NiO Samples Research Articles

Elevated asymmetric supercapacitor with the nickel-metal organic framework derived nickel oxide/nickel composite: Designed to optimize efficiency and reliability

In this study, a solvothermal method is used to synthesizes MOF-derived NiO/Ni composites and NiO samples at various calcination temperatures (350 °C, 450 °C, 550 °C, and 650 °C) and durations (4, 5, 6, and 7 h). The NiO/Ni sample calcined at 450 °C for 5 h (NiO/Ni-450) exhibits the highest specific capacitance (Cs) of 410 Fg⁻¹ at a scan rate of 5 mV s⁻¹. This superior performance results from the combined benefits of Ni's metallic conductivity and NiO's redox activity. In a 1 M KOH electrolyte, NiO/Ni-450 retains about 83 % of its initial capacitance after 2,000 cycles. The NiO sample formed by calcining NiO/Ni-450 for 5 h (H5) achieves a specific capacitance (Cs) of 371 Fg⁻¹ and retains 80 % of its capacitance after 2,000 cycles. NiO/Ni-450 and H5 demonstrate energy densities (Ed) of 8.58 and 5.48 Wh kg⁻¹, and power densities (Pd) of 608 and 500 W kg⁻¹, respectively. An asymmetric supercapacitor using NiO/Ni-450 and activated carbon (AC) shows an impressive 80 % capacitance retention after 9,000 cycles, with a Cs of 123 Fg⁻¹, and Ed and Pd values of 14.58 Wh kg⁻¹ and 4038 W kg⁻¹, respectively, highlighting the potential for industrial-scale production of these materials for energy storage.

Magnetic field-augmented photoelectrochemical water splitting in Co3O4 and NiO nanorod arrays

Effective charge separation is crucial for improving the sensitivity of photoelectrochemical studies. Here, we provide an immense magnetic field-based electron spin polarization approach for an efficient charge carrier separation. We have fabricated NiO and Co3O4 thin film and nanorod arrays by electron beam evaporation glancing angle method followed by annealing in a two-zone furnace. The photoelectrochemical performance was investigated for NiO and Co3O4 samples in the presence and absence of a magnetic field. The NiO and Co3O4 nanorods array samples exhibit better absorption compared with the thin film samples. The Co3O4 and NiO nanorod arrays showed the highest photocurrent density of 0.12 and 0.55 mA/cm2 in a magnetic field. The superior photoelectrochemical response of NiO and Co3O4 nanorods in a magnetic field could be ascribed to the limitation of non-radiative recombination of carriers manipulated by Lorentz force and spin polarization. Furthermore, the electrochemical impedance spectra of NiO and Co3O4 nanorod arrays in a magnetic field show the least charge transfer resistance. This study sheds light on the interaction process between external fields and radiative/non-radiative recombination of manipulating carriers. Thus, the application of a magnetic field presents an efficient and versatile approach to enhance the performance of photoelectrodes in solar water splitting.

NiO thin films fabricated using spray-pyrolysis technique: structural and optical characterization and ultrafast charge dynamics studies

Nickel (II) oxide, NiO, is a wide band gap Mott insulator characterized by strong Coulomb repulsion between d-electrons and displays antiferromagnetic order at room temperature. NiO has gained attention in recent years as a very promising candidate for applications in a broad set of areas, including chemistry and metallurgy to spintronics and energy harvesting. Here, we report on the fabrication of polycrystalline NiO using spray-pyrolysis technique, which is a deposition technique able to produce quite uniform films of pure and crystalline materials without the need of high vacuum or inert atmospheres. The composition and structure of the NiO thin films were then studied using x-ray diffraction, and atomic force and scanning electron microscopies (SEM). The phononic and magnonic properties of the NiO thin films were also studied via Raman spectroscopy, and the ultrafast electron dynamics by using optical pump probe spectroscopy. We found that the NiO samples display the same phonon and magnon excitations expected for single crystal NiO at room temperature, and that electron dynamics in our system is like those of previously reported NiO mono- and polycrystalline systems synthesized using different techniques. These results prove that spray-pyrolysis can be used as affordable and large-scale fabrication technique to synthesize strongly correlated materials for a large set of applications.

