Versatile photocatalytic systems for H2 generation in water based on an efficient DuBois-type nickel catalyst.

The generation of renewable H2 through an efficient photochemical route requires photoinduced electron transfer (ET) from a light harvester to an efficient electrocatalyst in water. Here, we report on a molecular H2 evolution catalyst (NiP) with a DuBois-type [Ni(P2(R')N2(R"))2](2+) core (P2(R')N2(R") = bis(1,5-R'-diphospha-3,7-R"-diazacyclooctane), which contains an outer coordination sphere with phosphonic acid groups. The latter functionality allows for good solubility in water and immobilization on metal oxide semiconductors. Electrochemical studies confirm that NiP is a highly active electrocatalyst in aqueous electrolyte solution (overpotential of approximately 200 mV at pH 4.5 with a Faradaic yield of 85 ± 4%). Photocatalytic experiments and investigations on the ET kinetics were carried out in combination with a phosphonated Ru(II) tris(bipyridine) dye (RuP) in homogeneous and heterogeneous environments. Time-resolved luminescence and transient absorption spectroscopy studies confirmed that directed ET from RuP to NiP occurs efficiently in all systems on the nano- to microsecond time scale, through three distinct routes: reductive quenching of RuP in solution or on the surface of ZrO2 ("on particle" system) or oxidative quenching of RuP when the compounds were immobilized on TiO2 ("through particle" system). Our studies show that NiP can be used in a purely aqueous solution and on a semiconductor surface with a high degree of versatility. A high TOF of 460 ± 60 h(-1) with a TON of 723 ± 171 for photocatalytic H2 generation with a molecular Ni catalyst in water and a photon-to-H2 quantum yield of approximately 10% were achieved for the homogeneous system.

Introduction

The sunlight-driven generation of the energy carrier H2 from water using earth-abundant materials is considered one of the key processes to generate more sustainable fuels in a postfossil age.[1] Synthetic first-row transition-metal complexes containing Co,[2] Fe,[3] and Ni[4] are under intense development as scalable alternatives to the benchmark H2 evolution catalysts platinum[5] and hydrogenases.[6] Complexes with a catalytic bis(1,5-R′-diphospha-3,7-R″-diazacyclooctane)nickel(II) core, [Ni(P2R′N2R″)2]2+, developed by DuBois and co-workers, emerged in the past decade as probably the most active synthetic 3d transition-metal electrocatalysts for proton reduction.[4b,4c,7] The catalytic cycle has been studied experimentally[8] and computationally,[9] and the catalyst mimics important features of the hydrogenase active site.[4c,7c,10]

Photocatalytic H2 generation requires the efficient coupling of an efficient proton reduction electrocatalyst with a light-harvesting component. In particular, the development and investigation of molecular H2 evolution catalysts made of abundant elements that show stability and activity in water are of major interest.[11] Many reports on homogeneous photocatalytic systems for water reduction using Fe[3a,3e,12] or Co[2b,13] complexes are available, but there are relatively few using Ni-based catalysts.[4f−4i,14] Heterogenization of well-defined molecular catalysts on semiconductor surfaces is an emerging approach for photocatalytic H2 generation. Systems that have been shown to have notable H2 production efficiencies include cobaloximes immobilized on TiO2 nanoparticles[13f−13i] or CdSe/ZnS quantum dots,[13j] and [FeFe]-hydrogenase mimics on CdTe[12a] or ZnS[15] quantum dots. Efforts have also been made to integrate molecular catalysts on photoelectrodes, such as an [FeFe] complex on InP,[12c] a cobaloxime catalyst on p-type GaP,[16] and Fe(dithiolato)(diphosphine)[17] or cobaloxime catalysts[18] on dye-sensitized NiO films. However, the frequent use of organic solvents, poor light-to-H2 conversion efficiencies, and photoinstabilities are drawbacks in these systems.

Despite the promising properties of DuBois-type catalysts, they were previously only applied in homogeneous photocatalytic schemes in the presence of organic solvents or a biological matrix.[4g,19] Surface immobilization of a [Ni(P2R′N2R″)2]2+ derivative on carbon nanotubes yielded highly active electrodes for H2 generation under strongly acidic conditions.[4e] When a [Ni(P2R′N2R″)2]2+ derivative was immobilized on a p-type silicon photoelectrode, no (photo)activity for H2 production was reported.[20] Water-soluble [Ni(P2R′N2R″)2]2+ derivatives were previously not shown to operate in aqueous homogeneous or heterogeneous environments for photocatalytic H2 generation.

In this work, we report on a novel DuBois-type catalyst (NiP; Figure 1), which not only contains a highly electroactive [Ni(P2R′N2R″)2]2+ core, but also is surrounded by an outer sphere containing four dangling phosphonic acid moieties. This outer-sphere feature provides the catalyst with good solubility in aqueous solutions and anchors for immobilization onto metal oxide surfaces.[21] Electrochemical studies of NiP in an aqueous electrolyte solution demonstrated sustained electrocatalytic activity in the absence of organic solvents. Subsequently, we studied NiP in aqueous photochemical systems with a phosphonated bipyridine-based Ru(II) photosensitizer (RuP; Figure 1) in both a homogeneous system and heterogeneous photocatalytic environments where RuP and NiP were immobilized on different metal oxide semiconductors.

Figure 1

Chemical structures of the doubly charged cationic complexes used in this study: the electrocatalyst NiP and dye RuP. Both complexes have bromide counterions.

The kinetics of charge separation and recombination were studied by time-correlated single photon counting (TC-SPC) and transient absorbance spectroscopy (TAS). This spectroscopic study reveals that distinct reaction mechanisms are possible in these remarkably versatile photochemical systems. In particular, we consider whether this coimmobilization strategy on metal oxide surfaces reduces the rapid electron/hole recombination losses typically observed when such photosensitizers and catalysts are covalently attached via molecular linker groups.[22] The visible-light-driven H2 generation of the different systems was also studied in bulk experiments and revealed that NiP can operate with a high photocatalytic turnover frequency for a Ni-based molecular proton reduction catalyst in purely aqueous systems.

