Modulating Hole Transport in Multilayered Photocathodes with

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Modulating Hole Transport in Multilayered Photocathodes with Derivatized p‑Type Nickel Oxide and Molecular Assemblies for SolarDriven Water Splitting Bing Shan,† Benjamin D. Sherman,† Christina M. Klug,‡ Animesh Nayak,† Seth L. Marquard,† Qing Liu,† R. Morris Bullock,‡ and Thomas J. Meyer*,† †

Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States Center for Molecular Electrocatalysis, Pacific Northwest National Laboratory, P.O. Box 999, K2-12, Richland, Washington 99352, United States

‡

S Supporting Information *

ABSTRACT: For solar water splitting, dye-sensitized NiO photocathodes have been a primary target. Despite marginal improvement in performance, limitations remain arising from the intrinsic disadvantages of NiO and insufficient catalysis. We report here a new approach to modifying NiO photocathodes with doped NiO bilayers and an additional layer of macro− mesoporous ITO. The trilayered electrode is functionalized with a surface-attached ruthenium polypyridyl dye and a covalently bridged nickel-based hydrogen evolution catalyst. The NiO film, containing a 2% K+-doped NiO inner layer and a 2% Cu2+-doped NiO outer layer, provides sufficient driving force for hole transport following hole injection by the molecular assembly. Upon light irradiation, the resulting photocathode generates hydrogen from water sustainably with enhanced photocurrents and a Faradaic efficiency of ∼90%. This approach highlights the value of modifying both the internal and surface structure of NiO and provides insights into a new generation of dye-sensitized photocathodes for solar-driven water splitting cells.

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reduction (Figure 1B). The resulting photocathode (Figure 1A) generates hydrogen with enhanced photocurrent and a Faradaic efficiency of ∼90% under mild bias/pH conditions and is catalytically stable for hours with a minor decrease in activity. As shown in the preparation procedure in Figure S1, the NiO layers contain different dopants selected to modify the valence band edges and establish an internal driving force for hole transport from the surface to the FTO back contact. The internal driving force created by the NiO layers separates photogenerated electrons at the molecular assemblies and holes at the electrode surface and thereby decreases interfacial charge recombination. A contribution to the high performance of the photocathodes also exists from the top macro−mesoporous ioITO loading layer, which provides a sufficient area of strongly basic loading sites for anchoring the dye−catalyst assemblies.20,22 The photocathode architecture featured in this work is significant to provide insights into the design of a new generation of NiO-based photocathodes for solar-driven water splitting. Direct loading of the dye−catalyst assemblies on mesoporous NiO films does not result in observable photocatalysis. There are physical constraints in the mesopores of the NiO film (5− 20 nm, Figure 2C) that may not be able to accommodate the

he dye-sensitized photoelectrosynthesis cell (DSPEC) integrates molecular-level photochemistry and catalysis with the band gap energetics of semiconducting materials for solar energy conversion.1−3 Relative to a single-junction DSPEC, a tandem DSPEC offers higher theoretical and experimentally demonstrated efficiencies for solar-driven water splitting.4−6 The relatively low efficiencies of dyesensitized photoanode/photocathode-based tandem devices are due in large part to photocurrent mismatching of the two electrodes, typically caused by the limited performance of the photocathode.7−10 Among the reported DSPEC photocathodes, NiO has been the dominant p-type semiconducting material.11−14 Strategies for improving photocathode efficiencies have mainly focused on modifying dye structures to spatially separate photogenerated electron/hole pairs at the interface.15−18 Despite marginal improvements, limitations remain due to the intrinsic disadvantages of NiO with low hole mobility, insufficient loading of the molecular assemblies, and catalyst leaching during photoelectrocatalysis. We report here a new NiO-based photocathode with the structure shown in Figure 1A for enhanced photoelectrocatalytic water reduction. It consists of a bilayered doped NiO film covered by an inverse-opal indium tin oxide (ioITO) layer with molecular assemblies anchored to the ioITO surface. Coupled with a molecular nickel catalyst (NiII),19−21 a series of chromophores, with/without a covalently linked electron donor, have been investigated for photoelectrocatalytic water © XXXX American Chemical Society

Received: July 24, 2017 Accepted: August 30, 2017 Published: August 30, 2017 4374

DOI: 10.1021/acs.jpclett.7b01911 J. Phys. Chem. Lett. 2017, 8, 4374−4379

Letter

The Journal of Physical Chemistry Letters

Figure 1. (A) Structure of the photocathode, NiK0.02O|NiCu0.02O|ioITO|−RuII−Zr−NiII; (B) sensitizing units.

