Slow Photon Research Articles

Preparation of Waterproof Organometal Halide Perovskite Photonic Crystal Beads

AbstractHerein, we report on an innovative method for the preparation of a series of organometal halide perovskite (OHP) photonic crystal beads with pronounced and tunable photonic stop bands by using self‐assembled polystyrene spheres as a mold. After infiltration of the mold with OHP precursor solution and slow drying, the OHPs crystallized in the voids of the polystyrene arrays. By controlling the diameter of the polystyrene spheres, the photonic stop band of the OHPs could be precisely tuned. The overlap between the photonic stop band of the beads and the band gap of the OHPs enhances the light harvesting of the perovskite because of the slow photon effect, which arises from the photonic crystal beads. Moreover, the stability of the composite was greatly enhanced by coating with the transparent polymer PDMS without blocking the light propagation. The coated OHP photonic beads kept their composition even after having been in contact with water for 24 h.

Stopband tuning of TiO2 inverse opals for slow photon absorption

The effect of photonic photocatalysis was explored employing TiO2 inverse opal structures. The degradation of methylene blue in aqueous phase was performed under visible light (λ=450nm) irradiation and showed an improved degradation rate over TiO2 inverse opals with respect to non-structured films. Moreover, tuning the stopband by means of varying the lattice parameter and the irradiation incident angle results in a further improvement of the photocatalytic rate due to the slow photon effect. The gas-phase photocatalytic degradation of acetaldehyde under UV irradiation and ISO-standard conditions showed a similar (0.6%) photonic efficiency in all the studied TiO2 inverse opals regardless the photonic lattice parameter. Gas phase degradations were useful to investigate diffusional properties, in particular for small pore size samples. The mechanical stability of the samples, impacting directly on the structure-dependent photocatalytic performance, was also studied as a function of the consecutive photocatalytic reaction cycles.

Slow-photon enhancement of dye sensitized TiO2 photocatalysis

Photonic band gap engineered TiO2 inverse opals were fabricated using self-assembled polystyrene films as sacrificial templates with controlled optical properties, aimed at the identification of the slow-photon effect on dye sensitized TiO2 photocatalysis. The materials’ photocatalytic efficiency was evaluated using Raman spectroscopy, on methylene blue photodegradation following both UVA and monochromatic visible light illumination. Contrary to UVA, where no photonic effect could be traced, laser irradiation within the slow-photon energy range of the TiO2 inverse opals, resulted in a marked increase of the dye photosensitized degradation rate, outperforming not only compact nanocrystalline films but also the benchmark mesoporous Aeroxide® P25 TiO2 films. This effect provides direct evidence for the presence of slow photons that amplify the interaction of visible light with the adsorbed dye molecules on the periodically structured TiO2 film.

On the Hidden Beauty of Trigonometric Functions

In the unit circle with radius R = E0 = mc2 = 1 we have defined the trigonometric function cos(Theta) = v/c. The known trigonometric functions revealed the hidden relationships between sensible energy, latent energy, sensible momentum and latent momentum of the moving object, and the absorbed momentum from outside and the available momentum in the outside of the moving object. We present the trigonometric concept inspired by the old Babylonian clay tablet IM 55357 and based on the knowledge of the School of Athens (the fresco of Raphael) and the work of many generations of the Masters of trigonometry. The concept of the Divided Line of Plato can be now quantitatively tested. For the experimental analysis of this concept we propose to study in details the very well known beta decay of RaE to determine the sensible and latent energy (heat) of those beta particles and the sensible and latent energy of the remaining nucleus. The longitudinal momentum and the transverse (latent) momentum can be studied on the effects of the slow neutrons. The longitudinal momentum and the transverse momentum of photons can be manipulated in a convenient medium in order to prepare slow photons. The photoormi effect might improve the efficiency of the light-to-electricity conversion and the efficiency of the light-to-heat conversion.

