Chemistry at the Interface: Polymer-Functionalized GaP

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Chemistry at the Interface: Polymer-Functionalized GaP Semiconductors for Solar Hydrogen Production Anna Mary Beiler, Diana Khusnutdinova, Samuel I. Jacob, and Gary F. Moore Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00478 • Publication Date (Web): 18 Apr 2016 Downloaded from http://pubs.acs.org on April 20, 2016

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Industrial & Engineering Chemistry Research

Chemistry at the Interface: Polymer-Functionalized GaP Semiconductors for Solar Hydrogen Production 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, AZ 85287-1604, United States

ABSTRACT: New opportunities for organizing and controlling molecular components arise with the use of a stabilizing organic layer composed of grafted polymer chains at a semiconductor surface. We highlight recent advances in our research efforts to use polymer brush coatings containing pendent ligands that can be used to used to direct and assemble molecular catalysts for fuel production to visible-light-absorbing substrates. We illustrate how the polymeric interface can be varied to control the structure and photoelectrochemical response of gallium phosphide (100) electrodes containing surface-immobilized pyridyl or imidazole ligands with attached cobaloxime catalysts for hydrogen production. Surface sensitive spectroscopic methods, including X-ray photoelectron spectroscopy, grazing angle total reflectance Fourier transform infrared spectroscopy, and ellipsometry provide structural information regarding the nanoscale molecular connectivity and mesoscale dimensions of the cobaloxime-containing polymer grafts. At the macroscale, three-electrode photoelectrochemical testing of the cobaloximemodified electrodes under simulated solar lighting conditions in pH neutral aqueous solutions show up to a three-fold increase of hydrogen production as compared to results obtained using polymer-grafted electrodes without attached cobaloximes tested under nearly identical conditions.

1. INTRODUCTION Controlling matter and information at the nanoscale 1 presents a challenge for science and the imagination. In this vein, integrated materials have been developed using a range 2-11 and recent of surface functionalization chemistries advances in polymer chemistry offer opportunities to tailor 12-20 their structure and macromolecular properties. Herein, we present and discuss results from our laboratory using molecular science to build photoactive materials that include applications to artificial photosynthesis as well as emerging 21-50 solar-to-fuels technologies. The constructs exploit the properties of olefins to undergo self-initiated photografting and photopolymerization at semiconductor surfaces as well as a variety of other hydrogen- or hydroxyl-terminated 51-54 Post-polymerization modification is then used materials. to direct and assemble molecular catalysts onto built-in recognition sites –ligands- on the surface-attached polymer. Thus, the polymeric brush serves as a structural interface between the soft molecular components and the hard semiconductor surface. The catalysts used in this work belong to the cobaloximes, a class of compounds that are well studied and have shown promise in electrocatalysis as 55–69 well as solar-to-fuel applications. Previous reports show that cobaloximes can be assembled onto GaP via a polyvinylpyridine (PVP) interface, resulting in a cathode material that uses photonic energy, and no

external electrochemical forward biasing or sacrificial electron donors, to generate a relatively stable current and associated rate of hydrogen production in pH neutral 42-45 aqueous solutions. In addition, treatment of PVPfunctionalized GaP using cobaloxime catalyst precursors with a ligand macrocycle modified at the molecular level has been shown to affect the photoelectrochemical (PEC) response to illumination and pH observed at the construct 44 Thus, the photoactivity of the modified level. semiconductor can be influenced by changes to the ligand environment of the attached cobalt complexes. We now show that the chemical identity of the polymeric functional groups used to assemble the cobaloximes to the polymer graft provide an additional design parameter for controlling the fuel production properties of the overall assembly. The polyvinylimidazole (PVI) grafts used in this work are prepared in a manner analogous to those previously reported using PVP grafts. To facilitate direct comparisons between the PVI- and PVP-functionalized materials in the work reported here, PVI and PVP grafts were assembled on GaP(100) wafers with identical physical properties including doping conditions, resistivity, mobility, carrier concentration and etch pit density (i.e. the wafers are cut from the same ingot). Structural characterization of the modified wafers is achieved using X-ray photoelectron spectroscopy (XPS), grazing angle attenuated total reflection Fourier transform

