Pair Transfer Research Articles

Dual Z-scheme 2D/3D carbon-bridging modified g-C3N4/BiOI-Bi2O3 composite photocatalysts for effective boosting visible-light-driven photocatalytic performance

• Carbon-bridging modified g-C 3 N 4 /BiOI-Bi 2 O 3 was successfully prepared via sample solvothermal and calcination methods. • The 10 % Carbon-bridged g-C 3 N 4 /BiOI-Bi 2 O 3 catalyst exhibits high photocatalytic activity. • The formation mechanism of Carbon-bridged g-C 3 N 4 /BiOI-Bi 2 O 3 were established. • The energy band structure and electronic density of states were calculated according to DFT. A dual Z-scheme carbon-bridged g-C 3 N 4 /BiOI-Bi 2 O 3 photocatalyst were synthesized by simple solvothermal and calcination methods. The physical and chemical properties of prepared samples were explored via a series of characterization and Density Functional Theory (DFT) calculations. The formation mechanism of the composite photocatalyst was systematically studied and the formation process was simulated. Under the irradiation of visible light, 10% carbon-bridged g-C 3 N 4 /BiOI-Bi 2 O 3 exhibited the best photocatalytic performance. And the kinetics reaction model was followed pseudo-first-order kinetics. Moreover, the possible TC degradation pathways were established based on LC-MS. The enhanced photocatalytic activity was mainly attributed to the dual Z-scheme heterojunction formed by carbon-bridged g-C 3 N 4 deposition on the surface of Bi 2 O 3 and BiOI and a higher charge transfer rate of carbon-bridged g-C 3 N 4 , which effectively promoted the separation and transfer of electron-hole pairs. The trapping experiments were carried out to further explore its photocatalytic mechanism. This work provided an in-depth understanding of g-C 3 N 4 and bismuth-based catalysts.

Green and efficient degradation of cefixime by 3D flower-like BiOBr: Performance and degradation pathway

Surface property and microstructure significantly affect the photocatalytic activity of photocatalysts. In this work, a facile precipitation method with simply controlling the pH value was employed to prepare 3D flower-like BiOBr photocatalyst. The well-ordered nano lamellae of this 3D flower-like BiOBr can not only help facilitate the photogenerated charge carrier pairs transfer and separation, but also promote the sunlight absorption between the plenty of lamellae, consequently enhance the photocatalytic activity of the BiOBr photocatalyst. According to the photocatalytic experiments, the BiOBr-5 photocatalyst (5 represents the pH values) showed superior photocatalytic activity than other BiOBr-x samples, and the BiOBr-5 can degrade Rhodamine B (RhB) completely under 30 min visible light irradiation, while the degradation rate of cefixime (CEFs) was about 86.79% after 120 min visible light irradiation. Moreover, cefixime can be degraded to small molecule then continue to degrade CO2 and H2O to avoid the secondary pollution. Based on the suitable conduction band (CB, −0.49 eV) and valence band (VB, 2.41 eV) positions of BiOBr photocatalyst,·O2- and·OH can generate and participate in photocatalytic process to improve the degradation efficiency of organic pollutants. This work has provided an example for preparation of morphology-dependent photocatalyst to degrade organic pollutants.

Bi2O2CO3/red phosphorus S-scheme heterojunction for H2 evolution and Cr(VI) reduction

Red phosphorus (RP) has a suitable energy band structure and excellent photocatalytic properties. However, there are some problems, such as low quantum efficiency and serious photogenerated electron-hole recombination. The S-scheme heterostructure shows great potential in facilitating the separation and transfer of photogenerated carriers and obtaining strong photo-redox ability. Herein, hydrothermally treated red phosphorus (HRP) was combined with Bi2O2CO3 to construct Bi2O2CO3/HRP S-scheme heterojunction composite. The Bi2O2CO3 content was optimized, and the 5 %Bi2O2CO3/HRP composite obtained at 5 %Bi2O2CO3 mass fraction exhibited the strongest photoreduction ability. The Cr(VI) photoreduction and photolytic hydrogen production rates were as high as 0.22 min−1 and 157.2 μmol •h−1, which were 7.3 and 3.0 times higher than those of HRP, respectively. The promoted photocatalytic activity could be attributed to the formation of S-scheme heterojunctions, which accelerated the separation and transfer of useful photogenerated electron-hole pairs, while enhancing the recombination of relatively useless photogenerated electron-hole pairs, thereby resulting in the highest photocurrent density (17.3 μA/cm2) of the 5 %Bi2O2CO3/HRP composite, which was 1.6 and 4.3 times higher than pure Bi2O2CO3 (10.5 μA/cm2) and pure HRP (4.0 μA/cm2), respectively. This work would provide an advanced approach to enhance the photocatalytic activity of RP.

