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•rGO-ZnO1-x nanosheet composites have been successfully in-situ synthesized.•rGO-ZnO1-x shows good response to ppb NO2 illuminated by white light at room temperature.•rGO-ZnO1-x ...exhibits much higher sensitivity, faster response and recovery than ZnO.•Molecular Dynamics and Langmuir adsorption model are used to build the sensing mechanism.
Recently, room-temperature gas sensors have become very attractive due to the fact that they can be operated without heating, and thus simplifying the sensor design, reducing the fabrication cost, decreasing the power consumption and increasing the long-term stability. In this study, we propose a facile one-step hydrothermal method to prepare a composite of reduced graphene (rGO)/oxygen-deficient zinc oxide (ZnO1-x), which exhibits obvious room-temperature gas sensing response. X-ray diffraction, Raman and X-ray photoelectron spectroscopy demonstrate that the rGO@ZnO1-x composite is successfully synthesized and large numbers of dual donor defects, oxygen vacancy and zinc interstitial, are introduced into the composite. Field-emission scanning electron microscopy and transmission electron microscopy results reveal that many nanoscale p-n junctions are in-situ formed between ZnO nanosheets and rGO sheets. UV–vis spectra show that the light absorption of the rGO@ZnO1-x composite is red-shifted and extended to the whole visible light region in comparison. The fraction of rGO in the composites plays an important role in the sensing performance. 2.0% is the optimal proportion to obtain the best sensing properties in terms of sensitivity, response and recovery times. The rGO@ZnO1-x composite exhibits significant responses to ppb-level NO2 with white LED light stimulation at room temperature. The enhanced sensing properties can be attributed to three factors: light activation, synergistic effects between rGO and ZnO1-x, and high concentration in donor defects. Molecular Dynamics (MD) is used to quantitively simulate the adsorption process, and the results show that the incorporation of rGO decreases the adsorption energy of NO2. It means that more NO2 species would adsorb on the rGO@ZnO1-x composites, which greatly improves the sensitivity. A new gas sensing mechanism based on the MD calculated results and Langmuir adsorption model is used to explain the reason that the rGO@ZnO1-x composites have a much faster response and recovery process. In addition, the rGO@ZnO1-x sensor shows a weaker response to other interference gases.
Plasma electrolytic oxidation (PEO) is a versatile and cost-effective technique to obtain protective oxide coatings in light metals, although its intrinsic porosity is a drawback for long-term ...corrosion resistance. Sol-gel layers are pointed as a useful tool to seal PEO porosity and increase its corrosion resistance, although little is known about its influence on the wear performance of PEO coatings. In this study, a PEO coating obtained on AA2024 was sealed with a hybrid sol-gel via dip-coating. Two withdrawal speeds were set in order to investigate the influence of the sol-gel filling in its sealing ability. Both sol-gel application conditions were able to fill the PEO pores and microcracks and change its wettability. EIS results showed samples sealed with sol-gel maintained the protective behavior upon 28 days and increased the resistance of the system by several orders of magnitude in comparison to unsealed PEO coatings. Pin-on-disk tests indicate the sol-gel decrease the shear stresses of the coating, decreasing its wear rate in 40% compared to the unsealed PEO.
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•Hybrid sol-gel coating sealed pores and cracks from the plasma electrolytic oxidationcoating.•Different withdrawal speeds resulted in different sealing morphologies.•No significant difference in wear and corrosion rate was observed between the sol-gel sealed PEO coatings.•The sol-gel sealed PEO coating reduced the wear rate by 40 % compared to the unsealedPEO coating.•The sol-gel sealing protected the PEO coating against corrosion upon 28 days ofimmersion in corrosive media.
The effect of cerium salts on the cut-edge of zinc-based sacrificial coated steel was investigated by theoretical approach (based on individual polarization curves on the top surface and by Scanning ...Vibrating Electrochemical Technique (SVET). In both approaches, cerium salts increase the galvanic corrosion of zinc-based sacrificial layer coupled with steel. Furthermore, the anion parts of cerium salts exhibited a synergistic accelerated effect of galvanic activity on the cross-section. The SVET results indicated that the addition of Al, Mg alloying elements promotes localized corrosion and influences the formation and distribution of corrosion products.
•Effect of CeCl3 and Ce(NO3)3 on the cut-edge of steel coated by zinc alloyed (HDG, ZnAl, and ZAM).•Cerium salts accelerated the corrosion of zinc coated steel on the cut-edge.•NO3- ion exhibited a synergistic accelerating effect on galvanic corrosion.•The presence of Al and Mg alloying elements promotes localized corrosion.
