Hierarchical Inorganic Assemblies for Artificial Photosynthesis

Aug 30, 2016 - Biography. Wooyul Kim received his Ph.D. from POSTECH in 2012 under the direction of Prof. Wonyong Choi. During his postdoctoral stay a...
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Hierarchical Inorganic Assemblies for Artificial Photosynthesis Wooyul Kim,† Eran Edri, and Heinz Frei* Molecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720, United States CONSPECTUS: Artificial photosynthesis is an attractive approach for renewable fuel generation because it offers the prospect of a technology suitable for deployment on highly abundant, non-arable land. Recent leaps forward in the development of efficient and durable light absorbers and catalysts for oxygen evolution and the growing attention to catalysts for carbon dioxide activation brings into focus the tasks of hierarchically integrating the components into assemblies for closing of the photosynthetic cycle. A particular challenge is the efficient coupling of the multi-electron processes of CO2 reduction and H2O oxidation. Among the most important requirements for a complete integrated system are catalytic rates that match the solar flux, efficient charge transport between the various components, and scalability of the photosynthetic assembly on the unprecedented scale of terawatts in order to have impact on fuel consumption. To address these challenges, we have developed a heterogeneous inorganic materials approach with molecularly precise control of light absorption and charge transport pathways. Oxo-bridged heterobinuclear units with metal-to-metal charge-transfer transitions absorbing deep in the visible act as single photon, single charge transfer pumps for driving multi-electron catalysts. A photodeposition method has been introduced for the spatially directed assembly of nanoparticle catalysts for selective coupling to the donor or acceptor metal of the light absorber. For CO2 reduction, a Cu oxide cluster is coupled to the Zr center of a ZrOCo light absorber, while coupling of an Ir nanoparticle catalyst for water oxidation to the Co donor affords closing of the photosynthetic cycle of CO2 conversion by H2O to CO and O2. Optical, vibrational, and X-ray spectroscopy provide detailed structural knowledge of the polynuclear assemblies. Time resolved visible and rapid-scan FT-IR studies reveal charge transfer mechanisms and transient surface intermediates under photocatalytic conditions for guiding performance improvements. Separation of the water oxidation and carbon dioxide reduction half reactions by a membrane is essential for efficient photoreduction of CO2 by H2O to liquid fuel products. A concept of a macroscale artificial photosystem consisting of arrays of Co oxide−silica core−shell nanotubes is introduced in which each tube operates as a complete, independent photosynthetic unit with built-in membrane separation. The ultrathin amorphous silica shell with embedded molecular wires functions as a proton conducting, molecule impermeable membrane. Photoelectrochemical and transient optical measurements confirm tight control of charge transport through the membrane by the orbital energetics of the wire molecules. Hierarchical arrangement of the components is accomplished by a combination of photodeposition, controlled anchoring, and atomic layer deposition methods.

1. INTRODUCTION The recent substantial progress toward more efficient and durable light absorbers and catalysts for solar fuel generation using molecular or heterogeneous approaches is setting the stage for addressing the challenges of integration of the components into complete artificial photosystems.1−9 The photon to fuel conversion efficiency of an integrated system, whether splitting water to hydrogen and oxygen gas or reducing carbon dioxide to carbon monoxide, formate, or more energyrich products relies on the quality of charge transport coupling of the components and their proper hierarchical arrangement. The latter is critical for directional transport of charges and efficient use at catalytic sites for water oxidation or carbon dioxide reduction. Systems for visible light driven reduction of carbon dioxide by water to hydrocarbon molecules require separation of the oxidation and reduction half reactions by a proton conducting, gas impermeable membrane to prevent © 2016 American Chemical Society

back and cross reaction of the emerging products. With this added complexity, meeting the challenge of scalability on the unprecedented scale of terawatts in order to have impact on global fuel consumption is daunting yet essential. Natural photosynthesis, which operates at the 100 terawatt scale, provides inspiration for systems design by closing the cycle of photosynthesis of the initial reduction product under oxidation of water on the shortest possible length scale, nanometers, which includes an ultrathin separation membrane. A nanoscale system for completing the cycle of CO2 reduction by H2O under membrane separation may offer an efficient, scalable approach for artificial photosynthesis because it requires minimal balance of systems components, avoids ion transport resistance losses associated with macroscale redox Received: April 12, 2016 Published: August 30, 2016 1634