Indium-nickel oxide nanocomposite for room temperature H2 gas detection and its sensing mechanism

Lithium-ion batteries (LIBs) have a wide range of applications in energy storage power stations. However, thermal runaway of LIBs can lead to serious safety accidents and cause significant losses. Therefore, it is particularly important to carry out early security warning of LIBs. And H2 is generated along with the growth of lithium dendrites, which can serve as an effective indicator for detecting LIBs before TR. In this paper, In2O3/NiO nanocomposites were prepared, and five groups of samples were systematically prepared by adjusting the molar ratio of In and Ni. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and other test methods were used to characterize the prepared nanocomposites. Through experiments, the best samples among several groups of samples were determined. When the test gas is introduced, the resistance change of this optimal sensor is the greatest, indicating the best performance, which is significantly better than pure In2O3 and pure NiO samples. Furthermore, compared to other gases, this nanocomposite sensor exhibits significant selectivity towards H2. The results show that In2O3/NiO is a promising candidate material for room temperature H2 detection, and provide suggestions for the development of gas sensors with heterogeneous structures.

Boosting the performance of dye-sensitized solar cells by employing Li-substituted NiO nanosheets as highly efficient electrocatalysts for reduction of triiodide

Dye-sensitized solar cells (DSSCs) exhibit considerable potential as a promising technology, particularly when addressing the challenge of replacing the costly Platinum (Pt) counter electrode (CE) with economically viable and chemically stable CE materials. This study examines the use of two-dimensional hexagonal-shaped nickel oxide nanosheets substituted with varying mol% of lithium (1, 3, and 5 %) (Li (1–5%)-NiO NSs) as the counter electrodes (CEs) for DSSCs. The facile hydrothermal method was employed for the preparation of NiO and Li (1–5 mol%)-NiO samples. The BET analysis of NiO and Li (1–5%)-NiO indicates that higher concentrations of Li1+ ions in Ni2+ ions sites lead to an increase in the overall surface area of NiO. This leads to an elevated number of exposed electrocatalytic active sites which resulted in enhancing the rate of reduction of I3− ions. The cyclic voltammetry (CV) result of 5 mol% of Li substituted NiO (5-LNO) shows outstanding electrocatalytic activity towards the redox reaction of I3−/I− redox mediator among the as-prepared CEs. The DSSCs assembled with 5-LNO CE show an excellent power conversion efficiency (η) of 5.84 % with short-circuit current (Jsc) of 18.76 mAcm−2, and open-circuit voltage (Voc) of 0.80 V which is higher than Pt-based CE (η of 4.05 % with Jsc of 10.16 mAcm−2, and Voc of 0.69 V). The DSSCs fabricated with 5-LNO CE exhibit excellent photovoltaic performance because of their high active surface area and enhanced electrical conductivity. The 5-LNO CE has low cost, significant electrocatalytic activity, less toxicity, and superior device efficiency than NiO, other Li (1–3%)-NiO, and Pt CEs, making it a suitable candidate for Pt-free DSSCs application.

Photocatalytic application of nickel oxide/zinc oxide nanoparticles decorated kaolinite nanocomposite for the degradation of crystal violet

The photocatalysis process, which is effective, economical, and environmentally friendly can overcome rapidly increasing environmental problems, has shown rapid development in recent years. Nontoxic, low-cost natural minerals are often used in photocatalysis processes that remove organic pollution from wastewater. In this study, a selective method for the removal of crystal violet (CV) from aqueous solutions using nanocomposite of nickel oxide/zinc oxide nanoparticles (NiO/ZnO) supported on kaolinite photocatalyst was investigated. The raw kaolinite, bare NiO, bare ZnO and nanocomposite samples were characterized by the XRD, TEM, SEM-EDX, XPS and FTIR techniques. The photocatalytic performances of kaolinite-NiOZnO (K-NiO/ZnO) nanocomposite and bare samples were also investigated and comparatively evaluated by the photodegradation of CV (cationic dye) dye under visible light irradiation. The effects of various parameters such as the dosage of photocatalysis, contact time, and initial dye concentration on the rate of removal of dye were investigated and optimized. The kinetics of decolorization by samples can be well described by the pseudo-first-order model. Finally, it can be used in advanced oxidative processes for the degradation of organic pollutants.

