Facet Coupling Research Articles

Crystal facet/interface anchored Janus activity of BiOBr in driving photocatalytic water splitting

Constructing Janus junctions is expected to provide a novel perspective for analyzing the intrinsic photocatalytic activity of bismuth oxyhalide. Herein, Janus-typed BiOBr(010)/BiOBr(001) homojunction with efficiently photocatalytic H2 evolution has been achieved by a hydrothermal synthesis. The experimental results show that the designed Janus-typed junction can effectively elevate the carrier lifetime, significantly enhance the photocurrent, and prominently reduce the overpotential. The potential difference (0.34 eV) between (010) facet and (001) facet can drive the carrier transport at the interface for Janus junctions. Theoretical calculations indicate that the designed Janus junction has a stable mechanical structure, providing certain support for the long lifetime of the photocatalyst in the photocatalytic process. The carrier distributions in the Janus junction exhibit the typical characteristic distribution. Interestingly, the photo-generated electrons are only localized in BiOBr(001) component, which embodies significantly a role of n-type semiconductor. This essentially activates HER activity (32 µmol/h g−1) of Janus junction. The holes preferentially localize in the BiOBr(010) component of Janus junction. This component undertaking p-type role reduces the free energy of the rate determining step (from OH* to O*) in the oxygen evolution process (0.88 eV). The Janus junction intensively suppresses the carrier recombination probability at the high symmetry point (reciprocal space) in BiOBr. Ab initio molecular dynamics calculations show that the adsorbed water is easy to decompose on the surface of Janus junction. The obtained results of the photocatalytic water splitting further confirm that the designed BiOBr(010)/BiOBr(001) has Janus characteristics, which are consistent with theoretical calculations and tracer tracing results. The diffusion characteristics of hydrogen also evidence that the interaction of the internal electric field in the Janus junction can drive the HER performance. Additionally, these results offer us a unique angle of view to regulate the charge carrier dynamics by the highly oriented facet coupling for the layered semiconductor.

In situ construction of Ag/Bi2O3/Bi5O7I heterojunction with Bi-MOF for enhance the photocatalytic efficiency of bisphenol A by facet-coupling and s-scheme structure

In this study, Ag/Bi2O3/Bi5O7I with s-scheme heterostructures were successfully synthesized in situ by nano-silver modification of CUA-17 and halogenated hydrolysis.The growth rate of Bi2O3 crystals was effectively controlled by adjusting the doping amount of Ag, resulting in the formation of a facet-coupling heterojunctions. Through the investigation of the microstructure and compositional of catalysts, it has been confirmed that an intimate facet coupling between the Bi2O3 (120) facet and the Bi5O7I (312) facet, which provides robust support for charge transfer. Under visible light irradiation, the AgBOI.3 heterojunction photocatalyst exhibited an outstanding degradation rate of 98.2% for Bisphenol A (BPA) with excellent stability. Further characterization using optical, electrochemical, impedance spectroscopy, and electron spin resonance techniques revealed significantly enhanced efficiency in photogenerated charge separation and transfer, and confirming the s-scheme structure of the photocatalyst. Density functional theory calculations was employed to elucidate the mechanism of BPA degradation and the degradation pathway of BPA was investigated by LC-MS. Finally, the toxicity of the degradation intermediates was evaluated using T.E.S.T software.

Divergence-Conforming Velocity and Vorticity Approximations for Incompressible Fluids Obtained with Minimal Facet Coupling

We introduce two new lowest order methods, a mixed method, and a hybrid discontinuous Galerkin method, for the approximation of incompressible flows. Both methods use divergence-conforming linear Brezzi–Douglas–Marini space for approximating the velocity and the lowest order Raviart–Thomas space for approximating the vorticity. Our methods are based on the physically correct viscous stress tensor of the fluid, involving the symmetric gradient of velocity (rather than the gradient), provide exactly divergence-free discrete velocity solutions, and optimal error estimates that are also pressure robust. We explain how the methods are constructed using the minimal number of coupling degrees of freedom per facet. The stability analysis of both methods are based on a Korn-like inequality for vector finite elements with continuous normal component. Numerical examples illustrate the theoretical findings and offer comparisons of condition numbers between the two new methods.

Auto-Calibrated Charge-Sensitive Infrared Phototransistor at 9.3 µm.

