Charge Separation Effects Research Articles

A new approach to detecting gravitational waves via the coupling of gravity to the zero-point energy of the phonon modes of a superconductor

The response of a superconductor to a gravitational wave is shown to obey a London-like constituent equation. The Cooper pairs are described by the Ginzburg–Landau free energy density embedded in curved spacetime. The lattice ions are modeled by quantum harmonic oscillators characterized by quasi-energy eigenvalues. This formulation is shown to predict a dynamical Casimir effect since the zero-point energy of the ionic lattice phonons is modulated by the gravitational wave. It is also shown that the response to a gravitational wave is far less for the Cooper pair density than for the ionic lattice. This predicts a “charge separation effect” which can be used to detect the passage of a gravitational wave.

Cosmological CP -odd axion field as the coherent Berry’s phase of the Universe

We consider a dark matter (DM) model offering a very natural explanation of the observed relation, $\Omega_{\rm dark} \sim \Omega_{\rm visible}$. This generic consequence of the model is a result of the common origin of both types of matter (DM and visible) which are formed during the same QCD transition. The masses of both types of matter in this framework are proportional to one and the same dimensional parameter of the system, $\Lambda_{\rm QCD}$. The focus of the present work is the detail study of the dynamics of the $\cal{CP}$-odd coherent axion field $a(x)$ just before the QCD transition. We argue that the baryon charge separation effect on the largest possible scales inevitably occurs as a result of merely existence of the coherent axion field in early Universe. It leads to preferential formation of one species of nuggets on the scales of the visible Universe where the axion field $a(x)$ is coherent. A natural outcome of this preferential evolution is that only one type of the visible baryons remains in the system after the nuggets complete their formation. This represents a specific mechanism on how the baryon charge separation mechanism (when the Universe is neutral, but visible part of matter consists the baryons only) replaces the conventional "baryogenesis" scenarios.

Development of photoanodes for photoelectrocatalytic solar cells based on copper-based nanoparticles on titania thin films of vertically aligned nanotubes

Titania nanotube (TNT)-array thin films well decorated by copper nanoparticles with average size of 3nm were prepared by spray coating of a solution containing size-controlled Cu0 nanoparticles. The consecutive calcination at 300°C and 450°C leads to the oxidation of these Cu NPs to CuO, with small amounts of Cu2O at the lower calcination temperature, but maintaining the high dispersion. Analogous materials prepared by copper electrodeposition lead to significantly larger Cu NPs. The TNT-array thin film shows significantly enhanced photocurrent (up to about 90%) with respect to a comparable thin film prepared by spin coating using a commercial TiO2 P25 sample. The behavior is similar by applying different filters to cut part of solar light simulator radiation. Particularly, using an UV B/C blocking filter, which permits to pass light in the 350–550nm range, an about two-fold intensification in the current-to-electrical energy conversion (normalized to the same total irradiance) is obtained. The presence of CuO nanoparticles decreases the photocurrent density with respect to the support alone (TNT-array 1h), but enhances the H2 photogeneration rate in the gas-phase photoreactor experiments. The results indicate that in the tested experimental conditions, the main role of CuO nanoparticles is to act as co-catalyst to improve the H2 photogeneration rate rather than to promote charge separation or other effects, which promote the photocurrent density.

Effect of collisions on drift instabilities in a field reversed configuration under conditions of magneto-inertial fusion

Drift instabilities in a field reversed configuration are studied under conditions of magneto-inertial fusion (MIF). Specifically, the collisional effect is taken into account because of high-density plasmas in MIF where the drift wave frequency is smaller than the electron-ion collision frequency. Dispersion relations are based on the two fluid equations including the collisional terms; meanwhile, the electromagnetic effect is also considered due to high β values (β is the ratio of plasma pressure to magnetic pressure). It is found that in the limit of low β, the behavior of instabilities described by the dispersion relations in the present paper would become like drift instabilities in tokamaks, where β ∼ 0.1. Therefore, in the MIF case, electromagnetic drift instabilities could be driven by electron-ion collisions due to the charge separation effect. The collisions also bring the phase difference between the perturbed density and the potential perturbation, which is significant for the particle transport.

