Enriching surface-ordered defects on WO3 for photocatalytic CO2-to-CH4 conversion by water.

Defect engineering has been widely applied in semiconductors to improve photocatalytic properties by altering the surface structures. This study is about the transformation of inactive WO3 nanosheets to a highly effective CO2-to-CH4 conversion photocatalyst by introducing surface-ordered defects in abundance. The nonstoichiometric WO3-x samples were examined by using aberration-corrected electron microscopy. Results unveil abundant surface-ordered terminations derived from the periodic {013} stacking faults with a defect density of 20.2%. The {002} surface-ordered line defects are the active sites for fixation CO2, transforming the inactive WO3 nanosheets into a highly active catalyst (CH4: O2 = 8.2: 16.7 µmol h-1). We believe that the formation of the W-O-C-W-O species is a critical step in the catalytic pathways. This work provides an atomic-level comprehension of the structural defects of catalysts for activating small molecules.

Ultrafine Pt Nanoparticles on Defective Tungsten Oxide for Photocatalytic Ethylene Synthesis.

The selective conversion of ethane (C2H6) to ethylene (C2H4) under mild conditions is highly wanted, yet very challenging. Herein, it is demonstrated that a Pt/WO3-x catalyst, constructed by supporting ultrafine Pt nanoparticles on the surface of oxygen-deficient tungsten oxide (WO3-x) nanoplates, is efficient and reusable for photocatalytic C2H6 dehydrogenation to produce C2H4 with high selectivity. Specifically, under pure light irradiation, the optimized Pt/WO3-x photocatalyst exhibits C2H4 and H2 yield rates of 291.8 and 373.4 µmol g-1 h-1, respectively, coupled with a small formation of CO (85.2 µmol g-1 h-1) and CH4 (19.0 µmol g-1 h-1), corresponding to a high C2H4 selectivity of 84.9%. Experimental and theoretical studies reveal that the vacancy-rich WO3-x catalyst enables broad optical harvesting to generate charge carriers by light for working the redox reactions. Meanwhile, the Pt cocatalyst reinforces adsorption of C2H6, desorption of key reaction species, and separation and migration of light-induced charges to promote the dehydrogenation reaction with high productivity and selectivity. In situ diffuse reflectance infrared Fourier transform spectroscopy and density functional theory calculation expose the key intermediates formed on the Pt/WO3-x catalyst during the reaction, which permits the construction of the possible C2H6 dehydrogenation mechanism.

Towards high-performance polarimeters with large-area uniform chiral shells: a comparative study on the polarization detection precision enabled by the Mueller matrix and deep learning algorithm.

Polarization detection and imaging technologies have attracted significant attention for their extensive applications in remote sensing, biological diagnosis, and beyond. However, previously reported polarimeters heavily relied on polarization-sensitive materials and pre- established mapping relationships between the Stokes parameters and detected light intensities. This dependence, along with fabrication and detection errors, severely constrain the working waveband and detection precision. In this work, we demonstrated a highly precise, stable, and broadband full-Stokes polarimeter based on large-area uniform chiral shells and a post-established mapping relationship. By precisely controlling the geometry through the deposition of Ag on a large-area microsphere monolayer with a uniform lattice, the optical chirality and anisotropy of chiral shells can reach about 0.15 (circular dichroism, CD) and 1.7, respectively. The post-established mapping relationship between the Stokes parameters and detected light intensities is established through training a deep learning algorithm (DLA) or fitting the derived mapping-relationship formula based on the Mueller matrix theory with a large dataset collected from our home-built polarization system. For the detection precision with DLA, the mean squared errors (MSEs) at 710 nm can reach 0.10% (S1), 0.41% (S2), and 0.24% (S3), while for the Mueller matrix theory, the corresponding values are 0.14% (S1), 0.46% (S2), and 0.48% (S3). The in-depth comparative studies indicate that the DLA outperforms the Mueller matrix theory in terms of detection precision and robustness, especially for weak illumination, small optical anisotropy and chirality. The averaged MSEs over a broad waveband ranging from 500 nm to 750 nm are 0.16% (S1), 0.46% (S2), and 0.61% (S3), which are significantly smaller than those derived from the Mueller matrix theory (0.45% (S1), 1% (S2), and 39.8% (S3)). The optical properties of chiral shells, the theory and DLA enabled mapping-relationships, the combination modes of chiral shells, and the MSE spectra have been systematically investigated.

