Xuemei Zhou

Black TiO2 nanotubes formed by high energy proton implantation show noble-metal-co-catalyst free photocatalytic H2-evolution

We apply high-energy proton ion-implantation to modify TiO2 nanotubes selectively at their tops. In the proton-implanted region, we observe the creation of intrinsic cocatalytic centers for photocatalytic H2-evolution. We find proton implantation to induce specific defects and a characteristic modification of the electronic properties not only in nanotubes but also on anatase single crystal (001) surfaces. Nevertheless, for TiO2 nanotubes a strong synergetic effect between implanted region (catalyst) and implant-free tube segment (absorber) can be obtained.

Hydrogenated Anatase: Strong Photocatalytic Dihydrogen Evolution without the Use of a Co-Catalyst†

The high-pressure hydrogenation of commercially available anatase or anatase/rutile TiO2 powder can create a photocatalyst for H2 evolution that is highly effective and stable without the need for any additional co-catalyst. This activation effect cannot be observed for rutile; however, for anatase/rutile mixtures, a strong synergistic effect can be found (similar to results commonly observed for noble-metal-decorated TiO2). EPR and PL measurements indicated the intrinsic co-catalytic activation of anatase TiO2 to be due to specific defect centers formed during hydrogenation. These active centers can be observed specifically for high-pressure hydrogenation; other common reduction treatments do not result in this effect.

Photocatalysis with TiO2 Nanotubes: “Colorful” Reactivity and Designing Site-Specific Photocatalytic Centers into TiO2 Nanotubes

Photocatalytic reactions on TiO2 have recently gained an enormous resurgence because of various new strategies and findings that promise to drastically increase efficiency and specificity of such reactions by modifications of the titania scaffold and chemistry. In view of geometry, in particular, anodic TiO2 nanotubes have attracted wide interest, as they allow a high degree of control over the separation of photogenerated charge carriers not only in photocatalytic reactions but also in photoelectrochemical reactions. A key advantage of ordered nanotube arrays is that nanotube modifications can be embedded site specifically into the tube wall; that is, cocatalysts, doping species, or junctions can be placed at highly defined desired locations (or with a desired regular geometry or pattern) along the tube wall. This allows an unprecedented level of engineering of energetics of reaction sites for catalytic and photocatalytic reactions, which target not only higher efficiencies but also the selectivity of re...

Aligned MoOx/MoS2 Core–Shell Nanotubular Structures with a High Density of Reactive Sites Based on Self‐Ordered Anodic Molybdenum Oxide Nanotubes

The present work demonstrates the self-organized formation of anodic molybdenum oxide nanotube arrays. The amorphous tubes can be crystallized to MoO2 or MoO3 and be converted fully or partially into molybdenum sulfide. Vertically aligned MoOx /MoS2 nanotubes can be formed when, under optimized conditions, defined MoS2 sheets form in a layer by layer arrangement that provide a high density of reactive stacking misalignments (defects). These core-shell nanotube arrays consist of a conductive suboxide core and a functional high defect density MoS2 coating. Such structures are highly promising for applications in electrocatalysis (hydrogen evolution) or ion insertion devices.In the present work we demonstrate the self-organized formation of anodic Mo-oxide nanotube arrays grown on a Mo sheet under suitable electrochemical conditions in glycerol/NH4F electrolytes. The resulting amorphous tubes can be crystallized by annealing to MoO2 or MoO3. The tube walls then can be further sulfurized fully or partially to Mo-sulfide to form well-ordered arrays of vertically aligned MoOx/MoS2 nanotubes. Under optimized conditions, defined MoS2 sheets form on the oxide walls in a layer by layer low angle zig-zag arrangement that provide a high density of reactive stacking faults. These core-shell nanotube arrays, consisting of tubes with a conductive suboxide core and a functional high defect density MoS2 coating, are highly promising for applications such as electrocatalysis (hydrogen evolution) or ion insertion devices.

Strongly Enhanced Water Splitting Performance of Ta3 N5 Nanotube Photoanodes with Subnitrides.

Subnitrides strongly enhance the efficiency of Ta3 N5 -nanotube photoanodes in photochemical water splitting. The fabrication of Ta3 N5 nanotube layers with a controlled subnitride layer at the interface to the back contact is demonstrated. The insertion of this subnitride layer has a strong influence on the electron transfer to the back contact, and as a result leads to a drastic shift in photocurrent onset potential and a considerable enhancement of photocurrent conversion efficiency.

