Sherdil Khan

On the crystallization of Ta2O5 nanotubes: structural and local atomic properties investigated by EXAFS and XRD

Metal oxide nanotubes (NTs) semiconductors prepared by anodization are promising materials due to their expected unique optical and electric properties. However, most of the work reported to date did not find photoelectrochemical devices with higher efficiency than those assembled with nanoparticles. Moreover, this behavior is due to the difficulty of having non-defective crystalline structures and the disruption of the tubular shape during thermal treatment while trying to reduce oxygen vacancies. This work describes in detail the local atomic configuration, surface area and morphology properties of Ta2O5 NTs prepared by anodization as a function of the temperature and the crystallization time by using X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). The crystallization process of adhered and freestanding Ta2O5 NTs is discussed. Adhered NTs crystallized at 550 °C due to the oxidation of the Ta metal during annealing in the air atmosphere and not the NT array itself. Freestanding Ta2O5 NTs crystallized after annealing at 800 °C. Rietveld refinements were performed to investigate the effects of the temperature and the annealing time on the grain size and microstrain and obtain information about Ta–O interatomic distances. The local structure of amorphous and crystalline Ta2O5 NTs was investigated with EXAFS. Low coordination numbers were found in the as-anodized samples as well as the samples annealed for 30 min at 800 °C. The coordination number increased when annealing was performed above 800 °C or when the annealing time was longer than 30 min. Moreover, the decrease of defects was followed by an increase in the crystal size and collapse of the tubular shape due to the increase in internal stress generated by the increase in the crystallinity of the tubes and the orthorhombic Ta2O5 crystal size.

Polymorphic phase study on nitrogen-doped TiO2 nanoparticles: effect on oxygen site occupancy, dye sensitized solar cells efficiency and hydrogen production

In this work we show that phase formation and oxygen substitution can be controlled by the source of nitrogen used during the synthesis of TiO2 nanoparticles. By performing a thorough study on the structure of the nanoparticles, the use of NH4+ or NO3− was found to influence not only the N-doping level but also the formation of the polymorphic phase. Structural and microstructural refinement obtained by XRD pattern and data processing performed by the Rietveld refinement revealed that TiO2 obtained with HNO3 presents ca. 98% of anatase and ca. 2% for rutile. Meanwhile TiO2 nanoparticles synthesized with NH4F and NH4Cl presents a single anatase phase with ca. 7.0% and 4.4% of nitrogen substitutional oxygen sites, respectively. The local structure of N-doped TiO2 around the Ti atoms was investigated by X-ray absorption spectroscopy. The XANES spectra show that N-doped TiO2 possesses a characteristic pre-edge of a single anatase structure. The coordination number decreased and the shrinking Ti–O bond distances are due to the N-doping in the TiO2 structure. The most efficient dye sensitized solar cell and the higher hydrogen production was obtained from the TiO2/NH4Cl, which was obtained as a single anatase phase with intermediary concentration of N substitutional oxygen sites.

Enhanced photocatalytic hydrogen production from Y2O3/TiO2 nano-composites: a comparative study on hydrothermal synthesis with and without an ionic liquid

Hetero-junction Y2O3/TiO2 nano-composite (NC) photocatalysts were synthesized using a conventional hydrothermal method (Y2O3/TiO2 NC(HM)) and an ionic liquid assisted hydrothermal method (Y2O3/TiO2 NC(ILAHM)). The composite nature and physico-chemical properties of the photocatalysts prepared through both routes as a function of thermal treatment were investigated via thorough characterization. A comparison of the photocatalytic hydrogen production via water splitting is provided. It was found that the concentration of Y2O3 in the TiO2 matrix and the post thermal treatment have obvious effects on the hydrogen production activity, and they were fine-tuned for enhancement of the hydrogen production. The required synergy between Y2O3 and TiO2 was found to occur at an optimum concentration of 25 wt% Y2O3 in the TiO2 matrix, with the sample prepared at 400 °C for 1 h. The optimized NC i.e. 25 wt% Y2O3/TiO2 NC(ILAHM) produced a promising hydrogen evolution of 1380 μmol g−1 that was almost 2-fold the production from 25 wt% Y2O3/TiO2 NC(HM). The enhancement was attributed to the porous surface morphology, higher surface area and quantum yield of the NC prepared using the ionic liquid assisted hydrothermal method.

