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Enhanced NO2 detection using hierarchical porous ZnO nanoflowers modified with graphene
Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Enhanced NO2 detection using hierarchical porous ZnO nanoflowers modified with graphene Jing Li, Weiguang Zhang, Jianbo Sun n The Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, School of Physics and Electronic Engineering, Harbin Normal University, Harbin 10025, China
art ic l e i nf o
a b s t r a c t
Article history: Received 24 February 2016 Received in revised form 10 March 2016 Accepted 10 March 2016
Because of their potential applications in gas sensing and catalysis, reduced graphene oxide (RGO) and ZnO have been the focus of much recent attention. However, few reported materials have been produced via the combination of hierarchical ZnO structures with RGO to achieve high sensing performances. In this paper, a hydrothermal method was used to synthesize hierarchical porous ZnO nanoflowers, which were then combined with graphene to enhance their sensing performances. The rapid detection of 1 ppm NO2 was achieved at 174 °C. The morphologies and structures of these materials were characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. Photoluminescence measurements and X-ray photoelectron spectroscopy were also used to investigate the mechanism of gas sensing by these materials. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Hierarchical and porous structures Graphene Gas sensing
1. Introduction Hierarchical architectures have promising characteristics for sensing applications. Three-dimensional (3D) hierarchical architectures such as hollow spheres, flower-like structures, and urchin-like structures have been assembled using nanoscale building blocks (e.g., 0D grain nanodots, 1D nanowires or nanorods, and 2D nanosheets) [1–4]. These architectures often possess large surface areas, which provide more active sites and favorable transport pathways for gas molecules. As a consequence, these structures have reduced response and recovery times and increased sensor sensitivities at low temperatures [5–7]. Sensors employing ZnO hierarchical structures have been a topic of much recent research [8–10]. However, high operating temperatures and low responses have limited their development for practical applications. Recently, the synergistic effects of hierarchical ZnO combined with different materials have been explored to enhance the sensing performance of ZnO. The use of dopants (e.g., Au [11], Ag [12], Pt [13], and Pd [14]) and heterogeneous semiconductor composites (e.g., SnO2 [15], CuO [16], and graphene [17]) has significantly improved the sensing performances of ZnO-based gas sensors. Currently composites of graphene and ZnO are the most promising due to the fact that graphene has a high electron mobility, a good chemical stability, and a low cost. Chemically-derived graphene is particularly expected to aid in the development of highly sensitive chemical sensors [18,19]. According to recent research on composites of graphene and n
Corresponding author. E-mail addresses: [email protected], [email protected] (W. Zhang).
ZnO, these material have increased sensitivities when they contain chemically reduced graphene oxide (GO) or when their GO surface is functionalized to increase the number of adsorption sites [20]. For example, Zou et al. reported the excellent field emission, gas sensing, and photocatalytic properties of ZnO nanorods on reduced graphene sheets [22]. Uddin et al. reported the synthesis and acetylene sensing properties of highly dispersed ZnO nanoparticles on graphene surfaces [23]. Abideen et al. synthesized ZnO nanofibers on reduced GO nanosheets for gas detection [24]. Overall, the combination of hierarchical ZnO with functionalized graphene is a promising route towards the development of gas sensitive materials with high sensitivities and low working temperatures. Such materials have rarely been reported in the literature. In this work, a hydrothermal method was used to synthesize hierarchical porous ZnO nanoflowers modified with reduced GO (RGO). These materials possessed improved sensing characteristics. The porous nature of the materials provided a higher specific surface area and more active sites. Their hierarchical structure also contained channels for rapid gas adsorption. Functionalization of the RGO at the RGO/ZnO interface further enhanced the performances of the ZnO nanoflowers.
2. Experimental section 2.1. Sample preparation All reagents were of analytical grade and were used without further purification.
