Ultrasensitive sensing performances of amphiphilic block copolymer induced gyrus-like In₂O₃ thick films to low-concentration acetone

Abstract

In the present work, an inducible assembly of di-block polymer compounds approach was employed for the synthesis of mesoscopic gyrus-like In₂O₃ by using lab-made high-molecular-weight amphiphilic di-block copolymer poly(ethylene oxide)-b-polystyrene (PEO-b-PS) as a revulsive, with indium chloride as an indium source and THF/ethanol as the solvent. The obtained mesoscopic gyrus-like In₂O₃ indium oxide materials exhibit a large surface area and a highly crystalline In₂O₃ nanostructure framework, and the gyrus distance is about 40 nm, which can facilitate the diffusion and transport of acetone vapor molecules. By using this material as a chemoresistance sensor, the obtained gyrus-like indium oxides were used as sensing materials, showing an excellent performance to acetone at a low operating temperature (150 °C) due to their high porosity and unique crystalline framework. The limit of detection of the thick-film sensor based on indium oxides is appropriate for diabetes exhaled breath acetone concentration detection. Moreover, the thick-film sensor shows a very fast response–recovery dynamics upon contacting acetone vapor due to its abundant open folds mesoscopic structure, and also to the large surface area of the nanocrystalline gyrus-like In₂O₃.

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1. Introduction

Acetone is widely used in many fields of industrial production because of its chemical activity, which is considered as one of the important raw materials, solvents and combustible gases.^1,2 In addition, in human breath there are hundreds of species of volatile organics which are exhaled from the blood through breath in the lungs. Expiratory analysis has attracted much attention among potential diagnostic techniques because of its noninvasive and real-time diagnostic advantages. In the breath of healthy people, the average concentration is below 0.9 ppm, while in diabetics, it is usually above 1.8 ppm.^3–8 Therefore, acetone can be used as a good biomarker for detecting diabetes in breath analysis. In order to detect diabetes mellitus by using an exhaled gas sensor, it is required that gas sensing material should show a large response under the condition that the concentration of acetone is very low, especially in a humid environment and under a complex background gas with different oxygen concentrations.

Many binary oxides or their composites (such as SnO₂,^9–12 ZnO,^13–16 WO₃,^17–19 NiO,^20,21 Fe₂O₃,^22,23 Co₃O₄,²⁴ TiO₂,^25–28 CuO–ZnO,²⁹ and ZnO–In₂O₃ (ref. 30)) have been found to be useful as acetone sensors. In recent years, indium oxide-based gas sensing materials have attracted extensive attention in the detection of low-concentration acetone gas. Gas sensors based on indium oxide and its derivatives react well to acetone. Different oxide morphologies (such as thick film, thin film, nanoparticles, bulk) have a great influence on the gas sensing performance of gas sensors.^31–40 In addition, the response to various reducing gases can be improved by compounding certain materials in inorganic oxides and perovskite oxides, such as LaFeO₃, SmFeO₃, NdFeO₃, Yb₂Fe₃O₇, CdSnO₃, La_1−xPb_xFeO₃ etc.^41–54 In addition, lots of efforts have been made in order to synthesize highly effective indium oxide to improve the performance of In₂O₃-based nanomaterials and devices with different morphologies and sizes to promote the guest gas molecules' diffusion. Meanwhile, increasing the surface area provides abundant exposed active sites for kinds of interface adsorption, electron transport, reactions and host–guest interactions.

Different forms of In₂O₃ nanostructures which have various forms and morphologies, such as nanosheets, nanowires, nanospheres, nanotubes, nanoflowers, and layered nanostructures, are prepared by hydrothermal and solvothermal preparation of In(OH)₃ or InOOH,^55–59 followed by calcination treatment. Methods of electrospinning, chemical vapor deposition, polymer templating, laser ablation, self-templating etc. can also be used to prepare large specific surface area In₂O₃ nanocrystalline materials.^60–70 Acetone sensing properties for various semiconductor sensors are shown in Table 1. However, the above methods often result in ill-defined convoluted structures and uncontrolled morphology and porosity. It was found that these materials have remarkable electrochemical and gas sensing properties due to their large specific surface area and interconnected mesoscopic structure. In addition, the induction synthesis method for the inducer is more suitable and flexible for the production of high-performance sensing materials with large surface area. However, due to the difficulty in controlling the self-assembly and crystallization process of organic templates and inorganic precursors, there are few studies on the synthesis of sensing materials with high sensing efficiency and large surface area by flexible soft template-induced synthesis.

