Facile Fabrication of Mesoporous Hierarchical Co-Doped ZnO for

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Materials and Interfaces

Facile Fabrication of Mesoporous Hierarchical Codoped ZnO for Highly Sensitive Ethanol Detection Yufan Mo, Feng Shi, Shuaiwei Qin, Pinggui Tang, Yongjun Feng, Yingying Zhao, and Dianqing Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00158 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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Industrial & Engineering Chemistry Research

Facile Fabrication of Mesoporous Hierarchical Co-doped ZnO for Highly Sensitive Ethanol Detection Yufan Mo, Feng Shi, Shuaiwei Qin, Pinggui Tang,* Yongjun Feng, Yingying Zhao,* and Dianqing Li State Key Laboratory of Chemical Resource Engineering, and Beijing Engineering Center for Hierarchical Catalysts, Beijing University of Chemical Technology, Beijing 100029, P.R. China

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ABSTRACT: The development of highly sensitive gas sensing materials for the detection of volatile organic compounds at low temperature is vital for gas sensors. Herein, novel mesoporous hierarchical Co-doped ZnO were successfully constructed by calcining layered zinc hydroxides precursors prepared through a facile hydrothermal route. The hierarchical architectures of the precursors were preserved after their conversion to mesoporous Co-doped ZnO through calcination, and the obtained mesoporous hierarchical Co-doped ZnO were systemically characterized by X-ray powder diffraction, scanning electron microscopy, high resolution transmission electron microscopy, nitrogen adsorption−desorption analysis, and X–ray photoelectron spectra. The hierarchical Co-doped ZnO were assembled by many thin nanosheets composed of small nanoparticles (8.6 nm), and the hierarchical Co-doped ZnO with 5 at.% doping amount has a specific surface area of 111.7 m2g-1 and abundant mesopores in the range of 2-50 nm. Gas sensing tests demonstrate that the hierarchical 5 at.% Co-doped ZnO showed a sensing response of 54 to 50 ppm ethanol at the low operating temperature of 180 °C, and the response followed a good linear relationship with 5-160 ppm ethanol. Moreover, the hierarchical Co-doped ZnO also exhibited distinctive selectivity, good response repeatability, and preferable long-term stability to ethanol, indicating that the nanosheet-assembled mesoporous hierarchical Co-doped ZnO has potential applications as high-performance gas sensing materials working at low temperature.

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1. INTRODUCTION Metal oxide semiconductors have drawn tremendous attention because they have the advantages of low cost, good sensitivity, controllable preparation and facile integration, and have been developed for gas detection over the past few decades.1-3 These metal oxide semiconductors can be classified as n-type semiconductors (such as In2O3,4,

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SnO2,6, 7 ZnO,8-12 Fe2O3,13, 14 WO3,15, 16 etc.) and p-type semiconductors (such as NiO,17-19 Co3O4,20-23, CuO,24 etc.) according to their major charge carrier. As a promising n-type semiconductor gas-sensing materials, ZnO has been intensively studied owing to its excellent characteristics of low cost, nontoxicity, abundant reserves and good stability,25, 26

and ZnO nanomaterials with morphologies of nanoparticles,27 nanorods,28 nanowires,29

nanobelts,30 and three-dimensional hierarchical structures have been devised for gas sensing.26 Recently, enormous efforts have been devoted to constructing three-dimensional hierarchical structures composed of nanoscale building blocks (nanoparticles, nanorods, nanowires, and nanosheets) with the aim of enhancing the gas sensing performance by utilizing their large specific surface area, abundant accessible pores, and good structural stability, which could provide plentiful adsorption sites for gas molecules, facilitate the adsorption and diffusion of gas molecules, and promote the gas sensing stability.31-33 For example, Fan et al.31 developed a facile and mild route to prepare dandelion-like ZnO with hierarchical porous structure, and the sensing response of such dandelion-like ZnO 3



to 50 ppm ethanol is about 34.5 at 250 °C. Lu’s group synthesized a novel double-shell hierarchical ZnO/ZnFe2O4 composite by in-situ growing ZnFe2O4 nanosheets on the surfaces of ZnO hollow microspheres, and they found that ZnO/ZnFe2O4 exhibited a response of 16.8 towards 100 ppm acetone at 250 °C.2 Alenezi et al.34 demonstrated that the response of 3D hierarchical ZnO nanowires with high surface-to-volume ratios to 100 ppm acetone reached 42 at 425 °C. Despite the prepared three-dimensional hierarchical ZnO samples showed good stability and fast response, they still suffer from high working temperature and relatively low gas sensitivity, which seriously restrict their application in gas sensors with low working temperature.28, 35, 36 It is well known that doping with metal elements is an efficient strategy to improve the gas sensing performance of metal oxide semiconductors by modifying the surface state.26, 37

