Charge Imbalance Research Articles

Reduction-Induced Uptake of Cs+ in Metal-Organic Frameworks Loaded with Polyoxometalates.

Materials for Cs+ adsorption continue to be important for the treatment of various solutions. Metal-organic frameworks (MOFs) with large specific surface areas promise adsorption properties for various gases, vapors, and ions. However, the utilization of MOFs for alkali ion capture, specifically, Cs+ capture is still in its infancy. Herein, MOFs are hybridized with polyoxometalates (POMs) to study the effect of i) MOF type, ii) POM type, and iii) POM loading amounts on Cs+ capture. In particular, the composite of ZIF-8 and [α-PMo12O40]3- (PMo12/ZIF-8) adsorbed Cs+ ions effectively when compared to pristine ZIF-8. In addition, the reduction of Mo within the POM from MoVI to MoV by ascorbic acid during the Cs+ uptake process doubled the Cs+ uptake capacity of PMo12/ZIF-8. This observation can be attributed to the increased overall negative charge of the POM facilitating Cs+ uptake to compensate for the charge imbalance. Hybridization with other MOFs (MIL-101 and UiO-66) largely suppresses the Cs+ uptake, highlighting the importance of hydrophobicity in Cs+ capture. Furthermore, PMo12/ZIF-8 led to an outstanding Cs+ uptake (291.5mg g-1) with high selectivity (79.6%) from quinary mixtures of alkali metal cations even among other representative porous materials (Prussian blue and zeolites).

Interfacial Passivation Engineering for Highly Efficient Quantum Dot Light-Emitting Diodes via Aromatic Amine-Functionalized Dipole Molecules.

Blue quantum dot (QD) light-emitting diodes (QLEDs) exhibit unsatisfactory operational stability and electroluminescence (EL) properties due to severe nonradiative recombination induced by large numbers of dangling bond defects and charge imbalance in QD. Herein, dipolar aromatic amine-functionalized molecules with different molecular polarities are employed to regulate charge transport and passivate interfacial defects between QD and the electron transfer layer (ETL). The results show that the stronger the molecular polarity, especially with the -CF3 groups possessing a strong electron-withdrawing capacity, the more effective the defect passivation of S and Zn dangling bonds at the QD surface. Moreover, the dipole interlayer can effectively reduce electron injection into QD at high current density, enhancing charge balance and mitigating Joule heat. Finally, blue QLEDs exhibit a peak external quantum efficiency (EQE) of 21.02% with an operational lifetime (T50 at 100 cd m-2) exceeding 4000 h.

Dynamics of charge imbalance resolved negativity after a local joining quench

In this paper, we consider the dynamics of charge imbalance resolved negativity after a local joining quench in the 1 + 1 dimensional free complex boson CFT. In the first part, we study the local joining quench by applying conformal maps, obtaining analytical universal results. We first calculate the quench dynamics of charged logarithmic negativity. Then using the Fourier transformation, we obtain the charge imbalance resolved negativity. The total negativity can be recovered from the charge-resolved ones. In the second part, we test our CFT predictions against the underlying lattice model numerically. Finally, we explain our results based on the quasi-particle picture.

Exploring the role of oxygen vacancies on the magnetic and electromagnetic absorption properties of La3+-modified M-type hexaferrite with Al3+ doping

