Controlling Vertical and Lateral Electron Migration Using a

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Controlling Vertical and Lateral Electron Migration Using a Bifunctional Chromophore Assembly in Dye-Sensitized Photoelectrosynthesis Cells Bing Shan, Animesh Nayak, M. Kyle Brennaman, Meichuan Liu, Seth L. Marquard, Michael S. Eberhart, and Thomas J. Meyer J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03453 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 27, 2018

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Journal of the American Chemical Society

Controlling Vertical and Lateral Electron Migration Using a Bifunctional Chromophore Assembly in Dye-Sensitized Photoelectrosynthesis Cells

Bing Shan, Animesh Nayak, M. Kyle Brennaman, Meichuan Liu, Seth L. Marquard, Michael S. Eberhart and Thomas J. Meyer*

Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA.

Abstract. Integration of photo-responsive chromophores that initiate multi-step catalysis is essential in dye-sensitized photoelectrosynthesis cells and related devices. We describe here an approach that incorporates a chromophore assembly surface-bound to metal oxide electrodes for light absorption with an over-layer of catalysts for driving the half-reactions of water splitting. The assembly is a combination of a core-twisted perylene diimide and a ruthenium polypyridyl complex. By altering the connection sequence of the two subunits in the assembly, in their surfacebinding to either TiO2 or NiO, the assembly can be tuned to convert visible light into strongly oxidizing equivalents for activation of an electrodeposited water oxidation catalyst (NiCo2Ox) at the photoanode, or reducing equivalents for activation of an electrodeposited water reduction catalyst (NiMo0.05Sx) at the photocathode. A key element in the design of the photoelectrodes comes from the synergistic roles of the vertical (inter-layer) charge transfer and lateral (intra-layer) charge hopping in determining overall cell efficiencies for photoelectrocatalysis.

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INTRODUCTION Understanding how electron transfer dynamics influence the performance of molecular-based devices such as dye-sensitized solar cells, organic solar cells, and artificial photosynthesis cells is an essential element in the development of solar energy conversion devices.1-6 The dye-sensitized photoelectrosynthesis cells operate by integrating light-induced electron transfer reactions for activation of catalysts and dark catalytic reactions for solar fuel generation.7-10 For those devices, considerable attention has been focused on photogeneration of redox equivalents that can be transported to catalysts.11-16 In addition to the photogenerated charges, efficient activation of catalysts also requires effective electronic coupling of the catalysts with the charge carriers and pathways for favorable electron migration. Ruthenium polypyridyl (RuLn2+) complexes have been found to mediate intrinsically slow activation of molecular water oxidation catalysts in solution.17-20 The mediating effect arises from the low barrier for self-exchange reactions of the RuLn3+/RuLn2+ couples, with self-exchange rate constants near the diffusion-controlled limit.21 Self-exchange reactions have also been observed for molecules immobilized on metal oxide surfaces where they undergo cross-surface, lateral charge-hopping.22-25 Application of the mediating effects of molecules with rapid lateral charge hopping kinetics in dye-sensitized photosynthetic electrodes can potentially enhance their efficiencies by facilitating catalyst activation. We report here an integrated design for dye-sensitized photoanodes and photocathodes based on a bi-functional chromophore assembly that consists of a ruthenium polypyridyl complex (RuC2+: [RuII(4,4’-(CH2PO3H2)2-2,2’-bipyridine)2(2,2’-bipyridine)]Cl2) and a covalently linked perylene diimide compound (PDI’: N,N’-Di(4-benzylphosphonic acid)-1,7-di(2,6-dimethoxyphenyl) perylene-3,4,9,10-tetracarboxylic acid bisimide), with structures shown in Figure 1a. In the photoelectrodes, the assembly was investigated as an oxidative dye for TiO2 photoanode and as a reductive dye for NiO photocathode. Control of the photogenerated electron flow was achieved by altering the connection sequence of the two subunits in the assembly. Following light excitation, the photogenerated redox equivalents in the assembly were transported to external layers of water oxidation or reduction catalysts electrodeposited on the photoelectrodes. Feasibility of the approach is tied to the dramatically different lateral charge hopping rates of PDI’ and RuC2+ during respective redox cycles. Our results show the importance of coupling lateral charge hopping with inter-layer charge transfer in activating the catalysts in dye-sensitized photoelectrosynthesis cells.


