Highly sensitive detection of acetone using mesoporous In2O3 nanospheres decorated with Au nanoparticles

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Highly sensitive detection of acetone using mesoporous In2 O3 nanospheres decorated with Au nanoparticles Su Zhang, Peng Song ∗ , Jia Zhang, Huihui Yan, Jia Li, Zhongxi Yang, Qi Wang ∗ School of Materials Science and Engineering, Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, University of Jinan, Jinan 250022, China

a r t i c l e

i n f o

Article history: Received 15 May 2016 Received in revised form 9 September 2016 Accepted 26 September 2016 Available online xxx Keywords: Mesoporous In2 O3 nanospheres Au nanoparticles Nanocompoistes Gas sensors

a b s t r a c t Surface modification with noble metal is considered as an effective strategy to enhance sensing performance of metal oxide-based gas sensors. In this work, mesoporous In2 O3 nanospheres decorated with gold nanoparticles (Au NPs) have been successfully synthesized by a two-step approach including a facile hydrothermal reaction and subsequent in situ reducing process. Various techniques were employed for the characterization of the structure and morphology of as-obtained Au/In2 O3 nanocomposites. The results reveal that Au NPs with average diameters of 3–5 nm are uniformly deposited on the surface of mesoporous In2 O3 nanospheres with a size range of 100–200 nm, specific surface area of 40.3 m2 /g, and average pore size of 5 nm. Importantly, the mesoporous structure, large specific surface area, and catalytic effect of Au NPs endow the Au/In2 O3 nanocomposites with highly sensitive performance for acetone detection. The response value to 10 ppm acetone is about 53.08 at an operating temperature of 320 ◦ C, and the response and recovery time are 4 and 9 s, respectively. The probable enhancing mechanism of as-prepared Au/In2 O3 nanocomposites is discussed as well. It is expected that Au NPs-decorated mesoporous In2 O3 nanosphere with excellent sensing performance is a promising functional material to actual application in monitoring and detecting acetone. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Acetone (C3 H6 O), a common reagent widely used in industries and laboratories, is easy to evaporate at room temperature. Inhalation of acetone may cause various health hazards and acetone is a selective breath marker for type-1 diabetes [1]. For a healthy human, the acetone concentration in the breath should be below 0.9 ppm, when the concentration of acetone is more than 1.8 ppm, which could indicate ketosis of insulin-dependent diabetes [2]. However, the current diagnosis of diabetes is blood test, which is inefficient and painful. In contrast, using gas sensors to detect acetone is a facial method to ensure the health and safety of individuals. Thus, fast response and highly selective sensors for acetone detection are required for the rapid assessment of diabetes and related diseases [3]. Metal oxide semiconductors (MOSs) have been extensively applied in various fields, such as solar cells, photocatalysis, fuel cells, biomedicine, etc [4–8], especially in the application of gas sensors owing to their high sensitivity, fast sensing, simple and

∗ Corresponding authors. E-mail addresses: mse [email protected] (P. Song), mse [email protected] (Q. Wang).

cheap fabrication [9–11]. Among the MOSs, indium oxide (In2 O3 ) is an important direct semiconductor with a wide direct band gap (3.55–3.75 eV) and high conductivity [12]. Because of its unique properties, such as lower resistivity, lower absorbance rate in the visible region, and prolific defects on the surface, it is widely used in gas sensor [13,14]. However, many studies are still working to improve the performances of gas sensors based on In2 O3 . According to lots of literature, we can propose that there are two universal strategies to enhance the performances of gas sensors. One method is to control the morphology, shape and size of sensing materials [15]. Various morphologies and sizes of In2 O3 were prepared by many different method, such as nanoparticles [16], nanowires [17], nanoflower [18], nanosheets [19], nanocubes [20], hollow [21], mesoporous [22] and hierarchical [23] structures, etc. Among these morphologies, the porous structures with high specific surface areas can significantly improve the properties of the sensing materials [24]. The reason can be attributed to the extensive pore structures on the surface provide large surface area and abundant reaction sites, which is beneficial for adsorptiondesorption process and gas diffusion on the material surface [25]. For instance, Li et al. successfully synthesized flower-like Co3 O4 nanostructures with porous structure by a facile hydrothermal method, which exhibited enhanced sensing performance for ethanol [26]. Likewise, Stefanie et al. reported mesoporous In2 O3

