Chromophore-Catalyst Assembly for Water Oxidation Prepared by

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Chromophore-Catalyst Assembly for Water Oxidation Prepared by Atomic Layer Deposition Leila Alibabaei, Robert J. Dillon, Caroline E. Reilly, M. Kyle Brennaman, KyungRyang Wee, Seth L. Marquard, John M. Papanikolas, and Thomas J. Meyer ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11905 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 17, 2017

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ACS Applied Materials & Interfaces

Chromophore-Catalyst Assembly for Water Oxidation Prepared by Atomic Layer Deposition

Leila Alibabaei, Robert J. Dillon, Caroline E. Reilly, M. Kyle Brennaman, Kyung-Ryang Wee†, Seth L. Marquard, John M. Papanikolas, and Thomas J. Meyer* Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599 (USA)

AUTHOR INFORMATION Corresponding Author * [email protected] Present Addresses † Department of Chemistry, Daegu University, Gyeongsan 38453, Republic of Korea. Author Contributions The manuscript was written through contributions of all authors.

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Abstract: Visible-light-driven water splitting was investigated in a dye sensitized photoelectrosynthesis cell (DSPEC) based on a photoanode with a phosphonic acid-derivatized donor–π–acceptor (D-π-A) organic chromophore, 1, and the water oxidation catalyst [Ru(bda)(4-O(CH2)3P(O3H2)2-pyr)2], 2, (pyr = pyridine; bda = 2,2’-bipyridine-6,6’-dicarboxylate). The photoanode was prepared by using a layering strategy beginning with the organic dye anchored to an FTO|core/shell electrode, atomic layer deposition (ALD) of a thin layer (< 1 nm) of TiO2, and catalyst binding through phosphonate linkage to the TiO2 layer. Device performance was evaluated by photocurrent measurements for core/shell photoanodes, with either SnO2- or nanoITO-core materials, in acetate-buffered, aqueous solutions at pH 4.6 or 5.7. Although the absolute magnitudes of photocurrent changes with the core material, TiO2 spacer layer thickness, or pH, observed photocurrents were 2.5-fold higher in the presence of catalyst. The results of transient absorption measurements and DFT calculations show that electron injection by the photo-excited organic dye is ultrafast promoted by electronic interactions enabled by orientation of the dye's molecular orbitals on the electrode surface. Rapid injection is followed by recombination with the oxidized dye which is 95% complete by 1.5 ns. Although chromophore decomposition limits the efficiency of the DSPEC devices toward O2 production, the flexibility of the strategy presented here offers a new approach to photoanode design.

Keywords: D-π-A Organic Dye, core/shell, Dye Sensitized Photoelectrosynthesis Cell, electron transfer, transient absorption, water splitting, artificial photosynthesis


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INTRODUCTION: Artificial photosynthesis has the same goal as natural photosynthesis, conversion of solar energy by driving high-energy chemical reactions that convert and store energy. The dye sensitized photoelectrosynthesis cell (DSPEC) integrates molecular light absorption and catalysis with high-band-gap metal-oxide semiconductors for water splitting and CO2 reduction.1-5 Multiple strategies have been adopted for the preparation of chromophore-catalyst assemblies for water oxidation at photoanodes with one of the simplest, and most straightforward, involving co-loading on metal oxide nanoparticle films6-13 as discussed in recent reviews.14-15 The co-loading approach shows promise, yet significant limitations must be overcome to enable enhanced solar absorption, increased cell efficiencies, and surface stabilization of the molecular assemblies. The stability of the molecular components is an issue especially for water splitting given the reactivity requirements for water oxidation to O2 and the required stability of molecular components in multiple oxidation states through multiple cycles. As an example, in a recent series of experiments, it was revealed that commonly used RuII(bpy)32+-based chromophores are unstable for extended periods as Ru(III) with decomposition occurring by ligand loss.16 Organic donor-acceptor dyes, such as 1 (Figure 1) shown below, also undergo decomposition upon oxidation under aqueous conditions with the thiophene units highly susceptible to oxidative decomposition.17



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Figure 1. Structures of the D-π-A organic chromophore (1) and the water oxidation catalyst (2), [Ru(bda)(4-pyCH2PO3H2)2], (pyr = pyridine; bda = 2,2’-bipyridine-6,6’-dicarboxylate).

We have initiated a series of studies designed to understand and overcome chromophorecatalyst instabilities in chromophore-catalyst assemblies. As part of those efforts, we introduce here a new strategy- utilization of a thin oxide layer, in this case TiO2, between the catalyst and the core/shell-bound chromophore. We describe the synthesis, film characterization, electrochemistry, photophysics, and photoelectrochemistry of this multi-layered co-loading approach as a potential basis for surface stability in molecular assemblies. The present study explores the impact of physically separating the chromophore and catalyst on mesoporous, core/shell (either SnO2/TiO2 or nanoITO/TiO2) electrodes by use of atomic layer deposition (ALD). In a first step, surfaces loaded with chromophore were stabilized by ALD overlayer addition of a thin (< 1 nm) layer of TiO2 with surface thicknesses sufficient to protect the oxidatively sensitive dithiolene links of the dye. The water oxidation catalyst, [Ru(bda)(4O(CH2)3P(O3H2)2-pyr)2], 2, (pyr = pyridine; bda = 2,2’-bipyridine-6,6’-dicarboxylate), was added in a second step by surface loading. Catalysts of the type [Ru(bda)(L)2], originally reported by Licheng Sun and co-workers,18 have low over-potentials making them ideal for lightdriven water oxidation.19 The resulting FTO|core/shell-1|TiO2|-2 assemblies were evaluated for light-driven water oxidation by photocurrent and product measurements.

