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2D-NiO nanosheet-based hybrid nanostructure and its use in highly sensitive NO2 sensors
Sensors and Actuators B 185 (2013) 701–705
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Short communication
Fabrication of a novel 2D-graphene/2D-NiO nanosheet-based hybrid nanostructure and its use in highly sensitive NO2 sensors Le Thuy Hoa, Huynh Ngoc Tien, Van Hoang Luan, Jin Suk Chung, Seung Hyun Hur ∗ School of Chemical Engineering and Bioengineering, University of Ulsan, Daehak-ro 93, Nam-gu, Ulsan 680-749, Republic of Korea
a r t i c l e
i n f o
Article history: Received 4 January 2013 Received in revised form 7 April 2013 Accepted 15 May 2013 Available online 27 May 2013 Keywords: NiO nanosheets Reduced graphene oxide NO2 gas sensors Hybrid structures
a b s t r a c t A highly sensitive gas sensor based on novel hybrid structures composed of 2D graphene and 2D NiO nanosheets (NSs) is fabricated using a low-cost, low temperature and large area scalable solution-based process. The highly developed hierarchically porous structures of 2D NiO sheets are grown on reduced graphene oxide (RGO) surfaces. Sensors fabricated with hybrid structures showed a responsivity and sensitivity two orders higher than that of a NiO NS alone toward NO2 even at 1 ppm level. This is attributed to the effective carrier transfer from NiO NS to graphene and to the well-developed 2D NiO structure. The sensing results are similar when reducing gases such as H2 , NH3 and H2 S are tested, but the responsivity toward NO2 was highest among all the gases tested. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The development of highly sensitive gas sensors has been regarded as the one of the most important issues in monitoring pollutant gases such as NOx and SOx emitted from vehicles. Due to its high resistance to various chemicals, nickel oxide (NiO) is widely used for gas sensors [1], supercapacitors [2,3], lithium ion batteries [4], and electrochromic devices [5,6] with various structures including thin films [1,5,6], nanotubes [4], hierarchical porous networks [2], and flake-like morphologies [3]. Various NiO nanostructures can be fabricated by sputtering [1,5], sol–gel processing [6], template-assisted forming [4], and hydrothermal synthesis [3]. Graphene has been extensively explored for solar cells [7] and organic light emitting diodes [8] due to its relatively low work function (∼4.4 eV) and excellent optical transparency as well as superior mechanical strength, thermal stability, and electrical conductivity. Recent studies showed that hybrid structures of graphene and inorganic semiconductors exhibited highly improved optical properties by the enhanced charge transfer effects [9,10]. In this paper, we report a highly sensitive NO2 gas sensor based on the new hybrid structures of 2D graphene and 2D NiO nanosheets (NSs) fabricated by a low-cost, low temperature, and large area scalable solution-based process [11]. The graphene oxide (GO) used in this study can provide numerous anchoring sites of Ni precursors due to its high number of functional groups [10], which
∗ Corresponding author. Tel.: +82 2 259 1028; fax: +82 52 259 1689. E-mail address: [email protected] (S.H. Hur). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.05.050
results in the uniform growth of NiO films on GO surfaces. The NO2 sensors fabricated in this study showed about hundred times greater NO2 gas responsivity and sensitivity even at a few ppm, as compared to those of the NiO NS alone.
2. Experimental As shown in Scheme 1, the hybrid structure was fabricated by two steps: (i) creating GO films on the electrodes and (ii) growing NiO nanosheets on GO films. The GO suspension was prepared by expanding graphite via the Hummers method followed by spray-coating on the sensing electrodes (Fig. S1, see supporting information), as described in a previous report [12]. A GO solution (10 ml of 10 mg/ml H2 O) was mixed with 1 ml of hydrazine and 5 ml of deionized water (DI water), followed by ultrasonication for 4 min. Then, the solution was sprayed on the sensing electrodes on the Si/SiO2 wafer at 250 ◦ C to convert GO to reduced graphene oxide (RGO). Next, the Ni seed solution was prepared by mixing nickel acetate tetrahydrate (Ni(OCOCH3 )2 ·4H2 O, Sigma Aldrich) with 2-methoxyethanol (CH3 OCH2 CH2 OH, Sigma Aldrich). Then, diethanolamine (HN(CH2 CH2 OH)2 , Sigma Aldrich) was dropped into the Ni seed solution under continuous magnetic stirring at 70 ◦ C for 2 h. The solution was spin-coated on the RGO and annealed in air at 400 ◦ C for 4 h. Then, the Ni seed layer was exposed to the mixed aqueous solution of 0.02 M nickel nitrate hexahydrate (Ni(NO3 )2 ·6H2 O 98%, Sigma Aldrich) and 0.02 M hexamethylenetetramine (C6 H12 N4 , Sigma Aldrich) at 90 ◦ C for 5 h to grow NiO nanosheets. Finally, the NiO NS/RGO on the sensing electrodes was rinsed and dried in a vacuum oven.
