Home
Search
Collections
Journals
About
Contact us
My IOPscience
Facile synthesis of Cr-decorated hexagonal Co3O4 nanosheets for ultrasensitive ethanol detection
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 275501 (http://iopscience.iop.org/0957-4484/26/27/275501) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 141.117.125.76 This content was downloaded on 19/06/2015 at 07:37
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 26 (2015) 275501 (7pp)
doi:10.1088/0957-4484/26/27/275501
Facile synthesis of Cr-decorated hexagonal Co3O4 nanosheets for ultrasensitive ethanol detection Pingping Zhang, Jiwei Wang, Xiaoxin Lv, Hui Zhang and Xuhui Sun Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu 215123, People’s Republic of China E-mail: [email protected] Received 7 March 2015, revised 19 April 2015 Accepted for publication 18 May 2015 Published 12 June 2015 Abstract
Cr-decorated hexagonal Co3O4 nanosheets were synthesized by a facile hydrothermal method on a SiO2/Si substrate, followed by a simple physical deposition of Cr film and a thermal annealing treatment. The Co3O4 nanosheets possess a porous and polycrystalline structure consisting of Co3O4 nanoparticles; Cr2O3 nanoparticles are uniformly formed on the surface of the Co3O4 nanosheets after the annealing treatment. The ethanol-sensing properties of the Cr-decorated hexagonal Co3O4 nanosheets were investigated in detail, and compared to pure hexagonal Co3O4 nanosheets, they show unique sensing properties toward ethanol, including high response (5.4) even when the ethanol concentration is as low as 10 ppm, ultrafast response (1 s) and recovery (7 s) rates, and good selectivity at a 300 °C operating temperature. These properties make the Crdecorated hexagonal Co3O4 nanosheets good candidates for ethanol detection. S Online supplementary data available from stacks.iop.org/NANO/26/275501/mmedia Keywords: Co3O4 nanosheets, ethanol sensor, Cr-decorated (Some figures may appear in colour only in the online journal) 1. Introduction
high selectivity and functionality due to their good catalysts for the oxidation of various volatile organic compounds [18– 21] and connectivity between primary particles [22–25]. In addition, the amount of oxygen adsorption in these p-type semiconductors is known to be significantly higher than in n-type oxide semiconductors [26], which is another advantage for sensing applications. Among these p-type oxide semiconductors, cobalt oxide (Co3O4), with its rich oxygen content, has proven potential as a promising gas-sensing material due to its catalytic activator behavior in oxidation reactions [15, 27–29]. It has also garnered extensive interest because of its different gas responses and good selectivity. Recently, a wide variety of nanostructured Co3O4 with different structures—including nanorods [30], hollow structures [31], nanoblets [32], nanokernels [33], and nanoflowers [34]—have been synthesized. These structures exhibited excellent sensing performance due to their high specific surface area, which enhances gas adsorption and the changes in conductivity
Because of its importance in industrial safety and environmental monitoring, gas sensing using metal oxides has attracted much attention since the pioneering work by Seiyama et al [1, 2]. Metal oxide semiconductor-based chemiresistive gas sensors, which show significant resistance change upon exposure to target gases such as toxic and explosive gases [3, 4], are among the most widely investigated candidates due to their high sensitivity, stability, low cost, and compatibility with microelectronic processes [5–8]. Among these sensing materials, n-type semiconductor metal oxides such as ZnO [9], SnO2 [10], In2O3 [11], TiO2 [12], and WO3 [13], which are attractive candidates for gas-sensor applications, have been widely studied. Over the past few years, studies have shown that p-type semiconductor metal oxides such as Cr2O3 [14], Co3O4 [15], NiO [16], Cu2O [17], and CuO [8] are also good candidates for gas sensing; they provide new and unique gas-sensing characteristics, offering 0957-4484/15/275501+07$33.00
1
© 2015 IOP Publishing Ltd Printed in the UK
P Zhang et al
Nanotechnology 26 (2015) 275501
Figure 1. Schematic diagram of the fabrication process of the Cr-decorated hexagonal Co3O4 NSs and sensor device.
