Hydrothermal synthesis and Cl₂ sensing performance of porous-sheets-like In₂O₃ structures with phase transformation

Abstract

A facile hydrothermal route was employed to synthesize the porous-sheets-like In₂O₃ structures without any surfactant and template. The morphologies of the porous-sheets-like In₂O₃ structures consisted of many thin sheets with length of 40–120 nm, and the amount of Fe-doped significantly affected the overall morphologies and the phase transformation of In₂O₃. Furthermore, the formation mechanism of the porous-sheets-like In₂O₃ structure is investigated, which revealed that the doping of Fe plays a significant role in the self-assembled and oriented attachment mechanism of In₂O₃, and the phase transformation of In₂O₃ (the pure bcc-In₂O₃ was transformed into the pure rh-In₂O₃) also contributed to the formation of the porous-sheets-like In₂O₃ structure. Finally, the gas sensing characteristics of the products were studied. The results demonstrated that the sensor based on porous-sheets-like In₂O₃ structures (the coexistence of bcc-In₂O₃ and rh-In₂O₃) exhibited a much higher response (54.7 ± 5.3 for 5 ppm Cl₂) to Cl₂ than those pure bcc-In₂O₃ without Fe (S1) and pure rh-In₂O₃ (S5 and S6) samples, so the phase transformation influences on the gas sensing performance of In₂O₃. The porous-sheets-like In₂O₃ structures (S4) had the biggest surface area (42.5 m² g⁻¹), which contributed to the improvement of the gas sensing characteristics, the gas sensing mechanism were also studied.

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Introduction

As n-type semiconductors with a wide band gap (E_g = 3.6 eV),¹ In₂O₃ has been proven to be a highly sensitive material for the detection of both reducing and oxidizing gases, such as trimethylamine (N(CH₃)₃),² C₂H₅OH,³ HCHO,⁴ CH₃COCH₃,⁵ CO,⁶ O₃,⁷ H₂S,⁸ Cl₂,⁹ and NO₂,¹⁰ et al. To enhance the functional properties of In₂O₃, the size, shape, surface chemistry, defect, and dispersion should be designed carefully. In particular, high surface area and nano-scale dimensions are the key factors to determine the gas response for application as gas sensors.^11–13 Up to now, In₂O₃ nano-/microstructure with different morphologies, such as nanocrystals,^14–16 1D nanowires/nanobelts,^17,18 2D thin films,^19,20 2D nanosheets,²¹ 3D hierarchical structures,^22–24 3D nanotowers^25,26 have been developed. And 2D nanosheets are considered to be the most effective and promising candidates as gas sensors due to their unique structures with low density and high surface area.²⁷

Meanwhile, doping is another simple and feasible approach to improve sensing performance by the way of catalytic effect, decreasing grain size, facilitating adsorption of gas, and so on.^28–30 For example, Zhang et al. found that the self-assembled hierarchical Au-loaded In₂O₃ hollow microspheres with superior ethanol sensing properties which is up to 9 times compared with the pure In₂O₃.³¹ Ding et al. indicate that the Ag doped In₂O₃ (1%) is almost 23 times higher than that of the sensor based on pure In₂O₃ toward 1 ppm NO₂.³² Moreover, the doping metal nanoparticles also can be integrated into the original hierarchical nanostructures, and the morphology would affect directly on the redox reaction at its surface by the doping elements. Wei et al. presented with the increase of Ce doping amount, the average sizes of the flower-like spheres were decrease, and the boundaries of as-prepared In₂O₃ microstructures gradually become more and more unconspicuous.³³ Li et al. found that by introduce of Fe, the flower-like structures (pure In₂O₃) collapsed into thin sheet-based structures (Fe doped In₂O₃).²¹

In₂O₃ has two phases: cubic In₂O₃ (bcc-In₂O₃) and rhombohedral In₂O₃ (rh-In₂O₃).^34,35 The stable form of In₂O₃ is body-centered cubic bixbyite-type crystal (bcc-In₂O₃), while the metastable corundum-type In₂O₃ (rh-In₂O₃) is rhombohedral.³⁶ The corundum In₂O₃ exhibits more stable conductivity than that of the cubic counterpart.³⁷ The rh-In₂O₃ transformed into the bcc-In₂O₃ phase under certain physical and chemical conditions, if the change of crystal structure can reduce the free energy of the system, which may affects the morphology and the gas sensing characteristic of In₂O₃.

Herein, we report a facile hydrothermal route for the phase transformation of In₂O₃ structures (the pure bcc-In₂O₃ was transformed into the coexistence of bcc-In₂O₃ and rh-In₂O₃, then transformed into the pure rh-In₂O₃) with doped an appropriate amount of Fe. The morphologies of the porous-sheets-like In₂O₃ structures consisted of many thin sheets with length of 40–120 nm. The obtained nanomaterials were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) characteristic techniques, ICP-MS instrument, and N₂ adsorption/desorption tests. The gas sensing properties of the resulting materials have also been investigated. The introduction of a small quantity of Fe in the reaction system was found to weigh heavily in the phase transformation of In₂O₃ structures, which affected the gas sensing properties of Cl₂.