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Scheme 1. The fabrication scheme of hybrid structures of 2D graphene and 2D NiO nanosheets.
The structures of NiO NS/RGO, NiO and RGO were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and Raman spectra. The sensing characteristics were characterized using a MST-5000 chamber (MS-Tech) at 700 torr. Gas flow was controlled by a mass flow controller (GMC 1200, ATOVAC). A semiconductor parameter analyzer (Hewlell-Packard-4155A) was used to measure the resistance of the sensing devices. 3. Results and discussion Fig. 1 shows SEM images of NiO NS grown on the SiO2 /Si wafer and RGO at the same growing conditions. The height of NiO NSs was about 1.167 m. NiO NS grown on the RGO exhibited better developed hierarchically porous structures and perpendicular alignment to the substrate surface over those grown on the SiO2 /Si wafer, which may be due to the high number of functional groups on the RGO surface. The thicknesses and height of NiO NS are 20-30 nm and 1–3 m, respectively, which are similar to those of previously reported values [11]. AFM images of RGO and the EDS results are shown in Figs. S2 and S3. The thickness of RGO used in this study was measured to be about 1 nm, and carbon was found only in the NiO NS grown on the RGO. The XRD spectra were used to verify the crystal structure and phase purity of the samples used in this study. As shown in Fig. 2a, both show the same peaks at 37.30◦ , 43.28◦ and 65.23◦ , which correspond to the (1 1 1), (2 0 0) and (2 2 0) planes of NiO nanosheets,
respectively [2,11]. In contrast, NiO NS grown on the RGO exhibits a highly enhanced crystallinity with a (2 2 0) orientation, which might contribute to more developed hierarchical structures of NiO NS. As depicted in Fig. 2b, the bond length of O O in the (220) NiO crystal structure is about 2.9 A˚ [13], which is similar to the distance ˚ in the graphene [14]. Instead between confronting C C (2.84 A) ˚ which is narrower than that bond length of Si O was about 1.56 A, of O O in the (2 2 0) NiO crystal structure [15]. Hence, it can be concluded that the NiO NS on the RGO surface can grow better in the (2 2 0) orientation than on the SiO2 /Si wafer. X-ray photoelectron spectroscopy (XPS) was used to examine the oxidation state of the samples. As shown in Fig. 3a, the XPS survey spectra of NiO NS grown on RGO sheets clearly indicate the existence of nickel, oxygen and carbon. The Ni 2p peak shows two edge splits by spin-orbital coupling (Fig. 3b), 2p1/2 at ∼872 eV and 2p3/2 at ∼845–869 eV, and a satellite peak of 2p3/2 at ∼861.8 eV, which are all similar to a previous report [16]. The XPS of GO (Fig. 3c) exhibits peaks at 284.6, 286.7, 287.7 and 288.5 eV corresponding to C C in aromatic rings, C O (epoxy and alkoxy), C O (carbonyl group) and O C O(carboxyl group), respectively [12]. After the reaction, the intensities of C O, C O and O C O all decreased, but that of C C increased, which indicates that the GO was thoroughly reduced to RGO [17]. The reduction of GO also can be confirmed by Raman spectra, as shown in Fig. 4. The two strong peaks of the D-band (∼1328 cm−1 ) and G-band (∼1595 cm−1 ) were observed, which correspond to the diamondoid and graphitic graphene structures, respectively. It is
Fig. 1. SEM images of NiO NS grown on (a) SiO2 /Si wafer and (b) RGO with the height of NiO NS (cross-sectional SEM image is shown in inset).