caused by the surface reaction between the Co3O4 surface and gas molecules. More recently, two-dimensional porous nanosheets (NSs) with unique sheet-like morphologies and porous structures have attracted great research interest because of their significantly enhanced gas-sensor application properties [35]. In addition to morphology control, the loading of dopant or metal/metal oxide catalysts [14, 36] can also be used to enhance the performance of the gas sensors due to both the synergistic effect of the decorators and sensing materials, and the decorators’ ability to facilitate electron transfer to the sensing material [37, 38]. Hence, the gas sensors based on Co3O4 NSs are necessarily more promising because of their unique structure and the catalytic activator behavior; the decoration of the Co3O4 NSs with noble metals or metal oxides can be adopted to further improve both their sensitivity and the selectivity of the Co3O4 NS-based sensors. Chromium oxide (Cr2O3) is a good additive to achieve functional improvement to gas sensors due to its fine conductivity and good catalytic reactivity [18, 37]. Thus, Cr-decorated Co3O4 NSs, with the Cr in the form of Cr2O3, are seen as promising candidate materials for the highly selective and sensitive detection of toxic gases. In this work, the Cr-decorated hexagonal Co3O4 NSs (HCo3O4 NS) were synthesized by a facile hydrothermal method on the SiO2-layered Si (SiO2/Si) substrate. The product exhibited a porous structure and shows a very high response to both high and low concentrations of ethanol due to its high specific surface area and the catalytic reactivity of Cr2O3 on the surface of the sample. Moreover, the Cr-decorated H-Co3O4 NS also exhibited good selectivity and ultrafast
response (1 s) and recovery (7 s) rates at an operating temperature of 300 °C, when compared to pure H-Co3O4 NS. These properties make the fabricated Cr-decorated H-Co3O4 nanosheet a good candidate for ethanol detection.
2. Experimental section The synthetic procedure for the Cr-decorated H-Co3O4 NS is shown in figure 1. In a typical synthesis process, 0.48 g cobalt chloride hexahydrate and 0.36 g hexamethylenetetramine were first dissolved in 70 ml of a 9:1 mixture of deionized water and ethanol to obtain the precursor solution of 2 mM under continuous stirring. Consequently, the resultant solution was then loaded into a Teflon-lined stainless steel autoclave with a cleaned SiO2/Si substrate (2 × 2 cm). Then, the reaction solution was heated up to 90 °C for 10 h. After completing the reaction in the desired time, the autoclave was naturally allowed to cool at room temperature, and a solid, green-colored product was obtained on the SiO2/Si substrate. Then it was washed with deionized water and anhydrous ethanol several times, and finally air-dried at room temperature. Afterward, the 10 nm Cr film was deposited on the substrate in a conventional radio-frequency power supply magnetron sputtering system (PVD 75, Kurt J Lesker) at room temperature using a Cr target. Finally, the product was calcined at 450 °C for 4 h to obtain the Cr-decorated H-Co3O4 NS. The interdigital electrodes were then deposited on the Crdecorated H-Co3O4 NS with an electron-beam evaporation deposition system (PVD 75, Kurt J Lesker) at a background pressure of 7 × 10−6 torr with Au (100 nm)/Ti (5 nm) 2
P Zhang et al
Nanotechnology 26 (2015) 275501
Figure 2. SEM images of (a), (d) the H-Co(OH)2 NS; (b), (e) the pure Co3O4 NS; and (c), (f) the Cr-decorated H-Co3O4 NS at different magnifications.
electrodes (finger spacing: 150 μm, 3.4 mm in length, 200 μm in width) to fabricate the sensing devices. The as-synthesized products were investigated in terms of their morphological, structural, and compositional properties. The general morphologies of the Cr-decorated H-Co3O4 NSs were examined by scanning electron microscopy (SEM, FEI Co., model Quanta-200), atomic force microscopy (AFM) (Multimode V system, Veeco), and transmission electron microscopy (TEM, FEI Tecnai G2 F20 S-TIWN) equipped with energy-dispersive x-ray spectroscopy (EDX) with an accelerating voltage of 200 keV. The crystal structure was investigated by x-ray diffraction (XRD, Empyrean, PANalytical B. V.) using a Philips X’pert PRO MPD diffractometer with Cu Kα radiation (λ = 0.15406 nm) in the 2θ range of 10–80°. Further investigation of the chemical composition and chemical state of these samples was carried out by x-ray photoelectron spectroscopy (XPS, Kratos AXIS UltraDLD) using Al K α radiation (1486 eV) as a probe. The core-level binding energies were corrected with the C 1s binding energy of 284.9 eV. The gas-sensing properties were measured using a homebuilt intelligent gas-sensing analysis system. The analysis system offered a substrate temperature control (from room temperature to 500 °C), which could adjust the sensor temperature with a precision of 1 °C. I–V characteristics of the sensor were measured by a computer-controlled Keithley2400 system. The sensor resistance and sensitivity were collected and analyzed by this system in real time. The gas sensitivity (S) was defined as the ratio of their resistances in air (Rg) to those in a mix of target gases (Ra); this ratio was given by S = Rg/Ra. The time taken by the sensor to achieve 90% of the total resistance change was defined as the
response time in the case of adsorption, and the recovery time in the case of desorption.
