Solar Hydrogen Production Using Molecular Catalysts Immobilized on

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Solar Hydrogen Production Using Molecular Catalysts Immobilized on Gallium Phosphide (111)A and (111)B Polymer-Modified Photocathodes Anna M. Beiler, Diana Khusnutdinova, Samuel I. Jacob, and Gary F. Moore* School of Molecular Sciences and the Biodesign Institute Center for Applied Structural Discovery (CASD), Arizona State University, Tempe, Arizona 85287-1604, United States S Supporting Information *

ABSTRACT: We report the immobilization of hydrogen-producing cobaloxime catalysts onto p-type gallium phosphide (111)A and (111)B substrates via coordination to a surface-grafted polyvinylimidazole brush. Successful grafting of the polymeric interface and subsequent assembly of cobalt-containing catalysts are confirmed using grazing angle attenuated total reflection Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Photoelectrochemical testing in aqueous conditions at neutral pH shows that cobaloxime modification of either crystal face yields a similar enhancement of photoperformance, achieving a greater than 4-fold increase in current density and associated rates of hydrogen production as compared to results obtained using unfunctionalized electrodes tested under otherwise identical conditions. Under simulated solar illumination (100 mW cm−2), the catalyst-modified photocathodes achieve a current density ≈ 1 mA cm−2 when polarized at 0 V vs the reversible hydrogen electrode reference and show near-unity Faradaic efficiency for hydrogen production as determined by gas chromatography analysis of the headspace. This work illustrates the modularity and versatility of the catalyst−polymer−semiconductor approach for directly coupling light harvesting to fuel production and the ability to export this chemistry across distinct crystal face orientations. KEYWORDS: photoelectrochemistry, hydrogen production, cobaloxime, gallium phosphide, polyvinylimidazole, solar fuels

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INTRODUCTION Human-engineered systems capable of converting sunlight and water to fuels offer a promising approach to obtaining a sustainable energy future.1−3 Biology provides inspiration for developing such future technologies in the process of photosynthesis which utilizes earth-abundant materials to capture, convert, and store solar energy in chemical bonds.4−9 Yet, many features of the biological counterpart are undesirable from the perspective of meeting technological energy demands, including their overall poor energy conversion efficiencies, fragility, and the relatively large size of enzymes. However, scientists have set out to re-design photosynthesis for industrial applications using top-down as well as bottom-up strategies.10 Molecular catalysts powered by visible-light-absorbing semiconductors represent one approach to developing an integrated system for generating solar fuels.11−16 This strategy offers an appealing integration of organic nanoarchitecture with inorganic foundations. Thus, finding new ways to immobilize catalysts onto semiconductor surfaces and characterize the hard-to-soft matter interfaces remains a major challenge.17,18 Gallium phosphide (GaP) is a III−V semiconductor with a midsize optical band gap (indirect Eg = 2.26 eV) used in optoelectronic applications. Additionally, GaP has promising features as a candidate material for applications in solar fuels generation,19−27 including a capacity for achieving relatively large photovoltages.28 The crystal face of GaP(100) terminates with a mixed phase of Ga and P sites, while GaP(111)A and © 2016 American Chemical Society

GaP(111)B consist of surfaces with predominantly atop Ga or atop P, respectively. Both faces are subject to oxidative and corrosive processes; however methods have been developed to chemically protect and passivate these otherwise structurally unstable surfaces, including treatment with (NH4)2Sx,29 alkylation or allylation with subsequent secondary functionalization of the A-side30,31 and alkyl halide reactions with the P sites via a Williamson ether synthesis on the B-side.32 An attachment strategy that can be utilized across multiple crystal face orientations may be useful in interfacing molecular catalysts to nanostructured materials that terminate with a range of indices, thereby permitting a relatively dense loading of catalysts and relieving the turnover frequency (TOF) of individual active sites required to achieve a selected current density. Reports describing strategies to directly integrate molecular catalysts for hydrogen production to visible-lightabsorbing semiconductors are limited, but include the following: [Fe2S2 (CO)6] adsorbed onto indium phosphide nanocrystals,33 a cobaloxime-containing polymer grafted onto GaP(100),34−37 trinuclear molybdenum cluster salts drop-cast on Si(100),38 ferrocenophane-containing polymers deposited on Si(100) and (111), 39 modified Dubois-type nickel bisdiphosphine catalysts covalently attached to amine-modified Received: February 5, 2016 Accepted: March 21, 2016 Published: March 21, 2016 10038

