Formation Of Internal Electric Field Research Articles

Steering photo-excitons towards active sites: Intensified substrates affinity and spatial charge separation for photocatalytic molecular oxygen activation and pollutant removal

Bismuth oxybromide-based heterostructure has been constructed widely for various photocatalytic application. However, the function of the exposed facets in heterostructure still remains to be elucidated. Herein, we chemically implanted bismuth oxybromide nano-units with dominantly exposed (010) facets on indium vanadate particles to form composites. Results showed that the absorption range of visible light was extended due to the high photosensitivity of InVO4 and the charge transfer was also effectively expedited by the formation of internal electric field. Contrast experiments manifested that BiOBr with dominant (010) facets was more suitable than that with (001) facets to act as electron acceptor in InVO4/BiOBr heterojunction, because the open channels on (010) facets provided large accommodation space for organic pollutants. Thus, the generated superoxide radicals on BiOBr could annihilate the adsorbed pollutants rapidly. Moreover, the hierarchical core-shell structure provided large surface area to further intensify the affinity of InVO4/BiOBr composites to pollutants. Consequently, the hierarchical InVO4/BiOBr heterojunction exhibited excellent photocatalytic activity and could degrade 92.59% of ciprofloxacin (CIP) within 1 h under visible light. The reaction data were fitted with multiple models to investigate the whole reaction process. Furthermore, the impacts of various simulated actual factors on CIP adsorption and degradation were explored. The intermediates and possible degradation pathways were also illustrated through Liquid Chromatography-Mass/Mass Spectrometry (LC-MS). This work not only provide a facet engineering concept for heterojunction construction, but also help us to further understand the relationship between adsorption and photocatalytic degradation.

Crystallization, cyanamide defect and ion induction of carbon nitride: Exciton polarization dissociation, charge transfer and surface electron density for enhanced hydrogen evolution

Exciton polarization dissociation, charge transfer and surface electron density in photocatalysts are crucial for photocatalytic hydrogen evolution reactions (HERs). In this study, crystalline carbon nitride with cyanamide defect edges (crystalline CCN) was synthesized by one step polymerization of urea in the presence of KCl. The texture and electronic band structure of carbon nitrides could be facilely tailored by changing KCl dosage. The light absorption edge of crystalline CCN extended to 736 nm due to n→π* electron transition. The enhanced dielectric constants of crystalline carbon nitrides promoted exciton polarization dissociation. The small effective electron mass (me*) in crystalline CCN facilitated me* diffusion. The efficient separation of electrons and holes benefited the formation of internal electric field, showing an 8.56-fold promotion in electron transfer compared to pristine CN. Significantly, femtosecond time-resolved transient absorption demonstrated that the surface electron density on crystalline CCN was enhanced in the presence of salt ions (NaCl). As a result, crystalline CCN exhibited 14.9 times higher HER rate than pristine CN under visible light irradiation. The apparent quantum yield for H2 evolution on crystalline CCN reached to 42% at 420 nm and 9% at 500 nm. This study gets a comprehensive understanding of photocatalytic HERs using carbon nitride photocatalysts.

New understanding on the different photocatalytic activity of wurtzite and zinc-blende CdS

In general, wurtzite CdS exhibits a higher photocatalytic activity than zinc-blende CdS. However, the underlying physicochemical reasons responsible for the differences of photocatalytic activity between the wurtzite and zinc-blende CdS are still unclear. In this work, the structural characteristics, band structures, density of states, bond populations, optical properties and charge carrier effective mass of wurtzite and zinc-blende CdS were investigated based on first-principle theoretical calculations. The calculated results indicate that the distortion of CdS4 tetrahedron units results in the formation of internal electric field in wurtzite CdS, which is beneficial for the efficient separation and diffusion of photogenerated charge carriers. Contrarily, the internal electric field is absent in zinc-blende CdS. Moreover, the effective masses of photogenerated charge carriers of wurtzite CdS are smaller than those of zinc-blende CdS, implying faster migration of photogenerated charge carriers to perform photocatalytic reactions on wurtzite CdS surfaces. All the above factors result in the lower recombination rate of photogenerated charge carriers within wurtzite CdS. Therefore, it is not surprising that wurtzite CdS usually shows a higher photocatalytic activity than zinc-blende CdS. This investigation will provide some new understanding on the difference of photocatalytic activity between wurtzite and zinc-blende CdS.

Identification of intrinsic deep level defects responsible for electret behavior in TlGaSe2 layered semiconductor

Unusual behavior of pyroelectric current signal polarity near the Curie point (Tc) was observed for TlGaSe2 a ferroelectric-semiconductor. It has been revealed that the polarity of the spontaneous polarization near Tc depends on the sample poling prehistory. In particular, applying an external electric field only in the temperature range of the paraelectric state during cooling regime in darkness brought to the depolarization current at Tc with the sign opposite to the external field polarity. Otherwise, if the sample was poled in the temperature interval of the incommensurate phase, pyroelectric current exhibits a peak at Tc with the polarity that is the same as for the external poling electric field. These observations indicate that internal electric field is present in the bulk and near-surface layer regions of the electrically poled single crystal TlGaSe2. Possible mechanisms and origins responsible for the internal electric fields in TlGaSe2 are discussed. It is shown that the formation of internal electric fields in TlGaSe2 is due to charging of intrinsic native defects during the poling process.Characteristics of electrically active intrinsic defects in TlGaSe2 were investigated by using of Photo-Induced Current Transient Spectroscopy (PICTS) technique. Six deep defect levels in the band gap of TlGaSe2 were determined, which were localized both in the bulk and on the surface of the sample and could be electrically charged. The correlation between polarization effects and PICTS results has been established.It was shown that native deep defects (A3–A6) localized in the bulk of crystal are responsible for hetero-charge formation and negative sign of the pyroelectric current peak observed around the Curie temperature after poling the sample in the temperature intervals well above Tc. It was also shown that the positive sign pyrocurrent observed near the Curie point is attributed to the homo-charge formed by native A2-trapping centers which are localized near the surface region of TlGaSe2. Native deep level trap having an activation energy of 0.26eV and the capture cross section of 2.8×10−13cm2 were established for A2 from PICTS measurements.

