The InSe/g-CN van der Waals hybrid heterojunction as a photocatalyst for water splitting driven by visible light

Designing and developing the highly efficient photocatalysts is full of significance to achieve spontaneous photolysis water. In this work, using the first-principles calculations, we have performed a systematic theoretical study of water splitting photocatalytic activity of the InS e/g-CN heterojunction. It is concluded that the In Se/g-CN heterojunction is a typical type-II semiconductor, whose electrons and holes can be effectively separated. And the potential of the conduction band minimum（C...     

The spectroscopy of InSe nanoparticles

The synthesis of several different sizes of InSe nanoparticles from organometallic precursors is reported. These particles are characterized by transmission electron microscopy, electron diffraction, and optical spectroscopy. The electron diffraction results and optical properties indicate that these particles are two-dimensional disks, consisting of single Se-In-In-Se tetralayer sheets. The absorption spectra indicate strong quantum confinement along the z axis and, for the smaller particles, in the x,y plane. The z-axis quantum confinement may be quantitatively understood in terms of the band structure of bulk InSe. The results indicate that the z-axis quantum confinement reverses the order of the direct and indirect transitions in the case of the largest particles. The smaller particles exhibit strong, polarized fluorescence, and the fluorescence polarization may also be understood in terms of the band structure of bulk InSe.     

Exciton dynamics of GaSe nanoparticle aggregates

Time-resolved and static spectroscopic results on GaSe nanoparticle aggregates are presented to elucidate the exciton relaxation and diffusion dynamics. These results are obtained in room-temperature TOP/TOPO solutions at various concentrations. The aggregate absorption spectra are interpreted in terms of electrostatic coupling and covalent interactions between particles. The spectra at various concentrations may then be interpreted in terms of aggregate distributions calculated from a simple equilibrium model. These distributions are used to interpret concentration-dependent emission anisotropy kinetics and time-dependent emission spectral shifts. The emission spectra are reconstructed from the static emission spectra and decay kinetics obtained at a range of wavelengths. The results indicate that the aggregate z axis persistence length is about 9 particles. The results also show that the one-dimensional exciton diffusion coefficient is excitation wavelength dependent and has a value of about 2 x 10(-5) cm(2)/s following 406 nm excitation. Although exciton diffusion results in very little energy relaxation, subsequent hopping of trapped electron/hole pairs occurs by a Forster mechanism and strongly red shifts the emission spectrum.     

Multifunctional silicene/CeO2 heterojunctions: Desirable electronic material and promising water-splitting photocatalyst

The first-principles calculations demonstrate that covalently bonded (cb) heterojunction and van der Waals (vdW) heterojunction can coexist in silicene/CeO₂ heterojunctions, due to the different stacking patterns. Especially, the cb heterojunction with band gap of 1.97 eV, forms a type-II heterojunction, exhibits good redox performance and has high-effective optical absorption spectra, thus it is a promising photocatalyst for overall water splitting. Besides, for the vdW heterojunction, the Dirac cone of silicene is well kept on CeO₂ semiconducting substrate, with a considerable energy gap of 0.43 eV, which can be an ideal material in building silicene-based electronic device. These results may open a new gateway in both of nanoelectronic device and energy conversion for silicene/CeO₂ nanocomposites.     

Exciton trapping at heterojunctions in polymer blends

Optoelectronic devices made from semiconductor polymers often employ partially phase-separated binary polymer blends with "distributed heterojunctions" in the polymer film, and the migration of bulk excitons towards these heterojunctions crucially influences the device performance. Here, we investigate exciton migration in blend films of two polyfluorene derivatives. Localized exciplex states form in electron-hole capture at the heterojunction between the two polymers and these can be thermally excited to transfer to bulk excitons. Rapid radiative emission from these excitons can then allow efficient light-emitting diode operation. We show here that when these excitons migrate to another heterojunction site within their lifetime they are re-trapped at the interface and again form exciplex states or dissociate completely. We demonstrate that in polymer blend light-emitting diodes this can reduce the exciton population by more than 54% and can strongly influence the emission spectrum. We then analyze exciton re-trapping in detail using time-resolved photoluminescence spectroscopy on blends with different morphologies and find that for nanometer-scale phases exciton emission is completely suppressed. We show that the data agree well with a simple kinetic model which confirms the importance of the blend morphology for the exciton trapping efficiency.     