3. Results and discussion The general morphologies of the as-synthesized hexagonal Co (OH)2 NSs (denoted as H-Co(OH)2 NSs), pure H-Co3O4 NSs, and Cr-decorated H-Co3O4 NSs were observed by SEM, with the results shown in figure 2. The low-magnification SEM image of the H-Co(OH)2 NS as the precursor of the H-Co3O4 NS (figure 2(a)) reveals that these NSs exhibit a perfectly hexagonal shape with an internal angle of approximately 120° and a uniform size ranging from 10 to 20 μm. The density of the NSs is very high, as most of the NSs are attached each other. From the side view of the high-resolution SEM image shown in the inset of figure 2(a), the typical thickness of the hexagonal NSs is approximately 30 nm. The high-magnification SEM image in figure 2(d) shows that the NSs have smooth, clean surfaces. Figures 2(b) and (c) show low-magnification SEM images of the pure H-Co3O4 NS and the Crdecorated H-Co3O4 NS obtained by the thermal oxidation of pure H-Co(OH)2 NS and Cr-deposited H-Co(OH)2 NS. It is clear that the synthesized pure H-Co3O4 NS and Cr-decorated H-Co3O4 NS both keep their hexagonal shape after thermal oxidation. However, it is clearly seen in the high-magnification SEM images in figures 2(e) and (f) that some cracks and porosity appear at the surface of both the pure H-Co3O4 NS and the Cr-decorated H-Co3O4 NS. This may be attributed to the impact of gas evolution during the thermal decomposition reaction from H-Co(OH)2 [39]. The AFM image of the pure H-Co3O4 NS is shown in figure S1(a) (all S-numbered figures 3
P Zhang et al
Nanotechnology 26 (2015) 275501
Figure 3. (a), (b) TEM images of Cr-decorated H-Co3O4 NS at different magnifications. (c) High-resolution TEM of Cr-decorated H-Co3O4 NS.
appear in the supporting information); the cracks and porosity can be observed in this figure. From a 10 × 10 μm AFM image of the pure H-Co3O4 NS, a thickness of 30 nm can be obtained from the line-scan profile of the AFM data in figure S1(b), which is consistent with the thickness of the NS from the SEM image. The thickness distribution of the NSs is also shown in figure S1(c). The thickness of the NSs is in the range of 20 nm to 70 nm, and the majority of the NSs are about 30 nm thick. Also, the morphology of the Cr-decorated H-Co3O4 NS is similar to that of the pure H-Co3O4 NS after Cr deposition. Apparently, the above results demonstrate that the Cr-decorated H-Co3O4 NS with a highly developed hexagonal morphology, uniform size, porous structure, and high aspect ratio can be produced on a large scale using the present synthetic scheme. For the further detail on the structural and compositional characterizations, the Cr-decorated H-Co3O4 NS was examined by TEM microscopy, as shown in figure 3. Figures 3(a) and (b) present the low-resolution TEM images of a single Crdecorated H-Co3O4 NS, where one can see the porous structure and many small nanoparticles on the surface. The concentration of the Cr is about 5 wt% on the surface of the H-Co3O4 NS, based on EDX measurement. Figure 3(c) shows the typical high-resolution TEM image of the Cr-decorated H-Co3O4 NS. The porous NSs are composed of the Co3O4 nanograins, which exhibit clear and well-defined lattice fringes in figure 3(c). The measured spacings between two lattice fringes are 0.28 nm and 0.46 nm, corresponding to the (2 2 0) and (1 1 1) crystal planes of cubic Co3O4, respectively. The well-defined lattice fringes of the grains confirm again that the porous Cr-decorated H-Co3O4 NS are polycrystalline structures. The crystal structure of the pure H-Co3O4 NS and Crdecorated H-Co3O4 NS were further examined by XRD (figure S2). For the H-Co3O4 NS, the obtained diffraction pattern in the top panel is well indexed to the typical cubic structure of Co3O4 (JCPDS Card no. 42-1467). For the Crdecorated H-Co3O4 NS, all the obtained diffraction peaks in the bottom panel are close to that of the pure H-Co3O4 NS. No other secondary phase is observed, which indicates the absence of a Cr-related crystalline phase in the Cr-decorated
H-Co3O4 NS sample. This may indicate that Cr2O3 on the surface of the H-Co3O4 NS is an amorphous structure. The detailed surface information and chemical states of Co and Cr in the Cr-decorated H-Co3O4 NS sample were studied by XPS analysis (figure S3). The survey scan spectrum of the Cr-decorated H-Co3O4 NS is presented in figure S3(a) and indicates the existence of cobalt, oxygen, and chromium elements on the sample surfaces. The high intensity of the Cr peak indicates that the Cr element is mainly on the surface instead of incorporated inside the H-Co3O4 NS due to the surface sensitivity of the XPS technique. Figure S3 (b) presents the detailed scan spectra of Co in the position of the main spin–orbit component of the Co 2p peaks. As shown in figure S3b, it exhibits two major peaks with binding energy values of 780.3 eV and 796.8 eV, corresponding to the Co 2p3/2 and Co 2p1/2 doublet core-level peaks in the phase of Co3O4 [40], respectively, which can be further confirmed by the occurrence of a weak satellite peak at a binding energy value of 787.6 eV between the main peaks, Co 2p3/2 and Co 2p1/2 [41, 42]. In addition, the nature of the O 1s peaks of the Cr-decorated H-Co3O4 NS is deconvoluted into three peaks at 530.1 eV, 531.8 eV, and 533.1 eV, which correspond to the lattice oxygen atoms in the metal oxides, -OH (hydroxyl) species adsorbed on the surface, and the adsorbed H2O [43, 44], respectively. Figure S3(d) presents the two peaks of Cr 2p at 577.2 eV and 586.7 eV, which correspond to Cr 2p3/2 and Cr 2p1/2, respectively, with the value for Cr in Cr2O3. Based on the XPS, XRD, and TEM data, we suggest that an amorphous Cr2O3 layer is present on the surface of the H-Co3O4 NS after thermal oxidation. The sensing responses of the Cr-decorated H-Co3O4 NS and pure H-Co3O4 NS toward 100 ppm ethanol at operating temperatures ranging from room temperature (25 °C) to 400 °C were investigated, and the results are shown in figure 4(a). The response of the Cr-decorated H-Co3O4 NS sensor increases initially with the increase in temperature, reaching a maximum response at an optimum temperature of 300 °C, and decreasing thereafter. This phenomenon can be explained by the balance between the speed of the chemical reaction and the speed of gas diffusion. At low temperatures, the sensitivity rises with the increase in the reaction 4
P Zhang et al
Nanotechnology 26 (2015) 275501
Figure 4. (a) Sensitivity of Co3O4 NS and Cr-decorated H-Co3O4 NS to 100 ppm ethanol at different temperatures. (b) Sensitivity of Crdecorated H-Co3O4 NS to different concentrations of ethanol at 300 °C.