Table 1 Acetone sensing properties for various semiconductor sensors

Author	Materials	T (°C)	Acetone concentration (ppm)	Response	Reference
Suparat Singkammo et al.	Ni-doped SnO2 composite film	350	200	54.2	3
S. B. Patil et al.	Co-doped SnO2 thin films	270	60	32	9
M. Punginsang et al.	Co-doped SnO2 thin films	250	20	36.9	10
R. K. Mishra et al.	SnO2 nanoparticles	250	10	42	11
W. X. Jin et al.	SnO2 nanoflowers	260	25	40	12
S. Wei et al.	ZnO hollow nanofibers	220	1	7.1	13
D. An et al.	ZnO nanomaterials	220	100	6.0	14
C. Peng et al.	ZnO hollow nanofibers	300	100	18.6	15
N. H. Al-Hardan et al.	Cr-doped ZnO films	400	500	90	16
D. Chen et al.	WO3 nanoparticle	300	2	2	17
S. Kim et al.	WO3 with both Pd and Au	300	200	152.4	19
L. Wang et al.	Au-doped NiO hybrid structure	240	20	7.6	20
C. Wang et al.	W-doped NiO films	250	100	198.1	21
Hao Shan et al.	La-doped α-Fe2O3 nanomaterials	240	50	26	22
Chang Su et al.	Sm-doped α-Fe2O3 nanomaterials	240	0.5	2.3	23
Z. Zhang et al.	Co3O4 nanosheets	150	10	1.7	24
M. Epifani et al.	TiO2 nanocrystals	400	100	1.2	26
B. Bhowmik et al.	TiO2 nanotubes	27	10	1.136	27
H. Bian et al.	TiO2 nanorods	500	10	9	28
C. Wang et al.	CuO–ZnO nanoparticles	340	10	3.7	29
X. Chi et al.	ZnO–In2O3 composite nanotubes	280	60	43.2	30
Z. L. Wu et al.	NdFeO3	120	50	300	47
T. Chen et al.	SmFeO3	250	380	2.6	48
X. Liu et al.	SmFe0.9Mg0.1O3	260	300	353	49
X. Liu et al.	LaFeO3	400	80	204	50
L. Zhang et al.	La0.68Pb0.32FeO3	200	50	7	51
S. Zhang et al.	In2O3 nanospheres	350	10	53.08	65
S. Park et al.	In2O3–TiO2	250	1	10	66
F. Chen et al.	In2O3	250	50	12	67
W. Liu et al.	Pt–In2O3	180	10	15.1	68
Y. Che et al.	In2O3	200	100	37.9	69
Z. Song et al.	Pt–In2O3	300	10	113	70

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In this work, mesoscopic gyrus-like indium oxide with two-dimensional porous nanowalls synthesized from a high-molecular-weight di-block copolymer (PEO-b-PS) induced the In₂O₃ simple. Due to the support from the rigidity of the PS segments during the process of synthesis, the mesoscopic structure can be well obtained after ordered annealing in the air. This results in the mesoscopic gyrus-like nano-In₂O₃ with large specific area structure. Mesoscopic gyrus-like In₂O₃ materials have high porosity and unique crystal structures, and have excellent sensing properties to low-concentration acetone, such as fast response and recovery time, detection limit of sub-ppb and good selectivity in different gases. This excellent performance makes possible the use of intelligent gas sensors based on this In₂O₃ structure, which can be used in many fields, especially the non-invasive diagnosis of diabetes.

2. Materials and methods

2.1. Synthesis of mesoscopic grooved indium oxide

The laboratory-prepared amphiphilic block copolymer PEO112-b-PS165 synthesized according to an existing related research report⁶⁴ was dissolved in 4.5 mL THF, and then a solution can be obtained. And then, 0.3 g of indium trichloride was dissolved in 3 mL ethanol. The two solutions were then mixed and stirred for half an hour to form a transparent and colorless solution. Then the solution was poured into a watch glass, the solvent was placed at room temperature for 2 hours (ambient humidity was controlled to <40% in order to prevent the InCl₃ hydrolysis reaction with the water in the air), and then heated continuously at 60 °C for 5 hours and at 100 °C for 12 hours. Then the translucent film in a glass container was calcined in a muffle furnace at 400 °C with appropriate amount of CaO₂ for 30 minutes at a rate of 4 °C min⁻¹. Finally, the rolled film was ground into a yellow uniform powder.

