Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
Pathways Following Electron Injection: Medium Effects and CrossSurface Electron Transfer in a Ruthenium-Based, Chromophore− Catalyst Assembly on TiO2 M. Kyle Brennaman,* Melissa K. Gish, Leila Alibabaei, Michael R. Norris,‡ Robert A. Binstead, Animesh Nayak, Alexander M. Lapides, Wenjing Song,∥ Robert J. Brown, Javier J. Concepcion,§ Joseph L. Templeton, John M. Papanikolas, and Thomas J. Meyer Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States S Supporting Information *
ABSTRACT: Interfacial dynamics following photoexcitation of the water oxidation assembly [((PO3H2)2bpy)2RuII(bpy-bimpy)RuII(tpy)(OH2)]4+, −[RuaII−RubII−OH2]4+, on nanocrystalline TiO2 electrodes, starting from either −[RuaII−RubII−OH2]4+ or −[RuaII−RubIII− OH2]5+, have been investigated. Transient absorption measurements for TiO2−[RuaII−RubII−OH2]4+ in 0.1 M HPF6 or neat trifluoroethanol reveal that electron injection occurs with high efficiency but that hole transfer to the catalyst, which occurs on the electrochemical time scale, is inhibited by local environmental effects. Back electron transfer occurs to the oxidized chromophore on the microsecond time scale. Photoexcitation of the once-oxidized assembly, TiO2−[RuaII− RubIII−OH2]5+, in a variety of media, generates −[RuaIII−RubIII−OH2]6+. The injected electron randomly migrates through the surface oxide structure reducing an unreacted −[RuaII−RubIII−OH2]5+ assembly to −[RuaII−RubII−OH2]4+. In a parallel reaction, −[RuaIII−RubIII−OH2]6+ formed by electron injection undergoes proton loss giving −[RuaII−RubIVO]4+ with possible conversion to −[RuaII−RubII−OH2]4+ by an electrolyte-mediated reaction. In the following slow step, re-equilibration on the surface occurs either by reaction with added FeIII/II or by cross-surface electron transfer between spatially separated −[RuaII− RubIVO]4+ and −[RuaII−RubII−OH2]4+ assemblies to give −[RuaII−RubIII−OH2]5+ with a half-time of t1/2 ∼ 68 μs. These results and analyses show that the transient surface behavior of the assembly and cross-surface reactions play important roles in producing and storing redox equivalents on the surface that are used for water oxidation.
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INTRODUCTION A key to devices for artificial photosynthesis is coupling visiblelight absorption with catalysis to drive component half reactions: water oxidation to O2; H+/H2O reduction to H2; and CO2 reduction to carbon-based fuels.1,2 In one approach, chromophore−catalyst molecular assemblies are integrated with wide band gap semiconductor oxides in a dye-sensitized photoelectrosynthesis cell (DSPEC).3−17 An advantage of using molecular assemblies in DSPEC applications is the ability to control system properties through systematic synthetic variations. Nonetheless, the multielectron/multiproton nature of the oxidative activation process for water oxidation, 2H2O → O2 + 4e− + 4H+, poses a significant challenge for electrode design that is based on coupled, single-photon, single-electron activation of single catalyst sites. A number of chromophore−catalyst assemblies have been prepared and characterized on oxide surfaces for studies of both water oxidation catalysis18−23 and interfacial electron transfer.4,5,23−28 In TiO2-bound, Ru-based chromophore−catalyst assemblies, chromophore excitation and excited-state electron injection lead to rapid electron-transfer activation of a linked molecular catalyst. Equation 1 shows the first photodriven cycle © XXXX American Chemical Society
for a Ru-based example with Rua and Rub referring to the chromophore and catalyst units, respectively.4−6 TiO2 −[Rua II−Rub II−OH 2]4 + hν
→ TiO2 −[Rua II*−Rub II−OH 2]4 +
(1a)
TiO2 −[Rua II*−Rub II−OH 2]4 + → TiO2 (e−)−[Rua III−Rub II−OH 2]5 +
(1b)
TiO2 (e−)−[Rua III−Rub II−OH 2]5 + → TiO2 (e−)−[Rua II−Rub III−OH 2]5 +
(1c)
Progression through cycles of sequential excitation/electron transfer events provides the four required oxidative equivalents for overall, light-driven water oxidation at the photoanode.3,6,17 In an earlier paper, visible-light-driven water splitting was reported for the chromophore−catalyst assembly shown in Figure 1, [(4,4′-(PO 3 H 2 ) 2 bpy) 2 Ru(bpy-bimpy)Ru(tpy)Received: May 22, 2018
A
DOI: 10.1021/acs.jpcc.8b04837 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
two additional excitation−injection−electron transfer cycles are required to initiate water oxidation catalysis. For maximum efficiency, three-cycle activation of the catalyst occurs before back electron transfer to any of the activated forms of the assembly (eq 2). TiO2 (e−)−[Rua II−Rub III−OH 2]5 + → TiO2 −[Rua II−Rub II−OH 2]4 +
(2a)
TiO2 (e−)−[Rua II−Rub IV O]4 + +2H+
⎯⎯⎯⎯⎯→ TiO2 −[Rua II−Rub III−OH 2]5 +
Figure 1. Structure of phosphonate-derivatized chromophore−catalyst assembly, 1.
