A stable dye-sensitized photoelectrosynthesis cellmediated by a NiO overlayer for water oxidationDegao Wanga, Fujun Niua,b, Michael J. Mortellitia, Matthew V. Sheridanc, Benjamin D. Shermand, Yong Zhue,James R. McBridef, Jillian L. Dempseya, Shaohua Shenb, Christopher J. Daresc, Fei Lia,e,1, and Thomas J. Meyera,1

aDepartment of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599; bInternational Research Centre for Renewable Energy, StateKey Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Shaanxi 710049, China; cDepartment of Chemistry and Biochemistry,Florida International University, Miami, FL 33199; dDepartment of Chemistry, Texas Christian University, Fort Worth, TX 76129; eState Key Laboratory of FineChemicals, Dalian University of Technology, Dalian 116024, China; and fVanderbilt Institute of Nanoscale Science and Engineering, Vanderbilt University,Nashville, TN 37235

Edited by Richard Eisenberg, University of Rochester, Rochester, NY, and approved August 7, 2019 (received for review March 18, 2019)

In the development of photoelectrochemical cells for water splittingor CO2 reduction, a major challenge is O2 evolution at photoelec-trodes that, in behavior, mimic photosystem II. At an appropriatesemiconductor electrode, a water oxidation catalyst must be inte-grated with a visible light absorber in a stable half-cell configura-tion. Here, we describe an electrode consisting of a light absorber,an intermediate electron donor layer, and a water oxidation cata-lyst for sustained light driven water oxidation catalysis. In assem-bling the electrode on nanoparticle SnO2/TiO2 electrodes, a Ru(II)polypyridyl complex was used as the light absorber, NiO was de-posited as an overlayer, and a Ru(II) 2,2′-bipyridine-6,6′-dicarboxylatecomplex as the water oxidation catalyst. In the final electrode,addition of the NiO overlayer enhanced performance toward wateroxidation with the final electrode operating with a 1.1 mA/cm2

photocurrent density for 2 h without decomposition under onesun illumination in a pH 4.65 solution. We attribute the enhancedperformance to the role of NiO as an electron transfer mediatorbetween the light absorber and the catalyst.

artificial photosynthesis | water oxidation | mediator | core/shell | NiO

The dye-sensitized photoelectrosynthesis cell (DSPEC) pro-vides an approach for converting solar energy into chemical

fuels by water splitting, 2H2O + 4 hν→ O2 + 2H2, or by reducingCO2 (1–6). In a typical DSPEC design, a TiO2-based semi-conductor photoanode incorporates surface-bound visible-light–absorbing chromophores with a water oxidation catalyst (WOC)and performs water oxidation by the surface reactions shown inEqs. 1–3. Electrons collected at TiO2 are transferred to a cathodefor hydrogen production. A nominal voltage (ΔV) is supplied by apotentiostat or second photojunction to supply electrons withsufficient energy for hydrogen production at the cathode (7–9).Examples of such photoanodes have demonstrated the first of the4 catalyst activation steps for water splitting (10, 11). The design ofwater oxidation DSPEC photoanodes is particularly challengingbecause of the need to integrate both light absorption and catal-ysis at the oxide interface. The resulting interfacial structure mustbe stable when irradiated and support a high number of turnoversin aqueous solution. Atomic layer deposition (ALD) and elec-tropolymerization have been used to stabilize surface chromo-phore and assembly structures on metal oxide electrodes (12, 13).However, the conditions required to achieve stable surface as-sembly structures that can sustain long-term catalytic behaviorremains a key challenge in this field:

TiO2j-Chrom-Cat+ hv→TiO2ðe−Þj-Chrom+-Cat ðelectron  injectionÞ, [1]

TiO2ðe−Þj-Chrom+-Cat→TiO2ðe−Þj-Chrom-Cat+ ðhole  transferÞ, [2]

H+ðcathodeÞ-ΔV-TiO2ðe−Þj-Chrom-Cat+

→ ðcathodeÞ-TiO2j-Chrom-Cat+

+ 1=2H2 ðcathode  activationÞ.[3]

Multistep electron transfer is a key element in water oxidationwith the 4-electron oxidation of water being kinetically demand-ing (14, 15). Water oxidation occurs on the millisecond timescalefor the oxygen-evolving complex (OEC) in photosystem II (PSII)(16, 17). A key element in PSII is a tyrosine–histidine pair thatmediates electron transfer between the P680 primary donor andthe manganese-based OEC (18–22). Synthetic mimics for thebiological relay have been investigated previously in moleculardyads and triads for water splitting electrodes (14, 23–26):

TiO2j-Chrom-Donor-Cat+ hv→TiO2ðe−Þj-Chrom-Donor+-Cat,[4]

TiO2ðe−Þj-Chrom-Donor+-Cat→TiO2ðe−Þj-Chrom-Donor-Cat+,

[5]

TiO2ðe−Þj-Chrom-Donor-Cat+

→TiO2j-Chrom-Donor-Cat charge  recombination.