Figure 5. (a) The sensitivity of the Cr-decorated H-Co3O4 NS on concentrations of ethanol ranging from 10 ppm to 100 ppm at 300 °C. (b) The response and recovery curves of the Cr-decorated NS-based H-Co3O4 to 100 ppm ethanol at 300 °C.
response and recovery curve of the Cr-decorated H-Co3O4 NS sensor upon exposure to 100 ppm ethanol gas at 300 °C. The response times (ts) and recovery times (tr) are approximately 1 s and 7 s, respectively. The sensor’s unusual, ultrafast response and recovery can be attributed to the porous structure of the Cr-decorated H-Co3O4 NS, which facilitates fast mass transfer of ethanol molecules to and from the interaction region, in addition to improving the rate for charge carriers to traverse the barriers introduced by molecular recognition events. Also, the Cr2O3 as a catalyst benefits the fast response and recovery rate. This clearly demonstrates that the Crdecorated H-Co3O4 NS sensor has great advantages in the real-time monitoring of ethanol. Reproducibility and stability are important parameters of a sensor. It is useful to have both a stable baseline and a reproducible signal change to a given ethanol concentration. The reproducibility of the Cr-decorated H-Co3O4 NS sensor was measured by repeating the sensing response test. As shown in figure 6(a), the Cr-decorated H-Co3O4 NS was periodically exposed to 100 ppm ethanol at 300 °C. The Cr-
temperature. However, the diffusion speed of the target gas is accelerated at high temperatures. Thus, the two processes tend to balance at a certain temperature, at which the sensitivity of the gas sensors is at a maximum [45, 46]. The pure H-Co3O4 NS sensor has an optimum operating temperature of 350 °C. Moreover, the Cr-decorated H-Co3O4 NS sensor has a much higher response to ethanol than the pure H-Co3O4 NS sensor under the same conditions. Figure 4(b) shows the dynamic sensing transients of the Cr-decorated H-Co3O4 NS sensor to different concentrations of ethanol, ranging from 10 ppm to 100 ppm, at 300 °C. The response varies from 5.4 to 24.6 as the concentration increases. It still shows a high response (5.4) when the concentration is as low as 10 ppm. The sensing response is found to increase almost linearly from 5.4 to 24.6 with the increase in ethanol concentration from 10 ppm to 100 ppm, as shown in figure 5(a), indicating the applicability of the sensor’s concentration detection to real-world applications. As for the gas-sensing applications, rapid response and recovery are of great importance. Figure 5(b) shows a typical 5
P Zhang et al
Nanotechnology 26 (2015) 275501
Figure 6. (a) Sensitivity of the Cr-decorated H-Co3O4 NS to 100 ppm ethanol at 300 °C. (b) Responses of the Cr-decorated H-Co3O4 NS sensor to 100 ppm ethanol, methanol, CO, H2S, and ammonia.
