ARTICLE pubs.acs.org/JPCC
Porous ZnO Polygonal Nanoflakes: Synthesis, Use in High-Sensitivity NO2 Gas Sensor, and Proposed Mechanism of Gas Sensing Mei Chen, Zhihua Wang, Dongmei Han, Fubo Gu,* and Guangsheng Guo* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China
bS Supporting Information ABSTRACT: Unique porous ZnO polygonal nanoflakes were synthesized by the microwave hydrothermal method. The structural properties of the products were investigated by using X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM), and high-resolution TEM techniques. In situ diffuse reflectance infrared Fourier transform spectroscopy technique was employed to investigate the mechanism of NO2 sensing. Free nitrate ions, nitrate ions, and nitrite anions were the main adsorbed species. N2O was formed via NO and N2O2 that were stemmed from NO. Comparative tests for gas sensing between gas sensors based on the as-prepared porous ZnO nanoflakes and purchased ZnO nanoparticles clearly showed that the former exhibited more excellent NO2 sensing performances. Photoluminescence and X-ray photoelectron spectroscopy spectra further proved that the intensities of donors (oxygen vacancy (VO) and/or zinc interstitial (Zni)) and surface oxygen species (O2 and O2), which were involved in the mechanism of gas sensing led to the different gas-sensing properties.
1. INTRODUCTION ZnO, as a functional n-type semiconductor, has already been utilized in gas sensors for detecting toxic or hazardous gases because of its typical properties, such as changeless resistivity, high electrochemical stability, no toxicity, and abundance in nature.1 Recent studies have demonstrated that morphology has a significant influence on the gas-sensing properties of nanomaterials.2,3 One-dimensional (1D) structures of ZnO, such as nanowires,4 nanorods,5 and nanobelts,6 and their hierarchical structures were widely used in gas sensors.5 Two-dimensional (2D) structures, such as nanoplates, are another common structure of ZnO.7,8 Most work has been reported about sheet-based 3D structure ZnO gas sensors or large thick sheet ZnO gas sensors.912 However, work has rarely been reported about polygonal nanoflake gas sensors. Several researchers have reported the sensing properties of the large 3D porous ZnO.13,14 In terms of gas-sensing properties, porous or hollow sphere materials are promising candidates, because their particular structures can usually greatly facilitate gas diffusion and mass transport in sensor materials, thereby improving sensor performance.15 Therefore, the porous ZnO polygonal nanoflakes have wide application in gas sensors. Both theoretical and experimental studies have been carried out in order to elucidate the mechanism of the reactions between target gases and semiconductor materials. The mechanism of gas sensing for nitrogen dioxide has been the emphasis to argue about all the time. It is accepted that the general mechanism is that a nitrogen dioxide molecule gains a conductive band electron from the surface of the semiconductive oxide and/or releases a nitrogen oxide and an O simultaneously.16,17 The D-L model was first established for explaining the sensing ability of semiconductors through the comparison between particle size (D) and surface charge layer (L):18 if D r 2011 American Chemical Society
is comparable to or less than 2 L, the gas sensitivity is expected to be high. However, the L value is hard to be measured by using current instruments and is usually estimated to be in the range of several nanometers. Furthermore, this model is not consistent with some reported studies.19,20 When the researchers reached deeper levels of investigation, they found that the nonstoichiometry (i.e., crystal defects) of ZnO was a key factor in determining the gas-sensing property.16,19,2123 Most researchers postulated that the relatively high gas sensitivities of ZnO might be attributed to higher content of oxygen vacancy,16,21,22 but the proofs gained from the characterization by X-ray diffraction (XRD), transmission electron microscopy (TEM), electron paramagnetic resonance (EPR), and diffused reflectance spectroscopy (DRS) are not enough. Han et al.19 then proposed that the gas-sensing reaction of NO2 with ZnO is related to not only oxygen vacancy (VO) but also zinc interstitial (Zni), by means of the characterization of far-infrared (IR) spectra, Raman spectra, photoluminescence (PL) spectra, and X-ray photoelectron spectrometry (XPS) spectra. In addition, the Fourier transform infrared (FT-IR) spectroscopy was utilized to test the surface adsorbed species, such as nitrites and nitrates, and a series of reactions of NO2 with MoOx-SnO223 were listed. However, further work is needed to ascertain a comprehensive and well-defined mechanism, and the effect of the morphology on the sensing performance of metal oxide based gas sensors has not yet been fully understood and requires further investigation. Received: February 24, 2011 Revised: May 23, 2011 Published: May 26, 2011 12763
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The Journal of Physical Chemistry C In this study, zinc nitrate hydroxide hydrate (ZNH) polygonal nanoflakes were first synthesized as a precursor by using a mild and environmentally benign microwave-assisted approach. Subsequently, porous ZnO polygonal nanoflakes were obtained by calcining the ZNH precursors. PL and XPS spectra were utilized to investigate the effect of crystal defect on the sensing performance of different ZnO nanomaterials. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was utilized to study the dynamic process of the reaction of NO2 with zinc oxide. A possible mechanism was proposed based on the DRIFTS signals and other catalysis results.2325 Moreover, a comparative study on the NO2 gas sensing between the as-prepared porous ZnO nanoflakes and purchased ZnO nanoparticles was carried out.