[6]

As illustrated in Eqs. 4 and 5, an important role for an electrontransfer mediator also appears in the design of artificial assemblies.Addition of the Donor+/0 couple as an electron transfer relay betweenthe primary donor (Chrom) and catalyst increases the timescale for

This paper results from the Arthur M. Sackler Colloquium of the National Academy ofSciences, “Status and Challenges in Decarbonizing our Energy Landscape,” held October10–12, 2018, at the Arnold and Mabel Beckman Center of the National Academies ofSciences and Engineering in Irvine, CA. NAS colloquia began in 1991 and have beenpublished in PNAS since 1995. From February 2001 through May 2019 colloquia weresupported by a generous gift from The Dame Jillian and Dr. Arthur M. Sackler Foundationfor the Arts, Sciences, & Humanities, in memory of Dame Sackler’s husband, Arthur M.Sackler. The complete program and video recordings of most presentations are availableon the NAS website at http://www.nasonline.org/decarbonizing.

Author contributions: D.W., F.L., and T.J.M. designed research; D.W., F.N., M.J.M., andJ.R.M. performed research; Y.Z. contributed new reagents/analytic tools; D.W., M.V.S.,B.D.S., J.L.D., C.J.D., and F.L. analyzed data; and D.W., M.V.S., B.D.S., J.L.D., S.S., F.L.,and T.J.M. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

Published under the PNAS license.1To whom correspondence may be addressed. Email: lifei@dlut.edu.cn or tjmeyer@unc.edu.

This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1821687116/-/DCSupplemental.

First published September 5, 2019.

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back electron transfer from the electrode to holes in the assembly(14, 27, 28). This enhances the likelihood of performing water ox-idation catalysis before charge recombination occurs at Eq. 6.The role of the tyrosine–histidine analogs has been recognized

experimentally in the design of surface-bound molecular assem-blies including added electron transfer mediators (29, 30). In thisrole, the mediator must have a redox potential between thechromophore and highest oxidation state couple of the catalyst totransfer reductive equivalents from the catalyst to the oxidizedchromophore. The mediator must also be a nonoverlapping lightabsorber with the chromophore in the visible region and be stableunder the conditions for water splitting.Here, we describe the use of an ultrathin NiO overlayer between

the chromophore and catalyst on nanoparticle SnO2/TiO2 surfacesto serve as an electron transfer analog of the Tyr–His pair in PSII.NiO films were inserted between the chromophore and catalyst inthe assemblies, which provide a redox level sufficient to drive elec-tron transfer between the oxidized chromophore and catalyst.The assembly utilized a derivative of the WOC Ru(II)(2,2′-bipyridine-6,6′-dicarboxylate)(L)2 (Cat, L is a pyridyl ligand), asfirst described by Sun and coworkers (31). In the photoanode,FTOjSnO2/TiO2j-RuP2+jNiOj-Cat, with 20 cycles deposited NiOlayer, a photocurrent density of 1.1 mA/cm2 was reached over aperiod of 2 h with an applied bias of 0.6 V vs. NHE and 1-sunillumination in a 0.1 M acetate buffer at pH 4.65. The Faradaicefficiency for O2 production was 87% with an incident photonconversion efficiency (IPCE) value of 18% at 440 nm. The perfor-mance characteristics of this DSPEC photoelectrode are impressive,even when compared to related semiconductor photoelectrodes.

Results and DiscussionAssemblies.Molecular structures of the chromophore andWOC areshown in Fig. 1. The chromophore RuP2+ ([Ru(bpy)2(4,4′-PO3H2-bpy)]2+: bpy = 2,2′-bipyridine; 4,4′-PO3H2-bpy) and catalyst (Ru(-bda)(4,4′-bpy)2 [4,4′-bpy = 4,4′-bipyridine; bda = 2,2′-bipyridine-6,6′-dicarboxylate]), were prepared by literature procedures (32,33). Preparation and characterization of nanoITO and SnO2/TiO2 core/shell structures followed known literature procedures(34, 35). In the current experiments, 4-μm-thick films of SnO2(composed of 15-nm nanoparticle) on FTO electrodes werecoated with a 5-nm layer of TiO2 to give the core/shell structures.To prepare the interface, the electrode as described above was

