Titanium and zinc oxide nanoparticles are proton-coupled electron transfer agents

A proton-coupled electron transfer process from functionalized carbon dots to molecular substrates: the role of pH

Multiple electron and proton transfers in nanomaterials pose significant demands and challenges across the various fields such as renewable energy, chemical processes, biological applications, and photophysics. In this context, pH-responsive functional group-enriched carbon dots (C-Dots) emerge as superior proton-coupled electron transfer (PCET) agents owing to the presence of multiple functional groups (-COOH, -NH2, and -OH) on the surface and redox-active sites in the core. Here, we elucidate the 2e-/2H+ transfer ability of carboxyl-enriched C-Dots (C-Dot-COOH) and amine-enriched C-Dots (C-Dot-NH2) with molecular 2e-/2H+ acceptor (benzoquinone, BQ) as a function of pKa, facilitated by the formation of new O-H bonds. The ground state and excited state pKa values of different functional groups on the surface of C-Dots are determined using steady-state absorbance and photoluminescence (PL) spectroscopy. The optical spectroscopy and electrochemical studies are employed to comprehend the influence of the surface and core of C-Dots on the proton and electron transfer processes as a function of pH. The cyclic voltammetry analysis reveals a standard Nernstian shift in E1/2 per pH unit of 30 mV, indicating that the functionalized C-Dots hold promise as candidates for the 2e-/2H+ transfer process. The calculated bond dissociation free energy (BDFE) of the electroactive O-H/N-H bonds provides a more nuanced and detailed understanding of PCET thermodynamic landscapes. These findings underscore the potential of nanoscale functionalized C-Dots for facilitating multiple PCET reactions in future energy technologies.

Phase-change VO2-based thermochromic smart windows

Thermochromic coatings hold promise in reducing building energy consumption by dynamically regulating the heat gain of windows, which are often regarded as less energy-efficient components, across different seasons. Vanadium dioxide (VO2) stands out as a versatile thermochromic material for smart windows owing to its reversible metal-to-insulator transition (MIT) alongside correlated structural and optical properties. In this review, we delve into recent advancements in the phase-change VO2-based thermochromic coatings for smart windows, spanning from the macroscopic crystal level to the microscopic structural level (including elemental doping and micro/nano-engineering), as well as advances in controllable fabrication. It is notable that hybridizing functional elements/materials (e.g., W, Mo/SiO2, TiN) with VO2 in delicate structural designs (e.g., core-shell, optical cavity) brings new degrees of freedom for controlling the thermochromic properties, including the MIT temperature, luminous transmittance, solar-energy modulation ability and building-relevant multi-functionality. Additionally, we provide an overview of alternative chromogenic materials that could potentially complement or surpass the intrinsic limitations of VO2. By examining the landscape of emerging materials, we aim to broaden the scope of possibilities for smart window technologies. We also offer insights into the current challenges and prospects of VO2-based thermochromic smart windows, presenting a roadmap for advancing this field towards enhanced energy efficiency and sustainable building design. In summary, this review innovatively categorizes doping strategies and corresponding effects of VO2, underscores their crucial NIR-energy modulation ability for smart windows, pioneers a theoretical analysis of inverse core-shell structures, prioritizes practical engineering strategies for solar modulation in VO2 films, and summarizes complementary chromogenic materials, thus ultimately advancing VO2-based smart window technologies with a fresh perspective.

CdS Quantum Dot Gels as a Direct Hydrogen Atom Transfer Photocatalyst for C-H Activation

Here, we report CdS quantum dot (QD) gels, a three-dimensional network of interconnected CdS QDs, as a new type of direct hydrogen atom transfer (d-HAT) photocatalyst for C-H activation. We discovered that the photoexcited CdS QD gel could generate various neutral radicals, including α-amido, heterocyclic, acyl, and benzylic radicals, from their corresponding stable molecular substrates, including amides, thio/ethers, aldehydes, and benzylic compounds. Its C-H activation ability imparts a broad substrate and reaction scope. The mechanistic study reveals that this reactivity is intrinsic to CdS materials, and the neutral radical generation did not proceed via the conventional sequential electron transfer and proton transfer pathway. Instead, the C-H bonds are activated by the photoexcited CdS QD gel via a d-HAT mechanism. This d-HAT mechanism is supported by the linear correlation between the logarithm of the C-H bond activation rate constant and the C-H bond dissociation energy (BDE) with a Brønsted slope α=0.5. Our findings expand the currently limited direct hydrogen atom transfer photocatalysis toolbox and provide new possibilities for photocatalytic C-H activation.

Selective Hydrogenation of Azobenzene to Hydrazobenzene via Proton-Coupled Electron Transfer from a Polyoxotungstate Cluster

In this report, we describe proton-coupled electron transfer (PCET) reactivity at the surface of the Keggin-type polyoxotungstate cluster [nBu4N]3[PWVI12O40] (PW12) in acetonitrile. Bond dissociation free energies (BDFEs) of the O-H groups generated upon reduction of PW12 in the presence of acid are determined through the construction of a potential-pKa diagram. The surface O-H bonds are found to be weak (BDFE(O-H)avg < 48 kcal mol-1), comparable to the BDFE of H2. This is consistent with the observed formation of H2 upon addition of a suitably strong organic acid, H2NPh2+ (pKa MeCN = 5.98), to the reduced form of the cluster. The one-electron reduced form of PW12 is isolated and used in conjunction with acid to realize the stoichiometric semihydrogenation of azobenzene via PCET from the surface of the reduced cluster.

