Extraction of Hydrophobic Analytes from Organic Solution into a Titanate 2D-Nanosheet Host: Electroanalytical Perspectives

Titanate nanosheets (single layer, typically 200 nm lateral size) deposited from aqueous colloidal solution onto electrode surfaces form lamellar hosts that bind redox active molecular redox probes. Here, hydrophobic redox systems such as anthraquinone, 1-amino-anthraquinone, deca-methylferrocene, 5,10,15,20-tetraphenyl-21H,23H-porphine manganese (III) chloride (TPPMnCl), and α-tocopherol are shown to bind directly from cyclopentanone solution (and from other types of organic solvents) into the titanate nanosheet film. For anthraquinone derivatives, stable voltammetric responses are observed in aqueous media consistent with 2-electron 2-proton reduction, however, independent of the pH of the outside solution phase environments. For decamethylferrocene a gradual decay of the voltammetric response is observed, but for TPPMnCl a more stable voltammetric signal is seen when immersed in chloride containing (NaCl) electrolyte. α-Tocopherol exhibits chemically irreversible oxidation and is detected with 1 mM–20 mM linear range and approximately 10 −3 M concentration limit of detection. All redox processes exhibit an increase in current with increasing titanate film thickness and with increasing external electrolyte concentration. This and other observations suggest that important factors are analyte concentration and mobility within the titanate host, as well as ion exchange between titanate nanosheets and the outside electrolyte phase to maintain electroneutrality during voltammetric experiments. The lamellar titanate (with embedded tetrabutyl-ammonium cations) behaves like a hydrophobic host (for hydrophobic redox systems) similar to hydrophobic organic microphase systems. Potential for analytical applications is discussed. </p

Extraction of Hydrophobic Analytes from Organic Solution into a Titanate 2D-Nanosheet Host: Electroanalytical Perspectives

Titanate nanosheets (single layer, typically 200 nm lateral size) deposited from aqueous colloidal solution onto electrode surfaces form lamellar hosts that bind redox active molecular redox probes. Here, hydrophobic redox systems such as anthraquinone, 1-amino-anthraquinone, deca-methylferrocene, 5,10,15,20-tetraphenyl-21H,23H-porphine manganese (III) chloride (TPPMnCl), and α-tocopherol are shown to bind directly from cyclopentanone solution (and from other types of organic solvents) into the titanate nanosheet film. For anthraquinone derivatives, stable voltammetric responses are observed in aqueous media consistent with 2-electron 2-proton reduction, however, independent of the pH of the outside solution phase environments. For decamethylferrocene a gradual decay of the voltammetric response is observed, but for TPPMnCl a more stable voltammetric signal is seen when immersed in chloride containing (NaCl) electrolyte. α-Tocopherol exhibits chemically irreversible oxidation and is detected with 1 mM–20 mM linear range and approximately 10−3 M concentration limit of detection. All redox processes exhibit an increase in current with increasing titanate film thickness and with increasing external electrolyte concentration. This and other observations suggest that important factors are analyte concentration and mobility within the titanate host, as well as ion exchange between titanate nanosheets and the outside electrolyte phase to maintain electroneutrality during voltammetric experiments. The lamellar titanate (with embedded tetrabutyl-ammonium cations) behaves like a hydrophobic host (for hydrophobic redox systems) similar to hydrophobic organic microphase systems. Potential for analytical applications is discussed. Keywords: 2D-nanosheet, Microphase, Hydrophobicity, Electrocatalysis, Sensor, Membran

Electroanalysis in 2D-TiO2 Nanosheet Hosts:Electrolyte and Selectivity Effects in Ferroceneboronic Acid – Saccharide Binding

