Essential Roles of Proton Transfer in Photocatalytic Redox Reactions

How can heteroatoms boost the performance of photoactive nanomaterials for wastewater purification?

Photocatalysis, as an alternative for treating persistent water pollutants, holds immense promise. However, limitations hinder sustained treatment and recycling under varying light conditions. This comprehensive review delves into the novel paradigm of metal and non-metal doping to overcome these challenges. It begins by discussing the fundamental principles of photocatalysis and its inherent limitations. Understanding these constraints is crucial for developing effective strategies. Band gap narrowing by metal and non-metal doping modifies the band gap, enabling visible-light absorption. Impurity energy levels and oxygen vacancies influenced the doping energy levels and surface defects. Interfacial electron transfer and charge carrier recombination are the most important factors that impact overall efficiency. The comparative analysis of nanomaterials are reviewed on various, including nanometal oxides, nanocarbon materials, and advanced two-dimensional structures. The synthesis process are narratively presented, emphasizing production yields, selectivity, and efficiency. The review has potential applications in the environment for efficient pollutant removal and water purification, economic cost-effective and scalable production and technological advancement catalyst design, in spite of its challenges in material stability, synthesis methods and optimizing band gaps. The novelty of the review paper is on the proposal of a new paradigm of heterojunctions of doped metal and non-metal photocatalysts to promise highly efficient water treatment. This review bridges the gap between fundamental research and practical applications, offering insights into tailored nano photocatalysts.

Reduction of Superoxide Radical Intermediate by Polydopamine for Efficient Hydrogen Peroxide Photosynthesis

The synthesis of hydrogen peroxide through artificial photosynthesis is a green and promising technology with advantages in sustainability, economy and safety. However, superoxide radical (⋅O2 -), an important intermediate in photocatalytic oxygen reduction to H2O2 production, has strong oxidizing properties that potentially destabilize the catalyst. Therefore, avoiding the accumulation of ⋅O2 - for its rapid conversion to H2O2 is of paramount significance in improving catalyst stability and H2O2 yield. In this work, a strategy was developed to utilize protonated groups for the rapid depletion of converted ⋅O2 -, thereby the efficiency of photocatalytic synthesis of H2O2 from CN was successfully enhanced by 47-fold. The experimental findings demonstrated that polydopamine not only improved carrier separation efficiency, and more importantly, provided the adsorption reduction active site for ⋅O2 - for efficient H2O2 production. This work offers a versatile approach for synthesizing efficient and stable photocatalysts.

Transition from Sequential to Concerted Proton-Coupled Electron Transfer of Water Oxidation on Semiconductor Photoanodes

Accelerating proton transfer has been demonstrated as key to boosting water oxidation on semiconductor photoanodes. Herein, we study proton-coupled electron transfer (PCET) of water oxidation on five typical photoanodes [i.e., α-Fe2O3, BiVO4, TiO2, plasmonic Au/TiO2, and nickel-iron oxyhydroxide (Ni1-xFexOOH)-modified silicon (Si)] by combining the rate law analysis of H2O molecules with the H/D kinetic isotope effect (KIE) and operando spectroscopic studies. An unexpected and universal half-order kinetics is observed for the rate law analysis of H2O, referring to a sequential proton-electron transfer pathway, which is the rate-limiting factor that causes the sluggish water oxidation performance. Surface modification of the Ni1-xFexOOH electrocatalyst is observed to break this limitation and exhibits a normal first-order kinetics accompanied by much enhanced H/D KIE values, facilitating the turnover frequency of water oxidation by 1 order of magnitude. It is the first time that Ni1-xFexOOH is found to be a PCET modulator. The rate law analysis illustrates an effective strategy for modulating PCET kinetics of water oxidation on semiconductor surfaces.

Changing the reaction pathway in TiO2 photocatalytic dehalogenation of halogenated aromatic pollutants by surface hydroxyl regulation

Understanding the photocatalytic reductive dehalogenation mechanism of halogenated aromatic pollutants is of great research value. However, the proton source in the photocatalytic dehalogenation process of representative halogenated aromatic pollutants by TiO₂ is not clear. In this study, the TiO₂ surface was modified by hydrochloric acid, sodium hydroxide, and sodium fluoride to obtain TiO₂ samples with different hydroxyl groups. It was found that the hydroxyl groups on the surface of TiO₂ affects the sequence of proton and electron transfer in dehalogenation. The abundance of hydroxyl groups on the surface of TiO₂ can accelerate the reductive dehalogenation process of representative halogenated aromatic pollutants. The kinetic solvent isotope effect was used to study the proton-coupled electron transfer process in the reaction. It shows that the enriching of protons on TiO₂ bridging oxygen (bridging hydroxyl groups) is conducive to the rapid step of protonation of the reactant, and subsequent proton and electron transfer. On the contrary, the bridging hydroxyl groups can be removed by reacting with strongly basic sodium hydroxide and sodium ions can occupy the bridging oxygen. The substitution of bridging oxygen by fluorine ions can also lead to the destruction of bridge hydroxyl groups. Significantly, the absence of bridging hydroxyl groups on titanium dioxide will lead to the dehalogenation of representative halogenated aromatic pollutants initiated by electron transfer. This study is helpful to understand dehalogenation reaction paths catalyzed by TiO₂.

