Graphene Oxide and Polymer Humidity Micro-Sensors Prepared by Carbon Beam Writing.

In this study, novel flexible micro-scale humidity sensors were directly fabricated in graphene oxide (GO) and polyimide (PI) using ion beam writing without any further modifications, and then successfully tested in an atmospheric chamber. Two low fluences (3.75 × 1014 cm-2 and 5.625 × 1014 cm-2) of carbon ions with an energy of 5 MeV were used, and structural changes in the irradiated materials were expected. The shape and structure of prepared micro-sensors were studied using scanning electron microscopy (SEM). The structural and compositional changes in the irradiated area were characterized using micro-Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Rutherford back-scattering spectroscopy (RBS), energy-dispersive X-ray spectroscopy (EDS), and elastic recoil detection analysis (ERDA) spectroscopy. The sensing performance was tested at a relative humidity (RH) ranging from 5% to 60%, where the electrical conductivity of PI varied by three orders of magnitude, and the electrical capacitance of GO varied in the order of pico-farads. In addition, the PI sensor has proven long-term sensing stability in air. We demonstrated a novel method of ion micro-beam writing to prepare flexible micro-sensors that function over a wide range of humidity and have good sensitivity and great potential for widespread applications.

2D Few-Layered PdPS: Toward High-Efficient Self-Powered Broadband Photodetector and Sensors.

Photodetectors and sensors have a prominent role in our lives and cover a wide range of applications, including intelligent systems and the detection of harmful and toxic elements. Although there have been several studies in this direction, their practical applications have been hindered by slow response and low responsiveness. To overcome these problems, we have presented here a self-powered (photoelectrochemical, PEC), ultrasensitive, and ultrafast photodetector platform. For this purpose, a novel few-layered palladium-phosphorus-sulfur (PdPS) was fabricated by shear exfoliation for effective photodetection as a practical assessment. The characterization of this self-powered broadband photodetector demonstrated superior responsivity and specific detectivity in the order of 33 mA W-1 and 9.87 × 1010 cm Hz1/2 W-1, respectively. The PEC photodetector also exhibits a broadband photodetection capability ranging from UV to IR spectrum, with the ultrafast response (∼40 ms) and recovery time (∼50 ms). In addition, the novel few-layered PdPS showed superior sensing ability to organic vapors with ultrafast response and a recovery time of less than 1 s. Finally, the photocatalytic activity in the form of hydrogen evolution reaction was explored due to the suitable band alignment and pronounced light absorption capability. The self-powered sensing platforms and superior catalytic activity will pave the way for practical applications in efficient future devices.

High-Entropy NASICON Phosphates (Na3M2(PO4)3 and NaMPO4Ox, M = Ti, V, Mn, Cr, and Zr) for Sodium Electrochemistry.

High-entropy materials, with complex compositions and unique cocktail characteristics, have recently drawn significant attention. Additionally, a family of sodium super ion conductors (NASICONs)-structured phosphates in energy storage areas shows a comprehensive application for traditional alkaline ion batteries and, in particular, solid-state electrolytes. However, there is no precedent in fabricating this kind of NASICON-type high-entropy phase. Here, we report the successful fabrication of two well-crystallized high-entropy phosphates, namely, Na3(Ti0.2V0.2Mn0.2Cr0.2Zr0.2)2(PO4)3 (HE-N3M2P3) and Na(Ti0.2V0.2Mn0.2Cr0.2Zr0.2)2PO4Ox (HE-NMP). The prepared materials in which the transition metals (TMs) of Ti, V, Mn, Cr, and Zr occupy the same 12c Wykoff position can form a structure analogous to R3̅c Na3V2(PO4)3 that is carefully determined by X-ray diffraction, neutron diffraction, and transmission electron microscopy. Further, their performance for sodium ion batteries and sodium-based solid-state electrolytes was evaluated. The HE-N3M2P3 might exhibit a promising electrochemical performance for sodium storage in terms of its structure resembling that of Na3V2(PO4)3. Meanwhile, the HE-NMP shows considerable electrochemical activity with numerous broad redox ranges during extraction and insertion of Na+, related to the coexistence of several TM elements. The evaluated temperature-dependent ionic conductivity for HE-NMP solid electrolyte varies from 10-6 to 10-5 S cm-1 from room temperature to 398.15 K, offering high potential for energy storage applications as a new high-entropy system.

