- Email: [email protected]
Porous corundum-type In2O3 nanosheets: Synthesis and NO2 sensing properties
Sensors and Actuators B 208 (2015) 436–443
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Porous corundum-type In2 O3 nanosheets: Synthesis and NO2 sensing properties Liping Gao a , Zhixuan Cheng b,∗∗ , Qun Xiang b , Yuan Zhang b , Jiaqiang Xu a,b,∗ a b
Department of Physics, College of Science, Shanghai University, Shanghai, 200444, China Department of Chemistry, College of Science, Shanghai University, Shanghai, 200444, China
a r t i c l e
i n f o
Article history: Received 14 July 2014 Received in revised form 7 November 2014 Accepted 10 November 2014 Available online 18 November 2014 Keywords: Rh-In2 O3 nanosheets Porous NO2 Gas-sensing properties
a b s t r a c t Porous corundum-type In2 O3 (rhombohedra In2 O3 , rh-In2 O3 ) nanosheets with unique shoeprint morphology are fabricated successfully by a facile, template-free solvent-thermal method. The products are characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), photoluminescence (PL) and N2 adsorption–desorption. The response of the porous rh-In2 O3 nanosheets sensor to 50 ppm NO2 is about 164, and the response/recovery times are not exceeding 5 s and 14 s, respectively. Porous lamellar structure could increase the number of the active sites and speed up the gas transportation, which is helpful for the enhancement of the response and rapid adsorption/desorption. The effect of morphology on the gas sensing responses is investigated. Compared with the nonporous rh-In2 O3 nanosheets sensor, the porous rh-In2 O3 nanosheets sensor exhibits significantly enhanced sensing performances toward NO2 . Porous rh-In2 O3 nanosheets sensors show a long-term stability, which might be ascribed to the avoidance of aggregation of the nanostructures. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Materials with porous structure have good applications in many fields, such as supercapacitors [1], lithium-ion batteries [2], solar cells [3], surface-enhanced Raman scattering (SERS) [4,5], photocatalysis [6], and gas sensing [7]. Porous nanosheets materials with unique structural advantages have led to their intriguing properties, attractive applications, as well as industrial and environmental benefits [8–10]. Different kinds of porous nanosheets material were obtained and achieved good success in some fields [11–15]. For example, porous NiO nanosheets prepared by Dong et al. showed enhanced photocatalytic activity for NO destruction [11]. Porous ZnO nanosheets were employed as high-performance surface-enhanced Raman scattering substrate [4]. As an important semiconductor material, In2 O3 nanostructure with unique physicochemical properties has attracted particular attention. Different In2 O3 porous structures such as porous nanospheres [16], porous nanotubes [17], porous nanobelts [18], porous nanoflowers [19,20] have been successfully fabricated. Most of the In2 O3 structures as
∗ Corresponding author at: Department of Physics, College of Science, Shanghai University, Shanghai, 200444, China. Tel.: +86 21 66132701; fax: +86 21 66132701. ∗∗ Corresponding author. Tel.: +86 21 66132701; fax: +86 21 66132701. E-mail addresses: [email protected], [email protected] (J. Xu). http://dx.doi.org/10.1016/j.snb.2014.11.053 0925-4005/© 2014 Elsevier B.V. All rights reserved.
mentioned above are in a cubic lattice, whereas controlled synthesis of rhombohedra In2 O3 (rh-In2 O3 ) has been rarely studied because of its synthesis usually needs high temperature and pressure. For example, Slerght group [21] synthesized rh-In2 O3 through the phase transition method under the temperature of 1000 ◦ C and the pressure of 65 kbar. Recently, solvothermal method was proved to synthesis rhombohedra In2 O3 under rather low temperature and pressure, such as rh-In2 O3 microspheres [22,23], rh-In2 O3 nanocrystals [24] and rh-In2 O3 nanofibers [25]. However, controlled synthesis of porous rh-In2 O3 nanosheet gas sensing material has been less reported. Lamellar nanosheet structure could endow the good stability of the material compared with nanoparticles due to the decreased surface energies. Stable nanosheet structure can ensure the stability of gas sensitive test results which can be seen in previous literatures [26,27]. Porous structure is beneficial to gas diffusion and mass transport [28,29], the existence of defects such as oxygen vacancies could increase the chance of electrostatic interaction between testing gas and the surface of sensing materials, benefitting to oxygen adsorption and further enhance the gas response of the sensors [30,31]. So, it is of significant value to develop porous In2 O3 nanosheets sensors. NO2 detection is of paramount importance in explosives detection as it is a decomposition product of many explosive formulations and improvised explosive devices [32]. In2 O3 as an important semiconductor with low resistance has been proved
L. Gao et al. / Sensors and Actuators B 208 (2015) 436–443
437
Fig. 1. (a) Sketch of the gas-sensor structure, (b) the measuring electric circuit for the gas sensor and (c) gas sensing test device.
to be an excellent gas-sensing material for detecting oxidizing gases, especially suitable for detecting NO2 [33–36]. The gas sensing properties of In2 O3 nanostructures to NO2 have been widely investigated by many research groups [37–39]. However, some of the nanostructures showed certain shortage in sensitivity, response or recovery. The lamellate and porous structure is favorable for gas adsorption and diffusion, resulting in the constructed sensor possessed excellent gas-sensing performance, making it a promising material for detection of NO2 gas. In this paper, a facile and eco-friendly self-assembly approach to obtain porous In2 O3 nanosheets with unique shoeprint morphology has been described. The as-prepared porous In2 O3 nanosheets sensor showed excellent gas-sensing properties toward NO2 . The effect of morphology on the gas sensing responses was also investigated. Compared with the nonporous rh-In2 O3 nanosheets sensor, the porous rh-In2 O3 nanosheets sensor exhibits significantly enhanced sensing performance toward NO2 . 2. Experimental sections All the reagents (analytical-grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and were used without further purification. Deionizer water was used throughout the experiments. 2.1. Synthesis In a typical synthesis, 6 mL In(NO3 )3 ·5H2 O (0.1 M) aqueous solution, 20 mL glycol, and 13 mL ethanol solution were mixed together. After vigorous stirring for 30 min, the mixture was transferred into a Teflon-lined stainless steel autoclave of 50 mL capacity. The autoclave was sealed and maintained at 180 ◦ C for 24 h, and
then naturally cooled down to room temperature, after which white precipitated powder was obtained. The powder was centrifuged and washed several times with distilled water and absolute ethanol alternatively, and then dried at 60 ◦ C for 8 h. Porous In2 O3 nanosheets were prepared by annealing the precursor in a muffle furnace at 500 ◦ C for 2 h under ambient pressure. 2.2. Characterization Powder X-ray diffraction measurements were performed with a Rigaku D-MAX/IIA X-ray diffractometer in a scanning range of 10–70◦ (2) at a rate of 1.8◦ (2)/min with Cu Ka radiation. Scanning electron microscopy (SEM) (JSM-6700F and S-4800) and transmission electron microscope (TEM) (JEM-2010F) was utilized for the investigation of the morphology of the prepared material. Surface groups were monitored by Fourier Transform Infrared Spectroscopy (FTIR) with an AVATRA370 FTIR instrument using KBr plates within 4500–400 cm−1 . Room temperature PL measurements were performed on a Hitachi RF-5301PC spectrofluophotometer using the 325 nm Xe laser line as the excitation source. The N2 adsorption–desorption analyses were carried out at liquid nitrogen temperature on an ASAP 2020 analyzer (Micromeritics Instrument, USA). The Brunauer–Emmett–Teller (BET) specific surface areas (SBET) were calculated using the BET equation. Desorption isotherm was used to determine the pore size distribution using the Barret–Joyner–Halender (BJH) method. 2.3. Gas response test Fig. 1a is a schematic of the sensor structures, in which a pair of Au electrodes was printed on both sides of the ceramic tube, while a Pt resistance heater inside of the ceramic tube was used to
438
L. Gao et al. / Sensors and Actuators B 208 (2015) 436–443
Fig. 2. XRD patterns of (a) the precursor InOOH and (b) the porous rh-In2 O3 nanosheets.
control the temperature of sensor. Firstly, the samples were mixed and ground with an adhesive in an agate mortar to form a gas sensing paste. Then, the paste was coated on an alumina ceramic tube and dried under IR radiation for 15 min. Subsequently the alumina ceramic tube was sintered at 500 ◦ C for 2 h to get good stability and welded onto the pedestal. Finally, the obtained sensors were aged at 300 ◦ C for 7 d to increase their stability before tested. The measuring electric circuit for the gas sensor is shown in Fig. 1b. Through varying heating voltage (Vh ), different working temperatures of a sensor could be obtained. In the measurement of electric circuit for gas sensors, a load resistor (RL ) was connected in series with a gas sensor. The circuit voltage (Vc ) was set at 5 V, and the output voltage (Vout ) was the terminal voltage of the load resistor. The resistance of a sensor in air (Ra ) or test gas (Rg ) was measured by monitoring Vout . The gas response of the sensor in this paper was defined as S = Ra /Rg (reducing gases) or S = Rg /Ra (oxidizing gases). The response or recovery time was expressed as the time taken for the sensor output to reach 90% of its saturation after applying or switching off the gas in a step function. Fig. 1c shows the gas sensing test device. The test was operated in a measuring system of HW-30A (Hanwei Electronics Co. Ltd., P.R. China). A stationary state gas distribution method was used for testing gas response. This HW-30A gas sensing test system was shown in the manuscript (Fig. 1c). According to the reference [40], the volumes of gas or liquid corresponding to different concentrations can be obtained. Under certain temperature, we could get the resistance of the sensor in the air (Ra ). When the gas was injected into the test chamber, it could mix with air and then adsorb on the surface of the sensor. For the liquid, it should be injected on an evaporator
Fig. 4. FTIR spectra of the precursor InOOH.
through the hole at the back of the chamber. Through the heating evaporator, the liquid sample was converted to its vapor phase, and then mixed with air. The resistance of the sensor changed and reached a stable value several seconds later through contacting and reacting with the gas. In the meantime, the resistance of the sensor in the test gas (Rg ) can be recorded. Finally open the test chamber, the sensor contacted with air again, and then the resistance of the sensor returned to the level before the gas injection. In this measurement, the humidity is controlled with 40–50% RH. 3. Results and discussion Fig. 2a shows the powder X-ray diffraction (XRD) pattern of the InOOH precursor. All of the peaks can be indexed to pure InOOH with the orthorhombic phase (JCPDS card, No. 17-0549). The XRD pattern of the annealed In2 O3 product is depicted in Fig. 2b. All of the detectable peaks in the pattern can be easily indexed to pure hexagonal In2 O3 phase (JCPDS Card, No. 73–1809). No other diffraction peaks related to impurities are observed, suggesting the high purity of the as-prepared hexagonal In2 O3 . The morphology of the In2 O3 was investigated by scanning electron microscopy (SEM) and transmission electron microscope (TEM), and the corresponding results are displayed in Fig. 3. As shown in Fig. 3a, we can clearly see that 2-D In2 O3 nanostructures are obtained on a large scale and the dispersion of nanosheets is good. The nanosheets are assembled by some small nanoparticles and there are some pores existed in the nanosheets. A higher magnification SEM image in the inset of Fig. 3a gives more information about the thickness of the In2 O3 nanosheets.
