Heterojunction Area Research Articles

Highly Active Ni Incorporated Co2p Heterostructure As Efficient Hydrogen Evolution Reaction Electrode for Alkaline Water Electrolysis

The development of efficient and cost-effective electrocatalyst for the hydrogen evolution reaction (HER) is essential for alkaline water electrolysis. Cobalt phosphide (Co2P) is a promising non-noble electrocatalyst material for HER due to its low cost and high stability in alkaline medium. Nevertheless, the HER activity of Co2P needs to be further improved as compared to noble metal based electrocatalysts. In this study, we have incorporated nickel (Ni) into Co2P using a co-sputtering system for enhanced HER performance, considering the junction of Ni/Co2P heterostructure. This junction serves as dual sites for water dissociation at Ni and hydrogen adsorption/desorption at Co2P, which synergistically enhances the HER activity. We also noticed that Ni concentration influences the heterojunction area as highly active site. Too low or high Ni concentration diminishes active sites by few Ni site formation or Ni agglomeration, respectively. Therefore, precise control over the Ni concentration is essential for maximizing the active site. Our optimized Ni incorporated Co2P sample showed outstanding HER performance with low overpotential of ~77 mV at 10 mA/cm2 and Tafel slope of ~47 mV/dec. The findings of this study will provide insight into the relationship between heterojunction formation and HER performance in transition metal-based heterostructure materials.

Thermally Stable Oxide-Capsulated Metal Nanoparticles Structure for Strong Metal-Support Interaction via Ultrafast Laser Plasmonic Nanowelding.

Strong metal-support interaction (SMSI) has drawn much attention in heterogeneous catalysts due to its stable and excellent catalytic efficiency. However, construction of high-performance oxide-capsulated metal nanostructures meets great challenge in materials thermodynamic compatibility. In this work, dynamically controlled formation of oxide-capsulated metal nanoparticles (NPs) structures is demonstrated by ultrafast laser plasmonic nanowelding. Under the strong localized electromagnetic field interaction, metal (Au) NPs are dragged by an optical force toward oxide NPs (TiO2). Intense energy is simultaneously injected into this heterojunction area, where TiO2 is precisely ablated. With the embedding of metal into oxide, optical force on Au gradually turned from attractive to repulsive due to the varied metal-dielectric environment. Meanwhile, local ablated oxides are redeposited on Au NP. Upon the whole coverage of metal NP, the implantation behavior of metal NP is stopped, resulting in a controlled metal-oxide eccentric structure with capsulated oxide layer thickness ≈0.72-1.30nm. These oxide-capsulated metal NPs structures can preserve their configurations even after thermal annealing in air at 600°C for 10min. This ultrafast laser plasmonic nanowelding can also extend to oxide-capsulated metal nanostructure fabrication with broad materials combinations (e.g., Au/ZnO, Au/MgO, etc.), which shows great potential in designing/constructing nanoscale high-performance catalysts.

Identifying and Amplifying the Spontaneously Formed Photo‐Charge Contribution in Opaque and Semitransparent Organic Photovoltaics

AbstractSpontaneous photo‐charge (SP) generation within the non‐fullerene (NFA) bulk helps avoid exciton (EX) related loss as compared to the traditional heterojunction route in organic solar cells (OSCs). Unfortunately, the promising SP utilization attracts little attention at this time due to the lack of knowledge on its specific contribution to the photovoltaic performance and rational optimization in high‐efficiency devices. To fill the gap, the NFAs’ SP characteristics are related with their photocurrent loss and power conversion efficiency (PCE) in state‐of‐the‐art materials. Though higher SP population is good for efficient EX utilization, it simultaneously acts as an additional photo‐charge recombination channel. The former can be further enhanced by increasing the NFAs’ crystallinity with preferred lamellar growth, and the latter can be suppressed via electric doping. With the aid of the two‐step optimization strategy, the total photocurrent loss is reduced from 14.4% to 4.8%, accompanying with PCE increment from 16.9% to 18.5%. The SP utilization strategy is finally applied in the semitransparent OSCs (ST‐OSCs), due to its less heterojunction area sensitive photo‐charge generation characteristic. A light utilization efficiency (LUE) is then increased from 3.21% to 3.77%. The findings highlight the pursue of efficient SP utilization as a new design philosophy in future OSCs’ progresses.

