Plasmon-Enhanced Sub-Bandgap Photocatalysis via Triplet-Triplet Annihilation Upconversion for Volatile Organic Compound Degradation

Photonic crystal-assisted sub-bandgap photocatalysis via triplet-triplet annihilation upconversion for the degradation of environmental organic pollutants

This study explores novel approaches to enhance photocatalysis efficiency by introducing a photonic crystal (PC)-enhanced, multi-layered sub-bandgap photocatalytic reactor. The design aims to effectively utilize sub-bandgap photons that might otherwise go unused. The device consists of three types of layers: (1) two polymeric triplet-triplet annihilation upconversion (TTA-UC) layers converting low-energy green photons (λEx = 532 nm, 2.33 eV) to high-energy blue photons (λEm = 425 nm, 2.92 eV), (2) a platinum-decorated WO3 layer (Eg = 2.8 eV) serving as a visible-light photocatalyst, and (3) a PC layer optimizing both TTA-UC and photocatalysis. The integration of the PC layer resulted in a 1.9-fold increase in UC emission and a 7.9-fold enhancement in hydroxyl radical (•OH) generation, achieved under low-intensity sub-bandgap irradiation (17.6 mW cm-2). Consequently, the combined layered structure of TTA/Pt-WO3/TTA/PC achieved a remarkable 38.8-fold improvement in •OH production, leading to outstanding degradation capability for various organic pollutants (e.g., 4-chlorophenol, bisphenol A, and methylene blue). This multi-layered sub-bandgap photocatalytic structure, which uniquely combines TTA-UC and PC layers, offers valuable insights into designing efficient photocatalytic systems for future solar-driven environmental remediation.

Time-resolved spectroscopic investigation for the practical application of a photocatalytic air purifier

The photocatalytic efficiency for removing volatile organic compounds (VOCs) is significantly influenced by operational parameters like humidity and flow velocity, exhibiting notable and inconsistent fluctuations in both lab-scale and large-scale demonstrations. In this study, operando spectroscopy and isotope analysis were employed to investigate the correlation between humidity levels and degradation of gaseous acetaldehyde using TiO2 photocatalysts, aiming to demonstrate the scaling-up of photocatalytic air purifier. It was observed that rate constants for the mineralization of acetaldehyde rapidly decreased by 30% as relative humidity increased from 25% to 80% in the flow system (with an air velocity, v = 0.78 m/s). However, batch system showed smaller change with only a 10% reduction of the rate constant. Humidity fluctuations were more pronounced under high-speed conditions and were amplified in air purifier (v = 3.8 m/s). Time-resolved operando spectroscopy using an 13C isotope of acetaldehyde revealed that humidity's distinct role in dark adsorption and photocatalytic reactions. Water was found to inhibit the formation of crotonaldehyde during aldol condensation reaction in dark condition. Moreover, water suppressed photocatalytic mineralization by inhibiting acetate oxidation to formate. These findings provide valuable insights for improving realistic air purification processes, underscoring the importance of identifying key intermediates and controlling humidity to enhance the selectivity of gaseous pollutant oxidation reactions.

Development of excitation power-responsive anti-stokes emission wavelength switching and their energy saving induced by localized surface plasmon resonance

We designed an external stimulus-responsive anti-Stokes emission switching using dual-annihilator-based triplet-triplet annihilation upconversion systems. This system, which was constructed by incorporating a palladium porphyrin derivative as a sensitizer and 9,10-diphenylanthracene (DPA) and 9,10-bis(triisopropylsilyl)ethynylanthracene (TIPS) as annihilators into polymer thin films, produced TIPS- and DPA-based anti-Stokes emission under low and high excitation powers, respectively. The mechanism involves the following: under low excitation power, triplet energy transfer from triplet-excited PdOEP to DPA is induced, followed by relay to TIPS. This results in the generation of triplet-excited TIPS, and the subsequent triplet-triplet annihilation between them produces TIPS-based anti-Stokes emission. Conversely, under high excitation power, the high-density triplet-excited DPA, generated through triplet energy transfer from PdOEP, undergoes triplet-triplet annihilation among themselves, resulting in the generation of DPA-based anti-Stokes emission. Additionally, we achieved energy savings by reducing the required excitation power for switching through the utilization of plasmonic metal nanoparticles. The strong local electromagnetic fields associated with the localized surface plasmon resonance of metal nanoparticles enhance the photoexcitation efficiency of PdOEP, subsequently increasing the density of triplet-excited DPA. As a result, anti-Stokes emission switching becomes feasible at lower excitation powers.