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with regular morphology by using various porous silica phases as structure matrices, which provide a new and simple path to obtain mesoporous In2 O3 nanomaterials [27]. Thus, mesoporous In2 O3 nanostructures are considered to be a promising sensing material. Another method to enhance the performances of MOSs gas sensor is surface modification, such as noble metals, including Au, Pt, and Ag. The metal additive acts as a catalyst to improve the surface reactions between the sensing materials and detected gases [28]. Au nanoparticles may enrich the conversion of oxygen species and the reaction rate between oxygen species and target gas molecules, which can ultimately improve the gas-sensing properties. Several studies on the enhancement of gas sensors using the decoration of Au have been reported. Wang et al. have successfully synthesized Au modified mesoporous SnO2 spheres through a solvothermal route, resulting in high gas-sensing properties toward CO and H2 [29]. Xu and her colleagues prepared Au-loaded In2 O3 nanofibers via an electrospinning method, they found that the Au-loaded In2 O3 sensor possessed the low power consumption, high response, fast response and recovery time to ethanol compared with the pure one [30]. Through the above investigation, we found that decorated with Au nanoparticles can substantially improve the gas-sensing performances of metal oxides based sensors. Therefore, to explore higher gas-sensing performance of In2 O3 , mesoporous nanostructure combined with Au-decorated is worthy of being fabricated and investigated. In this work, we prepared mesoporous In2 O3 nanospheres via a facile hydrothermal method followed by a thermal treatment process. The as-obtained mesoporous In2 O3 nanospheres were further uniformly decorated with Au nanoparticles on its surface, and the gas-sensing properties of mesoporous Au-loaded In2 O3 nanospheres and pure In2 O3 nanospheres were investigated using acetone as a detected gas. The results indicated that the as-obtained Au-loaded mesoporous In2 O3 nanospheres exhibited significantly enhanced gas-sensing performance to acetone, including high response, good selective and short response and recovery times, which is due to the high surface area, abundant active sites and the catalysis of the Au nanoparticles. 2. Experimental 2.1. Synthesis of mesoporous In2 O3 nanospheres All the reagents we used in this experiment are of analysis grade and used without further purification. Mesoporous In2 O3 nanospheres were prepared by a hydrothermal route. In a typical process, 4 mmol of NH4 HCO3 were dissolved into 20 mL deionized water to form a homogeneous solution, marked as solution A. 1 mmol of InCl3 ·4H2 O, 0.2 g Na2 SO4 and 1 mmol of citric acid (C6 H8 O7 ) were added into 20 mL deionized water under continuous stirring to form solution B. Under stirring, solution A was slowly added to solution B to form a mixed solution C. Then transferred the mixed solution into a 50 mL Teflon-lined stainless steel autoclave and maintained the temperature at 160 ◦ C for 8 h. After the autoclave cooled to room temperature naturally, the as-prepared precipitates were collected by centrifugation, washed several times with deionized water and absolute ethanol, respectively. The precipitates were dried at 60 ◦ C for 6 h. Finally the as-obtained In(OH)3 precursors were annealing in a muffle furnace at 500 ◦ C for 3 h to form porous In2 O3 conversions. 2.2. Synthesis of Au/In2 O3 nanocomposites At first, 50 mg as-obtained In2 O3 nanospheres, 1 mL of 0.01 M chloroauric acid (HAuCl4 ) and 1 mL of 0.01 M l-lysine (C11 H23 N3 O6 ) solution were dispersed into 15 mL deionized water by ultrasoni-

cation for 15 min, and l-lysine was used as an adhesives between Au nanoparticles and In2 O3 nanospheres. 0.1 mL of 0.1 M Na3 cit solution was added in the above solution dropwise under continually stirring for 30 min. The as-prepared samples were washed several times with deionized water and absolute ethanol, and dried at 60 ◦ C for 6 h. Finally the as-obtained Au/In2 O3 nanocomposites were annealing in a muffle furnace at 300 ◦ C for 30 min. 2.3. Characterization The phase composition, crystal structure and purity of asobtained pure mesoporous In2 O3 nanospheres and Au/In2 O3 nanocomposites samples were examined by powder X-ray diffractometer (XRD, Bruker D8 Advance, ␭ = 0.15406 nm) using CuKa1 radiation at 40 kV and 40 mA. The morphology and more information of the samples were observed by field-emission scanning electronic microscope (FESEM, FEI Company, QUANTA FEG 250) and transmission electron microscope (TEM, Hitachi H-800). The energy-dispersive X-ray spectroscopy (EDS) analysis was analyzed by the FESEM attachment. To identify the elements and its valence of the surface of as-prepared samples, the X-ray photoelectron spectrometer (PHI 5300) was employed to obtain the X-ray photoelectron spectra (XPS) of the Au/In2 O3 nanocomposites sample. The specific surface area and pore-size distribution were estimated by the Brunauer-Emmett-Teller (BET) method based on N2 adsorption-desorption test. 2.4. Fabrication and measurement of the gas sensors First, fabricated the sensors by an ordinary method, 50 mg asobtained pure mesoporous In2 O3 nanospheres and 50 mg Au/In2 O3 nanocomposites were dispersed into deionized water to form pastes with suitable viscosity, respectively. Then the pastes were uniformly coated on the ceramic tube with a pair of gold electrodes and four Pt wires by a small brush to form a thin layer. A Ni-Cr resistor, as a heater, was put in the inner of alumina ceramic tube to provide the operating temperature for as-fabricated sensors. The as-fabricated sensors were aged at 300 ◦ C for at least 24 h before tests to obtain a good stability. Therefore, indirectly-heated gas sensors have been produced, and the sensors were put into the test chamber in a measuring system of WS-30A by a staticprocess. In a typical testing procedure, first, the as-fabricated sensors based on pure mesoporous In2 O3 nanospheres and Au/In2 O3 nanocomposites were put into a glass chamber with 18 L of capacity. After the resistances of all the sensors were stable, a certain amount of target gas or liquid was injected into the chamber by a micro-injector. If the target gas was obtained from liquid, the concentration of target gas can be calculated by the following formula, C = (22.4 × ␳ × d × V1 )/(M × V2 )

