Existence Of Internal Electric Field Research Articles

Thermally assisted in situ synthesis of C3N5/TiO2 S-scheme heterojunctions with enhanced visible light response for photocatalytic hydrogen precipitation

Nitrogen-rich C3N5 has emerged as a promising visible light catalyst due to its narrow band gap. Nevertheless, its practical application is hindered by the rapid recombination of photogenerated carriers. To address this limitation and enhance the photocatalytic performance of C3N5-based photocatalysts, one effective measure involves the construction of heterojunctions in conjunction with other semiconductors. In this work, the C3N5/TiO2 S-scheme photocatalysts was constructed by matching C3N5 with a more negative conduction band (CB) and TiO2 with a more positive valence band (VB). Photoelectric tests exhibited that the prepared photocatalysts possessed enhanced visible light photoresponsivity as well as excellent charge separation and transport efficiencies. The characterization results of XPS and EPR confirmed that the existence of internal electric field and the unique S-scheme charge transfer mechanism, which achieves overall spatial separation of effective photogenerated carriers and accelerates the recombination of redundant electrons and holes. The photocatalytic hydrogen production performance of the TCN2 is 6.9 times and 30 times higher than that of C3N5 and TiO2, respectively, which is ascribed to the unique charge transfer mechanism of the S-scheme heterojunction and significantly enhanced photocatalytic activity. The simple construction of S-scheme heterojunctions in this work provides a new strategy for research in other fields.

Internal electric field induced photocarriers separation of nickel-doped pyrite/pyrite homojunction with rich sulfur vacancies for superior Cr(VI) reduction

Improving the separation efficiency and transfer ability of photoinduced electrons/holes in pyrite (FeS2)-based photocatalytic materials is significant for the photoreduction of hexavalent chromium (Cr(VI)) but still remains a challenge. Herein, a novel homojunction was prepared through in-situ growth of nickel (Ni) doped FeS2 nanoparticles on FeS2 nanobelts (denoted as Ni-FeS2/FeS2). Systematical characterizations revealed that Ni doped FeS2 nanoparticles have been successfully in situ grown along the lattice of FeS2 nanobelts. Photoreduction experiments demonstrated that the Ni-FeS2/FeS2 homojunction with 2 mmol Ni doping contents (denoted as 2Ni-FeS2/FeS2) exhibited the optimum Cr(VI) reduction efficiency among the studied catalysts. Density Functional Theory (DFT) calculated results verified that Ni doping could not only be advantageous for the formation of sulfur vacancies but also modify the band gap and band structure of FeS2 nanoparticles. Moreover, several doping energy levels caused by Ni doping have also appeared near the Fermi level of FeS2 nanoparticles. The migration paths of electrons and the existence of internal electric field (IEF) in homojunction were further verified by the calculation of work function. To sum up, the doping energy levels and IEF that produced by homojunction played important roles in accelerating the separation efficiency of its photogenerated carriers.

CsPbBr3 perovskite based tandem device for CO2 photoreduction

The design of efficient photocatalytic devices is of great significance to achieve low-cost carbon dioxide reduction into solar fuels. Herein, directional charge-carrier migration is realized by introducing a CO2 reduction device with a tandem structure made up of g-C3N4 and an inorganic lead-perovskite (CsPbBr3). This photocatalytic device shows a significantly increased CO2 photoreduction rate in a humid gaseous CO2 system when compared to the parental g-C3N4 layer. This “full artificial photosynthesis” thereby generates fuel molecules and oxygen from water and CO2, only, without any co-catalysts or sacrificial agents. To our surprise, the device maintains greater than 90% of the CO production yield after 5 days continuous irradiation. The existence of internal electric field is confirmed, which drive the directional movement of photogenerated charge-carriers with the perovskite being reductive, while the carbon nitride with its enormous oxidation stability is covering the oxidation processes. This represents a promising possibility for the practical application of CO2 photoreduction with scalable devices.

