Spectroscopic Studies of Light-driven Water Oxidation Catalyzed by

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Spectroscopic Studies of Light-driven Water Oxidation Catalyzed by Polyoxometalates Zhuangqun Huang, Yurii V. Geletii, Djamaladdin G. Musaev, Craig L. Hill, and Tianquan Lian* Department of Chemistry, and Cherry L. Emerson Center for Scientific Computation, Emory University, Atlanta, Georgia 30322, United States ABSTRACT: This review is focused on the spectroscopic studies of the charge transfer dynamics in polyoxometalate (POM)catalyzed artificial photodriven water oxidation systems. We first describe the general challenges in solar fuel production, showing that water oxidation catalysts (WOCs) are of central importance. After the introduction of a new class of highly efficient WOCs based on all-inorganic polyoxometalates, we summarize the performance of these WOCs in homogeneous water oxidation systems. We show that the individual steps involved in the overall light-driven water oxidation reaction can be investigated by spectroscopic techniques (fluorescence quenching, transient absorption, and stopped flow). These studies provide important insight into the factors that limit the overall conversion efficiency in these systems and suggest possible approaches for improving these devices.

1. INTRODUCTION The direct harvesting, conversion, and storage of solar energy in chemical bonds is a promising approach for meeting the global demand for clean energy.1−5 In particular, efficient sunlightdriven water splitting (H2O + 2hν → H2 + 1/2 O2) remains one of the most desirable ways to store energy by forming energydense H2.6−13 It can be also used to convert the greenhouse gas CO2 into liquid fuels. The splitting of H2O consists of two half reactions: water oxidation to form O2 and water reduction to form H2, 2H2O → O2 + 4H+ + 4e− 2H+ + 2e− → H2

E = 0.82V, at pH 7

E = 0.41V, at pH 7

(1) (2)

Figure 1. Schematic representation for a typical light-driven water splitting photo-electrochemical cell. Notations: SMO, semiconductor metal oxide; PS, photosensitizer; WOC, water oxidation catalyst; WRC, water reduction catalyst.

where E is the reduction potential. Both reactions proceed through multiple proton-coupled electron transfer steps. O2 formation in eq 1 is a four-electron oxidation process with a high redox potential of 0.82 V vs NHE at pH 7, which makes water oxidation significantly more difficult than water reduction, which requires only 0.41 V vs NHE at pH 7. Water oxidation can also proceed through the one-electron oxidation pathway leading to a hydroxyl radical intermediate (H2O → HO• + H + e−). However, the latter process is much more thermodynamically unfavorable (2.41 V vs NHE at pH 7).14 A general schematic for artificial photosynthesis systems is shown in Figure 1.9,13,15 At the photoanode, upon optical excitation, the excited photosensitizer transfers an electron to a nano/mesoporous semiconductor metal oxide (SMO) supported on a transparent conductive substrate.16−19 Meanwhile, the hole left behind in the photosensitizer sequentially oxidizes the attached water oxidation catalyst (WOC). This triadic anode is connected to an external circuit, such that the photoinjected electron can be removed from the anode and transported to the cathode for H2O and/or CO2 reduction. In some cases, narrow-band semiconductor metal oxides (α-Fe2O3, doped TiO2, WO3, etc.) are introduced as both the light absorber and the charge separation center to construct a © XXXX American Chemical Society

dyadic anode.13,20−22 After four of these processes, four holes are accumulated in a WOC, which can then oxidize water. In efficient artificial photosynthetic systems: (i) the light harvesting component (photosensitizer) should absorb a maximum portion of the solar spectrum, (ii) the photogenerated electrons and holes should be efficiently separated, (iii) charge recombination should be suppressed to accumulate the holes in the WOC, and (iv) the catalytic turnover frequency (TOF) of the WOC should be higher than the rate of charge recombination and the average solar photon flux. In addition, the desired photocatalytic system should be oxidatively and hydrolytically robust. It also should be cheap for large-scale production. Special Issue: Alternative Energy Systems Received: December 15, 2011 Revised: February 22, 2012 Accepted: March 5, 2012