Photoelectrochemical Proton-Coupled Electron Transfer of TiO2 Thin Films on Silicon

TiO2 thin films are often used as protective layers on semiconductors for applications in photovoltaics, molecule-semiconductor hybrid photoelectrodes, and more. Experiments reported here show that TiO2 thin films on silicon are electrochemically and photoelectrochemically reduced in buffered acetonitrile at potentials relevant to photoelectrocatalysis of CO2 reduction, N2 reduction, and H2 evolution. On both n-type Si and irradiated p-type Si, TiO2 reduction is proton-coupled with a 1e-:1H+ stoichiometry, as demonstrated by the Nernstian dependence of the Ti4+/3+ E1/2 on the buffer pKa. Experiments were conducted with and without illumination, and a photovoltage of ∼0.6 V was observed across 20 orders of magnitude in proton activity. The 4 nm films are almost stoichiometrically reduced under mild conditions. The reduced films catalytically transfer protons and electrons to hydrogen atom acceptors, based on cyclic voltammogram, bulk electrolysis, and other mechanistic evidence. TiO2/Si thus has the potential to photoelectrochemically generate high-energy H atom carriers. Characterization of the TiO2 films after reduction reveals restructuring with the formation of islands, rendering TiO2 films as a potentially poor choice as protecting films or catalyst supports under reducing and protic conditions. Overall, this work demonstrates that atomic layer deposition TiO2 films on silicon photoelectrodes undergo both chemical and morphological changes upon application of potentials only modestly negative of RHE in these media. While the results should serve as a cautionary tale for researchers aiming to immobilize molecular monolayers on "protective" metal oxides, the robust proton-coupled electron transfer reactivity of the films introduces opportunities for the photoelectrochemical generation of reactive charge-carrying mediators.

Unraveling a Concerted Proton-Coupled Electron Transfer Pathway in Atomically Precise Gold Nanoclusters

Metal nanoclusters (MNCs) represent a promising class of materials for catalytic carbon dioxide and proton reduction as well as dihydrogen oxidation. In such reactions, multiple proton-coupled electron transfer (PCET) processes are typically involved, and the current understanding of PCET mechanisms in MNCs has primarily focused on the sequential transfer mode. However, a concerted transfer pathway, i.e., concerted electron-proton transfer (CEPT), despite its potential for a higher catalytic rate and lower reaction barrier, still lacks comprehensive elucidation. Herein, we introduce an experimental paradigm to test the feasibility of the CEPT process in MNCs, by employing Au18(SR)14 (SR denotes thiolate ligand), Au22(SR)18, and Au25(SR)18- as model clusters. Detailed investigations indicate that the photoinduced PCET reactions in the designed system proceed via an CEPT pathway. Furthermore, the rate constants of gold nanoclusters (AuNCs) have been found to be correlated with both the size of the cluster and the flexibility of the Au-S framework. This newly identified PCET behavior in AuNCs is prominently different from that observed in semiconductor quantum dots and plasmonic metal nanoparticles. Our findings are of crucial importance for unveiling the catalytic mechanisms of quantum-confined metal nanomaterials and for the future rational design of more efficient catalysts.

Electrically driven proton transfer promotes Brønsted acid catalysis by orders of magnitude

Electric fields play a key role in enzymatic catalysis and can enhance reaction rates by 100,000-fold, but the same rate enhancements have yet to be achieved in thermochemical heterogeneous catalysis. In this work, we probe the influence of catalyst potential and interfacial electric fields on heterogeneous Brønsted acid catalysis. We observed that variations in applied potential of ~380 mV led to a 100,000-fold rate enhancement for 1-methylcyclopentanol dehydration, which was catalyzed by carbon-supported phosphotungstic acid. Mechanistic studies support a model in which the interfacial electrostatic potential drop drives quasi-equilibrated proton transfer to the adsorbed substrate prior to rate-limiting C-O bond cleavage. Large increases in rate with potential were also observed for the same reaction catalyzed by Ti/TiOyHx and for the Friedel Crafts acylation of anisole with acetic anhydride by carbon-supported phosphotungstic acid.

Heterometal Dopant Changes the Mechanism of Proton-Coupled Electron Transfer at the Polyoxovanadate-Alkoxide Surface

The transfer of two H-atom equivalents to the titanium-doped polyoxovanadate-alkoxide, [TiV5O6(OCH3)13], results in the formation of a V(III)-OH2 site at the surface of the assembly. Incorporation of the group (IV) metal ion results in a weakening of the O-H bonds of [TiV5O5(OH2)(OCH3)13] in comparison to its homometallic congener, [V6O6(OH2)(OCH3)12], resembling more closely the thermodynamics reported for the one-electron reduced derivative, [V6O6(OH2)(OCH3)12]1-. An analysis of early time points of the reaction of [TiV5O6(OCH3)13] and 5,10-dihydrophenazine reveals the formation of an oxidized substrate, suggesting that proton-coupled electron transfer proceeds via initial electron transfer from substrate to cluster prior to proton transfer. These results demonstrate the profound influence of heterometal dopants on the mechanism of PCET with respect to the surface of the assembly.

Direct Evidence for a Sequential Electron Transfer-Proton Transfer Mechanism in the PCET Reduction of a Metal Hydroxide Catalyst

The proton-coupled electron transfer (PCET) mechanism for the reaction Mox-OH + e- + H+ → Mred-OH2 was determined through the kinetic resolution of the independent electron transfer (ET) and proton transfer (PT) steps. The reaction of interest was triggered by visible light excitation of [RuII(tpy)(bpy')H2O]2+, RuII-OH2, where tpy is 2,2':6',2″-terpyridine and bpy' is 4,4'-diaminopropylsilatrane-2,2'-bipyridine, anchored to In2O3:Sn (ITO) thin films in aqueous solutions. Interfacial kinetics for the PCET reduction reaction were quantified by nanosecond transient absorption spectroscopy as a function of solution pH and applied potential. Data acquired at pH = 5-10 revealed a stepwise electron transfer-proton transfer (ET-PT) mechanism, while kinetic measurements made below pKa(RuIII-OH/OH2) = 1.3 were used to study the analogous interfacial reaction, where electron transfer was the only mechanistic step. Analysis of this data with a recently reported multichannel kinetic model was used to construct a PCET zone diagram and supported the assignment of an ET-PT mechanism at pH = 5-10. Ultimately, this study represents a unique example among Mox-OH/Mred-OH2 reactivity where the protonation and oxidation states of the intermediate were kinetically and spectrally resolved to firmly establish the PCET mechanism.

