A New Method to Prepare Few-Layers of Nanoclusters Decorated Graphene : Nb 2 O 5 / Graphene and Its Gas Sensing Properties †

During the last decade, due to its excellent electrical, mechanical and thermal properties of chemically modified graphene has been extensively studied for many applications, such as polymer composites, energy-related materials, biomedical applications and sensors. The aim of this work is to evaluate the gas sensing performance of niobium oxide (Nb2O5) nanoclusters deposited onto few-layers graphene powder by magneton sputtering. Two different samples were prepared by changing electrical power of deposition. The materials were deeply morphologically, structurally and chemically characterized. Finally, they were deposited onto alumina substrates and their sensing properties were investigated vs. different gases, showing good sensing performance vs. ppm concentrations of NO2 at room temperature.


Introduction
In the last years, the research in the gas sensor field experienced a significant boost.Gas sensors represent the crucial elements in gas detection systems and olfactory systems for several applications: environmental monitoring, safety and security, quality control of food production, medical diagnosis and so on [1,2].Among the various innovative gas sensing materials investigated so far, 2D materials shown interesting chemoresistive behaviour, due to their excellent electrical, mechanical and thermal properties [3,4].Graphene, an allotrope form of carbon, is the most famous 2D-material, consisted of a single layer of carbon atoms arranged in a hexagonal lattice [5].This configuration rends graphene a zero-band-gap material since it has a small overlap between the valence and the conduction bands [6].This property confers to the graphene unique chemical-physical characteristics, which have led it to be the most studied material of recent years [7,8].Among the various advantage of this semiconductor, the high electrical conductivity allowed to explore its gas sensing performance at room temperature, which opens up to the development of ultra-low power consumption gas sensors [9].However, gas-sensing responses vs. gases resulted to be low and the reaction kinetics slow, giving a high response and recovery times.To counteract these drawbacks, various investigations were carried out to verify the effect of adding functionalization on the graphene sensing properties [10].
In this work, we investigated the gas sensing performance of nanoclusters decorated few layer graphene (Nb2O5/graphene).This composite material was obtained by using magnetron-sputtering technique, in which a mixing system allow to deposit homogeneously Nb2O5 nanoclusters on graphene powder.Two different samples were prepared to investigate the influence of sputtering power deposition effect on the final Nb2O5 decorated graphene samples.The electrically characterization highlighted that Nb2O5/graphene sensors showed good and selective sensing properties vs. NO2 both in dry and wet air, at room temperature.

Materials and Methods
Nb2O5 deposits were grown at room temperature on graphene powder of 1.6 nm thickness purchased from Graphene Supermarket, by means of RF magnetron sputtering of a commercially available high purity Nb2O5 disc (99.99%) with 5 cm diameter.A powder vibration set up was employed to uniformly coat the graphene powder with Nb2O5.The deposition was carried out at a 6 Pa gas pressure, with a powder vibration frequency of 10 Hz, a deposition time of 30 min and varying the electrical power at the target (50 and 80) W.
Then, chemical, structural and morphological characterizations were carried out on samples.The morphology was analysed by Scanning Electron Microscopy (SEM, cold cathode JEOL Microscope, model JSM 7401-F) and Transmission Electron Microscopy (TEM, Hitachi H-800 model).The chemical characterization was performed by using a Kratos AXIS Ultra DLD instrument equipped with a monochromatic Al Kα (1486.6 eV) x-ray source.
Nb2O5/graphene, graphene and Nb2O5 nanostructured materials were mixed with ethanol and deposited onto alumina substrates, provided with gold interdigitated electrodes and platinum heater, by means of drop coating technique.Gas sensing performance of devices were investigated in a suitable gas chamber, both in dry and wet conditions, vs. various concentrations of different gases, i.e., CO, H2S, butanol and NO2.Gases were from certified cylinder, and they were injected into the gas chamber by using mass flow controller.Gas sensing responses was calculated as ( − )/ for reducing behaviour and ( − )/ , where Gair is the conductance of sensing material in air and Ggas in the presence of gas.

Results and Discussion
In the Figure 1 SEM and TEM characterizations are shown.In Figure 1a is reported an image of the graphene flakes before the Nb2O5 deposition, meanwhile Figure 1b highlights the change of the sample morphology after the magnetron sputtering deposition, due to the formation of Nb2O5 clusters over the graphene flakes surface.The XPS analysis was carried out to investigate how the deposition parameters influenced the Nb2O5 concentration on samples (Table 1).As it can be seen, the change in the deposition electrical power strongly affected the Nb2O5 concentration in the material, giving a major concentration by using 80 W compared to 50 W.
The electrical characterization, in dry air, highlighted that Nb2O5/graphene sensors were insensitive to all gases tested, except for NO2, at room temperature.The same measurements were repeated in presence of wet air, showing that the selectivity of the sensing material to NO2 was not affected by the presence of moisture.Furthermore, the response values obtained in dry and wet air were comparable up to a relative humidity of 40% (Figure 2).The response of sensors was strongly influenced by the concentration of Nb2O5 on the surface of the graphene flakes.Indeed, the sensor obtained by depositing the sample with the highest concentration of Nb2O5 showed responses 1.5 times higher than the less concentrated one (Figure 3).Further electrical characterizations have shown that sensors prepared using separately graphene and Nb2O5 showed a strong decrease in the sensing performance compared to Nb2O5/graphene sensors.

Conclusions
In this work, an innovative method to decorate homogenously graphene with Nb2O5 nanoparticles is presented, using magneton-sputtering instrument equipped with a powder mixing system.This material highlighted good sensing properties vs. NO2 both in dry and wet air, at room temperature.The electrical characterization showed that the gas sensing performance of Nb2O5/graphene layers depended on the deposition parameters used in the magnetron sputtering, which were improved by increasing the deposition electrical power and thus the concentration of Nb2O5 nanoparticles on graphene flakes.