Charge-sensitive infrared photo-transistors (CSIP) are quantum detectors of mid-infrared radiation (λ=4 µm-14 µm) which have been reported to have outstanding figures of merit and sensitivities that allow single photon detection. The typical absorbing region of a CSIP consists of an AlxGa1-xAs quantum heterostructure, where a GaAs quantum well, where the absorption takes place, is followed by a triangular barrier with a graded x(Al) composition that connects the quantum well to a source-drain channel. Here, we report a CSIP designed to work for a 9.3 µm wavelength where the Al composition is kept constant and the triangular barrier is replaced by tunnel-coupled quantum wells. This design is thus conceptually closer to quantum cascade detectors (QCDs) which are an established technology for detection in the mid-infrared range. While previously reported structures use metal gratings in order to couple infrared radiation in the absorbing quantum well, here, we employ a 45° wedge facet coupling geometry that allows a simplified and reliable estimation of the incident photon flux Φ in the device. Remarkably, these detectors have an "auto-calibrated" nature, which enables the precise assessment of the photon flux Φ solely by measuring the electrical characteristics and from knowledge of the device geometry. We identify an operation regime where CSIP detectors can be directly compared to other unipolar quantum detectors such as quantum well infrared photodetectors (QWIPs) and QCDs and we estimate the corresponding detector figure of merit under cryogenic conditions. The maximum responsivity R = 720 A/W and a photoconductive gain G~2.7 × 104 were measured, and were an order of magnitude larger than those for QCDs and quantum well infrared photodetectors (QWIPs). We also comment on the benefit of nano-antenna concepts to increase the efficiency of CSIP in the photon-counting regime.

Hexahedron Symmetry and Multidirectional Facet Coupling of Orthorhombic CsPbBr3 Nanocrystals

The cube shape of orthorhombic phase CsPbBr3 nanocrystals possesses the ability of selective facet packing that leads to 1D, 2D, and 3D nanostructures. In solution, their transformation with linear one-dimensional packing to nanorods/nanowires is extensively studied. Here, multifacet coupling in two directions of the truncated cube nanocrystals to rod couples and then to single-crystalline rectangular rods is reported. With extensive high-resolution transmission electron microscopy image analysis, length and width directions of these nanorods are derived. For the seed cube structures, finding {110} and {002} facets has remained difficult as these possess the hexahedron symmetry and their size remains smaller; however, for nanorods, these planes and the ⟨110⟩ and ⟨001⟩ directions are clearly identified. From nanocrystal to nanorod formation, the alignment directions are observed as random (as shown in the abstract graphic), and this could vary from one to the other rods obtained in the same batch of samples. Moreover, seed nanocrystal connections are derived here as not random and are rather induced by addition of the calculated amount of additional Pb(II). The same has also been extended to nanocubes obtained from different literature methods. It is predicted that a Pb-bromide buffer octahedra layer was created to connect two cubes, and this can connect along one, two, or even more facets of cubes simultaneously to connect other cubes and form different nanostructures. Hence, these results here provide some basic fundamentals of seed cube connections, the driving force to connect those, trapping the intermediate to visualize their alignments for attachments, and identifying and establishing the orthorhombic ⟨110⟩ and ⟨001⟩ directions of the length and width of CsPbBr3 nanostructures.

Bi2 S3 -In2 S3 Heterostructures for Efficient Photoreduction of Highly Toxic Cr6+ Enabled by Facet-Coupling and Z-Scheme Structure.

The construction of Z-scheme photocatalyst materials mimicking the natural photosynthesis system provides many advantages, including increased light harvesting, spatially separated reductive and oxidative active sites and strong redox ability. Here, a novel Bi2 S3 nanorod@In2 S3 nanoparticle heterojunction photocatalyst synthesized through one-pot hydrothermal method for Cr6+ reduction is reported. A systematic investigation of the microstructural and compositional characteristics of the heterojunction catalyst confirms an intimate facet coupling between (440) crystal facet of In2 S3 and (060) crystal facet of Bi2 S3 , which provides a robust heterojunction interface for charge transfer. When tested under visible-light irradiation, the Bi2 S3 -In2 S3 heterojunction photocatalyst with 15% Bi2 S3 loading content achieves the highest Cr6+ photoreduction efficiency of nearly 100% with excellent stability, which is among the best-reported performances for Cr6+ removal. Further examination using optical, photoelectrochemical, impedance spectroscopy, and electron spin resonance spectroscopy characterizations reveal greatly improved photogenerated charge separation and transfer efficiency, and confirm Z-scheme electronic structure of the photocatalyst. The Z-scheme Bi2 S3 -In2 S3 photocatalyst demonstrated here presents promise for the removal of highly toxic Cr6+ , and could also be of interest in photocatalytic energy conversion.