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2. HETEROBINUCLEAR LIGHT ABSORBERS ANCHORED ON SILICA SURFACE Matching the redox potential of the light absorber with the energetics of the catalyst is a challenging task especially on the water oxidation side when using stable inorganic semiconductor materials. Yet, potential matching is essential for converting a maximum fraction of the solar photon energy into chemical energy of a fuel. For this reason, we have developed allinorganic heterobinuclear light absorbers that allow for almost free choice of metals. About a dozen different heterobinuclear units, such as ZrOCuI, TiOMnII, or ZrOCoII, anchored on silica surfaces have been developed by our group and in the Hashimoto lab over the past ten years.11−18 As shown in Figure 2, acceptor centers are d0 elements Ti or Zr, while donor metals include most of the upper row transition elements and some group IIIA−VIA metals that represent a wide range of redox potentials. We have used the channels of mesoporous silica materials or the convex surfaces of spherical silica nanoparticles as high surface area supports for developing and photocatalytically characterizing these charge transfer chromophores. The synthetic approach for the selective assembly of oxobridged binuclear units is based on very mild (often room temperature) solution reactions of organometallic precursors featuring weakly bound ligands, for example, MnII(NCCH3)6, with tetrahedral Ti−OH or Zr−OH groups anchored in the silica surface. The latter are attached to silica surfaces according to established methods at typical loading levels of 1% (Ti/Si = 1/100). While the density of surface silanol OH groups by far exceeds that of Ti−OH sites, the reaction of the Mn complex is remarkably selective; around 80% of the Mn centers form an O bridge to Ti (or Zr), with the remainder anchoring as isolated Mn centers on the silica surface. The selective reaction of the precursor with a TiOH or ZrOH group is attributed to the higher acidity, hence higher reactivity, of these OH groups compared to much more abundant silanol groups.12 All heterobinuclear units possess a metal-to-metal chargetransfer absorption (MMCT) that extends into the red or green spectral region, as shown in Figure 2A for ZrOCoII and Figure 2B for TiOMnII. Detailed study of the structure of these heterobinuclear units by curve fitting analysis of EXAFS (extended X-ray absorption fine structure) measurements revealed distances between the oxo-bridged metal centers of 3.3−3.4 Å and a bridge angle of 110−115°.18,19 Observed MMCT optical absorptions shown in Figure 2 exhibit onsets that reflect ionization potentials and electron affinities of donor and acceptor metal, respectively. Since the MMCT transition accomplishes charge separation by the act of photon absorption, the chromophore serves as single photon, single charge transfer pump for driving multielectron catalysts for CO2 reduction or H2O oxidation, as will be discussed below. All-inorganic MMCT chromophores were first investigated in the field of mineralogy.20 For example, the mineral humite, which is a Ti−Fe mixed oxide, has edge sharing TiIVO6 and FeIIO6 octahedra with a Ti−Fe distance of 3.2 Å that gives rise to a visible TiIVOFeII → TiIIIOFeIII absorption. Extinction coefficients of around 1000 L mol−1 cm−1 are similar to those found for binuclear units anchored on silica reflecting the common electronic origin and similar geometry.21 Molecular oxo-bridged heterobinuclear systems have recently been reported, for example, the organometallic compound with a VIVOFeII moiety shown in Figure 3A whose VIVOFeII → VVOFeI MMCT chromophore is assigned to a band at 502 nm

cycles, and minimizes major side reactions and other efficiency degrading processes. By allowing operation under mild pH conditions or avoiding aqueous solution altogether by using the vapor phase, nanoscale photosynthetic systems will expand the materials choices for components, in particular to those made of earth abundant elements or ones that are unstable in extreme pH environments. To develop nanoscale photosynthetic assemblies for coupling light-driven multielectron catalysis for CO2 reduction with the H2O oxidation half reaction, we are taking an inorganic molecular approach that utilizes heterobinuclear units such as ZrOCoII as light absorbers coupled to metal oxide clusters as catalysts for H2O oxidation or CO2 reduction (sections 2−4). By focusing on inorganic oxide components of well-defined composition and structure in the form of oxo-bridged binuclear units for light absorption, crystalline nanoclusters as multielectron catalysts, and nanoscale silica layers with embedded molecular wires as separation membranes, the simultaneous requirements of robustness and tunability of electronic properties for converting the maximum fraction of the solar photon energy into chemical energy of the fuel are addressed. At the same time, this materials approach allows for the use of earth abundant components and a broad choice of morphology, which is the key to exploring and optimizing different types of hierarchical nanoscale architectures for integrated photosystems. Inorganic nanoscale units are chosen in the form of core−shell nanotubes for closing the photosynthetic cycle, and arrays of such nanotubes, operating as independent units, are developed as macroscale photosystems in which the fluids for CO2 reduction to fuel and water oxidation to oxygen are separated on all length scales, as sketched in Figure 1 (section 5).10