The demand for effective microwave-absorbing materials has recently surged due to rapid advancements in electromagnetic (EM) devices. Recently, engineering oxygen vacancies has also become one of the effective ways to develop efficient microwave-absorbing materials. So, understanding the EM absorption mechanism of these materials has become crucial for better engineering of such materials. This article investigates the magnetic properties along with the EM absorption mechanism of M-type hexaferrite, with optimal incorporation of rare-earth element La3+ and doping of transition metal Al3+ cation. The presence of La3+ ions at an optimal level promotes the reduction of Fe3+ to Fe2+ cations and creating oxygen vacancies to offset the electrical charge imbalance. This phenomenon impacts both the magnetic and EM characteristics of the materials. The presence of Fe2+ cations enhanced the spin-orbital interaction, resulting in a strong magnetic anisotropy field along the c-axis. The lowest reflection loss of −36.37 dB at 14.19 GHz, is observed with a bandwidth of 3.61 GHz below −10 dB for x = 0.6. These microwave absorption properties can be attributed to the adequate compensation between dielectric and magnetic losses, which arise from phenomena like dielectric relaxation, magnetic resonance, and conduction loss due to electron hopping between Fe3+ and Fe2+ with proper incorporation of the attenuating constant and excellent impedance matching, along with microstructure of the materials. Furthermore, the material’s exceptional absorption properties are also influenced by the rapid movement of oxygen vacancies from its interior to its surface when exposed to high frequencies, thereby impacting its conductivity. Therefore, it is believed that the regulation of oxygen vacancies can serve as a versatile strategy for developing materials with efficient microwave-absorbing capabilities.

Characterizing and controlling infrared phonon anomaly of bilayer graphene in optical-electrical force nanoscopy

We, for the first time, report the nanoscopic imaging study of anomalous infrared (IR) phonon enhancement of bilayer graphene, originated from the charge imbalance between the top and bottom layers, resulting in the enhancement of E1u mode of bilayer graphene near 0.2 eV. We modified the multifrequency atomic force microscope platform to combine photo-induced force microscope with electrostatic/Kelvin probe force microscope constituting a novel hybrid nanoscale optical-electrical force imaging system. This enables to observe a correlation between the IR response, doping level, and topographic information of the graphene layers. Through the nanoscale spectroscopic image measurements, we demonstrate that the charge imbalance at the graphene interface can be controlled by chemical (doping effect via Redox mechanism) and mechanical (triboelectric effect by the doped cantilever) approaches. Moreover, we can also diagnosis the subsurface cracks on the stacked few-layer graphene at nanoscale, by monitoring the strain-induced IR phonon shift. Our approach provides new insights into the development of graphene-based electronic and photonic devices and their potential applications.

Bimetallic Fe:Co metal–organic framework (MOF) with unsaturated metal sites for efficient Fenton-like catalytic degradation of oxytetracycline (OTC) antibiotics

Fe:Co MIL-88B nanorods with abundant unsaturated metal centers have been synthesized via a rapid microwave-assisted solvothermal process and used as a Fenton-like catalyst for oxytetracycline (OTC) degradation. The bimetallic Fe:Co MOFs outperformed their monometallic counterparts in catalytic degradation in both alkaline and acidic environments, with the highest catalytic activity in the alkaline pH region. The synthesis time, temperature, and bimetal ratios were essential in creating MOFs with high catalytic activity. Acid-base titration clearly revealed the resulting MOFs to possess high acidity, although no acid solution was used in the synthesis. The acidity arises probably from the charge imbalance in the mixed substitutions of Fe(III) and Co(II) in the MIL-88B topology under rapid synthesis, which leads to the metal centers having incomplete coordination and thus are very active. These highly active metal centers can catalyze hydrogen peroxide into reactive radicals to degrade OTC efficiently with 50 mg/L OTC being degraded completely within 90 min at pH 8, and 0.125 g/L catalyst loading. The obtained catalysts also displayed high stability and reusability. The metal ions leaching of the catalysts are less than 1.0 ppm in the fresh use and decrease to less than 0.1 ppm after 4 cycles, while still sustaining the catalytic activity over 90 % of its initial efficiency. This work demonstrates the use of mixed metals in synthesizing MOFs to achieve highly active, broad pH, Fenton-like catalysts that may be useful for advanced oxidation and environmental remediation.

