Applying Dynamic Strain on Thin Oxide Films Immobilized on a Pseudoelastic Nickel-Titanium Alloy.

Direct alteration of material structure/function through strain is a growing area of research that has allowed for novel properties of materials to emerge. Tuning material structure can be achieved by controlling an external force imposed on materials and inducing stress-strain responses (i.e., applying dynamic strain). Electroactive thin films are typically deposited on shape or volume tunable elastic substrates, where mechanical loading (i.e., compression or tension) can affect film structure and function through imposed strain. Here, we summarize methods for straining n-type doped titanium dioxide (TiO2) films prepared by a thermal treatment of a pseudo-elastic nickel-titanium alloy (Nitinol). The main purpose of the described methods is to study how strain affects electrocatalytic activities of metal oxide, specifically hydrogen evolution and oxygen evolution reactions. The same system can be adapted to study the effect of strain more broadly. Strain engineering can be applied for optimization of a material function, as well as for design of adjustable, multifunctional (photo)electrocatalytic materials under external stress control.

Dynamic Tuning of a Thin Film Electrocatalyst by Tensile Strain.

We report the ability to tune the catalytic activities for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) by applying mechanical stress on a highly n-type doped rutile TiO2 films. We demonstrate through operando electrochemical experiments that the low HER activity of TiO2 can reversibly approach those of the state-of-the-art non-precious metal catalysts when the TiO2 is under tensile strain. At 3% tensile strain, the HER overpotential required to generate a current density of 1 mA/cm2 shifts anodically by 260 mV to give an onset potential of 125 mV, representing a drastic reduction in the kinetic overpotential. A similar albeit smaller cathodic shift in the OER overpotential is observed when tensile strain is applied to TiO2. Results suggest that significant improvements in HER and OER activities with tensile strain are due to an increase in concentration of surface active sites and a decrease in kinetic and thermodynamics barriers along the reaction pathway(s). Our results highlight that strain applied to TiO2 by precisely controlled and incrementally increasing (i.e. dynamic) tensile stress is an effective tool for dynamically tuning the electrocatalytic properties of HER and OER electrocatalysts relative to their activities under static conditions.

Balancing the Hydrogen Evolution Reaction, Surface Energetics, and Stability of Metallic MoS2 Nanosheets via Covalent Functionalization.

We modify the fundamental electronic properties of metallic (1T phase) nanosheets of molybdenum disulfide (MoS2) through covalent chemical functionalization, and thereby directly influence the kinetics of the hydrogen evolution reaction (HER), surface energetics, and stability. Chemically exfoliated, metallic MoS2 nanosheets are functionalized with organic phenyl rings containing electron donating or withdrawing groups. We find that MoS2 functionalized with the most electron donating functional group (p-(CH3CH2)2NPh-MoS2) is the most efficient catalyst for HER in this series, with initial activity that is slightly worse compared to the pristine metallic phase of MoS2. The p-(CH3CH2)2NPh-MoS2 is more stable than unfunctionalized metallic MoS2 and outperforms unfunctionalized metallic MoS2 for continuous H2 evolution within 10 min under the same conditions. With regards to the entire studied series, the overpotential and Tafel slope for catalytic HER are both directly correlated with the electron donating strength of the functional group. The results are consistent with a mechanism involving ground-state electron donation or withdrawal to/from the MoS2 nanosheets, which modifies the electron transfer kinetics and catalytic activity of the MoS2 nanosheet. The functional groups preserve the metallic nature of the MoS2 nanosheets, inhibiting conversion to the thermodynamically stable semiconducting state (2H) when mildly annealed in a nitrogen atmosphere. We propose that the electron density and, therefore, reactivity of the MoS2 nanosheets are controlled by the attached functional groups. Functionalizing nanosheets of MoS2 and other transition metal dichalcogenides provides a synthetic chemical route for controlling the electronic properties and stability within the traditionally thermally unstable metallic state.

A facile solvothermal growth of single crystal mixed halide perovskite CH3NH3Pb(Br(1-x)Cl(x))3.