Figure 1. Cobalt oxide−silica core−shell nanotube array design for artificial photosynthesis. Each nanotube consists of a 5−10 nm thick inner Co3O4 tube where water oxidation takes place on the inside surface. The tube is surrounded by a 2−3 nm thick dense phase silica shell that acts as a proton conducting, O2 impermeable membrane separating the water oxidation catalysis on the inside from chargetransfer light absorber and CO2 reduction sites on the outside. Control of electron transport across the silica membrane is accomplished by embedded molecular wires. The spaces of O2 evolution and fuel generation do not intersect. Reproduced with permission from ref 10. Copyright 2016 Royal Society of Chemistry. 1635

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Figure 2. Optical spectra of heterobinuclear metal-to-metal charge-transfer light absorbers. (A) Diffuse reflectance spectrum (DRS) of the MMCT transition of the ZrOCoII unit anchored on SBA-15 silica mesopores. (B) Transmission spectrum of the MMCT transition of TiOMnII anchored on silica nanoparticles (12 nm). Reproduced with permission from refs 18 and 21. Copyright 2014 American Chemical Society.

Figure 3. Metal-to-metal charge-transfer absorptions of oxo-bridged heterobimetallic molecules. (A) VIVOFeII → VVOFeI MMCT absorption of complex [(TMTAA)VIVOFeIIPy5Me2]2+ (TMTAA = 7,16-dihydro-6,8,15,17-tetramethyldibenzo[b,i][1,4,8,11]tetraazacyclotetradecine; Py5Me2 = 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine). The red-circled section of the shaded area, with deconvolution shown in the inset, is assigned to the MMCT absorption. Reproduced with permission from ref 22. Copyright 2015 American Chemical Society. (B) CoIIOWVI → CoIIIOWV metal-topolyoxometalate absorption of complex [CoW12O40]6−. Reproduced with permission from ref 23. Copyright 2014 Wiley-VCH.

(1700 L mol−1 cm−1).22 Closely related molecular charge transfer systems include all-inorganic polyoxometalates like [CoIIWVI12O40]6− featuring a CoIIOWVI → CoIIIOWV metal-topolyoxo-metalate charge-transfer transition absorbing in the 400−500 nm region (Figure 3B).23 The excited state lifetime of MMCT units, an important property for efficiently driving multielectron catalysts, was

found to be microseconds at room temperature based on transient optical absorption measurements for the systems investigated so far.21,24 For the TiOMn unit, the kinetics of recovery of the MMCT ground state bleach and the decay of the transient TiIII absorption at 580 nm generated by TiIVOMnII → TiIIIOMnIII excitation gave a time constant for back electron transfer of 2.43 ± 0.20 μs (Figure 4). 1636

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Figure 4. (A) Transient absorption of TiIII center of excited TiOMn unit. (B) Decay of TiIII absorption at 580 nm upon excitation of MMCT with a 420 nm laser pulse of 8 ns duration (trace 1). Signal is not observed in the absence of MMCT as shown for Ti only (trace 2) or Mn only samples (trace 3). (C) Proposed mechanism of back electron transfer. Reproduced with permission from ref 21. Copyright 2014 American Chemical Society.

Figure 5. Spatially directed assembly of Cu oxide nanocluster catalyst coupled to ZrOCo light absorber. Optical DRS shows the reduction of the [CuII(NCCH3)4]2+ precursor loaded in ZrOCo SBA-15 (black trace) upon photo-excitation of the ZrOCo charge transfer state to CuI (red trace). High angle annular dark field image (HAADF) of the sample after calcination reveals the formation of 3 nm sized Cu oxide clusters (bright spots). EDX of bright spots confirm the presence of Cu oxide clusters. Reproduced with permission from ref 29. Copyright 2015 American Chemical Society. 1637

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Figure 6. (A) Photoreduction of CO2 gas by CuxOy−ZrOCo unit. (B) Adsorbed 12CO2 after loading of gas (blue trace). Subsequent traces show reactant depletion during photocatalysis. (C) FT-IR spectra of gas phase 13CO growth upon photolysis in the presence of diethylamine donor. (D) Growth kinetics of 13CO (2065.9 cm−1 band) for CuxOy−ZrOCo units with predominantly surface Cu0 (red) and surface CuII (black). Reproduced with permission from ref 29. Copyright 2015 American Chemical Society.