In the present study, the surface of AZ31 Mg alloy was pretreated by etching in nitric acid and plasma electrolytic oxidation (PEO). Then Mg-Al layered double hydroxide (Mg-Al/LDH) was synthesized on ...the pretreated surface of substrates via hydrothermal treatment to increase the corrosion resistance. The effect of surface pretreatment (etched substrate and PEO layer) on LDH formation, structure, and corrosion resistance was investigated. Benzoxazine resin (thermosets), has been used for coatings, however, the high curing temperature limits their applications. For this purpose, a new bio-based benzoxazine resin with a lower curing temperature (160 °C) was developed to be applicable for Mg alloys. Phenol (Phloretic acid), diol (Dodecandiol), and amine (monoethanolamine) were used to synthesize innovative benzoxazine with exchangeable ester functions and self-healing ability. This resin was further used for post-treatment of LDH samples. The morphology, chemical composition, and crystalline structure of LDH samples were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The corrosion resistance of the coatings was investigated by salt spray test as well as electrochemical impedance spectroscopy (EIS). The LDH layer on the etched substrates showed better corrosion resistance than the LDH on PEO layer due to having a thicker inner layer (11.8 ± 0.2 μm and 7.0 ± 0.6 μm for LDH on the etched substrate and PEO layer, respectively). Post-treatment of samples with benzoxazine resin showed the effective capability of this polymeric coating for enhancing the corrosion resistance of Mg alloy substrate. This benzoxazine coating increased the corrosion resistance of the coating/substrate system around 104 times in comparison to the bare substrate. Moreover, it was observed that the LDH layer can increase the adhesion and compatibility of the substrate with benzoxazine resin. These results were confirmed by EIS and cross-cut adhesion tests experimentally as well as theoretical calculations of the adsorption energy of benzoxazine molecule to the LDH layer using Density Functional Theory (DFT).
•Forming LDH on pretreated AZ31 Mg alloy (PEO, etched) by hydrothermal method•Post-treatment of Mg alloy substrates with a new bio-based benzoxazine resin•Corrosion protection of benzoxazine for Mg alloy substrates•Enhanced benzoxazine adhesion to LDH, validated experimentally and theoretically (DFT)•Improved corrosion resistance achieved by LDH and benzoxazine for Mg alloy
In this study, the localized corrosion susceptibility of AA2050‐T84, AA2198‐T851, and AA2024‐T3 alloys was compared by immersion in three test solutions, specifically sodium chloride solution (3.5% ...NaCl), EXCO solution, according to ASTM G34‐18, and a solution for intergranular corrosion (IGC) test recommended by ASTM G110‐15 and via scanning vibrating electrode technique (SVET). The results showed higher susceptibility of AA2050‐T84 alloy to all types of corrosion tested compared to the other studied 2XXX series alloys. Corrosion penetration was deeper in the AA2050‐T84 due to its microstructural characteristics as a difference in the micrometric Cu‐Rich intermetallic particle distribution on the surface and related to the distribution of the T1 (Al2CuLi) phase.
Due to the intrinsic porosity of the layers formed by the plasma electrolytic oxidation (PEO) process, the application of silane-based coatings as an eco-friendly layer is a promising way to diminish ...penetration of the corrosive species into the PEO coating by pores sealing. In this study, to enhance the corrosion protection of AA2024, the performance of the duplex system achieved by adding different concentrations in sodium montmorillonite (Na-MMT) dispersed into the silane coating was assessed. In this study, a hybrid sol-gel layer (30% V/V) obtained from tetraethoxysilane (TEOS) and 3-glycidoxypropyltrimethoxysilane (GPTMS) sol-gel solution was applied on AA2024 previously covered by an optimized PEO layer using a solution containing sodium silicate and potassium hydroxide solution as electrolyte. Electrochemical impedance spectroscopy (EIS) revealed the significant impact of the sol-gel/clay nanocomposite layer on the corrosion protection performance of the PEO layer on the AA2024 substrate in a 0.1 M NaCl solution. Regarding, the low-frequency impedance of different coating systems upon five weeks exposure to the aggressive solution reported the sealing ability of the sol-gel coatings in which the silane coating modified with 1000 ppm of clay nanoparticles had the maximum corrosion resistance among them (i.e., higher than 107 after 5 weeks immersion). The flake-like structure of sodium montmorillonite not only enhanced the barrier performance, but also FT-IR outcomes reflected the reticulation of the silane network through the interaction of nanoparticles with SiOH groups of the silane layer.
•Sol-gel/clay system improves sealing and protection of PEO coating on AA2024.•NaMMT nanoparticles interact with SiOH groups of the silane layer.•The clay concentration has a significant role on the corrosion performance.•1000 ppm of clay in sol-gel protect the PEO coating upon five weeks in 0.1 M NaCl.