TiO2 (e−)−[Rua III−Rub IV O]5 +
(OH2)] (1, [Rua −Rub −OH2] ; 4,4′-(PO3H2)2bpy is 4,4′bisphosphonic acid-2,2′-bipyridine; bpy-bimpy is N-(2-pyridyl)N′-(4-bipyridyl-4′-methyl)benzimidazole; tpy is 2,2′:6′,2″terpyridine), surface-bound to the TiO2 shell of a mesoporous, transparent-conducting-oxide-core/TiO2-shell photoanode. Sustained, visible-light-driven water splitting was observed with an absorbed-photon conversion efficiency of 4.4% with 445 nm photolysis.21 Incorporating 1 on a SnO2-core/TiO2shell photoanode stabilized by TiO2 or Al2O3 ALD overlayers yielded a photocurrent enhancement of ∼5 and established a key DSPEC design principle based on core/shell electrodes.29 Surface mechanistic studies on 1 on mesoporous, nanoparticle indium tin oxide (nanoITO) films have shown that oxidative activation occurs by initial, stepwise loss of 3e− and 2H+ to give nanoITO-[RuaIII−RubIVO]5+. Oxidative activation is followed by rate-limiting O atom transfer to a water molecule with concerted proton release (Scheme 1). The complexity of the electrocatalytic water-oxidation mechanism carries over into the light-driven analogue. For the latter, a three-photon-absorption sequence for catalyst activation is needed to reach the rate-limiting step. Following the initial sequence of events in eq 1 (excitation, electron injection, intra-assembly electron-transfer catalyst activation), 4+
II
II
(2b)
4+
→ TiO2 −[Rua II−Rub IV O]4 +
(2c)
Here, we detail the results of a transient photophysical study on 1 bound to TiO2 as −[RuaII−RubII−OH2]4+ and −[RuaII− RubIII−OH2]5+, from picoseconds to microseconds, along with complementary spectroelectrochemical experiments for 1 on nanoITO. The goal was to elucidate interfacial dynamics following visible-light excitation of the surface-bound assembly along the path to photodriven water oxidation.
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EXPERIMENTAL SECTION Materials and Methods. Distilled water was further purified by using a Milli-Q Ultrapure water purification system. The solvent 2,2,2-trifluoroethanol (99.8% purity) was purchased and used as received from Fluka Analytical. Perchloric acid (99.999% trace metals basis), tetrafluoroboric acid, acetic acid, nitric acid, triflic acid, and hexafluorophosphoric acid were purchased and used as received from Sigma-Aldrich. Be sure to take suitable precautions when handling these acids, particularly HPF6, as it is a superacid and its MSDS states that it is fatally toxic even upon skin contact! Sodium acetate was purchased and used as received from Sigma-Aldrich. [((PO3H2)2bpy)2Ru(bpybimpy)Ru(tpy)(OH2)](Cl)4 (1) was available from a previous study with (PO3H2)2bpy = 4,4′-bisphosphonic acid-2,2′bipyridine, bpy-bimpy = N-(2-pyridyl)-N′-(4-bipyridyl-4′methyl)benzimidazole, and tpy = 2,2′:6′,2″-terpyridine.19 Phosphonated catalyst and associated precursors were prepared and characterized as described in the Supporting Information including Scheme S1 and Figures S1−S4. All other reagents were ACS grade and used without additional purification. Fluoride-doped tin oxide (FTO)-coated glass (Hartford Glass; sheet resistance 15 Ω/cm2) was cut into 11 mm × 45 mm strips and used as the substrate for nanoparticle films. A previous study made available electrodes comprised of micron-thick, nanoporous films of indium tin oxide (nanoITO) on FTO glass, FTO|nanoITO.30 Screen-printed nanocrystalline TiO2 films, typically 2.2 μm thick, coating an area of 11 mm × 25 mm, were prepared on FTO-coated glass (Hartford Glass; sheet resistance 15 Ω/cm2) by using screen-printable TiO2 paste from Dyesol (18NR-T transparent titania paste, Dyesol, 20 nm particle diameter). Once TiO2 paste was printed onto FTO glass, the glass was placed in a box oven and gradually heated at 325 °C for 5 min, at 375 °C for 5 min, at 450 °C for 15 min, and at 500 °C for 15 min, as reported previously.31 Complete loading of assembly 1 on FTO|TiO2 slides, with maximum surface coverage, Γo, of 1 × 10−7 mol/cm2, was achieved by immersing the slides in a 1 mM solution of 1 in MeOH, or H2O, unless otherwise noted, overnight (∼16 h).
