Reactions of HF with uncoated and Al and Zn oxide-coated surfaces of LiCoO2 cathodes were studied... more Reactions of HF with uncoated and Al and Zn oxide-coated surfaces of LiCoO2 cathodes were studied using density functional theory. Cathode degradation caused by reaction of HF with the hydroxylated (101̅4) LiCoO2 surface is dominated by formation of H2O and a LiF precipitate via a barrierless reaction that is exothermic by 1.53 eV. We present a detailed mechanism where HF reacts at the alumina coating to create a partially fluorinated alumina surface rather than forming AlF3 and H2O and thus alumina films reduce cathode degradation by scavenging HF and avoiding H2O formation. In contrast, we find that HF etches monolayer zinc oxide coatings, which thus fail to prevent capacity fading. However, thicker zinc oxide films mitigate capacity loss by reacting with HF to form a partially fluorinated zinc oxide surface. Metal oxide coatings that react with HF to form hydroxyl groups over H2O, like the alumina monolayer, will significantly reduce cathode degradation.
Multi-exciton generation (MEG) has been observed in a variety of materials and might be exploited... more Multi-exciton generation (MEG) has been observed in a variety of materials and might be exploited in solar-cells to dramatically increase efficiency. In tetracene and pentacene MEG has been attributed to singlet fission (SF), however a fundamental mechanism for SF has not been previously described. Here, we use sophisticated ab initio calculations to show that MEG in pentacene proceeds by transition of the lowest optically allowed excited state S1 to a dark state (D) of multi-exciton character, which subsequently undergoes SF to generate two triplets (2´T0). D satisfies the energy requirement for SF (ED>2ET0) and lies just below S1 in pentacene, but above S1 in tetracene, consistent with the observed thermally activated SF process in tetracene, but no thermal activation in pentacene. While S1 exhibits single exciton character, D shows multi-exciton character comprising two separated electron-hole pairs. Dimer simulations predict S1 excimer formation and that fission of D into tri...
We have employed ab initio quantum chemical methods to model a set of both existing and new photo... more We have employed ab initio quantum chemical methods to model a set of both existing and new photoinitiators (PI) and synthesized and characterized several PIs that are predicted to be promising. Calculations of existing PIs are used to benchmark the specific computational methods as well as to develop a fundamental understanding of the various phenomena involved in the photoinitiation process. We then use quantum chemistry to computationally prototype a series of new PIs by calculating a set of properties for each case, including; absorption peak, oscillator strength/absorbtivity, bond dissociation energy, radical reactivity and properties related to the rate of intersystem crossing. We have synthesized and characterized several new PIs, including derivatives of the phosphine oxide and the benzoyl germanium PIs. These experiments confirm the ab initio predictions and provide additional feedback to the computational prototyping effort.
It has been shown that the dielectric constant of certain glassy materials can be changed with a ... more It has been shown that the dielectric constant of certain glassy materials can be changed with a dc electric field bias. Here we extend those studies to higher frequencies where both the real and imaginary part of the dielectric constant can be studied. We have designed a dc electric-field tunable LC resonator built from superconducting thin-film aluminum to test this effect in SiNx at GHz frequencies. We will report progress on measuring these devices down to single photon storage energies at temperatures of approximately 30 mK, where the dynamics are dominated by the tunneling of two-level defects.
We employ quantum chemical calculations to investigate the mechanism of homogeneous CO2 reduction... more We employ quantum chemical calculations to investigate the mechanism of homogeneous CO2 reduction by pyridine (Py) in the Py/p-GaP system. We find that CO2 reduction by Py commences with PyCOOH0 formation where: a) protonated Py (PyH+) is reduced to PyH0, b) PyH0 then reduces CO2 by one electron transfer (ET) via nucleophilic attack by its N lone pair on the C of CO2 and finally c) proton transfer (PT) from PyH0 to CO2 produces PyCOOH0. The predicted enthalpic barrier for this proton coupled ET (PCET) reaction is 45.7 kcal/mol for direct PT from PyH0 to CO2. However, when PT is mediated by one to three water molecules acting as a proton relay the barrier decreases to 29.5, 20.4 and 18.5 kcal/mol, respectively. The water proton relay reduces strain in the transition state (TS) and facilitates more complete ET. For PT mediated by a three water molecule proton relay, adding water molecules to explicitly solvate the core reaction system reduces the barrier to 13.6 - 16.5 kcal/mol, depen...
