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Doping and compositional tuning of Cs(2)AInCl(6) (A = Ag, Na) double perovskite nanocrystals (PNCs) is considered a promising strategy toward the development of light-emitting sources for applications in solution-processed optoelectronic devices. Oleic acid and oleylamine are by far the most often used surface capping ligands for PNCs. However, the undesirable desorption of these ligands due to proton-exchange reaction during isolation and purification processing results in colloidal and structural instabilities. Thus, the improvement of colloidal and optical stability of PNCs represents one of the greatest challenges in the field. Here, we report a trioctylphosphine-mediated synthesis and purification method toward Sb-alloyed Cs2NaInCl6 PNCs with excellent stability and optical features. Nuclear magnetic resonance spectroscopy enabled one to explain the role of trioctylphosphine and to reveal the reaction mechanism during crystal nucleation and growth. Under the optimized reaction conditions, in situ-generated trioctylphosphonium chloride and benzoyl trioctylphosphonium chloride serve as highly reactive halide sources, while benzoyl trioctylphosphonium and oleylammonium cations together with the oleate anion serve as surface capping ligands, which are bound strongly to the PNC surface. The tightly bound ionic pair of oleylammonium oleate and benzoyl trioctylphosphonium chloride/oleate ligands allows one to obtain monodispersed bright-blue-emitting PNCs with high photoluminescence quantum yields exceeding 50% at an optimum Sb content (0.5%), which also exhibit long-term colloidal stability. The approach based on dual cationic ligand passivation of double PNCs opens the doors for applications in other systems with a potential to achieve higher stability along with superior optical properties. Web of Science 13 40 47859 47845

Reducing the size of a catalyst to a single atom (SA) level can dramatically change its physicochemical properties and significantly boost its catalytic activity. However, the massive synthesis of SA catalysts still remains a grand challenge mainly because of the aggregation and nucleation of the generated atoms during the reaction. Here, we design and implement a spatially confined synthetic strategy based on a porous-hollow carbon nitride (p-CN) coordinated with 1-butyl-3-methylimidazole hexafluorophosphate, which can act as a nanoreactor and allow us to obtain metal SA catalysts (p-CN@M SAs). This relatively easy and highly effective method provides a way to massively synthesize single/multiple atoms (p-CN@M SAs, M = Pt, Pd, Cu, Fe, etc.). Moreover, the amorphous NiB-coated p-CN@Pt SAs can further increase the loading amount of Pt SAs to 3.7 wt %. The synthesized p-CN@Pt&NiB electrocatalyst exhibits an extraordinary hydrogen evolution reaction activity with the overpotential of 40.6 mV@10 mA/cm-2 and the Tofel slope of 29.26 mV/dec.

Highly ordered and uniform titanium dioxide (TiO2) nanotubes (NTs) with different morphologies (15 nm, 50 nm and 100 nm in diameter) were prepared by the electrochemical anodization of Ti substrates. The TiO2 NTs’ surface properties were characterized by X-ray diffraction (XRD) spectroscopy, Raman spectroscopy, scanning electron microscopy (SEM) and atomic force microscopy (AFM). The elastic modulus (E) and the Vickers hardness (HV) of the Ti foil and of the different-morphology TiO2 NTs were evaluated with the nano-indentation technique. E and HV increase with the decreasing length/diameter of the NTs, meaning that NTs with smaller diameters are more resistant to mechanical wear. The elastic modulus of the TiO2 NTs with 15-nm and 50-nm diameters is similar to that of the human bone.

A thin layer of gold nanoparticles (Au NPs) sputtered on cadmium sulfide quantum dots (CdS QDs) decorated anodic titanium dioxide nanotubes (TNTs) (Au/CdS QDs/TNTs) was fabricated and explored for the nonenzymatic detection of cholesterol and hydrogen peroxide (H2O2). Morphological studies of the sensor revealed the formation of uniform nanotubes decorated with a homogeneously dispersed CdS QDs and Au NPs layer. The electrochemical measurements showed an enhanced electrocatalytic performance with a fast electron transfer (∼2 s) between the redox centers of each analyte and electrode surface. The hybrid nanostructure (Au/CdS QDs/TNTs) electrode exhibited about a 6-fold increase in sensitivity for both cholesterol (10,790 μA mM-1 cm-2) and H2O2 (78,833 μA mM-1 cm-2) in analyses compared to the pristine samples. The hybrid electrode utilized different operational potentials for both analytes, which may lead to a voltage-switchable dual-analyte biosensor with a higher selectivity. The biosensor also demonstrated a good reproducibility, thermal stability, and increased shelf life. In addition, the clinical significance of the biosensor was tested for cholesterol and H2O2 in real blood samples, which showed maximum relative standard deviations of 1.8 and 2.3%, respectively. These results indicate that a Au/CdS QDs/TNTs-based hybrid nanostructure is a promising choice for an enzyme-free biosensor due to its suitable band gap alignment and higher electrocatalytic activities.

Titanium dioxide (TiO2) is the most frequently studied semiconducting material for photocatalytic water splitting. One of the favored forms of TiO2 for photocatalytic applications is layers of erected single-crystalline anatase nanosheets (NSs), while the most frequently reported substrate used for its synthesis is a fluorine-doped tin oxide (FTO). Herein we demonstrate that anatase TiO2 NS layers can be similarly grown on a Ta2O5 substrate. We found that a Ta2O5 back contact provides a remarkable improvement of the photocatalytic activity of the TiO2 NSs in comparison to a FTO back contact. The TiO2 NSs on Ta2O5 exhibit a 170-fold increase in photocatalytic H2 production rate than that obtained by TiO2 NSs on FTO substrate. The proposed mechanism reveals that such a drastic enhancement of optimized TiO2 NS arrays on the Ta2O5 substrate is attributed to the blocking nature of Ta2O5 for photo-generated electrons in the TiO2 NSs.