decorated H-Co3O4 NS shows a good, reproducible response and a stable baseline upon repeated exposure and removal of ethanol under same conditions, suggesting that the Cr-decorated H-Co3O4 NS can serve as a reusable and stable sensing material for the detection of ethanol. Selectivity is another important parameter for evaluating the sensing performance of the materials. To study the selectivity of the Cr-decorated H-Co3O4 NS, the interference gases of methanol, CO, H2S, and ammonia at 100 ppm were used to test the selective response at 300 °C. Figure 6(b) shows that the Cr-decorated H-Co3O4 NS exhibit the highest response to ethanol (24.6), but a considerably lower response (
Search
Collections
Journals
About
Contact us
My IOPscience
Facile synthesis of Cr-decorated hexagonal Co3O4 nanosheets for ultrasensitive ethanol detection
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 275501 (http://iopscience.iop.org/0957-4484/26/27/275501) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 141.117.125.76 This content was downloaded on 19/06/2015 at 07:37
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 26 (2015) 275501 (7pp)
doi:10.1088/0957-4484/26/27/275501
Facile synthesis of Cr-decorated hexagonal Co3O4 nanosheets for ultrasensitive ethanol detection Pingping Zhang, Jiwei Wang, Xiaoxin Lv, Hui Zhang and Xuhui Sun Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu 215123, People’s Republic of China E-mail: [email protected] Received 7 March 2015, revised 19 April 2015 Accepted for publication 18 May 2015 Published 12 June 2015 Abstract
Cr-decorated hexagonal Co3O4 nanosheets were synthesized by a facile hydrothermal method on a SiO2/Si substrate, followed by a simple physical deposition of Cr film and a thermal annealing treatment. The Co3O4 nanosheets possess a porous and polycrystalline structure consisting of Co3O4 nanoparticles; Cr2O3 nanoparticles are uniformly formed on the surface of the Co3O4 nanosheets after the annealing treatment. The ethanol-sensing properties of the Cr-decorated hexagonal Co3O4 nanosheets were investigated in detail, and compared to pure hexagonal Co3O4 nanosheets, they show unique sensing properties toward ethanol, including high response (5.4) even when the ethanol concentration is as low as 10 ppm, ultrafast response (1 s) and recovery (7 s) rates, and good selectivity at a 300 °C operating temperature. These properties make the Crdecorated hexagonal Co3O4 nanosheets good candidates for ethanol detection. S Online supplementary data available from stacks.iop.org/NANO/26/275501/mmedia Keywords: Co3O4 nanosheets, ethanol sensor, Cr-decorated (Some figures may appear in colour only in the online journal) 1. Introduction
high selectivity and functionality due to their good catalysts for the oxidation of various volatile organic compounds [18– 21] and connectivity between primary particles [22–25]. In addition, the amount of oxygen adsorption in these p-type semiconductors is known to be significantly higher than in n-type oxide semiconductors [26], which is another advantage for sensing applications. Among these p-type oxide semiconductors, cobalt oxide (Co3O4), with its rich oxygen content, has proven potential as a promising gas-sensing material due to its catalytic activator behavior in oxidation reactions [15, 27–29]. It has also garnered extensive interest because of its different gas responses and good selectivity. Recently, a wide variety of nanostructured Co3O4 with different structures—including nanorods [30], hollow structures [31], nanoblets [32], nanokernels [33], and nanoflowers [34]—have been synthesized. These structures exhibited excellent sensing performance due to their high specific surface area, which enhances gas adsorption and the changes in conductivity
Because of its importance in industrial safety and environmental monitoring, gas sensing using metal oxides has attracted much attention since the pioneering work by Seiyama et al [1, 2]. Metal oxide semiconductor-based chemiresistive gas sensors, which show significant resistance change upon exposure to target gases such as toxic and explosive gases [3, 4], are among the most widely investigated candidates due to their high sensitivity, stability, low cost, and compatibility with microelectronic processes [5–8]. Among these sensing materials, n-type semiconductor metal oxides such as ZnO [9], SnO2 [10], In2O3 [11], TiO2 [12], and WO3 [13], which are attractive candidates for gas-sensor applications, have been widely studied. Over the past few years, studies have shown that p-type semiconductor metal oxides such as Cr2O3 [14], Co3O4 [15], NiO [16], Cu2O [17], and CuO [8] are also good candidates for gas sensing; they provide new and unique gas-sensing characteristics, offering 0957-4484/15/275501+07$33.00
1
© 2015 IOP Publishing Ltd Printed in the UK
P Zhang et al
Nanotechnology 26 (2015) 275501
Figure 1. Schematic diagram of the fabrication process of the Cr-decorated hexagonal Co3O4 NSs and sensor device.
