Facile approach to prepare hierarchical Au-loaded In2O3 porous nanocubes and their enhanced sensing performance towards formaldehyde

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Facile approach to prepare hierarchical Au-loaded In2 O3 porous nanocubes and their enhanced sensing performance towards formaldehyde Su Zhang, Peng Song ∗ , Jia Li, Jia Zhang, Zhongxi Yang, Qi Wang ∗ School of Material 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 26 July 2016 Received in revised form 2 October 2016 Accepted 5 October 2016 Available online xxx Keywords: Hierarchical In2 O3 nanocubes Au nanoparticles Porous Gas sensors

a b s t r a c t Controlling the morphology of materials and surface modification with noble metal are considered as effective strategies to improve gas-sensing properties of sensors based on metal oxide. In this work, hierarchical Au-loaded In2 O3 porous nanocubes have been successfully synthesized by a two-step approach including a facile hydrothermal reaction and subsequent in situ reducing process. Various methods were employed to characterize the structure and morphology of as-obtained hierarchical Au-loaded In2 O3 porous nanocubes. The results reveal that the side length of In2 O3 nanocubes is about 150–200 nm and the Au nanoparticles attached on it is about 15 nm. The specific surface area of hierarchical Au-loaded In2 O3 porous nanocubes is 38.1 m2 /g, and main pore size is distributed in of 8–9 nm. In addition, the hierarchical Au-loaded In2 O3 porous nanocubes based sensor possesses highly sensing performance for formaldehyde detection. The response value to 100 ppm formaldehyde is about 37 at an operating temperature of 240 ◦ C, which is doubled than the pure one and reduced the operating temperature. The response and recovery time are 3 and 8 s, respectively. It is expected that hierarchical Au-loaded In2 O3 porous nanocubes with excellent gas-sensing performance is a promising functional material to actual application in formaldehyde detection. © 2016 Elsevier B.V. All rights reserved.

1. Introduction In recent years, with the rapid development of industrialization, pollution and atmospheric environment issues have become more and more serious, and the species and amount of emissions have raised, the atmospheric where people lives contains large amounts of imperceptible nanoparticles, dust and toxic gases. This situation not only harms people’s health, but also severely restricts the green economy and sustainable development. To solve the worsening environmental pollution and safety issues, more and more people are committed to develop high-performance gas sensors for detecting toxic gases, supervising air quality and protecting human health [1,2]. Sensing material, as an important component of a gas sensor, determines the performance of the sensor [3]. Metal oxide semiconductors (MOSs) with high response, fast response and recovery time, simple circuits and operation, low cost, etc, has been widely

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

applied to various fields of medical, aerospace, home life, emissions testing [4–10]. 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 [11–13]. 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 [14,15]. However, many studies are still working to improve the performances of gas sensors based on In2 O3 . Morphology and structure of In2 O3 materials is the main factor affecting the application performance, many people have committed to control its shape and nanostructure, or the development of new structures in the production process to achieve performance enhancements [16]. Various morphologies and sizes of In2 O3 were prepared by many different method, such as nanoparticles [17], nanowires [18], nanoflower [19], nanosheets [20], nanocubes [21], hollow [22], mesoporous [23] and hierarchical [24] structures, etc. In the past few years, the hierarchical material, especially micro/nanostructures, due to their potential in these unique properties of optical, electronic, magnetic, bionics and catalysis has been widespread concern. This structure includes not only characteristics of single nanostructures, but also