Aluminum ion aqueous batteries (AIBs) are among the most promising candidates for high energy density devices due to the multivalent redox processes associated with Al3+ ion intercalation. However, only a few stable AIB electrode materials have been reported so far. MoO3 is a very promising electrode material due to its octahedral layered crystal structure which can accommodate multivalent cation by intercalation. However, the poor electrochemical stability of MoO3 and the sluggish intercalation kinetics of Al3+ ion in Mo oxides electrodes limit its practical application. In this work, we propose a strategy to overcome such shortcomings of MoO3 by fabricating electrodes composed of self-ordered one-dimensional (1D) MoTaOx nanotubes synthesized via electrochemical anodization of Mo-Ta alloy substrates. We show that this approach allows for direct incorporation of Ta in the Mo oxide nanotubes. The resulting MoTaOx nanotubes, composed of octahedral MoO3 and rhombohedral Mo2Ta2O11 phases, exhibit remarkable electrochemical stability and Al-ion storage properties in aqueous electrolytes that are superior to that of pristine Mo oxide or other most efficient electrode materials reported to date. Such MoTaOx nanotube-based electrodes can achieve a specific capacity of 1180 mA h cm-3 (337 mA h g-1, 141 μA h cm-2) at 1.25 A cm-3 (∼0.35 A g-1, 0.15 mA cm-2). More importantly, the capacity retention of such nanotube array electrodes remains above 83% of the initial capacity after 3000 cycles.

Herein we systematically investigate the influence of hydrogenation time and temperature on defects formation, structural changes, and co-catalyst free photocatalytic activity for epitaxially grown anatase thin films on strontium titanate single crystals. The photocatalytic activity trend of the anatase epi layers is similar to that of anatase powders and brookite single crystals in reported recent literature, that is, the H2 evolution rate exhibits a typical maximum upon hydrogenation under moderate annealing temperatures (500 °C). By combining surface sensitive X-ray diffraction methods and analytical high-resolution electron microscopy techniques we reveal quantitatively the oxygen loss from the epi-layer ascribed to the hydrogenation (H2 reduction) treatment. We observe specific physicochemical changes associated with the introduction of oxygen vacancies: (i) the formation of a nanoscale strained crystal surface, (ii) the agglomeration of point defects in the bulk of the anatase epi layer, and (iii) a transition towards a Ti2O3 like symmetry at the film surface, i.e. an evident oxygen deficiency at the epi layer surface in particular prominent for the most active sample as a function of the hydrogenation parameters. These extensive experimental findings allow us to propose an empirical model, which links detrimental and beneficial effects of Ti3+ centers and oxygen vacancies in the bulk and at the surface and their abundance to an optimum point defect configuration for water splitting via “grey” anatase.

Highly ordered titanium dioxide nanotube (TiO2 NT) arrays were grown by electrochemical anodization of titanium in a molten NH4F/o-H3PO4 electrolyte. NTs with various diameters, lengths, wall thick...

TiO2-based catalyst supports in polymer electrolyte membrane fuel cells (PEMFCs) are widely investigated and provide an alternative to corrosion-susceptible carbon-based catalyst supports. For PEMF...

Photoelectrochemical (PEC) water splitting is a promising method for the conversion of solar energy into chemical energy stored in the form of hydrogen. Nanostructured hematite (α-Fe2 O3 ) is one of the most attractive materials for a highly efficient charge carrier generation and collection due to its large specific surface area and the short minority carrier diffusion length. In the present work, the PEC water splitting performance of nanostructured α-Fe2 O3 is investigated which was prepared by anodization followed by annealing in a low oxygen ambient (0.03 % O2 in Ar). It was found that low oxygen annealing can activate a significant PEC response of α-Fe2 O3 even at a low temperature of 400 °C and provide an excellent PEC performance compared with classic air annealing. The photocurrent of the α-Fe2 O3 annealed in the low oxygen at 1.5 V vs. RHE results as 0.5 mA cm-2 , being 20 times higher than that of annealing in air. The obtained results show that the α-Fe2 O3 annealed in low oxygen contains beneficial defects and promotes the transport of holes; it can be attributed to the improvement of conductivity due to the introduction of suitable oxygen vacancies in the α-Fe2 O3 . Additionally, we demonstrate the photocurrent of α-Fe2 O3 annealed in low oxygen ambient can be further enhanced by Zn-Co LDH, which is a co-catalyst of oxygen evolution reaction. This indicates low oxygen annealing generates a promising method to obtain an excellent PEC water splitting performance from α-Fe2 O3 photoanodes.

In electrocatalysis, layered double hydroxide (LDH) materials have attracted considerable interest in promoting the oxygen evolution reaction (OER). For the hydrogen evolution reaction (HER), these materials show a poor electrocatalytic activity. In this work, we take CoFe LDH grown on nickel foam as a model system, and for the first time we treat CoFe LDH under plasma etching conditions, which results in a highly active HER electrocatalyst in alkaline media. Massive defects and amorphous phase formation occur during plasma etching. More importantly, phase transformation of CoFe LDH into CoFe2O4 takes place. Additional introduction of Ce leads to more active sites and improved electrical conductivity, which provide an overpotential of 73 mV at a current density of 10 mA cm−2 for V-Ce/CoFe LDH. The experimental results match well with DFT calculations, proving that defect engineering, phase transformation, and electronic structure tailoring can result in a high efficiency of HER activity. Moreover, a cell constructed of CoFe LDH‖V-Ce/CoFe LDH is capable of overall water splitting with a cell voltage as low as 1.65 V at 10 mA cm−2 and a remarkable long-term stability of 60 h. The plasma etching strategy can also be effective for other LDH-based electrocatalysts (e.g. NiFe LDH and NiCo LDH), which demonstrates the universality of the strategy for improving HER performance. This work provides a simple but feasible pathway for designing alkaline HER electrocatalysts based on advanced LDH materials.

Anodic TiO2 nanotubes that are grown on Ti substrates by a simple anodization in various types of fluoride containing electrolytes have attracted scientific and technological interest due to their wide potential applications, and therefore, numerous research efforts have been dedicated to these self-ordered oxide structures in the past decade. The present mini-review highlights less known but important aspects, such as the formation of spaced nanotubes with adjustable interspacing that is achieved in a few specific organic electrolytes, and strong effects of the metallic Ti substrate which significantly affect the growth of the tubes. We discuss the formation of oxide nanotubes grown from adequately alloyed substrates and noble metal nanoparticle decoration of tubes. We describe how specific heat-treatment can introduce a nanotwinned boundary in the oxide tube walls of single-walled nanotubes obtained by a decoring process. All the facts and findings were studied in recent years and TiO2 nanotubes can be upgraded with more optimized functionalities for their applications.

