru/Pt
Si/Pt/Ru
PS-Si/Pt
PS-Si/Ru
Si/Pt
326–384
377–390
304–316
276–281
404–424
350–423
341–349
339–393
0.96–1.84
0.87–1.61
0.82–1.13
0.76–1.19
1.93–2.13
1.07–1.69
0.22–0.95
0.39–0.93
0.23
0.15
0.09
0.07
0.21
0.13
0.06
0.05
(7.14)
(2)
(2)
(2)
(14)
(1)
8.5
5.6
3.3
2.6
7.8
4.8
2.2
1.9
(1)
(1)
Si/Ru
◦
−
Fig. 8. SEM view (tilt 45 ) of the porous n-Si/Ru/Pt photo-anode obtained according
non-aqueous electrolyte containing the Br /2Br reversible redox
2
to the procedure from Section 3.2, after PEC cell experiments (four repeated series
of tests over a 2-week period—total ca. 1 h of effective operation in electrolyte).
couple, dissolved in TBAP acetonitrile solution, was used. The tests
were repeated over a 2-week period, in the 1st, 2nd, 7th, and 14th
day after photo-anode preparation (total ca. 1 h of effective opera-
tion in electrolyte).
The extents of the OCP and SCPC measurements show that
porous modifications resulted in higher maximum OCPs and SCPCs
compared to the non-porous modifications. In consequence, their
output powers and also conversion efficiencies were significantly
higher. Six anodes gained the best output power parameter in the
first or second day of tests. Two others were more stable. In the case
of the PS-Si/Pt anode, the stable PEC cell parameters were recorded
in all four tests, whereas in the case of the PS-Si/Ru/Pt anode, a
rise (Fig. 7) and stabilization of PEC cell parameters with time was
observed. It is possible that this phenomenon was connected with
the fact that some of bigger and less-adherent metal particles might
have broken off the surface during the PEC cell tests, whereas the
ultrafine metal structure, which gives the contribution to the pho-
tocurrent, remained unchanged and stable. Fig. 8 shows the view of
PS-Si/Ru/Pt anode after four repeated series of PEC cell tests. A not
destroyed structure of Ru/Pt nanoparticles and pores was observed.
The alloy anodes were distinguished by higher fill factors, but this
parameter was generally low and did not exceed 0.46 (Fig. 7a). In
case of PS-Si/Pt and PS-Si/Ru (1st and 2nd day) photo-anodes, the
highest (more than 0.42 V) OCPs were recorded.
Fig. 9. SEM views of the porous n-Si/Pt photo-anode: (a) after PEC cell experiments
(four repeated series of tests over a 2-week period—total ca. 1 h of effective operation
in electrolyte); (b) after additional 5 h permanent operation at the applied potential
difference of 200 mV.
Fig. 7. i–U and P curves for the PEC cell equipped with a porous n-Si/Ru/Pt photo-
anode obtained according to the procedure from Section 3.2: (a) 1st day of tests; (b)
7
th day of tests.

M. Macherzynski et al. / Electrochimica Acta 55 (2010) 4395–4401
4401
Fig. 9a shows a surface of the PS-Si/Pt anode after four repeated
Acknowledgement
series of tests over a 2-week period (total ca. 1 h of effective opera-
tion in electrolyte). Afterwards the anode was put to the test of 5 h
permanent operation at the applied potential difference of 200 mV,
which roughly corresponds to a maximum power output judged
from the i–U cell characteristics. During this test, the generated
photocurrent decreased by 30% in the first hour and another 7%
during the next 4 h. Fig. 9b shows a surface of the electrode after
this long-term test. The pore structure and Pt particles distribution
was not changed noticeably. The coverage is even ca. 10% higher
Many thanks to Dr. Andrew Parker from University of Reading
for fruitful comments and editorial help.
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(
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[
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1
0
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(
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[
[
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[
−
anodes in the PEC cell with non-aqueous Br /2Br electrolyte.
2
Porous photo-anodes exhibited higher OCPs, SCPCs, maximum out-
put powers and better conversion efficiencies when compared to
their non-porous counterparts. Two porous anodes (PS-Si/Pt and
PS-Si/Ru/Pt) produced much more stable and higher photocurrents.
They also maintained good electrical parameter values during the
[
(
1
[
6
2
-week repeated tests.