DOI: 10.1021/acsami.6b01557 ACS Appl. Mater. Interfaces 2016, 8, 10038−10047

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ACS Applied Materials & Interfaces Si(100) and GaP(100),40 Negishi coupling of phosphine ligands onto Si(111) followed by metalation,41 a metal−organic surface composed of a cobalt dithiolene polymer drop-cast onto Si(100),42 and cobaloxime catalysts adsorbed onto a TiO2coated GaInP2 semiconductor.43 Many of the cathode assemblies reported to date require an electrochemical bias poised negative of 0 V vs the reversible hydrogen electrode (0 V vs RHE) (i.e., an applied cathodic polarization beyond the thermodynamic potential of the H+/H2 half-reaction under the conditions tested to produce hydrogen) and some require extreme pH operating conditions or nonaqueous conditions using relatively strong organic acids as a proton source, features that may be undesirable in the context of large-scale deployment, environmental impact, and biocompatibility. Nonetheless, ion migration and mass transfer properties associated with water electrolysis schemes (i.e., the three terms in the Nernst−Plank equation, dif f usion, migration, and convection) do impose overall constraints that must be considered at the system design level.44−46 Although immobilization via drop-casting methods and other electrostatic-based deposition techniques used to affix molecules to surfaces offer ease of assembly, the development of more precise surface attachment chemistries47−56 could offer additional control over orientation of attached components, surface loading, passivation of the substrate, and interfacial energetics.57−59 Previous work by Moore and co-workers34−37 adopted a grafted polymer brush containing built-in ligand sites for assembling catalysts on the modified-GaP surface. In this construct, the polymer coat of the GaP surface likely provides protection against oxide layer passivation by restricting access to surface GaP(100) sites while providing attachment sites for assembly of the catalysts, allowing analyses of electrode performances depending on light intensity34 and pH of the electrolyte as well as the attachment of alternative cobaloxime catalysts.36 We now show, for the first time, the functionalization of GaP(111) with cobaloxime catalysts60−75 for hydrogen production using a chemical attachment strategy that can be deployed on both the A and B crystal faces. It has been shown that polyvinylimidazole (PVI) confers stability to metal surfaces76,77 and in our efforts it serves the intended dual purpose of providing a linkage to molecular catalysts as well as a protective layer for the underpinning GaP. As compared to unmodified electrodes, cobaloxime modification yields a greater than 4-fold increase in the photocurrent density measured for electrodes polarized at 0 V vs RHE. Chronoamperometry coupled with gas analysis measurements show the photocurrent densities are also relatively stable, with a drop off of less than 1% over 55 min following an initial decrease of up to 12% during the first 5 min of photoelectrochemical (PEC) testing, and that the current produced is associated with a nearly quantitative production of hydrogen gas. This work illustrates the capability of these constructs to produce a fuel using light as an energy input with no requirement of forward electrochemical biasing, sacrificial reagents, or extreme pH conditions. Additionally, the modular assembly method allows modification of components as new materials and discoveries emerge. In particular, and as illustrated in this report, the ability to export the grafting chemistry across a selection of crystal face orientations is a desirable feature in the context of functionalizing the inherently multifaceted surfaces of nanostructured light-harvesting materials which have shown great promise in emerging light capture

and conversion technologies due to their relatively large aspect ratios and short minority-carrier diffusion lengths.78−81

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RESULTS AND DISCUSSION Cobaloxime-modified GaP(111) wafers are assembled via a two-step process. First, PVI is grafted onto gallium phosphide wafers using self-initiated polymerization of the vinylimidazole monomer under shortwave UV light.82−85 Second, surfaceattached cobaloxime complexes are formed using a wet chemical treatment that exchanges one of two chloride ligands from the complex Co(dmgH2)(dmgH)Cl2 with an imidazole ligand on the PVI graft (Scheme 1). As described in previous Scheme 1. Schematic Representation of the Attachment Method Used To Assemble the Chemically Modified Gallium Phosphide (111) Electrodes

work,34−37 sample preparation starts with placing clean, freshly etched GaP wafers into a quartz flask containing the monomer under argon. The flask is then illuminated with UV light (254 nm) for 2 h before removing and cleaning the polymerfunctionalized wafers with successive solvent washes followed by drying under a stream of nitrogen (see Methods for details). The polymer-modified wafers are then placed in a sealed flask containing an argon-sparged methanolic solution that is 1 mM in triethylamine and 1 mM Co(dmgH2)(dmgH)Cl2. After 12 h, 10039


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Figure 1. FTIR transmission spectra of (a and c) N-vinylimidazole monomer in KBr (blue) as well as a powder sample of the model cobaloxime catalyst, Co(dmgH)2(meIm)Cl, in KBr (red). GATR-FTIR absorbance spectra of (b) the (111)A-face and (d) (111)B-face of unmodified GaP (black dash) polyvinylimidazole-grafted GaP (blue) and cobaloxime-modified GaP (red). The spectra shown in parts a and c are identical and included to facilitate comparisons.