loaded with RuP2+ to give FTOjSnO2/TiO2j-RuP2+ by soaking in0.5 mM solutions of the Cl− salt in methanol overnight. The slideswere rinsed with copious amounts of pure methanol to removephysically absorbed molecular and salts. After drying in air, theFTOjSnO2/TiO2j-RuP2+ slides were placed in an ALD chamber. Thetemperature in the ALD chamber was held at 150 °C under conditionswhere the adsorbed molecules were known to be stable during ALDdeposition conditions (34). The added metal oxide overlayer of 20cycles NiO provides new binding sites for additional mole-cules on the surface. The catalyst was subsequently added byplacing electrodes FTOjSnO2/TiO2j-RuP2+j20cycles NiO insolutions 3 mM in Cat in methanol for an additional 12 h in aglove box to obtain FTOjSnO2/TiO2j-RuP2+j20cycles NiOj-Cat.Relative configuration is shown in SI Appendix, Fig. S1.Surface coverages were evaluated by UV-visible spectral measure-

ments by using the molar absorptivity of the chromophore withe =13,000 M−1·cm−1 at 450 nm (36). Based on the data in Fig. 2, the20 cycle NiO layer has no obvious impact on the optical absorptionfeatures of the assembly with no significant changes from 400 to600 nm but with light scattering/absorption from SnO2/TiO2 below 400nm. Based on past results, surface loading of the chromophoreRuP2+ on these electrode occurs with close packing and saturationsurface coverages under the conditions for surface loading (36). Sur-face analysis of the final electrodes by absorption measurements wasbased on Eq. 7. In Eq. 7, Γ is the surface loading in moles per squarecentimeter with absorption measurements at wavelength λ and a molarextinction coefficient of eλ. Analysis of data at 450 nm gave a surfaceloading of Γ = 6 × 10−8 mol/cm2 for the 4-μm films. In the final films,the metal (ion)-to-ligand charge transfer (MLCT) absorption forRuP2+ is somewhat red shifted with peak broadening after ALD de-position (37, 38). The overall appearance of the RuP2+ absorptionfeature at 450 nm is relatively unchanged showing that RuP2+ isretained during the catalyst addition step without loss from the surfacesduring the ALD treatment. The 20 ALD deposition cycles used todeposit the NiO is estimated to produce 0.6-nm-thick films as de-termined by ellipsometric spectroscopy on a spectator Si wafer:

Γ�mol

�cm2�=Aλ=ðeλ1,000Þ. [7]

To further confirm the ratio of chromophore and catalyst on themetal oxide surface, X-ray photoelectron spectroscopy (XPS)

Fig. 1. (Top) Structure of the DSPEC photoanode FTOjSnO2/TiO2j-RuP2+jNiOj-Cat with a structural comparison to important residues and cofactors inPSII. The direction of electron transfer following excitation in PSII is shown below. The photoanode studied here integrates a molecular chromophore(RuP2+), an electron acceptor electrode (SnO2/TiO2), an electron transfer relay (NiO), and a WOC (Cat). (Bottom) Key components of the electron transportchain in PSII.

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was used to investigate the interfacial elemental composition.Based on the data in SI Appendix, Fig. S2, with the Ti ele-ments in the substrate as the reference, the Ru/Ti ratio was0.0193 in FTOjSnO2/TiO2j-RuP2+j20cycles NiO and 0.0434 inFTOjSnO2/TiO2j-RuP2+j20cycles NiOj-Cat. The increase of Rucontent is consistent with addition of the catalyst. Based on thedata, the chromophore/catalyst ratio was ∼1:1.2. Relative resultsof the assemblies FTOjSnO2/TiO2j-RuP2+j10cycles NiOj-Cat andFTOjSnO2/TiO2j-RuP2+j30cycles NiOj-Cat are shown in SI Ap-pendix, Fig. S2. XPS was also used to explore the oxidation stateof Ni in the 20cycles NiO layer deposited by ALD. As shown inSI Appendix, Fig. S3, the Ni 2p3/2 binding energy appears at 856eV, more positive than typical Ni2+ energies consistent with partialoxidation to Ni3+ as found in related samples (39).Fig. 3 shows high-angle annular dark-field (HAADF) images