caused by the surface reaction between the Co3O4 surface and gas molecules. More recently, two-dimensional porous nanosheets (NSs) with unique sheet-like morphologies and porous structures have attracted great research interest because of their significantly enhanced gas-sensor application properties [35]. In addition to morphology control, the loading of dopant or metal/metal oxide catalysts [14, 36] can also be used to enhance the performance of the gas sensors due to both the synergistic effect of the decorators and sensing materials, and the decorators’ ability to facilitate electron transfer to the sensing material [37, 38]. Hence, the gas sensors based on Co3O4 NSs are necessarily more promising because of their unique structure and the catalytic activator behavior; the decoration of the Co3O4 NSs with noble metals or metal oxides can be adopted to further improve both their sensitivity and the selectivity of the Co3O4 NS-based sensors. Chromium oxide (Cr2O3) is a good additive to achieve functional improvement to gas sensors due to its fine conductivity and good catalytic reactivity [18, 37]. Thus, Cr-decorated Co3O4 NSs, with the Cr in the form of Cr2O3, are seen as promising candidate materials for the highly selective and sensitive detection of toxic gases. In this work, the Cr-decorated hexagonal Co3O4 NSs (HCo3O4 NS) were synthesized by a facile hydrothermal method on the SiO2-layered Si (SiO2/Si) substrate. The product exhibited a porous structure and shows a very high response to both high and low concentrations of ethanol due to its high specific surface area and the catalytic reactivity of Cr2O3 on the surface of the sample. Moreover, the Cr-decorated H-Co3O4 NS also exhibited good selectivity and ultrafast
response (1 s) and recovery (7 s) rates at an operating temperature of 300 °C, when compared to pure H-Co3O4 NS. These properties make the fabricated Cr-decorated H-Co3O4 nanosheet a good candidate for ethanol detection.
2. Experimental section The synthetic procedure for the Cr-decorated H-Co3O4 NS is shown in figure 1. In a typical synthesis process, 0.48 g cobalt chloride hexahydrate and 0.36 g hexamethylenetetramine were first dissolved in 70 ml of a 9:1 mixture of deionized water and ethanol to obtain the precursor solution of 2 mM under continuous stirring. Consequently, the resultant solution was then loaded into a Teflon-lined stainless steel autoclave with a cleaned SiO2/Si substrate (2 × 2 cm). Then, the reaction solution was heated up to 90 °C for 10 h. After completing the reaction in the desired time, the autoclave was naturally allowed to cool at room temperature, and a solid, green-colored product was obtained on the SiO2/Si substrate. Then it was washed with deionized water and anhydrous ethanol several times, and finally air-dried at room temperature. Afterward, the 10 nm Cr film was deposited on the substrate in a conventional radio-frequency power supply magnetron sputtering system (PVD 75, Kurt J Lesker) at room temperature using a Cr target. Finally, the product was calcined at 450 °C for 4 h to obtain the Cr-decorated H-Co3O4 NS. The interdigital electrodes were then deposited on the Crdecorated H-Co3O4 NS with an electron-beam evaporation deposition system (PVD 75, Kurt J Lesker) at a background pressure of 7 × 10−6 torr with Au (100 nm)/Ti (5 nm) 2
P Zhang et al
Nanotechnology 26 (2015) 275501
Figure 2. SEM images of (a), (d) the H-Co(OH)2 NS; (b), (e) the pure Co3O4 NS; and (c), (f) the Cr-decorated H-Co3O4 NS at different magnifications.
electrodes (finger spacing: 150 μm, 3.4 mm in length, 200 μm in width) to fabricate the sensing devices. The as-synthesized products were investigated in terms of their morphological, structural, and compositional properties. The general morphologies of the Cr-decorated H-Co3O4 NSs were examined by scanning electron microscopy (SEM, FEI Co., model Quanta-200), atomic force microscopy (AFM) (Multimode V system, Veeco), and transmission electron microscopy (TEM, FEI Tecnai G2 F20 S-TIWN) equipped with energy-dispersive x-ray spectroscopy (EDX) with an accelerating voltage of 200 keV. The crystal structure was investigated by x-ray diffraction (XRD, Empyrean, PANalytical B. V.) using a Philips X’pert PRO MPD diffractometer with Cu Kα radiation (λ = 0.15406 nm) in the 2θ range of 10–80°. Further investigation of the chemical composition and chemical state of these samples was carried out by x-ray photoelectron spectroscopy (XPS, Kratos AXIS UltraDLD) using Al K α radiation (1486 eV) as a probe. The core-level binding energies were corrected with the C 1s binding energy of 284.9 eV. The gas-sensing properties were measured using a homebuilt intelligent gas-sensing analysis system. The analysis system offered a substrate temperature control (from room temperature to 500 °C), which could adjust the sensor temperature with a precision of 1 °C. I–V characteristics of the sensor were measured by a computer-controlled Keithley2400 system. The sensor resistance and sensitivity were collected and analyzed by this system in real time. The gas sensitivity (S) was defined as the ratio of their resistances in air (Rg) to those in a mix of target gases (Ra); this ratio was given by S = Rg/Ra. The time taken by the sensor to achieve 90% of the total resistance change was defined as the
response time in the case of adsorption, and the recovery time in the case of desorption.