Origination of the chiroptical effect in plasmonic nano-structures in the view of quasi-normal mode theory.

General chiroptical effects describe all of the interaction differences between light carrying opposite spins and chiral matters, such as circular dichroism, optical activity, and chiral Raman optical activity, and have been proven to hold great promise for extensive applications in physics, chemistry, and biology. However, the underlying physical mechanism is usually explained intangibly by the twisted currents in chiral geometry, where the cross coupling between the electric and magnetic dipoles breaks the degeneracy of the helicity eigenmodes. In this Letter, we construct a clear sight on the origination of the chiroptical effect in the view of the eigenstates of a non-Hermitian system, i.e., quasi-normal modes (QNMs). The intrinsic chiroptical effect comes from the chiral QNMs, which have distinct excitation and emission differences in both phase and intensity for lights carrying opposite spins, while the extrinsic chiroptical effect coming from the achiral QNMs requires specific illumination and observation conditions, where the low symmetrical QNM can generate chiroptical effects in both absorption and scattering, but the highly symmetrical QNMs can only generate chiroptical effects in scattering through the coherent superposition of several QNMs. Our findings offer an in-depth understanding of the chiroptical effect and have the potential to bring broad inspiration to the design and applications of chiroptical effects.

Semiconducting Polymers for Oxygen Evolution Reaction under Light Illumination.

Sunlight-driven water splitting to produce hydrogen fuel has stimulated intensive scientific interest, as this technology has the potential to revolutionize fossil fuel-based energy systems in modern society. The oxygen evolution reaction (OER) determines the performance of overall water splitting owing to its sluggish kinetics with multielectron transfer processing. Polymeric photocatalysts have recently been developed for the OER, and substantial progress has been realized in this emerging research field. In this Review, the focus is on the photocatalytic technologies and materials of polymeric photocatalysts for the OER. Two practical systems, namely, particle suspension systems and film-based photoelectrochemical systems, form two main sections. The concept is reviewed in terms of thermodynamics and kinetics, and polymeric photocatalysts are discussed based on three key characteristics, namely, light absorption, charge separation and transfer, and surface oxidation reactions. A satisfactory OER performance by polymeric photocatalysts will eventually offer a platform to achieve overall water splitting and other advanced applications in a cost-effective, sustainable, and renewable manner using solar energy.

The Quenching Mechanism and Suppressing Method of Single-Photon Emitters from Point Defects in GaN.

The defect-based single-photon emitters (SPEs) in gallium nitride (GaN) have attracted considerable research interest due to their high emission rate, narrow line width, and room-temperature operation. However, the quenching effect greatly restricts the applications of these SPEs, and the origin of the quenching mechanism is still unclear. Here, based on systematic ab initio calculations, we reveal a possible quenching mechanism originating from the transformation between two different structures of the defect-pair NGaVN in wurtzite GaN. Our results indicate that the defect-pair NGaVN possesses two stable detect-structures A and B, where the structure B has a small zero phonon line (ZPL) and long lifetime. The transformation barrier from structures A to B is only 0.097 eV. Thus, structure A can easily transform to structure B under laser illumination due to thermal fluctuations, causing a quenching phenomenon. Our work also predicts that the barrier energy between defect structures A and B could be effectively adjusted through tuning the triaxial compressive strain of the crystal structure. This provides an effective method to suppress the quenching effect of defect-pair NGaVN in GaN, paving the way for practical applications of SPEs.

S-scheme heterojunction of crystalline carbon nitride nanosheets and ultrafine WO3 nanoparticles for photocatalytic CO2 reduction.

