Three-dimensional ordered macroporous In₂O₃-supported Au for high-performance ethanol sensing

Abstract

Au-loaded three-dimensional ordered macroporous In₂O₃ (Au-3DOM In₂O₃) was successfully prepared by a colloidal crystal template method, which exhibited high ethanol-sensing response and improved selectivity due to its unique skeleton structure and highly dispersed Au nanoparticles.

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Conductometric gas sensors have promising applications in breath analysis and volatile organic compound (VOC) monitoring due to their low cost, easy portability, power-efficiency, high sensitivity and fast response/recovery time.^1–5 Developing highly efficient sensing nanomaterials is significant for realizing a high-performance conductometric gas sensor. Controlling the shape and size of the sensing nanomaterials is an important approach used for the improvement of sensor property. Different sensing nanomaterials with diverse morphologies have been synthesized and used in gas sensor, which exhibit excellent sensing performance.^6–11

Recently, much attention has been paid to well-ordered macroporous nanomaterials because of their periodically ordered/highly interconnected porous structure, controllable pore diameter and large surface area. Theoretical and experimental results indicated that the well-ordered macroporous materials exhibited interesting properties used as electrode materials, optical sensors and catalysts.^12–17 The periodic macroporous architectures of three-dimensional ordered macroporous (3DOM, a kind of inverse opal) Mo:BiVO₄ can provide long-range-ordered paths for faster electron transport throughout the electrode and further optimize the photoelectrochemical performances of BiVO₄.¹² Well-ordered macroporous materials also facilitated the ion transport phenomena and provided unprecedented membrane opportunity for the development of high-performance batteries.¹³ 3DOM semiconductor is well-known for its excellent performance on modulation of light propagation. Well-ordered macroporous TiO₂ with novel and improved photonic band gap dependent SERS sensitivity was reported.¹⁴ 3DOM Pd–LaMnO₃ catalysts were successfully prepared by our group, which exhibits excellent catalytic activity for methane combustion due to the ordered porous structure and large surface area.¹⁵ However, there are few reports about well-ordered macroporous semiconductor oxide based gas sensing materials.¹⁸ In terms of sensing mechanism, well-ordered macroporous semiconductor oxides are promising candidates because their particular structures can usually greatly facilitate gas diffusion and electron transport, thereby improving the performance of conductometric gas sensor.

In addition, noble metal has been widely used in sensing materials to improve the sensing sensitivity and response/recovery time because it can catalyze molecule dissociation.^6,19–22 In this paper, 3DOM In₂O₃ supported Au (Au-3DOM In₂O₃) was synthesized for the first time, and their gas sensing properties were investigated. In₂O₃ is an important n-type semiconductor material and exhibits excellent performance in gas sensor.^23–27 The combination of unique 3DOM skeleton structure of In₂O₃ and Au nanoparticles is advantage to enhance their sensing performance. Our experimental results indicated that high ethanol sensitivity, improved selectivity and good reproducibility are characteristic for the Au-3DOM In₂O₃ based sensor.

3DOM In₂O₃ and Au-3DOM In₂O₃ were prepared by a colloidal crystal template method. The template was shown in Fig. S1.† The detailed synthetic processes are presented in the ESI.† The typical XRD patterns of the samples are shown in Fig. 1. The calculated lattice constants a = 10.112 Å for 3DOM In₂O₃ and 10.118 Å for Au-3DOM In₂O₃ are consistent with the standard values (a = 10.118 Å) of cubic In₂O₃ (JCPDS card no. 06-0416), which shows the good crystallinity of the products. The mean grain sizes of In₂O₃ are estimated using the Debye–Scherrer equation and the values are 22.62 nm and 21.78 nm for 3DOM In₂O₃ and Au-3DOM In₂O₃ respectively. The recording of weak diffraction peaks of Au was due to the low loadings and good dispersion of Au nanoparticles.

Fig. 1 XRD patterns of 3DOM In₂O₃ and Au-3DOM In₂O₃.

The SEM images of 3DOM In₂O₃ are shown in Fig. S2.† Fig. 2a and b show the SEM images of Au-3DOM In₂O₃. 3DOM In₂O₃ has well-ordered pores with an average diameter of ca. 140 nm and a wall thickness of ca. 25 nm. Fig. 2c and d show the TEM and high-resolution TEM images of Au-3DOM In₂O₃. A lot of Au nanoparticles with a size of ca. 5 nm were highly dispersed on the surface of 3DOM In₂O₃ (shown in red circle). In addition, there is no obvious change after NaBH₄ reduction treatment, indicating the good stability of the 3DOM skeleton structure. The high-resolution TEM images of Au-3DOM In₂O₃ reveals that the average distances between the adjacent lattice planes are 0.29 nm and 0.23 nm, which corresponds well to the interplanar spacing of the (222) crystal planes of the standard In₂O₃ sample and the (111) crystal plane of the standard Au sample, respectively.^28,29

Fig. 2 (a and b) SEM images with different magnifications and (c) TEM image of Au-3DOM In₂O₃. (d) High-resolution TEM image of Au-3DOM In₂O₃. The inset in (d) EDS pattern of Au-3DOM In₂O₃.