Preparation of Surface Dispersed WO3/BiVO4 Heterojunction Arrays and Their Photoelectrochemical Performance for Water Splitting.

In this work, a surface dispersed heterojunction of BiVO4-nanoparticle@WO3-nanoflake was successfully prepared by hydrothermal combined with solvothermal method. We optimized the morphology of the WO3 nanoflakes and BiVO4 nanoparticles by controlling the synthesis conditions to get the uniform BiVO4 loaded on the surface of WO3 arrays. The phase composition and morphology evolution with different reaction precursors were investigated in detail. When used as photoanodes, the WO3/BiVO4 composite exhibits superior activity with photocurrent at 3.53 mA cm-2 for photoelectrochemical (PEC) water oxidation, which is twice that of pure WO3 photoanode. The superior surface dispersion structure of the BiVO4-nanoparticle@WO3-nanoflake heterojunction ensures a large effective heterojunction area and relieves the interfacial hole accumulation at the same time, which contributes to the improved photocurrents together with the stability of the WO3/BiVO4 photoanodes.

Hybrid Schottky and heterojunction vertical β-Ga2O3 rectifiers

Schematic of hybrid Schottky and Junction Barrier Schottky Ga2O3 rectifiers. Breakdown voltage increased as the proportion of heterojunction area did, from 1.2 kV for Schottky rectifiers to 6.2 kV for pure heterojunction devices.

Performance optimization of epitaxial-layer based Si/SiGe hetero-junction area scaled tunnel FET label-free biosensors considering steric hindrance

This paper explores Si/SiGe hetero-junction dielectrically modulated area-scaled tunnel FET (DM-ASTFET) for label-free bio-sensing applications. The proposed sensor can detect bioanalytics such as protein, APTES, Choline Oxidase and Uricase. We have also investigated the influence of quantum confinement on the proposed device, and the onset voltage shifted by 0.1–0.3 V, as reported, without affecting the sensitivity mechanism. Sensitivity improvement is observed when the gate oxide length is 20 nm and the cavity length is 50 nm for a given bio-molecule due to the increased cross-section area for tunneling. The impact of epitaxial engineering on the biosensor's performance is also investigated in this work. A physics-based in-depth explanation of steric hindrance on the performance of the proposed sensor is also presented in this work. Finally, the optimum device design improves the sensitivity of ION ∼×103 and Vth ∼×2 times.

Highly efficient and stable near-infrared photo sensor based on multilayer MoS2/p-Si integrated with plasmonic gold nanoparticles

The exceptional characteristics of two-dimensional materials make them highly efficient and stable for electronic and optoelectronic applications. These materials exhibit a range of beneficial properties, such as ultrafast carrier dynamics, layer-dependent energy bandgap, tunable optical properties, low power dissipation, high mobility, transparency, flexibility, simple fabrication, and ability to confine electromagnetic energy within extremely small volumes. In this work, infrared light (980 nm) photo sensors are fabricated based on a MoS2/p-Si substrate utilizing the plasmonic phenomenon of gold nanoparticles (AuNPs) to enrich the optoelectronic properties and to enhance the photoresponse. The infrared light response for (Au NPs MoS2) comes from the strong interlayer coupling, which narrow the energy gap in the heterojunction area, thus rendering heterostructures to longer wavelength detection ability. Considering the low light absorption due to indirect bandgap essence of multilayer MoS2, its infrared responsivity further enhanced by 100.21% with a response rate of 42.39/95.44 μs (1 kHz) at a bias of 3 V, a repeatability responsivity of up to 0.59 A/W, and a detectivity of 4.5 × 1010 Jones with a maximum of 9.57 mW/cm2 light intensity, which is maintained through surface plasmon resonance (SPR). The plasmon-assisted photo sensors can be seamlessly integrated into the semiconductor industry to boost the optoelectronic performance in practical applications.