Laser-generated plasmas by graphene nanoplatelets embedded into polyethylene

AbstractGraphene micrometric particles have been embedded into polyethylene at different concentrations by using chemical–physical processes. The synthesized material was characterized in terms of mechanical and optical properties, and Raman spectroscopy. Obtained targets were irradiated by using a Nd:YAG laser at intensities of the order of 1010 W/cm2 to generate non-equilibrium plasma expanding in vacuum. The laser–matter interaction produces charge separation effects with consequent acceleration of protons and carbon ions. Plasma was characterized using time-of-flight measurements of the accelerated ions. Applications of the produced targets in order to generate carbon and proton ion beams from laser-generated plasma are presented and discussed.

Effectiveness of charge separation from the long-lived second excited state of donors

In molecular zinc-porphyrin-based donor–acceptor systems, the electron transfer from the second singlet excited state S2 is accompanied by ultrafast recombination into the first excited state, resulting in a low quantum yield of the thermalized charge-separated state (20%). It is demonstrated that the quantum yield of ultrafast charge separation in donor–acceptor triads D–A1–A2 can be close to 100% in molecular systems with lifetimes of the S2 state longer than 150 ps. As prototypes of such systems, donor–acceptor diads D–A1 and triads D–A1–A2 are considered, wherein the xanthione molecule plays the role of a donor. The ranges of the model parameters are determined in which the efficiency of charge separation is high. The twostage photoinduced charge transfer is studied within the framework of a multichannel stochastic model that takes into account the reorganization of a polar solvent and a high-frequency intramolecular vibrational mode.

Electrostatic Surface Waves on Semi-Bounded Quantum Electron-Hole Semiconductor Plasmas

The electrostatic surface waves on semi-bounded quantum electron-hole semiconductor plasmas are studied within the framework of the quantum hydrodynamic model, including the electrons and holes quantum recoil effects, quantum statistical pressures of the plasma species, as well as exchange and correlation effects. The dispersion characteristics of surface electrostatic oscillations are investigated by using the typical values of GaAs, GaSb and GaN semiconductors. Numerical results show the existence of one low-frequency branch due to the mass difference between the electrons and holes in addition to one high-frequency branch due to charge-separation effects.

Palladium modified porous silicon as multi-functional MALDI chip for serum peptide detection.

Interest in using mesoporous materials for peptidomic research has increased recently. The present study reports a new type of matrix assisted laser desorption/ionization (MALDI) plate derived from electrochemically etched porous silicon (PSi) whose surface was modified with palladium nanoparticles (PdNPs). Owing to the well-tailored pore size and the molecular filtration effect of the PSi, peptides in serum samples can be selectively captured and enriched in the pore channel, thereby eliminating the interference from large proteins in subsequent MALDI-MS detection. On the other hand, the PdNPs with localized surface plasmon resonance (LSPR) effect can help to enhance the efficiency of energy absorption in the UV region. Meanwhile, the charge separation effect between the PSi semiconductor and PdNPs also can be applied to promote the accumulation of positive charges on PdNPs, resulting in an improvement in laser desorption/ionization (LDI) efficiency under positive linear detection mode. The interplay among these unique properties of PSi and PdNPs can synergistically increase the overall sensitivity in serum peptide detection. Using this technology, serum sample can be directly detected on the PSi-PdNPs chip without complicated pretreatment process. Therefore, a high fidelity serum peptide fingerprint can be acquired in a high throughput way. With the assistance of statistical analysis, colorectal cancer patients and healthy people can be accurately distinguished based on the serum peptide fingerprints.