Neutron-proton pair transfer reactions and corresponding Weisskopf-type units

We introduce the concept of neutron-proton two-particle units (np-Weisskopf units) to be used in the analysis of the (3He,p) and (p,3He) reactions on nuclei along the N=Z line. These are presented for the conditions relevant to the (n,j,ℓ) orbits expected from 16O to 100Sn. As is the case of the Weisskopf units for electromagnetic transitions, the np-WU's will provide a simple, yet robust, measure of isoscalar and isovector np pairing collective effects.

The improved spatial charge separation and antibiotic removal performance on Z-scheme Zn-Fe2O3/ZnIn2S4 architectures

In this study, Z-scheme Zn doped Fe 2 O 3 /ZnIn 2 S 4 (Zn-Fe 2 O 3 /ZnIn 2 S 4 ) heterojunction with improved spatial charge separation ability was successfully fabricated through solvothermal method. The obtained samples exhibited improved photocatalytic degradation performance containing ciprofloxacin (CIP) as simulated pollutant. The excellent photoactivity could be attributed to the tight contact between Zn-Fe 2 O 3 nanocubes and ZnIn 2 S 4 microflowers to form the interfaces which is convenient for charge transfer. Otherwise, the matched band gap structure of Zn-Fe 2 O 3 and ZnIn 2 S 4 could be the driving force of effective separation and transfer of photo-excited electron-hole pairs. The study of the photocatalytic mechanism demonstrates that the charge transfer path on Zn-Fe 2 O 3 /ZnIn 2 S 4 architectures conforms to the Z-scheme route. We hope this research could provide new ideas for the feasible design of Z-scheme heterojunction with high-performance in the field of photocatalysis. • Z-scheme Zn-Fe 2 O 3 /ZnFe 2 O 4 nanoarchitectures with nanocube/microflower structure were fabricated by hydrothermal method. • The Z-scheme Zn-Fe 2 O 3 /ZnFe 2 O 4 nanocomposites possessed excellent performance in ciprofloxacin (CIP) photodegradation. • The possible Z-scheme charge transfer on Zn-Fe 2 O 3 /ZnIn 2 S 4 was confirmed during photocatalytic process.

Sono-Chemical Synthesis of Silver Quantum Dots Immobilized on Exfoliated Graphitic Carbon Nitride Nanostructures Using Ginseng Extract for Photocatalytic Hydrogen Evolution, Dye Degradation, and Antimicrobial Studies.

Due to modernization and the scarcity of fossil fuel resources, energy demand is continuously increasing. In this regard, it is essential and necessary to create a renewable energy source that can meet future energy demands. Recently, the production of H2 by water splitting and removing pollutants from the water has been essential for issues of energy and environmental demands. Herein, g-C3N4 and Ag-g-C3N4 composite structures have been successfully fabricated by the ultrasonication method. The physio/photochemical properties of prepared g-C3N4 and Ag-g-C3N4 were examined with different analytical techniques such as FTIR, XRD, UV-DRS, SEM, TEM, PL, and XPS analyses. The silver quantum dots (QDS) anchored to g-C3N4 structures performed the profound photocatalytic activities of H2 production, dye degradation, and antimicrobial activity under visible-light irradiation. The Ag/g-C3N4 composite with an Ag loading of 0.02 mole has an optimum photoactivity at 335.40 μmol g−1 h−1, which is superior to other Ag loading g-C3N4 composites. The synthesized Ag/g-C3N4 nanoparticles showed potential microbial inhibition activity during the preliminary screening, and the inhibition zones were comparable to the commercial antibiotic chloramphenicol. The loading of Ag into g-C3N4 paves the suppression, recombination and transfer of photo-generated electron-hole pairs, leading to the enhancement of hydrogen production, the diminishment of pollutants in water under visible light irradiation, and antimicrobial activity against multidrug-resistant pathogens.