Exciton dissociation dynamics in model donor-acceptor polymer heterojunctions. I. Energetics and spectra

In this paper we consider the essential electronic excited states in parallel chains of semiconducting polymers that are currently being explored for photovoltaic and light-emitting diode applications. In particular, we focus upon various type II donor-acceptor heterojunctions and explore the relation between the exciton binding energy to the band offset in determining the device characteristic of a particular type II heterojunction material. As a general rule, when the exciton binding energy is greater than the band offset at the heterojunction, the exciton will remain the lowest-energy excited state and the junction will make an efficient light-emitting diode. On the other hand, if the offset is greater than the exciton binding energy, either the electron or hole can be transferred from one chain to the other. Here we use a two-band exciton to predict the vibronic absorption and emission spectra of model polymer heterojunctions. Our results underscore the role of vibrational relaxation and suggest that intersystem crossings may play some part in the formation of charge-transfer states following photoexcitation in certain cases.     

压力诱导相变原位构建BiVO4/Bi0.6Y0.4VO4型异质结增强光催化全解水性能

光催化完全分解水制氢是一个在粉末颗粒中实现多个串行物理化学步骤的复杂反应过程.这一过程在理论上具有体系简单、成本低、易操作等特点.然而,单步光激发系统中通常存在严重的光生载流子复合,这极大地制约了光催化的整体效率.利用能带结构不同的半导体合理构建异质结催化剂被认为是解决这一难题的重要途径之一.特别是近年来,S型异质结概念的提出为设计异质结结构以及分析不同半导体之间的载流子迁移问题提供了新的思路.本文以小粒径BiVO₄/Bi_0.6Y_0.4VO₄(BYV)为研究对象,首先利用"共沉淀-晶化"的方法制备了BYV固溶体纳米颗粒,随后利用压力诱导固溶体中四方相钒酸铋结构转变为单斜相,从而构建了BiVO₄/Bi_0.6Y_0.4VO₄复合光催化剂.XRD,Raman,HRTEM,HAADF-EDS的结果表明,经过高压后处理的BYV固溶体表面会出现粒径约为5 nm单斜钒酸铋纳米颗粒,实现了原位构建异质结结构.随后载流子动力学的相关表征以及Au选择性光沉积的结果表明,在光照条件下,所构建异质结中的光生电子主要分布在BYV固溶体上,而在表面形成的单斜相钒酸铋颗粒主要起到了类似"空穴"捕获的作用.这种在异质结中的载流子迁移路径符合S型异质结机理.电化学、稳态荧光光谱以及瞬态荧光光谱的表征结果表明,相比于单一固溶体,在S型异质结这种两步激发系统中所存在的载流子迁移路径能够大幅促进光生载流子分离,从而提高了小粒径BYV的光催化完全分解水性能.综上,构建S型异质结是一种解决小粒径光催化剂中载流子分离能力差的有效途径.同时,压力诱导材料晶型转变实现原位构建异质结的制备方法也为提高光生载流子分离效率提供了新的研究思路与机遇.     

Exciton energy transfer between optically forbidden states of molecular aggregates

3-ethyl-2-[3-(3-ethyl-2(3H)-benzoxazolylidene)-1-propenyl]benzoxazolium iodide (dye I) and pseudoisocyanine bromide are employed to form H aggregates as donors and J aggregates as acceptors. The energy of an H band of the H aggregates is higher than that of a J band of the J aggregates. It was confirmed that excitation of the H band does not emit fluorescence by comparison of excitation spectra of dye I H aggregates with that of dye I monomer. Absorption, fluorescence, and excitation spectra of spin-coated films of H aggregates mixed with various quantities of J aggregates have been observed. Excitation spectra probed at the J band are found to have a component of the H band. Fluorescence spectra originated from excitation of the H band are extracted and qualitatively analyzed. It is confirmed that excitation of the H band causes to emit fluorescence of a J band of the J aggregates. These phenomena show that exciton energy can transfer from the lowest energy in electronic states of the H aggregate, which state is optically forbidden, to electronic state of the J aggregate.     

Determination of Mass Attenuation Coefficients of Some Semiconductor and Biological Samples

The mass attenuation coefficients for TlGaSe₂, GaAs, GaSe, InSe, InSe:Al, InSe:Cd, InSe:Er, InSe:Ho, InSe:Sn, Si single crystals, human teeth, and different filling materials have been determined at different X-ray energies. We utilized X-ray absorption technique and a gravimetric method to determine these coefficients. X-ray spectra were collected using a Si(Li) detector with Camberra DSA-1000 desktop spectrum analyzer. The energy resolution of the spectrometer is 160 eV at 5.9 keV. The measured values were compared with the theoretical values using the WinXCom program. A good agreement was observed between the experimental theoretical values.     

Energy transfer via guest excitons in isotopic mixed naphthalene crystals

The temperature dependence of the fluorescence spectra of aggregates in naphthalene-perdeuteronaphthalene mixed crystals has been investigated between 1.4 and 70 K and for concentrations up to 50% naphthalene. It is shown that the most abundant traps — the monomer guest molecules — transfer energy like a guest exciton band 48 cm^?1 below the host exciton band. With increasing temperature, the excitation energy is redistributed between the different aggregate traps by thermal activation into the monomer states. The energy transfer constant within the monomer exciton band is measured as a function of concentration. It is suggested that dipole-dipole interaction between the monomer guests is responsible for the energy transfer via guest excitons.     

