Graphene functionalised by laser-ablated V2O5 for a highly sensitive NH3 sensor

Layered Heterostructure of Graphene and TiO2 as a Highly Sensitive and Stable Photoassisted NO2 Sensor

As an atomically thin electric conductor with a low density of highly mobile charge carriers, graphene is a suitable transducer for molecular adsorption. In this study, we demonstrate that the adsorption properties can be significantly enhanced with a laser-deposited TiO2 nanolayer on top of single-layer CVD graphene, whereas the effective charge transfer between the TiO2-adsorbed gas molecules and graphene is retained through the interface. The formation of such a heterostructure with optimally a monolayer thick oxide combined with ultraviolet irradiation (wavelength 365 nm, intensity <1 mW/mm2) dramatically enhances the gas-sensing properties. It provides an outstanding sensitivity for detecting NO2 in the range of a few ppb to a few hundred ppb-s in air, with response times below 30 s at room temperature. The effect of visible light (436 and 546 nm) was much weaker, indicating that the excitations due to light absorption in TiO2 play an essential role, while the characteristics of gas responses imply the involvement of both photoinduced adsorption and desorption. The sensing mechanism was confirmed by theoretical simulations on a NO2@Ti8O16C50 complex under periodic boundary conditions. The proposed sensor structure has significant additional merits, such as relative insensitivity to other polluting gases (CO, SO2, NH3) and air humidity, as well as long-term stability (>2 years) in ambient air. The results pave the way for an emerging class of gas sensor structures based on stacked 2D materials incorporating highly charge-sensitive transducer and selective receptor layers.

A Reconfigurable Remotely Epitaxial VO2 Electrical Heterostructure

The reconfigurability of the electrical heterostructure featured with external variables, such as temperature, voltage, and strain, enabled electronic/optical phase transition in functional layers has great potential for future photonics, computing, and adaptive circuits. VO₂ has been regarded as an archetypal phase transition building block with superior metal-insulator transition characteristics. However, the reconfigurable VO₂-based heterostructure and the associated devices are rare due to the fundamental challenge in integrating high-quality VO₂ in technologically important substrates. In this report, for the first time, we show the remote epitaxy of VO₂ and the demonstration of a vertical diode device in a graphene/epitaxial VO₂/single-crystalline BN/graphite structure with VO₂ as a reconfigurable phase-change material and hexagonal boron nitride (h-BN) as an insulating layer. By diffraction and electrical transport studies, we show that the remote epitaxial VO₂ films exhibit higher structural and electrical quality than direct epitaxial ones. By high-resolution transmission electron microscopy and Cs-corrected scanning transmission electron microscopy, we show that a graphene buffered substrate leads to a less strained VO₂ film than the bare substrate. In the reconfigurable diode, we find that the Fermi level change and spectral weight shift along with the metal-insulator transition of VO₂ could modify the transport characteristics. The work suggests the feasibility of developing a single-crystalline VO₂-based reconfigurable heterostructure with arbitrary substrates and sheds light on designing novel adaptive photonics and electrical devices and circuits.

Nanocomposite of polypyrrol and silica rods-gold nanoparticles core-shell as an ammonia sensor

Ammonia is widely needed in the chemical industry as well as in fertilizers for agriculture. However, in small as well as large quantities, it is not only hazardous for human health but also for our ecosystem. Therefore, ammonia sensing at low concentration with high sensitivity, selectivity and low response time as well as recovery time is important. Here, various nanosensors are fabricated using gold nanoparticles (∼15 nm), silica-gold nanoparticles coreshell particles and coreshell particles embedded in polypyrrol. Comparisons with bare polypyrrol and coreshell particles are also made. In fact, two types of coreshell particles with rod (∼300 nm × 2 μm) shape and spheres (200 nm) of silica were used to anchor gold nanoparticles on them. A comparison showed that silica-gold core-shell particle with silica rods had the highest sensitivity (∼166% @ 130 ppm) amongst all. The sensor is simple to operate (only resistance change is measured), requires no heater as the sensing occurs at room temperature, and showed no response, except for ammonia, to other gases or humidity. It also has a low response time (4 s) and recovery time (10 s) at the lowest (10 ppm) ammonia concentration measured here. Thus, a simple, economical ammonia sensor has been demonstrated here.