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.

The low penetration depth of UV light in mammalian tissue is the critical limitation for the use of TiO2-based photocatalysis in biomedical applications. In this work, we develop an effective near-infrared (NIR)-active photocatalytic platform that consists of a shell structure of upconversion nanocrystals decorated on a core of Au/dark-TiO2. The heart of this system is the strong photocatalytic activity in the visible region enabled by the gold surface-plasmon resonance on dark TiO2 (D-TiO2). Simulation and experiment demonstrate for an optimized Au/D-TiO2 combination a highly enhanced light absorption in the visible range. Using ampicillin sodium (AMP) as model drug, we exemplify the effective use of this principle by demonstrating a NIR light-triggered photocatalytic payload release. Importantly, the photocatalytically generated reactive oxygen species can effectively inactivate AMP-resistant bacteria strains, thus maintaining an antibacterial effect even after all drug is released. Overall, we anticipate that the here-introduced NIR-light-active photocatalytic cascade can considerably widen TiO2-based photocatalysis and its applications into the infrared range.

The primary focus of the paper is the fabrication of nanotubes via a two steps anodizing procedure on a TiZr substrate followed subsequent annealing treatments and their effect on surface and biocompatibility. The two step anodizing process is optimized for obtaining uniform and ordered nanotubular structures, that are then annealed in air or reduced in an Ar/H2 environment. The high aspect ratio nanotubular structures (diameter 160 nm, length 9.4 μm) maintained their morphology after annealing or reduction. While the thermal treatments burned-off the fluoride, they converted the tubes to ZrTiO4 (annealing in air) or (Zr0.333 Ti0.666)O2 (reducing). Surface roughness and adhesion forces evaluation of the nanostructures was included in the AFM investigations, and show that these properties can be tuned. In vitro cell response to TiZr substrate and nanotubular coatings were analyzed using HGF-1 gingival fibroblasts cell line and LDH, nitric oxide (NO) and reactive oxygen species (ROS) tests. Regarding the hydrophilic balance of the studied samples, as expected it decreased after annealing and but increased after reduction, with only small change in the cell response. The results suggest that these coatings are biocompatible with human gingival fibroblast and can be used in biomedical applications without generating any considerable inflammatory process. Keywords: Anodizing, Thermal treatments, Roughness, Adhesion, Biocompatibility

Single atom (SA) catalysis, over the last 10 years, has become a forefront in heterogeneous catalysis, electrocatalysis, and most recently also in photocatalysis. Pt SAs on titania have been reported to be a highly effective co-catalyst in photocatalytic H2 generation. Most recently, for the synthesis of SA loading on titania surfaces some simple immersion techniques have been described. Here we introduce a so far unexplored but evidently very effective approach to enhance the effectivity of Pt SA co-catalysts on anodic TiO2 layers. For this, we first decorate TiO2 surfaces with a partial coverage of headgroups of silane self-assembled monolayers (SAMs) and then deposit Pt SAs on this SAM modified surface. We observe a significantly enhanced photocatalytic activity for H2 generation using this sensitization of the surface with a modified SAM.

In the present work we introduce a simple approach to produce porous PtPdAg alloy nanoparticles on TiO2 nanotubes by a simple combined sputtering-dewetting-dealloying process. In a first step, three individual metals (Pt, Pd and Ag) were sequentially sputter-deposited on spaced TiO2 nanotubes, followed by a thermal dewetting treatment in Ar to convert these metal layers into ternary PtPdAg alloyed particles. Dealloying was carried out by a selective chemical dissolution of Ag in piranha solution that finally generated porous alloyed nanoparticles located on the TiO2 nanotube walls. The porous PtPdAg-decorated TiO2 nanotubes show enhanced photocatalytic H2 generation in comparison with porous Pt (67% increase) or any monometallic decoration (>120% increase). The photocatalytic enhancement is not only due to the increased number of active sites (larger surface area) of the porous particles but also due to the synergistic effects of the different alloying elements.

Titanium dioxide (TiO2) holds remarkable promises for developing current theranostic strategies. Anodic TiO2 nanostructures as a porous scaffold have offered a broad range of useful theranostic properties; however, previous attempts to generate single and uniform TiO2 one-dimensional nanocarriers from anodic nanotube arrays have resulted in a broad cluster size distribution of arbitrarily broken tubes that are unsuitable for therapeutic delivery systems due to poor biodistribution and the risk of introducing tissue inflammation. Here, we achieve well-separated, uniformly shaped anodic TiO2 nanotubes and nanocylinders through a time-varying electrochemical anodization protocol that leads to the generation of planar sheets of weakly connected nanotubes with a defined fracture point near the base. Subsequent sonication cleanly detaches the nanotubes from the base. Depending on the position of the fracture point, we can fabricate single-anodic nanocylinders that are open on both ends and nanotubes that are closed on one end. We proceed to show that anodic nanotubes and nanocylinders are nontoxic at therapeutic concentrations. When conjugated with the anticancer drug doxorubicin using a pH-responsive linker, they are readily internalized by cells and subsequently release their drug cargo into acidic intracellular compartments. Our results demonstrate that uniformly sized anodic TiO2 nanotubes and nanocylinders are suitable for subcellular delivery of therapeutic agents in cancer therapy.