Hydrochloric acid and dopamine hydrochloride combined regulation of the photocatalytic activity of heterogeneous DBSO microflowers in visible light degradation of various dye water and antibiotics

Heterostructures with intimate contact interfaces and well-matched band structure construction are considered to be one the effective methods to promote photogenerated carrier separation and photocatalytic semiconductor activity. Herein, a simple joint post-modification method of hydrochloric acid and dopamine hydrochloride mediated under acidic conditions was proposed. Bi12SiO20-Bi2O2SiO3-BiOCl curved microflower photocatalyst with double heterojunction and large specific surface area was prepared. Hydrochloric acid and dopamine hydrochloride act as reactants and halogen sources in the composite photocatalyst, and their amount precisely control the surface morphology and the thickness of nanosheets. Benefiting from the synergistic effect of morphology regulation and interface optimization, the DBSO-10 not only changes its morphology from the original bone rod structure to the curved microflower structure with a large specific surface area, but also formes a double type-II heterostructure with tight interface connection. And the new microflower photocatalyst can achieve a degradation rate of 99.7 % in 20 min, which is 40 times higher than that of Bi12SiO20-Bi2O2SiO3 photocatalyst. Notably, the material can also effectively degrade five catalysts including dyes and antibiotics within 1 h.

A high-performance quasi-vertical MoSe2 photodiode with ultra-low dark current

Van der Waals heterostructure based on 2D materials is a promising technology for high-performance optoelectronic devices because of its tunable bandgaps and optical properties. However, photodetectors with a low dark current and a fast response speed commonly lose their photoresponsivity. The recovery current induced by the Schottky barrier height variation cancels out the device's reverse bias current in this paper, resulting in a quasi-vertical MoSe2 photodiode with ultralow dark current (<1 pA). Simultaneously, rapid electron–hole pair separation occurs at the interface due to the large heterojunction area and the strong interlayer coupling of MoSe2/graphene heterojunction, resulting in a fast response time of 1.5 ms and a high photoresponsivity of 19.72 A/W. Furthermore, the Au/MoSe2 forms a Schottky contact, which is asymmetrical to the Ohmic contact formed by the MoSe2/graphene, enabling the proposed device to achieve high-performance self-powered photodetection. Our work shows an alternative approach to improve the performance of future electronic and optoelectronic applications.

One-dimensional rod-shaped Ag2Mo2O7/BiOI n-n junctions for efficient photodegradation of tetracycline and rhodamine B under visible light

One-dimensional rod-shaped Ag 2 Mo 2 O 7 /BiOI composite photocatalysts were prepared via a facile hydrothermal-calcining method. The structure, morphologies, and elemental nature of the photocatalysts were characterized. Ag 2 Mo 2 O 7 /BiOI was formed through the attachment of BiOI nanosheets on the surface of Ag 2 Mo 2 O 7 rods, resulting in a 1D/2D heterojunction with larger interface area, more surface-active sites, and faster charge transfer channel. The Ag 2 Mo 2 O 7 /BiOI heterogeneous exhibited excellent and stable photodegradation performances for Rhodamine B (RhB) and Tetracycline (TC) under visible light irradiation. At the optimum composition of 15 wt% BiOI, the Ag 2 Mo 2 O 7 /BiOI heterojunction exhibited a highest degradation rate for RhB and TC, which were 70 times and 16 times higher than those of Ag 2 Mo 2 O 7 , respectively. The improved photocatalytic performance can be attribute to the efficient separation of photoinduced charges caused by the formation of the n-n heterojunction between Ag 2 Mo 2 O 7 and BiOI. Additionally, the dominant active species of photocatalytic reaction were determined to be • O 2 − and h + by free radical capture experiments and ESR test. In addition, a toxicity test with E. coli was carried out, which shows that the photodegraded Tetracycline solution is basically harmless to E. coli. Based on the experimental results and hybrid density functional theory calculation, a plausible photocatalytic mechanism of n-n heterojunction was proposed. • One-dimensional rod-shaped Ag 2 Mo 2 O 7 /BiOI were synthesized by a solvothermal method. • Ag 2 Mo 2 O 7 /BiOI can efficiently degrade Tetracycline. • The toxicity of the degraded tetracycline solution was low by E. coli activity detection. • Charge carriers were separated due to the n-n heterojunction and large interface area.

