type-I Heterojunction Research Articles

Enhanced removal of p-nitrophenol by ꞵ-Ga2O3-TiO2 photocatalyst immobilized onto rice straw-based SiO2 via factorial optimization of the synergy between adsorption and photocatalysis

This work was undertaken to provide a new potent route for wastewater treatment by developing a hybrid photocatalyst from waste rice straw. On this basis, rice straw-based SiO2 was utilized as a platform to synthesize a ternary β-Ga2O3-TiO2-SiO2 nanocomposite with type-I heterojunction band alignment using the mechanochemical method. The categorical factorial design was employed to optimize the synergistic effect between adsorption and photocatalytic performance of the nanocomposite towards removing p-nitrophenol (PNP) from wastewater under various experimental conditions. The optimization results indicated the negligible impact of catalyst dose (Xc) and total dissolved solids (XTDS) on the photocatalysis with the increase of pre-adsorption time (Xads) from 60 to 180 min. In contrast, the adsorption/photocatalytic performance of ꞵ-Ga2O3-TiO2-SiO2 was found to be dependent on the solution pH (XpH, with a percentage contribution (PC) of 26.1 %) due to its effect on the photocatalyst surface charge and the dissociation process of PNP molecules. At optimum conditions (Xc = 4.0 g/L, XpH = 6.0, XTDS = 1000 mg/L, and Xads = 180 min), the β-Ga2O3-TiO2-SiO2 exhibited high efficiency to degrade 92.4 ± 3.8 % and 68.65 ± 4.3 % of PNP (25 mg/L) after 120 min UV and visible irradiation, with kinetic rates (k) of 0.62 and 0.53 mg/(g.min), respectively. This improved performance was due to the synergism between (i) the high adsorption of PNP (k = 0.017 mg/(g.min)) on SiO2 and (ii) the effective separation of photogenerated charges between ꞵ-Ga2O3 and TiO2. The high-performance stability (up to 3 cycles) of β-Ga2O3-TiO2-SiO2 was also demonstrated towards UV/visible-induced photodegradation of organic contaminants in real petrochemical wastewater.

Cu2Se-ZnSe heterojunction encapsulated in carbon fibers for high-capacity anodes of sodium-ion batteries

Transition metal chalcogenides have been one of the research hotspots in sodium-ion batteries (SIBs). In this work, Cu2Se-ZnSe heterojunction nanoparticles were embedded in carbon nanofibers to obtain the composites (Cu2Se-ZnSe-CNFs). As anodes for SIBs, Cu2Se-ZnSe-CNFs showed a reversible capacity of 310 mAh g−1 after 100 cycles at 0.1 A g−1. Their rate capacity is 237 mAh g−1 at 2 A g−1. The high specific capacities of Cu2Se-ZnSe-CNFs at a wide range of current density can be attributed to the type-I heterojunction between Cu2Se and ZnSe in composites, which can speed up electron transport. In addition, the protection from CNFs is another factor to ensure the long lifetime of Cu2Se-ZnSe-CNF anodes. The strategy of using heterojunction to promote the electrochemical properties of multi-metal sulfides may be helpful for the application of carbon composites in electrochemical catalysts and other batteries.

Two-Dimensional Perovskite Capping Layer Simultaneously Improves the Charge Carriers' Lifetime and Stability of MAPbI3 Perovskite: A Time-Domain Ab Initio Study.

Two- (2D) and three-dimensional (3D) heterostructured perovskites show enhanced stability and an extended charge lifetime compared to those of the 3D component. The mystery remains unexplored for both phenomena in the class of the typical type-I heterojunction. By using time-domain density functional theory combined with nonadiabatic (NA) molecular dynamics simulations for the MA3Bi2I9/MAPbI3 (MA = CH3NH3+) junction, we demonstrate that the formation of I-Pb chemical bonds at the junction suppresses the atomic motions. The inhibited charge recombination in the junction is ascribed to the increased band gap, reduced NA coupling, and shortened coherence time. By localizing the hole wave function, the NA coupling is decreased by about a factor of 1.4. The presence of multiple phonon modes, particularly the Bi-I vibrations, accelerates decoherence about twice as fast as that in the pristine MAPbI3. As a result, the 2D capping layer reduces the recombination in MAPbI3 by more than a factor of 2, decreasing charge and energy losses. The strategy can be applied to optimize the performance of other 2D/3D heterostructured perovskite solar cells.