2.2. Synthesis of mesoscopic grooved In₂O₃ nanocrystals

Fig. 1 illustrates the synthesis of indium oxide, firstly with the mixed solution including PEO112-b-PS165 di-block copolymer, ethanol, InCl₃, and THF. With the solvent (mainly THF) at an ambient temperature of about 25 °C, the dissolved PEO-b-PS copolymer then gradually aggregated into columnar micelles with the PS segment as the gyrus because of the hydrophilic and hydrophobic end-to-end arrangement. Simultaneously part of the hydrolyzed hydrophilic inorganic indium interacted with the end of PEO by weak coordination bonds and hydrogen bonds. With the THF in the solution evaporating, a uniform and transparent rolled film formation was observed in the watch glass. Finally, the organic–inorganic hybrid membrane attached on the watch plate was prepared at 400 °C for 30 min. Calcium oxide was used to promote the decomposition of PEO-b-PS organics, the mesostructure being protected by the rapid release of oxygen. A series of mesoscopic gyrus-like In₂O₃-x-y-z nanocrystalline materials were synthesized, where x, y, z are related to the proportion by weight of PEO-b-PS polymer and InCl₃, the annealing temperature and the time, respectively. Fig. 1 illustrates the induced assembly process for the mesoscopic gyrus-like In₂O₃ starting from the solution with the mixture containing PEO-b-PS di-block copolymer etc.

Fig. 1 Illustration of the synthesis of mesoscopic gyrus-like In₂O₃.

A certain amount of indium oxide product synthesized by induction was mixed with an appropriate amount of deionized water, and then the mixture was loaded on an Al₂O₃ ceramic tube with two electrodes at one end in the form of a thick film. The ceramic tube was approximately 10 mm long, with an outer diameter of approximately 8 mm and an inner diameter of approximately 5 mm (Fig. 2). To improve the repeatability and stability, the gas sensor should be calcined at 200 °C for 3 h. The gas sensing properties of the nanocrystalline material were measured in the temperature range of 100–200 °C. The resistance of the sensor was measured in the air and guest gas (test), dry air being used as the carrier gas. The sensitivity of the thick-film sensor to the test gas is defined as S = R_a/R_g, where R_a is the resistance in air and R_g is the resistance in the test gas.

Fig. 2 Schematic of the thick-film sensor.

3. Results and discussion

3.1. The X-ray diffraction (XRD) and the scanning electron microscope (SEM)

Wide angle X-ray diffraction (XRD) measurements showed that the In₂O₃-400-0.5 sample obtained after calcination of the prepared PEO-b-PS/In₂O₃ showed excellent resolved diffraction peaks in the 20–80° range (Fig. 3), consistent with the crystal-centered cubic phase of In₂O₃ (JCPDS no. 06-0416), which has no other diffraction peaks. Also, no other crystalline impurities were detected in the XRD pattern, indicating that In₂O₃ was purely crystalline. According to scanning electron microscopy (SEM), mesoscopic outer In₂O₃ indium oxide materials are observed, with an orderly walnut shape of the outer structure. With the change of calcination temperature, the nano structure of the samples shaped to be layer and block through carefully observation. According to the SEM images, the outer gap diameter is estimated at about 40 nm (Fig. 4).

Fig. 3 The X-ray diffraction patterns of In₂O₃ annealed at different temperatures.

Fig. 4 SEM images of mesoscopic gyrus-like In₂O₃ annealed at 400 °C for 30 min.