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has coupling effect and synergies after assembled by nano-building blocks [25,26]. Thus, complicated hierarchical structure will maintain the superior performances of its structural elements, and significantly enhance its performance, even obtained the unique properties that low-dimensional structures do not have [27]. In addition, porous structure facilitates the target gas into the interior of the material, and inter-connected pore structure and excellent crystallinity of sensitive materials are conducive to increasing the carrier mobility to obtain better gas-sensing property. For instance, Zhu et al. have reported a prepared method of hierarchically porous CuO architectures via copper basic carbonate precursor obtained with a facile hydrothermal route, which exhibit higher sensing response than that of commercial CuO powder towards a series of typical organic solvents and fuels [28]. Thus, In2 O3 sensing material combining the advantages of hierarchical and porous nanostructure, is considered to be a very promising gas-sensing material with high properties. Another method to enhance the gas-sensing performance of gas sensor is surface modification with noble metals, such as loaded Au, Pt, or Ag nanoparticles on the surface. The noble metals acts as a catalyst, which can enhance the reactions between the sensing materials and detected gases [29]. In our work, Au nanoparticles were loaded on the In2 O3 surface, they enrich the conversion of oxygen species and the reaction rate between oxygen species and target gas molecules, which can ultimately improve the gassensing properties [30]. 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 [31]. 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 [32]. 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, hierarchical Au-loaded In2 O3 porous nanocubes is worthy of being fabricated and investigated. In this work, we prepared hierarchical In2 O3 porous nanocubes via a facile hydrothermal method followed by a thermal treatment process of In(OH)3 precursors. The as-obtained hierarchical In2 O3 porous nanocubes were further uniformly loaded with Au nanoparticles on its surface, and the gas-sensing properties of hierarchical Au-loaded In2 O3 porous nanocubes and pure hierarchical In2 O3 porous nanocubes were investigated using formaldehyde as a target gas. The results indicated that the as-obtained hierarchical Au-loaded In2 O3 porous nanocubes exhibited significantly enhanced gas-sensing performance to formaldehyde, 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 hierarchical In2 O3 porous nanocubes All the reagents in this process are of analysis grade and used without further purification. Hierarchical In2 O3 porous nanocubes were prepared by a simply and environmental hydrothermal route. In a typical process, 0.8 mmol InCl3 ·4H2 O and 4.8 mmol Na3 cit were dissolved into 52 mL deionized water, stirring for 30 min to form a homogeneous solution, added 4 mmol CO(NH2 )2 into the above-mentioned solution and then stirring for 30 min to obtain a mixed solution. Then transferred the 40 mL of mixed solution into

a 50 mL Teflon-lined stainless steel autoclave and maintained the temperature at 140 ◦ C for 24 h. After the autoclave cooled to room temperature naturally, the as-prepared precipitates were collected by centrifugation and washed several times by 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 2 h in air to form hierarchical In2 O3 porous conversions. 2.2. Synthesis of hierarchical Au-loaded In2 O3 porous nanocubes Firstly, 50 mg as-obtained In2 O3 products, 1.0 mL of 0.01 M chloroauric acid (HAuCl4 ) solution and 1.0 mL of 0.01 M l-lysine solution were dispersed into 15 mL deionized water by ultrasonication for 15 min, 0.1 mL of 0.1 M fresh Na3 cit solution was dropped slowly into the above solution under continually stirring to reduce HAuCl4 to the Au nanoparticles. Then continuously stirred for 30 min, the precipitate was collected by centrifugation and washed with water/ethanol several times, and dried at 60 ◦ C overnight. Finally, the as-obtained samples were calcined at 300 ◦ C for 30 min in air. 2.3. Characterization The crystal structure and phase composition of as-prepared pure In2 O3 nanocubes and Au-loaded In2 O3 nanocubes were identified by powder X-ray diffraction (XRD, Bruker D8 Advance) using CuKa1 radiation (=0.15406 nm) at 30 kV and 40 mA. The morphology and nanostructure of the surface of samples were characterized by field-emission scanning electronic microscope (FESEM, FEI Company, QUANTA FEG 250), and the internal nanostructures and lattice fringes were observed by transmission electron microscopy (TEM, Hitachi H-800). The energy-dispersive X-ray spectroscopy (EDS) analysis was performed by the FESEM attachment. The rmogravimetry-differential scanning calorimetry (TG-DSC, TGA/DSC1/1600HT, Mettler) was used to analyze the variation of heat and weight while the precursors transformed to conversions. The specific surface area was estimated by Nitrogen adsorption isotherm instrument (ASiQC0000-4, Quantachrome) using the Brunauer-Emmett-Teller (BET) method based on the N2 adsorption-desorption tests. Pore-size distribution was calculated from the adsorption branch of the nitrogen isotherm, using the Barrett-Joyner-Halenda method applied to the desorption part of the adsorption-desorption isotherm. 2.4. Fabrication and measurement of the gas sensor Firstly, the as-obtained samples were mixed with suitable distilled water and stir by brush to form slurry, and then pasted onto a prefabricated alumina tube (7 mm in length and 1.5 mm in diameter, attached with a pair of gold electrodes and four platinum wires) by a small brush to form a thick film. Then, as a heater, a Ni-Cr resistor (diameter = 0.5 mm, resistance = 35 ) was put in the inner of alumina ceramic tube to provide the working temperature for the sensor device. After dried in the air and aged at 300 ◦ C for at least 24 h, a simple indirectly-heated gas sensor was accomplished. In measure process, the as- fabricated gas sensor was put into the test chamber in a measuring system of WS-30A. A typical testing procedure was as follows: At first, the sensors were put into a glass chamber (18 L), the calculated amount of the target gas or liquid was injected into glass chamber by a micro-injector and mixed with air. For the target gases obtained from liquid, the concentration of target gas was calculated by the following formula, C = (22.4∗∗d∗V 1 )/(M∗V 2 )