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RESULTS AND DISCUSSION Design and synthesis of the photoelectrodes. Perylene diimide dyes have been recognized as promising organic chromophores for solar energy conversion systems based on their long fluorescence lifetimes, high molar absorptivities, and chemical stabilities,26-29 with systematic structural variations at the peri-, bay- or ortho- positions of the perylene core.30,31 Most perylene diimides tend to π-stack cofacially, leading to aggregates that diminish the excited state lifetime and energy by self-quenching.31,32 It has been demonstrated that bay-functionalization of the perylene cores with sterically demanding substituents could substantially reduce aggregation of PDI molecules.30,31 In this work, we synthesized a perylene diimide chromophore (PDI’) with the structure shown in Figure 1a. A pronounced distortion of the skeleton exists originating from the 1,3-dimethoxybenzene substituents, which minimizes the formation of large aggregates from π-π stacking. On an insulating, mesoporous Al2O3 surface, PDI’ presents a dominant absorption band centered at 545 nm (Figure S1), which is characteristic for the allowed π→π* transition localized on the perylene core. An intense emission band was observed at 635 nm (Figure S2a). Based on the emission spectral fitting as detailed in the supplementary information, the excited state free energy (ΔGES) of PDI’ was estimated as 2.27 eV with E00 2.09 eV. The chromophore assembly was prepared through ZrIV bridging33,34 of RuC2+ and PDI’ on TiO2 or NiO surfaces in two forms. For the photoanode, PDI’ molecules were initially selfassembled on TiO2, followed by a two-step loading of the ZrIV bridge and another layer of RuC2+ to form the photoanode TiO2│-PDI’-ZrIV-RuC2+. For the photocathode, RuC2+ was loaded on NiO first, with PDI’ connected to the surface-bound RuC2+ by the ZrIV bridge, forming the photocathode NiO│-RuC2+-ZrIV-PDI’. The structures of the photoelectrodes are shown in Figure 1b. For this type of layer-by-layer assembly with the redox-inert bridging ion, ZrIV, the photogenerated charges transfer from one layer to another through electron tunneling.35,36 Light absorption efficiencies of the photoelectrodes were evaluated based on the UV-visible absorption spectra in Figure 1e,f. The fractions of light absorbed at 540 nm (ηF) by PDI’ or RuC2+ are summarized in Table S1. Based on the data, RuC2+ only absorbs ≤ 0.2% of the incident light at ~540 nm for the photocathodes and ≤ 6.6% for the photoanodes. Most of the light (at ~540 nm) is absorbed by PDI’, with 35% for the photocathodes and 60% for the photoanodes.



Figure 1. a, Structures of PDI and RuC2+. b. Structures of the photoanode TiO2│-PDI’-ZrIV-RuC2+│WOC and the photocathode NiO│-RuC2+-ZrIV-PDI’│WRC. c, d, Surface/cross-section SEM images of the photoanode and the photocathode, respectively. e, UV-Vis absorption spectra of TiO2 (dashed, I), TiO2│-PDI’ (orange, II), TiO2│-PDI’ZrIV-RuC2+ (red, III), TiO2│-PDI’-ZrIV-RuC2+│WOC (brown, IV). f, UV-Vis absorption spectra of NiO (dashed, I), NiO│-RuC2+ (gray, II), NiO│-RuC2+-ZrIV-PDI’ (green, III), NiO│-RuC2+-ZrIV-PDI’│WRC (black solid, IV). The insets of e and f show photographs of the electrodes.

