Green solvents to enhance hydrochar quality and clarify effects of secondary char.

Several limitations hinder the industrial-scale implementation of hydrothermal carbonization (HTC) of biomass, especially the quality of as-carbonized hydrochar. This work investigates solvent extraction of hydrochars to enhance their potential applications. Hydrochars were produced at several HTC temperatures (190, 220, 250 °C) from cellulose and extracted using combinations of green polar solvents (ethyl acetate, acetone, and methanol). Results show that the composition of the extractable fraction resembles that of the HTC liquor, rich in carboxylic acids and furan derivatives, while the non-extractable solid phase shows improved thermal profiles devoid of highly volatile compounds. Carbon microspheres (non-dissolvable secondary char) are unaffected by extraction. The organics adsorbed on the hydrochar surface comprise highly volatile species and solvent washing effectively removes them.

Electrodeposited PEDOT/Nafion as Catalytic Counter Electrodes for Cobalt and Copper Bipyridyl Redox Mediators in Dye-Sensitized Solar Cells.

PEDOT-based counter electrodes for dye-sensitized solar cells (DSSCs) are generally prepared by electrodeposition, which produces polymer films endowed with the best electrocatalytic properties. This translates in fast regeneration of the redox mediator, which allows the solar cell to sustain efficient photoconversion. The sustainable fabrication of DSSCs must consider the scaling up of the entire process, and when possible, it should avoid the use of large amounts of hazardous and/or inflammable chemicals, such as organic solvents for instance. This is why electrodeposition of PEDOT-based counter electrodes should preferably be carried out in aqueous media. In this study, PEDOT/Nafion was electrodeposited on FTO and comparatively evaluated as a catalytic material in DSSCs based on either cobalt or copper electrolytes. Our results show that the electrochemical response of PEDOT/Nafion toward Co(II/III-) or Cu(I/II)-based redox shuttles was comparable to that of PEDOT/ClO4 and significantly superior to that of PEDOT/PSS. In addition, when tested for adhesion, PEDOT/Nafion films were more stable for delamination if compared to PEDOT/ClO4, a feature that may prove beneficial in view of the long-term stability of solar devices.

Evaluation of the role of beam homogeneity on the mechanical coupling of laser-ablation-generated impulse.

The material emitted from a target surface during laser ablation generates a net thrust (propulsion) in the opposite direction. The momentum generation efficiency of this laser-driven propulsion is given by the mechanical coupling coefficient (Cm). In this work, we considered nanosecond UV laser ablation of the aluminum 6061 alloy to study the Cm behavior with different irradiating conditions. This is done by systematically changing fluence, uniform/nonuniform intensity, and incident angle of the laser beam. In particular, we found that when dealing with nonuniform laser intensity, characterizing Cm exclusively in terms of fluence is not fully satisfactory because the energy distribution over the irradiated area plays a key role in the way material is removed-interplay between vaporization and phase explosion-and thrust is generated.

Laser Ablation of Aluminum Near the Critical Regime: A Computational Gas-Dynamical Model with Temperature-Dependent Physical Parameters.

The complexity of the phenomena simultaneously occurring, from the very first instants of high-power laser pulse interaction with the target up to the phase explosion, along with the strong changes in chemical-physical properties of matter, makes modeling laser ablation a hard task, especially near the thermodynamic critical regime. In this work, we report a computational model of an aluminum target irradiated in vacuum by a gaussian-shaped pulse of 20 ns duration, with a peak intensity of the order of GW/cm2. This continuum model covers laser energy deposition and temperature evolution in the irradiated target, along with the mass removal mechanism involved, and the vaporized material expansion. Aluminum was considered to be a case study due to the vast literature on the temperature dependence of its thermodynamic, optical, and transport properties that were used to estimate time-dependent values of surface-vapor quantities (vapor pressure, vapor density, vapor and surface temperature) and vapor gas-dynamical quantities (density, velocity, pressure) as it expands into vacuum. Very favorable agreement is reported with experimental data regarding: mass removal and crater depth due to vaporization, generated recoil momentum, and vapor flow velocity expansion.

