Browsing by Author "Monteiro, O. C."
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- An AFM, TEM, XRD and luminescence study of carbon nanomaterials obtained from cork industry wastewaterPublication . Prata, José Virgílio; Monteiro, O. C.; Silvestre, António Jorge; Viana, A. S.; Costa, AlexandraQuercus suber L. (cork oak) contributes significantly to the economies of several European Union countries. The industrial processing of cork planks involves a cooking operation in which the textural and plastic properties of cork are improved. The resulting wastewater (ca. 400 L/ton of processed cork) is rich in water-soluble extractives, namely phenol- and polyphenol-based compounds and pectins. While the presence of these and other compounds usually defy several spent water treatment processes owing to their inherent poor biodegradability, they represente otherwise a huge opportunity to produce high-valued carbon-based materials.
- Influence of the Sodium/Proton replacement on the structural, morphological and photocatalytic properties of titanate nanotubesPublication . Bem, V.; Neves, M. C.; Nunes, M. R.; Silvestre, António Jorge; Monteiro, O. C.Titanate nanotubes (TNT) with different sodium contents have been synthesised using a hydrothermal approach and a swift and highly controllable post-washing processes. The influence of the sodium/proton replacement on the structural and morphological characteristics of the prepared materials was analysed. Different optical behaviour was observed depending on the Na+/H+ samples’ content. A band gap energy of 3.27±0.03 eV was estimated for the material with higher sodium content while a value of 2.81±0.02 eV was inferred for the most protonated material, which therefore exhibits an absorption edge in the near visible region. The point of zero charge of the materials was determined and the influence of the sodium content on the adsorption of both cationic and anionic organic dyes was studied. The photocatalytic performance of the TNT samples was evaluated in the rhodamine 6G degradation process. Best photodegradation results were obtained when using the most protonated material as catalyst, although this material has shown the lowest R6G adsorption capability.
- Magnetic properties of Co-doped TiO(2) anatase nanopowdersPublication . Pereira, L. C. J.; Nunes, M. R.; Monteiro, O. C.; Silvestre, António JorgeThis letter reports on the magnetic properties of Ti(1-x)Co(x)O(2) anatase phase nanopowders with different Co contents. It is shown that oxygen vacancies play an important role in promoting long-range ferromagnetic order in the material studied in addition to the transition-metal doping. Furthermore, the results allow ruling out the premise of a strict connection between Co clustering and the ferromagnetism observed in the Co:TiO(2) anatase system.
- Novel C-dots/titanate nanotubular hybrid materials with enhanced optical and photocatalytic propertiesPublication . Alves, D. M.; Prata, José Virgílio; Silvestre, António Jorge; Monteiro, O. C.Advanced nanomaterials with enhanced optical and photocatalytic properties for the photodegradation of organic pollutants, in particular pharmaceuticals and personal care products (PPCPs), were successfully prepared by a swift one-pot synthesis. Nanostructured materials were synthesized through an integrated hydrothermal procedure which generates titanate nanotubes (TNTs) with different carbon dots (C-dots) contents, from an amorphous titanium oxide-based source and cork industry wastewaters (CIWW) as carbon source. Their structural, microstructural, morphological, and optical properties were studied by XRD, TEM, XPS, UV-Vis diffuse reflectance and photoluminescence spectroscopies. As aimed, the hybrid C-dots/ TNT nanomaterials extend their light absorption towards the red, in comparison to pristine TNT, prompting them for a more efficient use of light in photocatalysis by widening the TNT energy uptake range. The decrease of bandgap energy with increasing sample's C-dots content seems to be originated from energy intermediate states formed within the TNTs' forbidden band resulting from Ti-O-C bonds established between the TNTs and the C-dots that form tails of states.The as-synthesized C-dots/TNT samples were tested in the photodegradation of caffeine as a pollutant model for PPCPs. Rewarding results were obtained, with the hybrid C-dots/TNT nanomaterials showing significant enhanced photocatalytic ability toward caffeine degradation in comparison to pristine TNTs. Photocatalysis assays in the presence of scavengers and/or in the absence of oxygen were also performed aiming to characterize the most reactive species formed during the semiconductor photo-activation process and thus assessing to possible reactive pathways underpinning the photocatalytic activity of the hybrid C-dots/TNT nanomaterials.