On the surfaces of chromophore-derivatized electrodes, the water oxidation catalyst (WOC), nickel cobalt oxide (NiCo2Ox),37,38 and the water reduction catalyst (WRC), Mo-doped nickel sulfide (NiMo0.05Sx),39-41 were electrodeposited at negative applied potentials (see the Supplementary Information for the experimental details) to give the photoanode TiO2│-PDI’-ZrIVRuC2+│WOC and the photocathode NiO│-RuC2+-ZrIV-PDI’│WRC, respectively. The microstructures of the photoelectrodes are shown in the surface and cross-section scanning electron microscopy (SEM) images in Figure 1c,d. For electrodeposition of the WOC on TiO2│PDI-ZrIV-RuC2+, a negative bias at -0.95 V vs NHE was applied to reduce the deposition


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components at the electrode/solution interface. On the photocathode side, the WRC was electrodeposited on NiO│-RuC2+-ZrIV-PDI’ by the procedure shown in Figure S3a, based on the fact that NiO is a p-type semiconductor and is insulating in its depletion region under negative bias. During electrodeposition, electrons were transported from the FTO back contact to the molecules directly attached to FTO, and further delivered to the molecules attached to NiO through cross-surface electron hopping. The deposition components (Ni2+, Mo6+ and coordinated thiourea (TU)) were reduced by electrons percolated over the NiO surface and formed a layer of NiMo0.05Sx on top of the dye-modified NiO. In Figure S3b, the increase in current density with deposition time indicates continuous growth of the catalyst on the photocathode surface. The optical transparency of the photoelectrodes drops dramatically with the added catalysts as shown by the dark curves in Figure 1e,f. In the photoelectrocatalytic experiments described in the following sections, the photoelectrodes were illuminated from the FTO│metal oxide sides, with the incident light absorbed mostly by the assembly (Table S1), much less by the catalysts.

Photoelectrocatalysis. The resulting photoelectrodes undergo light-driven water splitting with performances characterized by slow linear sweep voltammetry and chronoamperometry under intermittent green LED light illumination (λmax: 545 nm, 20-nm FWHM, 10 mW/cm2), as shown in Figure 2. Among the photocathodes investigated in Figure 2a, the largest photocurrent response was obtained with the photocathode NiO│-RuC2+-ZrIV-PDI’│WRC (green curve in Figure 2a) at the applied bias range. Inverting the connection sequence of the assembly to NiO│-PDI’-ZrIVRuC2+│WRC resulted in a substantial decrease of the photocurrents (pink curve in Figure 2a). For the photoanodes in Figure 2b, TiO2│-PDI’-ZrIV-RuC2+│WOC performs the best relative to the other two photoanodes. Likewise, greatly diminished photocurrents were observed by inverting the connection sequence of the assembly to TiO2│-RuC2+-ZrIV-PDI’│WOC (green curve in Figure 2b). As background observations, in the absence of PDI’ or the catalysts, the outputs of the photoelectrodes were negligible under these conditions. The small photocurrent responses for the photoelectrodes without PDI’, as shown by the gray curves in Figure 2a-b, result from low light absorption efficiencies of RuC2+ as listed in Table S1.



Figure 2. Photocurrent densities (j/ mA cm-2) under green LED light illumination. a, j as a function of applied bias at a scan rate of 5 mV/s for the photocathodes: NiO│-RuC2+-ZrIV-PDI’│WRC (green), NiO│-PDI’-ZrIV-RuC2+│WRC (pink), NiO│-PDI’│WRC (blue) and NiO│-RuC2+│WRC (gray), in pH 4.5, 0.1 M acetate buffer. The dashed curve shows the dark currents of NiO│-RuC2+-ZrIV-PDI’│WRC. b, As in a, j as a function of applied bias at a scan rate of 5 mV/s for the photoanodes: TiO2│-RuC2+-ZrIV-PDI’│WOC (green), TiO2│-PDI’-ZrIV-RuC2+│WOC (pink), TiO2│PDI’│WOC (blue) and TiO2│-RuC2+│WOC (gray), in pH 7.0, 0.1 M phosphate buffer. The dashed curve shows dark currents for TiO2│-PDI’-ZrIV-RuC2+│WOC. c, d, j as a function of time for the photocathodes and photoanodes, respectively, under zero applied bias (vs NHE) and intermittent green LED light illumination, with the same color designations as in a, b.

