Surface-Modified Anisotropic TiO2 Nanocrystals Immobilized in Membranes: A Biologically Inspired Solar Fuel Catalyst

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Surface-Modified Anisotropic TiO2 Nanocrystals Immobilized in Membranes: A Biologically Inspired Solar Fuel Catalyst James D. Hoefelmeyer Department of Chemistry, University of South Dakota, Vermillion, SD 57069, USA

11.1 INTRODUCTION Anthropogenic perturbations to the environment have increased exponentially and affect planetary biogeochemical cycles. The chemical composition of the atmosphere, hydrosphere, and soils has been altered and are often accompanied by significant changes to the morphological features of the planet. Human activities have increased the toxicity of the environment, and have set into motion (often through feedback mechanisms) a new set of climate patterns that arise from longer residence time of heat on the planet surface due to the influence of the greenhouse gases. The combustion of fossil fuels is the major factor contributing to anthropogenic environmental change. Globally, human activities demand about 15 TW power, and about 85% is derived from release of chemical energy in fossil fuels. Human population growth and economic development of populations in poverty will be positive pressures on power demand, and by the middle of the 21st century, global power demand is projected to be 30 TW. For these reasons, the development of clean, abundant power sources is a critical challenge—perhaps the greatest challenge of the 21st century [1]. Power sources with zero carbon dioxide emissions include nuclear, biofuels, geothermal, tidal, wind, and solar. Among these options, solar is attractive due to the abundance of sunlight incident on Earth—roughly 89 PW [2]. Utilization of only a small fraction of this would be enough to power human activity well beyond projected demands. This prospect has been

Solar Photocatalysis http://dx.doi.org/10.1016/B978-0-444-53872-7.00012-1

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© 2013 Elsevier B.V. All rights reserved.

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the motivation for research into solar energy utilization. The field was recently summarized in a US Department of Energy report that is a primer for solar energy research projects [3]. Solar energy utilization involves the interconversion between different forms of energy. Light energy can be converted to thermal, electrical, or chemical energy in solar heaters, photovoltaics, and photoelectrochemical cells, respectively. More efficient and low cost processes for solar energy are becoming economically more competitive with other technologies, which may be a requirement to disrupt the global energy status quo. In this paper, the focus will be on conversion of light energy to chemical energy. This process has advantage that chemical energy can be stored easily compared to thermal or electrical energy. If liquid chemical fuels can be produced, then current infrastructure could be retained for transport and storage. In nature, photosynthesis is a process wherein light energy is converted to chemical energy. In plants, photosynthesis light reactions occur in the thylakoid membranes of chloroplasts. A series of membrane protein complexes moderate a sequence of reactions that lead to synthesis of O2, NADPH (reducing equivalent), and ATP (energy equivalent). The NADPH and ATP are later utilized in the light-independent CO2 fixation steps, and eventually in synthesis of glucose chemical fuel. Glucose can be polymerized to starch for storage in the plant. The light reactions are driven by photosystem I (PS I) and photosystem II (PS II), which are membrane-spanning protein complexes that contain P700 (PS I) and P680 (PS II) chromophores that are at the heart of an extended electron transfer chain. The chromophores are polychlorophyll assemblies that, upon photon absorption to yield P*, donate an electron to a chlorophyll monomer to yield a P+ electron acceptor [4]. The P680*/P680+ potential is −0.57 V and P680/P680+ potential is +1.25 V [5]. The P700*/P700+ potential is −1.32 V and P700/ P700+ potential is +0.43 V [6]. Thus the redox potentials spanned by the photosystems is very large (P700* is strongly reducing and P680+ is strongly oxidizing), and sufficient to drive the endothermic reactions of water oxidation (+0.82 V) and carbon dioxide (or proton) reduction (−0.42 V (−0.41 V)). All potentials are expressed as standard reduction potentials at pH7 and 25 °C; the SHE = −0.42 V. It is instructive to study the efficiency of the light reactions of photosynthesis. The overall equation for the light reactions is:

2H2 O + 2NADP+ + 3ADP + 3Pi + light → 2NADPH + 2H+ + 3ATP + O2 . The synthesis of one molecule of water occurs at the oxygen-evolving complex of PS II, and requires four photons (680 nm) to produce the most oxidized state and turnover of the catalytic cycle [7]. Electrons donated from P680* (four electrons) are carried to PSI to regenerate the oxidized P700+ center. Synthesis of two molecules NADPH requires four electrons and four photons (700 nm). The light reactions create a proton gradient across the thylakoid membrane. The gradient is relieved as protons pass through ATP Synthase, and the free energy release is used to synthesize ATP. According to the balanced reaction equation, eight quanta of light lead to synthesis of three molecules of ATP. Therefore, the energy input is four photons (680 nm), four photons (700 nm), which totals 23.0 × 10−19 J. The energy out is the stored chemical energy within three ATP molecules, and two NADPH molecules. The free energy of hydrolysis of ATP under cellular conditions is approximately −50 kJ/mol, so this component contributes 2.5 × 10−19 J. The energy stored in NADPH can be calculated based on the reduction potentials of oxygen (E°’ = +0.82 V) and NADP+ (E°’ = −0.32 V), as the following reaction describes the oxidation of NADPH by O2:



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O2 + 2H+ + 2NADPH → 2H2 O + 2NADP+ ,

(E◦′ = +1.14 V).