Oxygen vacancies (V O ) in oxides are extensively used to manipulate vital material properties. A... more Oxygen vacancies (V O ) in oxides are extensively used to manipulate vital material properties. Although methods to predict defect formation energies have advanced significantly, an understanding of the intrinsic material properties that govern defect energetics lags. We use first-principles calculations to study the connection between intrinsic (bulk) material properties and the energy to form a single, charge neutral oxygen vacancy (E V ). We investigate 45 binary and ternary oxides and find that a simple model which combines (i) the oxide enthalpy of formation (ΔH f ), (ii) the midgap energy relative to the O 2p band center (E O 2p + (1/2)E g ), and (iii) atomic electronegativities reproduces calculated E V within ∼0.2 eV. This result provides both valuable insights into the key properties influencing E V and a direct method to predict E V . We then predict the E V of ∼1800 oxides and validate the predictive nature of our approach against direct defect calculations for a subset of 18 randomly selected materials.
The composition dependence of oxygen vacancy formation energies (E V ) in the complex perovskite ... more The composition dependence of oxygen vacancy formation energies (E V ) in the complex perovskite Sr x La 1−x Mn y Al 1−y O 3 , a material system demonstrated to significantly outperform ceria for solar thermochemical (STC) fuel production, was investigated using a combination of ab initio and experimental techniques. Density functional theory calculations predict that E V as a function of composition spans a large range from near 0 to over 3 eV under standard conditions and exhibits trends consistent with equilibrium oxygen nonstoichiometries we determined using thermogravimetric analysis. Our findings show that the strong sensitivity of E V to composition arises from the composition dependence of the Fermi energy relative to the O 2p band center. From these results, we identify the range of E V particularly suited for STC cycles for a given set of reduction and oxidation reaction conditions and present a materials design strategy for the optimization of E V through compositional tuning.
Electrochemical supercapacitors utilizing pseudocapacitive materials offer the possibility of bot... more Electrochemical supercapacitors utilizing pseudocapacitive materials offer the possibility of both high power density and high energy density. Unfortunately, the mechanism of electrochemical charge storage in pseudocapacitive materials and the effect of operating conditions on the charge storage mechanism are generally not well understood. Here, we present the first detailed pseudocapacitive charge storage mechanism of MnO 2 and explain the capacity differences between αand ß-MnO 2 using a combined theoretical electrochemical and band structure analysis. We identify the importance of the band gap, work function, the point of zero charge, and the tunnel sizes of the electrode material, as well as the pH and stability window of the electrolyte in determining the viability of a given electrode material. The high capacity of α-MnO 2 results from cation induced charge-switching states in the band gap that overlap with the scanned potential allowed by the electrolyte. The charge-switching states originate from interstitial cations (H + , Li + , Na + , and K + ) that stabilize Mn-O antibonding orbitals from the conduction band and substitutional cations that interact with O[2p] dangling bonds created by Mn vacancies that are destabilized from the valence band. We calculate the equilibrium electrochemical potentials at which these states are reduced and predict the effect of the electrochemical operating conditions on their contribution to charge storage. The mechanism and theoretical approach we report is general and can be used to computationally screen new materials for improved pseudocapacitive charge storage.