Material and methods

Materials

The chemical reagents indium nitrate hydrate (In(NO₃)₃·4.5H₂O), ferric nitrate hydrate (Fe(NO₃)₃·9H₂O), polyvinyl pyrrolidone (PVP), ethylene glycol were of analytical grade and used without further purification. All the chemical reagents were purchased from Shanghai Chemical Co.

Synthesis

In a typical synthesis, 10 mL ethylene glycol was added into 30 mL distilled water under constantly strong stirring for 15 min. Then, 1.5 mmol of In(NO₃)₃·4.5H₂O, 0.1500 g polyvinyl pyrrolidone, and different amounts of Fe(NO₃)₃·9H₂O were added into distilled water under constantly strong stirring. After being stirred for another 30 min, the solutions were transferred into 60 mL Teflon-lined stainless steel autoclaves. The autoclaves were heated at 140 °C for 12 h, and then cooled down to room temperature naturally. The reddish brown precipitates were separated by centrifugation, then washed with distilled water and anhydrous ethanol for several times, respectively, and dried at 80 °C for 6 h. Finally, porous-sheets-like Fe-doped In₂O₃ structures were obtained after a thermal treatment at 500 °C for 2 h. We refer to these samples as S1, S2, S3, S4, S5 and S6, representing pure In₂O₃, and the In/Fe molar ratios of 15 : 1, 12 : 1, 9 : 1, 7 : 1, and 5 : 1, respectively.

Characterization

The phase structure and purity of the as-synthesized products were examined by X-ray diffraction (XRD; X'pert PRO MPD, Philips, Eindhoven, The Netherlands) with Cu-Kα radiation (λ = 1.5406 Å) at 40 kV, 30 mA over the 2θ range 20–70°. The morphology of the obtained samples was investigated by using field emission scanning electron microscopy (FE-SEM; JSM-6701F, JEOL, Tokyo, Japan), and high resolution transmission electron microscopy (HRTEM; Tecnai F30G², FEI, Hillsboro, OR, USA) operated at 300 kV accelerating voltage. The chemical state is examined by high-resolution X-ray photoelectron spectroscopy (XPS) on a spectrometer (ESCALAB250Xi, Thermo Fisher Scientific, Waltham, MA, USA) with a monochromatic Al Kα source. An Agilent 7500a model ICP-MS system (Agilent Technologies, State of California, USA) was used for simultaneous multielement detection of Fe ions. The ICP-MS instrument is a Babington nebulizer with a double pass quartz spray chamber, the frequency RF generator is 10 MHz; the output power is 1220 W; the flow rate for Ar gas, auxiliary gas and nebuliser gas is 20 L min⁻¹, 0.9 L min⁻¹ and 1–1.2 L min⁻¹, respectively; the sample uptake rate is 400 μL min⁻¹; the integration time is 0.1 s; and the wavelength is 206.200 nm. The porous characterization of the In₂O₃ structures was obtained by full analysis of N₂ adsorption/desorption tests (V-Sorb 2800P, Gold APP Instruments Corporation, Beijing, China). The AC impedance spectroscopy was measured with a precision impedance analyzer (4294A, Agilent, Santa Clara, CA, USA).

Gas sensor fabrication and measurements

The basic fabricated process is the same of reference.³⁸ The as-obtained In₂O₃ products were slightly grinded with α-terpineol in an agate mortar to form a gas sensing paste. The paste was coated on an alumina tube with Au electrodes and platinum wires, dried under IR light for several minutes in air, and then sintered at 500 °C for 2 h. A Ni–Cr alloy crossing alumina tube was used as a heating resistor which ensured both substrate heating and temperature control. In order to improve their stability, the gas sensors were aged at 300 °C for 240 h in air.

The gas sensing properties were tested by using a gas response instrument (HW-30A, Hanwei Ltd, Zhengzhou, China). The gas sensing properties of the In₂O₃ structures were tested in a glass test chamber, and the volume of the test chamber was 15 L. In the measuring electric circuit of the gas sensor, a load resistor was connected in series with a gas sensor. The circuit voltage V_c was 5 V, and the output voltage (V_out) was the terminal voltage of the load resistor R_L. The working temperature of the sensor was adjusted by varying the heating voltage V_h. When a given amount of tested gas was injected into the chamber, the resistance of the sensor changed. As a result, the output voltage changed. The gas response (S) is defined as eqn (1) and (2):

For an oxidizing gases:

Response = R_g/R_a (1)

For a reducing gases:

Response = R_a/R_g (2)

which R_g and R_a were the resistance values measured in an oxidizing or reducing atmosphere and air, respectively. For each sample, three sensors were made by the same fabrication process, each sensor was tested three times in the gas sensing testing process, and the gas response values in the text were the average value.

