Improving the ammonia sensing of reduced graphene oxide film by using metal nanomaterials

Gas sensing is one of the most promising applications for reduced graphene oxide (rGO). High surface-to-volume ratio in conjunction with remaining reactive oxygen functional groups translates into sensitivity to molecular on the rGO surface. The response of the rGO based devices can be further improved by functionalizing its surface with metal nano-materials. In this paper, we report the ammonia (NH3) sensing behavior of rGO based sensors functionalized with nano-structured metal: silver (Ag) or platinum (Pt) or gold (Au) in air at room temperature and atmospheric pressure. The gas response is detected by the monitoring changes in electrical resistance of the rGO/metal hybrids due to NH3 gas adsorption. Compared to bare rGO, significantly improved NH3 sensitivity is observed with the addition of nanostructured metals. These materials are applied to play the small bridges role connecting many graphene islands together to improve electrical conduction of hybrids while maintaining the inherent advantage of rGO for NH3 gas sensitivity. Key word: reduced graphene oxide, silver nanowires, polyol method, NH3 gas sensing.


INTRODUCTION
Recent studies revealed that the reduced graphene oxide or chemically modified graphene (rGO) can be served as high performance molecular sensors because rGO contains a range of reactive oxygen functional groups.Many groups extensively studied molecular adsorption on rGO and proposed that the active defective sites provided by the residual oxygen or hydroxyl functional groups during the reduction of GO may improve the interaction of adsorbate and GO, thereby enhancing the sensor response [1][2][3].However, most of the rGO sensors were recovered very slowly after sensing NH 3 at room temperature.This shortcoming must be overcome

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to apply rGO to NH 3 detection at RT.One of methods to improve the recovery of these rGO based sensors was the decoration of nanomaterials on the surface of rGO [4,5].
For the synthesis of metal nanostructures, various methods have been successfully developed.Up to now, the polyol method has become widely used by many research groups because of its advantages such as cost, yield, and simplicity [6][7][8][9].
In this study, we report on the synthesis of rGO/metal hybrid nano-structures by using chemical method for making rGO thin films and polyol process for synthesis metal nano-materials (Ag, Au and Pt) and then these hybrids are applied in the NH 3 gas sensors.

Synthesis of reduced graphene oxide (rGO) and metal nano-materials
Synthesis of rGO.Graphite was oxidated to graphene oxide (GO) by using the mixture of KMnO 4 /NaNO 3 /H 2 SO 4 (modified Hummers method).This GO solution was spin-coated directly onto quartz substrate.The GO thin films were subsequently reduced to rGO using chemical agent (hydrazine) and heating (250 0 C).More details about the synthesis of rGO was presented in our previous papers [10,11].
Synthesis of metal nano-materials.The Ag, Au and Pt nano-materials were synthesized through polyol method.This polyol process is based on the reduction of an inorganic salt by a polyol at an elevated temperature and a surfactant is used to prevent the agglomeration of the colloidal particles.In our experiment, AgNO 3 , HAuCl 4 and H 2 PtCl 6 were used as Ag + , Au 3+ and Pt 4+ source, respectively.Ethylene glycol (EG) was used as both solvent and reducing agent for reduction of Ag + /Pt 4+ ions to Ag 0 /Pt 0 atoms and polyvinyl pyrrolidone (PVP) and NaCl were used as stabilizing agents.Small gold nanoparticles were prepared by the reduction of Au 3+ ions with sodium borohydride/ascorbic acid in the presence of a stabilizing agent (trisodium citrate or CTAB) [7][8][9].

Preparation of gas sensing devices and measurement system
After the rGO thin films were formed, two silver planar electrode arrays were deposited on the rGO films using thermal evaporation method with 6 mm distance between them.Finally, we used spray-coating method to disperse metal nano-materials on rGO surface area between two electrodes to complete our gas-sensing devices which is ready for NH 3 sensing signal measurement.More details about the preparation of gas sensing devices were presented in our previous paper [12].

RESULTS AND DISCUSSION
Fig. 1 shows the Energy-dispersive X-ray Spectroscopy (EDS) spectra of the Ag, Au and Pt thin films, spraying of their solutions onto quartz substrates, which contain strong peaks for elemental Ag, Au and Pt suggesting the formation of Ag, Au and Pt nano-materials in synthesis processes.Continuously, Fig. 3 shows the UV-visible absorption spectra of Ag, Au and Pt colloid solution products.These spectra fortify the formation of metal nano-materials in our experiment with the appearance of their typical peaks.The large peak around 445 nm suggests that the final product is AgNPs with a large range of different diameters while a peak at ∼380 nm and the shoulder around ∼350 nm indicate that the main product is AgNWs in solution (Fig 3 A) [13][14][15].Besides, the peaks at ~520 nm and ~250 nm show the presence of AuNPs and PtNPs in final products, respectively [9,16].The nanomaterials solutions are ready for combine with rGO and complete the gas sensors.Finally, we investigate the sensitivity ability NH 3 of bare rGO material and its hybrids with these metal nano-materials.The experimental processes are performed in the same condition (room temperature and atmospheric pressure).The data in Fig. 4 show that the sensitivity ability of original rGO material is improved significantly by nanomaterials.In comparison with the sensitivity of bare rGO material (10 %), the sensitivity of the rGO-AgNPs, rGO-AuNPs and rGO-PtNPs hybrids increases 15 %, 25 % and 12 %, respectively, although the recovery of these sensors remain uncompleted.Particularly, the combination of rGO and AgNWs with the length more than 5µm affords not only to improve NH 3 gas sensitivity (40 %) but also nearly complete recovery (Fig. 4).

CONCLUSION
In this study, we have investigated the effect of nanostructure materials (Ag, Au and Pt) with different sharp and size to NH 3 adsorption of hybrids between rGO (reduced graphene oxide) and these metals.The metal nanostructure materials play the role of bridges connecting together many rGO islands so that their contact resistance is reduced, resulting in strainght forward absorption and desorption signals.With addition of one-dimensional nanostructure (AgNWs), the enhancement of NH 3 gas sensitivity of rGO-AgNWs hybrid is the highest and in particular its recovery ability is the most efficient in comparison with rGO-NPs, rGO-AuNPs and PtNPs hybrids.We suggest that the work reported here is a significant step toward the practical application of rGO-based chemical sensors.