Animesh Nayak

Single catalyst electrocatalytic reduction of CO2 in water to H2+CO syngas mixtures with water oxidation to O2

A single Ru(II) polypyridyl carbene complex catalyzes the electrochemical reduction of CO2 in water with added bicarbonate to give syngas H2+CO mixtures at the cathode and water oxidation to O2 at the anode. The H2/CO ratio in the syngas product can be adjusted between 4 : 1 and 1 : 2 by varying the applied potential and solution pH with overall electrolysis energy efficiencies near 50%.

An aqueous, organic dye derivatized SnO2/TiO2 core/shell photoanode

Visible light driven water splitting in a dye-sensitized photoelectrochemical cell (DSPEC) based on a phosphonic acid-derivatized donor–π–acceptor (D–π–A) organic dye (P–A–π–D) is described with the dye anchored to an FTO|SnO2/TiO2 core/shell photoanode in a pH 7 phosphate buffer solution. Transient absorption measurements on FTO|TiO2|–[P–A–π–D] compared to core/shell, FTO|SnO2/TiO2(3 nm)|–[P–A–π–D], reveal that excitation of the dye is rapid and efficient with a decrease in back electron rate by a factor of ∼10 on the core/shell. Upon visible, 1 sun excitation (100 mW cm−2) of FTO|SnO2/TiO2(3 nm)|–[P–A–π–D] in a phosphate buffer at pH 7 with 20 mM added hydroquinone (H2Q), photocurrents of ∼2.5 mA cm−2 are observed which are sustained over >15 min photolysis periods with a current enhancement of ∼30-fold compared to FTO|TiO2|–[P–A–π–D] due to the core/shell effect. On surfaces co-loaded with both –[P–A–π–D] and the known water oxidation catalyst, Ru(bda)(pyP)2 (pyP = pyridin-4-methyl phosphonic acid), maximum photocurrent levels of 1.4 mA cm−2 were observed which decreased over an 10 min interval to 0.1 mA cm−2. O2 was measured by use of a two-electrode, collector–generator sandwich cell and was produced in low faradaic efficiencies with the majority of the oxidative photocurrent due to oxidative decomposition of the dye.

Synthesis and photophysical characterization of porphyrin and porphyrin–Ru(II) polypyridyl chromophore–catalyst assemblies on mesoporous metal oxides

A layer-by-layer procedure has been used to prepare chromophore–catalyst assemblies consisting of phosphonate-derivatized porphyrin chromophores and a phosphonate-derivatized Ru(II) water oxidation catalyst on the surfaces of SnO2 and TiO2 mesoporous, nanoparticle films. In the procedure, initial surface binding of the phosphonate-derivatized porphyrin is followed in sequence by reaction with ZrOCl2 and then with the phosphonate-derivatized water oxidation catalyst [RuII(2,6-bis-(1-methylbenzimidazole-2-yl)pyridine)(2,2′-bipyridine-4,4′-hydroxymethylphosphonate)(H2O)]2+, [RuII(Mebimpy)(4,4′-(PO(OH)2–CH2)2-bpy)(OH2)]2+. Fluorescence from both the free base and Zn(II) porphyrin derivatives on SnO2 is quenched; substantial emission quenching of the Zn(II) porphyrin occurs on TiO2. Transient absorption difference spectra provide direct evidence for appearance of the porphyrin radical cation on SnO2via excited-state electron injection. For the chromophore–catalyst assembly on SnO2, transient absorption difference spectra demonstrate rapid intra-assembly electron transfer oxidation of the catalyst following excitation and injection by the porphyrin chromophore.

Large Hyperpolarizabilities at Telecommunication-Relevant Wavelengths in Donor-Acceptor-Donor Nonlinear Optical Chromophores