Experimental Section

Materials and Methods

All synthetic procedures involving air- or moisture-sensitive materials were carried out under N2 by using either a glovebox or Schlenk techniques. Chemicals for the synthetic part of this work were purchased from commercial suppliers and used without further purification. Solvents were dried using standard purification procedures under an N2 atmosphere. Chemicals for analytical measurements were purchased in the highest available purity. TiO2 nanoparticles (Aeroxide TiO2 P25 particles; anatase/rutile (8/2) mixture, average particle size 21 nm) were a gift from Evonik Industries, and ZrO2 nanoparticles (99.9%, 20–30 nm) were obtained from Skyspring Nanomaterials Inc. Nanostructured anatase TiO2 and ZrO2 films were prepared by the Doctor Blading technique from colloidal pastes as reported previously.[13f,23] The films were annealed at 450 °C for 30 min prior to use, and the anatase phase for TiO2 was confirmed by X-ray diffraction studies after heating. The resulting film thicknesses, determined by profilometry (Tencor Instruments), were 4 μm. [Ni(P2N2)2](BF4)2 (NiP)[7a] (Figure S1, Supporting Information) and RuP(24) were prepared according to published procedures.

Physical Measurements

1H and 31P NMR spectra were recorded on a Bruker 400 MHz spectrometer. 1H NMR spectra were referenced to the solvent residual peaks as an internal reference,[25] and 31P NMR spectra were referenced to an external standard (85% H3PO4 in D2O). UV–vis spectra were recorded on a Varian Cary 50 UV–vis spectrophotometer using quartz glass cuvettes. High-resolution electrospray ionization mass spectra (HR-ESI-MS) were recorded on a Quattro LC spectrometer, and the theoretical and experimental isotope distributions were compared. Elemental analysis was carried out by the microanalytical laboratory in the Department of Chemistry of the University of Cambridge.

Synthesis of [Ni(P2N2)2]Br2·HBr (NiP)

Bromotrimethylsilane (0.25 mL, 1.86 mmol) was added dropwise to a deep red solution of NiP (250 mg, 0.143 mmol) in degassed dichloromethane (15 mL) under a N2 atmosphere at room temperature. The color changed to deep purple, and the solution was stirred for 2 days at room temperature. The solvent was removed, followed by addition of deoxygenated MeOH (HPLC grade, 15 mL) to the dark solid residue and stirring under N2 for 1 day at room temperature. The volume of the solvent was reduced to approximately one-third, and the product was precipitated with Et2O (∼25 mL). After the mixture was stirred for 30 min, the deep purple precipitate was filtered off under N2, washed with Et2O, and dried in vacuo. The solid was redissolved in MeOH, the solution was stirred for several minutes, and the solvent was evaporated under high vacuum to give a purple solid. Yield: 183 mg (81%). 1H NMR (400 MHz, CD3OD, δ): very broad signals, 8.23–6.70 (m, 36H, Ph), 3.60–4.70 (m, 16H, NCH2P), 2.97–3.22 (m, 8H, PhCH2PO(OH)2) ppm. 31P{1H} NMR (162 MHz, CD3OD, δ): 25.8 (PO(OH)2), 21.0 (NCH2P), −13.9 (NCH2P) ppm. HR-ESI-MS (MeOH, negative): m/z calcd for [M – 2H + Br]− 1419.1121, found 1419.1154; m/z calcd for [M – 3H]−, 1339.1895, found 1339.1903. Anal. Calcd for C60H68Br2N4NiO12P8·HBr·2H2O: C, 44.47; H, 4.54; N, 3.46; P, 15.29; Br, 14.79. Found: C, 44.45; H, 4.70; N, 3.24; P, 14.61; Br, 15.08. UV–vis (MeOH): λmax, nm (ε) 510 (1100).

Electrochemistry

Electrochemical measurements were performed on an IviumStat or Ivium CompactStat potentiostat under an argon atmosphere using a three-electrode configuration. A glassy-carbon disk (3 mm diameter) working electrode, a platinum-wire counter electrode, and a Ag/AgCl/KCl(sat) reference electrode were used for cyclic voltammetry (CV). All potentials were converted to the normal hydrogen electrode (NHE) by addition of +0.197 V.[26]

Controlled-potential electrolysis (CPE) was carried out in a three-necked flask with a three-electrode setup using glassy-carbon rods as working and counter electrodes (surface area in solution ∼2 cm2) and a Ag/AgCl/KCl(sat) reference electrode. A solution of NiP (0.18 mM) in ascorbic acid (AA, 0.1 M, pH 4.5) was purged with N2 containing CH4 (2%) as the internal gas chromatography (GC) standard, and a potential of −0.5 V vs NHE was applied during CPE for 2 h. The Faradaic yield was calculated from the amount of H2 accumulated in the headspace, as measured by GC. CPE of catalyst-free electrolyte at the same potential showed no production of H2.

Preparation of RuP-Sensitized TiO2 and ZrO2 Films Loaded with and without NiP for Spectroscopic Studies

To prepare TiO2 and ZrO2 films sensitized with RuP, an aqueous solution of RuP (10 μL of 4 μM) was spread onto the films (geometrical surface area 1.5 cm2), and the solvent was dried in air for 30 min. For TiO2 and ZrO2 films cofunctionalized with RuP and NiP, first a solution of NiP (10 μL of 8 μM) in MeOH was spread onto the surface of the film, followed by drying for 30 min in air. Then, an aqueous RuP solution (10 μL of a 4 μM) was spread onto the surface of the film and was dried for an additional 30 min.