CO2 reduction because of their unique ability to form more deeply reduced products beyond CO or formate, including species with two or more CC bonds.25 Remarkable progress in terms of the lowering of overpotentials and selectivity has recently been made through nanostructuring and oxidation/ reduction pretreatment of Cu materials.26−28 For proper coupling of the ZrOCoII light absorber to a Cu oxide nanocluster at the Zr acceptor end, we have exploited the directionality of electron transfer of MMCT excitation for assembling the catalyst nanoparticle adjacent to the Zr acceptor center in spatially controlled manner. Our hypothesis was that transient ZrIII of the ZrIIIOCoIII excited state will transfer an electron to a reducible Cu precursor, which, upon subsequent calcination, could act as nucleus for spontaneous growth of a nanocluster adjacent to the Zr center. Mononuclear precursors with weakly bound ligands such as the CuII(NCCH3)4 complex offered a logical starting point. As shown by the UV−visible spectra in Figure 5, visible light excitation of the ZrIVOCoII chromophore in the presence of trimethylamine as sacrificial electron donor results in the reduction of the Cu center to CuI, and FT-IR spectroscopy revealed concurrent loss of acetonitrile ligands.29 Subsequent calcination at 350 °C produced cupric oxide nanoclusters of 3 nm diameter (Figure 5), suggesting that the ligand deficient CuI species formed by photodeposition acts as nucleus for cluster growth. Continued illumination of the ZrOCoII MMCT unit with visible light led to efficient reduction of the cluster surface to Cu0.29 Exposure of the reduced CuxOyZrOCoII unit to CO2 gas gave a band at 1682 cm−1 (1656 cm−1 for 13CO2) for weakly adsorbed CO2 with carboxylate structure that was bleached upon illumination of the MMCT transition, as shown in Figure 6B. Therefore, CO2 adsorbed on surface Cu0 centers is the site of catalytic reduction, with gas phase CO product detected by

Semiclassical Marcus treatment of the temperature dependence of the decay kinetics revealed a very small electronic coupling constant of 0.22 cm−1, which contrasts sharply with an optical coupling constant of 4200 cm−1 derived from the absorption cross section. The large difference between the two coupling constants points to a spin forbidden process for the origin of the observed slow back electron transfer (kBET2 in Figure 4C), indicating that ultrafast intersystem crossing (kISC) that follows (spin-allowed) optical excitation of the S = 5/2 high spin MMCT state competes favorably with ultrafast back electron transfer on the initially excited MMCT surface (kBET1).21 With this long lifetime of the excited binuclear unit, charge transport from the donor metal to a catalyst may become competitive with back electron transfer of the light absorber, which renders the light absorber suitable for driving multielectron catalysts for CO2 reduction and H2O oxidation.

3. COUPLING BINUCLEAR LIGHT ABSORBER TO METAL OXIDE CATALYST FOR CO2 REDUCTION OR H2O OXIDATION Metal oxide nanoclusters are attractive as multielectron catalysts for H2O oxidation or CO2 reduction not only due to their robustness but because they offer a very large number of surface sites for mitigating the discrepancy between the high rate of incoming photons and subsequent charge delivery at maximum solar intensity and comparatively slow catalytic turnover frequency. The large number of reactive sites available at nanoparticle surfaces ready to start a new catalytic cycle even as chemical transformation at each site is substantially slower than the rate of photon absorption makes productive use of all delivered charges possible. Cu catalysts, in particular in the form of metallic Cu and cuprous oxide, play a special role in 1638

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Figure 7. (A) Photoreduction of 13CO2 by H2O at ZrOCo−IrOx units in SBA-15. (B) Rovibrational FT-IR spectra of growing gas phase 13CO product upon irradiation at 355 nm in the presence of 13CO2 and H2O gas mixture. (C) Mass spectroscopic monitoring of O2 product upon irradiation of ZrOCo−IrOx SBA-15 loaded with 13CO2 and H2O. (D) Oxygen monitoring of ZrOCo−IrOx SBA-15 loaded with 13C16O2 and H218O. Controls are shown. Reproduced with permission from ref 30. Copyright 2014 American Chemical Society.

