Stabilization of Ruthenium(II) Polypyridyl Chromophores on

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Research Article Cite This: ACS Cent. Sci. XXXX, XXX, XXX−XXX

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Stabilization of Ruthenium(II) Polypyridyl Chromophores on Mesoporous TiO2 Electrodes: Surface Reductive Electropolymerization and Silane Chemistry Lei Wu, M. Kyle Brennaman, Animesh Nayak, Michael Eberhart, Alexander J. M. Miller, and Thomas J. Meyer*

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Department of Chemistry, University of North Carolina at Chapel Hill, CB#3290, Chapel Hill, North Carolina 27599-3290, United States S Supporting Information *

ABSTRACT: Stabilization is a critical issue in the long term operation of dye-sensitized photoelectrosynthesis cells (DSPECs) for water splitting or CO2 reduction. The cells require a stable binding of the robust molecular chromophores, catalysts, and chromophore/catalyst assemblies on metal oxide semiconductor electrodes under the corresponding (photoelectro)chemical conditions. Here, an efficient stabilization strategy is presented based on functionalization of FTO|nanoTiO2 (mesoporous, nanostructured TiO2 deposited on fluorine-doped tin oxide (FTO) glass) electrodes with a vinylsilane followed by surface reductive electropolymerization of a vinyl-derivatized Ru(II) polypyridyl chromophore. The surface electropolymerization was dominated by a graftingthrough mechanism, and rapidly completed within minutes. Chromophore surface coverages were controlled up to three equivalent monolayers by the number of electropolymerization cycles. The silane immobilization and cross-linked polymer network produced highly (photo)stabilized chromophore-grafted FTO|nanoTiO2 electrodes. The electrodes showed significant improvements over structures based on atomic layer deposition and polymer dip-coating stabilization methods in a wide pH range from pH ≈ 1 to pH ≈ 12.5 under both dark and light conditions. Under illumination, with hydroquinone added as a sacrificial electron transfer donor, a photoresponse for sustained electron transfer mediation occurred for at least ∼20 h in a pH ≈ 7.5 phosphate buffer (0.1 M NaH2PO4/Na2HPO4, with 0.5 M NaClO4). The overall procedure provides an efficient way to fabricate highly stabilized molecular assemblies on electrode surfaces with potential applications for DSPECs in solar fuels.

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INTRODUCTION A variety of approaches have been exploited to harness solar energy for energy conversion and storage.1−11 Dye-sensitized photoelectrosynthesis cells (DSPECs) provide a promising technique that incorporates molecular components on metal oxide electrodes for water splitting and carbon dioxide reduction to solar fuels.3,12,13 A DSPEC photoanode consists of a chromophore and a water oxidation catalyst bound to a metal oxide surface, typically TiO2.13−15 The chromophore is photoexcited and injects electrons into TiO2 generating oxidative equivalents which drive the water oxidation catalyst. Integration of molecular components for the chromophore and catalyst offers the advantages of systematically controlling photoelectrochemical properties on the electrode surfaces.14,16−18 A critical requirement for a long-term DSPEC electrode over a range of pH values is stable binding of molecular components on electrode surfaces.13,14,19 The most extensively used binding strategies have involved carboxylic acid (−COOH) or phosphonic acid (−PO(OH)2) ester linkages.13,14 However, carboxylate binding is typically unstable and subject to surface © XXXX American Chemical Society

hydrolysis when exposed to water. Phosphonate binding is more robust in aqueous solution but unstable above pH ≈ 5 because of surface hydrolysis. On the basis of these binding strategies, progress has been made for aqueous stabilization including atomic layer deposition (ALD),20−22 polymer dipcoating,23,24 and electropolymerized overlayers.25−28 The first two methods produce a physical barrier to water/ion attack on the molecule-oxide link by either burying the linkage by ALD or by excluding water from the surface by the hydrophobicity of the polymer coating.13,21,23,29 The electropolymerized overlayer method is a chemical stabilization procedure in which the molecular components are cross-linked to form polymer network overlayers.26−28 In previous applications of the procedure, it has been shown that vinyl-derivatized Ru(II) chromophores preadsorbed on TiO2 through phosphonate linkages could be stabilized by reductive electropolymerization with divinylbenzene, or vinyl-derivatized Fe(II), Ru(II), or Zn(II) complexes, to create protective Received: December 9, 2018