Microporous α‐Fe2O3@NiO Heterostructures Synthesized for Enhanced Supercapacitor Performance

AbstractIn this present research, we synthesized pure α‐Fe2O3, NiO, and a microporous α‐Fe2O3@NiO heterostructure electrode via the spray pyrolysis method. The microporous α‐Fe2O3@NiO heterostructure electrode, with its highly porous structure, offers a larger electrode‐electrolyte interface, which results in improved electrochemical performance. Specifically, the microporous heterostructure exhibited a higher specific capacitance of 530.87 F/g at a current density of 7 mA/cm2 in a 1 M NaOH electrolyte, as compared to α‐Fe2O3 (187.12 F/g at 7 mA/cm2) and NiO (492.69 F/g at 7 mA/cm2). The microporous α‐Fe2O3@NiO heterostructures exhibited significantly enhanced electrochemical performance as compared to the individual α‐Fe2O3 and NiO samples, owing to the synergistic effect of the heterostructure. Furthermore, the microporous α‐Fe2O3@NiO heterostructure exhibited high specific capacitances and exceptional cycling stability for more than 2000 cycles, indicating their potential as excellent electrode for supercapacitor devices.

The influence of anionic surfactant (SDS) on the optical, magnetic and capacitive properties of NiO nanocrystals via gas-liquid diffusion synthesis

A facile gas–liquid diffusion route with magnetic stirring was used for the preparation of black fungus-like NiO nanostructure with the assistance of SDS surfactant. The NiO obtained without addition of surfactant was used in the control test. The structure, composition and morphology of NiO nanocrystals were characterized by various methods including XRD, Raman, SEM, HRTEM, BET and XPS techniques. The optical properties were analyzed with UV–Vis DRS. The contact angle measurement and vibrating sample magnetometer were used to investigate surface wettability and magnetic properties respectively. The electrochemical behavior of two NiO samples was studied by CV and GCD tests. The characterization and test results showed that compared with surfactant-free NiO sample, the prepared NiO sample using SDS surfactant as template agent exhibited specific morphology and higher surface area, better optical properties, magnetic susceptibility and electrochemical properties.

Synthesis and physical properties of pure NiO and Ni1–2xMgxMxO (M= Cu, Ru) nanoparticles: Role of growth temperature

Herein, low-cost chloride precursors, EDTA, and NaOH were employed through a facile coprecipitation synthesis route to prepare pure and codoped NiO nanoparticles: Ni1–2xMgxMxO, where M = Cu and Ru and x = 1, 4, and 8 at%. To examine the impact of reaction temperature on the thermal, structural, morphological, optical, and magnetic properties of the prepared samples, the precursor aqueous solutions were heated at 60, 70, and 80 ºC. Data analysis revealed the strong dependence of all the studied properties on reaction temperature and codoping. Thermogravimetric analysis (TGA) was performed to evaluate the thermal stability of the prepared samples. The face-centered cubic (fcc) crystallization of NiO was maintained at all the applied temperatures and doping concentrations. X-ray diffraction (XRD) patterns showed the good incorporation of Mg2+ and Cu2+ ions with the host Ni2+ ions in Ni1–2xMgxCuxO samples, while peaks of RuO2 appeared as an impurity phase in Ni1–2xMgxRuxO samples. Tiny spherical nanoparticles with a crystallite size of 6 nm were produced at 80 ºC and 8 at% (Mg, Ru) codoping. Energy dispersive X-ray (EDX) analysis confirmed that the elemental compositions of the synthesized samples were in stoichiometric ratios. The UV-Vis spectra demonstrated a monotonic increase in band gap with reaction temperature and codoping. The estimated Herve-Vandamme refractive index (nHV), the high (ε∞) and the static (εo) dielectric constants showed a decreasing trend with increasing reaction temperature. Weak ferromagnetic behavior was observed for the pure NiO samples, while the codoped samples with x > 4% showed linear M-H curves. The coercivety (Hc) decreased with reaction temperature, implying single-domain magnetization and sizes below the critical one. The magnetocrystalline anisotropy (K) indicated the softness of the prepared samples.

