152 research outputs found
Surface Modification of SnO<sub>2</sub> Photoelectrodes in Dye-Sensitized Solar Cells: Significant Improvements in Photovoltage via Al<sub>2</sub>O<sub>3</sub> Atomic Layer Deposition
We report here the exploitation of ultrathin layers of Al<sub>2</sub>O<sub>3</sub> deposited via atomic layer deposition (ALD) on SnO<sub>2</sub> photoanodes used in dye-sensitized solar cells featuring the I<sub>3</sub><sup>−</sup>/I<sup>−</sup> couple as the redox electrolyte. We find that a single ALD cycle of Al<sub>2</sub>O<sub>3</sub> increases the lifetimes of injected electrons by more than 2 orders of magnitude. The modified SnO<sub>2</sub> 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<sub><i>x</i></sub> Incorporation in a Metal–Organic Framework
Methyltrioxorhenium
(ReO<sub>3</sub>Me) 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–AlMe<sub>3</sub>). The growth rate of the smooth Re–Al oxide films,
with general stoichiometry Re<sub><i>x</i></sub>Al<sub><i>y</i></sub>O<sub>3<i>x</i></sub>, has been monitored
by in situ quartz crystal microbalance (QCM) and ex situ ellipsometry,
and found to be 3.2 Ã…/cycle. X-ray photoelectron spectroscopy
(XPS) revealed the mixed valent composition of the film with ReÂ(III)
species being the main component. In addition, ReO<sub>3</sub>Me 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 H<sub>2</sub> 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<sup>III</sup>–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 Co<sub>9</sub>S<sub>8</sub>. 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<sub><i>x</i></sub> 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 Zr<sub>6</sub>-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 SnO<sub>2</sub> and TiO<sub>2</sub> Photoanodes
Ultrathin
films of TiO<sub>2</sub>, ZrO<sub>2</sub>, and Al<sub>2</sub>O<sub>3</sub> were conformally created on SnO<sub>2</sub> and
TiO<sub>2</sub> photoelectrodes via atomic layer deposition (ALD)
to examine their influence upon electron transfer (ET) from the electrodes
to a representative molecular receptor, I<sub>3</sub><sup>–</sup>. 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 ZrÂ(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
Ni<sup>2+</sup> and S<sup>2–</sup> sites are deposited within
the MOF. In a pH 7 buffered aqueous solution, the porous catalyst
is able to produce H<sub>2</sub> gas at a rate of 3.1 mmol g<sup>–1</sup> h<sup>–1</sup> upon UV irradiation, whereas no H<sub>2</sub> is generated by irradiating bare NU-1000. Upon visible light irradiation,
little H<sub>2</sub> generation was observed; however, with the addition
of an organic dye, rose bengal, NiS-AIM can catalyze the production
of H<sub>2</sub> at an enhanced rate of 4.8 mmol g<sup>–1</sup> h<sup>–1</sup>. 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 m<sup>2</sup>/g and 2,500 m<sup>2</sup>/g respectively
Determining the Conduction Band-Edge Potential of Solar-Cell-Relevant Nb<sub>2</sub>O<sub>5</sub> 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 recombinationî—¸especially recombination at
semiconductor/solution, semiconductor/polymer, or semiconductor/perovskite
interfaces. In devices featuring TiO<sub>2</sub> 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 Nb<sub>2</sub>O<sub>5</sub>. Its conduction-band-edge potential <i>E</i><sub>CB</sub> 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><sub>CB</sub> have been reported for conventionally synthesized,
bulk Nb<sub>2</sub>O<sub>5</sub>, none have been described for ALD-fabricated
versions. Here, we specifically determine the conduction-band-edge
energy of ALD-fabricated Nb<sub>2</sub>O<sub>5</sub> relative to that
of TiO<sub>2</sub>. We find that, while the value for ALD-Nb<sub>2</sub>O<sub>5</sub> is indeed higher than that for TiO<sub>2</sub>, the
difference is less than anticipated based on measurements of conventionally
synthesized Nb<sub>2</sub>O<sub>5</sub> 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
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