Experimental General: All chemicals were purchased from Sigma-Aldrich or Alfa Aesar and used as received unless otherwise noted. The fluorine-doped tin oxide (FTO) electrodes (TEC 15) were purchased from Hartford glass (Hartford, IN). Deuterated chloroform, CDCl3, was obtained from Cambridge Isotope Laboratories, Inc. for NMR experiments. The 1H and 31P NMR spectra were recorded on a Bruker 400 MHz spectrometer. All proton chemical shifts were measured relative to internal residual chloroform (99.5% CDCl3) from the lock solvent.


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Detailed synthesis of organic chromophore, 1, may be found in supporting information. Synthesis of catalyst 2 was as described previously in the literature.20

Preparation of the core/shell electrodes: Mesoporous, nanocrystalline SnO2, ZrO2, and nanoITO electrodes were prepared as previously described.5, 21-22 To form the TiO2 shell, mesoporous SnO2 films with a thickness of ~5 µm were subjected to 75 cycles of atomic layer deposition (ALD) (Ultratech/Cambridge Nanotech, model S200) by using tetrakis(dimethylamido) titanium, Ti(NMe2)4 (TDMAT, 99.999%, SigmaAldrich), and water. The reactor temperature was 130 °C. The TDMAT reservoir was kept at 75 °C. The TDMAT was pulsed into the reactor for 0.3 s and then held for 10 s before opening the pump valve and purging for 20 s. ALD coating conditions were 130 °C and 20 torr of N2 carrier gas with a sequence of 0.3-s metal precursor dose, 10-s hold, 20-s N2 purge, 0.02-s H2O dose, 10-s hold, 20-s N2 purge.5 Ellipsometric measurements were conducted with a J. A. Woollam, variable-angle, spectroscopic ellipsometer to measure TiO2 layer thickness. The growth rate of TiO2 was estimated as ≈0.6 Å per ALD cycle by spectroscopic ellipsometry on Si wafers. The core/shell electrodes were sintered at 450 °C for 30 min after ALD deposition. SEM: Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) results were obtained on a FEI Helios 600 Nanolab Dual Beam System equipped with an INCA PentaFET-x3 detector from Oxford instruments. Cross-sectional SEM images were taken of FTO|nanoITO/3.3 nm TiO2-1|0.6 nm ALD TiO2|-2. Surface images were imaged at 20 kV with a 0.69 nA beam current. The EDS spectra were obtained at the interface of the cross section of films on FTO glass. Steady-State Emission: A sub-millimolar amount of 1 was dissolved in 4 mL of air-saturated dichloromethane contained in a fluorescence cuvette (quartz, 1 cm path length). As an alternate sample preparation method, FTO|ZrO2-1 electrodes were immersed in air-saturated, pH 5.7, acetate buffers in an open-



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topped, fluorescence cuvette (quartz, 1 cm path length). Emission spectra (500 – 800 nm, every 1 nm, 0.3 s integration time, 5 averages) were collected at room temperature by inserting the quartz cuvette into a PTI QuantaMaster 4SE-NIR emission spectrophotometer equipped with a thermoelectrically cooled (-20 oC) photomultiplier tube (R928P, Hamamatsu Inc). The output from a housed, 75 W Xe lamp was directed into a Czerny-Turner monochromator set to 2 nm spectral bandwidth. A 300 nm long-pass filter was placed into the excitation beam path, before the sample, to prevent illumination of the sample with deep-UV light passing through the monochromator by a second-order grating effect. A small portion of the excitation beam was directed onto a Si reference detector whose output was divided into the raw emission data to eliminate the impact of Xe lamp intensity fluctuations on observed emission intensity. Photoluminescence from the sample was collected in a 90° geometry and passed into a monochromator set to 2 nm spectral bandwidth. No color filters were used in the emission beam path except a 500 nm long-pass filter was inserted when measuring FTO|ZrO2-1 electrodes. Emission spectra were multiplied by instrument-specific correction factors (obtained in a separate experiment using a NIST-traceable standard tungsten lamp) to account for the wavelength-dependent response of the system. For solution-based experiments, correction factor distortions were subtracted following collection of the emission spectrum of neat dichloromethane under identical conditions as in the presence of 1. For electrode-based experiments, contributions from FTO|ZrO2 in the presence of pH 5.7 acetate buffer were subtracted following collection of the emission spectrum of FTO|ZrO2 under identical conditions as for electrodes derivatized with 1. Emission Spectral Fitting: Emission spectral intensities, corrected for the instrument’s wavelength-dependent response, were converted to units of quanta per second by multiplication by the square of the wavelength. 23

The transformed spectra were expressed in wavenumbers and fit to the single average mode,

Franck-Condon expression in equation 1.24-27


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− ℏω =

!