2. EXPERIMENTAL PROCEDURES All reagents employed were analytically pure and used as received from Beijing Yili Fine Chemical Research Institute (Beijing, China). The ZnO nanoparticles were purchased from Central Research Institute of China Chemical Science and Technology (Beijing, China) and used with calcination treatment at 500 °C for 3 h. Distilled water was used throughout the experiments. 2.1. Synthesis. In a typical procedure, a 0.2 M zinc nitrate solution was prepared by dissolving a certain amount of zinc nitrate hexahydrate in 25 mL of double-distilled water. The pH of the solution was maintained between 6.5 and 6.8 by using 1.5 M aqueous ammonia. The generated suspension was stirred for 20 min with a magnetic stirrer and then transferred into a microwave oven and irradiated at 90 °C for 10 min. After the completion of the reaction, white product was centrifuged and washed several times with deionized water to remove impurities. Then the product was dried by using an oven at 60 °C. Finally, the asprepared powder was calcined at different temperatures for later use. The microwave oven used for the sample preparation was capable of controlling temperature and pressure (WX-4000, maximal temperature 220 °C, maximal pressure 25 atm, Preekem, China). The power used during reaction was 800 W. 2.2. Characterization. The samples were characterized by XRD with a scanning rate of 10 °C/min on a D/max2500VB2þ/ PC X-ray diffractometer using graphite monochromatized Cu KR radiation (λ = 0.15406 nm). The morphologies of the samples were characterized by cold field-emission scanning electron microscopy (SEM, S-4700), TEM (Hitachi-800), and high-resolution TEM (HRTEM, JEOL-2010). Thermal analysis employed a TG/DTA Instruments (WCT-1D, Boif) working at a constant heating rate of 10 °C/min. PL spectra were recorded from 370 to 600 nm at room temperature by a 320-nm excitation (Hitachi Model F-7000 PL spectrophotometer). XPS measurements were carried out on a VG Scientific ESCALAB 250 spectrometer. 2.3. Sensing Property. The sensors were constructed by using the annealed ZnO nanocrystals as the building blocks, and the detailed fabrication were as follows. A certain amount of ZnO was ground together with ethanol in an agate mortar. The formed slurry was spread onto an alumina tube with a pair of Au electrodes and then aged at 300 °C for 24 h to improve the stability. A NiCr coil was employed as a heater to control the operation temperature by tuning the heating voltage. Gas sensing tests were performed on a WS-30A gas-sensing measurement
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system (WeiSheng Electronics Co., Ltd., Henan Province, China) at a relative humidity (RH) of 1020%. Test gases with calculated amounts were introduced into the test chamber via a syringe. Two electric fans installed in the chamber made the test gas homogeneous. After the test, the chamber was opened to diffuse test gases away. The scheme and working principle of SW30A gas-sensing measurement system are analogous to the one showed by Zhang et al.15 The gas sensitivity of the products, Sg, to oxidative gas, such as NOx, and reductive gases, such as ethanol and carbon monoxide, is defined as Rg/Ra and Ra/Rg, respectively, wherein Rg is the resistance of the sensor in a target gas and Ra is that in air. 2.4. In situ DRIFTS Measurement. DRIFT spectra were recorded on a Bruker Tensor 27 FTIR Spectrometer equipped with a KBr beam splitter and an MCT detector operating between 800 and 4000 cm1 using a Harrick reaction chamber with KBr windows and a Harrick DRA-2C1 diffuse reflectance accessory. The vacuum chamber was connected to gas lines for sample treatment, and its temperature was controlled by a programmer from room temperature to 450 °C. Spectra were recorded at a resolution of 2 cm1, and 128 scans were averaged for background spectrum resulting in a time resolution of 2 min, and 64 scans were averaged for sample spectrum resulting in a time resolution of 1 min. To improve the time resolution for the experiments with high NO2 concentrations during the initial stage, only 32 scans were averaged for the sample spectrum. In the DRIFTS experiment, a 100 mg sample (annealed at 500 °C for 3 h) was located in the sample holder of the vacuum cell by using a filling device to obtain reproducible reflecting planes. The sample layer was 3 mm thick. Spectra were measured in absorbance. During NO2 exposure, the in situ DRIFTS analyses were continuously performed. The samples were treated according to the following procedure: (a) pretreated in He flow (20 mL/min) at 300 °C for 1 h (heating rate 10 °C/min) and then cooled down to room temperature (RT) in He flow (20 mL/min). (b) Backgrounds were recorded at RT, 50, 100,150, 200, 250, and 300 °C by using sample powders in He flow (20 mL/min), and then cooled down to RT in He flow (20 mL/min). (c) Replacement of He flow by a NO2 (9800 ppm vol. ca.)/He flow (20 mL/min). After saturation, the NO2 flow was terminated and the cell was purged with pure He flow (20 mL/min) until the DRIFTS data would not change anymore. (d) Heating up in He flow to 50 °C and subsequently to 100, 150, 200, 250, and 300 °C in order to correlate the DRIFTS and gassensing properties.
3. RESULTS AND DISCUSSIONS 3.1. Structure and Morphology of Porous ZnO Nanoflakes. Figure 1a is the XRD pattern of the as-prepared precursor.
The diffraction peaks are indexed on the monoclinic crystal phase of ZNH (PDF card no. 241460). ZNH is a typical metal hydroxide salt, which is a lamellar structure compound. The detail morphology of the precursor was presented in the SEM image of Figure 1b and the TEM image of Figure 1c. Figure 1b reveals that the precursor is uniform polygonal flakes with thickness of about 5090 nm and an average size less than 900 nm. Figure 1c reveals that the polygonal nanoflakes have many shiny pores dispersed homogeneously in them. When the precursor of ZNH nanoflakes was calcined at 500 °C for 3 h, the ZnO nanoflakes were obtained. The SEM image of Figure 2a clearly shows that the ZnO nanoflakes can 12764
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Figure 2. SEM (a) and TEM (b) images of the porous ZnO nanoflakes obtained by calcining the as-prepared precursor nanoflakes at 500 °C (the inset image of (b) is the HRTEM image and the corresponding Fourier transform image).
