H2 Photogeneration Using a Phosphonate-Anchored Ni-PNP Catalyst

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H2 Photogeneration Using a Phosphonate-Anchored Ni-PNP Catalyst on a Band-Edge-Modified p‑Si(111)|AZO Construct Hark Jin Kim,† Junhyeok Seo,† and Michael J. Rose* Department of Chemistry, University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *

ABSTRACT: We report the fabrication of a {semiconductor}| {metal oxide}|{molecular catalyst} construct for the photogeneration of dihydrogen (H2) under illumination, including band-edge modulation of the semiconductor electrode depending on the identity of Si(111)−R and the metal oxide. Briefly, a synergistic band-edge modulation is observed upon (i) the introduction of a p-Si|n-AZO heterojunction and (ii) introduction of an organic dimethoxyphenyl (diMeOPh) group at the heterojunction interface; the AZO also serves as a transparent and conductive conduit, which was capped with an ultrathin layer (20 Å) of amorphous TiO2 for stability. A phosphonate-appended PNP ligand and its Ni complex were then adsorbed to the p/n heterojunction for photoelectrochemical H2 generation (figures of merit: Vonset ≈ + 0.03 V vs NHE, Jmax ≈ 8 mA cm−2 at 60 mM TsOH). KEYWORDS: photoelectrochemical H2 evolution, band-edge modulation, Si(111) semiconductor, AZO, TiO2, Ni-PNP

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Al2O3, TiO2, and AZO (aluminum-doped zinc oxide). Such a heterojunction has been shown to stabilize the function of the device in the presence of water and to expand the anodic window of operation.19,22 Additionally, we have shown that the Si|metal-oxide interface can beneficially modulate the bandedge position, effectively shifting the onset potential (VOC) for H2 evolution to positive potentials.25 These insulating (Al2O3, amorphous TiO2) or semiconducting (anatase TiO2) layers can at the same time retain effective charge transfer within certain parameters (e.g., thickness, interface type, tunneling “target”, etc.).19,25 In this work, we sought to expand the scope of our photoelectrode|molecular-catalyst constructs to include such metal oxide interfaces. The device reported herein utilizes aluminum-doped zinc oxide (AZO), followed by grafting of the molecular catalyst to the metal oxide layer using a phosphonate-appended PNP (where PNP = Ph2PCH2N((C6H4)PO3H2)CH2PPh2) catalyst ligand. Such phosphonate anchors have been shown to be an effective means of stably attaching molecular species26 such as catalysts,7 sensitizers,27 and electronics devices28,29 to metal oxide surfaces. We have prepared the AZO layer via atomic layer deposition (ALD),30 thus allowing precise control of the doping, which maximizes electron transfer through the material. We also report on the critical role of the Si|metal-oxide interface, wherein the type of surface linker (R = CH3, aryl) modulates the efficacy of the p/n

he specter of decreasing fossil fuel supplies, increasing environmental impact, and increasing energy costs has motivated researchers to develop new ways of generating and storing energy derived from renewable resources.1 One of the areas receiving a high extent of attention is the direct solar → chemical energy conversion processes, such as 2H+ → H2 and CO2 → CHxOy.2 A number of researchers have pursued the attachment of H2-generating, molecular electrocatalysts to electrodes such as glassy carbon,3,4 carbon nanotubes,5 graphene6 and ITO.7 In related research, water oxidation molecular catalysts have been attached to dye-sensitized metal oxides such as TiO2 and SnO2.8,9 However, instances of covalent attachment or grafting of molecular catalysts to lightresponsive semiconductors remain sparse.10−17 Critical analyses of the solar spectrum−and consideration of the water oxidation counter-reaction−have demonstrated that small bandgap (1− 1.5 eV) semiconductors such as silicon, indium phosphide, and gallium arsenide are ideally suited to absorb the red and infrared light that can drive the 2H+ → H2 reaction.18 Previously, we reported the attachment and functionality of a DuBois-type PNP-nickel catalyst covalently tethered to a Si(111) photoelectrode, which afforded one of the early reports of functional semiconductor|molecular-catalyst constructs.10 However, the residual susceptibility of the semiconductor surface−in that case, protected by an organic monolayer of aryl and methyl groups−to corrosion processes (i.e., Si−R → SiOx) motivated us to include further protection of the atop Si sites. We19 and others20−24 have reported on the utility of interfacing Si photoelectrodes with ultrathin (5−50 Å) or thin (5−50 nm), respectively, layers of metal oxides−such as © 2016 American Chemical Society