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Fig. 2. (a) XRD spectra of NiO NS grown on the (i) RGO and (ii) SiO2 /Si wafer. (b) Atomic structure of graphene and NiO (2 2 0).
well known that the intensity ratio of the D and G band (ID /IG ) is strongly related to the amount of functional groups of RGO, and the increased ID /IG value of RGO (1.137) compared to that of GO (1.051) indicates restoration of C C bonds after hydrazine reduction [18]. NO2 is emitted from vehicles and industry and is known as one of the very harmful gases for human, animals and the environment,
even at low concentrations [19,20]. Therefore, the fabrication of a highly sensitive gas sensor that can detect ppm level NO2 is regarded as a very important issue. Fig. 5 shows the NO2 responsivity (RS, (Ra − Rg )/Rg (%)) and sensitivity (RS/conc. of NO2 ) of the NiO NS/RGO and NiO NS alone, where Ra and Rg are the resistance of the sensing layer measured in an atmosphere of only N2
Fig. 3. XPS survey spectra of (a) NiO NS and NiO NS/RGO. (b) Ni 2p peaks of NiO NS/RGO. C 1s peaks of (c) GO and (d) RGO.
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Fig. 6. Responsivity of NiO NS and NiO NS/RGO sensors in various gases, where the concentrations of NO2 , H2 S, NH3 were 100 ppm, and H2 was 4%. Fig. 4. Raman spectra of GO and RGO.
and the NO2 /N2 gas mixture, respectively. The resistance of sensors decreased when exposed to NO2 and nearly returned to its initial value after NO2 was eliminated, which indicates a reversible bonding between the NO2 molecule and NiO surface. When the two sensors were compared, it can be clearly seen that a hybrid structure of NiO NS/RGO showed an about 100-fold higher responsivity than that of NiO NS alone even at 1 ppm. Moreover, the sensitivity was also about 90-fold higher when the hybrid structure was used over NiO NS alone (Fig. 5b). The enhanced response of the hybrid structure of the NiO NS/RGO may be due to the enhanced charge transfer between NiO and RGO as well as the more developed hierarchically porous structures of NiO NS on RGO. As proposed in the following reactions, when the NiO NS is exposed to NO2 gas molecules, the concentration of hole carriers on the surface of NiO increases due to the loss of an electron as the NO2 has a higher electron affinity (∼2.28 eV) than the pre-adsorbed oxygen (0.43 eV) [21], which results in a decrease in resistance in the NiO layer because the NiO is a p-type semiconductor [21]. Due to the low conductivity of NiO, the adsorption of further NO2 molecules is limited by the accumulation of holes. Instead, as shown in the inset of Fig. 5a, the transfer of hole carriers from the valence band of NiO NS (−4.64 eV [22]) to the RGO (work function: −4.40 eV [23]) may suppress the accumulation of holes in NiO NS and thus
further increase the adsorption of NO2 , which results in enhanced responsivity and sensitivity. NO2 (gas) + e− ↔ NO2 − (ads)
(a)
NO2 − (ads) + O− (ads) + 2e− ↔ NO(gas) + 2O2− (ads)
(b)
The responsivity of the fabricated gas sensors was measured with reducing gases such as H2 S, H2 and NH3 . As shown in Fig. 6, the responsivity of the fabricated NiO NS/RGO sensor was highest toward NO2 among all the gases used. The increase in sensor resistance when it was exposed to a reducing gas can be explained by the following reactions. H2 (gas) + O− (ads) → H2 O + e−
(c)
The interaction between reducing gases and adsorbed oxygen in the NiO surface releases free electrons that neutralize the holes in the NiO, which results in a decrease in the number of hole carriers in NiO and, consequently, an increase in sensor resistance. Similar to the NO2 sensing case, the NiO NS/RGO sensor exhibited enhanced responsivity compared to that of NiO NS for all tested reducing gases, which also can be attributed to the enhanced electron transfer from NiO NS to RGO.
Fig. 5. (a) Responsivity of NiO NS and NiO NS/RGO sensors in various NO2 concentrations at 200 ◦ C; the inset depicts the carrier transfer and (b) the sensitivity of NiO NS and NiO NS/RGO at different NO2 gas concentrations.
