Surface Modification of SnO₂ Photoelectrodes in Dye-Sensitized Solar Cells: Significant Improvements in Photovoltage via Al₂O₃ Atomic Layer Deposition

We report here the exploitation of ultrathin layers of Al₂O₃ deposited via atomic layer deposition (ALD) on SnO₂ photoanodes used in dye-sensitized solar cells featuring the I₃⁻/I⁻ couple as the redox electrolyte. We find that a single ALD cycle of Al₂O₃ increases the lifetimes of injected electrons by more than 2 orders of magnitude. The modified SnO₂ photoanode yields nearly a 2-fold improvement fill factor and a greater than 2-fold increase in open-circuit photovoltage, with a slight increase in short-circuit photocurrent. The overall energy conversion efficiency increases by roughly 5-fold. The effects appear to arise primarily from passivation of reactive, low-energy tin-oxide surface states, with band-edge shifts and tunneling based blocking behavior playing only secondary roles

Atomic Layer Deposition of Rhenium–Aluminum Oxide Thin Films and ReO_<i>x</i> Incorporation in a Metal–Organic Framework

Methyltrioxorhenium (ReO₃Me) is introduced as the first rhenium atomic layer deposition (ALD) precursor and used to grow rhenium–aluminum oxide thin films in combination with trimethylaluminum (TMA–AlMe₃). The growth rate of the smooth Re–Al oxide films, with general stoichiometry Re_<i>x</i>Al_<i>y</i>O_3<i>x</i>, has been monitored by in situ quartz crystal microbalance (QCM) and ex situ ellipsometry, and found to be 3.2 Å/cycle. X-ray photoelectron spectroscopy (XPS) revealed the mixed valent composition of the film with Re­(III) species being the main component. In addition, ReO₃Me has been successfully used to deposit rhenium oxide in NU-1000, a mesoporous zirconium-based metal–organic framework (MOF). The metalated MOF was found to retain porosity and crystallinity and to be catalytically active for ethene hydrogenation

Postassembly Transformation of a Catalytically Active Composite Material, Pt@ZIF-8, via Solvent-Assisted Linker Exchange

2-Methylimidazolate linkers of Pt@ZIF-8 are exchanged with imidazolate using solvent-assisted linker exchange (SALE) to expand the apertures of the parent material and create Pt@SALEM-2. Characterization of the material before and after SALE was performed. Both materials are active as catalysts for the hydrogenation of 1-octene, whereas the hydrogenation of <i>cis</i>-cyclohexene occurred only with Pt@SALEM-2, consistent with larger apertures for the daughter material. The largest substrate, β-pinene, proved to be unreactive with H₂ when either material was employed as a candidate catalyst, supporting the contention that substrate molecules, for both composites, must traverse the metal–organic framework component in order to reach the catalytic nanoparticles

Complete Double Epoxidation of Divinylbenzene Using Mn(porphyrin)-Based Porous Organic Polymers

A series of porphyrin-based porous organic polymers (PPOPs) were synthesized in excellent yields via the Yamamoto–Ullmann couplings of tetrabromo spirobifluorene with several brominated porphyrin monomers. After isolation and demetalation, the metal-free PPOP can be postsynthetically metalated to form a Mn^III–PPOP that is catalytically active toward the selective double-epoxidation of divinylbenzene to divinylbenzene dioxide

Atomically Precise Growth of Catalytically Active Cobalt Sulfide on Flat Surfaces and within a Metal–Organic Framework <i>via</i> Atomic Layer Deposition

Atomic layer deposition (ALD) has been employed as a new synthetic route to thin films of cobalt sulfide on silicon and fluorine-doped tin oxide platforms. The self-limiting nature of the stepwise synthesis is established through growth rate studies at different pulse times and temperatures. Additionally, characterization of the materials by X-ray diffraction and X-ray photoelectron spectroscopy indicates that the crystalline phase of these films has the composition Co₉S₈. The nodes of the metal–organic framework (MOF) <b>NU-1000</b> were then selectively functionalized with cobalt sulfide <i>via</i> ALD in MOFs (AIM). Spectroscopic techniques confirm uniform deposition of cobalt sulfide throughout the crystallites, with no loss in crystallinity or porosity. The resulting material, <b>CoS-AIM</b>, is catalytically active for selective hydrogenation of <i>m</i>-nitrophenol to <i>m</i>-aminophenol, and outperforms the analogous oxide AIM material (<b>CoO-AIM</b>) as well as an amorphous CoS_<i>x</i> reference material. These results reveal AIM to be an effective method of incorporating high surface area and catalytically active cobalt sulfide in metal–organic frameworks

Dual-Function Metal–Organic Framework as a Versatile Catalyst for Detoxifying Chemical Warfare Agent Simulants