(1)

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Fig. 2. TG-DSC curves of conversion process of In(OH)3 precursors.

Fig. 1. XRD patterns of In(OH)3 precursors, pure In2 O3 and Au-loaded In2 O3 nanocubes samples.

where C (ppm) is the target gas concentration,  (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 glass chamber, and M (g/mol) is the molecular weight of the liquid. The working temperature of the sensor can be controlled by adjusting the heating voltage (Vheating ) across a Ni-Cr alloy resistor inside the ceramic tube. A reference resistor (Rload ) is put in series with the sensor to form a complete measurement circuit. In the test process, a working voltage of 5 V (Vworking ) was applied. By monitoring the voltage across the reference resistor (Voutput ), the response of the sensor in air or in a test gas could be measured. The sensor response was defined as, Response = Rgas /Rair

(2)

Rair and Rgas are the resistances in the air and target gas, respectively. In addition, the response/recovery time was expressed o reach 90% of the overall resistance change. 3. Results and discussion 3.1. Structural and morphological characteristics In order to determine the phase composition crystal structure of as-prepared In(OH)3 precursors, pure In2 O3 nanocubes and Auloaded In2 O3 nanocubes, we employed X-ray diffractometer. Fig. 1 shows the XRD patterns of In(OH)3 precursors, pure In2 O3 and Auloaded In2 O3 nanocubes. As we can see from the XRD patterns of In(OH)3 precursors, all the main peaks can be accurately indexed to the cubic structure of In(OH)3 (JCPDS card No. 16-0161) and no impure peaks are detected. From the XRD patterns of pure In2 O3 samples we can see, 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. As for the XRD pattern of Au-loaded In2 O3 nanocubes, it can be obviously observed that the position of main peaks is same to that of In2 O3 samples. We can also observed that besides the diffraction peaks of In2 O3 , the XRD pattern of Au-loaded In2 O3 nanocubes also shows four small peaks compared with pure In2 O3 samples, which can be ascribed to the (111), (200) (220) and (311) planes of face-centered cubic (fcc) Au (JCPDS No. 65-8601).