(1)

In this formula, C (ppm) is the concentration we needed of target gas, ␳ (g/mL) is the density of the liquid, d is the purity of the liquid, V1 (L) is the volume of the liquid, V2 (L) is the volume of the test chamber, and M (g/mol) is the molecular weight of the liquid. By adjusting the heating voltage (Vheating ) of the Ni-Cr alloy resistor inside the ceramic tube can control the operating temperature of sensors. A reference resistor (Rload ) is put in series with the sensor to form a complete measurement circuit. In the test process, the working voltage (Vworking ) was 5 V. By monitoring the voltage across the reference resistor (Voutput ), the response of the sensor in air or in a target gas could be measured. The sensor response was defined as, Response = R gas /R air

(2)

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Fig. 1. XRD patterns of as-obtained In(OH)3 precursors, pure In2 O3 and Au/In2 O3 nanocomposites samples.

where Rair and Rgas is the resistance of sensor in air and the presence of the test gas, respectively. 3. Results and discussion 3.1. Structural and morphological characteristics The phase and crystal structure of the samples were obtained by X-ray diffractometer. Fig. 1 shows the XRD patterns of In(OH)3 precursors, pure In2 O3 and Au/In2 O3 nanocomposites. As we can see from the In(OH)3 pattern, all the main peaks can be perfectly indexed to the cubic structure of In(OH)3 (JCPDS card No. 16-0161)

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and no miscellaneous peaks were detected. The In2 O3 samples were synthesized by annealing hydrothermally obtained In(OH)3 precursors. The XRD pattern of pure In2 O3 is corresponding to JCPDS card No. 06-0416 with lattice constants of a = 10.77 Å. All the peaks are well sharp and no other peaks from impurities, which indicated that the In2 O3 we have obtained possesses high purity. As for the XRD pattern of Au/In2 O3 nanocomposites, it can be obviously observed that it is very similar to that of In2 O3 samples. Besides the diffraction peaks of In2 O3 , the XRD pattern of Au/In2 O3 nanocomposites also shows four small peaks, which can be ascribed to the (111), (200) (220) and (311) planes of face-centered cubic (fcc) Au (JCPDS No. 65-8601). In order to obtain more detailed chemical compositions and their chemical states of material surfaces, the as-synthesized Au/In2 O3 nanocomposites were further characterized by X-ray photoelectron spectroscopy (XPS). The XPS survey spectrum for Au/In2 O3 nanocomposites is shown in Fig. 2(a). The spectrum shows that the main constituent elements are In, O, and Au, except for additional peak resulting from carbon which is the charged correction calibration. Fig. 2(b) presents the XPS spectrum of In 3d. The peak positions of In3d3/2 (BE = 452 eV) and In3d5/2 (BE = 444.3 eV) peaks are about 451.7 eV and 444.1 eV, and the binding energies both declined, which indicated that the chemical environment around the indium atoms have changed. The possible explanation is In2 O3 adsorbed lots of oxygen on the surface and the chemisorption oxygen interacted with indium, weakened the bond of In-O. It can increase the electron density around indium atom, and enhanced the shielding effect [31]. The shift in binding energy suggests a charge in the binding state of cation which can physically be described by a loss in the number of oxygen inos in In2 O3 nanospheres. It has been argued that the increasing broken bonds tend to reduce the charge transfer from cation to oxygen. Thus increasing the shielding effect of the valence electrons causes decrease in the binding energy of the core electrons in the cation. Such shielding effect makes the transfer of free electrons easier,

Fig. 2. XPS spectra of Au/In2 O3 nanocomposites: (a) wide XPS spectrum of Au/In2 O3 nanocomposites; (b) In 3d spectrum; (c) O 1s spectrum; (d) Au 4f spectrum.