Hematite is a low band gap, earth abundant semiconductor and it is considered to be a promising choice for photoelectrochemical water splitting. However, as a bulk material its efficiency is low because of excessive bulk, surface, and interface recombination. In the present work, we propose a strategy to prepare a hematite (&alpha, Fe2O3) photoanode consisting of hematite nanorods grown onto an iron oxide blocking layer. This blocking layer is formed from a sputter deposited thin metallic iron film on fluorine doped tin oxide (FTO) by using cyclic voltammetry to fully convert the film into an anodic oxide. In a second step, hematite nanorods (NR) are grown onto the layer using a hydrothermal approach. In this geometry, the hematite sub-layer works as a barrier for electron back diffusion (a blocking layer). This suppresses recombination, and the maximum of the incident photon to current efficiency is increased from 12% to 17%. Under AM 1.5 conditions, the photocurrent density reaches approximately 1.2 mA/cm2 at 1.5 V vs. RHE and the onset potential changes to 0.8 V vs. RHE (using a Zn-Co co-catalyst).

In this work, we grow intrinsically Cu-doped TiO2 nanotubes (TiNTs) by self-organizing anodization of Ti–Cu binary alloys. We demonstrate that up to a copper concentration of 1.5 at.% in the alloy, self-ordered Cu2+-doped nanotubes can be grown. Under UV illumination the Cu2+ ion-doped oxide structures can be converted to nanotubes that carry metallic nanoparticles (NPs) uniformly decorated on top of the TiNTs. We investigate the formation of these metallic nanoparticles under UV illumination by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). The resulting intrinsic copper-doped and decorated TiNTs have a strongly enhanced photocatalytic activity for H2 evolution in comparison to pristine TiNTs. Key is the light-induced conversion of the intrinsic Cu dopant to metallic copper nanoparticles that act as a stable co-catalyst for H2 generation. Keywords: TiO2, Cu doped TiO2, Photocatalytic activity, Intrinsic copper decoration

Here we report that both-end open anodic TiO2 nanotube membranes, after sensitization with a Ru(II)-based dye, exhibit visible-light switching properties for flow-through the nanotube channels. Under illumination, the gate is in an open state providing ∼four-times faster permeation of small molecules through the membrane compared to a dark state. Switching is reversible with no apparent dye degradation being observed. Gating is possible not only of permeating dye molecules but also of nanoprobes such as polystyrene nanospheres. Supported by quantitative modelling, we attribute the switching mechanism to light-induced changes of the charge distribution at the dye/TiO2 interface which in turn alters the hydrodynamics within the anodic tube membranes. This demonstrates that these simple dye-sensitized nanotube membranes can be used as an optically addressable flow-through gate in nanofluidics.

The photocatalytic activity for H2 production from water, without presence of hole scavengers, of thermally reduced TiO2 nanoparticles (H-500, H-700) and neat anatase were followed by in-situ continuous-wave light-induced electron paramagnetic resonance technique (CW-LEPR), in order to correlate the H2 evolution rates with the electronic fingerprints of the photoexcited systems. Under UV irradiation, photoexcited electrons moved from the deep lattice towards the superficially exposed Ti sites. These photogenerated redox sites mediated (e- +h+ ) recombination and were the crucial electronic factor affecting catalysis. In the best-performant system (H-500), a synergic combination of mobile electrons was observed, which dynamically created diverse types of Ti3+ sites, including interstitial Ti3+ , and singly ionized electrons trapped in oxygen vacancies (VO. ). The interplay of these species fed successfully surface exposed Ti4+ sites, which became a catalytically active, fast reacting Ti4+ ⇄Ti3+ state that was key for the H2 evolution process.

Reduced titanium dioxide has recently attracted large attention, particularly for its unique co-catalyst-free H$_2$ heterogeneous photocatalytic application. The enhanced photocatalytic activity of the reduced TiO$_2$ was previously ascribed to the introduction of point crystal defects (mainly Ti$_3^+$ centers), which result in the formation of intrinsic co-catalytic centers and enhanced visible light absorption. In this work, we systematically investigate the effect of different defects in the TiO$_{2-x}$ lattice on photocatalytic H$_2$ evolution. To introduce different types of defects, thermal annealing in air (oxidative), Ar (inert), Ar/H$_2$ (reducing), and H$_2$ (reducing) atmospheres were performed on commercially available anatase nanopowder. Then, the powders were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) to clarify the effect of treatment on material properties. Furthermore, the defect types were characterized by electron paramagnetic resonance (EPR) spectroscopy. We show that thermal annealing in different atmospheres can form different amounts of different defect types in the TiO$_2$ structure. The highest photocatalytic activation is achieved by annealing the anatase powder in a reducing atmosphere for an appropriate temperature/annealing time. By combining the results from H$_2$ generation and EPR analysis we show that the simultaneous presence of two types of defects, i.e. surface exposed Ti$_3^+$ and lattice embedded Ti$_3^+$ centers, in an optimum low concentration, is the determining factor for an optimized photocatalytic H$_2$ evolution rate. In fact, annealing anatase powder under the so-reported optimized conditions in reducing atmosphere leads to the generation of a considerable amount of H$_2$, with rates as high as 338 $\mu$molh-1g-1.

Magneli phases of titanium dioxide (such as Ti4O7, Ti5O9, etc.) provide electronic properties, namely a stable metallic behavior at room temperature. In this manuscript, we demonstrate that nanoscopic Magneli phases, formed intrinsically in anatase during a thermal aerosol synthesis, can enable significant photocatalytic H2 generation. This without the use of any extrinsic co-catalyst in anatase. Under optimized conditions, mixed phase particles of 30 percent anatase, 25 percent Ti4O7 and 20 percent Ti5O9 are obtained that can provide, under solar light, direct photocatalytic H2 evolution at a rate of 145 micromol h-1 g-1. These anatase particles contain 5-10 nm size inter-grown phases of Ti4O7 and Ti5O9. Key is the metallic band of Ti4O7 that induces a particle internal charge separation and transfer cascade with suitable energetics and favorable dimensions that are highly effective for H2 generation.