Blue Ca4MgAl2Si3O14:Ce3+, Li+ phosphor with excellent performance for human-centered green plant growth lighting

Light-emitting diodes (LEDs) can regulate the light environment for plant growth to increase productivity. Deep blue light, which plays a pivotal role in plant photosynthesis, has received limited attention in research due to the challenges associated with synthesizing a blue phosphor exhibiting high thermal stability, high quantum efficiency and spectral similarity to plant response spectra. This study presents the synthesis and characterization of a blue phosphor Ca4MgAl2Si3O14:xCe3+,yLi+, which is characterized by remarkable thermal stability and quantum efficiency reached through charge compensation. The introduction of Li+ ions eliminates cation vacancy defects, mitigates charge imbalance due to heterovalent substitution, and acts as a flux, thereby effectively suppressing energy transfer between Ce3+ ions and significantly improving several key properties of the phosphor. Notably, the emission intensity is significantly enhanced, the internal quantum efficiency was increased from 74.2 to 91.6 %, and the external quantum efficiency was improved from 54.9 to 67.7 %. Moreover, the thermal stability at 423 K was augmented from 71 to 82 %. Of particular importance is the exceptional alignment of the phosphor's spectrum with the absorption spectrum of chlorophyll-a (with a spectral resemblance of SRchlorophyll-a = 94.39 %). These findings underscore the great potential of Ca4MgAl2Si3O14:xCe3+,yLi+ phosphors in applications related to plant growth lighting.

Preconditioning Operation of Membraneless Vanadium Micro Redox Flow Batteries

AbstractDevelopment of a Membraneless Vanadium Micro Redox Flow Battery (MVMRFB) with an automated closed‐loop control, using micro actuators and micro sensors, is presented for the first‐time during electrolyte preconditioning operation in recirculation mode. The progress of preconditioning is tracked with UV‐vis spectroscopy by 3D printed micro flow cuvettes. Influence of flow rate, reactor internal resistance, and presence of side reactions in the preconditioning process are studied. Optimal flow rate ratio between negative and positive electrolytes is determined and significant performance improvements achieved by operating at lower flow rates are obtained. Influence of reactor internal resistance, which is directly related with the maximum power density, is evaluated demonstrating that operating at a high‐power density can be a source of inefficiency due to the presence of side reactions. Finally, presence of side reactions is evaluated through a dual measurement of electrolytes concentrations in both negative and positive side, and it is demonstrated to be a cause for charge imbalance between the two half‐cells. This work lays a solid foundation for the successful implementation of a charge‐discharge cycle in MVMRFBs operating in recirculation mode.

Ionic Charge Imbalance in Perovskite Solar Cells

Ionic Charge Imbalance in Perovskite Solar Cells

Inhomogeneity of Cleaved Bulk MoS2 and Compensation of Its Charge Imbalances by Room‐Temperature Hydrogen Treatment (Adv. Mater. Interfaces 32/2023)

Inhomogeneity of Cleaved Bulk MoS₂ and Compensation of Its Charge Imbalances by Room‐Temperature Hydrogen Treatment (Adv. Mater. Interfaces 32/2023)

Hybrid Functional Study on Electronic and Optical Properties of the Dopants in Anatase TiO2.

TiO2 was known as a golden heterogeneous photocatalyst due to its chemical stability, low cost, nontoxicity, and strong oxidizing power. However, anatase TiO2 predominantly absorbs the photon energy in the ultraviolet region (λ < 387.5 nm); therefore, to increase the utilization of sunlight, the approach of doping of metals and nonmetals into pure TiO2 is implemented. Here we incorporate the dopants of Zr, Si, V, W, Ge, Cr, Sn, Mo, and Pb into the TiO2 lattice and study the optoelectronic properties, including the formation energies and the electron charge distributions, using the Vienna ab initio Simulation Package (VASP) from the hybrid functional of Heyd, Scuseria, and Erhzerhof (HSE06). We observed that V-, Mo-, and Cr-doped systems introduce shallow impurity states within the band gap, and those states influence the shift of the absorbance spectra to visible light by enhancing the photocatalytic efficiency. W-doped anatase TiO2 structure reduces the band gap of the pure anatase TiO2 by 0.7 eV. Notably, this reduction occurs without the introduction of any impurity states between the band edges. Additionally, the absorption edge of the solar spectrum shifts toward lower photon energy from 3.5 to 3.1 eV. From Bader charge analysis, we observed that mainly the charge transfer occurred from the dopants and charge accumulation happened around nearby oxygen atoms. The ferromagnetism was observed in V-, Cr-, Mo-, and W-doped anatase TiO2 structures due to the charge imbalance of the spin-up and spin-down states.