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Fig. 3. SEM images of In(OH)3 precursors (a, b), pure In2 O3 nanospheres (c, d) and Au/In2 O3 nanocomposites (e, f).

which leads a faster response of a sensor to target gas. Fig. 2(c) shows the O 1s XPS peaks. The ␣ peak is associated with lattice oxygen of In2 O3 and ␤ peak is caused by the surface hydroxyl oxy-

gen of In2 O3 [32]. As shown in Fig. 2(d), the chemical state of Au has two separate peaks located at 83.3 and 87.1 eV, respectively. The binding energy of Au 4f 5/2 and Au 4f 7/2 have a shift com-

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Fig. 4. EDS patterns of Au/In2 O3 nanocomposites.

pared to bulk Au, which can be attributed to the strong electronic interaction between Au nanoparticles and In2 O3 [33,34]. The morphology and internal structure of as-synthesized Au/In2 O3 nanocomposites were investigated by FESEM. Fig. 3 presents the FESEM images of In(OH)3 precursors, In2 O3 nanospheres and Au/In2 O3 nanocomposites samples. As we can see in Fig. 3(a), the shape and size of In(OH)3 precursors are very uniform, and the sample has good dispersibility. High-resolution FESEM image of In(OH)3 precursors is showed in Fig. 3(b), from this image we can find that the diameter of these nanospheres is about 100–200 nm. The surface of these In(OH)3 nanospheres is very smooth and without any impurities. Fig. 3(c) and Fig. 3(d) show the FESEM images of the In2 O3 nanospheres samples. It can be seen that most In2 O3 samples maintained the morphology of In(OH)3 precursors, while the surface was obviously became coarsened. Fig. 3(e) and Fig. 3(f) show the FESEM images of Au/In2 O3 nanocomposites. As we can see, After decorated with Au nanoparticles, the In2 O3 nanospheres have no significant change, which need further observation by transmission electron microscopy (TEM). And we can clearly see some of the Au nanoparticles attached on the In2 O3

surface. In addition, we obtained the EDS patterns of the Au/In2 O3 nanocomposites samples to determine the element composition. As we can see in Fig. 4, there are three elements: In, O, and Au in the as-prepared sample. We can also find that the Au nanoparticles are evenly loaded on the surface of In2 O3 nanospheres. The intriguing structure is also elucidated under TEM to provide further insight about the morphology and microstructure of the as-synthesized pure In2 O3 nanospheres Au/In2 O3 nanocomposites. From Fig. 5(a) we can see that the size of In2 O3 nanospheres is pretty uniform and there are lots of pores on the In2 O3 nanospheres. Because of the presence of large amounts of pores, the In2 O3 microsphere looks coarse compared with In(OH)3 precursors. Fig. 5(b) presents the TEM image of Au/In2 O3 nanocomposites and it can be clearly observed that there are lots of Au nanoparticles evenly loaded on the In2 O3 surface. Further increased the magnification and obtained Fig. 5(c), the shape and size of Au nanoparticles can be observed. From the image, the Au present as a sphere and the diameter is about 3–5 nm. It is tightly attached on the surface of In2 O3 . Fig. 5(d) exhibits the HRTEM image of lattice fringes of Au and In2 O3 . The spacing of lattice fringes of Au and In2 O3 are mea-

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Fig. 5. (a) TEM image of the pure In2 O3 nanospheres; (b, c) TEM images of the Au/In2 O3 nanocomposites and (d) high resolution TEM image of Au/In2 O3 nanocomposites.

sured to be about 0.204 and 0.416 nm, respectively. They can be assigned to the (200) crystal planes of cubic Au and the (211) crystal planes of cubic In2 O3 , respectively. This porous structure increases the surface area of the samples and so can improve gas-sensing performance. In addition, We used laser particle size analysis to obtain the size distribution of all samples, as shown in Fig. 6. As we can see the particle size of as-prepared In2 O3 nanospheres samples mainly distributed in the 80–175 nm, 99.995% of In2 O3 nanospheres are less than 200 nm. This result is corresponding with SEM and TEM results. In order to investigate the porosity and surface area, BET nitrogen adsorption–desorption measurements were carried out on the as-prepared Au/In2 O3 nanocomposites. Fig. 7 shows the N2 adsorption-desorption isotherm curve and pore size distribution plots (inset) of the mesoporous In2 O3 nanospheres decorated with Au nanoparticles. The curve shows a type-IV isotherm and a hysteresis loop from 0.4 to 1.0 (P/P0 ), and the N2 adsorption quantity

Fig. 6. The size distribution of In2 O3 nanospheres samples.