the catalyst-modified wafers are removed from the flask and cleaned with successive solvent washes followed by drying under a stream of nitrogen then under vacuum (see Methods for details). Following the PVI-grafting procedure, spectroscopic ellipsometry measurements, performed in air, yield a polymer thickness of ≈6 nm on both the (111)A and (111)B surfaces (see Methods and Supporting Information (SI) Figure S24 for details). This thickness is slightly lower than that previously reported using polyvinylpyridine (PVP) to functionalize 100 GaP, yet these results are consistent with lower grafting concentrations reported for the UV polymerization of PVI relative to PVP.86 An upper limit on the imidazole unit site density can be provided given the bulk PVI density of the polymer-modified samples (1.25 g cm−3)87 is representative of the solvent-free layer density. Thus, for a 6 nm thick PVI layer, the maximum site density is 5 × 1015 cm−2 = 8 nmol cm−2. Surface analysis using grazing angle attenuated total reflection (GATR-FTIR) spectroscopy provides compelling evidence for successful chemical functionalization of the GaP(111) surfaces. GATR-FTIR spectra of unmodified GaP(111)A and GaP(111)B as well as spectra collected following photochemical grafting of vinylimidazole, yielding PVI−GaP(111)A or PVI−GaP(111)B, and spectra collected following assembly of cobaloximes, yielding Co−PVI− GaP(111)A or Co−PVI−GaP(111)B, are shown in Figure 1. For the GaP samples functionalized only with PVI, and prior to attachment of cobaloximes, distinct IR absorbance bands associated with multimode in-ring CC as well as in-ring CN stretches of PVI are observed at 1500 cm−1. In addition, the distinct 1373 cm−1 peak of the CC stretch associated with the vinyl group of the monomer is not pronounced on the PVI-functionalized wafers. Instead, a new peak indicative of the

planar deformation vibration of the CH2 groups in the polymer chain88 and centered at 1454 cm−1, is present. Following treatment of PVI−GaP surfaces with the Co(dmgH2)(dmgH)Cl2 solution, unique vibrational modes characteristic of imidazole units coordinated to cobaloxime complexes are detected, including a peak at 1570 cm−1 characteristic of the CN stretches on the glyoximate ligands of cobaloximes coordinated to an axial imidazole.89 Furthermore, unlike the CN and CC vibrations of the imidazole ring observed at 1500 cm−1 on PVI−GaP samples, vibronic features appear at 1517 cm−1 on samples of Co−PVI− GaP(111)A and Co−PVI−GaP(111)B associated with the CN and CC vibrations of imidazole units coordinated to cobalt centers. This 17 wavenumber difference is consistent with previously reported IR spectra of metals coordinating to PVI.90 Finally, and as described in the following sentences, comparison of the NO− stretching region in spectra of Co− PVI−GaP with spectra of Co(dmgH2)(dmgH)Cl2 and Co(dmgH)2(meIm)Cl, a model cobaloxime compound bearing an axial N-methylimidazole unit, affords a direct spectroscopic method of confirming successful synthesis of intact cobaloxime complexes on the polymeric interface. The FTIR spectrum of the precursor material, Co(dmgH2)(dmgH)Cl2, used to assemble the attached cobalt complexes to the PVI-functionalized wafer includes a strong NO− stretch at 1225 cm−1 (SI Figure S3). However, this feature is not pronounced in the spectrum of PVI−GaP samples following treatment with Co(dmgH2)(dmgH)Cl2. Instead a strong NO− stretching frequency is observed at 1239 cm−1 on both the A and B cobaloxime-modified surfaces. Within the 4 cm−1 resolution of the spectroscopic scans, this value is consistent with that measured for the NO− stretch of the cobaloxime model compound, Co(dmgH)2(meIm)Cl, at 1237 cm−1. The lack of apparent peak features centered at 1225 cm−1 in spectra of 10040


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Figure 2. N 1s core level spectra of (a) PVI−GaP(111)A (b) Co−PVI−GaP(111)A (c) PVI−GaP(111)B and (d) Co−PVI−GaP(111)B. Circles represent the spectral data. Solid and dash lines are background (gray), component (red or blue), and overall (black) fits to the experimental data.