Low-temperature dynamics of weakly localized Frenkel excitons in disordered linear chains

We calculate the temperature dependence of the fluorescence Stokes shift and the fluorescence decay time in linear Frenkel exciton systems resulting from the thermal redistribution of exciton population over the band states. The following factors, relevant to common experimental conditions, are accounted for in our kinetic model: (weak) localization of the exciton states by static disorder, coupling of the localized excitons to vibrations in the host medium, a possible nonequilibrium of the subsystem of localized Frenkel excitons on the time scale of the emission process, and different excitation conditions (resonant or nonresonant). A Pauli master equation, with microscopically calculated transition rates, is used to describe the redistribution of the exciton population over the manifold of localized exciton states. We find a counterintuitive nonmonotonic temperature dependence of the Stokes shift. In addition, we show that depending on experimental conditions, the observed fluorescence decay time may be determined by vibration-induced intraband relaxation, rather than radiative relaxation to the ground state. The model considered has relevance to a wide variety of materials, such as linear molecular aggregates, conjugated polymers, and polysilanes.     

Vibronic fine structure in the absorption spectrum of oligothiophene thin films

A multimode Holstein Hamiltonian is used to describe optical excitations in quaterthiophene pinwheel aggregates. The Hamiltonian includes the coupling of excitons originating from the 1A(g)-->1B(u) electronic transition to phonons originating from the five intramolecular vibrational modes known from oligothiophene solution absorption/emission spectroscopy. The resulting eigenstates with lowest energy are best described as hybrid polaron phonons. The polarons are formed by coupling excitons with the higher frequency (688, 1235, and 1551 cm(-1)) vibrational modes, while the (optical) phonons arise from the lower frequency (161 and 333 cm(-1)) modes. The polaron phonons are responsible for the fine structure defining the A(1) band in the low-energy region of the absorption spectrum, ranging from the band origin to approximately 1500 cm(-1) beyond. The calculated A(1) band of quaterthiophene aggregates agrees favorably with that observed from thin films.     

CdS/聚合物梯形产氢光催化剂及其原位光照电子转移机制

氢气是公认的洁净、高效、可再生的能源载体,有望替代目前广泛使用的化石燃料.太阳能光催化产氢是实现高效、低成本、可持续生产氢能的一种途径.然而,单一组分光催化剂的性能受限于光生电子-空穴的快速复合,并且不能同时满足宽的太阳光吸收范围和强的氧化还原能力的要求,因此需要构建异质结来增强光生载流子的分离并保持高效光吸收和强氧化...     

Strongly coupled excitonic states in H-aggregated single crystalline nanoparticles of 2,5-bis(4-methoxybenzylidene) cyclopentanone

This paper reports that extremely strongly coupled excitonic states were formed in H-aggregated monocrystalline nanosheets and semicrystalline nanowires of coplanar organic molecules of 2,5-bis(4-methoxybenzylidene) cyclopentanone, due to the highly regular face-to-face stacking of molecular excitons. It was demonstrated that the spectral absorption and fluorescence emission behaviors are dependent on the routes of molecular aggregation and the ordered degree of molecular arrangement in aggregated nanoparticles. In particular, the H-type aggregation of molecules through a highly ordered molecular arrangement in the monocrystalline nanosheets led to the formation of a new exciton coupling state with an energy band higher than that in normal semi-/noncrystalline H-aggregation. A strong symmetric absorption at higher energy bands was thus observed in the solution of nanosheets. Furthermore, the strongly coupled excitonic state may hold all the oscillator strength, leading to the extinction of the original intramolecular electronic transitions of individual molecules and the appearance of new strong absorption and fluorescence emissions at high-energy bands. These results show a perspective that the ability to control the molecular structure and its arrangement in aggregates holds promise for creating novel optical properties in molecular materials.     

Long distance energy transfer in a polymer matrix doped with a perylene dye

Exciton migration over long distances is a key issue for various applications in organic electronics. We investigate a disordered material system which has the potential for long exciton diffusion lengths in combination with a high versatility. The perylene bisimide dye Perylene Red is incorporated in a polymer matrix with a high concentration. The dye molecules represent active sites with a narrow energy distribution for the electronically excited states. Excitons can be efficiently exchanged between them by F?rster resonance energy transfer (FRET). The narrow energy distribution reduces drastically the trapping probability of the excitons compared to polymers and allows for long transfer distances. To characterize the mobility of the excitons and their diffusion length the dye Oxazine 1 is added as an acceptor in low concentration and the transfer probability to the acceptor is determined by measuring the reduction of Perylene Red fluorescence. The quenched quantum yield is measured for dye concentrations varying from 0.05?M to 0.15 M for Perylene Red and from 0.3 mM to 3 mM for Oxazine 1. The experimental results are compared to a model which assumes that excitons can diffuse through the material by FRET between Perylene Red sites and are trapped at an acceptor with a final hetero FRET step. We find a quite good match between theory and experiment though the observed diffusion constant is about two times smaller than the calculated one. The exciton diffusion length extracted from the data is 30 nm for a Perylene Red concentration of 0.1 M and demonstrates that long distance energy transfer is possible in this disordered material system.     

