BiOBr Nanosheets Research Articles

In situ forming heterointerface in g-C3N4/BiOBr photocatalyst for enhancing the photocatalytic activity

Removing wastewater with organic pollutants or medicine using semiconductor photocatalysts has attracted increasing attention in recent decades. However, low visible light utilization, high charge carrier recombination and insufficient surface active sites are still the limitation of photocatalysts. Herein, an in situ-forming BiOBr nanosheets on the surface of g-C3N4 were successfully synthesized via water bath in ambient temperature. The carboxyl group of acetic acid plays a vital role in growing for BiOBr nanosheets. The particular construction of g-C3N4/BiOBr composites increases the active sites and improve separation efficiency of photogenerated carriers, which strengthen photocatalytic degradation for pollutants. Meanwhile, electrons were transmitted quickly in the layered graphite carbon of g-C3N4 which owns excellent electrical conductivity. As a result, this work provides a novel approach to in-situ construction of semiconductor heterojunctions and demonstrates the prepared g-C3N4/BiOBr photocatalysts behave great potential in the removal of pollutants.

Enhancement of green upconversion luminescence of Yb3+/Tb3+ co-doped BiOBr nanosheets and its potential applications in photocatalysis

In this work, a series of Yb3+/Tb3+ co-doped BiOBr nanosheets were synthesized by a simple solvothermal method, and the effect of Tb3+ and Yb3+ doping on the up-conversion (UC) luminescence was investigated. Under the excitation of 980 ​nm, BiOBr: Yb3+/Tb3+ nanosheets displayed emission bands at 494, 544, 584 and 622 ​nm arise from transitions 5D4 → 7FJ (J ​= ​6, 5, 4, 3) of Tb3+ ions, respectively. Furthermore, the concentration dependence of UC luminescence was comprehensively explored, and the BiOBr:15%Yb3+/3%Tb3+ nanosheets exhibited the most outstanding luminescent properties by the cooperative energy transfer. Moreover, the degradation rate of RhB by Yb3+/Tb3+ co-doped BiOBr reaches 98.9% for 30 ​min under full spectral irradiation, and 51.7% for 5 ​h under 980 ​nm irradiation, respectively. This work proves that Yb3+/Tb3+ co-doped BiOBr nanosheets have excellent green UC emission and have potential application in the field of near infrared activated photocatalysts.

Photocatalytic degradation of ammonium dinitramide over novel S-scheme g-C3N4/BiOBr heterostructure nanosheets

Photocatalytic remediation of explosive wastewater pollution gradually attracts our attention owing to strict environmental protection requirements. In this study, visible light driven photocatalytic degradation of ammonium dinitramide (ADN) over the novel solvothermal-synthesized 2D/2D g-C3N4(CN)/BiOBr(BB) heterostructure nanosheets was carried out. Multiple characterization techniques confirm the successful formation of tight contact interface between BiOBr and CN nanosheets. 30CN/BB composite exhibits the superior photocatalytic activity with degradation efficiency of 95.0 %, attributing to larger effective electrochemicalsurface area (ECSA), expanded light capture range and efficient S-scheme charge transfer ability. Besides, the main active species were determined as e− and •O2−, and generation level of free radicals was detected by EPR technique. Moreover, the reactive sites and possible degradation mechanism of ADN were analyzed by DFT calculation.

Enhancement of Photocatalytic Ammonia Production Via Photoreconstruction of AU Supported on Biobr Nanosheets

Enhancement of Photocatalytic Ammonia Production Via Photoreconstruction of AU Supported on Biobr Nanosheets

Structural engineering of BiOBr nanosheets for boosted photodegradation performance toward rhodamine B.

A series of BiOBr nanosheets were synthesized through a facile solvothermal method, whose structures were adjusted by changing solvent ratios. Their photodegradation properties toward rhodamine B (RhB) were further investigated under visible light irradiation. The photocatalytic results indicated that the B-1:3 sample showed superior photoactivity and the RhB removal efficiency attained 97% within 30 min. The outstanding photodegradation activity can be ascribed to the small particle size and thickness, suppressed e−–h+ pair recombination and more active electrons and holes. Moreover, free radical quenching experiments suggest that ·O2− and h+ play a crucial role in improving photoactivity. This work opens a new avenue to boost the removal rate of organic pollutants by engineering the solvent ratios of photocatalysts in the wastewater treatment field.