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60

In2O3

50

Response (Ra/Rg)

Au/In2O3 40 30 20 10 0 240

260

280

300

320

340

360

380

Time (s) Fig. 7. N2 adsorption-desorption isotherm curve and pore size distribution plots (inset) of Au/In2 O3 nanocomposites.

also increases with the increase of pressure, which indicated the asobtained sample possesses porous structures. The specific surface area of Au-loaded mesoporous In2 O3 nanospheres is calculated to be 40.3 m2 /g. It is much larger than normal In2 O3 particles (about 4–5 m2 /g) [35]. From the pore size distribution plots we can see that the main pore diameter is about 4 nm. As we know, the porous structure, because of its large specific surface area, can effectively improve the gas-sensing performances of materials. 3.2. Gas sensing properties The operating temperature has a significant impact for gas sensors. At the optimum detection temperature, the sensors show high response to detected gases. Relatively low response at low temperature is caused by the test gas molecules not having enough thermal energy to react with the surface-adsorbed oxygen species. The enhanced response with increasing of operating temperature can be attributed to two factors. One factor is that the thermal energy of the gas molecule obtained is high enough to overcome the activation energy barrier of the surface reaction. The second factor is that conversion of adsorbed oxygen species occurs by the reactions, which attract more electrons from the semiconductor. Namely, the increase of operating temperature would facilitate the chemical reaction, leading to the increase of response to acetone. However, with further increasing of operating temperature, the low gas adsorption ability of the gas molecule at high temperature causes the low utilization rate of the sensing material, which is the reason for the decrease of the response. To find the optimum operating temperature of the pure mesoporous In2 O3 and Au/In2 O3 nanocomposites sensors, we investigated the responses of these two sensors to 10 ppm acetone at the temperature from 240 ◦ C to 380 ◦ C. As we can see in Fig. 8, with the increase of the operating temperature, the response values of the pure mesoporous In2 O3 and Au/In2 O3 nanocomposites sensors both increase, especially the sensor based on Au/In2 O3 nanocomposites, the response values improve rapidly. At 320 ◦ C, the response values of these two sensors both reach the maximum value. So the result is that the optimum operating temperature of the sensors based on pure In2 O3 and Au/In2 O3 nanocomposites samples to acetone both are 320 ◦ C. Then the response decreases with the increase of temperature, the rate of desorption is faster than that of adsorption at high temperature causes the low utilization rate of the sensing material, which is the reason for the decrease of the response [36]. In addition, the maximum response values of these two sensors are 3.92 and 53.08,

Fig. 8. Response of sensors based on mesoporous In2 O3 nanospheres and Au/In2 O3 nanocomposites to 10 ppm of acetone at different operating temperature.

respectively. Obviously, after decorated with Au nanoparticles, the response of In2 O3 sensor has significant improved. The gas-sensing properties of sensors based on pure In2 O3 nanospheres and Au/In2 O3 nanocomposites are further investigated by detecting 10 ppm of acetone gas under the optimum operating temperature. Fig. 9(a) shows the response and recovery curves of these two sensors. We can see that the response and recovery time of pure In2 O3 and Au/In2 O3 nanocomposites sensors are almost same, both are about 4 s and 9 s, respectively. Although decorated with Au nanoparticles have no significant impact on the response and recovery time, but it is clearly that after decorated with the Au nanoparticles, the response of the as-fabricated sensor has significantly increased. The response value of Au/In2 O3 nanocomposites sensors is about 13 times higher than that pure one. Fig. 9(b) presents the response and recovery curves of sensors upon exposure to 5–500 ppm of acetone. It can be observed that with the increase of acetone concentration, the response of pure In2 O3 and Au/In2 O3 nanocomposites sensors gradually become higher, and Au/In2 O3 nanocomposites sensor shows much higher response than the pure In2 O3 sensor at every concentration. At low acetone concentration, such as 5 ppm, the response of Au/In2 O3 nanocomposites sensor is about 28, which indicates the sensor can be used to detect low concentration of the target gas. The linear relationship of log (S-1) − log (C) plot to acetone is shown in Fig. 9(c). The relationship of response of gas sensor and gas concentration can be represented as S = a[C]b + 1, where a and b are the constants, S is the gas response, C is the concentration of the target gas. Generally, the exponent b has an ideal value of 0.5-1, which is derived from the surface interaction between chemisorbed oxygen and reducing gas to n-type semiconductor [37]. From the image we can see that pure In2 O3 and Au/In2 O3 nanocomposites sensors exhibit good linear relationship with the concentration. The slopes of pure In2 O3 and Au/In2 O3 nanocomposites sensors are 0.37005 and 0.52368, respectively. It indicates that with the increase of gas concentration, the response of Au/In2 O3 nanocomposites sensor increase faster than that pure one. To investigate the stability, the two sensors were stored in air and kept working at 320 ◦ C to repeat testing cycles for five times. The stability of pure In2 O3 and Au/In2 O3 nanocomposites sensors are shown in Fig. 9(d). The response of these two sensors are reproducible and have no obvious change for the successional tests, especially the Au/In2 O3 nanocomposites sensor shows high response, which has great potential for practical applications. Table 1 presents the comparison of gas-sensing performances between Au/In2 O3 nanocomposites and other In2 O3 nanostructures toward acetone. It can be clearly

