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Title: Hierarchical Inorganic Assemblies for Artificial Photosynthesis

Abstract

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 CO 2 reduction and H 2O 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 in this paper 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 formore » selective coupling to the donor or acceptor metal of the light absorber. For CO 2 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 CO 2 conversion by H 2O to CO and O 2. 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 CO 2 by H 2O 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. Finally, hierarchical arrangement of the components is accomplished by a combination of photodeposition, controlled anchoring, and atomic layer deposition methods.« less

Authors:
 [1];  [1];  [1]
  1. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Publication Date:
Research Org.:
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1393075
DOE Contract Number:
AC02-05CH11231
Resource Type:
Journal Article
Resource Relation:
Journal Name: Accounts of Chemical Research; Journal Volume: 49; Journal Issue: 9
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE

Citation Formats

Kim, Wooyul, Edri, Eran, and Frei, Heinz. Hierarchical Inorganic Assemblies for Artificial Photosynthesis. United States: N. p., 2016. Web. doi:10.1021/acs.accounts.6b00182.
Kim, Wooyul, Edri, Eran, & Frei, Heinz. Hierarchical Inorganic Assemblies for Artificial Photosynthesis. United States. doi:10.1021/acs.accounts.6b00182.
Kim, Wooyul, Edri, Eran, and Frei, Heinz. Tue . "Hierarchical Inorganic Assemblies for Artificial Photosynthesis". United States. doi:10.1021/acs.accounts.6b00182.
@article{osti_1393075,
title = {Hierarchical Inorganic Assemblies for Artificial Photosynthesis},
author = {Kim, Wooyul and Edri, Eran and Frei, Heinz},
abstractNote = {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 in this paper 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. Finally, hierarchical arrangement of the components is accomplished by a combination of photodeposition, controlled anchoring, and atomic layer deposition methods.},
doi = {10.1021/acs.accounts.6b00182},
journal = {Accounts of Chemical Research},
number = 9,
volume = 49,
place = {United States},
year = {Tue Aug 30 00:00:00 EDT 2016},
month = {Tue Aug 30 00:00:00 EDT 2016}
}
  • The mechanism of photosensitization of colloidal TiO[sub 2] electrodes with chlorin e[sub 6] and copper chlorophyllin is deduced from static and time-resolved fluorescence quenching as well as laser flash photolysis and photocurrent/voltage transients in combination with cyclic voltammetry and spectroelectrochemistry. The fluorescence of chlorin e[sub 6] on TiO[sub 2] decays to 80% within 0.4 ns, indicating efficient electron injection from the singlet excited state with k[sub et] = 2.2 x 10[sup 9] s[sup [minus]1]. Copper chlorophyllin emits only from the triplet state, due to subpicosecond intersystem crossing in the presence of the paramagnetic Cu[sup 2+] center. Its phosphorescence is stronglymore » enhanced by immobilization on a nonquenching ZrO[sub 2] reference adsorbent, but quenched on TiO[sub 2], indicating electron transfer from the triplet state with k[sub et] = 3 x 10[sup 8] s[sup [minus]1]. The energy levels of the excited photosensitizers and the acceptor state density of TiO[sub 2] are determined by cyclic voltammetry. A strong tail of shallow surface states on the colloidal TiO[sub 2] electrodes modifies the classical picture of a conduction band edge. Transient absorption spectra of the electrodes show three equivalent contributions due to dye bleaching, cation radical formation, and conduction band electrons, in accordance with electron injection from the neutral excited dye. 36 refs., 6 figs., 1 tab.« less
  • Colloidal TiO[sub 2] electrodes were photosensitized with derivatives of chlorophyll and related natural porphyrins resulting in light harvesting and charge separation efficiencies comparable to those in natural photosynthesis. The photocurrent action spectra of the electrodes correlate well with the absorption spectra of the dyes in solution. Incident photon to current efficiencies up to 83% are reached in the Soret peak at 400 nm with a 12-[mu]m-thick TiO[sub 2] film sensitized by copper mesoporphyrin IX, which corresponds to nearly unity quantum efficiency of charge separation when light reflection losses are taken into account. Photocurrent/voltage curves of TiO[sub 2] solar cells sensitizedmore » with copper chlorophyllin show an energy conversion efficiency of 10% for