Figure 1. XRD pattern (a), SEM image (b), and TEM (c) image of the as-prepared precursor.
preserve the polygonal structure perfectly. The nanoflakes interlace and have certain gaps between each other. The highmagnification SEM image shown in the inset of Figure 2a reveals that the ZnO nanoflakes have porous structure and the pore size is in the range of 3060 nm. Moreover, it is obvious that the ZnO nanoflakes with a thickness of 4080 nm consist of no more than two layers of irregular ZnO primary nanoparticles. Further detailed analysis of the porous nanoflakes was carried out by using TEM and HRTEM. The TEM image of Figure 2b clearly displayed the porous structure of the ZnO nanoflakes. The ZnO nanoparticles, which are overlapped each other with a small bridge, have a diameter in the range of 4080 nm. The typical
HRTEM image of the nanoflakes is illustrated in the inset image of Figure 2b. The crystal lattice fringes are clear, and the average distance between the adjacent lattice planes is 0.26 nm, which corresponds well to the interplanar distance of the (002) crystal planes of wurtzite ZnO. This is supported by the fast Fourier transform (FFT) pattern (inserted in Figure 2(b)). 3.2. Effects of Calcination and Operation Temperatures on the Gas Sensitivities. The porous ZnO nanoflakes prepared by calcining the ZNH precursor at 300, 400, 500, and 600 °C were denoted as ZnO300, ZnO400, ZnO500, and ZnO600, respectively. Nitrogen dioxide, a major poisonous gas in vehicle exhaust and chemical industry, was selected for evaluating the sensing property of the ZnO nanoflakes. The gas sensitivities of the ZnO nanoflake sensors to 0.5 ppm NO2 are listed in Figure 3. It is evident that ZnO500 has the highest sensitivities at the operating temperature from 129 to 230 °C. To investigate the difference of the sensing performances resulting from the calcination temperatures, further characterizations were carried out. The XRD patterns of the porous ZnO nanoflakes annealed at 300600 °C are shown in Figure 4, which reveal that the ZnO nanoflakes prepared at different temperatures are of the wurtzite phase (ZnO PDF 361451). Moreover, the diffraction peaks of ZnO300 and ZnO400 are broader and lower compared to ZnO500 and ZnO600, which suggests small crystal size and poor crystallinity at lower calcination temperatures. By use 12765
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The Journal of Physical Chemistry C of the Scherrer formula (DXRD = 0.89γ/β cos θ, wherein DXRD is the crystallite size, γ = 0.15406 nm, and β is the full width at the halfmaximum peak at diffraction angle 2θ), the mean crystallite size is estimated to be 27, 29.2, 34, and 38 nm for ZnO300600, respectively. The morphologies of ZnO300600 nanoflakes were characterized by TEM (Figure 5). Statistical measurements indicate that the particle size (DTEM) of the porous nanoflakes increases from 45 nm of ZnO300 to 90 nm of ZnO600, which indicates that ZnO particles tend to coarsen and further grow at high temperatures. The primary particles shown in TEM images are comprised of one or more crystallites, which leads to the different size between DTEM and DXRD. However, the gradual increase trend of DTEM is consistent with DXRD (Figure 4). Moreover, the edge and the angle
Figure 3. Sensitivities of the porous ZnO nanoflakes working at different temperatures to 0.5 ppm NO2. The ZnO nanoflakes were obtained at different calcination temperatures.
Figure 4. XRD patterns of the porous ZnO nanoflakes annealed at different temperatures in air for 3 h: ZnO300 (a), ZnO400 (b), ZnO500 (c), and ZnO600 (d).
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of the nanoparticles become more and more obvious and finally, the columnar nanoparticles appear in ZnO600. The gas sensitivity and the particle (or crystallite) size estimated from TEM and XRD are coplotted in Figure 6, which indicates that there is no line relationship between the particle (or crystallite) size and the gas sensitivity. Therefore, the experimental results of ZnO nanoflake sensors should not be simply explained by surface area or D-L model. To the ZnO sensor, the key influence on the sensing property may be the electric state of ZnO rather than surface/volume ratio.19 It is well-known that the nonstoichiometry or crystal defect determines the electronic properties of semiconductors. Therefore, the defect investigation of the ZnO nanoflakes may explain the existing of the max sensitivity in Figure.6. The intrinsic defects of ZnO are categorized as zinc interstitial (Zni), zinc vacancy (VZn), oxygen interstitial (Oi), oxygen vacancy (VO), oxygen antisite (OZn) and Zn antisite (ZnO).26 Of the six defects, ZnO are unlikely to be stable under equilibrium conditions due to their high formation energies; Zni and VO give rise to free electrons (i.e., donor) in ZnO crystal, while VZn, Oi, and OZn consume free electrons (i.e., acceptor).26,27 In equilibrium, the amount of released free electrons is higher than that of trapped electrons making ZnO to be an n-type semiconductor. The relative content of donors and acceptors determines the semiconductive property of ZnO. The PL spectrum of ZnO originates from either the photoinduced electronhole recombination or the intrinsic defects.16,26,27 Therefore, we used the PL spectroscopy methods to study the defect structures of ZnO300600 nanoflakes. Parts ad of Figure 7 are the PL spectra and their deconvolutions by GaussianLorentz distribution of ZnO300600 nanoflakes. According to their origination, the ultraviolet (UV) peak at 3.33.4 eV (ca. 380 nm) is assigned to the exciton emission from conduction band to valence band (CB-VB);28,29 the emission at 3.15 eV (ca. 395 nm) is ascribed to a shallow donor (Zni or Zni related complex defect);30,31 the peak at ca. 2.9 eV (ca. 415 and 435 nm) is attributed to Zni;29,32 the peak
Figure 6. Sensitivity vs the particle (crystallite) size of the porous ZnO nanoflakes annealed at different temperatures.