For example, Wang et al.26 reported that the response of 5 at.% In-doped 3D ordered

macroporous ZnO hierarchical structure to 100 ppm ethanol reached 88 at 250 °C, which is approximately 3 times higher than that of pure ZnO. Jia’s group revealed that the response of 2 at.% Nd-doped ZnO nanorods to 100 ppm ethanol at 348 °C was 37.8, which is higher than that of pristine ZnO nanorods (20.3).38 Among the doping elements, cobalt (Co) has attracted considerable attention due to its catalytic effect arising from its special configuration of extra-nuclear electron. Until now, Co-doped ZnO with different nanostructures such as nanowires,39 nanoparticles,40 and plates41 have been reported. However, the reported Co-doped ZnO materials showed drawbacks of high working temperature and relatively low sensing response to the target gases because of their low 4


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specific surface area and sparse pores. In this regard, the synthesis of unique structural Co-doped ZnO with large specific surface area and abundant pores has importantly scientific and practical significance. To the best of our knowledge, there have been very few reports on the fabrication of nanosheet-assembled hierarchical Co-doped ZnO for gas sensing. Herein, a facile layered hydroxides precursor route combined with the subsequent calcining process was proposed for the preparation of mesoporous hierarchical Co-doped ZnO, which was assembled by lots of thin nanosheets composed of nanoparticles with size of approximately 8.6 nm. Owing to its large specific surface area, plentiful mesopores, and 3D hierarchical structure, the sensing response of the hierarchical Co-doped ZnO with 5 at.% doping amount reached 54 to 50 ppm ethanol at 180 °C and had a good linear relationship with 5-160 ppm ethanol gas. Besides, the hierarchical Co-doped ZnO also has the advantages of low detection limit, excellent selectivity, repeatability and long-term stability towards ethanol at 180 °C. Hence, this work provides a simple way to synthesize nanosheet-assembled mesoporous hierarchical Co-doped ZnO with excellent sensing performance at low working temperature, which has potential applications in the fields of gas sensors. 2. EXPERIMENTAL SECTION 2.1. Materials. The used materials including zinc nitrate (Zn(NO3)2·6H2O), cobalt nitrate (Co(NO3)2·6H2O), salicylic acid (C7H6O3), urea and ethanol (Beijing Chemical 5



Factory) with purity of ˃99.5% were purchased from Beijing Chemical Factory and used as received. 2.2. Preparation of Hierarchical Co-doped Zn(OH)2 Precursors. The salicylic intercalated hierarchical Co-doped Zn(OH)2 precursors with different Co doping amount were firstly synthesized through co-precipitating zinc and cobalt salts with salicylic from homogeneous solution. In detail, 0.02 mol of Zn(NO3)2·6H2O, 0.001 mol of Co(NO3)2·6H2O and 0.05 mol of urea were dissolved in 150 mL of deionized water by ultrasound method to form a solution with Co:Zn molar ratio of 5:100. The above solution was transferred into a four-necked flask and heated to 95 °C in a water bath. Then, 0.01 mol of salicylic acid was added into the solution and this solution was maintained at 95 °C for 6 h with stirring. The collected slight pink precipitate was dried in an oven at 90 °C for 10 h after centrifugation and washing with deionized water and ethanol for 6 times. Hierarchical Co-doped Zn(OH)2 precursors with different Co doping amount and pure Zn(OH)2 were prepared as well by the same method through keeping the molar amount of zinc nitrate to be 0.02 mol and changing the molar amounts of cobalt nitrate. Table 1 shows the detailed amounts of materials used in the preparation process, and the prepared Co-doped Zn(OH)2 precursors are denoted as xCo/Zn(OH)2 (x= 1, 3, 5 and 10%, indicating the Co doping amount used in the synthesis process).

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Table 1. Detailed Preparation Conditions for the Precursors (unit: mol) Samples