AuPd/3DOM-TiO2 catalysts for photocatalytic reduction of CO2: High efficient separation of photogenerated charge carriers

The photocatalytic conversion of CO2 and H2O into value-added chemicals using sunlight is significant to solve energy crisis and environmental problems. In this work, a series of novel bifunctional catalysts of core-shell structured AuPd nanoparticles decorated 3DOM TiO2 (AuPd/3DOM-TiO2) w were successfully fabricated via a facile one-pot method of gas bubbling-assisted membrane reduction (GBMR). AuPd/3DOM-TiO2 catalysts show uniform 3D ordered macroporous structure, and the slow photon effect of 3DOM-TiO2 as a photonic crystal can enhance light-harvesting efficiency. AuPd nanoparticles are highly dispersed on the surface of 3DOM-TiO2 carrier. Since bimetallic AuPd nanoparticles with the relatively low Fermi level have good capacity of trapping electron, they can efficiently promote the separation of photogenerated electron-hole pairs in TiO2. The AuPd/3DOM-TiO2 catalysts exhibit excellent photocatalytic activity for CO2 reduction with H2O to CH4 under light irradiation. Among the studied catalysts, Au3Pd1/3DOM-TiO2 catalyst exhibits the highest photocatalytic activity and selectivity for CO2 reduction, e.g., its formation rate of CH4 is 18.5μmolg−1h−1 and its selectivity to CH4 production by CO2 reduction is 93.9%. The possible mechanism of AuPd/3DOM-TiO2 catalysts for photocatalytic CO2 reduction is also proposed, and it would guide further design and synthesis of high efficient photocatalysts for CO2 reduction with H2O.

Slow Photons for Photocatalysis and Photovoltaics.

Solar light is widely recognized as one of the most valuable renewable energy sources for the future. However, the development of solar-energy technologies is severely hindered by poor energy-conversion efficiencies due to low optical-absorption coefficients and low quantum-conversion yield of current-generation materials. Huge efforts have been devoted to investigating new strategies to improve the utilization of solar energy. Different chemical and physical strategies have been used to extend the spectral range or increase the conversion efficiency of materials, leading to very promising results. However, these methods have now begun to reach their limits. What is therefore the next big concept that could efficiently be used to enhance light harvesting? Despite its discovery many years ago, with the potential for becoming a powerful tool for enhanced light harvesting, the slow-photon effect, a manifestation of light-propagation control due to photonic structures, has largely been overlooked. This review presents theoretical as well as experimental progress on this effect, revealing that the photoreactivity of materials can be dramatically enhanced by exploiting slow photons. It is predicted that successful implementation of this strategy may open a very promising avenue for a broad spectrum of light-energy-conversion technologies.

Sandwich-structured TiO2 inverse opal circulates slow photons for tremendous improvement in solar energy conversion efficiency

Slow photon circulation in a sandwich-structured TiO2 inverse opal results in tremendously improved photon absorption.

Slow light and slow acoustic phonons in optophononic resonators

Slow light has been exploited in optoelectronics to enhance the light-matter interaction, including light absorption and emission, nonlinear processes such as sum-frequency generation and phase modulation, and lasing. Modulated structures that modify the propagation of light can also be tailored to alter the propagation of sound. The question then arises: how can slow high-frequency acoustic waves and slow light in the same device perform with a perspective in optomechanics? This question becomes particularly relevant in view of recent reports of photon lifetimes extended to the millisecond range (lifetimes that are characteristic of phonons) by introducing slow-light effects in microresonators. With this application in mind, this work studies DBR-based GaAs/AlAs microcavities as structures that present both light confinement and slowing-down, depending on whether the laser is tuned resonantly to the cavity or stop-band edge modes. Interestingly, the authors show that for these kind of devices precisely the same happens for acoustic phonons, thus providing a rich playground to investigate confined and slow optomechanical effects. Time-resolved coherent phonon generation experiments using picosecond lasers are reported, showing a strong enhancement of the optomechanical coupling using both confined and also properly designed slow photon and phonon modes. The prospects for the use of these optoelectronic devices in confined and slow optomechanics is addressed.

Slow photon amplification of gas-phase ethanol photo-oxidation in titania inverse opal photonic crystals

Here we describe the successful fabrication of six titania inverse opal (TiO2 IO) photocatalysts with fcc[111] pseudo photonic band gaps (PBGs) tuned to span the UV–vis region. Photocatalysts were fabricated by a colloidal crystal templating and sol-gel approach – a robust and highly applicable bottom-up scheme which allowed for precise control over the geometric and optical properties of the TiO2 IO photocatalysts. Optical properties of the TiO2 IO thin films were investigated in detail by UV–vis transmittance and reflectance measurements. The PBG along the fcc[111] direction in the TiO2 IOs was dependent on the inter-planar spacing in the [111] direction, the incident angle of light and the refractive index of the medium filling the macropores in the IOs, in agreement with a modified Bragg’s law expression. Calculated photonic band structures for the photocatalysts revealed a PBG along the Γ→L direction at a/λ∼0.74, in agreement with the experimental optical data. By coupling the low frequency edge of the PBG along the [111] direction with the electronic absorption edge of anatase TiO2, a two-fold enhancement in the rate of gas phase ethanol photo-oxidation in air was achieved. This enhancement appears to be associated with a ‘slow photon’ effect that acts to both enhance TiO2 absorption and inhibit spontaneous emission (i.e. suppress electron-hole pair recombination).

