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Study of Ti, V and Their Oxides-Based Thin Films in the Search for Hydrogen Storage Materials
Abstract
Thin film series consisting of Ti, V, TiO2 and V2O5 layer with different layer geometries, sequences and thicknesses have been prepared by the sputtering technique. The hydrogen depth profile of selected films upon hydrogen charging at 1 bar and/or hydrogenation at pressure up to 102 bar was determined by using secondary ion mass spectrometry and nuclear reaction analysis using a N-15 beam. The highest hydrogen storage with a concentration up to 50 at.% was found in the pure Ti and Ti-contained layer, while it amounts to around 30% in the metallic Ti-V-Ni layer. Hydrogen can diffuse through the TiO2 layer without accumulation, but can be stored in the VO2 layer in some cases. Hydrogen can remove the preferential Ti orientation in the films and induce a complete transition of V2O5 into VO2 in the films.
... RBS and XRR experiments have been performed on films before and after each hydrogenation to underline the changes of the film composition and structure upon hydrogenation. Details of XRR and RBS data analysis, especially the estimate of composition and layer-thickness, have been reported in [15,19]. ...
... The hydrogen quantity in the thin film systems is very small to be qualified by desorption. Thus we used a secondary ion mass spectroscopy (SIMS) and Nuclear Reaction Analysis using the 15 N beam ( 15 N-NRA method with 1 H( 15 N, a,g) 12 C reaction) for determination of hydrogen concentration in the films [18,19]. Besides, unlike the bulk samples, for the thin film systems as in our case with very thin layers, it is convenient to use the atomic percent for the very small amount of hydrogen. ...
... For both films, the stoichiometric TiO 2 and/or V 2 O 5 layer was well preserved on the film surface: the layer thickness is almost unchanged upon hydrogen charging [19]. Thus the swelling effect seems to be related the mixed TiO 2 eVO 2 eSiO 2 layer (i.e. ...
Series of thin films of single-, bi- and tri-layered structure consisting of Ti, V, TiO2 and V2O5 layer and/or mixed Ti-V-Ni layer with different layer sequences and thicknesses were prepared by the sputtering technique on Si and SiO2 substrates. The layer chemical composition and thickness were determined by a combined analysis of X-ray diffraction, X-ray reflectometry, Rutherford backscattering and optical reflectivity spectra. The films were hydrogenated at 1 bar at 300oC and/or at high pressures up to 100 bar at room temperature. The hydrogen concentration and hydrogen profile was determined by means of a secondary ion mass spectroscopy and N-15 Nuclear Reaction Analysis. The highest hydrogen storage with a concentration up to 50 at.% was found in the pure Ti layers, while it amounts to about 30 at.% in the metallic Ti-V-Ni layers. A large hydrogen storage (up to 20 at. %) was also found in the V2O5 layers, while no hydrogen accumulation was found in the TiO2 layers. Hydrogen could remove the preferential orientation of the Ti films and induce a complete transition of V2O5 to VO2.
... The model used to calculate and fit the XRR pattern is V 2 O 5 /SiO 2 /Si, which gives a good fit and reveals that the film density is ∼3.15 g cm −3 which is reasonably close to the reported bulk value of 3.36 g cm −3 . 48,49 The data fitting revealed a film thickness of ∼33 nm, which is slightly thicker than our ellipsometry-measured values, and the film surface roughness of ∼1.7 nm was calculated. A similar fitting approach for VO 2 film is performed using VO 2 /SiO 2 /Si model and the XRR pattern is displayed in Fig. 7(b). ...
... The density found is smaller than the reported bulk density value of 4.57 g/cm 3 . 49 However, the higher roughness is attributed to the discontinuity of VO 2 film triggered by the postdeposition annealing, which can be overcome by growing thicker films as will be demonstrated with the HCP-ALD grown and postdeposition annealed samples. Figure 8 shows the elemental content and atomic concentrations of the ICP-ALD grown V 2 O 5 film for air-exposed as-received samples without any Ar sputtering, extracted via XPS measurements from the surface of the samples. ...
Vanadium oxide (VO x ) compounds feature various polymorphs, including V 2 O 5 and VO 2 , with attractive temperature-tunable optical and electrical properties. However, to achieve the desired material property, high-temperature post-deposition annealing of as-grown VO x films is mostly needed, limiting its use for low-temperature compatible substrates and processes. Herein, we report on the low-temperature hollow-cathode plasma-enhanced atomic layer deposition (ALD) of crystalline vanadium oxide thin films using tetrakis(ethylmethylamido)vanadium and oxygen plasma as a precursor and coreactant, respectively. To extract the impact of the type of plasma source, VO x samples were also synthesized in an inductively coupled plasma-enhanced ALD reactor. Moreover, we have incorporated in situ Ar-plasma and ex situ thermal annealing to investigate the tunability of VO x structural properties. Our findings confirm that both plasma-ALD techniques were able to synthesize as-grown polycrystalline V 2 O 5 films at 150 °C. Postdeposition thermal annealing converted the as-grown V 2 O 5 films into different crystalline VO x states: V 2 O 3 , V 4 O 9 , and VO 2 . The last one, VO 2 is particularly interesting as a phase-change material, and the metal-insulator transition around 70 °C has been confirmed using temperature-dependent x-ray diffraction and resistivity measurements.