A dual strategy to construct flowerlike S-scheme BiOBr/BiOAc1−xBrx heterojunction with enhanced visible-light photocatalytic activity

For broad-band-gap semiconductors like BiO(CH3COO) (denoted as BiOAc), constructing heterojunctions with a narrow-band-gap semiconductor or forming solid solution is an effective strategy to improve visible-light-driven photocatalytic activity. Herein, by combining two strategies, a novel step-scheme (S-scheme) heterojunction BiOBr/BiO(CH3COO)1−xBrx (denoted as BiOBr/BiOAc1−xBrx) was synthesized via a facile co-precipitation method at room temperature. The as-prepared BiOBr/BiOAc1−xBrx with feeding molar ratio of Br/Bi at 0.8 exhibited outstanding visible-light photocatalytic activity for tetracycline (TC) and rhodamine B (RhB) degradation, with degradation efficiency of 99.2% and 99.4%, respectively, which was higher than those of BiOAc (17.3% and 34.8%), BiOBr (57.2% and 62.7%), solid solution BiOAc1−xBrx(x = 0.4) (68.6% and 68.2%) and heterojunction BiOBr/BiOAc (69.9% and 88.1%). The enhanced photocatalytic activity could be attributed to two main aspects. Firstly, the formation of the solid solution BiOAc1−xBrx could not only enlarge visible light response of BiOAc, but also successfully transform type-I heterojunction of BiOBr/BiOAc to typical S-scheme heterojunction of BiOBr/BiOAc1−xBrx, which thus prolong the lifetime of charge carriers with stronger redox ability. Secondly, by adding urea to the reaction system as a morphology modifier, uniform ultrathin nanosheet-based flowerlike BiOAc was obtained, and more importantly, the heterojunction BiOBr/BiOAc1−xBrx inherited the flowerlike contour of BiOAc and thus improved the dispersion, which benefit to the transfer of carriers at the interfaces and onto the surface of nanoparticles. In addition, the as-prepared S-scheme heterojunction also possessed desirable photodegradation efficiency for TC and RhB in real wastewater or in the presence of some electrolytes. This study provides a simply and energy-saving strategy for simultaneously optimizing energy band structures and microstructure to highly efficient heterojunction photocatalysts.

Determination of the band offsets of the Ga2O3:Si/FTO heterojunction for current spreading applications

Because of the relatively low electron mobility of Ga2O3, it is important to identify suitable current spreading materials. Fluorine-doped SnO2 (FTO) offers superior properties to those of indium tin oxide (ITO), including higher thermal stability, a larger bandgap, and lower cost. However, the Ga2O3:Si/FTO heterojunction, including the important band offsets and the I–V characteristics, have not previously been reported. In this work, we have grown a Ga2O3:Si/FTO heterojunction and performed x-ray photoelectron spectroscopy measurements. The conduction and valence band offsets were determined to be 0.11 and 0.42 eV, indicating a minor barrier for electron transport and a type-I heterojunction. The subsequent I–V measurement of the Ga2O3:Si/FTO heterojunction exhibited pseudo-ohmic behavior. The results of this work support the potential of FTO for the current spreading layers of Ga2O3 devices for high temperature and ultraviolet applications.