Received: October 17, 2015 Accepted: December 23, 2015 Published: January 7, 2016 1061

DOI: 10.1021/acsami.5b09902 ACS Appl. Mater. Interfaces 2016, 8, 1061−1066

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ACS Applied Materials & Interfaces

Figure 1. (a) Sheet resistance for p-Si(111)−CH3|Al:ZnO substrate as a function of Al:Zn oxides ALD cycle ratio, e.g., (1:20) × 4. (b) Mott− Schottky plots for p-Si(111)−CH3 and p-Si(111)−CH3|AZO substrates. Experimental conditions: 0.2 M LiClO4 electrolyte in MeCN, under dark. (c) Band diagrams of the p/n junction equilibrium before (left) and after (right) deposition of the AZO layer.

heterojunction and resulting Vonset values. Ultimately, the final photoelectrode represents a semiconductor|molecular-catalyst construct that exhibits a positive onset potential versus NHE.11,14,31 First, methylated p-Si(111) substrates were prepared by known procedures (HF etch, PCl5 chlorination, CH3MgCl alkylation) to generate substrates that are thermally stable under inert atmosphere up to ∼250 °C, thus being well-suited to ALD-based metal oxide growth (AZO, amorphous TiO2 growth: 150 °C). The aluminum-doped zinc oxide (AZO) layer was introduced by an atomic layer deposition (ALD) process designed with varying Al:Zn pulse ratios.30 Figure 1a shows the sheet resistance of such AZO films on a p-Si(111)−CH3 substrate as a function of Al:Zn pulse ratios. The bare pSi(111)−CH 3 substrate shows 173.5 ± 1.06 ohm sq −1 (corresponding to ρ = 7.81 ohm cm, NA = 1.75 × 1015 cm−3). As shown in Figure 1a, the (1:20) × 4 ALD cycle ratio of Al:Zn oxides exhibited the lowest sheet resistance and thus was selected to form the p/n heterojunction on the Si substrate for the present work. Figure 1b shows the Mott− Schottky plot of bare p-Si(111)−CH3, as well as the AZO (1:20) × 4 modified p-Si(111)−CH3 substrate. The n-type behavior (positive slope) was not observed, likely due to the ultrathin nature of the AZO film. Notably, the p-type region (negative slope) shows distinct behavior before and after AZO deposition. After AZO deposition, the slope of the M-S plot was decreased, and the flat-band potential (Efb) was shifted to a more positive potential (+0.49 → +0.59 V vs NHE) − a beneficial result. This indicates a higher p-type barrier height and greater built-in potential, Vbi (related to Voc), as shown in Figure 1c. The calculated apparent NA of p-Si was changed from 2.07 × 1015 cm−3 to 5.82 × 1015 cm−3, which reflects the enhanced generation and capture of electrons induced by the p/n junction. To demonstrate both the conductivity and chemical functionality of the AZO layer, we opted to adsorb a molecular catalyst for H2 generation onto the metal oxide layer. DuBois and Bullock have described a series of PNP, P2N2, and P2N chelates that support H2 generation by molecular nickel and cobalt species.31−33 We thus prepared the phosphonate-