(1) )

− + ℏω × exp −4 ln 2 Δ,'/)

For eq 1, ħωM is the quantum vibrational energy spacing of the single acceptor mode of medium

frequency, Δ,'/) is the full-width-at-half-maximum (FWHM) of the gaussian-broadened 0-0

vibronic component, SM is the Huang-Rhys factor – the electron-vibrational coupling constant,

and E0 is the energy difference between the ground state and the lowest-energy excited-state. With emission spectra normalized to one, the four variable parameters (ħωM, Δ,'/), SM, and

E0), were iteratively optimized until a global minimum was reached by use of a trust-regionreflective, least-squares algorithm carried out using custom code executed in MatLab (The MathWorks, Inc.; version R2014b). The summation included 11 vibrational levels (νM = 0 → 10).23-27

Transient Absorption: Transient absorption (TA) measurements were performed with a Clark-MXR CPA2210, 1 kHz regeneratively amplified Ti:Sapphire laser system. The 450 nm pump beam was generated by optical parametric amplification of a portion of the laser fundamental (775 nm) to 1800 nm, followed by frequency doubling in two BBO crystals to produce 900 nm and then finally 450 nm. The white light probe was produced taking another portion of the laser fundamental and pumping a translating CaF2 window to generate a supercontinuum. Short-pass filters were employed on the pump and probe to eliminate residual fundamental. A waveplate was used to set the polarization of the pump beam to the magic angle, 54.7°, relative to the probe beam. The timing of the pump and probe was varied using a computer-controlled mechanical delay stage. The per-pulse fluence was 380 µJcm-2. TA measurements were performed with the electrodes immersed in the same buffer solution as the other experiments in a sealed cuvette. Samples were sparged with Ar for 45 min before sealing. During the measurement, the sample was continuously translated in the focal plane of the beams by a mechanical stage to sample a large 7 ACS Paragon Plus Environment


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area of the film and avoid photodamage. Optical chirp correction was achieved by determination of the frequency resolved optical gating of a CCl4 solution. Electrochemistry: A CH Instruments 601D potentiostat was used for electrochemical measurements and a CH Instruments 760E bipotentiostat was used for the photoelectrochemical measurements. Current-time (i-t), cyclic voltammetry (CV) and IPCE measurements were performed in a threeelectrode configuration, with Ag/AgCl (3 M in KCl) as a reference and platinum as a counter electrode.

Spectroelectrochemistry: Spectroelectrochemical measurements were performed by using a Princeton Applied Research VersaSTAT4 potentiostat and an HP 8453 UV-Vis spectrophotometer. The two instruments were synchronously controlled with LabView software. Samples were subjected to increasingly positive potentials in 50 mV increments, for a set period of time. Following each change in applied potential, the spectrophotometer recorded the spectrum after a set delay. In the absence of the TiO2 overlayer, dye desorption was problematic. Instead of a simple staircase scheme for the applied potential, in between each potential step, the sample was "rested" at the starting potential. By returning the sample to the neutral state after each increasingly positive potential, irreversible loss of the absorption signal due to dye desorption was also tracked. For each potential in the difference spectra plots, the difference spectrum was computed by subtraction of the average of the before and after "resting" potential spectra from the spectrum at the indicated potential. Samples for spectroelectrochemistry consisted of either the dye or the catalyst loaded to high-surface area ITO substrates. A three-electrode setup was employed with a Ag/AgCl (3M KCl) reference electrode, a Pt wire counter electrode, and the film sample as the working electrode. All three electrodes were fit into the cell, sparged with Ar for 15 min, and then sealed. Spectroelectrochemistry of 2 was performed in the acetate buffer solution while 1 was performed in 0.1 M LiClO4, pH 1 HClO4 solution in order to alleviate dye desorption occurring without a TiO2 overlayer. For 1, the starting/resting potential was +0.2 V, the duration of each 50 mV step was 30 s, and the spectrophotometer delay was 20 s. For 2, the starting/resting potential was 0.2 V, the duration of each 50 mV step was 20 s, and the spectrophotometer delay was 10 s. 8 ACS Paragon Plus Environment

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Density Functional Theory: Density functional theory (DFT) calculations were performed by using Gaussian '09 software using the B3LYP 6-31G** level of theory. The presence of solvent was simulated using the polarizable continuum model in the software. For the oxidized dye geometry, the sample was evaluated as a doublet. For the excited-state geometry, six excited-states were solved for with the first used to optimize the structure. HOMO-LUMO maps and dihedral angles were obtained using GaussView. RESULTS AND DISCUSSION: Photoanode preparation and characterization: Water-splitting photoanodes were prepared by first soaking FTO|MOx/TiO2 core/shell electrodes, with MOx as either SnO2 or nanoITO, in dichloromethane solutions that were 1 mM in 1 (16 hr). Following a dichloromethane rinse and nitrogen drying, the FTO|MOx/TiO2-1 electrodes were subjected to 5, 10 or 15 cycles of ALD to generate a thin stabilizing/scaffold layer directly on top of the surface-bound organic dyes to give FTO|MOx/TiO2-1|TiO2 electrodes. After addition of TiO2, the electrodes were soaked in a methanol/1% acetonitrile solution with 0.5 mM of 2 (16 hr). The resulting assembly structure, FTO|MOx/TiO2-1|TiO2|-2, is shown in Figure 2.