excess oxygen.10−15 Its band gap features and electronic structures can be modified by impurity doping with metal ions such as Li+,25,26 Co2+,27 K+,28 Al3+,29 Cu+/2+,30 and Mg2+.31 Dopants present in the NiO lattice alter the valence band maximum by changing the effective mass and carrier concentrations in the valence band.32 The addition of dopant ions during the solution processing of NiO allows facile synthesis of doped NiO. The two layers of NiO were examined with doping of 2 or 6% (atomic ratios of dopant ions/Ni2+) K+, Co2+, and Cu2+, resulting in changes in the density of states33 shown in Figure 3. Recent first-principle calculations have

Figure 2. SEM (scanning electron microscopy) images of the cross section (A) and surface (B) of NiK0.02O|NiCu0.02O|ioITO; (C) surface of NiK0.02O|NiCu0.02O.

strained coordination geometry of the dye−ZrIV-catalyst assemblies. As shown in the preparation procedure in Figure S1, an ioITO loading layer is applied on top of the NiO layers, which significantly enhances the loading of the assemblies (surface coverages in Table S1). Comparisons of the surface geometry and pore size of the mesoporous NiO and the ioITOcoated NiO are shown in the SEM images in Figure 2. The improved loading results from the unique architecture of the ioITO layer with macropores of ∼300 nm and mesopores of 10−30 nm (Figure 2B) and from the stronger basicity23 of ITO toward the phosphonate binding. Given the high charge mobility of degenerately doped ioITO, the direction of lightinduced electron transfer in the trilayered electrode is not affected by the ioITO layer (vide infra).22,24 The NiO electrodes were investigated with regard to the changes in the Fermi level with different dopants. Stoichiometric NiO, a Mott−Hubbard insulator at room temperature, shows p-type electrical conduction due to the formation of Ni2+ vacancies (VNi″) and self-doping of Ni3+ ions in the presence of

Figure 3. DOS of NiO with and without dopants (2 or 6% K+, Co2+, or Cu2+);.

shown that the features in the DOS at early applied potentials (more negative than ∼0.7 V vs NHE) are mostly from localized trap states, originating from Ni3+/NiOOH and undercoordinated oxygen atoms adjacent to VNi″ in the lattice.34,35 From the DOS in Figure 3, doping of NiO results in the following trend of the flat-band potentials (EF) of the films, 2% K < 2% Co < NiO < 6% Co ≈ 6% K < 6% Cu < 2% Cu, as summarized in Table S2, which was supported by the valence band spectra 4375

DOI: 10.1021/acs.jpclett.7b01911 J. Phys. Chem. Lett. 2017, 8, 4374−4379

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Figure 4. (A) Current density responses of NiO|NiO|ioITO|−RuII−Zr−NiII (blue), NiK0.02O|NiCu0.02O|ioITO|−RuII−Zr−NiII (pink), NiO|NiO| ioITO|−DA−Zr−RuII−Zr−NiII (green), and NiK0.02O|NiCu0.02O|ioITO|−DA−Zr−RuII−Zr−NiII (red) under light illumination (0.2 Sun) and an applied bias of −0.25 V vs NHE; (B) long-term photocurrent and (C) TON/mols of hydrogen evolved by irradiation of NiO|NiO|ioITO|−RuII− Zr−NiII (blue) and NiK0.02O|NiCu0.02O|ioITO|−RuII−Zr−NiII (pink) photocathodes; (D) faradaic yields of hydrogen.

probably influenced by proton intercalation to a greater extent relative to the one with undoped NiO. For the photocathode assemblies with the dianiline (DA) as an electron-donating mediator, the photocurrent of the undoped NiO photocathode is enhanced (green vs blue traces in Figure 4A). Addition of DA enhances the efficiency by decreasing the interfacial charge recombination between the reduced assembly and the oxidized NiO,18 which is consistent with our previous findings on a photocathode assembly with DA as an electron mediator.20 By contrast, the presence of DA negatively influences the performance of the doped NiO photocathode (red vs pink traces in Figure 4A). After the hole is transferred from the excited state RuII* to DA, the subsequent hole injection by DA+• to the doped NiO (NiCu0.02O) is not as favorable as that to the undoped NiO (vide infra). Without DA, the hole carrier in the dye−catalyst assembly is a stronger oxidant than DA+• (formal potentials in Table S3) for transferring holes into the doped NiO (NiCu0.02 O). The difference in E F between NiO and NiCu0.02O rationalizes the lower efficiency of the photocathode in the presence of both DA and the doped NiO (NiK0.02O| NiCu0.02O). The more positive EF of NiCu0.02O relative to the reduction potential of DA+• results in thermodynamically unfavorable hole injection from DA+• into NiCu0.02O. The related energetics are shown in Figure S4. Reversing the order of the NiO layers to form the photocathode NiCu0.02O|NiK0.02O|ioITO|−RuII−Zr−NiII results in a greatly diminished photocurrent response as compared with either NiO|NiO|ioITO|−RuII−Zr−NiII or NiK0.02O|NiCu0.02O|ioITO|−RuII−Zr−NiII, shown by the green trace in Figure 5. The much lower efficiency is consistent with unfavorable hole transport from NiK0.02O to NiCu0.02O, which causes accumulation of holes at the outer NiK0.02O layer, accelerating charge recombination between the oxidized electrode and the reduced assembly.18 As shown in Figure S5, the photocathodes prepared with singularly doped NiO layers, NiK 0. 0 2 O|NiK 0 .0 2 O|ioITO|−Ru I I −Zr−Ni I I and