Novel p-n type polyimide aerogels/BiOBr heterojunction for visible light activated high efficient photocatalytic degradation of organic contaminants

In this work, a novel p-n heterojunction polyimide aerogel/BiOBr (PI/BOB) photocatalyst was successfully synthesized by combining supercritical drying and in-situ precipitation method, to degrade organic pollutants in visible light. The synthesized PI/BOB photocatalysts exhibited excellent photocatalytic activity by establishing p-n heterojunction with improved charge separation and transfer performance. Moreover, the combination of BiOBr and PI can effectively reduce the hydrophobicity of PI and help to improve its application in water pollution treatment. After a comprehensive study of the microstructure and optical properties of PI/BiOBr heterojunction, it is found that the combination of PI and BiOBr nanosheets can expand the light absorption range, and the porous PI can greatly increase the specific surface area of PI/BiOBr heterojunctions. Furthermore, XPS results showed that the electron in PI transferred to BiOBr through the concentration differences, thus generating an internal electric field (IEF) between PI and BiOBr. Under the effect of IEF, photogenerated carriers can be separated between PI and BiOBr more efficiently, which can effectively improve the photocatalytic efficiency. Photocatalytic degradation experiments showed that PI/BOB has excellent photocatalytic degradation effect on RhB, and the degradation rate of 50 mg RhB was 99% in 60 min for 20 mg catalyst.

Synthesis of novel BiOBr/Bio-veins composite for photocatalytic degradation of pollutants under visible-light

A novel BiOBr/Bio-vein photocatalyst was synthesized with a wet drop addition method, where the substrate biological veins (Bio-veins) were obtained with a green and environmentally-friendly multiple route. BiOBr/Bio-veins composite show larger visible light adsorption compared with pure BiOBr, and the size of BiOBr nanosheet from BiOBr/Bio-veins composite is smaller than that of pure BiOBr. BiOBr/Bio-veins composite shows highly photocatalytic degradation of Rhodamine B (RhB) under visible light irradiation. Reactive species trapping experiments and electron paramagnetic resonance (EPR) tests were used to explore photodegradation mechanism of BiOBr/Bio-veins, which demonstrated that superoxide radicals (•O2−) and holes (h+) were the main active groups for photocatalytic degradation. Moreover, BiOBr/Bio-veins composite can be easily recovered without centrifugation and filtration. This study provides novel natural biological materials to immobilize photocatalyst particles with highly photocatalytic degradation of pollutants.

Crafting of plasmonic Au nanoparticles coupled ultrathin BiOBr nanosheets heterostructure: steering charge transfer for efficient CO2 photoreduction

Integrating semiconductor photocatalysts with outstanding visible light absorption and fast surface/interface charge transfer kinetics is still an enormous challenge for efficient CO2 photoreduction. In this work, the Au nanoparticles have been coupled with ultrathin BiOBr nanosheets, the formed heterostructure (Au/BiOBr) possesses a localized surface plasmon resonance (LSPR) and enhances the visible light absorption ability, as well as forms a fast charge transport channel on the interface between Au and BiOBr. Thus, the heterostructure photocatalyst exhibits higher photocatalytic CO2 to CO performance (135.3/16.43 μmol g−1) than that of BiOBr (89.0/6.46 μmol g−1) under 300 W Xe lamp and visible light (λ > 400 nm) irradiation for 5 h, respectively. Finally, the in situ FT-IR spectroscopy revealed CO2 photoreduction process and found that the ∗COOH is the key intermediate for CO2 to CO. This work provides an effective method to construct multielectron transfer scheme for efficient photocatalytic CO2 reduction.