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Figure 2. Illustration of the layered, co-loading strategy for the organic dye surface-bound to the core/shell surface with the water oxidation catalyst, 2, added following ALD addition of the TiO2 overlayer.

To ensure that the thin TiO2 overlayer was applied throughout the mesoporous film, and that subsequent loading of 2 was homogeneous, cross-sectional scanning electron microscope (SEM) images with superimposed elemental analyses were obtained. A representative elementmapped image is shown for FTO|nanoITO|3.3 nmTiO2-1|0.6 nmTiO2|-2 in Figure S7. Although Ti mapping to visualize the homogeneous formation of a thin TiO2 overlayer is obscured due to the more intense Ti signal from the shell, the Ru-mapped image reveals homogeneous penetration of the Ru-based catalyst. The presence of the TiO2 stabilizing/scaffold spacer layer over the film surface is inferred by the appearance of Ru which presumably binds directly to the TiO2 overlayer. Energetics: 10 ACS Paragon Plus Environment

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The ground state potential for the 1+/0 couple, E°ʹ(1+/0), on nanoITO in the presence of a pH 5.7 acetate buffer was investigated by cyclic voltammetry (CV). Figure 3a shows CVs of FTO|TiO2-1 in acetonitrile with 0.1 M TBAP as a function of scan rate. Reversible, kinetically distorted waves appear at E1/2 = 1.2 V and 1.5 V vs Ag/AgCl (3 M KCl). Compared to previous results with a related donor-π-acceptor organic dye, the first oxidation potential for 1 is higher by 135 mV.17 The excited-state oxidation potential for 1 was estimated from an average-single-mode, Franck-Condon analysis of the emission spectrum of 1 in dichloromethane, Figure 3b. The results of the emission fitting procedure are shown superimposed on the spectrum. The spectral

fitting parameters used to fit the spectrum were ħωM = 1122 cm-1, Δ,'/), = 1868 cm-1, SM, = 2.59 and E0 = 17427 cm-1. Nearly identical results were obtained for FTO|ZrO2-1 electrodes

immersed in air-saturated, pH 5.7, acetate buffer (data not shown).

(b)

(a)

Figure 3: (a) Scan-rate-dependent CVs for FTO|TiO2–1 (20 nm TiO2 particle diameter, ~1 µm film thickness) in acetonitrile with 0.1 M TBAP. (b) Solvent-subtracted, normalized, emission 11 ACS Paragon Plus Environment


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spectrum for 1 (black points) in dichloromethane following conversion to intensity units of quanta per second and plotted as a function of wavenumber (λexc = 450 nm). The emission spectral fit is shown as the red line. ° The free-energy difference between the ground and excited states, Δ*+, , was 2.35 eV as

calculated from E°ʹ(1+/*) + λo, with E°ʹ(1+/*) the formal excited-state oxidation potential and λo the reorganization energy as calculated from equation 2 with kB Boltzmann’s constant, T the

temperature and Δν/'⁄) , the FWHM of the gaussian-broadened 0-0 vibronic component, determined from emission spectral fitting. For 1, E°ʹ(1+/*) is -1.15 V vs NHE as calculated from equation 3 with F the Faraday constant and E°ʹ(1+/0) the first oxidation potential of ground state 1. )

1Δν/'⁄) 2 = 1656 T89 ln2

° E°ʹ(1+/*) = E°ʹ(1+/0) - Δ*+, /F

(2) (3)

Photoelectrochemistry: The photoelectrochemical performance upon 445 nm irradiation of photoanodes under aqueous conditions was evaluated as a function of ALD spacer layer thickness. Figure 4 shows photocurrents

generated

by

FTO|nanoITO/4.5 nm TiO2-1|x

TiO2

and

FTO|nanoITO/4.5 nm TiO2-1|x TiO2-2 electrodes. In the electrode sequences, x is 5, 10 or 15 cycles of ALD TiO2, corresponding to 0.3, 0.6 and 0.9 nm TiO2. For these experiments, electrodes were immersed in pH 4.6, 0.5 M LiClO4, 20 mM acetate buffer with an applied bias of Ebias = 0.2 V vs Ag/AgCl (3 M KCl). Three light intensities were used over the course of each measurement, in 15 s intervals. As shown in Figure 4a and Table 1, illumination of nanoITO/4.5 nm TiO2-1| 0.3 nm TiO2 gave photocurrent densities of 14.5 µAcm-2 at 7.8 mW light intensity (15 s), 68 µAcm-2 at 49 mW light intensity (45 s), and 82 µAcm-2 at 90 mW light intensity (75 s). For nanoITO/4.5 nm TiO2-1| 0.3 nm TiO2|-2, the values were 1.08, 45, and 61 µAcm-2 at 15, 45, and 75 s, respectively. The decreased photocurrents with surface-bound 2 most likely arise from partial desorption of 1, see Figure S8, due to insufficient protection by the ALD TiO2 layer with


