Efficiency and stability of spectral sensitization of boron-doped-diamond electrodes through covalent anchoring of a donor-acceptor organic chromophore (P1)

Article abstract of DOI:10.1039/c6cp02209j

A novel procedure is developed for chemical modification of H-terminated B-doped diamond surfaces with a donor-π-bridge-acceptor molecule (P1). A cathodic photocurrent near 1 μA cm^-2 flows under 1 Sun (AM 1.5) illumination at the interface betw

Full text of DOI:10.1039/c6cp02209j

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Efficiency and stability of spectral sensitization
of boron-doped-diamond electrodes through
covalent anchoring of a donor–acceptor organic
chromophore (P1)†
Cite this:DOI: 10.1039/c6cp02209j
a
bc
de
fg
fh
Hana Krysova, Jan Barton, Vaclav Petrak, Radek Jurok, Martin Kuchar,
b
a
Petr Cigler* and Ladislav Kavan*
A novel procedure is developed for chemical modification of H-terminated B-doped diamond surfaces with
ꢀ2
a donor–p-bridge–acceptor molecule (P1). A cathodic photocurrent near 1 mA cm flows under 1 Sun
AM 1.5) illumination at the interface between the diamond electrode and aqueous electrolyte solution
(
containing dimethylviologen (electron mediator). The efficiency of this new electrode outperforms that
of the non-covalently modified diamond with the same dye. The found external quantum efficiency of
the P1-sensitized diamond is not far from that of the flat titania electrode sensitized by a standard
organometallic dye used in solar cells. However, the P1 dye, both pure and diamond-anchored, shows
significant instability during illumination by solar light. The degradation is a two-stage process in which
the initially photo-generated products further decompose in complicated dark reactions. These findings
need to be taken into account for optimization of organic chromophores for solar cells in general.
Received 4th April 2016,
Accepted 23rd May 2016
DOI: 10.1039/c6cp02209j
www.rsc.org/pccp
2
–4
p,n-DSCs are still by an order of magnitude less efficient.
Boron-doped diamond (BDD) is a popular electrode material in
1
. Introduction
1
6
Dye-sensitized solar cells (DSCs) usually operate with n-type electrochemistry. It could be similarly attractive for the p-DSC
semiconductor photoanodes (e.g. TiO ), but there are also sporadic photocathode, because it outperforms p-NiO in chemical and
2
2
,3
7,8
9,10
reports on p-type DSCs with e.g. p-NiO photocathodes and on electrochemical stability, optical transparency
the tandem device (p,n-DSC). While the state-of-art n-DSC diffusion coefficient.
and hole
4
11,12
However, the so far reported photo-
5
achieves a solar conversion efficiency of 14.3% the p-DSCs or electrochemical performance of sensitized p-BDD electrodes is
not satisfactory.
Spectral sensitization of BDD by organic dyes was pioneered
in 2008 by Zhong et al. Photocurrents of ca. 4–6 mA cm were
a
J. Heyrovsk ´y Institute of Physical Chemistry, v.v.i.,
1
3
ꢀ2
Academy of Sciences of the Czech Republic, Dolejskova 3, 18223 Prague 8,
Czech Republic. E-mail: kavan@jh-inst.cas.cz
observed under 1 Sun illumination in aqueous electrolyte solution
b
10
Institute of Organic Chemistry and Biochemistry, v.v.i.,
Academy of Sciences of the Czech Republic, Flemingovo nam. 2,
with dimethylviologen acting as the electron carrier. Sensitization
0
0
14
of BDD by Ru(SCN) (2,2 -bipyridine, 4,4 -dicarboxylate) (coded N3)
2
2
1
66 10 Prague 6, Czech Republic. E-mail: Cigler@uochb.cas.cz
ꢀ2
provided photocurrents of the order of 1–10 nA cm under 1 Sun.
A biophotovoltaic variant of these systems was recently demon-
strated by Caterino et al. using BDD with grafted bacterial
c
d
e
f
Faculty of Science, Charles University, Hlavova 2030, 128 40 Prague 2,
Czech Republic
1
5
Institute of Physics, v.v.i., Academy of Sciences of the Czech Republic,
Na Slovance 2, 182 21, Prague 8, Czech Republic
photoactive centers. They observed photocurrents of about
ꢀ2
Czech Technical University in Prague, Faculty of Biomedical Engineering,
S ´ı tn a´ 3105, 272 01 Kladno, Czech Republic
0
.3 mA cm under NIR light (870 nm; unspecified intensity).
