- Email: [email protected]
Facile preparation of hierarchical structure based on p-type Co3O4 as toluene detecting sensor
Applied Surface Science 503 (2020) 144167
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
Facile preparation of hierarchical structure based on p-type Co3O4 as toluene detecting sensor ⁎
Rui Zhanga, Shang Gaoa, Tingting Zhoua, Jinchun Tub, , Tong Zhanga, a b
T
⁎
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, PR China State Key Laboratory of Marine Resource Utilization in South China Sea, College of Materials and Chemical Engineering, Hainan University, Haikou 570228, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Hierarchical nanostructure Porous surface Co3O4 Toluene Gas sensor
Rational structure design of sensing materials plays a crucial part in fabricating high performance gas sensors. In this work, three Co3O4 samples with different morphologies, including Co3O4-C (Cube-shaped Co3O4), Co3O4-R (Rod-shaped Co3O4) and Co3O4-S (Sheet-shaped Co3O4), were synthesized through a hydrothermal route. Hierarchical structures-dependent gas-sensing properties were further investigated. The toluene sensing performance of Co3O4-S hierarchical structure-based sensor outperforms the other two ones. It exhibited higher sensitivity, faster response-recovery speed and better selectivity to toluene at the working temperature of 180 °C. In addition, the Co3O4-S-based gas sensor can maintain its sensitivity to toluene after one month, suggesting it good reliability. Preliminary functional tests for the detection of toxic analytes indicated it can be considered as a promising candidate for applications in detecting toluene.
1. Introduction Toluene, as a typical aromatic compound, is widely used in the production of lacquers, adhesives, fingernail polish, rubber, leather. Human exposure to toluene may do damage to the skin, kidneys, liver, and central nervous system [1], which emphasizes the need for specific, sensitive, simple, affordable, and reproducible methods for detecting toluene gas. However, due to the low chemical reactivity of BTX (benzene, toluene and xylene) gases, it is really hard to detect them with high sensitivity using resistive-based gas sensors [2]. Compared with n-type metal oxide semiconductors (MOSs), p-type ones have been widely reported to be used as effective catalysts to improve selectivity towards volatile organic compounds (VOCs) [3,4]. Generally, (electron) intrinsic acceptor states exist in p-type MOS, which are caused by excess oxygen at interstitial sites and/or metal vacancies in the crystal lattice. Additional chemical-adsorbed oxygen species may accumulate on the surface of them [5]. Thus, they still own huge potential for the development of chemi-resistors with high performances. As a typical p-type mixed valence MOS, cobalt oxide (Co3O4) could be regarded as the mixture of CoO and Co2O3. Some reports focused on Co3O4 applications in many fields, like sensors, catalysis, energy storage and magnetism [6–9]. Wang and coworkers [10] fabricated four kinds of Co3O4 crystals exposed {0 0 1}, {1 1 0}, {1 1 1}, and {1 1 2} crystal
⁎
facets for electro-catalytic application toward water splitting, which highlighted the superior activity of {1 1 1} facet. Kannan et al. [11] used a single-step hydrothermal process to synthesize three dimensional (3D)-Co3O4 thorn-like and wire-like nanostructures for enzyme-less glucose detection. Compared to the wire-like morphology, the thornlike nanostructure exhibited higher non-enzymatic catalytic activity. Specially, some effort has been made on preparation of Co3O4 for gas sensors [12,13]. Lee et al. [14] reported that Co3O4 nanocage that was synthesized using metal-organic framework as template could show highly sensitive detection of methylbenzene. Moreover, Tian and coworkers [15] reported excellent ethanol sensing performance of Co3O4 mesostructures synthesized by templating with KIT-6 and SBA-15. Meanwhile, it is found that the sensing properties to gases heavily depend on the porosity, dimension and morphology of Co3O4 material [16–18]. Thus, it is of great value to explore the sensing performance of p-type Co3O4 with different nanostructures. Considering the advantages of material and structure, herein, three Co3O4 structures were synthesized by surfactant-assisted hydrothermal methods. The results demonstrate that the gas sensing activity towards toluene is ranked in the order of Co3O4-sheet (Co3O4-S) > Co3O4-rod (Co3O4-R) > Co3O4-cube (Co3O4-C). This study aims to explore affinity between morphology of sensing materials and their sensing performances.
Corresponding authors. E-mail addresses: [email protected] (J. Tu), [email protected] (T. Zhang).
https://doi.org/10.1016/j.apsusc.2019.144167 Received 26 July 2019; Received in revised form 2 September 2019; Accepted 23 September 2019 Available online 17 October 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 503 (2020) 144167
R. Zhang, et al.
2. Experimental section
(Beijing Alite). In this work, the ratio of resistance quantities in gas and air (S = Rg/Ra) was used to record response values. And response/recovery time was defined by the time of 90% resistance values variation when gas introduced or removed, respectively.
2.1. Chemical materials All chemicals are listed in follows: Cobalt acetate tetrahydrate (Co (CH3COO)2·4H2O), hexahydrated cobalt chloride (CoCl2·6H2O), cobalt sulfate seven hydrate (Co(SO4)2·7H2O), urea (CO(NH2)2), ethylene glycol (EG, (CH2OH)2), ethanol (C2H5OH) and deionized water. It should be noted that all chemicals were at analytical grade without purification after received.
3. Results and discussion 3.1. Structural and morphological characteristics Morphologies of three Co3O4 products were investigated and illustrated in Fig. 2. Noticeably, all of Co3O4 samples exhibited uniform hierarchical architectures, with many out-of-order aggregated cubes, rods and sheets assembly, respectively. Moreover, the microstructures of products were further investigated by TEM images. It can be clearly seen that the cube-like sample exhibits a uniform cube-assembly morphology with an edge length of approximately 2 µm (Fig. 2(a)). After heat treatment, the cubes exhibited strong agglomeration with compact dense surface (Fig. 2(a1)). The interior structure of cube-like sample was showed in Fig. 2(a3). A dense solid structure with rough surface was presented, which is consistent with SEM analysis. The cubes are observed to contain some cracked corners and edges with a right-angle side. As for Co3O4-R, a bundle of nanorods strongly is tied and considered as the building blocks to form straw-sheaf-like architectures (Fig. 2(b)). The rods have an approximately average diameter of 300 nm and are composed of small nanoparticles (Fig. 2(b1)). A typical TEM image of a single nanorod (Fig. 2(b3)) shows that the three dimensional straw-sheaf-like Co3O4 are composed of numerous interconnected elongated nanorods with porous nature (similar to thorn). Clearly, Fig. 2(c) exhibited the achieved structures were composed of randomly assembled nanosheets, resulting in a hierarchical morphology. A magnified FESEM image (Fig. 2(c1)) showed that the sheetlike product consisted of a large number of porous nanosheets with a thickness of about 30 nm. In Fig. 2(c3), a piece of nanosheet has been magnified to examine its surface structure, and porous walls were observed. The loose but stable structure contains continuous transmission path and sufficient holes, which is believed to be in favor of fast charge transport and gas diffusion processes. The diffraction rings in SAED patterns (Fig. 2(a4-c4)) revealed the polycrystalline Co3O4 assembly in the three nanostructures. Fig. S2 displays the XRD patterns of as-synthesized Co3O4-C, Co3O4-R and Co3O4-S samples. The observed diffraction peaks coincide with the standard corresponding peaks of the cubic Co3O4 (JCPDS No. 43-1003). Additionally, no other peaks were observed, which confirmed the purity of as-synthesized samples. Fig. 3(a-c) illustrates the pore size distributions of the three Co3O4 samples. It can be clearly seen that the most pore size of Co3O4-S sample is the smallest one of these products. Additionally, the Co3O4-S sample (69 m2g−1) shows the largest surface area among the three samples, which is approximately four and two times higher than those of the Co3O4-C (17 m2g−1), Co3O4-R (36 m2g−1) samples, respectively. The obtained mesoporous structure with large exposed surface was believed to supply sufficient gas adsorption sites and channels, which could enhance the sensing performance. It is known that there is a strong dependent relationship between chemical states of nanomaterial and their performances, thus, XPS analysis is also carried out. The full spectra, Co 2p and O 1s high resolution spectra of three samples are shown in Fig. S3 and Fig. 3(d-i),