appended PNP-type chelate 3 (see the Supporting Information for synthetic details) for adsorption to the metal oxide surface and subsequent metal binding. Following incubation of the pSi(111)−CH 3 |AZO|a-TiO 2 substrate with PNP−PO 3 H 2 (DMF, 24 h), the surface was metalated with [Ni(H2O)6](ClO4)2 in MeCN and then capped with a second PNP moiety to complete the coordination environment (Scheme 1). XPS analysis of the resulting construct confirmed the presence of N (ligand) and Ni2+ ions quite close to the ideal ∼2:1 ratio (i.e., 2.4:1; Figures S1 and S2). Covalent attachment of the Ni-PNP complex to the metal oxide surface was further studied by photoelectrochemical measurements. The cyclic voltammogram traces of the p-Si(111)−CH3|AZO|a-TiO2|Ni(PNP)2 substrate were obtained as a function of scan rate under Ar atmosphere (glovebox), 0.2 M LiClO4 electrolyte in MeCN, and illumination with broadband LED 33 mW cm−2 light. The Ni(II/I) cathodic wave appeared at −0.31 V vs NHE, and the linear scan rate dependence of the CV feature supports the assignment of a covalently attached Ni-PNP entity (Figure S3). Additionally, XPS analysis of the substrate after several CV scans (vide infra) demonstrated that the nickel ions remained bound to the surface in similar proportions compared with the starting device. Figures 2a, b show the ensuing catalytic cyclic voltammograms for photogeneration of H2 for the two constructs pSi(111)−CH3|a-TiO2|PNP−Ni−PNP and p-Si(111)−CH3| AZO|a-TiO2|PNP−Ni−PNP (respectively) in MeCN in the presence of increasing [TsOH]. The figures of merit for the PEC-H2 evolution by all the photocathodes prepared here are tabulated in Table 1. The pKa of tosylic acid (TsOH) is known to be 8.45 in acetonitrile,34 thus, the standard potential (E°TsOH) for the 2H+/H2 half reaction in acetonitrile was determined as −0.53 V vs Fc+/0 (≈ 0.11 V vs NHE), following the method of Appel and Helm.35 In case of with AZO, the trace tracking the maximum reduction current (red dashed line) occurs at a more positive potential and exhibits a greater slope as compared to the sample without AZO (i.e., TiO2 only). This suggests that (i) p-Si/n-AZO acts an effective heterojunction interface, and that (ii) the AZO layer is sufficiently conductive to facilitate electron transfer to the 1062

DOI: 10.1021/acsami.5b09902 ACS Appl. Mater. Interfaces 2016, 8, 1061−1066

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ACS Applied Materials & Interfaces Scheme 1. Device Fabrication: Alkylation of the Si(111) Substrate, ALD Metal Oxide Growth (AZO + TiO2), Attachment of the PNP-Phosphonate Catalyst Ligand, and Nickel Metalation Plus Capping Procedurea

Table 1. Summary of Photoelectrode|Catalyst Performance Metricsa for Light-Driven 2H+ → H2 Conversion Depending on R (surface linker), Metal (Ni or Co), and Capping Ligand (PNP, DPPE, and DFPPE) devices Si|CH3|AZO|a-TiO2|PNP-Ni-PNP Si|CH3|AZO|a-TiO2 + Ni(PNP(C6H4) Br)2 solutiond Si|CH3|a-TiO2|PNP-Ni-PNP Si|CH3|AZO|a-TiO2|Pte Si|CH3|AZO|a-TiO2|PNP-Co-PNPe Si|CH3|AZO|a-TiO2|PNP-Ni-DPPEe Si|CH3|AZO|a-TiO2|PNP-Ni-DFPPEe Si|diMeOPh|AZO|a-TiO2|PNP-NiPNPe Si|diMeOPh|AZO|a-TiO2e

Vonsetb V vs NHE (overpotential)f

Jmaxc mA cm−2

−0.12 (0.23) −0.05 (0.16)

7.77 1.00

−0.19 (0.30) 0.23 −0.18 (0.29) −0.06 (0.17) −0.41 (0.52) 0.03 (0.08)

4.71 8.57 5.34 4.32 2.37 7.14

0.01 (0.10)

5.81

a

The PEC performance was evaluated under broadband LED light 33 mW cm−2, 0.2 M LiClO4 electrolyte in MeCN, 100 mV s−1 scan rate, and argon atmosphere (glovebox). bVonset (V) = E vs Fc+/0 for −0.5 mA cm−2. cJmax (mA cm−2) = maximum photocurrent density expressed as |Jmax| value. d3 μM Ni(PNP(C6H4)Br)2 solution (Figure S4). eSee Supporting Information for full PEC−CVs. fOverpotential = (E°TsOH − Vonset of each photocathode).

electrolyte interface. For direct comparison, Figure 2c shows CVs for both samples at 30 mM TsOH in 0.2 M LiClO4/ MeCN solution. The onset potential (where Vonset = potential for J = −0.5 mA cm−2) of p-Si(111)−CH3|AZO|a-TiO2|PNP− Ni−PNP sample was −0.12 V vs NHE, which is slightly shifted +0.07 V compared to the p-Si(111)−CH3|a-TiO2|PNP−Ni− PNP sample (−0.19 V vs NHE). This value agrees with the shift in Efb as demonstrated in the Mott−Schottky plot (in Figure 1b). The greater rise-to-max slope in the AZO sample (red dashed line) also indicates faster electron transfer through the interface. Overall, the synergistic effects of the positive shift

a

The synthetic scheme for the PNP−PO3H2 ligand may be found in the Supporting Information.