Harnessing efficient in-situ H2O2 production via a KPF6/BiOBr photocatalyst for the degradation of polyethylene

• KPF 6 doped BiOBr were successfully synthesized using a simple one-pot hydrothermal method. • The photocatalytic H 2 O 2 production of 20 wt% KPF 6 /BiOBr is 1.96 times higher than pure BiOBr. • Plastic bags were decomposed by KPF 6 /BiOBr in conjunction with the in-situ generation of H 2 O 2 . • Hydroxyl radicals play a key role in the polyethylene degradation. • A potential mechanism for H 2 O 2 production and polyethylene degradation was proposed. The development of cost-effective dual-function photocatalytic materials to advance clean energy generation and storage combined with superior pollutant degradation performance has thus far remained challenging. For this study, potassium hexafluorophosphate (KPF 6 ) was introduced to modify BiOBr via a simple one-pot hydrothermal method, where uniformly dispersed KPF 6 on BiOBr nanosheets improved H 2 O 2 yields and the pollutant degradation performance. Such materials have proven to be critical for low bandgap and enhanced surface charge separation for efficient H 2 O 2 production. The optimized sample of 20 wt% KPF 6 /BiOBr showed a H 2 O 2 production rate of 53.36 mg·L –1 , which was much higher than that of pure BiOBr (27.19 mg·L –1 ). Nitroblue tetrazolium chloride (NBT) superoxide radical detection results suggested that two-step single-electron oxygen reduction was the primary pathway. More importantly, discarded polyethylene bags from supermarkets have been decomposed though the in-situ generation of H 2 O 2 using KPF 6 /BiOBr materials, and the mass loss of plastics was 2.33 times higher than that of the pure BiOBr. This research offers new perspectives toward the development of new functionalized materials for H 2 O 2 production, while improving capacities for the degradation of plastics.

Scalable Chemical Interface Confinement Reduction BiOBr to Bismuth Porous Nanosheets for Electroreduction of Carbon Dioxide to Liquid Fuel

AbstractElectrochemical reduction of carbon dioxide (CO2) toward chemical and fuel production is a compelling component of the new energy system. Two‐dimensional bismuth with a particular surface has been identified as a highly efficient electrocatalyst for converting CO2 to formate. However, the development of a controllable synthetic strategy for possible large‐scale production of such Bi materials remains highly challenging. Herein, a scalable chemical interface confinement reduction method is proposed for topotactic transformation of BiOBr (001) nanosheets to metallic Bi (001) porous nanosheets (PNS). As expected, the Bi (001) PNS exhibits excellent electrochemical performance on CO2 reduction to formate, with Faradaic efficiency of 95.2% and formate partial current density of 72 mA cm−2. Density functional theory calculations suggest that Bi PNS selectively exposes (001) surfaces with small‐angle grain boundaries can significantly lower the free energy barrier for the formation of *OCHO, which are responsible for the high activity and selectivity toward CO2‐to‐formate conversion.

Bismuth oxybromide/bismuth oxyiodide nanojunctions decorated on flexible carbon fiber cloth as easily recyclable photocatalyst for removing various pollutants from wastewater

Various semiconductor powders (such as bismuth oxybromide/bismuth oxyiodide (BiOBr/BiOI) nanojunctions) can photodegrade wastewater efficiently, but their practical application is limited by poor recovery performance. To address the problem, we report the construction of BiOBr/BiOI nanojunctions on flexible carbon fiber cloth (CFC) substrate as an easily recycled photocatalyst by the dipping-solvothermal-dipping-solvothermal four-step method. CFC/BiOBr/BiOI is composed of CFC substate and two layers of nanosheets, while BiOBr nanosheets (thickness: 10–30 nm, diameter: 200–400 nm) were grown in the inner layer and BiOI nanosheets (thickness: 50–80 nm, diameter:300–600 nm) were grown in the outer layer. CFC/BiOBr/BiOI (4 × 4 cm2) can effectively photodegrade 97.7% acid orange 7 (AO7), 91.3% levofloxacin (LVFX) and 97.8% tetracycline (TC) within 120 min under the illumination of visible-light, better than CFC/BiOBr (73.2% AO7, 71.6% LVFX and 81.6% TC). Furthermore, superoxide radical (•O2−) and hydroxyl radical (•OH) are the main active substances during removing LVFX by CFC/BiOBr/BiOI. Besides, CFC/BiOBr/BiOI can efficiently reduce 93.5% chemical oxygen demand (COD) concentration of acrylic resin production wastewater (ARPW) under visible-light illumination for 3 h, better than CFC/BiOBr (36.6% COD). Therefore, CFC/BiOBr/BiOI has broad application prospects in purifying wastewater as a new type of easily recycled photocatalyst.

