稀有金属(英文版) 2020,39(06),651-658

Au-decorated porous structure graphene with enhanced sensing performance for low-concentration NO₂ detection

Yan-Yan Fan Hai-Ling Tu Yu Pang Feng Wei Hong-Bin Zhao Yi Yang Tian-Ling Ren

State Key Laboratory of Advanced Materials for Smart Sensing,General Research Institute for Nonferrous Metals

GRIMAT Engineering Institute Co.,Ltd

Institute of Microelectronics,Tsinghua University

General Research Institute for Nonferrous Metals

Beijing National Research Center for Information Science and Technology(BNRist),Tsinghua University

作者简介：*Hai-Ling Tu,e-mail:tuhl@grinm.com;

收稿日期：5 July 2019

基金：financially supported by National Natural Science Foundation of China(No.61874137);

Au-decorated porous structure graphene with enhanced sensing performance for low-concentration NO₂ detection

Yan-Yan Fan Hai-Ling Tu Yu Pang Feng Wei Hong-Bin Zhao Yi Yang Tian-Ling Ren

State Key Laboratory of Advanced Materials for Smart Sensing,General Research Institute for Nonferrous Metals

GRIMAT Engineering Institute Co.,Ltd

Institute of Microelectronics,Tsinghua University

General Research Institute for Nonferrous Metals

Beijing National Research Center for Information Science and Technology(BNRist),Tsinghua University

Abstract：

A recent progress in new emerging two-dimensional(2 D) materials has provided promising opportunity for gas sensing in ultra-low detectable concentration.In this work,we have demonstrated a flexible NO₂ gas sensor with porous structure graphene on polyethylene terephthalate substrates operating at room temperature.The gas sensor exhibited good performance with response of 1.2%and a fast response time within 30 s after exposure to50 × 10^-9 NO₂ gas.As porous structure of graphene increased the surface area,the sensor showed high sensitivity of ppb level for NO₂ detection.Au nanoparticles were decorated on the surface of the porous structure graphene skeleton,resulting in an incensement of response compared with pristine graphene.Au nanoparticles-decorated graphene exhibits not only better sensitivity(1.5-1.6 times larger than pristine graphene) for NO₂ gas detection,but also fast response.The sensor was found to be robust and sensitive under the cycling bending test,which could also be ascribed to the merits of graphene.This porous structure graphene-based gas sensor is expected to enable a simple and inexpensive flexible gas sensing platform.

Keyword：

Graphene; Au nanoparticles; Porous structure; Gas sensor; Nitrogen dioxide(NO₂);

Received： 5 July 2019

1 Introduction

Gas sensors have been attracting considerable attention since detection of poisonous and detrimental gases is vital for environmental protection and human health monitoring [ 1, 2, 3] .NO₂ is one kind of typical air pollutants formed from the emissions of power plants and combustion engine [ 3] and can produce ground-level ozone in the atmospheric reaction [ 4] .In addition,an increase in the amount of NO₂ in the environment causes acid rain and photo chemical smog [ 5, 6] .Recent research shows that exposure to low concentration of NO₂ will lead to decrements in lung function and significant morbidity in asthmatic inpiduals [ 7, 8] .Therefore,it is necessary to develop high-performance gas sensing devices capable of detecting NO₂ at low concentrations for human health and the environmental protection.

Since Novoselov et al.firstly demonstrated graphenebased devices for detecting NO₂ gas [ 9] ,which has stimulated considerable interest in potential application in NO₂sensing [ 6, 10, 11, 12, 13] .Many groups devoted to the research in different aspects,including materials [ 6] ,structures [ 13] and techniques [ 12] .Jung et al. [ 14] elevated the gas sensing behavior of pristine graphene by using soft lithographic patterning method.Yuan et al. [ 15] fabricated a chemoresistor-type NO₂ sensors by hybrid sulfonated reduced graphene oxide (rGO).Paul et al. [ 16] applied lithography technique to pattern graphene film into graphene nanomesh for gas sensing.Cho et al. [ 17] reported a heterostructural gas sensor consisting of graphene and MoS₂.Although much advancement has been made recently,there is still a need,considering practical applications,for continued improvement of the sensing performance of graphene-based NO₂ gas sensors.The major sensing properties including sensitivity and detection limit,and fabrication cost are still needed to be further improved.It is well known that structure strongly impacts the gas sensing activity.In particular,three-dimensional microstructure-based graphene gas sensors have stimulated intensive research due to its many attractive characteristics [ 18, 19] .The enlarged surface-to-volume ratio can offer more reactive sites and increase mass transport promoting the adsorption/desorption process of gas molecules [ 20, 21] .Another promising option to enhance the sensing performance is modification of graphene by noble metal nanoparticles.Owing to high conductivity of noble metal,it has been proved that noble metal nanoparticles play an effectively selective and catalytic role in gas sensing process [ 20, 22, 23] .Decoration of graphene with metal nanoparticles can also provide extra absorption sites for gas molecules to some extent.And researches in both theory and experiments aspects have confirmed the fact that noble metal nanoparticles loading onto graphene sheet is valid in promoting gas sensing behavior [ 24, 25] .Furthermore,since the interest in flexible devices has been increasing in perse applications [ 26, 27] ,such as health monitoring and handheld products,graphene with excellent mechanical properties would also be a good candidate for flexible gas sensing [ 28] .