The free energy of the reaction is −440 kJ/mol, corresponding to 7.3 × 10−19 J. The total fuel energy out is 9.8 × 10−19 J. The maximum efficiency of light to chemical energy conversion is:

100 × (9.8 × 10−19 J/23.0 × 10−19 J) ∼ = 42%. This value is the monochromatic efficiency of photosynthesis. Plants do not utilize the entire solar spectrum, and the Shockley-Queisser limit of photosynthesis (1.8 eV) is ∼24%. Thus the efficiency becomes (0.42 × 0.24 = 10%). This upper limit of efficiency in photosynthesis is not achievable during full midday solar illumination. The rate of incident photons is greater than the rate of electron transfer in photosynthesis. Indeed, there are mechanisms to simply dissipate excess energy that cannot be captured within the electron transfer pathway [8]. Photosynthesis is not optimized for the purpose of fuel production and storage, as plants are living organisms that have evolved for the purpose of survival. Plants produce support structures, chemical defenses, structures to attract pollinators, seeds and means for their dispersal, etc. Concomitantly, the products of the light reactions of photosynthesis, NADPH and ATP, are biologically convenient due to the participation in numerous biochemical pathways that enable synthesis of multitude of biomolecules that serve various purposes in the organism. Even the fuel molecule glucose synthesized in plants offers advantages for survivability, such as high water solubility and ability to polymerize into starch (energy storage) or cellulose (structure). If one only counts the harvested unit of the plant as the energy product, then the efficiency of photosynthesis appears low (typically ∼1%). Despite apparent efficiency of photosynthesis of ∼1% (100 × fuel energy harvested by human agriculture/light energy), the value of ∼10% calculated above is more useful in the context of comparison to artificial devices. The synthesis of O2, NADPH, and ATP is analogous to synthesis of O2 and H2. In both cases an organism selects the fuel molecules for its specific application (plants-survival, humans-hydrogen economy). One method of hydrogen synthesis is water electrolysis. When coupled to a photovoltaic, the energy input for the system is sunlight, and the efficiency of the tandem system can be compared to efficiency of photosynthesis. While numerous photovoltaic devices exist, silicon-based solar cells are the most common. Efficiency of multi-crystalline silicon cells available for large-scale production is ∼18% [9]. Commercial electrolyzers have efficiency of ∼80%. The tandem photovoltaic-electrolyzer efficiency; therefore, is ∼14% [10]. Thus plant and human solar to fuel conversion efficiencies are ∼10% and ∼14%, respectively. The difference is in large part due to the Shockley-Queisser limits of the two processes (∼24% and ∼33%, respectively). Highly optimized photovoltaic devices such as single crystal silicon cells or multi-junction cells operate at still higher efficiency. The highest efficiency reported for a demonstration cell is ∼32% on a GaInP/GaAs/Ge multi-junction device, while laboratory cells have achieved efficiency of ∼43%. The multi-junction technology allows absorption of multiple photons over a greater range of wavelengths and minimization of thermal losses due to internal relaxation. The Shockley-Queisser limit of multi-junction cells is ∼85%. There is great promise in the field of “artificial photosynthesis.” Artificial systems can far exceed the efficiency of photosynthesis, primarily due to the ability to engineer broad-spectrum absorption devices. Moving forward, the (efficiency × cost/area) of competing technologies must be evaluated.

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It is due to the cost/area consideration that competing technologies are under development and evaluation. Production methods suddenly become as important as ShockleyQueisser limits. The cost/area figure may imply minimization of the moles of chromophore in the device, such as printing of thin-film cells. Another implication may be a shift away from highly refined ultrapure single crystalline silicon. Certainly earth-abundant materials must be utilized to keep costs low. Thus, research efforts may be focused on new PV devices, new electrocatalysts for electrolyzers, new “wireless” photoelectrochemical cells, or bioreactors to develop the best (efficiency × cost/area) technology for solar fuels. In this chapter, the focus will be narrowed to “wireless” photoelectrochemical cells. Key developments will be highlighted and followed by a discussion of a project underway in our laboratory to produce surface-modified anisotropic TiO2 nanocrystals immobilized in membranes.