... g/cm 3 . 60 The lower film density can be explained by the void structures in the film morphology confirmed via HR-SEM images [ Fig. 15(b)], which were formed during the post-deposition annealing process. Likely, the roughness increased from 1.70 to 2.87 nm due to the impact of post-deposition annealing. ...
Due to its different polymorphs, including vanadium pentoxide (V2O5) and vanadium dioxide (VO2), the vanadium oxide (VOX) compound is an immensely interesting material with many important applications. While atomic layer deposition (ALD) is among the possible VOX film synthesis methods, literature reports have majorly utilized thermal-ALD, which reveals as-grown amorphous VOX films. Further post-deposition annealing process is needed to crystallize these films. High-temperature crystallization indeed limits the use of low-temperature compatible materials, processes, and substrates. In this work, we report on the low-temperature crystalline VOX film growth in a hollow-cathode plasma-enhanced atomic layer deposition reactor using two different vanadium precursors, tetrakis(ethylmethylamino)vanadium and vanadium(V) oxytriisopropoxide. Oxygen plasmas were used as co-reactants at a substrate temperature of 150 °C. Along with the purpose of investing in the impact of metal precursors on VOX film growth, we also studied Ar-plasma in situ and thermal ex situ annealing to investigate possible structural enhancement and phase transformation. In situ Ar-plasma annealing was performed with 20 s, 20 SCCM Ar-plasma, while post-deposition ex situ annealing was carried out at 500 °C and 0.5 mTorr O2 pressure. In situ ellipsometry was performed to record instant film thickness variation and several ex situ characterizations were performed to extract the optical, structural, and electrical properties of the films. The outcomes of the study confirm that both metal precursors result in as-grown crystalline V2O5 films at 150 °C. On the other hand, post-deposition annealing converted the as-grown crystalline V2O5 film to VO2 film. Finally, we have also successfully confirmed the metal-to-insulator transition property of the annealed VO2 films via temperature-dependent structural and electrical measurements.
... Furthermore, we also note that when the pulse frequencies lower than 200 KHz, the deposition rate of the VOx films deposited in the reactive atmosphere was higher than the V films deposited in the pure argon atmosphere. This is because the densities of metal V (6.0 g/cm 3 ) was higher than its oxide (V 2 O 3 , 4.87/cm 3 ) (VO 2 , 4.57/cm 3 ) (V 2 O 5 , 3.36 g/cm 3 ), which leads to the oxide films possess a larger mole volume compare with the metal films [40]. And besides that, the oxygen will also incorporate into the growing film during the reactive sputtering, which further reduced the density of the film, and in fact, similar phenomena has also been observed in many reactive sputtering process, i.e. with the increase of the reactive gas flow, the deposition rate of the coatings first increase slightly when the reactive gas flow was low, and then it dropped significantly when the reactive gas flow exceed the critical level [39]. ...
In this study, vanadium oxide (VOx) thin films were prepared by reactive pulsed DC magnetron sputtering (PDMS). The influence of pulse frequency (Fp) on the deposition process and properties of VOx films was investigated. The obtained results indicate that with the increase of Fp, which not only can enhance the oxidation of vanadium but also can facilitate the formation of a denser microstructure and smoother surface morphology in the films. More importantly, films with composition from metal-rich to maximum stoichiometry can be accessed by tuning the Fp, without varying any other parameters, especially, the flow rate of reactive gas. And temperature dependent electrical properties of the VOx films can also be tailored via the Fp to fall in the desirable range for microbolometer applications. The impacts of Fp on the reaction between V and O, thin film growth and some dynamic processes occurring during reactive PDMS were analyzed. The corresponding underlying mechanisms were discussed in detail. Our present work may provide a practical and effective way to develop the reactive sputtering towards hysteresis-free, and also paves the way for optimizing the reactive sputtered VOx films with desirable properties via Fp.
The article describes deposition of copper oxide coatings using magnetron sputtering in a metallic mode enhanced by a radio-frequency inductively coupled plasma (RF-ICP) source. The role of substrate biasing (floating potential, −70 and −140 V) on structural properties and elemental composition of copper oxide coatings is determined using SEM with EDS, XRD and TEM. This study shows the change of coating microstructure from columnar to dense/compact and the increase in crystallinity degree of coatings, when the substrate is biased. EDS analysis reveals the presence of Ar and the increase in O content in the coating due to intensive ion bombardment. Deposition conditions are changed using substrate biasing, which suggests the contribution of ion bombardment to the coating deposition in the case of magnetron sputtering of copper oxide in the metallic mode.
