Evolution of nanomechanical properties and crystallinity of individual titanium dioxide nanotube resonators.

Herein a complete characterization of single TiO2 nanotube resonator was reported for the first time. The modal vibration response analysis allows a non-invasive indirect evaluation of the mechanical properties of the TiO2 nanotube. The effect of post-grown thermal treatments on nanotube mechanical properties was investigated and carefully correlated to the chemico-physical parameters evolution. The Young's modulus of TiO2 nanotube rises linearly from 57 GPa up to 105 GPa for annealing at 600 °C depending on the compositional and crystallographic evolution of the nanostructure. Considering the growing interest in single nanostructure devices, the reported findings allow a deeper understanding of the properties of individual titanium dioxide nanotubes extrapolated from their standard arrayed architecture.

Graphene as Barrier to Prevent Volume Increment of Air Bubbles over Silicone Polymer in Aqueous Environment.

The interaction of air bubbles with surfaces immersed in water is of fundamental importance in many fields of application ranging from energy to biology. However, many aspects of this topic such as the stability of surfaces in contact with bubbles remain unexplored. For this reason, in this work, we investigate the interaction of air bubbles with different kinds of dispersive surfaces immersed in water. The surfaces studied were polydimethylsiloxane (PDMS), graphite, and single layer graphene/PDMS composite. X-ray photoelectron spectroscopy (XPS) analysis allows determining the elemental surface composition, while Raman spectroscopy was used to assess the effectiveness of graphene monolayer transfer on PDMS. Atomic force microscopy (AFM) was used to study the surface modification of samples immersed in water. The surface wettability has been investigated by contact angle measurements, and the stability of the gas bubbles was determined by captive contact angle (CCA) measurements. CCA measurements show that the air bubble on graphite surface exhibits a stable behavior while, surprisingly, the volume of the air bubble on PDMS increases as a function of immersion time (bubble dynamic evolution). Indeed, the air bubble volume on the PDMS rises by increasing immersion time in water. The experimental results indicate that the dynamic evolution of air bubble in contact with PDMS is related to the rearrangement of surface polymer chains via the migration of the polar groups. On the contrary, when a graphene monolayer is present on PDMS, it acts as an absolute barrier suppressing the dynamic evolution of the bubble and preserving the optical transparency of PDMS.

New insights on laser-induced graphene electrodes for flexible supercapacitors: tunable morphology and physical properties.

In certain polymers the graphenization of carbon atoms can be obtained by laser writing owing to the easy absorption of long-wavelength radiation, which generates photo-thermal effects. On a polyimide surface this process allows the formation of a nanostructured and porous carbon network known as laser-induced graphene (LIG). Herein we report on the effect of the process parameters on the morphology and physical properties of LIG nanostructures. We show that the scan speed and the frequency of the incident radiation affect the gas evolution, inducing different structure rearrangements, an interesting nitrogen self-doping phenomenon and consequently different conduction properties. The materials were characterized by infrared and Raman spectroscopy, XPS elemental analysis, electron microscopy and electrical/electrochemical measurements. In particular the samples were tested as interdigitated electrodes into electrochemical supercapacitors and the optimized LIG arrangement was tested in parallel and series supercapacitor configurations to allow power exploitation.

Immobilization of Oligonucleotides on Metal-Dielectric Nanostructures for miRNA Detection.