To verify the conversion process of In(OH)3 precursors, the TGDSC analysis was characterized at heating rate of 10 ◦ C–min−1 in air surrounding, which controlled by program. Fig. 2 presents the TG-DSC curves of conversion process of In(OH)3 precursors. As it can be observed from the TG curve, there is an obvious weight loss accompanied with a sharp peak in the DSC curve between 400 and 550 ◦ C. This phenomenon can be interpreted as rapidly conversion of In(OH)3 precursors in this temperature range. The equation of this dehydration reaction of In(OH)3 is shown in Eq. (3): (3) In(OH)3 → In2 O3 + 3H2 O (loss 16.28 wt%) By the result of TG analysis, the actual mass loss of this reaction is about 16.5%, which is approached to the theoretical value. After 550 ◦ C, the TG and DSC curves are both become flat. From the DSC curve, we know that this reaction is a typical exothermic reaction. After we referred lots of references about annealing of In(OH)3 precursors and a series of experiments and combines the result of TG-DSC analysis, we choose 500 ◦ C, 2 h as the calcination condition of In(OH)3 precursors. The morphology and nanostructures of the as-obtained samples were observed by scanning electronic microscope (SEM). Fig. 3 shows the SEM images of In(OH)3 precursors, pure In2 O3 nanocubes and Au-loaded In2 O3 nanocubes. As we can see in Fig. 3(a), the morphology of as-prepared In(OH)3 precursors is nanocube. These nanocubes are evenly dispersed and have uniform size. High-resolution FESEM image of In(OH)3 precursors is showed in Fig. 3(b), as we can see, the side length of these nanocubes is about 150–200 nm. The surface of nanocubes is very smooth and has no impurities on it. Fig. 3(c) presents the SEM image of pure In2 O3 nanocubes. We can see from this image that the morphology of precursors has been maintained and the size is also similar to the precursors. Further increased the magnification, we can obtained Fig. 3(d). As it can be seen, the surface of these nanocubes has become coarse and consisted of nanoparticles. Besides, the In2 O3 nanocubes possess lots of pores after calcination at high temperature. Fig. 3(e) and (f) show the SEM images of Au-loaded In2 O3 nanocubes. From these images, the size and distribution of nanocubes are same as pure In2 O3 nanocubes. This means that the load of Au nanoparticles does not affect the morphology of In2 O3 . However, due to the limit of scanning electron microscope, we can’t clearly observe the Au nanoparticles, so we employed the EDS analysis to determine the element composition of Au-loaded In2 O3 sample. Fig. 3(g) shows the mapping patterns of as-obtained Au-loaded In2 O3 sample. From the spectrum we can know that there are three elements in this sample: Au, In, O. The weight percent of Au is 0.53%, which is approach to the theoretical value (0.6 wt%). The experiments showed, with the increase of weight ratio of Au nanoparticles, the performance of the gas sen-

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Fig. 4. TEM images of the Au-loaded In2 O3 nanocubes (a,b), high-resolution TEM image of the Au-loaded In2 O3 nanocubes (c) and lattice fringes image of Au-loaded In2 O3 nanocubes (d).

Fig. 3. SEM images of In(OH)3 precursors (a, b), pure In2 O3 nanocubes (c, d) and Au-loaded In2 O3 nanocubes (e, f), mapping patterns of Au-loaded In2 O3 nanocubes (g).

sors was gradually improved, once the weight ratio of Au is too high, the performances of the sensor began to decline. When the Au nanoparticles are partially or completely in contact with each other, gas-sensing characteristics of the materials would not be controlled by the sensitive In2 O3 , but by the insensitive Au nanoparticles, which suppresses the gas-sensing performances, therefore, control the weight ratio of Au nanoparticles plays an important role for improving the gas-sensing performances. In addition, the distribution of Au nanoparticles on the In2 O3 surface is very uniform. This result also proves that the sample does contain Au nanoparticles and dispersed uniformly. In order to obtain more detailed information about the internal nanostructure and morphology of the as-obtained Au-loaded In2 O3 samples, the TEM analysis was employed. Fig. 4 shows the

TEM images of Au-loaded In2 O3 samples. As we can see in Fig. 4(a) and (b), the size of In2 O3 nanocubes is pretty uniform and there are lots of pores on it. The side length of In2 O3 nanocubes is about 150–200 nm. According to the TEM images we can clearly see the In2 O3 nanocubes are hierarchical structure. The In2 O3 nanocubes are consisted of plenty of nanoparticles and there are a large number of pores between these nanoparticles. The diameter of pores is about 10 nm. Fig. 4(c) shows HRTEM image of the Au-loaded In2 O3 nanocubes. The Au nanoparticles can be observed. From the image, the Au present as a sphere and the diameter is about 15 nm. It is tightly attached on the surface of In2 O3 . Fig. 4(d) exhibits the HRTEM image of lattice fringes of Au and In2 O3 . The spacing of lattice fringes of Au is measured to be about 0.233 nm. It can be assigned to the (111) crystal planes of cubic Au. The spacing of lattice fringes of In2 O3 is about 0.416 nm and it can be assigned to the (211) crystal planes of cubic In2 O3 . The porous structure combines hierarchical possesses great surface area and can improve gas-sensing performances. To obtain the information of the specific surface area and pore size, we used BET and BJH analysis. Fig. 5 presents N2 adsorptiondesorption isotherm curve and pore size distribution plots (inset) of the hierarchical In2 O3 porous nanocubes loaded with Au nanoparticles. The curve shows a typical type-IV isotherm and a hysteresis loop from 0.6 to 1.0 (P/P0 ), and the N2 adsorption quantity also increases with the increase of pressure, which indicated the asobtained sample possesses mesoporous structure. The specific surface area of Au-loaded hierarchical In2 O3 porous nanocubes is calculated to be 38.1 m2 /g. It is much larger than solid In2 O3 particles. From the pore size distribution plots we can see that the main pore diameter is about 8–9 nm. We all know that large specific surface area is very conducive to adsorbed target gas, thereby improving the sensing properties. 3.2. Gas sensing properties To determine the optimum operating temperature of sensors based on the pure In2 O3 porous nanocubes and Au-loaded In2 O3 porous nanocubes, we investigated the responses of two sensors to

