Heterojunction Type Research Articles

Nanoscale engineering of semiconductor photocatalysts boosting charge separation for solar-driven H2 production: Recent advances and future perspective

Abstract Photocatalytic hydrogen (H₂) production is regarded as an efficient method for generating renewable energy. Despite recent advancements in photocatalytic water splitting, the solar-to-hydrogen conversion efficiency of photocatalysts remains well below the 10% target needed for commercial viability due to ongoing scientific challenges. This review comprehensively analyzes recent advancements in nanoscale engineering of photocatalytic materials, emphasizing techniques to enhance photogenerated charge separation for efficient solar hydrogen production. Here we highlight the nanoscale engineering strategies for effective charge separation including crystal engineering, junction engineering, doping-induced charge separation, tailoring optoelectronic properties, hierarchical architecture, defects engineering, various types of heterojunctions, and polarity-induced charge separation, and discuss their unique properties including ferroelectric on spatial charge separation along with the fundamental principles of light-induced charge separation/transfer mechanisms, and the techniques for investigation. This study, critically assesses strategies for effective photogenerated charge separation to enhance photocatalytic hydrogen production and offers guidance for future research to design efficient energy materials for solar energy conversion.

Construction of COF/COF Organic S-Scheme Heterostructure for Enhanced Overall Water Splitting.

Covalent organic frameworks (COFs) as a new type of photocatalysts have shown unique advantages in visible-light-driven hydrogen evolution, while the reported overall water-splitting systems are still very rare among various COF-based photocatalysts. Herein, two COFs are integrated to construct a type of organic S-scheme heterojunction for improved overall water splitting. In this system, TpBpy-COF and COF-316 serve as H2- and O2-evolving components, respectively, which are combined through π-π interaction between conjugated aromatic rings. By introducing ultra-small Pt nanoparticles (NPs) into the pores of the TpBpy-COF nanosheets (NS), the resultant COF-316/Pt@TpBpy-COF NS heterostructure achieves extremely high H2 and O2 evolution rates of 220.4 and 110.2µmolg-1h-1, respectively, under visible light irradiation (λ ≥ 420nm). The results of transient absorption spectra (TAS) and photoelectronic measurements indicate that the organic heterojunction interface notably facilitates the separation and transfer of photogenerated electron-hole pairs. Further, theoretical calculations and in situ experiments confirm the spontaneous formation of the COF/COF heterojunction interface and the active sites for overall water splitting.

Enhanced photoelectrochemical CO2 reduction activity towards selective generation of alcohols over CuxO/SrTiO3 heterojunction photocathodes

Photoelectrochemical reduction (PECR) of CO2 to value-added chemicals and fuels will reduce the dependency on fossil fuels, also it will solve environmental issues that arise due to greenhouse gases. A heterojunction of CuxO/SrTiO3 with varying post-annealing temperature ranges from 300 to 600 °C (CS300-600) was synthesized by overlying the spin-coated SrTiO3 on electrodeposited Cu2O over the FTO glass substrate. The synthesized photoelectrode's crystallinity and phase formation significantly varied by varying the post-annealing temperature, which was characterized via XRD and Raman spectroscopy. Optical, morphological, type of heterojunction formation and elemental surface oxidation states of photoelectrodes were studied through UV–visible DRS spectrum, FE-SEM, and XPS analysis respectively. Electrocatalytic analysis such as Linear Sweep Voltammetry (LSV), and Electrochemical Impedance Spectrometry (EIS) was employed, thus it conforms to the highest photocurrent density (−1.38 mA/cm2 at −0.6V vs. Ag/AgCl), and low charge transfer resistance (RCT=0.412 kΩ) at the electrode-electrolyte interfaces for CS500 photoelectrode as compared to others photoelectrodes. PECR of CO2 to liquid product formation was evaluated by applying different potential ranges (0 to −0.6 V vs. Ag/AgCl). Highest methanol formation of 48.69 μmol.cm−2hr−1, which is approximately 9 times enhanced as compared to the pure Cu2O (5.62 μmol.cm−2hr−1) photoelectrode at 0 V vs Ag/AgCl for CS500 heterojunction. Optimization of overlaying SrTiO3 synthesis temperature for crystallization formation on Cu2O and applied photoelectrode potential findings could pave the way for designing other new heterojunction types for selective liquid alcohol production.