Elucidating protonation pathways in CO2 photoreduction using the kinetic isotope effect

The surge in anthropogenic CO2 emissions from fossil fuel dependence demands innovative solutions, such as artificial photosynthesis, to convert CO2 into value-added products. Unraveling the CO2 photoreduction mechanism at the molecular level is vital for developing high-performance photocatalysts. Here we show kinetic isotope effect evidence for the contested protonation pathway for CO2 photoreduction on TiO2 nanoparticles, which challenges the long-held assumption of electron-initiated activation. Employing isotopically labeled H2O/D2O and in-situ diffuse reflectance infrared Fourier transform spectroscopy, we observe H+/D+-protonated intermediates on TiO2 nanoparticles and capture their inverse decay kinetic isotope effect. Our findings significantly broaden our understanding of the CO2 uptake mechanism in semiconductor photocatalysts.

Surface ligand length influences kinetics of H-atom uptake in polyoxovanadate-alkoxide clusters

The uptake of hydrogen atoms (H-atoms) at reducible metal oxide nanocrystal surfaces has implications in catalysis and energy storage. However, it is often difficult to gain insight into the physicochemical factors that dictate the thermodynamics and kinetics of H-atom transfer to the surface of these assemblies. Recently, our research group has demonstrated the formation of oxygen-atom (O-atom) defects in polyoxovanadate-alkoxide (POV-alkoxide) clusters via conversion of surface oxido moieties to aquo ligands, which can be accomplished via addition of two H-atom equivalents. Here, we present the dependence of O-atom defect formation via H-atom transfer at the surface of vanadium oxide clusters on the length of surface alkoxide ligands. Analysis of H-atom transfer reactions to low-valent POV-alkoxide clusters [V6O7(OR)12]1- (R = Me, Et, nPr, nBu) reveals that the length of primary alkoxide surface ligands does not significantly influence the thermodynamics of these processes. However, surface ligand length has a significant impact on the kinetics of these PCET reactions. Indeed, the methoxide-bridged cluster, [V6O7(OMe)12]1- reacts ∼20 times faster than the other derivatives evaluated. Interestingly, as the aliphatic linkages are increased in size from -C2H5 to -C4H9, reaction rates remain consistent, suggesting restricted access to available ligand conformers as a result of the incompatibility of the aliphatic ligands and acetonitrile may buffer further changes to the rate of reaction.

Tandem Hydroperoxyl-Alkylperoxyl Radical Quenching by an Engineered Nanoporous Cerium Oxide Nanoparticle Macrostructure (NCeONP): Toward Efficient Solid-State Autoxidation Inhibitors

The use of nanomaterials as inhibitors of the autoxidation of organic materials is attracting tremendous interest in petrochemistry, food storage, and biomedical applications. Metal oxide materials and CeO2 in particular represent one of the most investigated inorganic materials with promising radical trapping and antioxidant abilities. However, despite the importance, examples of the CeO2 material's ability to retard the autoxidation of organic substrates are still lacking, together with a plausible chemical mechanism for radical trapping. Herein, we report the synthesis of a new CeO2-derived nanoporous material (NCeONP) with excellent autoxidation inhibiting properties due to its ability to catalyze the cross-dismutation of alkyl peroxyl (ROO•) and hydroperoxyl (HOO•) radicals, generated in the system by the addition of the pro-aromatic hydrocarbon γ-terpinene. The antioxidant ability of NCeONP is superior to that of other nanosized metal oxides, including TiO2, ZnO, ZrO2, and pristine CeO2 nanoparticles. Studies of the reaction with a sacrificial reductant allowed us to propose a mechanism of inhibition consisting of H atom transfer from HOO• to the metal oxides (MOx + HOO• → MOx-H• + O2), followed by the release of the H atom to an ROO• radical (MOx-H• + ROO• → MOx + ROOH). Besides identifying NCeONP as a promising material for developing effective antioxidants, our study provides the first evidence of a radical mechanism that can be exploited to develop novel solid-state autoxidation inhibitors.

Photocatalytic Production of Syngas from Biomass

ConspectusAs a renewable solar energy and carbon carrier, biomass exploration has received global attention. Photocatalytic valorization of biomass into fuels and chemicals is a promising and sustainable method for future chemical production. Photocatalysis has the potential to accomplish reactions under ambient conditions due to the unique reaction mechanisms involving photoinduced charge carriers and has recently been recognized as an efficient and feasible technology for biomass conversion. Biomass is widely used as sacrificial agent to scavenge holes in photocatalytic hydrogen evolution, and the carbon is eventually degraded to CO₂ with a minor amount of CO. The generation of CO instead of CO₂ is more economical and promising but also a challenge under photoreforming conditions.This is a new research direction, while until now there has still been the lack of a comprehensive review article to summarize and provide prospects for this topic. This Account will highlight our contributions in the research direction of the photocatalytic reforming of biomass into syngas (CO + H₂). In 2020, we first reported the photocatalytic conversion of biopolyols and sugars into syngas by employing a defect-rich Cu-TiO₂ nanorod photocatalyst and found that formic acid is a key intermediate to CO. Further study revealed that a facet-dependent electron-trapping state on anatase TiO₂ will affect the photocatalytic dehydration activity for formic acid intermediates by regulating the electron transfer process during the reaction, and the selective generation of FA or CO from photocatalytic biomass reforming was achieved via exposing the (100) or (101) facets, respectively. Visible light-driven syngas generation was further achieved over a CdS-based photocatalyst. Sulfate modification of CdS ([SO₄]/CdS) was constructed as the proton acceptor, thus efficiently facilitating the proton-coupled electron transfer process. Besides, we put forward an oxygen-controlled strategy to increase the CO generation rate without a significant decrease in CO selectivity via controlling the O₂/substrate ratio. Based on this system, a Z-scheme CdS@g-C₃N₄ core-shell structure and CdO-CdS semicoherent interface were created to facilitate charge transfer and enhance the O₂ activation, thus increasing the CO generation rate. Moreover, we also developed a photoelectrochemical approach to separately produce CO and H₂ from biomass. Nitrogen doping of a hexagonal WO₃ nanowire array was used to produce the photoanode. The built-in electric field generated via nitrogen doping promoted charge transfer, hence improving the efficiency of PEC reforming of biopolyols and sugars. This Account will systematically analyze the challenges in this research direction, the reaction route in the photocatalytic biomass reforming, and the factors affecting CO selectivity and give insight into the design of efficient photocatalytic systems.