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an additional impact from incomplete loading of the catalyst due to the lack of binding surface area afforded by the thin, 0.3 nm, TiO2 spacer layer. For photoanodes with a 0.6 nm TiO2 spacer layer, enhanced photocurrent densities were observed with added 2, Figure 4b and Table 1. For nanoITO/4.5 nm TiO2-1| 0.6 nm TiO2, photocurrent densities of 2.26, 69, and 67 µAcm-2 were obtained and for nanoITO/4.5 nm TiO21| 0.6 nm TiO2|-2 the values were 3.88, 122, and 132 µAcm-2 at incident intensities of 7.8, 49 and

90 mW, an enhancement of ∼2× as compared to the core/shell electrodes without added catalyst.

This photocurrent enhancement observed upon addition of catalyst to nanoITO/4.5 nm TiO2-

1|0.6 nm TiO2 is a result of two key factors – 1) the significant enhancement in dye anchoring stability provided by the 0.6 nm TiO2 spacer layer, Figure S8, as compared to electrodes with a 0.3 nm TiO2 spacer layer which is too thin to provide effective stabilization, and 2) a much more complete catalyst coverage as compared to electrodes with a 0.3 nm TiO2 spacer layer. This improvement in photocurrent is achieved despite an increase in competitive light absorption by the catalyst which is shown to be non-productive by IPCE results shown below. Illumination of photoanodes prepared with a TiO2 spacer thickness of 0.9 nm gave the lowest photocurrent densities of the three spacer thicknesses investigated. Based on the data in Figure 4c and tabulated in Table 1, photocurrent densities for nanoITO/4.5 nm TiO2-1| 0.9 nm TiO2 were 6.55, 52, and 65 µAcm-2 and for nanoITO/4.5 nm TiO2-1| 0.9 nm TiO2|-2 were 1.72, 38, and 56 µAcm-2 at 7.8, 49 and 90 mW 445 nm illumination, respectively, revealing that there is no improvement in photocurrent with added catalyst. These observations are significant in showing that the increased photocurrent observed for 0.6 nm TiO2-based electrodes with added catalyst arises from the interaction of the chromophore and catalyst, which is greatly diminished for a 0.9 nm spacer thickness, with no difference in the composition of the films. This behavior is not a result of photoexcitation of the catalyst, a weak light-absorber at 445 nm. If light absorption by the catalyst led to enhancement in photocurrent, the incremental increase in spacer thickness from 0.6 to 0.9 nm is not expected to have such a dramatic effect.



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1| 0.3 nm

1| 0.3 nm

1| 0.6 nm

1| 0.6 nm

1| 0.9 nm

1| 0.9 nm

TiO2

TiO2|-2

TiO2

TiO2|-2

TiO2

TiO2|-2

7.8 mW

14.5

1.08

2.26

3.88

6.55

1.72

49 mW

68.85

45.7

69.89

122.5

52.51

38.35

90 mW

82.79

61.85

76.68

132

65.09

55.96

Intensity

Table 1. Photocurrent densities (µAcm-2) as a function of TiO2 spacer thickness for layered, coloaded electrodes as a function of TiO2 ALD layer thickness between 1 and 2 using nanoITO/4.5 nm TiO2 core/shell in argon-saturated, pH 4.6, 0.5 M LiClO4, 20 mM acetate buffers at Ebias = 0.2 V vs Ag/AgCl illuminated by a 445 nm blue LED during the following time intervals: 15-30 s (7.8 mW), 45-60 s (49 mW), and 75-90 s (90 mW).


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Figure 4. Photocurrent-time traces with 445 nm excitation of layered, co-loaded electrodes as a function of the TiO2 ALD layer thickness between 1 and 2: a) FTO|nanoITO/4.5 nm TiO2-1| 0.3 nmTiO2|-2, b) FTO|nanoITO/4.5 nm TiO2|-1|0.6 nmTiO2|-2, and c) FTO|nanoITO/4.5 nm TiO2-1|0.9 nm TiO2|-2; in argon-saturated, pH 4.6, 0.5 M LiClO4, 20 mM acetate buffers. Photoelectrochemical data were obtained at Ebias = 0.2 V vs Ag/AgCl at 15-30 s (7.8 mW), 4560 s (49 mW), and 75-90 s (90 mW). As a complement to the photoelectrochemical current vs time measurements, cyclic voltammograms under light and dark conditions were performed on core/shell-based 15 ACS Paragon Plus Environment