High-performance inkjet-printed inverted QD-LEDs based on cross-linkable electron regulation layers

In all-solution-processed quantum dot light-emitting diodes (QD-LEDs), zinc oxide nanoparticles (ZnO NPs) have been one of the most widely used electron transport layer materials. However, the high electron mobility and abundant surface defects of ZnO NPs usually entail not only charge imbalance in the QDs, but also exciton quenching at the QDs/ZnO interface. To overcome the shortcomings, a strategy is proposed by employing cross-linkable organics (isomeric TV-TmPY or TV-T2T) as an electron regulation layer (ERL) in inverted QD-LEDs. The spin-coated RGB QD-LEDs with TV-T2T inserts exhibit less sensitivity to interlayer thickness and great performance enhancements, with EQEs up to 20.2% for the red devices, 15.3% for the green devices, and 9.1% for the blue devices. The inkjet-printed RGB QD-LEDs with TV-T2T inserts also achieve EQEs of 18.4%, 13.3%, and 5.6%, respectively, representing the highest reported performance for inkjet-printed inverted QD-LEDs to date.

Vibrational properties of OH groups associated with divalent cations in corundum (α-Al2O3)

Abstract. The infrared spectra of synthetic corundum (α-Al2O3) samples either doped directly with divalent cations (Mg2+) or containing divalent cations formed by reduction of trivalent cations in H2 gas (Co2+, Ni2+) may display broad OH stretching bands at ∼3000 cm−1 due to the structural incorporation of trace amounts of hydrogen. Experimental spectra recorded from some natural sapphires display a similar absorption band associated with a dominant absorption at 3161 cm−1, and some beryllium-diffused corundum crystals show a band at 3060 cm−1. All of these also display smaller and generally narrower bands between 1900 and 2700 cm−1, whose natures are poorly defined. In this work, the atomic-scale structure, relative stability and infrared spectroscopic properties of a series of OH defects in corundum (α-Al2O3) are theoretically investigated at the density-functional-theory level. The investigated defects consist of interstitial H+ ions forming OH groups and compensating for the charge imbalance related to the presence of divalent cations (Be2+, Mg2+, Cr2+, Mn2+, Fe2+, Co2+, Ni2+) substituted for Al3+ at nearby octahedral sites. Bands occurring at ∼3000 cm−1 in experimental spectra are assigned to the OH stretching modes of some of these defects, with bands observed around 1900 and 2700 cm−1 being assigned to overtones of corresponding OH bending modes. The results also support the assignment of the so-called “3161 cm−1 series”, observed in experimental spectra of some rubies and yellow sapphires, to structural OH groups in association with Fe2+ ions, rather than Si4+, as has been previously proposed. These inferences are also supported by analysis of correlations between band areas in experimental infrared spectra extracted from a database of corundum gemstones. A qualitative explanation relating the anomalous intensity and the polarisation properties of the OH bending overtone bands to the electrical anharmonicity of OH groups involved in medium-strength H bonds is proposed.

Charge injection engineering at organic/inorganic heterointerfaces for high-efficiency and fast-response perovskite light-emitting diodes