of a typical FTOjSnO2/TiO2j-RuP2+j20cycles NiO electrode topresent the core/shell/shell structure and composition. A high-resolution transmission electron microscopy (TEM) image of theelectrode is shown in SI Appendix, Fig. S4. From Fig. 3, visualanalysis of the HAADF mapping shows the SnO2 nanoparticle(Fig. 3B) is coated by TiO2 (Fig. 3C), with a thin outer coating ofNiO (Fig. 3D) as proposed. Even with RuP2+ loaded on theSnO2/TiO2 surface, the robust ALD method introduced NiO as arelatively uniform thin coating. In addition, SI Appendix, Fig. S5A and B displays an electron diffraction pattern with 2 spots of0.340 nm, which closely matches the (110) plane of SnO2. As shownin SI Appendix, Fig. S5 C and D confirmed the SnO2/TiO2jNiOcore/shell/shell structure. SI Appendix, Fig. S6 showed energy-dispersive X-ray spectroscopy analysis of the selected area for theTEM images, and it showed the elements of O, Sn, Ti, and Ni wereall included in the area.Cyclic voltammetry (CV) measurements of the catalyst on

nanoITO electrodes and on the same electrode with an added20cycles layer of NiO are shown in Fig. 4. From the data, E1/2 forthe RuP3+/2+ couple was observed at 1.30 V vs. NHE in a 0.1 Macetate buffer, 0.4 M in NaClO4 at pH 4.65. For the 20cyclesNiO overlayer, an onset potential for oxidation of the oxide

occurred at ∼1.4 V. The close overlap between the 2 indicatesintermediate oxidation of the NiO film is feasible as a mecha-nism for RuP3+ oxidation of the catalyst. Fig. 4B shows CV scansof nanoITOj-Cat in the same medium at a series of scan rates. At5 mV/s, a wave for the Rucat

III/RucatII couple appears at 0.7 V vs.

NHE and for the RucatIV/Rucat

III couple at 0.9 V, as expected(40, 41). The Cat water oxidation onset potential under theseconditions is 1.0 V vs. NHE. A single wave for the RuIV/RuV

couple appears at ∼1.4 V vs. NHE.

Water Oxidation. The electrodes FTOjSnO2/TiO2j-RuP2+-Catand FTOjSnO2/TiO2j-RuP2+j20cycles NiOj-Cat were investi-gated for light-driven water oxidation in a standard 3-electrodephotoelectrochemical cell with 1-sun illumination (100 mW/cm2,400-nm cutoff filter). A platinum mesh electrode was used as thecounter electrode along with a Ag/AgCl (3 M KCl) reference elec-trode. Current density vs. time traces (j–t) under dark/light cyclingconditions are shown in Fig. 5 with, as noted previously, theSnO2/TiO2 core/shell providing enhanced charge separation atthe SnO2/TiO2 interface compared to TiO2 (42–44).Based on the data in Fig. 5, deposition of the 20cycles NiO

overlayer in FTOjSnO2/TiO2j-RuP2+j20cycles NiOj-Cat resultedin a photocurrent density of 1.15 mA/cm2 after 90 s of total il-lumination, slightly lower than 1.35 mA/cm2 observed forFTOjSnO2/TiO2j-RuP2+j-Cat over the same period. The roleof the thickness of the NiO overlayer and of other added over-layers was also investigated. Based on the data in SI Appendix,

400 500 600 7000.0

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Wavelength(nm)

FTO|SnO2/TiO

2|-RuP2+

FTO|SnO2/TiO

2|-RuP2+|20cycles NiO|

FTO|SnO2/TiO

2|-RuP2+|20cycles NiO|-Cat

FTO|SnO2/TiO

2

FTO|SnO2/TiO

2|20cycles NiO

Fig. 2. Absorption spectra for FTOjSnO2/TiO2j-RuP2+ before (dark blue)and after (light blue) the addition of 20 cycles of ALD-deposited NiO. Alsoshown are absorbance spectra for the blank FTOjSnO2/TiO2 electrode(black), an FTOjSnO2/TiO2 electrode after NiO deposition (red), and thecomplete FTOjSnO2/TiO2j-RuP2+j20cycles NiOj-Cat electrode (green). Allspectra were obtained with the electrodes at a 45° angle to the incidentlight and analyzed in aqueous N2-sparged pH 4.65 with 0.1 M acetic acid/acetate (HAC/Ac−) buffer and 0.4 M NaClO4.

Fig. 3. HAADF image of SnO2/TiO2 jRuP2+j20cycles NiO nanocomposite:(A) the area selected for elementary analysis; elemental mapping of (B)Sn, (C ) Ti, (D) Ni, and (E ) O; (F ) composite overlay of Sn, Ti, and Nimapping.

12566 | www.pnas.org/cgi/doi/10.1073/pnas.1821687116 Wang et al.