ꢀ2
This photocurrent increased to ca. 2 mA cm if the sensitizer
loading was enhanced through a polymer brush on top of the
BDD surface, but the photocurrent was quite unstable during
Forensic Laboratory of Biologically Active Substances,
University of Chemistry and Technology Prague, Technick a´ 5,
1
66 28 Prague 6, Czech Republic
g
15
16
Department of Organic Chemistry, University of Chemistry and Technology Prague,
Technick a´ 5, Prague 6, 166 28, Czech Republic
long-term (hours) illumination. Yeap et al. modified BDD
with thiophene derivatives, and observed a photocurrent of ca.
h
Department of Chemistry of Natural Compounds, University of Chemistry and
Technology Prague, Technick a´ 5, Prague 6, 166 28, Czech Republic
Electronic supplementary information (ESI) available: Experimental details,
ꢀ2
1
50 nA cm under 0.15 Sun. To enhance the dye-loading, a
standard compact BDD film was replaced by a diamond foam, while
†
17
ca. 3-times larger photocurrent was observed. It further increased
XPS, HPLC, optical and Raman spectra, and additional electrochemical data.
See DOI: 10.1039/c6cp02209j
ꢀ
2
to 15–22 mA cm (at 1 Sun) during long-term illumination.
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This is the largest value for a dye-sensitized diamond, but the After the mixture was concentrated under reduced pressure, the
photo-activation is, unfortunately, associated with unclear oily residue was distilled under reduced pressure, bp 70–73 1C
1
7
1
degradation of the dye.
(20 mbar). Yield (15.32 g, 91%). H NMR (300 MHz, CDCl ):
3
The donor–p-bridge–acceptor dye, 4-(bis-4-[5-(2,2-dicyano- d = 6.38 (s, 1H), 5.90–5.78 (m, 1H), 5.30–5.23 (m, 2H), 3.99
1
3
vinyl)-thiophene-2-yl]-phenyl-amino)-benzoic acid (denoted P1, (t, J = 5.8 Hz, 2H) ppm. C NMR (75 MHz, CDCl ): d = 157.3
3
see Scheme S1 in the ESI†), is one of the most efficient chromo- (q, J = 37.0 Hz, 1C), 131.7, 117.7, 115.8 (q, J = 287.0 Hz, 1C),
1
8,19
ꢀ1
phores which is frequently used for the sensitization of p-NiO
and some other p-type semiconductors.
42.0 ppm. IR (film, cm ): 3310, 3094, 1704, 1557, 1434, 1161,
In addition to applica- 994, 931, 726. Anal. calcd for C NO: C 39.22; H 3.95;
tions in solar cells, the P1@NiO electrode was also successfully F 37.23; N 9.15. Found: C 39.83; H 3.72; F 37.02; N 9.03%.
20,21
5 6 3
H F
22,23
tested for hydrogen production by photo-electrolysis of water.
N-Allyltrifluoromethanesulfonamide. Allylamine (5.00 g,
24
Krysova et al. reported on non-covalent anchoring of P1 to 87.6 mmol) and triethylamine (10.63 g, 105.1 mmol) were mixed
diamond (coated by polyethyleneimine). This electrode provided with 150 mL of chloroform at ꢀ40 1C, and trifluoromethanesulfonic
ꢀ
2
cathodic photocurrents of about 100–150 nA cm at 0.18 Sun. The anhydride (25.96 g, 92.0 mmol) was added dropwise to the mixture
photocurrent was constant during illumination for ca. 250 s, but no under a nitrogen atmosphere. The solution was stirred at room
long-term tests have been carried out yet. Here we report on an temperature for four hours. The chloroform was removed under
alternative strategy towards P1-sensitized diamond, leading to reduced pressure and the remaining viscous liquid was dissolved in
improved efficiency. Nevertheless, illumination of the P1 dye by aqueous solution of NaOH (50 mL, 4 M). The aqueous solution was
1-Sun light causes also some degradation, which is analysed here in washed three times with 30 mL of chloroform. These organic phases
detail.
were discarded and the aqueous solution was acidified with
hydrochloric acid by weakly acidic reaction followed by washing
three times with 40 mL of chloroform. The organic extract was
2
. Experimental details
4
dried over anhydrous MgSO and the solvent was removed
under reduced pressure. The liquid residue was distilled twice
under reduced pressure, bp 75–80 1C (15 mbar). Yield 14.10 g
2.1. Diamond growth
2
Silicon substrates (5 ꢁ 10 mm ) were nucleated in an aqueous
detonation nanodiamond colloid (NanoAmando, NanoCarbon
Institute, nominal particle size of 4.9 ꢂ 0.1 nm). Deposition of
BDD was performed in an ASTeX 5010 (Seki Technotron, Japan)
Microwave plasma enhanced chemical vapour deposition (MW
PECVD) reactor with the following process parameters: 1% of
methane in hydrogen, a gas pressure of 50 mBar, a microwave
power of 1250 W and a substrate temperature of 720 1C as
monitored using a Williamson Pro92 dual wavelength pyrometer.
