Accepted Manuscript Full Length Article Facile Route to Synthesize Porous Hierarchical Co3O4/CuO Nanosheets with High Porosity and Excellent NOx Sensing Properties at Room Temperature Siyu Liu, Lei Teng, Yiming Zhao, Zhi Liu, Jiawei Zhang, Muhammad Ikram, Afrasiab Ur Rehman, Li Li, Keying Shi PII: DOI: Reference:
S0169-4332(18)31110-3 https://doi.org/10.1016/j.apsusc.2018.04.150 APSUSC 39149
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
30 November 2017 6 April 2018 16 April 2018
Please cite this article as: S. Liu, L. Teng, Y. Zhao, Z. Liu, J. Zhang, M. Ikram, A. Ur Rehman, L. Li, K. Shi, Facile Route to Synthesize Porous Hierarchical Co3O4/CuO Nanosheets with High Porosity and Excellent NOx Sensing Properties at Room Temperature, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.04.150
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Facile Route to Synthesize Porous Hierarchical Co3O4/CuO Nanosheets with High Porosity and Excellent NOx Sensing Properties at Room Temperature Siyu Liu a, Lei Teng a, Yiming Zhao a, Zhi Liu a, Jiawei Zhang b, Muhammad Ikram a, Afrasiab Ur Rehman a, Li Li a, c*, Keying Shi a, b* a
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and
Material Science, Heilongjiang University, Harbin, 150080, P. R. China. b
Key Laboratory for Photonic and Electronic, Ministry of Education, Modern Experiment Center, Harbin Normal
University, Harbin 150025, P. R. China. c
Key Laboratory of Chemical Engineering Process & Technology for High-efficiency Conversion, School of
Chemistry and Material Science, Heilongjiang University, Harbin 150080, P. R. China.
Corresponding Author Fax: +86 451 8667 3647; Tel: +86 451 8660 9141 E-mail:
[email protected],
[email protected]
Abstract To fabricate sensors that are capable of ultrasensitive detection of NOx as well as optimize their synthetic route, highly porous and hierarchically structured Co3O4/CuO nanosheets were synthesized by a facile hydrothermal-calcination route. The CC2-1 sample synthesized with the 2:1 molar ratio of Co(NO3)2·6H2O and CuCl2·2H2O has the most abundant porosity. Structural measurements found that the size of pore is 3.37 nm, the specific surface area is 24.04 m2g-1, and the average slice thickness is about 5 nm. This optimum sample presented excellent NOx sensing performance at room temperature (RT = 21 ℃), which has not only the highest response (14.16 to 1000 ppm), the shortest response time (2 s to 1000 ppm), and the minimum detection limit (0.01 ppm), but also good reversibility and selectivity. The superior property arises from the appropriate CuO ratio and the addition of pore-forming agent NaHCO3, and all together resulted in the unique hierarchical heterojunction structure, endowed with abundant porosity and a large number of defects, which eventually engender the remarkable chemisorbed ability to oxygen species.
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Keywords: Co3O4/CuO semiconductors; porous hierarchical nanosheets; NOx sensors; room temperature 1. Introduction Nowadays, nitrogen oxides (NOx) detection has become a burgeoning area of research due to increasingly serious air pollution. NOx species are important components of PM2.5, which are difficult to be observed because of their high visibility [1-4]. Throughout the history of exploring and developing sensors, high temperature operating NOx sensors has achieved great success [5-7]; however, room temperature (RT) sensors remain underdeveloped. Therefore, studies of RT sensors for NOx are highly desirable. Porous metal oxide semiconductors have received much more attentions in recent years due to their advantages, such as abundant reactive sites around porosity and fast interfacial transport between adjacent crystallites [8-11]. Compared with the entire particle bulk, the porous hierarchical structure exhibits a gallery pathway that facilitates carriers’ diffusion/transportation [12]. Recent studies have been made to design and synthesize porous hierarchical materials with good performance. Among various synthetic strategies, the hydrothermal-calcination route is an alternative because of its simplicity, high efficiency and low cost. To obtain porous hierarchical metal oxides with high porosity, the addition of pore-forming agent is the crucial factor. The precursor can release a large amount of gas during the conversion process (such as CO2 or H2O) and produce a volume of pores in the products. Moreover, this manner is able to retain a stable original hierarchical structure [13]. Spinel cobalt oxide (Co 3O4) is known as a typical p-type semiconductors [14]. It has been widely used in the field of photocatalytic [15], lithium-ion batteries [16], supercapacitors [17], gas sensors [18, 19] and so on. Due to its excellent electrical conductivity, catalytic activity and chemical stability [20], Co3O4 has been an ideal candidate for atmospheric environmental monitoring with high response and short response time [21, 22]. Diverse Co3O4 porous nanostructures, including nanospheres, nanorods, nanotubes, and nanoflowers, have been synthesized by a variety of methods [23-26]. However, some formed porous Co 3O4 products remain unfavorable pore structure, and often are unable to maintain their original morphology. Most seriously, several Co3O4 nanostructures have poor response, long response time and instability; on the other hand they require high operating temperatures [24, 27]. Because investigations of Co3O4 gas sensors are mainly focused on the test under high temperature conditions, their application is severely limited to detect some special gases 2
[5-7, 28-31]. So, it is very important to fabricate Co3O4-type gas sensors that are suitable for RT operation. The most direct way to improve Co 3O4 sensing properties at RT is to incorporate with the other metal oxides. Up to date, there has been extensive research regarding Co3O4 modification. As another important p-type semiconductor, CuO is also used in gas sensors and exhibits good performances [32]. Thus, we have endeavored to investigate the porous nanostructure Co3O4 doped CuO sensors as shown in Scheme 1, and aimed to obtain the ultrasensitive sensors materials for detection NOx at room temperature. This work has been dedicated to fabricating two-dimensional (2D) hierarchical Co 3O4/CuO nanosheets (NSs) with abundant pores, large surface area and good NOx sensing activity. A simple hydrothermal-calcination method (Scheme 1) was applied. The molar ratio of composites was tuned on one hand; simultaneously the pore-forming agent NaHCO3 was added to construct more active sites. The samples with the optimum NO x gas sensing performance at RT were addressed. Detailed analysis of the synthesized materials was made to explain their excellent gas sensing property.