In the present work,we fabricated a high-performance NO₂ gas sensor based on the pristine graphene with porous structure.The gas sensing performance was enhanced by decorating the porous structure graphene-based gas sensor with Au nanoparticles by soaking method.The experiments were operating at room temperature.And both sensors demonstrated a low detection of 50×10^-9 and fast response within 30 s to NO₂,attributing to the high conductivity of material and large surface area of the structure.Moreover,the sensors were constructed on flexible polyethylene terephthalate (PET) substrates and showed outstanding mechanical stability for bending cycles,indicating great potential of application in flexible electronics.

2 Experimental

2.1 Fabrication of gas sensor

The sequential procedures of the device fabrication are schematically illustrated in Fig.1a-e which do not involve photolithography or high vacuum and all that strict and complex experiment processes.The PET functioning as flexible substrates was firstly washed with deionized (DI)water and ethanol and then dried in N₂ flow.The commercial multilayer graphene with porous structure was grown on nickel foam by chemical vapor deposition (CVD)method under ambient pressure,and CH₄ was used as a carbon source.The large-scale porous structure graphene was cut into rectangle samples with a size of 1.0 cm×0.5 cm,which were immersed in 18 wt%HCl solution for12 h to remove the Ni foam.After the Ni foam had been completely etched,the samples were washed by DI water several times to remove the possible impurities adhered to the surface of the graphene skeleton.In addition,surface modification was developed via Au nanoparticles.The graphene sample without Ni foam was immersed into Au solution for another 10 h in order to make Au nanoparticles and the graphene in full contact.After that,the samples were transferred to PET substrates and kept in the fuming hood drying at room temperature.Finally,two copper wires as electrodes were connected on either side of the rectangle samples by Ag paste,and the entire sensors were then dried under the infrared lamp.Figure lf shows a photograph of our flexible graphene-based gas sensor with a structure of copper wire/graphene/copper wire/PET.

2.2 Characterization and performance testing

The morphologies and structures of the graphene were performed by a field-emission scanning electron microscopy (FESEM,Zeiss Merlin,Germany).Raman spectroscopy was characterized using a laser with a wavelength of 532 nm (HORIBA Inc.).The gas sensing performance was conducted by our homemade gas sensor test system.The cyclic bending performance was performed with a universal testing machine (Shimadzu AGS-X),and the resistance of the sample was in situ recorded by a digital multimeter (GOGOL DM3068).

3 Results and discussion

3.1 Structural characterization

Raman spectroscopy is one of the most facile methods to identify the quality of graphene,and Fig.2a shows Raman spectrum of the porous structure graphene with and without Au nanoparticles decoration.It can be clearly observed that there were two characteristic peaks at the position around1586 cm^-1 and 2725 cm^-1,belonging to the G peak and2D peak of graphene,respectively [ 14] .The sharp shape of G peak indicated the good organization of in-plane carbon atoms of the graphene.Also,the intensity ratio of 2D/G was found to be 0.66,which was an indication that the porous structure graphene was comprised of mono-layer to multilayer graphene sheet.In addition,there was no typical D peak observed,which is usually located at～1351 cm^-1and associated with the structural defect,confirming the high quality of the graphene.The same two characteristic peaks can also be observed in approximately the same position of the graphene decorated with Au nanoparticles.The reinforced intensity of Raman spectrum of the latter was induced by the surface enhancement of Raman scattering [ 29] .The characterization curves of current versus voltage (Ⅰ-Ⅴ) of both pristine graphene and Au-decorated graphene are shown in Fig.2b.The linear relationship indicated that it was the ohmic contact of the sensing layer and electrodes during the sensing processes.Besides,the slopes of the curves demonstrated that the conductivity of the graphene sensing layer with Au nanoparticles was better than that of the pristine graphene,which could facilitate the sensing performance to some extent.