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Scheme 1: Schematic Representation of the Attachment Method Used to Assemble Cobaloxime-Modified Photocathodes

infrared spectroscopy (GATR-FTIR) and ellipsometry. PEC testing, including gas chromatography (GC) analysis of photochemical products, verifies the functionality of the hybrid constructs. 2. EXPERIMENTAL METHODS 2.1 Materials Single crystalline p-type gallium phosphide wafers were purchased from University Wafers. The material is single side polished to an epi-ready finish. The p-type Zndoped GaP(100) wafers have a resistivity of 0.2 Ω cm, a 2 -1 -1 mobility of 66 cm V s , and a carrier concentration of 4.7 x 17 -3 4 -2 10 cm , with an etch pit density of less than 8 x 10 cm . Preparation of all wafers began with initial degreasing with an acetone-soaked cotton swab followed by consecutive ultrasonic cleaning in acetone and isopropanol for 5 minutes each followed by drying under a nitrogen stream. The wafers were then exposed to an air-generated plasma (Harrick Plasma, USA) at 30 W for 2 minutes. Finally the wafers were etched in buffered hydrofluoric acid (6:1 HF:NH 4 F in H 2 O) for 5 minutes followed by rinsing with distilled methanol and drying under a nitrogen stream then storing under vacuum (-750 Torr). XPS analysis of the unmodified GaP wafers was performed using substrates that were thoroughly degreased and plasma cleaned but not acid-etched. All reagents were purchased from Aldrich. Isopropanol was cleanroom grade and used as received. Dichloromethane, methanol and vinylimidazole were freshly distilled before use. Methanol was stored over the appropriate molecular sieves prior to use. Milli-Q water (18.2 MΩ cm) was used to prepare all aqueous solutions. Synthesis and characterization methods of the cobaloxime precursor Co(dmgH 2 )(dmgH)Cl 2 as well as the model cobaloxime catalysts Co(dmgH) 2 (Py)Cl and Co(dmgH) 2 (meIm)Cl is available as SI Figures S1-S4. 2.2 Wafer Functionalization The freshly etched wafers were placed into a quartz flask containing the appropriate argon-sparged monomer (N-vinylimidazole or 4vinylpyridine) and purged with argon for 15 minutes, after which they were placed into a UV chamber and irradiated 70-72 Following the with shortwave (254 nm) light for 2 hours. UV-induced attachment, the wafers were ultrasonically cleaned in freshly distilled methanol and dried under a nitrogen stream then stored under vacuum (-750 Torr). Cobaloxime functionalization was achieved using wetchemical treatment of the polymer-modified substrate with a methanolic solution of Co(dmgH 2 )(dmgH)Cl 2 (1 mM) and 1 mM triethylamine (TEA). After 12 hours, the wafers were removed from the reaction mixture, ultrasonically cleaned in freshly distilled methanol, dried under a nitrogen stream and stored under vacuum (-750 Torr).

2.3 Wafer Characterization XPS was performed using a monochromatized Al Kα source (hν = 1486.6 eV) operated at 63 W with a takeoff angle of 0° relative to the surface normal and a pass energy for narrow scan spectra (40 scans) of 20 eV at an instrument resolution of 700 meV. Survey spectra were collected with a pass energy of 150 eV. A minimum of 2 samples was analyzed for each sample. We also note that these loadings and peak positions are consistent with 42-45,48 Spectral fit was analyzed using CASA previous work. software and all spectra were calibrated by adjusting adventitious carbon to 284.8 eV. Curves were fitted using a linear background for polymers and Shirley background for cobaloxime-functionalized films. N 1s components were constrained to a full-width half maximum (fwhm) of 1.3 eV. 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. A minimum of 4 individual wafers was tested for each sample. Samples were pressed against the Ge crystal to ensure effective optical coupling. Spectra were collected using 256 −1 scans under a dry nitrogen purge at 4 cm resolution with a 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 processed using OPUS software. GATR measurements were baseline corrected for rubberband scattering. Film thickness (FT) was determined using a J.A. Woollam Variable Angle Spectroscopic ellipsometer with a spectral range of 200 nm-1200 nm. Measurements were taken at 69⁰ and 74⁰ incidence angles and analysis performed using Complete EASE software. The model used to determine the FT of surface-grafted polyvinylimidazole was composed of a GaP substrate layer (Palik), a GaP oxide layer (Zollner) with a fixed thickness of 1.01 nm and a Cauchy layer for the polymer film of each sample. 2.4 Electrode fabrication GaP working electrodes were fabricated by applying an indium gallium eutectic (Aldrich) to the backside of a wafer, 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. 2.5 Photoelectrochemical Testing Three-electrode PEC −2 testing was performed using 100 mW cm illumination from a 100 W Oriel Solar Simulator equipped with an AM1.5 filter.

