Influence of Anchoring Groups on the Charge Transfer and the

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Influence of Anchoring Groups on the Charge Transfer and the Performance of p-Si/TiO/Cobaloxime Hybrid Photocathodes for Photoelectrochemical H Production 2

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Lunlun Gong, Heng Yin, Chengming Nie, Xuran Sun, Xiuli Wang, and Mei Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b12182 • Publication Date (Web): 27 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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ACS Applied Materials & Interfaces

Influence of Anchoring Groups on the Charge Transfer and the Performance of p-Si/TiO2/Cobaloxime Hybrid Photocathodes for Photoelectrochemical H2 Production Lunlun Gong,† Heng Yin,‡ Chengming Nie,† Xuran Sun,† Xiuli Wang*,‡ and Mei Wang*,† †State

Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024,

China ‡State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Dalian 116023, China ABSTRACT: Although hybrid photocathodes built by immobilizing molecular catalysts to the surface of semiconductors through chemical linkages have been reported in recent years, systematic and comparative studies remain scarce about the impact of various anchoring groups on the performance, stability, and charge-transfer kinetics of molecular catalyst-decorated hybrid photocathodes for photoelectrochemical (PEC) H2 production. In this study, the molecular cobaloxime catalysts, CoPy-4-X (Py = pyridine, X = PO3H2, COOH, CONH(OH)), bearing different anchoring groups were synthesized and covalently immobilized to the surface of the porous TiO2 layer coated on a p-Si plate or a fluorine-doped tin oxide (FTO) glass. The influence of the anchoring groups on the performance of p-Si/TiO2/CoPy-4-X photocathodes was comparatively studied for PEC H2 evolution. Among the tested hybrid photocathodes, the one with a hydroxamate as an anchoring group displayed higher activity and lower charge-transfer resistance than that observed for the electrode with a carboxylate or a phosphonate as the anchoring group. Notably, the catalytic current of p-Si/TiO2/CoPy-4-CONH(OH) was attenuated only by 2.9% in the controlled potential photoelectrolysis tests in pH 9 borate buffer solutions at 0 V vs RHE over 6 h. Moreover, the influence of anchoring groups on the interfacial electron transfer from the TiO2 layer to the immobilized cobaloxime catalyst and electron-hole recombination was studied by transient absorption spectroscopy. The results revealed that the hydroxamate as an anchoring group

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is superior to carboxylate and phosphonate groups for speeding up the interfacial electron transfer and firmly immobilizing molecular catalysts to the metal oxide semiconductors to build efficient and stable hybrid photoelectrodes.

KEYWORDS:

anchoring

group,

cobaloxime,

hydrogen

production,

photocathode,

photoelectrochemistry, silicon

INTRODUCTION The production of hydrogen by light-driven water splitting is considered to be an ideal approach to store intermittent solar energy as an eco-friendly and momentarily available chemical fuel. From the perspective of the solar-to-hydrogen conversion efficiency, cost, safety and availability, photoelectrochemical (PEC) water splitting is one of the most promising technologies,1‒4 which allows sunlight harvesting and water electrolysis to occur simultaneously at a single device in aqueous electrolytes. To build efficient bias-free PEC cells, the challenging work is to develop highly active, robust, inexpensive, and mutually compatible photoanodes and photocathodes. Visible-light-absorbing semiconductor/molecular catalyst (SC/MC) hybrid photoelectrodes without dye-sensitizing have attracted intensive attention in recent years.5‒7 For the assembly of hybrid photocathodes, the mostly used semiconductor materials are p-type Si,8‒15 InP,16 GaP,17‒21 and GaInP2.22 Compared with In- and Ga-containing semiconductors, Si is a much less expensive and more abundant material. Although Si with a band gap of 1.12 eV is a good sunlight absorber and has a large theoretical maximum photocurrent density of 44 mA cm−2,1,3 the pristine p-Si is inactive for the bias-free PEC proton reduction under 1 sun illumination because of the sluggish charge-transfer kinetics at the Si/electrolyte interface, and Si is not stable in aqueous solutions, especially in basic aqueous solutions. In recent years, extensive efforts have been devoted to 2


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protecting Si electrodes from corrosion by using a covering layer (generally with the TiO2 coating)23 and to improving the sluggish kinetics of Si for H2 evolution by loading catalysts on the surface of Si electrodes.24,25 The molecular catalysts used to modify the surface of p-type semiconductor photocathodes are cobaloximes,15‒20,22 DuBois’ nickel complexes,10‒13 iron and cobalt porphyrin complexes,21 bio-inspired diiron dithiolate complexes,16 and incomplete cubanelike Mo3S4 clusters.8,9 Many examples have demonstrated that chemically bonding molecular catalysts to the surface of semiconductor electrodes could effectively accelerate the kinetics of photoinduced charge separation,5‒7,26 and hence greatly improve the performance of the photocathodes for PEC hydrogen production. In addition to the properties and stabilities of semiconductors and catalysts employed, another key factor in designing and assembling efficient and durable hybrid photoelectrodes is the method used to immobilize molecular catalysts on the surface of semiconductors. Chemical linkage of molecular catalysts to a semiconductor surface generally gives higher active and more stable hybrid photoelectrodes compared to the immobilization of catalysts by physisorption or by polymer coating with the aid of Nafion film. Although the influence of anchoring groups on the photoinduced electron transfer and the stability of surface attachment has been revealed for some semiconductor/dye assemblies,27‒31 there are seldom reports on the impact of anchoring groups on the charge transfer and the performance of SC/MC hybrid photoelectrodes.26 The catalytic activity and stability of SC/MC hybrid photoelectrodes for PEC proton reduction and water oxidation can be influenced by the type and the number of surface anchoring groups, the length and conjugation property of linkers, and the orientation of anchoring groups toward the semiconductor surface. It is important to make systematic and comparative studies on the photoelectrodes which are assembled by grafting a series of molecular catalysts bearing various anchoring groups to a certain type of semiconductor substrate, to have a better understanding about the interfacial effects of 3



anchoring groups on the electron/hole transfer kinetics and charge recombination, and on the efficiency and stability of SC/MC hybrid photoelectrodes in PEC H2 and O2 evolution. In this work, we prepared three cobaloxime complexes, CoPy-4-X (X = PO3H2, COOH, CONH(OH)), with different anchoring groups at the para position of the axial pyridine ligand, and chemically attached them to the TiO2 layer coated on the surface of p-Si plates or to the fluorinedoped tin oxide (FTO) glasses to build hybrid photocathodes (Figure 1). The p-Si/TiO2/CoPy-4-X photocathodes, as well as the bare p-Si/TiO2 electrode in the presence and absence of the anchorfree cobaloxime catalyst (CoPy), were comparatively studied for PEC H2 production. Moreover, the impact of the anchoring groups on the kinetics of charge separation and recombination for FTO/TiO2/CoPy-4-X electrodes was explored by means of transient absorption (TA) spectroscopy. Among the cobaloxime-modified hybrid photocathodes, the one with a hydroxamate as an anchoring group displayed the lowest electrochemical impedance, the fastest electron transfer rate, the highest activity, and the best stability for PEC H2 evolution. Although the hydroxamate anchor has been employed to tether a DuBois-type nickel catalyst to the surface of an organic dyesensitized NiO electrode, the assembled hybrid photocathode was inactive for PEC proton reduction.32 To our knowledge, p-Si/TiO2/CoPy-4-CONH(OH) is the first example of a PEC active hybrid photoelectrode in which the hydroxamate group was used as an anchor to immobilize molecular catalysts to the electrode surface, and transient spectroscopic studies on the hydroxamate-linked SC/MC hybrid photoelectrodes have not been reported to date.