Zn(NO3)2·6H2O

Co(NO3)2·6H2O

Urea

Salicylic acid

1%Co/Zn(OH)2

0.02

0.0002

0.05

0.01

3%Co/Zn(OH)2

0.02

0.0006

0.05

0.01

5%Co/Zn(OH)2

0.02

0.001

0.05

0.01

10%Co/Zn(OH)2

0.02

0.002

0.05

0.01

Zn(OH)2

0.02

0

0.05

0.01

2.3. Preparation of Hierarchical Co-doped ZnO. Hierarchical Co-doped ZnO samples were obtained by calcining the prepared Co-doped Zn(OH)2 precursors at a given temperature in a muffle furnace. The powder of Co-doped Zn(OH)2 precursor was put in a porcelain boat which was then transfered into a temperature-programmed muffle furnace. The heating rate of the furnace was set to be 5 °C min-1, and the furnace was maintained at 250, 300, 350 or 400 °C for 2 h in air. The calcined powders are denoted to be xCo/ZnO-T (x = 1, 3, 5 and 10%; T =250, 300, 350 and 400, representing the calcining temperature). Pristine ZnO was obtained by calcining the Zn(OH)2 precursor at 300 °C for 2 h in air. 2.4. Characterization. The crystal phases were characterized by X-ray diffraction on a Rigaku D/max-Ultima III X-ray powder diffractometer in a scan range from 2 to 70° (working at 40 kV and 40 mA with Ni-filtered Cu Ka radiation). The morphology and structure analysis of the samples were performed on a field-emission scanning electron 7



microscope (SEM; Zeiss Supra 55) and a high resolution transmission electron microscope (HRTEM, JEOL JEM-2010 with an accelerating voltage of 200 kV). The specific surface area and pore structure of the samples were investigated by Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods with Micromeritics Surface Area and Porosity Gemini VII 2390 system. The surface elemental compositions analysis was carried out on an ESCALAB 250 X–ray photoelectron spectrometer (XPS, Al Kα excitation source). The electrical properties of samples were recorded on a Hall effect measurement system (RH2030, Phys Tech) by using 4-wire contact measurement under 0.5 T magnetic field. The powdered samples were pressed into a disk with a diameter of 1 cm and a thickness of 1 mm for Hall effect measurement. 2.5. Sensor Fabrication and Sensing Performance Test. The sensor used for sensing performance test was fabricated via coating sensing material on an alumina ceramic tube (length: 4 mm, external and internal diameter: 1.2 and 0.8 mm) with two parallel gold electrodes which were attached with two Pt wires to connect the measurement system (Figure S1a). A uniform paste was formed by grinding the calcined powders with ethanol and coated on the external surface of the alumina tubes to form a uniform film with a thickness of 20±2 μm on the external surface of the alumina tube by rotating. A Ni–Cr heating wire which serves as a heater was then inserted into the tube, and the Ni−Cr wire and Pt wires were welded on the special test base (Figure S1b). Four sensors for each sample were fabricated for testing. The whole sensors were heated at 200 °C for 24 hours before sensing performance tests, which were carried out at constant voltage of 5.6 V 8


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under laboratory conditions (temperature: 25–35 °C, relative humidity: 20–35%) on a CGS-8 intelligent gas sensing analysis system (Elite Tech Co., Ltd.). The gas sensing response to reducing gases is defined as Ra/Rg for an n-type semiconductor, where Ra and Rg are the electrical resistances of sensors in ambient air and analyte gas, respectively. The times taken to reach 90% of the resistance change upon exposure to the analyte gas and air are defined as the response and recovery times. 3. RESULTS AND DISCUSSION 3.1. Structure and Morphology of Samples. The crystalline phase structure of the precursors and the corresponding calcined hierarchical Co-doped ZnO samples were investigated by powder XRD. Figure 1a and b reveals the XRD patterns of the prepared layered Zn(OH)2 precursors and the calcined ZnO, respectively. In the XRD patterns of all Zn(OH)2 precursors, a series of (00l) feature diffraction peaks of layered hydroxide salts are observed at 2θ = 5.44, 10.89, and 16.34°, corresponding to the crystal planes of (003), (006), and (009) with interplanar spacing of 1.63, 0.82, and 0.54 nm, respectively.42,

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These results suggest that salicylic anions are intercalated into the

interlayer of layered zinc hydroxide, which is further confirmed by the FT-IR analysis as shown in Figure S2 in the supporting information. After calcination process, the feature diffraction peaks of layered Zn(OH)2 precursors completely disappear, while a series of new diffraction peaks which agree well with the standard reflections of hexagonal wurtzite ZnO (JCPDS card no.36-1451) appear, suggesting the complete transformation 9



of Zn(OH)2 precursors to ZnO. No other crystalline phase such as CoO, Co2O3, or Co3O4 was observed,23 indicating that the products had a high purity and Co was doped into the crystal phase of ZnO. It should be noted that cobalt and zinc atoms are evenly distributed within the nanosheets of the precursor, which is helpful for the formation of Co-doped ZnO nanosheets.12 The intensity of the diffraction peaks gradually decrease with the increase of Co doping amount, indicating that doping of Co into the lattice of ZnO decreases the crystallinity of ZnO. In addition, it can be observed that the diffraction peak of the (101)

Figure 1. XRD patterns of the layered Zn(OH)2 precursors (a) and the calcined ZnO (b). 10