http://dx.doi.org/10.1016/j.ceramint.2016.03.083 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: J. Li, et al., Enhanced NO2 detection using hierarchical porous ZnO nanoflowers modified with graphene, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.03.083i
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2.1.1. Synthesis of graphene oxide GO was prepared from a fine graphite powder using the modified Hummers’ method [25]. In brief, 3 g of KMnO4 was mixed with 0.5 g of NaNO3, 25 mL of H2SO4, and 0.2 g of graphite in an ice bath under stirring. The mixture was maintained at 0 °C for 24 h before being stirred at 35 °C for 30 min. Then, 50 mL of H2O2 was added to the mixture, which was heated at 95 °C for 15 min before 115 mL of distilled water was added to the mixture. Unreacted KMnO4 was removed by the addition of 10 mL of H2O2. The oxidized graphene was purified by washing with distilled water and absolute ethanol and dried in air at room temperature. 2.1.2. Synthesis of ZGO composites In a typical experiment, a 0.005 M equimolar mixture of Zn (NO3)2 6H2O and hexamethylenetetramine (HMT) was prepared in 50 mL of distilled water. H2C2O4 was then added to this solution to reach an oxalate ion-to-Zn2 þ molar ratio of 0.1. After 30 min of agitation, a homogeneous solution was obtained. This mixture was sealed in a Teflon-lined stainless-steel autoclave with a 40 mL capacity at 90 °C for 3 h. It was then allowed to cool to room temperature. The white precipitate product was collected and rinsed several times with distilled water and absolute ethanol. Finally, this hydrothermal product was filtered and dried at 60 °C for 6 h. In order to obtain porous crystalline ZnO nanosheets, the hydrothermal product was heated at 300 °C for 1 h before being further heated at 600 °C for 2 h. Different amounts of GO (0.02 mg, 0.05 mg, 0.1 mg, and 0.2 mg) were mixed with the hydrothermal product in 50 mL of distilled water and agitated for 5 min. Subsequently, the mixtures were transferred into a Teflon-lined stainless-steel autoclave and held at 90 °C for 3 h. After the hydrothermal procedure, the autoclave was allowed to cool to room temperature. The series of different Zn/GO (ZGO) composites containing 0.02 mg, 0.05 mg, 0.1 mg, and 0.2 mg of GO are referred to here as ZGO002, ZGO005, ZGO01, and ZGO02, respectively. 2.2. Characterization The crystallographic structures of the as-prepared products were determined using X-ray diffraction (XRD, D/max2600, Rigaku, Japan) with Cu Kα radiation (λ ¼1.5418 Å) at a scanning speed of 0.4° min 1 in the 2θ range from 10° to 80°. Their morphologies were investigated using scanning electron microscopy (SEM, SU70, Hitachi, Japan) at an accelerating voltage of 15 kV. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were performed using an FEI Tecnai F20 microscope operated at 200 kV. For TEM observation, the ZGO005 powder was ultrasonically dispersed in ethanol and dropped onto carbon-coated copper grids. Raman spectra of the products were recorded with a Micro-Raman spectrometer (J-Y;
HR800, France) using an excitation wavelength of 488 nm. X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCALAB MK IIX-ray photoelectron spectrometer, which used Mg as an excitation source. 2.3. Gas sensor fabrication and measurements For sensor fabrication, a paste was produced by mixing the grinded sample in deionized water. This paste was then coated on a small alumina tube containing two Pt electrodes. The response of the sensor was defined as Response ¼Rg/R0 where Rg and R0 were the resistances in the target gas and in air, respectively. In addition, the response time was defined as the time required for the resistance to reach 90% of its equilibrium value after the injection of a test gas. Recovery time was defined as the time required for a sensor to attain a resistance 10% below its original value after returning to air.
3. Results and discussion 3.1. Morphology and crystalline structure The structure and morphology of ZGO005 which was used as a representative sample were investigated using XRD, SEM, and TEM. As shown in a low magnification SEM image in Fig. 1a, ZGO005 contained 3D flower-like structures that were a few microns ( 6–10 μm) in size. These structures were assembled from porous layers as shown in Fig. 1b. The flower-like structures were composed of nanosheets and contained many pores. The diameters of these inhomogeneous pores ranged from 10 nm to150 nm, and the nanosheets were approximately 20–30 nm thick. The XRD pattern of ZGO005, which is shown in Fig. 2, confirmed the formation of pure RGO. The ZGO005 nanosheets produced the characteristic peaks of ZnO (XRD card no. 36-1451) including those of its (100), (001), and (101) facets from space group P63mc, which had lattice parameters of a ¼3.25 Å and c ¼5.21 Å. In the region from 10° to 30°, an amplification in the broad diffraction peak of RGO (d-pacing of 3.7 Å at 2θ ¼ 23.0°) was also observed for ZGO005 as shown in the inset of Fig. 2. The broad diffraction peak of RGO was close to the (002) diffraction peak of graphite (d-spacing of 3.35 Å at 2θ ¼26.6°) [26,27]. The XPS spectra of ZGO005 are shown in Fig. 3a and b. The investigation spectrum (Fig. 3a) indicated the presence of Zn, O, and C. According to the higher resolution spectrum of Zn2p shown in the inset to Fig. 3a, the binding energy of Zn2p3/2 was 1021.66 eV. The C1s spectrum of ZGO005 (Fig. 3b) indicated the presence of a considerable amount of GO. Three major
Fig. 1. Morphologies of the as-synthesized samples. (a) Low-magnification SEM image of the ZGO005 nanosheets. (b) High-magnification image of the ZGO005 nanosheets.