11.2  WIRELESS PHOTOELECTROCHEMICAL CELLS (PECs) The first PEC consisted of electrically connected TiO2 photoelectrode and Pt electrode immersed in aqueous cells [11]. Irradiation of the photoelectrode with UV radiation led to bandgap excitation and electron transfer from TiO2 to Pt through a Cu wire (attached to In-sputtered TiO2 with Ag paste). The electrochemical potential at the Pt electrode was very close to the potential of the proton reduction half-reaction. Without overpotential to drive the reaction, H2 evolution was low. Addition of reducible species such as O2 or Fe3+ to the Pt aqueous cell led to higher current, and quantum yield of ∼10%. Meanwhile, at the TiO2 photoelectrode, electron transfer from water to fill electron vacancies led to oxygen evolution. The thermodynamically favorable reaction was very slow, and the TiO2 surface is poor catalyst for electron transfer from water. The Fujishima-Honda PEC was a landmark experiment, and invited much work to improve the concept. The device was only capable of collecting UV photons, therefore much of the solar spectrum could not be utilized. The conduction band-edge potential of the photoelectrode was not sufficiently reducing for thermodynamically favorable H2 evolution. The rate of electron transfer from water to TiO2 was very slow. The device utilized the scarce element Pt. Furthermore, the structure of the TiO2 photoelectrode was not optimized to reduce recombination. Later, the laboratory of Professor Gratzel developed colloidal catalysts for the photochemical electrolysis of water. Ruthenium colloids (diameter ∼28 nm) stabilized with polyvinylalcohol were dispersed in acidic aqueous solution with ceric ion (Ce4+). Under such conditions, a ruthenium(IV) oxide layer was hypothesized to form on the surface of the colloidal particles and catalyze water oxidation [12]. Ceric ion was sacrificial oxidant in the reaction catalyzed by RuO2. Colloidal platinum particles were shown to evolve H2 from aqueous solution containing methylviologen. In later work, a colloidal photocatalyst was reported that consisted of Ru(IV)-doped TiO2 with adsorbed Pt particles and Ru(bpy)2+ 3 sensitizer [13]. The doping level of Ru(IV) (as RuO2) was 0.1%, which presumably led to distribution of Ru(IV) within the particle and on the surface. The TiO2 particles were anatase with diameter of ∼47 nm. It is not clear whether the Ru(IV) was isolated or if RuO2 domains could form in the system. The Ru(IV)-doped TiO2 particles were added to a dispersion of colloidal platinum and sonicated for several minutes.



11.2  WIRELESS PHOTOELECTROCHEMICAL CELLS (PECs)

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The colloidal Pt/RuO2–TiO2 particles were subjected to UV irradiation. Evolution of H2 and O2 in the expected stoichiometry was observed. The UV photons induced bandgap excitation of the semiconductor, followed by electron transfer from the TiO2 conduction band to Pt and from Ru(IV) to the TiO2 valence band. The Pt domain catalyzed H2 evolution while the highly oxidized Ru sites catalyzed O2 evolution. The reaction was completely suppressed with a 400 nm cutoff filter. Visible light photocatalysis was demonstrated in the presence of sensitizer and electron relay, Ru(bpy)2+ 3 derivatives, and methylviologen, respectively. With the cutoff filter in place, irradiation with the Xe lamp led to water splitting with quantum yield of H2 near 5%. Later studies in which the Pt/RuO2–TiO2 catalyst was prepared via irradiation of aqueous colloidal (20 nm) TiO2 in the presence of RuO4 and H2PtCl6 showed high activity for water splitting under bandgap excitation; however, the quantum yield of 5% under visible light irradiation in the presence of sensitizer and electron relay was never again matched [14]. More importantly, the mechanism of Pt/RuO2–TiO2 catalyst in the presence of Ru(bpy)2+ 3 and methylviologen (MV2+) was demonstrated. The MV2+ relay accepts an electron from the excited sensitizer, and then donates the electron to Pt or the TiO2 conduction band. The oxidized Ru(bpy)2+ 3 species was found to be a selective electron acceptor for RuO2. In a review of solar H2 catalysts, “microheterogeneous systems” such as the Pt/RuO2–TiO2 catalyst may not be viable due to several issues [15]. Potentially the Pt/RuO2–TiO2 catalyst could be improved if absolutely the Pt, RuO2, and sensitizer were attached to the TiO2. Furthermore, the size and shape of the semiconductor and catalyst particles could be selected for optimized electron transfer. Quantum confinement in semiconductor nanocrystals leads to partitioning and separation of the conduction and valence bands. The band-edge potentials as a function of particle size can effectively be selected to provide optimum overpotential. The shape or crystal phase of the TiO2 could also prove important variables in the design of the photocatalyst. Similarly, size and shape of the catalyst sites could be optimized, particularly if the electron transfer reactions are structure-sensitive [16]. Attachment of the catalyst domains to the semiconductor particle should allow for rapid electron transfer for catalysis. Orientation of sensitizer should be arranged, or pre-organized, to minimize the barrier of electron transfer (to TiO2 or from RuO2). Essentially, the bottlenecks for electron transfer should be identified and the system modified to overcome them. Finally, hierarchical preorganization of the catalyst within a reactor system may be required to produce separate streams of H2 and O2. Since these early investigations, many inorganic materials have been evaluated as catalysts for photochemical splitting of water [17]. The catalysts include a wide variety of semiconductor in combination with metals. The semiconductors are mostly made of earth-abundant materials; however, the metals are typically the rare and expensive platinum group metals. New materials should feature earth-abundant materials that catalyze H2 and O2 evolution [18]. Furthermore, the semiconductor morphology should be optimized to reduce charge carrier recombination. Very recently, the Nocera laboratory announced a wireless photoelectrochemical cell, or “artificial leaf” [19]. The device consists of a commercial triple-junction amorphous silicon solar cell interfaced to earth-abundant catalysts for water oxidation and proton reduction. The water oxidation catalyst is one of a new generation of living inorganic catalyst films that consist of cobalt(II) phosphate [20] or nickel(II) borate [21] that include earth-abundant firstrow transition metals that can cycle through several oxidation states and earth-abundant

