Incorporation of Reduced Graphene Oxides for Enhancing the Gas Sensing Characteristics
Authors: Kim Hyoun Woo , Choi Myung Sik , Bang Jae Hoon , Han Seungmin , Lee Ha Young , Na Han Gil , Mirzaei Ali , Majhi Sanjit Manohar , Kim Jae-Hun , Lee Jae-Hyoung , Kim Jin-Young , Kim Sang Sub 
Published: May 2020

Introduction

Recently, reduced graphene oxide (RGO) has been studied and used in many fields due to their intrinsic properties and its easy of fabrication. RGO has high surface area, lots of dangling bonds, oxygen functional groups and defects that contribute enhancement of absorption and interactions with target gases [1]. Also, RGO is retaining flake size and initial graphene oxide layer ratios. Because of those advantages, many researches have been conducted to improve gas sensing properties by synthesizing RGO nanosheets (NSs)-based nanocomposites. In this study, we discussed enhanced gas sensing abilities by incorporating RGO or graphene NSs to metal oxide nanofibers (NFs) and nanoparticles (NPs). First, RGO NSs were incorporated to SnO2 NFs for enhanced gas sensing abilities. By varying the weight percent of RGO NSs to SnO2 NFs, the optimal 0.44 wt% of RGO NSs showed highest response to NO2 gas. Also metal NPs such as Au, Pd, and Pt were functionalized to further increase their gas selectivity to CO, C6H6, and C7H8 gases. Likewise, RGO NSs were loaded to ZnO NFs and enhanced their H2 gas sensing ability. Then, for the selective detection of CO and C6H6 gases, Au and Pd NPs were functionalized on RGO NSs incorporated ZnO NFs. When RGO NSs were incorporated with CuO NFs, sensing abilities of CuO NFs to H2S gas were enhanced by balancing the electrons between CuO NFs and RGO NSs. At the end, by utilizing the enriched electrons in graphene NSs, SnO2 and ZnO powders were mixed with graphene NSs and irradiated with microwaves. As fabricated SnO2-graphene and ZnO-graphene composites were highly sensitive and selective to NO2 gas.

Method

For the fabrication of RGO, graphite powder was used as starting material and graphene oxide (GO) was produced using Hummers method. The process of graphite oxidation was conducted by stirring graphite (1g) with H2SO4 (46 ml) containing KMnO4 (12 g) and H3PO4 (12 g) to facilitate exfoliation and separation of graphene sheets. After repeating the washing and filtration obtained a very fine brown powder was dried in a vacuum oven. In order to reduce exfoliated GO NSs, the powder was mixed with hydrazine monohydrate and the mixture was heated at 150oC for 24h. A homogenous RGO NSs suspension was prepared in dimethylformamide (DMF) by sonication. The procedure to synthesize RGO NSs loaded SnO2 NFs using electrospinning is as follows. First, PVAc was dissolved in mixed solvent composed of ethanol (20 g) and DMF (15 g). Then, SnCl2∙2H2O (2.7 g) and RGO NSs 0.44 wt% were added to the PVAc solution, and stirred for 10 h. Prepared solution was loaded into a syringe equipped with a 21-gauge stainless steel needle with an inner diameter of 0.51 mm. Feed rate, applied voltage, and distance between the tip of needle and collector were fixed at 0.03 mL/h, 15 kV, and 20 cm. Similarly, for the fabrication of RGO NSs loaded ZnO and CuO NFs, zinc acetate (Zn(CH3CO2)2 and copper acetate (Cu(CH3CO2)2) were used as the source materials. For the functionalization of Au, Pd, and Pt NPs, aqueous solution of HAuCl4∙nH2O, PdCl2, and H2PtCl6∙nH2O were dissolved in a mixed solution of acetone and 2-propanol, and then exposed to UV radiation for 1 min. To the next, for the fabrication of graphene-SnO2 and graphene-ZnO nanocomposites, mixture of SnO2, ZnO nanopowders and graphene were uniformly dispersed in ethanol for 1 h and dried. The dried powder mixture s were put into an alumina crucible and treated by a microwave heating process with a power of 1 kW for 5 min. For sensing tests, double-layer electrode comprising an Au layer (300 nm thick) and a Ti layer (50 nm thick) were deposited on the specimens using sputtering at room temperature.

Results and Conclusions

In the gas sensing test, improved sensing performance was identified by using RGO-based nanocomposites. Sensor response to CO and NO2 gases of RGO NSs-loaded SnO2 NFs were superior to those of pure SnO2 NFs. Also, by incorporating Au NPs to RGO NSs-loaded SnO2 NFs, sensor response to 5 ppm CO gas was changed from 17.0 to 27.4. Likewise, functionalization of Pd and Pt NPs enhanced the sensing properties of RGO NSs-loaded SnO2 NFs and showed high response of 12.3 and 16.0 to 5 ppm of C6H6 and C7H8 gases. Incorporation of RGO NSs to ZnO NFs increased gas response of ZnO NFs to all gases and especially showed extremely high response of 2524.0 to 10 ppm H2 gas at 400oC. By decoration Au and Pd NPs on RGO NSs-loaded ZnO NFs, sensor response to 1 ppm CO and C6H6 gases showed high response of 23.5 and 11.8. When RGO NSs were loaded on CuO NFs, the sensor response of 0.5 wt% RGO NSs-loaded CuO NFs showed highest response to 10 ppm H2S gas. Finally, as the extremely enhanced selective NO2 sensor, SnO2-graphene and ZnO-graphene composites showed high response of 24.66 and 12.57 to 1ppm NO2 gas. These enhanced sensing properties will be attributed to the creation of defects due to microwave irradiation, increased surface area and generation of heterojunctions and homojunctions.

References

[1] J. H. Kim, A. Mirzaei, Y. Zheng, J. H. Lee, J. Y. Kim, H. W. Kim, S. S. Kim, Enhancement of H2S sensing performance of p-CuO nanofibers by loading p-reduced graphene oxide nanosheets, Sensors and Actuators B: Chemical 281, 453-461 (2019); doi: 10.106/j.snb.2018.10.144