We have investigated the hydrogen storage ability and the effect of hydrogenation on structure and physical properties of Ti/V and their oxides-based thin films. A series of Ti─TiO2 and VOx─TiO2 thin films with different layer structures, geometries and thicknesses have been prepared by the sputtering technique on different (Si(111), SiO2, C) substrates. For the Ti─TiO2─Ti films up to 50 at.% of hydrogen can be stored in the Ti layers, while the hydrogen can penetrate without accumulation through the TiO2 layer. A large hydrogen storage was also found in some V2O5─TiO2 films. Hydrogen could also remove the preferential orientation in the Ti films and induce a transition of V2O5 to VO2 in the films.Invited talk at the 7th International Workshop on Advanced Materials Science and Nanotechnology IWAMSN2014, 2-6 November, 2014, Ha Long, Vietnam.
Titanium and titanium dioxide thin films were deposited onto Si(111) substrates by magnetron sputtering from a metallic Ti target in a reactive Ar+O2 atmosphere, the composition of which was controlled by precision gas controllers. For some samples, 1/3 of the surface was covered with palladium using molecular beam epitaxy. Chemical composition, density, and layer thickness of the layers were determined by Auger electron spectroscopy (AES) and Rutherford backscattering spectrometry (RBS). The surface morphology was studied using high-resolution scanning electron microscopy (HRSEM). After deposition, smooth, homogenous sample surfaces were observed. Hydrogen charging for 5 hours under pressure of 1 bar and at temperature of 300°C results in granulation of the surface. Hydrogen depth profile was determined using secondary ion mass spectrometry (SIMS) and nuclear Reaction Analysis (N-15 method), using a 15N beam at and above the resonance energy of 6.417 MeV. NRA measurements proved a higher hydrogen concentration in samples with partially covered top layers, than in samples without palladium. The highest value of H concentration after charging was about 50% (in the palladium-covered part) and about 40% in titanium that was not covered by Pd. These values are in good agreement with the results of SIMS measurements.
Multilayered thin films of Ti–TiO2 system have been investigated, focusing on all of the important parameters in both photocatalysis and H storage. Numerous Ti–TiO2 thin films with a single-, bi- and tri-layered structure have been deposited on different substrates by means of dc pulsed magnetron sputtering from a metallic Ti target in an inert Ar or reactive Ar + O2 atmosphere. The film chemical composition, depth profile, layer thickness and structure were determined by combined analysis of x-ray diffraction, x-ray reflectometry, Rutherford back- scattering and optical reflectivity spectra. The results show that the Ti films deposited on Si(111) exhibit a strong preferred orientation with the (00.1) plane parallel to the substrate, while a columnar structure was developed for TiO2 films. H charging at 1 bar and at 300 °C revealed that, in the case of the tri-layered structure of Ti/TiO2/Ti/Si(111), H diffused through the TiO2 layer without any accumulation in it. Pd acts as a catalyst for gathering H in Ti layers and up to 50% of H is stored in the topmost and bottom Ti layers. The preferential orientation in the Ti films was found to be destroyed upon hydrogenation at 100 bar. The hydride TiHx phase (x < 0.66) was formed under such a high H pressure.
Doping with other elements is one of the efficient ways to modify the physical and chemical properties of TiO2 nanomaterials. In the present work, Ni-doped TiO2 nanotubes were fabricated through anodic oxidation of NiTi alloy and further annealing treatment. The hydrogen sensing properties of the nanotube sensor were investigated. It was found that the Ni-doped TiO2 nanotubes were sensitive to an atmosphere of 1,000 ppm hydrogen, showing a good response at both room temperature and elevated temperatures. A First-Principle simulation revealed that, in comparison with pure anatase TiO2 oxide, Ni doping in the TiO2 oxide could result in a decreased bandgap. When the oxide sensor adsorbed a certain amount of hydrogen the bandgap increased and the acceptor impurity levels was generated, which resulted in a change of the sensor resistance.
The electronic properties of stoichiometric, defective, and aluminum-doped rutile TiO2 have been investigated theoretically with periodic quantum-chemical calculations. Theoretical results obtained with the Perdew-Wang density functional method [ Phys. Rev. B 45 13244 (1992)] and with a density functional-Hartree-Fock hybrid method are compared. Occupied defect states are observed in the band gap of rutile due to the presence of oxygen vacancies, which is in accord with previous theoretical studies and the experimentally observed coloring. For the investigation of aluminum doping, three different situations have been considered: substitution of a single Ti atom by an Al atom, cosubstitution of Ti by Al and O by Cl, and substitution of two Ti by two Al combined with the formation of an O vacancy. In the last two cases, aluminum doping does not introduce band gap states, and the band gap is even increased compared to undoped rutile. We conclude that stoichiometric Al doping reduces pigment coloring induced by oxygen vacancies in rutile and also suppresses the photocatalytic activity of titania pigments.
