Preparation of superhydrophobic surfaces with micro/nano alumina molds

Polymer micro/nano hierarchical structures were successfully formed by photo-nanoimprinting using anodic porous alumina molds. Anodic porous alumina molds with hierarchical structures were prepared by the anodization of an Al substrate with a micro-concave array. The obtained surfaces with hierarchical structures exhibited superhydrophobicity. The hydrophobic properties of the obtained samples were dependent on the surface structures and could be optimized by changing the micro- and nanopatterns in the hierarchical structure.

Superhydrophobic surface with hierarchical structures prepared by nanoimprinting using anodic porous alumina molds.     

Difference in the deactivation of Au catalysts during ethanol transformation when supported on ZnO and on TiO2

Au nanoparticles of different sizes were supported by the deposition–precipitation method on two metal oxides: ZnO and TiO₂. The resulting catalysts were tested in the ethanol catalytic transformation reaction. Both metal oxide support materials exerted a different influence on the achieved Au particle size as well as on the behavior of the subsequent catalyst, with regard to their initial conversion values, product distribution and stability. While TiO₂ favors the formation of smaller nanoparticles, ZnO offers larger Au particle sizes when prepared under similar conditions. At the same time, TiO₂ produced catalysts which displayed higher initial conversions in comparison with AuZnO catalysts, even when observing catalysts of each series with similar particle sizes. At the same time, catalysts supported on ZnO exhibited higher resistance to deactivation caused by coke formation. These results were evidenced employing different characterization techniques on both used and fresh catalyst samples. The decline in deactivation was generally accompanied by an increase in the carbon content on the catalyst''s surface.

Au NPs of different sizes were supported on two metal oxides: ZnO and TiO₂. Differences in ethanol transformation for Au of similar particle size reveal that TiO₂ support induces condensation products while ZnO only gives place to dehydrogenation.     

Solid state interdigitated Sb2S3 based TiO2 nanotube solar cells

TiO₂ nanotubes generated by anodization of metallic titanium sputter-coated on indium tin oxide (ITO) substrates are used as a conductive scaffold for all solid-state Sb₂S₃-sensitized extremely thin absorber (ETA) solar cells. A blocking layer of TiO₂ placed between Ti and ITO in combination with optimized Ti deposition and anodization conditions enables the formation of crack-free layers of straight, cylindrical TiO₂ nanotubes of tunable length and diameter. ALD (atomic layer deposition) is subsequently used to coat this substrate conformally with a highly pure Sb₂S₃ light absorber layer under an inert atmosphere. The high absorption coefficient of Sb₂S₃ as compared to molecular dyes allows for the utilization of very short nanotubes, which facilitates the infiltration of the organic hole transport material and formation of a p–i–n heterojunction in an interdigitated and tunable geometry. We investigate the influence of nanotube length and of the absorber thickness to enhance the photocurrent value to twice that of planar reference structures.

TiO₂ nanotubes generated by anodization of metallic titanium sputter-coated on indium tin oxide (ITO) substrates are used as a conductive scaffold for all-solid-state Sb₂S₃-sensitized extremely thin absorber (ETA) solar cells.     

Au nanoparticle-decorated TiO2 hollow fibers with enhanced visible-light photocatalytic activity toward dye degradation

TiO₂ hollow fibers (THF) were prepared by a template method using kapok as a biotemplate and subsequently decorated by plasmonic Au nanoparticles using a solution plasma process. The THF exhibited an anatase phase and a hollow structure with a mesoporous wall. Au nanoparticles with a diameter of about 5–10 nm were uniformly distributed on the THF surface. Au nanoparticles-decorated TiO₂ hollow fibers (Au/THF) have enhanced photocatalytic activity toward methylene blue degradation under visible light-emitting diode (Vis-LED) as compared to pristine THF and P25. This could be attributed to combined effects including effective light-harvesting by a hollow structure, large surface area due to a mesoporous wall of THF, and visible-light absorption and efficient charge separation induced by Au nanoparticles. The Au/THF also showed good recyclability and separation ability.