Beyond WIMPs: the Quark (Anti) Nugget Dark Matter

We review a testable dark matter (DM) model outside of the standard WIMP paradigm. The model is unique in a sense that the observed ratio $\Omega_{\rm dark} \simeq \Omega_{\rm visible}$ for visible and dark matter densities finds its natural explanation as a result of their common QCD origin when both types of matter (DM and visible) are formed during the QCD transition and both are proportional to single dimensional parameter of the system, $\Lambda_{\rm QCD}$. We argue that the charge separation effect also inevitably occurs during the same QCD transition in the presence of the $\cal{CP}$ odd axion field $a(x)$. It leads to preferential formation of one species of nuggets on the scales of the visible Universe where the axion field $a(x)$ is coherent. A natural outcome of this preferential evolution is that only one type of the visible baryons (not anti- baryons) remain in the system after the nuggets complete their formation. Unlike conventional WIMP dark matter candidates, the nuggets and anti-nuggets are strongly interacting but macroscopically large objects. The rare events of annihilation of the anti-nuggets with visible matter lead to a number of observable effects. We argue that the relative intensities for a number of measured excesses of emission from the centre of galaxy (covering more than 11 orders of magnitude) are determined by standard and well established physics. At the same time the absolute intensity of emission is determined by a single new fundamental parameter of the theory, the axion mass, $10^{-6} {\rm eV} \lesssim m_a \lesssim 10^{-3}{\rm eV}$. Finally, we comment on implications of these studies for the axion search experiments, including microwave cavity and the Orpheus experiments.

Gradient doping of phosphorus in Fe2O3 nanoarray photoanodes for enhanced charge separation.

Hematite (α-Fe2O3) is a promising candidate for solar-to-hydrogen energy conversion. However, the low carrier mobility and extremely high charge recombination rate limit the practical application of hematite in solar water splitting. This paper describes the fabrication of a Fe2O3 photoanode with gradient incorporation of phosphorus (P) employing a facile dipping and annealing method to improve the charge separation for enhanced photoelectrochemical water oxidation. This gradient P incorporation increases the width of band bending over a large region in Fe2O3, which is crucial for promoting the charge separation efficiency in the bulk. Although both gradient and homogeneous P-incorporated Fe2O3 samples exhibit similar electrical conductivity, the Fe2O3 electrode with a gradient P concentration presents an additional charge separation effect. A photocurrent of ∼1.48 mA cm-2 is obtained at 1.23 V vs. reversible hydrogen electrode (vs. RHE) under air mass 1.5G illumination. Additionally, the H2O oxidation kinetics of Fe2O3 with gradient P incorporation was further improved upon loading cobalt phosphate as cocatalyst, reaching a photocurrent of ∼2.0 mA cm-2 at 1.23 V vs. RHE.

Complete photocatalytic mineralization of Nile blue on hetero-structured CoTiO3 nano-composite coated on glass in a sol–gel process using diethylene glycol as stabilizer: effect of charge separation and calcination temperature on activity

Hetero-structured CoTiO3 nano-composite photocatalyst was produced by a modified sol–gel process diethylene glycol as as stabilizer and coated on glass as a thin film by Doctor Blade coating. CoTiO3 nano-composites were calcinated at 550, 650 and 750 °C to study the structure and phase formation of CoTiO3 nano-composites. X-ray diffraction results showed that the pure CoTiO3 nano-crystalline phase is formed at 650 °C while Co3O4 and TiO2 were present as impurities at 550 and 750 °C annealing temperatures. Field emission scanning electron microscopy and energy dispersive X-ray spectroscopy analysis of hetero-structured CoTiO3 nano-composite coated on a glass reveal the formation of spherical shape of nano-crystalline phases with an average size of 75 nm. The Fourier transform infrared spectroscopy results showed the existence of Co–O, Ti–O, and Ti–O–Co bonds. Thermogravimetric and derivative thermogravimetric analysis show that the weight loss phases are completed at 750 °C with peak temperatures of 107 and 345 °C. The UV–vis diffuse reflectance spectra reveal the optical band gaps of 1.587 eV (781 nm) and 2.079 eV (596 nm). Photocatalytic activity of hetero-structured CoTiO3 nano-composite photocatalyst was examined for degradation of Nile blue dye and the results showed complete degradation in 2.5 h of light irradiation. This is attributed to the efficient separation of electron–hole pairs of the CoTiO3 nano-composite, narrow band gap, and visible-light harvesting ability.