A growth time of 2 hours yielded 0.5 mm thick films. Addition of
1
(
85%). H NMR (300 MHz, CDCl
3
): d = 5.81 (ddt, 1H, CHQ,
Q, J = 17.1 Hz),
.15 (d, 1H, cis-CH Q, J = 10.1 Hz), 5.08 (brs, 1H, NH), 3.78
2
3
J = 17.1, 10.1, J = 5.6 Hz), 5.24 (d, 1H, trans-CH
2
5
(
2
3
13
d, 2H, NCH , J = 5.6 Hz) ppm. C NMR (75 MHz, CDCl3):
2
d = 132.21 (Q CH), 119.73 (q, CF , J = 320.9 Hz), 118.76
3
CF
19
(
QCH
2
), 46.64 (NCH
2
) ppm. F NMR (300 MHz, CDCl
3
):
ꢀ1
d = ꢀ79.12 ppm. IR (film, cm ): 3310, 3180, 3094, 2888,
1
721, 1648, 1440, 1372, 1261, 1231, 1151, 1072, 994, 928, 859.
Anal. calcd for C NO S: C 25.40; H 3.20; F 30.13; N 7.40; S
6.95. Found: C 26.10; H 2.90; F 30.32; N 7.28; S 16.74%.
-(Bis-4-[5-(2,2-dicyano-vinyl)-thiophene-2-yl]-phenyl-amino)-
4
H
6
F
3
2
1
trimethylboron (B(CH
during deposition translated into the B concentration in the bulk
3 3
) ) to the gas in a B/C ratio of 4000 ppm
4
benzoyl chloride (P1-Cl). 4-(Bis-4-[5-(2,2-dicyano-vinyl)-thiophene-2-
yl]-phenyl-amino)-benzoic acid (P1) (75 mg, 0.124 mmol) was dried
for 30 min under vacuum in a 20 mL dry flask using a magnetic
stirrer. Under an Ar atmosphere, 5 mL of dry dichloromethane
21
ꢀ3
material of ca. 3 ꢁ 10 at cm (ref. 25). Hydrogen termination of
the surface of BDD samples was performed in hydrogen plasma
using the same reactor. The hydrogenation was carried out in a
hydrogen flow of 300 sccm, at a pressure of 60 mBar, and 1000 W
microwave power for 10 min. Samples were allowed to cool to room
temperature under constant hydrogen flux.
(DCM) was added, followed by triethylamine (43 mL, 0.310 mmol,
2
.5 eq.) and SOCl (18 mL, 0.248 mmol, 2 eq.). The reaction mixture
2
was stirred for 3 hours at room temperature. The solvent was
evaporated and the solids were dissolved in 8 mL of dry THF. This
dispersion was filtered using a PTFE 0.45 mm filter and the solution
2.2. Syntheses
The P1 dye (purity 495%) was purchased from Dyenamo AB, of P1-Cl was used directly in the next reaction step for modification
Sweden. Other chemicals were supplied by Sigma-Aldrich and of 20 diamond sample plates.
used as received. MilliQ water was used in all syntheses.
2
.3. Modification of a BDD surface
Organic solvents were HPLC-grade and were dried before use.
2
Sonication was performed using an Elmasonic P60 H ultra- An approximately 1 ꢁ 0.5 cm BDD plate was immersed in
sound bath for 1 min at 30% power and 85 kHz. Drying was N-allyltrifluoroacetamide or N-allyltrifluoromethanesulfonamide
carried out under vacuum for 1 hour at 60 1C, if not stated in a quartz tube and secured with Ar. The plate was irradiated
otherwise.
using a water-cooled mercury-vapor discharge lamp for 3 hours.