We demonstrate a facile synthetic approach for preparing mixed halide perovskite (CH3NH3)Pb(Br1-xClx)3 single crystals by the solvothermal growth of stoichiometric PbBr2 and [(1 - y)CH3NH3Br + yCH3NH3Cl] DMF precursor solutions. The band gap of (CH3NH3)Pb(Br1-xClx)3 single crystals increased and the unit cell dimensions decreased with an increase in Cl content x, consistent with previous theoretical predictions. Interestingly, the Cl/Br ratio in the (CH3NH3)Pb(Br1-xClx)3 single crystals is larger than that of the precursor solution, suggesting an unusual crystal growth mechanism.

Manganese as a substitute for rhenium in CO2 reduction catalysts: the importance of acids.

Electrocatalytic properties, X-ray crystallographic studies, and infrared spectroelectrochemistry (IR-SEC) of Mn(bpy-tBu)(CO)3Br and [Mn(bpy-tBu)(CO)3(MeCN)](OTf) are reported. Addition of Brönsted acids to CO2-saturated solutions of these Mn complexes and subsequent reduction of the complexes lead to the stable and efficient production of CO from CO2. Unlike the analogous Re catalysts, these Mn catalysts require the addition of Brönsted acids for catalytic turnover. Current densities up to 30 mA/cm(2) were observed during bulk electrolysis using 5 mM Mn(bpy-tBu)(CO)3Br, 1 M 2,2,2-trifluoroethanol, and a glassy carbon working electrode. During bulk electrolysis at -2.2 V vs SCE, a TOF of 340 s(-1) was calculated for Mn(bpy-tBu)(CO)3Br with 1.4 M trifluoroethanol, corresponding to a Faradaic efficiency of 100 ± 15% for the formation of CO from CO2, with no observable production of H2. When compared to the analogous Re catalysts, the Mn catalysts operate at a lower overpotential and exhibit similar catalytic activities. X-ray crystallography of the reduced species, [Mn(bpy-tBu)(CO)3](-), shows a five-coordinate Mn center, similar to its rhenium analogue. Three distinct species were observed in the IR-SEC of Mn(bpy-tBu)(CO)3Br. These were of the parent Mn(bpy-tBu)(CO)3Br complex, the dimer [Mn(bpy-tBu)(CO)3]2, and the [Mn(bpy-tBu)(CO)3](-) anion.

Structural investigations into the deactivation pathway of the CO2 reduction electrocatalyst Re(bpy)(CO)3Cl.

We report a series of complexes synthesized from the chemical reduction of the fac-tricarbonyl complex Re(bpy)(CO)(3)Cl. Synthesis and characterization of [Re(bpy)(CO)(3)](2), [Re(bpy)(CO)(3)](2)(-), and Re(bpy)(CO)(3)(-) are presented. The Re(bpy)(CO)(3)(-) anion has long been postulated as the active species that reacts with carbon dioxide in the electrochemical reduction of CO(2).

Kinetic and structural studies, origins of selectivity, and interfacial charge transfer in the artificial photosynthesis of CO.

The effective design of an artificial photosynthetic system entails the optimization of several important interactions. Herein we report stopped-flow UV-visible (UV-vis) spectroscopy, X-ray crystallographic, density functional theory (DFT), and electrochemical kinetic studies of the Re(bipy-tBu)(CO)(3)(L) catalyst for the reduction of CO(2) to CO. A remarkable selectivity for CO(2) over H(+) was observed by stopped-flow UV-vis spectroscopy of [Re(bipy-tBu)(CO)(3)](-1). The reaction with CO(2) is about 25 times faster than the reaction with water or methanol at the same concentrations. X-ray crystallography and DFT studies of the doubly reduced anionic species suggest that the highest occupied molecular orbital (HOMO) has mixed metal-ligand character rather than being purely doubly occupied d(z)(2), which is believed to determine selectivity by favoring CO(2) (σ + π) over H(+) (σ only) binding. Electrocatalytic studies performed with the addition of Brönsted acids reveal a primary H/D kinetic isotope effect, indicating that transfer of protons to Re -CO(2) is involved in the rate limiting step. Lastly, the effects of electrode surface modification on interfacial electron transfer between a semiconductor and catalyst were investigated and found to affect the observed current densities for catalysis more than threefold, indicating that the properties of the electrode surface need to be addressed when developing a homogeneous artificial photosynthetic system.