Spectroscopic Characterization

Spectroscopic measurements on homogeneous solutions and functionalized TiO2 and ZrO2 films were carried out in water or in aqueous AA solution (0.1 M) carefully adjusted to pH 4.5 with NaOH (0.1 M), unless otherwise stated. A fresh AA solution was prepared prior to any measurements. The aqueous solutions were purged with N2 for 15 min prior to the measurements. The UV–vis and fluorescence spectra of the solutions and films were recorded using a quartz cuvette (1 cm path length) on a Perkin–Elmer Lambda 35 UV–vis spectrophotometer and a Horiba Jobin Yvon Fluorolog luminescence spectrophotometer, respectively. TC-SPC measurements were performed by using a Horiba Jobin Yvon TBX Fluorocube system. As the excitation source, a pulsed laser with 467 nm nominal wavelength at a repetition rate of 100 kHz was used. The photoluminescence intensity of RuP at λem 650 nm was measured as a function of time after the excitation pulse, for a fixed data collection period to ensure matched densities of absorbed photons between samples (600 s). The instrument response was measured at the full width at half-maximum and showed typically a 200–250 ps value. All TC-SPC experiments undertaken on TiO2 and ZrO2 films were measured under aqueous conditions or a 0.1 M AA solution at pH 4.5 in air.

The microsecond to second transient absorption decays were measured using a Nd:YAG laser (Big Sky Laser Technologies Ultra CFR Nd:YAG laser system, 6 ns pulse width). The second and third harmonics of the laser (corresponding to 532 and 355 nm, respectively) were used to excite RuP. The laser intensity was adjusted using neutral density filters as appropriate, with experiments typically employing 350 μJ cm–2, and the frequency of the laser pulse was fixed to 1 Hz. A liquid light guide with a diameter of 0.5 cm was used to transmit the laser pulse to the sample. The probe light source was a 100 W Bentham IL1 tungsten lamp, and the probing wavelength was selected by using a monochromator (OBB-2001, Photon Technology International) placed prior to the sample. Several high-pass, low-pass and neutral-density filters (Comar Optics) were used to decrease the light arriving to the detector. Transient absorption data were collected with a Si photodiode (Hamamatsu S3071). The information was passed through an amplifier box (Costronics) and recorded using a Tektronics TDS 2012c oscilloscope (microsecond to millisecond time scale) and a National Instruments (NI USB-6211) DAQ card (millisecond to second time scale). The decays observed were the average of 500 laser pulses. The data were processed using home-built software based on Labview.

Photocatalytic H2 Evolution Experiments

All photocatalytic experiments were carried out using a Solar Light Simulator (Newport Oriel, 100 mW cm–2) equipped with an air mass 1.5 global filter (AM 1.5G). UV irradiation was filtered using a 420 nm cutoff filter (UQG optics), and IR irradiation was filtered by a water filter (path length 10 cm). The photoreactor was held at a constant temperature of 25 °C in all experiments. Samples were generally prepared in air protected from light by an Al foil. The reaction vessel was sealed with a rubber septum, and air was replaced by N2 containing 2% CH4 (internal GC standard). The irradiated cross section of the solution in the vials was approximately 3.3 cm2. H2 evolution was monitored by GC measurements with an Agilent 7890A Series GC equipped with a 5 Å molecular sieve column. The GC oven temperature was kept constant at 45 °C, N2 was used as a carrier gas at an approximate flow rate of 3 mL min–1, and a thermal conductivity detector (TCD) was used. Samples (15–20 μL) for headspace gas analysis were taken from the reaction vessel headspace in 30 min intervals for 2 h in the screening experiments. For long-term measurements longer intervals were chosen. The response factor of the thermal conductivity detector for H2 compared to CH4 was 1.91.

Preparation of Homogeneous System RuP-NiP

In a typical experiment, a freshly prepared AA solution was titrated to the desired pH using NaOH (0.1 M) or HBF4 (0.1 M) and diluted to a final AA concentration of 0.1 M (the final pH was confirmed). Stock solutions of RuP (1.0 or 2.0 mM in H2O) and NiP (2.0 mM in MeOH) were added in the desired ratio to the aqueous AA solution (0.1 M) to reach a final volume of 2.25 mL, leaving 5.59 mL of gas headspace in the vial. Control experiments in the absence of MeOH were carried out and confirmed a comparable photocatalytic activity.

Immobilization of RuP and NiP on TiO2 and ZrO2: RuP-TiO2-NiP and RuP-ZrO2-NiP

AA (0.1 M) was prepared as described above. ZrO2 or TiO2 nanoparticles (2.5 mg) were placed in a vial containing an AA solution and dispersed by sonication for 5 min. To load the particles, first NiP was added and after stirring for several min RuP was added, resulting in a final volume of 2.25 mL, leaving 5.59 mL of gas headspace in the vial.

Quantification of Catalyst Loading on the Nanoparticles by UV–Vis Spectrophotometry

The adsorption of NiP on TiO2 and ZrO2 nanoparticles was quantified by recording the difference UV–vis spectrum of a solution of NiP in AA (0.1 M, pH 4.5, 2.25 mL) before and after exposure to TiO2 or ZrO2 nanoparticles (2.5 mg). The solution was stirred with the nanoparticles for 30 min, followed by centrifugation (7000 rpm, 5 min) between the measurements. Adsorption of RuP on nanoparticles after coadsorption with NiP was estimated by adding RuP (0.05 μmol) to a suspension of NiP-sensitized TiO2 or ZrO2 particles (0.02 or 0.1 μmol NiP loading on 2.5 mg nanoparticles). The suspensions were purged with N2 during catalyst loading. Quantitative adsorption of 0.05 μmol RuP and 0.02 μmol NiP would result in an estimated surface coverage of approximately 400 RuP and 150 NiP molecules per nanoparticle and approximately 45% coverage of the TiO2 surface (see the Supporting Information).