Please cite this article as: J. Li, et al., Enhanced NO2 detection using hierarchical porous ZnO nanoflowers modified with graphene, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.03.083i
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Fig. 2. XRD patterns of the as-prepared ZGO005 nanosheets and RGO. Inset is an amplification in the broad RGO and ZGO005 diffraction peak from 10° to 40°.
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components of the spectrum corresponded to carbon atoms in different functional groups including the non-oxygenated C–C bonds (284.6 eV), the C–O bonds of epoxy and hydroxyl groups (286.7 eV), and the carbonyl C ¼ O bonds (288.5 eV) [28,29]. The results of these characterization methods confirmed the successful synthesis of ZGO005. While XRD analyses confirmed the presence of RGO in the material, structural information of these hierarchical porous ZGO005 nanostructures was obtained using TEM and HRTEM. Fig. 4 shows the HRTEM images of the as-prepared ZGO005 sample. As shown in Fig. 4a, the low magnification TEM images revealed flower-like structures. These structures consisted of selfassembled porous nanosheets. Fig. 4b shows a high-magnification image of a nanosheet. The pores in this nanosheet have a regular hexagonal structure and were 30–200 nm in size. The ZnO/RGO interface was clearly visible at the edge of pores. The detailed interfacial structure of ZnO and RGO is shown in Fig. 4c. The HRTEM image in Fig. 4d corresponds to the boundary of the hexagon on the left side of Fig. 4c. This HRTEM image revealed lattice fringes with a distance of 0.28 nm, which corresponded to the inter-plane spacing of the (100) facet of hexagonal ZnO. Fig. 4e shows a highresolution image from the right side of Fig. 4c. This region contained no clear crystal lattice and corresponded to the presence of RGO. Fig. 5 shows the Raman spectra of ZGO005 and GO. These spectra both contained peaks for the D band at approximately 1352 cm 1 and the G band at approximately 1598 cm 1, which were attributed to the breathing mode of k-point phonons with A1g symmetries and the first-order scattering of E2g phonons, respectively [30,31]. The relative intensities of the D and G bands of the ZGO005 hybrids (1.03) were larger than those of GO's D and G bands (0.88), indicating an increased D/G intensity ratio for ZGO005 compared to that of GO. This change in relative band intensity suggested a decrease in the average size of the sp2 domains upon the reduction of GO, which may have been caused by the creation of new graphitic domains that were smaller (but more numerous) than those in the unreduced GO [30–35]. Overall, these results confirmed the formation of RGO by the reduction of GO during the ZGO005 synthesis process. 3.2. Gas-sensing properties
Fig. 3. (a) XPS spectra of ZGO005. (b) The C1s spectra.
The sensing capabilities of the ZGO samples prepared with different concentrations of graphene were systematically investigated. Fig. 6a shows the responses of gas sensors prepared with ZGO002, ZGO005, ZGO01, and ZGO02 upon exposure to 10 ppm concentrations of NO2 at 174 °C. Following the introduction of NO2, the resistances of the different materials increased rapidly. As expected, the responses of the ZGO-based NO2 sensors outperformed those of the ZnO-based sensors. ZGO005 had the highest response of 84, while the response of ZnO was only 30. ZGO005 had an excellent response to NO2 and was selective for the detection of NO2 over other gases. The ZGO005-based sensor exhibited rapid response and recovery times of 26 s and 10 s, respectively. ZGO005 also exhibited a rapid response at 174 °C. Fig. 6b shows the response and recovery curves for ZGO005 sensors exposed to 10 ppm NO2 after 3 on–off cycles at 174 °C. After repeated cycling, the material's stability improved. Fig. 6c shows that the response of ZGO005 varied with concentrations of NO2 gas from 1 ppm to 10 ppm at 174 °C. When the concentration of NO2 was increased, the sensor's response also increased. Saturation did not occur in this concentration range, and the response varied linearly with concentration. Since selectivity is an important parameter for gas sensors, the sensitivity of the ZGO005 sensor was tested using various gases including nitrogen dioxide, carbonic oxide, ethylalcohol, ammonia, and hydrogen as shown in
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Fig. 4. (a) Low-magnification and (b) high-magnification TEM images of an individual ZGO005 nanosheet. (c) TEM images of the inside edge of the ZGO005 nanosheets. (d) HRTEM image of the ZGO005 nanosheets.
Fig. 5. Raman spectrum of ZGO005.