Figure 5. TEM images of the porous ZnO nanoflakes annealed at different temperatures in air for 3 h (the bar is 100 nm): ZnO300 (a), ZnO400 (b), ZnO500 (c), and ZnO600 (d). 12766
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Figure 7. Gaussian deconvolutions of the PL spectra of the porous ZnO nanoflakes annealed at different temperatures: ZnO300 (a), ZnO400 (b), ZnO500 (c), and ZnO600 (d) (correlation coefficient r2 > 0.999).
Table 1. Defect Percentages of the Porous ZnO Nanoflakes Calculated by Their Corresponding PL Spectraa origination
a
peak (nm)
ZnO300 (%)
ZnO400 (%)
ZnO500 (%)
ZnO600 (%)
(CB-VB) combination
∼380
5.0
14.8
10.2
8.6
Zni (3.163.11 eV)
∼395
19.5
21.1
22.2
19.4
Zni (∼2.9 eV)
∼415, ∼435
29.6
36.0
38.0
27.8
VZn (∼2.7 eV)
∼455, ∼486
22.7
16.4
12.8
22.3
VO (∼2.53 eV)
∼484, ∼500
13.1
7.3
13.1
14.5
OZn (∼2.38 eV) Oi (∼2.23 eV)
∼520 ∼550
3.2 6.9
0.6 4.1
2.9 0.9
4.4 3.0
DL (Zni þ VO)
62.2
64.4
73.2
61.6
AL (VZn þ OZn þ Oi)
32.8
21.1
16.6
29.7
The nanoflakes are obtained under different calcination temperatures.
at ca. 2.7 eV (ca. 455 and 486 nm) is assigned to VZn;33,34 the peak at ca. 2.53 eV (ca. 484 and 500 nm) is attributed to VO;35,36 ca. 2.38 eV (ca. 520 nm) is assigned to OZn,33,36,37 and the yellow and orange luminescence (>540 nm) are ascribed to Oi.33,37 The percentages of donors (Zni and VO) and acceptors (VZn, Oi and OZn) of ZnO300600 nanoflakes are summarized in Table 1. It is obvious that the Zni is the main defects of these four ZnO nanomaterials. The VO of ZnO600 is the most, while ZnO300 has the same amount of VO to ZnO500, and ZnO400 is the lowest. Furthermore, ZnO300 has much more Oi and OZn than other ZnO, which may be resulted from the incomplete release of oxygen in the decomposition process of the precursor (the details are shown in Figure S1 of Supporting Information). Overall, ZnO500 has the most donorsrelated PL (DL) content and the lowest acceptor-related PL (AL) content. More donors in crystal structure can release more electrons
for the redox reactions, which means that ZnO500 will have the best gas sensitivities than others. The operation temperature has a significant effect on the gas response. Figure 3 also reveals that the optimum operation temperatures for all of the sensors are 175 °C. Lee and Reedy theoretically summarized that the factors, such as oxygen species, the rates of adsorption and desorption, the charge carrier concentration, and the Debye length, contributed to the sensor response.38 These factors complicate the relationship between the conductance and the temperature of sensor. Some of the factors are demonstrated as the function of temperature, and all of the factors together determine the activities of the electrons needed in the sensing reaction.39 3.3. In situ DRIFTS Study of the NO2 Sensing Mechanism. DRIFTS technique was used to study the dynamic reaction 12767
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Figure 8. Time-resolved DRIFTS spectra of the porous ZnO nanoflakes after adsorption of 9800 ppm NO2/He at RT for 0.5, 1, and 5 min (a), 0.5, 1, 5, 10, 20, 34, and 35 min (b and c).