Stable self-assembly Cu2O/ZIF-8 heterojunction as efficient visible light responsive photocatalyst for tetracycline degradation and mechanism insight

Currently, the occurrence of tetracycline (TC) in the water environment has raised serious concerns. For utilizing the sunlight for degradation of TC is important to develop visible light semiconductor photocatalysts with high catalytic activity. In this paper, Cu2O/ZIF-8 particles were prepared by in situ growth method. The effects of the structures and performances of ZIF-8-doped Cu2O were examined via XRD, XPS, FTIR, SEM, TEM, BET, PL, EIS, ESR and UV–vis techniques. As result, Cu2O/ZIF-8 exhibited a better TC degradation rate constant (k ​= ​0.0102 ​min−1), 1.9 times higher than that of Cu2O (k ​= ​0.0053 ​min−1). The TC removal rate of Cu2O/ZIF-8 (84.1%) was higher than that of Cu2O (67.1%). Cu2O/ZIF-8 could still degrade TC effectively after four cycles, which proved that it had good stability. Also, the composite material showed superior removal efficacy in oxytetracycline (52%) and tetracycline hydrochloride (73%). The investigations on active species revealed that synergetic effects of Cu2O/ZIF-8 facilitated the production of oxygen radicals (•O2−) and hole (h+) for boosting TC degradation. The photoelectrochemical results also confirmed the improved separation and transfer of electron-hole pairs at the interface of Cu2O and ZIF-8. The photocatalytic mechanism and the factors responsible for the enhanced photocatalytic performance of the Cu2O/ZIF-8 were investigated. This visible light-driven system will ultimately reduce resource consumption, providing a sustainable and energy-saving environmental decontamination strategy.

Binary TiO2/RGO photocatalyst for enhanced degradation of phenol and its application in underground coal gasification wastewater treatment

In this work, we synthesized the TiO2/reduced graphene oxide (TiO2/RGO) binary nanohybrids through the sol-anaerobic calcination method, the TiO2 nanoparticles belonging to anatase type were evenly loaded on RGO surface in the form of Ti–O–C bond. The enhanced photocatalytic performance can be ascribed to the existence of RGO that can effectively promote the transfer of photo-induced electron-hole pairs to generate more hydroxyl radicals (·OH) and superoxide anion (O2·-) degrading phenol. The TiO2/RGO nanocomposite with weight ratio of RGO to TiO2 (37.0%–55.7%) had achieved maximum degradation of phenol from aqueous solution, and about 97.87% of phenol had been removed within 180 min under UV-light illumination. Through GC-MS tracking of phenol degradation process, it can be observed that long-chain intermediates will be generated, and these intermediates will eventually be degraded, confirming that phenol can be completely removed. Additionally, around 67% TOC of simulated underground coal gasification (UCG) wastewater can be removed after 10 h degradation using TiO2/RGO binary composites as photocatalysts. All these results demonstrate that the binary nanohybrids possess great potential for the organic contaminants destruction from the actual coal chemical wastewater.

Learning in Convolutional Neural Networks Accelerated by Transfer Entropy.

Recently, there is a growing interest in applying Transfer Entropy (TE) in quantifying the effective connectivity between artificial neurons. In a feedforward network, the TE can be used to quantify the relationships between neuron output pairs located in different layers. Our focus is on how to include the TE in the learning mechanisms of a Convolutional Neural Network (CNN) architecture. We introduce a novel training mechanism for CNN architectures which integrates the TE feedback connections. Adding the TE feedback parameter accelerates the training process, as fewer epochs are needed. On the flip side, it adds computational overhead to each epoch. According to our experiments on CNN classifiers, to achieve a reasonable computational overhead–accuracy trade-off, it is efficient to consider only the inter-neural information transfer of the neuron pairs between the last two fully connected layers. The TE acts as a smoothing factor, generating stability and becoming active only periodically, not after processing each input sample. Therefore, we can consider the TE is in our model a slowly changing meta-parameter.