Scheme 1. Design and synthesis of the porous hierarchical Co3O4/CuO NSs.
2. Experimental 2.1. Synthesis of Co3O4/CuO NSs In a typical experiment, 50 mL deionized water was added to a 250 mL beaker，then total 4 mmol of Co(NO3)2·6H2O and CuCl2·2H2O (mole ratio of 4:1, 2:1, 1:1) were dissolved together then the total amount of 4 mmol Co(NO3)2·6H2O and CuCl2·2H2O were dissolved together under magnetic stirring, and the molar ratios were set to 4:1, 2:1 and 1:1, respectively. After stirring for 10 minutes, 20 mL 1 M NaHCO3 aqueous solution was add slowly and continue stirring for 30 minutes. Subsequently, the mixture was transferred to a Teflon-lined stainless steel autoclave and the 3
hydrothermal synthesis was conducted at 200 ℃ for 4 h. After naturally cooled to room temperature, the precipitate was filtered and washed through deionized water and absolute ethanol for several times and then dried in vacuum for 12 h at 60 ℃, the mixed carbonate precursor was obtained which named CCP4-1, 2-1, 1-1, respectively. Finally, the precursor was calcined in air at 550 °C for 2 h, the obtained porous Co3O4/CuO NSs named as CC4-1, 2-1, 1-1, respectively. For comparison, pure Co3O4, CuO and its precursor was prepared using the same procedure only without CuCl2·2H2O, named as Co3O4, CuO and Co3O4P, respectively. 2.2. Material characterizations Characterization of crystal phase and composition of Co3O4/CuO by X-ray powder diffraction (XRD, D/max-IIIB-40 KV, Japan, Cu Kα radiation, λ=1.5406 Å). The Fourier transform infrared (FT-IR) spectrum was studied by FT-IR Spectrometer (Perkin Elmer Spectrometer, KBr pellet technique). The morphologies of the Co3O4/CuO were observed by scanning electron microscopy (SEM, HITACHI S-4800) and transmission electron microscopy (TEM, JEOL 2100). Thermal gravimetric differential scanning calorimetry (TG-DSC) analysis of the samples was carried out using TA-SDTQ600. The temperature of the reactor increases at a rate of 8 ℃·min-1, the air flow rate is maintained at 60 mL·min-1. The Brunauer-Emmett-Teller (BET) surface area of the samples was measured by N2 adsorption-desorption (TriStar II3020). X-ray photoelectron spectra (XPS) were recorded with an AXIS ULPRA DLD (Shimadzu Corporation) system. 2.3. Gas sensing tests The gas sensing test system used in this subject is designed and assembled by our research group. Firstly, the Au interdigitated electrodes were cleaned several times with ethanol and distilled water and dried. Secondly, 0.5 mg of CC sample was dissolved in 1 mL ethanol, dispersed by ultrasonication to form a suspension, and then 2-4 layers of this suspension were added dropwise to the surface of the Au interdigitated electrode (99.6%, 7 mm×5 mm×0.25 mm). Each Au electrode contains 100 interdigitated finger fringes, and the distance between the two fingers is 20 μm. Finally, the assembled sensor was dried at 60 °C for 6 hours to prepare a gas sensor. Secondly, the sensor is installed in the test chamber with inlet and outlet for contact with NOx. The resistance of the sensor is measured at room temperature (RT) with a relative humidity (RH) of about 24%. Finally, use a micro-injector to inject different concentrations of NOx in turn, and then measure the response of the sensor. The concentration of NOx gas from high to low order was 1000, 700, 500, 4
300, 100, 50, 30, 10, 5, 3, 1, 0.5, 0.3, 0.1, 0.05, 0.03, 0.01 ppm, respectively. The response of the gas sensor was defined as S=Rg/Ra, where Ra and Rg are the resistance in air and the NOx gas.
3. Results and Discussion 3.1. Morphology and Structure characterizations To study the structural features of the samples, the crystalline and phase of the pure Co 3O4, synthesized Co3O4/CuO and its precursors composites were characterized by X-ray diffraction (XRD) as shown in Fig. 1.