Formation of Mo2C/hollow tubular g-C3N4 hybrids with favorable charge transfer channels for excellent visible-light-photocatalytic performance

The pharmaceutical products have becoming ubiquitous in aquatic environment. Photocatalytic degradation is considered as a promising strategy to address this environmental threat. Here we showed new Mo2C/hollow tubular g-C3N4 hybrids (Mo2C/TCN) consisting of well-designed direct Z-scheme heterojunction with favorable charge transfer channels for efficient contaminants degradation. Compare to the traditional Mo2C/g-C3N4 type-I heterojunction reported in the previous literature, the powerful direct Z-scheme heterojunction retains the original redox ability of the component without changing its oxidation and reduction potential. By virtue of the hollow tubular architecture, more incident electrons are expected to be rapid trapped by Mo2C nanoparticles, which contributes to the effective separation of photoinduced hole-electron pairs. As a result, the optimized Z-scheme system exhibits impressive visible-light photocatalytic performance. Especially, the 2 wt% Mo2C/TCN photocatalysts exhibits superior photocatalytic performance for tetracycline degradation with a reaction rate of 0.0391 min−1, which is 3-times and 9-times higher than those of TCN and pristine g-C3N4, respectively. The outstanding performance strongly depends on the synergistic effects among the favorable electrical conductivity of Mo2C and the multitude of charge transfer channels provided by the Z-scheme heterojunction. This work provides a new idea of designing direct Z-scheme material and it sheds novel insight to establish photocatalytic model for environmental amendment.

An Electrically Modulated Single-Color/Dual-Color Imaging Photodetector.

An electrically modulated single-/dual-color imaging photodetector with fast response speed is developed based on a small molecule (COi 8DFIC)/perovskite (CH3 NH3 PbBr3 ) hybrid film. Owing to the type-I heterojunction, the device can facilely transform dual-color images to single-color images by applying a small bias voltage. The photodetector exhibits two distinct cut-off wavelengths at ≈544 nm (visible region) and ≈920 nm (near-infrared region), respectively, without any power supply. Its two peak responsivities are 0.16 A W-1 at ≈525 nm and 0.041 A W-1 at ≈860 nm with a fast response speed (≈102 ns). Under 0.6 V bias, the photodetector can operate in a single-color mode with a peak responsivity of 0.09 A W-1 at ≈475 nm, showing a fast response speed (≈102 ns). A physical model based on band energy theory is developed to illustrate the origin of the tunable single-/dual-color photodetection. This work will stimulate new approaches for developing solution-processed multifunctional photodetectors for imaging photodetection in complex circumstances.

Liquid exfoliating CdS and MoS2 to construct 2D/2D MoS2/CdS heterojunctions with significantly boosted photocatalytic H2 evolution activity

Fabrication of 2D/2D heterojunction photocatalysts have attracted more attentions due to their inherent merits involving the large contact interface, short charge migration distance and plentiful active sites, which are beneficial for the enhancement of photocatalytic activity. Herein, a series of 2D/2D MoS2/CdS type-I heterojunctions were prepared by incorporation the exfoliating of bulk CdS and MoS2 with post-sintering procedure. Multiple characteristic techniques were employed to corroborate the formation of heterojunctions. By optimizing the 2D MoS2 amounts in the heterojunction, the 7 wt.% 2D/2D MoS2/CdS heterojunction displayed the maximal photocatalytic H2 evolution rate of 18.43 mmol h−1 g−1 under visible light irradiation in the presence of lactic acid as the sacrificial reagent, which was 6 times higher than that of pristine 2D CdS. Based on the photoelectrochemical and photoluminescence spectra tests, it could be deduced that the charge separation and transfer of 2D/2D MoS2/CdS heterojunction was tremendously improved, and the recombination of photoinduced electron-hole pairs was effectively impeded. Moreover, the 2D MoS2 was used as a cocatalyst to provide the abundant active sites and lower the overpotential for H2 generation reaction. The current work would offer an insight to fabricate the 2D/2D heterojunction photocatalysts for splitting H2O into H2.

The Evolution from a Typical Type-I CdS/ZnS to Type-II and Z-Scheme Hybrid Structure for Efficient and Stable Hydrogen Production under Visible Light

An incompletely covered CdS/ZnS core/shell is synthesized via a simple hydrothermal method. Due to the defect energy levels introduced by zinc vacancies, the typical type-I heterojunction of CdS/Zn...