Figure 2. Catalytic CVs for (a) p-Si(111)−CH3|a-TiO2|PNP−Ni−PNP and (b) p-Si(111)−CH3|AZO|a-TiO2|PNP−Ni−PNP as a function of [TsOH]. Insets of a and b: maximum catalytic current, JRed‑max, as a function of [TsOH]. Experimental conditions: 0.2 M LiClO4 electrolyte in MeCN, 33 mW cm−2 broadband LED, 100 mV s−1 scan rate, argon atmosphere (glovebox). (c) Comparison of a and b at 30 mM TsOH. 1063

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ACS Applied Materials & Interfaces in Efb (vis a vis Vonset) and the AZO sample’s greater slope reaching to Jmax result in a ∼300 mV positive shift in the EJmax value. Furthermore, the AZO sample exhibits a higher saturation current (insets in Figure 2a and b), likely due to (i) the higher conductivity of the AZO layer in direct contact with the Si (compared to TiO2), and (ii) the enhanced electron transport across the Si|AZO p/n junction, as compared to the semiconductor|insulator junction present on the Si|a-TiO2 substrate. To obtain an even more positive Vonset (lower overpotential), we modified the organic “linker” on the silicon surface to 3,5dimethoxyphenyl (diMeOPh) units. In previous report,25 the diMeOPh-functionalized substrate exhibited a larger p-type barrier height (∼0.29 eV) compared to the methyl-functionalized surface. Additionally, the oxygen-containing methoxy moieties afford a better surface for the nucleation of ALD metal oxides such as the AZO used in this work. Structurally, AFM images (Figure S5) show that while the CH3-functionalized AZO|TiO2 surface is composed of a rough terrain of conjoined nanoparticles, the diMeOPh-functionalized AZO|TiO2 surface exhibits a more coherent, relatively smooth film. Both films were somewhat “elastic”, as some variation was observed between repeated scans of the same area, yet without permanent damage to the surfaces. Neither substrate exhibited any evidence of pinholes in the AFM images, suggesting that the coherent and conductive AZO layer, indeed, facilitates the electron transfer from Si to the PNP-Ni catalyst. With respect to PEC H2 performance, introduction of the diMeOPh linker (Figure 3) shifted the Vonset to −0.07 V vs NHE (at 10 mM

potential near +0.3 V (Figure S8). As such, we are now working to modulate the molecular interface (p/n junction), improve electron transfer through the metal oxide, as well as to electronically and/or structurally modify the PNP ligand to better match the performance of the Pt-functionalized device. In conclusion, we have demonstrated that a combination of organic linker (diMeOPh) and metal oxide (AZO) on pSi(111) can shift the band-edge and resulting onset potential ∼200 mV more positive in a {semiconductor}|{organic}| {metal-oxide}|{molecular-catalyst} construct. Synthetic derivation of a DuBois-type PNP-Ni complex with a phosphonate moiety allows for adsorption of the ligand and metal complex onto the metal oxide surface. Overall, the device exhibits an onset potential positive of 0 V vs NHE, which brings us closer to the possibility of a semiconductor|molecular-catalyst device with the desired efficiency (η) for solar → H2 conversion.22 Importantly, and compared with our previous work,10 we have demonstrated that decoupling the molecular catalyst from direct, covalent attachment to Si allows for band-edge modulation by additional means (e.g., identity of organic linker and metal oxide).