process of NO2 and ZnO in situ and the sensing mechanism. The information on the reactions process was noted by the appearance of adsorbed species in the initial stages of adsorption. The representative time-resolved DRIFTS spectra of ZnO500 recorded at RT after exposure to NO2 are displayed in Figure 8. The strong bands at ca. 1664, 1630, 1598, 1554, 1525, 1494, 1452, 1410, 1388, 1332, 1300, 1260, 1100, 1038, 1020, 988, 940, 926, and 884 cm1 and the weak bands at ca. 1222, 1187, and 1140 cm1 are observed in the spectra (Figure 8a) after half a minute. The intensity of the strong bands increases rapidly over the reaction time, while the intensity of the weak bands barely increase, and all of the absorbance at RT do not reach saturation until 35 min later (Figure 8c). Moreover, the development of the strong bands is accompanied by the increase of weak absorptions at ca. 2530, 2396, 2276, 2235, 2214, 2040, 1785, and 1746 cm1 (Figure 8b). Meanwhile, the weak bands at ca. 1904 and 1850 cm1 increase within the first several minutes of the reaction and then decrease gradually until reaching the saturation. Because of the important effect of intrinsic defects on the electrical properties of the semiconductor oxides, the defect species were introduced into our proposed gas-sensing mechanism. When the gas-sensing materials are exposed to air, oxygen molecules are adsorbed and become negatively charged via obtaining electrons from the conduction band of ZnO.40 The reaction of eqs 1 or 2 takes place on the surface of ZnO materials at first, and one or two electrons divert from the oxygen vacancies
of ZnO to the oxygen molecule V O þ O2 ðgÞ T O2 ðadsÞ þ V O 3 2V O þ O2 ðgÞ T O2 2 ðadsÞ þ 2V O 3 T 2O ðadsÞ þ 2V O 3
ð1Þ ð2Þ
wherein “g” and “ads” refer respectively to gas and adsorbate, VO oxygen vacancy, and VO 3 single electropositive oxygen vacancy. In the initial stages of adsorption, it is interesting that monodentate nitrites species (at ca. 1452 and 1064 cm1) are the main species and bidentate nitrites (at 1222 cm1) also can be clearly seen in Figure 9a.24,25,41 The formation of the nitrite species may originate from the reaction25,42 NO2 ðadsÞ þ Zni T NO2 ðadsÞ þ Zni 3
ð3Þ
3
wherein Zni is zinc interstitial and Zni single electropositive zinc interstitial. Furthermore, the intensity of the nitrite species increases very slowly with the reaction proceeding. This may be due to the transformation of nitrite to nitrate by the cooperative oxygen on the zinc sites through the reaction: NO2 ðadsÞ þ NO2 ðadsÞ þ Zni 3 :::O2 T 2NO3 ðadsÞ þ Zni 3 3 ð4Þ
wherein Zni 3 3 is double electropositive zinc interstitial. The nitrate species, including free nitrate ion bounded to Znxþ (at 1418 1388 cm1),23,25,41,43 bridging nitrates (at ca. 1554, 1260, and 988 cm1),2325,41,43,44 bidentate nitrates (at ca. 1525, 1332, and 1038 cm1),2325,43 and monodentate nitrates (at ca. 1494, 1300, and 1020 cm1),24,25,43 are the prevailing sorption species in the first 12768
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Figure 9. DRIFTS spectra of the porous ZnO nanoflakes after adsorption of 9800 ppm NO2/He at different temperatures.
minute of the reaction. The possible way for the formation of the nitrate species can also be via electron transfer from VO 3 donor center to NO2 molecule with a simultaneous reoxidation of an oxygen vacancy,23,24 which can be depicted as 2NO2 ðadsÞ þ O2 ðadsÞ þ V O 3 T 2NO3 ðadsÞ þ V O 3 3 ð5Þ wherein VO 3 3 is double electropositive oxygen vacancy. With the reaction proceeding, the intensity of the nitrate species increases sharply due to the reactions of eqs 4 and 5 and obscures the weak absorption of nitrite species. Moreover, all of the different species are present and overlap each other in the same region just after one minute of adsorption. The complexity of the absorption peaks for these nitrates and nitrites reflects the inhomogeneity of surface sites, which results in the coordinating of nitrate and nitrite species with the surface in different manners. Another interesting phenomenon is that two sharp bands (at ca. 1630 and 1598 cm1), which seem like a doublet, are observed after one minute of adsorption. The intensity of the two bands develops drastically over time. They are assigned to the NO2 gas or physically adsorbed NO2 gas.24,45 In addition, the low-intensity signals observed at ca. 2530, 2396, 2276, and 2040 cm1 are ascribed to the overtone and combination vibrations of these nitrate compounds.25,44 Meanwhile, a tiny amount of NO (at ca. 1904 and 1865 cm1)24,45,46 may come from the processes of desorption and decomposition of the main adsorbed nitrates (eq 6), which occur on the surface to release the nitric oxide and oxygen gases, and the sensing sites obtain electrons to revert to the original conditions.25,41 2NO3 ðadsÞ þ V O 3 3 =Zni 3 3 T 2NOðgÞ þ 2O2 ðadsÞ þ Zni =V O ð6Þ
Moreover, the appearance of N2O (at ca. 2235, 2220, and 2214 cm1)25,46 may be because of the further reactions of the NO on the surface via the electronegative NO (at 1186 and 1135 cm1) and N2O2 (at ca. 1375 and 1104 cm1) anion,25,43,46 which can be proposed as NOðadsÞ þ Zni T NO ðadsÞ þ Zni 3 NO ðadsÞ þ Zni 3 þ NOðadsÞ T N2 O2 ðadsÞ þ Zni 3
ð7Þ ð8Þ
N2 O2 ðadsÞ þ Zni 3 þ NO2 ðadsÞ T NO3 ðadsÞ þ Zni 3 þ N2 OðgÞ ð9Þ
On the whole, these processes result in the formation of nitrate species and the shift of one electron. It is noticeable that the
intensity of the adsorbed N2O is very low and increases just a little when the reactions reach equilibrium. Compared with the intensities of the nitrate and nitrite species, the formation of NO and N2O is minor. This may be caused by the weak adsorbability of NO and N2O gases, which leads to weak singles in the spectra. Figure 9 shows the in situ DRIFTS spectra of the ZnO500 sample when exposed to NO2 at different temperatures. The intensities of all of the adsorbed species on ZnO500 change with the temperature obviously. Because of the decrease of adsorbability, the intensity of the physically adsorbed NO2 decreases with the temperature increase. Meanwhile, when the temperature is raised from RT to 150 °C, the intensity of the main nitrate species reaches the maximun. Nevertheless, when the temperature rises up to 200 °C, there is a sharp decline in the spectra. And at 250 and 300 °C, there are very weak adsorptions on the surface of ZnO500. On the one hand, the formation of the different adNO2 species is via electron transfer from donor center, such as VO 3 , Zni, or Zni 3 , to a NO2 molecule with a simultaneous reoxidation of an oxygen vacancy or interstitial zinc. When the temperature increases, the activities of the electrons are enhanced, and as a result, more electrons can jump over the potential barrier of the crystal to join the adsorption of NO2. Therefore, the donor sites (i.e., sensing sites) increase with the temperature. On the other hand, when the temperatures increase continuously, the process of thermal decomposition of the main adsorbed nitrates occurs on the surface (eqs 6 and 10) and the sensing sites obtain electrons to revert to the original conditions.23,44 2NO3 ðadsÞ þ V O 3 3 =Zni 3 3 T 2NO2 ðgÞ þ O2 ðadsÞ þ Zni =V O
ð10Þ
The increasing of the intensity of NO gas can also prove this process (shown in Figure 9a). Therefore, when the adsorption rate of NO2 is higher than the decomposition rate of NO3, the intensity of the ad-NO2 species increase; when equal, the intensity of the ad-NO2 species reaches the maximum; when lower, the intensity of the ad-NO2 species decreases. According to the proposed reactions above, we can conclude that the higher intensities of the adsorbed nitrate and nitrite species the more conductive band electrons transfer from crystal to nitrogen dioxide molecules. Consequently, ZnO500 has an optimum operating temperature at ca. 150 °C. However, in our gas-sensing test, the optimum operation temperature of ZnO sensors is ca. 12769
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Figure 11. Gas sensitivities of the porous ZnO nanoflakes (square) and the purchased ZnO nanoparticles (triangle) to 0.5 ppm and 1 ppm NO2 at different operation temperatures.
