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The preparation of nanoparticles represents a powerful tool for lignin valorization, as it combines easy methodologies with high application potential. Different synthetic strategies and various lignin sources have been employed in the process. However, the great variability in the lignin structure prevents a direct comparison of the so far reported lignin nanoparticles (LNPs), especially as regards their physicochemical and functional properties. To this purpose, two green protocols, that is, solvent-antisolvent and hydrotropic, were optimized and used to generate LNPs from the same softwood kraft lignin. The nanomaterials were fully characterized to extrapolate structure/property relationships and reveal any differences in the mechanism of self-assembly. Furthermore, tests on methylene blue entrapment capacity and release behavior at two different pH values (2.0 and 7.4) evidenced a clear dependence on the LNPs characteristics and thus on the strategy adopted for their production.

Catalysts capable of improving the performance of oxygen evolution reaction (OER) and oxygen reduction reactions (ORR) are essential for the advancement of renewable energy technologies. Herein, Ag-decorated vertically aligned MoS2 nanoflakes are developed via magnetron co-sputtering and investigated as electrocatalyst towards OER and ORR. Due to the presence of silver, the catalyst shows more than 1.5 times an increase in the roughness-normalized rate of OER, featuring a very low Tafel slope (58.6 mv dec(-1)), thus suggesting that the catalyst surface favors the thermodynamics of hydroxyl radical (OH center dot) adsorption with the deprotonation steps being the rate-determining steps. The improved performance is attributed to the strong interactions between OOH intermediates and the Ag surface which reduces the activation energy. Rotating ring disk electrode (RRDE) analysis shows that the net disk currents on the Ag-MoS2 sample are two times higher at 0.65 V compared to MoS2, demonstrating the co-catalysis effect of silver doping. Based on the rate constant values, Ag-MoS2 proceeds through a mixed 4 electron and a 2 + 2 serial route reduction mechanism, in which the ionized hydrogen peroxide is formed as a mobile intermediate. The presence of silver decreases the electron transfer number and increases the peroxide yield due to the interplay of a 2 + 2 electron reduction pathway. A 2.5-6 times faster conversion rate of peroxide to OH- observed due to the presence of silver, indicating its effective cocatalyst nature. This strategy can help in designing a highly active bifunctional catalyst that has great potential as a viable alternative to precious-metal-based catalysts. Funding Agencies|Knut & Alice Wallenberg foundation, Sweden; Swedish foundation consolidator fellowship, Sweden; European UnionEuropean Commission [654002, 785219]; Lulea University of Technology laboratory fund program, Sweden; Kempe Foundation, Sweden; VINNOVA under the VINNMER Marie cure incoming Grant, Sweden [2015-01513]; Swedish Research Council, SwedenSwedish Research Council [VR 2019-05577]; MIUR-PON TARANTO, Italy [ARS01_00637]

The electrical properties of an all-oxide core–shell ZnO–Co3O4 nanorod heterojunction were studied in the dark and under UV-vis illumination. The contact potential difference and current distribution maps were obtained utilizing new methods in dynamic multifrequency atomic force microscopy (AFM) such as electrostatic and conductive intermodulation AFM. Light irradiation modified the electrical properties of the nanorod heterojunction. The new techniques are able to follow the instantaneous local variation of the photocurrent, giving a two-dimensional (2D) map of the current–voltage curves and correlating the electrical and morphological features of the heterostructured core–shell nanorods.

Transparent nanoscale-film heterojunctions based on Cu2O and ZnO were fabricated by atomic layer deposition and reactive magnetron sputtering. The constitutive layers exhibit high crystalline quali...

Atomic layer deposition (ALD) was used to deposit ZnO/Al2O3/V2O5 nanolaminate coatings to demonstrate a coating system with temperature adaptive frictional behaviour. The nanolaminate coating exhibited excellent conformity and crack-free coating of thickness 110 nm over Inconel 718 substrate. The ALD trilayer coating showed a hardness and elastic modulus of 12 GPa and 193 GPa, respectively. High-temperature tribology of the nanolaminate trilayer was tested against steel ball in dry sliding condition at 25 °C (room temperature, RT), 200 °C, 300 °C, and 400 °C. It was found that the nanolaminate coating showed a low coefficient of friction (COF) and wear rate at RT and 300 °C. The trilayer coating was found intact and stable at all temperatures during the friction tests. The adaptability of nanolaminate coating with the temperature was verified by performing the cyclic friction test at 300 °C and RT. The low COF and wear rate had been attributed to the (100) and (002) basal plane sliding of ZnO top layer, and the interlayer sliding of weakly bonded planes parallel to (001) plane in V2O5 bottom layer. Furthermore, even after the removal of ZnO coating during the tribotest, the bottom V2O5 layer coating stabilized the COF and wear rate at RT and 300 °C.

