Carbon-modified titanium oxide materials for photocatalytic water and air decontamination

Titanium oxide-based materials with different physical and chemical features were synthetized aiming at removing organic pollutants from both water and air media. The materials were produced employing two different heating methodologies (thermal, T and hydrothermal, H) at distinct temperatures resulting in porous materials. These materials were also modified with either graphene oxide (GO) or carbon nanotubes (CNT), using an in-situ approach. All materials were tested as photocatalysts using ultra-violet (UV), visible (Vis) and solar radiation. Rhodamine B (RhB) and benzene were used as representative pollutants in water and air, respectively. The addition of carbon to the catalysts improved the removal of both pollutants. In the case of the photocatalytic degradation of rhodamine B, under both UV and Vis light, it was found that, the materials containing carbon nanostructures allowed the highest degradation degree, while the photosensitisation phenomenon became negligible. The best catalyst is the one containing CNT (2.98 wt% of C) and thermally treated at 300 °C (T300_CNT). This material showed higher degradation ability than the commercial TiO2 nanopowder Degussa P25 (P25) under Vis light. Regarding benzene removal, the samples thermally treated at 300 °C and modified with CNT and GO (T300_CNT and T300_GO, respectively) outperformed Degussa P25. The former material was successfully reused in the photocatalytic degradation of benzene over 6 consecutive cycles.publishe

Enhancement of thermoelectric performance of donor-doped ZnO ceramics by involving an In situ aluminothermic reaction during processing

This work explores the possibility of involving aluminothermy in processing donor-doped zinc oxide-based thermoelectrics by relying on local, strong exothermic effects developed during sintering, with a potential positive impact on the electrical and thermal transport properties. The strategy was exemplified by using aluminium as a dopant, due to its recognized ability to generate additional, available charge carriers in ZnO, and by using two different metallic Al powders and conventional Al2O3 as precursors. Nanosized aluminium powder was involved in order to evaluate the possible desirable effects of the particles size, as compared to aluminium micropowder. A significant enhancement of the electrical and thermoelectric performance of the samples prepared via metallic Al precursors was observed and discussed in terms of the potential impacts provided by the aluminothermic reaction on the microstructure, charge carrier concentration and mobility during sintering. Although the presented results are the first to show evidence of how aluminothermic reactions can be used for boosting the thermoelectric performance of zinc oxide materials, the detailed mechanisms behind the observed enhancements are yet to be understood.publishe

High colouring efficiency, optical density and inserted charge in sol–gel derived electrochromic titania nanostructures

A pure TiO2 thin film (100–120 nm) was made from a green aqueous sol–gel precursor on FTO glass and calcined at 430 °C. It was a mix of amorphous, anatase, rutile and brookite TiO2 phases, and exhibited very good electrochromic properties over visible and NIR wavelengths with an applied bias of +0.1 V to −1.5 V. It was highly transparent showing excellent coloration with applied voltage, with transmittance modulation (ΔT) = 69.7% at 550 nm, 86% at 700 nm and an overall ΔT between 400–1650 nm of 60%, giving a very large change in optical density (ΔOD) of 1.4 at 550 nm and 2.4 at 700 nm. Cyclic voltammograms had typical peaks for TiO2 at −1.3 V for colouration and −0.9 V for bleaching, with a high separation of 0.37 V between peaks, and a charge density after charging for 25 min of Qc = 50 mC cm−2. After only 60 s and 120 s at −1.5 V, inserted charge values of 17.6 and 22 mC cm−2 were observed, leading to a high colouration efficiency (CE) of 55.9 cm2 C−1 at 550 nm. These ΔOD, ΔT, Qc and CE values are superior to any previously reported for crystalline sol–gel TiO2 films. They also possessed rapid switching times for bleaching and colouring of τb90% = 10 s and τc90% = 55 s, comparable to the best previously reported sol–gel anatase-based TiO2 films. This makes this nanomaterial an excellent candidate for smart windows and other electrochromic devices and applications

Sol gel graphene/TiO2 nanoparticles for the photocatalytic-assisted sensing and abatement of NO2

Abstract Human exposure to volatile organic compounds and NO2 can lead to health problems, therefore strategies to mitigate against the risks are required. Abatement and sensing are approaches which could both neutralise and monitor these species thus providing a safer environment and warning occupants of harmful levels. This paper presents pure TiO2 and TiO2/graphene hybrids synthesized through a sol-gel route. Electron optical, helium ion microscopy, X-ray diffraction and spectroscopic methods have been applied to elucidate the physical and chemical behaviour. NO2 sensing properties of TiO2/graphene hybrids formed by the addition of graphene to the reaction vessel prior to initiating the sol gel reaction followed by annealing (GTiO2S), and an alternative manufacturing method involving the addition of graphene to TiO2 nanoparticles which had already been annealed (GTiO2M) were compared and evaluated. A conductometric sensor based on TiO2/graphene prepared using material GTiO2S showed a higher response to NO2 compared to sensors based on pure TiO2 and TiO2/graphene prepared with material GTiO2M. Under UV irradiation generated by a low power LED, the sensor showed a remarkably enhanced response to 1750 ppb NO2, about double the response in the dark, and a limit of detection of about 50 ppb of NO2 (Signal/Noise = 3). Photocatalytic tests to assess the degradation of NOx showed that TiO2/graphene hybrids using material GTiO2S were the most active amongst the whole series of TiO2-based materials. Our data highlights the unique characteristics of material GTiO2S TiO2/graphene and the suitability for multi-purpose applications in the field of environmental monitoring and remediation. The capability of the material for both sensing and abatement of NOx could be exploited to offer a safer environment through providing a warning of the presence of NOx whilst also reducing levels

