Supporting Information
Electro-Assembly of a Chromophore–Catalyst Bilayer for Water
Oxidation and Photocatalytic Water Splitting**
Dennis L. Ashford, Benjamin D. Sherman, Robert A. Binstead, Joseph L. Templeton, and
Thomas J. Meyer*
anie_201410944_sm_miscellaneous_information.pdf
Supporting Information
Contents
Experimental Section…………………………………………………………………………….………….S3
Figure S1. Upper pane (black traces), current traces measured at a nTiO2-RuPdvb2+-poly1 (W1) electrode
with the electrode illuminated from 60 to 960 s during the trace (solid trace) and without a light phase
(dashed trace) performed at same bias. Current measured with an applied bias of 0.2 V vs. SCE in 0.1
M phosphate buffer with 0.02 M hydroquinone and 0.4 M NaClO4 supporting electrolyte. Lower
pane (blue traces, solid trace with illumination, dashed trace in dark for all time at same bias), current
traces measured concurrently with the above trace at an FTO electrode (W2) with an applied bias of 0.5 V vs. SCE. Fraction of the charge passed at W2 with that at W1 from 60 s to the end of the trace
(corrected for background current, dashed traces) used to determine the collection efficiency of the
dual electrode assembly. ....................................................................................................................... 6
Figure S2. Upper pane (black trace), current trace measured at a nTiO2-RuPdvb2+-poly1 (W1) electrode
with the electrode poised at 1.1 V vs. SCE from 0 to 900 s and 0.4 V vs. SCE from 900 to 1800 s.
Lower pane (blue trace), current trace measured concurrently with the above trace at an FTO
electrode (W2) with an applied bias of -0.8 V vs. SCE. Comparison of the charge passed from 0 to
900 s in the black trace with the total charge passed in the blue trace gave a faradaic efficiency for O 2
production of 36%. Current traces were recorded 0.1 M phosphate buffer at pH 7 with 0.4 M NaClO 4.
.............................................................................................................................................................. 7
Figure S3. Photocurrent vs. applied bias for (red) nTiO2-RuPdvb2+-poly1 and (black) nTiO2-RuPdvb2+.
Solid lines show measurements taken under illumination and dash traces show dark measurements.
The electrolyte contained 0.1 M phosphate buffer at pH 7 with 0.4 M NaClO4. .................................. 7
Figure S4. Poly1 surface coverage on GC versus the number of multi-step potential cycles held at -2.4 V
(vs Ag/AgNO3) for 1 s followed by 0 V for 10 s in dry PC solution of 1 (0.5 mM in complex, 0.1 M
TBAPF6), Pt counter, and Ag/AgNO3 reference electrode. 1 monolayer = 1 x 10 -10 mol cm-2. ............ 8
Figure S5. Peak current for the RuIII/II redox couple versus the scan rate for GC-poly1 in aqueous 0.1 M
HClO4, Pt-wire counter, and Ag/AgCl reference electrode. ................................................................. 8
Figure S6. Cyclic voltammograms of GC-poly1 in aqueous 0.1 M HClO4 (black) and with 4% CH3CN
added (red). ........................................................................................................................................... 9
Figure S7. Cyclic voltammograms of GC-poly1 (red) and 2 in solution (black) in (A) 0.1 M HClO4, pH 4
(0.1 M acetate, 0.5 M NaClO4), and pH 7 (0.1 M phosphate, 0.5 M NaClO4) with 30% CF3CH2OH.
