Nanoconfinement boosts affinity of hydrated zirconium oxides to arsenate: Surface complexation modeling study

Nanoscale hydrated zirconium oxide (HZO) holds great potential in groundwater purification due to its ability to form inner-sphere coordination with arsenate. Despite being frequently used, especially as encapsulations in host materials for practical application in water treatment, the adsorption mechanisms of solutes on HZO are not appropriately explored, in particular for arsenate adsorption. In this study, we investigated the Zr-As coordination configuration and identified the most credible Zr-As configuration using surface complexation modeling (SCM), XPS and FT-IR analysis. The corresponding intrinsic coordination constants (Kintr) values was calculated by SCM, and the nanoconfinement effects were distinguished by comparing bare HZO with the HZO nanoparticles (NPs) encapsulated inside the strongly basic anion exchanger D201. Potentiometric titration suggests that the surface Zirconium hydroxyl groups (≡ZrOH) mainly exist in protonated form (≡ZrOH2+). Batch adsorption experiments demonstrate that the D201 hosts could adsorb As(V) through ion exchange by the quaternary ammonium groups under the low ionic strength (≤0.01 M NaNO3) and at pH > 6. The nanocomposite (HZO@D201) exhibits a higher adsorption capacity in a wide range of pH (3-10) and ionic strength (0.001-0.1 M NaNO3) than bare HZO. SCM simulations reveal that the coordination configuration of diprotonated monodentate mononuclear (MM-H2) dominates at pH 3-6, while deprotonated bidentate binuclear (BB-H0) dominates at pH > 7. For each configuration, the intrinsic coordination constants (Kintr) of HZO@D201 (10-0.66 and 10-16.10, respectively) are significantly higher than those of bare HZO (10-12.24 and 10-44.42, respectively), indicating a superior chemical bonding affinity caused by nanoconfinement. The obtained Kintr values are used to predict arsenate adsorption isotherms in pH 3 and 9, and the results align with the SCM simulation outcomes. This study may offer a feasible method for investigating the nanoconfinement effect of emerging nanocomposite adsorbents from a thermodynamic perspective, and provide reference coordination equilibrium constants of HZO for research and practical application.

Ideal Vacuum-Based Efficient and High-Throughput Computational Screening of Type II Heterojunctions

Heterojunctions featuring a type II band alignment play a crucial role in a wide range of devices, particularly in the realm of solar cells. However, the design of such heterojunctions with a specific type of band alignment poses a substantial challenge due to the immense number of potential combinations of bulk semiconductors and their relative orientations. In this study, we propose an efficient, high-throughput computational screening method tailored for heterojunctions. Our approach, using the ideal vacuum level as a reference energy, eliminates the need for explicit electronic structure calculations for junctions. Through this protocol, we identify 1041 type II heterojunctions out of 2692 structures constructed from 86 selected inorganic compounds with appropriate band gaps sourced from the Inorganic Crystal Structure Database. For potential application in solar cells, we assess these heterojunctions, and remarkably, 58 of them exhibit a power conversion efficiency (PCE) exceeding 15%, with 13 surpassing the 20% threshold. Test calculations with expensive interface models confirm the reliability of PCE predictions based on ideal vacuums. These predictions will be of benefit in assessing the material applicability for solar cell applications. Furthermore, the versatility of our proposed screening method extends beyond solar cells, making it a valuable theoretical design tool that can be applied to a wide range of heterojunction devices.

Charged species redistribution at electrochemical interfaces: a model system of the zirconium oxide/water interface

Quantifying the local distribution of charged defects in the solid state and charged ions in liquid solution near the oxide/liquid interface is key to understanding a range of important electrochemical processes, including oxygen reduction and evolution, corrosion and hydrogen evolution reactions. Based on a grand canonical approach relying on the electrochemical potential of individual charged species, a unified treatment of charged defects on the solid side and ions on the water side can be established. This approach is compatible with first-principles calculations where the formation free energy of individual charged species can be calculated and modulated by imposing certain electrochemical potential. Herein, we apply this framework to a system of monoclinic ZrO₂(1̄11)/water interface. The structure, defect chemistry and dynamical behavior of the electric double layer and space charge layer are analyzed with different pH values, water chemistry and doping elements in zirconium oxide. The model predicts ZrO₂ solubility in water and the point of zero charge consistent with the experimentally-measured values. We reveal the effect of dopant elements on the concentrations of oxygen and hydrogen species at the surface of the ZrO₂ passive layer in contact with water, uncovering an intrinsic trade-off between oxygen diffusion and hydrogen pickup during the corrosion of zirconium alloys. The solid/water interface model established here serves as the basis for modeling reaction and transport kinetics under doping and water chemistry effects.