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photoanodes as a function of TiO2 overlayer thickness. Figure S9 compares the CV responses of FTO|nanoITO/4.5 nm TiO2|-1|x nmTiO2

and

FTO|nanoITO/4.5 nm TiO2|-1|x

nmTiO2-2

electrodes with x = 0.3, 0.6 or 0.9. The CVs were measured for photoanodes in pH 4.6, 0.5 M LiClO4 in 20 mM acetate buffers in the dark and under 445 nm illumination. The results mirror the photoelectrochemical current vs time measurements in Figure 4 and Table 1, both of which show an optimal TiO2 spacer layer thickness of 0.6 nm. We have also explored the generality of the assembly/stabilization strategy which, in principle, should be applicable to other semiconductor and core/shell materials. A series of SnO2/6.6 nm TiO2 core/shell-based photoanodes were fabricated, sensitized with 1, and then coated with TiO2 spacer layers of varying thicknesses. UV-Vis absorption spectra collected following each step of the stepwise synthesis of an FTO|SnO2/6.6 nm TiO2-1|0.6 nm TiO2|-2 electrode is shown in Figure S10. The data show that the stepwise addition of each component occurs, although with partial loss of 1 during the final two steps. Photoelectrochemical measurements, in pH 4.6, 0.5 M LiClO4, 20 mM acetate buffer with Ebias = 0.4 V vs Ag/AgCl, were carried out for the nanoITO/TiO2-based photoanodes as a function of TiO2 layer thickness. Figure S10b shows the photoelectrochemical response upon 445 nm (49 mW) irradiation of FTO|SnO2/6.6 nm TiO2-1|0.6 nmTiO2 (535 µAcm–2) and FTO|SnO2/6.6 nm TiO2-1|0.6 nm TiO2|-2 (214 µAcm–2). Based on the data, an enhancement of ∼2.5 was observed with the added catalyst despite partial loss of 1 from the surface during the fabrication of the assembly. Incident photon to current efficiency (IPCE) measurements were obtained for SnO2/3.3 nmTiO2 core/shell-based photoanodes in argon-saturated, pH 5.6, 0.1 M NaClO4, 0.1 M acetate buffer solutions with Ebias = 0.4 V vs Ag/AgCl, Figure S11. At 400 nm, the IPCE value for FTO|SnO2/3.3 nmTiO2-1|0.6 nmTiO2|-2 was ~1.5 times higher than the sample without catalyst, FTO|SnO2/3.3 nmTiO2-1|0.6 nmTiO2. Other than the increased amplitude, the shape of the IPCE action spectrum is otherwise unchanged by the addition of the catalyst, peaking at the peak absorption for the chromophore. This again demonstrates that the increased efficiency arises from the interaction of the chromophore with the catalyst. Other configurations without the 0.6 nm TiO2 outer layer failed to show related enhancements. A collector−generator (C−G) dual working electrode was used to examine light-driven O2 production by these electrodes. Collector-generator method based on a reported method for the detection of photochemically produced O2 from dye sensitized photoanodes 4. In brief, The 16 ACS Paragon Plus Environment

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FTO|SnO2/6.6 nm TiO2-1|0.6 nm TiO2 and FTO|SnO2/6.6 nm TiO2-1|0.6 nm TiO2|-2 electrodes as generator and a FTO collector electrodes, been placed about 1 mm from the photoanode and kept under an applied bias of -0.85 V vs Ag/AgCl, The FTO|SnO2/6.6 nm TiO2-1|0.6 nm TiO2 and FTO|SnO2/6.6 nm TiO2-1|0.6 nm TiO2|-2 electrodes, was illuminated, and the charge passed measured concurrently, Figure S12. The charge passed at each electrode during the experiment was used to determine faradaic efficiency. The FTO|SnO2/6.6 nm TiO2-1|0.6 nm TiO2 produced a minimal quantity of O2. For FTO|SnO2/6.6 nm TiO2-1|0.6 nm TiO2|-2, during the illumination period, the faradaic efficiency for O2 production was 10.5%, comparable with the results from a previous study.17

Density Functional Theory: Density functional theory (DFT) calculations were performed to determine the geometries and electronic distributions of the ground, excited, and one-electron oxidized states of 1. In the ground state geometry, the HOMO is delocalized over the complex, and the LUMO is localized on the arm of the dye containing the thiophene, nitrile, and phosphonate groups, with the latter added for binding the dye to the electrodes, Figure 5a. The lowest energy transition for 1 is the HOMO→LUMO transition yielding a twisted intramolecular charge-transfer (TICT) excited-state. Following photoexcitation, population of π-bonding orbitals in the anchoring arm of the dye results in its planarization, Figure 5b. The planar structure is retained in the oneelectron oxidized dye, Figure 5c, with oxidation subtly depleting the electron density about the tri-aryl amine moiety in the HOMO, while the LUMO is relatively unchanged.



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Figure 5. Optimized geometries and molecular orbitals for the ground state (a), excited state (b), and one-electron oxidized (c) dye. Planarization of the excited and one-electron oxidized states occurs about the indicated bond; the dihedral angle for each structure is provided.


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Transient Absorption: The photophysics following visible excitation of the layered, co-loaded assemblies were explored by ultrafast transient absorption measurements. The excited-state properties for 1 were evaluated by using FTO|ZrO2-1 with the conduction band of ZrO2 is energetically inaccessible toward electron-injection. In these experiments, surface loading of FTO|ZrO2-1 was matched to the loading of the core/shell samples by adjusting solution soaking times.