Solvent-Triggered Long-Range Proton Transport in 7-Hydroxyquinoline Using a Sulfonamide Transporter Group

The ability of long-range proton transport by substitution of 7-hydroxyquinoline at the eighth position with sulfonamide and sulfonylhydrazone rotor units to act as a crane-arm has been studied. Different proton transport pathways triggered by different stimuli have been established depending on the structure of the crane-arms. Solvent-driven proton switching from OH to the quinoline nitrogen (N_quin) site, facilitated by a sulfonamide transporter group in polar protic and aprotic solvents, has been confirmed by optical (absorption and fluorescence) and NMR spectroscopies as well as by single-crystal X-ray structure analysis. Photoinduced long-range proton transport to the N_quin site upon 340 nm UV light irradiation has been estimated in sulfonylhydrazone, which is not sensitive to solvent-driven switching. Both compounds have exhibited acid-triggered switching by trifluoroacetic acid due to the formation of a stable six-membered intramolecular hydrogen bonding interaction between the protonated N_quin and crane-arm. The structures of acid-switched form were confirmed by NMR spectroscopy and single-crystal X-ray structure analysis. The behavior of the compounds suggests a big step forward in the advanced proton pump-switching architecture because they cover three distinct driving forces in the switching process: solvent, light, and acid.

Fundamental Flaw in the Current Construction of the TiO2 Electron Transport Layer of Perovskite Solar Cells and Its Elimination

The top-performing perovskite solar cells (efficiency > 20%) generally rely on the use of a nanocrystal TiO₂ electron transport layer (ETL). However, the efficacies and stability of the current stereotypically prepared TiO₂ ETLs employing commercially available TiO₂ nanocrystal paste are far from their maximum values. As revealed herein, the long-hidden reason for this discrepancy is that acidic protons (∼0.11 wt %) always remain in TiO₂ ETLs after high-temperature sintering due to the decomposition of the organic proton solvent (mostly alcohol). These protons readily lead to the formation of Ti-H species upon light irradiation, which act to block the electron transfer at the perovskite/TiO₂ interface. Affront this challenge, we introduced a simple deprotonation protocol by adding a small amount of strong proton acceptors (sodium ethoxide or NaOH) into the common TiO₂ nanocrystal paste precursor and replicated the high-temperature sintering process, which wiped out nearly all protons in TiO₂ ETLs during the sintering process. The use of deprotonated TiO₂ ETLs not only promotes the PCE of both MAPbI₃-based and FA_0.85MA_0.15PbI_2.55Br_0.45-based devices over 20% but also significantly improves the long-term photostability of the target devices upon 1000 h of continuous operation.

Pseudo Jahn-Teller Origin of the Proton-transfer Energy Barrier in the Hydrogen-bonded [FHF]-System

The results of ab initio calculations of the adiabatic potential energy surfaces for the proton-bound [FHF]- system at different F-F distances have been rationalized in the framework of the vibronic theory. It is shown that the instability of the symmetric D∞h structure at increased F∙∙∙F distances and the proton displacement to one of the fluorine atoms is due to the pseudo Jahn–Teller mixing of the ground 1Σg electronic state with the lowest excited state of 1Σu symmetry through the asymmetric σu vibrational mode.

Proton-assisted electron transfer and hydrogen-atom diffusion in a model system for photocatalytic hydrogen production

Solar energy can be converted into chemical energy by photocatalytic water splitting to produce molecular hydrogen. Details of the photo-induced reaction mechanism occurring on the surface of a semiconductor are not fully understood, however. Herein, we employ a model photocatalytic system consisting of single atoms deposited on quantum dots that are anchored on to a primary photocatalyst to explore fundamental aspects of photolytic hydrogen generation. Single platinum atoms (Pt₁) are anchored onto carbon nitride quantum dots (CNQDs), which are loaded onto graphitic carbon nitride nanosheets (CNS), forming a Pt₁@CNQDs/CNS composite. Pt₁@CNQDs/CNS provides a well-defined photocatalytic system in which the electron and proton transfer processes that lead to the formation of hydrogen gas can be investigated. Results suggest that hydrogen bonding between hydrophilic surface groups of the CNQDs and interfacial water molecules facilitates both proton-assisted electron transfer and sorption/desorption pathways. Surface bound hydrogen atoms appear to diffuse from CNQDs surface sites to the deposited Pt₁ catalytic sites leading to higher hydrogen-atom fugacity surrounding each isolated Pt₁ site. We identify a pathway that allows for hydrogen-atom recombination into molecular hydrogen and eventually to hydrogen bubble evolution.

