Article pubs.acs.org/JPCC
Bias-Dependent Oxidative or Reductive Quenching of a Molecular Excited-State Assembly Bound to a Transparent Conductive Oxide Byron H. Farnum,† Akinobu Nakada,‡ Osamu Ishitani,‡ and Thomas J. Meyer*,† †
Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290, United States Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1-NE-1 O-okayama, Meguro-ku, Tokyo 152-8550, Japan
‡
S Supporting Information *
ABSTRACT: Visible light induced electron or hole injection by the surface-bound molecular assembly [(4,4′-(Me)2bpy)(4,4′-(CH2PO3H2)2bpy)RuII(MebpyCH2CH2bpyMe)ReI(CO)3Br]2+ (Me = CH3, bpy =2,2′-bipyridine) into In2O3:Sn nanoparticles (nanoITO) has been investigated as a function of applied bias by transient absorption spectroscopy. The metallic properties of degenerately doped nanoITO allowed the driving force for electron or hole injection to be varied systematically by controlling the Fermi level of the oxide through an applied bias. At Eapp > 0.4 V vs SCE, electron injection occurred by oxidative quenching of the Ru-based metal-to-ligand charge-transfer (MLCT) excited state to yield oxidized RuIII. At Eapp < 0.4 V, hole injection by reductive quenching of the MLCT excited state yielded reduced RuII(bpy•−) followed by rapid intra-assembly electron transfer to generate ReI(bpy•−).
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electronic levels in the conduction band.15,16 Our group has described photoanodic electron transfer between nanoITO and [RuII((4,4′-(PO3H 2) 2bpy)(bpy) 2]2+ chromophores where changes in the applied bias to the nanoITO film were used to control the Fermi level and thus the driving force and activation barrier to electron transfer.17−19 Rate constants for electron injection and back electron transfer were well modeled by use of Marcus−Gerischer interfacial electron-transfer theory. Photocathodic electron transfer (excited-state hole injection into the oxide) has also been reported for molecular assembly derivatized nanoITO by Huang et al. in p-type DSSCs.12 They also found an important role for driving force on electron transfer. Their results were important in demonstrating the use of degenerately doped n-type metal oxides as a p-type oxide. Wide band gap p-type metal oxides are essential to solar energy conversion in p-type DSSCs and for tandem applications in solar fuel DSPEC devices, but the number of known p-type metal oxides is relatively small with NiO and Cu-based delafossites most commonly used.20−23 Here we report bias controlled photoanodic and photocathodic electron transfer between nanoITO and the molecular assembly [(4,4′-(Me)2bpy)(4,4′-(CH2PO3H2)2bpy)RuII(4,4′MebpyCH2CH2bpyMe)ReI(CO)3Br]2+ (Ru−Re; Me = CH3, bpy = 2,2′-bipyridine). We demonstrate that the direction of photoinduced electron transfer is controllable by varying the applied bias (Eapp); with electron injection dominating at Eapp > 0.4 V vs SCE and hole injection becoming competitive at Eapp
1020 cm−3) within the wide band gap parent oxide result in metallic properties including high electron mobility and conductivity over a wide range of applied potentials while retaining the visible transparency of the parent oxide. The Fermi level of a TCO exists within the conduction band and can be readily adjusted with an external bias, much like a metal, allowing for anodic and cathodic electron-transfer reactions. TCOs are traditionally used as thin films on glass, yet many examples of high surface area, nanocrystalline 1−10 μm films (nanoTCO) consisting of 10−50 nm particles have been utilized for applications in electrocatalysis,5,6 spectroelectrochemistry,7−9 electrochromic displays,10,11 dye-sensitized solar cells (DSSC),12,13 and dye-sensitized photoelectrosynthesis cells (DSPEC).14 Photoinduced electron transfer between nanoTCOs and excited-state chromophores can be a valuable tool for understanding electron-transfer reactions at metal oxides by allowing for control of dopant density/distribution and driving force. The Lian group has reported photoanodic electron transfer (excited-state electron injection into the oxide) for chromophore derivatized nanoATO films. In these films an increased Sb-dopant density led to a pronounced decrease in the electron injection yield and increase in the electron-transfer recombination rate constant due to a higher occupancy of © 2015 American Chemical Society
Received: June 17, 2015 Revised: October 8, 2015 Published: October 12, 2015 25180
DOI: 10.1021/acs.jpcc.5b05801 J. Phys. Chem. C 2015, 119, 25180−25187
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The Journal of Physical Chemistry C