Visible-Light-Induced Phenoxyl Radical-based Metal-Organic Framework for Selective Photooxidation of Sulfides

Phenoxyl radicals originating from phenols through oxidation or photoinduction are relatively stable and exhibit mild oxidative activity, which endows them with the potential for photocatalysis. Herein, a stable and recyclable metal-organic framework Zr-MOF-OH constructed of a binaphthol derivative ligand has been synthesized and functions as an efficient heterogeneous photocatalyst. Zr-MOF-OH shows fairly good catalytic activity and substrate compatibility toward the selective oxidation of sulfides to sulfoxides under visible light irradiation. Such irradiation of Zr-MOF-OH converts the phenolic hydroxyl groups of the binaphthol derivative ligand to phenoxyl radicals through excited state intramolecular proton transfer, and the excited state photocatalyst triggers the single-electron oxidation of the sulfide. No reactive oxygen species are produced in the photocatalytic process, and triplet O₂ directly participates in the reaction, endowing Zr-MOF-OH with wide substrate compatibility and high selectivity, which also proposes a promising pathway for the direct activation of substrates via phenoxyl radicals.

Photoactivation of titanium-oxo cluster [Ti6O6(OR)6(O2C t Bu)6]: mechanism, photoactivated structures, and onward reactivity with O2 to a peroxide complex

The molecular titanium-oxo cluster [Ti₆O₆(OⁱPr)₆(O₂C ^t Bu)₆] (1) can be photoactivated by UV light, resulting in a deeply coloured mixed valent (photoreduced) Ti (iii/iv) cluster, alongside alcohol and ketone (photooxidised) organic products. Mechanistic studies indicate that a two-electron (not free-radical) mechanism occurs in this process, which utilises the cluster structure to facilitate multielectron reactions. The photoreduced products [Ti₆O₆(OⁱPr)₄(O₂C ^t Bu)₆(sol)₂], sol = ⁱPrOH (2) or pyridine (3), can be isolated in good yield and are structurally characterized, each with two, uniquely arranged, antiferromagnetically coupled d-electrons. 2 and 3 undergo onward oxidation under air, with 3 cleanly transforming into peroxide complex, [Ti₆O₆(OⁱPr)₄(O₂C ^t Bu)₆(py)(O₂)] (5). 5 reacts with isopropanol to regenerate the initial cluster (1) completing a closed cycle, and suggesting opportunities for the deployment of these easily made and tuneable clusters for sustainable photocatalytic processes using air and light. The redox reactivity described here is only possible in a cluster with multiple Ti sites, which can perform multi-electron processes and can adjust its shape to accommodate changes in electron density.

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.

Photocatalytic Oxo-Amination of Aryl Cyclopropanes through an Unusual SN2-Like Ring-Opening Pathway: Won >99% ee

One-pot oxo-amination of unactivated cyclopropanes with safe, green dioxygen as an oxidant and low-cost amines as nitrogen sources has generated interest since this can directly result in uniform β-located difunctional units. Formation of the three-electron cation radical followed by the nucleophilic attack of amines to open the strained ring of cyclopropanes catalyzed by classic noble-complex photocatalysts was a promising strategy. However, this ring-opening pathway could not maintain the entire second-order nucleophilic substitution (S_N2) conversion, which generally led to unsatisfactory enantioselectivity (enantiomeric excess (ee) value ∼60%). Here, we demonstrate that for such a one-step oxo-amination of cyclopropanes with benign dioxygen and pyrazoles, a highly uniform inversion of configuration could be first accomplished through a TiO₂ photocatalyst. This strategy features low-cost, semiheterogeneous photocatalysis and environmentally friendly reaction conditions, without using any sacrificial reagent or additive. Importantly, our protocol not only provides a relatively broad substrate scope tolerant to a certain range of substituted cyclopropanes and pyrazoles, resulting in various β-amino ketone products (∼50 examples) with excellent conversions and yields, but also retains excellent enantioselectivity (ee value ∼99%). A concerted S_N2 ring opening raised from an oxetane cation intermediate rather than a conventional three-electron cation radical prior to attaching to dioxygen was proposed.

Connecting Thermodynamics and Kinetics of Proton Coupled Electron Transfer at Polyoxovanadate Surfaces Using the Marcus Cross Relation

Here, we evaluate the efficacy of multiple methods for elucidating the average bond dissociation free energy (BDFE) of two surface hydroxide moieties in a reduced polyoxovanadate cluster, [V₆O₁₁(OH)₂(TRIOL^NO2)₂]^-2. Through cyclic voltammetry, individual thermochemical parameters describing proton coupled electron transfer (PCET) are obtained, without the need for synthetic isolation of intermediates. Further, we demonstrate that a method involving a series of open circuit potential measurements with varying ratios of reduced to oxidized clusters is most attractive for the direct measurement of BDFE(O-H) for polyoxovanadate clusters as this approach also determines the stoichiometry of PCET. We subsequently connect the driving force of PCET to the rate constant for the transfer of hydrogen atoms to a series of organic substrates through the Marcus cross relation. We show that this method is applicable for the prediction of reaction rates for multielectron/multiproton transfer reactions, extending the findings from previous work focused on single electron/proton reactions.

