The preparation of Cr₂O₃@WO₃ hierarchical nanostructures and their application in the detection of volatile organic compounds (VOCs)

Abstract

Cr₂O₃@WO₃ and WO₃ hierarchical nanostructures are prepared using a two-step water bath method, which just needs mild conditions and takes little time. The response to xylene of the Cr₂O₃@WO₃-based sensor is much higher (about 4 times) than that of a WO₃-based sensor, and both of the response and recovery processes are quick. As for the selectivity, the Cr₂O₃@WO₃-based sensor shows a better response to xylene, while the WO₃-based sensor shows a better response to ethanol. The reason for the response and selectivity change is considered to be that a heterostructure is formed between Cr₂O₃ (a P-type semiconductor) and WO₃ (an N-type semiconductor). Moreover, Cr₂O₃ is reported to show catalytic oxidation of methyl groups.

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1. Introduction

We mostly breathe indoor air and volatile organic compounds (VOCs) including formaldehyde, ethanol, acetone, xylene, etc. are typical pollutants of indoor air. Even a small concentration of VOCs can damage the health of the human body and a big accumulation can increase the risk of cancer or death. For example, formaldehyde, a common raw material in the timber industry, textile industry, and anticorrosive industry, is also a carcinogenic source which is often released from new furniture and can irritate the human skin, eyes and throat, and even threaten reproductive performance. Therefore, monitoring the content of VOCs in indoor air is important for people’s living standards. A gas sensor based on a metal oxide semiconductor (MOS) stands out from many traditional air monitoring techniques for its advantages such as small size, easy-operation and low-price. As functional materials, MOSs including ZnO,^1–5 SnO₂,^6–9 Fe₂O₃,^10–12 CuO,^13–15 TiO₂,^16–18 Co₃O₄,^19–21 etc., play such a significant part in many fields due to their superior character, cheapness and easy accessibility, that many experimental and theoretical efforts have been made on such research. For example, in the gas sensing field, Qu et al. prepared Cu₂O–Co₃O₄ core–shell composites²² and Fe₃O₄@Co₃O₄ core–shell microspheres²³ to detect ethanol and acetone.

In the past few years, with the rapid development of nanotechnology, nanomaterials have been increasingly applied in the field of sensors for their superior properties including a large surface to volume ratio and excellent chemical and electrical performances, so related reports have emerged. As for the study of WO₃, many researchers have demonstrated its excellent photocatalytic,^24–28 electrocatalytic,^29,30 and electrochromic properties,^31–35 and it has also been applied to many fields, for example as an absorbing material in the solar energy field,^36,37 a stealth material in the military field, and a significant anticorrosive and colorant material for industrial production. In recent years, the investigation of WO₃’s gas sensing character has also expanded, such as WO₃ hollow spheres for NO₂ detection,³⁸ TiO₂(B) nanoparticle-functionalized WO₃ nanorods for acetone detection,³⁹ TiO₂–WO₃ nanocomposites for benzene detection,⁴⁰ hierarchical Fe₂O₃@WO₃ for H₂S detection,⁴¹ and so on. All these reports give a promising prospect of WO₃ for scientific and practical applications. As for Cr₂O₃, there are some articles reporting its significant effect as a catalyst,^42–44 among which the catalytic oxidation of methyl groups is attractive,⁴⁵ and it provides an idea for improving the performance of a material.

It should be noted that the response of an oxide semiconductor sensor to ethanol is known to be usually relatively high,⁴⁶ compared to the responses to other VOCs. But here we want to obtain a sensor whose response to xylene or toluene is higher than that to ethanol with a quick response/recovery speed. In this work, WO₃ was synthesized via a water bath method, and then Cr₂O₃ was developed on the surface of WO₃ through a second water bath method. As far as we know, such novel Cr₂O₃@WO₃ hierarchical nanostructures obtained using a simple method have rarely been reported. For further understanding of their gas sensing properties, a series of experiments were carried out. The result of the gas sensing properties test is not far from our expectation: the sensor made using pure WO₃ shows a high response to ethanol, and the Cr₂O₃@WO₃ sensor to xylene. For the selectivity change, both the conformation of the PN junction between Cr₂O₃ and WO₃, and the catalytic oxidation of the methyl group for the joining of Cr in the system, are considered to be significant factors which bring about the promotion of the response to xylene and toluene.