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polyprotic acid buffers. These two features are critical in the proton-coupled electron transfer reactions. The active catalyst is a freshly deposited film that is consumed and regenerated during catalysis, enabling long lifetime with no loss of activity. The hydrogen evolving catalyst is a Ni–Mo–Zn alloy deposited on the stainless steel backing of the triple-junction silicon cell. Ni–Mo–Zn alloy is an innovative material that includes earth-abundant metals, as opposed to the traditional and expensive catalyst, platinum. The design could potentially be adapted to colloidal nanoparticles, in effect, creating “artificial algae” [22]. The artificial algae concept may be quite close to fruition due to the incredible advances in synthesis of nanostructured materials. The synthesis of well-defined, near-monodisperse nanocrystals is based on ideas of nucleation and growth [23], size focusing [24], and the utilization of surface stabilizers and other reaction variables to precisely control facet passivation and growth direction [25]. Nanocrystal synthesis has rapidly evolved such that it may become possible to engineer the synthesis of desired materials. In particular, two advances may hold significant promise in nanostructured solar fuels catalyst materials: synthesis of anisotropic semiconductor nanocrystals (nanorods, nanowires), and synthesis of polydomain nanostructured materials.

11.3  SEMICONDUCTOR NANOWIRES AND NANORODS Nanowires and nanorods are anisotropic nanocrystals that exhibit growth in one direction. A defining characteristic of the material is its aspect ratio (major axis/minor axis), and nanowires generally have higher aspect ratio than nanorods. Anisotropic nanocrystals can be grown in solution or from gaseous precursors onto a solid substrate. In solution, surfactants passivate facets of the growing nanocrystal, and strongly inhibit growth by increasing the activation energy of growth in a symmetry equivalent set of directions. Anisotropic growth occurs in the direction with the lowest activation energy only. The catalytic growth of nanorods on solid surfaces from gaseous precursors typically involves a liquid catalyst that propagates anisotropic growth of the nanocrystal. Oriented nanowire arrays can be grown on substrates using chemical vapor deposition. An oriented zinc oxide nanowire array was synthesized using a seeded growth process in which a zinc complex was decomposed in the presence of thin layer of ZnO seed nanocrystals on F:SnO2 substrate [26]. The nanowire film was incorporated as anode of a dye-sensitized solar cell. The electrical properties of the nanowire present several advantages over nanoparticle films. The nanowires exhibited good electrical conductivity along the growth axis, they are single crystalline, and the internal electric field favors migration of electrons to the wire core. The nanowire length can be adjusted to efficiently absorb solar radiation while the diameter can be separately adjusted for efficient exciton separation. Vertically aligned single crystal TiO2 nanowire array were grown directly on transparent conducting oxide substrates [27]. An FTO substrate was combined with titanium precursors, Ti(OBu)4, and TiCl4 in toluene with 37% HCl and heated to 180 °C up to 2 days to grow the thin films. The nanowires were single crystalline rutile 10–35 nm in diameter and 2–5 μm in length. The film was incorporated into dye-sensitized solar cell (DSSC) with N719 dye, and the cell had area of ∼0.5 cm2. The device achieved an AM1.5 photoconversion efficiency of 5%.



