Solar-energy-enabled photocatalysis is a sustainable process to destruct persistent environmental pollutants via the attack of photogenerated reactive species (e.g., holes, reactive oxygen species such as OH, O2− /HO2 , H2O2, and 1O2). Graphitic carbon nitride (g-C3N4) has emerged as a promising polymeric photocatalyst, and its highly tunable properties allow the material with an enhanced photocatalytic activity for environmental remediation. In this study, by taking advantage of simulation and experimental tools, we systematically evaluated the production of reactive species of undoped and carbon (C)-doped g-C3N4 samples, and identify the role of these species in contaminant oxidation under simulated visible sunlight irradiation (xenon lamp, λ > 400 nm). Both g-C3N4 samples produced negligible OH or triplet-excited states (3g-C3N4*), but the C-doped g-C3N4 sample generated more 1O2, O2− , and H2O2 compared to the undoped counterpart. Surprisingly, all these oxidative species did not contribute substantially for the degradation kinetics of contaminant phenol and atrazine, at least for the initial oxidation of the parent compounds; and the results otherwise highlighted the important role of the holes for contaminant transformation. The surface-mediated hole oxidation of the contaminants, including the adsorption of the contaminants on the photocatalyst surface (quantified by the binding free energy) and electron transfer kinetics from the contaminants to the excited photocatalysts (quantified by the concerted proton-coupled electron transfer (CPCET) rate) were investigated by molecular dynamics (MD) and density functional theory (DFT) simulations. The simulation results indicated that C-doping could favor the binding of atrazine but not of phenol on g-C3N4. The C-doping also significantly decreased the CPCET rate of phenol on g-C3N4 but could have little to no adverse impact for the CPCET rate of atrazine. These simulation and experimental results could explain the selective oxidation of phenol and atrazine on undoped and C-doped g-C3N4 in our previous study, and the results also highlight the dominant role of the holes for contaminant transformation that was largely overlooked before. This study generates mechanistic insights of photocatalytic oxidation, and will provide guidelines for the rational design of g-C3N4 as an effective visible-light-responsive photocatalyst for many applications, including contaminant degradation, chemical synthesis, and beyond.

Looking at the overlooked hole oxidation: Photocatalytic transformation of organic contaminants on graphitic carbon nitride under visible light irradiation

Singlet fission is a multiple exciton generation process that splits a singlet exciton (S0S1) into a correlated triplet pair (T1T1), affording a route to overcome the long-standing Shockley–Queisser thermodynamic limit for solar energy conversion. A new theory, based on multiconfiguration-constrained density functional theory and functional mode analysis, has been developed to model intermolecular singlet fission in organic photovoltaics. Specifically, constrained density functional theory is first employed to construct molecular orbitals for the six spin configurations comprising T1T1, the diabatic product state. In a subsequent step, linear response time-dependent density functional theory is utilized to formulate the S0S1 diabatic reactant state. Functional mode analysis is then applied to a thermalized ensemble of diabatic energy gaps to ascertain the reaction coordinate for the S0S1 → T1T1 transition. If singlet fission is assumed to follow a direct route, its rate may be evaluated using a modified J...

Functional Mode Singlet Fission Theory

https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1838&context=chem_facpub

Anisotropic Singlet Fission in Single Crystalline Hexacene.

Fast hole hopping in Ti17cat4, a 1 nm diameter molecular polyoxotitanate cluster bearing four catechol ligands (Ti17(μ4-O)4(μ3-O)16(μ2-O)4(cat)4(OPri)16), was investigated by ultrafast spectroscopy and quantum dynamics simulations. The catechol moieties coupled to the TiO2 core of the cluster give rise to a charge-transfer band, the excitation of which promotes an electron from the highest occupied molecular orbital of the ligand to the inorganic core, resulting in the formation of {cat+•,Ti3+}, a vibrationally hot polaronic exciton. Dynamic depolarization measurements indicate that within less than 100 fs the Franck–Condon polaronic state formed at the interface evolves into a fully charge-separated state and the injected electron delocalizes over the quasi conduction band of the cluster. The positive charge (hole) resulting from the injection does not remain static either. The initial hole hopping between the catechol sites occurs with the rate of ∼5 × 1011 s–1 or more and competes with the intramolecul...

Hot Hole Hopping in a Polyoxotitanate Cluster Terminated with Catechol Electron Donors

We have applied our functional mode framework for singlet fission to pentacene, a prototypical organic material for multiple exciton generation. It was found that singlet fission in pentacene occurs predominantly through a coherent process mediated by a virtual charge-transfer (CT) intermediate, which lies slightly above the photoexcited S1S0 state. This energetic near-degeneracy facilitates a substantial vibronic superposition, leading to a rapid transition rate of 25.1 ps–1. By contrast, the direct S1S0 → T1T1 path constitutes a much more sluggish route with a rate of 2.6 ps–1, largely due to the weak diabatic coupling between participant states. These data collectively afford an experimentally consistent rate of 27.7 ps–1 for the entire singlet fission process. The presence of this low-lying CT intermediate suggests that enhanced electronic coupling between S1S0 and T1T1 states may collude with coherent vibrational mixing to expedite the formation of triplet pairs. The knowledge gleaned from our invest...

Computer Simulation of Singlet Fission in Single Crystalline Pentacene by Functional Mode Vibronic Theory

Charge carriers that have been driven out of thermal equilibrium by their excessive vibrational energies are termed hot carriers. A theory has been developed to model the injection of these vibrationally excited electrons by explicitly accounting for the time-dependent thermal relaxation of the electron-transfer driving vibrational mode, as ascertained using functional mode analysis. Specifically, the thermal relaxation rate of the driving mode is first determined through the so-called frozen-phonon approach after which the energy-dependent line shape function is revisited to include memory effects for the vibrational quanta within the framework of Fermi’s golden rule. As shown by the numerical simulations of a 6-methyl-azulene-2-carboxylic acid dye molecule bound to an anatase TiO2[101] surface, our new theory not only yields persistently faster electron injection rates with higher incident photon energy but also exhibits a sharp increase when the vibrational quanta of the photoexcited dye molecule chang...

Functional Mode Hot Electron Transfer Theory

Strong electric fields can accelerate molecular dissociation reactions. The phenomenon known as the Wien effect was previously observed using high-voltage electrolysis cells that produced fields of about 107 V m-1, sufficient to accelerate the dissociation of weakly bound molecules (e.g., organics and weak electrolytes). The observation of the Wien effect for the common case of water dissociation (H2O [Formula: see text] H+ + OH-) has remained elusive. Here we study the dissociation of interfacial water adjacent to proton-permeable graphene electrodes and observe strong acceleration of the reaction in fields reaching above 108 V m-1. The use of graphene electrodes allows measuring the proton currents arising exclusively from the dissociation of interfacial water, while the electric field driving the reaction is monitored through the carrier density induced in graphene by the same field. The observed exponential increase in proton currents is in quantitative agreement with Onsager's theory. Our results also demonstrate that graphene electrodes can be valuable for the investigation of various interfacial phenomena involving proton transport. 

/pdf/wien-effect-in-interfacial-water-dissociation-through-proton-3ckqngyr.pdf

Jesse Y. Cai

Papers

Functional Mode Hot Electron Transfer Theory

Wien effect in interfacial water dissociation through proton-permeable graphene electrodes