The formation of O2 was directly monitored mass spectrometrically (Figure 7C). Furthermore, isotopic labeling experiments using 13CO2 and H218O resulted in evolution of 18O2, which confirmed water as the electron source. Concurrent formation of unlabeled 16O2 stems from dissociation of 13C16O2 (Figure 7D). The quantum efficiency upon illumination at 355 nm was estimated at 17%, which implies that hole transfer from transient CoIII of the excited MMCT state to the Ir oxide cluster (Figure 7A) competes remarkably well with back electron transfer of the ZrOCo unit. The ZrOCoIIIrOx assembly constitutes the first system for closing the photosynthetic cycle of CO2 reduction by H2O with an all-inorganic polynuclear cluster featuring a molecularly defined light absorber. The complete cycle of CO2 reduction by H2O has been demonstrated recently for semiconductor light absorber based systems such as SrTiO3/InP monolith functionalized with a molecular CO2 reduction catalyst to generate formate at 0.1% efficiency.31 Using triple junction PV elements, 4.6% power efficiency was reported for amorphous Si−Ge coupled to noble metal cocatalysts for CO2 reduction and H2O oxidation for making formate,32 while perovskite driving noble metal electrodes achieved conversion of CO2 and H2O to CO and O2 with power efficiency of 6.5%.33

its rovibrational infrared bands upon excitation of the ZrOCo chromophore (Figure 6C). Interestingly, the rate of photoreduction is substantially larger (by a factor of 3) for such prereduced Cu oxide clusters in the early stages of the catalysis than for nonreduced clusters, as shown in Figure 6D. The surface Cu oxidation state dependence of the activity confirms that the binuclear unit acts as single electron, single charge transfer chromophore driving the CuxOy multielectron catalyst as illustrated by the cartoon of Figure 6A. Search for deeper reduction products beyond CO, which are expected based on electrocatalytic results,27 is in progress. By analogy to the chemistry developed for the coupling of binuclear light absorbers to a Cu oxide catalyst for CO2 reduction, a photodeposition method was introduced for the directed assembly of an Ir oxide cluster (2 nm), known to be an efficient water oxidation catalyst, on the CoII donor end of the binuclear unit using IrIII(acac)3 as precursor.30

4. PHOTOREDUCTION OF CO2 BY H2O AT AN ALL-INORGANIC POLYNUCLEAR UNIT Because ZrOCoII sites split CO2 to CO upon excitation of the MMCT chromophore according to our previous studies,18 the ZrOCoII−IrOx assembly allowed us to explore not only MMCT driven water oxidation, but photocatalytic CO2 reduction using electrons from H 2 O (Figure 7A). When the MMCT chromophore of ZrOCoII−IrOx units in SBA-15 loaded with 1 atm of a vapor mixture of CO2 and H2O was illuminated, gaseous CO evolved. Growth of the product shown in Figure 7B was only observed with H2O present, indicating that water acts as electron source. In light of our previous finding that split-off O of CO2 forms H2O when receiving an electron from MMCT excited ZrOCuI units anchored on silica surfaces (assisted by SiOH groups),11 the same mechanism is expected for ZrOCoII units pulling electrons from IrOx catalyst clusters.

5. TOWARD CLOSING THE PHOTOSYNTHETIC CYCLE ON THE NANOSCALE UNDER MEMBRANE SEPARATION OF THE HALF REACTIONS For developing artificial photosystems capable of reducing CO2 by H2O to liquid products such as CH3OH or even multicarbon molecules, separation of the oxidation and reduction half reactions by a membrane is indispensable because cross and back reactions would otherwise severely degrade the photocatalytic efficiency. Therefore, we seek to 1639

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Figure 8. Time-resolved ATR FT-IR spectroscopy of visible light driven water oxidation at Co3O4 nanoparticles. (A) (left) Charge injection by visible light-generated [Ru(bpy)3]3+; (right) TEM image of single crystal Co3O4 particle (4 nm). (B) Laser excitation and FT-IR probe geometry. Reproduced with permission from refs 37 and 38. Copyright 2014 Nature Publishing Group and 2014 Elsevier B. V.

Figure 9. Surface intermediates of water oxidation on Co3O4 catalyst in aqueous solution observed by rapid-scan FT-IR spectroscopy. (A) (left) Infrared absorption of O−O mode of superoxide in (a) H216O, (b) H218O, (c) D2O, and (d) control run without catalyst; (right) CoIV oxo surface species at 450 ms after onset of 300 ms laser photolysis pulse in (a) H216O, (b) H218O, and (c) D2O. (B) Kinetics of intermediates. Gray area shows illumination period. Reproduced with permission from ref 37. Copyright 2014 Nature Publishing Group.

arrays of separated core−shell nanotubes, with each tube operating as an independent photosynthetic unit as shown in Figure 1.

develop hierarchical assemblies that incorporate Nature’s design principle of closing the photosynthetic cycle on the nanoscale under separation of the half reactions by a membrane. We have chosen a metal oxide−silica nanotube construct for accomplishing the redox cycle. The core is a crystalline Co3O4 nanotube that functions as an efficient water oxidation catalyst, with O2 gas evolving on the inner surface of the tube. Grown on the outside of the Co3O4 nanotube is a dense phase, 2−3 nm thick silica shell that acts as a proton conducting, molecule impermeable membrane separating the H2O oxidation space on the inside from the light absorber and CO2 reduction sites on the outside. Molecular wires embedded in the silica allow for tightly controlled electron transport between the light absorber and the Co3O4 water oxidation catalyst across the insulating silica layer, as will be discussed below. A macroscale integrated photosystem will consist of