Plasmonic Au nanoparticles-decorated TiO₂ hollow fibers with enhanced visible-light photocatalytic activity have been successfully prepared by a two-step process: (i) template method using kapok and (ii) solution plasma process.     

A visible-light phototransistor based on the heterostructure of ZnO and TiO2 with trap-assisted photocurrent generation

Visible-light phototransistors have been fabricated based on the heterojunction of zinc oxide (ZnO) and titanium oxide (TiO₂). A thin layer of TiO₂ was deposited onto the spin-coated ZnO film via atomic layer deposition (ALD). The electrical characteristics of the TiO₂ layer were optimized by controlling the purge time of titanium isopropoxide (TTIP). The optimized TiO₂ layer could absorb the visible-light from the sub-gap states near the conduction band of TiO₂, which was confirmed via photoelectron spectroscopy measurements. Therefore, the heterostructure of TiO₂/ZnO can absorb and generate photocurrent under visible light illumination. The oxygen-related-states were investigated via X-ray photoelectron spectroscopy (XPS), and the interfacial band structure between TiO₂ and ZnO was evaluated via ultraviolet photoelectron spectroscopy (UPS). Oxygen-related states and subgap-states were observed, which could be used to generate photocurrent by absorbing visible light, even with TiO₂ and ZnO having a wide bandgap. The optimized TiO₂/ZnO visible-light phototransistor showed a photoresponsivity of 99.3 A W⁻¹ and photosensitivity of 1.5 × 10⁵ under the illumination of 520 nm wavelength light. This study provides a useful way to fabricate a visible-light phototransistor based on the heterostructure of wide bandgap oxide semiconductors.

Visible-light phototransistors have been fabricated based on the heterojunction of zinc oxide (ZnO) and titanium oxide (TiO₂).     

Polyacrylamide exotemplate-assisted synthesis of hierarchically porous nanostructured TiO2 macrobeads for efficient photodegradation of organic dyes and microbes

Nano/microscale TiO₂ materials and their composites have reached the pinnacle of their photocatalytic performances to destroy persistent organic pollutants and waterborne microorganisms, but their practical applications are limited by the drawbacks associated with their stability, leaching, processing and separation. To overcome these shortcomings, we have prepared hierarchically porous nanostructured TiO₂ macrobeads via an exotemplating or nanocasting strategy by infiltrating the TiO₂ sol into the emulsion-templated porous polyacrylamide scaffold followed by its gelation, drying and calcination. The nanoscale TiO₂ building units tailor the shape of the porous polymeric network after calcination thereby retaining the macroscale morphology of polymer beads after template removal. A novel combination of the hierarchical macroporosity, orderly crystalline anatase nature, nanoscale feature and good surface area revealed by the relevant characterization tools makes these TiO₂ scaffolds particularly effective for superior degradation of methylene blue with the enhanced rate constant and efficient disinfection of E. coli and S. aureus under UV light. The macrosize and mechanical stability of these purely TiO₂ beaded architectures have several potential advantages over conventional TiO₂ nanocomposites and slurry systems to address the inherent bottlenecks of secondary contamination, difficult operation and energy-intensive post-recovery processes that are indeed deemed to be the barriers to develop practically useful water treatment technologies.

Hierarchically porous nanostructured anatase TiO₂ macrobeads prepared by using emulsion-templated polyacrylamide beads as the sacrificial exotemplates for efficient photodegradation of dyes/microbes.     