Results and discussion

Crystalline structure and morphology

Fig. 1a shows the XRD patterns of the pure In₂O₃ and Fe-doped In₂O₃ structures of S1, S2, S3, S4, S5 and S6 (from bottom to top). All the strong and sharp diffraction peaks marked by a diamond can be readily indexed to a cubic lattice [space group Ia3̄(206)] of pure In₂O₃ according to the Joint Committee on Powder Diffraction Standards (JCPDS) data card no. 06-0416. All diffraction peaks marked by a circle can be readily indexed to a rhombohedral lattice [space group R3̄c(167)] of pure In₂O₃ according to the JCPDS data card no. 22-0336. The XRD results show that with the increasing of Fe concentration, the pure bcc-In₂O₃ (S1) tended to be transformed into the coexistence of bcc-In₂O₃ and rh-In₂O₃ (S2, S3 and S4), and then transformed into the pure rh-In₂O₃ (S5 and S6).

Fig. 1 (a) XRD patterns of pure In₂O₃ and Fe-doped In₂O₃ structures, and (b) relative concentration of the bcc-In₂O₃ phase with respect to the rh-In₂O₃ phase as a function of In/Fe molar ratios.

According to the three Gaussian profiles fitting of the curves,³⁹ the intensities were determined for the bcc-In₂O₃ (400), bcc-In₂O₃ (440), and rh-In₂O₃ (012), rh-In₂O₃ (110) peaks. The relative phase concentration of the bcc-In₂O₃ phase in respect of the rh-In₂O₃ phase was estimated from profile fitting. The fractions of bcc-In₂O₃ (bcc) and rh-In₂O₃ (rh) phases were determined using the relations of eqn (3) and (4):

rh = (1 − bcc) (4)

As expected, the bcc-In₂O₃ phase fraction decreased with the decreasing of In/Fe molar ratios, as shown in Fig. 1b. Pure bcc-In₂O₃ phase was presence without Fe doped, while the coexistence of bcc-In₂O₃ and rh-In₂O₃ appeared by introducing of Fe (the In/Fe molar ratios were 15 : 1, 12 : 1 and 9 : 1), with further increasing of Fe content (the In/Fe molar ratios were 7 : 1 and 5 : 1), sole rh-In₂O₃ phase was presence. When the In/Fe molar ratio was 9 : 1 (S4), the bcc-In₂O₃ fraction was 36.8%, while the intensity of the rh-In₂O₃ reflection was 63.2%, indicating the presence of the coexistence of bcc-In₂O₃ and rh-In₂O₃. The degree of bcc-In₂O₃ remained unchanged throughout the two-phase region, only the relative amounts of the bcc-In₂O₃ and rh-In₂O₃ phases changed.

The cell parameters for bcc-In₂O₃ and rh-In₂O₃ and the size of the crystallites determined with the Scherrer formula were listed in Table 1. As can be seen, the above calculated lattice constants compare well with the literature values of a = b = c = 10.118 Å (bcc-In₂O₃, JCPDS 06-0416), and a = b = 5.487 Å, c = 14.510 Å (rh-In₂O₃, JCPDS 22-0336). The size of the crystallites were 12.62, 11.97, 13.63, 10.84, 12.90, 13.11 nm for S1, S2, S3, S4, S5 and S6, respectively.

Table 1 Cell parameters and the size of pure In₂O₃ and Fe-doped In₂O₃ structures, and the actual Fe³⁺ contents

Sample	bcc-In2O3 (cell parameters)	rh-In2O3 (cell parameters)		D (nm)	Theoretical In/Fe (molar ratio)	Actual Fe^3+ contents (wt%)
	a = b = c (Å)	a = b (Å)	c (Å)
S1	10.0956	—	—	12.62	—	—
S2	10.1109	5.4406	14.3219	11.97	15 : 1	1.20
S3	10.1201	5.4542	14.3207	13.63	12 : 1	1.49
S4	10.1268	5.4623	14.3560	10.84	9 : 1	1.97
S5	—	5.4590	14.3614	12.90	7 : 1	2.46
S6	—	5.4516	14.4008	13.11	5 : 1	3.41

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The concentration of Fe element in Fe-doped In₂O₃ structures was determined by using ICP-MS instrument, as shown in Table 1. The results indicate that the Fe³⁺ content (wt%) is very low in different amounts of Fe-doped samples, and the Fe³⁺ content increased with the decreasing of In/Fe molar ratios. The actual Fe³⁺ contents were 0, 1.20%, 1.49%, 1.97%, 2.46%, 3.41% (wt%) for S1, S2, S3, S4, S5 and S6, respectively.