Electronic State Engineering in Perovskite-Cerium-Composite Nanocrystals toward Enhanced Triplet Annihilation Upconversion

Wavelength conversion based on hybrid inorganic-organic sensitized triplet-triplet annihilation upconversion (TTA-UC) is promising for applications such as photovoltaics, light-emitting-diodes, photocatalysis, additive manufacturing, and bioimaging. The efficiency of TTA-UC depends on the population of triplet excitons involved in triplet energy transfer (TET), the driving force in TET, and the coupling strength between the donor and acceptor. Consequently, achieving highly efficient TTA-UC necessitates the precise control of the electronic states of inorganic donors. However, conventional covalently bonded nanocrystals (NCs) face significant challenges in this regard. Herein, a novel strategy to exert control over electronic states is proposed, thereby enhancing TET and TTA-UC by incorporating ionic-bonded CsPbBr3 and lanthanide Ce3+ ions into composite NCs. These composite-NCs exhibit high photoluminescence quantum yield, extended single-exciton lifetime, quantum confinement, and uplifted energy levels. This engineering strategy of electronic states engendered a comprehensive impact, augmenting the population of triplet excitons participating in the TET process, enhancing coupling strength and the driving force, ultimately leading to an unconventional, dopant concentration-dependent nonlinear enhancement of UC efficiency. This work not only advances fundamental understanding of hybrid TTA-UC but also opens a door for the creation of other ionic-bonded composite NCs with tunable functionalities, promising innovations for next-generation optoelectronic applications.

Optimizing the Distance between Upconversion Thin Films and Silver Nanoprisms for the Design of a High-Performance Plasmonic Triplet-Triplet Annihilation Upconversion System

While the distance dependence of metal-enhanced fluorescence has been extensively studied for composite systems comprising fluorophores and metal nanoparticles, the corresponding distance dependence of triplet-triplet annihilation upconversion (TTA-UC) systems remains unexplored. Herein, we investigated the influence of the spatial distance between Ag nanoprisms (AgPRs) and TTA-UC thin films consisting of a palladium octaethylporphyrin (PdOEP) sensitizer and a 9,10-diphenylanthracene (DPA) emitter, aiming at enhancing the upconverted (UC) emission as efficiently as possible. Results indicated that the optimal distance for the examined system was significantly longer (12.6 nm) than those of typical metal-enhanced fluorescence systems (about 2 nm). We demonstrated that the UC emission enhancement factor can be expressed as a product including factors of the PdOEP photoexcitation rate, triplet-triplet energy transfer (TTET) efficiency from PdOEP to DPA, triplet excited DPA lifetime, and fluorescence efficiency of singlet excited DPA. We discovered that the AgPRs play a beneficial role in enhancing the PdOEP photoexcitation, whereas they exert detrimental effects on the other three factors. Among these three factors, quenching contributions by the decrease of the triplet excited DPA lifetime and DPA fluorescence efficiency were significant, making these the primary and secondary factors, respectively, for the UC emission quenching, particularly at short distances. These results demonstrate that the characteristic distance dependence of the UC emission enhancement is determined by the competing effects of beneficial PdOEP photoexcitation enhancement and the detrimental localized surface plasmon (and/or AgPR)-induced nonradiative decay of the triplet- and singlet excited DPA molecules. The findings offer valuable guidelines for the design of high-performance plasmonic TTA-UC systems.