Electrochemistry. A custom-made three-electrode electrochemical cell was used to measure the redox potentials for Ru− Re and Ru surface bound to nanoITO electrodes. The cell was assembled such that the working electrode was either nanoITO|-Ru−Re or nanoITO|-Ru, the counter electrode was Pt mesh, the reference electrode was a pseudoreference electrode (Ag wire/KCl (satd.)/H2O), and the electrolyte was 0.1 M tetra-n-butylammonium perchlorate (TBAClO4, ≥99.0%, Fluka) MeCN. The reference potential was calibrated before and after all experiments using ferrocene (Fc). All applied potentials were referenced versus SCE using the conversion factor Fc+/0 = 0.380 V vs SCE.30 Coincidentally, the pseudoreference electrode was found to be stable at the same potential as SCE such that Eapp (V vs ref) = Eapp (V vs SCE). Cyclic voltammetry was performed at a scan rate of 50 mV/s using a CH Instruments 601D potentiostat. All samples were purged with N2 for 30 min prior to experimentation. Spectroelectrochemistry. Steady-state spectroelectrochemical studies on nanoITO|-Ru and bare nanoITO films were performed in 0.1 M TBAClO4 MeCN using an electrochemical cell identical to that described for electrochemical studies. For nanoITO|-Ru, a CH Instruments 601D potentiostat was used to apply a constant potential of Eapp = 1.5 V vs SCE to the working electrode, leading to oxidation of nanoITO|-Ru(II) → nanoITO|-Ru(III), and a Cary 50 spectrophotometer was used to record the UV−visible difference absorption spectrum. For bare nanoITO, a constant potential of Eapp = −0.1 V vs SCE was used to generate the difference absorbance spectrum. Samples were purged with N2 for at least 30 min prior to experimentation. Spectroelectrochemical studies using a flow-electrolysis method31 were used to determine the one-electron reduced spectra for Re and [RuII(4,4′-(Me)2bpy)3]2+ in 0.1 M tetraethylammonium tetrafluoroborate (TEABF4) MeCN. The cell was assembled such that the working electrode was a glassy-carbon disk, the counter electrode was Pt wire, and the reference electrode was Ag/AgNO3 (0.01 M). All applied potentials were referenced versus SCE using the conversion factor Ag/AgNO3 (0.01 M) = 0.298 V vs SCE.30 A CH Instruments 760ES potentiostat was used to apply a constant potential of Eapp = −1.70 V vs SCE for Re and Eapp = −1.49 V vs SCE for [RuII(4,4′-(Me)2bpy)3]2+, and a Photal MCPD2000 spectrophotometer was used to record the UV−visible difference absorption spectra. Samples were purged with N2 for at least 30 min prior to experimentation. Transient Absorption. All samples utilized in transient absorption measurements consisted of either nanoITO|-Ru−Re or nanoITO|-Ru electrodes immersed in 0.1 M LiClO4 (99.99%, Sigma-Aldrich) MeCN solution and connected in the same fashion as those discussed for electrochemistry studies above. Samples were purged with N2 for at least 30 min prior to experimentation. During experimentation, the externally applied bias was controlled by a Pine WaveNow potentiostat and held constant for the duration of each TA experiment. Nanosecond transient absorption measurements were performed by using 488 nm nanosecond laser pulses (5 mJ/ pulse) produced by a Spectra-Physics Quanta-Ray Lab-170 Nd:YAG laser combined with a VersaScan OPO (5−7 ns, operated at 1 Hz, beam diameter 0.5 cm) integrated into a commercially available Edinburgh LP920 laser flash photolysis spectrometer system. A white light probe pulse was generated by a pulsed 450 W Xe lamp and passed through a 375 nm long pass filter before reaching the sample, to avoid direct band gap
0.4 V. The bias-controlled selectivity of n-type or p-type behavior with nanoITO is discussed in terms of the effect of the applied bias on the barrier to electron transfer.
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EXPERIMENTAL SECTION Synthesis. [(4,4′-(Me) 2 bpy)(4,4′-(CH 2 PO 3 H 2 ) 2 bpy)RuII(4,4′-MebpyCH2CH2bpyMe)ReI(CO)3Br](PF6)2 (Ru− Re),24 [RuII(4,4′-(Me)2bpy)2(4,4′-(CH2PO3H2)2bpy)](PF6)2 (Ru),25 ReI(4,4′-(Me)2bpy)(CO)3Br (Re),26 and [RuII(4,4′(Me)2bpy)3](PF6)2,27 where Me = CH3 and bpy = 2,2′bipyridine, were synthesized according to reported procedures. Photoluminescence. Steady-state and time-resolved photoluminescence studies were performed on Ru−Re and Ru in acetonitrile (MeCN, Optima LC/MS grade, Fisher) solution at room temperature using an Edinburgh FLS920 spectrometer. Samples were purged for 30 min with N2 in a custom 1 cm2 glass cuvette whose top had been adapted with a #15 O-ring sealing joint, side arm, and Kontes valve prior to experimentation. Spectra were collected using monochromatic excitation at 488 and 370 nm. Photoluminescence was passed through a 495 nm long-pass filter and a single Czerny−Turner monochromator before being detected by a Peltier-cooled Hamamatsu R2658P