(Figure S2) from X-ray photoelectron microscopy (XPS) with ultraviolet photoelectron spectroscopy (UPS) measurements. On the basis of the results of EF, the use of NiK0.02O (2% K doped NiO) as the inner layer and NiCu0.02O as the outer layer creates 0.2 eV of driving force for interlayer hole transport. Once the holes are injected into the Fermi level of the outer NiCu0.02O layer by the assembly, they are driven to the inner NiK0.02O layer and separated from the electrons at the external assembly. The doped NiO exhibits high transparency without significant variations in UV−vis absorption, as shown in Figure S3, indicating the absence of phase separation within the doping levels (≤6%). The trilayered photocathodes are sensitized by RuII or porphyrin dyes (Por1, Por2)36,37 in Figure 1B. The hydrogen evolution catalyst, NiII, is bridged to the dye through a ZrIV− phosphonate bond22 to form the dye−catalyst assembly on the ioITO surface. The photoelectrocatalytic performance of the resulting photocathodes is presented in Figure 4. Upon light excitation, the photocathode NiK0.02O|NiCu0.02O|ioITO|−RuII−Zr−NiII is able to generate a photocurrent with twice the magnitude as that of the photocathode NiO|NiO|ioITO|−RuII− Zr−NiII (pink vs blue traces in Figure 4A). The doped NiO layers facilitate hole transport after the photoexcited assembly injects holes into the electrode surface, enhancing the light conversion efficiency. As shown in Figure 4B,C, during 2 h of photoelectrocatalysis, the photocathodes, NiO|NiO|ioITO|−RuII−Zr−NiII and NiK0.02O|NiCu0.02O|ioITO|−RuII−Zr−NiII, generate 0.37 ± 0.05 and 0.70 ± 0.08 μmol of hydrogen, corresponding to Faradaic efficiencies of 92 ± 6 and 86 ± 3%, respectively. The changes in Faradaic efficiency with time are shown in Figure 4D. During the second hour of photoelectrocatalysis, the faradaic efficiencies decrease by ∼14% for the doped photocathode and ∼8% for the undoped photocathode. The differences in the changes may originate from different degrees of proton intercalation effects38 on the two photocathodes. The photocathode with doped NiO was 4376

DOI: 10.1021/acs.jpclett.7b01911 J. Phys. Chem. Lett. 2017, 8, 4374−4379

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attributed to the improved hole transport (eq 4 in Scheme S1A) from NiCu0.02O to NiK0.02O after the hole injection. Expedited hole transport between the two layers of NiO aids in hole extraction from the interface, decreasing charge recombination between the reduced catalyst and the oxidized surface. The energy diagram in Figure 6 illustrates those steps in blue

Figure 5. Current density responses of ioITO|−Ru II−Zr−NiII (orange) and NiCu0.02O|NiK0.02O|ioITO|−RuII−Zr−NiII (green) under light illumination (0.2 Sun) with an applied bias of −0.25 V vs NHE.