2. Experimental

2.1 Chemicals

Sodium tungstate dehydrate (Na₂WO₃·2H₂O, Xilong Chemical Reagent Co.), oxalic acid dehydrate (H₂C₂O₄·2H₂O, Beijing Chemicals Co.), hydrochloric acid (HCl, Beijing Chemicals Co.), and chromium nitrate nonahydrate (Cr(NO₃)₂·9H₂O, Xilong Chemical Reagent Co.) were all of analytical grade and used without any further purification.

2.2 The preparation of Cr₂O₃@WO₃ hierarchical nanostructures

Firstly, the WO₃ nanostructures were prepared using a water bath method. Na₂WO₃·2H₂O (10 mmol) was dissolved in deionized water (60 ml), and stirred to form a clear solution, then the pH value was adjusted to 1.5 with HCl, followed by the addition of H₂C₂O₄·2H₂O in a ratio of 1 : 1. The mixture was stirred for 30 min and then transferred to a beaker (100 ml). The beaker was placed in a water bath pot to be heated and maintained at 92 °C for 2 h. After that, a yellow precipitate was obtained in the beaker. When cooled to room temperature, the precipitate was centrifuged and washed with deionized water and ethanol 2 or 3 times. Finally, the precipitate was dried at 60 °C overnight and then calcined at 300 °C for 2 h in air.

Secondly, the Cr₂O₃@WO₃ hierarchical nanostructures were synthesized via a water bath route following a reported method with some modification.²⁵ To obtain the Cr₂O₃@WO₃ hierarchical nanostructures, 0.1 g of the previously obtained WO₃ powders were dispersed into 100 ml of deionized water under vigorous stirring. Hereafter, 20 ml of the fresh Cr(NO₃)₃·9H₂O solution (3 g l⁻¹) and 0.02 g of hexamethylenetetramine were added into the suspension. The above mixture was kept at 80 °C for 2 h with stirring. Then the precipitate was separated using centrifugation, and washed with deionized water and ethanol. Finally, the sample was dried at 60 °C for 24 h, and then calcined at 400 °C for 5 h in air.

2.3 Sample characterization

The X-ray diffraction (XRD) analysis was conducted on a Scintag XDS-2000 X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were performed on a SHIMADZU SSX-550 (Japan) instrument. Transmission electron microscopy (TEM) was conducted on a FEI Tecnai F20. The surface area was evaluated via the Brunauer–Emmett–Teller (BET) method (Micromeritics Gemini VII 2390).

2.4 Gas sensor fabrication and response test

The as-prepared sample was mixed with deionized water in an approximate weight ratio of 100 : 25 and ground in a mortar to shape it into a paste. Then the paste was coated on a ceramic tube to form a sensing film (with a thickness of about 300 μm). The ceramic tube had two Au electrodes previously printed, each of which was attached using a Pt lead wire and a spiral heating wire made of Ni–Cr alloy inserted through the hollow centre of the ceramic tube to control the operating temperature. The structure of the device is shown in Fig. 1. The completed sensor was aged by heating it for two days.

Fig. 1 The schematic structure of the gas sensor.

2.5 The gas sensing measurement system

All the gas sensing performance data of the samples were obtained from the gas sensing test using a Chemical gas sensor-8 intelligent gas sensing analysis system (Beijing Elite Tech Co., Ltd., China) under ambient conditions (about 20 °C, 40 RH%). The values of the sensor resistance in air (R_o) and in the detecting atmosphere (a mixture of air and the target gas in a certain concentration) (R_g) were acquired by the analysis system automatically. The response value (R) was defined as R = R_o/R_g (N-type semiconductor). The time taken for the sensor to achieve 90% of the total resistance change was defined as the response time (target gas adsorption time) or the recovery time (target gas desorption time).

The measurement process of the gas sensing properties is similar to previous reports.^22,47 Typically, a certain volume of the organic vapour of the sample under standard atmospheric pressure was introduced using an injecting needle into the testing chamber. After a while, when the sample gas diffused uniformly in the testing chamber, a response step followed. The detected concentration of the gases inside the testing chamber was determined using the concentration and injected volume of its saturated vapour. Once the gas sensing measurement was over, the gases in the testing chamber were finally released by inputting fresh air.