Molecular-Scale Characterization of Photoinduced Charge Separation in Mixed-Dimensional InSe–Organic van der Waals Heterostructures

Layered indium selenide (InSe) is an emerging two-dimensional semiconductor that has shown significant promise for high-performance transistors and photodetectors. The range of optoelectronic applications for InSe can potentially be broadened by forming mixed-dimensional van der Waals heterostructures with zero-dimensional molecular systems that are widely employed in organic electronics and photovoltaics. Here, we report the spatially resolved investigation of photoinduced charge separation between InSe and two molecules (C70 and C8-BTBT) using scanning tunneling microscopy combined with laser illumination. We experimentally and computationally show that InSe forms type-II and type-I heterojunctions with C70 and C8-BTBT respectively, due to an interplay of charge transfer and dielectric screening at the interface. Laser-excited scanning tunneling spectroscopy reveals a ~0.25 eV decrease in the energy of the lowest unoccupied molecular orbital of C70 with optical illumination. Furthermore, photoluminescence spectroscopy and Kelvin probe force microscopy indicate that electron transfer from InSe to C70 in the type-II heterojunction induces a photovoltage that quantitatively matches the observed downshift in the tunneling spectra. In contrast, no significant changes are observed upon optical illumination in the type-I heterojunction formed between InSe and C8-BTBT. Density functional theory calculations further show that, despite the weak coupling between the molecular species and InSe, the band alignment of these mixed-dimensional heterostructures strongly differs from the one suggested by the ionization potential and electronic affinities of the isolated components. Self-energy corrected density functional theory indicates that these effects are the result of the combination of charge redistribution at the interface and heterogeneous dielectric screening of the electron-electron interactions in the heterostructure. In addition to providing specific insight for mixed-dimensional InSe-organic van der Waals heterostructures, this work establishes a general experimental methodology for studying localized charge transfer at the molecular scale that is applicable to other photoactive nanoscale systems.

Black Phosphorus-Based Semiconductor Heterojunctions for Photocatalytic Water Splitting.

Solar-to-hydrogen (H2 ) conversion has been regarded as a sustainable and renewable technique to address aggravated environmental pollution and global energy crisis. The most critical aspect in this technology is to develop highly efficient and stable photocatalysts, especially metal-free photocatalysts. Recently, black phosphorus (BP), as a rising star 2D nanomaterial, has captured enormous attention in photocatalytic water splitting owing to its widespread optical absorption, adjustable direct band gap, and superior carrier migration characteristics. However, the rapid charge recombination of pristine BP has seriously limited its practical application as photocatalyst. The construction of BP-based semiconductor heterojunctions has been proven to be an effective strategy for enhancing the separation of photogenerated carriers. This Minireview attempts to summarize the recent progress in BP-based semiconductor heterojunctions for photocatalytic water splitting, including type-I and type-II heterojunctions, Z-Scheme systems, and multicomponent heterojunctions. Finally, a brief summary and perspective on the challenges and future directions in this field are also provided.

Vertical van der Waals Heterostructure of Single Layer InSe and SiGe

We present a first-principles investigation on the stability, electronic structure and mechanical response of ultra-thin heterostructure composed of single layers of InSe and SiGe. First, by per- forming total energy optimization and phonon calculations, we show that single layers of InSe and SiGe can form dynamically stable heterostructures in 12 different stacking types. Valence and conduction band edges of the heterobilayers form a type-I heterojunction having a tiny bandgap ranging between 0.09-0.48 eV. Calculations on elastic-stiffness tensor reveal that two mechanically soft single layers form a heterostructure which is stiffer than constituent layers due to relatively strong interlayer interaction. Moreover, phonon analysis show that the bilayer heterostructure has highly Raman active modes at 205.3 and 43.7 cm −1 , stemming from out-of-plane interlayer mode and layer breathing mode, respectively. Our results show that, as a stable type-I heterojunction, ultra-thin heterobilayer of InSe/SiGe holds ...