Facet junction of BiOBr nanosheets boosting spatial charge separation for CO2 photoreduction

Understanding on the photogenerated charge separation from a microscopic level remains a challenge and is highly desirable as it provides a cornerstone for designing high-performance photocatalysts. Herein, facet engineering is chosen as a tool to reveal the relationship between the charge separation/transfer and crystal structure. A series of BiOBr nanosheets with dominantly exposed facet of (001) or (010) as well as different lateral facet exposure ratios are constructed via adjusting pH value during the hydrothermal process. It is found that exposure of anisotropic crystal facets allows the separative transfer of photogenerated electrons and holes onto the lateral facets and dominantly exposed facets, respectively, which is attributed to the junction formed between distinct facets (i.e., facet junction). In the case of BiOBr-5 with (010)/(102) facet junction, the electron transfer rate (kET) and efficiency (ηET) are 3.658 × 106s−1 and 54.09%, which are superior than the counterpart of BiOBr-1 with (001)/(110) facet junction. The fast electron transfer rate and high transfer efficiency of BiOBr-5 result in the high CO evolution rate from CO2 photoreduction under artificial sunlight. Our work may bring some new insights into the mechanism of facet junction and rational design of photocatalysts with high performance for solar energy storage in future.

Self-assembled ultrathin closely bonded 2D/2D heterojunction for enhanced visible-light-induced photocatalytic oxidation and reaction mechanism insights

Two-dimensional (2D) layered heterojunctions with a staggered band structure and unique interface properties exhibit promising application prospects in photocatalytic pollutant removal, water splitting, and CO2 reduction. Ultrathin 2D/2D heterojunctions with a large specific surface area and a short migration path of the photogenerated charge always illustrate a better photocatalytic performance than non-ultrathin 2D heterojunction photocatalysts. In this study, a novel ultrathin 2D/2D heterojunction of the Bi2O2(OH)(NO3)/BiOBr nanosheet composite (ultrathin BION/BiOBr) was in situ self-assembled though a cetyltrimethylammonium bromide assisted one-step hydrothermal method. Benefiting from the advantage of the unique ultrathin heterojunction structure, the ultrathin 2D/2D BION/BiOBr heterojunctions exhibit a greatly improved photocatalytic removal effect for multiple pollutants compared to the nanocrystal BION/BiOBr, pure BION. As a representative, the ultrathin 2D/2D Br-modified BION/BiOBr heterojunction shows an enhanced tetracycline degradation rate of 76%, which corresponded to a higher photodegradation rate constant of 0.01116 min−1 when compared to pure BION (17%, 0.00161 min−1) and nanocrystal BION/BiOBr (24%, 0.00223 min−1) under visible-light irradiation for 2 h. A series of characterization and density functional theory calculations demonstrate the enhanced separation and migration efficiency of the photogenerated electrons and holes over the ultrathin heterojunction, facilitating the formation of oxidizing groups for the organic pollutant removal. The possible mechanism of the TC photodegradation and the possible photodegradation pathway are also investigated in detail. This work provides a feasible method for constructing ultrathin 2D/2D heterojunction materials for environmental purification.

Photocatalytic activity and degradation mechanism of in-situ reduction Bi-doped self-assembled 3D flower-sphere UiO-66-NH2/BiOBr

The 3D flower-sphere UiO-66-NH2/Bi/BiOBr with good photocatalytic performance was successfully prepared by in-situ reduction and self-assembly process. The formation of semi-metal Bi was identified by XRD, XPS, HR-TEM and other methods. Transient photocurrent response and EIS spectra proved that compared with BiOBr nanosheets, the 3D flower-sphere structure, the reduction of metal Bi and the construction of Z-scheme heterojunction significantly improved the separation efficiency of photogenerated carriers in composite materials. The degradation rates of 50% UiO-66-NH2/Bi/BiOBr on tetracycline (30 mg/L TC) and norfloxacin (10 mg/L NOR) reached about 80% and 100%, respectively. Cycling experiments and XRD and XPS characterization results before and after the photocatalytic reaction verified that the composite had excellent reusability and stability. The experiment of free radical trapping agent and EPR spectrum analysis showed that in the UiO-66-NH2/Bi/BiOBr system, superoxide radical (·O2-) was the major active substance in the photocatalytic reaction, and Z-scheme degradation mechanism using metal Bi as electron conduction bridge was surmised.