Determination of Photon to H Quantum Efficiency

The external quantum (photon to H2 conversion) efficiency (EQE) was determined by an LED light source (Modulight, Ivium) using blue light (λ 460 nm, 5 mW cm–2). The light intensity was measured with a Newport thermopile detector (818P-020-12) coupled with an optical power meter (1916-R). Samples of RuP (0.3 μmol) and NiP (0.1 μmol) in solution were used. Aliquots of headspace gas were subjected to GC analysis during irradiation. The efficiency was determined from the amount of H2 produced after 2 h of irradiation using the equation

Treatment of Data

All analytical measurements were repeated at least three times. The obtained data were treated as follows: for a sample of n observations x, the unweighted mean value x0 and the standard deviation σ were calculated using the equationsA minimum σ of 10% was assumed in all experiments. The light sources and gas chromatographs were calibrated regularly to ensure reproducibility throughout all experiments.

Results and Discussion

Synthesis and Characterization of NiP

The catalyst NiP was synthesized through hydrolysis of the octaethyl phosphonate ester analogue NiP [7a] by dealkylation of the ethyl ester groups with trimethylsilyl bromide in dichloromethane[27] (Figure S1, Supporting Information). NiP was obtained in 81% yield and characterized by 1H and 31P NMR spectroscopy, high-resolution mass spectrometry, and elemental analysis for C, H, N, P, and Br. NiP is soluble in water (∼0.3 mg mL–1) and dissolves well in an aqueous ascorbic acid solution (0.1 M, pH 4.5) with a solubility of more than 5 mg mL–1. NiP therefore is an example of a DuBois-type Ni catalyst that is soluble in aqueous solutions and can be employed homogeneously in organic solvent free aqueous catalytic systems.

Electrocatalytic Activity of NiP in Aqueous Solution

Before studying NiP in a purely aqueous electrolyte solution, we compared NiP with the previously studied complex NiP (Figure S1)[7a] in acetonitrile/water mixtures and in the presence of added triflic acid. On a glassy-carbon working electrode under Ar at a scan rate of 100 mV s–1, CV measurements of NiP (0.3–0.5 mM) show two quasi-reversible waves at E1/2 = −0.38 and −0.54 V vs NHE in H2O (0.1 M Na citrate pH 5)/acetonitrile (0.1 M TBABF4) (1/1). The two waves are assigned to the NiII/I and NiI/0 redox couples, respectively (Figure 2A and Figures S2 and S3 (Supporting Information)).[4g] Addition of triflic acid induced a catalytic current attributed to proton reduction below wspH[28] ∼4.9 following the reduction of NiII to NiI with an onset potential of −0.3 V vs NHE (Figure 2A). The small cathodic shift in potential of the reduction wave upon acidification is presumably due to protonation of the dangling phosphonate moieties. Thus, the [Ni(P2R′N2R″)2]2+ core and electrocatalytic proton reduction activity are intact in NiP and the electrochemical response is indeed comparable to that of NiP in the presence of an organic acid (Figures S2–S4, Supporting Information).

Figure 2

(A) CV of NiP (0.5 mM, black trace) in H2O (0.1 M Na citrate pH 5)/acetonitrile (0.1 M TBABF4) (1/1), followed by titration with increasing amounts of triflic acid (0.678 M in H2O). wspH is the concentration of protons in a mixed solvent (water/organic solvent) where the pH was measured in water against an aqueous reference.[28] (B) CV of NiP (0.3 mM, solid red trace) in an aqueous AA solution (0.1 M, pH 4.5). Wave (a) shows the first reduction wave of NiP, and wave (b) indicates the onset of catalytic proton reduction. Reduction potentials of possible reactive intermediates of RuP in a photocatalytic process and the conduction band potential of TiO2 (ECB(TiO2) are also shown. A glassy-carbon working electrode, a Ag/AgCl/KCl(sat.) reference electrode, and a platinum-wire counter electrode were employed at room temperature with a scan rate of 100 mV s–1 in (A) and (B). Control experiments in the absence of NiP are shown as dashed traces.

Electrocatalytic proton reduction by NiP was also observed in the absence of organic solvent in an aqueous solution buffered with ascorbic acid (AA) or Na citrate (0.1 M, pH 4.5). A single, irreversible reduction wave (a) was observed at Ep = −0.35 V vs. NHE, followed by the onset of a catalytic wave (b) at approximately −0.48 V vs NHE (Figure 2B and Figure S5 (Supporting Information)). Therefore, the catalyst operates with a small overpotential requirement of approximately 0.2 V in comparison to the thermodynamic potential for proton reduction of −0.27 V vs NHE at pH 4.5. The reduction potentials of intermediates of RuP, which are likely to be formed during the quenching process after photoexcitation (see below) are also indicated in Figure 2B. The position of these potentials and the conduction band edge of TiO2 suggest that enough driving force for photocatalytic proton reduction with NiP would be available when using RuP and RuP-sensitized TiO2 as a light harvester, as described in detail in the kinetic and photocatalytic sections.

Controlled-potential electrolysis with NiP (0.18 mM) in an aqueous ascorbic acid solution (0.1 M, pH 4.5) on a glassy-carbon-rod working electrode (surface area ∼2 cm2) at −0.5 V vs NHE for 2 h confirmed the generation of H2 gas with a Faradaic yield of 85 ± 4% (H2 in the headspace quantified by GC). Rinsing the electrode with H2O after CPE and immersing it in a fresh electrolyte solution (in the absence of NiP catalyst) did not result in the formation of H2 when continuing with CPE at the same potential. Thus, H2 production originates from a dissolved catalyst and not from electrodeposited decomposition products on the electrode. Thus, the water-soluble NiP displays electroactivity for the reduction of aqueous protons with a high Faradaic yield in the absence of organic solvents, a prerequisite for the use of NiP in photocatalytic schemes in water.