A

dx.doi.org/10.1021/ie202950h | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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and charge).54−57 There has been a great deal of research on the fundamental principles and the applications of this family of WOCs in solar fuel production. The design, synthesis, and characterization of these WOCs have been reviewed recently and will not be discussed in this paper.34 So far, six polyoxometalate WOCs have been reported: [{Ru 4 O 4 (OH) 2 (H 2 O) 4 }(γ-SiW 10 O 36 ) 2 ] 10− (henceforth Ru4POM, Figure 2a), [{Ru4O5(OH)(H2O)4}(γ-PW10O36)2]9− (the isostructural phosphorus analogue of the Ru4POM, henceforth Ru4-(P)-POM, Figure 2a), [Co4(H2O)2(PW9O34)2]10− (henceforth Co4POM, Figure 2b), the hydrolytically unstable [(IrCl4)KP2W20O72]14−, and two single site Ru Keggin POMs, [Ru I I I (H 2 O)SiW 1 1 O 3 9 ] 5 − and [Ru I I I (H 2 O)GeW11O39]5−.23−32,34−37Co4POM constitutes a breakthrough in WOC development because it contains only earth-abundant elements, which has several implications including the possibility of large-scale production. While seven independent lines of experimental evidence unequivocally demonstrate that Co4POM is stable under homogeneous water oxidation conditions at low concentration (180) and Co4POM (>220). These numbers are lower limits because the amount of O2 generated in a catalytic single run is limited by depletion of the sacrificial electron acceptor (i.e., Na2S2O8) and does not reflect the stability of the catalysts.34 It is also interesting to comment on the physical

Figure 4. Schematic presentation of the homogeneous light-driven water oxidation systems catalyzed by polyoxometalate-based WOCs. Components and typical concentrations: photosensitizers, [Ru(bpy)3]2+ (1 mM); sacrificial electron acceptors, [S2O8]2− (5 mM); WOC, Ru4POM/Ru4-(P)-POM/Co4POM (5 μM). Illumination: 420−470 nm.

This system uses [Ru(bpy)3]2+ as a photosensitizer and [S2O8]2− as a sacrificial electron acceptor.25,32,34 The overall reaction can be expressed as 2[S2O8]2 − + 2H2O + 2hν → 4SO4 2 − + O2 + 4H+ (3)

Photodriven water oxidation using this scheme was initially tested with our Ru4POM catalyst, as shown in Figure 5.25 Using

Figure 5. Kinetics of O2 formation (○) and persulfate consumption (Δ) for photodriven water oxidation by Ru4POM.25 The two arrows indicate the corresponding y-axes of the plots. (Reprinted with permission from ref 25. Copyright 2010, American Chemical Society).

5 μM Ru4POM and 5 mM Na2S2O8 in sodium phosphate buffer (20 mM, initial pH 7.2), we reported a quantum efficiency of 9%,25 which was later improved to 27% under optimized conditions.34 The quantum efficiency reported here is the initial stoichiometric yield, ΦQY(0) = 2 [Δ(O2)/Δ(hν)], where Δ(O2) and Δ(hν) are the change in the total amount of C



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quantum efficiency of O2 generation can be related to the yields of individual steps,34

meaning of TOF reported in water oxidation studies. In biochemistry, Michaelis−Menten kinetics is expressed as kf

ΦCY = 2[O2 ]f /[Na2S2O8]0 = 0.5(1 + ϕr)ϕc

kcat

E + S ⇄ (ES) ⎯⎯⎯→ E + P kr

(5)

ΦQY (t ) = 2(Δ[O2 ]/Δ(hν))t = 0.5ϕq (t )(1 + ϕr)ϕc

where E is an enzyme and S is a substrate. In this model, TOF is defined as kcat and is an intrinsic property of the catalyst. The TOF reported here is defined as the ratio of initial rate per catalyst concentration, which is the same as TON per second or TOF = d(TON)/dt. This definition follows the common practice of the field and differs slightly from TOF in enzyme kinetics.63 The formation of the ES adduct might be rate limiting. In this case, TOF as defined for water oxidation is limited by kf[S], which is not related to kcat. Therefore, TOF can appear as an experimental-condition-dependent quantity and should be considered as a lower limit of the kcat value.