EXPERIMENTAL SECTION Materials and Instruments. Planar p-Si wafers (P-100, 11-13 Ω cm, 450 μm thickness) were purchased from Hangzhou Bojing Science and Technology Limited Company. Titanium oxide nanoparticles (TiO2 NPs, 5‒10 nm, anatase, hydrophilicity) were purchased from Aladdin 4


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Industrial Corporation and calcined at 500 C for 12 h in air before use. Commercially available isonicotinic acid (Py-4-COOH) was purchased from Damao Chemical Reagent Factory in Tianjin and used as received. The other two functionalized pyridine ligands, pyridin-4-ylphosphonic acid (Py-4-PO3H2) and N-hydroxyisonicotinamide (Py-4-CONH(OH)), were synthesized according to previously reported procedures.27,28,33 The catalysts, CoPy-4-PO3H2, CoPy-4-COOH, and CoPy, were prepared as literature protocols.34 All chemicals for synthesis of the catalysts and for photoelectrochemical measurements were used without further purification. The water used for electrochemical and photoelectrochemical experiments was deionized with a Millipore AFS-E system (18.2 MΩ cm resistivity). Field-emission scanning electron microscopy (FESEM) images were recorded on a Nova NanoSEM 450 instrument. X-ray photoelectron spectroscopy (XPS) measurements were taken on a Thermo VG ESCALAB 250 surface analysis system. Liquid UV-vis spectra were collected at a Lambda 35 instrument (PerkinElmer, America). Optical diffuse reflection spectra were characterized using an UV−vis light spectrophotometer (Thermo Scientific, America). Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra were carried out on a Thermo Fisher Nicolet iN10 spectrometer. 1H NMR spectra were analyzed by a 400 MHz/54 mm system (Bruker Avance II 400). Mass spectra were recorded on an HP 1100 HPL/ESI-DAD-MS instrument. The loading amounts of the cobaloxime catalysts with different anchoring groups on the p-Si/TiO2 electrode were determined by the inductively coupled plasma optical emission spectroscopy (ICPOES) analysis (PerkinElmer 2000 DV). Preparation of CoPy-4-CONH(OH). The new complex, CoPy-4-CONH(OH), was prepared by referring to the protocol for the preparation of CoPy.34 Complex [Co(dmgH2)(dmgH)Cl2] (dmgH = dimethylglyoximate) (0.36 g, 1.0 mmol) was dissolved in MeOH (30 mL)/Et3N (0.18

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mL) under N2, and the solution was stirred for 15 min. Afterward, Py-4-CONH(OH) (138 mg, 1.0 mmol) was added to the solution, and the mixture was stirred for 12 h at 35 C. After reaction, the solvent was removed in vacuum, and water (about 2 mL) was added to the flask, the product was precipitated as light brown solid, which was filtered and washed several times with water. The product was dried in vacuum, and the yield obtained was 186 mg (40%). 1H-NMR (400 MHz, DMSO-d6): 2.32 (s, 12H, 4CH3), 7.69 (d, J = 6.3 Hz, 2H, py), 8.14 (d, J = 6.0 Hz, 2H, py), 9.46 (s, 1H, N-H), 11.52 (s, 1H, O-H). MS: calcd for C14H20ClCoN6O6, m/z 462.05; found for [M ‒ H]+, 460.86 (Figure S3). Preparation of the p-Si/TiO2/CoPy-4-X Hybrid Photocathodes. The p-Si wafer (1.1  1.1 cm2) was cleaned and etched as previously reported procedures.35 Nafion solution (30 μL) was added to MeOH (600 μL), and the mixture was sonicated for 2.5 h. Then TiO2 NPs (10 mg) were dispersed in the nafion/MeOH solution by being sonicated for 10 min and then stirred for 6 h. The dispersion solution of TiO2 NPs (20, 40, or 60 μL) was drop-casted onto the surface of the polished side of a freshly cleaned p-Si wafer. After drying in air, the p-Si/TiO2 was immersed in the MeOH solution of the selected cobaloxime catalyst (1 mM) for 16 h and then washed several times with MeOH to remove the non-bonded catalyst. Afterward, Ga-In eutectic (Aldrich) was scratched onto the unpolished side of the Si electrode to make an ohmic contact. The Cu wire was connected to the Ga-In eutectic film by Ag conductive adhesive (SPI supplies, PA, USA) and threaded into a glass tube. Upon drying, a hysol epoxy (Loctite 9462) was applied on both sides of the electrode except for the intended illumination area (geometric surface area ~ 1 cm2) of the polished side of Si. The photocathodes were then allowed to dry thoroughly in air before testing. Preparation of the FTO/TiO2/CoPy-4-X Hybrid Photoelectrodes. The anatase TiO2 (20 nm, from Heptachroma company in Dalian, China) film was made on an FTO glass by doctor blading

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and calcined at 450 C for 1 h in a flattening oven. The area of the deposited TiO2 film is approx. 2 cm2 with a thickness of about 5 μm. The FTO/TiO2 electrodes were dipped in the MeOH solutions of 0.2 mM functionalized cobaloxime catalyst in the dark for different dipping time. Afterward, the electrodes were thoroughly washed with MeOH to remove the non-anchored molecular catalysts on the TiO2 surface. The loading amounts of the cobalt catalysts were detected by ICPOES analyses, which showed that similar amounts of grafted cobalt catalysts, that is 12.6, 12.9, and 13.0 nmol cm−2 for CoPy-4-PO3H2, CoPy-4-COOH, and CoPy-4-CONH(OH), respectively, were obtained for the FTO/TiO2 electrodes being dipped in the MeOH solutions of CoPy-4-PO3H2 for 20 min, CoPy-4-COOH for 9 h, and CoPy-4-CONH(OH) for 14 h. The as-prepared FTO/TiO2/CoPy-4-X hybrid electrodes were used for exploring the effect of the anchoring groups on electron-transfer and recombination kinetics of the photoelectrodes by means of TA spectroscopy. Photoelectrochemical Measurements. All photoelectrochemical measurements were performed in a three-electrode cell under Ar at room temperature using an electrochemical workstation (CHI660E) with the as-fabricated p-Si/TiO2/CoPy-4-X photocathodes as working electrodes, Ag/AgCl as the reference electrode, and a Pt foil (1 cm2) as the counter electrode. A 300 W Xenon arc lamp (100 mW cm−2, AM1.5G, λ > 400 nm) was employed as the light source. The potassium borate buffer solution was used as an electrolyte, which was prepared by adjusting the pH of the 0.1 M boric acid solution with KOH (0.05 M) to the desired pH value. All experimentally measured potentials were converted to the ones versus RHE by using the equation: E(RHE) = E(Ag/AgCl) + Eϴ(Ag/AgCl) + 0.059pH V (Eϴ(Ag/AgCl) = 0.197 V at 25 °C). The LSVs were measured under illumination from a 300 W Xenon arc lamp (AM1.5G) passed through an optical filter (λ > 400 nm) at 5 mV s−1, and the CVs were recorded in the dark at 50 mV