We use quantum chemical calculations to elucidate a viable mechanism for pyridine-catalyzed reduc... more We use quantum chemical calculations to elucidate a viable mechanism for pyridine-catalyzed reduction of CO 2 to methanol involving homogeneous catalytic steps. The first phase of the catalytic cycle involves generation of the key catalytic agent, 1,2-dihydropyridine (PyH 2 ). First, pyridine (Py) undergoes a H + transfer (PT) to form pyridinium (PyH + ), followed by an e − transfer (ET) to produce pyridinium radical (PyH 0 ). Examples of systems to effect this ET to populate PyH + 's LUMO (E 0 calc ∼ −1.3 V vs SCE) to form the solution phase PyH 0 via highly reducing electrons include the photoelectrochemical p-GaP system (E CBM ∼ −1.5 V vs SCE at pH 5) and the photochemical [Ru(phen) 3 ] 2+ / ascorbate system. We predict that PyH 0 undergoes further PT−ET steps to form the key closed-shell, dearomatized (PyH 2 ) species (with the PT capable of being assisted by a negatively biased cathode). Our proposed sequential PT−ET−PT−ET mechanism for transforming Py into PyH 2 is analogous to that described in the formation of related dihydropyridines. Because it is driven by its proclivity to regain aromaticity, PyH 2 is a potent recyclable organo-hydride donor that mimics important aspects of the role of NADPH in the formation of C−H bonds in the photosynthetic CO 2 reduction process. In particular, in the second phase of the catalytic cycle, which involves three separate reduction steps, we predict that the PyH 2 /Py redox couple is kinetically and thermodynamically competent in catalytically effecting hydride and proton transfers (the latter often mediated by a proton relay chain) to CO 2 and its two succeeding intermediates, namely, formic acid and formaldehyde, to ultimately form CH 3 OH. The hydride and proton transfers for the first of these reduction steps, the homogeneous reduction of CO 2 , are sequential in nature (in which the formate to formic acid protonation can be assisted by a negatively biased cathode). In contrast, these transfers are coupled in each of the two subsequent homogeneous hydride and proton transfer steps to reduce formic acid and formaldehyde.
The roles of deposited Pt clusters and adsorbed O 2 in the photoactivity of anatase TiO 2 (101) s... more The roles of deposited Pt clusters and adsorbed O 2 in the photoactivity of anatase TiO 2 (101) surfaces have been studied using density functional theory. O 2 only adsorbs to TiO 2 surfaces when excess negative charge is available to form O−Ti bonds, which can be provided by a photoexcited electron or subsurface oxygen vacancy, in which cases the adsorption energies are −0.94 and −2.52 eV, respectively. When O 2 adsorbs near a subsurface defect, it scavenges extra electron density and creates a hole that can annihilate excited electrons. In aqueous solutions, O 2 interactions with the TiO 2 surface are rare because water preferentially adsorbs at the surface. Pt clusters on TiO 2 significantly enhance O 2 adsorption providing many adsorption sites with adsorption energies up to −1.69 eV, stronger than the −0.52 eV adsorption energy of H 2 O on the Pt cluster. Consequently, Pt increases the rate of electron scavenging by O 2 relative to that of undoped TiO 2 leading to enhanced photocatalytic performance. Pt states completely bridge the band gap and act as electron−hole recombination centers, which are deleterious to the photoactivity of TiO 2 . The initial rise and subsequent fall in TiO 2 's photoactivity with Pt loading results from the competition between enhanced electron scavenging due to increased O 2 adsorption and increased electron−hole recombination.
We perform ab initio calculations of hydrogen-based tunneling defects in alumina to identify dele... more We perform ab initio calculations of hydrogen-based tunneling defects in alumina to identify deleterious two-level systems (TLS) in superconducting qubits. The defects analyzed include bulk hydrogenated Al vacancies, bulk hydrogen interstitial defects, and a surface OH rotor. The formation energies of the defects are first computed for an Al-and O-rich environment to give the likelihood of defect occurrence during growth. The potential energy surfaces are then computed and the corresponding dipole moments are evaluated to determine the coupling of the defects to an electric field. Finally, the tunneling energy is computed for the hydrogen defect and the analogous deuterium defect, providing an estimate of the TLS energy and the corresponding frequency for photon absorption. We predict that hydrogenated cation vacancy defects will form a significant density of GHz frequency TLSs in alumina.