Hydrothermal synthesis of highly dispersed Bi–Ni–Sb–O pyrochlore for catalytic oxidation of carbon monoxide

The highly dispersed Bi–Ni–Sb–O pyrochlore was prepared under hydrothermal conditions with and without microwave treatment. Composite samples of NiO/(Bi–Ni–Sb–O pyrochlore) demonstrated consistently high activity without pretreatment in CO oxidation

Synthesis, Structural, Optical and Electrical Properties of Pure and Doped NiO Nanostructures Prepared via The Co-precipitation Method

Here, we report the synthesized high purity NiO nanostructures using simple and inexpensive chemical route. NiO NPs and (Cu, Zn) single and dual doped NiO NPs prepared by co-precipitation technique and annealed at 350 ℃ for 2 hours. Structural properties of prepared samples were investigated by X-ray diffraction (XRD). The optical characterizations and electrical conductivity were characterized by UV-vis spectrophotometer and I-V measurement. The crystallite size for dual doped NiO samples was 7.9 ± 0.9 nm compared to pure NiO, which had 9.52 nm. These results are consistent with the ionic radii of the doped metals. The obtained values of the band gap energy increased from 3 eV for pure sample to higher values (3.14 ± 0.06 eV) for doped samples due to the quantum confinement effect. These results agreed with the DC conductivity of samples. The ionic conductivity σ_ionic was developed with dual doping. The characterization of the doped samples makes them good candidates for photoelectronic applications.

Synthesis, Electrochemistry, and Thermal Stability of High‐Energy Ball‐Milled Silicon‐based Alloy Anodes in Lithium‐Ion Batteries**

AbstractThe fast capacity degradation of silicon‐based anodes significantly limits the application in lithium‐ion battery (LIB) industries. Recently, Si−CuO composites have been reported as promising anodes in terms of being cost‐effective and technically feasible, but improved cycle stability is still desired. This work introduces a proper amount of NiO into the Si−CuO composites via a facile high‐energy ball‐milling method. The study reveals that compared to the binary Si‐CuO composites, Si−CuO−NiO samples have less pronounced volume change during the cycles due to the formation of rich‐Si NiSi2. More specifically, Si87.5(CuO)3.4(NiO)9.1 shows the highest 100‐cycle capacity retention of ∼86.9 % at 0.2 C with an average coulombic efficiency of ∼99.4 %. Moreover, the thermal stability investigation demonstrates that the temperature of 600 °C is suitable to coat a carbon layer on Si87.5(CuO)3.4(NiO)9.1, where the microstructure and the uniform element distribution produced in the milling process as well as the suppression to the cr‐Li3.75Si formation can be maintained to the maximum extent, thus with further enhanced electrochemical performance.

Effect of thermal annealing on electrochemical and morphological properties of printed NiO electrodes for flexible asymmetric micro-supercapacitor applications

We report the effects of thermal annealing on screen-printed NiO electrodes for asymmetric battery-type supercapacitors on flexible substrates. Ni(OH)2 was synthesized and annealed at 300 °C, 400 °C, 500 °C, 600 °C, 700 °C and 800 °C to obtain six morphologically diverse NiO samples. FE-SEM, XRD, BET, and four-probe measurements were used to investigate their morphology, crystallinity, surface area, and conductivity, respectively. Six symmetric supercapacitors were screen-printed on a PET substrate with 1 M H2SO4 electrolyte to probe their electrochemical properties. NiO annealed at 400 °C (NiO-400) demonstrated the highest areal capacitance of 19.54 mF/cm2 coupled with reduced charge-transfer resistance, consistent with the highly porous yet continuous structure observed via FE-SEM and BET surface analysis. With this, an inter-digital asymmetric supercapacitor with NiO-400 cathode and graphene anode was fabricated to demonstrate high areal capacitance of 26.42 mF/cm2, a wide potential window of 1.5 V (at 97.3% coulombic efficiency) and excellent flexibility with 96.4% retention (180°) after 1000 cycles.

Iron-doped nickel oxide as a potential biosensor of urea determination via voltammetry

Biosensor made from metal oxide is reported to be useful for sensing urea levels in the human body, NiO nanoparticles have successfully been utilized to make urea biosensors. Ni-based electrodes are favorable for making a low-cost urea biosensor. Fe-doped NiO nanoparticles were prepared by the sol-gel method. Different concentrations of Iron doping were done in NiO and its sensing response was measured. XRD characterization was performed to determine the material's structure, phase purity, and crystallinity. Also, this characterization showed that the NiO compound is pure and single phase with a cubic structure. CV, time response FTIR, and UV–Visible spectroscopy were performed on the pure and Fe-doped NiO samples. The CV curve of a Fe-doped NiO-modified electrode in the presence of urea showed an excellent electrochemical response. Furthermore, it was also observed that with the increase in Fe doping, the sensing response and absorption properties of the samples were enhanced.