Figure 1 X-ray diffraction patterns of (a) Co3O4/CuO 2:1 precursor and (b) Co3O4/CuO NSs; FT-IR spectra of (c) Co3O4/CuO 2:1 precursor, (d) Co3O4 and Co3O4/CuO NSs.
Fig. 1a exhibited the XRD pattern of Co 3O4/CuO 2:1 precursor. The sharp peaks at 14.7°, 17.4°, 24.0°,30.2°, 35.4°, 36.6°, 38.5°, 41.3°, 45.0°, 61.4° and 65.0° of CCP2-1 could be indexed to the (020), (120), (220), (021), (240), (330), (150), (420), (060), (-451) and (280) planes of monoclinic phases of (Cu, Co)2(OH)2CO3, and the corresponding interplanar spacing was 6.04 Å, 5.08 Å, 3.70 Å, 2.96 Å, 2.54 Å, 2.45 Å, 2.34 Å, 2.19 Å, 2.01 Å, 1.51 Å and 1.44 Å, respectively (JCPDS 29-1416). Fig. 1b showed XRD patterns of Co 3O4/CuO in different molar ratios. The curve exhibited eight peaks at 2θ = 19.0°, 31.3°, 36.9°,44.8°, 55.7°, 59.4°, 65.2° and 77.3°, which were indexed to the (111), (220), (311), (400), (422), (511), (440) and (533) planes of Co 3O4 with d spacing of 4.67 Å, 2.86 Å, 2.44 Å, 2.02 Å, 1.65 Å, 1.56 Å, 1.43 Å, and 1.23 Å, respectively (JCPDS 42-1467). And the particularly narrow and high intensity peak at (311) plane was a very typical characteristic peak of 5
Co3O4 which indicating the sample has high purity and crystallinity. The other five peaks at 2θ = 35.5°, 38.7°, 48.7°, 58.3° and 75.0° assigned to the (-111), (111), (-202), (202) and (004) reflections of CuO with the corresponding interplanar spacing of 2.53 Å, 2.32 Å, 1.87 Å, 1.58 Å, and 1.26 Å, respectively (JCPDS 45-0937). And the (-111) plane was a very typical characteristic peak of CuO. The composite comprised monoclinic phase of CuO and cubic phase of Co 3O4 two crystal phases. XRD patterns of pure Co3O4 as shown in Fig. S1. The peak of the Co3O4 at 38.5° disappeared after doped CuO. As shown in Fig. 1a, the relative intensity of all these eight peaks of Co3O4 were gradually weakened with the proportion of CuO gradual increased, and relative intensity and the number of CuO peaks significantly increased. When the molar ratio of Co to Cu reaches 1: 1, the peak of Co3O4 at 55.7° disappears and a new CuO peak appears at 58.3°. These results suggested that CuO is well doped into Co 3O4, and incorporation of different proportions of CuO will inhibit the growth of Co3O4 in varying degrees. The narrow and high intensity peaks in XRD showed that the composites have good purity and crystallinity [33]. Moreover, the compositions of the composites were further investigated by FT-IR spectra. Fig. 1c presented FT-IR spectra of the CC2-1 precursor. The broad peak around 3300 and 3499 cm-1 corresponded to -OH stretching vibration of hydroxyl ions in (Cu, Co) 2(OH)2CO3. The four bands in the region of 1551-836 cm-1 were assigned to the ν2 and ν3 mode of the carbonate ion [34] in (Cu, Co)2(OH)2CO3. The bands at 661 and 423 cm-1 were attributed to Co-OH and Cu-OH bonds (strong Co/Cu-OH interactions), respectively [35]. Following XRD, FT-IR results can further confirm that the precursor is mixed carbonate (Cu, Co)2(OH)2CO3 combined with Co2(OH)2CO3 and Cu2(OH)2CO3. As shown in Fig. 1d, the broad peak around 3390 cm-1 corresponded to the -OH stretching vibration of the hydrated water of Co 3O4 [36]. The FT-IR spectrum displayed two intensive bands around at 660 cm-1 and 580 cm-1 were assigned to the stretching vibration of (Co Ⅱ-O) and (CoⅢ-O) [37-39], which were caused by the Co2+ and Co3+ vibrations in the tetrahedron and octahedral gaps in the spinel lattice of Co 3O4, respectively [36]. Furthermore, as the doping amount of CuO (CuII-O) gradually increased, the absorption band at about 580 cm-1 shifted to a low wavenumber of 560 cm-1, and the peak became wider. This could further confirm the presence of Co 3O4 and the incorporation of CuO into spinel phase Co 3O4 [40]. 6
Figure 2. (a) TG-DSC curves of Co2(OH)2CO3 (Co3O4) and (b) TG-DSC curves of (Cu, Co)2(OH)2CO3 (CCP2-1); (c) The BET and (d) BJH curve of CC4-1, CC2-1 and CC1-1.