Fig. 6d. Clearly, the ZGO005 sensor exhibited a larger response to NO2 than to the other gases tested, indicating that the prepared ZGO005 sensor displayed a superior selectivity for NO2 over other interfering gases at 174 °C. 3.3. Gas-sensing mechanism The sensing mechanism of the ZGO materials might have resulted from several different factors, such as those depicted in Fig. 7. One possible sensing mechanism was impedance modulation. In this case, the material's resistance was influenced by the adsorption of gas on its surface. Oxygen from the air was adsorbed on the material's surface, and this oxygen accepted electrons from the surface at the optimum working temperature, forming O−2 . This electron transfer created an electronic depletion layer on the surface. When NO2, which is oxidizing, was then added at a low
temperature (174 °C), NO2 adsorption occurred more readily than O−2 adsorption. Therefore, NO2 gained more electrons from the material, leading to a larger depletion layer thickness and a higher material resistance. Oxygen in a reducing atmosphere reacts with ethanol, ammonia, CO, hydrogen, and many other gases, transferring electrons to the materials and decreasing the material's resistance. The high adsorption capacity of oxygen and NO2 is important for achieving a high sensitivity. Ultimately, the high adsorption of oxygen and NO2 increased the material's sensitivity. The reaction mechanism was also influenced by the nanoflowers’ porous structures, which allowed for the rapid infiltration and diffusion of gases. A unit of the monocrystalline nanosheet decreased the transport barrier at the grain boundaries, lowering the possibility of compound during the transmission of carriers and increasing its speed. Meanwhile, carriers in the thin nanosheet were more likely to gather on the surface, enabling a higher utilization of these carriers. Therefore, by working at a suitably low temperature, the porous nanoflower structures were able to achieve faster response times. ZnO and RGO combined to form complex materials during the hydrothermal reduction process, and the presence of RGO likely enhanced the composite material's gas-sensing performances. During the reduction of GO, its surface functional groups were altered, allowing the graphene and ZnO to bond and leading to an increase in the number of ZnO surface defects, the appearance of more active surface sites, and an improvement in the adsorption of oxidizing gas molecules. Each of these outcomes likely contributed to the materials’ enhanced sensing properties. The effect of the amount of graphene added on the conductivity of the composites is shown in Fig. 8. To prepare sensors, 10 mg of the various samples (ZnO, ZGO002, ZGO005, ZGO01, and ZGO02) were used, and the resistances of three devices were measured in air at the optimum working temperature. The sensors’ resistances increased significantly when their graphene content was increased. Their resistance increased by approximately two orders of magnitude in some cases. The change in resistance was caused by the adsorption of oxygen on the surface of the PN heterojunctions. When less graphene was added, the resistance was influenced
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Fig. 6. (a) Responses of the ZGO-based sensors in gas containing 10 ppm NO2 at 174 °C. (b) Response and recovery curves of the ZGO005 sensor device to 10 ppm NO2 after 3 cycles of gas addition at 174 °C. (c) Response of the ZGO005 sensor exposed to different concentrations of NO2 from 1 to 10 ppm at 174 °C. (d) Responses of ZGO005 sensor to various gases at 174 °C.
Fig. 8. Resistance of the different graphene-containing materials.
Fig. 7. Possible gas-sensing reaction mechanism.
more by the adsorption of oxygen. The materials containing less graphene had more defects on their ZnO surfaces, while the materials containing more graphene adsorbed more oxygen.
Therefore, the resistances and sensitivities of the graphene-rich sensors were higher. When the graphene content was further increased in sample ZGO01, scattered graphene regions on the material's surface became interconnected, decreasing the number of surface defects. The effect of the PN heterojunctions was most likely dominant for this material. The formation of many PN
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[2]
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Fig. 9. PL spectra of the different samples.
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heterojunctions increased the electric-field intensity, reducing the number of carriers. Although the material was still in a high impedance state, its sensitivity began to decline under these conditions. In order to further determine the influence of graphene content on the material's defects, PL spectra of the samples were obtained as shown in Fig. 9. Excitation peaks of ZnO, ZGO002, ZGO005, ZGO01, and ZGO02 were observed at approximately 421 nm, 425 nm, 429 nm, 420 nm, and 413 nm, respectively. When the graphene content was below that of ZGO005, the material's PL peak underwent a redshift. The PL peaks of impurities were also stronger in these samples, indicating that the addition of graphene resulted in peaks associated with ZnO that contained more impurities and/or defects. When the graphene content exceeded that of ZGO10, the materials’ PL peaks underwent a blue shift. This blue shift might have been caused by the large amount of p-type graphene covering the surface of the n-type ZnO. This layering of graphene on ZnO decreased the number of surface defects and the intensities of the impurities’ peaks.
4. Conclusions Hierarchical flower-like porous ZnO modified with RGO was synthesized and displayed great potential for gas sensing applications. ZGO achieved the best sensing performance for the detection of NO2 gas when the optimal amount of RGO (0.05 mg) was added. This ZGO005 sample was sensitive to and selective for NO2 gas, and it had a rapid response time at 174 °C. Experimental results indicated that the addition of RGO greatly improved the sensing characteristics of the hierarchical flower-like porous ZnO. This method is a promising route for the fabrication of practical sensors with improved sensing capabilities.
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This work was partially supported by the National Natural Science Foundation of China, China (No. 61403110).