Ferromagnetic Ni1-xVxO1-y Nano-Clusters for NO Detection at Room Temperature: A Case of Magnetic Field-Induced Chemiresistive Sensing.

Surface modulation of functional nanostructures is an efficient way of improving gas sensing properties in chemiresistive materials. However, synthesis methods employed so far in achieving desired performances are cumbersome and energy intensive. Moreover, nano-engineering-induced magnetic properties of these materials which are expected to enhance sensing responses have not been utilized until now in improving their interaction with target gases. In particular for gasses with paramagnetic nature such as NO or NO2, the inherent magnetic property of the chemiresistor might assist in enabling superior sensing performance. In this work, vanadium-doped NiO nano-clusters with ferromagnetic behavior at room temperature have been synthesized by a simple and effective combination of soft chemical routes and employed in efficient and selective detection of paramagnetic NO gas. While NiO is typically anti-ferromagnetic, the nanoscale engineering of NiO- and V-doped NiO samples have been found to tune the inherent anti-ferromagnetic behavior into room-temperature ferromagnetism. Surface modification in terms of formation of nano-clusters led to an increased Brunauer-Emmett-Teller surface area of ∼120 m2/g. The sample Ni0.636V0.364O has been observed to exhibit a selective and high response of ∼98% to 1 ppm NO at room temperature with fast response (14 s) and recovery (95 s). The improved sensing response of this sample compared to other doped NiO variants could be explained in terms of lower remnant magnetic moment of the sample accompanied with higher excess negative charge at the surface. The sensing response of this sample was increased by 30% in the presence of an external magnetic field of 280 gauss, highlighting the importance of magnetic ordering in chemiresistive gas sensing between the magnetic sensor material and target analyte. This material stands as a potential gas sensor with excellent NO detection properties.

Fabrication of a Tubular CuO/NiO Biomimetic Nanozyme with Synergistically Promoted Peroxidase-like Performance for Isoniazid Sensing.

Isoniazid is an antibiotic primarily used in clinical treatment of tuberculosis, but excessive usage can lead to serious consequences such as hepatotoxicity, neurotoxicity, and even coma and death. Therefore, it is critical to exploit a quick, facile, and acute way for isoniazid analysis. In this work, we have demonstrated an efficient electrospinning-carbonation-wet chemistry reaction-calcination process to fabricate CuO/NiO nanotubes (NTs) as a promising nanozyme for peroxidase (POD) mimicking. In virtue of the distinct tubular structure and synergy between CuO and NiO from the mechanisms of both electron transfer and hydroxyl radical generation, a remarkably improved catalytic activity is realized for the CuO/NiO NTs compared with bare CuO and NiO samples. According to the admirable POD-like property, a rapid colorimetric detection for isoniazid is accomplished with a detection limit of 0.4 μM (S/N = 3) and favorable selectivity. In addition, the sensing capability of isoniazid in a real sample is also investigated with satisfactory results. This work offers a novel tactic to fabricate high-performance nanozymes with efficient isoniazid sensing capabilities to address challenges in disease treatment efficacy and public safety monitoring.

Enhanced non-radical activation of persulfate with pompon-like NiO microspheres for removing sulfamethoxazole in water.

The development of efficient heterogeneous catalyst for non-radical activation of persulfate (PS) is highly desired for removing organic pollutants in water. Herein, four NiO samples were prepared by different methods, and their performance for PS activation was investigated using sulfamethoxazole (SMX) as the target pollutant. The structure, surface chemical state, and redox ability of these samples were measured by various characterization techniques, and the key property affecting PS activation efficiency was explored. The results showed that the degradation of SMX by these samples all followed the non-radical mechanism, and the activated PS was the dominant active species. Among them, pompon-like NiO microspheres exhibited the highest activity due to its large surface area and especially high oxidation ability. Catalyst with high oxidation ability or reducing ability should facilitate the non-radical or radical activation of PS, respectively. SMX was completely removed by pompon-like NiO microspheres within 10min, and the reaction rate constant was calculated to be 0.4199min-1. An adsorption-degradation experiment was designed to verify the high stability and oxidation potential of the adsorbed PS on NiO surface. Pompon-like NiO microspheres exhibited good reusability, and its performance was barely affected by water quality, demonstrating its potential application in water treatment.