photomultiplier tube. Comparative actinometry using [RuII(bpy)3](PF6)2 in MeCN (φPL = 0.062)28 as a standard was used to calculate photoluminescence quantum yields for Ru−Re and Ru with λex = 488 nm. The time-resolved photoluminescence of Ru−Re and Ru in MeCN was also measured at 620 nm following 484 nm pulsed laser diode excitation. Excited-state lifetimes were extracted from a firstorder decay function and found to be 784 ns (Ru−Re) and 700 ns (Ru). Excited-state free energies (ΔGes) for Ru−Re and Ru were calculated by modeling the photoluminescence spectra (λex = 488 nm) using a single-mode Franck−Condon line shape analysis described in the Supporting Information.29 nanoITO Film Preparation/Derivatization. The preparation of nanoITO film electrodes was based on a previously published method.18 Briefly, a 10 wt % suspension of hydroxypropyl cellulose (HPC, average MW = 80 000, 20 mesh particle size, Sigma-Aldrich) in ethanol (200 Proof, Decon Laboratories) was prepared by adding 1.3 g of HPC to 15 mL of ethanol and stirred overnight. A volume of 5 mL was transferred from a 20 wt % ITO stock dispersion in ethanol (In2O3:Sn, TC8 DE, Evonik Industries) to a clean 22 mL scintillation vial and bath sonicated for 20 min. To this freshly mixed ITO dispersion was added 5 mL of the 10 wt % HPC suspension, and the final mixture stirred overnight. nanoITO films were deposited onto conducting FTO glass substrates (SnO2:F, 15 Ω/cm2, Hartford Glass, Inc.) by a doctor blade technique. Film thickness was controlled by the numbered layers of scotch tape used to define the exposed area and measured using a Bruker Dektak XT profilometer. One layer of tape yielded 2.5 μm thick nanoITO films. Finally, nanoITO films were annealed (Thermo Scientific Lindberg/Blue M) at 500 °C for 1 h in air to remove HPC, leaving behind a mesoporous, sintered nanoparticle film of ITO. Previous studies have shown that nanoITO films prepared with this method result in a doping density of ND = 3.1 × 1020 cm−3.17,18 nanoITO films were derivatized by soaking in either 0.5 mM Ru−Re or Ru MeCN solutions overnight. Surface coverages were evaluated by UV−visible absorption measurements (Cary 50 Spectrophotometer) on nanoITO|-Ru−Re and nanoITO|-Ru in air by using the expression, Γ = Abs460nm/(ε460nm1000) where ε460 nm = 15 300 M−1 cm−1. 25181
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The Journal of Physical Chemistry C excitation of nanoITO, and then detected by a photomultiplier tube (Hamamatsu R928). A 488 nm notch filter was placed before the detector to reject unwanted scattered laser light. Single wavelength kinetic data were averaged over 60 laser shots.
same emission features as excitation at 488 nm, despite overlapping absorbance from Ru and Re at 370 nm. On the basis of this observation, intra-assembly energy transfer from Re* to Ru, [RuII(bpy)−ReII(bpy•−)*]2+ → [RuIII(bpy•−)*− ReI(bpy)]2+, is rapid and efficient, in line with previous reports.32 nanoITO films (2.5 μm thick, 10−20 nm particles) deposited on FTO glass electrodes were prepared by a standard procedure.17−19 The doping density was previously determined to be ND = 3.1 × 1020 cm−3 with 9.7% Sn.18 Films were derivatized by soaking in either 0.5 mM Ru−Re or Ru MeCN solutions overnight, conditions that result in complete surface loading.33 Surface coverages were evaluated with UV−visible absorption measurements by using the expression, Γ = Abs460nm/(ε460nm1000), which gave Γ = 3.3 and 3.6 × 10−8 mol cm−2 (1.3 and 1.4 × 10−8 mol cm−2 μm−1) for Ru−Re and Ru, respectively. Table 1 summarizes the results of cyclic voltammetry measurements on nanoITO|-Ru−Re and nanoITO|-Ru electro-
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RESULTS AND DISCUSSION Structures of the assembly, Ru−Re, and chromophore model, [RuII(4,4′-(Me)2bpy)2(4,4′-(CH2PO3H2)2bpy)]2+ (Ru), are shown in Scheme 1. The assembly was synthesized and Scheme 1. Chemical Structures of Ru−Re and Ru Complexes
Table 1. Summary of Electrochemical Data for Ru−Re and Ru Surface-Bound to nanoITOa characterized by previously reported methods.24,25 Absorption spectra for Ru−Re, Ru, and ReI(4,4′-(Me)2bpy)(CO)3Br (Re) in MeCN are shown in Figure 1. These spectra are dominated
Ru−Re Ru
Re0/+
Ru3+/2+
Re0/−
Ru2+/+
Ru3+/2+*
Ru2+*/+
1.25
1.14 1.11
−1.44
−1.62 −1.61
−0.99 −1.04
0.51 0.54
a All potentials reported as V vs SCE in 0.1 M TBAClO4 MeCN electrolyte; Re0/+ = Ep,a(ReII/ReI); Ru3+/2+ = E°′(RuIII/RuII); Re0/− = E°′(Re I (bpy) 0 /Re I (bpy • − ) − ); Ru 2 + / + = E°′(Ru I I (bpy) 2 + / RuII(bpy•−)+); Ru3+/2+* = E°′(RuIII(bpy)3+/RuIII(bpy•−)2+*) = E°′(Ru3+/2+) − ΔGes; Ru2+*/+ = E°′(RuIII(bpy•−)2+*/RuII(bpy•−)+) = E°′(Ru2+/+) + ΔGes. ΔGes = 2.13 eV (Ru−Re) and 2.15 eV (Ru).
