Core-shell Heterojunction Research Articles

A ZnO/ZnFe2O4 uniform core-shell heterojunction with a tubular structure modified by NiOOH for efficient photoelectrochemical water splitting.

It is known that heterojunction photoelectrodes can improve light absorption and accelerate the separation of photogenerated carriers in the field of photoelectrochemical (PEC) water splitting. However, the key to efficient photoelectrochemical performance is to build heterojunctions with a narrow band gap semiconductor loaded on a wide band gap semiconductor uniformly and densely. Herein, a ZnO/ZnFe2O4 uniform core-shell heterojunction photoelectrode is prepared by a simple ion etching method. Moreover, the chemical reaction process and mechanism are discussed in detail. The ZnO/ZnFe2O4 photoelectrode shows a higher photocurrent density (0.29 mA cm-2 at 1.23 V vs. RHE) compared with that of ZnO (0.17 mA cm-2 at 1.23 V vs. RHE). Furthermore, the PEC performance of ZnO/ZnFe2O4 is optimized by depositing NiOOH; the photocurrent density of ZnO/ZnFe2O4/NiOOH is 0.48 mA cm-2 at 1.23 V vs. RHE. The remarkable PEC performance of ZnO/ZnFe2O4/NiOOH benefits from the following important factors: (i) the uniformity and nanotube structure of the ZnO/ZnFe2O4 heterojunction; (ii) the excellent light harvesting ability; and (iii) the reduced photogenerated electron-hole pair recombination rate. This work provides new insights into designing heterojunction systems for efficient photoelectrochemical water splitting.

In-depth understanding of core-shell nanoarchitecture evolution of g-C3N4@C, N co-doped anatase/rutile: Efficient charge separation and enhanced visible-light photocatalytic performance

Herein, we demonstrated the simultaneous formation of multi-component heterojunction consisting graphitic carbon nitride (g-C3N4) and C, N co-doped anatase/rutile mixed phase by using facile sol-gel assisted heat treatment. The evolution of core-shell nanostructures heterojunction formation was elucidated by varying the temperature of heat treatment from 300°C to 600°C. Homogeneous heterojunction formation between g-C3N4 and anatase/rutile mixed phase was observed in gT400 with C and N doping into TiO2 lattice by O substitution. The core-shell nanoarchitectures between g-C3N4 as shell, and anatase/rutile mixed phase as core with C and N atoms are doped at the interstitial positions of TiO2 lattice was observed in gT500. The result indicated that core-shell nanoarchitectures photocatalyst (gT500) prepared at 500 ◦C exhibited the highest photocatalytic activity in the degradation of methyl orange under visible light irradiation. Meanwhile, the possible mechanisms of charge generation, migration, action species and reaction that probably occur at the gT500 sample were also proposed. The photodegradation results of gT500 correlated completely with the results of the PEC and photoluminescence analysis, which directly evidenced improved charge separation and migration as the crucial parameters governing photocatalysis. It is worthy to note that, the simultaneous formation of multicomponent heterojunction with core-shell structure provided an enormous impact in designing highly active photocatalyst with superior interfacial charge transfer.

A-Axis GaN/AlN/AlGaN Core-Shell Heterojunction Microwires as Normally Off High Electron Mobility Transistors.

Micro/nanowire-based devices have been envisioned as a promising new route toward improved electronic and optoelectronic applications, which attracts considerable research interests. However, suffering from applicable strategies to synthesize uniform core-shell structures to meet the requirement for the investigations of electrical transport behaviors along the length direction or high electron mobility transistor (HEMT) devices, heterojunction wire-based electronics have been explored limitedly. In the present work, GaN/AlN/AlGaN core-shell heterojunction microwires on patterned Si substrates were synthesized without any catalyst via metalorganic chemical vapor deposition. The as-synthesized microwires had low dislocation, sharp, and uniform heterojunction interfaces. Electrical transport performances were evaluated by fabricating HEMTs on the heterojunction microwire channels. Results demonstrated that a normally off operation was achieved with a threshold voltage of 1.4 V, a high on/off current ratio of 108, a transconductance of 165 mS/mm, and a low subthreshold swing of 81 mV/dec. The normally off operation may attribute to the weak polarization along semipolar facets of the heterojunction, which leads to weak constrain of 2DEG.