as anticipated for the 1:1 ratio of amine and imine nitrogens associated with imidazole units on the PVI graft (Figure 2a,c). This is in sharp contrast to spectra reported on GaP samples following grafting of PVP where only a single nitrogen feature is observed at 398.7 eV.34−37 Survey XP spectra of Co−PVI−GaP samples confirm the presence of additional Co and Cl elements associated with attached cobaloximes (SI Figures S20−S21). Analysis of the Co and Cl spectral intensities obtained from core level XP spectra, following the application of appropriate sensitivity factors, yields a measured Co:Cl ratio of 1:1. However, the measured Co:Cl ratio using a Co(dmgH2)(dmgH)Cl2 powder sample is 1:2. These ratios are consistent with the proposed attachment mechanism invoking a base-promoted conversion of the dimethylglyoxime ligand of Co(dmgH2)(dmgH)Cl2 to a dimethylglyoximate monoanion and the associated replacement of one of the axial X-type chloride ligands with an L-type nitrogen ligand on the PVI−polymer brush. Additional structural information is provided by comparison of the N 1s and Co 2p core level spectra of Co−PVI−GaP samples which yields a Co:N spectral intensity ratio of 1:11 and 1:10 on the (111)A and (111)B modified surfaces, indicating that 28%− 32% of the imidazole units on the polymer graft are coordinated to a Co center provided there is an evenly distributed loading of Co sites along the ≈6 nm depth polymer coat. Further, unlike the N 1s core level spectra obtained for PVI−GaP surfaces which show only two distinct nitrogen features with approximately equal spectral intensity, N 1s core level spectra of Co−PVI−GaP samples can be fit with three additional nitrogen contributions assigned to imine, amine, and glyoximate nitrogens coordinated to the Co centers of the cobaloximes (Figure 2b,d). As measured in previous reports,34−37 coordination of cobalt centers to a surface-grafted PVP−polymer brush is associated with displacement of the N 1s feature assigned to attached pyridyl nitrogens to higher

Co−PVI−GaP surfaces indicate that minimal to no residual Co(dmgH2)(dmgH)Cl2 remains following ultrasonic cleaning of the samples (see Methods). In addition, the close alignment of the spectral features of Co−PVI−GaP samples with those obtained for powders of the model cobaloxime compound indicates that that surface-assembled cobaloxime species have a vibrational structure similar to that of the non-surface-attached model compound. Sample analysis using X-ray photoelectron spectroscopy (XPS) provides further evidence of successful surface functionalization. Survey XP spectra of PVI−GaP samples show the presence of additional O, N, and C elements (SI Figures S12−S13) as compared to spectra of unfunctionalized GaP. We also note that, in contrast to the Ga 2p and P 2p core level XP spectra of untreated wafers with a native oxide layer (SI Figures S8−S11), the spectral features associated with gallium oxide and phosphate are significantly reduced following the H2SO4 etch and subsequent polymer grafting, despite the samples being exposed to air following the polymer-grafting step (SI Figures S14−S17). For the untreated GaP(111)A and GaP(111)B wafers the Ga 2p3/2 spectral intensity ratios, AGa−O/ AGa−P, are 0.08 and 0.49, and the P 2p spectral intensity ratios, AP−O/AP−Ga, are 0.04 and 0.12, respectively. By contrast, inspection of the XP core level spectra for PVI−GaP surfaces show significant attenuation of substrate signals due to the presence of the grafted molecular layer. On the PVI−GaP Aface, the AGa−O/AGa−P ratio is 0.06 and there is no detectable phosphate feature. On the PVI−GaP B-face, both the Ga signal and phosphate feature are unapparent. In the C 1s core level XP spectra of the polymer-functionalized wafers, a shakeup satellite, appearing 6 eV higher in binding energy than the C 1s feature centered at 284.8 eV (SI Figures S18−S19), is indicative of π/π* bonding of aromatic polymers. Lastly, the N 1s core level XP spectra of PVI−GaP samples show two features centered at ∼400 and 398 eV with near-equal spectral intensity 10041


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ACS Applied Materials & Interfaces binding energies. For the Co−PVI−GaP samples, the relative spectral contributions used to obtain the fit (imine:amine:Co− imine:Co−amine:Co−glyoximate) are consistent with the cobaloxime:imidazole unit loading ratios estimated from the Co:N spectral intensities (SI Table S2). Finally, the Co 2p core level spectra of Co−PVI−GaP samples show peaks centered at 780 eV (2p3/2) and 795 eV (2p1/2), a 2:1 branching, and no apparent shakeup satellite structures, all features indicative of the Co3+ species (Figure 3).91,92 Detailed spectra of all functionalized samples as well as untreated GaP wafers are available as SI Figures S6−S23.

Figure 4. Linear sweep voltammograms of (a) GaP(111)A and (b) GaP(111)B working electrodes following polyvinylimidazole grafting (blue) and cobaloxime attachment (red). All measurements were performed at pH 7 in the dark (dashed-dotted lines) or under 100 mW cm−2 illumination (solid lines).

measured at 0 V vs RHE are significantly higher than those achieved testing unmodified or polymer-only treated electrodes with the PVI−GaP electrodes having a current density of 0.32 ± 0.04 and 0.30 ± 0.08 mA cm−2 and the Co−PVI−GaP electrodes having a current density of 0.89 ± 0.02 and 0.89 ± 0.03 mA cm−2 for the A- and B-faces, respectively (Table 1).

