Synthesis of Zn-doped In₂O₃ nano sphere architectures as a triethylamine gas sensor and photocatalytic properties

Abstract

Zn-doped In₂O₃ nano spheres (ZIO NSs) were synthesized by calcining the precipitates prepared through a facile one-step hydrothermal synthesis method. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), X-ray photo-electron spectroscopy (XPS) and energy dispersive X-ray spectrometry (EDX) were respectively used to characterize surface crystallinity and morphologies. Gas sensors were made based on the as-synthesized materials to investigate their gas sensing properties of triethylamine (TEA). The ZIO NSs microstructures showed that the sensor response (S = R_a/R_g) reached 36 at 50 ppm, which was much larger than the previous results, and this combined with a low operating temperature of 280 °C, suggested a promising application of ZIO NSs in TEA gas sensing. In addition, the photocatalytic activities of ZIO NSs have been evaluated by the degradation measurement of methylene blue (MB), methylene orange (MO), and rhodamine B (RhB). As a result, the ZIO NSs demonstrate a much higher selectivity on the degradation of MB (90%) than that of MO (20%) and RhB (35%) in 60 min of measurement.

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1 Introduction

Over the past decades, nanoscale metal oxides with good gas sensitivity and selectivity have received great attention in lots of fields such as the chemical industries, environmental protection and human health.^1,2 Volatile organic compounds (VOCs), such as benzene, toluene, acetone, formaldehyde, aliphatic and alicyclic amines, are toxic pollutants that are very harmful to human health. Among the different amines, triethylamine (TEA) is one of the main environmental pollutants. TEA is widely used in pharmaceuticals, pesticides, surfactants, biological buffer substances and colorants.³ In addition, chronic exposure to triethylamine gas may cause adverse health effects such as irritation to the dermis, asthma, visual disturbances, vascular headaches and abnormal embryos.^4–6 Therefore, it is very necessary to detect TEA momentarily. Nowadays, many analytical methods have been established for the determination of amines in water, including gas chromatography, electrochemistry analysis, immunoassays, high-performance liquid chromatography (HPLC) and so on.⁷ However, most of these means are limited for the detection of volatile TEA gas for its non-portable character and are often accompanied with a high testing cost. For solving this problem, it is important to seek more suitable gas sensor materials to TEA.

In₂O₃, as an important oxide semiconductor oxide with a direct band gap energy of 3.6 eV, has been widely used in ultrasensitive toxic gas detectors, transparent conductors, and solar cells.^8–10 In₂O₃ has been widely used to detect low-concentration gases, such as methanol, nitrogen dioxide, carbon monoxide, acetone, chlorine, ethanol gas and other species, due to its range of conductance variability and response towards both oxidative and reductive gases.^11–17 However, In₂O₃ nanomaterial sensors for detecting TEA are rarely reported, and investigations on the photocatalytic activity of In₂O₃ nano-/microstructured materials in the degradation of organic pollutants are also quite rare.

Recently, some strategies, such as doping and the formation of semi-conductor heterostructures, have been demonstrated to be efficient ways to improve gas-sensing properties, such as sensitivity, selectivity, and especially lower working temperatures.^18–20 Therefore, Zn-doped In₂O₃ nano sphere architectures would be expected to exhibit excellent gas sensing and photocatalytic properties. During the past decades, In₂O₃ nano-/microstructured materials with various geometric morphologies, including nanowires, hollow spheres, nanotubes and nanocuboids, have been synthesized via the electrospinning technique, carbon spheres template method, solvothermal route, and situ growth method.^21–25 So far, little information is available on the Zn doping effect for gas sensitivity to TEA gas synthesized by a hydrothermal method.