Fig. 2a–f shows the SEM images of the pure bcc-In₂O₃, the coexistence of bcc-In₂O₃ and rh-In₂O₃ structures, and the pure rh-In₂O₃ structures. They indicate that Fe doping plays an important role in controlling the phase transformation and the morphology of In₂O₃. As shown in Fig. 2a, pure bcc-In₂O₃ consists of cubes with a size of 80–400 nm. Upon the introduction of Fe, the morphology of In₂O₃ changed into a mixture of cubes and porous thin sheets. With the increase of the amount of Fe doping, the number of cubes decreased, while the number of porous thin sheets increased, as shown in Fig. 2b–f. As depicted in Fig. 2b, very few porous thin sheets were present in S2 sample, with a size of 30–50 nm. Upon further increasing the Fe doping concentration (S3–S6), the amount and the size of cubes decreased sharply, while most of the In₂O₃ nanostructures consisted of porous thin sheets with larger size. When the In/Fe molar ratio was 9 : 1 (S4), as shown in Fig. 2d, a mass of porous thin sheets was appeared, with length of 40–120 nm, meanwhile, there existed very few cubes with length of 70 nm. On further increasing the Fe doping amount (S5 and S6), sole rh-In₂O₃ phase was presence. As can been seen from Fig. 2(e and f), some of the porous thin sheets agglomerated and grown into larger flakes, the size of the flakes increased further to 0.6–1 mm, which may destroy the morphology and affect the Cl₂ sensing performance of In₂O₃.

Fig. 2 SEM images of (a) S1, (b) S2, (c) S3, (d) S4, (e) S5 and (f) S6 (inset: high-magnification SEM images of S1–S6).

Transmission electron microscopy (TEM) is then employed to gain further insight into the porous-sheets-like Fe-doped In₂O₃ structures. Fig. 3a shows the TEM image of the porous-sheets-like In₂O₃ structures (S4), it shows that the diameter of the porous thin sheets is 50–100 nm, which is consistent with the value estimated in the SEM image (Fig. 2d). The corresponding HRTEM image (Fig. 3b) exhibits well-defined lattice fringes, and two kinds of lattice spacing can be observed. The lattice spacing of 0.291 nm corresponds to the (222) plane of bcc-In₂O₃, and the lattice spacing of 0.396 nm corresponds to the (012) plane of rh-In₂O₃.

Fig. 3 (a) TEM and (b) HRTEM images of the porous-sheets-like Fe-doped In₂O₃ (S4); SAED patterns taken from the corresponding areas marked (c) A1 and (d) A2; and (e) the EDX spectrum of S4.

Fig. 3c and d show the SAED patterns taken from the corresponding marked areas of A1 and A2, respectively. Fig. 3c indicates that the porous-sheets-like Fe-doped In₂O₃ grows along the [012] direction for rh-In₂O₃, while Fig. 3d demonstrates that the crystals grew along the [222] direction for bcc-In₂O₃, which is consistent with the values estimated from the HRTEM image (Fig. 3b). The EDX spectroscopy (Fig. 3e) shows that the porous-sheets-like Fe-doped In₂O₃ structures are elementally composed of In, Fe and O. The atomic ratio for In, Fe and O calculated from the EDX analysis was In/Fe/O = 31.01 : 1.66 : 67.31 (atomic ratio).

To further investigate the chemical state of the containing elements in the porous-sheets-like Fe-doped In₂O₃ structures (S4), the XPS data were collected and are presented in Fig. 4. The fully scanned spectra (Fig. 4a) shows the survey spectrum of Fe-doped In₂O₃, which indicates that the surface area of the synthesized material has elements of In, O, C and Fe. The C element is ascribed to adventitious carbon-based additives and the C 1s, whose banding energy peak locating at 284.6 eV, is used as reference for calibration.¹⁶ The high resolution XPS spectrum of O 1s in Fig. 4b could be resolved to two Gaussian function peaks with the energy of 530.3 eV and 531.5 eV,³² which are attributed to two kinds of oxygen species on the surface of the material. The O 1s core level spectrum recorded on the sample was a little asymmetric, because of α peaks are associated with lattice oxygen of In₂O₃ and the β peaks are arisen from the surface hydroxyl oxygen of In₂O₃.³¹ The In 3d spectrum (shown in Fig. 4c) has two strong peaks at binding energy of 444.6 and 452.0 eV. They can be respectively indexed to the characteristic spin–orbit split states of In 3d_5/2 and In 3d_3/2 originated from In–O in In₂O₃ lattice,⁴⁰ indicating an In oxidation state of +3. Fig. 4d shows the high-resolution XPS spectrum of Fe 2p. It reveals the doublet Fe 2p_3/2 and 2p_1/2 with binding energies of 710.9 and 724.8 eV, respectively. Both peaks are accompanied by satellite structures with higher binding energy (approximately 8 eV), which is characteristic of the Fe³⁺ species.⁴¹ The XPS analysis results show the atomic ratio for In, Fe and O was In/Fe/O = 31.15 : 1.58 : 67.27 (atomic ratio). Hence, we have successfully synthesized Fe-doped In₂O₃ structures.

Fig. 4 XPS spectra of the porous-sheets-like Fe-doped In₂O₃ (S4): (a) fully scanned spectra, (b) O 1s, (c) In 3d, (d) Fe 2p.