Self-powered photodetectors operating in the UV-visible-NIR window made of environmentally friendly, earth abundant, and cheap materials are appealing systems to exploit natural solar radiation without external power sources. In this study, we propose a new p-n junction nanostructure, based on a ZnO-Co3O4 core-shell nanowire (NW) system, with a suitable electronic band structure and improved light absorption, charge transport, and charge collection, to build an efficient UV-visible-NIR p-n heterojunction photodetector. Ultrathin Co3O4 films (in the range 1-15 nm) were sputter-deposited on hydrothermally grown ZnO NW arrays. The effect of a thin layer of the Al2O3 buffer layer between ZnO and Co3O4 was investigated, which may inhibit charge recombination, boosting device performance. The photoresponse of the ZnO-Al2O3-Co3O4 system at zero bias is 6 times higher compared to that of ZnO-Co3O4. The responsivity (R) and specific detectivity (D*) of the best device were 21.80 mA W-1 and 4.12 x 10(12) Jones, respectively. These results suggest a novel p-n junction structure to develop all-oxide UV-vis photodetectors based on stable, nontoxic, low-cost materials.

Hybrid structures composed of layered one-dimensional (1D) and two-dimensional (2D) materials opened new perspectives and opportunities through the build-up of hetero-junctions with versatile layered structures and led to fascinating fundamental phenomena and advanced devices. We succeeded in depositing by magnetron sputtering vertically aligned 1D ZnO nanorods on 2D MoS2 flakes obtained by exfoliation, preserving the structure of the 2D materials. The photoluminescence (PL) optical properties of the hybrid structure were assessed towards developing a contactless optical chemical sensor.

Nanostructured SnO2 is a promising material for the scalable production of portable gas sensors. To fully exploit their potential, these gas sensors need a faster recovery rate and higher sensitivity at room temperature than the current state of the art. Here we demonstrate a chemiresistive gas sensor based on vertical SnOx nanopillars, capable of sensing 2 at room temperature and 10 ppt at 230 °C. We test the sample both in vacuum and in air and observe an exceptional improvement in the performance compared to commercially available gas sensors. In particular, the recovery time for sensing NH3 at room temperature is more than one order of magnitude faster than a commercial SnO2 sensor. The sensor shows an unique combination of high sensitivity and fast recovery time, matching the requirements on materials expected to foster widespread use of portable and affordable gas sensors.

α-Fe2O3 nanorods have been grown by hydrothermal method, dispersed in ethanol and drop casted on a pre-patterned alumina substrate with Pt electrodes. Their morphology, crystalline and electronic properties have been investigated by Scanning Electron Microscopy, Raman and X-ray Photoelectron Spectroscopies and X-ray Diffraction. The so-fabricated devices have been used for hydrogen gas sensing, showing their ability to detect H2 at operating temperatures > 200 °C, at relative humidity values comprised from 0% to 50%. The sensing behavior of α-Fe2O3 nanorods is compatible with an n to p conductivity transition when the operating temperature is increased up to 300 °C. Outstanding p-type hydrogen sensing performances of α-Fe2O3 have been observed and reported. Besides H2 detection, the α-Fe2O3 nanorods-based device is a good humidity sensor, at room temperature (n-type) and at 400 °C (p-type). CO and ethanol sensing performances have been investigated at different operating temperatures and relative humidity values. CO and ethanol anomalous acceptor-like behaviors at 200 °C in humid air has been explained by the interactions of these target gases with the water molecules adsorbed on the metal oxide surfaces. An explanation of the n–p behavior transition at T > 200 °C in terms of band bending is reported.

Area-selective atomic layer deposition (AS-ALD) has attracted much attention in recent years due to the possibility of achieving accurate patterns in nanoscale features, which render this technique compatible with the continuous downscaling in nanoelectronic devices. The growth selectivity is achieved by starting from different materials and results (ideally) in localized growth of a single material. We propose here a new concept, more subtle and general, in which a property of the substrate is modulated to achieve localized growth of different materials. This concept is demonstrated by selective growth of high-quality metallic Cu and semiconducting Cu2O thin films, achieved by changing the type of majority point defects in the ZnO underneath film exposed to the reactive species using a patterned bilayer structure composed of highly conductive and highly resistive areas, as confirmed by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). The selective growth of these mater...