Octopolar D2-symmetric chromophores, based on the MPZnM supermolecular motif in which (porphinato)zinc(II) (PZn) and ruthenium(II) polypyridyl (M) structural units are connected via ethyne linkages, were synthesized. These structures take advantage of electron-rich meso-arylporphyrin or electron-poor meso-(perfluoroalkyl)porphyrin macrocycles, unsubstituted terpyridyl and 4′-pyrrolidinyl-2,2′;6′,2″-terpyridyl ligands, and modulation of metal(II) polypyridyl-to-(porphinato)zinc connectivity, to probe how electronic and geometric factors impact the measured hyperpolarizability. Transient absorption spectra obtained at early time delays (tdelay < 400 fs) demonstrate fast excited-state relaxation, and formation of a highly polarized T1 excited state; the T1 states of these chromophores display expansive, intense T1 → Tn absorption manifolds that dominate the 800–1200 nm region of the NIR, long (μs) triplet-state lifetimes, and unusually large NIR excited absorptive extinction coefficients [ε(T1 → Tn) ∼ 105 M–1 cm–1]. Dynamic hyperpolarizability (βλ) values were determined from hyper-Rayleigh light scattering (HRS) measurements, carried out at multiple incident irradiation wavelengths spanning the 800–1500 nm spectral domain. The measured βHRS value (4600 ± 1200 × 10–30 esu) for one of these complexes, RuPZnRu, is the largest yet reported for any chromophore at a 1500 nm irradiation wavelength, highlighting that appropriate engineering of strong electronic coupling between multiple charge-transfer oscillators provides a critical design strategy to realize octopolar NLO chromophores exhibiting large βHRS values at telecom-relevant wavelengths. Generalized Thomas–Kuhn sum (TKS) rules were utilized to compute the effective excited-state-to-excited-state transition dipole moments from experimental linear-absorption spectra; these data were then utilized to compute hyperpolarizabilities as a function of frequency, that include two- and three-state contributions for both βzzz and βxzx tensor components to the RuPZnRu hyperpolarizability spectrum. This analysis predicts that the βzzz and βxzx tensor contributions to the RuPZnRu hyperpolarizability spectrum maximize near 1550 nm, in agreement with experimental data. The TKS analysis suggests that relative to analogous dipolar chromophores, octopolar supermolecules will be likely characterized by more intricate dependences of the measured hyperpolarizability upon irradiation wavelength due to the interactions among multiple different β tensor components.

Layer-by-Layer Molecular Assemblies for Dye-Sensitized Photoelectrosynthesis Cells Prepared by Atomic Layer Deposition

In a dye sensitized photoelectrosynthesis cell (DSPEC), the relative orientation of the catalyst and chromophore plays an important role in determining the device efficiency. Here we introduce a new, robust atomic layer deposition (ALD) procedure for the preparation of molecular chromophore-catalyst assemblies on wide bandgap semiconductors. In this procedure, solution deposited, phosphonate derivatized metal complexes on metal oxide surfaces are treated with reactive metal reagents in the gas phase by ALD to form an outer metal ion bridging group, which can bind a second phosphonate containing species from solution to establish a R1-PO2-O-M-O-PO2-R2 type surface assembly. With the ALD procedure, assemblies bridged by Al(III), Sn(IV), Ti(IV), or Zr(IV) metal oxide units have been prepared. To evaluate the performance of this new type of surface assembly, intra-assembly electron transfer was investigated by transient absorption spectroscopy, and light-driven water splitting experiments under steady-state illumination were conducted. A SnO2 bridged assembly on SnO2/TiO2 core/shell electrodes undergoes light-driven water oxidation with an incident photon to current efficiency (IPCE) of 17.1% at 440 nm. Light-driven water reduction with a ruthenium trisbipyridine chromophore and molecular Ni(II) catalyst on NiO films was also used to produce H2. Compared to conventional solution-based procedures, the ALD approach offers significant advantages in scope and flexibility for the preparation of stable surface structures.

Modulating Hole Transport in Multilayered Photocathodes with Derivatized p-Type Nickel Oxide and Molecular Assemblies for Solar-Driven Water Splitting

For solar water splitting, dye-sensitized NiO photocathodes have been a primary target. Despite marginal improvement in performance, limitations remain arising from the intrinsic disadvantages of NiO and insufficient catalysis. We report here a new approach to modifying NiO photocathodes with doped NiO bilayers and an additional layer of macro-mesoporous ITO. The trilayered electrode is functionalized with a surface-attached ruthenium polypyridyl dye and a covalently bridged nickel-based hydrogen evolution catalyst. The NiO film, containing a 2% K+-doped NiO inner layer and a 2% Cu2+-doped NiO outer layer, provides sufficient driving force for hole transport following hole injection by the molecular assembly. Upon light irradiation, the resulting photocathode generates hydrogen from water sustainably with enhanced photocurrents and a Faradaic efficiency of ∼90%. This approach highlights the value of modifying both the internal and surface structure of NiO and provides insights into a new generation of dye-sensitized photocathodes for solar-driven water splitting cells.