3. Results and discussion

3.1 Characterization of the Cr₂O₃@WO₃ hierarchical nanostructures

The phase and crystallinity of the powders were analysed using X-ray diffraction (XRD), as shown in Fig. 2.

Fig. 2 The XRD patterns of the Cr₂O₃@WO₃ hierarchical nanostructures.

The curve exhibits sharp diffraction peaks, which indicate the good crystallization of the WO₃ hierarchical nanostructures. All the diffraction peaks can be indexed to WO₃ and Cr₂O₃, which agree with the reported values from the Joint Committee on Powder Diffraction Standards cards (JCPDS file no. 38-1479 and JCPDS file no. 20-1324, respectively). There are no other clear peaks coinciding with other peaks from impurities. These results confirm that the sample has a relatively high crystal purity.

The microscopic morphology and structure of the samples were observed using scanning electron microscopy (SEM). Fig. 3(a) and (c) show the low-magnification SEM images of the pure WO₃ and the Cr₂O₃@WO₃ hierarchical structures, both of which exhibit uniform particle sizes. Fig. 3(b) and (d) show the corresponding high-magnification SEM images. Both of the samples appear to be flower-like particles with a diameter of about 5 μm, and their petals, the secondary structure of the particle, are thin sheets with a thickness of about 100 nm. This microscopic structure has a large surface to volume ratio, thus the target gas can come into contact with the sensing material as much as possible. When comparing Fig. 3(b) and (d), it can be found that the joining of Cr₂O₃ introduces little change in the surface morphology of the material, so Cr₂O₃ is thought to grow on the surface of the WO₃ sheet in the form of a nanoparticle, and the element distribution shown in Fig. 4 provides evidence for this. But Cr₂O₃ is not observed clearly because its ratio is small. The distribution of W and O is crowded, while Cr is scarce. The spectrum shown in Fig. 4(e) shows the peaks of every element, and the result of intelligent quantitative analysis shows that the atom ratios of O, Cr and W are 77.61%, 1.63% and 20.76%, respectively. It can be figured out that the mole ratio of Cr₂O₃ : WO₃ is about 4%, and it is reasonable that the content of O is a little high, because the material attracts oxygen from air. For the macroscopic character, the two materials grow as nearly the same yellow powders, but the colour of the Cr₂O₃@WO₃ powder is lighter than the other.

Fig. 3 (a and b) The low-magnification and high-magnification SEM images of the WO₃ hierarchical nanostructures; (c and d) the low-magnification and high-magnification SEM images of the Cr₂O₃@WO₃ hierarchical nanostructures.

Fig. 4 (a) The SEM image of the Cr₂O₃@WO₃ hierarchical nanostructures; (b–d) the EDX element maps and (e) the EDX spectrum of the Cr₂O₃@WO₃ hierarchical nanostructures.

For characterizing the microscopic morphology of Cr₂O₃, the TEM image and HRTEM image of the Cr₂O₃@WO₃ hierarchical nanostructures are shown as Fig. 5(a) and (b). The whole size of the particle is too large for transmission scanning, so only the TEM image of a petal is given here. The HRTEM image shows the lattices of Cr₂O₃ and WO₃, and according to the lattice spacing, Cr₂O₃ is distinguished and marked on Fig. 5(b).

Fig. 5 (a) The TEM image of the Cr₂O₃@WO₃ hierarchical nanostructures; (b) the high-resolution TEM image of the Cr₂O₃@WO₃ hierarchical nanostructures.

In order to study the sample’s surface area, the BET surface area was determined by measuring the corresponding nitrogen adsorption–desorption isotherms, which are shown in Fig. 6. It is observed at a high relative pressure that the curve exhibits a type IV isotherm with an H3 hysteresis loop according to the Brunauer–Deming–Deming–Teller (BDDT) classification. The BET surface area of the Cr₂O₃@WO₃ hierarchical nanostructures is determined to be 21.6 m³ g⁻¹.

Fig. 6 Typical N₂ adsorption–desorption isotherms.