Wastewater remediation with ZnO photocatalysts: Green synthesis and solar concentration as an economically and environmentally viable route to application.

Green-synthesized materials and solar concentration technology for advanced oxidation processes (AOPs) offer important opportunities in water remediation by giving value to clean, renewable and potentially low-cost resources. Here, Zinc Oxide (ZnO) nanostructures (NSs) were prepared via a green synthesis method based on garlic bulbs (Allium Sativum) extract (ZnO-Green), resulting in crystalline (wurtzite) nanorods (NRs). ZnO nanoparticles (NPs) were also chemically prepared through a standard co-precipitation (ZnO-Chem) for comparative solar photocatalytic (PC) studies. The green-synthesized ZnO NRs exhibited a favorable photocatalytic activity (PCA) in colloidal suspension for the methylene blue (MB) dye degradation upon exposure to concentrated sunlight. Comparison with the chemically synthesized ZnO results in almost equal degradations of 94% in optimal loading condition. To explore the possibility to use immobilized photocatalyst in heterogeneous condition, green-synthesized ZnO NRs coatings were fabricated and compared with a 135 nm thick ZnO thin film produced by pulsed laser deposition (PLD) (ZnO-PLD). PCA on MB degradation (120 min experiments) resulted in degradations of 69% and 73%, respectively, proving the feasibility of the immobilized photocatalyst approach. Finally, an economic analysis of the process shows that the combination of green-synthesis and concentrated sunlight significantly reduces costs, paving the way for large-scale photocatalytic wastewater remediation.

Laser-Synthesis of NV-Centers-Enriched Nanodiamonds: Effect of Different Nitrogen Sources.

Due to the large number of possible applications in quantum technology fields-especially regarding quantum sensing-of nitrogen-vacancy (NV) centers in nanodiamonds (NDs), research on a cheap, scalable and effective NDs synthesis technique has acquired an increasing interest. Standard production methods, such as detonation and grinding, require multistep post-synthesis processes and do not allow precise control in the size and fluorescence intensity of NDs. For this reason, a different approach consisting of pulsed laser ablation of carbon precursors has recently been proposed. In this work, we demonstrate the synthesis of NV-fluorescent NDs through pulsed laser ablation of an N-doped graphite target. The obtained NDs are fully characterized in the morphological and optical properties, in particular with optically detected magnetic resonance spectroscopy to unequivocally prove the NV origin of the NDs photoluminescence. Moreover, to compare the different fluorescent NDs laser-ablation-based synthesis techniques recently developed, we report an analysis of the effect of the medium in which laser ablation of graphite is performed. Along with it, thermodynamic aspects of the physical processes occurring during laser irradiation are analyzed. Finally, we show that the use of properly N-doped graphite as a target for laser ablation can lead to precise control in the number of NV centers in the produced NDs.

Realization of a solar hydrothermal carbonization reactor: A zero-energy technology for waste biomass valorization.

Research around hydrothermal carbonization (HTC) has seen a huge development in recent years, materializing in the first pilot and industrial plants. Even though HTC reactions are slightly exothermic, the overall process entails energy consumption to both reach operating conditions and tackle heat losses. To face this issue and to develop a zero-energy process, this work proposes an innovative solution: the coupling of an HTC reactor with a solar concentrator, designed to fully cover the HTC energy needs. A 300 ml stainless steel HTC reactor was constructed and positioned on the focus of a parabolic dish concentrator (PDC), consisting of one parabolic mirror of 0.8 m2. To maximize the light absorption, the illuminated side of the HTC reactor was coated with a thin layer of nanostructured copper oxide, realized via electron beam deposition. Then, the effectiveness of the hybrid solar-HTC solution was demonstrated by carrying out an experimental campaign on a residual agro-biomass (grape seeds), which was treated at 180, 220, and 250 °C for 2 h. The coating confers excellent absorbing performances to the system, exhibiting an absorptance of up to 95.6% (at 300 nm wavelength). Heating times, yields, composition, and energy properties of "solar hydrochars" resemble those of studies performed in traditional HTC systems. This research work proves the feasibility of the solar-HTC prototype apparatus and opens the way to the development of a zero-energy solar-HTC technology.