Photoelectron Storages in Functionalized Carbon Nitrides for Colorimetric Sensing of Oxygen

Colorimetric sensors have been widely used for centuries across diverse fields, thanks to their easy operation and uncompromisingly high sensitivity with no need for electricity. However, it is still a great challenge for conventional chromogenic systems to perform multiple measurements meanwhile maintaining high robustness. Here, we reported that carbon nitrides (CNs), the raw materials that are abundant, structure-tunable, and stable semiconductors with photoelectron storage capability, can be developed as a chromogenic system for colorimetric sensors. Beyond conventional metal oxides that only demonstrated a single blue-color switch after photoelectron storage, CN exhibited a multicolor switch under identical conditions owing to the unusual multiple photoelectron storage pathways. Mechanism studies revealed cyano and carbonyl groups in CN crucially elongated the centroid distance of electrons/holes, which exclusively stabilized the specific excited states that have different light absorption; meanwhile, the counter cations strengthened these processes. As a result, O₂, a proof-of-concept analyte, was quantitatively detected by the CN-derived colorimetric sensor, showing high reversibility in hundreds of cycles and adaptable sensitivity/detection range, outperforming most reported and commercial oxygen sensors. These intriguing features of CN are highly envisioned for the next generation of colorimetric sensors, especially in developing countries or fieldworks, to improve the detection reliability and lower the sensing cost.

Photochromism of colloidal ZnO nanocrystal powders under ambient conditions

Zinc oxide (ZnO) nanocrystals (NCs) exhibit photochromic reactions under specific conditions upon ultraviolet light irradiation. Since the color is originated from the excited electrons at the conduction band of ZnO NCs, the photoinduced absorption is observed only in the solution with hole acceptors under inert conditions. ZnO is earth-abundant and less toxic than many other substances, and has been widely used in various industrial fields. If the photochromic reaction of ZnO can be observed consistently under ambient conditions, the material may pave the way for large-scale photochromic applications such as in pigments, windows, and building materials in addition to conventional photochromic applications. In this study, we synthesize hydrophilic ZnO NCs and observe the solid-state photochromic reactions in the visible to mid-infrared regions even in humid-air conditions. We reveal that the coloration of powders of ZnO NCs under ambient conditions originates mainly from two factors: (1) charge separation induced by hole trapping by water molecules adsorbed on the surface of NCs, and (2) deceleration of the reactions involving the electrons in the conduction band of ZnO NCs with molecular oxygen and the adsorbed water molecules.

High Photoreactivity on a Reconstructed Anatase TiO2(001) Surface Predicted by Ab Initio Nonadiabatic Molecular Dynamics

Anatase TiO₂(001) surface with (4 × 1) reconstruction is proposed to be a highly active catalytic surface. In this work, using time-domain ab initio nonadiabatic molecular dynamics, we reveal that the ridge structure formed by anatase(001) surface reconstruction is the photoreactive site for hole migration and trapping. Moreover, the ridge structure is destroyed by low-coverage CH₃OH adsorption, leading to the suppression of its high photoreactivity. However, when the CH₃OH coverage is increased and intermolecular hydrogen bonds (H-bonds) form, the ridge structure and its high photoreactivity are restored. Furthermore, the hole trapping dynamics is strongly coherent with intermolecular proton transfer in structures with intermolecular H-bonds. Our study proves that anatase TiO₂(001)-(4 × 1) is a highly photoreactive surface where the ridge is the photoreactive site for hole trapping, which is coherent with the proton transfer process.

Continuous flow scale-up of biofunctionalized defective ZnO quantum dots: A safer inorganic ingredient for skin UV protection

The versatility of ZnO quantum dots (QDs) exhibiting size-tunable visible photoluminescence has propelled them to the forefront of leading-edge innovations in healthcare. At the nano-bio interface, enhancing the singly-ionized oxygen vacancy defects (V_O^•) through holistic, sustainable synthesis protocols driven by the synergistic influence of QDs' nucleation-growth kinetics has implications on their bioactivity, physiochemical, and optical performance. Recently, robust continuous flow platforms have transcended the conventional batch reactors by alleviating the concerns of "hot-spot" formation due to inhomogeneous heat distribution, acute energy consumption, poor quality, and yield. However, complexities exist in translating batch chemistries into flow processes. Here, a unique, rationally designed continuous flow synthesis of luminescent defect-engineered ZnO QDs (E-QDs) via helical-reactor assembly that can adequately synthesize on a large scale is reported. The crux of this lies in the amalgamation of "green chemistry" and flow synthesis, which results in Lamer-mechanism mediated monodispersed E-QDs demonstrating high photoluminescence quantum yield (PLQY) of 89% under an accurately regulated synthesis environment. Process intensification corroborated that the bio-stable E-QDs manifested admirable photostability, broad-spectrum UV-shielding (400-250 nm), colloidal stability, in vitro biocompatibility against L929 and HaCaT cells, and antioxidant activity. These attributes were better compared to the commercial ZnO nanoparticles (ZnO_C-NPs) used for skin UV protection. Delving deeper, the main drivers for the high density of intrinsic V_O^• formation (I_v/I_o∼42.5) were revealed to be the reactor's hydrodynamic performance and the improvised heating rate (2.5°C/sec). Hence, these E-QDs have potential as a new, safe, and economical multifunctional active ingredient for skin UV protection and antioxidants for treating ROS-mediated disorders. STATEMENT OF SIGNIFICANCE: UV filters exhibiting questionable UV-attenuation efficacy and phototoxicity are significant impediments to the healthcare industry emphasizing skin cancer prevention. Although least explored, V_O^•-governed aberrant photoactive, biological, and surface-reactive qualities of engineered ZnO QDs (E-QDs) have created ample room to investigate these hallmarks for skin UV protection. However, the bottlenecks in stereotypical ZnO QDs production confined by inefficient process control are annihilated by continuous flow strategies. Herein, the high-throughput continuous flow helical reactor assembly was designed and fabricated to successfully showcase optimized transport properties, reproducibility, yield, and quality E-QDs. Anticipating a skyrocketing demand for E-QDs as bioactive-sunscreen components, the comprehensive investigation has demonstrated unprecedented biofunctionality and ROS-scavenging behaviour, even upon UVR exposure, contrary to the traditional nanoparticulate ZnO UV filters.