The anodic oxidation of niobium is investigated in pure molten ortho-phosphoric acid (o-H3PO4). At applied potentials in the 2.5–60 V range and for electrolyte temperatures in the 60–110 °C range, porous niobium oxide layers are formed. Pore ordering and morphology depend on the anodization voltage, time and temperature, i.e. the anodic oxide develops different morphologies depending on low- or high-field anodizing conditions. At 100 °C and low voltages, e.g. ≤ 5 V, vertically oriented, amorphous oxide nanopores with cylindrical shape (“nanochannels”) grow with an average diameter of 5–8 nm. Higher voltages (10–20 V) lead to a less ordered pore morphology, resembling a “fish bone” structure. At 40–60 V, and after a sufficiently long anodization time (≥30 min), an orthorhombic Nb2O5 hierarchical structure forms that shows a bimodal pore size distribution with some 100 nm wide main pores that branch out into ∼ 10 nm wide side nanochannels. A partial crystallization of the anodic oxide is observed at high voltages (≥40 V). We propose that this is due to a high field-induced crystallite nucleation in the barrier oxide at the oxide/metal interface. With time, the crystalline oxide is incorporated in the porous layer. The main pores, partially crystalline and hence more stable in the anodizing electrolyte, result from the gradual dissolution of the amorphous side nanochannels.

Noble metal cocatalysts are conventionally a crucial factor in oxide-semiconductor-based photocatalytic hydrogen generation. In the present work, we show that optimized high-temperature hydrogenation of commercially available strontium titanate (SrTiO3) powder can be used to engineer an intrinsic cocatalytic shell around nanoparticles that can create a photocatalyst that is highly effective without the use of any additional cocatalyst for hydrogen generation from neutral aqueous methanol solutions. This intrinsic activation effect can also be observed for SrTiO3[100] single crystal as well as Nb-doped SrTiO3 (100) single crystal. For all types of SrTiO3 samples (nanopowders and either of the single crystals), hydrogenation under optimum conditions leads to a surface-hydroxylated layer together with lattice defects visible by transmission electron microscopy, electron paramagnetic resonance (EPR), and photoluminescence (PL). Active samples provide states in a defective matrix -- this is in contrast to the inactive defects formed in other reductive atmospheres. In aqueous media, active SrTiO3 samples show a significant negative shift of the flatband potential (in photoelectrochemical as well as in capacitance data) and a lower charge-transfer resistance for photoexcited electrons. We therefore ascribe the remarkable cocatalyst-free activation of the material to a synergy between thermodynamics (altered interface energetics induced by hydroxylation) and kinetics (charge transfer mediation by suitable Ti3+ states).

Titanium dioxide (TiO2) is the most investigated semiconductor in photocatalysis, but the photocatalytic performance can be further improved by rational design of the material as nanostructure with special surface and structural properties. In this chapter, we discuss methods for site-selective decoration of anodic TiO2 nanotube (NT) arrays, particularly to introduce stable defects and investigate the mechanism for enhanced activity and stability of these noble metal–free photocatalysts. The investigated methods include proton implantation and nitrogen implantation into the TiO2 NTs and decoration of alternative cocatalysts (e.g., molybdenum-based cocatalysts). Such modification on TiO2 NT layers leads to the creation of efficient photocatalysts for H2 evolution, which is ascribed to the synergistic effect of light harvesting by TiO2 NTs coupled with cocatalytic centers atop the tube layers. The described methods for selective decoration of TiO2 NTs can be further extended and potentially applied to the activation of a range of one-dimensional (1D) semiconductor materials.

In this work, we describe a simple anodization process of titanium in a hot phosphoric acid/ethylene glycol electrolyte that forms a hierarchically structured photocatalytically active TiO2 layer that consists directly of anatase nanocrystallites. Such anodic layers show, without any annealing, a high photocatalytic degradation activity for AO7 (an azo‐dye used as model pollutant) that is higher than the activity of comparable fully crystallized TiO2 nanotube layers. Furthermore, the hierarchically structured layers have a remarkable mechanical bending stability. These features make these novel anodic layers highly promising for applications in flexible substrate based photocatalytic devices.

The need to satisfy the growing global population’s enormous energy demands is a major challenge for modern societies. Photoelectrochemical (PEC) water splitting (WS) is seen as a leading strategy for producing an extremely promising renewable store of energy – hydrogen (H2). However, PEC-WS is a complex process involving several sequential physicochemical reaction steps including light absorption, separation of photoexcited charges, and surface redox reactions. At present, FeO-based semiconductors represent a unique class of materials known to exhibit very high performance in all these processes. This review summarizes and critically discusses the major components of PEC-WS systems incorporating FeO-based light-harvesting systems, and outlines the progress that has been made, particularly over the last decade. Emphasis is placed on materials used as photoanodes (including hematite and nonhematite iron oxides, spinel iron ferrites, and pseudobrookite iron titanates) as well as materials used as cocatalysts and passivation layers – notably iron hydroxyoxides and their composites. We discuss strategies for overcoming the main limitations of the aforementioned materials via nanostructuring, elemental doping, surface decoration, and the formation of advanced hybrid nanoarchitectures. Finally, we use this knowledge to present a critical overview of the field and the future prospects of Fe-O semiconductors in PEC-WS applications.

With a large-scale usage of portable electric appliances, a high demand for increasingly high density energy storage devices has emerged. MoO3 has, in principle, a large potential as negative electrode material in supercapacitive devices, due to high charge densities that can be obtained from its reversible redox reactions. Nevertheless, the extremely poor electrochemical stability of MoO3 in aqueous electrolytes prevents a practical use in high capacitance devices. In this work, we describe how to overcome this severe stability issue by forming amorphous molybdenum oxide/tantalum oxide nanotubes by anodic oxidation of a Mo-Ta alloy. The presence of a critical amount of Ta-oxide (> 20 at-%) prevents the electrochemical decay of the MoO3 phase and thus yields an extremely high stability. Due to the protection provided by tantalum oxide, no capacitance losses are measureable after 10000 charg-ing/discharging cycles.

Au and Pt do not form homogeneous bulk alloys as they are thermodynamically not miscible. However, we show that anodic TiO2 nanotubes (NTs) can in situ be uniformly decorated with homogeneous AuPt alloy nanoparticles (NPs) during their anodic growth. For this, a metallic Ti substrate containing low amounts of dissolved Au (0.1 atom %) and Pt (0.1 atom %) is used for anodizing. The matrix metal (Ti) is converted to oxide, whereas at the oxide/metal interface direct noble metal particle formation and alloying of Au and Pt takes place; continuously these particles are then picked up by the growing nanotube wall. In our experiments, the AuPt alloy NPs have an average size of 4.2 nm, and at the end of the anodic process, these are regularly dispersed over the TiO2 nanotubes. These alloyed AuPt particles act as excellent co-catalyst in photocatalytic H2 generation, with a H2 production rate of 12.04 μL h–1 under solar light. This represents a strongly enhanced activity as compared to TiO2 NTs decorated with mon...