Scheme 1. Electrocatalytic Water-Oxidation Mechanism by [((PO3H2)2bpy)2RuII(bpy-bimpy)RuII(tpy)(OH2)]4+ (1) Bound to Mesoporous, Nanoparticle Films of Indium Tin Oxide (nanoITO) Showing the Rate-Determining Step (RDS)17,19a
a
Adapted with permission from ref 17. Copyright 2016 American Chemical Society. B
DOI: 10.1021/acs.jpcc.8b04837 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C The extent of dye loading, Γ/Γ0, is defined as the ratio of the actual surface coverage to the maximum surface coverage. Halfloaded (Γ/Γ0 = 0.5) slides were loaded by soaking slides in the 1 mM loading solution for ∼45 min. Electrochemical measurements were performed on a (CH Instruments model 660D or 601D) potentiostat/galvanostat. Voltammetric measurements were made with an FTO|1 or FTO|nanoITO-1 working electrode, a platinum wire (CH Instruments model 115) counter electrode, and a Ag/AgCl (CH Instruments model 111) reference electrode (3 M NaCl, 0.207 V vs NHE). UV−Visible Absorbance Spectroscopy. Optical absorbance measurements were recorded on an Agilent 8453 UV− vis−NIR dual beam absorption spectrophotometer. Derivatized films were inserted into a 10 mm path length cuvette with added electrolyte solution. Spectroelectrochemical UV−Visible Absorbance Spectroscopy. Spectroelectrochemical experiments were performed by controlled potential electrolysis of nanoITO slides fully loaded with 1 or the phosphonated catalyst in 0.1 M HPF6 from 0.0 to 1.5 V vs NHE in 0.02 V increments with UV−visible absorption (Agilent 8453A) monitoring at each potential step. Each incremental electrolysis step was performed for 60 s, which was sufficient time for the current to plateau, signaling complete oxidation at that potential step. UV−visible absorbance spectra were recorded after each controlled potential electrolysis step. Contributions from nanoITO absorbance changes were subtracted using data from an analogous experiment without surface-bound molecular assemblies or catalysts. Nanosecond Transient Absorption Spectroscopy. Transient absorption spectroscopy was performed by using a commercially available laser flash photolysis apparatus (Edinburgh Instruments, Inc., model LP920) with laser excitation provided by a pulsed Nd:YAG (5−7 ns fwhm; Spectra-Physics model Quanta-Ray LAB-170−10) and OPO (VersaScan-MB) laser combination. For nearly all of the experiments described here, the laser was tuned to produce 3.2 mJ, 425 nm laser pulses. The repetition rate of the laser was matched to the rate at which the probe source was pulsed (i.e., intensified 50× compared to nonpulsed output), typically 1 Hz, although the laser flashlamps were always fired at 10 Hz. Timing of the experiment was PC controlled via Edinburgh software (L900). The white light output of the LP920 probe source, a 450 W Xe lamp, was passed through a 380 nm longpass color filter before passing through the sample to minimize bandgap excitation of TiO2. The LP920 was equipped with a multigrating detection monochromator outfitted with a Hamamatsu R928 photomultiplier tube (PMT) in a noncooled housing. The PMT detector was used for monitoring transient absorption kinetics at a single wavelength (10 ns fwhm IRF, 300 nm−900 nm). For PMT measurements, spectral bandwidth was typically 1.15 V, and to nanoITO− [RuaIII−RubIVO]5+at >1.35 V. Water oxidation catalysis occurs past 1.4 V. As expected, spectral changes are dominated by loss of Ru(II)-based MLCT absorptions in the visible. The −Ru(III) and −Ru(IV) sites in the oxidized assemblies are only weakly absorbing in the visible and characterized by small positive absorption features that appear beyond ∼600 nm. Notable features in the difference spectra in Figure 2a include the bleach at 515 nm arising from oxidation of the catalyst to nanoITO− [RuaII−RubIII−OH2]5+ that is complete by 1 V. Further D