In this paper, we demonstrate a simple novel hydrothermal method for ZIO NSs synthesis. The uniform ZIO NSs exhibit excellent sensitivity and selectivity to TEA gas, in which a high detection response, 36, has been achieved for under 50 ppm TEA and the detection limit can be lowered to 5 ppm at a detection response of 4.92. In addition, the operating temperature is as low as 280 °C. Moreover, the photocatalytic properties of ZIO NSs have been evaluated by the degradation test of MB, MO, and RhB under UV light irradiation. A high photocatalytic degradation selectivity of ZIO NSs on MB (90%), in contrast to that on MO (20%) and RhB (35%), has been observed in 60 min of measurement. These studies are important for pushing the potential applications of Zn-doped In₂O₃ nanostructures in gas sensing and/or for waste water treatments.

2 Experimental

2.1 Material synthesis

All the chemicals were of analytical grade reagents without any further purification. In a typical synthesis, 1.06 mmol of InCl₃·4H₂O and 0.21 mmol of Zn(NO₃)₂·6H₂O was firstly dissolved in 60 mL of deionized water and ethylene glycol binary solvent (water : ethylene glycol = 1 : 1). Subsequently, 16 mmol of urea and 3.53 mmol of sodium citrate were added to the solution successively. Under vigorous stirring for 30 min, this transparent solution was transferred to a 100 mL Teflon-lined stainless steel autoclave, which was sealed and then heated at 200 °C for 16 h. After cooling down to room temperature in air, the obtained precipitates were collected by centrifugation and washed several times with deionized water and ethanol, followed by drying at 60 °C for 24 h. The Zn-doped In₂O₃ porous spheres were obtained by annealing the sample at 400 °C for 2 h in a muffle furnace. An undoped In₂O₃ sample was obtained by the same process except for the addition of Zn(NO₃)₂·6H₂O.

2.2 Characterization and testing

X-ray powder diffraction (XRD) patterns of as-prepared samples were analyzed with a German X-ray diffractometer (D8-Advance, Bruker AXS, Inc., Madison, WI, USA) equipped with Cu K_α radiation (λ = 0.15406 nm). The morphologies of the as-synthesized products were observed by a field emission scanning electron microscope (FESEM; FEI QUANTA FEG250, FEI, Hillsboro, USA), equipped with energy dispersive X-ray spectroscope (EDX, INCA MAX-50). X-ray photo-electron spectroscopy (XPS) was performed on a Thermo ESCALAB 250XI electron spectrometer equipped with Al Kα X-ray radiation (hν = 1486.6 eV) as the source for excitation.

Gas sensors were fabricated as follows: the as-synthesized Zn-doped In₂O₃ samples were mixed with deionized water and a paste formed. Next, the paste of the obtained sample was coated onto an Al₂O₃ tube to form a thick film between two parallel Au electrodes. A Ni–Cr coil heater was used to keep the sensor at the operating temperature. The gas sensor properties were measured by a gas sensing test system (WS-60A, Weisheng Electronics, Zhengzhou, China) under laboratory conditions (25% ± 10% RH, 23 ± 1 °C). The schematic electronic circuit of the test system is shown in Fig. 1. In the circuit, V_H is the heating voltage, R_L is the constant resistance (470 KΩ), V_RL is the voltage on R_L and V_C is the constant voltage (5 V) applied to the R_L and the sensor. A given amount of target gas was injected into the test chamber; when the response reached a constant value, the upper cover was removed and the sensor began to recover. The sensor response (S) is defined as S = R_a/R_g, where R_a and R_g are the sensor resistances in air and in the target gas. The response and recovery times are defined by the time spent on the sensor to achieve 90% of the total resistance change in the case of adsorption and desorption, respectively.

Fig. 1 The electronic circuit of the gas sensor measurement system. V_H is the heating voltage, R_L is the constant resistance (470 KΩ), V_RL is the voltage on R_L and V_C is the constant voltage (5 V) which is applied on the R_L and the sensor.