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crystal plane shifts toward low angle (Figure S3). The average crystal sizes of Co-doped ZnO with doping amount of 1, 3, 5, and 10% calculated from the Scherrer formula are about 9.9, 9.0, 8.6 and 8.5 nm, respectively, which is smaller than that of ZnO (10.8 nm), suggesting that doping of Co into the lattice of ZnO can decrease the crystal size of ZnO. Figure S4 and 2 display the SEM images of the Co-doped Zn(OH)2 precursors and their calcined product, respectively. It is obvious that the precursors have a hierarchical structure, which consists of very thin flakes with size of about 1 μm. This hierarchical structure and two-dimensional nanosheets were maintained after calcination at 300 °C for 2 h, indicating that the hierarchical structures have good stability at 300 °C. As gas sensing process involves the surface reaction, such hierarchical structure could be in favor of the diffusion and adsorption of gas molecules.2, 44 In addition, the thickness of a nanosheet of 5%Co/ZnO-300 measured by SEM is about 8.6 nm (Figure 2k), indicating that the thickness of the prepared ZnO nanosheets is in the order of 10 nm. The spatial distribution and the element contents of Zn and Co in 5%Co/ZnO-300 sample were investigated by EDX analysis, and the obtained elemental mapping images of this sample is illustrated in Figure 2l and Figure S5 in the supporting information, which clearly show that Zn and Co elements are evenly distributed within the sample and the molar percent of Co is about 5.78%. HRTEM analysis was carried out to get more detailed structural information about the hierarchical Co-doped ZnO sample. As shown in Figure 3a, thin flakes can be found in 11



the sample of 5%Co/ZnO-300, and the flakes consist of small nanoparticles with average size of about 6 nm and lots of nanopores were observed (Figure 3b, d and S6). As displayed in Figure 3c, the lattice fringes of 5%Co/ZnO-300 show interplanar spacings of 0.286 and 0.247 nm in the particle, which are in good agreement with those of the (100)

Figure 2. SEM images of ZnO-300 (a, b), 1%Co/ZnO-300 (c, d), 3%Co/ZnO-300 (e, f), 5%Co/ZnO-300 (g, h), 10%Co/ZnO-300 (i, j), enlarged SEM image (k) and SEM images with EDX mapping (l) of 5%Co/ZnO-300.

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Figure 3. HRTEM images of 5%Co/ZnO-300.

and (101) planes of the hexagonal wurtzite ZnO, respectively.45 No lattice fringes of any Co oxides appear, indicating that no separate Co oxides were formed in the conversion process of 5%Co/Zn(OH)2 to 5%Co/ZnO-300.39, 40 Therefore, it further confirms that Co atoms successfully get into the crystal lattice of ZnO, forming Co-doped ZnO composites. The specific surface area and pore structure of hierarchical Co-doped ZnO samples were investigated by nitrogen adsorption-desorption measurement and BJH pore size distribution analysis. As shown in Figure 4a, all of hierarchical Co-doped ZnO samples manifest a type III adsorption isotherm and H3 hysteresis loops, indicating that there are lots of mesopores in the obtained samples.3 The specific surface areas of ZnO-300, 1%Co/ZnO-300, 3%Co/ZnO-300, 5%Co/ZnO-300, and 10%Co/ZnO-300 calculated by 13



the BET method are 101.1, 105.8, 107.3, 111.7, and 104.9 m2 g-1, respectively. It is interesting that the specific surface area of 5%Co/ZnO-300 is larger than those of other four samples, implying that 5%Co/ZnO-300 can offer much more adsorption sites for oxygen and target gases. Figure 4b reveals that the major pore sizes of hierarchical Co-doped ZnO samples are in the range of 2–10 nm, further confirming the presence of ample mesopores in the obtained hierarchical Co-doped ZnO. It can be found that the pore volume of 5%Co/ZnO-300 with pore size in range of 2-10 nm is larger than those of other four samples, implying that 5%Co/ZnO-300 has more mesopores.

Figure 4. Nitrogen adsorption-desorption isotherms (a) and pore size distribution (b) of ZnO-300, and Co-doped ZnO-300. 14


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3.2. Gas Sensing Performance. As we all know, the gas sensing performance of the sensor is greatly affected by the composition of semiconductor metal oxides and the working temperature. Hence, the responses of the sensors based on hierarchical ZnO and hierarchical Co-doped ZnO samples with different Co doping amounts obtained by calcining the corresponding precursors at 300 °C to 50 ppm ethanol gas were investigated first at working temperature in the range of 145-290 °C. As shown in Figure 5a, the sensing response of ZnO-300 gradually increases from 2.8 to 8.3 when the working temperature elevated from 145 to 180 °C, and then decreases with further elevated temperature. The sensing responses of hierarchical Co-doped ZnO samples are different from each other. The sensing response increases markedly from 16.4 to 54 as the doping amount of Co increases from 1% to 5% and then decrease to 20.5 when the Co doping amount further increases to 10%. It should be noted that, when the working temperature increases to 160, 170 and 180 °C, the sensing response of 5%Co/ZnO-300 increases from 10 to 14.5, 21.3 and 54, which is about 6 time higher than that of pristine ZnO-300 at 180 °C. It is well known that the sensing process of metal oxides semiconductor involves the adsorption and reaction of O2 and target gas on the active sites on the surface of the sensing material.1, 35 Sufficient thermal energy is needed to overcome the energy barrier for activating the chemisorption and surface reaction. Therefore, the response is very low at low working temperature because the supplied thermal energy is not enough to surmount the activation energy barrier. However, less gas molecules would be adsorbed on the surface of metal oxide when the working temperature is higher than the optimal 15