Visible-Light-Triggered Drug Release from TiO2 Nanotube Arrays: A Controllable Antibacterial Platform

In this work, we use a double-layered stack of TiO2 nanotubes (TiNTs) to construct a visible-light-triggered drug delivery system. The key for visible light drug release is a hydrophobic cap on the nanotubes containing Au nanoparticles (AuNPs). The AuNPs allow for a photocatalytic scission of the hydrophobic chain under visible light. To demonstrate this principle, we loaded ampicillin (AMP) into the lower part of the TiO2 nanotube stack, triggered visible-light-induced release, and carried out antibacterial studies. The release from the platform becomes most controllable if the drug is silane-grafted in the hydrophilic bottom layer for drug storage. Thus, visible light photocatalysis can also determine the release kinetics of the active drug from the nanotube wall.

Plasmon‐Enhanced Photoelectrochemical Water Splitting Using Au Nanoparticles Decorated on Hematite Nanoflake Arrays

Hematite nanoflake arrays were decorated with Au nanoparticles through a simple solution chemistry approach. We show that the photoactivity of Au-decorated Fe2 O3 electrodes for photoelectrochemical water oxidation can be effectively enhanced in the UV/Visible region compared with the bare Fe2 O3 . Au-nanoparticle-decorated Fe2 O3 nanoflake electrodes exhibit a significant cathodic shift of the onset potential up to 0.6 V [vs. reversible hydrogen electrode (RHE)], and a two times increase in the water oxidation photocurrent is achieved at 1.23 VRHE . A maximum photocurrent of 2.0 mA cm(-2) at 1.6 VRHE is obtained in 1 M KOH under AM 1.5 (100 mW cm(-2) ) conditions. The enhancement in photocurrent can be attributed to the Au nanoparticles acting as plasmonic photosensitizers that increase the optical absorption.

Enhanced Charge Transport in Tantalum Nitride Nanotube Photoanodes for Solar Water Splitting

In the present work we grow anodic self-organized Ta2O5 nanotube layers, which are converted by ammonolysis to Ta3 N5 nanotubes, and then are used as photoanodes for photoanalytic water splitting. We introduce a two-step anodization process that not only improves order (reduced growth defects) and overall light absorption in the nanotube layers, but also provides a significantly reduced interface charge resistance at the nitride/metal interface due to subnitride (TaNx ) formation. As a result, such nanotube anodes afford a 15-fold increase of the photocurrent compared with conventional nanotubular Ta3 N5 electrodes under AM 1.5 G simulated sunlight (100 mW cm(-2)) conditions.

Stable Co‐Catalyst‐Free Photocatalytic H2 Evolution From Oxidized Titanium Nitride Nanopowders

A simple strategy is used to thermally oxidize TiN nanopowder (∼20 nm) to an anatase phase of a TiO2:Ti(3+):N compound. In contrast to the rutile phase of such a compound, this photocatalyst provides activity for hydrogen evolution under AM1.5 conditions, without the use of any noble metal co-catalyst. Moreover the photocatalyst is active and stable over extended periods of time (tested for 4 months). Importantly, to achieve successful conversion to the active anatase polymorph, sufficiently small starting particles of TiN are needed. The key factor for catalysis is the stabilization of the co-catalytically active Ti(3+) species against oxidation by nitrogen present in the starting material.

Spaced TiO2 nanotube arrays allow for a high performance hierarchical supercapacitor structure

In this work, we describe the synthesis and electrochemical properties of nitridated hierarchical TiO2 nanotubes as an electrode for supercapacitors. The hierarchical TiO2 nanostructures are formed by a controlled layer-by-layer TiO2 nanoparticle decoration on self-organized spaced TiO2 nanotubes. These structures are then annealed in a NH3 atmosphere at elevated temperature to convert the material to a nitride structure – this drastically enhances their electron-transport properties. The areal capacitance of hierarchical structures can be tuned by changing the number of decorated TiO2 nanoparticle layers. The capacitance enhancement of the hierarchical structures reaches a maximum when the surface area through nanoparticle deposition is highest and the conductivity via nitridation is optimized.