Figure 3. Co 2p core level spectra of (a) Co−PVI−GaP(111)A and (b) Co−PVI−GaP(111)B. Circles represent the spectral data. Solid lines are background (gray) and component (black) fits to the experimental data.

Table 1. Open-Circuit Photovoltages and the Photocurrent Densities Measured at 0 V vs RHE for Unmodified, PolymerGrafted, and Cobalt-Functionalized Gallium Phosphide Wafers

Photoactivity of the electrode assemblies is assessed via electrochemical testing in aqueous solutions buffered at pH 7 (0.1 M phosphate buffer) in the dark and upon illumination using a 100 mW cm−2 solar simulator (see Methods for details). For the (111) samples reported here, all A- and B-face constructs are prepared using wafers cut from the same ingot resulting in samples that achieve a similar saturating photocurrent when under equal illumination conditions, thus simplifying the comparison of results obtained using the photoelectrodes. We note that the doping conditions of these wafers as well as the light source used for illumination differ from previous reports from our group34−37 and that both of these factors can affect the maximum attainable current density (SI Figure S27). Figure 4 shows the three-electrode polarization curves obtained using surface-modified GaP working electrodes (see Methods for testing details and SI Figure S26 for data collected using unmodified GaP working electrodes). For the cobaloxime-modified electrodes, the photocurrents

sample GaP(111)A PVI−GaP(111)A Co−PVI−GaP(111)A GaP(111)B PVI−GaP(111)B Co−PVI−GaP(111)B

Voc (V vs RHE) 0.02 0.56 0.64 0.02 0.58 0.65

± ± ± ± ± ±

0.02 0.02 0.02 0.06 0.04 0.02

J at 0 V vs RHE (mA cm−2) 0.22 0.32 0.89 0.22 0.30 0.89

± ± ± ± ± ±

0.06 0.04 0.02 0.06 0.08 0.03

Consistent with the increased photoactivity of the catalystmodified electrodes, the measured open-circuit photovoltages of the PVI−GaP electrodes are 0.56 ± 0.02 and 0.58 ± 0.04 V vs RHE for the A- and B-faces, respectively; with an increase to 0.64 ± 0.02 V vs RHE following cobaloxime-modification of Aface electrodes and to 0.65 ± 0.02 V vs RHE following cobaloxime-modification of B-face electrodes (Table 1). The 10042


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was reported by Artero and co-workers using an analogous cobaloxime species surface grafted to an electroactive carbon support polarized at −0.59 V vs RHE,50 and a TOF of 1.9/s was reported by Turner et al.43 using a cobaloxime catalyst assembled on a TiO2-coated GaInP2 semiconductor poised at 0 V vs RHE.

minimum sample size of each photocathode type reported (GaP, PVI−GaP, and Co−PVI−GaP) included testing of three distinct electrode assemblies. We note that the polymer-only functionalized electrodes show a slightly greater deviation in their current−voltage responses as compared to results obtained using Co−PVI−GaP assemblies especially in the Bface samples. It has previously been shown that the B-face of GaP(111) is more prone to residual oxide coverage following initial etching and cleaning, 93 which may provide an explanation for the slightly increased variability in the data reported using PVI-GaP(111)B electrodes. However, following cobaloxime attachment the electrodes prepared on either face show nearly identical current and voltage responses with minimal variability of results between individually fabricated electrodes. For all catalyst-modified photocathodes reported here, hydrogen production is confirmed via gas chromatography analysis of the headspace, showing near-unity Faradaic efficiency (Figure 5). Thus, the photocurrent response and