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

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

Enhancing electron density of bulk g-C3N4 through phosphorus doping for promoting photocatalytic hydrogen evolution reaction

Graphite-phase carbon nitride (g-C3N4) has attracted extensive attention due the ubiquity of the raw materials, low price, and excellent visible light photocatalytic hydrogen evolution reaction (HER) activity. Bulk g-C3N4 is more advantageous for industrial production and application compared with g-C3N4 nanostructures such as nanosheets. Polyphosphate cyanogen was used herein as a phosphorus source to prepare phosphorus-doped bulk g-C3N4 with a coral-like hierarchical structure. This structure affords more extensive contact between the photocatalyst and reaction medium, and is thus beneficial for the photocatalytic hydrogen evolution reaction (HER). Density functional theory calculations showed that phosphorus-doping in bulk g-C3N4 can increase the electron density, thereby enhancing the separation and transfer of photon-generated carriers. The rate of the photocatalytic HER is thus promoted due to the faster separation and transfer of photoexcited electron-hole pairs. Bulk g-C3N4 doped with 20% phosphorous afforded a photocatalytic hydrogen production rate of 2.96 mmol g−1h−1, which is ∼ 3 times higher than that of the original bulk g-C3N4 sample with 1% Pt loading (1.01 mmol g−1h−1) under visible light. This study may inspire the further development of phosphorus-doped g-C3N4 for harvesting and converting solar energy to hydrogen energy.

Nonthermal plasma sulfurized CuInS2/S-doped MgO nanosheets for efficient solar-light photocatalytic degradation of tetracycline

To enhance the adsorption-photocatalytic capacity of tetracycline hydrochloride (TCH) under solar light irradiation, ultrafine CuInS2 nanoparticles of 5 nm modified MgO with S doping (CIS/S-MgO) nanosheets were in-situ sulfurized in the nonthermal plasma. Compared with pure MgO, In2S3/S-MgO and CuS/S-MgO, CIS/S-MgO exhibited the better photocatalytic activity while worse adsorption capacity. The optimal 2-CIS/S-MgO could remove 98.4% of 200 mg L−1 TCH solution within 180 min, and declined to 92.3% after five cycles. The attenuated surface discharge and small BET surface area of CIS/S-MgO induced to the inferior adsorption capacity, while the Z-structured junction formed between CuInS2 and S-MgO was favorable for the charge transfer and separation of electron-hole pairs. •O2− was vital for enhancing the solar-light photocatalytic activity of 2-CIS/S-MgO.

An effective CuO/Bi2WO6 heterostructured photocatalyst: Analyzing a charge-transfer mechanism for the enhanced visible-light-driven photocatalytic degradation of tetracycline and organic pollutants

Over the past few years, industrial pollution has had a negative impact on aquatic life by releasing significant amounts of hazardous chemicals into the ecosystem. Therefore, it is imperative to develop photocatalytic materials with good photocatalytic activity and easy separation. Photocatalytic degradation has been employed for the removal of such contaminants using binary hybrid nanocomposites as photocatalysts. In the present study, binary CuO/Bi2WO6 (CuBW) nanocomposites with different loadings of Bi2WO6 (~5, 10, and 15 mg) were successfully constructed using a simple hydrothermal method and used as a potential photocatalyst for the degradation of tetracycline (TC) and methylene blue (MB) under visible-light irradiation. The structure, surface morphology, and optical properties were studied to investigate the formation of the heterostructure. Among the prepared samples, the CuBW nanocomposite containing the optimum content of Bi2WO6 (~10 mg) exhibited superior activity toward the photocatalytic degradation of TC (97.72%) in 75 min and MB (99.43%) in 45 min under visible-light illumination. Radical trapping experiments suggested that holes and •OH radicals were the dominant reactive species during the photocatalytic process. The photoelectrochemical results also confirmed the improved separation and transfer of electron-hole pairs at the interface of Bi2WO6 and CuO. Our results demonstrate that the binary CuO/Bi2WO6 nanocomposite has significant potential applications in the field of photocatalysis due to its enhanced separation of the photoexcited charge carriers and strong synergistic interactions.