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s−1. The chopped light LSVs and the controlled potential photoelectrolysis (CPP) tests were conducted at on-off every 5 s and 50 s, respectively. Electrochemical impedance spectra were measured at 0 V under illumination with the sweeping of frequency from 100 kHz to 0.1 Hz and with a 5 mV amplitude. The long-time CPP experiments were carried out in 0.1 M potassium borate buffer solutions at pH 9 at an applied potential of 0 V under illumination. The amount of evolved H2 in the headspace of the cell was analyzed by a gas chromatograph (GC, Ceaulight GC-7920) equipped with a 5 Å molecular sieve column (2 mm × 2 m) during the CPP experiments. The faradaic efficiencies were determined from the CPP experiments at 0 V over 5.5 h illumination. The IPCE was detected under illumination from a 300 W Xenon arc lamp (AM1.5G) passed through an optical filter. The monochromatic light was excited by using a Ceaulight CEL-IS151 monochromator with a 20-nm bandpass, and the light intensity at each wavelength was obtained through a Ceaulight CEL-NP2000 photometer. The IPCE value of the photocathode was calculated according to the following equation: IPCE (%) =

1240 x (Jlight  Jdark)

 x Pin

x 100%

where Jlight and Jdark are the measured photocurrent density and dark current density; λ and Pin are the wavelength and the power density of incident illumination. TA Spectra Measurements. A Nd:YAG laser (EKSPLA, NT 342B, 5 ns pulse width) was used as pump beam source in the transient absorption experiments. The wavelength of 355 nm (third harmonic) was chosen to excite TiO2 samples. Laser frequency was set to be 0.9 Hz and laser intensity was adjusted to be 160 μJ cm−2. A xenon lamp (Bentham, IL 1) was used as the probe beam source with a monochromator (Zolix, omni–λ 300). The transmitted probe light was detected by a detector of silicon photodiode, then the signal was recorded by an oscilloscope (Tektronix, TDS 2012C) on μs‒ms time scale and a DAQ card (National Instruments, NI USB-6211) on ms‒s

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time scale. All TA experiments were operated on back-illuminated mode in air (laser illuminated from FTO side of sample) in order to ensure the identical absorption of 355 nm laser by TiO2.

RESULTS AND DISCUSSION Fabrication and Characterization of the Cobaloxime-Modified Hybrid Photocathodes. Cobaloxime complexes with different anchoring groups were prepared by referring to the reported procedures30,34 and characterized by MS and 1H NMR spectroscopy (Figures S1‒S3). The TiO2 films (geometric area 1 cm2) with different thicknesses of 4.3, 6.9, and 12.2 μm (Figure S4a‒c†) were prepared by coating the Si substrates with 20, 40, and 60 μL TiO2 nanoparticulate paste (anatase, d = 5‒10 nm), respectively, by drop-casting method. The SEM image of p-Si/TiO2 (Figure S4d) showed that the TiO2 layer had a porous microstructure. The cobaloxime-modified hybrid photoelectrodes (Figure 1) were fabricated by dipping the p-Si/TiO2 electrodes in the MeOH solutions of corresponding catalysts for 16 h in the dark (for determination of optimal soaking time, see the PEC performance section). Afterward, the electrode surface was rinsed several times with distilled water and MeOH to remove the catalyst molecules which were not covalently bound to the surface of TiO2 layer. The FTO/TiO2/CoPy-4-X hybrid electrodes were also fabricated for investigating the effect of anchoring groups on the charge dynamics by TA spectroscopy. To ensure that the cobaloxime catalysts bearing different anchors were immobilized on the TiO2 film in a similar amount, the dipping processes were monitored by the inductively coupled plasma optical emission spectroscopy (ICP-OES) analyses to track the loading amount of the catalyst on the FTO/TiO2 electrode at different time of the dipping period. It was found that approximately identical amounts of the functionalized cobaloxime catalysts (12.6‒13.0 nmol cm−2) were grafted on the electrode surface when the FTO/TiO2 electrodes were dipped in the MeOH solutions of 0.2

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mM CoPy-4-PO3H2 for 20 min, CoPy-4-COOH for 9 h, and CoPy-4-CONH(OH) for 14 h in the dark (see Experimental section for details).

Figure 1. Schematic illustration of the p-Si/TiO2/CoPy-4-X hybrid photocathodes.

The as-prepared p-Si/TiO2/CoPy-4-X photocathodes displayed binding energy peaks of Co 2p and N 1s in their XPS (Figures 2a,b, S5a‒c and S6a,b), supporting the successful assembly of the cobaloxime catalyst to the TiO2 surface. Here p-Si/TiO2/CoPy-4-CONH(OH) is taken as an example for the XPS analysis. As shown in Figure 2a, two peaks at 796.5 and 781.5 eV appear in the Co 2p region, which correspond to the Co 2p1/2 and Co 2p3/2 core levels, respectively.18,36,37 In the N 1s region, the broad peak in the range of 398.5‒403.5 eV originates from both the N−O and N−C of the dimethylglyoxime and the functionalized pyridine ligands (Figure 2b). For pSi/TiO2/CoPy-4-PO3H2, the peak at around 133.3 eV in the region of the P 2P core level arises from the anchoring group, PO3H2 (Figure S5c). Notably, the intensities of Co 2p and N 1s peaks are in a decreasing order of p-Si/TiO2/CoPy-4-CONH(OH) > p-Si/TiO2/CoPy-4-PO3H2 > pSi/TiO2/CoPy-4-COOH, indicating the difference in the amount of immobilized cobaloxime catalysts on the TiO2 films. This observation is consistent with the results obtained from the ICP

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analysis (Table S1), which shows that the amounts of catalysts immobilized on the photocathodes are 22.0(± 1.4), 18.1(± 0.8), and 14.1(± 2.4) nmol cm−2 for CoPy-4-CONH(OH), CoPy-4-PO3H2, and CoPy-4-COOH, respectively. The disparate bonding ability of anchoring groups leads to discrepancy in the achievable loading amount of catalyst on the TiO2 film,27‒30 which would influence the performance of the hybrid photocathodes for PEC H2 production.

Figure 2. (a,b) XPS spectra of Co 2p and N 1s for p-Si/TiO2/CoPy-4-CONH(OH) before and after 2 and 6 h of CPP tests at 0 V vs RHE. (c) ATR-FTIR spectra of p-Si/TiO2/CoPy-4-CONH(OH) before and after 6 h of the CPP tests, compared with that of CoPy-4-CONH(OH) (KBr disc). (d) UV-vis reflection spectra of p-Si/TiO2 and p-Si/TiO2/CoPy-4-X.