Photocatalytic degradation of organic dyes (MB, MO and RhB) was carried out in the presence of the products as photocatalysts under a mercury lamp (500 W) illumination. Generally, a glass reactor with 25 ± 2 °C circulating water flowing outside was employed for the secondary test group. For each test, 0.05 g of the sample was added to 50 mL of organic dye aqueous solutions (15 mg L⁻¹). The solutions were stirred for 40 min in the dark to establish the adsorption–desorption equilibrium. Then, the solutions were exposed to UV irradiation. During the irradiation, a 3 mL sample of the reaction suspension was taken every 10 min and centrifuged at 8000 rpm for 6 min. The photocatalytic degradation was estimated by measuring the absorbance of the dye solution in the presence of a photocatalyst at different time intervals. The degradation efficiencies of organic dyes were analyzed by monitoring the dye decolorization at the maximum absorption wavelength using a UV-vis spectrophotometer (TU-1901).

3 Results and discussion

The structures and morphologies of the ZIO NS samples have been characterized by SEM and the results are shown in Fig. 2. The low magnification SEM image indicates that the products possess well-dispersed spherical structures with uniform diameters in the range of 350–400 nm (Fig. 2a and b). Fig. 2b reveals the rough surface of an isolated sphere. It is speculated that the architecture was composed of small nanoparticles with dimensional sizes from several tens of nanometers to a hundred nanometers. Numerous 0D-nanoparticles arranged irregularly and self-assembled to form a structure of uniform spheres. EDX analysis (Fig. 2c) demonstrates that the uniform nanoscale spherical structures are composed of Zn, In and O elements. The peak of Zn can be clearly observed in the spectrum and the content ratio of Zn and In is about 2.10 : 15.64. Fig. 2d shows typical XRD patterns of the as-prepared Zn-doped In₂O₃ architectures. All the diffraction peaks could be well indexed to the tetragonal rutile structure of h-In₂O₃, which was consistent with the standard data file (JCPDS file no. 21-0406).^26,27

Fig. 2 (a and b) Low and high magnification SEM images; (c) EDX spectra and (d) XRD pattern of Zn-doped In₂O₃.

The XPS spectra of Zn-doped In₂O₃ are depicted in Fig. 3 to further confirm the composition and the elemental states. From the high resolution XPS spectrum in the In 3d_3/2 and In 3d_5/2 binding energy region, the two main peaks located at about 451.62 eV and 444.07 eV were in agreement with the reported value of In₂O₃,^11,28,29 respectively, which confirms the main composition of the structure is In₂O₃. The O 1s region shown in Fig. 3b could be fit into two main peaks locating at 529.47 eV and 531.27 eV, which were ascribed to the lattice oxygen of In₂O₃ and to surface-absorbed oxygen species.¹¹ In the Zn 2p spectrum (Fig. 3c), there are two peaks located at 1022.07 and 1045.17 eV, which are assigned to Zn 2p_3/2 and Zn 2p_1/2, respectively, and are characteristic of Zn²⁺, indicative of the oxidation state of Zn in the Zn-doped In₂O₃.

Fig. 3 XPS spectra of Zn-doped In₂O₃: high-resolution XPS spectra of (a) In 3d, (b) O 1s and (c) Zn 2p.

In order to find the optimum working temperature of our samples, gas sensing experiments were firstly operated at different temperatures in the range of 200–400 °C. Fig. 4 shows the responses of the ZIO NS sensors to 50 ppm TEA at different operating temperatures. It is observed that the response increases with increments of the working temperature before achieving a maximum. When the temperature is 280 °C, the sensor exhibits a maximum response of 36 at 50 ppm TEA, which is higher than that of pristine In₂O₃, and then decreases rapidly with increasing temperature. The optimal operating temperature we reported here is as low as 280 °C, about 70–80 °C less than in other previous reports,^30–32 which can lead to a lower power consumption. This phenomenon can be attributed to the balance between the formation of O⁻ ions (formation rate increases with a temperature increase) and the adsorptive reaction (reaction rate decreases with a temperature increase). When the working temperature is higher than 280 °C, with an increase in operating temperature, the rate of desorption is much higher than that of adsorption, limiting the reaction between the adsorption oxygen and target gas molecules and further reducing the response.³³

Fig. 4 Relationship between working temperature and response at 50 ppm TEA gas.