In order to determine the weight change of precursors during calcination process, thermogravimetric differential scanning calorimetry analysis (TG-DSC) was performed in air atmosphere, as shown in Fig. 2. XRD patterns and FT-IR spectra of CCP2-1 proved that the precursor of Co3O4 and Co3O4/CuO NSs were Co2(OH)2CO3 and mixed carbonate (Cu, Co)2(OH)2CO3 which combined with Co2(OH)2CO3 and Cu2(OH)2CO3, respectively. As shown in Fig. 2a, b, the weight losses of Co2(OH)2CO3 and (Cu, Co)2(OH)2CO3 divided into three stages. The first little weight losses of the two materials below 230 ℃ was caused by the evaporation of the adsorbed water, and also has the formation of a sharp endothermic peak around 50 ℃. For pure Co3O4 precursors Co2(OH)2CO3, the second stage of the distinct weight losses (ca. 22%) from 250 ℃ to 385 ℃ owing to the removal of carbonate anions, and there was a transition between 300 ℃ and 385 ℃ on the TG curve since Co2+ began to oxidized. The two weight losses corresponded to two exothermic/oxidation peaks at 263 ℃ and 333 ℃, respectively [41]. For Co3O4/CuO precursors, the weight loss (ca. 18%) of the second stage from 225 °C to 370 °C was owing to the removal of hydroxyl and carbonate anions which eventually generated CO2 and H2O etc., and corresponding to two exothermic/oxidation peaks at 274 ℃ and 389 ℃. The slight weight losses of Co2(OH)2CO3 and (Cu, Co)2(OH)2CO3 about 380 ℃ to 700 ℃ in the final stage owing to the complete oxidation of Co 2+ 7
[13]. The overall peak position of (Co, Cu)2(OH)2CO3 was shifted to a higher temperature than Co2(OH)2CO3 in the DSC curve. The TG-DSC curve showed that the precursor had been completely oxidized at about 450 ℃, this indicating that it was reasonable to select 550 ℃ as the calcination temperature. Fig. 2c, d showed the N2 adsorption-desorption isotherms (BET) and pore size distributions (BIH) of CC4-1, CC2-1 and CC1-1, respectively. Remarkably, all of these samples displayed typical type IV adsorption isotherms with H3-type hysteresis loops, indicating the presence of mesoporous structures. And the hysteresis loops are really narrow, this indicating that the adsorption and desorption process was fast. The specific surface area of these three samples was 16.71, 24.04 and 17.93 m2g-1, respectively. As shown in the pore size distribution curves, CC4-1, CC2-1 and CC1-1 NSs contained a large number of mesoporous in the 3-55 nm with the main mesoporous sizes of 17.46, 3.37 and 17.45 nm. It was worth noting that the pore size of CC2-1 in BJH coincided with the TEM image. The specific surface area of CC2-1 was the largest and the main pore size is the smallest among these three samples. And the smallest pore size is the reason for its larger surface area, which means high porosity. The smallest pore size and the largest specific surface area provided a large number of transmission channels for the NSs to increase the reaction rate [12] and accelerate the adsorption and desorption processes between gas molecules and material surfaces, which ultimately enhanced the response and reduced the response time of the material [33].
Figure. 3 (a) The low magnification TEM image and (b) HRTEM image of pure Co3O4.
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Figure. 4 (a) The low magnification TEM image and (b) HRTEM image of pure CuO.
Figure. 5 (a), (b), (c) The low magnification TEM image of CC2-1 and the SAED image of (a); (d), (e), (f) HRTEM images of CC2-1.
The microstructure and morphology of pure Co3O4, pure CuO and CC samples were observed by TEM and HRTEM as shown in Fig. 3, Fig. 4, Fig. 5, Fig. S2 and Fig. S3. The yellow rectangles and circles indicated the defects, and the red circles indicated the pores. Fig. 5a and Fig. S2a, b clearly 9
exhibited the porous hierarchical nanosheets structure of CC2-1, and demonstrated the CC2-1 porous hierarchical structure which was built by the homogeneously distributed mesoporous with an average pore size of 3-4 nm, and the rectangular nanosheets with an average thickness about 5 nm, width of 200-400 nm and length of 1-3µm. While Fig. S3 presented the CC1-1 porous hierarchical structure built by pores size of 4-18 nm. It was cleared that CC2-1 has a smaller pore size and a higher porosity than CC1-1. And it can be seen that there are containing many small cross-linking on the submicron-sized sheets. That is, nanocrystals are interconnected to construct a 2D nanosheet. As shown in Fig. 5b-f, Fig. 5c, f are enlarged views of b, e, respectively, the HRTEM images presented the lattice fringes with the d spacing of 0.244 and 0.286 nm of CC2-1 corresponded to the (311) and (220) planes of Co 3O4, respectively, and the (-111) plane of CuO corresponded to 0.253 nm can be also observed. Moreover, (220) is an active plane with a high catalytic activity of Co 3O4, which was presented in CC samples and help to improve the surface reaction rate of the sample. HRTEM images of CC2-1 and CC1-1 demonstrated that there were many heterojunctions and defects in the interface between submicron Co 3O4 and CuO in nanosheets. In addition, the selected area electron diffraction patterns were shown in Fig. 5a. It was clearly presented that CC2-1 not only have polycrystalline diffraction rings but also a regular single crystal lattice structure, which corresponding to Co3O4 (311), (511), (533) and CuO (-111) crystal planes from inside to outside. All of these results are consistent with XRD results. Appropriate proportion of Cu doping leads to amount of lattice defects that allowed more oxygen species was adsorbed on the surface of CC2-1. This made capture electrons process more efficiently [33]. It meant that a large number of mesoporous, defects and heterojunctions could provide more reactive sites and enhance the ability of the sample to adsorb and desorb gas. The synthesis process of porous hierarchical Co 3O4/CuO NSs was proposed in Scheme 2.