L.T. Hoa et al. / Sensors and Actuators B 185 (2013) 701–705
4. Conclusions In summary, we showed a highly sensitive hybrid NO2 sensor composed of 2D NiO NS and 2D RGO fabricated using a facile solution based process. More fully developed (2 2 0) oriented hierarchically porous structures of 2D NiO sheets were grown on the RGO surfaces compared to those grown on the SiO2 /Si wafer. With the large surface area of NiO NS and the improved carrier transport from NiO NS to the highly conductive RGO, NiO NS/RGO hybrid sensors showed about 100-fold greater responsivity and 90-fold increased sensitivity compared to that of a NiO NS sensor even at 1 ppm concentration. We believe that this type of hybrid structure can be effectively used in many types of gas-sensing applications due to its simple processing and superior performance.
[11]
[12]
[13]
[14]
[15]
[16]
Acknowledgement
[17]
This work was supported by the 2013 Research Fund of University of Ulsan.
[18]
Appendix A. Supplementary data
[19]
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2013.05.050.
[20]
[21]
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Biographies Le ThuyHoa is a Ph. D. student in the School of Chemical Engineering, University of Ulsan, Korea. He is currently investigating nano-structured sensors for gas and biomaterial detection. Huynh Ngoc Tien is a Ph. D. student in the School of Chemical Engineering, University of Ulsan, Korea. He is currently studying the preparation process of graphene and its use in the electronic devices. Van Hoang Luan is a Ph. D. student in the School of Chemical Engineering, University of Ulsan, Korea. He is currently studying the fabrication of 3D graphenenetworks and its use in the energy and environmental applications. Jin Suk Chung is a professor in the School of Chemical Engineering, University of Ulsan, Korea. He received his Ph. D. degree from the Department of Chemical Engineering, Seoul National University, Korea in 1997. His current scientific interests are graphene and related materials. Seung Hyun Hur is anassociate professor in the School of Chemical Engineering, University of Ulsan, Korea. He received his Ph. D. degree from the Department of Chemical Engineering and Biomolecular Engineering, KAIST, Korea in 2005. His current scientific interests are nanostructures and their applications on the electronic devices such as sensors and transistors.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Short communication
Fabrication of a novel 2D-graphene/2D-NiO nanosheet-based hybrid nanostructure and its use in highly sensitive NO2 sensors Le Thuy Hoa, Huynh Ngoc Tien, Van Hoang Luan, Jin Suk Chung, Seung Hyun Hur ∗ School of Chemical Engineering and Bioengineering, University of Ulsan, Daehak-ro 93, Nam-gu, Ulsan 680-749, Republic of Korea
a r t i c l e
i n f o
Article history: Received 4 January 2013 Received in revised form 7 April 2013 Accepted 15 May 2013 Available online 27 May 2013 Keywords: NiO nanosheets Reduced graphene oxide NO2 gas sensors Hybrid structures
a b s t r a c t A highly sensitive gas sensor based on novel hybrid structures composed of 2D graphene and 2D NiO nanosheets (NSs) is fabricated using a low-cost, low temperature and large area scalable solution-based process. The highly developed hierarchically porous structures of 2D NiO sheets are grown on reduced graphene oxide (RGO) surfaces. Sensors fabricated with hybrid structures showed a responsivity and sensitivity two orders higher than that of a NiO NS alone toward NO2 even at 1 ppm level. This is attributed to the effective carrier transfer from NiO NS to graphene and to the well-developed 2D NiO structure. The sensing results are similar when reducing gases such as H2 , NH3 and H2 S are tested, but the responsivity toward NO2 was highest among all the gases tested. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The development of highly sensitive gas sensors has been regarded as the one of the most important issues in monitoring pollutant gases such as NOx and SOx emitted from vehicles. Due to its high resistance to various chemicals, nickel oxide (NiO) is widely used for gas sensors [1], supercapacitors [2,3], lithium ion batteries [4], and electrochromic devices [5,6] with various structures including thin films [1,5,6], nanotubes [4], hierarchical porous networks [2], and flake-like morphologies [3]. Various NiO nanostructures can be fabricated by sputtering [1,5], sol–gel processing [6], template-assisted forming [4], and hydrothermal synthesis [3]. Graphene has been extensively explored for solar cells [7] and organic light emitting diodes [8] due to its relatively low work function (∼4.4 eV) and excellent optical transparency as well as superior mechanical strength, thermal stability, and electrical conductivity. Recent studies showed that hybrid structures of graphene and inorganic semiconductors exhibited highly improved optical properties by the enhanced charge transfer effects [9,10]. In this paper, we report a highly sensitive NO2 gas sensor based on the new hybrid structures of 2D graphene and 2D NiO nanosheets (NSs) fabricated by a low-cost, low temperature, and large area scalable solution-based process [11]. The graphene oxide (GO) used in this study can provide numerous anchoring sites of Ni precursors due to its high number of functional groups [10], which
∗ Corresponding author. Tel.: +82 2 259 1028; fax: +82 52 259 1689. E-mail address: [email protected] (S.H. Hur). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.05.050
results in the uniform growth of NiO films on GO surfaces. The NO2 sensors fabricated in this study showed about hundred times greater NO2 gas responsivity and sensitivity even at a few ppm, as compared to those of the NiO NS alone.
