2D nanosheet-assembled PdZnO microflowers for acetone sensor with enhanced performances

Journal of Physics and Chemistry of Solids 124 (2019) 330–335

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2D nanosheet-assembled PdeZnO microflowers for acetone sensor with enhanced performances

T

Yong-Hui Zhang∗, Xiao-Li Cai, Lian-Zhi Song, Fu-Yong Feng, Jun-Yi Ding, Fei-Long Gong∗∗ College of Materials and Chemical Engineering, Collaborative Innovation Center of Environmental Pollution Control and Ecological Restoration, Zhengzhou University of Light Industry, Zhengzhou, 450002, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Microflowers PdeZnO Porous materials Sensors

Hierarchical microflowers architecture assembled with 2D porous ZnO nanosheets were fabricated by a one-step solvothermal method and subsequent annealing process. A series of PdeZnO have been produced by injecting various volumes of Pd(NO3)2·2H2O solution. The ZnO and PdeZnO microflowers are applied to fabricate gas sensors to investigate their sensing characteristics for various gases at different operating temperatures. The 0.05 wt% PdeZnO exhibit the highest selective enhancement of acetone (165%) than ammonia (0.9%), methanal (38%), methanol (71%), H2 (35%) and CO (15%). In addition, it possesses the lower operating temperature and shorter response/recovery times (11s, 5s) toward acetone gas compared with the pure ZnO microflowers.

1. Introduction Acetone is an important raw material for organic synthesis, the good organic solvents and the extracting agent of industry. In particular, the concentration of acetone in breath could be a vital index for diabetes mellitus [1,2]. Hence, it is urgent to find an easier, more effective solution to detect the acetone. Recently, acetone chemical sensor based on the metal-oxide semiconductors has been widely investigated for its high sensitivity, excellent selectivity and long-time stability [3,4]. Zinc oxide (ZnO), with a band gap (3.37eV), has been widely applied in photocatalysis, solar cell, transistor devices, water splitting and gas sensors [2,5–8]. Till now, diverse morphologies of ZnO, such as nanoparticle, nanowire, nanorod, and nanosheet [9–13], have been carried out by using various synthesis methods. ZnO nanosheet, for example, has been extensively used to detect chemicals such as acetone (CH3COCH3), ethanol (C2H5OH), nitrogen dioxide (NO2), methanal (HCHO) and carbonic oxide (CO) [14–18]. In recent years, the high level 3D supercrystals evolved from low dimensional nano-architectures has caused great attention [19–21]. Due to the large specific surface areas, 3D porous architectures or hollow microspheres materials were always applied to fabricate sensors and exhibited enhanced responses for toxic gas [22,23]. However, gas sensors still meet testing requirements under some rigorous conditions like low selectivity and high operating temperature [24]. To overcome the issues, many efforts to improve the sensitivity of ZnO sensors have been made for examples, ∗

preparation of nanostructures with different morphologies [17,25], metal oxide heterostructures [20,26], and noble metals adding (such as Ag, Au, Pd, Pt) [27–29]. Pd nanoparticles (NPs) is one of the best gas sensor candidate owing to their excellent chemical stability, high surface area and low orbital energy [30–32]. Hence, PdeZnO materials could improve the sensing response, selectivity, response time and recovery of chemical sensors. However, only a few articles have been concerned with the synthesis of PdeZnO microflowers and focused on the selective enhancement to acetone vapor. In this paper, porous nanosheet-assembled hierarchical ZnO microflowers are synthesized via a novel one-step solvothermal route and subsequently a calcination procedure. Pd nanoparticles in size of 10 nm are uniformly decorated on the surface of ZnO through injecting the solutions of Pd(NO)3·2H2O. Sensing performance based on pristine ZnO and PdeZnO are systematically investigated. 2. Experimental procedure 2.1. Synthesis of ZnO microflowers 1 mmol of ZnCl2 and 1 mmol of urea were dissolved into 40 mL of mixture solution which containing ethanol and deionized water with a ratio of 3:1. After the solution became homogeneous, PVP (0.2000 g) was added which acting as an surfactant to help the structure forming of ZnO precursors. The mixtures were transferred into 50 mL teflon-

Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y.-H. Zhang), [email protected] (F.-L. Gong).

∗∗

https://doi.org/10.1016/j.jpcs.2018.09.006 Received 13 June 2018; Received in revised form 13 August 2018; Accepted 6 September 2018 Available online 07 September 2018 0022-3697/ © 2018 Elsevier Ltd. All rights reserved.

