Unveiling the role of carbon defects in the exceptional narrowing of m-ZrO2 bandgap for enhanced photoelectrochemical water splitting performance

The development of efficient photoelectrodes via defect engineering of wide-band gap metal oxides has been the prime focus for many years. Specifically, the effect of carbon defects in wide-band gap metal oxides on their performance in photoelectrochemical (PEC) applications raised numerous controversies and still elusive. Herein, the effect of various carbon defects in m-ZrO2 was investigated using the density functional theory to probe the thermodynamic, electronic, and optical properties of the defective structures against pristine m-ZrO2. The defect formation energies revealed that elevating the temperature promotes and facilitates the formation of carbon defects. Moreover, the binding energies confirmed the stability of all studied complex carbon defects. Furthermore, the band edge positions against the redox potentials of water species revealed that all the studied defective structures can serve as photoanodes for water splitting. Additionally, CO3c (carbon atom substituted O3c site) was the only defective structure that exhibited slight straddling of the redox potentials of water. Importantly, all investigated defective structures enhanced light absorption with different optical activities. Finally, CO3cVO3c (carbon atom substituted O3c associated with O3c vacancy) defective m-ZrO2 enjoyed low direct band gap (1.9 eV), low defect formation energy, low exciton binding energy, high mobility of charge carriers, fast charge transfer, and low recombination rate. Concurrently, its optical properties were exceptional in terms of high absorption, low reflectivity and improved static dielectric constant. Hence, the study recommends CO3cVO3c defective m-ZrO2 as the leading candidate to serve as a photoanode for PEC applications

TiO2 nanotubes with ultrathin walls for enhanced water splitting

We demonstrate, for the first time, the synthesis of titania nanotubes with ultrathin (3–5 nm) wall thickness. As revealed by the incident photon-to-current collection efficiency (IPCE) and electrochemical impedance spectroscopy measurements, the ultrathin walls, less than the charge carrier diffusion length, were essential to ensure fast and efficient charge carrier collection.Qatar National Research Fund, a member of Qatar Foundation NPRP Grant no. 6-569-1-11

Computational and experimental elucidation of the boosted stability and antibacterial activity of ZIF-67 upon optimized encapsulation with polyoxometalates

Water microbial purification is one of the hottest topics that threats human morbidity and mortality. It is indispensable to purify water using antimicrobial agents combined with several technologies and systems. Herein, we introduce a class of nanosized metal organic framework; Zeolitic imidazolate framework (ZIF-67) cages encapsulated with polyoxometalates synthesized via facile one-step co-precipitation method. We employed two types of polyoxometalates bioactive agents; phosphotungstic acid (PTA) and phosphomolybdic acid (PMA) that act as novel antibacterial purification agents. Several characterization techniques were utilized to investigate the morphological, structural, chemical, and physical properties such as FESEM, EDS, FTIR, XRD, and N₂ adsorption/desorption isotherms techniques. The antibacterial assessment was evaluated using colony forming unit (CFU) against both Escherichia coli and Staphylococcus aureus as models of Gram-negative and Gram-positive bacteria, respectively. The PTA@ZIF-67 showed higher microbial inhibition against both Gram-positive and Gram-negative bacteria by 98.8% and 84.6%, respectively. Furthermore, computational modeling using density functional theory was conducted to evaluate the antibacterial efficacy of PTA when compared to PMA. The computational and experimental findings demonstrate that the fabricated POM@ZIF-67 materials exhibited outstanding bactericidal effect against both Gram-negative and Gram-positive bacteria and effectively purify contaminated water

Rapid and controlled electrochemical synthesis of crystalline niobium oxide microcones

We demonstrate the fabrication by anodization of niobium oxide microcones, several microns long, from aqueous solutions of 1 wt% hydrogen fluoride (HF) with varied sodium fluoride (NaF) concentration (0–1 M). Raman spectroscopy and x-ray diffractometer analysis revealed the as-grown microcones to be crystalline Nb_2O_(5−x) with preferred (1 0 0) and (0 1 0) orientations. The overall Nb_2O_(5−x) formation rate increased with the increasing NaF concentration, and structures as tall as 20 μm were achieved in just 20 min of anodization at 1 M NaF. Rapid formation of niobia microcones was even observed in the absence of HF at this NaF concentration. Photocatalytic activity for water oxidation was highest for microcones grown under the highest NaF concentration