The development of nanostructured metal-dielectric materials, suitable for biodetection based on surface plasmon resonance and surface enhanced Raman scattering (SERS), requires the refinement of proper biological protocols for their effective exploitation. In this work, the immobilization of DNA probes on nanostructured metal-dielectric/semiconductor substrates has been optimized, to develop a bioassay for the detection of miRNA. To ensure a broad relevance, the proposed biological protocol was applied to different silver-decorated functional supports: porous silicon (pSi), TiO2 nanotube arrays, and polydimethylsiloxane (PDMS). The efficiency and the stability of the substrates were carefully analyzed by Raman spectroscopy and electron microscopy after the incubation in buffers with the appropriate combination of pH, ionic strength, and surfactant content. The customized protocol, initially developed on multiwell plates, was transferred and refined on the nanostructured substrates. The nonspecific interaction of the biological species with the surface was evaluated and reduced thanks to a tailored surface pretreatment. SERS analysis was applied to check the immobilization of DNA probes on pretreated samples. Silvered PDMS-supported pSi membranes, the most promising substrates in terms of stability, were subjected to further optimizations. Concentrations, volume, and duration of incubations were finely adapted with respect to the surface probe density and to the corresponding hybridization of the complementary miRNA. The optimized ELISA-like assay shows sensitivities comparable to those of commercial plates for the detection of miRNA222 (LOD: 485 pM), paving the way for the application of the developed protocol on metal-dielectric/semiconductor nanostructures for ultrasensitive SERS biosensing applications.

Memristive behaviour in poly-acrylic acid coated TiO2 nanotube arrays.

This work investigates titanium dioxide nanotube arrays (TiO2-NTA) grown by anodic oxidation as an active material for memristive applications. In particular, metal-insulator-metal structures made of vertically oriented amorphous TiO2-NTA grown on titanium foil were exploited in Ti/TiO2-NTA/Pt devices. The deposition of a polymeric thin film between NTA and top electrodes significantly improved the stability of the devices and increased by more than double the off/on resistance ratio. The resistive switching of TiO2-NTA samples crystallised by thermal annealing was also studied. Such devices displayed nonlinear I-V curves characterised by a smooth rectifying behaviour, without any evident resistive switching (RS). Also in this case, the interposition of the polymeric layer enhanced the RS behaviour of TiO2-NTA samples, remarkably increasing the devices' off/on ratio and endurance. The rise of high resistance states can be simply related to the addition of the polymer as resistance in series, while the variation of the low resistance states is here attributed to the occurrence of surface chemical reactions between polymer functional groups and the metal oxide, which increase the charge carriers available for conduction.

Comparison of photocatalytic and transport properties of TiO2 and ZnO nanostructures for solar-driven water splitting.

Titanium dioxide (TiO2) and zinc oxide (ZnO) nanostructures have been widely used as photo-catalysts due to their low-cost, high surface area, robustness, abundance and non-toxicity. In this work, four TiO2 and ZnO-based nanostructures, i.e. TiO2 nanoparticles (TiO2 NPs), TiO2 nanotubes (TiO2 NTs), ZnO nanowires (ZnO NWs) and ZnO@TiO2 core-shell structures, specifically prepared with a fixed thickness of about 1.5 µm, are compared for the solar-driven water splitting reaction, under AM1.5G simulated sunlight. Complete characterization of these photo-electrodes in their structural and photo-electrochemical properties was carried out. Both TiO2 NPs and NTs showed photo-current saturation reaching 0.02 and 0.12 mA cm(-2), respectively, for potential values of about 0.3 and 0.6 V vs. RHE. In contrast, the ZnO NWs and the ZnO@TiO2 core-shell samples evidence a linear increase of the photocurrent with the applied potential, reaching 0.45 and 0.63 mA cm(-2) at 1.7 V vs. RHE, respectively. However, under concentrated light conditions, the TiO2 NTs demonstrate a higher increase of the performance with respect to the ZnO@TiO2 core-shells. Such material-dependent behaviours are discussed in relation with the different charge transport mechanisms and interfacial reaction kinetics, which were investigated through electrochemical impedance spectroscopy. The role of key parameters such as electronic properties, specific surface area and photo-catalytic activity in the performance of these materials is discussed. Moreover, proper optimization strategies are analysed in view of increasing the efficiency of the best performing TiO2 and ZnO-based nanostructures, toward their practical application in a solar water splitting device.

Consistent static and small-signal physics-based modeling of dye-sensitized solar cells under different illumination conditions.