Construction of 3D sponge-like hierarchical pore Ag10Si4O13 microblock photocatalyst with enhanced photocatalytic activities

Designing efficient visible light photocatalysts is targeted to improve the solar utilization to ameliorate environmental problems. Ag10Si4O13 has great application potential due to a narrow band gap and the existence of internal electric field（spontaneous polarization electric field inside the crystal）. Here, a novel and convenient synthetic strategy based on sol-gel method and template method was proposed to fabricate 3D sponge-like hierarchical pore Ag10Si4O13 (HPA) microblock with enhanced photocatalytic activities. The group multiple coordination is conducive to further improve the dispersion of Ag10Si4O13 precursor around polystyrene (PS) microsphere. Weak sintering between nano-Ag10Si4O13 particles ensured the formation of sponge skeleton structure. Compared with Ag10Si4O13, HPA showed enhanced separation efficiency of electron hole pairs due to the carbon doping. And the uniform and well-developed pore structures further improved the photocatalytic performance. As a result, HPA (P-5) exhibited a degradation efficiency of 98.98% at 30 min under visible light, and an excellent cycle performance were obtained. The proposed strategy provides an important reference for the preparation of easily recoverable microblock photocatalyst.

Theoretical study on the improvement of optoelectronic properties of Al_xGa_(1-x) As nanowires with variable Al content adsorbed by metal nanoparticles

GaAs nanowire arrays with variable Al content can effectively improve the photoelectric emission of nanowires due to the existence of internal electric field. Metal particles adsorb nanowires, which can excite local plasmons and enhance the optical absorption of nanowires. In this paper, the Al content change of Al_xGa_ (1-x) As material and metal particle adsorption are combined to study the light absorption and quantum efficiency of the Al_xGa_ (1-x) As nanowire array with metal particles adsorbed on the surface. Based on the finite element method and COMSOL multiphysics software, the effects of the number of metal nanoparticles, the distribution of particles, array period and particle size on the optical properties of the array are analyzed. The results show that metal particle adsorption can enhance the light capture ability of Al_xGa_(1-x) As nanowires arrays, the absorption of short wave light can be improved by adding adsorption particles layer by layer. Higher quantum efficiency can be obtained by appropriately increasing the array period and the diameter of metal particles of the structure.

Eliminating the electric field response in a perovskite heterojunction solar cell to improve operational stability

Intrinsic and extrinsic ion migration is a very large threat to the operational stability of perovskite solar cells and is difficult to completely eliminate due to the low activation energy of ion migration and the existence of internal electric field. We propose a heterojunction route to help suppress ion migration, thus improving the operational stability of the cell from the perspective of eliminating the electric field response in the perovskite absorber. A heavily doped p-type (p+) thin layer semiconductor is introduced between the electron transporting layer (ETL) and perovskite absorber. The heterojunction charge depletion and electric field are limited to the ETL and p+ layers, while the perovskite absorber and hole transporting layer remain neutral. The p+ layer has a variety of candidate materials and is tolerant of defect density and carrier mobility, which makes this heterojunction route highly feasible and promising for use in practical applications.

Sonochemical synthesis of BiOI-TiO2heterojunction with enhanced visible-light-driven photocatalytic activity

Construction of p-n heterostructured semiconductor photocatalysts is of great attention. Herein, BiOI-TiO2p-n heterojunction photocatalysts with different molar ratios have been successfully synthesized by ultrasonic-assisted method. On the basis of the XPS analysis, Ti-O-Bi bonds are formed between BiOI nanosheets and TiO2nanoparticles rather than a simple overlap of single material. Compared to the pure BiOI and TiO2, the BiOI-TiO2p-n heterojunctions show prominently enhanced photocatalytic activity for degradation of methyl orange (MO) under visible light irradiation (λ > 420 nm). Especially, 80% BiOI-TiO2possesses the highest photocatalytic reaction rate constant, which is 3.9 and 12.2 times higher than that of BiOI and TiO2, respectively. The superoxide radical species (•O2−) has been regarded as the predominant active species in the reaction process, confirmed by the trapping experiments and Electron spin resonance (ESR) results. In addition, efficient separation and transfer of photogenerated electron-hole pairs rendered by the heterojunctions are proved employing photocurrent experiments, electrochemical impedance spectroscopy (EIS) and photoluminescence (PL). Moreover, the positions of the conduction bands (CB) and valence bands (VB) of the samples are investigated by Mott–Schottky measurements. It can be deduced that the existence of internal electric field caused by p-n junction leads to effective separation of electron hole pairs, resulting in the production of a large number of active species for the MO degradation.