Peroxymonosulfate-assisted for facilitating photocatalytic degradation performance of 2D/2D WO3/BiOBr S-scheme heterojunction

The Sulfate-mediated photocatalysis (SR-photo) technology was considered to be a rapid, efficient and green environmental treatment technology. In this study, the two-dimensional/two-dimensional WO3/BiOBr S-scheme heterojunction (2D/2D WOB) were prepared through the hydrothermal method. Subsequently, multiple characterization techniques confirmed that WO3 was successfully loaded on the surface of BiOBr nanosheets and an intimate interaction was formed. Tetracycline (TC) and enrofloxacin (ENR) were used as target pollutants to evaluate the degradation performance of the WOB/SR-photo system. The 1:2 WOB heterojunction exhibited the highest degradation activity, and the remove rate of TC and ENR reached 98% and 87%, respectively, within 60 min. Moreover, radical quenching experiments and electron paramagnetic resonance (EPR) results implied that the 1O2, h+, and ·O2 − played critical roles in the WOB/SR-photo system. The intermediate products of TC degradation were analyzed by liquid chromatography-mass spectrometer (LC-MS), and three possible degradation pathways were inferred. The photoelectrochemical measurement, photoluminescence (PL) and time-resolved PL confirmed that 1:2 WOB composite exhibited the highest transfer rate of charge carriers. Finally, based on the results of UV-DRs, Mott-Schottky curve and Density functional theory (DFT) calculations, the S-scheme WOB heterostructure was determined, and the degradation mechanism of the WOB/SR-photo system was also clarified. This study confirmed the superiority of the SR-photo system and brought dawn to the application of numerous photocatalysts with insufficient performance.

Highly selective photocatalytic synthesis of ethylene-derived commodity chemicals on BiOBr nanosheets

The synthesis of ethylene-derived commodity chemicals is an important process in industry. However, the current manufacturing process is energy-intensive and contributes a significant amount of greenhouse gas emissions. Photocatalytic conversion of ethylene to target products is a more sustainable and green alternative. In this work, we designed a bromide-mediated photocatalytic approach for highly selective synthesis of ethylene oxide or ethylene glycol from ethylene. The bromine/hypobromous acid (Br2/HOBr) generated in photocatalytic process are effective active species for conversion of gaseous ethylene to liquid intermediates, bromoethanol and dibromoethane. These intermediates were further converted to ethylene oxide or ethylene glycol via a facile alkali treatment in different temperatures, giving rise to a high selectivity of 92% and 100%, respectively. Interestingly, the BiOBr could oxidize its crystal Br− to generate Br2/HOBr efficiently, which is a more favorable process thermodynamically than oxidation of aqueous Br−. Furthermore, long-term stability (> 40 h) of BiOBr is maintained by adding KBr in the reaction system to automatically replenish Br− in BiOBr. This work provides a potential alternative to current industrial method for green synthesis of ethylene-derived commodity chemicals.

Efficient photocatalytic degradation of commercial pharmaceutical contaminants of carbamazepine using BiOBr nanosheets under visible-light irradiation

Removing pharmaceutical contaminants is the growing concerns for the public people. Herein, commercial pharmaceutical drug of carbamazepine (CBZ) was degraded by photocatalysis. Highly efficient visible-light-driven BiOBr nanosheets were fabricated using water mediated hydrolysis route with the assistment of tetra (n-butyl) ammonium hydroxide (TBAOH). In particular, BiOBr nanosheets prepared with 0.1 mmol TBAOH (0.1 mmol TBAOH + KBr) performed the optimum photocatalytic degradation activity, enabling 88.7% removals of CBZ after 20 min irradiation under visible-light. Furthermore, 0.1 mmol TBAOH + KBr BiOBr nanosheets displayed fast degradation kinetics, with a degradation rate constant of 6.2 times than BiOBr prepared without TBAOH (0 mmol TBAOH + KBr). The enhanced photocatalytic activity was originated from the improvement of adsorption performance, photo absorption, and separation rate of photoinduced electron-hole pair. The major active species participated in CBZ degradation were proved to be h+, •O2-, and •OH using quenching experiments. The present results can throw light on the development of visible-light-driven photocatalyst with considerable catalytic activity for removing pharmaceutical contaminants.