Study on the regulation mechanism of Al doping on the controllable photocatalytic performance of the ZnO@MgAl-LDH nanocomposite

The ZnO hollow nanospheres were initially prepared using the template method. Subsequently, MgAl-LDH nanosheets were in-situ grown on the surface of the nanospheres via the hydrothermal method, resulting in the synthesis of a type-I heterojunction-based ZnO@MgAl-LDH composite photocatalyst. However, the introduction of Al dopant into the ZnO hollow nanosphere component resulted in an elevation of the Fermi level of ZnO, which led to a change in the heterojunction type in the ZnO@MgAl-LDH composite from type-I to type-II. Consequently, the separation of photo-generated carriers in the Al-doped ZnO@MgAl-LDH composite was significantly enhanced, thereby improving its photocatalytic performance. Moreover, the mass ratio of Al-doped ZnO hollow nanospheres and MgAl-LDH was examined in greater detail to ascertain its influence on the photocatalytic performance of the composite photocatalyst. All Al-doped ZnO@MgAl-LDH composite samples exhibited enhanced photocatalytic performance in comparison to the composite sample without Al doping. Furthermore, when the mass of Al-doped ZnO hollow nanospheres was maintained at 0.100 g and the Mg2+ concentration in the precursor was 0.006530 mol/L, the resulting Al-doped ZnO@MgAl-LDH composite sample demonstrated the most optimal photocatalytic performance among all the investigated samples.

Fabrication of InP Vertical Gate-All-Around Transistors Using Crystal Phase Transition Heterojunction

The crystal phase heterojunction (CPHJ) is essentially a new type of heterojunction without misfit dislocations and lattice relaxation. The transistor structure using the CPHJ could significantly impact the transistor technologies. However, CPHJ has difficulty in fabricating a gate structure that modulates the electric field normal to the the CPHJ. Here, we report on the selective-area growth of WZ-InP NWs for InP NWs and demonstration of vertical gate-all-around transistors using the InP CPHJ.

Polyacid-Modulated Carrier Dynamic Behavior at the Interface of 0D/2D Heterojunctions.

Heterojunctions formed by polyoxometalates and 2D materials draw attention owing to their remarkable photoelectric and catalytic properties. However, the intrinsic mechanisms of polyoxometalates regulating the heterojunction photoelectric properties are unclear. Herein, we constructed two types of heterojunctions by integrating polyoxometalates (Keggin-type H3PW12O40 and Lindqvist-type H2W6O19) on g-C3N4 monolayers, exploring photoexcited carrier dynamics in these heterojunctions by ab initio calculations combined with nonadiabatic molecular dynamics (NAMD) simulations. Our results show that electrons and holes in H3PW12O40 on g-C3N4 monolayers relax within 583 and 760 fs, respectively. The electron-hole recombination occurs at 342 fs, faster than carrier separation, aligning with the behavior of Z-type heterojunctions. Contrarily, the H2W6O19/g-C3N4 heterojunction exhibits the typical characteristics of type II heterojunctions, with a long photogenerated carrier lifetime reaching 652 fs. These findings show tunable band alignment in polyoxometalate-supported systems by modulating polyoxometalate type, influencing hot electron dynamics, and guiding 0D/2D heterojunction design.

Wafer-Scale Transfer of MXene Films with Enhanced Device Performance via 2D Liquid Intercalation.

Wafer-scale transfer processes of 2D materials significantly expand their application space in scalable microelectronic devices with excellent and tunable properties through van der Waals (vdW) stacking. Unlike many 2D materials, wafer-scale transfer of MXene films for vdW contact engineering has not yet been reported. With their rich surface chemistry and tunable properties, the transfer of MXenes can enable enormous possibilities in electronic devices using interface engineering. Taking advantage of the MXene hydrophilic surface, a straightforward, green, and fast process for the transfer of MXene films at the wafer scale (4-inch) is developed. Uniform vdW stacking of several types of large-area heterojunctions including MXene/MXene (Ti3C2Tx, Nb2CTx, and V2CTx), MXene/MoS2, and MXene/Au is further demonstrated. Multilayer support is applied to minimize damage or deformation in the transfer process of patterned Ti3C2Tx film. It allows us to fabricate thin film transistors and manipulate the MXene/MoS2 interface through the intercalation of various 2D liquids. Particularly noteworthy is the significant enhancement of the interfacial carrier transfer efficiency by ≈2 orders of magnitude using hydrogen iodide (HI) intercalation. This finding indicates a wide range of possibilities for interface engineering by transferring MXene films and employing liquid-assisted interfacial intercalation.