Graphene-Based Ammonia Sensors Functionalised with Sub-Monolayer V₂O₅: A Comparative Study of Chemical Vapour Deposited and Epitaxial Graphene †

Graphene in its pristine form has demonstrated a gas detection ability in an inert carrier gas. For practical use in ambient atmosphere, its sensor properties should be enhanced with functionalisation by defects and dopants, or by decoration with nanophases of metals or/and metal oxides. Excellent sensor behaviour was found for two types of single layer graphenes: grown by chemical vapour deposition (CVD) and transferred onto oxidized silicon (Si/SiO₂/CVDG), and the epitaxial graphene grown on SiC (SiC/EG). Both graphene samples were functionalised using a pulsed laser deposited (PLD) thin V₂O₅ layer of average thickness ≈ 0.6 nm. According to the Raman spectra, the SiC/EG has a remarkable resistance against structural damage under the laser deposition conditions. By contrast, the PLD process readily induces defects in CVD graphene. Both sensors showed remarkable and selective sensing of NH₃ gas in terms of response amplitude and speed, as well as recovery rate. SiC/EG showed a response that was an order of magnitude larger as compared to similarly functionalised CVDG sensor (295% vs. 31% for 100 ppm NH₃). The adsorption site properties are assigned to deposited V₂O₅ nanophase, being similar for both sensors, rather than (defect) graphene itself. The substantially larger response of SiC/EG sensor is probably the result of the smaller initial free charge carrier doping in EG.

Ruthenium-decorated vanadium pentoxide for room temperature ammonia sensing

Layer structured vanadium pentoxide (V₂O₅) microparticles were synthesized hydrothermally and successfully decorated by a facile wet chemical route, with ∼10–20 nm sized ruthenium nanoparticles. Both V₂O₅ and ruthenium nanoparticle decorated V₂O₅ (1%Ru@V₂O₅) were investigated for their suitability as resistive gas sensors. It was found that the 1%Ru@V₂O₅ sample showed very high selectivity and sensitivity towards ammonia vapors. The sensitivity measurements were carried out at 30 °C (room temperature), 50 °C and 100 °C. The best results were obtained at room temperature for 1%Ru@V₂O₅. Remarkably as short a response time as 0.52 s @ 130 ppm and as low as 9.39 s @ 10 ppm recovery time at room temperature along with high selectivity towards many gases and vapors have been noted in the 10 to 130 ppm ammonia concentration range. Short response and recovery time, high reproducibility, selectivity and room temperature operation are the main attributes of the 1%Ru@V₂O₅ sensor. Higher sensitivity of 1%Ru@V₂O₅ compared to V₂O₅ has been explained and is due to dissociation of atmospheric water molecules on 1%Ru@V₂O₅ as compared to bare V₂O₅ which makes hydrogen atoms available on Brønsted sites for ammonia adsorption and sensing. The presence of ruthenium with a thin layer of oxide is clear from X-ray photoelectron spectroscopy and that of water molecules from Fourier transform infrared spectroscopy.

Layer structured vanadium pentoxide (V₂O₅) microparticles were synthesized hydrothermally and successfully decorated by a facile wet chemical route, with ∼10–20 nm sized ruthenium nanoparticles.

Electrodeposition of reduced graphene oxide with chitosan based on the coordination deposition method

The electrodeposition of graphene has drawn considerable attention due to its appealing applications for sensors, supercapacitors and lithium-ion batteries. However, there are still some limitations in the current electrodeposition methods for graphene. Here, we present a novel electrodeposition method for the direct deposition of reduced graphene oxide (rGO) with chitosan. In this method, a 2-hydroxypropyltrimethylammonium chloride-based chitosan-modified rGO material was prepared. This material disperses homogenously in the chitosan solution, forming a deposition solution with good dispersion stability. Subsequently, the modified rGO material was deposited on an electrode through codeposition with chitosan, based on the coordination deposition method. After electrodeposition, the homogeneous, deposited rGO/chitosan films can be generated on copper or silver electrodes or substrates. The electrodeposition method allows for the convenient and controlled creation of rGO/chitosan nanocomposite coatings and films of different shapes and thickness. It also introduces a new method of creating films, as they can be peeled completely from the electrodes. Moreover, this method allows for a rGO/chitosan film to be deposited directly onto an electrode, which can then be used for electrochemical detection.