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Herein we illustrate the functionality as pseudocapacitive material of tungsten trioxide (WO 3 ) nanochannel layers fabricated by electrochemical anodization of W metal in pure hot ortho-phosphoric acid ( o -H 3 PO 4 ). These layers are characterized by a defined nanochannel morphology and show remarkable pseudocapacitive behaviour in the negative potential (−0.8–0.5 V) in neutral aqueous electrolyte (1 M Na 2 SO 4 ). The maximum volumetric capacitance of 397 F cm −3 is obtained at 2 A cm −3 . The WO 3 nanochannel layers display full capacitance retention (up to 114%) after 3500 charge-discharge cycles performed at 10 A cm −3 . The relatively high capacitance and retention ability are attributed to the high surface area provided by the regular and defined nanochannel morphology. Kinetic analysis of the electrochemical results for the best performing WO 3 structures, i.e. , grown by 2 h-long anodization, reveals the occurrence of pseudocapacitance and diffusional controlled processes. Electrochemical impedance spectroscopy measurements show for the same structures a relatively low electrical resistance, which is the plausible cause for the superior electrochemical behaviour compared to the other structures. [ABSTRACT FROM AUTHOR]

Abstract: TiO2 powders modified through the deposition of surfactant-stabilized preformed metal (Pt, Pd, Au and Ag) nanoparticles (NPs) were tested as photocatalysts for batch ammonia photo-oxidation in aqueous suspension, under UVA irradiation. NH3 conversion and the selectivity toward mildly oxidized N2 and highly oxidized nitrite and nitrate anions were monitored during the runs, in order to study the effects of noble metal NPs deposition on the reaction mechanism. This involves parallel and consecutive paths, also depending on the type of metal deposited on TiO2, as also highlighted by the nitrite photo-oxidation tests performed in the present study. Metal NPs acted as electron traps ensuring better separation of the photoproduced charge couples and consequently higher rates of hole-mediated oxidation reactions. Apart from Au/TiO2, the metal-modified powders exhibited higher photoactivity with respect to that of naked titania, with Ag/TiO2 being the best performing photocatalyst in terms of ammonia conversion. On the other hand, Pd NPs on TiO2 ensured a considerable increase of the selectivity toward N2 and may thus be considered the best co-catalysts of TiO2 for environmentally friendly photocatalytic NH3 abatement. [Copyright &y& Elsevier]

Abstract: TiO2 nanotube (NT) arrays of different lengths were prepared by electrochemical anodization of ca. 10cm2 area titanium disks in NH4F–H2O–formamide solution for different times, followed by annealing at 450°C. After Pt deposition on the opposite side of the disk, the so obtained Ti-supported crystalline anodic oxides were employed as photoactive electrodes in a two compartments cell for separate H2 and O2 production through water photosplitting and characterized by SEM, XRD analysis and photocurrent measurements. The anodization time affected the phase composition and morphology of the growing NTs, which strictly influenced their photocatalytic activity. Short anodization times (40–60min) resulted in well aligned short tubes composed of anatase-rutile mixed phases. Longer anodization (>2.5h) yielded thicker NT arrays covered on top by a preferentially oriented anatase layer that limited their photoactivity. Photocurrent intensity measurements perfectly paralleled the water splitting activity results obtained with the different NT arrays. In particular, a square-shaped fast photoresponse was recorded with ordered and fully top-open nanotubular structures. On the other hand, clogged tubes not only yielded low current densities, but also showed delayed photocurrent transient signals due to the reduced mobility of the charge carriers within the preferentially oriented anatase layer. NT arrays obtained under optimized conditions had a ca. 80:20 anatase:rutile composition and ensured a hydrogen production rate of 83mmolh−1 m−2 (i.e. 1.9NLh−1 m−2) in the absence of any hole scavenger or external bias. [Copyright &y& Elsevier]

We present a photoanode concept for photoelectrochemical (PEC) water splitting based on metal nanoparticles (Au, Ag, Cu) deposited on a Ti mirror (NPoM) to photosensitize an intermediate TiO 2 layer to induce the harvesting of light with sub-bandgap photon energies. The generation of hot electron-hole pairs in metallic nanostructures can occur either by intraband excitation, i.e., plasmon electrons, or by the interband transition of the d-band electrons to the unoccupied conduction band states. Our results demonstrate that the underlying Ti mirror significantly amplifies the PEC activity of such NPoM systems in the visible spectral range. We show that the PEC enhancement in the visible spectral range is not limited to the intraband excitations but is affected mainly by the light trapping pathway within the TiO 2 layer, i.e. due to a favorable interplay between thin-film interference cavity modes with both intraband and interband excitations. Among tested metals, Cu and Ag NPs demonstrate a ~3-fold higher enhancement factor than that of Au NPs, while in both former cases the nature of the excited electrons is different. The experimentally determined internal quantum efficiency demonstrates alternating behavior with wavelength showing higher efficiencies at short wavelengths, which is attributed to the reduced Schottky barrier of the NPoMs at constructive interference maxima. [Display omitted] A photoanode concept for photoelectrochemical water splitting based on metal nanoparticles (Au, Ag, Cu) deposited on a Ti mirror to photosensitize an intermediate TiO 2 layer is developed to induce the harvesting of light with sub-bandgap photon energies. The photoelectrochemical enhancement is mainly affected by a favorable interplay between thin-film interference cavity modes with both intraband and interband excitations. • A new photoanode concept of metallic NanoParticles-on-Mirror (NPoM) for photoelectrochemical water splitting by harvesting light with sub-bandgap photon energies is demonstrated. • The photoanode is made of metal nanoparticles either Au, Ag or Cu deposited on a Ti mirror and back contact to photosensitize an intermediate TiO 2 layer. • The NPoM configuration significantly amplifies the photoelectrochemical activity of TiO 2 in the visible spectral range due to a favorable interplay between thin-film interference with both intraband and interband electronic excitations. • Cu and Ag NPs demonstrate a ~3-fold higher incident photon-to-current efficiency enhancement than Au NPs, while in both these cases the nature of the excited electrons, i.e. , intraband or interband, is different. [ABSTRACT FROM AUTHOR]

