TiO2 Passivation Layer Research Articles

TiO2 Passivated Zno Nanoarray Layer Based Fluoroalkylsilane Film for Photovoltaic Optical Glass: Achieving UV Shielding, Acid Rain Resistance, and Self‐Cleaning Properties

AbstractPhotovoltaic technology is a prominent source of renewable energy, but maintenance costs and efficiency attenuation of large photovoltaic devices are significant issues due to their vast energy conversion area. To reduce costs and facilitate maintenance, superhydrophobic surfaces with self‐cleaning properties have been developed for photovoltaic glass. In this study, transparent ZnO nanoarrays (NAs) are synthesized on photovoltaic glass, with Eu3+ doping enhancing the ultraviolet radiation resistance of photovoltaic devices and slightly increasing visible transmittance. A TiO2 passivation layer is introduced on the ZnO NAs surface to enhance acid resistance and mitigate corrosion caused by acidic rainwater. Fluoroalkylsilane (POTS) modification achieves superhydrophobicity with a water contact angle of 160.25°, as demonstrated by droplet rolling experiments. Micro‐interaction of superhydrophobic properties is further investigated by force curve measurement using atomic force microscope, showing almost no nanometer water moisture on the superhydrophobic surface, but conspicuous water moisture on the control glass surface. Finally, simulated models and practical silicon crystal solar cells fabricated using the self‐cleaning glass show excellent acid rain resistance, self‐cleaning, and long service life properties, with no power conversion efficiency degeneration during a 40‐day outdoor application test.

Improving corrosion resistance of pure titanium by simple and controlled oxidation for biomedical applications

AbstractIn this study, the TiO2 passive layer was formed on a pure titanium substrate by simple oxidation in an aqueous solution of H2O2 35 wt.%. In order to determine the appropriate time for the oxidation of the pure titanium substrate, the corrosion rate of the metal in the solution of H2O2 was investigated. The electrochemical behaviour of the TiO2 passive layer on the Ti substrate was investigated in Hank's solution at 37.5°C. The results show that Tafel‐plot extrapolations from the potentiodynamic curves revealed a substantial improvement in the corrosion potentials for the TiO2 coatings. According to the theory calculation, to obtain the optimal surface roughness values, around 100 nm to 300 nm, the oxidation time in H2O2 35 wt.% solution is controlled in the range of 2‐5 hours. The results were explained by considering the effects of crystal morphology and roughness. The results indicated that the controlled oxidation‐prepared TiO2 coatings have attractive prospects for use in biomedical applications since better corrosion protection of pure Ti can be expected.

Fabrication of Cu oxide/TiO2 p–n nanojunctions by stress-induced migration

Many Cu oxide/TiO2 p–n nanojunctions were fabricated by introducing the concept of using n-type TiO2 layers to the fabrication method of p-type Cu oxide nanowires by stress-induced migration. Cu oxide/TiO2 nanojunctions were formed along with nanowire growth by heating a Cu thin film with TiO2 passivation layers of different thicknesses. The presence of p–n nanojunctions at the Cu oxide/TiO2 interface was analyzed by current–voltage and electrochemical impedance measurements. The sheet resistance of the samples decreased as the TiO2 thickness increased from 0 to 20 nm and then increased with increasing TiO2 thickness. The shapes of Nyquist plots consisted of two semicircles, one distorted semicircle, and two distorted semicircles for samples without a TiO2 layer, with 2 or 20 nm TiO2 layer, and with 100 or 200 nm TiO2 layer, respectively. The TiO2 thickness dependence of these electrical characteristics suggests that electrical conduction in the sample heated with a TiO2 passivation layer was through the Cu oxide grain surface, Cu oxide/TiO2 interface, and Cu oxide nanowires. In the equivalent circuit representing this electrical conduction, the capacitance values at the Cu oxide/TiO2 interface decreased with increasing TiO2 thickness. These results suggest that a thicker TiO2 passivation layer led to the formation of more p–n nanojunctions at the Cu oxide/TiO2 interface.