A

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ACS Central Science polymeric overlayers.25−28 The systems were ultimately limited by the stability of the phosphonate attachment,25−28 and the added overlayer structure introduced new components that could be deleterious to DSPEC performance.25,26 In addition, no clear mechanism for the surface growth of the polymer overlayer has been elucidated.25,26,28 Here we report an electrode fabrication procedure that integrates silane immobilization and surface reductive electropolymerization for preparing surface assemblies of Ru(II) polypyridyl chromophores stably bound on mesoporous metal oxide surfaces in a wide pH range from pH ≈ 1.0 to pH ≈ 12.5. The fabrication was carried out in two steps (Scheme 1).

effects of silane immobilization and the polymer network provided a sustained photoresponse with added hydroquinone as a sacrificial electron transfer donor for at least ∼20 h at pH ≈ 7.5 with a gradual photocurrent decrease to ∼45% of the initial value originating from ligand loss photochemistry at the Ru(II) light absorber.

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RESULTS AND DISCUSSION Synthesis and Characterization of Monomer 12+. The structure of 12+ is shown in Scheme 1. Synthesis of the complex as a chloride salt was based on a modified version of an earlier procedure (Experimental Section in the Supporting Information)30 with one bpy ligand containing two vinyl groups on the 5,5′-positions for a highly efficient crosslinking.13 Cyclic voltammograms (CVs) of complex 12+ in CH3CN containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) showed three ligand-based reductions at −1.50, −1.82, and −2.06 V vs Ag/AgNO3 (Figure S1). Scanning the electrode potential between −1.00 vs −2.30 V vs Ag/AgNO3 resulted in ligand-based reductions which caused anionic initiation of the vinyl groups followed by radical− radical coupling for polymerization.31,32 The ultraviolet−visible (UV−vis) absorption spectrum of 12+ was collected in CH3CN (Figure S2). The characteristic intense π−π* absorption below 350 nm and metal-to-ligand charge transfer (MLCT) bands from 400 to 500 nm were discerned in the spectrum. Chromophore 12+ has an MLCT absorption maximum at λmax ≈ 456 nm with ε ≈ 11 700 M−1· cm−1. A red shift in the MLCT absorption by ∼6 nm occurred compared to [Ru(II)(bpy)3]2+ (λmax ≈ 450 nm) due to the stabilizing effect of the electron-withdrawing vinyl groups in lowering the π* levels.13,18 VTMS-Functionalized FTO|nanoTiO2. To perform surface reductive electropolymerization, the FTO|nanoTiO2 electrode was derivatized with VTMS (i in Scheme 1). The functional silane contains methoxy groups on one end for attachment onto electrode surfaces by −O−Si−O−Si−O− bonding (i in Scheme 1) from condensation with surface −OH groups and neighboring silane groups, which leads to a closedpacked, self-assembled single molecular layer in aprotic solvents such as toluene.33−35 The remaining vinyl group on the other side was available for surface electropolymerization. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) elemental mapping confirmed the success of silane functionalization on the FTO|nanoTiO2 electrode surface. As shown in the XPS analysis, the characteristic peaks for Si 2p and Si 2s from the silane attachment were observed at 103.5 and 154.0 eV (Figure S3a).36 Since the XPS probing depth was less than ∼20 nm,37 SEM/EDX elemental mapping was performed to investigate the Si composition in the bulk mesoporous structure (Figure S3b,c). From the SEM/EDX analysis, silane functionalization was shown to have been successful throughout the entire mesoporous structures. Mechanism for Surface Reductive Electropolymerization. Surface polymerization has been extensively used to grow polymers on the nanostructure surfaces in the preparation of polymer-grafted nanostructure composites based on graftingto, grafting-from, and grafting-through mechanisms.35,38−40 In grafting-to, the polymer chain is synthesized and then attached onto the surface by functional linkages. In the grafting-from pathway, functional initiators are first anchored onto the surface which directly grow polymer chains from the initiating

Scheme 1. Fabrication of FTO|nanoTiO2|-g-poly12+ by (i) VTMS Surface Functionalization of FTO|nanoTiO2 and (ii) Surface Reductive Electropolymerization of 12+a

a

The structure of the complex 12+ is simplified for a clear illustration.