AlN interlayer-induced reduction of dislocation density in the AlGaN epilayer

The emerging ultrawide-bandgap AlGaN alloy system holds promise for the development of advanced materials in the next generation of power semiconductor and UV optoelectronic devices. Within this context, heterostructures based on III-nitrides are very popular in view of their applications as electronic and optoelectronic components. AlGaN-based deep UV emitters are gaining visibility due to their disinfection capabilities. Likewise, high electron mobility transistors are attracting increasing attention owing to their superior electron transport which yields high-speed and high-power applications. These devices are conventionally made of AlGaN/GaN heterostructures grown on foreign substrates. However, structural defects, including stress induced by a mismatch in unit cell parameters and the presence of dislocations, can not only decrease the efficiency of the light emitters (by facilitating the non-radiative recombination of electron-hole pairs), but also impede electron mobility within the two-dimensional electron gas at the AlGaN/GaN interface. Therefore, the significance of obtaining high-quality AlGaN layers becomes evident. Including a thin AlN interlayer between the GaN buffer layer and AlGaN is a possible answer to address these drawbacks. Not only do we show that a thin AlN layer, approximately ≤3 nm in thickness, between the GaN buffer and AlGaN layers, is effective in decreasing the dislocation densities in the AlGaN layer by around 30%, but also this is responsible for an increase in the electron mobility (approximately 33%) compared to a classical AlGaN/GaN heterostructure. Additionally, the resulting heterostructure exhibits better optical quality, with a 7-fold increase in intensity as well as a 20% reduction in full-width at half-maximum in the AlGaN emission

Synergistic effects of zirconium- and aluminum co-doping on the thermoelectric performance of zinc oxide

This work aims to explore zirconium as a possible dopant to promote thermoelectric performance in bulk ZnO-based materials, both within the single-doping concept and on simultaneous co-doping with aluminum. At 1100–1223 K mixed-doped samples demonstrated around ∼2.3 times increase in ZT as compared to single-doped materials, reaching ∼0.12. The simultaneous presence of aluminum and zirconium imposes a synergistic effect on electrical properties provided by their mutual effects on the solubility in ZnO crystal lattice, while also allowing a moderate decrease of the thermal conductivity due to phonon scattering effects. At 1173 K the power factor of mixed-doped Zn0.994Al0.003Zr0.003O was 2.2–2.5 times higher than for single-doped materials. Stability tests of the prepared materials under prospective operation conditions indicated that the gradual increase in both resistivity and Seebeck coefficient in mixed-doped compositions with time may partially compensate each other to maintain a relatively high power factorpublishe

Graphene-TiO2 hybrids for photocatalytic aided removal of VOCs and nitrogen oxides from outdoor environment

No abstract available

Graphene-modified g-C3N4/ a-Fe2O3 systems for light-induced hydrogen generation

Photocatalysis represents an advanced and efficient technology for harnessing light energy. The non-toxicity, affordability, and versatility of this technique render it particularly attractive for hydrogen production via water splitting. Nevertheless, the primary challenge lies in identifying materials capable of efficiently catalyzing the water splitting reaction upon exposure to light. This study presents the influence of the quantity of hematite and graphene on g-C3N4 in the context of hydrogen generation from methanol-water decomposition under UVC irradiation. Pure g-C3N4 exhibits the highest hydrogen generation efficiency. However, adding hematite decreases photocatalytic efficiency, likely due to the formation of a type II heterojunction between α-Fe2O3 and g-C3N4, which reduces the overall reduction capacity of the system. While incorporating graphene into the g-C3N4/α-Fe2O3 system enhances photocatalytic efficiency by improving electron mobility and prolonging the lifetime of photo-generated excitons, the highest yield was achieved with BUF10/GNP0.5. This research offers valuable insights into charge transfer and separation processes for photo-generated excitons within the g-C3N4/α-Fe2O3 and g-C3N4/α-Fe2O3/graphene systems in the context of light-induced hydrogen production

Exploring tantalum as a potential dopant to promote the thermoelectric performance of zinc oxide

Zinc oxide (ZnO) has being recognised as a potentially interesting thermoelectric material, allowing flexible tuning of the electrical properties by donor doping. This work focuses on the assessment of tantalum doping effects on the relevant structural, microstructural, optical and thermoelectric properties of ZnO. Processing of the samples with a nominal composition Zn1-xTaxO by conventional solid-state route results in limited solubility of Ta in the wurtzite structure. Electronic doping is accompanied by the formation of other defects and dislocations as a compensation mechanism and simultaneous segregation of ZnTa2O6 at the grain boundaries. Highly defective structure and partial blocking of the grain boundaries suppress the electrical transport, while the evolution of Seebeck coefficient and band gap suggest that the charge carrier concentration continuously increases from x = 0 to 0.008. Thermal conductivity is almost not affected by the tantalum content. The highest ZT~0.07 at 1175 K observed for Zn0.998Ta0.002O is mainly provided by high Seebeck coefficient (-464 µV/K) along with a moderate electrical conductivity of ~13 S/cm. The results suggest that tantalum may represent a suitable dopant for thermoelectric zinc oxide, but this requires the application of specific processing methods and compositional design to enhance the solubility of Ta in wurtzite lattice