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Fig. 5. N2 adsorption-desorption isotherm curve and pore size distribution plots (inset) of Au-loaded In2 O3 nanocubes.

Fig. 6. Response of sensors based on pure In2 O3 porous nanocubes and Au-loaded In2 O3 porous nanocubes to 100 ppm of formaldehyde at different operating temperature.

100 ppm formaldehyde at the temperature from 160 ◦ C to 300 ◦ C. As we can see in Fig. 6, with the increase of working temperature, the responses of pure In2 O3 nanocubes and Au-loaded In2 O3 nanocubes based sensors are both raise. We can clearly see that the maximum response values of pure In2 O3 nanocubes and Au-loaded In2 O3 nanocubes based sensors appeared at 280 ◦ C and 240 ◦ C, respectively. The response value of Au-loaded In2 O3 nanocubes based sensor is about 37, which is doubled compared to the pure In2 O3 nanocubes based sensor. Obviously, after loaded with Au nanoparticles, the response of In2 O3 nanocubes sensor has significant improved and the optimum operating temperature is also lower. Then the response decreases with the increase of temperature after appeared of optimum operating 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 [33]. More gas-sensing properties of pure In2 O3 nanocubes and Auloaded In2 O3 nanocubes based sensors are further investigated by detecting 100 ppm of formaldehyde gas under the optimum operating temperature. Fig. 7(a) shows the response and recovery curves of these two sensors. As we can see, the response time of these two sensors is about 3 s, the recovery time is about 8 s. According to this result, loaded with Au nanoparticles have no significant impact on

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the response time, but it is clearly that after loaded with the Au nanoparticles, the response of the as-fabricated sensor has significantly increased. The response value of Au-loaded In2 O3 nanocubes based sensor is about 2 times higher than that pure one. Fig. 7(b) shows the response curves of the two sensors upon exposure to 10–500 ppm formaldehyde. With the increase of the formaldehyde concentration, the responses of two sensors are both become higher. At low concentration, such as 10 ppm, the sensors have good response, and it can be used to detect low concentration of the target gas. Fig. 7(c) shows the linear relationship of log (S-1) – log (C) plot to formaldehyde. 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 formaldehyde. 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 [34]. As we can see that pure In2 O3 nanocubes and Au-loaded In2 O3 nanocubes based sensors exhibit good linear relationship with the concentration. The slopes of pure In2 O3 nanocubes and Au-loaded In2 O3 nanocubes based sensors are 0.760 and 0.848, respectively. It indicates that the response of Au-loaded In2 O3 nanocubes based sensor increase faster than that pure one accompanied with the increase of gas concentration. To investigate the stability, the two sensors were stored in air and kept working at 240 ◦ C and 280 ◦ C, respectively. The stability of pure In2 O3 nanocubes and Au-loaded In2 O3 nanocubes based sensors are shown in Fig. 7(d). In the fifth cycle test, the response of two sensors are reproducible and have no obvious change, especially the Au-loaded In2 O3 nanocubes based sensor shows higher response, which has great potential for practical applications. From the gas-sensing test results we can clearly see that after loaded with Au nanoparticles, the In2 O3 nanocubes displays higher response, lower operating temperature compared with the pure one, it can be used in many fields. Fig. 8(a) displays the histogram of the response of pure In2 O3 nanocubes and Au-loaded In2 O3 nanocubes based sensors to six kinds of tested gases with a concentration of 100 ppm. As we can see, the sensor based on Au-loaded In2 O3 nanocubes shows higher response to formaldehyde compared with other gases, which indicates the Au-loaded In2 O3 nanocubes based sensor has good selectivity to formaldehyde at 240 ◦ C. After a series of gas-sensing tests, the study of Au-loaded In2 O3 nanocubes based sensor is a very promising object, due to its high response and selectivity, good stability, and lower operating temperature to formaldehyde. As we know, the selectivity is a very important parameter of a gas sensor, the response of a sensor has a significant relationship with the adsorption and reaction of gas molecules on the materials surface [42]. The response is related to the surface structure of the sensing material. Stability is also one of the most important characteristics for the sensors. To investigate the time stability of Au-loaded porous In2 O3 nanocubes based sensor, the sensor was stored in air and kept working at 280 ◦ C for subsequent sensing property tests after the first measurement. The response decreased slightly after 30 days (Fig. 8(b)), suggesting a good stability of the sensing materials originating from the as-prepared Au-loaded porous In2 O3 nanocubes. Table 1 presents the comparison of gas-sensing performances between hierarchical Au-loaded In2 O3 porous nanocubes and other sensing materials toward formaldehyde. It can be clearly observed that the as-obtained hierarchical Au-loaded In2 O3 porous nanocubes based sensor possesses excellent sensing properties, including fast response and recovery time, high response value, compared with other sensors [35–41]. Therefore, it can infer that the sensors based on the hierarchical Au-loaded In2 O3 porous nanocubes displays excellent gas-sensing performances towards formaldehyde, it may use as a potential material in many fields. As it is known, In2 O3 is an important n-type metal oxide semiconductor, when it is exposed to the reducing gas, the resistance