2. Experimental As shown in Scheme 1, the hybrid structure was fabricated by two steps: (i) creating GO films on the electrodes and (ii) growing NiO nanosheets on GO films. The GO suspension was prepared by expanding graphite via the Hummers method followed by spray-coating on the sensing electrodes (Fig. S1, see supporting information), as described in a previous report [12]. A GO solution (10 ml of 10 mg/ml H2 O) was mixed with 1 ml of hydrazine and 5 ml of deionized water (DI water), followed by ultrasonication for 4 min. Then, the solution was sprayed on the sensing electrodes on the Si/SiO2 wafer at 250 ◦ C to convert GO to reduced graphene oxide (RGO). Next, the Ni seed solution was prepared by mixing nickel acetate tetrahydrate (Ni(OCOCH3 )2 ·4H2 O, Sigma Aldrich) with 2-methoxyethanol (CH3 OCH2 CH2 OH, Sigma Aldrich). Then, diethanolamine (HN(CH2 CH2 OH)2 , Sigma Aldrich) was dropped into the Ni seed solution under continuous magnetic stirring at 70 ◦ C for 2 h. The solution was spin-coated on the RGO and annealed in air at 400 ◦ C for 4 h. Then, the Ni seed layer was exposed to the mixed aqueous solution of 0.02 M nickel nitrate hexahydrate (Ni(NO3 )2 ·6H2 O 98%, Sigma Aldrich) and 0.02 M hexamethylenetetramine (C6 H12 N4 , Sigma Aldrich) at 90 ◦ C for 5 h to grow NiO nanosheets. Finally, the NiO NS/RGO on the sensing electrodes was rinsed and dried in a vacuum oven.
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Scheme 1. The fabrication scheme of hybrid structures of 2D graphene and 2D NiO nanosheets.
The structures of NiO NS/RGO, NiO and RGO were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and Raman spectra. The sensing characteristics were characterized using a MST-5000 chamber (MS-Tech) at 700 torr. Gas flow was controlled by a mass flow controller (GMC 1200, ATOVAC). A semiconductor parameter analyzer (Hewlell-Packard-4155A) was used to measure the resistance of the sensing devices. 3. Results and discussion Fig. 1 shows SEM images of NiO NS grown on the SiO2 /Si wafer and RGO at the same growing conditions. The height of NiO NSs was about 1.167 m. NiO NS grown on the RGO exhibited better developed hierarchically porous structures and perpendicular alignment to the substrate surface over those grown on the SiO2 /Si wafer, which may be due to the high number of functional groups on the RGO surface. The thicknesses and height of NiO NS are 20-30 nm and 1–3 m, respectively, which are similar to those of previously reported values [11]. AFM images of RGO and the EDS results are shown in Figs. S2 and S3. The thickness of RGO used in this study was measured to be about 1 nm, and carbon was found only in the NiO NS grown on the RGO. The XRD spectra were used to verify the crystal structure and phase purity of the samples used in this study. As shown in Fig. 2a, both show the same peaks at 37.30◦ , 43.28◦ and 65.23◦ , which correspond to the (1 1 1), (2 0 0) and (2 2 0) planes of NiO nanosheets,