Oxidation behavior of the TiAlN hard coating in the process of recycling coated hardmetal scrap

Recycling coated hardmetal scraps is becoming increasingly important for tungsten resource recovery. However, the coatings in these materials are one of the biggest problems, especially Al-containing coatings. In this study, discarded TiAlN-coated WC–Co hardmetal tool tips were isothermally oxidized at 900 °C in air, during which the final oxide, phase transition and microstructure evolution were investigated. Milled powders below 0.15 mm were completely oxidized in 180 min, and pieces of coatings were found in the final oxides. White WO₃ was mainly distributed on defect-rich areas of oxide scale surfaces. Furthermore, the final oxide scale was triple-layered, mainly consisting of the WO₃-concentrated outmost layer, the Al₂O₃-concentrated middle layer, and the TiO₂-concentrated inner layer. It is different from the bi-layered Al₂O₃/TiO₂ oxide scale that appeared for a new TiAlN-coated hardmetal during an oxidation resistance test. This was attributed to the defects in hardmetal scraps, which provided a fast pathway for element diffusion and volatilization of WO₃. Consequently, it was impossible to remove Al₂O₃ completely.

The final oxide scale was triple-layered, consisting of a WO₃-rich outmost layer, Al₂O₃-concentrated middle layer and TiO₂-concentrated inner layer.     

Plasmonic-enhanced photocatalysis reactions using gold nanostructured films

This work shows the enhancement of the visible photocatalytic activity of TiO₂ NPs film using the localized surface plasmonic resonance of Au nanostructures. We adopted a simple yet effective surface treatment to tune the size distribution, and plasmonic resonance spectrum of Au nanostructured films on glass substrates, by hot plate annealing in air at low temperatures. A hybrid photocatalytic film of TiO₂:Au is utilized to catalyse a selective photodegradation reaction of Methylene Blue in solution. Irradiation at the plasmonic resonance wavelength of the Au nanostructures provides more effective photodegradation compared to broadband artificial sunlight of significantly higher intensity. This improvement is attributed to the active contribution of the plasmonic hot electrons injected into the TiO₂. The broadband source initiates competing photoreactions in the photocatalyst, so that carrier transfer from the catalyst surface to the solution is less efficient. The proposed hybrid photocatalyst can be integrated with a variety of device architectures and designs, which makes it highly attractive for low-cost photocatalysis applications.

This work shows the enhancement of the visible photocatalytic activity of TiO₂ NPs film using the localized surface plasmonic resonance of Au nanostructures.     

Novel blended catalysts consisting of a TiO2 photocatalyst and an Al2O3 supported Pd–Au bimetallic catalyst for direct dehydrogenative cross-coupling between arenes and tetrahydrofuran

Dehydrogenative cross-coupling (DCC) is a clean methodology to make C–C bonds by using abundant C–H bonds. The blended catalyst, developed in this study, consists of a TiO₂ photocatalyst and an Al₂O₃ supported Pd–Au bimetallic catalyst and shows superior activity to the conventional TiO₂ photocatalyst loaded with the corresponding metal co-catalyst for the direct DCC between various arenes and tetrahydrofuran, with concomitant evolution of hydrogen gas. The reactions were done under mild conditions without consuming any oxidising agent or other additional chemicals. This new approach of separating the photocatalyst and the metal catalyst parts allows their independent modification to improve the overall catalytic performance.

A TiO₂ photocatalyst physically mixed with a supported Pd–Au bimetallic catalyst is more efficient than Pd loaded TiO₂ sample for the photocatalytic DCC between arene and THF.     

Dealloyed porous gold anchored by in situ generated graphene sheets as high activity catalyst for methanol electro-oxidation reaction

A novel one-step method to prepare the nanocomposites of reduced graphene oxide (RGO)/nanoporous gold (NPG) is realized by chemically dealloying an Al₂Au precursor. The RGO nanosheets anchored on the surface of NPG have a cicada wing like shape and act as both conductive agent and buffer layer to improve the catalytic ability of NPG for methanol electro-oxidation reaction (MOR). This improvement can also be ascribed to the microstructure change of NPG in dealloying with RGO. This work inspires a facile and economic method to prepare the NPG based catalyst for MOR.

A novel one-step method to prepare the nanocomposites of reduced graphene oxide (RGO)/nanoporous gold (NPG) is realized by chemically dealloying an Al₂Au precursor.     