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Fig. 7. (a) Response and recovery curves of the sensors based on pure In2 O3 nanocubes and Au-loaded In2 O3 nanocubes to 100 ppm formaldehyde at respective optimum operating temperature. (b) The response and recovery curves of sensors upon exposure to 10–500 ppm of formaldehyde. (c) The linear relationship of log (S-1)-log (C) plot to formaldehyde. (d) Response and recovery curves of sensors to 100 ppm formaldehyde after 5 cycles of gas in and off.

Table 1 Responses to formaldehyde of some sensors reported in the literature. Sensing materials

Operating temperature (◦ C)

HCHO (ppm)

Response

Response/Recovery times (s)

Refs.

Hierarchical Au-loaded In2 O3 porous nanocubes TiO2 -Ag nanocomposite Au@ZnO core-shell structure Pd-decorated hollow SnO2 nanofibers Au@SnO2 core-shell structure Ag-functionalized In2 O3 /ZnO nanocomposites Au/TiO2 nanocomposite Ag-In2 O3 composite nanorods

240 360 room temperature 160 room temperature 300 room temperature 300

100 200 5 100 50 1000 20 85

37 3.7 10.57 18.8 2.9 283 42 34

3/8 30/45 138/104 2/10 80/62 20/4 26/226 135/160

This work [35] [36] [37] [38] [39] [40] [41]

of In2 O3 will reduce, in contrast, the resistance increases when it is exposed to the oxidizing gas. Based on this principle, the gassensing mechanism of hierarchical In2 O3 porous nanocubes based sensors can be proposed. The detected gas molecules are adsorbed and desorbed on the surface of the sensing materials causes the change of resistance [43,44]. During the test process, the sensors based on pure In2 O3 nanocubes and Au-loaded In2 O3 nanocubes were exposed to air at first, there will be plenty of oxygen molecules adsorbed on the In2 O3 surface, these absorbed oxygen molecules will capture free electrons from the conduction band of In2 O3 to form chemisorbed oxygen ions, such as O2 − , O− , O2− [45]. This progress can be expressed as the following reactions: O2(gas) → O2(ads) −

(4)

O2(ads) + e → O2 O2

−

−

O

(ads)

(ads)

−

(ads)

(5)

−

(6)

−

+ e → 2O −

+e → O

2−

(ads)

(ads)

(7)

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 formaldehyde was injected into the chamber, the formaldehyde molecules reacted 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, decreases the resistance of In2 O3 [47]. The reaction process is as follows [48]: HCHO + 2O− (ads) → CO2 + H2 O + 2e−

(8)