Taken at 50 mV/s, 23ºC, Pt-wire counter, and SCE reference electrode. CVs were normalized to the
RuIII/II redox couple for comparison purposes. ...................................................................................... 9
Figure S8. Cyclic voltammograms of GC-poly1 (red) and 2 in solution (black) in (A) 0.1 M HClO4, pH 4
(0.1 M acetate, 0.5 M NaClO4), and pH 7 (0.1 M phosphate, 0.5 M NaClO4) with 4% CH3CN. Taken
at 50 mV/s, 23 ℃, Pt-wire counter, and SCE reference electrode. CVs were normalized to the RuIII/II
redox couple for comparison purposes. ...............................................................................................10
Figure S9. Linear voltammograms collected at a RRDE with a GC disk with electropolymerized poly1
(solid lines) and corresponding currents measured at a Pt ring (dashed lines). Presented in this figure
are the results for: (solid red) GC disk with electropolymerized poly1, (dashed red) Pt ring poised at 0.445 V vs. SCE measured simultaneously with the solid red trace, (solid blue) GC disk with
electropolymerized poly1, (dashed blue) Pt ring poised at 0.065 V vs. SCE measured simultaneously
with the solid blue trace. Linear voltammograms (solid lines) were recorded at a scan rate of 5 mV/s
with an electrode rotation rate of 500 RPM and the electrolyte contained 0.1 M phosphate buffer at
S1
pH 7 with 0.4 M NaClO4. The Pt ring currents are corrected for the collection efficiency (23%)
determined for the ring-disk electrode assembly. .................................................................................10
S2
Experimental
Materials. [Ru(𝜂 6-benzene)(Cl)2]2,[1] RuPdvb2+,[2] and [2,2'-bipyridine]-6,6'-dicarboxylic acid[3] were synthesized as
previously reported. Distilled water was further purified by using a Milli-Q Ultrapure water purification system. All
other reagents were ACS grade and used without further purification. Fluorine-doped tin oxide (FTO)-coated glass
-2
Hartford Glass; sheet resistance 15 Ω cm ), was cut into 10 mm × 40 mm strips and used as the substrate for TiO 2
nanoparticle films.
Upon purification and isolation, both complexes 1 and 2 were kept in a glovebox. Both of these complexes
decompose both in the solid state and when dissolved in solution likely to a dimeric or trimeric species.
Ru(bda)(4-vinylpyridine)2 (1)
[Ru(Bz)(Cl)2]2 (0.3 g, 0.6 mmol) and [2,2'-bipyridine]-6,6'-dicarboxylic acid (0.29 g, 1.19 mmol) were dissolved in ~
30 mL of methanol. The solution was thoroughly degassed with argon and heated to reflux for 2.5 hours under an
atmosphere of argon. To the solution was added 4-vinylpyridine (2 mL) and triethylamine (1.5 mL). The reaction was
then refluxed overnight under an atmosphere of argon. The reaction was cooled to room temperature, filtered, and taken
to dryness. Acetone was added to the slurry and the mixture was taken to dryness again (likely helping remove the
excess vinylpyridine). The solid was stirred in ether overnight and collected by suction filtration. The crude product
was purified by size exclusion chromatography (Sephadex LH- 20) with 1:1 MeOH:H2O as eluent. Similar fractions
(based on UV/Vis absorption spectroscopy) were combined, and the solvent was removed by rotary evaporation. The
dark-red solid was triturated with ether and collected. 1H NMR (400 MHz, DMSO-d6 with a small amount of Lascorbic acid) 𝛿(ppm) 8.68 (d, 2H), 7.93 (t, 4H), 7.67 (d, 4H), 7.32 (d, 4H), 6.60 (dd, 2H), 6.06 (d, 2H), 5.53 (d, 2H).
HR-ESI-MS (MeOH): m/z = 555.067+, [M + H+]+ = 555.060, m/z = 1109.115, [M2 + H+]+ = 1109.11. Anal. Found
(Calc) for C26H26N4O9Ru: C, 48.12 (48.83); H, 3.88 (4.10); N, 8.23 (8.76).