Catalyst-free fabrication of novel ZnO/CuO core-Shell nanowires heterojunction: Controlled growth, structural and optoelectronic properties

Development of controlled growth and vertically aligned ZnO/CuO core-shell heterojunction nanowires (NWs) with large area by a catalyst free vapor deposition and oxidation approach has been investigated. Structural characterization reveals successful fabrication of a core ZnO nanowire having single crystalline hexagonal wurtzite structure along [002] direction and CuO nanostructure shell with thickness (8–10nm) having polycrystalline monoclinic structure. The optical property analysis suggests that the reflectance spectrum of ZnO/CuO heterostructure nanowires is decreased by 18% in the visible range, which correspondingly shows high absorption in this region as compared to pristine ZnO nanowires. The current-voltage (I–V) characteristics of core-shell heterojunction nanowires measured by conductive atomic force microscopy (C-AFM) shows excellent rectifying behavior, which indicates the characteristics of a good p-n junction. The high-resolution transmission electron microscopy (HRTEM) has confirmed the sharp junction interface between the core-shell heterojunction nanowire arrays. The valence band offset and conduction band offset at ZnO/CuO heterointerfaces are measured to be 2.4±0.05 and 0.23±0.005eV respectively, using X-ray photoelectron spectroscopy (XPS) and a type-II band alignment structure is found. The results of this study contribute to the development of new advanced device heterostructures for solar energy conversion and optoelectronics applications.

The flexible SiC nanowire paper electrode as highly efficient photocathodes for photoelectrocatalytic water splitting

An ultralong SiC@g-C3N4 core-shell nanowires heterojunction photocatalyst was synthesized by the direct pyrolysis of melamine on the surface of SiC nanowires (NWs). The flexible heterojunction used as a photocathode for photoelectrochemical (PEC) water splitting shows a photocurrent density of −0.62mAcm−2 at −0.6V vs Ag/AgCl in 0.1M Na2SO4 solution, which is due to obviously enhanced visible light collection and charge separation. The photoexcited electrons on the lowest unoccupied molecular orbital (LUMO) level of g-C3N4 can be easily transported to the conduction band (CB) of SiC, whereas the photoexcited holes on the valence band (VB) of SiC can be transferred to the highest occupied molecular orbital (HOMO) level of g-C3N4. The synergic effect between SiC and g-C3N4 can decrease the recombination rate of photogenerated carriers and improve the photocatalytic hydrogen production activity. This work presents a novel SiC@g-C3N4 core-shell flexible photocathode for highly efficient PEC water splitting, which can be extended to fabricate various other flexible core-shell nanoheterostructures.

Heterojunctions Based on II-VI Compound Semiconductor One-Dimensional Nanostructures and Their Optoelectronic Applications

Wide band gap II-VI semiconductor nanostructures have been extensively studied according to their great potentials for optoelectronic applications, while heterojunctions are fundamental elements for modern electronic and optoelectronic devices. Subsequently, a great deal of achievements in construction and optoelectronic applications of heterojunctions based on II-VI compound semiconductor one-dimensional nanostructures have been obtained in the past decade. Herein, we present a review of a series of progress in this field. First, construction strategies towards different types of heterojunctions are reviewed, including core-shell heterojunctions, one-dimensional axial heterojunctions, crossed nanowires heterojunctions, and one-dimensional nanostructure/thin film or Si substrate heterojunctions. Secondly, optoelectronic applications of these constructed heterojunctions, such as photodetectors, solar cells, light emitting diodes, junction field effect transistors, etc., are discussed briefly. This review shows that heterojunctions based on II-VI compound semiconductor 1-D nanostructures have great potential for future optoelectronic applications.

Low Power Consumption Gas Sensor Created from Silicon Nanowires/TiO2 Core-Shell Heterojunctions.

Silicon nanowires/TiO2 (SiNWs/TiO2) array with core-shell nanostructure was created by sol-gel and drop-casting methods. The hybrid material displayed excellent sensing performance for CH4 detection at room temperature. The chemiresistor sensor has a linear response toward CH4 gas in the 30-120 ppm range with a detection limit of 20 ppm, which is well below most CH4 sensors reported before. The enhanced gas sensing performance at room temperature was attributed to the creation of heterojunctions that form a depletion layer at the interface of SiNWs and TiO2 layer. Adsorption of oxygen and corresponding gas analyte on TiO2 layer could induce the change of depletion layer thickness and consequently the width of the SiNWs conductive channel, leading to a sensitive conductive response toward gas analyte. Compared to conventional metal oxide gas sensors, the room temperature gas sensors constructed from SiNWs/TiO2 do not need an additional heating device and work at power at the μW level. The low power consumption feature is of great importance for sensing devices, if they are widely deployed and connected to the Internet of Things. The innovation of room temperature sensing materials may push forward the integration of gas sensing element with wireless device.