N-Allyltrifluoroacetamide. To an ice-cold solution of methyl The product was washed with methanol, sonicated and dried.
trifluoroacetate (15.00 g, 117.1 mmol) in THF (40 mL) was For removal of the trifluoroacetyl protecting group, the plate was
added allylamine (6.28 g, 110.0 mmol). The reaction mixture was rinsed in 25% methanolic solution of tetramethylammonium
allowed to warm to room temperature and stirred for 3 hours at RT. hydroxide for 1 hour on a gel rocker, then washed with methanol,
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sonicated and dried. For removal of the trifluoromethylsulfonyl source (Oriel Xenon lamp, model 6269) in a dark room. The
group, the plate was covered with sodium bis(2-methoxyethoxy)- solar radiation (direct and diffuse) was simulated using an
aluminumhydride and heated overnight at 105 1C, quenched by Oriel AM 1.5 Global (81088) filter. The light intensity was
slow addition of methanol, the newly formed precipitate was measured using a standard Si photodiode (PV Measurements,
removed with 2 mL of 20% HCl, sonicated, washed with methanol, Inc., USA). For the quantum efficiency measurements (IPCE),
then again sonicated and dried. The dry plate was immersed in the light was monochromatized using a Newport 1/4 m grating
1
mL of dry tetrahydrofuran (THF) containing 10 mL of triethyl- monochromator (model 77200). Photoelectrochemical measure-
amine. P1-Cl solution obtained in the previous step (2 mL) was ments were performed in an Ar-saturated 0.1 M Na SO solution
2
4
2+
added and left overnight at room temperature under an Ar atmo- containing 5 mM dimethylviologen (MV ), pH E 7. High-
sphere on a gel rocker. The plate was then washed and sonicated resolution mass spectra were measured using an LTQ Orbitrap XL
with THF, DCM and methanol, and finally dried. To improve the (Thermo Fisher Scientific) instrument. Analysis of P1 degradation
s
grafting conjugation yield and properties of the sensitized BDD, was performed on an Agilent 1260 Infinity HPLC using Luna C8
the acylation with P1-Cl was repeated. The plate was sonicated for column (Phenomenex LC 150 ꢁ 4.6 mm, particle size: 5 mm).
5
minutes at 100% at 37 kHz then rinsed in a mixture of solvents Mobile phase: water (A) and acetonitrile (B), both with 0.1%
(
THF, DCM, and methanol) at room temperature for 3 days, trifluoroacetic acid. The linear gradient program was as follows:
washed and dried. 0–0.5 min: 95% of A and 5% of B; 16–17.5 min: 5% of A and
5% of B.
9
2.4. Methods
The X-ray photoelectron spectroscopy (XPS) spectra were recorded
using an Omicron Nanotechnology instrument equipped with a
monochromatized AlKa source (1486.7 eV) and a hemispherical
3
. Results and discussion
3.1. Dye-sensitization of diamond
analyzer operating in a constant analyzer energy mode with a Our synthetic strategy started from the hydrogen-terminated
multichannel detector. The CasaXPS program was used for spectral diamond surface, which was modified by photochemically-
2
6
analysis. Raman spectra were measured using a Renishaw InVia triggered grafting of alkenes. Due to their direct C–C coupling
Raman microscope, interfaced to an Olympus microscope to diamond, this type of modification is known to be chemically
2
7
(
objective 50ꢁ) with an excitation wavelength of 488 nm and resistant and stable. For instance, long chain (C10) alkenes
28
a power at the sample of 2.6 mW. The spectrometer was were used for the attachment of biomolecules to diamond.
ꢀ
1
calibrated by the F1g mode of Si at 520.2 cm . UV-Vis absorp- However, long spacers are not appropriate for sensitized photo-
tion spectra were recorded using a Perkin Elmer Lambda 1050 electrodes, because the dye should be located in the closest
UV-Vis-NIR spectrometer. The solution of P1 in absolute ethanol vicinity of the surface for efficient hole-injection. Here, we used
was sealed under ca. 0.1 bar He into a vacuum-tight quartz a short C3 allyl bearing a protected amino group (see Fig. 1).
optical cell (1 cm optical length).
The linker was attached to BDD under UV illumination, followed
Photoelectrochemical experiments were carried out in a one- by removal of the protecting groups which provided an amine-
compartment cell using a (micro-Autolab III, Metrohm) potentiostat modified surface. Notably, the linkers are transparent at the
controlled using the NOVA software. The BDD film was used as spectral maximum of the UV lamp (254 nm) which facilitates
a working electrode (Ag contact with a Au wire insulated by their anchoring. Sensitization of the BDD surface was carried out
TorrSeal epoxy coating), platinum was used as the counter by acylation of the free amines using the P1-Cl dye. As the
electrode and an Ag/AgCl electrode (sat. KCl) was used as the multilayer adsorption of P1 decreases the photoelectrochemical
reference electrode. The cell was equipped with a quartz optical activity, excessive dye was washed out under sonication (see the
window, and the electrode was illuminated using a white light Experimental section).