Simple Fabrication of Visible Light-Responsive Bi-BiOBr/BiPO4 Heterostructure with Enhanced Photocatalytic Activity

A Bi-BiOBr/BiPO4 heterojunction structure was successfully synthesized via a two-step solvothermal method with ethylene glycol as a reducer. Little BiPO4 irregular polyhedrons and little metal Bi spherical nanoparticles were uniformly dispersed on the surface of BiOBr nanosheets with intimate contact and formed a heterojunction structure between BiPO4 and BiOBr. It was found that Bi-BiOBr/BiPO4 had a significant improvement in photocatalytic performance for RhB degradation compared to bare BiOBr and BiPO4. The photocatalytic degradation rate constant of 0.2-Bi/BiOBr/BiPO4 was 1.44 h-1, which was 3.8 times and 14.2 times more than that of bare BiOBr and BiPO4, respectively. This is attributed to the formation of a ternary heterojunction, which benefits the separation of photogenerated electron-hole pairs. Furthermore, with the introduction of metal Bi, the SPR effect of metal Bi can effectively improve the absorption ability of Bi-BiOBr/BiPO4 photocatalyst, resulting in enhanced photoactivity. In this work, the mechanism of photocatalytic degradation was studied by using the photochemical technique and the capture experiment of active species, and it was revealed that h+ and ⋅O2- played a major role in the photocatalytic process.

Fabrication of three-dimensional hierarchical BiOBr/Bi2O4 p–n heterojunction with excellent visible light photodegradation performance for 4-chlorophenol

Construction of heterojunction and design of its architecture are effective strategies to improve the catalytic performance of photocatalyst. Herein, BiOBr/Bi2O4 p-n heterojunction was fabricated by a co-precipitation method, in which two-dimensional (2D) BiOBr nanosheets sheathed well on the surface of quasi-one-dimensional (1D) Bi2O4 submicrorods, forming a three-dimensional (3D) hierarchical structure. Such structural feature can not only enhance the utilization of incident photons through multi-reflection within the hierarchical structure but also raise the specific surface areas to supply more surface-active sites to participate in photocatalytic reaction. More importantly, BiOBr/Bi2O4 p-n junction could accelerate the interfacial electron-hole pairs transfer and separation via the built-in electric field, extending the lifetime of charge carriers. Thus, as-prepared BiOBr/Bi2O4 heterojunctions exhibit superior visible light catalytic activity for the photodegradation of 4-chlorophenol (4-CP). The catalytic activity of optimal junction is 2.03-fold as high as that of pristine Bi2O4. The trapping experiments demonstrate that hole (h+) and superoxide radical (•O2−) are the major active species during the catalytic photodegradation process of 4-CP. This work presents a unique tactic for the construction of high-performance Bi2O4-based heterojunction for the removal of organic pollutants.

Engineering an Interfacial Facet of S-Scheme Heterojunction for Improved Photocatalytic Hydrogen Evolution by Modulating the Internal Electric Field.

Constructing a step-scheme (S-scheme) heterojunction represents a promising route to overcome the drawbacks of single-component and traditional heterostructured photocatalysts by simultaneously broadening the optical response range and optimizing the redox ability of the photocatalytic system, the efficiency of which greatly lies in the separation behaviors of photogenerated charge carriers with strong redox capabilities. Herein, we demonstrate interfacial facet engineering as an effective strategy to manipulate the charge transfer and separation for substantially improving the photocatalytic activities of S-scheme heterojunctions. The facet engineering is performed with the growth of ZnIn2S4 on (010) and (001) facet-dominated BiOBr nanosheets to fabricate ZIS/BOB-(010) and ZIS/BOB-(001) S-scheme heterojunctions, respectively. It is disclosed that a larger Fermi level difference between BiOBr-(001) and ZnIn2S4 enables the formation of a stronger built-in electric field with more serious band bending in the space charge region around the interface. As a result, the directional migration and recombination of pointless photoexcited electrons in the conduction band (CB) of BiOBr and holes in the valence band (VB) of ZnIn2S4 with weak redox ability are speeded up enormously, thereby contributing to more efficient spatial separation of powerful CB electrons of ZnIn2S4 and VB holes of BiOBr for participating in overall redox reactions. Profiting from these merits, the ZIS/BOB-(001) displays a significant superiority in photocatalytic H2 evolution over ZIS/BOB-(010) and mono-component counterparts. This work provides new deep insights into the rational construction of a S-scheme photocatalyst based on an interfacial facet design from the viewpoint of internal electric field regulation.