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CONCLUSIONS

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METHODS

In conclusion, we report successful assembly of cobaloxime catalysts on both crystal faces of gallium phosphide (111) by employing a vinylimidazole polymer brush interface. Structural analysis of the reported assemblies is performed using spectroscopic ellipsometry, GATR-FTIR and XPS. Threeelectrode PEC testing of the modified cathodes shows significant improvement of the photoperformance for hydrogen production of GaP electrodes following polymeric cobaloxime modification, obtaining a current density of ≈1 mA cm−2 when polarized at 0 V vs RHE. Our measurements indicate that a slightly higher loading of catalysts can be achieved via functionalization of the B-face with respect to results obtained on A-face-modified samples. Yet, the photoelectrochemical testing of samples using A- or B-face modification show nearly identical photoperformance under the conditions tested, illustrating that factors other than catalyst loading can limit photoactivity. We also note that previous light intensity dependence measurements obtained using Co−PVP−GaP samples show a near-linear response of the current measured at 0 V vs RHE,34 indicating that, in such hybrid constructs, photocarrier transport to the interface may in part limit the performance of the Co-modified GaP photocathodes. Although the mechanistic details of vinyl-group attachment chemistry are not settled,17,95−100 molecular binding appears to occur over bridging oxygen atoms on GaP(100) surfaces.40 For the GaP(111) constructs reported in this work, we speculate that a similar chemistry occurs on the A and B crystal faces, consistent with the catalysts−polymer loadings and photoelectrochemical performance achieved using these substrates as well as the analysis of surface oxygen content performed prior to and following surface functionalization. Efforts to further analyze the loading, mesoscale architecture, surface energetics (thermodynamics), and activity of catalysts (kinetics) are underway. Nonetheless, we demonstrate that the attachment chemistry reported here is not limited to a specific crystal face of gallium phosphide, a promising feature regarding the applicability of this attachment strategy across varying crystal face orientations.

Figure 5. Gas chromatogram of a 5 mL aliquot of the headspace drawn from a sealed photo-electrochemical cell using a Co−PVI− GaP(111)B working electrode polarized at 0 V vs RHE (pH = 7) before (dashed-dotted line) and after (solid line) 10 min of illumination at 100 mW cm−2. A similar gas chromatogram was obtained using a Co−PVI−GaP(111)A electrode (data not shown).

associated rate of hydrogen production at both the A- and Bfaces of cobalt-functionalized GaP(111) wafers are nearly identical. Our measurements also show that the photocurrent over an hour is relatively stable, with a decrease of less than 1% over 55 min following an initial drop off of up to 12% during the first 5 min of PEC testing (SI Figure S28). Previous computational and experimental studies show that redox cycling of cobaloxime solutions can result in the disassociation of N-containing ligands from cobalt centers. We speculate that the loss of current during the initial 5 min of PEC testing of the cobaloxime-modified assemblies reported here may in part be due to loss of loosely bound catalysts located at electrolyteexposed edges of the polymer that are not protected by the encapsulating environment of the polymer.34−37 The data obtained from ellipsometry, XPS, and PEC measurements allow an estimate94 of the TOF of the Co− PVI−GaP construct per cobalt center. For example, an electrode operating at a 0.9 mA cm−2 current density (a value representative of that measured using Co−PVI−GaP electrodes polarized at 0 V vs RHE) with a 6 nm PVI graft and 30% attachment to cobaloxime catalysts yields a TOF of 7,000/ h per cobalt center, assuming that all Co are photoelectrochemically active. For comparison, a TOF of 8,000/h

Materials. Single crystalline p-type gallium phosphide wafers were purchased from University Wafers. The material was single side polished to an epi-ready finish. The transmission spectra, relative specular reflectance, and diffuse reflectance spectra are included as SI Figure S1. The p-type GaP(111)A wafers have a resistivity of 0.046 Ω cm, a mobility of 50 cm2 V−1 s−1, and a carrier concentration of 2.7 × 1018 cm−3. The etch pit density was less than 9.5 × 104 cm−2. The ptype Zn-doped GaP(111)B(±0.5°) wafers have a resistivity of 0.054 Ω cm, a mobility of 46 cm2 V−1 s−1, and a carrier concentration of 2.5 × 1018 cm−3. The etch pit density was less than 2.0 × 104 cm−2. Synthesis. All syntheses were carried out under an argon atmosphere using Schlenk techniques unless otherwise stated. All reagents were purchased from Aldrich. Isopropanol was cleanroom grade and used as received. Dichloromethane, methanol, and vinylimidazole were freshly distilled before use. All solvents were 10043