Compared to many other metal oxides, anatase TiO2 shows relatively lower reactivity toward carboxylic acid anchor groups. The latter is crucial for applications, for example, in dye-sensitized solar cells (DSSCs), where the most used dyes bind to the metal oxide surface through carboxylic acid terminations. To improve the surface reactivity, metal-phosphate bilayers of Ni or Co were synthesized on anatase TiO2 compact oxide and nanotubes. In both cases, time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) results showed that the bilayers were successfully formed and that the phosphate layer works as an intermediate between TiO2 and the other species. ToF-SIMS depth profiles of modified nanotubes showed that Ni and Co are present through the whole tube length and reduce in content after heat treatment, in agreement with XPS results. Phosphate groups, on the other hand, are more present in the tubes' depth, and their content on the surface is reduced upon exposure to temperature. The reactivity of the modified surfaces toward carboxylic acid-terminated molecules, as stearic acid and Ru-based N719 dye, was evaluated. Contact angle measurements together with dye desorption experiments demonstrated that the Co-phosphate bilayers heat-treated at 300 °C resulted in the largest enhancement compared to the reference. Bilayer-modified compact anatase TiO2 and anatase TiO2 nanotubes were utilized as photoanodes in DSSCs. An increase in efficiency was observed for all modified electrodes with phosphate-Co treatment, leading to the highest JSC values and an efficiency improvement of 48%.

One prominent cause of implant failure is infection; therefore, research is focusing on developing surface coatings that render the surface resistant to colonization by micro-organisms. Permanently attached coatings of antimicrobial molecules are of particular interest because of the reduced cytoxicity and lower risk of developing resistance compared to controlled release coatings. In this study, we focus on the chemical grafting of bioactive molecules on titanium. To concentrate the molecules at the metallic implant surface, we propose electrophoretic deposition (EPD) applying alternating current (AC) signals with an asymmetrical wave shape. We show that for the model molecule bovine serum albumin (BSA), as well as for the clinically relevant antifungal lipopeptide caspofungin (CASP), the deposition yield is drastically improved by superimposing a DC offset in the direction of the high-amplitude peak of the AC signal. Additionally, in order to produce immobilized CASP coatings, this experimental AC/DC-EPD method is combined with an established surface activation protocol. Principle component analysis (PCA) of time-of-flight secondary ion mass spectrometry (ToF-SIMS) data confirm the immobilization of CASP with higher yield as compared to a diffusion-controlled process, and higher purity than the clinical CASP starting suspensions. Scratch testing data indicate good coating adhesion. Importantly, the coatings remain active against the fungal pathogen C. albicans as shown by in vitro biofilm experiments. In summary, this paper delivers a proof-of-concept for the application of AC-EPD as a fast grafting tool for antimicrobial molecules without compromising their activities.

In the present work we show how to achieve different nanotube morphologies (namely, not only single- and double-walled tubes, but also a tube-in-tube configuration) by combining specific anodization parameters. We characterize as-grown tube layers in terms of morphology, chemical composition and properties. As-grown tubes exhibit a double-walled morphology, that is, they consist of an inner and an outer shell. The low quality inner shell can be removed by an optimized chemical treatment thus leading to nanotubes of a higher quality oxide and with a single-walled nature. Double-walled tubes grown at low electrolyte temperature provide a thick inner shell that after adequate annealing can form a unique tube-in-tube morphology.

Solar driven photoelectrochemical water splitting (PEC-WS) using semiconductor photoelectrodes represents a promising approach for a sustainable and environmentally friendly production of renewable energy vectors and fuel sources, such as dihydrogen (H2). In this context, titanium dioxide (TiO$_2$) and iron oxide (hematite, $\alpha$-Fe$_2$O$_3$) are among the most investigated candidates as photoanode materials, mainly owing to their resistance to photocorrosion, non-toxicity, natural abundance, and low production cost. Major drawbacks are, however, an inherently low electrical conductivity and a limited hole diffusion length that significantly affect the performance of TiO$_2$ and $\alpha$-Fe$_2$O$_3$ in PEC devices. To this regard, one-dimensional (1D) nanostructuring is typically applied as it provides several superior features such as a significant enlargement of the material surface area, extended contact between the semiconductor and the electrolyte and, most remarkably, preferential electrical transport that overall suppress charge carrier recombination and improve TiO$_2$ and $\alpha$-Fe$_2$O$_3$ photo-electrocatalytic properties. The present review describes various synthetic methods, properties and PEC applications of 1D-photoanodes (nanotubes, nanorods, nanofibers, nanowires) based on titania, hematite, and on $\alpha$-Fe$_2$O$_3$/TiO$_2$ heterostructures. Various routes towards modification and enhancement of PEC activity of 1D photoanodes are also discussed including doping, decoration with co-catalysts and heterojunction engineering. Finally, the challenges related to the optimization of charge transfer kinetics in both oxides are highlighted.

We report on free-standing Ta3N5 nanotubular membranes with open top and bottom, used as visible-light-active, flow-through photocatalytic micro-reactors. We grow first a robust anodic Ta2O5 layer, lift-off a membrane and convert to Ta3N5. Such membranes can easily, in a flow-through mode, degrade methylene blue under visible light (wavelength >400 nm) or solar illumination.