Ru(bda)(4-picoline)2 (2)
[Ru(𝜂 6-benzene)(Cl)2]2 (0.30 g, 0.60 mmol) and [2,2'-bipyridine]-6,6'-dicarboxylic acid (0.292 g, 1.2 mmol) were
dissolved in ~30 mL anhydrous CH3OH and the solution was thoroughly degassed with argon. The solution was heated
to reflux under an atmosphere of argon for 6 hrs and to the solution was added 4-methylpyridine (2 mL, 20 mmol) and
S3
anhydrous triethylamine (1.5 mL). The solution was refluxed overnight under an atmosphere of argon, cooled, and the
solution was cannula filtered to remove an orange precipitate that forms during the reaction, presumably trans-Ru(4methylpyridine)4Cl2. The CH3OH was removed from the filtrate under vacuum and anhydrous acetone (~40 mL) was
added to the slurry. The acetone was removed under vacuum to yield a dark red slurry, likely due to excess 4methylpyridine. The crude product was precipitated by the addition of ether (~60 mL) and the solid was collected by
suction filtration. The crude product was redissolved in CH2Cl2 and loaded onto Al2O3, washed with 1:1
hexanes:CH2Cl2, CH2Cl2, and CH2Cl2 with 1% CH3OH added. The pure product was then washed off of the column
using CH2Cl2 with 2% CH3OH added. Like fractions (based on TLC) were collected, taking to dryness by rotary
evaporation, triturated with ether, and collected (0.15 g, 23%). Characterization matches that of previously reported. [4]
1H NMR (600 MHz, DMSO-d with a small amount of L-ascorbic acid) 𝛿(ppm) 8.66 (d, 2H), 7.92 (d, 2H), 7.85 (t, 2H),
6
7.53 (d, 4H), 7.09 (d, 4H), 2.18 (s, 6H).
Metal Oxide Films. nTiO2 films, typically 4 - 7 μm thick (~20 nm particle diameter), with a coating area of roughly 10
mm × 15 mm, were synthesized according to a literature procedure.[5]
Electrochemical and Photophysical Measurements.
Absorption spectra were obtained by placing the dry derivatized films perpendicular to the detection beam path of the
spectrophotometer. The expression, Γ= A(𝜆)/𝜀(𝜆)/1000, was used to calculate surface coverages.[6] Molar extinction
coefficients (𝜀) in H2O were used; A(λ) was the absorbance at the MLCT λmax. All measurements were carried out of
films loaded from methanol solutions of 150 𝜇M in ruthenium complex, which gave complete surface coverage (Γ= 8 ×
10-8 mol cm-2).
Electrochemical measurements were conducted on a CH Instruments 660D potentiostat with a Pt-mesh or Pt-wire
counter electrode, and a Ag/AgNO3 (0.01 M AgNO3/0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF6) in
CH3CN (-0.09 V vs. Fc+/0)[7] or Ag/AgCl (3 M NaCl; 0.197 V vs. NHE) reference electrode. E1/2 values were obtained
from the peak currents in square wave voltammograms or from averaging cathodic and anodic potentials at peak
current values (Ep,c and Ep,a) in cyclic voltammograms.
Electropolymerization of 1 was carried out in a three-compartment electrochemical cell under an atmosphere
of argon. All solutions were dried over MgSO4, filtered, and deaerated with argon for 10 min before
electropolymerization. Electropolymerization to give film-based structures was conducted on both glassy carbon
(GC) electrodes and, for photoelectrochemical studies, electro-assembly formation on mesoporous, nanoparticle
films of titanium dioxide (nTiO2). In a typical electropolymerization experiment, the working electrode was
cycled in a solution of 1 (0.5 mM in complex, 0.1 M TBAPF6/PC; PC = propylene carbonate) from 0 to -2.4 V
(vs Ag/AgNO3) either by cyclic voltammetry or chronocoulometry with the potential stepped from 0 V to -2.4 V
S4
in successive scans. Solutions were continually stirred during the reductive procedure to promote percolation of 1
throughout the electrode films.[2, 8]
Surface coverages on planar FTO electrodes were calculated using Eq. S1 where Q is the integrated current
under the RuIII/II redox couple of poly1, F is Faraday’s constant (96,485 C), n is the number of electrons transferred (n =
1), and A is the area of the electrode (~ 1 cm2).