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Fig. 9. (a) Response and recovery curves of the sensors based on pure In2 O3 microspheres and Au/In2 O3 nanocomposites to 10 ppm acetone at 320 ◦ C. (b) The response and recovery curves of sensors upon exposure to 5–500 ppm of acetone at 320 ◦ C. (c) The linear relationship of log (S-1)-log (C) plot to acetone at 320◦C. (d) Response and recovery curves of sensors to 10 ppm acetone after 5 cycles of gas in and off at 320 ◦ C.

Table 1 Comparison of gas-sensing properties of various In2 O3 nanostructures to acetone in the literatures and this work. In2 O3 nanostructures

Operating temperature (◦ C)

Acetone (ppm)

Response

Response/Recovery times (s)

Refs.

Mesoporous In2 O3 nanospheres decorated with Au NPs ZnO-In2 O3 composited nanotubes Mesoporous In2 O3 nanostructures In2 O3 hollow spheres Porous In2 O3 -CeO2 binary oxide nanotubes Pd NPs@nest-In2 O3 materials In2 O3 -WO3 heterojunction nanofibers

320 280 300 300 300 370 275

10 60 50 100 200 100 100

53.08 43.2 29.8 30 ∼30 30.6 20.3

4/9 5/25 0.7/14 8/15 9/80 9/7 –

present study [38] [39] [40] [41] [42] [43]

observed that the as-obtained Au/In2 O3 nanocomposites based sensor possesses excellent sensing properties compared with other In2 O3 sensors [38–43]. Therefore, it can infer that the sensors based on the Au/In2 O3 nanocomposites based sensor displays excellent gas-sensing performances towards acetone, it may use as a potential material in many fields. Fig. 10(a) displays the histogram of the response of pure In2 O3 nanospheres and Au/In2 O3 nanocomposites sensors to six kinds of target gases with a concentration of 10 ppm. As we can see the sensor based on Au/In2 O3 nanocomposites shows better gassensing properties to acetone compared with other gases, which indicates the Au/In2 O3 nanocomposites sensor have good selectivity to acetone at 320 ◦ C. However, the response of the sensor based on pure In2 O3 nanospheres to acetone, trimethylamine, and ethanol is similar, so after decorated with Au nanoparticles, the selectivity of mesoporous In2 O3 nanospheres sensor have substantial increase. The interference of humidity is an important problem that must consider in breath sensing, because the VOCs signals could be screened by the high humidity levels in breath, and the small fluctuations in humidity also have big effect on the sensitivity of VOCs. Here, the influence of the high humidity level was considered in our research. As to the influence of humidity, we have measured 100 ppm acetone gas sensing response under different

humidity of 25–80%. Fig. 10(b) shows the humidity effect on the asprepared Au/In2 O3 nanocomposites. No significant degradation in response magnitude was observed with the relative humidity (RH), which indicates that our sensor is practical under high humidity. After a series of gas-sensing tests, the study of Au/In2 O3 nanocomposites sensor is a very promising object, due to its high response and selectivity, good stability to acetone. As it is known, In2 O3 is a typical n-type metal oxide semiconductor, when it is exposed to the reducing gas, the resistance of In2 O3 reduces, in contrast, the resistance increases in oxidizing gas. The gas-sensing mechanism of mesoporous In2 O3 nanospheres sensors can be proposed. The detected gas molecules are adsorbed and desorbed on the surface of the materials causes the change of resistance of In2 O3 [44]. During the test, the sensors based on pure In2 O3 nanospheres and Au/In2 O3 nanocomposites were exposed to air at first, there will be a large number of oxygen molecules adsorbed on the In2 O3 surface to, these absorbed oxygen molecules on the In2 O3 surface will capture free electrons from the conduction band to form chemisorbed oxygen ions, such as O2 − , O− , O2− [45]. This progress can be expressed as the following reactions: O2(gas) → O2(ads)

(3)

O2(ads) + e− → O2 − (ads)

(4)

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50

In2O 3

40

Au/In O 2 3

9

30 20 10

(a)

M et ha no Tr l im et hy la m in e Et ha no l

ue ne To l

on i m

A m

A

ce to n

e

a

0

70

Relative humidity (RH%) Response (Ra/Rg)

60

25 %

40 %

Fig. 11. The relationship between resistance and time of sensors based on pure In2 O3 nanospheres and Au/In2 O3 nanocomposites.