Transient absorption spectra and dynamics of InSe nanoparticles

Time-resolved fluorescence and transient absorption results have been obtained for small (approximately 3 nm) and large (approximately 5-8 nm) InSe nanoparticles in room-temperature solutions. The large particles are nonfluorescent, indicating that the conduction band is at M and the optical transition is forbidden. For some fraction of the small particles, the bottom of the conduction band is at Gamma and the optical transition is allowed. The small particle fluorescence measurements indicate that hole trapping occurs on the 200-300 ps time scale. The transient absorption spectra are featureless throughout the visible with a broad maximum at 600-650 nm. The transient absorption kinetics of both small and large particles show a 200-300 ps decay component that is assigned to hole trapping. These kinetics also show a 15 ps decay that has a larger amplitude in the case of the large particles and is assigned to an electron Gamma to M relaxation. The amplitude of this decay indicates that the initial electron and hole intraband transitions result in roughly comparable intensities of the initial transient absorption.     

TiO2/CuS异质结的制备及光催化降解染料废水的研究

以原位沉淀法和水热法混合的合成手段,制备了TiO_2/CuS异质结光催化剂。这种异质结改善了单一TiO_2半导体光催化剂的缺陷,明显提高了太阳光下光催化降解甲基橙的效率。TiO_2与CuS之间形成的异质结结构和合适的能带结构能够扩展材料对太阳光谱的响应范围并且很好地收集和传输光诱导载流子,从而提高了载流子的分离效率,最终使半导体的光催化活性明显增强。结果表明,太阳光照射25min后,相比于TiO_2/MnS、TiO_2/CdS和TiO_2/ZnS异质结,TiO_2/CuS异质结(TiO_2和CuS的摩尔比为3∶1)对甲基橙的降解效果最佳,降解效率能达到97.3%。为提高半导体的光催化活性提供了一条可行的路径。     

Electronic properties of the interface between p-CuI and anatase-phase n-TiO2 single crystal and nanoparticulate surfaces: a photoemission study

We present a study of the growth of the p-type inorganic semiconductor CuI on n-type TiO2 anatase single crystal (101) surfaces and on nanoparticulate anatase surfaces using synchrotron radiation photoemission spectroscopy. Core level photoemission data obtained using synchrotron radiation reveal that both the substrate (TiO2) and the overlayer (CuI) core levels shift to a lower binding energy to different degrees following the growth of CuI on TiO2. Valence band photoemission data show that the valence band maximum of the clean substrate differs from that of the dosed surface which may be interpreted qualitatively as due to the introduction of a new density of states within the band gap of TiO2 as a result of the growth of CuI. The valence band offset for the heterojunction n-TiO2p-CuI has been measured using photoemission for both nanoparticulate and single crystal TiO2 surfaces, and the band energy alignment for these heterojunction interfaces is presented. With the information obtained here, it is suggested that the interface between p-CuI and single crystal anatase-phase n-TiO2 is a type-II heterojunction interface, with significant band bending. The measured total band bending matches the work function change at the interface, i.e., there is no interface dipole. In the case of the nanoparticulate interface, an interface dipole is found, but band bending within the anatase nanoparticles remains quite significant. We show that the corresponding depletion layer may be accommodated within the dimension of the nanoparticles. The results are discussed in the context of the functional properties of dye-sensitized solid state solar cells.     

Enhanced Raman scattering and photocatalytic activity of Ag/ZnO heterojunction nanocrystals

In this work, we study the enhancement of Raman signals and photocatalytic activity of Ag/ZnO heterojunctions with an Ag content of 1 at.%, which were synthesized by photochemical deposition of Ag nanoparticles onto pre-synthesized ZnO nanorods. A strong interaction between Ag and ZnO nanocrystals were evidenced by XPS and UV-vis spectroscopy. The binding energy of Ag nanoparticles shifts toward lower energy compared to that of pure Ag nanoparticles, revealing that electrons transfer from Ag to the ZnO nanocrystals. The red shift of the plasmon absorption peak of Ag nanoparticles in Ag/ZnO heterojunctions further confirms the strong interaction between the two components. This strong interaction, arising from the coupling between Ag and ZnO nanocrystals, is responsible for the enhancement of Raman signals and photocatalytic activity of the Ag/ZnO heterojunctions.