Γ=
𝑄
𝑛𝐹𝐴
(Eq. S1)
Rotating ring disk electrochemical (RRDE) measurements were performed using a Pine rotator (model AFMSRCE)
and Pine AFCBP1 bipotentiostat. The rotating ring-disk electrode (Pine model AFEGR1PT) contained a glassy carbon
disk and platinum ring. A platinum counter electrode and Ag/AgCl reference electrode were used. The Ag/AgCl
reference was separated from the working solution with a vycor junction to prevent Cl - contamination of the working
electrolyte. The electrolyte used for RRDE experiments consisted of 0.1 M phosphate buffer at pH 7 with 0.4 M
NaClO4 or 0.1 M HClO4 (pH 1) with 0.4 M NaClO4 as indicated. Potentials are reported vs. SCE (saturated calomel
electrode) by subtracting 45 mV from the potential measured versus Ag/AgCl. For turnover frequency (TOF)
measurements, the potential at the Pt ring electrode was set at -0.5 V vs. Ag/AgCl at pH 7 and -0.2 V vs. Ag/AgCl at
pH 1 for detection of O2 generated at the glassy carbon disk. These potentials were selected from linear sweep
voltammetry experiments carried out using the Pt ring surface in the same conditions by comparing the currents with
air saturated solution and N2 saturated solution. An applied potential at -0.5 V vs. Ag/AgCl at pH 7 and -0.2 V vs.
Ag/AgCl at pH 1 gave higher cathodic current in the presence of O2 without substantial background current from the
production of H2 at the platinum surface. For determination of the faradaic efficiency for the production of O 2, the
collection efficiency of the ring-disk electrode was measured using potassium ferricyanide and found to be 23%.
Turnover frequencies (TOF) for O2 production were also determined by RRDE measurements. In these
measurements, the current arising from O2 reduction at the ring at a fixed potential at the GC disk was measured over
known time intervals. . The TOF was determined using eq S2 where Q is the charged passed, 𝜑 is the collection
efficiency of the RRDE electrode (23%), mol 1 is the moles of 1 present on the GC disk as determined by integrating
CV scans for the catalyst Ru(III/II) wave, and t is the measurement interval for the O2 reduction experiment (120 s). In
this analysis it is assumed that the 4 e- reduction of O2 occurs at the Pt ring. When evaluated in this way, the TOF
values are a direct measure of the rate of O2 generation.
TOF =
Q
4φ(mol𝟏)𝑡
(Eq. S2)
Two Electrode Method for Measurement of O2
A duel working electrode setup, similar in design to that described by Mallouk et al., was used for the electrochemical
detection of oxygen.[9] This approach consisted of a four electrode setup including a Pt counter electrode, saturated
calomel reference electrode, and two FTO based working electrodes. Of the two FTO electrodes, one was prepared as a
photoanode utilizing a mesoporous TiO2 semiconducting layer (working electrode 1, W1), and the other was a clean
FTO electrode with no surface modification (working electrode 2, W2). The two FTO electrodes were placed with the
conductive sides facing with a thin, 1 mm thick glass spacer placed on both lateral edges between the electrodes
creating an even internal gap between the two. Epoxy (Hysol E-00CL) was applied along the edges to secure the
assembly. Using conductive silver epoxy (Chemtronics CW2400), wire leads were secured to the FTO electrodes with
care taken to avoid any contact between the two. The detection of photogenerated oxygen at W1 was carried out by
monitoring the current at W2 when this electrode was poised at a sufficiently negative potential to reduce O 2.
Previous work has established the observation of oxygen reduction on FTO,[10] however, the observation of cathodic
current at W2 cannot be unambiguously assigned to the reduction of O2 here without consideration of other possible
electrochemically active species in the system. To determine the optimal potential for carrying out the reduction of O2
on FTO, CV scans were taken in air saturated and N2 saturated buffer. In 0.1 M phosphate buffer at pH 7 with 0.4 M
NaClO4, at an applied potential of -0.8 V vs. SCE the observed current showed a clear dependence on the presence of
O2 with limited background current. Applied potentials negative of -0.9 V vs. SCE generated substantial background
current in the presence or absence of O2 likely resulting from the reductive decomposition of the FTO film and possibly
H2 generation.[11] Therefore potentials negative of -0.8 V vs. SCE at W2 were avoided. Control experiments, both with
W1 poised to generate O2 and W2 poised at more positive potential (-0.2 V vs. SCE) and with W1 poised negative of
S5
the onset of O2 production and W2 biased at -0.8 V vs. SCE, did not show substantial currents at W2 and led to the
assignment of the of the cathodic current at W2 as due to the reduction of O 2.