Controlled thermal nitridation resulting in improved structural and photoelectrochemical properties from Ta3N5 nanotubular photoanodes

Ta3N5 nanotubular photoanodes were synthesized by thermal nitridation of anodized Ta2O5 nanotubes (NTs) in a temperature range from 650 °C to 1000 °C. XRD diffractograms and Rietveld refinements show that the crystalline structure is strongly dependent on thermal nitridation that triggers defects in the orthorhombic structure of Ta3N5 NTs. A non-stoichiometric TaN0.1 phase was observed at the bottom of the Ta3N5 NTs at the Ta–Ta3N5 interface. Electrochemical impedance spectroscopy revealed that nitridation conditions such as temperature and time strongly influence the interfacial charge transportation; affecting the photoelectrochemical (PEC) activities of the photoanodes. Improved PEC performance was obtained from the NTs synthesized at higher temperature for shorter nitridation time. This result is related to the preservation of the tubular morphology obtained at short nitridation time, high crystallinity and lower charge transfer resistance across the semiconductor–electrolyte interface.

Pristine Ta3N5 Nanotubes: Trap-Driven High External Biasing Perspective in Semiconductor/Electrolyte Interfaces

Ta3 N5 is a promising photoelectrode for solar hydrogen production; however, to date pristine Ta3 N5 electrodes without loading co-catalysts have presented limited photoelectrochemical (PEC) performance. In particular, large external biasing has been required to run water oxidation, the origin of which is investigated herein. Ta3 N5 nanotubes (NTs) prepared by nitridation were characterized by a wide range of techniques. The bandgap was confirmed by a novel PEC technique. Nondestructive synchrotron-excited XPS has shown the presence of reduced Ta species deeper in the Ta3 N5 surface. Lower photocurrent and transient spikes that were intense at lower applied biasing were observed under water oxidation; however, spikes were inhibited in the presence of a sacrificial agent and photocurrent was improved even at low biasing. It was observed for the first time that the lower PEC performance under water oxidation can be attributed to the presence of interband trapping states associated with pristine Ta3 N5 NTs/electrolyte junction. These states correspond to the structural defects in Ta3 N5 , devastate PEC performance, and present the necessity to apply higher biasing. The key to circumvent them is to use a sacrificial agent in the electrolyte or to load a suitable co-catalyst to avoid hole accumulation under water oxidation, thereby improving the phootocurrent. The findings on the interband states could also provide guidance for the investigation of PEC properties of new types of semiconducting devices.

Liquid-crystalline coumarin derivatives: contribution to the tailoring of metal-free sensitizers for solar cells

ABSTRACT Coumarins have been used in a wide range of applications, such as dye-sensitised solar cells, laser dyes and optical sensors. In order to further explore the properties of these materials, three new coumarin derivatives were obtained with different terminal arylalkyne linkages to the 6-position of the coumarin core. The synthesised materials were characterised by NMR, absorption and emission spectroscopy, and the liquid crystal properties were investigated through differential scanning calorimetry and polarised optical microscopy. In addition, dye-sensitised solar cells were assembled to evaluate the photoelectrochemical properties of the materials. Only the coumarin with a naphthyl group exhibited stable smectic A and nematic mesophases. All the coumarins were photoemissive in the range 420–461 nm. The adsorption of these dyes on TiO2 was observed by UV–vis spectroscopy; in addition, by incident photon-to-electron conversion efficiency and I–V curves, photocurrent generation was observed. GRAPHICAL ABSTRACT

Photoelectrochemical study of Ta3N5 nanotubes for water splitting

Nanotubes (NTs) of Ta3N5 were synthesized by nitridation of Ta2O5 NTs. The samples were studied by scanning electron microscopy, UV-VIS spectrophotometry, x-ray diffraction and photoelectrochemical (PEC) measurements carried out in aqueous solutions of Na2SO4 and Fe(CN)6 3-/4-. The results show the presence of trapping states on illuminated pristine Ta3N5 NTs in Na2SO4 (aq). These trapping states act as recombination center for photogenerated holes, affecting the photocatalytic performance of Ta3N5 NTs. On the other hand, by using Fe(CN)6 3-/4- (aq) the photogenerated holes were scavenged efficiently without giving rise to the trapping states at Ta3N5 NTs/solution interface. The results obtained by cyclic voltammetry, linear sweep voltammetry and electrochemical impedance spectroscopy have shown that the presence of these trapping states is a limiting step for water oxidation using pristine Ta3N5 NTs.

