Multihierarchical electrodes based on titanate nanotubes and zinc oxide nanorods for photoelectrochemical water splitting

Studies involving water splitting to form hydrogen and oxygen have attracted attention because H2 is considered the fuel of the future. Photoelectrocatalysts have been widely used for this application, and several metal oxides can be applied as catalysts. Among them, we highlight zinc oxide nanorods (ZnONRs) and titanate nanotubes (TiNTs); however, their individual nanostructures exhibit disadvantages. For example, ZnONR shows rapid recombination of the photogenerated charges, and TiNT gives rise to randomly orientated films; these disadvantages limit their application as photoanodes. In this study, for the first time, we present a new class of multihierarchical electrodes based on TiNT-decorated ZnONR films that exhibited superior results to the individual species. The TiNTs are homogenously dispersed over the surface of the rods without forming agglomerates, giving rise to a heterojunction that exhibits lower recombination rates. It was found that the results are better when the contents of TiNT in the electrode are higher; thus, glycine was successfully used as a bridge to link both of the structures, increasing the amount of TiNT decorating the rods. As a result, the photocurrent generated with these multihierarchical electrodes is higher than that obtained for pure ZnONR electrodes (0.9 mA and 0.45 mA, respectively), and the electrode potentials for O2 evolution is lower than that observed for pure TiNT electrodes (0 V and 0.8 V vs. ERHE, respectively). The IPCE values are also higher for the multihierarchical electrodes

The effect of ionic charge on the adsorption of organic dyes onto titanate nanotubes

The adsorption of dye molecules from aqueous solution onto the surface of titanate nanotubes (which have been synthesized by an alkaline hydrothermal treatment) has been studied. The ionic charge on the dye molecules was found to affect their ability to adsorb onto the titanate nanotube surface. In case of (cationic) methylene blue, the adsorption was favourable on the negatively charged surface of titanate nanotubes rather than on positively charged P25 TiO2 nanoparticles. In case of (anionic) eriochrome black T dye, the opposite trend was found. The dynamics of dye adsorption and the effect of pH on the adsorption capacity are considered

The oxidation of borohydride ion at titanate nanotube supported gold electrodes

Steady state voltammetry was used to study the oxidation of 0.02 mol dm3 borohydride ion (in 3 mol dm3 NaOH at 298 K) at three gold-based electrodes, namely: (a) a commercially available gold on Vulcan carbon Au/C, supported by carbon felt resulting in a 0.5 mg cm2 of Au loading, (b) gold nanoparticles deposited on titanate nanotubes with approximately 0.1 mg cm2 loading Au and (c) gold foil. Gold nanoparticles which are well dispersed onto titanate nanotubes can produce currents within the same order of magnitude as the Au/C and Au foil electrodes and represent an option for anode materials in a direct borohydride fuel cell

An aqueous, alkaline route to titanate nanotubes under atmospheric pressure conditions

A significant reduction in the temperature for alkaline synthesis of titanate nanotubes is reported. Nanotubular titanates have been synthesized from TiO2 at 100 ?C in a mixture of concentrated, aqueous solutions of KOH and NaOH. After 48 h of refluxing, essentially complete conversion was achieved. For a given temperature, the composition of KOH and NaOH in the binary mixture was chosen to achieve a particular concentration of Ti(IV), which favoured the formation of nanotubes. The morphology, surface area and crystal structure of the titanate nanotubes produced were similar to those produced in a solution of pure NaOH at a higher temperature (?140 ?C)

Synthesis of novel composite materials via the deposition of precious metals onto protonated titanate (TiO2) nanotubes

Methods for the deposition of precious metals (Au, Pt, Pd and ruthenium hydrated oxide) onto the surface of nanotubular titanates are considered. Viable techniques include preliminary ion exchange of precious metal cations onto the nanotubes followed by chemical, electrochemical or photochemical reduction to the metal. The morphology and size of the metal nanoparticles ranged from spheroidal particles of a few nanometres to larger, rod like particles. The deposits, which were densely loaded onto the surface and were uniformly distributed, had a high surface area and good chemical stability. The size of metal nanoparticles ranged from 1 to 50 nm

Sorption of hydrogen onto titanate nanotubes decorated with a nanostructured Cd3[Fe(CN)6]2 Prussian Blue analogue