Highly crystalline carbon nitride polymers have shown great opportunities in overall water photosplitting; however, their mission in light-driven CO2 conversion remains to be explored. In this work, crystalline carbon nitride (CCN) nanosheets of poly triazine imide (PTI) embedded with melon domains are fabricated by KCl/LiCl-mediated polycondensation of dicyandiamide, the surface of which is subsequently deposited with ultrafine WO3 nanoparticles to construct the CCN/WO3 heterostructure with a S-scheme interface. Systematic characterizations have been conducted to reveal the compositions and structures of the S-scheme CCN/WO3 hybrid, featuring strengthened optical capture, enhanced CO2 adsorption and activation, attractive textural properties, as well as spatial separation and directed movement of light-triggered charge carriers. Under mild conditions, the CCN/WO3 catalyst with optimized composition displays a high photocatalytic activity for reducing CO2 to CO in a rate of 23.0 µmol/hr (i.e., 2300 µmol/(hr·g)), which is about 7-fold that of pristine CCN, along with a high CO selectivity of 90.6% against H2 formation. Moreover, it also manifests high stability and fine reusability for the CO2 conversion reaction. The CO2 adsorption and conversion processes on the catalyst are monitored by in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), identifying the crucial intermediates of CO2*-, COOH* and CO*, which integrated with the results of performance evaluation proposes the possible CO2 reduction mechanism.

Hydroxyl-Bonded Ru on Metallic TiN Surface Catalyzing CO2 Reduction with H2O by Infrared Light.

Synchronized conversion of CO2 and H2O into hydrocarbons and oxygen via infrared-ignited photocatalysis remains a challenge. Herein, the hydroxyl-coordinated single-site Ru is anchored precisely on the metallic TiN surface by a NaBH4/NaOH reforming method to construct an infrared-responsive HO-Ru/TiN photocatalyst. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (ac-HAADF-STEM) and X-ray absorption spectroscopy (XAS) confirm the atomic distribution of the Ru species. XAS and density functional theory (DFT) calculations unveil the formation of surface HO-RuN5-Ti Lewis pair sites, which achieves efficient CO2 polarization/activation via dual coordination with the C and O atoms of CO2 on HO-Ru/TiN. Also, implanting the Ru species on the TiN surface powerfully boosts the separation and transfer of photoinduced charges. Under infrared irradiation, the HO-Ru/TiN catalyst shows a superior CO2-to-CO transformation activity coupled with H2O oxidation to release O2, and the CO2 reduction rate can further be promoted by about 3-fold under simulated sunlight. With the key reaction intermediates determined by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and predicted by DFT simulations, a possible photoredox mechanism of the CO2 reduction system is proposed.

High performance laser-driven flyers based on a refractory metamaterial perfect absorber.

Laser-driven flyers (LDFs), which can drive metal particles to ultra-high speeds by feeding high-power laser, have been widely used in many fields, such as ignition, space debris simulation, and dynamic high-pressure physics. However, the low energy-utilization efficiency of the ablating layer hinders the development of LDF devices towards low power consumption and miniaturization. Herein, we design and experimentally demonstrate a high-performance LDF based on the refractory metamaterial perfect absorber (RMPA). The RMPA consists by a layer of TiN nano-triangular array, a dielectric layer and a layer of TiN thin film, and is realized by combing the vacuum electron beam deposition and colloid-sphere self-assembled techniques. RMPA can greatly improve the absorptivity of the ablating layer to about 95%, which is comparable to the metal absorbers, but obviously larger than that of the normal Al foil (∼10%). This high-performance RMPA brings a maximum electron temperature of ∼7500 K at ∼0.5 µs and a maximum electron density of ∼1.04 × 1016 cm-3 at ∼1 µs, which are higher than that the LDFs based on normal Al foil and metal absorbers due to the robust structure of RMPA under high-temperature. The final speed of the RMPA-improved LDFs reaches to about 1920 m/s measured by the photonic Doppler velocimetry system, which is about 1.32 times larger than the Ag and Au absorber-improved LDFs, and about 1.74times larger than the normal Al foil LDFs under the same condition. This highest speed unambiguously brings a deepest hole on the Teflon slab surface during the impact experiments. The electromagnetic properties of RMPA, transient speed and accelerated speed, transient electron temperature and density have been systematically investigated in this work.

Deep learning-enabled broadband full-Stokes polarimeter with a portable fiber optical spectrometer.