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Three-electrode linear sweep voltammetry and bulkelectrolysis experiments (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, polymer-grafted GaP and cobaloximemodified GaP) in a custom PEC cell containing a quartz window. The supporting electrolyte was 0.1 M phosphate buffer (pH 7). Linear sweep voltammograms were recorded −1 at sweep rates of 100 mV s 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 at -0.610 V vs. Ag/AgCl, slightly positive of 0 V vs. RHE, where E vs. RHE = E vs. NHE + 0.05916 V x pH = E vs. Ag/AgCl + 0.05916 V x pH +0.21 V. 2.6 Hydrogen Detection 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 using argon as the carrier gas. Gas samples were syringe-injected using 5 mL aliquots of headspace gas collected with a gas-tight Hamilton syringe from a sealed PEC cell both prior to and following 60 min of threeelectrode 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 calibrated source obtained from a standard lecture bottle containing a hydrogen and argon mixture. 3. RESULTS AND DISCUSSION 3.1 Wafer Preparation and Characterization A two-step method is used to assemble catalysts at the semiconductor surface. First, UV-induced grafting of the appropriate polymer to the GaP(100) surface yields the polyvinylpyridinegrafted GaP (PVP-GaP) or polyvinylimidazole-grafted GaP (PVI-GaP) constructs. In the second step, wet chemical treatment of the polymer-modified surface is used to assemble cobaloximes on the polymer graft. This chemical transformation exploits the base-promoted ligand exchange of axial X-type chloride ligands on cobalt catalyst precursors with pendent N-type ligands on the surface-grafted polymers, yielding cobaloxime-modified PVP-GaP (Co-PVPGaP) or cobaloxime-modified PVI-GaP (Co-PVI-GaP) (Scheme 1, see Methods for details). XPS analysis of the chemically modified surfaces provides evidence of successful functionalization with the desired molecular connectivity. Survey spectra of the PVP-GaP and PVI-GaP samples show C, N and O elements (SI Figure S8), and cobaloxime-functionalized samples include additional C, N, O, Cl and Co elements associated with attached cobaloximes (SI Figure S12). N 1s core level XP spectra of polymer-functionalized GaP surfaces (Figure 1) show a single nitrogen feature centered at 398.7 for PVP-GaP corresponding to the pyridyl nitrogens of the graft. Conversely, N 1s core level spectra of PVI-GaP surfaces exhibit two distinct nitrogen features centered at 398.3 and 400.2 and having a 1:1 spectral intensity ratio arising from the 73-75 with the two types of nitrogen environments in PVI, amine nitrogen feature at a higher binding energy than that assigned to the imine nitrogen.

higher binding energies in the N 1s core level spectra. Spectral fitting of the XP data (see Methods) facilitates assignment of the individual contributions (Figure 2 and Table 1). For Co-PVP-GaP, a spectral contribution at 400.6 eV is assigned to pyridyl nitrogens on the graft that are coordinated to cobalt centers and the contribution centered at 400.0 eV is indicative of the N 1s features of the cobaloxime glyoximate ligands. The remnant contribution at 398.6 eV is assigned to

Figure 1. N 1s core level XP spectra of (top) PVI-GaP and (bottom) PVP-GaP. The solid blue (b) and solid dark blue (c) lines represent the component fit of the imine nitrogen, and the dashed blue (a) line the component fit of the amine nitrogen. The circles represent the spectral data, the gray line is the linear background, and the black line the overall fit.