We employ quantum chemical calculations to investigate the mechanism of homogeneous CO 2 reductio... more We employ quantum chemical calculations to investigate the mechanism of homogeneous CO 2 reduction by pyridine (Py) in the Py/p-GaP system. We find that CO 2 reduction by Py commences with PyCOOH 0 formation where: (a) protonated Py (PyH + ) is reduced to PyH 0 , (b) PyH 0 then reduces CO 2 by one electron transfer (ET) via nucleophilic attack by its N lone pair on the C of CO 2 , and finally (c) proton transfer (PT) from PyH 0 to CO 2 produces PyCOOH 0 . The predicted enthalpic barrier for this proton-coupled ET (PCET) reaction is 45.7 kcal/mol for direct PT from PyH 0 to CO 2 . However, when PT is mediated by one to three water molecules acting as a proton relay, the barrier decreases to 29.5, 20.4, and 18.5 kcal/mol, respectively. The water proton relay reduces strain in the transition state (TS) and facilitates more complete ET. For PT mediated by a three water molecule proton relay, adding water molecules to explicitly solvate the core reaction system reduces the barrier to 13.6−16.5 kcal/mol, depending on the number and configuration of the solvating waters. This agrees with the experimentally determined barrier of 16.5 ± 2.4 kcal/mol. We calculate a pK a for PyH 0 of 31 indicating that PT preceding ET is highly unfavorable. Moreover, we demonstrate that ET precedes PT in PyCOOH 0 formation, confirming PyH 0 's pK a as irrelevant for predicting PT from PyH 0 to CO 2 . Furthermore, we calculate adiabatic electron affinities in aqueous solvent for CO 2 , Py, and Py·CO 2 of 47.4, 37.9, and 66.3 kcal/mol respectively, indicating that the anionic complex PyCOO − stabilizes the anionic radicals CO 2 − and Py − to facilitate low barrier ET. As the reduction of CO 2 proceeds through ET and then PT, the pyridine ring becomes aromatic, and thus Py catalyzes CO 2 reduction by stabilizing the PCET TS and the PyCOOH 0 product through aromatic resonance stabilization. Our results suggest that Py catalyzes the homogeneous reductions of formic acid and formaldehyde en route to formation of CH 3 OH through a series of one-electron reductions analogous to the PCET reduction of CO 2 examined here, where the electrode only acts to reduce PyH + to PyH 0 .
We employ quantum chemical calculations to discover how frustrated Lewis pairs (FLP) catalyze the... more We employ quantum chemical calculations to discover how frustrated Lewis pairs (FLP) catalyze the reduction of CO 2 by ammonia borane (AB); specifically, we examine how the Lewis acid (LA) and Lewis base (LB) of an FLP activate CO 2 for reduction. We find that the LA (trichloroaluminum, AlCl 3 ) alone catalyzes hydride transfer (HT) to CO 2 while the LB (trimesitylenephosphine, PMes 3 ) actually hinders HT; inclusion of the LB increases the HT barrier by ∼8 kcal/mol relative to the reaction catalyzed by LAs only. The LB hinders HT by donating its lone pair to the LUMO of CO 2 , increasing the electron density on the C atom and thus lowering its hydride affinity. Although the LB hinders HT, it nonetheless plays a crucial role by stabilizing the active FLP·CO 2 complex relative to the LA dimer, free CO 2 , and free LB. This greatly increases the concentration of the reactive complex in the form FLP·CO 2 and thus increases the rate of reaction. We expect that the principles we describe will aid in understanding other catalytic CO 2 reductions.
Bottom-up approaches to nanofabrication are of great interest because they can enable structural ... more Bottom-up approaches to nanofabrication are of great interest because they can enable structural control while minimizing material waste and fabrication time. One new bottom-up nanofabrication method involves excitation of the surface plasmon resonance (SPR) of a Ag surface to drive deposition of sub-15 nm Au nanoparticles from MeAuPPh3. In this work we used density functional theory to investigate the role of the PPh3 ligands of the Au precursor and the effect of adsorbed solvent on the deposition process, and to elucidate the mechanism of Au nanoparticle deposition. In the absence of solvent, the calculated barrier to MeAuPPh3 dissociation on the bare surface is <20 kcal/mol, making it facile at room temperature. Once adsorbed on the surface, neighboring MeAu fragments undergo ethane elimination to produce Au adatoms that cluster into Au nanoparticles. However, if the sample is immersed in benzene, we predict that the monolayer of adsorbed solvent blocks the adsorption of MeAuP...