The development of advanced perovskite emitters has considerably improved the performance of perovskite light-emitting diodes (LEDs). However, the further development of perovskite LEDs requires ideal device electrical properties, which strongly depend on its interfaces. In perovskite LEDs with conventional p-i-n structures, hole injection is generally less efficient than electron injection, causing charge imbalance. Furthermore, the popular hole injection structure of NiOx/poly(9-vinylcarbazole) suffers from several issues, such as weak interfacial adhesion, high interfacial trap density and mismatched energy levels. In this work, we insert a self-assembled monolayer of [2-(9H-carbazol-9-yl)ethyl]phosphonic acid between the NiOx and poly(9-vinylcarbazole) layers to overcome these challenges at the organic/inorganic heterointerfaces by establishing a robust interface, passivating interfacial trap states and aligning the energy levels. We successfully demonstrate blue (emission at 493 nm) and green (emission at 515 nm) devices with external quantum efficiencies of 14.5% and 26.0%, respectively. More importantly, the self-assembled monolayer also gives rise to devices with much faster response speeds by reducing interfacial capacitance and resistance. Our results pave the way for developing more efficient and brighter perovskite LEDs with quick response, widening their potential application scope.

Design of Fuzzy Logic Controller for Inductor Based Cell Balancing in Battery Management System

Lithium-ion batteries are a popular energy storage used in various applications including electronic devices, electric vehicles, renewable energy systems, and microgrids. The limitations in capacity and voltage of individual battery cells necessitate arranging them in series and parallel configurations to meet the energy demands of the system. However, connecting cells in series can potentially lead to charge imbalance among cells, resulting in reduced battery pack capacity and triggering safety concerns. Numerous topologies and control methods for battery cell balancing systems have been explored in previous research. In this study, two fuzzy logic controllers with different membership function shapes are designed to drive the duty cycle of PWM signals for inductor-based cell to cell balancing. Based on simulation results, the designed controllers demonstrate good performance compared to conventional methods.

Modulation of electronic and magnetic properties of graphene-triangular h-BN hybrid through monovacancy: Carbon vacancy-BN size coupling

The effects of monovacancy on the stability, electronic and magnetic properties of the triangular h-BN embedded graphene nanosheet (G) were investigated by density functional theory (DFT) method. The interface is the energetically most favorable site for creating a vacancy in the GBN nanosheet. Defective GBN hybrids are either metallic or exhibit a small band gap energy ranging from 0.01 to 0.23 eV. The dangling bonds induced a localized magnetic moment ranging from 0.02 to 3 μB in the defective hybrid. The magnetic moment induced by the carbon vacancy in the G layout of the N-rich hybrid interestingly increases with island size. This coupling effect can be explained by the local charge imbalance between charge centers of opposite sign. We could not observe similar behavior in B- rich hybrids. Our results showed that the narrow bandgap and induced magnetic moment of the vacancy-defected GBN hybrid could be easily modulated and thus could be potential materials for spintronic /electronic applications.

Surface engineering to tailor the active sites of SrTi0.9Co0.1O3-δ perovskite for CO oxidation

We successfully engineered the surface properties of SrTiO3 perovskite for a carbon monoxide (CO) oxidation catalyst by substituting Co ions (10 at.%) into Ti-sites in SrTiO3 and subsequently applying a reductive thermal treatment. The substitution of Co ions in SrTiO3 induced charge imbalance, which was compensated by the generation of oxygen vacancies. During the subsequent reductive thermal treatment process, lattice oxygen species were released from the lattice of SrTiO3, leading to the further formation of oxygen vacancies and the segregation of Co cations on the catalyst surface. Thus, SrTi0.9Co0.1O3-δ catalyst showed superior activity compared to the pristine SrTiO3 in CO oxidation, and its catalytic activity was further enhanced after the thermal treatment. This improvement was due to the abundant surface oxygen vacancies generated by Co substitution and reductive thermal treatment. The presence of a surface Co-enriched phase increased the concentration of Co at the catalyst surface, providing more adsorption sites for CO. These findings demonstrate the effective control of the active sites (oxygen vacancies and B-site cations) in SrTiO3 using the two-step surface engineering method. This research contributes to the design of perovskite-oxide catalysts with reaction-tailored active sites by exploiting the synergistic effect of substitution and reductive thermal treatment methods.