des in 0.1 M TBAClO4 MeCN. As shown in Figure S2, reversible RuIII/RuII waves were observed for Ru−Re and Ru at 1.14 and 1.11 V vs SCE, respectively. For the Ru−Re assembly, an irreversible anodic wave was observed at Ep,a = 1.25 V, arising from the ReII/ReI couple. Quasi-reversible ligand-based reductions were observed at E°′(ReI(bpy)0/ReI(bpy•−)−) = E°′(Re0/−) = −1.44 V (Ru−Re) and E°′(RuII(bpy)2+/ RuII(bpy•−)+) = E°′(Ru2+/+) = −1.62 V (Ru−Re) and −1.61 V (Ru). Reduction potentials for the Ru-based MLCT excitedstate, E°′(RuIII(bpy)3+/RuIII(bpy•−)2+*) = E°′(Ru3+/2+*) = E°′(Ru3+/2+) − ΔGes = −0.99 V and E°′(RuIII(bpy•−)2+*/ RuII(bpy•−)+) = Eo′(Ru2+*/+) = E°′(Ru2+/+) + ΔGes = 0.51 V, were calculated from ground-state potentials and excited-state free energies obtained by photoluminescence spectral fitting (ΔGes; Supporting Information). Nanosecond transient absorption measurements were conducted as a function of applied bias (Eapp) in a threeelectrode spectroelectrochemical cell in 0.1 M LiClO4 MeCN electrolyte at room temperature. In these experiments, the Fermi level of the nanoITO|-Ru−Re or nanoITO|-Ru photoelectrode was held at a constant potential by using an external bias for the duration of the transient absorption measurements. Figure 2a shows transient absorption spectra for nanoITO|-Ru− Re and nanoITO|-Ru following laser flash excitation at 488 nm with a constant applied bias of Eapp = 1.0 V vs SCE. For both photoelectrodes, the earliest time-resolved difference spectrum (30 ns delay) was consistent with the appearance of RuIII(bpy) and nanoITO(e−) as a result of electron injection and oxidative quenching of the MLCT excited state, eqs 1 and 2.
Figure 1. Absorbance spectra reported as molar absorptivity of Ru (black), Re (gray), and Ru−Re (red) in MeCN at room temperature. The blue dashed spectrum is the sum of individual Ru and Re spectra.
by intense absorption bands at 460 and 368 nm arising from metal-to-ligand charge-transfer (MLCT) transitions at the Ru and Re molecular units, respectively. The sum of the constituent spectra (Ru + Re) is nearly identical to the spectrum of the Ru−Re assembly, consistent with weak electronic coupling across the −CH2CH2− bridge. Steady-state and time-resolved photoluminescence measurements on Ru−Re and Ru were conducted in MeCN at room temperature. Emission spectra with 488 and 370 nm excitation are shown in Figure S1. Selective excitation at λex = 488 nm, into the Ru-based MLCT absorption band, for both complexes yielded the typical RuIII(bpy•−)* triplet excited-state emission spectrum with λem,max = 635 nm. Emission quantum yields (λex = 488 nm) were 0.046 (Ru−Re) and 0.051 (Ru) with no evidence for intramolecular quenching by the Re complex. Similarly, emission lifetimes (Figure S1, λex = 484 nm) monitored at 620 nm were found to be similar at 780 ns (Ru−Re) and 700 ns (Ru). Excitation at 370 nm yielded the 25182
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Figure 2. (a) Transient absorption difference spectra obtained 30 ns after 488 nm laser excitation of nanoITO|-Ru−Re (red circles) and nanoITO|Ru (black squares) at Eapp = 1.0 V vs SCE in 0.1 M LiClO4 MeCN at room temperature. Overlaid on these data are simulations based on a linear summation of the known absorbance difference spectra for RuIII(bpy) and nanoITO(e−). (inset) Single wavelength traces at 450 nm showing the kinetics of back electron transfer. (b) Molar absorptivity difference spectrum (Δε vs λ) for RuIII(bpy) on nanoITO and the absorbance difference spectrum (ΔAbs) for nanoITO(e−) obtained by spectroelectrochemical measurements.
Figure 3. (a) Transient absorption difference spectra obtained 30 ns after 488 nm laser excitation of nanoITO|-Ru−Re (red circles) and nanoITO|Ru (black squares) at Eapp = −1.0 V vs SCE in 0.1 M LiClO4 MeCN at room temperature. Overlaid on the transient data are simulations based on the known difference spectra for ReI(bpy•−) (red line) and RuIII(bpy•−)* + RuIII(bpy) (black line). (inset) Single wavelength traces at 450 nm for nanoITO|-Ru and 510 nm for nanoITO|-Ru−Re. (b) Molar absorptivity difference spectra for reduced model complexes RuII(bpy•−) and ReI(bpy•−) obtained from stopped-flow spectroelectrochemistry along with the ΔAbsλ spectrum for excited-state RuIII(bpy•−)* measured for Ru in MeCN.