Doping with other elements is one of the efficient ways to modify the physical and chemical properties of TiO2 nanomaterials. In the present work, anatase TiO2 nanofilms doped with Al and V elements were fabricated through anodic oxidation of Ti6Al4V alloy and further annealing treatment. The first principle simulation revealed that, in comparison with undoped TiO2 oxide, Al and V doping in the TiO2 oxide could result in a decreased bandgap. Hydrogen sensing behavior of the crystallized Ti-Al-V-O nanofilms at various working temperatures was investigated through exposure to 1,000 ppm H2. Different from n-type hydrogen sensing characteristics of undoped TiO2 nanotubes, the Al- and V-doped nanofilms presented a p-type hydrogen sensing behavior by showing increased resistance upon exposure to the hydrogen-containing atmosphere. The Ti-Al-V-O nanofilm annealed at 450[degree sign]C was mainly composed of anatase phase, which was sensitive to hydrogen-containing atmosphere only at elevated temperatures. Annealing of the Ti-Al-V-O nanofilm at 550[degree sign]C could increase the content of anatase phase in the oxide nanofilm and thus resulted in a good sensitivity and resistance recovery at both room temperature and elevated temperatures. The TiO2 nanofilms doped with Al and V elements shows great potential for use as a robust semiconducting hydrogen sensor.
Nano-sized TiO2 photocatalytic water-splitting technology has great potential for low-cost, environmentally friendly solar-hydrogen production to support the future hydrogen economy. Presently, the solar-to-hydrogen energy conversion efficiency is too low for the technology to be economically sound. The main barriers are the rapid recombination of photo-generated electron/hole pairs as well as backward reaction and the poor activation of TiO2 by visible light. In response to these deficiencies, many investigators have been conducting research with an emphasis on effective remediation methods. Some investigators studied the effects of addition of sacrificial reagents and carbonate salts to prohibit rapid recombination of electron/hole pairs and backward reactions. Other research focused on the enhancement of photocatalysis by modification of TiO2 by means of metal loading, metal ion doping, dye sensitization, composite semiconductor, anion doping and metal ion-implantation. This paper aims to review the up-to-date development of the above-mentioned technologies applied to TiO2 photocatalytic hydrogen production. Based on the studies reported in the literature, metal ion-implantation and dye sensitization are very effective methods to extend the activating spectrum to the visible range. Therefore, they play an important role in the development of efficient photocatalytic hydrogen production.
To use solar irradiation or interior lighting efficiently, we sought a photocatalyst with high reactivity under visible light.
Films and powders of TiO2-xNxhave revealed an improvement over titanium dioxide (TiO2) under visible light (wavelength < 500 nanometers) in optical absorption and photocatalytic activity such as photodegradations
of methylene blue and gaseous acetaldehyde and hydrophilicity of the film surface. Nitrogen doped into substitutional sites
of TiO2 has proven to be indispensable for band-gap narrowing and photocatalytic activity, as assessed by first-principles calculations
and x-ray photoemission spectroscopy.
Vanadium pentoxide is one of the most widely investigated multifunctional materials for electro-chromic and electro-chemical device applications. Its layered and open structure makes this material in thin film form well suited for electro-chemical insertion reactions with the Li ions. In the present investigation, V2O5 thin films have been prepared by electron beam evaporation technique at a substrate temperature of 523 K in an oxygen partial pressure of 2 × 10−4 mbar. The XRD patterns exhibited predominant (001) diffraction orientation along with (200) and (400) characteristic peaks corresponding to orthorhombic phase of V2O5 with Pmnm space group. The estimated average grain size is about 90 nm. The optical absorption coefficient ‘α’ determined from the experimentally measured transmittance and reflectance data for V2O5 films was fund to give a better fit for the exponent n = 3/2 suggesting the direct forbidden transitions with an estimated optical band gap of 2.36 eV. Two sets of films were prepared with thickness of 300 nm on ITO coated glass for electrochromic analysis and 600 nm on metalized silicon for electrochemical insertion studies. The electron beam evaporated V2O5 thin films with thickness 300 nm showed a good reversible cyclic voltammogram on sweeping the potential downward and upward. A two step electrochromism i.e. yellow to green and green to deep blue was observed from reversible current—voltage curves. The electrochemical characteristics of V2O5 thin films with thickness 600 nm were examined in non-aqueous region. The film exhibited a discharge capacity of 70 μAh/(cm2–μm) at a current density of 10 μA/cm2. Annealing of these films at 673 K exhibited a better discharge capacity of 90 μAh/(cm2–μm).
Nanosized titanium dioxide (nano-TiO2) particles are used in diverse products and devices, including photocatalytic water splitting and solar cells whose successful commercialization is still facing scientific and technologi- cal challenges. Particularly, dye-sensitized solar cells (DSSCs) have recently attracted a lot of attention. The number of papers and patents published in this area has grown exponentially over the last ten years. However, at the present, commercial devices are of small sizes and produced in limited quantities, thus addressing niche markets. Research efforts have largely focused on the optimization of the dye, but the TiO2 nanocrystalline electrode itself also needs to be optimized. It has been shown that particle size and shape, crystallinity, surface morphology and chemistry of the TiO2 material are key parameters to be controlled for enhanced performance of the cell. This article gives an overview of the state-of-the-art on nano-TiO2 for applications in photocatalytic water splitting and, more specifically, in DSSCs. After a brief analysis of the commercial perspectives of DSSCs and of the remaining challenges for their successful commercialization, our approach to the control of the nano-TiO2 surface chemistry for improvement of the DSSC performance is briefly introduced.