The faradaic efficiency for the light driven production of O2 was determined by comparison of the charge passed at W1
with that at W2. For such an experiment (Figure 7 in text), the assembly would be illuminated for a 10 min period
followed by a 15 min dark period. The charge collected (background corrected for residual O 2 in solution) at W2 during
the entire trace divided by charge collected at W1 during illumination, divided by the collection efficiency of the setup,
gives the faradaic efficiency for O2 production. This analysis assumes both the 4 e- oxidation of water at W1 and the 4
e- reduction of O2 at W2. The collection efficiency of each two electrode assembly was determined with the use of
hydroquinone (QH2) in solution. A similar experiment to that described above was carried out with 20 mM QH 2 in
solution, such that the current under illumination resulted from the oxidation of QH 2; the collector electrode, W2 was
poised at -0.5 V vs. SCE to reduce the photogenerated (semi)quinone and the charge passed at W1 and W2 compared to
derive the collection efficiency. Figure S1 shows a characteristic experiment with QH2. Collection efficiencies were
found between 60% and 80%. In addition, the collection efficiency of the dual electrode assembly was investigated
electrochemically with an FTO-FTO cell with K3Fe(CN)6 in solution. The charge resulting from reduction to Fe(II) at
one electrode was compared to the charge collected from the oxidation back to Fe(III) at the second electrode and gave
an average collection efficiency of 70%.
Figure S1. Upper pane (black traces), current traces measured at a nTiO2-RuPdvb2+-poly1 (W1) electrode with the
electrode illuminated from 60 to 960 s during the trace (solid trace) and without a light phase (dashed trace) performed
at same bias. Current measured with an applied bias of 0.2 V vs. SCE in 0.1 M phosphate buffer with 0.02 M
hydroquinone and 0.4 M NaClO4 supporting electrolyte. Lower pane (blue traces, solid trace with illumination, dashed
trace in dark for all time at same bias), current traces measured concurrently with the above trace at an FTO electrode
(W2) with an applied bias of -0.5 V vs. SCE. Fraction of the charge passed at W2 with that at W1 from 60 s to the end
of the trace (corrected for background current, dashed traces) used to determine the collection efficiency of the dual
electrode assembly.
Along with the light driven measurements outlined above, electrochemically driven experiments were also conducted
using the dual working electrode assembly. Figure S2 shows the results using the same electrode assembly as in Figure
7 in the text, however with the nTiO2-RuPdvb2+-poly1 poised at 1.1 V vs. SCE from 0 to 900 s, followed by 0.4 V vs.
SCE from 900 to 1800 s. The blue trace shows the currents measured at the FTO collector (W2) poised at -0.8 V vs.
SCE. The measurements were taken in 0.1 M phosphate buffer at pH 7 with 0.4 M NaClO4. Comparison of the charge
passed at each electrode with correction for the measured collection efficiency of the setup gave a faradaic efficiency of
36% for O2 production. Under the same solution conditions, measurements by RRDE with only poly1 gave a faradic
efficiency of 62%. While this method gave a lower efficiency as compared with RRDE, it should be noted this
measurement was taken on an FTO-TiO2 electrode as opposed to GC surface and also contained RuPdvb2+ in addition
to poly1.
S6
Figure S2. Upper pane (black trace), current trace measured at a nTiO2-RuPdvb2+-poly1 (W1) electrode with the
electrode poised at 1.1 V vs. SCE from 0 to 900 s and 0.4 V vs. SCE from 900 to 1800 s. Lower pane (blue trace),
current trace measured concurrently with the above trace at an FTO electrode (W2) with an applied bias of -0.8 V vs.