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EXPERIMENTAL SECTION

Full experimental and synthetic details can be found in the Supporting Information documents. Device Fabrication. A single side polished p-type Si wafer (Virginia Semiconductor Inc., VA, B-doped Czochralski (CZ) grown p-type Si wafer (450 ± 25 μm thickness), 1.4−9 Ω cm resistivity) was chlorinated and alkylated as previously described.36 Atomic layer deposition (ALD) was performed using a Savannah S100 apparatus (Cambridge Nanotechnology Inc., USA). The layers of AZO, TiO2, and Pt were achieved using the metal precursors diethylzinc (DEZ) and trimethylaluminum (TMA); tetrakis(dimethylamido)titanium (TDMAT); and trimethyl-(methylcyclopentadienyl)-platinum ([(MeCp)Pt(Me)3]. The AZO layer was constructed at 150 °C to achieve consistent film thicknesses: the unit AZO ALD process consisted of x cycles of ZnO and 1 cycle of Al2O3 that was repeated y times [(x:1) × y = (16:1) × 5, (20:1) × 4, (27:1) × 3, (41:1) × 2, as well as pure ZnO 83 cycles]. From the growth rate supported by instrument supplier − 1.66 and 1.01 Å for ZnO and Al2O3, respectively − the estimated film thickness was 138 ± 0.2 Å. The growth of Pt nanoparticles was achieved at 240 °C using the Pt precursor and O2 as counter-reactant (20 cycles). Molecular Synthesis. Full synthetic details can be found in Supporting Information. Briefly, p-bromoaniline was functionalized with triethylphosphite via Pd-coupled Suzuki-type reaction, followed by Mannich condensation with diphenylphosphine (HPPh2) and paraformaldehyde. The final phosphonate-appended PNP ligand was obtained by hydrolyzing the triethylphosphite moiety with Me3SiBr followed by workup in MeOH. Incubation of PNP−PO3 with the selected substrate occurred in DMF for 24 h, followed by thorough rinsing with DMF and MeCN. The nickel ions were introduced by incubation of the Si|PNP construct with an MeCN solution of [Ni(H2O)6](ClO4)2 that had been dried over Na2SO4 for several hours. Physical and Spectroscopic Characterization. XPS spectra were obtained using a Kratos Axis Ultra X-ray photoelectron spectrometer with a monochromated Al Kα X-ray source (hv = 1486.5 eV) maintained at ∼2 × 10−9 Torr during measurements. The sheet resistance data set was obtained using a Lucas Laboratories SP4 four-point probe head combined with a Keithley 2400 source meter. The Park Scientific CP Research AFM (force constant of 0) was used for comparison of surface roughness of the CH3- and diOMePhfunctioalized p-Si|R|metal oxide surfaces. Vapor transfer of dodecylamine was performed for 10 min before analysis in a small glass chamber, and WSxM was used to analyze the images.37

Figure 3. Catalytic CVs for comparison of p-Si(111)−CH3|AZO|aTiO2|PNP−Ni−PNP (red) and p-Si(111)−diMeOPh|AZO|a-TiO2| PNP−Ni−PNP (blue) at 10 mM TsOH. Experimental conditions: 0.2 M LiClO4 electrolyte in MeCN, 33 mW cm−2 broadband LED, 100 mV s−1 scan rate, argon atmosphere (glovebox).

TsOH), which is shifted +100 mV compared with the CH3functionalized substrate (−0.17 V vs NHE), and a similar Jmax value (∼8 mA cm−2) (see Figure S6 in ESI for full CVs). The inclusion of a different metal (Co2+) or various capping ligands other than PNP (e.g., DPPE or perfluoro-DPPE) did significantly modulate the Vonset and Jmax values, consistent with the molecular nature of the catalysis (ESI, Figure S7). However, these tested variations did not lead to any beneficial change in the performance metrics (Vonset, Jmax) compared with the original PNP-Ni-PNP combination. Lastly, to compare this Si|molecular-catalyst device against a “gold standard”, we functionalized the same Si|CH3|AZO|aTiO2 photoelectrodes with ALD-deposited platinum nanoparticles analogous to that described previously.25 Under the same catalytic conditions (MeCN, 0.2 LiClO4 electrolyte, 1−50 mM TsOH), the Pt-functionalized device exhibited an onset 1064