Journal of Physics and Chemistry of Solids 124 (2019) 330–335

Y.-H. Zhang et al.

performed at temperature from 25 to 1000 °C in a flowing nitrogen environment. A sharp endothermic peak centered at 252 °C is observed and the weight lose is 26%. It is in well fit with the conversion from precursor to ZnO by release of H2O and CO2 when elevating temperature. The decomposition can be described by the following equation (eq:1). The results display that two stages occurred during the process of Zn5(OH)6(CO3)2 precursor convert to crystalline ZnO. The second stage with a weight loss of 2.8% happened above 300 °C. It could be attributed to the oxidation of organic residues and evaporation of chemisorbed water.

lined stainless steel autoclaves and kept at 180 °C for 4 h. The white precipitates were obtained after hydrothermal reaction and washing several times, dried at 60 °C overnight. The products were calcined in air at 500 °C to yield crystalline ZnO microstructures. 2.2. Preparation of Pd decorating ZnO microflowers To prevent the ZnO surface structure from being affected by chemically active precursors such as nitrates, the PdeZnO were prepared by injecting palladium II acetate salt onto the surface of ZnO, followed by drying at 80 °C for 8 h and calcined at 300 °C for 2 h in argon atmosphere. For each series, the Pd decorating amount (wt %) was varied at 0, 0.01, 0.05, and 0.1 wt%.

Zn5(OH)6(CO3)2 → 5ZnO + 2CO2 + 3H2O

(1)

FESEM images of the hydrothermally synthesized Zn5(OH)6(CO3)2 precursor before calcination (Fig. 2a–c). It shows that the flower-like Zn5(OH)6(CO3)2 with an average particle size of approximately 10 μm were successfully prepared through the one-pot hydrothermal method. As shown in Figs. 2b and 3D Zn5(OH)6(CO3)2 microstructures are further consisted of orderly arranged nanosheets with the average thickness of about 15 nm. Compared with smooth structure, 3D ZnO assembled with 2D porous nanosheets were obtained after annealed at 500 °C (Fig. 2d–f). It's clearly to see that the morphologies of Zn5(OH)6(CO3)2 are entirely retained without heavy damage, which can be attributed to loss of volatile gases such as H2O and CO2. These pores in different sizes may significantly improve the chemical properties or serve as transport paths for small molecules. In addition, the FESEM images of 0.01 wt%, 0.05 wt% and 0.1 wt% Pd-doping ZnO microfolwers were shown in Fig. S5. After decorating Pd nanoparticles, the morphologies of ZnO microfolwers have not been obviously changed. TEM was further used to explore the structure of the precursor in Fig. S1. It can be readily observed that smooth nanosheets of the precursor alternately connect with each other to form a flower-like surface. The HR-TEM shows the lattice fringe spacing of 0.27 nm, indexed to the (401) plane of Zn5(OH)6(CO3)2. The ring-like SAED pattern reveals polycrystalline of precursor. The microstructure of porous ZnO is further investigated by the TEM images (Fig. 3a). The lattice fringe of 0.24 nm (Fig. 3b) was corresponding with the (101) plane of ZnO. The SAED image in Fig. 3c suggests the single crystal characteristic after annealed. In Fig. 3d, it is clearly observed the Pd NPs successfully decorated onto the surfaces of the nanosheets. The size of the Pd NPs is ca. 10 nm. Fig. 3e shows a HRTEM image of the interface between Pd and ZnO, the lattice fringes with spacing of 0.24 nm corresponds to (101) crystal planes of ZnO, and the

2.3. Characterization The materials crystal structures were investigated by powder X-ray diffraction (XRD) patterns on a D/max 2550 V diffractometer with monochromatized Cu Kα (λ = 1.54056 Å) incident radiation. Morphologies and sizes of the samples were characterized by fieldemission scanning electron microscopy (FESEM, JSM-7001F, 10 kV) images, transmission electron microscopy (TEM, JEM-2100, operating at 200 kV), high-resolution TEM (HRTEM, 200 kV) and selected-area electron diffraction (SAED). Thermal analysis was performed by a TG/ DTA instrument(TA, Q100). X-ray photoelectron spectroscopy (XPS) result was analyzed by ESCALAB-250Xi spectrometer. The specific surface area was measured using a Belsorp-Mini adsorption apparatus (Bel Japan Inc.). Gas sensing properties were carried out by WS-30A (SI). The sensor response S(S]Ra/Rg) was defined as the ratio of sensor resistance where Ra is in air and Rg is in target gas. 3. Results and discussion XRD was firstly applied to investigate the phase structure of precursor. All the peaks match well with Zn5(OH)6(CO3)2 (JCPDS card No. 72-1100) in Fig. 1a. The product of Zn5(OH)6(CO3)2 after annealing is indexed as hexagonal wurtzite ZnO (JCPDS card No. 36-1451) shown in Fig. 1a. No peak for other impurities can be detected, indicating a complete conversion of the precursor into pure ZnO. Sharp diffraction peaks indicate the high degree crystalline of ZnO after dealing with higher temperature. To investigate the decomposition progress of the as-prepared Zn5(OH)6(CO3)2 precursor, TG-DTA (Fig. 1b) was