Fabrication of UiO-66/g-C3N4 Heterostructure Nanocatalyst for Efficient Visible-Light Photocatalytic Performance

It is well known that synthetic heterojunction photocatalysts can promote photocatalytic activity. Herein, we successfully combine UiO-66 with [Formula: see text]-C3N4 as ultrathin layer coating via a convenient synthesis method, the photocatalytic performance through the degradation of RhB is then shown efficient. The heterojunction and high surface area give the photocatalysts an ability to promote visible light absorption and strengthen the separation and migration of photogenerated electrons, which ultimately leads to the improvement of photocatalytic degradation. The heterostructure nanocatalyst helps the degradation rate of RhB reach 98.98% in 140[Formula: see text]min under visible light irradiation. In addition, the hole (h[Formula: see text]) plays a major role in the reaction. This work offers an efficient method for the application in the field of environmental degradation.

Unraveling the Origin of Dark Current in Organic Bulk Heterojunction Photodiodes for Achieving High Near-Infrared Detectivity

Organic infrared materials are attractive due to their high absorption coefficients and facile tuning of the absorption spectrum beyond that of silicon. Although bulk heterojunctions (BHJs) are widely used in organic photodetectors (OPDs), achieving a high signal-to-noise ratio in the near-infrared spectrum is often challenging due to the inherently large dark currents in low-bandgap polymers and their blending morphology effect. Herein, we investigate the origin of dark currents in near-infrared OPDs and reveal a strategy to improve the detectivity in terms of two aspects: (1) donor–acceptor blending morphology and (2) effects of carrier injection from outer electrodes. To do so, a series of random terpolymers were synthesized in a similar band structure and their blending morphology with the PC71BM acceptor was gradually controlled. As the phase separation was smaller, the dark current gradually reduced while the responsivity increased, leading to a higher detectivity. Complex charge-transporting paths formed under the fine percolating network account for the dark current reduction, while the increased heterojunction area facilitates the dissociation of the photogenerated excitons. From our thermal admittance spectroscopy, deeper sub-bandgap states were found under the smaller phase separation, further contributing to the dark current reduction. Second, the carrier injection effect was revealed by inserting a charge-blocking layer at the active layer/electrode interface. While manipulating both parameters could yield a high near-infrared detectivity up to 1012 Jones at −0.5 V, the blending morphology effect turns out to be more dominant over the carrier injection effect in suppressing the dark current and achieving higher detectivity in BHJ OPDs.

Enhanced photoelectric responsivity of bilayer graphene/GaAs photodetector using plasmon resonance grating structures

High-efficiency integrated photodetectors are the core supporting components for the miniaturization of optoelectronic equipment. The design of a grating structure on the surface of bilayer p-graphene/n-GaAs can excite a plasmon mode to enhance the photoelectric performance of photodetectors. Here, we simulate a heterojunction photodetector which is highly sensitive to wavelengths ranging from 550 nm to 850 nm to evaluate the optical and electrical performance of the grating/graphene/GaAs structure. Such a high performance is attributed to the efficient carrier separation and transfer between graphene and GaAs, which is created by the strong electromagnetic field produced at the heterojunction area. The results indicate that through sophisticated design, heterojunctions using plasmon resonance can become potential candidates for various essential electronic and optoelectronic applications.

Van der Waals MoS2/Two-Dimensional Perovskite Heterostructure for Sensitive and Ultrafast Sub-Band-Gap Photodetection.

Two-dimensional (2D) hybrid perovskites have been extensively studied as the promising light-sensitive materials in the photodetectors owing to their improved structural stability over that of their three-dimensional counterparts. However, the application of the 2D perovskite-based photodetector in the near-infrared (NIR) region is obstructed by the large intrinsic optical band gap. Herein, we develop a novel van der Waals heterostructure composed of few-layer 2D perovskite/MoS2 nanoflakes, which exhibits high-sensitivity detection performance over a broad spectral region, from the visible region to the telecommunication wavelength (i.e., 1550 nm). In particular, the photoresponsivity and specific detectivity under an 860 nm laser reach 121 A W-1 and 4.3 × 1014 Jones, respectively, whereas the individual nanoflakes show no response under the same wavelength. Meanwhile, the response time at the microsecond (μs) level is obtained, shortened by around 3 orders of magnitude compared to that of the constituting layers. The sensitive and ultrafast photoresponse at the NIR wavelength stems from the strong interlayer transition of sub-band-gap photons and the rapid separation of the photogenerated carriers by the built-in field within the heterojunction area. Our results not only provide an effective approach to achieve sub-band-gap photodetection in 2D perovskite-based structures but also suggest a universal strategy to fabricate high-performance optoelectronic devices.