Figure 10. SEM (a) and TEM (b) images of the purchased ZnO nanoparticles.
175 °C. The disagreement of the results of gas-sensing tests and DRIFTS spectra may be due to the variations of the test conditions and the special calculation methods of gas sensitivity. 3.4. Comparison Studies of the Nanoflake and the Nanoparticle ZnO Sensors. To further verify our gas mechanism, the ZnO500 nanoflake sensor was compared with a purchased ZnO nanoparticle sensor. The detailed structural analysis of the purchased ZnO nanoparticles was carried out by using SEM and TEM, as shown in parts a and b of Figure 10, respectively. It is clear that the irregular ZnO particles with an average diameter of ca. 45 nm connect each other without obvious boundary. What is interesting is that the morphology of the purchased ZnO nanoparticles is very similar to ZnO500 nanoflakes except that the purchased nanoparticles aggregate more seriously than ZnO500 nanoflakes. Figure 11 shows the gas sensitivities of the porous ZnO500 nanoflakes and the purchased ZnO nanoparticles to 1 or 0.5 ppm NO2 at the operation temperatures from 100 to 240 °C, which indicates that the optimum operation temperatures are ca. 175 °C. When exposed to 0.5 ppm NO2, the gas sensitivity of ZnO500 nanoflake gas sensor is 79, while that of the purchased ZnO nanoparticle gas sensor is only 14. When exposed to 1 ppm NO2, the gas sensitivity of ZnO500 nanoflake gas sensor can reach 134. Parts a and b of Figure 12 show the corresponding sensitivity plots of ZnO500 nanoflakes and the purchased ZnO nanoparticles vs NO2 in the concentration range of 0.05 10 ppm. The sensitivities of ZnO500 nanoflake gas sensor to 0.05, 0.1, 0.2, 0.5, 1, 2, 4, 5, 7, and 10 ppm NO2 are 5.6, 30, 56, 79, 134, 205, 445, 560, 760, and 1060, respectively. The sensitivities of the
Figure 12. Gas sensitivities of the porous ZnO nanoflakes and the purchased ZnO nanoparticles vs different concentrations of NO2.
purchased ZnO nanoparticle gas sensor are much lower than those of ZnO500 nanoflake gas sensor, and those are 1.5, 4, 7, 12, 26, 38, 63, 81, 115, and 156, respectively. The result of ZnO500 nanoflake gas sensor is much better than previous reported results.15,47,48 In addition, each sensitivity curve can fit a straight line, and the correlation coefficients R of ZnO500 nanoflakes and the purchased ZnO nanoparticles are 0.9993 and 0.9983, respectively. The gas sensitivity (Sg) of semiconducting oxide sensors can be empirically represented by Sg = RCgβ, wherein Cg is the concentration of the target gas, and the sensitivity is 12770
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Table 2. Defect Percentages of the Porous ZnO Nanoflakes and the Purchased ZnO Nanoparticles Calculated by Their Corresponding PL Spectra origination (CB-VB) combination Zni (3.163.11 eV) Zni (∼2.9 eV) VZn (∼2.7 eV) VO (∼2.53 eV) OZn (∼2.38 eV) Oi (∼2.23 eV) DL (Zni þ VO) AL (VZn þ OZn þ Oi)
Figure 13. Gaussian deconvolutions of PL spectra of the porous ZnO nanoflakes (a) and the purchased ZnO nanoparticles (b).