The nanocrystals of a porphyrin-based zirconium(IV) metal–organic framework (MOF) are used as a dual-function catalyst for the simultaneous detoxification of two chemical warfare agent simulants at room temperature. Simulants of nerve agent (such as GD, VX) and mustard gas, dimethyl 4-nitrophenyl phosphate and 2-chloroethyl ethyl sulfide, have been hydrolyzed and oxidized, respectively, to nontoxic products <i>via</i> a pair of pathways catalyzed by the same MOF. Phosphotriesterase-like activity of the Zr₆-containing node combined with photoactivity of the porphyrin linker gives rise to a versatile MOF catalyst. In addition, bringing the MOF crystals down to the nanoregime leads to acceleration of the catalysis

Systematic Modulation of Quantum (Electron) Tunneling Behavior by Atomic Layer Deposition on Nanoparticulate SnO₂ and TiO₂ Photoanodes

Ultrathin films of TiO₂, ZrO₂, and Al₂O₃ were conformally created on SnO₂ and TiO₂ photoelectrodes via atomic layer deposition (ALD) to examine their influence upon electron transfer (ET) from the electrodes to a representative molecular receptor, I₃^–. Films thicker than 2 Å engender an exponential decrease in ET time with increasing film thickness, consistent with tunneling theory. Increasing the height of the barrier, as measured by the energy difference between the transferring electron and the bottom of the conduction band of the barrier material, results in steeper exponential drops in tunneling rate or probability. The variations are quantitatively consistent with a simple model of quantum tunneling of electrons through square barriers (i.e., barriers of individually uniform energy height) that are characterized by individually uniform physical thickness. The findings demonstrate that ALD is a remarkably uniform and precise method for modifying electrode surfaces and imply that standard tunneling theory can be used as a quantitative guide to intentionally and predictively modulating rates of ET between molecules and electrodes

Toward Inexpensive Photocatalytic Hydrogen Evolution: A Nickel Sulfide Catalyst Supported on a High-Stability Metal–Organic Framework

Few-atom clusters composed of nickel and sulfur have been successfully installed into the Zr­(IV)-based metal–organic framework (MOF) NU-1000 via ALD-like chemistry (ALD = atomic layer deposition). X-ray photoelectron spectroscopy and Raman spectroscopy are used to determine that primarily Ni²⁺ and S^2– sites are deposited within the MOF. In a pH 7 buffered aqueous solution, the porous catalyst is able to produce H₂ gas at a rate of 3.1 mmol g^–1 h^–1 upon UV irradiation, whereas no H₂ is generated by irradiating bare NU-1000. Upon visible light irradiation, little H₂ generation was observed; however, with the addition of an organic dye, rose bengal, NiS-AIM can catalyze the production of H₂ at an enhanced rate of 4.8 mmol g^–1 h^–1. These results indicate that ALD in MOFs (AIM) can engender reactivity within high surface area supports for applications in the solar fuels field

Thermally Enhancing the Surface Areas of Yamamoto-Derived Porous Organic Polymers

Thermal treatment of highly stable porous organic polymers based upon the Yamamoto polymerization of 2,2′,7,7′-tetrabromo-9,9′-spirobifluorene was done. The polymers are shown to be thermally and chemically stable. Upon thermal treatment the polymers are shown to have BET surface areas of ca. 2,000 m²/g and 2,500 m²/g respectively

Determining the Conduction Band-Edge Potential of Solar-Cell-Relevant Nb₂O₅ Fabricated by Atomic Layer Deposition

Often key to boosting photovoltages in photoelectrochemical and related solar-energy-conversion devices is the preferential slowing of rates of charge recombinationespecially recombination at semiconductor/solution, semiconductor/polymer, or semiconductor/perovskite interfaces. In devices featuring TiO₂ as the semiconducting component, a common approach to slowing recombination is to install an ultrathin metal oxide barrier layer or trap-passivating layer atop the semiconductor, with the needed layer often being formed via atomic layer deposition (ALD). A particularly promising barrier layer material is Nb₂O₅. Its conduction-band-edge potential <i>E</i>_CB is low enough that charge injection from an adsorbed molecular, polymeric, or solid-state light absorber and into the semiconductor can still occur, but high enough that charge recombination is inhibited. While a few measurements of <i>E</i>_CB have been reported for conventionally synthesized, bulk Nb₂O₅, none have been described for ALD-fabricated versions. Here, we specifically determine the conduction-band-edge energy of ALD-fabricated Nb₂O₅ relative to that of TiO₂. We find that, while the value for ALD-Nb₂O₅ is indeed higher than that for TiO₂, the difference is less than anticipated based on measurements of conventionally synthesized Nb₂O₅ and is dependent on the thermal history of the material. The implications of the findings for optimization of competing interfacial rate processes, and therefore photovoltages, are briefly discussed