working temperature, resulting in the decrease of response.4,

35, 37

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The above analyses

give good explanation for the sensing behavior of hierarchical ZnO and Co-doped ZnO at different operating temperatures. Hence, doping hierarchical ZnO with 5 at.% Co can greatly improves the sensing response of hierarchical ZnO towards ethanol.

Figure 5. (a) Response of ZnO-300 and Co-doped ZnO to 50 ppm ethanol at different operating temperature; (b) Response of 5%Co/ZnO samples calcined at different temperature to 50 ppm ethanol; (c) Dynamic sensing transients of ZnO-300 and 5%Co/ZnO-300 to 50 ppm ethanol at their optimal working temperature; (d) Response of ZnO-300 and 5%Co/ZnO-300 to 50 ppm various gases.

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The calcination temperature is a key factor influencing the structure and performance of materials. To evaluate the influence of calcination temperature on the sensing performance of hierarchical Co-doped ZnO, the sensing responses of hierarchical 5 at.% Co-doped ZnO samples calcined at 250, 300, 350 and 400 °C towards 50 ppm ethanol were tested at the working temperature from 145 to 290 °C. As shown in Figure 5b, the 5 at.% Co-doped ZnO calcined at 300 °C demonstrated the highest response at 180 ºC among the four samples, which may result from the suitable crystallinity, particle size, pore structure and large specific surface area of 5%Co/ZnO-300.22, 40, 46 Therefore, 300 ºC is regarded as the best calcination temperature, and ZnO-300 and 5%Co/ZnO-300 were employed for further test. Figure 5c shows the dynamic response and recovery curves of ZnO-300 and 5%Co/ZnO-300 to 50 ppm ethanol at 180 ºC, respectively. It can be obviously seen that the response of hierarchical 5%Co/ZnO-300 (54) is greatly higher than that of ZnO-300 (8.3), suggesting that doping Co into the lattice of ZnO can markedly enhance the gas sensing response. The response and recovery times, which corresponds to the times to reach 90% of the resistance change values when ethanol gas is injected or removed, for hierarchical 5%Co/ZnO-300 and ZnO-300 is shown in Figure 5c. The response and recovery times for 5%Co/ZnO-300 (22 and 53 s, respectively) are slightly shorter than those for ZnO-300 (40 and 57 s, respectively), which may be attributed to the existence of more mesopores in hierarchical 5%Co/ZnO-300 than ZnO-300 as displayed in the pore size distribution curves.

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The sensing response of hierarchical 5%Co/ZnO-300 and ZnO-300 to ethanol, n-butyl alcohol, methanol, acetone, toluene, formaldehyde and cyclohexene were investigated to evaluate the selectivity of hierarchical 5%Co/ZnO-300 to ethanol. As shown in Figure 5d, the response of hierarchical 5%Co/ZnO-300 to ethanol (54) is much higher than the responses to n-butyl alcohol (16.3), methanol (8.7), acetone (13.4), toluene (3.7), formaldehyde (5.8) and cyclohexene (1.2). The selective sensing coefficients of hierarchical 5%Co/ZnO-300 for ethanol (Rethanol/Rx) to n-butyl alcohol, methanol, acetone, toluene, formaldehyde and cyclohexene are about 3.3, 6.3, 4.1, 14.7, 9.4 and 45.4, respectively. In contrast, the response of pristine hierarchical ZnO-300 toward ethanol is very low (8.6) and its selective sensing coefficients for ethanol to the other six gases are about 2.0, 3.2, 1.4, 3.1, 2.4, and 6.2 respectively, which are much lower than those of 5%Co/ZnO-300. Therefore, the doping of hierarchical ZnO-300 with 5 at.% Co not only markedly improves the sensing response but also greatly enhances the selectivity to ethanol at the optimal working temperature of 180 ºC. The dynamic sensing characteristics of 5%Co/ZnO-300 to different concentrations of ethanol gas (5-160 ppm) were investigated at the optimal working temperature of 180 ºC, and the obtained results are presented in Figure 6a. It can be seen that the responses of 5%Co/ZnO-300 sharply increased from 4.5 to 163 as the ethanol concentration increased from 5 to 160 ppm, indicating that the as-synthesized 5%Co/ZnO-300 is very sensitive to ethanol. The relationship between the response and the ethanol concentration was further investigated. As shown in Figure 6b. the response of 5%Co/ZnO-300 to ethanol increases 18