Figure 6. Transient absorption difference spectra (a) and decay kinetics (b) for FTO|ZrO2-1. All times in (a) are in picoseconds. In (b), normalized kinetics for the ground state recovery, at 430 nm, and excited-state decay, at 680 nm, are indicated by red squares and black circles, respectively. The solid line in (b) is the result of a global fit of the 430 and 680 nm kinetics to the

expression, yt = A' e>?⁄@A + A) e>?⁄@B + A e>?⁄@C , with τ1= 2.27 ps, τ2=130 ps, τ3=1743 ps, and A1=0.41, A2=0.29, and A3=0.29.

Time-resolved spectra, following 450 nm excitation of FTO|ZrO2-1, Figure 6a, are dominated by the bleach of the ground state absorption centered at 440 nm with a shoulder at 540 nm, and a broad absorption with peaks near 600 and 700 nm. The spectra evolve rapidly in time. The 540 nm bleach, attributed to π-stacked aggregates, relaxes within 16 ps coinciding with the appearance of an isosbestic point at 514 nm, a feature which persists for the remainder 19 ACS Paragon Plus Environment


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of the decay. Normalized kinetics for the excited-state relaxation of FTO|ZrO2-1* monitored at 430 and 680 nm, Figure 6b, reveal rapid excited-state decay. The kinetics are multi-exponential with half of the excited state relaxing within 40 ps and only 12% of the initial amplitude remaining for the full duration of the TA experiment (1.5 ns).

Figure 7. Transient absorption difference spectra for FTO|nanoITO/4.5 nmTiO2-1|0.6 nm TiO2 (a), and decay kinetics at 504 nm (b). All times in (a) are in picoseconds. In (b) the 504 nm kinetics for the dye on ZrO2 and the final co-loaded system area included for comparison.

Injection kinetics were monitored on the 1-derivatized core/shell architecture, FTO|nanoITO/4.5 nmTiO2-1|0.6 nm TiO2. While the SnO2/TiO2-based electrodes exhibited higher photocurrents, the optical properties of those samples were not conducive to photophysical characterization. Time-resolved difference spectra for FTO|nanoITO/4.5 nmTiO21|0.6 nmTiO2 are shown in Figure 7a. The earliest observable spectrum, at 170 fs, resembles the spectrum of the oxidized dye obtained by spectroelectrochemical experiments, Figure S13a. This comparison suggests that the majority of excited chromophores undergo electron injection within the instrument response time. A small contribution from the dye’s excited state is noticeable at 600 - 700 nm in the 170 fs spectrum. However, this component is quenched within the first picosecond as noted by the appearance of an isosbestic point at 475 nm. The comparison in 20 ACS Paragon Plus Environment

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lifetimes between the conducting and non-conducting oxides is revealing. It shows that even with the relatively short lifetime for the excited-state of 1, injection is far more rapid than the excitedstate lifetime and not the origin of a significant decrease in the efficiency of the cell. Following the photophysics beyond 1 ps for FTO|nanoITO/4.5 nm TiO2-1|0.6 nm TiO2, the kinetics at 504 nm (the isosbestic point for the equilibrated excited-state, red trace in Figure 7b) reveal rapid multi-exponential traces from electron recombination with the oxidized dye. Half of the oxidized dye signal decays within 10 ps, and only 5.5% of the original signal remains at 1.5 ns. In the DFT analysis, it is notable that the LUMO of the oxidized dye resides on the arm of the dye in contact with the electrode surface, Figure 5c. It is apparent that the ligand energetics and orbital orientations that enable fast electron-injection on the conducting oxide likely also facilitate fast recombination.

Figure 8. Transient absorption difference spectra of FTO|nanoITO/4.5 nm TiO2-1|0.6 nmTiO2|2. The spectra shown correspond to the picosecond delay times indicated in the legend. Small sharp peaks near 600 nm in some of the spectra are artifacts from pump scatter (sum frequency of the 1800 nm and 900 nm beams used to generate the 450 nm pump).

Time-resolved spectra for the full system, FTO|nanoITO/4.5 nm TiO2-1|0.6 nm TiO2|-2, are shown in Figure 8. The spectra of the full system are a combination of excited state, 1*, and oxidized 1 with no evidence for the oxidized catalyst, Figure S13b. The positive absorption 21 ACS Paragon Plus Environment


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signal in the 500 – 600 nm spectral range appearing at our earliest observation time is consistent with ultrafast formation of oxidized dye with overlap from features of 1*. The loss of the chargeseparated state is more rapid than excited-state decay and, over time, the spectra increasingly resemble the excited-state dye (1*) spectrum observed on ZrO2, shown in Figure 6a. The overall intensity of the transient signal in the full system is also diminished compared to the samples without catalyst, which is likely due to loss of 1 during the loading of 2. In addition to the outright loss of dye, the catalyst loading procedure also presents the opportunity for dye redistribution from the annealed crystalline electrode to the amorphous ALD overlayer. Amorphous TiO2 from ALD has different electronic properties which might also account for the decreased appearance of transiently formed oxidized dye in the full system. Despite these issues, the amplitudes at 504 nm for FTO|nanoITO/4.5 nm TiO2-1|0.6 nmTiO2|-2 and FTO|nanoITO/4.5 nm TiO2-1|0.6 nmTiO2 are comparable by 1.5 ns., Figure 6b. Although this could be purely coincidental, this suggests that the local dynamics of the subset of chromophores that ultimately produce a long-lived charge-separated state are unchanged by the addition of 2. For the dye+catalyst system, presumably there is a contribution to the 504 nm signal from a small subset of oxidized catalysts despite the absence of a prominent bleach feature in the 500-600 nm region expected upon formation of oxidized catalyst. This suggests that the local dynamics of the subset of chromophores that ultimately produce a long-lived charge-separated state are unchanged by the addition of 2. Scheme 1 summarizes the insight gained from the photophysical characterization. On nanoITO, photoexcitation of 1 produces an excited state which undergoes rapid electron injection despite a short-lived excited-state and this is followed by back electron transfer. On the full electrode, FTO|nanoITO/4.5 nmTiO2|-1|0.6 nmTiO2, excitation of the dye is followed dominantly by injection and back electron transfer with activation of the catalyst largely unobserved by TA. In this system, rapid recombination outpaces electron transfer from the catalyst to the chromophore with the slow catalyst-activation kinetics a major contributing factor that decreases the ability of the assembly to undergo water oxidation.

