Publications by authors named "Stefano Lettieri"

5 Publications

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Charge Carrier Processes and Optical Properties in TiO and TiO-Based Heterojunction Photocatalysts: A Review.

Materials (Basel) 2021 Mar 27;14(7). Epub 2021 Mar 27.

Department of Physics "E. Pancini", University of Naples "Federico II", Complesso Universitario di Monte S. Angelo, Via Cupa Cintia 21, 80126 Napoli, Italy.

Photocatalysis based technologies have a key role in addressing important challenges of the ecological transition, such as environment remediation and conversion of renewable energies. Photocatalysts can in fact be used in hydrogen (H) production (e.g., via water splitting or photo-reforming of organic substrates), CO reduction, pollution mitigation and water or air remediation via oxidation (photodegradation) of pollutants. Titanium dioxide (TiO) is a "benchmark" photocatalyst, thanks to many favorable characteristics. We here review the basic knowledge on the charge carrier processes that define the optical and photophysical properties of intrinsic TiO. We describe the main characteristics and advantages of TiO as photocatalyst, followed by a summary of historical facts about its application. Next, the dynamics of photogenerated electrons and holes is reviewed, including energy levels and trapping states, charge separation and charge recombination. A section on optical absorption and optical properties follows, including a discussion on TiO photoluminescence and on the effect of molecular oxygen (O) on radiative recombination. We next summarize the elementary photocatalytic processes in aqueous solution, including the photogeneration of reactive oxygen species (ROS) and the hydrogen evolution reaction. We pinpoint the TiO limitations and possible ways to overcome them by discussing some of the "hottest" research trends toward solar hydrogen production, which are classified in two categories: (1) approaches based on the use of engineered TiO without any cocatalysts. Discussed topics are highly-reduced "black TiO", grey and colored TiO, surface-engineered anatase nanocrystals; (2) strategies based on heterojunction photocatalysts, where TiO is electronically coupled with a different material acting as cocatalyst or as sensitizer. Examples discussed include TiO composites or heterostructures with metals (e.g., Pt-TiO, Au-TiO), with other metal oxides (e.g., CuO, NiO, etc.), direct Z-scheme heterojunctions with g-CN (graphitic carbon nitride) and dye-sensitized TiO.
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http://dx.doi.org/10.3390/ma14071645DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8036967PMC
March 2021

Growth Mechanisms of ZnO Micro-Nanomorphologies and Their Role in Enhancing Gas Sensing Properties.

Sensors (Basel) 2021 Feb 13;21(4). Epub 2021 Feb 13.

Istituto di Scienze e Tecnologie per l'Energia e la Mobilità Sostenibili (CNR-STEMS), Via Canal Bianco 28, 44124 Ferrara, Italy.

Zinc oxide (ZnO) is one of the main functional materials used to realize chemiresistive gas sensors. In addition, ZnO can be grown through many different methods obtaining the widest family of unique morphologies. However, the relationship between the ZnO morphologies and their gas sensing properties needs more detailed investigations, also with the aim to improve the sensor performances. In this work, seven nanoforms (such as leaves, bisphenoids, flowers, needles, etc.) were prepared through simple wet chemical synthesis. Morphological and structural characterizations were performed to figure out their growth mechanisms. Then, the obtained powders were deposited through screen-printing technique to realize thick film gas sensors. The gas sensing behavior was tested toward some traditional target gases and some volatile organic compounds (acetone, acetaldehyde, etc.) and compared with ZnO morphologies. Results showed a direct correlation between the sensors responses and the powders features (morphology and size), which depend on the specific synthesis process. The sensors can be divided in two behavioral classes, following the two main morphology kinds: aggregates of nanocrystals (leaves and bisphenoids), exhibiting best performances versus all tested gases and monocrystal based (stars, needle, long needles, flowers, and prisms).
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http://dx.doi.org/10.3390/s21041331DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7918259PMC
February 2021

(Ti,Sn) Solid Solution Based Gas Sensors for New Monitoring of Hydraulic Oil Degradation.

Materials (Basel) 2021 Jan 28;14(3). Epub 2021 Jan 28.

Istituto di Scienze e Tecnologie per l'Energia e la Mobilità Sostenibili (CNR-STEMS), 44124 Ferrara, Italy.

