Natural hydrophobicity and reversible wettability conversion of flat anatase TiO₂ thin film

Photoelectrochemical N2 -to-NH3 Fixation with High Efficiency and Rates via Optimized Si-Based System at Positive Potential versus Li0/

As a widely used commodity chemical, ammonia is critical for producing nitrogen-containing fertilizers and serving as the promising zero-carbon energy carrier. Photoelectrochemical nitrogen reduction reaction (PEC NRR) can provide a solar-powered green and sustainable route for synthesis of ammonia (NH₃ ). Herein, an optimum PEC system is reported with an Si-based hierarchically-structured PdCu/TiO₂ /Si photocathode and well-thought-out trifluoroethanol as the proton source for lithium-mediated PEC NRR, achieving a record high NH₃ yield of 43.09 µg cm^-2 h^-1 and an excellent faradaic efficiency of 46.15% under 0.12 MPa O₂ and 3.88 MPa N₂ at 0.07 V versus lithium(0/+) redox couple (vs Li^0/+ ). PEC measurements coupled with operando characterization reveal that the PdCu/TiO₂ /Si photocathode under N₂ pressures facilitate the reduction of N₂ to form lithium nitride (Li₃ N), which reacts with active protons to produce NH₃ while releasing the Li⁺ to reinitiate the cycle of the PEC NRR. The Li-mediated PEC NRR process is further enhanced by introducing small amount of O₂ or CO₂ under pressure by accelerating the decomposition of Li₃ N. For the first time, this work provides mechanistic understanding of the lithium-mediated PEC NRR process and opens new avenues for efficient solar-powered green conversion of N₂ -to-NH₃ .

Compression Stress-Induced Internal Magnetic Field in Bulky TiO2 Photoanodes for Enhancing Charge-Carrier Dynamics

Enhancing charge-carrier dynamics is imperative to achieve efficient photoelectrodes for practical photoelectrochemical devices. However, a convincing explanation and answer for the important question which has thus far been absent relates to the precise mechanism of charge-carrier generation by solar light in photoelectrodes. Herein, to exclude the interference of complex multi-components and nanostructuring, we fabricate bulky TiO₂ photoanodes through physical vapor deposition. Integrating photoelectrochemical measurements and in situ characterizations, the photoinduced holes and electrons are transiently stored and promptly transported around the oxygen-bridge bonds and 5-coordinated Ti atoms to form polarons on the boundaries of TiO₂ grains, respectively. Most importantly, we also find that compressive stress-induced internal magnetic field can drastically enhance the charge-carrier dynamics for the TiO₂ photoanode, including directional separation and transport of charge carriers and an increase of surface polarons. As a result, bulky TiO₂ photoanode with high compressive stress displays a high charge-separation efficiency and an excellent charge-injection efficiency, leading to 2 orders of magnitude higher photocurrent than that produced by a classic TiO₂ photoanode. This work not only provides a fundamental understanding of the charge-carrier dynamics of the photoelectrodes but also provides a new paradigm for designing efficient photoelectrodes and controlling the dynamics of charge carriers.

Nanostructured versus flat compact electrode for triboelectric nanogenerators at high humidity

The triboelectric nanogenerator (TENG) is a promising technology for mechanical energy harvesting. TENG has proven to be an excellent option for power generation but typically TENGs output power drops significantly in humid environments. In this work, the effect of electrode's material on power output, considering smooth and nanostructured porous structures with various surface hydrophobicity, is investigated under various humidity conditions. A vertical contact-separation mode TENG is experimentally and numerically studied for four surface morphologies of Ti foil, TiO2 thin film, TiO2 nanoparticulated film, and TiO2 nanotubular electrodes. The results show that the TENG electrical output in the flat structures such as Ti foil and TiO2 thin film at 50% RH is reduced to 50% of its initial state, while in the nanoporous structures such as nanoparticle and nanotube arrays, this is observed at RH above 95%. The results show that the use of porous nanostructures in TENG due to their high surface-to-volume, and that the process of water adsorption on the pore leads to better performance than the flat surface in humid environments. Based on our study, employing nanoporous layers is vital for nanogenerators either for power generation or active sensor applications at high humidity conditions.