Ru, respectively. This corresponds to ∼3% of the total surface concentration of RuII(bpy) molecules at both photoelectrodes. The coefficients for nanoITO(e−) derived from the data were α = 0.16 and 0.17 for nanoITO|-Ru−Re and nanoITO|-Ru, respectively. These values correspond to a transient change in the Fermi level of ΔEF = −17 mV at 30 ns based on the linear response in ΔAbsλ vs Eapp, Figure S3. This small shift is due to the small increase in electron density from electron injection. Given Eapp = 1.0 V vs SCE in these experiments, the degenerately doped ITO nanoparticles are expected to be partially depleted of electrons on the basis of their ability to sustain band bending at the particle surface. Electron injection gives rise to small fluctuations in EF on the nanosecond time scale despite the constant Eapp.7,18 Decay of the nanoITO(e−) and RuIII(bpy) features occurred on the nanosecond time scale and were consistent with the back electron-transfer reaction shown in eq 3. The single wavelength traces at 450 nm in the inset of Figure 2a decayed nonexponentially. The kinetics were characterized as the time required for 1/2 of the ΔAbs at 450 nm to decay to zero, k1/2 = 1/τ1/2 = 3.3 × 107 s−1.
nanoITO|‐[Ru II(bpy)−Re I(bpy)]2 + + hν → nanoITO|‐[Ru III(bpy •−)*−Re I(bpy)]2 +
(1)
nanoITO|‐[Ru III(bpy •−)* − Re I(bpy)]2 + → nanoITO(e−)|‐[Ru III(bpy)−Re I(bpy)]3 +
(2)
Overlaid on the data in Figure 2a are simulations based on a linear summation of the known difference spectra of RuIII(bpy) and nanoITO(e−), Figure 2b, calculated from ΔAbsλ = ΓRuIIIΔελ(RuIII(bpy))1000 + αΔAbsλ(e−). Here, Δελ(RuIII(bpy)) is the molar absorptivity difference between RuIII(bpy) and RuII(bpy), ΓRuIII is the surface concentration of RuIII(bpy), ΔAbsλ(e−) is the difference spectrum for a blank nanoITO electrode at a change in applied potential of ΔEapp = −100 mV, and α is a coefficient for the ΔAbsλ(e−) spectrum. The difference spectrum for nanoITO(e−) results from a blue shift of the optical band gap as the nanoparticle film is reduced as reported earlier in transient absorption electron injection experiments.7,17−19 From the simulations in Figure 2a, the surface concentration of RuIII(bpy) from electron injection at 30 ns was ΓRuIII = 8.3 and 9.3 × 10−10 mol cm−2 for nanoITO|-Ru−Re and nanoITO|25183
DOI: 10.1021/acs.jpcc.5b05801 J. Phys. Chem. C 2015, 119, 25180−25187
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The Journal of Physical Chemistry C nanoITO(e−)|‐[Ru III(bpy)−Re I(bpy)]3 + → nanoITO|‐Ru II(bpy)−Re I(bpy)]2 +
nanoITO(h+)|‐[Ru II(bpy)−Re I(bpy •−)]+ → nanoITO|‐[Ru II(bpy)−Re I(bpy)]2 +
(3)
I
Decay of the transient Re (bpy ) intermediate on the nanosecond time scale was consistent with the back electrontransfer reaction in eq 6. Kinetics of back electron transfer were nonexponential with k1/2 = 1.4 × 107 s−1. Given the remote nature of the ReI(bpy•−) site from the nanoITO surface, back electron transfer may occur by direct ReI(bpy•−) → nanoITO(h+) electron-transfer tunneling or by ReI(bpy•−) → RuII(bpy) → nanoITO(h+) electron hopping. It is notable that reversible Ru to Re electron-transfer hopping in related Ru−Re complexes has been observed under steady-state irradiation conditions.35,36 To investigate the crossover behavior between electron and hole injection, transient absorption traces at 500 nm were recorded and compared over a range of applied potentials with the results shown in Figure 4. From these data, a decrease in
As Eapp was decreased, the magnitude of the characteristic bleach features for nanoITO(e−) and RuIII(bpy) decreased. At the most negative applied potential of Eapp = −1.0 V, there was no evidence for nanoITO(e−) or RuIII(bpy) from transient absorption measurements of nanoITO|-Ru−Re. Instead, positive features at 375 and 520 nm appeared in the transient spectrum at 30 ns, Figure 3a. This observation was consistent with the appearance of ReI(bpy•−) in the reduced assembly, −[RuII(bpy)−ReI(bpy•−)]+, as shown by the Δελ spectrum obtained from spectroelectrochemical reduction of the Re model complex, Figure 3b.34 The transient spectrum in Figure 3 at 30 ns was simulated by using the relation ΔAbsλ = ΓReΔελ(ReI(bpy•−))1000 where ΓRe = 7.5 × 10−10 mol cm−2. In marked contrast, the transient spectrum for nanoITO|-Ru in Figure 3a was most consistent with excited-state RuIII(bpy•−)* and minor contributions from RuIII(bpy) as a result of electron injection. The spectrum recorded at 30 ns could be accurately