Two-Dimensional Gallium Sulfide Nanoflakes for UV-Selective Photoelectrochemical-type Photodetectors.

Two-dimensional (2D) transition-metal monochalcogenides have been recently predicted to be potential photo(electro)catalysts for water splitting and photoelectrochemical (PEC) reactions. Differently from the most established InSe, GaSe, GeSe, and many other monochalcogenides, bulk GaS has a large band gap of ∼2.5 eV, which increases up to more than 3.0 eV with decreasing its thickness due to quantum confinement effects. Therefore, 2D GaS fills the void between 2D small-band-gap semiconductors and insulators, resulting of interest for the realization of van der Waals type-I heterojunctions in photocatalysis, as well as the development of UV light-emitting diodes, quantum wells, and other optoelectronic devices. Based on theoretical calculations of the electronic structure of GaS as a function of layer number reported in the literature, we experimentally demonstrate, for the first time, the PEC properties of liquid-phase exfoliated GaS nanoflakes. Our results indicate that solution-processed 2D GaS-based PEC-type photodetectors outperform the corresponding solid-state photodetectors. In fact, the 2D morphology of the GaS flakes intrinsically minimizes the distance between the photogenerated charges and the surface area at which the redox reactions occur, limiting electron-hole recombination losses. The latter are instead deleterious for standard solid-state configurations. Consequently, PEC-type 2D GaS photodetectors display a relevant UV-selective photoresponse. In particular, they attain responsivities of 1.8 mA W-1 in 1 M H2SO4 [at 0.8 V vs reversible hydrogen electrode (RHE)], 4.6 mA W-1 in 1 M Na2SO4 (at 0.9 V vs RHE), and 6.8 mA W-1 in 1 M KOH (at 1.1. V vs RHE) under 275 nm illumination wavelength with an intensity of 1.3 mW cm-2. Beyond the photodetector application, 2D GaS-based PEC-type devices may find application in tandem solar PEC cells in combination with other visible-sensitive low-band-gap materials, including transition-metal monochalcogenides recently established for PEC solar energy conversion applications.

Self-Powered Broadband Photodetector and Sensor Based on Novel Few-Layered Pd3(PS4)2 Nanosheets.

Optoelectronics and sensing devices are of enormous importance in our modern lives, which has propelled the scientific community to explore new two-dimensional (2D) nanomaterials to meet the requirements of future devices. Herein, we present the exfoliation of palladium thiophosphate (Pd3(PS4)2) by mechanical shear force exfoliation. The Pd3(PS4)2-based photoelectrochemical (PEC) device demonstrated self-powered broadband photodetection in the range of 385-940 nm with an unprecedented responsivity of 2 A W-1 and a specific detectivity of about 8.67 × 1011 cm Hz1/2 W-1 under the illumination of 420 nm LED light. The crucial parameters such as photoresponsivity, response, and recovery time of the device can be controlled by an externally applied voltage and the analyte concentration. Moreover, Pd3(PS4)2-based vapor-sensing devices exhibited frequency-dependent selective acetone sensing in the presence of other organic vapors with an ultrafast response and a recovery time of less than 1 s. Finally, the photocatalytic activity of Pd3(PS4)2 was revealed, which can be attributed to the presence of an appropriate band alignment with the catalytic activity of Pd. This novel material with the aforementioned fascinating phenomenon will pave the way toward practical future applications in optoelectronics and sensing.

Single-Step Synthesis of Platinoid-Decorated Phosphorene: Perspectives for Catalysis, Gas Sensing, and Energy Storage.