Reactions of HF with uncoated and Al and Zn oxide-coated surfaces of LiCoO2 cathodes were studied... more Reactions of HF with uncoated and Al and Zn oxide-coated surfaces of LiCoO2 cathodes were studied using density functional theory. Cathode degradation caused by reaction of HF with the hydroxylated (101̅4) LiCoO2 surface is dominated by formation of H2O and a LiF precipitate via a barrierless reaction that is exothermic by 1.53 eV. We present a detailed mechanism where HF reacts at the alumina coating to create a partially fluorinated alumina surface rather than forming AlF3 and H2O and thus alumina films reduce cathode degradation by scavenging HF and avoiding H2O formation. In contrast, we find that HF etches monolayer zinc oxide coatings, which thus fail to prevent capacity fading. However, thicker zinc oxide films mitigate capacity loss by reacting with HF to form a partially fluorinated zinc oxide surface. Metal oxide coatings that react with HF to form hydroxyl groups over H2O, like the alumina monolayer, will significantly reduce cathode degradation.
Multi-exciton generation (MEG) has been observed in a variety of materials and might be exploited... more Multi-exciton generation (MEG) has been observed in a variety of materials and might be exploited in solar-cells to dramatically increase efficiency. In tetracene and pentacene MEG has been attributed to singlet fission (SF), however a fundamental mechanism for SF has not been previously described. Here, we use sophisticated ab initio calculations to show that MEG in pentacene proceeds by transition of the lowest optically allowed excited state S1 to a dark state (D) of multi-exciton character, which subsequently undergoes SF to generate two triplets (2´T0). D satisfies the energy requirement for SF (ED>2ET0) and lies just below S1 in pentacene, but above S1 in tetracene, consistent with the observed thermally activated SF process in tetracene, but no thermal activation in pentacene. While S1 exhibits single exciton character, D shows multi-exciton character comprising two separated electron-hole pairs. Dimer simulations predict S1 excimer formation and that fission of D into tri...
We have employed ab initio quantum chemical methods to model a set of both existing and new photo... more We have employed ab initio quantum chemical methods to model a set of both existing and new photoinitiators (PI) and synthesized and characterized several PIs that are predicted to be promising. Calculations of existing PIs are used to benchmark the specific computational methods as well as to develop a fundamental understanding of the various phenomena involved in the photoinitiation process. We then use quantum chemistry to computationally prototype a series of new PIs by calculating a set of properties for each case, including; absorption peak, oscillator strength/absorbtivity, bond dissociation energy, radical reactivity and properties related to the rate of intersystem crossing. We have synthesized and characterized several new PIs, including derivatives of the phosphine oxide and the benzoyl germanium PIs. These experiments confirm the ab initio predictions and provide additional feedback to the computational prototyping effort.
It has been shown that the dielectric constant of certain glassy materials can be changed with a ... more It has been shown that the dielectric constant of certain glassy materials can be changed with a dc electric field bias. Here we extend those studies to higher frequencies where both the real and imaginary part of the dielectric constant can be studied. We have designed a dc electric-field tunable LC resonator built from superconducting thin-film aluminum to test this effect in SiNx at GHz frequencies. We will report progress on measuring these devices down to single photon storage energies at temperatures of approximately 30 mK, where the dynamics are dominated by the tunneling of two-level defects.
We employ quantum chemical calculations to investigate the mechanism of homogeneous CO2 reduction... more We employ quantum chemical calculations to investigate the mechanism of homogeneous CO2 reduction by pyridine (Py) in the Py/p-GaP system. We find that CO2 reduction by Py commences with PyCOOH0 formation where: a) protonated Py (PyH+) is reduced to PyH0, b) PyH0 then reduces CO2 by one electron transfer (ET) via nucleophilic attack by its N lone pair on the C of CO2 and finally c) proton transfer (PT) from PyH0 to CO2 produces PyCOOH0. The predicted enthalpic barrier for this proton coupled ET (PCET) reaction is 45.7 kcal/mol for direct PT from PyH0 to CO2. However, when PT is mediated by one to three water molecules acting as a proton relay the barrier decreases to 29.5, 20.4 and 18.5 kcal/mol, respectively. The water proton relay reduces strain in the transition state (TS) and facilitates more complete ET. For PT mediated by a three water molecule proton relay, adding water molecules to explicitly solvate the core reaction system reduces the barrier to 13.6 - 16.5 kcal/mol, depen...