Portable fiber optical spectrometers (PFOSs) have been widely used in the contemporary industrial and agricultural production and life due its low cost and small volume. PFOSs mainly combine one fiber to guide light and one optical spectrometer to detect spectra. In this work, we demonstrate that PFOSs can work as a broadband full-Stokes polarimeter through slightly bending the fiber several times and establishing the mapping relationship between the Stokes parameters S^ and the bending-dependent light intensities I^, i.e., S^=f(I^). The different bending geometries bring different birefringence effects and reflection effects that change the polarization state of the out-going light. In the meanwhile, the grating owns a polarization-depended diffraction efficiency especially for the asymmetric illumination geometry that introduces an extrinsic chiroptical effect, which is sensitive to both the linear and spin components of light. The minimum mean squared error (MSE) can reach to smaller than 1% for S1, S2, and S3 at 810 nm, and the averaged MSE in the wave band from 440 nm to 840 nm is smaller than 2.5%, where the working wavelength can be easily extended to arbitrary wave band by applying PFOSs with proper parameters. Our findings provide a convenient and practical method for detecting full-Stokes parameters.

Bottom-up Synthesis of Single-Crystalline Poly (Triazine Imide) Nanosheets for Photocatalytic Overall Water Splitting.

Poly (triazine imide) (PTI/Li+ Cl- ), one of the crystalline versions of polymeric carbon nitrides, holds great promise for photocatalytic overall water splitting. In principle, the photocatalytic activity of PTI/Li+ Cl- is closely related to the morphology, which could be reasonably tailored by the modulation of the polycondensation process. Herein, we demonstrate that the hexagonal prisms of PTI/Li+ Cl- could be converted to hexagonal nanosheets by adjusting the binary eutectic salts from LiCl/KCl or NaCl/LiCl to ternary LiCl/KCl/NaCl. Results reveal that the extension of in-plane conjugation is preferred, when the polymerisation was performed in the presence of ternary eutectic salts. The hexagonal nanosheets bears longer lifetimes of charge carriers than that of hexagonal prisms due to lower intensity of structure defects and shorter hopping distance of charge carriers along the stacking direction of triazine nanosheets. The optimized hexagonal nanosheets exhibits a record apparent quantum yield value of 25 % (λ=365 nm) for solar hydrogen production by one-step excitation overall water splitting.

The Directional Crystallization Process of Poly (triazine imide) Single Crystals in Molten Salts.

Poly (triazine imide) photocatalysts prepared via molten salt methods emerge as promising polymer semiconductors with one-step excitation capacity of overall water splitting. Unveiling the molecular conjugation, nucleation, and crystallization processes of PTI crystals is crucial for their controllable structure design. Herein, microscopy characterization was conducted at the PTI crystallization front from meso to nano scales. The heptazine-based precursor was found to depolymerize to triazine monomers within molten salts and KCl cubes precipitate as the leading cores that guide the directional stacking of PTI molecular units to form aggregated crystals. Upon this discovery, PTI crystals with improved dispersibility and enhanced photocatalytic performance were obtained by tailoring the crystallization fronts. This study advances insights into the directional assembling of PTI monomers on salt templates, placing a theoretical foundation for the ordered condensation of polymer crystals.

Rh/Cr2 O3 and CoOx Cocatalysts for Efficient Photocatalytic Water Splitting by Poly (Triazine Imide) Crystals.

In situ photo-deposition of both Pt and CoOx cocatalysts on the facets of poly (triazine imide) (PTI) crystals has been developed for photocatalytic overall water splitting. However, the undesired backward reaction (i.e., water formation) on the noble Pt surface is a spontaneously down-hill process, which restricts their efficiency to run the overall water splitting reaction. Herein, we demonstrate that the efficiency for photocatalytic overall water splitting could be largely promoted by the decoration of Rh/Cr2 O3 and CoOx as H2 and O2 evolution cocatalysts, respectively. Results reveal that the dual cocatalysts greatly extract charges from bulk to surface, while the Rh/Cr2 O3 cocatalyst dramatically restrains the backward reaction, achieving an apparent quantum efficiency (AQE) of 20.2 % for the photocatalytic overall water splitting reaction.

