Light-Driven Water Splitting with a Molecular Electroassembly-Based

Letter pubs.acs.org/JPCL

Light-Driven Water Splitting with a Molecular Electroassembly-Based Core/Shell Photoanode Benjamin D. Sherman, Dennis L. Ashford, Alexander M. Lapides, Matthew V. Sheridan, Kyung-Ryang Wee, and Thomas J. Meyer* Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States S Supporting Information *

ABSTRACT: An electrochemical procedure for preparing chromophore-catalyst assemblies on oxide electrode surfaces by reductive vinyl coupling is described. On core/shell SnO2/TiO2 nanoparticle oxide films, excitation of the assembly with 1 sun (100 mW cm−2) illumination in 0.1 M H2PO4−/HPO42− at pH 7 with an applied bias of 0.4 V versus SCE leads to water splitting in a DSPEC with a Pt cathode. Over a 5 min photolysis period, the core/shell photoanode produced O2 with a faradaic efficiency of 22%. Instability of the surface bound chromophore in its oxidized state in the phosphate buffer leads to a gradual decrease in photocurrent and to the relatively modest faradaic efficiencies. and layered structures,19,22,23 and electrocatalytic films,21,24,25 including films for electrocatalytic water oxidation.22 Photoelectrochemical oxidation of iodide and hydroquinone in electropolymerized Ru(II) polypyridyl films has also been reported.26 In a recent report, we described an extension of the vinyl reduction/C−C coupling chemistry used in cross-linked films to the preparation of electroassemblies within the cavities of nanoparticle and mesoscopic oxide films and demonstrated visible light-driven water oxidation.27 Sun et al. have recently described a related result based on electro-oligomerization of catalyst 2 following separate phosphonate-surface binding of the chromophore.28 We describe here an extension of the previous results with a dramatic improvement in DSPEC performance by using a core−shell nanoparticle oxide electrode substrate and provide insight into both stability and functional behavior of the resulting surface assembly as a photoanode for visible light-driven water splitting. Figure 1 shows structures of chromophore 1 ([Ru(5,5′divinyl-2,2′-bipyridine) 2 (2,2′-bipyridine-4,4′-diylbis(phosphonic acid))]2+) and catalyst 2 (Ru(2,2′-bipyridine-6,6′dicarboxylic acid)(4-vinylpyridine)2). Complex 2 belongs to a class of relatively rapid water oxidation catalysts of the type Ru(bda)(L)2 (bda = 2,2′-bipyridine-6,6′-dicarboxylic acid where L is a neutral donor ligand such as pyridine or isoquinoline) described by Sun and coworkers12,29,30 with additional mechanistic insights into the presence of added buffer bases reported recently.31,32 Photoanodes for DSPEC

T

he dye-sensitized photoelectrosynthesis cell (DSPEC), which incorporates electrode architectures similar to those used in dye-sensitized solar cells (DSSCs),1,2 integrates molecular chromophores and catalysts with a high band gap semiconductor oxide electrode for water splitting into O2 and H2 or for CO2 reduction to a reduced carbon fuel.3−7 In exploiting the initial, seminal work of Fujishima and Honda,8 a DSPEC integrates molecular light absorption and catalysis with the bandgap properties of oxide semiconductors to extend light absorption into the visible and utilize chemical catalysis of solar fuel half reactions. A variety of DSPEC configurations have appeared in the literature,9−15 but low overall solar energy conversion efficiencies as well as poor long-term stability remain a central challenge. Examples of DSPEC water splitting have been reported based on carboxylate-,13 phosphonate-,14,16 or siloxyl-derivatized12 surface binding and by preformed, covalently linked chromophore-catalyst assemblies.9 Additional surface binding strategies have been explored including embedding molecular components in polymer film coatings10,15 and “layer-by-layer” assemblies with Zr(IV)-phosphonate bridges.17,18 Use of preformed assemblies offers synthetic control and well-defined structures but, typically, requires laborious multiple-step synthetic procedures resulting in low overall yields. Based on earlier procedures for preparing cross-linked electropolymerized films by reductive coupling of vinylderivatized polypyridyl complexes,19 electroassembly offers the advantage of on-surface synthesis without prior covalent bond formation. Electropolymerization has been used to form electroactive thin films on a variety of conducting and semiconducting substrates, including metal oxides.20,21 It provides a basis for preparing controlled surface coverages,22,23 multicomponent © XXXX American Chemical Society

Received: June 26, 2015 Accepted: August 3, 2015

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DOI: 10.1021/acs.jpclett.5b01370 J. Phys. Chem. Lett. 2015, 6, 3213−3217

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The Journal of Physical Chemistry Letters

shows representative cyclic voltammograms before and after electroassembly formation.