Figure 2. N 1s core level spectra of (top) Co-PVI-GaP and (bottom) Co-PVP-GaP. The solid blue (b) and solid dark blue (c) lines are the component fits for the unbound imine, the dotted blue line (a) the component fit for the unbound amine, the solid red (f) and solid dark red (h) lines the component fit for the bound imine, and the dotted red line (e) the component fit for the bound amine. The dashed red (d) and dashed dark red (g) line are the component fits for the glyoximate contributions. The circles represent the spectral data, the gray line is the Shirley background, and the black line the overall fit.

The assembly of cobaloximes on the PVP or PVI grafts gives rise to additional nitrogen signal contributions at


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Table 1: Summary of the peak positions of N 1s core level spectra shown in Figures 1 and 2.

Peak Assignments

Imine nitrogen

PVP-GaP Co-PVP-GaP PVI-GaP Co-PVI-GaP

398.7 398.6 398.3 398.4

Binding energy (eV) Amine Co-imine nitrogen nitrogen ---400.6 400.2 -400.0 399.0

Co-amine nitrogen ---400.6

Glyoximate nitrogen -400.0 -400.0

pyridine units on the polymer chains that are not coordinated to cobalt centers. For Co-PVI-GaP surfaces, assembly of Co centers to imidazole units on the polymer graft contribute to the components centered at 399.0 eV and 400.6 eV while those centered at 398.4 eV and 400.0 eV are assigned to remnant imidazole nitrogens on the polymer that are not coordinated to cobalt centers. The prominent peak at 400.0 eV is assigned to glyoximate nitrogens of the cobaloximes.

Methods) with 4-vinylpyridine or N-vinylimidazole show -1 distinct C-H and C-N modes in the 1400 to 1600 cm range indicative of successful polymerization, and unique from the C-H and C-N vibrations associated with monomeric 4vinylpyridine or N-vinylimidazole. (Figure 3, SI Table S2). -1 Additionally, a peak observed at 1453 and 1454 cm for PVPGaP and PVI-GaP samples, respectively, is indicative of the planar deformation vibration of the CH 2 groups along the 78 polymer chain backbone.

In our proposed mechanism of assembly and attachment of cobaloximes to PVI-GaP and PVP-GaP substrates, charge balance of the cobalt complex when replacing chloride, an anionic X-type ligand, with a neutral L-type ligand is accounted for by the base-promoted conversion of the dimethylglyoxime ligand to the dimethylglyoximate monoanion. As anticipated, the Co 2p core level spectra signal shows a 15 eV splitting and 2:1 ratio of the 2p 3/2 and 3+ 2p 1/2 peaks (SI Figure S14), consistent with the Co oxidation 76-77 In addition, analysis of the state of the grafted complex. Co and Cl spectral intensities obtained from core level XP spectra following the application of appropriate sensitivity factors indicate a Co:Cl ratio of 1:1 for both the Co-PVP-GaP and Co-PVI-GaP constructs. This ratio contrasts the 2:1 Co:Cl ratio obtained for the Co(dmgH 2 )(dmgH)Cl 2 precursor used to functionalize the polymer-modified GaP samples. This collective information regarding the elemental identity and chemical composition obtained from XPS analysis provides confirmation of our synthetic efforts.

Treatment of the polymer-grafted surfaces with a solution of the catalyst precursor, Co(dmgH 2 )(dmgH)Cl 2 , gives rise to new vibrational modes on the surface that are ascribed to the formation of intact cobaloxime catalyst. These modes are distinct from those observed in the FTIR spectrum of the Co(dmgH 2 )(dmgH)Cl 2 precursor but are nearly identical to

Surface analysis using GATR-FTIR spectroscopy provides further structural characterization of the modified GaP substrates, including direct observation of vibrational modes associated with the grafted polymers and intact cobaloxime

Figure 3. GATR-FTIR spectra of PVI-GaP (blue) and PVPGaP (dotted dark blue) grafted onto gallium phosphide, as well as unmodified GaP (dashed black). Spectra are normalized for comparison.