In this work, we present all-oxide p-n junction core-shell nanowires (NWs) as fast and stable self-powered photodetectors. Hydrothermally grown n-type ZnO NWs were conformal covered by different thicknesses (up to 420 nm) of p-type copper oxide layers through metalorganic chemical vapor deposition (MOCVD). The ZnO NWs exhibit a single crystalline Wurtzite structure, preferentially grown along the [002] direction, and energy gap Eg = 3.24 eV. Depending on the deposition temperature, the copper oxide shell exhibits either a crystalline cubic structure of pure Cu2O phase (MOCVD at 250 °C) or a cubic structure of Cu2O with the presence of CuO phase impurities (MOCVD at 300 °C), with energy gap of 2.48 eV. The electrical measurements indicate the formation of a p-n junction after the deposition of the copper oxide layer. The core-shell photodetectors present a photoresponsivity at 0 V bias voltage up to 7.7 µA/W and time response ≤ 0.09 s, the fastest ever reported for oxide photodetectors in the visible range, and among the fastest including photodetectors with response limited to the UV region. The bare ZnO NWs have slow photoresponsivity, without recovery after the end of photo-stimulation. The fast time response for the core-shell structures is due to the presence of the p-n junctions, which enables fast exciton separation and charge extraction. Additionally, the suitable electronic structure of the ZnO-Cu2O heterojunction enables self-powering of the device at 0 V bias voltage. These results represent a significant advancement in the development of low-cost, high efficiency and self-powered photodetectors, highlighting the need of fine tuning the morphology, composition and electronic properties of p-n junctions to maximize device performances.

International audience; Plasmonic Cu nanoparticles (NP) were successfully deposited on ZnO substrates by atomic layer deposition (ALD) owing to the Volmer–Weber island growth mode. An evolution from Cu NP to continuous Cu films was observed with an increasing number of ALD cycles. Real and imaginary parts of the NP dielectric functions, determined by spectroscopic ellipsometry using an effective medium approach, evidence a localized surface plasmon resonance that can be tuned between the visible and near-infrared ranges by controlling the interparticle spacing and size of the NP. The resulting Cu NP/ZnO device shows an enhanced photoresponse under white light illumination with good responsivity values, fast response times, and stability under dark/light cycles. The significant photocurrent detected for this device is related to the hot-electron generation at the NP surface and injection into the conduction band of ZnO. The possibility of tuning the plasmon resonance together with the photoresponsivity of the device is promising in many applications related to photodetection, photonics, and photovoltaics.

There is need for reliable, low cost detectors to continuously monitor the air quality in presence of hazardous gases, harmful for human health. In this work we demonstrate how the use of different type of metal oxide one-dimensional sensors based on ZnO and SnO 2 could provide high sensing capabilities together with differentiated responses to gases. The sensors can then be used together into a cost-effective portable sensor systems for quantitative detection of harmful emissions and contaminations. The gases tested in the present work are H 2 a highly explosive gas (4% is the lower explosion limit), H 2 S, one of the principal compounds involved in the natural cycle of sulphur in the environment, and DMMP, a simulant of the nerve agent Sarin. ZnO and SnO 2 nanowires were prepared by Physical Vapour Deposition and ZnO polycrystalline nanorods were prepared by RF sputtering. The optimum response to low concentrations of DMMP at 500°C was observed for ZnO materials, with respect to SnO 2 at 500°C. At 400°C H 2 is better sensed by ZnO. Detection of H 2 S is better obtained with ZnO NRs compared to the other sensors.

International audience; A low-cost method for carbon nanotubes (CNTs) network production from solutions on flexible polyethylene naphthalate substrates has been adopted to prepare high quality and well characterized SWCNT bundle layers to be used as the active layer in chemiresistor gas sensors. Two types of SWCNTs have been tested: pristine SWCNTs, deposited from a surfactant solution, and covalently functionalized SWCNTs, deposited from a dimethyl-acetamide solution. The humidity effects on the sensitivity of the SWCNTs network to NH3 have been investigated. The results show that relative humidity favors the response to NH3, confirming recent theoretical predictions. The COOH-functionalized sample displays the largest response owing to both its hydrophilic nature, favoring the interaction with H2O molecules, and its largest surface area. Compared to data available in the literature, the present sensors display a remarkable sensitivity well below the ppm range, which makes them quite promising for environmental and medical applications, where NH3 concentrations (mostly of the order of tens of ppb) have to be detected.

A combination of the functionalities of carbon nanotube (CNT)-Si hybrid heterojunctions is presented as a novel method to steer the efficiency of the photovoltaic (PV) cell based on these junctions, and to increase the selectivity and sensitivity of the chemiresistor gas sensor operated with the p-doped CNT layer. The electrical characteristics of the junctions have been tracked by exposing the devices to oxidizing (NO2) and reducing (NH3) molecules. It is shown that when used as PV cells, the cell efficiency can be reversibly steered by gas adsorption, providing a tool to selectively dope the p-type layer through molecular adsorption. Tracking of the current-voltage curve upon gas exposure also allowed to use these cells as gas sensors with an enhanced sensitivity as compared to that provided by a readout of the electrical signal from the CNT layer alone. In turn, the chemiresistive response was improved, both in terms of selectivity and sensitivity, by operating the system under illumination, as the photo-induced charges at the junction increase the p-doping of CNTs making them more sensitive to NH3 and less to NO2.