Oxygen-Atom Defect Formation in Polyoxovanadate Clusters via Proton-Coupled Electron Transfer

The uptake of hydrogen atoms (H-atoms) into reducible metal oxides has implications in catalysis and energy storage. However, outside of computational modeling, it is difficult to obtain insight into the physicochemical factors that govern H-atom uptake at the atomic level. Here, we describe oxygen-atom vacancy formation in a series of hexavanadate assemblies via proton-coupled electron transfer, presenting a novel pathway for the formation of defect sites at the surface of redox-active metal oxides. Kinetic investigations reveal that H-atom transfer to the metal oxide surface occurs through concerted proton-electron transfer, resulting in the formation of a transient VIII-OH2 moiety that, upon displacement of the water ligand with an acetonitrile molecule, forms the oxygen-deficient polyoxovanadate-alkoxide cluster. Oxidation state distribution of the cluster core dictates the affinity of surface oxido ligands for H-atoms, mirroring the behavior of reducible metal oxide nanocrystals. Ultimately, atomistic insights from this work provide new design criteria for predictive proton-coupled electron-transfer reactivity of terminal M═O moieties at the surface of nanoscopic metal oxides.

Direct decarboxylative Giese reactions

The quest to find milder and more sustainable methods to generate highly reactive, carbon-centred intermediates has led to a resurgence of interest in radical chemistry. In particular, carboxylic acids are seen as attractive radical precursors due their availability, low cost, diversity, and sustainability. Moreover, the corresponding nucleophilic carbon-radical can be easily accessed through a favourable radical decarboxylation process, extruding CO₂ as a traceless by-product. This review summarizes the recent progress on using carboxylic acids directly as convenient radical precursors for the formation of carbon-carbon bonds via the 1,4-radical conjugate addition (Giese) reaction.

Quantitative U=O bond activation in uranyl complexes via silyl radical transfer

Reductive silylation of the uranyl dication with 1,4-bis(trimethylsilyl)dihydropyrazine, or “Mashima’s Reagent”, is detailed. The substrate simultaneously delivers silylium ions and electrons to multiple uranyl complexes (e.g. pyridine dipyrrolide uranyl complex...

Aqueous TiO2 Nanoparticles React by Proton-Coupled Electron Transfer

Redox reactions of aqueous colloidal TiO₂ 4 nm nanoparticles (NPs) have been examined, including both citrate-capped and uncapped NPs (c-TiO₂ and uc-TiO₂). Photoreduction gave stable blue colloidal c-TiO₂^R NPs with 10-60 electrons per particle. Equilibration of these reduced NPs with soluble redox reagents such as methylviologen (MV²⁺) provided measurements of the colloid reduction potential as a function of pH. The potentials of c-TiO₂ from pH 2-9 varied linearly with pH, with a slope of -60 ± 5 mV/pH. Estimates of the potential at pH 12 were consistent with extrapolating that line to high pH. The reduction potentials did not correlate with the zeta potentials (ζ) or the surface charge of the NPs across this pH range. Similar reduction potentials were observed for c- and uc-TiO₂ at low pH even though they have quite different ζ potentials. These results show that the common surface-charging explanation of the pH dependence is not tenable in these systems. Oxidation of reduced c-TiO₂^R with the electron-transfer oxidant potassium triiodide (KI₃) occurred with a significant drop in pH, showing that protons were released when the electrons were removed from the NPs. Smaller pH drops were observed for the proton-coupled electron transfer (PCET) reagents O₂ (air) and 4-MeO-TEMPO (4-methoxy-2,2,6,6-tetramethylpiperine-1-oxy radical). The difference in the number of protons released with KI₃ vs O₂ and 4-MeO-TEMPO was roughly one proton per electron removed. Thus, the thermodynamically preferred reactivity of these colloidal TiO₂ NPs is PCET over the pH 2-13 range studied. The measured redox potentials refer to the chemical process TiO₂ + H⁺ + e^- → TiO₂·e^-,H⁺; and therefore they do not correspond with an electronic energy such as a conduction band edge or flat band potential. The 1e^-/1H⁺ stoichiometry means that the TiO₂ reduction potentials correspond to a TiO₂-H bond dissociation free energy (BDFE), determined to be 49 ± 2 kcal mol^-1. The PCET description is consistent with the pH dependence of E(TiO₂/TiO₂·e^-,H⁺), the release of protons upon oxidation, the lack of correlation with ζ potentials, the similarity of capped and uncapped NPs, and the small change in the potential and BDFE from the first to the last electron/proton pair (H atom) removed. This behavior is suggested to be the norm for redox-active oxide/water interfaces.

Multisite PCET with photocharged carbon nitride in dark

A combination of photochemistry and proton coupled electron transfer (PCET) is a primary strategy employed by biochemical systems and synthetic chemistry to enable uphill reactions under mild conditions. Degenerate nanometer-sized n-type semiconductor nanoparticles (SCNPs) with the Fermi level above the bottom of the conduction band are strongly reducing and act more like metals than semiconductors. Application of the degenerate SCNPs is limited to few examples. Herein, we load microporous potassium poly(heptazine imide) (K-PHI) nanoparticles with electrons (e‒) and charge balancing protons (H+) in an illumination phase using sacrificial agents. e‒/H+ in the K-PHI nanoparticles are weakly bound and therefore could be used in a range of PCET reactions in dark, such as generation of aryl radicals from aryl halides, ketyl radicals from ketones, and 6e‒/6H+ reduction of nitrobenzene to aniline. The integration of several features that until now were intrinsic for plants and natural photosynthesis into a transition metal free nanomaterial composed of abundant elements (C, N, and K) offers a powerful tool for synthetic organic chemistry.