Plasmonic Metal Nanostructures Meet Triplet-Triplet Annihilation-Based Photon Upconversion Systems: Performance Improvements and Application Trends

Improving the performance of upconversion systems based on triplet-triplet annihilation (TTA-UC) can have far-reaching implications for various fields, including solar devices, nano-bioimaging, and nanotherapy. This review focuses on the use of localized surface plasmon (LSP) resonance of metal nanostructures to enhance the performance of TTA-UC systems and explores their potential applications. After introducing the basic driving mechanism of TTA-UC and typical sensitizers used in these systems, we discuss recent studies that have utilized new sensitizers with distinct characteristics. Furthermore, we confirm that the enhancement in upconverted emission can be explained, at least in part, by the mechanism of "metal-enhanced fluorescence", which is attributed to LSP resonance-induced fluorescence enhancement. Next, we describe selected experiments that demonstrate the enhancement in upconverted emission in plasmonic TTA-UC systems, as well as the emerging trends in their application. We present specific examples of studies in which the enhancement in upconverted emission has significantly improved the performance of photocatalysts under both sunlight and indoor lighting. Additionally, we discuss the potential for future developments in plasmonic TTA-UC systems.

Recent Advances in the Photoreactions Triggered by Porphyrin-Based Triplet–Triplet Annihilation Upconversion Systems: Molecular Innovations and Nanoarchitectonics

Triplet–triplet annihilation upconversion (TTA-UC) is a very promising technology that could be used to convert low-energy photons to high-energy ones and has been proven to be of great value in various areas. Porphyrins have the characteristics of high molar absorbance, can form a complex with different metal ions and a high proportion of triplet states as well as tunable structures, and thus they are important sensitizers for TTA-UC. Porphyrin-based TTA-UC plays a pivotal role in the TTA-UC systems and has been widely used in many fields such as solar cells, sensing and circularly polarized luminescence. In recent years, applications of porphyrin-based TTA-UC systems for photoinduced reactions have emerged, but have been paid little attention. As a consequence, this review paid close attention to the recent advances in the photoreactions triggered by porphyrin-based TTA-UC systems. First of all, the photochemistry of porphyrin-based TTA-UC for chemical transformations, such as photoisomerization, photocatalytic synthesis, photopolymerization, photodegradation and photochemical/photoelectrochemical water splitting, was discussed in detail, which revealed the different mechanisms of TTA-UC and methods with which to carry out reasonable molecular innovations and nanoarchitectonics to solve the existing problems in practical application. Subsequently, photoreactions driven by porphyrin-based TTA-UC for biomedical applications were demonstrated. Finally, the future developments of porphyrin-based TTA-UC systems for photoreactions were briefly discussed.

Performance Improvement of Triplet-Triplet Annihilation-Based Upconversion Solid Films through Plasmon-Induced Backward Scattering of Periodic Arrays of Ag and Al

The performance improvement of solid-state triplet-triplet annihilation-based photon upconversion (TTA-UC) systems is required for the application to various solar devices. The performance can be improved by making use of the local strong electric field generated through the excitation of localized surface plasmon (LSP) resonance of metal nanostructures. However, since the improvement is effective only within the limited nanospace around nanoparticles (i.e., the near-field effect), a methodology for improving the performance over a wider spatial region is desirable. In this study, a significant improvement in the threshold light excitation intensity (I_th) (77% decrease) as the figure of merit and the upconverted emission intensity (6.3 times enhancement) in a solid-state TTA-UC film with a thickness of 3 μm was achieved by stacking the film with periodic Ag half-shell arrays. The highest-enhanced upconverted emission was obtained by tuning the diffuse reflectance peak, which results from the excitation of LSP resonance of the Ag half-shell arrays, to overlap well with the photoexcitation peak of the sensitizer in the TTA-UC film. The intensity of the enhanced upconverted emission was independent of the distance between the lower edge of the TTA-UC film and the surface of half-shell arrays in the nanometer order. These results suggest that the performance improvement was attributed to the photoexcitation enhancement of the sensitizer by elongating the excitation light path length inside the TTA-UC film, which was achieved through a strong backward scattering of the incident light based on the LSP resonance excitation (i.e., the far-field effect). In addition, the upconverted emission was improved using half-shell arrays comprising low-cost Al, although the enhancement factor was 3.5, which was lower than that of Ag half-shell arrays. The lower enhancement may be attributed to a decrease in the backward scattering of the excitation light owing to the intrinsic strong interband transition of Al at long visible wavelengths.