simulated from the ΔAbsλ spectrum for RuIII(bpy•−)*, Figure 3b, and the Δελ spectrum for RuIII(bpy), Figure 2b, from the relation ΔAbsλ = αΔAbsλ(RuIII(bpy•−)*) + ΓRuIIIΔελ(RuIII(bpy))1000 with α = 0.5 and ΓRuIII = 1.8 × 10−10 mol cm−2. No spectral features could be assigned to nanoITO(e−) or nanoITO(h+) in either spectrum shown in Figure 3a at Eapp = −1.0 V. The lack of spectral evidence may be due to the magnitude of the applied potential with the density of electrons sufficiently large to mask changes in electron density due to electron and hole injection. Given that there was no evidence for intra-assembly excitedstate quenching from the photoluminescence data of Ru−Re, the appearance of reduced ReI(bpy•−) provides indirect evidence for sequential hole injection from excited-state RuIII(bpy•−)* into nanoITO followed by intra-assembly electron transfer from RuII(bpy•−) to ReI(bpy), eqs 4 and 5. The lack of evidence for the intermediate RuII(bpy•−), Δελ spectrum shown in Figure 3b, at 30 ns suggested that intraassembly electron transfer was faster than we could time resolve. The reduced RuII(bpy•−) species was also not observed for the model nanoITO|-Ru sample on the nanoseconds time scale where only excited-state RuIII(bpy•−)* and RuIII(bpy) were detected and all spectral features decayed rapidly within 60 ns. However, hole injection is expected to occur given the similar thermodynamics between the Ru and Ru−Re excited states. The absence of spectral features associated with RuII(bpy•−) may indicate that back electron transfer, nanoITO(h+)|-[RuII(bpy•−)]+ → nanoITO|-[RuII(bpy)]2+, occurs on a similar time scale as for hole injection, leading to the appearance of rapid excited-state decay. The comparison of transient spectra for nanoITO|-Ru−Re and nanoITO|-Ru at Eapp = −1.0 V highlights the ability of the Re unit to act as an electron trap and allow for the detection of hole injection on the nanoseconds time scale.
Figure 4. Single wavelength traces at 500 nm for nanoITO|-Ru−Re as a function of applied bias in 0.1 M LiClO4 MeCN at room temperature. (inset) ΔAbs change at 500 nm, obtained 40 ns after 488 nm laser excitation, plotted versus bias applied for nanoITO|-Ru− Re (red circles) and nanoITO|-Ru (black squares).
applied bias from +1.0 to −1.0 V resulted in a change in the transient absorption response from a bleach feature, consistent with electron injection and formation of RuIII(bpy), to a positive feature, consistent with hole injection and formation of ReI(bpy•−). The inset in Figure 4 shows the variation in ΔAbs at 500 nm, 40 ns after the laser pulse, with applied bias for both nanoITO|Ru−Re and nanoITO|-Ru. At Eapp > 0.4 V, the ΔAbs changes at 500 nm were similar for Ru−Re and Ru and reflected electron injection into nanoITO. For Eapp < 0.4 V, the variation in ΔAbs for the two diverges due to the onset of hole injection and the appearance of ReI(bpy•−). Attempts to measure quantum yields for electron and hole injection were complicated by the rapid and bias-dependent time scales for back electron-transfer reactions that resulted in a significant loss of signal within the instrument response time. The bias dependence for the kinetics of injection and back electron transfer at the nanoITO interface have been shown to be consistent with the free energy dependence predicted by classical Marcus−Gerischer electron-transfer theory.17,18 The rate constant for electron transfer is governed by the free energy distribution function, (4πλkT)−1/2 exp(−(ΔG°′ + λ)2/ 4λkT), which describes the activation barrier for electron
nanoITO|‐[Ru III(bpy •−)*−Re I(bpy)]2 + → nanoITO(h+)|‐[Ru II(bpy •−)−Re I(bpy)]+
(4)
nanoITO(h+)|‐[Ru II(bpy •−)−Re I(bpy)]+ → nanoITO(h+)|‐[Ru II(bpy)−Re I(bpy •−)]+
(6)
•−
(5) 25184
DOI: 10.1021/acs.jpcc.5b05801 J. Phys. Chem. C 2015, 119, 25180−25187
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Scheme 2. Energy Level Diagrams Describing Photoinduced Electron and Hole Injection from Ru−Re into nanoITO in MeCN with 0.1 M LiClO4a
a
In the left panel at Eapp = 1.0 V vs SCE, electron injection is favored due to overlap between unfilled conduction band levels in nanoITO and the Marcus−Gerischer free energy distribution function related to electron injection (green). In the right panel at Eapp = −1.0 V, hole injection is favored due to overlap between filled electronic levels in nanoITO and Marcus−Gerischer distribution for hole injection (red). Arrows indicate the direction of electron transfer.