respectively [2,11]. In contrast, NiO NS grown on the RGO exhibits a highly enhanced crystallinity with a (2 2 0) orientation, which might contribute to more developed hierarchical structures of NiO NS. As depicted in Fig. 2b, the bond length of O O in the (220) NiO crystal structure is about 2.9 A˚ [13], which is similar to the distance ˚ in the graphene [14]. Instead between confronting C C (2.84 A) ˚ which is narrower than that bond length of Si O was about 1.56 A, of O O in the (2 2 0) NiO crystal structure [15]. Hence, it can be concluded that the NiO NS on the RGO surface can grow better in the (2 2 0) orientation than on the SiO2 /Si wafer. X-ray photoelectron spectroscopy (XPS) was used to examine the oxidation state of the samples. As shown in Fig. 3a, the XPS survey spectra of NiO NS grown on RGO sheets clearly indicate the existence of nickel, oxygen and carbon. The Ni 2p peak shows two edge splits by spin-orbital coupling (Fig. 3b), 2p1/2 at ∼872 eV and 2p3/2 at ∼845–869 eV, and a satellite peak of 2p3/2 at ∼861.8 eV, which are all similar to a previous report [16]. The XPS of GO (Fig. 3c) exhibits peaks at 284.6, 286.7, 287.7 and 288.5 eV corresponding to C C in aromatic rings, C O (epoxy and alkoxy), C O (carbonyl group) and O C O(carboxyl group), respectively [12]. After the reaction, the intensities of C O, C O and O C O all decreased, but that of C C increased, which indicates that the GO was thoroughly reduced to RGO [17]. The reduction of GO also can be confirmed by Raman spectra, as shown in Fig. 4. The two strong peaks of the D-band (∼1328 cm−1 ) and G-band (∼1595 cm−1 ) were observed, which correspond to the diamondoid and graphitic graphene structures, respectively. It is
Fig. 1. SEM images of NiO NS grown on (a) SiO2 /Si wafer and (b) RGO with the height of NiO NS (cross-sectional SEM image is shown in inset).
L.T. Hoa et al. / Sensors and Actuators B 185 (2013) 701–705
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Fig. 2. (a) XRD spectra of NiO NS grown on the (i) RGO and (ii) SiO2 /Si wafer. (b) Atomic structure of graphene and NiO (2 2 0).
well known that the intensity ratio of the D and G band (ID /IG ) is strongly related to the amount of functional groups of RGO, and the increased ID /IG value of RGO (1.137) compared to that of GO (1.051) indicates restoration of C C bonds after hydrazine reduction [18]. NO2 is emitted from vehicles and industry and is known as one of the very harmful gases for human, animals and the environment,
even at low concentrations [19,20]. Therefore, the fabrication of a highly sensitive gas sensor that can detect ppm level NO2 is regarded as a very important issue. Fig. 5 shows the NO2 responsivity (RS, (Ra − Rg )/Rg (%)) and sensitivity (RS/conc. of NO2 ) of the NiO NS/RGO and NiO NS alone, where Ra and Rg are the resistance of the sensing layer measured in an atmosphere of only N2
Fig. 3. XPS survey spectra of (a) NiO NS and NiO NS/RGO. (b) Ni 2p peaks of NiO NS/RGO. C 1s peaks of (c) GO and (d) RGO.
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Fig. 6. Responsivity of NiO NS and NiO NS/RGO sensors in various gases, where the concentrations of NO2 , H2 S, NH3 were 100 ppm, and H2 was 4%. Fig. 4. Raman spectra of GO and RGO.