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Fig. S7, under the same measurement conditions, the elec-trodes FTOjSnO2/TiO2j-RuP2+j10cycles NiOj-Cat andFTOjSnO2/TiO2j-RuP2+j30cycles NiOj-Cat gave photocurrentdensities of 1.3 and 0.73 mA/cm2, respectively. The data showthat introduction of the ALD NiO overlayer did not contributesignificantly to a decrease in photocurrent for short time periods.The decrease in photocurrent with increasing separation dis-tance from the electrode from 10cycles to 20cycles is presumablya consequence of the increased electron transfer distance fromthe chromophore to the catalyst and an increase in the proba-bility for back electron transfer from the electrode.Other external layers were also explored. The electrodes

FTOjSnO2/TiO2j10cycles TiO2j-Cat and FTOjSnO2/TiO2j5cyclesAl2O3j-Cat were prepared by using related ALD procedures. Forthese electrodes, the ALD deposition of Al2O3 and TiO2 wereapplied over RuP2+ on the surface as a replacement for the NiOoverlayer. From SI Appendix, Fig. S8, photocurrent densities of1.2 mA/cm2 for the 10cycles TiO2 layered electrode and 0.75 mA/cm2

for the 5cycles Al2O3 electrode were obtained.

Linear sweep voltammograms (LSVs) for both FTOjSnO2/TiO2j-RuP2+-Cat and FTOjSnO2/TiO2j-RuP2+j20cycles NiOj-Catare shown in Fig. 5B. The current–time results are consistent withthe onset potentials for other core/shell electrodes near 0.1 V vs.NHE with maximum photocurrents obtained at 0.4 to 0.5 V vs.NHE. These results also show that the addition of 20cycles NiO didnot change the onset potential for DSPEC water oxidation.O2 measurements for water oxidation from FTOjSnO2/

TiO2j-RuP2+j20cycles NiOj-Cat following illumination with a100 mW·cm−2 white light and a 400-nm cutoff filter from 30 to7,230 s at a bias of 0.6 V vs. NHE are shown in Fig. 6A. Thecurrent–time response in red is for the O2 collector electrodeheld 1 mm from the photoanode and held at an applied po-tential of −0.65 V vs. NHE. The use of the collector genera-tor allows for verification of the production of O2 as well as

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

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Cur

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(mA

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Potential V vs (NHE)

nano ITO|-RuP2+

nano ITO|-20cycles NiO

RuP2+/3+

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

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nanoITO|-Cat Cat(RuIV/V)

A

B

Fig. 4. (A) Cyclic voltammograms taken with nanoITOj-RuP2+ (black)and nanoITOj20cycles NiO (red) electrodes at a scan rate of 25 mV/s in a0.1 M acetate buffer, with 0.4 M NaClO4 at pH 4.65. (B) CV scans fornanoITOj-Cat at a scan rate of 5 mV/s in 0.1 M acetate buffer with 0.4 MNaClO4 at pH 4.65.

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FTO|SnO2/TiO

2|-RuP2+-Cat

FTO|SnO2/TiO

2|-RuP2+-|20cycles NiO|-Cat

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j (m

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0cycle NiO dark 0cycle NiO light20cycles NiO dark 20cycles NiO light

A

B

Fig. 5. (A) Current density−time (j–t) traces over 30-s dark−light cycles forwater oxidation by FTOjSnO2/TiO2j-RuP2+-Cat (black) and FTOjSnO2/TiO2j-RuP2+j20cycles NiOj-Cat (red) under an applied bias of 0.6 V vs. NHE. (B) Lin-ear sweep voltammograms (LSVs) for FTOjSnO2/TiO2j-RuP2+j20cycles NiOj-Catunder dark (blue) and light (cyan) conditions and for FTOjSnO2/TiO2j-RuP2+-Catunder dark (black) and light (red) conditions recorded with a scan rate of50 mV/s from positive to negative potential. All scans shown were recordedwith electrolyte containing pH 4.65 0.1 M acetate buffer with 0.4 MNaClO4.