Facile interface treatment for simultaneous enhancing the quantum dots anchoring and charge transport performances of CdS/TiO2 photoelectrodes

The effectiveness of simultaneous etching and W-doping has been demonstrated in improving the quantum-dots anchoring and charge transport properties of CdS/TiO2 heterojunction films, thereby enhancing their photoelectrochemical performances. After the process of etching and doping, a rough and amorphous W-doped TiO2 shell is formed on the TiO2 nanorods surface. It can not only provide more active sites and larger surface areas for quantum-dots loading, light-harvesting and photoelectrochemical reaction, but also protect the electron-hole pairs from recombination at the interface. Furthermore, W-doping plays a role in motivating the separation and transfer of photo-generated carriers by introducing the intermediate energy level and inducing the positive shift of TiO2 conduction band, meanwhile prominently increase the amount of electrons due to the intrinsic valence difference between W6+ and Ti4+. Based on the above advantages, the uniform and compact deposition of CdS quantum-dots on the surface of etched and W-doped TiO2 nanorods pose a great influence on the establishment of the built-in electric field, heterojunction areas, light-capture ability, and the separation and injection efficiency of photo-generated carriers. Therefore, the optimal photocurrent density of improved CdS/TiO2 photoelectrodes is 7.05 mA cm−2, approximately 12 and 2.6-fold enhancement compared with conventional TiO2 and CdS/TiO2 photoelectrodes, respectively.

0D/2D CdS/ZnO composite with n-n heterojunction for efficient detection of triethylamine

It is imperative to seek for novel materials with pronounced gas sensing performance towards triethylamine for the sake of human health. Herein, we successfully fabricate an outstanding triethylamine sensor based on CdS/ZnO composite with 0D/2D structure, which are prepared by in-situ growth of CdS quantum dots on ultra-thin ZnO nanosheets. The ratios between the two ingredients are adjusted and their effect is evaluated. The optimal sample exhibits the lowest operating temperature of 200 °C, the highest response value of ~20 and the fastest response time of 2 s. Besides, it also has the virtues of durable stability, excellent selectivity and superior anti-interference ability. The mechanism behind the aforementioned intriguing performance is investigated by X-ray photoelectron spectroscopy, Kelvin probe and density function theory (DFT) simulation. All the results verify that the enhanced gas sensing properties are derived from splendid 0D/2D structure, n-n heterojunction and large specific surface area. Additionally, this study opens an avenue for designing sensors with 0D/2D structure.

Research progress in improving the performance of PEDOT:PSS/Micro- and Nano-textured Si heterojunction for hybrid solar cells

Silicon-based hybrid solar cells (HSCs), especially PEDOT:PSS/Si HSC, have attracted the interest of researchers because they combine the advantages of organic and inorganic materials. A high quality PEDOT:PSS/Si heterojunction is the key to the good performance of PEDOT:PSS/Si HSC. However, as generally requisite to enhance light absorption for HSCs, Si Micro/Nano structures will reduce the interface contact quality between PEDOT:PSS and Si surface. The inferior interface contact quality will limit the separation efficiency of the photogenerated carriers. In this paper, we summarize the research progress in improving the interface contact between Si Micro/Nano structures and PEDOT:PSS film from three aspects: the optimization of Si Micro/Nano structures aimed to improve the fluid properties of PEDOT:PSS solution, the material modification of PEDOT:PSS and interface modification with the purpose to enlarge the heterojunction area and improve the electrical contact, and the specific deposition process of PEDOT:PSS solution developed to achieve the high filling rate of PEDOT:PSS on Si Micro/Nano structures. The insight of this paper is helpful for the preparation of high-quality heterojunction, which is vitally important for the development of high efficiency PEDOT:PSS/Si HSCs.