Hot electrons generated by intraband and interband transition detected using a plasmonic Cu/TiO2 nanodiode

The use of metal nanoparticles, including Au, Ag, Cu, and Al, can increase the efficiency of photovoltaic devices by electromagnetic field enhancement, which is driven by the excitation of localized surface plasmon resonance. Direct energy conversion from light into electricity via the decay of surface plasmons causing the excitation of hot electron–hole pairs is also a feasible channel. The generation of hot electrons in metal nanostructures can occur through intraband excitation within the conduction band or through interband transition, which is caused by transitions from other bands to the unoccupied conduction band states. Here, we show the distinction between hot electron generation induced by intraband excitation and interband transition on a plasmonic Cu/TiO₂ nanodiode by measuring the current conversion efficiency with a monochromator system. We also show the dependence of the production of photocurrent on the thickness of the Cu layer and the effect of an aluminum oxide protection layer on the hot electron flux versus oxidation of the Cu layer. Our results can provide a better understanding for copper-based hot electron photovoltaics, which could lead to more efficient plasmonic energy conversion.

Better hot electron extraction via intraband and interband excitation was discerned on Cu compared with Au using plasmonic nanodiode.     

Laser-induced crystallization of anodic TiO2 nanotube layers

In this study, crystallization of amorphous TiO₂ nanotube (TNT) layers upon optimized laser annealing is shown. The resulting anatase TNT layers do not show any signs of deformation or melting. The crystallinity of the laser annealed TNT layers was investigated using X-ray diffraction, Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM). The study of the (photo-)electrochemical properties showed that the laser annealed TNT layers were more defective than conventional TNT layers annealed in a muffle oven at 400 °C, resulting in a higher charge recombination rate and lower photocurrent response. However, a lower overpotential for hydrogen evolution reaction was observed for the laser annealed TNT layer compared to the oven annealed TNT layer.

Laser treatment of amorphous TiO₂ nanotube layers leads to their crystallization without deformation or formation of the thermal oxide layer.     

Studies of adsorption of α,β-unsaturated carbonyl compounds on heterogeneous Au/CeO2, Au/TiO2 and Au/SiO2 catalysts during reduction by hydrogen

Our research focuses on phenomena accompanying adsorption of mesityl oxide (4-methylpent-3-en-2-one) on the surface of heterogeneous supported gold catalysts: Au/CeO₂, Au/TiO₂ and Au/SiO₂. We have studied reduction in the gas phase of (volatile) α,β-unsaturated carbonyl compounds (R-(V)ABUCC) which mesityl oxide is a basic model of. In situ infrared (IR) spectroscopy was employed to establish that the most active catalysts allow adsorption of conjugated ketones or aldehydes in the enolate (i.e. bridge-like adsorption through the oxygen from the carbonyl group and the β-carbon) and carboxylic form or with the ^αC ^βC double bond on a Lewis acidic site. Reductive properties of the catalysts and pure supports were studied by temperature-programmed reduction (TPR). We show that cerium(iv) oxide (CeO₂, ceria) and titanium(iv) oxide (TiO₂, titania) when decorated with gold nanoparticles (AuNP) can interact with hydrogen at temperatures approx. 150 °C lower than typical for pure oxides what includes even cyclic adsorption and instant release of H₂ below 100 °C in the case of gold–ceria system. Morphology and structure characterisation by transmission electron microscopy (TEM) and powder X-ray diffraction (PXRD) confirms that, with the obtained Au loadings, we achieved excellent dispersion of AuNPs while maintaining their small size, preferably below 5 nm, even though the Au/CeO₂ catalyst contained broad distribution of AuNPs sizes.

We deliver spectroscopic IR data describing the adsorption phenomena accompanying reduction of conjugated carbonyl compounds aided by heterogeneous catalysts.     