Corrosion behavior of in situ-synthesized (TiC+TiB)/Ti composites by laser powder-bed fusion: Role of scan strategy

The in-situ TiC and TiB reinforced titanium matrix composites were fabricated by laser powder fusion (LPBF) of B4C/Ti powder mixture, and the influence of scan strategy on the microstructure morphologies and corrosion resistance in NaCl solution were also investigated. The experimental results reveal that anodic dissolution of titanium matrix is accelerated and thus facilitates the formation of the TiO2 passive layer on the surface of the LPBF-produced (TiC+TiB)/Ti composites owing to the in-situ TiC and TiB reinforcements acting as micro-cathode in NaCl aqueous solution. The island scan strategy tends to result in a net-shaped distribution of TiC and TiB reinforcements and pore defects, which causes the inhomogeneity of galvanic couples and resultant TiO2 passive layer. Fortunately, the scan strategies of XY+ 30° and XY+ 67° can alleviate the thermal accumulation and leads to the transformation of refinements distribution to radial-shaped arrangement, which favors for the homogeneity of galvanic couples and resultant TiO2 passive layer. Moreover, the scan strategy of XY+ 90° generates a refinement and increased volume fraction of reinforcements and micro-galvanic couples, which significantly facilitates the formation of a thicker and less-defective TiO2 passive layer, thus leading to better corrosion resistance with a larger value of Rf. Furthermore, the corrosion mechanism of LPBF-produced (TiC+TiB)/Ti composites has also been discussed in detail based on the Electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy results.

Atomic layer deposition technique refining oxygen vacancies in TiO2 passivation layer for photoelectrochemical ammonia synthesis

Photoelectrochemically (PEC) converting environmental pollutant nitrate back to valuable ammonia is a powerful complement to NH3 synthesis from Haber-Bosch process. Herein, atomic layer deposition (ALD) technique is employed to refine oxygen defect of TiO2 passivation layer on p-type silicon nanowire arrays to introduce active sites and improve the stability of silicon. Si@TiO2-50-Vo nanowire arrays photocathode delivers a greatly enhanced NH3 yield rate of 1074 μgNH3 h−1 cm−2 and Faradaic efficiency (FE) of 94.3% at −0.6 V, as well as with excellent cycling stability. Theoretical calculations indicate that oxygen defect in passivation layer promotes the adsorption of nitrate and lowers the energy barrier of primary steps in NTRR.

ZnFe2 O4 Dendrite/SnO2 Helix 3D Hetero-Structure Photoanodes for Enhanced Photoelectrochemical Water Splitting: Triple Functions of SnO2 Nanohelix.

An array of SnO2 nanohelix structures is employed to fabricate a SnO2 helix@ZnFe2 O4 dendrite core-shell 3D heterostructure photoanode for photoelectrochemical (PEC) water splitting. The SnO2 helix provides triple critical functions to enhance the PEC performance of the photoanode. First, it scatters the incident light to achieve a higher light harvesting efficiency. Second, it provides a facile electron pathway as an electron transfer layer (ETL) while blocking hole transport to mitigate charge recombination in the bulk of ZnFe2 O4 . Finally, it becomes a template for the formation of ZnFe2 O4 dendrite nanostructure shell. The ZnFe2 O4 dendrite/SnO2 helix photoanode exhibits a remarkable increase in incident photon-to-electron conversion efficiency compared to unmodified ZnFe2 O4 with no ETL and modified one with "flat" SnO2 ETL. The surface of the ZnFe2 O4 /SnO2 helix photoanode is further modified with TiO2 passivation layer and NiFeOx oxygen evolution co-catalyst to achieve one of the best PEC performances among reported ZnFe2 O4 -based photoanodes.

Part I.: Friction stir welding of equiatomic nickel titanium shape memory alloy – microstructure, mechanical and corrosion behavior