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Enhanced NO2 detection using hierarchical porous ZnO nanoflowers modified with graphene Jing Li, Weiguang Zhang, Jianbo Sun n The Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, School of Physics and Electronic Engineering, Harbin Normal University, Harbin 10025, China
art ic l e i nf o
a b s t r a c t
Article history: Received 24 February 2016 Received in revised form 10 March 2016 Accepted 10 March 2016
Because of their potential applications in gas sensing and catalysis, reduced graphene oxide (RGO) and ZnO have been the focus of much recent attention. However, few reported materials have been produced via the combination of hierarchical ZnO structures with RGO to achieve high sensing performances. In this paper, a hydrothermal method was used to synthesize hierarchical porous ZnO nanoflowers, which were then combined with graphene to enhance their sensing performances. The rapid detection of 1 ppm NO2 was achieved at 174 °C. The morphologies and structures of these materials were characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. Photoluminescence measurements and X-ray photoelectron spectroscopy were also used to investigate the mechanism of gas sensing by these materials. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Hierarchical and porous structures Graphene Gas sensing
1. Introduction Hierarchical architectures have promising characteristics for sensing applications. Three-dimensional (3D) hierarchical architectures such as hollow spheres, flower-like structures, and urchin-like structures have been assembled using nanoscale building blocks (e.g., 0D grain nanodots, 1D nanowires or nanorods, and 2D nanosheets) [1–4]. These architectures often possess large surface areas, which provide more active sites and favorable transport pathways for gas molecules. As a consequence, these structures have reduced response and recovery times and increased sensor sensitivities at low temperatures [5–7]. Sensors employing ZnO hierarchical structures have been a topic of much recent research [8–10]. However, high operating temperatures and low responses have limited their development for practical applications. Recently, the synergistic effects of hierarchical ZnO combined with different materials have been explored to enhance the sensing performance of ZnO. The use of dopants (e.g., Au [11], Ag [12], Pt [13], and Pd [14]) and heterogeneous semiconductor composites (e.g., SnO2 [15], CuO [16], and graphene [17]) has significantly improved the sensing performances of ZnO-based gas sensors. Currently composites of graphene and ZnO are the most promising due to the fact that graphene has a high electron mobility, a good chemical stability, and a low cost. Chemically-derived graphene is particularly expected to aid in the development of highly sensitive chemical sensors [18,19]. According to recent research on composites of graphene and n
Corresponding author. E-mail addresses: [email protected], [email protected] (W. Zhang).
ZnO, these material have increased sensitivities when they contain chemically reduced graphene oxide (GO) or when their GO surface is functionalized to increase the number of adsorption sites [20]. For example, Zou et al. reported the excellent field emission, gas sensing, and photocatalytic properties of ZnO nanorods on reduced graphene sheets [22]. Uddin et al. reported the synthesis and acetylene sensing properties of highly dispersed ZnO nanoparticles on graphene surfaces [23]. Abideen et al. synthesized ZnO nanofibers on reduced GO nanosheets for gas detection [24]. Overall, the combination of hierarchical ZnO with functionalized graphene is a promising route towards the development of gas sensitive materials with high sensitivities and low working temperatures. Such materials have rarely been reported in the literature. In this work, a hydrothermal method was used to synthesize hierarchical porous ZnO nanoflowers modified with reduced GO (RGO). These materials possessed improved sensing characteristics. The porous nature of the materials provided a higher specific surface area and more active sites. Their hierarchical structure also contained channels for rapid gas adsorption. Functionalization of the RGO at the RGO/ZnO interface further enhanced the performances of the ZnO nanoflowers.
2. Experimental section 2.1. Sample preparation All reagents were of analytical grade and were used without further purification.