Synthesis of CdS/CoP hollow nanocages with improved photocatalytic water splitting performance for hydrogen evolution

Photocatalysts with hollow structure have obvious advantages in photocatalytic hydrogen production. Herein, CoP was modified on the surface of CdS by sacrificial template method to prepare CdS/CoP composite photocatalyst with hollow structures. Compared with pure CdS, CdS/CoP nanocage has improved photocatalytic hydrogen production performance due to its hollow structure, excellent light absorption capacity, and electron-hole pair separation and transfer efficiency. The hydrogen production rate of CdS/CoP can reach to 15.74 mmol g−1 h−1, which is about 9 times than that of pure CdS. It is found that the formation of a p-n heterojunction between CdS and CoP is the key factor to improve the photocatalytic performance of the composites. This study provides a simple strategy for the construction of more hollow nanocomposite catalysts with excellent photocatalytic performance.

In-situ growth UiO-66 on Bi2O3 to fabrication p-p heterojunction with enhanced visible-light degradation of tetracycline

A p-p heterostructure has been constructed through loading UiO-66 on Bi2O3 as a composite photocatalyst to degrade the antibiotic tetracycline (TC) under visible light. The Bi2O3@UiO-66 composites were characterized by XRD, SEM, UV, BET, PL and FTIR. The formed p-p heterojunction improves the separation efficiency of electrons and holes, which caused the degradation rate constant of the composite photocatalyst for TC reaching 0.0492 min-1. The UiO-66 is added to improve the TC adsorption capacity and photocatalytic stability of the composite photocatalyst, so that the composite photocatalyst can have a photodegradation effect on TC under different anion and cation environments or a strong acid-base solution environment with the pH value of 1~13. The photoelectrochemical analyses is the evidence of the fast transfer of electron and hole pairs with inhibiting recombination.

Fabrication of trimodal porous silica/g-C[formula omitted]N[formula omitted] nanotubes for efficient visible light photocatalytic reduction of CO[formula omitted] to ethanol

The novel composite photocatalyst of trimodal porous silica (TPS)/g-C3N4 nanotubes was fabricated via a two-step hydrothermal synthesis method with intermediate pretreatment. The TPS was extracted from rice husk, which is agricultural waste, to further reduce the cost of photocatalyst preparation. A series of characterization results, including X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, N2 adsorption–desorption, and pore size analysis, illustrated the properties of crystal, pore structure, and morphology of the composite photocatalyst. The TPS/g-C3N4 presented a hollow tubular shape with a large surface area and CO2 adsorption capacity, so there were more catalytic active reaction sites. The photocatalytic activity measurement was operated, and only ethanol was detected when g-C3N4 NT and composite TGNT were used as photocatalysts. In particular, the ethanol yield of TGNT2 was 196μm/g, and after 10 cycles of running, the photocatalytic activity decreased by less than 5%. UV–Vis diffuse reflectance spectra, photoluminescence spectra, and photoelectrochemical analysis revealed the mechanism of the photocatalytic activity enhancement from the aspects of visible light absorption, photo-generated electron–hole pair separation and transfer, and charges recombination. It theoretically explained the prospect of this fabrication strategy. It demonstrated that TGNT is a low-cost, stable, efficient, and promising photocatalyst to reduce CO2 to ethanol under visible light.