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almost linearly in the concentration range of 5–160 ppm, indicating that the active sites on the surface of 5%Co/ZnO-300 was not fully occupied by ethanol molecules even when the concentration of ethanol is up to 160 ppm. Therefore, the prepared hierarchical 5%Co/ZnO-300 can accurately monitor the ethanol concentration in a wide linear range, which is of great significance for its practical application. The detection limit (DL), which indicates the lowest concentration in which the response of the sensor is considered to be accurate, is an important indexes reflecting the sensitivity of a sensor and generally defined according to equation 1, where RMSnoise is defined as equation 2. DL = 3

RMS noise Slope

RMSnoise =

S2 N

(1)

(2)

Where S and N are the standard deviation and the number of data point, respectively. 50 data obtained from Figure 6a before exposure to ethanol were averaged and a standard deviation (S) was calculated to be 5.81×10-2 using the root-mean-square deviation. The RMSnoise was calculated to be 8.28×10-3 according to the above equation 2, and the slope obtained from Figure S7 is about 0.547. Hence, the DL of 5%Co/ZnO-300 to ethanol was calculated to be approximately 45.4 ppb in this work. The repeatability of 5%Co/ZnO-300 to 50 ppm ethanol at 180 ºC shown in Figure 6c suggests that 5%Co/ZnO-300 has good sensing reproducibility. The stability of the sensor based on the hierarchical 5%Co/ZnO-300 was investigated by testing its response towards 50 ppm ethanol at 180 ºC for 17 times in 30 days. As shown in Figure 6d, the 19



Figure 6. (a) Dynamic sensing curve of 5%Co/ZnO-300 to 5-160 ppm ethanol at 180 ºC; (b) Linear relationship between the response of 5%Co/ZnO-300 and ethanol concentration at 180 ºC; (c) Response repeatability curve of 5%Co/ZnO-300 to 50 ppm ethanol at 180 ºC; (d) Response stability curve of 5%Co/ZnO-300 to 50 ppm ethanol at 180 ºC.

response of the sensor remains at around 50 after 30 days, indicating that 5%Co/ZnO-300 has excellent stability. The comparison of the responses to ethanol gas between the hierarchical 5%Co/ZnO-300 and other ZnO based materials reported in the literature is present in Table 2. Although a variety of ZnO based materials have been developed and have very high responses to ethanol at a temperature higher than 200 ºC, ZnO based materials which can work at a low temperature of 180 ºC is still unusual. It is reported that 20


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In-doped 3DOM ZnO showed a response of 88 to 100 ppm ethanol at the optimal working temperature of 250 ºC,26 and Co-doped ZnO nanorods showed a response as high as 129 to 100 ppm ethanol at 350 ºC.47 SnO2-ZnO hetero ninofibers and Fe-doped lotus-like ZnO showed responses of 78 and 24 to 100 ppm ethanol at 300 and 400 ºC, 36, 48

respectively. A few of ZnO based sensors with working temperature lower than 200 ºC

were also reported. Li et al. revealed that ZnO/ZnCo 2 O 4 tube in tube nanostructures showed a response of 58 to 100 ppm ethanol at 150 ºC. 49 Obviously, the working Table 2. Sensing Performance of Varied ZnO Based Materials to Ethanol Materials

Temperature

Concentration

Response

( ºC )

(ppm)

(Ra/Rg)

350

100

129

47

150

100

58

49

SnO2–ZnO hetero nanofiber

300

100

78

36

In-doped 3DOM ZnO

250

100

88

26

Sn-doped ZnO microrods

300

100

52.4

46

ZnO/SnO2 Core–shell sphere

270

50

7.5

50

V-doped burger-like ZnO

350

100

10

51

Fe-doped lotus-like ZnO

400

100

24

48

hierarchical Co-doped ZnO

180

50

54

This work

Co-ZnO nanorods

Ref.