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examining the stability of the catalytic activity over extended illu-mination periods (7). Eq. 8 enables the calculation of the Faradaicefficiency for O2 production, where QCollector/QGenerator is the ratioof charge at the collector and generator electrodes and 0.7 is theobserved collection efficiency for the cell design:

ðQCollector=QGeneratorÞ=0.70× 100%=FEð%Þ. [8]

Based on the results in Fig. 6 after 2 h of continuous illumina-tion, the assembly FTOjSnO2/TiO2j20cycles NiOj-Cat main-tained a photocurrent density of ∼1 mA/cm2 with a slightincrease in the first hour. Analysis of O2 production gave a Far-adaic efficiency for O2 generation of 87% with some loss of O2noted at the edge of the glass between the generator and collectorelectrodes.The results of a series of collector–generator experiments are

summarized in Table 1. These experiments were performedwith an applied bias of 0.6 V vs. NHE, under 1-sun illumination, in

0.1 M acetate buffer in 0.4 NaClO4 at pH 4.65. It summarizes dataon the effect of variations in the thickness of the NiO overlayerwith results shown in SI Appendix, Figs. S9–S11. From the data,introduction of 10cycles NiO overlayer between the chromo-phore and catalyst resulted in a stabilization of the DSPECphotoanode with a photocurrent density of 0.67 mA/cm2

obtained over a 2-h period. The maximum photocurrent wasreached for a 20cycles NiO overlayer. As noted above, the de-creases in photocurrent with increasing NiO overlayer thick-nesses of 30cycles and 50cycles are most likely due to largerseparation distance to the catalyst from the electrode, makingback electron transfer to the oxidized chromophore, SnO2/TiO2(e

−)j-RuP23+- → SnO2/TiO2j-RuP2

2+-, competitive withelectron transfer activation of the catalyst, j-RuP3+j-xNiOj-Cat →j-RuP2+j-xNiOj-Cat(h+). For reference, the results from previousDSPEC water oxidation studies are summarized in SI Appendix,Table S1. Comparison with these previous studies highlightsthe dramatic improvement to the long-term photocurrent sta-bility after introducing the NiO overlayer.The importance of the NiO layer in the performance of the

electrode was also illustrated by data without the mediator inFTOjSnO2/TiO2j-RuP2+-jCat. In the absence of NiO, the pho-tocurrent density fell to 0.37 mA/cm2 with an O2 efficiency of 88%(SI Appendix, Fig. S12).The role of the overlayer was investigated with other elec-

trode configurations. For TiO2 and Al2O3 as the overlayers inFTOjSnO2/TiO2j-RuP2+-j10cyclesTiO2j-Cat and FTOjSnO2/TiO2j-RuP2+-j5cycles Al2O3j-Cat, 2 h of continuous illumination(SI Appendix, Figs. S13 and S14) resulted in photocurrent densitiesof 0.14 mA/cm2 for FTOjSnO2/TiO2j-RuP2+-j10cycles TiO2j-Cat and0.089 mA/cm2 for FTOjSnO2/TiO2j-RuP2+-j5cycles Al2O3j-Cat.Bothhighlight the importance of the NiO layer for engendering stablephotocurrent densities. After a 2-h photolysis of the electrodeFTOjSnO2/TiO2j-RuP2+-j20cycles NiOjCat, XPS analysis wasconducted to check for surface composition changes. From SIAppendix, Fig. S15, it was observed that the signal shape of theNiO overlayer remains the same but the signal intensity de-creased slightly, indicative that no chemical change of theALD NiO layer occurred during the photochemical water ox-idation process. There is roughly 15% ALD NiO loss of theelectrode FTOjSnO2/TiO2j-RuP2+j20cycles NiOj-Cat after the2-h photolysis. The importance of the catalyst is shown bythe data for FTOjSnO2/TiO2j-RuP2+j20cycles NiO in SI Ap-pendix, Fig. S16. In that case, over a 2-h period, the photo-current density was only 40 uA/cm2 with a 37% Faradaic effi-ciency for O2.For the electrode FTOjSnO2/TiO2j-RuP2+j20cycles NiO, the

poorly understood nature of the NiO overlayer prepared byALD accounts for the low overall performance. Nevertheless,this early result is encouraging with past attempts to integrate

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Fig. 6. (A) O2 measurements for water oxidation from (black) FTOjSnO2/TiO2j-RuP2+j20cycles j-Cat on 1-cm2 slide illuminated with a 100 mW·cm−2

white light with a 400-nm cutoff filter over a 120-min period at a bias of0.6 V vs. NHE and (red) an FTO electrode positioned 1 mm from the pho-toanode poised at −0.65 V vs. NEH. The experiment was performed in 0.1 Macetic acid/acetate buffer at pH 4.65 with 0.4 M NaClO4. (B) The integratedcurrent over time (charge) used to calculate Faradaic efficiencies for thesame collector and generator electrodes shown in A.