Rational Design Combining Morphology and Charge-Dynamic for Hematite/Nickel-Iron Oxide Thin-Layer Photoanodes: Insights into the Role of the Absorber/Catalyst Junction.

Water oxidation represents the anodic reaction in most of the photoelectrosynthetic setups for artificial photosynthesis developed so far. The efficiency of the overall process strongly depends on the joint exploitation of good absorber domains and interfaces with minimized recombination pathways. To this end, we report on the effective coupling of thin-layer hematite with amorphous porous nickel-iron oxide catalysts prepared via pulsed laser deposition. The rational design of such composite photoelectrodes leads to the formation of a functional adaptive junction, with enhanced photoanodic properties with respect to bare hematite. Electrochemical impedance spectroscopy has contributed to shed light on the mechanisms of photocurrent generation, confirming the reduction of recombination pathways as the main contributor to the improved performances of the functionalized photoelectrodes. Our results highlight the importance of the amorphous catalysts' morphology, as dense and electrolyte impermeable layers hinder the pivotal charge compensation processes at the interface. The direct comparison with all-iron and all-nickel catalytic counterparts further confirms that control over the kinetics of both hole transfer and charge recombination, enabled by the adaptive junction, is key for the optimal operation of this kind of semiconductor/catalyst interfaces.

Anomalous molecular infiltration in graphene laminates.

Graphene laminated (GL) coatings formed by stacked few layer graphene (FLG) nanocrystals were deposited on low-density polyethylene (PE) films by the mechanical rubbing technique. Molecular transport through the bilayer membrane was studied by the gas phase permeation technique by monitoring the CO2, N2 and 2H2 transport fluxes in transient conditions. The results evidenced that the transport exhibited anomalous character. The experimental data could be reproduced assuming that the penetrant concentration in the GL coating, cint(t), reached a saturation value cs following compressed exponential kinetics cint(t) = cs[1 - e-(λrelt)ß]. The relaxation time τrel = 1/λrel showed thermally activated behavior, and its value increased with the kinetic diameter of the penetrant molecules. The critical exponent ß = 1.5 ± 0.1 for CO2 and N2 and ß = 2.0 ± 0.1 for 2H2 did not change with temperature. Positron annihilation lifetime spectroscopy (PALS) analysis indicated that the average cross-section (hg) of the cavities in the GL coating exhibited comparable size to the kinetic diameter (σk) of the penetrant molecules. The results could be explained by assuming that the molecular infiltration in the GL structure occurred in nano-channels having distributed path lengths where the penetrant transport obeyed a configurational diffusion mechanism.

Roles of Vanadium and Nitrogen in Photocatalytic Activity of VN-Codoped TiO2 Photocatalyst.

In the present manuscript, we explore the VN-codoped TiO2 system intended to understand the role played by each dopant in synergistic enhancement in performance of TiO2 photocatalyst. The photocatalytic decomposition of organic pollutants in aqueous solution under visible light was used as a probe reaction to evaluate the performance of VN-codoped TiO2 samples with different V and N concentrations. An optical measurement of VN-codoped TiO2 shows considerable improved visible light absorption with increase in V-concentration as compared to increase in N concentration, which was due to the effective narrowing of the band gap by V-doping. The energy levels formed by N-dopants act as the trapping centers for photogenerated holes to suppress recombination process as indicated by PL and TRPL results. It is also observed that at high V-concentrations recombination centers are created in the form of oxygen vacancies as indicated by XPS and PL. In VN-codoped TiO2 , addition of N partially fills these oxygen vacancies to reduce the number of recombination centers and prolong the lifetime of charge carriers. Thus, V improves the visible light absorption while N reduces the recombination of electron-hole pairs, thus creating the synergistic effect to produce three times better performance than pure TiO2 .

XANES study of vanadium and nitrogen dopants in photocatalytic TiO2 thin films.