Oxygen vacancies (V O ) in oxides are extensively used to manipulate vital material properties. A... more Oxygen vacancies (V O ) in oxides are extensively used to manipulate vital material properties. Although methods to predict defect formation energies have advanced significantly, an understanding of the intrinsic material properties that govern defect energetics lags. We use first-principles calculations to study the connection between intrinsic (bulk) material properties and the energy to form a single, charge neutral oxygen vacancy (E V ). We investigate 45 binary and ternary oxides and find that a simple model which combines (i) the oxide enthalpy of formation (ΔH f ), (ii) the midgap energy relative to the O 2p band center (E O 2p + (1/2)E g ), and (iii) atomic electronegativities reproduces calculated E V within ∼0.2 eV. This result provides both valuable insights into the key properties influencing E V and a direct method to predict E V . We then predict the E V of ∼1800 oxides and validate the predictive nature of our approach against direct defect calculations for a subset of 18 randomly selected materials.
The composition dependence of oxygen vacancy formation energies (E V ) in the complex perovskite ... more The composition dependence of oxygen vacancy formation energies (E V ) in the complex perovskite Sr x La 1−x Mn y Al 1−y O 3 , a material system demonstrated to significantly outperform ceria for solar thermochemical (STC) fuel production, was investigated using a combination of ab initio and experimental techniques. Density functional theory calculations predict that E V as a function of composition spans a large range from near 0 to over 3 eV under standard conditions and exhibits trends consistent with equilibrium oxygen nonstoichiometries we determined using thermogravimetric analysis. Our findings show that the strong sensitivity of E V to composition arises from the composition dependence of the Fermi energy relative to the O 2p band center. From these results, we identify the range of E V particularly suited for STC cycles for a given set of reduction and oxidation reaction conditions and present a materials design strategy for the optimization of E V through compositional tuning.
Electrochemical supercapacitors utilizing pseudocapacitive materials offer the possibility of bot... more Electrochemical supercapacitors utilizing pseudocapacitive materials offer the possibility of both high power density and high energy density. Unfortunately, the mechanism of electrochemical charge storage in pseudocapacitive materials and the effect of operating conditions on the charge storage mechanism are generally not well understood. Here, we present the first detailed pseudocapacitive charge storage mechanism of MnO 2 and explain the capacity differences between αand ß-MnO 2 using a combined theoretical electrochemical and band structure analysis. We identify the importance of the band gap, work function, the point of zero charge, and the tunnel sizes of the electrode material, as well as the pH and stability window of the electrolyte in determining the viability of a given electrode material. The high capacity of α-MnO 2 results from cation induced charge-switching states in the band gap that overlap with the scanned potential allowed by the electrolyte. The charge-switching states originate from interstitial cations (H + , Li + , Na + , and K + ) that stabilize Mn-O antibonding orbitals from the conduction band and substitutional cations that interact with O[2p] dangling bonds created by Mn vacancies that are destabilized from the valence band. We calculate the equilibrium electrochemical potentials at which these states are reduced and predict the effect of the electrochemical operating conditions on their contribution to charge storage. The mechanism and theoretical approach we report is general and can be used to computationally screen new materials for improved pseudocapacitive charge storage.