(12)

ΦQY (t ) = ϕq (t ) ΦCY

5. CHARGE TRANSFER DYNAMICS IN THE HOMOGENEOUS SYSTEM The reactions proposed in eqs 6−10 can be observed directly by spectroscopy, providing verification of the proposed mechanism and insight into factors that limit the overall conversion efficiency. In this section, we summarize some of our efforts to directly characterize some of the reactions steps. 5.1. Steady-State Fluorescence. Photoinduced oxidation of [Ru(bpy)3]2+ to [Ru(bpy)3]3+ by persulfate ion (eq 6 and 7) has been found to proceed via a quenching of the metal-toligand charge transfer (MLCT) excited state, where the quenching efficiency (ϕq) depends on the solution conditions.64−66 Because [Ru(bpy)3]2+ and [S2O8]2− have opposite charges, quenching by both unimolecular processes, involving intra-ionpair electron transfer, and bimolecular processes are operable. Based on this model, the Stern−Volmer relationship, the ratio of steady emission without (I0) and with (I) quencher, can be derived,64−66

photoexcitation: 2[Ru(bpy)3 ]2 + + 2hν → 2[Ru(bpy)3 ]2 + * (6)

oxidant generation: 2[Ru(bpy)3 ]2 + * + 2[S2O8]2 − (7)

2SO4−• + 2[Ru(bpy)3 ]2 + → 2[Ru(bpy)3 ]3 + + 2SO4 2 − (8)

catalyst oxidation: 4[Ru(bpy)3]3 + + [M 4POM] → 4[Ru(bpy)3 ]2 + + [M+4POM]

−1 ⎧ ⎫ ⎪ I0 ⎪ α 1−α ⎬ =⎨ + 2− ⎪ I 1 + kET τ0 + kq′τ0[S2O82 −] ⎪ ⎩ 1 + kq τ0[S2O8 ] ⎭

(9)

water oxidation:[M+4POM] + 2H2O → [M 4POM] + 4H+ + O2

(13)

These equations provide insight into the factors that limit the overall quantum efficiency of the photodriven water oxidation system.25,34 As will be described below, the initial ϕq(0) value is measured to be 67% by steady-state fluorescence spectroscopy. ΦCY was found to be low (40−45%) for both Co4POM and Ru4POM. Therefore, the low overall O2 formation quantum yield can be attributed to the low chemical yield (due to the presence of noncatalytic reaction pathways) and the non-unity quenching efficiency of the excited sensitizer (because the slow rate of electron transfer cannot complete with the intrinsic rate of excited state relaxation). In addition, ϕr is reported to be less than unity and highly dependent on the solution environment.67−69 According to eq 11, ϕc is larger than ΦCY and can be estimated to be >40% and >45% for Ru4POM and Co4POM, respectively. These estimated values are consistent with those measured in dark reactions: 66% for Ru4POM at pH 7.223 and 64% for Co4POM at pH 8.29

4. MECHANISMS IN HOMOGENEOUS LIGHT-DRIVEN SYSTEMS In the homogeneous photodriven water oxidation system shown in Figure 4, the reaction is initiated upon the absorption of two photons by two [Ru(bpy)3]2+ complexes. The excited [Ru(bpy)3]2+* can undergo intrinsic relaxation or be quenched by S2O82− through both bimolecular or unimolecular electron transfer pathways (quenching efficiency, ϕq).64−66 The photoinduced electron transfer produces two [Ru(bpy)3]3+ and two SO4−•. The latter radicals subsequently oxidize two additional [Ru(bpy)3]2+ to two [Ru(bpy)3]3+ (radical yield, ϕr). The resulting four [Ru(bpy)3]3+ complexes oxidize water to make one O2 (catalytic yield, ϕc) and regenerate the [Ru(bpy)3]2+. The individual steps of the reaction are shown in eqs 6−10 (M is Co or Ru).