Fig.1 a-e Schematic of gas sensor fabrication process with porous structure graphene;f photograph of flexible porous structure graphene-based gas sensor with a size of 1.0 cm×0.5 cm

Fig.2 a Raman spectra of porous structure pristine graphene and Au nanoparticles-decorated graphene (Au-graphene);bⅠ-Ⅴcharacteristics of porous structure pristine graphene and Au-graphene

SEM images of the pristine graphene network and graphene network decorated with Au nanoparticles are presented in Fig.3a,b,respectively.From the top-down SEM images,it can be observed that the continuous porous structure of graphene was well maintained without obvious damage after Ni foam skeleton was chemically etched.The graphene sheets interconnected with each other without upholder,forming a free-standing network structure.According to the SEM image,the size of the macro-pores of the graphene network ranged from 100μm to 350μm,and the width of the skeleton was about 60μm.The detailed information of structure of the graphene was characterized under high magnification.The thickness of each layer was about 1.5μm,as shown in Fig.3c.This porous structure enhanced the graphene’s special surface area to a great extent and provided more adsorption sites for gas molecules.Figure 3c also shows EDS test result of the pristine graphene sample,indicating that the elemental component was carbon.Figure 3d shows a magnified SEM image of porous structure graphene decorated with Au nanop articles.From both SEM images (Fig.3b,d),the Au nanoparticles with diameters from 150 nm to 500 nm were distributed on the surface of the whole graphene framework obviously.EDS mapping results (Fig.3d) confirmed the existence of the Au element in the sensor,mainly located at the surface of the graphene skeleton.

Fig.3 a SEM images of a pristine graphene with porous structure and b Au nanoparticles-decorated graphene with porous structure;c magnified SEM images and EDS results of c pristine graphene with porous structure and d Au nanoparticles-decorated graphene with porous structure

3.2 Gas sensing performance

To demonstrate the sensing performance of the devices,a homemade testing system was used.It should be noted that all the gas sensing measurements were operated at room temperature.The graphene samples were placed in a gastight chamber in turn and then the chamber was vacuumized.The N₂ was used as carrier gas,and the NO₂ gas was purged into the chamber at different concentrations controlled by mass flow controllers.The sensors were kept in the N₂ atmosphere for 10 min to achieve a steady state each time.The mass flow controllers were adjusted between N₂ and NO₂ gases periodically.The resistance variations of the device exposed to NO₂ gases from50×10^-9 to 1000×10^-9 in discrete steps were recorded.At first,the sensing performance of the flexible graphenebased gas sensors was examined without any bending.The transient resistance changes in pristine graphene based on porous structure as a function of time for different concentrations of NO₂ are shown in Fig.4a.Response is defined as:response=(R-R₀)/R₀×100%=ΔR/R₀×100%,where R₀ is the initial resistance value of the sensor in N₂,and R is the resistance value of the sensor in analyte gas.As exhibited in the diagram,the resistance variation was about 1.2%when the sensor was exposed to as low as50×10^-9 NO₂ which was the lowest reliably achievable concentration in our setup.The device underwent～3.4%resistance change at room temperature at the NO₂ concentration of 1000×10^-9.Overall,the resistance variation exhibited an increase with the ascending of the NO₂concentration,and the resistance recovered to original value after the chamber was purged with N₂ again.

Fig.4 a Resistance variation results of pristine porous structure graphene exposed to various concentrations of NO₂ (50×10^-9,100×10^-9,200×10^-9,1000×1^0-9);b response and recovery curve of pristine porous structure graphene exposed to 50×10^-6 NO₂;resistance variation results of Au-decorated porous structure graphene exposed to various concentrations of c NO₂ (50×10^-9,60×10^-9,80×10^-9,100×10^-9,200×10^-9,1000×10^-9) and d NH₃ (50×10^-9,100×10^-9,200×10^-9,500×10^-9,1000×10^-9,2000×10^-9)

Figure 4b shows the magnified gas sensing process upon exposure to 50×10^-9 NO₂ gas.An immediate change in resistance was observed as soon as the NO₂ was purged into the chamber.Notably,it only took 21 s for the sensor to respond to the gas and reached a steady state.Once NO₂gas was turned off,the resistance would change nearly to its initial value.The recovery time was measured to be20 s.Obviously,it is a proof that our porous structure graphene-based gas sensor is superior to other flexible NO₂gas sensors (Table 1) in terms of both detection limit and response time [ 29, 30, 31, 32, 33, 34] .