In this study, friction stir welding (FSW) process is utilized to join equiatomic nickel-titanium (NiTi) shape memory alloys as a solid-state welding alternative to fusion welding. In the studied range of process parameters (spindle speeds – 350 – 450 rpm and constant translation speed 75 mm/min), combination of 400 rpm spindle speed and 75 mm/min tool translation speed produced a defect-free joint without any discernable voids. The weld microstructure consisted of a typical stir zone with fine grains; a rather small thermo-mechanically affected zone and heat affected zone. Vickers microhardness measurements after appropriate thermal cycling, showed uniform hardness across the weld zone, indicating a joint free of detrimental precipitates. The transformation temperatures measured by differential scanning calorimetry, showed little to no change across the weld zones. Under tensile loading, at ambient temperature a maximum joint efficiency of 93% was obtained in the martensitic phase while at elevated temperature a joint efficiency of 84% was obtained in the austenitic phase. The welded specimen showed martensitic detwinning plateau, characteristic of SMAs tested in martensitic phase, indicating that martensitic detwinning was the dominant mode of deformation at low strains, even within the weld zone. The weld zone showed comparatively lower corrosion resistance than the base material, owing to less dense TiO2 passive layer formation. Galvanic effect between the weld zone and base materials was negligible, indicating good compatibility between the different weld zones. Our results show that friction stir welding can be successfully applied for joining NiTi SMAs sheets.

A multiparametric study on the dissolution of synthetic brannerite

Brannerite, UTi2O6 is reported to occur in various uraniferous deposits worldwide. Natural brannerite specimens are found in the amorphous state and are usually considered to be refractory to dissolution due to the formation of TiO2 passivation layer. In the present work, brannerite was synthesized by wet chemistry route, then characterized prior the development of multiparametric dissolution experiments. The evolution of U and Ti concentrations was followed in 0.1–2 mol/L H2SO4 solutions, for temperatures ranging from 25 to 80 °C, in the presence (or not) of 2.8 g/L of dissolved Fe(III). The dissolution of synthetic brannerite was congruent in the whole experimental domain. The formation of Ti-enriched secondary phase at the surface of the brannerite grains was not evidenced. The dissolution rate constants, activation energies and partial orders of the overall dissolution reaction relative to proton activity were determined in the presence (or absence) of Fe(III). The introduction of Fe(III) in sulfuric acid solutions increased the dissolution rate constant by 5 orders of magnitude and induced significant modifications of the apparent activation energy (from 71 ± 4 to 91 ± 6 kJ/mol) and of the partial order relative to proton activity (from 0.42 ± 0.09 to 0.84 ± 0.08). This study suggested that the uncongruency of the brannerite dissolution and the changes usually observed in the rate-controlling step with temperature could be linked to the loss of the crystal structure in natural samples.

High-performance normally off p-GaN gate high-electron-mobility transistor with In0.17Al0.83N barrier layer design

This paper proposes a novel normally-off p-GaN gate InAlN/GaN HEMT to replace p-GaN gate AlGaN/GaN HEMT for improving the device stability, enhancing saturation current, reducing the on-state resistance, improving the cut off frequency and decreasing the subthreshold swing. Due to the higher two-dimensional electron gas induced by the polarization, the on-state resistance for p-GaN gate InAlN HEMT can be lowered by 23% while the on-state current can be enhanced by 161% comparing with the common p-GaN gate AlGaN HEMT at Vgs = 5 V. The cut off frequency of InAlN HEMT is 11.5 GHz, which is 39% higher than that of AlGaN HEMT. While the low breakdown voltage (BV) of InAlN HEMT restricts the device performance and power application. The thickness and permittivity of the passivation layer are found to influence the BV of the device significantly, due to the alleviation of peak electric field at the drain-side gate edge, leading to a much smoother distribution of electric field in the channel. A proposed structure with TiO2 passivation layer exhibits a breakdown of 1028 V, showing 25% increase in breakdown voltage compared to the TiO2 passivation layer AlGaN HEMT. These simulation results revealed that the p-GaN gate InAlN/GaN HEMT is a reliable candidate in power application.