Figure 2. (Left) Cyclic voltammogram of FTO|nanoSnO2|TiO2(3 nm)|-1 before (dashed trace) and after (solid trace) electroassembly formation by addition of 2. (Upper right) Current−time trace showing the first five potential step cycles during electroassembly formation in acetonitrile 0.1 M in N(n-Bu)4PF6. (Lower right) Total charge passed during a representative electroassembly procedure. The electrode area in solution was 1.8 cm2.

Figure 1. (a) Structure of chromophore 1 ([Ru(5,5′-divinyl-2,2′bipyridine)2(2,2′-bipyridine-4,4′-diylbis(phosphonic acid))]2+) and (b) catalyst 2 ([Ru(2,2′-bipyridine-6,6′-dicarboxylic acid)(4-vinylpyridine)2]). (c) Structure of a 1:1 C−C linked electro-assembly between 1 and 2 on a SnO2/TiO2 core−shell electrode, FTO| nanoSnO2|TiO2(3 nm)|-1-2.

As shown in the current−time traces in Figure 3, the FTO| nanoSnO2|TiO2(3 nm)|-1-2 (FTO|nanoSnO2|TiO2(3 nm)|applications have used derivatives of these catalysts for short photolysis periods with oxygen generation from water.12,18,33 Figure 1c illustrates the proposed C−C bond coupled assembly, 1-2, prepared as previously described by reductive electroassembly34 by using conditions known to give a 1:1 chromophore−catalyst ratio.27 Given the presence of multiple vinyl groups on both 1 and 2, the final surface structure may contain local assemblies with more than one C−C linked catalyst per chromophore. Core/shell SnO2/TiO2 photoanodes were prepared on fluorine-doped tin oxide (FTO)-coated glass electrodes. A colloidal SnO2 paste was synthesized and applied to FTO electrodes by protocols described elsewhere.35 After sintering, the mesoporous SnO2 layer measured 8 μm thick. As a final step, an overlayer of TiO2 was deposited on the SnO2 surface by atomic layer deposition (ALD) using the Ti(IV) precursor TDMAT (tetrakis-(dimethylamido)titanium(IV)) to form 3 nm shells of TiO2.9,36 The core/shell electrodes then underwent annealing at 450 °C in air, which reduces both light absorption and light scattering by the TiO2 shell. In forming the electroassembly, the initial step involved the surface binding of 1 by soaking the core/shell electrode in a 400 μM solution of 1 in methanol overnight, resulting in monolayer coverage of the SnO2/TiO2 surface. In the subsequent vinyl reduction procedure, a prederivatized electrode, FTO|nanoSnO2|TiO2(3 nm)|-1, was immersed in a 500 μM solution of 2 in acetonitrile 0.1 M in N(n-Bu)4PF6. Electroassembly formation was induced by using a potential step method with the potential at the FTO|nanoSnO2|TiO2(3 nm)|-1 electrode held at −2 V versus Ag+/Ag for 1 s, followed by a positive step to 0.2 V versus Ag+/Ag for 5 s, over a total of 200 cycles. During the electroassembly procedure, the solution containing 2 was stirred under a N2 atmosphere. Figure 2

Figure 3. Current−time traces over 30 s dark−light cycles for FTO| nanoSnO2|TiO2(3 nm)|-1-2 (red, 1.7 cm2 in solution), FTO| nanoSnO2|TiO2(3 nm)|-1 (black, 2 cm2), and FTO|nanoSnO2|-1 (gray, 2 cm2 in solution) at an applied bias of 0.4 V versus SCE in 0.1 M H2PO4−/HPO42− buffer at pH 7 in 0.4 M NaClO4: 100 mW cm−2 white light source with a 400 nm cutoff filter.