2.2. Chemical synthesis Synthesis of three hierarchical Co3O4 nanostructures [19]: Typically, three-dimensional hierarchical Co3O4 structures assembled with cubes, rods and sheets were all synthesized by a facile hydrothermal method and followed by heat treatment in air. First, 15 mmol of cobalt salt was dissolved in 70 mL solvent under magnetic stirring at room temperature. After 30 mins, a specific amount of urea as precipitating reagent was mixed with the above solution. Then, the homogeneous mixture was transferred to a Teflon-lined stainless-steel autoclave (volume: 100 mL) for hydrothermal reaction of 12 h. After natural cooling to ambient temperature, the obtained precipitates were collected and rinsed with ethanol and deionized water for three times. Finally, the dried precursors were further heated in air at 300 °C for 3 h (1 °C/min) to obtain the final cube, rod and sheet-shaped Co3O4 products, denoted as Co3O4-C, Co3O4-R and Co3O4-S in this study. The detailed synthetic conditions for the three Co3O4 nanostructures are shown in Table 1 and Fig. 1. Three samples with different morphologies were obtained using different cobalt salts, probably because the capping of the nuclei’s surface could be tailored by the anions of cobalt salt, resulting in controlling the shapes of Co3O4 microcrystals [20]. 2.3. Characterization The morphologies of the products were confirmed by field emission scanning electron microscopy (FESEM) using a JEOL 7500F microscope. The interior structure of the products were further examined on a high resolution transmission electron microscopy (HRTEM) equipped with Tecnai G2 20S-Twin microscope at an accelerating voltage of 200 kV. X-ray diffraction (XRD) patterns were used to investigate the crystal structure of samples and recorded on a Scintag XDS-2000 X-ray diffractometer (Cu Kα radiation equipped with λ = 1.54 Å) in a 2θ range of 10–80° (7° min−1).The composition of the samples was analyzed by a PREVAC X-ray photoelectron spectrometer (XPS). A JWBK132F analyzer was used to record the specific surface area and pore size distribution of three products. 2.4. Measurement of the gas sensor The as-obtained Co3O4 product was dispersed with deionized water in a mortar to form a paste. Then, it was pasted onto a hollow ceramic tube that was printed two gold electrodes on each side. A Ni-Cr heater wire was used to supply heating source and inserted into the ceramic tube. The structure of the sensor was shown in Fig. S1. The electric curves of the sensor were collected by CGS-8 intelligent test system Table 1 Detailed synthetic conditions for the preparation of three samples. Samples
Structure
Cobalt source
Urea
Solvent
Tem.(°C)/Time(h)
Co3O4-C Co3O4-R Co3O4-S
Cube Rod Sheet
Co(CH3COO)2·4H2O CoCl2·6H2O Co(SO4)2·7H2O
30 mmol 75 mmol 75 mmol
glycol deionized water deionized water
180/12 100/12 100/12
♦Tem.: Temperature. 2
Applied Surface Science 503 (2020) 144167
R. Zhang, et al.
Fig. 1. Schematic diagram of the formation of cube, rod, and sheet-shaped Co3O4.
Fig. 2. (a-c) SEM, (a1-c1) enlarged SEM images, (a2-c2) schematic illustration, (a3-c3) TEM images, (a4-c4) SAED patterns and (a5-c5) HRTEM images of Co3O4-C, Co3O4-R and Co3O4-S samples.
3
Applied Surface Science 503 (2020) 144167
R. Zhang, et al.
Fig. 3. (a-c) Pore size distribution, (d-f) Co 2p spectrum and (g-i) O 1s spectrum of XPS spetrum of Co3O4-C, Co3O4-R and Co3O4-S samples, respectively.
performances of three sensors in this work. It is known that working temperature range is one important parameter for MOS-based gas sensor [14]. Thus, responses of three sensors towards 200 ppm toluene were investigated at different working temperatures and shown in Fig. 4(d). It could be clearly seen that all of the response curves are parabola-like, and the response values slowly rise as the operating temperature from 160 to 180 °C. However, the response gradually decreases with the temperature further increasing. The maximum values of three sensors are 2.4, 4.1 and 8.5, respectively. In order to further evaluate the sensing performances of three sensors, selectivity of three sensors were tested by exposing to 200 ppm different interfering gases at 180 °C (Fig. 4(e)). It is clearly seen that all the sensors showed the highest response to toluene, which indicated the good selectivity to toluene. Moreover, the response of Co3O4-S-based sensor to toluene is at least twice than those of other gases. The good selectivity makes Co3O4-S-based sensor good candidate for fabricating practical toluene sensor. Then, a reproducible characteristic of Co3O4-Sbased sensor was investigated with the sensor being orderly exposed to different concentrations of toluene at 180 °C (Fig. 4(f)). When the gas concentration increasing from 50 to 200 ppm, an increasing response amplitude of the sensor was observed. Additionally, the rapid response and recovery process are almost reproducible, indicating repeatable property while exposing different concentrations of toluene. In order to describe clearly, the sensor responses vs. toluene concentration are plotted in Fig. 4(g). It is clearly seen that the response gradually increased with the increasing concentration of toluene, and it tends to a stable value. But, the responses of Co3O4-S-based sensor increase
respectively. Take Co3O4-C as example, four sub-peaks with two satellite peaks were observed in Fig. 3(d), wherein the fitting peaks at 781.7 and 796.9 eV are indexed to Co2+, and the fitting peaks at 780.0 and 795.1 eV prove the chemical nature of Co3+ [21]. Those results confirm that Co2+ and Co3+ ions coexist in the three samples. More importantly, it is worth mentioning that the integral area ratio of Co2+ to (Co2++Co3+) of Co3O4-C, Co3O4-R and Co3O4-S gradually increased, demonstrating that catalysis to oxygen becomes much easier [22]. The asymmetric O 1s peaks are attributed to three peaks (OC, OV and OL): adsorbed oxygen, vacancy oxygen and crystal lattice oxygen, respectively. Obviously, similar chemisorbed oxygen contents (20.1%, 18.8%, 21.8% for Co3O4-C, Co3O4-R, Co3O4-S) were observed from Fig. 3(g-i). However, the content of vacancy oxygen on Co3O4-S (20.0%, 31.0% and 37.2% for Co3O4-C, Co3O4-R and Co3O4-S) is the biggest of the three samples, which indicated that sheet-assembly Co3O4 has a better ability to attract oxygen species than the others. 3.2. Toluene sensing properties Response/recovery behavior was firstly investigated and showed in Fig. 4(a-c). All of the curves ascend or descend when gas vapor is introduced or removed, which revealed the MOS characteristic of the sensors [4,23]. The results indicated that response/recovery characteristic was greatly influenced by morphologies of sensing materials. Among these three sensors, Co3O4-S-based sensor showed the highest response value and the fastest response/recovery speed. The resistances of all the sensors were at kilo-ohm level. Table 2 listed the sensing 4
Applied Surface Science 503 (2020) 144167
R. Zhang, et al.
Fig. 4. Resistance curves of the sensors based on (a) Co3O4-C, (b) Co3O4-R, (c) Co3O4-S towards 200 ppm toluene at 180 °C, respectively; (d) responses of three sensors towards 200 ppm toluene at different working temperatures; (e) responses of three sensors to 200 ppm different target gases at 180 °C; (f) dynamic resistance curve of the sensor based on Co3O4-S to different concentration of toluene; (g) relationship between response and toluene concentration; (h) response and morphology stability of Co3O4-S-based sensor to 200 ppm toluene during 30 days (measurement number = 3); (i) schematic of sensor exposed to air and target gas.
humidity (RH) in environment. It is widely considered that the water molecules may interact with the adsorbed oxygen on the sensing surface, resulting in forming hydroxyl groups and release electrons [31–32]. The negligible effect may be attributed to the reason that the hydroxyl groups are almost completed at low RH [31].
rapidly when the toluene concentration is less than 50 ppm. And, the Co3O4-S exhibits a recognizable response to 5 ppm xylene (1.2) and 1.2 was used as the criterion of detection limit for gas sensing [24,25]. Long-term stability of gas sensors was further investigated and gas response evolution was shown in Fig. 4(h) through repeatedly measuring the response for a number of times during one month. The 30 days-later responses slightly changed from 8.5 ± 0.2 to 7.5 ± 0.7. Moreover, sheet-like morphologies still existed after 30 days tests. The results illustrated good reliability and stability of Co3O4-S for potentially commercial application as sensing material. Table 3 listed toluene-sensing performances of Co3O4-S-based sensor compared with the previous works, which highlighted the advancement of this work [26–30]. Moreover, for the purpose of better practical application, the effect of moisture on sensing performance was also investigated (Fig. S4). It could be clearly seen that the humidity has a slightly negative impact on the gas sensing response with the increasing relative
3.3. Gas sensing mechanism The possible reasons of such excellent characteristic of hierarchical Co3O4 nanostructures are studied in the following sections. Gas sensing mechanism on metal oxide semiconductors have been widely discussed by lots of previous works [4,5,33]. Most believe that the electrical conductivity of sensing materials would be changed by the chemical reactions taken place on the surface of semiconductors. In this work, Co3O4 is p-type semiconductor and holes are the charge carriers. Once the sensor exposed in air, oxygen molecules may adsorb on the surface
Table 2 Comparison of toluene-sensing properties of three Co3O4-based sensors. Samples
Structure
BET/[m2g−1]
Co2+/(Co2++Co3+)
R0/[kΩ]
Res.