Fig. 3. (a) SEM and (b) TEM images of porous rh-In2 O3 nanosheets.
L. Gao et al. / Sensors and Actuators B 208 (2015) 436–443
439
Fig. 5. SEM images of rh-In2 O3 with different amounts of glycol, low (a) and high (b) magnification SEM image of In2 O3 (10 mL glycol), low (c) and high (d) magnification SEM image of In2 O3 (without glycol).
From the image, we can clearly see that the thickness of the nanosheets is less than 50 nm. Fig. 3b is a typical TEM image of the porous In2 O3 nanosheets, which is in accordance with the SEM results. Porous In2 O3 nanosheets with unique shoeprint morphology clearly showed that there are lots of pores on them (Fig. 3b). Glycol has been widely used in the polyol synthesis of metal (both pure and alloyed) nanoparticles due to its strong reducing power and relatively high boiling point (197 ◦ C) [41,42]. Recently, it has been used to fabricate mesostructures of titania, tin dioxide,
zirconia and niobium oxide by forming glycolate precursors because of its coordination ability with transition metal ions [43,44]. Xia and co-workers have synthesized porous SnO2 nanowires through a route that involved complexation with glycol, followed by polymerization [45]. For porous In2 O3 nanosheets, In3+ may combine with glycol and ethanol units through the formation of In –O– covalent and In–OH coordination bonds. Fig. 4 is the FT-IR studies of the precursor InOOH, in which we observed a strong broad absorption bands at 3450 cm−1 and a narrow band
Fig. 6. Photoluminescence spectra of different In2 O3 samples at room temperature, ex = 325 nm.
Fig. 7. N2 adsorption and desorption isotherms for the porous rh-In2 O3 nanosheets.
440
L. Gao et al. / Sensors and Actuators B 208 (2015) 436–443
Fig. 8. NO2 gas sensing property of porous rh-In2 O3 nanosheets and nonporous rh-In2 O3 nanosheets at different test temperatures.
at 1637 cm−1 , confirming the presence of crystallization water molecules. The peaks at 472 cm−1 are the In–O feature modes. The 2800–3000 cm−1 mode in the InOOH sample are assigned to the CH2 and CH3 symmetric and asymmetric stretching vibrations, and the 1465 cm−1 peak is the CH2 bending vibration mode. The peak position is consistent with the literatures [46,47]. The 1000–1300 cm−1 mode in the InOOH sample are assigned to the C–O stretching vibration. The appearance of CH2 – and C–O bands represents the combination of In3+ with glycol and ethanol unit which led to the formation of longer chains, and finally could further self-assemble into ordered bundles (nanosheets) through van der Waals interactions. Glycol as a cross-linking reagent is beneficial for the synthesis of porous structures [28,44]. In order to demonstrate the important role of glycol in synthesizing porous In2 O3 nanosheets, we performed the reaction through changing the amount of glycol with ethanol instead of it when other conditions keep the same. When the amount of glycol is 10 mL, broken In2 O3 nanosheets can be observed (Fig. 5a and b), which are different from Fig. 3. When glycol was completely replaced by ethanol, some small irregular nanoparticles could be found (Fig. 5c and d). We speculated that a moderate amount of glycol could promote the formation of the uniform porous nanosheets. Due to only a fraction of In3+ complexation with glycol, nanoparticles cannot complete assembly into nanosheets. Without glycol as the solvent, only small nanoparticles could be formed due to the disappearance of complexation. In order to illustrate the advantage of pore structure, we synthesized nonporous rh-In2 O3 nanosheets by changing the conditions of the preparation. As shown in Fig. 1s, the obtained nanosheets are assembled of nanoparticles. The nanoparticles are closely connected and there are no holes between the particles. The photoluminescence (PL) spectra of the porous In2 O3 nanosheets
Fig. 9. Response–recovery curves of the sensors to 50 ppm NO2 .
Fig. 10. Responses of porous rh-In2 O3 nanosheets and nonporous rh-In2 O3 nanosheets sensors to various gases (the concentration of all gases was 50 ppm).
and nonporous In2 O3 nanosheets were measured using Xe laser (325 nm) as excitation source (Fig. 6). The two samples both displayed UV emission at about 360 nm, which might be the characteristic near band-edge emission of the wide band gap from the recombination of the free excitons [48]. The blue-green light emission peaks from 450 nm to 490 nm for the porous In2 O3 nanosheets is stronger than that of the nonporous In2 O3 nanosheets. The stronger blue-green light emission intensity suggests that there is a greater fraction of oxygen vacancies in the porous In2 O3 nanosheets. Defects at metal oxide surfaces are believed to significantly influence the surface properties, such as heterogeneous catalysis, corrosion inhibition and gas sensing [49]. More oxygen vacancies may result in high adsorptions of oxygen, and then enhance the gas response of the sensor. The In2 O3 nanosheets exhibit a porous structure and it is therefore interesting to study the surface area and the pore size of these nanosheets. Fig. 7 shows a typical N2 adsorption and desorption isotherm for porous In2 O3 nanosheets. The N2 sorption isotherm of the porous In2 O3 nanosheets can be categorized as type IV based on IUPAC classification. The BET surface area of the product was calculated to be 24.2 m2 /g. The corresponding BJH pore size distribution plot (inset of Fig. 8) of the porous In2 O3 nanosheets shows that the average pore size of such a sample is around 10–35 nm which is corresponding with the SEM and TEM in Fig. 3. The BET surface area of the nonporous In2 O3 nanosheets (19.8 m2 /g) is a little smaller than porous In2 O3 nanosheets. Therefore, we suspect that the performance of the gas-sensing properties of porous In2 O3 nanosheets would better than nonporous In2 O3 nanosheets due to larger specific surface area and more pore. 4. Gas-sensing properties In this paper, we studied the NO2 gas-sensing properties of porous In2 O3 nanosheets, and discussed the effect of morphology on the gas sensing responses. The gas-sensing properties of porous In2 O3 nanosheets were studied by using the method mentioned above. The relation between the response and the operating temperature of the sintered sensors were investigated, which is shown in Fig. 8. The result showed that both porous nanosheets sensor and nonporous nanosheets sensor obtain the highest response to 10 ppm NO2 at 250 ◦ C, so we choose the temperature of 250 ◦ C as the best working temperature of the NO2 sensors. When bulk particles are exposed to a target gas, the gas is difficult to diffuse into the interior, only react with the outer surface. For porous structures, the gas is easy to be diffused into the inside surface [28], and thus promote the rapid response/recovery of the sensor. The response and recovery of porous nanosheets and nonporous nanosheets were shown in Fig. 9. For porous In2 O3 nanosheets, the response and the recovery time to 50 ppm NO2
L. Gao et al. / Sensors and Actuators B 208 (2015) 436–443
441
Table 1 NO2 sensing performances based on different structures. Materials
NO2 concentration (ppm)
Working temperature (◦ C)
Response (Rg /Ra )
References
Lotus root slice-like In2 O3 microspheres In2 O3 hollow spheres Zn-doped In2 O3 hollow sphere Zn-doped In2 O3 cubes WO3 nanoplates SnO2 microrods CuO nanosheets Nonporous In2 O3 nanosheets Porous In2 O3 nanosheets
50 100 50 50 30 50 100 50 50
250 215 300 300 ∼25 (RT) 300 240 250 250
∼90 ∼2 <100 ∼70 ∼3 <100 <10 57 164
[19] [50] [51] [39] [52] [53] [54] This work This work
Fig. 11. (a) Typical response curves of the sensor to 1–50 ppm NO2 and (b) the corresponding response values to 1–50 ppm NO2 .
were not exceeding 5 s and 14 s, respectively. For nonporous In2 O3 nanosheets, the response and the recovery time were about 5 s and 23 s. To investigate the selectivity of the sensor, we also measured the response of the sensors to other gases such as C2 H5 OH, C3 H6 O, HCHO, CH3 OH, NH4 OH and C7 H8 . The responses of porous rhIn2 O3 nanosheets and nonporous rh-In2 O3 nanosheets to different gases are shown in Fig. 10. For these two structures, both of them showed excellent selectivity on NO2 . Obviously, the response to NO2 was at least four times higher than the responses to the other gases, meaning that the porous rh-In2 O3 nanosheets sensor had a more satisfactory selectivity for NO2 . Moreover, the response of the porous In2 O3 nanosheets sensor to 50 ppm NO2 was about 164, which was two times more than the nonporous In2 O3 nanosheets sensor (to 50 ppm NO2 was about 57). The porous nanosheets obtained a higher response and faster recovery due to the diffusion is not hampered toward the entire sensing surface. Table 1 shows
NO2 sensing performances based on different structures. Obviously, the response of the porous rh-In2 O3 nanosheets was much higher than those of lotus root slice-like In2 O3 [19], In2 O3 hollow spheres [50], Zn-doped In2 O3 hollow sphere/cubes [39,51], WO3 nanoplates [52], SnO2 microrods [53] and CuO Nanosheets [54]. The gas sensing response enhancement may be attributed to porous structure and more active centers obtained from the enhanced oxygen vacancy defects on the porous nanostructure as shown in PL spectra. The relationship between resistance and NO2 concentration of the porous rh-In2 O3 nanosheets can be observed from Fig. 11a. In the range of 1–50 ppm, we can see that the resistance increased with the increase of the NO2 concentration. Fig. 11b shows the response value of the sensor with the changing of NO2 concentration. The gas response (y) as a function of NO2 concentration ranging from 1 to 50 ppm (x) was well fitted by using the equation y = abx/(1 + bx), where a is the maximum adsorption amount, b is adsorption constant and x is gas concentration. This can be understood as the surface coverage of adsorbed molecules followed the Langmuir isotherm model, and the specific equation is showed as followed: y=
Fig. 12. Long-term stability of porous rh-In2 O3 nanosheets sensor to 50 ppm NO2 . Inset is the typical response curve cycling between 50 ppm NO2 and ambient air at 250 ◦ C.