The originality of phosphorene is suppressed by its structural defects, irreproducibility, and sensitivity to the ambient environment. To preserve phosphorene's essential characteristics, for example, influencing the charge redistribution and generating the formation of active centers, noble-metal decoration is found to be an efficient approach. Herein, we demonstrate a single-step electrochemical synthesis of platinoid-decorated few-layer phosphorene (FP). The material's structure and effects of metal (Ru, Rh, and Pd) deposition on the FP nanosheets were first explored by numerous analytical techniques and theoretical calculations. Platinoid-decorated FPs demonstrate high quality and consist of one to five layers modified with round- and heptagon-shaped metal nanoparticles with the most intense distribution of Pd. The high-rate Rh deposition provides the enhanced electrocatalytic efficiency for hydrogen evolution (79 mV·dec-1-Tafel slope) and almost 20 times increased capacity for the Li-ion batteries in comparison to bare and Ru-decorated FP. The chemosensing of platinoid-decorated FP indicates a response to methanol plus ethanol and shows inertness to acetone. The incorporation of Ru and Rh nanoparticles increases FP's selectivity toward methanol. This research provides a new approach for the in situ FP functionalization during top-down synthesis and thus broadens the material's feasibility for advanced nanotechnology.

Autogenous Formation of Gold on Layered Black Phosphorus for Catalytic Purification of Waste Water.

Layered black phosphorus (BP) is a member of a layered material family with anisotropic properties and layer-dependent band gaps that can be exfoliated down to single-layered phosphorene. Compared with graphene, few-layered BP and its single-layer phosphorene are significantly more reactive, and this reactivity can be applied for the autogenous reduction of gold ions to metallic gold nanoparticles supported by few-layered BP (Au/BP). Few-layered BP and gold are well-known oxidation catalysts important in organic synthesis and also in the catalytic treatment and purification of industrial wastewater. The treatment of organic contamination present in industrial wastewater presents serious problems and is an important issue for current catalysis. Here, we show the high catalytic activity of the gold supported on few-layered black phosphorus (Au/BP) for wet oxidation of acrylic acid, including samples of industrial wastewater with complex composition. The catalyst Au/BP exhibits high stability, which allows utilization of its easily accessible 2D surface for the preparation of 2D material-supported metal catalysts.

Free-Standing Black Phosphorus Foils for Energy Storage and Catalysis.

Few-layered black phosphorus (BP) is a two-dimensional material that has attracted intensive attention for applications in energy storage and catalysis due to its large surface area and good electrical and thermal conductivity. Herein, a comparable study of BP electrochemical exfoliation in various solutions of tetrabutylammonium salts (TBAX; X is PF6 - , BF4 - , and ClO4 - ) in DMSO is reported. Based on morphological and structural analyses, it is shown that TBAPF6 /DMSO medium was specifically appropriate for the production of high-quality BP nanosheets with micrometer lateral size and a thickness of about 2.4 nm. TBAPF6 /DMSO-processed, few-layered BP exhibits enhanced hydrogen evolution reaction (HER) catalytic activity compared with that of samples exfoliated with the assistance of BF4 - and ClO4 - ions. Finally, the fabrication of flexible, free-standing BP films and their performance in an all-solid-state supercapacitor device are demonstrated.

TaS2, TaSe2, and Their Heterogeneous Films as Catalysts for the Hydrogen Evolution Reaction.

Metallic two-dimensional transition-metal dichalcogenides (TMDs) of the group 5 metals are emerging as catalysts for an efficient hydrogen evolution reaction (HER). The HER activity of the group 5 TMDs originates from the unsaturated chalcogen edges and the highly active surface basal planes, whereas the HER activity of the widely studied group 6 TMDs originates solely from the chalcogen- or metal-unsaturated edges. However, the batch production of such nanomaterials and their scalable processing into high-performance electrocatalysts is still challenging. Herein, we report the liquid-phase exfoliation of the 2H-TaS2 crystals by using 2-propanol to produce single/few-layer (1H/2H) flakes, which are afterward deposited as catalytic films. A thermal treatment-aided texturization of the catalytic films is used to increase their porosity, promoting the ion access to the basal planes of the flakes, as well as the number of catalytic edges of the flakes. The hybridization of the H-TaS2 flakes and H-TaSe2 flakes tunes the Gibbs free energy of the adsorbed atomic hydrogen onto the H-TaS2 basal planes to the optimal thermo-neutral value. In 0.5 M H2SO4, the heterogeneous catalysts exhibit a low overpotential (versus RHE, reversible hydrogen electrode) at the cathodic current of 10 mA cm-2 (η10) of 120 mV and high mass activity of 314 A g-1 at an overpotential of 200 mV. In 1 M KOH, they show a η10 of 230 mV and a mass activity of 220 A g-1 at an overpotential of 300 mV. Our results provide new insight into the usage of the metallic group 5 TMDs for the HER through scalable material preparation and electrode processing.