In order to investigate the crystallization behavior of In₂O₃ gyrus walls during the annealing process, non-PEO-b-PS-In₂O₃ hybrid nanocrystalline material was prepared and calcined at 400 °C and 450 °C for 0.5 h in air with a low-humidity environment. The results show that increasing of the annealing temperature at 400 °C can make In₂O₃ nanograins (without PEO-b-PS) grow large into nanosheets which have larger size about (400 nm × 300 nm), larger thickness (20–25 nm) and higher crystallinity (Fig. 5a). During this heating-up annealing process, the two-dimensional lamella layer morphology remains intact. By comparison, with the calcination temperature increasing from 400 °C to 450 °C, the nano-lamella layer In₂O₃ rapidly disappeared and discrete rhomboid octahedral particles with diameters of 500 nm were formed (Fig. 5b). This means that a calcination temperature as high as 450 °C can cause a significant change in the structure and morphology of gyrus indium oxide, and this is a typical thermodynamic transformation reaction process. And a further extension of the annealing time to 30 min at 450 °C resulted in the formation of In₂O₃ discrete particles with the shape of unique octahedra, which also is a typical stable morphology for In₂O₃ nanocrystals (Fig. 5a and b). Besides, the In₂O₃ particles' inhomogeneity of size was mainly because of the inhomogeneous mass transport between the conversion reactions from nanosheets to lamella layer and octahedral particles (Fig. 5). The XRD patterns of these samples all calcined at 400 °C with different times also confirm the morphological transformation phenomenon (Fig. 3). With the annealing temperature increasing, the diffraction intensity of these samples is gradually increased, indicating that the crystallinity of In₂O₃ is enhanced.

Fig. 5 SEM images of In₂O₃ precursor (without PEO-b-PS) annealed at (a) 400 °C for 30 min and (b) 450 °C for 30 min.

The results above indicate that the copolymer (PEO-b-PS) strongly interacts with indium species (In³⁺) during the assembly process, which reduces the condensation and hydrolysis rates of indium species throughout this synthesis process. Even its presence reduces the indium species' crystallization rate.

3.2. Gas sensing properties of gyrus-like In₂O₃ crystal materials

In₂O₃ has an ordered mesoscopic gyrus-like structure. Because of the low resistivity (high conductivity) of In₂O₃, it has great advantages in high-performance gas sensing (chemical) for its (gyrus structure) high surface area. This not only provides large numbers of O vacancies for the adsorption sites of the sensing gas, but also facilitates the diffusion of guest gas molecules in the surface structure. Besides, due to the charge carriers' fast transport between the surface of gyrus-like structure and the bulk, the prepared In₂O₃ material can provide excellent response/recovery as a chemical gas sensor.

The sensing response of the gyrus-like In₂O₃ sensing film to acetone in the concentration range of 0.5–5 ppm was measured at 150 °C (the content of acetone in diabetic exhaled breath). Based on the definition of S = R_a/R_g, the responses for 0.5, 1, 2, and 5 ppm acetone were calculated to be 1.73, 2.67, 3.12 and 3.75, respectively. Limit of detection (LOD) of exhaled breath for the thick-film sensor is below 0.5 ppm for the detection of acetone, which set the threshold for acetone at greater than 0.8 ppm. In addition, the experimental results show that there is a good concentration saturation relationship between the sensing response (S = R_a/R_g) and the acetone concentration.

When the gas test box was removed, the exposure of the sensor is from acetone vapor to pure air, the acetone gas molecules immediately being desorbed from the indium trioxide surface and releasing the electrons pre-trapped on the surface. The resistance of the gyrus-like In₂O₃ thick-film sensor can almost return to the original value, indicating that the gas sensor has good reversibility (Fig. 6b). The response–recovery time of the thick-film sensor is also very fast. Taking the gyrus-like In₂O₃ sensor with 1 ppm acetone as an example: the response time is 20 seconds and the recovery time is 16 seconds. Compared to previous reports of acetone sensors based on different In₂O₃ nanostructures, this work's sensors showed better overall sensing performance, such as having a much lower operating temperature (150 °C) which is more amenable for fabricating flexible devices, and also higher sensitivity and much lower LOD, which are because of the following factors. Firstly, the In₂O₃ nanocrystalline material has a uniform nano-gyrus structure (about 40 nm) and the meso-grooved In₂O₃ materials not only facilitate the diffusion of guest gas molecules, but also can provide a large number of activation sites for the adsorption and desorption of acetone gas molecules. Besides, the crystalline particles in the mesoscopic In₂O₃ nanocrystalline structure provide fast transport of charge carriers (electrons) between the sensing film surface and the bulk. So the open mesoscopic gyrus-like structure and high crystallinity lead to excellent low concentration acetone sensing performance.