In this work we introduce the use of TiN/Ti2N layers as a back contact for Ta3N5 membranes of lifted-off anodic Ta3N5 nanotubular layers. In photoelectrochemical H2 generation experiments under simulated AM 1.5G light, a shift in the onset potential of anodic photocurrents to lower potentials is observed, as well as a higher magnitude of the photocurrents compared to a conventional Ta3N5 nanotubular layer (Ta3N5/Ta, ~0.5 VRHE). We ascribe this beneficial effect to the improved conductive properties of the TiNx-based back contact layer that enables a facilitated electron transport from the tantalum nitride based materials to the conductive substrate. Keywords: Tantalum nitride, Nanotubular membranes, Titanium nitride, Water splitting

Hollow titanium dioxide (TiO2) nanotubes offer substantially higher drug loading capacity and slower drug release kinetics compared to solid drug nanocarriers of comparable size. In this report, we load TiO2 nanotubes with iron oxide nanoparticles to facilitate site-specific magnetic guidance and drug delivery. We generate magnetic TiO2 nanotubes (TiO2NTs) by incorporating a ferrofluid containing Ø ≈ 10 nm iron oxide nanoparticles in planar sheets of weakly connected TiO2 nanotubes. After thermal annealing, the magnetic tubular arrays are loaded with therapeutic drugs and then sonicated to separate the nanotubes. We demonstrate that magnetic TiO2NTs are non-toxic for HeLa cells at therapeutic concentrations (≤200 µg/mL). Adhesion and endocytosis of magnetic nanotubes to a layer of HeLa cells are increased in the presence of a magnetic gradient field. As a proof-of-concept, we load the nanotubes with the topoisomerase inhibitor camptothecin and achieve a 90% killing efficiency. We also load the nanotubes with oligonucleotides for cell transfection and achieve 100% cellular uptake efficiency. Our results demonstrate the potential of magnetic TiO2NTs for a wide range of biomedical applications, including site-specific delivery of therapeutic drugs.

Cu-doping in titania is usually detrimental to the material’s photoconductivity which prevents the use of this combination in photoanodes. In this work, we produce TiO2 nanotube arrays intrinsically doped with copper and establish sufficient conductivity to use them as efficient photoanodes for methanol oxidation in a photoelectrochemical hydrogen generation setting. Firstly, Cu doped TiO2 nanotubes were produced by anodizing a Ti-Cu binary alloy. By subsequent thermal reduction of the structure in an Ar/H2 environment, conductive copper doped TiO2 nanotubes (TiCuTN-Ar/H2) can be achieved with an approximately 103 times higher conductivity than the non-reduced material. When these reduced Cu doped TiO2 nanotubes are used as photoanode, copper species embedded in the TiO2 wall catalyze the methanol oxidation reaction. As a result of the combined effect of conductivity and catalytic effect of Cu, such a reduced Cu:TiO2 nanotubes can generate a photocurrent of 0.76 mA.cm-2 at 1 V vs. RHE, under AM1.5 (100 mW/Cm2) irradiation – in a 50:50 MeOH/water solution – this is 33 times higher than for pristine Cu:TiO2 nanotubes.

TiO2 nanotubes have been investigated in photoelectrochemistry and photocatalysis for more than a decade. However, up to now, a systematic investigation of different hole scavengers is still lacking. Here we investigate the effect of the most relevant sacrificial hole scavengers on the photoelectrochemical properties and photocatalytic H2 evolution performance of pristine and Pt-decorated anodic TiO2 nanotubes. We examine methanol, isopropanol, ethylene glycol, EDTA-Na2, as well as Na2SO3, and find that the incident photocurrent conversion efficiency (IPCE) of the TiO2 nanotubes in 0.1 M Na2SO4 electrolytes increases by 1.8–3.1 times, depending on the used hole scavenger. The efficiency increases in the sequence Na2SO3 < isopropanol < MeOH < ethylene glycol < EDTA-Na2. In presence of any hole scavenger, for nanotubes in the length-range of 2-10 μm, the photocurrent spectra and the ICPE magnitude are independent of the tube length. The photocurrent onset potential (optical flatband potential) is significantly affected by the different type of scavengers, in line with their red-ox potential. Under open circuit conditions (photocatalytic conditions), organic hole scavengers lead to a 10.0 - 28.8 times higher H2 production by TiO2 nanotubes than the scavenger-free case, with a sequence MeOH > i-PrOH > EDTA-Na2 > EG, while a detrimental effect of Na2SO3 is observed. These results are compared to results obtained for TiO2 particles, and discussed in terms of various concepts in the literature.

Solutions that simulate hydrometallurgical base metal process streams with high nickel (Ni) and minor platinum (Pt) concentrations were used to create Pt/Ni nanoparticles on TiO2 nanotube surfaces. For this, electrochemical deposition – redox replacement (EDRR) was used that also allowed to control the nanoparticle size, density and Pt/Ni content of the deposited nanoparticles The Pt/Ni nanoparticle decorated titanium dioxide nanotubes (TiO2 nanotubes) become strongly activated for photocatalytic hydrogen (H2) evolution. Moreover, EDRR facilitates nanoparticle formation without the need for any additional chemicals and is more effective than electrodeposition alone. Actually, a 10,000-time enrichment level of Pt took place on the TiO2 surface when compared to Pt content in the solution with the EDRR method. The results show that hydrometallurgical streams offer great potential as an alternative raw material source for industrial catalyst production when coupled with redox replacement electrochemistry.

TiS2 nanotubular structures were synthesized by a high temperature H2S treatment of anodized TiO2 nanotube layers, and their electrochemical activity for a use as counter electrodes in dye sensitized solar cells (DSSCs) was evaluated. During conversion to TiS2 compositional, morphological and structural transformations were monitored. The fully converted TiS2 nanostructures show a high electrocatalytic activity for the I−/I3− oxidation comparable to a nanoparticular platinum layer. For a simplified model a DSSC solar cell efficiency of 6.1% was obtained using the TiS2 nanotube layer as counter electrode, which is very close to values obtained for a Pt reference (6.2%).