Engineering a strong and stable ultraviolet chiroptical effect in a large-area chiral plasmonic shell.

Ultraviolet chiral metamaterials (UCM) are highly desired for their strong interaction with the intrinsic resonance of molecules and ability in manipulating the polarization state of high energy photons, but rarely reported to date due to their small feature size and complex geometry. Herein, we design and fabricate a kind of novel ultraviolet chiral plasmonic shell (UCPS) by combing the stepwise Al deposition and colloid-sphere assembled techniques. The cancellation effect originated from the disorder lattices of micro-domains in the colloid monolayer has been successfully overcome by optimizing the deposition parameters, and a strong CD signal of larger than 1 deg in the UV region is demonstrated both in simulation and experiment. This strong ultraviolet chiroptical resonances mainly come from the surface chiral lattice resonance mode, the whispering gallery mode and also the interaction between neighbor shells, and can be effectively tuned by changing structural parameters, for example, the sphere diameter, or even slightly increasing the deposition temperature in experiment. To improve the stability, the fabricated UCPSs are protected by N2 in the deposition chamber and then passivated by UV-ozone immediately after each deposition step. The formed UCPS show an excellent stability when exposing in the atmospheric environment. The computer-aided geometrical model, electromagnetic modes, and the tunable chiroptical resonance modes have been systematically investigated.

Enhanced Metal-Semiconductor Interaction for Photocatalytic Hydrogen-Evolution Reaction.

The selective immobilization of noble metals right at the place where photogenerated electrons migrate through the photodeposition approach is a unique strategy to load cocatalysts on semiconductors for solar hydrogen production. However, a poor metal-semiconductor interaction is often formed, which not only hinders the interfacial charge transfer, but also results in the easy aggregation and shedding of cocatalysts during photocatalytic reactions. Herein, it is demonstrated that the photodeposited ultrafine metals, such as nanosized Au, can be well stabilized on TiO2 nanocrystallines without sintering by employing a sacrificial carbon coating annealing strategy to strengthen the metal-support interaction. Benefiting from the improved interfacial contact between Au and TiO2 for fast charge transfer and the well-preserved size-dependent catalytic behavior of Au nanoparticles toward hydrogen evolution reaction, the annealed Au/TiO2 exhibits a significant enhanced activity toward photocatalytic H2 production with good durability.

The Hole-Tunneling Heterojunction of Hematite-Based Photoanodes Accelerates Photosynthetic Reaction.

Single-atom metal-insulator-semiconductor (SMIS) heterojunctions based on Sn-doped Fe2 O3 nanorods (SF NRs) were designed by combining atomic deposition of an Al2 O3 overlayer with chemical grafting of a RuOx hole-collector for efficient CO2 -to-syngas conversion. The RuOx -Al2 O3 -SF photoanode with a 3.0 nm thick Al2 O3 overlayer gave a >5-fold-enhanced IPCE value of 52.0 % under 370 nm light irradiation at 1.2 V vs. Ag/AgCl, compared to the bare SF NRs. The dielectric field mediated the charge dynamics at the Al2 O3 /SF NRs interface. Accumulation of long-lived holes on the surface of the SF NRs photoabsorber aids fast tunneling transfer of hot holes to single-atom RuOx species, accelerating the O2 -evolving reaction kinetics. The maximal CO-evolution rate of 265.3 mmol g-1 h-1 was achieved by integration of double SIMS-3 photoanodes with a single-atom Ni-doped graphene CO2 -reduction-catalyst cathode; an overall quantum efficiency of 5.7 % was recorded under 450 nm light irradiation.

Asymmetric chiroptical effect from chiral medium filled golden slit grating on substrate.