Electronic structural effects in self-exchange reactions.

We report rate constants for electron self-exchange reactions of a series of trinuclear ruthenium clusters substituted with three pyridines [Ru(3)O(OAc)(6)(L)(3)](+/0) or one carbonyl and two pyridines [Ru(3)O(OAc)(6)(CO)(L)(2)](0/-). For the 0/- couple in the latter series, the observed rate constant is determined by orbital overlap. More electron-withdrawing pyridine ligands increase the donor-acceptor overlap and more effectively reduce a large reorganization barrier (λ ≈ 10,000 cm(-1)), leading to faster exchange. Larger aromatic ligands also increase the rate by increasing the coupling. For the +/0 couple in tris-pyridyl clusters, there is no observable trend based on pyridine electron-withdrawing ability, and we conclude that charge density on peripheral ligands is not a determining factor for this exchanging pair. From structural and vibrational data, the inner sphere reorganization barrier, λ(is), is estimated to be 1520 cm(-1) and outer sphere reorganization energy, λ(os) is estimated to be between 1800 and 3600 cm(-1) in CD(2)Cl(2). The large range is due to uncertainty in the electron transfer distance, r. λ(tot) for the +/0 pair is then estimated to be between 3320 and 5120 cm(-1), well less than the 10,000 cm(-1) estimated for the 0/- pair. The similar rate constants observed despite very different reorganization energies is explained by donor-acceptor orbital overlap in the 0/- pair, which is consistent with greater delocalization allowed by donor-acceptor orbital symmetry.

Synthesis and characterization of 6,6'-(2,4,6-triisopropylphenyl)-2,2'-bipyridine (tripbipy) and its complexes of the late first row transition metals.

The synthesis of tripbipy, a new substituted bipyridine ligand (6,6'-(2,4,6-triisopropylphenyl)-2,2'-bipyridine), and the syntheses, structures, and magnetic properties of the first coordination compounds based on this ligand are described. Tripbipy was synthesized by the Suzuki coupling of 2,4,6-triisopropylphenyl boronic acid and 6,6'-dibromo-2,2'-bipyridine. Reported here are the tripbipy complexes of five late first row transition metal chlorides (MCl(2); M = Fe, Co, Ni, Cu, Zn). Four of the complexes MCl(2)tripbipy (M = Fe, Co, Ni, Zn) crystallize in the space group P2(1)/c and are isomorphous with one solvent molecule of crystallization. The complex CuCl(2)tripbipy crystallizes in the space group P2(1)2(1)2(1) with two solvent molecules of crystallization. All MCl(2)tripbipy complexes are four coordinate and contain distorted tetrahedral metal centers. CuCl(2)tripbipy shows a pseudo Jahn-Teller distortion, and X-band electron paramagnetic resonance (EPR) in a toluene glass gives approximate g( perpendicular, parallel) values of 2.2 and 2.1. Magnetic measurements (M = Fe, Co, Ni, Cu) are consistent with high spin d(n) configurations (n = 6-9, S = 2, 3/2, 1, 1/2) tetrahedral complexes and give chi(M)T values at 300 K of 3.56, 2.10, 1.01, and 0.37 cm(3) M(-1) K, respectively.

Electrocatalytic and homogeneous approaches to conversion of CO2 to liquid fuels.

Research in the field of catalytic reduction of carbon dioxide to liquid fuels has grown rapidly in the past few decades. This is due to the increasing amount of carbon dioxide in the atmosphere and a steady climb in global fuel demand. This tutorial review will present much of the significant work that has been done in the field of electrocatalytic and homogeneous reduction of carbon dioxide over the past three decades. It will then extend the discussion to the important conclusions from previous work and recommendations for future directions to develop a catalytic system that will convert carbon dioxide to liquid fuels with high efficiencies.