Vertically Phase Separated Photomultiplication Organic Photodetectors with p-n Heterojunction Type Ultrafast Dynamic Characteristics.

Photomultiplication (PM)-type organic photodetectors (OPDs), which typically form a homogeneous distribution (HD) of n-type dopants in a p-type polymer host (HD PM-type OPDs), have achieved a breakthrough in device responsivity by surpassing a theoretical limit of external quantum efficiency (EQE). However, they face limitations in higher dark current and slower dynamic characteristics compared to p-n heterojunction (p-n HJ) OPDs due to inherent long lifetime of trapped electrons. To overcome this, a new PM-type OPD is developed that demonstrates ultrafast dynamic properties through a vertical phase separation (VPS) strategy between the p-type polymer and n-type acceptor, referred to as VPS PM-type OPDs. Notably, VPS PM-type OPDs show three orders of magnitudeincrease in -3dB cut-off frequency (120kHz) and over a 200-fold faster response time (rising time = 4.8 µs, falling time = 8.3 µs) compared to HD PM-type OPDs, while maintaining high EQE of 1121% and specific detectivity of 2.53 × 1013 Jones at -10V. The VPS PM-type OPD represents a groundbreaking advancement by demonstrating the coexistence of p-n HJ and PM modes within a single photoactive layer for the first time. This innovative approach holds the potential to enhance both static and dynamic properties of OPDs.

TiO2-based heterojunctions for photocatalytic hydrogen evolution reaction

Solar-driven photocatalysis hydrogen evolution is a promising method to generate hydrogen from water, a green and clean energy source, using solar and semiconductors. Up to now, TiO2 still represents the most inexpensive and widely studied metal oxide semiconductors for photocatalysis. TiO2 coupling with other semiconductors to form heterojunctions is considered an efficient way to improve photocatalytic performances. In this review, TiO2-based heterojunctions are classified into conventional, p-n type, Z-scheme, S-scheme, and other heterojunctions based on their band structures. The photocatalytic mechanisms of various types of heterojunctions are described in detail. In order to rationally design and better synthesize heterojunctions with excellent performance, the contribution of theoretical calculations to the field of TiO2-based heterojunction photocatalysts and the key role of theoretical prediction are also discussed. Finally, the opportunities and current challenges to promote photocatalytic performance are provided to assist the design of TiO2-based heterojunction photocatalysts with superior performance.

Full solar-spectrum available Z-scheme MOF-on-MOF heterostructure for highly efficient photocatalytic VSCs removal

The construction of efficient photocatalysts with strong response to sunlight is crucial for clean and sustainable pollutant degradation and environmental remediation. Herein, a novel MOF-on-MOF structural photocatalyst named HKU@MIL with Z-scheme heterojunction was successfully designed. The light response of the composites effectively extends to the whole solar spectrum through a simple activation process, addressing the issue of the low utilization of sunlight by photocatalysts. The photocatalytic degradation rate of methyl mercaptan by activated HKU@MIL is 2.2, 3.2, and 1.8 times higher than that of NH2-MIL-125, HKUST-1, and HKU/MIL-5 (physical mixtures), respectively. Under simulated sunlight irradiation, the rate constant increases to 0.507 min−1, which is 1.4 times higher than that under visible light. Additionally, the composites exhibit good photocatalytic performance when irradiated with near-infrared light alone. The heterojunction type of the photocatalyst was investigated according to in-depth analysis of X-ray photoelectron spectroscopy. Additionally, the interfacial charge transmission mode of the composites and the probable pathways for methyl mercaptan degradation were explored through band structure analysis, radical quenching tests and electron spin resonance analysis. The improved photocatalytic performance of activated HKU@MIL can be attributed to the full solar spectral response and the successful creation of the Z-scheme electron migration channel, which serves to reinforces the electron pump effect. It achieves this by improving the transmission of photogenerated carriers and ensuring the strongest redox potential. This work presents a new perspective on enhancing the utilization of sunlight by photocatalysts and improving the photocatalytic performance of heterojunction photocatalysts used in environmental remediation.