Improving the Electrical Performance of a Quantum Well FET With a Shell Doping Profile by Heterojunction Optimization

This paper investigates the impacts of typical semiconductor material properties—electron affinity, bandgap, and dielectric constant, on the electrical performance of a p-type core–shell heterojunction nanowire FET by numerical simulations. At the heterojunction, a valence band offset of 200 meV forms a sufficient energy barrier confining the holes in the quantum well, resulting in the optimal OFF-state current. A higher dielectric constant of the shell region is found to be able to decrease the leakage current of the device. The optimum conditions from the parameter analysis are demonstrated by a realistic and achievable material combination of Si/SiGe for the core–shell configuration. This paper provides physical insights into the materialwise impacts for designing the proposed transistor showing the reduced OFF-current and a better subthreshold swing for low-power applications.

Design of Core–Shell‐Structured ZnO/ZnS Hybridized with Graphite‐Like C3N4 for Highly Efficient Photoelectrochemical Water Splitting

AbstractNumerous efforts have been focused on the heterojunction structure for enhancing the electron injection across the interface in application of photocatalysis or photoelectrochemical (PEC) catalysis. The electrochemically deposited ZnO nanorods arrays are sulfurated following hydrothermal reaction to form core–shell heterojunction structure. Furthermore, the graphite‐like C3N4 (g‐C3N4) is added into the formed ZnO/ZnS core–shell nanorods during the sulfuration process to get ZnO/ZnS/g‐C3N4 photoanode. The heterojunction structure is characterized via X‐ray diffraction, scanning electron microscope, X‐ray photoelectron spectroscopy, transmission electron microscopy, UV‐vis diffuse‐reflectance spectra, time‐resolved photoluminescence, and Raman. The ZnO/ZnS/g‐C3N4 photoanode yields a photocurrent of ≈0.66 mA cm−2 at 1.23 V versus reversible hydrogen electrode, which is fourfold as large as pure ZnO electrode. The enhanced photocurrent is attributed to the improved separation efficiency of photogenerated electron–hole pairs and accelerated transport of hole to the electrode surface for the oxidation of water. These results suggest substantial potential of metal oxide nanorods arrays with controlled heterojunction construction in PEC water splitting applications.

Vertically Aligned Core–Shell PbTiO3@TiO2 Heterojunction Nanotube Array for Photoelectrochemical and Photocatalytic Applications

Vertically aligned core–shell PbTiO3@TiO2 heterojunction nanotube arrays are fabricated on F:SnO2 (FTO) glass substrate via a unique three-step process: an electron beam vapor deposition of Ti thin film on FTO, anodic oxidation of the Ti film to TiO2 nanotubes, and finally formation of PbTiO3 perovskite layer at the inner wall of the TiO2 nanotubes. The PbTiO3@TiO2 nanotube array exhibits dramatically improved photoactivity relative to TiO2 nanotubes or PbTiO3/TiO2 composite powders in photoelectrochemical water splitting and photocatalytic isopropyl alcohol decomposition under visible light irradiation (>420 nm). In the core–shell heterojunction electrodes, PbTiO3 serves as a visible light responsive inorganic photosensitizer with its small band gap and forms a heterojunction with TiO2 for effective charge separation.

Compact and uniform TiO2@g-C3N4 core-shell quantum heterojunction for photocatalytic degradation of tetracycline antibiotics

Optimizing the heterojunction structure of semiconductor photocatalysts is significant for taking full advantage of their abilities for organic molecules degradation. Here, we demonstrate a feasible strategy of polymerizing the quantum-thick graphitic carbon nitride (g-C3N4) on to the surface of anatase titanium dioxide (TiO2) nanosheets with exposed {001} facets to form the TiO2@g-C3N4 (TCN) core-shell quantum heterojunction for improving photocatalytic tetracycline degradation activity. 100mg of TCN photocatalyst shows the highest tetracycline degradation rate of 2.2mg/min, which is 36% higher than that of the TiO2/g-C3N4 random mixture (TCN(mix)), 2 times higher than that of TiO2, and 2.3 times higher than that of bulk g-C3N4. Results also indicate that h+ and ·O2− are the main oxidant species for the efficient photocatalytic reaction. The decisive factors in improving the photocatalytic activity of TCN is the unique structural advantages of quantum-thick g-C3N4 shell, compact and uniform contact interface, richly available reaction sites, more surface adsorbed hydroxyl (OH) groups. Efficient electron transfer between TiO2 and g-C3N4 is also demonstrated by the significant enhancement of photocurrent response of TCN electrodes and decrement of fluorescence emission spectra. This work demonstrates new sights for synthesizing high-efficient and environment-stable photocatalysts by engineering the surface heterojunction.