characterized by the prefactor R and the exponent β. Generally, the exponent β has an ideal value of either 0.5 or 1. The available adsorption sites of the ZnO nanoflakes and the purchased ZnO nanoparticles are sufficient for the sensing reactions when the concentration of NO2 is in the range of 0.0510 ppm, which results in the values of β to be 1 (as shown in Figure 12).14 However, the different surface structures and adsorption species may lead to the different R values. Next, the PL spectroscopic methods were used to study the differences of the defect structures of ZnO500 nanoflakes and the purchased ZnO nanoparticles. According to the sensing mechanism, defect structures relate to the gas sensitivity. Parts a and b of Figure 13 show the PL spectra and their deconvolutions of ZnO500 nanoflakes and the purchased ZnO nanoparticles, respectively. The results of the curve fitting of the PL spectra are summarized in Table 2. It is obvious that ZnO500 nanoflakes have more VO than the purchased ZnO nanoparticles, and the Zni are the main defects to both ZnO. Overall, ZnO500 nanoflakes have more donors and fewer acceptors. According to the mechanism of gas sensing deduced from the DRIFTS, more donors in the crystal surface structures can release more electrons to the redox reactions of the target gases. Consequently, the PL spectra verify that ZnO500 nanoflakes have higher gas sensitivities than the purchased ZnO nanoparticles. Moreover, the XPS technology was used to further investigate the nonstoichiometry of the ZnO samples surface. Parts a and b of Figure 14 are GaussianLorentz fitting curves of Zn L3M45M45 Auger electron spectra (AES). From them one can see that the main peak is at ca. 498 eV, which is attributed to the
peak (nm)
ZnO nanoflakes (%)
ZnO nanoparticles (%)
∼380 ∼395 ∼415, ∼435 ∼455, ∼486 ∼484, ∼500 ∼520 ∼550
10.2 22.2 38.0 12.8 13.1 2.9 0.9 73.2 16.6
8.2 20.7 25.0 22.0 12.5 5.8 5.7 58.2 33.6
lattice Zn2þ, and the shoulder peak at ca. 495 eV is usually assigned to Zni.49 Parts c and d of Figure 14 present the Gauss fitting curves of O1s spectra of ZnO500 nanoflakes and the purchased ZnO nanoparticles. Three species, centered at ca. 530.2 eV (O1), 531.4 eV (O2), and 532.5 eV (O3), were indexed. The species of O1 on the lower binding energy side of the O1s spectrum belong to O2 ions in wurtzite structure in a fully oxidized stoichiometric surrounding. The medium binding energy species of O2, centered at ∼531.9, belong to Ox ions (O and O2 ions) in the oxygen-deficient regions caused by VO, Oi, and OZn. The high binding energy species of O3 are usually attributed to the chemisorbed or dissociated oxygen or OH species on the surface of ZnO, such as the absorbed O2 or H2O.50 The comparisons of the Zn L3M45M45 AES data and the O1s XPS data between ZnO500 nanoflakes and the purchased ZnO nanoparticles are presented in Table 3. It is obvious that ZnO500 nanoflakes have larger Zni content than the purchased ZnO nanoparticles, which is consistent with the PL results. Additionally, by comparison with the purchased ZnO nanoparticles, ZnO500 nanoflakes have more O2 and almost an equal amount of O3. According to the results of the PL spectra, O2 species of ZnO500 nanoflakes are composed of VO (13.1%) and a little OZn and Oi (3.8% in total), while the O2 species of the purchased ZnO nanoparticles are composed of 12.5% VO and a great amount of OZn and Oi (11.5% in total). This indicates that most of O2 species (Ox ions) of ZnO500 nanoflake are resulted from VO which plays an important role in the gas-sensing reactions, and few from Oi and OZn. Consequently, ZnO500 has more donor content (Zni and VO) than that of the purchased ZnO nanoparticles. As indicated above, the porous ZnO500 nanoflakes have higher gas sensitivity because of the more donors (Zni and VO) indicated by the PL and the XPS spectra. Therefore, spectroscopy method has provided a satisfactory explanation on the difference of the gas-sensing properties resulted from the different morphologies of ZnO. 3.5. Selectivity of the Porous ZnO Nanoflake Sensor. In conslucion from the foregoing experiment results, the unique porous ZnO nanoflake gas sensor has high sensitivity to NO2. This means that the NO2 gas has good “opportunities” to be detected by the porous ZnO nanoflake gas sensor. Selectivity is one of the basic properties of gas sensor. Figure 15 shows the sensitivity of the porous ZnO nanoflake gas sensor to 0.5 ppm NO2 and 85 ppm other gases including ethanol, carbon monooxide, propylene, methane, diethyl ether, formaldehyde, acetone, cyclohexane, and dimethylbenzene. Some of them are the air pollutant produced by automotive engines. Obviously, 12771
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Figure 14. High-resolution O1s XPS spectra of the porous ZnO nanoflakes (a) and the purchased ZnO nanoparticles (b). Zn L3M45M45 Auger peaks of the porous ZnO nanoflakes (c) and the purchased ZnO nanoparticles (d).
Table 3. Comparison of the XPS Data of O1s and ZnL3M45M45 Auger Peaks of the Porous ZnO Nanoflakes and the Purchased ZnO Nanoparticles ZnO nanoflakes species 2þ
Zn Zni O1 O2 O3
ZnO nanoparticles
peak
percentage (%)
peak
percentage (%)
498.21 495.18 530.28 531.53 532.63
67.2 32.8 53.9 23.8 22.3
498.03 494.98 530.14 531.26 532.36
71.7 28.7 57.6 20.0 22.4
Figure 15. Sensitivities of the porous ZnO nanoflakes gas sensor to NO2 (0.5 ppm) and other gases (85 ppm) under the optimum conditions.
without any special treatments, the porous ZnO polygonal nanoflake gas sensor has more excellent selectivity to NO2 than to other gases. Thus, it can be concluded that the porous ZnO polygonal nanoflake gas sensor may be useful in detecting NO2.