DOI: 10.1021/acs.jpcc.8b04837 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C Similar femtosecond transient absorption results were obtained when the acid was changed from HPF6 to either HClO4 or HNO3 (see Figures S8 and S9, respectively). Nanosecond Transient Absorption. TiO2−[RuaII−RubII− OH2]4+. Interfacial dynamics of TiO2−1 in argon-saturated 0.1 M HPF6 were also investigated by nanosecond transient absorption measurements (10 ns fwhm instrument response time). Transient absorption spectra of TiO2−1 (Γ/Γo = 1.0) in 0.1 M HPF6 recorded at various times after 3.3 mJ, 5 ns, 425 nm excitation are shown in Figure 4. At 15 ns, a bleach appears
Figure 5. Transient absorption kinetic traces following 3.3 mJ, 425 nm excitation for a TiO2−1 film (Γ/Γo = 1.0) in 0.1 M HPF6 at 22 ± 2 °C followed for 9 μs and probed at 450 nm (blue squares) and 515 nm (green circles). A triexponential fit (515 nm) and quad-exponential fit (450 nm) are shown as black lines with the lifetimes indicated in the figure. In the fits, the relative contributions of the components for 450 nm were a1 = 0.54, a2 = 0.22, a3 = 0.12, and a4 = 0.11 and for 515 nm were a1 = 0.62, a2 = 0.22, and a3 = 0.16, respectively.
electron transfer sequences summarized in eq 1.4,24−28 Documenting the transient, early time behavior of this assembly is important given its role in photoanodes for DSPEC water splitting.21,29 In this assembly, rapid electron injection is expected following photoexcitation of the chromophore. Electron injection is favored by 0.8 eV with Eo′(RuIII/II*) ∼ −0.6 eV based on the known redox potential for the RuIII/II couple and excited-state energy of the parent chromophore39 and given the conduction band edge for TiO2 at pH 1 of ∼0.2 eV.34,40,41 Photoexcitation of TiO2−[RuaII−RubII−OH2]4+ in 0.1 M HPF6, neat trifluoroethanol, or 0.1 M HClO4 with the electrode held at 0 V (vs NHE) is followed by rapid, subnanosecond electron injection to give TiO2(e−)−[RuaIII− RubII−OH2]5+ (eqs 1a and 1b) with no evidence for hole transfer to the remote catalyst site (eq 1c), despite previous observations on related assemblies.4,25−28,42 Based on our experimental observations, and by the observations of others in related structures, we conclude that the assembly TiO2(e−)− [RuaIII−RubII−OH2]5+ is transiently stabilized on the surface by local medium and ionic strength effects.43−47 In water oxidation cycles, the break in the overall redox cycle at the first stage by transient stabilization of −[RuaIII−RubII− OH2]5+ has little impact on the subsequent steps for water oxidation. Oxidation of the assembly is followed by conversion to −[RuaII−RubIII−OH2]5+ on the longer time scales required for water oxidation. Even on the rapid time scales reported here, injection by −[RuaIII−RubII*−OH2]5+ to give −[RuaIII− RubIII−OH2]6+ would lead to the second intermediate in the water oxidation cycle.19 Spectroelectrochemistry Revisited. −[RuaII−RubIII− OH2]5+ Baseline. As an aid in elucidating the photophysics following photoexcitation of TiO2−[RuaII−RubIII−OH2]5+ presented below, Figure 6 displays the spectroelectrochemical data of nanoITO−1 in aqueous 0.1 M HPF6 (data from Figure 2) recalculated to recast the data relative to −[RuaII−RubIII− OH2]5+ rather than −[RuaII−RubII−OH2]4+. This was accomplished by subtraction of the deconvoluted −[RuaII−RubIII−
Figure 4. Transient absorption spectra following 3.2 mJ, 5 ns, 425 nm excitation of TiO2−1 (Γ/Γo = 1.0) in 0.1 M HPF6 at 22 ± 2 °C.
at ∼440 nm along with a small, positive absorption feature in the low-energy visible (> ∼580 nm). At longer times, both features decay with complex kinetics over the course of several microseconds (Figure 5 and eq 3). TiO2 (e−)−[Rua III−Rub II−OH 2]5 + → TiO2 −[Rua II−Rub II−OH 2]4 +
(3)
The results of the transient experiments on the nanosecond− microsecond time scale were surprising given the wellestablished redox chemistry of the assembly on analogous oxide electrodes on longer time scales as investigated electrochemically. Based on the transient data, it is clear that excitation and injection by the assembly gives −[RuaIII−RubII− OH2]5+ on the