RuII-RuIIcat) core/shell photoanode produced a dramatically higher photocurrent response than FTO|nanoSnO2|TiO2(3 nm)|-1 or FTO|nanoSnO2|-1. Photocurrents were also considerably higher than for the same electroassembly on TiO2 (FTO| nanoTiO2|-1-2) as reported in our previous study with sustained photocurrent densities of 40 μA cm−2 observed under the same conditions.27 The large initial current spikes after illumination for FTO| nanoSnO2|-1 and FTO|nanoSnO2|TiO2(3 nm)|-1 in Figure 3 arise from injection, surface oxidation, and dynamic local electric field effects in the double layer.16,37 Notably, the 3214

DOI: 10.1021/acs.jpclett.5b01370 J. Phys. Chem. Lett. 2015, 6, 3213−3217

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The Journal of Physical Chemistry Letters absence of a photocurrent spike for FTO|nanoSnO2|TiO2(3 nm)|-1-2 under water oxidation conditions indicates rapid catalysis without significant buildup of charge at the interface. As documented elsewhere, the improved photocurrent response for the core/shell structure arises from its impact on local interfacial dynamics. With the core/shell, the ∼0.4 V offset between the conduction band potentials of SnO2 and TiO2 creates a barrier to back electron transfer after injection and electron transfer to the core has occurred. Control of local dynamics provides a basis for accumulating multiple oxidative equivalents at the catalyst.36,38−40 The results of incident photon to current efficiency (IPCE) measurements on FTO|nanoSnO2|TiO2(3 nm)|-1-2 in 0.1 M H2PO4−/HPO42− buffer at pH 7 in 0.4 M NaClO4 are shown in Figure S1. The match with the absorption spectrum of FTO| nanoSnO2|-1 from 400 to 600 nm, with nearly complete light absorption by the dye, shows that chromophore injection results in the appearance of photocurrents and water photoelectrolysis without complications from direct bandgap excitation of TiO2 in the oxide core/shell. Photoelectrolysis experiments over longer time periods were conducted to investigate O2 production. Measurement of evolved O2 utilized a previously described generator/collector electrode technique.27 In this technique, a negative potential was applied to an FTO electrode positioned 1 mm from the face of the photoanode to detect dissolved O2 by current versus time measurements, providing a basis for quantitative evaluation of O2 produced. The technique also provides timeresolved profiles for O2 production. Figure 4 shows the

reduction of photogenerated O2. After correcting for the collection efficiency of the collector electrode (70%), an average faradaic efficiency of 22% was obtained for the production of O2 from water over a 5 min illumination period as an average of five separate freshly prepared samples (Figure S3). From the current−time profile at the FTO collector in Figure 4, a gradual reduction in O2 evolution occurs following an initial burst upon exposure to light. This pattern parallels the loss in photocurrent during the course of the experiment. At the end of the 15 min illumination period, repetition of the photoelectrochemical experiment (dashed traces in Figure 4) results in a steady-state photocurrent of 0.15 mA cm−2 but with negligible O2 production. Sustained photocurrents without O2 production demonstrate a competing anodic process (or processes) for both the electroassembly in FTO|nanoSnO2|TiO2(3 nm)|-1-2 and for FTO|nanoSnO2|TiO2(3 nm)|-1. As shown by the data in Figure 3 for the latter and by O2 measurements at the generator− collector electrode in Figure 4, the appearance of a photocurrent under these conditions continues to occur without O2 evolution. These observations are consistent with light-driven, redox decomposition of the chromophore on the surface following injection and oxidation to −RuIII. Similar behavior has been reported for related assemblies.12−16 In addition, we have studied the chemical and photochemical stability of Ru(bpy)32+ complexes on oxide surfaces,41,42 and a recent study has addressed the fate of oxidized, phosphonate-derivatized RuIII polypyridyl complexes and an assembly on oxide electrodes in acidic solution.43 It was found that oxidation to RuIII initiates bpy ligand loss and anion and water coordination. More relevant to the conditions used here, previous experiments at higher pH on RuIII polypyridyl complexes showed that bpy ligand hydrolysis occurs followed by ligand oxidation.44−46 In a recent mechanistic study on water oxidation by an analogous RuII(bda) catalyst, water oxidation was shown to occur following oxidation to RuV(O)+ and rate-limiting O atom transfer to water.31 In the surface-bound assembly under steady-state photolysis conditions, assuming the same ratelimiting step, the assembly would exist as FTO|nanoSnO2| TiO2(3 nm)|-RuIII-RuV(O)+, eq 1. Under these conditions, chromophore decomposition as −RuIII presumably competes with rate-limiting water oxidation, explaining the loss in O2 evolution while maintaining decreased but sustained photocurrents. The decomposition chemistry is currently under investigation in further detail.