Review of roles for photonic crystals in solar fuels photocatalysis

We cover the intersection of nanophotonics, materials such as photonic crystals (PC), but also irregular templated light scattering interfaces, with their application to solar fuels photocatalysis. We describe the fundamental principles of adapting nanophotonics to photocatalysis, particularly slow photon effects and how appropriate choice of stop band and edge position of the PC can be exploited for light harvesting. We also review several representative examples of nanophotonic design applied to photocatalytic semiconductor materials. We include the most heavily investigated photocatalytic materials (such as TiO2), as well as inherently visible light active semiconductors, and materials sensitized with semiconductor nanocrystals or plasmonic metal nanoparticles. Finally, we review alternative scattering interfaces useful for improving the performance of solar fuels photocatalysis.

Novel three-dimensionally ordered macroporous SrTiO3 photocatalysts with remarkably enhanced hydrogen production performance

Three-dimensionally ordered macroporous SrTiO3 (3DOM-SrTiO3) materials with different pore sizes were originally prepared and employed as photocatalysts in the water splitting for hydrogen generation. These 3DOM-SrTiO3 photocatalysts were characterized by XRD, SEM, TEM, EDS, DR UV–vis and N2 sorption. Because of the slow photon effect of 3DOM-SrTiO3, notably enhanced efficiency of water splitting for hydrogen evolution was obtained compared with the solid-state SrTiO3 (SS-SrTiO3) and disordered porous SrTiO3 (Disorder-SrTiO3). The effect of stop-band on the efficiency of photocatalytic hydrogen production was studied by tuning the pore diameter of 3DOM-SrTiO3. It was found that higher hydrogen evolution efficiency could be achieved when the photonic stop-band of 3DOM-SrTiO3 was overlapped with its band gap, which was further confirmed by the control experiments under certain wavelength light irradiation. 3DOM-SrTiO3(270nm) exhibited the highest photocatalytic hydrogen evolution rate (up to 3599μmol/g·h), which was 31 times as high as that of SS-SrTiO3 and 11 times higher than that of Disorder-SrTiO3. Furthermore, these 3DOM-SrTiO3 catalysts were very stable and no obvious loss in photocatalytic efficiency of hydrogen production was observed after five cycles.

Ga doped ZnO photonic crystals with enhanced photocatalytic activity and its reaction mechanism

In this study, Ga doped ZnO photonic crystals were developed for the purpose of achieving the cooperative enhancement from the photonic crystals structure and Ga doping. The Ga doped ZnO photonic crystals with highly ordered skeleton structure could be obtained via a dip-coating infiltration method. By varying the pore diameter of photonic crystals, the slow photon effect could be found over the sample with a pore diameter of 217nm and the corresponding electronic band gap absorption was enhanced. Moreover, the introduction of Ga increased the specific surface area of Ga doped ZnO photonic crystals while the concentration of hydroxyl oxygen on the surface decreased simultaneously, which formed a competitive relationship in determining the photocatalytic performance of Ga doped ZnO photonic crystals. The photocatalytic activities for the degradation of rhodamine B showed that 3 at% Ga doped ZnO photonic crystals with a pore diameter of 217nm exhibited higher activity compared with other samples. The higher activity could be attributed to the slow photon effect from the photonic crystal structure and increased specific surface area after Ga doping. Furthermore, based on the results of the detection of radicals, the degradation mechanism was discussed in details.