227.526 × 0.053x , (1 + 0.053x)
R∧ 2 = 0.964
(1)
At a lower concentration, the sensor exhibits a linear relation between the sensor response and the NO2 concentration as shown in Fig. 11b. At higher concentration, the surface coverage tends to saturate and hence leads to a saturation response. Stability is also an important factor for gas sensor. Good stability needs the reliability guarantee of the material. Fig. 12 shows gas sensing stability of the porous In2 O3 nanosheets sensor. It can be seen that the responses of the sensor had an acceptable change after a pulse test for 45 days, indicating good stability of the sensor. The inset picture is the typical response curve cycling between 50 ppm NO2 and ambient air on the seventh day, from which we can know that the sensor has good reversibility and repeatability.
442
L. Gao et al. / Sensors and Actuators B 208 (2015) 436–443
the porous In2 O3 nanosheets sensor is a promising candidate for detecting NO2 . Acknowledgments The research is supported by National Natural Science Foundation of China (61371021 and 51301101). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2014.11.053. Fig. 13. Schematic drawing for microstructure of thick film.
References Gas-sensing characteristics of the sensors were investigated by measuring the resistance changes of the sensors. When In2 O3 sensors are exposed to air at high temperature, oxygen molecules would adsorb on the In2 O3 surface and generate ionized oxygen species, such as O2 − , O− and O2− through trapping electrons from the conductance band of non-stoichiometric In2 O3 − x [18]. Once oxidized gases, such as NO2 , diffuse onto the In2 O3 surface, they will react with the chemisorbed oxygen on the surface of In2 O3 or they will directly adsorb on the surface of In2 O3 in the form of NO2 − , resulting in the increase of resistance [37,55]. Sensing body should be porous enough and thin enough in order to obtain a high gas response [56]. In the sensing bodies, primary particle (oxide grains) gather together to form secondary particles, leaving macropores among them, as shown in Fig. 13. Gas diffusion through macropores (molecular diffusion) is rapid so that the target gas can reach the surface of each secondary particle very easily. Porous rh-In2 O3 nanosheets which are assembled by small nanoscale particles have large number of pores on them. Gas could also diffuse from the macropores to the pores in the nanosheets, and then the gas sensitive region could spread to the outside surface of the primary particles. Porous lamellar structure could increase the activity site of material and speed up the gas transmission, which is helpful for the enhancement of the response and rapid adsorption/desorption. Porous nanosheets have been an aggregate assembled by nanoparticles, and their surface energies have been decreased largely compared with nanoparticles without self-assembly. So the twice aggregation of the nanosheets is difficult. If the nanosheets are easy to agglomerate each other in a working state, their sensitivities would change resulting from the variation of contact resistance (Ra ), further affecting the stability of gas sensors. It can be proved from Fig. 12 that the responses of the sensor had an acceptable change after a pulse test for 45 days, indicating a good stability of the sensor. So, choosing porous rh-In2 O3 nanosheets as sensitive material is a good choice for the detecting of NO2 . 5. Conclusion In summary, a facile and eco-friendly self-assembly approach to obtain porous In2 O3 nanosheets with unique shoeprint morphology is achieved. The results prove that glycol plays an important role in governing the porous rh-In2 O3 nanosheets. Compared with the nonporous rh-In2 O3 nanosheets sensor, the porous rh-In2 O3 nanosheets sensor exhibits significantly enhanced response toward NO2 . The porous In2 O3 nanosheets sensor could detect 50 ppm NO2 at about 250 ◦ C with a response value of 164. The response and the recovery time of the sensor to 50 ppm NO2 are not exceeding 5 s and 14 s, respectively and the response of the sensor had an acceptable change after a pulse test for 45 days. The high sensitivity, short response and recovery time, good stability suggests that
[1] C.Z. Yuan, X.G. Zhang, L.H. Su, B. Gao, L.F. Shen, Facile synthesis and selfassembly of hierarchical porous NiO nano/micro spherical superstructures for high performance supercapacitors, J. Mater. Chem. 19 (2009) 5772–5777. [2] W.Y. Li, L.M. Xu, J. Chen, Co3 O4 nanomaterials in lithium-ion batteries and gas sensors, Adv. Funct. Mater. 15 (2005) 851–857. [3] Q.X. Zhang, X.Z. Guo, X.M. Huang, S.Q. Huang, D.M. Li, Y.H. Luo, Q. Shen, T. Toyoda, Q.B. Meng, Highly efficient CdS/CdSe-sensitized solar cells controlled by the structural properties of compact porous TiO2 photoelectrodes, Phys. Chem. Chem. Phys. 13 (2011) 4659–4667. [4] Q. Liu, L. Jiang, L. Guo, Precursor-directed self-assembly of porous ZnO nanosheets as high-performance surface-enhanced Raman scattering substrate, Small 10 (2014) 48–51. [5] C. Qiu, L. Zhang, H. Wang, C.Y. Jiang, Surface-enhanced Raman scattering on hierarchical porous cuprous oxide nanostructures in nanoshell and thin-film geometries, J. Phys. Chem. Lett. 3 (2012) 651–657. [6] D.F. Hou, W. Luo, Y.H. Huang, J.C. Yu, X.L. Hu, Synthesis of porous Bi4 Ti3 O12 nanofibers by electrospinning and their enhanced visible-light-driven photocatalytic properties, Nanoscale 5 (2013) 2028–2035. [7] Z.H. Jing, J.H. Zhan, Fabrication and gas-sensing properties of porous ZnO nanoplates, Adv. Funct. Mater. 20 (2008) 4547–4551. [8] N. Pinna, G. Neri, M. Antonietti, M. Niederberger, Nonaqueous synthesis of nanocrystalline semiconducting metal oxides for gas sensing, Angew. Chem. Int. Ed. 43 (2004) 4345–4349. [9] P. Innocenzi, L. Malfatti, G.J.A.A. Soler-Illia, Hierarchical mesoporous films: from self-assembly to porosity with different length scales, Chem. Mater. 23 (2011) 2501–2509. [10] K.R. Prasad, K. Koga, N. Miura, Electrochemical deposition of nanostructured indium oxide: high-performance electrode material for redox supercapacitors, Chem. Mater. 16 (2004) 1845–1847. [11] Q. Dong, S. Yin, C.S. Guo, X.Y. Wu, N. Kumada, T. Takei, A. Miura, Y. Yonesaki, T. Sato, Single-crystalline porous NiO nanosheets prepared from -Ni(OH)2 nanosheets: magnetic property and photocatalytic activity, Appl. Catal. B: Environ. 147 (2014) 741–747. [12] H. Dai, Y. Zhou, L. Chen, B.L. Guo, A.D. Li, J.G. Liu, T. Yu, Z.G. Zou, Porous ZnO nanosheet arrays constructed on weaved metal wire for flexible dye-sensitized solar cells, Nanoscale 5 (2013) 5102–5108. [13] W.W. Lei, D. Portehault, D. Liu, S. Qin, Y. Chen, Porous boron nitride nanosheets for effective water cleaning, Nat. Commun. 4 (2013) 1777. [14] G.S. Gund, D.P. Dubal, D.S. Dhawale, S.S. Shinde, C.D. Lokhande, Porous CuO nanosheet clusters prepared by a surfactant assisted hydrothermal method for high performance supercapacitors, RSC Adv. 3 (2013) 24099–24107. [15] W.W. Zhao, C.B. Liu, L.M. Cao, X.G. Yin, H.L. Xu, B. Zhang, Porous singlecrystalline CdS nanosheets as efficient visible light catalysts for aerobic oxidative coupling of amines to imines, RSC Adv. 3 (2013) 22944–22948. [16] Z. Guo, J.Y. Liu, J. Y, X. Chen, F.L. Meng, M.Q. Li, J.H. Liu, Template synthesis, organic gas-sensing and optical properties of hollow and porous In2 O3 nanospheres, Nanotechnology 19 (2008) 345704. [17] N. Du, H. Zhang, B.D. Chen, X.Y. Ma, Z.H. Liu, J.B. Wu, D.R. Yang, Porous indium oxide nanotubes: layer-by-layer assembly on carbon-nanotube templates and application for room-temperature NH3 gas sensors, Adv. Mater. 19 (2007) 1641–1645. [18] Y.S. Li, J. Xu, J.F. Chao, D. Chen, S.X. Ouyang, J.H. Ye, G.Z. Shen, High-aspect-ratio single-crystalline porous In2 O3 nanobelts with enhanced gas sensing properties, J. Mater. Chem. 21 (2011) 12852–12857. [19] Z.X. Cheng, L.Y. Song, X.H. Ren, Q. Zheng, J.Q. Xu, Novel lotus root slice-like selfassembled In2 O3 microspheres: synthesis and NO2 -sensing properties, Sens. Actuators B: Chem. 176 (2013) 383–389. [20] C.Q. Wang, D.R. Chen, X.L. Jiao, Flower-like In2 O3 nanostructures derived from novel precursor: synthesis, characterization, and formation mechanism, J. Phys. Chem. C 113 (2009) 7714–7718. [21] C.T. Prewitt, R.D. Shannon, D.B. Rogers, A.W. Slerght, C rare earth oxidecorundum transition and crystal chemistry of oxides having the corundum structure, Inorg. Chem. 8 (1969) 1985–1993. [22] H.X. Dong, Z.H. Chen, L.X. Sun, L. Zhou, Y.J. Ling, C.Z. Yu, H.H. Tan, C. Jagadish, X.C. Shen, Nanosheets-based rhombohedral In2 O3 3D hierarchical microspheres:
L. Gao et al. / Sensors and Actuators B 208 (2015) 436–443
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38] [39]
[40]
[41]
[42] [43] [44]
synthesis, growth mechanism, and optical properties, J. Phys. Chem. C 113 (2009) 10511–10516. W.H. Zhang, W.D. Zhang, Synthesis and optical properties of nanosheet-based rh-In2 O3 microflowers by triethylene glycol-mediated solvothermal process, J. Phys. Chem. Solids 74 (2013) 1271–1274. H.H. Jiang, L.C. Zhao, L.G. Gai, L. Ma, Y. Ma, M. Li, Hierarchical rh-In2 O3 crystals derived from InOOH counterparts and their sensitivity to ammonia gas, Cryst. Eng. Comm. 15 (2013) 7003–7009. D. Yu, S.H. Yu, S. Zhang, J. Zuo, D. Wang, Y.T. Qian, Metastable hexagonal In2 O3 nanofibers templated from InOOH nanofibers under ambient pressure, Adv. Funct. Mater. 13 (2003) 497–501. J.Y. Liu, Z. Guo, F.L. Meng, T. Luo, M.Q. Li, J.H. Liu, Novel porous single-crystalline ZnO nanosheets fabricated by annealing ZnS(en)0.5 (en = ethylenediamine) precursor. Application in a gas sensor for indoor air contaminant detection, Nanotechnology 20 (2009) 125501. L.X. Zhang, J.H. Zhao, H.Q. Lu, L. Li, J.F. Zheng, H. Li, Z.P. Zhu, Facile synthesis and ultrahigh ethanol response of hierarchically porous ZnO nanosheets, Sens. Actuators B: Chem. 161 (2012) 209–215. L.L. Wang, T. Fei, Z. Lou, T. Zhang, Three-dimensional hierarchical flowerlike ␣-Fe2 O3 nanostructures: synthesis and ethanol-sensing properties, ACS Appl. Mater. Interfaces 3 (2011) 4689–4694. Y.H. Li, W. Luo, N. Qin, J.P. Dong, J. Wei, W. Li, S.S. Feng, J.C. Chen, J.Q. Xu, A.A. Elzatahry, M.H. Es-Saheb, Y.H. Deng, D.Y. Zhao, Highly ordered mesoporous tungsten oxides with a large pore size and crystalline framework for H2 S sensing, Angew. Chem. Int. Ed. 53 (2014) 9035–9040. C.H. Zhao, J.C. Fu, Z.X. Zhang, E.Q. Xie, Enhanced ethanol sensing performance of porous ultrathin NiO nanosheets with neck-connected networks, RSC Adv. 3 (2013) 4018–4023. C.L. Shao, L. Tu, A. Yu, B. Li, X.F. Zhou, From Zn4 (CO3 )(OH)6 ·H2 O curling nanopetals to ZnO stretching porous nanosheets: growth mechanism and gas sensing property, Thin Solid Films 525 (2012) 148–153. S. Deng, V. Tjoa, H.M. Fan, H.R. Tan, D.C. Sayle, M. Olivo, S. Mhaisalkar, J. Wei, C.H. Sow, Reduced graphene oxide conjugated Cu2 O nanowire mesocrystals for high-performance NO2 gas sensor, J. Am. Chem. Soc. 134 (2012) 4905–4917. A. Gurlo, N. Barsan, M. Ivanovskaya, U. Weimar, W. Gopel, In2 O3 and MoO3 In2 O3 thin film semiconductor sensors: interaction with NO2 and O3 , Sens. Actuators B: Chem. 47 (1998) 92–99. D.H. Zhang, Z.Q. Liu, T. Tang, X.L. Liu, S. Han, B. Lei, C.W. Zhou, Detection of NO2 down to ppb levels using individual and multiple In2 O3 nanowire devices, Nano Lett. 4 (2004) 1919–1924. P.C. Xu, Z.X. Cheng, Q.Y. Pan, J.Q. Xu, Q. Xiang, W.J. Yu, Y.L. Chu, High aspect ratio In2 O3 nanowires: synthesis, mechanism and NO2 gas-sensing properties, Sens. Actuators B: Chem. 130 (2008) 802–808. X.M. Xu, P.L. Zhao, D.W. Wang, P. Sun, L. You, Y.F. Sun, X.S. Liang, F.M. Liu, H. Chen, G.Y. Lu, Preparation and gas sensing properties of hierarchical flower-like In2 O3 microspheres, Sens. Actuators B: Chem. 176 (2013) 405–412. X.M. Xu, D.W. Wang, J. Liu, P. Sun, Y. Guan, H. Zhang, Y.F. Sun, F.M. Liu, X.S. Liang, Y. Gao, G.Y. Lu, Template-free synthesis of novel In2 O3 nanostructures and their application to gas sensors, Sens. Actuators B: Chem. 185 (2013) 32–38. P.S. khiabani, E. Marzbanrad, H. Hassani, B. RaissiFast, Fast response NO2 gas sensor based on In2 O3 nanoparticles, J. Am. Ceram. Soc. 96 (2013) 2493–2498. P. Li, H.Q. Fan, Y. Cai, M.M. Xu, C.B. Long, M.M. Li, S.H. Lei, X.W. Zou, Phase transformation (cubic to rhombohedral): the effect on the NO2 sensing performance of Zn-doped flower-like In2 O3 structures, RSC Adv. 4 (2014) 15161–15170. N. Qin, X.H. Wang, Q. Xiang, J.Q. Xu, A biomimetic nest-like ZnO: controllable synthesis and enhanced ethanol response, Sens. Actuators B: Chem. 191 (2014) 770–778. P. Toneguzzo, G. Viau, O. Acher, F. Fievet-Vincent, F. Fievet, Monodisperse ferromagnetic particles for microwave applications, Adv. Mater. 10 (1998) 1032–1035. Y.G. Sun, Y.N. Xia, Shape-controlled synthesis of gold and silver nanoparticles, Science 298 (2002) 2176–2179. R.W.J. Scott, N. Coombs, G.A. Ozin, Non-aqueous synthesis of mesostructured tin dioxide, J. Mater. Chem. 13 (2003) 969–974. X.C. Jiang, Y.L. Wang, T. Herricks, Y.N. Xia, Ethylene glycol-mediated synthesis of metal oxide nanowires, J. Mater. Chem. 14 (2004) 695–703.
443
[45] Y.L. Wang, X.C. Jiang, Y.N. Xia, A solution-phase, precursor route to polycrystalline SnO2 nanowires that can be used for gas sensing under ambient conditions, J. Am. Chem. Soc. 125 (2003) 16176–16177. [46] X.X. Xu, X. Wang, Size- and surface-determined transformations: from ultrathin InOOH nanowires to uniform c-In2 O3 nanocubes and rh-In2 O3 nanowires, Inorg. Chem. 48 (2009) 3890–3895. [47] H.L. Zhu, K.H. Yao, H. Zhang, D.R. Yang, InOOH hollow spheres synthesized by a simple hydrothermal reaction, J. Phys. Chem. B 109 (2005) 20676–20679. [48] H.Q. Cao, X.Q. Qiu, Y. Liang, Q.M. Zhu, M.J. Zhao, Room-temperature ultravioletemitting In2 O3 nanowires, Appl. Phys. Lett. 83 (2003) 761–763. [49] Y. Zhang, J.Q. Xu, Q. Xiang, H. Li, Q.Y. Pan, P.C. Xu, Brush-Like hierarchical ZnO nanostructures: synthesis, photoluminescence and gas sensor properties, J. Phys. Chem. C 113 (2009) 3430–3435. [50] T. Zhang, F.B. Gu, D.M. Han, Z.H. Wang, G.S. Guo, Synthesis, characterization and alcohol-sensing properties of rare earth doped In2 O3 hollow spheres, Sens. Actuators B: Chem. 177 (2013) 1180–1188. [51] P. Li, H.Q. Fan, Y. Cai, M.M. Xu, Zn-doped In2 O3 hollow spheres: mild solution reaction synthesis and enhanced Cl2 sensing performance, Cryst. Eng. Comm. 16 (2014) 2715–2722. [52] M. Bao, Y.J. Chen, F. Li, J.M. Ma, T. Lv, Y.J. Tang, L.B. Chen, Z. Xu, T.H. Wang, Platelike p–n heterogeneous NiO/WO3 nanocomposites for high performance room temperature NO2 sensors, Nanoscale 6 (2014) 4063–4066. [53] S.W. Choi, A. Katoch, G.J. Sun, P. Wu, S.S. Kim, NO2 -sensing performance of SnO2 microrods by functionalization of Ag nanoparticles, J. Mater. Chem. 1 (2013) 2834–2841. [54] F. Zhang, A.W. Zhu, Y.P. Luo, Y. Tian, J.H. Yang, Y. Qin, CuO nanosheets for sensitive and selective determination of H2 S with high recovery ability, J. Phys. Chem. C 114 (2010) 19214–19219. [55] L. Francioso, A. Forleo, S. Capone, M. Epifani, A.M. Taurino, P. Siciliano, Nanostructured In2 O3 -SnO2 sol-gel thin film as material for NO2 detection, Sens. Actuators B: Chem. 114 (2006) 646–655. [56] N. Yamazoe, G. Sakai, K. Shimanoe, Oxide semiconductor gas sensors, Catal. Surv. Asia 7 (2003) 63–75.