Atomic-level charge transport mechanism in gate-tunable anti-ambipolar van der Waals heterojunctions

van der Waals p–n heterojunctions using both 2D–2D and mixed-dimensional systems have shown anti-ambipolar behavior. Gate tunability in anti-ambipolar characteristics is obtained in special heterojunction geometries, such as self-aligned black phosphorus/MoS2 p–n heterojunctions. Although the device physics of anti-ambipolar characteristics has been investigated using finite-element or semi-classical device models, an atomic-level description has not yet been developed. This work models the interface physics with quantum transport including incoherent scattering and carrier recombination. Densities of electrons and holes are calculated in DFT-based maximally localized Wannier functions with 2% strain. Qualitative agreement with our experiments is found for both the anti-ambipolar (or Gaussian) behavior and the tunability of Gaussian function in a dual-gated geometry. Carrier recombination is found to determine the overall current density. The two gates control the recombination by regulating the density of electrons in MoS2 and holes in black phosphorus reaching the heterojunction area.

Transformation synthesis of heterostructured SnS2/ZnS microspheres for ultrafast triethylamine detection

At present, gas sensors have promising prospects in terms of gas monitoring. Heterostructure with high carrier transport property is an alternative strategy to enhance the gas sensitivity. Herein, we directly transformed ZnSn(OH)6 microspheres to obtain SnS2/ZnS heterostructures through a low-cost hydrothermal treatment. Hierarchical SnS2/ZnS flowerlike microspheres consisting of ultrathin nanoflakes subunits were successfully synthesized without any impurities. The more attractive finding is the great improvement of triethylamine gas-sensing properties of the SnS2/ZnS composite compared to the pure SnS2 at 180 °C. Especially, the proposed SnS2/ZnS sensor demonstrates ultrafast responding and recovering time (2/8 s) to 50 ppm TEA. Such significantly enhanced sensing properties can be ascribed to the merit of heterojunction and high specific surface area provided by flakes-assembled flowerlike structure. Moreover, the convenient synthetic route paves the way to the future design of various heterojunctions for diverse gas detection.

NiO Decorated Ti/TiO2 Nanotube Arrays (TiO2NT)/TiO2/g-C3N4 Step-Scheme Heterostructure Thin Film Photocatalyst with Enhanced Photocatalytic Activity for Water Splitting

A new strategy to enhance the photocatalytic properties of graphitic carbon nitride is reported via constructing an effective NiO decorated Ti/TiO2NT/TiO2/g-C3N4 S-scheme heterostructure photocatalyst thin film (Ti/TiO2NT/TiO2/g-C3N4/NiO). This thin film photocatalyst can separate hydrogen gas and oxygen gas into different sides of the Ti substrate. Owing to the synergistic effect of the NiO as co-catalyst and the formation of Step-scheme (S-scheme) heterojunction between g-C3N4 and TiO2, the electrons and holes were successfully separated to their opposing active centers, which is beneficial to inhibiting the recombination of the charge carriers. The gas evolution rate of Ti/TiO2NT/TiO2/g-C3N4/NiO is 7.97 μmol h−1 cm−2 under the irradiation of high-pressure mercury lamp, which was 3 and 1.3 times as high as that of the Ti/TiO2NT/g-C3N4 and Ti/TiO2NT/TiO2/g-C3N4, respectively. Firstly, benefiting from the well-matched band structure, a direct S-scheme Ti/TiO2NT/g-C3N4 heterojunction thin film is constructed easily driven by the built-in electric field across the Ti/TiO2NT/g-C3N4 interface. Furthermore, the introduction of the TiO2 layer on the TiO2NT will broaden light absorption range and increase the heterojunction area, which in turn conduces to effective charge separation. In addition, NiO as a co-catalyst cannot only accelerate the charge separation but also provide plentiful catalytic sites and decompose H2O2 produced by g-C3N4. The thin film photocatalysts can be recycled again, which is different from traditional powdered photocatalysts. This work constructs a NiO decorated Ti/TiO2NT/TiO2/g-C3N4 S-scheme heterostructure photocatalyst thin film, and provides a new insight to promote photocatalytic activity of g-C3N4 under the synergetic impact of S-scheme heterostructure and NiO co-catalyst. A novel NiO decorated Ti/TiO2NT/TiO2/g-C3N4 S-scheme heterostructure photocatalyst thin film was fabricated to enhance photocatalytic activity for water splitting.