We report an X-ray absorption near edge structure (XANES) study of vanadium (V) and nitrogen (N) dopants in anatase TiO2 thin films deposited by radio-frequency magnetron sputtering. Measurements at the Ti K and V K edges were combined with soft X-ray experiments at the Ti L2,3, O K and N K edges. Full potential ab initio spectral simulations of the V, O and N K-edges were carried out for different possible configurations of substitutional and interstitial dopant-related point defects in the anatase structure. The comparison between experiments and simulations demonstrates that V occupies substitutional cationic sites (replacing Ti) irrespective of the film structure and dopant concentration (up to 4.5 at%). On the other hand, N is found both in substitutional anionic sites (replacing O) and as N2 dimers within TiO2 interstices. The dopants' local structures are discussed with reference to the enhanced optical absorption and photocatalytic activity achieved by (co)doping.

Tungsten-doped TiO2/reduced Graphene Oxide nano-composite photocatalyst for degradation of phenol: A system to reduce surface and bulk electron-hole recombination.

Recombination of photogenerated charges is the main factor affecting the photocatalytic activity of TiO2. Here, we report a combined strategy of suppressing both the bulk as well as the surface recombination processes by doping TiO2 with tungsten and forming a nanocomposite with reduced graphene oxide (rGO), respectively. Sol-gel method was used to dope and optimize the concentration of W in TiO2 powder. UV-Vis, XPS, PL and time resolved PL spectra along with DFT calculations indicate that W6+ in TiO2 lattice creates an impurity level just below the conduction band of TiO2 to act as a trapping site of electrons, which causes to improve the lifetime of the photo-generated charges. Maximum reduction in the PL intensity and the improvement in charge carrier lifetime was observed for TiO2 doped with 1 at.% W (1W-TiO2), which also displayed the highest photo-activity for the degradation of p-nitro phenol pollutant in water. Tuning of rGO/TiO2 ratio (weight) disclosed that the highest activity can be achieved with the composite formed by taking equal amounts of TiO2 and rGO (1:1), in which the strong interaction between TiO2 and rGO causes an effective charge transfer via bonds formed near the interface as indicated by XPS. Both these optimized concentrations were utilized to form the composite rGO/1W-TiO2, which showed the highest activity in photo-degradation of p-nitro phenol (87%) as compared to rGO/TiO2 (42%), 1W-TiO2 (62%) and pure TiO2 (29%) in 180 min. XPS and PL results revealed that in the present nanocomposite, tungsten species traps the excited electron to reduce the interband recombination in the bulk, while the interaction between TiO2 and rGO creates a channel for fast transfer of excited electrons towards the latter before being recombined on the surface defect sites.

Porous versus Compact Nanosized Fe(III)-Based Water Oxidation Catalyst for Photoanodes Functionalization.

Integrated absorber/electrocatalyst schemes are increasingly adopted in the design of photoelectrodes for photoelectrochemical cells because they can take advantage of separately optimized components. Such schemes also lead to the emergence of novel challenges, among which parasitic light absorption and the nature of the absorber/catalyst junction features prominently. By taking advantage of the versatility of pulsed-laser deposition technique, we fabricated a porous iron(III) oxide nanoparticle-assembled coating that is both transparent to visible light and active as an electrocatalyst for water oxidation. Compared to a compact morphology, the porous catalyst used to functionalize crystalline hematite photoanodes exhibits a superior photoresponse, resulting in a drastic lowering of the photocurrent overpotential (about 200 mV) and a concomitant 5-fold increase in photocurrents at 1.23 V versus reversible hydrogen electrode. Photoelectrochemical impedance spectroscopy indicated a large increase in trapped surface hole capacitance coupled with a decreased charge transfer resistance, consistent with the possible formation of an adaptive junction between the absorber and the porous nanostructured catalyst. The observed effect is among the most prominent reported for the coupling of an electrocatalyst with a thin layer absorber.

Free volumes and gas transport in polymers: amine-modified epoxy resins as a case study.