Deciphering photocarrier dynamics for tuneable high-performance perovskite-organic semiconductor heterojunction phototransistors

Looking beyond energy harvesting, metal-halide perovskites offer great opportunities to revolutionise large-area photodetection technologies due to their high absorption coefficients, long diffusion lengths, low trap densities and simple processability. However, successful extraction of photocarriers from perovskites and their conversion to electrical signals remain challenging due to the interdependency of photogain and dark current density. Here we report hybrid hetero-phototransistors by integrating perovskites with organic semiconductor transistor channels to form either “straddling-gap” type-I or “staggered-gap” type-II heterojunctions. Our results show that gradual transforming from type-II to type-I heterojunctions leads to increasing and tuneable photoresponsivity with high photogain. Importantly, with a preferential edge-on molecular orientation, the type-I heterostructure results in efficient photocarrier cycling through the channel. Additionally, we propose the use of a photo-inverter circuitry to assess the phototransistors’ functionality and amplification. Our study provides important insights into photocarrier dynamics and can help realise advanced device designs with “on-demand” optoelectronic properties.

Direct/indirect band gap tunability in van der Waals heterojunctions based on ternary 2D materials Mo1−xWxY2

Artificial van der Waals (vdW) heterojunctions assembled by atomically-thin two-dimensional (2D) materials have demonstrated new physical phenomena and unusual properties, thus triggering new electronic, optoelectronic, valleytronic and photocatalytic application. Herein, the electronic band structures of different vdW heterojunctions based on ternary Mo1−xWxY2 (Y = S, Se; x = 0–1) monolayer with five stacking orders (AA, AA, AB, AB, AB) have been investigated using first principle calculations. The direct/indirect band gap has been obtained in the AA stacking type-II heterojunctions, ranging from 0.538 eV to 1.260 eV, that are determined by the interlayer distances and stoichiometries. The estimated power conversion efficiency of the AA stacking type-II heterojunction varied from 9.1% to 23.4%. The type-I heterojunctions have also been predicted when semiconducting 2H-MoTe2 monolayer stacks with the specific Mo1−xWxSe2 monolayer, which are MoTe2/Mo0.25W0.75Se2 and MoTe2/Mo0.5W0.5Se2. The reported theoretical results can provide broader 2D materials design possibility for the functional devices.

Interfacial Engineering Determines Band Alignment and Steers Charge Separation and Recombination at an Inorganic Perovskite Quantum Dot/WS2 Junction: A Time Domain Ab Initio Study.

Using time-domain density functional theory and nonadiabatic (NA) molecular dynamics, we demonstrate that interfacial interaction between WS2 and CsPbBr3 quantum dots (QDs) determines the band alignment, leading to a type-II and type-I heterojunction for the WS2 contacting with Cs/Br- and PbBr2-terminated facet QD, respectively. In the type-II heterojunction, electron transfer is faster than hole transfer arising due to the stronger NA coupling, higher density of electron acceptor states, and more and higher phonon modes involved. Both the electron and hole transfer times are subpicosecond, in agreement with experiments. The energy lost by the electron and hole is slower than charge transfer by several times, facilitating keeping charge carriers sufficiently "hot". Particularly, the electron-hole recombination occurs over 1 ns, favoring a long-lived charge-separated state. Detailed atomistic insights into the photoinduced charge and energy dynamics at the WS2/QD interface provide valuable guidelines for improving performance of perovskite/transition-metal dichalcogenide solar cells.

Excitation and Emission Transition Dipoles of Type-II Semiconductor Nanorods

The mechanisms of exciton generation and recombination in semiconductor nanocrystals are crucial to the understanding of their photophysics and for their application in nearly all fields. While many studies have been focused on type-I heterojunction nanocrystals, the photophysics of type-II nanorods, where the hole is located in the core and the electron is located in the shell of the nanorod, remain largely unexplored. In this work, by scanning single nanorods through the focal spot of radially and azimuthally polarized laser beams and by comparing the measured excitation patterns with a theoretical model, we determine the dimensionality of the excitation transition dipole of single type-II nanorods. Additionally, by recording defocused patterns of the emission of the same particles, we measure their emission transition dipoles. The combination of these techniques allows us to unambiguously deduce the dimensionality and orientation of both excitation and emission transition dipoles of single type-II semiconductor nanorods. The results show that in contrast to previously studied quantum emitters, the particles possess a 3D degenerate excitation and a fixed linear emission transition dipole.