Two-Step Laser Post-Processing for the Surface Functionalization of Additively Manufactured Ti-6Al-4V Parts

Laser powder bed fusion (LPBF) is one of the additive manufacturing methods used to build metallic parts. To achieve the design requirements, the LPBF process chain can become long and complex. This work aimed to use different laser techniques as alternatives to traditional post-processes, in order to add value and new perspectives on applications, while also simplifying the process chain. Laser polishing (LP) with a continuous wave laser was used for improving the surface quality of the parts, and an ultrashort pulse laser was applied to functionalize it. Each technique, individually and combined, was performed following distinct stages of the process chain. In addition to removing asperities, the samples after LP had contact angles within the hydrophilic range. In contrast, all functionalized surfaces presented hydrophobicity. Oxides were predominant on these samples, while prior to the second laser processing step, the presence of TiN and TiC was also observed. The cell growth viability study indicated that any post-process applied did not negatively affect the biocompatibility of the parts. The presented approach was considered a suitable post-process option for achieving different functionalities in localized areas of the parts, for replacing certain steps of the process chain, or a combination of both.

Effect of the Titanium Isopropoxide:Acetylacetone Molar Ratio on the Photocatalytic Activity of TiO2 Thin Films

TiO₂ thin films with different titanium isopropoxide (TTIP):acetylacetone (AcacH) molar ratios in solution were prepared by the chemical spray pyrolysis method. The TTIP:AcacH molar ratio in spray solution varied from 1:3 to 1:20. TiO₂ films were deposited onto the glass substrates at 350 °C and heat-treated at 500 °C. The morphology, structure, surface chemical composition, and photocatalytic activity of the obtained TiO₂ films were investigated. TiO₂ films showed a transparency of ca 80% in the visible spectral region and a band gap of ca 3.4 eV irrespective of the TTIP:AcacH molar ratio in the spray solution. TiO₂ films consist of the anatase crystalline phase with a mean crystallite size in the range of 30-40 nm. Self-cleaning properties of the films were estimated using the stearic acid (SA) test. A thin layer of 8.8-mM SA solution was spin-coated onto the TiO₂ film. The degradation rate of SA as a function of irradiation time was monitored by Fourier-transform infrared spectroscopy (FTIR). An increase in the TTIP:AcacH molar ratio from 1:4 to 1:8 resulted in a ten-fold increase in the photodegradation reaction rate constant (from 0.02 to the 0.2 min^-1) under ultraviolet light and in a four-fold increase under visible light.

Photocatalytic Degradation of Different VOCs in the Gas-Phase over TiO2 Thin Films Prepared by Ultrasonic Spray Pyrolysis

In this study, we deposited TiO2 thin films onto borosilicate glass by ultrasonic spray pyrolysis at 350 and 450 °C. The aim of study is to determine the effect of deposition temperature on photocatalytic activity of TiO2 thin films and to investigate the performance of TiO2 thin films on photocatalytic degradation of methyl tert-butyl ether (MTBE), acetone, acetaldehyde, and heptane as functions of different operating parameters. TiO2 thin films deposited at 350 and 450 °C have a thickness value of 190 and 330 nm, respectively. All as-prepared TiO2 films possess an anatase crystalline structure. According to the X-ray photon spectroscopy (XPS) study, the TiO2 thin film deposited at 350 °C showed a higher amount of oxygen vacancies and hydroxyl groups on the film surface after UV treatment. The aged-TiO2 thin film deposited at 350 °C showed a water contact angle (WCA) value of 0° after 10 min UV irradiation, showing superhydrophilic surface behavior. The TiO2 film deposited at 350 °C exhibited the highest amount of conversion of MTBE (100%). The results also showed that TiO2 films are capable of photocatalytic degradation of MTBE (100%) and acetaldehyde (approx. 80%) in humid air conditions and high airflow rate. The visible-light-activity of TiO2 thin films was tested with 5 ppm MTBE and acetone. TiO2 thin films deposited at 350 °C with a surface area of 600 cm2 showed 60% of MTBE and 33% of acetone degradation under VIS light.

Visible Light-Activated Self-Recovery Hydrophobic CeO2/Black TiO2 Coating Prepared Using Air Plasma Spraying

Hydrophobic coatings are widely used in many areas from home to industry. However, the hydrophobicity can be easily degraded by adsorption of the atmospheric contaminations. In this study, a novel CeO₂/black TiO₂ hydrophobic coating with self-recovery capability is prepared using air plasma spraying without any chemical modification. The water contact angles (WCAs) of the coatings decreased after oleic acid contamination. Because of the presence of black TiO₂, the composite coating has the photodegradation property under irradiation of visible light, and the part of the black TiO₂ transforms to be superhydrophilic after irradiation for the generation of the surface oxygen vacancies. The oleic acid was decomposed and the WCAs changed depending on the volume percentage of the CeO₂. The coating exhibits hydrophilicity when the volume percentage of the CeO₂ is less than 60%, and hydrophobic when higher. After storage in a dark and clean environment, all the coatings can recover their hydrophobicity for the black TiO₂ that returned back to its origin state. It is believed that this hydrophobic self-cleaning ceramic coating should have potential in engineering applications.