We use quantum chemical calculations to elucidate a viable mechanism for pyridine-catalyzed reduc... more We use quantum chemical calculations to elucidate a viable mechanism for pyridine-catalyzed reduction of CO 2 to methanol involving homogeneous catalytic steps. The first phase of the catalytic cycle involves generation of the key catalytic agent, 1,2-dihydropyridine (PyH 2 ). First, pyridine (Py) undergoes a H + transfer (PT) to form pyridinium (PyH + ), followed by an e − transfer (ET) to produce pyridinium radical (PyH 0 ). Examples of systems to effect this ET to populate PyH + 's LUMO (E 0 calc ∼ −1.3 V vs SCE) to form the solution phase PyH 0 via highly reducing electrons include the photoelectrochemical p-GaP system (E CBM ∼ −1.5 V vs SCE at pH 5) and the photochemical [Ru(phen) 3 ] 2+ / ascorbate system. We predict that PyH 0 undergoes further PT−ET steps to form the key closed-shell, dearomatized (PyH 2 ) species (with the PT capable of being assisted by a negatively biased cathode). Our proposed sequential PT−ET−PT−ET mechanism for transforming Py into PyH 2 is analogous to that described in the formation of related dihydropyridines. Because it is driven by its proclivity to regain aromaticity, PyH 2 is a potent recyclable organo-hydride donor that mimics important aspects of the role of NADPH in the formation of C−H bonds in the photosynthetic CO 2 reduction process. In particular, in the second phase of the catalytic cycle, which involves three separate reduction steps, we predict that the PyH 2 /Py redox couple is kinetically and thermodynamically competent in catalytically effecting hydride and proton transfers (the latter often mediated by a proton relay chain) to CO 2 and its two succeeding intermediates, namely, formic acid and formaldehyde, to ultimately form CH 3 OH. The hydride and proton transfers for the first of these reduction steps, the homogeneous reduction of CO 2 , are sequential in nature (in which the formate to formic acid protonation can be assisted by a negatively biased cathode). In contrast, these transfers are coupled in each of the two subsequent homogeneous hydride and proton transfer steps to reduce formic acid and formaldehyde.
The roles of deposited Pt clusters and adsorbed O 2 in the photoactivity of anatase TiO 2 (101) s... more The roles of deposited Pt clusters and adsorbed O 2 in the photoactivity of anatase TiO 2 (101) surfaces have been studied using density functional theory. O 2 only adsorbs to TiO 2 surfaces when excess negative charge is available to form O−Ti bonds, which can be provided by a photoexcited electron or subsurface oxygen vacancy, in which cases the adsorption energies are −0.94 and −2.52 eV, respectively. When O 2 adsorbs near a subsurface defect, it scavenges extra electron density and creates a hole that can annihilate excited electrons. In aqueous solutions, O 2 interactions with the TiO 2 surface are rare because water preferentially adsorbs at the surface. Pt clusters on TiO 2 significantly enhance O 2 adsorption providing many adsorption sites with adsorption energies up to −1.69 eV, stronger than the −0.52 eV adsorption energy of H 2 O on the Pt cluster. Consequently, Pt increases the rate of electron scavenging by O 2 relative to that of undoped TiO 2 leading to enhanced photocatalytic performance. Pt states completely bridge the band gap and act as electron−hole recombination centers, which are deleterious to the photoactivity of TiO 2 . The initial rise and subsequent fall in TiO 2 's photoactivity with Pt loading results from the competition between enhanced electron scavenging due to increased O 2 adsorption and increased electron−hole recombination.
We perform ab initio calculations of hydrogen-based tunneling defects in alumina to identify dele... more We perform ab initio calculations of hydrogen-based tunneling defects in alumina to identify deleterious two-level systems (TLS) in superconducting qubits. The defects analyzed include bulk hydrogenated Al vacancies, bulk hydrogen interstitial defects, and a surface OH rotor. The formation energies of the defects are first computed for an Al-and O-rich environment to give the likelihood of defect occurrence during growth. The potential energy surfaces are then computed and the corresponding dipole moments are evaluated to determine the coupling of the defects to an electric field. Finally, the tunneling energy is computed for the hydrogen defect and the analogous deuterium defect, providing an estimate of the TLS energy and the corresponding frequency for photon absorption. We predict that hydrogenated cation vacancy defects will form a significant density of GHz frequency TLSs in alumina.