55 %

50

70 % 80 %

40 30 20 10 0 0

100

200

300

400

Time (s)

(b) Fig. 12. The schematic of sensing mechanism of Au/In2 O3 nanocomposites. Fig. 10. (a) Response of sensors based on pure In2 O3 nanospheres and Au/In2 O3 nanocomposites to 10 ppm various gases. (b) Sensor response versus humidity concentration curve of the Au/In2 O3 nanocomposites to 100 ppm acetone at 320 ◦ C.

O2 − (ads) + e− → 2O− (ads) −

O

(ads)

−

+e → O

2−

(ads)

(5) (6)

The above process will form a depletion layers on the In2 O3 surface and causes the increase of resistance of the sensors [46]. Then the acetone gas was injected into the chamber, the acetone molecules can react with the chemisorbed oxygen ions absorbed on the In2 O3 surface and release the trapped electrons back to the conduction band of In2 O3 , thus, the resistance of In2 O3 decreases [47]. The reaction process is as follows: CH3 COCH3 + 8O− → 3CO2 + 3H2 O + 8e−

(7)

Fig. 11 shows the relationship between resistance and time of sensors based on pure In2 O3 nanospheres and Au-decorated In2 O3 nanospheres. From the figure we can see that Au-decorated In2 O3 nanospheres sensor possesses higher resistance compared with pure In2 O3 nanospheres sensor in air, which means sensors based on Au-decorated In2 O3 nanospheres adsorbed more oxygen molecules and increase the conversion rate of oxygen molecules. The schematic of sensing mechanism of Au/In2 O3 nanocomposites is depicted in Fig. 12. From the above results we can clearly observed that Au nanoparticles are dispersed uniformly on the In2 O3 surface and sufficiently expose their active surface. As we know, Au is a kind of noble metal, it can be used as catalysts to enhance the performances of sensing materials [48]. Au as a catalyst, due to its electronic sensitization, it can enhance the effect of gas-sensing properties [49]. For the effects of the noble met-

als on the improvement of sensor response at present, there are two types of mechanisms are discussed commonly to explain, the chemical and electronic sensitization. The electronic sensitization mechanism supposes that the negative charged adsorbed oxygen at the noble metal/gas interface induces an electrical perturbation at the noble metal/oxide interface and results in an electron deficit in the oxide. When a reducing gas is oxidized on the noble metal surface, an electron is given back to the noble metal and then to the oxide [49]. The electronic sensitization greatly improves the direct electrons exchange between the In2 O3 and the noble metal (Au) additives. In the electronic sensitization mechanism, the Au nanoparticles plays chemical and electronic sensitization role in the gas-sensing performances, the reaction between Au nanoparticles and gas molecules will take place at the Au additives [50]. The Au nanoparticles on the In2 O3 surface can increase the conversion rate of oxygen molecules, thus, adsorbed more oxygen, which result in faster electron depletion between the interface of Au nanoparticles and In2 O3 nanospheres, lead to a higher reaction rate and improving the gas-sensing properties. In addition, Au additives can prompt In2 O3 to form more active sites on their surface, it can faster the process of acetone gas diffusion and the formation of oxygen species, which results in stronger electron depletion on the In2 O3 surface, which is considered to be an important reason for the enhanced response compared with pure In2 O3 . In our work, the response of Au/In2 O3 nanocomposites is significantly higher than the pure one. The selectivity of a gas sensor is a very important parameter. From the above results, we can obviously observed that the response of Au/In2 O3 nanocomposites sensor toward acetone is much higher that other gases. As we know that the response of a sensor has a significant relationship with the adsorption and reac-