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ACS Applied Materials & Interfaces Photoelectrochemical Measurements. PEC measurements were performed using an Interface 1000 (Gamry Instruments, USA) potentiostat. A three-electrode setup was composed of a Si wafer working electrode, a Pt-wire counter electrode, and an Ag-wire quasireference electrode. Ferrocene was used as an internal reference, and the obtained potentials were converted versus NHE (E1/2 Fc0/+ = 0.64 V vs NHE). To assemble the PEC cell, copper tape (Electron Microscopy Sciences, USA) was attached on a stainless steel base, where the Si wafer was placed after scratching the back side with a diamond scribe. The ohmic contact was made with Ga/In eutectic (99.99%, Alfa Aesar). A broadband LED bulb (Osram Sylvania Inc., Ultra LED 50 W) was used as a light source, with the light intensity measured as ∼33 mW cm−2 at the sample. All PEC measurements were performed under argon atmosphere (glovebox) and at room temperature. The datasets for Mott−Schottky plots were obtained under analogous (but dark) conditions with a 10 kHz modulation frequency in 0.2 M LiClO4/MeCN solution. Electrochemical impedance spectroscopy (EIS) was carried out at potentials between 0 V to −0.8 V vs Ag under illumination with 10 mV AC amplitude over a frequency range of 1 × 105 to 0.1 Hz. For the extraction of charge transfer resistance (Rct) from the EIS results, a Randle’s equivalent circuit was applied with Zview software (version 2.8d, Scribner Associate Inc.).

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(5) Andreiadis, E. S.; Jacques, P.-A.; Tran, P. D.; Leyris, A.; ChavarotKerlidou, M.; Jousselme, B.; Matheron, M.; Pécaut, J.; Palacin, S.; Fontecave, M.; Artero, V. Molecular Engineering of a Cobalt-Based Electrocatalytic Nanomaterial for H2 Evolution under Fully Aqueous Conditions. Nat. Chem. 2013, 5, 48−53. (6) Eady, S. C.; Peczonczyk, S. L.; Maldonado, S.; Lehnert, N. Facile Heterogenization of a Cobalt Catalyst via Graphene Adsorption: Robust and Versatile Dihydrogen Production Systems. Chem. Commun. 2014, 50, 8065−8068. (7) Muresan, N. M.; Willkomm, J.; Mersch, D.; Vaynzof, Y.; Reisner, E. Immobilization of a Molecular Cobaloxime Catalyst for Hydrogen Evolution on a Mesoporous Metal Oxide Electrode. Angew. Chem., Int. Ed. 2012, 51, 12749−12753. (8) Brimblecombe, R.; Koo, A.; Dismukes, G. C.; Swiegers, G. F.; Spiccia, L. Solar Driven Water Oxidation by a Bioinspired Manganese Molecular Catalyst. J. Am. Chem. Soc. 2010, 132, 2892−2894. (9) Chen, Z.; Concepcion, J. J.; Jurss, J. W.; Meyer, T. J. Single-Site, Catalytic Water Oxidation on Oxide Surfaces. J. Am. Chem. Soc. 2009, 131, 15580−15581. (10) Seo, J.; Pekarek, R. T.; Rose, M. J. Photoelectrochemical Operation of a Surface-Bound, Nickel-Phosphine H2 Evolution Catalyst on p-Si(111): a Molecular Semiconductor|Catalyst Construct. Chem. Commun. 2015, 51, 13264−13267. (11) Krawicz, A.; Yang, J.; Anzenberg, E.; Yano, J.; Sharp, I. D.; Moore, G. F. Photofunctional Construct That Interfaces Molecular Cobalt-Based Catalysts for H2 Production to a Visible-Light-Absorbing Semiconductor. J. Am. Chem. Soc. 2013, 135, 11861−11868. (12) Lattimer, J. R. C.; Blakemore, J. D.; Sattler, W.; Gul, S.; Chatterjee, R.; Yachandra, V. K.; Yano, J.; Brunschwig, B. S.; Lewis, N. S.; Gray, H. B. Assembly, Characterization, and Electrochemical Properties of Immobilized Metal Bipyridyl Complexes on Silicon(111) Surfaces. Dalton Trans. 2014, 43, 15004−15012. (13) Nann, T.; Ibrahim, S. K.; Woi, P.-M.; Xu, S.; Ziegler, J.; Pickett, C. J. Water Splitting by Visible Light: A Nanophotocathode for Hydrogen Production. Angew. Chem., Int. Ed. 2010, 49, 1574−1577. (14) Hou, Y.; Abrams, B. L.; Vesborg, P. C. K.; Björketun, M. E.; Herbst, K.; Bech, L.; Setti, A. M.; Damsgaard, C. D.; Pedersen, T.; Hansen, O.; Rossmeisl, J.; Dahl, S.; Nørskov, J. K.; Chorkendorff, I. Bioinspired Molecular Co-Catalysts Bonded to a Silicon Photocathode for Solar Hydrogen Evolution. Nat. Mater. 2011, 10, 434−438. (15) Moore, G. F.; Sharp, I. D. A Noble-Metal-Free Hydrogen Evolution Catalyst Grafted to Visible Light-Absorbing Semiconductors. J. Phys. Chem. Lett. 2013, 4, 568−572. (16) Krawicz, A.; Cedeno, D.; Moore, G. F. Energetics and Efficiency Analysis of a Cobaloxime-Modified Semiconductor under Simulated Air Mass 1.5 Illumination. Phys. Chem. Chem. Phys. 2014, 16, 15818− 15824. (17) Cedeno, D.; Krawicz, A.; Doak, P.; Yu, M.; Neaton, J. B.; Moore, G. F. Using Molecular Design to Control the Performance of Hydrogen-Producing Polymer-Brush-Modified Photocathodes. J. Phys. Chem. Lett. 2014, 5, 3222−3226. (18) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446−6473. (19) Kim, H. J.; Kearney, K. L.; Le, L. H.; Pekarek, R. T.; Rose, M. J. Platinum-Enhanced Electron Transfer and Surface Passivation through Ultrathin Film Aluminum Oxide (Al2O3) on Si(111)−CH3 Photoelectrodes. ACS Appl. Mater. Interfaces 2015, 7, 8572−8584. (20) Liang, W.; Weber, K. J.; Suh, D.; Phang, S. P.; Yu, J.; McAuley, A. K.; Legg, B. R. Surface Passivation of Boron-Diffused p-Type Silicon Surfaces with (100) and (111) Orientations by ALD Al2O3 Layers. IEEE J. Photovolt. 2013, 3, 678−683. (21) Dingemans, G.; Kessels, W. M. M. Status and Prospects of Al2O3-Based Surface Passivation Schemes for Silicon Solar Cells. J. Vac. Sci. Technol., A 2012, 30, 040802. (22) Kalanyan, B.; Parsons, G. Atomic Layer Deposited Oxides for Passivation of Silicon Photoelectrodes for Solar Photoelectrochemical Cells. ECS Trans. 2011, 41, 285−292.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b09902. Experimental details, full XPS survey and high-resolution spectra, CV data, and AFM images (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions †

H.J.K and J.S. contributed equally

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the US Office of Naval Research (N00014-13-1-0530), the Robert A. Welch Foundation (F-1822), the American Chemical Society PRF program (53542-DN13), and the UT Austin College of Natural Sciences. Funding for the Kratos Axis Ultra XPS was provided by a grant from the National Science Foundation (MRI0618242), and we acknowledge Mr. Ryan Pekarek for assistance in obtaining and analyzing the AFM images.

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REFERENCES

(1) Lewis, N. S.; Nocera, D. G. Powering the Planet: Chemical Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15729−15735. (2) Herron, J. A.; Kim, J.; Upadhye, A. A.; Huber, G. W.; Maravelias, C. T. A General Framework for the Assessment of Solar Fuel Technologies. Energy Environ. Sci. 2015, 8, 126−157. (3) Das, A. K.; Engelhard, M. H.; Bullock, R. M.; Roberts, J. A. S. A Hydrogen-Evolving Ni(P2N2)2 Electrocatalyst Covalently Attached to a Glassy Carbon Electrode: Preparation, Characterization, and Catalysis. Comparisons with the Homogeneous Analogue. Inorg. Chem. 2014, 53, 6875−6885. (4) Berben, L. A.; Peters, J. C. Hydrogen Evolution by Cobalt Tetraimine Catalysts Adsorbed on Electrode Surfaces. Chem. Commun. 2010, 46, 398−400. 1065

DOI: 10.1021/acsami.5b09902 ACS Appl. Mater. Interfaces 2016, 8, 1061−1066

Letter

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DOI: 10.1021/acsami.5b09902 ACS Appl. Mater. Interfaces 2016, 8, 1061−1066