Identification of deactivation-resistant origin of In(OH)3 for efficient and durable photodegradation of benzene, toluene and their mixtures

Aromatic hydrocarbon is a representative type of VOCs, which causes adverse effects to human health. The degradation stability of aromatic hydrocarbon is of vital importance to commercializing a photocatalyst for its practical application. The most commonly used titanium dioxide photocatalyst (P25) was deactivated rapidly in the photocatalytic VOCs degradation process. In this work, the indium hydroxide (In(OH)₃) photocatalyst was developed, which exhibited not only higher efficient activity but also ultra-stable stability for degradation of benzene, toluene and their mixtures. The origin of the activity difference between two catalysts was investigated by combined experimental and theoretical ways. Based on in situ DRIFTS and GC-MS, it was revealed that benzoic acid and carbonaceous byproducts were specifically formed and accumulated on P25, which were responsible for deactivation of photocatalyst. In contrast, as revealed by both DFT calculations and experimental results, the reaction pathway with byproducts blocking the active sites can be thermodynamically avoided on In(OH)₃. This rendered high durability to In(OH)₃ photocatalyst in degradations of aromatic pollutants. The elucidation of deactivation-resistant effect and reaction mechanism as an ideal photocatalyst for practical usage were provided.

Photon Upconversion for Photovoltaics and Photocatalysis: A Critical Review

Opportunities for enhancing solar energy harvesting using photon upconversion are reviewed. The increasing prominence of bifacial solar cells is an enabling factor for the implementation of upconversion, however, when the realistic constraints of current best-performing silicon devices are considered, many challenges remain before silicon photovoltaics operating under nonconcentrated sunlight can be enhanced via lanthanide-based upconversion. A photophysical model reveals that >1-2 orders of magnitude increase in the intermediate state lifetime, energy transfer rate, or generation rate would be needed before such solar upconversion could start to become efficient. Methods to increase the generation rate such as the use of cosensitizers to expand the absorption range and the use of plasmonics or photonic structures are reviewed. The opportunities and challenges for these approaches (or combinations thereof) to achieve efficient solar upconversion are discussed. The opportunity for enhancing the performance of technologies such as luminescent solar concentrators by combining upconversion together with micro-optics is also reviewed. Triplet-triplet annihilation-based upconversion is progressing steadily toward being relevant to lower-bandgap solar cells. Looking toward photocatalysis, photophysical modeling indicates that current blue-to-ultraviolet lanthanide upconversion systems are very inefficient. However, hope remains in this direction for organic upconversion enhancing the performance of visible-light-active photocatalysts.

Examining the influence of bilayer structure on energy transfer and molecular photon upconversion in metal ion linked multilayers

Metal ion linked multilayers is a unique motif to spatially control and geometrically restrict molecules on a metal oxide surface and is of interest in a number of promising applications. Here we use a bilayer composed of a metal oxide surface, an anthracene annihilator molecule, Zn(II) linking ion, and porphyrin sensitizers to probe the influence of the position of the metal ion binding site on energy transfer, photon upconversion, and photocurrent generation. Despite being energetically similar, varying the position of the carboxy metal ion binding group (i.e. ortho, meta, para) of the Pt(II) tetraphenyl porphyrin sensitizer had a large impact on energy transfer rates and upconverted photocurrent that can be attributed to differences in their geometries. From polarized attenuated total reflectance measurements of the bilayers on ITO, we found that the orientation of the first layer (anthracene) was largely unperturbed by subsequent layers. However, the tilt angle of the porphyrin plane varies dramatically from 41° to 64° to 57° for the para-, meta-, and ortho-COOH substituted porphyrin molecules, which is likely responsible for the variation in energy transfer rates. We go on to show using molecular dynamics simulations that there is considerable flexibility in porphyrin orientation, indicating that an average structure is insufficient to predict the ensemble behavior. Instead, even a small subset of the population with highly favorable energy transfer rates can be the primary driver in increasing the likelihood of energy transfer. Gaining control of the orientation and its distribution will be a critical step in maximizing the potential of the metal ion linked structures.