Multi-charge transfer from photodoped ITO nanocrystals

Metal oxide nanocrystals are emerging as an extremely versatile material for addressing many of the current challenging demands of energy-conversion technology. Being able to exploit their full potential is not only an advantage but also a scientific and economic ambition for a more sustainable energy development. In this direction, the photodoping of metal oxide nanocrystals is a very notable process that allows accumulating multiple charge carriers per nanocrystal after light absorption. The reactivity of the photodoped electrons is currently the subject of an intense study. In this context, the possibility to extract efficiently the stored electrons could be beneficial for numerous processes, from photoconversion and sunlight energy storage to photocatalysis and photoelectrochemistry. In this work we provide, via oxidative titration and optical spectroscopy, evidence for multi-electron transfer processes from photodoped Sn : In₂O₃ nanocrystals to a widely employed organic electron acceptor (F4TCNQ). The results of this study disclose the potential of photodoped electrons to drive chemical reactions involving more than one electron.

Highly efficient visible light active Cu-ZnO/S-g-C3N4 nanocomposites for efficient photocatalytic degradation of organic pollutants

The photocatalytic activity of photocatalysts is severely hampered by limited visible light harvesting and unwanted fast recombination of photogenerated e^- and h⁺. In the current study, the photocatalytic efficiency of Cu-ZnO/S-g-C₃N₄ (CZS) nanocomposites was investigated against MB dye. The composite materials were designed via chemical co-precipitation method and characterised by important analytical techniques. Distinctive heterojunctions developed between S-g-C₃N₄ and Cu-ZnO in the CZS composite were revealed by TEM. The synthesized composites exhibit a huge number of active sites, a large surface area, a smaller size and better visible light absorption. The considerable enhancement in the photocatalytic activity of CZS nanocomposites might be accredited to the decay in the e-h pair recombination rate and a red shift in the visible region, as observed by PL and optical analysis, respectively. Furthermore, the metal (Cu) doping into the S-g-C₃N₄/ZnO matrix created exemplary interfaces between ZnO and S-g-C₃N₄, and maximized the photocatalytic activity of CZS nanocomposites. In particular, CZS nanocomposites synthesized by integrating 25% S-g-C₃N₄ with 4% Cu-ZnO (CZS-25 NCs) exhibited the 100% photocatalytic degradation of MB in 60 minutes under sunlight irradiation. After six cycles, the photocatalytic stability of CZS-25 NCs was excellent. Likewise, a plausible MB degradation mechanism is proposed over CZS-25 NCs based on photoluminescence and reactive species scavenger test observation. The current research supports the design of novel composites for the photocatalytic disintegration of organic contaminants.

Photochemical reduction of nanocrystalline maghemite to magnetite

We present a method for thephotochemical conversion of the inverse spinel iron oxides in which the mixed-valent magnetite phase (Fe₃O₄) is accessed from the maghemite phase (γ-Fe₂O₃) via a stable, colloidal nanocrystal-to-nanocrystal transformation. Anaerobic UV-irradiation of colloidal γ-Fe₂O₃ nanocrystals in the presence of ethanol as a sacrificial reductant yields reduction of some Fe³⁺ to Fe²⁺, resulting in a topotactic reduction of γ-Fe₂O₃ to Fe₃O₄. This reduction is evidenced by the emergence of charge-transfer absorption and increased d-spacing in UV-irradiated nanocrystals. Redox titrations reveal that ∼43% of Fe in <d> = 4.8 nm nanocrystals can be reduced with this method and comparison of optical data indicates similar reduction levels in <d> = 7.3 and 9.0 nm nanocrystals. Addition of excess acetaldehyde during photoreduction shows that the extent of reduction is likely pinned by the hydrogenation of acetaldehyde back to ethanol and can be increased with the use of an alkylborohydride sacrificial reductant. Photochemical reduction is accompanied by increased magnetization and emergence of magnetic features characteristic of Fe₃O₄. Overall, this work provides a reversible, post-synthetic strategy to obtain Fe₃O₄ nanocrystals with well-controlled Fe²⁺ compositions.

Modelling local structural and electronic consequences of proton and hydrogen-atom uptake in VO2 with polyoxovanadate clusters

We report the synthesis and characterisation of a series of siloxide-functionalised polyoxovanadate-alkoxide (POV-alkoxide) clusters, [V6O6(OSiMe3)(OMe)12] n (n = 1-, 2-), that serve as molecular models for proton and hydrogen-atom uptake in vanadium dioxide, respectively. Installation of a siloxide moiety on the surface of the Lindqvist core was accomplished via addition of trimethylsilyl trifluoromethylsulfonate to the fully-oxygenated cluster [V6O7(OMe)12]2-. Characterisation of [V6O6(OSiMe3)(OMe)12]1- by X-ray photoelectron spectroscopy reveals that the incorporation of the siloxide group does not result in charge separation within the hexavanadate assembly, an observation that contrasts directly with the behavior of clusters bearing substitutional dopants. The reduced assembly, [V6O6(OSiMe3)(OMe)12]2-, provides an isoelectronic model for H-doped VO2, with a vanadium(iii) ion embedded within the cluster core. Notably, structural analysis of [V6O6(OSiMe3)(OMe)12]2- reveals bond perturbations at the siloxide-functionalised vanadium centre that resemble those invoked upon H-atom uptake in VO2 through ab initio calculations. Our results offer atomically precise insight into the local structural and electronic consequences of the installation of hydrogen-atom-like dopants in VO2, and challenge current perspectives of the operative mechanism of electron-proton co-doping in these materials.