Initially, a mesoporous FTO|nanoTiO2 electrode (∼20−30 nm diameter TiO2 nanoparticle film on FTO glass, ∼6.5 μm film thickness) was functionalized with vinyltrimethoxysilane (VTMS). The latter established robust silane surface attachment sites on TiO2 while introducing a vinyl group for surface electropolymerization (step i in Scheme 1). The chromophore, [Ru(II)(bpy)2(dvbpy)]2+ (12+; bpy = 2,2′-bipyridine, dvbpy = 5,5′-divinyl-2,2′-bipyridine), was subsequently reductively electropolymerized onto the VTMS-functionalized FTO|nanoTiO2 electrode, resulting in the poly12+-grafted FTO|nanoTiO2 electrodes (FTO|nanoTiO2|-g-poly12+; with g an abbreviation for grafted), step ii in Scheme 1. Surface reductive electropolymerization of 12+ was mechanistically dominated by a grafting-through pathway and rapidly accomplished within minutes. The surface coverage of the chromophore was controlled by the number of electropolymerization cycles up to three equivalent monolayers. The stability of FTO|nanoTiO2|-g-poly12+ under both dark and light conditions was evaluated. The cooperative stabilization B

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ACS Central Science sites. For the grafting-through mechanism, polymerizable groups, usually vinylic for radical polymerization, are immobilized on the surface. Polymerization is initiated in the solution. During the polymerization process, the preattached polymerizable groups are integrated into the growing polymeric chains leading to grafted polymers on the surface. In our sequence, we utilized the surface polymerization technique to graft poly-chromophores onto the mesoporous, nanostructured TiO2 electrodes, which mechanistically occurred by grafting-through as discussed below. CVs of VTMS in 0.1 M TBAPF6/CH3CN showed no redox couples between −1.0 and −2.3 V vs Ag/AgNO3 (Figure S1). These results show that, during electropolymerization, the vinyl group in the surface-bound VTMS on FTO|nanoTiO2 electrodes is not reducible to form radicals for initiating polymerization. Reduction is limited to the polypyridyl ligands in 12+ in the solution phase for the anionic generation of radicals on the vinyl groups by inter- or intraligand electron transfer, leading to radical−radical coupling for chain propagation.31 During polymeric chain growth in solution, the surface-bound vinyl groups are integrated into the propagating chains resulting in a permanent tethering of the polymers on the electrode surface (i in Scheme 2). The Scheme 2. Surface Reductive Electropolymerization by a Grafting-through Pathway, Consisting of (i) the Tethering Step and (ii or iii) Additional Polymer Chain Growth on the Surface

surface-tethered chains continue to propagate by integrating additional units (ii or iii in Scheme 2).41 Mechanistically, surface reductive electropolymerization of 12+ on the VTMSfunctionalized FTO|nanoTiO2 electrodes is dominated by a grafting-through pathway (Scheme 2). Surface Reductive Electropolymerization of 12+ on VTMS-Functionalized FTO|nanoTiO2. Chromophore 12+ was grafted onto VTMS-functionalized FTO|nanoTiO2 electrodes (∼1 cm2) by surface reductive electropolymerization (ii in Scheme 1 and Figure 1a). With monomer 12+ at 1 mM in 0.1 M TBAPF6/CH3CN and VTMS-functionalized FTO|nanoTiO2 as the working electrode, cyclic voltammetry (CV) was used to cycle the applied bias between −1.0 and −2.3 V vs Ag/ AgNO3 at a scan rate of 100 mV/s for 10 cycles (Figure 1a). The potential range was sufficiently negative to transport electrons into the conduction band of TiO2 from the FTO

Figure 1. (a) Surface reductive electropolymerization (10 cycles) of 1 mM 12+ on VTMS-functionalized FTO|nanoTiO2 to produce FTO| nanoTiO2|-g-poly12+. Conditions: scan rate 100 mV/s in 0.1 M TBAPF6/CH3CN electrolyte; Pt-wire counter electrode; Ag/AgNO3 reference electrode (0.32 vs NHE); under nitrogen protection. (b) UV−vis absorption spectra for FTO|nanoTiO2|-g-poly12+ following increasing numbers of reductive electropolymerization cycles (1, 2, 3, 4, 5, 7, 10, and 20; yellow to red). The absorption of FTO|nanoTiO2 was subtracted from each UV−vis absorption measurement. (c) The surface coverage of 12+ versus the number of reductive electroC