A 2D-TiO2 nanosheet material (as a film deposit of approximately 1 μm thickness on glassy carbon) is employed to host ferroceneboronic acid receptor molecules. It is suggested that the negative surface charge on 2D-TiO2 nanosheets allows weak binding of ferroceneboronic acid, which can then be employed to detect fluoride, glucose, or fructose. The nature of the aqueous electrolyte is shown to strongly affect the ferroceneboronic acid – host interaction. In the presence of di-sodium sulfate stable reversible voltammetric responses are observed. In the presence of fluoride loss of the ferroceneboronic acid occurs probably due to weakening of the boron-titanate interaction. For glucose and for fructose “bound” and “unbound” states of the ferroceneboronic acid are observed as long as fast square wave voltammetry is employed to capture the “bound” state before decomplexation can occur. It is shown that this kinetic selectivity is highly biassed towards fructose and essentially insensitive to glucose.</p

Processes associated with ionic current rectification at a 2D-titanate nanosheet deposit on a microhole poly(ethylene terephthalate) substrate

Films of titanate nanosheets (approx. 1.8-nm layer thickness and 200-nm size) having a lamellar structure can form electrolyte-filled semi-permeable channels containing tetrabutylammonium cations. By evaporation of a colloidal solution, persistent deposits are readily formed with approx. 10-μm thickness on a 6-μm-thick poly(ethylene-terephthalate) (PET) substrate with a 20-μm diameter microhole. When immersed in aqueous solution, the titanate nanosheets exhibit a p.z.c. of − 37 mV, consistent with the formation of a cation conducting (semi-permeable) deposit. With a sufficiently low ionic strength in the aqueous electrolyte, ionic current rectification is observed (cationic diode behaviour). Currents can be dissected into (i) electrolyte cation transport, (ii) electrolyte anion transport and (iii) water heterolysis causing additional proton transport. For all types of electrolyte cations, a water heterolysis mechanism is observed. For Ca 2+ and Mg 2+ ions, water heterolysis causes ion current blocking, presumably due to localised hydroxide-induced precipitation processes. Aqueous NBu 4 + is shown to ‘invert’ the diode effect (from cationic to anionic diode). Potential for applications in desalination and/or ion sensing are discussed. [Figure not available: see fulltext.]. </p

Kinetics of alkali metal ion exchange into nanotubular and nanofibrous titanates

The kinetics of intercalation of Li+, Na+, K+ and Cs+ cations between the layers of titanate nanotubes and nanofibres have been studied in an aqueous suspension of nanotubes at 25ºC. The rate of intercalation was found to be similar for different cations and depended on the length of the nanotubes. The decrease in nanotube length resulted in a higher rate of ion-exchange, indicating that the transport of cations in titanate nanotubes occurred probably along their length. In contrast, the transport of cations in titanate nanofibres probably dominated in the direction perpendicular to length. Correlations between the rate of intercalation and the crystal structure modification following intercalation have been established for nanotubular and nanofibrous titanates