Determination of specific surface area and porosity

Fig. 5a and ESI Fig. A(1–5)† show the N₂ adsorption/desorption isotherms of the pure In₂O₃ and Fe-doped In₂O₃ structures. According to the IUPAC classification, the similar N₂ adsorption/desorption isotherms of the six samples can be classified as a type IV isotherm, with a hysteresis loop where desorption required definitively higher energy than adsorption. The isotherms of the porous-sheets-like Fe-doped In₂O₃ (S4) show a hysteresis loop at a relatively high pressure indicating the largest surface area. The amount of N₂ adsorbed was higher for S4 (459.8 cm³ g⁻¹) than S1 (178.8 cm³ g⁻¹), S2 (232.5 cm³ g⁻¹), S3 (323.9 cm³ g⁻¹), S5 (402.3 cm³ g⁻¹), or S6 (366.3 cm³ g⁻¹).

Fig. 5 N₂ adsorption–desorption curves (a) and the pore size distribution (b) of the porous-sheets-like Fe-doped In₂O₃ (S4).

The pore size distributions are reported in Fig. 5b, apparently, the pore size of the porous-sheets-like Fe-doped In₂O₃ (S4) concentrate between 2 and 10 nm. The results suggest that a small pore size distribution and uniform pore structure were obtained. The steepness of desorption branch verified the uniformity of the pore diameter with narrow distribution. Moreover, it is evident that both small and large pore diameters were in the mesopore region. The pores in the porous-sheets-like Fe-doped In₂O₃ structure also has been proved by the SEM image (Fig. 2d) and the TEM image (Fig. 3a).

Phase transformation mechanism and formation mechanism of the porous-sheets-like Fe-doped In₂O₃ structures

We found that the concentration of Fe-doped determine both crystal phase and morphology of In₂O₃ in the final products.

According to some literatures,^38,42,43 the two phases of In₂O₃ (bcc-In₂O₃ and rh-In₂O₃) will transform to each other when sufficient energy is made available, although the rh-In₂O₃ is the so-called metastable states and the bcc-In₂O₃ is stable states. In certain physical and chemical conditions, if the change of crystal structure reduced the free energy of the system, the polymorphism transformation was inevitable. The added Fe³⁺ ions changed the growth rate in the crystal plane of bcc-In₂O₃ phase, so the displacive transformation of bcc-In₂O₃ structure happened and transformed into the rh-In₂O₃ phase. In result, some bcc-In₂O₃ phase is transformed into the rh-In₂O₃ phase, with further increasing of Fe content, sole rh-In₂O₃ phase was presence. XRD patterns of the samples obtained at different In/Fe molar ratios of 0, 15 : 1, 12 : 1, 9 : 1, 7 : 1, and 5 : 1 reveals the tendency of phase transformation, which is shown in Fig. 1a. Without Fe, the obtained sample was bcc-In₂O₃ (S1), when In/Fe molar ratios was increased to 15 : 1–9 : 1 (S2, S3 and S4), the prepared samples were mixture of bcc-In₂O₃ and rh-In₂O₃, and upon the increase of the amount of Fe, relative amounts of the rh-In₂O₃ increased. Pure rh-In₂O₃ was obtained when In/Fe molar ratios reached to 7 : 1 and 5 : 1 (S5 and S6).

The formation mechanism of the porous sheets-like Fe-doped In₂O₃ structures was also proposed. The whole growth process was illustrated in the scheme of Fig. 6. The concentration of Fe-doped also determine the morphology of In₂O₃, and the corresponding SEM images are shown in Fig. 2a–f. Without Fe (S1), cubes were formed. By increasing the In/Fe molar ratios to 15 : 1 (S2), some porous thin sheets appeared with large number of cubic. Upon further increasing the Fe doping concentration of (S3–S6), the amount of cubes decreased sharply, while most of the In₂O₃ sample consisted of porous thin sheets. However, more Fe resulted in the agglomeration of the porous thin sheets (S5 and S6). These experimental results reveal that the amount of Fe affects the morphology.

Fig. 6 Formation schematic of the porous-sheets-like Fe-doped In₂O₃ structures.

Without Fe, the obtained sample was pure bcc-In₂O₃, the bcc-In₂O₃ was simply enclosed by {001} faces because these faces have the slowest growth rate and lowest surface energy. The cubic shape is consistent with the cubic crystal structure of In₂O₃.⁴⁴ In the In₂O₃ cubic structure, the {001} family of planes contain three equivalent planes, (100), (010), and (001), which are perpendicular to the three directions [100], [010], and [001], respectively. The In₂O₃ nanocrystallites grow in all three directions at an equal speed.^45,46 Consequently, the cubic morphology of the product enclosed with crystal faces of {001} is obtained (S1 in Fig. 2a).