ZnO/ZnCo2O4 tube in tube nanostructures

21



temperature of the hierarchical 5%Co/ZnO-300 in this work is lower than most of the reported ZnO-based materials and the hierarchical 5%Co/ZnO-300 showed a much higher response to ethanol than most of them. In a word, the obtained hierarchical 5%Co/ZnO-300 possesses the advantages of low working temperature, high sensing response, low detection limit, excellent selectivity, satisfactory repeatability and good long-term stability, which make it a more ideal sensing candidate for the detection of ethanol gas. 3.3. Gas Sensing Mechanism. Up to now, it is widely accepted that the sensing principle of semiconductor metal oxide is based on the change of sensor resistance arising from the adsorption/desorption of gases. In the case of hierarchical 5%Co/ZnO-300, Co3+ ions were doped into the lattice of ZnO and substituted a portion of Zn2+ ions in the lattice, resulting in the release of free electrons into the conduction band of doped ZnO to compensate the positive valence charge of substituted Zn2+ site by Co3+.26,

35, 40

Therefore, in comparison with pristine ZnO, the concentration of free

electrons will increase in Co-doped ZnO, decreasing the resistance of 5%Co/ZnO-300. This theoretical analysis was confirmed by the Hall measurements. As shown in Table 3, 5%Co/ZnO-300 has higher electron carrier concentration, larger Hall mobility, and lower resistivity than ZnO-300. The higher electron concentration is beneficial to the adsorption of more O2, which can contribute to the enhancement of the gas sensing performance.1, 48

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Table 3. Hall Measurements Results of ZnO-300 and 5%Co/ZnO-300 Materials

Electron concentration

Hall mobility

Resistivity

(1020 cm-3)

(102 cm2/VS)

(105 Ω·cm)

ZnO-300

4.30

3.99

3.64

5%Co/ZnO-300

13.92

9.04

2.26

XPS studies were carried out to analyze the surface elemental compositions of ZnO-300 and 5%Co/ZnO-300, and the XPS fitting was carried out by fixing the location and quantity of the peaks and Gaussian-Lorentzian ratio (20%). As shown in Figure 7a and b, only C, Zn, O, and Co elements are detected, indicating that no impurities are present in the obtained samples. The Zn 2p1/2 and Zn 2p3/2 spin-orbit peaks for 5%Co/ZnO-300 (Figure 7c) appear at 1044.5 and 1021.4 eV, which are a little larger than those of ZnO-300 (1044.2 for Zn 2p1/2 and 1021.2 eV for Zn 2p3/2, respectively), implying that there may exist electronic interactions between Zn2+ and Co3+.35, 37 The two peaks located at about 796 eV and 780 eV in Figure 7d can be ascribed to Co 2p3/2 and Co 2p1/2 spin-orbit peaks, which can be further fitted to Co3+ 2p3/2, Co2+ 2p3/2, Co3+ 2p1/2, and Co2+ 2p1/2 spin-orbit peaks, indicating that both Co3+ and Co2+ are present in 5%Co/ZnO-300 sample.21, 39 Figure 7e and f display the Gauss fitting curves of the O 1s spectra of ZnO-300 and 5%Co/ZnO-300. The asymmetric O 1s peak can be coherently fitted into Olat, Odef, and Oabs, corresponding to the O2− ions in the lattice, O2– ions in oxygen deficient regions within the matrix, and chemisorbed and dissociated oxygen 23



species (O2−, O2−, or O−) and OH−, respectively.21, 35, 52 The larger relative percentage of Oabs in 5%Co/ZnO-300 (13.71%, Table S1) than that of ZnO-300 (12.62 %) could be ascribed to the doping of Co into the lattice of ZnO and its larger specific surface area. It is well-known that gas sensing process is related to the adsorption and reaction of O2 and gases on the surface of sensing materials. As shown in Figure 8, oxygen molecules are adsorbed on the surface of ZnO-300 and 5%Co/ZnO-300 in air, and ionized to the chemisorbed oxygen species of O2− (T ˂ 150 °C), O− (150 °C ˂ T ˂ 300 °C), and O2− (T

Figure 7. XPS spectra of ZnO-300 (a), 5%Co/ZnO-300 (b), enlarged spectra of Zn 2p (c), Co 2p (d) and O 1s (e, f). 24


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˃ 300 °C) by capturing free electrons from their conduction band.37, 53 Highly reactive O− ions may be generated on the surface of ZnO-300 and 5%Co/ZnO-300 by capturing electrons from conduction band according to equation 5 because the working temperature is 180 °C,54 and electron depletion layers are formed on the surface domains of ZnO-300 and 5%Co/ZnO-300 nanoparticles at the same time. Therefore, the resistance of ZnO-300 and 5%Co/ZnO-300 will increase upon on exposure to air. When the reducing gas ethanol is introduced, ethanol molecules are adsorbed onto the surface of ZnO-300 and 5%Co/ZnO-300 nanoparticles and then react with O− ions on the surface, giving rise to CO2, H2O and free electrons according to equation 6.47 The generated electrons are reinjected into the conduction band, leading to decrease of the thickness of the depletion layer on the surface domains and thereby decreasing the resistance of ZnO-300 and 5%Co/ZnO-300. O2(gas) → O2(ads)