Ferroelectric properties study for nanocgrain barium titanate ceramics

Dense nanocrystalline BaTiO 3 ceramics with 50 nm average grain size obtained by spark plasma sintering were investigated. The dielectric data show a ferro-para phase transition with a maximum permittivity of 1602 at 120 °C and at 1 kHz. The polarization-reversal characteristics and the local ferroelectric switching behavior were measured; the typical piezoelectric hysteresis loops were recorded. The asymmetry of permanent polarization and coercive field in polarization–electric field curves were obtained and attributed to the existence of internal electric field. The present results provide experimental evidence, indicating that if a critical grain size exists for ferroelectricity it is less than 50 nm for polycrystalline BaTiO 3 ceramics.

Electromodulation of photoluminescence in vacuum-evaporated films of fac-tris(2-phenylpyridine)iridium(III)

Electromodulation of photoluminescence (EML) has been investigated in vacuum-evaporated films of Ir(ppy) 3 (tris(2-phenylpyridine)iridium) commonly used as a phosphorescent emitter in organic light-emitting diodes. The quenching of photoluminescence up to 30% is observed in the external electric field applied to sandwich cells Al/Ir(ppy) 3/Al/glass. The electric field characteristics, the emission and excitation spectra of EML signals are interpreted in terms of field-induced dissociation of excited states into free and/or trapped charge carriers according to the Onsager theory. In addition, the existence of internal electric fields of (1–3) × 10 5 V/cm in Ir(ppy) 3 films is inferred from the (1 ω)EML signals measured at the first harmonic of the applied electric field frequency, in accordance with previous electroabsorption results.

Recent progress in growth and physics of GaN/AlN quantum dots

We report our recent results on hexagonal and cubic GaN/AlN quantum dots grown by molecular beam epitaxy. It is shown that the growth of GaN on AlN can occur either in a layer-by-layer mode to form quantum wells or in the Stranski–Krastanow mode to form self-assembled quantum dots. High resolution transmission electron microscopy reveals that quantum dots are truncated pyramids (typically 3 nm high and 15 nm wide), nucleating on top of a wetting layer. The existence of internal electric fields of 7 MV/cm in hexagonal quantum dots is evidenced by observations of various physical effects related to the quantum confined Stark effect, e.g. redshift of the interband transition energy, decrease of its oscillator strength, or enhancement of the exciton interaction with LO phonons. Preliminary results on rare-earth doping of GaN/AlN quantum dots will be discussed also in this report. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Charge transfer and photogeneration process in device consisting of safranine O dye and TiO 2 nano-particles

We have studied the effect of titanium dioxide (TiO 2) nano-particles on charge transfer processes in safranine O (SAF) dye photovoltaic device. The photoconductivity is increased by two orders and a sharp saturation is also observed. The device shows a sharp saturation in photocurrent in forward bias which indicates that exciton diffusion and dissociation at the TiO 2 interface, due the existence of internal electric field is dominant process for the collection of charge. The value of diode quality factor has been calculated also by fitting the experimental results with modified Shockley equation. The internal electric field is mainly due to the asymmetry between the work functions of the materials. Electron injection from the excited state of dye on illumination, into the conduction band of TiO 2 is followed by subsequent hole transfer from the photooxidized dye to the collecting Al electrode, regenerating the dye's original ground state. The charge transfer and photogeneration process in device have been discussed on the basis of current–voltage characteristics and photoaction action spectra of the device.

Profiling electric fields in GaN/InGaN/GaN single quantum wells by electron holography

ABSTRACTWe use electron holography to profile the local internal potential due to spontaneous polarization and piezoelectric effects in strained quantum well structures of wurtzitic group III nitrides. Profiles of the electrostatic potential across a GaN/InxGa1−xN/GaN quantum well structure show the existence of internal electric fields of about –2.2 ± 0.6 MV/cm, and a potential drop across the quantum well of 0.6 ± 0.16 V. The electric fields indicate an average indium composition of 15% in the quantum well, for a thickness of 2.7 nm. This indium composition compares well with measurements by energy-disperse spectroscopy of 18 ± 2 %. Screening effect is not observed under these experimental conditions.

Absorption edge and internal electric fields in amorphous semiconductors

A simple model based on the existence of internal electric fields is suggested to explain the exponential part of the absorption edge observed in many amorphous semiconductors. Some possible origins of these fields are discussed: frozen-in LA and LO phonons, and charged centers.