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On the basis of the UV−vis spectra, the surface coverage (Γ in nmol/cm2) of grafted 12+ was determined from the expression Γ = Abs(λmax)/(1000·ε(λmax)), with Abs(λmax) the background-corrected absorbance and ε(λmax) the molar absorptivity at the λmax for the solution analogue.13,42 During the first three cycles, the surface coverage of 12+ increased linearly with the number of reductive electropolymerization cycles (Figure 1c). Approximately 76 nmol/cm2 of 12+ were grafted on the VTMS-functionalized FTO|nanoTiO2 electrode surface after three cycles, equivalent to one monolayer of surface-bound Ru(II) polypyridyl chromophore based on phosphonate linkages.23,26,27 From 4 to 20 cycles, surface coverages continued to grow but at a gradually decreased rate, demonstrating a decreasing polymerization rate. The rate decrease presumably arises from a decrease in the interfacial electron transfer rate from TiO2 to the polymerizable monomer 12+ in solution due to the presence of the surfacegrafted Ru(II) layers. There may also be an influence from a decrease in available pore volume in the internal cavities of the mesoporous structures which diminishes monomer diffusion to the surface. After 20 cycles, the surface coverage reached ∼200 nmol/cm2 , equivalent to three monolayers of Ru(II) chromophore grafted on the surface. Electrochemical properties of FTO|nanoTiO2|-g-poly12+, following different surface reductive electropolymerization cycles, were evaluated by CV (Figure 1d). Although the Ru(III/II) redox couple for the Ru(II) polypyridyl chromophore falls within the band gap of TiO2, direct detection of

Figure 1. continued polymerization cycles from 1 to 20. (d) CVs for FTO|nanoTiO2|-gpoly12+ from 0 to 1.3 V after 1, 2, 3, 4, 5, 7, 10, and 20 electropolymerization cycles. Conditions: scan rate 5 mV/s in 0.1 M TBAPF6/CH3CN electrolyte; Pt-wire counter electrode; Ag/AgNO3 reference electrode (0.32 vs NHE); under nitrogen protection; electrode area ∼1 cm2.

substrate through the nanofilm, enabling reductive electropolymerization to occur on the surface.26 During electropolymerization, the ligand reduction waves for 12+ were obscured by the large background current due to charging of TiO2.25 However, a gradual increase of the cathodic current appeared with proceeding potential cycles, consistent with surface reductive electropolymerization to form FTO|nanoTiO2|-g-poly12+ (Figure 1a). The growth of poly12+ on the electrode surface as a function of the number of electropolymerization cycles was monitored spectrophotometrically by UV−vis measurements (Figure 1b). As expected, the absorbance increased with the number of electropolymerization cycles (Figure 1b). Upon electropolymerization, a ∼10 nm blue-shift (λmax ≈ 446 nm) in the MLCT band was observed for the surface-grafted Ru(II) complex in FTO|nanoTiO2|-g-poly12+ compared to monomer 12+ in solution (Figure 1b and Figure S2). The shift is due to conversion of the electron-withdrawing vinyl groups in 12+ to saturated alkyl groups in the polymeric network during the reductive electropolymerization process.

Figure 2. (a) SEM image of FTO|nanoTiO2|-g-poly12+ with 20 electropolymerization cycles. (b) Zoom-in surface view of the mesoporous electrode. (c−e) Zoom-in cross-sectional views of top, middle, and bottom in (a) with the corresponding elemental ratios of Ru to Si (f). D

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and S5−S8) in the dark. Desorption was monitored by recording absorbance−time changes at 480 nm (Figure 3b).