An easy transfer procedure to obtain graphene-based gas sensing devices operating at room temperature (RT) is presented. Starting from chemical vapor deposition-grown graphene on copper foil, we obtained single layer graphene which could be transferred onto arbitrary substrates. In particular, we placed single layer graphene on top of a SiO2/Si substrate with pre-patterned Pt electrodes to realize a chemiresistor gas sensor able to operate at RT. The responses to ammonia (10, 20, 30 ppm) and nitrogen dioxide (1, 2, 3 ppm) are shown at different values of relative humidity, in dark and under 254 nm UV light. In order to check the sensor selectivity, gas response has also been tested towards hydrogen, ethanol, acetone and carbon oxide. Finally, a model based on linear dispersion relation characteristic of graphene, which take into account humidity and UV light effects, has been proposed.

In this work, we prepared zinc stannate (ZTO) thin films by RF sputtering starting from a sintered ceramic target with Zn:Sn ratio 2:1, by varying the argon/oxygen pressures during the deposition process. Thin films were deposited on alumina substrates at 400 °C, and subsequently annealed at 600 °C in the air. The transparency typical for the as-deposited films was preserved even after their annealing at 600 °C in the air, where the high temperature was crucial for the improvement of the crystalline Zn 2 SnO 4 , ZTO phase, as confirmed by Raman measurements. Additionally, the post-deposition thermal treatment resulted in porous ZTO films suitable for gas sensing application. The structural and surface morphological properties of thin films were investigated by Raman spectroscopy, field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray analysis (EDX). Finally, the gas sensing properties towards nitrogen dioxide, ethanol and acetone have been tested, showing the high potentiality of this material as the gas sensor for ethanol and acetone at 400 °C, and nitrogen dioxide at 200 °C.

A novel carbon-based nanostructured material, which includes carbon nanotubes (CNTs), porous carbon, nanostructured ZnO and Fe nanoparticles, has been synthetized using catalytic chemical vapour deposition (CVD) of acetylene on vertically aligned ZnO nanorods (NRs). The deposition of Fe before the CVD process induces the presence of dense CNTs in addition to the variety of nanostructures already observed on the process done on the bare NRs, which range from amorphous graphitic carbon up to nanostructured dendritic carbon films, where the NRs are partially or completely etched. The combination of scanning electron microscopy and in situ photoemission spectroscopy indicate that Fe enhances the ZnO etching, and that the CNT synthesis is favoured by the reduced Fe mobility due to the strong interaction between Fe and the NRs, and to the presence of many defects, formed during the CVD process. Our results demonstrate that the resulting new hybrid shows a higher sensitivity to ammonia gas at ambient conditions (∼60 ppb) than the carbon nanostructures obtained without the aid of Fe, the bare ZnO NRs, or other one-dimensional carbon nanostructures, making this system of potential interest for environmental ammonia monitoring. Finally, in view of the possible application in nanoscale optoelectronics, the photoexcited carrier behaviour in these hybrid systems has been characterized by time-resolved reflectivity measurements.

The ideal chemical gas sensors would provide a device capable of being sensible, selective and stable. To improve the state of the art metal oxide gas sensors two approaches are pursued: obtaining nanosized, single crystalline metal oxide nanorods, and introducing innovative transduction principle. The first approach ensures high surface area for gas interaction coupled with the higher stability. The second approach could overcome limitations on the performances achievable and set a new milestone in the field. In this work magnetic gas sensing tests were carried out, using a MOKE (magneto-optical Kerr effect) magnetometer, showing that it is possible to exploit a new mechanism for sensing devices. The sensing layer, based on Co layer covered by ZnO nanorods, was entirely deposited by RF sputtering. The device showed very good H2 detection at room temperature. The current work focuses on the characterization of the sensing heterostructure based on Co/ZnO nanorods by volumetric magnetization measurements and by MOKE measurements in air and in gas. A model for H2 sensing is also formulated.

The gas response of single-wall carbon nanotubes (SWCNT) functionalized with indium tin oxide (ITO) nanoparticles (NP) has been studied at room temperature and an enhanced sensitivity to ammonia and nitrogen dioxide is demonstrated. The higher sensitivity in the functionalized sample is related to the creation of nano-heterojunctions at the interface between SWCNT bundles and ITO NP. Furthermore, the different response of the two devices upon NO2 exposure provides a way to enhance also the selectivity. This behavior is rationalized by considering a gas sensing mechanism based on the build-up of space-charge layers at the junctions. Finally, full recovery of the signal after exposure to NO2 is achieved by UV irradiation for the functionalized sample, where the ITO NP can play a role to hinder the poisoning effects on SWCNT due to NO2 chemisorption.