3. Results and discussion The general morphologies of the as-synthesized hexagonal Co (OH)2 NSs (denoted as H-Co(OH)2 NSs), pure H-Co3O4 NSs, and Cr-decorated H-Co3O4 NSs were observed by SEM, with the results shown in figure 2. The low-magnification SEM image of the H-Co(OH)2 NS as the precursor of the H-Co3O4 NS (figure 2(a)) reveals that these NSs exhibit a perfectly hexagonal shape with an internal angle of approximately 120° and a uniform size ranging from 10 to 20 μm. The density of the NSs is very high, as most of the NSs are attached each other. From the side view of the high-resolution SEM image shown in the inset of figure 2(a), the typical thickness of the hexagonal NSs is approximately 30 nm. The high-magnification SEM image in figure 2(d) shows that the NSs have smooth, clean surfaces. Figures 2(b) and (c) show low-magnification SEM images of the pure H-Co3O4 NS and the Crdecorated H-Co3O4 NS obtained by the thermal oxidation of pure H-Co(OH)2 NS and Cr-deposited H-Co(OH)2 NS. It is clear that the synthesized pure H-Co3O4 NS and Cr-decorated H-Co3O4 NS both keep their hexagonal shape after thermal oxidation. However, it is clearly seen in the high-magnification SEM images in figures 2(e) and (f) that some cracks and porosity appear at the surface of both the pure H-Co3O4 NS and the Cr-decorated H-Co3O4 NS. This may be attributed to the impact of gas evolution during the thermal decomposition reaction from H-Co(OH)2 [39]. The AFM image of the pure H-Co3O4 NS is shown in figure S1(a) (all S-numbered figures 3
P Zhang et al
Nanotechnology 26 (2015) 275501
Figure 3. (a), (b) TEM images of Cr-decorated H-Co3O4 NS at different magnifications. (c) High-resolution TEM of Cr-decorated H-Co3O4 NS.
appear in the supporting information); the cracks and porosity can be observed in this figure. From a 10 × 10 μm AFM image of the pure H-Co3O4 NS, a thickness of 30 nm can be obtained from the line-scan profile of the AFM data in figure S1(b), which is consistent with the thickness of the NS from the SEM image. The thickness distribution of the NSs is also shown in figure S1(c). The thickness of the NSs is in the range of 20 nm to 70 nm, and the majority of the NSs are about 30 nm thick. Also, the morphology of the Cr-decorated H-Co3O4 NS is similar to that of the pure H-Co3O4 NS after Cr deposition. Apparently, the above results demonstrate that the Cr-decorated H-Co3O4 NS with a highly developed hexagonal morphology, uniform size, porous structure, and high aspect ratio can be produced on a large scale using the present synthetic scheme. For the further detail on the structural and compositional characterizations, the Cr-decorated H-Co3O4 NS was examined by TEM microscopy, as shown in figure 3. Figures 3(a) and (b) present the low-resolution TEM images of a single Crdecorated H-Co3O4 NS, where one can see the porous structure and many small nanoparticles on the surface. The concentration of the Cr is about 5 wt% on the surface of the H-Co3O4 NS, based on EDX measurement. Figure 3(c) shows the typical high-resolution TEM image of the Cr-decorated H-Co3O4 NS. The porous NSs are composed of the Co3O4 nanograins, which exhibit clear and well-defined lattice fringes in figure 3(c). The measured spacings between two lattice fringes are 0.28 nm and 0.46 nm, corresponding to the (2 2 0) and (1 1 1) crystal planes of cubic Co3O4, respectively. The well-defined lattice fringes of the grains confirm again that the porous Cr-decorated H-Co3O4 NS are polycrystalline structures. The crystal structure of the pure H-Co3O4 NS and Crdecorated H-Co3O4 NS were further examined by XRD (figure S2). For the H-Co3O4 NS, the obtained diffraction pattern in the top panel is well indexed to the typical cubic structure of Co3O4 (JCPDS Card no. 42-1467). For the Crdecorated H-Co3O4 NS, all the obtained diffraction peaks in the bottom panel are close to that of the pure H-Co3O4 NS. No other secondary phase is observed, which indicates the absence of a Cr-related crystalline phase in the Cr-decorated
H-Co3O4 NS sample. This may indicate that Cr2O3 on the surface of the H-Co3O4 NS is an amorphous structure. The detailed surface information and chemical states of Co and Cr in the Cr-decorated H-Co3O4 NS sample were studied by XPS analysis (figure S3). The survey scan spectrum of the Cr-decorated H-Co3O4 NS is presented in figure S3(a) and indicates the existence of cobalt, oxygen, and chromium elements on the sample surfaces. The high intensity of the Cr peak indicates that the Cr element is mainly on the surface instead of incorporated inside the H-Co3O4 NS due to the surface sensitivity of the XPS technique. Figure S3 (b) presents the detailed scan spectra of Co in the position of the main spin–orbit component of the Co 2p peaks. As shown in figure S3b, it exhibits two major peaks with binding energy values of 780.3 eV and 796.8 eV, corresponding to the Co 2p3/2 and Co 2p1/2 doublet core-level peaks in the phase of Co3O4 [40], respectively, which can be further confirmed by the occurrence of a weak satellite peak at a binding energy value of 787.6 eV between the main peaks, Co 2p3/2 and Co 2p1/2 [41, 42]. In addition, the nature of the O 1s peaks of the Cr-decorated H-Co3O4 NS is deconvoluted into three peaks at 530.1 eV, 531.8 eV, and 533.1 eV, which correspond to the lattice oxygen atoms in the metal oxides, -OH (hydroxyl) species adsorbed on the surface, and the adsorbed H2O [43, 44], respectively. Figure S3(d) presents the two peaks of Cr 2p at 577.2 eV and 586.7 eV, which correspond to Cr 2p3/2 and Cr 2p1/2, respectively, with the value for Cr in Cr2O3. Based on the XPS, XRD, and TEM data, we suggest that an amorphous Cr2O3 layer is present on the surface of the H-Co3O4 NS after thermal oxidation. The sensing responses of the Cr-decorated H-Co3O4 NS and pure H-Co3O4 NS toward 100 ppm ethanol at operating temperatures ranging from room temperature (25 °C) to 400 °C were investigated, and the results are shown in figure 4(a). The response of the Cr-decorated H-Co3O4 NS sensor increases initially with the increase in temperature, reaching a maximum response at an optimum temperature of 300 °C, and decreasing thereafter. This phenomenon can be explained by the balance between the speed of the chemical reaction and the speed of gas diffusion. At low temperatures, the sensitivity rises with the increase in the reaction 4
P Zhang et al
Nanotechnology 26 (2015) 275501
Figure 4. (a) Sensitivity of Co3O4 NS and Cr-decorated H-Co3O4 NS to 100 ppm ethanol at different temperatures. (b) Sensitivity of Crdecorated H-Co3O4 NS to different concentrations of ethanol at 300 °C.