Figure 4. (a) FTIR transmission spectra of Co(dmgH) 2 Cl 2 in KBr (green), Co(dmgH) 2 (Py)Cl in KBr (dashed dark red), and Co(dmgH) 2 (meIm)Cl in KBr (red), as well as (b) GATR-FTIR absorbance spectra of Co-PVI-GaP (red), Co-PVP-GaP (dashed dark red) and GaP following a fresh HF etch (dashed black). Insets show the NO region. Spectra are normalized for comparison.

complexes. FTIR spectra following the photochemical treatment of freshly cleaned and etched GaP surfaces (see


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Industrial & Engineering Chemistry Research photocathodes were assembled using functionalized and unmodified GaP wafers and used as working electrodes in three-electrode potentiostatic measurements (see Methods section and SI Figures S16). All measurements were performed in aqueous solutions buffered at pH 7 (0.1 M phosphate buffer) in the dark and at 1 sun illumination using a 100 W solar simulator (SI Figure S21) with a sample size consisting of at least 4 individually tested electrodes. We note that the doping conditions of these wafers as well as the light source used for illumination differ from previous 42-45 and that both of these factors reports from our group, can affect the maximum attainable current density (SI Fig S17).

Figure 5. Spectra obtained from ellipsometry measurement of PVI-GaP at 69° and 74° (red) and model fit to data (dashed black). the vibrational modes observed in spectra of the model molecular catalysts Co(dmgH) 2 (Py)Cl or (Figure 4a). In particular, Co(dmgH) 2 (meIm)Cl measurements of the NO stretching frequency of the cobaloxime-functionalized surfaces provide direct spectroscopic evidence supporting assembly of the cobaloxime units at the pendent pyridyl or imidazole sites of the polymer graft. For the Co(dmgH 2 )(dmgH)Cl 2 precursor -1 complex the NO stretch occurs at 1225 cm , however for the and model cobaloximes Co(dmgH) 2 (Py)Cl Co(dmgH) 2 (meIm)Cl, this mode is centered at 1240 and 1237 -1 cm , respectively. FTIR spectra of the polymerfunctionalized GaP surfaces following wet chemical treatment with a solution of Co(dmgH 2 )(dmgH)Cl 2 show a -1 strong NO vibration at 1241 cm for Co-PVP-GaP and at 1240 -1 cm for Co-PVI-GaP (Figure 4b), values that closely match -1 those of the model catalysts within the 4 cm resolution of the spectroscopic scans. Additionally, there is no -1 pronounced peak at 1225 cm , indicating that minimal to no precursor complex remains in or on the cobaloximemodified GaP following ultrasonic cleaning of the samples. Thus FTIR analysis indicates effective assembly of cobaloxime units using the built-in recognition sites of the polymer grafts, in agreement with our surface analysis using XPS. Information on the film thickness (FT) of the PVP and PVI polymer grafts is provided by spectroscopic ellipsometry measurements, yielding a PVP thickness of 10 ± 1 nm and a PVI thickness of 6 ± 1 nm on the GaP(100) substrates (Figure 5). Using these FT values, the corresponding pyridyl and imidazole site density can be estimated given the bulk -3 -3 polymer density (1.15 g cm for PVP and 1.25 g cm for PVI) is representative of the solvent-free layer density. For a 10 nm thick solvent-free PVP layer the corresponding maximum 15 -2 -2 site density is 6.6 x 10 cm or 11 nmol cm . In the case of a 6 nm thick solvent-free PVI layer the maximum site density 15 -2 -2 is 4.8 x 10 cm or 8 nmol cm . We note that the larger FT of PVP grafts with respect to those obtained for the PVI grafts is consistent with previous reports of lower grafting concentrations reported for the UV polymerization of PVI 79 relative to PVP. The identity of the polymer graft thus can be used to control the physical characteristics at the mesoscale in addition to the chemical identity of the ligand sites at the nanoscale. 3.2 Photoelectrochemical Performance To evaluate the PEC performance of the assembled constructs,