SCE. Comparison of the charge passed from 0 to 900 s in the black trace with the total charge passed in the blue trace
gave a faradaic efficiency for O2 production of 36%. Current traces were recorded 0.1 M phosphate buffer at pH 7 with
0.4 M NaClO4.
Photoelectrochemical Measurements
A Thor Labs HPLS-30-04 light source was used for all photochemical experiments. Samples were positioned to receive
100 mW cm-2 (400 to 700 nm) with the light intensity determined with an Oriel Instruments 91150V reference cell. A
380 nm longpass filter was used to avoid direct bandgap excitation of the TiO 2. A CHI 760E or Pine AFCBP1
potentiostat was used with a Pt counter and SCE reference. The geometric illuminated area was measured for
photocurrents reported as current densities.
Figure S3. Photocurrent vs. applied bias for (red) nTiO2-RuPdvb2+-poly1 and (black) nTiO2-RuPdvb2+. Solid lines
show measurements taken under illumination and dash traces show dark measurements. The electrolyte contained 0.1
M phosphate buffer at pH 7 with 0.4 M NaClO4.
S7
Figure S4. Poly1 surface coverage on GC versus the number of multi-step potential cycles held at -2.4 V (vs
Ag/AgNO3) for 1 s followed by 0 V for 10 s in dry PC solution of 1 (0.5 mM in complex, 0.1 M TBAPF6), Pt counter,
and Ag/AgNO3 reference electrode. 1 monolayer = 1 x 10-10 mol cm-2.
Figure S5. Peak current for the RuIII/II redox couple versus the scan rate for GC-poly1 in aqueous 0.1 M HClO4, Pt-wire
counter, and Ag/AgCl reference electrode.
S8
Figure S6. Cyclic voltammograms of GC-poly1 in aqueous 0.1 M HClO4 (black) and with 4% CH3CN added (red).
Figure S7. Cyclic voltammograms of GC-poly1 (red) and 2 in solution (black) in (A) 0.1 M HClO4, pH 4 (0.1 M
acetate, 0.5 M NaClO4), and pH 7 (0.1 M phosphate, 0.5 M NaClO4) with 30% CF3CH2OH. Taken at 50 mV/s, 23ºC,
Pt-wire counter, and SCE reference electrode. CVs were normalized to the Ru III/II redox couple for comparison
purposes.
S9
Figure S8. Cyclic voltammograms of GC-poly1 (red) and 2 in solution (black) in (A) 0.1 M HClO4, pH 4 (0.1 M
acetate, 0.5 M NaClO4), and pH 7 (0.1 M phosphate, 0.5 M NaClO4) with 4% CH3CN. Taken at 50 mV/s, 23 ℃, Ptwire counter, and SCE reference electrode. CVs were normalized to the Ru III/II redox couple for comparison purposes.
Figure S9. Linear voltammograms collected at a RRDE with a GC disk with electropolymerized poly1 (solid lines) and
corresponding currents measured at a Pt ring (dashed lines). Presented in this figure are the results for: (solid red) GC
disk with electropolymerized poly1, (dashed red) Pt ring poised at -0.445 V vs. SCE measured simultaneously with the
solid red trace, (solid blue) GC disk with electropolymerized poly1, (dashed blue) Pt ring poised at 0.065 V vs. SCE
measured simultaneously with the solid blue trace. Linear voltammograms (solid lines) were recorded at a scan rate of
S10
5 mV/s with an electrode rotation rate of 500 RPM and the electrolyte contained 0.1 M phosphate buffer at pH 7 with
0.4 M NaClO4. The Pt ring currents are corrected for the collection efficiency (23%) determined for the ring-disk
electrode assembly.
Figure S9 shows the results from RRDE obtained with a GC disk with an electropolymerized layer of poly1 and a Pt
ring poised at -0.445 V vs. SCE (red) and 0.065 V vs. SCE (blue). The corresponding increase in cathodic current at the
Pt cathode when poised at -0.445 V vs. SCE (red) with the increase in anodic current at GC-poly1 evidences the
catalytic production of O2 by poly1 at sufficiently positive potential. When poised at more positive potential (0.065 V
vs. SCE), no corresponding increase in cathodic current is observed at the Pt ring. Control experiments showed this
potential insufficient to carry out O2 reduction while sufficient to carry out the reduction of H2O2. This in turn implies
no hydrogen peroxide formation by poly1 at pH 7.
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