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To provide more evidence for the immobilization of intact cobaloxime catalysts, the as-prepared hybrid photocathodes were detected by ATR-FTIR spectroscopy. The IR spectrum of p-Si/TiO2 has been deducted from the ATR-FTIR spectra of p-Si/TiO2/CoPy-4-X (Figures 2c, S5d and S6c). The characteristic bands of the cobaloxime complex17,18 were observed in the region of 1100 to 1700 cm−1 in the IR spectra of the as-fabricated hybrid photocathodes. Again p-Si/TiO2/CoPy-4CONH(OH) is taken as an example for the FTIR spectra analysis. The typical ν(C=O) absorption of the hydroxamate anchoring group is observed at 1620‒1627 cm−1, which may also cover the ν(C=C) absorption of the aromatic ring.13,38 The bands in the region of 1520‒1580 cm−1 originate from the C=N stretches of both the pyridine and glyoximate ligands, and the bands around 1230 and 1154 cm−1 are attributed to the characteristic absorptions of the C‒C and oxime stretches, respectively, in the glyoximate ligand.13,15,17‒20 The UV-vis reflection spectra showed the reflection of p-Si/TiO2/CoPy-4-X decreased at about λmax = 400 nm compared to that of bare p-Si/TiO2 (Figure 2d), because of the absorption of anchored cobaloxime catalysts tethered on the TiO2 layer (Figure S7). The IR and UV-vis spectra gave further proofs for the attachment of the cobaloxime catalyst to the TiO2 film. PEC Performance of the Hybrid Photocathodes with Different Anchoring Groups. In the initial electrochemical studies, the influences of the pH of electrolyte, the thickness of TiO2 layer, and the dipping duration of p-Si/TiO2 on the photoelectrocatalytic performance of p-Si/TiO2/CoPy4-X were explored. The linear scan voltammograms (LSVs) of p-Si/TiO2/CoPy-4-CONH(OH) were carried out in 0.1 M borate buffer solutions (BBSs) at different pH values (pH 7‒10) under simulated AM1.5G illumination (λ > 400 nm). The j‒E plots showed that when the pH value of electrolyte was increased from 7 to 9 the short-circuit photocurrent density (jscpc) enhanced from 83 to 319 μA cm−2 (Figure S8), while further increase of pH to 10 led to a diminution of jscpc to 115 12


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μA cm−2. Figure S9 shows that the p-Si/TiO2/CoPy-4-CONH(OH) electrode with an approx. 6.9 μm thickness of the TiO2 layer displays higher photocurrent than the electrodes with 4.3 or 12.2 μm thickness of the TiO2 layer (Figure S4a,c). Moreover, the jscpc was increased from 81 to 319 μA cm−2 as the dipping period of p-Si/TiO2 in the MeOH solution of CoPy-4-CONH(OH) was extended from 8 to 16 h, while the identical jscpc values were obtained for the electrodes fabricated by dipping p-Si/TiO2 for 16 and 24 h (Figure S10). Therefore the photocathodes, fabricated with the optimal thickness of the TiO2 layer (ca. 6.9 μm) and the dipping duration of 16 h, were employed for the following PEC measurements in 0.1 M BBSs at pH 9. The LSVs under illumination clearly manifested that the cobaloxime-decorated p-Si/TiO2 hybrid photoelectrodes displayed a higher photocurrent density and a more positive onset potential than bare p-Si/TiO2 did (Figure 3a, Table S2). The jscpc values of 319, 180, and 158 μA cm−2 were achieved for the photocathodes having a hydroxamate, a phosphonate, and a carboxylate anchoring group, respectively, with an increase of 42‒203 μA cm−2 in jscpc relative to that of bare p-Si/TiO2. To compare the mass-specific activity of the as-prepared hybrid photocathodes, the photocurrent density were normalized by the amount of the grafted catalysts. The plots of the normalized current density against applied potential indicated that the mass-specific activities were in an increasing order of p-Si/TiO2/CoPy-4-PO3H2 < p-Si/TiO2/CoPy-4-COOH < p-Si/TiO2/CoPy-4-CONH(OH) (Figure S11). The difference in the mass-specific activities of photoelectrodes could be attributed to the distinct interfacial interaction between the anchoring group and the surface of the TiO2 layer, because these pyridine-functionalized cobaloxime complexes displayed very similar reduction potentials in 0.1 M BBS at pH 9 (Figure S12†). The p-Si/TiO2/CoPy-4-PO3H2 electrode exhibited the lowest mass-specific photocurrent density probably because phosphonic acids are not as good as hydroxamic acids and carboxylic acids in electron injection,31 which was evidenced by the transient spectroscopic studies (vide infra). Figure 3b shows LSVs of p-Si/TiO2/CoPy-4-X, as well 13



as the reference electrodes, p-Si and p-Si/TiO2, in the region of −0.23 to 0.38 V (all potentials given in this paper are versus RHE) under chopped illumination. The jscpc values of the catalyst-modified electrodes are enhanced by a factor of 2.8‒5.5 compared to that of bare p-Si/TiO2. It is worth to note that an apparent anodic dark current is observed in the LSVs of bare p-Si/TiO2, which arises from the photoelectrons stranded in the conduction band (CB) of TiO2. More details on this issue are discussed later in the paper.

Figure 3. LSVs of the as-prepared photocathodes and the reference electrodes under (a) continuous and (b) chopped illumination at 5 mV s−1.

Stability, Faradaic Efficiency, and Incident Photon-to-Current Efficiency (IPCE). The CPP experiments under continuous illumination at 0 V showed that the photocathodes with surfacebound cobaloxime catalysts exhibited higher and more stable photocurrent than the bare p-Si/TiO2 did, and the stability of as-prepared hybrid photocathodes depended strongly on their anchoring groups. For the photocathode with cobaloxime catalyst tethered to the surface of p-Si/TiO2 through a hydroxamate group, the photocurrent density was maintained constant over 2 h of the CPP experiment (Figure 4a), and only a slight attenuation (ca. 3%) in photocurrent was observed over

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6 h of the CPP experiment (Figure S13). The jumps of the curves in Figures 4a and S13 were caused by the detachment of large hydrogen bubbles from the electrode surface. The stability of pSi/TiO2/CoPy-4-CONH(OH) was superior to the previously reported cobaloxime-modified hybrid photocathodes for long-term PEC hydrogen production.13,17‒20,22 With a phosphonate and a carboxylate as anchoring groups, the photocurrents of the hybrid electrodes were decayed by about 19% and 30%, respectively, over 2 h of the CPP experiments. These comparative studies demonstrate that the hydroxamate anchor is much more stable than commonly used phosphonate and carboxylate anchoring groups, which is in accord with previous reports for the dye-sensitized TiO2 systems.27,28,39,40

Figure 4. (a) CPP curves of the p-Si/TiO2/CoPy-4-X photocathodes as well as p-Si/TiO2 under continuous illumination at 0 V. (b) Plots of the amount of evolved hydrogen and the faradaic efficiency for p-Si/TiO2/CoPy-4-CONH(OH).