Kinetics and Mechanisms of ET in Photocatalytic Schemes

The suitability of NiP in photocatalytic H2 generation was studied with RuP,[24,29] AA (0.1 M) as a sacrificial electron donor, and buffer (pKa 4.17) in aqueous solution.[30] Photoexcitation of RuP to RuP* (λmax 455 nm, MLCT) can result in either oxidative or reductive quenching of the photoexcited state (Figures 3 and 4).[31] First, the kinetics and mechanisms of ET between RuP and NiP in homogeneous aqueous solution (RuP-NiP system) and anchored onto metal oxide surfaces (RuP-ZrO2-NiP and RuP-TiO2-NiP systems) were studied by TC-SPC and TAS. The three photocatalytic systems are illustrated in Figure 3, and the mechanistic and kinetic details are summarized in Figure 4 and described below. The RuP-NiP and RuP-ZrO2-NiP systems generate H2 through a reductive quenching mechanism of RuP in solution or “on the particle”, respectively. RuP-TiO2-NiP undergoes oxidative quenching and electrons are transferred “through the particle”.

Figure 3

Three distinct photosystems with homogeneous and heterogenized catalysts studied herein. See Figure 4 for kinetic and mechanistic details.

Figure 4

Summary of ET kinetics for the three photocatalytic systems studied as determined by TC-SPC and TAS (defined as t50% times). Recombination reactions are represented with dashed gray arrows.

TiO2 was observed to cause oxidative quenching of the immobilized RuP*, resulting in the oxidized intermediate RuP (E(RuP/RuP*) = −0.95 V vs NHE)[32] by electron injection into the conduction band of TiO2 (ECB = −0.55 V vs NHE at pH 4.5).[33] These kinetics were measured by TC-SPC to take place in approximately 180 ps, with an injection efficiency of >95% (Figure 5).[13f]

Figure 5

Time-resolved luminescence measurements of RuP with and without NiP in water and in the presence of AA (0.1 M) at pH 4.5: (A) anchored onto the surface of a TiO2 and ZrO2 film (10 μL of 4 μM RuP, 10 μL of 8 μM NiP); (B) in a homogeneous solution ([RuP] = 4 μM, [NiP] = 8 μM).

The transient absorption spectrum of a photoexcited RuP-TiO2 film shows a maximum transient absorption peak at 700 nm, corresponding to the absorption spectrum of the oxidized RuP (Figure S6, Supporting Information). In the absence of AA, the TiO2 conduction band electrons recombine with RuP+ within approximately 1 ms. Upon reduction of RuP by AA (t50% ≈ 50 μs), corresponding to the regeneration of RuP, the resulting photoinjected TiO2 electrons exhibit a lifetime of 0.5 s. Following the codeposition of NiP (molecular ratio RuP/NiP of 1/2), the decay of these electrons is accelerated to approximately 1 ms, assigned to ET to NiP (Figure S7, Supporting Information).

The oxidative ET mechanism from RuP to NiP through the semiconductor (“through particle” system, Figure 3C), is possible with TiO2 but not with ZrO2, due to the energetic mismatch between RuP* and the conduction band of ZrO2 (ECB = −1.26 V vs NHE at pH 4.5).[34] ZrO2 is therefore unable to accept electrons from RuP* and can only be used as a matrix to immobilize the compounds in close proximity on the particle (“on particle” system, Figure 3B).

The reductive quenching through intermolecular ET from AA (E = 1.17 V vs NHE)[35] to RuP* (E(RuP*/RuP) = 1.08 V vs NHE)[31] was measured by TC-SPC to take place in approximately 250 ns with an estimated efficiency of 70% in homogeneous RuP-NiP and heterogenized “on particle” RuP-ZrO2-NiP systems (Figure 5).[36] Photoexcitation of an aqueous RuP solution (4 μM) containing AA (0.1 M, pH 4.5) resulted in the appearance of a transient absorption peak at 500 nm (Figure S8, Supporting Information) assigned to the formation of the reactive intermediate RuP.[37] This reduced state of the dye, RuP–, is a strong reducing agent and has a large driving force for the reduction of NiP (E(RuP/RuP) = −1.09 V vs NHE, Figure 2).[31]

The addition of increasing amounts of NiP (from 0 to 16 μM) to a solution containing RuP (4 μM) and AA (0.1 M, pH 4.5) results in the linear decrease of the lifetime of RuP from 700 to 29 μs, following first-order kinetics with respect to NiP concentration for the intermolecular ET between RuP and NiP (Figure 6). The second-order rate constant of this ET is kET = 1.4 × 109 M–1 s–1, indicating that the ET kinetics are diffusion limited (Figure 6B and Table S1 (Supporting Information)). In addition, the presence of NiP resulted in the appearance of a long-lived (t50% ≈ 0.1 s) bleaching signal assigned to the reduction of NiII species (Figure S9 (Supporting Information)) upon reduction by RuP–. Further experiments are ongoing to monitor the nickel catalytic species involved in the H+ reduction reaction.

Figure 6

(A) Transient absorption decays of RuP (4 μM) in an aqueous AA solution (0.1 M, pH 4.5) after addition of different concentrations of NiP (0, 2, 4, 8, and 16 μM). The excitation wavelength was λex 532 nm, and the decays were probed at λprobe 500 nm. The data were fitted to stretched exponential equations. (B) Calculated pseudo-first-order rate constant (kET[NiP]) for the ET from RuP to NiP as a function of the concentration of NiP catalyst (Table S1 (Supporting Information)). See the caption to Table S1 for details of the analysis.