Biographies Liping Gao received her BS degree in applied chemistry in 2009 from Zhengzhou Institute of Light Industry and obtained her MS degree in Materials Science and Engineering in 2012 from Beijing University of Chemical Technology. She has been pursuing her Ph.D. degree in condensed matter physics in Shanghai University since 2012. Currently her main efforts are taken to rational synthesis of Nano oxide materials with various morphologies as well as to exploit their potential application involving gas sensors. Zhixuan Cheng received her MS and Ph.D. degrees in chemistry from Shanghai University in 2001 and 2008 in material science from Shanghai University. Her current interests include the synthesis of nanoscale materials and their application in gas sensors. Qun Xiang received her MS degrees in physical chemistry from Shanghai University in 2001. Her current research concentrates on development of gas sensor and photocatalysts. Yuan Zhang obtained her Ph.D. degree in condensed matter physics in 2012 and M.S. degree in inorganic chemistry in 2009 from Shanghai University. She is currently a postdoctoral fellow at Shanghai University. Her research interests include nanomaterials synthesis, electro-chemical biosensors and fuel cells. Jiaqiang Xu obtained his MS degree in inorganic chemistry from University of Science and Technology of China in 1988 and Ph.D. in engineering in 2005 from Shanghai University. He was a professor in applied chemistry at Zhengzhou Institute of Light Industry from 2001 to 2006. He is a professor in Department of Chemistry at Shanghai University and a Director of China Special Committee of Gas and Humidity Sensor Technology. His research interests include the synthesis of nanoscale materials and their applications in gas sensor and other fields.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Porous corundum-type In2 O3 nanosheets: Synthesis and NO2 sensing properties Liping Gao a , Zhixuan Cheng b,∗∗ , Qun Xiang b , Yuan Zhang b , Jiaqiang Xu a,b,∗ a b
Department of Physics, College of Science, Shanghai University, Shanghai, 200444, China Department of Chemistry, College of Science, Shanghai University, Shanghai, 200444, China
a r t i c l e
i n f o
Article history: Received 14 July 2014 Received in revised form 7 November 2014 Accepted 10 November 2014 Available online 18 November 2014 Keywords: Rh-In2 O3 nanosheets Porous NO2 Gas-sensing properties
a b s t r a c t Porous corundum-type In2 O3 (rhombohedra In2 O3 , rh-In2 O3 ) nanosheets with unique shoeprint morphology are fabricated successfully by a facile, template-free solvent-thermal method. The products are characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), photoluminescence (PL) and N2 adsorption–desorption. The response of the porous rh-In2 O3 nanosheets sensor to 50 ppm NO2 is about 164, and the response/recovery times are not exceeding 5 s and 14 s, respectively. Porous lamellar structure could increase the number of the active sites and speed up the gas transportation, which is helpful for the enhancement of the response and rapid adsorption/desorption. The effect of morphology on the gas sensing responses is investigated. Compared with the nonporous rh-In2 O3 nanosheets sensor, the porous rh-In2 O3 nanosheets sensor exhibits significantly enhanced sensing performances toward NO2 . Porous rh-In2 O3 nanosheets sensors show a long-term stability, which might be ascribed to the avoidance of aggregation of the nanostructures. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Materials with porous structure have good applications in many fields, such as supercapacitors [1], lithium-ion batteries [2], solar cells [3], surface-enhanced Raman scattering (SERS) [4,5], photocatalysis [6], and gas sensing [7]. Porous nanosheets materials with unique structural advantages have led to their intriguing properties, attractive applications, as well as industrial and environmental benefits [8–10]. Different kinds of porous nanosheets material were obtained and achieved good success in some fields [11–15]. For example, porous NiO nanosheets prepared by Dong et al. showed enhanced photocatalytic activity for NO destruction [11]. Porous ZnO nanosheets were employed as high-performance surface-enhanced Raman scattering substrate [4]. As an important semiconductor material, In2 O3 nanostructure with unique physicochemical properties has attracted particular attention. Different In2 O3 porous structures such as porous nanospheres [16], porous nanotubes [17], porous nanobelts [18], porous nanoflowers [19,20] have been successfully fabricated. Most of the In2 O3 structures as
∗ Corresponding author at: Department of Physics, College of Science, Shanghai University, Shanghai, 200444, China. Tel.: +86 21 66132701; fax: +86 21 66132701. ∗∗ Corresponding author. Tel.: +86 21 66132701; fax: +86 21 66132701. E-mail addresses: [email protected], [email protected] (J. Xu). http://dx.doi.org/10.1016/j.snb.2014.11.053 0925-4005/© 2014 Elsevier B.V. All rights reserved.
mentioned above are in a cubic lattice, whereas controlled synthesis of rhombohedra In2 O3 (rh-In2 O3 ) has been rarely studied because of its synthesis usually needs high temperature and pressure. For example, Slerght group [21] synthesized rh-In2 O3 through the phase transition method under the temperature of 1000 ◦ C and the pressure of 65 kbar. Recently, solvothermal method was proved to synthesis rhombohedra In2 O3 under rather low temperature and pressure, such as rh-In2 O3 microspheres [22,23], rh-In2 O3 nanocrystals [24] and rh-In2 O3 nanofibers [25]. However, controlled synthesis of porous rh-In2 O3 nanosheet gas sensing material has been less reported. Lamellar nanosheet structure could endow the good stability of the material compared with nanoparticles due to the decreased surface energies. Stable nanosheet structure can ensure the stability of gas sensitive test results which can be seen in previous literatures [26,27]. Porous structure is beneficial to gas diffusion and mass transport [28,29], the existence of defects such as oxygen vacancies could increase the chance of electrostatic interaction between testing gas and the surface of sensing materials, benefitting to oxygen adsorption and further enhance the gas response of the sensors [30,31]. So, it is of significant value to develop porous In2 O3 nanosheets sensors. NO2 detection is of paramount importance in explosives detection as it is a decomposition product of many explosive formulations and improvised explosive devices [32]. In2 O3 as an important semiconductor with low resistance has been proved
L. Gao et al. / Sensors and Actuators B 208 (2015) 436–443
437
Fig. 1. (a) Sketch of the gas-sensor structure, (b) the measuring electric circuit for the gas sensor and (c) gas sensing test device.
to be an excellent gas-sensing material for detecting oxidizing gases, especially suitable for detecting NO2 [33–36]. The gas sensing properties of In2 O3 nanostructures to NO2 have been widely investigated by many research groups [37–39]. However, some of the nanostructures showed certain shortage in sensitivity, response or recovery. The lamellate and porous structure is favorable for gas adsorption and diffusion, resulting in the constructed sensor possessed excellent gas-sensing performance, making it a promising material for detection of NO2 gas. In this paper, a facile and eco-friendly self-assembly approach to obtain porous In2 O3 nanosheets with unique shoeprint morphology has been described. The as-prepared porous In2 O3 nanosheets sensor showed excellent gas-sensing properties toward NO2 . The effect of morphology on the gas sensing responses was also investigated. Compared with the nonporous rh-In2 O3 nanosheets sensor, the porous rh-In2 O3 nanosheets sensor exhibits significantly enhanced sensing performance toward NO2 . 2. Experimental sections All the reagents (analytical-grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and were used without further purification. Deionizer water was used throughout the experiments. 2.1. Synthesis In a typical synthesis, 6 mL In(NO3 )3 ·5H2 O (0.1 M) aqueous solution, 20 mL glycol, and 13 mL ethanol solution were mixed together. After vigorous stirring for 30 min, the mixture was transferred into a Teflon-lined stainless steel autoclave of 50 mL capacity. The autoclave was sealed and maintained at 180 ◦ C for 24 h, and
then naturally cooled down to room temperature, after which white precipitated powder was obtained. The powder was centrifuged and washed several times with distilled water and absolute ethanol alternatively, and then dried at 60 ◦ C for 8 h. Porous In2 O3 nanosheets were prepared by annealing the precursor in a muffle furnace at 500 ◦ C for 2 h under ambient pressure. 2.2. Characterization Powder X-ray diffraction measurements were performed with a Rigaku D-MAX/IIA X-ray diffractometer in a scanning range of 10–70◦ (2) at a rate of 1.8◦ (2)/min with Cu Ka radiation. Scanning electron microscopy (SEM) (JSM-6700F and S-4800) and transmission electron microscope (TEM) (JEM-2010F) was utilized for the investigation of the morphology of the prepared material. Surface groups were monitored by Fourier Transform Infrared Spectroscopy (FTIR) with an AVATRA370 FTIR instrument using KBr plates within 4500–400 cm−1 . Room temperature PL measurements were performed on a Hitachi RF-5301PC spectrofluophotometer using the 325 nm Xe laser line as the excitation source. The N2 adsorption–desorption analyses were carried out at liquid nitrogen temperature on an ASAP 2020 analyzer (Micromeritics Instrument, USA). The Brunauer–Emmett–Teller (BET) specific surface areas (SBET) were calculated using the BET equation. Desorption isotherm was used to determine the pore size distribution using the Barret–Joyner–Halender (BJH) method. 2.3. Gas response test Fig. 1a is a schematic of the sensor structures, in which a pair of Au electrodes was printed on both sides of the ceramic tube, while a Pt resistance heater inside of the ceramic tube was used to
438
L. Gao et al. / Sensors and Actuators B 208 (2015) 436–443
Fig. 2. XRD patterns of (a) the precursor InOOH and (b) the porous rh-In2 O3 nanosheets.
control the temperature of sensor. Firstly, the samples were mixed and ground with an adhesive in an agate mortar to form a gas sensing paste. Then, the paste was coated on an alumina ceramic tube and dried under IR radiation for 15 min. Subsequently the alumina ceramic tube was sintered at 500 ◦ C for 2 h to get good stability and welded onto the pedestal. Finally, the obtained sensors were aged at 300 ◦ C for 7 d to increase their stability before tested. The measuring electric circuit for the gas sensor is shown in Fig. 1b. Through varying heating voltage (Vh ), different working temperatures of a sensor could be obtained. In the measurement of electric circuit for gas sensors, a load resistor (RL ) was connected in series with a gas sensor. The circuit voltage (Vc ) was set at 5 V, and the output voltage (Vout ) was the terminal voltage of the load resistor. The resistance of a sensor in air (Ra ) or test gas (Rg ) was measured by monitoring Vout . The gas response of the sensor in this paper was defined as S = Ra /Rg (reducing gases) or S = Rg /Ra (oxidizing gases). The response or recovery time was expressed as the time taken for the sensor output to reach 90% of its saturation after applying or switching off the gas in a step function. Fig. 1c shows the gas sensing test device. The test was operated in a measuring system of HW-30A (Hanwei Electronics Co. Ltd., P.R. China). A stationary state gas distribution method was used for testing gas response. This HW-30A gas sensing test system was shown in the manuscript (Fig. 1c). According to the reference [40], the volumes of gas or liquid corresponding to different concentrations can be obtained. Under certain temperature, we could get the resistance of the sensor in the air (Ra ). When the gas was injected into the test chamber, it could mix with air and then adsorb on the surface of the sensor. For the liquid, it should be injected on an evaporator
Fig. 4. FTIR spectra of the precursor InOOH.
through the hole at the back of the chamber. Through the heating evaporator, the liquid sample was converted to its vapor phase, and then mixed with air. The resistance of the sensor changed and reached a stable value several seconds later through contacting and reacting with the gas. In the meantime, the resistance of the sensor in the test gas (Rg ) can be recorded. Finally open the test chamber, the sensor contacted with air again, and then the resistance of the sensor returned to the level before the gas injection. In this measurement, the humidity is controlled with 40–50% RH. 3. Results and discussion Fig. 2a shows the powder X-ray diffraction (XRD) pattern of the InOOH precursor. All of the peaks can be indexed to pure InOOH with the orthorhombic phase (JCPDS card, No. 17-0549). The XRD pattern of the annealed In2 O3 product is depicted in Fig. 2b. All of the detectable peaks in the pattern can be easily indexed to pure hexagonal In2 O3 phase (JCPDS Card, No. 73–1809). No other diffraction peaks related to impurities are observed, suggesting the high purity of the as-prepared hexagonal In2 O3 . The morphology of the In2 O3 was investigated by scanning electron microscopy (SEM) and transmission electron microscope (TEM), and the corresponding results are displayed in Fig. 3. As shown in Fig. 3a, we can clearly see that 2-D In2 O3 nanostructures are obtained on a large scale and the dispersion of nanosheets is good. The nanosheets are assembled by some small nanoparticles and there are some pores existed in the nanosheets. A higher magnification SEM image in the inset of Fig. 3a gives more information about the thickness of the In2 O3 nanosheets.