NiCu0.02O|NiCu0.02O|ioITO|−RuII−Zr−NiII, do not show enhanced efficiency as the doubly doped photocathode, NiK 0.02 O|NiCu 0.02O|ioITO|−Ru II−Zr−Ni II, which further underlines the impact from the created internal driving force by the doubly doped NiO layers. As a background observation, in the absence of any NiO layers, the assembly −RuII−Zr−NiII on ioITO only generates anodic photocurrent (orange trace in Figure 5). That emphasizes the importance of the p-type NiO in the photocathode which provides hole carriers for the external assembly. With only ioITO as the electrode substrate, excitation of RuII is followed by rapid electron injection into ioITO, resulting in anodic photocurrent. For the assembly −RuII−Zr−NiII on NiO|NiO|ioITO, two possible pathways for the light-induced electron transfer are illustrated in Scheme S1. The excited state RuII*, formed upon light excitation, can be quenched either by the NiII catalyst with a driving force of 0.82 eV (E(NiII/I)19,20 − E(RuIII/II*)) or by NiO through hole injection with a driving force of ∼0.24 eV (E(RuII*/I) − EF39). The differences in driving force result in different kinetics for the two pathways. Quenching of RuII* by the catalyst NiII was investigated on mesoporous ZrO2, which is inert toward injection.40 The rate of oxidative quenching was determined by probing time-resolved emission decay profiles, as shown in Figure S6. In the assembly −RuII−Zr−NiII, quenching of RuII* by NiII has a rate constant (kq) of 1.2 × 108 s−1, regardless of the type of oxide support used. If the initial quenching of RuII* occurs through hole injection, the rate should be faster or at least comparable to the quenching by NiII. However, the injection by RuII* is considerably slower, with rate constants (kinj) of 3.9 × 106 s−1 to NiO and 3.4 × 106 s−1 to NiCu0.02O, based on the nanosecond transient absorption (TA) spectra in Figure S7. Excited-state quenching by NiII is ∼30 times faster than the hole injection from RuII*. The relatively large difference in the two rate constants indicates that direct injection from RuII* into NiO or NiCu0.02O is thermodynamically less favorable than the intra-assembly hole transfer from NiII to RuII*. After initial quenching by NiII, the oxidized RuIII injects holes into NiO or NiCu0.02O with a driving force of 0.34 and 0.27 eV, respectively. The intra-assembly recombination between RuIII and NiI has a time scale of ∼39 μs (, Figure S8), far slower than hole injection. The quenching product RuIII lives sufficiently long to inject holes into NiO. The higher photocurrent observed for NiK0.02O|NiCu0.02O|ioITO|−RuII−Zr−NiII is

Figure 6. Energy diagram for the light-induced electron/hole transfer in NiK0.02O|NiCu0.02O|ioITO|−RuII−Zr−NiII. Blue and gray arrows illustrate the hole transfer steps in Scheme S1A,B, respectively. The solid and dashed arrows represent the forward and back hole transfer. Light excitation of RuII is shown by the orange arrows.

and gray arrows that correlate with Scheme S1A,B, respectively. For the photocathode assemblies with DA present, the lightinduced electron-transfer mechanism is shown in Scheme S2, which involves reductive quenching of RuII* by DA and hole injection to NiO by DA+•. DA+• is a less powerful oxidant than RuIII in oxidizing NiCu0.02O, which induces lower photocatalytic efficiencies. Photocathodes with porphyrins as the light absorber are less efficient than those with RuII dye, as shown in Figure S9A. The differences in performance of these photocathodes are most likely due to rapid intra-assembly charge recombination between the oxidized porphyrins and the reduced catalyst. The highly conjugated nature of the surface assembly with the porphyrin dyes36,37 may explain the much more rapid rates of recombination based on the TA spectra in Figure S9B. For this type of photocathode to function effectively, the intra-assembly charge recombination between the oxidized dye and the reduced catalyst has to be relatively slow, so that the holes at the oxidized dye have a sufficiently long lifetime to be injected into NiO. We describe here the design and fabrication of a trilayered photocathode that consists of two layers of doped NiO to provide an internal driving force for hole transport and an additional layer of macro−mesoporous ITO to improve loading of molecular assemblies. The Ru II−Ni II (dye−catalyst) assembly enables light capture and subsequent catalytic hydrogen evolution from water on the oxide surface. The resulting photocathode NiK0.02O|NiCu0.02O|ioITO|−RuII−Zr− NiII is capable of generating hydrogen with enhanced photocurrent and a high Faradaic efficiency without significant decrease in activity for several hours. The results of our study highlight the value of modifying both the internal structure of NiO to facilitate hole transport and its surface structure to enhance light capturing and photocatalysis for DSPECs. 4377

DOI: 10.1021/acs.jpclett.7b01911 J. Phys. Chem. Lett. 2017, 8, 4374−4379

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b01911. Experimental details, general information on the photocathodes, additional steady-state and transient absorption and emission spectra, and supplementary photocurrent results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Benjamin D. Sherman: 0000-0001-9571-5065 R. Morris Bullock: 0000-0001-6306-4851 Thomas J. Meyer: 0000-0002-7006-2608 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research on the photoelectrodes (at UNC Chapel Hill) was supported as part of 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 Number DE-SC0001011. Synthesis of the catalyst (at PNNL) was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy. The scanning electron microscopy experiment was 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 ECCS1542015, as part of the National Nanotechnology Coordinated Infrastructure, NNCI.

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DOI: 10.1021/acs.jpclett.7b01911 J. Phys. Chem. Lett. 2017, 8, 4374−4379

Letter

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DOI: 10.1021/acs.jpclett.7b01911 J. Phys. Chem. Lett. 2017, 8, 4374−4379