Dendritic CuBi2O4 Array Photocathode Coated with Conformal TiO2 Protection Layer for Efficient and Stable Photoelectrochemical Hydrogen Evolution Reaction

Short carrier diffusion length and poor photostability are two major bottlenecks limiting the application of CuBi2O4 as a competitive candidate for photoelectrochemical (PEC) hydrogen evolution reaction (HER). To overcome the bottlenecks, we develop a novel template-assisted synthesis strategy to prepare the CuBi2O4 dendrites and then protect the dendritic structure with a conformal nanometer-thick TiO2 passivation layer. The obtained CuBi2O4/TiO2 composite dendrites exhibit high activity for the PEC HER with a photocurrent density of ca. 0.90 mA cm–2 at 0.200 V vs reversible hydrogen electrode (RHE) in neutral solution, and this value is among the highest reported on CuBi2O4-based photocathodes at the same potential. Moreover, the CuBi2O4/TiO2 dendrites show a high photostability with the photocurrent density being stabilized at ca. 0.36 mA cm–2 (88.8% of its original value) after 6 h measurements at a high applied potential of 0.600 V vs RHE, which is superior to all CuBi2O4-based photocathodes reported recently in the literature. This work provides an effective strategy to improve the activity and stability of the copper-oxide-based materials for the HER via constructing a dendritic structure and protecting the structure with a nanoscale TiO2 passivation layer.

Enhancing the plasmonic component of photonic–plasmonic resonances in self-assembled dielectric spheres on Ag

The plasmonic component of photonic–plasmonic hybrid resonances from a dielectric sphere array on Ag is substantially strengthened by introducing a thin Ag coating and TiO2 passivating layer above and below the PS sphere array, respectively.

Large-Signal Linearity and High-Frequency Noise of Passivated AlGaN/GaN High-Electron Mobility Transistors.

This study proposes AlGaN/GaN/silicon high-electron mobility transistors (HEMTs) grown by a metallorganic chemical vapor deposition (MOCVD) system. The large-signal linearity and high-frequency noise of HEMTs without and with different passivation layers are compared. The experimental data show that the addition of a TiO2 passivation layer to undoped AlGaN/GaN HEMT’s increases the value of the third-order intercept point (OIP3) by up to 70% at 2.4 GHz. Furthermore, the minimum noise figure (NFmin) of the HEMT with TiO2 passivation is significantly reduced.

Effects of substrate preparation on TiO2 morphology and topography during anodization of biomedical Ti6Al4V

TiO2 coatings were fabricated by anodizing unpolished and polished Ti6Al4V substrates in 1–3 M H2SO4 for 10 and 20 min at 120 V. Anatase was the principal polymorph and oxygen dissolution in the Ti6Al4V substrates caused αTi lattice contraction. Dielectric breakdown and associated heating of the insulating TiO2 promoted trace rutile formation. The surface preparation of the Ti6Al4V substrates was the principal factor that determined not only the grain and pore sizes but also the morphology and topography of the coating. Consequently, the presence of the amorphous TiO2 passivating layer (unpolished) resulted in thicker and more irregular coatings than when anodization was done on the bare metal surface (polished). Large-scale delamination steps were observed for the coatings on the unpolished substrates while localized delaminations occurred for the coatings on polished substrates; the amounts of both increased with increasing anodization time. 3D laser scanning confocal microscopy allowed distinction between the fine-scale morphology and the coarse-scale topography. These data showed that increasing time and acid concentration resulted in similar trends of increasing roughness (morphology) and unevenness (topography). The present work focusses on the effect of initial substrate surface characteristics and its modification to improve the quality of the coatings formed by anodization of titanium biomedical implants.

TiO2 Passivation Layer on ZnO Hollow Microspheres for Quantum Dots Sensitized Solar Cells with Improved Light Harvesting and Electron Collection.

Light harvesting and electron recombination are essential factors that influence photovoltaic performance of quantum dots sensitized solar cells (QDSSCs). ZnO hollow microspheres (HMS) as architectures in QDSSCs are beneficial in improving light scattering, facilitating the enhancement of light harvesting efficiency. However, this advantage is greatly weakened by defects located at the surface of ZnO HMS. Therefore, we prepared a composite hollow microsphere structure consisting of ZnO HMS coated by TiO2 layer that is obtained by immersing ZnO HMS architectures in TiCl4 aqueous solution. This TiO2-passivated ZnO HMS architecture is designed to yield good light harvesting, reduced charge recombination, and longer electron lifetime. As a result, the power conversion efficiency (PCE) of QDSSC reaches to 3.16% with an optimal thickness of TiO2 passivation layer, which is much higher when compared to 1.54% for QDSSC based on bare ZnO HMS.