http://dx.doi.org/10.1016/j.ceramint.2016.03.083 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: J. Li, et al., Enhanced NO2 detection using hierarchical porous ZnO nanoflowers modified with graphene, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.03.083i
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2.1.1. Synthesis of graphene oxide GO was prepared from a fine graphite powder using the modified Hummers’ method [25]. In brief, 3 g of KMnO4 was mixed with 0.5 g of NaNO3, 25 mL of H2SO4, and 0.2 g of graphite in an ice bath under stirring. The mixture was maintained at 0 °C for 24 h before being stirred at 35 °C for 30 min. Then, 50 mL of H2O2 was added to the mixture, which was heated at 95 °C for 15 min before 115 mL of distilled water was added to the mixture. Unreacted KMnO4 was removed by the addition of 10 mL of H2O2. The oxidized graphene was purified by washing with distilled water and absolute ethanol and dried in air at room temperature. 2.1.2. Synthesis of ZGO composites In a typical experiment, a 0.005 M equimolar mixture of Zn (NO3)2 6H2O and hexamethylenetetramine (HMT) was prepared in 50 mL of distilled water. H2C2O4 was then added to this solution to reach an oxalate ion-to-Zn2 þ molar ratio of 0.1. After 30 min of agitation, a homogeneous solution was obtained. This mixture was sealed in a Teflon-lined stainless-steel autoclave with a 40 mL capacity at 90 °C for 3 h. It was then allowed to cool to room temperature. The white precipitate product was collected and rinsed several times with distilled water and absolute ethanol. Finally, this hydrothermal product was filtered and dried at 60 °C for 6 h. In order to obtain porous crystalline ZnO nanosheets, the hydrothermal product was heated at 300 °C for 1 h before being further heated at 600 °C for 2 h. Different amounts of GO (0.02 mg, 0.05 mg, 0.1 mg, and 0.2 mg) were mixed with the hydrothermal product in 50 mL of distilled water and agitated for 5 min. Subsequently, the mixtures were transferred into a Teflon-lined stainless-steel autoclave and held at 90 °C for 3 h. After the hydrothermal procedure, the autoclave was allowed to cool to room temperature. The series of different Zn/GO (ZGO) composites containing 0.02 mg, 0.05 mg, 0.1 mg, and 0.2 mg of GO are referred to here as ZGO002, ZGO005, ZGO01, and ZGO02, respectively. 2.2. Characterization The crystallographic structures of the as-prepared products were determined using X-ray diffraction (XRD, D/max2600, Rigaku, Japan) with Cu Kα radiation (λ ¼1.5418 Å) at a scanning speed of 0.4° min 1 in the 2θ range from 10° to 80°. Their morphologies were investigated using scanning electron microscopy (SEM, SU70, Hitachi, Japan) at an accelerating voltage of 15 kV. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were performed using an FEI Tecnai F20 microscope operated at 200 kV. For TEM observation, the ZGO005 powder was ultrasonically dispersed in ethanol and dropped onto carbon-coated copper grids. Raman spectra of the products were recorded with a Micro-Raman spectrometer (J-Y;
HR800, France) using an excitation wavelength of 488 nm. X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCALAB MK IIX-ray photoelectron spectrometer, which used Mg as an excitation source. 2.3. Gas sensor fabrication and measurements For sensor fabrication, a paste was produced by mixing the grinded sample in deionized water. This paste was then coated on a small alumina tube containing two Pt electrodes. The response of the sensor was defined as Response ¼Rg/R0 where Rg and R0 were the resistances in the target gas and in air, respectively. In addition, the response time was defined as the time required for the resistance to reach 90% of its equilibrium value after the injection of a test gas. Recovery time was defined as the time required for a sensor to attain a resistance 10% below its original value after returning to air.
3. Results and discussion 3.1. Morphology and crystalline structure The structure and morphology of ZGO005 which was used as a representative sample were investigated using XRD, SEM, and TEM. As shown in a low magnification SEM image in Fig. 1a, ZGO005 contained 3D flower-like structures that were a few microns ( 6–10 μm) in size. These structures were assembled from porous layers as shown in Fig. 1b. The flower-like structures were composed of nanosheets and contained many pores. The diameters of these inhomogeneous pores ranged from 10 nm to150 nm, and the nanosheets were approximately 20–30 nm thick. The XRD pattern of ZGO005, which is shown in Fig. 2, confirmed the formation of pure RGO. The ZGO005 nanosheets produced the characteristic peaks of ZnO (XRD card no. 36-1451) including those of its (100), (001), and (101) facets from space group P63mc, which had lattice parameters of a ¼3.25 Å and c ¼5.21 Å. In the region from 10° to 30°, an amplification in the broad diffraction peak of RGO (d-pacing of 3.7 Å at 2θ ¼ 23.0°) was also observed for ZGO005 as shown in the inset of Fig. 2. The broad diffraction peak of RGO was close to the (002) diffraction peak of graphite (d-spacing of 3.35 Å at 2θ ¼26.6°) [26,27]. The XPS spectra of ZGO005 are shown in Fig. 3a and b. The investigation spectrum (Fig. 3a) indicated the presence of Zn, O, and C. According to the higher resolution spectrum of Zn2p shown in the inset to Fig. 3a, the binding energy of Zn2p3/2 was 1021.66 eV. The C1s spectrum of ZGO005 (Fig. 3b) indicated the presence of a considerable amount of GO. Three major
Fig. 1. Morphologies of the as-synthesized samples. (a) Low-magnification SEM image of the ZGO005 nanosheets. (b) High-magnification image of the ZGO005 nanosheets.
Please cite this article as: J. Li, et al., Enhanced NO2 detection using hierarchical porous ZnO nanoflowers modified with graphene, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.03.083i
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Fig. 2. XRD patterns of the as-prepared ZGO005 nanosheets and RGO. Inset is an amplification in the broad RGO and ZGO005 diffraction peak from 10° to 40°.