The photoelectrodes were also investigated by chronoamperometry measurements under intermittent green LED light illumination at zero applied bias (vs NHE), with results shown in Figure 2c,d. The performances of both the NiO photocathodes (Figure 2c) and the TiO2 photoanodes (Figure 2d) varied with the connection sequence of PDI’ and RuC2+ in the assembly. The NiO photocathode with -RuC2+-ZrIV-PDI’ and the TiO2 photoanode with -PDI’-ZrIV-RuC2+ show much higher efficiencies relative to the photoelectrodes with inversely connected PDI’ and RuC2+. During long-term photoelectrocatalysis, the photoanode TiO2│-PDI’-ZrIV-RuC2+│WOC and the photocathode NiO│-RuC2+-ZrIV-PDI’│WRC catalyze water oxidation and reduction,


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respectively, under the green light illumination with photocurrents shown in Figure S4a,b. Faradaic yields of hydrogen and oxygen from two-hour photoelectrocatalysis experiments by the photocathode

NiO│-RuC2+-ZrIV-PDI’│WRC

and

the

photoanode

TiO2│-PDI’-ZrIV-

RuC2+│WOC are 79% and 64%, respectively.

Light-driven intra-assembly charge separation. On the studies of metal oxide electrodes modified with self-assembling molecular building blocks, we have explored the direct relationships

between

inter-layer

electron

transfer

dynamics

and

the

observed

photoelectrocatalytic performances.12,13,42 In the chromophore assemblies (-PDI’-ZrIV-RuC2+ or RuC2+-ZrIV-PDI’) on metal oxide electrodes, green light illumination creates the excited state, PDI’*, which initiates a sequence of electron transfer reactions. The intra-assembly electron transfer between PDI’* and RuC2+ was investigated with the assembly PDI’-ZrIV-RuC2+ attached to Al2O3. As shown in the steady state emission spectra in Figure 3a, the decreased emission intensity of PDI’* in Al2O3│-PDI’-ZrIV-RuC2+ compared to that in Al2O3│-PDI’ results from reductive quenching of PDI’* by RuC2+ (equation (1)). The quenching efficiency is 63.4%, determined from the relative emission intensities of the two samples. Al2O3│-PDI’*-ZrIV-RuC2+ → Al2O3│-PDI’-•-ZrIV-RuC3+

(1)

Electron transfer from RuC2+ to PDI’* is thermodynamically favored with a driving force of 0.37 eV as estimated from the redox potentials in Table 1. Details in determining the potential values are shown in Figure S5 in the Supplementary Information. The electron-deficient nature of PDI’* drives the electron migration from RuC2+ to the perylene core, forming the stable perylene diimide anion radical, PDI’-•. In the assemblies, electron transfer between the two excited states, PDI’* and RuC2+*, is not expected to play a role, given the mismatched lifetimes (τes) of the two excited states (τes(PDI’) ~5.8 ns from Figure 3b, and τes(RuC2+) ~250 ns43) and the small fraction of light absorbed by RuC2+ at ~540 nm (Table S1).



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Figure 3. a, Emission spectra for PDI’ and PDI’-ZrIV-RuC2+ on Al2O3 surfaces upon excitation at 532 nm. The emission intensities are normalized with respect to the surface coverages of PDI’. b, Time-resolved emission (dots) with single exponential fitting (line) for PDI’ on Al2O3, from which, τes(PDI’) is estimated to be 5.8 ns.