→ 2[Ru(bpy)3 ]3 + + 2SO4 2 − + 2SO4−•

(11)

(14)

(10)

where α is the percentage of [Ru(bpy)3] existing in the form of an ion pair; kq and kq′ are the bimolecular reaction rate constants for free and ion pairing, [Ru(bpy)3]2+*, respectively; and kET is the unimolecular electron transfer rate. In the limit of no or 100% ion pairing of [Ru(bpy)3]2+, eq 14 is reduced to two linear equations, 2+

We define the average stoichiometric dioxygen chemical yields, ΦCY = 2[O2]f/[Na2S2O8]0, where [O2]f and [Na2S2O8]0 are based on the final yield of O2 and the initial concentration of persulfate, respectively. Figure 5 shows that the ratio of [O2] formed to [Na2S2O8] consumed changes negligibly throughout the course of the catalytic runs, and thus, only the average value for 2[O2]/[Na2S2O8] (over the course of the reaction) is used. The ΦQY(t) is related to the slope of the plot of O2 vs illumination time shown in Figure 5, with the largest value at the onset of the reaction. Its value decreases with time due to the gradual consumption of S2O82−. The chemical yield and D

I0 = 1 + kq τ0[S2O82 −] I

(15)

I0 = 1 + kET τ0 + kq′τ0[S2O82 −] I

(16)



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As seen from eqs 14−16, plotting the measured quenching efficiency vs persulfate concentration allows the differentiation of static and dynamic quenching mechanisms and the determination of quenching rate constants. In the case that both free [Ru(bpy)3]2+ and ion pairs are present, the Stern− Volmer plot yields a curve with a downward curvature, as shown in Figure 6.

Figure 7. Schematic representation of pump−probe measurements in transient absorption spectroscopy. The pump pulse was introduced to excite the sample and trigger the photoreactions. After certain delay times (on fs-μs time scales), the probe pulse was used to take snapshot transient absorption spectra of the systems.

Figure 6. Stern−Volmer plot of the quenching efficiency of Ru[bpy]32+ as a function of persulfate concentration. The solid curve is a fit according to eq 14.65 All solutions contained 1 mM of [Ru(bpy)3]2+ in 20 mM pH 7.2 phosphate buffer and were purged with Ar.

From the plot and the fit (Figure 6), the quenching efficiency (ϕq) increases with [S2O8]2− quencher concentration and is found to be 67% at 5 mM [S2O8]2−, the initial concentration used in our photocatalytic water oxidation system. In the lightdriven water oxidation system, as the reaction proceeds, ϕq(t), and thus ΦQY(t), decreases as a result of the depletion of [S2O8]2− (eq 13). Furthermore, the non-unity value of ϕq suggests that the overall catalytic quantum efficiency can be improved with better approaches for generating [Ru(bpy)3]3+. The fit to the Stern−Volmer plot also shows that 61% [Ru(bpy)3]2+ forms an ion-pair with [S2O8]2−. Most importantly, unimolecular electron transfer (1/kET, 200 ns) is ∼2.5 times faster than the bimolecular one (1/(kq[Na2S2O8]), 500 ns). This suggests that the quenching efficiency can be further improved by constructing closely interacting catalyst−sensitizer− electron acceptor complexes. 5.2. Transient Absorption Spectroscopy. The fluorescence quenching study described above probes the first step of the overall process, that is, the quenching of the excited sensitizer molecules by charge transfer (eq 6 and 7). To follow the charge transfer dynamics, we carried out transient absorption measurements based on the ultrafast (fs-μs) pump−probe method, as shown in Figure 7. A short pump pulse excites the sample and triggers the photoinduced events under investigation. The second pulse, a panchromic probe pulse with a certain spectral range, probes the sample after a given delay time. With broad-band probe pulses, this technique allows for simultaneous spectroscopic identification of multiple species (ground, excited, oxidized, and reduced states) of the electron donor and acceptor present at a given time delay after optical excitation. The measured transient spectra as a function of time enables the direct measurement of charge transfer kinetics. The transient spectra of [Ru(bpy)3]2+ in the presence of [S2O8]2− after 400 nm excitation is shown in Figure 8a. The bleach band at 450 nm is due to the photoinduced depopulation of ground state [Ru(bpy)3]2+ (eq 6). The absorption