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Synthesis of the morphological inverse of a nanowire array has been developed. TiO2 nanotube arrays can be grown from anodization of Ti foil [28]. The electrochemical etching produces pits that increasingly penetrate the foil depth with reaction time. Highly uniform and ordered arrays of nanotubes can be synthesized and lifted from the Ti substrate. The thin film can be incorporated into DSSC [29] or PEC [30]. A Fujishima-Honda-type PEC in which TiO2 nanotube array was anode gave quantum yield of H2 (from water splitting) of 7.9% under UV illumination (320–400 nm) [31]. Advantages of the nanotube array include small crystallite size to minimize bulk recombination, more effective absorption of incident photons due to light scattering within the nanostructure, large surface area, and separation of photogenerated charges assisted by action of the depletion region electric field. The films are polycrystalline in nature; however, and the crystal grain boundaries are sites for recombination. Colloidal semiconductor nanorods typically have lower aspect ratio than nanowires and are synthesized in solution in the presence of surfactants. Quantum confinement within the nanorod occurs when the diameter is less than the Bohr exciton radius of the charge carrier. So the electronic structure of the nanorod is tunable just like a quantum dot. Due to the ease of synthesis and post-synthesis processing, semiconductor nanorods are promising for solar energy utilization. Typically nanorods are single crystalline, therefore, photogenerated charge carriers are more likely to migrate to the surface before recombination. Earth-abundant semiconductor materials as nanorods that have band-edge potentials that match the energy requirements for excitonic water splitting include TiO2, ZnO, and Si. We have taken an interest in TiO2 due to the large body of knowledge in the synthesis of anisotropic TiO2 nanocrystals. The nonhydrolytic synthesis of anisotropic TiO2 nanocrystals can be achieved through solvothermal method using Ti(OR)4 or TiCl4. Anatase nanocrystals with [0 0 1] growth direction were synthesized from Ti(OBu)4 and linoleic acid stabilizer in acid digestion bomb [32]. The gram-scale synthesis of anatase TiO2 nanorods with [0 0 1] growth was found using Ti(OiPr)4 and oleic acid [33]. Anatase TiO2 nanorods with [0 0 1] growth were found in a low temperature hydrolytic synthesis using Ti(OiPr)4, oleic acid, and aqueous Me3NO [34]. Interestingly, the use of Ti(OR)4 as precursor seems to yield exclusively anatase TiO2 with [0 0 1] growth. In contrast, TiCl4 as precursor leads to large range of anisotropic TiO2 nanocrystals. Decomposition of TiCl4 in oleylamine and oleic acid mixture leads to anatase TiO2 nanorods with [1 0 0] growth direction [35]. If the oleylamine:TiCl4 ratio is adjusted, it is possible to obtain brookite TiO2 nanorods with [0 0 1] growth direction [36]. At higher temperatures, the thermodynamic TiO2 product can be isolated as V-shaped rutile twinned nanorods with [0 0 1] growth direction and (1 0 1) or (3 0 1) twin plane [37]. We see an opportunity to use anisotropic TiO2 nanocrystals to improve on the concept of surface-modified TiO2 colloid for photochemical water splitting.

11.4  COLLOIDAL HYBRID NANOSTRUCTURES Let’s examine closely a set of materials within the mesoscale that have well-defined structure and interface between two or more substances. In the mesoscale, we seek to understand the “middle” or changes that occur between atomic/molecular electronic structure and bulk electronic structure (a size range of ∼1–30 nm, which falls within the scale of nanomaterials).

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Here, the electronic structure and surface topology of a material could be selected for optimal function in some application. Colloidal hybrid nanostructures are exceptional because the specific attachment of nanocrystals to one another presents functional interfaces. For example, the interface could be computed precisely, degree of lattice match can be quantified, electronic structure can be simulated, catalysis or electron transfer at the interface can be measured, etc. Excellent reviews on the topic of colloidal hybrid nanostructures can be found [62–65] with important examples summarized here. Colloidal hybrid nanostructures are formed by epitaxial growth of one crystal phase onto another. The epitaxial growth can be achieved by atom diffusion from a nanocrystal upon addition a second precursor that forms more stable phase with the diffusing atom. For example, Pd nanocrystals in a S-rich environment were synthesized in the presence of Co2+ ion. The Co2+ added to the PdSx surface and formed Co9S8 crystal phase joined through the (0 0 1) plane to led to “acorns” [38]. A similar strategy led to the formation of CdS–FePt bifunctional heterodimers of nanoparticles [39]. First, FePt nanocrystals were synthesized, and elemental S was added to form an FePt@S core-shell structure. Addition of Cd2+ led to formation of a CdS shell, and thermal annealing led to dewetting to form the CdS–FePt heterodimer. Interestingly, oxidation of a Pt@Co core-shell structure leads to Pt–CoO yolk-shell particles. In this case, instead of dewetting of the core, a nanoscale Kirkendall effect was observed in which interior Co atoms diffuse to the surface to form the CoO shell [40]. Alternatively, epitaxial growth can be achieved by direct nucleation of one crystal on the surface of another. Dumbbell-like bifunctional Au–Fe3O4 nanoparticles were synthesized by addition of Fe(CO)5 to Au nanocrystal followed by air-oxidation [41]. The structure arises from the epitaxial growth of Fe3O4 (a = 8.35 Å) on Au (a = 4.08 Å), which is within 3% of 2:1 ratio. Similar dumbbell-like structures were also formed after epitaxial growth of Fe3O4 (d-spacing of (400) = .21 nm) on Pt (d-spacing of (200) = .20 nm). Heterodimer nanocrystals UO2/In2O3 and FePt/In2O3 were synthesized by epitaxial growth of In2O3 on the UO2 or FePt nanocrystal seed [42]. Silicon nanorods with gold particle at one tip were synthesized by Au-seeded solution-liquidsolid growth [43]. Finally, epitaxial growth can arise upon selective nucleation of nanocrystals on nanorod tips. The first demonstration of the concept was selective nucleation of Au on the tips of CdSe nanorods [44]. The high surface free energy of the nanorod tips, and other defect sites on the nanorod initiate nucleation of the metal, which is followed by nanocrystal growth. Furthermore, prolonged heating of the dumbbell structure led to intrarod Ostwald ripening and formation of one-sided metal-tipped Au–CdSe nanorods [44]. Since the initial discovery of selective nucleation of nanocrystals on nanorod tips, other groups have reproduced the phenomenon. Co–TiO2 dumbbell structures were synthesized upon decomposition of Co2(CO)8 in the presence of TiO2 nanorods [45]. The interface between Co and TiO2 was present in more than one epitaxial configuration. Selective nucleation of Pt, Pd, or PtPd bimetallic particles on CdS nanorods led to dumbbell structures [46]. The growth of Co on the tips of CdSe nanorods was demonstrated [47]. The nanodumbbell structures are a specific set of materials that arise from nucleation of nanocrystals on the defect sites of another nanocrystal. The more general nucleation reaction was observed in the Au–CdSe and Co–TiO2 systems above and other materials. Photodeposition of Ag on TiO2 nanorods led to Ag–TiO2 hybrid nanocrystals in which Ag clusters were uniformly deposited on the TiO2 nanorod surface [48]. The recent advances in well-defined colloidal hybrid nanostructures are truly exciting. Multiple applications can be envisioned with such materials due to the combination of