After the doping of Fe, the bcc-In₂O₃ structure was transformed into the rh-In₂O₃ phase. During the hydrolyzation process of rh-In₂O₃, the generation rate of the In(OH)₃ nanoparticles was slow in solution. The relative slow generation rate of In(OH)₃ is favorable for the subsequent growth of 2D nanosheets-like-structures along with the determined direction. Then, these primary nanoparticles self-assembly by oriented attachment aggregated into sheets (S2–S6 in Fig. 2b–f). As the larger ionic radius induces the higher diffusion barrier, the diffusion coefficient is lower with a bigger radius.⁴⁷ Because of the larger size of indium ions (In³⁺: 80 pm, Fe³⁺: 64 pm), the diffusion of iron ions through the In₂O₃–Fe interface is faster than that of indium ions to the In₂O₃–Fe interface, the gradual inward diffusion of iron ions leads to the increase of the overall size of the porous thin sheets. So, with increasing of Fe-added amount, the diameter of porous thin sheets increased, as shown in Fig. 2b–d. With further increasing of Fe-doped amount (S5 and S6), the diameter of porous thin sheets continues to increase, some of the porous thin sheets agglomerated and grown into larger flakes, as shown in Fig. 2e and f.

Gas sensing properties

Fig. 7 displays the plots of gas response versus the gas concentration when the sensors based on the samples with different amounts of Fe were exposed to Cl₂ with the concentration ranging from 5 to 100 ppm at 300 °C. The results show that the response increased for each sensor with the increasing of the concentration of Cl₂. The sensors based on Fe-doped structures exhibit a much higher response than those based on the pure In₂O₃ structures (S1), and the In/Fe molar ratio 9 : 1 (S4) sensor has the highest response. The appropriate addition of Fe would be beneficial to the improvement of the gas sensing properties, but superabundant addition may reduce the available adsorption sites and worse the gas sensing properties. It reveals that the porous-sheets-like Fe-doped In₂O₃ structures (S4) sensor has the highest response which can reach 54.7 ± 5.3, 180.3 ± 18.8, 517.3 ± 52.2, 1186.8 ± 117.1, 1752.5 ± 169.9 for 5, 10, 30, 50, and 100 ppm Cl₂, respectively. Therefore, the following study of the selectivity was focused on the S4 sensor.

Fig. 7 Gas response of sensors based on samples with different amounts of Fe doping, exposed to Cl₂ at concentrations ranging from 5 to 100 ppm at 300 °C, and the single point surface area of S1–S6. The error bars show the standard deviation between the three gas sensing tests of the three sensors made by the same fabrication process.

The single point surface area was clearly the largest for S4 (42.5 m² g⁻¹) than S1 (18.3 m² g⁻¹), S2 (24.6 m² g⁻¹), S3 (28.1 m² g⁻¹), S5 (36.9 m² g⁻¹), or S6 (33.7 m² g⁻¹), which is shown in Fig. 7. The increase in surface area for S4 is due to its porous-sheets-like structural features, which was evidenced by the SEM images, TEM images and the pore size distribution (BET). With the support of the pores in the surface of the porous-sheets-like structures, the BET specific surface became larger. The larger the surface area, the easier the mass transport of Cl₂ in the material. So, the porous-sheets-like Fe-doped In₂O₃ (S4) possess excellent gas sensing characteristics.

As we know, the gas-sensing properties of a sensor have an important relationship with the operating temperature. To find the optimum detection temperature of the sensors based on the porous-sheets-like Fe-doped In₂O₃ structures (S4), we investigated the sensor responses to 50 ppm Cl₂ at the operating temperature from 80 °C to 300 °C, as indicated in Fig. 8a. From which it can be obviously observed accompanied by the increasing operating temperature, the response values of the sensor increasing. It is mainly owing to activation energy barrier of chem-sorption and surface reactions overcome by the increasing thermal energy.⁴⁸ Such behaviour can be understood by considering the role of the kind of adsorption oxygen and the characteristic of Cl₂, the oxygen adsorption depends on the particle size, large specific area of the material, and the operating temperature of the sensor.⁴⁹ In₂O₃ is typical of the performance of a surface-controlled gas sensor. With increasing the temperature in ambience, the state of oxygen adsorbed on the surface of the as-prepared porous-sheets-like In₂O₃ structures material. The species of physical adsorption oxygen (O₂⁻ (ads)) or chemical adsorption oxygen (O⁻ (ads), O₂²⁻ (ads)) depends on the material,^50,51 while the surface adsorbed oxygen changes with the change of the operating temperature. When the working temperature is lower (<160 °C), most of the adsorbed oxygen is O₂⁻ (ads), indicated as physical adsorption; with the increase of the working temperature (160 °C < T < 300 °C), the O₂⁻ (ads) is transformed into the O⁻ (ads), showed as the chemical adsorption. The reaction rate of chemical oxygen adsorption (O⁻ (ads)) is higher than the physical adsorption (O₂⁻ (ads)).⁵² As the amount of adsorbed oxygen increase with the operation temperature, the responses increase with operating temperature. When the Cl₂ was injected in the test chamber, the Cl₂ was adsorbed on the surface of the gas sensing materials, and then reacted with the oxygen adsorbed on the surface of the In₂O₃, leading to an increase in sensor resistance.