The reductive quenching of RuP* also occurs through intermolecular ET from AA to RuP* in RuP-ZrO2-NiP. The photoluminescence intensity and lifetime of RuP* on nanostructured ZrO2 films decrease upon the addition of an aqueous AA solution (0.1 M, pH 4.5), and TC-SPC measurements reveal that reductive quenching of RuP* to form RuP occurs in approximately 250 ns with an estimated ET efficiency of 70% (Figure 5B). The cofunctionalization of ZrO2 with NiP (8 μM) and RuP (4 μM) in water does not affect the luminescence of RuP, indicating that ET between the two molecules requires the formation of RuP in the presence of a sacrificial electron donor. A transient absorption peak at 500 nm corresponding to the signal of RuP– was also observed in the RuP-ZrO2 system in the presence of AA (0.1 M at pH 4.5), with the decay of RuP– accelerating following the codeposition of NiP (Figure S10 (Supporting Information)). It can be concluded that the same reductive ET mechanism and similar kinetics (dependent upon component loadings/concentrations) take place both in homogeneous media and on anchoring of the molecules onto the surface of a material that does not allow for dye electron injection. Although the reductive ET mechanism is the same for both homogeneous and the “on-particle” ZrO2 systems, charge separation is achieved through two different processes: while in homogeneous media charge separation occurs through the diffusion of molecules into the solution, when the dye and catalyst are immobilized in RuP-ZrO2-NiP charge separation can take place through the intermolecular ET between neighboring molecules.[38]

Standard Conditions for Photocatalytic H2 Evolution

The spectroscopic studies revealed that all three photocatalytic systems drive the efficient photoreduction of NiP. We subsequently studied photocatalytic generation with NiP in bulk experiments. All three systems were indeed efficient in producing H2 during irradiation with visible light. Consequently, the systems were studied and optimized by varying the pH value of the AA solution and concentrations of RuP and NiP (Table 1 and Tables S2–S4 (Supporting Information)). The photocatalytic activity and longevity of H2 production was dependent on the conditions and type of system employed. The photocatalytic performance of NiP generally varied in all three environments, and the rate of photogenerated H2 was constant over at least 1 h in all experiments (Figure 7A). The turnover frequencies based on NiP (TOFNiP) were calculated from the amount of H2 accumulated in a photoreactor after 1 h irradiation (Tables S3 and S4).

Table 1

Visible-Light-Driven H2 Production with RuP and NiP in Homogenous Solution and Coimmobilized on TiO2 and ZrO2

conditionsa	TOFNiP ± σ/h–1	H2 ± σ/μmol (after 2 h)	TONNiP	lifetime/h
RuP Dependenceb
0.025 μmol of RuP	64 ± 10	14.3 ± 1.3	>142	>2
0.3 μmol of RuP	236 ± 21	50.0 ± 2.3	>500	>2
0.5 μmol of RuP	297 ± 48	62.1 ± 6.3	>620	>2
Replacement of RuP by Organic Dye
Eosin Y (0.3 μmol), NiP (0.1 μmol), AA (0.1 M, pH 4.5)	189 ± 31f	12.3 ± 3.5
pH Dependencec
pH 4.0	185 ± 25	41.2 ± 3.4	>412	>2
pH 4.5	236 ± 21	50.0 ± 2.3	>500	>2
pH 5.0	210 ± 24	33.4 ± 0.4	>334	>2
NiP Dependenced
RuP-NiP (0.02 μmol of NiP)	460 ± 60	14.5 ± 3.4	723 ± 171	2
RuP-NiP (0.1 μmol of NiP)	104 ± 10	22.0 ± 1.2	651 ± 30	30
RuP-ZrO2-NiP (0.02 μmol of NiP)	27 ± 3	0.9 ± 0.03	>43	>2
RuP-ZrO2-NiP (0.1 μmol of NiP)	92 ± 26	16.1 ± 1.4	524 ± 36	30
RuP-TiO2-NiP (0.02 μmol of NiP)	51 ± 7	1.7 ± 0.2	>85	>2
RuP-TiO2-NiP (0.1 μmol of NiP)	72 ± 5	13.8 ± 0.3	278 ± 19	30
Control Experiments – Homogeneous System
no RuP, NiP (0.1 μmol) in AA (0.1 M, pH 4.5)		g
RuP (0.3 μmol), NiP (0.1 μmol), Na citrate (0.1 M, pH 4.5), no AA		g
Control Experiments – Heterogenized Systemse
no NiP, TiO2, RuP (0.05 μmol), AA (0.1 M, pH 4.5)		g
no NiP, ZrO2, RuP (0.05 μmol), AA (0.1 M, pH 4.5)		g

All samples were irradiated with visible light (AM 1.5 G filter, 100 mW cm–2, λ >420 nm, 25 °C) under an N2 (2% CH4) atmosphere and a standard solvent volume of 2.25 mL, leaving a gas headspace volume of 5.59 mL. Standard screening samples were irradiated for 2 h in AA (0.1 M, pH 4.5), and the TOF was determined after 1 h irradiation.

Homogeneous system with NiP (0.1 μmol) and different amounts of RuP.

Homogeneous system with NiP (0.1 μmol), RuP (0.3 μmol), and different pH values.

RuP (0.05 μmol); in heterogenized systems TiO2 or ZrO2 (2.5 mg per sample) nanoparticles were used as dispersions.

TiO2 or ZrO2 nanoparticles (2.5 mg per sample).

TOFNiP after 15 min visible light irradiation reported due to rapid bleaching of Eosin Y.

No H2 detected in GC measurements (limit of detection <0.01%).

Figure 7

(A) Visible-light-driven generation of H2 in a homogeneous aqueous system (0.1 M AA, pH 4.5) comprised of (A) NiP (0.1 μmol) with different amounts of RuP and (B) RuP (0.05 μmol) with low (0.02 μmol, squares) and high loadings of NiP (0.1 μmol, circles).

Homogeneous system with n class="Gene">NiP (0.1 μmol) and difn class="Chemical">ferent amounts of RuP.

Homogeneous system with n class="Gene">NiP (0.1 μmol), RuP (0.3 μmol), and difn class="Chemical">ferent pH values.