Crystalline TiO2 protective layer with graded oxygen defects for efficient and stable silicon-based photocathode

The trade-offs between photoelectrode efficiency and stability significantly hinder the practical application of silicon-based photoelectrochemical devices. Here, we report a facile approach to decouple the trade-offs of silicon-based photocathodes by employing crystalline TiO₂ with graded oxygen defects as protection layer. The crystalline protection layer provides high-density structure and enhances stability, and at the same time oxygen defects allow the carrier transport with low resistance as required for high efficiency. The silicon-based photocathode with black TiO₂ shows a limiting current density of ~35.3 mA cm^-2 and durability of over 100 h at 10 mA cm^-2 in 1.0 M NaOH electrolyte, while none of photoelectrochemical behavior is observed in crystalline TiO₂ protection layer. These findings have significant suggestions for further development of silicon-based, III-V compounds and other photoelectrodes and offer the possibility for achieving highly efficient and durable photoelectrochemical devices.

Crystalline TiO2 protective layer with graded oxygen defects for efficient and stable silicon-based photocathode

The trade-offs between photoelectrode efficiency and stability significantly hinder the practical application of silicon-based photoelectrochemical devices. Here, we report a facile approach to decouple the trade-offs of silicon-based photocathodes by employing crystalline TiO₂ with graded oxygen defects as protection layer. The crystalline protection layer provides high-density structure and enhances stability, and at the same time oxygen defects allow the carrier transport with low resistance as required for high efficiency. The silicon-based photocathode with black TiO₂ shows a limiting current density of ~35.3 mA cm⁻² and durability of over 100 h at 10 mA cm⁻² in 1.0 M NaOH electrolyte, while none of photoelectrochemical behavior is observed in crystalline TiO₂ protection layer. These findings have significant suggestions for further development of silicon-based, III–V compounds and other photoelectrodes and offer the possibility for achieving highly efficient and durable photoelectrochemical devices.

While silicon-based materials can convert sunlight directly to fuel and electricity, balancing their stability and efficiency constrains usage. Here, authors protect silicon photocathodes with crystalline titanium dioxide layers with graded oxygen defects to improve both durability and efficiency.

Plasma sputtering depositions with colloidal masks for fabrication of nanostructured surfaces with enhanced photocatalytic activity

Titanium oxide/silicon oxide (TiO₂/SiO₂) 2D patterns were obtained by magnetron sputtering depositions of Ti on close-packed and size-reduced colloidal masks on Si and quartz substrates, followed by mask lift-off and ending with thermal oxidation. The physical processes involved in growing 2D Ti patterns and their oxidation are analyzed. For the magnetron sputtering deposition, two regimes are considered: the low-pressure regime when the flux of sputtered atoms is anisotropic, and the high-pressure regime, when the flux of sputtered atoms is isotropic due to frequent collisions. Moreover, magnetron sputtering operation modes, such as dc sputtering and high power impulse sputtering, are compared. The changes in pattern size and morphology determined by the oxidation of the Ti patterns and Si substrate are analyzed. The hydrophilicity induced by UV-light irradiation and the visible-light photocatalytic activity towards the degradation of the methylene blue of the fabricated TiO₂/SiO₂ patterns were considerably higher when compared to the performances of uniform TiO₂ films.

Internal stress induced natural self-chemisorption of ZnO nanostructured films

The energetic particles bombardment can produce large internal stress in the zinc oxide (ZnO) thin film, and it can be used to intentionally modify the surface characteristics of ZnO films. In this article, we observed that the internal stress increased from −1.62 GPa to −0.33 GPa, and the naturally wettability of the textured ZnO nanostructured films changed from hydrophobicity to hydrophilicity. According to analysis of surface chemical states, the naturally controllable wetting behavior can be attributed to hydrocarbon adsorbates on the nanostructured film surface, which is caused by tunable internal stress. On the other hand, the interfacial water molecules near the surface of ZnO nanostructured films have been identified as hydrophobic hydrogen structure by Fourier transform infrared/attenuated total reflection. Moreover, a remarkable near-band-edge emission peak shifting also can be observed in PL spectra due to the transition of internal stress state. Furthermore, our present ZnO nanostructured films also exhibited excellent transparency over 80% with a wise surface wetting switched from hydrophobic to hydrophilic states after exposing in ultraviolet (UV) surroundings. Our work demonstrated that the internal stress of the thin film not only induced natural wettability transition of ZnO nanostructured films, but also in turn affected the surface properties such as surface chemisorption.