Tres/[s]
Trec/[s]
Co3O4-C Co3O4-R Co3O4-S
Cube Rod Sheet
17 36 69
42% 59% 62%
2.3 4.7 4.9
2.4 4.1 8.5
116 22 10
107 47 30
♦R0: Resistance in air; ♦Res.: Response; ♦Tres: Response time; ♦Trec: Recovery time. 5
Applied Surface Science 503 (2020) 144167
R. Zhang, et al.
Table 3 Comparison of toluene-sensing performances of the sensors based different materials. Samples
Structure
Tem.(°C)
Res.
Tres/[s]
Trec/[s]
Ref.
La/SnO2 SnO2 NiO/SnO2 Au-ZnO Co3O4 Co3O4
Nanorod array Nanofiber Nanofiber Nanowire Network Sheet
290 350 300 340 150 180
3 (200 ppm) 4.7 (100 ppm) 11 (50 ppm) 7.5 (50 ppm) 60.8 (100 ppm) 8.5 (200 ppm)
– 1 11.2 45 150 10
– 5 4 39 200 30
[26] [27] [28] [29] [30] This work
♦Tem.: Working temperature; ♦Res.: Response; ♦Tres: Response time; ♦Trec: Recovery time; ♦Ref: Reference.
surface with the largest exposed specific surface area among the three materials, which may contribute to the mass transportation and gas diffusion across the sensing materials. Thus, the gas sensor based on Co3O4-S showed the highest response and fastest response/recovery speed. 4. Conclusion In summary, the uniform hierarchical spinel-type Co3O4 with cube(C), rod- (R) and sheet- (S) shaped morphologies were successfully synthesized using a hydrothermal method. The sensing performance of toluene detection increased in the order C < R < S. The Co3O4-S nanostructure showed a highest response (8.5) with a fast response/ recovery speed (10/30 s) at a working temperature of 180 °C. The superior properties are attributed to the largest specific surface area, highest cobalt ion (Co2+) and oxygen vacancy content, leading to attracting more surface adsorbed oxygen species. Moreover, for the S sample, no significant decrease in response was observed over one month, which indicates that the 3D hierarchical sheet-stacked Co3O4 structures exhibit excellent sensing activity and stability for toluene detection. Therefore, it might be a potential effective sensing material in practical applications. Acknowledgements
Fig. 5. Schemtic of sensing mechanism (a) in air and (b) in toluene; (c) spinel structure of Co3O4 crystal; (d) a potion of the porous sheet-shaped hierarchical Co3O4 nanostructures.
This work was supported by the Natural Science Foundation Committee (NSFC, Grant No. 61673191) and the Science and Technology Development Plan of Jilin Province (No. 20180414025GH).
of the Co3O4 materials, resulting in formation of adsorbed oxygen species. During this process, electron will be trapped in Co3O4 and hole accumulation layers (HALs) are formed, resulting in low resistance value in air. Then, when the sensor exposed to reducing gas (toluene), adsorbed oxygen species may react with target gas molecules. Thus, electrons were released back to sensing materials, resulting in high resistance value in gas. The related reactions are proposed in follows (Fig. 5(a-b)):
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.144167. References
O2 + 2e− → 2O−
[1] A. Mirzaei, S.G. Leonardi, G. Neri, Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors: A review, Ceram. Int. 42 (2016) 15119–15141. [2] A. Mirzaei, J.-H. Kim, H.W. Kim, S.S. Kim, Resistive-based gas sensors for detection of benzene, toluene and xylene (BTX) gases: a review, J. Mater. Chem. C 6 (2018) 4342–4370. [3] S.-Y. Cho, H.-W. Yoo, J.Y. Kim, W.-B. Jung, M.L. Jin, J.-S. Kim, H.-J. Jeon, H.T. Jung, High-resolution p-type metal oxide Semiconductor nanowire array as an ultrasensitive sensor for volatile organic compounds, Nano Lett. 16 (2016) 4508–4515. [4] H.J. Kim, J.H. Lee, Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview, Sens. Actuators B 192 (2014) 607–627. [5] J.M. Xu, J.P. Cheng, The advances of Co3O4 as gas sensing materials: A review, J. Alloys Compd. 686 (2016) 753–768. [6] H.Q. Sun, H.M. Ang, M.O. Tadé, S.B. Wang, Co3O4 nanocrystals with predominantly exposed facets: synthesis, environmental and energy applications, J. Mater. Chem. A 1 (2013) 14427–14442. [7] X. Wang, W. Tian, T.Y. Zhai, C.Y. Zhi, Y. Bando, D. Golberg, Cobalt(II, III) oxide hollow structures: fabrication, properties and applications, J. Mater. Chem. 22 (2012) 23310–23326. [8] G.R. Patzke, Y. Zhou, R. Kontic, F. Conrad, Oxide nanomaterials: synthetic developments, mechanistic studies, and technological innovations, Angew. Chem. Int. Ed. 50 (2011) 826–859.
C7H8 + 18O− → 4H2O + 7CO2 + 18e− Many reports have clarified the good catalytic performance of Co3O4 to toluene [34–36]. Co(II) ions and Co(III) ions occupy the cobalt sites simultaneously according to the spinel Co3O4 crystal in Fig. 5(c). The oxidation sites of cobalt ions could easily switch between Co2+ and Co3+. Thus, more oxygen ions could be adsorbed and gathered easier, resulting in enriching active oxygen species on the surface of the material. Furthermore, the morphological appearance of the sensing materials has a serious impact on gas sensing performances [5,37–39]. Generally, a porous hierarchical structure could facilitate the diffusion of target gases and contribute to increasing the surface reactive sites in the meanwhile [37,40,41]. Although Co3O4-C, Co3O4-R and Co3O4-S are all well hierarchical structures, the interior structure of Co3O4-C and Co3O4-R samples are really compact compared with the sheet-assembly one. The unique nanosheet-shaped hierarchical structure has porous 6