Photo-H2 Generation with Homogeneous RuP-NiP

In a homogeneous RuP-NiP system, a TOFNiP value of 460 ± 60 h–1 was observed when RuP (0.05 μmol) was used with a low amount of NiP (0.02 μmol) in aqueous AA (0.1 M, 2.25 mL). This RuP-NiP system was photoactive for 2 h with a final TONNiP value of 723 ± 171 (15 ± 3 μmol of H2). An increasing amount of NiP (0.1 μmol) resulted in a decreased initial TOFNiP concomitant with an increased system lifetime and a comparable overall TONNiP value of 651 ± 30 (65 ± 3 μmol of H2) after 30 h irradiation (Table 1 and Figure 7B). This observation might be explained by the less likely double reduction of a single molecular catalyst at high NiP concentrations. Addition of RuP or AA to a photodegraded system did not result in reactivation, suggesting that decomposition of NiP occurred after approximately 700 TONs. Addition of NiP to a deactivated homogeneous system did result in reactivation, but full photoactivity was not restored, indicating that photodegradation affected not only NiP but also other system components such as RuP. Indeed, photobleaching of RuP became evident after 1 h irradiation by recording an electronic absorption spectrum of the homogeneous solution after irradiation in a gastight quartz cuvette under standard conditions (Figure S11 (Supporting Information)).

Several sets of control experiments were carried out, which showed that no or only negligible amounts of H2 were produced in the absence of RuP, NiP, AA or light. The presence of additional buffers such as citrate and acetate did not impede the photocatalytic activity of the original system. Replacement of NiP by different Ni salts such as NiCl2 and NiBr2 in combination with 4 equiv of a water-soluble phosphine ([2-(dicyclohexylphosphino)ethyl]trimethylammonium chloride) resulted only in negligible amounts of H2. Homogeneous systems using commercially available dyes such as [Ru(bipy)3]Cl2 (TOFNiP = 220 ± 20 h–1) or the organic, noble-metal-free dye Eosin Y (used as the disodium salt; TOFNiP = 189 ± 31 h–1) also resulted in efficient photocatalytic H2 production in an aqueous AA solution (0.1 M, pH 4.5) (Table 1 and Table S6 (Supporting Information)).

Photon-to-H2 Conversion Efficiency

The external quantum efficiency (EQE) of the homogeneous photocatalytic system with RuP (0.3 μmol) and NiP (0.1 μmol) in AA (0.1 M, pH 4.5) was measured using an LED light source (λ 460 nm, 5 mW cm–2). An EQE value of 9.7 ± 1.2% was determined after 2 h irradiation. We note that this external yield assumes that all photons emitted by the light source were absorbed by RuP and therefore represents a lower estimate of the quantum efficiency of the system. The photon-to-H2 quantum yield observed for our homogeneous RuP-NiP system is remarkable in comparison to other photocatalytic systems with molecular 3d transition-metal catalysts operating homogeneously or immobilized in aqueous solution. Recently reported quantum yields for homogeneous photocatalytic systems ranged from 0.23 to 0.6% for a Co-pentapyridine catalyst with [Ru(bpy)3]2+ as photosensitizer[39] in an aqueous system at neutral pH to 4.6% for a cobaloxime-based H2 generation system with an Al-porphyrin dye in a water/acetone solvent system.[13k] In systems with a molecular catalyst immobilized on a solid-state material, EQEs of approximately 1–1.5% for a cobaloxime immobilized on RuP-sensitized TiO2[13f] and a Ni(TEOA)32+ (TEOA = triethanolamine) complex on graphitic carbon nitride,[40] respectively, were reported.

EQEs for self-assembled photocatalytic systems using Ni salts and 2-mercaptoethanol (24.5%)[4f] or dihydrolipoic acid (36% QE)[14] ligands are still benchmarks; however, a defined catalytically active species has not been reported for these systems.

Benchmark for Synthetic Photochemical Systems

Various homogeneous photocatalytic schemes have been reported for the reduction of aqueous protons with molecular catalysts. Examples of efficient photo-H2 generating systems include a molecular dye combined with [Ni(P2R′N2R″)2]2+-type[4g] and Fe-based molecular catalysts,[3c,15] but in these systems water/organic solvent mixtures were employed. Photocatalytic schemes for H2 generation which operate in pure aqueous systems are highly desirable.[11] Very recently, the water-insoluble [Ni(P2PhN2Ph)2](BF4)2 was incorporated into the photosynthetic protein Photosystem I (PSI) for photocatalytic H2 production in aqueous solution,[19b] but only a very limited amount of Ni catalyst can be loaded onto dilute enzyme systems and the photostability of the hybrid assembly remained an issue (more than half of the photoactivity ceased after 30 min of irradiation).

Nickel-thiolate[4i] and cobalt-dithiolene[13l] catalysts have been reported to achieve several thousand turnovers in organic solvent/water mixtures. Cobaloximes have been reported to evolve H2 in a homogeneous system[13g] and on RuP-sensitized TiO2 in water,[13f] but the TOFCo value never exceeded 20 h–1 and the photon-to-H2 efficiency was only 1%. The higher photocatalytic activity and EQE of NiP in comparison to those of cobaloxime catalysts may be related to the similar reduction potentials required for both reduction steps in NiP. We have previously shown that the photocatalytic activity of a cobaloxime complex is significantly limited by its relatively slow second reduction kinetics, attributed at least in part to the relatively unfavorable energetics for this second reduction.[41] The use of a Co-pentapyridine catalyst[39] and a [FeFe]-hydrogenase mimic[12a] in photocatalytic schemes in an aqueous solution resulted in TOFs of approximately 20 and 50 h–1, respectively. Thus, our RuP-NiP system compares favorably with previously reported systems in organic solvent free aqueous solution.

Photo-H2 Generation with Heterogenized Catalysts

The immobilization of a molecular catalyst on a photoelectrode is a prerequisite for fuel generation in a photoelectrochemical device, and the attachment on a semiconductor material is particularly desirable. Therefore, we extended our studies using NiP for H2 generation in heterogeneous photocatalytic assemblies with nanoparticle suspensions as a first step towards an electrode assembly.