Photo-induced reduction of CO2 to CO with hydrogen over plasmonic Ag-NPs/TiO2 NWs core/shell hetero-junction under UV and visible light

Ag-NPs-promoted TiO2 nanowires (TiO2 NWs) core/shell hetero-junction with plasmonic properties for selective CO production in photo-induced CO2-hydrogen system has been investigated. Ag-NPs were photo-deposited over the TiO2 NWs and characterized by XRD, TEM, N2-adsorption-desorption, XPS, UV–vis and PL spectroscopy. Ag-NPs in metal-state were successfully coated on the TiO2 NWs surface, producing core-shell hetero-junction. Ag-NPs coated over TiO2 NWs exhibited strong absorption of visible light due to localized surface plasmon resonance (LSPR) excitation, trapped electrons and hindered charges recombination rate. The synergistic effect of Ag-NPs coated over TiO2 NWs for CO2 conversion was evaluated in a gas-phase system under UV and visible light irradiation. The plasmonic Ag-NPs/TiO2 NWs demonstrated excellent photoactivity in the reduction of CO2 into CO, CH4 and CH3OH under visible light irradiation. The results show that 3wt.% Ag-NPs-loaded TiO2 NWs was found to be the most active, giving the highest CO evolution of 983μmole-g-catal.−1h−1 at selectivity 98%. This amount of CO produced was 23 times more than the TiO2 NWs and 109 times larger than the yield of CO produced over the pure TiO2. More importantly, the quantum yield was substantially enhanced for CO evolution. The LSPR excitation and synergic effect of Ag-NPs which can effectively accelerate the charge separation was proposed to be responsible for the enhancement of photocatalytic activity. The photo-stability of Ag-NPs/TiO2 NWs evidenced in cyclic runs for selective CO production under visible light, yet photoactivity declined over the irradiation time under UV-light. The reaction mechanism to describe the reaction pathways is also presented.

Efficient photovoltaic devices based on p-ZnSe/n-CdS core–shell heterojunctions with high open-circuit voltage

Large band-gap ZnSe nanowires and CdS films are used to construct core–shell heterojunction solar cells with high open circuit voltage and efficiency.

Ag3PO4@holmium phosphate core@shell composites with enhanced photocatalytic activity

In this study, Ag3PO4@holmium phosphate (HP) composite photocatalysts were firstly prepared. The core–shell heterojunction structure of Ag3PO4@HP significantly improved the photocatalytic activity of Ag3PO4.

An α-Bi2O3/BiOBr core-shell heterojunction with high photocatalytic activity.

We prepared an α-Bi2O3/BiOBr core-shell heterojunction via a facile in situ chemical transformation method. Ultrathin BiOBr nanosheets were observed and these were found to be evenly distributed on the surface of α-Bi2O3/BiOBr by SEM and TEM. The results revealed that the prepared α-Bi2O3/BiOBr photocatalysts were porous, and their specific surface areas were found to be raised on comparing with α-Bi2O3 and BiOBr. In our experiments, the photocatalytic activity of α-Bi2O3 was obviously enhanced when assembling with ultrathin BiOBr compared with α-Bi2O3/BiOBr and the individual phase. It is considered that the surface porous structure increases the specific surface areas, which improved the adsorption characteristics of α-Bi2O3/BiOBr. The suitable band alignment (between α-Bi2O3 and BiOBr) and oxygen vacancy effect can be attributed to the increased photocatalytic activity under visible irradiation.

Preparation of the TiO2/Graphic Carbon Nitride Core-Shell Array as a Photoanode for Efficient Photoelectrochemical Water Splitting.