A numerical device-level model of dye-sensitized solar cells (DSCs) is presented, which self-consistently couples a physics-based description of the photoactive layer with a compact circuit-level description of the passive parts of the cell. The opto-electronic model of the nanoporous dyed film includes a detailed description of photogeneration and trap-limited kinetics, and a phenomenological description of nonlinear recombination. Numerical simulations of the dynamic small-signal behavior of DSCs, accounting for trapping and nonlinear recombination mechanisms, are reported for the first time and validated against experiments. The model is applied to build a consistent picture of the static and dynamic small-signal performance of nanocrystalline TiO2-based DSCs under different incident illumination intensity and direction, analyzed in terms of current-voltage characteristic, Incident Photon to Current Efficiency, and Electrochemical Impedance Spectroscopy. This is achieved with a reliable extraction and validation of a unique set of model parameters against a large enough set of experimental data. Such a complete and validated description allows us to gain a detailed view of the cell collection efficiency dependence on different operating conditions. In particular, based on dynamic numerical simulations, we provide for the first time a sound support to the interpretation of the diffusion length, in the presence of nonlinear recombination and non-uniform electron density distribution, as derived from small-signal characterization techniques and clarify its correlation with different estimation methods based on spectral measurements.

Charge transport improvement employing TiO2 nanotube arrays as front-side illuminated dye-sensitized solar cell photoanodes.

TiO(2) nanotube (NT) arrays with different lengths were fabricated by anodic oxidation of Ti foil and free-standing NT membranes were detached by the metal substrate and bonded on the fluorine-doped tin oxide surface implementing an easy procedure. Morphology of the as-grown material and of the prepared photoanode was investigated by means of electron microscopy, deepening the investigation on the thermal treatment effect. Crystalline orientation and exposed surface area were studied by X-ray diffraction and Brunauer-Emmett-Teller measurements, showing suitable characteristics for the application in dye-sensitized solar cells (DSCs). DSCs were assembled employing a microfluidic housing system. The cell performances and the electron transport properties as a function of the tube length, before and after a TiCl(4) treatment, were characterized by I-V electrical measurements, incident photon-to-electron conversion efficiency, electrochemical impedance spectroscopy and open circuit voltage decay. Fitting the impedance spectra with an equivalent circuit, it was possible to obtain information on the electron diffusion properties into the TiO(2) nanotubes. A comparison with the charge transport properties evaluated in nanoparticle-based photoanodes witnesses a noteworthy increase of electron lifetime and diffusion length, yielding an overall power conversion efficiency up to 7.56%.

Combined experimental and theoretical investigation of the hemi-squaraine/TiO2 interface for dye sensitized solar cells.

A simple hemi-squaraine dye (CT1) has been studied as a TiO2 sensitizer for application in dye sensitized solar cells (DSCs) by means of a combined experimental and theoretical investigation. This molecule is a prototype dye presenting an innovative anchoring group: the squaric acid moiety. Ab initio calculations based on Density Functional Theory (DFT) predict that this acid spontaneously deprotonates at the anatase (101) surface forming chemical bonds that are stronger than the ones formed by other linkers (e.g. cathecol and isonicotinic acid). Moreover an analysis of the electronic structure of the hybrid interface reveals the formation of a type II heterostructure ensuring adiabatic electron transfer from the molecule to the oxide. DSCs containing hemi-squaraine dyes were assembled, characterized and their performances compared to state of the art cells. Experimental results (large incident photon-to-electron conversion efficiency and an efficiency of 3.54%) confirmed the theoretical prediction that even a simple hemi-squaraine is an effective sensitizer for TiO2. Our study paves the way to the design of more efficient sensitizers based on a squaric acid linker and specifically engineered to absorb light in a larger part of the visible range.

Green Methods for the Fabrication of Graphene Oxide Membranes: From Graphite to Membranes.

Graphene oxide (GO) has shown great potential as a membrane material due to its unique properties, including high mechanical strength, excellent thermal stability, versatility, tunability, and outperforming molecular sieving capabilities. GO membranes can be used in a wide range of applications, such as water treatment, gas separation, and biological applications. However, the large-scale production of GO membranes currently relies on energy-intensive chemical methods that use hazardous chemicals, leading to safety and environmental concerns. Therefore, more sustainable and greener approaches to GO membrane production are needed. In this review, several strategies proposed so far are analyzed, including a discussion on the use of eco-friendly solvents, green reducing agents, and alternative fabrication techniques, both for the preparation of the GO powders and their assembly in membrane form. The characteristics of these approaches aiming to reduce the environmental impact of GO membrane production while maintaining the performance, functionality, and scalability of the membrane are evaluated. In this context, the purpose of this work is to shed light on green and sustainable routes for GO membranes' production. Indeed, the development of green approaches for GO membrane production is crucial to ensure its sustainability and promote its widespread use in various industrial application fields.