oxidation of Ru(II) to Ru(III) on the TiO2 surface is still possible by cross-surface electron transfer at the FTO interface followed by site-to-site, cross-TiO2 electron hopping between the individual Ru sites.13,23 CVs for FTO|nanoTiO2|-g-poly12+ in a fresh 0.1 M TBAPF6/CH3CN solutions exhibited a reversible Ru(III/II) wave at E1/2 = 1.28 V vs NHE with integrated anodic currents increasing with the number of electropolymerization cycles (Figure 1d). The appearance of the Ru(III/II) couple points to successful electropolymerization of 12+ on the surface of the VTMS-functionalized FTO|nanoTiO2 electrode to form FTO| nanoTiO2|-g-poly12+ without decomposition of the Ru(II) complex. Morphology of FTO|nanoTiO2|-g-Poly12+. The morphology and composition of FTO|nanoTiO2|-g-poly12+ were investigated by SEM/EDX. Figures S4 and 2 show SEM images of the mesoporous electrodes with 3 and 20 electropolymerization cycles, corresponding to one and three monolayers, respectively. After three electropolymerization cycles, the electrode retained the mesoporous structure as observed from zoom-in views of surface and cross sections of top, middle, and bottom (Figure S4b−e). In contrast, after 20 cycles of surface reductive electropolymerization, a thin film appeared on the top of the mesoporous structures (Figure 2b), while the porosity was maintained in the bulk (Figure 2c−e). Unlike small molecules, the sphere-of-action radius of [Ru(II)(bpy)3]2+ is approximately 0.75 nm,27 and mass diffusion within the mesoporous structures is uneven.35 During surface electropolymerization, monomer 12+ diffuses more easily into the upper section of the mesoporous structures for polymerization. Surface-polymerized monomers inhibit mass diffusion into the deeper sections of the mesoporous structures, and a gradient content of surface-polymerized Ru(II) chromophores appears. With more polymerization cycles, the gradient increases and becomes visible by morphological measurements (Figure 2b). Homogeneous coverage of vinylsilane throughout the mesoporous FTO|nanoTiO2 film after VTMS surface functionalization was shown by SEM/EDX elemental mapping of Si (Figure S3). EDX was also utilized to estimate the composition of the Ru(II) chromophore at different depths within the mesoporous structures (Figures S4f and 2f). Both films after 3 and 20 cycles showed inhomogeneities in Ru content throughout the mesoporous structure, revealing a gradient in the grafting of the Ru(II) complex. The Ru content was highest on the top section and decreased with depth in the mesoporous structure toward the FTO surface (Figures S4f and 2f). This result matches well with the mass diffusion analysis above and is consistent with a previous polymer overlayer study in which [Fe(II)(v-tpy)2]2+ (v-tpy = 4′-vinyl2,2′:6′,2″-terpyridine) was electropolymerized onto preadsorbed Ru(II) chromophores on TiO2.26 Stability of FTO|nanoTiO2|-g-Poly12+ in the Dark. The stability of FTO|nanoTiO2|-g-poly12+ was investigated in pH ≈ 1.0 aqueous HClO4 (0.1 M), pH ≈ 4.5 acetate buffer (0.1 M CH3COOH/CH3COONa, with 0.5 M LiClO4), pure water, pH ≈ 7.5 phosphate buffer (0.1 M NaH2PO4/Na2HPO4, with 0.5 M NaClO4), and pH ≈ 12.5 phosphate buffer (0.1 M Na2HPO4/Na3PO4, with 0.5 M KNO3) under nitrogen protection. The poly12+-grafted electrodes were immersed in the solutions, and UV−vis absorption spectra were monitored in situ during a period of 14 h at 15 min intervals (Figures 3a

Figure 3. (a) Absorption-time changes for FTO|nanoTiO2|-g-poly12+ (three electropolymerization cycles) in pH ≈ 7.5 phosphate buffer (0.1 M NaH2PO4/Na2HPO4, with 0.5 M NaClO4) from 0 to 14 h, black to green. Note: the different absorption intensity from surface coverage characterization in Figure 3 arises from a variation in the experimental setup for an in situ UV−vis monitoring during stability investigation where a smaller slide was used with a tilt position. (b) Relative At/A0 values for FTO|nanoTiO2|-g-poly12+ (three electropolymerization cycles) at 480 nm versus time in solutions with different pH values.

In a pH ≈ 7.5 phosphate buffer (0.1 M NaH2PO4/ Na2HPO4, with 0.5 M NaClO4), the UV−vis spectra of FTO| nanoTiO2|-g-poly12+ (three electropolymerization cycles) remained constant during a 14 h soaking period, indicating no chromophore desorption from the surface (Figure 3a). The surface grafting of poly12+ was robust over a wide pH range even in a strongly basic buffer solution at pH ≈ 12.5 (Figure 3b and Figure S8). Under the condition, bleaching occurs rapidly within ∼15 min with phosphonate and carboxylate linkages.14,23,24 The electrode was exposed to organic solvents such as DMF, methanol, ethanol, and CH3CN, with no sign of changes in the UV−vis absorption spectra even after a ∼1 day period (Figure S9). Stabilization was achieved regardless of the number of electropolymerization cycles (Figure S10). Photostability of FTO|nanoTiO2|-g-Poly12+. The photostability of FTO|nanoTiO2|-g-poly12+ electrodes at different pH values was investigated under nitrogen protection by a previously established protocol.21 UV−vis spectra of FTO| nanoTiO2|-g-poly12+ in pH ≈ 1.0 aqueous HClO4 (0.1 M), pH E