The gradual attenuation of photocurrent density is possibly caused by detaching of the grafted catalysts from the surface of TiO2 and by degradating of the cobaloxime catalysts under the CPP conditions as previously reported,13,17‒20,22 as well as by the gradual corrosion of the Si surface during the long-time CPP experiment.41 After used for 6 h of the CPP experiment, p-Si/TiO2/CoPy15



4-CONH(OH) was detected by SEM, XPS, and ATR-FTIR spectroscopy. The SEM images (Figure S14) illustrated that there was no noticeable change for the porous microstructure and the thickness of the TiO2 layer after the long-term CPP experiments. Evident Co 2p and N 1s peaks were detected by the XPS spectra of p-Si/TiO2/CoPy-4-CONH(OH) measured after 2 and 6 h of the CPP tests (Figure 2a,b), and all characteristic IR absorptions for CoPy-4-CONH(OH) were observed after the photocathode was used for 6 h of the CPP test (Figure 2c). These results demonstrated that most of the grafted-cobaloxime catalysts still remained on the used p-Si/TiO2/CoPy-4-CONH(OH) electrode and were kept intact. Accordingly, after used for 6 h of the CPP test, the p-Si/TiO2/CoPy4-CONH(OH) electrode displayed only a slight decrease in photocurrent compared to the asprepared p-Si/TiO2/CoPy-4-CONH(OH), and no dark current was observed with an anodic scanning (Figure S15). By contrast, the Co 2p signals in the XPS spectrum and the characteristic absorptions of the cobaloxime catalysts in the ATR-FTIR spectra (Figures S5 and S6) were apparently decreased for the p-Si/TiO2/CoPy-4-PO3H2 and p-Si/TiO2/CoPy-4-COOH electrodes after used for 2 h of the CPP tests, indicating that a considerable amount of the cobaloxime catalysts released from the electrode surfaces. These observations further demonstrated the much better stability of the hydroxamate anchor than phosphonate and carboxylate anchors for molecular catalysts attached to the p-Si/TiO2 electrode in PEC hydrogen production. The H2 evolved from the CPP experiment of p-Si/TiO2/CoPy-4-CONH(OH) was measured by gas chromatography analysis of the gaseous phase in the sealed cell. The amount of H2 evolved was 16.2 μmol cm−2 over 5.5 h of CPP at 0 V, and the cumulated charges having passed through the external circuit was 3.9 C, indicative of a faradaic efficiency of 76(±4)% (Figure 4b). The moderate faradaic efficiency is most possibly due to the following two reasons: (i) a small part of the evolved H2 was dissolved in the electrolyte, and (ii) some of the photoelectrons were consumed at the electrode/electrolyte interface because of the recombination of photogenerated charges. 16


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The measurements of incident photon-to-current efficiency (IPCE) of p-Si/TiO2/CoPy-4CONH(OH) were conducted at 0 V under illumination (Figure S16). The calculated IPCE values (see the Experimental Section) were 3.9‒9.0% across the wavelengths of 400‒800 nm, wherein the maximum of IPCE appeared at 720 nm. The good IPCE in the visible light region around 720 nm indicates that such a Si-based cheap hybrid photocathode could effectively convert low-energy photons to current to be used in situ for proton reduction by molecular catalysts. Influence of the Anchoring Groups on Interfacial Electron Transfer. To further explore the charging and discharging feature of the CB of TiO2, LSVs were scanned in both reducing and oxidizing directions under chopped illumination. The charging behavior of the CB of TiO2 was manifested by the anodic dark current observed at around 0.4 V in the LSV of bare Si/TiO2 with scanning from the positive to negative direction (Figure 5a). In the subsequent reverse scan, the electrons trapped in the CB of TiO2 were gradually discharged at the applied potentials more positive than −0.4 V, which caused an anodic dark current. Similar charging and discharging feature of the TiO2 layer on p-Si has been reported previously.13 By contrast, when the cobaloxime catalyst is chemically bound to the TiO2 surface, such an anodic dark current was not observed in the reverse scan (Figures 5b and S17a), regardless of the anchoring group. For comparison, we also measured the LSVs of p-Si/TiO2 in the BBSs containing CoPy under chopped illumination with scanning in both reducing and oxidizing directions. In the presence of CoPy, the reverse scan of pSi/TiO2 displayed an apparent anodic dark current in the range of −0.3 to 0.3 V (Figure S17b), which was only slightly reduced as compared to that observed for p-Si/TiO2 measured in the absence of CoPy. This observation signifies that the surface-attached catalyst can substantially facilitate the electron transfer at the semiconductor/electrolyte interface of the p-Si/TiO2 photocathode, while the electrons trapped in the CB of TiO2 cannot be effectively extracted out and used for proton reduction by the free cobaloxime catalyst. 17



Figure 5. LSVs of (a) p-Si/TiO2 and (b) p-Si/TiO2/CoPy-4-CONH(OH) under chopped illumination at 5 mV s−1 in both reducing and oxidizing directions. (c) Plots of current density versus time under chopped illumination at 0 V and (d) Nyquist plots under continuous illumination at 0 V for the as-prepared photocathodes and the p-Si/TiO2 reference electrode in the presence and absence of CoPy (inset: Randles equivalent circuit diagram).

The apparent anodic dark current of p-Si/TiO2 was also observed in the j‒t plot of CPP under chopped illumination at 0 V (Figure 5c), which illustrated that some long-lived excited electrons have been trapped in the CB of TiO2 and discharged in the dark. By contrast, all three p-Si/TiO2 electrodes with surface-bound cobaloxime catalysts did not display the anodic dark current in their

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j‒t plots, while p-Si/TiO2 in the BBSs containing the unfunctionalized cobaloxime catalyst displayed a similar photoresponsive behavior to p-Si/TiO2 in the absence of catalyst (Figure 5c). These results demonstrate fast electron transfer from the CB of TiO2 to the attached catalysts but sluggish to the free cobaloxime in solution, which has been further demonstrated by TA spectroscopic studies (vide infra). Subsequently, the electrochemical impedance spectroscopy (EIS) was applied to investigate the influence of the anchoring groups on electron-transfer kinetics of the hybrid photocathodes. Figure 5d shows that the semicircles corresponding to the p-Si/TiO2 electrodes with surface-bound cobaloxime catalysts have much smaller diameters than that corresponding to bare p-Si/TiO2, indicating that the charge-transfer resistance (Rct) at the semiconductor/electrolyte interface of photoelectrodes is greatly reduced by chemically bonding cobaloxime catalysts to the surface of TiO2. The Rct values obtained from data fitting of the Nyquist plots using a simplified Randles equivalent circuit (inset in Figure 5d) are 461, 330, and 214 Ω for the CoPy-4-COOH-, CoPy-4PO3H2-, and CoPy-4-CONH(OH)-decorated p-Si/TiO2 electrodes, respectively, which are significantly smaller than that (ca. 1500 Ω) for bare p-Si/TiO2 in the absence and presence of the free-cobaloxime catalyst. The decreasing order of the Rct values obtained for the hybrid photocathodes with different anchoring groups, namely COOH > PO3H2 > CONH(OH), is consistent with the increasing order of the photoelectrocatalytic activities of these photocathodes for PEC H2 production as shown in Figure 3. The EIS studies demonstrated that the interfacial charge-transfer resistances of the Si-based hybrid photocathodes with chemically attached CoPy4-X on the surface were significantly reduced as compared to that observed for bare p-Si/TiO2 in the presence of the anchor-free cobaloxime catalyst in electrolytes. As such, the photogenerated electrons trapped in the CB of TiO2 could be effectively extracted out by the surface-bound