Photocatalytic Water Splitting by Suspended Semiconductor Particles

Starting and emphasizing the importance for future demand on Earth energy feed, this chapter discusses the state-of-the-art of photocatalytic water splitting (PWS) using suspended particles for hydrogen evolution. Herein, the thermodynamics requirements for photocatalyst semiconductors for water splitting for efficient hydrogen and oxygen production, to date overview on important photocatalysts and related fine-tuning processes adopted to meet the desired properties for PWS with improved photocatalytic activities, a profound discussion and review about tandem photocatalyst systems are covered. A descriptive discussion on efficiency determination including solar-to-hydrogen efficiency and apparent quantum efficiency to compare the photocatalytic performance of the photocatalysts is presented.

Synergistic interplay of ionic liquid and dodecyl sulphate driving the oxidation state of polypyrrole based electrodes

Herein, polypyrrole (PPy) films are prepared via electrochemical deposition in a mixture of 1-n-butyl-3-methylimidazolium methanesulfonate (BMI·CH3SO3) ionic liquid (IL) and dodecyl sulfate (DS). The physico-chemical properties of the films have been investigated by a wide range of characterization techniques. PPy films synthesized in DS or IL have shown larger and irregular granules as compared to PPy films prepared in the mixture of DS and IL. This result is related to the preferential dissolution of pyrrole monomers in micelle templates formed by the IL and a co-assembly of IL and dodecyl-sulfate (DS), thereby decreasing the granule size and affecting the structural arrangement of the polymer chain. The template behavior of the IL in combination with DS promotes the selective formation of polaron and bipolaron states in the PPy films. This effect has been investigated by UV-Vis spectrophotometry, Raman spectroscopy, electrochemical impedance spectroscopy (EIS), cyclic voltammetry, scanning electron microscopy, transmission electron microscopy, and electrical conductivity measurements. The synergy of IL–DS has helped to decrease the resistivity of the PPy film from 2.17 × 102 Ω cm for PPy–IL to 2.44 Ω for PPy–IL–DS. EIS has also shown a decreased interfacial charge transfer resistance for PPy–IL–DS when compared to PPy–IL. The cyclic voltammetry curves have shown that the PPy films are electrocatalytically active for the I−/I3− redox reaction, and therefore, can be applied as a counter electrode (CE) in dye sensitised solar cells. The PPy based CEs resulted in nearly the same photocurrent and energy conversion efficiencies as that obtained from a conventional Pt CE.

Syntheses and structural understanding of a Ti–Ta alloy-based nanotubular oxide photocatalyst

Herein, we have synthesized Ti–Ta based mixed oxide vertically oriented and highly ordered freestanding nanotubes having an average length of 60 μm by anodic oxidation of a homemade Ti–Ta (50–50 at%) alloy. The effect of heat treatment on the morphology, phase transformation to TiTa2O7 nanostructures, crystalline structures, optical and surface properties has been investigated by a variety of characterization techniques. The samples remained mainly amorphous at temperatures up to 700 °C, and according to the Rietveld refinement analyses the bulk formation of crystalline Ta2O5 and TiO2-anatase phases occurred only at 800 °C in the tubular matrix followed by a slight solid to solid transformation of TiTa2O7 and TiO2-rutile phases starting from 850 °C. At 1000 °C, the content of TiTa2O7 increased followed by a decrease in TiO2 and Ta2O5 content; however, the nanotubes appeared to be interconnected particles imitating the verticality of the nanotubes. Compared to the literature (∼1350 °C), in this work the transformation of TiTa2O7 occurred at a lower temperature which is related to the amorphous nature of the starting nanotubular samples. The HAADF-STEM images and localized EDS mapping showed the distribution of Ti and Ta elements along the tubular matrix demonstrating that the as-anodized nanotubes consist of a uniform mixture of TiO2 and Ta2O5 whereas for the calcined samples random distribution of Ti was observed indicating the migration of Ti inside the tubular matrix for the formation of mixed oxide and TiTa2O7 phases. X-ray photoelectron spectroscopy (XPS) analysis identified the dislocation in the binding energies of Ti 2p and Ta 4f regions for the heat treated samples as compared to the as-anodized nanotubes; related to the formation of TiTa2O7. The optical band gap of TiTa2O7 was found to be 3.08 eV. These oxide mixtures were applied in photocatalytic H2 generation under solar irradiation; where the sample heat treated at 1000 °C has resulted in a production rate of 41.58 μmol h−1 g−1 which was higher than all the other samples synthesized in this work. For continuous 48 h irradiation the photocatalytic activity of the sample remained highly stable. This result is related to the higher content and lower bandgap energy of TiTa2O7 in the mixed oxide matrix indicating that TiTa2O7 is a promising photocatalyst.