Fig. 9 displays the schematic of sensing mechanism of Au-loaded In2 O3 nanocubes. From the above results we can clearly see that Au nanoparticles are dispersed uniformly on the In2 O3 surface and sufficiently expose their active surface to enhance the gas-sensing properties of In2 O3 . As we know, Au is a kind of noble metal, it can be used as catalysts [49]. For the effects of the noble metals on the improvement of sensor response at present, there are two types of mechanisms are discussed commonly to explain, the chemical and electronic sensitization [50]. 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. As for the electric effects, due to the work function of Au, which is 5.1 eV, is larger than that of In2 O3 , some of the electrons from the conduction band of In2 O3 must enter the Au nanoparticles to equalize the Fermi levels [51]. The transferred electrons will form a faster electron depletion region of In2 O3 at the inter-

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metal and then to the oxide [52]. 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 electronic sensitization role in the gas-sensing performances, the reaction between Au nanoparticles and gas molecules will take place at the Au [53]. The Au nanoparticles can increase the conversion rate of oxygen molecules, thus, adsorbed more oxygen, thus leads to a higher reaction rate and improving the gas-sensing properties. In addition, Au additives can promote In2 O3 to form more active sites on their surface, it can faster the process of formaldehyde gas diffusion and the formation of oxygen species, which is considered to be an important reason for the enhanced response compared with pure In2 O3 nanocubes. In our work, the gas-sensing performance of Au-loaded hierarchical In2 O3 porous nanocubes is significantly better than the pure one. 4. Conclusions In summary, we report the preparation of Au-loaded hierarchical In2 O3 porous nanocubes via an environmentally harmless hydrothermal method with subsequent calcination of In(OH)3 precursors. The side length of hierarchical In2 O3 porous nanocubes is about 150–200 nm with good monodispresity and contains a large number of pores. The size of Au nanoparticles is about 15 nm. The BET surface area of the Au-loaded hierarchical In2 O3 porous nanocubes is calculated to be 38.081 m2 /g and the main pore diameter is 8–9 nm. The pure hierarchical In2 O3 porous nanocubes and Au-loaded hierarchical In2 O3 porous nanocubes based sensors were tested by formaldehyde. The sensor based on Au-loaded In2 O3 nanocubes show highly improved and excellent sensing performance compared with the pure one. It has the lower operating temperature (240 ◦ C), higher response value (about 37), fast response and recovery time (3 s and 8 s, espectively), and good selectivity and stability towards formaldehyde. The In2 O3 nanocubes with porous and hierarchical nanostructures provides more active sites, bigger specific surface area and channels for the gas diffusion, and the Au nanoparticles loaded on the surface promote the reaction between target gas molecules and oxygen ions, resulting in the improvement of the gas-sensing properties to formaldehyde. Fig. 8. (a) Response of sensor based on pure In2 O3 nanocubes and Au-loaded In2 O3 nanocubes to 100 ppm formaldehyde various gases at 280 ◦ C and 240 ◦ C, and (b) stability test of the sensor to 100 ppm formaldehyde over a period of 30 days.

Acknowledgements This work was financially supported by National Natural Science Foundation of China (No. 61102006 and 51672110), Natural Science Foundation of Shandong Province, China (No. ZR2015EM019 and ZR2014EL006), and Shandong Province Higher Educational Science and Technology Program (No. J15LA56). 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.2016.10.023. References

Fig. 9. The schematic of sensing mechanism of Au-loaded In2 O3 nanocubes.

face, which causes a greater band bending at the interface of Au nanoparticles and In2 O3 , which leads to the Schottky barrier and preventing the transportation of electrons. In air, the Schottky barrier height (SBH) is magnified by absorbed oxygen species, and thus the electrical resistance increases. When a reducing gas is oxidized on the noble metal surface, an electron is given back to the noble

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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 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

Please cite this article in press as: S. Zhang, et al., Facile approach to prepare hierarchical Au-loaded In2 O3 porous nanocubes and their enhanced sensing performance towards formaldehyde, Sens. Actuators B: Chem. (2016), http://dx.doi.org/10.1016/j.snb.2016.10.023

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research interests focus on the nanostructures functional materials and ceramics matrix composites. 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.

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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.

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.

Please cite this article in press as: S. Zhang, et al., Facile approach to prepare hierarchical Au-loaded In2 O3 porous nanocubes and their enhanced sensing performance towards formaldehyde, Sens. Actuators B: Chem. (2016), http://dx.doi.org/10.1016/j.snb.2016.10.023