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ACS Central Science ≈ 4.5 acetate buffer (0.1 M CH3COOH/CH3COONa, with 0.5 M LiClO4), pure water, pH ≈ 7.5 phosphate buffer (0.1 M NaH2PO4/Na2HPO4, with 0.5 M NaClO4), and pH ≈ 12.5 phosphate buffer (0.1 M Na2HPO4/Na3PO4, with 0.5 M KNO3) were recorded in situ every 15 min under continuous illumination at 455 nm (30 nm fwhm, 475 mW/cm2) over a 16 h period. Results are shown in Figures 4a and S11−S14, respectively.

Photodesorption kinetics were quantified by monitoring absorption-time traces at 480 nm (Figure 4b) with fitting to a triexponential function (eq 1). The photodesorption rate constant (kphotodes) was calculated as the inverse of the weighted average lifetime (1/⟨τ⟩) as shown in eq 2. The rate constants obtained as a function of pH are summarized in Table 1. y = A1e−(1/ τ1)x + A 2 e−(1/ τ2)x + A3e−(1/ τ3)x + y0

(1)

ΣAi τi2 ΣAi τi

(2)

1/k photo ‐ des = ⟨τ ⟩ =

Under illumination, poly12+-grafted electrodes were the most highly stabilized under all pH conditions (Table 1). In solutions over a wide pH range, more than ∼70% of the initial Ru(II) chromophore remained on the poly12+ -grafted electrode surface after 16 h of illumination (Figure 4b). Photodesorption for FTO|nanoTiO2|-RuP2+ was too rapid to be quantified above pH ≈ 4.5 and was greatly decreased for FTO|nanoTiO2|-g-poly12+ (Table 1). As shown in Table 1, kphotodes for FTO|nanoTiO2|-g-poly12+ was much lower than for all other stabilization strategies over the entire pH range. As an example, in a pH ≈ 7.5 phosphate buffer, kphotodes for FTO| nanoTiO2|-g-poly12+ was ∼25 times lower than for the poly[Fe(II)(v-tpy)2]2+ overlayer method. To the best of our knowledge, the combination of silane attachment and surface electropolymerization provides the best route for stabilizing surface-bound Ru(II) chromophores on nanoTiO 2. The origin of the significant improvement presumably lies in the cooperative stabilization of silane immobilization and electropolymerization from (1) desorption by hydrolysis at the surface linkage, especially under illumination, is diminished by silane immobilization; (2) electropolymerization binds chromophores onto the surface and to each other, significantly preventing (photo)desorption by cross-linking and a decreased solubility for poly chromophores into the aqueous solutions; and (3) chromophore loss by photoinduced ligand dissociation is reduced by binding ligands to neighboring complexes. The kphotodes values changed little with the number of electropolymerization cycles, consistent with the cooperative stabilization by silane attachment and electropolymerized network (Table S1). Sustainable Light-Driven Hydroquinone Dehydrogenation. To investigate the long-term stability of the FTO|

Figure 4. (a) Photostability of FTO|nanoTiO2|-g-poly12+ electrodes (three electropolymerization cycles) in a pH ≈ 7.5 phosphate buffer (0.1 M NaH2PO4/Na2HPO4, with 0.5 M NaClO4) under constant 455 nm LED illumination (475 mW/cm2) from 0 to 16 h, black to green, recorded every 15 min. (b) Absorbance at 480 nm versus time with desorption rate constants (kphotodes) at different pH values.

Table 1. Summary of Values for FTO|nanoTiO2|-g-poly12+ Electrodes and Comparison to Literature Data for Unprotected [Ru(II)(bpy)2(4,4′-(PO3H2)2bpy)]2+ (RuP2+), with ALD, PMMA Dip-Coating, and poly[Fe(II)(v-tpy)2]2+ Overlayer Protection under Constant Illumination at 455 nm (30 nm fwhm, 475 mW/cm2) in pH ≈ 1.0 Aqueous HClO4 (0.1 M), pH ≈ 4.5 Acetate Buffer (0.1 M CH3COOH/CH3COONa, with 0.5 M LiClO4), Pure Water, pH ≈ 7.5 Phosphate Buffer (0.1 M NaH2PO4/Na2HPO4, with 0.5 M NaClO4), and pH ≈ 12.5 Phosphate Buffer Solution (0.1 M Na2HPO4/Na3PO4, with 0.5 M KNO3)a kphotodes (× 10−5 s−1) solvent pH 1.0 pH 4.5 pure water pH 7.5 pH 12.5