Surface plasmon resonance of gold nanocrystals coupled with slow-photon-effect of biomorphic TiO2 photonic crystals for enhanced photocatalysis under visible-light

The slow photon effect in a photonic crystal (PC) can lead to delay and storage of light in the PC and has an immense potential for improving solar-to-energy efficiency. A series of biomorphic TiO2 photonic crystal (TiO2-BMPC) structures are fabricated through a sol–gel method by using butterfly wings as templates, and subsequently gold nanoparticles (Au NPs) with an average size of about 9nm are deposited in situ into the TiO2-BMPC. Varying structural characteristics of the PCs by choosing different butterfly wings, the slow photon region of the photonic band gap could be purposely tuned to overlap with strong localized surface plasmon resonance (SPR) region of the Au NPs. The matching slow photon of the TiO2-BMPC templated from the butterfly wings of Euploea mulciber intensifies the SPR responses (central at 550nm) of the Au NPs. Consequently, the visible-light harvesting capability of the Au/TiO2-BMPC is significantly improved due to this unique biomorphic architecture. As a result, the designed photocatalyst exhibits a photocatalytic activity that is several time higher than conventional Au/TiO2-P25 material, as illustrated by the example of the photocatalytic decomposition of methyl orange (MO) under visible-light illumination.

3D nanospherical CdxZn1−xS/reduced graphene oxide composites with superior photocatalytic activity and photocorrosion resistance

Herein, a series of CdxZn1−xS and sulfide/graphene photocatalysts with 3D nanospherical framework have been successfully fabricated by one-pot solvothermal method for the first time. The morphology and structure of samples were confirmed by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray (EDX) spectrometry, N2 adsorption, Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS). The as-prepared samples exhibit excellent photocatalytic activities and photocorrosion resistance in the degradation of dyes under visible light. The Cd0.5Zn0.5S/rGO sample shows the most efficient in the photodegradation of methyl orange (MO). It takes about 30min for degradation completely. The enhanced photocatalytic activity is mainly attributed to the slow photon enhancement of the 3D structure, and the heterojunction between the 3D nanospherical Cd0.5Zn0.5S solid solutions and a high quality 2D rGO support, which can greatly promote the separation of light-induced electrons and holes. Moreover, the large SBET and extended light absorption range also play an important role for improving the photocatalytic activity. The high photocatalytic stability is due to the successful inhibition of the photocorrosion of Cd0.5Zn0.5S/rGO by forming heterojunction between CdS and ZnS, and transferring the photogenerated electrons of Cd0.5Zn0.5S to rGO. The present work can provide rational design of graphene-based photocatalysts with large contact interface and strong interaction between the composites for other application.

Demonstration of slow photon chemistry on multilayer inverse opals

We demonstrate the use of atomic layer deposited multilayered alumina/titania photonic crystals for stearicacid (SA) degradation using broadband light illumination. The degradation of SA was monito ...

Efficient light harvesting over a CdS/In2O3 photonic crystal photocatalyst for hydrogenation of 4-nitroaniline to p-phenylenediamine.

Efficient light harvesting was observed over CdS photodeposited on In2O3 photonic crystals during the photocatalytic hydrogenation of 4-nitroaniline to p-phenylenediamine. The highest conversion of 4-nitroaniline and selectivity of p-phenylenediamine over the In2O3 photonic crystal supported CdS were ∼93% and ∼99%, respectively, which were better than that achieved for commercial hexagonal CdS. The existence of the photonic crystal structure was responsible for the higher efficiency of In2O3 photonic crystal supported CdS. The ordered structure facilitated the mass transport. The elaborate tuning of the photonic band gap activated the slow photon enhancement effect on the blue edge for intensifying the light harvesting efficiency of CdS. Moreover, CdS supported on In2O3 photonic crystals exhibited higher photocatalytic stability than that on the In2O3 nanocrystals. The mechanism of photocatalytic hydrogenation over In2O3 photonic crystal supported CdS was discussed. Our results provided guidance for efficiently utilizing light for photocatalysis by applying photonic crystals as support.

Inverse Opal Photonic Crystals as a Strategy to Improve Photocatalysis: Underexplored Questions.

The structuring of materials in the form of photonic crystals for photocatalytic applications is a quite new strategy aiming to enhance the performance of the photocatalysts at wavelength ranges where their absorption is poor. It is of particular interest to successfully manufacture an efficient photocatalytic system that could make use of solar light. Thus, the key of the strategy is the "slow photon effect", occurring at the edges of a forbidden band for photons. In this Perspective we have chosen some questions that we consider of relevance and that are well worth being addressed both theoretically and experimentally. It is the aim of this discussion to eventually lead to a more productive use of inverse opals as photonic photocatalytic materials.