Applied Surface Science 503 (2020) 144167
R. Zhang, et al.
[25] R. Zhang, X.P. Liu, T.T. Zhou, L.L. Wang, T. Zhang, Carbon materials-functionalized tin dioxide nanoparticles toward robust, high-performance nitrogen dioxide gas sensor, J. Colloid Interf. Sci. 524 (2018) 76–83. [26] F. Gao, G.H. Qin, Y.H. Li, Q.P. Jiang, L. Luo, K. Zhao, Y.J. Liu, H.Y. Zhao, One-pot synthesis of La-doped SnO2 layered nanoarrays with an enhanced gas-sensing performance toward acetone, RSC Adv. 6 (2016) 10298–10310. [27] Q. Qi, T. Zhang, L. Liu, X. Zheng, Synthesis and toluene sensing properties of SnO2 nanofibers, Sens. Actuators B 137 (2009) 471–475. [28] Y.L. Liu, G. Zhang, S. Wang, L. Li, Y. Wang, X. Han, A.W. Jiang, High toluene sensing properties of NiO-SnO2 composite nanofiber sensors operating at 330 °C, Sens. Actuators B 160 (2011) 448–454. [29] L. Wang, S. Wang, M. Xu, X. Hu, H. Zhang, Y. Wang, W. Huang, A Au-functionalized ZnO nanowire gas sensor for detection of benzene and toluene, Phys. Chem. Chem. Phys. 15 (2013) 17179–17186. [30] C. Zhao, B. Huang, J. Zhou, E. Xie, Synthesis of porous Co3O4 nanonetworks to detect toluene at low concentration, Phys. Chem. Chem. Phys. 16 (2014) 19327–19332. [31] X.H. Ma, H.Y. Li, S.-H. Lweon, S.-Y. Jeong, J.-H. Lee, S. Nahm, Highly sensitive and selective PbTiO3 gas sensors with negligible humidity interference in ambient atmosphere, ACS Appl. Mater. Interfaces 11 (2019) 5240–5246. [32] D.X. Ju, H.Y. Xu, Z.W. Qiu, Z.C. Zhang, Q. Xu, J. Zhang, J.Q. Wang, B.Q. Cao, Near room temperature, fast-response, and highly sensitive triethylamine sensor assembled with Au-loaded ZnO/SnO2 core-shell nanorods on flat alumina substrates, ACS Appl. Mater. Interfaces 7 (2015) 19163–19171. [33] R. Zhang, T.T. Zhou, L.L. Wang, T. Zhang, Metal−organic frameworks-derived hierarchical Co3O4 structures as efficient sensing materials for acetone detection, ACS Appl. Mater. Interfaces 10 (2018) 9765–9773. [34] G.M. Bai, H.Q. Dai, J.G. Deng, Y.X. Liu, F. Wang, Z.X. Zhao, W.G. Qiu, C.T. Au, Porous Co3O4 nanowires and nanorods: Highly active catalysts for the combustion of toluene, Appl. Catal., A: General 450 (2013) 42–49. [35] Y.X. Liu, H.X. Dai, J.G. Deng, S.H. Xie, H.G. Yang, W. Tan, W. Han, Y. Jiang, G.S. Guo, Mesoporous Co3O4-supported gold nanocatalysts: Highly active for the oxidation of carbon monoxide, benzene, toluene, and o-xylene, J. Catal. 309 (2014) 408–418. [36] Y.X. Liu, H.X. Dai, J.G. Deng, L. Zhang, Z.X. Zhao, X.W. Li, Y. Wang, S.H. Xie, H.G. Yang, G.S. Guo, Controlled generation of uniform spherical LaMnO3, LaCoO3, Mn2O3, and Co3O4 nanoparticles and their high catalytic performance for carbon monoxide and toluene oxidation, Inorg. Chem. 52 (2013) 8665–8676. [37] J.H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures: overview, Sens. Actuators B 140 (2009) 319–336. [38] L.L. Wang, S. Chen, W. Li, K. Wang, Z. Lou, G.Z. Shen, Grain-boundary-induced drastic sensing performance enhancement of polycrystalline-microwire printed gas sensors, Adv. Mater. 31 (2019) 1804583. [39] X.H. Liu, T.T. Ma, N. Pinna, J. Zhang, Two-dimensional nanostructured materials for gas sensing, Adv. Funct. Mater. 27 (2017) 1702168-(30). [40] R. Zhang, T. Zhang, T.T. Zhou, L.L. Wang, Rapid sensitive sensing platform based on yolk-shell hybrid hollow sphere for detection of ethanol, Sens. Actuators B 256 (2018) 479–487. [41] S.H. Choi, I.S. Hwang, J.H. Lee, S.G. Oh, I.I.-D. Kim, Microstructural control and selective C2H5OH sensing properties of Zn2SnO4 nanofibers prepared by electrospinning, Chem. Commun. 47 (2011) 9315–9317.
[9] M.B. Gawande, A. Goswami, T. Asefa, H.Z. Guo, A.V. Biradar, D.L. Peng, R. Zboril, R.S. Varma, Core–shell nanoparticles: synthesis and applications in catalysis and electrocatalysis, Chem. Soc. Rev. 44 (2015) 7540–7590. [10] L. Liu, Z.Q. Jiang, L. Fang, H.T. Xu, H.J. Zhang, X. Gu, Y. Wang, Probing the crystal plane effect of Co3O4 for enhanced electrocatalytic performance toward efficient overall water splitting, ACS Appl. Mater. Interfaces 9 (2017) 27736–27744. [11] P. Kannan, T. Maiyalagan, E. Marsili, S. Ghosh, L.H. Guo, Y.J. Huang, J.A. Rather, D. Thiruppathi, J. Niedziolka-Jönsson, M. Jönsson-Niedziolka, Highly active 3-dimensional cobalt oxide nanostructures on the flexible carbon substrates for enzymeless glucose sensing, Analyst 142 (2017) 4299–4307. [12] Q. Zhou, W. Zeng, Shape control of Co3O4 micro-structures for high-performance gas sensor, Physica E 95 (2018) 121–124. [13] J.M. Xu, J. Zhang, B.B. Wang, F. Liu, Shape-regulated synthesis of cobalt oxide and its gas-sensing property, J. Alloys Compd. 619 (2015) 361–367. [14] Y.-M. Jo, T.-H. Kim, C.-S. Lee, K. Lim, C.W. Na, F. Abdel-Hady, A.A. Wazzan, J.H. Lee, Metal-organic framework-derived hollow hierarchical Co3O4 nanocages with tunable size and morphology: ultrasensitive and highly selective detection of methylbenzenes, ACS Appl. Mater. Interfaces 10 (2018) 8860–8868. [15] Q. Li, Y. Du, X.J. Li, G.Y. Lu, W.Y. Wang, Y.F. Geng, Z.Q. Liang, X.Q. Tian, Different Co3O4 mesostructures synthesised by templating with KIT-6 and SBA-15 via nanocasting route and their sensitivities toward ethanol, Sens. Actuators, B 235 (2016) 39–45. [16] S. Chen, L. Zhang, Y.T. Han, X.W. Chen, S.T. Wang, M. Zeng, N.T. Hu, Y.J. Su, Z.H. Zhou, H. Wei, Z. Yang, Glucose-assisted synthesis of hierarchical flower-like Co3O4 nanostructures assembled by porous nanosheets for enhanced acetone sensing, Sens. Actuators, B 235 (2016) 39–45. [17] K. Zhao, H.T. Li, S.Q. Tian, W.J. Yang, X.X. Wang, A.M. Pang, C.S. Xie, D.W. Zeng, A facile low-temperature synthesis of hierarchical porous Co3O4 micro/nano structures derived from ZIF-67 assisted by ammonium perchlorate, Inorg. Chem. Front. 6 (2019) 715–722. [18] M.J. Wang, Z.R. Shen, X.D. Zhao, F.P. Duanmu, H.J. Yu, H.M. Ji, Rational shape control of porous Co3O4 assemblies derived from MOF and their structural effects on n-butanol sensing, J. Hazard. Mater. 371 (2019) 352–361. [19] Q.M. Ren, S.P. Mo, R.S. Peng, Z.T. Feng, M.Y. Zhang, L.M. Chen, M.L. Fu, J.L. Wu, D.Q. Ye, Controllable synthesis of 3D hierarchical Co3O4 nanocatalysts with various morphologies for the catalytic oxidation of toluene, J. Mater. Chem. A 6 (2018) 498–509. [20] L.L. Wang, T.T. Zhou, R. Zhang, Z. Lou, J.N. Deng, T. Zhang, Comparison of toluene sensing performances of zinc stannate with different morphology-based gas sensors, Sens. Actuators, B 227 (2016) 448–455. [21] J.F. Marco, J.R. Gancedo, M. Gracia, J.L. Gautier, E. Rios, F.J. Berry, Characterization of the nickel cobaltite, NiCo2O4 prepared by several methods: An XRD, XANES, EXAFS, and XPS study, J. Solid State Chem. 153 (2000) 74–81. [22] J.W. Xiao, Q. Kuang, S. Yang, F. Xiao, S. Wang, L. Guo, Surface structure dependent electrocatalytic activity of Co3O4 anchored on graphene sheets toward oxygen reduction reaction, Sci. Rep. 3 (2013) 2300. [23] D.R. Miller, S.A. Akbar, P.A. Morris, Nanoscale metal oxide-based heterojunctions for gas sensing: A review, Sens. Actuators B 204 (2014) 250–272. [24] S.-Y. Jeong, J.-W. Yoon, T.-H. Kim, H.-M. Jeong, C.-S. Lee, Y.C. Kang, J.-H. Lee, Ultra-selective detection of sub-ppm-level benzene using Pd-SnO2 yolk-shell microreactors with a catalytic Co3O4 overlayer for monitoring air quality, J. Mater. Chem. A 5 (2017) 1446–1454.