Fig. 1. (a) XRD pattern of Zn5(OH)6(CO3)2 precursor and crystalline ZnO microflowers annealed at 500 °C; (b) TG-DTA curves of Zn5(OH)6(CO3)2 precursor in range of 25–1000 °C in a nitrogen atmosphere. 331

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Fig. 2. FESEM images of as-prepared precursors (a, b, c), and crystalline ZnO microstructures after annealing at 500 °C (d, e, f).

Fig. 3. (a) TEM, (b) HR-TEM and (c) SAED images of ZnO microflowers after calcination; (d) TEM, (e) HR-TEM, (f) SAED patterns, (g)EDS spectra of 0.05 wt% PdeZnO microflowers.

X-ray photoelectron spectroscopy (XPS) analysis was conducted to examine the distribution and chemical states in pristine ZnO and PdeZnO materials. Fig. 4a shows the XPS survey spectra of the pristine ZnO and PdeZnO microflowers. The results confirm that Pd NPs have been successfully decorated on the surface of ZnO microflowers. Two peaks were located at the position of 1021.2 and 1044.3 eV, which are assigned to the Zn 2p3/2 and Zn 2p1/2, respectively (Fig. 4b). It indicated that the presence of Zn2+ in the PdeZnO [33]. In Fig. 4c, two peaks are detected at 334.5 and 339.8 eV corresponding to Pd 3d5/2 and Pd 3d3/2 in PdeZnO microflowers, respectively. Compared to the standard binding energy of Pd 3d5/2 and Pd 3d3/2 (335 and 340 eV), the peak 3d moved to the lower binding energy implying the electron density of Pd decreased. The presence of Pd2+ species was due to interaction between Pd and ZnO [30,34]. Table S1 lists the elemental composition (wt%) of elements present in the pristine ZnO and 0.05 wt % PdeZnO sensor.

d-spacing of 0.22 nm is attributed to the (111) lattice planes of Pd NPs. The SAED of the Pd decorated ZnO nanosheets (Fig. 3f) was consisted of two different natures of crystals. One regular bright spots of the singlecrystalline ZnO, and the other irregular diffraction spots which were marked with white arrows of the Pd NPs. It indicates that ZnO nanosheets are uniformly decorated with Pd NPs and both of them are single-crystal structures. The EDS of Pd decorated ZnO sample was investigated shown in Fig. 3g. The results indicated the existence of C, Zn, O and Pd elements in the material. The peak intensity of Pd was lower than other elements, it is confirmed that tiny amounts of Pd were decorated into the ZnO microflowers surfaces. The BET specific surface area of the ZnO microflower was calculated to be 13.02 m2/g with average pore size of 24.12 nm, which is larger than PdeZnO with specific surface areas of 11.21 m2/g and average pore size of 21.33 nm. After decorating Pd NPs, the BET values have no obvious change (Fig. S2). 332

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Fig. 4. (a) XPS of survey spectra of pure and PdeZnO, (b) Zn 2p, (c) Pd 3d XPS spectrum of 0.05 wt% PdeZnO.

microflowers, thus affecting the functionalities of chemical sensors. In comparison, Table S2 lists the comparison investigation on the sensing performances of the ZnO sensors to acetone from our work to the earlier reports. Fig. 5d shows the selective responses of ZnO and 0.05 wt% PdeZnO sensors, which were exposed to toxic gas (370 °C, 200 ppm). As expected, the response of these two sensors for acetone is much higher than other gases. The 0.05 wt% PdeZnO sensor exhibited the highest response towards acetone gas, which may be attributed to the following reasons: firstly, the 0.05 wt% PdeZnO microflowers could provide more available active sites for O2 adsorption and sufficient free electrons because of the introduction of Pd NPs; secondly, the proper amount of palladium can make the electron-depleted layers forming a continuous band on the surface of ZnO microflowers, which improving the sensing performance. Further studies were carried out in the insert of Fig. 5d to investigate the growth rate of selectivity towards toxic gases. It was defined as S = △S/S1 = |S2e S1|/S1, where S2 and S1 are the sensitivities in the different gases of the 0.05 wt% PdeZnO microflowers and pure ZnO microflowers sensors, respectively. The 0.05 wt% PdeZnO sensor exhibit the highest selective enhancement of acetone (165%) than ammonia (0.9%), methanal (38%), methanol (71%), H2 (35%) and CO (15%). The results indicate that the sensor based on PdeZnO shows excellent selectivity to acetone against other gases towards. To investigate the stability of the sensor toward lower acetone concentration, we measured the fabricated sensors at regular intervals