Fig. 6 (a) The response of In₂O₃ nanocrystalline thick-film sensor to 100 ppm acetone at different operating temperatures. (b) The response and recovery curves of the gyrus-like In₂O₃ nanocrystalline sensor to acetone with different concentrations (0.5–5 ppm) at 150 °C. (c) The response (R_a/R_g) of the sensor vs. different acetone concentrations. (d) The selectivity of the In₂O₃ sensor to different guest gases (10 ppm) at 150 °C.

Gas selectivity is an important parameter of gas sensors, especially in the complex environment of human exhaled breath. In this study, acetone, carbon monoxide, ethanol, ammonia, formaldehyde, and other typical gas molecules were chosen as interference and mixed gases. As shown in Fig. 6d, the response to acetone of the sensor was mainly 5 times higher than that to 5 other selected interfering gases. The phenomenon of the excellent selectivity can be explained as follows: as a typical reducibility gas, the adsorbed oxygen with different valence states on the surface of In₂O₃ is substituted by acetone, which has a stronger ability to capture electrons of n-type In₂O₃ semiconductor than other gases.

The mechanism of a chemoreceptive gas sensor at suitable operating temperature is the conversion (resistance becomes larger and smaller) of electrical resistance between the surface reactions of the gas sensing materials. In₂O₃ is a typical n-type semiconductor. When the sensor was exposed to the air, the oxygen in the air can adsorb on the surface of In₂O₃ to form ionic oxygen species (O⁻, O²⁻) by capturing electrons from the conduction band of In₂O₃. Reducing/oxidizing gases can change the surface carrier concentration by capturing electrons from or releasing electrons to the conduction band of In₂O₃, acting as electron-donating groups. It is obvious that the amount of adsorbed oxygen species plays an indispensable role in the performance of the sensor. As the operating temperature increases, the In₂O₃-based sensors exhibit different sensitivity to acetone vapor at >150 °C, which may be due to the small coverage of adsorbed oxygen as well as a small change in resistivity below the LOD.

Therefore, when the gas sensor is exposed to acetone gas, the resistance of the sensor increases sharply. By contrast, the oxygen species may oxidize the interfering gas molecules, and such oxygen species (O⁻, O²⁻) are expended by the interfering gas molecules, and the electrons captured by oxygen are returned to In₂O₃ during the oxidation process. Consequently, setting the same gas sensor exposed to one kind of interfering gas, a response in the negative direction (decrease in resistance value) can be obtained. Under this sensing mechanism, it is obvious that all the typical interfering gases have a limited effect on the detection of acetone gas by the In₂O₃ nanocrystalline sensor. A possible mechanism of In₂O₃ and the sensing gas reaction process is shown in Fig. 7.

Fig. 7 Schematic illustration of the acetone sensing mechanism of the sensor based on gyrus-like In₂O₃ in air and target gas–air mixture.

4. Conclusion

In summary, a di-block copolymer-induced method has been developed for the synthesis of gyrus-like indium oxides, which used high-molecular-weight di-block copolymer PEO-b-PS as a revulsive, and the indium source is indium chloride. THF/ethanol was used as the solvent. By the support of PS segments during the reaction, ordered gyrus-like In₂O₃ was obtained with a crystalline nanostructure of about 40 nm with high surface area. The obtained gyrus-like In₂O₃ nanocrystalline material was fabricated into thick-film sensors which can work at a relatively low temperature of about 150 °C. Also, the thick-film sensor exhibits an excellent sensing performance with a fast response–recovery time. And the LOD of 0.5 ppm is perfect for use as a diabetic exhaled gas (healthy range is 0.3–0.8 ppm) sensor, and shows very fine selectivity, which may be due to the high porosity of the gyrus-like In₂O₃ and the unique nanocrystalline framework. In conclusion, such excellent performance of the material in this work shows promise for fabricating In₂O₃-based sensors for application in non-invasive diagnosis of diabetes mellitus and preliminary screening. Furthermore, the nanocrystalline gyrus-like In₂O₃ with special structure can be expected to be used in many other applications, such as lithium storage, photocatalysis and so on. It has an important practical significance.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by Shandong Natural Science Foundation (no. ZR2019BEM036) and Shandong Jianzhu University Doctor’s Foundation (X19009Z).

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