Figure 8. (a) Transient absorption spectra averaged over different delay times for [Ru(bpy)3]2+ in the presence of 5 mM [S2O8]2− after 400 nm excitation. (b) Transient kinetics monitored at 450 nm for (1) 50 μM [Ru(bpy)3]2+; (2) 50 μM [Ru(bpy)3]2+ with 5 mM [S2O8]2−. (3−5) 50 μM [Ru(bpy)3]2+ with 5 mM [S2O8]2− and different concentrations of [Ru4POM]: (3) 12.5 μM; (4) 35 μM; and (5) 50 μM. All samples contain 20 mM phosphate buffer (initial pH 7.2).70 (Part b is adopted with permission from Figure 2b in ref 70. Copyright 2011, Society of Photo Optical Instrumentation Engineers.)

feature above ∼500 nm results from the new species: [Ru(bpy)3]2+* and [Ru(bpy)3]3+. The spectral evolution in this area is due to the conversion from [Ru(bpy)3]2+* to [Ru(bpy)3]3+ (eqs 7 and 8). Because the radical reaction (eq 8) further depopulates the ground state of [Ru(bpy)3]2+, it results in the further growth of the bleach band in Figure 8a. To understand the charge transfer process, we monitored the transient kinetics of the [Ru(bpy)3]2+ ground state bleach (Figure 8b) as a function of persulfate and catalyst concentration.70 In the absence of the persulfate and catalyst (curve 1 in Figure 8b), the kinetic trace reflects the instantaneous depopulation of the ground state due to photoexcitation (eq 6) and recovery with a time constant (the excited state lifetime) of E



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390 ns64 due to the intrinsic relaxation of the excited sensitizer to regenerate the ground state. In the presence of the persulfate (curve 2 in Figure 8b), the further growth of the bleach can be attributed to the reaction of persulfate radical with the sensitizer (eq 8). The presence of the WOC results in the recovery of the ground-state bleach (trace 3−5 in Figure 8b) and the recovery rate increases with the WOC concentration, indicating a more efficient oxidation of the catalysts by the photo-oxidized sensitizer molecules. This trend is consistent with the observed dependence of O2 formation rate on the catalyst concentration measured in the photocatalytic system (eq 4). The maximum concentration of the in situ photogenerated oxidant [Ru(bpy)3]3+ is estimated to be 5 μM, using the extinction coefficient of [Ru(bpy)3]2+ (ε450 nm = 14200 M−1 cm−1) and the maximum bleach on the kinetic trace of [Ru(bpy)3]3+/ [S2O8]2−. Because this concentration is lower than that of the catalysts (>10 μM), the kinetics shown in Figure 8b likely probe the first of the four hole accumulation steps on the WOC. This step is believed to be the most thermodynamically favorable, since it has the largest driving force compared to the subsequent hole accumulation. However, as shown in Figure 8b, even in this first step, the electron transfer occurs on the μs time scale. The slow electron transfer kinetics are attributed to the diffusion-controlled electron transfer processes in solution at low catalyst concentrations (

Spectroscopic Studies of Light-driven Water Oxidation Catalyzed by

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