- Synthesis and properties of Co-doped titanate nanotubes and their optical sensitization with methylene bluePublication . Ferreira, V. C.; Nunes, M. R.; Silvestre, António Jorge; Monteiro, O. C.Here we report on a novel chemical route to synthesize homogenous cobalt-doped titanate nanotubes (CoTNT), using an amorphous Co-doped precursor. The influence of the synthesis temperature, autoclave dwell time and metal doping on the structural and microstructural as well as on the optical properties of the synthesized titanate nanotubes is studied and discussed. The optical band gaps of the CoTNT samples are red shifted in comparison with the values determined for the undoped samples, such red shifts bringing the absorption edge of the CoTNT samples into the visible region. CoTNT materials also demonstrate particular high adsorption ability for methylene blue, the amount of the adsorbed dye being higher than the one predictable for a monolayer formation. This suggests the possibility of intercalation of the dye molecule between the TiO6 layers of the TNT structure. It is also shown that the methylene blue sensitized Co-doped nanostructures are highly stable under UV radiation and present a strong and broad absorption in the visible region. (C) 2013 Elsevier B.V. All rights reserved.
- Synthesis of Sub-5 NM Co-Doped SnO2 nanoparticles and their structural microstructural, optical and photocatalytic propertiesPublication . Entradas, T.; Cabrita, J. F.; Dalui, S.; Nunes, M. R.; Monteiro, O. C.; Silvestre, António JorgeA swift chemical route to synthesize Co-doped SnO2 nanopowders is described. Pure and highly stable Sn1-xCoxO2-delta (0 <= x <= 0.15) crystalline nanoparticles were synthesized, with mean grain sizes <5 nm and the dopant element homogeneously distributed in the SnO2 matrix. The UV-visible diffuse reflectance spectra of the Sn1-xCoxO2-delta samples reveal red shifts, the optical bandgap energies decreasing with increasing Co concentration. The samples' Urbach energies were calculated and correlated with their bandgap energies. The photocatalytic activity of the Sn1-xCoxO2-delta samples was investigated for the 4-hydroxylbenzoic acid (4-HBA) degradation process. A complete photodegradation of a 10 ppm 4-HBA solution was achieved using 0.02% (w/w) of Sn0.95Co0.05O2-delta nanoparticles in 60 min of irradiation. (C) 2014 Elsevier B.V. All rights reserved.
- Synthesis of titanate nanofibers co-sensitized with ZnS and Bi2S3 nanocrystallites and their application on pollutants removalPublication . Entradas, T.J.; Cabrita, J. F.; Barrocas, B.; Nunes, M. R.; Silvestre, António Jorge; Monteiro, O. C.The synthesis of nanocomposite materials combining titanate nanofibers (TNF) with nanocrystalline ZnS and Bi2S3 semiconductors is described in this work. The TNF were produced via hydrothermal synthesis and sensitized with the semiconductor nanoparticles, through a single-source precursor decomposition method. ZnS and Bi2S3 nanoparticles were successfully grown onto the TNF's surface and Bi2S3-ZnS/TNF nanocomposite materials with different layouts. The samples' photocatalytic performance was first evaluated through the production of the hydroxyl radical using terephthalic acid as probe molecule. All the tested samples show photocatalytic ability for the production of this oxidizing species. Afterwards, the samples were investigated for the removal of methylene blue. The nanocomposite materials with best adsorption ability were the ZnS/TNF and Bi2S3ZnS/TNF. The dye removal was systematically studied, and the most promising results were obtained considering a sequential combination of an adsorption-photocatalytic degradation process using the Bi2S3ZnS/TNF powder as a highly adsorbent and photocatalyst material. (C) 2015 Elsevier Ltd. All rights reserved.
- The effect of ionic Co presence on the structural, optical and photocatalytic properties of modified cobalt–titanate nanotubesPublication . Barrocas, B.; Silvestre, António Jorge; Rolo, A. G.; Monteiro, O. C.With the aim of producing materials with enhanced optical and photocatalytic properties, titanate nanotubes (TNTs) modified by cobalt doping (Co-TNT) and by Na+ -> Co ion-exchange (TNT/Co) were successfully prepared by a hydrothermal method. The influence of the doping level and of the cobalt position in the TNT crystalline structure was studied. Although no perceptible influence of the cobalt ion position on the morphology of the prepared titanate nanotubes was observed, the optical behaviour of the cobalt modified samples is clearly dependent on the cobalt ions either substituting the Ti4+ ions in the TiO6 octahedra building blocks of the TNT structure (doped samples) or replacing the Na+ ions between the TiO6 interlayers (ion-exchange samples). The catalytic ability of these materials on pollutant photodegradation was investigated. First, the evaluation of hydroxyl radical formation using the terephthalic acid as a probe was performed. Afterwards, phenol, naphthol yellow S and brilliant green were used as model pollutants. Anticipating real world situations, photocatalytic experiments were performed using solutions combining these pollutants. The results show that the Co modified TNT materials (Co-TNT and TNT/Co) are good catalysts, the photocatalytic performance being dependent on the Co/Ti ratio and on the structural metal location. The Co(1%)-TNT doped sample was the best photocatalyst for all the degradation processes studied.