The anodic oxidation of niobium is investigated in pure molten ortho-phosphoric acid (o-H 3 PO 4). 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 Nb 2 O 5 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. Image 1 [ABSTRACT FROM AUTHOR]

Anodic TiO 2 nanotube membranes of various thicknesses (1.5–60 μm) fixed on FTO slides were site-selectively decorated with Pt nanoparticles at only their upper extremity. Their photocatalytic H 2 evolution ability (with and without Pt) is investigated under UV light (325 nm) illumination through the FTO slide. The irradiation side is opposite to the Pt decorations in order to avoid Pt shading effects. This design is key to study the H 2 evolution in relation to membrane thickness, light absorbance, and electron diffusion length in the nanotubes. The Pt decoration improves the H 2 evolution, and the photocatalytic enhancement, quantified as ratio of H 2 evolution rate with Pt vs. that without Pt, varies with the membrane thickness. The highest photocatalytic enhancement is for 5-10 μm-long nanotubes, owing to maximized photon harvesting and efficient electron trapping/transfer to reactants. This is enabled by a short pathway of electron diffusion towards Pt. The photocatalytic enhancement drops dramatically for membrane >30 μm while membranes of intermediate lengths (∼10–30 μm) show similar photocatalytic enhancement. This can be ascribed to an electron diffusion length in Pt-TiO 2 nanotubes of 20–30 μm. [ABSTRACT FROM AUTHOR]

Abstract Hematite-based photoanodes were used for the photoelectrocatalysed batch oxidation of As(III) in water at pH 7 and 10. Tests were carried out at different As(III) initial concentrations, ranging from 150 μg/L to 30 mg/L. With no pre-treatments, an initial inactive period of 40–50 min was always observed at the beginning of every test. This initial inactive period is completely removed by a surface modification of surface active sites induced by a pre-polarisation at 800 mV (vs. Ag/AgCl). The water splitting active sites related to the Fe(III)-Fe(IV) redox couple were proved to be not active towards As(III) oxidation. The modifications induced by the anodic prepolarisation were deeply studied: no evidence of the formation of surface highly oxidised iron sites (Fe(V) or Fe(VI)) and/or of highly reactive oxygen vacant sites emerges from XPS analysis. Rather, all collected characterisation data support the conclusion that more reactive terminal oxygen species are necessary for As(III) oxydation. The As(III) abatement reaction was modelled by two subsequent first order kinetics in As(III), independently from the initial As(III) concentration: this behaviour was explained suggesting that the highly reactive sites, being formed after prepolarisation, are gradually depleted during the reaction. As this reaction was proved to be very sensitive to the presence of highly reactive iron sites, its utilisation as a probe reaction to study hematite photoanodes is suggested. Graphical abstract Image 1 Highlights • Hematite photoanodes catalyse the oxidation of As(III) under solar illumination. • Up to 90% As(III) abatements are observed within 24 h independently from the initial As(III) concentration. • The proposed process is in principle environmentally sustainable, as the disposal of chemicals is almost totally absent. • The production of hydrogen during As(III) oxidation provides a suitable power source to sustainably drive the process. [ABSTRACT FROM AUTHOR]

Vertically aligned Nb 2 O 5 nanochannel layers are grown on Nb metal substrates by self-organizing electrochemical anodization in a pure molten o- H 3 PO 4 electrolyte. The capacitive behavior of these structures when used as negative electrodes is investigated in aqueous 1 M Na 2 SO 4 electrolyte, in a potential range from −0.2 to −1.25 V vs. SCE. Surface chemistry, morphology and crystallographic features of the Nb 2 O 5 nanochannel electrodes are tailored by adjusting the synthesis parameters, namely anodization time and crystallization temperature, which have a significant effect on the electrode performance. 8 μm thick Nb 2 O 5 nanochannel layers that are converted into orthorhombic phase by crystallization at 450 °C, display a maximized areal capacitance of ∼100 mF cm -2  at a current density of 1 mA cm −2 . These electrodes retain 63% of the initial capacitance at 10 mA cm −2 and 81% after 1500 charge-discharge cycles at a current density of 1.3 mA cm −2 . Kinetic analysis of the electrochemical results reveals the occurrence of pseudocapacitive and diffusion-controlled processes. Electrochemical impedance spectroscopy evidences for these structures a low resistance across the electrode and at the electrode/substrate interface. These results are associated with the nanochannel morphology (high active area) of the Nb 2 O 5 layers, and are ascribed to their crystalline nature, which provides an “oriented porosity” for ion diffusion and directional pathways for charge transport and collection. [ABSTRACT FROM AUTHOR]