In-situ growth of Ag/AgBr nanoparticles on a metal organic framework with enhanced visible light photocatalytic performance

Recently, the continuous release of antibiotics has resulted in a serious environmental pollution. Tetracycline (TC), one of the most commonly used antibiotics due to the effective antimicrobial effects, is hard to be decomposed in living organisms. Photocatalysis technology, regarded as one of the promising methods has been widely studied to degrade the antibiotics. A series of Ag/AgBr@UiO-66 composites have been constructed to obtain materials with enhanced photocatalytic activities of tetracycline (TC) degradation. The Ag/AgBr@UiO-66 composites were characterized by XRD, SEM, TEM, XPS and BET. Decorated with 25% Ag/AgBr, the composite exhibited the best photocatalytic efficiency thanks to improved adsorption as well as the spatial separation of photogenerated carriers between Ag/AgBr and UiO-66 particles. The photocatalytic mechanism was also studied by free radical capture experiments, revealing that ·O2 is the predominant active specie generated from the electrons on the CB of UiO-66. The PL and photoelectrochemical analyses are the evidence of the fast transfer of electron and hole pairs with inhibiting recombination.

NiO/g-C3N4 2D/2D heterojunction catalyst as efficient peroxymonosulfate activators toward tetracycline degradation: Characterization, performance and mechanism

To promote the degradation efficiency of tetracycline (TC) by G-C3N4 photocatalysts activating peroxymonosulfate (PMS) under visible light, in this study, NiO 2D porous nanosheets/g-C3N4 2D nanosheets (NiO/g-C3N4) heterojunction catalyst was prepared via calcining the uniform mixture of MOF-Ni and g-C3N4. This special structure benefited in rapid electron-hole pairs transfer and separation, and NiO coupling not only promoted the photon energy absorption but also provided an additional pathway for PMS activation. As expected, the obtained optimum 3NiO/g-C3N4 had a superior performance for TC removal. 100% of TC can be degraded in 60 min by 3NiO/g-C3N4 activating PMS activation under visible light, being 6.64-time that of single g-C3N4. 3NiO/g-C3N4 also shows excellent stability during recycling tests. The mechanism and pathway of TC degradation were investigated in detail. This study provides a new insight into the construction of g-C3N4-based photocatalysts with excellent photocatalysis and PMS activation efficiency toward TC removal under visible light for efficient antibiotic degradation.

Effects of dynamical screening on the BCS-BEC crossover in double bilayer graphene: Density functional theory for exciton bilayers

We derive a gap equation for bilayer excitonic systems based on density functional theory and benchmark our results against quantum Monte Carlo simulations and recent experiments on double bilayer graphene. The gap equation has a mean-field form but includes a consistent treatment of dynamical screening. We show that the gap survives at much higher densities than previously thought from mean-field estimates which gives strong indications that the double-bilayer graphene systems at zero magnetic field can be used as model systems to investigate the BCS-BEC crossover. Furthermore, we show that Josephson-like transfer of pairs can be substantial for small band gaps and densities.

Type-I Energy Level Alignment at the PTCDA-Monolayer MoS2 Interface Promotes Resonance Energy Transfer and Luminescence Enhancement.

Van der Waals heterostructures consisting of 2D semiconductors and conjugated molecules are of increasing interest because of the prospect of a synergistic enhancement of (opto)electronic properties. In particular, perylenetetracarboxylic dianhydride (PTCDA) on monolayer (ML)‐MoS2 has been identified as promising candidate and a staggered type‐II energy level alignment and excited state interfacial charge transfer have been proposed. In contrast, it is here found with inverse and direct angle resolved photoelectron spectroscopy that PTCDA/ML‐MoS2 supported by insulating sapphire exhibits a straddling type‐I level alignment, with PTCDA having the wider energy gap. Photoluminescence (PL) and sub‐picosecond transient absorption measurements reveal that resonance energy transfer, i.e., electron–hole pair (exciton) transfer, from PTCDA to ML‐MoS2 occurs on a sub‐picosecond time scale. This gives rise to an enhanced PL yield from ML‐MoS2 in the heterostructure and an according overall modulation of the photoresponse. These results underpin the importance of a precise knowledge of the interfacial electronic structure in order to understand excited state dynamics and to devise reliable design strategies for optimized optoelectronic functionality in van der Waals heterostructures.