Highly efficient photocatalytic performances of SnO2-deposited ZnS nanorods based on interfacial charge transfer

SnO2/ZnS nanocomposites of SnO2 quantum dots (QDs)-deposited ZnS nanorods having highly enhanced photocatalytic activity and photostability have been fabricated via a facile two-step hydrazine-assisted hydrothermal process without involving any surface treatments. The photocatalytic activity of SnO2/ZnS nanocomposites with a Sn-to-Zn molar ratio (RSn/Zn) of 0.1 is 3 times higher than that of pristine ZnS nanorods and 17 times higher than that of commercial ZnS. The incorporation of SnO2 QDs increases the photocatalytic efficiency of ZnS nanorods due to the following reasons: photogenerated charge carriers are readily separated owing to type II band configuration and direct contact at interfaces without having any linker molecules; active surface sites are increased to adsorb more dye molecules; the light absorption range is extended to the visible region, generating more charge carriers on the surfaces of heterojunction structures. The decay time, as well as the intensity, of the band-edge emission of SnO2/ZnS nanocomposites at 325nm decreases progressively and rapidly with the increase of RSn/Zn, indicating that fast electron transfer takes place from photoexcited ZnS nanorods to SnO2 QDs. Thus, the higher photocatalytic degradation efficiencies of SnO2/ZnS nanocomposites are considered to result mainly from the increased separation rates of photogenerated charges. The photostability of SnO2/ZnS nanocomposites is also improved due to the protection and charge-separation effects of decorating SnO2 QDs. Our prepared SnO2/ZnS nanocomposites are suggested to have great potential for photodegradation nanocatalysts in the field of waste-water treatment.

Photocatalytic performance of PMN-PT@TiO2 highly enhanced by alternative spatial electric field induced charge separation effect

Efficient charge separation is a vital factor to gain high photocatalytic activity. Alternative spatial electric field generated by piezoelectric effect could significantly accelerate charge separation when introduced into photocatalyst system. In this work, famous piezoelectric lead magnesium niobate-lead titanate 0.675Pb(Mg1/3Nb2/3)O3-0.325PbTiO3 (PMN-PT) was firstly constructed with TiO2 to prepare composite photocatalyst PMN-PT@TiO2. The results indicate that the alternative spatial electric field, excited by ultrasonic wave vibration, greatly improves the photocatalytic degradation efficiency of rhodamine B (RhB) over PMN-PT@TiO2 by about 50%. The durability experiments suggest that PMN-PT@TiO2 can keep excellent photocatalytic stability even after several runs. Moreover, in the photocatalytic degradation of methylene blue (MB), the enhancement effect of alternative spatial electric field on photocatalytic performance is also observed. Importantly, this study provides a general method for constructing piezoelectric-based photocatalyst to capture solar energy, acoustic wave energy and wind energy, which will realize multi-field energy coupling and high photocatalysis.

Efficient Solar-Induced Photoelectrochemical Response Using Coupling Semiconductor TiO₂-ZnO Nanorod Film.

Efficient solar driven photoelectrochemical (PEC) response by enhancing charge separation has attracted great interest in the hydrogen generation application. The formation of one-dimensional ZnO nanorod structure without bundling is essential for high efficiency in PEC response. In this present research work, ZnO nanorod with an average 500 nm in length and average diameter of about 75 nm was successfully formed via electrodeposition method in 0.05 mM ZnCl2 and 0.1 M KCl electrolyte at 1 V for 60 min under 70 °C condition. Continuous efforts have been exerted to further improve the solar driven PEC response by incorporating an optimum content of TiO2 into ZnO nanorod using dip-coating technique. It was found that 0.25 at % of TiO2 loaded on ZnO nanorod film demonstrated a maximum photocurrent density of 19.78 mA/cm2 (with V vs. Ag/AgCl) under UV illumination and 14.75 mA/cm2 (with V vs. Ag/AgCl) under solar illumination with photoconversion efficiency ~2.9% (UV illumination) and ~4.3% (solar illumination). This performance was approximately 3–4 times higher than ZnO film itself. An enhancement of photocurrent density and photoconversion efficiency occurred due to the sufficient Ti element within TiO2-ZnO nanorod film, which acted as an effective mediator to trap the photo-induced electrons and minimize the recombination of charge carriers. Besides, phenomenon of charge-separation effect at type-II band alignment of Zn and Ti could further enhance the charge carrier transportation during illumination.