V2O5 thin films were prepared by reactive rf sputtering and used in the application as electrodes in rechargeable battery with Li as a counterelectrode. The V2O5 films were deposited onto uncoated and SnO2 coated glass by sputtering a V cathode using Ar+O2 gas mixtures. The structural properties of the V2O5 films were characterized by x-ray diffraction, infrared spectroscopy, and photoelectron spectroscopy (XPS: x-ray induced and UPS: UV induced photoelectron spectroscopy). Microbatteries were constructed using the V2O5 thin films as cathode, LiClO4+propylene carbonate as the electrolyte and Li metal as the anode. The V2O5 electrodes were characterized by there electrochemical behavior (current-voltage curves during charging and recharging). X-ray diffraction measurement demonstrated that unannealed V2O5 films were amorphous with no sharp reflexes. Annealing to 350 °C was sufficient to produce crystalline beta-V2O5 (monoclinic) on uncoated glass or orthorhombic V2O5 on SnO2 coated glass. XPS spectra revealed the expected V/O ratio and line positions. However, the O 1s peak was split into two components, one from the V2O5 component (530.4 eV), the other at 532.9 eV is attributed to the uptake of water from the atmosphere. The charged microbattery resulted in a maximum voltage of 3.4 V for the currentless circuit and a capacity of 34.5 mC/cm2. With XPS we could demonstrate the migration of Sn (from the conductive coating of the glass substrate) into the V2O5 layer for the Li intercalated electrodes. Further, UPS spectra from charged and uncharged V2O5 indicate band-gap changes due to the Li uptake.
The polycrystalline V(2)O(5) films as the anode in V(2)o(5) /LiPON /LiCoO(2) lithium microbattary were prepared by RF magnetron sputtering system. The V(2)O(5) films' crystal structures, surface morphologies and composition were characterized and analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The microbatteries were fabricated by micro electro-mechanical system (MEMS) technology. The battery active unit area is 500 mu m X 500 mu m, and the thickness of V(2)O(5), LiPON and LiCoO(2) films was estimated to be 200, 610, and 220nm, respectively. The discharge volumetric capacity is between 9.36 mu Ahcm(-2)mu m(-1) and 9.63 mu Ahcm(-2)mu m(-1) after 40 cycles.
In this work, we have chosen oxidation of TiN thin films as a feasible method for preparation of nitrogen-doped titanium dioxide thin films, TiO2:N, for photocatalytic applications. DC reactive magnetron sputtering with the plasma emission control was used for deposition of stoichiometric TiN thin films. The microstructure and chemical composition of films before and after oxidation were investigated by means of RBS, X-ray diffraction (XRD) in grazing incidence diffraction (GID) configuration, AFM and XPS techniques. The electrical conductivity was measured by the van der Pauw method as a function of the oxidation temperature. The optical transmittance and reflectance spectra of the films were measured over the visible and UV ranges of the light spectrum. GID diffraction patterns of as-sputtered TiN thin films and those after oxidation indicate that TiO2 rutile is formed at around 300°C. Nitrogen is still present as indicated by XPS studies even when XRD detects the rutile only. Optical absorption of thin films oxidized at 450°C is shifted towards the visible range of the light spectrum.
In the present investigation, high energy ball milling and annealing were used to manipulate the grain size of AB3 based Ml0.7Mg0.3Ni3.2 multi-phase alloys (here Ml denotes La-rich mish metal). X-ray diffraction, transmission electron microscopy and pressure–composition–isothermal measurements were used to characterize the microstructure and hydrogen storage properties of the alloys after different milling and annealing treatments. It has been found that hydrogen storage capacity decreases with decreasing of grain size caused by milling. However, the hydrogen storage capacity of the milled alloy recovers gradually to the value of the as-cast state after annealing in a temperature range of 973–1173K. The effect of grain size on hydrogen storage property has been discussed based on the interaction of hydrogen with the inter- and intra-grain region of nanocrystalline alloys.
The present work considers quasi-metallic electrical properties for TiO2. The quasi- metallic effect may be produced by the incorporation of a small amount of donor-type ions, such as Nb, into the TiO2 lattice forming a solid solution. The metallic properties can be obtained in strongly reduced conditions, which may be achieved by annealing of the specimen at elevated temperatures in the gas phase of very low oxygen activities. The observed effect paves the way for processing photoelectrode with substantially enhanced charge transport.
Photocatalysis has recently become a common word and various products using photocatalytic functions have been commercialized. Among many candidates for photocatalysts, TiO2 is almost the only material suitable for industrial use at present and also probably in the future. This is because TiO2 has the most efficient photoactivity, the highest stability and the lowest cost. More significantly, it has been used as a white pigment from ancient times, and thus, its safety to humans and the environment is guaranteed by history. There are two types of photochemical reaction proceeding on a TiO2 surface when irradiated with ultraviolet light. One includes the photo-induced redox reactions of adsorbed substances, and the other is the photo-induced hydrophilic conversion of TiO2 itself. The former type has been known since the early part of the 20th century, but the latter was found only at the end of the century. The combination of these two functions has opened up various novel applications of TiO2, particularly in the field of building materials. Here, we review the progress of the scientific research on TiO2 photocatalysis as well as its industrial applications, and describe future prospects of this field mainly based on the present authors' work.