Generating a tunable narrow electron beam comb via laser-driven plasma grating

We propose a novel approach for generating a high-density, spatially periodic narrow electron beam comb (EBC) from a plasma grating induced by the interference of two intense laser pulses in subcritical-density plasma. We employ particle-in-cell (PIC) simulations to investigate the effects of cross-propagating laser pulses with specific angles overlapping in a subcritical plasma. This overlap results in the formation of a transverse standing wave, leading to a spatially periodic high-density modulation known as a plasma grating. The electron density peak within the grating can reach several times the background plasma density. The charge imbalance between electrons and ions in the electron density peaks causes mutual repulsion among the electrons, resulting in Coulomb expansion and acceleration of the electrons. As a result, some electrons expand into vacuum, forming a periodic narrow EBC with an individual beam width in the nanoscale range. To further explore the formation of the nanoscale EBC, we conduct additional PIC simulations to study the dependence on various laser parameters. Overall, our proposed method offers a promising and controlled approach to generate tunable narrow EBCs with high density.

Low-Temperature Plasma-Assisted Catalytic Dry Reforming of Methane over CeO2 Nanorod-Supported NiO Catalysts in a Dielectric Barrier Discharge Reactor.

Nonthermal plasma (NTP)-assisted catalytic dry reforming of methane (DRM) is considered a powerful single-stage reaction mechanism because of its ability to activate normally stable CO2 and CH4 at a low temperature under ambient conditions. The thermodynamic barrier of DRM requires a high operating temperature (>700 °C), which can be reduced by nonequilibrium plasma. Herein, we present a method for the wet-impregnation synthesis of CeO2 nanorod (NR)-supported 5 and 15 wt % NiO catalysts for efficient NTP-promoted DRM with an applied power in the range of 24.9-25.8 W (frequency: 20 kHz), a CH4:CO2 feed gas ratio of 100:250 sccm, and a total flow rate of 350 sccm. The presence of NTP dramatically increased the reaction activity, even at 150 °C, which is usually inaccessible for thermally catalyzed DRM. The CH4 and CO2 conversion reaches a maximum of 66 and 48%, respectively, at 500 °C with the 15 wt % NiO/CeO2 NR catalyst, which are much higher than the values obtained for the 5 wt % NiO/CeO2 NR catalyst under the same conditions. According to the X-ray photoelectron spectroscopy profile for 15 wt % NiO/CeO2 NR, a higher concentration of NiO on CeO2 increases the proportion of Ce3+ in the catalyst, suggesting enhanced oxygen vacancy concentration with an increased amount of NiO loading. Additionally, a higher NiO loading promotes a higher rate of replacement of Ce4+ with Ni2+, which generates more oxygen vacancies due to the induced charge imbalance and lattice distortion within the CeO2 lattice. As a result, it can be inferred that the incorporation of Ni ions into the CeO2 structure resulted in inhibited growth of CeO2 crystals due to the creation of a NixCe1-xO2-α solid solution and the production of oxygen vacancies.

Mechanistic insights into photo-assisted DMF cleavage mediated by charge transfer reaction

Mechanistic insights into the synthetically-valued metal-free sustainable N,N-dimethylamination of perfluoroarenes have been investigated. Density functional theory (DFT) and time-dependent DFT (TD-DFT) have been applied to provide details regarding photo-assisted cleavage of N, N-dimethylformamide (DMF) for the valued N, N-dimethylamination reaction. The radical reaction mediated by the electron transfer process causes DMF cleavage, which is responsible for the amination reaction. It was found that the energy barrier for the ground state reaction is thermally prohibitive. On the other hand, visible-light irradiation activates the electron-donor properties of DMF and acceptor properties of perfluoronaphthalene (PFN), so that the energy barrier for electron transfer reduces to ∼ 14 kcal/mol. The redistribution of the electronic charge between the DMF and PFN generates an imbalance of charge on the overall molecule. These events prompt the relocation of extraneous charges to some antibonding molecular orbitals so that one of the C-F bonds in PFN gets weakened and is easily broken. The reductive cleavage of the C-F bond enables the radical ions to react quickly to afford the amination products, which eventually deactivate to the ground state.