Photocatalytic CO2 reduction over B4C/C3N4 with internal electric field under visible light irradiation

Boron carbide/graphitic carbon nitride (B4C/g-C3N4) p–n hetero-junction photocatalyst with an internal electric field was synthesized by a facile solvent evaporation method and characterized by field emission scanning electron microscope (FESEM), UV–Vis diffuse reflectance spectra (UV–Vis DRS), photoluminescence spectra (PL), etc. Photocatalytic activity of the composite B4C/g-C3N4 loaded with Pt co-catalyst was evaluated using CO2 conversion to CH4 with H2 as the hydrogen source and reductant under visible light irradiation. The coupling of p-type B4C with n-type g-C3N4 significantly improved the performance of photocatalytic CO2 reduction; with the optimum B4C mass fraction of 1/6, the composite photocatalyst showed approximately 6 and 8 times higher CH4 generation rate than g-C3N4 and B4C, respectively. The enhancement was attributed to efficient photo-excited electron/hole separation due to the formation of internal electric field at the p-B4C/n-C3N4 interface.

Effect of internal electric field on ferroelectric polarization in multiferroic TbMnO3

Pyro-current measurements have been widely used to study ferroelectric properties in multiferroic materials. However, determination of intrinsic polarization by this method is not straightforward because of leakage current and trapped charge carriers. Here, we demonstrate the formation of internal electric field due to thermally stimulated charge carriers and its influence on ferroelectric polarization in a polycrystalline sample of the well-known multiferroic TbMnO3. While an electric field (Eext) poling across the ferroelectric transition (TC ~ 26 K) is essential to obtain depolarization current at TC, the sample poled only in the paraelectric state (TPole=130–50K) also exhibits a pyro-current peak at TC but with the same polarity (−IPyro) as that of the external field (−Eext). We demonstrate that these unusual behavior of pyro-current are caused by a positive internal electric field (+Eint) which in turn are created by thermally stimulated free charge carriers during the poling process in the paraelectric state. We also show that a combination of DC-biased current and pyro-current measurements is a promising method to study the intrinsic ferroelectric properties in multiferroic materials.

Electroabsorption Study of Internal Electric Fields in Vacuum-Evaporated Layers of Organic Materials

Abstract The Stark effect in opfical absorption of some organic compounds has been analyzed to yield information on the internal electric fields (IEF) occurring in their vacuum-evaporated thin Myers. IEF strengths in excess 105V/cm have been observed with films of Epindolidione (EPI: C18H12N2O2) and linear trans — Quinacridone (QAC: C20H12N2O2) at room temperature. The lower values of IEF have been obtained for QAC, where, in contrast to EPI, the IEF are directed from the non-substrate to substrate side of the samples. No external field dependence of IEF was detected. However, the exact value of IEF depends on the preparation procedure of the layers. The hypothesis has been confirmed that formation of IEF involves a charge generation process on non-identified generation centres associated with defects showing an asymmetry in their characteristics on the substrate and non-substrate side of the samples.

The significance of thermalization of [formula omitted]-level in formation of photoelectric structure in ruby

A microscopic model explaining the formation of the electric domain structure in a heavily doped ruby under intensive laser irradiation is suggested. The model is based on the following facts: the thermalization of a metastable level Ē of Cr3+; the inhomogeneous Cr3+-ions distribution in the A and B sublattices during the crystal growth; the hopping conductivity in the system of Cr3+ ions excited into a metastable level. The suggested thermalizational model decribes the starting mechanism of the formation of internal electric fields in domains and explains qualitatively most of the known experimental results.

Thermalizational mechanism of photoinduced electric domain structure formation in ruby

A model explaining the formation of electric domain structure in heavily doped ruby under intensive laser irradiation is suggested. The model is based on the following facts: thermalization of metastable level E of Cr3+; distribution in the A and B sublattices during crystal growth hopping conductivity in a system of Cr3+ ions excited into a metastable level. It is demonstrated experimentally that the number of Cr3+ ions decreases during the formation of internal photoelectrical structure in ruby, the distribution of Cr3+ ions in the A and B sublattices is inhomogeneous in the sample, and the rate of formation of the domain structure depends on the irradiation prehistory of the sample. Observing the dynamics of the electric domain structure in ruby, conclusions on the microscopic nature of the formation of internal electric fields and the close arrangement of the excess charged chromium ions in domain walls are drawn. The suggested thermalizational model describes a starting mechanism for the formation of internal electric fields in domains and explains qualitatively most of the known experimental results.