The EDS pattern (inset of Fig. 2d) shows the presence of In and Au in the samples. The presence of the Cu peaks in the EDS pattern comes from the copper grids used as a support of the samples. Furthermore, a practical content of Au in the samples was obtained by inductively coupled plasma mass spectroscopy (ICP-MS). The testing results indicate that the actual Au content of Au-3DOM In₂O₃ was 0.39 wt% which is a little decrease compared with the theoretical contents (0.50 wt%). The surface compositions of Au-3DOM In₂O₃ were also characterized by XPS as shown in Fig. 3. Fig. 3a is the characteristic spin–orbit split for In 3d_5/2 (444.3 eV) and In 3d_3/2 (451.8 eV) of trivalent indium. No variations in the In 3d energy position were recorded after the doping of Au species. Fig. 3b is the Au 4f XPS spectrum of Au-3DOM In₂O₃, which is featured by the recording signals at BE = 83.6 eV (Au 4f_7/2) and 87.2 eV (Au 4f_5/2). Fig. S3† shows the Gaussian decomposition spectra of oxygen species. The lowest bonding energy peak at ∼530 eV is ascribed to the lattice oxygen (OI) of In₂O₃ and the binding energy at ∼531 eV is ascribed to O⁻ and O₂⁻ ions (OII).³⁰ The highest peak at ∼532 eV is ascribed to chemisorbed or dissociated oxygen or OH species (OIII). The contents of the different oxygen species are shown in Table S1.† Compared with 3DOM In₂O₃, the OII content of Au-3DOM In₂O₃ is larger. When the sensing materials are exposed to ethanol, OII will react with ethanol molecule. Larger OII content will result in higher sensing response.

Fig. 3 XPS spectra of Au-3DOM In₂O₃: (a) In 3d and (b) Au 4f.

The combination of 3DOM metal oxides and activated Au nanoparticles makes the Au-3DOM In₂O₃ promising sensing material for gas detection. Here, the ethanol sensing abilities of Au-3DOM In₂O₃ were evaluated. The effects of operating temperature on their sensing performance were investigated. The responses of 3DOM In₂O₃ and Au-3DOM In₂O₃ to 100 ppm C₂H₅OH were tested as a function of operating temperature and the results are shown in Fig. 4a. The maximum responses of Au-3DOM In₂O₃ and 3DOM In₂O₃ appear at ca. 230 °C. This can be explained as follows: when the temperature is lower than 230 °C, the adsorbed ethanol molecules are not activated enough to overcome the activation energy barrier to react, while when the temperature is higher than 230 °C, the gas adsorption is too difficult to be adequately compensated for the increased surface reactivity.⁷ Previous literature reported that the responses of ordered mesoporous In₂O₃ and porous In₂O₃ nanospheres were 64 and 22 respectively.^23,27 The response of our synthesized 3DOM In₂O₃ is 75, which is higher than those of the ordered mesoporous In₂O₃ and the porous In₂O₃ nanospheres. After loading Au nanoparticles, the response is enhanced remarkably than those of pure In₂O₃. The differences in the sensing response presented by the two sensors should be directly related to the supported Au nanoparticles. Fig. 4b illustrates the response of 3DOM In₂O₃ and Au-3DOM In₂O₃ to 100 ppm of various gases, including ethanol, diethyl ether, acetone, isopropanol, cyclohexane, xylene and methanol at the operating temperature of 230 °C. Obviously, the responses of Au-3DOM In₂O₃ to seven gases are all improved compared with those of the pure 3DOM In₂O₃, and the largest increase is observed for ethanol, implying the good selective detection of the Au-3DOM In₂O₃ sensor to ethanol.

Fig. 4 (a) Responses of 3DOM In₂O₃ and Au-3DOM In₂O₃ to 100 ppm ethanol at different operating temperatures. (b) Selectivity of 3DOM In₂O₃ and Au-3DOM In₂O₃ sensors.

It is common knowledge that development of gas sensor with wide concentration detection range and good reproducibility is of practical interest. Fig. 5a shows the representative dynamic responses as a function of ethanol concentration. It reveals that the sensor maintains its initial response without a clear decrease and has a satisfying stability and reproducibility when the ethanol concentration is in the range of 5–100 ppm. Fig. 5b displays the dynamic response–recovery curves of Au-3DOM In₂O₃ for sensing 100 ppm ethanol. It is observed that the response and recovery time of the material are 15 s and 21 s, respectively.

Fig. 5 (a) Response to different concentration ethanol and (b) the response/recovery curve of Au-3DOM In₂O₃.