Table 1. Formal redox potentials of PDI’ and RuC2+ in V vs NHE at room temperature. PDI’ a

a

RuC2+ b

+•/0

0/-•

*/-•

+•/*

3+/2+

2+/+

1.46

-0.66

1.61

-0.81

1.24

-1.12

Determined from the method detailed in Figure S5, with PDI’ attached

on nanoITO electrodes which were immersed in N2-degassed acetonitrile with 0.1 M LiClO4 under room temperature. b From the reference43.

Light-driven interfacial charge separation in the photoanode. Photoinduced vertical (interlayer) electron transfer reactions for the photoelectrodes in the absence of the catalysts were investigated by nanosecond transient absorption (TA) measurements. Excitation of the photoelectrodes by 532 nm laser pulses generates the transient absorptive changes as shown in Figure 4 that were analyzed based on the spectroelectrochemical data for PDI’ and RuC2+ in Figure 5. For the photoanode TiO2│-PDI’-ZrIV-RuC2+, the excited state PDI’* forms upon light excitation and subsequently injects an electron into TiO2 to give PDI’+• with a positive absorptive feature at 750 nm and a bleach at 560 nm, Figure 4a. Oxidation of RuC2+ by PDI’+• is shown by the bleach of RuC2+ at 450 nm due to the formation of RuC3+. The underlying electron transfer steps are summarized in equations (2)-(3). With the added WOC, RuC3+ transfers holes to the WOC, equation (4), forming the ultimate redox-separated state, TiO2(e-)│-PDI’-ZrIV-RuC2+│WOC(h+).


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TiO2│-PDI’*-ZrIV-RuC2+ → TiO2(e-)│-PDI’+•-ZrIV-RuC2+

(2)

TiO2(e-)│-PDI’+•-ZrIV-RuC2+ → TiO2(e-)│-PDI’-ZrIV-RuC3+

(3)

TiO2(e-)│-PDI’-ZrIV-RuC3+│WOC → TiO2(e-)│-PDI’-ZrIV-RuC2+│WOC(h+)

(4)

For TiO2│-PDI’ in the absence of RuC2+, the TA in Figure 4c shows the absorption features of PDI’+• which lives for ~0.5 μs before the completion of charge recombination with TiO2(e-). For the photoanode TiO2│-PDI’-ZrIV-RuC2+, the forward hole migration from PDI’+• to RuC2+ is complete within ~0.1 μs (Figure 4a), which is more rapid than the recombination between TiO2(e-) and PDI’+• (~0.5 μs). The redox-separated state, TiO2(e-)│-PDI’-ZrIV-RuC3+, can therefore be favorably generated. Light-driven interfacial charge separation in the photocathode. For the photocathode NiO│-RuC2+-ZrIV-PDI’ in Figure 4b, light excitation initiates the intra-assembly, reductive quenching of PDI’* by RuC2+, forming RuC3+ and PDI’-•, equation (5). The persistent absorption feature at 740 nm and a small bleach at 560 nm are attributed to PDI’-•. In the spectra, RuC3+ appears as the bleach at ~450 nm, which decays within 0.25 μs due to the subsequent hole injection into NiO, equation (6). In the presence of the WRC, the terminal PDI’-• reduces the WRC, forming the interfacial redox-separated state NiO(h+)│-RuC2+-ZrIV-PDI’│WRC(e-) towards water reduction. The reaction sequence is summarized in equations (5)-(7). NiO│-RuC2+-ZrIV-PDI’* → NiO│-RuC3+-ZrIV-PDI’-•

(5)

NiO│-RuC3+-ZrIV-PDI’-• → NiO(h+)│-RuC2+-ZrIV-PDI’-•

(6)

NiO(h+)│-RuC2+-ZrIV-PDI’-•│WRC → NiO(h+)│-RuC2+-ZrIV-PDI’│WRC(e-)

(7)

The TA spectra in Figure 4d for the assembly -RuC2+-ZrIV-PDI’ on Al2O3 shows the intraassembly quenching and the subsequent charge recombination. The initial absorptive changes following excitation of Al2O3│-RuC2+-ZrIV-PDI’ are similar to those of the photocathode NiO│RuC2+-ZrIV-PDI’. For both samples, the first photoinduced electron transfer step is the intraassembly quenching reaction. For Al2O3│-RuC2+-ZrIV-PDI’, the quenching products undergo charge recombination to the initial state within 0.5 μs. For the photocathode with added WRC, following the quenching step, the holes in RuC3+ are injected into NiO and the electrons in PDI’-• are transferred to the WRC.