7
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
Facile preparation of hierarchical structure based on p-type Co3O4 as toluene detecting sensor ⁎
Rui Zhanga, Shang Gaoa, Tingting Zhoua, Jinchun Tub, , Tong Zhanga, a b
T
⁎
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, PR China State Key Laboratory of Marine Resource Utilization in South China Sea, College of Materials and Chemical Engineering, Hainan University, Haikou 570228, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Hierarchical nanostructure Porous surface Co3O4 Toluene Gas sensor
Rational structure design of sensing materials plays a crucial part in fabricating high performance gas sensors. In this work, three Co3O4 samples with different morphologies, including Co3O4-C (Cube-shaped Co3O4), Co3O4-R (Rod-shaped Co3O4) and Co3O4-S (Sheet-shaped Co3O4), were synthesized through a hydrothermal route. Hierarchical structures-dependent gas-sensing properties were further investigated. The toluene sensing performance of Co3O4-S hierarchical structure-based sensor outperforms the other two ones. It exhibited higher sensitivity, faster response-recovery speed and better selectivity to toluene at the working temperature of 180 °C. In addition, the Co3O4-S-based gas sensor can maintain its sensitivity to toluene after one month, suggesting it good reliability. Preliminary functional tests for the detection of toxic analytes indicated it can be considered as a promising candidate for applications in detecting toluene.
1. Introduction Toluene, as a typical aromatic compound, is widely used in the production of lacquers, adhesives, fingernail polish, rubber, leather. Human exposure to toluene may do damage to the skin, kidneys, liver, and central nervous system [1], which emphasizes the need for specific, sensitive, simple, affordable, and reproducible methods for detecting toluene gas. However, due to the low chemical reactivity of BTX (benzene, toluene and xylene) gases, it is really hard to detect them with high sensitivity using resistive-based gas sensors [2]. Compared with n-type metal oxide semiconductors (MOSs), p-type ones have been widely reported to be used as effective catalysts to improve selectivity towards volatile organic compounds (VOCs) [3,4]. Generally, (electron) intrinsic acceptor states exist in p-type MOS, which are caused by excess oxygen at interstitial sites and/or metal vacancies in the crystal lattice. Additional chemical-adsorbed oxygen species may accumulate on the surface of them [5]. Thus, they still own huge potential for the development of chemi-resistors with high performances. As a typical p-type mixed valence MOS, cobalt oxide (Co3O4) could be regarded as the mixture of CoO and Co2O3. Some reports focused on Co3O4 applications in many fields, like sensors, catalysis, energy storage and magnetism [6–9]. Wang and coworkers [10] fabricated four kinds of Co3O4 crystals exposed {0 0 1}, {1 1 0}, {1 1 1}, and {1 1 2} crystal
⁎
facets for electro-catalytic application toward water splitting, which highlighted the superior activity of {1 1 1} facet. Kannan et al. [11] used a single-step hydrothermal process to synthesize three dimensional (3D)-Co3O4 thorn-like and wire-like nanostructures for enzyme-less glucose detection. Compared to the wire-like morphology, the thornlike nanostructure exhibited higher non-enzymatic catalytic activity. Specially, some effort has been made on preparation of Co3O4 for gas sensors [12,13]. Lee et al. [14] reported that Co3O4 nanocage that was synthesized using metal-organic framework as template could show highly sensitive detection of methylbenzene. Moreover, Tian and coworkers [15] reported excellent ethanol sensing performance of Co3O4 mesostructures synthesized by templating with KIT-6 and SBA-15. Meanwhile, it is found that the sensing properties to gases heavily depend on the porosity, dimension and morphology of Co3O4 material [16–18]. Thus, it is of great value to explore the sensing performance of p-type Co3O4 with different nanostructures. Considering the advantages of material and structure, herein, three Co3O4 structures were synthesized by surfactant-assisted hydrothermal methods. The results demonstrate that the gas sensing activity towards toluene is ranked in the order of Co3O4-sheet (Co3O4-S) > Co3O4-rod (Co3O4-R) > Co3O4-cube (Co3O4-C). This study aims to explore affinity between morphology of sensing materials and their sensing performances.
Corresponding authors. E-mail addresses: [email protected] (J. Tu), [email protected] (T. Zhang).
https://doi.org/10.1016/j.apsusc.2019.144167 Received 26 July 2019; Received in revised form 2 September 2019; Accepted 23 September 2019 Available online 17 October 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 503 (2020) 144167
R. Zhang, et al.
2. Experimental section
(Beijing Alite). In this work, the ratio of resistance quantities in gas and air (S = Rg/Ra) was used to record response values. And response/recovery time was defined by the time of 90% resistance values variation when gas introduced or removed, respectively.
2.1. Chemical materials All chemicals are listed in follows: Cobalt acetate tetrahydrate (Co (CH3COO)2·4H2O), hexahydrated cobalt chloride (CoCl2·6H2O), cobalt sulfate seven hydrate (Co(SO4)2·7H2O), urea (CO(NH2)2), ethylene glycol (EG, (CH2OH)2), ethanol (C2H5OH) and deionized water. It should be noted that all chemicals were at analytical grade without purification after received.
3. Results and discussion 3.1. Structural and morphological characteristics Morphologies of three Co3O4 products were investigated and illustrated in Fig. 2. Noticeably, all of Co3O4 samples exhibited uniform hierarchical architectures, with many out-of-order aggregated cubes, rods and sheets assembly, respectively. Moreover, the microstructures of products were further investigated by TEM images. It can be clearly seen that the cube-like sample exhibits a uniform cube-assembly morphology with an edge length of approximately 2 µm (Fig. 2(a)). After heat treatment, the cubes exhibited strong agglomeration with compact dense surface (Fig. 2(a1)). The interior structure of cube-like sample was showed in Fig. 2(a3). A dense solid structure with rough surface was presented, which is consistent with SEM analysis. The cubes are observed to contain some cracked corners and edges with a right-angle side. As for Co3O4-R, a bundle of nanorods strongly is tied and considered as the building blocks to form straw-sheaf-like architectures (Fig. 2(b)). The rods have an approximately average diameter of 300 nm and are composed of small nanoparticles (Fig. 2(b1)). A typical TEM image of a single nanorod (Fig. 2(b3)) shows that the three dimensional straw-sheaf-like Co3O4 are composed of numerous interconnected elongated nanorods with porous nature (similar to thorn). Clearly, Fig. 2(c) exhibited the achieved structures were composed of randomly assembled nanosheets, resulting in a hierarchical morphology. A magnified FESEM image (Fig. 2(c1)) showed that the sheetlike product consisted of a large number of porous nanosheets with a thickness of about 30 nm. In Fig. 2(c3), a piece of nanosheet has been magnified to examine its surface structure, and porous walls were observed. The loose but stable structure contains continuous transmission path and sufficient holes, which is believed to be in favor of fast charge transport and gas diffusion processes. The diffraction rings in SAED patterns (Fig. 2(a4-c4)) revealed the polycrystalline Co3O4 assembly in the three nanostructures. Fig. S2 displays the XRD patterns of as-synthesized Co3O4-C, Co3O4-R and Co3O4-S samples. The observed diffraction peaks coincide with the standard corresponding peaks of the cubic Co3O4 (JCPDS No. 43-1003). Additionally, no other peaks were observed, which confirmed the purity of as-synthesized samples. Fig. 3(a-c) illustrates the pore size distributions of the three Co3O4 samples. It can be clearly seen that the most pore size of Co3O4-S sample is the smallest one of these products. Additionally, the Co3O4-S sample (69 m2g−1) shows the largest surface area among the three samples, which is approximately four and two times higher than those of the Co3O4-C (17 m2g−1), Co3O4-R (36 m2g−1) samples, respectively. The obtained mesoporous structure with large exposed surface was believed to supply sufficient gas adsorption sites and channels, which could enhance the sensing performance. It is known that there is a strong dependent relationship between chemical states of nanomaterial and their performances, thus, XPS analysis is also carried out. The full spectra, Co 2p and O 1s high resolution spectra of three samples are shown in Fig. S3 and Fig. 3(d-i),