Growth and Post-Deposition Treatments of SrTiO3 Films for Dye-Sensitized Photoelectrosynthesis Cell Applications.

Sensitized SrTiO3 films were evaluated as potential photoanodes for dye-sensitized photoelectrosynthesis cells (DSPECs). The SrTiO3 films were grown via pulsed laser deposition (PLD) on a transparent conducting oxide (fluorine-doped tin oxide, FTO) substrate, annealed, and then loaded with zinc(II) 5,10,15-tris(mesityl)-20-[(dihydroxyphosphoryl)phenyl] porphyrin (MPZnP). When paired with a platinum wire counter electrode and an Ag/AgCl reference electrode these sensitized films exhibited photocurrent densities on the order of 350 nA/cm(2) under 0 V applied bias conditions versus a normal hydrogen electrode (NHE) and 75 mW/cm(2) illumination at a wavelength of 445 nm. The conditions of the post-deposition annealing step-namely, a high-temperature reducing atmosphere-proved to be the most important growth parameters for increasing photocurrent in these electrodes.

Near IR nonlinear absorption of an organic supermolecule [Invited]

The photophysics of bis(terpyridyl)osmium-(porphinato)zinc-bis(terpyridyl)osmium (OsPZnOs), a D-π-A-π-D symmetric supermolecule, were investigated in the femtosecond and nanosecond regimes. The supermolecule exhibits a two-photon absorption (δpeak ~900 GM) in the near IR (900-1300 nm) and optical pumping by two-photon absorption leads to a broad excited state absorption (σpeak ~1.1 × 10−16 cm2) in the same near IR region. Since the excited state has a long lifetime, OsPZnOs exhibits a strong nanosecond nonlinear absorption in this region. That nonlinear absorption is substantially enhanced when OsPZnOs is incorporated into a multimode waveguide. When two-photon pumping is the dominant mechanism, an additional enhancement of up to ~100 × in the nonlinear absorption is observed in a microchannel waveguide. OsPZnOs is a promising material for photonic applications such as optical noise suppression and optical limiting in the near IR.

Fluoropolymer-Stabilized Chromophore–Catalyst Assemblies in Aqueous Buffer Solutions for Water-Oxidation Catalysis

Here, the application of the fluorinated polymer [Dupont AF, a copolymer of 4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole and tetrafluoroethylene] is described in stabilizing phosphonate-derivatized molecular assemblies on oxide electrodes. In the procedure, the polymer was dip-coated onto the surfaces of oxide electrodes with pre-bound, phosphonate-derivatized chromophores and assemblies, including assemblies for water oxidation. The results of the experiments showed a high degree of stabilization by the added polymer and a demonstration of its use in stabilizing surface-bound assemblies for water-oxidation catalysis.

Controlling Vertical and Lateral Electron Migration Using a Bifunctional Chromophore Assembly in Dye-Sensitized Photoelectrosynthesis Cells

Integration of photoresponsive chromophores that initiate multistep catalysis is essential in dye-sensitized photoelectrosynthesis cells and related devices. We describe here an approach that incorporates a chromophore assembly surface-bound to metal oxide electrodes for light absorption with an overlayer of catalysts for driving the half-reactions of water splitting. The assembly is a combination of a core-twisted perylene diimide and a ruthenium polypyridyl complex. By altering the connection sequence of the two subunits in the assembly, in their surface-binding to either TiO2 or NiO, the assembly can be tuned to convert visible light into strongly oxidizing equivalents for activation of an electrodeposited water oxidation catalyst (NiCo2O x) at the photoanode, or reducing equivalents for activation of an electrodeposited water reduction catalyst (NiMo0.05S x) at the photocathode. A key element in the design of the photoelectrodes comes from the synergistic roles of the vertical (interlayer) charge transfer and lateral (intralayer) charge hopping in determining overall cell efficiencies for photoelectrocatalysis.