Polarization curves obtained using the functionalized and unmodified GaP wafers are shown in figure 6. For the polymer-grafted GaP electrodes (PVP-GaP and PVI-GaP) a -2 photocurrent density of 0.68 ± 0.18 mA cm and 0.43 ± 0.02 -2 mA cm , respectively, is achieved for electrodes polarized at zero volts versus the reversible hydrogen electrode (0 V vs. RHE). For the cobaloxime-modified GaP electrodes polarized

Figure 6. Linear sweep voltammograms of PVP-GaP (dark blue), PVI-GaP (blue), Co-PVP-GaP (dark red), Co-PVI-GaP (red), and unmodified GaP (black) working electrodes at pH -2 7 in the dark (dotted lines) and with 100 mW cm illumination (solid and dashed lines).

Figure 7. Gas chromatography data obtained using 5 mL aliquots of the headspace collected before (dashed) and after -2 (solid) 60 minutes of illumination at 100 mW cm from a sealed photoelectrochemical cell using a Co-PVI-GaP working electrode polarized at 0 V vs. RHE (pH=7).


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Table 2: PEC Characteristics Construct

V oc (V vs RHE)

V on * (V vs RHE)

GaP(100) PVP-GaP Co-PVP-GaP PVI-GaP Co-PVI-GaP

0.57 ± 0.03 0.57 ± 0.01 0.61 ± 0.05 0.53 ± 0.05 0.58 ± 0.02

-0.04 ± 0.06 -0.07 ± 0.09 +0.24 ± 0.04 -0.14 ± 0.02 +0.07 ± 0.05

J at 0 V vs RHE (mA cm-2) 0.86 ± 0.01 0.68 ± 0.18 1.3 ± 0.1 0.43 ± 0.02 1.2 ± 0.1

*where Von is defined here as the potential required to achieve a 1 mA cm-2 current density -2

at 0 V vs. RHE the photocurrent density is 1.3 ± 0.1 mA cm -2 for PVP-GaP and 1.2 ± 0.1 mA cm for PVI-GaP (Table 2). In addition, the cobaloxime-modified samples are 80 characterized by positive shifts in V on as compared to those measured using the respective polymer-modified electrodes prior to cobaloxime attachment. For example, the cobaloxime modification of PVP-grafted electrodes yields a +310 ± 100 mV shift in V on . For PVI-grafted electrodes, cobaloxime modification yields a +210 ± 50 mV shift in V on as compared to polymer-only electrodes. Likewise, the V on of Co-PVP-GaP electrodes is 170 mV positive of that measured using Co-PVI-GaP electrodes (Figure 6 and Table 2). The open-circuit photovoltages (V oc ) of the electrodes are also consistent with the increased photoactivity of the electrodes following cobaloxime modification, with a V oc of 0.61 ± 0.05 V vs. RHE for Co-PVP-GaP and 0.58 ± 0.02 V vs. RHE for Co-PVI-GaP electrodes. In contrast, the polymeronly modified electrodes (i.e. prior to catalyst functionalization) have a V oc of 0.57 ± 0.01 V vs. RHE for the PVP-GaP electrodes and 0.53 ± 0.05 V vs. RHE for the PVI-GaP electrodes. For both Co-PVP-GaP and Co-PVI-GaP photocathodes, analysis of the headspace of the PEC cell by gas chromatography (Figure 7) after 60 minutes of bulk photoelectrolysis (chronoamperometry) performed at 0 V vs. RHE confirms the formation of H 2 gas with a near-unity Faradaic efficiency (see Methods). Electrochemical measurements using solutions of the cobaloxime model compounds (Co(dmgH) 2 (meIm)Cl and Co(dmgH) 2 (Py)Cl recorded in acetonitrile solutions (see Methods) show that structural modifications to the ligand environment (axial coordination to imidazole vs. pyridine) II I result in a 60 mV shift of the Co /Co redox couple to more negative potentials (SI Figure S2), illustrative of the ligand environment’s effect on the redox properties of the cobaloxime catalysts. For the cobaloxime-modified GaP substrates reported here, the differences in PEC performance in aqueous environments may in part be due to the inherent difference in activity of the cobaloximes in the grafted polymer environment (PVI vs. PVP). 3.3 Turnover frequency An estimate of the per cobalt center turnover frequency (TOF) of the cobaloximemodified electrodes can be arrived at using the following assumptions 1) the FT determined by ellipsometry is representative of the solvent-free thickness of the film, 2) there is uniform surface coverage of the graft and uniform distribution of cobaloximes along the depth of the polymer graft and 3) all cobalt centers are active and are the only sites of hydrogen production. Given the first and second set of conditions, cobalt loading of the films can be estimated