Fabrication of inverse opal TiO2-supported Au@CdS core–shell nanoparticles for efficient photocatalytic CO2 conversion

The photocatalytic conversion of CO2 with H2O into renewable hydrocarbon fuels by solar energy is of significance in solving both energy and environmental problems. Herein, we firstly report a practical fabrication of all-solid-state three-component Z-scheme system with the core–shell structured Au@CdS core–shell nanoparticles on inverse opal TiO2 (Au@CdS/IO-TiO2) via the gas bubbling-assisted membrane reduction-precipitation (GBMR/P) method. All the catalysts possess well-defined inverse opal structure with the interconnected networks of spherical voids, and the Au@CdS core–shell nanoparticles with different molar ratios of Cd/Au are well dispersed and supported on the inner wall of uniform macropore. The slow photon effect of inverse opal structure with moderate macropore sizes can enhance the light-harvesting efficiency. And the all-solid-state Z-scheme system with CdS(shell)-Au(core)-TiO2(support) nanojunction is favourable for the separation of photogenerated electrons and holes due to the vectorial electron transfer of TiO2→Au→CdS. The Au@CdS/IO-TiO2 catalysts exhibit super photocatalytic performance for CO2 reduction to CH4 under the simulated solar irradiation. Among the as-prepared catalysts, Au@CdS/IO-TiO2-1 catalyst with the moderate thickness of CdS nanolayer shell shows the highest photocatalytic activity and selectivity for CO2 reduction, e.g., its formation rate of CH4 is 41.6μmolg−1h−1 and its selectivity to CH4 production by CO2 reduction is 98.6%. The design and versatile synthetic approach of all-solid-state Z-scheme system on the surface of inverse opal oxides are expected to throw new light on the fabrication of highly efficient photocatalyst for CO2 reduction to hydrocarbon.

Three-dimensionally ordered macroporous WO3 supported Ag3PO4 with enhanced photocatalytic activity and durability

Ag3PO4 nanoparticles were firstly deposited into the three-dimensionally ordered macroporous WO3 (3DOM-WO3) with different pore sizes. The resulted Ag3PO4/3DOM-WO3 composites were characterized by XRD, SEM, TEM and DR UV–vis. These composite photocatalysts showed extraordinarily excellent efficiencies in the visible light degradation of organic contaminants and water splitting for oxygen evolution, which were mainly due to the synergic effect of Ag3PO4 and 3DOM-WO3 as well as the periodic macroporous structure of 3DOM-WO3. Especially, the catalyst A5W5 with Ag3PO4:WO3 mass ratio of 5:5 exhibited the highest catalytic activity, and complete degradation could be achieved within 4min for phenol and three typical dyes investigated in this work. In addition, the slow photon effect in Ag3PO4/3DOM-WO3 was demonstrated, and the pore size of 3DOM-WO3 was found to have a significant influence on the catalytic performance. The catalyst A5W5(270) showed notably higher photocatalytic activity than A5W5(150) and A5W5 (420), which was mainly attributed to the exact overlap between the stop-band of 3DOM-WO3(270) and the electronic absorption band of Ag3PO4/3DOM-WO3 composite. Moreover, these composite catalysts were very stable and could be recycled ten times without any loss in photocatalytic activity for RhB degradation. More excitingly, remarkably improved activity and durability were also obtained over Ag3PO4/3DOM-WO3 in water splitting for oxygen evolution. According to the XPS and HRTEM characterizations of used catalyst, the enhanced durability was mainly attributed to the effective transfer of photogenerated electrons from Ag3PO4 to 3DOM-WO3.

Fabrication and photocatalysis of BiFeO3 with inverse opal structure

The bismuth ferrite (BiFeO3, BFO)with a face-centered cubic inverse opal structure was prepared by the sol–gel method, involving the infiltration of liquid precursors into polystyrene (PS) colloidal crystals and the post-annealing for the remove of PS and the crystallization of BFO. This inverse opals exhibit a continuous 3D photonic crystal structure consisting of single phase BFO. The photocatalytic investigation reveals that the degradation rate of Rhodamine B under UV–vis light irradiation is 1.4 times that of the conventional BFO film without the high periodic porous structure. The high surface area, slow photon and light scattering effects due to the photonic crystal structure are responsible for the enhanced photocatalysis of BFO inverse opal. This work is expected to improve the comprehensive performance of BFO by integrating the photonic effect together with its ferromagnetic and visible-light-driving catalysis properties.