Permselectivity and Ionic Conductivity Study of Na+ and Br- Ions in Graphene Oxide-Based Membranes for Redox Flow Batteries.

Permselectivity of a membrane is central for the development of electrochemical energy storage devices with two redox couples, such as redox flow batteries (RFBs). In RFBs, Br3-/Br- couple is often used as a catholyte which can cross over to the anolyte, limiting the battery's lifetime. Naturally, the development of permselective membranes is essential to the success of RFBs since state-of-the-art perfluorosulfonic acid (PFSA) is too costly. This study investigates membranes of graphene oxide (GO), polyvinylpyrrolidone (PVP), and imidazole (Im) as binder and linker, respectively. The GO membranes are compared to a standard PFSA membrane in terms of ionic conductivity (Na+) and permselectivity (exclusion of Br-). The ionic conduction is evaluated from electrochemical impedance spectroscopy and the permselectivity from two-compartment diffusion cells in a four-electrode system. Our findings suggest that the GO membranes reach conductivity and permselectivity comparable with standard PFSA membranes.

Perspective on the use of methanogens in lithium recovery from brines.

Methanogenic archaea stand out as multipurpose biocatalysts for different applications in wide-ranging industrial sectors due to their crucial role in the methane (CH4) cycle and ubiquity in natural environments. The increasing demand for raw materials required by the manufacturing sector (i.e., metals-, concrete-, chemicals-, plastic- and lubricants-based industries) represents a milestone for the global economy and one of the main sources of CO2 emissions. Recovery of critical raw materials (CRMs) from byproducts generated along their supply chain, rather than massive mining operations for mineral extraction and metal smelting, represents a sustainable choice. Demand for lithium (Li), included among CRMs in 2023, grew by 17.1% in the last decades, mostly due to its application in rechargeable lithium-ion batteries. In addition to mineral deposits, the natural resources of Li comprise water, ranging from low Li concentrations (seawater and freshwater) to higher ones (salt lakes and artificial brines). Brines from water desalination can be high in Li content which can be recovered. However, biological brine treatment is not a popular methodology. The methanogenic community has already demonstrated its ability to recover several CRMs which are not essential to their metabolism. Here, we attempt to interconnect the well-established biomethanation process with Li recovery from brines, by analyzing the methanogenic species which may be suitable to grow in brine-like environments and the corresponding mechanism of recovery. Moreover, key factors which should be considered to establish the techno-economic feasibility of this process are here discussed.

3D Printed Supercapacitor Exploiting PEDOT-Based Resin and Polymer Gel Electrolyte.

Renewable energy-based technologies and increasing IoT (Internet of Things) objects population necessarily require proper energy storage devices to exist. In the view of customized and portable devices, Additive Manufacturing (AM) techniques offer the possibility to fabricate 2D to 3D features for functional applications. Among the different AM techniques extensively explored to produce energy storage devices, direct ink writing is one of the most investigated, despite the poor achievable resolution. Herein, we present the development and characterization of an innovative resin which can be employed in a micrometric precision stereolithography (SL) 3D printing process for the fabrication of a supercapacitor (SC). Poly(3,4-ethylenedioxythiophene) (PEDOT), a conductive polymer, was mixed with poly(ethylene glycol) diacrylate (PEGDA), to get a printable and UV curable conductive composite material. The 3D printed electrodes were electrically and electrochemically investigated in an interdigitated device architecture. The electrical conductivity of the resin falls within the range of conductive polymers with 200 mS/cm and the 0.68 µWh/cm2 printed device energy density falls within the literature range.