from the associated XPS and ellipsometry data. In the case of the Co-PVP-GaP construct, the spectral intensity ratio of the nitrogen contribution at 400.6 eV, assigned to pyridyl nitrogen coordinated to cobalt centers, and the remnant peak at 398.6 equate to a 30% loading of cobalt centers to pyridyl groups on the PVP graft. A similar analysis of spectral intensity ratio of the nitrogen contributions using Co-PVI-GaP samples indicates that 35% of the available imidazole sites on the PVI graft are associated with an attached cobaloxime. An analysis of the total Co:N ratios yields loadings that are consistent with those obtained from the spectral intensity ratio of the nitrogen contributions. If the cobaloximes are, however, not distributed evenly along the nitrogen sites of the grafted brushes but are instead concentrated toward the upper portions of the polymer (i.e. away from the underpinning semiconductor surface), then our analysis results in an overestimate of the loading due to signal attenuation of nitrogen sites located at lower depths along the polymer. Likewise, if solvent remains in the brush following cleaning and drying, then the calculated FT represents an upper limit on the site density. Using cobaloxime loadings obtained from the analysis described above, the current densities measured in threeelectrode polarization experiments using cobaloximemodified GaP electrodes give information on the activity of the electrodes per number of cobaloximes assembled on the polymer grafts. For example, the current density of 1.3 -2 mA cm , measured at 0 V vs. RHE and under 1 sun illumination, for the Co-PVP-GaP electrode yields a TOF of -1 2 2.1 s , and the current density of 1.2 mA/cm for the CoPVI-GaP electrode, measured under similar conditions, -1 gives a TOF of 2.4 s . This analysis represents the activity of the electrode per amount of cobalt present, regardless of the activity of individual centers. If not all cobaloxime centers are active or equally active, as could likely be the case for the inherently heterogenized molecular components reported here, our analysis could underestimate the TOF of individual cobaloxime sites. Conversely, if there are other sites of hydrogen production on the surface the TOF could be overestimated for individual cobaloxime sites. For comparison, a 1.0 mM solution Co(dmgH) 2 (Py)Cl at a graphite electrode poised at -0.90 V vs. Ag/AgCl in 1,2-dichlorethane in the presence of 0.2 M Et 3 NH(BF 4 ) (a sacrificial reductant) yielded ≈ 100 turnovers in 2.5 hours with no visible degradation of the 57 catalyst. Previous reports, however, indicate that immobilized catalysts can have significantly higher turnover ability compared to their solution counterpart. In a report by Artero and coworkers using cobalt diiminedioxime catalysts grafted onto carbon nanotubes polarized


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at -0.59 V vs. RHE in acetate buffer (pH 4.5) a TOF of ≈2.2 -1 81 s is reported. The authors speculate that immobilization of the catalyst prevents inactivation through dimerization and enhances electron transfer dynamics from the conductive electrode to the catalytic site. Similarly, a more recent report by Turner and coworkers assigns a TOF of 1.9 -1 -1 s to 3.4 s using an isonicotinic acid-modified cobaloxime catalysts adsorbed on a TiO 2 -coated GaInP 2 electrode operating at 0 V vs. RHE in a basic aqueous solution (pH 50 13).

constructs. This work illustrates the potential to control and optimize the properties of visible-light-absorbing semiconductors using polymeric overlay techniques coupled with the principles of synthetic molecular design. Key features of the construct include the use of polymerfunctionalized semiconductors, the assembly of molecular components at the polymeric interface, a modular architecture, and PEC operation in aqueous conditions (an imperative for the photoelectrochemical reduction of water).