In this Letter, we report a giant and robust asymmetric chiroptical effect (ACOE) in the chiral medium filled golden slit grating on glass substrate (CMGSG-GS). This ACOE comes from the influence of interface asymmetry on the electromagnetic cross-coupling in the CMGSG-GS, and it is inherently different than that reported in the Faraday medium and the planar anisotropic chiral metamaterials. Both the polarization eigenstate and the transmission matrix are highly dependent on the metal structure used in the CMGSG-GS. The polarization eigenstates of the CMGSG-GS are two co-rotating elliptical states with ellipticity of nearly 0, and they remain mostly unchanged for opposite directions. The transmission matrices of opposite directions are normal matrices, which do not show any symmetric law although the geometry of the CMGSG-GS owns a high rotational symmetry. The reported ACOE gives a measurable physical parameter to reveal the events happening at interface.

N-Rich Carbon Catalysts with Economic Feasibility for the Selective Oxidation of Hydrogen Sulfide to Sulfur.

The efficient removal of hydrogen sulfide (H2S) from exhaust emissions is a great challenge to chemical industries. Selective catalytic oxidation of H2S into elemental sulfur is regarded as one of the most promising approaches to alleviate environmental pollution, while recycling sulfur resources. It is therefore highly desirable to develop efficient catalysts for the conversion of H2S to sulfur under mild reaction conditions. Here we present a nitrogen-rich carbon obtained by the direct thermal treatment of commercial polyaniline (PANI) for the selective oxidation of H2S in a continuous way at relatively low temperature (180 °C). The efficient conversion of H2S over the N-rich carbon catalysts was attributed to the in situ generation of pyridine-N on the carbon matrix, which served as the active sites to promote the absorption and dissociation of H2S molecules, achieving a superior catalytic conversion rate of 99% and selectivity up to 95% at 180 °C.

Active perfect absorber based on planar anisotropic chiral metamaterials.

Active chiral plasmonics have attracted a considerable amount of research interest for their power to switch the handedness of chiral metamaterials and the potential applications in highly integrated polarization sensitive devices, stereo display fields, and so on. In this work, we propose a kind of active chiral metamaterial absorber (ACMA) composed by planar anisotropic chiral metamaterials (PACMs) and a metal layer. Our in-depth theoretical analysis indicates that the circular conversion dichroism (CCD) from PACMs plays a crucial role to achieve the active chiroptical effect. The CCD effect can enable a differentiated microcavity-interference effect between the left and right circular incident lights and results in a chiroptical effect related to the equivalent optical length between the PACMs and the metal layer. In simulations, a high-performance ACMA, which are composed by the 'Z'-shaped PACMs, is designed, and the maximum reflection CDR from ACMA can reach 0.882. Meanwhile, the minimum reflection CDR can reach to 0, resulting a very large adjustable range of from 0 to 0.882. The maximum modulation sensitivity, which is defined as Mn=∂CDR/∂n and Md=∂CDR/∂d, can reach to about 1368.252 for d=100um and 0.06157 nm-1 for n=4.5,respectively. In addition to the active chiroptical effect, the designed ACMA also shows excellent performance as a sensor, such as when it is being used as a highly-sensitive temperature sensor. In that case, the minimum detected precision can reach approximately 3.067 * 10-8 °C, if VO2 is used to fill the FP cavity.

Quantitative Determination of Contribution by Enhanced Local Electric Field, Antenna-Amplified Light Scattering, and Surface Energy Transfer to the Performance of Plasmonic Organic Solar Cells.

Plasmonic metal nanostructures are widely used as subwavelength light concentrators to enhance light harvesting of organic solar cells through two photophysical effects, including enhanced local electric field (ELEF) and antenna-amplified light scattering (AALS), while their adverse quenching effect from surface energy transfer (SET) should be suppressed. In this work, a comprehensive study to unambiguously distinguish and quantitatively determine the specific influence and contribution of each effect on the overall performance of organic solar cells incorporated with Ag@SiO2 core-shell nanoparticles (NPs) is presented. By investigating the photon conversion efficiency (PCE) as a function of the SiO2 shell thickness, a strong competition between the ELEF and SET effects in the performance of the devices with the NPs embedded in the active layers is found, leading to a maximum PCE enhancement of 12.4% at the shell thickness of 5 nm. The results give new insights into the fundamental understanding of the photophysical mechanisms responsible for the performance enhancement of plasmonic organic solar cells and provide important guidelines for designing more-efficient plasmonic solar cells in general.