Titanium dioxide nanotubes, made by anodization, are highly sensitive to hydrogen; for example, cycling between nitrogen atmosphere and 1000ppm hydrogen a variation in measured resistance of 103 is seen for 46nm diameter nanotubes at 290°C. The hydrogen sensors are completely reversible and have response times of approximately 150s. Field emission scanning electron microscopy and Glancing angle X-ray diffraction (GAXRD) are used to study the surface morphology and crystal structure of the nanotubes.
Epitaxial Fe3O4(001) thin films (with a thickness in the range of 10–20nm) grown on MgO substrates were characterized using low-energy electron diffraction (LEED), conversion electron Mössbauer spectroscopy (CEMS) and investigated using Rutherford backscattering spectrometry (RBS), channeling (RBS-C) experiments and X-ray reflectometry (XRR). The Mg out-diffusion from the MgO substrate into the film was observed for the directly-deposited Fe3O4/MgO(001) films. For the Fe3O4/Fe/MgO(001) films, the Mg diffusion was prevented by the Fe layer and the surface layer is always a pure Fe3O4 layer. Annealing and ion beam mixing induced a very large interface zone having a spinel and/or wustite formula in the Fe3O4-on-Fe film system.
The possibility for the dispersion of V2O5 on different oxidic supports by heating physical mixtures of them to 773 K in dry air has been studied by means of X-ray diffraction and laser Raman spectroscopy. It is shown that monolayer dispersion occurs on TiO2 (anatase) as a support. This dispersion is considered to occur by a surface transport process. By contrast, no evidence for V2O5 dispersion was obtained with γ-Al2O3, SiO2, SiO2-Al2O3, SnO2, ZrO2, and Nb2O5 as supports, whereas a V2O5/MgO mixture led to the formation of the "three-dimensional" binary oxide Mg2V2O7.
Until now, photovoltaics--the conversion of sunlight to electrical power--has been dominated by solid-state junction devices, often made of silicon. But this dominance is now being challenged by the emergence of a new generation of photovoltaic cells, based, for example, on nanocrystalline materials and conducting polymer films. These offer the prospect of cheap fabrication together with other attractive features, such as flexibility. The phenomenal recent progress in fabricating and characterizing nanocrystalline materials has opened up whole new vistas of opportunity. Contrary to expectation, some of the new devices have strikingly high conversion efficiencies, which compete with those of conventional devices. Here I look into the historical background, and present status and development prospects for this new generation of photoelectrochemical cells.
The field of photocatalysis can be traced back more than 80 years to early observations of the chalking of titania-based paints and to studies of the darkening of metal oxides in contact with organic compounds in sunlight. During the past 20 years, it has become an extremely well researched field due to practical interest in air and water remediation, self-cleaning surfaces, and self-sterilizing surfaces. During the same period, there has also been a strong effort to use photocatalysis for light-assisted production of hydrogen. The fundamental aspects of photocatalysis on the most studied photocatalyst, titania, are still being actively researched and have recently become quite well understood. The mechanisms by which certain types of organic compounds are decomposed completely to carbon dioxide and water, for example, have been delineated. However, certain aspects, such as the photo-induced wetting phenomenon, remain controversial, with some groups maintaining that the effect is a simple one in which organic contaminants are decomposed, while other groups maintain that there are additional effects in which the intrinsic surface properties are modified by light. During the past several years, powerful tools such as surface spectroscopic techniques and scanning probe techniques performed on single crystals in ultra-high vacuum, and ultrafast pulsed laser spectroscopic techniques have been brought to bear on these problems, and new insights have become possible. Quantum chemical calculations have also provided new insights. New materials have recently been developed based on titania, and the sensitivity to visible light has improved. The new information available is staggering, but we hope to Offer an overview of some of the recent highlights, as well as to review some of the origins and indicate some possible new directions. (C) 2008 Elsevier B.V. All rights reserved
Thermochromic films of VO2, as well as multilayer films of VO2 and TiO2, were made by reactive DC magnetron sputtering. Spectrophotometrically measured transmittance and reflectance were used to determine optical constants pertinent to temperatures below and above a temperature-induced structural change at τc≈60 °C. We then used computations to optimize multilayer films for specific applications and, specifically, demonstrated that TiO2/VO2/TiO2 films could display a luminous transmittance significantly higher than that of bare VO2 films, and that TiO2/VO2/TiO2/VO2/TiO2 films could yield a large change of solar transmittance for temperatures above and below τc. Our data can serve as starting points for developing novel coatings for windows with superior energy efficiency.
One of the advantageous functional properties of Mg alloys (or compounds) is to exhibit the reversible hydriding reaction.
In this paper, we present our systematic studies regarding the relationship between nanometer- or atomistic-scale structures
and the specific hydriding properties of the Mg-Ni binary system, such as(1) nanostructured (n)-Mg2Ni, (2) a mixture of n-Mg2Ni and amorphous (a)-MgNi,(3) pure a-MgNi, and(4) n-MgNi2. Further studies on(5) an a-MgNi-based system for clarifying the effect of the short-range ordering on the structural and
hydriding properties and(6) a MgNi2-based system for synthesizing the new Laves phase structure are also presented. The materials science of Mg-Ni-based new
hydrides will provide indispensable knowledge for practically developing the Mg alloys as hydrogen-storage materials.