Figure 4. Top: Current−time traces for (red) FTO|nanoSnO2|TiO2(3 nm)|-1-2 illuminated with 100 mW cm−2 white light with a 400 nm cutoff filter from 60 to 960 s with a bias of 0.4 V versus SCE. Bottom: Current−time response at a FTO collector electrode positioned 1 mm from the photoanode biased at −0.85 V versus SCE in 0.1 M H2PO4−/ HPO42− buffer at pH 7 in 0.4 M NaClO4. The solid traces were from the initial photolysis period and the dashed traces from a subsequent photolysis period with the same electrode.

FTO|nanoSnO2 |TiO2 (3nm)|‐Ru II‐Rucat II +4hν , +H2O, −4e−, −2H+

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ FTO|nanoSnO2 |TiO2 (3nm)|‐Ru III‐Rucat V(O)+

(1)

To test the efficiency of photoelectrochemical H2 production under illumination, we employed a two electrode cell with an FTO|nanoSnO2|TiO2(3 nm)|-1-2 photoanode and Pt cathode and applied a forward bias of 600 mV to stabilize the photocurrent response and sustain H2 evolution. The time variation of the H2 partial pressure in the headspace over the Pt counter electrode during a 10 min photolysis period is shown in Figure S4. In this experiment, evolved H2 was measured by a Unisense H2 sensor in the sealed cathode compartment separated from the anode by a nafion membrane. With LED illumination (450 nm; 14 mW cm−2), the solar efficiency for

photocurrent response of a FTO|nanoSnO2|TiO2(3 nm)|-1-2 photoanode biased at 0.4 V versus SCE (saturated calomel electrode) during a 15 min illumination period with a 100 mW cm−2 white light source and current monitoring at the collector electrode at −0.85 V versus SCE in 0.1 M H2PO4−/HPO42− buffer at pH 7 in 0.4 M NaClO4. For freshly prepared FTO|nanoSnO2|TiO2(3 nm)|-1-2 photoanodes, the integrated charge passed at the photoanode during an illumination period was compared with the current measured at the FTO collector electrode resulting from the 3215

DOI: 10.1021/acs.jpclett.5b01370 J. Phys. Chem. Lett. 2015, 6, 3213−3217

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The Journal of Physical Chemistry Letters DSPEC production of hydrogen was ∼0.3%. (See the SI for details.) As described here, the electroassembly procedure provides a new approach to surface assembly preparation avoiding complications arising from the synthesis of preformed assemblies. It offers control of surface coverage, an interface stabilized toward desorption, and the facile preparation of layered assembly structures. The impact of the core/shell metal oxide structure on performance in a DSPEC photoanode for water oxidation is significant. The appearance of competitive chromophore decomposition over extended photolysis periods in the 0.1 M H2PO4−/HPO42− buffer at pH 7 highlights the need for either stabilization of the oxidized chromophore or minimization of its residence time in photocatalytic cycles.

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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.5b01370. Experimental details, incident photo to current (IPCE) spectra, current versus voltage (IV) plots, and H2 evolution traces. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported primarily 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. DESC0001011, supporting B.D.S., M.V.S., and K.-R.W. D.L.A. acknowledges support from the Department of Energy Office of Science Graduate Fellowship Program under contract no. DE-AC05-06OR23100. A.M.L. acknowledges a graduate fellowship supported by the Department of Defense (DoD) through the National Defense Science & Engineering Graduate Fellowship (NDSEG) Program.

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REFERENCES

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The Journal of Physical Chemistry Letters

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DOI: 10.1021/acs.jpclett.5b01370 J. Phys. Chem. Lett. 2015, 6, 3213−3217