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components of the spectrum corresponded to carbon atoms in different functional groups including the non-oxygenated C–C bonds (284.6 eV), the C–O bonds of epoxy and hydroxyl groups (286.7 eV), and the carbonyl C ¼ O bonds (288.5 eV) [28,29]. The results of these characterization methods confirmed the successful synthesis of ZGO005. While XRD analyses confirmed the presence of RGO in the material, structural information of these hierarchical porous ZGO005 nanostructures was obtained using TEM and HRTEM. Fig. 4 shows the HRTEM images of the as-prepared ZGO005 sample. As shown in Fig. 4a, the low magnification TEM images revealed flower-like structures. These structures consisted of selfassembled porous nanosheets. Fig. 4b shows a high-magnification image of a nanosheet. The pores in this nanosheet have a regular hexagonal structure and were 30–200 nm in size. The ZnO/RGO interface was clearly visible at the edge of pores. The detailed interfacial structure of ZnO and RGO is shown in Fig. 4c. The HRTEM image in Fig. 4d corresponds to the boundary of the hexagon on the left side of Fig. 4c. This HRTEM image revealed lattice fringes with a distance of 0.28 nm, which corresponded to the inter-plane spacing of the (100) facet of hexagonal ZnO. Fig. 4e shows a highresolution image from the right side of Fig. 4c. This region contained no clear crystal lattice and corresponded to the presence of RGO. Fig. 5 shows the Raman spectra of ZGO005 and GO. These spectra both contained peaks for the D band at approximately 1352 cm 1 and the G band at approximately 1598 cm 1, which were attributed to the breathing mode of k-point phonons with A1g symmetries and the first-order scattering of E2g phonons, respectively [30,31]. The relative intensities of the D and G bands of the ZGO005 hybrids (1.03) were larger than those of GO's D and G bands (0.88), indicating an increased D/G intensity ratio for ZGO005 compared to that of GO. This change in relative band intensity suggested a decrease in the average size of the sp2 domains upon the reduction of GO, which may have been caused by the creation of new graphitic domains that were smaller (but more numerous) than those in the unreduced GO [30–35]. Overall, these results confirmed the formation of RGO by the reduction of GO during the ZGO005 synthesis process. 3.2. Gas-sensing properties
Fig. 3. (a) XPS spectra of ZGO005. (b) The C1s spectra.
The sensing capabilities of the ZGO samples prepared with different concentrations of graphene were systematically investigated. Fig. 6a shows the responses of gas sensors prepared with ZGO002, ZGO005, ZGO01, and ZGO02 upon exposure to 10 ppm concentrations of NO2 at 174 °C. Following the introduction of NO2, the resistances of the different materials increased rapidly. As expected, the responses of the ZGO-based NO2 sensors outperformed those of the ZnO-based sensors. ZGO005 had the highest response of 84, while the response of ZnO was only 30. ZGO005 had an excellent response to NO2 and was selective for the detection of NO2 over other gases. The ZGO005-based sensor exhibited rapid response and recovery times of 26 s and 10 s, respectively. ZGO005 also exhibited a rapid response at 174 °C. Fig. 6b shows the response and recovery curves for ZGO005 sensors exposed to 10 ppm NO2 after 3 on–off cycles at 174 °C. After repeated cycling, the material's stability improved. Fig. 6c shows that the response of ZGO005 varied with concentrations of NO2 gas from 1 ppm to 10 ppm at 174 °C. When the concentration of NO2 was increased, the sensor's response also increased. Saturation did not occur in this concentration range, and the response varied linearly with concentration. Since selectivity is an important parameter for gas sensors, the sensitivity of the ZGO005 sensor was tested using various gases including nitrogen dioxide, carbonic oxide, ethylalcohol, ammonia, and hydrogen as shown in
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Fig. 4. (a) Low-magnification and (b) high-magnification TEM images of an individual ZGO005 nanosheet. (c) TEM images of the inside edge of the ZGO005 nanosheets. (d) HRTEM image of the ZGO005 nanosheets.
Fig. 5. Raman spectrum of ZGO005.