The selective detection of target gases remains a challenge and plays a major role in the applications of metal oxide semiconductor based gas sensors.³⁴ In order to identify their selectivity, the cross response properties were examined by exposing the sensor to 50 ppm TEA and other gases like trimethylamine (TMA), mono ethylene glycol (MEG), iso-propyl alcohol (IPA), ethanol, acetone, methyl alcohol, ammonia water and 500 ppm water at 280 °C, as summarized in Fig. 5. It was seen that the response of the ZIO NS sensor to 50 ppm TEA is much higher than those to other gases. The response can reach up to 36, which was about more than twice that of other gases, indicating an excellent selectivity to TEA. The results shown in Fig. 5 also confirm that the ZIO NS sensor has a higher sensitivity to all of the test gases than pristine In₂O₃. That different kinds of gases have different responses under the same conditions can be attributed to different gases possessing different inherent energy levels for adsorption, desorption and reaction with the active sites on the surface of the sensing materials.³⁵

Fig. 5 Selectivity of three sensors for different target gases with same concentration at 280 °C.

Fig. 6a shows the dynamic response and recovery curve of the ZIO NS sensor to TEA with variable concentrations from 5 ppm to 500 ppm at 280 °C. With increasing TEA concentration, the responses of sensors increase gradually. The gradually increasing response reaches up to 267 when the TEA concentration is 500 ppm. This also indicates that the detection limit could be down to 5 ppm-level with a response of about 4.92. Compared with the performance of the sensors to TMA, as shown in Fig. 6b, TEA gas is more active than TMA, which leads to a higher response than for TMA. The stability of the ZIO NS gas sensor over one month was also checked as shown in Fig. 6c. Clearly, the sensors show a nearly constant response to 50 ppm TEA, which indicates a high stability of the sensor devices. Compared with TMA gas, the sensitivity increases twofold while maintaining a fast response speed (Fig. 6d). The response and recovery times of the ZIO NS sensor are 9 and 7 s, respectively. The sensors recover extremely faster after the response returns 90% of the response variation, which could be explained as follows. When the sensor is exposed in air again after the reaction with TEA gas, oxygen molecules will diffuse into the surface, so the complete desorption reaction of outer space takes less time. Therefore it leads to a shorter recovery time. A comparison between the sensing performance of the Zn-doped In₂O₃ sensor and some other gas sensors previously reported is summarized in Table 1. As can be seen, the Zn-doped In₂O₃ sensor has a lower working temperature and a higher response than others,^36–39 which confirms that ZIO NSs have an obvious advantage over the others for TEA gas sensing.

Fig. 6 Zn-doped In₂O₃ gas sensor: (a) responses to TEA of different concentrations, (b) responses to TMA of different concentrations, (c) the response repeatability of 50 ppm TMA and TEA, (d) response and recovery times at 50 ppm TEA and TMA gas at an operating temperature of 280 °C.

Table 1 Sensing properties of ZIO NSs and other reported gas sensors working under different operating temperatures

Sensing material	TEA (ppm)	Response (Ra/Rg)	Temperature at the maximum response (Tms, °C)	Reference
Mesoporous α-Fe2O3 microrods	100	11.8	275	36
ZnO foam structure	100	79.5	350	30
SnO2 flowerlike	100	4	350	31
V2O5 hollow sphere	100	7.2	370	32
TiO2 nanorod	100	4.9	290	37
Zn2SnO4	100	37	200	38
Our work	50	36	280	Present study