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Scheme 2. Schematic illustration of porous hierarchical Co3O4/CuO NSs via a hydrothermal-calcination route.
In order to investigate the formation process of porous structure, the structure and morphology of CC precursors and CC samples were analyzed by XRD, FT-IR, TG-DSC, EDS mapping and TEM. Firstly, Co(NO3)2·6H2O and CuCl2·2H2O was dissolved in deionized water to produce Co2+ and Cu2+, respectively. Then, a mixture of Co 2(OH)2CO3 and Cu2(OH)2CO3 was prepared by co-precipitation at room temperature using saturated NaHCO3 solution as a precipitant. Secondly, the carbonate mixtures (Cu, Co)2(OH)2CO3 with different pore size and hierarchical structures were synthesized by the co-assembly of Co2(OH)2CO3 and Cu2(OH)2CO3 under hydrothermal conditions, as shown in XRD and FT-IR, the precursor was (Cu, Co)2(OH)2CO3 (equation (1)) [13]. Finally, when the calcination temperature reached and exceeded the decomposition temperature, the mixed carbonate precursor and other guest molecules in the Co3O4/CuO precursors began to decompose in air atmosphere and released CO2 and H2O, thereby a large number of mesoporous in the nanosheets structure were creating (equation (2)). At the same time, Co II and CuII ions in the precursors reacted with O2 and transformed new porous Co 3O4/CuO hybrid nanosheets structure [41]. The reaction equation was as follows: Co(NO3)2·6H2O + CuCl2·2H2O + NaHCO3 → Co2(OH)2CO3 + (Cu)2(OH)2CO3 + NaCl + NaNO3 (Cu, Co)2(OH)2CO3 + O2 → Co3O4 + CuO + H2O ↑ + CO2 ↑
(1) (2)
As mentioned above, the pore size of porous Co3O4/CuO NSs would increase with the different Cu ratio. After calcinations, CuO nanoparticles and Co 3O4 nanoparticles were formed and interconnected. The formation of the Co 3O4/CuO NSs is probably due to the multiple interactions between metal ions [42]. Fig. 6, Fig. S4 and Table 1 exhibited the STEM image and EDS mapping of CC2-1. Fig. 6a is the bright field image of CC2-1, it was clear that the rectangular nanosheets were longitudinally distributed with a plenty of mesoporous with a diameter about 4 nm and the thickness of the porous nanosheets is about 5 nm. And Fig. 6 b-d displayed a uniform distribution of these three elements (Co, Cu and O) within the sample, respectively.
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Figure 6. STEM image/EDS mapping of CC2-1 sample. (a) STEM bright field image; ( b), (c), (d) corresponding to Co, Cu, and O single elemental mapping, respectively.
Fig. S4 and Table 1 exhibited the EDS spectra of CC2-1. It could be seen that the atom percentages (at. %) of these three elements Co, Cu, and O were 38.06%, 15.67% and 46.26%, respectively. This result showed the ratio of Co to Cu is 2: 1 exactly. Table 1 EDS elements content of CC2-1 Elements
Atomic
Series
Number
unn. C
norm. C
Atom. C
Error (1 Sigma)
[wt.%]
[wt.%]
[at.%]
[wt.%]
Co
27
K-series
39.63
56.37
38.06
1.34
Cu
29
K-series
17.60
25.03
15.67
0.74
O
8
K-series
13.08
18.60
46.26
2.03
12
Figure. 7 Mott-Schottky plots of the pristine Co3O4, CC4-1, CC2-1, CC1-1 electrodes in 1 mol·L-1 KOH electrolyte measured at a frequency of 1 kHz.
To further investigate the doping effects on the carrier densities of four samples, the Mott-Schottky test was carried out. As shown in Fig. 7, the negative slope of the MS curves indicated that the four samples exhibiting the characteristics of a typical p-type semiconductor. The lower the slope in the MS plot, the higher the carrier concentration. The carrier densities (Na) of the samples were calculated by using the following Eq: Na (2 / e0 0 )[d (1 / C 2 ) / dV ]1
(3)
Where d(1/C2)/dV is the linear slope of the sample MS curve, e0 is the fundamental charge constant (e0=1.60×10-19 C), Ɛ0 is the permittivity of vacuum (Ɛ0=8.85×10-14 F·cm-1), Ɛ is the relative permittivity of Co3O4 (Ɛ= 12.9). The carrier densities of pristine Co3O4, CC4-1, CC2-1, CC1-1were 2.30×1017, 2.81×1017, 4.01×1017 and 5.67×1017 cm-3, respectively. The lower the slope in the MS curve, the higher the carrier concentration of the sample. MS result showed that with CuO doping into the sample, the carrier density increased, the CC2-1 had a much higher carrier density, that is, CC2-1 had the best electrical conductivity [42]. Moreover, the order of the carrier density of each sample corresponded to their below gas sensing results. This further indicated that the electrochemical performance was closely related to the gas sensing properties. To observe the effect of element peak shift and confirm the influence of the adsorbed oxygen species on the sensing properties of the sensors, the XPS spectra of three Co3O4 samples with 13
different Cu rations were further measured as shown in Fig. 8.