Fig. 6d. Clearly, the ZGO005 sensor exhibited a larger response to NO2 than to the other gases tested, indicating that the prepared ZGO005 sensor displayed a superior selectivity for NO2 over other interfering gases at 174 °C. 3.3. Gas-sensing mechanism The sensing mechanism of the ZGO materials might have resulted from several different factors, such as those depicted in Fig. 7. One possible sensing mechanism was impedance modulation. In this case, the material's resistance was influenced by the adsorption of gas on its surface. Oxygen from the air was adsorbed on the material's surface, and this oxygen accepted electrons from the surface at the optimum working temperature, forming O−2 . This electron transfer created an electronic depletion layer on the surface. When NO2, which is oxidizing, was then added at a low
temperature (174 °C), NO2 adsorption occurred more readily than O−2 adsorption. Therefore, NO2 gained more electrons from the material, leading to a larger depletion layer thickness and a higher material resistance. Oxygen in a reducing atmosphere reacts with ethanol, ammonia, CO, hydrogen, and many other gases, transferring electrons to the materials and decreasing the material's resistance. The high adsorption capacity of oxygen and NO2 is important for achieving a high sensitivity. Ultimately, the high adsorption of oxygen and NO2 increased the material's sensitivity. The reaction mechanism was also influenced by the nanoflowers’ porous structures, which allowed for the rapid infiltration and diffusion of gases. A unit of the monocrystalline nanosheet decreased the transport barrier at the grain boundaries, lowering the possibility of compound during the transmission of carriers and increasing its speed. Meanwhile, carriers in the thin nanosheet were more likely to gather on the surface, enabling a higher utilization of these carriers. Therefore, by working at a suitably low temperature, the porous nanoflower structures were able to achieve faster response times. ZnO and RGO combined to form complex materials during the hydrothermal reduction process, and the presence of RGO likely enhanced the composite material's gas-sensing performances. During the reduction of GO, its surface functional groups were altered, allowing the graphene and ZnO to bond and leading to an increase in the number of ZnO surface defects, the appearance of more active surface sites, and an improvement in the adsorption of oxidizing gas molecules. Each of these outcomes likely contributed to the materials’ enhanced sensing properties. The effect of the amount of graphene added on the conductivity of the composites is shown in Fig. 8. To prepare sensors, 10 mg of the various samples (ZnO, ZGO002, ZGO005, ZGO01, and ZGO02) were used, and the resistances of three devices were measured in air at the optimum working temperature. The sensors’ resistances increased significantly when their graphene content was increased. Their resistance increased by approximately two orders of magnitude in some cases. The change in resistance was caused by the adsorption of oxygen on the surface of the PN heterojunctions. When less graphene was added, the resistance was influenced
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Fig. 6. (a) Responses of the ZGO-based sensors in gas containing 10 ppm NO2 at 174 °C. (b) Response and recovery curves of the ZGO005 sensor device to 10 ppm NO2 after 3 cycles of gas addition at 174 °C. (c) Response of the ZGO005 sensor exposed to different concentrations of NO2 from 1 to 10 ppm at 174 °C. (d) Responses of ZGO005 sensor to various gases at 174 °C.
Fig. 8. Resistance of the different graphene-containing materials.
Fig. 7. Possible gas-sensing reaction mechanism.
more by the adsorption of oxygen. The materials containing less graphene had more defects on their ZnO surfaces, while the materials containing more graphene adsorbed more oxygen.
Therefore, the resistances and sensitivities of the graphene-rich sensors were higher. When the graphene content was further increased in sample ZGO01, scattered graphene regions on the material's surface became interconnected, decreasing the number of surface defects. The effect of the PN heterojunctions was most likely dominant for this material. The formation of many PN
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[2]
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Fig. 9. PL spectra of the different samples.
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heterojunctions increased the electric-field intensity, reducing the number of carriers. Although the material was still in a high impedance state, its sensitivity began to decline under these conditions. In order to further determine the influence of graphene content on the material's defects, PL spectra of the samples were obtained as shown in Fig. 9. Excitation peaks of ZnO, ZGO002, ZGO005, ZGO01, and ZGO02 were observed at approximately 421 nm, 425 nm, 429 nm, 420 nm, and 413 nm, respectively. When the graphene content was below that of ZGO005, the material's PL peak underwent a redshift. The PL peaks of impurities were also stronger in these samples, indicating that the addition of graphene resulted in peaks associated with ZnO that contained more impurities and/or defects. When the graphene content exceeded that of ZGO10, the materials’ PL peaks underwent a blue shift. This blue shift might have been caused by the large amount of p-type graphene covering the surface of the n-type ZnO. This layering of graphene on ZnO decreased the number of surface defects and the intensities of the impurities’ peaks.
4. Conclusions Hierarchical flower-like porous ZnO modified with RGO was synthesized and displayed great potential for gas sensing applications. ZGO achieved the best sensing performance for the detection of NO2 gas when the optimal amount of RGO (0.05 mg) was added. This ZGO005 sample was sensitive to and selective for NO2 gas, and it had a rapid response time at 174 °C. Experimental results indicated that the addition of RGO greatly improved the sensing characteristics of the hierarchical flower-like porous ZnO. This method is a promising route for the fabrication of practical sensors with improved sensing capabilities.
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Acknowledgments [27]
This work was partially supported by the National Natural Science Foundation of China, China (No. 61403110).
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