An Efficient and Robust Method for Selective Conversion of Aniline to Azobenzene Using nano-TiO2 -P25-SO3 H, under Visible Light Irradiation

Nano-TiO₂ -P25-SO₃ H as our previous report has successfully been utilized to synthesize azobenzene through the selective conversion of aniline under visible light irradiation. According to PL emission spectra, the immobilizing a solid Brønsted acid of -SO₃ H groups on the pure-TiO₂ -P25 surface with a close interface is an approach to amplify the nano-TiO₂ -P25 response to visible light, which can productively hinder the recombination rate of photogenerated electrons and holes as carriers. Therefore, the photocatalytic activity of the semiconductor is highly likely to increase. Photooxidation of aniline to azobenzene was achieved by applying nano-TiO₂ -P25-SO₃ H (E_g  = 2.6 eV) that activated by blue photons (λ_max  = 460 nm), green photons (λ_max  = 510 nm) and red photons (λ_max  = 630 nm) which is introducing as a sustainable procedure. Central composite design (CCD) was employed for evaluating the effects of photocatalyst amount, oxidant concentration and irradiation time on the synthesis of azobenzene by this approach. Easily synthesizing, recyclability of the photocatalyst, mild reaction condition and short reaction time could be considered as plus points of this process.

Efficient Photocatalysis of Composite Films Based on Plasmon-Enhanced Triplet-Triplet Annihilation

To avoid secondary environmental pollution caused by photocatalysts in their applications, our work offers a new strategy for fabricating photocatalytic films based on plasmon-enhanced triplet-triplet annihilation upconversion (TTA-UC). Polydimethylsiloxane (PDMS) films containing platinum (II)-octaethylporphyrin and 9,10-diphenylanthracene (PtDPAP), and gold nanoparticles (AuNPs) were prepared. While graphene (G) was used as an adhesive and conductive layer, CdS nanoparticles were deposited onto the films (AuNPs-PtDPAP/G/CdS) by plasma glow discharge pretreatment. The AuNPs-PtDPAP film had an enhancement in the green-to-blue upconversion compared with the pristine PtDPAP film. CdS can utilize the AuNPs plasmon-enhanced TTA-UC photons to realize efficient photocatalytic reactions. The pseudo-first-order rate constant (k_pfo) of the optimized active and stable photocatalytic film, 0.3 AuNPs-PtDPAP/G/CdS, reached 0.294 h^-1 for tetracycline degradation under green light irradiation. Its k_pfo in decomposing tetracycline under visible light is 2.62 times higher than that of the PtDPAP/G/CdS. The reported composite films provide a strategy to improve the photocatalytic activity and promote the practical applications of nanosize photocatalysts.

Leaping across the visible range: near-infrared-to-violet photon upconversion employing a silyl-substituted anthracene

The first example of near-infrared (NIR, λ > 700 nm)-to-violet (λ < 450 nm) photon upconversion based on triplet–triplet annihilation is achieved by developing a silyl-substituted anthracene violet emitter with a low triplet energy level.

Enhanced triplet–triplet annihilation upconversion by photonic crystals and Au plasma resonance for efficient photocatalysis

The coupling electromagnetic field of AVS structure effect and AuNPs LSPR can synergistically improve TTA-UC efficiency, thereby enhancing the photocatalytic activity of g-C₃N₄@CdS.