Figure 8. XPS analysis of (a) full spectra, (b) Co 2p, (c) Cu 2p and (d, e and f) O 1s of porous Co3O4, CC1-1and CC2-1 NSs.
Fig.8a showed XPS survey spectrum of porous Co3O4, CC1-1and CC2-1 NSs which indicating the presence of Co, Cu, O elements, Cu successfully incorporated into Co3O4 NSs [43]. As shown in Fig. 8b, the distribution of Co 2p XPS peaks in three samples is similar, and the two major peaks intensity increased after doped CuO. The bands at 779.4, 779.8 and 780.2 eV (Co 2p
3/2
binding
energy), 794.5, 794.8 and 795.2 eV (Co 2p 1/2 binding energy) were same as spin-orbit splitting of Co3O4. Noticeably, a weak satellite peak around at 787.8 eV was an important characteristic for the Co3O4 phase [44]. The Co 2p3/2 and Co 2p1 /2 peak of CC1-1 and CC2-1 has a slight negative chemical shift to 0.4 eV, as summarized in Table S1. This is due to the electron interaction between Cu and Co, where electrons are transferred from Cu in CuO to Co in Co3O4 [42]. 14
Fig. 8c showed Cu 2p XPS spectra of the samples Co3O4, CC1-1 and CC2-1. The binding energies of two peaks of CC2-1 (Cu2p3/2 =933.7 eV, Cu2p1/2 = 953.6 eV) are lower than those of CC1-1 (Cu2p3/2 = 934.0 eV, Cu2p1/2 = 953.9 eV). The difference in the binding energy of satellite peaks between Cu2p3/2 and Cu2p1/2 is about 19.9 eV, which proves the presence of Cu2+ [45]. Moreover, Cu 2p3/2 and Cu2p1/2 peaks of CC1-1 and CC2-1 all have a slight negative chemical shift of 0.3 eV, as summarized in Table S1. In addition, the weak satellite peak of 940–945 eV is consistent with the reference value of CuO [46]. From Fig. 8d-f, it could be seen that there was a slight 0.2 eV shift between O 1s XPS peaks of the three CC samples. The three peaks of O1, O2 and O3 in the asymmetric O 1s spectrum correspond to the lattice oxygen (around 529.6±0.2 eV), the oxygen vacancies (531.2 ± 0.1 eV) and the chemisorbed oxygen (include O2−, O2−, or O−, around at 532.0 ± 0.1 eV) in Co3O4/CuO, respectively. Table S1 listed the percentage of each O 1s component of the Co 3O4 samples. The percentages of O1 content were about 66.0% (Co3O4), 51.6% (CC1-1), and 50.3% (CC2-1), and the percentages of O3 content were about 18.8% (Co3O4), 24.7% (CC1-1), and 24.9% (CC2-1), respectively. It was noteworthy that the CC2-1 has the highest chemisorbed oxygen (O3) content, and the lowest lattice oxygen (O1) content. Obviously, CC2-1 has better chemisorbed oxygen species ability than the others, so the enhancement of porous Co3O4/CuO NSs sensing properties is inevitable [47]. 3.2. Enhanced Gas Sensing Properties of the Porous Hierarchical CC2-1 NSs The NOx gas sensing properties of porous hierarchical Co3O4/CuO sensors were investigated in air at room temperature, and presented in Fig. 9, 10, S5 and Table S2.
15
Figure 9. (a and b) The response–recovery cycle curves and response-response time of CC2-1 to NOx at room temperature (RT). (c and d) Bar graphs present the response and response time of pure Co3O4, CC4-1 CC2-1, and CC1-1 to 1000 ppm NOx at RT.
Fig. 9a, S5 showed the relationship between resistance response and response time of Co 3O4, CuO, CCP2-1, CC4-1, CC2-1 and CC1-1 to different concentrations NOx, respectively. Obviously, when NOx gas was injected, the resistance dropped abruptly and then rose to the initial position after the sensor had been in contacted with air for several seconds, this presented a typical p-type response of p-type semiconductor to oxidizing gas, and it consistented with the result of Mott-Schottky in Fig. 7. As shown in Fig. 9b, the response was proportional to the NOx concentration, while the response time is inversely proportional to the NOx concentration which finally leads to the response is inversely proportional to the response time. It is clear that the CC2-1-based thin film sensor exhibited a fast and reversible response signal during both adsorption and desorption processes to NOx even at an extremely low concentrations of NO x (0.01 ppm). As presented in Fig. 9c, d and Table S2, the maximum response of CC2-1 to 1000 ppm NOx was 14.16, which was the highest among the five sensors and almost 3 times higher than pure Co3O4, 1~4 16
times higher than that of our other sensors. At the same time, the response time was only 2.0 s which was 78 times faster than pure Co 3O4 to 1000 ppm NOx and 1~120 times faster than other sensors. Particularly, the NOx detection limit of CC4-1, CC2-1 and CC1-1 decreased to a minimum of 0.001 ppm and the response reached to 1.06. However, only the response time of CC2-1 can still reach 9.3 s, this noted that to all NOx concentrations, the response time of CC2-1 was within 10 s. At the same gas concentration, CC2-1 possesses the highest response followed by CC2-1 precursor, CC1-1, pure Co3O4, CC4-1 and pure CuO. Compared with the other five NOx sensors, the CC2-1 sensor not only has the highest and fastest response but also the minimum detection limit.