Nanostructured films of cadmium hexacyanoferrate (III), Cd3[Fe(CN)6]2 have been deposited on the surface of titanate nanotubes (TiNT) by ion exchange with CdSO4, followed by reaction with K3[Fe(CN)6] in an aqueous suspension. The composite demonstrates a significantly higher hydrogen storage uptake than pure Cd3[Fe(CN)6]2 and TiNT. At a temperature of 77 K and a pressure 100 bar, the hydrogen uptake for the composite is approximately 12.5 wt %, whereas only 4.5 wt % and 4 wt % are achieved for the TiNT and Cd3[Fe(CN)6]2 respectively. Electron microscopy and infrared spectroscopy show that Cd3[Fe(CN)6]2 is uniformly distributed on the surface of the nanotubes forming a discontinuous nanostructured film with a well developed interface, which allows efficient interaction with the support. The possible reasons for the high uptake of hydrogen in the composite are discusse

Electrodeposition of polypyrrole–titanate nanotube composites coatings and their corrosion resistance

Polypyrrole (PPy) films (2 micro-m) containing titanate nanotubes (TiNT) were deposited from 0.5 mol dm-3 pyrrole (Py) and 1 g dm-3 of TiNT in 0.1 mol dm-3 aqueous oxalic acid on 904L stainless steel (SS) 0.1 mm thickness at 298 K. Electron microscopy showed that the nanotubes were adsorbed on the PPy surface and uniformly dispersed in the polymer matrix. The PPy/TiNT composite contained < 10 % wt. titanates which showed an increase of 53% hardness compared with polypyrrole alone. The TiNT provide nucleation centres to catalyze the polymerization of pyrrole and can adsorb up to 240 mg g-1 of the monomer. The corrosion rates for SS, SS/PPy and SS/PPy/TiTN composites, evaluated by linear sweep voltammetry and open-circuit potential measurements in 3% w/v NaCl, were 1.61, 0.008 and 0.004 mg dm-2 day-1, respectively, indicating that corrosion rates of stainless steel decreased by up to three orders of magnitude in the presence of the composite film

The preparation of PbO2 coatings on reticulated vitreous carbon for the electro-oxidation of organic pollutants

The preparation of PbO2 coatings on reticulated vitreous carbon (RVC) has been carried out at constant current from electrolytic baths containing aqueous Pb(II) and methanesulfonic acid (MSA, CH3SO3H). The morphological and structural analysis of the RVC/PbO2 deposits carried out by scanning electron microscopy (SEM) and X-ray diffraction revealed that a thick (100 ?m), homogeneous, nanostructured ?-PbO2 film can be successfully formed. As a result, three-dimensional ?-PbO2 structures were obtained, being particularly interesting for their use as anodes in wastewater treatment. The high oxidation ability of these anodes has been verified by the electro-oxidation of Methyl Orange aqueous solutions. Quick decolourisation was achieved, with total colour removal in less than 60 min at 600 mA due to the production of large amounts of reactive OH radicals from the oxidation of water at high anodic potentials. The progressive mineralisation of the solutions was also ascertained from the total organic carbon (TOC) removal, which was much quicker at a higher applied current. All the coated RVC electrodes exhibited excellent long-term stability and remained unaltered after prolonged electrolyses. In addition, novel PbO2 composite coatings were prepared in the presence of hydrothermally synthesized titanate nanotubes (TiNT). The SEM images showed the presence of TiNT agglomerates along the PbO2 surface, which led to higher anodic current in the cyclic voltammetries carried out with Methyl Orange solutions. It is suggested that TiNT favour the adsorption of the organic molecules, facilitating the contact with the OH radicals and thus accelerating the electro-oxidation process. This was confirmed by the faster TOC removal compared to that yielded by the RVC/PbO2, being 45% instead of 24% at 120 mi

A novel cation-binding TiO2 nanotube substrate for electro- and bioelectro-catalysis

TiO2 nanotubes (8–20 nm outer diameter and 3–5 nm inner diameter) grown via alkaline hydrothermal synthesis are characterised and compared to 6 nm diameter TiO2 (anatase) nanoparticles. Zeta potential, voltammetric, and titration experiments reveal that, in contrast to anatase nanoparticles (p.z.c. ca. 6), TiO2 nanotubes carry a stronger negative surface charge (p.z.c. ca. 3, acidic protons ca. 2 × 10?3 mol g?1, electrostatic cation adsorption sites in neutral solution ca. 7 × 10?5 mol g?1) and, under neutral conditions, offer electrostatic binding sites for cations.When immobilised onto an inert boron-doped diamond substrate, TiO2 nanotubes show electrochemical reactivity due to reversible Ti(IV) reduction, which is very similar to that observed for anatase nanoparticles. Three cationic redox systems, Meldola’s blue, Ni2+, and cytochrome c, are immobilised on the TiO2 nanotube surface; the binding ability and the number of binding sites are quantified voltammetrically. Redox proteins, such as cytochrome c, adsorb readily and irreversibly. Well-defined voltammetric signals for the immobilised protein are observed in an aqueous buffer. TiO2 nanotubes are shown to be novel, inert substrates for both inorganic and biological electrocatalysts