Titanate and Titania Nanotubes: Synthesis, Properties and Applications

ContentsAbbreviations ixList of symbols xiChapter 1 Introduction and Scope 1 1.1 The history of nanomaterials 1 1.1.1 The importance of TiO2 and titanate nanomaterials 3 1.2 Classification of the structure of nanomaterials 5 1.3 Synthesis of important elongated nanomaterials 7 1.3.1 Metal oxide nanotubes 7 1.3.2 Metal chalcogenide nanotubes 12 1.3.3 Mixed oxides, silicates and other compounds as nanotubes 13 1.4 Techniques and instruments used to study nanomaterials 15 References: 16Chapter 2 Synthesis Techniques and the Mechanism of Growth 19 2.1 Template methods 19 2.2 Alkaline hydrothermal synthesis of elongated titanates 23 2.2.1 Alkaline hydrothermal synthesis of titanate nanotubes and nanofibres 24 2.2.2 Mechanism of nanostructure growth 26 2.2.3 Methods to control the morphology of nanostructures 33 2.3 Electrochemical (anodic) oxidation 35 2.3.1 Principles and examples 35 2.3.2 Mechanism of nanotube growth 38 2.3.3 Methods to the control the morphology of nanotubes 40 2.4 Conclusions 42 References 43Chapter 3 Structural and Physical Properties of Elongated TiO2 and Titanate Nanostructures 47 3.1 Crystallography 47 3.1.1 Crystallography of titanate nanotubes 47 3.1.2 Crystallography of titanate nanofibres, nanorods and nanosheets 51 3.1.3 Crystallography of anodized and template assisted TiO2 52 3.1.4 Conclusions 52 3.2 Adsorption, surface area and porosity 53 3.2.1 Surface area of nanotubes 53 3.2.2 Pore volume of nanotubes 56 3.2.3 Effect of ionic charge on adsorption from aqueous solutions 59 3.3 Electronic structure of titanate nanotubes 61 3.3.1 Spectroscopy of titanate nanotubes: UV-Vis, Pl, ESR, XPS, NMR, Raman and FTIR 63 3.3.2 Electrical-, proton- and thermal conductivities of titanate nanotubes 70 3.4 Physical properties of TiO2 nanotube arrays 71 References 73Chapter 4 Chemical Properties, Transformation and Functionalization of Elongated Titanium Oxide Nanostructures 77 4.1 Thermodynamic equilibrium between the nanotube and its environment 77 4.2 Ion-exchange properties of nanostructured titanates 80 4.2.1 Kinetic characteristics of ion-exchange 80 4.2.2 Decoration of nanotubes using using the ion-exchange method 86 4.2.3 Decoration of substrates with nanotubes 88 4.3 Surface chemistry and functionalization of nanostructured titanates 91 4.4 Stability of nanotubes and phase transformations 92 4.4.1 Thermal stability 92 4.4.2 Acidic environments 95 4.4.3 Mechanical treatment 95 References 95Chapter 5 Potential Applications 98 5.1 Energy conversion and storage 98 5.1.1 Solar cells 98 5.1.2 Lithium batteries 101 5.1.3 Fuel cells and batteries 104 5.1.4 Hydrogen storage and sensing 107 5.2 Catalysis, electrocatalysis and photocatalysis 108 5.2.1 Reaction catalysis 108 5.2.2 Supercapacitors and general electrochemistry 115 5.2.3 Photocatalysis in elongated titanates and TiO2 117 5.3 Magnetic materials 124 5.4 Drug delivery and bio-applications 125 5.5 Composites, surface finishing and tribological coatings 126 5.6 Other applications 128 References 128<br/

Elongated titanate nanostructures and their applications

Recent advances in the synthesis, characterisation and applications of elongated titanates and TiO2 nanostructures (including nanotubes, nanofibres and nanorods) are reviewed. The physico-chemical properties of nanostructures, such as high surface area, efficient ion-exchanged properties, electron and proton conductivity and high aspect ratio, are described in connection with a particular application. Practical aspects of the preparation, stability and transformation of elongated titanates are considered. A critical survey of the literature is provided together with the development of prospective energy applications of elongated titanates in catalysis, photocatalysis, electrocatalysis, solar cells, fuel cells, lithium batteries and hydrogen storage. Other applications utilising the high aspect ratio of elongated nanostructures include biomedical implants, sensors, drug delivery systems and smart, tribological composite coatings

The stability of halloysite nanotubes in acidic and alkaline aqueous suspensions

The long term stability of natural halloysite nanotubes was studied at room temperature (22 ± 2 ºC) in pure water, acidic and basic aqueous suspensions. The structural and morphological transformations of nanotubes were studied by TEM, SEM, nitrogen adsorption, XRD Raman and FTIR spectroscopy accompanied by monitoring the concentration of dissolved Si(IV) and Al(III) in solution. It has been revealed that, in 1 mol dm-3 H2SO4 solution, the dissolution of halloysite is initiated on the inner surface of nanotubes leading to formation of amorphous spheroidal nanoparticles of SiO2 whereas, in 1 mol dm-3 NaOH solution, dissolution of inner surface of nanotubes is accompanied by formation of Al(OH)3 nanosheets

Hierarchical tube-in-tube structures prepared by electrophoretic deposition of nanostructured titanates into TiO2 nanotubes array

Multiwalled nanotubular titanates have been incorporated inside the pores of a wide TiO2 nanotube array using electrophoretic deposition under vigorous stirring. The resulting hierarchical electrodes combine both benefits of open channels for rapid transport of ions and high specific surface area