11.5  New Designs That Build On the “Gratzel Colloid” and “Artificial Leaf”

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functional properties, such as optical, magnetic, catalytic, etc. In particular, the metal-semiconductor nanodumbbell materials are attractive for solar fuels catalysis. Two advantages are immediately apparent. The electronic properties of the anisotropic nanocrystal can facilitate exciton separation and the epitaxial attachment of the metal particles to the semiconductor ensures efficient electron transfer.

11.5  NEW DESIGNS THAT BUILD ON THE “GRATZEL COLLOID” AND “ARTIFICIAL LEAF” Based on the ideas presented so far, new materials for solar fuels catalysis can be proposed. We are pursuing a straightforward idea to synthesize a sensitized heteronanodumbbell composed of TiO2 nanorod and catalyst nanoparticles at opposite tips. The catalytic nanocrystals should catalyze the component half-reactions of an endothermic reaction for the production of a chemical fuel, such as conversion of water to hydrogen and oxygen. The first synthesis target is Pt–TiO2–RuO2 heteronanodumbbell; however, in a push to utilize earth-abundant materials a Co–TiO2 dumbbell or Ni–TiO2–NiMoZn heterodumbbell may be more desirable. Heteronanodumbbell structures will have significant advantages for solar fuels catalysis that include improved exciton separation and electron transfer catalysis with reduced recombination, and self-assembly to yield functional higher order structures due to the anisotropic geometry. There are few [66,67] heteronanodumbbell structures reported to date in which a nanorod has two different materials attached to opposite tips. A hypothesized synthetic route is to use selective nucleation method to produce a nanodumbbell, follow with intrarod Ostwald ripening to produce a matchstick structure, and then attach a different nanocrystal on the free tip. One potential barrier to this method is instead of selective nucleation of the second nanocrystal on the free tip, a shell could grow on the already modified tip. There are some possible strategies to overcome this issue. One may be to align the matchstick structures as a film in order to expose only one side of the film to conditions for selective nucleation of the second reactant. Another strategy may be to compare the interfacial energy of the two desired catalyst materials and with TiO2 and choose the order of addition of the two materials to the nanorod such that the matchstick possesses the material with less compatible interface than that of the second material and TiO2. The interfacial energy strategy may require that the two catalyst domains are [68] incompatible with each other while highly compatible with TiO2. This idea will depend on the ability to compare lattice constants between crystal phases and quantify the surface free energy of each interface. Alignment of matchstick structures may be possible in more than one method. One possibility may be to attach matchstick structures to a substrate in a manner similar to the formation of a self-assembled monolayer (SAM). This would align the free nanorod tip away from the substrate, and potentially allow reaction for selective nucleation of different material exclusively on the free tip. For an added level of protection of the substrate-side tip, an organic film could be deposited with thickness such that the free tips just protrude from the film surface. Another possibility for matchstick alignment could be to use a magnetic field. To test the hypothesis, a Langmuir-Blodgett (LB) film of matchstick structures could be formed in the presence of a magnetic field. Of course, the matchstick must consist of magnetic material at