Fig. 8 Gas response of sensor as a function of sample (a) typical response and recovery curve of the sensor based on the porous-sheets-like Fe-doped In₂O₃ structures (S4) exposed to 50 ppm Cl₂ at different working temperatures. (b) Temperature dependence of the sensor gas response to 50 ppm Cl₂. The error bars mean the standard deviation between the three times in the gas sensing testing of all the three sensors made by the same fabricated process.

Moreover, it can be seen from Fig. 8b that the response increased with the operating temperature, when the operating temperature was 300 °C, the response was 1186.8 ± 117.1 for 50 ppm Cl₂. Therefore, we chose 300 °C as the operating temperature for the subsequent detections of the porous-sheets-like Fe-doped In₂O₃ structures.

Under the optimum operating temperature of 300 °C, the typical response/recovery curve of the porous-sheets-like Fe-doped In₂O₃ structures (S4) to various concentrations of Cl₂ (5–100 ppm) is displayed in Fig. 9a. This response transient indicated that the interaction between the porous-sheets-like Fe-doped In₂O₃ structures and Cl₂ was reversible with a fast equilibration time. The porous-sheets-like Fe-doped In₂O₃ structures (S4) exhibited excellent response in the range of 5–100 ppm Cl₂. With the increase of the Cl₂ concentration, the responses of the sensor become higher. At low concentration, such as 5 ppm, the sensors have good response (S = 54.7 ± 5.3), indicating that a high gas response can be achieved in detecting low concentration Cl₂ using the porous-sheets-like Fe-doped In₂O₃ structures as sensing material. Furthermore, the sensor showed a quick response and short recovery time. When exposed to 50 ppm Cl₂, the response and recovery time (defined as the time required to reach 90% of the final equilibrium value) is 2 s and 5 s, respectively, indicating the fast response and quick recovery of the porous-sheets-like Fe-doped In₂O₃ structures (S4) sensor, as shown in Fig. 9b.

Fig. 9 Gas response of the sensor based on the porous-sheets-like Fe-doped In₂O₃ structures (S4) exposed to Cl₂ at (a) concentrations ranging from 5 to 100 ppm at 300 °C and (b) 50 ppm.

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.⁵³ Fig. 10 displays the histogram of the response of porous-sheets-like Fe-doped In₂O₃ structures (S4) based sensors to eight kinds of tested gases with a concentration of 50 ppm at 300 °C. The tested gases or vapours include toluene, acetone, ammonia, nitrogen dioxide, hydrogen sulfide, formaldehyde, and gasoline, respectively. The porous-sheets-like Fe-doped In₂O₃ structures (S4) sensor showed the highest response to Cl₂ (1186.8 ± 117.1), while its response to toluene, acetone, ammonia, nitrogen dioxide, hydrogen sulfide, formaldehyde, gasoline is 3.5 ± 0.3, 1.8 ± 0.1, 5.3 ± 0.5, 101.9 ± 11.2, 8.1 ± 0.8, 7.2 ± 0.7, and 6.4 ± 0.6, respectively. Clearly, the gas response to Cl₂ is significantly higher than that to the other tested gases, with a magnitude about 11.6–659.3 times greater to 50 ppm Cl₂ than that for the other tested gases under the same concentration. The above results indicates the porous-sheets-like Fe-doped In₂O₃ structures (S4) sensor has good selectivity to Cl₂ at 300 °C.

Fig. 10 Selectivity of the porous-sheets-like Fe-doped In₂O₃ structures (S4) sensor to Cl₂ with a concentration of 50 ppm at 300 °C.

The stability of the porous-sheets-like Fe-doped In₂O₃ structures (S4) sensor to Cl₂ with a concentration of 50 ppm at 300 °C is shown in Fig. 11. The sensor was stored in air and kept working at 300 °C for subsequent sensing property tests after the first measurement. The results show that the response decreased over time, but the response was still very high even after 30 days, indicating a good stability in a natural environment.

Fig. 11 Stability of the porous-sheets-like Fe-doped In₂O₃ structures (S4) sensor to Cl₂ with a concentration of 50 ppm at 300 °C. (Inset: (a) gas response of the sensor for the first day (b) the 30^th) the error bars mean the standard deviation between the three times in the gas sensing testing of all the three sensors made by the same fabrication process.

Based on the above results, it is reasonable to believe that the porous-sheets-like Fe-doped In₂O₃ structures (S4) sensor is potentially applicable to detecting the Cl₂ concentration in our living environment, due to its high response, short response–recovery time, excellent selectivity and good stability.

Gas sensing mechanism

The gas sensing mechanism of the porous-sheets-like Fe-doped In₂O₃ structures was also discussed.