Enhanced n-butanol sensing performance of metal-organic frameworks-derived Cr2O3/MXene composites

Cr2O3 nanoparticles derived from metal-organic frameworks (MOFs) are prepared, furtherly Cr2O3/MXene composites are synthesized via two different methods (hydrothermal and mechanical stirring, respectively). X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) are used to characterize the structure and morphology of these samples. The gas sensing performance of these materials is being studied. The sensors based on Cr2O3/MXene composites which was synthesized by the hydrothermal method exhibits higher response (130.1) to 100 ppm n-butanol at a lower operating temperature (160 °C), lower detection limit (10.1 ppb), shorter response/recovery time (90 s/210 s), better repeatability, and higher selectivity. It has been confirmed that most of the MXene in the Cr2O3/MXene composites synthesized by the hydrothermal method is oxidized to TiO2 and C-dots, as opposed to the mechanical stirring method. In addition, different types of heterojunctions and electron sensitization of C-dots in Cr2O3/MXene composites provides additional pathways for electron transfer, as well as more active sites for gas diffusion and adsorption, significantly enhancing gas sensing performance. This work offers promising materials for designing and preparing gas sensors for detecting n-butanol.

Cadmium doping induced type-II to Z-scheme switching in CdSe/BiVO4 heterojunction for enhancing photocatalytic hydrogen production

Heterojunction type in composite photocatalysts determines their charge carrier transfer path, which is critical to the redox capacity of their charge carriers and needs careful modulation for their designed photocatalytic reactions. In this work, a transit from the type-II (BVO/CdSe) to Z-scheme (CdSe/BVO) in CdSe/BiVO4 heterojunction happened by changing the deposition sequence of CdSe and BiVO4 layers to introduce Cd-doping into the BiVO4 layer, which fundamentally changed their charge carrier separation and transfer behaviors as demonstrated by both experimental and calculation results. So, the charge carrier separation efficiency in CdSe got largely enhanced, more photogenerated electrons with high reduction potential were allowed to participate in the photocatalytic H2 production, and photogenerated hole accumulation in CdSe reduced. As a result, the CdSe/BVO photoelectrode demonstrated the best photocatalytic hydrogen production rate (2.59 μmol cm−2 h−1), which is about 136.3 and 5.9 times higher than that of BVO/CdSe and pure CdSe, and the stability under visible light irradiation is also greatly improved.

InP Crystal Phase Heterojunction Transistor with a Vertical Gate-All-Around Structure.

Crystal phase transitions can form a new type of heterojunction with different atomic arrangements in the same material: crystal phase heterojunction (CPHJ). The CPHJ has an inherently strong impact on band engineering without concerns over critical thicknesses with misfit dislocations and a semiconductor-metal transition. In-plane CPHJ was recently demonstrated in two-dimensional (2D) transition-metal dichalcogenide (TMD) materials and utilized for conventional planar field-effect transistor applications. However, scalability such as gate electrostatic control, miniaturization, and multigate structure have been limited because of the geometrical issue. Here, we demonstrated a transistor using the CPHJ with a vertical gate-all-around structure by forming a CPHJ in conventional III-V semiconductors. The CPHJ, composed of wurtzite InP nanowires with zincblende InP substrates, showed an atomically flat heterojunction without dislocations and indicated a Type-II band discontinuity across the junction. The CPHJ transistor had moderate to good gate electrostatic controllability with high on-state currents and transconductance. The CPHJ offer will provide a new switching mechanism and add a new junction and device design choice to the long history of transistors.