The DFT+U: Approaches, Accuracy, and Applications

This chapter introduces the Hubbard model and its applicability as a corrective tool for accurate modeling of the electronic properties of various classes of systems. The attainment of a correct description of electronic structure is critical for predicting further electronic-related properties, including intermolecular interactions and formation energies. The chapter begins with an introduction to the formulation of density functional theory (DFT) functionals, while addressing the origin of bandgap problem with correlated materials. Then, the corrective approaches proposed to solve the DFT bandgap problem are reviewed, while comparing them in terms of accuracy and computational cost. The Hubbard model will then offer a simple approach to correctly describe the behavior of highly correlated materials, known as the Mott insulators. Based on Hubbard model, DFT+U scheme is built, which is computationally convenient for accurate calculations of electronic structures. Later in this chapter, the computational and semiempirical methods of optimizing the value of the Coulomb interaction potential (U) are discussed, while evaluating the conditions under which it can be most predictive. The chapter focuses on highlighting the use of U to correct the description of the physical properties, by reviewing the results of case studies presented in literature for various classes of materials

Electrical Characterization of Nanopolyaniline/Porous Silicon Heterojunction at High Temperatures

Nanopolyaniline/p-type porous silicon (NPANI/PSi) heterojunction films were chemically fabricated via in situ polymerization. The composition and morphology of the nanopolymer were confirmed using Fourier transform infrared, scanning electron microscopy, UV-visible, and transmission electron microscopy techniques. The results indicated that the polymerization took place throughout the porous layer. The I-V measurements, performed at different temperatures, enabled the calculation of ideality factor, barrier height, and series resistance of those films. The obtained ideality factor showed a nonideal diode behavior. The series resistance was found to decrease with increasing temperature

Recent advances in the design of cathode materials for Li-ion batteries

The Li-ion battery (LIB) industry has rapidly developed and dominates the market of electric vehicles and portable electronic devices. Special attention is devoted to achieving higher power and energy densities, along with enhancing safety and reducing cost. Therefore, critical insights should be made on the understanding of the behavior of the components of LIBs under working conditions in order to direct future research and development. The present review discusses the literature on the properties and limitations of different cathode materials for LIBs, including layered transition metal oxides, spinels, and polyanionic positive electrode materials, with critical insights on the structural, thermal, and electrochemical changes that take place during cycling. Besides, the strategies and techniques capable of overcoming current limitations are highlighted

An alternative, low-dissolution counter electrode to prevent deceptive enhancement of HER overpotential

Electrochemical hydrogen evolution reaction (HER) is typically studied in three-electrode system. In this system, several counter electrodes are commonly used to ensure fast kinetics, including Pt, gold, and glassy carbon. However, the extensive application of such electrodes has raised caveats on the contribution of the redox-active species dissolving from such electrodes and redepositing on the surface of the working electrode to the measured overpotential. Consequently, this has been frequently confused with the actual electrochemical signature of the working electrode catalyst, resulting in a deceptive enhancement in the recorded overpotential. This issue becomes more critical when the electrolysis measurements involve an activation step, necessitating the need for alternative counter electrodes that are stable, especially in acidic medium, which is commonly used as the electrolyte in HER studies. Herein, while we systematically unveil such problems, an alternative counter electrode that overcomes those problems is demonstrated. Specifically, the correlation between the working electrode area to that of the counter electrode, the dissolution rate of the counter electrode, and the potential range used in the activation/cleaning of the surface on accelerating the dissolution rate is explored and discussed in detail. Finally, commercial Ti mesh is demonstrated as an alternative emerging counter electrode, which is proven to be very stable and convenient to study the HER in acidic media

Untapped potential of 2D charge density wave chalcogenides as negative supercapacitor electrode materials

Two-dimensional (2D) materials have opened new avenues for the fabrication of ultrathin, transparent, and flexible functional devices. However, the conventional inorganic graphene analogues are either semiconductors or insulators with low electronic conductivity, hindering their use as supercapacitor electrode materials, which require high conductivity and large surface area. Recently, 2D charge density wave (CDW) materials, such as 2D chalcogenides, have attracted extensive attention as high performance functional nanomaterials in sensors, energy conversion, and spintronic devices. Herein, TaS2 is investigated as a potential CDW material for supercapacitors. The quantum capacitance (CQ) of the different TaS2 polymorphs (1T, 2H, and 3R) was estimated using density functional theory calculations for different numbers of TaS2 layers and alkali-metal ion (Li, Na and K) intercalants. The results demonstrate the potential of 2H- and 3R-polymorphs as efficient negative electrode materials for supercapacitor devices. The intercalation of K and Na ions in 1T-TaS2 led to an increase in the CQ with the intercalation of Li ions resulting in a decrease in the CQ. In contrast, Li ions were found to be the best intercalant for the 2H-TaS2 phase (highest CQ), while K ion intercalation was the best for the 3R-TaS2 phase. Moreover, increasing the number of layers of the1T-TaS2 resulted in the highest CQ. In contrast, CQ increases upon decreasing the number of layers of 2H-TaS2. Both 1T-MoS2 and 2H-TaS2 can be combined to construct a highly performing supercapacitor device as the positive and negative electrodes, respectively