2+21

RuP 5.0 >20 >30 N/A N/A

ALD 0.33 nm 2.3 3.2 9.5 N/A

Al2O321

1.0 wt % PMMA dip-coating 0.44 3.68 N/A

23

poly[Fe(II)(v-tpy)2]2+ overlayer26 0.6 1.3 0.9 5.5 N/A

current work 0.15 0.16 0.23 0.19 2.99

a

N/A, desorption was too rapid to obtain reliable kinetics; PMMA is poly(methyl methacrylate). F

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ACS Central Science nanoTiO2|-g-poly12+ electrodes, the electrode (∼1 cm2) was used to undergo sustained electron transfer mediation in a pH ≈ 7.5 phosphate buffer solution (0.1 M NaH2PO4/Na2HPO4, with 0.5 M NaClO4) with added hydroquinone (H2Q; 50 mM) as a sacrificial electron transfer donor. A three-electrode configuration was utilized with an Ag/AgCl reference electrode and a Pt-wire counter electrode in a two-compartment cell separated from the FTO|nanoTiO2|-g-poly12+ working electrode by a Nafion membrane. The photoresponse was recorded at an applied bias of 0.2 V (vs Ag/AgCl) under illumination with a 100 mW/cm2 white light source (400 nm long-pass filter).13,23 The measurements were conducted with vigorous stirring under nitrogen protection. With added H2Q, photoexcitation of the surface-bound Ru(II) chromophore results in electron injection into TiO2 yielding the oxidized assembly FTO|nanoTiO2(e−)|-g-poly13+, which, in turn, is rapidly reduced to FTO|nanoTiO2|-g-poly12+ by H2Q (Scheme 3). Scheme 3. Electron Transfer during Photolysis with H2Q Added as a Sacrificial Electron Transfer Donor

The photoelectrochemical response of FTO|nanoTiO2|-gpoly12+ with different reductive electropolymerization cycles was monitored by measuring off/on current−time traces over 30 s dark/light intervals (Figure 5a). Under illumination, currents from FTO|nanoTiO2|-g-poly12+ were enhanced consistent with light-driven photo-oxidation of H2Q by the surface-grafted Ru(II) chromophore (Figure 5a). The photocurrent varied with the number of electropolymerization cycles (Figure 5b). It increased linearly with the number of electropolymerization cycles up to four cycles, demonstrating an increase of the chromophore surface coverage. From 5 to 20 cycles, the photocurrent no longer increased linearly but kept little change, while the surface coverage continued to increase with the number of electropolymerization cycles. With polymerization past a monolayer, not all of the chromophores are directly attached to the surface, and the increase in the distance between the photoexcited chromophore and the electrode surface may decrease the internal electron transfer rate and the dehydrogenation efficiency. Photo-oxidation of H2Q was observed over a ∼20 h period (Figure 5c). After ∼10 h, a new stock buffer solution was added to provide a fresh source of H2Q. During the ∼20 h irradiation period, the photocurrent gradually decreased to ∼120 μA/cm2 from ∼220 μA/cm2 with the initially transparent buffer turning into light brown consistent with a sustained production of (semi)quinone.43 The decrease in photocurrent is presumably due to ligand loss photochemistry as shown by the loss in MLCT absorption with absorption band shifting to longer wavelengths (Figure S15). The fabrication strategy provides a striking new method for enhanced stability compared to phosphonic acid or carboxylic acid binding strategies. In the latter two approaches, more than ∼30% photocurrent loss occurred during a period of less than ∼15 min due to rapid surface desorption of Ru(II) chromophore from the surface.23 Best to our knowledge, the