transfer as ΔG‡ = (ΔG°′ + λ)2/4λ. Here, λ is the sum of the intramolecular and medium reorganization energies and ΔGo′ is the free energy for electron transfer, defined as ΔGo′ = −(EF − E°′(Ru 3+/2+ * )) for electron injection and ΔG o ′ = −(E°′(Ru2+*/+) − EF) for hole injection. Electron transfer is barrierless at the condition −ΔG°′ = λ or EF = E°′(Ru3+/2+*) + λ for electron injection and EF = E°′(Ru2+*/+) − λ for hole injection. At this condition, the rate constant for electron transfer is maximized as indicated by a peak in the free energy distribution function described above. The Fermi level, and thus ΔGo′ for electron transfer, are controlled directly by the applied bias to the nanoITO electrode. Due to the large doping density within each ITO nanoparticle, the conduction band edge Ecb is pinned with respect to EF in the core or “bulk” of each particle such that under an applied bias, band bending on the order of 1−3 nm occurs at the nanoITO surface.18,37 Boschloo et al. have further shown that a significant fraction of the applied bias dissipates across the Helmholtz layer.7 The combined effect of the two creates a metallic response for nanoITO that results in a near uniform shift of all energy levels within the oxide with respect to EF as Eapp is changed. Scheme 2 illustrates an energy level diagram to describe electron injection and hole injection into nanoITO as a function of Eapp. The relevant redox potentials for Ru−Re and the free energy distribution function for electron injection are shown in green with the reorganization energy taken to be λ(Ru3+/2+*) = 0.8 eV on the basis of previous estimates for Ru−polypyridyl complexes in the range of 0.71−0.83 eV.17,18,38 Redox potentials and the free energy distribution related to hole injection are shown in red with λ(Ru2+*/+) = 0.5 eV. To our knowledge, λ(Ru2+*/+) has not been directly measured with estimates of λ(Ru2+*/+) = λ(Ru3+/2+) ∼ 0.5 eV cited in the literature.39 For nanoITO, unfilled electronic levels in the conduction band are shown in gray and filled levels in the conduction band and within the band gap are shown in blue. The presence of electronic levels within the band gap are due to
Sn-doping on the order of 10% and oxygen vacancies in the ITO lattice.40,41 Ultraviolet photoelectron spectroscopy has provided evidence for filled band gap states well below the Fermi level in planar ITO films.42 Our own studies have suggested there are filled electronic levels as far as 1 eV below EF on the basis of the absence of an inverted region effect for back electron transfer between nanoITO(e−) and [RuIII((4,4′(PO3H2)2bpy) (bpy)2]3+ at large driving forces.17,18 The left panel in Scheme 2 illustrates the condition Eapp = EF = 1.0 V where electron injection is highly favored (ΔGo′ = −1.99 eV) and many unfilled electronic levels in the nanoITO conduction band are able to accept injected electrons, as shown by the overlap with the free energy distribution for electron injection (green). At this applied potential, hole injection is unfavorable by 0.49 eV. The right panel, with Eapp = EF = −1.0 V, illustrates the other extreme where hole injection is favored with ΔGo′ = −1.51 eV and a strong overlap between filled electronic levels in nanoITO and the free energy distribution for hole injection (red) promotes rapid hole transfer. From Figure 4, the onset for hole injection appears at Eapp = EF = 0.4 V, just past the excited-state reduction potential Eo′(Ru2+*/+) = 0.51 V where the free energy distribution begins to be significantly large. This result is consistent with the report by Huang et al., who demonstrated that hole injection into nanoITO was dependent on the reduction potential for the excited-state couple, E°′(C*/C−).12 In their study, the largest cathodic photocurrents for p-type DSSCs were observed for chromophores with E°′(C*/C−) slightly more positive than EF. Our results on nanoITO are significantly different from previous reports on the bias dependence of electron injection at undoped mesoporous TiO2 and SnO2 nanoparticle films.43−45 In those studies, as the external bias was decreased, electron injection decreased with a concomitant enhancement in excited-state photoluminescence and no evidence for the intervention of alternate quenching pathways. On nanoITO, electron injection also decreased as the applied bias was decreased; however, under these conditions, hole injection 25185