The operating temperature curves (Fig. 5a) suggest that the optimal operating temperature of the PdeZnO sensors (370 °C) are lower than the pure ZnO sensors (420 °C). It is clearly demonstrated that the addition of Pd NPs can effectively reduce the working temperature. the curve between resistance and operating temperature of the ZnO and 0.05 wt% PdeZnO sensor are shown in Fig. S3. In addition, the response of PdeZnO sensors is two-fold higher than the pure ZnO sensors. Compared with the pure ZnO (Fig. 5b), PdeZnO sensors (0.05 wt%) exhibits the highest response value (81) and the fastest response and recovery time (10 s and 5s). The response and recovery time of the 0.01 wt% PdeZnO to acetone are 11s and 6s, respectively, smaller than that of 0.1 wt% PdeZnO sensor (19s and 19s). The significantly fast response and recovery time of the 0.05 wt% PdeZnO sensors could be ascribed to the unique porous structures and the effects of Pd decorating, which facilitated the adsorption and desorption rate of the acetone vapor. Fig. 5c illustrates the dynamic response and recovery curves of the Pd decorated ZnO sensor at different concentrations of acetone vapors at 370 °C from 5 ppm to 200 ppm. It is clearly demonstrated that the 0.05 wt% PdeZnO sensor is more sensitive than the pure ZnO at various concentrations to acetone vapor. The proper amount of palladium can make the electron-depleted layers forming a continuous band on the surface of ZnO microflowers, thus improving the sensing performance. In contrast, if the amount of palladium is too low, it will not be able to form a continuous band. The excessive amount of palladium will affect the surface activity of ZnO 333

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Fig. 5. (a) Operating temperature curves at 200 ppm acetone; (b) Response-recovery curves to 200 ppm acetone at 370 °C; (c) Dynamic sensing characteristics towards acetone at various concentrations at 370 °C; (d) Responses to different kinds of gases in the presence of 200 ppm at the optimum operating temperature of 370 °C; Growth rate of ZnO and 0.05 wt% ZnO microflowers (insert of Fig. 5d).

reaction.

of time (∽9 days). Compared to pure ZnO sensor, the sensors made with 0.05 wt% PdeZnO exhibits highly improved stability as shown in Fig. S4. According to the experimental data, we suggest that the superior functionality of porous PdeZnO microflowers sensors could be attribute to the band-gap structures of ZnO. As is known that ZnO is an n-type semiconductor, the sensing mechanism is due to the gas molecules on the surface of the ZnO microflowers by the adsorption and desorption that cause the remarkable change in the electrical conductivity of the sensor. In the air, O2 is absorbed onto the surface of ZnO microflowers and form the depletion layer (O2−, O2−and O−) by capturing electrons from the conduction band of ZnO. When the sensor is exposed to acetone gas, the oxygen molecules will react with acetone species, the depletion layer will decrease or disappear, which results the sensor resistance decreases. Meanwhile, acetone molecules can be oxidized to CO2 and H2O. Pd NPs on the surfaces of ZnO microflowers can provide more active sites for O2 adsorption and increase the conversion rate of oxygen molecules form into ionized species including O2− and O−. When the Pd nanoparticles were uniformly decorated on the surfaces of ZnO microflowers, the electron depletion layers will form a continuous band. The high sensors response and decreasing operating temperature were owing to the easy adsorption and conversion process of oxygen molecules [21]. Finally, it was concluded that the superior sensor performance based on PdeZnO could be attributed to the high amount of chemisorbed oxygen species and the easy of interface chemical

4. Conclusions In summary, the porous PdeZnO microflowers were successfully synthesized by injecting the solution of Pd2+ onto the surface of ZnO thin films. In particular, the method can effectively control the concentration of the Pd NPs by adding various volumes of the Pd2+ solutions. Compared with the pure ZnO sensors and PdeZnO, the gas sensing performances of PdeZnO (0.05 wt%) sensors demonstrated excellent sensitivity, low operation temperature, and the superior selectivity toward acetone.

Acknowledgements The authors are grateful to the National Nature Science Foundation of China (21771166, 21301158), Outstanding Young Scholars Program of Henan Province (164100510011).

Appendix A. Supplementary data Supplementary data related to this article can be found at https:// doi.org/10.1016/j.jpcs.2018.09.006. 334

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