Fig. 1 Synthetic scheme for photochemically triggered modification of BDD using short amine-terminated linkers followed by sensitization by the P1-Cl
dye. Two different linkers (1 and 2) were used.
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Fig. 2 Raman spectra (at 488 nm excitation) of pure BDD at the Si
ꢀ1
substrate (black dashed curve; the Si signal from the substrate at 520 cm
is labeled ‘x’) and the pure P1 dye (red dashed curve). The spectra of
P1-modified BDD are shown for three samples: P1@BDD_a (black curve) is
for a fresh sample; P1@BDD_b (blue curve) is for an aged sample after
passing long-term photoelectrochemical tests (45 hours of chopped
illumination at 1 Sun, see Section 3.2); P1@BDD_c (red curve) is for an
as-received sample, which passed long-term irradiation using a 488 nm
laser. The strong Raman intensities of P1 are ascribed to resonance
enhancement, because the dye has the main optical absorption peak at
the 488 nm wavelength (cf. Fig. 7).
Fig. 3 XPS spectra of the studied samples. P1@BDD is the optimized
sample with linker 1; P1@BDD (2) is another sample with linker 2. Curves
are offset for clarity, but the intensity scale is identical for all spectra.
Because the electronic structure of the protecting group is
29
important for the grafting yield, we synthesized trifluoroacetamide
and trifluoromethylsulfonamide 2 derivatives of allylamine (Fig. 1).
Although 2 contains an excellent electron acceptor and should
1
The Raman spectrum of the product, P1@BDD (Fig. 2), is
roughly a superposition of the spectra of pure BDD and P1, with
29
therefore enhance the grafting conversion, we observed actually a
ꢀ1 smaller photoelectrochemical activity of the sensitized BDD (see
Section 3.2 and Fig. S4; ESI†). This can be explained by higher
chemical stability of the protecting group, which requires harsher
conditions for deprotection. In the case of a dye anchored through
linker 1, the surface concentration of sulfur is diagnostic for P1 only.
The concentration of S found by XPS (2.9 at%; Table S2a in the ESI†)
is not far from 4.5 at% S which is expected for the pure P1 molecule
ꢀ
1
small (o7 cm ) shifts of certain modes (e.g. the 1436 cm
line) and interestingly strong Raman intensities of the surface
anchored P1 dye which are, presumably, due to resonance
enhancement at the used laser line (488 nm). However, both
P1@BDD and pure P1 decompose under the laser light (see
Fig. 2 and discussion below). Photoelectron spectra (XPS; Fig. 3)
provide complementary information about the surface chemistry
of the products and reaction intermediates. The found surface
concentration of the boron dopant decreases from 2.5% in pure
BDD to 1.6%, 1.1% and 0.7% in the modified surfaces with
linker 1, deprotected intermediate and P1@BDD, respectively
(without H-atoms). This would indicate quite high surface coverage,
but the experimental error of XPS analysis and the found non-
stoichiometric S/N ratio (Table S2a in the ESI†) must be also taken
into account.
(
Table S2a; ESI†). The attenuation of B1s to about half of its
3
.2. Photoelectrochemical characterization
original value was observed for a monolayer of phenyl-based
linker. Hence, we estimate a nearly monolayer coverage in our Fig. 4 shows the response of our optimized P1@BDD electrode
case too. This matches the earlier work by Strother et al. who (linker 1) to chopped white light in an aqueous electrolyte
reported on photochemically functionalized diamond by various solution with methylviologen redox mediator. The reference
long-chain alkenes under similar experimental conditions. (non-sensitized BDD) provided small cathodic photocurrent too
The anchoring was reported to be covalent and unperturbed (ca. 20 nA cm ), which is ascribed to either sp impurities or
by polymerization under UV light; the surface coverage was specific B-states in the lattice. Our P1@BDD electrode exhibited
ca. one molecule per 10 surface C-atoms, i.e. E0.3 monolayer.