A hollow core–shell structure material NiCo2S4@Ni2P with uniform heterojunction for efficient photocatalytic H2 evolution reaction

Based on a bimetallic sulfide, a hollow core–shell structure material, NiCo2S4@Ni2P, with a uniform type-I heterojunction achieved efficient photocatalytic H2 evolution.

Valence and Conduction Band Offsets for InN and III-Nitride Ternary Alloys on (−201) Bulk β-Ga2O3

Valence and conduction band offsets of the InN/β-Ga2O3 type-I heterojunction have been determined to be −0.55 ± 0.11 eV and −3.35 ± 0.11 eV, respectively, using X-ray photoelectron spectroscopy. The InN layers were grown using atomic layer epitaxy on (−201) oriented commercial β-Ga2O3 substrates. Combining this data with published band offsets for the GaN and AlN heterojunctions to β-Ga2O3 has allowed us to predict the band offsets for the AlGaN, AlInN, and InGaN ternary alloys to β-Ga2O3. The conduction band offsets for InGaN and AlInN to β-Ga2O3 increased for high In concentration and, similarly, the valence band offsets for AlGaN and AlInN to β-Ga2O3 decreased at high Al concentration.

Self-sacrifice transformation for fabrication of type-I and type-II heterojunctions in hierarchical BixOyIz/g-C3N4 for efficient visible-light photocatalysis

Construction of semiconductor heterojunction with hierarchical architectures is highly effective for improving photocatalytic performance. Different heterojunction types with distinct mechanisms lead to different photocatalytic activity enhancement level, and thus the control on heterojunction type is meaningful. Herein, we achieve the fabrication of a series of different types of hierarchical heterojunctions in BixOyIz/g-C3N4, namely, g-C3N4/BiOI, g-C3N4/Bi4O5I2, and g-C3N4/Bi5O7I. g-C3N4/BiOI is prepared by a direct precipitation method, and g-C3N4/Bi4O5I2 and g-C3N4/Bi5O7I are obtained by in situ calcination transformation of g-C3N4/BiOI at different temperature. Among them, g-C3N4/BiOI and g-C3N4/Bi4O5I2 are type-I heterojunction, and g-C3N4/Bi5O7I belongs to type-II heterojunction. The photocatalyitc activity is surveyed by decomposition of diverse industrial contaminants, including methyl orange, bisphenol A and tetracycline hydrochloride under visible light irradiation (λ > 420 nm). It is found that g-C3N4/Bi5O7I shows largely enhanced photodegradation performance compared to g-C3N4/BiOI and g-C3N4/Bi4O5I2. The much higher photocatalytic activity of g-C3N4/Bi5O7I is attributed to the enhanced specific surface area, more efficient charge separation and surface transfer efficiency and increased density of charge carriers owing to the formation of type-II heterojunction. The study provides a reference for in situ fabrication of hierarchical photocatalysts with diverse heterojunction types for optimizing photocatalytic activity.

Under vacuum synthesis of type-I heterojunction between red phosphorus and graphene like carbon nitride with enhanced catalytic, electrochemical and charge separation ability for photodegradation of an acute toxicity category-III compound

In this study, we reported a novel type-I heterojunction between red phosphorus and graphitic carbon nitride under vacuum condition, which is a novel synthesis process. For preparing the red phosphorus composites, the vacuum condition is appropriate to prevent it from self-ignition. The prepared materials were analyzed with different analytic techniques for the confirmation of their heterojunctions and structure formation. The introduction of red phosphorus into graphitic carbon nitride, reduce the structural defects and increase the charge separation ability. After medication, the synthesized composite shows an enhanced photocatalytic activity against acute toxicity category-III non-color (MTSM, Metsulfuron methyl) and color (RhB), organic pollutants. The photocatalytic degradation mechanism (Mott-Schottky and VB-XPS) study reveal that red phosphorus /graphitic carbon nitride possess type-I heterojunction with enhanced catalytic behavior, due to the effective transformation of photo-generated electrons from CB of GCN to CB of RP with the help of heterojunctions and due to unmoved photo holes on VBs. Synthesized composites also maintained their performance after reused them in three cyclic form.