We employ quantum chemical calculations to investigate the mechanism of homogeneous CO 2 reductio... more We employ quantum chemical calculations to investigate the mechanism of homogeneous CO 2 reduction by pyridine (Py) in the Py/p-GaP system. We find that CO 2 reduction by Py commences with PyCOOH 0 formation where: (a) protonated Py (PyH + ) is reduced to PyH 0 , (b) PyH 0 then reduces CO 2 by one electron transfer (ET) via nucleophilic attack by its N lone pair on the C of CO 2 , and finally (c) proton transfer (PT) from PyH 0 to CO 2 produces PyCOOH 0 . The predicted enthalpic barrier for this proton-coupled ET (PCET) reaction is 45.7 kcal/mol for direct PT from PyH 0 to CO 2 . However, when PT is mediated by one to three water molecules acting as a proton relay, the barrier decreases to 29.5, 20.4, and 18.5 kcal/mol, respectively. The water proton relay reduces strain in the transition state (TS) and facilitates more complete ET. For PT mediated by a three water molecule proton relay, adding water molecules to explicitly solvate the core reaction system reduces the barrier to 13.6−16.5 kcal/mol, depending on the number and configuration of the solvating waters. This agrees with the experimentally determined barrier of 16.5 ± 2.4 kcal/mol. We calculate a pK a for PyH 0 of 31 indicating that PT preceding ET is highly unfavorable. Moreover, we demonstrate that ET precedes PT in PyCOOH 0 formation, confirming PyH 0 's pK a as irrelevant for predicting PT from PyH 0 to CO 2 . Furthermore, we calculate adiabatic electron affinities in aqueous solvent for CO 2 , Py, and Py·CO 2 of 47.4, 37.9, and 66.3 kcal/mol respectively, indicating that the anionic complex PyCOO − stabilizes the anionic radicals CO 2 − and Py − to facilitate low barrier ET. As the reduction of CO 2 proceeds through ET and then PT, the pyridine ring becomes aromatic, and thus Py catalyzes CO 2 reduction by stabilizing the PCET TS and the PyCOOH 0 product through aromatic resonance stabilization. Our results suggest that Py catalyzes the homogeneous reductions of formic acid and formaldehyde en route to formation of CH 3 OH through a series of one-electron reductions analogous to the PCET reduction of CO 2 examined here, where the electrode only acts to reduce PyH + to PyH 0 .
We employ quantum chemical calculations to discover how frustrated Lewis pairs (FLP) catalyze the... more We employ quantum chemical calculations to discover how frustrated Lewis pairs (FLP) catalyze the reduction of CO 2 by ammonia borane (AB); specifically, we examine how the Lewis acid (LA) and Lewis base (LB) of an FLP activate CO 2 for reduction. We find that the LA (trichloroaluminum, AlCl 3 ) alone catalyzes hydride transfer (HT) to CO 2 while the LB (trimesitylenephosphine, PMes 3 ) actually hinders HT; inclusion of the LB increases the HT barrier by ∼8 kcal/mol relative to the reaction catalyzed by LAs only. The LB hinders HT by donating its lone pair to the LUMO of CO 2 , increasing the electron density on the C atom and thus lowering its hydride affinity. Although the LB hinders HT, it nonetheless plays a crucial role by stabilizing the active FLP·CO 2 complex relative to the LA dimer, free CO 2 , and free LB. This greatly increases the concentration of the reactive complex in the form FLP·CO 2 and thus increases the rate of reaction. We expect that the principles we describe will aid in understanding other catalytic CO 2 reductions.
Bottom-up approaches to nanofabrication are of great interest because they can enable structural ... more Bottom-up approaches to nanofabrication are of great interest because they can enable structural control while minimizing material waste and fabrication time. One new bottom-up nanofabrication method involves excitation of the surface plasmon resonance (SPR) of a Ag surface to drive deposition of sub-15 nm Au nanoparticles from MeAuPPh3. In this work we used density functional theory to investigate the role of the PPh3 ligands of the Au precursor and the effect of adsorbed solvent on the deposition process, and to elucidate the mechanism of Au nanoparticle deposition. In the absence of solvent, the calculated barrier to MeAuPPh3 dissociation on the bare surface is <20 kcal/mol, making it facile at room temperature. Once adsorbed on the surface, neighboring MeAu fragments undergo ethane elimination to produce Au adatoms that cluster into Au nanoparticles. However, if the sample is immersed in benzene, we predict that the monolayer of adsorbed solvent blocks the adsorption of MeAuP...
Uploads
Papers by Aaron Holder