Figure 4. TA spectra as a function of time delay after laser excitation at 532 nm for the photoelectrodes: a, TiO2│PDI’-ZrIV-RuC2+; b, NiO│-RuC2+-ZrIV-PDI’; c, TiO2│-PDI’; d, Al2O3│-RuC2+-ZrIV-PDI’. e, Energy diagram for the photoanode, TiO2│-PDI’-ZrIV-RuC2+│WOC, showing the photo-induced electron transfer initiated by electron injection to TiO2 by PDI’* (solid red arrow) and the following electron transfer steps towards oxidation of WOC (dashed red arrows). f, Energy diagram for the photocathode, NiO│-RuC2+-ZrIV-PDI’│WRC, showing the photoinduced electron transfer initiated by reductive quenching of PDI’* by RuC2+ (solid green arrow), the subsequent electron transfer from NiO to RuC3+ and from PDI’-• to the WRC (dashed green arrows).


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Figure 5. Spectroelectrochemistry of RuC2+ and PDI’. a, Steady-state absorptive changes following one-electron oxidation (RuC3+, green) and reduction (RuC+, orange) of RuC2+. b, Steady-state absorptive changes following oneelectron oxidation (PDI’+., blue) and reduction (PDI’-., red) of PDI’.

Charge transfer activation of the catalysts. In order for catalysis to occur, potentials of the charge carriers in the assembly must exceed the onset potentials of the catalysts. Based on the linear sweep voltammograms in Figure 6, the onset potentials of the WRC and WOC are -0.19 V and 1.40 V vs RHE, respectively. For the photoanode TiO2│-PDI’-ZrIV-RuC2+, the hole carried by RuC3+ has an oxidizing power of 1.65 V vs RHE (pH 7.0; 1.24 V vs NHE), far surpassing the onset potential of WOC for water oxidation (driving force for oxidation of WOC: 0.25 eV). For the photocathode NiO│-RuC2+-ZrIV-PDI’, the electron carried by PDI’-• has a reducing power of -0.39 vs RHE (pH 4.5; -0.66 V vs NHE; driving force for reduction of WRC: 0.20 eV). Relative energetics are shown in the energy diagrams in Figure 4e for the photoanode and in Figure 4f for the photocathode.

Figure 6. Linear sweep voltammograms for a, nanoITO|NiCo2Ox in pH 7.0 phosphate buffer and b, nanoITO|NiMo0.05Sx in pH 4.5 acetate buffer. Scan rate: 2 mV/s. The measured potentials versus Ag/AgCl were



converted to a reversible hydrogen electrode (RHE) scale by ERHE = EAg/AgCl + 0.059 pH + 0.197. The vertical dashed lines show the estimated onset potentials for the WOC, NiCo2Ox, in a, and for the WRC, NiMo0.05Sx, in b.44