The proper operation of a fluid power system in terms of efficiency and reliability is directly related to the fluid state; therefore, the monitoring of fluid ageing in real time is fundamental to prevent machine failures. For this aim, an innovative methodology based on fluid vapor analysis through metal oxide (shortened: MOX) gas sensors has been developed. Two apparatuses were designed and realized: (i) A dedicated test bench to fast-age the fluid under controlled conditions; (ii) a laboratory MOX sensor system to test the headspace of the aged fluid samples. To prepare the set of MOX gas sensors suitable to detect the analytes' concentrations in the fluid headspace, different functional materials were synthesized in the form of nanopowders, characterizing them by electron microscopy and X-ray diffraction. The powders were deposited through screen-printing technology, realizing thick-film gas sensors on which dynamical responses in the presence of the fluid headspace were obtained. It resulted that gas sensors based on solid solution TiSnO with x = 0.9 and 0.5 offered the best responses toward the fluid headspace with lower response and recovery times. Furthermore, a decrease in the responses (for all sensors) with fluid ageing was observed.
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http://dx.doi.org/10.3390/ma14030605DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7865283PMC
January 2021

Luminescent -Iridium(III) Complex Based on a Bis(6,7-dimethoxy-3,4-dihydroisoquinoline) Platform Featuring an Unusual cis Orientation of the CN Ligands: From a Theoretical Approach to a Deep Red LEEC Device.

ACS Omega 2019 Jan 25;4(1):2009-2018. Epub 2019 Jan 25.

Department of Chemical Sciences and Department of Physics "E. Pancini", University of Naples Federico II, via Cintia 4, I-80126 Napoli, Italy.

By pursuing the strategy of manipulating natural compounds to obtain functional materials, in this work, we report on the synthesis and characterization of a luminescent cationic iridium complex (-), designed starting from the catecholic neurotransmitter dopamine, exhibiting the unusual cis arrangement of the CN ligands. Through an integrated experimental and theoretical approach, it was possible to delineate the optoelectronic properties of -. In detail, (a) a series of absorption maxima in the range 300-400 nm was assigned to metal-to-ligand charge transfer and weak and broad absorption maxima at longer wavelengths (400-500 nm) were ascribable to spin-forbidden transitions with a mixed character; (b) there was an intense red phosphorescence with emission set in the range 580-710 nm; and (c) a highest occupied molecular orbital was mainly localized on the metal and the 2-phenylpiridine ligand and a lowest unoccupied molecular orbital was localized on the NN ligand, with a Δ set at 2.20 eV. This investigation allowed the design of light-emitting electrochemical cell (LEEC) devices endowed with good performance. The poor literature reporting on the use of -iridium(III) complexes in LEECs prompted us to investigate the role played by the selected cathode and the thickness of the emitting layer, as well as the doping effect exerted by ionic liquids on the performance of the devices. All the devices exhibited a deep red emission, in some cases, quite near the pure color (devices #1, #4, and #8), expanding the panorama of the iridium-based red-to-near-infrared LEEC devices. The characteristics of the devices, such as the brightness reaching values of 162 cd/m for device #7, suggested that the performances of - are comparable to those of trans isomers, opening new perspective toward designing a new set of luminescent materials for optoelectronic devices.
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http://dx.doi.org/10.1021/acsomega.8b02859DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6648618PMC
January 2019

Giant O-Induced Photoluminescence Modulation in Hierarchical Titanium Dioxide Nanostructures.

ACS Sens 2017 Jan 4;2(1):61-68. Epub 2017 Jan 4.

Institute of Applied Sciences and Intelligent Systems (CNR-ISASI) , Via Campi Flegrei 34, I-80078 Pozzuoli, Italy.

We demonstrate exceptionally large modulation of PL intensity in hierarchical titanium dioxide (TiO) nanostructures exposed to molecular oxygen (O). Optical responsivities up to about 1100% at 20% O concentrations are observed in hyperbranched anatase-phase hierarchical structures, outperforming those obtainable by commercial TiO nanopowders (up to a factor of ∼7 for response to synthetic air) and significantly improving the ones typically reported in PL-based opto-chemical gas sensing using MOXs. The improved PL response is discussed in terms of the specific morphology of hierarchical structures, characterized by simultaneous presence of small nanoparticles, large surface areas, and large voids. These characteristics guarantee an optimal interplay between photogenerated charges, PL-active centers, and adsorbed gas molecules. The results highlight the potentialities offered by hierarchical structures based on TiO or other MOXs and open interesting scenarios toward the development of all-optical and/or hybrid (opto/electrical) chemical sensors with improved sensitivity.
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http://dx.doi.org/10.1021/acssensors.6b00432DOI Listing
January 2017
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