Modeling the electrical double layer at solid-state electrochemical interfaces

Models of the electrical double layer (EDL) at electrode/liquid-electrolyte interfaces no longer hold for all-solid-state electrochemistry. Here we show a more general model for the EDL at a solid-state electrochemical interface based on the Poisson-Fermi-Dirac equation. By combining this model with density functional theory predictions, the interconnected electronic and ionic degrees of freedom in all-solid-state batteries, including the electronic band bending and defect concentration variation in the space-charge layer, are captured self-consistently. Along with a general mathematical solution, the EDL structure is presented in various materials that are thermodynamically stable in contact with a lithium metal anode: the solid electrolyte Li7La3Zr2O12 (LLZO) and the solid interlayer materials LiF, Li2O and Li2CO3. The model further allows design of the optimum interlayer thicknesses to minimize the electrostatic barrier for lithium ion transport at relevant solid-state battery interfaces.

Highly Efficient InGaN Nanorods Photoelectrode by Constructing Z-scheme Charge Transfer System for Unbiased Water Splitting

Unbiased photoelectrochemical water splitting for the promising InGaN nanorods photoelectrode is highly desirable, but it is practically hindered by the serious recombination of charge carrier in bulk and surface of InGaN nanorods. Herein, an unbiased Z-scheme InGaN nanorods/Cu₂ O nanoparticles heterostructured system with boosted interfacial charge transfer is constructed for the first time. The introduced Cu₂ O nanoparticles pose double-sided effect on photoelectrochemical (PEC) performance of InGaN nanorods, which enables a robust hybrid structure and induces weakened light absorption capability simultaneously. As a result, the optimized InGaN/Cu₂ O-1.5C photoelectrode with the uniform morphology exhibits an enhanced photocurrent density of ≈170 µA cm^-2 at 0 V versus Pt, with 8.5-fold enhancement compared with pure InGaN nanorods. Comprehensive investigations into experimental results and theoretical calculations reveal that the electrons accumulation and holes depletion of Cu₂ O facilitate to form a typical Z-scheme band alignment, thus providing a large photovoltage to drive unbiased water splitting and enhancing the stability of Cu₂ O. This work provides a novel and facile strategy to achieve InGaN nanorods and other catalyst-based PEC water splitting without external bias, and to relieve the bottlenecks of charge transfer dynamics at the electrode bulk and electrode/electrolyte interface by constructing Z-scheme heterostructure.

Enhanced Adhesion Energy at Oxide/Ag Interfaces for Low-Emissivity Glasses: Theoretical Insight into Doping and Vacancy Effects

Low-emissivity glasses rely on multistacked architectures with a thin silver layer sandwiched between oxide layers. The mechanical stability of the silver/oxide interfaces is a critical parameter that must be maximized. Here, we demonstrate by means of quantum-chemical calculations that a low work of adhesion at interfaces can be significantly increased via doping and by introducing vacancies in the oxide layer. For the sake of illustration, we focus on the ZrO₂(111)/Ag(111) interface exhibiting a poor adhesion in the pristine state and quantify the impact of introducing n-type dopants or p-type dopants in ZrO₂ and vacancies in oxygen atoms (nV_O; with n = 1, 2, 4, 8, 10, 16), zirconium atoms (mV_Zr; with m = 1, 2, 4, 8), or both (nV_O + mV_Zr; with m/n = 1:2, 1:4, 2:2, 2:4). In the case of doping, interfacial electron transfer promotes an increase in the work of adhesion, from initially 0.16 to ∼0.8 J m^-2 (n-type) and ∼2.0 J m^-2 (p-type) at 10% doping. A similar increase in the work of adhesion is obtained by introducing vacancies, e.g., V_O [V_Zr] in the oxide layer yields a work of adhesion of ∼1.5-2.0 J m^-2 at 10% vacancies. An increase is also observed when mixing V_O and V_Zr vacancies in a nonstoichiometric ratio (nV_O + mV_Zr; with 2n ≠ m), while a stoichiometric ratio of V_O and V_Zr has no impact on the interfacial properties.