2.2. Chemical synthesis Synthesis of three hierarchical Co3O4 nanostructures [19]: Typically, three-dimensional hierarchical Co3O4 structures assembled with cubes, rods and sheets were all synthesized by a facile hydrothermal method and followed by heat treatment in air. First, 15 mmol of cobalt salt was dissolved in 70 mL solvent under magnetic stirring at room temperature. After 30 mins, a specific amount of urea as precipitating reagent was mixed with the above solution. Then, the homogeneous mixture was transferred to a Teflon-lined stainless-steel autoclave (volume: 100 mL) for hydrothermal reaction of 12 h. After natural cooling to ambient temperature, the obtained precipitates were collected and rinsed with ethanol and deionized water for three times. Finally, the dried precursors were further heated in air at 300 °C for 3 h (1 °C/min) to obtain the final cube, rod and sheet-shaped Co3O4 products, denoted as Co3O4-C, Co3O4-R and Co3O4-S in this study. The detailed synthetic conditions for the three Co3O4 nanostructures are shown in Table 1 and Fig. 1. Three samples with different morphologies were obtained using different cobalt salts, probably because the capping of the nuclei’s surface could be tailored by the anions of cobalt salt, resulting in controlling the shapes of Co3O4 microcrystals [20]. 2.3. Characterization The morphologies of the products were confirmed by field emission scanning electron microscopy (FESEM) using a JEOL 7500F microscope. The interior structure of the products were further examined on a high resolution transmission electron microscopy (HRTEM) equipped with Tecnai G2 20S-Twin microscope at an accelerating voltage of 200 kV. X-ray diffraction (XRD) patterns were used to investigate the crystal structure of samples and recorded on a Scintag XDS-2000 X-ray diffractometer (Cu Kα radiation equipped with λ = 1.54 Å) in a 2θ range of 10–80° (7° min−1).The composition of the samples was analyzed by a PREVAC X-ray photoelectron spectrometer (XPS). A JWBK132F analyzer was used to record the specific surface area and pore size distribution of three products. 2.4. Measurement of the gas sensor The as-obtained Co3O4 product was dispersed with deionized water in a mortar to form a paste. Then, it was pasted onto a hollow ceramic tube that was printed two gold electrodes on each side. A Ni-Cr heater wire was used to supply heating source and inserted into the ceramic tube. The structure of the sensor was shown in Fig. S1. The electric curves of the sensor were collected by CGS-8 intelligent test system Table 1 Detailed synthetic conditions for the preparation of three samples. Samples
Structure
Cobalt source
Urea
Solvent
Tem.(°C)/Time(h)
Co3O4-C Co3O4-R Co3O4-S
Cube Rod Sheet
Co(CH3COO)2·4H2O CoCl2·6H2O Co(SO4)2·7H2O
30 mmol 75 mmol 75 mmol
glycol deionized water deionized water
180/12 100/12 100/12
♦Tem.: Temperature. 2
Applied Surface Science 503 (2020) 144167
R. Zhang, et al.
Fig. 1. Schematic diagram of the formation of cube, rod, and sheet-shaped Co3O4.
Fig. 2. (a-c) SEM, (a1-c1) enlarged SEM images, (a2-c2) schematic illustration, (a3-c3) TEM images, (a4-c4) SAED patterns and (a5-c5) HRTEM images of Co3O4-C, Co3O4-R and Co3O4-S samples.
3
Applied Surface Science 503 (2020) 144167
R. Zhang, et al.
Fig. 3. (a-c) Pore size distribution, (d-f) Co 2p spectrum and (g-i) O 1s spectrum of XPS spetrum of Co3O4-C, Co3O4-R and Co3O4-S samples, respectively.
performances of three sensors in this work. It is known that working temperature range is one important parameter for MOS-based gas sensor [14]. Thus, responses of three sensors towards 200 ppm toluene were investigated at different working temperatures and shown in Fig. 4(d). It could be clearly seen that all of the response curves are parabola-like, and the response values slowly rise as the operating temperature from 160 to 180 °C. However, the response gradually decreases with the temperature further increasing. The maximum values of three sensors are 2.4, 4.1 and 8.5, respectively. In order to further evaluate the sensing performances of three sensors, selectivity of three sensors were tested by exposing to 200 ppm different interfering gases at 180 °C (Fig. 4(e)). It is clearly seen that all the sensors showed the highest response to toluene, which indicated the good selectivity to toluene. Moreover, the response of Co3O4-S-based sensor to toluene is at least twice than those of other gases. The good selectivity makes Co3O4-S-based sensor good candidate for fabricating practical toluene sensor. Then, a reproducible characteristic of Co3O4-Sbased sensor was investigated with the sensor being orderly exposed to different concentrations of toluene at 180 °C (Fig. 4(f)). When the gas concentration increasing from 50 to 200 ppm, an increasing response amplitude of the sensor was observed. Additionally, the rapid response and recovery process are almost reproducible, indicating repeatable property while exposing different concentrations of toluene. In order to describe clearly, the sensor responses vs. toluene concentration are plotted in Fig. 4(g). It is clearly seen that the response gradually increased with the increasing concentration of toluene, and it tends to a stable value. But, the responses of Co3O4-S-based sensor increase
respectively. Take Co3O4-C as example, four sub-peaks with two satellite peaks were observed in Fig. 3(d), wherein the fitting peaks at 781.7 and 796.9 eV are indexed to Co2+, and the fitting peaks at 780.0 and 795.1 eV prove the chemical nature of Co3+ [21]. Those results confirm that Co2+ and Co3+ ions coexist in the three samples. More importantly, it is worth mentioning that the integral area ratio of Co2+ to (Co2++Co3+) of Co3O4-C, Co3O4-R and Co3O4-S gradually increased, demonstrating that catalysis to oxygen becomes much easier [22]. The asymmetric O 1s peaks are attributed to three peaks (OC, OV and OL): adsorbed oxygen, vacancy oxygen and crystal lattice oxygen, respectively. Obviously, similar chemisorbed oxygen contents (20.1%, 18.8%, 21.8% for Co3O4-C, Co3O4-R, Co3O4-S) were observed from Fig. 3(g-i). However, the content of vacancy oxygen on Co3O4-S (20.0%, 31.0% and 37.2% for Co3O4-C, Co3O4-R and Co3O4-S) is the biggest of the three samples, which indicated that sheet-assembly Co3O4 has a better ability to attract oxygen species than the others. 3.2. Toluene sensing properties Response/recovery behavior was firstly investigated and showed in Fig. 4(a-c). All of the curves ascend or descend when gas vapor is introduced or removed, which revealed the MOS characteristic of the sensors [4,23]. The results indicated that response/recovery characteristic was greatly influenced by morphologies of sensing materials. Among these three sensors, Co3O4-S-based sensor showed the highest response value and the fastest response/recovery speed. The resistances of all the sensors were at kilo-ohm level. Table 2 listed the sensing 4
Applied Surface Science 503 (2020) 144167
R. Zhang, et al.
Fig. 4. Resistance curves of the sensors based on (a) Co3O4-C, (b) Co3O4-R, (c) Co3O4-S towards 200 ppm toluene at 180 °C, respectively; (d) responses of three sensors towards 200 ppm toluene at different working temperatures; (e) responses of three sensors to 200 ppm different target gases at 180 °C; (f) dynamic resistance curve of the sensor based on Co3O4-S to different concentration of toluene; (g) relationship between response and toluene concentration; (h) response and morphology stability of Co3O4-S-based sensor to 200 ppm toluene during 30 days (measurement number = 3); (i) schematic of sensor exposed to air and target gas.
humidity (RH) in environment. It is widely considered that the water molecules may interact with the adsorbed oxygen on the sensing surface, resulting in forming hydroxyl groups and release electrons [31–32]. The negligible effect may be attributed to the reason that the hydroxyl groups are almost completed at low RH [31].
rapidly when the toluene concentration is less than 50 ppm. And, the Co3O4-S exhibits a recognizable response to 5 ppm xylene (1.2) and 1.2 was used as the criterion of detection limit for gas sensing [24,25]. Long-term stability of gas sensors was further investigated and gas response evolution was shown in Fig. 4(h) through repeatedly measuring the response for a number of times during one month. The 30 days-later responses slightly changed from 8.5 ± 0.2 to 7.5 ± 0.7. Moreover, sheet-like morphologies still existed after 30 days tests. The results illustrated good reliability and stability of Co3O4-S for potentially commercial application as sensing material. Table 3 listed toluene-sensing performances of Co3O4-S-based sensor compared with the previous works, which highlighted the advancement of this work [26–30]. Moreover, for the purpose of better practical application, the effect of moisture on sensing performance was also investigated (Fig. S4). It could be clearly seen that the humidity has a slightly negative impact on the gas sensing response with the increasing relative
3.3. Gas sensing mechanism The possible reasons of such excellent characteristic of hierarchical Co3O4 nanostructures are studied in the following sections. Gas sensing mechanism on metal oxide semiconductors have been widely discussed by lots of previous works [4,5,33]. Most believe that the electrical conductivity of sensing materials would be changed by the chemical reactions taken place on the surface of semiconductors. In this work, Co3O4 is p-type semiconductor and holes are the charge carriers. Once the sensor exposed in air, oxygen molecules may adsorb on the surface
Table 2 Comparison of toluene-sensing properties of three Co3O4-based sensors. Samples
Structure
BET/[m2g−1]
Co2+/(Co2++Co3+)
R0/[kΩ]
Res.
