Self-ordered Nanotubular TiO2 Multilayers for High-Performance Photocatalysts and Supercapacitors

Efficient activation of peroxymonosulfate by nanotubular Co3O4 for degradation of Acid Orange 7: performance and mechanism

Morphology optimization of catalysts has been considered as a viable strategy to improve the catalytic efficiency for peroxymonosulfate (PMS) activation by providing high surface area and abundant active sites. In this study, nanotubular Co₃O₄ (NT-Co₃O₄) was successfully synthesized as a PMS activator for the rapid removal of acid orange 7 (AO7) in aqueous solutions. Characterization results showed that NT-Co₃O₄ presented as aggregated nanotubes, with an average pore diameter of 10 nm. The specific surface area of NT-Co₃O₄ was as high as 41.8 m² g^-1. Catalytic experiments demonstrated that the degradation rate of AO7 in the NT-Co₃O₄/PMS system was 15 times greater than that in commercially available Co₃O₄/PMS system. The effects of various experimental parameters, including catalyst dose, PMS dose, pH, and temperature, were comprehensively investigated. The reactive species in the NT-Co₃O₄/PMS system were identified as sulfate radical (SO₄^•-) through both quenching tests and electron paramagnetic resonance (EPR) technology, and ≡CoOH⁺ played an important role in PMS activation. N atoms in the AO7 molecule were found to be preferentially attacked by SO₄^•-. Moreover, the good stability and reusability of NT-Co₃O₄ were confirmed by a five-cycle AO7 removal experiment. This study provides a broader view of the potential applications of nanotubular materials to achieve highly efficient PMS activation in treating dyes in wastewater.

Polarization-enhanced cell walls etching of anodic titanium oxide

Anodic titanium oxide (ATO) photonic crystals (PhCs) are promising for photonics, photocatalysis, and solar cells. A refractive index modulation in ATO PhCs is caused by the modulation of porosity and, thus, the pore diameter should be controlled precisely. The ATO cell walls etching in electrolyte solution during anodizing increases the porosity of the PhC structure and shifts the photonic band gap (PBG) position to shorter wavelengths. Until now, the ATO cell walls etching in organic based electrolytes has been associated solely with the chemical dissolution of ATO in fluoride-containing solutions. Here, a significant enhancement of cell walls etching is observed when electric current flows under anodic polarization. This effect leads to the blue shift of the PBG position with the number of periods of ATO PhC structure. Therefore, it is essential for the synthesis of ATO PhCs with a precise PBG position.

In situ synthesis of nanostructured Fe3O4@TiO2 composite grown on activated carbon cloth as a binder-free electrode for high performance supercapacitors

Transition metal oxide (TMO) nanomaterials with regular morphology have received widening research attention as electrode materials due to their improved electrochemical characteristics. In this study we present the successful fabrication of an Fe₃O₄/TiO₂ nanocomposite grown on a carbon cloth (Fe₃O₄/TiO₂@C) used as a high-efficiency electrochemical supercapacitor electrode. Flexible electrodes are directly used for asymmetric supercapacitors without any binder. The increased specific surface area of the TiO₂ nanorod arrays provides sufficient adsorption sites for Fe₃O₄ nanoparticles. An asymmetric supercapacitor composed of Fe₃O₄/TiO₂@C is tested in 1 M Na₂SO₃ electrolyte, and the synergistic effects of fast reversible Faraday reaction on the Fe₃O₄/TiO₂ surface and the highly conductive network formed by TiO₂@C help the electrode to achieve a high areal capacitance of 304.1 mF cm⁻² at a current density of 1 mA cm⁻² and excellent cycling stability with 90.7% capacitance retention at 5 mA cm⁻² after 10 000 cycles. As a result, novel synthesis of a binder-free Fe₃O₄/TiO₂@C electrode provides a feasible approach for developing competitive candidates in supercapacitor applications.

Transition metal oxide (TMO) nanomaterials with regular morphology have received widening research attention as electrode materials due to their improved electrochemical characteristics.

Voltage-Driven Room-Temperature Resistance and Magnetization Switching in Ceramic TiO2/PAA Nanoporous Composite Films

Voltage control of room-temperature ferromagnetism has remained a big challenge which will greatly influence the multifunctional memory devices. In this paper, porous TiO₂ thin films were deposited by dc-reactive magnetron sputtering onto ordered porous anodic alumina (PAA) substrates. Voltage-driving room-temperature resistance and magnetization switching without external magnetic field are simultaneously found in an Ag/TiO₂/PAA/Al (Ag/TP/Al) device. Further analysis indicates that the formation/rupture of oxygen vacancy defect-based conductive filaments would be responsible for the changes of resistivity and magnetization. Our present results suggest that the TP nanoporous composite film material may therefore be used to achieve voltage control of magnetism and resistance switching in the future multifunctional memory devices. The Ag/TP/Al devices can also be used for new spintronic devices, neuromorphic operations, and alternative logic circuits and computing.