5.1. Earth Abundant Metal Oxide Catalyst for H2O Oxidation

The selection of Co3O4 as catalytic material for water oxidation is based on the recent finding that nanostructured particles with their very large surface area afford substantially higher water oxidation activity per projected area of the catalyst than larger, micrometer sized particles.34 Originally demonstrated for Co3O4 nanoclusters supported in mesoporous silica scaffolds, turnover frequency (TOF) enhancements for water oxidation per projected area of over 1000 at comparable overpotential were achieved at close to pH neutral conditions. The enhancement includes an over 10-fold increase of the intrinsic 1640

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Figure 10. (A) (top) Electrochemical monitoring of O2 in solution induced by a single 476 nm light pulse (300 ms pulse, red trace; 1 s pulse, black trace; shaded section indicates the missing O2 in the 300 ms experiment due to incomplete catalytic cycles arrested at an intermediate stage); (bottom) Isotopic composition of O2 gas accumulated in the headspace of H218O solution measured by mass spectroscopy. (B) Proposed reaction mechanism of water oxidation on Co3O4 surface for fast and slow site. Reproduced with permission from ref 37. Copyright 2014 Nature Publishing Group.

Figure 11. Molecular wires for controlled charge transport embedded in nanometer thick silica membrane. (A) Schematic of Co3O4−SiO2 core− shell nanotube with ZrOCo light absorber coupled to p-oligo(phenylene vinylene) wire cast into silica shell. (B) Energetics of HOMO and LUMO of light absorber, molecular wire, and catalyst. Reproduced with permission from ref 35. Copyright 2014 Royal Society of Chemistry.

for monitoring transient surface intermediates and their kinetics under reaction conditions with the visible light sensitized method shown in Figure 8. Upon initiation of catalysis with a 300 ms light pulse two surface intermediates were detected, namely, a superoxide absorbing at 1013 cm−1 (18O16O, 995 cm−1; 18O18O, 966 cm−1), and a CoIVO species at 840 cm−1 (Figure 9A; the structure of this species is thought to have strong oxyl character because of the “oxo wall”1).37 For the experiments shown, cubic Co3O4 crystalline nanoparticles were used.38 As can be seen from the slow decay kinetics of the 840 cm−1 band of CoIVO (lifetime 1 s) and the fast rise of the 1013 cm−1 superoxide species (most of the growth occurs within the 300 ms duration of the sensitization pulse), Figure 9B, the CoIVO site observed at 840 cm−1 is not a precursor of the superoxide species and belongs to a much slower catalytic site. At the fast site, which possesses the CoIII(−OH)−

catalytic rate per surface Co center compared to macroscopic Co3O4 particles.34 In the case of Co3O4 nanotubes, oxygen evolution activity measured by a visible light sensitization method shown in Figure 8 revealed a TOF of 0.02 O2 molecules s−1 per surface Co center.35 Due to the very large number of surface metal centers of nanoparticle catalysts per geometrical area, this rate is adequate for the productive use of the majority of solar photons shining on the catalyst even at maximum solar intensity.36 If TOF of 0.02 s−1 represents an average value that includes sites with much higher catalytic rates while others are much slower or inactive, knowledge of the structure of the most active sites and the detailed reaction mechanism would provide guidance for further catalyst improvement. We sought to find out by using time-resolved rapid-scan FTIR spectroscopy in the attenuated total reflection mode (ATR) 1641

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Figure 12. (A) TEM of Co3O4−SiO2 core−shell nanoparticles. (B) FT-Raman spectra of (a) wire molecules embedded in the silica shell of Co3O4− SiO2 core−shell particles or (b) free wire molecules. Bands with asterisk are Co3O4. (C) Schematic of visible light induced hole transfer process from sensitizer on the outside to Co3O4 on the inside of the core−shell particle. (D) (black trace) Recovery of the bleach of the Ru sensitizer complex (470 nm) upon hole injection into embedded wire molecules; (gray trace) control with Co3O4−SiO2 particles with no wires. Reproduced with permission from ref 42. Copyright 2013 American Chemical Society.