Figure 10. (a) Reversibility of CC2-1 to 100 ppm NOx response five cycles at RT, (b) Selectivity of CC2-1 to 1000 ppm gas at RT, (c) Stability of CC2-1 to 1000 ppm NOx over 55 days at RT.
For further investigate the excellent gas sensing properties of CC2-1, reversibility, selectivity, and a 50-day stability test were carried out. Reproducibility is another key feature of gas sensor. Fig. 10a showed the dynamic sensing curve of CC2-1, which distinctly displayed the response-recovery performances of the sensors. It is worth noting that the sensor exhibited a very stable start/stop signal when switching between air and NOx. Besides that, the symmetry of the on/off curves showed no slightly change in the course of five cycles to the response and response time, this revealed good sensing repeatability and reproducibility of the CC2-1 sensor towards NOx at the same 100 ppm concentration. As shown in Fig. 10b, the selectivity to the 1000ppm gases including H2S, NH3, CO and H2 at RT were also measured. The response of NOx was 14.16, the response of the other four gases only up to 2.75, 1.63, 1.06 and 1, respectively, and almost no response to H2 was observed. In contrast, the response order of CC2-1 gas sensor to different gases is H2S > NH3 > CO > H2. For comparison, at the same concentration, the response for NOx was 4 times higher than H2S, 8 times than NH3, 12 times than CO, and 13 times than H2, respectively. Obviously, CC2-1 sensor has remarkable selectivity for NOx. 17
Due to stability of the sensor device is a very important factor for its further application, as shown in Fig. 10c, the stability of CC2-1 sensor was measured for 50 days to 1000 ppm of NOx and the response was maintained at 12-14.5 for 50 days at RT, which demonstrated its favorable stability. Generally, the porous hierarchical Co 3O4/CuO 2:1 NSs exhibited excellent sensing properties contain high response, short response time, good reversibility, selectivity and high stability in gas detection. The high response and short response/recovery time of CC sensor to NOx should be attributed to the smaller pore size and more holes of CC2-1 sensor which leads to the extremely abundant adsorption sites on the surface. This means that the porous hierarchical structure not only allowed gas to be absorbed on the surface, but also absorbed gas throughout the bulk deeply. 3.3. Discussion of sensing mechanism The gas sensing mechanism of Co 3O4/CuO essentially belongs to the surface-resistance controlled mode which caused by the chemical interaction between the gas molecules and the sensor, usually involving surface reactions and gas adsorption/desorption processes [48]. Therefore, the porous hierarchical structure increases the specific surface area and defects of the sensor, thus increasing oxygen vacancies and chemisorbed oxygen content, resulting in enhanced gas sensing properties. The gas sensing mechanism of the porous hierarchical CC NSs sensor, both of the porous hierarchical structure interaction and electron transport mechanism were presented in Scheme 3.
Scheme 3 The gas sensing mechanism of the porous hierarchical CC NSs sensor (a) The model of CC sensor and HRTEM image of CC NSs structure image; (b) structural model of CC sensor exposed in O2 and the adsorbed O2 capturing; (c) CC sensor exposed in NOx and adsorbed reaction with O2- and e-.
On the one hand, the superior performances of the Co 3O4/CuO sensors could be attributed to the chemical modification of Co 3O4 by appropriate proportion of CuO, and 18
its clearly link to the
unique porous hierarchical heterojunction structure of Co 3O4/CuO NSs as shown in Scheme 3a. Since the proper ratio of CuO made the material has hierarchical porous structure, and the suitable and smaller pore size, larger specific surface area, and defects at the interface ultimately produced extremely abundant reactive sites in the sensing materials which in favor of the adsorption -desorption. As active centers in gas sensing process, surface defects could attract oxygen molecules and induce trapping of carrier electrons which resulting in the formation of HALs in the interface regions [49]. This implied that gases not only adsorbed on the interior and exterior surfaces but also throughout the bulk deeply. And the porous hierarchical structure provided a faster and efficient diffusion channel of the electrons transmission between the gas molecular and the materials [12]. Moreover, the adsorption oxygen capacity of the sensitive material is closely related to its gas sensing properties [42]. As shown in XRD and XPS, CC2-1 exhibited the highest adsorption oxygen content. It means that CC2-1 has better chemisorption oxygen capacity compared with others, this resulting in better gas sensing performance of CC2-1 than the other sensors. On the other hand, Co 3O4 and CuO are p-type semiconductors, and its conductivity mainly depends on the holes in the valence band [50]. When the Co3O4/CuO sensors is exposed to air at RT, the oxygen species could absorbed to the sensor surface by capturing free electrons from the conduction band of Co 3O4 and CuO and then formed a hole-accumulation layer (HAL), as shown in Scheme 3b and equation (4-5). Due to the unique porous hierarchical structure of Co 3O4/CuO NSs, multiple layer of HALs could be developed and formed on both of the interior and exterior surfaces. A large amount of oxygen ions (O2-) adsorbed on the surface play a vital role