1
6
2
6
ꢀ
2
2
3
0
2
6
ꢀ2
a cathodic photocurrent of ca. 0.9 mA cm during the first
Deconvolution of the N1s line in our detailed XPS spectra allows ca. 5 minutes of chopped illumination of 1 Sun intensity. (The
distinction of two chemically-shifted species, which are tentatively photocurrent was proportional to the light intensity, amounting
ꢀ
2
assigned to the –CN group from the P1 dye and the amino groups to e.g. 0.2 mA cm at 0.2 Sun for the fresh electrode).
from the linker and the triphenylamino group from the P1 dye
Table S2b and Fig. S3; ESI†).
The mechanism of photocurrent generation starts from
photon absorption in P1, creating electron–hole pairs, which
(
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Fig. 5 Long-term chronoamperometric measurement of the diamond
electrode sensitized with P1 (red curve) and the blank, non-sensitized diamond
ꢀ
2
electrode (black curve). Chopped white light illumination (100 mW cm
;
simulated AM 1.5G solar spectrum, 10 min dark/light interval). Electrolyte
2 4
solution: 5 mM dimethylviologen in 0.1 M Na SO , pH 7. Applied potential
bias: ꢀ0.3 V vs. Ag/AgCl.
Fig. 4 Chronoamperometric plot for a blank BDD electrode and that
sensitized with P1. Freshly prepared P1@BDD electrode (red curve, top)
3
1
and that after passing long-term chopped irradiation (45 hour at 1 Sun). single crystal TiO
Electrolyte solution: 0.1 M Na SO containing 5 mM dimethylviologen,
applied bias voltage ꢀ0.3 V vs. Ag/AgCl. Illumination with simulated AM 1.5
solar light of varying intensity. Curves are offset for clarity, but the current
density scale is identical in all cases.
anatase was 0.11%. Our P1@BDD shows
2
2
4
roughly half of this value at lmax E 350 nm (where the P1 dye
7
2
ꢀ1 19
has an optical peak with e350 = 3.47 ꢁ 10 cm mol
)
and the
estimated roughness factor of our BDD is ca. 4 (ref. 17).
Our found maximum IPCE (Fig. 6) is ca. 4-times larger than
that reported previously for a non-covalently anchored P1 to
2
4
subsequently dissociate in the donor–p-bridge–acceptor mole- BDD. This confirms that the covalent anchoring is beneficial
cular structure of P1 (Scheme S1, ESI†). The separated holes are for the efficient sensitization of diamond electrodes. The blank
injected into the valence band of BDD, and the electrons flow to (non-sensitized) BDD expectedly shows negligible IPCEs. The
the MV (dimethylviologen) electron carrier in the electrolyte IPCEs of the 5 h-aged electrode are similar or larger (particularly
solution which is finally regenerated by dark charge transfer at in the UV region) than those of the fresh electrode, but drop
2
+
1
0,13,14,16,24
the counter electrode (Fig. S5, ESI†).
The HOMO significantly for the 45 h-aged electrode. This is qualitatively
1
8
level of P1 (ꢀ5.8 eV) is well below the valence band of the consistent with the performance under white light (Fig. 5). The
H-terminated diamond (ca. ꢀ4.2 eV for diamond in a vacuum, IPCE maximum is blue-shifted against that of the P1-sensitized
7
18
and ca. ꢀ5.5 eV for a diamond contacting electrolyte solution).
p-NiO, which is reminiscent of the same spectrum of non-
2
4
The LUMO level of P1 corresponds to a potential of ꢀ0.87 V vs. covalently anchored P1 to BDD. Our maximal photocurrent
1
8
2+/+
ꢀ2
SHE, which is more negative compared to the MV
redox under simulated AM 1.5 solar light (E1.5 mA cm ) is similar or
potential (ꢀ0.45 V vs. SHE). Hence, both the hole injection into
2
+
the VB of diamond and the electron transfer to MV have a
reasonable driving force of ca. 0.5 eV.
The long-term stability of our P1@BDD was tested under
chopped 1 Sun illumination. Interestingly, the photocurrent
ꢀ
2
ꢀ2
initially increased during E5 hours from 1 mA cm to 1.5 mA cm
,
ꢀ
2
but then dropped to ca. 0.6 mA cm after about 40 hours (Fig. 5).
These changes were further addressed by the external quantum
efficiency (also called IPCE = incident photon to current conversion
efficiency) vs. wavelength at various stages of long-term illumina-
tion. It is defined as
IPCE = iphhv/eP = Zinj(1ꢀ10ꢀ
GRe
)
(1)
where iph is the photocurrent density, h is Planck’s constant, v
is the photon frequency, P is the incident light power, e is
electron charge, Zinj is the quantum yield of charge injection
into the semiconductor, G is the dye’s surface coverage, R is
the electrode roughness factor and e extinction coefficient.