4. CONCLUSIONS In summary, the unique porous ZnO polygonal nanoflakes were obtained by using microwave hydrothermal method that was combined with a subsequent annealing process. The gassensing properties of ZnO nanoflakes obtained by calcining the precursor at different temperatures were found to be independent of particle size and greatly rely on crystal defect structure. The PL spectra and the XPS spectra further proved that the intensity of the donors (VO and Zni) and the surface oxygen species (O2 and O2-) involved in the mechanism of gas sensing on different structure surfaces (as-prepared porous ZnO nanostructures and purchased ZnO nanoparticles) was capable of leading to the different gas-sensing properties to NO2 gas. The mechanism of NO2 sensing was proposed according to the results of DRIFTS experiments. It was verified that the NO2 response proceeded mainly via electron transfer on the donor sites and/or the participation of surface oxygen species to form the nitrate species, and free nitrate ions, nitrite anions, nitroxyl anions and N2O also formed in the reactions. The DRIFTS experiments further explained the existing of the optimum operating temperature of gas sensor. In addition, the porous ZnO nanoflakes gas sensor exhibited a much higher sensitivity to NO2 than to other gases, suggesting the good selectivity of the sensors on detecting NO2 and potential application thereof. 12772
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’ ASSOCIATED CONTENT
bS
Supporting Information. The detailed thermogravimetric and differential thermal analysis of the as-prepared ZNH precursor is contained in this section. This information is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: þ86 10 64445927. E-mail:
[email protected] (G.G);
[email protected].
’ ACKNOWLEDGMENT This work was supported by the China National Natural Science Funds (No. 20875007 and No. 20935002) and the Excellent Ph.D. Thesis Fund of Beijing (YB20091001002). ’ REFERENCES (1) Ismail, B.; Abaab, M.; Rezig, B. Thin Solid Films 2001, 383, 92–94. (2) Lee, J. H. Sens. Actuators B 2009, 140, 319–336. (3) Yamazoe, N.; Shimanoe, K. J. Electrochem. Soc. 2008, 155, 85–92. (4) Hernandez-Ramirez, F.; Prades, J. D.; Morante, J. R. Sens. Mater. 2009, 21, 219–227. (5) Xu, J. Q.; Chen, Y. P.; Chen, D. Y.; Shen, J. N. Sens. Actuators B 2006, 113, 526–531. (6) Xi, Y.; Hu, C. G.; Han, X. Y.; Xiong, Y. F.; Gao, P. X.; Liu, G. B. Solid State Commun. 2007, 141, 506–509. (7) Han, X. G.; He, H. Z.; Kuang, Q.; Zhou, X.; Zhang, X. H.; Xu, T.; Xie, Z. X.; Zheng, L. S. J. Phys. Chem. C 2009, 113, 584–589. (8) Wang, Y. D.; Zhang, S.; Ma, C. L.; Li, H. D. J. Lumin. 2007, 126, 661–664. (9) Xu, X. Y.; Cao, C. B. J. Alloys Compd. 2010, 501, 265–268. (10) Duan, J. X.; Huang, X. T.; Wang, H.; Zhong, Q.; Sun, F. L.; He, X. Mater. Chem. Phys. 2007, 106, 181–186. (11) Liu, Y.; Dong, J.; Hesketh, P. J.; Liu, M. L. J. Mater. Chem. 2005, 15, 2316–2320. (12) Wang, H. H.; Xie, C. S. J. Cryst. Growth 2006, 291, 187–195. (13) Xue, L. H.; Mei, X. T.; Zhang, W. X.; Yuan, L. X.; Hu, X. L.; Huang, Y. H.; Yanagisawa, K. Sens. Actuators B 2010, 147, 495–501. (14) Li, J.; Fan, H. Q.; Jia, X. H. J. Phys. Chem. C 2010, 114, 14684–14691. (15) Zhang, J.; Wang, S. R.; Wang, Y.; Xu, M. J.; Xia, H. J.; Zhang, S. M.; Huang, W. P.; Guo, X. Z.; Wu, S. H. Sens. Actuators B 2009, 139, 411–417. (16) Epifani, M.; Prades, J. D.; Comini, E.; Pellicer, E.; Avella, M.; Siciliano, P.; Faglia, G.; Cirera, A.; Scotti, R.; Morazzoni, F.; Morante, J. R. J. Phys. Chem. C 2008, 112, 19540–19546. (17) Fan, S. W.; Srivastava, A. K.; Dravid, V. P. Sens. Actuators B 2010, 144, 159–163. (18) Xu, C.; Tamaki, J.; Miura, N.; Yamazoe, N. Sens. Actuators B 1991, 3, 147–155. (19) Han, N.; Wu, X. F.; Chai, L. Y.; Liu, H. D.; Chen, Y. F. Sens. Actuators B 2010, 150, 230–238. (20) Wang, C. H.; Chu, X. F.; Wu, M. W. Sens. Actuators B 2006, 113, 320–323. (21) Chen, L.; Tsang, S., Ch. Sens. Actuators B 2003, 89, 68–75. (22) Naval, S. C.; Srinivas, V.; Ravi, D.; Mulla, I. S.; Gosavi, S. W.; Kulkarni, S. K. Sens. Actuators B 2008, 130, 668–673. (23) Chiorino, A.; Ghiotti, G.; Prinetto, F.; Carotta, M. C.; Gnani, D.; Martinelli, G. Sens. Actuators B 1999, 58, 338–349. (24) Stasio, S. D.; Santo, V. D. Appl. Surf. Sci. 2006, 253, 2899–2910. (25) Azambre, B.; Zenboury, L.; Koch, A.; Weber, J. V. J. Phys. Chem. C 2009, 113, 13287–13299.
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