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Photocatalytic activities were evaluated by the degradation tests of MB, MO and RhB. Fig. 7a shows the spectrum of the MB dye at different time intervals. The characteristic absorption peaks at 625 nm decrease quickly with the increase of the illumination time. MB is degraded about 90% when the irradiation time is 60 min. However, the degradation of MO is slower, as shown in Fig. 7b. From Fig. 7c, it can be seen obviously that ZIO NSs exhibit a more excellent photocatalytic activity toward MB with the degradation efficiency of 90% than that of In₂O₃. In order to illustrate the selectivity difference of the Zn-doped In₂O₃ catalysts among those dyes, the degradation efficiencies are compared for different dyes in 60 min as shown in Fig. 7d. The results show that the degradation efficiencies of MB, MO, and RhB are about 90, 20, and 35% in 60 min, respectively. The changes in the organic pollutant concentrations under ultraviolet irradiation are calculated by the equation I = (c/c₀) × 100%, where c₀ is the initial concentration of the organic pollutants, while the real time concentration of organic pollutants under ultraviolet light irradiation is expressed by c. The photocatalytic efficiency derived from the changes in the organic dye concentration can be represented by the relative ratio c/c₀, which obviously demonstrates that the Zn-doped In₂O₃ catalysts have an excellent selectivity on the photocatalytic degradation of MB dye.

Fig. 7 (a) Absorption spectra of MB solutions in the presence of irradiation using a UV lamp for different times; (b) absorption spectra of MO solutions, (c) comparison of photocatalytic degradation rate of MB, (d) photocatalytic degradation rate of MB, MO, and RhB.

When In₂O₃ is illuminated by visible light, the photoelectrons would be excited from the valance band to the conducting band and then a transient photocurrent is generated. However, in most instances, the valence band holes and conduction band electrons simply recombine liberating heat or light, a process known as recombination, which is responsible for the low photon conversion efficiency.^40,41 On the basis of the photocatalytic mechanism (Fig. 8), hydroxyl radicals (OH⁻) are generated from the oxidization of H₂O by photogenerated holes (h⁺) under UV irradiation. When ZIO NSs are irradiated, they are stimulated to produce h⁺ and e⁻. Then, h⁺ is trapped by H₂O at the catalyst surface to yield hydroxyl radicals (OH⁻) with high oxidability and reactivity. Here, remarkable photocatalytic activities of Zn-doped In₂O₃ architectures may be attributed to the uniform nanoscale spherical structure. The structure can provide more active sites to adsorb reactive species and O₂. This means that the structure can improve the photon application efficiency, which leads to a high photocatalytic efficiency. These different degradation ratios may be attributed to different molecular structures of the three dyes, which result in different degradation mechanisms.

Fig. 8 Schematic of the photocatalytic mechanism of the as synthesized Zn-doped In₂O₃ nanostructure.

4 Conclusions

In summary, a simple synthesis route was developed for the fabrication of monodispersed and uniform Zn-doped In₂O₃ nano spheres, which were constructed by many small nanocrystallites with the help of citric acid and urea. Morphologies, microstructures, and compositions of the samples were characterized by a series of techniques. The gas sensing tests exhibited that the ZIO NSs showed a superior gas sensing performance toward TEA, in comparison to the previous reports, which was mainly attributed to their structure. ZIO NSs were used for highly sensitive and selective TEA gas sensing, in which the sensor response can reach up to 36 at 50 ppm gas, and the detection limit can reach down to the 5 ppm level at an operating temperature as low as 280 °C. ZIO NSs were also used as an efficient photocatalyst for the degradation of MB, MO and RhB, in which the ZIO NSs exhibited much higher degradation efficiency for MB (90%) than those for MO (20%) and RhB (35%). This work demonstrates that Zn-doped In₂O₃ nano spheres (ZIO NSs) have a potential application in TEA gas sensing and photocatalytic degradation of an MB dye.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 61205175, 61575081 and 11304120), the Encouragement Foundation for Excellent Middle-aged and Young Scientist of Shandong Province, China (Grant No. BS2011CL008) and the Doctoral Foundation of University of Jinan, China (Grant No. XBS0920).

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