Principles, mechanism, and identification of S‐scheme heterojunction for photocatalysis: A critical review

AbstractSemiconductor photocatalysis has been extensively used in the degradation of pollutants and the production of hydrogen fuel. The main drawback in the application of semiconductor photocatalysis is the rapid recombination of charge carriers. Several strategies have been applied to improve charge carrier separation to preserve them for imparting in photocatalytic reactions. Among the modifications that are made in the photocatalytic systems, the construction of different types of heterostructures, including type II, Z‐scheme, p–n junction, and Schottky junction, has received great attention. Recently, emerging S‐scheme heterojunctions have been shown to be able to preserve powerful charge carriers for photocatalytic reactions, which is not the case in other heterostructures. In this review, principles and mechanisms of charge transfer in S‐scheme heterostructures are discussed, and important semiconductors that have been used in the construction of this type of heterojunctions are reviewed. Methods for identification of S‐scheme heterojunction, challenges, and prospects have been addressed.

Photogenerated carriers transfer in Pd/TiO2-AgO heterojunction and its photocatalytic behaviors on oxidative dehydrogenation of ethane to ethylene

Photocatalytic oxidative dehydrogenation of ethane to ethylene with a high selectivity is an attractive reaction. In this study, different types of heterojunctions between Pd/TiO2 and a second metal oxide semiconductor were designed and synthesized to realize high activity conversion of C2H6 to C2H4. Pd/TiO2-AgO heterojunction was proved with best reaction performance. The addition of AgO facilitated the activation of reactant molecules. It enhanced directional transfer of electron-hole pairs and separation efficiency of photogenerated carriers. Pd/TiO2-5 % AgO heterojunction was proved with the best separation ability of photogenerated carriers and the largest number of surface hydroxyl groups. Detailed charge transfer path and reaction mechanism were proposed and discussed.

Construction of Z-scheme ZnO/g-C3N4 photocatalytic systems for degradation of dyeing wastewater

A novel heterogeneous nanocomposite photocatalyst ZnO/g-C3N4 was fabricated by secondary high-temperature calcination using g-C3N4 and ZnO as raw materials and characterized by XRD, SEM, TEM, EDS, FT-IR, Zeta potential, UV-Vis and XPS. The results displayed that the ZnO/g-C3N4 components were tightly bound, evenly distributed and significantly reduced in size, which could provide more active sites. The heterogeneous types, photocatalytic performance, degradation mechanism of ZnO/g-C3N4 for dyeing wastewater were speculated by REDOX potential, heterojunction, and quenching experiments. The degradation efficiency of ZnO/g-C3N4 on MB was much higher compared with that of single ZnO, g-C3N4 and the mechanical mixture of ZnO and g-C3N4, which can reach more than 95% at 60 min. The degradation rate was also significantly improved, with the degradation rate exceeding 50% at 20 min. The composite ZnO/g-C3N4 could degraded 90% MB after 5 cycles. The quenching experiment showed that hydroxyl (·OH) and photogenerated hole (h+) played the main role in the reaction process. It was confirmed that the prepared ZnO/g-C3N4 formed a Z-scheme photocatalytic system by examining the transfer paths of e- and h+ in various types of heterojunction and contrasting the CB and VB energy levels with the REDOX potentials of ZnO and g-C3N4. The Z-Scheme ZnO/g-C3N4 photocatalyst has the characteristics of stability, reusability, and no by-product, making it an ideal choice for environmental purification.

Narrow Band Organic Photodiode with Photoresponse at 808 nm for Photoplethysmography

The fabrication and characterization of an all polymers‐based bulk heterojunction type organic photodiode having a narrowband response in the near‐infrared spectral region with full width at half‐maxima of 63 nm is reported. The active layer of the photodiode constitutes a 1:1 (by weight) blend of poly[4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b;4,5‐b']dithiophene‐2,6‐diyl‐alt‐(4‐(2‐ethylhexyl)‐3‐fluorothieno[3,4‐b]thiophene‐)‐2‐carboxylate‐2‐6‐diyl)] (PCE10) and poly{[N,N′‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diy1]‐alt‐5,5′‐(2,2′‐dithiophene)} (N2200) deposited using solution processing. The device exhibits a linear dynamic range of 35 dB, response speed of 893 kHz, and specific‐detectivity of 109 Jones at 808 nm excitation wavelength. The narrowband response is achieved using the charge collection narrowing mechanism in a 2.7 μm thick junction device.