Fig. 3. (a) SEM and (b) TEM images of porous rh-In2 O3 nanosheets.
L. Gao et al. / Sensors and Actuators B 208 (2015) 436–443
439
Fig. 5. SEM images of rh-In2 O3 with different amounts of glycol, low (a) and high (b) magnification SEM image of In2 O3 (10 mL glycol), low (c) and high (d) magnification SEM image of In2 O3 (without glycol).
From the image, we can clearly see that the thickness of the nanosheets is less than 50 nm. Fig. 3b is a typical TEM image of the porous In2 O3 nanosheets, which is in accordance with the SEM results. Porous In2 O3 nanosheets with unique shoeprint morphology clearly showed that there are lots of pores on them (Fig. 3b). Glycol has been widely used in the polyol synthesis of metal (both pure and alloyed) nanoparticles due to its strong reducing power and relatively high boiling point (197 ◦ C) [41,42]. Recently, it has been used to fabricate mesostructures of titania, tin dioxide,
zirconia and niobium oxide by forming glycolate precursors because of its coordination ability with transition metal ions [43,44]. Xia and co-workers have synthesized porous SnO2 nanowires through a route that involved complexation with glycol, followed by polymerization [45]. For porous In2 O3 nanosheets, In3+ may combine with glycol and ethanol units through the formation of In –O– covalent and In–OH coordination bonds. Fig. 4 is the FT-IR studies of the precursor InOOH, in which we observed a strong broad absorption bands at 3450 cm−1 and a narrow band
Fig. 6. Photoluminescence spectra of different In2 O3 samples at room temperature, ex = 325 nm.
Fig. 7. N2 adsorption and desorption isotherms for the porous rh-In2 O3 nanosheets.
440
L. Gao et al. / Sensors and Actuators B 208 (2015) 436–443
Fig. 8. NO2 gas sensing property of porous rh-In2 O3 nanosheets and nonporous rh-In2 O3 nanosheets at different test temperatures.
at 1637 cm−1 , confirming the presence of crystallization water molecules. The peaks at 472 cm−1 are the In–O feature modes. The 2800–3000 cm−1 mode in the InOOH sample are assigned to the CH2 and CH3 symmetric and asymmetric stretching vibrations, and the 1465 cm−1 peak is the CH2 bending vibration mode. The peak position is consistent with the literatures [46,47]. The 1000–1300 cm−1 mode in the InOOH sample are assigned to the C–O stretching vibration. The appearance of CH2 – and C–O bands represents the combination of In3+ with glycol and ethanol unit which led to the formation of longer chains, and finally could further self-assemble into ordered bundles (nanosheets) through van der Waals interactions. Glycol as a cross-linking reagent is beneficial for the synthesis of porous structures [28,44]. In order to demonstrate the important role of glycol in synthesizing porous In2 O3 nanosheets, we performed the reaction through changing the amount of glycol with ethanol instead of it when other conditions keep the same. When the amount of glycol is 10 mL, broken In2 O3 nanosheets can be observed (Fig. 5a and b), which are different from Fig. 3. When glycol was completely replaced by ethanol, some small irregular nanoparticles could be found (Fig. 5c and d). We speculated that a moderate amount of glycol could promote the formation of the uniform porous nanosheets. Due to only a fraction of In3+ complexation with glycol, nanoparticles cannot complete assembly into nanosheets. Without glycol as the solvent, only small nanoparticles could be formed due to the disappearance of complexation. In order to illustrate the advantage of pore structure, we synthesized nonporous rh-In2 O3 nanosheets by changing the conditions of the preparation. As shown in Fig. 1s, the obtained nanosheets are assembled of nanoparticles. The nanoparticles are closely connected and there are no holes between the particles. The photoluminescence (PL) spectra of the porous In2 O3 nanosheets
Fig. 9. Response–recovery curves of the sensors to 50 ppm NO2 .
Fig. 10. Responses of porous rh-In2 O3 nanosheets and nonporous rh-In2 O3 nanosheets sensors to various gases (the concentration of all gases was 50 ppm).
and nonporous In2 O3 nanosheets were measured using Xe laser (325 nm) as excitation source (Fig. 6). The two samples both displayed UV emission at about 360 nm, which might be the characteristic near band-edge emission of the wide band gap from the recombination of the free excitons [48]. The blue-green light emission peaks from 450 nm to 490 nm for the porous In2 O3 nanosheets is stronger than that of the nonporous In2 O3 nanosheets. The stronger blue-green light emission intensity suggests that there is a greater fraction of oxygen vacancies in the porous In2 O3 nanosheets. Defects at metal oxide surfaces are believed to significantly influence the surface properties, such as heterogeneous catalysis, corrosion inhibition and gas sensing [49]. More oxygen vacancies may result in high adsorptions of oxygen, and then enhance the gas response of the sensor. The In2 O3 nanosheets exhibit a porous structure and it is therefore interesting to study the surface area and the pore size of these nanosheets. Fig. 7 shows a typical N2 adsorption and desorption isotherm for porous In2 O3 nanosheets. The N2 sorption isotherm of the porous In2 O3 nanosheets can be categorized as type IV based on IUPAC classification. The BET surface area of the product was calculated to be 24.2 m2 /g. The corresponding BJH pore size distribution plot (inset of Fig. 8) of the porous In2 O3 nanosheets shows that the average pore size of such a sample is around 10–35 nm which is corresponding with the SEM and TEM in Fig. 3. The BET surface area of the nonporous In2 O3 nanosheets (19.8 m2 /g) is a little smaller than porous In2 O3 nanosheets. Therefore, we suspect that the performance of the gas-sensing properties of porous In2 O3 nanosheets would better than nonporous In2 O3 nanosheets due to larger specific surface area and more pore. 4. Gas-sensing properties In this paper, we studied the NO2 gas-sensing properties of porous In2 O3 nanosheets, and discussed the effect of morphology on the gas sensing responses. The gas-sensing properties of porous In2 O3 nanosheets were studied by using the method mentioned above. The relation between the response and the operating temperature of the sintered sensors were investigated, which is shown in Fig. 8. The result showed that both porous nanosheets sensor and nonporous nanosheets sensor obtain the highest response to 10 ppm NO2 at 250 ◦ C, so we choose the temperature of 250 ◦ C as the best working temperature of the NO2 sensors. When bulk particles are exposed to a target gas, the gas is difficult to diffuse into the interior, only react with the outer surface. For porous structures, the gas is easy to be diffused into the inside surface [28], and thus promote the rapid response/recovery of the sensor. The response and recovery of porous nanosheets and nonporous nanosheets were shown in Fig. 9. For porous In2 O3 nanosheets, the response and the recovery time to 50 ppm NO2
L. Gao et al. / Sensors and Actuators B 208 (2015) 436–443
441
Table 1 NO2 sensing performances based on different structures. Materials
NO2 concentration (ppm)
Working temperature (◦ C)
Response (Rg /Ra )
References
Lotus root slice-like In2 O3 microspheres In2 O3 hollow spheres Zn-doped In2 O3 hollow sphere Zn-doped In2 O3 cubes WO3 nanoplates SnO2 microrods CuO nanosheets Nonporous In2 O3 nanosheets Porous In2 O3 nanosheets
50 100 50 50 30 50 100 50 50
250 215 300 300 ∼25 (RT) 300 240 250 250
∼90 ∼2 <100 ∼70 ∼3 <100 <10 57 164
[19] [50] [51] [39] [52] [53] [54] This work This work
Fig. 11. (a) Typical response curves of the sensor to 1–50 ppm NO2 and (b) the corresponding response values to 1–50 ppm NO2 .
were not exceeding 5 s and 14 s, respectively. For nonporous In2 O3 nanosheets, the response and the recovery time were about 5 s and 23 s. To investigate the selectivity of the sensor, we also measured the response of the sensors to other gases such as C2 H5 OH, C3 H6 O, HCHO, CH3 OH, NH4 OH and C7 H8 . The responses of porous rhIn2 O3 nanosheets and nonporous rh-In2 O3 nanosheets to different gases are shown in Fig. 10. For these two structures, both of them showed excellent selectivity on NO2 . Obviously, the response to NO2 was at least four times higher than the responses to the other gases, meaning that the porous rh-In2 O3 nanosheets sensor had a more satisfactory selectivity for NO2 . Moreover, the response of the porous In2 O3 nanosheets sensor to 50 ppm NO2 was about 164, which was two times more than the nonporous In2 O3 nanosheets sensor (to 50 ppm NO2 was about 57). The porous nanosheets obtained a higher response and faster recovery due to the diffusion is not hampered toward the entire sensing surface. Table 1 shows
NO2 sensing performances based on different structures. Obviously, the response of the porous rh-In2 O3 nanosheets was much higher than those of lotus root slice-like In2 O3 [19], In2 O3 hollow spheres [50], Zn-doped In2 O3 hollow sphere/cubes [39,51], WO3 nanoplates [52], SnO2 microrods [53] and CuO Nanosheets [54]. The gas sensing response enhancement may be attributed to porous structure and more active centers obtained from the enhanced oxygen vacancy defects on the porous nanostructure as shown in PL spectra. The relationship between resistance and NO2 concentration of the porous rh-In2 O3 nanosheets can be observed from Fig. 11a. In the range of 1–50 ppm, we can see that the resistance increased with the increase of the NO2 concentration. Fig. 11b shows the response value of the sensor with the changing of NO2 concentration. The gas response (y) as a function of NO2 concentration ranging from 1 to 50 ppm (x) was well fitted by using the equation y = abx/(1 + bx), where a is the maximum adsorption amount, b is adsorption constant and x is gas concentration. This can be understood as the surface coverage of adsorbed molecules followed the Langmuir isotherm model, and the specific equation is showed as followed: y=
Fig. 12. Long-term stability of porous rh-In2 O3 nanosheets sensor to 50 ppm NO2 . Inset is the typical response curve cycling between 50 ppm NO2 and ambient air at 250 ◦ C.