O−CoIII−OH motif, the O−O bond forming step is proposed to be nucleophilic attack of H2O on one of the CoIVO moieties, a process too fast for detection on the millisecond time scale.37,39 Because O2 is detected electrochemically following a 300 ms sensitization pulse (Figure 10A), the TOF of the fast site whose mechanism is shown in Figure 10B is at least 3 s−1, which is 150 times faster than the average TOF value per surface Co center. This result indicates that catalytic improvement may be achieved, for example, by catalyst surface treatment to generate higher concentrations of CoIII(−OH)− O−CoIII−OH sites. The slow sites marked by the 840 cm−1 CoIVO band are proposed to be CoIII−OH centers that lack an adjacent CoIII−OH and, therefore, are unable to yield a transient CoIV(O)−O−CoIVO moiety. For these sites, nucleophilic attack of H2O to form an OO bond occurs on the much slower time scale of 1 s.37

with delocalized HOMO of Co 3 O4 (a charge-transfer insulator)40 and the HOMO of the light absorber for efficient hole hopping upon excitation of the ZrOCoII unit (or a sensitizer with similar redox potential). By contrast, the LUMO of PV3 is at a too negative potential for the excited electron to transfer to the wire molecule, thus preventing the electron from hopping to the water oxidation catalyst that otherwise would degrade the photocatalytic efficiency.41 To explore synthetic methods for the covalent attachment of the molecular wires to the Co oxide surface and casting them into silica, we have prepared spherical Co3O4 particles and attached PV3 molecules via tripodal anchors (1,3-dihydroxy-2(hydroxymethyl)propan-2-yl)carbamoyl). Wire molecules functionalized with these anchoring groups arrange vertically on the metal oxide surface, further enforced by SO3− groups introduced on the opposite end to achieve repulsion by the negatively charged Co3O4 surface (Figure 11A).41 Casting of the wires into silica was conducted with a modified solvothermal method.42 Silica shells are uniform as can be seen by TEM, and FT-Raman spectra of embedded wire molecules confirm the structural integrity upon casting into silica (Figure 12A,B). Aqueous colloidal solutions of these core−shell particles allowed us to monitor visible light sensitized hole injection by transient optical absorption spectroscopy as shown in Figure 12C,D. Regeneration of the reduced sensitizer upon hole injection into embedded wire molecules occurred at diffusion controlled rate, indicating that hole injection into the silica embedded wire is highly efficient. Furthermore, the absence of the characteristic absorption band of a hole on PV3 at 600 nm, which can readily be observed for wire molecules not attached to Co3O4, indicates hole transfer

5.2. Nanoscale Silica Membrane with Embedded Molecular Wires for Artificial Photosynthesis

Nanoscale amorphous silica is impermeable to small molecules including oxygen, while transmitting protons.35 Therefore, this ultrathin insulating oxide material may be suitable as a membrane for nanoscale artificial photosynthetic units if methods can be found for tightly controlled electron (hole) transport between the light absorber on one side and the Co3O4 catalyst on the other side (Figure 11A). We have introduced short (2 nm) organic molecular wires in the form of p-oligo(phenylene vinylene) featuring three aryl units (1,3di((E)-styryl)benzene, abbreviated PV3) with properly positioned HOMO and LUMO, which, when coupled to a light absorber, afford rectifying behavior as illustrated in the energy diagram of Figure 11B. The HOMO potential of PV3 is aligned 1642

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Figure 13. Spectroscopic characterization of planar Co3O4−SiO2 constructs with embedded wires. (A) EDX maps of Co (top left), Si (bottom left), and S (top right) and dark field image (bottom right). (B) Sulfur S(2p) XPS (top) and nitrogen N(1s) XPS (bottom) of planar Co3O4-wire sample before (blue) and after deposition of 2.3 nm of silica (green). Reproduced with permission from ref 43. Copyright 2015 American Chemical Society.

Figure 14. Photoelectrochemical and CV measurements of charge transport across silica membrane with embedded wires. (A) Short circuit photocurrent observed for visible light sensitized hole injection into bare Co oxide, Co oxide coated with SiO2−PV3 membrane, and Co oxide coated with silica only, using [Ru(bpy)3]2+-persulfate (blue) and Sn porphyrin-triethylamine (red) systems. (B) Schematic of experiment. (C) CV of ferrocene using bare Co3O4 on Pt electrode (red curve), Co3O4−PV3−SiO2 (blue curve), and Co3O4−SiO2 (green curve). Reproduced with permission from ref 43. Copyright 2015 American Chemical Society.