The ethanol-sensing properties of the noble metal–metal oxide based sensors reported in the literature were summarized in Table 1.^31–36 Table 1 indicates that the combination of 3DOM In₂O₃ and highly dispersed Au nanoparticles has great effect on its gas sensing properties. In general, our synthesized Au-3DOM In₂O₃ have higher sensing response at relative low working temperature. Therefore, Au-3DOM In₂O₃ is a good candidate for detecting low concentration of ethanol.

Table 1 Noble metal–metal oxide based gas sensors for 100 ppm ethanol detection

Materials	Noble metal content	Work temperature	Response	Ref.
Au/In2O3 nanofibers	0.2%	140 °C	6.5	31
In2O3/Au nanorods	—	400 °C	75	32
Au@In2O3 nanoparticles	—	160 °C	36.1	33
Pt/WO3 particles	2.36%	140 °C	87	34
Au/ZnO nanoplates	1.1%	300 °C	36	35
Au/ZnO nanorods	3.4%	380 °C	33.6	36
Au-3DOM In2O3	0.39%	230 °C	205	This work

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The response of the conductometric gas sensor is determined by the change of resistance and its sensing mechanism can be described by modulation model of the depletion layer.^7,37,38 The experimental results show that the resistance of Au-3DOM In₂O₃ decreases upon exposure to reduction gases, which is indicative of n-type behavior. The dominant charge carriers are electrons in Au-3DOM In₂O₃. When the sensor is exposed to air, oxygen molecule will adsorb on the surface of Au-3DOM In₂O₃ and form active oxygen by capturing electrons from the sensing materials. After the electrons are captured from the sensing materials, a depletion layer is formed, which results in the increase of sensor resistance. When the sensor is exposed to ethanol, the reaction between ethanol molecule and active oxygen will happen and release the trapped electrons back to the sensing materials, which leads to the decrease of sensor resistance (shown in Scheme 1).

Scheme 1 Schematic diagram of the proposed sensing mechanism of Au-3DOM In₂O₃ to ethanol: in air (left), in ethanol (right).

In order to explore the sensing mechanism, temperature-programmed surface reaction (TPSR) technique was used to investigate the dynamic reaction process of ethanol and sensing materials in situ. The changes of the main species with the increase of temperatures are displayed in Fig. 6, in which three temperature ranges are observed: 100–200 °C, 200–310 °C, 310–500 °C. When the temperature is below 200 °C, no obvious change happens. In the temperature range of 200–310 °C, the main products are acetaldehyde, CO₂ and H₂O. In the temperature range of 310–500 °C, a large amount of H₂ is detected for Au-3DOM In₂O₃, which may result from the dehydrogenation reaction of ethanol. In addition, CO and ethyl acetate (EA) are also detected because of the anaerobic reaction. According to the results above, the sensing mechanism is proposed: firstly, ethanol molecule would react with O⁻ and form acetaldehyde and H₂O (eqn (1)). Then, intermediate acetaldehyde could be further react with O⁻ and form CO₂ and H₂O (eqn (2)). The overall reaction can be depicted as eqn (3).

CH₃CH₂OH + O⁻ → CH₃CHO + H₂O + e⁻ (1)

CH₃CHO + 5O⁻ → 2CO₂ + 2H₂O + 5e⁻ (2)

CH₃CH₂OH + 6O⁻ → 2CO₂ + 3H₂O + 6e⁻ (3)

Fig. 6 TPSR curves of Au-3DOM In₂O₃.

Compared to pure 3DOM In₂O₃ (Fig. S4†), C₂H₅OH was oxidized to acetaldehyde at relatively low temperature on Au-3DOM In₂O₃, which is suggested as the first step of C₂H₅OH oxidation due to the Au nanoparticles on the In₂O₃ surface. Moreover, Au nanoparticles can serve as effective adsorption sites to bind and dissociate oxygen, which could result in high degree of electron extraction from Au-3DOM In₂O₃ and the formation of deep depletion layer. When Au-3DOM In₂O₃ was exposed to ethanol, the elections will be fed back to Au-3DOM In₂O₃ through eqn (3). In addition, the ordered macroporous structure facilitates gas diffusion and electron transport. Therefore, the combination of unique skeleton structure of In₂O₃ and Au nanoparticles resulted in the high-performance ethanol sensing of the Au-3DOM In₂O₃ sensor.

In summary, a novel sensing material of Au-3DOM In₂O₃ was successfully synthesized. Ordered macroporous structure facilitated gas diffusion and electron transport. Highly dispersed Au nanoparticles effectively catalyzed C₂H₅OH oxidation. Moreover, the Au nanoparticles could serve as adsorption sites to bind and dissociate oxygen, which further resulted in the formation of deep depletion layer. These reasons may result in the high-performance ethanol sensing of the Au-3DOM In₂O₃ sensor.

Acknowledgements

This work was financial support by the National Natural Science Foundation of China (No. 21275016, 21575011).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Materials preparation, characterization, gas-sensing measurements. See DOI: 10.1039/c5ra19525j

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