Crucial Effect of Ti-H Species Generated in the Visible-Light-Driven Transformations: Slowed-Down Proton-Coupled Electron Transfer

Despite the fact that proton-coupled electron transfer (PCET) has been hypothesized to play a pivotal role in the power conversion efficiency (PCE) of TiO₂-based solar-energy applications, the specific relationship between the intrinsic nature of visible-light (Vis)-driven PCET reactions and limited PCE gains has not yet been well revealed. Here we studied the detailed kinetics of reactions between various alcohols and radicals (^tBu₃ArO^•/TEMPO) on a TiO₂ photocatalyst under dye-sensitization Vis irradiation versus direct ultraviolet (UV) irradiation. We found that the rates of Vis-driven reactions were much slower than those of UV-driven reactions under identical light intensity. A similar phenomenon was observed under the off-line dark-reaction conditions in which TiO₂ was prereduced by alcohols. The rapid formation and difficult breakage of the stable "Ti-H" intermediate were proposed to account for the slowed-down PCET effect. This finding revealed an inherent bottleneck in Vis-driven energy conversion applications.

Theoretical study of kinetics of proton coupled electron transfer in photocatalysis

Photocatalysis induced by sunlight is one of the most promising approaches to environmental protection, solar energy conversion, and sustainable production of fuels. The computational modeling of photocatalysis is a rapidly expanding field that requires to adapt and to further develop the available theoretical tools. The coupled transfer of protons and electrons is an important reaction during photocatalysis. In this work, we present the first step of our methodology development in which we apply the existing kinetic theory of such coupled transfer to a model system, namely, methanol photodissociation on the rutile TiO₂(110) surface, with the help of high-level first-principles calculations. Moreover, we adapt the Stuchebrukhov-Hammes-Schiffer kinetic theory, where we use the Georgievskii-Stuchebrukhova vibronic coupling to calculate the rate constant of the proton coupled electron transfer reaction for a particular pathway. In particular, we propose a modified expression to calculate the rate constant, which enforces the near-resonance condition for the vibrational wave function during proton tunneling.

Theoretical Insights into Proton-Coupled Electron Transfer from a Photoreduced ZnO Nanocrystal to an Organic Radical

Proton-coupled electron transfer (PCET) at metal-oxide nanoparticle interfaces plays a critical role in many photocatalytic reactions and energy conversion processes. Recent experimental studies have shown that photoreduced ZnO nanocrystals react by PCET with organic hydrogen atom acceptors such as the nitroxyl radical TEMPO. Herein, the interfacial PCET rate constant is calculated in the framework of vibronically nonadiabatic PCET theory, which treats the electrons and transferring proton quantum mechanically. The input quantities to the PCET rate constant, including the electronic couplings, are calculated with density functional theory. The computed interfacial PCET rate constant is consistent with the experimentally measured value for this system, providing validation for this PCET theory. In this model, the electron transfers from the conduction band of the ZnO nanocrystal to TEMPO concertedly with proton transfer from a surface oxygen of the ZnO nanocrystal to the oxygen of TEMPO. Moreover, the proton tunneling at the interface is gated by the relatively low-frequency proton donor-acceptor motion between the TEMPO radical and the ZnO nanocrystal. The ZnO nanocrystal and TEMPO are found to contribute similar amounts to the inner-sphere reorganization energy, implicating structural reorganization at the nanocrystal surface. These fundamental mechanistic insights may guide the design of metal-oxide nanocatalysts for a wide range of energy conversion processes.

Facet-dependent solar ammonia synthesis of BiOCl nanosheets via a proton-assisted electron transfer pathway

Under the pressure of a fossil fuels shortage and global climate change, solar ammonia synthesis and the need to develop N2 fixation under mild conditions is becoming more urgent need; however, their intrinsic mechanisms still remain unclear. Herein, we demonstrate that the kinetic inertia of N2 can be overcome using oxygen vacancies (OVs) of BiOCl as the catalytic centers to create lower energy molecular steps, which are amendable for the solar light driven N-N triple bond cleavage via a proton-assisted electron transfer pathway. Moreover, the distinct structures of OVs on different BiOCl facets strongly determine the N2 fixation pathways by influencing both the adsorption structure and the activation level of N2. The fixation of terminal end-on bound N2 on the OVs of BiOCl {001} facets follows an asymmetric distal mode by selectively generating NH3, while the reduction of side-on bridging N2 on the OVs of BiOCl {010} facets is more energetically favorable in a symmetric alternating mode to produce N2H4 as the main intermediate.

The plausible role of carbonate in photo-catalytic water oxidation processes

DFT calculations point out that the photo-oxidation of water on GaN is energetically considerably facilitated by adsorbed carbonate.