The presence of RuC2+ in both photoelectrodes has a significant impact on their performances, which can be induced by changes in the yields of photogenerated redox equivalents or the intrinsic differences between the charge carriers in the assemblies. The quantum yields of the photogenerated redox equivalents were estimated from the TA spectra for the photoanodes, TiO2│-PDI’-ZrIV-RuC2+ (Figure 4a) and TiO2│-PDI’ (Figure 4c), and for the photocathodes, NiO│-RuC2+-ZrIV-PDI’ (Figure 4b) and NiO│-PDI’ (Figure S6). The results are summarized in Tables S2 and S3. The quantum yields of redox equivalents do not vary significantly between the photoelectrodes with and without RuC2+. It is unlikely that the improved efficiencies for the photoelectrodes with RuC2+ are due to enhanced quantum yields of redox equivalents. The other factor in activating the catalysts, the intrinsic differences between the charge carriers, RuC2+ and PDI’, is expected to play the dominant role in influencing the overall efficiencies. Cross-surface lateral charge hopping in the assembly. Excitation of the photoanode TiO2│PDI’-ZrIV-RuC2+ generates holes carried by RuC3+. For the photoanode TiO2│-PDI’ without RuC2+, the photogenerated holes are carried by PDI’+•. Based on the formal potentials in Table 1, both RuC3+ and PDI’+• are thermodynamically capable of oxidizing WOC with comparable driving forces. The notably different photoelectrocatalytic performances between the two photoanodes, TiO2│-PDI’-ZrIV-RuC2+│WOC and TiO2│-PDI’│WOC, point to the fact that activation of WOC is kinetically more favored by RuC3+ than by PDI’+•. In previous studies,17-20 electron transfer mediating effects of rutheniumII polypyridyl complexes (Ru(bpy)32+, typically) were investigated for activation of water oxidation catalysts dissolved in solution or immobilized on nanoITO electrode surfaces. The well-defined Ru(bpy)33+/Ru(bpy)32+ redox couples facilitate the intrinsically slow oxidation of the catalysts by lowering barriers during multi-electron activation of the catalysts due to rapid self-exchange of the Ru(bpy)33+/Ru(bpy)32+ couples21. Given the structures of the photoelectrodes used here, similar mediating effects are expected in activation of the catalysts by the assembly. The lateral hole hopping behaviors of the redox couples, RuC3+/RuC2+ and PDI’+•/PDI’0, on TiO2 electrodes were characterized electrochemically with the apparent diffusion coefficients (Dapp)22 determined from chronoabsorptometry (see the Supplementary Information for the method) with results shown in


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Figure 7a,b. The lateral hole hopping rate constant (khop)23 of PDI’+•/PDI’0 is slower than that of RuC3+/RuC2+ on TiO2 surface, which probably contributes to the much lower efficiency for TiO2│PDI’│WOC compared to TiO2│-PDI’-ZrIV-RuC2+│WOC. In the photoanode TiO2│-PDI’-ZrIVRuC2+│WOC, the rapid hole hopping between RuC3+ and RuC2+ facilitates activation of the catalyst. In combination with the favorable vertical hole transfer from PDI’+• to RuC2+ and from RuC3+ to WOC, and the extended lifetime of the redox-separated state (TiO2(e-)│-PDI’-ZrIVRuC3+), the overall catalytic performance is greatly enhanced as observed in Figure 2b.

Figure 7. Results of lateral (cross-surface) charge hopping measurements. a, Absorptive changes at 580 nm for TiO2│PDI’ under forward bias at 1.7 V vs NHE followed by reverse bias at -0.05 V vs NHE. b, Absorptive changes at 460 nm for TiO2│-RuC2+ under forward bias at 1.5 V vs NHE followed by reverse bias at -0.10 V vs NHE. c, Absorptive changes at 510 nm for NiO│-PDI’ under forward bias at -0.5 V vs NHE followed by reverse bias at 0.05 V vs NHE.



d, Absorptive changes at 510 nm for NiO│-RuC2+ under forward bias at -1.3 V vs NHE followed by reverse bias at 0.10 V vs NHE. The insets show the related optical Anson plots45. e, Summary of Dapp, khop and surface coverages of PDI’ or RuC2+ on TiO2 or NiO.