Fig. 6 IPCE spectrum for the fresh P1@BDD electrode and that after
passing 4 or 45 hours of photoelectrochemical test with chopped illumination
at 1 Sun (aged). A reference spectrum for pristine, non-sensitized BDD
7
2
ꢀ1
For instance, the N3 dye (e530 = 1.2710 cm mol , G E
ꢀ
2
0
.55 molecules nm ) provides the theoretical IPCE = 0.27% on a
2 4
electrode is also shown. Electrolyte solution 0.1 M Na SO + 5 mM
flat surface (R E 1) assuming Zinj = 100%. Experimental IPCE for a dimethylviologen; bias voltage ꢀ0.3 V vs. Ag/AgCl.
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better compared to other previously reported values on flat (non- of the chemical structures. In the aged solution, we observed
1
0,13–16
textured) BDD electrodes.
Nevertheless, the photoelectro- oligomers originating most likely from polymerization of dye frag-
chemical performance of the sensitized BDD is still far from that ments containing heterocyclic thiophene rings (data not provided).
of sensitized p-NiO, the latter exhibiting photocurrents in the
mA cm domain for good samples.
Although this model experiment gave evidence of photo-
chemical instability of P1 in ethanolic solution, the conditions
occurring on the sensitized diamond surface can be dissimilar,
and the stability of the surface-grafted dye can be better. For
example, we observed a substantial photocurrent on a P1-modified
ꢀ
2
2–4,18,19
3.3. Stability of the P1 dye
The Raman spectra of the photoelectrochemically treated electrode diamond electrode even after 440 hours irradiation by chopped
see Section 3.2) show distinct changes resembling the photo- light at 1 Sun intensity (see Fig. 5) indicating the presence of some
(
chemically degraded pure P1@BDD and solid P1, see Fig. 2 and photoelectrochemically active dye molecules on the surface.
Fig. S6 (ESI†), respectively. These changes were further investigated This conclusion is also supported by Raman spectra (Fig. 2).
by optical spectra and by liquid chromatography (HPLC) analysis of The covalent attachment of P1 to the diamond surface provides
the degradation products in a model system, i.e. ethanolic solution obviously different electronic conditions, driving the fate of the
of P1.
P1 excited state (with hole injection into the diamond electrode).
The P1 dye has two absorption maxima in tetrahydrofuran Furthermore, the slower diffusion of solvent molecules, impurities
1
9
ꢀ1
ꢀ1
solution: 348 nm (e = 34 720 M cm ) and 481 nm (e = and radicals from the bulk solution to the interface are other
ꢀ
1
ꢀ1
5
7 900 M cm ) or 345 nm and 468 nm in acetonitrile. In possible factors contributing to better stability of the P1 dye on the
ꢀ1
ꢀ1
ethanol, we found 349 nm (e = 26 300 M cm ) and 488 nm diamond surface, as compared to its strikingly fast photochemical
ꢀ
1
ꢀ1
(
e = 51 500 M cm ). To test the photochemical stability of degradation in the solution.
the ethanolic P1 solution, its spectrum was recorded after
For comparison, we also tested the photochemical stability
and 24 hours of continuous illumination at 1 Sun in a closed, of another typical dye, which is frequently used in dye-sensitized
vacuum-sealed optical cell (Fig. S7; ESI†). Interestingly, the Vis solar cells, the N719 dye: di-tetrabutylammonium cis-bis(iso-
band bleached completely, while the UV band attenuated and thiocyanato)bis(2,2 -bipyridyl-4,4 -dicarboxylato)ruthenium(II)
blue-shifted (Fig. 7). This effect is reminiscent of the photo- (purchased from Dyesol, SA, Switzerland). This complex is very
chemical degradation of some other diamond-sensitizing dyes similar to the N3 dye which was also previously applied for
thiophene-based).
The complete decomposition of P1 in ethanolic solution optical spectra of N719 are shown in Fig. S9 (ESI†). It is
upon 24 hours of illumination is clearly confirmed by HPLC obvious that this dye is fairly stable in ethanolic solution
5
0
0
2
4
14
(
diamond sensitization, albeit with moderate success. The
(
Fig. S8; ESI†). The degradation products manifest themselves under the conditions, when P1 degrades totally. Our results on
3
2,33
by several chromatographic peaks upon UV detection. More- N719 essentially match those of earlier studies.