Figure 5. (a) The sensitivity of the Cr-decorated H-Co3O4 NS on concentrations of ethanol ranging from 10 ppm to 100 ppm at 300 °C. (b) The response and recovery curves of the Cr-decorated NS-based H-Co3O4 to 100 ppm ethanol at 300 °C.
response and recovery curve of the Cr-decorated H-Co3O4 NS sensor upon exposure to 100 ppm ethanol gas at 300 °C. The response times (ts) and recovery times (tr) are approximately 1 s and 7 s, respectively. The sensor’s unusual, ultrafast response and recovery can be attributed to the porous structure of the Cr-decorated H-Co3O4 NS, which facilitates fast mass transfer of ethanol molecules to and from the interaction region, in addition to improving the rate for charge carriers to traverse the barriers introduced by molecular recognition events. Also, the Cr2O3 as a catalyst benefits the fast response and recovery rate. This clearly demonstrates that the Crdecorated H-Co3O4 NS sensor has great advantages in the real-time monitoring of ethanol. Reproducibility and stability are important parameters of a sensor. It is useful to have both a stable baseline and a reproducible signal change to a given ethanol concentration. The reproducibility of the Cr-decorated H-Co3O4 NS sensor was measured by repeating the sensing response test. As shown in figure 6(a), the Cr-decorated H-Co3O4 NS was periodically exposed to 100 ppm ethanol at 300 °C. The Cr-
temperature. However, the diffusion speed of the target gas is accelerated at high temperatures. Thus, the two processes tend to balance at a certain temperature, at which the sensitivity of the gas sensors is at a maximum [45, 46]. The pure H-Co3O4 NS sensor has an optimum operating temperature of 350 °C. Moreover, the Cr-decorated H-Co3O4 NS sensor has a much higher response to ethanol than the pure H-Co3O4 NS sensor under the same conditions. Figure 4(b) shows the dynamic sensing transients of the Cr-decorated H-Co3O4 NS sensor to different concentrations of ethanol, ranging from 10 ppm to 100 ppm, at 300 °C. The response varies from 5.4 to 24.6 as the concentration increases. It still shows a high response (5.4) when the concentration is as low as 10 ppm. The sensing response is found to increase almost linearly from 5.4 to 24.6 with the increase in ethanol concentration from 10 ppm to 100 ppm, as shown in figure 5(a), indicating the applicability of the sensor’s concentration detection to real-world applications. As for the gas-sensing applications, rapid response and recovery are of great importance. Figure 5(b) shows a typical 5
P Zhang et al
Nanotechnology 26 (2015) 275501
Figure 6. (a) Sensitivity of the Cr-decorated H-Co3O4 NS to 100 ppm ethanol at 300 °C. (b) Responses of the Cr-decorated H-Co3O4 NS sensor to 100 ppm ethanol, methanol, CO, H2S, and ammonia.
decorated H-Co3O4 NS shows a good, reproducible response and a stable baseline upon repeated exposure and removal of ethanol under same conditions, suggesting that the Cr-decorated H-Co3O4 NS can serve as a reusable and stable sensing material for the detection of ethanol. Selectivity is another important parameter for evaluating the sensing performance of the materials. To study the selectivity of the Cr-decorated H-Co3O4 NS, the interference gases of methanol, CO, H2S, and ammonia at 100 ppm were used to test the selective response at 300 °C. Figure 6(b) shows that the Cr-decorated H-Co3O4 NS exhibit the highest response to ethanol (24.6), but a considerably lower response (