Epitaxially-grown Fe3O4(0 0 1) thin films by reactive deposition on MgO(1 0 0) substrates were studied using low-energy electron diffraction (LEED), conversion electron Mössbauer spectroscopy (CEMS), Rutherford backscattering spectrometry (RBS), channeling (RBS-C) experiments and X-ray reflectometry (XRR). No visible influence from the ion irradiation of the samples on the CEMS spectra was found, while surface oxidation of the samples was observed after exposure to the atmospheric pressure. RBS analysis indicated the presence of magnesium with an average amount of 3% in the films. RBS-C experiments yielded a value of 22% for the minimum yield of Fe and a value of 0.62° for the half-angle for Fe in the film indicating a good crystal quality of the films. The value for film-thickness obtained from XRR is in a good agreement with that from RBS and the nominal value.
Scientific studies on photocatalysis started about two and a half decades ago. Titanium dioxide (TiO2), which is one of the most basic materials in our daily life, has emerged as an excellent photocatalyst material for environmental purification. In this review, current progress in the area of TiO2 photocatalysis, mainly photocatalytic air purification, sterilization and cancer therapy are discussed together with some fundamental aspects. A novel photoinduced superhydrophilic phenomenon involving TiO2 and its applications are presented.
Vertically oriented nanotubular TiO2 arrays were formed by a simple anodization process. Hydrogen storage studies were carried out on the TiO2 nanotubular arrays having different diameters by charging and discharging hydrogen with potentiostatic/galvanostatic control. The hydrogen storage capacities of the nanotubes were only marginally affected by the tube diameter. Concentration of oxygen vacancies as defects influenced the hydrogen storage of the nanotubes. Annealing of the TiO2 nanotubes in argon atmosphere increased the defect density and decreased the hydrogen discharge during initial charge–discharge cycles. Hydrogen storage studies through electrochemical route did not show significant storage capacity of TiO2 nanotubes. Diffusion of hydrogen as protons and interference of the double layer capacitance of nanotubes could be attributed to the lower hydrogen storage capacity.
Reliable measurement of the photoconversion efficiency for semiconductor electrodes is essential to the assessment of electrode performance. In this paper, the influence of the choice of light source on measured photoconversion efficiencies for semiconductor photoelectrodes is examined. Measurements of efficiency performed under xenon lamp and solar illumination are compared with efficiencies calculated by integrating the incident photon conversion efficiency (IPCE) over the lamp and solar spectra. It is shown that use of a xenon lamp as the light source can lead to a large overestimate of the photoconversion efficiency, relative to that obtained under standard AM1.5 solar illumination. The overestimate is greater when a water filter is fitted to the xenon lamp, and when a wide-band gap semiconductor such as TiO2 is used as the photoelectrode. Achievable photoconversion efficiencies using rutile TiO2 are calculated taking into account the losses due to imperfect absorption, reflection and charge-carrier recombination; these calculated efficiencies agree with the measurements to within experimental uncertainties. It is demonstrated that many photoconversion efficiencies presented in the literature are overestimated. It is concluded that reliable estimation of efficiency under standard conditions is best obtained by measuring the IPCE as a function of wavelength, and integrating over the AM1.5 solar spectrum, or by measuring under sunlight with a similar zenith angle to that of the AM1.5 spectrum.
Water splitting and environmental cleanup are two active fields in heterogeneous photocatalysis, which are both closely related to the research in semiconductor photoelectrochemistry since the 1960s. The present review paper will attempt to describe some of the progress and resulting achievements in these two fields, and to briefly discuss the future prospects. We will cover the major developments worldwide but will highlight work carried out in Japan over the last several years. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
The present work considers the concept of photoelectrochemical generation of hydrogen through water splitting using solar energy (solar-hydrogen). The focus is on functional material properties that are essential for the performance of photoelectrochemical cell for solar-hydrogen. The performance of the cell is discussed in terms of the energy conversion efficiency (ECE). It is argued that TiO2 and TiO2-based materials are the most promising candidates for photoelectrodes for solar-hydrogen. The modification of TiO2 in order to achieve desired performance parameters is discussed in terms of the electronic structure, concentration of charge carriers and segregation-induced surface properties, which are critical to the ECE. Challenges to the development of a bi-photoelectrode cell, equipped with both n-type and p-type TiO2, forming photoanode and photocathode, respectively, are discussed. The research strategies and pressing issues related to the optimization of key functional properties necessary for the commercialization of solar-hydrogen are outlined. It is shown that defect chemistry is the most appropriate framework for tailoring the functional properties of TiO2-based oxide systems in order to obtain high-performance photoelectrodes. The present work provides an overview of the research progress on solar-hydrogen.