Further enhancement on the sensing performance was achieved by decorating noble metal.When the Au-decorated porous structure graphene-based gas sensor was exposed to the same concentration range of NO₂,the resistance change exhibited a similar variation tendency,as shown in Fig.4c.The major difference was that it had larger resistance variation by the introduction of Au nanoparticles,while the fast response and recovery performance were still as good as pristine graphene sensor.The response to NO₂ was promoted to 1.8%and 5.6%for concentrations of 50×10^-9 and 1000×10^-9,respectively,which were about 1.5-1.6 times that of pristine graphene sensor.The improvement of NO₂ gas sensing behavior may be explained from two main aspects.On one hand,since the resistance of the Au-decorated graphene was smaller than that of the pristine graphene,the introduction of Au contributed to the improvement of electrical conductivity and promotion of the transfer process of free electrons.On the other hand,the decorating of Au nanoparticles on the porous structure graphene can be regarded as the combination of zero dimension and three dimensions.The micro-particles adhered on the surface of graphene skeleton further enlarged the specific surface area,providing additional effective absorption sites for NO₂ molecules.Besides,the proved catalytic property in sensing process was not that obvious because the porous structure graphene-based gas sensor’s response was fast enough.

  下载原图

Table 1 NO₂ sensing performance of different flexible sensor devi-ces at room temperature

Fig.5 a Photograph of gas sensor with flexibility;b photograph of bending experiment;c resistance change of Au-decorated porous structure graphene sensor bending 250 cycles with a bending radius of～7 mm;d magnified resistance change during bending experiment;e resistance variation results of Au-decorated porous structure graphene exposed to various concentrations of NO₂ (50×10^-9,60×10^-9,80×10^-9,100×10^-9,200×10^-9,1000×10^-9) after bending 250 cycles

In addition,NH₃ sensing performance was also tested for comparison at room temperature.Figure 4d presents the resistance changes of the Au-decorated graphene-based gas sensor responded to different concentrations of NH₃ ranging from 50×10^-9 to 2000×10^-9.The comparatively poor sensing performance of the sensor for NH₃ gas detection was observed due to the relatively smaller charge transfer.The response was only～1.8%even the concentration of NH₃ had been raised to 2×10^-9 and the sensing state was not as stable as NO₂ detection.

Flexible electronics devices have been receiving extensive attention due to their application in human health care for real-time detecting of toxic gases.For this demonstration,the mechanical flexibility performance with high bending was investigated,which would contribute to applications of the porous structure graphene sensor in flexible electronics.An image of the flexible sensor is exhibited in Fig.5a.The durability of the flexible porous structure graphene-based gas sensor was valuated.Figure 5b shows a bending cycle experiment operated at radius curvature～7 mm of the Au-decorated graphene gas sensor upon 250 bending cycles.The measured resistance values and the magnified cycles of bending test results are shown in Fig.5c,d,respectively,which exhibited comparatively stable state without obvious resistance variation during the bending cycle process.The gas sensing behavior of the flexible sensor after bending250 times was also investigated with various NO₂ concentrations,and the results are illustrated in Fig.5e.It is revealed that the responses of this flexible sensor after bending experiment were almost consistent with the characteristics before bending test,indicating its excellent sensing stability for practical use.

4 Conclusion

In conclusion,we developed a high-performance gas sensor based on the porous structure of graphene.First,we showed that pristine graphene device could detect NO₂even at a concentration of 50×10^-9 within 30 s at room temperature.Second,the response was enhanced after Au nanoparticles was decorated onto the surface of the porous graphene by an immersed method.It is generally considered that Au NPs act as effective absorption sites for NO₂ molecules.Furthermore,the flexible characteristic of the sensor was kept stable after bending cycles.This porous structure graphene-based gas sensor with simple and efficient fabrication processes exhibited several remarkable sensing features,such as fast response,good reversibility and mechanical stability,and low detection limit down to ppb level at room temperature.Therefore,this would offer a new possible approach to gas sensing applications in environmental monitoring and flexible electronics.

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