Near-infrared to violet triplet-triplet annihilation fluorescence upconversion of Os(ii) complexes by strong spin-forbidden transition

Three Os(ii) complexes were synthesized with ligands 2,2'-dipyridyl (dipy), 1,10-phenanthroline monohydrate (phen), and 4,7-diphenyl-1,10-phenanthroline (diphen), and applied as triplet photosensitizers for triplet-triplet annihilation (TTA) fluorescence upconversion. The strong spin-orbital coupling made direct spin-forbidden transition of S₀-T₁ feasible. Lifetimes of the lowest triplet state of these complexes were determined to be 107 ns, 373 ns, and 386 ns for Os-dipy, Os-phen, and Os-diphen, respectively, using nanosecond transient absorption spectra. From steady-state phosphorescence emission spectra, energies of the triplet states were derived to be 1.75 eV, 1.80 eV, and 1.74 eV for Os-dipy, Os-phen, and Os-diphen, respectively. Using these photosensitizers, strong upconverted fluorescence of the triplet acceptors, 9,10-diphenylanthracene (DPA), perylene, and 9,10-bis(phenethynyl) anthracene (BPEA), was observed in the visible to violet range. In particular, fluorescence emission with the largest anti-Stokes shift of 1.14 eV was observed for the Os-phen/DPA system, and the upconverted quantum yield was determined as 5.9% in deoxygenated dichloroethane. Additionally, upconversion was determined in air using mixtures of dichloroethane and DMSO solvents, and the maximal quantum yield was measured to be 4.5% for Os-phen/DPA.

Preparation and application of solid-state upconversion materials based on sodium polyacrylate

By loading a microemulsion containing both sensitizer and emitter into porous sodium polyacrylate (PAAS), a water-absorbent resin (WAR) upconversion (UC) material was fabricated for photocatalysis applications. This WAR UC material showed a highly efficient UC process in the ambient environment owing to its liquid/solid encapsulation structure. In the application measurement, the UC emission from WAR UC materials can excite the catalyst Pt/WO3 to produce hydroxyl radicals, yielding 7-hydroxycoumarin by reacting with coumarin. In another case, since the band gap of ZnCdS matches the energy of UC emission, hole-electron pairs can be obtained under the UC irradiation and capture electrons from rhodamine B, leading to the degradation of rhodamine B. The maximum of the photocatalysis efficiency can be up to 97%. This work solves the oxygen quenching problem by preparing a triplet-triplet annihilation upconversion (TTA-UC) O/W microemulsion and loading it into PAAS WAR, and opens a new avenue to solid-state devices for TTA-UC. The applications of photocatalytic synthesis and photocatalytic degradation lay a foundation for future practical applications for TTA-UC materials.

Triplet-Triplet Annihilation Upconversion in Broadly Absorbing Layered Film Systems for Sub-Bandgap Photocatalysis

Upconversion (UC) of sub-bandgap photons extends the effective light absorption range of photovoltaic and photocatalytic devices, allowing them to reach higher conversion efficiencies. Recent advances in polymer host materials make it possible to translate triplet-triplet annihilation (TTA)-UC, the UC mechanism most suitable for this purpose, to solid films that can be integrated into devices. The promise of these films is currently limited by the narrow light absorption of TTA-UC sensitizer chromophores, but incorporating multiple sensitizers into layered film systems presents a promising strategy for producing UC materials with broadened light absorption. This strategy is herein applied for photocatalytic air purification, demonstrating its use in a real-world application for the first time. We superimpose optimized red-to-blue and green-to-blue UC films within dual-layer systems and develop a new photocatalyst compatible with their fluorescence emission. By integrating the dual-layer UC film systems with films of this photocatalyst, we produce the first devices that use TTA-UC to harness both red and green sub-bandgap photons for hydroxyl radical generation and photocatalytic degradation of gaseous acetaldehyde, a model volatile organic compound (VOC). Under white light-emitting diode excitation, the dual-layer film systems' broadened light absorption enhances their devices' photocatalytic degradation efficiency, enabling them to degrade twice as much acetaldehyde as their single-sensitizer counterparts. We show that as a result of the different absorption profiles of the two sensitizers, the film order significantly impacts UC fluorescence and VOC degradation. By probing the influence of the excitation light source, excitation geometry, and chromophore spectral overlap on the film systems' UC performance, we propose a framework for the design of multilayer TTA-UC film systems suitable for integration with a variety of photovoltaic and photocatalytic devices.