For the photocathodes, the efficiency of NiO│-PDI’│WRC was greatly enhanced by adding a layer of RuC2+ in between NiO and PDI’. Since the WRC in both NiO│-RuC2+-ZrIV-PDI’│WRC and NiO│-PDI’│WRC was activated by the reduced PDI’ (PDI’-•) which forms at equal yields (Table S2), variations in efficiencies should result from different local kinetics induced by the intermediate RuC2+ layer. Rapid, lateral charge hopping between surface-attached molecules has been observed to induce faster interfacial charge recombination and thus limits device efficiencies.46 For the photocathode NiO│-RuC2+-ZrIV-PDI’│WRC, RuC+/RuC2+ with a slower lateral electron hopping rate (9.0×103 s-1 in Figure 7e) spatially separates PDI’-•/PDI’0 with a much faster electron hopping rate (9.5×104 s-1) from the NiO surface. The better performance of the photocathode with the intermediate RuC2+ layer is probably due to the slower interfacial charge recombination as revealed by the TA spectra in Figure 4b for NiO│-RuC2+-ZrIV-PDI’ and Figure S6 for NiO│-PDI’. For both the photoanode TiO2│-PDI’-ZrIV-RuC2+│WOC and the photocathode NiO│-RuC2+-ZrIV-PDI’│WRC, the inner monolayers (PDI’+/PDI’0 for the photoanode, and RuC+/RuC2+ for the photocathode) undergo slower lateral charge hopping than the outer monolayers (RuC3+/RuC2+ for the photoanode, and PDI’-•/PDI’0 for the photocathode), as shown by the Dapp and kapp values in Figure 7e, which also contributes to the enhanced photoelectrocatalytic performances in Figure 2.

CONCLUSIONS In this work, we have investigated the dynamics of photoinduced, vertical (inter-layer) and lateral (cross-surface) electron migration for a bi-functional chromophore assembly on semiconducting nanostructured films in photoelectrosynthesis cells. Both types of dynamics play well-defined and microscopically important roles in overall cell performances. The cells are based on TiO2 photoanodes and NiO photocathodes with the surface-bound assembly, RuC2+-ZrIV-PDI’, terminally functionalized with electrodeposited catalysts, nickel cobalt oxide (NiCo2Ox) for water oxidation and Mo-doped nickel sulfide (NiMo0.05Sx) for water reduction. The configuration of the assembly varies with the photoelectrode and the target, water oxidation or water reduction.


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Detailed examinations on the local dynamics of vertical electron transfer and lateral electron hopping reveal a close relationship between the photoelectrocatalytic performances and the electron migration kinetics in the assembly layers. The vertical charge transfer determines the quantum yield and the direction of photogenerated charge flow from the metal oxide electrode to the external catalyst. Rapid, lateral charge hopping of the outer redox species in the assembly facilitates the intrinsically slow activation of the catalysts by lowering barriers during multielectron oxidation or reduction of the catalysts. Slow, lateral charge hopping of the inner redox species in the assembly contributes to the enhanced photoelectrocatalytic performances by slowing down the interfacial charge recombination. The results in this work are significant in describing a new approach to the design of molecular assembly-functionalized photoelectrodes. It is based on the exploitation of a flexible organic dye coupled with a ruthenium polypyridyl complex, with accessibility through vertical and lateral electron migration for external catalyst activation towards water oxidation and reduction. ASSOCIATED CONTENT Supporting Information All synthetic and experimental details, and supplementary spectroscopic and photoelectrocatalytic data. AUTHOR INFORMATION Corresponding Author

* [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS This material is based on work supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, under Award No. DE-SC0015739. The experiments with nanosecond TA spectrometer, fluorimeter, profilometer and solid-state light source were performed with the instruments in the UNC EFRC Instrumentation Facility established 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 DE-SC0001011. The SEM and ALD experiments were performed at the Chapel Hill Analytical and Nanofabrication Laboratory, CHANL, a member of the North Carolina Research Triangle Nanotechnology Network, RTNN, supported by the National Science Foundation, Grant ECCS-1542015, as part of the National Nanotechnology Coordinated Infrastructure, NNCI. The authors thank Byron H. Farnum of Auburn University for discussions on the manuscript.

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Controlling Vertical and Lateral Electron Migration Using a

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