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one tip. No experiment of this type has been reported. Upon formation of the aligned matchstick film, it may be possible to crosslink the nanostructures to form a more rigid film. If the aligned matchstick LB film can be produced, then the side with free tips can be subjected conditions for selective nucleation reaction. A proposed synthesis of Pt–TiO2–RuO2 heteronanodumbbell is from selective nucleation of RuO2 on the TiO2 tip of a Pt–TiO2 matchstick structure. The matchstick structure can conceivably be synthesized via selective nucleation of Pt on TiO2 nanorods followed by intrarod Ostwald ripening. Potentially, the synthesis could be improved using an alignment method for the matchstick intermediate. A proposed route is the synthesis of Co–TiO2 nanodumbbell structures, followed by intrarod Ostwald ripening to make Co–TiO2 matchstick. Then the Co–TiO2 matchstick could be subjected to conditions for Pt growth only on the Co particle to form a matchstick with Co/Pt core-shell particle. Due to the magnetic properties of cobalt, perhaps the matchstick could be aligned magnetically in a thin film, followed by selective nucleation of RuO2 on the non-magnetic side of the film. The Co–TiO2 dumbbell structure has been reported in the literature [44], and may be useful in the context of photochemical water splitting. If one cobalt tip can be selectively oxidized, then the material could be converted to a heteronanodumbbell. The concept is intriguing in the context of the recent discovery that cobalt(II) phosphate films efficiently catalyze water oxidation [19]. Metallic cobalt is known to catalyze proton reduction [49,50]. It may be possible to form an LB film of vertically aligned Co–TiO2 dumbbells followed by oxidation to make a bifunctional film. So far, the Co–TiO2 dumbbell structures have not been reproduced in any report in the literature. We have spent significant resources in order to reproduce the reported synthesis, and unfortunately could not obtain the Co–TiO2 dumbbells. A proposed synthesis of Ni–TiO2–NiMoZn heterodumbbell structure is from addition of Mo and Zn precursors to one side of a Ni–TiO2 dumbbell. So far, there is no report of a Ni– TiO2 dumbbell structure; however, upon its synthesis, the objective is to form a vertically aligned LB film of the material. Then one side of the film can be exposed to reaction conditions to incorporate Mo and Zn selectively on one side of the dumbbell film to yield a bifunctional thin film. The proposed earth-abundant heteronanodumbbell includes a Ni–Mo–Zn alloy particle as potential H2 evolution catalyst inspired by the recent discovery of this catalysis by thin films of the same alloy [18]. Furthermore, nickel(II) borate films efficiently catalyze water oxidation [20]. The proposed synthesis strategies may be risky with some unproven steps; however, the potential prize of low-cost materials that efficiently catalyze photochemical water splitting is great. More steps are required beyond the proposed synthesis of polydomain nanostructured materials in order to realize their full potential for solar fuels catalysis. Additionally, a visible/NIR chromophore must be attached to harvest the solar spectrum, inject electrons to the TiO2 NR conduction band, and accept electrons from the O2 evolving catalyst. There are well-established protocols for dye-sensitization of TiO2 [51,52]. Finally, the material should be organized such that the H2 and O2 evolution are kept separate, having benefits of avoiding the back-reaction to reform water and avoiding dangerous, less valuable gas mixture.



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11.6  SELF-ASSEMBLY OF NANOSTRUCTURED CATALYST TO FORM ORGANIZED SYSTEMS FOR WATER SPLITTING AT THE MACROSCALE The heteronanodumbbell featuring H2 evolving catalyst and O2 evolving catalyst attached to dye-sensitized TiO2 nanorod may be an efficient catalyst for photochemical water splitting on the nanometer scale; however, significant benefits likely emerge upon organization of the material on longer length scales. One of the obvious problems is the catalysis of H2O synthesis from H2 and O2. Related to this problem, a mixture of H2 and O2 gas as the product of water splitting is far less useful than the pure gases separately. Organization of the nanoscale component in a way such that the gaseous products can be synthesized and transported separately can prevent these problems. Furthermore, compartmentalizing the half-reactions could allow pH optimization of each reaction environment. For instance the catalyst sites for H2 and O2 synthesis could be maintained at low and high pH, respectively. Molecules can spontaneously self-assemble in numerous ways. In biology, phospholipids self-assemble to form lipid bilayers that are the basis of cell membranes, proteins can selfassemble to form protein superstructures, etc. In water, lipids can form lamellar phase, micelles, or liposomes. Polymers can self-assemble in a myriad of ways including lamellar structures and micelles. The remarkable ability to synthesize polymers with low polydispersity and precise functionality allows for unprecedented design of polymers as functional materials. Nanocrystals can spontaneously self-assemble into colloidal crystals. Self-assembly is evident on larger scales, such as formation of opals from polystyrene [53] or silica spheres [54]. Self-assembly is potentially a low cost method to produce organized functional structures (devices) at the macroscale from components orders of magnitude smaller. Complex systems such as the thylakoid membrane of chloroplast are assembled from small molecule (lipid) and macromolecular (protein) complexes. The highly organized nature of the complex ensemble allows the system to carry out its function. In photosynthesis, protons are captured in the thylakoid lumen that creates the pH gradient and is driving force for ATP synthesis; whereas NADPH and O2 freely diffuse away. Inspired by the biological design, we hypothesize that nanostructured catalysts could self-assemble in the presence of lipids to form a system in which the surface modified nanorods are immobilized as membrane-spanning units in a lipid bilayer. In such a system, the most benefit arises when the alignment of the nanorods is such that the H2 evolving catalysts are only found on one side of the membrane (and O2 evolving catalysts only on the opposite side). Colloidal TiO2 nanorods are typically synthesized using surfactant molecules such as oleic acid or oleylamine to stablize the TiO2 surface and provide steric protection via the long hydrophobic hydrocarbon chain. In this case the colloid disperses in nonpolar solvent and flocculates in polar solvent. In nonvolatile nonpolar solvent, TiO2 nanorods can self-assemble to form colloidal crystal. Potentially, ligand stabilized nanorods could be dispersed in polar solvent with surfactant if the amphilic molecule formed a micelle around the particle or interdigitated with the nonpolar shell to present polar headgroups to the solvent. If an experiment were conducted to introduce ligand stabilized nanoparticles with hydrophobic surface to aqueous medium with surfactant in conditions favorable for the self-assembly of a lipid bilayer, then the nanorods would become encased within the bilayer. The nanorod in this case would be oriented with the major axis parallel to the plane of the bilayer, and a dumbbell