Tres/[s]
Trec/[s]
Co3O4-C Co3O4-R Co3O4-S
Cube Rod Sheet
17 36 69
42% 59% 62%
2.3 4.7 4.9
2.4 4.1 8.5
116 22 10
107 47 30
♦R0: Resistance in air; ♦Res.: Response; ♦Tres: Response time; ♦Trec: Recovery time. 5
Applied Surface Science 503 (2020) 144167
R. Zhang, et al.
Table 3 Comparison of toluene-sensing performances of the sensors based different materials. Samples
Structure
Tem.(°C)
Res.
Tres/[s]
Trec/[s]
Ref.
La/SnO2 SnO2 NiO/SnO2 Au-ZnO Co3O4 Co3O4
Nanorod array Nanofiber Nanofiber Nanowire Network Sheet
290 350 300 340 150 180
3 (200 ppm) 4.7 (100 ppm) 11 (50 ppm) 7.5 (50 ppm) 60.8 (100 ppm) 8.5 (200 ppm)
– 1 11.2 45 150 10
– 5 4 39 200 30
[26] [27] [28] [29] [30] This work
♦Tem.: Working temperature; ♦Res.: Response; ♦Tres: Response time; ♦Trec: Recovery time; ♦Ref: Reference.
surface with the largest exposed specific surface area among the three materials, which may contribute to the mass transportation and gas diffusion across the sensing materials. Thus, the gas sensor based on Co3O4-S showed the highest response and fastest response/recovery speed. 4. Conclusion In summary, the uniform hierarchical spinel-type Co3O4 with cube(C), rod- (R) and sheet- (S) shaped morphologies were successfully synthesized using a hydrothermal method. The sensing performance of toluene detection increased in the order C < R < S. The Co3O4-S nanostructure showed a highest response (8.5) with a fast response/ recovery speed (10/30 s) at a working temperature of 180 °C. The superior properties are attributed to the largest specific surface area, highest cobalt ion (Co2+) and oxygen vacancy content, leading to attracting more surface adsorbed oxygen species. Moreover, for the S sample, no significant decrease in response was observed over one month, which indicates that the 3D hierarchical sheet-stacked Co3O4 structures exhibit excellent sensing activity and stability for toluene detection. Therefore, it might be a potential effective sensing material in practical applications. Acknowledgements
Fig. 5. Schemtic of sensing mechanism (a) in air and (b) in toluene; (c) spinel structure of Co3O4 crystal; (d) a potion of the porous sheet-shaped hierarchical Co3O4 nanostructures.
This work was supported by the Natural Science Foundation Committee (NSFC, Grant No. 61673191) and the Science and Technology Development Plan of Jilin Province (No. 20180414025GH).
of the Co3O4 materials, resulting in formation of adsorbed oxygen species. During this process, electron will be trapped in Co3O4 and hole accumulation layers (HALs) are formed, resulting in low resistance value in air. Then, when the sensor exposed to reducing gas (toluene), adsorbed oxygen species may react with target gas molecules. Thus, electrons were released back to sensing materials, resulting in high resistance value in gas. The related reactions are proposed in follows (Fig. 5(a-b)):
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.144167. References
O2 + 2e− → 2O−
[1] A. Mirzaei, S.G. Leonardi, G. Neri, Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors: A review, Ceram. Int. 42 (2016) 15119–15141. [2] A. Mirzaei, J.-H. Kim, H.W. Kim, S.S. Kim, Resistive-based gas sensors for detection of benzene, toluene and xylene (BTX) gases: a review, J. Mater. Chem. C 6 (2018) 4342–4370. [3] S.-Y. Cho, H.-W. Yoo, J.Y. Kim, W.-B. Jung, M.L. Jin, J.-S. Kim, H.-J. Jeon, H.T. Jung, High-resolution p-type metal oxide Semiconductor nanowire array as an ultrasensitive sensor for volatile organic compounds, Nano Lett. 16 (2016) 4508–4515. [4] H.J. Kim, J.H. Lee, Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview, Sens. Actuators B 192 (2014) 607–627. [5] J.M. Xu, J.P. Cheng, The advances of Co3O4 as gas sensing materials: A review, J. Alloys Compd. 686 (2016) 753–768. [6] H.Q. Sun, H.M. Ang, M.O. Tadé, S.B. Wang, Co3O4 nanocrystals with predominantly exposed facets: synthesis, environmental and energy applications, J. Mater. Chem. A 1 (2013) 14427–14442. [7] X. Wang, W. Tian, T.Y. Zhai, C.Y. Zhi, Y. Bando, D. Golberg, Cobalt(II, III) oxide hollow structures: fabrication, properties and applications, J. Mater. Chem. 22 (2012) 23310–23326. [8] G.R. Patzke, Y. Zhou, R. Kontic, F. Conrad, Oxide nanomaterials: synthetic developments, mechanistic studies, and technological innovations, Angew. Chem. Int. Ed. 50 (2011) 826–859.
C7H8 + 18O− → 4H2O + 7CO2 + 18e− Many reports have clarified the good catalytic performance of Co3O4 to toluene [34–36]. Co(II) ions and Co(III) ions occupy the cobalt sites simultaneously according to the spinel Co3O4 crystal in Fig. 5(c). The oxidation sites of cobalt ions could easily switch between Co2+ and Co3+. Thus, more oxygen ions could be adsorbed and gathered easier, resulting in enriching active oxygen species on the surface of the material. Furthermore, the morphological appearance of the sensing materials has a serious impact on gas sensing performances [5,37–39]. Generally, a porous hierarchical structure could facilitate the diffusion of target gases and contribute to increasing the surface reactive sites in the meanwhile [37,40,41]. Although Co3O4-C, Co3O4-R and Co3O4-S are all well hierarchical structures, the interior structure of Co3O4-C and Co3O4-R samples are really compact compared with the sheet-assembly one. The unique nanosheet-shaped hierarchical structure has porous 6