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tion of gas molecules on the materials surface [51]. On the one hand, the gas-sensing properties of sensing materials are strongly relying on the surface structures [52]. The polar property of the Au/In2 O3 nanocomposites will promote the adsorption of polar molecules. In addition, the polarity of acetone molecules is higher than other gas we detected. So acetone is much easier to be adsorbed on the Au/In2 O3 nanocomposites surface. On the other hand, the bond dissociation energy of CH3 -COCH3 (352 kJ/mol) is smaller than that of C2 H5 O-H, CH3 O-H (462 kJ/mol), H-NH2 (452 kJ/mol), H-CH2 C6 H5 (371 kJ/mol), which indicates that acetone is easier to react with the adsorbed oxygen species than other gas molecules [53,54]. Furthermore, according to the literature survey [55–59], the presence of Au clearly promotes the catalytic conversion of acetone molecules, which is likely to be associated with the selective acetone detection by the Au/metal oxide nanocomposites. Although, the above results still do not fully account for the superior sensitivity of the Au/In2 O3 nanocomposites toward acetone molecules, and this needs further investigation for clarifying, but this result is very interesting in fabrication of high sensitive acetone gas sensors. 4. Conclusions In summary, Au/In2 O3 nanocomposites were prepared via a pollution-free hydrothermal route followed by a thermal treatment process. Through characterized by various methods, we found that the diameter of as-prepared mesoporous In2 O3 nanospheres is about 100–200 nm and the size of Au nanoparticles is about 3–5 nm. The BET surface area of the mesoporous In2 O3 nanospheres is calculated to be 40.3 m2 /g and the main pore diameter is about 4 nm. The sensors were fabricated by coating the as-obtained In2 O3 samples on the ceramic tube. The pure mesoporous In2 O3 nanospheres and Au/In2 O3 nanocomposites based sensors were tested by acetone. It is found that the sensors based on Au/In2 O3 nanocomposites presented high response (about 53.08), fast response and recovery times (4 s and 9 s, respectively) and excellent selectivity to 10 ppm acetone. The In2 O3 nanospheres with mesoporous nanostructures provides more active sites and channels for the gas diffusion, and the Au nanoparticles decorated on the surface promote the reaction between test gas molecules and oxygen ions, resulting in the improvement of the gas-sensing properties to acetone. Acknowledgements This work was financially supported by National Natural Science Foundation of China (No. 61102006 and 51172095), Natural Science Foundation of Shandong Province, China (No. ZR2015EM019 and ZR2014EL006), and Shandong Province Higher Educational Science and Technology Program (No. J15LA56). References [1] P. Song, Q. Wang, Z. Yang, Preparation, characterization and acetone sensing properties of Ce-doped SnO2 hollow spheres, Sens. Actuators B 173 (2012) 839–846. [2] S. Singkammo, A. Wisitsoraat, C. Sriprachuabwong, A. Tuantranont, S. Phanichphant, C. Liewhiran, Electrolytically exfoliated graphene-loaded flame-made Ni-doped SnO2 composite film for acetone sensing, ACS Appl. Mater. Interfaces 7 (2015) 3077–3092. [3] T.I. Nasution, I. Nainggolan, S.D. Hutagalung, K.R. Ahmad, Z.A. Ahmad, The sensing mechanism and detection of low concentration acetone using chitosan-based sensors, Sens. Actuators B 177 (2013) 522–528. [4] Q. Yue, M. Wang, J. Wei, Y. Deng, T. Liu, A template carbonization strategy tosynthesize ordered mesoporous silica microspheres with trapped sulfonated carbon nanoparticles for efficient catalysis, Angew. Chem. Int. Ed. 51 (2012) 10514–10518. [5] H.P. Cong, X.C. Ren, H.B. Yao, P. Wang, H. Colfen, Synthesis and optical properties of mesoporous ␤-Co(OH)2 /brilliant blue G (G250) hybrid hierarchical structures, Adv. Mater. 24 (2012) 1309–1315.

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Biographies Su Zhang majored in materials science and engineering and received her BS degree in 2015 at University of Jinan. She is currently studying for MS degree at School of Material Science and Engineering, University of Jinan. Now, she is engaged in the synthesis and characterization of the motel oxide semiconducting functional materials and gas sensors. Peng Song received his B.Sc. and Ph. D degree in chemistry and materials physics and chemistry, Shandong University, in 2001, and 2006, respectively. Currently, he is an associate professor in School of Material Science and Engineering, University of Jinan. His research focuses on the synthesis of new functional nanostructure materials and their application in gas sensors. Jia Zhang is currently studying for MS degree at School of Material Science and Engineering, University of Jinan. Her research subject is synthesis and gas-sensing properties of one–dimensional nanostructures. Huihui Yan majored in materials science and engineering and received her BS degree of Engineering in 2014 at University of Jinan. She is currently studying for MS degree at School of Material Science and Engineering, University of Jinan. Her research subject is synthesis and gas-sensing properties of porous nanomaterials. Jia Li received her PhD degree in 2003 from Shandong University. Now, she is a professor at School of Material Science and Engineering, University of Jinan. Her research interests focus on the nanostructures functional materials and ceramics matrix composites. Zhongxi Yang received his BS Degree from China University of Geosciences in 1994, and MS Degree from Wuhan University of Technology in 1997. Now he is an associate professor of School of Material Science and Engineering, University of Jinan, majored in Metal Material and organic-inorganic composite materials. Qi Wang received his BS Degree from Shandong Institute of Building Materials in 1985. Then he got his MS and Ph.D. Degree from Wuhan University of Technology in 1995 and 2004, respectively. Currently, he is a professor at the School of Material Science and Engineering, University of Jinan. His work is devoted to building materials, organic-inorganic composite materials and their application.

Please cite this article in press as: S. Zhang, et al., Highly sensitive detection of acetone using mesoporous In2 O3 nanospheres decorated with Au nanoparticles, Sens. Actuators B: Chem. (2016), http://dx.doi.org/10.1016/j.snb.2016.09.155