CONCLUSION Cobaloxime catalysts for hydrogen production have been assembled on the surface of GaP(100) using both PVP and PVI grafts. The variation of the polymeric interface affects the photoelectrochemical performance of the constructs. Surface sensitive spectroscopic analyses verify the successful synthesis of cobaloximes on the polyvinylpyridine or polyvinylimidazole grafts using molecular assembly at recognition sites on the surfaceattached polymers. Photoelectrochemical testing of the cobaloxime-modified assemblies shows a marked increase in per geometric area hydrogen production rates as compared to results obtained using electrodes without cobaloxime functionalization. This work illustrates the versatility of the polymer graft as an interface for controlling structure at the nano and meso scales as well as functional performance at the macroscale as evidenced by PEC testing of the photocathodes assembled from the modified wafers. Although the precise attachment mechanism achieved through alkene moieties is not 3-5,82-85 molecular binding of PVP appears to occur settled 40 over bridging oxygen atoms on GaP(100) surfaces. For the PVI-GaP constructs reported in this work, we speculate that a similar surface attachment chemistry occurs, consistent with the analysis of surface oxygen content performed prior to and following surface functionalization of GaP(100) substrates. The imidazole- and pyridyl-grafted cobaloxime constructs are both capable of achieving a current density -2 greater than 1 mA cm when polarized at potentials -2 positive of RHE, achieving 1 mA cm at +0.24 V vs. RHE for the pyridyl-based construct and +0.07 V vs. RHE for the imidazole-based assembly. However, unlike the conditions at more anodic polarization, Co-PVP-GaP and Co-PVI-GaP electrodes polarized at 0 V vs RHE operate at more similar current densities (Table 2) due to the limiting photon flux under 1-sun illumination. Yet the FT obtained for the imidazole graft is ~60% that of the pyridyl graft and based on our loading estimates described in this report the CoPVI-GaP assembly contains ~80% the number of cobalt centers per geometric area, indicating that the C0-PVI-GaP assembly is more conservative in overall use of cobaloxime complexes as compared to the Co-PVP-GaP construct. This more effective use of cobalt is evidenced in the per-cobalt TOF analysis of the Co-PVP-GaP and Co-PVI-GaP -1 -1 photocathodes, estimated at 2.1 s and 2.4 s respectively for electrodes polarized at 0 V vs. RHE. Future work will focus on an improved understanding of surface coverage and distribution of cobaloximes along the polymer graft using angle-resolved XPS measurements, which should enable a refined model and assessment of the loadings and per-cobalt TOFs achieved using these hybrid

ASSOCIATED CONTENT Supporting Information Synthesis and characterization of the cobaloximes, XPS data, chronoamperometry scans, optical characteristics of GaP, spectral output of solar simulator, and a table of the FTIR peaks are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources 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.

ACKNOWLEDGMENT This contribution was identified by Amitav Sanyal (Bogazici University) as the Best Presentation in the session “Biological Inspiration for Environmental Sustainability: Bioinspired Approaches for Energy Conversion, Storage and Materials” of the 2015 ACS Fall National Meeting in Boston, MA. The authors gratefully acknowledge the use of facilities within the LeRoy Eyring Center for Solid State Science at Arizona State University and 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.

ABBREVIATIONS



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dmgH = dimethylglyoximate monoanion; dmgH 2 = dimethylglyoxime; meIm = N-methylimidazole; FT = film thickness; GATR-FTIR = grazing angle attenuated total reflectance- Fourier transform infrared spectroscopy; meIm = methylimidazole; NHE = Normal Hydrogen Electrode; py = pyridine; PEC = photoelectrochemical; PVI = polyvinylimidazole; PVP = polyvinylpyridine; RHE = Reversible Hydrogen Electrode; TOF = turnover frequency; XPS = X-ray photoelectron spectroscopy. REFERENCES

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TOC Figure


Chemistry at the Interface: Polymer-Functionalized GaP

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