and the NO2 /N2 gas mixture, respectively. The resistance of sensors decreased when exposed to NO2 and nearly returned to its initial value after NO2 was eliminated, which indicates a reversible bonding between the NO2 molecule and NiO surface. When the two sensors were compared, it can be clearly seen that a hybrid structure of NiO NS/RGO showed an about 100-fold higher responsivity than that of NiO NS alone even at 1 ppm. Moreover, the sensitivity was also about 90-fold higher when the hybrid structure was used over NiO NS alone (Fig. 5b). The enhanced response of the hybrid structure of the NiO NS/RGO may be due to the enhanced charge transfer between NiO and RGO as well as the more developed hierarchically porous structures of NiO NS on RGO. As proposed in the following reactions, when the NiO NS is exposed to NO2 gas molecules, the concentration of hole carriers on the surface of NiO increases due to the loss of an electron as the NO2 has a higher electron affinity (∼2.28 eV) than the pre-adsorbed oxygen (0.43 eV) [21], which results in a decrease in resistance in the NiO layer because the NiO is a p-type semiconductor [21]. Due to the low conductivity of NiO, the adsorption of further NO2 molecules is limited by the accumulation of holes. Instead, as shown in the inset of Fig. 5a, the transfer of hole carriers from the valence band of NiO NS (−4.64 eV [22]) to the RGO (work function: −4.40 eV [23]) may suppress the accumulation of holes in NiO NS and thus
further increase the adsorption of NO2 , which results in enhanced responsivity and sensitivity. NO2 (gas) + e− ↔ NO2 − (ads)
(a)
NO2 − (ads) + O− (ads) + 2e− ↔ NO(gas) + 2O2− (ads)
(b)
The responsivity of the fabricated gas sensors was measured with reducing gases such as H2 S, H2 and NH3 . As shown in Fig. 6, the responsivity of the fabricated NiO NS/RGO sensor was highest toward NO2 among all the gases used. The increase in sensor resistance when it was exposed to a reducing gas can be explained by the following reactions. H2 (gas) + O− (ads) → H2 O + e−
(c)
The interaction between reducing gases and adsorbed oxygen in the NiO surface releases free electrons that neutralize the holes in the NiO, which results in a decrease in the number of hole carriers in NiO and, consequently, an increase in sensor resistance. Similar to the NO2 sensing case, the NiO NS/RGO sensor exhibited enhanced responsivity compared to that of NiO NS for all tested reducing gases, which also can be attributed to the enhanced electron transfer from NiO NS to RGO.
Fig. 5. (a) Responsivity of NiO NS and NiO NS/RGO sensors in various NO2 concentrations at 200 ◦ C; the inset depicts the carrier transfer and (b) the sensitivity of NiO NS and NiO NS/RGO at different NO2 gas concentrations.
L.T. Hoa et al. / Sensors and Actuators B 185 (2013) 701–705
4. Conclusions In summary, we showed a highly sensitive hybrid NO2 sensor composed of 2D NiO NS and 2D RGO fabricated using a facile solution based process. More fully developed (2 2 0) oriented hierarchically porous structures of 2D NiO sheets were grown on the RGO surfaces compared to those grown on the SiO2 /Si wafer. With the large surface area of NiO NS and the improved carrier transport from NiO NS to the highly conductive RGO, NiO NS/RGO hybrid sensors showed about 100-fold greater responsivity and 90-fold increased sensitivity compared to that of a NiO NS sensor even at 1 ppm concentration. We believe that this type of hybrid structure can be effectively used in many types of gas-sensing applications due to its simple processing and superior performance.
[11]
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Acknowledgement
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This work was supported by the 2013 Research Fund of University of Ulsan.
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Appendix A. Supplementary data
[19]
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2013.05.050.
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Biographies Le ThuyHoa is a Ph. D. student in the School of Chemical Engineering, University of Ulsan, Korea. He is currently investigating nano-structured sensors for gas and biomaterial detection. Huynh Ngoc Tien is a Ph. D. student in the School of Chemical Engineering, University of Ulsan, Korea. He is currently studying the preparation process of graphene and its use in the electronic devices. Van Hoang Luan is a Ph. D. student in the School of Chemical Engineering, University of Ulsan, Korea. He is currently studying the fabrication of 3D graphenenetworks and its use in the energy and environmental applications. Jin Suk Chung is a professor in the School of Chemical Engineering, University of Ulsan, Korea. He received his Ph. D. degree from the Department of Chemical Engineering, Seoul National University, Korea in 1997. His current scientific interests are graphene and related materials. Seung Hyun Hur is anassociate professor in the School of Chemical Engineering, University of Ulsan, Korea. He received his Ph. D. degree from the Department of Chemical Engineering and Biomolecular Engineering, KAIST, Korea in 2005. His current scientific interests are nanostructures and their applications on the electronic devices such as sensors and transistors.