Electrical and Thermal Conductivities of Single CuxO Nanowires.

Copper oxide nanowires (NWs) are promising elements for the realization of a wide range of devices for low-power electronics, gas sensors, and energy storage applications, due to their high aspect ratio, low environmental impact, and cost-effective manufacturing. Here, we report on the electrical and thermal properties of copper oxide NWs synthetized through thermal growth directly on copper foil. Structural characterization revealed that the growth process resulted in the formation of vertically aligned NWs on the Cu growth substrate, while the investigation of chemical composition revealed that the NWs were composed of CuO rather than Cu2O. The electrical characterization of single-NW-based devices, in which single NWs were contacted by Cu electrodes, revealed that the NWs were characterized by a conductivity of 7.6 × 10-2 S∙cm-1. The effect of the metal-insulator interface at the NW-electrode contact was analyzed by comparing characterizations in two-terminal and four-terminal configurations. The effective thermal conductivity of single CuO NWs placed on a substrate was measured using Scanning Thermal Microscopy (SThM), providing a value of 2.6 W∙m-1∙K-1, and using a simple Finite Difference model, an estimate for the thermal conductivity of the nanowire itself was obtained as 3.1 W∙m-1∙K-1. By shedding new light on the electrical and thermal properties of single CuO NWs, these results can be exploited for the rational design of a wide range of optoelectronic devices based on NWs.

High efficiency dye-sensitized solar cells exploiting sponge-like ZnO nanostructures.

Sponge-like nanostructured ZnO layers were successfully employed as photoanodes for the fabrication of highly efficient dye-sensitized solar cells. The sponge-like ZnO layers were obtained by room temperature radio-frequency magnetron sputtering deposition of metallic zinc, followed by thermal oxidation treatment in an ambient atmosphere. The porous films show a 3D branched nanomorphology, with a feature similar to natural coral. The morphological and optical properties of these layers were studied through field emission scanning electron microscopy, specific surface area measurements, ultraviolet-visible transmittance and absorption spectroscopy. The sponge-like ZnO film presents a high density of branches, with a relatively high specific surface area value, and fine optical transmittance. The morphology of the porous structure provides a high number of adsorption sites for the anchoring of sensitizer molecules, making it suitable for the fabrication of ZnO-based photoanodes for dye-sensitized solar cells. The light harvesting performance of the sensitized semiconductor was evaluated by current density vs. voltage measurements, incident photon-to-electron conversion efficiency, open circuit voltage decay and impedance spectroscopy. The modelling of the electrical characteristics evidences a higher electron lifetime and a longer charge diffusion length, if compared to standard TiO(2) nanoparticle based photoanodes. For ZnO films with a thickness up to 18 µm, a photoconversion efficiency as high as 6.67% and a maximum value of the incident photon-to-electron collection efficiency equal to 87% at 530 nm were demonstrated.

Crown-Ether Functionalized Graphene Oxide Membrane for Lithium Recovery from Water.

The massive worldwide transition of the transport sector to electric vehicles has dramatically increased the demand for lithium. Lithium recovery by means of ion sieves or supramolecular chemistry has been extensively studied in recent years as a viable alternative approach to the most common extraction processes. Graphene oxide (GO) has also already been proven to be an excellent candidate for water treatment and other membrane related applications. Herein, a nanocomposite 12-crown-4-ether functionalized GO membrane for lithium recovery by means of pressure filtration is proposed. GO flakes were via carbodiimide esterification, then a polymeric binder was added to improve the mechanical properties. The membrane was then obtained and tested on a polymeric support in a dead-end pressure setup under nitrogen gas to speed up the lithium recovery. Morphological and physico-chemical characterizations were carried out using pristine GO and functionalized GO membranes for comparison with the nanocomposite. The lithium selectivity was proven by both the conductance and ICP mass measurements on different sets of feed and stripping solutions filtrated (LiCl/HCl and other chloride salts/HCl). The membrane proposed showed promising properties in low concentrated solutions (7 mgLi/L) with an average lithium uptake of 5 mgLi/g in under half an hour of filtration time.