Engineering of Charged Defects at Perovskite Oxide Surfaces for Exceptionally Stable Solid Oxide Fuel Cell Electrodes

Cation segregation, particularly Sr segregation, toward a perovskite surface has a significant effect on the performance degradation of a solid oxide cell (solid oxide electrolysis/fuel cell). Among the number of key reasons generating the instability of perovskite oxide, surface-accumulated positively charged defects (oxygen vacancy, V_o^··) have been considered as the most crucial drivers in strongly attracting negatively charged defects (Sr_A - site^') toward the surface. Herein, we demonstrate the effects of a heterointerface on the redistribution of both positively and negatively charged defects for a reduction of V_o^·· at a perovskite surface. We took Sm_0.5Sr_0.5CoO_3-δ (SSC) as a model perovskite film and coated Gd_0.1Ce_0.9O_2-δ (GDC) additionally onto the SSC film to create a heterointerface (GDC/SSC), resulting in an ∼11-fold reduction in a degradation rate of ∼8% at 650 °C and ∼10-fold higher surface exchange (k^q) than a bare SSC film after 150 h at 650 °C. Using X-ray photoelectron spectroscopy and electron energy loss spectroscopy, we revealed a decrease in positively charged defects of V_o^·· and transferred electrons in an SSC film at the GDC/SSC heterointerface, resulting in a suppression of negatively charged Sr (Sr_Sm^') segregation. Finally, the energetic behavior, including the charge transfer phenomenon, O p-band center, and oxygen vacancy formation energy calculated using the density functional theory, verified the effects of the heterointerface on the redistribution of the charged defects, resulting in a remarkable impact on the stability of perovskite oxide at elevated temperatures.

Anomalous diffusion along metal/ceramic interfaces

Interface diffusion along a metal/ceramic interface present in numerous energy and electronic devices can critically affect their performance and stability. Hole formation in a polycrystalline Ni film on an α-Al₂O₃ substrate coupled with a continuum diffusion analysis demonstrates that Ni diffusion along the Ni/α-Al₂O₃ interface is surprisingly fast. Ab initio calculations demonstrate that both Ni vacancy formation and migration energies at the coherent Ni/α-Al₂O₃ interface are much smaller than in bulk Ni, suggesting that the activation energy for diffusion along coherent Ni/α-Al₂O₃ interfaces is comparable to that along (incoherent/high angle) grain boundaries. Based on these results, we develop a simple model for diffusion along metal/ceramic interfaces, apply it to a wide range of metal/ceramic systems and validate it with several ab initio calculations. These results suggest that fast metal diffusion along metal/ceramic interfaces should be common, but is not universal.

Little is known about diffusion along metal/ceramic interfaces even though it controls the physical behavior and lifetimes of many devices (including batteries, microelectronics, and jet engines). Here, the authors show that diffusion along a nickel/sapphire interface is abnormally fast due to nickel vacancies and generalise their findings to a wide-range of metal/ceramic systems.

Polarizing Oxygen Vacancies in Insulating Metal Oxides under a High Electric Field

We demonstrate a thermodynamic formulation to quantify defect formation energetics in an insulator under a high electric field. As a model system, we analyzed neutral oxygen vacancies (color centers) in alkaline-earth-metal binary oxides using density functional theory, Berry phase calculations, and maximally localized Wannier functions. The work of polarization lowers the field-dependent electric Gibbs energy of formation of this defect. This is attributed mainly to the ease of polarizing the two electrons trapped in the vacant site, and secondarily to the defect induced reduction in bond stiffness and softening of phonon modes. The formulation and analysis have implications for understanding the behavior of insulating oxides in electronic, magnetic, catalytic, and electrocaloric devices under a high electric field.