Photoelectrochemical hydrogen production at neutral pH phosphate buffer solution using TiO2 passivated InAs Nanowire/p-Si heterostructure photocathode

Abstract Formation of heterojunctions with other light absorbing semiconductors is one of the attractive approaches to realize efficient silicon-based solar water splitting. As one of the promising candidates for heterojunction with silicon, InAs compound semiconductor has two major advantages of narrow bandgap and high electron mobility. However, the integration of Si with InAs is challenging due to several physical issues. Here, we successfully demonstrate the integration of InAs nanowires (NWs) and p-Si via a catalyst-free nanowire growth method. The presence of InAs NWs effectively improved the photoelectrochemical activities of the Si photocathode in neutral pH condition. The longer NWs are more advantageous for the proton reduction at the photoelectrode/electrolyte interface, since the InAs NWs act like a highway for photogenerated electrons due to their excellent electron mobility. The TiO2 layer grown by atomic layer deposition (ALD) effectively passivated the vulnerable InAs NWs and Pt co-catalyst prevented the photoelectron accumulation on the TiO2 surface, suppressing the self-reduction of TiO2 passivation layer to ensure device stability over 20 h. The Pt/TiO2/InAs NWs/p-Si photocathode exhibited an onset potential shift of 0.91 V (versus reversible hydrogen electrode) compared to bare p-Si and a photocurrent density as high as 8.6 mA/cm2 at 0 V vs. RHE. Furthermore, we precisely analyzed the heterojunction band diagram of the Pt/TiO2/InAs NWs/p-Si photocathode to investigate the band offsets and electronic barriers occurring at each heterojunction interface.

Microstructure and electrochemical properties of IrO2+RhOx+ZrO2 coated titanium anodes

The electrodes of Ti/IrO2+RhOx + ZrO2 were prepared with varying proportions of RhOx using thermal decomposition treatment at 550 °C. The microstructure of these coatings was investigated by SEM and XRD methods. The electrochemical behavior of coatings was assessed through accelerated life test (ALT), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The results showed that increasing the RhOx content led to superior service lifetime and lower electrical conductivity of the coatings. Also, the results of cyclic voltammetry tests confirmed that the anodic voltammetric charge was enhanced by increasing the concentration of RhOx in coatings. The EIS examination of the coatings indicated that the charge transfer resistance at the external active surface of the coatings was increased by adding the RhOx content. Furthermore, it was found that the formation of TiO2 passive layer at the interface of the substrate/oxides layer was the main destruction mechanism of the coated anodes. Among the tested anodes, the anode with 10%IrO2-60%RhOx-30%ZrO2 coating exhibited larger electrochemically active surface area, higher stability and inferior electrocatalytic activity for oxygen evolution.

Surface Activation and Pretreatments for Biocompatible Metals and Alloys Used in Biomedical Applications.

To improve the biocompatibility of medical implants, a chemical composition of bone-like material (e.g., hydroxyapatite) can be deposited on the surface of various substrates. When hydroxyapatite is deposited on surfaces of orthopedic implants, several parameters must be addressed including the need of rapid bone ingrowth, high mechanical stability, corrosion resistance, biocompatibility, and osseointegration induction. However, the deposition process can fail due to poor adhesion of the hydroxyapatite coating to the metallic substrate. Increasing adhesion by enhancing chemical bonding and minimizing biocoating degradation can be achieved through surface activation and pretreatment techniques. Surface activation can increase the adhesion of the biocoating to implants, providing protection in the biological environment and restricting the leaching of metal ions in vivo. This review covers the main surface activation and pretreatment techniques for substrates such as titanium and its alloys, stainless steel, magnesium alloys, and CoCrMo alloys. Alkaline, acidic, and anodizing techniques and their effects on bioapatite deposition are discussed for each of the substrates. Other chemical treatment and combination techniques are covered when used for certain materials. For titanium, the surface pretreatments improve the thickness of the TiO2 passive layer, improving adhesion and bonding of the hydroxyapatite coating. To reduce corrosion and wear rates on the surface of stainless steel, different surface modifications enhance the bonding between the bioapatite coatings and the substrate. The use of surface modifications also improves the morphology of hydroxyapatite coatings on magnesium surfaces and limits the concentration of magnesium ions released into the body. Surface treatment of CoCrMo alloys also decreased the concentration of harmful ions released in vivo. The literature covered in this review is for pretreated surfaces which then undergo deposition of hydroxyapatite using electrodeposition or other wet deposition techniques and mainly limited to the years 2000-2019.