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(4) Robertson, J.; Falabretti, B. Electronic Structure of Transparent Conducting Oxides (Ch. 2). Handbook of Transparent Conductors 2011, 27−50. (5) Chen, Z.; Concepcion, J. J.; Luo, H.; Hull, J. F.; Paul, A.; Meyer, T. J. Nonaqueous Catalytic Water Oxidation. J. Am. Chem. Soc. 2010, 132, 17670−17673. (6) Chen, Z.; Concepcion, J. J.; Hu, X.; Yang, W.; Hoertz, P. G.; Meyer, T. J. Concerted O Atom-Proton Transfer in the O-O Bond Forming Step in Water Oxidation. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 7225−7229. (7) Boschloo, G.; Fitzmaurice, D. Spectroelectrochemistry of Highly Doped Nanostructured Tin Dioxide Electrodes. J. Phys. Chem. B 1999, 103, 3093−3098. (8) Hoertz, P. G.; Chen, Z.; Kent, C. A.; Meyer, T. J. Application of High Surface Area Tin-Doped Indium Oxide Nanoparticle Films as Transparent Conducting Electrodes. Inorg. Chem. 2010, 49, 8179− 8181. (9) Zum Felde, U.; Haase, M.; Weller, H. Electrochromism of Highly Doped Nanocrystalline SnO2:Sb. J. Phys. Chem. B 2000, 104, 9388− 9395. (10) Llordés, A.; Garcia, G.; Gazquez, J.; Milliron, D. J. Tunable nearInfrared and Visible-Light Transmittance in Nanocrystal-in-Glass Composites. Nature 2013, 500, 323−327. (11) Schwab, P. F. H.; Diegoli, S.; Biancardo, M.; Bignozzi, C. A. Novel Ru-Dioxolene Complexes as Potential Electrochromic Materials and NIR Dyes. Inorg. Chem. 2003, 42, 6613−6615. (12) Huang, Z.; He, M.; Yu, M.; Click, K.; Beauchamp, D.; Wu, Y. Dye-Controlled Interfacial Electron Transfer for High-Current Indium Tin Oxide Photocathodes. Angew. Chem., Int. Ed. 2015, 54, 6857− 6861. (13) Alibabaei, L.; Farnum, B. H.; Kalanyan, B.; Brennaman, M. K.; Losego, M. D.; Parsons, G. N.; Meyer, T. J. Atomic Layer Deposition of TiO2 on Mesoporous nanoITO: Conductive Core − Shell Photoanodes for Dye-Sensitized Solar Cells. Nano Lett. 2014, 14, 3255−3261. (14) Alibabaei, L.; Brennaman, M. K.; Norris, M. R.; Kalanyan, B.; Song, W.; Losego, M. D.; Concepcion, J. J.; Binstead, R. A.; Parsons, G. N.; Meyer, T. J. Solar Water Splitting in a Molecular Photoelectrochemical Cell. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 20008−20013. (15) Guo, J.; She, C.; Lian, T. Ultrafast Electron Transfer Between Molecule Adsorbate and Antimony Doped Tin Oxide (ATO) Nanoparticles. J. Phys. Chem. B 2005, 109, 7095−7102. (16) Guo, J.; She, C.; Lian, T. Ultrafast Electron Transfer Between Conjugated Polymer and Antimony-Doped Tin Oxide (ATO) Nanoparticles. J. Phys. Chem. C 2008, 112, 4761−4766. (17) Farnum, B. H.; Morseth, Z. A.; Brennaman, M. K.; Papanikolas, J. M.; Meyer, T. J. Driving Force Dependent, Photoinduced Electron Transfer at Degenerately Doped, Optically Transparent Semiconductor Nanoparticle Interfaces. J. Am. Chem. Soc. 2014, 136, 15869−15872. (18) Farnum, B. H.; Morseth, Z. A.; Brennaman, M. K.; Papanikolas, J. M.; Meyer, T. J. Application of Degenerately Doped Metal Oxides in the Study of Photoinduced Interfacial Electron Transfer. J. Phys. Chem. B 2015, 119, 7698−7711. (19) Farnum, B. H.; Morseth, Z. A.; Lapides, A. M.; Rieth, A. J.; Hoertz, P. G.; Brennaman, M. K.; Papanikolas, J. M.; Meyer, T. J. Photoinduced Interfacial Electron Transfer within a Mesoporous Transparent Conducting Oxide Film. J. Am. Chem. Soc. 2014, 136, 2208−2211. (20) Yu, M.; Natu, G.; Ji, Z.; Wu, Y. P-Type Dye-Sensitized Solar Cells Based on Delafossite CuGaO2 Nanoplates with Saturation Photovoltages Exceeding 460 mV. J. Phys. Chem. Lett. 2012, 3, 1074− 1078. (21) Yu, M.; Draskovic, T. I.; Wu, Y. Cu(I)-Based Delafossite Compounds as Photocathodes in P-Type Dye-Sensitized Solar Cells. Phys. Chem. Chem. Phys. 2014, 16, 5026−5033.
became competitive and no photoluminescence was observed. This difference in results for undoped metal oxides and degenerately doped metal oxides can be attributed to the wide distribution of electronic levels below the conduction band edge in nanoITO that are able to reductively quench the molecular excited state. Finally, there is a well-established electrocatalytic and photocatalytic chemistry toward CO2 reduction by ReI(bpy) (CO)3Br and related derivatives.34,46 We are currently investigating photoactivation of the surface-bound Ru−Re assembly, and related assemblies, with an applied bias and hole injection as a basis for photoelectrochemical CO2 reduction.
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CONCLUSION The results described here are important in demonstrating the versatility of mesoporous, nanoparticle films of transparent conducting oxides for exploring interfacial electron-transfer dynamics. Closely related semiconductor oxides such as TiO2 and NiO play an important role as wide band gap n-type and ptype substrates, respectively, in dye-sensitized solar energy conversion strategies.23,47 Our results show that mesoporous nanoITO films can act as either n-type (oxidative quenching; electron injection) or p-type (reductive quenching; hole injection) substrates depending on the applied bias. The ability to tune the direction of excited-state electron transfer with applied bias and strategically designed molecular assemblies may presage applications in sensing, photoconductivity, and energy conversion.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05801. Spectral fitting analysis, photoluminescence modeling data, photoluminescence spectra, cyclic voltammograms, UV−vis absorbance difference spectra (PDF)
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AUTHOR INFORMATION
Corresponding Author
*T. J. Meyer. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS B.H.F. and T.J.M. acknowledge support from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-FG02-06ER15788. A.N. and O.I. thank the Academy for Co-creative Education of Environment and Energy Science (ACEEES) for the Leading Program Educational Research Fund.
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