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catalyst in this orientation cannot be effective in the context of product separation. It would be preferable to modify the nanorod to have a hydrophobic shaft and hydrophilic tips. The amphiphilic structure may be more likely to self-assemble within a lipid bilayer in an orientation with the major axis perpendicular to the plane of the membrane. To our knowledge, this experiment has not been done. However, membrane-spanning proteins are ubiquitous. The proteins have membrane-spanning α-helix bundle or β-barrel motifs wherein hydrophobic residues are found that lead to preferred immobilization in the hydrophobic interior of the membrane [55]. Potentially, an amphiphilic nanostructure may become immobilized within a lipid bilayer with preferential orientation. One issue that arises is the size match of the nanostructure (nanorods have length on the order of 10s of nm) and the lipid bilayer. The hydrophobic region of a lipid bilayer is 3–5 nm [56,57]. Potentially, other self-assembled lamellar structures could be adapted instead of the phospholipid bilayer. Amphiphilic block copolymers can self-assemble into micelles and lamellar phases, including vesicle structures that resemble liposomes. The diblock copolymer vesicles are called polymersomes [58–60]. The properties of the polymersomes depend on the hydrophobicity/hydrophilicity of the copolymer blocks and the chain lengths. Right away, one can see the vast potential to synthesize polymersomes from a wide possibility of block copolymers. Vesicles no longer need to be confined by the phase, shear, and geometric limitations of phospholipid liposomes. Polymersomes can have much larger membrane thickness. Polyethyleneoxide-polyethylethylene [EO40-EE37; number-average molecular weight ∼3900 g/mol] forms mixture of micelle and lamellar phases in water. Polymersomes up to 200 nm in diameter formed. Cryogenic transmission electron microscopy was used to collect micrographs of the polymersomes that were found to have lamellar thickness of ∼8 nm (thickness of the hydrophobic portion of the bilayer). Polymersomes with larger hydrophobic thickness have been reported. The OB19 polyethyleneoxide-polybutadiene [EO150-BD250] in water led to polymersome with hydrophobic thickness of 21 nm [61]. Clearly, it is possible to fabricate soft lamellar materials with membrane thickness that matches closely with the length of nanorods. The next step is to incorporate amphiphilic nanorods into polymersomes or lamellar membrane of block copolymer and attempt to orient them as membrane-spanning units. This experiment has not been reported. If a pH gradient can be established across the membrane, then it may be possible to optimize pH for the component half-reactions of water splitting. A property of lamellar block copolymer materials with thick hydrophobic membranes is the reduced diffusion of polar/charged species across the membrane.

11.7 CONCLUSION In this chapter, artificial systems for solar fuels catalysis were compared with natural photosynthesis. The greatest difference appears to be the degree of complexity and organization in biology is much greater than artificial systems. The most straightforward way to reproduce such order and complexity is controlled self-assembly with careful control of the individual organizing units. Colloidal hybrid nanostructure that consist of dye-sensitized TiO2 nanorod onto which catalyst nanoparticles are attached to the tips could serve as the photocatalyst tecton. While heteronanodumbbell structures are not developed so far, it is conceivable that



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these could be synthesized. Examples for syntheses of Pt–TiO2–RuO2, Co/Pt–TiO2–RuO2, Co–TiO2 –Co(II), and Ni(II) –TiO2 –NiMoZn heteronanodumbbells were presented. The full potential of these materials can only be realized upon their organization into larger scale materials. Inspired by the membrane-spanning protein complexes that mediate photosynthesis, we propose to immobilize the heteronanodumbbell structures as membrane-spanning units in membranes. Phospholipids self-assemble to lipid bilayers and liposomes that are suitable for biological structures have limitations, namely, the thickness is restricted. The very new and exciting field of polymersomes offers huge potential for artificial lamellar structures, including polymersomes, with hydrophobic membrane thickness on the order of 10s of nm. These appear to be quite viable structure into which modified nanorods can be immobilized as membrane spanning unites. Preferential orientation of the heteronanodumbbell is a significant challenge, and a few strategies to achieve this were presented. Ultimately, the proposed composite lamellar functional material may have incredible potential for solar fuels catalysis. A pH gradient across the membrane can be imposed with high proton concentration at the catalyst site for H2 evolution and low proton concentration at the catalyst site for O2 evolution that would accelerate the water splitting reaction. Furthermore, separation of the H2 and O2 products occurs at the point of synthesis, thereby avoiding the dangerous combustible mixture and expensive gas separation. While many of the strategies presented are untested, undemonstrated, and potentially high risk experiments, there are many exciting developments in nanostructured and soft materials that may allow new advances. Clearly, the work will be multidisciplinary in nature and require large collaborative efforts to be successful.

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