We first determined the maximum loading capacity of RuP(6c) and NiP on the metal oxide particles by spectrophotometry. Approximately 0.05 μmol of RuP or NiP can be immobilized per milligram of TiO2 or ZrO2 when adding an excess of phosphonated catalyst (see the Supporting Information).[6c,13f]

The metal oxide nanoparticles were loaded by the following procedure: NiP was added to a suspension of TiO2 or ZrO2 in aqueous AA solution (2.5 mg in 2.25 mL), and then RuP was added. NiP was loaded first due to the optimized geometry of adsorption of phosphonated bipyridine ligands of RuP.[29b] The photoactivity of the suspensions was studied under irradiation with visible light (AM1.5G, 100 mW cm–2, λ >420 nm). Upon investigating the heterogeneous photocatalytic systems, notably different trends in performance were observed in comparison to the homogeneous system at lower NiP loadings (Figure 8).

Figure 8

Visible-light-driven H2 evolution rate with different amounts of NiP and RuP (0.05 μmol): (triangle) RuP-NiP; (square) RuP-TiO2-NiP; (circle) RuP-ZrO2-NiP.

In a RuP-ZrO2-NiP system, a low amount of NiP (0.02 μmol) with RuP (0.05 μmol) on 2.5 mg of ZrO2 resulted in a TOFNiP value of 27 ± 3 h–1. The results obtained in TAS measurements suggest that direct interaction of the quenched dye (RuP) with NiP is required to drive the reaction. ET between RuP and NiP “on the particle” as observed by the spectroscopic studies above becomes difficult under such dilute conditions due to the spatial separation of the compounds on the ZrO2 surface. When the amount of NiP added to ZrO2 is increased to 0.1 μmol, a significant enhancement in TOFNiP to 92 ± 26 h–1 was observed (Figure 8 and Table S4 (Supporting Information)).

We note that, at low concentrations of compounds carrying phosphonic acid groups, attachment on ZrO2 or TiO2 is not quantitative, presumably due to competitive binding of AA to metal oxides.[42] Spectrophotometric studies indicate that NiP is almost quantitatively (>80%) adsorbed on ZrO2, whereas only approximately 10% of RuP adsorbs on NiP-modified ZrO2 (Table S7 (Supporting Information)). When the ZrO2 nanoparticles were loaded with NiP (0.02 μmol) and RuP (0.05 μmol), separated by centrifugation, and redispersed in fresh AA (0.1 M, pH 4.5), the amount of H2 produced was 0.16 μmol after 1 h irradiation (in comparison to 0.54 μmol of H2 before centrifugation). Thus, the photodriven H2 production in bulk experiments can best be described as a mixture of ET between surface-immobilized catalysts through an “on particle” mechanism[38] and from solubilized RuP to surface-bound NiP.

A RuP-TiO2-NiP system displays a TOFNiP value of 51 ± 7 h–1 at a low NiP loading of 0.02 μmol on 2.5 mg of TiO2 (Table 1). The rate of H2 production reached a maximum with a NiP loading of 0.1 μmol, whereupon a TOFNiP value of 72 ± 5 h–1 and an overall TONNiP value of 278 ± 19 h–1 (after 30 h) was obtained (Table S5 (Supporting Information)).

Under these conditions, at least 80% of NiP and more than 20% of RuP are attached on TiO2, as measured by spectrophotometry (Figures S12 and S13 (Supporting Information)). Once NiP or RuP is bound to the TiO2 surface, it cannot easily be removed from the solid-state material. Redispersion of loaded particles in a fresh AA solution did not result in the detection of significant amounts of RuP or NiP in solution. Loading the TiO2 nanoparticles with NiP (0.02 μmol) and RuP (0.05 μmol), centrifugation, and resuspension in fresh AA (0.1 M, pH 4.5) resulted in a TOFNiP value of 24 h–1 during irradiation. Thus, 50% of the photocatalytic activity remained, thereby establishing the importance of the role of the conduction band and the “through particle” mechanism observed in the spectroscopic study. This experiment and the significantly higher rate of photocatalytic H2 production for RuP-TiO2-NiP in comparison to RuP-ZrO2-NiP at very low loading of the particles with NiP (Table 1) support a preferential “through particle” ET mechanism for TiO2: a mechanism which does not require the direct electronic communication of RuP and NiP as needed on ZrO2.

These results show that photocatalytic H2 production can be achieved with a DuBois-type catalyst attached on a solid-state semiconductor. However, to this point limitations by the loading capacity of materials used and by competitive binding of electron donor or electrolyte are still being faced when investigating such hybrid materials. Work is in progress to overcome these limitations by investigating the binding modes of the molecular components and photocatalytic activity of RuP-NiP systems on thin films and electrode materials.

Conclusions

In summary, we describe a novel [Ni(PR′2NR″2)2]2+-type H2 evolution catalyst (NiP), which is soluble in water and can be immobilized on metal oxide surfaces. NiP is electrocatalytically active in organic solvent free aqueous solution and evolves H2 with an onset potential of only −0.48 V vs NHE under mild conditions (pH 4.5). Photocatalytic and spectroscopic studies were performed with NiP in three different systems in a purely aqueous solution containing AA. A homogeneous RuP-NiP system operates through reductive quenching of RuP* in solution. The heterogeneous RuP-ZrO2-NiP system shows the same ET mechanism, and ZrO2 acts merely as a matrix to retain the attached molecules closely together, hence facilitating ET “on the particle”. In RuP-TiO2-NiP, ET occurs via a “through particle” mechanism, where RuP* is oxidatively quenched upon injection of an electron into the conduction band of TiO2, which can subsequently be harvested by NiP.

A high TOF value of 460 ± 60 h–1 for light-driven H2 evolution with a molecular 3d transition metal catalyst in pure aqueous solution was obtained, with TONs of approximately 700 for NiP. Advanced spectroscopic methods (TC-SPC and TAS) confirmed that directed ET from RuP to NiP occurs efficiently in all systems on the nano- to microsecond time scale. Losses due to charge recombination are minimized, as ET occurs efficiently within the lifetimes of the excited species. The highly efficient ET from dye to proton reduction catalyst is also reflected in the high photon to H2 quantum yield of the homogeneous system of almost 10% in the presence of the sacrificial electron donor AA. Work is in progress to assemble a photoelectrode with NiP for use in a photoelectrochemical water splitting cell.