Fabrication of plasmonic dye-sensitized solar cells using ion-implanted photoanodes

Plasmonic dye-sensitized solar cells containing metal nanoparticles suffer from stability issues due to their miscibility with liquid iodine-based electrolytes. To resolve the stability issue, herein, an ion implantation technique was explored to implant metal nanoparticles inside TiO₂, which protected these nanoparticles with a thin coverage of TiO₂ melt and maintained the localized surface plasmon resonance oscillations of the metal nanoparticles to efficiently enhance their light absorption and make them corrosion resistant. Herein, Au nanoparticles were implanted into the TiO₂ matrix up to the penetration depth of 22 nm, and their influence on the structural and optical properties of TiO₂ was studied. Moreover, plasmonic dye-sensitized solar cells were fabricated using N719 dye-loaded Au-implanted TiO₂ photoanodes, and their power conversion efficiency was found to be 44.7% higher than that of the unimplanted TiO₂-based dye-sensitized solar cells due to the enhanced light absorption of the dye molecules in the vicinity of the localized surface plasmon resonance of Au as well as the efficient electron charge transport at the TiO₂@Au@N719/electrolyte interface.

Ion implantation technique can resolve the stability issue of metal nanoparticles with liquid iodine-based electrolyte to improve PCE of plasmonic dye-sensitized solar cells.     

Synergistic effect of hierarchical nanopores in Co-doped cobalt oxide 3D flowers for electrochemical energy storage

Hybridizing hierarchical porous transition oxides composed of nanoscale building blocks is highly desirable for improving the electrochemical performance of energy storage. Herein, we contribute a fabrication of novel hierarchically nanoporous flower-shaped metal/transition oxide (Co/Co₃O₄–CoO) with controllable three-dimensional structure. The designed Co/Co₃O₄–CoO 3D flowers (3DFs) are made of petal-shaped nanoporous Co₃O₄–CoO nanosheets with tunable pore sizes, in which metallic Co nanoparticles tend to attach to the edge of larger ones. The hierarchically nanoporous 3DFs with bimodal pore size distribution and higher fraction of small nanopores exhibit a higher specific capacitance (902.3 F g⁻¹ at current density of 2 A g⁻¹) and better cyclability than the uniformly nanoporous 3DFs with unimodal pore size distribution and larger BET surface area. The enhanced capacitance is mainly derived from the synergistic effect of hierarchical nanopores, in which large nanopores disproportionately facilitate osmotic solution flux and diffusive solute transport, whilst small nanopores supply faster channels for electron transportation and ion diffusion. Our work should provide a strategy to fabricate a smart functional hierarchical nanoporous architecture with 3DF structures for the development of electrochemical energy storage materials.

Hybridizing hierarchical porous transition oxides composed of nanoscale building blocks is highly desirable for improving the electrochemical performance of energy storage.     

Reduced graphene oxide/TiO2(B) nanocomposite-modified separator as an efficient inhibitor of polysulfide shuttling in Li–S batteries

The shutting effect in lithium–sulfur (Li–S) batteries hinders their widespread application, which can be restrained effectively by a modified separator. In this work, a composite of reduced graphene oxide and beta-phase TiO₂ nanoparticles (RGO/TiO₂(B)) is designed as a separator modification material for improving the electrochemical behavior of Li–S batteries. The TiO₂(B) nanoparticles are in situ prepared and tightly adhere to the RGO layer. A series of examinations demonstrated that the RGO/TiO₂(B)-coated separator efficiently inhibits the polysulfide shuttling phenomenon by the cooperative effect of physical adsorption and chemical binding. Specifically, as modified separators, a comparison between TiO₂(B) and anatase TiO₂(A) each composited with RGO has been conducted. The TiO₂(B) sample not only exhibits a superior blocking character of migrating polysulfides, but also enhances battery electrochemical kinetics by fast Li ion diffusion.

Beta-phase TiO₂ nanoparticles were adhered onto RGO in situ to fabricate a multi-functional separator for high-performance lithium–sulfur (Li–S) batteries.     