They
over, the irradiated solution was unstable too, and transformed found that the degradation of N719 occurred mostly through
during long-term (2 month) storage to more than 10 different S/N thiocyanate isomerization and through exchange of NCS
products (Fig. S8, ESI†). We attempted to analyze both mixtures ligands with e.g. solvent molecules, but without massive
using high-resolution mass spectroscopy and HPLC-MS and to decomposition of the organometallic skeleton as a whole.
resolve the mechanism of P1 decomposition. Unfortunately, The reported degradation products exhibited slightly blue-shifted
the MS analysis did not provide reliable data for reconstruction optical bands which is in accord with our spectrum in Fig. S9
(ESI†). Nevertheless, the decomposition of N719 and similar dyes
like N3 and other organometallic complexes under the conditions
32,33
of illuminated solar cells
is obviously not that dramatic, like the
degradation of the P1 dye which is studied here.
To our knowledge, there are no long-term stability tests of P1
1
8–21
in p-DSCs available.
1
Photocurrent decay at a timescale of
2
0 s was reported for the P1@NiO electrode during H generation
by photo-electrolysis of water.
decomposition or to its delamination from the electrode surface.
2
22,23
It was ascribed to co-catalyst
23
However, our results show that also the inherent photochemical
instability of organic dyes must be considered for the design of
p-DSCs and for sensitized photo-electrolytic water splitting.
4
. Conclusions
Fig. 7 UV-Vis spectrum of the solution of the P1 dye in absolute ethanol
ꢀ5 ꢀ1
A novel synthetic procedure is developed for covalent anchoring
of a P1 dye to the surface of a H-terminated B-doped diamond
electrode. The sensitized diamond electrode exhibits cathodic
(starting concentration 10 mol L ). Optical length 1 cm. The spectrum
of a fresh solution (red curve) and that after illumination with a white light
of 1 Sun intensity for 5 hours (blue curve) or 24 hours (black curve).
Phys. Chem. Chem. Phys.
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Paper
photocurrent upon illumination with solar light, which pre- 12 S. Mori, S. Fukuda, S. Sumikura, Y. Takeda, Y. Tamaki,
destines it for application in p-type dye-sensitized solar cells.
E. Suzuki and T. Abe, J. Phys. Chem. C, 2008, 112, 16134.
The photoelectrochemical efficiency considerably outperforms 13 Y. L. Zhong, K. P. Loh, A. Midya and Z. K. Chen, Chem.
that of non-covalently derivatized BDD with P1. The found external
Mater., 2008, 20, 3137.
quantum efficiency (IPCE) of the P1-sensitized diamond is not 14 W. S. Yeap, X. Liu, D. Bevk, A. Pasquarelli, L. Lutsen,
far from that of the flat titania electrode sensitized by a standard
Ru–bipyridine complex, which has been frequently used in the
M. Fahlman, W. Maes and K. Haenen, ACS Appl. Mater.
Interfaces, 2014, 6, 10322.
n-type solar cells. The sensitized photocurrent is reasonably 15 R. Caterino, R. Csiki, A. Lyuleeva, J. Pfisterer, M. Wiesinger,
stable during ca. 40 hours of illumination at 1 Sun (AM 1.5),
but there are certain photo-initiated changes of the P1 dye, both
pure and diamond-anchored.
Model experiments in ethanolic solutions show that P1 is
sensitive to photochemical degradation in solar light, as com-
S. D. Janssens, K. Haenen, A. Cattani-Scholz, M. Stutzmann
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16 W. S. Yeap, D. Bevk, X. Liu, H. Krysova, A. Pasquarelli,
D. Vanderzande, L. Lutsen, L. Kavan, M. Fahlman, W. Maes
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pared to another generic dye for solar cells, i.e. N-719. The 17 H. Krysova, L. Kavan, Z. Vlckova-Zivcova, W. S. Yeap,
former dye is totally degraded upon 24 hours of illumination in
solution to more than ten (yet unidentified) products. The
P. Verstappen, W. Maes, K. Haenen, F. Gao and C. E. Nebel,
RSC Adv., 2015, 5, 81069.
degradation is a two-stage process in which the initially 18 P. Qin, H. Zhu, T. Edvinsson, G. Boschloo, A. Hagfeldt and
photo-generated products further decompose in complicated dark
L. Sun, J. Am. Chem. Soc., 2008, 130, 8570.
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Acknowledgements
This work was supported by the Czech Science Foundation,
contract No. 13-31783S.
2 F. Li, K. Fan, B. Xu, E. Gabrielsson, Q. Daniel, L. Li and
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