Polypropylene (PP) powders are coated with silica nanoparticles in a fluidized bed to improve the flow behavior of the powders and the processability in powder bed fusion. The nanoparticles are produced in situ via dusty plasma‐enhanced chemical vapor deposition (PECVD) in an atmospheric‐pressure Ar/O2 plasma jet fixed at the distributor plate of the fluidized bed. Hexamethyldisiloxane is used as a precursor of the nanoparticles. The influence of the oxygen concentration in the plasma gas and the number of treatment cycles on the chemical composition of the nanoparticles, the amount of nanoparticles deposited, and the flow properties of the coated PP powders is investigated. The chemical composition of the formed silica particles is determined by X‐ray photon spectroscopy and infrared spectroscopy. The results reveal that the composition of the nanoparticles is SiOxCy, that is, the portion of organic residues introduced by the precursor can be controlled by changing the oxygen concentration in the plasma gas. The mass of nanoparticles deposited on the polymer powder's surface, as determined by inductively coupled optical emission spectroscopy, shows a linear dependence of the number of cycles and the oxygen concentration in the plasma gas. A considerable improvement of the flow behavior of the PP powders is observed after PECVD treatment.

Bismuth vanadate (BiVO4) is a promising material for photoelectrochemical (PEC) water splitting, however, its PEC performance is limited by the high surface and bulk charge recombination rates. Here we present a comprehensive study to elucidate a recombination phenomenon of BiVO4 that arises with Mo doping. The Mo doping produces multiple effects including the formation of MoOx (reduced form of Mo6+) species and oxygen vacancies (VOs) on the surface of the BiVO4 that work in tandem with V4+ species (and MoOx) acting as surface-active intermediates (i-SS) providing improved hole transfer to the electrolyte. In contrast, in the absence of V4+ species, the VOs can act as recombination centers (r-SS). Further, CoOOH co-catalyst coating is used to minimize such recombination centers. Eventually, a photocurrent enhancement of ~37 times (1.1 mA/cm2 at 1.23 V vs. RHE) and a cathodic shift in onset potential of ~500 mV compared to that of pristine BiVO4 (0.03 mA/cm2 at 1.23 V vs. RHE) is obtained. We carried out in-depth PEC analysis using hole scavenger measurements, PEC impedance spectroscopy, and intensity-modulated photocurrent spectroscopy to elucidate the effect of the surface reduction process upon doping, the impact of Vos, MoOx species and CoOOH layer on the enhanced PEC performance.

In this work, we design a TiO2 nanomembrane (TiNM) that can be used as a nanofilter platform for a selective enrichment of specific proteins. After use the photocatalytic properties of TiO2 allow to decompose unwanted remnant on the substrate and thus make the platform reusable. To construct this platform we fabricate a free-standing TiO2 nanotube array and remove the bottom oxide to form a both-end open TiNM. By pyrolysis of the natural tube wall contamination (C/TiNM), the walls become decorated with graphitic carbon patches. Owing to the large surface area, the amphiphilic nature and the charge adjustable character, this C/TiNM can be used to extract and enrich hydrophobic and charged biomolecules from solutions. Using human serum albumin (HSA) as a model protein as well as protein mixtures, we show that the composite membrane exhibits a highly enhanced loading capacity and protein selectivity and is reusable after a short UV treatment.

TiO2 nanotube arrays grown by anodization were coated with thin layers of polydopamine as visible light sensitizer. The PDA-coated TiO2 scaffolds were used as photocatalyst for selective oxidation of benzyl alcohol under monochromatic irradiation at 473 nm. Benzaldehyde was selectively formed and no by-products could be detected. A maximized reaction yield was obtained in O2-saturated acetonitrile. A mechanism is proposed that implies firstly the charge carrier generation in polydopamine as a consequence of visible light absorption. Secondly, photo-promoted electrons are injected in TiO2 conduction band, and subsequently transferred to dissolved O2 to form oxygen radical anions. These radicals react with benzyl alcohol and lead to its selective dehydrogenation oxidation towards benzaldehyde.

In the present work we use TiO2 nanotube arrays, deposit onto them a thin layer of Pt, dewet the Pt to nanoparticles, and use the structures as photocatalyst to generate H2. In order to achieve a maximum H2-generation efficiency, on the one hand an optimal thermal dewetting of the Pt layer is needed (this requires an oxygen-free heat treatment), and on the other hand an optimal crystallization of the TiO2 nanotubes into a suitable crystalline oxide is required (achieved by annealing in an O2-containing environment). To combine the two requirements, we provide an adequate multiple annealing treatment under reducing and oxidizing conditions and obtain Pt-nanoparticle-decorated TiO2 nanotubes, composed of an anatase–rutile mixed phase, that show a significantly enhanced photocatalytic H2 generation ability compared to tube layers that underwent any single-step annealing process.

During self-organized nanotubular oxide growth by electrochemical anodization of Ti in lactic acid (LA) containing dimethyl sulfoxide (DMSO) electrolytes, we observe the occurrence of regular current oscillations (COs). Key factors that influence COs are the LA concentration, the surface roughness and diffusion limitation. The oscillation characteristics (amplitude and period) depend on both LA concentration and applied voltage. Under optimized oscillation conditions a sequential growth/detachment of the TiO2 layers is observed that leads to a self-stacking of TiO2 nanostructured multilayers. Keywords: Anodization, TiO2 nanotubes, Electrochemical oscillations, Multilayers

Nanoscale topography in improving vascular response in vitro was established previously on various titanium surfaces. In the present study different surface nanotopographies that is different diameters of titanium dioxide (TiO2 ) nanotubes (NTs) were fabricated by electrochemical anodization and conditioned with highly reactive gaseous oxygen plasma. The morphology of different diameter NTs was studied by scanning electron microscopy and atomic force microscopy, while changes in chemical composition on the surface before and after plasma treatment were determined by X-ray photoelectron spectroscopy. Performance of human coronary artery endothelial cells (HCAEC) on those conditioned surfaces was studied in regard to cell proliferation, released IL-6 protein and immunofluorescence microscopy (IFM). We show that HCAEC function is dependent on the diameter of the TiO2 NTs, functioning far less optimally when bound to 100 nm TiO2 NTs as compared to Ti foil, 15 nm NTs or 50 nm NTs. There were improved, morphological cell shape changes, observed with IFM, between HCAEC growing on oxygen-rich plasma-treated versus nontreated 100 nm NTs. These endothelialized conditioned Ti nanosurfaces could elucidate optimization conditions necessary for vascular implants in coronary arteries.