Application of a bodipy-C70 dyad in triplet-triplet annihilation upconversion of perylene as a metal-free photosensitizer

A bodipy-C70 dyad was synthesized and applied in triplet-triplet annihilation (TTA) upconversion of perylene as a novel metal-free organic photosensitizer. The photophysical processes were investigated by the methods of steady-state UV-Vis absorption and fluorescence spectroscopy, nanosecond time-resolved transient absorption spectroscopy, cyclic voltammetry, and density functional theory calculations. The bodipy-C70 dyad showed an increased molar extinction coefficient up to 82 300 mol-1 cm-1 at 518 nm compared with the C70 monomer. With photo-excitation of the bodipy moiety at 532 nm, the intramolecular singlet-singlet energy transfer between bodipy and C70 units was efficient with a quantum yield of nearly 100%, and the lowest triplet state of the dyad was subsequently populated via ISC of the C70 moiety, with a lifetime of ca. 80 μs in toluene. Electrochemical investigation suggested that the intramolecular electron transfer of the excited dyad was thermodynamically prohibited in toluene due to the positive ΔGCS for charge-separation. With the presence of perylene in solution as the triplet energy acceptor and emitter, the TTA upconverted fluorescence was observed with a maximum quantum yield of 10.3%. The overall upconversion capability of 4417 M-1 cm-1 exceeded that of C70 approximately two-fold. Moreover, the bodipy-C70 dyad also exhibited an enhanced optical stability under intense irradiation. All data indicated that the dyad was another ideal photosensitizer for TTA upconversion of perylene in the fullerene derivative family.

Improving Photocatalytic Water Treatment through Nanocrystal Engineering: Mesoporous Nanosheet-Assembled 3D BiOCl Hierarchical Nanostructures That Induce Unprecedented Large Vacancies

Vacancy control can significantly enhance the performance of photocatalytic semiconductors for water purification. However, little is known about the mechanisms and approaches that could generate stable large vacancies. Here, we report a new mechanism to induce vacancy formation on nanocrystals for enhanced photocatalytic activity: the introduction of mesopores. We synthesized two nanosheet-assembled hierarchical 3D BiOCl mesoporous nanostructures with similar morphology and exposed facets but different nanosheet thickness. Positron annihilation analysis detected unprecedentedly large V_Bi^‴ V_O^•• V_Bi^‴ V_O^•• V_Bi^‴ vacancy associates (as well as V_Bi^‴ V_O^•• V_Bi^‴) on BiOCl assembled from 3-6 nm nanosheets but only V_Bi^‴ V_O^•• V_Bi^‴ vacancy associates on BiOCl assembled from thicker (10-20 nm) nanosheets. Comparison of vacancy properties with 2D ultrathin 2.7 nm nanosheets (with V_Bi^‴ V_O^•• V_Bi^‴ and V_Bi^‴) indicates that nanosheet thinness alone cannot explain the formation of such large atom vacancies. On the basis of density functional theory computations of formation energy of isolated Bi vacancy, we show that mesopores facilitate the formation of large vacancies to counterbalance thermodynamic instability caused by incompletely coordinated Bi and O atoms along the mesopore perimeters. We corroborate that the extraordinarily large V_Bi^‴ V_O^•• V_Bi^‴ V_O^•• V_Bi^‴ vacancy associates facilitate photoexcitation of electrons and prevent the recombination of electron-hole pairs, which significantly enhances photocatalytic activity. This is demonstrated by the rapid mineralization of bisphenol A (10^-5 M) with low photocatalyst loading (1 g L^-1), as well as enhanced bacterial disinfection. Improved electron-hole separation is also corroborated by enhanced photocatalytic reduction of nitrate.