Determinants of corrosion resistance of Ti-6Al-4V alloy dental implants in an In Vitro model of peri-implant inflammation

BackgroundTitanium (Ti) and its alloys possess high biocompatibility and corrosion resistance due to Ti ability to form a passive oxide film, i.e. TiO2, immediately after contact with oxygen. This passive layer is considered stable during function in the oral cavity, however, emerging information associate inflammatory peri-implantitis to vast increases in Ti corrosion products around diseased implants as compared to healthy ones. Thus, it is imperative to identify which factors in the peri-implant micro-environment may reduce Ti corrosion resistance.MethodsThe aim of this work is to simulate peri-implant inflammatory conditions in vitro to determine which factors affect corrosion susceptibility of Ti-6Al-4V dental implants. The effects of hydrogen peroxide (surrogate for reactive oxygen species, ROS, found during inflammation), albumin (a protein typical of physiological fluids), deaeration (to simulate reduced pO2 conditions during inflammation), in an acidic environment (pH 3), which is typical of inflammation condition, were investigated. Corrosion resistance of Ti-6Al-4V clinically-relevant acid etched surfaces was investigated by electrochemical techniques: Open Circuit Potential; Electrochemical Impedance Spectroscopy; and Anodic Polarization.ResultsElectrochemical tests confirmed that most aggressive conditions to the Ti-6Al-4V alloy were those typical of occluded cells, i.e. oxidizing conditions (H2O2), in the presence of protein and deaeration of the physiological medium.ConclusionsOur results provide evidence that titanium’s corrosion resistance can be reduced by intense inflammatory conditions. This observation indicates that the micro-environment to which the implant is exposed during peri-implant inflammation is highly aggressive and may lead to TiO2 passive layer attack. Further investigation of the effect of these aggressive conditions on titanium dissolution is warranted.

Quantification of Electronic Activity Inside Photo-Activated TiO2 Layers through a New Electrical Model Supported by Electrokinetic Data

Surface electrical characterization of metals and semiconductors with extensive geometry continues to be of interest. This work explores the electrical information that can be extracted from the electrokinetic (zeta) potential of metallic surfaces, and proposes an analytical model based on streaming potential/current data. In particular, our study focuses on the alloy Ti6Al4V, which is capable of modifying its surface electrical properties after excitation with ultraviolet irradiation due to the semiconductor properties of its TiO2 passivation layer. Evidence is presented for the first time of electronic activity inside the passivation layer following UV excitation. Charge displaced through sample (Q), electrons density (evol), surface charge density (σ) and current density (J) are extracted from the proposed model. The amount of displaced charge through Ti6Al4V disks not exposed to UV irradiation was similar to that displaced inside the thin TiO2 excited layer. Relative conductivity of the TiO2 excited layer versus Ti6Al4V showed a difference of up to five orders of magnitude between the two samples. The model is suitable for application to any metal and any excited semiconductor, thus increasing the usefulness of electrokinetic characterization based on streaming potential/current measurements.

Resistance Switching Capable Polymer Nanocomposites Employing Networks of One-Dimensional Nanocarbon Wrapped by TiO2 Conformal Layer

In order to impart resistance switching capability to polymer-based composite, one-dimensional conductive nanomaterials such as carbon nanotube (CNT) and carbon nanofiber (CNF) were wrapped with TiO2 conformal layer for passivation, and embedded in polymer matrix to form a networklike distribution within it. The CNT-TiO2 and CNF-TiO2-embedded composites, respectively, exhibited the reproducible resistance switching behavior of the high on/off ratio, along with the good switching stability under repetitive switching measurements. Furthermore, it is notable that the presence of defect site or incomplete formation of the TiO2 passivation layer on the conductive component would significantly alter the switching performance. The advantages of our approach include the simple and mass-production capable fabrication procedure along with the sustainable switching performance suitable to promising nonvolatile memory device applications.