Firstly, the as-prepared porous-sheets-like In₂O₃ structures adsorbs oxygen from the air and captures free electrons from the conduction band which causes the chemisorbed negatively charged oxygen ions (O₂⁻, O⁻ and O²⁻) and electron-depleted region generated on the surface, thus leading to the formation of a thick space charge layer and an increase of surface band bending. However, only O₂⁻, O⁻ could be formed when the temperature is lower than 300 °C.⁵⁴ Then it would result an increase on the resistance. These adsorption processes can be expressed as eqn (5) and (7).

O₂ (gas) ⇔ O₂ (ads) (5)

O₂ (ads) + e⁻ → O₂⁻ (ads) (6)

O₂⁻ (ads) + e⁻ → 2O⁻ (ads) (7)

When Cl₂ is introduced in this condition, as the highly reactive oxidizing gas, Cl₂ intensely capture electrons from the conduction band due to its higher electrophilic property, and reacted with the adsorbed oxygen species leading to the formation of adsorbed Cl⁻ (ads), while the electron-depleted region is then further thickened. As a result, the resistance of the In₂O₃ sensor greatly increases. For the resistance increase, Cl₂ molecule is negatively adsorbed on In₂O₃ to attract electrons from In₂O₃ (eqn (8)). On the other hand, Cl₂ molecule is substituted with adsorbed oxygen (O²⁻(ads)) or lattice oxygen (O_x²⁻) to release electrons into In₂O₃ for resistance decrease (eqn (9) and (10)).⁵⁵

Resistance increase:

Resistance decrease:

or

The above reactions decrease the carrier concentration and electron mobility on the sensor surface, which led to the increase of depletion layer width accompanied by an increase in resistance. The electron transfer between In₂O₃ and Fe also led to the formation of an accumulation layer on the surface of the porous-sheets-like Fe-doped In₂O₃ structures. On the other hand, the trapped electrons were released to the porous-sheets-like Fe-doped In₂O₃ structures by Cl₂ after the supply of Cl₂ was stopped, leading to a decrease of the resistance.

The enhanced sensing performance of the porous-sheets-like Fe-doped In₂O₃ structures can be ascribed to its large BET surface area. The porous-sheets-like Fe-doped In₂O₃ structures could provide more available active surface areas because of the unique porous microstructure and its own good physicochemical properties, thus enhancing the reaction between Cl₂ and the adsorbed oxygen at the optimum operating temperature of sensor. The porous microstructure also increased the BET surface area of Fe-doped In₂O₃ structures. As the porous-sheets-like Fe-doped In₂O₃ structures (S4) had the largest BET surface area (42.5 m² g⁻¹), the sensor could absorb more Cl₂, the resistance's increasing and the resistance's decreasing became more notable, which can enhance its sensing performance.

In order to observe clearly dielectric response of the pure In₂O₃ and Fe-doped In₂O₃, AC impedance spectroscopy of In₂O₃/Fe sensor with different amounts of Fe doping in the frequency range of 100 Hz to 10 MHz at 300 °C (50 ppm Cl₂) are shown in Fig. 12. Upon the introduction of Fe, the diameter of semicircle of AC impedance spectroscopy enlarged, the impedance increased, too. The AC impedance spectroscopy of the sensor based on S4 shows the largest semicircle (the inserted in Fig. 12), which is far larger than the value of sensors based on S1, S2, S3, S5, and S6. This is mainly due to the largest surface area of the S4 sample, which leading to the largest resistance. With further increasing of Fe doping concentration, the surface area decrease, and the resistance declined. It agrees well with SEM images, N₂ adsorption/desorption curve and the BET surface area value. More details of the enhancing effect of the porous-sheets-like Fe-doped In₂O₃ structures on sensing properties need further investigation.

Fig. 12 AC impedance spectroscopy of S1, S2, S3, S4 (inset), S5 and S6 based sensors.

Conclusions

A facile hydrothermal route for the phase transformation of In₂O₃ structures (the pure bcc-In₂O₃ was transformed into the pure rh-In₂O₃) without any surfactant and template was discussed. SEM images showed the molar ratio of In/Fe profoundly affected the morphologies of the In₂O₃/Fe composites and the porous-sheets-like Fe-doped In₂O₃ structures was obtained when the molar ratio of In/Fe was 9 : 1 (S4). A possible growth mechanism of the porous-sheets-like Fe-doped In₂O₃ structures has been proposed. The gas sensing measurements demonstrated that the porous-sheets-like Fe-doped In₂O₃ structures (S4) sensor exhibited the highest response of 54.7 ± 5.3 for 5 ppm Cl₂ at 300 °C. That is due to the large specific surface area of the porous-sheets-like Fe-doped In₂O₃ structures. Moreover, the sensor showed quick response–recovery behaviour, excellent selectivity and stability. Therefore, it is expected that this facile route to prepare the porous-sheets-like Fe-doped In₂O₃ structures would be an ideal candidate for applications in Cl₂ sensors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Science Foundation of Department of Education of JiangXi Province (No. GJJ150677), the National Natural Science Foundation (61463020).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra10201a

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