Table 1. Faradaic efficiencies calculated after 120 min ofphotolysis and photocurrents after 10 and 120 min of photolysisfor the electrodes FTOjSnO2/TiO2j-RuP22+-jxNiOj-Cat

Electrodes FTOj-SnO2/TiO2j-RuP2+j-

Faradaicefficiency, %

Initialphotocurrent

density, j, mA/cm2After 120 min,

j, mA/cm2

0 NiOj-Cat 88 1.22 0.3810cycles NiOj-Cat 84 1.1 0.6720cycles NiOj-Cat 86 1.02 0.9830cycles NiOj-Cat 89 0.51 0.3250cycles NiOj-Cat 88 0.27 0.2210cycles TiO2j-Cat 78 1.12 0.145cycles Al2O3j-Cat 81 1.07 0.0920cycles NiO 37 0.15 0.04

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metal-oxide–based WOCs with chromophores in photochemicalcells proving to be exceptionally difficult (10, 45). The promise ofpreparing a high-activity WOC from NiO with ALD and in-tegration into a DSPEC warrants further investigation. These re-sults indicate that ALD NiO is able to trap the holes fromRuP3+ and presumably act as an electron transfer mediator be-tween RuP3+ and the catalyst in the electron delivery chain.Together, these data highlight the importance of all 3 com-ponents for water oxidation in the assembly—the chromo-phore, the electron transfer mediator, and the WOC. All arerequired for efficient and stable water oxidation.

Mechanism. IPCE measurements as a function of excitation wave-length were investigated for FTOjSnO2/TiO2j-RuP2+j20cyclesNiOj-Cat. Results are shown in Fig. 7. IPCE values were calcu-lated by using Eq. 9, with I the photocurrent in milliamperes,P the incident light intensity in milliwatts, and λ the incidentwavelength in nanometers. An IPCE value of 18.1% was obtainedat a bias of 0.6 V vs. NHE, in 0.1 M acetate buffer at pH4.65, 0.4 M in NaClO4. The IPCE profiles demonstrate that thecurrent response overlaps with the visible absorption spectrumof RuP2+, consistent with excitation and photoresponse by itsMLCT chromophore:

IPCE= 1,240 × I=ðP× λÞ. [9]

In evaluating the role of added NiO, the open-circuit photo-voltage (OCP) of the electrode FTOjSnO2/TiO2j-RuP2+j-20cyclesNiO was investigated at pH 4.65 in 0.1 M HAc/AC−, 0.4 M inNaClO4 buffer solutions both in the dark and under illumination.As illustrated in Fig. 8, under illumination, ALD-deposited NiObehaves as a p-type site with electron transfer occurring fromNiO to theoxidized chromophore, FTOjSnO2/TiO2(e

−)j-RuP3+j-20cycles NiO →FTOjSnO2/TiO2(e

−)j-RuP2+j-20cycles NiO(h+). This result showsthat NiO facilitates charge separation following excitation.When the illumination is stopped, the junction would be a pref-erable site for the photogenerated electrons and holes pairs torecombine, and thus charge recombination is expected whenthe light is off (39, 46). As shown by earlier results, open-circuit voltage decays should be more rapid with, rather thanwithout, the NiO overlayer. Similar results were found for theelectrodes, FTOjSnO2/TiO2j-RuP2+j10cycles NiO, and FTOjSnO2/

TiO2jRuP2+j30cycles NiO, in SI Appendix, Fig. S17. From the data,open-circuit voltage decays was only minor variations as the thicknessof the ALDNiO overlayer was varied from 10cycles to 30cycles. The open-circuit potential decay for FTOjSnO2/TiO2j-RuP2+j20cycles NiO is morerapid than for the electrode FTOjSnO2/TiO2j-RuP2+j10cycles NiO.A variation in the amount of NiO deposited on the TiO2 could bethe origin of this difference.An obvious role for NiO surface passivation arises from a

decrease in the rate of back electron transfer to the electrodefollowing excitation and electron transfer to RuP3+. To explorethe point, we applied 5 ALD cycles of Al2O3 and 10 cycles ofTiO2 to FTOjSnO2/TiO2j-RuP2+. As shown in SI Appendix, Fig.S18, the resulting electrode, FTOjSnO2/TiO2j-RuP2+j-10cyclesTiO2, showed only a slight change in OCP with similarelectron decay rates compared to FTOjSnO2/TiO2j-RuP2+.For the electrode, FTOjSnO2/TiO2j-RuP2+j-5cycles Al2O3,the OPV was decreased in the light with slower electrondecay in the dark.Consistent with the experimental results, a proposed mecha-

nism for the appearance of photocurrents in the electrode,FTOjSnO2/TiO2j-RuP2+j-20cycles NiOj-Cat, is summarized inEqs. 10–13. In the mechanism, light absorption by RuP2+ isfollowed by injection from the excited state, -RuP2+*- into TiO2.Following injection, the oxidized state, -RuP3+- oxidizes NiO