Chalcogenide vacancies drive the electrocatalytic performance of rhenium dichalcogenides.

The hydrogen evolution reaction (HER) is one of the most promising ways to produce clean energy. However, its wide-spread use is hindered by the price of the state-of-the-art catalysts based on precious metals. Transition metal dichalcogenide (TMD) nanomaterials are a cheap alternative, but a relatively large portion of them remains unexplored in terms of the HER. Here we report the HER performance of rhenium disulfide and diselenide in an acidic environment. We used sodium naphthalenide as an exfoliation agent. In comparison with other TMDs, the degree of exfoliation was relatively low. On the other hand, rhenium disulfide and diselenide both exhibit high chemical stability towards oxidation even after the exfoliation. The reported HER performance of the exfoliated rhenium dichalcogenides was strongly dependent on electrochemical treatment and to further enhance the performance, we used a series of electrochemical pretreatments at potentials as high as -2 V in sulfuric acid. While the morphology of the samples remained unchanged, the surface was found out to be chalcogen deficient, pointing out the formation of chalcogen vacancies. Consequentially, the HER performance was substantially enhanced. These results were corroborated by theoretical calculations showing improved bonding of hydrogen by chalcogenide vacancies at the surface. Our results show that a very simple electrochemical procedure can be used to improve the electrocatalytic performance of rhenium dichalcogenides and, possibly, also other TMDs.

MoS2 versatile spray-coating of 3D electrodes for the hydrogen evolution reaction.

A facile one-step MoS2 spray-coating method was applied to a range of rigid, flexible, porous and 3D printed carbon-based surfaces, yielding high loadings in MoS2 flakes. The characterization of MoS2 flakes from a commercial lubricant spray reveals up to micron-scale bulk sheets of the layered material, constituted in its majority by the semiconducting 2H polymorph, in the presence of the metallic 1T phase. Consequently, the process generates MoS2 spray-coated surfaces with improved hydrogen evolution reaction (HER) catalytic performance. In the case of carbon-based screen printed electrodes (SPE), a short-term thermal post-treatment of the MoS2 spray-coated SPE had a further beneficial effect in the HER overpotential. The MoS2 spray-coated 3D metallic meshes held the lowest HER overpotential of the series. Finally, MoS2 spray-coated 3D printed electrodes yielded improved heterogeneous charge transfer and a 500 mV shift in the required overpotential at a current density of -10 mA cm-2. The MoS2 spray-coated 3D printed electrode displayed an abundant coverage at the inner, external and planar zones of electrodes by the MoS2 sheets, even after long-term operation conditions. These outcomes can be beneficial for future tailoring of MoS2 spray-coated surfaces and their implementation in energy conversion technologies.

Shear-force exfoliation of indium and gallium chalcogenides for selective gas sensing applications.

Layered chalcogenides AIIIBVI of gallium and indium form a group of semiconducting nanomaterials with huge potential in electronic, sensor and energy storage applications. However, the preparation method predetermines the usage of the prepared nanomaterial. In this paper, we investigated shear-force milling exfoliation in a surfactant free water/ethanol mixture on indium and gallium chalcogenides and their utilization in the gas sensing of volatile organic compounds (VOCs). The exfoliation of bulk materials in a surfactant-free environment helped to avoid any surface contamination and allowed the preparation of materials without non-covalently bonded large organic molecules. Furthermore, the gas-sensing properties were evaluated by electrical impedance spectroscopy on VOCs. Our results showed high sensitivity and selectivity towards methanol. This suggests that shear-force milling is an effective method for the exfoliation of indium and gallium chalcogenides which can find application in the selective gas sensing of VOCs.