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ACS Applied Materials & Interfaces stored over the appropriate molecular sieves prior to use. Milli-Q water (18.2 MΩ cm) was used to prepare all aqueous solutions. Co(dmgH2)(dmgH)Cl2. This complex was prepared according to a previously reported procedure.101 Co(dmgH)2(meIm)Cl. A suspension of Co(dmgH2)(dmgH)Cl2 (1.1 g, 3.0 mmol) and N-methylimidazole (0.58 g, 7.0 mmol) was vigorously agitated in 60 mL of chloroform. After 10 min, water (30 mL) was added and the mixture was stirred for 2.5 h in open air. The mixture was filtered and washed with 60 mL portions of water using a separatory funnel until the aqueous layer was clear. The organic layer was concentrated under reduced pressure and precipitated with the addition of ethanol. The crude product was recrystallized from dichloromethane and ethanol yielding a brown crystalline solid (45% yield). 1H NMR (400 MHz, CDCl3): δ 2.39 (12H, s, CH3), 3.60 (3H, s, ArCH3), 6.66−6.67 (1H, m, J = 1.60 Hz ArH), 6.73−6.74 (1H, m, J = 1.47 Hz, ArH), 7.27 (1H, s, ArH), 18.43−18.51 (2H, brs, OH). UV−vis (CH2Cl2): 252, 297, 355 nm. The absorbance spectrum, FTIR spectrum and NMR spectra are included as SI Figures S2, S3, and S5, respectively. Wafer Preparation. All wafers were initially degreased with an acetone-soaked cotton swab. GaP(111) wafers were further cleaned using consecutive ultrasonic treatments in a series of solvents (acetone, 1 min; methanol, 1 min; dichloromethane, 30 s; methanol, 1 min; and water, 1 min), followed by a 30 s etch in concentrated sulfuric acid.93 Unmodified wafers used for XPS analysis were thoroughly degreased but not acid-etched. Wafer Functionalization. The freshly etched wafers were put into an argon-sparged solution of N-vinylimidazole and exposed to 254 nm UV light for 2 h. After thoroughly rinsing with methanol the wafers were dried under N2 and stored under vacuum. Cobaloxime functionalization was achieved by covering the PVI−GaP wafers with an argon-degassed solution of Co(dmgH)2(dmgH)Cl2 and triethylamine (1 mM in each) in methanol and allowing them to react overnight. The wafers were rinsed with methanol, then ultrasonically treated in methanol for 1 min, followed by rinsing with isopropanol, and drying under N2 then vacuum. Electrode Fabrication. GaP working electrodes were fabricated by applying an indium−gallium eutectic (Aldrich) to the backside of a wafer and then fixing a copper wire to the back of the wafer using a conductive silver epoxy (Circuit Works). The copper wire was passed through a glass tube, and the wafer was insulated and attached to the glass tube with Loctite 615 Hysol Epoxi-patch adhesive. The epoxy was allowed to fully cure before testing the electrodes. Instrumentation. UV−Vis Spectroscopy. Ultraviolet−visible (UV−vis) optical spectra were recorded on a Shimadzu SolidSpec3700 spectrometer with a D2 (deuterium) lamp for the ultraviolet range and a WI (halogen) lamp for the visible and near-infrared regions. Transmission and reflectance measurements were performed with an integrating sphere. NMR Spectroscopy. Nuclear magnetic resonance (NMR) spectra were recorded on a Varian NMR spectrometer operating at 400 MHz. Unless otherwise stated, all spectra were collected at room temperature. FTIR Spectroscopy. Grazing angle attenuated total reflection Fourier transform infrared spectroscopy (GATR-FTIR) was performed using a VariGATR accessory (Harrick Scientific) with a Ge crystal plate installed in a Bruker Vertex 70. Samples were pressed against the Ge crystal to ensure effective optical coupling. Spectra were collected using 256 scans under a dry nitrogen purge with a 4 cm−1 resolution, GloBar MIR source, a broadband KBr beamsplitter, and a liquid nitrogen cooled MCT detector. Background measurements were obtained from the bare Ge crystal, and the data were processed using OPUS software. GATR measurements were baseline-corrected using an ATR correction for a germanium crystal and a sample with a refractive index of 1.43. Spectra from model compounds in pressed KBr pellets were acquired with the same settings but using transmission mode. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS) was performed using a monochromatized Al Kα source (hν = 1486.6 eV), operated at 63 W, on a Kratos system at a takeoff