the red peak at 630 nm. Under simulated sunlight illumination, an open circuit photovoltage of 0.52 V and a short circuit current density of 9.4 mA/cm[sup 2] are measured. The overall energy conversion efficiency of the cell is 2.6% under these conditions, in part limited by ohmic losses at such high current densities. The comparison of different chlorophyll derivatives indicates that free carboxyl groups are important for adsorption and sensitization on TiO[sub 2]. However, conjugation of the carboxyl groups with the [pi] electron system of the chromophore is not necessary for efficient electron transfer. Free bases, zinc, and even the nonfluorescent copper complexes of chlorophyllins and mesoporphyrin IX are efficient sensitizers for TiO[sub 2]. Cholanic acids as coadsorbates were found to be unique in improving both photocurrent and voltage of copper chlorophyllin sensitized cells. This effect is discussed by comparison with other coadsorbates. 23 refs., 3 figs., 1 tab.« less
  • In this report we discuss the combined application of scanning electron microscopy and quasielastic light scattering as well as UV/vis titrations employing [Ru(bpy)[sub 3]][sup 2+] and especially designed bis-heteroleptic sensor ruthenium complexes [Ru(bpy)[sub 2](MS-R)][sup 2+], [Ru(bpy)[sub 2](DS-R)][sup 2+], [Ru(tap)[sub 2](MS-R)][sup 2+], and [Ru(tap)[sub 2](DS-R)][sup 2+] (bpy = 2,2[prime]-bipyridine, tap = tetraazaphenanthrene) possessing mono- and bis-styrene-attached tetraazaphenanthrene units as third ligands for the characterization of Ru, RuO[sub 2], IrO[sub 3], and MnO[sub 2] colloids, as they are commonly used in model systems for artificial photosynthesis. The elucidation of the binding mechanisms and orientation and the arising supramolecular photoelectron transfer properties ofmore » ruthenium sensitizers adsorbed on metal colloids can be regarded as tools for the generation of highly efficient systems for artificial photosynthesis. We describe in this paper a simple and application-oriented method for the evaluation of the catalyst's effective surface in water solution and characteristic differences in the binding and interaction modes of [Ru(bpy)[sub 3]][sup 2+] to the four colloidal catalysts and the bis-heteroleptic sensor complexes listed above with the ruthenium colloid. 30 refs., 12 figs., 2 tabs.« less
  • In this Account the authors focus on {open_quotes}water splitting,{close_quotes} the photodriven conversion of liquid water to gaseous hydrogen and oxygen. Beyond the intellectual challenge of designing and fabricating such a system, there are several practical implications. H{sub 2} could serve directly as a fuel, e.g., for transportation or for the production of electricity in fuel cells, without producing pollutants or greenhouse gases upon combustion. For some purposes, however, it might be useful to use the H{sub 2} as a reactant to produce a different fuel, such as one that is liquid at the usual temperatures and pressures. Thus, the authorsmore » seek as a {open_quotes}Holy Grail{close_quotes} a renewable energy source driven by solar energy that produces a clean and storable fuel. 33 refs., 2 figs.« less
  • The effective design of an artificial photosynthetic system entails the optimization of several important interactions. Herein we report stopped-flow UV-visible (UV-vis) spectroscopy, X-ray crystallographic, density functional theory (DFT), and electrochemical kinetic studies of the Re(bipy-tBu) (CO)3(L) catalyst for the reduction of CO2 to CO. A remarkable selectivity for CO2 over Hþ was observed by stopped-flow UV-vis spectroscopy of [Re(bipy-tBu)3]-1. The reaction with CO2 is about 25 times faster than the reaction with water or methanol at the same concentrations. X-ray crystallography and DFT studies of the doubly reduced anionic species suggest that the highest occupied molecular orbital (HOMO) has mixedmore » metal-ligand character rather than being purely doubly occupied dz2, which is believed to determine selectivity by favoring CO2 (σ+π) over H+ (σ only) binding. Electrocatalytic studies performed with the addition of Brönsted acids reveal a primary H/D kinetic isotope effect, indicating that transfer of protons to Re-CO2 is involved in the rate limiting step. Lastly, the effects of electrode surface modification on interfacial electron transfer between a semiconductor and catalyst were investigated and found to affect the observed current densities for catalysis more than threefold, indicating that the properties of the electrode surface need to be addressed when developing a homogeneous artificial photosynthetic system.« less