Light Absorption and Emission by Defects in Doped Nickel Oxide

Nickel oxide is a versatile p‐type semiconducting oxide with many applications in optoelectronic devices, but high doping concentrations are often required to achieve necessary electrical conductivity. In contrast to many other transparent oxide semiconductors, even moderate doping levels in NiO can lead to significant optical absorption in the visible spectral range, limiting the application range of the material. This correlation has been reported extensively in the literature, but its origin has been unknown until now. This work combines experimental data on optical properties from a variety of NiO samples with results from hybrid density functional theory calculations. It shows that strong electron–phonon interaction leads to a significant blueshift (0.6–1 eV) of electronic transitions from the valence band maximum to defect states by light absorption with respect to the thermodynamic charge transition levels. This essentially renders NiO a narrow‐gap semiconductor by defect band formation already at moderate doping levels, with strong light absorption for photon energies of approximately 1 eV. The calculations are also shown to be fully consistent with experimental data on defect‐related light emission in NiO.

Enhanced electrochemical performance of facilely synthesized cobalt doped cubic NiO nanoflakes for supercapacitor application

Being inexpensive, nontoxic and environmental friendly, NiO has caught special attention as a promising electrode material for supercapacitors. We recently reported facile synthesis of porous NiO nanocrystallites for energy storage application. A sophisticated strategy to improve the electrochemical properties of NiO which is a p-type semiconductor is the incorporation of 3d metal ions into it. In this article, we present successful fabrication of highly porous cobalt doped NiO nanostructures through a facile, low cost, one step solution combustion method without a post calcination treatment and demonstrate the influence of cobalt content on structural properties and electrochemical behavior. The thus prepared Co doped NiO samples were systematically characterized by X-ray diffraction, Fourier transform infrared spectroscopy, Field emission scanning electron microscopy, High resolution transmission electron microscopy, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller measurements, cyclic voltammetry, chronopotentiometry and electrochemical impedance spectroscopy. It was found that the phase structure of pristine NiO remains unaffected by the process of doping. But doping obviously influences the textural and morphological properties, more importantly, enhances the electrochemical properties of pristine NiO. A maximum specific capacity of 746 C/g (specific capacitance of 1244 F/g) was obtained for an optimum cobalt dopant ratio, which is superior to undoped NiO (295 C/g). Moreover, cobalt doped NiO nanostructures exhibited a good rate capability (71 % capacity retention from 1 to 15 A/g) and long term cycling stability (83.7 % after 5000 cycles at 100 mV/s). An asymmetric device fabricated using Co doped NiO electrode as the positive electrode and activated carbon as the negative electrode delivered an energy density of 32.7 Wh/kg at a power density of 261 W/kg.

High frequency resistive switching behavior of amorphous TiO2 and NiO

Resistive switching (RS) of Transition Metal Oxides (TMOs) has become not only an attractive choice for the development of next generation non-volatile memory, but also as a suitable family of materials capable of supporting high-frequency and high-speed switching needed for the next generation wireless communication technologies, such as 6G. The exact mechanism of RS is not yet clearly understood; however, it is widely accepted to be related to the formation and rupture of sub-stoichiometric conductive filaments (Magnéli phases) of the respective oxides upon activation. Here, we examine the switching behaviour of amorphous TiO2 and NiO both under the DC regime and in the high frequency mode. We show that the DC resistance of amorphous TiO2 is invariant of the length of the active region. In contrast, the resistance of the NiO samples exhibits a strong dependence on the length, and its DC resistance reduces as the length is increased. We further show that the high frequency switching characteristics of TiO2, reflected in insertion losses in the ON state and isolation in the OFF state, are far superior to those of NiO. Fundamental inferences stem from these findings, which not only enrich our understanding of the mechanism of conduction in binary/multinary oxides but are essential for the enablement of widespread use of binary/multinary oxides in emerging non-volatile memory and 6G mm-wave applications. As an example of a possible application supported by TMOs, is a Reflective-Type Variable Attenuator (RTVA), shown here. It is designed to operate at a centre frequency of 15 GHz. The results indicate that it has a dynamic range of no less than 18 dB with a maximum insertion loss of 2.1 dB.