19



cobaloxime catalysts and used for proton reduction, while the free cobaloxime catalyst in electrolyte did not display a considerable role in facilitating the interfacial electron transfer. Transient Spectroscopic Studies on Charge Separation and Recombination. In terms of the actual PEC processes, the interfacial electron transfer and the electron-hole recombination are competitive processes. Speeding the interfacial electron transfer and in the meantime retarding the back recombination is the effective engineering strategy to let more excited electrons transfer to the catalyst and participate in the redox reactions. To have an insight into the influence of different anchoring groups on the electron/hole transfer rates, we studied the dynamics of photogenerated electrons and holes in the TiO2 layer for the as-prepared FTO/TiO2/CoPy-4-X electrodes and also for bare FTO/TiO2 in the presence and absence of CoPy by TA spectroscopy. For fabrication of FTO/TiO2/CoPy-4-X electrodes, the dipping processes were monitored by the inductively coupled plasma optical emission spectroscopy (ICP-OES) analyses to track the loading amount of the catalyst on the FTO/TiO2 electrode at different time of the dipping period, to ensure that the cobaloxime catalysts bearing different anchors were immobilized on the TiO2 film in a similar amount. It was found that approximately identical amounts of the functionalized cobaloxime catalysts (12.6‒13.0 nmol cm−2) were grafted on the electrode surface when the FTO/TiO2 electrodes were dipped in the MeOH solutions of 0.2 mM CoPy-4-PO3H2 for 20 min, CoPy-4COOH for 9 h, and CoPy-4-CONH(OH) for 14 h in the dark (see Experimental section for details). First, to study the electron transfer dynamics, the decay of the photogenerated electrons in the TiO2 layer was monitored by TA spectroscopy with triethanolamine (TEOA) as a hole scavenger in BBS at pH 9, with a probing wavelength of 900 nm (Figure 6a, Table S3).26 The excited electrons for bare FTO/TiO2 were long-lived with t50% ~ 151 ms in the presence of TEOA. The decay of the excited electrons was considerably accelerated by the immobilized cobaloxime complexes, indicative of an effective electron transfer from TiO2 to the tethered catalysts. The excited electrons 20


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in the TiO2 layer decorated with CoPy-4-CONH(OH) displayed the shortest lifetime, with t50% ~ 10 μs, while the lifetimes of photogenerated electrons were t50% ~ 94 and 197 μs for the FTO/TiO2 electrodes decorated with CoPy-4-COOH and CoPy-4-PO3H2, respectively. The difference in t50% is caused by the anchoring groups employed. Figure 6a clearly shows that the t50% values for the cobaloxime-immobilized FTO/TiO2 electrodes are more than 766 times shorter than that for bare FTO/TiO2. When bare FTO/TiO2 was measured in the presence of free CoPy, t50% was only slightly shortened to about 20 ms. The observations demonstrated that the interfacial electron transfer from TiO2 to the anchored cobaloxime catalysts was significantly faster than to the free catalyst in the bulk solution. And the more important finding was that the electron transfer from TiO2 to the hydroxamate-functionalized cobaloxime catalyst was an order of magnitude faster than to the carboxylate- and phosphonate-functionalized cobaloxime catalysts. According to the TAS results reported in the literature,42 the TA decays (Figure 6a) measured for FTO/TiO2/CoPy-4-X in the presence of a hole scavenger result predominantly from electron transfer from TiO2 to CoIII, while the one from TiO2 to CoII could also make a small contribution to the TA decays. The detailed analysis on the electron transfer from TiO2 to CoIII and further to CoII species is beyond the scope of this paper.

21



Figure 6. (a) TA decays corresponding to the photoexcited electrons in the TiO2 layer for the asprepared FTO/TiO2/CoPy-4-X electrodes and bare FTO/TiO2 in the presence and absence of CoPy, measured at pH 9 with TEOA (0.1 M) as a hole scavenger. All t50% values were calculated using the transient absorption of bare FTO/TiO2 at 10 μs as the initial value. TA decays corresponding to photoexcited holes and electrons in the TiO2 layers for FTO/TiO2/CoPy-4-CONH(OH) (b), FTO/TiO2/CoPy-4-PO3H2 (c), FTO/TiO2/CoPy-4-COOH (d), and bare FTO/TiO2 in the presence (e) and absence (f) of CoPy, measured in the absence of hole scavenger.

Subsequently, the effect of the anchoring groups on the electron-hole recombination was investigated by TA measurements of the as-prepared photocathodes in BBSs at pH 9 in the absence of hole scavenger, with the probing wavelengths of 460 nm for holes and 900 nm for electrons (Figure 6b‒f, Table S4).26 The results obtained from the TA measurements in the absence of hole scavenger showed that the signal amplitudes of excited electrons at 10 μs in the cobaloximeimmobilized FTO/TiO2 electrodes were lower than that of bare FTO/TiO2, resulting from the 22


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effective electrons transfer from TiO2 to immobilized catalysts. In accordance with these results, the signal amplitudes of excited holes at 10 μs for the cobaloxime-decorated FTO/TiO2 electrodes were higher than that for bare FTO/TiO2, indicating the retardation of charge recombination in FTO/TiO2/CoPy-4-X. As a result, the excited holes in the valence band of cobaloxime-decorated TiO2 displayed t50% of ~ 572, ~ 712, and ~ 926 μs for the FTO/TiO2 electrodes decorated with CoPy-4-COOH, CoPy-4-PO3H2, and CoPy-4-CONH(OH), respectively, which are considerably slower than t50% (~194 μs) for bare FTO/TiO2. When bare FTO/TiO2 was measured in the presence of free CoPy in solution, t50% was slightly increased to ~ 313 μs, indicating that the anchored cobaloxime catalysts were much more effective to retard back recombination than the free catalyst in the bulk solution. Importantly, the FTO/TiO2 electrode with the cobaloxime catalyst immobilized by a hydroxamate anchor exhibited not only the shortest lifetime of the excited electrons, but also the longest lifetime of the photogenerated holes in the TiO2 layer, resulting in the best charge separation in the hybrid photocathode. It manifests that the hydroxamate is much more effective as an anchor than the carboxylate and phosphonate groups to greatly accelerate the interfacial electron transfer and to effectively retard the electron-hole recombination. On the basis of these results, the hydroxamate is an appropriate anchoring group to immobilize molecular catalysts for designing of efficient and stable SC/MC hybrid photoelectrodes in future.