Insight into sulfamethoxazole degradation, mechanism, and pathways by AgBr-BaMoO4 composite photocatalyst

A composite photocatalyst, AgBr-BaMoO4 was fabricated by two step method; microwave hydrothermal and precipitation-deposition. The as prepared photocatalyst samples were characterized by various techniques. The facet coupling was seen between the (204) plane of BaMoO4 and (200)/(222) planes of AgBr on the basis of XRD/HRTEM analysis. The pharmaceutical pollutant, sulfamethoxazole was adopted to investigate the photocatalytic performances of samples under UV–vis irradiation. The AgBr-BaMoO4 composite degraded the aqueous sulfamethoxazole drug in UV–vis light about 64% within 75 min, which was attributed to efficient separation of photogenerated electron–hole pairs across the interface between Ag/AgBr and BaMoO4. The multi-electron induced oxygen reduced reaction (ORR) was observed. The radical trapping experiment indicates that OH• has major role for sulfamethoxazole degradation. The four successive photodegradation of sulfamethoxazole in UV–vis light indicates the stability of composite photocatalyst. Furthermore, the three different degradation pathways were designed on the basis of retention time and molecular masses of 18 degraded organic fragments that was confirmed by high-performance liquid chromatography photodiode array (HPLC-PDA) and high resolution-quadruple time of flight electrospray ionization mass spectroscopy (HR-QTOF ESI/MS) techniques. The total organic carbon (TOC) analysis suggested the mineralization of SMZ by composite photocatalyst. This study not only demonstrates the enhancement of photocatalytic performance of wide band gap semiconductor by making composite with narrow band gap semiconductor but also detail degradation pathways and mechanisms of sulfamethoxazole.

Ultrafast terahertz detectors

Terahertz (THz) technologies have broad application prospects in ultrafast space communication, heterodyne detection, biological detection, non-destructive testing and national security. Ultrafast THz detectors, which can respond to the THz light with modulation rate larger than 1 GHz, are the key component of fast imaging, space communication, ultrafast spectroscopy and THz heterodyne applications. Theoretically, the traditional THz detectors based on heat effects are difficult to meet the requirements for fast detections, while the semiconductor based THz detectors can work under the condition of ultrafast detection. Photoconductive antennas with ultrafast response time are suitable for room-temperature broad-spectrum THz detections. Schottky barrier diodes, superconductor-insulator-superconductor mixers and hot electron bolometers are promising candidates for high-speed THz spatial heterodyne and direct detections attributable to their high conversion efficiency and low noise. High-mobility field effect transistors based on two-dimensional graphene material have the advantages of high sensitivity and low impedance, which make this kind of device have great potential applications in room-temperature high-speed detections. THz quantum well detectors (THz QWPs) based on inter-subband transitions are very suitable for the applications in high-frequency and high-speed detections because of the advantages of high responsivity, small value and integrated packaging. Recently, we have demonstrated 6.2 GHz bandwidth modulation by using THz QWPs, the fast THz receiving device. On the other hand, low working temperature and low coupling efficiency are the main factors that restrict the applications of THz QWPs. From the Brewster angle, 45 polished facet coupling structure, to one-or two-dimensional metal grating and surface Plasmon polariton coupling configuration, researchers often explore the appropriate coupling mechanism which can not only couple the normal incidence THz light, but also improve the coupling efficiency substantially. The sub-wavelength double-metal micro-cavity array coupling structure has two advantages which make THz QWPs a key candidate for fast imaging and detection in THz band:firstly, the patch antennas on the device surface can effectively increase the light absorption region, and the periodic structure can make the normal incidence THz light fulfill the rule of intersubband transition. Secondly, the sub-wavelength size double metal structure can restrict the light within a very small volume, and the electric current will be enhanced by the resonance effect when the cavity mode is equal to the peak response frequency, which can suppress the dark current and improve the optical coupling efficiency of the device. In this paper, several ultrafast THz detectors are reviewed and the advantages and disadvantages of various detectors are also analyzed.