Charge separation effects in time-resolved photoluminescence of Cu(In,Ga)Se2 thin film solar cells

Time-resolved photoluminescence (TRPL) is a powerful tool for investigating the charge carrier dynamics in semiconductors. Many mechanisms may contribute to the PL decay characteristics, such as radiative band-to-band recombination, non-radiative Shockley-Read-Hall recombination, capture and re-emission of minority carriers via trap states, as well as separation of the generated electron-hole pairs by gradients in their electrochemical potentials combined with different conductivities. In Cu(In,Ga)Se2 thin film solar cells charge separation has been shown to dominate the TRPL signal at low injection levels in complete devices whereas the PL decay in CdS-covered absorbers seemed unaffected by charge separation and has been reported to equal that of the bare absorber layer. In this study we investigate the PL decay of complete cells, CdS-covered absorbers and bare absorbers as a function of injection level. We show that the PL decay of not only the complete cell but also the CdS-covered absorber is dominated by charge separation at sufficiently low injection levels. In this injection regime the TRPL signal of the CdS-covered absorber significantly differs from that of the bare absorber, but also at higher injection levels different decay time constants are observed. By means of time-resolved device simulations we analyse the mechanisms of charge separation and their impact on the transient PL decay.

Photo-depositing Ru and RuO2 on Anatase TiO2 Nanosheets as Co-catalysts for Photocatalytic O2 Evolution from Water Oxidation

TiO2 nanosheets mainly exposed (001) facet were prepared through a hydrothermal process with HF as the morphology-directing agent. Ru and RuO2 species were loaded by photo-deposition methods to prepare the photocatalysts. The structural features of the catalysts were characterized by X-ray diffraction, transmission electron microscopy, inductively coupled plasma atomic emission spectrum, and H2 Temperature-programmed reduction. The photocatalytic property was studied by the O2 evolution from water oxidation, which was examined with respect to the influences of Ru contents as well as the oxidation and reduction treatments, suggesting the charge separation effect of the Ru species co-catalysts on different facets of TiO2 nanosheets. In contrast to Ru/TiO2 and RuO2/TiO2 with the single deposited co-catalyst, the optimized catalyst 0.5%Ru-1.0%RuO2/TiO2 with dual co-catalysts achieved a much improved catalytic performance, in terms of the synergetic effect of dual co-catalysts and the enhanced charge separation effect.

Exploitation of a spontaneous spatial charge separation effect in plasmonic polyhedral α-Fe2O3 nanocrystal photoelectrodes for hydrogen production

Abstract High specific surface area bipyramidal and pseudocubic polyhedral α-Fe 2 O 3 nanocrystals were fabricated for hydrogen reduction in a water-splitting process. Two different facets of polyhedral α-Fe 2 O 3 nanocrystals were synthesized via a versatile hydrothermal route. Typically, photocatalytic activity of hematite is limited by its relatively poor absorptivity, very short excited-state lifetime, and short hole diffusion length. To address these issues, a pseudocubic polyhedral α-Fe 2 O 3 photoelectrode was fabricated that achieved a spontaneous charge spatial separation during the water-splitting process. Additionally, Sn from the FTO substrate was diffused and doped into polyhedral α-Fe 2 O 3 during the sintering process to serve as an electron donor and increase the carrier density. To further exploit the surface plasmon resonance (SPR) properties and enhance the water-splitting efficiency of polyhedral α-Fe 2 O 3 photoelectrodes, Au nanoparticles were decorated on the surface of Fe 2 O 3 . This increased incident light absorption and suppressed charge recombination by the strong field generated from collective oscillations of surface electrons in the Au nanoparticles. Subsequently, this Au/pseudocubic Fe 2 O 3 photoelectrode demonstrated a 9.05 μmol/min hydrogen production rate.