Figure 5. (a) Current−time traces over 30 s dark/light intervals for FTO|nanoTiO2|-g-poly12+ following different surface reductive electropolymerization cycles using H2Q as a sacrificial electron transfer donor in pH ≈ 7.5 phosphate buffer (0.1 M NaH2PO4/ Na2HPO4, with 0.5 M NaClO4) under a 100 mW/cm2 white light source (400 nm long-pass filter). (b) Photocurrent for light-driven H2Q dehydrogenation with FTO|nanoTiO2|-g-poly12+ as a function of the number of electropolymerization cycles. (c) Long-term photolysis for FTO|nanoTiO2|-g-poly12+ (20 electropolymerization cycles) with H2Q as a sacrificial electron transfer donor in pH ≈ 7.5 phosphate buffer (0.1 M NaH2PO4/Na2HPO4, with 0.5 M NaClO4) at an applied bias of 0.2 V vs Ag/AgCl at 100 mW/cm2 with a 400 nm longpass filter. The break after ∼10 h is adding a new buffer solution to give a fresh H2Q source for continuing the photolysis. Note: the electrode area is ∼1 cm2.

combined silane immobilization and surface reductive electropolymerization for FTO|nanoTiO2|-g-poly12+ appears to maximize surface (photo)stability for the Ru(II) chromophores on the electrode surfaces.13 In perspective, to extend the success of the stabilization method into a water oxidation catalyst study, a Ru(II)polypyridyl-aqua-complex was stably bound on mesoporous G

DOI: 10.1021/acscentsci.8b00914 ACS Cent. Sci. XXXX, XXX, XXX−XXX

ACS Central Science

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ACKNOWLEDGMENTS The research was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, under Award No. DE-SC0015739. Experimental work using UV-vis spectrometers and photoelectrochemical equipment, was performed in the Instrumentation Facility established by the Alliance for Molecular PhotoElectrode Design for Solar Fuels (AMPED), 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. This work made use of XPS and SEM/EDX instrumentation at the Chapel Hill Analytical and Nanofabrication Laboratory (CHANL), a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), which is supported by the National Science Foundation (Grant ECCS-1542015) as part of the National Nanotechnology Coordinated Infrastructure (NNCI).

nanostructured indium tin oxide (nanoITO) for sustained heterogeneous catalytic water oxidation (Figure S16). The combined silane immobilization and cross-linked polymer network stabilized the molecular Ru(II) catalyst on the electrode surface under a variety of conditions especially at pH > ∼6, making the catalyst stable toward redox cycling at pH ≈ 7.5 over a ∼4 h period. Sustained heterogeneous electrocatalytic water oxidation by the electrode gave steadystate currents for ∼3 h with a Faradaic efficiency of ∼68% for O2 production, detected by the Generator-Collector cell configuration (Figure S17). More details were submitted as a separate work. Safety Statement. No unexpected or unusually high safety hazards were encountered.

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CONCLUSIONS The combination of silane immobilization and electropolymerization has provided a basis for preparing exceptionally stabilized chromophore-grafted nanoTiO2 electrodes in DSPEC photoanode configurations. They have resulted in a significantly improved stability under both dark and light conditions over a wide pH range. The procedure described here establishes a new strategy for surface binding and stabilization of molecular chromophores, catalysts, and chromophore/catalyst assemblies on DSPEC electrodes for long-term operations.

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REFERENCES

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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/acscentsci.8b00914. Experimental details with NMR spectra, CVs of complex 12+ and VTMS, UV−vis spectrum of complex 12+, XPS survey and SEM/EDX elemental mapping analysis of VTMS-functionalized FTO|nanoTiO2, SEM image of FTO|nanoTiO2|-g-poly12+ with three electropolymerization cycles, stability and photostability studies of FTO| nanoTiO2|-g-poly12+ electrodes in pH ≈ 1.0 aqueous HClO4 (0.1 M), pH ≈ 4.5 acetate buffer (0.1 M CH3COOH/CH3COONa, with 0.5 M LiClO4), pure water, and pH ≈ 12.5 phosphate buffer (0.1 M Na2HPO4/Na3PO4, with 0.5 M KNO3), kphotodes values for FTO|nanoTiO2|-g-poly12+ electrodes (3 and 20 electropolymerization cycles, respectively) in different pH conditions, UV−vis spectra of FTO|nanoTiO2|-gpoly12+ electrodes before and after ∼20 h photolysis, fabrication of molecular Ru(II)-polypyridyl-aqua-complexes bound on conductive electrodes for sustained heterogeneous catalytic water oxidation (PDF)

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Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Thomas J. Meyer: 0000-0002-7006-2608 Notes

The authors declare no competing financial interest. H

DOI: 10.1021/acscentsci.8b00914 ACS Cent. Sci. XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acscentsci.8b00914 ACS Cent. Sci. XXXX, XXX, XXX−XXX