Concerted Multiproton-Multielectron Transfer for the Reduction of O2 to H2O with a Polyoxovanadate Cluster

The concerted transfer of protons and electrons enables the activation of small-molecule substrates by bypassing energetically costly intermediates. Here, we present the synthesis and characterization of several hydrogenated forms of an organofunctionalized vanadium oxide assembly, [V₆O₁₃(TRIOL^NO₂)₂]^2-, and their ability to facilitate the concerted transfer of protons and electrons to O₂. Electrochemical analysis reveals that the fully reduced cluster is capable of mediating 2e^-/2H⁺ transfer reactions from surface hydroxide ligands, with an average bond dissociation free energy (BDFE) of 61.6 kcal/mol. Complementary stoichiometric experiments with hydrogen-atom-accepting reagents of established bond strengths confirm that the electrochemically established BDFE predicts the 2H⁺/2e^- transfer reactivity of the assembly. Finally, the reactivity of the reduced polyoxovanadate toward O₂ reduction is summarized; our results indicate a stepwise reduction of the substrate, proceeding through H₂O₂ en route to the formation of H₂O. Kinetic isotope effect experiments confirm the participation of hydrogen transfer in the rate-determining step of both the reduction of O₂ and H₂O₂. This work constitutes the first example of hydrogen atom transfer for small-molecule activation with reduced polyoxometalates, where both electron and proton originate from the cluster.

Covalent Organic Frameworks: New Materials Platform for Photocatalytic Degradation of Aqueous Pollutants

Covalent organic frameworks (COFs) are highly porous and crystalline polymeric materials, constructed by covalent bonds and extending in two or threedimensions. After the discovery of the first COF materials in 2005 by Yaghi et al., COFs have experienced exciting progress and exhibitedtheirpromising potential applications invarious fields, such as gas adsorption and separation, energy storage, optoelectronics, sensing and catalysis. Because of their tunablestructures, abundant, regular and customizable pores in addition to large specific surface area, COFs can harvest ultraviolet, visible and near-infrared photons, adsorb a large amount of substrates in internal structures and initiate surface redox reactions to act as effective organic photocatalysts for water splitting, CO₂ reduction, organic transformations and pollutant degradation. In this review, we will discuss COF photocatalysts for the degradation of aqueous pollutants. The state-of-the-art paragon examples in this research area will be discussed according to the different structural type of COF photocatalysts. The degradation mechanism will be emphasized. Furthermore, the future development direction, challenges required to be overcome and the perspective in this field will be summarized in the conclusion.

Applications of electron spin resonance spectroscopy in photoinduced nanomaterial charge separation and reactive oxygen species generation

Nano-metals, nano-metal oxides, and carbon-based nanomaterials exhibit superior solar-to-chemical/photo-electron transfer properties and are potential candidates for environmental remediations and energy transfer. Recent research effort focuses on enhancing the efficiency of photoinduced electron-hole separation to improve energy transfer in catalytic reactions. Electron spin resonance (ESR) spectroscopy has been used to monitor the generation of electron/hole and reactive oxygen species (ROS) during nanomaterial-mediated photocatalysis. Using ESR coupled with spin trapping and spin labeling techniques, the underlying photocatalytic mechanism involved in the nanomaterial-mediated photocatalysis was investigated. In this review, we briefly introduced ESR principle and summarized recent advancements using ESR spectroscopy to characterize electron-hole separation and ROS production by different types of nanomaterials.

Origin of the Anomalous Temperature Dependence of the Photochromic Reaction of Cu-Doped ZnS Nanocrystals

The temperature dependence of the color fading process of thermally reversible photochromic reactions is one of the most important challenges for their industrial applications. Generally, photochromic reactions of organic molecules have a strong temperature dependence due to the occurrence of large conformational changes during the reactions. In contrast, we recently reported that the photochromic reaction of Cu-doped ZnS nanocrystals (NCs) exhibits a very small temperature dependence around room temperature. However, the mechanism underlying this phenomenon has not been clarified yet. Here, we reveal that the anomalous temperature dependence of Cu-doped ZnS NCs originates from the balance between the temperature dependence of the charge recombination and that of the adsorption/desorption of water molecules on the surface of the NCs, which act as hole acceptors. Exploring temperature-insensitive photochromic reactions is important not only for gaining fundamental insight into nanomaterials but also for developing novel photochromic materials for outdoor applications.

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.

A titanium(iii) phosphite exhibits polymorph-distinct redox activity involving proton-coupled electron transfer

A novel titanium(iii) phosphite with intriguing polymorphism and solid-state proton-coupled electron transfer (PCET) oxidation is presented. The polymorphs show structure-dependent PCET reactivity, interpretable by proton distribution in channels. Combined with subsequent photoreduction, the redox cycle initiated with Ti^III can produce H₂ and transform organics.

Performance and mechanism of nickel hydroxide catalyzed reduction of N-nitrosodimethylamine by iron

Since iron (Fe) was first proven to have a strong reduction ability, it has been successfully applied to remove pollutants from water. In this study, nickel hydroxide (Ni(OH)₂), a catalyst commonly used in hydrogen evolution reactions, was added to improve the activity of Fe to remove N-nitrosodimethylamine (NDMA). The results showed that with the increasing Ni(OH)₂ dosages, the reactions accelerated. The NDMA removal rates increased when the pH value was 6 or 7. Further, when the dissolved oxygen concentration was in the range of 0-12.0 mg∙L^-1, it had little effect on the Fe/Ni(OH)₂ system, and all the reactions obeyed pseudo-first-order kinetics. 1,1-dimethylhydrazine and dimethylamine were formed during NDMA degradation. The capture of active substances and electron spin resonance method confirmed that the main active species were active hydrogen atoms, which participated in the removal of NDMA. Ni(OH)₂ acting as a catalyst was confirmed using wide-angle X-ray diffraction, X-ray photoelectron spectroscopy and Ni²⁺ dissolution. Further, catalytic hydrogenation was proposed as the main removal mechanism as Ni(OH)₂ promotes the corrosion of Fe and dissociation of water, thereby generating more active hydrogen atoms. In addition, Ni(OH)₂ may activate both Fe and NDMA. This technique could be employed as an alternative for NDMA reduction and expand the application field of Ni(OH)₂.