3.2 Gas sensing performance

It is well known that the response of a semiconductor sensor is usually dependent on the sensor’s operating temperature. The response of the sensors based on the pure WO₃ or the Cr₂O₃@WO₃ hierarchical nanostructures to 100 ppm of the target gas was tested to determine the optimum operating temperature, as shown in Fig. 7(a). It can be observed that the responses of the sensors varied with the operating temperature. For xylene detection, the optimum operating temperatures of the sensors based on Cr₂O₃@WO₃ and WO₃ were both suggested to be 300 °C and the maximum response of the Cr₂O₃@WO₃-based sensor was 26.60, which was about four times higher than that of the WO₃-based sensor (6.68).

Fig. 7 (a) Response of the sensors based on the as-prepared hierarchical nanostructures to 100 ppm of xylene as a function of the operating temperature. (b) Response of the sensors to 100 ppm of various gases at 300 °C. (c) Response transients of the sensors varying from 1 ppm to 1000 ppm. (d) Response of the sensor based on the Cr₂O₃@WO₃ hierarchical nanostructures versus different xylene concentrations at 300 °C.

The selectivity of the sensors to several interference gases with concentrations of 100 ppm is shown in Fig. 7(b). It can be seen that the sensor based on the pure WO₃ exhibited a very high response (61.00) to ethanol, which was about 4 and 8 times higher than those to acetone and xylene, respectively. While the sensor based on Cr₂O₃@WO₃ displayed a good selectivity for xylene, to which the response increased to 26.00, at the same time, the response to ethanol dropped to 8.10 which is closely related to the presence of Cr₂O₃.

It is of great significance for practical sensors to have a fast response/recovery rate. Fig. 7(c) shows the absorption and desorption behaviours of the two sensors in xylene at different concentrations at 300 °C. All the absorption processes last about 5 s (response time), while the desorption processes last about 20 s (recovery time). This result demonstrates that the sensors have good response/recovery speed properties. In addition, compared with the WO₃-based sensor, the Cr₂O₃@WO₃-based sensor always exhibits a higher response to the same concentration of xylene. The introduction of Cr₂O₃ seems to promote some concern reactions about xylene, such as the oxidation of methyl linked to benzene ring.

Fig. 7(d) shows the response of the sensor based on the Cr₂O₃@WO₃ hierarchical nanostructures versus the different xylene concentrations at 300 °C. According to the chart, we can observe that the response increases with an increase in the xylene concentration from 1 to 1000 ppm. When the concentration is over 100 ppm, with a further increase in xylene concentration, the response of the sensor tends to saturate gradually. A linear relationship (y = 0.24265x + 1.3547) between the response and the xylene concentration in the range of 1 to 100 ppm can be built. The detection limit of xylene for the sensor based on the Cr₂O₃@WO₃ hierarchical nanostructures is estimated to be 3 ppm, when the criterion for gas detection is set to R_a/R_g > 2.

3.3 A possible gas sensing mechanism

As mentioned above, the WO₃ hierarchical nanostructures show a selective response to ethanol, while the Cr₂O₃@WO₃ nanostructures to xylene. In general, it depends on the change of the surface resistance of the material due to the different contactants that MOS sensors respond to in the gas. WO₃ is an N-type semiconductor, and when exposed to air, it will adsorb oxygen, which will transform into oxygen ions (O⁻, O²⁻ or O₂²⁻) by means of seizing electrons from the conduction band of WO₃. The main chemical equations involved in the process are shown as follows:⁴⁸

O_2(gas) ↔ O_2(ads) (1)

O_2(ads) + e⁻ ↔ O_2(ads)⁻ (2)

O_2(ads)⁻ + e⁻ ↔ 2O_(ads)⁻ (3)

O_(ads)⁻ + e⁻ ↔ O_(ads)²⁻ (4)

The processes above generate a depletion layer on the surface of WO₃, which decreases the conductivity of the material, or namely, brings about a high-resistance state. When WO₃ is exposed to an ethanol atmosphere, an electronic exchange will arise between the ethanol molecules and WO₃ or the oxygen ions attached to WO₃. So, a conducting channel emerges and a low-resistance state appears. The possible main chemical equations are listed as follows:⁴⁹

C₂H₅OH_(vap) + W_(lattice) → W–C₂H₅O_(ads)⁻ + H_(ads)⁺ (5)

2H_(ads)⁺ + O_(ads)⁻ → H₂O_(vap) (6)

C₂H₅O_(ads)⁻ + H_(ads)⁺ → C₂H_4(vap) + H₂O_(vap) (7)