Visible photocatalytic and photoelectrochemical activities of TiO2 nanobelts modified by In2O3 nanoparticles

Novel In2O3 nanoparticle/TiO2 nanobelt heterostructures with enhanced visible-light photocatalytic and photoelectrochemical (PEC) performance were successfully prepared via a facile hydrothermal method. Well-dispersed In2O3 nanoparticles with small sizes are uniformly attached on the surface of TiO2 nanobelts to form In2O3 nanoparticle/TiO2 nanobelt heterostructures. The TiO2 nanobelts as backbones restrict the aggregation of In2O3 nanoparticles, resulting in the formation of smaller In2O3 nanoparticles with more interaction sites for pollutants. The visible photocatalytic activity of as-prepared heterostructures for degradation of methyl blue (MB) is higher than those of TiO2 nanobelts and In2O3 nanoparticles alone. Moreover, the In2O3 nanoparticle/TiO2 nanobelt heterostructure shows an enhanced PEC performance under irradiation of visible light. The enhanced photocatalytic and PEC activities are mainly ascribed to the synergic effect of efficient charge separation of heterostructure, visible-light harvesting ability of In2O3, and the formation of preferential adsorption sites by the small size of In2O3 nanoparticles. Finally, based on the experimental results of Mott-Schottky, UV–vis DRS, photocurrent and open-circuit voltage response, a possible photocatalytic mechanism over the In2O3 nanoparticle/TiO2 nanobelt heterostructure is proposed.

Numerical study on the effect of charge separation at low cloud temperature and effective water content on thunderstorm electrification

In the present study, a numerical model is used to evaluate the effects of low effective water content and low cloud temperature on graupel charging, charge structure and lightning activity in regions of thunderstorms. Two idealized cloud cases were simulated with MesoNH using different configurations of the main known parameterizations for noninductive charging involving ice crystal/graupel rebounding collisions. Simulations in regions with very low effective cloud water content were performed with the parameterization proposed in Mitzeva et al. (2006) based on the “Relative Growth Rate” hypothesis, while for simulations in regions with low cloud temperature, charge values from Avila et al. (2011) were used. Results showed that the inclusion of the charge separation at very low effective water content influences more the simulated cloud charge structure than does the inclusion of the charge separated at low temperatures. Also, the effect of the charge separated at very low effective water content is more significant when the original parameterization for non-inductive charging is based on the effective water content rather than on the rime accretion rate.

Accessing the charge separation effects in dye-sensitized solar cells based on a vectorial planning of supramolecular ruthenium dyes

Charge recombination has been postulated a critical point in dye sensitized solar cells (DSSC), however the role of the electron-hole separation distance from the TiO2 surface and the oxidized dye has actually never been experimentally demonstrated for this type of device, operating under normal working conditions. In order to elucidate this point, supramolecular ruthenium dyes encompassing vectorial electron transfer properties have been especially devised, consisting of electronically coupled [Ru(dcbH2)2(B)Cl]+ and [Ru(dmb)2(B)Cl]+ complexes acting as acceptor–donor pairs, respectively, where dcbH2=2,2′-bipyridine-4,4′-dicarboxylic acid, and dmb=4,4′-dimethyl-2,2′-bipyridine. In these systems, the two ruthenium complexes are connected by linear bridging ligands (B) of programmed lengths, e.g. 1.85, 2.06 and 2.70nmfor 4,4′-bipyridine (bpy), trans-1,2-bis(4-pyridyl)-ethylene (bpe) or trans-1,4-bis[2-(4-pyridyl)ethenyl]-benzene) (bpeb), respectively, thus allowing a precise control of the interfacial charge-separation distance. By working under similar, reproducible conditions, the overall efficiency of the corresponding DSSCs increased from 2.78 (bpy) to 2.89 (bpe) and 3.25% (bpeb), paralleling the increase of the bridging ligand lengths. Such results were corroborated by impedance measurements, highlighting the important role of the electron-hole separation effect in DSSCs, and also demonstrating how a suitable supramolecular design can improve the overall cell efficiency.