Curtis P. Berlinguette

Photochemical Route for Accessing Amorphous Metal Oxide Materials for Water Oxidation Catalysis

Amorphous and More Active The electrochemical generation of hydrogen from water could help in the storage of energy generated by renewable resources at off-peak times. However, catalysts for the slow step of this reaction, the oxygen evolution reaction (OER), are based on oxides of noble metals (iridium and ruthenium) that have limited abundance. A strategy for improving the performance of earth-abundant elements is to explore mixed-metal oxides and to prepare these as amorphous phases. Smith et al. (p. 60, published online 28 March) developed a general method for preparing amorphous oxides, based on photodecomposition of organometallic precursors. Amorphous mixed-metal oxides of iron, nickel, and cobalt were more active than comparable crystalline materials and provided OER performance comparable to noble metal oxides. Amorphous oxides of earth-abundant metals catalyze water oxidation with performance approaching that of noble metal catalysts. Large-scale electrolysis of water for hydrogen generation requires better catalysts to lower the kinetic barriers associated with the oxygen evolution reaction (OER). Although most OER catalysts are based on crystalline mixed-metal oxides, high activities can also be achieved with amorphous phases. Methods for producing amorphous materials, however, are not typically amenable to mixed-metal compositions. We demonstrate that a low-temperature process, photochemical metal-organic deposition, can produce amorphous (mixed) metal oxide films for OER catalysis. The films contain a homogeneous distribution of metals with compositions that can be accurately controlled. The catalytic properties of amorphous iron oxide prepared with this technique are superior to those of hematite, whereas the catalytic properties of a-Fe100-y-zCoyNizOx are comparable to those of noble metal oxide catalysts currently used in commercial electrolyzers.

Water oxidation catalysis: electrocatalytic response to metal stoichiometry in amorphous metal oxide films containing iron, cobalt, and nickel.

Photochemical metal-organic deposition (PMOD) was used to prepare amorphous metal oxide films containing specific concentrations of iron, cobalt, and nickel to study how metal composition affects heterogeneous electrocatalytic water oxidation. Characterization of the films by energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy confirmed excellent stoichiometric control of each of the 21 complex metal oxide films investigated. In studying the electrochemical oxidation of water catalyzed by the respective films, it was found that small concentrations of iron produced a significant improvement in Tafel slopes and that cobalt or nickel were critical in lowering the voltage at which catalysis commences. The best catalytic parameters of the series were obtained for the film of composition a-Fe20Ni80. An extrapolation of the electrochemical and XPS data indicates the optimal behavior of this binary film to be a manifestation of iron stabilizing nickel in a higher oxidation level. This work represents the first mechanistic study of amorphous phases of binary and ternary metal oxides for use as water oxidation catalysts, and provides the foundation for the broad exploration of other mixed-metal oxide combinations.

A Trisheteroleptic Cyclometalated RuII Sensitizer that Enables High Power Output in a Dye-Sensitized Solar Cell†

The low embodied energy and high power-conversion efficiency (h) over disparate light intensities renders the dyesensitized solar cell (DSSC) a promising alternative to conventional photovoltaic technologies. Significant penetration of the DSSC into the photovoltaic market, however, is hindered predominantly by the long-term stability of dyes and electrolytes under practical conditions. The instability of champion (i.e., h> 10%) dyes (which, until recently, all were derivatives of [Ru(dcbpy)2(NCS)2] (N3 ; dcbpy= 4,4’dicarboxy-2,2’-bipyridine)) in the DSSC is caused primarily by desorption of the dyes from the surface and/or liberation of the NCS ligands from the metal centre. While the rate of dye desorption from TiO2 can be manipulated by replacing the CO2H moiety with other anchoring groups, this strategy typically compromises electron injection into the TiO2. [8] An alternative approach is to replace the dcbpy ligands that comprise N3 with bidentate ligands bearing aliphatic substituents (e.g., Scheme 1a), which serve to hinder water from reaching the surface to hydrolytically cleave the TiO2–dye ester linkage. These groups provide the additional benefit of suppressing recombination between the electrolyte and the electrons in TiO2, thus leading to higher efficiencies (Scheme 1a). Chemical strategies for avoiding the labile Ru NCS bond have been realized recently; indeed, we and others have now documented remarkably high h values for DSSCs containing NCS -free Ru sensitizers. Cyclometalated Ru complexes such as [Ru(dcbpy)2(ppy)] 1+ (ppy= 2-phenylpyridine) provide a versatile platform in this respect because: 1) the highest occupied molecular orbital (HOMO) is extended over the metal and anionic ring thus enabling its modulation through judicious installation of substituents at the R2 site in Scheme 1b; and 2) the low-lying excited states, which contain orbital character that resides on the p* framework of the dcbpy ligand(s), are poised for electron injection into the TiO2. [10,11,15–18] This scenario leaves open the opportunity to replace one dcbpy with a bidentate ligand capable of suppressing recombination and enhancing the optical properties as per the aforementioned protocol (Scheme 1). While we recently demonstrated synthetic access to trisheteroleptic Ru sensitizers (e.g., 1 and 2 ; Scheme 2), we learned that removing the acid linkers