Although extensive studies have been carried out on n-type semiconductors for room-temperature gas sensor applications, some intrinsic problems remain. Therefore, other interesting attempts should be ...adopted to solve these issues, like p-type semiconductors. Previous studies have demonstrated that p-type semiconductor gas sensors exhibit better selectivity and less humidity dependence due to the distinctive oxygen adsorption and surface reactivity. Visible light is used as the external activation source to accelerate the sensing kinetics instead of heating. Stoichiometric NiO cannot absorb visible lights. Inspired by the works of black TiO2, we adopted three methods to prepare black NiO. XPS characterizations reveal that the presence of Ni3+ ions leads to the formation of black NiO. However, not all black NiO samples show good responses to NO2 at room temperature. Three main routes: synthesizing specific morphology with large specific surface area and porosity, introduction of Ni3+ ions and oxygen vacancies, are needed to get the enhanced sensing performance. The black NiO samples with large specific surface area and oxygen vacancies and Ni3+ ions show obvious response towards ppb-level NO2 with visible light irradiation at room temperature. Furthermore, light wavelength is found to play a vital role in the sensing characteristics, and blue light is the optimal choice. Different from traditional NiO sensors operated at high temperatures exhibiting superior response to reducing gases, the black NiO show excellent selectivity towards oxidizing gas, ppb-level NO2, at room temperature illuminated by blue light. In contrast with n-type semiconductors, the black NiO samples also exhibit less humidity dependence.
•Black NiO samples were synthesized by three different routes by introducing Ni3+ ions.•The light absorption range of black NiO was extended to the whole visible light region.•Three main factors enhance the sensing performance of black NiO.•Visible-light-assisted black NiO shows excellent response to NO2 at room temperature.•Light wavelength plays a crucial role in sensing performance and blue light is optimal.
•Cerium modified montmorillonite (CeMMT) is an efficient corrosion inhibitor for AA2024.•60 wt.% of cerium ions are released in 0.1 M NaCl from modified nanoclays.•CeMMT improves the barrier ...properties of the doped sol–gel film applied on AA2024.•Cerium ions provide an active corrosion protection to the doped sol–gel film.
This work presents a new approach to develop protective pre-treatment for AA2024 based on the inhibitor efficiency of cerium ions which are considered as a promising alternative to replace chromate compounds. The inhibition properties of cerium salts for improving the anti-corrosion performance of AA2024 have been characterized by polarization curves and EIS measurements. Cerium ions were inserted into nanoclays platelets by cationic-exchange reactions (CeMMT). XRD analysis proved the presence of cerium ions intercalated into clay structure. EIS measurements and polarization curves highlighted a high corrosion inhibition effect of cerium-modified nanoclays. This effectiveness can be related to the release of 60% of cerium ions from CeMMT structure in sodium chloride as determined by UV–vis measurements. Salt spray test made on scratched sol–gel samples indicated a self-healing effect of cerium ions which provide an active corrosion protection to aluminum substrate. EIS measurements revealed that hybrid sol–gel films doped with CeMMT improve barrier properties and anticorrosion protection of aluminum 2024 substrate.
The inherent porous structure of PEO coatings is regarded as a drawback for long-term protection. In this investigation, the presence of pores and defects in the PEO coating was exploited as ...reservoirs for corrosion inhibitors to generate self-healing properties on AA2024. Smart coating systems were fabricated on the PEO layer in which 8-hydroxyquinoline (8-HQ) and benzotriazole (BTA) were employed as corrosion inhibitive layers followed by a sol-gel sealing. The protective performance relies on the mechanical interlocking between the sol-gel coating and the sublayer plus the sealing ability of the sol-gel in the presence of the intermediate layer.
•The synergistic inhibitive behavior in 0.1 M NaCl was found for 8-HQ and BTA with a ratio of 7/3.•The impregnation of the PEO layer by inhibitors reduced its porosity.•The optimal concentration to form the inhibitive layer was 5 g/L.•Mechanical interlocking and self-healing properties in the PEO/intermediate/sol-gel coating system were evidenced.•High corrosion protection performance was obtained for the optimized system till five weeks of immersion in 0.1 M NaCl.
In this paper, the effects of visible-light illumination on sensing properties of tungsten trioxide (WO3) sensors have been investigated. WO3 sensitive films are prepared on alumina substrates by ...screen-printing. WO3 sensors illuminated by visible/ultra-violet lights are submitted to sensing tests with 160 and 320ppb nitrogen dioxide (NO2) at room temperature. NO2 sensing characteristics of the WO3 sensor are significantly improved by illumination as compared to that in the dark. Influences of light wavelength and light intensity on the sensing characteristics are further studied. The results show that visible light can effectively activate the WO3 sensors and blue light is a good choice to produce room-temperature devices which are suited for NO2 sensing applications. A sensing mechanism is proposed according to a simple adsorption–photodesorption model.