C₂H₅OH_(ads) → C₂H_4(vap) + H₂O_(vap) (8)

C₂H₅OH_(ads) + O_(ads)⁻ → CH₃CHO_(ads) + H₂O_(vap) + e⁻ (9)

H₃CHO_(ads) + O_(ads)⁻ → CH₃COOH_(vap) + e⁻ (10)

Cr₂O₃ is a P-type semiconductor, and when it is developed on the surface of the WO₃ hierarchical nanostructures, a PN junction automatically appears. Electrons from WO₃ go to Cr₂O₃ via diffusion, thus the concentration of the major carriers (electrons) of WO₃ decreases, a barrier forms and the depletion on the surface of WO₃ expands (when it is balanced) which causes a high-resistance state. Thus, the Cr₂O₃@WO₃-based sensor has a higher inherent resistance than the pure WO₃. When reacting with ethanol, the magnitude of the reduction in the resistance of Cr₂O₃@WO₃ appears to be small compared to its inherent resistance, that is to say, the response of Cr₂O₃@WO₃ to ethanol becomes low. In addition, the covering of the Cr₂O₃ particles narrows the surface area of WO₃, but the reaction between Cr₂O₃ and ethanol is far less efficient than that between WO₃ and ethanol, even inefficient.

Table 1 gives some parameters related to WO₃ and Cr₂O₃, according to which a possible electronic transport schematic diagram is drawn, shown in Fig. 8. The methyl connected to the benzene ring is oxidized by the oxygen ions attached to the surface of the Cr₂O₃@WO₃ hierarchical nanostructures. At the same time, the electrons generated in the above process are captured by Cr₂O₃, and then passed to the conduction band of WO₃. Thus the electron concentration of WO₃ becomes larger than before, and as an N-type semiconductor, the resistance of WO₃ goes down obviously so the response appears to be higher.

Table 1 The conduction band and valence band values of WO₃ and Cr₂O₃

	χ	Eg	Ec	Ev
WO3	6.59 eV	2.70 eV	−5.24 eV	−7.74 eV
Cr2O3	5.68 eV	3.50 eV	−3.93 eV	−7.74 eV

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Fig. 8 A schematic showing the possible electron transport in the detection of methyl benzene for the sensor based on Cr₂O₃@WO₃.

Besides, Cr₂O₃ is reported to have a good performance in the catalytic oxidation of methyl groups,⁴⁵ which should be considered to be the significant reason for the change in selectivity. The Cr₂O₃, scattered on the surface of WO₃, provides many active spots, decreases the required energy, and prompts the oxidation process of the methyl groups. When considering why Cr₂O₃ does not enhance the response of ethanol since the latter also has a methyl group, maybe it does, but more weakly than WO₃, and for that matter, the active spots from Cr₂O₃ just amount to passive spots for ethanol. A difference exists inevitably, and it is thought to result from the distinction between the alcohol group and the benzene ring. The plane structure and the π-conjugate system of the benzene ring makes it more easily adsorbed than ethanol. There has already been a report referring to this phenomenon that the introduction of another material causes a change in selectivity.⁴⁷

The reaction of xylene is complex, and here are some possible equations:

In addition, a flower-like hierarchical structure makes the gas sensor more sensitive for the reason that it is easy for a gas to diffuse into the film via molecular diffusion.⁵⁰ Moreover, the hierarchical structure introduces some macropores, which make the adsorption and desorption processes of gas molecules faster, so the sensor made using materials with hierarchical nanostructures often has short response and recovery times.^51,52

4. Conclusions

In summary, WO₃ and Cr₂O₃@WO₃ hierarchical nanostructures were successfully prepared via a facile two-step water bath process and their sensing properties were investigated. The introduction of Cr₂O₃ brings a great improvement in the response to xylene. In short, the presently constructed Cr₂O₃@WO₃ hierarchical nanostructures have potential in the gas sensor field, and the means of doping to improve performance has a promising future as it can provide more materials meeting particular demands.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 61274068), Chinese National Programs for High Technology Research and Development (Grant No. 2013AA030902), Project of Science and Technology Plan of Changchun City (Grant Nos 14KG020 and 14KG019) and the Opened Fund of the State Key Laboratory on Integrated Optoelectronics (No. IOSKL2013KF10).

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