CONCLUSIONS In summary, a series of hybrid photocathodes were constructed by chemically immobilizing cobaloxime complexes to the surface of p-Si/TiO2 through different anchoring groups. Comparative studies on the PEC performance of p-Si/TiO2/CoPy-4-X (X = PO3H2, COOH, CONH(OH)) revealed that the hybrid photocathode with a hydroxamate as an anchoring group exhibited a higher photocurrent, a lower interfacial charge-transfer resistance, and a better stability 23



than the ones with a carboxylate and a phosphonate as anchoring groups for the PEC H2 production under test conditions. Notably, the stability of p-Si/TiO2/CoPy-4-CONH(OH) is also better than the cobaloxime-decorated inexpensive semiconductor photocathodes, either sensitized by dyes and quantum dots or not, reported to date for PEC H2 production. The superior stability of pSi/TiO2/CoPy-4-CONH(OH) is thanks to the good binding affinity of CONH(OH) to metal oxides to resist the detachment of molecular catalysts. The TA spectroscopic studies of FTO/TiO2/CoPy4-X manifested that (i) the anchoring groups have an important impact on the kinetics of electron transfer and back recombination in the hybrid photocathodes, and (ii) the FTO/TiO2 electrode with cobaloxime catalyst immobilized by hydroxamate displayed much faster t50% of excited electrons and slower t50% of photogenerated holes compared to the electrodes with a carboxylate and a phosphonate as anchoring groups. In short, this part of work unveiled the remarkable merits of hydroxamate as an anchor of molecular catalysts in the aspects of reducing charge transport resistance, facilitating interfacial electron transfer, retarding electro-hole recombination, enhancing photocurrent, and more importantly, improving the stability of VLAS/MC hybrid photoelectrodes. Encouraged by these results, we will use hydroxamate as a replacement of commonly used carboxylate and phosphonate anchors in our future studies for building highly efficient and stable SC/MC hybrid photoelectrodes for PEC H2 production.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.xxxxxxx.

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Additional MS and 1H NMR spectra; SEM images; XPS spectra; ATR-FTIR spectra; UVvis spectra; LSVs and CVs; long-time CPP curve; IPCE spectrum; summarization of ICP−OES analysis; important photoelectrocatalytic data and dynamic data.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (M.W.) *E-mail: [email protected] (X.W.) ORCID Mei Wang: 0000-0002-5531-5056 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 21673028 and 21373040) and the Basic Research Program of China (Grant No. 2014CB239402).

REFERENCES (1) 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. (2) Pinaud, B. A.; Benck, J. D.; Seitz, L. C.; Forman, A. J.; Chen, Z.; Deutsch, T. G.; James, B. D.; Baum, K. N.; Baum, G. N.; Ardo, S.; Wang, H.; Miller, E.; Jaramillo, T. F. Technical and Economic

25



Page 26 of 32

Feasibility of Centralized Facilities for Solar Hydrogen Production via Photocatalysis and Photoelectrochemistry. Energy Environ. Sci. 2013, 6, 1983−2002. (3) Ager, J. W.; Shaner, M. R.; Walczak, K. A.; Sharp, I. D.; Ardo, S. Experimental Demonstrations of Spontaneous, Solar-Driven Photoelectrochemical Water Splitting. Energy Environ. Sci. 2015, 8, 2811−2824. (4) Kaneko, H.; Minegishi, T.; Domen, K. Recent Progress in the Surface Modification of Photoelectrodes toward Efficient and Stable Overall Water Splitting. Chem.-Eur. J. 2018, 24, 5697−5706. (5) Queyriaux, N.; Kaeffer, N.; Morozan, A.; Chavarot-Kerlidou, M.; Artero, V. Molecular Cathode and Photocathode Materials for Hydrogen Evolution in Photoelectrochemical Devices. J. Photochem. Photobiol C: Photochem. Rev. 2015, 25, 90−105. (6) Wang,

M.;

Yang,

Y.;

Shen,

J.;

Jiang,

J.;

Sun,

L.

Visible-Light-Absorbing

Semiconductor/Molecular Catalyst Hybrid Photoelectrodes for H2 or O2 Evolution: Recent Advances and Challenges. Sustainable Energy Fuels 2017, 1, 1641−1663. (7) Dalle, K. E.; Warnan, J.; Leung, J. J.; Reuillard, B.; Karmel, I. S.; Reisner, E. Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes. Chem. Rev. 2019, 4, 2752−2875. (8) 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. (9) Seger, B.; Herbst, K.; Pedersen, T.; Abrams, B.; Vesborg, P. C. K.; Hansen, O.; Chorkendorff, I. Mo3S4 Clusters as an Effective H2 Evolution Catalyst on Protected Si Photocathodes. J. Electrochem. Soc. 2014, 161, H722−H724. 26


Page 27 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(10) 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. (11) 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. (12) Kim, H. J.; Seo, J.; Rose, M. J. H2 Photogeneration Using a Phosphonate-Anchored Ni-PNP Catalyst on a Band-Edge-Modified p-Si(111)|AZO Construct. ACS Appl. Mater. Interfaces 2016, 8, 1061−1066. (13) Leung, J. J.; Warnan, J.; Nam, D. H.; Zhang, J. Z.; Willkomm, J.; Reisner, E. Photoelectrocatalytic H2 Evolution in Water with Molecular Catalysts Immobilised on p-Si via a Stabilising Mesoporous TiO2 Interlayer. Chem. Sci. 2017, 8, 5172−5180. (14) Downes, C. A.; Marinescu, S. C. Efficient Electrochemical and Photoelectrochemical H2 Production from Water by a Cobalt Dithiolene One-Dimensional Metal–Organic Surface. J. Am. Chem. Soc. 2015, 137, 13740−13743. (15) Chandrasekaran, S.; Kaeffer, N.; Cagnon, L.; Aldakov, D.; Fize, J.; Nonglaton, G.; Baleras, F.; Mailley, P.; Artero, V. A Robust ALD-Protected Silicon-Based Hybrid Photoelectrode for Hydrogen Evolution under Aqueous Conditions. Chem. Sci. 2019, 10, 4469−4475. (16) 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. (17) 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-LightAbsorbing Semiconductor. J. Am. Chem. Soc. 2013, 135, 11861−11868.