Recent Advances in the Lithium Recovery from Water Resources: From Passive to Electrochemical Methods.

The ever-increasing amount of batteries used in today's society has led to an increase in the demand of lithium in the last few decades. While mining resources of this element have been steadily exploited and are rapidly depleting, water resources constitute an interesting reservoir just out of reach of current technologies. Several techniques are being explored and novel materials engineered. While evaporation is very time-consuming and has large footprints, ion sieves and supramolecular systems can be suitably tailored and even integrated into membrane and electrochemical techniques. This review gives a comprehensive overview of the available solutions to recover lithium from water resources both by passive and electrically enhanced techniques. Accordingly, this work aims to provide in a single document a rational comparison of outstanding strategies to remove lithium from aqueous sources. To this end, practical figures of merit of both main groups of techniques are provided. An absence of a common experimental protocol and the resulting variability of data and experimental methods are identified. The need for a shared methodology and a common agreement to report performance metrics are underlined.

Photocurable 3D-Printable Systems with Controlled Porosity towards CO2 Air Filtering Applications.

Porous organic polymers are versatile platforms, easily adaptable to a wide range of applications, from air filtering to energy devices. Their fabrication via vat photopolymerization enables them to control the geometry on a multiscale level, obtaining hierarchical porosity with enhanced surface-to-volume ratio. In this work, a photocurable ink based on 1,6 Hexanediol diacrylate and containing a high internal phase emulsion (HIPE) is presented, employing PLURONIC F-127 as a surfactant to generate stable micelles. Different parameters were studied to assess the effects on the morphology of the pores, the printability and the mechanical properties. The tests performed demonstrates that only water-in-oil emulsions were suitable for 3D printing. Afterwards, 3D complex porous objects were printed with a Digital Light Processing (DLP) system. Structures with large, interconnected, homogeneous porosity were fabricated with high printing precision (300 µm) and shape fidelity, due to the addition of a Radical Scavenger and a UV Absorber that improved the 3D printing process. The formulations were then used to build scaffolds with complex architecture to test its application as a filter for CO2 absorption and trapping from environmental air. This was obtained by surface decoration with NaOH nanoparticles. Depending on the surface coverage, tested specimens demonstrated long-lasting absorption efficiency.

Tragacanth Gum as Green Binder for Sustainable Water-Processable Electrochemical Capacitor.

Enabling green fabrication processes for energy storage devices is becoming a key aspect in order to achieve a sustainable fabrication cycle. Here, the focus was on the exploitation of the tragacanth gum, an exudated gum like arabic and karaya gums, as green binder for the preparation of carbon-based materials for electrochemical capacitors. The electrochemical performance of tragacanth (TRGC)-based electrodes was thoroughly investigated and compared with another water-soluble binder largely used in this field, sodium-carboxymethyl cellulose (CMC). Apart from the higher sustainability both in production and processing, TRGC exhibited a lower impact on the obstruction of pores in the final active material film with respect to CMC, allowing for more available surface area. This directly impacted the electrochemical performance, resulting in a higher specific capacitance and better rate capability. Moreover, the TRGC-based supercapacitor showed a superior thermal stability compared with CMC, with a capacity retention of about 80 % after 10000 cycles at 70 °C.

Binder Free and Flexible Asymmetric Supercapacitor Exploiting Mn3O4 and MoS2 Nanoflakes on Carbon Fibers.

Emerging technologies, such as portable electronics, have had a huge impact on societal norms, such as access to real time information. To perform these tasks, portable electronic devices need more and more accessories for the processing and dispensation of the data, resulting in higher demand for energy and power. To overcome this problem, a low cost high-performing flexible fiber shaped asymmetric supercapacitor was fabricated, exploiting 3D-spinel manganese oxide Mn3O4 as cathode and 2D molybdenum disulfide MoS2 as anode. These asymmetric supercapacitors with stretched operating voltage window of 1.8 V exhibit high specific capacitance and energy density, good rate capability and cyclic stability after 3000 cycles, with a capacitance retention of more than 80%. This device has also shown an excellent bending stability at different bending conditions.