(3)

O2(ads) + e− → O2−(ads)

(4)

O2−(ads) + e− → 2 O−(ads)

(5)

CH3CH2OH(ads) + 6 O−(ads) → 2 CO2(g) + 3 H2O(g) + 6 e−

(6)

According to above analysis, the hierarchical 5%Co/ZnO-300 has richer pores, larger specific surface area, and higher electron concentration than the hierarchical ZnO-300, and thus more oxygen would be adsorbed onto the surface of the hierarchical 5%Co/ZnO-300 and more chemisorbed oxygen species are formed. Hence, the increased times of the resistance in air for the hierarchical 5%Co/ZnO-300 would be larger than 25



that for ZnO-300 at their optimal working temperature, which was confirmed by the results obtained in this study. As shown in Figure 9 and Table S2, the R a value of ZnO-300 at 180 °C is about 1.4 times of the value at 145 ºC, but the R a value of 5%Co/ZnO-300 at 180 °C is about 1.5 times of the value at 145 °C, implying that more chemisorbed oxygen species were formed on the surface of 5%Co/ZnO-300 than ZnO-300. Besides, in comparison with the Rg value at 145 °C, the Rg value at 180 °C for 5%Co/ZnO-300 shrink by 3.4 times, which is larger than that of ZnO-300 (2.1 times), suggesting that more ethanol molecules are adsorbed on the the surface of

Figure 8. The schematic diagram of the proposed sensing mechanism of pristine ZnO and Co-doped ZnO gas sensors.

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Figure 9. The resistance values of ZnO-300 (a) and 5%Co/ZnO-300 (b) in air (Ra) and 50 ppm ethanol (Rg) at different temperatures.

5%Co/ZnO-300 and react with chemisorbed oxygen species, and thereby more electrons are released back to the conduction band. Therefore, the thickness of the depletion layer of 5%Co/ZnO-300 nanoparticles would be much thinner than that of ZnO-300. The doping of the hierarchical ZnO with 5 at.% Co not only endows it with larger specific surface area and more efficient active sites for the adsorption of O2 and ethanol, but also decreases the resistance of ZnO by releasing free electrons into the conduction band and 27



lowers the activation energy for the formation of chemisorbed oxygen species and the reaction of chemisorbed oxygen species and ethanol, resulting in enhanced sensing performance toward ethanol. 4. CONCLUSIONS In summary, a simple route has been developed to prepare novel mesoporous hierarchical Co-doped ZnO materials which are assembled by very thin nanosheets composed of nanoparticles with size of about 8.6 nm. The hierarchical Co-doped ZnO with 5 at.% cobalt doping amount exhibits a sensing response of 54 to 50 ppm ethanol at 180 °C, which is about 6 times higher than that of the pristine ZnO. The greatly improved sensing response could result from the hierarchical structure with more mesopores and higher specific surface area and the doping of cobalt into the lattice of ZnO, giving rise to much more active adsorption sites for adsorption of oxygen and target gases and facilitating the reaction between chemisorbed oxygen species and ethanol. Besides, the hierarchical 5 at.% Co-doped ZnO also shows distinctive selectivity towards ethanol, excellent response repeatability, good linear relationship between the response and the ethanol concentration (5−160 ppm, R2=0.9955), and preferable stability. Therefore, the hierarchical 5 at.% Co-doped ZnO can be used as a promising candidate for ethanol gas sensing, and layered hydroxides can be employed as a good precursor to prepare high-performance gas sensing materials with low working temperature. ASSOCIATED CONTENT 28


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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:. Photos of gold electrode and fabricated sensor, FT-IR spectra of precursors; Enlarged XRD patterns of ZnO-300 and Co-doped ZnO; SEM images of layered Zn(OH)2 precursors; EDX spectrum of 5%Co/ZnO-300; Particle size statistics of 5%Co/ZnO-300; Response of 5%Co/ZnO-300 to low concentration ethanol gas; The relative percentage of O species on the surface of ZnO-300 and 5%Co/ZnO-300; Resistance values of sensors based on ZnO-300 and 5%Co/ZnO-300 in air and ethanol. AUTHOR INFORMATION Corresponding Author Fax and Tel: + 86-10-64436992. E-mail: [email protected], [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

29



ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundations of China (U1507119, 21521005, 21627813), National Key Research and Development Program of China (2016YFB0301600). Pinggui Tang particularly appreciates the aids of China Scholarship Council.

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Plasmonic and Chemoresistive Gas Sensors. ACS Appl. Mater. Interfaces 2016, 8, 30440-30448.

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For Table of Contents Only

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Facile Fabrication of Mesoporous Hierarchical Co-Doped ZnO for

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