Fabrication of ideally ordered TiO2 through-hole membranes by two-layer anodization

Ideally ordered TiO₂ through-hole membranes were obtained by a combination of Ti substrate pre-patterning and two-layer anodization. The Ti substrate was pre-patterned by Ar ion milling using ideally ordered porous alumina as an etching mask. Each concave pit formed by dry etching acted as an initiation site for hole development during anodization, and ideally ordered anodic porous TiO₂ was produced by anodization using an electrolyte containing fluoride ions. Two-layered anodic porous TiO₂ samples with different solubilities were formed by heat treatment for the crystallization of the first anodized oxide layer and a subsequent second anodization. The selective dissolution of the lower part of the two-layered porous TiO₂ produced an ideally ordered TiO₂ through-hole membrane. Using the present process, the repeated preparation of ideally ordered TiO₂ membranes can be conducted because the ideally ordered concave array, which corresponds to the hole arrangement at the bottom of the detached TiO₂ membrane, is maintained even after detaching the membrane. The obtained samples can be applied to various functional devices, such as photonic crystals and filtration membranes with photocatalytic properties.

Ideally ordered TiO₂ through-hole membrane obtained through a two-layer anodization process.     

Efficient fabrication of ordered alumina through-hole membranes using a TiO2 protective layer prepared by atomic layer deposition

Ordered alumina through-hole membranes were obtained by a combination of the anodization of Al, formation of a TiO₂ protective layer, and subsequent etching. Two-layered anodic porous alumina materials composed of TiO₂-coated and noncoated alumina were prepared by the combination of the anodization of Al and the formation of a TiO₂ protective layer by atomic layer deposition (ALD). The obtained two layers of anodic porous alumina have different solubilities because the TiO₂ thin layer formed by ALD acts as a protective layer that prevents the dissolution of the alumina layer during wet etching of the sample in an etchant. After the selective dissolution of the bottom layer of porous alumina without the TiO₂ layer, an ordered alumina through-hole membrane could be detached from the Al substrate. This process allows the repeated preparation of ordered alumina through-hole membranes from a single Al substrate. By this process, ordered alumina through-hole membranes with large interhole distances could also be obtained. The obtained alumina through-hole membrane can be used in various applications.

Ordered alumina through-hole membrane with an interhole distance of 1 μm.     

Plasmonic enhanced photocatalytic activity of Ag/TiO2 tube-in-tube fibers

Ag nanoparticle was found to significantly enhance the photocatalytic activity of self-organized TiO₂ nanotube structures. Herein, novel Ag/TiO₂ tube-in-tube fibers have been prepared by a facile electrospinning technology and calcination process. Employed as the photocatalyst, the composite could efficiently catalyze the photodegradation of the model organic pollutant, rhodamine B under visible light irradiation, exhibiting a superior photocatalytic activity than the undoped TiO₂ tube-in-tube fibers. This enhanced activity has been ascribed to plasmonic characteristics of Ag nanoparticles, which promote the light absorption and charge transfer feasibility. The simple, low-cost and green fabrication route of the composite provides a novel means for preparing similar materials, holding great promise for wider application in the future.

Ag nanoparticle was found to significantly enhance the photocatalytic activity of self-organized TiO₂ nanotube structures.     

Carboxylation of sodium arylsulfinates with CO2 over mesoporous K-Cu-20TiO2

A mesoporous ternary metal oxide (K-Cu-20TiO₂) from a simple sol–gel method was prepared to catalyze heterogeneously the carboxylation reaction of various sodium arylsulfinates under atmospheric carbon dioxide. The catalyst showed excellent selectivity and good functional group tolerance to carboxylation recycle. The oxidation state of active copper(i) by characterization using FTIR, XRD, TG, XPS and TEM techniques proved to be efficacious to conduct atom economical reactions.

A mesoporous ternary metal oxide (K-Cu-20TiO₂) from a simple sol–gel method was prepared to catalyze heterogeneously the carboxylation reaction of various sodium arylsulfinates under atmospheric carbon dioxide.