Scheme 1.


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hν FTO|nanoITO-TiO2|-1|TiO2|-2 → FTO|nanoITO-TiO2|-1*|TiO2|-2 (photoexcitation)

→ FTO|nanoITO-TiO2|-1|TiO2|-2 (excited-state decay) FTO|nanoITO-TiO2|-1*|TiO2|-2  → FTO|nanoITO-TiO2 (e-)|-1+|TiO2|-2 (electron injection) FTO|nanoITO-TiO2|-1*|TiO2|-2  → FTO|nanoITO-TiO2|-1|TiO2|-2 (back electron transfer) FTO|nanoITO-TiO2(e-)|-1+|TiO2|-2  FTO|nanoITO-TiO2(e-)|-1+|TiO2|-2  → FTO|nanoITO-TiO2(e-)|-1|TiO2|-2+ (catalyst activation) Conclusion: Our results on mixed photoanodes of the phosphonic acid-derivatized donor–π–acceptor chromophore and the water oxidation catalyst [Ru(bda)(4-pyCH2PO3H2)2] open a new area for photoanode design. The design is based on surface structures that integrate a light-absorbing organic dye and an inorganic catalyst for water oxidation, here linked by ALD deposition of a TiO2 overlayer. A virtue of this approach to assembly formation is the absence of a requirement for synthesis of a covalently linked chromophore-catalyst assembly. The modular approach presented here can be applied to any core- or core/shell-based electrode and presumably to any 23 ACS Paragon Plus Environment


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type of dye (organic, polypyridyl, porphyrin, etc.) and coupled water oxidation catalyst to create assemblies for photoelectrochemical water splitting. In the experiments described here, a TiO2 spacer layer thickness of 0.6 nm was found to optimize the structure in terms of delivery of redox equivalents through the assembly to the external catalyst as shown by photocurrent response of the cell. A comparison of the electrodes with and without catalyst 2 showed an enhanced photocurrent of ~2x for nanoITO/TiO2-based and ~2.5x for SnO2/TiO2-based electrodes, despite some loss of 1 during catalyst loading. The detailed photophysical analysis revealed a significant limitation in this specific assembly caused by the short-lived nature of the dye's charge-separated state, which we attribute to the structure and energetics of the dye. This shortcoming inadvertently highlights a key feature of the multi-layer method taken in this work. This new approach is a modular one that enables each molecular component to be tailored independently to optimize chromophore light absorption, chromophore redox potential, catalytic activity, etc. before incorporation of the different components into surface-bound assemblies. Further studies, employing different dyes for example, in this layered, co-loading strategy for fabrication of DSPEC photoanodes are underway.

Supplementary Information: Synthesis of the organic chromophore donor–π–acceptor 1, (Figure S1), H1-NMR and P31-NMR spectra of 1 and its intermediates (Figure S2 - Figure S6), cross-sectional SEM and elemental analysis (Figure S7), absorption spectra of the FTO|nanoITO-4.5 nm TiO2 core/shell electrodes (Figure S8), cyclic-voltammograms collected in the dark and under illumination as a function of TiO2 spacer layer thickness between 1 and 2 (Figure S9), absorption spectra and photocurrent for the FTO|SnO2/6.6 nm TiO2 core/shell electrodes (Figure S10), incident photon-to-current efficiency for FTO|SnO2/3.3 nm TiO2-1|0.6 nm TiO2 and FTO|SnO2/3.3 nm TiO2-1|0.6 nm TiO2|-2 electrodes (Figure S11), collector-generator charge vs. time measurements (Figure S12), and spectroelectrochemical oxidation of 1 and 2 on nanoITO electrodes (Figure S13).


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Acknowledgements This research was wholly supported by the UNC EFRC Center for Solar Fuels, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. DE-SC0001011. This work made use of instrumentation at the Chapel Hill Analytical and Nanofabrication Laboratory (CHANL), a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), which is supported by the National Science Foundation (Grant ECCS-1542015) as part of the National Nanotechnology Coordinated Infrastructure (NNCI). References

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Chromophore-Catalyst Assembly for Water Oxidation Prepared by

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