400 450 500 550 600 650

0

5

10

15

20

25

IPC

E(%

)

Wavelegth (nm)

FTO|SnO2/TiO

2|-RuP2+-|20cycles NiO|-Cat

Fig. 7. IPCE measurements on FTOjSnO2/TiO2j-RuP2+j20cycles NiOj-Cat atan applied bias of 0.6 V vs. NHE at pH = 4.65 in 0.1 M in acetate buffer with0.4 M in NaClO4 with a 400-nm cutoff filter.

0 10 20 30 40 50 60-0.1

0.0

0.1

0.2

0.3

0.4

Light onPote

ntia

l (V

vs N

HE)

Time(s)

FTO|SnO2/TiO2|-RuP2+

FTO|SnO2/TiO2|-RuP2+|20cycles NiO

Light off

0.15 0.10 0.05 0.00 -0.05 -0.10 -0.150.1

1

10

100

t(s)

Potential (V vs NHE)

FTO|SnO2/TiO2|-RuP2+|20cycles NiO

FTO|SnO2/TiO2|-RuP2+

A

B

Fig. 8. (A) Open-circuit voltage decays for the electrodes FTOjSnO2/TiO2j-RuP2+ (black) and FTOjSnO2/TiO2j-RuP2+j-20cycles NiO (red). (B) Semi-logarithmic time constant decays plotted vs. potential in V vs. NHE.

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(Eq. 12), and the latter activates the catalyst, Cat, in the terminalstep in the electrode activation sequence, Eq. 13:

FTOjSnO2=TiO2j-RuP2+-jNiOj-Cat+ hv

→FTOjSnO2=TiO2j-RuP2+p-jNiOj-Cat; [10]

FTOjSnO2=TiO2j-RuP2+p-jNiOj-Cat→FTOðe−ÞjSnO2=TiO2j-RuP3+-jNiOj-Cat; [11]

FTOðe−ÞjSnO2=TiO2j-RuP3+-jNiOj-Cat→FTOðe−ÞjSnO2=TiO2j-RuP3+-jNiOðh+Þj-Cat;

[12]

FTOðe−ÞjSnO2=TiO2j-RuP3+-jNiOðh+Þj-Cat→FTOðe−ÞjSnO2=TiO2j-RuP3+-jNiOj-Catðh+Þ.

[13]

ConclusionsThe results described here introduce the value of an ultrathinALD NiO layer between the chromophore and catalyst for me-diating electron transfer in a DSPEC photoanode for water ox-idation. Our data suggest that the NiO overlayer acts as anintermediate redox layer. As such, it connects the surface-bounddye with an external catalyst playing a role that is reminiscent the

electron transfer behavior of tyrosine in the electron transferchain in PSII.As shown by the systematic variations in the thickness of the

NiO overlayer, the thickness of the overlayer plays an importantrole in dictating electrode stability. A maximum stability wasreached at a NiO thickness of 20cycles, which we attribute tocompetition between catalyst activation and back electrontransfer to the electrode.The overall efficiency of NiO-stabilized electrodes is im-

pressive with per-photon absorbed quantum efficiencies of 18%at the MLCT maximum of 450 nm. Compared to comparablemolecular-based dye photoanodes, the stable performance overa period of 2 h with efficient O2 evolution is notable. An im-portant key in the long-term applicability of this approach isachieving electrode stabilization, which is currently under in-vestigation and appears to arise from the long-term instabilityof the catalyst.

ACKNOWLEDGMENTS. D.W. (IPCE, CV, LSV, and O2 measurements, and ALDdeposition) acknowledges support from a US Department of Energy NuclearEnergy University Program award, under Contract DE-NE0008539. M.J.M.(UV-visible absorption measurements) and M.V.S. and B.D.S. (data analysis)acknowledge support from the Alliance for Molecular PhotoElectrode De-sign for Solar Fuels, an Energy Frontier Research Center funded by the USDepartment of Energy, Office of Science, Office of Basic Energy Sciencesunder Award DE-SC0001011. This work was performed in part (ALD, ellips-ometry, XPS) at the Chapel Hill Analytical and Nanofabrication Laboratory, amember of the North Carolina Research Triangle Nanotechnology Network,which is supported by the NSF, Grant ECCS-1542015, as part of the NationalNanotechnology Coordinated Infrastructure.

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