angle of 0° relative to the surface normal and a pass energy for narrow scan spectra of 20 eV, at an instrument resolution of approximately 700 meV. Survey spectra were collected with a pass energy of 150 eV. Spectral fitting was performed using Casa XPS analysis software. Spectral positions were corrected by shifting the primary C 1s core level position to 284.8 eV, and curves were fit with quasi-Voigt lines following Shirley background subtraction. All N 1s fits were constrained to a full width at half-maximum (fwhm) value of less than 1.3 eV. Electrochemistry. Cyclic voltammetry was performed with a Biologic potentiostat using a glassy carbon (3 mm diameter) disk, a platinum counter electrode, and a silver wire pseudoreference electrode in a conventional three-electrode cell. Anhydrous acetonitrile (Aldrich) was used as the solvent for electrochemical measurements. The supporting electrolyte was 0.10 M tetrabutylammonium hexafluorophosphate. The solution was deoxygenated by bubbling with argon. The working electrode was cleaned between experiments by polishing with alumina (50 nm diameter) slurry, followed by solvent rinses. The potential of the pseudoreference electrode was determined using the ferrocenium/ferrocene redox couple as an internal standard and adjusting to the normal hydrogen electrode (NHE) scale (with E1/2 taken as 0.40 V vs NHE). Photoelectrochemistry. Photoelectrochemical (PEC) testing was performed using 100 mW cm−2 illumination from a 100 W Oriel Solar Simulator equipped with an AM 1.5 filter (SI Figure S29). Linear sweep voltammetry and three-electrode electrolysis (chronoamperometry) were performed with a Biologic potentiostat using a platinum coil counter electrode, a Ag/AgCl, NaCl (3 M) reference electrode (0.21 V vs NHE), and GaP working electrodes (including GaP following acid etch, PVI-grafted GaP, and cobaloxime-modified GaP) in a modified cell containing a quartz window. The supporting electrolyte was 0.1 M phosphate buffer (pH 7). Linear sweep voltammograms were recorded at sweep rates of 100 mV s−1 under a continuous flow of 5% hydrogen in nitrogen. Open-circuit photovoltages were determined by the zero current value in the linear sweep voltammograms. Chronoamperometry was performed with the working electrode polarized at −0.610 V vs Ag/AgCl, slightly positive of 0 V vs RHE, where Evs RHE = Evs NHE + (0.05916 V) (pH) (V) = Evs Ag/AgCl + (0.05916 V) (pH) + 0.21 V. GC Analysis. Gas analysis was performed via gas chromatography (GC) using an Agilent 490 Micro GC equipped with a 5 Å MolSieve column at a temperature of 80 °C and argon as the carrier gas. Gas samples were syringe-injected using 5 mL aliquots of headspace gas collected with a gastight Hamilton syringe from a sealed PEC cell both prior to and following 10 min of three-electrode photoelectrolysis using a cobaloxime-modified working electrode polarized at 0 V vs RHE. Prior to the experiment the cell was purged for 30 min with argon before sealing. The retention time of hydrogen was confirmed using a known source of hydrogen obtained from a standard lecture bottle containing a hydrogen and argon mixture. Ellipsometry. Film thickness (FT) was determined using a J. A. Woollam variable angle spectroscopic ellipsometer with a spectral range of 200−1200 nm. Measurements were at 70°, 75°, and 80° incidence angles, and analysis was done using VASE software. The model used to determine the FT of surface-grafted polyvinylimidazole was composed of a GaP substrate layer (Aspnes), a GaP oxide layer (Zollner) with a fixed thickness of 1.01 nm, and a Cauchy layer for the polyvinylimidazole film of each sample. The Cauchy coefficients for GaP(111)A were A = 1.414 and B = −0.034, and the FT of the Cauchy layer was determined to be 6.2 ± .2 nm with an MSE of 8.356. The Cauchy coefficients for GaP(111)B were A = 1.371 and B = 0.008, and the FT of the Cauchy layer was determined to be 6.5 ± .2 nm with an MSE of 3.745. In this analysis, the thickness of the oxide layers is based on ellipsometry measurements performed on unfunctionalized GaP(111)A and GaP(111)B surfaces. However, the vast majority of the oxide is likely formed during transportation and handling of the sample in air (up to 10 min) following the etching process and subsequent ellipsometry measurement. The process of photografting and polymerizing N-vinylimidazole onto freshly etched GaP, under inert conditions, should give an oxide layer much thinner than the 10044


Research Article

ACS Applied Materials & Interfaces average oxide FT of 1.01 nm; thus these values provide an upper limit of any oxide formation on polyvinylimidazole-grafted GaP.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01557. UV−vis absorbance measurements, NMR data, FTIR data, XPS data, electrochemical data, and ellipsometry data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

This material is based upon work supported through the College of Liberal Arts and Sciences at Arizona State University and Biodesign Institute Center for Applied Structural Discovery (CASD). A.M.B. was supported by an IGERT-SUN fellowship funded by the National Science Foundation (Award 1144616). S.I.J. was supported by the Arizona State University College of Liberal Arts and Sciences (CLAS) Undergraduate Summer Enrichment program. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the use of facilities within the LeRoy Eyring Center for Solid State Science at Arizona State University. We thank Timothy Karcher for assistance with XPS experiments, and Diana Convey for assistance with ellipsometry measurements. NMR studies were performed using the Magnetic Resonance Research Center at Arizona State University.

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ABBREVIATIONS dmgH = dimethylglyoximate monoanion dmgH2 = dimethylglyoxime meIm = N-methylimidazole FT = film thickness GATR-FTIR = grazing angle attenuated total reflectance Fourier transform infrared spectroscopy NHE = normal hydrogen electrode PEC = photoelectrochemical PVI = polyvinylimidazole PVP = polyvinylpyridine RHE = reversible hydrogen electrode TOF = turnover frequency XPS = X-ray photoelectron spectroscopy

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Solar Hydrogen Production Using Molecular Catalysts Immobilized on

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