The CO2 transport process was studied in a series of amine-modified epoxy resins having different cross-linking densities but the same chemical environment for the penetrant molecules. Positron Annihilation Lifetime Spectroscopy (PALS) was used to monitor the free volume structure of the samples and experimentally evaluate their fractional free volume fh(T) and its temperature evolution. The analysis of the free volume hole size distribution showed that all the holes have a size large enough to accommodate the penetrant molecules at temperatures T above the glass transition temperature Tg. The measured gas diffusion constants at T > Tg have been reproduced in the framework of the free volume theory of diffusion using a novel procedure based on the use of fh(T) as an input experimental parameter.

Improvement of the electron collection efficiency in porous hematite using a thin iron oxide underlayer: towards efficient all-iron based photoelectrodes.

Different approaches have been explored to increase the water oxidation activity of nanostructured hematite (α-Fe2O3) photoanodes, including doping with various elements, surface functionalization with both oxygen evolving catalysts (OEC) and functional overlayers and, more recently, the introduction of ultrathin oxide underlayers as tunneling back contacts. Inspired by this latter strategy, we present here a photoanode design with a nanometric spin-coated iron oxide underlayer coupled with a mesoporous hematite film deposited by electrophoresis. The electrodes equipped with the thin underlayer exhibit a four-fold improvement in photoactivity over the simple hematite porous film, reaching a stable photocurrent density of ca. 1 mA cm(-2) at 0.65 V versus the saturated calomel electrode (SCE) at pH 13.3 (NaOH 0.1 M) under air mass (AM) 1.5G illumination. A further improvement to 1.5 mA cm(-2) is observed after decoration of the hematite surface with a Fe(iii)-OEC. These results demonstrate that by combining different iron oxide morphologies, it is possible to improve the selectivity of the interfaces towards both electron collection at the back contact and hole transfer to the electrolyte, obtaining an efficient all-iron based photoelectrode entirely realized with simple wet solution scalable procedures.

Liquid nanodroplet formation through phase explosion mechanism in laser-irradiated metal targets.

Some quantitative aspects of laser-irradiated pure metals, while approaching phase explosion, are still not completely understood. Here, we develop a model that describes the main quantities regulating the liquid-vapor explosive phase transition and the expulsion of liquid nanodroplets that, by solidifying, give rise to nanoparticle formation. The model combines both a thermodynamics description of the explosive phase change and a Monte Carlo simulation of the randomly generated critical vapor bubbles. The calculation is performed on a set of seven metals (Al, Fe, Co, Ni, Cu, Ag, and Au) which are frequently used in pulsed laser ablation experiments. Our final predictions about the size distribution of the liquid nanodroplets and the number ratio of liquid/vapor ejected atoms are compared, whenever possible, with available molecular dynamics simulations and experimental data.

Pulsed-laser deposition of nanostructured iron oxide catalysts for efficient water oxidation.

Amorphous iron oxide nanoparticles were synthesized by pulsed-laser deposition (PLD) for functionalization of indium-tin oxide surfaces, resulting in electrodes capable of efficient catalysis in water oxidation. These electrodes, based on earth-abundant and nonhazardous iron metal, are able to sustain high current densities (up to 20 mA/cm2) at reasonably low applied potential (1.64 V at pH 11.8 vs reversible hydrogen electrode) for more than 1 h when employed as anodes for electrochemical water oxidation. The good catalytic performance proves the validity of PLD as a method to prepare nanostructured solid-state materials for catalysis, enabling control over critical properties such as surface coverage and morphology.

Synthesis of lead nanowires in a single co-sputtering deposition step.

In the present work Pb NWs were grown in a single step by co-sputtering of an Al bulk target partially covered with Pb-metal pieces on its surface and without using extra catalyst. NWs have been characterized by X-ray diffraction technique and Secondary Electron Microscopy. Substrate materials, Pb concentration, and deposition time have been varied in order to establish their effects on NWs growth. In-situ single NW growth has been observed and analyzed by Secondary Electron Microscopy. The driving force that supports the growth of NWs is provided by compressive stress induced in these composite thin films during co-deposition. The present synthesis method was able to produce metal NWs over large area of the Al film with diameter ranging from 50 to 100 nm. The maximum achieved length of NWs is about 25 microm.