The field of surface science provides a unique approach to understanding bulk, surface and interfacial phenomena occurring during TiO2 photocatalysis. This review highlights, from a surface science perspective, recent literature that provides molecular-level insights into photon-initiated events occurring at TiO2 surfaces. Seven key scientific issues are identified in the organization of this review. These are: (1)Â photon absorption, (2)Â charge transport and trapping, (3)Â electron transfer dynamics, (4)Â the adsorbed state, (5)Â mechanisms, (6)Â poisons and promoters, and (7)Â phase and form. This review ends with a brief examination of several chemical processes (such as water splitting) in which TiO2 photocatalysis has made significant contributions in the literature.
Vanadium oxide supported over titanium oxide is an important example of a transition-metaloxide 'monolayer' catalyst with practical relevance for various industrial applications. After a brief introduction on the concept of monolayer oxide catalysts, this review examines three main topics: (i) the nature of vanadium oxide species over the titanium oxide surface, (ii) their reactivity and structure/activity-selectivity relationships and (iii) modification of the surface properties during the catalytic reactions of o-xylene oxidation and alkylaromatic ammoxidation. An analysis of selected literature data leads to the following main conclusions: (i) the support promotes the formation of an amorphous, hydrated active layer of titanium oxide which may extend up to 3-5 layers; (ii) the reactivity of titanium oxide in this form is enhanced with respect to crystalline vanadium oxide, but the selectivity properties which are a function of the oxidation state of vanadium ion and of the modifications occurring during the catalytic reaction are not significantly different; (iii) in the presence of ammonia, the characteristics of the catalyst surface must be tuned to limit the side reaction of NH3 combustion. The optimal catalyst composition for ammoxidation reactions is thus different from that for oxidation catalysts. Simultaneous O and N insertion requires instead an intermediate catalyst composition between those optimal for oxidation and ammoxidation of alkylaromatics.
Owing to the limited resources of fossil fuels, hydrogen is proposed as an alternative and environment-friendly energy carrier. However, its potential is limited by storage problems, especially for mobile applications. Current technologies, as compressed gas or liquefied hydrogen, comprise severe disadvantages and the storage of hydrogen in lightweight solids could be the solution to this problem. Since the optimal storage mechanism and optimal material have yet to be identified, this first handbook on the topic provides an excellent overview of the most probable candidates, highlighting both their advantages as well as drawbacks. From the contents: Physisorption. Clathrates. Metal hydrides. Complex hydrides. Amides, imides, and mixtures. Tailoring Reaction Enthalpies. Borazan. Aluminum hydride. Nanoparticles. A one-stop reference on all questions concerning hydrogen storage for physical and solid state chemists, materials scientists, chemical engineers, and physicists.
Considerable effort has been made to design, fabricate, and manipulate nanostructured materials by innovative approaches. The precise control of nanoscale structures will pave the way not only for elucidating unique size/shape-dependent physicochemical properties but also for realizing new applications in science and technology. Nanotechnology offers unprecedented opportunities for improving our daily lives and the environment in which we live. This review mainly describes our recent progress in the design, fabrication, and modification of nanostructured semiconductor materials for environmental applications. Their potential applications in the field of energy are briefly introduced. The scope of this article covers a variety of semiconductor materials, focusing particularly on TiO(2)-based nanostructures (e.g., pure, doped, coupled, nanoporous, mesoporous, hierarchically porous, and ordered mesoporous TiO(2)). The preparation of nanoparticles, hierarchical nanoarchitectures, thin films, and single crystals by sol-gel, microemulsion, hydrothermal, sonochemical, microwave, photochemical, and nanocasting methods is discussed.
ALTHOUGH the possibility of water photolysis has been investigated by many workers, a useful method has only now been developed. Because water is transparent to visible light it cannot be decomposed directly, but only by radiation with wavelengths shorter than 190 nm (ref. 1).
TiO(2) nanotubes can reproducibly store up to approximately 2 wt % H(2) at room temperature and 6 MPa. However, only about 75% of this stored hydrogen can be released when the hydrogen pressure is lowered to ambient conditions, suggesting that both physisorption and chemisorption are responsible for the hydrogen uptake. FTIR spectroscopy, temperature-programmed desorption (TPD), and pressure-composition (P-C) isotherms suggest that 75% of the H(2) is physisorbed and can be reversibly released upon pressure reduction. Approximately 13% is weakly chemisorbed and can be released at 70 degrees C as H(2), and approximately 12% is bonded to oxide ions and released only at temperatures above 120 degrees C as H(2)O.
The sorption of hydrogen between the layers of the multilayered wall of nanotubular TiO2 was studied in the temperature range of -195 to 200 degrees C and at pressures of 0 to 6 bar. Hydrogen can intercalate between layers in the walls of TiO2 nanotubes forming host-guest compounds TiO2 x xH2, where x < or = 1.5 and decreases at higher temperatures. The rate of hydrogen incorporation increases with temperature and the characteristic time for hydrogen sorption in TiO2 nanotubes is several hours at 100 degrees C. The rate of intercalate formation is limited by the diffusion of molecular hydrogen inside the multilayered walls of the TiO2 nanotube. 1H NMR-MAS and XRD data confirm the incorporation of hydrogen between the layers in the walls of TiO2 nanotubes. The nature and possible applications of the observed intercalates are considered.
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