Enhanced Capacitance of TiO2 Nanotubes with a Double-Layer Structure Fabricated in NH4F/H3PO4 Mixed Electrolyte

TiO₂ is an attractive electrode material in fast charging/discharging supercapacitors because of its high specific surface area. However, the low capacitance of TiO₂ nanotubes as-anodized in the classical electrolyte restricts their further application in supercapacitors. Here, we study the performances of larger-diameter nanotubes with a double-layer structure fabricated in an NH₄F/phosphoric acid (H₃PO₄) mixed electrolyte. Results show that the double-layer structure increased the specific surface area of nanotubes owing to the cavities between the double layers and the porous structure on walls. After soaking in H₃PO₄ aqueous solution for 40 min, the nanotubes anodized in the mixed electrolyte containing 6 wt % H₃PO₄ show a specific capacitance of 13.89 mF cm^-2, ∼3.11 times that of the pristine nanotubes in the classical electrolyte. The specific surface area of the soaked nanotubes is up to 113.2 m² g^-1, which is ∼2.94 times that of the pristine nanotubes. The values of specific surface area of the anodized nanotubes and the soaked nanotubes fabricated in the mixed electrolyte containing 6 wt % H₃PO₄ are roughly equal. It demonstrated that the specific surface area increased mainly due to the double-layer structure. The double-layer structure reveals a new strategy to enhance the specific capacitance of TiO₂ nanotubes.

Phase-pure VO2 nanoporous structure for binder-free supercapacitor performances

Vanadium oxides are anticipated as a high-performance energy storage electrode due to their coupled double layer and pseudo-capacitative charge storage mechanism. In the present work, we investigated the influence of different structural phases of as-grown VO₂ nanoporous structure and corresponding oxidation states on the supercapacitor performance. This nanoporous structure facilitates fast ion diffusion and transport. It is shown that stoichiometric monoclinic VO₂, with V oxidation state of +4, provides superior charge storage capacity with a capacitance value of 33 mF/cm², capacitance retention of 93.7% and Coulombic efficiency of 98.2%, to those for VO₂ structures with mixed oxidation states of V⁵⁺ and V⁴⁺. A comparable high energy density is also recorded for the sample with all V⁴⁺. Scanning Kelvin probe microscopy results clarify further the formation of space charge region between VO₂ and carbon paper. These key findings indicate the potentiality of binder-free single phase monoclinic VO₂ porous structure towards the next-generation micro-supercapacitor application.

On the Interfacial Adhesion between TiO2 Nanotube Array Layer and Ti Substrate

Anodic titania nanotube arrays (TNTAs) with higher aspect ratio are observed to be liable to spontaneous curling or delamination from the underlying titanium (Ti) metal once dried because of the poor interfacial adhesion of the TNTA layer to the underlying Ti, especially when a thin Ti sheet is used. The interfacial adhesion strength was shown to decrease with increasing thickness of the TNTA layer. In this work, although the preparation of TNTAs in a frequently used fluoride-containing solution was completed, different anodization processes were further performed at lower current densities or at lower voltages for a short time in the same electrolyte to increase the adhesion. The mechanical test demonstrated that better adhesion properties have been achieved by applying these anodization posttreatment processes. It is believed that during the fabrication of TNTAs, a large residual stress at the interface of the nanotube layer and the underlying Ti is created. It is the residual stress that leads to the weak interfacial adhesion. The anodization posttreatment processes can reduce or eliminate the residual stress, thereby improving the interfacial adhesion. Further, these processes can also boost the performances of TNTAs for supercapacitors. When the anodization posttreatment processes are implemented at 1 mA cm^-2 or at 10 V for 5 min, considerable improvements in the interfacial adhesion are observed. Particularly, both posttreatment processes are also applicable to the very thin Ti sheet (∼18 μm). The realization of robust and adherent TNTAs grown on very thin Ti sheets can not only significantly improve the volumetric capacitances of TNTAs but also make TNTAs an attractive material in flexible supercapacitor applications.

One-pot synthesis of CoFe2O4/rGO hybrid hydrogels with 3D networks for high capacity electrochemical energy storage devices

CoFe₂O₄/reduced graphene oxide (CoFe₂O₄/rGO) hydrogel was synthesized in situ via a facile one-pot solvothermal approach. The three-dimensional (3D) network structure consists of well-dispersed CoFe₂O₄ nanoparticles on the surfaces of graphene sheets. As a binder-free electrode material for supercapacitors, the electrochemical properties of the CoFe₂O₄/rGO hybrid hydrogel can be easily adjusted by changing the concentration of the graphene oxide (GO) precursor solution. The results indicate that the hybrid material made using 3.5 mg mL⁻¹ GO solution exhibits an outstanding specific capacitance of 356 F g⁻¹ at 0.5 A g⁻¹, 68% higher than the pure CoFe₂O₄ counterpart (111 F g⁻¹ at 0.5 A g⁻¹), owing to the large specific surface area and good electric conductivity. Additionally, an electrochemical energy storage device based on CoFe₂O₄/rGO and rGO was assembled, which exhibits a high energy density of 17.84 W h kg⁻¹ at a power density of 650 W kg⁻¹ and an excellent cycling stability with 87% capacitance retention at 5 A g⁻¹ after 4000 cycles. This work takes one step further towards the development of 3D hybrid hydrogel supercapacitors and highlights their potential application in energy storage devices.

CoFe₂O₄/reduced graphene oxide (CoFe₂O₄/rGO) hydrogel was synthesized in situ via a facile one-pot solvothermal approach.

Formation mechanism of multilayer TiO2 nanotubes in HBF4 electrolyte

Multilayer anodic TiO₂ nanotubes with A-shaped sidewalls are first fabricated in HBF₄-containing electrolyte by a one-step galvanostatic anodization.

Simulation of anodizing current–time curves and the morphology evolution of TiO2 nanotubes obtained in phosphoric electrolytes

The simulation and separation of anodizing current density–time curves obtained in mixed electrolytes.