Applied Surface Science 503 (2020) 144167
R. Zhang, et al.
[25] R. Zhang, X.P. Liu, T.T. Zhou, L.L. Wang, T. Zhang, Carbon materials-functionalized tin dioxide nanoparticles toward robust, high-performance nitrogen dioxide gas sensor, J. Colloid Interf. Sci. 524 (2018) 76–83. [26] F. Gao, G.H. Qin, Y.H. Li, Q.P. Jiang, L. Luo, K. Zhao, Y.J. Liu, H.Y. Zhao, One-pot synthesis of La-doped SnO2 layered nanoarrays with an enhanced gas-sensing performance toward acetone, RSC Adv. 6 (2016) 10298–10310. [27] Q. Qi, T. Zhang, L. Liu, X. Zheng, Synthesis and toluene sensing properties of SnO2 nanofibers, Sens. Actuators B 137 (2009) 471–475. [28] Y.L. Liu, G. Zhang, S. Wang, L. Li, Y. Wang, X. Han, A.W. Jiang, High toluene sensing properties of NiO-SnO2 composite nanofiber sensors operating at 330 °C, Sens. Actuators B 160 (2011) 448–454. [29] L. Wang, S. Wang, M. Xu, X. Hu, H. Zhang, Y. Wang, W. Huang, A Au-functionalized ZnO nanowire gas sensor for detection of benzene and toluene, Phys. Chem. Chem. Phys. 15 (2013) 17179–17186. [30] C. Zhao, B. Huang, J. Zhou, E. Xie, Synthesis of porous Co3O4 nanonetworks to detect toluene at low concentration, Phys. Chem. Chem. Phys. 16 (2014) 19327–19332. [31] X.H. Ma, H.Y. Li, S.-H. Lweon, S.-Y. Jeong, J.-H. Lee, S. Nahm, Highly sensitive and selective PbTiO3 gas sensors with negligible humidity interference in ambient atmosphere, ACS Appl. Mater. Interfaces 11 (2019) 5240–5246. [32] D.X. Ju, H.Y. Xu, Z.W. Qiu, Z.C. Zhang, Q. Xu, J. Zhang, J.Q. Wang, B.Q. Cao, Near room temperature, fast-response, and highly sensitive triethylamine sensor assembled with Au-loaded ZnO/SnO2 core-shell nanorods on flat alumina substrates, ACS Appl. Mater. Interfaces 7 (2015) 19163–19171. [33] R. Zhang, T.T. Zhou, L.L. Wang, T. Zhang, Metal−organic frameworks-derived hierarchical Co3O4 structures as efficient sensing materials for acetone detection, ACS Appl. Mater. Interfaces 10 (2018) 9765–9773. [34] G.M. Bai, H.Q. Dai, J.G. Deng, Y.X. Liu, F. Wang, Z.X. Zhao, W.G. Qiu, C.T. Au, Porous Co3O4 nanowires and nanorods: Highly active catalysts for the combustion of toluene, Appl. Catal., A: General 450 (2013) 42–49. [35] Y.X. Liu, H.X. Dai, J.G. Deng, S.H. Xie, H.G. Yang, W. Tan, W. Han, Y. Jiang, G.S. Guo, Mesoporous Co3O4-supported gold nanocatalysts: Highly active for the oxidation of carbon monoxide, benzene, toluene, and o-xylene, J. Catal. 309 (2014) 408–418. [36] Y.X. Liu, H.X. Dai, J.G. Deng, L. Zhang, Z.X. Zhao, X.W. Li, Y. Wang, S.H. Xie, H.G. Yang, G.S. Guo, Controlled generation of uniform spherical LaMnO3, LaCoO3, Mn2O3, and Co3O4 nanoparticles and their high catalytic performance for carbon monoxide and toluene oxidation, Inorg. Chem. 52 (2013) 8665–8676. [37] J.H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures: overview, Sens. Actuators B 140 (2009) 319–336. [38] L.L. Wang, S. Chen, W. Li, K. Wang, Z. Lou, G.Z. Shen, Grain-boundary-induced drastic sensing performance enhancement of polycrystalline-microwire printed gas sensors, Adv. Mater. 31 (2019) 1804583. [39] X.H. Liu, T.T. Ma, N. Pinna, J. Zhang, Two-dimensional nanostructured materials for gas sensing, Adv. Funct. Mater. 27 (2017) 1702168-(30). [40] R. Zhang, T. Zhang, T.T. Zhou, L.L. Wang, Rapid sensitive sensing platform based on yolk-shell hybrid hollow sphere for detection of ethanol, Sens. Actuators B 256 (2018) 479–487. [41] S.H. Choi, I.S. Hwang, J.H. Lee, S.G. Oh, I.I.-D. Kim, Microstructural control and selective C2H5OH sensing properties of Zn2SnO4 nanofibers prepared by electrospinning, Chem. Commun. 47 (2011) 9315–9317.
[9] M.B. Gawande, A. Goswami, T. Asefa, H.Z. Guo, A.V. Biradar, D.L. Peng, R. Zboril, R.S. Varma, Core–shell nanoparticles: synthesis and applications in catalysis and electrocatalysis, Chem. Soc. Rev. 44 (2015) 7540–7590. [10] L. Liu, Z.Q. Jiang, L. Fang, H.T. Xu, H.J. Zhang, X. Gu, Y. Wang, Probing the crystal plane effect of Co3O4 for enhanced electrocatalytic performance toward efficient overall water splitting, ACS Appl. Mater. Interfaces 9 (2017) 27736–27744. [11] P. Kannan, T. Maiyalagan, E. Marsili, S. Ghosh, L.H. Guo, Y.J. Huang, J.A. Rather, D. Thiruppathi, J. Niedziolka-Jönsson, M. Jönsson-Niedziolka, Highly active 3-dimensional cobalt oxide nanostructures on the flexible carbon substrates for enzymeless glucose sensing, Analyst 142 (2017) 4299–4307. [12] Q. Zhou, W. Zeng, Shape control of Co3O4 micro-structures for high-performance gas sensor, Physica E 95 (2018) 121–124. [13] J.M. Xu, J. Zhang, B.B. Wang, F. Liu, Shape-regulated synthesis of cobalt oxide and its gas-sensing property, J. Alloys Compd. 619 (2015) 361–367. [14] Y.-M. Jo, T.-H. Kim, C.-S. Lee, K. Lim, C.W. Na, F. Abdel-Hady, A.A. Wazzan, J.H. Lee, Metal-organic framework-derived hollow hierarchical Co3O4 nanocages with tunable size and morphology: ultrasensitive and highly selective detection of methylbenzenes, ACS Appl. Mater. Interfaces 10 (2018) 8860–8868. [15] Q. Li, Y. Du, X.J. Li, G.Y. Lu, W.Y. Wang, Y.F. Geng, Z.Q. Liang, X.Q. Tian, Different Co3O4 mesostructures synthesised by templating with KIT-6 and SBA-15 via nanocasting route and their sensitivities toward ethanol, Sens. Actuators, B 235 (2016) 39–45. [16] S. Chen, L. Zhang, Y.T. Han, X.W. Chen, S.T. Wang, M. Zeng, N.T. Hu, Y.J. Su, Z.H. Zhou, H. Wei, Z. Yang, Glucose-assisted synthesis of hierarchical flower-like Co3O4 nanostructures assembled by porous nanosheets for enhanced acetone sensing, Sens. Actuators, B 235 (2016) 39–45. [17] K. Zhao, H.T. Li, S.Q. Tian, W.J. Yang, X.X. Wang, A.M. Pang, C.S. Xie, D.W. Zeng, A facile low-temperature synthesis of hierarchical porous Co3O4 micro/nano structures derived from ZIF-67 assisted by ammonium perchlorate, Inorg. Chem. Front. 6 (2019) 715–722. [18] M.J. Wang, Z.R. Shen, X.D. Zhao, F.P. Duanmu, H.J. Yu, H.M. Ji, Rational shape control of porous Co3O4 assemblies derived from MOF and their structural effects on n-butanol sensing, J. Hazard. Mater. 371 (2019) 352–361. [19] Q.M. Ren, S.P. Mo, R.S. Peng, Z.T. Feng, M.Y. Zhang, L.M. Chen, M.L. Fu, J.L. Wu, D.Q. Ye, Controllable synthesis of 3D hierarchical Co3O4 nanocatalysts with various morphologies for the catalytic oxidation of toluene, J. Mater. Chem. A 6 (2018) 498–509. [20] L.L. Wang, T.T. Zhou, R. Zhang, Z. Lou, J.N. Deng, T. Zhang, Comparison of toluene sensing performances of zinc stannate with different morphology-based gas sensors, Sens. Actuators, B 227 (2016) 448–455. [21] J.F. Marco, J.R. Gancedo, M. Gracia, J.L. Gautier, E. Rios, F.J. Berry, Characterization of the nickel cobaltite, NiCo2O4 prepared by several methods: An XRD, XANES, EXAFS, and XPS study, J. Solid State Chem. 153 (2000) 74–81. [22] J.W. Xiao, Q. Kuang, S. Yang, F. Xiao, S. Wang, L. Guo, Surface structure dependent electrocatalytic activity of Co3O4 anchored on graphene sheets toward oxygen reduction reaction, Sci. Rep. 3 (2013) 2300. [23] D.R. Miller, S.A. Akbar, P.A. Morris, Nanoscale metal oxide-based heterojunctions for gas sensing: A review, Sens. Actuators B 204 (2014) 250–272. [24] S.-Y. Jeong, J.-W. Yoon, T.-H. Kim, H.-M. Jeong, C.-S. Lee, Y.C. Kang, J.-H. Lee, Ultra-selective detection of sub-ppm-level benzene using Pd-SnO2 yolk-shell microreactors with a catalytic Co3O4 overlayer for monitoring air quality, J. Mater. Chem. A 5 (2017) 1446–1454.
7