Atomic Level Resolution of Dye Regeneration in the Dye-Sensitized Solar Cell

Two donor-acceptor organic dyes have been synthesized that differ only by a two-heteroatom change from oxygen to sulfur within the donor unit. The two dyes, (E)-3-(5-(4-(bis(4-(hexyloxy)phenyl)amino)phenyl)thiophen-2-yl)-2-cyanoprop-2-enoic acid (Dye-O) and (E)-3-(5-(4-(bis(4-(hexylthio)phenyl)amino)phenyl)thiophen-2-yl)-2-cyanoprop-2-enoic acid) (Dye-S), were tested in solar cell devices employing both I(3)(-)/I(-)-based and [Co(bpy)(3)](3+/2+) redox mediators. Power conversion efficiencies over 6% under simulated AM 1.5 illumination (1 Sun) were achieved in both electrolytes. Despite similar optical and redox properties for the two dyes, a consistently higher open-circuit voltage (V(oc)) was measured for Dye-S relative to Dye-O. The improved efficiency observed with Dye-S in an iodide redox mediator is against the commonly held view that sulfur atoms promote charge recombination attributed to inner-sphere interactions. Detailed mechanistic studies revealed that this is a consequence of a 25-fold enhancement of the regeneration rate constant that enhances the regeneration yield under open circuit conditions. The data show that a high short circuit photocurrent does not imply optimal regeneration efficiency as is often assumed.

Interrogation of electrocatalytic water oxidation mediated by a cobalt complex

Examination of the aqueous electrochemistry of a Co(II) complex bearing a pentadentate ligand suggests that the catalytic current corresponding to water oxidation is molecular in origin, and does not emanate exclusively from Co-oxide phases formed in situ.

Physicochemical Analysis of Ruthenium(II) Sensitizers of 1,2,3-Triazole-Derived Mesoionic Carbene and Cyclometalating Ligands

A series of heteroleptic bis(tridentate) ruthenium(II) complexes bearing ligands featuring 1,2,3-triazolide and 1,2,3-triazolylidene units are presented. The synthesis of the C^N^N-coordinated ruthenium(II) triazolide complex is achieved by direct C-H activation, which is enabled by the use of a 1,5-disubstituted triazole. By postcomplexation alkylation, the ruthenium(II) 1,2,3-triazolide complex can be converted to the corresponding 1,2,3-triazolylidene complex. Additionally, a ruthenium(II) complex featuring a C^N^C-coordinating bis(1,2,3-triazolylidene)pyridine ligand is prepared via transmetalation from a silver(I) triazolylidene precursor. The electronic consequences of the carbanion and mesoionic carbene donors are studied both experimentally and computationally. The presented complexes exhibit a broad absorption in the visible region as well as long lifetimes of the charge-separated excited state suggesting their application in photoredox catalysis and photovoltaics. Testing of the dyes in a conventional dye-sensitized solar cell (DSSC) generates, however, only modest power conversion efficiencies (PCEs).

A comparison of several nanoscale photocatalysts in the degradation of a common pollutant using LEDs and conventional UV light.

A comparative study on the photocatalytic activities of four different catalysts, P-25 TiO(2), TiO(2) nanofibers, tin-doped TiO(2) nanofibers under UV light irradiation at 350 nm, and coumarin (C-343) coated TiO(2) nanofibers at 436 nm light emitting diodes (LED) is reported. Catalysts performance has been compared based on their reflectance spectrum and activity. A common water contaminant 4-chlorophenol was used as a substrate to compare the activity of the different catalysts under both direct and dye sensitized conditions. Results indicated that amongst the four different catalysts the activity of P-25 was the highest. However the activity of C-343 coated TiO(2) nanofibers in the LED (436 nm) based reactor was competitive. Identification of reaction intermediates implied that the reaction pathways under UV (band gap) and visible (dye sensitized) irradiation were different. Nonetheless, ring opening took place in all reactions with both maleic and dihydroxymaleic have been identified as intermediates. The study indicates that ordered arrays of TiO(2) irradiated by panels of arrays of low cost high intensity LEDs might be used for the design of reactors. The near monochromaticity, long life, and operation under direct currents are advantages of using LEDs.