Flexible and Micropatternable Triplet-Triplet Annihilation Upconversion Thin Films for Photonic Device Integration and Anticounterfeiting Applications

Triplet-triplet annihilation upconversion (TTA-UC) has recently drawn widespread interest for its capacity to harvest low-energy photons and to broaden the absorption spectra of photonic devices, such as solar cells. Although conceptually promising, effective integration of TTA-UC materials into practical devices has been difficult due to the diffusive and anoxic conditions required in TTA-UC host media. Of the solid-state host materials investigated, rubbery polymers facilitate the highest TTA-UC efficiency. To date, however, their need for long-term oxygen protection has limited rubbery polymers to rigid film architectures that forfeit their intrinsic flexibility. This study introduces a new multilayer thin-film architecture, in which scalable solution processing techniques are employed to fabricate flexible, photostable, and efficient TTA-UC thin films containing layers of oxygen barrier and host polymers. This breakthrough material design marks a crucial advance toward TTA-UC integration within rigid and flexible devices alike. Moreover, it introduces new opportunities in unexplored applications such as anticounterfeiting. Soft lithography is incorporated into the film fabrication process to pattern TTA-UC host layers with a broad range of high-resolution microscale designs, and superimposing host layers with customized absorption, emission, and patterning ultimately produces proof-of-concept anticounterfeiting labels with advanced excitation-dependent photoluminescent security features.

The Myth of Visible Light Photocatalysis Using Lanthanide Upconversion Materials

Upconversion luminescence is a nonlinear optical process achieved by certain engineered materials, which allows conversion of low energy photons into higher energy photons. Of particular relevance to environmental technology, lanthanide-based upconversion phosphors have appeared in dozens of publications as a tool for achieving visible light activation of wide-band gap semiconductor photocatalysts, such as TiO₂, for degradation of water contaminants. Supposedly, the phosphor particles act to convert sub-band gap energy photons (e.g., solar visible light) into higher energy ultraviolet photons, thus driving catalytic aqueous contaminant degradation. Herein, however, we reexamined the photophysical properties of the popular visible-to-UV converters Y₂SiO₅:Pr³⁺ and Y₃Al₅O₁₂:Er³⁺, and found that their efficiencies are not nearly high enough to induce catalytic degradations under the reported excitation conditions. Furthermore, our experiments indicate that the false narrative of visible-to-UV upconversion-sensitized photocatalysis likely arose due to coincidental enhancements of dye degradation via direct electron injection that occur in the presence of dielectric-semiconductor (phosphor-catalyst) interfaces. These effects were unrelated to upconversion and only occurred for dye solutions illuminated within the chromophore absorption bands. We conclude that upconversion using Pr³⁺ or Er³⁺-activated systems is not a technologically appealing mechanism for visible light photocatalysis, and provide experimental guidelines for avoiding future misinterpretation of these phenomena.

Photon upconversion in organic nanoparticles and subsequent amplification by plasmonic silver nanowires

The development of photonic materials with high photoluminescence is always a challenge in photochemistry and photophysics. Here we present a general approach for enhancing photon upconversion through aggregation and further via surface plasmon resonance (SPR). Luminescent nanoparticles from a tetraphenylethylene derivative were fabricated, showing excellent aggregation-induced emission (AIE) behavior. By mixing with a triplet sensitizer platinum octaethylporphyrin (PtOEP), aggregation-induced photon upconversion (iPUC) could be achieved, resulting in an enhancement of the emission. Blending such iPUC nanoparticles with silver nanowires (AgNWs), the upconverted emission intensity could be significantly amplified due to the SPR of AgNWs. Thus, the concepts of aggregation-induced emission (AIE), metal enhanced fluorescence (MEF) and aggregation-induced photon upconversion (iPUC) were successfully integrated and achieved.

Examining the role of acceptor molecule structure in self-assembled bilayers: surface loading, stability, energy transfer, and upconverted emission

This report provides insights into the TTA-UC process with respect to the structure of the acceptor molecule in self-assembled bilayer films.

Sulfur vacancy-rich CdS loaded on filter paper-derived 3D nitrogen-doped mesoporous carbon carrier for photocatalytic VOC removal

A new strategy is explored to fabricate a sulfur vacancy-rich CdS composite photocatalyst with a nitrogen-doped 3D porous carbon matrix for VOC removal.

Nonstoichiometric tungsten oxide residing in a 3D nitrogen doped carbon matrix, a composite photocatalyst for oxygen vacancy induced VOC degradation and H2 production

An oxygen vacancy rich WO_3−x based composite material with 3D nitrogen doped carbon as a matrix is synthesized and it exhibits photocatalytic VOC removal and H₂ production activities.