Microtetrahedronal Bi12TiO20/g-C3N4 composite with enhanced visible light photocatalytic activity toward gaseous formaldehyde degradation: Facet coupling effect and mechanism study

Uniform Bi12TiO20 microtetrahedrons exposed by {111} facets were synthesized by a controlled hydrothermal method. Well resolved density functional theory (DFT) calculations showed that {111} facet was the most active facet with the highest surface energy among the low index facets of Bi12TiO20. Series of facet coupled microtetrahedronal Bi12TiO20/g-C3N4 composites were prepared via a simple two-step method. The interface structure of composite was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS), and fourier transform infrared (FT-IR) spectroscopy analyses, showing that the interface of Bi12TiO20/g-C3N4 composite was connected by the {111} facets of Bi12TiO20 and the {002} facets of g-C3N4. Owing to this typical structure, the Bi12TiO20/g-C3N4 composite with mass ratio 0.5:0.02 of Bi12TiO20 to g-C3N4 presented higher photocatalytic activity for degradation of gaseous formaldehyde under visible light irradiation (λ≥400nm) as compared to the pure g-C3N4 and the as-obtained Bi12TiO20. Furthermore, the band structure calculations were employed to investigate the photocatalytic mechanism of Bi12TiO20/g-C3N4 composite.

Synthesis and enhanced photocatalytic activity of a BiOI/TiO2 nanobelt array for methyl orange degradation under visible light irradiation

A novel heterojunction by facet coupling of BiOI onto TiO2 nanobelt arrays (NBAs) as a visible light photocatalyst was achieved through a hydrothermal method.

Synthesis of BiOI@(BiO)2CO3 facet coupling heterostructures toward efficient visible-light photocatalytic properties.

In this paper, BiOI@(BiO)2CO3 facet coupling heterostructures were synthesized via exfoliation and ion exchange, and characterized by X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), UV-vis diffuse reflectance spectra (DRS), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectra and valence-band XPS spectra. With the reaction time increasing, more BiOI can be transformed to (BiO)2CO3, and BiOI@(BiO)2CO3 facet coupling heterostructures were obtained. The photocatalytic results showed that BiOI@(BiO)2CO3 displays much higher photocatalytic activity than BiOI and (BiO)2CO3 under visible-light. The photocatalytic mechanism study revealed that BiOI@(BiO)2CO3 has strong adsorption for RhB molecules due to the ultrathin nanosheets and higher BET, and displays better separation efficiency of photoinduced charge carriers and higher photocurrent due to the {001}/{001} facet coupling.

Terahertz quantum well photo-detectors: grating versus 45° facet coupling

We demonstrate two GaAs/AlGaAs-based terahertz (THz) quantum well photo-detectors (QWPs), with one-dimensional (1D) metal grating and 45° facet light coupling configurations. The two THz QWPs have the same multi-quantum well structure parameters. The different light coupling configurations do not affect the peak response frequency (5.85 THz). The background limited infrared performance temperatures and responsivities of the two devices are measured. The effects of different light coupling configurations on the main features of the devices are investigated in detail. The experimental results show that the performance of the THz QWP with 1D metal grating is a little better but there is still a lot of room for improvement.

Heterojunction of facet coupled g-C3N4/surface-fluorinated TiO2 nanosheets for organic pollutants degradation under visible LED light irradiation

Novel construction of a heterojunction structure by facet coupling of surface-fluorinated TiO2 (F-TiO2) nanosheet onto g-C3N4 nanosheet as a visible light photocatalyst was achieved through a simple hydrothermal method. Facet coupled structure between F-TiO2-{001} nanosheet and g-C3N4-{002} nanosheet was evidently investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), Fourier transform infrared (FT-IR) spectroscopy and UV–vis diffuse reflectance spectroscopy (DRS). The g-C3N4/F-TiO2 facet coupled hybrid with remarkably increased interfacial area presented a significantly enhanced photocatalytic performance in degrading methylene blue (MB) under 410nm LED light irradiation. The obviously reduced electron–hole recombination rate of hybrid was demonstrated from photoluminescence (PL) spectroscopy measurements and the photoelectrochemical evaluation. An optimal g-C3N4 content has been determined to be 30wt%, corresponding to apparent pseudo-first-order rate constant kapp of 0.0374min−1. It is 4.5 times and 13.9 times more than that of pure F-TiO2 nanosheets and commercial P25 photocatalyst, respectively.