Curing BiVO4 Photoanodes with Ultraviolet Light Enhances Photoelectrocatalysis

Exposure of BiVO4 photoanodes to ultraviolet (UV) radiation for extended time periods (e.g., 20 h) produces a morphological change and concomitant improvement in photo-electrocatalytic (PEC) efficiency for driving water splitting directly by sunlight. The ∼230 mV cathodic shift in onset potential and doubling of the photocurrent at 1.23 V vs. RHE after UV curing are comparable to the effects engendered by the presence of a secondary catalyst layer. PEC measurements and absorption spectra indicate that the cathodic shift after UV curing corresponds to a suppression of charge recombination and a greater photovoltage generation caused by the shift of the flat-band potential, and not an improvement in electrocatalytic activity or light absorption. Spectroscopic surface analysis suggests that surface defect sites, which are eliminated by UV curing, for the differences in observed charge recombination.

Sol–gel synthesis of linear Sn-doped TiO2 nanostructures

The sol–gel synthesis of linear Sn-doped TiO2 (TDT) nanostructures with high aspect ratios is reported. These binary metal-oxide nanostructures are readily accessed by treating titanium isopropoxide (Ti(OiPr)4) with appropriate quantities of acetic acid (AcOH) and Sn(OAc)4 in heptanes to generate linear macromolecules that form nanofibers upon calcination. While these linear nanostructures are isolated at low R values (0.05–0.20), where R is defined as the molar ratio of Sn(OAc)4 to Ti(OiPr)4, axially directional growth is not favored at higher R values (0.30–0.50). Scanning and transmission electron microscope (SEM and TEM) imaging of the nanofibers revealed diameters in the 10–20 nm range and lengths in excess of 1 mm. Elemental mapping by STEM-EDS techniques indicates a homogeneous distribution of Sn ions throughout the linear TDT structures. The molecular intermediates that form during the early stages of the reaction were monitored using electrospray-ionization mass spectrometry to confirm the presence of metal-oxide clusters containing both Sn and Ti ions. These intermediates then undergo polycondensation reactions to form the final linear product, thereby indicating the homogeneous incorporation of Sn into the TiO2 lattice and rules out the possibility of independent SnO2 and TiO2 aggregates. Powder X-ray diffraction data indicate that pure TiO2 nanostructures are anatase when calcined at 500 °C, but show a propensity to adopt the rutile phase at progressively higher Sn concentrations.

Solution growth of anatase TiO2 nanowires from transparent conducting glass substrates

Sol–gel reaction conditions that enable the growth of one-dimensional (1D) anatase titanium dioxide (TiO2) nanostructures from fluorine-doped indium tin oxide (FTO) substrates are described. The generation of these linear nanostructures is achieved using acetic acid (HOAc) and titanium isopropoxide (Ti(OiPr)4) in anhydrous heptane in the absence of an external bias or template. The procedure requires the functionalization of base-treated substrates with Ti-oxide nucleation sites, which serve as a foundation for the growth of linear Ti-oxide macromolecules. Calcination of these macromolecules at 450 °C under an ambient atmosphere produce uniform films of randomly oriented anatase TiO2 nanowires. The nucleation and growth processes are both acutely sensitive to the relative molar ratio (R) of HOAc to Ti(OiPr)4. Optimal surface coverage of the nucleation sites is observed when the R value utilized for the nucleation phase (denoted Ri) is equal to 1.3. The highest quality nanowire films were obtained when the R value employed during the gelation phase (denoted Rf) was held between 8.5 and 14. Characterization of the films by electron microscopy revealed a uniform film of disordered anatase TiO2 nanowires with high aspect ratios. The dimensions of the nanostructures correspond to lengths of ca. 1–10 μm and widths of 54 ± 10 nm. High-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) techniques demonstrate that the anatase nanowires are a linear arrangement of crystallites ranging in size from 13 to 19 nm. A systematic evaluation of how reaction conditions (e.g., solvent volume, stoichiometry of reagents, substrate base treatment) affect the generation of these TiO2 films is presented.