The photoelectrochemical (PEC) oxygen evolution reaction over a photoanode is a promising process for renewable energy. The fascinating properties of graphic carbon nitride (g-CN) in water splitting make the photoelectrode engineering of it for PEC use quite meaningful. In this work, we report the fabrication of the core-shell-structured TiO2/g-CN composite film by hydrothermal growth for TiO2 nanorod arrays and solvothermal growth for the g-CN layer. Herein, TiO2 is used as an effective electron-transfer layer, and g-CN is used as a visible light absorption layer. Different reaction conditions were investigated in order to obtain the uniform TiO2/g-CN nanorod core-shell structure. Outstanding photoelectrochemical performances of the optimized composites were obtained compared to that of pristine TiO2 or g-CN because the high-quality heterojunction between g-CN and TiO2 turned out to effectively reduce the recombination of charge carriers and improve the photoelectric conversion ability. Thus, the photocurrent density under visible light of TiO2/g-CN reached 80.9 μA cm-2, which is 21 times that of g-CN under 0.6 V (vs SCE). Finally, a systematic photoelectrocatalytic mechanism of charge carrier migration and the recombination path in the TiO2/g-CN nanorod core-shell heterojunction was proposed, which can be considered to be a probable explanation of efficient PEC performance.

Perovskite heterojunction based on CH3NH3PbBr3 single crystal for high-sensitive self-powered photodetector

Perovskite single crystals exhibit extraordinary optoelectronic performances due to their advantages such as low trap-state densities, long carrier diffusion, and large absorption coefficient, and thus, photodetectors based on perovskite single crystals have attracted much research interest. Unlike the reported one-component single-crystal perovskite photodetectors, here, we have developed a facile two-step approach to fabricate a core-shell heterojunction based on the CH3NH3PbBr3 single crystal. A photodetector made of the as-prepared perovskite heterojunction renders the feature of self-power attributed to a built-in electric field in the junction and exhibits a wavelength-dependent responsivity with a peak responsivity up to 11.5 mA W−1 under 450 nm irradiation at zero bias, which is one order of magnitude higher than the CH3NH3PbBr3 single crystal and shows a maximum external quantum efficiency of 3.17%, also higher than the reported 0.2% of the CH3NH3PbBr3 single crystal. Our work may lead to more efficient self-powered heterojunction systems based on perovskite single crystals.

Ag3PO4@LaPO4 core–shell composite with high photocatalytic activity

In this study, Ag3PO4@LaPO4 compositephotocatalysts were successfully fabricated by silver-ammonia-solution-assisted solution co-deposition. During co-deposition, Ag3PO4 particles were encapsulated by a flocculent amorphous substance layer of LaPO4, affording a Ag3PO4@LaPO4 core–shell heterojunction structure. The LaPO4 shell structure improved the crystallization of the Ag3PO4 core, as well as the dispersion and separation efficiency of photoelectrons and holes, which significantly promoted photocatalytic activity.

Synthesis of core-shell ZnO/oxygen doped g-C3N4 visible light driven photocatalyst via hydrothermal method

Abstract Core-shell ZnO/oxygen-doped g-C 3 N 4 nanocomposite was prepared via hydrothermal method with the addition of hydrogen peroxide (H 2 O 2 ) and its visible light photocatalytic performance was investigated. In the process of hydrothermal reaction, g-C 3 N 4 nanosheets were oxidized by hydrogen peroxide and then dehydration reaction occurred between C-OH groups to form the C-O-C groups, resulting in a relatively stable oxygen doped g-C 3 N 4 nanosheets wrapped on the surface of ZnO nanoparticles. The photocatalytic performance of ZnO/oxygen-doped g-C 3 N 4 sample showed significant photocatalytic activity, higher than ZnO, g-C 3 N 4 nanosheets and ZnO/g-C 3 N 4 samples. The PL spectra indicated that the stable core-shell heterojunction of ZnO/oxygen-doped g-C 3 N 4 composite could block the recombination rate of electron−hole pairs effectively and remarkably enhanced photocatalytic activity.

Engineered WO3 nanorods for conformal growth of WO3/BiVO4 core–shell heterojunction towards efficient photoelectrochemical water oxidation

High aspect ratio WO3 nanorods were obtained by post growth transformation of ammonium tungsten bronze ((NH4)0.33·WO3) nanorods which are grown via a facile solvothermal method. Influence of capping agents, reaction temperature and annealing temperature on the morphologies and performances of (NH4)0.33·WO3 films have been investigated and the growth mechanism of (NH4)0.33·WO3 nanorods was also explored. The results show that NH4 + plays a key role for formation of rod-like morphology at appropriate reaction temperature. Monoclinic WO3 nanorods were obtained via removal of ammonium by annealing the synthesized (NH4)0.33·WO3 at 500 °C in air. Conformal WO3/BiVO4 heterojunction was also synthesized using PLD to extend its light harvest range as well as enhance the charge separation. The heterojunction was characterized to demonstrate efficient photocatalytic water splitting by core–shell arrays constructed based on high aspect ratio nanowires.