from the wire molecule to the Co3O4 catalyst in less than a microsecond.42 Quantitative understanding of electron flow through the wires embedded in the silica membrane was obtained by photoelectrochemical studies using cm2 sized, planar Co oxide−silica/wire constructs prepared by atomic layer deposi-

tion (ALD) under mild conditions. Figure 13A shows STEM EDX images (scanning transmission electron microscopy, energy dispersive X-ray spectroscopy) of a Co3O4 layer of 7 nm thickness grown on a 100 nm thick Pt electrode, followed by anchoring of PV3 wire molecules and casting of 2 nm silica.43 The Co and Si images confirm that the layers are 1643

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Accounts of Chemical Research Funding

uniform, and wire coverage of 0.2 monolayer (one molecule nm−2) is indicated by the sulfur EDX image of the SO3− groups. The presence of embedded wire molecules is further confirmed by sulfur and nitrogen XPS (N of amide group) (Figure 13B). Visible light sensitized photocurrent measurements with this planar Co3O4−SiO2/PV3 assembly in short circuit configuration allowed us to quantify the controlled electron flow through the embedded wires of the silica membrane, as illustrated schematically in Figure 14A. For the [Ru(bpy)3]2+ photosensitizer, photocurrent was observed upon blue light excitation at a rate of 27 electrons s−1 per wire molecule. The role of the wires as exclusive electron conduits was confirmed by the absence of current for Co3O4−SiO2 electrodes without embedded wires (Figure 14B). Tight control of electron transport by the orbital energetics of the wires was established by using Sn porphyrin as visible light sensitizer whose redox properties do not match the PV3 orbital energy as no current was observed upon generation of Sn porphyrin radical anion. The result is consistent with the fact that the reduction potential of SnP radical anion is not sufficient for injection of an electron into LUMO of PV3 (Figure 14B). Confirmation of pinhole-free silica membranes was further established by CV measurements using the ferrocene/ferrocenium redox couple (Figure 14C).43 The photoelectrochemical characterization combined with the transient optical electron transfer measurements establish the silica nanolayers with embedded wires construct as a novel membrane for nanoscale artificial photosynthesis.

This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Chemical, Geological and Biosciences of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Portions of this work were performed as a User Project at the Molecular Foundry, Lawrence Berkeley National Laboratory, which is supported by the Office of Science, Office of Basic Energy Sciences. Notes

The authors declare no competing financial interest. Biographies Wooyul Kim received his Ph.D. from POSTECH in 2012 under the direction of Prof. Wonyong Choi. During his postdoctoral stay at LBNL, he developed synthetic methods and mechanistic understanding of inorganic polynuclear structures for artificial photosynthesis. He recently joined Sookmyung Women’s University as Assistant Professor. Eran Edri received his Ph.D. at the Weizmann Institute of Science in 2014 under the guidance of Prof. Gary Hodes. In his current postdoctoral period at LBNL, he is developing synthetic and physicochemical methods for nanoscale photocatalytic units and assemblies. Heinz Frei is a Senior Scientist at LBNL and received his Dr.Sc. from the Swiss Federal Institute of Technology (ETH Zuerich) in 1977. His current research focuses on the synthetic, spectroscopic, and mechanistic challenges of the conversion of carbon dioxide and water to a liquid fuel by artificial photosynthesis.

6. OUTLOOK All-inorganic binuclear light absorbers, Co3O4 nanotube catalysts, and silica/embedded wire membranes, combined with the synthetic methods developed for the hierarchical arrangement of these components, open up the pursuit of macroscale integrated systems based on nanoscale photosynthetic units. Macroscale arrays in the form of Co oxide− silica nanotubes as depicted in Figure 1 offer geometry for integration that separates water oxidation catalysis from light absorber and carbon dioxide reduction to fuel while closing the photocatalytic cycle at each of the independently operating nanotubes. The nanotube array design provides flexibility for maximizing photocatalytic efficiency by adjusting structural parameters such as nanotube size, space between the tubes, and electronic properties of molecular wires and light absorbers. The pursuit of a diversity of ideas for building hierarchical structures based on nanoscale photosynthetic units is needed for converging toward the most efficient integrated system for scalable solar light driven reduction of carbon dioxide by water. The capabilities of nanoscience and synthetic chemistry combined with the recent substantial progress in the understanding of molecular mechanisms of photosynthetic processes make this a particularly promising time to explore scalable artificial photosystems.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

Dept. of Chemical and Biological Engineering, Sookmyung Women’s University, Yongsan-gu, Seoul 04310, Korea. 1644

DOI: 10.1021/acs.accounts.6b00182 Acc. Chem. Res. 2016, 49, 1634−1645

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DOI: 10.1021/acs.accounts.6b00182 Acc. Chem. Res. 2016, 49, 1634−1645