Innovation of BiOBr/BiOI@Bi5O7I Ternary Heterojunction for Catalytic Degradation of Sodium P-Perfluorous Nonenoxybenzenesulfonate.

As an alternative for perfluorooctane sulfonic acid (PFOS), sodium p-perfluorononyloxybenzene sulfonate (OBS) has been widely used in petroleum, fire-fighting materials, and other industries. In order to efficiently and economically remove OBS contaminations from water bodies, in this study, a ternary heterojunction was constructed by coupling BiOBr and BiOI@Bi5O7I for improving the redox capacity and carrier separation ability of the material and investigating the effect of the doping ratios of BiOBr and BiOI@ Bi5O7I on the performance of the catalysts. Furthermore, the effects on the degradation of OBS were also explored by adjusting different catalyst doping ratios, OBS concentrations, catalyst amounts, and pH values. It was observed that when the concentration of OBS was 50 mg/L, the amount of catalyst used was 0.5 g/L, and the pH was not changed. The application of BiOBr/BiOI@ Bi5O7I consisting of 25% BiOBr and 75% BiOI@ Bi5O7I showed excellent stability and adsorption degradation performance for OBS, and almost all of the OBS in the aqueous solution could be removed. The removal rate of OBS by BiOBr/BiOI@ Bi5O7I was more than 20% higher than that of OBS by BiOI@Bi5O7I and BiOBr when the OBS concentration was 100 mg/L. In addition, the reaction rate constants of BiOBr/BiOI@ Bi5O7I were 2.4 and 10.8 times higher than those of BiOI@ Bi5O7I and BiOBr, respectively. Therefore, the BiOBr/BiOI@ Bi5O7I ternary heterojunction can be a novel type of heterojunction for the efficient degradation of OBS in water bodies.

Effect of proton irradiation on the sensitivity of CO2 sensors based on SnO2 and SnO–SnO2 heterojunctions

Gas sensors are integral to space exploration and development projects. However, few studies have examined the effects of proton irradiation on the performance of semiconductor gas sensors. This study fills this gap by investigating the effect of proton irradiation on the sensitivity of CO2 semiconducting sensors, specifically SnO2 and SnO–SnO2 heterojunction types. In SnO2-based sensors, sensitivity was indicated to remain stable at low fluence and increase at higher fluences owing to proton-induced oxygen vacancy formations, mainly. Meanwhile, in SnO–SnO2 heterojunction sensors, it was found to decrease at low fluences and increase significantly at higher fluences owing to changes in the electrical properties of SnO. These findings suggest that proton irradiation can enhance sensor sensitivity, enabling potential applications in radiation-prone environments, such as outer space. This study contributes to the understanding of the effects of proton irradiation on semiconductor gas sensors and paves the way for their development.

In-situ surface bismuth assembled amorphous BiOI nanoplatforms for enhancing NIR-triggered bacterial inactivation

A large number of antibiotics show inadequate antibacterial activity and lead to the emergence of resistant-drug bacteria. Thus, exploring new antibacterial agents with antibiotics-free is a crucial task for decreasing further environmental and health risks. In this work, an amorphous BiOI nanoparticle with in-situ nanoscale bismuth growth and oxygen vacancy construction (BiOI@Bi-V) is rationally designed to perform an high-performance antibacterial activity. Benefits from the reasonable heterojunction and amorphous type, BiOI@Bi-V with an enhanced heterojunction-assistant internal electric field can improves electron transfer to effectively promote the separation of carriers and holes, further heightening photocatalytic activity. Meanwhile, the satisfactory photothermal ability is also driven by tuning crystal structural integrity and surface Bi growth, the photothermal effect synergistically develops the catalytic activity through a rapid molecular thermal motion reaction. Ultimately, the prepared BiOI@Bi-V achieves satisfactory photo-active antibacterial capability by employing the synergistic advantages of photothermal and photocatalytic activity. This work provides a promising consultation for designing highly efficient hybrid NIR-driven bactericidal agents by using an amorphous and in-situ surface growth strategy of nanoparticles.