227.526 × 0.053x , (1 + 0.053x)
R∧ 2 = 0.964
(1)
At a lower concentration, the sensor exhibits a linear relation between the sensor response and the NO2 concentration as shown in Fig. 11b. At higher concentration, the surface coverage tends to saturate and hence leads to a saturation response. Stability is also an important factor for gas sensor. Good stability needs the reliability guarantee of the material. Fig. 12 shows gas sensing stability of the porous In2 O3 nanosheets sensor. It can be seen that the responses of the sensor had an acceptable change after a pulse test for 45 days, indicating good stability of the sensor. The inset picture is the typical response curve cycling between 50 ppm NO2 and ambient air on the seventh day, from which we can know that the sensor has good reversibility and repeatability.
442
L. Gao et al. / Sensors and Actuators B 208 (2015) 436–443
the porous In2 O3 nanosheets sensor is a promising candidate for detecting NO2 . Acknowledgments The research is supported by National Natural Science Foundation of China (61371021 and 51301101). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2014.11.053. Fig. 13. Schematic drawing for microstructure of thick film.
References Gas-sensing characteristics of the sensors were investigated by measuring the resistance changes of the sensors. When In2 O3 sensors are exposed to air at high temperature, oxygen molecules would adsorb on the In2 O3 surface and generate ionized oxygen species, such as O2 − , O− and O2− through trapping electrons from the conductance band of non-stoichiometric In2 O3 − x [18]. Once oxidized gases, such as NO2 , diffuse onto the In2 O3 surface, they will react with the chemisorbed oxygen on the surface of In2 O3 or they will directly adsorb on the surface of In2 O3 in the form of NO2 − , resulting in the increase of resistance [37,55]. Sensing body should be porous enough and thin enough in order to obtain a high gas response [56]. In the sensing bodies, primary particle (oxide grains) gather together to form secondary particles, leaving macropores among them, as shown in Fig. 13. Gas diffusion through macropores (molecular diffusion) is rapid so that the target gas can reach the surface of each secondary particle very easily. Porous rh-In2 O3 nanosheets which are assembled by small nanoscale particles have large number of pores on them. Gas could also diffuse from the macropores to the pores in the nanosheets, and then the gas sensitive region could spread to the outside surface of the primary particles. Porous lamellar structure could increase the activity site of material and speed up the gas transmission, which is helpful for the enhancement of the response and rapid adsorption/desorption. Porous nanosheets have been an aggregate assembled by nanoparticles, and their surface energies have been decreased largely compared with nanoparticles without self-assembly. So the twice aggregation of the nanosheets is difficult. If the nanosheets are easy to agglomerate each other in a working state, their sensitivities would change resulting from the variation of contact resistance (Ra ), further affecting the stability of gas sensors. It can be proved from Fig. 12 that the responses of the sensor had an acceptable change after a pulse test for 45 days, indicating a good stability of the sensor. So, choosing porous rh-In2 O3 nanosheets as sensitive material is a good choice for the detecting of NO2 . 5. Conclusion In summary, a facile and eco-friendly self-assembly approach to obtain porous In2 O3 nanosheets with unique shoeprint morphology is achieved. The results prove that glycol plays an important role in governing the porous rh-In2 O3 nanosheets. Compared with the nonporous rh-In2 O3 nanosheets sensor, the porous rh-In2 O3 nanosheets sensor exhibits significantly enhanced response toward NO2 . The porous In2 O3 nanosheets sensor could detect 50 ppm NO2 at about 250 ◦ C with a response value of 164. The response and the recovery time of the sensor to 50 ppm NO2 are not exceeding 5 s and 14 s, respectively and the response of the sensor had an acceptable change after a pulse test for 45 days. The high sensitivity, short response and recovery time, good stability suggests that
[1] C.Z. Yuan, X.G. Zhang, L.H. Su, B. Gao, L.F. Shen, Facile synthesis and selfassembly of hierarchical porous NiO nano/micro spherical superstructures for high performance supercapacitors, J. Mater. Chem. 19 (2009) 5772–5777. [2] W.Y. Li, L.M. Xu, J. Chen, Co3 O4 nanomaterials in lithium-ion batteries and gas sensors, Adv. Funct. Mater. 15 (2005) 851–857. [3] Q.X. Zhang, X.Z. Guo, X.M. Huang, S.Q. Huang, D.M. Li, Y.H. Luo, Q. Shen, T. Toyoda, Q.B. Meng, Highly efficient CdS/CdSe-sensitized solar cells controlled by the structural properties of compact porous TiO2 photoelectrodes, Phys. Chem. Chem. Phys. 13 (2011) 4659–4667. [4] Q. Liu, L. Jiang, L. Guo, Precursor-directed self-assembly of porous ZnO nanosheets as high-performance surface-enhanced Raman scattering substrate, Small 10 (2014) 48–51. [5] C. Qiu, L. Zhang, H. Wang, C.Y. Jiang, Surface-enhanced Raman scattering on hierarchical porous cuprous oxide nanostructures in nanoshell and thin-film geometries, J. Phys. Chem. Lett. 3 (2012) 651–657. [6] D.F. Hou, W. Luo, Y.H. Huang, J.C. Yu, X.L. Hu, Synthesis of porous Bi4 Ti3 O12 nanofibers by electrospinning and their enhanced visible-light-driven photocatalytic properties, Nanoscale 5 (2013) 2028–2035. [7] Z.H. Jing, J.H. Zhan, Fabrication and gas-sensing properties of porous ZnO nanoplates, Adv. Funct. Mater. 20 (2008) 4547–4551. [8] N. Pinna, G. Neri, M. Antonietti, M. Niederberger, Nonaqueous synthesis of nanocrystalline semiconducting metal oxides for gas sensing, Angew. Chem. Int. Ed. 43 (2004) 4345–4349. [9] P. Innocenzi, L. Malfatti, G.J.A.A. Soler-Illia, Hierarchical mesoporous films: from self-assembly to porosity with different length scales, Chem. Mater. 23 (2011) 2501–2509. [10] K.R. Prasad, K. Koga, N. Miura, Electrochemical deposition of nanostructured indium oxide: high-performance electrode material for redox supercapacitors, Chem. Mater. 16 (2004) 1845–1847. [11] Q. Dong, S. Yin, C.S. Guo, X.Y. Wu, N. Kumada, T. Takei, A. Miura, Y. Yonesaki, T. Sato, Single-crystalline porous NiO nanosheets prepared from -Ni(OH)2 nanosheets: magnetic property and photocatalytic activity, Appl. Catal. B: Environ. 147 (2014) 741–747. [12] H. Dai, Y. Zhou, L. Chen, B.L. Guo, A.D. Li, J.G. Liu, T. Yu, Z.G. Zou, Porous ZnO nanosheet arrays constructed on weaved metal wire for flexible dye-sensitized solar cells, Nanoscale 5 (2013) 5102–5108. [13] W.W. Lei, D. Portehault, D. Liu, S. Qin, Y. Chen, Porous boron nitride nanosheets for effective water cleaning, Nat. Commun. 4 (2013) 1777. [14] G.S. Gund, D.P. Dubal, D.S. Dhawale, S.S. Shinde, C.D. Lokhande, Porous CuO nanosheet clusters prepared by a surfactant assisted hydrothermal method for high performance supercapacitors, RSC Adv. 3 (2013) 24099–24107. [15] W.W. Zhao, C.B. Liu, L.M. Cao, X.G. Yin, H.L. Xu, B. Zhang, Porous singlecrystalline CdS nanosheets as efficient visible light catalysts for aerobic oxidative coupling of amines to imines, RSC Adv. 3 (2013) 22944–22948. [16] Z. Guo, J.Y. Liu, J. Y, X. Chen, F.L. Meng, M.Q. Li, J.H. Liu, Template synthesis, organic gas-sensing and optical properties of hollow and porous In2 O3 nanospheres, Nanotechnology 19 (2008) 345704. [17] N. Du, H. Zhang, B.D. Chen, X.Y. Ma, Z.H. Liu, J.B. Wu, D.R. Yang, Porous indium oxide nanotubes: layer-by-layer assembly on carbon-nanotube templates and application for room-temperature NH3 gas sensors, Adv. Mater. 19 (2007) 1641–1645. [18] Y.S. Li, J. Xu, J.F. Chao, D. Chen, S.X. Ouyang, J.H. Ye, G.Z. Shen, High-aspect-ratio single-crystalline porous In2 O3 nanobelts with enhanced gas sensing properties, J. Mater. Chem. 21 (2011) 12852–12857. [19] Z.X. Cheng, L.Y. Song, X.H. Ren, Q. Zheng, J.Q. Xu, Novel lotus root slice-like selfassembled In2 O3 microspheres: synthesis and NO2 -sensing properties, Sens. Actuators B: Chem. 176 (2013) 383–389. [20] C.Q. Wang, D.R. Chen, X.L. Jiao, Flower-like In2 O3 nanostructures derived from novel precursor: synthesis, characterization, and formation mechanism, J. Phys. Chem. C 113 (2009) 7714–7718. [21] C.T. Prewitt, R.D. Shannon, D.B. Rogers, A.W. Slerght, C rare earth oxidecorundum transition and crystal chemistry of oxides having the corundum structure, Inorg. Chem. 8 (1969) 1985–1993. [22] H.X. Dong, Z.H. Chen, L.X. Sun, L. Zhou, Y.J. Ling, C.Z. Yu, H.H. Tan, C. Jagadish, X.C. Shen, Nanosheets-based rhombohedral In2 O3 3D hierarchical microspheres:
L. Gao et al. / Sensors and Actuators B 208 (2015) 436–443
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38] [39]
[40]
[41]
[42] [43] [44]
synthesis, growth mechanism, and optical properties, J. Phys. Chem. C 113 (2009) 10511–10516. W.H. Zhang, W.D. Zhang, Synthesis and optical properties of nanosheet-based rh-In2 O3 microflowers by triethylene glycol-mediated solvothermal process, J. Phys. Chem. Solids 74 (2013) 1271–1274. H.H. Jiang, L.C. Zhao, L.G. Gai, L. Ma, Y. Ma, M. Li, Hierarchical rh-In2 O3 crystals derived from InOOH counterparts and their sensitivity to ammonia gas, Cryst. Eng. Comm. 15 (2013) 7003–7009. D. Yu, S.H. Yu, S. Zhang, J. Zuo, D. Wang, Y.T. Qian, Metastable hexagonal In2 O3 nanofibers templated from InOOH nanofibers under ambient pressure, Adv. Funct. Mater. 13 (2003) 497–501. J.Y. Liu, Z. Guo, F.L. Meng, T. Luo, M.Q. Li, J.H. Liu, Novel porous single-crystalline ZnO nanosheets fabricated by annealing ZnS(en)0.5 (en = ethylenediamine) precursor. Application in a gas sensor for indoor air contaminant detection, Nanotechnology 20 (2009) 125501. L.X. Zhang, J.H. Zhao, H.Q. Lu, L. Li, J.F. Zheng, H. Li, Z.P. Zhu, Facile synthesis and ultrahigh ethanol response of hierarchically porous ZnO nanosheets, Sens. Actuators B: Chem. 161 (2012) 209–215. L.L. Wang, T. Fei, Z. Lou, T. Zhang, Three-dimensional hierarchical flowerlike ␣-Fe2 O3 nanostructures: synthesis and ethanol-sensing properties, ACS Appl. Mater. Interfaces 3 (2011) 4689–4694. Y.H. Li, W. Luo, N. Qin, J.P. Dong, J. Wei, W. Li, S.S. Feng, J.C. Chen, J.Q. Xu, A.A. Elzatahry, M.H. Es-Saheb, Y.H. Deng, D.Y. Zhao, Highly ordered mesoporous tungsten oxides with a large pore size and crystalline framework for H2 S sensing, Angew. Chem. Int. Ed. 53 (2014) 9035–9040. C.H. Zhao, J.C. Fu, Z.X. Zhang, E.Q. Xie, Enhanced ethanol sensing performance of porous ultrathin NiO nanosheets with neck-connected networks, RSC Adv. 3 (2013) 4018–4023. C.L. Shao, L. Tu, A. Yu, B. Li, X.F. Zhou, From Zn4 (CO3 )(OH)6 ·H2 O curling nanopetals to ZnO stretching porous nanosheets: growth mechanism and gas sensing property, Thin Solid Films 525 (2012) 148–153. S. Deng, V. Tjoa, H.M. Fan, H.R. Tan, D.C. Sayle, M. Olivo, S. Mhaisalkar, J. Wei, C.H. Sow, Reduced graphene oxide conjugated Cu2 O nanowire mesocrystals for high-performance NO2 gas sensor, J. Am. Chem. Soc. 134 (2012) 4905–4917. A. Gurlo, N. Barsan, M. Ivanovskaya, U. Weimar, W. Gopel, In2 O3 and MoO3 In2 O3 thin film semiconductor sensors: interaction with NO2 and O3 , Sens. Actuators B: Chem. 47 (1998) 92–99. D.H. Zhang, Z.Q. Liu, T. Tang, X.L. Liu, S. Han, B. Lei, C.W. Zhou, Detection of NO2 down to ppb levels using individual and multiple In2 O3 nanowire devices, Nano Lett. 4 (2004) 1919–1924. P.C. Xu, Z.X. Cheng, Q.Y. Pan, J.Q. Xu, Q. Xiang, W.J. Yu, Y.L. Chu, High aspect ratio In2 O3 nanowires: synthesis, mechanism and NO2 gas-sensing properties, Sens. Actuators B: Chem. 130 (2008) 802–808. X.M. Xu, P.L. Zhao, D.W. Wang, P. Sun, L. You, Y.F. Sun, X.S. Liang, F.M. Liu, H. Chen, G.Y. Lu, Preparation and gas sensing properties of hierarchical flower-like In2 O3 microspheres, Sens. Actuators B: Chem. 176 (2013) 405–412. X.M. Xu, D.W. Wang, J. Liu, P. Sun, Y. Guan, H. Zhang, Y.F. Sun, F.M. Liu, X.S. Liang, Y. Gao, G.Y. Lu, Template-free synthesis of novel In2 O3 nanostructures and their application to gas sensors, Sens. Actuators B: Chem. 185 (2013) 32–38. P.S. khiabani, E. Marzbanrad, H. Hassani, B. RaissiFast, Fast response NO2 gas sensor based on In2 O3 nanoparticles, J. Am. Ceram. Soc. 96 (2013) 2493–2498. P. Li, H.Q. Fan, Y. Cai, M.M. Xu, C.B. Long, M.M. Li, S.H. Lei, X.W. Zou, Phase transformation (cubic to rhombohedral): the effect on the NO2 sensing performance of Zn-doped flower-like In2 O3 structures, RSC Adv. 4 (2014) 15161–15170. N. Qin, X.H. Wang, Q. Xiang, J.Q. Xu, A biomimetic nest-like ZnO: controllable synthesis and enhanced ethanol response, Sens. Actuators B: Chem. 191 (2014) 770–778. P. Toneguzzo, G. Viau, O. Acher, F. Fievet-Vincent, F. Fievet, Monodisperse ferromagnetic particles for microwave applications, Adv. Mater. 10 (1998) 1032–1035. Y.G. Sun, Y.N. Xia, Shape-controlled synthesis of gold and silver nanoparticles, Science 298 (2002) 2176–2179. R.W.J. Scott, N. Coombs, G.A. Ozin, Non-aqueous synthesis of mesostructured tin dioxide, J. Mater. Chem. 13 (2003) 969–974. X.C. Jiang, Y.L. Wang, T. Herricks, Y.N. Xia, Ethylene glycol-mediated synthesis of metal oxide nanowires, J. Mater. Chem. 14 (2004) 695–703.
443
[45] Y.L. Wang, X.C. Jiang, Y.N. Xia, A solution-phase, precursor route to polycrystalline SnO2 nanowires that can be used for gas sensing under ambient conditions, J. Am. Chem. Soc. 125 (2003) 16176–16177. [46] X.X. Xu, X. Wang, Size- and surface-determined transformations: from ultrathin InOOH nanowires to uniform c-In2 O3 nanocubes and rh-In2 O3 nanowires, Inorg. Chem. 48 (2009) 3890–3895. [47] H.L. Zhu, K.H. Yao, H. Zhang, D.R. Yang, InOOH hollow spheres synthesized by a simple hydrothermal reaction, J. Phys. Chem. B 109 (2005) 20676–20679. [48] H.Q. Cao, X.Q. Qiu, Y. Liang, Q.M. Zhu, M.J. Zhao, Room-temperature ultravioletemitting In2 O3 nanowires, Appl. Phys. Lett. 83 (2003) 761–763. [49] Y. Zhang, J.Q. Xu, Q. Xiang, H. Li, Q.Y. Pan, P.C. Xu, Brush-Like hierarchical ZnO nanostructures: synthesis, photoluminescence and gas sensor properties, J. Phys. Chem. C 113 (2009) 3430–3435. [50] T. Zhang, F.B. Gu, D.M. Han, Z.H. Wang, G.S. Guo, Synthesis, characterization and alcohol-sensing properties of rare earth doped In2 O3 hollow spheres, Sens. Actuators B: Chem. 177 (2013) 1180–1188. [51] P. Li, H.Q. Fan, Y. Cai, M.M. Xu, Zn-doped In2 O3 hollow spheres: mild solution reaction synthesis and enhanced Cl2 sensing performance, Cryst. Eng. Comm. 16 (2014) 2715–2722. [52] M. Bao, Y.J. Chen, F. Li, J.M. Ma, T. Lv, Y.J. Tang, L.B. Chen, Z. Xu, T.H. Wang, Platelike p–n heterogeneous NiO/WO3 nanocomposites for high performance room temperature NO2 sensors, Nanoscale 6 (2014) 4063–4066. [53] S.W. Choi, A. Katoch, G.J. Sun, P. Wu, S.S. Kim, NO2 -sensing performance of SnO2 microrods by functionalization of Ag nanoparticles, J. Mater. Chem. 1 (2013) 2834–2841. [54] F. Zhang, A.W. Zhu, Y.P. Luo, Y. Tian, J.H. Yang, Y. Qin, CuO nanosheets for sensitive and selective determination of H2 S with high recovery ability, J. Phys. Chem. C 114 (2010) 19214–19219. [55] L. Francioso, A. Forleo, S. Capone, M. Epifani, A.M. Taurino, P. Siciliano, Nanostructured In2 O3 -SnO2 sol-gel thin film as material for NO2 detection, Sens. Actuators B: Chem. 114 (2006) 646–655. [56] N. Yamazoe, G. Sakai, K. Shimanoe, Oxide semiconductor gas sensors, Catal. Surv. Asia 7 (2003) 63–75.
Biographies Liping Gao received her BS degree in applied chemistry in 2009 from Zhengzhou Institute of Light Industry and obtained her MS degree in Materials Science and Engineering in 2012 from Beijing University of Chemical Technology. She has been pursuing her Ph.D. degree in condensed matter physics in Shanghai University since 2012. Currently her main efforts are taken to rational synthesis of Nano oxide materials with various morphologies as well as to exploit their potential application involving gas sensors. Zhixuan Cheng received her MS and Ph.D. degrees in chemistry from Shanghai University in 2001 and 2008 in material science from Shanghai University. Her current interests include the synthesis of nanoscale materials and their application in gas sensors. Qun Xiang received her MS degrees in physical chemistry from Shanghai University in 2001. Her current research concentrates on development of gas sensor and photocatalysts. Yuan Zhang obtained her Ph.D. degree in condensed matter physics in 2012 and M.S. degree in inorganic chemistry in 2009 from Shanghai University. She is currently a postdoctoral fellow at Shanghai University. Her research interests include nanomaterials synthesis, electro-chemical biosensors and fuel cells. Jiaqiang Xu obtained his MS degree in inorganic chemistry from University of Science and Technology of China in 1988 and Ph.D. in engineering in 2005 from Shanghai University. He was a professor in applied chemistry at Zhengzhou Institute of Light Industry from 2001 to 2006. He is a professor in Department of Chemistry at Shanghai University and a Director of China Special Committee of Gas and Humidity Sensor Technology. His research interests include the synthesis of nanoscale materials and their applications in gas sensor and other fields.