in the enhancement of charge transfer. With free electrons are transferred from the conduction band to chemisorbed oxygen, the density of holes in the valence band increased which leads to an increase in the electrical conductivity and the sensors resistance finally decreased [47]. Owing to NOx is a lone-pair electron so it is an electron-withdrawing molecule, tending to adsorbed on electron-rich sites. It was known as a strong oxidizing gas, when the HALs (result in O2 adsorption in the air) of Co3O4/CuO exposed to NOx (similar to that of O2), electrons were transferred from Co 3O4 and CuO to the NOx [51], thus NOx adsorbed on Co3O4/CuO and produced NOx - not only by captured the electrons from accept electron level of Co3O4/CuO but also reacted with O2- (Scheme 3c and equation (6-7)), which resulted in increased hole density [52], eventually lead to a rapid decrease in resistance of p-type Co3O4/CuO (Fig. 9a), so that the sensors exhibited a 19
much stronger saturated sensing signal and a higher response. Since the combination of NOx molecules and the lone-pair electrons was weaker, NOx molecules could be desorbed from the sensing materials by flowing air. In addition, NOx has a strong oxidizing property and it is an electron-withdrawing gas, so the conductance of the CC2-1 could not be strongly changed for the electron donating reducing gas, such as NH3, H2S, CO [53]. This is why Co3O4/CuO exhibits superior gas sensing response and selectivity to highly oxidized NOx compared with other gases. The reaction of the gas sensing process was described as follows: O2 (gas) → O2(adsorbed)
(4)
O2(adsorbed) + e- → O2-
(5)
NOx + e- → NOx -
(6)
NOx + O2-+ 2e- → NOx -+ 2O-
(7)
In conclusion, CC2-1 sample with appropriate proportion of CuO have smaller pore size and a larger specific surface area due to its unique porous hierarchical structure, together with plenty of heterojunctions and defects on the surface, lead to the rich adsorption-desorption and reactive sites and excellent adsorption oxygen capacities which effectively enhanced the gas sensing performance. So that CC2-1 has extremely high and fast response and low detection limit.
4. Conclusions A facile hydrothermal-calcination route has been applied to successfully fabricate porous hierarchical Co3O4/CuO NSs. Controlling the appropriate proportion of CuO and adding NaHCO3 as pore-forming agent are found to be crucial to access the hierarchical structure with abundant porosity and high specific surface area. Consequently, a large number of heterojunctions and defects pose the outstanding chemisorbed ability to oxygen species. It was indicated that the as-synthesized Co3O4/CuO NSs exhibited excellent gas sensing properties at room temperature, including very high response (as high as 14.16), low detection limit (as low as 0.01 ppm), short response/recovery time (within 10 s), and good selectivity to NOx. Compared with pure Co 3O4, CC2-1 precursor and other sensor samples, the CC2-1 sensor behaved 1~4 times higher response and 1~120 times faster response time. In brief, our chemical modification presents an effective approach to improve the performance of Co 3O4/CuO in gas-sensing applications. We expected that the newly-synthesized sample has a promising application in the field of ultrasensitive NOx gas sensors. 20
Acknowledgments This work was supported by the Program for Innovative Research Team in Chinese Universities (IRT1237); the National Natural Science Foundation of China (No.21671060); and the National Natural Science Foundation of Heilongjiang Province (No. D2015003)
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Facile Route Synthesized Porous Hierarchical Co3O4/CuO Nanosheets with High Porosity and Excellent NOx Sensing Properties at Room Temperature Siyu Liu a, Lei Teng a, Yiming Zhao a, Zhi Liu a, Jiawei Zhang b, Muhammad Ikram a, Afrasiab Ur Rehman a, Li Li a, c*, Keying Shi a, b* a
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and
Material Science, Heilongjiang University, Harbin, 150080, P. R. China. b
Key Laboratory for Photonic and Electronic, Ministry of Education, Modern Experiment Center, Harbin Normal
University, Harbin 150025, P. R. China. c
Key Laboratory of Chemical Engineering Process & Technology for High-efficiency Conversion, School of
Chemistry and Material Science, Heilongjiang University, Harbin 150080, P. R. China. Corresponding Author Fax: +86 451 8667 3647; Tel: +86 451 8660 9141 E-mail:
[email protected],
[email protected]
In this work, porous hierarchically structured Co3O4/CuO nanosheets were synthesized by a facile hydrothermal-calcination route. The CC2-1 sample synthesized with the 2:1 molar ratio of Co(NO3)2·6H2O and CuCl2·2H2O presented excellent NOx sensing performance at room temperature, which has not only the highest response (14.16 to 1000 ppm), the shortest response time (2 s to 1000 ppm), and the minimum detection limit (0.01 ppm), but also good reversibility and selectivity. It is could be associated with the unique hierarchical heterojunction structure which endowed with high porosity.
26
Highlights
•Simple and low cost method to synthesized Co3O4/CuO sensor was presented. •Molar ratio effects of Co3O4: CuO on NOx sensing properties were investigated. •CC2-1 exhibited the best sensing properties to NOx at room temperature. •Mechanism of unique structure on NOx sensing properties was investigated.
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