One-step fabrication of recyclable and robust fluorine/polymer-free superhydrophobic fabrics

Without using any low-surface-energy fluoro-containing groups or long alkyl groups, via a simple vacuum heating process, we prepared a robust superhydrophobic TiO₂/PET fabric.

Understanding and Tuning the Intrinsic Hydrophobicity of Rare-Earth Oxides: A DFT+U Study

Rare-earth oxides (REOs) possess a remarkable intrinsic hydrophobicity, making them candidates for a myriad of applications. Although the superhydrophobicity of REOs has been explored experimentally, the atomistic details of the structure at the oxide-water interface are still not well understood. In this work, we report a density functional theory study of the interaction between water and CeO2, Nd2O3, and α-Al2O3 to explain their different wettability. The wetting of the metal oxide surface is controlled by geometric and electronic factors. While the electronic term is related to the acid-base properties of the surface layer, the geometric factor depends on the matching between adsorption sites and oxygen atoms from the hexagonal water network. For all the metal oxides considered here, water dissociation is confined to the first oxide-water layer. Hydroxyl groups on α-Al2O3 are responsible for the strong oxide-water interaction, and thus, both Al- and hydroxyl-terminated wet. On CeO2, the intrinsic hydrophobicity of the clean surface disappears when lattice hydroxyl groups (created by the reaction of water with oxygen vacancies) are present as they dominate the interaction and drive wetting. Therefore, hydroxyls may convert a intrinsic nonwetting surface into a wetting one. Finally, we also report that surface modifications, like cation substitution, do not change the acid-base character of the surface, and thus they show the same nonwetting properties as native CeO2 or Nd2O3.

Robust biomimetic-structural superhydrophobic surface on aluminum alloy

The following facile approach has been developed to prepare a biomimetic-structural superhydrophobic surface with high stabilities and strong resistances on 2024 Al alloy that are robust to harsh environments. First, a simple hydrothermal treatment in a La(NO3)3 aqueous solution was used to fabricate ginkgo-leaf like nanostructures, resulting in a superhydrophilic surface on 2024 Al. Then a low-surface-energy compound, dodecafluoroheptyl-propyl-trimethoxylsilane (Actyflon-G502), was used to modify the superhydrophilic 2024 Al, changing the surface character from superhydrophilicity to superhydrophobicity. The water contact angle (WCA) of such a superhydrophobic surface reaches up to 160°, demonstrating excellent superhydrophobicity. Moreover, the as-prepared superhydrophobic surface shows high stabilities in air-storage, chemical and thermal environments, and has strong resistances to UV irradiation, corrosion, and abrasion. The WCAs of such a surface almost remain unchanged (160°) after storage in air for 80 days, exposure in 250 °C atmosphere for 24 h, and being exposed under UV irradiation for 24 h, are more than 144° whether in acidic or alkali medium, and are more than 150° after 48 h corrosion and after abrasion under 0.98 kPa for 1000 mm length. The remarkable durability of the as-prepared superhydrophobic surface can be attributed to its stable structure and composition, which are due to the existence of lanthanum (hydr)oxides in surface layer. The robustness of the as-prepared superhydrophobic surface to harsh environments will open their much wider applications. The fabricating approach for such robust superhydrophobic surface can be easily extended to other metals and alloys.

Superhydrophobic surface formation and modulation in a biphenyltetracarboxylic dianhydride derivative self-assembly system via changing alkyl chain lengths

A Superhydrophobic surface with lotus effect was formed via low weight molecule self-assembly.

Activation and enhancement of room-temperature ferromagnetism in Cu-doped anatase TiO₂ films by bound magnetic polaron and oxygen defects

Cu-doped anatase TiO2 films grown by magnetron sputtering at room temperature showed the unexpected observation of room-temperature ferromagnetism, which was enhanced or destroyed corresponding to low or high impurity concentration via vacuum annealing. On the basis of the analysis of composition and structure, the most important factor for activating ferromagnetism can be identified as the creation of grain boundary defects. In addition, oxygen defects can be the dominating factor for increasing the saturation moment of the 0.19 at. % Cu-doped TiO2 film from 0.564 to 26.41 emu/cm(3). These results help elucidate the origin of ferromagnetism and emphasize the role of oxygen defects for the application of ferromagnetic films.