27



Page 28 of 32

(18) 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. (19) Beiler, A. M.; Khusnutdinova, D.; Jacob, S. I.; Moore, G. F. Chemistry at the Interface: Polymer-Functionalized GaP Semiconductors for Solar Hydrogen Production. Ind. Eng. Chem. Res. 2016, 55, 5306−5314. (20) Beiler, A. M.; Khusnutdinova, D.; Jacob, S. I.; Moore, G. F. Solar Hydrogen Production Using Molecular Catalysts Immobilized on Gallium Phosphide (111)A and (111)B Polymer-Modified Photocathodes. ACS Appl. Mater. Interfaces 2016, 8, 10038−10047. (21) Khusnutdinova, D.; Beiler, A. M.; Wadsworth, B. L.; Jacob, S. I.; Moore, G. F. Metalloporphyrin-Modified Semiconductors for Solar Fuel Production. Chem. Sci. 2017, 8, 253−259. (22) Gu, J.; Yan, Y.; Young, J. L.; Steirer, K. X.; Neale, N. R.; Turner, J. A. Water Reduction by a p-GaInP2 Photoelectrode Stabilized by an Amorphous TiO2 Coating and a Molecular Cobalt Catalyst. Nat. Mater. 2016, 15, 456−460. (23) Bae, D.; Seger, B.; Vesborg, P. C.; Hansen, O.; Chorkendorff, I. Strategies for Stable Water Splitting via Protected Photoelectrodes. Chem. Soc. Rev. 2017, 46, 1933−1954. (24) Zhang, D.; Shi, J.; Zi, W.; Wang, P.; Liu, S. Recent Advances in Photoelectrochemical Applications of Silicon Materials for Solar-to-Chemicals Conversion. ChemSusChem 2017, 10, 4324−4341. (25) Luo, Z.; Wang, T.; Gong, J. Single-Crystal Silicon-Based Electrodes for Unbiased Solar Water Splitting: Current Status and Prospects. Chem. Soc. Rev. 2019, 48, 2158−2181. (26) Reynal, A.; Willkomm, J.; Muresan, N. M.; Lakadamyali, F.; Planells, M.; Reisner, E.; Durrant,

J.

R.

Distance

Dependent

Charge 28

Separation


and

Recombination

in

Page 29 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Semiconductor/Molecular Catalyst Systems for Water Splitting. Chem. Commun. 2014, 50, 12768−12771. (27) Brewster, T. P.; Konezny, S. J.; Sheehan, S. W.; Martini, L. A.; Schmuttenmaer, C. A.; Batista, V. S.; Crabtree, R. H. Hydroxamate Anchors for Improved Photoconversion in Dye-Sensitized Solar Cells. Inorg. Chem. 2013, 52, 6752−6764. (28) Martini, L. A.; Moore, G. F.; Milot, R. L.; Cai, L. Z.; Sheehan, S. W.; Schmuttenmaer, C. A.; Brudvig, G. W.; Crabtree, R. H. Modular Assembly of High-Potential Zinc Porphyrin Photosensitizers Attached to TiO2 with a Series of Anchoring Groups. J. Phys. Chem. C. 2013, 117, 14526−14533. (29) Brennan, B. J.; Portoles, M. J. L.; Liddell, P. A.; Moore, T. A.; Moore, A. L.; Gust, D. Comparison of Silatrane, Phosphonic Acid, and Carboxylic Acid Functional Groups for Attachment of Porphyrinsensitizers to TiO2 in Photoelectrochemical Cells. Phys. Chem. Chem. Phys. 2013, 15, 16605−16614. (30) Takijiri, K.; Morita, K.; Nakazono, T.; Sakai, K.; Ozawa, H. Highly Stable Chemisorption of Dyes with Pyridyl Anchors over TiO2: Application in Dye-Sensitized Photoelectrochemical Water Reduction in Aqueous Media. Chem. Commun. 2017, 53, 3042−3045. (31) Materna, K. L.; Crabtree, R. H.; Brudvig, G. W. Anchoring Groups for Photocatalytic Water Oxidation on Metal Oxide Surfaces. Chem. Soc. Rev. 2017, 46, 6099−6110. (32) Bosch, B.; Rombouts, J. A.; Orru, R. V. A.; Reek, J. N. H.; Detz, R. J. Nickel-Based DyeSensitized Photocathode: Towards Proton Reduction Using a Molecular Nickel Catalyst and an Organic Dye. ChemCatChem 2016, 8, 1392−1398. (33) Konar, S.; Zon, J.; Prosvirin, A. V.; Dunbar, K. R.; Clearfield, A. Synthesis and Characterization of Four Metal−Organophosphonates with One-, Two-, and Three-Dimensional Structures. Inorg. Chem. 2007, 46, 5229−5236. 29



(34) Trogler, W. C.; Stewart, R. C.; Epps, L. A.; Marzilli, L. G. Cis and Trans Effects on the Proton Magnetic Resonance Spectra of Cobaloximes. Inorg. Chem. 1974, 13, 1564−1570. (35) Kast, M. G.; Enman, L. J.; Gurnon, N. J.; Nadarajah, A.; Boettcher, S. W. Solution-Deposited F:SnO2/TiO2 as a Base-Stable Protective Layer and Antireflective Coating for Microtextured Buried-Junction H2-Evolving Si Photocathodes. ACS Appl. Mater. Interfaces 2014, 6, 22830−22837. (36) 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. (37) Reuillard, B.; Warnan, J.; Leung, J. J.; Wakerley, D. W.; Reisner, E. A Poly(cobaloxime)/Carbon Nanotube Electrode: Freestanding Buckypaper with Polymer-Enhanced H2-Evolution Performance. Angew. Chem. Int. Ed. 2016, 55, 3952−3957. (38) Yang, J.; Bremer, P. J.; Lamont, I. L.; McQuillan, A. J. Infrared Spectroscopic Studies of Siderophore-Related Hydroxamic Acid Ligands Adsorbed on Titanium Dioxide. Langmuir 2006, 22, 10109−10117. (39) McNamara, W. R.; Snoeberger III, R. C.; Li, G.; Richter, C.; Allen, L. J.; Milot, R. L.; Schmuttenmaer, C. A.; Crabtree, R. H.; Brudvig, G. W.; Batista, V. S. Hydroxamate Anchors for Water-Stable Attachment to TiO2 Nanoparticles. Energy. Environ. Sci. 2009, 2, 1173−1175. (40) McNamara, W. R.; Milot, R. L.; Song, H.; Snoeberger III, R. C.; Batista, V. S.; Schmuttenmaer, C. A.; Brudvig, G. W.; Crabtree, R. H. Water-Stable, Hydroxamate Anchors for Functionalization of TiO2 Surfaces with Ultrafast Interfacial Electron Transfer. Energy. Environ. Sci. 2010, 3, 917−923. (41) Bae, D.; Shayestehaminzadeh, S.; Thorsteinsson, E. B.; Pedersen, T.; Hansen, O.; Seger, B.; Vesborg, P. C. K.; Ólafsson, S.; Chorkendorff, I. Protection of Si Photocathode Using TiO2 30


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Deposited by High Power Impulse Magnetron Sputtering for H2 Evolution in Alkaline Media. Sol. Energy Mater. Sol. Cells 2016, 144, 758−765. (42) A. Reynal, F. Lakadamyali, M. A. Gross, E. Reisner and J. R. Durrant, Energy Environ. Sci., 2013, 6, 3291−3300.

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Graphic Abstract:

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Influence of Anchoring Groups on the Charge Transfer and the

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