Facets coupling of BiOBr-g-C3N4 composite photocatalyst for enhanced visible-light-driven photocatalytic activity

BiOBr-g-C3N4 inorganic–organic composite photocatalysts were synthesized by a one-step chemical bath method at low temperature and characterized by X-ray diffraction patterns (XRD), Fourier transform infrared (FT-IR), thermo gravimetric (TG), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and UV–vis diffuse reflectance spectra (DRS). The BiOBr-g-C3N4 composite showed much higher visible-light-driven (VLD) photocatalytic activity than pure g-C3N4 and BiOBr for rhodamine B (RhB) degradation. The photocatalytic mechanism analysis revealed that the interreaction between BiOBr and g-C3N4 is a kind of facet coupling between BiOBr-{001} and g-C3N4-{002}. The active species trapping and quantification experiments indicated that the photoinduced charges transfer between these facets resulted in the efficient charge separation.

Surface Plasmon-Enhanced Absorption in Metal Grating Coupled Terahertz Quantum Well Photodetectors

Effects of surface plasmon polaritons (SPPs) on the absorption efficiencies of metal-grating-coupled terahertz quantum well photodetectors (QWPs) are theoretically investigated. The thin top contact layers of the devices are utilized to support the SPPs, and the metal gratings couple p-polarized incident light into SPP modes. The electric fields related to the SPP modes have large components in the quantum well growth direction and are mostly localized in the multi-quantum-well regions of the terahertz QWPs. The dependences of the resonant absorption behaviors on the device parameters are systematically studied. A maximum value of about 30-fold enhancement of inter-sub-band absorption efficiency is achieved in comparison with that of a traditional 45-degree facet coupling scheme. Our proposed metal-grating-coupled terahertz QWPs is expected to significantly improve the device performance.

Numerical Study on Metal Cavity Couplers for Terahertz Quantum-Well Photodetectors

Metal cavities composed of metal gratings, layered semiconductor structures, and bottom metal layers are proposed as light couplers for terahertz quantum-well photodetectors (QWPs). Due to a strong waveguide effect, the coupling properties of the metal cavities are significantly changed in comparison with the case of metal-grating-coupled terahertz QWPs. Numerical calculations show that there are two resonant coupling peaks for the sub-wavelength metal cavity couplers, with one located at the lower side of the first grating diffraction mode and the other at the upper side. A simple ray propagation picture explains the behaviors of the two resonant coupling peaks on a qualitative level. Our numerical data show that the coupling efficiencies of metal cavity couplers are several tens of times higher than that of the 45-degree facet coupling scheme.

Silicon–on–insulator waveguide photodetector with Ge/Si self-assembled islands

We have investigated a silicon-based near-infrared photodetector using a waveguide with strong optical confinement. The high-difference index waveguide is obtained with a silicon–on–insulator substrate. The optically active region consists of self-assembled Ge/Si islands embedded in a p-i-n junction. The Ge/Si islands grown by high-pressure chemical-vapor deposition exhibit a broad photoluminescence and electroluminescence which are resonant around 1.5 μm. The photoluminescence and electroluminescence energies are correlated to the island size and to the island composition using a six-band k⋅p calculation. The spectral responsivity of the detectors is measured in a front facet coupling geometry with a broadband source and with semiconductor laser diodes. For a 0 V applied bias, responsivities of 25 and 0.25 mA/W are measured at room temperature at 1.3 and 1.55 μm, respectively.

The symmetry of the amplified spontaneous emission spectrum in complex-coupled DFB lasers

We present relations between facet reflections and coupling coefficient for the amplified spontaneous emission (ASE) spectrum of a complex-coupled, an index-coupled, and a gain-coupled distributed feedback (DFB) laser to have a mirror image with respect to the Bragg wavelength. We show that the ASE spectrum of a complex-coupled DFB laser cannot be symmetric in general with respect to the Bragg wavelength. The index-coupled and gain-coupled DFB lasers can have symmetric ASE spectrum for some values of the phase of the facet reflection coefficient that are determined.

Dependence of mode stability and linewidth of DFB lasers on facet reflectivity and coupling coefficient

The effect on dominant emission mode and linewidth of 1550 nm DFB lasers of antireflection coating of a single facet has been investigated. The probability of a mode jump caused by the coating has been correlated with coupling coefficient magnitude. Linewidth increased by a factor of 1.6 when facet reflectivity decreased to 5%, compared with a calculated change of 2.3.